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We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred to be about 33 meV. This small value of the bandgap renormalization rules out the formation of small polarons and points to weak electron–phonon coupling. The experiments are performed on the nanosheets of the archetypal halide perovskite, CsPbBr3.

We describe a simple and accurate differential thermal analysis set up to measure the latent heat of solid state materials undergoing abrupt phase transitions in the temperature range from 77 K to above room temperature. We report a numerical technique for the absolute calibration of the latent heat of the transition, without the need of a reference sample. The technique is applied to three different samples -- vanadium sesquioxide undergoing the Mott transition, bismuth barium ruthenate undergoing a magnetoelastic transition, and an intermetallic Heusler compound. In each case, the inferred latent heat value agrees with the literature value to within its error margins. To further demonstrate the importance of absolute calibration, we show that the changes in the latent heat of the Mott transition in vanadium sesquioxide (V$_2$O$_3$) stays constant to within 2% even as the depth of supersaturation changes by about 10 K, in non-equilibrium dynamic hysteresis measurements. We also apply this technique for the measurement of the temperature-dependent specific heat., This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Review of Scientific Instruments (volume 92, Issue 7) and may be found at this link (https://doi.org/10.1063/5.0056857)

We report that the thermally-induced Mott transition in vanadium sesquioxide shows critical-slowing-down and enhanced variance ('critical opalescence') of the order parameter fluctuations measured through low-frequency resistance-noise spectroscopy. Coupled with the observed increase of also the phase-ordering time, these features suggest that the strong abrupt transition is controlled by a critical-like singularity in the hysteretic metastable phase. The singularity is identified with the spinodal point and is a likely consequence of the strain-induced long-range interaction., 14 pages, 16 figures

Lead and tin-based chalcogenide semiconductors like PbTe or SnSe have long been known to exhibit an unusually low thermal conductivity that makes them very attractive thermoelectric materials. An apparently unrelated fact is that the excitonic bandgap in these materials increases with temperature, whereas for most semiconductors one observes the opposite trend. These two anomalous features are also seen in a very different class of photovoltaic materials, namely the halide-perovskites such as CsPbBr3. It has been previously proposed that emphanisis, a local symmetry-breaking phenomenon, is the one common origin of these unusual features. Discovered a decade ago, emphanisis is the name given to the observed displacement of the lead or the tin ions from their cubic symmetry ground state to a locally distorted phase at high temperature. This phenomenon has been puzzling because it is unusual for the high-temperature state to be of a lower symmetry than the degenerate ground state. Motivated by the celebrated vibration-inversion resonance of the ammonia molecule, we propose a quantum tunneling-based model for emphanisis where decoherence is responsible for the local symmetry breaking with increasing temperature. From the analytic expression of the temperature dependence of the tunnel splitting (which serves as an order parameter), we provide three-parameter fitting formulae which capture the observed temperature dependence of the ionic displacements as well as the anomalous increase of the excitonic bandgap in all the relevant materials., Comment: The following article has been submitted to Applied Physics Letters

The emission spectrum of lanthanide-rich upconverters consists of optically or thermally excited transition bands involving different pathways. We investigate multiphoton processes in Yb3+–Er3+ codoped β − NaGdF4 to demonstrate that the emission from the phosphor involves both two- and three-photon mechanisms, where the sample temperature, and the laser excitation power play crucial roles in determining the leading pathway. The two-photon processes dominate at room temperature, and the effect of higher-order (three-photon) processes becomes increasingly important as the temperature is lowered. The Yb3+–Er3+ systems are reported to be among the best performing ratiometric luminescent thermometers in a very wide range. Our experimental findings demand a careful elimination of errors that may appear from the laser-induced heating of the sample and contributions from higher-order multiphoton processes when using such thermometers below room temperature.

Upconversion luminescence bands from Yb3+/Er3+ codoped into a matrix such as NaGdF4 can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.

