High-speed InAlAs digital alloy avalanche photodiode

Avalanche photodiodes are widely used in various fields, such as optical communication and laser radar, because of their high multiplication. In order to adapt to very weak signal detection applications, devices are required to have higher gain values. The existing avalanche photodiodes generally use single carrier multiplication mode of operation, its multiplication effect is limited. In this paper is designed an InP/In<sub>0.53</sub>Ga<sub>0.47</sub>As/In<sub>0.52</sub>Al<sub>0.48</sub>As avalanche photodiode structure with electrons and holes jointly involved in multiplication. In this structure, In<sub>0.53</sub>Ga<sub>0.47</sub>As material is used for the absorption layer, InP material is used for the hole multiplication layer, In<sub>0.52</sub>Al<sub>0.48</sub>As is used for the electron multiplication layer, and the two multiplication layers are distributed on the upper side and lower side of the absorber layer. Under the reverse bias, the photogenerated electrons and the absorber-layer generated holes can enter into the respective multiplier layers in different directions and create the avalanche multiplication effect, so that the carriers are fully utilized. This structure and the conventional single multiplication layer structure are simulated by Silvaco TCAD software. Comparing the single InP multiplication layer structure with the single In<sub>0.52</sub>Al<sub>0.48</sub>As multiplication layer structure, the gain value of the double multiplication layer structure at 95% breakdown voltage is about 2.3 times and about 2 times of the former two, respectively, and the device has a larger gain value because both carriers are involved in multiplication in both multiplication layers at the same time. The structure has a dark current of 1.5 nA at 95% breakdown voltage, which does not increase in comparison with the single multiplication layer structure, owing to the effective control of the electric field inside the structure by multiple charge layers. Therefore, this structure is expected to improve the detection sensitivity of the system.

We report on a new platform for separate absorption and multiplication avalanche photodiodes (SAM APDs), developed using 6.1A materials lattice matched to GaSb. Materials and quantum structures grown on GaSb substrates have become of great technological significance for infrared detectors, with absorber options covering the important SWIR, MWIR and LWIR wavebands. The new APD platform is able to exploit these well-established absorber materials, by linking them to a new avalanche multiplication medium, Al0.9Ga0.1As0.08Sb0.92 in a SAM architecture. This opens up the potential for linear and Geiger mode APDs operating across the infrared wavebands for applications such as environmental monitoring, gas sensing and active imaging. In particular, extension of the operating wavelength for active systems with single photon sensitivity, beyond the currently available 1.55µm, is compelling due to the commensurate reduction in solar background photon flux. Recent characterisation has shown that Al0.9Ga0.1As0.08Sb0.92 exhibits desirable avalanche multiplication characteristics. Most notably tunnelling current is negligible due to the material’s large indirect bandgap, while the ionisation coefficients are found to exceed those of other III-V materials. In this work these characteristics are exploited to design and fabricate SAM APDs with breakdown voltages below 15V. This is significantly lower than alternative technologies such as AlInAs/InGaAs APDs, which typically breakdown in excess of 25V in order to limit the magnitude of deleterious tunnelling current. Such a low breakdown voltage is desirable because it greatly aids the design of ROICs enabling future 2D imaging APD and SPAD arrays. In this work Al0.9Ga0.1As0.08Sb0.92/In0.22Ga0.78As0.19Sb0.81 SAM APDs are designed, fabricated and characterised. The detector’s spectral response covers the SWIR window, reaching 2.5µm. Grading between the absorption and multiplication layers is refined to achieve high quantum efficiencies both at room temperature and more challengingly at 77K. Operation below breakdown as a linear mode APD and above breakdown as a Geiger mode SPAD when cooled, are studied and reported. As a further proof of the new SAM APD platform, an Al0.9Ga0.1As0.08Sb0.92/InAs0.91Sb0.09 detectors is also realised, extending responsivity into the MWIR window.

