High quality AlN epilayers were grown on sapphire substrates by metal organic vapor deposition and exploited as active deep ultraviolet (DUV) optoelectronic materials through the demonstration of AlN metal-semiconductor-metal (MSM) photodetectors. DUV photodetectors with peak responsivity at 200nm with a very sharp cutoff wavelength at 207nm have been attained. The AlN MSM photodetectors are shown to possess outstanding features that are direct attributes of the fundamental properties of AlN, including extremely low dark current, high breakdown voltage, and high DUV to visible rejection ratio and high responsivity. The results demonstrate the high promise of AlN as an active material for DUV device applications.

/pdf/200-nm-deep-ultraviolet-photodetectors-based-on-aln-2wj8ggifjk.pdf

200 nm deep ultraviolet photodetectors based on AlN

Laser-induced graphene (LIG) is a simple, environmentally friendly, efficient, and less costly method, as well as can form various shapes on a flexible substrate in situ without the use of masks. More importantly, it can tune the work function of LIG easily by changing laser parameters to control the transportation of carriers. In this work, the work functions of LIG were controlled by adjusting the frequency or speed of the laser, and a series of LIG/GaOx Schottky photodetectors were formed. When the work function of the graphene increases, the Fermi energy is shifted below the crossing point of the Π and Π* bands, and then more electrons or holes can be activated to participate in the conduction process, resulting in low resistance. Meanwhile, a large built-in electric field can be formed when using a high work function LIG, which is more beneficial to separate photo-generated carriers. Enabled by the controllable LIG, LIG/GaOx Schottky photodetectors can be modulated to have high photoresponsivity or self-powered characteristics. Our work provides a high-performance photodetector with excellent mechanical flexibility and long-life stability, promising applications in the flexible optoelectronic fields.

Work function tunable laser induced graphene electrodes for Schottky type solar-blind photodetectors

In this work, we demonstrate a high-performance ultraviolet phototransistor (UVPT) based on the AlGaN/GaN high-electron mobility transistor (HEMT) configuration. When the device is biased at off state, the peak photoresponsivity (R) of 3.6 × 107 A/W under 265 nm illumination and 1.0 × 106 A/W under 365 nm illumination can be obtained. Those two R values are one of the highest among the reported UVPTs at the same detection wavelength under off-state conditions. In addition, we investigate the gate-bias (VGS) dependent photoresponse of the fabricated device with the assistance of band structure analysis. It was found that a more negative VGS can significantly reduce the rise/decay time for 265 nm detection, especially under weak illumination. This can be attributed to a largely enhanced electric field in the absorptive AlGaN barrier that pushes the photo-generated carriers rapidly into the GaN channel. In contrast, the VGS has little impact on the switching time for 365 nm photodetection, since the GaN channel has a larger absorption depth and the entire UVPT simply acts as a photoconductive-type device. In short, the proposed AlGaN/GaN HEMT structure with the superior photodetection performance paves the way for the development of next generation UVPTs.

Demonstration of AlGaN/GaN-based ultraviolet phototransistor with a record high responsivity over 3.6 × 107 A/W

The growing demand for scalable solar-blind image sensors with remarkable photosensitive properties has stimulated the research on more advanced solar-blind photodetector (SBPD) arrays. In this work, the authors demonstrate ultrahigh-performance metal-semiconductor-metal (MSM) SBPDs based on amorphous (a-) Ga2 O3 via a post-annealing process. The post-annealed MSM a-Ga2 O3 SBPDs exhibit superhigh sensitivity of 733 A/W and high response speed of 18 ms, giving a high gain-bandwidth product over 104 at 5 V. The SBPDs also show ultrahigh photo-to-dark current ratio of 3.9 × 107 . Additionally, the PDs demonstrate super-high specific detectivity of 3.9 × 1016 Jones owing to the extremely low noise down to 3.5 fW Hz-1/2 , suggesting high signal-to-noise ratio. Underlying mechanism for such superior photoelectric properties is revealed by Kelvin probe force microscopy and first principles calculation. Furthermore, for the first time, a large-scale, high-uniformity 32 × 32 image sensor array based on the post-annealed a-Ga2 O3 SBPDs is fabricated. Clear image of target object with high contrast can be obtained thanks to the high sensitivity and uniformity of the array. These results demonstrate the feasibility and practicality of the Ga2 O3 PDs for applications in solar-blind imaging, environmental monitoring, artificial intelligence and machine vision.

Ultra-High Performance Amorphous Ga 2 O 3 Photodetector Arrays for Solar-Blind Imaging

Gallium oxide (Ga2O3) is rapidly emerging as a material of choice for the development of solar blind photodetectors and power electronic devices which are particularly suitable in harsh environment applications, owing to its wide bandgap and extremely high Baliga figure of merit (BFOM). The Ga2O3 based devices show robustness against chemical, thermal and radiation environments. Unfortunately, the current Ga2O3 technology is still not mature for commercial usage., Thus, extensive research on the growth of various polymorph of Ga2O3 materials has been carried out. This article aims to provide an overview of the current understanding of epitaxial growth of different phases of Ga2O3 by various growth techniques including pulsed laser deposition (PLD), molecular beam epitaxy (MBE), metal-organic chemical vapour deposition (MOCVD), sputtering, mist chemical vapour deposition (Mist CVD) and atomic layer deposition (ALD).The review also investigates the factors such as the growth temperature, pressure, carrier gas, III/V ratio, substrate as well as doping which would influence the synthesis and the stability of meta stable phases of Ga2O3. In addition, a through discussion of growth window is also provided using phase diagrams for aforementioned epitaxial deposition methods.

