R J Nicholas

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.

A study of the deep acceptor levels of iron in InP

The measurement of photoconductivity in samples of semi-insulating n-type Fe-doped InP are reported. Optical transitions are interpreted as between Fe3+, Fe2+ and Fe+ ions and the conduction and valence bands, as well as an internal transition of the Fe2+ level. The relative strengths of the transitions observed are found to depend strongly upon the doping of the samples due to the change of the concentration of the different ion species present. A level scheme of the energy of the acceptors at 6K (300K) is deduced to consist of an Fe2+ ground state 0.64 eV (0.66 eV) below the Gamma -point conduction minimum with an excited state of Fe2+ 0.351 eV (0.351 eV) above the ground state. A two-electron trap state Fe+ is believed to lie 0.34 eV (0.28 eV) below the conduction band. Transitions are also thought to occur to higher satellite valleys of the conduction band located at 0.62 eV (0.42 eV) above the Gamma -point minimum.

The magnetophonon effect

Abstract This article is a review of the magnetophonon effect, covering its origins, uses and the information which derives from it. The magnetophonon effect arises due to resonant phonon emission or absorption by free charge carriers in a solid in a high magnetic field. The most usual example of this is when longitudinal optic (L.O.) phonons are absorbed by electrons or holes in a semiconductor, causing resonant transitions between Landau levels at magnetic fields given by the resonance condition B = (1/N)m∗ωL.O./e, where ωL.O. is the optic phonon frequency, and m∗ is the effective mass of the carriers in question. These resonant transitions cause resonances in a wide variety of different transport parameters, such as resistivity, Hall and Seebeck coefficients. The article reviews the observation of normal magnetophonon resonances in a wide variety of semiconducting and semi-metallic materials, including alloys, p-type materials and semiconductor heterostructures. A section is devoted to the hot-electron magnetophonon effect, which results from resonant electron cooling, and gives information on the electron energy loss mechanisms. This review was completed in December 1984.

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 .

Raman scattering in InP1-xAsx alloys

Phonons in the alloy system InP1-xAsx have been studied by Raman spectroscopy. Long-wavelength optical phonons display a two-mode behaviour throughout the whole composition range. The frequencies of the local and gap modes found experimentally at 77K are given. Short-wavelength phonons proceed from second-order processes near x=0 and 1 and from first-order disorder-induced scattering around x=0.5. This latter mechanism explains the two features around 55 cm-1 and 145 cm-1. They are attributed to the maxima of the density of transverse and longitudinal acoustic bands.

Evidence for a reduction in the momentum matrix element P2 due to alloy disorder in InAp1-xPx

Measurements of cyclotron resonance and the infrared absorption in the region of the direct band gap are reported for the complete range of alloys InAs1-xPx. The experimentally determined values of the effective mass are found to be almost linear functions of the alloy composition. This is in contrast to the predictions of k.p theory which, using a linear interpolation of the momentum matrix element (P2) between the values for the two alloy constituents, predicts a strong bowing. The discrepancy is thought to arise from a reduction in P2 brought about by a mixing of the conduction and valence bands by random local disorder.

High field magneto-transport measurements in GaAs-GaAlAs multilayers

Abstract Measurements have been made of magneto-transport in GaAs - Ga 1− x Al x As multi-lavers with magnetic fields up to 35 T. At high fields (22 T) a strong, temperature dependent spin splitting of the N = 0 Landau level is observed. This indicates a strong many-body enhancement of the g factor, since the bare spin splitting (0.7 mV) is much less than the broadening parameter Γ (5.3 meV) while the strong temperature dependence below 4.2 K is clear evidence for a temperature dependent enhancement. In the ultra-quantum regime ( ν = 2 πnl 2 0 ≲ 1) the magnetoresistance rises very rapidly in contrast to the predictions of independent electron theories and exhibits a weak thermal activation.

Cyclotron resonance linewidth in a two-dimensional electron gas

Abstract A density matrix formulation has been developed to study cyclotron resonance lineshape for two-dimensional electron-impurity and electron-phonon systems. Cyclotron resonance lineshape has been calculated for several different impurity and phonon scattering mechanisms including short range, Gaussian and ionised impurities, and acoustic phonons.

Energy relaxation mechanisms in n-type GaAs from magnetophonon spectroscopy

Magnetophonon resonances have been observed in the warm-electron coefficient beta in n-GaAs, at temperatures between 55K and 77K. The resonances seen indicates a rapid change in importance of the energy relaxation processes involving LO phonons from impurity site capture at 60K to inter-Landau level transitions at 70K. The relative amplitudes of the different magnetophonon series was measured precisely by the use of Fourier analysis. The observed behaviour is interpreted in terms of a strong distortion of the electron distribution at energies one LO phonon distant from the donor ground state. At 77K it is found that high electric fields can change the sign of the extrema and also produce a small shift in the position of the extrema.