Band gap engineering provides an opportunity to not only provide higher overall conversion efficiencies of the reference AM1.5 spectra but also customize PV device design for specific geographic locations and microenvironments based on... more
Band gap engineering provides an opportunity to not only provide higher overall conversion efficiencies of the reference AM1.5 spectra but also customize PV device design for specific geographic locations and microenvironments based on atmospheric conditions characteristic to that particular location. Indium gallium nitride and other PV materials offer the opportunity for limited bandgap engineering to match spectra. The effects of atmospheric conditions such as aerosols, cloud cover, water vapor, and air mass have been shown to cause variations in spectral radiance that alters PV system performance due to both overrating and underrating. Designing PV devices optimized for spectral radiance of a particular region can result in improved PV system performance. This paper presents a new method for designing geographically optimized PV cells with using a numerical model for bandgap optimization. The geographic microclimate spectrally resolved solar flux for twelve representative atmospheric conditions for the incident radiation angle (zenith angle) of 48.1° and fixed array angle of 40° is used to iteratively optimize the band gap for tandem, triple, and quad-layer of InGaN-based multijunction cells. The results of this method are illustrated for the case study of solar farms in the New York region and discussed.
We report on the nonlinear optical absorption of InN/InxGa1-xN (x=0.8,0.9) multiple-quantum-well structures characterized at 1.55 μm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the... more
We report on the nonlinear optical absorption of InN/InxGa1-xN (x=0.8,0.9) multiple-quantum-well structures characterized at 1.55 μm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the samples at high repetition rate. Saturable absorption is observed for the sample with x=0.9, with an effective α2~-9×103 cm/GW for the studied optical regime. For lower In content in the barrier, reverse saturable absorption is observed, which is attributed to two-photon absorption.
The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found... more
The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence
emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a
model that the “green gap” problem might relate to localized strain relaxation occurring through defects.
Laser engraving is an advanced technology in material processing that involves removal of material layer by layer. The aim of this research paper is to obtain digitally designed patterns and pictures on materials using InGaN (indium... more
Laser engraving is an advanced technology in material processing that involves removal of material layer by layer. The aim of this research paper is to obtain digitally designed patterns and pictures on materials using InGaN (indium gallium nitride) laser module and to measure certain characteristic features of this advance laser technology. A novel tabletop CNC laser milling machine is specially built up to for using the laser engraving. The time taken to complete the engraving operation is measured using different techniques for different resolution in the design. The adaption of internet of things and Industry 4.0 featured a low-cost CNC laser engraver is special useful for learners to explore further research on the laser field.
The quantum efficiency of GaN-based light-emitting diodes (LEDs) is investigated at temperatures 77–300 K. It is found that the efficiency droop is due to a decrease in the internal quantum efficiency (IQE) in the low-energy part of the... more
The quantum efficiency of GaN-based light-emitting diodes (LEDs) is investigated at temperatures 77–300 K. It is found that the efficiency droop is due to a decrease in the internal quantum efficiency (IQE) in the low-energy part of the emission spectrum. The efficiency starts to decrease at a temperature independent forward voltage of Umax≈2.9 V. At this voltage tunneling current through the LED-structure begins to dominate. It is suggested that the external quantum efficiency droop is related to reduction of the IQE due to tunneling leakage of carriers from the quantum well (QW) to defect states in barriers, and to reduction of the injection efficiency by excess tunneling current under QW through deep defect states in barriers.
A breakthrough in the development of 4D scanning ultrafast electron microscopy is described for real-time and space imaging of secondary electron energy loss and carrier diffusion on the surface of an array of nanowires as a model system,... more
A breakthrough in the development of 4D scanning ultrafast electron microscopy is described for real-time and space imaging of secondary electron energy loss and carrier diffusion on the surface of an array of nanowires as a model system, providing access to a territory that is beyond the reach of either static electron imaging or any time-resolved laser spectroscopy.
