MSM PDs Research Articles

AlGaN-based metal–semiconductor–metal photodiodes fabricated with nonplanar structure geometry for ultraviolet light detection

Based upon GaN-on-Si wafers with a Al0.24Ga0.76N/AlN/GaN heterostructure, metal–semiconductor–metal photodetectors (MSM PDs) with nonplanar structure geometry are proposed for ultraviolet (UV) light detection. Although the presence of a highly conductive two-dimensional electron gas (2DEG) channel in the Al0.24Ga0.76N/AlN/GaN heterostructure is beneficial for realizing a high-electron-mobility transistor with a drain current of 600 mA/mm and a transconductance of 96 mS/mm, a high dark current is also observed in Schottky-barrier based MSM PDs. Using the nonplanar structure design for the MSM PDs (i.e., one Schottky contact formed on the top surface of the AlGaN mesa while the other is on the GaN buffer) achieves a significant decrease in PD’s dark current (<6 × 10−10 A). In addition, an anomalous increase in dark current of the proposed MSM PDs was found at forward bias, which could be attributed to the reduced potential barrier of the electrons in 2DEG and/or the lowering of the Schottky barrier height by hole trapping at the metal/semiconductor interface. Due to the improved signal to noise ratio and increased dissociation of the photogenerated carriers at the reverse junction (i.e., the reversely biased Schottky contact), the proposed MSM PDs operating at reverse bias exhibit enhanced UV photoresponsivity at λ < 360 nm. In addition to being used for UV light detection, these MSM PDs with a 3-dB bandwidth of 29.6 Hz are also useful for potential optical communications applications.

Solar-blind UV photodetector with low-dark current and high-gain based on ZnO/Au/Ga2O3 sandwich structure

High performance ultraviolet photodetectors (UV PDs) have attracted significant attention due to their great potential applications in both military and civilian fields. In this work, dual-band ultraviolet PDs with two peak responses at solar blind (255 nm) and near UV (360 nm) region were successfully fabricated by constructing ZnO/Au/Ga2O3 sandwich structure. The dark current of the ZnO/Au/Ga2O3 sandwich structure PD was reduced by 35.6 times and 5.9 times in comparison with ZnO/Au and Au/Ga2O3 MSM PDs, respectively. The responsivity of the ZnO/Au/Ga2O3 PD to 255 nm light was 336.3 A/W, which is 1443 times higher than that of the Au/Ga2O3 PD and corresponds to a gain of 1637. While, the responsivity of the ZnO/Au/Ga2O3 PD to 360 nm light was 21.6 A/W, which is only 1.2 times higher than that of the ZnO/Au PD and corresponds to a gain of 73. Based on the dependence of photocurrent on incident light intensity, the suppression of dark current in the ZnO/Au/Ga2O3 PD can be attributed to the reduction of the unsaturated bonds and the trap states at the interface of ZnO and Ga2O3. The high gain at 255 nm light in the sandwiched PD has been attributed to the migration of the photogenerated hole from Ga2O3 to ZnO and the higher electron mobility and the longer hole lifetime of ZnO than that of Ga2O3. Such ZnO/Au/Ga2O3 sandwich structure PDs with low dark current and high gain hold promise for developing high performance UVA and UVC dual-band PDs.

Metal-Semiconductor-Metal GeSn Photodetectors on Silicon for Short-Wave Infrared Applications.

Metal-semiconductor-metal photodetectors (MSM PDs) are effective for monolithic integration with other optical components of the photonic circuits because of the planar fabrication technique. In this article, we present the design, growth, and characterization of GeSn MSM PDs that are suitable for photonic integrated circuits. The introduction of 4% Sn in the GeSn active region also reduces the direct bandgap and shows a redshift in the optical responsivity spectra, which can extend up to 1800 nm wavelength, which means it can cover the entire telecommunication bands. The spectral responsivity increases with an increase in bias voltage caused by the high electric field, which enhances the carrier generation rate and the carrier collection efficiency. Therefore, the GeSn MSM PDs can be a suitable device for a wide range of short-wave infrared (SWIR) applications.

