Cubic GaN Light Emitting Diode Grown by Metalorganic Vapor-Phase Epitaxy

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1 TANAKA IEICE TRANS. and NAKADAIRA: ELECTRON., VOL. CUBIC E83-C, GaN LIGHT NO. 4 APRIL EMITTING 2000 DIODE 585 PAPER Special Issue on Blue Laser Diodes and Related Devices/Technologies Cubic GaN Light Emitting Diode Grown by Metalorganic Vapor-Phase Epitaxy Hidenao TANAKA a), Member and Atsushi NAKADAIRA, Nonmember SUMMARY We studied Si and Mg doping characteristics in cubic GaN and fabricated a light emitting diode of cubic GaN on a GaAs substrate by metalorganic vapor-phase epitaxy. The diode structure consisted of undoped and Mg-doped GaN stacking layers deposited on Si-doped GaN and AlGaN layers. The electron-beam-induced-current signal and current injection characteristics of this diode structure were measured. There was a peak at the interface between the Mg-doped and undoped GaN in the electron-beam-induced-current signal. This shows successful growth of the p-n junction. Light emitting operation was achieved by currents injected through the conducting GaAs substrate of this diode at room temperature. We observed electroluminescence below the bandgap energy of cubic GaN with a peak at 2.6 ev. key words: cubic GaN, light emitting diode, Si-doping, Mg-doping, p-n junction 1. Introduction Although the hexagonal structure is stable for group III nitrides, the cubic structure (the zinc-blende structure) can be grown under certain growth conditions. Cubic group III nitrides are now attracting interest because of their easy cleavability for making laser facets. We have reported improving the crystal quality of cubic GaN [1], growing cubic AlGaN alloy [2], [3], and observing optically pumped stimulated emission from a cubic GaN/AlGaN heterostructure [4], [5]. We have also reported on the optical characteristics of Si-donor and Mg-acceptor doped cubic GaN [6], [7]. Recently, electroluminescence from cubic GaN was reported [8], [9]. However, despite the possible optoelectronic applications of cubic GaN, there are few reports on its impurity doping and electrical characteristics. To fabricate a practicable device, further study is needed. In particular, p-type doping is the key to making an electrical device with a wide bandgap semiconductor. In this paper, we report on the doping characteristics of Si and Mg in the growth of cubic GaN by metalorganic vapor-phase epitaxy (MOVPE). We also report the growth of a p-n diode structure made of cubic GaN, and its characterization by electron-beam-induced-current (EBIC) and current injection measurements. 2. Experiment We grew cubic GaN and AlGaN films by low-pressure (76 torr) MOVPE with H 2 as the carrier gas. The total H 2 gas Manuscript received September 27, Manuscript revised October 20, The authors are with NTT Cyber Space Laboratories, Musashino-shi, Japan. a) tanaka.hidenao@lab.ntt.co.jp flow was 7 l/min. The N precursor was NH 3. The NH 3 gas flow was in the range between 2 and 100 cc/min. Ga precursors were trimethylgallium (TMGa) and triethylgallium (TEGa), and the Al precursor was triethylaluminum. We used monomethylsilane (MeSiH 3 ) for the Si donor-dopant, and bisethylcyclopentadienyl-magnesium (EtCp 2 Mg) for the Mg acceptor-dopant. The surfaces of the GaAs substrates were thermally cleaned in a H 2 and AsH 3 ambient at 720 C for 5 min before the deposition of a low-temperature-grown GaN buffer layer. After a 10-nm-thick had been deposited at 570 C, the substrate temperature was raised to grow the main GaN layers. The growth temperature was 950 C except when the Si and Mg concentration dependences on growth temperature were examined. The diode structure was grown on a Si-doped GaAs substrate. The schematic device structure is shown in Fig. 1. After deposition of the buffer layer, Si-doped GaN and AlGaN layers were grown. The total thickness of these two layers was 300 nm. Then undoped and Mg-doped GaN layers, each 800 nm thick, were deposited. After annealing the specimen in N 2 ambient at 900 C, we deposited an 80-µm-diameter Au electrode on the GaN surface. Then the Mg-doped and undoped GaN layers were etched by reactive ion etching using Cl 2 to isolate the Mgdoped GaN layer. A Au-Ge-Ni electrode was deposited on the rear side of the GaAs substrate and annealed in a N 2 ambient at 400 C. The wafer was then cleaved into chips. To compare the current injection properties of this diode device, we made other two devices. One was a Ni-electrode device using the same wafer. Before making the Au electrode, we deposited Ni and annealed the specimen again at 900 C in a N 2 ambient to decrease the contact resistance. The other was n-type GaN on a GaAs iso-type heterostructure device. The Au GaN:Mg GaN:undoped AlGaN:Si GaN:Si GaAs:Si Fig. 1 Schematic structure of the cubic GaN diode device grown on a GaAs substrate.