The influence of interface donor and acceptor traps on the static and dynamic behavior of Ge/Si separate absorption, charge, and multiplication avalanche photodiodes (APDs) is modeled. The effects of different trap types, densities, and carrier capture cross sections on the dark current level, breakdown voltage, dc gain, and electric field profile, as well as on the frequency response and gain-bandwidth product of the device, are investigated for the first time. Our results show that the interface traps significantly increase the dark current and reduce the gain from 290 to less than 10. We also show that the acceptor traps reduce the APD bandwidth considerably to 0.5 GHz whereas the donor traps increase the bandwidth to around 10 GHz.

Resonant-cavity photodetectors (RECAPs) can circumvent the tradeoff between quantum efficiency and bandwidth that can limit the performance of conventional photodiode structures. For example, a Si-based RECAP has achieved 65% external quantum efficiency with almost 10x improvement in bandwidth compared to commercially-available Si p-i-n photodiodes. In addition, the wavelength selective spectral response offers potential advantages for applications where filtering is needed such as wavelength division multiplexing (WDM). In this paper, we demonstrate, for the first time, a resonant-cavity, separate absorption and multiplication (SAM) avalanche photodiode (APD). The motivation for using the SAM-APD structure is to achieve single carrier injection into the multiplication region and thus obtain low excess multiplication noise. We also demonstrate a long-wavelength, resonant-cavity photodetector that exhibits a high quantum efficiency and the narrowest spectral-linewidth reported to date.

Waveguide-coupled Ge/Si separate absorption, charge and multiplication avalanche photodiodes (SACM APDs) have shown significant potential as high-sensitivity, low-noise, and high-speed photo-detection for optical communications. Here we present a nanoscale single-mode waveguide-integrated vertical Ge absorption, lateral Si charge and single multiplication configuration for a waveguide Ge/Si SACM APD. This device can achieve 90% absorption at 1550 nm wavelengths with a 10 μm-long Ge layer. The device exhibits a seven times reduction in device length compared to conventional waveguide structures and a 29% increase compared to multi-mode interference coupling. Meanwhile, a 3-dB bandwidth can achieve 47 GHz, which is five times higher than the conventional vertical Ge absorption, lateral Si charge and multiplication devices.

A p-i-i-n type AlGaN heterostructure avalanche photodiodes (APDs) is proposed to decrease the avalanche breakdown voltage and to realize higher gain by using high-Al-content AlGaN layer as multiplication layer and low-Al-content AlGaN layer as absorption layer. The calculated results show that the designed APD can signifi-cantly reduce the breakdown voltage by almost 30%, and about sevenfold increase of maximum gain compared to the conventional AlGaN APD. The noise in designed APD is also less than that in conventional APD due to its low dark current at the breakdown voltage point. Moreover, the one-dimensional (1D) dual-periodic photonic crystal (PC) with anti-reflection coating filter is designed to achieve the solar-blind characteristic and cutoff wavelength of 282 nm is obtained.

We present a general methodology for device-level simulation of multiplication, noise, and receiver sensitivity in separate absorption, grading, charge, and multiplication avalanche photodiodes (SAGCM APDs) with a thick charge layer. According to the best of our knowledge, it is the first time to report the experimental evidence for impact ionization in combination of absorption, grading, charge layer with multiplication layers for SAGCM APDs. A thick charge layer was adopted in our InP/InGaAs APDs, which provides us a chance to elucidate the contribution of the electric field in the charge layer to the overall performance for SAGCM APDs. In addition, thereby performance dependence of SAGCM APDs on the thickness of charge layer has also been discussed. An empirical formula is proposed to predict the thickness of the InP multiplication layer of APDs with different charge doping levels for optimization in noise performance. The technological significance of the device-level simulation on the design and development of SAGCM APDs has also been addressed.