Post-oxidation effects on MgxZn1-xO/ZnO bi-layer metal-semiconductor-metal photodetectors

Post-oxidation was conducted for various times (0–8 min) in MgxZn1−xO/ZnO bi-layers and metal–semiconductor–metal photodetectors (MSM PDs) were fabricated. The effects of post-oxidation on the MgxZn1−xO/ZnO bi-layers and the optoelectronic characteristics of the MSM PDs were studied in detail. The leakage current of the MSM PDs decreased as post-oxidation time increased owing to the improvement of the crystal structure of the MgxZn1−xO/ZnO bi-layers. However, a long post-oxidation time (8 min) destroyed the crystal structure and raised the leakage current. After post-oxidation, the visible response of the MSM PDs was greatly reduced, therefore enhancing the ultraviolet (UV) (320 nm)/visible (500 nm) rejection ratio from 77 to 331 (approximately 4.3 times) for the MSM PDs with 5-min post-oxidation compared to the one without post-oxidation. X-ray photoelectron spectroscopy showed that, after post-oxidation, the introduced oxygen atoms compensated the oxygen-vacancy-related defects and that the thermal energy forced interstitial oxygen atoms (Oi) to fill in oxygen vacancies (OV). As a result, the number of lattice oxygen atoms (OL) increased, whereas the number of OV and Oi decreased. The improved crystal structure reduced the leakage current and enhanced the UV/visible rejection ratio. However, for a long post-oxidation time (8 min), the weakly-bonded oxygen atoms leaves the MgxZn1−xO surface, hence increasing the number of OV and decreasing that of OL. Increasing the number of defect degrades the performance of MSM PDs.

Metal-germanium-metal photodetector grown on silicon using low temperature RF-PECVD.

In this paper, germanium metal-semiconductor-metal photodetectors (MSM PDs) are fabricated on Si using a low-temperature two-step deposition technique by RF-PECVD. The photodetectors are optimized to effectively suppress the dark current through the insertion of n-type a-Si:H interlayer between the metal/Ge interface. Tuning the Schottky Barrier Height (SBH) by inserting different thickness of the interlayer is investigated. Results revealed that SBH for electrons and holes can effectively be enhanced by 0.3eV and 0.54eV, respectively. Furthermore, the dark-current (IDark) is suppressed significantly by more than four orders of magnitude. The measured IDark is ∼76 nA for an applied reverse bias of 1.0 V. Additionally, the Ge MSMs structure exhibited a photo responsivity of 0.8A/W at that bias. The proposed low-temperature (<550°C) Ge-on-Si MSM PD demonstrates a great potential for high-performance Ge-based photodetectors in monolithically integrated CMOS platform.

UV photocurrent responses of ZnO and MgZnO/ZnO processed by atmospheric pressure plasma jets

This paper reports the 368-nm UV photocurrent responses of rf-sputter-deposited ZnO and MgZnO/ZnO metal–semiconductor–metal photodetectors (MSM PDs) treated using atmospheric pressure plasma jets (APPJs). In ZnO and MgZnO/ZnO PDs, the dark current and photocurrent levels, photoresponsivities, and photocurrent response times in the fast rising and decay transition regions increase with the APPJ treatment duration. The MgZnO capping layer also increase the dark current and photocurrent levels, photoresponsivity, and fast decay transition time. These observations can be attributed to the shielding of ambient oxygen, defect and surface states passivation, and introduction of highly conductive interfaces at the MgZnO/ZnO heterojunctions.

