C. J. Kao

GaN metal-semiconductor-metal photodetectors with low-temperature-GaN cap layers and ITO metal contacts

Nitride-based metal-semiconductor-metal (MSM) photodetectors (PDs) with low-temperature (LT) gallium nitride (GaN) cap layers and indium-tin-oxide (ITO) metal contacts were successfully fabricated. It was found that we could achieve three orders of magnitude smaller dark current by the introduction of the LT-GaN layer. For the PDs with LT-GaN cap layers, the maximum responsivity at 350 nm was found to be 0.1 and 0.9 A/W when the device was biased at 1 and 5 V, respectively. Operation speed of PDs with LT-GaN cap layers was also found to be faster than that of conventional PDs without LT-GaN cap layers.

GaN Schottky barrier photodetectors with a low-temperature GaN cap layer

By using organometallic vapor phase epitaxy, we have prepared i-GaN/low-temperature (LT) GaN/Ni/Au (sample A) and i-GaN/Ni/Au (sample B) Schottky barrier UV photodiodes (PDs). It was found that we could significantly reduce the leakage current and achieve a much larger photocurrent to dark current contrast ratio by introducing a LT GaN on top of the conventional nitride-based UV PDs. With incident light wavelength of 350 nm and a −1 V reverse bias, it was found that the measured responsivity was around 0.1 and 0.37 A/W for samples A and B, respectively. Furthermore, it was found that the operation speed of sample A is slower than that of sample B due to the highly resistive LT–GaN layer induced large RC time constant.

Fabrication of Hybrid n-ZnMgO/n-ZnO/p-AlGaN/p-GaN Light-Emitting Diodes

We report on the fabrication of UV light-emitting diodes (LEDs) based on a p–n junction n-ZnMgO/n-ZnO/ p-AlGaN/ p-GaN semiconductor triple-heterostructure. Radio-frequency plasma-assisted molecular beam epitaxy was used to grow the complete heterostructure on p-AlGaN/ p-GaN c-plane sapphire templates. Cross-sectional transmission electron microscopy showed single-crystal quality of the pseudomorphically grown ZnO active region of the device. The LEDs were fabricated by a process involving both wet and dry etching. Electroluminescence emission most likely associated with ZnO excitonic transition was observed up to 370°C. The results show the potential of ZnO-based materials for UV emitters of potentially lower cost and with comparable or higher emission intensity than comparable AlGaN/GaN devices.

Planar GaN n(+)-p photodetectors formed by Si implantation into p-GaN

GaN n+–p junction diodes were fabricated by implanting Si atoms into p-GaN. It was shown that we could use these diodes as GaN-based planar photodetectors. It was found that the dark current density of the diodes was around 1.5 μA/cm2 and 50 nA/cm2 at reverse biases of 3 and 1 V, respectively. Spectra response measurements revealed a cutoff wavelength at around 365 nm and a peak responsivity of 0.33 mA/W at 365 nm for the GaN planar n+–p photodetectors. It was also found that the visible rejection ratio was around 260. Furthermore, temporal response measurements revealed that the fall times of these GaN planar n+–p photodetectors were found to be shorter than 0.4 μs.

Characterization of GaN Schottky barrier photodetectors with a low-temperature GaN cap layer

By using organometallic vapor phase epitaxy we have prepared i-GaN/low temperature (LT) GaN/Ni/Au (sample A) and i-GaN/Ni/Au (sample B) Schottky barrier ultraviolet (UV) photodiodes (PDs). It was found that we could significantly reduce leakage current and achieve a much larger photocurrent to dark current contrast ratio by introducing a LT GaN on top of the conventional nitride-based UV PDs. With an incident light wavelength of 350 nm and a −1 V reverse bias, it was found that the measured responsivity was around 0.1 and 0.37 A/W for samples A and B, respectively. Furthermore, it was found that the operation speed of sample A is slower than that of sample B due to the highly resistive LT GaN layer induced large RC time constant.

