Deep Ultraviolet Photodetectors Research Articles

Fabrication of cerium-doped β-Ga2O3 epitaxial thin films and deep ultraviolet photodetectors.

High-quality cerium-doped β-Ga2O3 (Ga2O3:Ce) thin films could be achieved on (0001)α-Al2O3 substrates using a pulsed-laser deposition method. The impact of dopant contents concentration on crystal structure, optical absorption, photoluminescence, and photoelectric properties has been intensively studied. X-ray diffraction analysis results have shown that Ga2O3:Ce films are highly (2¯01) oriented, and the lattice spacing of the (4¯02) planes is sensitive to the Ce doping level. The prepared Ga2O3:Ce films show a sharp absorption edge at about 250nm, meaning a high transparency to deep ultraviolet (DUV) light. The photoluminescence results revealed that the emissions were in the violet-blue-green region, which are associated with the donor-acceptor transitions with the Ce3+ and oxygen vacancies related defects. A simple DUV photodetector device with a metal-semiconductor-metal structure has also been fabricated based on Ga2O3:Ce thin film. A distinct DUV photoresponse was obtained, suggesting a potential application in DUV photodetector devices.

Electrodeposited CuSCN metal-semiconductor-metal high performance deep-ultraviolet photodetector

This study reports a low-cost electrodeposited copper(I) thiocyanate (CuSCN) film as a wide band-gap absorber layer for deep-ultraviolet (DUV) photodetector (PD) applications. Electrodeposited CuSCN films deposited with electrolyte concentrations of 24 mM and 33 mM were evaluated as the absorber layers for planar metal-semiconductor-metal PDs. The best photo-responsivity of the CuSCN PD was found to be as high as 70.3 A/W at a −1 V bias under DUV illumination at 300 nm, which corresponded to an external quantum efficiency of 3.1 × 104%. Furthermore, the DUV-to-visible rejection ratio (R300 nm/R400 nm) of ∼103 was realized. This study demonstrated that the CuSCN film has great potential for low-cost DUV PD applications.

Two dimensional hole gas induced by the heterointerface of nonpolar plane AlN(1 [formula omitted] 0 0)/H-terminated (1 0 0) diamond

Tailoring the electronic states of the AlN/diamond (AlN/C) interface is critical to develop the next generation semiconductor devices such as the deep-ultraviolet light-emitting diode, photodetector, and high-power high-frequency field-effect transistor. In this work, we investigate the electronic properties of the nonpolar plane AlN(1 1¯ 0 0)/H terminated (1 0 0) diamond heterointerface by using first-principles method with regard to different surface structures of diamond (1 0 0) plane, a semiconductor feature is predicted at the interface of nonpolar plane AlN(1 1¯ 0 0)/C(1 0 0), and a two dimensional hole gas (2DHG) is generated when diamond surface is terminated by hydrogen. The charge transfer at the interface strongly depends on the surface termination of diamond, on which hydrogen suppresses the charge exchange at the interface. The band alignments of AlN/C heterostructures show a typical electronic character of the type-II staggered band configuration. The hydrogen-termination of diamond markedly increases the band offsets with a maximum valence band offset of 2.7 eV and a conduction band offset of 2.0 eV for the AlN(1 1¯ 0 0)/C(1 0 0). The observation of the 2DHG at the nonpolar AlN(1 1¯ 0 0)/HC(1 0 0) interface and the larger band offsets open the avenue for the development of novel high-power, high-frequency diamond power devices.

