Room Temperature Dark Current Research Articles

An Ultrahigh‐Contrast Violet Phosphorus Van der Waals Phototransistor

AbstractUltrahigh‐contrast photodetection with low background noise is of critical importance for accurate image‐sensing in deep‐sea and deep‐space exploration. The state‐of‐the‐art silicon and III–V semiconductor‐based photodetectors usually require extended exposure to incident light for high‐quality sensing in darkness, which unfortunately results in large room temperature dark currents (RT‐Idark) and deteriorates the expected imaging contrasts. Herein, a high‐performance violet phosphorus (VP) phototransistor is reported by constructing a trap‐free interface between the VP channel and hexagonal boron nitride (h‐BN) dielectric via perfect van der Waals stacking. The device shows an extremely low RT‐Idark of 80 fA and gate‐tunable high ON/OFF ratio over 105, which is 2–4 orders of magnitude superior to that of conventional counterparts. A VP photodetector array has been fabricated to demonstrate the high‐contrast image‐sensing capability. The findings demonstrate the effectiveness of VP in low background noise and ultrahigh‐contrast image‐sensing applications, while also presenting exciting opportunities to enhance interface qualities through van der Waals architectures for high‐performance optoelectronics.

Complementary metal oxide semiconductor (CMOS) compatible gallium arsenide metal-semiconductor-metal photodetectors (GaAs MSMPDs) on silicon using ultra-thin germanium buffer layer for visible photonic applications

The monolithic integration of III–V materials on silicon appears as the most promising, cost-effective, and versatile method for next-generation optoelectronic devices. Here, we report on GaAs metal-semiconductor-metal photodetectors integrated on an Si substrate by metal-organic chemical vapor deposition. The device architecture is based on a GaAs active layer grown on Si via ultrathin, low-temperature Ge buffer layers. The Ge-on-Si acts as a “virtual” substrate to reduce the overall structural defects in the GaAs device layers. The metal-semiconductor junction characteristics were optimized to effectively suppress the dark current and passivate the interface defects. This was achieved through the insertion of an ultrathin Al2O3 interlayer at the metal/GaAs interface. The results show that a Schottky barrier height of 0.62 eV and 0.8 eV for electrons and holes, respectively, can be achieved. Circular devices with diameters ranging from 30 to 140 μm were fabricated. The measured room temperature dark current is ∼48 nA for an applied reverse bias of 1.0 V and a device diameter of 30 μm. Additionally, the GaAs metal-semiconductor-metal structure exhibited a remarkable photoresponsivity and detectivity values of (0.54 ± 0.15) A/W and ∼4.6 × 1010 cm Hz1/2 W−1 at 5 V reverse bias, 850 nm, respectively. The proposed method offers great potential for the monolithic integration of GaAs on an Si platform. Furthermore, this technique can be extended to other III–V materials and lattice mismatched systems for high-performance multiple band optoelectronics.

Night Vision for Small Telescopes

We explore the feasibility of using current generation, off-the-shelf, indium gallium arsenide (InGaAs) near-infrared (NIR) detectors for astronomical observations. Light-weight InGaAs cameras, developed for the night vision industry and operated at or near room temperature, enable cost-effective new paths for observing the NIR sky, particularly when paired with small telescopes. We have tested an InGaAs camera in the laboratory and on the sky using 12 and 18 inch telescopes. The camera is a small-format, 320 × 240 pixels of 40 μm pitch, shortwave infrared (SWIR) device from Sensors Unlimited. Although the device exhibits a room-temperature dark current of 5.7 × 104 per pixel, we find observations of bright sources and low-positional-resolution observations of faint sources remain feasible. We can record unsaturated images of bright (J = 3.9) sources due to the large pixel well-depth and resulting high dynamic range. When mounted on an 18 inch telescope, the sensor is capable of achieving milli-magnitude precision for sources brighter than J = 8. Faint sources can be sky-background-limited with modest thermoelectric cooling. We can detect faint sources (J = 16.4 at 10σ) in a one-minute exposure when mounted to an 18 inch telescope. From laboratory testing, we characterize the noise properties, sensitivity, and stability of the camera in a variety of different operational modes and at different operating temperatures. Through sky testing, we show that the (unfiltered) camera can enable precise and accurate photometry, operating like a filtered J-band detector, with small color corrections. In the course of our sky testing, we successfully measured sub-percent flux variations in an exoplanet transit. We have demonstrated an ability to detect transient sources in dense fields using image subtraction of existing reference catalogs.

