Metal-semiconductor-metal Research Articles

ZnO:Ca MSM ultraviolet photodetectors

Photoresponse of calcium doped zinc oxide (ZnO:Ca) metal-semiconductor-metal (MSM) UV detectors are reported. Photosensitive ZnO:Ca films exhibit high transparency in the visible region, show polycrystalline, hexagonal wurtzite structure, having c-axis preferred growth. Noticed redshift of 60 meV in the optical band gap for ZnO:Ca (3 wt%) thin film is attributed to increased crystallite size. Improved n-type conductivity in ZnO with Ca doping is due to increased carrier concentration, MSM photodetector fabricated with, 3 wt% Ca doped ZnO exhibit higher responsivity of 3.05A/W at 365 nm UV illumination. Transient response measurements show reasonably fast switching with a rise and fall time of 15s and 26s respectively. The obtained results suggest, ZnO:Ca can be used as active material in the fabrication of UV photodetectors.

Metal–Semiconductor–Metal Metasurface for Multiband Infrared Stealth Technology Using Camouflage Color Pattern in Visible Range

AbstractOptical stealth technology is being developed to cope with high‐sensitivity infrared image detectors and various guided detectors. Infrared stealth technology has remarkable performance; however, future advancements in multi‐band stealth technology covering the entire optical frequency range, from visible to wide infrared, are key challenges regarding unmanned surveillance systems. Thus, a metal–semiconductor–metal (MSM) metasurface with Fabry–Pérot (F–P) and multiple plasmonic resonant modes is introduced to realize multiband stealth technology. Different colors are obtained for printing camouflage patterns in the visible range using localized surface plasmon modes in Al disks on an opaque Ge layer. The F–P resonance of the Ge layer induces a strong absorption of >92% at 1.06 µm, reducing the guidance signal of the infrared laser‐guided detector. With an additional plasmonic resonance in the MSM metasurface, infrared signature reductions of >34%, >94.4%, and >97.7% are obtained for short‐wave, mid‐wave, and long‐wave infrared bands, respectively.

Characterization of porous InGaN-based metal-semiconductor-metal

In this study, the characteristics of metal-semiconductor-metal (MSM) photodetector based on a porous In0.27Ga0.73N thin film were reported. Nanostructured porous film was synthesized using the UV-assisted electrochemical etching technique. The formed pores were dissimilar in terms of shape and size. The effect of annealing in the range of 300e500 C on Pt/In0.27Ga0.73N was investigated by IeV measurements. Schottky barrier height was at maximum value under 500 C. The fabricated MSM photodetector shows photovoltaic characteristics in the green region of the electromagnetic spectrum. The device responsivity increased with increasing the bias voltage. Moreover, the rise and recovery times of the device were investigated at 10 mW cm 2 of a 550 nm chopped light. Finally, the sensitivity and quantum efficiency were also investigated.

Fast-Response Amorphous Ga₂O₃ Solar-Blind Ultraviolet Photodetectors Tuned by a Polar AlN Template

Gallium oxide (Ga₂O₃) is an emerging semiconductor for advanced optoelectronic applications owing to its suitable bandgap and advantageous electronic characteristics. While amorphous Ga₂O₃ is highly desired for low-cost and large-scale device application, it suffers from high dark current and slow response speed for practical ultraviolet photodetection. Here we integrate a polar aluminum nitride (AlN) template with amorphous Ga₂O₃ to fabricate a fast-response metal semiconductor metal (MSM) photodetector. It is revealed that the carrier transport and device performance strongly depend on the polarization direction of the AlN template. With a downward polarization from an Al-polar AlN layer, the Ga₂O₃ photodetector achieves an optimized performance with a lower dark current of 0.015 nA, a higher photo-to-dark current ratio of 10⁴, a faster response speed of 31 ms and a recovery time of 22 ms. This study demonstrates an effective route of engineering carrier transport via the external spontaneous polarization for fabricating high-performance solar-blind photodetectors.

(INVITED) Opto-electronic properties of solution-synthesized MoS2 metal-semiconductor-metal photodetector

We report a simple fabrication method for metal-semiconductor-metal (MSM) photodetectors using solution-synthesized MoS2 films as the semiconductor channel and pure indium for the metal contacts. Resonance Raman spectroscopy has been used to confirm the growth of ultra-thin MoS2 films. Due to the low work function of indium, the back-to-back indium-MoS2 Schottky junctions formed relatively low Schottky barrier height (0.43 eV), which has been calculated using the thermionic field emission model. Moreover, the MSM device shows good responsivity (22 mA/W) under white light illumination. The MoS2-MSM device possesses higher photoelectrical response in the visible and near IR region compared to UV due to the indirect bandgap of ultra-thin MoS2 film which has been estimated to be 1.38 eV. These findings reveal the significance of using low work function metals like indium as contacts for layered transition metal dichalcogenides (TMDC) films (produced by low cost solution-based techniques), particularly MoS2, in order to design semiconductor devices with efficient opto-electronic performance.