2 586 IEICE TRANS. ELECTRON., VOL. E83-C, NO. 4 APRIL 2000 thickness of the Si-doped n-type GaN layer grown at 950 C was 800 nm. An Al electrode was used to make ohmic contact with the Si-doped GaN. We examined the crystal quality of these specimens by X-ray diffraction. The impurity concentrations of Mg and Si in the films were measured by secondary ion mass spectrometry (SIMS). The sensitivity was corrected using ion implanted standards. We also examined their optical properties by photoluminescence (PL) measurement at room temperature. A frequency-doubled Ar ion laser (wavelength: 257 nm) was used as the excitation light source. The EBIC signal was obtained using a conventional scanning electron microscope (SEM). We measured currents from the devices during SEM measurement at the cleaved facet of the chip. The energy of the induced electrons was 25 kev. Electroluminescence (EL) from this diode was measured at room temperature under direct current injection. The EL was dispersed using a 100-cm focal length grating monochromator and detected using a photomultiplier tube in conjunction with a lock-in amplifier. 3. Results and Discussion 3.1 Doping Characteristics We measured the x-ray rocking curve of the cubic (004) diffraction of the Si-doped GaN wafer and found the full width at half-maximum (FWHM) of the peak to be about 40 min. This is the same as the value for the undoped cubic GaN/ AlGaN heterostructure used in an earlier experiment on stimulated emission [4]. We also measured the hexagonal ( 1011) diffraction of the film for four directions using an asymmetric configuration, but the peak signal was lower than the noise level in our experimental equipment. The noise level was less than 2.5% of the cubic (002) diffraction intensity. This means that the hexagonal phase in cubic GaN is less than 10% on the assumption that the structure factor of (002) is almost equal to that of ( 1011 ) [10]. Our undoped cubic GaN films were always n-type. High T ( C) doping was thus necessary to get low resistivity. First we examined the n-type Si doping. Figure 2 shows Si concentration versus the reciprocal of substrate temperature. The Si concentration decreased as the temperature was decreased. This dependency indicates that the Si doping mainly depends on the decomposition of MeSiH 3. Figure 3 shows Si concentration versus V/III molar ratio during growth. In this experiment the supplies of TEGa and MeSiH 3 were kept constant, and only the supply of NH 3 was changed. The Si concentration slightly increased when the V/III molar ratio was increased. Figure 4 shows the Si concentration versus MeSiH 3 flow during growth. The Si concentration was almost proportional to the MeSiH 3 supply. We could control the Si concentration in the range from 5 to 1 cm -3 by controlling the MeSiH 3 flow. For p-type doping we used Mg as an acceptor. Though the surface morphology of Mg-doped GaN directly grown on the low-temperature-grown was very rough, it was improved when an Mg-doped GaN layer was deposited after the 300-nm-thick undoped GaN layer. We think that Mg-doping prevented the smoothing during the initial growth on low-temperature-grown buffer layer. Therefore we always grew Mg-doped GaN layers on an undoped Fig. 3 Si concentration (cm -3 ) Si concentration versus V/III molar ratio during growth V/III molar ratio Si concentration (cm -3 ) Si concentration (cm -3 ) /T (K -1 ) Fig. 2 Si concentration as a function of the reciprocal of substrate temperature. The V/III molar ratio during growth was MeSiH 3 /H 2 10-ppm flow (ccm) Fig. 4 Si concentration as a function of MeSiH 3 flow during growth. The V/III molar ratio during growth was 30.

