Band Gap Shift of GaN under Uniaxial Strain Compression
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Band Gap Shift of GaN under Uniaxial Strain Compression H. Y. Peng, M. D. McCluskey, Y. M. Gupta, M. Kneissl1, and N. M. Johnson1 Institute for Shock Physics and Department of Physics, Washington State University, Pullman, WA 99164-2816, U.S.A. 1 Xerox PARC, 3333 Coyote Hill Rd., Palo Alto, CA 94304, U.S.A. ABSTRACT The band-gap shift of GaN:Mg epilayers on (0001)-oriented sapphire was studied as a function of uniaxial strain compression along the c-axis using time-resolved, optical absorption measurements in shock wave experiments. For longitudinal stresses ranging from 4 to 14 GPa, the band gap shift is approximately 0.026 eV/GPa. Combining this result with the known behavior of wurtzite GaN under hydrostatic pressure and biaxial stress, a new set of deformation potentials has been estimated: acz-D1 = -10.2 eV, act-D2 = -7.9 eV, D3 = 1.33 eV and D4 = -0.74 eV. A slow band gap shift is also observed following the immediate band gap increase upon impact. This phenomenon can be explained by a time-dependent screening of the piezoelectric field. INTRODUCTION In recent years, much attention has been devoted to the study of the properties of GaN-based III-V semiconductors. The effect of strain on the optical properties of wurtzite GaN thin films and quantum-well structures is of great interest. Strain is always present in such heterostructures due to the large differences in lattice parameters and expansion coefficients between the substrate and III-nitride epilayers. In addition to its role in band-gap engineering, strain influences the electrical and optical properties of the devices through the piezoelectric effect [13]. The values of the deformation potentials for wurtzite GaN are important for device modeling. However, the values found in the literature are scattered over a relatively large range [4-7]. In this study, we present band-gap measurements of GaN:Mg under uniaxial strain compression along the c-axis using time-resolved, optical absorption spectroscopy in shock-wave experiments. This unique method enables us to obtain the hydrostatic deformation potential along the c-axis (acz-D1) directly. EXPERIMENTAL DETAILS The samples used in this work were thin (4 µm), semi-insulating, Mg-doped GaN epilayers on c-cut sapphire substrates (420 µm) grown by metalorganic chemical vapor deposition. Shock waves were generated by the impact of an optically transparent sapphire impactor onto the sample. The impactor was mounted on a projectile that was accelerated by a light gas gun. The GaN:Mg sample was backed by a sapphire buffer window. The transmitted light was collected by a UV-transparent lens and focused into an optical fiber. This light was spectrally dispersed by a spectrometer (ARC SpectraPro 150, 600 grooves/mm grating blazed at 300 nm), temporally dispersed by an electronic streak camera (Imacon 500) and digitally recorded on a chargecoupled device (CCD) detector as a series of transmission spectra, each separated in time by 20 I11.49.1
ns. The shock wave produced a constant stress at the center of the sample
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