While time-resolved pump-probe differential reflectivity and transmitivity measurements are routinely used to monitor the population relaxation dynamics on the subpicosecond time scale, it is also known that the signal in the negative delay can yield direct signatures of the perturbed-free-induction-decay of polarization. Yet this technique, especially in reflection geometry, has never been popular because the experiment is conceptually not very intuitive. Coherent dynamics is therefore usually studied using the more complex four-wave-mixing experiments. Here we derive from first principles the simplest possible but mathematically complete framework for the negative delay signal in both the time and the spectral domains. The calculation involving the optical Bloch equations to describe the induced polarization and the Ewald-Oseen idea to calculate the reflected signal as a consequence of the free oscillations of perturbed dipoles, also explicitly includes the process of lock-in detection of a double-chopped signal after it has passed through a monochromator. The theory is compared with experiments on high quality GaAs quantum well sample. The dephasing time inferred experimentally at 4 K compares remarkably well with the inverse of the absorption linewidth of the continuous-wave photoluminescence excitation spectrum. Spectrally resolved signal at negative delay calculated from our theoretical expression nicely reproduces the coherent spectral oscillations, although exact fitting of the experimental spectra with the theoretical expression is difficult. This is on account of additional resonances present in the sample corresponding to lower energy bound states., 20 pages, 5 figures

We present the first systematic observation of scaling of thermal hysteresis with the temperature scanning rate around an abrupt thermodynamic transition in correlated electron systems. We show that the depth of supercooling and superheating in vanadium sesquioxide (${\mathrm{V}}_{2}{\mathrm{O}}_{3}$) shifts with the temperature quench rates. The dynamic scaling exponent is close to the mean field prediction of $2/3$. These observations, combined with the purely dissipative continuous ordering seen in ``quench-and-hold'' experiments, indicate departures from classical nucleation theory toward a barrier-free phase ordering associated with critical dynamics. Observation of critical-like features and scaling in a thermally induced abrupt phase transition suggests that the presence of a spinodal-like instability is not just an artifact of the mean field theories but can also exist in the transformation kinetics of real systems, surviving fluctuations.

We have closely examined the emission spectrum at the heavy-hole exciton resonance in a high-quality GaAs multi-quantum well (MQW) sample using picosecond excitation-correlation photoluminescence (ECPL) spectroscopy. Dynamics of the ECPL signal at low and high energy sides of the excitonic photoluminescence (PL) peak shows complementary behavior. The ECPL signal is positive (negative) below (above) the PL peak and it changes sign within a narrow band of energy lying between excitonic absorption and emission peaks. The energy at which this sign change takes place is interpreted as the excitonic mobility edge as it separates localized excitons in quantum dot-like states from mobile excitons in quantum well-like states., Comment: 12 pages, 4 figures

We analyze the applicability of transient thermal lens (TL) z-scan technique as a sensitive tool to measure heat diffusivity of liquids. Suitable dyes at very low concentrations were added to the host liquid to enhance the TL effect through improved optical absorption. We investigate if these dye impurities, besides improving light absorption, have any effect on the thermal properties of the host liquid. We find that even a trace amount of impurity significantly alters the thermal properties of a solvent. Time-evolution of TL showed pronounced asymmetry about laser focus revealing anomalous behavior in thermal blooming of the laser beam. Heat transport was strongly dependent on the rise in sample temperature by light absorption. Important effects of nonlinear heat transport in time-resolved TL z-scan experiments were revealed.

The energy gap of simple band insulators like GaAs is a strong function of temperature due to the electron-phonon interactions. Interestingly, the perturbation from zero-point phonons is also predicted to cause significant (a few percent) renormalization of the energy gap at absolute zero temperature but its value has been difficult to estimate both theoretically and, of course, experimentally. Given the experimental evidence [Bhattacharya, et al., Phys. Rev. Lett. 114, 047402 (2015)] that strongly supports that the exponential broadening (Urbach tail) of the excitonic absorption edge at low temperatures is the manifestation of this zero temperature electron-phonon scattering, we argue that the location of the Urbach focus is the zero temperature unrenormalized gap. Experiments on GaAs yield the zero temperature bare energy gap to be 1.581 eV and thus the renormalization is estimated to be 66 meV., Comment: Includes Supplementary Data. To appear in Journal of Physics: Condensed Matter as a Letter

We review the photoluminescence of semiconductor nanostructures in high magnetic fields, concentrating on the effects of the applied magnetic field on orbital motion (wave function extent), which is probed in experiments on large ensembles. We present an overview of the physics of excitons in high magnetic fields in 3- and 2-D before introducing the zero-dimensional case. We then discuss the physics of quantum-dot excitons in high magnetic fields with particular attention to the approximate analytical models used to interpret experimental results. This is followed by a brief description of a typical high-field magneto-photoluminescence setup. We then present four examples of magneto-photoluminescence experiments on different materials systems chosen from our own research to illustrate how high magnetic fields can be used to reveal new insights into the physics of semiconductor nanostructures.

