T. K. Sharma

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.

Strain-driven light-polarization switching in deep ultraviolet nitride emitters

Residual strain plays a critical role in determining the crystalline quality of nitride epitaxial layers and in modifying their band structure; this often leads to several interesting physical phenomena. It is found, for example, that compressive strain in AlxGa1-xN layers grown on AlyGa1-yN (x<y) templates results in an anti-crossing of the valence bands at considerably much higher Al composition than expected. This happens even in the presence of large and negative crystal field splitting energy for AlxGa1-xN layers. A judicious magnitude of the compressive strain can support vertical light emission (out of the c-plane) from AlxGa1-xN quantum wells up to xapprox 0.80, which is desirable for the development of deep ultraviolet light-emitting diodes designed to operate below 250nm with transverse electric polarization characteristics.

Application-oriented nitride substrates: The key to long-wavelength nitride lasers beyond 500 nm

We present results based on quantum mechanical estimates of the longest emission wavelength for nitride laser diodes grown on c-plane GaN/sapphire substrates. The results indicate that the absence of polarization-induced electric fields in nonpolar/semipolar GaN substrates does not necessarily guarantee that nitride lasers will operate at the longest possible wavelength for a given set of parameters. Our calculations suggest that the limit on the longest possible wavelength of nitride lasers is constrained by the lattice mismatch rather than by the strength of the polarization-induced electric field. Although it may be possible to develop lasers that approach the green portion of the electromagnetic spectrum (∼520u2002nm) by growing the structures on nonpolar/semipolar GaN substrates, the development of red and near-infrared nitride lasers appears extremely difficult by merely growing the structures on any crystallographic orientation of the GaN substrate. We suggest that efficient lasers emitting at the gree...

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.

On ternary nitride substrates for visible semiconductor light-emitters

No nitride or other substrate material exists for growing lattice-matched nitride device structures. Use of bulk GaN or sapphire substrates is complicated by lattice and thermal mismatches that lead to defect and dislocation generation. To alleviate this problem, we recently proposed ternary nitride substrates on which lattice-matched structures could be grown for lasers within specified spectral bands. These proposed application-oriented nitride substrates have one drawback: several would be required to cover the visible spectrum. By taking advantage of the complex (but feature-rich) valence band structure of nitrides, we have determined that a single substrate (In0.15Ga0.85N) could be used for the development of efficient blue, green, and red laser diodes.

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 photo-induced inverse spin Hall effect in Au/InP hybrid structures

Photo-induced Inverse Spin Hall Effect (ISHE) measurements on Au/InP hybrid structures are performed over a temperature range of 45 to 300u2009K. Dependence of the spin current density on the degree of circular polarization and also on the angle of incidence of laser beam confirms the ISHE origin of measured signal. The magnitude of ISHE increases with sample cooling. A numerical model based on the spin relaxation of non-equilibrium spin-polarized electrons is proposed for predicting the temperature dependence of ISHE. Our results indicate that the proposed device can be used as a spin photodetector over a wide temperature range.

A method for evaluating the ground state excitonic band gaps of strained InxGa1−xN/GaN quantum wells

A simple method for calculating the ground state excitonic band gaps of strained wurtzite InxGa1−xN/GaN quantum wells (QWs) for the entire composition range is proposed. The modification of the electronic band structure due to strain becomes significant for high values of indium concentration. It is observed that confinement effects become less important for electrons (due to a shallower well) while the opposite is true for holes (because of a deeper well) in InxGa1−xN/GaN QW with large residual strain. Following the proposed method, one can obtain a satisfactory agreement between the theoretical predictions and experimental observations for ground state excitonic band gaps in InGaN/GaN QWs and InN/GaN ultrathin QWs.

Dislocation-assisted tunnelling of charge carriers across the Schottky barrier on the hydride vapour phase epitaxy grown GaN

Barrier height and Ideality factor of Ni/n-GaN Schottky diodes are measured by performing temperature dependent current-voltage measurements. The measured value of barrier height is found to be much smaller than the theoretically calculated Schottky-Mott barrier height for the Ni/n-GaN diodes. Furthermore, a high value of ideality factor (>2) is measured at low temperatures. In order to understand these results, we need to consider a double Gaussian distribution of barrier height where the two components are related to the thermionic emission and thermionic filed emission mediated by dislocation-assisted tunnelling of carriers across the Schottky barrier. Thermionic emission is seen to dominate at temperatures higher than 170u2009K while the dislocation-assisted tunnelling dominates at low temperatures. The value of characteristic tunnelling energy measured from the forward bias current-voltage curves also confirms the dominance of dislocation-assisted tunnelling at low temperatures which is strongly corrobor...

Dislocations limited electronic transport in hydride vapour phase epitaxy grown GaN templates: A word of caution for the epitaxial growers

GaN templates grown by hydride vapour phase epitaxy (HVPE) and metal organic vapour phase epitaxy (MOVPE) techniques are compared through electronic transport measurements. Carrier concentration measured by Hall technique is about two orders larger than the values estimated by capacitance voltage method for HVPE templates. It is learnt that there exists a critical thickness of HVPE templates below which the transport properties of epitaxial layers grown on top of them are going to be severely limited by the density of charged dislocations lying at layer-substrate interface. On the contrary MOVPE grown templates are found to be free from such limitations.