ZnO ultraviolet random laser diode on metal copper substrate C. Y. Liu,1 H. Y. Xu,1,* Y. Sun,1 J. G. Ma,1 and Y. C. Liu1,2 1
Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China 2 [email protected] * [email protected]
Abstract: Direct fabrication of light emitting devices on metal substrates is highly desirable due to their advantages of high thermal conductivity and light reflection. In this work, we demonstrated a feasibility of directly fabricating ZnO-based ultraviolet laser diodes on metal substrates. By introducing an anti-oxidation buffer layer, Au/MgO/ZnO metal-insulatorsemiconductor heterojunction devices are successfully fabricated on the copper substrate. Electrically pumped ultraviolet random lasing was achieved from ZnO active layer. The use of copper substrate offers some merits, including lower thermal effect and higher stability of emission wavelength. ©2014 Optical Society of America OCIS codes: (140.3610) Lasers, ultraviolet; (140.5960) Semiconductor lasers; (160.6000) Semiconductor materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
D. S. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4(5), 359–367 (2008). J. Huang, S. Chu, J. Kong, L. Zhang, C. M. Schwarz, G. Wang, L. Chernyak, Z. Chen, and J. Liu, “ZnO p–n homojunction random laser diode based on nitrogen-doped p-type nanowires,” Adv. Opt. Mater. 1(2), 179–185 (2013). H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chang, “Random laser action in semiconductor powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). J. Fallert, R. J. B. Dietz, J. Sartor, D. Schneider, C. Klingshirn, and H. Kalt, “Co-existence of strongly and weakly localizedrandom laser modes,” Nat. Photon. 3(5), 279–282 (2009). H. Y. Yang, S. F. Yu, G. P. Li, and T. Wu, “Random lasing action of randomly assembled ZnO nanowires with MgO coating,” Opt. Express 18(13), 13647–13654 (2010). S. F. Yu, C. Yuen, S. P. Lau, and H. W. Lee, “Zinc oxide thin-film random lasers on silicon substrate,” Appl. Phys. Lett. 84(17), 3244–3246 (2004). H. Zhu, C. X. Shan, J. Y. Zhang, Z. Z. Zhang, B. H. Li, D. X. Zhao, B. Yao, D. Z. Shen, X. W. Fan, Z. K. Tang, X. Hou, and K. L. Choy, “Low-threshold electrically pumped random lasers,” Adv. Mater. 22(16), 1877–1881 (2010). H. Long, G. Fang, S. Li, X. Mo, H. Wang, H. Huang, Q. Jiang, J. Wang, and X. Zhao, “A ZnO/ZnMgO multiplequantum-well ultraviolet random laser diode,” IEEE Electron Device Lett. 32(1), 54–56 (2011). C. Y. Liu, H. Y. Xu, J. G. Ma, X. H. Li, X. T. Zhang, Y. C. Liu, and R. Mu, “Electrically pumped nearultraviolet lasing from ZnO/MgO core/shell nanowires,” Appl. Phys. Lett. 99(6), 063115 (2011). Y. Li, X. Ma, L. Jin, and D. Yang, “A chemical strategy to reinforce electrically pumped ultraviolet random lasing from ZnO films,” J. Mater. Chem. 22(33), 16738–16741 (2012). C. Y. Liu, H. Y. Xu, Y. Sun, C. Zhang, J. G. Ma, and Y. C. Liu, “Ultraviolet electroluminescence from Au/MgO/MgxZn1-xO heterojunction diodes and the observation of Zn-rich cluster emission,” J. Lumin. 148, 116– 120 (2014). S. J. Jiao, Z. Z. Zhang, Y. M. Lu, D. Z. Shen, B. Yao, J. Y. Zhang, B. H. Li, D. X. Zhao, X. W. Fan, and Z. K. Tang, “ZnO p-n junction light-emitting diodes fabricated on sapphire substrates,” Appl. Phys. Lett. 88(3), 031911 (2006). Ya. I. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, “Fabrication and characterization of n-ZnO/p-AlGaN heterojunction light-emitting diodes on 6HSiC substrates,” Appl. Phys. Lett. 83(23), 4719–4721 (2003). K. Chung, C. H. Lee, and G. C. Yi, “Transferable GaN layers grown on Zno-coated graphene layers for optoelectronic devices,” Science 330(6004), 655–657 (2010). T. Doan, C. Chu, C. Chen, W. Liu, J. Chu, J. Yeh, H. Chen, F. Fan, and C. Tran, “Vertical GaN based light emitting diodes on metal alloy substrate for solid state lighting application,” Proc. SPIE 6134, 61340G (2006).
