Separate Absorption And Multiplication Research Articles

AlGaN‐based solar‐blind avalanche photodetectors with gradually doped charge layer

AbstractTo improve the performances of AlGaN solar‐blind ultraviolet (UV) avalanche photodetectors (APDs), we propose a separate absorption and multiplication (SAM) AlGaN APD with a gradually doped charge layer. The calculation results indicate that the designed APD can significantly increase the maximum gain by almost 40%, and reduce the breakdown voltage by about 9% compared with the conventional AlGaN APD. The parameters of multiplication layer and p‐type layer are optimized, and the physical mechanism is described in detail, which provides theoretical guidance for high‐performance device structure design and experimental preparation.

High-Performance Waveguide-Integrated Ge/Si Avalanche Photodetector with Lateral Multiplication Region

High-performance waveguide-integrated Ge/Si APDs in separate absorption, charge, and multiplication (SACM) schemes have been exploited to facilitate energy-efficient optical communication and interconnects. However, the charge layer design is complex and time-consuming. A waveguide-integrated Ge/Si avalanche photodetector (APD) is proposed in a separate absorption and multiplication (SAM) configuration. The device can work at low voltage and high speed with a lateral multiplication region without complexity of the charge layer. The proposed device is implemented by the complementary metal-oxide-semiconductor (CMOS) process in the 8-inch Si photonics platform. The device has a low breakdown voltage of 12 V and shows high responsivity of 15.1 A/W at 1550 nm wavelength under optical power of −22.49 dBm, corresponding to a multiplication gain of 18.1. Moreover, an opto-electrical bandwidth of 20.7 GHz is measured at 10.6 V. The high-speed performance at low voltage shows a great potential to implement high-energy-efficient Si optical communications and interconnections.

AlGaN solar-blind ultraviolet avalanche photodiodes with a p-graded AlxGa1-xN layer and high/low Al-content AlGaN multiplication layer

We design a back-illuminated p–i–n–i–n separate absorption and multiplication (SAM) AlGaN solar-blind avalanche photodiode (APD) with a low Al-content p-graded AlxGa1-xN layer and a high/low Al-content AlGaN multiplication layer. Simultaneously, an III-nitride AlN/Al0.64Ga0.36 N distributed Bragg reflector (DBR) structure is inserted to improve the solar-blind photoresponse for the designed APD. The simulation results show that the designed APD exhibits enhanced optoelectronic characteristics compared to the conventional APD, which is attributed to the higher holes impact ionization coefficient and generated polarization electric field in the same direction as the applied bias field of the designed APD. The designed APD exhibits significant enhanced avalanche gain and reduced avalanche breakdown voltage compared with the conventional APD.

Direct observation of reach-through behavior in back-illuminated algan avalanche photodiode with separate absorption and multiplication structure

A back-illuminated heterostructure AlGaN avalanche photodiode (APD) was fabricated with a separate absorption and multiplication (SAM) structure. The SAM structure is able to modulate the electric field distribution of the photodetector and induce a single carrier to trigger an avalanche event. The fabricated p-i-n-i-n APD exhibited a superior maximum gain of 1.8 × 105 at 70 V and distinct solar-blind responsivity peak at 280 nm. However, before the avalanche occurs, the SAM APD needs to be reached through for facilitating carrier transport. By investigating the capacitance characteristics of the SAM APD, the reach-through behavior was directly observed. The capacitance of the detector demonstrated evident declining trends with the applied voltage in the range of 18 to 24 V at different frequencies. The simulated electric field and energy band also revealed that the absorption region was initially reached through at 18 V reverse bias, corresponding to the depletion of n-type interlayer into the absorption region. Meanwhile, there were large differences in capacitances between low and high frequencies, which indicated the effects of impurity and defect states. These findings explicitly reveal the reach-through mechanism of the SAM APD, which is beneficial to the better understanding of device physics and further APD design.

