Photoluminescence, Magnetospectroscopy, and Resonant Electronic Raman Studies of Heteroepitaxial Gallium Nitride
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Internet Journal Nitride Semiconductor Research
Photoluminescence, Magnetospectroscopy, and Resonant Electronic Raman Studies of Heteroepitaxial Gallium Nitride B.J. Skromme1 1Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University,
(Received Friday, October 8, 1999; accepted Friday, December 3, 1999)
Optical spectroscopy, including low and room temperature photoluminescence (PL), reflectance, PL measurements in high magnetic fields up to 12 T, and resonantly-enhanced electronic Raman scattering (RERS) in zero and high magnetic field, has been used to investigate exciton and impurity states and surface recombination in high quality heteroepitaxial GaN grown on sapphire and SiC. Theoretical finite-difference calculations of the donor states as a function of magnetic field have been carried out for comparison, including the effects of anisotropy in the effective mass and dielectric constant. Up to six residual donor species are observed in material grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) from their n=2 and n=3 two-electron satellites observed in PL and by RERS. The donor-related nature of the relevant transitions is confirmed from their magnetic field dependence, and the spectral resolution is improved at high fields. The Si donor level is determined to have a binding energy of about 21 meV from observation of its two-electron satellite in lightly Si-doped HVPE material. The free exciton binding energy is shown to be about 26.4 meV, independent of strain, based on observations of the n=2 free exciton. The room temperature band-edge PL peak is confirmed to be free excitonic in nature, based on its linewidth and on comparison with simple reflectance measurements. Reflectance from the edge of a thick HVPE layer shows clear evidence of A, B, and C excitons obeying the relevant selection rules at both low and room temperature. Surface chemical treatments are shown to have substantial effects on room temperature PL efficiency. Passivation with ammonium or sodium sulfide solutions, in particular, yields increases in PL efficiency by a factor of five to seven over air-exposed surfaces. The passivation effect is stable in air, lasting at least one month.
1
Introduction
Many important materials properties of GaN, such as impurity and exciton binding energies, effective masses, defect properties, and surface characteristics remain poorly known at present, in spite of considerable technological interest in this material. Optical spectroscopy is a highly useful tool to determine many of these properties [1]. In the following [a] , we describe some results of recent experimental and theoretical investigations of donor, exciton, and surface properties of GaN. 2 Theoretical Calculation Of Donor States In GaN In a direct gap material such as GaN, hydrogenic effective mass theory ordinarily gives a good description of shallow donor levels. The wurtzite symmetry of GaN,
however, requires some modification of this theory
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