Ultraviolet Avalanche Photodiodes Research Articles

Dark Count Analysis in 4H-SiC Based UV Avalanche Photodiodes With Effective After-Pulse Suppression

Dark Count Analysis in 4H-SiC Based UV Avalanche Photodiodes With Effective After-Pulse Suppression

Experimental identification of defect-induced destructive breakdown of AlGaN ultraviolet avalanche photodiodes

The issue of destructive breakdown and high dark current in AlGaN ultraviolet avalanche photodiodes has conventionally been attributed to material defects, yet direct evidence supporting this claim has been absent. Examining damaged devices that experienced destructive breakdown revealed an intriguing finding: post-treatment with a potent alkali resulted in the emergence of corrosion pits on the device’s surface. Subsequently, a dry etching process was implemented in the targeted area to eliminate the corrosion pit. Remarkably, the current–voltage measurement subsequent to this treatment showcased a complete restoration of the damaged device’s previous performance. This unequivocally indicates that the destructive breakdown was localized, and the point of punch-through could be precisely identified through the corrosion pit. Consequently, it is now firmly believed that material defects constitute the primary cause of destructive breakdown in these instances.

Unipolar carrier multiplication high-gain and low-noise AlGaN ultraviolet avalanche photodiode with periodically stacked structure

Highly sensitive avalanche photodetector (APD) has become a promising candidate for detecting extremely weak target signals. However, the impact ionization multiplication simultaneously triggered by electrons and holes will lead to large excess noise, thus significantly influencing device avalanche performance. Herein, we propose a distinctive AlGaN-based ultraviolet avalanche photodiode with AlN/Al0.2Ga0.8N periodically stacked multiplication region. The higher effective masses and density of states in valence band renders holes limited in the quantum-well region, where thermalization plays a dominant role during carrier transport process. On the contrary, in the atomic-scale AlN/AlGaN stacked structure with a periodic thickness of 10 nm, the electron mutualization motion is conductive to electron obtaining sufficient energy to induce impact ionization. Hence, the mechanism of unipolar carrier induced avalanche multiplication effectively reduces device noise and improving multiplication gain. Meanwhile, the high electric field intensity and tilted energy band in the AlGaN/AlN periodically stacked region significantly contribute to the carrier impact ionization. Consequently, the device achieves a superior avalanche gain of more than 105 at 74 V reverse bias. It is envisioned that the unipolar carrier triggering avalanche events offers a viable route to build high-performance APDs.

Observation of photoelectric-induced microplasma avalanche breakdown in AlGaN ultraviolet photodiode with separate absorption and multiplication structure

Amplification of weak ultraviolet signals has always been a challenging issue to design and fabricate high-performance ultraviolet photodetectors. Here, we observe a distinctive microplasma breakdown behavior in AlGaN-based ultraviolet avalanche photodiodes with artificial mesa architecture. At 107 V breakdown voltage, the photocurrent increases sharply whereas dark current intriguingly remains at the extremely low level of 0.1 nA as the applied voltage increases. Simultaneously, a significant blue luminescence phenomenon is observed at the mesa edge of photodiode at breakdown voltage, indicating the occurrence of microplasma breakdown. Ultimately, the microplasma avalanche photodiode achieves a record-high avalanche gain of 3 × 106 with light–dark current ratio readily exceeding 107. Kelvin probe force microscopy was employed to reveal the physical mechanism of localized avalanche breakdown induced by photoelectric effects and elaborate the microplasma discharge process, which is related to surface states. The unprecedented detection mode of photocurrent triggering avalanche events while remaining low dark current is anticipated to effectively shield the background noise and amplify ultraviolet signals. It is worth further research to explore its possibility on high-sensitivity ultraviolet photodetection.

Analysis of high-temperature performance of 4H-SiC avalanche photodiodes in both linear and Geiger modes

The high-temperature performance of 4H-SiC ultraviolet avalanche photodiodes (APDs) in both linear and Geiger modes is extensively investigated. During the temperature-dependent measurements, a fixed bias voltage is adopted for the device samples, which is much more practical and important for high-temperature applications. The results show that the fabricated 4H-SiC APDs are very stable and reliable at high temperatures. As the temperature increases from room temperature to 425 K, the dark current at 95% of the breakdown voltage increases slightly and remains lower than 40 pA. In Geiger mode, our 4H-SiC APDs can be self-quenched in a passive-quenching circuit, which is expected for high-speed detection systems. Moreover, an interesting phenomenon is observed for the first time: the single-photon detection efficiency shows a non-monotonic variation as a function of temperature. The physical mechanism of the variation in high-temperature performance is further analyzed. The results in this work can provide a fundamental reference for researchers in the field of 4H-SiC APD ultraviolet detectors.

