Transitions between Be acceptor levels in GaAs bulk

Abstract

The doping is one of important means in the semiconductor manufacturing techniques, by which the optical and electric properties of semiconductor materials can be significantly improved. The doping level and energy level structure of dopants have a great influence on the operating performances of micro-electronic devices. Beryllium is one of acceptors, which is frequently used to be doped in GaAs bulk, because it is very stable with respect to diffusion at higher temperatures. Therefore, it is significant for the application to optoelectronic devices that the energy-state structure of Be acceptors in GaAs bulk can be investigated in detail. The sample GaAs:Be used in experiment is a 5-μm-thick epitaxial single layer doped uniformly by Be acceptors with a doping level of 2 × 10¹⁶ cm^–3, and grown by molecular beam epitaxy on 450-μm-thick semi-insulating (100) GaAs substrates in a VG V80 H reactor equipped with all solid sources. The transitions between the energy states of Be acceptors are studied experimentally by different spectroscopy techniques. The far-infrared absorption experiments are performed by using a Fourier-transform spectrometer equipped with a tungsten light source and a multilayer wide band beam splitter. Prior to the absorption spectrum measurement, the sample is thinned, polished and wedged to approximately a 5° angle to suppress optical interference between the front and back faces. Then, the sample is placed into the cryostat with liquid helium (4.2 K). The photoluminescence and Raman spectra are also measured at 4.2 K by a Renishaw Raman imaging microscope. The optical excitation to the sample is provided by an argon-ion laser with a wavelength of 514.5 nm, and the excited power is typically 5 mW. The odd-parity transitions from the Be acceptor ground state 1S_3/2<i>Γ</i>₈ to three excited states, i.e. 2P_3/2<i>Γ</i>₈, 2P_5/2<i>Γ</i>₈ and 2P_5/2<i>Γ</i>₇ are clearly observed in the far-infrared absorption spectra, then the respective transition energy values are obtained, which are in excellent agreement with the experimental results reported previously. In the photoluminescence spectrum, the emission peak labelled two holetransition, originating from the two-hole transition of recombination of the neutral-accptor bound excitons, is seen obviously, thus the energy of the even-parity transition between 1S_3/2<i>Γ</i>₈ and 2S_3/2<i>Γ</i>₈ states is found indirectly. Furthermore, in the Raman spectrum measured, the transition peak between 1S_3/2<i>Γ</i>₈ and 2S_3/2<i>Γ</i>₈ states is well resolved, and the transition energy between them is gained directly. By comparison, the transition energy values gained directly and indirectly are found to be consistent with each other.