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16731
16. T. Doan, C. Tran, C. Chu, C. Chen, W. H. Liu, J. Chu, K. Yen, H. Chen, and F. Fan, “Vertical GaN based light emitting diodes on metal alloy substrate boosts high power LED performance,” Proc. SPIE 6669, 666903 (2007). 17. C. F. Chu, F. I. Lai, J. T. Chu, C. C. Yu, C. F. Lin, H. C. Kuo, and S. C. Wang, “Study of GaN light-emitting diodes fabricated by laser lift-off technique,” J. Appl. Phys. 95(8), 3916–3922 (2004). 18. B. S. Tan, S. Yuan, and X. J. Kang, “Performance enhancement of InGaN light-emitting diodes by laser lift-off and transfer from sapphire to copper substrate,” Appl. Phys. Lett. 84(15), 2757–2759 (2004). 19. Y. Sun, T. Yu, Z. Chen, X. Kang, S. Qi, M. Li, G. Lian, S. Huang, R. Xie, and G. Zhang, “Properties of GaNbased light-emitting diode thin film chips fabricated by laser lift-off and transferred to Cu,” Semicond. Sci. Technol. 23(12), 125022 (2008). 20. Y. F. Dong, Q. S. Li, L. C. Zhang, and L. K. Song, “Contact properties of Zno/Cu films with MSM structure,” Chin. J. Lumin. 33, 412–416 (2012). 21. C. Y. Liu, H. Y. Xu, L. Wang, X. H. Li, and Y. C. Liu, “Pulsed laser deposition of high Mg-content MgZnO films: Effects of substrate temperature and oxygen pressure,” J. Appl. Phys. 106(7), 073518 (2009). 22. C. Liu, H. Xu, J. Ma, and Y. Liu, “Origin of ultraviolet electroluminescence in n-ZnO/p-GaN and n-MgZnO/pGaN heterojunction light-emitting diodes,” Phys. Status Solidi A 210(12), 2751–2755 (2013). 23. C. H. Chen, S. J. Chang, S. P. Chang, M. J. Li, I. C. Chen, T. J. Hsueh, and C. L. Hsu, “Electroluminescence from n-ZnO nanowires/p-GaN heterostructure light-emitting diodes,” Appl. Phys. Lett. 95(22), 223101 (2009). 24. C. Y. Liu, Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory for UV LightEmitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China, H. Y. Xu, J. G. Ma, and Y. C. Liu are preparing a manuscript to be called “Enhanced ultraviolet random lasing from p-Cu2O inserted ZnO-based metal-insulator-semiconductor heterostructure device.” 25. E. F. Schubert, Light-Emitting Diodes (Cambridge University, 2003). 26. S. Gottardo, S. Cavalieri, O. Yaroshchuk, and D. S. Wiersma, “Quasi-two-dimensional diffusive random laser action,” Phys. Rev. Lett. 93(26), 263901 (2004).
1. Introduction Random laser is attracting increasing attention due to its potential applications in broad angular display, remote temperature sensing, document encoding, medical diagnostics, etc [1,2]. The formation conditions of random laser are not so strict as those of conventional laser modes, and a disorder system can act as both gain medium and resonant cavity, which offers advantages of simple processing technique and low cost. With a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, ZnO is considered as a promising candidate for ultraviolet light-emitting devices. Since last 90s, optically pumped random laser has been observed in various ZnO structures including nanoparticles, nanowires and thin films [3–6]. Recently, electrically pumped random laser were also achieved from ZnO homo- and heterostructures [2,7–11]. These laser diodes (LDs) were usually fabricated on the Al2O3, SiC and GaN substrates [8,12,13]. Though these substrates have an epitaxial relationship with ZnO, their low thermal conductivity and high transparency bring some negative effects, such as poor heat dissipation and light loss in the direction of substrates. These problems can be mitigated by the use of metal substrates [14–19]. However, the device fabrication on metal substrates is very difficult because of thermal mismatch and lattice mismatch. At the present stage, a laser lift-off technique is usually employed to transfer light emitting devices from epitaxial wafers to metal substrates [17–19], which not only increases the technical complexity, but also may degrade the device performance. Thereby, it is highly desirable to directly grow ZnO LDs on metal substrates. We know that instead of single-crystalline film, a disorder polycrystalline film is usually formed on metal substrates, and such a microstructural feature just favors the generation of random laser. That is, not strict conditions for random lasing offer the possibility of achieving ZnO random LDs on metal substrates. Herein, we demonstrated a feasibility of directly fabricating ZnO ultraviolet random LDs on metal Cu substrates based on an Au/MgO/ZnO metal-insulator-semiconductor (MIS) heterostructure. To our knowledge, almost no report of ZnO light emitting devices on metal substrate is available before. Compared to p-n heterojunction devices such as p-GaN/n-ZnO, pure ZnO emission is easier to obtain from these MIS heterostructure [7,9–11]. The devices built on the Cu substrates showed some improved performance, including lower thermal effect and better stability of emission wavelength. The introduction of an anti-oxidation layer between the Cu substrate and the MIS heterostructure was observed to be a key factor to activate the device’s emission.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16732
2. Experiments
Fig. 1. (a) A structure diagram of Au/MgO/ZnO MIS-heterojunction LDs on the Cu substrate. (b), (c) and (d) XRD patterns, I-V characteristics and PL spectra of ZnO films or LDs on the Cu substrate with and without anti-oxidation layer.