Demonstration of a Solar-Blind Single-Photon Imaging Lidar Based on SiC Ultraviolet Avalanche Photodiodes

In this work, a prototype solar-blind single-photon imaging lidar based on SiC ultraviolet (UV) avalanche photodiodes (APD) is demonstrated. The SiC APD with a peak responsivity at ~280 nm can greatly lessen the strict solar-blind filtering requirement for the system. When characterized by using a gated quenching circuit, the SiC APD operating in Geiger mode could exhibit a room temperature single photon detection efficiency of ~27% with a dark count probability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.59\times 10 ^{-2}$ </tex-math></inline-formula> . Based on the SiC APD, a 266 nm nanosecond pulsed laser, a galvanometer scanner and a time-correlated single-photon counter, 3-dimensional depth image with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$128\times 128$ </tex-math></inline-formula> resolution is realized by the prototype system in daylight. The average ranging accuracy of the system is found within 28 mm based on current setup.

Self-Assembly Nanopillar/Superlattice Hierarchical Structure: Boosting AlGaN Crystalline Quality and Achieving High-Performance Ultraviolet Avalanche Photodetector.

As a burgeoning wide-band gap semiconductor material, AlxGa1-xN alloy has attracted great attention for versatile applications due to its superior properties. However, its poor crystalline quality has restricted the employment of AlGaN on electronic devices for a long time. Herein, we proposed a nanopillar/superlattice hierarchical structure for AlGaN epitaxy to boost the crystalline quality. The scale-controllable AlN nanopillar template is fabricated from a nickel self-assembly process. AlGaN initiates the epitaxial laterally overgrowth mode based on the nanopatterned template. In addition, the AlxGa1-xN/AlyGa1-yN superlattice structure could effectively block the propagation of threading dislocation segments. The kinetics of the dislocation and epitaxy process in the hierarchical structure is intuitively demonstrated and analyzed. Consequently, the dislocation density of AlGaN grown by this method is significantly reduced by more than 30 times compared to the AlN template. No threading dislocation segments were observed in the 4 μm TEM field of view. Moreover, based on the hierarchical structure, we also fabricated an AlGaN ultraviolet avalanche photodiode (APD). The APD exhibits superior performance, achieving a maximum gain of 1.3 × 105 and high responsivity of 1.46 A/W at 324 nm. The reliability of the nanopillar/superlattice AlGaN epitaxial procedure is anticipated to shed new light on the nitride semiconductor material, further bringing a breakthrough to wide-band gap electronic devices.

Uniform and High Gain GaN p-i-n Ultraviolet APDs Enabled by Beveled-Mesa Edge Termination

In this letter, a beveled-mesa edge termination technology was developed to improve the performance of GaN pi-n ultraviolet avalanche photodiodes (APDs). Simulation results showed that the beveled-mesa is effective in reduction of the electric field at the mesa sidewall. With a photoresist thermal-reflow process, 12°-angle beveled-mesa APDs were fabricated from homoepitaxial p-i-n structures on GaN substrate. The most beneficial property of the beveled-mesa termination is the uniform breakdown-characteristic of the beveled-mesa APDs compared to the vertical-mesa APDs. Record-high VBR uniformity of 95.4 V ± 0.2 V and average avalanche gain up to 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> were observed for the beveled-mesa APDs within a linear array across 4 mm side-length. These results indicate the potential of the beveledmesa GaN APD array for ultraviolet light imaging applications.

128-pixel arrays of 4H-SiC UV APD with dual-frequency PECVD SiNx passivation.

In this paper, high-performance 1×128 linear arrays of 4H-SiC ultraviolet (UV) avalanche photodiode (APD) with dual-frequency plasma enhanced chemical vapor deposition (PECVD) passivation are demonstrated for the first time. The results show that SiNx dielectric deposited by dual-frequency PECVD can effectively reduce the leakage current at high bias voltages. Due to the improved 4H-SiC epi-layer material and SiNx passivation, the fabricated 22 mm-long 1×128 4H-SiC APD linear arrays exhibit an excellent performance with a high pixel yield of 100% and a small breakdown voltage variation of 0.2 V, which is the best result ever reported. At room temperature, the pixels have a gain of over 105 and a maximum quantum efficiency of 53.5% @ 285 nm. Besides the high uniformity of breakdown voltage for 128 pixels, the dark currents at 95% of breakdown voltage are all below 1 nA.

