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.
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.
Recent theoretical predictions of ferromagnetic behavior in transition metal (TM)-doped ZnO have focused significant
attention on these materials for use as spintronic materials. Moreover, rare earth (RE) elements in wide bandgap
semiconductors would be useful not only in spintronics but also in optoelectronic applications. This work presents
results obtained from an investigation into the optical, magnetic, and structural properties of transition-metal (TM)-
doped ZnO and rare earth (RE) doped ZnO (TM = Mn, Co, Ni, and Fe; RE = Gd, Eu, and Tb) bulk crystals and thin
films. Properties of TM- and RE-doped ZnO bulk crystals and thin films were studied and compared in order to better
understand the nature of these dopant centers and their effects on the properties of the host crystal. Optical properties
confirm the incorporation of substitutional transition metal ions on cation sites. While most thin film samples show
ferromagnetic behavior, the magnetic response of the bulk crystals varies. This suggests that the magnetic behavior of
TM-doped ZnO is highly dependent on growth conditions, and growth conditions which favor the formation of grain
boundaries and interfaces may be more likely to result in ferromagnetic behavior. Origins of this ferromagnetic behavior
are still under investigation. Defect luminescence observed in the RE-doped samples suggests that these materials may
prove useful in optoelectonic applications as well.
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.
ZnO and N-doped ZnO thin films were grown by MOCVD on sapphire and ZnO substrates. Diethyl zinc and O2 were used as sources for Zn and O, respectively. A specially designed plasma system was employed to produce atomic N dopant for in-situ doping. Proper disk rotation speeds were found for ZnO growth on different size wafers. High crystal quality N-doped ZnO films were grown based on optimized growth conditions. Wet chemical etch of ZnO was investigated by using NH4Cl, and etch activation energy was calculated to be 463meV. Ohmic contact on N-doped ZnO film was achieved by using Ni/Au/Al multiple layers. ZnO based p-n junction has demonstrated rectification. Electroluminescence at about 384nm was obtained from ZnO based LED.
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.
This paper investigates the properties of homoepitaxial growth of ZnO. High quality bulk ZnO crystals have been produced by melt growth techniques in addition to ZnO thin films grown by Metalorganic Chemical Vapor Deposition (MOCVD) on bulk ZnO substrates (Zinc side and Oxygen side). The photoluminescence showed the dominance of strong and narrow band due to the band edge emissions for undoped ZnO. UV transmission showed sharp transition indication good crystal quality. High resolution x-ray diffraction measurements (HRXRD) along with rocking curve showed excellent crystal quality with full width at half maximum values close to 100 arc seconds.
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