In our experiment, the polished polycrystalline Cu wafer is used as the substrate, which is known as an ideal choice for the heat-sink of commercial chips due to its high thermal conductivity. The work function of Cu (~4.65 eV) is close to the Fermi level of ZnO (depending on its electron concentration), and it has been reported that an ohmic contact can be formed between Cu and ZnO by thermal diffusion [20]. Figure 1(a) shows the schematic illustration of MIS diodes on the Cu substrate. In our experiment, two different programs are employed. For the first one, the MIS heterojunction is directly prepared on the Cu substrate. ZnO (~330 nm) and MgO (~170 nm) layers are sequentially grown by pulsed laser deposition (PLD) in oxygen atmosphere, and the chamber pressure and substrate temperature are 20 Pa and 600 °C, respectively. Then, very thin Au electrodes are thermally evaporated on the topside, and patterned into 1 mm circular pad by shadow mask. The electroluminescence (EL) is collected from these semitransparent Au electrodes and their edges. Detailed growth conditions can be found in our previous work [11,21,22]. In this program, the device structure is almost identical to that of the reported LDs on conventional substrates like Si and ITO glass [7,11]. However, such a simple transfer to the metal Cu substrate does not yield a workable LD device. Instead of the smooth surface observed in epitaxial substrates, this sample on the Cu substrate looks dark and rough. X-ray diffraction (XRD) measurement [Fig. 1(b)] shows that besides Cu diffraction peaks, the signals of Cu2O and CuO can also be detected, indicating that the Cu substrate surface is oxidized during the deposition of ZnO film. Since the oxygen activity can be enhanced in the plasma plume of PLD, the oxidation of Cu surface is inevitable at the high substrate temperature. The oxidation deteriorates the heterojunction device. It is known that Cu2O is usually p-type, therefore an unintended p-Cu2O/n-ZnO heterojunction would form underlying the MIS heterojunction, and the two diodes are connected back to back. As shown in current-voltage (I-V) curve of the oxidized device [inset of Fig. 1(c)], the turn-on characteristic is observed at both forward and reverse bias.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16733
Symmetrical rectifying behavior demonstrates the formation of p-Cu2O/n-ZnO/MgO/Au heterostructure [23]. The injection current remains at a low level due to the bidirectional blocking effect, and this heterostructure is easy to break down as the current gets higher. No EL emission could be obtained. To prevent the Cu substrate from being oxidized, the gas ambient is changed at the initial stage of ZnO growth. A two-step growth method is introduced in the second program. Firstly, a ~240 nm-thick oxygen-deficient ZnO layer is deposited on the Cu substrate in ultrapure argon ambient at a lower temperature of 300 °C. Then, high-quality ZnO film (~90 nm) is grown in oxygen atmosphere at 600 °C. The other fabrication steps and conditions are the same as those described in the first program. There are no diffraction signals from Cu2O and CuO observed in XRD pattern, indicating the inserted oxygen-deficient ZnO layer indeed plays a role of anti-oxidation coating. Typical diode behavior is observed in the I-V characteristic of Fig. 1(c). XRD studies [Fig. 1(b)] reveal that ZnO active layer in the present device is preferentially oriented along c-axis direction, differing from the multiple orientations observed on the oxidized Cu substrate, and the diffraction peak becomes more intense and sharp. Figure 1(d) presents the photoluminescence (PL) spectra of the two devices with and without anti-oxidation layer. The emission intensity of directly grown ZnO is very weak, it could be attributed to its inferior crystalline quality as well as the additional pCu2O/ZnO heterojunction, where the built-in field of this p-n junction can separate photogenerated electrons and holes. While the ZnO device with anti-oxidation layer exhibits a nearly two orders of magnitude stronger near-band-edge (NBE) emission at ~377 nm, even though the excitation power reduces to one tenth. All these observations suggest that the antioxidation ZnO layer also performs as a buffer layer to improve the crystalline and optical quality of ZnO active layer, though the related physical mechanism cannot be fully understood at present. 3. Results and discussions
Fig. 2. (a) EL spectra of ZnO LDs on the Cu substrate with an anti-oxidation layer under different injection currents, and its inset shows the energy-band alignment of Au/MgO/ZnO heterostructure under forward bias, which is plotted according to Anderson model. (b) Superlinear dependence of integrated EL intensity on the injection current density, the inset illustrates the process of population inversion and stimulated emission. (c) EL spectrum of ZnO MIS LDs on the Si substrate [Ref. 11, Copyright 2014, with permission from Elsevier].
Figure 2(a) presents a set of EL spectra of ZnO LDs on the Cu substrate with an antioxidation layer. Under the injection current density of 25.5 A/cm2, a spontaneous emission emerges at ~381 nm, which may be associated with ZnO exciton recombination, and no detectable EL signal is observed in the visible region. The recombination mechanism can be understood in terms of the band alignment [inset of Fig. 2(a)]. Under forward bias, electrons would be blocked and accumulated at ZnO/MgO interface due to the large conduction band offset. Considering its dielectric nature, large portion of the bias is applied on MgO layer. The electric field strength could be up to ~107 V/m, thus electrons and holes can be generated #211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16734
through a so-called impact-ionization process [7,9–11]. The holes from MgO are driven into ZnO and recombine with the accumulated electrons radiatively, resulting in a ZnO NBE emission. As the injection reaches 31.8 A/cm2, some sharp peaks appear and superimpose on the spontaneous emission band. With further increasing injection to 50.9 A/cm2, more distinct sharp peaks are observed. The appearance of sharp peaks demonstrates lasing action, which is further confirmed by the superlinear dependence of integrated emission intensity on current density, as shown in Fig. 2(b). The lasing threshold is determined as 29.3 A/cm2, which is larger than that of ZnO LDs on conventional substrates reported by our group [9,11]. Figure 2(c) shows a typical lasing spectrum of ZnO MIS LDs on the Si substrate in Ref [11]. for comparison. Distinct lasing spikes could be observed under 5.09 A/cm2, while the emission shifts towards longer wavelength several nanometers more than the LDs on the Cu substrate. However, it is indicated that the present ZnO LDs on the Cu substrate has large lasing threshold and low external quantum efficiency. It may be mainly attributed to the following factors: (i) A lot of carrier traps, generated in the anti-oxidation layer due to low growth temperature and non-stoichiometry, may act as nonradiative recombination centers and decrease effective carrier injection. (ii) Ascribing to the rough surfaces of used Cu wafers, the root-mean-square roughness of ZnO film is up to 34.4 nm. Post-grown MgO can hardly cover ZnO surface uniformly, many leak channels would form. (iii) The hole generation rate of MIS heterojunctions is very low. Overall, the improvement of threshold and efficiency is most important in our further work, possible plans are proposed herein: (1) The growth ambience of anti-oxidation layer can be changed as nitrogen/hydrogen mixed gas to passivate native defects, and its thickness and growth temperature could be further optimized. (2) Singlecrystalline Cu wafers (Rq < 10 nm) would be employed instead. (3) It is reasonably expected that the performance of MIS device could be improved by introducing a hole-injection layer [24]. It provides us another feasible route to realize ZnO LDs on metal Cu substrate, that is, an inverted MIS heterostructure of Cu/p-Cu2O/MgO/ZnO/In, where the p-Cu2O layer is prepared by thermal oxidation. For this inverted stack structure, the oxidation of Cu substrate is no longer a disadvantage and the native defects could also be suppressed to some extent.