Effects of Interdigitated Platinum Finger Geometry on Spectral Response Characteristics of Germanium Metal-Semiconductor-Metal Photodetectors

We fabricated interdigitated germanium (Ge) metal-semiconductor-metal photodetectors (MSM PDs) with interdigitated platinum (Pt) finger electrodes and investigated the effects of Pt finger width and spacing on their spectral response. An increase in the incident optical power enhances the creation of electron-hole pairs, resulting in a significant increase in photo current. Lowering of the Schottky barrier could be a main cause of the increase in both photo and dark current with increasing applied bias. The manufactured Ge MSM PDs exhibited a considerable spectral response for wavelengths in the range of 1.53-1.56 μm, corresponding to the entire C-band spectrum range. A reduction in the area fraction of the Pt finger electrode in the active region by decreasing and increasing finger width and spacing, respectively, led to an increase in illuminated active area and suppression of dark current, which was responsible for the improvement in responsivity and quantum efficiency of Ge MSM PDs.

Transparent Ga2O3 Photodetectors for Harsh Optoelectronics

We fabricate fully transparent solar-blind DUV PD devices employing β-Ga2O3 as an active layer and indium zinc oxide (IZO) as the transparent electrodes, exhibiting the average transmittance up to 80% from visible to near IR wavelength region without image blurring. The β-Ga2O3 MSM PDs for the use in harsh environments show a very low dark current (~1 nA), no sign of breakdown even at a bias higher than 200 V, excellent thermal stability, and intrinsic solar blindness. The dark current of β-Ga2O3 MSM PDs under significantly different oxygen concentration in the ambiences is similar, indicating that the high inertness to surface effect due to superior crystallinity nature of β-Ga2O3. The working temperature is up to 700 K and the responsivity is 0.32 mA/W at 10 V bias under 185-nm illumination. β-Ga2O3 PDs can be fully recovered after 700-K operation, showing excellent thermal reliability and robustness. The intriguing optoelectronic and electrical properties of β-Ga2O3 promise a new generation of stable, solar-blind DUV PDs for the extremely harsh electronic applications, such as sensing, imaging, and intrachip optical interconnects in high-temperature environments.

Assessment of amplifying effects of ridges spacing and height on nano-structured MSM photo-detectors

This paper presents the light transmission efficiency of metal–semiconductor–metal photo-detectors (MSM-PDs) using finite difference time-domain method. The absorption of MSM PDs can be enhanced with the aid of nano-grating coupled surface plasmon resonances at the design wavelength. The simulated results show that the light absorption enhancement of nano-grating assisted MSM-PD is $$\sim $$ 9-times better than the conventional MSM-PD.

Influence of two-tier structuring on the performance of black silicon-based MSM photodetectors

Black silicon, fabricated by alkaline anisotropic etching along with metal-assisted etching, consists of regular distributed micro-scale pyramids and irregular distributed nano-scale pores, in which the pore size plays an important role in the performance of the black silicon-based metal–semiconductor–metal photodetectors (BS MSM PDs). It is found that the dark current characteristic of BS MSM PD is different from that of traditional silicon MSM PD, and the former has negative differential resistance in part of its operating range because of the quantum tunneling effect exists in black silicon. Moreover, it is interesting to note that BS MSM PD with longer metal-assisted etching time (larger nanopore size) has higher responsivity at high bias voltage but lower responsivity at low bias voltage.

Few-Layer MoS2 with High Broadband Photogain and Fast Optical Switching for Use in Harsh Environments

Few-layered MoS2 as Schottky metal-semiconductor-metal photodetectors (MSM PDs) for use in harsh environments makes its debut as two-dimensional (2D) optoelectronics with high broadband gain (up to 13.3), high detectivity (up to ~10(10) cm Hz(1/2)/W), fast photoresponse (rise time of ~70 μs and fall time of ~110 μs), and high thermal stability (at a working temperature of up to 200 °C). Ultrahigh responsivity (0.57 A/W) of few-layer MoS2 at 532 nm is due to the high optical absorption (~10% despite being less than 2 nm in thickness) and a high photogain, which sets up a new record that was not achievable in 2D nanomaterials previously. This study opens avenues to develop 2D nanomaterial-based optoelectronics for harsh environments in imaging techniques and light-wave communications as well as in future memory storage and optoelectronic circuits.

