A Study of the Avalanche Multiplication and Excess Noise in Al x In1-x AsγSb1‑ γ Avalanche Photodiodes Lattice-Matched to GaSb.

The 2-μm optical window has recently become an area of great interest for imaging and LIDAR applications due to improved ranging capability and eye safety compared to common telecommunications wavelengths. Avalanche photodiodes (APDs) operating in this spectrum are highly desirable, as their intrinsic gain offers increased sensitivity over traditional photodiodes, further improving receiver sensitivity. HgCdTe, InAs, and InSb, as well as various superlattice materials have been used for this purpose, however, the combination of high electric field and narrow-bandgap absorber yields high dark current. As a result, these APDs are operated at cryogenic temperatures to suppress recombination mechanisms. At the high electric fields required for impact ionization, narrow bandgap materials are also susceptible to band-to-band tunneling, which cannot be suppressed by cooling. The separate absorption, charge, and multiplication (SACM) APD was designed to address this challenge by reducing the electric field in the absorber while maintaining sufficiently high enough field in the multiplication region for impact ionization [1] . This design spatially separates the absorption and multiplication layers, controlling the electric field in the absorber and multiplication layers through an intermediate charge layer. SACM APDs have been widely deployed in the InGaAs/InP and InGaAs/InAlAs materials systems for use in near-infrared telecommunications applications.

AlxGa1-xAsySb1-y grown lattice-matched to InP has attracted significant research interest as a material for low noise, high sensitivity avalanche photodiodes (APDs) due to its very dissimilar electron and hole ionization coefficients, especially at low electric fields. All work reported to date has been on Al concentrations of x = 0.85 or higher. This work demonstrates that much lower excess noise (F = 2.4) at a very high multiplication of 90 can be obtained in thick Al0.75Ga0.25As0.56Sb0.44 grown on InP substrates. This is the lowest excess noise that has been reported in any III-V APD operating at room temperature. The impact ionization coefficients for both electrons and holes are determined over a wide electric field range (up to 650 kV/cm) from avalanche multiplication measurements undertaken on complementary p-i-n and n-i-p diode structures. While these ionization coefficients can fit the experimental multiplication over three orders of magnitude, the measured excess noise is significantly lower than that expected from the β/α ratio and the conventional local McIntyre noise theory. These results are of importance not just for the design of APDs but other high field devices, such as transistors using this material.

A theoretical model incorporating the mechanism of resonant absorption of the multiple reflected lightwaves is presented for the frequency response of resonant-cavity (RC) separate absorption, charge, and multiplication (SACM) avalanche photodiodes (APDs). The derived theoretical expressions are general and can be readily applied to many other RC and non-RC APDs. These analytical expressions also allow for fast computation of the frequency response and bandwidth characteristics. Combining this frequency response theory with expressions of multiplication gain and ionization coefficients, an efficient approach is proposed for modeling the general performance characteristics of RC APDs. The modeling approach is applied to an InGaAs-AlGaAs RC SACM APD. The computed results are demonstrated, and the results of -3 dB bandwidth are comparable to experimental work. The validity of the modeling parameters is also discussed. It is further found that the normalized frequency response is unaffected when the value of the absorption coefficient is changed, suggesting that the standing-wave effect within the RC structure may not influence the bandwidth characteristics.

In this work, we present the study on Separate Absorption, Charge and Multiplication (SACM) APDs utilising 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 as the multiplication layer and In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.51</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.49</sub> periodic heterostructures as the absorption layer. 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 lattice matched to InP has been shown to have superior excess noise characteristics and multiplication with relatively low temperature dependence compared to InP. Furthermore, the type-II staggered band line-up of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.51</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.49</sub> heterostructures leads to a narrower effective bandgap of approximately 0.49 eV corresponding to the APD cut off wavelength of 2.4 μm. The SACM APD exhibited low dark current densities near breakdown. The device also exhibited multiplication in excess of 100 at 200 K. The excess noise of the APD was low as expected, and is comparable to that of a 1 μm 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 PIN diode.

