Low Reverse Bias Voltage Research Articles

DLTS Study of Defects in HgCdTe Heterostructure Photodiode

Deep-level transient spectroscopy (DLTS) measurements were performed on HgCdTe heterostructure photodiode grown by metal-organic chemical vapor deposition (MOCVD) on GaAs substrate. In order to extract defects from individual layers of the heterostructure, three consecutive etchings were performed. In the first experiment, the N+/T/p/T/P+/n+ structure was chemically etched to the N+ bottom contact to obtain a mesa-type detector. Six localized defects were extracted across the entire photodiode. In the second experiment, the bottom contact was made to the p-type absorber. Two localized defects were found in the p/T/P+/n+ structure. In the third experiment, the top layers were removed and N+/T/p type detector was made—the cap contact was made to the p-type absorber, the bottom to the N+ layer. Five defect levels were identified, three of which overlap with the first experiment. A deep-trap level located at 183 meV above the top of the valence band was identified within the absorber bandgap. This energy coincides with the activation energy determined from the Arrhenius plot for the dark currents. A defect at the level of ∼0.5Eg suggests that the dark current at low reverse bias voltage is dominated by the Shockley–Read–Hall (SRH) mechanism.

Realizing improved performance of metal-insulator-semiconductor diodes with high-k MgO/SiOx stack

With the development of metal-insulator-semiconductor (MIS) field-effect transistors, the thickness of silicon dioxide (SiO2) must be scaled down; however, it causes a large gate leakage current and a low on/off current ratio. Magnesium oxide (MgO) exhibits a dielectric constant as high as 11.2, which is about three times larger than that (3.9) of SiO2; thus the thickness of MgO can be three times greater than that of SiO2 and significantly reduces the gate leakage current. Nevertheless, a large lattice mismatch (22.5%) exists between MgO and Si materials. In the study, the MgO/SiOx stack dielectrics are employed to suppress the large lattice mismatch and improve the performance of MIS diodes. The MgO/SiOx stack largely reduces the oxygen vacancy of MgO from 75% to 52.3%; also the fixed oxide charge and interface trap charge density are drastically decreased from 1.3 × 1012 and 1.8 × 1013 to 8.2 × 1011 cm−2 and 7.8 × 1012 cm−2eV−1, respectively. Compared to the MIS diodes with a single MgO, the MgO/SiOx stack dielectrics suppress the gate leakage current by about two orders in MIS diodes; hence the rectification ratio is enhanced from 114 to 1032 at ±2V. Band-diagram shows that Fowler-Nordheim tunneling occurs at a high reverse-bias voltage, and the barrier height increases from 0.18 to 0.42 eV in the MIS diodes with single MgO and MgO/SiOx stack dielectrics, respectively. Nevertheless, the MIS diodes operate through a direct tunneling at low reverse-bias voltages.

Voltage-dependent extended shortwave infrared (e-SWIR) photodetection-band tuning utilizing the Moss–Burstein effect

Extended shortwave infrared (e-SWIR) photodetectors and imaging focal plane arrays covering the wavelength beyond the conventional In0.53Ga0.47As cutoff wavelength of 1.65 micrometers (µm) can find numerous applications in infrared sensing and imaging. This paper reports voltage-tunable e-SWIR photodetectors based on the conventional gallium antimonide (GaSb) n–i–p and p–i–n homojunctions on GaSb substrates, which offer bias-dependent photodetection band tuning with a simple structure and high material crystal quality due to the perfect lattice matching on the substrates. Detection bands between the cutoff wavelengths of 1.7 µm and 1.9 µm can be tuned with a low reverse bias voltage of <0.1 volts (V). The mechanism of the voltage-dependent band-tuning was analyzed and attributed to the Moss–Burstein effect, which changes the electron and hole filling factors under different reverse bias voltages. This analysis agreed with the experimental data. The Moss–Burstein effect-induced voltage-dependent band-tuning mechanism can provide useful guidance for the designs of e-SWIR photodetectors.