High-performance deep ultraviolet photodetectors based on few-layer hexagonal boron nitride

Hexagonal boron nitride (h-BN), an isomorph of graphene, has attracted great attention owing to its potential applications as an ultra-flat substrate or gate dielectric layer in novel graphene-based devices. Besides, h-BN appears to be a promising material for deep ultraviolet (DUV) optoelectronic applications because of its extraordinary physical properties, such as wide band gap and high absorption coefficient. In this work, two-dimensional h-BN with controllable layers was synthesized on Cu foils by ion beam sputtering deposition, and DUV photodetectors were fabricated from the transferred h-BN layers on SiO2/Si substrates. The h-BN layers synthesized at the higher substrate temperature possess a lower density of domain boundaries and higher crystalline quality, and the photodetectors based on a 3 nm h-BN layer exhibited high performance with an on/off ratio of >103 under DUV light illumination at 212 nm and a cutoff wavelength at around 225 nm. This work demonstrates that two-dimensional h-BN layers are promising for the construction of high-performance solar-blind photodetectors.

High-performance self-powered deep ultraviolet photodetector based on MoS2/GaN p–n heterojunction

Self-powered MoS2/GaN p–n heterojunction photodetectors exhibited high sensitivity to deep-UV light with high responsivity, specific detectivity and fast response speeds.

Solution-processed solar-blind deep ultraviolet photodetectors based on strongly quantum confined ZnS quantum dots

Strongly quantum confined ZnS quantum dots were synthesized and solar-blind deep ultraviolet photodetectors were fabricated via a solution process.

High Gain β-Ga2O3 Solar-Blind Schottky Barrier Photodiodes via Carrier Multiplication Process

β-Ga2O3, which is a ultra-wide band-gap semiconductor, is an attractive material for next-generation solar-blind photodetectors. A high gain solar-blind Schottky barrier photodetector using an exfoliated single crystalline β-Ga2O3 nano-layer was demonstrated by employing internal carrier multiplication process. Excellent spectral selectivity with high responsivity was obtained between UV-A and UV-C wavelengths with fast response/decay characteristics. The gain of our β-Ga2O3 solar-blind PD was ∼3.78 × 103 under the multiplication mode at the reverse bias of −60 V, where the peak electric field was estimated to be 4.3 MV/cm (equivalent to impact ionization coefficient of 5 × 103 cm−1). Compared with non-multiplication mode, outstanding photo-sensing performances were achieved under the multiplication mode, including a responsivity of 8.18 A/W, a photocurrent-to-dark-current ratio of ∼103 and external quantum efficiency of ∼4 × 103%. High gain via carrier multiplication process in a β-Ga2O3 photodiode proposes a new route toward high performance solar-blind deep-UV photodetectors.

High-Performance and Self-Powered Deep UV Photodetectors Based on High Quality 2D Boron Nitride Nanosheets.

High-quality two-dimensional (2D) crystalline boron nitride nanosheets (BNNSs) were grown on silicon wafers by using pulsed plasma beam deposition techniques. Self-powered deep ultraviolet (DUV) photodetectors (PDs) based on BNNSs with Schottky contact structures are designed and fabricated. By connecting the fabricated DUV photodetector to an ammeter, the response strength, response time and recovery time to different DUV wavelengths at different intensities have been characterized using the output short circuit photocurrent without a power supply. Furthermore, effects of temperature and plasma treatment on the induced photocurrent response of detectors have also been investigated. The experimental data clearly indicate that plasma treatment would significantly improve both induced photocurrent and response time. The BNNS-based DUV photodetector is demonstrated to possess excellent performance at a temperature up to 400 °C, including high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability, which is better than almost all reported semiconducting nanomaterial-based self-powered photodetectors.

Deep-ultraviolet photodetector based on exfoliated n-type β-Ga2O3 nanobelt/p-Si substrate heterojunction

Low-dimensional semiconductor p-n junctions as components for optoelectronic devices are considered to be more promising than thin film equivalents. We fabricated heterojunction p-n solar blind photodiodes with the configuration of n-type β-Ga2O3 nanobelts contacted onto p-Si substrates. The junction between β-Ga2O3 and Si was formed by van der Waals interactions. The fabricated heterojunction p-n diodes exhibited typical rectifying current–voltage characteristics, with a rectification ratio as high as 1.56×104 at ±20 V and an ideality factor of approximately eight. Photoresponsive measurements showed that the heterojunction p-n diodes had a high sensitivity and selectivity for light at a wavelength of 254 nm, with fast response and decay characteristics. For the fast-response components, the response time constant was 4.06 s and the decay time constant was 0.16 s. The exfoliated β-Ga2O3 nanobelt/Si p-n heterojunction presented here constitutes a functional unit for low-dimensional ultra-wide bandgap electronic and optoelectronic devices.