Regulating interface Schottky barriers toward a high-performance self-powered imaging photodetector.

Two-dimensional (2D) layered organic–inorganic hybrid perovskites have attracted wide attention in high-performance optoelectronic applications due to their good stability and excellent optoelectronic properties. Here, a high-performance self-powered photodetector is realized based on an asymmetrical metal–semiconductor–metal (MSM) device structure (Pt-(PEA)2PbI4 SC-Ag), which introduces a strong built-in electric field by regulating interface Schottky barriers. Benefitting from excellent built-in electrical potential, the photodetector shows attractive photovoltaic properties without any power supply, including high photo-responsivity (114.07 mA W−1), fast response time (1.2 μs/582 μs) and high detectivity (4.56 × 1012 Jones). Furthermore, it exhibits high-fidelity imaging capability at zero bias voltage. In addition, the photodetectors show excellent stability by maintaining 99.4% of the initial responsivity in air after 84 days. This work enables a significant advance in perovskite SC photodetectors for developing stable and high-performance devices.

Surface Modification of AlGaN Solar-Blind Ultraviolet MSM Photodetectors With Octadecanethiol

We successfully fabricate the AlGaN solar-blind ultraviolet metal-semiconductor-metal (MSM) photodetectors (PDs) with high Al composition of 0.6, and an effective method by modifying organic molecules of Octadecanethiol (ODT) on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> N surface is proposed to enhance the optoelectronic performance of the MSM PDs. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the ODT organic molecules are chemically adsorbed on the surface of the AlGaN active layer successfully. The Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> N MSM PD modified with ODT exhibits a cutoff wavelength of ~250 nm, and the dark current is reduced compared to the referential PD without ODT modification, indicating the decreased surface state density due to suppressible oxidation of AlGaN surface by using ODT modification. Moreover, the ODT-modified AlGaN PD shows a high peak responsivity of 2.75 A/W at 10 V, which is enhanced by ~3 times than that of the referential PD without modification.

The Gold Nanoparticles Enhanced ZnO/GaN UV Detector

Ultraviolet detectors can be used in ultraviolet disinfection, missile guidance and short-wave communication fields. Here we have grown GaN (002) film by chemical vapor deposition (CVD), through depositing 100 nm ZnO film as buffer layers on sapphire substrates by magnetron sputtering. Using Ni as the electrode, the metal-semiconductor-metal (MSM) ultraviolet (UV) detector was prepared. The device dark current, photocurrent and UV on/off current ratio were 5.19×10-9A, 2.52×10-7A and 54, under 2V bias. Then, using Au nanoparticles as plasmons, the photocurrent and photoresponsivity of the device were increased by 5.32 times and 5.25 times, respectively, and the UV on/off current ratio reached 176. The photoresponsivity of the device reaches the maximum value 0.42A/W at 370nm, the response time and relaxation time reach 0.1s and 0.12s, respectively.

Investigating the effect of number of metal electrodes on performance parameters of AlGaN MSM photodetectors

Abstract In most past studies, MSM (Metal–Semiconductor–Metal) detectors with the varying area have been investigated for varying number of fingers (metal electrodes) of equal width (W) and spacing (S). Therefore, there is a need to investigate fixed area MSM detectors with varying number of fingers as there are few reports on electrical analysis of larger electrode spacing dimensions. In the current work effect of variation in the number of fingers is studied for two types of Al0.5Ga0.5N/AlN/Sapphire based fixed area MSM detectors. Comparative performance analysis between (S = W) and (S = 2W) based detectors is carried out for photocurrent, dark current density and transient response. I–V characteristics, structure diagram plots are generated using the TCAD Silvaco simulator. It has been observed that S = W detectors exhibit higher photocurrent and lower dark current density is shown by S = 2W designs. Therefore, simulation outcomes can be beneficial for selecting suitable MSM detector for reliable, high-speed optical communication and switching applications.