In this work, we present the study on In 0.53 Ga 0.47 As/GaAs 0.51 Sb 0.49 type-II heterojunction PIN diodes and Separate Absorption, Charge and Multiplication (SACM) APDs utilising In 0.52 Al 0.48 As as the multiplication layer and In 0.53 Ga 0.47 As/GaAs 0.51 Sb 0.49 type-II heterostructures as the absorption layer. In 0.52 Al 0.48 As lattice matched to InP has been shown to have superior excess noise characteristics and multiplication with relatively low temperature dependence compared to InP. Furthermore, the type-II staggered band line-up leads to a narrower effective bandgap of approximately 0.49 eV corresponding to the APD cut off wavelength of 2.4 P m. The device exhibited low dark current densities near breakdown. The device also exhibited mu ltiplication in excess of 100 at 200 K. Keywords : Superlattice, infrared, photodetector, InGaAs/GaAsSb 1. INTRODUCTION There are many applications such as thermal imaging and chemical sensing where the ability to detect weak optical signals beyond 2 P m at room temperature would be of value. Photodiodes operating in midwave infrared range (2-5 P m) wavelength region find many applications in remote sensing, thermal imaging, and chemical sensing in industrial and military operations and also in medical diagnostics. Avalanche photodiodes can achieve 5-10 dB higher sensitivity compared to normal PIN photodiodes because of its intrinsic gain mechanism of the avalanche process. Research efforts have been made to explore alternative material systems to make devices that operate beyond the 2 P m range at room temperature. Devices based on Si and In

The avalanche photodiode (APD) is widely used in optical fibre communications (Campbell, 2007) due to its ability to achieve high internal gain at relatively high speeds and low excess noise (Wei et al., 2002), thus improving the system signal-to-noise ratio. Its internal mechanism of gain or avalanche multiplication is a result of successive impact ionisation events. In an optical receiver system, the advantage of internal gain, in the APD, is experienced when the amplifier noise dominates that of a unity-gain photodiode. This increases the signal-to-noise ratio (SNR) and ultimately improves the receiver sensitivity as the gain increases until the APD noise rises to become dominant. Indium Phosphide (InP) is widely used as the multiplication layer material in commercially available APDs for applications in the 0.9–1.7μm wavelength region with In0.53Ga0.47As grown lattice-matched to it as the absorption layer. It has been predicted that Indium Alluminium Arsenide (In0.52Al0.48As) will replace InP, as a more favourable multiplication layer material due to its lower excess noise characteristics (Kinsey et al., 2000). In comparison to InP, tunnelling currents remain lower in InAlAs due to its larger bandgap. While holes ionise more readily than electrons in InP, the opposite holds true for InAlAs and InGaAs, as electrons ionise more readily than holes; thus making the InGaAs/InAlAs combination superior to InGaAs/InP in a SAM APD, in terms of lower excess noise, higher gain-bandwidth product, and improved sensitivity. Studies have also shown that the breakdown voltage of InAlAs APDs is less temperature dependent compared to InP (Tan et al., 2010), which would be useful in temperature sensitive applications, thus making temperature control less critical. The sensitivity performance criterion for digital receivers is its bit-error rate (BER), which is the probability of an error in the bit-identification by the receiver. The receiver sensitivity is defined as the minimum average optical power to operate at a certain BER; 10-12 being a common standard for digital optical receivers. The sensitivity of APD-based high speed optical receivers is governed by three main competing factors, namely the excess noise, avalanche-buildup time and dark current of the APD. Generally, the excess noise and avalanche-buildup time increases with APD gain. Thus, for a fixed multiplication layer thickness, there is a sensitivity-optimised gain that offers a balance between SNR while keeping the degrading contributions from the excess noise factor and intersymbolinterference (ISI) at a minimum. More importantly, changing the thickness of the multiplication layer strongly affects the receiver sensitivity, as the aforementioned three

