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Field-Effect Transistors Based on Amorphous Black Phosphorus Ultrathin Films by Pulsed Laser Deposition Zhibin Yang, Jianhua Hao,* Shuoguo Yuan, Shenghuang Lin, Hei Man Yau, Jiyan Dai, and Shu Ping Lau* Elemental phosphorus possesses three main allotropes, namely, white, red, and black phosphorus. Black phosphorus (BP) is the most thermodynamically stable allotrope of phosphorus at ambient temperature and pressure. It has three crystalline phases (orthorhombic, rhombohedral, and simple cubic), and one amorphous form.[1,2] The crystalline of bulk BP is a layered structure with eight atoms per unit cell. The layers are held together by van der Waals forces and strong covalent bonds form between phosphorus atoms within the layers. Bulk BP is a direct band gap (Eg) semiconductor with Eg ≈ 0.30 eV, and it possesses good electrical properties with electron and hole mobility at room temperature of 220 and 350 cm2 V−1 s−1, respectively.[1,3] Although the discovery of BP can be dated back over one century long ago, there has been much less study on the use of BP compared to those elemental semiconductors, such as Si and Ge. The reason for this is mainly from the significantly difficult synthesis process of BP involved in high temperature and high pressure conditions, which limits the applications and commercial value of BP material.[4] Since the discovery of graphene, 2D materials have received much attention for both electronic and photonic applications.[5–7] Until the year of 2014, few-layer BP named as phosphorene has been isolated successfully by both mechanical exfoliation [8–15] and liquid exfoliation techniques.[16,17] Phosphorene can be regarded as a new generation of elemental 2D material after graphene and it has shown intrinsic tunable direct band gap and relatively high carrier mobility (µ). Over the past one year, electrical and optical properties of few-layer BP have already been studied, and various 2D BP based devices, such as field-effect transistor (FET) and photodetector, have been demonstrated.[8–15] Unlike graphene and other 2D materials, so far there has been no report on the fabrication of phosphorene through the route of vapor growth, such as chemical vapor deposition (CVD). Hence, the size of few-layer BP obtained up to now is mainly limited to micrometer scale, which cannot fully afford the requirement of manufacturing practical devices. It is noticeable that the discovery of phosphorene has ignited the research interests in BP material. Besides the 2D BP, the black phosphorus quantum dots (BPQDs) have also been presented according to the most recent work done by Zhang’s group.[18] The as-prepared BPQDs Z. Yang, Prof. J. Hao, S. Yuan, Dr. S. Lin, H. M. Yau, Prof. J. Dai, Prof. S. P. Lau Department of Applied Physics The Hong Kong Polytechnic University, Hung Hom Kowloon, Hong Kong, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201500990
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with the lateral size of around 4.9 nm and thickness of around 1.9 nm were synthesized from bulk crystal by a facile solution based method. Here, we report another unexplored nanoscale form of BP, namely amorphous BP (a-BP) ultrathin films with the thickness ranging from 2 to 10 nm. Here, we specifically define a-BP as highly disordered form with resemblance to BP. Such wafer-scale a-BP ultrathin films with tunable direct band gap can be made at the temperature as low as 150 °C by conventional pulsed laser deposition (PLD),[19,20] which is in contrast with extreme conditions of high temperature and high pressure for the preparation of BP bulk. It is well-known that thin films of hydrogenated amorphous silicon (a-Si:H) have extensively been employed in fabricating thin film transistors (TFTs) which have already become building blocks for various wide applications in state-of-the-art microelectronics and photonics.[21] In the device application, amorphous semiconductors are preferred over polycrystalline ones for serving as active layers when considering low processing temperature and high uniformity of device characteristics. However, one of the drawbacks of a-Si:H suffers from its low carrier mobility limited to less than 1 cm2 V−1 s−1, which is associated with the intrinsic nature of the chemical bonding.[21] In this Communication, we have fabricated and demonstrated novel a-BP ultrathin film FETs for the first time. Interestingly, the fabricated prototype FET with 2 nm thick a-BP active channel layer exhibits relatively high µ of 14 cm2 V−1 s−1 and moderate on/off current ratio (Ion/Ioff) of 102. Thickness dependence of µ and Ion/Ioff has also been observed. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to examine the structural properties of the PLD grown ultrathin films prepared from a BP crystal target. As the results of energy-dispersive X-ray spectroscopy (EDX) illustrated in Figure 1a, the chemical composition of the thin films deposited on SiO2/Si substrates at 150 °C can be confirmed to be pure phosphorus (P) element. The minor peaks corresponding to carbon and copper elements shown in EDX spectrum are brought by TEM grid, while small silicon and oxygen peaks are caused by SiO2/Si substrates used in the film deposition. Hence, the EDX result rules out the possibility of forming any phosphorus based compound (e.g., P2O5) in the film. We have performed the comparison of crystal structure and phase identification between BP crystal target and the as-grown film (Figure 1b). Compared to the XRD pattern from the crystalline BP target, only (400) peak of Si substrate is shown and no observable characteristic peak of BP can be seen from the 53 nm thick BP film, revealing the amorphous nature of our deposited film. In order to confirm the crystallinity of the film, the film/SiO2/Si was investigated by TEM with the
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COMMUNICATION Figure 1. Characterization of amorphous nature of phosphorus thin films grown on SiO2 (300 nm)/Si substrates at 150 °C. a) EDX analysis of film/ SiO2/Si. b) XRD results of film/SiO2/Si and bulk BP crystal. c) Cross-sectional TEM images of thin films. d) High-resolution TEM image with selected area electron diffraction pattern (inset), indicating amorphous phases of the deposited films.