Electron-initiated low noise 1064 nm InGaAsP/InAlAs avalanche photodetectors.

We report an electron-initiated 1064 nm InGaAsP avalanche photodetectors (APDs) with an InAlAs multiplier. By utilizing a tailored digital alloy superlattice grading structure, a charge layer and a p type InAlAs multiplier, an unity gain quantum efficiency of 48%, a low room temperature dark current of 470 pA at 90% breakdown voltage, and a low multiplication noise with an effective k ratio of ∼0.2 are achieved. The measured maximum gain factor is 5 at room temperature, which is currently limited by the non-optimized electric field profiles, and can be readily enhanced by modifying the doping and thickness parameters for the multiplier and the charge layer.

Room-temperature InP/InAsP Quantum Discs-in-Nanowire Infrared Photodetectors.

The possibility to engineer nanowire heterostructures with large bandgap variations is particularly interesting for technologically important broadband photodetector applications. Here we report on a combined study of design, fabrication, and optoelectronic properties of infrared photodetectors comprising four million n+-i-n+ InP nanowires periodically ordered in arrays. The nanowires were grown by metal-organic vapor phase epitaxy on InP substrates, with either a single or 20 InAsP quantum discs embedded in the i-segment. By Zn compensation of the residual n-dopants in the i-segment, the room-temperature dark current is strongly suppressed to a level of pA/NW at 1 V bias. The low dark current is manifested in the spectrally resolved photocurrent measurements, which reveal strong photocurrent contributions from the InAsP quantum discs at room temperature with a threshold wavelength of about 2.0 μm and a bias-tunable responsivity reaching 7 A/W@1.38 μm at 2 V bias. Two different processing schemes were implemented to study the effects of radial self-gating in the nanowires induced by the nanowire/SiOx/ITO wrap-gate geometry. Summarized, our results show that properly designed axial InP/InAsP nanowire heterostructures are promising candidates for broadband photodetectors.

4H-SiC SACM Avalanche Photodiode With Low Breakdown Voltage and High UV Detection Efficiency

In this paper, a high-performance 4H-SiC separated absorption charge multiplication ultraviolet avalanche photodiode (APD) with low breakdown voltage is designed and fabricated. The room temperature dark current of the APD remains at ∼0.1 pA level ( $\rm{\sim}29\;\text{pA/ cm}^{- 2}$ ) when reverse bias is below 50 V and then rises by several orders of magnitude before avalanche breakdown occurs. An avalanche gain up to 106 is achieved at 67 V. Based on secondary ion mass spectroscopy measurement, the electric field distribution within the as-fabricated APD is numerically simulated. The ionization ratio of Al in the p-type charge control layer is estimated to be 40%. The frequency dispersion phenomenon in capacitance–voltage (C-V) measurement should be caused by structural defects existing within the APDs, which may also be the source of the enhanced dark current before avalanche breakdown. The room-temperature maximum quantum efficiency is ∼80% at 270 nm with an ultraviolet (UV)/visible rejection ratio more than 10 4.

Investigation of planar photodiodes of a focal plane array based on a heteroepitaxial InGaAs/InP structure

The results of investigation of planar photodiodes (PDs) in a focal plane array (FPA) based on a heteroepitaxial InGaAs/InP structure are reported. The FPA has a size of 320 × 256 elements with a pitch of 30 μm, which are hybridized with various ROIC readout circuits. It is demonstrated that the PD reverse bias should be no lower than 2 V in order to suppress the FPA intercoupling at the room temperature. It is found that the dark current may be reduced considerably by cooling the FPA to–20°С with a two-stage thermoelectric cooler. The best average room-temperature dark current over the FPA planar photodiodes is 0.22 pA at an optimum PD bias of–2.4 V.