InP/InGaAs Uni-Traveling-Carrier Photodiode (UTC-PD) With Improved EM Field Response

In this article, a unique structure of InP/InGaAs uni-traveling-carrier photodiode (UTC-PD) is proposed. Compared with the one-sided junction photodiode, the UTC-PD has the advantages of a simpler epitaxial layer structure while maintaining the characteristics of high bandwidth and high output power loss profile. Simulated results show that a large built-in electric field can be generated under illumination, which aids UTC carrier velocity. The merits of the new structure are compared to a standard UTC-PD photodiode in terms of improved electric field and carrier concentration response. It is demonstrated that the photogeneration and light absorption of UTC-PDs are improved by incorporating a step-like internal texture of cones and dots profile in the photo absorption layer. The simulated device shows a peak electrical 3-dB bandwidth of 65 GHz at a low light intensity and reverse bias voltage. The performance characteristics of 1-D and 2-D UTC-PD simulations, including internal electric field distribution, energy band diagram, carrier concentration, and power loss distribution, are carefully studied. A theoretical discussion of the working principle and key performance characteristics of a UTC-PD enhanced by heterojunction design is reported. The entire physical environment is modeled and simulated through the finite element method (FEM) using commercial software. The proposed photodiode structure configuration is designed and optimized for photodetectors for high RF frequencies at different light intensities. To the best of our knowledge, the obtained bandwidth and electric field response is the fastest among those reported for other various higher wavelength photodiodes.

The Blue Light Defects Activation in A-Si:H Pin Photodiode as a Biosensor

The microfluidic Lab-On-Chip (LOC) systems, based on the CMOS technology, today grow rapidly based on requirement of the Point-of-care-testing (POCT). It is a need for a high sensitive biotransducers, as a part of biosensors to be integrated on LOC system. To detect low-level of light emitted by an analyte, promising material and devices are a p-i-n a-Si:H photodiodes. The observed absorbance of blue light in human cells HeLa (cervical carcinoma) induct H2O2 in same cells and consequently, chemical reaction with NO, detected as chemiluminescence signal by the photodiode, as well as formation of cytotoxic singlet oxygen. On the other side a-Si:H p-i-n photodiode has a high sensitivity on blue light at low-light intensity, good spectral responsivity and small reflectance for blue light, low dark current, low-noise in the range of low reverse bias voltages. The photoconductivity of a-Si:H p-i-n photodiode is influenced by the native and light induced localized state density and their energy distribution in the energy gap of intrinsic a-Si:H. It is observed that the defect states of i-layer at various bias voltages contribute to the detection of HeLa cells chemiluminescence. The optical bias dependence of modulated photocurrent method (OBMPC) using the blue LED light is applied to clarify the energy gap density of state nature and energy distribution, respectively in a-Si:H p-i-n photodiode i-layer.

Electron and ion transmission of the gating foil for the TPC in the ILC experiment

We have developed a gating foil for the time projection chamber envisaged as a central tracker for the international linear collider experiment. It has a structure similar to the Gas Electron Multiplier (GEM) with a higher optical aperture ratio and functions as an ion gate without gas amplification. The transmission rate for electrons was measured in a counting mode for a wide range of the voltages applied across the foil using an 55Fe source and a laser in the absence of a magnetic field. The upper limit of the transmission rate for positive ions was estimated to be 3.36 ± 0.05 (stat. only)) × 10−4 from the measured electron transmission at a relatively low reverse bias voltage applied to the foil (ΔV =−15.5 V). The blocking power of the gating foil was confirmed to be high enough to suppress the influence of ion backflow.

Photoelectric and magnetic properties of Fe-hyperdoped Si layers formed by the recoil-atom implantation

The doping of near-surface region of single crystalline p-type Si by Fe impurity under irradiation by the low-energy and high-current Xe+ ion beam is investigated. The recoil-atom implantation method was applied which utilizes simultaneous sputtering of Fe target with irradiation of the deposited Fe atoms on the Si substrate surface by Xe+ ion beam. The resulting incorporation of Fe atoms into Si leads to formation of very thin (~5 nm) highly doped (>1022 at/cm3) surface layer (Si:Fe) containing Si and α-Fe nanoparticles with sizes of 5–20 nm. Such a layer demonstrates ferromagnetism at T = 10 K and superparamagnetism at 300 K. Inversion of the conductivity type (from p-to n-type) in the heavily doped Si:Fe layer and formation of n-p junction to the substrate is observed. A photoresponse of thus obtained n-Si:Fe/p-Si diode structure demonstrates an intense signal in the wavelength range of 500–1200 nm with a maximum at about 950 nm under the low reverse bias voltage (U = 1 V), whose integral intensity is comparable with that for commercial silicon photodiode at U = 10 V.

