Solar-blind Ultraviolet Photodetectors Research Articles

High-performance AlGaN metal–semiconductor–metal solar-blind ultraviolet photodetectors by localized surface plasmon enhancement

AlGaN-based solar-blind ultraviolet photodetectors have attractive potential applications in the fields of missile plume detection, biochemical sensing, solar astronomy, etc. In this work, significant deep ultraviolet detection enhancement is demonstrated on AlGaN-based metal–semiconductor–metal (MSM) solar-blind ultraviolet photodetectors by introducing the coupling of localized surface plasmon from Al nanoparticles with the high-Al-content AlGaN epilayer. The size-controlled Al nanoparticle arrays fabricated by nanosphere lithography can not only reduce the detectors' dark current but also bring about greatly enhanced responsivity. The peak responsivity of AlGaN-based MSM solar-blind ultraviolet photodetectors with Al nanoparticles can reach 2.34 A/W at 269 nm under 20 V bias, enhanced more than 25 times than that without Al nanoparticles. Our approach shows an efficient fabrication technique of high-performance and low-cost plasmonic enhanced AlGaN solar-blind MSM ultraviolet photodetectors.

Enhanced spectral response of an AlGaN-based solar-blind ultraviolet photodetector with Al nanoparticles.

An enhanced spectral response was realized in an AlGaN-based solar-blind ultraviolet (SB-UV) detector using aluminum (Al) nanoparticles (NPs) of 20-60 nm. The peak responsivity of the detector (about 288 nm) with 60 nm Al NPs is more than two times greater than that of a detector without Al NPs under a 5-V bias, reaching 0.288 A/W. To confirm the enhancement mechanism of the Al NPs, extinction spectra were simulated using time-domain and frequency-domain finite-element methods. The calculation results show that the dipole surface plasmon resonance wavelength of the Al NPs is localized near the peak responsivity position of AlGaN-based SB-UV detectors. Thus, the improvement in the detectors can be ascribed to the localized surface plasmon resonance effect of the Al NPs. The localized electric field enhancement and related scattering effect result in the generation of more electron-hole pairs and thus a higher responsivity. In addition, the dark current of AlGaN-based SB-UV detectors does not increase after the deposition of Al nanoparticles. The results presented here is promising for applications of AlGaN-based SB-UV detectors.

Effects of thermal treatment on the MgxZn1 − xO films and fabrication of visible-blind and solar-blind ultraviolet photodetectors

A series of single-phase MgxZn1−xO films with different Mg contents were prepared on quartz substrates by RF magnetron sputtering technique using different MgZnO targets, and annealed under the atmospheric environment. The absorption edges of MgxZn1−xO films can cover the whole near ultraviolet and even the whole solar-blind spectra range, and the solar-blind wurtzite/cubic MgxZn1−xO films have been realized successfully by the same method. In addition, the absorption edges of annealed films shift to a long wavelength, which is caused by the diffusion of Zn atoms gathering at the surface during the thermal treatment process. Finally, the truly solar-blind metal-semiconductor-metal structured photodetectors based on wurtzite Mg0.445Zn0.555O and cubic Mg0.728Zn0.272O films were fabricated. The corresponding peak responsivities are 17mA/W at 275nm and 0.53mA/W at 250nm under a 120V bias, respectively.

High-performance solar-blind ultraviolet photodetector based on mixed-phase ZnMgO thin film

High Mg content mixed-phase Zn0.38Mg0.62O was deposited on a-face sapphire by plasma-assisted molecular beam epitaxy, based on which a metal-semiconductor-metal solar-blind ultraviolet (UV) photodetector was fabricated. The dark current is only 0.25 pA at 5 V, which is much lower than that of the reported mixed-phase ZnMgO photodetectors. More interestingly, different from the other mixed-phase ZnMgO photodetectors containing two photoresponse bands, this device shows only one response peak and its −3 dB cut-off wavelength is around 275 nm. At 10 V, the peak responsivity is as high as 1.664 A/W at 260 nm, corresponding to an internal gain of ∼8. The internal gain is mainly ascribed to the interface states at the grain boundaries acting as trapping centers of photogenerated holes. In view of the advantages of mixed-phase ZnMgO photodetectors over single-phase ZnMgO photodetectors, including easy fabrication, high responsivity, and low dark current, our findings are anticipated to pave a new way for the development of ZnMgO solar-blind UV photodetectors.

