S. D. Singh

Observation of electron confinement in InP/GaAs type-II ultrathin quantum wells

The issue of type-II band alignment for InP/GaAs heterostructure is addressed by means of simple layer architecture of ultrathin quantum wells (QWs). From specific signatures of the radiative recombination in type-II QWs especially the cube root dependence of blueshift in the lowest excitonic transition energy on excitation power in photoluminescence measurements indicates that the observed luminescence is originating from spatially separated electrons and holes. Such a blueshift is seen to increase with the QW thickness again confirming a type-II band alignment. A direct evidence of electron confinement in the conduction band of InP is provided by the capacitance voltage measurements.

A versatile phenomenological model for the S-shaped temperature dependence of photoluminescence energy for an accurate determination of the exciton localization energy in bulk and quantum well structures

Temperature dependence of the photoluminescence (PL) peak energy of bulk and quantum well (QW) structures is studied by using a new phenomenological model for including the effect of localized states. In general an anomalous S-shaped temperature dependence of the PL peak energy is observed for many materials which is usually associated with the localization of excitons in band-tail states that are formed due to potential fluctuations. Under such conditions, the conventional models of Varshni, Vi?a and Passler fail to replicate the S-shaped temperature dependence of the PL peak energy and provide inconsistent and unrealistic values of the fitting parameters. The proposed formalism persuasively reproduces the S-shaped temperature dependence of the PL peak energy and provides an accurate determination of the exciton localization energy in bulk and QW structures along with the appropriate values of material parameters. An example of a strained InAs0.38P0.62/InP QW is presented by performing detailed temperature and excitation intensity dependent PL measurements and subsequent in-depth analysis using the proposed model. Versatility of the new formalism is tested on a few other semiconductor materials, e.g. GaN, nanotextured GaN, AlGaN and InGaN, which are known to have a significant contribution from the localized states. A quantitative evaluation of the fractional contribution of the localized states is essential for understanding the temperature dependence of the PL peak energy of bulk and QW well structures having a large contribution of the band-tail states.

Temperature dependence of the lowest excitonic transition for an InAs ultrathin quantum well

Temperature dependent photoluminescence and photoreflectance techniques are used to investigate the lowest excitonic transition of InAs ultrathin quantum well. It is shown that the temperature dependence of the lowest energy transition follows the band gap variation of GaAs barrier, which is well reproduced by calculated results based on the envelope function approximation with significant corrections due to strain and temperature dependences of the confinement potential. A redshift in photoluminescence peak energy compared to photoreflectance is observed at low temperatures. This is interpreted to show that the photoluminescence signal originates from the recombination of carriers occupying the band-tail states below the lowest critical point.

Temperature dependence of the photoluminescence from InP/GaAs type-II ultrathin quantum wells

Temperature dependence of the photoluminescence (PL) spectra has been investigated for InP/GaAs type-II ultrathin quantum wells (QWs). Room temperature PL has been observed for 1.43 monolayer thick ultrathin QW. Fitting parameters of the Bose–Einstein empirical relation for ultrathin QWs show that the temperature dependence of PL peak energy is similar to the temperature dependence of the band gap for InP and GaAs materials. In addition, we have also determined the PL quenching mechanism from the Arrhenius-like plot of integrated PL intensity. Thermal escape of carriers from these ultrathin QWs into the GaAs barrier is mainly responsible for the PL quenching with temperature. This is also supported by the observation that the PL intensity related to the GaAs barrier increases with increasing temperature.

Sexual co-transmission of HIV, hepatitis B, and hepatitis C viruses

The spread of human immunodeficiency virus (HIV) infection has reached pandemic proportions. Hepatitis B (HBV) and hepatitis C (HCV) viruses, although predominantly blood borne can also be transmitted sexually with several studies pointing towards enhanced spread of HCV along with HIV. We have not come across any study in India that has studied the interrelation of these viruses. A total of 270 patients attending the STD clinic of JIPMER, Pondicherry, India, were randomly included in the study irrespective of age and sex. A detailed examination was done to rule out any underlying STD. All the patients were screened for syphilis (VDRL), HIV infection (enzyme immunoassay for …

Determination of band offsets in strained InAsxP1−x/InP quantum well by capacitance voltage profile and photoluminescence spectroscopy

InAsxP1−x/InP quantum wells (QWs) with excellent crystalline and interfacial quality are grown by metal organic vapor phase epitaxy as confirmed from the cross-sectional transmission electron microscopy, high resolution x-ray diffraction and photoluminescence measurements. The electron confinement in InAsxP1−x/InP QW states is determined by capacitance voltage measurements, where we find that the electron accumulation increases with increasing QWs thickness and arsenic composition. This is explained by the variation of the band offset and hence the effective change in the position of the electronic energy level from Fermi level with QWs composition and thickness. The conduction band offset (ΔEc) for InAsxP1−x/InP QWs has been obtained by solving the self consistent set of Schrodinger and Poisson equations and fitting the theoretical carrier density profile with the apparent carrier density measured from experiments. The ΔEc values in strained InAsxP1−x/InP QWs have been obtained which fits to the expressi...

