GaN Ultraviolet Photodetector Research Articles

Optimized photoresponse performances in vertical and horizontal photodetectors based on freestanding GaN membranes

GaN has become an important material in the optoelectronics due to its suitable band gap and excellent physical and chemical properties, however, the grown substrates limit device geometry owing to their poor electrical and thermal conductivity. Herein, horizonal and vertical GaN ultraviolet photodetectors (UV PDs) were fabricated using the freestanding GaN membranes obtained by an epitaxial lift-off technique. The photoresponse behaviors in the fabricated PDs were then optimized by the piezo-phototronic effect. Owing to the synergistic effect of spontaneous polarization and asymmetric electrodes, the vertical UV PD exhibits unique self-powered characteristics with the light on/off of 3.3 × 103 and the responsivity (R) of 114 mA/W at the bias of 0 V. Optimized by the piezo-phototronic effect, the R increases to 168 mA/W at a tension strain of 0.25 %. In comparison, the R of horizontal UV PD decreases, regardless of the compression or tension. Physical mechanisms of the performances optimization in the vertical and horizontal UV PDs are proposed and analyzed. This study opens a pathway to fabricate GaN UV PDs with different device configurations and provides fundamental support for their performance modulation by the piezo-phototronic effect.

Fabrication of GaN nanocrystalline thin films Schottky metal-semiconductor-metal ultraviolet photodetectors

Gallium nitride nanocrystalline thin films (GaN NCTH) were prepared on Si substrates using the radio frequency magnetron sputtering (RF) method. XRD patterns with narrow peaks intensity and field emission scanning electron microscopy (FESEM) images exhibits of the GaN nanoparticles have covered surfaces of the substrate with a regular distribution smooth and an average particles size proximately between are 10–20 nm. AFM indicates a smooth surface and RMS is 1.38 nm of GaN NCTH and the optical band gap was calculated to be 3.38 eV. The current-voltage characteristic of metal-semiconductor-metal (M-S-M) GaN ultraviolet photodetectors (UV-PD) in the dark and irradiated with 370 nm light and intensity was (1.4) mW/cm2 evaluated under bias voltages of −1 to 5 V. The average rise time speed of the photodetector is about 22 ms when current increases from 10 % to 90 % of its value of saturation, while the average fall time is 17 ms when current decreases from 90 % to 10 % of its value of saturation. UV-PD displays 19–261 sensitivity at a different voltage bias; internal photodetector gain between the low value is 1.19 at 8 V and 1.59 at 2 V.

High gain GaN ultraviolet Schottky photodetector with Al-doped ZnO interlayer

We fabricated an GaN Schottky photodiode (PD) with Al-doped ZnO (AZO) interlayer for ultraviolet (UV) photodetection, and its optoelectrical properties with a comparison to the GaN Schottky PD without AZO interlayer. The AZO-interlayered GaN Schottky PD was highly sensitive to 350 nm laser irradiation, having a photoresponsivity of 11.1 AW–1 and detectivity of 1.04 × 1012 cm·Hz1/2·W–1 at -2 V as against 0.28 AW−1 and 2.23 × 1010 cm·Hz1/2·W–1 for the GaN Schottky PD without AZO interlayer. The transient photoresponse revealed a rise and decay time of 203 and 293 ms, respectively for the AZO-interlayered GaN Schottky PD. The introduction of AZO interlayer resulted in a reduction in the noise spectral density, leading to an increase in the detectivity by more than an order, as compared to GaN Schottky PD without AZO interlayer. An improved photodetection behavior of the AZO-interlayered GaN Schottky PD could be ascribed to effective GaN surface passivation and reduction of the surface states by the AZO interlayer. The utilization of the AZO in GaN-based UV photodetectors can provide a potential and simplistic approach towards the fabrication of high performance GaN-based UV photodetectors.

