Zinc oxide (ZnO) single crystal films and periodic structures have potential applications that include energy saving light emitting diodes (LEDs), transparent electrodes for today?s multi-billion LED industry based on GaN alloys, and dye-sensitized solar cells. Due to its relatively high index of refraction (n = 2.05, lambda= 500 nm), and the ability to produce 2D and 3D periodic structures, ZnO has become an important candidate for photonic crystal applications that confine and direct the light used for optical communication. As an optical-electronic material, it has a wide band gap (~3.4 eV), an excitonic binding energy twice that of GaN, a high saturation velocity, a high radiation hardness, and a high optical transparency. In fact, most of these properties of ZnO are either similar or superior to those of GaN, which today is the material used for LEDs that has and will light our world at significant energy reduction. As a wide band gap semiconductor, ZnO faces the same technical hurdle as GaN prior to the innovations made by Shuji Nakamura in the early 1990?s. Due to advantages in properties, raw material availability and cost, ZnO is expected to displace GaN in the multi-billion dollar solid-state lighting industry once this technical hurdle is overcome The research will involve undergraduate interns and a graduate student. Emphasis will be placed on selecting underrepresented students.
TECHNICAL DETAILS: ZnO epitaxial films will be synthesized, under steady-state, equilibrium conditions, in a low temperature (≤ 90°C) continuous, aqueous reactor that has been constructed based on thermodynamic calculations and the retrograde solubility of ZnO. (JJ Richardson and FF Lange, ?Controlling Low Temperature Aqueous Synthesis of ZnO Part I and II,? Crystal Growth & Design, 9 [6], 2570-81 (2009)). The continuous aqueous reactor has also been used to synthesize periodic 2D and 3D nanostructures, with dimensions equivalent to the wavelength of blue to red light, with potential applications as photonic crystals. The research emphasizes the optimization of synthesis parameters, namely, pH, temperature, ammonia concentration, and additions of rate controlling growth agents. The primary goal is to use the continuous reactor to understand the point defects that produce unintentional n-type conductivity, the dislocations at low angle grain boundaries introduced by the coalescence of epitaxial nuclei, and void formation observed during heat treatments at ≥ 250°C. This understanding will, in turn, be used to optimize the optoelectronic properties of aqueous synthesized ZnO so that significant steps can be taken towards band gap engineering and intentional n- and p-type doping.
Heteroepitaxial ZnO transparent current spreading layers with low sheet resistances were deposited on GaN-based blue light emitting diodes using aqueous solution phase epitaxy at temperatures below 90°C. The performance of the light emitting diodes was analyzed and compared to identical devices using electron-beam evaporated indium tin oxide transparent current spreading layers. The devices with ZnO layers provided better current spreading, lower voltages and had higher external quantum efficiency than the devices with ITO layers. A white light emitting diode fabricated using an aqueous solution deposited ZnO current spreading layer displayed a 40% improvement in luminous efficacy over a device using ITO when operated at a current density of 35A/cm2. White LEDs with ZnO layers provided high luminous efficacy–157 lm/W at 0.5A/cm2, and 84.8 lm/W at 35A/cm2, 24% and 50% higher, respectively, than devices with ITO layers. The improvement appears to be due to enhanced current spreading provided by the low 26Ω/? sheet resistance of the ZnO, versus 91Ω/? for ITO.This demonstrates that ZnO current spreading layers could be a viable indium-free alternative to ITO in the context of GaN LEDs. Conclusion GaN LEDs with solution-deposited ZnO and electron-beam evaporated ITO current-spreading layers were fabricated. It was found that the operating voltage, EQE and current spreading were improved in the devices with ZnO current spreading layers, compared to those with ITO. This was primarily due to the lower sheet resistance of the thicker ZnO layers, which allowed current to be spread and injected into the active region over a larger area, while at the same time allowing more light to escape the LED die
