Thin Multiplication Regions Research Articles

Gain and noise in GaAs/AlGaAs avalanche photodiodes with thin multiplication regions

Avalanche photodiodes based on GaAs/AlGaAs with separated absorption and multiplication regions (SAM-APDs) will be discussed in terms of capacitance, response to light (gain and noise) and time response. The structures have been fabricated by molecular beam epitaxy introducing a δ p layer doped with carbon to separate the multiplication and the absorption regions. The thickness of the latter layer defines the detection efficiency and the time resolution of the structure, which in turn allows tailoring the device for specific scientific applications. Within the multiplication region a periodic modulation of the bandgap is obtained by growing alternating nanometric layers of AlGaAs and GaAs with increasing Al content; this staircase structure enables the tuning of the bandgap and subsequently provides a well-defined charge multiplication. The use of such staircase hetero-junctions enhances electron multiplication and conversely reduces—at least in principle—the impact of the noise associated to hole multiplication, which should result in a decreased overall noise, when compared to p-i-n diodes composed by a single material. The first part of this paper focuses on the electrical characteristics of the grown structure and on the comparison with the simulated behaviour of such devices. In addition, gain and noise measurements, which have been carried out on these devices by utilizing photons from visible light to hard X-rays, will be discussed and will be compared to the results of a nonlocal history-dependent model specifically developed for staircase APDs.

25 Gbps low-voltage waveguide Si–Ge avalanche photodiode

Silicon-germanium (Si–Ge)-based avalanche photodiodes (APDs) have shown a significant improvement in receiver sensitivity compared to their III–V counterparts due to the superior impact ionization property of silicon. However, conventional Si–Ge APDs typically operate at high voltages and low speed, limiting the application of this technology to data communication. In this paper, we present a waveguide Si–Ge avalanche photodiode using a thin silicon multiplication region with a breakdown voltage of −10 V, a speed of 25 GHz, and a gain-bandwidth product (GBP) of 276 GHz. At 1550 nm, sensitivities of −25 dBm and −16 dBm are achieved at 12.5 Gbps and 25 Gbps, respectively. This design will enable implementation of Si–Ge APDs for optical interconnects in data centers and high-performance computers, allowing significant reductions in aggregate system laser power (and therefore cost).

Plasmonic field confinement for separate absorption-multiplication in InGaAs nanopillar avalanche photodiodes.

Avalanche photodiodes (APDs) are essential components in quantum key distribution systems and active imaging systems requiring both ultrafast response time to measure photon time of flight and high gain to detect low photon flux. The internal gain of an APD can improve system signal-to-noise ratio (SNR). Excess noise is typically kept low through the selection of material with intrinsically low excess noise, using separate-absorption-multiplication (SAM) heterostructures, or taking advantage of the dead-space effect using thin multiplication regions. In this work we demonstrate the first measurement of excess noise and gain-bandwidth product in III–V nanopillars exhibiting substantially lower excess noise factors compared to bulk and gain-bandwidth products greater than 200 GHz. The nanopillar optical antenna avalanche detector (NOAAD) architecture is utilized for spatially separating the absorption region from the avalanche region via the NOA resulting in single carrier injection without the use of a traditional SAM heterostructure.

Study of Excess Noise Factor Under Nonlocal Effect in Avalanche Photodiodes

Avalanche photodiodes with thin multiplication regions exhibit a linear relationship between F(M) and M, which differs from the local-field model. This letter presents a study of the relationship between F(M) with M when nonlocal effects are significant. Systematic Monte Carlo simulations on different avalanche photodiodes have been carried out to support the analytical model.

Dual-Carrier High-Gain Low-Noise Superlattice Avalanche Photodiodes

In this paper, novel avalanche photodiode structures with alternate carrier multiplication nanometer regions, placed next to a wider electron multiplication region, to create dual-carrier feedback systems, are proposed. Gain and excess noise factor of these structures are calculated based on the dead space multiplication theory under uniform electric field. In addition, the equivalent impact ionization ratios are derived and compared. It is observed that the proposed structures can generate much higher gain compared with conventional pure electron multiplication structures under the same electric field without severely degrading the excess noise quality. Excess noise is further optimized with careful adjustment of thin multiplication regions' thicknesses. These high-gain structures can operate under low-bias (<; 5 V) conditions, making it possible to integrate infrared avalanche photodiodes (APDs) directly into silicon read-out circuits. In this paper, type-II mid-wavelength infrared InAs/GaSb strained layer superlattice is used for simulation. However, the concept of dual-carrier APDs, with carrier feedback to generate high gain and control of excess noise through confining impact ionization in thin layers, is general and can also be applied to other wavelength APDs with different materials and thicknesses. Type II InAs/GaSb strain layer superlattice allows for versatile band structure design leading to impact ionization coefficient engineering.