A general theory of noise in avalanche photodiodes (APDs), photomultipliers, and other cascaded multiplication devices in which ionization is independent of a carrier's history is presented. The theory is based on a composition law that allows the calculation of the gain distribution of a multilayer structure from the known distributions for each layer. The composition law is used to establish some well-known results and a number of new results. New formulas are derived for the gain and excess noise of a general double-carrier structure composed of multiple identical stages. A set of differential equations is derived for the case of continuous multiplication, as in a conventional APD. New results are reported for the noise of the staircase APD and for an APD with two regions of constant but different ionization ratios. The latter can be used to model the InGaAs/InP separated absorption and multiplication APD.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

4H-SiC is an attractive material for ultraviolet detection owing to its wide band gap and a matured material technology. This paper reports the mean multiplication gain and excess noise factor with electron- and hole-initiated multiplication of a thin 4H-SiC APDs. The impact ionization coefficients for electron (α) and hole (β) using Monte Carlo method over an electric field ranging from 2000 kV/cm up to 5000 kV/cm are simulated in our work. The results show that β > α, and the ratio remains large even at very high electric field region. The electric field dependence of the impact ionization coefficients equations have been deduced from our model. Based on these equations, the avalanche breakdown voltage, multiplication gain and excess noise factor at different avalanche width have been investigated by considering the effect of dead space. As the width is increasing, the breakdown voltage and multiplication gain also increases proportionally. We observe a significantly higher multiplication gain for hole- than that of electron-initiated multiplication. The excess noise factor of electron-initiated multiplication is greater than that of hole-initiated due to higher number of feedback carriers. Thus, pure hole injection is necessary in order to ensure low excess noise in 4H-SiC APD owing to the large β > α.

In this paper, a new high optical gain amorphous avalanche photodiode has been successfully fabricated on ITO/glass substrate by plasma enhanced chemical vapor deposition (PECVD). All of these superlattice (SL) structures are made of an a-Si/SiC:H heterojunction to spatially confine the carrier ionization events within the narrow bandgap material (a-Si:H). As the carriers move across the heterobarrier step, they gain kinetic energy from the band edge discontinuity thus effectively reducing the impact ionization threshold energy. In this way each carrier will impact ionize a new electron-hole pair and the ionization rate can be enhanced. The performance of high gain, high sensitivity and low noise can be achieved. Due to the multiplication mechanism in the Si/SiC:H layer, the optical gain will be enhanced. For the a-SiC:H graded-gap superlattice structure, the room-temperature electron and hole impact ionization rates, α and β, have been determined by the photocurrent multiplication measurements. The ratio of the electron to hole ionization rates is 7.743 at a fixed electric field of 2.19×10^5Vcm^(-1). Avalanche multiplication in the superlattice layer yields a high optical gain of 206 at a reverse bias V(subscript R)=20V and an incident light power P(subscript in)=5μW and has a switching time of 7.2μs at a load resistance R(subscript L)=2.2KΩ. Multiplication excess noise is also measured, and is used to evaluate the ionization rate ratio. It was found that the a-SiC:H graded-gap superlattice avalanche photodiode generates high optical gain and low excess noise during the photocurrent multiplication. These results indicate that the a-SiC:H graded-gap superlattice avalanche photodiode is a promising device for the high sensitive imaging sensor. Besides, a variety of peak response wavelength is achievable by changing the applied bias and the well width in the SL structure, making the device useful as a wavelength selective multimedia color management photosensor.

The excess noise and ionization coefficient ratio of InAlAs avalanche photodiodes (APDs) with heterojunctions has been measured. To obtain these values accurately, we used a differential amplifier and determined the optical current at which the avalanche multiplication equals 1. These made it possible to conduct measurements on excess noise including that in the low multiplication region. Measured ionization coefficient ratios at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> =10 were 0.18 and 0.17 with multiplication layer widths of 0.2 and 0.7 ¿m , respectively. Excess noise was reduced with an increase in the optical input power. This tendency appeared more prominently in APDs with a thin multiplication layer.