Fig. 3. (a) A comparison between PL and EL spectra of ZnO LDs on the Cu substrate. (b) and (c) I-V characteristics of ZnO LDs with the same MIS heterostructures on Cu and n+-Si substrates.
It is known that the ZnO LDs on the conventional substrates with low heat conductivity usually suffer from the heat dissipation problem, which causes the thermal instability of emission wavelength. The EL peak would continuously shift towards longer wavelengths with the injection current increasing [7], and a large relative red-shift is also observed between the EL and PL spectra [7,9]. For example, the EL emission even red-shifted to ~400 nm in our reported ZnO MIS LDs on ITO glass [9]. In contrast, the present ZnO LD on the Cu substrate exhibits much smaller thermal effect. As shown in Figs. 2(a) and 3(a), the central wavelength of EL emission band remains almost invariable as the injection current density increases from 25.5 to 63.7 A/cm2, and the relative shift between EL and PL peaks is negligible. In addition, though the threshold gets larger, the enhanced heat dissipation through the metal Cu substrate allows a high current injection without device failure.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16735
The use of Cu substrate offers additional benefits. Figures 3(b) and 3(c) show the I-V characteristics of the same MIS heterojunction LDs on Cu and n+-Si substrates. Herein, the Si wafer is n-type, high conductive, whose resistivity is less than 0.02 Ω·cm. By a linear fit to the I-V curves beyond the turn-on voltage [25], the series resistances are determined as 7.8 and 241 Ω. Smaller resistance produces less joule heat, further reducing the thermal effect of ZnO LDs on the Cu substrates. In addition, the use of Cu substrate not only simplifies the electrode preparation, but also can suppress the current-crowding effect, because the vertical configuration of anode and cathode can yield uniform current spreading [see the inset of Figs. 3(b) and 3(c)] [15,16].
Fig. 4. (a) A schematic diagram of the angle-dependent EL measurement configuration. (b) The lasing spectra of ZnO LDs on the Cu substrate taken from different angles. (c) Crosssectional TEM image of the ZnO film grown on the Cu substrate. (d) Schematic illustration of the formation process of random laser.
Now, let us briefly discuss the mode and formation mechanism of ZnO lasing. Figures 4(a) and 4(b) shows the schematic configuration of the angle-dependent EL measurement and the lasing spectra collected from different space angles, respectively. Distinct lasing spikes are observed in different directions, which is a strong evidence of random lasing action [7– 9,11]. As mentioned above, disorder medium provides the optical gain in random laser process. To further investigate the microstructure, cross-sectional transmission electron microscopy (TEM) characterization is performed and the image is shown in Fig. 4(c). The ZnO film grown on the Cu substrate is observed to be textured and consist of nanocolumns with different sizes, which act as both gain medium and scattering units in random lasing [9]. It is mentioned that the film surface roughness is dozens of nanometers, and many voids exist among the top. The post-grown MgO could fill up some of these voids. Thus, the ZnO/MgO interface is highly disordered, and the spatial variation of refractive index yields a twodimensional strong scattering media [9,11,26]. As illustrated in Fig. 4(d), the emitted light undergoes multiple coherent scatterings with lateral facets of ZnO grains, and returns to the starting point, forming a closed loop. As the optical gain exceeds the loss, random lasing oscillation occurs.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16736
4. Conclusions In summary, ZnO-based MIS LDs were directly built on the Cu substrate by introducing an anti-oxidation layer. The high thermal and electrical conductivity of Cu substrate increases the heat dissipation and decreases series resistance, thus resulting in lower thermal effect and higher wavelength stability. Besides, the formation mechanism of random laser is also discussed based on micro-structural studies. However, the present ZnO LDs on the Cu substrates still suffer from some problems, such as large lasing threshold and low device efficiency. The disadvantageous factors of present LDs are analyzed, some possible solutions related to these problems are proposed herein, and further work is in progress. Acknowledgments The work is supported by NSFC (Nos. 51172041 and 51372035), the Program for New Century Excellent Talents in University (No. NCET-11–0615), 973 Program (No. 2012CB933703), “111”project (No. B13013), the Fund from Jilin Province (Nos. 20121802 and 201201061), and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130043110004).