InAs/GaSb type-II superlattice (T2SL) avalanche photodiodes (APDs) are particularly well-suited for low-light detection and quantum communication due to their enhanced sensitivity. However, their performance is significantly impacted by dark current and breakdown voltage characteristics. Here, we explore the performance of T2SL APDs by analysing the relationship between the structural parameters of the absorption, charge, and multiplication layers, utilizing Silvaco software and the equivalent materials method. To enhance the device's performance, we integrated a high-doping AlAsSb charge layer into the separate absorption and multiplication (SAM) structure, constructing a SACM (separate absorption, charge, and multiplication) architecture. Simulation results show that the optimized SAM APD achieves a penetration voltage of 24.7 V and a breakdown voltage of 36.5 V. Notably, the insertion of the charge layer effectively reduced the device's dark current from 10-7 to 10-9 A. At an operating temperature of 300 K, the SACM APD demonstrates a gain of 73.4 with a reverse bias voltage of 35 V, surpassing the performance of the SAM structure. These findings provide critical insights for the design of high-performance mid-wave infrared detectors, highlighting the potential of T2SL-APDs in achieving high gain and low dark current.

In recent years, InAlAs avalanche photodiodes (APDs) have reported high sensitivities in high bit rate 10 Gb/s [1] and even 40 Gb/s [2] optical communication applications. InAlAs is able to supersede InP as the multiplication layer in SAM-structure APDs due to its superior ionization characteristics that include larger bandgap and very dissimilar electron and hole ionization coefficients (? and s respectively), while remaining lattice matched to InP substrates. Preliminary studies also suggest relatively small temperature dependence of breakdown voltages compared to InP, which reduces the need for temperature stabilization. The significance of a large electron and hole ionization coefficient ratio is lower excess noise and increased sensitivity, without sacrificing the gain-bandwidth product. Furthermore, InAlAs/InGaAs APDs are electron initiated, which further improves the excess noise performance [3] should there be low-field ionization in the InGaAs absorption layer [4]. Accurate characterization of the InAlAs ionization properties such as excess noise and ionization coefficients are essential when designing and optimizing the InAlAs APD performance using simulators and analytical models. It is therefore surprising that the only data on submicron avalanche structures is by Saleh et al. [5] and even this only reports on electron-initiated multiplication. We present a systematic study of avalanche multiplication and excess noise characteristics of InAlAs on a series of p+-i-n+ and n+-i-p+ diodes with nominal intrinsic region widths from 0.1 ?m to 2.5 ?m.

We present gain, dark current and excess noise characteristics of PIN Al_0.85Ga_0.15As_0.56Sb_0.44 (hereafter AlGaAsSb) avalanche photodiodes (APDs) on InP substrates with 1000 nm thick multiplier layers. The AlGaAsSb APDs were grown by molecular beam epitaxy using a digital alloy technique (DA) to avoid phase separation. Current-voltage measurements give a peak gain of ~ 42, a breakdown voltage of – 54.3 V, and a dark current density at a gain of 10 of ~ 145 &mu;A/cm². Excess noise measurements of multiple AlGaAsSb APDs show that k (the ratio of electron and hole impact ionization coefficients) is ~ 0.01. This k-value is comparable to Si, which is widely used for visible and near-infrared APDs. The low dark current density and low excess noise suggest that such thick AlGaAsSb layers are promising multipliers in separate absorption, charge and multiplication (SACM) structures for short-wavelength infrared applications such as optical communication and LIDAR, particularly on a commercial InP platform.

