GaN buffer layer

 

Background

Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on foreign substrates like SiC and sapphire (Al₂O₃). Both substrate materials are badly matched to GaN with respect to their lattice constant and the thermal expansion coefficients. In addition, the epitaxially deposited atoms are very mobile on the plain substrate, resulting in difficulties in homogeneously wetting the substrate. This problem is circumvented by depositing an AlN or GaN layer at a comparatively low temperature before the growth of the main layer at a higher temperature is initiated. The lower temperature significantly reduces the adatom mobility, leading to three-dimensional growth and complete coverage of the substrate with crystallites. The main layer growth involves a transition to two-dimensional layer-by-layer growth of GaN. Consequently the structural quality of the main layer, noteworthy the density of threading dislocations, is intimately related with the buffer layer properties.

LT-GaN Buffer Layers

We have performed many studies on the impact of the details of the buffer layer on the quality of the resulting GaN main layer using our MBE machine. Our studies concentrated mainly on c-plane sapphire substrates, although we also considered other orientations of Al₂O₃ as well as Ge, SiC, and Si as substrate materials. The actual sequence of growth steps is sapphire nitridation, leading in part to a conversion of Al₂O₃ into AlN, low-temperature (LT) GaN deposition, and GaN main layer deposition.

 

Our main interest was the stoichiometry of the LT buffer layer, and the explanation of the main layer properties in terms of the strain during the growth. The main finding was that variations of the stoichiometry towards an increased Ga content reduced the strain during the main layer growth and resulted in improved structural quality (less dislocations) and higher carrier mobility.

 

Ga-Metal Buffer Layers

As a natural limit of the stochiometry of LT-GaN buffer layers we studied plain Ga metal films deposited as wetting layers onto the sapphire substrates (LBNL patent IB-1461). The layer is converted into GaN by exposure to the Nitrogen from the plasma source during the heating to the main-layer growth temperature. Superior quality GaN main layers were achieved for a specific thickness of the deposited Ga metal layer. Using X-ray photoelectron spectroscopy we were able to detect Ga-N as well as Ga-Ga bonds in these buffer layers. We assume that metallic inclusions mediate a strong strain relief mechanism in these buffer layers. Experimental evidence for the strain relief action was provided by ex-situ biaxial straining of the samples using a pressure cell. Herein, the main layer strain was monitored as a function of the amount of bending of the sample. The strain transport from the sapphire to the GaN main layer is found to be strongly reduced for the Ga metal buffer layer as compared with a rigid stoichiometric LT-GaN buffer layer. Consequently the superior main layer quality is explained by the resulting reduction of strain during the main layer growth.

 

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Related Publications

Elastic strain relief in nitridated Ga metal buffer layers for epitaxial GaN growth
Y. Kim, N.A. Shapiro, H. Feick, R. Armitage, E.R. Weber
Appl. Phys. Lett. 78, 895 (2001). ONLINE

GaN thin films by growth on Ga-rich GaN buffer layers
Y. Kim, Sudhir G. S., H. Siegle, J. Krüger, P. Perlin, E.R. Weber, S. Ruvimov and Z. Liliental-Weber
J. Appl. Phys. 88, 6032 (2000). ONLINE

Improved Heteroepitaxial MBE GaN Growth with a Ga Metal Buffer Layer
Y. Kim, Sudhir G.S., J. Krüger, H Siegle, N.A. Shapiro, R. Armitage, H. Feick, E.R. Weber, C. Kisielowski, Y. Yang
Mat. Res. Soc. Symp. Vol. 622, T4.10 (2000). ONLINE

Effect of N/Ga flux ratio in GaN buffer layer growth by MBE on (0001) sapphire on defect formation in the GaN main layer
S. Ruvimov, Z. Lilienthal-Weber, J. Washburn, Y. Kim, Sudhir G.S., J. Krüger, E.R. Weber
Mat. Res. Soc. Symp. Vol. 572, 295 (1999).

Chemical and structural transformation of sapphrie (Al₂O₃) surface by plasma source nitridation
Y.Cho, Y. Kim, E.R. Weber, S. Ruvimov, Z. Liliental-Weber
J. Appl. Phys. 85, 7909 (1999). ONLINE

Chemical and structural analysis of nitridated sapphire
Y. Cho, S. Rouvimov, Y. Kim, Z. Lilienthal-Weber, E.R. Weber
Mat. Res. Soc. Symp. Vol. 482, 45 (1998).

Stress controlled MBE-growth of GaN:Mg and GaN:Si
Y. Kim, R. Klockenbrink, C. Kisielowski, J. Krüger, D. Corlatan, Sudhir G.S., Y. Peyrot, Y. Cho, M. Rubin, and E.R. Weber
Mat. Res. Soc. Symp. Vol. 482, 217 (1998). DOWNLOAD (228K)

Comparative analysis of strain and stress in MBE and MOCVD grown GaN thin films on sapphire
J. Krüger, Sudhir G.S., D. Corlatan, Y. Cho, Y. Kim, R. Klockenbrink, S. Ruvimov, Z. Lilienthal-Weber, C. Kisielowski, M. Rubin, E.R. Weber, B. McDermott, R. Pittman, E. R. Gertner
Mat. Res. Soc. Symp. Vol. 482, 447 (1998). DOWNLOAD (195K)

Impact of growth temperature, pressure and strain on the morphology of GaN films
H. Fujii, C. Kisielowski, J. Krüger, M.S.H. Leung, R. Klockenbrink, M. Rubin, E.R. Weber
Mat. Res. Soc. Symp. Vol. 449, 227 (1997). DOWNLOAD (943K)

Intrinsic and thermal stress in gallium nitride epitaxial films
J.W. Ager, T. Suski, S. Ruvimov, J. Krüger, G. Conti, E.R. Weber, M.D. Bremser, R. Davis, C.P. Kuo
Mat. Res. Soc. Symp. Vol. , 775(1997).

Strain-related phenomena in GaN thin films
C. Kisielowski, J. Krüger, S. Ruvimov, T. Suski, J.W. Ager III, E. Jones, Z. Liliental-Weber, M. Rubin, E.R.Weber, M.D. Bremser, R.F. Davis
Phys. Rev. B 54, 17745 (1996). ONLINE

Effect of Si doping on the dislocation structure of GaN grown on the A-face of sapphire
S. Ruvimov, Z. Liliental-Weber, T. Suski, J. W. Ager III, J. Washburn, J. Krüger, C. Kisielowski, E. R. Weber, H. Amano and I. Akasaki
Appl. Phys. Lett. 69, 990 (1996). ONLINE

Strain effects in GaN thin film growth
J. Krüger, C. Kisielowski, T. Suski, S. Ruvimov, Z. Liliental-Weber, J.W. Ager III, M. Rubin, and E.R. Weber
Proc. IEEE SIMC-9, Conf. on Semi-Insulating and Semiconducting Materials, Toulouse, 1996, p. 89. DOWNLOAD (163K) ONLINE

Origin of strain in GaN thin films
C. Kisielowski, J. Krüger, M. Leung, R. Klockenbrinck, H. Fujii, T. Suski, Sudhir G.S., J. W. Ager III, M. Rubin, and E.R. Weber
Proc. 23rd Internat. Conf. Phys. Semicond. Vol. 4, World Scientific, Singapore, p. 513 (1996).

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