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16737
Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China 2 [email protected] * [email protected]
Abstract: Direct fabrication of light emitting devices on metal substrates is highly desirable due to their advantages of high thermal conductivity and light reflection. In this work, we demonstrated a feasibility of directly fabricating ZnO-based ultraviolet laser diodes on metal substrates. By introducing an anti-oxidation buffer layer, Au/MgO/ZnO metal-insulatorsemiconductor heterojunction devices are successfully fabricated on the copper substrate. Electrically pumped ultraviolet random lasing was achieved from ZnO active layer. The use of copper substrate offers some merits, including lower thermal effect and higher stability of emission wavelength. ©2014 Optical Society of America OCIS codes: (140.3610) Lasers, ultraviolet; (140.5960) Semiconductor lasers; (160.6000) Semiconductor materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
D. S. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4(5), 359–367 (2008). J. Huang, S. Chu, J. Kong, L. Zhang, C. M. Schwarz, G. Wang, L. Chernyak, Z. Chen, and J. Liu, “ZnO p–n homojunction random laser diode based on nitrogen-doped p-type nanowires,” Adv. Opt. Mater. 1(2), 179–185 (2013). H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chang, “Random laser action in semiconductor powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). J. Fallert, R. J. B. Dietz, J. Sartor, D. Schneider, C. Klingshirn, and H. Kalt, “Co-existence of strongly and weakly localizedrandom laser modes,” Nat. Photon. 3(5), 279–282 (2009). H. Y. Yang, S. F. Yu, G. P. Li, and T. Wu, “Random lasing action of randomly assembled ZnO nanowires with MgO coating,” Opt. Express 18(13), 13647–13654 (2010). S. F. Yu, C. Yuen, S. P. Lau, and H. W. Lee, “Zinc oxide thin-film random lasers on silicon substrate,” Appl. Phys. Lett. 84(17), 3244–3246 (2004). H. Zhu, C. X. Shan, J. Y. Zhang, Z. Z. Zhang, B. H. Li, D. X. Zhao, B. Yao, D. Z. Shen, X. W. Fan, Z. K. Tang, X. Hou, and K. L. Choy, “Low-threshold electrically pumped random lasers,” Adv. Mater. 22(16), 1877–1881 (2010). H. Long, G. Fang, S. Li, X. Mo, H. Wang, H. Huang, Q. Jiang, J. Wang, and X. Zhao, “A ZnO/ZnMgO multiplequantum-well ultraviolet random laser diode,” IEEE Electron Device Lett. 32(1), 54–56 (2011). C. Y. Liu, H. Y. Xu, J. G. Ma, X. H. Li, X. T. Zhang, Y. C. Liu, and R. Mu, “Electrically pumped nearultraviolet lasing from ZnO/MgO core/shell nanowires,” Appl. Phys. Lett. 99(6), 063115 (2011). Y. Li, X. Ma, L. Jin, and D. Yang, “A chemical strategy to reinforce electrically pumped ultraviolet random lasing from ZnO films,” J. Mater. Chem. 22(33), 16738–16741 (2012). C. Y. Liu, H. Y. Xu, Y. Sun, C. Zhang, J. G. Ma, and Y. C. Liu, “Ultraviolet electroluminescence from Au/MgO/MgxZn1-xO heterojunction diodes and the observation of Zn-rich cluster emission,” J. Lumin. 148, 116– 120 (2014). S. J. Jiao, Z. Z. Zhang, Y. M. Lu, D. Z. Shen, B. Yao, J. Y. Zhang, B. H. Li, D. X. Zhao, X. W. Fan, and Z. K. Tang, “ZnO p-n junction light-emitting diodes fabricated on sapphire substrates,” Appl. Phys. Lett. 88(3), 031911 (2006). Ya. I. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, “Fabrication and characterization of n-ZnO/p-AlGaN heterojunction light-emitting diodes on 6HSiC substrates,” Appl. Phys. Lett. 83(23), 4719–4721 (2003). K. Chung, C. H. Lee, and G. C. Yi, “Transferable GaN layers grown on Zno-coated graphene layers for optoelectronic devices,” Science 330(6004), 655–657 (2010). T. Doan, C. Chu, C. Chen, W. Liu, J. Chu, J. Yeh, H. Chen, F. Fan, and C. Tran, “Vertical GaN based light emitting diodes on metal alloy substrate for solid state lighting application,” Proc. SPIE 6134, 61340G (2006).