cross-sectional sample prepared by a method using focused ion beam and lift-off. As shown in the low-magnification TEM (Figure 1c), a very uniform film can be seen. Furthermore, a dense and disordered morphology without long-range order is apparent at the interface between the grown film and substrate as illustrated in high-resolution TEM image with the scale bar of 20 nm (Figure 1d). It indicates that a-BP ultrathin film has a disordered structure with no observable layered structure. Moreover, a halo ring pattern is shown in the selected area electron diffraction (SAED) pattern (Inset of Figure 1d). The results further indicate that the obtained thin films are amorphous phase in nature. In addition to the film grown on SiO2/Si, the chemical composition and amorphous nature of the film grown on graphene/copper substrates have also been characterized by EDX and TEM (see Figure S1, Supporting Information). As aforementioned phosphorus characteristic, there are three main allotropes of phosphorus. Figure 2a (upper) shows the photographs of four wafer-scale films grown at different substrate temperatures, including sample A (film/graphene/ copper), sample B (film/SiO2/Si) deposited at 150 °C, sample C (film/SiO2/Si) deposited at 250 °C, and sample D (film/SiO2/Si) deposited at 300 °C. Similar to the previous reports on the PLD growth of graphene,[22] when each laser pulse hits on the surface of bulk BP target, the P–P bonds of BP are expected to be broken up to nanoscale phosphorus species, which forms the plasma plume propagating in the direction of the substrates in principle. Hence, the film thickness can be roughly controlled by counting the number of laser pulse. Afterward, atomic force microscopy (AFM) was employed to determine the film’s thickness and
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root-mean-square roughness (Rq) precisely (see Figure S2, Supporting Information). In order to characterize the allotrope type of the grown phosphorus films, Raman spectroscopy and its dependence on the substrate temperature and film’s thickness have been carried out. As shown in Figure 2b, Raman spectrum of sample A presents similar three feature peaks along with the bulk BP, including 360 cm−1 (A1g), 438 cm−1 (B2g), and 468 cm−1 (A2g). Comparatively, the A2g peak shift to 480 cm−1 and broadened full-width-at-half-maximum (FWHM) are visible from the measured spectrum of the film. Therefore, by combining the results of structural analysis and Raman spectroscopy, one can conclude that the sample A deposited at 150 °C is BP film possessing amorphous phase. The deviation of Raman spectra observed in a-BP film from the bulk BP is attributed to their different atomic configurations as shown in Figure 2a (lower). Compared to the crystallized structure of bulk BP, a-BP retains shortrange order and distorted lattices with variations in bonding length and angle, resulting in the change in the vibrational behaviors of the phonons. Besides the CVD grown graphene/ copper substrate, we focus on the use of SiO2/Si substrates in order to further measure electrical transport properties of a-BP thin films which will be discussed in later part of this Communication. It is noticeable that the profile of Raman spectra from film/SiO2/Si varies greatly when increasing substrate temperature from 100 to 300 °C as shown in Figure 2c. For the growth temperature at or below 100 °C, Raman peaks are too weak to be recognized, implying the as-prepared sample constitutes little phosphorus element. Similar to the result in Figure 2b, the Raman feature peaks of BP can be obtained when the growth
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Figure 2. Raman characterization of thin films. a) (top) Photographs of samples deposited under different conditions (from left to right: film/graphene/ copper deposited at 150 °C; film/SiO2/Si deposited at 150 °C; film/SiO2/Si deposited at 250 °C; film/SiO2/Si deposited at 300 °C), (bottom) schematic of top view atomic structures of BP crystal and a-BP thin film. b) Raman spectra of thin films grown on graphene/copper substrates deposited at 150 °C and bulk BP. c) Raman spectra of thin films grown on SiO2/Si substrates under different temperatures. d) Raman spectra of thin films with different thickness grown on SiO2/Si substrates deposited at 150 °C.
