Modulation-doped GaAs Quantum Wells Research Articles

Hot-electron transport in GaAs quantum wells: effect of non-drifting hot phonons and interface roughness

The influence of a non-equilibrium population of LO phonons (hot phonons) on hot-electron energy and momentum relaxation in modulation-doped GaAs quantum wells are studied both experimentally and theoretically. The experimental results on high-field parallel transport indicate strongly that: (i) non-equilibrium phonons in GaAs quantum wells, contrary to the assumptions made in the conventional theories, are non-drifting; (ii) therefore, the production of hot phonons not only reduces the energy relaxation rate but also enhances the momentum relaxation rate; (iii) the enhancement of the momentum relaxation at high fields inhibits negative differential conductivity via real space transfer or intervalley transfer; (iv) the enhancement of the momentum relaxation rate also reduces the drift velocity at high fields, which is detrimental to the speed of the hot electron devices; (v) hot phonon effects increases with increasing 3D carrier concentration. The results are compared with a comprehensive theoretical model involving non-drifting hot phonons and scattering from remote impurities and interface roughness. The agreement between the theory and the experiments is excellent.

Optical singularities of the one-dimensional electron gas in semiconductor quantum wires

The optical properties of a one-dimensional (1D) electron gas with only one or two occupied subbands have been studied in semiconductor quantum wires, obtained by electron beam lithography and subsequent low-energy ion bombardment of modulation-doped GaAs quantum wells. Large optical singularities have been observed at the Fermi level, both in optical absorption and emission. They disappear at temperatures comparable to the Fermi energy. The 1D singularities are much larger and sharper than in 2D systems due to the lack of certain hole recoil effects in 1D. The experimental data are in qualitative agreement with theoretical results based on the exact diagonalization of finite chains.

A study of band-gap renormalization in N- and P- type modulation doped GaAs-quantum wells

A systematic investigation of the band-gap renormalization observed in thin n- and p- modulation doped GaAs-quantum wells (QWs) is reported. Photoluminescence and photoluminescence excitation spectroscopy on undoped and modulation doped QWs of the same width provide reliable data for the band-gap shifts and the actual carrier concentrations. A careful lineshape analysis is crucial for the separation of intrinsic and extrinsic luminescence components. The experimental data compare favorably with our calculations of band-gap renormalization within the Local-Density approximation as well as with the self-energy shifts of electron and hole subbands found by a random phase approach. In particular, we observe that carrier confinement enhances BGR.

Single particle excitations and plasmons in a single asymmetric modulation-doped GaAs quantum well

We present and analyse electronic Raman scattering results on a single two-dimensional electron gas confined in a single quantum well, measured as a function of the scattering geometry and in the presence of additional illumination.

Coupling of excitons with free electrons in light scattering from GaAs quantum wells.

We observe unexpectedly narrow peaks (\ensuremath{\sim}1--2 meV) in the resonant enhancement of inelastic light scattering by a two-dimensional electron gas. The experiments are carried out in high-mobility modulation-doped GaAs quantum wells. Scattering by spin-density and charge-density intersubband excitations is enhanced through incoming and outgoing channels. Such excitonic resonances are much sharper than the Fermi energy (10--20 meV). To explain these observations we propose new mechanisms for light scattering by excitations of the electron gas. They are based on the coupling between excitons and the Fermi sea through Coulomb exchange and direct interactions.

Extremely high negative photoconductivity in p-modulation-doped GaAs quantum wells

In p-modulation-doped quantum wells of GaAs/AlGaAs extremely high negative photoconductivity is observed at low temperatures. The in-plane sheet resistance can be increased by a factor of more than 60 with illumination of less than 1 W/cm2. Spectral analysis shows that the effect is mainly due to hole trapping in the potential minima of AlGaAs and subsequent recombination of minority electrons.

High density femtosecond excitation of nonthermal carrier distributions in intrinsic and modulation doped Gaas quantum wells

We investigate the dynamics of non-thermal carrier distributions in undoped and modulation doped GaAs Quantum Well Structures (layer thickness ≈ 100 Å) with near bandgap-resonant intense femtosecond light pulses. We are able to study selectively the carrier-carrier scattering process up to very high densities and we carry out the first optical investigation of non-thermal carrier generation in the presence of a thermalized Fermi-sea of electrons. In undoped quantum wells we find a reduction of the thermalization time from about 100 fs to about 30 fs as the photocarrier density increases from N ch ≈ 2 × 10 10 cm −2 to ≈ 10 12 cm −2. The thermalization time is found to be sensitive to background doping with excess electrons in modulation doped samples.

Negative absolute mobility of minority electrons in GaAs quantum wells.

We report the observation of negative absolute mobility of electrons (i.e., a drift toward the negative electrode) in $p$-modulation-doped GaAs quantum wells. This unusual effect results from carrier drag on the electrons by the high-mobility hole plasma via electron-hole scattering.

Time-resolved photoluminescence for quantum well semiconductor heterostructures

Abstract Time-resolved photoluminescence spectroscopy with picosecond time resolution is currently being used to study the dynamics of photoexcited hot carriers confined in quantum well (QW) heterostructures. The energy relaxation of hot carriers in modulation-doped GaAs QW has been measured and the electron-LO phonon scattering rate is found to be substantially smaller than in the bulk. The experimental evidence is reviewed, and theoretical models incorporating screening of the e-LO phonon interaction and hot phonon effects are discussed. When a magnetic field is applied normal to the heterojunction the carrier motion is fully quantized. Recent time-resolved experiments which have measured the effect of magnetic fields on the energy relaxation rate are described. In undoped QW structures excitonic recombination dominates the emission spectrum; time-resolved photoluminescence experiments show that the exciton energy relaxation rate is significantly slower than that predicted by deformation potenial scattering.