Volume 17 Number 21 7 June 2015 Pages 13773–14242
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Physical Chemistry Chemical Physics www.rsc.org/pccp
ISSN 1463-9076
PAPER Yuan-Tao Zhang, Bao-Lin Zhang et al. Photoluminescence performance enhancement of ZnO/MgO heterostructured nanowires and their applications in ultraviolet laser diodes
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Photoluminescence performance enhancement of ZnO/MgO heterostructured nanowires and their applications in ultraviolet laser diodes†
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Zhi-Feng Shi,a Yuan-Tao Zhang,*a Xi-Jun Cui,a Shi-Wei Zhuang,a Bin Wu,a Xian-Wei Chu,a Xin Dong,a Bao-Lin Zhang*a and Guo-Tong Duab Vertically aligned ZnO/MgO coaxial nanowire (NW) arrays were prepared on sapphire substrates by metal– organic chemical vapor deposition combined with a sputtering system. We present a comparative investigation of the morphological and optical properties of the produced heterostructures with different MgO layer thicknesses. Photoluminescence measurements showed that the optical performances of ZnO/MgO coaxial NWs were strongly dependent on the MgO layer thickness. The intensity of deep-level emission (DLE) decreased monotonously with the increase of MgO thickness, while the enhancement of ultraviolet (UV) emission showed a critical thickness of 15 nm, achieving a maximum intensity ratio (B226) of IUV/IDLE at the same time. The significantly improved exciton emission efficiency of the coaxial NW structures allows us to study the surface passivation effect, photogenerated carrier confinement and transfer in terms of energy band theory. More importantly, we achieved an ultralow threshold (4.5 mA, 0.58 A cm 2) Received 3rd February 2015, Accepted 16th March 2015
electrically driven UV lasing action based on the ZnO/MgO NW structures by constructing an Au/MgO/ ZnO metal/insulator/semiconductor diode, and the continuous-current-driven diode shows a good
DOI: 10.1039/c5cp00674k
temperature tolerance. The results obtained on the unique optical properties of ZnO/MgO coaxial NWs shed light on the design and development of ZnO-based UV laser diodes assembled with nanoscale
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building blocks.
1. Introduction Recently, one-dimensional (1-D) semiconductor nanowires (NWs) have been actively studied as promising and powerful building blocks in nanoscale optoelectronic or photonic devices.1–5 And the ability to assemble and electrically drive these building blocks is especially critical since it opens the door to integrated photonic sources and other interesting novel functionalities.6,7 The progress in the electroluminescence (EL) devices based on 1-D NWs, such as light-emitting diodes (LEDs) and laser diodes (LDs), strongly relies on their natural advantages including structural flexibility, superior electrical transport, defect-free crystallinity and nanoscale carrier injection, which contributes to tackle the challenging problem of emission efficiency drop in LEDs and LDs.8 ZnO, owing to its large direct band gap (3.37 eV) and high exciton binding energy a
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Qianjin Street 2699, Changchun, 130012, China. E-mail: [email protected], [email protected]; Fax: +86-431-8516-8270; Tel: +86-136-3058-6267 b School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian, 116023, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp00674k
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(60 meV), has already been deemed a promising emitter in short-wavelength LEDs and LDs.1,9–13 However, the large surfaceto-volume ratio of ZnO NWs presumably deteriorates the studied device performance to some extent since the performance of a nanodevice critically depends on the surface of nanomaterials. The undesirable surface states serving as nonradiative recombination (NRR) centers can substantially quench the excitonrelated near-band-edge (NBE) emission,14 which is likely to impede UV lasing due to the reason that the required pumping energy for UV stimulated emission could be consumed with the undesirable surface NRR. A common technique to improve the luminescence efficiency of ZnO NWs is to create heterostructures by coating polymers or other materials with larger band gaps compared to ZnO.15–17 This method can modify the original electronic nature of ZnO by enhancing the interaction with the surrounding coating layer optically and electrically. In detail, the surface passivation/modification effect in heterostructures can substantially prevent photogenerated carrier migration toward nonradiative defects and, therefore, contributes to an enhanced optical gain within a designed laser structure due to high exciton emission efficiency. The inherent advantages of such structures certainly show potential for making electrically driven ZnO NW-based LDs.
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By combining the ideas on surface modification and design of a ZnO-based coaxial NW LD, we firstly present a comparative investigation on optical properties of ZnO NWs by covering the MgO layer with different thicknesses, and further, we proposed and constructed a metal/insulator/semiconductor (MIS) junction diode based on the ZnO/MgO coaxial NW structures. The desired coaxial NWs serving as ideal building blocks are expected to act as gain media and support the realization of low-threshold exciton lasing. In this respect, MgO material, with a larger band gap and lower refraction index than those of ZnO, makes itself a good candidate for the surface modification of ZnO NWs, generating a desired type-I band alignment. Other major advantages provided by the coaxial MgO layer geometry are strong carrier confinement and unique waveguide capabilities. As is well known, an improved luminescence efficiency in heterostructures requires a very careful selection of coating layer thickness in order to acquire a satisfactory and viable photo- and electro-excited carrier transport and recombination in the active region,14 which directly determines both the optical properties of ZnO/MgO coaxial NWs and the device performance of the studied diode. In this work, we discussed the impact of surface states on the optical properties of 1D ZnO NWs, and a significant improvement of the exciton emission efficiency from ZnO was realized by introducing the MgO material as the surface passivation layer. Depending on the thickness of the MgO layer, the intensity ratio of IUV/IDLE was tuned to achieve the best value at a critical MgO thickness of 15 nm. More importantly, a coherent random lasing was observed from the studied ZnO/MgO coaxial NW diode featuring an ultralow threshold (4.5 mA, 0.58 A cm 2) and good temperature tolerance, which are mostly ascribed to the higher optical gain provided by the studied coaxial configuration because of the surface passivation effect. Besides, other factors were also proposed and discussed, such as an optimized carrier injection and confinement configuration, a strong light scattering capability induced by the large refraction index fluctuation in the plane perpendicular to the ZnO NWs, and an increased junction area unique to coaxial NW structures.
2. Experimental 2.1
Fabrication of ZnO/MgO coaxial NW heterostructures
The preparation of ZnO/MgO coaxial NW structures starts with the epitaxy of ZnO NWs on commercially available c-Al2O3 substrates, which involves a custom designed photoassisted metal–organic chemical vapor deposition system using diethylzinc (DEZn) and ultrahigh purity oxygen gas (O2) as the reactants, and details about the reactor have been reported elsewhere.18 In this experiment, a two-stage growth method was employed. First, the growth was conducted at a low temperature of 565 1C for 30 s with the flow rates of DEZn and O2 of 8 and 180 sccm, respectively. Then, the ZnO main layer was prepared at a high temperature of 800 1C for 30 min, and DEZn and O2 were supplied at flow rates of 14 and 180 sccm, respectively. Note that the reaction pressure during both growth stages was maintained at B0.6 Torr.
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Consequently, vertically aligned and highly uniform ZnO NWs were produced. The deposition of the MgO coating layer was carried out by a radio-frequency (RF) magnetron sputtering system using high purity MgO ceramic target (99.999%). Before deposition, the sputtering chamber was evacuated to a base pressure below 10 4 Pa using a turbo molecular pump, then filled with the working gas (argon) to a pressure of 1.0 Pa. RF power, argon flow rate and temperature of the sample holder were kept constant at 110 W, 40 sccm, and 500 1C, respectively. During deposition, the sample holder was rotated to obtain good uniformity of the MgO layer. In this process, the thickness of the MgO layer can be controlled precisely by modulating the deposition time, allowing a comparison of the optical properties of ZnO/MgO coxial NWs with different layer thicknesses. 2.2
Characterization
The morphology and crystallinity of the products were characterized by field emission scanning electron microscopy (FE-SEM; Jeol-7500F) and X-ray diffraction (XRD; Rigaku Ultima IV) analysis. An energy-dispersive X-ray (EDX) spectroscope attached to a SEM was used to study the chemical composition characterization of the products. The photoluminescence (PL) spectra were recorded using a Zolix Omni-l 500 monochromator/spectrograph at room temperature (RT). And a He–Cd laser with a wavelength of 325 nm and power of 30 mW was utilized as an excitation light source. The current–voltage (I–V) characteristics were measured by using a semiconductor characterization analyser (Agilent B2900A). The EL spectra were recorded using a custom acquisition equipment including a photomultiplier tube and a lock-in amplifier system.
