Objective The undoped GaSb has a donor defect and exhibits p-type conduction with good electrical conductivity. GaSb is usually used as the substrate for Type-Ⅱ superlattice materials prepared by molecular beam epitaxy, and the thickness of GaSb substrate is much larger than the thickness of the superlattice material. The thickness of GaSb substrate tends to have a great influence on the electrical properties of Sb based Type-Ⅱ superlattice during Hall test (Fig.1). In the preparation process of infrared detector, in order to increase the absorption of infrared radiation by the material, the substrate is usually thinned after the preparation of the device, and the infrared radiation is detected by back-side illumination. Therefore, exploring the electrical characteristics of Type-Ⅱ superlattice influenced by GaSb thickness could provide theoretical basis for the structural design of superlattice.
Methods The effect of GaSb substrate thickness on the electrical properties of n-type and p-type superlattice films is discussed. Molecular beam epitaxy technology is used to grow Type-Ⅱ superlattice. After the pdoped GaSb buffer layer is grown on the n-type GaSb substrate, Si-doped n-type InAs/GaSb Type-Ⅱ superlattice (Fig.5) and Be-doped p-type InAs/GaSb Type-Ⅱ superlattice (Fig.6) are grown respectively. The substrates are thinned by mechanical polishing with different thicknesses and Hall tests are performed immediately.
Results and Discussions The results show that in the Hall test at 77 K temperature, the electrical properties of n doped superlattice and p-type superlattice vary with the thickness of the substrate. The carrier concentration and mobility of n-type InAs/GaSb Type-Ⅱ superlattice grown on GaSb substrate and buffer layer vary with the substrate thickness, but the variation is small within the same order of magnitude, which is mainly due to the fact that the Be-doped GaSb buffer layer attenuates the electrical influence of GaSb substrate on the superlattice material (Fig.7). The carrier concentration and mobility of p-type InAs/GaSb Type-Ⅱ superlattice grown on GaSb substrate and buffer layer change with substrate thickness in the same way as that of n-type superlattice films with substrate thickness: due to the presence of Be-doped GaSb buffer layer, the electrical influence of GaSb substrate on the superlattice material is attenuated, and the overall change changes are small, especially the mobility. The carrier concentration decreases with the thickness of GaSb substrate, and the mobility increases with the thickness of GaSb substrate (Fig.8). This result occurs for the following reasons: the decreasing of the superlattice material carrier concentration reduces the possibility of electron scattering result from the increased surface recombination effect and redistribution of impurity concentration, so the mobility increases with the thinning of the substrate thickness. The changes of carrier concentration and mobility of n-doped superlattice are of the same order of magnitude, and the electrical properties of thin film materials with opposite polarity to the buffer layer before thinning can be calibrated for the electrical properties of the materials after thinning. The change of carrier concentration in p-doped superlattice is relatively large.
Conclusions Although the buffer layer attenuates the effect of the substrate on the superlattice film, the effect of the substrate on the electrical properties of the superlattice film cannot be completely eliminated. When considering the carrier concentration of materials with the same polarity as the buffer layer material, high concentration doping is required during the growth process of materials that require precise doping to ensure the carrier concentration of the thin film material after thinning. This change needs to be taken into account, and the change in carrier mobility could be treated as a constant. This paper is of reference significance for the carrier concentration calibrations of Sb based Type-Ⅱ superlattice materials with different doping concentrations.
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