Near-infrared photodiodes (PDs) of Ge on Si have been widely studied in Si photonics for the optical communications (1.3–1.6 μm). Ge-based avalanche PDs (APDs) have been also studied for sensitive photodetections. There are mainly two different types of Ge APDs on Si. One utilizes Ge as the multiplication layer, i.e., Ge homojunction APDs [1], while the other one utilizes Si, i.e., Ge/Si heterojunction APDs [2]. Si is one of the best materials for the multiplication layer in terms of low excess noises, although relatively large applied voltage more than 20 V, corresponding to the electric field strength more than 300 kV/cm, is required to obtain the multiplication gain due to the small ionization coefficients. In order to realize low-noise and low-voltage Ge APDs, we have recently proposed a new structure utilizing Ge/graded-SiGe heterojunction multiplication layer [3]. As in Fig. 1(a), photo-generated holes are injected via the graded SiGe to the Ge multiplication layer. The band discontinuity at the graded-SiGe/Ge interface can enhance the impact ionization due to the sudden increase of kinetic energy across the interface. This is effective to realize low-noise and low-voltage Ge APDs [3]. In this work, such Ge/SiGe heterostructure APDs are fabricated, and the multiplication gain is measured. As a result, the growth temperature in ultrahigh-vacuum chemical vapor deposition (UHV-CVD) is found to be a key parameter to enhance the multiplication gain. UHV-CVD was used to grow Ge/graded-SiGe/Ge heterostructures for free-space APDs on Si. GeH4 and Si2H6 were used as the source gases. First, 350-nm-thick undoped Ge layer was directly grown on p+-Si using low/high-temperature two-step growth. The Ge buffer layer (~50 nm) was grown at 370°C, followed by the growth at an increased temperature. As an important parameter, we compared the growth temperature, 510°C and 580°C. Then, a 24-nm-thick SiGe layer with the graded Ge composition from 70% to 100% was grown at the same temperature of 510°C or 580°C. Finally, a 50-nm-thick Ge layer was grown, and P atoms were implanted to form the n-Ge optical absorption layer. The valence band discontinuity at the bottom Si0.3Ge0.7/Ge interface is as large as 0.1 eV, assisting holes to reach the threshold energy (~1.0 eV) required for the impact ionization. As a reference, a Ge homojunction APD without the SiGe layer was also fabricated. Figure 1(b) shows typical hole-initiated multiplication gain as a function of applied voltage. Photo-generated holes can be selectively injected from the top n-Ge absorption layer to the bottom SiGe/Ge multiplication layer, using the free-space illumination of 532 nm laser light whose penetration depth in Ge as large as 20 nm. The use of SiGe/Ge multiplication layer is found to increase the multiplication gain in comparison with the Ge homojunction APD, being effective for the low-voltage APD operation. Furthermore, the Ge/SiGe APDs formed at the lower growth temperature of 510°C showed larger multiplication gain than that for 580°C. This is ascribed to the formation of abrupt SiGe/Ge interface at the lower growth temperature, as confirmed by the x-ray diffraction measurements. Excess noise characteristics will be also presented. [1] L. Virot et al., Nature Commun. 5, 4957 (2014). [2] Y. Kang et al., Nature Photon. 3, 59 (2009). [3] Y. Miyasaka et al., Jpn. J. Appl. Phys. 55, 04EH10 (2016). Figure 1

The avalanche photodiode (APD) is experiencing rapid technological advancements and expanding applications. Effective electric field engineering in APDs is critical for enhancing performance. Traditional methods for the modulation of the electric field using the charge layer often cause the carrier accumulation and non-uniform electric field distribution, limiting the performance of the APD. In this work, groove rings are introduced in the multiplication layer to efficiently modulate the electric field in the InGaAs/Si APD. This modulation is similar to that produced by the charge layer but is more effective than traditional methods. The electric field in the absorption layer can be decreased by groove rings to decrease the dark current and increase the 3-dB bandwidth. The electric field in the multiplication layer can be increased by the groove rings, leading to the better avalanche multiplication and the improvement of the gain. Additionally, the avalanche voltage is reduced by groove rings, refining the electric field distribution at the bonded interface. Importantly, groove rings enable more efficient carrier transport by avoiding carrier accumulation associated with the doping concentration of the charge layer. These findings provide theoretical guidance to achieve a higher gain-bandwidth product (GBP) of the InGaAs/Si APD.&amp;#xD;

