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Electroluminescence properties of epitaxially grown n+-ZnO/p-GaN pn+-heterojunctions are investigated as functions of applied bias and temperature. This study reveals the existence of indirect interfacial excitons at sufficiently low temperatures. Electroluminescence feature associated with these excitons redshifts with increasing forward bias. It has been found that the binding energy of these entities can be controlled through applied forward bias and can even be made higher than that of the excitons in ZnO bulk (60 meV). However, the formation of these excitons becomes unsustainable when either the applied bias or the temperature crosses a threshold. This has been explained in terms of leakage and thermal escape of electrons (holes) into the GaN (ZnO) side. Calculations for the band diagram and the binding energy of these spatially indirect electron–hole coulomb-coupled entities are carried out. Theoretical results are found to explain the experimental findings quite well.

Here, the underlying mechanisms behind valley de-polarization is investigated in chemical vapor deposited 1L-MoS$_{2}$. Temperature dependent polarization resolved photoluminescence spectroscopy was carried out on as-grown, transferred and capped samples. It has been found that the momentum scattering of the excitons due to the sulfur-vacancies attached with air-molecule defects has a strong influence on the valley de-polarization process. Our study reveals that at sufficiently low densities of such defects and temperatures, long range electron-hole exchange mediated intervalley transfer due to momentum scattering via Maialle-Silva-Sham (MSS) mechanism of excitons is indeed the most dominant spin-flip process as suggested by T. Yu et al. The rate of momentum scattering of the excitons due to these defects is found to be proportional to the cube root of the density of the defects. Intervalley transfer process of excitons involving $\Gamma$-valley also has significance in the valley de-polarization process specially when the layer has tensile strain or high density of $V_S$ defects as these perturbations reduce $K$ to $\Gamma$-energy separation. Band-structural calculations carried out within the density functional theory framework validate this finding. Experimental results further suggest that exchange interactions with the physisorbed air molecules can also result in the intervalley spin-flip scattering of the excitons, and this process gives an important contribution to valley depolarization, specially at the strong scattering regime.

Electroluminescence properties of epitaxially grown n-ZnO/p-GaN pn-heterojunctions are investigated as functions of applied bias and temperature. The study reveals the existence of indirect interfacial excitons at sufficiently low temperatures. Electroluminescence feature associated with these excitons redshifts with increasing forward bias. It has been found that the binding energy of these entities can be controlled through applied forward bias and can even be made higher than that of the excitons in ZnO bulk (60 meV). However, formation of these excitons becomes unsustainable when either the applied bias or the temperature crosses a threshold. This has been explained in terms of leakage and thermal escape of electrons (holes) into GaN (ZnO) side. Calculations for the band diagram and the binding energy of these spatially indirect electron-hole coulomb-coupled entities are carried out. Theoretical results are found to explain the experimental findings quite well.

Incorporation of lithium in (111) NiO epitaxial layers grown using the pulsed layer deposition technique on c-sapphire substrates is studied as functions of growth conditions. The effect of Li-inclusion on the structural, morphological, electrical and optical properties of the films have been systematically investigated. It has been found that the concentration of Li in the film is more at lower growth temperatures. However, the crystalline quality deteriorates as the growth temperature is lowered. The investigation suggests that there is a miscibility limit of Li in nickel oxide (NiO). Beyond a critical concentration of lithium, Li-clusters are detected in the films. Further, it has been found that inclusion of Li gives rise to hydrostatic tensile strain in the NiO lattice that results in the reduction of the bandgap. The study also suggests that Li incorporation improves the electrical conductivity of NiO layers. Ni-vacancy defects also play an important role in governing the conductivity of these samples.

In this work, we demonstrate the potential of a gallium nitride (GaN)-based visible-blind ultraviolet (UV) photodetector (PD) on a commercially viable 150-mm Si wafer. The influence of thermionic field emission (TFE) and Poole–Frenkel (PF) mechanisms on the current transport of the PD has been analyzed. Conduction due to the TFE mechanism dominates in the moderate electric fields (1.25 kV/cm ${ kV/cm), while the influence of PF is prominent at higher electric fields. A bulk trap energy level of 0.374 eV is obtained with PF conduction analysis. A high responsivity of 33.3 A/W at 15 V with a 362-nm incident wavelength has been achieved in the presence of an internal gain. The internal gain of the PD is also assisted by TFE and PF mechanisms. The PD exhibits a low dark current of 4.7 nA as well as high detectivity of $4.6\times 10^{12}$ Jones at the abovementioned bias. The demonstrated robustness and high performance show the promise of III–nitride PDs for commercial applications.

A two-dimensional electron gas (2DEG), which has recently been shown to develop in the central vertical plane of a wedge-shaped c-oriented GaN nanowall due to spontaneous polarization effect, offers a unique scenario, where the symmetry between the conduction and valence band is preserved over the entire confining potential. This results in the suppression of Rashba coupling even when the shape of the wedge is not symmetric. Here, for such a 2DEG channel, relaxation time for different spin projections is calculated as a function of donor concentration and gate bias. Our study reveals a strong dependence of the relaxation rate on the spin-orientation and density of carriers in the channel. Most interestingly, relaxation of spin oriented along the direction of confinement has been found to be completely switched off. Upon applying a suitable bias at the gate, the process can be switched on again. Exploiting this fascinating effect, an electrically driven spin-transistor has been proposed.

