C. L. Yu

GaN-Based Schottky Barrier Photodetectors With a 12-Pair Mg

GaN-based ultraviolet (UV) photodetectors (PDs) separately prepared with a conventional single low-temperature (LT) GaN buffer layer and a 12-pair Mg_xN_y-GaN buffer layer were both fabricated. It was found that we could reduce threading dislocation (TD) density and thus improve crystal quality of the GaN-based UV PDs by using the 12-pair Mg_xN_y-GaN buffer layer. With a -2-V applied bias, it was found that the reverse leakage currents measured from PDs with a single LT GaN buffer layer and that with a 12-pair Mg_xN_y-GaN buffer layer were 4.57 times 10^-6 and 1.44 times 10^-12 A, respectively. It was also found that we could use the 12-pair Mg_xN_y-GaN buffer layer to suppress photoconductive gain, enhance UV-to-visible rejection ratio, reduce noise level, and enhance the detectivity.

_{\rm x}

Here, we present the characteristics of a novel GaN- based ultraviolet (UV) photodiode (PD) with a low-temperature (LT) AIN cap layer. The dark leakage current for the PD with the LT-AIN cap layer was shown to be about four orders of magnitude smaller than that for the conventional PDs. It was found that we could achieve larger UV to visible rejection ratio by inserting an LT-AIN cap layer. It was also found that we could improve minimum noise equivalent power and maximum normalized detectivity of the PD by inserting an LT-AIN cap layer.

N

AlGaN/GaN metal–semiconductor–metal (MSM) ultraviolet (UV) photodetectors with photo-chemical vapour deposition (photo-CVD) annealed Ni/Au semi-transparent contacts were fabricated. It was found that the transmittances of Ni/Au films increased while the photodetector dark currents became significantly lower after annealing. With a 5 V applied bias, it was found that the photocurrent to dark current contrast ratio and the maximum responsivity were 2.3 × 104 and 0.166 A W−1 for photodetector photo-CVD annealed at 550 °C.

_{\rm y}

ZnO epitaxial films were grown on sapphire (0001) substrates by using rf plasma-assisted molecular beam epitaxy. Metal-semiconductor-metal (MSM) photodetectors with Iridium (Ir) electrodes were then fabricated. It was found that Schottky barrier heights at the non-annealed and 5008C-annealed Ir/ZnO interfaces were around 0.65 and 0.78 eV, respect- ively. With an incident wavelength of 370 nm and 1 V applied bias, it was found that the maximum responsivities for the Ir/ZnO/Ir MSM photodetectors with and without thermal anneal- ing were 0.18 and 0.13 A/W, respectively. From transient response measurement, it was found that time constant t of the fabricated photodetectors was 22 ms. For a given bandwidth of 100 Hz and 1 V applied bias, we found that noise equivalent power and corresponding detectivity Dwere 6 � 10 213 W and 1.18 � 10 12 cm Hz 0.5 /W, respectively. In recent years, a lot of research has been focused on semiconductor-based ultraviolet (UV) photodiodes. Photodiodes operating in the short-wavelength UV region are vitally important for various commercial and military applications. Specifically, visible-blind UV photodiodes are being used in space communications, ozone layer moni- toring and flame detection. Currently, light detection in the UV spectral range still relies on Si-based optical photo- diodes. Even though these devices are quite sensitive to visible and infrared radiations, nevertheless, the responsiv- ity in the UV region is considered relatively low because of the fact that the room temperature bandgap energy of Si is only 1.2 eV. With the optoelectronic devices start being fabricated on direct wide bandgap materials, being able to fabricate high-performance solid-state photodiodes

–GaN Buffer Layer

InGaN-GaN multiquantum-well (MQW) metal-semiconductor-metal (MSM) photodetectors (PDs) with the unactivated Mg-doped GaN cap layer were successfully fabricated. It was found that we could achieve a dark current by as much as six orders of magnitude smaller by inserting the unactivated Mg-doped GaN cap layer. For MSM photodetectors with the unactivated Mg-doped GaN cap layer, the responsivity at 380 nm was found to be 0.372 A/W when the device was biased at 5 V. The UV-to-visible rejection ratio was also estimated to be around 1.96 times 103 for the photodetectors with the unactivated Mg-doped GaN cap layer. With a 5-V applied bias, we found that minimum noise equivalent power and normalized detectivity of our PDs were 4.09 times 10-14 W and 1.18 times 1013 cmmiddotHz0.5W-1, respectively. Briefly, incorporating the unactivated Mg-doped GaN layer into the PDs beneficially brings about the suppression of dark current and a corresponding improvement in the device characteristics.

