S. Porwal

Room temperature photoluminescence from ZnO quantum wells grown on (0001) sapphire using buffer assisted pulsed laser deposition

Efficient room temperature (RT) photoluminescence (PL) is achieved on ZnO multiple quantum wells (MQWs) grown on sapphire by pulsed laser deposition using a buffer assisted growth scheme. Absorption spectra of these MQWs at RT showed the excitonic features entwined with the band edges, which pointed to the excitonic nature of the PL transitions. At RT the band edge of these MQWs shifted from ∼3.36to3.78eV on decreasing the well layer thickness from ∼4to1nm. In the range from 10K to RT, the PL spectral linewidth increased and the peak shifted monotonically towards red with increasing temperature.

Absorption edge determination of thick GaAs wafers using surface photovoltage spectroscopy

A procedure for choosing the appropriate chopping frequency (f) for the surface photovoltage spectroscopy (SPS) measurements in order to obtain the absorption related features is presented. We could obtain the absorption edge of thick n+ GaAs wafer (thickness ≈700 μm) by performing SPS measurements at f⩾1 kHz at room temperature (300 K). The similar information for semi-insulating (SI) GaAs could not be obtained due to the carrier trapping at deep levels or surface states at 300 K. However, we could obtain the absorption edge of SI-GaAs by performing SPS measurements at 395 K at f=3 kHz. Here, we demonstrate the capability of the SPS technique to measure large absorption coefficient (α) values for thick wafers by performing SPS measurements and normalizing this with the reported α value at one wavelength in the above band gap region. For comparison, we also perform quasisimultaneous SPS and transmission spectroscopy (TS) measurements. The SPS technique could provide α values up to 104 cm−1 for 700-μm-thic...

Photoluminescence study of β-Ga2O3 nanostructures annealed in different environments

β-Ga2O3 nanostructures (nanowires, nanoribbons, and nanosheets) were synthesized via vapor transport method on gold coated silicon substrate in N2 ambient and these β-Ga2O3 nanostructures grown on silicon substrates were taken as the starting material to study the effect of annealing in the different environments (oxygen, water vapour, and ammonia solution) on the structural front and photoluminescence (PL) properties. The PL spectra of β-Ga2O3 nanostructures exhibit a UV-blue emission band whose intensity is strongly affected by the annealing in different environments. Annealing modifies the surface of the nanostructures by creating surface states which quench the PL by creating competitive nonradiative paths. This study also indicates the dominance of the formation of water induced surface states over ammonia induced surface states. The irreversible nature of these defects significantly affects the applicability of this system in moist high temperature environments.

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.

Water-vapour-assisted growth of ZnO nanowires on a zinc foil and the study of the effect of synthesis parameters

Dense and large-aspect-ratio ZnO nanowires have been synthesized by thermal oxidation of a Zn foil under the flow of moist nitrogen. The effects of annealing temperature and time have been systematically studied in detail. It was observed that the length, diameter and number density increase with time and temperature before they saturate at the optimum synthesis condition of 600 °C annealing temperature and 16 h annealing time. The nanowires have been characterized by a scanning electron microscope, a transmission electron microscope, UV–Vis absorption and room temperature photoluminescence. The nanowires grow uniquely along the [1 1 0] direction up to ~14–16 µm length and are of good crystalline and optical quality. It was found that the formation of nanowires enhances greatly in the presence of water vapour. We have also shown by TEM observations that nanowires grow by surface diffusion of Zn ions from root to tip.

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.

A comparative study on nanotextured high density Mg-doped and undoped GaN

Nanotextured high density Mg-doped and undoped GaN were obtained using photoelectrochemical etching. Interesting features are observed in the temperature dependent photoluminescence (PL) studies of these nanotextured materials. First, the PL intensity of the excitonic emissions shows more than three orders of enhancement. At low temperature, the peak energy shows a blueshift with temperature. This phenomenon is attributed to the formation of excitonic band-tail states. Second, the excitonic emissions in the nanotextured samples are redshifted compared to the as-grown GaN suggesting strain relaxation. Third, the blue luminescence band (2.7–2.9eV in Mg-doped GaN) shows a large redshift, which is not consistent with strain relaxation calculated from excitonic band. Furthermore, temperature dependence of the blue luminescence band energy shows an asymmetric S-shaped behavior in nanotextured GaN. All these observations are explained by invoking an increase in carrier localization due to an increase in potentia...

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...