3 TANAKA and NAKADAIRA: CUBIC GaN LIGHT EMITTING DIODE 587 Mg concentration (cm -3 ) 1050 T ( C) /T (K -1 ) Fig. 5 Mg concentration as a function of the reciprocal of substrate temperature. The V/III molar ratio during growth was 180. Fig. 6 Mg concentration (cm -3 ) V/III molar ratio Mg concentration versus V/III molar ratio during growth. GaN layer grown at 950 C. The FWHM of the (004) x-ray rocking curve of these Mg-doped wafers was similar to that of Si-doped GaN. We could grow Si and Mg-doped cubic GaN layers with crystal qualities were similar to those of undoped GaN. Figure 5 shows Mg concentration versus the reciprocal of substrate temperature. The Mg concentration decreased as the temperature was increased. This dependency is opposite to the results of Si doping. We think it indicates that the Mg doping mainly depends on the adsorption of Mg at the surface. Figure 6 shows Mg concentration versus V/III molar ratio during growth. In this experiment only the supply of NH 3 was changed. The Mg concentration was increased when the V/III molar ratio was increased. The inclination is very large in comparison with the Si doping. Therefore, we must control the V/III molar ratio strictly in order to control the Mg concentration precisely. Figure 7 shows the Mg concentration versus EtCp 2 Mg carrier gas flow during growth. The Mg concentration increased monotonically as the EtCp 2 Mg carrier gas flow was increased, but was not proportional to the supply. We could control the Mg concentration in the range from to 1 cm -3 by controlling the V/III molar ratio and the EtCp 2 Mg carrier gas flow. The bubbling temperature of the EtCp 2 Mg was 15 C in these experiments. The vapor pressure of EtCp 2 Mg around room temperature has not been clarified, so the molar flow of Mg is not clear. However, the results of our experiments show that EtCp 2 Mg is applicable as a Mg-dopant precursor for cubic GaN as well as for hexagonal GaN [11]. The resistivity of the as-deposited Mg-doped layers was very high. After the specimens were annealed in N 2 ambient at 900 C, some of them exhibited electrical conduction. It was difficult to estimate the carrier type of the Mg-doped layer by Hall effect measurement because of the effect of the carriers in the lower layers. Therefore, we were unable to clarify the carrier concentration and activation ratio of these Mg-doped layers. 3.2 Diode Structure To get a clear junction, an abrupt impurity profile is important. Abnormal diffusion along pinholes and defects destroys the p-n junction when the crystal quality is low. Thus, we examined the doping profile by SIMS measurement. Figure Mg concentration (cm -3 ) EtCp 2 Mg carrier gas flow (ccm) Fig. 7 Mg concentration as a function of EtCp 2 Mg carrier gas (H 2 ) flow during growth. The V/III molar ratio during growth was 60. Concentration (cm -3 ) Mg + Si - As - GaN - As + GaN Depth (µm) Ion counts (a.u.) Fig. 8 Dependence of the SIMS ion counts of the diode structure on depth. Si and Mg are shown as concentrations.

4 588 IEICE TRANS. ELECTRON., VOL. E83-C, NO. 4 APRIL 2000 (a) Intensity (a.u.) Intensity (a.u.) Energy (ev) Fig. 9 Photoluminescence spectrum of (a) undoped cubic GaN, and (b) annealed diode-structure film at room temperature. 8 shows the Si and Mg concentration dependences on the depth. The Si concentration of Si-doped GaN and AlGaN layers was about 2 cm 3. The Si concentration of the undoped layer was lower than 1 cm 3. The Mg concentration of the Mg-doped layer was 4 cm 3. To increase EtCp 2 Mg flow, we increased its bubbling temperature to 25 C during the growth of this diode structure. The Mg concentration of the undoped layer was lower than cm 3. We think that the clear impurity profile shown in Fig. 8 demonstrates that the crystal quality of our cubic GaN is enough to make a p-n junction. Photoluminescence from the as-deposited film was very weak, but clear emission was observed after annealing. The photoluminescence spectra of undoped and annealed diodestructure wafers at room temperature are shown in Fig. 9. The emission peak at 3.2 ev is the same peak as the nearband-edge emission of undoped cubic GaN. There are additional emissions between 2.8 and 3.1 ev in the spectrum of the diode structure. We think that these emissions are related to the Mg acceptor level in the top layer and that Mg acceptors were activated by annealing in the N 2 ambient. We measured EBIC to investigate the carrier types of the Mg-doped layer in