Conduction band offset and quantum states probed by capacitance–voltage measurements for InP/GaAs type-II ultrathin quantum wells

Quantum states in InP/GaAs type-II ultrathin quantum wells (QWs) are investigated through temperature dependent capacitance–voltage (C–V) measurements. We observe a well-defined peak in the apparent carrier density (ACD) profile for the ultrathin QWs at low temperatures in the vicinity of QWs. ACD peak value is found to decrease with the reduction in QW thickness, indicating quantum confinement effect. Decrease in the ACD peak value and increase in its width with increasing temperature confirms that the observed peak in the ACD profiles is related to the two dimensional electrons occupying the quantum states formed in the ultrathin QWs. We do not observe appreciable peak shift in ACD profiles with temperature, which is attributed to the less temperature dependence of the Debye length because of the high doping density used in the barrier region of InP/GaAs ultrathin QWs. We determine a strained value of 180 ± 30 meV for the conduction band discontinuity by simulating the C–V profile through the self-consi...

Effect of light-hole tunnelling on the excitonic properties of GaAsP/AlGaAs near-surface quantum wells

Light-hole tunnelling to the surface states is studied using photoluminescence (PL) spectroscopy and transient reflectivity measurements in the tensile-strained GaAsP/AlGaAs near-surface quantum well (NSQW) samples by reducing the top barrier layer thickness from 275 to 5 nm. The ground state transition (e1–lh1) remains excitonic even at room temperature (RT) for a buried quantum well sample with 275 nm thick top barrier. When the top barrier thickness is reduced to 50 nm the same transition is found to be excitonic only at low temperatures but changes to free-carrier recombination at higher temperatures. When the top barrier layer thickness is further reduced to 5 nm, the ground state transition is no longer excitonic in nature, where it shows free-carrier behaviour even at 10 K. We therefore find a clear relationship between the character of the ground state transition and the top barrier layer thickness. Light-hole excitons cannot be formed in NSQW samples when the top barrier layer thickness is kept reasonably low. This is attributed to the quantum mechanical tunnelling of free light holes to the surface states, which is found to be faster than the exciton formation process. A tunnelling time of ~500 fs for light holes is measured by the transient reflectivity measurements for the NSQW sample with a 5 nm top barrier. On the other hand, heavy-hole-related transitions in NSQW samples are found to be of excitonic nature even at RT because of the relatively large tunnelling time. It supports the dominance of excited state feature over the ground state transition in PL measurements at temperatures higher than 150 K.

Blueshift in sulfur treated GaAsP/AlGaAs near surface quantum well

Large blueshift was observed in a near-surface GaAs0.86P0.14/Al0.7Ga0.3As quantum well upon treatment with Na2S·xH2O solution. Very slow etching with simultaneous surface passivation of the quantum well was obtained using this chemical treatment. Photoreflectance (PR) spectra exhibit maximum blueshift of 28 meV after treating the quantum well surface with Na2S·xH2O solution for 30 min (top layer thickness reduced to 10 A). The blueshift is attributed to an increase in the confinement and/or an image charge effect due to the penetration of the wave function into vacuum. The blueshift is accompanied by a significant reduction in the broadening parameter of the observed e1-lh1 transition in PR spectra indicating effective passivation along with an increase in the confinement.

Effect of built-in electric field on the temperature dependence of transition energy for InP/GaAs type-II superlattices

Built-in electric field in InP/GaAs type-II superlattice structures considerably modifies the temperature dependence of the ground state (GS) transition energy in photoreflectance measurements. For moderate electric fields, the temperature dependence of the GS transition energy follows the bandgap of the GaAs barrier layer, and it decreases at a faster rate than that of the GaAs material in the case of larger values of built-in electric field. The GS excitonic feature red shifts with quantum well thickness, confirming that it originates from the superlattice structure. Further, the variation of the broadening parameter with temperature is governed by the scattering of electrons with longitudinal optical phonons.