The influence of the sapphire substrate on the temperature dependence of the GaN bandgap
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Department of Materials Science, University of California at Berkeley and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California94720, USA William S. Wong and Timothy Sands
Departmentof MaterialsScience, University of California at Berkeley Berkeley, California94720, USA Nathan W. Cheung
Department of ElectricalEngineeringand Computer Science, University of California, Berkeley, California94720, USA Richard J. Molnar
Lincoln Laboratory,Massachusetts Institute of Technology, Lexington, Massachusetts 02173, USA ABSTRACT This paper analyses the influence of the sapphire substrate on stress in GaN epilayers in the temperature range between 4K and 600K. Removal of the substrate by a laser assisted liftoff technique allows, for the first time, to distinguish between stress and other material specific temperature dependencies. In contrast to the prevailing assumption in the literature, that the difference in the thermal expansion coefficients is the main cause for stress it is found that the substrate has a rather small influence in the examined temperature range. The measured temperature dependence of stress is in contradiction to the published values for the thermal expansion coefficients for sapphire and GaN.
INTRODUCTION In recent years, GaN and related compounds have attracted a lot of academic as well as commercial interest. This is due to the potential applications for UV-based optoelectronic applications as well as high-temperature electronics [Kahl]. Very bright blue and green InGaN single quantum well diodes light-emitting diodes have been developed and commercialized [Nakl], and a laser diode consisting of 4 InGaN multi quantum wells has been reported to have a room temperature cw-operational lifetime of more than 10.000 hrs [Nak2]. Since large-scale GaN substrates are not available, epitaxial layers of GaN are deposited for the most part on foreign substrate materials like sapphire and SiC. These materials are known to result in stress in the GaN main layer which can reach values of up to 1.2 GPa [Krul], either compressive or tensile. It is commonly argued that the lattice mismatch between layer and substrate and the difference in thermal expansion coefficient (TEC) are the main causes of stress in the GaN layer at room temperature. Consequently, considerable efforts have been spent on the exploration of alternative substrate materials. These have been either focused on matching the lattice constant 289 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
(Si, GaAs, ZnO, LiAlO3) or matching the thermal expansion coefficient (Ge [Siel]). As to date, however, best results for GaN are still achieved for growth on sapphire. Though methodologies have been empirically developed to control stress to some extent, very little fundamental understanding of the exact causes of stress has been gained. Based upon the linear thermal expansion coefficients for GaN (a = 5.59 x 106/K), for sapphire (a - 7.5 x 106/K), and for SiC (a = 4.2 x 10'/K), all values are measured at
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