GaAs/Al0.8Ga0.2As separate absorption and multiplication region x-ray spectroscopic avalanche photodiodes

A GaAs/Al0.8Ga0.2As separate absorption and multiplication (SAM) x-ray avalanche photodiode (APD) structure was grown by metalorganic vapor phase epitaxy. Mesa photodiodes of different diameters (200 μm and 400 μm) were fabricated from the structure. Two of the photodiodes (one of each diameter) were characterized at 20 °C for their electrical properties and response to x rays using an 55Fe radioisotope x-ray (Mn Kα = 5.9 keV; Mn Kβ = 6.49 keV) source. An energy resolution of 508 eV ± 5 eV full width at half maximum (FWHM) at 5.9 keV was achieved at an apparent avalanche gain, M, of 1.1. This is the best energy resolution so far reported for GaAs/AlxGa1−xAs x-ray SAM APDs. The noise components associated with the achievable spectroscopic energy resolutions are reported. Comparisons between the 200 μm and 400 μm diameter GaAs/AlxGa1−xAs SAM x-ray APDs and recently studied GaAs p+-i-n+ detectors were made, showing that the inclusion of the avalanche layer improves the achievable energy resolution; energy resolutions of 508 eV FWHM at 5.9 keV at M = 1.1 and 603 eV FWHM at 5.9 keV at M = 1.2 were achieved with the 200 μm and 400 μm diameter GaAs/AlxGa1−xAs SAM x-ray APDs, respectively; this is better than was previously reported for similar devices without avalanche layers: 690 eV FWHM at 5.9 keV and 730 eV FWHM at 5.9 keV for 200 μm and 400 μm diameter GaAs p+-i-n+ detectors, respectively [Lioliou et al., J. Appl. Phys. 122, 244506 (2017)].

Theoretical analysis for AlGaN avalanche photodiodes with mesa and field plate structure**Project supported by the Natural Science Research Project of Anhui University, China (Grant No. KJ2019A0644), the National Natural Science Foundation of China (Grant Nos. 61634002 and 61804089), the Natural Science Alliance Foundation, China (Grant No. U1830109), the Natural Science Foundation of Anhui Province, China (Grant No. 1708085MF149), the Chuzhou

To suppress the electric field crowding at sidewall and improve the detection sensitivity of the AlGaN separate absorption and multiplication (SAM) avalanche photodiodes (APDs), we propose the new AlGaN APDs structure combining a large-area mesa with a field plate (FP). The simulated results show that the proposed AlGaN APDs exhibit a significant increase in avalanche gain, about two orders of magnitude, compared to their counterparts without FP structure, which is attributed to the suppression of electric field crowding at sidewall of multiplication layer and the reduction of the maximum electric field at the p-type GaN sidewall in p–n depletion region. Meanwhile, the APDs can produce an obviously enhanced photocurrent due to the increase in cross sectional area of multiplication region.

Back-illuminated AlGaN heterostructure solar-blind avalanche photodiodes with one-dimensional photonic crystal filter.

AlGaN heterostructure solar-blind avalanche photodiodes (APDs) were fabricated on a double-polished AlN/sapphire template based on a separate absorption and multiplication (SAM) back-illuminated configuration. By employing AlGaN heterostructures with different Al compositions across the entire device, the SAM APD achieved an avalanche gain of over 1×105 at an operated reverse bias of 92 V and a low dark current of 0.5 nA at the onset point of breakdown. These excellent performances were attributed to the acceleration of holes by the polarization electric field with the same direction as the reverse bias and higher impact ionization coefficient of the low-Al-content Al0.2Ga0.8N in the multiplication region. However, the Al0.2Ga0.8N layer produced a photocurrent response in the out of the solar-blind band. To retain the solar-blind detecting characteristic, a periodic Si3N4/SiO2 photonic crystal was deposited on the back of the AlN/sapphire template as an optical filter. This significantly improved the solar-blind characteristic of the device.

