Г-L intervalley separation and electron mobility in GaAsSb grown on InP: Transport comparison with the GaInAs and GaInAsSb alloys

Abstract

GaAs0.51Sb0.49 is lattice-matched to InP and finds electron transport applications in base or absorber layers in high-speed heterostructure bipolar transistors or photodiodes, because its staggered (“type-II”) band alignment with InP favors electron injection across abrupt heterojunctions. Little remains known about electron transport properties and band structure details of GaAsxSb1−x near x = 0.5. Particularly, based on the Γ-L intervalley separation in binary constituents (ΔΓL = 84 meV in GaSb and 290 meV in GaAs at 300 K), interpolation suggests a low Γ-L separation in GaAs0.51Sb0.49 before considering energy gap bowing effects. To gain insight into electron transport in GaAs0.51Sb0.49, we characterized experimental Hall electron mobilities vs carrier concentration at 300 and 77 K in n-type GaAs0.51Sb0.49, Ga0.47In0.53As, and Ga0.76In0.24As0.67Sb0.33 alloys nearly matched to InP. In marked contrast to the other two alloys, GaAs0.51Sb0.49 exhibits a sharp rise in 77 K electron mobility, which evidences L-valley de-population for lower electron concentrations. A two-band transport analysis reveals a Γ-L valley separation ΔΓL = 91 meV at 77 K, significantly lower than values recommended in the literature. Based on the reported temperature variations of ΔΓL in GaAs and GaSb, 84 < ΔΓL < 95 meV is expected at 300 K. The corresponding GaAsxSb1−x L-valley bowing parameter is cL = 1.63 eV, significantly higher than the 1.1–1.2 eV recommended in the literature. In contrast to GaAsxSb1−x, GaInAsSb grown on InP displays a strong alloy scattering, which limits its low-temperature electron mobility. The direct gap GaAs0.51Sb0.49 mixed group-V ternary alloy lattice-matched to InP is characterized by a staggered (“type-II”) band alignment that allows direct electron injection into InP across abrupt heterojunctions.1,2 As such, it finds electron transport applications in devices such as NpN double heterojunction bipolar transistors (DHBTs)3 and unitraveling carrier photodiodes (UTC-PDs).4 Despite increasing device uses over the last two decades, relatively little remains known about electron transport properties and the band structure of GaAsxSb1−x near x ∼ 0.5. Based on the known Γ-L intervalley separation in binary constituents (ΔΓL = 84 meV in GaSb5 and 290 meV in GaAs6 at 300 K, Table I), interpolation suggests a low Γ-L separation in GaAs0.51Sb0.49, even before considering potential alloy gap bowing effects. While linear interpolation yields ΔΓL = 187 meV, Adachi7 and Vurgaftman et al.8 recommend 181 and 223 meV, respectively. Due to the lack of experimental data around x ∼ 0.5, theoretical works involving GaAs0.51Sb0.49 rely on a variety of values: Tea and Aniel9 and Ferry10 used ΔΓL = 121 meV, while Wen et al. selected 196 meV.11 Bennett and Hung showed that because of the low ΔΓL separation and the high L-valley density of states in GaSb, more carriers are always present in the L-valley than the Г-valley at 300 K.12 We find that GaAs0.51Sb0.49 behaves similarly. Simple estimates using non-degenerate statistics at 300 K suggest the L-valley in GaAs0.51Sb0.49 would be significantly populated, even with a Γ-L separation as high as 220 meV (which is more than two times higher than our experimentally extracted ΔΓL value).