A single crystalline InP nanowire photodetector

Single crystalline nanowires are critical for achieving high-responsivity, high-speed, and low-noise nanoscale photodetectors. Here, we report a metal-semiconductor-metal photodetector based on a single crystalline InP nanowire. The nanowires are grown by a self-catalyzed method and exhibit stacking-fault-free zinc blende crystal structure. The nanowire exhibits a typical n-type semiconductor property and shows a low room temperature dark current of several hundred pA at moderate biases. A photoresponsivity of 6.8 A/W is obtained at a laser power density of 0.2 mW/cm2. This work demonstrates that single crystalline InP nanowires are good candidates for future optoelectronic device applications.

4H-SiC p-i-n Ultraviolet Avalanche Photodiodes Obtained by Al Implantation

In this letter, we report the first demonstration of a 4H-SiC p-i-n ultraviolet (UV) avalanche photodiode (APD) with its p-layer formed by Al implantation. In order to alleviate the electric field crowding around mesa edge, an appropriately doped p sub-layer with high lateral resistance is added beneath the top p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> contact layer, which are both formed by Al implantation into a lightly doped n-SiC layer. The room temperature (RT) dark current of the APD remains lower than 0.1 pA when reverse bias is <;200 V, and then increases by several orders of magnitude before avalanche breakdown occurs at ~260 V. A maximum avalanche gain larger than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> can be achieved, while the related breakdown voltage exhibits a positive temperature coefficient of ~25 mV/°C. The activation energy of dark current as well as the relationship between the dark current and the device active area is studied, which suggests that bulk leakage assisted by residual implantation-induced deep-level defects is the dominant source of dark current at high bias. The RT maximum quantum efficiency of the APD under 0 V bias is 44.4% at 270 nm with a UV/visible rejection ratio larger than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> .

Enhanced low-noise gain from InAs avalanche photodiodes with reduced dark current and background doping

We reduced the room temperature dark current in an InAs avalanche photodiode by increasing the p-type contact doping, resulting in an increased energetic barrier to minority electron injection into the p-region, which is a significant source of dark current at room temperature. In addition, by improving the molecular beam epitaxy growth conditions, we reduced the background doping concentration and realized depletion widths as wide as 5 μm at reverse biases as low as 1.5 V. These improvements culminated in low-noise InAs avalanche photodiodes exhibiting a room temperature multiplication gain of ∼80, at a record low reverse bias of 12 V.

Epitaxial InGaAsP/InP photodiode for registration of InP scintillation

Operation of semiconductor scintillators requires optically tight integration of the photoreceiver system on the surface of the scintillator slab. We have implemented an efficient and fast quaternary InGaAsP pin photodiode, epitaxially grown on the surface of an InP scintillator wafer and sensitive to InP luminescence. The diode is characterized by an extremely low room-temperature dark current, about 1nA/cm2 at the reverse bias of 2V. The low leakage makes possible a sensitive readout circuitry even though the diode has a large area (1×1mm2) and therefore large capacitance (50pF). Results of electrical, optical and radiation testing of the diodes are presented. Detection of individual α-particles and γ-photons is demonstrated.

