Nodestructive optical methods, based on measurements of the 'plasma edge' and the Moss-Burstein shift, are investigated as contactless reflection and transmission spectra of undoped and Sidoped lnAs grown on GaAs by MBE are studied. A curve-fitting procedure is developed to fit the reflectivity spectra with or without phonon-plasmon coupling. The range of carrier concentrations over which these optical methods can provide useful characterization is evaluated. The effective mass determined from 'plasma edge' measurements agrees well with the simple Kane model for n below 2.7 x 10'' c t r 3 . For n above 4 x I O ' ' cm-3, the sample effective mass deviates considerably from the simple Kane model. Excitonic structure in the absorption edge is reported for high-purity undoped samples.
Raman scattering by coupled plasmon-phonon modes is studied with Sidoped InAs epilayers grown by MBE with carrier concentrations from 7.5 x l o ' ' c m -3 to 4 x iO'9cm-3. Unexpectedly, an unscreened LO line is observed throughout the whole carrier concentration range together with a low frequency (L-) line arising from wavevector dependent LO phonon-plasmon coupling. The frequency of the L -branch lies between the LO and TO phonon frequencies and approaches the TO frequency asymptotically from the high-frequency side as the carrier concentration increases. This behaviour is attributed to competition between screening (dominant in the high-density limit) and large-wavevector induced decoupling. The L, branch of the plasmon-phonon system is observed for the first time in Raman experiments with InAs.
Phonon energies in inAs,,Sb, ternary alloys, grown on GaAs by molecular beam epitaxy, have been studied by Raman scattering. The microstructure for this alloy system depends strongly on the growth temperature For growth temperatures above 400"C, transmission electron microscopy (TEM) shows the alloy epilayer to be homogeneous. Raman spectra of these homogeneous InAsSb alloys show a strong InAs-like longitudinal optical (LO) phonon line, as well as an InSb-like LO line. throughout the composition range. The frequency of the InAs-like LO phonon varies linearly with composition. For growth temperatures below 400°C and compositions near t h e middle of the range, an interleaved platelet structure, arising from phase separation, is observed in TEM. Effects of phase separation in these alloys have been observed in the Raman spectra.
Two new peaks are observed in resonant Raman scattering from GaSb/lnAs quantum wells grown on (001) GaAs by MBE. These two lines are assigned to the coupled LO phonon-intersubband plasmon modes originating from the lnAs wells. The lower-frequency branch of the coupled system (L-mode) lies between the LO and the TO frequencies of InAs, and the line intensity depends strongly on the two-dimensional carrier concentration and the laser excitation energies (resonant near the E, gap of InAs). The L, line'is weak and relatively broad. Its frequency increases with increasing carrier concentration in the lnAs wells and is not observed when the carrier concentration is low.
Articles you may be interested inThe electronic structure of InGaAs/InP quantum wells measured by Fourier transform photoluminescence excitation spectroscopy InAs/InAs 0.865 Sb 0.135 quantum wells are characterized using magneto-photoluminescence. Bandto-band transitions are found at energies lower than the band gaps of either the InAs or the InAs 0.865 Sb 0.135 with photoluminescence emission at wavelengths up to 4.8 m. By modeling the quantum size shifts of the photoluminescence transitions and their energy shift in a magnetic field, the valence band offset between InAs and In͑As,Sb͒ is deduced to be type II with electron confinement in the In͑As,Sb͒ alloy and hole confinement in InAs.
Raman scattering in GaSb/InAs quantum wells has been studied near the E 1 resonance for a series of samples where two subbands are occupied. Up to three high-frequency branches of the coupled LO phonon-intersubband plasmon modes, i.e. L 12+ , L 23+ and L 13+ , are observed. The results are analysed using three alternative models to calculate the energies of the coupled LO phonon-intersubband plasmon modes. Calculations of the intersubband plasmon energies without the inclusion of band non-parabolicity consistently give mode energies which are 8 to 18% higher than experiment. Two theoretical methods have been developed which take into account the subband occupancies and strong band non-parabolicity. The first provides simple analytical solutions and the second is based on a detailed numerical calculation to calculate the energies of the coupled LO phonon-intersubband plasmon modes. Good agreement is achieved with the experimental data.
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