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Cyclotron resonance and the magnetophonon effect in GaxIn1−xAsyP1−y

Cyclotron resonance and the magnetophonon effect have been used to measure the effective mass as a function of alloy composition for GaxIn1−xAsyP1−y samples grown lattice matched to InP. Values of ωτ of up to 6 allow an accurate measurement of effective mass, which is found to depend linearly upon alloy composition y with the relation m*/m0=0.080−0.039y. The observation of shallow impurity transitions in a quaternary alloy is also reported.

Cyclotron resonance studies on bulk and two-dimensional conduction electrons in InSe

Abstract Cyclotron resonance is reported for both bulk electrons and electrons bound to charged defect planes in the layer compound InSe. At temperatures below 20 K all carriers present are bound to defect planes and behave as two-dimensional accumulation layers. At higher temperatures the electrons are excited into bulk regions between the defects, and show three-dimensional, bulk behaviour. The conduction band is shown to exhibit an “anomalous anisotropy” with m ∥ = 0.08 m 0 and m ⊥ = 0.14 m 0 . The bound, two-dimensional carriers exhibit a strong non-parabolicity, and show a band edge mass m ⊥ = 0.13 which is considerably lower than the bulk value possibly due to a reduction in the polaron constant in the degenerate electron gas.

Quantum transport in GaInAs-AlInAs heterojunctions, and the influence of intersubband scattering

Abstract Shubnikov-de Haas and cyclotron resonance results are presented for GaInAs-AlInAs heterojunctions in both perpendicular and tilted magnetic fields. Two electric subbands are occupied in zero magnetic field. Magnetic depopulation of the higher (E 1 ) subband is observed in both perpendicular and tilted orientations. This enables a demonstration of the importance of intersubband scattering in both resistivity and cyclotron resonance. A shift of the relative positions of the E o and E 1 subbands by parallel magnetic fields is measured to be 0.26 meV/T 2 .

Effect masses and non-parabolicity in GaxIn1-xAs

Measurements are reported of frequency- and temperature-dependent cyclotron resonance in the ternary alloy GaxIn1-xAs, close to its lattice match with InP. The band-edge effective mass is found to be 0.041 m0 for x=0.47, in agreement with earlier measurements, but it is found that the non-parabolicity of the conduction band is approximately double that expected on the basis of k.p perturbation theory. This is thought to be due to alloy disorder. Magnetophonon resonance was also observed, giving a further measurement of the effective mass, together with the first observation of two-phonon-mode behaviour in this material.

Two-dimensional magnetophonon resonance. II. GaInAs-AlInAs heterojunctions

For pt.I see ibid., vol.16, p.L573 (1983). Reports magnetophonon oscillations in the two-dimensional electron gas formed in GaInAs at GaInAs-AlInAs heterojunctions. One series of oscillations is seen, corresponding to the energy of the degenerate InAs-like LO phonon modes in GaInAs and AlInAs. There is also an interface phonon mode at the same energy. In bulk GaAInAs scattering is dominated by the GaAs-like mode, and so as long-range phonon interaction between the electrons in the GaInAs and the interface and/or AlInAs InAs-like modes must be invoked to explain the dominance of scattering by these modes. Comparing the results with those for GaInAs-InP superlattices, it can be seen that changing the electrically inactive component of the heterostructure produces a radical change in the electron-phonon interactions.

The magnetophonon effect in InAs1-xPx

Magnetophonon oscillations have been observed in the alloy system InAs1-xPx. Two magnetophonon series were detected across the complete range of alloy compositions, resulting from the two LO phonon modes present in mixed crystals of InAs1-xPx. The fundamental fields have been used to calculate the LO phonon energies and to determine the temperature dependence of the effective mass. Magnetophonon oscillations were also observed in the Hall coefficient and these exhibited phaseshifts and reversals of sign with respect to the extrema observed in the transverse magnetoresistance.

Evidence for a contribution to the extrinsic photoconductive signal by hopping through excited states of the donors in silicon and CdTe

Abstract The intensities of the bound to bound transitions of the shallow donors in silicon and CdTe are found to be strongly electric field dependent when observed by photoconductivity techniques at low temperatures. Under these conditions significant differences can be detected between the photoconductive spectrum and the same transitions observed in absorption. The photoconductive peaks are shifted consistently to lower frequency compared with the absorption spectrum and additional peaks appear which are not significant features in absorption. These additional peaks are believed to result from the formation of molecular complexes of donors. The strong electric field dependence of the bound transitions observed in photoconductivity, the shift to lower frequency of the peaks and the enhancement of the additional lines can all be explained qualitatively on the assumption that the mechanism responsible for the generation of the photosignal involves hopping between the excited states of the molecular complexes

Shallow donor spectroscopy and polaron coupling in Ga0.47In0.53As

An investigation of shallow donor energy levels in magnetic fields in the ternary alloy Ga0.47In0.53As, lattice matched to InP, has been made. Transitions between 1s and 2p energy levels of shallow hydrogenic donors have been observed in photoconductivity as a function of magnetic field. Transition energies and their magnetic field dependence are well predicted by the use of an effective Rydberg, however it is found that the 1s to 2p+ transition energy deviates from the theoretical value at high fields. This clearly indicates strong polaron pinning of the 2p+ level to two LO phonon modes which are GaAs-like and InAs-like, arising from the GaInAs.

An investigation of the valence bands of germanium, InSb and GaAs by means of the magnetophonon effect

The exact form of the valence bands in III-V compounds has long been controversial and it is demonstrated that quantitative information concerning the valence band structure can be obtained from the magnetophonon effect. The valence band anisotropy deduced for InSb from the low-field peaks is comparable with that of Ge. For GaAs the heavy hole peaks were interspersed with light hole peaks and were insufficiently numerous to determine the exact valence band anisotropy. The high field peaks of Ge showed a peak splitting and a reduction in anisotropy attributable to the unequal spacing of Landau levels close to the valence band edge which is caused by an admixture of light and heavy hole states. The high-field peaks of InSb showed a similar reduction in anisotropy but no peak splitting. The light hole series in both InSb and GaAs showed an additional peak arising from the admixture of light and heavy hole bands.

The electric sub-band structure of electron accumulation layers in InSe from Shubnikov-de Haas oscillations and inter-sub-band resonance

In bulk InSe crystals grown by the Bridgman method, electron accumulation layers are formed in the vicinity of stacking faults with charged impurities adsorbed to the defect plane. Shubnikov-de Haas experiments show that the 2D electron gas in InSe observed at low temperatures has a density of 1012 cm-2 and that several dielectric sub-bands are occupied. The g-factor is enhanced by electron-electron interactions. The optical absorption was studied in the frequency range 3-20 meV in perpendicular and tilted magnetic fields up to 8T. The cyclotron resonance at Eres=h(cross) omega c, the intersubband resonance at Eres=E01 and the combined resonance at Eres=E01+h(cross) omega c, are all observed for both parallel and tilted polarisation, where E01 is equal to the sub-band separation as deducted from the Shubnikov-de Haas periodicities. With increasing magnetic fields the exciton-and depolarisation-shifted resonance E01 approaches the true sub-band splitting E01, and is the same time reduced in amplitude. Absorption due to bulk 1s-2p transitions in isolated shallow donors is also observed. The 2p level shows a three-dimensional Zeeman splitting (h(cross) omega c)3D with a weak anomalous mass anisotropy as observed in cyclotron resonance measurements of bulk carriers at temperatures of 30-100K. The effective electronic Rydberg is in agreement with results of exciton spectra.