Wen-Yin Weng

p-Cu2O-shell/n-TiO2-nanowire-core heterostucture photodiodes

This study reports the deposition of cuprous oxide [Cu2O] onto titanium dioxide [TiO2] nanowires [NWs] prepared on TiO2/glass templates. The average length and average diameter of these thermally oxidized and evaporated TiO2 NWs are 0.1 to 0.4 μm and 30 to 100 nm, respectively. The deposited Cu2O fills gaps between the TiO2 NWs with good step coverage to form nanoshells surrounding the TiO2 cores. The p-Cu2O/n-TiO2 NW heterostructure exhibits a rectifying behavior with a sharp turn-on at approximately 0.9 V. Furthermore, the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiodes exhibit reasonably large photocurrent-to-dark-current contrast ratios and fast responses.

Deep UV

The authors report the fabrication of a deep-ultraviolet sensitive a-ZITO thin-film-transistor (TFT) with a Ta₂O₅ gate dielectric. It is found that carrier mobility, threshold voltage, and sub-threshold swing are 106.2 cm²/Vs, 0.75 V, and 0.45 V/decade, respectively, measured in the dark. It is also found that measured current increased from 2.3 × 10^-9 A to 7.97 × 10^-5 A, as we illuminated the sample with λ = 250-nm UV light when V<i>G</i> is biased at 0 V. Furthermore, it is found that deep-UV-to-visible rejection ratio could reach 2.3 × 10⁵ for the fabricated Ta₂O₅/a-ZITO TFT.

{\rm Ta}_{2}{\rm O}_{5}

InGaN/GaN multiquantum-well (MQW) metal-semiconductor-metal photodetectors with a beta-Ga₂O₃ cap layer formed by the furnace oxidation of a GaN epitaxial layer are fabricated and characterized. The beta-Ga₂O₃ cap layer is found to suppress the reverse leakage current by at least about two orders of magnitude with a 5-V applied bias because it creates a thicker and higher potential barrier. The reverse leakage current can be further reduced and a 90-fold larger ultraviolet-to-visible rejection ratio can be achieved by using InGaN/GaN MQW layers, which confine the electron-hole pairs generated by lower-energy photons. In addition, the noise level is reduced and detectivity is increased. With a 5-V applied bias, the noise-equivalent power and normalized detectivity are 8.4 × 10^-13 W and 7.9 × 10¹² Hz^0.5 W^-1, respectively.

/Zinc-Indium-Tin-Oxide Thin Film Photo-Transistor

Si nanowire (NW)-based photovoltaic devices were fabricated using NWs grown at various temperatures. It was found that the average length and average diameter of the NWs increased as the growth temperature was increased from 450°C to 620°C. It was also found that the NWs became sparser with increasing growth temperature. The reflectance spectra and I-V characteristics indicate that the average reflectances were about 9%, 15%, and 18% and that the photovoltaic conversion efficiencies were 4.1%, 2.33%, and 1.87% for Si NWs grown at 450°C, 550°C, and 620°C, respectively. The experiment results show that the largest fill factor (74%) and external quantum efficiency (40%) (for a wavelength of 740 nm) were achieved for Si NWs grown at 450°C.

InGaN/GaN Multiquantum-Well Metal-Semiconductor-Metal Photodetectors With Beta-

The electrical performance of amorphous indium gallium oxide (a-IGO) thin-film transistors applied as deep-ultraviolet (DUV) phototransistors is investigated. It was found that the bandgap of a-IGO can be engineered by altering its chemical composition. The performance of the phototransistors depended strongly on In2O3 content in the IGO film. When the indium content increases from 21% to 31%, the phototransistor cutoff red-shifted from 280 to 320 nm. The DUV-to-visible rejection ratio and photoresponsivity of the fabricated phototransistors were ~105 and 0.18 A/W.

{\rm Ga}_{2}{\rm O}_{3}

The authors report the growth of TiO₂ nanowires by heating the Ti/glass template and the fabrication of a TiO₂ nanowire metal-insulator-semiconductor (MIS) photodetector (PD). Compared with a TiO₂ nanowire metal-semiconductor- metal PD, it was found that we could achieve a photocurrent 29 times larger. It was also found that the dynamic response of the TiO₂ nanowire MIS PD was stable and reproducible with an on/off current contrast ratio of around 180. With an incident light wavelength of 390 nm and an applied bias of 5 V, it was found that the measured responsivity of the PD was 2.46 × 10^-3 A/W. Furthermore, the noise equivalent power and the detectivity of the fabricated TiO₂ nanowire MIS PD were 7.49 × 10^-12 W and 8.86 × 10¹¹ cm · Hz^0.5 · W^-1, respectively.

Cap Layers

The fabrication and characteristics of laterally grown ZnO nanowire/p-GaN heterojunction light emitting diodes are reported. The ZnO nanowires were grown using a direct single-step method on a prepared p-GaN patterned substrate. The rectifying current-voltage characteristics indicate that a p-n junction was formed with a heterostructure of n-ZnO nanowire/p-GaN. The room-temperature electroluminescent emission peak at 430 nm was attributed to the band offset at the interface between n-ZnO nanowire and p-GaN and defect-related emission from GaN.

Si Nanowire-Based Photovoltaic Devices Prepared at Various Temperatures

We report the fabrication of amorphous (In_xGa_1-x)₂O₃ metal-semiconductor-metal ultraviolet (UV) photodetectors on glass substrate by co-sputtering. It was found that, we could change the cutoff wavelength of the fabricated photodetectors by changing the RF sputtering power of the In₂O₃ target. With 5 V applied bias, it was found that the measured dark currents were 2 × 10^-12, 1 × 10^-11, and 2.3 × 10^-11 A for sample A prepared with 40 W In₂O₃ sputtering power, sample B prepared with 50 W In₂O₃ sputtering power, and sample C prepared with 60 W In₂O₃ sputtering power, respectively. It was also found that the UV-to-visible rejection ratios were 3 × 10³, 5 × 10³, and 1.5 × 10⁴ for samples A, B, and C, respectively. Furthermore, it was found that the response speeds of the fabricated devices were good.

Bandgap-Engineered in Indium–Gallium–Oxide Ultraviolet Phototransistors

GaN-based metal-semiconductor-metal photodetectors (PDs) with a β-Ga2O3 layer were fabricated. To increase performance, the Ga2O3/GaN-based PDs were covered with Au nanoparticles. The reverse leakage current decreased by more than two orders of magnitude with a 10-V applied bias and a 95-fold increase of the rejection ratio (250/360 nm) was achieved with a 1-V applied bias after the PDs were covered with Au nanoparticles. There was an obvious response at 490 nm because of the surface plasmon resonance in Au nanoparticles. The results indicate that Au nanoparticles can be used to improve the performance of optoelectronic devices.

A

The authors present the fabrication of a TiO2 nanowire ultraviolet (UV) photodetector (PD) with Iridium Schottky contact electrodes. UV-to-visible rejection ratio of the sample is around 107 when biased at 5 V, and the fabricated PD is visible-blind with a sharp cutoff at 390 nm. With an incident light wavelength of 390 nm and an applied bias of 5 V, the measured responsivity of the PD is 9.73 × 10-4 A/W.