Low noise silicon reach-through avalanche photodiodes are designed and implemented through 0.35 μm high voltage CMOS process. Seperated absorption charge multiplication (SACM) structure with vertical n++/p+/pi/p four layers is adopted. The light sensitive area of two different size element detectors is 200 μm and 500 μm in diameter, respectively. Regardless of the size of the light sensitive area, the typical reach-through voltage and the breakdown voltage for both element detectors is tested to be 60 V, and 172 V, respectively. The distribution of the breakdown voltage across the wafer is like a superposition of two Gaussian distributions of one peak at 168 V corresponding to the fewer detectors close to the rim and the other at 172 V corresponding to the most detectors in the center. The temperature coefficient of the breakdown voltage is tested down to be 0.32 V/K. The dark current at the gain M=100 is tested to be 50 pA and 500 pA for each detector. The responsive wavelength is 400-1100 nm. The peak responsivity of the 500 &mu;m diameter detector is tested to be 57 A/W at 900 nm wavelength and with gain 100 to show the successful near infrared enhanced reponsivity. The excess noise factor is tested to be 3 - 4, much lower than those in the reported high voltage CMOS avalanche photodiodes. The yield is 100%. The devices are applied to a multiple-line Lidar to show the feasibility. Two 16×1 linear arrays of different pixel pitches are also designed and fabricated in the same way.

Low frequency noise spectra of five SAGCM APDs were measured. The experimental noise current spectra were fitted with S/sub I/=A/sub F/I/sub d//sup x//f/sup y/+2qI/sub d/M/sub eff/. Any enhancement in shot noise was included in M/sub eff/. The A/sub F/ factors were found to vary from 10/sup -7/ to 10/sup -5/ and were related to the I-V characteristics of these APDs.

The thinning of the multiplication (M) layers in high-speed avalanche photodiodes (APDs) is an effective way to boost the gain-bandwidth product (GBP) and reduce excess noise. However, such downscaling usually comes at the price of a huge leakage current (>1 μA) induced by direct tunneling through the final thin M-layer (<100 nm) and degradation of the RC-limited bandwidth due to the decrease in the thickness of the depletion layer. In this work, we demonstrate how a p-side up top-illuminated APD structure with a thick InP collector layer buried below the thin and cascaded In<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.52</sub>Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.48</sub>As based multiplication layer can effectively relax the trade-offs among the dark current, RC-limited bandwidth, multiplication gain, and avalanche delay time. This advanced device structure is then applied and the performance of APDs with different thin M-layer thicknesses (<50 nm) explored. Under 1.55 μm wavelength excitation, a device fabricated with a large active window (mesa) diameter of 10 (20) μm and an optimized M-layer thickness exhibits a dark current as low as ∼0.4 μA and a high responsivity (2.8 A/W; gain = 9.3) at 0.9 V<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub>. This device also exhibits excellent dynamic performance, including a wide optical-to-electrical bandwidth (44 GHz at 0.84 A/W), an extremely large GBP of 1.03 THz, and a high saturation current (12 mA), which corresponds to a large millimeter-wave (MMW) output power (∼0 dBm) at 45 GHz. The exceptional speed performance coupled with the wide dynamic range and simple top-illuminated structure opens up new possibilities to further enhance the sensitivity of 50 G passive optical networks (PONs).

This study introduces a digital alloy AlAsSb/GaAsSb avalanche photodiode (APD) on InP, demonstrating low dark current and low noise performance. The lattice-matched AlAsSb/GaAsSb APD structure exhibits a dark current density of 16 μA/cm2 at a gain of 10 and an excess noise factor near 2 at room temperature, attributed to high crystal quality and effective passivation. The dark current level achieved in this work is the lowest reported for Sb-based p-i-n APDs with a 1000 nm thick intrinsic region. These results make it highly promising for separate absorption, charge, and multiplication (SACM) structures in optical communication and LIDAR applications.