Effects of annealing temperature on a ZnO thin film-based ultraviolet photodetector

In this paper, the effects of annealing temperature on the performance of a ZnO thin film-based Metal-Semiconductor-Metal (MSM) type ultraviolet (UV) photodetector is reported. ZnO thin films were grown on a glass substrate using the Pulsed Filtered Cathodic Vacuum Arc Deposition (PFCVAD) technique at room temperature and after the deposition process the samples were annealed at 400, 450 and 550 °C in air condition to investigate the annealing effect on the structural, electrical, and optical properties of the photodetector. ZnO thin films which have grains in nanometer range has an increasing in the diameter of grains from 10.5 to 18.3 nm as a function of annealing temperature results in a red shift in the cut-off wavelength of the photodetector from 3.25 eV (381 nm) to 3.23 eV (383 nm). It is demonstrated that the sensitivity and the speed (rise/fall times) of the ZnO thin film based MSM photodetectors enhances with increasing post growth annealing temperature of ZnO thin film due to the increase in the absorption coefficient and the decrease of the total area of the grain boundaries due to the larger grain sizes formation in ZnO thin films with increasing thermal annealing temperature.

Broad Range (254–302 nm) and High Performance Ga2O3:SnO2 Based Deep UV Photodetector

The harmful UV radiation leaking out of the ozone hole can have a detrimental effect on mother nature. To monitor any UV rays leaking out of the ozone hole requires an electronic device such as deep UV photodetectors. In this context, Sn-doped Ga₂O₃ incorporated with SnO₂ nanostructures has been grown on a c-plane sapphire substrate using low-pressure chemical vapor deposition (LPCVD) followed by the fabrication of metal-semiconductor-metal (MSM) based deep ultraviolet (UV) photodetector (PD) using Pt as electrodes with interdigitated geometry. The PD possesses a low dark current of 21 nA even at 50 V bias with a very high photo-to-dark current ratio of 9 &#x00D7; 10⁴ and exceptionally large responsivity of 1532 and 262 A&#x002F;W under 254 nm and 302 nm UV-illumination respectively. Consequently, an extremely high detectivity of 1.7 &#x00D7; 10¹⁵ Jones and external quantum efficiency of 7.4 &#x00D7; 10⁵&#x0025; has been recorded under 254 nm illumination with a fast fall time of 0.2 sec. The PD works well in UV-B range with high responsivity and is attributed to the long wavelength absorption by the SnO₂ nanostructures accompanied by a charge transfer from SnO₂ to the Ga₂O₃ layer. The high gain has been attributed to the photoconductive gain due to interface trapped charges and self-trapped holes, along with light trapping on the textured Ga₂O₃ surface.

Natural Boron and 10B-Enriched Hexagonal Boron Nitride for High-Sensitivity Self-Biased Metal-Semiconductor-Metal Neutron Detectors.

Metal–semiconductor–metal (MSM) detectors based on Ti/Au and Ni/Au interdigitated structures were fabricated using 2.5 micrometer thick hexagonal boron nitride (h-BN) layer with both natural and 10B-enriched boron. Current–voltage (I–V) and current–time (I–t) curves of the fabricated detectors were recorded with (IN) and without (Id) neutron irradiation, allowing the determination of their sensitivity (S = (IN– Id)/Id= ΔI/Id). Natural and 10B-enriched h-BN detectors exhibited high neutron sensitivities of 233 and 367% at 0 V bias under a flux of 3 × 104 n/cm2/s, respectively. An imbalance in the distribution of filled traps between the two electric contacts could explain the self-biased operation of the MSM detectors. Neutron sensitivity is further enhanced with electrical biasing, reaching 316 and 1192% at 200 V and a flux of 3 × 104 n/cm2/s for natural and 10B-enriched h-BN detectors, respectively, with dark current as low as 2.5 pA at 200 V. The increased performance under bias has been attributed to a gain mechanism based on neutron-induced charge carrier trapping at the semiconductor/metal interface. The response of the MSM detectors under thermal neutron flux and bias voltages was linear. These results clearly indicate that the thin-film monocrystal BN MSM neutron detectors can be optimized to operate sensitively with the absence of external bias and generate stronger signal detection using 10B-enriched boron.

Enhancing the Performance of NiO-Based Metal-Semiconductor-Metal Ultraviolet Photodetectors Using a MgO Capping Layer

Nickel oxide (NiO) based metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs) are fabricated using a radio-frequency (RF) magnetron sputtering system. In this study, the surface of NiO is capped with magnesium oxide (MgO) using the same RF sputtering system. The effects of the MgO capping layer on the characteristics of the prepared MSM-PDs are studied. Using X-ray diffraction, a smaller grain size and tensile strain on the MgO-capped NiO is observed, which is due to the introduction of O atoms from MgO. The MgO capping layer effectively reduced the dark current by three orders of magnitude compared to the MSM-PDs without the MgO capping layer. In the MSM-PDs without the MgO capping layer, the photo-to-dark current ratio is only 100, and the ratio is enhanced by three orders of magnitude upon capping the MSM-PDs with MgO. Both the MSM-PDs, with and without the MgO capping layer, exhibit a UV/visible rejection ratio as high as 800, demonstrating the fabricated MSM-PDs are good visible-blind photodetectors. X-ray photoelectron spectroscopy illustrates that the introduced Mg atoms from MgO react with NiO to form strong MgxNi1-xO and/or MgO bonding, which passivate the surface defects on NiO surface, thus significantly reducing the dark current and enhancing the performance of the prepared MSM-PDs.