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16731
16. T. Doan, C. Tran, C. Chu, C. Chen, W. H. Liu, J. Chu, K. Yen, H. Chen, and F. Fan, “Vertical GaN based light emitting diodes on metal alloy substrate boosts high power LED performance,” Proc. SPIE 6669, 666903 (2007). 17. C. F. Chu, F. I. Lai, J. T. Chu, C. C. Yu, C. F. Lin, H. C. Kuo, and S. C. Wang, “Study of GaN light-emitting diodes fabricated by laser lift-off technique,” J. Appl. Phys. 95(8), 3916–3922 (2004). 18. B. S. Tan, S. Yuan, and X. J. Kang, “Performance enhancement of InGaN light-emitting diodes by laser lift-off and transfer from sapphire to copper substrate,” Appl. Phys. Lett. 84(15), 2757–2759 (2004). 19. Y. Sun, T. Yu, Z. Chen, X. Kang, S. Qi, M. Li, G. Lian, S. Huang, R. Xie, and G. Zhang, “Properties of GaNbased light-emitting diode thin film chips fabricated by laser lift-off and transferred to Cu,” Semicond. Sci. Technol. 23(12), 125022 (2008). 20. Y. F. Dong, Q. S. Li, L. C. Zhang, and L. K. Song, “Contact properties of Zno/Cu films with MSM structure,” Chin. J. Lumin. 33, 412–416 (2012). 21. C. Y. Liu, H. Y. Xu, L. Wang, X. H. Li, and Y. C. Liu, “Pulsed laser deposition of high Mg-content MgZnO films: Effects of substrate temperature and oxygen pressure,” J. Appl. Phys. 106(7), 073518 (2009). 22. C. Liu, H. Xu, J. Ma, and Y. Liu, “Origin of ultraviolet electroluminescence in n-ZnO/p-GaN and n-MgZnO/pGaN heterojunction light-emitting diodes,” Phys. Status Solidi A 210(12), 2751–2755 (2013). 23. C. H. Chen, S. J. Chang, S. P. Chang, M. J. Li, I. C. Chen, T. J. Hsueh, and C. L. Hsu, “Electroluminescence from n-ZnO nanowires/p-GaN heterostructure light-emitting diodes,” Appl. Phys. Lett. 95(22), 223101 (2009). 24. C. Y. Liu, Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory for UV LightEmitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China, H. Y. Xu, J. G. Ma, and Y. C. Liu are preparing a manuscript to be called “Enhanced ultraviolet random lasing from p-Cu2O inserted ZnO-based metal-insulator-semiconductor heterostructure device.” 25. E. F. Schubert, Light-Emitting Diodes (Cambridge University, 2003). 26. S. Gottardo, S. Cavalieri, O. Yaroshchuk, and D. S. Wiersma, “Quasi-two-dimensional diffusive random laser action,” Phys. Rev. Lett. 93(26), 263901 (2004).
1. Introduction Random laser is attracting increasing attention due to its potential applications in broad angular display, remote temperature sensing, document encoding, medical diagnostics, etc [1,2]. The formation conditions of random laser are not so strict as those of conventional laser modes, and a disorder system can act as both gain medium and resonant cavity, which offers advantages of simple processing technique and low cost. With a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, ZnO is considered as a promising candidate for ultraviolet light-emitting devices. Since last 90s, optically pumped random laser has been observed in various ZnO structures including nanoparticles, nanowires and thin films [3–6]. Recently, electrically pumped random laser were also achieved from ZnO homo- and heterostructures [2,7–11]. These laser diodes (LDs) were usually fabricated on the Al2O3, SiC and GaN substrates [8,12,13]. Though these substrates have an epitaxial relationship with ZnO, their low thermal conductivity and high transparency bring some negative effects, such as poor heat dissipation and light loss in the direction of substrates. These problems can be mitigated by the use of metal substrates [14–19]. However, the device fabrication on metal substrates is very difficult because of thermal mismatch and lattice mismatch. At the present stage, a laser lift-off technique is usually employed to transfer light emitting devices from epitaxial wafers to metal substrates [17–19], which not only increases the technical complexity, but also may degrade the device performance. Thereby, it is highly desirable to directly grow ZnO LDs on metal substrates. We know that instead of single-crystalline film, a disorder polycrystalline film is usually formed on metal substrates, and such a microstructural feature just favors the generation of random laser. That is, not strict conditions for random lasing offer the possibility of achieving ZnO random LDs on metal substrates. Herein, we demonstrated a feasibility of directly fabricating ZnO ultraviolet random LDs on metal Cu substrates based on an Au/MgO/ZnO metal-insulator-semiconductor (MIS) heterostructure. To our knowledge, almost no report of ZnO light emitting devices on metal substrate is available before. Compared to p-n heterojunction devices such as p-GaN/n-ZnO, pure ZnO emission is easier to obtain from these MIS heterostructure [7,9–11]. The devices built on the Cu substrates showed some improved performance, including lower thermal effect and better stability of emission wavelength. The introduction of an anti-oxidation layer between the Cu substrate and the MIS heterostructure was observed to be a key factor to activate the device’s emission.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16732
2. Experiments
Fig. 1. (a) A structure diagram of Au/MgO/ZnO MIS-heterojunction LDs on the Cu substrate. (b), (c) and (d) XRD patterns, I-V characteristics and PL spectra of ZnO films or LDs on the Cu substrate with and without anti-oxidation layer.