temperature was set at 150 °C. When further increasing the growth temperature up to 200 and 250 °C, one may observe that the Raman feature peaks of BP are gradually evolved. Finally, the Raman spectrum of the films deposited at 250 °C shows three peaks at about 350, 380, and 440 cm−1, which accord with the Raman characteristic peaks of red phosphorus (RP).[23] When the temperature is increased at or above 300 °C, except for the Raman spectrum from SiO2/Si substrates, no detectable Raman signal from the sample was found. The reason for this could be related to the vaporization of phosphorus, which is consistent with the observed photograph image in Figure 2 where no film can be seen from sample D. Therefore, the PLD growth window of BP thin films is small. Figure 2d shows the dependence of Raman spectra on the film thickness of a-BP grown on SiO2/ Si substrates. For thinner sample, Raman spectra peaks become weaker and broader. Such a phenomenon has been frequently observed in previous studies.[24,25] Many studies have shown that the bulk BP is a narrow band gap material with 0.3 to 0.35 eV direct band gap,[1,9–11] although a few earlier reports claimed large Eg in unclassified film samples.[26] Recently, few layer phosphorene fabricated by mechanical exfoliation can show layer number dependent band gap based on photoluminescence (PL) measurement.[27] To explore whether such size dependence of band gap is applicable to our
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a-BP ultrathin films or not, we have used PL spectroscopy to measure 2 nm thick a-BP. With an excitation of 980 nm diode laser, near-infrared (NIR) PL emission is present, which covers from 1470 nm (0.84 eV) to 1620 nm (0.77 eV) with a band-toband emission peak at 1550 nm corresponding to the energy of 0.80 eV (Figure 3a). The measured PL peak is likely the lower bound on the fundamental band gap value of BP because of its nature of excitons.[10,27] This value is similar to the reported Eg from five-layered phosphorene produced by mechanical exfoliation.[27] For the thicker a-BP films, PL emission peak should be shifted toward long wavelength, and hence no PL emission was observed in the NIR region as expected. Therefore, optical reflectance measurement was performed within the range of 0.15–0.50 eV by using Fourier transform infrared spectroscopy (FTIR). Figure 3b shows the absorption spectrum of an 8 nm thick a-BP film grown on SiO2/Si substrate. The obtained optical absorption is increased along with the incident photon energy. The band gap of a-BP thin films can be approximately calculated by linear fitting the absorption data according to Tauc equation ahv = B (hv − E Tauc )
2
(1)
where α, hv, B are the absorption coefficient, energy of incident light and a constant, respectively. It is known that there is no
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clear cutoff for the absorption edge in amorphous material due to the complex structure which has been evident in amorphous semiconductors such as a-Si. The black and blue dashed lines show the approximate linear fit and Eg is estimated to be in the range of 0.21–0.26 eV, which is slightly smaller than the pre-
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Figure 3. Optical properties characterization of a-BP/SiO2/Si. a) Photoluminescence of 2 nm thick a-BP thin films. b) Optical absorption spectrum of 8 nm thick a-BP thin films measured by FTIR.
viously reported Eg ≈ 0.3 eV from exfoliated 30 nm thick BP flakes measured by infrared spectroscopy as well.[11] Such slight difference is compatible with other report in which the band gap of amorphous materials estimated from the Tauc equation is also smaller than their counterpart crystalline phases.[21] Therefore, the band gap decreases from 0.80 to 0.21–0.26 eV as the thickness of a-BP ultrathin films becomes thicker from 2 to 8 nm, respectively. The change trend is consistent with the reported layer thickness dependence of Eg found in 2D BP.[1,27] The observed thickness dependence of Eg in a-BP ultrathin films was also reported in previously studied amorphous semiconductors such as amorphous Ge, which can be explained by quantum confinement effect.[28] One of most attractive features from recently studied fewlayer BP is its remarkable electrical properties with high carrier mobility and current switching ratio, which may provide an alternative to graphene for the future nanoscale FET. Hence, it is intriguing to investigate the electrical transport properties of the obtained a-BP ultrathin films here and build a novel nanoscale FET accordingly. Figure 4a shows the schematic of FET based on PLD-grown a-BP thin film on SiO2/Si substrates. The doped silicon wafer serves as a back gate electrode. For the typical current–voltage (I–V) output characteristics of a-BP sample (Figure 4b), the on-state current decreases as the gate voltage sweeps from −30 to 0 V in ambient condition at room temperature. The linear relationship of drain–source current (Ids) and drain–source voltage (Vds) implies an Ohmic-like contact at the interface of metal/a-BP with a small Schottky barrier. Figure 5a shows the transfer characteristic of a 2 nm thick a-BP based FET biased at Vds = 1 V. The designed electrodes of FET are illustrated in the inset of Figure 5a. It is worthwhile to note that the a-BP FET has a gate modulation exceeding two orders of magnitude under negative Vg, while the FET yields negligible gate modulation under positive Vg. Such asymmetrical gate modulation indicates that the conductivity type of active a-BP layer is hole and hence the transistor shows a p-type behavior. The measured results of current switching can be illustrated by using the schematic of energy band diagrams (Figure 5b). Compared to crystalline semiconductor, it has been known that amorphous semiconductor induces localization of electron states. As a result, band-tail and mid-gap trap states are routinely invoked in band model of amorphous semiconductor as shown in Figure 5b. Similar to 2D BP transistor,[1] since the
Figure 4. FET based on PLD grown a-BP ultrathin film and its output characteristic. a) 3D schematic illustration of a-BP FET structure . b) Id–Vds for 8 nm a-BP FET.
