Metal-semiconductor-metal Photodetectors Research Articles

The Effect of Oxygen Partial Pressure and Annealing Process on the Characteristics of ZnGa2O4 MSM UV Photodetector

In this work, ZnGa2O4 solar-blind photodetectors (PD) based on a metal-semiconductor-metal (MSM) structure was fabricated by radio frequency (RF) magnetron sputtering method. The transmittance of the material shows larger than 80% cross the visible light region. As a wide bandgap and high transparency semiconducting material, ZGO is a potential candidate for UV-detection applications. The ZGO MSM PD with no oxygen flow during sputtering exhibits a responsivity of 4.46 × 10−2 A/W under illumination at wavelength of 260 nm, rejection ratio of 1.75 × 104. The performance can be enhanced with proper thermal annealing process. With an annealing process at temperature of 200°C, the responsivity and rejection ratio are 0.203 A/W and 1.12 × 105, respectively.

Effects of annealing temperature of spin-coated ZnO seed-layer on UV photo-sensing properties of PLD grown ZnO: Mg thin films

Here, we have synthesized nanostructured wurtzite ZnO: Mg thin films by pulsed laser deposition on sol-gel spin coated ZnO seed layers which are annealed at 325 °C, 400 °C, and 475 °C. The influence of seed layer annealing process temperature on its growth and hence the structural, surface morphological and UV photodetection properties of the ZnO: Mg nanostructured thin film were investigated and described for the first time. All the ZnO: Mg films and seed layers exhibit the hexagonal wurtzite phase with a preferred (0 0 2) orientation and the optimal crystallization of the ZnO: Mg film takes place when it is deposited on seed layer annealed at 400 °C. The FESEM studies revealed a modification of the film surface morphology from dense ridge link to petal-like surface structures as a result of the changes in nanofibrous structures of beneath seed layer caused by annealing temperature variation. AFM revealed that the film growth is greatly influenced by the seed layer annealing temperature while confirming the highest grain size for the film deposited on a seed layer annealed at 400 °C. Finally, we tested ZnO: Mg films for their photoconductive UV detection performance in metal–semiconductor–metal (MSM) formation, and are found to be greatly influenced by seed layer annealing temperature. As a result of its better crystalline and surface morphological properties, UV photodetector with ZnO: Mg film deposited on seed layer annealed at 400 °C exhibits the highest responsivity of about 0.0122 A/W upon 2 mW/cm2 light exposure at 365 nm peak wavelength and 5 V applied bias. The reliable UV detection competency of the MSM photodetector was also established by intermittently turning UV light on and off at static time intervals.

Electrical, optical and spectral characteristics of type-II ZnSe/ZnTe/GaAs superlattice and MSM-photodetector on their base

We report on growth, fabrication and characterization of the metal–semiconductor–metal (MSM) photodiode based on type-II ZnSe/ZnTe heterostructure. Heterostructure was grown on semi-insulating GaAs substrates by MOVPE. For the first time we present the results of experimental investigations of the MSM photodetector on the base of type-II ZnSe/ZnTe superlattice. The MSM-photodetector demonstrates very low dark current, high current sensitivity and external quantum efficiency. The maximum photoresponse of the MSM-detector at the wavelength 620 nm corresponds to current sensitivity 0.22 A/W and external quantum efficiency 44%. Photoresponse of the MSM-detector shows two peaks of response located at 620 nm and 870 nm. ZnSe/ZnTe type-II superlattice structure reduces the MSM-diode dark current significantly. For the MSM-diode with finger width and gap of 3 µm and 100 × 100 µm2 photosensitive area we have obtained dark current density 10−8 A/cm2 at room temperature.

Self‐Driven Metal–Semiconductor–Metal WSe2 Photodetector with Asymmetric Contact Geometries

AbstractSelf‐driven photodetectors have wide applications in wireless sensor networks and wearable physiological monitoring systems. While 2D materials have different bandgaps for potential novel application fields, the self‐driven photodetectors are mainly built on PN junctions or heterostructures, whose fabrication involves doping or reliable multiple transfer steps. In this study, a novel metal–semiconductor–metal (MSM) WSe2 photodetector with asymmetric contact geometries is proposed. A high responsivity of 2.31 A W−1 is obtained under zero bias, and a large open‐circuit voltage of 0.42 V is achieved for an MSM photodetector with a large contact length difference. The MSM photodetector can overcome the disadvantage of high dark current in traditional MSM photodetectors. A small dark current of ≈1 fA along with a high detectivity of 9.16 × 1011 Jones is achieved. The working principles and finite element analysis are presented to explain the origin of the self‐driven property and its dependence on the degree of asymmetry.

