Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells

In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron-neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n=2 subband in GaAs/Al_0.37Ga_0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm.     

Spectroscopy studies of highly acceptor doped GaAs/AlGaAs quantum wells

We present a theoretical and experimental study of optical properties on highly acceptor doped QWs. Steady state photoluminescence (PL) and PL excitation (PLE) results are compared with theoretical calculations involving exchange and correlation effects for the electron-hole system and their interaction with acceptor ions. We have studied the effects of impurity doping at levels varying from 10⁸ up to 10¹³ cm^-2. Excitons can still be detected at high hole concentrations above the degenerate limit. They survive due to the inefficiency of screening in the 2D system.     

Radiative coupling of intersubband transitions in GaAs/AlGaAs multiple quantum wells

Radiative coupling of resonantly excited intersubband transitions in GaAs/AlGaAs multiple quantum wells can have a strong impact on the coherent nonlinear optical response, as is shown by phase and amplitude resolved propagation studies of ultrashort electric field transients. Upon increasing the driving field amplitude, strong radiative coupling leads to a pronounced self-induced absorption, followed by a bleaching due to the onset of delayed Rabi oscillations. A many-particle theory including light propagation effects accounts fully for the experimental results.     

Time resolved studies of intersubband relaxation in GaAs/AlGaAs quantum wells below the optical phonon energy using a free electron laser

The problem of intersubband relaxation in GaAs/GaAs quantum wells, where the energy separation of the two lowest subbands is smaller than the optical phonon energy, is considered. Time resolved pump-and-probe measurements are performed with a far-infrared free-electron laser on two multiquantum well samples with similar thicknesses (≈300 Å), but different doping and mobilities. The measured lifetimes are shorter than could be explained by acoustic phonon emission alone. Monte-Carlo calculations show the importance of electron–electron scattering for thermalization of the hot electron distribution function and subsequent optical phonon emission from the long thermal tail.     

Spin relaxation of conduction electrons in GaAs

We have used optical spin orientation techniques to measure T₁ of conduction electrons in GaAs (NinA ≈ 10¹⁷ cm^-3) for 4.7 K ? T ? 200 K. From Hall effect measurements we estimated the electron momentum relaxation time τ_p. For 50 K ? T ? 200 K, the product T₁τ_p agrees with our earlier order of magnitude estimate of the D'yakonov-Perel' mechanism, in which band structure induced precession is strongly narrowed by momentum relaxation. The Elliott mechanism is one to two orders of magnitude weaker.     

Mean free path of ballistic electrons in GaAs/AlGaAs superlattices

The mean free path of ballistic electrons in GaAs/AlGaAs superlattices was measured using the technique of hot electron spectroscopy in magnetic fields perpendicular to the growth direction. We utilize the fact that the total effective path of an injected hot electron is a function of the applied magnetic field. For a superlattice with 6.5 nm GaAs wells and 2.5 nm GaAlAs barriers we measure a mean free path of 80 nm. The experimental results of a ten-period SL sample are compared to a fully three-dimensional calculation of the transmission including interface roughness with island sizes of 10 nm. We demonstrate that the observed mfp is limited due to interface roughness scattering for temperatures up to 50 K.     

Saturation absorption studies of intersubband relaxation rates in a p-GaAs/AlGaAs QW using a free electron laser

We present saturation absorption studies of the HH1→LH1 intersubband transition in a 70 Å p-type GaAs/AlGaAs multi-quantum well. The subband separation in this sample, with strongly non-parabolic bands, varies from 26 meV at k=0 to 16 meV at the Fermi wave vector. The absorption behaviour of this transition is studied over a range of four orders of magnitude of intensity of incident THz radiation. The results are analysed within the contexts of a simple two-level model and an instantaneous thermalization model. The results indicate that the lifetime obtained with these models is limited by the pulsewidth, i.e. is greater than 1.7 ps. This places a lower limit on the energy relaxation time for transitions below the LO-phonon energy.     

Carrier transport effects on the photocurrent spectra of an intrinsic GaAs/AlGaAs multiple quantum well with highly doped AlGaAs contact layers

In this study, we demonstrate how electroreflectance (ER) measurements as a function of bias, and of angle of incidence (θ₀), together with bias dependent photocurrent (PC) measurements, can be used to provide understanding of the complex electric field profile and carrier transport effects in a GaAs/Al_0.3Ga_0.7As multiple quantum well (MQW), grown inside n⁺ contact layers. The PC measurements exhibit split excitonic features, the components of which change in strength with the applied bias. The effect is explained by absorption in the front of the MQW stack, with the back of the stack acting as detector. We examine the θ₀-dependence of the ER lineshape, to determine the depth of the layers responsible for each feature. The ER and PC lineshapes and their bias dependence are explained by the unusual electric field profile across the stack. The field profile appears to be determined by tunnelling of the dark current.     

Diffusion process of electrons injected from STM tip into AlGaAs/GaAs quantum wells

We have investigated the electron diffusion process in Al_0.3Ga_0.7As/GaAs quantum well (QW) structures by means of scanning tunneling microscope light emission (STM-LE) spectroscopy. The optical measurements were performed on a cleaved (1 1 0) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants. From comparison with Monte Carlo simulations for hot electron relaxation, we found that the intervalley scattering from the Γ valley to the L and X valleys has the most significant effect on the diffusion process of the injected electrons.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Effective mass anisotropy of gamma electrons in GaAs/AlGaAs quantum wells

Resonant magnetotunneling in GaAs/Al(0.28)Ga0.72As double barrier structures is used to demonstrate that the effective mass of confined Gamma conduction electrons becomes anisotropic when an electric field is applied perpendicular to the interfaces. Although several authors have previously reported Gamma-related optical anisotropy, this is the first example of a corresponding electrical anisotropy. The results are explained using a quantum mechanical model involving interface band mixing that contains additional features not found in the optical case.     

Resistivity of dilute 2D electrons in an undoped GaAs heterostructure

We report resistivity measurements from 0.03 to 10 K in a dilute high mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a gated field-effect transistor geometry, a wide range of densities, 0.16 x 10(10) to 7.5 x 10(10) cm(-2), are explored. For high densities, the results are quantitatively shown to be due to scattering by acoustic phonons and impurities. In an intermediate range of densities, a peak in the resistivity is observed for temperatures below 1 K. This nonmonotonic resistivity can be understood by considering the known scattering mechanisms of phonons, bulk, and interface ionized impurities. Still lower densities appear insulating to the lowest temperature measured.