The results of studying the influence of the finite tunneling transparency of injection barriers in light-emitting diodes with InGaN/GaN quantum wells on the dependences of the current, capacitance, and quantum efficiency on the p-n... more
The results of studying the influence of the finite tunneling transparency of injection barriers in light-emitting diodes with InGaN/GaN quantum wells on the dependences of the current, capacitance, and quantum efficiency on the p-n junction voltage and temperature are presented. It is shown that defectassisted hopping tunneling is the main transport mechanism through the space charge region (SCR) and makes it possible to lower the injection barrier. It is shown that, in the case of high hopping conductivity through the injection barrier, the tunnel-injection current into InGaN band-tail states is limited only by carrier diffusion from neutral regions and is characterized by a close-to-unity ideality factor, which provides the highest quantum and power efficiencies. An increase in the hopping conductivity through the space charge region with increasing frequency, forward bias, or temperature has a decisive effect on the capacitance-voltage characteristics and temperature dependences of the high-frequency capacitance and quantum efficiency. An increase in the density of InGaN/GaN band-tail states and in the hopping conductivity of injection barriers is necessary to provide the high-level tunnel injection and close-to-unity power efficiency of high-power light-emitting diodes.
The quantum efficiency of GaN LED structures has been studied at various temperatures and biases. It was found that an efficiency falloff is observed with increasing current density and, simultaneously, the tunnel component of the current... more
The quantum efficiency of GaN LED structures has been studied at various temperatures and biases. It was found that an efficiency falloff is observed with increasing current density and, simultaneously, the tunnel component of the current through the LED grows and the quasi-Fermi levels reach the mobility edge in the InGaN active layer. It is shown that the internal quantum efficiency falloff with increasing current density is due to the carrier leakage from the quantum well as a result of tunnel transitions from its band-tail states to local defect-related energy levels within the energy gaps of the barrier layers.
Synchrotron radiation has been utilized for x-ray diffraction and reciprocal space mapping of InGaN / GaN multiple-quantum-well MQW structures grown on the sidewalls of 10-m-wide triangular GaN ridges with 1−1.1 facets. Samples were... more
Synchrotron radiation has been utilized for x-ray diffraction and reciprocal space mapping of InGaN / GaN multiple-quantum-well MQW structures grown on the sidewalls of 10-m-wide triangular GaN ridges with 1−1.1 facets. Samples were produced by lateral overgrowth through a patterned dielectric mask by using metal-organic vapor-phase epitaxy. Global MQW strain, period, and the tilt of the 00. 1 crystallographic planes have been measured across the sidewall facets using a 240 nm x-ray beam. Results of this study are interpreted in terms of suppressed intrafacet migration of In and Ga precursors during the MQW growth.
We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures T_S , shows that there is a strong tendency of GaN to form a separate phase as T_S is increased... more
We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures T_S , shows that there is a strong tendency of GaN to form a separate phase as T_S is increased from 600C to 650C. Concomitant with the phase separation, the PL from the InGaN phase broadens, which indicates that indium composition in this phase becomes increasingly non-uniform. Indium compositions measured by Rutherford backscattering (RBS) are consistent with these results. We also observed an increase in PL intensity for InGaN layers grown at higher temperatures. In this paper, we also report on preparing a top-contact InGaN/GaN light emitting diode. The device was operated at 447 nm and had the emission line width of 37 nm with no observable impurity related features. The turn-on voltage was 3.0 V. The output power was 20 mW at 60 mA drive current.
quantum wells (QWs) against (0001) and (112̅0). In incorporation, growth rate and the critical thickness of (112̅2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112̅0) is reduced by a factor of three. A small... more
quantum wells (QWs) against (0001) and (112̅0). In incorporation, growth rate and the critical thickness of (112̅2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112̅0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112̅0). The slight deviation of (112̅2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112̅2) and (0001) while (112̅0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4´ lower on (112̅2) and more than 10´ lower on (112̅0).
The structural properties of InGaN/GaN multiple quantum wells (MQW) were studied using synchrotron based high resolution x-ray diffraction (HRXRD). MQW structures were grown on the top and sidewall facets of triangular and trapezoidal... more
The structural properties of InGaN/GaN multiple quantum wells (MQW) were studied using synchrotron based high resolution x-ray diffraction (HRXRD). MQW structures were grown on the top and sidewall facets of triangular and trapezoidal shaped GaN ridges by metalorganic vapour phase epitaxy (MOVPE) in the regime of selective area growth (SAG). Period and strain variations as a function of oxide mask width were determined for both the sidewall and the top facet growth. Oxide mask widths ranged between 2 and 20 μm with openings between adjacent masks of 4 and 6 μm. Analysis of the x-ray diffraction curves revealed a sidewall / vertical growth rate ratio of ~0.3 through a comparison of the top to sidewall facet MQW periods. Masks orientated along the <11-20> crystallographic direction showed stronger growth enhancement along with large global strain for MQW growth on the top (0001) plane. Interpreting our results within the framework of vapour phase diffusion revealed that inter-facet migration of group-III species needs to be taken into account.