Characterization of Si implants in p-type GaN

Si ion implantation into p-type GaN followed by rapid thermal annealing (RTA) in N/sub 2/ has been performed. X-ray diffraction analyses indicate that ion-implanted damage remains even with 1050/spl deg/C, 60 s RTA. By varying implantation and postimplantation annealing conditions, we could convert carrier concentration from p-type 3 /spl times/ 10/sup 17/ cm/sup -3/ into n-type 2 /spl times/ 10/sup 17/ cm/sup -3/ /spl sim/2 /spl times/ 10/sup 19/ cm/sup -3/. It was found that typical activation energies of Si implants in p-GaN are lower than 10 meV. Such activation energies are smaller than those observed from epitaxially grown Si-doped GaN films. A deep donor level with activation energy of 60 meV was also found from some samples. Photoluminescence studies show that the peak appears at 372 nm might be related to implantation-induced defects. It was also found that a green emission band could be observed from Si-implanted GaN. It was shown that such a green emission is related to the yellow band observed from epitaxially grown Si-doped GaN. The transport properties of these Si-implanted samples were also studied.

Comparison of ZnO metal–oxide–semiconductor field effect transistor and metal–semiconductor field effect transistor structures grown on sapphire by pulsed laser deposition

ZnO thin film field effect transistors with 1.5–20μm gate width were fabricated using either a metal gate [metal–semiconductor field effect transistor (MESFET)] or a metal–oxide–semiconductor (MOS) gate. In both cases we found that use of a thick (∼0.8–0.9μm) ZnO buffer was necessary on the sapphire or glass substrate prior to growing the active layers in order to reduce gate leakage current. Source/drain contacts of e-beam deposited Ti∕Al∕Pt∕Au showed specific contact resistances of 2.18×10−6Ωcm2 without annealing and the interdevice isolation currents were ∼10μA at 40V bias. The MOS structure with 50nm (Ce,Tb)MgAl11O19 gate dielectric showed a 1 order of magnitude lower gate leakage current than the MESFET, due to the relatively low barrier height of metals on n-type ZnO (0.6–0.8eV). Good drain–source current characteristics were obtained from MOS gate structures using P-doped ZnO channels, whereas the metal structures showed very poor modulation.

W2B-based rectifying contacts to n-GaN

Schottky contact formation on n-GaN using a novel W2B∕Ti∕Au metallization scheme was studied using current-voltage, scanning electron microscopy and Auger electron spectroscopy measurements. A maximum barrier height of 0.55 eV was achieved on as-deposited samples, with a negative temperature coefficient of 8×10−4eV∕°C over the range 25–150 °C. There was also a negative temperature coefficient for the reverse breakdown of the Schottky diodes with W2B contacts. The barrier height was essentially independent of annealing temperature up to 500 °C and decreased thereafter due to the onset of metallurgical reactions with the GaN. The Ti began to out-diffuse to the surface at temperatures of >500°C. The reverse current magnitude was larger than predicted by thermionic emission alone.

Visible–blind GaN p–i–n photodiodes with an Al0.12Ga0.88N/GaN superlattice structure

Abstract GaN ultraviolet photodiodes with (i.e. sample 1) and without (i.e. sample 2) Mg-doped AlGaN/GaN strained-layer superlattice structure were both fabricated. It was found that we could achieve a near constant dark current of around 3 and 7 nA/cm 2 for sample 1 and sample 2, respectively. It was also found that we could achieve a larger photocurrent and a larger photocurrent to dark current contrast ratio from sample 1. The zero-bias peak responsivity was found to be around 0.05 A/W at 356 nm and 0.006 A/W at 363 nm for sample 1 and sample 2, respectively. However, the transient response full-width-half-maximum (FWHM) of sample 1 was found to be longer. For devices with a 500 μm diameter, it was found that the transient response FWHM was 0.6 μs for sample 1 and 0.2 μs for sample 2. The difference in response time between sample 1 and sample 2 should be attributed to the distribution of depletion layer in between the p-layer and the n-type i-layer.

Planar GaN p-i-n photodiodes with n+-conductive channel formed by Si implantation

In this study, planar GaN p-i-n photodiodes (PDs) were fabricated by Si implantation into GaN-based p-i-n structure grown by metal organic vapor-phase epitaxy. Triple silicon implantation was performed to form a selective n+ channel through the multilayer p-i-n structure. As a result of n+-channel formation, a planar GaN p-i-n detector could be obtained. With the reverse bias below 2V, the dark current density was well below 50pA. The dark current increased significantly with an increase of reverse bias, which may be attributed to the incomplete damage (from implantation) removal and thereby result in the higher leakage current. The typical peak responsivity and the cutoff wavelength for the Si-implanted planar p-i-n PDs were around 0.13A∕W and 365nm, respectively. In addition, the visible (450nm)-to-UV (365nm) rejection ratio of around three to four orders could be extracted from the spectra response. Furthermore, the transient response measurements revealed that the full width at half maximum (FWHM) of ...