Magnetotransport in modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterojunctions: in-plane effective mass, quantum and transport mobilities of 2D electrons

Shubnikov-de Haas (SdH) and Hall effect measurements, performed in the temp erature range between 3.3 and 20 K and at magnetic fields up to 2.3 T, have been used to investigate the electronic transport properties of lattice-ma tched In0.53Ga0.47As/In0.52Al0.48As heterojunctions. The spacer layer thick ness (t(S)) in modulation-doped samples was in the range between 0 and 400 Angstrom. SdH oscillations indicate that two subbands are already occupied for all samples except for that with t(S) = 400 Angstrom. The carrier densi ty in each subband, Fermi energy and subband separation have been determine d from the periods of the SdH oscillations. The in-plane effective mass (m* ) and the quantum lifetime (tau (q)) of 2D electrons in each subband have b een obtained from the temperature and magnetic field dependences of the amp litude of SdH oscillations, respectively. The 2D carrier density (N-1) in t he first subband decreases rapidly with increasing spacer thickness, while that (N-2) in the second subband, which is much smaller than N-1, decreases slightly with increasing spacer thickness from 0 to 200 Angstrom. The in-p lane effective mass of 2D electrons is similar to that of electrons in bulk In0.53Ga0.47As and show no dependence on spacer thickness. The quantum mob ility of 2D electrons is essentially independent of the thickness of the sp acer layer in the range between 0 and 200 Angstrom. It is, however, markedl y higher for the samples with a 400 Angstrom thick spacer layer. The quantu m mobility of 2D electrons is substantially smaller than the transport mobi lity which is obtained from the Hall effect measurements at low magnetic fi elds. The transport mobility of 2D electrons in the first subband is substa ntially higher than that of electrons in the second subband for all samples with double subband occupancy. The results obtained for transport-to-quant um lifetime ratios suggest that the scattering of electrons in the first su bband is, on average, forward displaced in momentum space, while the electr ons in the second subband undergo mainly large-angle scattering. (C) 2001 A cademic Press.