After-Pulse Characterizations of Geiger-Mode 4H-SiC Avalanche Photodiodes

In this work, the after-pulse properties of 4H-SiC ultraviolet (UV) avalanche photodiodes (APDs) working in gated quenching mode are firstly characterized by using a double gate method. The after-pulse probability is determined as a function of delay time at various overbias voltages and temperatures. It is found that the after-pulse probability of 4H-SiC APDs would decrease at higher chip temperature, supporting the carrier trapping/releasing mechanism. Compared to state-of-the-art Geiger-mode Si APDs, the 4H-SiC APD exhibits considerably larger after-pulsing effect, which should be mainly caused by high density point defects within the SiC epilayer. The maximum signal repetition rate of the SiC APD in gated quenching operation mode is estimated, which is a key performance parameter for future UV laser radar and quantum communication applications.

Effect of a Single Threading Dislocation on Electrical and Single Photon Detection Characteristics of 4H-SiC Ultraviolet Avalanche Photodiodes**Supported by the National Key R&D Program of China under Grant No. 2016YFB0400902, the National Natural Science Foundation of China under Grant No. 61921005, and the Natural Science Foundation of Jiangsu Province under Grant No. BK20190302

We fabricated 4H-SiC ultraviolet avalanche photodiode (APD) arrays and systematically investigated the effect of threading dislocations on electrical and single photon detection characteristics of 4H-SiC APDs. Based on a statistical correlation study of individual device performance and structural defect mapping revealed by molten KOH etching, it is determined with high confidence level that even a single threading dislocation within APD active region would lead to apparent device performance degradation, including increase of dark current near breakdown voltage, premature breakdown and reduction of single photon detection efficiency at fixed dark count rate.

High-Gain Ultraviolet Avalanche Photodiodes Using a ZnSe-Based Organic–Inorganic Hybrid Structure

We have developed high-gain and highly sensitive ZnSe-based organic–inorganic hybrid ultraviolet avalanche photodiodes (UV-APDs). The inorganic ZnSe-based wafers (i-ZnSSe active layer/n-ZnSSe) were grown by molecular beam epitaxy (MBE) on n-type GaAs substrates. The inorganic UV-transparent conducting polymer window layers of poly 3,4-ethylenedioxythiophene:poly-styrenesulfonate (PEDOT:PSS) were formed by spin-coating and a photolithography technique instead of the inkjet printing technique, which we previously reported. We have obtained a thin uniform window layer with a mesa-shaped edge by an optimized photolithography process. The leakage current before the breakdown voltage was suppressed to < 10−10 A/mm2, which is lower than that of the APD device fabricated by inkjet printing. The maximum external quantum efficiency was improved to ηmax = 70% (λ = 340 nm) using the photolithography technique compared with the inkjet printing (ηmax = 50%). The maximum responsivity was improved from 3 A/W to 10 A/W. The maximum multiplication factor was improved from M = 90 to M = 3100.

$8\times8$ 4H-SiC Ultraviolet Avalanche Photodiode Arrays With High Uniformity

In this letter, high-uniformity $8\times 8$ arrays of 4H-SiC ultraviolet (UV) avalanche photodiode (APD) are reported. In order to improve the detectivity, large active area ( $300~\mu \text{m}$ diameter) was designed for APD pixels. Thick SiN x dielectric deposited by plasma enhanced chemical vapor deposition (PECVD) was adopted as insulator and passivation layer to suppress the reverse leakage current and premature breakdown. Despite the large active area, a high yield of 97% is achieved for the pixels in the 4H-SiC APD array. At room temperature, the pixels exhibit a high gain of over 105, a maximum quantum efficiency of larger than 68% @ 282 nm and an excellent UV/visible rejection ratio of 104. In addition, temperature-stable avalanche breakdown voltage with a positive coefficient of 8 mV/°C is obtained. Furthermore, the APD array shows a high uniformity of breakdown voltage with a standard deviation of 0.1 V for all the 64 pixels, and the dark currents at 95% of breakdown voltage are as low as ~1 nA except for two pixels. The performance improvements indicate a new milestone for 4H-SiC APD arrays in UV imaging applications.

Surface smoothing with BCl3 plasma post-treatment to improve the performance of GaN avalanche photodiodes

This paper reports a plasma post-treatment technique to improve the performance of GaN ultraviolet avalanche photodiodes (APDs). A BCl3-based plasma post-etching technique was developed to smooth the roughened GaN surface after inductively-coupled-plasma etching. Atomic-scale surface roughness around 0.278 nm rms and photoluminescence intensity more than doubled were achieved after the post-treatment. The surface smoothing technique was applied to the fabrication process of double-mesa structure GaN APDs grown on a sapphire substrate. Compared to the non-treated APDs, the post-treated GaN APDs show high field leakage current suppressed more than two orders and the average gain increased from 2 × 104 to 1 × 105, indicating the low surface damage after BCl3 plasma post-treatment. For the GaN APDs fabricated into a 3 × 3 array, the devices show uniform distribution of the breakdown voltages after the plasma post-treatment.

