James Cheng

Ultrahigh Responsivity Visible and Infrared Detection Using Silicon Nanowire Phototransistors

Nanowire photodetectors can perform exceptionally well due to their unique properties arising from the nanowire geometry. Here we report on the phenomenal responsivity and extended spectral range of scalable, vertically etched, silicon nanowire photodetector arrays defined by nanoimprint lithography. The high internal gain in these devices allows for detection at below room temperatures of subfemtowatt per micrometer visible illumination and picowatt infrared illumination resulting from band to surface state generation.

Self-quenching and self-recovering InGaAs∕InAlAs single photon avalanche detector

To prevent device damage through thermal runaway, conventional III–V single photon avalanche diodes (SPADs) operate in gated mode where the device is biased above breakdown only for a short gating period. Here a free-running In0.53Ga0.47As∕InAlAs SPAD with built-in negative feedback mechanism is reported. A physical model is also developed to formulate the avalanche process with negative feedback. Introducing negative feedback enables the device to possess self-quenching and self-recovering capabilities. Such devices have demonstrated free-running single photon detection at 1550nm wavelength with single photon detection efficiency of 11.5%, dark count rate of 3.3M∕s, and a self-recovery time of 60ns at 160K.

Bias Dependence of Sub-Bandgap Light Detection for Core–Shell Silicon Nanowires

We experimentally demonstrate a vertically arrayed silicon nanowire-based device that exhibits voltage dependence of photoresponse to infrared sub-bandgap optical radiation. The device is fabricated using a proximity solid-state phosphorus diffusion method to convert the surface areas of highly boron-doped silicon nanowires into n-type, thus forming a radial core-shell p-n junction structure. Prominent photoresponse from such core-shell Si nanowires is observed under sub-bandgap illumination at 1310 nm. The strong bias dependence of the photoresponse and other device characteristics indicates that the sub-bandgap absorption is attributed to the intrinsic properties of core-shell Si nanowires rather than the surface states. The attractive characteristics are based on three physical mechanisms: the Franz-Keldysh effect, quasi-quantum confinement effect, and the impurity-state assisted photon absorption. The first two effects enhance carrier tunneling probability, rendering a stronger wave function overlap to facilitate sub-bandgap absorption. The last effect relaxes the k-selection rule by involving the localized impurity states, thus removing the limit imposed by the indirect bandgap nature of Si. The presented device uses single-crystal silicon and holds promise of fabricating nanophotonic systems in a fully complementary metal-oxide-semiconductor (CMOS) compatible process. The concept and approach can be applied to silicon and other materials to significantly extend the operable wavelength regime beyond the constraint of energy bandgap.

Self-quenching InGaAs/InP single photon avalanche detector utilizing zinc diffusion rings

InGaAs single photon avalanche detectors have previously been fabricated with a negative-feedback mechanism, which allows for free-running Geiger-mode operation and improves the signal noise. To reduce the dark count and improve the detection efficiency, zinc diffusion is necessary to define the p-i-n junction and separate the high-field region from any mesa surface. Here, we demonstrate the benefits of a simple Zn-diffused geometry, yielding 1550 nm single-photon detection efficiencies of 20% with a dark count rate of 8 kHz at 140 K for a 22 μm diameter device.

Self-Quenched InGaAs Single-Photon Detector

The requirement for external quenching circuits adds substantially to the complexity and processing difficulty for InGaAs single-photon detectors, particularly in array configurations. Using bandgap engineering, we have developed InGaAs SPADs with self-quenching and self-recovering capabilities. The quenching process occurs in less than 100 ps, determined by the gain buildup time and the magnitude of device overbias. On the other hand, the recovery time is determined by the carrier escape time over an energy barrier that is typically tens of meVs. The recovery time can range from 1 ns to > 100 ns from the design of device and material structures. The optimal recovery time is a function of dark count rate and afterpulsing rate. Our data show that a recovery time of around 10 ns is near the optimum in most operation conditions. The self-quenched SPADs also show great suppression in excess noise, yielding a very uniform intensity distribution of output response to single photons. This unique property favors resolving photon number in an array device. As in conventional InGaAs SPADs, the single-photon detection efficiency increases with the amount of overbias (bias above breakdown voltage) and so does the dark count rate. A detection efficiency of 13-16% is obtained while still keeping the dark count and afterpulsing rates low. To our knowledge, the self-quenched InGaAs SPAD is the only device in its class to be able to operate under DC bias without gating or external circuits. As a result, the device is particularly suitable for array structures often used in communications, sensing, and imaging.