A new high performance MSM hybrid plasmonic photodetector based on nanogratings and dual mode horn shape waveguide

A new metal–semiconductor–metal photodetector (MSM-PD) based on a hybrid plasmonic and extra ordinary transmission (EOT) phenomenon is proposed. In the designed structure, we demonstrate that light transmission is improved by means of a new shape of surface nanograting and by optimizing the physical parameters of the metal. In this work the optical transfer parameters are strongly interrelated and all the parameters are optimized and the light transmission is enhanced by 55 times compared to the reference structure. Furthermore, the output port of the subwavelength slit in the presented MSM-PD has a dual horn shape and improves absorption light in the active layer of the structure. Also, by filling this port with the same type of material as that of the active layer, the absorption becomes greater than before. Moreover, the confinement region of the light in the active layer is reduced by using the hybrid plasmonic effect and therefore, the probability of the photon absorption is 101 times greater than that of the reference structure. Thus, the transmission and absorption enhancement improves the quantum efficiency and the electrical response of the presented MSM-PD. A high performance new photodetector is identified as an ultra-fast MSM-PD and because of its thin film active layer can be used as a low-noise one.

Metal-Semiconductor-Metal Photodetector with Interdigitated Graphene Finger Electrodes Fabricated on Ge Epilayer Grown on Si Substrate

Based on Ge epitaxial layer grown on Si substrate, we have fabricated metal-semiconductor-metal (MSM) photodetectors (PD) with interdigitated graphene finger electrodes and demonstrated its opto-electrical properties. High quality Ge epitaxial layers were grown on n-type Si (100) wafers with a resistivity of 5 - 15 Ω•cm by using a rapid thermal chemical vapor deposition (RTCVD) method. The monolayer graphene grown on Cu foils using CVD method was transferred onto the cleaned Ge epilayer and was patterned by O2 plasma reactive-ion etching (RIE) process to form interdigitated finger electrodes. From temperature dependent current-voltage (I-V) characteristics, the barrier height of graphene/Ge epilayer Schottky junction was measured to be 0.42 eV, of which value was comparable to the barrier height of graphene Schottky contact to bulk-Ge. The manufactured MSM PD with interdigitated graphene finger electrodes exhibited a considerable spectral response for wavelengths in the range of 1530 – 1560 nm, demonstrating effective photo-detection for the entire C-band (conventional band) spectrum range. On the other hand, for wavelengths > 1560 nm, its responsivity fell off rapidly, which was consistent with the Ge absorption coefficient trend. Under the illumination of incident light with a power of 3.5 mW, the maximum values of responsivity and detectivity at - 2V reverse bias were measured to be 0.35 A/W and 3.86×1010 Jones (cmHz1/2/W) at the wavelength of 1545 nm, respectively.

Effect of Metal Contacts on a GaN/Sapphire-Based MSM Ultraviolet Photodetector

We report the fabrication of a GaN/sapphire ultraviolet (UV) photodetector (PD) with asymmetric (Pt-Ag) and symmetric (Pt-Pt) metal–semiconductor–metal (MSM) structure. The fabricated devices display Schottky behavior. Time-dependent photoresponse analysis reveals a significant enhancement in photocurrent leading to a photoresponsivity of 37 mA/W and 267 mA/W under 13 mW optical power at 5 V bias for Pt-Pt and Pt-Ag devices, respectively. Further, power-dependent measurements reveal a peak responsivity of 633 mA/W at 4mW optical power and 5 V bias for the Pt-Ag asymmetric MSM photodetector. The significant enhancement in responsivity for asymmetric MSM UV PD is ascribed to the built-in potential gradient in the GaN semiconductor, which drives a large number of charge carriers for enhanced charge collection. Further, the noise equivalent power was lowest for the Pt-Ag MSM structure, which was calculated to be 4.6 × 10−14 WHz−1/2. This high performance asymmetric MSM GaN UV PD can be integrated into efficient UV PD-based applications.

