Recent advances in mid-ultraviolet light-emitting diodes grown pseudomorphically on bulk AlN substrates have led to improved efficiencies and lifetimes. For a 266 nm device an output power of 66 mW at 300 mA has been achieved with an external quantum efficiency of 4.5%. More importantly, the lifetimes of these devices have been increased substantially. Testing of LEDs in both surface mount design (SMD) and TO-39 packages show L50 lifetimes well in excess of 1,000 hours under a variety of case temperatures and currents. Package-related catastrophic failures are eliminated through encapsulation and hermetic sealing, further reducing failure rates and extending the lifetime.
Thomas Edison is widely regarded as the greatest inventor in history and the most prominent individual behind the invention of the electric light. His impressive characteristics as an individual that led to his amazing success as an innovator continue to be an inspiration for researchers today. This paper considers how Edison might proceed in developing solid state lighting into a technology capable of displacing incumbent light sources, including his own incandescent lamps, then reviews some of the "Edison-like" contributions made to solid state lighting by the Next Generation Lighting research program at Georgia Tech.
Results on the achievable growth temperature as a function of the reactor pressure for the growth of InN by high-pressure CVD are presented. As the reactor pressure was increased from 1 bar to 19 bar, the optimal growth temperature raised from 759°C to 876°C, an increase of 6.6 °C/bar. The InN layers were grown in a horizontal flow channel reactor, using a pulsed precursor injection scheme. The structural and optical properties of the epilayers have been investigated by Raman spectroscopy, X-ray diffraction, and IR reflectance spectroscopy.
High quality InGaN-based LEDs have been grown on Si (111) substrates using an Al2O3 transition layer. Freestanding,
fabricated LED devices were achieved by removing the Si substrate using selective area wet etching.
Conventional device design was used for LED fabrication, in which p-type and n-type contacts are located at the same
side of the epilayers. These LED devices were bonded to a dual in-line package (DIP), and epoxy was used to protect the
front side of the epilayers thin films as well as the bonding wires. The Si substrate was removed by wet etching while the
chip was mounted in the DIP which prevented the thin film from cracking or warping.
Electroluminescence (EL) characteristics of the LED devices grown on ALD/Si were measured before and after
substrate removal. No significant change in peak emission wavelength was observed, nor any change in EL intensity
versus drive current. No degradation of electrical and optical properties was observed. This indicates that the devices
were not damaged by the wet etching process. However, the luminescence intensity of devices both before and after wet
etching did not increase beyond a drive current of ~60 mA due to inefficient heat dissipation. The process developed
and the challenges involved in the larger area substrate removal process will be discussed which could be substantially
beneficial to the future substrate transfer and packaging in the industrial fabrication of LED on silicon substrate.
KEYWORDS: Solar cells, Indium gallium nitride, Gallium nitride, Absorption, Tandem solar cells, Sapphire, Polishing, Indium, Metalorganic chemical vapor deposition, Scattering
In this work InGa0.85N p-n homojunction solar cells were grown by MOCVD on GaN/sapphire substrates and fabricated
using standard techniques. When illuminated from the backside, these devices showed 65.9% improvement in JSC and
4.4% improvement in VOC as compared to identical illumination from the front. These improvements arise from removal
of the losses from electrical contact shading on the front of the devices (11.7% of active area), as well as significant
optical absorption by the top current spreading layer. These improvements can likely be further enhanced by utilizing
double-side polished wafers, which would eliminate scattering losses on the back surface. In addition to improving
electrical characteristics of single cells, backside illumination is necessary for the realization of monolithic tandem
InGaN solar cells.
ZnO is a promising substrate for GaN growth due to a lattice match with In.18Ga.82N, similar thermal expansion
coefficient, and its ability to be easily chemically etched, which results in improved light extraction. A transition layer of
Al2O3 was also grown by ALD prior to MOCVD growth to prevent Zn diffusion, protect the ZnO substrate from H2 back
etching, and promote high quality nitride growth. Thick InGaN layers (~200-300nm) were grown in this study on bare
ZnO substrates and ALD/ZnO substrates. Various buffer layers were attempted, such as SLs of AlGaN/GaN, MQWs of
InGaN/GaN, and LT-GaN. These results are significant as previous studies showed decomposition of the layer at InGaN
thicknesses of 100nm or less. These layers allowed for the first LEDs to be grown on bare ZnO substrates. This study
demonstrated that InGaN LEDs showed emission in optical measurements as well as a high IQE of ~60%. The data
shows promise for LED structures on ZnO using InGaN as n- and p-type LED layers. Etching of the ZnO substrate also
showed that removal of the substrate can be performed easily.
The optical and structural properties of InN layers grown by 'High Pressure Chemical Vapor
Deposition' (HPCVD) using a pulsed precursor approach have been studied. The study focuses on
the effect of ammonia precursor exposure time and magnitude on the InN layer quality. The samples
have been analyzed by X-ray diffraction, Raman scattering, infra red reflectance spectroscopy and
photoluminescence spectroscopy. Raman measurements and X-ray diffraction showed the grown
layers to be single phase InN of high crystalline quality. The E2(high) Raman mode showed
FWHM's as small as 9.2 cm-1. The FWHM's of the InN(0002) X-ray Bragg reflex in the 2Θ-Ω-
scans were around 350 arcsec, with rocking curve values as low as 1152 arcsec Photoluminescence
features have been observed down to 0.7 eV, where the low energy cutoff might be due to the
detector limitation. The analysis of the IR reflectance spectra shows that the free carrier
concentrations are as low as as 3.3•1018 cm-3 for InN layers grown on sapphire substrates.