This paper examines the optical properties of self-assembled InAs/InP quantum dots grown by metalorganic vapour phase epitaxy (MOVPE). In a non-equilibrium regime of relatively low temperature (⩽450 °C) and higher growth rates (⩾1.5 monolayers/s), a rich variation in the peak emission wavelength, QD sizes, density and modality of the size distribution can be accomplished by changing the growth parameters. For example, the low-temperature peak emission wavelength of the ensemble can be tuned anywhere between 1.4 and 1.9 μm. Similarly, broadband emission with 250 meV bandwidth can be obtained from samples with bimodal dot distributions. The temperature dependent photoluminescence spectra show interesting dynamics associated with the thermally activated carrier transfer between dots of different ground state energies.

Dilute GaAsN layers are grown by liquid phase epitaxy from saturated melts containing polycrystalline GaN as the source of nitrogen. From photoluminescence measurements the nitrogen content in the material is obtained. Low temperature photocurrent and photocapacitance measurements reveal the presence of an electron trap with an ionization energy of 0.65–0.67 eV in the as-grown layers, whose origin is related to interstitial (N–N)As defects. High temperature annealing of the material almost removed the trap and new electron traps at 0.8 and 0.9 eV are produced. It is suggested that during the annealing process the (N–N)As defects, due to their lower energy of formation, are converted to more thermally stable (AsGa–NAs) or (AsN)As defects which might be the source of the new electron traps. High temperature treatment of the growth melt with erbium is found to remove nitrogen from the grown layer with complete annihilation of all the electron traps.

The growth of InAsxSb1−x∕GaAs (x⩽0.06) heterostructures has been achieved using liquid phase epitaxy. High resolution x-ray diffraction studies reveal that the films are single crystalline and structurally coherent with the substrate. It is also inferred from these measurements that the in-plane and out-of-plane strain arising out of mismatched epitaxy is almost completely relaxed, leading to a high dislocation density. The room temperature energy gap is measured to be 0.13eV for InAs0.06Sb0.94∕GaAs. Temperature dependence of the energy gap is studied between 93 and 433K through the absorption spectra. Temperature dependent Hall and mobility measurements carried out between 10 and 370K on these samples are discussed.

We briefly review the growth and structural properties of InAs x Sb 1−x (x⩽0.05) bulk single crystals and InAs x Sb 1−x (x⩽0.06) epitaxial films grown on semi-insulating GaAs substrates. Temperature-dependent transport measurements on these samples are then correlated with the information obtained from structural (XRD, TEM, SEM) and optical (FTIR absorption) investigations. The temperature dependence of mobility and the Hall coefficient are theoretically modelled by exactly solving the linearized Boltzmann transport equation by inversion of the collision matrix and the relative role of various scattering mechanisms in limiting the low temperature and 300 K mobility is estimated. Finally, the first observation of Shubnikov oscillations in InAsSb is discussed.

We study a specific type of lifetime broadening resulting in the well-known exponential "Urbach tail" density of states within the energy gap of an insulator. After establishing the frequency and temperature dependence of the Urbach edge in GaAs quantum wells, we show that the broadening due to the zero-point optical phonons is the fundamental limit to the Urbach slope in high-quality samples. In rough analogy with Welton's heuristic interpretation of the Lamb shift, the zero-temperature contribution to the Urbach slope can be thought of as arising from the electric field of the zero-point longitudinal-optical phonons. The value of this electric field is experimentally measured to be 3 kV cm(-1), in excellent agreement with the theoretical estimate.

A key feature of epitaxial semiconductor crystal growth is the possibility of realizing different surface morphological features when growing one material on another. This is driven by many factors, of which the relative lattice mismatch between the materials and the nature of the resultant strain is particularly crucial. Under appropriate conditions, elastic strain relaxation can lead to spontaneous generation of coherent three-dimensional (3D) islands with relatively small size dispersion. These “self-assembled” islands can be embedded within a material of larger bandgap resulting in 3D quantum confinement of electrons within the island and the formation of quantum dots (QDs). In this chapter we review the strain-driven self-assembly process during semiconductor epitaxy, looking at thermodynamic and kinetic factors that influence the growth as well as specific features of QDs in various materials systems. We highlight the very fundamental correlations between structure and functionality, discuss various characterization techniques, and examine the salient features of the electronic and optical properties of QDs which make them useful for device applications. We provide a summary of the state of the art in technological applications where the use of QD-based devices has led to improved performance and functionality. Specifically QD-based lasers, superluminescent diodes, infrared photodetectors, memories and single photon sources are discussed, with a focus on materials and growth issues.