Improving detectivity of self-powered GaN ultraviolet photodetector by nickel nanoparticles

We have demonstrated significant improvement on the performance of a self-powered (zero power consumption) GaN ultraviolet photodetector (GaN-UVPD) under a 254-nm UV light exposure through the application of nickel nanoparticles (NiNPs). Indeed, the detectivity of the device increased from 2.81 × 1013 to 7.19 × 1013 cm Hz1/2 W−1 at 0 V, which is the highest reported detectivity of the self-powered UV photodetectors, with the application of 5-µl Ni nanoparticle solution to the device surface under UV exposure. Furthermore, the GaN-UVPD revealed very high sensitivity values at zero bias as 5.04 × 106 and 6.67 × 106 before and after the application of the Ni nanoparticle solution, respectively. The preeminent improvement in detectivity and sensitivity from the Ni nanoparticles is likely due to localized surface plasmons leading to a strong light absorption around the Ni nanoparticles resulting an increase in number of photocarriers. Thus, this work will contribute towards developing high-performance self-powered UV photodetectors that can potentially replace traditional high-energy-consuming UV detection and secure ultraviolet communication devices.

Impact of gold nanoparticles on low-voltage operating GaN ultraviolet photodetector

We report, in this study, the impact of gold nanoparticles on low-voltage operating GaN ultraviolet (UV) photodetector through parameters such as detectivity, responsivity, repeatability, stability, and photo-to-dark current ratios. These studies revealed prominent increases in detectivity and responsivity of the device due to Au nanoparticle application, thus allowing it to detect weaker signals and obtain higher on/off current ratios. After the application of different quantities of Au nanoparticles to the device surface, the investigated photoelectric properties were enhanced reaching a peak detectivity of 1.09 × 1011 Jones and a photo-to-dark current ratio of 486 at 2-V bias. Moreover, the photocurrent increased by two-fold (from 35.3 to 70.6 nA) at a low bias of 0.5 V following the application of the Au nanoparticles to the device surface. The modified GaN photodetector with a remarkable detectivity offers solutions toward improving detectivity of UV photodetectors. In consequence, having established such prosperous properties under relatively low-voltage levels with this practical and cost-effective fabrication route provides a good approach toward a wide range of UV optoelectronic devices.

GaN ultraviolet photodetector with petal-like β-Ga2O3 microcrystalline layer

A GaN ultraviolet photodetector with a petal-like β-Ga2O3 microcrystalline layer was prepared on the GaN template using electrochemical anodizing and annealing processes. The petal-like β-Ga2O3 microcrystalline layer was found to enhance the absorption of ultraviolet light and suppress the dark current, and a high responsivity from 230 nm (responsivity 8.5 A/W) to 400 nm (responsivity 0.1 A/W) was achieved by the photodetector. The rejection ratio of ultraviolet–visible light is greater than three orders of magnitude representing a high selectivity of ultraviolet light detection. The responsivity slopes of the photodetector under different biases were found to be strongly correlated with the wavelength of light, and the responsivity is much higher than that of conventional metal/insulator/metal wavelength identification photodetectors. This effective method of synthesizing β-Ga2O3 crystallites on GaN can be used to enhance the ultraviolet absorption of GaN photodetectors and improve the detection performance.

Dual-surface lens with ring-shaped structures for optical tuning of GaN ultraviolet photodetectors at low temperature

This study aims to enhance the optical sensitivity of gallium nitride ultraviolet (UV) photodetectors at low temperatures using a multifunctional microlens with ring-shaped structures. This dual-surface lens with ring-shaped structures (i.e., DSLR), a unique microlens, was fabricated by employing a curable polymer and three-dimensional printed mold, and it has two primary regions: An upper and a lower region. The upper region of the DSLR serves as a smooth lens surface to focus the incident UV rays by delaying frost formation at low temperatures. The lower region captures condensed water droplets, thus allowing for only local frost formation. When compared to a photodetector without the DSLR, the photodetector covered by the DSLR generated a higher photocurrent at low temperatures (corresponding to a ∼13.85% increase in generated photocurrent in the temperature range of −20 °C to −1.2 °C). Thus, the findings of this study support the use of a facile, cost-effective, and multifunctional dual-surface polymer lens for higher-sensitivity UV detection in various low-temperature environments, such as the Arctic, outer space, and the environment within cryostats.

