Si-Chen Lee

Near-unity photoluminescence quantum yield in MoS2

Brighter molybdenum layers The confined layers of molybdenum disulphide (MoS2) exhibit photoluminescence that is attractive for optolectronic applications. In practice, efficiencies are low, presumably because defects trap excitons before they can recombine and radiate light. Amani et al. show that treatment of monolayer MoS2 with a nonoxidizing organic superacid, bis(trifluoromethane) sulfonimide, increased luminescence efficiency in excess of 95%. The enhancement mechanism may be related to the shielding of defects, such as sulfur vacancies. Science, this issue p. 1065 Superacid treatment enhances the luminescence efficiency of monolayer molybdenum disulfide from 1% to >95%. Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low. The prototypical 2D material molybdenum disulfide (MoS2) is reported to have a maximum QY of 0.6%, which indicates a considerable defect density. Here we report on an air-stable, solution-based chemical treatment by an organic superacid, which uniformly enhances the photoluminescence and minority carrier lifetime of MoS2 monolayers by more than two orders of magnitude. The treatment eliminates defect-mediated nonradiative recombination, thus resulting in a final QY of more than 95%, with a longest-observed lifetime of 10.8 ± 0.6 nanoseconds. Our ability to obtain optoelectronic monolayers with near-perfect properties opens the door for the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.

Super-gain AlGaAs/GaAs heterojunction bipolar transistors using an emitter edge-thinning design

A novel emitter edge‐thinning structure was adopted for Npn Al0.5Ga0.5As/GaAs single heterojunction bipolar transistors grown by liquid phase epitaxy. In this structure, the emitter edge was etched down to approximately 0.1 μm thick such that the surface and emitter‐base junction depletion regions could touch each other. As a result, the current is blocked from the emitter periphery and the surface leakage current is reduced which improves the current gain especially at low operating current. The best device thus obtained shows a common emitter current gain of 12 500 at a collector current of 50 mA which is the highest gain reported to date for the heterojunction bipolar transistors. The current gain characteristics were indeed improved especially at a collector current below 10 μA.

Near-room-temperature operation of an InAs/GaAs quantum-dot infrared photodetector

A ten-stacked self-assembled InAs/GaAs quantum-dot infrared photodetector operated in the 2.5–7 μm range by photovoltaic and photoconductive mixed-mode near-room-temperature operation (⩾250 K) was demonstrated. The specific peak detectivity D* is 2.4×108 cm Hz1/2/W at 250 K. The use of high-band-gap Al0.3Ga0.7As barriers at both sides of the InAs quantum-dot structure and the long carrier recombination time are the key factors responsible for its near-room-temperature operation.

Effect of porosity on infrared‐absorption spectra of silicon dioxide

The infrared‐absorption spectra of silicon dioxide have been studied for many years and most of the peaks have been identified. During the investigation of silicon dioxide deposited by the liquid‐phase‐deposition technique, an interesting phenomenon was observed. It was found that the intensity ratio between the side lobe (1200 cm−1) and main peak (1090 cm−1) varies and depends on both the concentration of boric acid and the dilution ratio of the growth solution. Measurement of the refractive index shows that the material with larger absorption at 1200 cm−1 has a smaller index and thus more porous structure; therefore, the peak at 1200 cm−1 is suggested to arise from porous oxide, i.e., Si—O—Si, in a large void.

Amorphous SiC/Si three‐color detector

A hydrogenated amorphous SiC/hydrogenated amorphous Si heterojunction photodetector whose peak response could be voltage adjusted to three wavelengths, i.e., 480, 530, and 575 nm, by applying a small bias within ±2 V has been successfully fabricated. The basic principle is to use two back‐to‐back p‐i‐n junction diodes (or an n‐i‐p‐i‐n transistor) in which photons with wavelength λ<500 nm (blue) are mainly collected in the front a‐SiC:H/a‐Si:H heterojunction and the rest (green and red) are absorbed in the rear a‐Si:H homojunction. To further distinguish the green from the red, two undoped a‐Si:H layers, deposited at different conditions, were used in the rear homojunction to obtain two distinct collection regions. It is found that the required voltage to select one of the collection regions is less than 2 V. This detector shows a very high rejection ratio at various responses and thus is good for distinguishing the entire color spectrum.

