Nitride-based UV Research Articles

Efficiency Breakthrough for Nitride-Based Blue and UV LEDs using Electrically Metal-Doped Glass-Film Electrodes

Efficiency Breakthrough for Nitride-Based Blue and UV LEDs using Electrically Metal-Doped Glass-Film Electrodes

The detrimental effect of AlGaN barrier quality on carrier dynamics in AlGaN/GaN interface

Carrier recombination and scattering at the semiconductor boundaries can substantially limit the device efficiency. However, surface and interface recombination is generally neglected in the nitride-based devices. Here, we study carrier recombination and diffusivity in AlGaN/GaN/sapphire heterointerfaces with AlGaN barriers of different quality. We employ the light induced transient grating and time-resolved photoluminescence spectroscopy techniques to extract carrier lifetime in different depths of the GaN buffer as well as in the AlGaN barrier, and to evaluate the carrier diffusion coefficient in the buffer. Moreover, we assess interface recombination velocity, Shockley-Read-Hall and radiative recombination rates. We reveal the adverse barrier influence on carrier dynamics in the underlying buffer: AlGaN barrier accelerates the nonradiative carrier recombination in the GaN buffer. The interface recombination velocity in the GaN buffer increases with decreasing AlGaN barrier quality, and the dominating recombination mechanism switches from Shockley-Read-Hall to interface recombination. These phenomena are governed by a cumulative effect of various interface-deteriorating barrier defects. Meanwhile, the carrier diffusivity in the GaN buffer is not affected by the AlGaN barrier. We conclude that barrier-accelerated interface recombination can become a major carrier loss mechanism in AlGaN/GaN interface, and may substantially limit the efficiency in nitride-based UV LEDs.

Observation of space-charge-limited current in AlGaN/GaN ultraviolet light-emitting diodes

AlGaN-containing ultraviolet light-emitting diodes (UV LEDs) emitting at 285 nm show abnormal diode characteristics below the sub-threshold input voltage region, indicating the existence of a space-charge-limited (SCL) current. Distinct transitions from Ohmic to SCL and further diode currents are observed in UV LEDs, unlike GaInN-based LEDs. Under varied temperatures, the transition voltages from Ohmic to SCL currents and the exponents of the power functions in the current–voltage dependency are influenced by the deactivated trap density in the AlGaN film, indicating that the charge injection and compensation limit for the current within the films in the low-voltage regimes for nitride-based UV LEDs.

AlGaN/GaN Ultraviolet Detector with Dual Band Response

The AlGaN/GaN ultraviolet detector with dual band response is investigated by a self-consistent solution of the Poisson-Schrödinger equation. Because of the polarization effect, the AlGaN/GaN UV detector with dual band response can be realized by varying the external voltage. At a low external voltage, the detector is mainly sensitive to the short wavelength. When the increasing external voltage is larger than the critical voltage, the detector has an obvious dual band response characteristic and the critical voltage is calculated. This characteristic makes nitride-based UV detector a larger potential application to develop multi band response by adjusting the Aluminum mole fraction in the AlGaN layer or even by using the AlGaN/GaN/InGaN heterostructure.

Ga-Doped ZnO/GaN Schottky Barrier UV Band-Pass Photodetector with a Low-Temperature-Grown GaN Cap Layer

Ga-doped ZnO (GZO) films were deposited onto low-temperature-grown (LTG) GaN/i-GaN (PD-I) and i-GaN (PD-II) epitaxy layers to form Schottky barrier UV band-pass photodetectors (PDs). The UV PDs exhibited a narrow band-pass spectral response ranging from 330 to 380 nm. It was also found that by using an LTG GaN layer on top of conventional nitride-based UV PDs, the leakage current was significantly reduced and a much larger photocurrent-to-dark-current contrast ratio was achieved. The short-wavelength cutoff at around 330 nm can be attributed to the marked absorption of the GZO top contact layer. The zero-bias peak responsivities were estimated to be 0.13 and 0.08 A/W at 360 nm for PD-I and PD-II, respectively. When the reverse bias was below -10 V, the dark current of PD-I was considerably below 20 pA.

Improved performance of GaN-based Schottky barrier photodetectors by annealing Ir/Pt Schottky contact in O 2

Nitride-based UV Schottky barrier photodetectors (PDs) with Ir/Pt after annealing in O 2 at 600 °C were fabricated successfully. With −5 V applied bias, the reverse leakage current of the annealed PD was 3.65 × 10 −12 A. It was found that we could achieve the larger Schottky barrier height, the smaller dark current and the larger photocurrent to dark current contrast ratio by annealing Ir/Pt. After annealing, the Schottky barrier height increased from 0.91 eV to 1.03 eV and the ideality factor decreased from 1.58 to 1.16. Such a result indicated that the dominant current transport mechanism may be thermionic emission and the Schottky contact is near-ideal after annealing. These results could be attributed to the formation of IrO x phase. It was also found that responsivity and the UV-to-visible rejection ratio were 0.19 A/W and 1.05 × 10 3 after annealing with −6 V applied bias.

Visible-Blind Metal–Semiconductor–Metal Photodetectors by Capping an In Situ Low-Temperature AlN Layer

We present the characteristics of a nitride-based UV metal–semiconductor–metal photodetector (MSM PD) with an in situ low-temperature (LT) grown AlN cap layer. From atomic-force microscopy scan images, we could clearly see that surface pits on the sample surface are almost invisible with an LT AlN cap layer but can be observed in a conventional cap layer. Compared with conventional MSM PDs, it was found that we achieved smaller dark current and larger UV-to-visible rejection ratio for the PDs with an LT AlN cap layer. With a applied bias, the UV-to-visible rejection ratio between 360 and was for the MSM PDs with an LT AlN cap layer. The calculated noise equivalent power and normalized detectivity biased at for the MSM PDs with an LT AlN cap layer was and , respectively.

Nitride-based MSM UV photodetectors with photo-chemical annealing Schottky contacts

The undoped GaN (u-GaN) and two-dimensional electron gas (2DEG) metal–semiconductor–metal (MSM) photodetectors with semi-transparent Ni/Au Schottky barrier contact electrodes were fabricated. It was found that we could achieve a larger Ni/Au transmittance, higher Schottky barrier heights and larger photocurrent to dark current contrast ratios by photo-chemical annealing of these photodetectors in O 2. It was also found that the maximum quantum efficiencies were 13% and 57% for the photo-chemical annealing u-GaN and 2DEG photodetectors, respectively. Furthermore, it was found that we could achieve a larger responsivity, a lower noise level and a larger detectivity by using the 2DEG structure.

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.

Group III nitride-based UV light emitting devices

The effect of the non-radiative recombination center at or around the dislocations on the performance of the AlGaN-based UV light emitting diode (LED) is discussed. For high-efficiency emission, it is necessary to reduce the density of dislocations to less than 5 × 10 7 cm -2 .

Design and Performance of Nitride-based UV LEDs

ABSTRACTWe overview several of the challenges in achieving high efficiency nitride-based UV (< 400 nm) LEDs. The issue of optical efficiency is presented through temperature-dependent photoluminescence studies of various UV active regions. These studies demonstrate enhanced optical efficiencies for active regions with In-containing alloys ( InGaN, AlInGaN). We compare the performance of two distinct UV LED structures. GaN/AlGaN quantum well LEDs with λ < 360 nm emission have demonstrated output powers > 0.1 mW, but present designs suffer from internal absorption effects. InGaN/AlInGaN quantum well LEDs with 370 nm< λ < 390 nm emission and > 1 mW output power are also presented.