Numerical investigations into polarization-induced self-powered GaN-based MSM photodetectors.

Traditional GaN-based metal-semiconductor-metal (MSM) photodetector (PD) features a symmetric structure, and thus a poor lateral carrier transport can be encountered, which can decrease the photocurrent and responsivity. To improve its photoelectric performance, we propose GaN-based MSM photodetectors with an AlGaN polarization layer structure on the GaN absorption layer. By using the AlGaN polarization layer, the electric field in the metal/GaN Schottky junction can be replaced by the electric fields in the metal/AlGaN Schottky junction and the AlGaN/GaN heterojunction. The increased polarization electric field can enhance the transport for the photogenerated carriers. More importantly, such polarization electric field cannot be easily screened by free carriers, thus showing the detectability for the even stronger illumination intensity. Moreover, we also conduct in-depth parametric investigations into the impact of different designs on the photocurrent and the responsivity. Hence, device physics regarding such proposed MSM PDs has been summarized.

Enhanced sensitivity of low-cost fabricated fluorine doped ZnO metal semiconductor metal photodetector

The literature mostly reports on thin-film fluorine-doped zinc oxide (FZnO) and has yet to fabricate metal semiconductor metal (MSM) photodetectors (PDs) based on FZnO nanostructures for the enhancement of photoresponse parameters despite their huge potential. In this work, ZnO film of 32 nm-thick was deposited and used as a seed layer on silicon (Si) substrates with the aid of the radio frequency (RF) magnetron sputtering method. Vertically well-aligned FZnO nanorods (NRs) were later grown on the seeded substrates via a hydrothermal method and annealed for 1 h at 400 °C. The formation of well-aligned hexagonal NRs at low fluorine doping was revealed by field emission scanning microscopy (FESEM), and energy-dispersive X-ray (EDX) analysis confirmed the presence of fluorine doping. X-ray diffraction (XRD) investigation confirmed the growth of good crystal quality hexagonal wurtzite F–ZnO NRs along the (002) direction, and bandgaps were estimated using UV–Vis–NIR spectroscopy. Photoluminescence (PL) measurement has revealed the main emission peak for ultraviolet (UV) is centered at 375 nm and a red-shifted emission with fluorine doping has been identified. An F2 UV MSM PD device is fabricated. Upon exposure to UV light (395 nm) at 4 V, it displayed a high sensitivity of 1.1 × 105%, responsivity of 0.66 A/W, detectivity of 1.30 × 1010 Jones, a response time of 90 ms, and recovery time of 110 ms.

Mechanism of defects and electrode structure on the performance of AlN-based metal semiconductor metal detectors

We used the metal-organic chemical vapor deposition(MOCVD) method to grow AlN material on a c-plane sapphire substrate and fabricate an AlN-based metal-semiconductor-metal (MSM) detector. Analyzing the influence mechanism of different dislocation densities in AlN materials and detector electrode structure on the detector performance, it was found that the lower the dislocations can effectively reduce the dark current of the detector under zero bias voltage, and help improve the performance of the detector. The study also found that when the finger spacing of the detector remained the same and the finger width increased, the efficiency of the detector decreased, while the response time of the detector increased, when the finger width of the detector electrodes remained unchanged and the finger spacing increased, the response time of the detector increased. Therefore, the electrode finger width and finger spacing must be compromised in the design of the electrode structure to improve the performance of the AlN-based MSM detector.