3. Results and discussion 3.1
Characterization analysis of ZnO/MgO coaxial NWs
Fig. 1a and b show the typical SEM images of the as-produced ZnO NWs grown on c-Al2O3 substrates. One can see that highly ordered and vertically aligned ZnO NWs, with a length of B1.3 mm and a uniform diameter distribution of B36 nm, grew up independently from the underlying substrate. Fig. 1c shows the XRD patterns of the as-grown ZnO NWs. Besides the (0006) plane of the c-Al2O3 substrate, only the respective (0001) reflection family of ZnO appears in the XRD profile, indicating that the NWs are preferentially oriented in the c-axis direction. Meanwhile, XRD-o scans were also performed to check the degree of alignment to the normal direction of the substrate surface. A relatively narrow full width of half-maximum (FWHM) of 1492 arcsec implies the excellent ordering of the resulting ZnO NWs along the growth direction. Fig. 1d shows the representative SEM images (head region) of the as-fabricated ZnO/MgO coaxial NWs with different MgO layer thicknesses. With the increase of MgO thickness, a regular variation of morphological features can be observed after a comparative analysis of four SEM images. On the one hand, the effective diameter of ZnO NWs at their heads is thickened gradually, evolving into a round-caplike structure finally, as clearly displayed in the bottom section of Fig. 1d. On the other hand, the MgO layer does not completely
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Fig. 1 (a) 251-tilted and (b) cross-sectional SEM image of well-aligned ZnO NWs. (c) Wide-range XRD 2y scans of ZnO NWs. The inset shows the o-rocking curves of the ZnO (0002) reflection. (d) Cross-sectional SEM images of ZnO NWs coated with different MgO thicknesses of 0, 10, 20 and 40 nm, respectively. (e) 251-tilted SEM image of ZnO/MgO coaxial NWs with a MgO layer thickness of 20 nm.
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further investigate the coaxial coating configuration. The elemental mapping results shown in Fig. 2b and c vividly show the spatial distributions of Zn and Mg elements exclusively. In the case of the Zn element, a homogeneous signal can be detected throughout the entire NW, while that of Mg element was different, which gradually reduces and terminates at a certain depth. In addition, the distribution of the Mg signal is wider than that of Zn in the radial direction of the NW at all equal and comparable positions, indicating the formation of ZnO/MgO coaxial coated NWs and the decreasing trend of MgO thickness along the axial direction. Additional analysis using the spot scanning EDX measurements conducted on the head, middle and bottom regions of the NW marked in Fig. 2a also indicates the nonuniform distribution of the MgO coating layer, as shown in Fig. 2d. The variation in optical properties of ZnO/MgO coaxial NWs with different layer thicknesses was investigated by PL measurements. Here, all of the positions of the emission peaks have been calibrated by the laser line (He–Cd laser of 325 nm). As displayed in Fig. 3a, six samples presented excellent optical quality with a dominant NBE excitonic emission at around 378 nm and a relatively weak DLE at B500 nm, which was frequently attributed to the intrinsic oxygen vacancy defects in the ZnO epilayers.18,19 For a clear and specific comparison, the DLE performances of six samples were magnified by 20 times simultaneously. It is worth noting that the intensity of DLE
fill up the blank zones around the ZnO NWs, but covers only their heads and side facets with the MgO thickness decreasing from the NW’s top to its bottom gradually. As shown in Fig. 1e (corresponding to the product with a 20 nm MgO coating layer), a geometrical configuration of ZnO/MgO coaxial NWs with rough, globular heads and well-separated nano-units can be clearly observed by a 251-tilted SEM image, and an enlarged view is shown in the inset. As shown in Fig. 2a, a typical ZnO NW coated with a 20 nm thick MgO layer was selected for the EDX measurements to
Fig. 2 (a) SEM image of a single ZnO/MgO coaxial NW, and (b, c) the corresponding elemental mapping images. (d) EDX spectra obtained from different areas of a single NW as marked in (a).
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Fig. 3 (a) RT PL spectra of the MgO-coated ZnO NWs with different thicknesses. The inset shows the schematic configuration for the PL measurement. (b) UV enhancement and DLE reduction factors of ZnO/MgO coaxial NWs in the case of different MgO layer thicknesses. (c) Plot of the intensity ratio of IUV/IDLE as a function of MgO thickness.
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decreases monotonously with the increase of MgO thickness from 0 to 25 nm, and the reduction of undesirable DLE results from the surface passivation effect of the MgO coating layer, which substantially suppresses the surface-mediated NRR and DLE, and the detailed carrier tunneling–trapping–recombination processes will be discussed later. A minimum DLE reduction factor (defined as the ratio of integrated emission intensity of ZnO NWs with and without MgO coating) of 0.13 was obtained as the MgO thickness increases to 25 nm, as shown in Fig. 3b. While the trend is somewhat dissimilar in the case of UV emission, and the critical thickness of the MgO coating layer can be inferred. A maximum of B7-fold UV enhancement ratio was achieved as the MgO thickness increases to 15 nm, and a further increase of MgO thickness induced the reduction of UV emission instead. In spite of this, the coaxial NWs with a 25 nm thick MgO coating still featured an acceptable UV enhancement factor of 1.28. Undoubtedly, an enhanced UV emission is reasonably beneficial for the realization of high-performance UV-LEDs and LDs. Here, we make a hypothesis that the surface passivation effect of MgO coating cannot be improved further once the coating layer exceeds a critical thickness of 15 nm because a certain amount of surface states has already been passivated by such a coating layer, and one possibility of absorption and scattering of the exciting light (325 nm) by too thick and nano-rough MgO layer ought to have mostly liability in the UV emission decay. The above arguments could be confirmed by the plot of the intensity ratio of IUV/IDLE as a function of MgO layer thickness shown in Fig. 3c, and an interesting and regular change was observed. The ratio of IUV/ IDLE reaches a maximum value of B226 at a MgO thickness of 15 nm, greatly superior to previous reports using MgO and other sheathing materials as the coating layer,14,15,20 and thereupon drops. However, such a reduction of IUV/IDLE at thicker MgO layers was maintained at a certain value around 89, rather than declining constantly. The above observations imply that the number of photogenerated carriers inside the ZnO NWs also strongly depends on the thickness of the MgO layer. Above a critical thickness of 15 nm, two independent radiative recombination pathways involved NBE and DLE that can be weakened simultaneously. And an almost constant IUV/IDLE also verifies our aforementioned hypothesis that the surface passivation effect of MgO coating above a critical value (15 nm) is no more effective. Besides, the existence of an optimum MgO layer thickness could be explained in terms of the effective modulation effect of MgO coating resulting from the combined action of the surface passivation effect and nonradiative thermal absorption in the amorphous MgO layer. With the increase of MgO coating layer thickness, the density of surface states can be reduced gradually, resulting in an increased number of excitons in ZnO NWs, and thus UV emission is enhanced. However, upon further increasing the MgO layer thickness above 15 nm, the nonradiative thermal transition owing to the scattering in the nano-rough MgO layer has to be taken into account. And the thermal transition intensity would increase gradually with the thickness of the MgO coating layer, inducing the decay of UV emission consequently.