Thin 3D Multiplication Regions in Plasmonically Enhanced Nanopillar Avalanche Detectors

We demonstrate a nanopillar (NP) device structure for implementing plasmonically enhanced avalanche photodetector arrays with thin avalanche volumes (∼ 310 nm × 150 nm × 150 nm). A localized 3D electric field due to a core-shell PN junction in a NP acts as a multiplication region, while efficient light absorption takes place via surface plasmon polariton Bloch wave (SPP-BW) modes due to a self-aligned metal nanohole lattice. Avalanche gains of ∼216 at 730 nm at -12 V are obtained. We show through capacitance-voltage characterization, temperature-dependent breakdown measurements, and detailed device modeling that the avalanche region is on the order of the ionization path length, such that dead-space effects become significant. This work presents a clear path toward engineering dead space effects in thin 3D-confined multiplication regions for high performance avalanche detectors for applications in telecommunications, sensing and single photon detection.

Ultra Low Dark Current, High Responsivity and Thin Multiplication Region in InGaAs/InP Avalanche Photodiodes

A separate absorption, grading, charge and multiplication InGaAs/InP avalanche photodiode with ultra low dark current and high responsivity is demonstrated. It has a thin multiplication layer and a planar structure. Through the use of a well and a single floating guard ring to suppress edge breakdown, the device can easily be fabricated by one step epitaxial growth and one step diffusion. The dark current of a 30 μm diameter device is as low as 0.028 nA at punch-through and 0.1 nA at 90% of the breakdown voltage. The responsivity at 1.55 μm is 0.93 A/W at unity gain and the multiplication layer is estimated to be less than 300 nm.

Bit error rates for ultrafast APD based optical receivers: exact and large deviation based asymptotic approaches

Exact analysis as well as asymptotic analysis, based on large-deviation theory (LDT), are developed to compute the bit-error rate (BER) for ultrafast avalanche-photodiode (APD) based optical receivers assuming on-off keying and direct detection. The effects of intersymbol interference (ISI), resulting from the APD's stochastic avalanche buildup time, as well as the APD's dead space are both included in the analysis. ISI becomes a limiting factor as the transmission rate approaches the detector's bandwidth, in which case the bit duration becomes comparable to APD's avalanche buildup time. Further, the effect of dead space becomes significant in high-speed APDs that employ thin avalanche multiplication regions. While the exact BER analysis at the generality considered here has not been reported heretofore, the asymptotic analysis is a major generalization of that developed by Letaief and Sadowsky [IEEE Trans. Inform. Theory, vol. 38, 1992], in which the LDT was used to estimate the BER assuming APDs with an instantaneous response (negligible avalanche buildup time) and no dead space. These results are compared with those obtained using the common Gaussian approximation approach showing the inadequacy of the Guassian approximation when ISI noise has strong presence.

Detection Efficiencies and Generalized Breakdown Probabilities for Nanosecond-Gated Near Infrared Single-Photon Avalanche Photodiodes

A rigorous model is developed for determining single-photon quantum efficiency (SPQE) of single-photon avalanche photodiodes (SPADs) with simple or heterojunction multiplication regions. The analysis assumes nanosecond gated-mode operation of the SPADs and that band-to-band tunneling of carriers is the dominant source of dark current in the multiplication region. The model is then utilized to optimize the SPQE as a function of the applied voltage, for a given operating temperature and multiplication-region structure and material. The model can be applied to SPADs with In/sub 0.52/Al/sub 0.48/As or InP multiplication regions as well as In/sub 0.52/Al/sub 0.48/As--InP heterojunction multiplication regions for wavelengths of 1.3 and 1.55 /spl mu/m. The predictions show that the SPQE generally decreases with decreasing the multiplication-region thickness. Moreover, an InP multiplication region requires a lower breakdown electric field (and, hence, offers a higher SPQE) than that required by an In/sub 0.52/Al/sub 0.48/As layer of the same width. The model also shows that the fractional width of the In/sub 0.52/Al/sub 0.48/As layer in an In/sub 0.52/Al/sub 0.48/As--InP heterojunction multiplication region can be optimized to attain a maximum SPQE that is greater than that offered by an InP multiplication region. This effect becomes more pronounced in thin multiplication regions as a result of the increased significance of dead space.