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Figure 5. Transfer characteristics and derived parameters of a-BP FET. a) Transfer characteristics of the FET based on 2 nm thick a-BP ultrathin films. The inset shows the photograph of electrode pattern used for the measurement. b) The off- and on-state band diagrams of a-BP FET. c) The field-effect mobility and switching ratio of the FET as a function of the thickness of a-BP films. d) Carrier concentration p and sheet resistance Rs as a function of the thickness of a-BP films.
Fermi level of the Au contact at the vicinity of the valence band (Ev) in a-BP as well, p-type conduction is preferable in a-BP transistor because of a smaller Schottky barrier for holes than electrons. Based on Ids–Vg curve, the threshold voltage (VT) can be deduced by linear extrapolation. The VT = −4.5 V is obtained for the FET with 2 nm thick a-BP. Apart from current switching ratio, mobility µ of active channel layer is another important parameter in determining FET’s device performance. Fieldeffect mobility (µEF) can be calculated from the transfer curve dI , where L, W are the by using an equation μEF = wc oxLv DS dV channel length and width, respectively, Cox is the capacitance dI of the back gate oxide. can be obtained by calculating the dV slope of fitted line of linear regime of the transfer curves. As a result, the µEF and Ion/Ioff for the 2 nm a-BP thin film are determined to be 14 cm2 V−1 s−1 and 102, respectively. We have also characterized the transport characteristics of FETs based on a-BP ultrathin films with different thickness ranging from 3 to 10 nm (see Figure S3, Supporting Information). Accordingly, the thickness dependent carrier mobility and current switching ratio of a-BP FETs are shown in Figure 5c. When increasing the thickness of a-BP ultrathin films from 2 to 10 nm, the fieldeffect mobility increases gradually, while on/off ratio becomes smaller. Generally speaking, the carrier transport in amorphous material is controlled by hopping between localized states. The mobility of the amorphous semiconductor may vary with several factors, such as microstructure, parasitic source and drain
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series resistance, and so on. Moreover, both µ and Ion/Ioff are usually related to Eg of materials. There is a trend that carrier mobility will increase as decreasing the Eg of the materials, which has been evident in semiconductors, carbon nanotubes, and graphene nanoribbons.[29] Therefore, it is understandable that thinner a-BP film yields lower µ when taking into account the fact that thinner sample exhibits larger Eg as shown in Figure 3a. In addition, thinner sample should be more susceptible to charge impurities at the interface.[9] On the other hand, as the thicker sample has smaller band gap as indicated in Figure 3b, the corresponding FET may remain conducting even when switched off, leading to a low switching ratio for the device with thicker a-BP. Another possible cause resulting in the low Ion/Ioff for the FET using thicker a-BP film is due to the screening of the gate electric field already described in few layer BP.[9] Table 1 shows the comparison of electrical properties of a-BP with conventional elemental amorphous materials, keeping in mind that our a-BP sample is much thinner that the previously reported materials. It shows that the values of field-effect mobility of a-BP thin films are much superior to those found in amorphous materials, such as amorphous silicon (a-Si), carbon (a-C), and germanium (a-Ge),[30–32] and even comparable to the reported μ from amorphous compound semiconductors,[21,30] such as In-Ga-Zn-O systems. At the same time, the material’s processing temperature of a-BP is lowest compared to the three
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film exhibits a moderate on/off current ratio up to 102, and high field-effect mobility up to 14 cm2 V−1 s−1 which is much superior to Refs. Materials Field-effect mobility On/off ratio Carrier type Process temperature those found in conventional elemental amor[°C] [cm2 V−1 s−1] phous materials (e.g., a-Si:H and a-carbon). a-BP 10–100 100–102 P This work ≈150 Both mobility and switching ratio depend on the thickness of active a-BP layer served N [30] a-Si 1 max 108 ≈250 in the devices. This work provides a new a-C 1–5
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Field-Effect Transistors Based on Amorphous Black Phosphorus Ultrathin Films by Pulsed Laser Deposition Zhibin Yang, Jianhua Hao,* Shuoguo Yuan, Shenghuang Lin, Hei Man Yau, Jiyan Dai, and Shu Ping Lau* Elemental phosphorus possesses three main allotropes, namely, white, red, and black phosphorus. Black phosphorus (BP) is the most thermodynamically stable allotrope of phosphorus at ambient temperature and pressure. It has three crystalline phases (orthorhombic, rhombohedral, and simple cubic), and one amorphous form.[1,2] The crystalline of bulk BP is a layered structure with eight atoms per unit cell. The layers are held together by van der Waals forces and strong covalent bonds form between phosphorus atoms within the layers. Bulk BP is a direct band gap (Eg) semiconductor with Eg ≈ 0.30 eV, and it possesses good electrical properties with electron and hole mobility at room temperature of 220 and 350 cm2 V−1 s−1, respectively.