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3.2
Mechanistic analysis of the improved PL performance
The substantial improvement of PL performance by MgO coating can be understood in terms of the models shown in Fig. 4a and b. In general, the surface states including the dangling bonds and surface adsorption, such as O2, H2O, and OH ,21,22 could have powerful influence on the PL performance of ZnO nanostructures, especially for those with large surface-to-volume ratios. For the bare ZnO NWs, the adsorbates localized in their surface may readily act as the acceptors, which trap the free electrons in the conduction band of ZnO due to the short relaxation time for carrier tunneling from the interior of ZnO to the surface, resulting in a depletion layer and band bending near the surface. The possible mechanistic pathways for such a capture process can be described as 2H2O(g) + O2 + 4e - 4OH (adsorption) and O2(g) + e - O2 (adsorption).23 On the one hand, the band bending is expected to facilitate the separation of photogenerated electron– hole pairs and the photogenerated holes will accumulate near the surface, as illustrated in Fig. 4a, thus a low probability of radiative recombination and the quenching of NBE excitonic emission are favored consequently. On the other hand, the trapped carriers by the undesirable surface-trapping channel will recombine nonradiatively at the surface, or part of the trapped holes tunnel back into the DL centers to contribute to the DLE, as proposed by Dijken et al.21 For the MgO coated ZnO NWs, the density of surface states can be reduced substantially, thereby decreasing the width of the depletion layer, built-inbarrier height (qVB) and surface band bending, as illustrated in Fig. 4b. The separation of photogenerated electron–hole pairs is therefore alleviated, resulting in an increased number of excitons in ZnO NWs, which makes the enhancement of exciton-related NBE emission possible. In addition, the band alignment of ZnO/ MgO heterostructures may also be an important factor for the improvement of PL performance. Considering the larger band gap of MgO (7.8 eV) than that of ZnO (3.37 eV), the suppression of
Fig. 4 Schematic band diagrams of (a) bare and (b) MgO-coated ZnO NWs. (c) Energy band diagram of a ZnO/MgO heterojunction at thermal equilibrium. (d) Schematic diagram of a ZnO/MgO coaxial NW and a proposed quantum-well-like structure for the explanation of enhancement of carrier radiation recombination.
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tunneling of charge carriers from the ZnO NW to the MgO layer or the air/MgO interface takes effect. In reality, the ZnO/MgO coaxial NWs can be approximatively regarded as a quasiquantum-well-like structure (Fig. 4d) because of the small diameter of ZnO NWs, thus the photogenerated carriers can be confined in the ZnO NW and recombine radiatively and efficiently. Moreover, it was recently reported that it is also likely to generate electron–hole pairs in MgO nanostructures upon laser irradiation with its energy smaller than the band gap of MgO due to the intrinsic defects of oxygen vacancies existing in MgO, which produce the corresponding energy levels in the forbidden energy band of MgO.24 As illustrated in Fig. 4c, the conduction and valence band offsets (DEC and DEV) at the MgO/ZnO interface can be determined to be 3.55 and 0.88 eV, respectively, considering that the electron affinities of MgO and ZnO are 0.80 and 4.35 eV, respectively. Therefore, photogenerated electrons and holes in MgO are transferred from the MgO coating layer to the ZnO NW, meaning that the carrier injection from the MgO barrier to ZnO contributes well to the NBE emission to some extent.
3.3
Electrically pumped random lasing from the MIS diode
To realize the preparation of a MIS-type LD based on ZnO/MgO coaxial NWs, an Au monolayer (B30 nm) was thermally evaporated on the MgO coating layer and patterned into a circular pad (1 mm) by a customized shadow mask, and the schematic diagram of the device structure is shown in Fig. 5a. Fig. 5b shows the I–V characteristics of the constructed NW diode with the positive voltage connected to the Au electrode, and a nonlinear rectifying behavior was observed as the reverse current increases gradually with the bias voltage, as reported in another work.25 EL characterization of the studied diode was performed to demonstrate the lasing action. As shown in Fig. 6, a series of surface EL spectra under a continuous current injection mode are plotted. Note that the UV emission can be detected even at a low excitation current of 2.5 mA. As the current was increased to 4.0 mA, a broad and featureless spontaneous emission peak is observed peaking at B380 nm, with a FWHM of B18 nm. Upon increasing the current to 5.5 mA, some dramatic sharp peaks are observed superposing on the broad emission band. The FWHM of these peaks are as narrow as 0.5 nm, more than 35 times smaller than that obtained at 4.0 mA. Following the increase in the current input to 6.4 mA and above, the number
Fig. 5
(a) Schematic diagram and (b) I–V characteristics of the studied LD.
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Fig. 6 Surface EL spectra of the LD under different driving currents. The inset shows the dependence of the integrated EL intensity on the injection current of the studied LD.
and intensity of such sharp peaks are increased correspondingly spanning from 370 to 410 nm. The narrow linewidth and a rapid increase in the emission intensity are indications of the appearance of lasing action, which implies that the optical gain obtained along the laser cavities is sufficiently high to achieve lasing at such a low current injection level. In addition, the dependence of the integrated EL intensity against the injection current is plotted in the inset of Fig. 6, from which an ultralow threshold current of B4.5 mA (0.58 A cm 2) can be derived, much lower than previously reported blue-/UV-light LDs employing ZnO and/or other semiconductors with small exciton binding energies as the gain media.26–28 Note that the ‘‘softer’’ threshold in our case is likely due to the large scattering loss and a relatively strong background of spontaneous emission, which has been frequently observed in ZnO micro- and nanolasers.29,30 The realization of lasing action in the studied diode without any artificially designed resonant cavity prompts us to consider the possible resonant model of the lasing action. Although ZnO NWs have been proven to be capable of achieving Fabry–Perot (F–P) type resonator cavities defined by the head facet of ZnO NWs and the epitaxial ZnO/substrate interface, such possibility in our case is almost negligible. It is generally accepted that the laser oscillation action depends strongly on the diameter of NWs; in theory, the NWs with the diameter smaller than the oscillation wavelength cannot play the role of light confinement and amplification substantially because the optical field could readily spill out of the NWs. The direct consequence is that the population inversion conditions cannot be satisfied, or satisfied only at an elevated pumping level because the round-trip gain inside the NWs must equal the round-trip loss. Experimentally, Zimmler et al. observed that the laser oscillation threshold in F–P cavities of ZnO NWs is greatly influenced by the diameter of the NW, with no laser oscillation realized for diameters smaller than B150 nm.31 More importantly, the as-prepared ZnO NWs do not behave like a smooth top surface (shown in Fig. 1d) serving as a mirror. Therefore, a sufficient optical gain enabling a F–P laser cannot be provided in these ZnO NWs, excluding the possibility of a F–P cavity mode established by two end facets of NWs reasonably. The resonant mode herein is therefore attributed to the random lasing, implying the random scattering and amplification of photons between adjacent NWs rather than a single NW. That is to say,
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the ZnO NWs act as both the gain media and scattering units in the studied diode. On some occasions, the emitted photons can return to a scattering path from which it was scattered before, thus the quasi-two-dimensional closed-loop laser cavities can be formed. When the optical gain exceeds the loss, lasing oscillation occurs. In addition, two aspects of this figure are important to note. Firstly, the irregular evolution of observed sharp peaks shows direct evidence of the determined random lasing action. With the increase of driving current, the position of lasing peaks is randomly changed, and the wavelength spacing between two adjacent peaks does not own a fixed value. These features elucidate the intrinsic traits of a random lasing mode.32–35 Secondly, three successive EL measurements of the studied diode at the same driving current were carried out for a further verification, with an interval of B120 s. As shown in Fig. 7a, the three EL spectra display distinct lasing spikes with different peak positions and FWHMs although the injection current and measurement conditions are the same, manifesting a random lasing action rather than a specific resonant mode. Besides, near-field optical microscope images of the studied LD were also recorded across the entire contact area for a vivid presentation of a random nature. As shown in Fig. 7b, many randomly distributed bright spots, which represented the isolated random resonators formed in the proposed ZnO/MgO coaxial heterostructure, were observed from the circular Au contact during the three successive EL measurements. It should be mentioned herein that the light emitted at B380 nm is beyond the response range of the digital microscope (Aigo, GE-5), inducing color aberration between the captured color (red) and the actual emission color (UV). Importantly, it was found that the locations of almost all bright spots irregularly changed in real time, which is typical in the case of a random lasing action and provides additional evidence for our arguments. Actually, the
Fig. 7 (a) EL spectra of the LD for three successive measurements under the same injection current of 7.9 mA. (b) Optical microscope image of the LD corresponding to the three successive measurements, and the scale bar is 250 mm. Note that the left image was taken with lamp illumination and zero bias.