In our experiment, the polished polycrystalline Cu wafer is used as the substrate, which is known as an ideal choice for the heat-sink of commercial chips due to its high thermal conductivity. The work function of Cu (~4.65 eV) is close to the Fermi level of ZnO (depending on its electron concentration), and it has been reported that an ohmic contact can be formed between Cu and ZnO by thermal diffusion [20]. Figure 1(a) shows the schematic illustration of MIS diodes on the Cu substrate. In our experiment, two different programs are employed. For the first one, the MIS heterojunction is directly prepared on the Cu substrate. ZnO (~330 nm) and MgO (~170 nm) layers are sequentially grown by pulsed laser deposition (PLD) in oxygen atmosphere, and the chamber pressure and substrate temperature are 20 Pa and 600 °C, respectively. Then, very thin Au electrodes are thermally evaporated on the topside, and patterned into 1 mm circular pad by shadow mask. The electroluminescence (EL) is collected from these semitransparent Au electrodes and their edges. Detailed growth conditions can be found in our previous work [11,21,22]. In this program, the device structure is almost identical to that of the reported LDs on conventional substrates like Si and ITO glass [7,11]. However, such a simple transfer to the metal Cu substrate does not yield a workable LD device. Instead of the smooth surface observed in epitaxial substrates, this sample on the Cu substrate looks dark and rough. X-ray diffraction (XRD) measurement [Fig. 1(b)] shows that besides Cu diffraction peaks, the signals of Cu2O and CuO can also be detected, indicating that the Cu substrate surface is oxidized during the deposition of ZnO film. Since the oxygen activity can be enhanced in the plasma plume of PLD, the oxidation of Cu surface is inevitable at the high substrate temperature. The oxidation deteriorates the heterojunction device. It is known that Cu2O is usually p-type, therefore an unintended p-Cu2O/n-ZnO heterojunction would form underlying the MIS heterojunction, and the two diodes are connected back to back. As shown in current-voltage (I-V) curve of the oxidized device [inset of Fig. 1(c)], the turn-on characteristic is observed at both forward and reverse bias.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16733
Symmetrical rectifying behavior demonstrates the formation of p-Cu2O/n-ZnO/MgO/Au heterostructure [23]. The injection current remains at a low level due to the bidirectional blocking effect, and this heterostructure is easy to break down as the current gets higher. No EL emission could be obtained. To prevent the Cu substrate from being oxidized, the gas ambient is changed at the initial stage of ZnO growth. A two-step growth method is introduced in the second program. Firstly, a ~240 nm-thick oxygen-deficient ZnO layer is deposited on the Cu substrate in ultrapure argon ambient at a lower temperature of 300 °C. Then, high-quality ZnO film (~90 nm) is grown in oxygen atmosphere at 600 °C. The other fabrication steps and conditions are the same as those described in the first program. There are no diffraction signals from Cu2O and CuO observed in XRD pattern, indicating the inserted oxygen-deficient ZnO layer indeed plays a role of anti-oxidation coating. Typical diode behavior is observed in the I-V characteristic of Fig. 1(c). XRD studies [Fig. 1(b)] reveal that ZnO active layer in the present device is preferentially oriented along c-axis direction, differing from the multiple orientations observed on the oxidized Cu substrate, and the diffraction peak becomes more intense and sharp. Figure 1(d) presents the photoluminescence (PL) spectra of the two devices with and without anti-oxidation layer. The emission intensity of directly grown ZnO is very weak, it could be attributed to its inferior crystalline quality as well as the additional pCu2O/ZnO heterojunction, where the built-in field of this p-n junction can separate photogenerated electrons and holes. While the ZnO device with anti-oxidation layer exhibits a nearly two orders of magnitude stronger near-band-edge (NBE) emission at ~377 nm, even though the excitation power reduces to one tenth. All these observations suggest that the antioxidation ZnO layer also performs as a buffer layer to improve the crystalline and optical quality of ZnO active layer, though the related physical mechanism cannot be fully understood at present. 3. Results and discussions
Fig. 2. (a) EL spectra of ZnO LDs on the Cu substrate with an anti-oxidation layer under different injection currents, and its inset shows the energy-band alignment of Au/MgO/ZnO heterostructure under forward bias, which is plotted according to Anderson model. (b) Superlinear dependence of integrated EL intensity on the injection current density, the inset illustrates the process of population inversion and stimulated emission. (c) EL spectrum of ZnO MIS LDs on the Si substrate [Ref. 11, Copyright 2014, with permission from Elsevier].
Figure 2(a) presents a set of EL spectra of ZnO LDs on the Cu substrate with an antioxidation layer. Under the injection current density of 25.5 A/cm2, a spontaneous emission emerges at ~381 nm, which may be associated with ZnO exciton recombination, and no detectable EL signal is observed in the visible region. The recombination mechanism can be understood in terms of the band alignment [inset of Fig. 2(a)]. Under forward bias, electrons would be blocked and accumulated at ZnO/MgO interface due to the large conduction band offset. Considering its dielectric nature, large portion of the bias is applied on MgO layer. The electric field strength could be up to ~107 V/m, thus electrons and holes can be generated #211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16734
through a so-called impact-ionization process [7,9–11]. The holes from MgO are driven into ZnO and recombine with the accumulated electrons radiatively, resulting in a ZnO NBE emission. As the injection reaches 31.8 A/cm2, some sharp peaks appear and superimpose on the spontaneous emission band. With further increasing injection to 50.9 A/cm2, more distinct sharp peaks are observed. The appearance of sharp peaks demonstrates lasing action, which is further confirmed by the superlinear dependence of integrated emission intensity on current density, as shown in Fig. 2(b). The lasing threshold is determined as 29.3 A/cm2, which is larger than that of ZnO LDs on conventional substrates reported by our group [9,11]. Figure 2(c) shows a typical lasing spectrum of ZnO MIS LDs on the Si substrate in Ref [11]. for comparison. Distinct lasing spikes could be observed under 5.09 A/cm2, while the emission shifts towards longer wavelength several nanometers more than the LDs on the Cu substrate. However, it is indicated that the present ZnO LDs on the Cu substrate has large lasing threshold and low external quantum efficiency. It may be mainly attributed to the following factors: (i) A lot of carrier traps, generated in the anti-oxidation layer due to low growth temperature and non-stoichiometry, may act as nonradiative recombination centers and decrease effective carrier injection. (ii) Ascribing to the rough surfaces of used Cu wafers, the root-mean-square roughness of ZnO film is up to 34.4 nm. Post-grown MgO can hardly cover ZnO surface uniformly, many leak channels would form. (iii) The hole generation rate of MIS heterojunctions is very low. Overall, the improvement of threshold and efficiency is most important in our further work, possible plans are proposed herein: (1) The growth ambience of anti-oxidation layer can be changed as nitrogen/hydrogen mixed gas to passivate native defects, and its thickness and growth temperature could be further optimized. (2) Singlecrystalline Cu wafers (Rq < 10 nm) would be employed instead. (3) It is reasonably expected that the performance of MIS device could be improved by introducing a hole-injection layer [24]. It provides us another feasible route to realize ZnO LDs on metal Cu substrate, that is, an inverted MIS heterostructure of Cu/p-Cu2O/MgO/ZnO/In, where the p-Cu2O layer is prepared by thermal oxidation. For this inverted stack structure, the oxidation of Cu substrate is no longer a disadvantage and the native defects could also be suppressed to some extent.
