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Optical and electrical properties of dilute GaN(x)As(1-x) alloys Strohm, Eric

Abstract

The bandgap and optical absorption edge are measured in semi-insulating and p-type GaNAs as a function of nitrogen content using a photoconductivity technique. The bandgap is found to decrease from 1.42 eV with 0% nitrogen to 1.20 eV with 0.9% nitrogen, and to 1.14 eV with 1.73% nitrogen content. The characteristic energy of the exponentional absorption edge (Urbach parameter) for p-type GaNAs is found to increase from 6.7 meV with 0% nitrogen to 14 meV with 0.8% nitrogen content. The mobility and carrier concentration are measured as a function of nitrogen content in p-type and n-type GaNAs using Hall measurements. The electron mobility decreases from 3000 cm²/Vs with 0% nitrogen, to 650 cm²/Vs with 0.1% nitrogen, and to 300 cm²/Vs with 1.0% nitrogen content. The hole mobility is relatively unaffected by nitrogen and stays constant around 300 cm²/Vs for up to 1% nitrogen content. The carrier concentration in p-type GaNAs is found to decrease for highly doped (2.5xl0 ¹⁶ cm ⁻³ ) GaNAs and increase for low doped (4.5xl0 ¹⁴ cm ⁻³ ) GaNAs with increasing nitrogen content. The carrier concentration converges to 7xl0 ¹⁵ cm ⁻³ for both low and high doping at greater than 0.8% nitrogen content, which suggests there is a trap that is pinning the Fermi level. This behaviour is modeled using conservation of charge in the band gap, and a trap level at 0.18 eV above the top of the valence band is found to explain the experimental data. A new method for measuring the bandgap and Urbach edge in epitaxial semiconductor films using photoconductivity is presented. In this method the sample is illuminated with monochromatic light, and the photoconductivity is measured as a function of incident wavelength. Light with energy greater than the band gap is absorbed by the sample and increases the photoconductivity signal. The absorption coefficient is determined from the photoconductivity signal. The electrical properties, such as the mobility and carrier concetration are obtained from Hall measurements.

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