Photo-Hall studies of modulation-doped field-effect transistors with short-period superlattice channels rather than alloy channels
We studied the photo-Hall mobility and the photo-Hall density of modulation-doped field-effect transistor structures using either an InGaAs alloy channel or a short-period superlattice channel. In defining the short-period superlattice channel we changed the thicknesses of the InAs and GaAs layers and the number of InAs/CaAs interfaces, The thicknesses of the InAs layers varied from 0.5 to 1.6 monolayers, those of GaAs layers from 4 to 20 monolayers, and the number of InAs/GaAs interfaces varied from 3 to 7. As a consequence the total short-period superlattice channel thicknesses varied too, but they were kept below the critical layer thickness to relax the pseudomorphic structure. The persistent photoconductivity effect at 77 K and the use of a red light emitting diode as the illumination source were employed to induce variation of the Hall mobility and the Hall density in the conduction channel. For the short-period superlattice samples our results indicate that the Hall mobility increases (a) with a reduction in the number of InAs/GaAs interfaces; (b) by employing an integral number of InAs monolayers; and (c) by increasing the thickness of the GaAs layers. These results suggest that the three effects are effective in producing surface smoothing. When the short-period superlattice samples are compared with samples having an InGaAs channel, we have shown that for an indium content around 8% the electrical properties are basically the same. On the other hand, for an indium content around 25% the mobility increases up to 35% proving that use of the short-period superlattice channel is useful in reducing the atomic disorder. The scattering associated with the alloy disorder increases with the indium content, and the deleterious effect of disorder can be reduced by replacing the alloy channel by a short-period superlattice channel.