the diode structure. Figure 10 shows an EBIC signal of the diode device. There is a peak at the interface between Mg-doped GaN and undoped GaN. There is a small shoulder near the Au electrode. Conversely, the EBIC signal decreased smoothly on the Si-doped GaN side. We think that the shoulder indicates the influence of a depletion layer caused by the Schottky junction between Au-metal and GaN. Therefore, we think that the main peak indicates the existence of a depletion layer caused by the p-n junction. This EBIC result indicates the successful growth of p-type cubic GaN by MOVPE. (b) Fig. 10 Electron-beam-induced-current signal and scanning electron microscope photograph of cubic GaN on GaAs. Au GaAs:Si GaN:Mg GaN:undoped AlGaN/GaN:Si Fig. 11 Electron-beam-induced-current signal and scanning electron microscope photograph of cubic GaN on GaAs. Figure 11 shows the current versus voltage characteristics of this diode. Current was injected through the GaAs substrate. This conductive substrate is advantageous when using cubic group III nitrides for diodes compared with hexagonal group III nitrides on an insulating sapphire substrate. The turn-on-voltage was about 6 V, which is higher than the value of 3.2 V expected from the bandgap energy of cubic GaN. We thought that this large discrepancy was caused by barriers at the Au electrode and at the iso-type heterostructure between the GaN and the GaAs substrate. To confirm this, we compared the current versus voltage characteristics with those of other devices. Figure 12 shows the current versus voltage characteristics of an Ni electrode device using the same diode-structure wafer. The turn-on-voltage was about 4 ev, which is less than that of a Au-electrode device. The contact of the Ni electrode with p-type GaN was ohmic-like in comparison with the Au electrode. Therefore, we thought this discrepancy was the built-in potential between p-type GaN and the Au electrode. To confirm the barrier at the interface between the GaN and the GaAs substrate, we examined an iso-type heterostructure device. Figure 13 shows the current versus voltage characteristics in the region between the Sidoped GaN and the Si-doped GaAs substrate. The diode characteristics indicate a barrier at the iso-type heterostructure. However, the barrier was as small as 0.8 V when current was injected from GaN to the GaAs substrate. To decrease this barrier even more, we think higher doping in the GaN and

5 TANAKA and NAKADAIRA: CUBIC GaN LIGHT EMITTING DIODE 589 Electroluminescence spectra of a cubic GaN p-n junction di- Fig. 14 ode. Au GaAs:Si Al GaN:Si GaAs:Si Fig. 13 Current versus voltage characteristics in the region between Si-doped cubic GaN and Si-doped GaAs. Intensity (a.u.) Ni GaN:Mg GaN:undoped AlGaN/GaN:Si Fig. 12 Current versus voltage curve of a cubic GaN diode with a Ni/ Au electrode on Mg-doped GaN. 100 ma 80 ma 60 ma 40 ma R.T. Band Gap Energy (ev) GaAs is necessary. From the position of the shoulder in the EBIC signal near the Au electrode, the depletion width is about 0.25 µm, which enables us to roughly estimate the hole carrier concentration using the calculation for the depletion width of an abrupt one-sided junction. For the unknown dielectric constant of cubic GaN, we used 10, which is the value for wurtzite GaN [12]. We used 2 ev for the built-in potential, as mentioned. The estimated acceptor concentration of Mg-doped GaN was cm 3. The activation ratio of the Mg acceptor seemed to be less than 0.1% in this experiment. Therefore, we think it is necessary to improve crystal growth for doping. Figure 14 shows the spectra for different injection currents. The Ni-electrode device was somewhat leaky during I- V measurement, so we used a Au-electrode device to measure EL. Emission below the bandgap energy of cubic GaN was observed. The peak energies were about 2.6 ev, and they increased with the injection current. Furthermore, in the emission spectra with high injection currents, a shoulder beneath the bandgap was obvious. The peak energies were lower than those of the Mg-related PL peaks. Thus, the origin of this emission peak has not been clearly identified. Tentatively, we think that the shoulder was near band edge emission and the main peak was only that part of the D-A pair emission not absorbed by the upper Mg-doped layer. 