Transient response of low-temperature ALD-grown ZnO thin film-based p–i–n UV photodetector

We report the fabrication and characterization ofAlxGaixN pifl photodiodes (0.05  x  0.30) grown on sapphireby low-pressure metalorganic chemical vapor deposition. The devices present a visible-rejection of about four ordersofmagnitude with a cutoffwavelength that shifts from 350 nm to 291 nm. They also exhibit a constant responsivityfor five decades (30 mW/rn2 to 1 kW/m2) of optical power density. Using capacitance measurements, the values forthe acceptor concentration in the p-A1Ga1N region arid the unintentional donor concentration in the intrinsic regionare found. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds tothe RC product ofthe system. For low load resistances the transient response becomes non-exponential, with a decaytime longer than the RC constant.Keywords: UV photodetectors, solar-blind, A1GaN, p-i-n photodetectors 1. INTRODUCTION High caliber ultraviolet (UV) and visible blind photodetectors are desired for many applications, both terrestrial andspace-based.' Notable progress in the device design, material quality and processing of wide bandgap 111-Nitridematerial, such as GaN and AlGa,N, has yielded visible-blind photodetectors with high responsivities, low darkcurrents, and fast response times,2'3 suitable for applications such as flame sensors, combustion process monitoring,and solar UV detection. While these visible-blind detectors can detect light at lower wavelengths and distinguish itfrom visible and infrared radiation, improvement is needed in the selectivity of the bandedge to more effectivelyfunction as a true solar-blind detector (?<3OO nm).Photoconductors based on the entire range of the A1Ga,N ternary material, from 365

Photodetectors based on the AlGaN/GaN heterostructure suffer from persistent photoconductivity (PPC) in which recovery from the optical stimulus can take days. This behavior is unsuitable for many applications where reliable and consistent optical response is required. This letter presents a method for suppressing PPC in AlGaN/GaN photodetectors by employing device suspension and in situ heating. The highly conductive two-dimensional electron gas (2DEG) at the interface of AlGaN and GaN serves as both a sensor and a heater (via Joule heating). Microfabricated AlGaN/GaN-on-Si ultraviolet (UV) photodetectors (suspended and unsuspended) were exposed to UV (365 nm) for 60 s and the transient responses were measured under various in situ heating conditions. The measured transient response showed a decay time of ~39 h when the photodetector was not heated and 24 s for a suspended photodetector with in situ 2DEG heating (270°C with a power of 75 mW). This remarkable suppression of the PPC in AlGaN/GaN UV photodetectors can be attributed to the novel device architecture and in situ heating capability, which enables acceleration of the carrier capture rate during operation.

In this work, a low leakage current and high responsivity GaN p-i-n ultraviolet (UV) photodetector (PD) is fabricated for a photocurrent instability study. When the illumination condition shifts from dark to constant UV illumination, the PD’s photocurrent is found first to increase sharply and then go through a slow rising process until reaching its saturation value in tens of seconds. The degree of photocurrent instability lessens as UV illumination intensity increases. Meanwhile, when the PD is illuminated by periodic square-wave UV light, its transient response time is measured to be ∼1–2 μs. The observed photocurrent instability behavior is likely due to photocarrier trapping by yellow luminescence band related defects in GaN, which is supported by multiwavelength light illumination and photocurrent decay measurements.

The fabrication of a superior-performance ultraviolet (UV) photodetector utilizing graphene quantum dots (GQDs) as a sensitization agent on a ZnO-nanorod/GaN-nanotower heterostructure has been realized. GQD sensitization displays substantial impact on the electrical as well as the optical performance of a heterojunction UV photodetector. The GQD sensitization stimulates charge carriers in both ZnO and GaN and allows energy band alignment, which is realized by a spontaneous time-correlated transient response. The fabricated device demonstrates an excellent responsivity of 3.2 × 103 A/W at -6 V and displays an enhancement of ∼265% compared to its bare counterpart. In addition, the fabricated heterostructure UV photodetector exhibits a very high external quantum efficiency of 1.2 × 106%, better switching speed, and signal detection capability as low as ∼50 fW.

ABSTRACTAn Ultraviolet (UV) photodetector with high responsivity and relative fast response speed was fabricated from three dimensional WO3 nanowires/reduced graphene oxide (3D WO3 NWs/RGO) composite materials. The 3D WO3 NDs/GN composite was synthesized using a facile three-step synthesis. First, the Na2WO4/Graphene Oxide (GO) precursor was synthesized by homogeneous precipitation. Second, the Na2WO4/GO precursor was transformed into H2WO4/GO composites by acidification. Finally, the H2WO4/GO composites were reduced to 3D WO3 NWs/RGO via hydrothermal reduction process. A maximum photoresponsivity of 4.2 A/W at 374 nm was observed under 20 V bias. The UV photodetector showed relative fast transient response, which is at least 2 orders of magnitude faster than UV photodetectors fabricated from WO3 nanowires. The good photoresponsivity and fast transient response are attributed to improved carrier transport and collection efficiency through graphene.