Performance improvement of 4H-SiC PIN ultraviolet avalanche photodiodes with different intrinsic layer thicknesses**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400902) and the Priority Academic Program Development of Jiangsu Higher Education Institutions, China.

Four 4H-SiC p–i–n ultraviolet (UV) avalanche photodiode (APD) samples PIN-0.1, PIN-0.35, PIN-0.5, and PIN-1.0 with different intrinsic layer thicknesses (, , , and , respectively) are designed and fabricated. Single photon detection efficiency (SPDE) performance becomes better as the intrinsic layer thickness increases, which is attributed to the inhibitation of tunneling. Dark count origin is also investigated, an activation energy as small as 0.22 eV of the dark count rate (DCR) confirms that the trap-assisted tunneling (TAT) process is the main source of DCR. The temperature coefficient ranges from −2.6 mV/°C to 18.3 mV/°C, demonstrating that the TAT process is dominant in APDs with thinner intrinsic layers. Additionally, the room temperature maximum quantum efficiency at 280 nm differs from 48% to 65% for PIN-0.35, PIN-0.5, and PIN-1.0 under 0 V bias, and UV/visible rejection ratios higher than 104 are obtained.

Performance Modulation for Back-Illuminated AlGaN Ultraviolet Avalanche Photodiodes Based on Multiplication Scaling

Back-illuminated Al0.1Ga0.9N ultraviolet avalanche photodiodes (APDs) of various multiplication widths were fabricated on AlN templates with a separate absorption and multiplication structure. The impacts of an increased multiplication scale on a device performance were investigated. The avalanche breakdown voltage was found to increase as the multiplication layer thickness (MLT) increases. The APD with 230-nm-MLT achieved a superior maximum multiplication gain of 5.4 × 10 4 , higher than that obtained in devices with 150-nm- and 300-nm-MLT. Theoretical simulations demonstrated that the critical electric field intensity in an avalanche region would decrease as the rising of MLT, indicating the modulating ability of multiplication scaling on the AlGaN APD performance. In addition, APDs fabricated on different AlN templates were employed to study the effects of crystalline quality on device properties.

Enhanced front-illuminated p-i-p-i-n GaN/AlGaN ultraviolet avalanche photodiodes

We proposed an improved front-illuminated GaN/AlGaN separate absorption and multiplication (SAM) ultraviolet avalanche photodiode (APD) based on physical analysis and device simulations. By inserting an Al0.2Ga0.8N p-type layer into the p-i-p-i-n APD structure, a built-in electric field resulted from polarization effect will be introduced to facilitate the transport of electrons from absorption layer to multiplication region. The enhanced heterostructure APD presents significantly higher multiplication gain compared with the homogeneous one. Moreover, the doping concentration and thickness of the interlayer were optimized according to device performances, because the interlay has important effects on the electric field distribution and energy-band profile of the front-illuminated SAM APD.

High-performance 4H-SiC p-i-n ultraviolet avalanche photodiodes with large active area

Ultraviolet (UV) detectors with large photosensitive areas are more advantageous in low-level UV detection applications. In this Letter, high-performance 4H-SiC p-i-n avalanche photodiodes (APDs) with large active area (800 μm diameter) are reported. With the optimized epitaxial structure and device fabrication process, a high multiplication gain of 1.4 × 106 is obtained for the devices at room temperature, and the dark current is as low as ∼10 pA at low reverse voltages. In addition, record external quantum efficiency of 85.5% at 274 nm is achieved, which is the highest value for the reported SiC APDs. Furthermore, the rejection ratio of UV to visible light reaches about 104. The excellent performance of our devices indicates a tremendous improvement for large-area SiC APD-based UV detectors. Finally, the UV imaging performance of our fabricated 4H-SiC p-i-n APDs is also demonstrated for system-level applications.

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.

AlGaN ultraviolet Avalanche photodiodes based on a triple-mesa structure

A high-performance Al0.1Ga0.9N ultraviolet (UV) avalanche photodiode (APD) with a separate absorption and multiplication structure grown on AlN templates is fabricated by employing a triple-mesa structure. The fabricated AlGaN UV-APD exhibits a maximum gain up to 2.3 × 104 at the reverse bias of 67 V and a low avalanche breakdown voltage (&amp;lt;70 V). The triple-mesa structure is confirmed to significantly lower the avalanche breakdown voltage and reduce the sidewall leakage current in comparison with the conventional double-mesa one. These improvements are explained by the simulation of the electric field which shows a significant improvement in the distribution uniformity in the active regions and enhancement in the intensity in the multiplication region. In addition, the scaling effects of various anodes and mesas are investigated, and the dark current is found to decrease with a decrease in the mesa size thanks to the reduced amount in the high-conductivity threading dislocation that crosses the multiplication region.