Variation of elastic scattering across a quantum well

Abstract

The Drude scattering times of electrons in two subbands of a parabolic quantum well have been studied at constant electron sheet density and different positions of the electron distribution along the growth direction. The scattering times obtained by magnetotransport measurements decrease as the electrons are displaced towards the well edges, although the lowest-subband density increases. By comparing the measurements with calculations of the scattering times of a two-subband system, new information on the location of the relevant scatterers and the anisotropy of intersubband scattering is obtained. It is found that the scattering time of electrons in the lower subband depends sensitively on the position of the scatterers, which also explains the measured dependence of the scattering on the carrier density. The measurements indicate segregation of scatterers from the substrate side towards the quantum well during growth.The striking success of Ga [Al]As semiconductor heterostructures originates from the extremely high mobilities obtained in these materials. One key ingredient for the fabrication of such samples is modulation doping, where dopants and electrons are spatially separated. At low temperatures, impurity scattering, alloy scattering and interface roughness scattering limit the electron mobility [1]. If more than one subband is occupied, intersubband scattering takes place in addition [2,3].Information on the relevant scattering processes is usually obtained by measuring how quantum (τ q ) and Drude scattering times (τ ) vary with carrier density n S . For two-dimensional electron gases (2DEGs) realized in AlGaAs heterostructures, it is found that impurity scattering is dominant. In this case, one finds τ ∝ n γ S , with γ between 1 and 1.5, depending on the distance between the dopants and the 2DEG [1].In a two-subband system with subband densities n 1 and n 2 , the Drude scattering times τ i of subband i are usually found to increase monotonically with n i [4,5]. Recent results show that in a parabolic quantum well (PQW), τ 1 may also slowly decrease, i.e. γ < 0, when a second subband is occupied [6]. In this paper, we investigate this unusual dependence and show that it may be due to a certain arrangement of the ionized impurities.The PQW, grown by molecular beam epitaxy (MBE), is a 760Å wide Al x Ga 1−x As layer with x varying parabolically between 0 and 0.1 [7] (inset of Fig. 1a). In the center of the well, a three monolayer thick Al 0.05 Ga 0.95 As layer forms a potential spike. The well is embedded symmetrically in 200Å of undoped Al 0.3 Ga 0.7 As spacer layers and remote Si-doping layers on both sides. On the surface side, the donors are provided by 11 sheets, each with a Si donor density of nominally 5·10 15 m −2

Magnetotransport in C-doped AlGaAs heterostructures

“…Intersubband scattering is not taken into account. The conductivity in the Drude model develops a parabolic B dependence in the classical regime, 21,22 where Landau quantization is not yet important. Figure 4 shows measured low-field magnetoresistance (solid line) together with the parabolic fit (dotted line).…”

Lateral superlattices on parabolic quantum wells

“…We notice that a positive magnetoresistance can also arise if two subbands with rather different carrier mobilities are occupied with different densities. 15 This effect occurs at low magnetic fields and is weak in the present case, since the carrier densities in the two subbands here are of the same order of magnitude.…”

Many-body effects in wide parabolic AlGaAs quantum wells

“…21͒, our interpretation is based on the fact that, during the growth process, Si atoms in the atmosphere of the growth chamber can be incorporated as a residual impurity inside the PQWs as already observed for the growth of samples with similar structure to the ones analyzed here. 22 Moreover, it is important to notice that the PL spectra of the undoped samples ͑see Fig. 2͒ does not show any emission peak or any low energy broadening around the energy of 1.513 eV ͑with exception of the intrinsic emission of the GaAs͒, which reinforces the hypothesis that this peak has extrinsic origin.…”