The electronic properties of modulation-doped GaAs/Ga 1−x Al x As multiple quantum wells (MWQ) with well width (L z ) in the range between 51 and 145 Å have been investigated by using the Shubnikov-de Haas (SdH) oscillations technique. The carrier density and the Fermi energy have been determined from the period of the SdH oscillations. The in-plane effective mass (m * ) and the quantum lifetime (τ q ) of 2D electrons have been obtained from the temperature and magnetic field dependences of the SdH amplitude. For narrow MQW samples (L z = 51, 75 and 78 Å), m * increases with decreasing L z ; for the samples with L z = 106 and 145 Å, m * is approximately equal to that of electrons in bulk GaAs. The values obtained for τ q show no clear well-width dependence and suggest that interface roughness is the dominating scattering mechanism in GaAs/Ga 1−x Al x As MQWs.

No abstract

The quantum and transport mobilities of electrons in modulation‐doped GaAs/Ga1−xAlxAs multiple quantum wells with well widths in the range between 51 and 145 Å and carrier density of about 1×1016 m−;2 have been investigated by magnetotransport measurements. The magnetic field dependence of the amplitude of the quantum oscillations in both magnetoresistance and Hall resistance have been used to determine the quantum (τq) and transport (τt) lifetimes (and hence the quantum (μq) and transport (μt) mobilities) of 2D electrons. The values thus found for μt are substantially smaller than those of the Hall mobility (μH) as obtained in the ohmic regime at low magnetic fields. The discrepancy between μt and μH has been explained in terms of a transport lifetime τt that depends on the electron energy due to the scattering of electrons by interface roughness in the quantum wells.

The well-width dependence of the two-dimensional (2D) electron energy relaxation associated with acoustic-phonon emission in GaAs/Ga 1−x Al x As multiple quantum wells (MQWs) has been investigated using Shubnikov-de Haas (SdH) effect measurements in the temperature range of 3.3-15 K and at applied electric fields up to 300 V m −1. The modulation-doped MQW samples studied have quantum-well widths (L z) in the range between 40 and 145Å; only the lowest subband in each sample is populated with a 2D carrier density in the range from 1.04 × 10 16 m −2 to 1.38 × 10 16 m −2. The power loss-electron temperature characteristics of the samples have been obtained from the lattice temperature and electric field dependences of the amplitude of the SdH oscillations. It is found that the power loss decreases markedly when L z increases from 40Å to about 120Å and it increases for L z > 120Å. The experimental results are compared with the current 2D and three-dimensional (3D) theoretical models for power loss, which include both piezoelectric and deformation-potential scattering. The electron-temperature dependence of power loss determined experimentally fits well to both the 2D and 3D theoretical models in the intermediatetemperature regime but with different values for the acoustic deformation potential. It is shown that for samples with L z in the range of 40-120Å, the 2D model predicts a dependence of power loss on the well width, which is similar to that observed experimentally. The 3D model, however, predicts a power loss which increases with increasing well width and describes well the well-width dependence of the experimental power loss for wide wells (L z 120Å). The results provide useful information about the relative magnitude of the deformation-potential and piezoelectric contributions to power loss.

The measurements of resistivity and low-field Hall effect made in the temperature range 3.3-295 K have been used to investigate the transport properties of modulation-doped, lattice-matched In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As heterojunctions as a function of the spacer thickness in the range from 0 to 400 A. It is found that the sheet carrier density determined at temperatures below about 80 K decreases rapidly with increasing spacer thickness. The low-temperature Hall mobility increases substantially when increasing the spacer thickness from 0 to 100 A, and decreases gradually with further increase in spacer thickness. The results suggest that, in addition to alloy scattering, remote ionized-impurity scattering is a major scattering mechanism at low temperatures in the samples with thin spacer layer and that background impurity scattering prevails in the samples with spacer thickness larger than about 100 A. The effect of modulation doping on the mobility decreases progressively with increasing temperature: the electron mobility becomes practically independent of spacer thickness in the temperature range above about 200 K. The variation of Hall mobility with temperature in the range above 90 K has been used to determine the energy of longitudinal optical phonons that limit the mobility of electrons at high temperatures.On the Transport Properties of Modulation-Doped In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As