This paper describes a new application of the photoacoustic (PA) elfectl in the characterization of powdered ZnS(Mn,Cu) which is the active phosphor material in electroluminescence cellsz'5.In addition to applying conventional PAS to obtain the optical absorption spectrum of this material, we have exploited a sim ple double beam arrangement in which a probe light beam is used to monitor the

Far-infrared absorption spectroscopy in the energy range 100-400cm(-1) is reported for donors in indium phosphide at 4.2K in magnetic fields of up to 30 T. Transitions are observed from the bound-electron ground state to both stable and metastable excited states in good agreement with the effective-mass theory. At the highest magnetic fields employed, corresponding to 2 < y < 3, several absorption features are observed just above the TO-phonon energy, similar to 5 effective Rydberg. These are attributed to the magnetopolaron interaction and cyclotron resonance. (c) 2006 Elsevier B.V. All rights reserved.

Broadband far-infrared absorption spectroscopy is used to investigate n -type indium phosphide in magnetic fields of up to 30 T. The large energy range (cid:2)(cid:4) 2–14 Ry (cid:3) and the large magnetic field range (cid:2) 0 (cid:2)(cid:3)(cid:4) 3 (cid:3) employed permit the observation of a rich variety of magnetopolaron interactions. We report on the magnetopolaron effect for bound states in InP, beginning with the coupling of the 2 p + state with the 1 s +LO phonon state. We further observe the magnetopolaron effect associated with the metastable state (cid:2) 210 (cid:3) . In addition we report (cid:2) i (cid:3) the re-emergence of the (cid:2) 210 (cid:3) impurity state transition beyond the LO phonon manifold, (cid:2) ii (cid:3) the possible coupling of an impurity transition with the 1 s +2LO phonon state, and (cid:2) iii (cid:3) both low- and high-energy one-phonon transitions of impressive richness and detail.

This article focuses on the characterisation of three versions of a custom designed silicon photodetector using nuclear spectroscopy techniques. Room temperature gamma ray spectroscopy experiments have been performed using an LYSO excited by /sup 137/Cs and /sup 22/Na sources and read out by the new photodiodes. We have measured an energy resolution of 13.1% FWHM for the 511 keV gamma rays from a /sup 22/Na source and 11.2% FWHM for the 662 keV gamma rays from a /sup 137/Cs source. This is one of the best ever-reported energy resolutions for a lutetium based scintillator crystal read out by a silicon photodetector.

In this work we report on spectral response data and gamma ray spectroscopy measurements using two newly developed silicon photodetectors that are designed to have enhanced sensitivity in the blue spectral region. The enhanced sensitivity is a result of our newly developed ion implantation profile used to create the active area of the photodetector. The quantum efficiency of the new photodetectors (without any optimised antireflective coating) has been measured to be /spl sim/40% at a wavelength of 420 nm. Gamma ray spectroscopy experiments have been performed using a CsI(Tl) and a LSO crystal excited by a /sup 137/Cs or /sup 22/Na source and read out by the new photodetectors. We have measured an energy resolution of 7.7% and 22.7% FWHM for the 662 keV gamma rays from a /sup 137/Cs for the CsI(Tl) and LSO scintillator crystal respectively. We intend to use the photodetectors, in the form of a detector array optically coupled to CsI(Tl) or LSO, in the development of a new scintillator detector module for use in positron emission tomography.

The central-cell correction has been determined experimentally for the two donor impurities S and Si in GaAs. Data have been obtained for magnetic fields to 39 T, corresponding to γ≈6. The observed behavior is in good agreement with theory. The analysis permits accurate evaluation of zero-field central-cell corrections, yielding 0.110 and 0.059 meV for S and Si, respectively.

We present a theory of photoluminescence in the presence of a quantizing magnetic field for a quasi-two-dimensional electron gas interacting with the lattice assuming a weak Fr\"ohlich interaction with bulk longitudinal optical (LO) phonons. Unlike in the conventional cyclotron resonance, the calculated photoluminescence spectrum for high electron concentrations exhibits strong renormalization due to the resonant electron-phonon coupling between two quasiholes in conduction Landau levels whenever the LO-phonon energy is close to the energy difference between any two occupied Landau levels. The high electron concentration is necessary to push the Fermi energy well above that of LO phonons. The screening of the electron-phonon interaction was included within the static random-phase approximation and the finite extent of the electron wave function in the quantum well was accounted for. The calculated spectra are in excellent agreement with experimental photoluminescence data for GaAs/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$As quantum wells.

When a double barrier semiconductor structure is biased near a tunneling resonance, charge can accumulate in the quantum well. Coupling between this two dimensional electron gas and the tunneling current is investigated. Experimental data taken inside a region of apparent bistability in one device reveal a satellite on the high energy side of the current resonance in the I(V) characteristic. A theoretical model based on the many-body transfer Hamiltonian formalism shows that a plasmon excitation has a remarkably similar structure. Magnetic field data support the plasmon satellite interpretation.