Ga Doped ZnO Nanorods Ultraviolet Photodetectors on Flexible Substrate

In this study, high-density single crystalline Ga doped ZnO (GZO) nanorods were grown on flexible substrate by the hydrothermal method. From the FESEM image, it was found that GZO nanorods have a hexagonal wurtzite structure and the average length and diameter of the GZO nanorods were 1.35 μm and 31 nm, respectively. It was also found that high density vertically oriented nanorods are grown on the seed layer/flexible substrate. Further, Ga-doped ZnO nanorods metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PD) were fabricated. Under 1 V applied bias, the dark current and photocurrent of GZO MSM PDs were 8.60 × 10-9 A and 3.92 × 10-6 A, respectively. The responsivity of the synthesized GZO PDs were around 0.031 A/W with a sharp cutoff wavelength at 370 nm. The UV-to-visible rejection ratio of the GZO PDs sample was 23.87 as the measured responsivity at UV and visible peaks were 370 and 450 nm, respectively.

Spectrum Selectivity and Responsivity of ZnO Nanoparticles Coated Ag/ZnO QDs/Ag UV Photodetectors

Photoresponsivity and spectrum selectivity of ZnO nanoparticles (NPs)-coated colloidal quantum dots (CQDs)-based Ag/ZnO QDs/Ag interdigitated metal-semiconductor-metal (MSM) photodetectors under front and back illuminations have been investigated in this letter. It is shown that the ZnO NPs film coated over the interdigitated MSM photodetector acts as a filter layer to enhance the spectrum selectivity at the cost of reduced responsivity of the detector under front illumination. Whereas, the same NPs layer acts as a scattering medium to enhance the responsivity at the cost of reduced selectivity of the detector under back illumination condition. The measured responsivity under front illumination is 6 A/W with a full width at half maxima (FWHM) of ~25 nm, whereas it is increased to 20 A/W with an FWHM of ~49 nm under back illumination at an applied bias of 2 V. Thus, the ZnO NPs layer can be used to optimize the photoresponse characteristics of the ZnO CQDs-based MSM photodetector by changing the direction of illumination on the device.

GaSb MSM Photodetectors on Si Waveguides by Rapid Melt Growth Method

Metal-semiconductor-metal (MSM) photodetectors made of gallium antimonide (GaSb) on silicon waveguides by the rapid melt growth method are investigated. By controlling the thermodynamics of crystal regrowth and optimizing the process condition, single-crystal GaSb stripes are monolithically integrated on the silicon substrate. The MSM GaSb waveguide photodetector shows a responsivity of 0.57 A/W at 1550 nm.

Effect of Annealing of Silver Electrodes on Photolithographically Fabricated Silicon Trenches for Photodetection Applications

The photosensitivity of metal–semiconductor–metal (MSM) ultraviolet photodetectors based on recessed electrodes on silicon (Si) substrate has been enhanced. Recessed electrodes were deposited in photolithography fabricated silicon trenches. Such recessed electrodes can collect photogenerated electrons beneath the silicon surface, hence increasing the photocurrent. For metallization of Si substrates to fabricate MSM photodetectors, two interdigitated silver (Ag) contacts were deposited for each electrode by sputtering. The morphology of the sputtered contacts was studied by scanning electron microscopy. The effect of postdeposition annealing on the recessed electrodes in silicon trenches was studied. The annealing process was carried out at 300°C for 10 min. The optoelectronic characteristics of MSM ultraviolet (UV) photodetectors with such trench electrodes in silicon substrates were studied in the dark and under UV illumination, comparing photodetectors based on annealed versus as-deposited samples. The electrical characteristics of photodetectors fabricated on annealed substrates revealed enhanced sensitivity to ultraviolet radiation. It was shown that annealing silver electrodes in silicon trenches can improve photodetector quality.