Deep Ultraviolet Copper(I) Thiocyanate (CuSCN) Photodetectors Based on Coplanar Nanogap Electrodes Fabricated via Adhesion Lithography.

Adhesion lithography (a-Lith) is a versatile fabrication technique used to produce asymmetric coplanar electrodes separated by a <15 nm nanogap. Here, we use a-Lith to fabricate deep ultraviolet (DUV) photodetectors by combining coplanar asymmetric nanogap electrode architectures (Au/Al) with solution-processable wide-band-gap (3.5-3.9 eV) p-type semiconductor copper(I) thiocyanate (CuSCN). Because of the device's unique architecture, the detectors exhibit high responsivity (≈79 A W-1) and photosensitivity (≈720) when illuminated with a DUV-range (λpeak = 280 nm) light-emitting diode at 220 μW cm-2. Interestingly, the photosensitivity of the photodetectors remains fairly high (≈7) even at illuminating intensities down to 0.2 μW cm-2. The scalability of the a-Lith process combined with the unique properties of CuSCN paves the way to new forms of inexpensive, yet high-performance, photodetectors that can be manufactured on arbitrary substrate materials including plastic.

High-performance solar-blind Al0.6Ga0.4N/Al0.5Ga0.5N MSM type photodetector

An Al0.6Ga0.4N/Al0.5Ga0.5N metal–semiconductor–metal (MSM) deep-ultraviolet (DUV) photodetector was developed. It possesses both high photosensitivity and high rejection ratios in comparison to photomultiplier tubes (PMTs). The photodetector was designed to have a two-dimensional electron gas layer at the Al0.6Ga0.4N/Al0.5Ga0.5N hetero-interface, which plays an important role in increasing the photosensitivity. Additionally, for reducing the dark current, a V/Al/Mo/Au electrode was employed as a Schottky electrode with an appropriate barrier height. Upon irradiation with 10 μW/cm2, an extremely low dark current (10−11 A) and high photocurrent (5 × 10−5 A) were achieved at a bias voltage of 5 V. The photodetector was true solar-blind with a cut-off wavelength of 280 nm. A high photosensitivity of 106 A/W and a rejection ratio of 106 were realized under the irradiation of 10 nW/cm2 DUV photons. The present results revealed that the AlGaN/AlGaN MSM DUV photodetector is one of the most suitable candidates for an all solid-state photodetector with a performance superior to PMTs.

High-Responsivity Deep-Ultraviolet-Selective Photodetectors Using Ultrathin Gallium Oxide Films

Wavelength-selective photodetectors responding to deep-ultraviolet (DUV) wavelengths (λ = 200–300 nm) are drawing significant interest in diverse sensing applications, ranging from micrometer biological molecules to massive military missiles. However, most DUV photodetectors developed thus far have suffered from long response times, low sensitivity, and high processing temperatures, impeding their practical use. Here, we report fast, high-responsivity, and general-substrate-compatible DUV photodetectors based on ultrathin (3–50 nm) amorphous gallium oxide (GaOX) films grown by low-temperature (∼<250 °C) atomic layer deposition (ALD) for the first time. ALD-grown GaOX films on glass substrates display a typical amorphous nature, which is identified by electron beam diffraction and X-ray diffraction measurements, while their band gap is sharply featured at ∼4.8 eV. Metal–semiconductor–metal photodetectors (active area of 30 × 30 μm2) using the 30-nm-thick GaOX films work reliably only for DUV wavelengths; t...

