Photoluminescent properties of InN epifilms
We report a detailed investigation of the photoluminescent properties of InN epifilms with free-electron concentrations ranging from 3.5 × 1017 cm−3 to 5 × 1019 cm−3. It is found that the photoluminescence (PL) peak energy strongly depends on the electron concentration. We show that the broadening of the PL spectra with increasing free-electron concentration arises from the breaking of the k = 0 selection rule. The large asymmetric line shape of the photoluminescence spectra can be well described by the free-electron recombination band model. We establish an empirical relation between the full-width at half-maximum (FWHM) value of the PL spectra and the free-electron concentration, which provides a convenient formula to determine the free-electron concentration in InN epifilms by PL measurement. We point out that the peak energy of the PL spectra does not reflect the real band gap of InN epifilms. Calculations based on the effects of Burstein–Moss absorption, band tail and band renormalization were used to analyse the PL spectra, and the fundamental band gap of the intrinsic InN film was obtained. The corresponding expression for the band gap narrowing effect of the InN film is found to be ΔEBGN = 1 × 10−8n1/3 + 3.6 × 10−7n1/4 + 2.3 × 10−11n1/2 eV. The temperature-dependent band gap of the intrinsic InN was fitted by the Passler equation. The Passler parameters of the intrinsic InN are α = 0.55 meV K−1, Θ = 576 K and p = 2.2. It is found that the band gap energies at T = 0 K and room temperature are close to 0.68 eV and 0.62 eV, respectively. In addition, we show that the band gap obtained from the PL spectra is in excellent agreement with that obtained from infrared absorption.