In a "thought experiment," now a classic in physics pedagogy, Feynman visualizes Young's double-slit interference experiment with electrons in magnetic field. He shows that the addition of an Aharonov-Bohm phase is equivalent to shifting the zero-field wave interference pattern by an angle expected from the Lorentz force calculation for classical particles. We have performed this experiment with one slit, instead of two, where ballistic electrons within two-dimensional electron gas diffract through a small orifice formed by a quantum point contact (QPC). As the QPC width is comparable to the electron wavelength, the observed intensity profile is further modulated by the transverse waveguide modes present at the injector QPC. Our experiments open the way to realizing diffraction-based ideas in mesoscopic physics.

State-filling dynamics in self-assembled InAs/GaAs quantum dots (QDs) is studied through their steady-state photoluminescence (PL) using a variant of picosecond excitation-correlation (EC) spectroscopy. Steady-state PL showed an interesting transition from bimolecular recombination at low-excitation fluence to excitonic recombination at higher fluence. As for the EC signal, while the ground-state response is always snubbed when the two excitation pulses are temporally nearly coincident, the excited-state response can either be enhanced or reduced, depending on the excitation fluence. The time evolution of this response is studied for the first three levels in a QD ensemble. A minimal theoretical model, which combines carrier loss kinetics with the principle of detailed balance and the Pauli exclusion principle, quantitatively reproduces the observations. DOI:10.1103/PhysRevB.87.115317

International audience; The GaP/AlP/GaP heterostructure has an indirect gap both in real as well as momentum space, making the first order radiative recombination doubly forbidden. Nevertheless, we have observed relatively efficient emission from these structures. This paper comprehensively studies the origin of this improved light emission through a detailed analysis of the photoluminescence (PL) spectra. Our observations suggest that localized excitons within the acceptor states in GaP close to the heterostructure interface are enough for efficient light emission in these structures, doing away with the need for more complicated structures (superlattices or neighboring confinement structures). This real space localization of holes, close to the interface, apart from increasing the wave function overlap, also relaxes the delta-function momentum selection rule. Independent experimental evidence for this assertion comes from (i) the PL spectrum at high excitation power where transitions from both the localized as well as extended states are independently observed, (ii) the observation that extended states emission has the expected band-bending-induced blue-shift with increase in excitation power, whereas the localized states do not, (iii) observation of phonon replicas for PL from localized states, and (iv) observation of persistent photoconductivity at low temperature. Finally, we propose a simple analytical model that accounts for both the type-II nature as well as the indirect bandgap to explain the improvement of radiative recombination efficiency with increased localization. The experimental observations are reproduced within an order of magnitude. The model is very general and it also provides a framework to study the optical properties of other such (type-II and/or indirect gap) heterostructures.

GaMnN films with 1–3% Mn deposited on Si(100) and Al 2 O 3 (0001) substrates, by the technique of nebulized spray pyrolysis by employing acetylacetonate precursors, have been characterized by X-ray diffraction, photoluminescence spectra and other techniques. The films are ferromagnetic and show magnetic hysteresis. The ferromagnetic T C increases with the Mn content, with the 3% Mn film showing a T C of ∼250 K. Anomalous Hall effect is observed below T C where the films exhibit a small negative magnetoresistance.

The growth of epitaxial InSb layers on highly lattice-mismatched semi-insulating GaAs substrates has been achieved via traditional liquid-phase epitaxy. Scanning and transmission electron microscopy show sharp interfaces even at 35 nm resolution. High-resolution x-ray diffraction studies reveal reflections even up to 2 theta =153° with distinct layer and substrate peaks, indicating structural coherence. The films grown were n type and the highest electron mobility obtained was 3.963104 cm2/Vs at room temperature. The band gap varies from 0.17 to 0.23 eV in the temperature range of 300–10 K and is consistent with the expected variation. These results indicate that the films grown are comparable to those grown by other sophisticated techniques in terms of structural, optical and electrical properties.