Back-illuminated AlxGa1−xN-based dual-band solar-blind ultraviolet photodetectors

Back-illuminated AlxGa1−xN-based dual-band solar-blind ultraviolet (UV) photodetectors (PDs) are realized by a three-terminal n—i–p–i–n heterojunction structure which is grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD). The two p—i—n junctions contained in the heterojunction structure can work separately and independently. Working in the photovoltaic mode, the PDs display peak responsivity of ∼10.8 mA/W at 242 nm and ∼5.0 mA/W at 257 nm, respectively. The two junctions with different size, whose diameters are 500 μm and 800 μm, exhibit almost the same leakage current of ∼1.3 × 10−9 A at a reverse bias of 10 V. Therefore, dark current densities of the two junctions are close to 6.6 × 10−7 A/cm2 and 2.6 × 10−7 A/cm2 at −10 V respectively.

An efficient fast response and high-gain solar-blind flexible ultraviolet photodetector employing hybrid geometry

We report high performance flexible hybrid ultraviolet photodetector with solar-blind sensitivity using nanocomposite film of plasma polymerized aniline-titanium dioxide. A facile solvent-free plasma technique is used to synthesize superior quality hybrid material with high yield. The hybrid photodetector exhibited high photoconductive gain of the order of ∼105 and fast speed with response and recovery time of 22.87 ms and 34.23 ms. This is an excellent result towards getting a balance in the response speed and photoconductive gain trade-off of the photodetectors reported so far. In addition, the device has the advantages of enhanced photosensitivity ((Ilight − Idark)/Idark) of the order of ∼102 and high responsivity of ∼104 AW−1. All the merits substantiates that, to prepare hybrid material, plasma based method holds potential to be an easy way for realizing large scale nanostructured photodetectors for practical applications.

Performance comparison of front- and back-illuminated modes of the AlGaN-based p-i-n solar-blind ultraviolet photodetectors

The authors report the comparison of front- and back-illuminated mode operations of Al0.4Ga0.6N positive-intrinsic-negative solar-blind photodetectors (PDs) grown on the double-side polished sapphire substrates by metalorganic chemical vapor deposition. It is shown that the responsivity in back-illumination mode of fabricated PDs can be almost three times as that in front-illumination mode under the same reverse bias. In addition, a wide spectral response between 300 nm and 370 nm is observed, which is not expected for solar-blind PDs in both illumination modes, while the PDs in back-illumination mode have a stronger ability to restrain the long-wavelength response, showing larger solar-blind/ultraviolet rejection ratio than front-illumination mode. The reasons for the performance differences are discussed.

Dark current suppression of MgZnO metal-semiconductor-metal solar-blind ultraviolet photodetector by asymmetric electrode structures

The application of asymmetric Schottky barrier and electrode area in an MgZnO metal-semiconductor-metal (MSM) solar-blind ultraviolet photodetector has been investigated by a physical-based numerical model in which the electron mobility is obtained by an ensemble Monte Carlo simulation combined with first principle calculations using the density functional theory. Compared with the experimental data of symmetric and asymmetric MSM structures based on ZnO substrate, the validity of this model is verified. The asymmetric Schottky barrier and electrode area devices exhibit reductions of 20 times and 1.3 times on dark current, respectively, without apparent photocurrent scarification. The plots of photo-to-dark current ratio (PDR) indicate that the asymmetric MgZnO MSM structure has better dark current characteristic than that of the symmetric one.

AlxGa1-xN-based back-illuminated solar-blind photodetectors with external quantum efficiency of 89%

We report on high performance AlxGa1−xN-based solar-blind ultraviolet photodetector (PD) array grown on sapphire substrate. First, high quality, crack-free AlN template layer is grown via metalorganic chemical vapor deposition. Then, we systematically optimized the device design and material doping through the growth and processing of multiple devices. After optimization, uniform and solar-blind operation is observed throughout the array; at the peak detection wavelength of 275 nm, 729 μm2 area PD showed unbiased peak external quantum efficiency and responsivity of ∼80% and ∼176 mA/W, respectively, increasing to 89% under 5 V of reverse bias. Taking the reflection loses into consideration, the internal quantum efficiency of these optimized PD can be estimated to be as high as ∼98%. The visible rejection ratio measured to be more than six orders of magnitude. Electrical measurements yielded a low-dark current density: <2 × 10−9 A/cm2, at 10 V of reverse bias.

