Extremely low excess noise and high sensitivity AlAs0.56Sb0.44 avalanche photodiodes

Abstract

Fast, sensitive avalanche photodiodes (APDs) are required for applications such as high-speed data communications and light detection and ranging (LIDAR) systems. Unfortunately the InP and InAlAs used at the gain material in these APDs have similar electron and hole impact ionisation coefficients (α and β respectively) at high electric fields, giving rise to relatively high excess noise, limiting their sensitivity and gain bandwidth product (GBP) 1. Here, we report on extremely low excess noise in AlAs0.56Sb0.44 lattice matched to InP. The deduced β/α ratio as low as 0.005 in the avalanche region of 1550 nm is close the theoretical minimum and is significantly smaller than Silicon, with modelling suggesting that vertically illuminated APDs with a sensitivity of-25.7 dBm at a bit-error rate (BER) of 1×10-12 at 25 Gb/s at 1550 nm can be realised. The findings could yield a new breed of high performance receivers for applications in networking and sensing. The advantages of using InP based APDs at traditional telecommunication wavelengths of 1310 nm and 1550 nm as a way of increasing sensitivity and speed of communication networks is well documented 2,3,4. Such APDs utilise InGaAs lattice matched to InP substrates as the absorber and an InP or InAlAs gain (or multiplication) region in the Separate Absorption and Multiplication (SAM)-APD configuration. However due to the broadly similar β/α ratio in these materials, particularly at high fields, their sensitivity and GBP are limited, hampering their use as the bit rate increases beyond 10 Gb/s. The best results at 25 Gb/s to 50 Gb/s currently utilise ≤ 100 nm thick InAlAs multiplication regions and these provide a sensitivity of between-22.6 dBm to-10 dBm respectively at a BER of 1×10-125-7. There have been many attempts at improving the performance of APDs for telecommunications, for example utilising Ge/Si 8-10 , nanopillars 11 , AlInAsSb 12-14 or InAs 15 , however these face problems such as limited wavelength operation, often requiring difficult growth procedures, complex fabrication technologies or the use of more expensive substrates. In this letter we demonstrate that the InP lattice matched alloy system AlAs0.56Sb0.44 with its very small β/α ratio 16 leads to extremely low excess noise at room temperature, even at high gains, exceeding the performance of all materials lattice matched to InP and even silicon. This extremely small β/α ratio changes the paradigm whereby high speed APDs always use very thin avalanching structures to one where both high speed and sensitivity can be achieved with thicker avalanching structures. Three AlAs0.56Sb0.44 structures with avalanche region thicknesses of 1550 nm (P1), 660 nm (P2) and 1150 nm (P3) in a PIN configuration and two NIP structures with avalanche region thicknesses of 1550 nm (N1) and 660 nm (N2) were investigated. Fig. 1a shows a schematic diagram of P1 with

Impact Ionization Coefficients in (Al x Ga1-x )0.52In0.48P and Al x Ga1-x As Lattice-Matched to GaAs

“…Hole impact ionization is also likely to be suppressed due to the flattening of the heavy hole band as aluminum content increases [29], which may cause the interband scattering rate to increase and prevent holes from accumulating sufficient energy to impact ionize [30]. It is notable that AlAs 0.56 Sb 0.44 , Al 0.85 Ga 0.15 As 0.56 Sb 0.44 , and Al x In 1−x As y Sb 1−y , all of which have recently been reported to have very low β/α ratios [12], [13], [31] are also indirect bandgap semiconductors with the lowest conduction band edge in the X valley.…”

“…There is also interest in GaAs-based SAM-APDs with wide bandgap multiplication regions, using GaNAsSb and GaSb absorption regions for SWIR detection [10], [11]. The understanding gained from this alloy system may also be useful in determining the optimum compositions of other Al/Ga containing alloy systems such as Al x Ga 1−x As 0.56 Sb 0.44 , for which extremely low-noise APDs have been recently reported [12], [13].…”

Extremely low-noise avalanche photodiodes based on AlAs0.56Sb0.44

“…We previously modelled the performance of 30 μm diameter InGaAs-AlAsSb based SAM-APDs [30] where we showed that it was possible to achieve a sensitivity of -32.1 dBm at BER=10 -12 at 1550 nm for a 10 Gb/s optical system. The prediction there assumed an InGaAs absorption region that is 600 nm thick with a maximized-induced current (MIC) design [31] and capable of a unity gain responsivity of 0.92 A/W.…”

Engineering of impact ionization characteristics in In0.53Ga0.47As/Al0.48In0.52As superlattice avalanche photodiodes on InP substrate

“…Thick AlAs 0.56 Sb 0.44 APDs on InP substrates have also been successfully grown as AlAs/ AlSb SLs for a 1550 nm thick M-layer. The SLs exhibited an extremely low k-value of 0.005 16 . To investigate the origin of lower k-value in a SL APD, AlAs/InAs SL (compositionally the same as Al 0.48 In 0.52 As RA) APDs were grown on an InP substrate.…”

“…However, an APD technology at least needs to be comparable to Si APDs, and the AlGaAsSb APDs cannot achieve such a low k-value with relatively thin layers. A way to attain the low k-value is to have an M-layer that is ~ 1500 nm thick 16 . Unfortunately, AlGaAsSb APDs cannot be randomly grown with such large thicknesses due to the existence of a large miscibility gap, causing phase separation during the growth.…”