Low-noise Avalanche Photodiodes Research Articles

Comparison of different models of non‐local impact ionization for low noise avalanche photodiodes

AbstractThe dead space model of non‐local impact ionization has been developed independently by three different research groups in order to include a ‘soft’ dead space. This paper seeks to compare these models for the first time. The models are fitted to Monte Carlo simulation data (both non‐local ionization coefficients and the ionization pathlength pdf) and it is found that there are no significant differences between the models except that the Liverpool model cannot be used to fit one data set which has a very soft dead space. Likewise the models give very similar results when used to calculate the excess noise factor for low noise avalanche photodiodes. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-speed and low-noise SACM avalanche photodiodes with an impact-ionization-engineered multiplication region

A separate absorption, charge, and multiplication In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As avalanche photodiode with an impact-ionization-engineered multiplication region is reported. By implementing an electric field gradient in the multiplication region, better control of impact-ionization can be achieved. Gain-bandwidth product of 160 GHz and excess noise factor with an equivalent k value of 0.1 are demonstrated.

Band-structure engineering to control impact ionisation and related high-field processes

Alloying with even small amounts of highly mismatched atoms can have dramatic and beneficial effects on the electronic band structure of semiconductors. For example, dilute amounts of nitrogen in GaAsN or GaInAsN may cause considerable disruption of the conduction band while leaving the valence band relatively unaltered. This means that the impact ionisation rate of electrons can be made much smaller than the rate for holes leading, for example, to the production of low-noise avalanche photodiodes.

Recent Advances in Avalanche Photodiodes

The development of high-performance optical receivers has been a primary driving force for research on III-V compound avalanche photodiodes (APDs). The evolution of fiber optic systems toward higher bit rates has pushed APD performance toward higher bandwidths, lower noise, and higher gain-bandwidth products. Utilizing thin multiplication regions has reduced the excess noise. Further noise reduction has been demonstrated by incorporating new materials and impact ionization engineering with beneficially designed heterostructures. High gain-bandwidth products have been achieved waveguide structures. Recently, imaging and sensing applications have spurred interest in low noise APDs in the infrared and the UV as well as large area APDs and arrays. This paper reviews some of the recent progress in APD technology.

Monte Carlo simulation of low-noise avalanche photodiodes with heterojunctions

A Monte Carlo model is developed to simulate avalanche photodiodes with AlGaAs/GaAs heterojunctions. The experimentally observed ultralow-noise behavior of a center-well avalanche photodiode is successfully reproduced in the model. It is found that the arrangement of different materials in the intrinsic region can modulate the positional dependence of impact ionization events, and hence the gain distribution. Consequently, the noise is sensitive to the structural parameters such as well thickness. Hot and energetic electrons are distinguished by their distribution in k space. This distinction is used to explain why the noise behavior is sensitive to the initial carrier excess energy from photogeneration but relatively insensitive to carrier energy gained from the electric field.

Multiplication and excess noise characteristics of thin 4H-SiC UV avalanche photodiodes

The avalanche multiplication and excess noise characteristics of thin 4H-SiC avalanche photodiodes with an i-region width of 0.1 /spl mu/m have been investigated. The diodes are found to exhibit multiplication characteristics which change significantly when the wavelength of the illuminating light changes from 230 to 365 nm. These multiplication characteristics show unambiguously that /spl beta/>/spl alpha/ in 4H-SiC and that the /spl beta///spl alpha/ ratio remains large even in thin 4H-SiC diodes. Low excess noise, corresponding to k=0.1 in the local model where k=/spl alpha///spl beta/ for hole injection, was measured using 325-nm light. The results indicate that 4H-SiC is a suitable material for realizing low-noise UV avalanche photodiodes requiring good visible-blind performance.

Low-noise visible-blind UV avalanche photodiodes with edge terminated by 2° positive bevel

4H-SiC avalanche photodiodes edge terminated by a 2° positive bevel have been fabricated and characterised. Low leakage current, positive temperature dependence of breakdown voltage, high avalanche gain and very low noise have been achieved.

Low-noise silicon avalanche photodiodes fabricated in conventional CMOS technologies

We present a simple design technique that allows the fabrication of UV/blue-selective avalanche photodiodes in a conventional CMOS process. The photodiodes are fabricated in a twin tub 0.8 /spl mu/m CMOS technology. An efficient guard-ring structure is created using the lateral diffusion of two n-well regions separated by a gap of 0.6 /spl mu/m. When operated at a multiplication gain of 20, our photodiodes achieve a very low dark current of only 400 pA/mm/sup 2/, an excess noise factor F=7 at /spl lambda/=400 nm and a good gain uniformity. At zero bias voltage, the responsivity peaks at /spl lambda/=470 nm, with 180 mA/W. It corresponds to a 50% quantum efficiency. Successive process steps are simulated to provide a comprehensive understanding of this technique.

