GaN -based light-emitting diodes (LEDs) with truncated micropyramids surfaces performed by metalorganic chemical vapor deposition were demonstrated. In this study, a growth-interruption step and an Mg-treatment process were simultaneously performed to create multiple truncated micropyramids on LED surface. Experimental results indicated that GaN-based LEDs with the truncated micropyramids on the top surface demonstrate improved external efficiency of around 60% at 20mA. It is worth noting that the typical 20mA driven forward voltage is only 0.15V higher than that of conventional LEDs (LEDs with specular surface).

Ultraviolet Al0.17Ga0.83N∕GaN-based heterojunction phototransistors (HPTs) grown by metal-organic vapor-phase epitaxy were demonstrated. The HPTs showed a bandpass spectral responsivity ranging from 280to390nm. With a bias voltage of 6V, the responsivity at an incident of 340nm was as high as 1500A∕W, corresponding to a quantum gain of 5.47×103. In contrast to GaN-based photoconductors, the HPTs also featured high contrast in spectral response. With a bias voltage of 3V, the spectral response showed high rejection ratios of approximately 4×105 and 1×104 for the long-wavelength side (340∕400nm) and the short-wavelength side, respectively. The high contrast of spectral response for the long-wavelength side could be due to the long trapping time of holes blocked by the base-emitter heterojunction and the low defect-to-band response that is caused by defect-related gap states.

In this study, Al-doped ZnO ͑AZO͒ and Ni/AZO films were deposited on p-type GaN films followed by thermal annealing to form ohmic contacts. After thermal annealing, the resistivities of AZO films reduced from 5 ϫ 10 −3 to 4-6 ϫ 10 −4 ⍀ cm. Both as-deposited AZO and Ni/AZO contacts on p-GaN displayed a non-ohmic characteristic. Only the 800°C-annealed Ni/AZO contacts exhibited a linear current-voltage characteristic, showing a specific contact resistance of around 1.2 ϫ 10 −2 ⍀ cm 2 . After undergoing the annealing in nitrogen ambience, the light transmittance of the Ni/AZO films increased from 70% to higher than 90% in the visible range. These results revealed that the Ni/AZO contact can serve as a suitable transparent current spreading layer for the fabrication of GaN-based light-emitting devices.Reliable and transparent contact layers are essential elements used in the development of high-brightness, GaN-based light emitting diodes ͑LEDs͒. Hitherto, bilayer metallization, such as Ni/Au, 1 and Pt/Au, 2 have been the subject of extensive research and investigation. In order to enhance the light-extraction efficiency, the Ni/Au thin film serving as anode electrode is commonly annealed in oxygen-containing ambient to form a low-resistivity and semitransparent ohmic contact to p-GaN. 3 In general, the transmittance of such semitransparent contacts is only around 60-75% in the visible region and lower than 60% in the ultraviolet ͑UV͒ region. Thus, in order to further improve the light-extraction efficiency of LEDs, high-transparency and low-resistivity top contact layer, such as indium tin oxide ͑ITO͒, 4,5 is necessary. However, the ITO films in direct contact with the p-GaN layer, showed a non-ohmic currentvoltage ͑I-V͒ behavior even after thermal annealing. 5 In order to eliminate this behavior, a Ni layer was inserted between the ITO film and p-GaN layer to improve electrical properties. 6 Aluminumdoped zinc oxide ͑AZO͒ is a well-known wide-bandgap material. It has good potential for utilization in transparent contact layers. Song et al. 7 reported that Ni/AZO bilayer contacts deposited onto p-type GaN followed by thermal annealing in ambient air could possess linear I-V characteristics, but the light transmittance was only about 76% with wavelengths in the range of 400-550 nm. Jung et al. revealed that significant increase in resistivity could be observed in the O 2 ambient annealed AZO films. 8 The increase could be attributed to the reduction of oxygen vacancies ͑V O ͒. 8 The increased resistivity can enhance the current crowding effect when utilized as p-type contacts of GaN-based LEDs. In this study, the AZO and Ni/AZO schemes were deposited on p-GaN followed by nitrogen ambient thermal annealing to form high-transparency and lowresistivity ohmic contacts. ExperimentalThe wafers used in this study were all grown on c-face sapphire ͑0001͒ substrates by metallorganic vapor phase epitaxy ͑MOVPE͒. The Mg-doped GaN films with a thickness of 1 m were grown on a 2-m-thick GaN template followed by a N 2 -ambient th...

In this study, indium tin oxide (ITO) or ITO/ZnO films, which were prepared by magnetron sputtering, were deposited onto p-GaN epitaxial films to form ultraviolet photodetectors (PDs). The ITO/ZnO/p-GaN heterojunction PDs behave like the p-i-n photodiodes, which characteristically exhibit low dark current, as opposed to the ITO/p-GaN PDs, which exhibit a marked bias-dependent dark current. The zero-bias rejection ratio can be improved up to 4×105 due to a further reduction in the dark current compared to the ITO/p-GaN PDs. When the incident wavelength is 360 nm, the ITO/ZnO/p-GaN heterojunction PD exhibits a zero-bias photocurrent/dark current ratio and a responsivity of approximately 8×104 and 0.015 A/W, respectively.

Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical (PEC) cells made of Mn-doped GaN photoelectrodes. In addition to its absorption of the ultraviolet spectrum, Mn-doped GaN photoelectrodes could absorb photons in the visible spectrum. The photocurrents measured from PEC cells made of Mn-doped GaN were at least one order higher than those measured from PEC cells made of undoped GaN-working electrodes. Under the visible light illumination and a bias voltage below 1.2 V, the Mn-doped GaN photoelectrodes could drive the water splitting reaction for hydrogen generation. However, hydrogen generation could not be achieved under the same condition wherein undoped GaN photoelectrodes were used. According to the results of the spectral responses and transmission spectra obtained from the experimental photoelectrodes, the enhanced photocurrent in the Mn-doped GaN photoelectrodes, compared with the undoped GaN photoelectrodes, was attributable to the Mn-related intermediate band within the band gap of GaN that resulted in further photon absorption.