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above-mentioned scenario was frequently observed in the optically pumped random lasing,36 similar to the situation in the case of electrically pumped behavior. In the studied diode, ZnO NWs can be regarded as both the random gain media and multiple scattering units. The light emission obtained at each circle can be scattered with random walks, corresponding to different feedback loops. For certain modes of emitted light which acquire a high optical gain to sustain the lasing, they vary from time to time in terms of the position, intensity and FWHM of the lasing peak because of the real-time change in the light feedback path. In addition, the evolution of the spatial distribution of the lasing action was also studied by angular-dependent EL measurements (shown in Fig. S1, ESI†), as frequently performed in the determination of a random lasing. Herein, we consider that the multidirectional light output is not the definitive measure of therandom nature because the ZnO NW F–P laser device also demonstrated a broadened emission pattern with angular oscillation,1 which is presumably due to the feature that the ZnO NW laser works as nanoscale units. Another important reason is that nano-rough ZnO/MgO interfaces were formed after the MgO coating, which favored an increased light scattering effect, making the large spatial distribution of the lasing action possible. In order to verify the sensitivity of the lasing action to operating temperature, the LD was pumped at an elevated temperature point (353 K). As shown in Fig. 8a, two typical lasing spectra captured at 295 and 353 K, respectively, operated at 7.9 and 13.9 mA were presented. It is remarkable to find that the lasing action can be still sustained at 353 K, which is evident from the low linewidth of the sharp peaks. The above observation suggests that the studied ZnO/ MgO coaxial NW diode has good temperature tolerance. Theoretically, the heating effect will inevitably give rise to a rapid proliferation of structural defects, and thus the probability of nonradiative recombination is likely to increase in proportion to operating temperature. The direct consequence is that the lasing action ceases or no reliable light output can be observed normally. Because no artificially designed resonant cavity is formed in the structure to provide a constant and stable optical gain operating
Fig. 8 (a) EL spectra of the LD measured at RT and 353 K with the driving currents of 7.9 and 13.9 mA, respectively. (b) Dependence of the lasing threshold on working temperature.
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at high-temperature, and moreover, a high scattering loss in a random fashion requires a high optical gain to sustain the random lasing. Therefore, it is reasonably believed that the high operating temperature of the LD is evident of the rational design of the device configuration, and the ideal building blocks of ZnO/MgO coaxial NWs towards the design of low-threshold LDs. As we stated above, at elevated temperatures the reduced carrier injection efficiency and radiative recombination probability will impede the UV lasing, and a higher pumping intensity is therefore required to compensate for the heating-induced loss. As shown in Fig. 8b, the lasing threshold of the LD is found to be 4.5 mA at RT, and it increases to 7.2 mA at 323 K, further increasing to 11.7 mA at 353 K, featuring a regular sensitivity to the temperature change. 3.4
Analyses of laser behavior
One of the features of the proposed laser model is the ultralow threshold current density, and it seems very worthwhile for us to clarify the inherent mechanisms because no complicated structures such as double heterojunction or quantum well layers were introduced. By overall analyses of our device structure and experiment results, possible reasons could be summarized and listed as follows. Firstly, the intrinsic characteristics of ZnO, large exciton-binding energy and high optical gain, make a great contribution to the ultralow lasing threshold. Moreover, the lasing action through the exciton interaction process is essentially more favorable for a low threshold laser than that through the electron–hole-plasma process. Secondly, an optimized carrier injection and confinement configuration greatly contribute to the ultralow lasing threshold. In our case, the dielectric MgO coating layer functions as the electron blocking layer and the hole supplying layer simultaneously. Due to the existence of high DEC, electrons are available in abundance at the MgO/ZnO interface under forward bias, as depicted in Fig. S2 (ESI†). And meanwhile, a sufficiently high bias enables the generation of holes in the MgO layer due to the high-electric-field-induced impact ionization process considering that almost all the voltage is applied to the insulating layer.37–39 The generated holes can be swept into the valence band of ZnO above a critical driving voltage, and recombine radiatively with the electrons accumulated at the MgO/ZnO interface, resulting in the NBE emission and even a lasing action. Thirdly, the effective surface passivation effect of the MgO layer ensures high-quality and optically active NWs, thus the surface-mediated NRR and DLE can be suppressed substantially (The wide-range EL spectrum in the UV-visible region is presented in Fig. S3, ESI†). It is also a general consensus that the defect emission and surface NRR could impede the UV lasing due to the reason that the required pumping energy for UV stimulated emission could be consumed with the spontaneous defect emission and surface NRR. Last but not least, the MgO coating, in the proposed ZnO/MgO coaxial NW structure, extends to a certain depth along the axial direction of ZnO NWs, resulting in a large refraction index fluctuation in the plane perpendicular to the ZnO NWs, the direct consequence of which is an enhanced light scattering capability of the active layer. In theory, Apalkov et al. predicted that random variation of the
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refraction index favors the formation of resonant cavities due to the inherent waveguiding structures in the plane.40 Therefore, it is reasonably deduced that the studied coaxial heterostructure featuring a strong light scattering capability can be electrically pumped into random lasing with a relatively low input power. Besides, an increased junction area offered by the studied heterostructures exclusively provides more chance for an enhanced carrier recombination efficiency, which further supports and facilitates the realization of a low-threshold lasing.
4. Conclusions In conclusion, we have successfully demonstrated the feasibility of utilizing ZnO/MgO coaxial NW structures to obtain highly efficient excitonic UV emission. Sheathing ZnO NWs with an appropriate MgO layer thickness results in substantial suppression of DLE, and a significant enhancement of UV emission. The intensity ratio of IUV/IDLE can also be tuned to achieve the best value at a critical thickness of 15 nm for MgO coating. The improved exciton emission efficiency by MgO coating can be attributed to the desirable surface passivation effects, which leads to a quasi-flat-band effect near the ZnO surface and an increased number of excitons in ZnO, enhancing the probability of carrier radiative recombination consequently. Besides, the typical band alignment of ZnO/MgO heterostructures favors the photogenerated carrier confinement and transfer behavior, which also blocks the undesirable surface-trapping channel and thereby improves the UV emission efficiency. The excellent optical properties of the as-produced ZnO/MgO coaxial NWs were further confirmed by constructing a MIS-type LD, which featured an ultralow threshold of 4.5 mA (0.58 A cm 2) and good temperature tolerance. Our work here provides a feasible way to improve the excitonic emission efficiency of ZnO, and demonstrates that ZnO/MgO coaxial NWs can be employed as ideal building blocks for the design of low-threshold LDs, which could promote existing applications and suggest new opportunities.
Acknowledgements This work was supported by the National Basic Research Program of China (2011CB302005), the National Natural Science Foundation of China (No 61106003, 61376046 and 61223005), the Science and Technology Developing Project of Jilin Province (20130204032GX), and the Program for New Century Excellent Talents in University (NCET-13-0254).