Buffer-Free Ge/Si by Rapid Melting Growth Technique for Separate Absorption and Multiplication Avalanche Photodetectors

Herein, we report the formation of buffer-free germanium (Ge) on silicon by a two-step rapid melting growth technique for separate absorption and multiplication (SAM) avalanche photodetectors (APDs) as well as its characteristic measurements. The quality of the Ge film was verified by standard electron microscopy and Raman spectroscopy. The high-quality Ge functioned as the absorption layer, whereas the multiplication layer was made with the Si layer. The electrical measurement identified that the photodetector shows high responsivity and gain to the near-infrared spectrum before the breakdown. The nanocrystallites and the intermixing SiGe layer at the interface will be the recombination centers for the photogenerated electron–hole pairs, resulting in a low photocurrent in a low-bias range. This letter demonstrated that buffer-free Ge films and SAM APDs with good photoelectric conversion efficiency would be obtained using a CMOS-compatible process.

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.

Polarization and p-type doping effects on photoresponse of separate absorption and multiplication AlGaN solar-blind avalanche photodiodes

The photoresponse characteristics of separate absorption and multiplication (SAM) AlGaN solar-blind avalanche photodiodes (APDs) were investigated in detail. The p-i-n-i-n avalanche photodiodes were examined using the newly designed model of avalanche photodiodes in AlGaN. The research results showed that the dark current density was about 3.51 × 10−8 A/cm2, the light current density was 5.86 × 10−5 A/cm2 under near-zero bias, and the avalanche breakdown occurred at about 135.0 V under reverse bias, which were all consistent with the experimental data. To investigate the effects influencing the photoresponse characteristics of the APDs, their photo responsivity spectra under different biases were simulated. The APD featured a window region over the wavelength range from 260 to 280 nm with a high rejection ratio on the short-wavelength side. Meanwhile, the dependence of APD responsivity on the polarization charge revealed that the negative polarization charges strongly affected the responsivity. Increased negative polarization charges at the Al0.4Ga0.6N/Al0.6Ga0.4N interface markedly lowered the responsivity, whereas charges of the same polarity at the GaN/Al0.4Ga0.6N interface enhanced the responsivity. Furthermore, the dependence of responsivity on p-type doping was analyzed by comparison with the effects of negative polarization charges on the conduction band of the APDs. Finally, the inversion layer models are used to interpret the effects of these on the APD responsivity. This research is useful for exploring polarization and p-type doping effects in SAM AlGaN structures and realization of high responsivity solar-blind APDs.

P-i-p-i-n Separate Absorption and Multiplication Ultraviolet Avalanche Photodiodes

Front-illuminated GaN-based separate absorption and multiplication (SAM) ultraviolet (UV) avalanche photodiodes (APDs) with various photon detection areas are demonstrated grown by metalorganic chemical vapor deposition on bulk GaN native substrates with low dislocation density. By adopting a front-illuminated UV-APD structure with a thin AlGaN window layer, no additional etching of the substrate for the reduction of strong UV absorption is required. The epitaxial layer structure of the p-i-p-i-n SAM UV-APDs consists of a Mg-doped p-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.05</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95</sub> N window layer to minimize UV absorption at the top surface region and a Mg-graded p-GaN charge layer to serve as a field-termination layer. The onset point of the breakdown voltage (VBR) is around 73 V for all SAM-APDs with different mesa areas ranging from 1963 to 10000 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is a lower VBR than the typical p-i-n UV-APDs with the similar thickness of undoped layer, where the photon absorption and multiplication processes take place simultaneously. Under UV-light illumination at λ = 340 nm, the SAM-APDs exhibit high avalanche gains greater than 8.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> at a reverse bias of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> > 72.5 V.

