Heavily doped GaAs:Se. I. Photoluminescence determination of the electron effective mass

Abstract

A systematic study of the photoluminescence (PL) of Se-doped n-type GaAs grown by metalorganic chemical vapor deposition is reported. A new method is presented to determine the electron effective mass of n+-direct-gap semiconductors from the PL spectrum. GaAs samples with electron densities from 1015 to 8×1018 cm−3 were investigated over the temperature range of 13 to 353 K. The PL spectra of n+-GaAs are analyzed using a physical model which for the first time explains in a consistent manner both the energy of the peak and the full width at half-maximum, and accounts for the electron density. An accurate fit of the PL spectra is obtained by invoking band-to-band transitions without k selection. The electron exchange and correlation interactions account for all the observed band shrinkage, which reaches 48 meV for n=8.0×1018 cm−3. No significant density of band-tail states is observed. The Fermi energy is obtained directly from the PL fitting and is used with the measured Hall electron density n to determine the energy-dependent effective mass m*. An increase in m* beyond the value expected from the nonparabolicity of the conduction band in pure GaAs is observed. The mass at the minimum of the conduction band m*0 increases from 0.0636 me for pure GaAs to 0.073 me for n=8.0×1018 cm−3, where me is the free electron mass. The increase in m*0 is given empirically as m*0/me =0.0635+2.06×10−22n+1.16×10−40n2. The change in m* is interpreted as a distortion of the conduction band produced by the impurities. The small dilation of the lattice, 0.0035% at n=8.0×1018 cm−3, indicates that this distortion is not mechanical, but electrical in nature. Also, the PL linewidth is abnormally small for samples in which inhomogeneities are believed to exist.