Low-noise avalanche photodiodes with graded impact-ionization-engineered multiplication region

A novel impact-ionization-engineered multiplication region for avalanche photodiodes is reported. By pseudograding the multiplication region with materials of different ionization threshold energies, the impact ionization of the injected carrier type is localized, while that of the feedback carrier type is suppressed. Low noise (k/sub eff/<0.1), low dark current, and high gain were achieved.

Very low noise avalanche detection

Recent theoretical work has shown that the excess noise of an avalanche can be quenched in high-frequency fields. In this brief, we propose a circuit to exploit this result and achieve very low noise avalanche photodiodes. The circuit also has the potential to discriminate against dark counts.

Impact ionization coefficients of Al0.8Ga0.2As

The impact ionization coefficients in bulk Al0.8Ga0.2As have been determined from photomultiplication measurements over the electric field range of 328–519 kV/cm. Unlike in AlxGa1−xAs (x⩽0.6), where the electron to hole ionization coefficients ratios (1/k) are less than 2, the 1/k value in Al0.8Ga0.2As was found to be greater than 10. Excess noise measurements corroborated the multiplication results, suggesting that this material may be a suitable multiplication medium for low noise avalanche photodiodes.

High-speed and low-noise avalanche photodiode operating at 1.06 /spl mu/m

Previously, it has been demonstrated that resonant-cavity-enhanced, quantum-dot avalanche photodiodes can achieve a good gain and high quantum efficiency at 1.06 /spl mu/m. In our new effort, these devices have shown RC-limited bandwidths of 35 GHz at low gain and gain-bandwidth products as high as 220 GHz. The achievable gain has been increased from /spl sim/18 to greater than 50 while keeping the quantum efficiency high. These photodiodes also exhibited low avalanche noise (k=0.24), low dark current (less than 100 nA at 90% of the breakdown voltage), and low-breakdown voltage (/spl sim/17 V).

Instrumentation for the joint European torus motional Stark effect diagnostic

A motional Stark effect magnetic field pitch angle diagnostic has been implemented on the joint European torus (JET) tokamak. The instrumentation designed following the study by Hawkes et al. (these proceedings) is described. Dα emission from the Octant 4 neutral beams is collected by optics which transport the plasma image outside the vacuum vessel and through a pair of photoelastic modulators (PEMs) and a linear polarizer. The light is fiber-optically coupled to interference filter spectrometers, which incorporate a remotely controlled filter tilting mechanism. This allows the center wavelength of the filter bandpass to be tuned over a range sufficient for observation of the σ and π lines of the Stark spectrum emitted by the full- and half-energy components of the beam, providing flexibility to make measurements with a variety of beam configurations. The detectors are low-noise avalanche photodiode modules. Fast digital signal processing techniques are used to extract the Fourier components of the signal at the PEM first and second harmonic frequencies. Analysis of these signals will yield the magnetic field pitch angle, which will be used as a constraint on EFIT equilibrium reconstruction modeling to obtain the q(r) profile. The system has 25 spatial channels covering the outer-half of a JET plasma with spatial resolution of 0.03–0.07 m per channel with ∼0.05 m channel-to-channel separation. Time resolution is expected to be 1–10 ms.

Integrated multi-sensor control of II–VI MBE for growth of complex IR detector structures

Next-generation HgCdTe infrared detectors and detector arrays require the growth of multilayer heterojunction structures with precisely controlled alloy composition and doping levels and minimal defect densities. Molecular beam epitaxy (MBE) provides the ability to produce such structures. However, in the absence of a real-time, in situ control methodology the extreme sensitivity of HgCdTe layer quality and doping efficiency on fundamental MBE variable such a substrate temperature and effusion cell flux provide serious challenges to the uniform and reproducible growth of such structures. In this paper, we describe an integrated, multi-sensor approach for monitoring and controlling the variables that are most important for MBE growth of HgCdTe device structures used in advanced multi-color infrared detectors and high speed, low-noise avalanche photodiodes. Substrate temperature, effusion cell flux, and layer composition are monitored using absorption-edge spectroscopy (ABES), optical flux monitoring (OFM), an spectroscopic ellipsometry (SE), respectively. Flexible, custom software has been developed and implemented for analysis of sensor inputs and feedback control of the MBE system in response to those inputs. The sensors and their application to growth of HgCdTe will be described, and the use of a custom software framework for data analysis and system control will be discussed.

