We present here growth temperature dependence of crystallinity of low-temperature-grown (LTG) InxGa1-xAs on InP substrates and its electrical and optical properties. LTG-GaAs-based III-V compound semiconductors have been studied actively because they are candidate materials to realize photoconductive antennas (PCAs) activated by femtosecond fiber lasers with wavelengths of 1.5 μm for terahertz (THz) wave emission and detection. These lasers are less expensive and more compact than conventional optical sources, and therefore, THz time-domain spectroscopy systems with these PCAs will be put into widespread use. The materials for the PCAs are required to have three properties of high carrier mobility, an ultrashort carrier lifetime, and high resistivity. However, there are some difficulties to obtain them in LTG-GaAs-based compounds. For LTG GaAs, the high density of defects such as excess As, Ga vacancy and the formation of As precipitates which is induced by low-temperature growth and thermal annealing generate the high resistivity and short carrier lifetime. Therefore, revealing the key defects is essential for efficient THz-wave emission and detection in LTG-GaAs-based compounds. For this reason, we focus on defects, and electrical and optical properties of one of the candidate materials, LTG InxGa1-xAs, in this study.
LTG-InxGa1-xAs samples (x = 0.42–0.54), with thicknesses of 1.0-2.0 μm, were grown on (100)InP substrates by molecular beam epitaxy at temperatures of 130 and 240 °C. Be was doped into InxGa1-xAs layers with a concentration in the range of 3.00×1017–18 cm-3. After the growth, the samples were annealed at 550 °C for 1 h in an H2 atmosphere. We performed X-ray diffraction (XRD) measurements, transmission electron microscope (TEM) observations, and optical absorption and Hall effect measurements at a temperature in the range of 120–300 K for these samples.
The (400) reflection XRD showed that the sample grown at 220 °C exhibited clear XRD peak while the samples grown at 200 °C did broad spectra with low intensities, suggesting that crystalline quality deteriorates drastically at a growth temperature between 200 and 220 °C. High-resolution XRD reciprocal space mapping measured around (115)InP and cross-sectional TEM images for InxGa1-xAs demonstrated that single crystalline InxGa1-xAs layers can be grown by solid-phase epitaxy at the substrate temperatures of 130-180 °C. The measured carrier densities for the LTG-InxGa1-xAs samples using Hall effect measurements were fitted by simulation based on the charge neutrality of carriers and impurities. These simulation fitting showed that the localized level lay 100 meV below the conduction band minimum (CBM) for the sample grown at 220 °C. As for the sample grown at 200 °C, it was shown that it lay 10-18 meV below the CBM. Depth of these localized levels were evaluated by optical absorption, and the results implied that there were fluctuations of CBM for these LTG InxGa1-xAs layers.