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22 Z. Shi, Y. Zhang, B. Wu, X. Cai, J. Zhang, X. Xia, H. Wang, X. Dong, H. Liang, B. Zhang and G. Du, Appl. Phys. Lett., 2013, 102, 161101. 23 Q. H. Li, Q. Wan, Y. X. Liang and T. H. Wang, Appl. Phys. Lett., 2004, 84, 4556. 24 F. L. Deepak, P. Saldanha, S. R. C. Vivekchand and A. Govindaraj, Chem. Phys. Lett., 2006, 417, 535. 25 J. Yu, S. J. Ippolito, W. Wlodarski, M. Strano and K. Kalantar-zadeh, Nanotechnology, 2010, 21, 265502. 26 X. F. Duan, Y. Huang, R. Agarwal and C. M. Lieber, Nature, 2003, 421, 241. 27 H. Yoshida, Y. Yamashita, M. Kuwabara and H. Kan, Appl. Phys. Lett., 2008, 93, 241106. 28 K. Okamoto, H. Ohta, S. F. Chichibu, J. Ichihara and H. Takasu, Jpn. J. Appl. Phys., 2007, 46, L187. 29 J. Dai, C. X. Xu and X. W. Sun, Adv. Mater., 2011, 23, 4115. 30 H. K. Liang, S. F. Yu and H. Y. Yang, Appl. Phys. Lett., 2010, 96, 101116. ¨ller and C. Ronning, 31 M. A. Zimmler, J. Bao, F. Capasso, S. Mu Appl. Phys. Lett., 2008, 93, 051101. 32 X. Y. Ma, J. W. Pan, P. L. Chen, D. S. Li, H. Zhang, Y. Yang and D. R. Yang, Opt. Express, 2009, 17, 14426. 33 H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang and R. P. H. Chang, Phys. Rev. Lett., 1999, 82, 11. 34 D. S. Wiersma, Nat. Phys., 2008, 4, 359. 35 D. S. Wiersma, Nature, 2000, 406, 132. 36 H. Cao, Y. G. Zhao, H. C. Ong, S. T. Ho, J. Y. Dai, J. Y. Wu and R. P. H. Chang, Appl. Phys. Lett., 1998, 73, 3656. 37 S. M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. 38 H. Zhu, C. X. Shan, B. Yao, B. H. Li, J. Y. Zhang, Z. Z. Zhang, D. X. Zhao, D. Z. Shen, X. W. Fan, Y. M. Lu and Z. K. Tang, Adv. Mater., 2009, 21, 1613. 39 P. L. Chen, X. Y. Ma and D. R. Yang, Appl. Phys. Lett., 2006, 89, 111112. 40 V. M. Apalkov, M. E. Raikh and B. Shapiro, Phys. Rev. Lett., 2002, 89, 016802.
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Physical Chemistry Chemical Physics www.rsc.org/pccp
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PAPER Yuan-Tao Zhang, Bao-Lin Zhang et al. Photoluminescence performance enhancement of ZnO/MgO heterostructured nanowires and their applications in ultraviolet laser diodes
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Cite this: Phys. Chem. Chem. Phys., 2015, 17, 13813
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Photoluminescence performance enhancement of ZnO/MgO heterostructured nanowires and their applications in ultraviolet laser diodes†
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Zhi-Feng Shi,a Yuan-Tao Zhang,*a Xi-Jun Cui,a Shi-Wei Zhuang,a Bin Wu,a Xian-Wei Chu,a Xin Dong,a Bao-Lin Zhang*a and Guo-Tong Duab Vertically aligned ZnO/MgO coaxial nanowire (NW) arrays were prepared on sapphire substrates by metal– organic chemical vapor deposition combined with a sputtering system. We present a comparative investigation of the morphological and optical properties of the produced heterostructures with different MgO layer thicknesses. Photoluminescence measurements showed that the optical performances of ZnO/MgO coaxial NWs were strongly dependent on the MgO layer thickness. The intensity of deep-level emission (DLE) decreased monotonously with the increase of MgO thickness, while the enhancement of ultraviolet (UV) emission showed a critical thickness of 15 nm, achieving a maximum intensity ratio (B226) of IUV/IDLE at the same time. The significantly improved exciton emission efficiency of the coaxial NW structures allows us to study the surface passivation effect, photogenerated carrier confinement and transfer in terms of energy band theory. More importantly, we achieved an ultralow threshold (4.5 mA, 0.58 A cm 2) Received 3rd February 2015, Accepted 16th March 2015
electrically driven UV lasing action based on the ZnO/MgO NW structures by constructing an Au/MgO/ ZnO metal/insulator/semiconductor diode, and the continuous-current-driven diode shows a good
DOI: 10.1039/c5cp00674k
temperature tolerance. The results obtained on the unique optical properties of ZnO/MgO coaxial NWs shed light on the design and development of ZnO-based UV laser diodes assembled with nanoscale
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building blocks.
1. Introduction Recently, one-dimensional (1-D) semiconductor nanowires (NWs) have been actively studied as promising and powerful building blocks in nanoscale optoelectronic or photonic devices.1–5 And the ability to assemble and electrically drive these building blocks is especially critical since it opens the door to integrated photonic sources and other interesting novel functionalities.6,7 The progress in the electroluminescence (EL) devices based on 1-D NWs, such as light-emitting diodes (LEDs) and laser diodes (LDs), strongly relies on their natural advantages including structural flexibility, superior electrical transport, defect-free crystallinity and nanoscale carrier injection, which contributes to tackle the challenging problem of emission efficiency drop in LEDs and LDs.8 ZnO, owing to its large direct band gap (3.37 eV) and high exciton binding energy a
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Qianjin Street 2699, Changchun, 130012, China. E-mail: [email protected], [email protected]; Fax: +86-431-8516-8270; Tel: +86-136-3058-6267 b School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian, 116023, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp00674k
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(60 meV), has already been deemed a promising emitter in short-wavelength LEDs and LDs.1,9–13 However, the large surfaceto-volume ratio of ZnO NWs presumably deteriorates the studied device performance to some extent since the performance of a nanodevice critically depends on the surface of nanomaterials. The undesirable surface states serving as nonradiative recombination (NRR) centers can substantially quench the excitonrelated near-band-edge (NBE) emission,14 which is likely to impede UV lasing due to the reason that the required pumping energy for UV stimulated emission could be consumed with the undesirable surface NRR. A common technique to improve the luminescence efficiency of ZnO NWs is to create heterostructures by coating polymers or other materials with larger band gaps compared to ZnO.15–17 This method can modify the original electronic nature of ZnO by enhancing the interaction with the surrounding coating layer optically and electrically. In detail, the surface passivation/modification effect in heterostructures can substantially prevent photogenerated carrier migration toward nonradiative defects and, therefore, contributes to an enhanced optical gain within a designed laser structure due to high exciton emission efficiency. The inherent advantages of such structures certainly show potential for making electrically driven ZnO NW-based LDs.
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By combining the ideas on surface modification and design of a ZnO-based coaxial NW LD, we firstly present a comparative investigation on optical properties of ZnO NWs by covering the MgO layer with different thicknesses, and further, we proposed and constructed a metal/insulator/semiconductor (MIS) junction diode based on the ZnO/MgO coaxial NW structures. The desired coaxial NWs serving as ideal building blocks are expected to act as gain media and support the realization of low-threshold exciton lasing. In this respect, MgO material, with a larger band gap and lower refraction index than those of ZnO, makes itself a good candidate for the surface modification of ZnO NWs, generating a desired type-I band alignment. Other major advantages provided by the coaxial MgO layer geometry are strong carrier confinement and unique waveguide capabilities. As is well known, an improved luminescence efficiency in heterostructures requires a very careful selection of coating layer thickness in order to acquire a satisfactory and viable photo- and electro-excited carrier transport and recombination in the active region,14 which directly determines both the optical properties of ZnO/MgO coaxial NWs and the device performance of the studied diode. In this work, we discussed the impact of surface states on the optical properties of 1D ZnO NWs, and a significant improvement of the exciton emission efficiency from ZnO was realized by introducing the MgO material as the surface passivation layer. Depending on the thickness of the MgO layer, the intensity ratio of IUV/IDLE was tuned to achieve the best value at a critical MgO thickness of 15 nm. More importantly, a coherent random lasing was observed from the studied ZnO/MgO coaxial NW diode featuring an ultralow threshold (4.5 mA, 0.58 A cm 2) and good temperature tolerance, which are mostly ascribed to the higher optical gain provided by the studied coaxial configuration because of the surface passivation effect. Besides, other factors were also proposed and discussed, such as an optimized carrier injection and confinement configuration, a strong light scattering capability induced by the large refraction index fluctuation in the plane perpendicular to the ZnO NWs, and an increased junction area unique to coaxial NW structures.
2. Experimental 2.1
Fabrication of ZnO/MgO coaxial NW heterostructures
The preparation of ZnO/MgO coaxial NW structures starts with the epitaxy of ZnO NWs on commercially available c-Al2O3 substrates, which involves a custom designed photoassisted metal–organic chemical vapor deposition system using diethylzinc (DEZn) and ultrahigh purity oxygen gas (O2) as the reactants, and details about the reactor have been reported elsewhere.18 In this experiment, a two-stage growth method was employed. First, the growth was conducted at a low temperature of 565 1C for 30 s with the flow rates of DEZn and O2 of 8 and 180 sccm, respectively. Then, the ZnO main layer was prepared at a high temperature of 800 1C for 30 min, and DEZn and O2 were supplied at flow rates of 14 and 180 sccm, respectively. Note that the reaction pressure during both growth stages was maintained at B0.6 Torr.