Validity of the Padovani–Stratton formulas for analysis of reverse current–voltage characteristics of 4H–SiC Schottky barrier diodes

In this present study, we test the accuracy of the Padovani–Stratton models in terms of the percent error in the barrier height extracted by this model from the reverse characteristics I-V simulated by the Tsu-Esaki model over a range −1000 V at room temperature. The thermionic field emission model without image force barrier lowering is much less accurate for low doping concentration (<1015 cm−3), in particular at low reverse bias voltages (<−200 V). While the field emission model is less accurate for high doping concentrations (>1017 cm−3) in particular for high reverse bias voltages. For medium doping concentration, thermionic field emission or field emission models provide an acceptable accurate for medium and high reverse bias voltages (>−100 V), however, it is less accurate at lower reverse bias voltages (<−100 V) and the best accuracy is occurred at about 1016 cm−3. When the Padovani- Stratton models is combined with image force barrier lowering model, the percent error in extracted barrier height takes large values; hence, it is inaccurate on entire range bias.

Large-Area 4H-SiC Ultraviolet Avalanche Photodiodes Based on Variable-Temperature Reflow Technique

In this letter, we report high-performance 4H-SiC separated absorption charge multiplication ultraviolet (UV) avalanche photodiodes (APDs) with a large active area (800- $\mu \text{m}$ diameter). For the first time, a variable-temperature photoresist reflow technique is presented to obtain very smooth sidewalls for positive beveled mesa, which is very useful for suppressing the reverse leakage and premature edge breakdown. At room temperature, the dark current of our fabricated large-area APD remains at ~1-pA level (0.2 nA/cm2) at low reverse bias voltage, and a high multiplication gain of over 106 is achieved. The peak responsivity at the wavelength of 274 nm reaches 0.18 A/W, corresponding to a maximum external quantum efficiency of 81.5% at unity gain. Moreover, an excellent UV/visible rejection ratio of more than 103 is obtained. To the best of our knowledge, this is the best result reported for visible-blind UV detectors based on the large-area 4H-SiC APDs.

Photovoltaic Action in Graphene–Ga2O3 Heterojunction with Deep‐Ultraviolet Irradiation

We demonstrate the fabrication of a monolayer graphene/β‐Ga2O3 heterostructure and its interesting prospect of producing a suitable Schottky barrier potential for deep‐ultraviolet (DUV) responsive photovoltaic device. The transient response behavior shows a faster response time for photovoltaic mode operation of the photodiode. The fast response at a zero bias is due to generation of photocurrent under an internal built‐in field in the graphene/Ga2O3 interface without any contribution from the trapped carriers. The fabricated device also shows an excellent photoresponsivity of 6.1 A W−1 with a slower response time at a low reverse bias voltage (−1.5 V). The high photoresponsivity at a bias voltage can be related to carrier multiplication due to carriers trapping/release process. Our findings show that the graphene/β‐Ga2O3 heterostructure can be significant for self‐powered/low power consuming DUV detector applications.

Numerical analysis of SiGeSn/GeSn interband quantum well infrared photodetector

In this paper, detailed theoretical investigation on the frequency response and responsivity of a strain balanced SiGeSn/GeSn quantum well infrared photodetector (QWIP) is made. Rate equation and continuity equation in the well are solved simultaneously to obtain photo generated current. Quantum mechanical carrier transport like carrier capture in QW, escape of carrier from the well due to thermionic emission and tunneling are considered in this calculation. Impact of Sn composition in the GeSn well on the frequency response, bandwidth and responsivity are studied. Results show that Sn concentration in the GeSn active layer and applied bias have important role on the performance of the device. Significant bandwidth is obtained at low reverse bias voltage, e.g., 200 GHz is obtained at 0.28 V bias for a single Ge 0.83 Sn 0.17 layer. Whereas, the maximum responsivity is of 8.6 mA/W at 0.5 V bias for the same structure. However, this can be enhanced by using MQW structure.

Silicon trench photodiodes on a wafer for efficient X-ray-to-current signal conversion using side-X-ray-irradiation mode

In this paper, we report a direct-conversion-type X-ray sensor composed of trench-structured silicon photodiodes, which achieves a high X-ray-to-current conversion efficiency under side X-ray irradiation. The silicon X-ray sensor with a length of 22.6 mm and a trench depth of 300 µm was fabricated using a single-poly single-metal 0.35 µm process. X-rays with a tube voltage of 80 kV were irradiated along the trench photodiode from the side of the test chip. The theoretical limit of X-ray-to-current conversion efficiency of 83.8% was achieved at a low reverse bias voltage of 25 V. The X-ray-to-electrical signal conversion efficiency of conventional indirect-conversion-type X-ray sensors is about 10%. Therefore, the developed sensor has a conversion efficiency that is about eight times higher than that of conventional sensors. It is expected that the developed X-ray sensor will be able to markedly lower the radiation dose required for X-ray diagnoses.

