V. K. Dixit

Observation of anisotropic distribution of microstructure in GaP/GaAs epitaxial layers

Anisotropic distribution of microstructure in GaP/GaAs epitaxial layers is investigated by performing high resolution x-ray diffraction (HRXRD) experiments for symmetric reflections at different azimuths. The observed anisotropy leads to the different values of lattice relaxation along the [011¯] and [01¯1¯] directions as revealed by the reciprocal space maps recorded at 0° and 90° azimuths for (42¯2¯) and (422¯) reflections. The anisotropic relaxation process causes large differences in the full width at half maximum of (400) diffraction peaks of omega scans along the [011¯] and [01¯1¯] directions. It therefore provides large differences in the values of microstructural parameters of the GaP epitaxial layer along the two in-plane orthogonal directions. A systematic Williamson-Hall analysis of HRXRD data confirms the presence of large anisotropy of lattice relaxation process along the [011¯] and [01¯1¯] directions. Due to the large lattice mismatch, the GaP/GaAs epilayer presented here possesses grain bou...

Electrical and optical characterization of photooxidized TPD

Changes in current–voltage characteristics of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′diamine (TPD) dispersed in polymer films are studied when solutions of TPD are exposed to UV light. Higher current density is observed in films cast with solutions exposed under UV for an optimum time. Observations are rationalized on the basis of the photoinduced oxidation of TPD when chlorinated solvents are used. Photoinduced generation of the TPD cation (TPD+) is confirmed using linear absorption spectroscopy when compared with salt-based generation (using CuCl2) of TPD+. Results suggest another possible route for generating TPD+ and the possibility of unintentional oxidation of TPD when chlorinated solvents like chloroform or dichloromethane are used for making devices.

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 170 K 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.

Study of the microstructure information of GaAs epilayers grown on silicon substrate using synchrotron radiation.

Williamson-Hall (WH) analysis is a well established method for studying the microstructural properties of epilayers grown on foreign substrates. However, the method becomes inapplicable in specific cases where the structure factor considerations and the presence of anti-phase domains forbid the data acquisition for certain reflections in conventional high-resolution X-ray diffraction (HRXRD) measurements. Here, this limitation is overcome by exploiting the large intensity (25 µW mm(-2)) and high photon energy (15.5 keV) of the X-ray beam obtained from a synchrotron radiation source. The lateral coherence length, vertical coherence length, tilt and micro-strain of GaAs epilayers grown on Si substrate have been successfully measured using the conventional WH analysis. The microstructure information obtained from the conventional WH analysis based on the data acquired at the synchrotron radiation source is in reasonable agreement with the results obtained from atomic force microscope and surface profiler measurements. Such information cannot be obtained on a laboratory-based HRXRD system where modification of the WH method by involving a set of parallel asymmetric crystallographic planes is found to be essential. However, the information obtained from the modified WH method is along a different crystallographic orientation.

An Accurate Measurement of Carrier Concentration in an Inhomogeneous GaN Epitaxial Layer from Hall Measurements

An appropriate method for an accurate determination of carrier concentration from Hall measurements for samples having large inhomogeneities is presented. Parasitic contributions in such samples generally limit the capabilities of Hall experiments where even the measurement of carrier type in some samples becomes doubtful. Here, we eliminate the major parasitic contributions from the measured Hall voltage through a systematic averaging procedure over the current and magnetic field polarities. Further, the carrier concentration values are unambiguously determined from the magnetic field dependent Hall measurements, where the slope of true Hall voltage versus magnetic field plot provides realistic values of carrier concentrations.

Estimation of electron spin polarization from circularly polarized photoluminescence in strained quantum wells

A general method based on the valence band mixing is proposed to establish a correlation between electron spin polarization and the degree of circular polarization of luminescence in a strained quantum well (QW). Key issues that are critical for establishing such a correlation are highlighted by invoking 4 band k.p calculations in a strained QW system. The proposed method takes care of the complex valence band mixing in strained QWs by simultaneously analyzing the heavy hole (e-hh) and light hole (e-lh) related luminescence transitions along the quantization direction. It is shown that the ratio of the degree of circular polarizations measured for the e-hh and e-lh features is a sensitive probe to estimate the values of band mixing. The method is reasonably supported by carefully conducted experiments on a tensile strained GaAsP/AlGaAs QW sample in the temperature range of 10 to 150 K. The proposed method shall be helpful for understanding the spin injection/transfer across the interfaces of QW, especiall...

A comparison of inverse spin hall spectra in Pt/III-V hybrid structures

The inverse spin Hall spectra in Pt/III-V semiconductor hybrid structures are estimated using numerical calculations. The numerical model used here takes account of both energy and spin relaxation phenomena, tunneling and thermionic transport effects at the interface and circular dichroism for the light transmitted through the Pt layer. The magnitude of the inverse spin hall field (EISHE) is found to be of similar order of magnitude for all the structures except Pt/InP structure, which has marginally higher EISHE.

Quantum States Probed By Temperature Dependence Capacitance‐Voltage Measurements For InP/GaAs Type‐II Ultrathin Quantum Well

We report on the temperature dependent capacitance‐voltage measurements on InP/GaAs ultrathin quantum well (QW). We observe a peak in apparent carrier distribution (ACD) profile at around the geometrical position of the QW. Peak value of the ACD profile increases, while its width decreases with reducing temperature indicating that the peak in ACD is due to the accumulation of two dimensional electrons occupying the quantum states in ultrathin QW.

Studies of Valence Band Alignment Between Nitrided GaPN/GaP (111) Interface Using X‐ray Photoelectron Spectroscopy

The analysis of core levels positions of Ga3d, N1s and P2p at different etching depth from the plasma nitrided GaP (111) surface shows that the nitrogen ions interact with both Gallium and Phosphorous ions with nearly equal probability. The analysis of valance band spectra shows the type‐II band alignment between GaPN0.22/GaP and the valence band offset is ∼2.2±0.1 eV.