Low-temperature, time-resolved magneto-photoluminescence (TRPL) measurements of 2D electron-free-va-lence-hole recombination have been carried out in the IQHE, FQHE and magnetically induced electron solid regimes of a GaAs/AlGaAs single heterojunction. Clear differences, not present at elevated temperatures, are found in the measured PL decay time (~ 1 ns) for components of multiple-peak structure observed beyond v = 15 used previously to construct a phase diagram associated with the Wigner solid. The new TRPL results and earlier PL data, taken together, are consistent with a mechanism in which the spectral feature associated with crystallisation derives from the trapping of excitons within a few Bohr radii (z separation) of the heterointerface and their subsequent (x,y) localisation by the presence of a 2D electron system with a degree of spatial order at this interface.

Strong enhancements of the exciton binding energy (E(x)), compared to bulk GaAs, are deduced from high-resolution spectroscopic studies of shallow GaAs-AlyGa1-yAs quantum wells, with aluminum concentrations of (1-4.5)%. A clear increasing trend in E(x) from 5.9 meV at 1% aluminum to 7.0 meV at 4.5% is deduced, in good agreement with the predictions of variational calculations. Even at 1%, the value of E(x) represents an enhancement of 40% over that in three-dimensional GaAs. The similarity of all the spectra supports strongly the marked two dimensionality of the lowest exciton states, even for very low barrier heights.

Longitudinal optic (LO) phonon assisted indirect exciton creation (X(LO)), hot carrier relaxation ((e-h)(LO)) and Raman scattering phenomena are reported in the optical spectra of GaAs-AlGaAs superlattices. Structures of the same dimensions both with and without double heterostructure confining barriers are studied. For the structures without confining barriers, continuum transitions are suppressed in photoluminescence excitation (PLE) spectra, and as a result the X(LO) (e-h)(LO) and Raman peaks are observed, The X(LO) absorption peaks are identified from the observation of a dear threshold in PLE at h omega(LO) (36.4 meV) above the heavy hole exciton peak. The intensity of X(LO) is a maximum at similar to 6 meV above the threshold, probably due to dissociation into free carriers at the exciton binding energy (6meV) above h omega(LO). The influence of non-radiative processes on incoherent (PLE) and coherent (Raman) processes is compared.

Bandgap magneto-photoluminescence (PL) has been used to investigate the optical signature of high quality p-type GaAs—AlxGa1−xAs single heterojunction samples in the quantum Hall effect (QHE) regime. The PL spectra show a series of sharp lines, with a number of bulk features attributed to free exciton emission and acceptor-related bound excitons evident. Significantly, however, one line within the complex PL spectrum, at slightly lower energy to the free exciton, becomes a dominant feature at low temperatures and has an intensity maximum which exhibits superlinear behaviour with excitation power that correlates with the v = 1 QHE. This line is assigned to an excitonic process at the heterointerface reminiscent of 2D excited subband recombination in n-type structures.

A strained ${\mathrm{In}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$As layer is incorporated adjacent to the emitter barrier of an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As double-barrier resonant-tunneling structure (DBRTS), so that it forms a prewell for the electrons that accumulate prior to tunneling into the GaAs quantum well (QW). Observation of photoluminescence (PL) and photoluminescence excitation (PLE) from the prewell then enables a direct optical determination of the charging behavior in the emitter-accumulation region to be achieved. We show in addition that an optical probe of the prewell can, by consideration of the electrostatics, provide a reliable determination of the charge distribution in the whole DBRTS at the peak of the tunneling resonance. These results are shown to be in agreement with separate determinations of the charge in the GaAs QW by direct PL measurements, and of the charge in the emitter-accumulation layer from magnetotransport studies. The electron density in the prewell can be varied continuously over a wide range, from 0 to 9\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The second subband is populated at high applied bias, with the density varying from 0 to 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. This structure is then very well suited to the study of the many-body nature of the excitonic enhancement near the absorption threshold, over a much wider range of electron densities than previously studied on an individual sample. Utilizing temperature-dependent PLE measurements, we have monitored the variation from an atomic exciton in n=1, to a Fermi-energy edge singularity (FEES) in n=1, through to a FEES in n=2, in the same sample.

A new technique has been developed to probe the region of apparent bistability due to a tunneling resonance in the characteristic of a semiconductor asymmetric double-barrier structure. The measuring circuit uses a voltage supply designed to have a load line with positive slope, equivalent to a voltage source and negative series resistance. The appearance of bistability and hysteresis in the characteristic is an artifact of the conventional measuring technique, which employs a load line with negative slope. The complete characteristic is found to be a continuous Z-shaped curve between 50 and 150 K, corresponding to tristability. Equivalent circuit models for the device and voltage supply predict a narrow range of circuit parameters for which a static operating point exists inside the tristable region.

A new measurement technique employing a positively sloping load line has been used to probe the region of apparent bistability near a tunneling resonance in the electrical characteristic of a resonant tunneling diode. This technique is equivalent to using a voltage source and negative series resistance. The appearance of bistability is an artifact of the conventional measuring technique which uses a load line with negative slope. The complete characteristic is found to be a continuous Z shaped curve between 20 and 150 K, corresponding to tristability and in accordance with theoretical models based on the effects of charge accumulation in the central quantum well of the diode. The width of the tristable region passes through a maximum at 40 K and, at 150 K, disappears as the resonance broadens. Above this temperature the resonance develops a region of negative differential resistance (NDR). As the device is cooled below 20 K additional structure develops in the central arm of the Z, with some portions of the characteristic exhibiting five stable current states at temperatures below 15 K. At 4.2 K, the effect of an in plane magnetic field mimics that of increasing temperature.