Li-doped NiO/[Formula: see text]-Ga2O3 polycrystalline bilayer thin-film pn-heterojunctions with different Li-doping concentrations are grown on Si-substrates using the pulsed laser deposition technique. Resistive switching property of these devices has been investigated in detail. This study shows that the Li-doping concentration in NiO layer significantly influences the performance of these devices. For an optimum Li-doping of 1.5%, a stable memory window of ∼102 with endurance of more than 100 cycles and long retention time can be achieved. The coefficient of variation ([Formula: see text]) of SET and RESET voltages also found to ∼ 20% and ∼ 40%, respectively, satisfying the acceptability benchmark. A transition from complementary resistive switching (CRS) to bipolar resistive switching (BRS) after multiple sweeping operations has been observed in devices with intermediate Li-doping concentrations. Observation of CRS has been explained in terms of the formation of Li-rich metallic layer at the NiO/Ga2O3 interface as a result of out-diffusion of Li. Redistribution of the Li-ions from the Li-rich interfacial zone to whole of the NiO layer after first few sweeping cycles must be the reason for CRS-to-BRS transition. Results further suggest that return to high resistive state via Poole–Frenkel (PF) pathway during the RESET process might be the key to achieve high performance in p–n junction based resistive switching devices. This study, thus, presents Li-doping as a possible route to modulate the resistive switching property of bilayer Li:NiO/Ga2O3 based memory devices.

Recently, the electron mobility in wedge-shaped c-GaN nanowall networks has been estimated to cross the theoretical mobility limit for bulk GaN. Significant blue-shift of the bandgap has also been observed. Both the findings are explained in terms of two-dimensional electron gas (2DEG) formed at the central vertical plane of the walls due to the polarization charges at the two inclined faces. Carrier concentration and mobility have earlier been determined from thermoelectric power and conductivity measurements with the help of a statistical model. Due to the network nature of the system, direct measurements of these quantities from Hall experiments are not possible. Search for a better way to estimate mobility in this system thus becomes important. Since, strain can also lead to the blue-shift of the bandgap, it is also imperative to evaluate carefully the role of strain. Here, using Raman spectroscopy, we have estimated carrier concentration and mobility in these nanowall networks with varied average tip-widths. Depth distribution of strain and luminescence characteristics are also studied. The study reveals that strain has no role in the bandgap enhancement. Moreover, the electron mobility, which is determined from the lineshape analysis of the A1(LO)-plasmon coupled mode in Raman spectra, has been found to be significantly higher than the theoretical limit of mobility for bulk GaN for the same electron concentration. These results thus corroborate the picture of polarization induced vertical 2DEG formation in these walls as predicted theoretically.

NiO and Ni-cluster-embedded NiO epitaxial films grown on c-sapphire substrates using pulsed laser deposition technique are annealed at different temperatures in an oxygen environment. The structural, morphological, and electrical properties of these samples are investigated as functions of the annealing temperature. It is observed that, in all cases, the sample surface becomes smoother with the increase in annealing temperature. The resistivity of the Ni-cluster-free NiO epilayers is found to decrease with the increase in annealing temperature beyond ∼300 °C. Annealing at 450 °C results in about a two orders of magnitude decrease of resistivity from the highly insulating state of the as-grown. The resistivity shows a gradual increase when annealed at temperatures higher than 450 °C. A reduction of resistivity with annealing can be attributed to the enhancement of the density of Ni vacancies, which are known to act as shallow acceptors in NiO. The formation energy of the Ni vacancies, which has been estimated from the ln ( ρ − 1 ) versus 1 / T A plot, comes out to be 623 meV. The density of 60° dislocations in these films increases sharply when annealed above 500 °C. A reduction of conductivity of films annealed above 500 °C can thus be explained in terms of the enhancement of the rate of scattering of carriers by these defects. The resistivity of the Ni-cluster-embedded highly conducting NiO epilayers is found to increase rapidly by more than several orders of magnitude beyond an annealing temperature of ∼200 °C. This has been ascribed to percolation-driven metal-to-insulator transition arising as a result of the reduction of the cluster density with annealing.

The development of future valley based electronics or valleytronics requires a high degree of valley polarization (VP) in large area monolayer (1L)-MoS2. Though it is possible to synthesize 1L-MoS2 films with large area coverage, VP property of as-grown films is found to be very poor. Here, we investigate the role of physisorbed air molecules and strain on the luminescence and the VP characteristics of large area monolayer MoS2 grown on various substrates by a microcavity based chemical vapor deposition (CVD) technique. The study shows that the removal of adsorbates from sulfur vacancy ( VS) sites not only suppresses the broad sub-bandgap luminescence feature that typically dominates low temperature photoluminescence (PL) spectra of these films but also significantly enhances VP. Post-growth transfer of the 1L-MoS2 film from sapphire to a SiO2/Si substrate by a polystyrene assisted process is found to be highly effective in improving the polarization characteristic (∼80%) of K-valleys through relaxation of the biaxial tensile strain and the removal of physisorbed air molecules from the VS sites. The process is also found to provide long lasting protection for MoS2 films from air. The finding, thus, creates much needed opportunity to use CVD grown large area 1L-MoS2 for realization of valleytronics of the future.

Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy techniques we investigate the incorporation of Manganese (Mn) in monolayer (1L)-MoS$_2$ grown on sapphire substrates by microcavity based chemical vapor deposition (CVD) method. These layers are coated with different amount of Mn by pulsed laser deposition (PLD) technique and temperature dependent photo-luminescence (PL) spectroscopic study has helped us in understanding how such deposition affects the dynamics of excitonic recombination in this system. The study further reveals two distinctly different Mn-incorporation regimes. Below a certain critical deposition amount of Mn, thin Mn-coating with large area coverage is found on MoS$_2$ layers and in this regime, substitution of Mo ions by Mn is detected through XPS. Dewetting takes place when Mn-deposition crosses the critical mark, which results in the formation of Mn-droplets on MoS$_2$ layers. In this regime, substitutional incorporation of Mn is suppressed, while the Raman study suggests an enhancement of disorder in the lattice with the Mn-deposition time. From PL investigation, it has been found that the increase of the amount of Mn-deposition not only enhances the density of non-radiative recombination channels for the excitons but also raises the barrier height for such recombination to take place. The study attributes these non-radiative transitions to certain Mo related defects (either Mo-vacancies or distorted Mo-S bonds), which are believed to be generated in large numbers during Mn-droplet formation stage as a result of the withdrawal of Mn ions from the Mo-substitutional sites., Comment: Page 1-19 main text with 6 figures. Page 20-24 supplementary material with 4 figures

(111) NiO epitaxial layers are grown on c-sapphire substrates by pulsed laser deposition technique. Structural and morphological properties of the films are studied using in-plane as well as out-of-plane high resolution x-ray diffraction and atomic force microscopy techniques as functions of growth temperature, oxygen pressure and the pulses count of the laser. The study shows that continuous epitaxial films of thickness as low as 3 nm with high crystalline quality, smooth surface and interface morphology can be grown by this technique. The study also reveals the co-existence of 60°-rotated (111) triangular domains of NiO in the film. The study also evidences the presence of a very low density of 60° dislocations in these films. Density of screw and edge dislocations are also estimated to be quite low. It has been found that growth-temperature, oxygen partial pressure and the film thickness can influence differently the density of various dislocation types. These parameters are also found to affect significantly the strain developed in the films.

The electrical and thermoelectrical properties of strictly monolayer ${\mathrm{Mo}\mathrm{S}}_{2}$ films, grown using a microcavity-based CVD growth technique, are studied under diverse environmental and annealing conditions. The resistance of a monolayer ${\mathrm{Mo}\mathrm{S}}_{2}$ device fabricated on an as-grown continuous single-layer film is found to be reduced by about six orders of magnitude upon annealing in vacuum at 525 K. The Seebeck coefficient of the layer is also reduced by almost an order of magnitude upon annealing. When the sample is exposed to an oxygen atmosphere, these parameters return to their previous values. It is found that the electron concentration, the mobility, and the thermoelectric power of the material can be varied significantly by annealing. Once a particular resistive state is established, it remains unchanged as long as the layer is not exposed to an oxygen environment. This may offer a unique way to control doping in the material provided an effective encapsulation method is devised. Such control is an important step forward for device applications. The effect is attributed to the passivation of disulfur-vacancy donors present in the ${\mathrm{Mo}\mathrm{S}}_{2}$ film by physisorbed oxygen molecules. Band-structure calculations using density-functional theory are carried out, the results of which validate this picture.

Optical properties of strictly monolayer MoS 2 films, which are grown on c-sapphire substrates using microcavity based CVD route, have been investigated by temperature dependent absorption and temperature as well as excitation intensity dependent photoluminescence (PL) spectroscopy. Our study reveals a high intensity broad luminescence band appearing at ∼ 1.7 eV along with the usual free exciton/trion peak ( ∼ 1.86 eV). The investigation furthermore attributes this broad transition to excitons bound to two types of defects, whose binding energies are found to be ∼ 11 and ∼ 118 meV. Integrated intensity of this feature decreases with the increase in the impinging sulfur flux during growth and by post-growth annealing in sulfur atmosphere suggesting that these defects must be related to sulfur deficiency. Interestingly, exciton-phonon coupling for these defect bound excitons is found to be much stronger than that is associated with free excitons/trions.

The photo-response properties of vapor-liquid-solid (VLS) grown [101[combining macron]0] oriented individual GaN nanowires of the diameter ranging from 30 to 100 nm are investigated under the joint illumination of above and sub-bandgap lights. When illuminated with above-bandgap light, these wires show persistent photoconductivity (PPC) effects with long build-up and decay times. The study reveals the quenching of photoconductivity (PC) upon illumination with an additional sub-bandgap light. PC recovers when the sub-bandgap illumination is withdrawn. A rate equation model attributing the PPC effect to the entrapment of photo-generated holes in the surface states and the PC quenching effect on the sub-bandgap light driven release of the holes from the trapped states has been proposed. The average height of the capture barrier has been found to be about 400 meV. The study also suggests that the capture barrier has a broad distribution with an upper cut-off energy of ∼2 eV.

It has been shown theoretically that an electromotive force (EMF) can be developed between two points on the surface of InN epitaxial films when illuminated with above-band gap light. This is possible if certain surface-attached groups/adatoms, which result in the formation of donor-like-surface states that lead to the formation of a two-dimensional electron gas (2DEG) channel just below the film surface, have a gradient of density between the two contacts. The magnitude and the orientation of the EMF are governed solely by the spatial variation of the surface potential between the two points. Experimentally, we have investigated several c-axis-oriented InN epitaxial films grown on c-sapphire substrates by various techniques. The study shows downward bending of the band at the surface suggesting the formation of a 2DEG channel there. In these layers, surface photo-EMF has indeed been found to develop between contact points. At the same time, the measurement of surface potential across these points shows the existence of a gradient.