Avalanche photodiodes (APDs) are critical components for a variety of remote sensing applications, particularly for 3D imaging using light detection and ranging (LiDAR). APDs can provide higher sensitivity and faster response times than traditional PIN diodes due to their internal gain. To apply LiDAR to gas monitoring applications, including greenhouse gases, APDs need to be sensitive further into the infrared than Si APDs can detect. This work investigates an absorber that is sensitive to 2 &mu;m and compatible with an APD. A separate absorption, charge, and multiplication (SACM) heterostructure is often used to reduce the dark current of an infrared APD. In a SACM design, the absorber is placed in a low field region to minimize tunneling and the multiplier is placed in a high field region to maximize impact ionization. We have previously explored high performance multipliers that are lattice matched to InP substrates. In this work, we explore a candidate lattice-matched absorber, an In_0.53Ga_0.47As/GaAs_0.51Sb_0.49 Type II superlattice (T2SL). We have demonstrated photoluminescence at 2 &mu;m using a 5 nm InGaAs/5 nm GaAsSb T2SL structure. We have grown and fabricated 1-micron thick PIN diodes with this absorber material and obtained an n-type background carrier concentration of 5&times;10¹⁵ cm^-3 . We are currently undertaking the radiometric characterization of these devices to support their integration into a SACM APD.

Digital alloy (DA) InAlAs on the InP substrate exhibits a lower excess noise compared to a traditional In0.52Al0.48As random alloy as the multiplication layer in avalanche photodiodes (APDs). This work implements DA InAlAs as the multiplication layer in a 1550 nm separate absorption, grading, charge, and multiplication APD and characterizes the performances through various analyses. The device reaches a maximum gain of 221 before avalanche breakdown, with a maximum gain-bandwidth product of more than 140 GHz. At 90% breakdown voltage, the dark current density is 4.1 mA/cm2, and the responsivity is 0.48 A/W at unit gain. Excess noise factors were identified, yielding an effective k value of around 0.15, which is lower than that of random alloy In0.52Al0.48As APDs (k ∼0.2). These findings show that DA InAlAs has the potential to be a promising material for high-performance APDs.

The relationship between the performance of avalanche photodiode (APD) and structural parameters of the absorption, grading, and multiplication layers has been thoroughly simulated and discussed using the equivalent materials approach and Crosslight software. Based on separate absorption, grading, charge, and multiplication (SAGCM) structure, the absorption layer of APD was replaced with InGaAs/GaAsSb superlattice compared to conventional InGaAs/InP SAGCM APD. The results indicated that the breakdown voltage increased with the doping concentration of the absorption layer. When the thickness of the multiplication layer increased from 0.1 μm to 0.6 μm, the linear range of punchthrough voltage increased from 16 V to 48 V, and the breakdown voltage decreased at first and then increased when the multiplication layer reached the critical thickness at 0.35 μm. The grading layer could not only slow down the hole carrier, but also adjust the electric field. The dark current was reduced to about 10 nA and the gain was over 100 when the APD was cooled to 240 K. The response wavelength APD could be extended to 2.8 μm by fine tuning the superlattice parameters. The simulation results indicated that the APD using superlattice materials has potential to achieve a long wavelength response, a high gain, and a low dark current.

This paper presents a theoretical analysis of npBp infrared (IR) barrier avalanche photodiode (APD) performance operating at 300 K based on a quaternary compound made of AIIIBV—InGaAsSb, lattice-matched to the GaSb substrate with a p-type barrier made of a ternary compound AlGaSb. Impact ionization in the multiplication layer of InGaAsSb separate absorption, grading, charge, and multiplication avalanche photodiodes (SAGCM APDs) was studied using the Crosslight Software simulation package APSYS. The band structure of the avalanche detector and the electric field distribution for the multiplication and absorption layers were determined. The influence of the multiplication and charge layer parameters on the impact multiplication gain and the excess noise factor was analyzed. It has been shown that with the decrease in the charge layer doping level, the gain and the breakdown voltage increase, but the punch-through voltage decreases, and the linear range of the APD operating voltages widens. The multiplication layer doping level slightly affects the detector parameters, while increasing its width, the photocurrent and the breakdown voltage also increase. The detector structure proposed in this work allows us to obtain a comparable gain and lower dark currents to the APD detectors made of InGaAsSb previously presented in the literature. The performed simulations confirmed the possibility of obtaining APDs with high performance at room temperatures made of InGaAsSb for the SWIR range.