Role of ZrO2 Passivation Layer Thickness in the Fabrication of High‐Responsivity GaN Ultraviolet Photodetectors

The importance of a ZrO2 passivation layer in the fabrication of high‐responsivity GaN‐based ultraviolet (UV) photodetectors (PDs) is discussed. It is found that an optimum thickness of the ZrO2 layer exists, which plays a critical role in controlling the photoresponse and transient response of the device. Beyond the optimal thickness, the performance of PDs deteriorates, which is limited by the restricted tunneling of photogenerated carriers across the oxide layer. At an optimum ZrO2 thickness of 3 nm, a spectral responsivity of 27 A W−1 at 361 nm is achieved at 4 V applied bias along with the fast response of the device with a rise (fall) time of 28 ms (178 ms), respectively. Such characteristics are found to be similar or better than the recently reported state‐of‐the‐art values for visible blind metal–semiconductor–metal PDs fabricated on GaN. The results confirm that the surface passivation with an optimal thickness of an oxide layer can be used to develop high‐responsivity GaN‐based UV PDs irrespective of having a large dark current, which is often inevitable due to the presence of a large density of dislocations in GaN epitaxial layers grown on foreign substrates.

Hybrid graphene/unintentionally doped GaN ultraviolet photodetector with high responsivity and speed

Ultraviolet (UV) photodetectors with high responsivity and speed are highly desirable for imaging and remote sensing applications. Limited by the crystalline quality of a GaN-based material, which is ideal for UV photodetection, the further improvement of the performance is minimal. A hybrid graphene/unintentionally doped (UID) GaN UV photodetector with both high responsivity and high speed is reported. Holes in graphene, which are induced by the photogenerated electrons trapped at the graphene/UID GaN interface according to the capacitive effect, have a long lifetime owing to the electron-hole pair separation in space. Graphene acts as a carrier transport channel and greatly increases the charge collection efficiency under an external bias voltage. The responsivity of a hybrid graphene/UID GaN photodetector with a photosensitive area of 2 mm2 reaches 5.83 A/W at −10 V with a specific detectivity of ∼1011 Jones. The response time is ∼5 ms, which is faster than that of traditional GaN photodetectors. These results will provide a feasible route to UV detection with high performance.

GaN-UV photodetector integrated with asymmetric metal semiconductor metal structure for enhanced responsivity

Fabrication of very thin GaN ultraviolet photodetectors on Si (111) substrate integrated with asymmetric (Pt–Ag, Pt–Cr) metal–semiconductor–metal (MSM) structure have been illustrated. Designed GaN photodetection device displays significant enhancement in responsivity for asymmetric (Pt–Ag) MSM structure (280 mA/W) in comparison to symmetric (Pt–Pt) MSM structure (126 mA/W) at 10 V bias. The fabricated asymmetric and symmetric devices also exhibit fast response time in the range of 30–59 ms. The enhancement in responsivity using asymmetric MSM structure ascribed to large difference in work function which lead to change in Schottky barrier height of the metal semiconductor junction. Additionally, power dependent photoresponse analysis of GaN asymmetric (Pt–Ag) ultraviolet photodetector was showing a responsivity of 116 mA/W at low optical power of 1 mW. Such GaN asymmetric MSM ultraviolet photodetectors having high responsivity can extensively be used for low power, high speed ultraviolet photo detection applications.

