Control of Dislocations and Stress in AlGaN on Sapphire Using a Low Temperature Interlayer

Transcription

1 H. Amano et al.: Dislocations and Stress in AlGaN on Sapphire 683 phys. stat. sol. (b) 216, 683 (1999) Subject classification: Jk; S7.14; S7.15 Control of Dislocations and Stress in AlGaN on Sapphire Using a Low Temperature Interlayer H. Amano 1 ) (a, b), M. Iwaya (b), N. Hayashi (b), T. Kashima (b), S. Nitta (b), C. Wetzel (a), and I. Akasaki (a, b) (a) High-Tech Research Center, Meijo University, Shiogamaguchi, Tempaku-ku, Nagoya , Japan (b) Department of Electrical and Electronic Engineering, Meijo University, Shiogamaguchi, Tempaku-ku, Nagoya , Japan (Received July 4, 1999) In organometallic vapor phase epitaxial growth of AlGaN on sapphire, the role of the low-temperature-deposited interlayers on a high-temperature-grown GaN layer was investigated by in-situ stress measurement, X-ray diffraction, and transmission electron microscopy. Crack-free and lowdislocation-density AlGaN with the whole compositional range has been realized on the sapphire substrate. 1. Introduction Al x Ga 1 ±x N thick films are inevitable for application to key parts in many novel devices such as window layers for UV light emitting diodes [1, 2], cladding layers for violet and UV laser diodes, insulating gate for FETs, widegap emitters for HBTs and in major parts of UV photodetectors [3 to 6]. In order to fabricate such novel devices, films with high-crystalline quality free of cracks are essential. There are several reports concerning the crystalline quality of AlGaN films. Koide et al. reported that the crystalline quality of AlGaN films on sapphire has been greatly improved by growing on a low-temperature-deposited AlN buffer layer (LT-AlN) [7]. Itoh reported in his doctoral thesis that the crystalline quality of AlGaN films grown on sapphire covered with LT-AlN becomes progressively worse with increasing AlN molar fraction [8]. He also reported that by growing AlGaN on GaN/LT-AlN/Sapphire, the optical quality could be significantly improved, which had been proved by cathodoluminescence (CL). At the same time, however, cracks were generated at high density due to the lattice mismatch between AlGaN and GaN. Recently, we found that the insertion of a low-temperature-deposited interlayer between high-temperature-grown GaN layers significantly reduces the density of threading dislocations in the upper GaN layer [9, 10]. In this paper, details of the process of lowtemperature interlayers are shown. Next, we applied a technique wherein Al x Ga 1 ±x N films with x ranging from 0 to 1 were grown on an LT-AlN interlayer. The effect of the low-temperature interlayer on the crystalline quality of AlGaN is discussed. 1 ) Corresponding author; Tel.: ; Fax: ; amano@meijo-u.ac.jp

2 684 H. Amano et al. 2. Process of GaN Growth on a Low-Temperature Interlayer At first, AlN about 25 nm in thickness was deposited at 500 C on a sapphire (0001) substrate by organometallic vapor phase epitaxy. After the deposition, the wafer was heated up to C. During ramping of the wafer temperature, the LT-AlN was annealed in a flow of hydrogen and ammonia. Figs. 1a and b show the cross sectional TEM pictures of the as-deposited AlN (Fig. 1a) and that of annealed LT-AlN (Fig. 1b). As-deposited LT-AlN is composed of very fine crystallites with a size of about a few nm in diameter. When it is annealed, grain growth occurred and the size of the crystallites becomes larger. The surface of both as-deposited and annealed LT-AlN layers is rough. GaN 1mm in thickness was grown at around C to cover LT-AlN as shown in Fig. 1b. Then, the wafer was cooled down to about 500 C to deposit either AlN or GaN about 25 nm in thickness. After the deposition, the wafer temperature was raised to C. Figs. 1c and d show the cross sectional TEM picture of the as-deposited AlN on GaN (Fig. 1c) and that of annealed AlN on GaN. The structure of as-deposited AlN on GaN is almost the same as that deposited on sapphire. Clear difference is observed when it is annealed. Annealed AlN shows an atomically flat surface. A lattice image observation showed that several threading dislocations in the underlying GaN Fig. 1. a) As-deposited AlN on sapphire. b) Annealed AlN on sapphire. HT-GaN has been already grown on it. HT-GaN growth does not affect the structure of annealed LT-AlN. c) As-deposited AlN on GaN. d) Annealed AlN on GaN

