Wide band gap dilute magnetic semiconductors have recently been of interest due to theoretical predictions of room
temperature ferromagnetism in these materials. In this work Ga1-xGdxN thin films were grown by Metalorganic
Chemical Vapor Deposition. These films were found to be ferromagnetic at room temperature and electrically
conducting. However, only GaN:Gd layers and devices grown with a TMHD3Gd precursor that contained oxygen
showed strong ferromagnetism, while materials grown with an oxygen-free Cp3Gd precursor did not show ferromagnetic behavior. This experimental observation was consistent with first-principles calculations based on density functional theory calculations that we completed that showed the ferromagnetism was mediated by interstitial oxygen. The results confirmed the first successful realization of Ga1-xGdxN-based spin-polarized LED with 14.6% degree of polarization at 5000 Gauss is obtained.
The influence of structural and optoelectronic properties of InN epilayers on the duration of initial nucleation
has been studied. High pressure chemical vapor deposition (HPCVD) has been utilized to deposit InN epilayers on
GaN/sapphire (0001) templates at a reactor pressure of 15 bar. The initial nucleation period was varied between 10 s and 60 s, leaving all other growth parameters constant. The structural properties of the grown samples have been investigated by X-ray diffraction (XRD) spectroscopy and Raman spectroscopy. The optoelectronic properties were analyzed by Fourier transform infra-red (FTIR) spectroscopy. The layer thickness, free carrier concentration and void fraction were obtained by simulating IR spectra, using multi-layer stack model for epilayers and Lorentz-Drude model for dielectric function. Raman, X-ray diffraction (XRD) and void fraction calculation results suggest that the optimum nucleation time is between 10 - 20 s. However, simulation results revealed that the free carrier concentration of the bulk layer does not show any significant dependency on the duration of initial nucleation.
The dependency of the structural and optoelectronic properties of InN thin films grown by high-pressure chemical
vapor deposition technique on the group V/III molar precursor ratio has been studied. X-ray diffraction, Raman
spectroscopy, and IR reflectance spectroscopy have been utilized to study local- and long-range structural ordering as
well as optoelectronic properties of the InN epilayers grown on crystalline sapphire substrates. The investigated InN
epilayers were grown with group V/III molar precursor ratio varying from 900 to 3600, while all other growth
parameters were kept constant. For a group V/III precursor ratio of 2400, the full width-half maximum of the Raman
E2(high) mode and XRD (0002) Bragg reflex exhibit minimums of 7.53 cm⁻¹ and 210 arcsec, respectively, with
maximized grain size and reduced in-plane strain effect. FTIR data analysis reveals a growth rate of 120 nm/hr, a carrier
mobility of 1020 cm²V⁻¹s⁻¹, and a free carrier concentration of 1.7×1018 cm⁻³ for a V/III ratio of 2400. The Raman
analysis indicate that non-polar E2(high) mode position remains unaffected from a changing V/III ratio; whereas, polar
A1(LO) mode position significantly changes with changing V/III ratio. Optical analysis also suggests that LO-phonon
correlates with free carrier concentration (ne) and TO-phonon correlates with free carrier mobility (μ) in the InN
epilayers.
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.
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.
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.
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.
GaN epilayers and AlGaN/GaN superlattice structures have been deposited on (0001) ZnO substrates by
metalorganic vapor phase epitaxy (MOVPE) using GaN and AlN buffer layers. The growth conditions were first
optimized on GaN templates using N2 as carrier gas at relatively low temperature (<800 °C), which is suitable for
GaN growth on a ZnO substrate. Experimental results show that high interfacial quality can be achieved in the
superlattice by using TMIn as a surfactant. The optimized growth conditions were subsequently transferred to ZnO
substrates. The influence of growth temperature on the material quality was studied. A proper growth temperature for
both GaN cover layer and AlGaN/GaN superlattice can improve the structural and optical properties of the structures
on ZnO. This improvement is verified using x-ray diffraction, atomic force microscopy and photoluminescence
characterizations. The growth temperature must be chosen with these two factors in mind, with too low a growth
temperature leading to poor quality material and too high a temperature causing reactions at the GaN/ZnO interface
that degrade quality. AlN buffer layers on ZnO were also studied to increase subsequent GaN epilayer quality.
Effects of buffer layer growth conditions on optical and structural quality were studied.
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