Ga2O3 and In2O3 bilayers are grown on c-plane sapphire substrate by pulsed laser deposition technique. Atomic force microscopy shows atomically flat surface morphology for both the layers. Cross-sectional imaging using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) have revealed the bilayer structure with abrupt interface quality. High-resolution x-ray diffraction (HR-XRD) measurement shows that oriented films of In2O3 and Ga2O3 can be grown individually on c-sapphire substrates. However, bilayer samples always show the top layer to be either amorphous or polycrystalline.Ga2O3 and In2O3 bilayers are grown on c-plane sapphire substrate by pulsed laser deposition technique. Atomic force microscopy shows atomically flat surface morphology for both the layers. Cross-sectional imaging using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) have revealed the bilayer structure with abrupt interface quality. High-resolution x-ray diffraction (HR-XRD) measurement shows that oriented films of In2O3 and Ga2O3 can be grown individually on c-sapphire substrates. However, bilayer samples always show the top layer to be either amorphous or polycrystalline.

The structural and electronic properties of In2O3 epitaxial films grown on c-sapphire substrates by a pulsed laser deposition technique are studied as functions of various growth conditions. Our study shows that growth temperature, growth time, oxygen partial pressure, and post-growth cooling conditions play important roles in governing the magnitude and character of the overall change in unit cell volume (volume strain) of the lattice. It is found that biaxial strain, which is developed due to lattice and thermal expansion coefficient mismatches between the layer and the substrate, leads to overall compression of the lattice. Interestingly, a hydrostatic tensile strain is found to coexist with biaxial strain in samples, which, following growth, are cooled to room temperature at a rate much slower than the natural rate of cooling. It has been observed that the overall strain can be varied from compressive to tensile by changing the growth parameters. The study further shows a systematic variation of the bandgap (from 3.8 to 3.1 eV) with a change in the unit cell volume of these layers, demonstrating the strong influence of strain on the band structural properties of the material.

Here, we have carried out a magnetic field dependent polarization selective photoluminescence (PL) study at 1.5 K on Gd-doped GaN epitaxial layers grown on c-SiC substrates, where the incorporation of Gd has been found to generate three types of defects, which results in bound excitonic features in low temperature PL spectra. The study reveals that the external magnetic field dependence of the spin-flip scattering rates for the three excitonic features shows an increasing tendency of saturation at high fields as the density of these defects increases in these layers. This suggests that the signals must be stemming from defects which are ferromagnetically coupled with each other. This is further confirmed by the study carried out on a GaN sample co-doped with Si and Gd, where defects are found to be ferromagnetically coupled, while the Si-donors do not.

Ba doped ZnO epitaxial layers (Ba:ZnO) with different Ba concentrations are grown on c-GaN/sapphire templates by chemical vapor deposition technique, where mixture of Ba and Zn powders is used as the source for these metals. Concentration of Ba in these layers is determined by secondary ion mass spectrometry and Rutherford backscattering techniques. X-ray diffraction studies demonstrates high epitaxial quality of these c-axis oriented films. c-lattice constant of these layers is found to increase with the Ba concentration, while a-lattice constant does not vary much. This suggests the strain developed in the film is uniaxial in nature. The width of the rocking curves taken at (0006) reflection is also found to increase with Ba concentration indicating the enhancement in the structural defects. This is also supported by the photoconductivity measurements. Thermally stimulated current spectroscopy reveals certain trap states with the activation energy of ∼ 192 meV in Ba doped layers. These layers also show ferromagnetic like behavior and the saturation magnetization is found to increase with the Ba-concentration, which may indicate the involvement of the generated defects in ferromagnetic ordering.

We report superconductivity in Chemical Vapor Deposition (CVD) and Plasma-Assisted Molecular Beam Epitaxy (PA-MBE) grown epitaxial InN films having carrier density ∼ 1019 − 1020cm-3. The superconducting phase transition starts at temperatures around Tc,onset∼3 K and the resistance goes to zero completely at Tc0 ∼ 1.6 K. The temperature dependence of the critical field HC2(T) does not obey a two fluid Casimir-Gorter (C-G) model rather it is well explained by the 2-D Tinkham model. The extrapolated value of the zero-temperature perpendicular critical field HC2(0) is found to be between 0.25 − 0.9 T, which is ten times greater than that of Indium metal. It may indicate the intrinsic nature of superconductivity in InN films. The angle dependence of critical field is well described by Lawrence-Doniach (L-D) model, which suggest the existence of quasi-2D superconducting layers.We report superconductivity in Chemical Vapor Deposition (CVD) and Plasma-Assisted Molecular Beam Epitaxy (PA-MBE) grown epitaxial InN films having carrier density ∼ 1019 − 1020cm-3. The superconducting phase transition starts at temperatures around Tc,onset∼3 K and the resistance goes to zero completely at Tc0 ∼ 1.6 K. The temperature dependence of the critical field HC2(T) does not obey a two fluid Casimir-Gorter (C-G) model rather it is well explained by the 2-D Tinkham model. The extrapolated value of the zero-temperature perpendicular critical field HC2(0) is found to be between 0.25 − 0.9 T, which is ten times greater than that of Indium metal. It may indicate the intrinsic nature of superconductivity in InN films. The angle dependence of critical field is well described by Lawrence-Doniach (L-D) model, which suggest the existence of quasi-2D superconducting layers.

Schrödingerand Poisson equations are solved self-consistently in order to obtain the potential and charge density distribution in [Formula: see text]-type GaN nanowalls tapered along [Formula: see text]-axis by different angles. The study shows two-dimensional (2D) quantum confinement of electrons in the central vertical plane of the wall for the entire range of tapering. Calculation of room temperature electron mobility in the 2D channel shows a steady decrease with the increase of the inclination angle of the side facets with respect to the base. However, it is interesting to note that the mobility remains to be much larger than that of bulk GaN even for the inclination angle of 65[Formula: see text]. The properties of high mobility and the vertical orientation of the 2DEG plane in this system can be exploited in fabricating highly conducting transparent interconnects and field effect transistors, which can lead to large scale integration of 2D devices in future.

