High Performance Deep Ultraviolet Research Articles

Highly Wavelength-Selective Self-Powered Solar-Blind Ultraviolet Photodetector Based on Colloidal Aluminum Nitride Quantum Dots.

Colloidal quantum dots are semiconductor nanocrystals endowed with unique optoelectronic properties. A major challenge to the field is the lack of methods for synthesizing quantum dots exhibit strong photo-response in the deep-ultraviolet (DUV) band. Here, a facile solution-processed method is presented for synthesizing ultrawide bandgap aluminium nitride quantum dots (AlN QDs) showing distinguished UV-B photoluminescence. Combined with the strong optical response in solar blind band, a solution-processed, self-powered AlN-QDs/β-Ga2O3 solar-blind photodetector is demonstrated. The photodetector is characterized with a high responsivity of 1.6mAW-1 under 0V bias and specific detectivity 7.60 × 10-11 Jones under 5V bias voltage with good solar blind selectivity. Given the solution-processed capability of the devices and extraordinary properties of AlN QDs, this study anticipates the utilization of AlN QDs will open up unique opportunities for cost-effective industrial production of high-performance DUV optoelectronics for large-scale applications.

Enhancing external quantum efficiency of deep ultraviolet micro-leds through geometry design and multi-physics field coupling analysis

Abstract High-efficiency deep-ultraviolet (DUV) micro light-emitting diodes (LEDs) are explored for inspiring development in numerous fields, such as non-line-of-sight solar-blind communication, optical pumping, and maskless lithography. In this study, we performed FDTD and SimuLED calculations to investigate the optimized DUV micro-LED structure geometry for high light extraction efficiency (LEE) by designing different mesa structures, including square, hexagonal, and circular geometries of micro-LEDs emitted at a wavelength of 275 nm. The results showed that a circular mesa of 5 μm diameter achieved a LEE of 27% from the bottom and sidewall emissions of as-prepared DUV micro-LED. And both the near- and far-field transverse magnetic polarized light intensities were enhanced by a factor of 1.5 over the square and hexagonal mesas. Meanwhile, the transverse electric (TE) polarized light of the circular mesa structure was enhanced and concentrated along the normal direction. Moreover, the internal quantum efficiency (IQE) of circular mesas with varied sizes was comprehensively investigated in the interactions of the thermal and electric fields. An AlGaN-based DUV micro-LED with a diameter of 5 μm was found to obtain the highest IQE owing to a high current-density distribution and its self-heating properties, thereby achieving a sufficiently high external quantum efficiency of 26.75%. This study provides a comprehensive technical report, including electrical, thermal, and optical analyses, and a new perspective for developing high-efficiency, high-performance DUV micro-LEDs in practical applications.

Low temperature growth of MgGa2O4 films for deep ultraviolet photodetectors

MgGa2O4 films were grown on the (0001) sapphire substrates by using pulsed laser deposition at substrate temperatures ranging from 100 to 500 °C. The influences of substrate temperature on the structural and optical properties of the films have been systematically investigated. Deep ultraviolet (DUV) photodetectors were demonstrated by using the MgGa2O4 films as wavelength-selective absorber layers. The photodetectors based on the MgGa2O4 films grown at 500 °C exhibited excellent peak responsivity (0.18 A/W) around 220 nm, high rejection ratio (R220 nm/R350 nm = 104), and reproducibility. Our results obtained in this work indicate the pulsed laser deposition is a promising technique for obtaining high quality MgGa2O4 film at low growth temperature, paving the way to realize high performance DUV photodetectors based on MgGa2O4 film for novel optoelectronic applications.

Nanoscale-Thick CuPc/β-Ga2O3 p–n Junctions for Harsh-Environment-Resistant Self-Powered Deep-UV Photodetectors

Due to their crucial role in ultraviolet communication and monitoring, deep-ultraviolet (DUV) photodetectors have garnered much interest. Recently, Ga2O3 has emerged as the best material for DUV photodetectors because of its ultrawide bandgap (4.5–4.9 eV), excellent UV photon absorption coefficient, high structural stability, and affordability. However, there are several difficulties in realizing high-performance Ga2O3-based DUV photodetectors with a high tolerance for harsh environments. In this work, nanoscale-thick CuPc/β-Ga2O3 p–n junctions were used to build high-performance DUV photodetectors by a straightforward solution-processing approach. The p–n junction photodetectors exhibit improved photoelectric performance compared to a single device made of β-Ga2O3 or CuPc, with a photo-to-dark current ratio of 3700 and a fast response time of ∼20 ms under a bias of 0 V. Due to the excellent stability of the nanoscale-thick CuPc film, the device can maintain a high photocurrent even at high temperatures or under long-term DUV irradiation. Our work provides an effective strategy toward highly harsh-environment-resistant DUV photodetectors.