Optoelectrical and low-frequency noise characteristics of flexible ZnO–SiO_2 photodetectors with organosilicon buffer layer

The present study demonstrates the optoelectrical and low-frequency noise characteristics of ZnO-SiO(2) nanocomposite solar-blind metal-semiconductor-metal photodetectors (MSM PDs) on flexible polyethersulfone (PES) substrate with and without an organosilicon [SiO(x)(CH(3))] buffer layer. For a given bandwidth of 100 Hz and a -5 V applied bias, the noise equivalent powers of the ZnO-SiO(2) nanocomposite MSM PD on PES with and without the SiO(x)(CH(3)) buffer layer were 1.39 × 10(-14) and 5.72 × 10(-14) W at 240nm, respectively, corresponding to the normalized detectivities of 5.04 × 10(14) and 1.22 × 10(14) Hz(0.5) W(-1), respectively. These findings indicate that a lower noise level and a higher detectivity can be achieved for ZnO-SiO(2) nanocomposite MSM PDs on PES by introducing a SiO(x)(CH(3)) buffer layer.

Low-Temperature, Ion Beam-Assisted SiC Thin Films With Antireflective ZnO Nanorod Arrays for High-Temperature Photodetection

This work demonstrates high-temperature operation of metal-semiconductor-metal photodetectors (MSM PDs) using low-temperature, ion beam-assisted deposition of nanocrystalline SiC thin films and hydrothermal synthesis of ZnO nanorod arrays (NRAs). Due to the incorporation of ZnO NRAs, the photo-to-dark current ratio of SiC MSM PDs is increased from 4.9 to 13.3 at 25 °C and from 4.9 to 7.6 at 200 °C. The enhancement in the sensitivity suggests that the ZnO NRAs could serve as an effective antireflective layer to guide more light into the SiC MSM PDs. This was confirmed through the characterization of reflectance measurements and finite-difference time-domain analysis. These results support the integration of nanocrystalline SiC thin films and ZnO NRAs for use in high-temperature photodetection applications.

Characterization of AlGaN/GaN Metal- Semiconductor-Metal Photodetectors With a Low-Temperature AlGaN Interlayer

AlGaN/GaN metal-semiconductor-metal photodetectors (MSM PDs) with a low-temperature (LT) AlGaN interlayer (IL) were fabricated. Compared with the conventional AlGaN/GaN MSM PD, it was found that leakage current can be suppressed by insertion of a LT AlGaN IL due to the reduction of surface pits and improvement of crystalline quality. It was also found that larger photoresponsivity can be achieved due to the enhanced electric field strength as a result of inserting a LT AlGaN IL. Furthermore, suppressed photoconductive gain, lower noise level, and larger detectivity of MSM PD can also be achieved by using a LT AlGaN IL.

The substrate-induced effect of GaN MSM photodetectors on silicon substrate

GaN metal–semiconductor–metal photodetectors (MSM PDs) on silicon substrates and sapphire substrates were fabricated and characterized. We found that the current–voltage (I–V) characteristics of MSM PDs on the silicon substrate could be approximated by the Poole–Frenkel conduction behavior. This phenomenon was attributed to the presence of micro-grain structure in the silicon-substrate epitaxy layer. The voltage-dependent responsivity of GaN MSM PDs on the silicon substrate was also evidence of micro-grains inside the epitaxy layer. At a low frequency, the 1/f-form noise was a main contribution to both PDs. Moreover, the extremely low β (∼0.7) extracted from GaN MSM PDs on the silicon substrate was first reported. Based on the current–voltage behavior, the extremely low β was believed to originate from the silicon-substrate-induced micro-grain.

