Research Highlights
1. Light Absorption Engineering for Enhanced Performance of Optoelectronic Devices
ACS Nano, In Press (2018) | Science Advances 3(7), e1602783 (2017) | Optics Express 24(15), 16894 (2016)
1. Light Absorption Engineering for Enhanced Performance of Optoelectronic Devices
ACS Nano, In Press (2018) | Science Advances 3(7), e1602783 (2017) | Optics Express 24(15), 16894 (2016)
Figure 1. (1) Array of pyramid on Ge surface using inverse-metal assisted chemical etching (I-MacEtch) and enhanced performance of Ge photoiodes. (2) Ultrathin Ge nanomembrane (Ge NM, < 20 nm) on pre-designed nanocavities for enhanced light absorption.
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Surface antireflection micro and nanostructures, normally formed by conventional reactive ion etching, offer advantages in photovoltaic and optoelectronic applications, including wider spectral wavelength ranges and acceptance angles. One challenge in incorporating these structures into devices is that optimal optical properties do not always translate into electrical performance due to surface damage, which significantly increases surface recombination. In this research, we present a simple approach for fabricating antireflection structures, with self-passivated amorphous Ge (α-Ge) surfaces, on single crystalline Ge (c-Ge) surface using the inverse metal-assisted chemical etching technology (I-MacEtch). Vertical Schottky Ge photodiodes fabricated with surface structures involving arrays of pyramids or periodic nano-indentations show clear improvements not only in responsivity, due to enhanced optical absorption, but also in dark current. The dark current reduction is attributed to the Schottky barrier height increase and self-passivation effect of the i-MacEtch induced α-Ge layer formed on top of the c-Ge surface. The results demonstrated in this work show that MacEtch can be a viable technology for advanced light trapping and surface engineering in Ge and other semiconductor based optoelectronic devices.
Miniaturization of optoelectronic devices offers tremendous performance gain. As the volume of photoactive material decreases, optoelectronic performance improves, including the operation speed, the signal-to-noise ratio, and the internal quantum efficiency. Over the past decades, researchers have managed to reduce the volume of photoactive materials in solar cells and photodetectors by orders of magnitude. However, two issues arise when one continues to thin down the photoactive layers to the nanometer scale (for example, <50 nm). First, light-matter interaction becomes weak, resulting in incomplete photon absorption and low quantum efficiency. Second, it is difficult to obtain ultrathin materials with single-crystalline quality. We introduce a method to overcome these two challenges simultaneously. It uses conventional bulk semiconductor wafers, such as Si, Ge, and GaAs, to realize single-crystalline films on foreign substrates that are designed for enhanced light-matter interaction. We use a high-yield and high-throughput method to demonstrate nanometer-thin photodetectors with significantly enhanced light absorption based on nanocavity interference mechanism. These single-crystalline nanomembrane photodetectors also exhibit unique optoelectronic properties, such as the strong field effect and spectral selectivity.
Miniaturization of optoelectronic devices offers tremendous performance gain. As the volume of photoactive material decreases, optoelectronic performance improves, including the operation speed, the signal-to-noise ratio, and the internal quantum efficiency. Over the past decades, researchers have managed to reduce the volume of photoactive materials in solar cells and photodetectors by orders of magnitude. However, two issues arise when one continues to thin down the photoactive layers to the nanometer scale (for example, <50 nm). First, light-matter interaction becomes weak, resulting in incomplete photon absorption and low quantum efficiency. Second, it is difficult to obtain ultrathin materials with single-crystalline quality. We introduce a method to overcome these two challenges simultaneously. It uses conventional bulk semiconductor wafers, such as Si, Ge, and GaAs, to realize single-crystalline films on foreign substrates that are designed for enhanced light-matter interaction. We use a high-yield and high-throughput method to demonstrate nanometer-thin photodetectors with significantly enhanced light absorption based on nanocavity interference mechanism. These single-crystalline nanomembrane photodetectors also exhibit unique optoelectronic properties, such as the strong field effect and spectral selectivity.
2. Free-Standing Single Crystalline Wide-Bandgap Semiconductor Nanomembranes
Journal of Materials Chemistry C 5(2), 264 (2017) | Journal of Materials Chemistry C 5(33), 8338 (2017)
Journal of Materials Chemistry C 5(2), 264 (2017) | Journal of Materials Chemistry C 5(33), 8338 (2017)
Figure 2. (1) Fabrication process of 4H-SiC on insulator (4H-SiCOI) using Smart-Cut technique and subsequent transfer-printing of 4H-SiC nanomembranes (4H-SiC NMs). (2) Optical images of the fabricated 4H-SiCOI and transfer-printed 4H-SiC NMs
Free-standing single crystalline semiconductor membranes have gained intensive attention over the last few years due to their versatile usage in many applications. This material platform possesses a high level of material quality similar to their bulk counterparts because single crystallinity is maintained. Si, Ge, and III–V based membranes have been widely studied for flexible electronic and optoelectronic devices such as thin-film transistors and photodetectors. However, the current status of research and development on free-standing single crystalline wide band-gap membranes is at a relatively early stage compared to IV and III–V based membranes. We have fabricated single crystalline 4H-SiC on insulator (4H-SiCOI) using Smart-Cut technique and successfully transfer 4H-SiC nanomembranes onto flexible substrates. This material platform has high potential for applications on ultraviolet (UV) optoelectronics and high power electronic devices.
3. High-Performance Green Flexible Electronics
Nature Communications 6, 7170 (2015)
Nature Communications 6, 7170 (2015)
Figure 3. Optical images of green flexible GaAs heterojunction bipolar transistors (HBTs) and Si digital circuits on biodegradable celluose nanofibril paper.
Today’s consumer electronics, such as cell phones, tablets and other portable electronic devices, are typically made of non-renewable, non-biodegradable, and sometimes potentially toxic (for example, gallium arsenide) materials. These consumer electronics are frequently upgraded or discarded, leading to serious environmental contamination. Thus, electronic systems consisting of renewable and biodegradable materials and minimal amount of potentially toxic materials are desirable. In this research, we report high-performance flexible microwave and digital electronics that consume the smallest amount of potentially toxic materials on biobased, biodegradable and flexible cellulose nanofibril papers. Furthermore, we demonstrate gallium arsenide microwave devices, the consumer wireless workhorse, in a transferrable thin-film form. Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using ecofriendly materials.