Recent advances in the growth of gallium oxide thin films employing various growth techniques- A review

Review of polymorphous Ga2O3 materials and their solar-blind photodetector applications

Gallium oxide (Ga2 O3 ), with an ultrawide bandgap, is currently regarded as one of the most promising materials for solar-blind photodetectors (SBPDs), which are greatly demanded in harsh environment, such as the situations of space exploration and flame prewarning. However, realization of high-performance SBPDs with high tolerance towards harsh environments based on low-cost Ga2 O3 material faces great challenges. Here, defect and doping (DD) engineering towards amorphous GaOX (a-GaOX ) has been proposed to obtain ultrasensitive SBPDs for harsh condition application. Serious oxygen deficiency and doping compensation of the engineered a-GaOX film ensure the high response currents and low dark currents, respectively. Annealing item in nitrogen of DD engineering also incurs the recrystallization of material, formation of nanopores by oxygen escape, and suppression of sub-bandgap defect states. As a result, the tailored GaOX SBPD based on DD engineering not only harvests a record-high responsivity rejection ratio (R254 nm /R365 nm ) of 1.8×107 , 102 times higher detectivity, and 2×102 times faster decay speed than the control device, but also keeps a high responsivity, high photo-to-dark current ratio, and sharp imaging capability even at high temperature (280°C) or high bias (100 V). The proposed DD engineering provides an effective strategy towards highly harsh-environment-resistant GaOX SBPDs. This article is protected by copyright. All rights reserved.

High-Performance Harsh-Environment-Resistant GaOX Solar-Blind Photodetectors via Defect and Doping Engineering.

This letter reports a high-performance solar-blind phototransistor based on N2-annealed $\beta $ -Ga2O3 microflake for weak light detection. The phototransistor exhibits an ultra-low dark current of 27 fA, a high external quantum efficiency of $8.36\times 10^{7}$ %, a photo-to-dark-current ratio of $1.08\times 10 ^{7}$ , a low power consumption of 2.92 pW, and a narrow-band response with a cut off wavelength of 263 nm. In addition, the spectral selectivity can be well modulated by the gate bias and reaches a maximum ${R} _{240}/{R} _{400}$ ratio of $2.4\times 10 ^{4}$ . Especially, an ultra-high responsivity ( ${R}$ ) of $1.71\times 10^{5}$ A/W and a record-high detectivity ( ${D} ^{\ast }$ ) of $1.19\times 10^{18}$ Jones have been achieved under 254nm illumination of $4.1~\mu \text{W}$ /cm2. The superior weak-light-detection performance of the device makes it one of the best Ga2O3 detectors towards solar-blind photodetection application.

High-Detectivity β -Ga₂O₃ Microflake Solar-Blind Phototransistor for Weak Light Detection

Metal oxide thin-films transistors (TFTs) produced from solution-based printing techniques can lead to large-area electronics with low cost. However, the performance of current printed devices is inferior to those from vacuum-based methods due to poor film uniformity induced by the “coffee-ring” effect. Here, we report a novel approach to print high-performance indium tin oxide (ITO)-based TFTs and logic inverters by taking advantage of such notorious effect. ITO has high electrical conductivity and is generally used as an electrode material. However, by reducing the film thickness down to nanometers scale, the carrier concentration of ITO can be effectively reduced to enable new applications as active channels in transistors. The ultrathin (~10-nm-thick) ITO film in the center of the coffee-ring worked as semiconducting channels, while the thick ITO ridges (>18-nm-thick) served as the contact electrodes. The fully inkjet-printed ITO TFTs exhibited a high saturation mobility of 34.9 cm2 V−1 s−1 and a low subthreshold swing of 105 mV dec−1. In addition, the devices exhibited excellent electrical stability under positive bias illumination stress (PBIS, ΔVth = 0.31 V) and negative bias illuminaiton stress (NBIS, ΔVth = −0.29 V) after 10,000 s voltage bias tests. More remarkably, fully printed n-type metal–oxide–semiconductor (NMOS) inverter based on ITO TFTs exhibited an extremely high gain of 181 at a low-supply voltage of 3 V, promising for advanced electronics applications.

/pdf/fully-printed-high-performance-n-type-metal-oxide-thin-film-rp6prczne3.pdf

Mengfan Ding

Papers

Review of polymorphous Ga2O3 materials and their solar-blind photodetector applications

High-Performance Harsh-Environment-Resistant GaOX Solar-Blind Photodetectors via Defect and Doping Engineering.

High-Detectivity β -Ga₂O₃ Microflake Solar-Blind Phototransistor for Weak Light Detection

Fully Printed High-Performance n-Type Metal Oxide Thin-Film Transistors Utilizing Coffee-Ring Effect.

Balancing the Transmittance and Carrier-Collection Ability of Ag Nanowire Networks for High-Performance Self-Powered Ga2O3 Schottky Photodiode