High performance midinfrared narrow-band plasmonic thermal emitter

The blackbody radiation spectrum is fundamental to any thermal emitter. However, by properly designing the emitter structure, a narrow bandwidth and high power infrared source can be achieved. This invention consists of a triple layer structure by sandwiching a dielectric SiO2 layer between two Ag metal films on the Si substrate. The top Ag layer is perforated by periodic holes. When the device was heated, the background thermal radiation was suppressed by the bottom Ag whose emissivity is very low. The thermal radiation generated in the SiO2 layer resonant between two metal films and the Ag∕SiO2 and the Ag/air surface plasmon polaritons are induced and converted to light radiation. Strong resonance at Ag∕SiO2 (1,0) degenerate modes results in the coherent light radiation at the wavelength associated with the dielectric constant of SiO2 and the lattice constant of the perforated hole array. The ratio of the full width at half maximum to the peak wavelength is 0.114. This narrow bandwidth and high power in...

Origin of high offset voltage in an AlGaAs/GaAs heterojunction bipolar transistor

The high offset voltage of an Npn AlGaAs/GaAs heterojunction bipolar transistor prepared by liquid phase epitaxy is proved to be equal to the turn‐on voltage difference between emitter‐base heterojunction and base‐collector homojunction. The potential spike at the emitter‐base heterointerface is experimentally confirmed by observing an activated transport process and a reachthrough effect under reverse operation. It is believed that the electron thermionic emission process plays an important role in determining the p‐n junction I‐V characteristics.

Oxidation of silicon nitride prepared by plasma‐enhanced chemical vapor deposition at low temperature

Hydrogenated amorphous silicon nitride (a‐SiNx:H) films have been fabricated by plasma‐enhanced chemical vapor deposition at temperatures ranging from 50 to 250 °C. It is found that as soon as the samples are taken out from the reaction chamber and exposed to the atmosphere, the a‐SiNx:H films start to oxide. The oxidation processes are monitored using infrared absorption spectroscopy. A model of porous ‘‘fractal‐like network’’ structure, which is probably inherent in low‐temperature deposition, is proposed to explain why moisture (H2O) in the air can percolate through numerous microvoids into these films. The H2O molecules which percolate into these porous films are active to react with the —Si—N—Si—, —Si—N—H, and —N—Si—‐H bonds and to form more chemically stabilized —Si—O—Si—, —Si—O—H, and H—O—H bond configurations with the result of eventual oxidization of the entire nitride films.

High-temperature operation normal incident 256/spl times/256 InAs-GaAs quantum-dot infrared photodetector focal plane array

In this letter, a 256/spl times/256 midwavelength infrared focal plane array (FPA) based on 30-period InAs-GaAs quantum-dot infrared photodetectors (QDIPs) is fabricated. The demonstrated original real-time nonuniformity corrected thermal images of hot soldering iron head with 30-Hz frame rate for the FPA are observed. Without additional light-coupling scheme, the QDIP FPA module is first operated at temperatures higher than 135 K under normal-incident condition with a 30/spl deg/ field of view and f/2 optics. For single device performances, a similar QDIP device with a 30-period InAs-GaAs QD structure is fabricated under the same processing procedure. High specific detectivity D/sup */ 1.5/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W and low noise current density 5.3/spl times/10/sup -13/ A/Hz/sup 1/2/ at applied voltage 0.3 V are observed.

Structural, optical, and electrical properties of hydrogenated amorphous silicon germanium alloys

Hydrogenated amorphous silicon germanium alloys (a-SiGe:H) have been prepared by rf glow discharge of silane, germane, and hydrogen gas mixture at substrate temperature of 200 and 250 °C. The structural properties of the films have been investigated by infrared, Raman, and secondary ion mass spectroscopy. It is found that there is a preferential incorporation of germanium into the film relative to silicon and the films with high germane gas phase composition Xg>0.4 tend to oxidize in atmosphere. Besides, polysilane is enhanced in the films with low germane gas phase composition. The electrical properties including dark, photo conductivities, and conduction activation energy are measured. As for the optical properties, optical transmission is adopted to determine the optical gap while photoluminescence spectra together with temperature variation are used to study the band tail states of the films. By applying Brodsky’s quantum well model, the various optical and electrical properties could be explained suc...