We describe an optical fiber based setup for performing polarization resolved magneto-optical spectroscopy measurements under low temperatures (similar to 4 K) and high magnetic fields (similar to 8 T). The measurements are performed in a windowless helium Dewar. Circularly polarized light is produced inside the Dewar by inserting the polarizing elements between the fiber end and the sample. Photoconductivity spectra of a GaAs/AlGaAs multiquantum-well sample have been measured over the photon energy range of 1.5-1.7 eV in left and right circularly polarized light under crossed magnetic and electric fields. It is shown that reversing the direction of magnetic field produces the same spectral changes as caused by changing the direction of circular polarization with the optical components. (C) 2010 [doi:10.1063/1.3462977]

The generation of pulsed magnetic fields is limited by the Lorentz force on the conductor and by ohmic heating. A large coil can give higher field (and/or longer pulse duration), but for a single coil this may require high power that cannot be supplied by a feasible source. Power can be kept within acceptable limits by using a system of multiple coils. Pulsed field coils can be energized either by a pulsed power supply or a pulsed energy source. The energy or power supplies tend to be the most expensive part of the installation and a combination of high power and high energy is extremely expensive. In a multi-coil system , the increased design freedom allows to optimize the strength, pulse duration and heating of the coil, and to optimize the selection of materials and power supplies; one can so also minimize damage in case of coil failure. Because of the increased number of parameters, systematic insight into their mutual dependence is helpful in order to converge to an optimized design. Since the local optima are relatively weak, for the final design any standard method of pulsed coil design can be used. In this paper we will discuss strategies to determine the optimum choice for the design of inner- and outer-coil and how to optimize their design in relation to the supply type used. In particular, we will consider energy-limited capacitor banks and power-limited supplies. The approach will use scaling arguments and modeling tools as the PMDS package originally developed in Leuven. Optimization of coil systems is demonstrated with practical examples, such as the successful 87 T pulsed dual coil system in Dresden, and the design of the future ‘ARMS’ successor in Toulouse

We have used magneto-photoluminescence measurements to establish that InP/GaAs quantum dots have a type-II (staggered) band alignment. The average excitonic Bohr radius and the binding energy are estimated to be 15nm and 1.5 meV respectively. When compared to bulk InP, the excitonic binding is weaker due to the repulsive (type-II) potential at the hetero-interface. The measurements are extended to over almost six orders of magnitude of laser excitation powers and to magnetic fields of up to 50 tesla. It is shown that the excitation power can be used to tune the average hole occupancy of the quantum dots, and hence the strength of the electron-hole binding. The diamagnetic shift coefficient is observed to drastically reduce as the quantum dot ensemble makes a gradual transition from a regime where the emission is from (hydrogen-like) two-particle excitonic states to a regime where the emission from (helium-like) four-particle biexcitonic states also become significant.

Photoluminescence spectra from disordered InN were studied in very high magnetic fields. The samples had Gaussian spectra with low temperature emission peaks at 0.82 and 0.98eV respectively. The average spatial extent of the excitonic wave functions, inferred from the diamagnetic shift, is only 2-3nm. This shows that the recombination is from an ensemble of highly localized states within a landscape of a smooth (classical) disorder potential of strength of the order of 10meV. The anomalies in the temperature dependence of the photoluminescence peak and linewidth give further support to the picture of trapped photoexcited carriers., 3 figures

Photoluminescence spectra measured on a type-II GaSb/GaAs quantum dot ensemble at high excitation power indicate a Mott transition from the low density state comprising of spatially-indirect excitons to a high density electron-plasma state. Under the influence of a very high magnetic field, the electron-plasma that is formed at high excitation powers is `frozen-out' into a state of optically inactive magneto-excitons., 4 figures

Photo-excited carriers, distributed among the localized states of self-assembled quantum dots, often show very anomalous temperature dependent photoluminescence characteristics. The temperature dependence of the peak emission energy may be non-monotonic and the emission linewidth can get narrower with increasing temperature. This paper describes a quasi-thermodynamic model that naturally explains these observations. Specifically, we introduce a temperature dependent function to parameterize the degree of thermalization of carriers. This function allows us to continuously interpolate between the well-defined low and high temperature limits of the carrier distribution function and describe the observed anomalies in the photoluminescence spectra with just two fitting parameters. We show that the description is equivalent to assuming that the partially thermalized carriers continue to be described by equilibrium statistics, but with a higher effective temperature. Our treatment of the problem is computationally simpler than the usually employed rate equations based analyses [e.g. Sanguinetti, et al. Phys. Rev. B 60, 8276 (1999)], which typically also have many more under-determined fitting parameters. The model is extended to quantum dots with a bimodal size distribution.

