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Photoelectrochemical Properties of InxGa1 xN/GaN Multiquantum Well Structures in Depletion Layers Katsushi Fujii,*,†,‡ Shinichiro Nakamura,§ Satoshi Yokojima,^,§ Takenari Goto,‡ Takafumi Yao,‡ Masakazu Sugiyama,|| and Yoshiaki Nakano†,|| †
Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8904, Japan Center for Interdisciplinary Research, Tohoku University, Aramaki aza Aoba 6-3, Aoba-ku, Sendai, 980-8578, Japan § Nakamura Laboratory, Research Cluster for Innovation, RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan ^ School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo, 192-0392, Japan School of Engineering, Univeristy of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
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‡
ABSTRACT: Photoelectrochemical conversion from light to chemical energy is one of the most promising candidates for renewable energy generation. The most crucial of the many serious problems to be solved for this conversion is the slow carrier transfer at the semiconductor electrolyte interface. We propose one of solutions using quantum wells, and in the work reported here we evaluated the electrochemical and photoelectrochemical properties of InxGa1 xN/GaN multi quantum well (MQW) structures in the depletion layer. The structures were grown by conventional metal organic vapor phase epitaxy on (0001) sapphire substrates. Their electrochemical and photoelectrochemical properties differ from those of the bulk materials.
’ INTRODUCTION The electrochemistry of the surface chemical reactions in GaAs/AlxGa1 xAs single quantum well (SQW) with multiquantum state structures in a depletion layer was investigated theoretically and experimentally in the 1980s.1 4 The structures were investigated aiming to entirely change the efficiency of photoelectrochemical reactions from their theoretical evaluation. However, the experimental reports of those investigations suggested that the limited results of photoelectrochemical reactions in a semiconductor were not shown the results expected theoretically. The investigations were limited, however, by the absence of semiconductor materials suitable for evaluation. The studies in the 1980s implied that the efficiency of electrochemical reactions in SQW structures would be increased by quantum tunneling, but this was not supported by experimental results. We have found that the results of our recently reported study of the photoelectrochemical properties of n-type GaN with thin p-type GaN junctions are not explained by the conventional Poisson equation model of a Schottky barrier with a p-n junction.5 The problems raised by SQW and p-n junction indicated that the simple model commonly used in bulk semiconductors cannot always explain physical properties of the depletion layer in a device structure. This is probably because of the existence of electrolyte attached to the semiconductor surface. Analyzing the details of this interface and its electrical double layer is thus of critical importance.6 In the work reported here we evaluated the electrochemical and photoelectrochemical properties in InxGa1 xN/GaN multiquantum well (MQW) structures in the depletion layer. r 2011 American Chemical Society
’ EXPERIMENTAL METHODS The structure of the 5QW samples used in the experiments is shown schematically in Figure 1. The InxGa1 xN wells were 5 nm thick and the GaN barriers were either 2, 5, or 10 nm thick. The well growth conditions were the same for all samples, but the barrier growth time was proportional to the thickness of the GaN. Bulk n-type GaN (2 μm) and bulk undoped InxGa1 xN (0.2 μm) were used as references. The samples were grown by conventional metal organic vapor phase epitaxy. That is, the 5QW structures were grown on (0001) sapphire substrates after first depositing a thin GaN buffer layer grown at a low temperature and two thick buffer layers of undoped and Si-doped GaN. The crystal quality was evaluated by measuring X-ray diffraction (XRD) patterns and room-temperature photoluminescence (PL). Samples for electrochemical evaluation were cut from the grown epitaxial wafer. The contact electrodes were indium and were covered with epoxy resin to keep them from dissolving in the electrolyte (0.5 mol/L H2SO4 aqueous solution, pH ≈ 0.2). The electrochemical properties were measured using an SI1280B potentiostat, platinum counter electrode, and Ag/ AgCl/NaCl (sodium chloride saturated silver chloride electrode, the electrode potential E(AgCl/Ag) = +0.212 V vs normal hydrogen electrode (NHE)) reference electrode. The reference electrode was not used in the photoelectrochemical measurements in some cases, in which the light source was a 500-W Xe Received: September 13, 2011 Revised: November 12, 2011 Published: November 15, 2011 25165
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Figure 1. Cross-sectional schematic diagram of the 5QW samples with InxGa1 xN wells and GaN barriers. The well width was 5 nm and the barrier width was either 2, 5, or 10 nm.