[1,3] Although the discovery of BP can be dated back over one century long ago, there has been much less study on the use of BP compared to those elemental semiconductors, such as Si and Ge. The reason for this is mainly from the significantly difficult synthesis process of BP involved in high temperature and high pressure conditions, which limits the applications and commercial value of BP material.[4] Since the discovery of graphene, 2D materials have received much attention for both electronic and photonic applications.[5–7] Until the year of 2014, few-layer BP named as phosphorene has been isolated successfully by both mechanical exfoliation [8–15] and liquid exfoliation techniques.[16,17] Phosphorene can be regarded as a new generation of elemental 2D material after graphene and it has shown intrinsic tunable direct band gap and relatively high carrier mobility (µ). Over the past one year, electrical and optical properties of few-layer BP have already been studied, and various 2D BP based devices, such as field-effect transistor (FET) and photodetector, have been demonstrated.[8–15] Unlike graphene and other 2D materials, so far there has been no report on the fabrication of phosphorene through the route of vapor growth, such as chemical vapor deposition (CVD). Hence, the size of few-layer BP obtained up to now is mainly limited to micrometer scale, which cannot fully afford the requirement of manufacturing practical devices. It is noticeable that the discovery of phosphorene has ignited the research interests in BP material. Besides the 2D BP, the black phosphorus quantum dots (BPQDs) have also been presented according to the most recent work done by Zhang’s group.[18] The as-prepared BPQDs Z. Yang, Prof. J. Hao, S. Yuan, Dr. S. Lin, H. M. Yau, Prof. J. Dai, Prof. S. P. Lau Department of Applied Physics The Hong Kong Polytechnic University, Hung Hom Kowloon, Hong Kong, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201500990
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with the lateral size of around 4.9 nm and thickness of around 1.9 nm were synthesized from bulk crystal by a facile solution based method. Here, we report another unexplored nanoscale form of BP, namely amorphous BP (a-BP) ultrathin films with the thickness ranging from 2 to 10 nm. Here, we specifically define a-BP as highly disordered form with resemblance to BP. Such wafer-scale a-BP ultrathin films with tunable direct band gap can be made at the temperature as low as 150 °C by conventional pulsed laser deposition (PLD),[19,20] which is in contrast with extreme conditions of high temperature and high pressure for the preparation of BP bulk. It is well-known that thin films of hydrogenated amorphous silicon (a-Si:H) have extensively been employed in fabricating thin film transistors (TFTs) which have already become building blocks for various wide applications in state-of-the-art microelectronics and photonics.[21] In the device application, amorphous semiconductors are preferred over polycrystalline ones for serving as active layers when considering low processing temperature and high uniformity of device characteristics. However, one of the drawbacks of a-Si:H suffers from its low carrier mobility limited to less than 1 cm2 V−1 s−1, which is associated with the intrinsic nature of the chemical bonding.[21] In this Communication, we have fabricated and demonstrated novel a-BP ultrathin film FETs for the first time. Interestingly, the fabricated prototype FET with 2 nm thick a-BP active channel layer exhibits relatively high µ of 14 cm2 V−1 s−1 and moderate on/off current ratio (Ion/Ioff) of 102. Thickness dependence of µ and Ion/Ioff has also been observed. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to examine the structural properties of the PLD grown ultrathin films prepared from a BP crystal target. As the results of energy-dispersive X-ray spectroscopy (EDX) illustrated in Figure 1a, the chemical composition of the thin films deposited on SiO2/Si substrates at 150 °C can be confirmed to be pure phosphorus (P) element. The minor peaks corresponding to carbon and copper elements shown in EDX spectrum are brought by TEM grid, while small silicon and oxygen peaks are caused by SiO2/Si substrates used in the film deposition. Hence, the EDX result rules out the possibility of forming any phosphorus based compound (e.g., P2O5) in the film. We have performed the comparison of crystal structure and phase identification between BP crystal target and the as-grown film (Figure 1b). Compared to the XRD pattern from the crystalline BP target, only (400) peak of Si substrate is shown and no observable characteristic peak of BP can be seen from the 53 nm thick BP film, revealing the amorphous nature of our deposited film. In order to confirm the crystallinity of the film, the film/SiO2/Si was investigated by TEM with the
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COMMUNICATION Figure 1. Characterization of amorphous nature of phosphorus thin films grown on SiO2 (300 nm)/Si substrates at 150 °C. a) EDX analysis of film/ SiO2/Si. b) XRD results of film/SiO2/Si and bulk BP crystal. c) Cross-sectional TEM images of thin films. d) High-resolution TEM image with selected area electron diffraction pattern (inset), indicating amorphous phases of the deposited films.
