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.
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.
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.
Al2O3 films were deposited on the Zn face of ZnO (0001) substrates as a transition layer by atomic layer
deposition (ALD). The as-deposited 20 and 50nm Al2O3 films were transformed to polycrystalline α-Al2O3 phase
after optimal annealing at 1100°C after 10 and 20 minutes, respectively, as identified by high resolution x-ray
diffraction (HRXRD). Furthermore, GaN and InGaN layers were grown on annealed 20 and 50nm Al2O3 deposited
ZnO substrates by metalorganic chemical vapor deposition (MOCVD) using NH3 as a nitrogen source at high growth
temperature. Wurtzite GaN was only seen on the 20nm Al2O3/ZnO substrates. Room temperature
photoluminescence (RT-PL) shows the near band-edge emission of GaN red-shifted, which might be from oxygen
incorporation forming a shallow donor-related level in GaN. Raman scattering also indicated the presence of a wellcrystallized
GaN layer on the 20nm Al2O3/ZnO substrate. InGaN was grown on bare ZnO as well as Al2O3
deposited ZnO substrates. HRXRD measurements revealed that the thin Al2O3 layer after annealing was an effective
transition layer for the InGaN films grown epitaxially on ZnO substrates. Auger Electron Spectroscopy (AES)
atomic depth profile shows a decrease in Zn in the InGaN layer. Moreover, (0002) InGaN layers were successfully
grown on 20nm Al2O3/ZnO substrates after 10min annealing in a high temperature furnace.
ZnO has been considered as a substrate for epitaxial growth of III-Nitrides due to its close lattice and stacking order
match. This paper will cover growth of InxGa1-xN epitaxial layer on lattice-matched ZnO substrates by metal-organic
chemical vapor deposition (MOCVD). InGaN of various indium compositions from different growth temperatures were
well controlled in the InGaN films on ZnO substrates. High-resolution X-ray diffraction (HRXRD) confirmed the
epitaxial growth of InGaN film on ZnO. The optical and structural characterization of InGaN epilayer on ZnO substrates
was measured by room temperature photoluminescence, temperature-dependent photoluminescence, and field-emission
secondary electron microscope. In addition, a transition layer of Al2O3 on ZnO substrates have been employed for
InGaN growth to help prevent Zn and O diffusion into the epilayers as well as assist nitride epilayer growth. HRXRD
results show a single crystal InGaN film has been successfully grown on annealed Al2O3 coated ZnO substrates.
Light emitting diodes (LEDs) based on indium gallium nitride (InGaN) have been used widely in lighting
applications and have shown great potential with high wall-plug efficiency and low manufacturing cost. However,
one of the roadblocks in achieving higher light-extraction efficiency is total internal reflection, which prevents a
majority of the photons from being extracted from the LEDs.
The objective of this paper is to investigate measures for improving LED chip efficiency. Here, a high efficiency
LED structure has been theoretically demonstrated using an infiltrated opal layer. A layer of indium-tin oxide (ITO)
infiltrated opals was deposited on the surface of the LED chips as the light extraction layer in order to enhance the
light extraction efficiency. Opals of similar radius periodically arranged on the structure will work as a rough
surface. By enhancing the light extraction angle, the spontaneous emission from the active region can be emitted
vertically. At the same time, opals with different radius and different refractive indexes are also studied. ITO, as the
infiltration, is used to provide good lateral current spreading.
The plane wave expansion (PWE), finite difference time domain (FDTD), and rigorous coupled wave analysis
(RCWA) methods are used to calculate the energy band structures and the extraction efficiency of the infiltrated
opals layer. The scanning method and the Monte Carlo method are employed to optimize the structural parameters of
the infiltrated opal layer. The infiltrated opals layer on the LED shows a high light extraction efficiency of over
60%.
The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) grown on sapphire by metalorganic
chemical vapor deposition (MOCVD) have been investigated by optical measurements of temperature-dependent
photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL), and
structural analysis methods of high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron
microscopy (HRTEM). Two typical samples are studied, both consisting of five periods of InGaN wells with 1.6 nm
thickness with different indium compositions of 21 and 24%, respectively. The thickness of the GaN barrier was 7.9
nm. According to the PL and PLE measurement results, large values of activation energy and Stokes' shift are obtained.
This indicates that higher In composition results in the increase of composition fluctuation in the InGaN MQW region,
showing the stronger carrier localization effect. The lifetime at the low-energy side of the InGaN peaks is longer for
higher indium composition, as expected from the larger Stokes shift.
