Abstract
We performed first-principles calculations based on density functional theory (DFT) to investigate the role of point defects in the structural, electronic, and optical properties of the GaAs(001)- β2(2x4). In terms of structural properties, AsGa is the most stable defect structure, consistent with experiments. With respect to the electronic structure, band structures revealed the existence of sub-band and midgap states for all defects. The induced sub-bands and midgap states originated from the redistributions of charges towards these defects and neighboring atoms. The presence of these point defects introduced deep energy levels characteristic of EB3 (0.97 eV), EL4 (0.52 eV), and EL2 (0.82 eV) for AsGa, GaAs, GaV, respectively. The optical properties are found to be strongly related to these induced gap states. The calculated onset values in the absorption spectra, corresponding to the energy gaps, confirmed the absorption below the known bulk band gap of 1.43 eV. These support the possible two-step photoabsorption mediated by midgap states as observed in experiments.
Highlights
The low-temperature-grown gallium arsenide (LT-GaAs) has been used as the photoconductive (PC) antenna substrate material for the generation and detection of terahertz (THz) radiation
We proved that, the presence of defects (AsGa, GaAs, and Ga vacancy (GaV)) allowed the shifting of the energy band gap by inducing midgap states which mediates the excitation of electrons at energies lower than the excitation on bulk GaAs upon absorption of incident radiation
DOS and band structure calculations show that AsGa induced an energy gap of 0.97 eV which is related to the EB3 deep level defects in experiments
Summary
The low-temperature-grown gallium arsenide (LT-GaAs) has been used as the photoconductive (PC) antenna substrate material for the generation and detection of terahertz (THz) radiation. One of the remarkable properties of LT-GaAs, based on experiments, is the ability to absorb sub-bandgap wavelength photons (e.g., 1.56 μm).[1,2,3] The observed near linear but not completely linear excitation intensity dependence of photoconductivity suggests the two-step photon absorption mediated by midgap states, possibly formed by defects.[1] Very recently, this sub-bandgap-optical-excitation induced photoconductivity (SOEP) using a 1.56 μm probe was observed and confirmed the sequential charge carrier excitation via midgap states.[4] These midgap states are referred to in other literatures as defect states, deep energy level (EL) defects, deep traps, deep defects, and midgap defects.[3,5,6,7,8] Defects in GaAs grown by molecular beam epitaxy (MBE) occur as a result of low-temperature growth in a range from 180 ◦C to 300 ◦C. This is lower than the normal epitaxial growth temperatures at around 580 ◦C to 600 ◦C.9 Particle-induced x-ray (PIXE) analysis revealed that LT-GaAs consists of 1-1.25 at % excess As10 which has been suggested as the source of the deep EL2-like defect (0.68 eV-0.92 eV).[8,11,12,13]
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