Figure 3. PL spectra excited at room temperature by a 325-nm He Cd laser. The labels InGaN and GaN designate the InxGa1 xN/GaN MQW and GaN peaks for 5QW samples with various barrier thicknesses. The occilation of the InxGa1 xN peaks is due to the interference of the periodic structures. The InxGa1 xN and GaN peaks of the sample with 2-nm GaN barriers are almost the same as those of the bulk InxGa1 xN sample. The absorption spectrum of bulk InxGa1 xN sample is shown by the bold black line. Neither the PL nor absorption of a bulk GaN sample is shown here.
Figure 2. The XRD (2θ ω scan) of 5QW sample with 2-nm barriers. Satellite peaks (MQW 1 and +1) are observed with the GaN and InxGa1 xN/GaN MQW (indicated as InGaN in the Figures) peaks. The periodicity is also indicated in the graph.
lamp with a neutral-density filter that reduced the light intensity at the sample to 90 mW/cm2.
’ RESULTS AND DISCUSSION MQW Structure Evaluation. Sample structure was evaluated by analyzing XRD patterns obtained using the Cu Kα1 line. The diffraction data (2θ ω scan) for the sample with 2-nm GaN barriers is shown for example in Figure 2. The reflection plane was the (0002) growth direction of the nitrides. The GaN buffer peak and InxGa1 xN/GaN MQW peak were clearly evident, and the positions of InxGa1 xN/GaN MQW peaks obviously differed from the position of bulk InxGa1 xN peak. The In composition of the bulk InxGa1 xN, calculated from the difference between the InxGa1 xN and GaN peaks, was 0.194. With increasing GaN barrier thickness the InxGa1 xN/GaN MQW peak approaches the GaN peak.7 Coherent growth of the MQW structure, which contains strain due to the difference between the lattice constants of InxGa1 xN and GaN, is assumed to be the reason for the change of the peak positions. As shown in Figure 2, satellite peaks due to the MQW periodicity are seen on both sides of the InxGa1 xN/GaN MQW peak. The thickness of the InxGa1 xN and GaN pair calculated from the separations of these peaks was 7.5 nm, which is very close to the designed thickness of 7 nm (5 nm for the InxGa1 xN well plus 2 nm for the GaN barrier). The thickness of the InxGa1 xN/GaN pairs in the other samples was also close to the designed thickness: 10.2 nm for the sample with 5-nm barriers
Figure 4. Mott Schotky plot of bulk GaN sample, bulk InxGa1 xN sample, and the 5QW sample with 5 nm InxGa1 xN wells and 2 nm GaN barriers. The capacitances were calculated from the model for impedance measurements in 0.5 mol/L H2SO4 aqueous solution.
and 14.5 nm for the sample with 10-nm barriers. The appearance of the satellite peaks indicates that the interfaces between InxGa1 xN and GaN are relatively flat. Thus, the thickness and structure of the MQW were grown almost as designed. The PL spectra measured at room temperature using a 5-mW He Cd laser (λ = 325.0 nm) as the excitation source are shown in Figure 3. The In composition was estimated from the PL peak of bulk InxGa1 xN to be 0.12.7 This composition is identical to that calculated from the absorption edges shown in Figure 3 but different from the composition calculated from the XRD data. The strain is probably the reason that the In composition estimated from the measured PL of bulk InxGa1 xN is lower than that estimated from the XRD data. The PL intensity of the MQW samples increases as the GaN barrier width increases, whereas their peak energy (horizontal axis in Figure 3) decreases. This peak intensity increasing with the GaN barrier width shows that the carrier confinement in the well increases as the GaN 25166
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Figure 5. Series resistance as a function of the bias vs the Ag/AgCl/ NaCl reference electrode. The resistances were calculated from the model for impedance measurements in 0.5 mol/L H2SO4 aqueous solution.