3 Dislocations and Stress in AlGaN on Sapphire 685 bend horizontally at the annealed AlN. By using a temperature controllable sample holder during TEM observation, mass transport and step flow reconstruction on the (0001) surface was observed. In case of annealed GaN, the process is almost the same except for the temperature at which each event occurred. Sometimes, a phase transition from amorphous-like to cubic, and then from cubic to wurtzite was observed. Next, GaN 1 mm in thickness was grown on annealed AlN shown in Fig. 1d or on annealed GaN. The process of low-temperature deposition and high-temperature growth (LT/HT) was repeated several times. The macroscopic stress of the film was either monitored in-situ by a multi-beam optical stress sensor system (MOSS) [10 to 13] or characterized at room temperature by high resolution X-ray diffraction (XRD). Fig. 2 shows the stress of the HT-GaN layer grown as the top layer. Thermal stress caused by the difference between nitrides and sapphire is the same for all the samples [12], therefore we can plot the results of MOSS and XRD in the same picture. In the case of LT-GaN, a series of LT-interlayers increases the stress during growth, which resulted in the cracking of the film [10]. On the contrary, the stress is almost constant in the case of the LT-AlN interlayers. Fig. 3 schematically shows the diagram of the change of the in-plane-lattice-constant of GaN grown on sapphire using LT-GaN interlayers. Solid lines denoted ªfree-standing GaNº and ªsapphireº are the change of inplane-lattice-constant of free-standing GaN and sapphire, respectively. In this figure, the as-deposited LT-buffer layer on sapphire is assumed to be free of strain. The MOSS observation showed that constant stress is applied to the top GaN layer during growth [10, 12]. In the following we describe the stress cycle during the process. Position 1 to 2: When the wafer is heated to the growth temperature, the LT-buffer layer is tensilely strained due to the difference of the thermal expansion between buffer layer and sapphire. This annealed buffer layer acts as the template of the upper GaN. Position 2 to 3: If the thickness of GaN is much smaller than that of sapphire, lines 1 to 2 and 2 to 3 should be anti-paralleled. At position 3, the LT-interlayer is deposited. If the LT-interlayer is epitaxially grown, it should follow line 3 to 2. However, actually, tensile stress is accumulated and it follows line 3 to 4. Fig. 2. Biaxial stress of the top GaN layer as a function of composite of LT-interlayer and number of series of LT-interlater/HTgrowth. XRD: Biaxial stress measured by X-ray diffraction. MOSS: Biaxial stress measured by multi beam optical stress sensor system

4 686 H. Amano et al. Fig. 3. Diagram of the change of in-plane lattice constant of GaN on sapphire using a) LT-GaN interlayer and b) LT-AlN interlayer. The as-deposited low temperature buffer layer on sapphire is assumed to be strain free. In case of LT-GaN interlayer, the process proceeds from the position labeled 1! 2! 3! 4! 5! 6, while in case of LT-AlN interlayer, the process is repeated from 1! 2! 3! 2! 3! 2 In the case of LT-AlN interlayers, the stress changes during the growth of HT-GaN. At the initial stage of the growth, HT-GaN is under compressive stress. After growing HT-GaN less than one micron thick, it changes to tensile stress [13]. This process is repeated during the series of the LT-interlayer/HT growth, therefore the final stress is the same for all the GaN layers. This LT-interlayer method tailors the film stress and is suitable to optimize the film quality by reducing the threading dislocations. For example, six series of this method achieved the reduction of threading dislocations by two orders of magnitude [10]. 3. The Growth of AlGaN Using Low Temperature Interlayers Al x Ga 1 ±x N films with various composition x, ranging from 0 to 1, were also grown on the HT-GaN/LT-AlN/Sap. For comparison, two other series of Al x Ga 1 ±x N films were grown. One is grown on sapphire covered with a LT-AlN buffer layer, and the other is