A systematic manipulation of the morphology and the optical emission properties of MOVPE grown ensembles of InAs/InP quantum dots is demonstrated by changing the growth kinetics parameters. Under non-equilibrium conditions of a comparatively higher growth rate and low growth temperature, the quantum dot density, their average size and hence the peak emission wavelength can be tuned by changing efficiency of the surface diffusion (determined by the growth temperature) relative to the growth flux. We further observe that the distribution of quantum dot heights, for samples grown under varying conditions, if normalized to the mean height, can be nearly collapsed onto a single Gaussian curve., 2 figures

We have investigated the specific heat and resistivity of a single crystal of ${\mathrm{Pr}}_{0.63}{\mathrm{Ca}}_{0.37}\mathrm{Mn}{\mathrm{O}}_{3}$ around the charge ordering (CO) transition temperature, ${T}_{\mathit{CO}}$, in the presence of high magnetic fields $(\ensuremath{\leqslant}12\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ which can melt the charge-ordered state. At low magnetic fields $(\ensuremath{\leqslant}10\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, the manganite transforms from a charge-disordered paramagnetic insulating (PI) state to a charge-ordered insulating (COI) state as the temperature is lowered. The COI state becomes unstable beyond a threshold magnetic field and melts to a ferromagnetic metallic phase (FMM). This occurs for $Tl{T}_{\mathit{CO}}$. However, above a critical field ${\ensuremath{\mu}}_{0}{H}_{\ensuremath{\rho}}^{*}$, the sample shows the onset of a metallic phase for $Tg{T}_{\mathit{CO}}$ and the COI transition occurs from a metallic phase. The onset temperature of the high-field metallic behavior decreases with an increase in the field and above a field ${\ensuremath{\mu}}_{0}{H}^{*}$, the COI transition does not occur and the CO state ceases to occur at all $T$. The entropy change involved in the CO transition, $\ensuremath{\Delta}{S}_{\mathit{CO}}\ensuremath{\approx}1.6\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}\mathrm{K}$ at $0\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, decreases with increasing field and eventually vanishes for a field ${\ensuremath{\mu}}_{0}{H}^{*}$. The collapse of the CO state above ${\ensuremath{\mu}}_{0}{H}^{*}$ is thus associated with a collapse of the entropy that stabilizes the CO state.

The effect of Landau level formation on the population of intrinsic electrons in InSb is probed near room temperature in magnetic fields upto 16 Tesla. Although the measured magnetic field dependence of the Hall coefficient is qualitatively similar to published results, it is shown that the data may also be explained by simply including ambipolar conduction. Thus the inference on band depopulation drawn from previous measurements on InSb is inconclusive unless both the Hall and the magnetoresistive components of the resistivity tensor are simultaneously measured and modelled. When the model includes both depopulation and ambipolar conduction, a reasonable agreement with theory can be established., Comment: 5 figs, to appear in Journal of Physics : Condensed Matter

The scattering of carriers by charged dislocations in semiconductors is studied within the framework of the linearized Boltzmann transport theory with an emphasis on examining consequences of the extreme anisotropy of the scattering potential. A new closed-form approximate expression for the carrier mobility valid for all temperatures is proposed. The ratios of quantum and transport scattering times are evaluated after averaging over the anisotropy in the relaxation time. The value of the Hall scattering factor computed for charged dislocation scattering indicates that there may be a factor of two error in the experimental mobility estimates using the Hall data. An expression for the resistivity tensor when the dislocations are tilted with respect to the plane of transport is derived. Finally an expression for the isotropic relaxation time is derived when the dislocations are located within the sample with a uniform angular distribution., 3 figures

Temperature dependence of the energy gap and free carrier absorption in a high-quality $InAs_0_._0_5Sb_0_._9_5$ single crystal was studied between 90 K and 430 K through the absorption spectra. At this alloy concentration, the room-temperature energy gap was measured to be 0.15 eV. Varshni- and the Bose-Einstein-type fit parameters were obtained from the measured temperature dependence of the energy gap, and the latter gave the zero-temperature gap to be 0.214 eV. It was found that although Weider's empirical formula for the dependence of the energy gap on temperature and the alloy concentration agrees with the value of the gap at room temperature, it is inaccurate in describing its temperature dependence. From the free carrier absorption measurements, the phonon limited cross section of $7.35X10^-^1^6 cm^2$ at 15 \mu m was deduced at room temperature.

