Shailesh K. Khamari

Investigation of crystalline and electronic band alignment properties of GaP/Ge(111) heterostructure

Gallium phosphide (GaP) epitaxial layer and nanostructures are grown on n-Ge(111) substrates using metal organic vapour phase epitaxy. It is confirmed by high resolution x-ray diffraction measurements that the layer is highly crystalline and oriented with the coexistence of two domains, i.e., GaP(111)A and GaP(111)B, with an angle of 60° between them due to the formation of a wurtzite monolayer at the interface. The valence band offset between GaP and Ge is 0.7 ± 0.1 eV as determined from the valence band onsets and from Krauts method. A band alignment diagram for GaP/Ge/GeOx is also constructed which can be used to design monolithic optoelectronic integrated circuits.

Numerical simulation of inverse spin Hall spectra in Pt/GaAs hybrid structure

The inverse spin Hall spectra in Pt/GaAs hybrid structure in the range 1?3?eV photon energy are estimated using numerical calculations. The numerical model used here takes account of both energy and spin relaxation phenomena for optically induced hot electrons, tunnelling and thermionic transport effects at Pt/GaAs interface and circular dichroism for the light transmitted through the Pt layer. The optically excited electrons in GaAs induces a spin current in the Pt layer after crossing the Schottky barrier that leads to transverse electrical voltage due to large spin?orbit coupling in Pt. The magnitude and polarity of this voltage depend on photon energy, angle of incidence of light, and barrier height. It is found that the magnitude of this voltage is maximum for 60? angle of incidence irrespective of photon energy and barrier height.

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 300 K. 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.

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...

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.

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.

Evaluation of structural and microscopic properties of tetragonal ZrO2 for the facet coating of 980 nm semiconductor laser diodes

ZrO2 based antireflection coatings are expected to show a high laser induced damage threshold in the facet coating applications of laser diodes. A single layer of undoped ZrO2 and multilayers of ZrO2/SiO2 are deposited at 80 °C by an e-beam evaporation technique on GaAs and Si substrates. ZrO2 layers deposited under the optimized conditions are of a tetragonal nature and the grain size increases with reduced atomistic configuration mismatch between the surfaces of the layer and substrate. The electron/optical density profiles of the tetragonal ZrO2 single layer and multilayers of ZrO2/SiO2 are obtained from x-ray reflectivity measurements which confirm a uniform surface without any evidence of interfacial diffusion. The refractive index spectrum of tetragonal ZrO2 is found to be different from its other stable monoclinic allotropes. This is due to the possible differences in the atomistic structure of tetragonal ZrO2 that might be caused by oxygen vacancies and impurities. The measured values of refractive index, surface and interface roughness are used to engineer the layer structures for achieving 2 and 90% reflectivity at 980 nm at the front and the rear facets of laser diodes. The slope efficiency of the facet coated laser diode increases from 0.5 to 0.91 W A−1 without any measurable difference in the threshold current density.

Peculiarities of the current-voltage and capacitance-voltage characteristics of plasma etched GaN and their relevance to n-GaN Schottky photodetectors

The impact of reactive ion etching (RIE) induced damage on the optoelectronic properties of GaN epitaxial layers and the photoresponse of Schottky detectors is investigated. Plasma induced surface damage in epitaxial layers is noticed which leads to a significant reduction of the intensity of the photoluminescence signal and also the photoresponse of detector devices post dry etching process. Electrical characterization of Au/Ni/GaN Schottky diodes along with secondary ion mass spectroscopy results indicate that the ion bombardment induced damage is mostly confined close to the surface of the GaN layer. It is found that the current-voltage characteristics of Schottky contacts on pristine n-GaN layers can be understood by considering a model based on the thermionic emission of carriers across the junction. However, the same is not possible in the case of plasma etched samples where the involvement of the thermionic field emission of carriers is essential. It is proposed that the RIE process leads to the generation of nitrogen vacancies in strongly localized domains near the surface. Such vacancies act as shallow donors shifting the Fermi level into the conduction band, thus enabling the tunnelling of carriers across the junction. However, this is not evident in capacitance-voltage characteristics since the damage is much prior to the depletion edge and is confined to extremely small domains. A method for the recovery of dry etch induced damage through O2 plasma treatment is demonstrated which is found to be very effective in improving the post-etch surface morphology and also the optoelectronic properties of etched GaN epitaxial layers. The spectral response of the Schottky photodetector is seen to degrade by 90% due to the plasma etching process. However, the same can be recovered along with an enhancement of the deep ultraviolet response of the detector after O2 plasma treatment of etched layers within the RIE chamber. The understanding developed here is crucial for the optimization of the RIE process and is found to be very helpful in recovery of damage caused by the dry etching process.The impact of reactive ion etching (RIE) induced damage on the optoelectronic properties of GaN epitaxial layers and the photoresponse of Schottky detectors is investigated. Plasma induced surface damage in epitaxial layers is noticed which leads to a significant reduction of the intensity of the photoluminescence signal and also the photoresponse of detector devices post dry etching process. Electrical characterization of Au/Ni/GaN Schottky diodes along with secondary ion mass spectroscopy results indicate that the ion bombardment induced damage is mostly confined close to the surface of the GaN layer. It is found that the current-voltage characteristics of Schottky contacts on pristine n-GaN layers can be understood by considering a model based on the thermionic emission of carriers across the junction. However, the same is not possible in the case of plasma etched samples where the involvement of the thermionic field emission of carriers is essential. It is proposed that the RIE process leads to the ge...

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...