Atomically abrupt semiconductor heterointerfaces: their role in advanced electronic and photonic devices and quantum phenomena

Atomically abrupt heterojunction interfaces have played a unique role in the emergence of a new generation of electronic and photonic devices. These include quantum-well lasers, low-noise superlattice avalanche photodiodes and solid-state photomultipliers, quantum-well infrared photodetectors, ultrahigh-speed heterojunction bipolar transistors and resonant-tunneling devices. The monolayer abruptness of heterojunction interfaces has also made possible the observation of exciting quantum phenomena including resonant tunneling, giant nonlinear susceptibilities associated with the quantum size effect and electron Bragg reflection phenomena. The latter have been observed in transport and in the creation of new bound states at energies in the classical continuum above potential wells. A brief discussion of recent work on tunable band discontinuities concludes this paper.

Realization of a staircase photodiode: Towards a solid-state photomultiplier

Abstract A few years ago a low noise avalanche photodiode with a near unity excess noise factor was proposed. This detector achieves a high ratio of ionization coefficient κ via a suitable staircase band diagram. This APD can in principle exhibit photomultiplier-like noise behavior. In this paper we report the first realization of a staircase APD, grown in AlGaAs/GaAs by MBE, consisting of a ten-stage undoped staircase high-field region between p+ and n+ GaAs layers. Typical dark currents were 200 pA at the breakdown onset. The electron initiated avalanche gain was greater than 100 at the same voltage. Ionization ratios greater than 10 were estimated. Pulse responses with a full width at half maximum (FWHM) of 200 ps were demonstrated.

Impact ionisation thresholds in silicon and germanium under hydrostatic pressure and strain

We have studied experimentally and theoretically impact ionisation thresholds in the indirect band-gap semiconductors, silicon and germanium. The threshold energies for electron- and hole-initiated ionisation processes were calculated numerically using an empirical pseudopotential band-structure which includes spin-orbit interactions. In silicon, the threshold energy for holes is significantly greater than that for electrons, whereas in germanium the thresholds are almost equal. We point out the band-structure features responsible for this behaviour, and its influence on the multiplication noise of avalanche photodiodes fabricated in these materials. We have measured the breakdown voltage in silicon and germanium avalanche photodiodes while varying the band-structure using hydrostatic pressure. For silicon, the results are consistent with impact ionisation dominated by electron-initiated umklapp processes associated with the Δ minima, in agreement with the calculations. The results for germanium show experimental evidence for impact ionisation above the threshold energy (‘soft’ threshold), and for multiple ionisation processes contributing to the carrier multiplication. We have also calculated the thresholds for impact ionisation in strained silicon grown on (100) germanium, and for strained germanium grown on (100) silicon. The results show that strained layers fabricated in indirect band-gap semiconductors are potentially attractive for use in low-noise avalanche photodiodes.

Design techniques for subcarrier multiplexed broadcast optical networks

Design techniques which enable the performance of subcarrier multiplexed FM broadcast optical networks to be optimized at the planning stage are described. At the transmitter node, criteria for maximizing the subscriber carrier-to-noise ratio (CNR) are presented. At the subscriber node, it is shown that under the majority of circumstances, the optimum direct detection receiver consists of a low-noise III-V avalanche photodiode and high-impedance front-end preamplifier. Sensitivities approaching -40 dB (1 mW) for 16.5 dB CNR in 36 MHz subscriber bandwidth are predicted for a 60-channel system. Preliminary experimental tests on an optical-feedback subcarrier receiver showed -34 dB (1 mW) sensitivity at 1.55 mu m from 16.5 dB CNR in a 36 MHz bandwidth centered on 1.2 GHz with a 12% modulation index single-channel FM test signal.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Growth and properties of GaSb, Ga 1- xIn xSb and Ga 1- xAl xSb epilayers by MOCVD

GaSb-based compound semiconductors are suitable materials for low-noise avalanche photo-diodes(APD's) and long wavelength laser diodes and photodetectors. In this paper, we review the metal organic chemical vapor deposition (MOCVD) growth conditions and the properties of GaSb, GaInSb and AlGaSb epitaxial layers, including the effects of III/V ratio, growth temperature, pressure and growth rate on electrical quality, optical properties, surface morphology and solid distribution coefficients.

Theory of the GaAs-doped p-i-n quantum well APD

A low-noise, high-gain, and high-bandwidth avalanche photodiode (APD) structure is described. The device is a variation of the p-i-n doped quantum well structure that is expected to show four orders of magnitude enhancement in the carrier ionization rates. In practice, p-i-n doped quantum well devices are difficult to realize owing to the difficulty in achieving highly doped n-type AlGaAs. A structure in which the doped layers are formed in GaAs rather than in AlGaAs, but in which the performance features of the doped AlGaAs devices are retained, is described. The device consists of repeated unit cells of an intrinsic Al/sub 0.45/Ga/sub 0.55/As layer followed by p-i-n-i doped GaAs layers. Calculations based on many-particle ensemble Monte Carlo simulation of the electron and hole ionization rates as a function of the device parameters are presented, illustrating the basic design criteria. >