Growth and characterization of β-Ga2O3 thin films by molecular beam epitaxy for deep-UV photodetectors

The growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O3) substrates. The compound source provides oxidized gallium molecules in addition to oxygen when heated from an iridium crucible in a high temperature effusion cell enabling a lower heat of formation for the growth of Ga2O3, resulting in a more efficient growth process. This source also enabled the growth of crystalline β-Ga2O3 without the need for additional oxygen. The influence of the substrate temperatures on the crystal structure and quality, chemical bonding, surface morphology, and optical properties has been systematically evaluated by x-ray diffraction, scanning transmission electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, spectroscopic ellipsometry, and UV-vis spectroscopy. Under optimized growth conditions, all films exhibited pure 2¯01 oriented β-Ga2O3 thin films with six-fold rotational symmetry when grown on a sapphire substrate. The thin films demonstrated significant absorption in the deep-ultraviolet (UV) region with an optical bandgap around 5.0 eV and a refractive index of 1.9. A deep-UV photodetector fabricated on the high quality β-Ga2O3 thin film exhibits high resistance and small dark current (4.25 nA) with expected photoresponse for 254 nm UV light irradiation suggesting that the material grown using the compound source is a potential candidate for deep-ultraviolet photodetectors.

High Operating Temperature and Low Power Consumption Boron Nitride Nanosheets Based Broadband UV Photodetector

We extend our work on the use of digitally controlled pulsed laser plasma deposition (PLPD) technique to synthesize high quality, 2-dimensional single crystalline boron nitride nanosheets (BNNSs) at a low substrate temperature for applications in high-performance deep UV photodetectors. The obtained sample consists of a large amount of BNNSs partially overlapping one another with random orientations. Each sheet is composed of a few (from 2 to 10) stacked atomic layers exhibiting high transparency due to its highly ordered hBN crystallinity. Deep UV detectors based on the obtained BNNSs were designed, fabricated, and tested. The bias and temperature effects on the photocurrent strength and the signal-to-noise ratio have been carefully characterized and discussed. A significant shift in the cut off wavelength of the BNNSs based photodetectors was observed suggesting a band gap reduction as a result of the BNNSs’ collective structure. The newly designed photodetector presented exceptional properties: a high sensitivity to weak intensities of radiation in both UVC and UVB range while remaining visible-blind, and a high signal-to-noise ratio operation even at temperatures as high as 400 °C. In addition, the BNNSs based photodetector exhibited potential for self-powered operation.

A self-powered deep-ultraviolet photodetector based on an epitaxial Ga2O3/Ga:ZnO heterojunction

An epitaxial β-Ga2O3/Ga:ZnO heterojunction based self-powered DUV photodetector with an excellent wavelength selectivity and a high DUV/visible rejection ratio.

Impurity Compensation Effect Induced by Tin Valence Change in α-Ga1.4Sn0.6O3 Thin Films.

Corundum-structured α-phase Ga1.4Sn0.6O3 thin films have been deposited on m-plane Al2O3(300) substrates using laser molecular beam epitaxy technology. With increasing of the oxygen partial pressure, the crystal lattice of Ga1.4Sn0.6O3 films expands due to tin ions valence changes from Sn4+ to Sn2+. The resistivity of the film deposited under 3 × 10-5 Pa is 3.54 × 104 Ω·cm, which decreases by about 2 orders of magnitude than that fabricated under 3 × 10-1 Pa. The mixture valence of Sn2+ and Sn4+ ions leads to the impurity altitude compensation effect. The deep ultraviolet photodetector based on α-phase Ga1.4Sn0.6O3 thin films was fabricated. With the oxygen partial pressure reducing gradually, the dark current and the photocurrent increase, and the relaxation time constants diminish, respectively.