AlxGa1−xN-based solar-blind ultraviolet photodetector based on lateral epitaxial overgrowth of AlN on Si substrate

We report on AlxGa1−xN-based solar-blind ultraviolet (UV) photodetector (PD) grown on Si(111) substrate. First, Si(111) substrate is patterned, and then metalorganic chemical vapor deposition is implemented for a fully-coalesced ∼8.5 μm AlN template layer via a pulsed atomic layer epitaxial growth technique. A back-illuminated p-i-n PD structure is subsequently grown on the high quality AlN template layer. After processing and implementation of Si(111) substrate removal, the optical and electrical characteristic of PDs are studied. Solar-blind operation is observed throughout the array; at the peak detection wavelength of 290 nm, 625 μm2 area PD showed unbiased peak external quantum efficiency and responsivity of ∼7% and 18.3 mA/W, respectively, with a UV and visible rejection ratio of more than three orders of magnitude. Electrical measurements yielded a low-dark current density below 1.6 × 10−8 A/cm2 at 10 V reverse bias.

Ultraviolet photodiodes based on AlGaN solid solutions

We present the characteristics of ultraviolet photodiodes based on Schottky barriers to the Aluminum Gallium Nitride (AlGaN) solid solution. The paper is devoted to the creation of ohmic and rectifying contacts to AlxGa1−xN solid solutions with a high (x > 0.5) proportion of AlN. The most important property of (AlxGa1−xN) materials is their solar blindness: the wide bandgap (Eg = 3.4 eV for GaN; 6.2 eV for AlN) makes these materials insensitive to the visual and IR light. The study of solid solutions with various mole fractions of Al has created solar-blind and see-blind ultraviolet photodetectors.

Steady-state characteristics and transient response of MgZnO-based metal-semiconductor-metal solar-blind ultraviolet photodetector with three types of electrode structures

Detailed studies of MgZnO-based metal-semiconductor-metal (MSM) solar-blind ultraviolet photodetector with different electrode structures are performed. A two-dimensional physical model is established based on the Poisson's equation and time-dependent continuity equations, which is verified by our experimental data of conventional electrode MSM detector. The steady-state characteristics and transient response of semicircular and triangular electrode MSM detectors are also investigated by this model. Compared with the conventional electrode, semicircular and triangular electrode devices exhibit a substantial improvement on the photocurrent. At a bias of 10 V, the steady-state saturated photocurrents for semicircular and triangular electrode devices are 14.69 nA and 24.37 nA respectively, corresponding to a 20.5% and 100% increase over the conventional electrode detector. Meanwhile, the transient peak photocurrents reach 31.38 nA and 52.09 nA respectively, both of which are notably larger than that of conventional device.

Epitaxial growth of cubic Mg0.45Zn0.55O thin films on SrTiO3 (001) substrate with MgO buffer layer

Cubic Mg0.45Zn0.55O thin films with MgO buffer layer were epitaxially grown on SrTiO3 substrate by radio frequency oxygen plasma assisted molecular beam epitaxy. X-ray diffraction measurements indicate high quality single crystal films of (200) orientation. The growth follows cubic-on-cubic alignment with epitaxial relationship of Mg0.45Zn0.55O (001)[100]//MgO (001)[100]//SrTiO3 (001)[100]. Atomic force microscopy measurements reveal a smooth surface with the rms roughness of 0.36nm in an area of 5×5μm2. The band gap of the film is calculated to be 4.8eV and in the solar blind range. Since the MgO buffer layer can suppress the sub-band-gap photoresponse from the SrTiO3 substrate and does not absorb moisture like the bare MgO substrate, the Mg0.45Zn0.55O thin films with MgO buffer layer grown on SrTiO3 substrate have promising applications in solar blind ultraviolet photodetectors.

Thermal annealing effect on material characterizations of β-Ga2O3 epilayer grown by metal organic chemical vapor deposition

In this work, a single-crystalline β-Ga2O3 epilayer was grown on (0001) sapphire at low temperature by low-pressure metal organic chemical vapor deposition. The optimized parameters for the chamber pressure, oxygen flow, and growth temperature were 15 Torr, 200 sccm, and 500 °C, respectively. The β-Ga2O3 epilayer was fabricated as a metal-semiconductor-metal solar-blind deep ultraviolet photodetector. Due to the gallium oxide grown at low temperature, the as-grown β-Ga2O3 epilayer was annealed at 800 °C in atmosphere or in a nitrogen environment. The effects of defects of the β-Ga2O3 epilayer before and after N2 annealing were studied using x-ray diffraction system, cathodoluminescence at differential temperature, and Hall measurement. The β-Ga2O3 epilayer that was N2 annealed for 15 min presented better photodetector performance than the as-grown β-Ga2O3 epilayer. The annealed epilayer exhibited a dark current of 1.6 × 10−13 A under 5 V bias.