Computation of bit-error probabilities for optical receivers using thin avalanche photodiodes

The large-deviation-based asymptotic-analysis and importance-sampling methods for computing bit-error probabilities for avalanche-photodiode (APD) based optical receivers, developed by Letaief and Sadowsky [IEEE Trans. Inform. Theory, vol. 38, pp. 1162-1169, 1992], are extended to include the effect of dead space, which is significant in high-speed APDs with thin multiplication regions. It is shown that the receiver's bit-error probability is reduced as the magnitude of dead space increases relative to the APD's multiplication-region width. The calculated error probabilities and receiver sensitivities are also compared with those obtained from the Chernoff bound.

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.

Performance of thin 4H-SiC UV avalanche photodiodes

The large bandgap of 4H-SiC (3.25 eV) makes it a suitable material for visible-blind UV detection. In the paper, the performance of 4H-SiC avalanche photodiodes (APDs) with a thin avalanche width of 0.1 µm is evaluated. Avalanche photodiodes with thin multiplication regions can greatly improve the signal-to-noise ratio of photoreceiver systems by providing internal gain while maintaining a high operating speed and low operating voltage. The diodes exhibit a peak unity-gain responsivity of 144 mA/W at a wavelength of 265 nm. Photomultiplication measurements carried out on these diodes showed that β>α in 4H-SiC, where β and α are the hole and electron ionisation coefficients, respectively. The 4H-SiC APDs also exhibit very low excess noise corresponding to k=0.1 (where k=α/β for hole multiplication) in the local model when illuminated by 325 nm light. This is much lower than that of commonly used Si APDs with identical thickness and indicates that 4H-SiC is well suited for high gain, low noise UV detection. In view of the large β/α ratio measured in these thin 4H-SiC APDs, multiplication must be initiated by hole injection to ensure a low excess-noise performance.

Analysis of breakdown probabilities in avalanche photodiodes using a history-dependent analytical model

The breakdown probabilities of avalanche photodiodes (APDs) working in the Geiger mode are analyzed using a history-dependent analytical impact-ionization model [R. J. McIntyre, IEEE Trans. Electron Devices 46, 1623 (1999)]. The breakdown sharpness in devices with thin and thick multiplication regions is found to follow the same trend in GaAs, InAlAs, and InP material systems. Breakdown characteristics of InP and InAlAs are compared for their applications in photon counting at telecommunication wavelengths.

Effect of thermal annealing of radiation defects on the noise characteristics of silicon p-n structures with a thin multiplication region

Specific features of changes in the noise spectral density and the lifetime of minority charge carriers in Si p-n structures with a thin multiplication region under thermal annealing of radiation defects were studied. It is shown that the change in the frequency characteristics of noise in p-n structures is related to the recovery of surface states. A qualitative explanation of self-limitation of the avalanche process in p-n structures with a thin multiplication region is suggested.

Multiplication noise of AlxGa1−xAs avalanche photodiodes with high Al concentration and thin multiplication region

We report that homojunction AlxGa1−xAs avalanche photodiodes (APDs) exhibit very low multiplication noise when the Al content is ⩾80%. It was also found that, due to nonlocal effects, the multiplication noise decreased as the ionization region thickness was reduced from 0.8 μm to ⩽0.2 μm for Al ratios (from 0 to 0.9). The excess noise factor of the thin (140 nm) Al0.9Ga0.1As APDs is the lowest reported to date for III–V compounds and is comparable to that of Si avalanche photodiodes.