In this study, ZnO nanorod ultraviolet (UV) photodetectors (PDs) with dye-sensitised solar cells (DSSCs) were prepared on ITO conductive glass. The average diameter and length of the ZnO nanorods were ∼99 nm and ∼1.6 μm, respectively. After the growth of the ZnO nanorods, the TiO2 film with the TiO2 nanoparticle size of ∼25 nm was coated using doctor-blade method. Then, sensitiser N3 was used for the charge transfer in the DSSCs. The solar photovoltaic conversion efficiency of the DSSCs was approximately 4.75%. The performance of the UV PDs was also measured by exposing it with UV light at 365 nm. Results showed that the transient responses of the ZnO nanorod UV PDs integrated with DSSCs were stable and reproducible.

Role of threading dislocations and point defects in the performance of GaN-based metal-semiconductor-metal ultraviolet photodetectors

Low power ZnO nanorod-based ultraviolet photodetector: Effect of alcoholic growth precursor

Pulsed laser deposition (PLD) is used to grow ‐oriented single‐crystalline β‐gallium oxide (β‐Ga2O3) thin films on c‐plane sapphire substrates by optimized growth temperature and pressure. The morphology and crystallinity of the thin films are examined using X‐ray diffraction and atomic force microscopy. The thin films are used as the semiconductor layer for metal–semiconductor–metal (MSM) photodetector (PD) devices with various electrode designs. The ultraviolet photodetectors are characterized under 250 nm illumination, showing a high current amplitude increase over dark current conditions that approaches three orders of magnitude at a 6 V bias for an optimized growth pressure of 1 × 10−3 torr. The photodetectors' transient response is also measured, allowing for the defect analysis to be performed. A peak spectral responsivity of 30.45 A W−1 is measured at 250 nm incident illumination.

A novel integrated ultraviolet (UV) photodetector has been proposed, which realizes a high UV selectivity by combining a conventional UV-selective photodiode with an extra infrared (IR) photodiode. The IR photodiode is designed for compensating the photocurrent response of the UV photodiode in the infrared band and is 15 times smaller than the UV one. The integrated photodetector has been fabricated in a 0.35 μm standard CMOS technology. Some critical performance indices of this new structure photodetector, such as spectral responsivity, breakdown voltage, quenching waveform, and transient response, are measured and analyzed. Test results show that the complementary UV–IR photodetector has a maximum spectral responsivity of 0.27 A·W−1 at the wavelength of 400 nm. The device has a high UV selectivity of 3000, which is much higher than that of the single UV photodiode.

Gain and response speed are two key performance parameters for high sensitivity photodetectors (PDs). However, the effect of incident light intensity on gain and transient response properties of AlGaN/GaN hetero-junction based PDs are still not fully understood. Here, we design and fabricate an AlGaN/GaN hetero-junction based ultraviolet (UV) PD with interdigitated electrodes formed by a conductive two-dimensional electron gas (2DEG) channel, which exhibits a low dark current of 2.92 × 10−11 A and a high responsivity of 3060 A/W at 10 V bias. The high-gain AlGaN/GaN 2DEG PD has a similar working mechanism to those of traditional phototransistors, but its device architecture is evidently simplified. By investigating the variation of gain and transient response characteristics of the 2DEG PD as a function of incident UV light intensity, it has been concluded that the gain of the PD in a low-light intensity region is dominated by hole accumulation-induced electron escape from the 2DEG, while in a high-light intensity region, the gain is dominated by photoconductivity effect and limited by carrier recombination. This study provides guidance for future practical applications of AlGaN/GaN-based PDs in complex UV illumination environments.

Ultraviolet photodetectors have attracted considerable interest for a variety of applications in health, industry, and science areas. Self-driven visible-blind photodetectors represent an appealing type of sensor, due to the reduced size and high flexibility. In this work, we employed BaTiO3 (BTO) single crystals with a bandgap of 3.2 eV for the realization of a self-driven ultraviolet detector, by utilizing the ferroelectric properties of BTO. We found that the sign of the photocurrent can be reversed by flipping the ferroelectric polarization, which makes the photodetector suitable for electrical manipulation. The photoelectric performance of this photodetector was systematically investigated in terms of rectification character, stability of short-circuit photocurrent, spectral response, and transient photoelectric response. Particularly, the self-driven photodetectors based on BTO showed an ultrafast response time about 200 ps. It is expected that the present work can provide a route for the design of photodetectors based on ferroelectric oxides.

High-performance ultraviolet photodetector based on p-PEDOT:PSS film/p-ZnO:Sb microwire/n-Si double heterojunction

Structural, optical and photoresponse characteristics of metal-insulator-semiconductor (MIS) type Au/Ni/CeO2/GaN Schottky barrier ultraviolet photodetector

GaN has been widely used in the fabrication of ultraviolet photodetectors because of its outstanding properties. In this paper, we report a graphene–GaN nanorod heterostructure photodetector with fast photoresponse in the UV range. GaN nanorods were fabricated by a combination mode of dry etching and wet etching. Furthermore, a graphene–GaN nanorod heterostructure ultraviolet detector was fabricated and its photoelectric properties were measured. The device exhibits a fast photoresponse in the UV range. The rising time and falling time of the transient response were 13 and 8 ms, respectively. A high photovoltaic responsivity up to 13.9 A/W and external quantum efficiency up to 479% were realized at the UV range. The specific detectivity D * = 1.44 × 1010 Jones was obtained at –1 V bias in ambient conditions. The spectral response was measured and the highest response was observed at the 360 nm band.