We report the metal organic chemical vapor deposition growth of dislocation-free 100 nm thick hexagonal InGaN nanopyramid arrays with up to 33% of indium content by nano-selective area growth on patterned AlN/Si (111) substrates. InGaN... more
We report the metal organic chemical vapor deposition growth of dislocation-free 100 nm thick hexagonal InGaN nanopyramid arrays with up to 33% of indium content by nano-selective area growth on patterned AlN/Si (111) substrates. InGaN grown on SiO2 patterned templates exhibit high selectivity. Their single crystal structure is confirmed by scanning transmission electron microscope combined with an energy dispersive X-ray analysis, which also reveals the absence of threading dislocations in the InGaN nanopyramids due to elastic strain relaxation mechanisms. Cathodoluminescence measurements on a single InGaN nanopyramid clearly show an improvement of the optical properties when compared to planar InGaN grown under the same conditions. The good structural, morphological, and optical quality of the InGaN nanostructures grown on AlN/Si indicates that the nano-selective area growth technology is attractive for the realization of site-controlled indium-rich InGaN nanostructure-based devices and can also be transfer ed to other highly mismatched substrates.
M anaging trap states and understanding their role in ultrafast charge-carrier dynamics, particularly at surface and interfaces, remains a major bottleneck preventing further advancements and commercial exploitation of nanowire (NW)-based... more
M anaging trap states and understanding their role in ultrafast charge-carrier dynamics, particularly at surface and interfaces, remains a major bottleneck preventing further advancements and commercial exploitation of nanowire (NW)-based devices. A key challenge is to selectively map such ultrafast dynamical processes on the surfaces of NWs, a capability so far out of reach of time-resolved laser techniques. Selective mapping of surface dynamics in real space and time can only be achieved by applying four-dimensional scanning ultrafast electron microscopy (4D S-UEM). Charge carrier dynamics are spatially and temporally visualized on the surface of InGaN NW arrays before and after surface passivation with octadecylthiol (ODT). The time-resolved secondary electron images clearly demonstrate that carrier recombination on the NW surface is signifi cantly slowed down after ODT treatment. This observation is fully supported by enhancement of the performance of the light emitting device. Direct observation of surface dynamics provides a profound understanding of the photophysical mechanisms on materials’ surfaces and enables the formulation of effective surface trap state management strategies for the next generation of high-performance NW-based optoelectronic devices.
Laser engraving is an advanced technology in material processing that involves removal of material layer by layer. The aim of this research paper is to obtain digitally designed patterns and pictures on materials using InGaN (indium... more
Laser engraving is an advanced technology in material processing that involves removal of material layer by layer. The aim of this research paper is to obtain digitally designed patterns and pictures on materials using InGaN (indium gallium nitride) laser module and to measure certain characteristic features of this advance laser technology. A novel tabletop CNC laser milling machine is specially built up to for using the laser engraving. The time taken to complete the engraving operation is measured using different techniques for different resolution in the design. The adaption of internet of things and Industry 4.0 featured a low-cost CNC laser engraver is special useful for learners to explore further research on the laser field.
This paper is an effort to analyze the performance of InGaN/GaN solar cell. InGaN/GaN solar cell contains p and n-type layer of GaN and intrinsic layer of InGaN. The proposed structure of solar cell also contains front TCO, back TCO and... more
This paper is an effort to analyze the performance of InGaN/GaN solar cell. InGaN/GaN solar cell contains p and n-type layer of GaN and intrinsic layer of InGaN. The proposed structure of solar cell also contains front TCO, back TCO and back reflector layers. This paper represents the mechanism of designing and analysis of InGaN/GaN solar cell. Performance of the designed solar cell was checked based on electric field, current density and electric potential generated in the designed cell. J-V curve is the most important factor to analyze the performance of the solar cell. Power conversion efficiency (PCE) and fill factors have been calculated from this graph. According to the analysis of the simulation results PCE (η) of the designed solar cell is 12.91% with Jsc of 19.3 mA/cm 2 and Voc of 0.78 V. At the end, this research work is about designing successfully with an efficient InGaN/GaN solar cell for further use in solar applications.