Van der Waals heterojunction diode composed of WS2 flake placed on p-type Si substrate

P–N junctions represent the fundamental building blocks of most semiconductors for optoelectronic functions. This work demonstrates a technique for forming a WS2/Si van der Waals junction based on mechanical exfoliation. Multilayered WS2 nanoflakes were exfoliated on the surface of bulk p-type Si substrates using a polydimethylsiloxane stamp. We found that the fabricated WS2/Si p–n junctions exhibited rectifying characteristics. We studied the effect of annealing processes on the performance of the WS2/Si van der Waals p–n junction and demonstrated that annealing improved its electrical characteristics. However, devices with vacuum annealing have an enhanced forward-bias current compared to those annealed in a gaseous environment. We also studied the top-gate-tunable rectification characteristics across the p–n junction interface in experiments as well as density functional theory calculations. Under various temperatures, Zener breakdown occurred at low reverse-bias voltages, and its breakdown voltage exhibited a negative coefficient of temperature. Another breakdown voltage was observed, which increased with temperature, suggesting a positive coefficient of temperature. Therefore, such a breakdown can be assigned to avalanche breakdown. This work demonstrates a promising application of two-dimensional materials placed directly on conventional bulk Si substrates.

Theoretical analysis of tin incorporated group IV alloy based QWIP

Detailed theoretical investigation on the frequency response, responsivity and detectivity of tin incorporated GeSn based quantum well infrared photodetector (QWIP) is presented in this paper. Rate equation and continuity equation in the well are solved simultaneously to obtained photo generated current. Quantum mechanical carrier transport like carrier capture in QW, escape of carrier from the well due to thermionic emission and tunneling are considered in this calculation. Impact of Sn composition in the GeSn well on the frequency response, bandwidth, responsivity and detectivity are studied. Results show that Sn concentration and applied bias have important role on the performance of the device. Significant bandwidth is obtained at low reverse bias voltage, e.g., 150 GHz is obtained at 0.14 V bias for single Ge0.83Sn0.17 layer. Detectivity, in the range of 107 cm Hz1/2 W−1 is obtained for particular choice of Sn-composition and bias.

Franz–Keldysh effect in n-type GaN Schottky barrier diode under high reverse bias voltage

The photocurrent of GaN vertical Schottky barrier diodes was investigated under sub-bandgap wavelength light irradiation. Under a low reverse bias voltage, the photocurrent is induced by internal photoemission, while under a high reverse bias voltage, the photocurrent increases significantly with the bias voltage. This is due to sub-bandgap optical absorption in a depletion region due to the Franz–Keldysh effect. The voltage and wavelength dependences of the photocurrent are successfully explained quantitatively.

A detailed analysis of the energy levels configuration existing in the band gap of supersaturated silicon with titanium for photovoltaic applications

The energy levels created in supersaturated n-type silicon substrates with titanium implantation in the attempt to create an intermediate band in their band-gap are studied in detail. Two titanium ion implantation doses (1013 cm-2 and 1014 cm-2) are studied in this work by conductance transient technique and admittance spectroscopy. Conductance transients have been measured at temperatures of around 100 K. The particular shape of these transients is due to the formation of energy barriers in the conduction band, as a consequence of the band-gap narrowing induced by the high titanium concentration. Moreover, stationary admittance spectroscopy results suggest the existence of different energy level configuration, depending on the local titanium concentration. A continuum energy level band is formed when titanium concentration is over the Mott limit. On the other hand, when titanium concentration is lower than the Mott limit, but much higher than the donor impurity density, a quasi-continuum energy level distribution appears. Finally, a single deep center appears for low titanium concentration. At the n-type substrate, the experimental results obtained by means of thermal admittance spectroscopy at high reverse bias reveal the presence of single levels located at around Ec-425 and Ec-275 meV for implantation doses of 1013 cm−2 and 1014 cm−2, respectively. At low reverse bias voltage, quasi-continuously distributed energy levels between the minimum of the conduction bands, Ec and Ec-450 meV, are obtained for both doses. Conductance transients detected at low temperatures reveal that the high impurity concentration induces a band gap narrowing which leads to the formation of a barrier in the conduction band. Besides, the relationship between the activation energy and the capture cross section values of all the energy levels fits very well to the Meyer-Neldel rule. As it is known, the Meyer-Neldel rule typically appears in processes involving multiple excitations, like carrier capture and emission in deep levels, and it is generally observed in disordered systems. The obtained Meyer-Neldel energy value, 15.19 meV, is very close to the value obtained in multicrystalline silicon samples contaminated with iron (13.65 meV), meaning that this energy value could be associated to the phonons energy in this kind of substrates.