5 Dislocations and Stress in AlGaN on Sapphire 687 Fig. 4. SEM photographs of the surface of the following structures: a) Al 0.45 Ga 0.55 N/LT-AlN/HT- GaN/LT-AlN/Sap., b) Al 0.45 Ga 0.55 N/LT-GaN/HT-GaN/LT-AlN/Sap., c) Al 0.45 Ga 0.55 N/LT-AlN/Sap., d) Al 0.45 Ga 0.55 N/HT-GaN/LT-AlN/Sap. grown on HT-GaN/LT-AlN/Sap. The thickness of each layer are as follows: Al x Ga 1 ±x N: 1 mm, LT-AlN: 25 nm, HT-GaN: 1 mm. Scanning electron micrographs of Al 0.45 Ga 0.55 N layers grown on different surfaces are shown in Figs. 4a through d. An Al 0.45 Ga 0.55 N layer with a flat surface free of cracks was obtained when it was grown on a LT-AlN layer (Figs. 4a and c). When Al 0.45 Ga 0.55 N was grown on a GaN surface, a crack network was formed (Figs. 4b and d). The high-density crack network shown in Figs. 4b and d originates from lattice mismatches between the AlGaN layer and the underlying GaN layer. Itoh compared the near-band-edge (NBE) CLs of Al x Ga 1 ±x N (0 x 0.3) grown on LT-AlN/Sap. and on HT-GaN/LT-AlN/Sap., and found that the FWHM of NBE-CL of AlGaN grown on LT-AlN/Sap. becomes progressively wider with increase of x, while that of AlGaN grown on HT-GaN/LT-AlN/Sap. is almost the same in the compositional range studied [8]. The result showed that CL properties were improved by growing AlGaN on HT-GaN/LT-AlN/Sap. instead of LT-AlN/Sap. He also pointed out that high-density

6 688 H. Amano et al. Fig. 5. FWHM of XRC from (0002) diffraction of Al x Ga 1 ±x N. Solid squares show XRC-FWHM of Al x Ga 1 ±x N grown on LT-AlN/Sap.; solid triangles show those of Al x Ga 1 ±x N grown on LT-AlN/HT- GaN/LT-AlN/Sap. cracks were formed in the AlGaN/HT-GaN/ LT-AlN/Sap. He measured CL from the very tiny portion of the crack-free area of AlGaN islands. Although CL properties were improved by growing AlGaN on HT-GaN/LT- AlN/Sap., from the practical point of view, generation of crack networks should be avoided. The crystalline quality of crack-free AlGaN films with two different structures, examples of which are shown in Figs. 4a and c, was characterized by X-ray diffraction. Fig. 5 shows the compositional dependence of the XRC-FWHM of (0002) diffraction by the w-scan from Al x Ga 1 ±x N. When Al x Ga 1 ±x N was grown on sapphire covered with a LT-AlN layer, the FWHM becomes progressively wider with increase in x, which means that the tilting component of the mosaicity increases with increase in the AlN molar fraction. On the contrary, if Al x Ga 1 ±x N was grown on the LT-AlN interlayer deposited on a HT-GaN, the FWHM of the XRC remains unchanged over the entire compositional range. The same is true for twisting components, which are confirmed by measuring XRC of the (10 10) diffraction. These results clearly show that the crystalline quality of an Al x Ga 1 ±x N film grown on an LT-AlN interlayer is comparable to that of HT-GaN. AFM and cross sectional TEM observation showed that Al x Ga 1 ±x N grown on LT-AlN/ Sap. is composed of three-dimensional stacking of small angle grain boundaries with a size of a few or less than one micrometer, while Al x Ga 1 ±x N grown on LT-AlN interlayer/ht-gan/sap. showed almost the same structure as HT-GaN. Therefore, LT-AlN interlayers transfer the crystalline information of the underlying GaN layer, while they suppress the generation of cracks. 4. Summary The effect of using a technique wherein Al x Ga 1 ±x N films are grown on the LT-AlN interlayer on the improvement in the crystalline quality of Al x Ga 1 ±x N ternary alloys was characterized and confirmed by XRD, AFM and TEM. This technique will surely play a key role in opening the application of nitrides to the UV and vacuum-uv region. Acknowledgements This work was supported in part by the Japan Society for the Promotion of Science Research for the Future Program in the Area of Atomic Scale Surface and Interface Dynamics under the project of ``Dynamical Process and Control of