Device-quality GaN thin films have been grown on Si(111) substrates using an Al2O3 transition layer, and initial
devices show performance similar to comparable devices on sapphire. X-ray diffraction rocking curve scans show a
linewidth of 378 arcsec for the GaN (0002) reflection. Comparison of these layers to GaN layers grown on bare Si
substrates shows a significant reduction in strain with the use of the Al2O3 transition layer. Raman spectroscopy
measurements verify this reduction in strain, as shown by the shift of the GaN E2(high) with variations in Al2O3 layer thickness. GaN-based devices were also grown and fabricated using this process. Devices on Si showed an IQE of
~32%, which is comparable to the ~37% observed for similar devices on sapphire. The devices on Si also showed better
efficiency at high current densities compared to the devices on sapphire, despite the longer peak emission wavelength on
Si, which may be due to a difference in thermal conductivity between Si and sapphire. A growth process has been developed for high-quality GaN on Si, and initial device results show that Si is a viable substrate technology for MOCVD growth of GaN-based devices.
The growths of droplet-free narrow-bandgap InN semiconductors on Ga-polar and N-polar GaN templates on c-plane sapphire substrates were performed by pulsed-MOVPE growth techniques. Under the optimum In-polar InN growth conditions, the carrier mobility and n-type carrier concentration were measured as 681 cm2/(V.sec) and 1.5×1019 cm-3, respectively. The room-temperature photoluminescence measurements of optimized In-polar grown by pulsed-MOVPE technique resulted in peak wavelength at 0.76 eV. The growth of N-polar InN grown on the N-polar GaN template is discussed and compared to that of the In-polar InN.
Al2O3 layers have been deposited by atomic layer deposition (ALD) on both silicon and zinc oxide (ZnO) substrates
as a transition layer for MOCVD growth of GaN. These Al2O3 layers have been shown to reduce tensile strain and
cracking in GaN thin films on Si, and they have also been shown to help suppress impurity diffusion from the ZnO
substrate into the GaN layers. Surface morphology of the ALD-grown layers was investigated using scanning electron
microscopy (SEM), and structural properties were studied using high resolution x-ray diffraction (HR-XRD). GaN thin
films were then grown on these layers to determine the effects of the Al2O3 layer on subsequent GaN quality. The
optical and structural properties of these films were studied, as well as surface morphology. GaN layers grown using the
Al2O3 layers on Si in particular exhibit structural and optical properties approaching those of typical GaN thin films on
sapphire, which shows significant promise for high performance GaN-based devices on Si substrates.
Relatively intense deep-green/yellow photoluminescence emission at ~600 nm is observed for InGaN/GaN multi quantum well (MQW) structures grown on bulk AlN substrates, demonstrating the potential to extend commercial III-Nitride LED technology to longer wavelengths. Optical spectroscopy has been performed on InGaN MQWs with an estimated In concentration of greater than 50% grown by metalorganic chemical vapor phase epitaxy at 750oC. Temperature- and power-dependence, time-resolved photoluminescence as well as spatially resolved cathodoluminescence measurements and transmission electron microscopy have been applied to understand and elucidate the nature of the mechanism responsible for radiative recombination at 600nm as well as higher energy emission band observed in the samples. A comparison between samples grown on bulk AlN and sapphire substrates indicate a lower degree of compositional and/or thickness fluctuation in the latter case. Our results indicate the presence of alloy compositional fluctuation in the active region despite the lower strain expected in the structure contrary to that of low In composition active regions deposited on bulk GaN substrates. Transient photoluminescence measurements signify a stretched exponential followed by a power decay to best fit the luminescence decay indicative of carrier hopping in the active region. Our results point to the fact that at such high In composition (>30%) InGaN compositional fluctuation is still a dominant effect despite lower strain at the substrate-epi interface.
It is of great technological importance to develop high quality III-Nitride layers and optoelectronic devices on Si substrates due to its low cost and wide availability as well as use of the highly matured Si microtechnology. Here we report on a novel scheme of substrate engineering to obtain high quality GaN layers on Si substrates. An ion implanted defective layer is formed in the substrate that partially isolates the III-Nitride layer from Si substrate and helps to reduce the strain in the film. The experimental results show substantial decrease in crack density, indicative of high interfacial tensile strain, with an average increase in the crack separation of 190 μm with crack free regions of 0.18 mm2 for a 2 μm thick GaN film. The optical quality and strain reduction in GaN film show strong dependence on the implantation conditions and the thickness of buffer layer. Moreover the GaN film grown on implanted AlN/Si substrate has better optical properties as compared to non implanted AlN/Si. In this paper we will show how the above mentioned scheme can resolve the issues related to cracks and dislocation density in the film that are detrimental to GaN based optoelectronic devices.
The growth of violet light emitting diodes (LEDs) was optimized using a statistical design of experiment (DOE) approach and several important interaction effects were found. The DOEs studied the effect of several variables on the well layer, the barrier layer, and the pAlGaN cladding layer. These variables included the gallium flow rate, the indium flow rate, the growth temperature, and the growth time for the well layer, the ammonia flow in the active region, the barrier growth time, and the Si doping of the barrier, as well as the growth time, growth temperature and Mg doping of the pAlGaN cladding layer. The LEDs were optimized based on combinations of several responses from photoluminescence and electroluminescence measurements. An overall process desirability was obtained, based on achieving the desired wavelength and maximizing the PL intensity and optical output power. Significant interactions between variables played a major role in the optimization of optical output power as well the emission wavelength. The understanding of these interactions led to the optimization of the LEDs both by improvements in the structure and improvements in the quality of the layers. Several of the interactions will be explained based on kinetic models of GaN growth by MOCVD.
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