Low-Dark-Current Heterojunction Phototransistors with Long-Term Stable Passivation Induced by Neutralized (NH4)2S Treatment

To investigate the effects of surface leakage on the temperature-dependent dark and optical performance of a heterojunction phototransistor (HPT), three sets of HPTs were prepared. They were unpassivated (HPT A), passivated with diluted (NH4)2S (HPT B), and passivated with neutralized (NH4)2S (HPT C). Passivation treatment successfully suppresses surface-defect-induced effects. The passivated HPTs exhibit a reduced dark current and an enhanced signal-to-noise ratio (SNR). HPT A exhibits a room-temperature collector dark current (ICdark) of 0.59 nA at a VCE of 1 V, while those of HPTs B and C are 0.03 and 0.89 pA, respectively. The room-temperature SNR values for HPTs A, B, and C at a Pin of 8.3 (107.6) nW are 5.9 (42), 59 (91), and 91 (122) dB, respectively. After a three-week air exposure, the room-temperature SNR at a Pin of 107.6 nW decreases to 25 (67) dB for HPT A (B), while it is 116 dB for HPT C. The decrease for HPT C is only 6 dB. A long-term stable passivation with good thermal stability has been achieved by neutralized (NH4)2S treatment.

Monolithic Integrated a-Si:H based pin-Diodes with Orthogonal Liquid Light Guidance Structures for Lab-on-Microchip Applications

AbstractWe report the fabrication of an amorphous silicon based fluorescence sensor for miniaturized total analysis systems along with experimental results on optical excitation and detection elements. The pin-photodiode exhibits a dynamic range of 110dB and a room temperature dark current of less than 3000 charge carriers per ms according to a detector area of 0.1256mm2. The spectral response is ranging from 320nm to 780nm with a maximum at 600nm @ 80% quantum efficiency. To provide high sensitivity, the excitation light irradiates the fluid orthogonally to the active sensor detection direction by means of specifically designed microfluidic capillaries filled with e.g. methylene iodide or 1,2-o-dibrombenzene. The liquid core, which is enclosed by solid cladding materials, has been calculated to dimensions of a width of 16.75µm or 59.67µm with a height from 15µm to 50µm according to a number of propagating modes inside of 16 or 57, respectively.

InSb high-speed photodetectors grown on GaAs substrate

We report InSb-based high-speed photodetectors grown on GaAs substrate. The p-i-n type photodetectors can operate at room temperature. Room-temperature dark current was 4 mA at 1 V reverse bias, and the differential resistance at zero bias was 65 Ω. At liquid nitrogen temperature, the dark current was 41 μA at 1 V reverse bias and the differential resistance at zero bias was 150 kΩ. Responsivity measurements were performed at 1.55 μm wavelength at room temperature. The responsivity increased with applied bias. At 0.6 V, responsivity was 1.3 A/W, where unity quantum efficiency was observed with internal gain. Time-based high-speed measurements were performed using a pulsed laser operating at 1.55 μm. The detectors showed electrical responses with 40 ps full width at half maximum, corresponding to a 3 dB bandwidth of 7.5 GHz.

A novel photoconductive detector for single photon detection

We report on first results of an ultra sensititve photo-conducting detector. It basically represents a p–i–n diode and junction FET integrated into a single device. The extremely high detectivity can be achieved by reducing the electrically active area and hence, the capacitance of a p–i–n photo-conductive detector without reducing the optical area. For this purpose, we have fabricated p–i–n-structures consisting of crossed p- and n-doped stripes of a few micrometer width. While the top n-stripe can easily be defined by wet-chemical etching, focussed Be ion beam implantation is used to define a buried p-doped stripe in a GaAs wafer. We show that room temperature dark currents at a few volts reverse bias are in the low pA- and capacitances in the low fF-range and the expected large photo-conductive gain is observed.

Molecular beam epitaxy of highly mismatched In0.73Ga0.27As on InP for near-infrared photodetectors

We demonstrate that high-quality lattice-mismatched In0.73Ga0.27As:InP photodetectors for use at 2.2 μm can be grown by molecular beam epitaxy using compositionally graded buffer layers. By investigating various growth conditions, we found that the quality of the active layer material depends on the thickness of the compositionally graded buffer layers and the substrate temperature during the growth of the compositionally graded buffer layers and active layers. The best device performance was obtained from a sample with a 1.0 μm compositionally graded buffer grown at a substrate temperature of 350 °C combined with active layers grown at 400 °C. The typical room temperature dark current at a reverse bias of 1 V for 100×100 μm2 photodetectors on this sample was 2 mA/cm2. This dark current is a factor of 10 better than that of commercially available devices fabricated from material grown using vapor phase epitaxy.