Radially-homogeneous and single-phase InAsx Sb(1–x) crystals, up to 5.0 at. % As concentration, have been grown using the rotatory Bridgman method. Single crystallinity has been confirmed by x-ray and electron diffraction studies. Infrared transmission spectra show a continuous decrease in optical energy gap with the increase of arsenic content in InSb. The measured values of mobility and carrier density at room temperature (for x =.05) are 5.6×10 4 cm2/V s and 2.04×10 16 cm–3, respectively.

Anti-Stokes photoluminescence is measured in high-quality GaAs quantum wells. The primary pathway for interband optical absorption and hence emission under subbandgap photoexcitation is the optical phonon-mediated second-order electric dipole transition. This conclusion is drawn from the remarkable agreement between predictions of second-order perturbation calculation and the measured intensity of anti-Stokes photoluminescence, both as function of the detuning wavelength and temperature. The results are of direct relevance to laser cooling of solids where phonon-assisted upconversion is a necessary condition.

Optically active states in liquid phase epitaxy-grown ultra-dilute GaAsN are studied. The feature-rich low temperature photoluminescence spectrum has contributions from excitonic band states of the GaAsN alloy, and two types of defect states—localized and extended. The degree of delocalization for extended states both within the conduction and defect bands, characterized by the electron temperature, is found to be similar. The degree of localization in the defect band is analyzed by the strength of the phonon replicas. Stronger emission from these localized states is attributed to their giant oscillator strength.

The problem of conversion of experimentally measured luminescence spectrum into the absorption cross section is revisited. The common practice of using the van Roosbroeck-Shockley (or Kubo-Martin-Schwinger or Kennard-Stepanov) relation in this context is incorrect because luminescence from semiconductors is essentially all due to the spontaneous emission component of the recombination of carriers distributed far-from-equilibrium. A simple, physically consistent, and practical prescription for converting the luminescence spectra into absorption is presented and its relation to the so-called nonequilibrium generalization of the van Roosbroeck-Shockley relationship is discussed., 3 pages, 2 figures

Photoluminescence from highly disordered GaAs quantum wells is studied in magnetic fields up to 50T. The monotonic decrease of the photoluminescence linewidth with increasing quantum well thickness indicates that interface roughness is the primary source of line broadening. The magnetic field-dependent exciton linewidth shows an unexpected behavior. We observe not only just a monotonic increase in linewidth but also a field-dependent decreasing linewidth in thicker quantum wells. These observations are understood by postulating the existence of two correlation lengths for the interface fluctuations, one much smaller than the exciton size and the other one of the order of the exciton size.

InAsxSb1−x alloys show a strong bowing in the energy gap, the energy gap of the alloy can be less than the gap of the two parent compounds. The authors demonstrate that a consequence of this alloying is a systematic degradation in the sharpness of the absorption edge. The alloy disorder induced band-tail (Urbach tail) characteristics are quantitatively studied for InAs0.05Sb0.95.

The effect of alloy disorder on the optical density of states and the average room temperature carrier statistics in GaInNAs quantum wells is discussed. A red shift between the peak of the room temperature photoluminescence and the surface photovoltage spectra, that systematically increases with the nitrogen content within the quantum wells is observed. The relationship between this Stokes' shift and the absorption linewidth in different samples suggests that the photoexcited carriers undergo a continuous transition, from being in quasi-thermal equilibrium with the lattice to being completely trapped by the quantum dot-like potential fluctuations, as the nitrogen fraction in the alloy is increased. The values of the 'electron temperature' inferred from the photoluminescence spectra are found to be consistent with this interpretation.

A low cost 7 tesla pulsed magnet system is described. The high magnetic fields generated are shown to be useful in reliably detecting parallel conduction in low mobility two dimensional quantum well samples of Corbino disc like (FET) geometry. The true mobility of the two dimensional channel is extracted for p-type Si/SiGe heterostructures having a large amount of parallel conduction