cross-sectional sample prepared by a method using focused ion beam and lift-off. As shown in the low-magnification TEM (Figure 1c), a very uniform film can be seen. Furthermore, a dense and disordered morphology without long-range order is apparent at the interface between the grown film and substrate as illustrated in high-resolution TEM image with the scale bar of 20 nm (Figure 1d). It indicates that a-BP ultrathin film has a disordered structure with no observable layered structure. Moreover, a halo ring pattern is shown in the selected area electron diffraction (SAED) pattern (Inset of Figure 1d). The results further indicate that the obtained thin films are amorphous phase in nature. In addition to the film grown on SiO2/Si, the chemical composition and amorphous nature of the film grown on graphene/copper substrates have also been characterized by EDX and TEM (see Figure S1, Supporting Information). As aforementioned phosphorus characteristic, there are three main allotropes of phosphorus. Figure 2a (upper) shows the photographs of four wafer-scale films grown at different substrate temperatures, including sample A (film/graphene/ copper), sample B (film/SiO2/Si) deposited at 150 °C, sample C (film/SiO2/Si) deposited at 250 °C, and sample D (film/SiO2/Si) deposited at 300 °C. Similar to the previous reports on the PLD growth of graphene,[22] when each laser pulse hits on the surface of bulk BP target, the P–P bonds of BP are expected to be broken up to nanoscale phosphorus species, which forms the plasma plume propagating in the direction of the substrates in principle. Hence, the film thickness can be roughly controlled by counting the number of laser pulse. Afterward, atomic force microscopy (AFM) was employed to determine the film’s thickness and
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root-mean-square roughness (Rq) precisely (see Figure S2, Supporting Information). In order to characterize the allotrope type of the grown phosphorus films, Raman spectroscopy and its dependence on the substrate temperature and film’s thickness have been carried out. As shown in Figure 2b, Raman spectrum of sample A presents similar three feature peaks along with the bulk BP, including 360 cm−1 (A1g), 438 cm−1 (B2g), and 468 cm−1 (A2g). Comparatively, the A2g peak shift to 480 cm−1 and broadened full-width-at-half-maximum (FWHM) are visible from the measured spectrum of the film. Therefore, by combining the results of structural analysis and Raman spectroscopy, one can conclude that the sample A deposited at 150 °C is BP film possessing amorphous phase. The deviation of Raman spectra observed in a-BP film from the bulk BP is attributed to their different atomic configurations as shown in Figure 2a (lower). Compared to the crystallized structure of bulk BP, a-BP retains shortrange order and distorted lattices with variations in bonding length and angle, resulting in the change in the vibrational behaviors of the phonons. Besides the CVD grown graphene/ copper substrate, we focus on the use of SiO2/Si substrates in order to further measure electrical transport properties of a-BP thin films which will be discussed in later part of this Communication. It is noticeable that the profile of Raman spectra from film/SiO2/Si varies greatly when increasing substrate temperature from 100 to 300 °C as shown in Figure 2c. For the growth temperature at or below 100 °C, Raman peaks are too weak to be recognized, implying the as-prepared sample constitutes little phosphorus element. Similar to the result in Figure 2b, the Raman feature peaks of BP can be obtained when the growth
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Figure 2. Raman characterization of thin films. a) (top) Photographs of samples deposited under different conditions (from left to right: film/graphene/ copper deposited at 150 °C; film/SiO2/Si deposited at 150 °C; film/SiO2/Si deposited at 250 °C; film/SiO2/Si deposited at 300 °C), (bottom) schematic of top view atomic structures of BP crystal and a-BP thin film. b) Raman spectra of thin films grown on graphene/copper substrates deposited at 150 °C and bulk BP. c) Raman spectra of thin films grown on SiO2/Si substrates under different temperatures. d) Raman spectra of thin films with different thickness grown on SiO2/Si substrates deposited at 150 °C.