Voice over Internet Protocol (VoIP) is actually a link between the nodes which communicate with each other through VoIP link. VoIP faces many threats and attacks. Among those attack is Eavesdropping attack. Voice packets suffer due to Eavesdropping attack because of noise and interference of the channel. Hence the receiver cannot receive all the packets at the same duration of time. Jitter buffer is an important device which can be used to eliminate noise and interference of the channel. In this paper, we have studied about VoIP codecs performance over IEEE 802.11n. Voice quality can be improved simply by increasing the average MOS and reducing the delay factor. There are different VoIP codecs such as G.711, G.729, G.726ar24 and G.726ar32. Out of all these codecs, G.711 has shown good results since the call quality for G.711 is excellent.

Multiwall carbon nanotubes (MWNTs) were melt-mixed in polyamide 6 (PA6) and acrylonitrile-butadiene-styrene (ABS) copolymer blends using a simultaneous mixing protocol in order to investigate the state of dispersion of MWNTs in PA6/ABS blends. The blend composition was varied from 40/60 (wt/wt) to 60/40 (wt/wt) in PA6/ABS blends, which showed co-continuous' morphology in the presence of MWNTs. State of dispersion of MWNTs in these blends was assessed through bulk electrical conductivity measurements, morphological analysis, solution experiments, and UV-vis spectroscopic analysis. MWNTs were subsequently modified with a novel organic modifier, sodium salt of 6-aminohexanoic acid (Na-AHA), to improve the state of dispersion of MWNTs. Blends with unmodified MWNTs exhibited the DC electrical conductivity in the range approximate to 10(-11) to approximate to 10(-5) S/cm, whereas blends with Na-AHA-modified MWNTs exhibited DC electrical conductivity in the range approximate to 10(-7) to approximate to 10(-5) S/cm. The reduction in MWNTs agglomerate' size (approximate to 73.7 m for 40/60 blend with unmodified MWNTs to approximate to 59.9 m in the corresponding blend with Na-AHA-modified MWNTs) was observed through morphological analysis. The rheological studies showed increased complex viscosity and storage moduli in lower frequency region in case of blends with Na-AHA-modified MWNTs confirming a refined network-like' structure of MWNTs. POLYM. ENG. SCI., 55:443-456, 2015. (c) 2014 Society of Plastics Engineers

Aluminum oxide (Al 2 O 3 ) thin films are used for passivation of p-type silicon surface in solar cells. In this work we investigate how atomic layer deposition (ALD) and pulsed-dc (p-DC) reactive sputtered Al 2 O 3 film respond to UltraViolet (UV) illumination. UV exposure of the passivation is inevitable in the field and stability of the films to UV is an important criteria for solar cell application. We find that the negative fixed charge in the film increases upon UV illumination, leading to a better passivation. In conventional process, the film has to be annealed after deposition for the activation of the passivation. Fixed oxide charge density is found to be 5.8 × 1012 cm−2 and 9 × 1012 cm−2 for annealed and UV illuminated film respectively. So, it is proposed that annealing step can be replaced by UV illumination for activation of passivation.

Small changes in electrical conductance of quasi-2 dimensional samples often need to be measured at low temperatures, T ∼ 1 K and lower. Care needs to be taken to minimize self-heating due to the excitation current itself and the possibility of any unexpected source of variation of the electrochemical potential around the contacts need to be understood. It is not only the low resistance of the ohmic contacts but also their “noninvasive” character that must be ensured. While the importance of the “noninvasiveness” of the voltage probes is well appreciated for mesoscopic devices—it turns out that these considerations continue to remain important even in macroscopic samples that are a few millimeters in size. We have done low temperature measurements on different types of gallium nitride samples with contacts made of pure indium (popularly called indium dots), which have a superconducting transition and contacts made of Ti/Au, which remain nonsuperconducting. We show that there are qualitative differences in the measured four-terminal resistance in the two cases. Obviously, correct conclusions about phase relaxation times, localization lengths, etc., will require ensuring that there are no effects arising from the “invasiveness” of the probes.

The persistence of photoconductivity after switching off the photoexcitation is investigated in individual m-axial n-GaN nanowires as a function of temperature. At room temperature, photoconductivity is found to decay with a time scale of several hours. The capture barrier height is estimated to be ∼ 450 meV from the stretched exponential fitting of the decay characteristics recorded at different temperatures. This energy value is found to be much less than the surface band-bending energy of ∼ 770 meV, which is believed to act as the capture barrier in this system. This finding indicates the tunneling of electrons through the top part of the band-bending barrier. Interestingly, the decay rate of photoconductivity is observed to reduce significantly when the photoconductivity in these wires is quenched by an additional sub-bandgap illumination prior to the switching off the photoexcitation. A rate equation model is proposed to explain the upward band bending at the surface as well as the persistent photoconductivity effect in terms of the transfer of holes between the valence band and acceptor-type surface states of the nanowires. Photoconductivity decay profiles simulated from the model are found to match very well with the experimental data recorded at different temperatures in both quenched and unquenched cases.The persistence of photoconductivity after switching off the photoexcitation is investigated in individual m-axial n-GaN nanowires as a function of temperature. At room temperature, photoconductivity is found to decay with a time scale of several hours. The capture barrier height is estimated to be ∼ 450 meV from the stretched exponential fitting of the decay characteristics recorded at different temperatures. This energy value is found to be much less than the surface band-bending energy of ∼ 770 meV, which is believed to act as the capture barrier in this system. This finding indicates the tunneling of electrons through the top part of the band-bending barrier. Interestingly, the decay rate of photoconductivity is observed to reduce significantly when the photoconductivity in these wires is quenched by an additional sub-bandgap illumination prior to the switching off the photoexcitation. A rate equation model is proposed to explain the upward band bending at the surface as well as the persistent p...