Analysis of n-type IBC solar cells with diffused boron emitter locally blocked by implanted phosphorus

Interdigitated back-contact (IBC) solar cells were fabricated with a process sequence combining local ion implantation of phosphorus and full area BBr3 furnace diffusion resulting in conversion efficiencies of up to 22.4%. The highly doped emitter and BSF are in direct contact to each other (p+n+ junction) leading to a controlled junction breakdown at low reverse-bias voltages of around 5V. The breakdown was located at the p+n+ junction and found to be homogeneously distributed over the whole cell area. This is not critical for module integration as the absolute temperature rise of a reverse-biased cell was determined to be less than 35K. After reverse breakdown, the conversion efficiency degraded by 1–2% absolute due to additional recombination at the p+n+ junction. The cell performance could be fully recovered by a short annealing at 300°C indicating that the Al2O3 passivation was altered by the reverse breakdown. This might be a fundamental issue for Al2O3 passivated IBC solar cells without gap between emitter and BSF, independent from the doping method.

InP-Based p-i-n-Photodiode Array Integrated With 90&lt;inline-formula&gt;&lt;tex-math&gt;$^\circ$&lt;/tex-math&gt; &lt;/inline-formula&gt; Hybrid Using Butt-Joint Regrowth for Compact 100 Gb/s Coherent Receiver

This paper reports on the performance of an InP-based p-i-n-photodiode array monolithically integrated with a 90 $^\circ$ hybrid consisting of multimode interference structures using the butt-joint regrowth and its application to compact 100 Gb/s coherent receivers. A responsivity including total loss of 8.2 dB in the waveguide was as high as 0.14 A/W, and responsivity imbalance of all channels was less than ±0.5 dB. The wide 3-dB bandwidth of 26 GHz at a low reverse bias voltage of 1.6 V was obtained under high optical input power conditions (photocurrent = 2.5 mA). The stable dark current was achieved in the accelerated aging test (test condition: −5 V at 175 $^\circ$ C) over 2000 h. Finally, the excellent common mode rejection ratio with low loss imbalance of 90 $^\circ$ hybrids (less than −20 dB up to 25 GHz) and demodulation of 128 Gb/s dual polarization quadrature phase-shift keying modulated signals were demonstrated for the compact coherent receiver (package body size: 15 mm × 26 mm × 5.5 mm).

Electrical characterization of CdTe pixel detectors with Al Schottky anode

Pixelated Schottky Al/p-CdTe/Pt detectors are very attractive devices for high-resolution X-ray spectroscopic imaging, even though they suffer from bias-induced time instability (polarization). In this work, we present the results of the electrical characterization of a (4×4) pixelated Schottky Al/p-CdTe/Pt detector. Current–voltage (I–V) characteristics and current transients were investigated at different temperatures. The results show deep levels that play a dominant role in the charge transport mechanism. The conduction mechanism is dominated by the space charge limited current (SCLC) both under forward bias and at high reverse bias. Schottky barrier height of the Al/CdTe contact was estimated by using the thermionic-field emission model at low reverse bias voltages. Activation energy of the deep levels was measured through the analysis of the reverse current transients at different temperatures. Finally, we employed an analytical method to determine the density and the energy distribution of the traps from SCLC current–voltage characteristics.

Low-voltage-operation avalanche photodiode based on n-gallium oxide/p-crystalline selenium heterojunction

In this study, we demonstrate the avalanche multiplication phenomenon in a crystalline-selenium (c-Se)-based heterojunction photodiode. The carrier injection from an external electrode, which is considered to be the major factor contributing to dark current at a high electric field, was significantly decreased by employing a thin n-type Ga2O3 layer with a high hole-injection barrier. The fabricated Ga2O3/c-Se diode exhibited extremely high external quantum efficiency of over 100% in the short-wavelength region at a relatively low reverse-bias voltage of ∼20 V. Furthermore, Sn-doping of the Ga2O3 layer increases the carrier concentration; hence, the resulting device has a lower threshold voltage for avalanche multiplication.