Oxide Based optical and Electronic Devices
 
Band-Edge Electroluminescence From N+-Implanted Bulk ZnO LEDs

ZnO system has some advantages because of the high exciton binding energy of ZnO relative to GaN, the availability of high quality ZnO substrates (enabling the fabrication of vertical geometry devices with low threading dislocation densities) and the simpler processing relative to GaN for which convenient wet etches are not available.

Ion implantation is an attractive process for low-cost, high throughput device manufacturing and in this work we showed that N+ implantation into bulk single-crystal ZnO substrates can be used to achieve bandedge electroluminescence (EL) in simple diode structures. The samples were (0001) undoped grade I quality bulk, single-crystal ZnO crystals. Ion implantation was performed at 300K with N+ ions of energy 5 keV (dose of 1.5 × 1013 cm-2), 20 keV (dose of 5 × 1013 cm-2) plus 50 keV (dose of 1.3×1014 cm-2) and 130 keV (dose of 3.5 × 1014 cm-2), followed by rapid thermal annealing (RTA)for 2 mins under a flowing oxygen ambient.
Figure Caption: A schematic of ZnO MIS diode formed by N+ implantation into a bulk single crystal substrate.
I-V characteristics and forward bias current dependence of integrated EL intensity measured by a Si photodiode from a structure annealed at 800°C showed as an diode characteristics. The device has an apparent threshold of about 4.5V and the forward current above this threshold is limited by a series resistance of about 25 O. The EL intensity increases almost linearly with drive current above threshold. The diodes emit a yellowish light due to the dominance of the deep level emission. We would expect a more uniform emission if we add a transparent conducting layer on the implanted layer to obtain improved current spreading.
Figure Caption: EL spectrum of N+ implanted LED
 
Room Temperature Deposited Indium Zinc Oxide Thin-Film Transistors
The deposition of amorphous indium zinc oxides films by co-sputtering from In2O3 and ZnO targets near room temperature was investigated as a function of power, process pressure and oxygen partial pressures in the sputtering ambient. The main influence on the resistivity of the films is found to be the oxygen partial pressure during deposition. The amorphous films exhibit carrier mobilities in the range 5-20 cm2V-1s-1.
Figure Caption: Drain IV characteristics of a depletion mode IZO FET.
 
Measurement of Zn0.95Cd0.050/ZnO (0001) heterojunction band offsets by x-ray photoelectron spectroscopy

Zinc oxide (ZnO) is an attractive candidate for UV light emission since it is an environmentally friendly material which be grown at low temperatures on cheap transparent substrates and has both a direct wide band gap of 3.3 eV and a very large exciton binding energy of 60meV,important for robust light emission. These properties make ZnO light emitting diodes (LEDs) potentially useful in efficient solid state lighting where white light can be achieved by pumping of an appropriate polymer overlayer, as is used in GaN white LEDs. In addition, it has been suggested that semiconducting nanowires may offer additional advantages for light emission due to the increased junction area, reduced temperature sensitivity, enhanced polarization dependence of reflectivity and improved carrier confinement in 1-D nanostructures.

Ternary ZnCdO seems to be a good candidate for the narrow bandgap active region for the ZnO or ZnMgO based LED because of the smaller bandgap of CdO (2.3 eV). In designing LED structures in this material system, there is a need to have available such basic information as the valence and conduction band offsets. To date, little is known for the ZnCdO/ZnO system, although the bandgap energies have been reported as a function of Cd composition. We report an X-Ray Photoelectron Spectroscopy (XPS) study of the valence band offset (?EV) in a Zn0.95Cd0.050/ZnO (0001)

heterojunction. The XPS spectrometer was calibrated using a polycrystalline Au foil. The XPS Zn 2p3 narrow scan and valence band spectrum from the 0.1 µm ZnCdO/0.1 µmZnO/MOCVD GaN/C-plane sapphire as well as ZnO substrate samples were taken using a pass energy of 11.75 eV and step size of 0.025 eV. The valence band value (EV) was determined by linearly fitting the leading edge of the valence band and linearly fitting the flat energy distribution. Core level survey spectra of ZnCdO, 1nm layer of ZnCdO on ZnO, and a ZnO substrate were also taken with a pass energy of 187.85 eV at take-off angle of 65o. These values were then inserted into the following equations to calculate ?Ev, namely ?Ev = (EZn-2p-EV)ZnO-( EZn-2p-EV)thick

Figure Caption: (Left) AlGaN/GaN diode based Hydrogen sensorintegrated with a wireless transmitter. (center) Hydrogen sening system including a transmitter and a receiver. (right) A wireless receiver connected with a laptop.

 
DNA Sensing
Au-gated AlGaN/GaN HEMTs functionalized in the gate region with label free 3'-thiol modified oligonucleotides, which serves as a binding layer to the AlGaN surface can detect the hybridization of matched target DNAs. XPS shows that immobilization of thiol modified DNA covalently bonded with gold on the gated region. Hybridization between probe DNA and matched or mismatched target DNA on the Au-gated HEMT was detected. The HEMT drain-source current showed a clear decrease of 115 µA as this matched target DNA was introduced to the probe DNA on the surface, showing the promise of the DNA sequence detection for biological sensing.
Figure Caption: High resolution Ga 3s and S 2p3 XPS peaks before and after thiol-modification of Au coated GaN samples
 
Protein Sensing
Ungated AlGaN/GaN High Electron Mobility Transistor structures were functionalized in the gate region with aminopropyl silane, which served as a binding layer to the AlGaN surface for attachment of biotin. Biotin has very high affinity to streptavidin proteins. Each time the chemicals attached to the AlGaN/GaN HEMT the charges on the attached chemicals affected the HEMT drain-source current. The HEMT showed a clear decrease of 4 µA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing.
Figure Caption: A cross sectional view of the AlGaN/GaN HEMT protein Sensor.