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Consequently, vertically aligned and highly uniform ZnO NWs were produced. The deposition of the MgO coating layer was carried out by a radio-frequency (RF) magnetron sputtering system using high purity MgO ceramic target (99.999%). Before deposition, the sputtering chamber was evacuated to a base pressure below 10 4 Pa using a turbo molecular pump, then filled with the working gas (argon) to a pressure of 1.0 Pa. RF power, argon flow rate and temperature of the sample holder were kept constant at 110 W, 40 sccm, and 500 1C, respectively. During deposition, the sample holder was rotated to obtain good uniformity of the MgO layer. In this process, the thickness of the MgO layer can be controlled precisely by modulating the deposition time, allowing a comparison of the optical properties of ZnO/MgO coxial NWs with different layer thicknesses. 2.2
Characterization
The morphology and crystallinity of the products were characterized by field emission scanning electron microscopy (FE-SEM; Jeol-7500F) and X-ray diffraction (XRD; Rigaku Ultima IV) analysis. An energy-dispersive X-ray (EDX) spectroscope attached to a SEM was used to study the chemical composition characterization of the products. The photoluminescence (PL) spectra were recorded using a Zolix Omni-l 500 monochromator/spectrograph at room temperature (RT). And a He–Cd laser with a wavelength of 325 nm and power of 30 mW was utilized as an excitation light source. The current–voltage (I–V) characteristics were measured by using a semiconductor characterization analyser (Agilent B2900A). The EL spectra were recorded using a custom acquisition equipment including a photomultiplier tube and a lock-in amplifier system.
3. Results and discussion 3.1
Characterization analysis of ZnO/MgO coaxial NWs
Fig. 1a and b show the typical SEM images of the as-produced ZnO NWs grown on c-Al2O3 substrates. One can see that highly ordered and vertically aligned ZnO NWs, with a length of B1.3 mm and a uniform diameter distribution of B36 nm, grew up independently from the underlying substrate. Fig. 1c shows the XRD patterns of the as-grown ZnO NWs. Besides the (0006) plane of the c-Al2O3 substrate, only the respective (0001) reflection family of ZnO appears in the XRD profile, indicating that the NWs are preferentially oriented in the c-axis direction. Meanwhile, XRD-o scans were also performed to check the degree of alignment to the normal direction of the substrate surface. A relatively narrow full width of half-maximum (FWHM) of 1492 arcsec implies the excellent ordering of the resulting ZnO NWs along the growth direction. Fig. 1d shows the representative SEM images (head region) of the as-fabricated ZnO/MgO coaxial NWs with different MgO layer thicknesses. With the increase of MgO thickness, a regular variation of morphological features can be observed after a comparative analysis of four SEM images. On the one hand, the effective diameter of ZnO NWs at their heads is thickened gradually, evolving into a round-caplike structure finally, as clearly displayed in the bottom section of Fig. 1d. On the other hand, the MgO layer does not completely
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Fig. 1 (a) 251-tilted and (b) cross-sectional SEM image of well-aligned ZnO NWs. (c) Wide-range XRD 2y scans of ZnO NWs. The inset shows the o-rocking curves of the ZnO (0002) reflection. (d) Cross-sectional SEM images of ZnO NWs coated with different MgO thicknesses of 0, 10, 20 and 40 nm, respectively. (e) 251-tilted SEM image of ZnO/MgO coaxial NWs with a MgO layer thickness of 20 nm.
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further investigate the coaxial coating configuration. The elemental mapping results shown in Fig. 2b and c vividly show the spatial distributions of Zn and Mg elements exclusively. In the case of the Zn element, a homogeneous signal can be detected throughout the entire NW, while that of Mg element was different, which gradually reduces and terminates at a certain depth. In addition, the distribution of the Mg signal is wider than that of Zn in the radial direction of the NW at all equal and comparable positions, indicating the formation of ZnO/MgO coaxial coated NWs and the decreasing trend of MgO thickness along the axial direction. Additional analysis using the spot scanning EDX measurements conducted on the head, middle and bottom regions of the NW marked in Fig. 2a also indicates the nonuniform distribution of the MgO coating layer, as shown in Fig. 2d. The variation in optical properties of ZnO/MgO coaxial NWs with different layer thicknesses was investigated by PL measurements. Here, all of the positions of the emission peaks have been calibrated by the laser line (He–Cd laser of 325 nm). As displayed in Fig. 3a, six samples presented excellent optical quality with a dominant NBE excitonic emission at around 378 nm and a relatively weak DLE at B500 nm, which was frequently attributed to the intrinsic oxygen vacancy defects in the ZnO epilayers.18,19 For a clear and specific comparison, the DLE performances of six samples were magnified by 20 times simultaneously. It is worth noting that the intensity of DLE
fill up the blank zones around the ZnO NWs, but covers only their heads and side facets with the MgO thickness decreasing from the NW’s top to its bottom gradually. As shown in Fig. 1e (corresponding to the product with a 20 nm MgO coating layer), a geometrical configuration of ZnO/MgO coaxial NWs with rough, globular heads and well-separated nano-units can be clearly observed by a 251-tilted SEM image, and an enlarged view is shown in the inset. As shown in Fig. 2a, a typical ZnO NW coated with a 20 nm thick MgO layer was selected for the EDX measurements to
Fig. 2 (a) SEM image of a single ZnO/MgO coaxial NW, and (b, c) the corresponding elemental mapping images. (d) EDX spectra obtained from different areas of a single NW as marked in (a).
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Fig. 3 (a) RT PL spectra of the MgO-coated ZnO NWs with different thicknesses. The inset shows the schematic configuration for the PL measurement. (b) UV enhancement and DLE reduction factors of ZnO/MgO coaxial NWs in the case of different MgO layer thicknesses. (c) Plot of the intensity ratio of IUV/IDLE as a function of MgO thickness.
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decreases monotonously with the increase of MgO thickness from 0 to 25 nm, and the reduction of undesirable DLE results from the surface passivation effect of the MgO coating layer, which substantially suppresses the surface-mediated NRR and DLE, and the detailed carrier tunneling–trapping–recombination processes will be discussed later. A minimum DLE reduction factor (defined as the ratio of integrated emission intensity of ZnO NWs with and without MgO coating) of 0.13 was obtained as the MgO thickness increases to 25 nm, as shown in Fig. 3b. While the trend is somewhat dissimilar in the case of UV emission, and the critical thickness of the MgO coating layer can be inferred. A maximum of B7-fold UV enhancement ratio was achieved as the MgO thickness increases to 15 nm, and a further increase of MgO thickness induced the reduction of UV emission instead. In spite of this, the coaxial NWs with a 25 nm thick MgO coating still featured an acceptable UV enhancement factor of 1.28. Undoubtedly, an enhanced UV emission is reasonably beneficial for the realization of high-performance UV-LEDs and LDs. Here, we make a hypothesis that the surface passivation effect of MgO coating cannot be improved further once the coating layer exceeds a critical thickness of 15 nm because a certain amount of surface states has already been passivated by such a coating layer, and one possibility of absorption and scattering of the exciting light (325 nm) by too thick and nano-rough MgO layer ought to have mostly liability in the UV emission decay. The above arguments could be confirmed by the plot of the intensity ratio of IUV/IDLE as a function of MgO layer thickness shown in Fig. 3c, and an interesting and regular change was observed. The ratio of IUV/ IDLE reaches a maximum value of B226 at a MgO thickness of 15 nm, greatly superior to previous reports using MgO and other sheathing materials as the coating layer,14,15,20 and thereupon drops. However, such a reduction of IUV/IDLE at thicker MgO layers was maintained at a certain value around 89, rather than declining constantly. The above observations imply that the number of photogenerated carriers inside the ZnO NWs also strongly depends on the thickness of the MgO layer. Above a critical thickness of 15 nm, two independent radiative recombination pathways involved NBE and DLE that can be weakened simultaneously. And an almost constant IUV/IDLE also verifies our aforementioned hypothesis that the surface passivation effect of MgO coating above a critical value (15 nm) is no more effective. Besides, the existence of an optimum MgO layer thickness could be explained in terms of the effective modulation effect of MgO coating resulting from the combined action of the surface passivation effect and nonradiative thermal absorption in the amorphous MgO layer. With the increase of MgO coating layer thickness, the density of surface states can be reduced gradually, resulting in an increased number of excitons in ZnO NWs, and thus UV emission is enhanced. However, upon further increasing the MgO layer thickness above 15 nm, the nonradiative thermal transition owing to the scattering in the nano-rough MgO layer has to be taken into account. And the thermal transition intensity would increase gradually with the thickness of the MgO coating layer, inducing the decay of UV emission consequently.