Impact-ionization and noise characteristics of thin III-V avalanche photodiodes

It is, by now, well known that McIntyre's localized carrier-multiplication theory cannot explain the suppression of excess noise factor observed in avalanche photodiodes (APDs) that make use of thin multiplication regions. We demonstrate that a carrier multiplication model that incorporates the effects of dead space, as developed earlier by Hayat et al. provides excellent agreement with the impact-ionization and noise characteristics of thin InP, In/sub 0.52/Al/sub 0.48/As, GaAs, and Al/sub 0.2/Ga/sub 0.8/As APDs, with multiplication regions of different widths. We outline a general technique that facilitates the calculation of ionization coefficients for carriers that have traveled a distance exceeding the dead space (enabled carriers), directly from experimental excess-noise-factor data. These coefficients depend on the electric field in exponential fashion and are independent of multiplication width, as expected on physical grounds. The procedure for obtaining the ionization coefficients is used in conjunction with the dead-space-multiplication theory (DSMT) to predict excess noise factor versus mean-gain curves that are in excellent accord with experimental data for thin III-V APDs, for all multiplication-region widths.

Avalanche noise characteristics of thin GaAs structures with distributed carrier generation [APDs

It is known that both pure electron and pure hole injection into thin GaAs multiplication regions gives rise to avalanche multiplication with noise lower than predicted by the local noise model. In this paper, it is shown that the noise from multiplication initiated by carriers generated throughout a 0.1 /spl mu/m avalanche region is also lower than predicted by the local model but higher than that obtained with pure injection of either carrier type. This behavior is due to the effects of nonlocal ionization brought about by the dead space; the minimum distance a carrier has to travel in the electric field to initiate an ionization event.

High Speed Resonant-Cavity InGaAs/InAlAs Avalanche Photodiodes

The evolution of long-haul optical fiber telecommunications systems to bit rates greater than 10 GB/s has created a need for avalanche photodiodes (APDs) with higher bandwidths and higher gain-bandwidth products than are currently available. It is also desirable to maintain good quantum efficiency and low excess noise. At present, the best performance (f3dB ~ 15 GHz at low gain and gain-bandwidth product ~ 150 GHz) has been achieved by AlInAs/InGaAs(P) multiple quantum well (MQW) APDs. In this paper we report a resonant-cavity InAlAs/InGaAs APD that operates near 1.55 μm. These APDs have achieved very low noise (k equivalent to 0.18) as a result of the very thin multiplication regions that were utilized. The low noise is explained in terms of a new model that accounts for the non-local nature of impact ionization. A unity-gain bandwith of 24 GHz and a gain-bandwidth-product of 290 GHz were achieved.

HIGH SPEED RESONANT-CAVITY InGaAs/InAlAs AVALANCHE PHOTODIODES

The evolution of long-haul optical fiber telecommunications systems to bit rates greater than 10 GB/s has created a need for avalanche photodiodes (APDs) with higher bandwidths and higher gain-bandwidth products than are currently available. It is also desirable to maintain good quantum efficiency and low excess noise. At present, the best performance (f3dB ~ 15 GHz at low gain and gain-bandwidth product ~ 150 GHz) has been achieved by AlInAs/InGaAs(P) multiple quantum well (MQW) APDs. In this paper we report a resonant-cavity InAlAs/InGaAs APD that operates near 1.55 μm. These APDs have achieved very low noise (k equivalent to 0.18) as a result of the very thin multiplication regions that were utilized. The low noise is explained in terms of a new model that accounts for the non-local nature of impact ionization. A unity-gain bandwith of 24 GHz and a gain-bandwidth-product of 290 GHz were achieved.

Dead-space-based theory correctly predicts excess noise factor for thin GaAs and AlGaAs avalanche photodiodes

The conventional McIntyre carrier multiplication theory for avalanche photodiodes (APDs) does not adequately describe the experimental results obtained from APDs with thin multiplication-regions. Using published data for thin GaAs and Al/sub 0.2/Ga/sub 0.8/As APDs, collected from multiplication-regions of different widths, we show that incorporating dead-space in the model resolves the discrepancy. The ionization coefficients of enabled carriers that have traveled the dead space are determined as functions of the electric field, within the confines of a single exponential model for each device, independent of multiplication-region width. The model parameters are determined directly from experimental data. The use of these physically based ionization coefficients in the dead-space multiplication theory, developed earlier by Hayat et al. provide excess noise factor versus mean gain curves that accord very closely with those measured for each device, regardless of multiplication-region width. It is verified that the ratio of the dead-space to the multiplication-region width increases, for a fixed mean gain, as the width is reduced. This behavior, too, is in accord with the reduction of the excess noise factor predicted by the dead-space multiplication theory.