Temperature Dependent Optical Band Gap Measurements of III-V films by Low Temperature Photoluminescence Spectroscopy
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1195-B08-24
Temperature Dependent Optical Band Gap Measurements of III-V films by Low Temperature Photoluminescence Spectroscopy Linda M. Casson, Francis Ndi and Eric Teboul HORIBA Scientific, 3880 Park Avenue, Edison, NJ, 08820, USA ABSTRACT Photoluminescence (PL) spectroscopy is a powerful technique for probing the structures of many types of III-V semiconductor materials. When a semiconductor material is excited at a particular wavelength, electron-hole pairs are generated. The most intense radiative transition is between the conduction band and valence band, and this measurement is used to determine the material band gap. Radiative and non-radiative transitions in semiconductors also involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration, and thus predict device quality. At ambient temperatures, the PL signal is typically broad, as much as 100 nm in width. When cooled, structural details may be resolved, and a small spectral shift between 2 samples may represent a change in a structural parameter. Thus a system with high spectral resolution is required. In this paper, a modular Low Temperature Photoluminescence system (LTPL) for measuring optical band gap as a function of temperature is described. Results show that the optical band gap shifts towards higher energy as the sample temperature decreases. INTRODUCTION Temperature dependent photoluminescence spectroscopy (PL) is a powerful optical method used for characterizing materials. It can be used to identify defects and impurities in Silicon and group III-V element semiconductors, and determine semiconductor band gaps (Eg). When light with sufficient energy hits a semiconductor, the material absorbs light, creating an electron-hole pair. An electron from the valence band jumps to the conduction band leaving a hole in the valence band. The electron relaxes back down to the lowest energy level in the Conduction Band. The electron drops back across the bandgap recombines with the hole. A photon is emitted during this recombination process; the emission of light is called photoluminescence. The photon emitted upon recombination corresponds to the energy difference between the valence and conduction bands or band gap (Eg), and is hence lower in energy than the excitation photon, so that the luminescence is red-shifted with respect to the excitation light. This is also called a Stokes shift. In addition to the main emission band, especially at low temperatures side bands may be observed, which may correspond to emission from defect states, indicating defects or impurities in the material.
EXPERIMENTS Two separate samples were characterized on the photoluminescence system described below. Sample 1 was made of a silicon doped (1 x 1018 cm-3) 400 micron thick InP substrate, a 1 micron thick InP (undoped) buffer layer, a 3 micron thick absorbing layer of lattice-matched In0.53Ga0.47As deposited by organometallic
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