Highly sensitive and cost-effective metal-semiconductor-metal asymmetric type Schottky metallization based ultraviolet photodetecting sensors fabricated on n-type GaN

Cost effective and highly sensitive Cr/Ni/Cu/n-GaN/Ti/Al asymmetric type metal-semiconductor-metal (MSM) Schottky barrier ultraviolet photodetector has been fabricated at room temperature. Surface morphological and oxidation states of Cr and Ni metal layers were investigated using atomic force microscopy and X-ray absorption spectroscopy, respectively. Current-voltage (I–V) measurements were performed in dark and at various wavelength illuminations in the range 350–370 nm indicated a very good rectification ratio of 1.5–4.6 × 104 (at ± 7 V) due to the utilisation of two metal stacks with different workfunctions. The Schottky diode parameters such as Schottky barrier height, ideality factor and series resistance were calculated at different wavelength illuminations using different reliable approaches. The spectral responses measured from the fabricated GaN based UV PD at several biases in the wavelength range 300–450 nm were found to be bias dependent due to the internal gain effect. Using time dependent photoresponses the rise/fall time values were evaluated at 5 V of different wavelengths and were found to be in the range 1.55–2.05 s and 2.92–3.35 s, respectively. Consequently, from current-voltage (I–V), photoresponsivity and time-dependent photoresponse studies, the 365 nm was found to be the optimum and reflects the high wavelength selectivity for the fabricated GaN based A-type MSM UV PD under study.

Solar-blind ultraviolet photodetectors with thermally reduced graphene oxide formed on high-Al-content AlGaN layers

Solar-blind deep-ultraviolet (UV) photodetectors (PDs) with high responsivity and fast response have attracted significant attention in environmental, industrial, biological, and military applications. AlGaN is a representative semiconductor material in the field of solar-blind detection; semiconductor performance can be accelerated by combining it with high-transparency, high-stability contact electrode materials. In this study, solar-blind deep-UV metal–semiconductor–metal (MSM) PDs were fabricated based on two-dimensional reduced graphene oxide (rGO) contacts formed on various high-Al-content AlGaN semiconductors. A low dark current in the order of a few picoamperes and a fast photoresponse time of a few tens of milliseconds were confirmed. The investigation of the effects of front- and back-side illumination showed that the photocurrents and corresponding responsivities of the PDs drastically improved under back-side illumination. In detail, the peak locations of the responsivity–wavelength curves were downshifted from 290 nm with a responsivity of 0.0518 A/W for the rGO/Al0.5Ga0.5N MSM PD to 250 nm with a responsivity of 0.0113 A/W for the rGO/Al0.7Ga0.3N MSM PD under back-side illumination. These results indicate that rGO contacts on AlGaN provide a viable approach for developing solar-blind deep-UV PDs.

Observation of negative photoconductivity at bandgap and super bandgap excitations in GaN nanorods

We report on the surprising observation of negative photoconductivity (NPC) in GaN nanorods (NRs) by bandgap and super bandgap excitations. A metal-semiconductor-metal (MSM) type of device configuration using Ni/Au as an electrode at both ends, has been used to study the NPC phenomenon. On exposing the device to the light sources with 363 nm (bandgap excitation) and 250 nm (super bandgap excitation) wavelengths, NPC with a drop of approximately 50 and 60% was observed, respectively. To explain this phenomenon, a model of trapping of photogenerated electrons by the trap states is proposed. Interestingly, a significant drop in the illumination current is observed at low values of power density (up to 3 μW/mm2) of the incident light. It has been interpreted that on increasing the power density, the density of trapped electrons starts approaching the density of available trap states. The de-trapping process is observed to be very slow as the illumination current reached the dark current value in nearly 2 h after switching off the incident light. The observation of NPC and saturation of trap states with high intensity of incident light are also validated by Kelvin Probe Force Microscopy (KPFM) characterization performed in the dark and on illumination with varying intensity of the incident light. Furthermore, on increasing the temperature from room temperature (RT) to 300 °C, the NPC was found to reduce to almost zero, which indicates the de-trapping of trapped electrons on increasing the temperature.

Elimination of Interfacial-Electrochemical-Reaction-Induced Polarization in Perovskite Single Crystals for Ultrasensitive and Stable X-Ray Detector Arrays.

Organic-inorganic halide perovskites have exhibited bright prospects in high-sensitivity X-ray detection. However, they generally suffer from the severe field-driven polarization issue that remarkably deteriorates the detection performance. Here, it is demonstrated that the interfacial electrochemical reaction between Au electrodes and halogen in MAPbI3 single crystals (SCs) is the major source of the dark current polarization in the metal-semiconductor-metal (MSM)-structured perovskite X-ray detectors at the initial stage of biasing. By introducing the p- and n-type charge transport layers to isolate the electrodes from contacting the SC surface, the polarization is fully eliminated under a large electric field up to 1000V cm-1 . Moreover, the resultant lateral p-i-n heterojunction suppresses the dark current of the devices by nearly 3 orders of magnitude as compared to the MSM counterparts and therefore enables a high sensitivity of 5.2 × 106 µC Gy-1 air cm-2 and a record low X-ray detection limit down to 0.1 nGyair s-1 . The excellent biasing stability and sensitivity of the devices allow to prepare the linear detector arrays for X-ray imaging applications.