4. Conclusions Si and Mg doping characteristics in cubic GaN were studied and we achieved high doping of over 1 cm 3. The Si concentration was almost proportional to the MeSiH 3 supply, so we could control the Si concentration by controlling the MeSiH 3 flow. The Mg concentration increased monotonically as the EtCp 2 Mg carrier gas flow was increased, but the Mg concentration was not proportional to the supply. The Mg concentration increased when the V/III molar ratio was increased. Therefore, we must control the V/III molar ratio strictly in order to control the Mg concentration. We fabricated a diode structure made of cubic GaN. The EBIC signal had a peak at the interface between the Mg-doped GaN and the undoped GaN. This is evidence of a depletion layer caused by the p-n junction. The turn-on-voltage of this diode device was about 6 V, which is larger than the value of 3.2 V expected from the bandgap energy of cubic GaN. This large turn-on-voltage can be explained as the sum of the barriers at the Au electrode, at the iso-type heterostructure, and at the p-n junction. We also measured the luminescence from the diode device produced by current injection. Emission below the bandgap energy of cubic GaN was observed. Although much improvement is needed to make a practicable device, our findings increase the likelihood of being able to use cubic group-iii nitrides for laser diodes. References [1] A.Nakadaira and H.Tanaka, Growth of zinc-blende GaN on GaAs (100) substrates at high temperature using low-presure MOVPE with low V/III molar ratio, J. Electronic Materials, vol.26, no.3, pp , [2] A.Nakadaira and H.Tanaka, Metalorganic vapor-phase epitaxial growth and characterization of cubic Al x Ga 1-x N alloy on a GaAs (100) substrate, Appl. Phys. Lett., vol.70, no.19, pp , May [3] A.Nakadaira and H.Tanaka, Metalorganic vapor-phase epitaxy of cubic Al x Ga 1-x N alloy, Jpn. J. Appl. Phys., vol.37, no.3b, pp , March [4] A.Nakadaira and H.Tanaka, Stimulated emission at 34 K from an optically pumped cubic GaN/AlGaN heterostructure grown

6 590 IEICE TRANS. ELECTRON., VOL. E83-C, NO. 4 APRIL 2000 by metalorganic vapor-phase epitaxy, Appl. Phys. Lett., vol.71, no.11, pp , Aug [5] A.Nakadaira and H.Tanaka, Optically pumped stimulated emission from cubic GaN/AlGaN double heterostructure grown on GaAs (100) using metalorganic vapor-phase epitaxy, J. Cryst. Growth, vol.189/190, pp , [6] H.Tanaka and A.Nakadaira, Growth and characterization of Si-doped cubic GaN, Inst. Phys. Conf. Ser., no.162, pp , [7] A.Nakadaira and H.Tanaka, Photoluminescence from Mg-doped cubic GaN grown by MOVPE, Inst. Phys. Conf. Ser., no.162, pp , [8] H.Tanaka and A.Nakadaira, EL spectra from a p-n junction diode made of cubic GaN, Extended Abstracts (The 46th Spring Meeting) The Japan Society of Applied Physics and Related Societies, p.429, March [9] H.Yang, L.X.Zheng, J.B.Li, X.J.Wang, D.P.Xu, Y.T.Wang, X.W.Hu, and P.D.Han, Cubic-phase GaN light-emitting diodes, Appl. Phys. Lett., vol.74, no.17, pp , April [10] H.Tsuchiya, K.Sunaba, S.Yonemura, T.Suemasu, and F.Hasegawa, Cubic dominant GaN growth on (001) GaAs substrates by hydride vapor phase epitaxy, Jpn. J. Appl. Phys., vol.36, no.1a/b, pp.l1-l3, Jan [11] Y.Ohuchi, K.Tadatomo, H.Nakayama, N.Kaneda, T.Detchprohm, K.Hiramatsu, and N.Sawaki, New dopant precursors for n-type and p-type GaN, J. Cryst. Growth, vol.170, no.1, pp , [12] A.S.Barker and M.Ilegems, Infrared lattice vibrations and freeelectron dispersion in GaN, Phys. Rev. B, vol.7, no.2, pp , Jan Hidenao Tanaka was born in Tokyo, Japan in He received his B.S. and M.S. degrees in applied physics from Waseda University, Tokyo in 1977 and 1979, respectively. He joined the laboratories of Nippon Telegraph and Telephone Corporation in He has been engaged in research on AlGaInP visible-light laser diodes and laser diodes with dry etched mirrors. His current research interests are integrated optical components for micro systems and short-wavelength laser diodes applied to storage systems. He is a member of the Japan Society of Applied Physics, and the Institute of Electrical and Electronics Engineers. Atsushi Nakadaira was born in Aichi, Japan in He received his B.S. degree from Kyoto University in 1992 and his M.S. degree from the University of Tokyo in In 1994 he joined Nippon Telegraph and Telephone Corporation (NTT). Since then, he has been engaged in research on short-wavelength laser diodes for optical data storage, including crystal growth of nitride semiconductors. He is a member of the Japan Society of Applied Physics (JSAP). He received the JSAP Award for Research Paper Presentation in 1998.