GaInAsSb/GaSb pn photodiodes for detection to 2.4 μm

Ga0.77In0.23As0.20Sb0.80/GaSb pn heterojunction photodiodes have been prepared by liquid phase epitaxy. They exhibit a long-wavelength threshold of 2.4 μm. The room-temperature dark current at V = −0.5 V is 3 μA (10mA/cm2) and the external quantum efficiency is around 40% in the wavelength range 1.75–2.25 μm. The estimated detectivity D* at 2.2 μm is 8.8 × 109 cm Hz½W−1.

Current-voltage characteristics of metalorganic chemical vapor deposition InP/InGaAs p-i-n photodiodes: The influence of finite dimensions and heterointerfaces

The electrical and optoelectronic characteristics of planar InP/InGaAs p-i-n photodiodes grown by metalorganic chemical vapor deposition were extensively investigated. A typical room-temperature dark current of 10 pA for 100-μm-diam devices was measured at −10-V bias on several wafers. A low capacitance between 0.42–0.46 pF (for 100-μm-diam devices), responsivities at 1.3 μm between 0.84–0.94 A/W (corresponding to external quantum efficiencies between 80% and 90%), and rise/fall times of less than 150 ps were measured at −5-V bias on several wafers. Excellent uniformity in dark current, junction capacitance, and optoelectronic characteristics was found over large wafer areas. The diode dark current was studied as a function of applied voltage, temperature, diode dimensions, and junction depth. The carrier transport mechanisms in different ranges of bias voltage and temperature have been identified and interpreted both in form and magnitude. At low to mid reverse bias, bulk generation-recombination currents dominate below 320 K. At higher temperatures, diffusion currents become dominant. Band-to-band tunneling current near the InP/InGaAs upper heterointerface dominates the dark current at high reverse bias. Under forward bias, the diffusion current dominates over the entire temperature range studied. Theoretical models have been developed to account for the magnitude of the reverse and forward diffusion currents and their dependence on diode dimensions. It is found that the lowest dark current measured at room temperature is very close to the theoretical limit achievable for planar InP/InGaAs/InP diodes with n doping of the InGaAs ∼1×1015 cm−3.

2.6 μm InGaAs photodiodes

We have developed In0.82Ga0.18As p-n homojunction photodiodes that have a long-wavelength threshold at about 2.65 μm. A compositionally graded InxGa1−xAs layer accommodates the 2% lattice mismatch between the InP substrate and the In0.82Ga0.18As active layers of the device. At −2 V reverse bias the room-temperature dark current is 3.5 μA (32 mA/cm2), and the quantum efficiency is 70–75% over the wavelength interval of 2.1–2.6 μm.

A low dark-current, planar InGaAs p-i-n photodiode with a quaternary InGaAsP cap layer

In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> As p-i-n photodiodes have become the most suitable photodectectors for long wavelength ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-1.65 \mu</tex> m) optical fiber communication systems due to their low dark-currents. Further reduction of the dark-current to subnanoampere range will bring significant improvement in receiver sensitivity at low bit rates. In this paper, we report the successful fabrication of In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> p-i-n diodes with room temperature dark-currents as low as 0.1 nA at -10 V bias by introducing a quaternary InGaAsP cap layer on the ternary InGaAs. In this new structure the InGaAsP cap layer undergoes the surface treatment and faces the dielectric silicon nitride film, but the p-n junction is still located inside the InGaAs layer. Since the addition of the wide bandgap cap layer results in low dark-current, the reduction of dark-current may be attributed to surface effects. Analysis of the temperature and voltage dependence of the dark-current indicates that at room temperature the ohmic conduction through the shunt formed on the surface limits the dark-current in this new diode structure.