Effects of metallic interlayers on the performance of nanocrystalline diamond metal-semiconductor-metal photodetectors

We have designed photodetectors based on a combination of nanocrystalline diamond (NCD) and bilayer metallic layers in a metal-semiconductor-metal (MSM) structure. The NCD films were deposited on silicon substrates which nucleated by different metallic layers (tungsten, molybdenum) and nanodiamond suspension. We found that the metallic layers not only significantly improve the nucleation density of the substrates but also reduce the surface roughness of the fabricated NCD films from 8.3 nm to 4.7 nm. Furthermore, the one-step fabrication of MSM photodetectors adopts metallic interlayers consisting of two functions (diamond nucleation site and back electrodes), which shows high detection efficiency and rapid response to UV irradiation in air ambient. The W/NCD/W structure exhibits good Ohmic contact characteristics and significant changes (∼3 orders of magnitude) in photocurrent and stable reproducibility. The achievement of this research indeed demonstrates great prospects of photodetector applications of NCD films in the future.

Post Annealing Effects on the Electrical Properties of Polysilicon Metal-Semiconductor-Metal Photodetectors

This study investigated the effects of the post annealing temperatures on the electrical and interfacial properties of a metal-semiconductor-metal photodetector(MSM-PD) device. The interdigitate type MSM-PD devices had the structure Al(500 nm) / Ti(200 nm) / poly-Si(500 nm). Structural analyses of the MSM-PD devices were performed by employing X-ray diffraction(XRD), scanning electron microscopy(SEM) and transmission electron microscope(TEM). Electrical characteristics of the MSM-PD were also examined using current-voltage(I-V) measurements. The optimal post annealing condition for the Schottky contact of MSM-PD devices are 350°C-30minutes. However, as the annealing temperature and time are increased, electrical characteristics of MSM-PD device are degraded. Especially, for the annealing conditions of 400°C-180minutes and 500°C-30minutes, the I-V measurement itself was impossible. These results are closely related to the solid phase reactions at the interface of MSM-PD device, which result in the formation of intermetallic compounds such as Al3Ti and Ti7Al5Si12.

Flexible Amorphous GeSn MSM Photodetectors

We demonstrate amorphous Ge0.92Sn0.08 surface illuminated metal–semiconductor–metal (MSM) photodetectors on flexible substrates for visible wavelength. Photodetection at 633 nm is achieved with an I–V response up to 10–4 A for a 2 V bias voltage. Different evaporation rate effects on the amorphous GeSn MSM photodetector are examined. Bending/strain effects on device performance were studied by evaluating the current density versus voltage characteristics. Amorphous GeSn thin film deposition on polyethylene terephthalate flexible substrate and Ni/Au–GeSn–Ni/Au MSM photodetector finger pattern deposition were performed via thermal evaporation. Photocurrent and dark current densities of amorphous GeSn MSM photodetectors were obtained at 1.36 A/cm2 and 0.24 A/cm2, respectively, where the photocurrent to dark current contrast ratio was found to be equal to 5.6. We also examined the evaporation rate, strain, and bending effect.

Impact of open-core threading dislocations on the performance of AlGaN metal-semiconductor-metal photodetectors

The influence of open-core threading dislocations on the bias-dependent external quantum efficiency (EQE) of bottom-illuminated Al0.5Ga0.5N/AlN metal-semiconductor-metal (MSM) photodetectors (PDs) is presented. These defects originate at the Al0.5Ga0.5N/AlN interface and terminate on the Al0.5Ga0.5N surface as hexagonal prisms. They work as electrically active paths bypassing the Al0.5Ga0.5N absorber layer and therefore alter the behavior of the MSM PDs under bias voltage. This effect is included in the model of carrier collection in the MSM PDs showing a good agreement with the experimental data. While such dislocations usually limit the device performance, the MSM PDs benefit by high EQE at a reduced bias voltage while maintaining a low dark current.

High Responsivity β-Ga2O3 Metal–Semiconductor–Metal Solar-Blind Photodetectors with Ultraviolet Transparent Graphene Electrodes

We demonstrated high responsivity metal–semiconductor–metal (MSM) solar-blind photodetectors by integrating exfoliated β-Ga2O3 microlayers with graphene, which is a deep ultraviolet (UV) transparent and conductive electrode. Photodetectors with MSM structures commonly suffer from low responsivity, although they feature a facile fabrication process, low dark current, and fast response speed. The β-Ga2O3 MSM solar-blind photodetectors with graphene electrodes exhibited excellent operating characteristics including higher responsivity (∼29.8 A/W), photo-to-dark current ratio (∼1 × 106%), rejection ratio (R254nm/R365nm, ∼9.4 × 103), detectivity (∼1 × 1012 Jones), and operating speed to UV-C wavelengths, compared with MSM photodetectors with conventional metal electrodes. Absence of shading by the integration of graphene with β-Ga2O3 allows maximum exposure to the incident photons, suggesting a great potential for deep UV optoelectronic applications.