Trap-assisted tunneling in AlGaN avalanche photodiodes

We fabricated AlGaN solar-blind avalanche photodiodes (APDs) that were based on separate absorption and multiplication (SAM) structures. It was determined experimentally that the dark current in these APDs is rapidly enhanced when the applied voltage exceeds 52 V. Theoretical analyses demonstrated that the breakdown voltage at 52 V is mainly related to the local trap-assisted tunneling effect. Because the dark current is mainly dependent on the trap states as a result of modification of the lifetimes of the electrons in the trap states, the tunneling processes can be modulated effectively by tuning the trap energy level, the trap density, and the tunnel mass.

High-Gain N-Face AlGaN Solar-Blind Avalanche Photodiodes Using a Heterostructure as Separate Absorption and Multiplication Regions**Supported by the State Key Project of Research and Development Plan of China under Grant No 2016YFB0400903, the National Natural Science Foundation of China under Grant Nos 61634002, 61274075 and 61474060, the Key Project of Jiangsu Province under Grant No BE2016174, the Anhui University Natural Science

It is well known that -nitride semiconductors can generate the magnitude of MV/cm polarization electric field which is comparable with their ionization electric fields. To take full advantage of the polarization electric field, we design an N-face AlGaN solar-blind avalanche photodiode (APD) with an Al0.45Ga0.55N/Al0.3Ga0.7N heterostructure as separate absorption and multiplication (SAM) regions. The simulation results show that the N-face APDs are more beneficial to improving the avalanche gain and reducing the avalanche breakdown voltage compared with the Ga-face APDs due to the effect of the polarization electric field. Furthermore, the Al0.45Ga0.55N/Al0.3Ga0.7N heterostructure SAM regions used in APDs instead of homogeneous Al0.45Ga0.55N SAM structure can increase significantly avalanche gain because of the increased hole ionization coefficient by using the relatively low Al-content AlGaN in the multiplication region. Meanwhile, a quarter-wave AlGaN/AlN distributed Bragg reflector structure at the bottom of the device is designed to remain a solar-blind characteristic of the heterostructure SAM-APDs.

Back-Illuminated GaN/AlGaN Solar-Blind Avalanche Photodiodes

The separate absorption and multiplication (SAM) GaN/AlGaN solar-blind avalanche photodiodes (APDs) integrated with the specific design of 1-D dual-periodic photonic crystal on the back of the sapphire substrate are investigated numerically. The calculated results show that the proposed APDs can increase significantly the avalanche gain compared with the conventional SAM AlGaN solar-blind APDs using GaN instead of AlGaN as the multiplication layer. The enhanced performance can be explained by the larger hole ionization coefficient in the multiplication layer, the higher hole injection efficiency, and the reduced total dark current in this APDs. Meanwhile, the cutoff wavelength of 282 nm is achieved due to the optical filter effect of 1-D dual-periodic photonic crystal.

Role of n-type AlGaN layer in photoresponse mechanism for separate absorption and multiplication (SAM) GaN/AlGaN avalanche photodiode

The detailed procedure of modeling spectral responsivity characteristics for separate absorption and multiplication (SAM) $$\hbox {GaN/Al}_{0.15}\hbox {Ga}_{0.85}\hbox {N}$$ avalanche photodiode (APD) is presented. In order to verify the validity of numerical models, the photoresponse spectra under different biases are first shown. It is found that a window region with wavelength range from 330 to 360 nm is formed regardless of reverse bias, and the responsivities with wavelength range from 240 to 380 nm notably increase with bias. The role of n-type AlGaN layer in the photoresponse mechanism is then primarily studied. Our discussion focuses on the dependence of responsivity on the thickness and doping concentration of n-type layer, respectively. It is demonstrated that the rejection ratio of the detector increases exponentially with the thickness of n-type layer as long as its thickness exceeds 50 nm. Finally, the optimal doping concentration of n-type layer is obtained for optimizing SAM GaN/AlGaN APD.