Figure 6. Cyclic voltammogram for bulk GaN sample, bulk InxGa1 xN sample, and the 5QW sample with 5 nm InxGa1 xN wells and 2 nm GaN barriers. The scans were perfomed with illuminated by 90 mW/cm2 Xe lamp in 0.5 mol/L H2SO4 aqueous solution. The scan speed was 20 mV/s.
barrier width increases. The difference would have been attributed to the quantum confined Stark effect (QCSE) due to the (0001) growth direction and to the strain due to difference between the lattice constants of InxGa1 xN and GaN.8 Electrochemical Properties Using Impedance Measurements. Electrochemical properties were investigated by measuring the impedance at biases between 1.0 and +1.0 V vs Ag/ AgCl/NaCl. The equivalent circuit used in this analysis was the simple model consisting of a parallel resistance Rp and a capacitance C for the semiconductor electrolyte interface and a series resistance Rs for the rest of the resistance.5 The Mott Schottky plots (1/C2 V) show a linear relationship for the bulk InxGa1 xN and the bulk GaN reference samples as shown in Figure 4. The flatband potentials (vs Ag/AgCl/NaCl) obtained from the Mott Schottky plot (1/C2 f 0) were 0.11 V for InxGa1 xN and 0.68 V for GaN. The Mott Schottky plots for the QW structures did not show linear relationships. The plot for the QW structure with the 2-nm GaN barrier thickness is also shown in Figure 4 as an example. Interestingly, the value of 1/C2 keeps almost constant during the bias showing above the flatband potential of InxGa1 xN. However, the value approaches to zero when the bias is below the flatband potential of InxGa1 xN. The bias at the 1/C2 changing to zero was negative shift at the QW structure with the 10-nm GaN barrier. This indicates that the InxGa1 xN well acts as a carrier trap. This carrier trap is a kind of a capacitance; thus, the C is independent of any applied bias when the bias is above around the flatband potential of InxGa1 xN. However, the trapped carrier in the InxGa1 xN well spills out from the well when the bias is below around the flatband potential of InxGa1 xN because the 1/C2 approaches to zero for the bias region. Although the Mott Schottky plot for the QW structure is not obtained, the flatband potential is thought to be near the bias at the 1/C2 changing to zero from the result. This also shows that the trapped carrier of electron near the conduction band edge in quantum well (due to n-type semiconductor without light illumination) moves out to electrolyte easily by a small negative amount of electric field applied to the quantum well region. As shown in Figure 5, there is a clear relation between the Rs and the thickness of the GaN barrier. The Rs at the bias of 0.0 V vs Ag/AgCl/NaCl was lowest for the bulk GaN and next lowest for the bulk InxGa1 xN. Interestingly, the Rs increased with the
barrier thickness and did not depend on the bias potential except at large negative biases (left side of Figure 5). This shows that the majority carrier electrons flow easily even though the existence of well and barrier structures due to the steep gradient of the electric field of the depletion layer (near the semiconductor-electrolyte interface). As shown in Figure 5, the profile of bias dependence of the Rs of the bulk InxGa1 xN differs from the profiles of bias dependence of the Rs of the other samples. This difference is probably due to the interface between the bulk InxGa1 xN and the GaN buffer layer involving the depletion layer, since the thickness of the bulk InxGa1 xN is only 0.2 μm. For the MQW samples, the Rs clearly increases with the GaN barrier thickness. This indicates that the carrier confinement in the InxGa1 xN well increases with the barrier thickness. The result is consistent with the increased PL intensity seen with increasing barrier thickness because that intensity increase is probably due to strong carrier confinement. Photocurrent Densities vs Bias. One of the important properties affected to photocurrent density is the photo absorption. To evaluate the effect of photo absorption, the cyclic voltammetry vs Ag/AgCl/NaCl reference electrode were observed as shown in Figure 6. The turn on voltage from negative bias is in order of bulk GaN, the QW structure with 5-nm GaN barrier thickness, and bulk InxGa1 xN. This result is expected from the impedance measurement. Flat photocurrent regions at the higher positive bias, where the current is limited by the absorbed light, are observed for the sample of the bulk GaN and the QW structure with 5-nm GaN barrier thickness. In addition, the photocurrents for the both samples are