—A study of the current and capacitance dependences on the forward voltage in Au/n-GaN Schottky diodes, the sub-band optical absorption spectra, and the defect photoluminescence in n-GaN bulk crystals and thin layers is reported. It is... more
—A study of the current and capacitance dependences on the forward voltage in Au/n-GaN Schottky diodes, the sub-band optical absorption spectra, and the defect photoluminescence in n-GaN bulk crystals and thin layers is reported. It is shown that defect-assisted tunneling is the dominant transport mechanism for forward-biased Schottky contacts on n-GaN. The dependences of the current and capacitance on forward bias reflect the energy spectrum of defects in the band gap of n-GaN: the rise in the density of deep states responsible for yellow photoluminescence in GaN with increasing energy and the steep exponential tail of states with an Urbach energy of E U = 50 meV near the conduction-band edge. A decrease in the frequency of electron hops near the Au/n-GaN interface results in a wide distribution of local dielectric relaxation times and in a dramatic transformation of the electric-field distribution in the space-charge region under forward biases.
III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by... more
III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar {1–100} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar {1–100} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.
We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and... more
We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and spectroscopy, transmission electron microscopy), and its morphological optical and structural properties directly correlated. We achieved this across an ensemble of defects large enough to be statistically significant. Our results provide evidence that carrier localization occurs in the direct vicinity of the dislocation through the enhanced formation of In-N chains and atomic condensates, thus limiting non-radiative recombination of carriers at the dislocation core. We highlight that the localization properties in the vicinity of threading dislocations arise as a consequence of the strain field of the individual dislocation and the additional strain field building between interacting neighboring dislocations. Our study therefore suggests that careful strain and dislocation distribution engineering may further improve the resilience of InGaNbased devices to threading dislocations. Besides providing a new understanding of dislocations in InGaN, this paper presents a proof-of-concept for a methodology which is relevant to many problems in materials science.
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Applied Physics and may be found at http://dx.doi.org/10.1063/1.4973278. Copyright 2017. This article is distributed under a Creative Commons Attribution (CC BY) License.
Nanodots, nanowires, and semi-polar quantum well structures of GaN-based material have been grown by nano-selective area growth (NSAG). The growth evolution of the nanostructure has been studied. Cross-sectional transmission electron... more
Nanodots, nanowires, and semi-polar quantum well structures of GaN-based material have been grown by nano-selective area growth (NSAG). The growth evolution of the nanostructure has been studied. Cross-sectional transmission electron microscopy (TEM) shows that the nanostructures are free of threading dislocations. The growth of AlGaN/GaN layers is uniform and shows sharp interfaces between the AlGaN and GaN epilayers. AlGaN nanodots/nanowires, which are formed at the apexes of the nano-pyramids/nano-ridges, are found to be homogeneous in size and to have a higher aluminum mole fraction than the surrounding material. In contrast, the InGaN/GaN growth shows no quantum dots at the apexes of the nanostructures. We found that the growth facets of different Miller's indices are formed on the InGaN/GaN nano-ridges. Energy dispersive X-ray spectroscopy (EDX) shows higher indium incorporation at the intersection of the growth facets. Cathodoluminescence measurements show enhanced luminescence intensity from InGaN multi-quantum wells (MQWs) grown on the nanostructure compared to that from InGaN MQWs grown on an unpatterned area.
InGaN structures epitaxially grown on a-plane or m-plane GaN exhibit in-plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of... more
InGaN structures epitaxially grown on a-plane or m-plane GaN exhibit in-plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of linear polarization for m-plane than a-plane InGaN. Using density functional theory, we study the response of InGaN random alloys to finite biaxial strains on both nonpolar planes. The calculated m-plane InGaN valence band splitting is larger than that of the a-plane, due to a greater degree of structural relaxation in a-plane InGaN. We provide a parametrization of the valence band splitting of InGaN strained to a-plane and m-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a-plane and m-plane polarization.
Indium gallium nitride films with nanocolumnar microstructure were deposited with varying indium content and substrate temperatures using plasma-enhanced evaporation on amorphous SiO2 substrates. FESEM and XRD results are presented,... more
Indium gallium nitride films with nanocolumnar microstructure were deposited with varying indium content and substrate temperatures using plasma-enhanced evaporation on amorphous SiO2 substrates. FESEM and XRD results are presented, showing that more crystalline nanocolumnar microstructures can be engineered at lower indium compositions. Nanocolumn diameter and packing factor (void fraction) was found to be highly dependent on substrate temperature, with thinner and more closely packed nanocolumns observed at lower substrate temperatures.