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3.2
Mechanistic analysis of the improved PL performance
The substantial improvement of PL performance by MgO coating can be understood in terms of the models shown in Fig. 4a and b. In general, the surface states including the dangling bonds and surface adsorption, such as O2, H2O, and OH ,21,22 could have powerful influence on the PL performance of ZnO nanostructures, especially for those with large surface-to-volume ratios. For the bare ZnO NWs, the adsorbates localized in their surface may readily act as the acceptors, which trap the free electrons in the conduction band of ZnO due to the short relaxation time for carrier tunneling from the interior of ZnO to the surface, resulting in a depletion layer and band bending near the surface. The possible mechanistic pathways for such a capture process can be described as 2H2O(g) + O2 + 4e - 4OH (adsorption) and O2(g) + e - O2 (adsorption).23 On the one hand, the band bending is expected to facilitate the separation of photogenerated electron– hole pairs and the photogenerated holes will accumulate near the surface, as illustrated in Fig. 4a, thus a low probability of radiative recombination and the quenching of NBE excitonic emission are favored consequently. On the other hand, the trapped carriers by the undesirable surface-trapping channel will recombine nonradiatively at the surface, or part of the trapped holes tunnel back into the DL centers to contribute to the DLE, as proposed by Dijken et al.21 For the MgO coated ZnO NWs, the density of surface states can be reduced substantially, thereby decreasing the width of the depletion layer, built-inbarrier height (qVB) and surface band bending, as illustrated in Fig. 4b. The separation of photogenerated electron–hole pairs is therefore alleviated, resulting in an increased number of excitons in ZnO NWs, which makes the enhancement of exciton-related NBE emission possible. In addition, the band alignment of ZnO/ MgO heterostructures may also be an important factor for the improvement of PL performance. Considering the larger band gap of MgO (7.8 eV) than that of ZnO (3.37 eV), the suppression of
Fig. 4 Schematic band diagrams of (a) bare and (b) MgO-coated ZnO NWs. (c) Energy band diagram of a ZnO/MgO heterojunction at thermal equilibrium. (d) Schematic diagram of a ZnO/MgO coaxial NW and a proposed quantum-well-like structure for the explanation of enhancement of carrier radiation recombination.
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tunneling of charge carriers from the ZnO NW to the MgO layer or the air/MgO interface takes effect. In reality, the ZnO/MgO coaxial NWs can be approximatively regarded as a quasiquantum-well-like structure (Fig. 4d) because of the small diameter of ZnO NWs, thus the photogenerated carriers can be confined in the ZnO NW and recombine radiatively and efficiently. Moreover, it was recently reported that it is also likely to generate electron–hole pairs in MgO nanostructures upon laser irradiation with its energy smaller than the band gap of MgO due to the intrinsic defects of oxygen vacancies existing in MgO, which produce the corresponding energy levels in the forbidden energy band of MgO.24 As illustrated in Fig. 4c, the conduction and valence band offsets (DEC and DEV) at the MgO/ZnO interface can be determined to be 3.55 and 0.88 eV, respectively, considering that the electron affinities of MgO and ZnO are 0.80 and 4.35 eV, respectively. Therefore, photogenerated electrons and holes in MgO are transferred from the MgO coating layer to the ZnO NW, meaning that the carrier injection from the MgO barrier to ZnO contributes well to the NBE emission to some extent.
3.3
Electrically pumped random lasing from the MIS diode
To realize the preparation of a MIS-type LD based on ZnO/MgO coaxial NWs, an Au monolayer (B30 nm) was thermally evaporated on the MgO coating layer and patterned into a circular pad (1 mm) by a customized shadow mask, and the schematic diagram of the device structure is shown in Fig. 5a. Fig. 5b shows the I–V characteristics of the constructed NW diode with the positive voltage connected to the Au electrode, and a nonlinear rectifying behavior was observed as the reverse current increases gradually with the bias voltage, as reported in another work.25 EL characterization of the studied diode was performed to demonstrate the lasing action. As shown in Fig. 6, a series of surface EL spectra under a continuous current injection mode are plotted. Note that the UV emission can be detected even at a low excitation current of 2.5 mA. As the current was increased to 4.0 mA, a broad and featureless spontaneous emission peak is observed peaking at B380 nm, with a FWHM of B18 nm. Upon increasing the current to 5.5 mA, some dramatic sharp peaks are observed superposing on the broad emission band. The FWHM of these peaks are as narrow as 0.5 nm, more than 35 times smaller than that obtained at 4.0 mA. Following the increase in the current input to 6.4 mA and above, the number
Fig. 5
(a) Schematic diagram and (b) I–V characteristics of the studied LD.
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Fig. 6 Surface EL spectra of the LD under different driving currents. The inset shows the dependence of the integrated EL intensity on the injection current of the studied LD.
and intensity of such sharp peaks are increased correspondingly spanning from 370 to 410 nm. The narrow linewidth and a rapid increase in the emission intensity are indications of the appearance of lasing action, which implies that the optical gain obtained along the laser cavities is sufficiently high to achieve lasing at such a low current injection level. In addition, the dependence of the integrated EL intensity against the injection current is plotted in the inset of Fig. 6, from which an ultralow threshold current of B4.5 mA (0.58 A cm 2) can be derived, much lower than previously reported blue-/UV-light LDs employing ZnO and/or other semiconductors with small exciton binding energies as the gain media.26–28 Note that the ‘‘softer’’ threshold in our case is likely due to the large scattering loss and a relatively strong background of spontaneous emission, which has been frequently observed in ZnO micro- and nanolasers.29,30 The realization of lasing action in the studied diode without any artificially designed resonant cavity prompts us to consider the possible resonant model of the lasing action. Although ZnO NWs have been proven to be capable of achieving Fabry–Perot (F–P) type resonator cavities defined by the head facet of ZnO NWs and the epitaxial ZnO/substrate interface, such possibility in our case is almost negligible. It is generally accepted that the laser oscillation action depends strongly on the diameter of NWs; in theory, the NWs with the diameter smaller than the oscillation wavelength cannot play the role of light confinement and amplification substantially because the optical field could readily spill out of the NWs. The direct consequence is that the population inversion conditions cannot be satisfied, or satisfied only at an elevated pumping level because the round-trip gain inside the NWs must equal the round-trip loss. Experimentally, Zimmler et al. observed that the laser oscillation threshold in F–P cavities of ZnO NWs is greatly influenced by the diameter of the NW, with no laser oscillation realized for diameters smaller than B150 nm.31 More importantly, the as-prepared ZnO NWs do not behave like a smooth top surface (shown in Fig. 1d) serving as a mirror. Therefore, a sufficient optical gain enabling a F–P laser cannot be provided in these ZnO NWs, excluding the possibility of a F–P cavity mode established by two end facets of NWs reasonably. The resonant mode herein is therefore attributed to the random lasing, implying the random scattering and amplification of photons between adjacent NWs rather than a single NW. That is to say,
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the ZnO NWs act as both the gain media and scattering units in the studied diode. On some occasions, the emitted photons can return to a scattering path from which it was scattered before, thus the quasi-two-dimensional closed-loop laser cavities can be formed. When the optical gain exceeds the loss, lasing oscillation occurs. In addition, two aspects of this figure are important to note. Firstly, the irregular evolution of observed sharp peaks shows direct evidence of the determined random lasing action. With the increase of driving current, the position of lasing peaks is randomly changed, and the wavelength spacing between two adjacent peaks does not own a fixed value. These features elucidate the intrinsic traits of a random lasing mode.32–35 Secondly, three successive EL measurements of the studied diode at the same driving current were carried out for a further verification, with an interval of B120 s. As shown in Fig. 7a, the three EL spectra display distinct lasing spikes with different peak positions and FWHMs although the injection current and measurement conditions are the same, manifesting a random lasing action rather than a specific resonant mode. Besides, near-field optical microscope images of the studied LD were also recorded across the entire contact area for a vivid presentation of a random nature. As shown in Fig. 7b, many randomly distributed bright spots, which represented the isolated random resonators formed in the proposed ZnO/MgO coaxial heterostructure, were observed from the circular Au contact during the three successive EL measurements. It should be mentioned herein that the light emitted at B380 nm is beyond the response range of the digital microscope (Aigo, GE-5), inducing color aberration between the captured color (red) and the actual emission color (UV). Importantly, it was found that the locations of almost all bright spots irregularly changed in real time, which is typical in the case of a random lasing action and provides additional evidence for our arguments. Actually, the
Fig. 7 (a) EL spectra of the LD for three successive measurements under the same injection current of 7.9 mA. (b) Optical microscope image of the LD corresponding to the three successive measurements, and the scale bar is 250 mm. Note that the left image was taken with lamp illumination and zero bias.