temperature was set at 150 °C. When further increasing the growth temperature up to 200 and 250 °C, one may observe that the Raman feature peaks of BP are gradually evolved. Finally, the Raman spectrum of the films deposited at 250 °C shows three peaks at about 350, 380, and 440 cm−1, which accord with the Raman characteristic peaks of red phosphorus (RP).[23] When the temperature is increased at or above 300 °C, except for the Raman spectrum from SiO2/Si substrates, no detectable Raman signal from the sample was found. The reason for this could be related to the vaporization of phosphorus, which is consistent with the observed photograph image in Figure 2 where no film can be seen from sample D. Therefore, the PLD growth window of BP thin films is small. Figure 2d shows the dependence of Raman spectra on the film thickness of a-BP grown on SiO2/ Si substrates. For thinner sample, Raman spectra peaks become weaker and broader. Such a phenomenon has been frequently observed in previous studies.[24,25] Many studies have shown that the bulk BP is a narrow band gap material with 0.3 to 0.35 eV direct band gap,[1,9–11] although a few earlier reports claimed large Eg in unclassified film samples.[26] Recently, few layer phosphorene fabricated by mechanical exfoliation can show layer number dependent band gap based on photoluminescence (PL) measurement.[27] To explore whether such size dependence of band gap is applicable to our
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a-BP ultrathin films or not, we have used PL spectroscopy to measure 2 nm thick a-BP. With an excitation of 980 nm diode laser, near-infrared (NIR) PL emission is present, which covers from 1470 nm (0.84 eV) to 1620 nm (0.77 eV) with a band-toband emission peak at 1550 nm corresponding to the energy of 0.80 eV (Figure 3a). The measured PL peak is likely the lower bound on the fundamental band gap value of BP because of its nature of excitons.[10,27] This value is similar to the reported Eg from five-layered phosphorene produced by mechanical exfoliation.[27] For the thicker a-BP films, PL emission peak should be shifted toward long wavelength, and hence no PL emission was observed in the NIR region as expected. Therefore, optical reflectance measurement was performed within the range of 0.15–0.50 eV by using Fourier transform infrared spectroscopy (FTIR). Figure 3b shows the absorption spectrum of an 8 nm thick a-BP film grown on SiO2/Si substrate. The obtained optical absorption is increased along with the incident photon energy. The band gap of a-BP thin films can be approximately calculated by linear fitting the absorption data according to Tauc equation ahv = B (hv − E Tauc )
2
(1)
where α, hv, B are the absorption coefficient, energy of incident light and a constant, respectively. It is known that there is no
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clear cutoff for the absorption edge in amorphous material due to the complex structure which has been evident in amorphous semiconductors such as a-Si. The black and blue dashed lines show the approximate linear fit and Eg is estimated to be in the range of 0.21–0.26 eV, which is slightly smaller than the pre-
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Figure 3. Optical properties characterization of a-BP/SiO2/Si. a) Photoluminescence of 2 nm thick a-BP thin films. b) Optical absorption spectrum of 8 nm thick a-BP thin films measured by FTIR.
viously reported Eg ≈ 0.3 eV from exfoliated 30 nm thick BP flakes measured by infrared spectroscopy as well.[11] Such slight difference is compatible with other report in which the band gap of amorphous materials estimated from the Tauc equation is also smaller than their counterpart crystalline phases.[21] Therefore, the band gap decreases from 0.80 to 0.21–0.26 eV as the thickness of a-BP ultrathin films becomes thicker from 2 to 8 nm, respectively. The change trend is consistent with the reported layer thickness dependence of Eg found in 2D BP.[1,27] The observed thickness dependence of Eg in a-BP ultrathin films was also reported in previously studied amorphous semiconductors such as amorphous Ge, which can be explained by quantum confinement effect.[28] One of most attractive features from recently studied fewlayer BP is its remarkable electrical properties with high carrier mobility and current switching ratio, which may provide an alternative to graphene for the future nanoscale FET. Hence, it is intriguing to investigate the electrical transport properties of the obtained a-BP ultrathin films here and build a novel nanoscale FET accordingly. Figure 4a shows the schematic of FET based on PLD-grown a-BP thin film on SiO2/Si substrates. The doped silicon wafer serves as a back gate electrode. For the typical current–voltage (I–V) output characteristics of a-BP sample (Figure 4b), the on-state current decreases as the gate voltage sweeps from −30 to 0 V in ambient condition at room temperature. The linear relationship of drain–source current (Ids) and drain–source voltage (Vds) implies an Ohmic-like contact at the interface of metal/a-BP with a small Schottky barrier. Figure 5a shows the transfer characteristic of a 2 nm thick a-BP based FET biased at Vds = 1 V. The designed electrodes of FET are illustrated in the inset of Figure 5a. It is worthwhile to note that the a-BP FET has a gate modulation exceeding two orders of magnitude under negative Vg, while the FET yields negligible gate modulation under positive Vg. Such asymmetrical gate modulation indicates that the conductivity type of active a-BP layer is hole and hence the transistor shows a p-type behavior. The measured results of current switching can be illustrated by using the schematic of energy band diagrams (Figure 5b). Compared to crystalline semiconductor, it has been known that amorphous semiconductor induces localization of electron states. As a result, band-tail and mid-gap trap states are routinely invoked in band model of amorphous semiconductor as shown in Figure 5b. Similar to 2D BP transistor,[1] since the
Figure 4. FET based on PLD grown a-BP ultrathin film and its output characteristic. a) 3D schematic illustration of a-BP FET structure . b) Id–Vds for 8 nm a-BP FET.