Deep ultraviolet photodetectors based on p-Si/i-SiC/n-Ga2O3 heterojunction by inserting thin SiC barrier layer

Deep ultraviolet photodetectors based on p-Si/n-Ga2O3 and p-Si/i-SiC/n-Ga2O3 heterojunctions were fabricated by laser molecular beam epitaxial (L-MBE), respectively. In compare with p-Si/n-Ga2O3 heterostructure-based photodetector, the dark current of p-Si/i-SiC/n-Ga2O3-based photodetector decreased by three orders of magnitude, and the rectifying behavior was tuned from reverse to forward. In order to improve the quality of the photodetector, we reduced the oxygen vacancies of p-Si/i-SiC/n-Ga2O3 heterostructures by changing the oxygen pressure during annealing. As a result, the rectification ratio (I F/I R) of the fabricated photodetectors was 36 at 4.5 V and the photosensitivity was 5.4 × 105% under the 254 nm light illumination at −4.5 V. The energy band structure of p-Si/n-Ga2O3 and p-Si/i-SiC/n-Ga2O3 heterostructures was schematic drawn to explain the physic mechanism of enhancement of the performance of p-Si/i-SiC/n-Ga2O3 heterostructure-based deep UV photodetector by introduction of SiC layer.

Vertical metal-semiconductor-metal deep UV photodetectors based on hexagonal boron nitride nanosheets prepared by laser plasma deposition

We report on the synthesis and characterization of hexagonal boron nitride nanosheets (BNNSs) for deep ultraviolet (DUV) photodetector applications. The vertical structure metal-semiconductor-metal (MSM) solar-blind DUV photodetector based on BNNSs has been fabricated and tested. The detector performance is stable under UV radiation at temperatures exceeding 100°C. The bandgap engineered material has a cutoff wavelength of approximately 250 nm and does not respond to any longer wavelength or visible light. The photodetector demonstrates the advantages of hexagonal BNNSs as a wide band gap material for DUV detection in harsh environments.

Highly sensitive and ultrafast deep UV photodetector based on a β-Ga2O3 nanowire network grown by CVD

We investigated a high-performance deep ultraviolet photodetector based on a β-gallium oxide (β-Ga2O3) nanowire network. β-Ga2O3 nanowires were grown at different temperatures by a chemical vapor deposition method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive spectrum analysis were utilized to characterize the structure and morphology. With increasing the temperature, the nanowires became thicker and their surface appeared more rough with many kinks and branch-like shapes. It is proposed that the growth mechanism is dominated by a combination of vapor–liquid–solid and vapor–solid growth. A photodetector based on the best quality β-Ga2O3 nanowire network was fabricated by a simple and cheap mask method, which exhibited excellent photoelectric performance. The responsive spectrum presented a peak response located at 231 nm with a sharp cutoff at 270 nm. The response rejection ratio of I231 nm/I290 nm is more than three orders of magnitude, demonstrating an intrinsic solar-blindness. The full width at half maximum of the response curve is only 1.2 ns under pulsed laser irradiation. The high sensitivity, superior selectivity, ultrafast response speed and simple fabrication technology show that the β-Ga2O3 nanowire network has promising application in solar-blind photodetectors.

Growth of hexagonal boron nitride on (111) Si for deep UV photonics and thermal neutron detection

Hexagonal boron nitride (hBN) growth was carried out on (111) Si substrates at a temperature of 1350 °C using a cold wall chemical vapor deposition system. The hBN phase of the deposited films was identified by the characteristic Raman peak at 1370 cm−1 with a full width at half maximum of 25 cm−1, corresponding to the in-plane stretch of B and N atoms. Chemical bonding states and composition of the hBN films were analyzed by X-ray photoelectron spectroscopy; the extracted B/N ratio was 1.03:1, which is 1:1 within the experimental error. The fabricated metal-hBN-metal devices demonstrate a strong deep UV (DUV) response. Further, the hBN growth on the vertical (111) surfaces of parallel trenches fabricated in (110) Si was explored to achieve a thermal neutron detector. These results demonstrate that hBN-based detectors represent a promising approach towards the development of DUV photodetectors and efficient solid-state thermal neutron detectors.