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above-mentioned scenario was frequently observed in the optically pumped random lasing,36 similar to the situation in the case of electrically pumped behavior. In the studied diode, ZnO NWs can be regarded as both the random gain media and multiple scattering units. The light emission obtained at each circle can be scattered with random walks, corresponding to different feedback loops. For certain modes of emitted light which acquire a high optical gain to sustain the lasing, they vary from time to time in terms of the position, intensity and FWHM of the lasing peak because of the real-time change in the light feedback path. In addition, the evolution of the spatial distribution of the lasing action was also studied by angular-dependent EL measurements (shown in Fig. S1, ESI†), as frequently performed in the determination of a random lasing. Herein, we consider that the multidirectional light output is not the definitive measure of therandom nature because the ZnO NW F–P laser device also demonstrated a broadened emission pattern with angular oscillation,1 which is presumably due to the feature that the ZnO NW laser works as nanoscale units. Another important reason is that nano-rough ZnO/MgO interfaces were formed after the MgO coating, which favored an increased light scattering effect, making the large spatial distribution of the lasing action possible. In order to verify the sensitivity of the lasing action to operating temperature, the LD was pumped at an elevated temperature point (353 K). As shown in Fig. 8a, two typical lasing spectra captured at 295 and 353 K, respectively, operated at 7.9 and 13.9 mA were presented. It is remarkable to find that the lasing action can be still sustained at 353 K, which is evident from the low linewidth of the sharp peaks. The above observation suggests that the studied ZnO/ MgO coaxial NW diode has good temperature tolerance. Theoretically, the heating effect will inevitably give rise to a rapid proliferation of structural defects, and thus the probability of nonradiative recombination is likely to increase in proportion to operating temperature. The direct consequence is that the lasing action ceases or no reliable light output can be observed normally. Because no artificially designed resonant cavity is formed in the structure to provide a constant and stable optical gain operating
Fig. 8 (a) EL spectra of the LD measured at RT and 353 K with the driving currents of 7.9 and 13.9 mA, respectively. (b) Dependence of the lasing threshold on working temperature.
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at high-temperature, and moreover, a high scattering loss in a random fashion requires a high optical gain to sustain the random lasing. Therefore, it is reasonably believed that the high operating temperature of the LD is evident of the rational design of the device configuration, and the ideal building blocks of ZnO/MgO coaxial NWs towards the design of low-threshold LDs. As we stated above, at elevated temperatures the reduced carrier injection efficiency and radiative recombination probability will impede the UV lasing, and a higher pumping intensity is therefore required to compensate for the heating-induced loss. As shown in Fig. 8b, the lasing threshold of the LD is found to be 4.5 mA at RT, and it increases to 7.2 mA at 323 K, further increasing to 11.7 mA at 353 K, featuring a regular sensitivity to the temperature change. 3.4
Analyses of laser behavior
One of the features of the proposed laser model is the ultralow threshold current density, and it seems very worthwhile for us to clarify the inherent mechanisms because no complicated structures such as double heterojunction or quantum well layers were introduced. By overall analyses of our device structure and experiment results, possible reasons could be summarized and listed as follows. Firstly, the intrinsic characteristics of ZnO, large exciton-binding energy and high optical gain, make a great contribution to the ultralow lasing threshold. Moreover, the lasing action through the exciton interaction process is essentially more favorable for a low threshold laser than that through the electron–hole-plasma process. Secondly, an optimized carrier injection and confinement configuration greatly contribute to the ultralow lasing threshold. In our case, the dielectric MgO coating layer functions as the electron blocking layer and the hole supplying layer simultaneously. Due to the existence of high DEC, electrons are available in abundance at the MgO/ZnO interface under forward bias, as depicted in Fig. S2 (ESI†). And meanwhile, a sufficiently high bias enables the generation of holes in the MgO layer due to the high-electric-field-induced impact ionization process considering that almost all the voltage is applied to the insulating layer.37–39 The generated holes can be swept into the valence band of ZnO above a critical driving voltage, and recombine radiatively with the electrons accumulated at the MgO/ZnO interface, resulting in the NBE emission and even a lasing action. Thirdly, the effective surface passivation effect of the MgO layer ensures high-quality and optically active NWs, thus the surface-mediated NRR and DLE can be suppressed substantially (The wide-range EL spectrum in the UV-visible region is presented in Fig. S3, ESI†). It is also a general consensus that the defect emission and surface NRR could impede the UV lasing due to the reason that the required pumping energy for UV stimulated emission could be consumed with the spontaneous defect emission and surface NRR. Last but not least, the MgO coating, in the proposed ZnO/MgO coaxial NW structure, extends to a certain depth along the axial direction of ZnO NWs, resulting in a large refraction index fluctuation in the plane perpendicular to the ZnO NWs, the direct consequence of which is an enhanced light scattering capability of the active layer. In theory, Apalkov et al. predicted that random variation of the
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refraction index favors the formation of resonant cavities due to the inherent waveguiding structures in the plane.40 Therefore, it is reasonably deduced that the studied coaxial heterostructure featuring a strong light scattering capability can be electrically pumped into random lasing with a relatively low input power. Besides, an increased junction area offered by the studied heterostructures exclusively provides more chance for an enhanced carrier recombination efficiency, which further supports and facilitates the realization of a low-threshold lasing.
4. Conclusions In conclusion, we have successfully demonstrated the feasibility of utilizing ZnO/MgO coaxial NW structures to obtain highly efficient excitonic UV emission. Sheathing ZnO NWs with an appropriate MgO layer thickness results in substantial suppression of DLE, and a significant enhancement of UV emission. The intensity ratio of IUV/IDLE can also be tuned to achieve the best value at a critical thickness of 15 nm for MgO coating. The improved exciton emission efficiency by MgO coating can be attributed to the desirable surface passivation effects, which leads to a quasi-flat-band effect near the ZnO surface and an increased number of excitons in ZnO, enhancing the probability of carrier radiative recombination consequently. Besides, the typical band alignment of ZnO/MgO heterostructures favors the photogenerated carrier confinement and transfer behavior, which also blocks the undesirable surface-trapping channel and thereby improves the UV emission efficiency. The excellent optical properties of the as-produced ZnO/MgO coaxial NWs were further confirmed by constructing a MIS-type LD, which featured an ultralow threshold of 4.5 mA (0.58 A cm 2) and good temperature tolerance. Our work here provides a feasible way to improve the excitonic emission efficiency of ZnO, and demonstrates that ZnO/MgO coaxial NWs can be employed as ideal building blocks for the design of low-threshold LDs, which could promote existing applications and suggest new opportunities.
Acknowledgements This work was supported by the National Basic Research Program of China (2011CB302005), the National Natural Science Foundation of China (No 61106003, 61376046 and 61223005), the Science and Technology Developing Project of Jilin Province (20130204032GX), and the Program for New Century Excellent Talents in University (NCET-13-0254).
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