We have investigated defects in large area monolayer MoS2 films using thermally stimulated current (TSC) spectroscopy. Films are grown on c-sapphire substrates using a microcavity based chemical vapor deposition technique. A theoretical framework to analyze TSC data for a two dimensional semiconductor has been developed. The study reveals the existence of two traps with average activation energies of 670 and 850 meV. The density of these traps shows an increase followed by saturation as the sample goes through repeated thermal cycles in vacuum. Interestingly, the density returns to its initial level when the sample is exposed to the ambient condition for a sufficiently long time, suggesting that these defects are passivated by certain adsorbate groups/molecules in the ambient condition. It has been found that annealing in a sulfur environment substantially reduces the concentration of these traps. This indicates that the traps must be related to sulfur deficiency related defects.We have investigated defects in large area monolayer MoS2 films using thermally stimulated current (TSC) spectroscopy. Films are grown on c-sapphire substrates using a microcavity based chemical vapor deposition technique. A theoretical framework to analyze TSC data for a two dimensional semiconductor has been developed. The study reveals the existence of two traps with average activation energies of 670 and 850 meV. The density of these traps shows an increase followed by saturation as the sample goes through repeated thermal cycles in vacuum. Interestingly, the density returns to its initial level when the sample is exposed to the ambient condition for a sufficiently long time, suggesting that these defects are passivated by certain adsorbate groups/molecules in the ambient condition. It has been found that annealing in a sulfur environment substantially reduces the concentration of these traps. This indicates that the traps must be related to sulfur deficiency related defects.

High quality c-axis oriented n-type ZnO epitaxial films are grown on p-type c-GaN/sapphire templates using a chemical vapor deposition technique where metallic zinc is used as the Zn source. n-ZnO/p-GaN heterojunctions were thus formed and show rectifying behaviour with a turn-on voltage of similar to 2.4 V and a very low leakage current of 1 x 10(-11) A. Study of the spectral distribution of the photo response properties of these heterostructures shows a maximum at a wavelength of 366 nm with a peak responsivity of similar to 0.4 mA/W at zero bias condition. The peak responsivity increases further with the applied forward bias and reaches similar to 191 mA/W at 1 V. The spectral profile shows a sharp reduction in responsivity for wavelengths larger than approximate to 400 nm, making these devices suitable for application in solar blind UV detection. These devices are also found to show a fast response with a rise/decay time of only a few milliseconds. The study also reveals that the photo-responsivity of these devices depends crucially on the microcrystalline quality of the ZnO layers.

ZnO nanoparticle samples, which are synthesized by a sol-gel route and collected after various stages of a washing procedure, are systematically investigated using a variety of experimental techniques. The study shows that while the surface charge polarity is positive for the as-grown (unwashed) particles, the surface turns negatively charged when the particles are sufficiently washed. This dissimilar surface charge polarity has been found to result in markedly different luminescence characteristics for the two cases (washed and unwashed). Our study of the luminescence properties under different surrounding conditions furthermore reveals that the major source for the GL emission for these nanoparticles is the hydroxyl groups, which are physisorbed on the surface. (C) 2013 Elsevier B. V. All rights reserved.

Despite a tremendous interest on molybdenum disulfide as a thinnest direct band gap semiconductor, single step synthesis of a large area purely monolayer MoS2 film has not yet been reported. Here, we report a CVD route to synthesize a continuous film of strictly monolayer MoS2 covering an area as large as a few cm(2) on a variety of different substrates without using any seeding material or any elaborate pretreatment of the substrate. This is achieved by allowing the growth to take place in the naturally formed gap between a piece of SiO2 coated Si wafer and the substrate, when the latter is placed on top of the former inside a CVD reactor. We propose a qualitative model to explain why the MoS2 films are always strictly monolayer in this method. The photoluminescence study of these monolayers shows the characteristic excitonic and trionic features associated with monolayer MoS2. In addition, a broad defect related luminescence band appears at similar to 1.7 eV. As temperature decreases, the intensity of this broad feature increases, while the band edge luminescence reduces. (C) 2016 AIP Publishing LLC.

ZnO epitaxial layers are grown on c-plane GaN (p-type)/sapphire substrates using a chemical vapor deposition technique. Structural and luminescence properties of these layers have been studied systematically as a function of various growth parameters. It has been found that high quality ZnO epitaxial layers can indeed be grown on GaN films at certain optimum conditions. It has also been observed that the growth temperature and growth time have distinctly different influences on the screw and edge dislocation densities. While the growth temperature affects the density of edge dislocations more strongly than that of screw dislocations, an increase of growth duration leads to a rapid drop in the density of screw dislocation, whereas the density of edge dislocation hardly changes. Densities of both edge and screw dislocations are found to be minimum at a growth temperature of 500 degrees C. Interestingly, the defect related visible luminescence intensity also shows a minimum at the same temperature. Our study indeed suggests that the luminescence feature is related to threading edge dislocation. A continuum percolation model, where the defects responsible for visible luminescence are considered to be formed under the influence of the strain field surrounding the threading edge dislocations, is proposed. The theory explains the observed variation of the visible luminescence intensity as a function of the concentration of the dislocations. Published by AIP Publishing.

