Molecular-beam epitaxial growth and characterization of modulation-doped field-effect transistor heterostructures using InAs/GaAs superlattice channels
The molecular-beam epitaxial growth conditions of (N + 1)(InAs)m/N(GaAs)n short period superlattices (SPSs) on GaAs substrates have been optimized. Hall electrical properties measured by the van der Pauw method were compared to low-temperature photoluminescence (77 K PL) spectra of GaAs/SPS/AlGaAs modulation-doped field-effect transistor-type heterostructures. By using these two characterization methods, the influences of the growth temperature T(s), of the SPS channel thickness d(ch) and of its average indium composition y(m) were studied. Interesting correlations were established between their optical and their transport properties measured at 77 K either in the dark or under white-light illumination. The thickness m of the InAs layers was varied in the range 0.57 to 1.7 and sharp optimum properties were obtained slightly above m = 1 monolayer. The PL spectra exhibit one or two bands which are attributed to transitions from electronic states belonging to the first or to the second subband formed in the conduction quantum well, the second transition at higher energy being observed only when the two-dimensional (2D) concentration exceeds a critical value n(c) which, in the dark, is of the order of 1.85 X 10(12) cm-2 (i.e., d(ch) almost-equal-to 119 angstrom) for our pulse-doped SPS heterostructures with y(m)=0.13. The electrical properties in the dark for modulation-doped structures having a (N + 1) (InAs) 1.14/N (GaAs)n SPS channel have been compared with those obtained on InGaAs alloy channels and it can be observed that the mobilities at 77 K are slightly lower than the mobilities of InGaAs alloys of equivalent In compositions, but the 2D electron gas concentration n(so,d) Seems to be slightly higher in the SPSs.