7 Dislocations and Stress in AlGaN on Sapphire 689 the Buffer Layer at the Interface in a Highly-Mismatched System (JSPS96P00204)º, the Ministry of Education, Science, Sports and Culture of Japan (contract number ), and the Murata Science Foundation. One of the authors (H. A.) would like to show sincere thanks to Drs. J. Han, S. Heane, J. A. Floro and E. Chason of Sandia National Laboratories (USA) for MOSS measurement and fruitful discussions. References [1] A. V. Sakharov, W. V. Lundin, A. Usikov, U. I. Ushakov, Yu. A. Kudriavtsev, A. V. Lunev, Y. M. Sherniakov, and N. N. Ledentsov, MRS Internet J. Nitride Semicond. Res. 3, 28 (1998). [2] J. Han, M. H. Crawford, R. J. Shul, J. J. Figiel, M. Banas, L. Zhang, Y. K. Song, H. Zhou, and A. V. Nurmikko, Appl. Phys. Lett. 73, 1688 (1998). [3] A. Osinsky, S. Gangopadhyay, B. W. Lim, M. Z. Anwar, M. A. Khan, D. Kuksenkov, and H. Tempkin, Appl. Phys. Lett. 72, 742 (1998). [4] D. Walker, X. Zhang, P. Kung, A. Saxler, S. Javadpour, J. Xu, and M. Razeghi, Appl. Phys. Lett. 68, 2100 (1996). [5] G. Y. Zhu, A. Salvador, W. Kim, Z. Fan, C. Lu, H. Tang, H. Morkoc, G. Smith, M. Estes, B. Goldenberg, W. Yang, and S. Krishnankutty, Appl. Phys. Lett. 7, 2154 (1997). [6] E. Monroy, F. Calle, E. MunÄoz, F. Omnes, B. Beaumont, P. Gibart, J. A. MunÄoz, and F. Cusso, MRS Internet J. Nitride Semicond. Res. 3, 9 (1998). [7] Y. Koide, Y. Koide, N. Itoh, K. Itoh, N. Sawaki, and I. Akasaki, Jpn. J. Appl. Phys. 27, 1156 (1988). [8] K. Itoh, Doctor Thesis, School of Engineering, Nagoya University, Nagoya [9] M. Iwaya, T. Takeuchi, S. Yamaguchi, C. Wetzel, H. Amano, and I. Akasaki, Jpn. J. Appl. Phys. 37, L316 (1998). [10] H. Amano, M. Iwaya, T. Kashima, M. Katsuragawa, I. Akasaki, J. Han, S. Hearne, J. A. Floro, E. Chason, and J. Figiel, Jpn. J. Appl. Phys. 37, L1540 (1998). [11] J. Floro, E. Chason, S. Lee, R. Twesten, R. Hwang, and L. Freud, J. Electron. Mater. 26, 969 (1997). [12] S. Hearne, E. E. Chason, J. Han, J. A. Floro, J. Figiel, J. Hunter, H. Amano, and I. Tsong, Appl. Phys. Lett. 74, 356 (1999). [13] J. Han, M. H. Crawford, R. J. Shul, S. J. Hearne, E. Chason, J. J. Figiel, and M. Banas, MRS Internet J. Nitride Semicond. Res. 4S1, G7.7 (1999).

8