Dual-band and dual-mode ultraviolet photodetection characterizations of Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N homo-type heterojunction

The deep-ultraviolet (DUV) photodetectors (PDs) have important applications in lots of fields. Thus, developing self-powered DUV PDs and excavating the inherent mechanism seem seriously crucial to achieving further actual applications. The construction of heterojunction can lead to many desired characteristics in optoelectronic devices. In the field of DUV photodetection, Ga<sub>2</sub>O<sub>3</sub> has been a popular subject for constructing DUV PDs. So, it is necessary to develop self-powered Ga<sub>2</sub>O<sub>3</sub>-based DUV PDs through fabricating its heterogeneous structure. Therefore, in this work, the Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction DUV PD is fabricated and discussed, which can achieve 254 and 365 nm DUV light photodetection. At positive voltages and negative voltages, the heterojunction PD can operate in a photoconductive mode or a depletion mode, respectively. In view of the PD performance, it displays decent dark current and DUV photoresponses. At voltage of 5 and –5 V, under 254 nm DUV light illumination, the photoresponsivity (<i>R</i>) is 2.09 and 66.32 mA/W, respectively, while under 365 nm DUV light illumination, <i>R</i> is 0.22 and 34.75 mA/W, respectively. In addition, under the built-in electric field (<i>E</i><sub>built-in</sub>), <i>R</i> is 0.13 and 0.01 mA/W for 254 nm and 365 nm DUV light illumination, respectively. In all, the fabricated heterojunction PD displays promising prospects in the coming next-generation semiconductor photodetection technology. The results in this work indicate the potential of Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction with high performance DUV photodetection. Furthermore, except for the characterizations of the materials and photodetector, in the end of this paper, the operating mechanism of the dual-band dual-mode heterojunction PD is analyzed through its heterogeneous energy-band diagram. It is concluded that the illustrated dual-band dual-mode Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction can be sensitive to UVA waveband and UVC waveband in the electromagnetic spectrum, extending its photodetection region. And, the dual-mode (photoconductive mode and depletion mode) photodetection indicates two kinds of carrier transports in one PD, which can be attributed to the successful construction of the N-N tomo-type Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction.

High-sensitive, self-powered deep UV photodetector based on p-CuSCN/n-Ga2O3 thin film heterojunction

Herein, a CuSCN/Ga2O3 heterojunction device was fabricated by spin-coating and metal-organic chemical vapor deposition (MOCVD) methods. Under the irradiation of 254 nm deep ultraviolet (DUV) light with an intensity of 1000 μW/cm2, the device shown high sensitivity and favorable self-powered performances with photo-to-dark current ratio (PDCR) of 1.29 × 104, photo responsivity (R) of 5.5 mA/W, rejection ratio (R270nm/R600nm) of 4.33 × 103, and specific detectivity (D∗) of 3.8 × 1011 cm Hz1/2W−1 (Jones) at −5 V. In addition, the device has a rise time of 0.45 s and 3.80 s, and a decay time of 0.26 s and 0.26 s. In general, the CuSCN/Ga2O3 heterojunction prepared in this work may well be a potential candidate for achieving a self-powered and high-performance DUV photodetector.

Band-Engineered Structural Design of High-Performance Deep-Ultraviolet Light-Emitting Diodes

In this study, systematic structural design was investigated numerically to probe into the cross-relating influences of n-AlGaN layer, quantum barrier (QB), and electron-blocking layer (EBL) on the output performance of AlGaN deep-ultraviolet (DUV) light-emitting diodes (LEDs) with various Al compositions in quantum wells. Simulation results show that high-Al-composition QB and high-Al-composition EBL utilized separately are beneficial for the enhancement of carrier confinement, while the wall-plug efficiency (WPE) degrades dramatically if both high-Al-composition QB and EBL are existing in a DUV LED structure simultaneously. DUV LEDs may be of great optical performance with appropriate structural design by fine-tuning the material parameters in n-AlGaN layer, QB, and EBL. The design curves provided in this paper can be very useful for the researchers in developing the DUV LEDs with a peak emission wavelength ranging from 255 nm to 285 nm.