A novel route to achieve two dimensional (2D) carrier confinement in a wedge shaped wall structure made of a polar semiconductor has been demonstrated theoretically. Tapering of the wall along the direction of the spontaneous polarization leads to the development of charges of equal polarity on the two inclined facades of the wall. Polarization induced negative (positive) charges on the facades can push the electrons (holes) inward for a n-type (p-type) material which results in the formation of a 2D electron (hole) gas at the central plane and ionized donors (acceptors) at the outer edges of the wall. The theory shows that this unique mode of 2D carrier confinement can indeed lead to a significant enhancement of carrier mobility. It has been found that the reduced dimensionality is not the only cause for the enhancement of mobility in this case. Ionized impurity scattering, which is one of the major contributer to carrier scattering, is significantly suppressed as the carriers are naturally separated from the ionized centers. A recent experimental finding of very high electron mobility in wedge shaped GaN nanowall networks has been analyzed in the light of this theoretical reckoning.

The high light-output efficiencies of In(x)Ga(1-x)N quantum-well (QW)-based light-emitting diodes (LEDs) even in presence of a large number of non-radiative recombination centers (such as dislocations) has been explained by localization of carriers in radiative potential traps, the origins of which still remain unclear. To provide insights on the highly efficient radiative traps, spectrally resolved photoluminescence (PL) microscopy has been performed on green-light-emitting In(0.22)Ga(0.78)N QW LEDs, by selectively generating carriers in the alloy layers. PL imaging shows the presence of numerous inhomogeneously distributed low-band-gap traps with diverse radiative intensities. PL spectroscopy of a statistically relevant number of individual traps reveals a clear bimodal distribution in terms of both band-gap energies and radiative recombination efficiencies, indicating the presence of two distinct classes of carrier localization centers within the same QW sample. Disparity in their relative surface coverage and photoemission "blinking" characteristics suggests that the deep traps originate from local compositional fluctuations of indium within the alloy, while the shallow traps arise from nanometer-scale thickness variations of the active layers. This is further supported by Poisson-Schrodinger self-consistent calculations and implies that radiative traps formed due to both local indium content and interface-morphology-related heterogeneities can coexist within the same QW sample.

Coherent transmission of Cooper pairs through a non-superconducting medium is a key challenge for hybrid electronics with superconductors, normal metals and semiconductors. While superconductor–normal metal–superconductor (SNS) junctions have been known for quite sometime, including a low carrier density region or a two-dimensional electron gas (2DEG) in the path of superconducting electrons is relatively less explored. Indeed, this is due to the limited choice of materials that would make ohmic contacts to such systems, while simultaneously supporting a superconducting phase. In this paper we show a coherent transmission of supercurrent through a degenerate semiconductor over a length ≈2 μm with a critical magnetic field B c ≈ 8 T at 1.6 K and T c ≈ 5 K at zero magnetic field. This length scale is much larger than the typical thickness of a Josephson junction. Our system is a fragment of a GaN nanowall network that has been shown to support a high mobility 2DEG (μ n > 104 cm2 V−1 s−1 ). The current and voltage probes were superconducting tungsten–gallium composite electrodes and the measurements could be done in four-probe geometry. We demonstrate ballistic type carrier transport with a near ideal transparency of 1 and a critical current (I c ) large enough such that the Josephson coupling parameter . Some features in magneto-transport data suggest that there is possibly a small magnetic moment forming in the semiconductor fragment. In addition the combination of a T c typical of elemental metallic superconductors, but a critical field that appears to be higher than the Clogston–Chandrasekhar limit, may be indicative of the emergence of a triplet pairing mechanism in these structures.

We report the effect of screw dislocations and oxygen vacancy defects on the optical nonlinear refraction response of ZnO films grown on a sapphire substrate at various oxygen flow rates using the chemical vapor deposition technique. The nonlinear refraction response was investigated in the off-resonant regime using a CW He-Ne laser source to examine the role of the intermediate bandgap states. It has been observed that the structural defects strongly influence the optical nonlinearity in the off-resonant regime. Nonlinearity has been found to improve as the oxygen flow rate is lowered from 2 sccm to 0.3 sccm. From photoluminescence studies, we observe that the enhanced defect density of the electronic defect levels due to the increased concentration of structural defects (with the decrease in the oxygen flow rate) is responsible for this improved optical nonlinearity along with the thermal effect. This suggests that defect engineering is an effective way to tailor the nonlinearity of ZnO films and their utility for optoelectronic device applications.

Magnetic resonance studies were preformed on the dilute magnetic semiconductor Gd:GaN. In samples grown by molecular beam epitaxy resonances were observed at temperatures below 50 K. The temperature dependence of the resonance field and the line width suggest a ferromagnetic origin. No similar signals were found in Gd implanted samples even for a Gd concentration ten times higher. At temperatures close to 5 K additional signals were observed near g = 2 in these samples. They are superimposed by a strong substrate signal which is observable in all samples and is attributed to nitrogen donors on sites of different symmetry in the SiC substrate crystal. Different possible explanations of our findings are discussed with special regard to a model suggesting magnetically polarized spheres of influence. By means of magnetic resonance we can not corroborate ferromagnetism at room temperature in Gd: GaN as suggested by SQUID data.