Physics of Single Photon Avalanche Detectors With Built-In Self-Quenching and Self-Recovering Capabilities

A single photon avalanche detector featuring a transient carrier buffer layer to form an energy barrier that tentatively stops avalanche-generated carriers, demonstrates self-quenching and self-recovering capabilities. The escape rate of those stopped avalanche carriers from the barrier determines the self-recovery time and thus the count rate of the single photon detector. A physical model has been developed to simulate the dynamic characteristics of the detector. The simulation results agree well with the experimental data, and the self-recovery time is found to be reduced with the increase of the temperature and the overbias magnitude as well as the decrease of the dosage in the charge layer and the barrier height. In addition, thermionic emission shows a stronger dependence on temperature and a weaker dependence on device bias and charge layer dosage than tunneling. The model contains no fitting parameters and therefore can be used to model and predict the device behaviors.

High-sensitivity visible and IR (1550nm) Si nanowire photodetectors

Vertical silicon nanowire detectors with high phototransistive gain have been demonstrated and the principles responsible for the high gain have been reported in recent publications. The emphasis of this paper is (a) the fabrication technology of silicon nanowire array detectors that can be integrated with Si VLSI and (b) the ability of sub-bandgap detection to achieve ultrawide band (from UV to IR) responsivity. We have demonstrated responsivity of greater than 100 A/W at 1550 nm for single crystal silicon nanowires to detect picowatts of IR light, the highest record ever reported for single crystal silicon detectors.

Characterization and physics of top-down silicon nanowire phototransistors

Nanowire photodetectors of a variety of materials have been attracting increased attention due to their potential for very high sensitivity detection. Silicon photodetectors are of particular interest for detection in the visible spectrum, having many benefits including cost of substrate, ease of processing, and ability for integration with conventional fabrication techniques. Using top-down fabrication techniques results in additional benefits of precise control of number, geometry, and placement of these wires. To demonstrate the potential of these devices, top-down, vertical silicon nanowire phototransistor arrays have been fabricated using ebeam lithography and deep reactive ion and inductively coupled plasma etching. These devices show a much higher phototransistive gain over nanowire photodiodes with similar geometry under illumination from a 635nm laser. Low temperature measurements also show the dependence of dark current and sensitivity on temperature. The mechanism responsible for this gain is shown to be dominated by the large surface-to-volume ratio of nanowires where charge capture and recombination at the surface creates a radial gate bias which is modulated with light intensity. 3D numerical simulations validate this mechanism and further show the dependence of device behavior on nanowire doping, geometry, and surface state density. This will allow for the precise engineering of these devices to achieve the maximum sensitivity obtainable as we strive for the ultimate goal of single photon resolution.

Patterned zinc-diffused structures for improved avalanche probabilities in InGaAs/InP single photon detectors

Optimal fabrication of the p-i-n junction in a high performance InGaAs/InP single photon avalanche photodiode is challenging. We present a novel design with patterned Zn-diffusion to greatly improve the detection efficiency and reduce dark count.

Self-recovered InGaAs single photon avalanche detector with patterned Zn-diffused structure

In conventional InGaAs/InP single photon avalanche detectors, zinc diffusion is used to define the multiplication junction to reduce the dark count and maximize the detection efficiency. The device performance is very sensitive to process variations, and the diffusion process must be carefully calibrated and analyzed to minimize any edge breakdown effects. Here we present a much simpler design utilizing patterned zinc diffusion rings. The processing is simplified - a single diffusion compared to two diffusions in a conventional device; and the device performance is not as critical to the processing variations. The diffusion is performed on a self-quenching self-recovering epitaxial structure, resulting in free-running single photon detection efficiencies of 20% at 140 K, with a dark count rate of 8 kHz for a 22μm diameter device.