Controlled bunching approach for achieving high efficiency active region in AlGaN-based deep ultraviolet light-emitting devices with dual-band emission

Developing efficient active region structures with low sensitivity to threading dislocation density (TDD) is of great technical significance for AlGaN-based deep ultraviolet (DUV) light-emitting devices. Here, we propose an active region strategy by introducing bunching effect to form self-assembled sidewall quantum wells (SQWs) with much stronger carrier confinement, resulting in a significant enhancement of internal quantum efficiency (from 46% to 59%) compared to the commonly adopted multiple quantum wells (MQWs) due to the lower sensitivity to TDD. As a demo, an AlGaN-based DUV light-emitting diode (LED) with the proposed active region involving both SQWs and MQWs presents dual-band emission and a consequent 68% enhancement in light output power compared to the DUV-LED with only MQWs as the active region, suggesting that the proposed architecture is fully suitable for the development of high performance DUV light-emitting devices even based on poor or medium quality materials.

Ultrawide Band Gap Oxide Nanodots (Eg > 4.8 eV) for a High-Performance Deep Ultraviolet Photovoltaic Detector.

Recently, deep ultraviolet (DUV) detectors based on gallium oxide (Ga2O3) have become promising in industrial and aerospace applications because of their inherently ultrawide band gaps (4.5-4.9 eV). Because most of them are difficult to be prepared, the lattice mismatch with the substrate and the expensive cost have to be taken into consideration. Because of such problems, the solution-processible nanodots (NDs) with ultrasmall size provide a solution. Here, we propose to use γ-Ga2O3 NDs as the DUV-sensitive layer to construct a DUV p-i-n-type detector with photovoltaic properties (p-graphene/γ-Ga2O3 NDs/n-SiC). The device exhibits a high photoresponsivity (5.8 mA/W) and detectivity (7.6 × 1010 jones) with a 250 nm source illumination under 0 V bias. Moreover, the DUV/UV injection ratio (R250/R360) reaches 103. These results demonstrate a new way to manufacture low-cost, high-performance DUV detectors based on γ-Ga2O3.

Sub-microsecond response time deep-ultraviolet photodetectors using α-Ga2O3 thin films grown via low-temperature atomic layer deposition

A photodetector responding to deep-ultraviolet (DUV) wavelengths (λ = 200–300 nm) is developed via the growth of crystalline α-Ga2O3 thin films with a bandgap of 5.2 eV on sapphire (0006) substrates using atomic layer deposition (ALD), at a low temperature of approximately 250 °C. The achievement of crystalline α-Ga2O3 films at such a low temperature is viable owing to the lattice match between α-Ga2O3 and sapphire crystals, which is confirmed by X-ray diffraction measurements and high-resolution transmission electron microscopy analysis. Metal–semiconductor–metal photodetectors (active area of 30 × 30 μm2) using 10-nm-thick α-Ga2O3 films exhibit a rise time (time required for the photocurrent to increase from 10% to 90% of its final value under illumination) of 539 ns at λ = 266 nm. Such an ultrafast response to DUV with λ = 266 nm is maintained for 3-nm-thick α-Ga2O3 photodetectors, suggesting that our ALD process is adequate for obtaining high-quality ultrathin α-Ga2O3 films. Measurements of the wavelength-resolved photocurrents reveal that the α-Ga2O3 photodetectors respond selectively to DUV wavelengths (λ = 200–300 nm), without responding to other longer wavelengths (λ > 300 nm). The responsivity is maximized to 0.76 A/W at λ = 253 nm, and drops off at λ ≈ 300 nm (i.e., a cutoff wavelength). The dark current measured at 10 V is as low as 0.5 pA, and the signal-to-noise ratio reaches 104, both of which underpin the pristine material quality of the ALD-grown α-Ga2O3 films. We believe that the fabrication of photodetectors using α-Ga2O3 thin films at such a low temperature will provide an economically feasible solution for high-performance DUV detection and sensing applications.

High-performance solar-blind flexible deep-UV photodetectors based on quantum dots synthesized by femtosecond-laser ablation

High-performance deep ultraviolet (DUV) photodetectors operating at ambient conditions with < 280 nm detection wavelengths are in high demand because of their potential applications in diverse fields. We demonstrate for the first time, high-performance flexible DUV photodetectors operating at ambient conditions based on quantum dots (QDs) synthesized by femtosecond-laser ablation in liquid (FLAL) technique. Our method is facile without complex chemical procedures, which allows large-scale cost-effective devices. This synthesis method is demonstrated to produce highly stable and reproducible ZnO QDs from zinc nitride target (Zn3N2) without any material degradation due to water and oxygen molecule species, allowing photodetectors operate at ambient conditions. Carbon-doped ZnO QD-based photodetector is capable of detecting efficiently in the DUV spectral region, down to 224 nm, and exhibits high photo-responsivity and stability. As fast-response of DUV photodetector remains significant parameter for high-speed communication; we show fast-response QD-based DUV photodetector. Such surfactant-free synthesis by FLAL can lead to commercially available high-performance low-cost optoelectronic devices based on nanostructures for large scale applications.