Visible-Blind Metal–Semiconductor–Metal Photodetectors by Capping an In Situ Low-Temperature AlN Layer

We present the characteristics of a nitride-based UV metal–semiconductor–metal photodetector (MSM PD) with an in situ low-temperature (LT) grown AlN cap layer. From atomic-force microscopy scan images, we could clearly see that surface pits on the sample surface are almost invisible with an LT AlN cap layer but can be observed in a conventional cap layer. Compared with conventional MSM PDs, it was found that we achieved smaller dark current and larger UV-to-visible rejection ratio for the PDs with an LT AlN cap layer. With a applied bias, the UV-to-visible rejection ratio between 360 and was for the MSM PDs with an LT AlN cap layer. The calculated noise equivalent power and normalized detectivity biased at for the MSM PDs with an LT AlN cap layer was and , respectively.

GaN MSM Photodetectors with an Unactivated Mg-Doped GaN Cap Layer and Sputtered ITO Electrodes

GaN UV metal–semiconductor–metal photodetectors (MSM PDs) with an unactivated Mg-doped cap layer and sputtered indium tin oxide (ITO) were fabricated. Compared with conventional MSM PDs without a cap layer, it was found that we could achieve a significantly much smaller dark current, larger UV to visible rejection ratio, and larger normalized detectivity by inserting an unactivated Mg-doped GaN cap layer. The dark leakage current for the MSM PDs with an unactivated Mg-doped GaN cap layer was shown to be about ten orders of magnitude smaller than that for the conventional MSM PDs. Under a 0.5 V bias, the measured responsivity and UV-to-visible rejection ratio were 0.017 A/W and , respectively, for the MSM PDs with an unactivated Mg-doped GaN cap layer. This result could be attributed to the thicker and higher potential barrier and effective surface passivation after inserting this in situ grown cap layer. With a 1 V applied bias, it was also found that we could achieve a lower noise level and a higher normalized detectivity of by inserting an unactivated Mg-doped GaN cap layer into MSM PDs.

Design and Fabrication of Monolithic Distributed Traveling-Wave Photodetectors Integrated With Polymer Optical Waveguides

We have designed and fabricated monolithic distributed balanced traveling-wave metal-semiconductor-metal photodetectors (MSM PDs) distributed along single-mode polymer optical waveguides. We demonstrate that monolithically integrated polymer optical waveguides on a semiconductor can provide efficient optical interconnects in an optoelectronic circuit. A responsivity of 0.44 A/W and a 3-dB bandwidth of 10 GHz were obtained in a PD array with five MSM PDs. A high photocurrent of 7.3 mA was measured in an array having eight MSM PDs. Impulse response measurements were performed and the results were compared with frequency domain results.

Mixing characteristics of InGaAs metal–semiconductor–metal photodetectors with Schottky enhancement layers

We report on the optoelectronic (OE) mixing characteristics of a Schottky-enhanced InGaAs-based metal–semiconductor–metal photodetector (MSM–PD). The measured frequency bandwidth of such a mixer is less than that of a corresponding photodetector. The mixing efficiency depends on the light modulation, local oscillator, and mixed signal frequencies and decreases nonlinearly with decrease in optical power. This is not observed in GaAs-based and non-Schottky-enhanced InGaAs MSM–PDs. We present a circuit model of the OE mixer to explain the experimental results.

Measurements of InGaAs metal–semiconductor–metal photodetectors under high-illumination conditions

We report on temporal response measurements of InGaAs metal–semiconductor–metal photodetectors (MSM–PDs) under high-illumination conditions. The peak current efficiency of the MSM–PDs decreases as optical pulse energy increases due to space-charge-screening effects. The screening effect begins to occur at an optical pulse energy as low as 1.0 pJ, as predicted by a recent two-dimensional model. The fall time and full width at half maximum of the impulse response increase as the optical pulse energy increases and decrease as the bias voltage increases. For optical pulse energies between 1.0 and 100 pJ, the rise time displays a U-shaped behavior as the bias voltage increases. This may be associated with the shape of the electron velocity-field characteristic in conjunction with the screening of the dark field by optical generated carriers.