Ching-Ying Lu

Microring bio-chemical sensor with integrated low dark current Ge photodetector

An integrated Ge photodetector of very low dark current density is demonstrated in an optoelectronic integrated circuit label-free biosensing system. The sensor system consists of a microring for optical sensing and a monolithically integrated Ge detector. For point-of-care applications, integration of Ge detector increases the reliability of measurement by eliminating mechanical-optical alignment of output signals. Optimizing Ge detector performance will further enhance system signal-noise ratio and reliability. For homogeneous sensing, the system has a sensitivity of ∼18.8 nm/RIU and a detection limit of 3.50 × 10−5.

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.

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.

Titanium oxide electron-selective layers for contact passivation of thin-film crystalline silicon solar cells

In crystalline silicon (c-Si) solar cells, carrier selective contacts are among the remaining issues to be addressed in order to reach the theoretical efficiency limit. Especially in ultra-thin-film c-Si solar cells with small volumes and higher carrier concentrations, contact recombination is more critical to the overall performance. In this paper, the advantages of using TiOX as electron-selective layers for contact passivation in c-Si solar cells are analyzed. We characterize the metal/TiOX/n-Si electron-selective contact with the contact recombination factor J0c and the contact resistivity ρc for the first time. Experimental results show that both J0c and ρc decrease after the insertion of TiOX. In addition, the effect of post-deposition rapid-thermal-annealing (RTA) at different temperatures is also evaluated. The best J0c of 5.5 pA/cm2 and the lowest ρc of 13.6 mΩ·cm2 are achieved after the RTA process. This work reveals the potential of TiOX as an electron-selective layer for contact passivation to enable high-efficiency ultra-thin c-Si solar cells with a low cost.

A new electro-absorption modulator structure based on Ge/SiGe coupled quantum wells for on-chip optical interconnects

In this paper, a novel electro-absorption modulation mechanism based on coupled-quantum-wells (CQWs) is proposed and demonstrated. Compared to a quantum-confined-stark-effect (QCSE) modulator with multiple fully decoupled single-QWs, the newly designed CQW modulator has two sub-quantum-wells partially coupled with a small barrier in between. Modulation is based on the change of electron and hole wave-function overlap in the CQWs, which requires a small bias electric field of <10 kV/cm) compared to the operation of a typical QCSE modulator which requires >50 kV/cm bias electrical field. Theoretically, the power consumption of this new CQW modulator can be lower than 20 fJ/bit and the speed can be higher than 10 Gbps, which outperforms the best Ge/SiGe QCSE modulator that has been previously demonstrated. A proof-of-concept Ge/SiGe CQW modulator based on this novel modulation mechanism was designed and fabricated. Instead of a traditional PIN diode structure, the new CQW modulator uses a PIP structure.

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.

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.