almost the same. On the other hand, the photocurrent density at the higher positive bias of the bulk InxGa1 xN is higher than the other two and does not approach the flat region. From the result, the amounts of photo absorption can be estimated. The photo absorption of bulk InxGa1 xN sample is higher than the other two samples. The photo absorption for the sample of the QW structure with 5-nm GaN barrier thickness is similar to that of bulk GaN. Although the wavelength dependence measurement is expected for the detailed analysis, the contribution of the absorption of quantum well is not enough large for the samples of QW structures from the result. Another important property for photocurrent density is depletion layer thickness.9 The depletion layer thickness is proportional to the reverse square of carrier concentration. Dielectric constant also 25167
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Figure 7. Photocurrent density as a function of the bias vs the Pt counterelectrode (CE). The data were obtained 60 s after applying the bias with illuminated by 90 mW/cm2 Xe lamp in 0.5 mol/L H2SO4 aqueous solution.
Figure 8. Photocurrent density without bias vs the counterelectrode (CE) as a function of barrier thickness. Also shown are the data for the bulk InxGa1 xN and bulk GaN samples. All the data were obtained 60 s after applying the bias.
affects the depletion layer thickness. However, the In composition of InxGa1 xN is not so high; thus, the dielectric constant is not much different between GaN and InxGa1 xN samples. The layers of bulk InxGa1 xN and QW structure were undoped; thus, the carrier concentration of InxGa1 xN layer is estimated up to 1 1018 cm 3 and that of GaN layer in QW would be much smaller. Since the carrier concentration of bulk GaN has been measured as 8.4 1017 cm 3, the carrier concentration is not so much different. In addition, the photocurrent density show the similar value9 in the carrier concentration region between 0.1 and 1 1018 cm 3. Thus, the GaN layer of QW may not be affected so much. Taking these arguments into account, we conclude that the photocurrent density is not affected by the depletion layer thickness. The photocurrent density, measured without using the reference electrode, is shown in Figure 7 as a function of the bias versus the counter electrode (CE). This condition is much closer to the real device use than that using reference electrode. Since the CE acts as hydrogen generation electrode and the pH is almost 0, the potential of the CE is basically regarded as the NHE. Thus, the bias of Figure 7 is regarded to shift with +0.212 V from the bias of Figure 6. Since the photocurrent density in Figure 7
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has been measured under static conditions, the results are different from the results of Figure 6. The turn-on voltages for the bulk InxGa1 xN and the bulk GaN sample shifted about +0.8 V compared to the flatband potential measured vs Ag/ AgCl/NaCl. This shift was previously observed in the case of an HCl electrolyte as +0.5 V.10 A barrier to the carrier transfer between semiconductor and electrolyte is responsible for this shift. The turn-on voltages of the QW structures with the 5-nm and 10-nm GaN barrier thicknesses were almost the same as the turn-on voltage of the bulk InxGa1 xN. The turn-on voltage of the structure with 2-nm barriers was between that of the bulk InxGa1 xN and the bulk GaN. The relation between the barrier thickness and the photocurrent density without bias vs CE is shown in Figure 8. The photocurrent density was lowest for the bulk GaN and was highest for the samples with the GaN barriers 2 and 5 nm thick. Note that, as described before, the Rs of the samples with 2-nm and 5-nm barriers were higher than those of the bulk InxGa1 xN and the bulk GaN samples. Usually, the photocurrent density decreases with increasing the Rs due to the photogenerated carrier is consumed by the resistance.9 It is noteworthy that the photocurrent densities of the samples with the 2-nm and 5-nm GaN barriers are the highest without bias vs CE even though the Rs of those samples are higher than the Rs of the others. Considering these data with the effects of photo absorption and depletion layer thickness as discussed earlier, we can expect that a new phenomenon, such as the tunneling effect, is occurring in the depletion layer when the GaN barrier thickness is around 2 and 5 nm. This effect is also supported by the carrier confinent decreasing with the GaN barrier width as shown in Figure 3, previously.
