S J Bass

Raman scattering study of the electron-phonon coupling in GaInAs-InP quantum wells

Recent Raman scattering studies of Ga0.47In0.53As-InP quantum wells have shown that the form of the alloy optical phonons can be quite different from that observed in the bulk due to coupling between the phonons and the two-dimensional electron gas (2DEG). However, the limited range of samples studied so far has made the elucidation of the precise coupling mechanism difficult. The authors report here a Raman scattering study of the GaInAs optical phonons in a wide range of undoped GaInAs-InP quantum wells with well widths in the range 50-300 AA. The frequencies and intensities of the two LO phonons of the alloy are studied as a function of both well width and photoexcited carrier density and a resonant coupling between the electrons and the GaAs-like LO phonon is observed when the lowest electronic inter-subband spacing approaches the phonon energy. These results are discussed in terms of the coupling between phonons and intersubband collective excitations.

Inter-sub-band absorption in InP/In0.53Ga0.47As multiple quantum wells

Inter-sub-band absorption is observed in InP/In0.53Ga0.47As modulation-doped quantum wells. A multiple internal reflection geometry is used to demonstrate a strong absorption which only occurs for p-polarised light. A quantitative analysis is performed on the weaker absorption observed with a single pass of light incident at the Brewster angle. The energy of the inter-sub-band transition is found to agree with an effective-mass model. The effects of free carriers and built in electric fields on the inter-sub-band absorption are discussed.

Localisation effects caused by the remote donors in modulation-doped quantum wells

In modulation-doped quantum wells (QWS) electrons are introduced into the QW by doping only the barrier region with an undoped spacer layer nearest the quantum well. The electrons flow into the QW forming a two-dimensional (2D) gas which is separated from the ionised donors by the spacer layer. The authors have developed a theoretical model to determine the size and scale of potential fluctuations 2 eta created in the 2D electron gas in the QW by the random distribution of ionised donors modelled as a 2D sheet. The interaction is through a 2D statistically screened coulomb interaction. The size of the fluctuations grow as square root Ns where Ns is the two-dimensional density of ionised donors which compares with a 2D level Fermi energy EFE which grows as n2D, where n2D is the two-dimensional density of electronics. They show that for EFE< eta all electrons are localised at low temperature, while for EFE< eta there are both localised and free electrons. This behaviour manifests itself in experimental results of mobility measurements using persistent photoconductivity which exhibit a mobility edge. Photoluminescence results where the oscillator strength for the localised electrons is suppressed compared with free electrons recombining with the few photo-excited localised holes also support this theory since the fluctuations separate the localised electrons and holes in real space.

Magneto-optical studies of GainAsInP quantum wells

Abstract The optical properties of GainAsInP quantum wells are studied in magnetic fields of up to 16T. A comparison of the absorption and photoluminescence spectra of a series of multiple quantum wells provides evidence that the photoluminescence occurs from excitons in which the hole is localised. This localisation is shown to be present in a highly doped sample with a sheet carrier density of ∼10 12 cm −2 , indicating that the localisation is not screened out by high free carrier densities. A theoretical fit to measured Landau level transitions in a 100A multiple quantum well allows values for the carrier masses, electron non-parabolicity and exciton binding energy to be determined.

Magnetic resonance of 2D electrons in a single quantum well of InP/GaInAs

The authors have measured , by magnetic resonance, for the first time the g-factor for electrons in the two-dimensional (2D) electron gas of a quantum well using the method of optical pumping. The value of mod ge mod =5.6+or-0.3 obtained for a single InP/GaInAs lattice-matched quantum well is approximately 70% greater than the accepted bulk value of mod ge mod =3.38 for GaInAs which implies that this value is too low or that, if exchange effects are negligible as expected for this low field measurement, the modification of the band structure in the 2D system results in an enhanced g-value.

Resonant Raman scattering in In.53Ga.47AsInP(100) quantum wells

Abstract We report on recent Resonant Raman scattering measurements of MOCVD grown InGaAs/InP quantum wells with well thicknesses in the range 50A to 150A. Unusually intense features are observed in the frequency range 200 – 280 cm−1. The GaInAs modes are red — shifted by ∼ 10 cm−1 from their bulk frequencies and the polarisation dependence is changed. Evidence of interface modes and of interface — induced forbidden TO scattering is presented.

High-quality InP/InGaAs Fabry-Perot etalons grown by AP MOCVD

The growth of a high-quality all-epitaxial semiconductor Fabry-Perot etalon in InP and lattice-matched In0.53Ga0.47As by atmospheric pressure metal-organic chemical vapour deposition (AP MOCVD) is reported. The etalon comprises multi-layer mirrors of InP and InGaAs (16 double layers) either side of a central InP (m lambda /2) region where m=4. An optical matrix method is used to model the reflectivity spectrum. Close agreement is demonstrated between modelled and actual reflectivity over a wide wavelength range. The etalon transmission, deliberately tuned away from the material band gaps, is centred on a wavelength of 1880 nm and has a FWHM of 13.5 nm (5 meV), 0.7% of the resonant wavelength.

Magneto-optics of wide modulation doped quantum wells: Landau level pinning phenomena

Landau level pinning phenomena in the photoluminescence spectra of an asymmetric, wide modulation doped quantum well are reported. The processes are driven by the need to maintain a self-consistent potential and charge distribution under conditions of high field Landau level quantization.

Conduction-band offset and differential-conductance studies of a 20 nm wide, In0.53Ga0.47As/InP, single-barrier structure

The authors report here a study of the conduction-band offset and differential conductance of a 20 nm wide, In0.53Ga0.47As/InP, single-barrier structure grown by atmospheric-pressure MOCVD. For processed samples with 50 or 100 mu m diameter mesas, comparison between experiment and numerical calculations shows that the conductance at 77 K, normalised with respect to the mesa area, is governed by the intrinsic thermionic emission and normal tunnelling transport mechanisms. The conduction-band offset for these samples is measured to be 180-190 meV. For processed samples with mesa diameters greater than about 50 or 100 mu m, the normalised conductance at zero bias is enhanced by several orders of magnitude compared with that of the smaller, well behaved mesas. This is attributed to parallel conduction via structural defects such as pin-holes or point/extended defects within the barrier which are spatially distributed on the scale of 50-100 mu m.