Fig. 3. (a) A comparison between PL and EL spectra of ZnO LDs on the Cu substrate. (b) and (c) I-V characteristics of ZnO LDs with the same MIS heterostructures on Cu and n+-Si substrates.
It is known that the ZnO LDs on the conventional substrates with low heat conductivity usually suffer from the heat dissipation problem, which causes the thermal instability of emission wavelength. The EL peak would continuously shift towards longer wavelengths with the injection current increasing [7], and a large relative red-shift is also observed between the EL and PL spectra [7,9]. For example, the EL emission even red-shifted to ~400 nm in our reported ZnO MIS LDs on ITO glass [9]. In contrast, the present ZnO LD on the Cu substrate exhibits much smaller thermal effect. As shown in Figs. 2(a) and 3(a), the central wavelength of EL emission band remains almost invariable as the injection current density increases from 25.5 to 63.7 A/cm2, and the relative shift between EL and PL peaks is negligible. In addition, though the threshold gets larger, the enhanced heat dissipation through the metal Cu substrate allows a high current injection without device failure.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16735
The use of Cu substrate offers additional benefits. Figures 3(b) and 3(c) show the I-V characteristics of the same MIS heterojunction LDs on Cu and n+-Si substrates. Herein, the Si wafer is n-type, high conductive, whose resistivity is less than 0.02 Ω·cm. By a linear fit to the I-V curves beyond the turn-on voltage [25], the series resistances are determined as 7.8 and 241 Ω. Smaller resistance produces less joule heat, further reducing the thermal effect of ZnO LDs on the Cu substrates. In addition, the use of Cu substrate not only simplifies the electrode preparation, but also can suppress the current-crowding effect, because the vertical configuration of anode and cathode can yield uniform current spreading [see the inset of Figs. 3(b) and 3(c)] [15,16].
Fig. 4. (a) A schematic diagram of the angle-dependent EL measurement configuration. (b) The lasing spectra of ZnO LDs on the Cu substrate taken from different angles. (c) Crosssectional TEM image of the ZnO film grown on the Cu substrate. (d) Schematic illustration of the formation process of random laser.
Now, let us briefly discuss the mode and formation mechanism of ZnO lasing. Figures 4(a) and 4(b) shows the schematic configuration of the angle-dependent EL measurement and the lasing spectra collected from different space angles, respectively. Distinct lasing spikes are observed in different directions, which is a strong evidence of random lasing action [7– 9,11]. As mentioned above, disorder medium provides the optical gain in random laser process. To further investigate the microstructure, cross-sectional transmission electron microscopy (TEM) characterization is performed and the image is shown in Fig. 4(c). The ZnO film grown on the Cu substrate is observed to be textured and consist of nanocolumns with different sizes, which act as both gain medium and scattering units in random lasing [9]. It is mentioned that the film surface roughness is dozens of nanometers, and many voids exist among the top. The post-grown MgO could fill up some of these voids. Thus, the ZnO/MgO interface is highly disordered, and the spatial variation of refractive index yields a twodimensional strong scattering media [9,11,26]. As illustrated in Fig. 4(d), the emitted light undergoes multiple coherent scatterings with lateral facets of ZnO grains, and returns to the starting point, forming a closed loop. As the optical gain exceeds the loss, random lasing oscillation occurs.
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16736
4. Conclusions In summary, ZnO-based MIS LDs were directly built on the Cu substrate by introducing an anti-oxidation layer. The high thermal and electrical conductivity of Cu substrate increases the heat dissipation and decreases series resistance, thus resulting in lower thermal effect and higher wavelength stability. Besides, the formation mechanism of random laser is also discussed based on micro-structural studies. However, the present ZnO LDs on the Cu substrates still suffer from some problems, such as large lasing threshold and low device efficiency. The disadvantageous factors of present LDs are analyzed, some possible solutions related to these problems are proposed herein, and further work is in progress. Acknowledgments The work is supported by NSFC (Nos. 51172041 and 51372035), the Program for New Century Excellent Talents in University (No. NCET-11–0615), 973 Program (No. 2012CB933703), “111”project (No. B13013), the Fund from Jilin Province (Nos. 20121802 and 201201061), and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130043110004).
#211958 - $15.00 USD (C) 2014 OSA
Received 13 May 2014; revised 21 Jun 2014; accepted 22 Jun 2014; published 30 Jun 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016731 | OPTICS EXPRESS 16737
