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 H<sub>2</sub> 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 H<sub>2</sub> 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 O<sub>2</sub> 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 NH<sub>4</sub>Cl, 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 10<sup>18</sup>cm<sup>-3</sup> with a mobility of 113cm<sup>2</sup>/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.