High response solar-blind ultraviolet photodetector based on Zr0.5Ti0.5O2 film

In this letter, high response solar-blind Zr0.5Ti0.5O2 ultraviolet detectors with Au and Pt electrodes were fabricated. The stability and photoelectric characteristics of the devices at different temperature and relative humidity were studied. The detectors covered the whole solar-blind spectrum range and responded mainly in 200–290nm. At 5V bias, 25°C and 33% RH, the dark current of the detector with Pt electrodes was only 17pA. Under the radiation of 250nm UV light, a high responsivity of 620mA/W was obtained for Pt electrodes device. Moreover, the detector also exhibited a fast response time: the rise and fall time were 424.1 and 154ms, respectively.

Performance comparison of front- and back-illuminated AlGaN-based metal–semiconductor–metal solar-blind ultraviolet photodetectors

In this work, AlGaN-based metal–semiconductor–metal solar-blind ultraviolet photodetectors (PDs) with low dark current were fabricated on sapphire substrates. In both front- and back-illumination operation modes, the PDs exhibited sharp photoresponse cutoffs at ∼280 nm with solar-blind/ultraviolet rejection ratios of more than 103. The quantum efficiency of the back-illuminated PD was observed to generally be higher than that of the front-illuminated PD. Nevertheless, at very low bias range, the front-illuminated PD exhibited greater photoresponsivity. The observed performance differences of the PDs in the different illumination modes are discussed in terms of surface reflectivity and photocarrier collection efficiency.

Wide band-gap investigation of modulated BeZnO layers via photocurrent measurement

We grew modulated BeZnO layers by hybrid plasma-assisted molecular-beam epitaxy/electron-beam deposition. A wide band-gap investigation of the modulated BexZn1−xO by means of photocurrent (PC) spectroscopy was conducted. The band-gap energy was directly acquired from the wavelength of the PC peak, caused by the band-to-band transition. By increasing x, which was the rate of the Be elements, the optical band-gap energy was empirically fitted by EBeZnO(x) = 6.32(x–1)x + 7.305x + 3.295. We expect that this finding can open new possibilities for wide band-gap engineering of BeZnO layers, which can be utilized as barrier layers in active layers consisting of ZnO/BeZnO quantum well structures and solar-blind ultraviolet photodetectors.

Photoconductive gain in solar-blind ultraviolet photodetector based on Mg0.52Zn0.48O thin film

An ultraviolet photodetector was fabricated on MgZnO thin film grown by metal-organic chemical vapor deposition. The peak response of the device centers at 238 nm and cutoff wavelength is 253 nm. The peak responsivity is 129 mA/W at 15 V bias, and the UV/visible reject ratio is 4 orders of magnitude. Internal gain is due to the hole trapping at interface that brings low response speed. Native defects at the Au/MgZnO interface degrade the barrier effect, which caused large dark current and high visible response.

Interface engineering of high-Mg-content MgZnO/BeO/Si for p-n heterojunction solar-blind ultraviolet photodetectors

High-quality wurtzite MgZnO film was deposited on Si(111) substrate via a delicate interface engineering using BeO, by which solar-blind ultraviolet photodetectors were fabricated on the n-MgZnO(0001)/p-Si(111) heterojunction. A thin Be layer was deposited on clean Si surface with subsequent in situ oxidation processes, which provides an excellent template for high-Mg-content MgZnO growth. The interface controlling significantly improves the device performance, as the photodetector demonstrates a sharp cutoff wavelength at 280 nm, consistent with the optical band gap of the epilayer. Our experimental results promise potential applications of this technique in integration of solar-blind ultraviolet optoelectronic device with Si microelectronic technologies.

Single-crystalline cubic MgZnO films and their application in deep-ultraviolet optoelectronic devices

By employing a relatively low growth temperature and oxygen-rich conditions, single-crystalline cubic MgZnO films were prepared. A solar-blind deep ultraviolet (DUV) photodetector was finished on the MgZnO film. The maximum responsivity of the photodetector is 396 mA/W at 10 V bias, which is almost three orders of magnitude larger than the highest value ever reported in MgZnO-based solar-blind photodetectors. The dark current density is 1.5×10−11 A/cm2, comparable with the smallest value ever reported in solar-blind photodetectors. The improved performance reveals that the single-crystalline cubic MgZnO films have great potential applications in DUV optoelectronic devices.