Matthew Morea

Passivation of multiple-quantum-well Ge0.97Sn0.03/Ge p-i-n photodetectors

We study the effect of surface passivation on pseudomorphic multiple-quantum-well Ge0.97Sn0.03/Ge p-i-n photodetectors. A combination of ozone oxidation to form GeOx and GeSnOx on the surface of the diodes followed by atomic layer deposition of Al2O3 for protection of these native oxides provides reduced dark current. With a temperature-dependent investigation of dark current, we calculate the activation energy to be 0.26 eV at a bias of −0.1 V and 0.05 eV at −1 V for the sample passivated by this ozone method. Based on these activation energy results, we find that the current is less dominated by bulk tunneling at lower reverse bias values; hence, the effect of surface passivation is more noticeable with nearly an order-of-magnitude improvement in dark current for the ozone-passivated sample compared to control devices without the ozone treatment at a voltage of −0.1 V. Passivation also results in a significant enhancement of the responsivity, particularly for shorter wavelengths, with 26% higher respons...

High-sensitivity silicon ultraviolet p+-i-n avalanche photodiode using ultra-shallow boron gradient doping

Photo detection of ultraviolet (UV) light remains a challenge since the penetration depth of UV light is limited to the nanometer scale. Therefore, the doping profile and electric field in the top nanometer range of the photo detection devices become critical. Traditional UV photodetectors usually use a constant doping profile near the semiconductor surface, resulting in a negligible electric field, which limits the photo-generated carrier collection efficiency of the photodetector. Here, we demonstrate, via the use of an optimized gradient boron doping technique, that the carrier collection efficiency and photo responsivity under the UV wavelength region have been enhanced. Furthermore, the ultrathin p+-i-n junction shows an avalanche gain of 2800 and an ultra-low junction capacitance (sub pico-farad), indicating potential applications in the low timing jitter single photon detection area.

Electrical and optical 3D modelling of light-trapping single-photon avalanche diode

Single-photon avalanche diodes (SPADs) have been widely used to push the frontier of scientific research (e.g., quantum science and single-molecule fluorescence) and practical applications (e.g., Lidar). However, there is a typical compromise between photon detection efficiency and jitter distribution. The light-trapping SPAD has been proposed to break this trade-off by coupling the vertically incoming photons into a laterally propagating mode while maintaining a small jitter and a thin Si device layer. In this work, we provide a 3D-based optical and electrical model based on practical fabrication conditions and discuss about design parameters, which include surface texturing, photon injection position, device area, and other features.

Silicon single-photon avalanche diodes with nano-structured light trapping

Silicon single-photon avalanche detectors are becoming increasingly significant in research and in practical applications due to their high signal-to-noise ratio, complementary metal oxide semiconductor compatibility, room temperature operation, and cost-effectiveness. However, there is a trade-off in current silicon single-photon avalanche detectors, especially in the near infrared regime. Thick-junction devices have decent photon detection efficiency but poor timing jitter, while thin-junction devices have good timing jitter but poor efficiency. Here, we demonstrate a light-trapping, thin-junction Si single-photon avalanche diode that breaks this trade-off, by diffracting the incident photons into the horizontal waveguide mode, thus significantly increasing the absorption length. The photon detection efficiency has a 2.5-fold improvement in the near infrared regime, while the timing jitter remains 25 ps. The result provides a practical and complementary metal oxide semiconductor compatible method to improve the performance of single-photon avalanche detectors, image sensor arrays, and silicon photomultipliers over a broad spectral range.The performance of silicon single-photon avalanche detectors is currently limited by the trade-off between photon detection efficiency and timing jitter. Here, the authors demonstrate how a CMOS-compatible, nanostructured, thin junction structure can make use of tailored light trapping to break this trade-off.

Germanium Quantum Well QCSE Waveguide Modulator With Tapered Coupling in Distributed Modulator–Detector System

Optical interconnections (interconnects) have been proposed as solutions to the ever-increasing bandwidth requirements and energy consumption in communication systems. Among possible photonic modulation strategies, the Ge quantum well (QW) based quantum-confined Stark effect (QCSE) stands out, as its strong electro-absorption effect allows for potentially lower power consumption and smaller device sizes compared to other modulation mechanisms. Here, we experimentally demonstrate a thin buffer layer Ge QW QCSE waveguide modulator that evanescently couples to and from an Si waveguide through an adiabatic three-dimensional (3D) taper. Simulations confirm that this 3-D taper yields higher coupling efficiency and improved maintenance of the fundamental mode after coupling compared to a 2-D taper. We also demonstrate that this geometry could potentially work in an integrated modulator-detector system. The combination of thin SiGe epitaxy (i.e., the buffer and device layers) with Si waveguides paves the way to easier integration of Si photonic integrated circuits.

Tensile-strained GeSn photodetectors with conformai nitride stressor

Applying tensile strain with silicon nitride is demonstrated to improve the responsivity of germanium-tin (Ge_1-xSn_x) PIN photodetectors at longer wavelengths. Such external stressor films show promise for extending the application of Ge_1-xSn_x optoelectronic devices into the mid-infrared range.

Optimization of TCR and heat transport in group-IV multiple-quantum-well microbolometers

Group-IV semiconductors have the opportunity to have an equivalent or better temperature coefficient of resistance (TCR) than other microbolometer thermistor materials. By using multiple-quantum-well (MQW) structures, their TCR values can be optimized due to a confinement of carriers. Through two approaches – an activation energy approximation and a custom Monte Carlo transfer matrix method – we simulated this effect for a combination of Group-IV semiconductors and their alloys (e.g., SiGe and GeSn) to find the highest possible TCR, while keeping in mind the critical thicknesses of such layers in a MQW epitaxial stack. We calculated the TCR for a critical-thickness-limited Ge0.8Sn0.2/Ge MQW device to be about -1.9 %/K. Although this TCR is lower than similar SiGe/Si MQW thermistors, GeSn offers possible advantages in terms of fabricating suspended devices with its interesting etch-stop properties shown in previous literature. Furthermore, using finite element modeling of heat transport, we looked at another key bolometer parameter: the thermal time constant. The dimensions of a suspended Ge microbolometer’s supporting legs were fine-tuned for a target response time of 5 ms, incorporating estimations for the size effects of the nanowire-like legs on thermal conductivity.