Low-Noise and High-Detectivity GaN UV Photodiodes With a Low-Temperature AlN Cap Layer

GaN-based ultraviolet photodiodes with a semi-insulating Mg-doped GaN cap layer were fabricated and characterized. Dark leakage current of the aforementioned photodiodes was much smaller than that of the conventional ones without the Mg-doped GaN cap layer due to a thicker and higher potential barrier and less amounts of interface states after inserting the Mg-doped GaN cap layer. The ultraviolet to visible rejection ratio is 3.44 x 103 by inserting a semi-insulating Mg-doped GaN cap layer with a -IV applied bias. In this study, we also discuss the noise characteristics. It was found that minimum noise equivalent power and maximum detectivity of our photodiode were 1.2 x 10-12 W and 9.34 x 1011 cmHz0.5 W-1, respectively.

AlGaN/GaN MSM photodetectors with photo-CVD annealed Ni/Au semi-transparent contacts

Nitride-based metal-insulator-semiconductor (MIS)-like photodiodes (PDs) with in situ grown 30-nm-thick unactivated semiinsulating Mg-doped GaN cap layers were fabricated. The authors found that the reverse leakage current of the aforementioned PD was comparably much smaller than that of conventional PD without the semiinsulating layer due to the facts that inserting a semiinsulating layer would result in a thicker and higher potential barrier, and also less amounts of interface states introduced. To sum up, it was determined that the benefits of incorporating a semiinsulating Mg-doped cap layer into the PD would encompass a larger photocurrent-to-dark-current contrast ratio and larger ultraviolet-to-visible rejection ratio

GaN Metal-Semiconductor-Metal Ultraviolet Photodetectors with Ir ∕ Pt Contact Electrodes

We report an AlGaN/GaN high electron mobility transistors (HEMTs) based on InGaN/GaN multiquantum-well (MQW) structure. When InGaN/GaN MQW structure was inserted, InGaN layer has an opposite piezoelectric polarization field compared to AlGaN, which results in a very sharp rise of the conduction band. The raised potential barrier can help to improve carrier confinement and obtain a larger main peak transconductance of 111 mS/mm and satellite peak transconductance of 24 mS/mm, corresponding to AlGaN/GaN heterojunction and InGaN layer. MQW-based metal-oxide-semiconductor-HEMT was also fabricated and significantly reduced the leakage current and increased transconductance as a result of passivation by Ta2O5 gate oxide.

High-Detectivity Nitride-Based MSM Photodetectors on InGaN–GaN Multiquantum Well With the Unactivated Mg-Doped GaN Layer

AlGaN/GaN metal-semiconductor-metal photodetectors (MSM PDs) with a low-temperature (LT) AlGaN interlayer (IL) were fabricated. Compared with the conventional AlGaN/GaN MSM PD, it was found that leakage current can be suppressed by insertion of a LT AlGaN IL due to the reduction of surface pits and improvement of crystalline quality. It was also found that larger photoresponsivity can be achieved due to the enhanced electric field strength as a result of inserting a LT AlGaN IL. Furthermore, suppressed photoconductive gain, lower noise level, and larger detectivity of MSM PD can also be achieved by using a LT AlGaN IL.

Low-Noise and High-Detectivity GaN-Based UV Photodiode With a Semi-Insulating Mg-Doped GaN Cap Layer

GaN UV metal-semiconductor-metal photodetectors (MSM PDs) with an unactivated Mg-doped cap layer and sputtered indium tin oxide (ITO) were fabricated. Compared with conventional MSM PDs without a cap layer, it was found that we could achieve a significantly much smaller dark current, larger UV to visible rejection ratio, and larger normalized detectivity by inserting an unactivated Mg-doped GaN cap layer. The dark leakage current for the MSM PDs with an unactivated Mg-doped GaN cap layer was shown to be about ten orders of magnitude smaller than that for the conventional MSM PDs. Under a 0.5 V bias, the measured responsivity and UV-to-visible rejection ratio were 0.017 A/W and 1.44 X 10 4 , respectively, for the MSM PDs with an unactivated Mg-doped GaN cap layer. This result could be attributed to the thicker and higher potential barrier and effective surface passivation after inserting this in situ grown cap layer. With a 1 V applied bias, it was also found that we could achieve a lower noise level and a higher normalized detectivity of 2.67 X 1010 cm Hz 0.5 W -1 by inserting an unactivated Mg-doped GaN cap layer into MSM PDs.