’ CONCLUSIONS The photoelectrochemical properties of MQW structures of MQW with InxGa1 xN wells and GaN barriers in the depletion layer were evaluated. The Rs increased remarkably with GaN barrier thickness and was higher than that of bulk InxGa1 xN and bulk GaN. The photocurrent density without bias was highest, even in comparison with the reference bulk samples, for samples with GaN barrier thicknesses of 2 and 5 nm. The result shows that tunneling between wells would occur in depletion-layer MQW structures. This result is also supported by the carrier confinement estimation from the PL emissions. ’ AUTHOR INFORMATION Corresponding Author
*Phone: 03-5452-5428. Fax: 03-5452-5428. E-mail: k.fujii@rcast. u-tokyo.ac.jp.
’ ACKNOWLEDGMENT We thank Professor Takashi Itoh for his technical assistance. ’ REFERENCES (1) Boudreaux, D. S.; Williams, F.; Nozik, A. J. J. Appl. Phys. 1980, 51, 2158. (2) Ross, R. T.; Nozik, A. J. J. Appl. Phys. 1982, 53, 3813. (3) Cooper, G.; Turner, J. A.; Parkinson, B. A.; Nozik, J. A. J. Appl. Phys. 1983, 54, 6463. (4) Parsons, C. A.; Thacker, B. R.; Szmyd, D. M.; Peterson, M. W.; McMahon, W. E.; Nozik, A. J. J. Chem. Phys. 1990, 93, 7706. 25168
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(5) Fujii, K.; Ono, M.; Iwaki, Y.; Sato, K.; Ohkawa, K.; Yao, T. J. Phys. Chem. C 2010, 114, 22727. (6) Fujii, K.; Iwaki, Y.; Masui, H.; Baker, T. J.; Iza, M.; Sato, H.; Kaeding, J.; Yao, T.; Speck, J. S.; DenBaars, S. P.; Nakamura, S.; Ohkawa, K Jpn. J. Appl. Phys. 2007, 46, 6573. (7) Wu, J.; Walukiewicz, W.; Yu, K. M.; Ager, J. W., III; Li, S. X.; Haller, E. E.; Lu, H.; Schaff, W. J. Solid State Commun. 2003, 127, 411. (8) Chichibu, S. F.; Abare, A. C.; Mack, M. P.; Minsky, M. S.; Deguchi, T.; Cohen, D.; Kozodoy, P.; Fleischer, S. B.; Keller, S.; Speck, J. S.; Bowers, J. E.; Hu, E.; Mishra, U. K.; Coldren, L. A.; DenBaars, S. P.; Wada, K.; Sota, T.; Nakamura, S. Mater. Sci. Eng., B 1999, 59, 298. (9) Ono, M.; Fujii, K.; Ito, T.; Iwaki, Y.; Hirako, A.; Yao, T.; Ohkawa, K. J. Chem. Phys. 2007, 126, 054708. (10) Fujii, K.; Ono, M.; Ito, T.; Ohkawa, K. Mater. Res. Soc. Proc. 2006, 0885, A11-04.1.
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