3-D Dual-Gate Photosensitive Thin-Film Transistor Architectures Based on Amorphous Silicon

In contrast to the conventional planar p-i-n photodiode and a metal-semiconductor–metal photodetector, the 3-D dual-gate photosensitive thin-film transistor (TFT) architectures presented here attain excellent photoresponse characteristics. Operating the device in the subthreshold regime further boosts the photoconductive gain as a result of light-induced decrease in the threshold voltage. This paper presents design considerations along with a performance comparison between 3-D photosensitive TFTs that have $\pi $ - and FIN-shaped channels and conventional TFT with a planar channel. Our paper shows that the $\pi $ -shaped structure tends to have a higher sensitivity while the FIN-shaped counterpart is more responsive with wider dynamic range. For both structures, a measured photoconductive gain of $10^{\textsf {4}} \sim 10^{\textsf {6}}$ % is obtained with spectral responsivity ranging from near UV to near IR, and the photoresponse time in the range of tens of milliseconds. The 3-D dual-gate photosensitive TFT architecture appears to be very promising for large-area, low-level UV, visible, and IR detection applications.

Performance improvement of plasmonic-based thin film assisted MSM-PDs

This paper presents numerical study on a novel type of plasmonic-based metal-semiconductor-metal photodetector (MSM-PD) for extraordinary light absorption, which can drastically modify transmission spectra when a thin film is designed inside the central subwavelength slit. This strategy assists to recover non-propagating evanescent waves at nano-scale. Furthermore, by focusing on metallic nanostructures and surface plasmon polaritons (SPPs), and their contribution to the confinement of light in subwavelength scale, we introduce the concept of plasmonic lens that significantly reduces metal induced energy losses. The proposed novel photodetector with a thin film on the central slit assists the device to absorb ∼280-times more light than the bare slit plasmonic-based MSM-PDs.

Effect of a SiO₂ Anti-reflection Layer on the Optoelectronic Properties of Germanium Metal-semiconductor-metal Photodetectors

The interdigitated germanium (Ge) metal-semiconductor-metal (MSM) photodetectors (PDs) with and without an SiO₂ anti-reflection (AR) layer was fabricated, and the effect of SiO₂ AR layer on their optoelectronic response properties were investigated in detail. The lowest reflectance of 15.6% at the wavelength of 1550 nm was obtained with a SiO₂ AR layer with a thickness of 260 nm, which was in a good agreement with theoretically calculated film thickness for minimizing the reflection of Ge surface. The Ge MSM PD with 260 nm-thick SiO₂ AR layer exhibited enhanced device performance with the maximum values of responsivity of 0.65 A/W, the quantum efficiency of 52.2%, and the detectivity of 2.49 × 10SUP9/SUP cm HzSUP0.5/SUPWSUP-1/SUP under the light illumination with a wavelength of 1550 nm. Moreover, timedependent switching analysis of Ge MSM PD with 260 nm- thick SiO₂ AR layer showed highest on/off ratio with excellent stability and reproducibility. All this investigation implies that 260 nm-thick SiO₂ AR layer, which is effective in the reduction in the reflection of Ge surface, has a great potential for Ge based optoelectronic devices.

(Invited) Transition Metal Dichalcogenide Semiconductor Growth and Large Area Devices for Optoelectronics and Sensing

Atomically-thin transition metal dichalcogenides, such as MoS2, are topological materials with physical characteristics of great interest for the realization of nano-electronic and photonic devices, with promising applications for chemical and biological sensing. We first report here the growth and characterization of continuous monolayer MoS2 films over large areas by chemical vapor deposition on both sapphire and silicon substrates. The effects of growth temperature were studied. The materials were subsequently characterized using Raman spectroscopy, photoluminescence, fluorescence imaging, optical absorption, atomic force microscopy, and electron microscopy. Large area metal-2D semiconductor-metal photodetector and gas sensor devices were then realized by conventional photolithography using an interdigitated pattern. The gas sensor was able to detect CO2 concentrations as low as 200 ppm, while the photodetector exhibited a detectivity of ~8x1011 Jones at room temperature.