Network of wedge shaped ZnO nanowalls are grown on c-sapphire by Chemical Vapor Deposition (CVD) technique. Structural studies using x-ray diffraction show much better crystallinity in the nanowall sample as compared to the continuous film. Moreover, the defect related broad green luminescence is found to be suppressed in the nanowall sample. The low temperature photoluminescence study also suggests the quantum confinement of carriers in nanowall sample. Electrical studies performed on the nanowalls show higher conductivity, which has been explained in terms of the reduction of scattering cross-section as a result of 1D quantum confinement of carriers on the tip of the nanowalls.

The depth distribution of the transport properties as well as the temperature dependence of the low field magneto-conductance for several c-axis oriented GaN nanowall network samples grown with different average wall-widths are investigated. Magneto-conductance recorded at low temperatures shows clear signature of weak localization effect in all nanowall samples studied here. The scattering mean free path and the phase coherence time, are extracted from the magneto-conductance profile. Electron mobility estimated from scattering mean free path is found to be comparable with those estimated previously from room temperature conductivity data for these samples [Appl. Phys. Lett. 101, 132109 (2012); AIP Conf. Proc. 1583, 252 (2014)], confirming independently the substantial mobility enhancement in these nanowalls as compared to bulk. Our study furthermore reveals that the high electron mobility region extends down to several hundreds of nanometer below the tip of the walls. Like mobility, phase coherence length is found to increase with the reduction of the average wall width. Interestingly, for samples with lower values of the average wall width, phase coherence length is estimated to be as high as 60 micron, which is much larger than those reported for GaN/AlGaN heterostructure based two dimensional electron gas (2DEG) systems., Comment: 15 pages, 4 Figures. arXiv admin note: text overlap with arXiv:1410.1295

We have studied by X-ray absorption spectroscopy the local environment of Mn in highly homogeneous Ga 1-x Mn x N (0.06

We present an experimental investigation on temperature, excitation intensity and spectral dependence of persistent photoconductivity (PPC) in GaN. A grain boundary induced potential barrier is predicted to be responsible for PPC. The non-exponential nature of the PPC decay has been explained by a Gaussian distribution of capture barriers, arising from the trapped charges at the grain boundary interface. The spectral dependence of the PPC suggests the origin of PPC and the yellow luminescence band may arise from the same intrinsic defect.

We investigate the structural and optical properties of M-plane GaN(1100) films grown on LiAlO2(100) with nucleation layers grown at high and low temperatures. Samples with a high temperature nucleation layer are found to exhibit a highly anisotropic surface morphology with pronounced corrugation, which basically replicates the surface morphology of the substrate. Photoluminescence spectra of these layers are dominated by a transition at 3.356 eV, which is absent for samples with a low-temperature nucleation layer. In conjunction with scanning electron microscopy, cathodoluminescence maps reveal that this transition predominantly stems from regions below the trenches of the surface corrugation. Transmission electron microscopy shows an abundance of stacking faults within these regions. Excitation-dependent and time-resolved photoluminescence demonstrates the intrinsic character of the 3.356 eV emission, which is thus attributed to excitons bound to stacking faults acting as ultrathin vertical quantum wel...

Experimental and theoretical studies of temperature and doping dependencies of electron mobility in 6H-SiC are reported. Low field electron mobility has been calculated by iterative technique and compared with experimental data. The following scattering mechanisms, i.e., impurity, polar optical phonon, acoustic phonon, and intervalley optical phonon are included in the calculation. Ionized and neutral impurity scattering have been treated beyond the Born approximation using a phase-shift analysis. We have found that neutral impurity scattering is very important in 6H-SiC due to large donor activation energy. Acceptor concentration is used as a parameter for fitting experimental data. Comparisons with published as well as our experimental data confirm the present calculation over a wide range of temperatures and electron concentrations.

Density and energetic distributions of interface states between metal-semiconductor rectifying contacts in sub-micron GaAs MESFET and AlGaAs/InGaAs pseudomorphic high electron mobility transistors (HEMT's) have been studied. Electrical properties of the interface states between gate metal and semiconductor in sub-micron devices depend on growth technique, associated processing parameters and surface states on III-V semiconductors. Correlation between nonideal current-voltage (I-V) characteristics and interface states has been established through the bias dependence of ideality factor. Ideality factor determined from I-V characteristics of MESFET and HEMT increases with bias and then decreases after reaching a maximum. A theoretical model based on nonequilibrium approach has been used to determine the density of interface states and their energetic distribution from ideality factor. Essentially, Fermi level shifts with applied bias and Schottky barrier height changes due to trapping and detrapping of electrons by the interface states, and from these changes, density of interface states and their energetic distributions have been determined.

We present a systematic study of the growth, structural, magnetic and magneto-optical properties of GaN:Gd layers grown directly on 6H-SiC(0001) substrates by reactive molecular-beam epitaxy with a Gd concentration ranging from 7 × 1015 to 2 × 1019 cm−3. The structural properties of these layers are found to be identical to those of undoped GaN layers. However, the magnetic characterization reveals an unprecedented effect. The average value of the magnetic moment per Gd atom is found to be as high as 4000 µB as compared to its atomic moment of 8 µB. This colossal magnetic moment can be explained in terms of a long-range spin polarization of the GaN matrix by the Gd atoms which is also reflected by the circular polarization of the excitonic emission in magneto-photoluminescence measurements. Moreover, the material system is found to exhibit ferromagnetism well above room temperature in the entire concentration range under investigation. We propose a phenomenological model to understand the macroscopic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd. This model also provides a natural explanation for the coexistence of two ferromagnetic phases observed in this material. The microscopic origin for the longrange spin polarization remains unclear at the moment, but recent studies in the literature strongly suggest that native point defects play a crucial role for this phenomenon.