Self-powered GaN ultraviolet photodetectors with p-NiO electrode grown by thermal oxidation

Self-powered ultraviolet photodetectors based on n-GaN and p-NiO were fabricated through thermal oxidation. The transparent NiO was mainly dominated by [111] texture with an optical band gap of approximately 3.69eV. Compared with the conventional Ni/GaN photodetector, the p-NiO/GaN heterostructure photodetector possess a larger turn-on voltage and a smaller dark current because of the relatively higher effective barrier height. At an external bias of 0V, the photo responsivity and the UV to visible rejection ratio of the NiO/GaN photodetector are enhanced to 0.15A/W and 406, respectively. The improvement in UV photodetector performance is attributed to that the depletion region in NiO/GaN heterostructure can effectively eliminate the trapping charge carriers at the metal/semiconductor interface. The high-performance NiO/GaN photodetectors without driving power are potential for portable UV detectors application.

A Highly Responsive Self‐Driven UV Photodetector Using GaN Nanoflowers

The rising demand for optoelectronic devices to be operable in adverse environments necessitates the sensing of ultraviolet (UV) radiation. Here, a self‐driven, highly sensitive, fast responding GaN nanoflower based UV photodetector is reported. By developing unique structures, the light absorption increases efficiently and a maximum responsivity of 10.5 A W−1 is achieved at 1 V bias. The reported responsivity is the highest among the GaN UV photodetectors on Si substrates and commercially available Si‐based UV photodetectors. Under self‐driven condition, the photodetector exhibits very low dark current (≈nA) with a very high responsivity (132 mA W−1) and detectivity (2.4 × 1010 Jones). A remarkably high light‐to‐dark current ratio of ≈260 signifies extremely high photodetection gain compared to planar GaN‐based photodetectors. The self‐driven and biased photodetector device yields highly stable rise and decay time response. A model based on band theory elucidates the origin of self‐driven photodetectors. Implementation of the innovative growth design structures assures an exceptionally high sensitivity toward UV signal, which is capable of substituting the existing technology of UV photodetectors. High responsivity and detectivity from devices based on the GaN nanoflower‐like structure with the advantage of high surface/volume ratio can have numerous applications in fabrication of nanoscale optoelectronic high performance devices such as self‐driven UV photodetectors.

Highly antireflective AlGaN/GaN ultraviolet photodetectors using ZnO nanorod arrays on inverted pyramidal surfaces

Highly antireflective heterostructured aluminum gallium nitride (AlGaN)/GaN ultraviolet (UV) photodetectors were demonstrated using a combination of inverted pyramidal surfaces and zinc oxide nanorod arrays (i.e., antireflective surface modification) to enhance the optical sensitivity. The microfabricated hierarchical surfaces significantly reduced the average surface reflectance to less than 0.3% in the UV region and less than 1% in the visible light region, allowing near-perfect absorption of incident light regardless of the angle of incidence (5–80°). As a result, the photodetectors fabricated on highly antireflective AlGaN/GaN surfaces showed higher sensitivity and responsivity over a broad range of incidence angles compared to photodetectors on planar AlGaN/GaN surfaces, supporting the use of a hierarchically modified sensing surface for omnidirectional UV monitoring with higher sensitivity.

High Responsivity in GaN Ultraviolet Photodetector Grown on a Periodic Trapezoid-Column Patterned Sapphire Substrate

High Responsivity in GaN Ultraviolet Photodetector Grown on a Periodic Trapezoid-Column Patterned Sapphire Substrate

Atomic layer deposited HfO2 based metal insulator semiconductor GaN ultraviolet photodetectors

A report on GaN based metal insulator semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with atomic layer deposited (ALD) 5-nm-thick HfO2 insulating layer is presented. Very low dark current of 2.24 × 10−11 A and increased photo to dark current contrast ratio was achieved at 10 V. It was found that the dark current was drastically reduced by seven orders of magnitude at 10 V compared to samples without HfO2 insulating layer. The observed decrease in dark current is attributed to the large barrier height which is due to introduction of HfO2 insulating layer and the calculated barrier height was obtained as 0.95 eV. The peak responsivity of HfO2 inserted device was 0.44 mA/W at bias voltage of 15 V.