In this work, ZnO has been investigated as a substrate technology for GaN-based devices due to
its close lattice match, stacking order match, and similar thermal expansion coefficient. Since
MOCVD is the dominant growth technology for GaN-based materials and devices, there is a need
to more fully explore this technique for ZnO substrates. Our aim is to grow low defect density
GaN for efficient phosphor free white emitters. However, there are a number of issues that need
to be addressed for the MOCVD growth of GaN on ZnO. The thermal stability of the ZnO
substrate, out-diffusion of Zn from the ZnO into the GaN, and H2 back etching into the substrate
can cause growth of poor quality GaN. Cracks and pinholes were seen in the epilayers, leading to
the epi-layer peeling off in some instances. These issues were addressed by the use of H2 free
growth and multiple buffer layers to remove the cracking and reduce the pinholes allowing for a
high quality GaN growth on ZnO substrate.
Wide bandgap nitrides and oxides have been heralded as a possible platform for future semiconductor spintronics applications based on the inherent compatibility of these materials with existing semiconductors as well as theoretical predictions of room temperature ferromagnetism. Experimental reports of room temperature ferromagnetism in these materials are complicated by disparate crystalline quality and phase purity in these materials, as well as
conflicting theoretical predictions as to the nature of ferromagnetic behavior in this system. A complete understanding of these materials, and ultimately intelligent design of spintronic devices, will require an exploration of the relationship between the processing techniques, resulting transition metal atom configuration, defects, and electronic compensation as related to the structure, magnetic, and magneto-optical properties of this material. This work explores the growth and properties of Ga1-xMnxN films by metalorganic chemical vapor deposition on cplane sapphire substrates with varying thickness, Mn concentration, and alloying elements. Homogenous Mn incorporation throughout the films was verified with Secondary Ion Mass Spectroscopy (SIMS), and no macroscopic second phases were detected using X-ray diffraction (XRD). SQUID and vibrating sample magnetometry measurements showed an apparent room temperature ferromagnetic hysteresis, whose strength can be altered considerably through annealing and introduction of either Si or Mg during the growth process. Three sets of Raman modes appeared to be sensitive to Mn incorporation. The intensities of a broad band around 300cm-1 and sharper modes near 669cm-1 increased with increasing Mn concentration. The rise of the former is attributed to a decrease in long-range lattice ordering for higher Mn concentration. The second mode is due to nitrogen vacancy-related local vibrational modes of the GaN host lattice. Si co-doped Ga1-xMnxN results in shallow donor states in GaN suppress the formation of nitrogen vacancies by compensating the p-type deep level defects introduced by substitutional Mn. The formation of a Mn-related midgap impurity band is observed via optical transmission measurement in Ga1-xMnxN with strong magnetic signatures, but not for Si co-doped samples. Initial studies on light emitting diodes (LEDs) containing a Mn-doped active region have also been produced. Devices were fabricated with different Mn-doped active layer thicknesses, and I-V characteristics show that the devices become highly resistive as thickness of the Mn-doped active layer increases. The electroluminescence of these devices is dominated by a high suppressed band-edge recombination and a midgap defect-related emission, leading to an orange-colored but weakly emitting LED. These results suggest that traditional theoretical and device approaches akin to those realized in Ga1-xMnxN may be difficult to realize in Ga1-xMnxN, and exploitation of these materials will require further novel device approaches taking into account the nature of this material.
Intentionally doped n-type bulk ZnO has been grown by patented melt technique at Cermet and was used as a substrate for homo-epitaxial growth of p-type ZnO films. The n-type ZnO has a carrier concentration on the order of 1018cm-3 with a mobility of 113cm2/Vs, which is good for optical devices. Secondary ion mass spectroscopy (SIMS) profile shows a very uniform distribution of n-type dopant in the ZnO. Excellent transmission from the sharp absorption edge through the visible portion of the spectrum indicates that as grown n-type ZnO is perfect for any optical device applications. P-type ZnO thin films were successfully grown by MOCVD technique on n-type ZnO substrate to form ZnO based p-n junction structure. Cadmium and magnesium doped ZnO films were also grown by MOCVD and resulted in tunable bad gap energy of ZnO based alloy. Ohmic contact layer on n-type ZnO was formed by using Ti/Au and on p-type ZnO was formed by using Ni/Au. The current-voltage (I-V) characteristics of the ZnO based p-n junction exhibited rectification when reverse biased with a breakdown voltage of 10 V and turn-on voltage of 3.3 V. Post anneal of p-type ZnO films showed big improvement on the I-V characteristics. Electroluminescence (EL) spectra obtained from devices driven to 40mA are dominated by a peak at 384nm.
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