Muyu Xue

Contact Selectivity Engineering in a 2 μm Thick Ultrathin c-Si Solar Cell Using Transition-Metal Oxides Achieving an Efficiency of 10.8%

In this paper, the integration of metal oxides as carrier-selective contacts for ultrathin crystalline silicon (c-Si) solar cells is demonstrated which results in an ∼13% relative improvement in efficiency. The improvement in efficiency originates from the suppression of the contact recombination current due to the band offset asymmetry of these oxides with Si. First, an ultrathin c-Si solar cell having a total thickness of 2 μm is shown to have >10% efficiency without any light-trapping scheme. This is achieved by the integration of nickel oxide (NiOx) as a hole-selective contact interlayer material, which has a low valence band offset and high conduction band offset with Si. Second, we show a champion cell efficiency of 10.8% with the additional integration of titanium oxide (TiOx), a well-known material for an electron-selective contact interlayer. Key parameters including Voc and Jsc also show different degrees of enhancement if single (NiOx only) or double (both NiOx and TiOx) carrier-selective contacts are integrated. The fabrication process for TiOx and NiOx layer integration is scalable and shows good compatibility with the device.

Tensile-strained Ge/SiGe multiple quantum well microdisks

An efficient monolithically integrated laser on Si remains the missing component to enable Si photonics. We discuss the design and fabrication of suspended and tensile-strained Ge/SiGe multiple quantum well microdisk resonators on Si for laser applications in Si photonics using an all-around SiNx stressor. An etch-stop technique in the Ge/SiGe system is demonstrated and allows the capability of removing the defective buffer layer as well as providing precise thickness control of the resonators. Photoluminescence and Raman spectroscopy indicate that we have achieved a biaxial tensile strain shift as high as 0.88% in the microdisk resonators by adding a high-stress SiNx layer. Optical gain calculations show that high positive net gain can be achieved in Ge quantum wells with 1% external biaxial tensile strain.

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.

Titanium oxide contact passivation layer for thin film crystalline silicon solar cells

Thin film crystalline silicon (c-Si) solar cells have been a hot topic of photovoltaic research recently because its lower material consumption could potentially lead to lower capital expenditure. However, contact recombination is more prominent in thin-film c-Si solar cells compared with it in traditional c-Si solar cells due to higher carrier concentration. To address such a challenge, this work presents a design of metal-insulator-semiconductor (MIS) contact, based on thin TiOx layer that is grown by atomic layer deposition (ALD). Transmission line measurement (TLM) was conducted to study the conducting behavior of the TiOx MIS contact structure. Experimental results demonstrate that with the same doping density in silicon, the TiOx MIS contact forms an Ohmic contact to n-type silicon with good conductivity while cannot form Ohmic contact with p-silicon. This result demonstrates that the ALD TiOx layer can conduct electrons while blocking holes, thereby potentially reduce the contact recombination for thin-film c-Si solar cells, leading to an improvement of cell efficiency.

Tensile-strained Ge/SiGe quantum-well microdisks with overlying SiN x stressors

We demonstrate Ge/SiGe multiple-quantum-well microdisks on Si substrates with SiNx stressors on top. The strain transferred from the SiNx to the Ge quantum wells are determined by photoluminescence and Raman measurements, and are in agreement with simulation results.

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.

Comprehensive modeling on luminescent coupling dependency in multi-junction solar cells

In this work, we first proposed a comprehensive numerical simulation for luminescent coupling efficiency (LCE) in a complete multi-junction (MJ) configuration and discussed its dependencies on temperature, incident light spectrum and bias voltage. We used finite difference time domain (FDTD) method and detailed balance principle to establish an optical transfer model. Based on that, we revealed the predominant influence factors of the simulation results in detail. This work will certainly fill a knowledge gap in understanding LCE and provide guidance to MJ photovoltaic device design and optimization as well.

Carrier-selective interlayer materials for silicon solar cell contacts

This work presents titanium oxide (TiOx) and nickel oxide (NiOx) as promising carrier-selective interlayer materials for metal-interlayer–semiconductor contacts for silicon solar cells. The electron-conducting, hole-blocking behavior of TiOx and the opposite carrier-selective behavior of NiOx are investigated using the transmission-line-method. The Fermi level depinning effect and the tunneling resistance are demonstrated to be dependent on the interlayer oxide thickness and annealing temperature. NiOx is furthermore experimentally demonstrated to be capable of improving the effective minority carrier lifetime by quasi-steady-state photoconductance method. Our study demonstrates that TiOx and NiOx can be effective carrier-selective materials for Si solar cells and provides a framework for characterizing carrier-selective contacts.

Simulations of taper designs for integrated Ge/SiGe waveguide system

We simulated different taper designs and demonstrated that 3D tapers improved the coupling efficiencies and maintained the fundamental mode as they provided a more gradual optical transition between a Si waveguide and Ge/SiGe device layers.

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.