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Figure 5. Transfer characteristics and derived parameters of a-BP FET. a) Transfer characteristics of the FET based on 2 nm thick a-BP ultrathin films. The inset shows the photograph of electrode pattern used for the measurement. b) The off- and on-state band diagrams of a-BP FET. c) The field-effect mobility and switching ratio of the FET as a function of the thickness of a-BP films. d) Carrier concentration p and sheet resistance Rs as a function of the thickness of a-BP films.
Fermi level of the Au contact at the vicinity of the valence band (Ev) in a-BP as well, p-type conduction is preferable in a-BP transistor because of a smaller Schottky barrier for holes than electrons. Based on Ids–Vg curve, the threshold voltage (VT) can be deduced by linear extrapolation. The VT = −4.5 V is obtained for the FET with 2 nm thick a-BP. Apart from current switching ratio, mobility µ of active channel layer is another important parameter in determining FET’s device performance. Fieldeffect mobility (µEF) can be calculated from the transfer curve dI , where L, W are the by using an equation μEF = wc oxLv DS dV channel length and width, respectively, Cox is the capacitance dI of the back gate oxide. can be obtained by calculating the dV slope of fitted line of linear regime of the transfer curves. As a result, the µEF and Ion/Ioff for the 2 nm a-BP thin film are determined to be 14 cm2 V−1 s−1 and 102, respectively. We have also characterized the transport characteristics of FETs based on a-BP ultrathin films with different thickness ranging from 3 to 10 nm (see Figure S3, Supporting Information). Accordingly, the thickness dependent carrier mobility and current switching ratio of a-BP FETs are shown in Figure 5c. When increasing the thickness of a-BP ultrathin films from 2 to 10 nm, the fieldeffect mobility increases gradually, while on/off ratio becomes smaller. Generally speaking, the carrier transport in amorphous material is controlled by hopping between localized states. The mobility of the amorphous semiconductor may vary with several factors, such as microstructure, parasitic source and drain
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series resistance, and so on. Moreover, both µ and Ion/Ioff are usually related to Eg of materials. There is a trend that carrier mobility will increase as decreasing the Eg of the materials, which has been evident in semiconductors, carbon nanotubes, and graphene nanoribbons.[29] Therefore, it is understandable that thinner a-BP film yields lower µ when taking into account the fact that thinner sample exhibits larger Eg as shown in Figure 3a. In addition, thinner sample should be more susceptible to charge impurities at the interface.[9] On the other hand, as the thicker sample has smaller band gap as indicated in Figure 3b, the corresponding FET may remain conducting even when switched off, leading to a low switching ratio for the device with thicker a-BP. Another possible cause resulting in the low Ion/Ioff for the FET using thicker a-BP film is due to the screening of the gate electric field already described in few layer BP.[9] Table 1 shows the comparison of electrical properties of a-BP with conventional elemental amorphous materials, keeping in mind that our a-BP sample is much thinner that the previously reported materials. It shows that the values of field-effect mobility of a-BP thin films are much superior to those found in amorphous materials, such as amorphous silicon (a-Si), carbon (a-C), and germanium (a-Ge),[30–32] and even comparable to the reported μ from amorphous compound semiconductors,[21,30] such as In-Ga-Zn-O systems. At the same time, the material’s processing temperature of a-BP is lowest compared to the three
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film exhibits a moderate on/off current ratio up to 102, and high field-effect mobility up to 14 cm2 V−1 s−1 which is much superior to Refs. Materials Field-effect mobility On/off ratio Carrier type Process temperature those found in conventional elemental amor[°C] [cm2 V−1 s−1] phous materials (e.g., a-Si:H and a-carbon). a-BP 10–100 100–102 P This work ≈150 Both mobility and switching ratio depend on the thickness of active a-BP layer served N [30] a-Si 1 max 108 ≈250 in the devices. This work provides a new a-C 1–5
