Jheng-Tai Yan

Mechanisms of high quality i-ZnO thin films deposition at low temperature by vapor cooling condensation technique

A comparative mechanism investigation on the structural and optoelectronic properties of i-ZnO thin films, deposited on the silicon substrates at various temperatures were conducted. The experimental results verified that the i-ZnO films deposited at a low temperature have better quality over the conventional high temperature deposited ones. This low temperature deposition by using vapor cooling condensation technique has been successfully used to fabricate optoelectronic devices, such as UV light-emitting diodes and UV photodetectors. The mechanisms responsible for the fact that the low temperature deposited samples had better quality were analyzed in terms of the adsorption/desorption and diffusion of ZnO particles in the growth process.A comparative mechanism investigation on the structural and optoelectronic properties of i-ZnO thin films, deposited on the silicon substrates at various temperatures were conducted. The experimental results verified that the i-ZnO films deposited at a low temperature have better quality over the conventional high temperature deposited ones. This low temperature deposition by using vapor cooling condensation technique has been successfully used to fabricate optoelectronic devices, such as UV light-emitting diodes and UV photodetectors. The mechanisms responsible for the fact that the low temperature deposited samples had better quality were analyzed in terms of the adsorption/desorption and diffusion of ZnO particles in the growth process.

Emission mechanisms of passivated single n-ZnO:In/i-ZnO/p-GaN-heterostructured nanorod light-emitting diodes

The single n-ZnO:In/i-ZnO/p-GaN-heterostructured n-i-p nanorod was deposited using a vapor cooling condensation system. The photoelectrochemical system was used to directly passivate the nanorod sidewall surface with a Zn(OH)2 layer. The electrical performance of the passivated and unpassivated single nanorod was measured using a conductive atomic force microscopy. The resulting nanorod light-emitting diodes were investigated for understanding the relevant light emission mechanisms. Since the nonradiative recombination centers, native defects, and dangling bonds existed on the nanorod sidewall surface were effectively passivated, the resultant surface leakage current was reduced and the near-band emission intensity of the nanorod light-emitting diode was increased accordingly.

Ultraviolet ZnO Nanorod/P-GaN-Heterostructured Light-Emitting Diodes

Both i-ZnO and n-ZnO : In nanorod arrays were grown on a p-GaN layer with an anodic alumina membrane template using a vapor cooling condensation method. Electroluminescence emissions were observed from the resulting p-n (p-GaN/n-ZnO : In nanorod array) and p-i-n (p-GaN/i-ZnO nanorod array/n-ZnO : In nanorod array) heterostructured light-emitting diodes (LEDs). The ultraviolet emission peak at 386 nm observed in the p-i-n heterostructured LEDs was attributed to radiative recombination of the near-band edge in the i-ZnO nanorods. Using power-law fitted current-voltage relationships, it was shown that a space-charge-limited current and associated effects occurred in the p-n and p-i-n nanorod heterostructured LEDs.

Ultraviolet Electroluminescence From ZnO-Based n-i-p Light-Emitting Diodes

The p-type ZnO films were obtained using codeposition of ZnO and sources by the vapor cooling condensation system. With an adequate postannealing temperature, the p-type conductive behaviors of the resulting ZnO films could be achieved due to the activation of Li-N dual acceptor. According to the emission energy of free electron to acceptor hole level, the acceptor binding energy of 137 meV was obtained. Furthermore, the ZnO-based n-i-p ultraviolet light-emitting diodes were deposited on sapphire substrates using the vapor cooling condensation system. A rectifying diode-like behavior and ultraviolet emission were observed from the ZnO-based n-i-p light-emitting diodes.

Investigation of a Metal―Insulator―Semiconductor Pt/Mixed Al2O3 and Ga2O3 Insulator/AlGaN Hydrogen Sensor

Pt/mixed Al 2 O 3 and Ga 2 O 3 reactive insulator/AlGaN metal―insulator―semiconductor (MIS)-type hydrogen sensors were fabricated using a photoelectrochemical oxidation method to directly oxidize AlGaN employed for increasing the sensor response to hydrogen gas. When hydrogen gas was sensed, the MIS hydrogen sensors exhibited a larger reduction of the barrier height and series resistances than that of the Pt/AlGaN metal―semiconductor (MS)-type hydrogen sensors. As a result, the MIS hydrogen sensors could detect the hydrogen concentration lower than 100 ppm H 2 /air while the MS hydrogen sensors could not detect such low hydrogen concentration. This provides a promising potential candidate of using a mixed Al 2 O 3 and Ga 2 O 3 insulator for AlGaN-based hydrogen gas sensors.

Light enhancement of Al nanoclusters embedded in Al-doped ZnO films of GaN-based light-emitting diodes

The aluminum (Al)-doped ZnO (AZO) films embedded with Al nanoclusters were employed to enhance the light output power of III-nitride-based light-emitting diodes (LEDs). The ZnO and Al targets were sputtered using a magnetron cosputtering system. Al nanoclusters embedded in AZO films were found in the AZO films deposited with Al dc power of 10W and ZnO rf power of 100W using high resolution transmission electron microscopy. An increase of 20% in the light output power of the GaN-based LEDs with AZO films embedded with Al nanoclusters can be obtained compared to the conventional LEDs operated at 500mA.

AZO films with Al nano-particles to improve the light extraction efficiency of GaN-based light-emitting diodes

The co-sputtering Al-doped ZnO (AZO) films with Al nano-particles were used to increase the extraction efficiency of GaN-based light-emitting diodes (LEDs). Fixing the ZnO radio frequency (RF) power of 100W and changing the Al DC power from 0 to 13W, the AZO films with various Al contents can be obtained. In the experimental results, the AZO films deposited with Al DC power of 0, 4.5 and 7W do not have Al segregation. However, the segregated Al nano-particles can be found in the AZO films deposited by Al DC power of 10W and 13W. The co-sputtering 170 nm-thick AZO films with and without Al nano-particles were deposited on the transparent area of LEDs and compared the light output intensity of conventional LEDs. The light intensity of LEDs with AZO films with Al DC power 0, 4.5 and 7W increased 10% than that of conventional LEDs. This was due to the AZO film played a role of anti-reflection coating (ARC) layer. The light intensity of LEDs with AZO film deposited using Al DC power of 10W and 13W increased about 35% and 30%, respectively. It can be deduced that the output light is scattered by the Al nano-particles existed in the AZO film.

Improved hydrogen detection sensitivity of a Pt/Ga2O3/GaN diode

The Pt/Ga2O3/GaN diodes were fabricated in which the Ga2O3 oxide layers were directly grown on GaN layer using a photoelectrochemical method. Then, the Ga2O3 oxide films were annealed in O2 ambiance at 700 °C for 2 hours to perform the β-Ga2O3 crystalline phases. The hydrogen sensing characteristics of Pt/GaN (metal-semiconductor, MS) and Pt/β-Ga2O3/GaN (metal-insulator-semiconductor, MIS) diodes under hydrogen-containing ambiance were studied in an air atmosphere. Compared with the MS devices, the MIS devices exhibited better hydrogen sensing ability. The result demonstrates that the β-Ga2O3 layer plays an important role in the hydrogen sensing of the GaN based MIS diodes.