Enhanced performance of GaN nanobelt-based photodetectors by means of piezotronic effects

GaN ultraviolet (UV) photodetectors (PDs) have attracted tremendous attention due to their chemical stability in harsh environments. Although Schottky-contacted GaN-based UV PDs have been implemented with better performance than that of ohmic contacts, it remains unknown how the barrier height at local Schottky contacts controls the sensors’ performance. In this work, the piezotronic effect was employed to tune the Schottky barrier height (SBH) at local contacts and hence enhance the performances of Schottky-contacted metal-semiconductor-metal (MSM) structured GaN nanobelt (NB)-based PDs. In general, the response level of the PDs was obviously enhanced by the piezotronic effect when applying a strain on devices. The responsivity of the PD was increased by 18%, and the sensitivity was enhanced by from 22% to 31%, when illuminated by a 325 nm laser with light intensity ranging from 12 to 2 W/cm2. Carefully studying the mechanism using band structure diagrams reveals that the observed enhancement of the PD performance resulted from the change in SBH caused by external strain as well as light intensity. Using piezotronic effects thus provides a practical way to enhance the performance of PDs made not only of GaN, but also other wurtzite and zinc blende family materials. Open image in new window

A new method to measure the carrier concentration of p-GaN

A new method to measure the carrier concentration of p-GaN is proposed. The main idea is as follows: the difference between p-n+ structure GaN ultraviolet photodetector’s quantum efficiency at two different wavelengths varies remarkably with increasing reversed bias, and the most characteristic change occurs at a reversed voltage under which the p-GaN layer starts to be completely depleted, consequently the carrier concentration of p-GaN can be derived basing on this effect. The simulation results prove the validity of the idea even under the condition of high surface recombination velocity and bad ohmic contact. The thickness choice of p-GaN samples during the carrier concentration test experiment using this method is investigated. It is shown that the optimized thickness of p-GaN decreases with the increase of carrier concentration of p-GaN samples.

Hole concentration test of p-type GaN by analyzing the spectral response of p–n+ structure GaN ultraviolet photodetector

A new method to test the hole concentration of p-type GaN is proposed, which is carried out by analyzing the spectral response of p-n(+) structure GaN ultraviolet photodetector. It is shown that the spectral response of the photodetector changes considerably with reversed bias. It is found that the difference between photodetector's quantum efficiency at two wavelengths, i.e. 250 and 361 nm, varies remarkably with increasing reversed bias. According to the simulation calculation, the most characteristic change occurs at a reversed voltage under which the p-GaN layer starts to be completely depleted. Based on this effect the carrier concentration of p-GaN can be derived.

Effects of thermal annealing on the properties of GaN MSM UV photodetectors

Abstract Metal–semiconductor–metal-structured GaN ultraviolet photodetectors have been fabricated on sapphire substrates by metalorganic chemical vapor deposition. The properties of GaN photodetectors have been improved through thermal annealing. With a 3 V bias, the very low dark current is about 200 pA, the maximum responsivity of 0.19 A/W is achieved at 362 nm, and the corresponding detectivity is 1.2×1011 cm Hz1/2/W. The physical mechanism of the effects of thermal annealing also has been studied.

Changes in surface state density due to chlorine treatment in GaN Schottky ultraviolet photodetectors

A chlorination surface treatment was used to reduce the surface density of states of a n-type GaN surface, which improves the Schottky performances of the resultant metal-semiconductor contact. Using capacitance-frequency measurement, the surface state density of the chlorine-treated GaN surface was about one order less than that without chlorination treatment. The dark current of the chlorine-treated GaN ultraviolet photodetectors (UV-PDs) is 1.5 orders of magnitude lower than that of those without chlorination treatment. The products of quantum efficiency and internal gain of the GaN Schottky UV-PDs without and with chlorination treatment under conditions of −10 V reverse bias voltage at a wavelength of 330 nm were 650% and 100%, respectively. The internal gain in chlorine-treated GaN UV-PDs can therefore be reduced due to a decrease in the surface state density.