Correlation between 193nm absorption and photoluminescence-related defects for fused silica materials

We reported on the study of the correlation between 193nm absorption under 1.5~5.0mJ/cm2 fluence irradiation and photoluminescence (PL) related defects for deep-ultraviolet (DUV) fused silica samples with different H2 and OH contents (0~1200ppm). Experimental results showed strong correlations between apparent nonlinear absorption at 193nm to 650nm PL band originated from a non-bridging oxygen hole center (NBOHC), and between apparent linear absorption at 193nm to 550nm PL band. In addition, only 650nm PL defects showed reversible concentration change under 193nm laser irradiation, indicating a possible link to the rapid damage process (RDP) under DUV irradiation. Experimental observation and theoretical calculations on the dependence of 650nm PL intensity on the laser fluence further demonstrated that the generation and annealing processes of NBOHC in these DUV fused silica samples are mainly due to two-photon excitation induced breakage of SiOH bond and combination of NBOHC with H2. These results give new insight into the influence of NBOHC and SiOH on fused silica’s DUV absorption and transmission properties, and therefore are helpful to the development of high-performance DUV fused silica materials.

Fabrications and application of single crystalline GaN for high-performance deep UV photodetectors

High-quality single crystalline Gallium Nitride (GaN) semiconductor has been synthesized using molecule beam epitaxy (MBE) technique for development of high-performance deep ultraviolet (UV) photodetectors. Thickness of the films was estimated by using surface profile meter and scanning electron microscope. Electronic states and elemental composition of the films were obtained using Raman scattering spectroscopy. The orientation, crystal structure and phase purity of the films were examined using a Siemens x-ray diffractometer radiation. The surface microstructure was studied using high resolution scanning electron microscopy (SEM). Two types of metal pairs: Al-Al, Al-Cu or Cu-Cu were used for interdigital electrodes on GaN film in order to examine the Schottky properties of the GaN based photodetector. The characterizations of the fabricated prototype include the stability, responsivity, response and recovery times. Typical time dependent photoresponsivity by switching different UV light source on and off five times for each 240 seconds at a bias of 2V, respectively, have been obtained. The detector appears to be highly sensitive to various UV wavelengths of light with very stable baseline and repeatability. The obtained photoresponsivity was up to 354 mA/W at the bias 2V. Higher photoresponsivity could be obtained if higher bias was applied but it would unavoidably result in a higher dark current. Thermal effect on the fabricated GaN based prototype was discussed.

Large Optical Gain AlInN-Delta-GaN Quantum Well for Deep Ultraviolet Emitters.

The optical gain and spontaneous emission characteristics of low In-content AlInN-delta-GaN quantum wells (QWs) are analyzed for deep ultraviolet (UV) light emitting diodes (LEDs) and lasers. Our analysis shows a large increase in the dominant transverse electric (TE) polarized spontaneous emission rate and optical gain. The remarkable enhancements in TE-polarized optical gain and spontaneous emission characteristics are attributed to the dominant conduction (C)-heavy hole (HH) transitions achieved by the AlInN-delta-GaN QW structure, which could lead to its potential application as the active region material for high performance deep UV emitters. In addition, our findings show that further optimizations of the delta-GaN layer in the active region are required to realize the high performance AlInN-based LEDs and lasers with the desired emission wavelength. This work illuminates the high potential of the low In-content AlInN-delta-GaN QW structure to achieve large dominant TE-polarized spontaneous emission rates and optical gains for high performance AlN-based UV devices.

Solution-processed graphene quantum dot deep-UV photodetectors.

Fast-response and high-sensitivity deep-ultraviolet (DUV) photodetectors with detection wavelength shorter than 320 nm are in high demand due to their potential applications in diverse fields. However, the fabrication processes of DUV detectors based on traditional semiconductor thin films are complicated and costly. Here we report a high-performance DUV photodetector based on graphene quantum dots (GQDs) fabricated via a facile solution process. The devices are capable of detecting DUV light with wavelength as short as 254 nm. With the aid of an asymmetric electrode structure, the device performance could be significantly improved. An on/off ratio of ∼6000 under 254 nm illumination at a relatively weak light intensity of 42 μW cm(-2) is achieved. The devices also exhibit excellent stability and reproducibility with a fast response speed. Given the solution-processing capability of the devices and extraordinary properties of GQDs, the use of GQDs will open up unique opportunities for future high-performance, low-cost DUV photodetectors.