Significant Performance Improvement in AlGaN Solar-Blind Avalanche Photodiodes by Exploiting the Built-In Polarization Electric Field

We present improved AlGaN solar-blind avalanche photodiodes (APDs) with a separate absorption and multiplication (SAM) structure by introducing a polarization electric field with the same direction as reverse bias field in the multiplication region. This polarization electric field can be realized by reducing the Al composition of the p-AlGaN layer in a conventional p-i-n-i-n SAM-APD structure. After employing a reduced-Al-composition p-AlGaN instead of the commonly used p-AlGaN, the polarization enhanced APD exhibits a markedly lower avalanche breakdown voltage and a near two times higher avalanche gain up to 2.1 × 10 4 compared with its conventional counterpart. In addition, X-ray diffraction and transmission electron microscopy results show that a moderate reduction of Al composition in the p-AlGaN layer does not degrade the crystalline quality of the polarization enhanced APD structure resulted from lattice mismatch, which guarantees the polarization enhanced effect.

GaAs/Al0.8Ga0.2As avalanche photodiodes for soft X-ray spectroscopy

The soft X-ray spectroscopic performance of a GaAs/Al0.8Ga0.2As Separate Absorption and Multiplication (SAM) APD was assessed at room temperature using a 55Fe source. An energy resolution of 1.08 keV (FWHM) was achieved for the 5.9 keV X-rays, at an avalanche gain of 3.5. The avalanche gain also improved the minimum detectable energy from 4.8 keV at unity gain to about 1.5 keV at a gain of 5. Through avalanche statistics analyses, we confirmed that (i) the APD's FWHM was degraded by X-ray photon absorption within the avalanche region, and (ii) photon absorption in/near the n-cladding layer contributed to an undesirable secondary peak in the spectrum.

Study of gain and photoresponse characteristics for back-illuminated separate absorption and multiplication GaN avalanche photodiodes

The gain and photoresponse characteristics have been numerically studied for back-illuminated separate absorption and multiplication (SAM) GaN avalanche photodiodes (APDs). The parameters of fundamental models are calibrated by simultaneously comparing the simulated dark and light current characteristics with the experimental results. Effects of environmental temperatures and device dimensions on gain characteristics have been investigated, and a method to achieve the optimum thickness of charge layer is obtained. The dependence of gain characteristics and breakdown voltage on the doping concentration of the charge layer is also studied in detail to get the optimal charge layer. The bias-dependent spectral responsivity and quantum efficiency are then presented to study the photoresponse mechanisms inside SAM GaN APDs. It is found the responsivity peak red-shifts at first due to the Franz-Keldysh effect and then blue-shifts due to the reach-through effect of the absorption layer. Finally, a new SAM GaN/AlGaN heterojunction APD structure is proposed for optimizing SAM GaN APDs.

Distribution of electric field and design of devices in GaN avalanche photodiodes

We have investigated the distribution of the electric field in p-i-n type and separate absorption and multiplication (SAM) type GaN avalanche photodiodes under different reverse bias values. We have also analyzed the influences of the parameters of each layer, including width and concentration, on the distribution of the electric field, especially on the breakdown voltage. It is found that a relatively high concentration of p-GaN (higher than 1×1018 cm−3) and low carrier concentration of i-GaN (lower than 5×1016 cm−3) are helpful to restrict the electric field and reduce the breakdown voltage. In a SAM (p-i-n-i-n) structure, a suitable choice should be made for the concentration and thickness of the intermediate n-GaN layer in order to decrease breakdown voltage and prevent the device from degenerating into a p-i-n structure. Finally, the optimized material parameters of each layer are proposed.

Solar-Blind 4H-SiC Avalanche Photodiodes

In this work, solar-blind UV 4H-SiC avalanche photodetectors were fabricated and tested in linear and Geiger modes. APDs with both PIN and separate absorption and multiplication (SAM) structures were investigated. PIN structures demonstrated higher quantum efficiencies while the SAM structure exhibit lower leakage currents. Deposition of a thin film optical filter on top of the devices was used to provide a high photon rejection ratio of (Stas add value here). However, an external filter showed a better photon rejection ratio compared to the deposited one by about one order of magnitude.