Liqin Su

Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-layer MoS2 Films

Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research.

Equally Efficient Interlayer Exciton Relaxation and Improved Absorption in Epitaxial and Nonepitaxial MoS2/WS2 Heterostructures

Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS(2)/WS(2) heterostructures consisting of monolayer MoS(2) and WS(2) stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS(2)/WS(2) heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS(2) and WS(2). Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.

Surface-Energy-Assisted Perfect Transfer of Centimeter-Scale Monolayer and Few-Layer MoS2 Films onto Arbitrary Substrates

The transfer of synthesized 2D MoS2 films is important for fundamental and applied research. However, it is problematic to translate the well-established transfer processes for graphene to MoS2 due to different growth mechanisms and surface properties. Here we demonstrate a surface-energy-assisted process that can perfectly transfer centimeter-scale monolayer and few-layer MoS2 films from original growth substrates onto arbitrary substrates with no observable wrinkles, cracks, and polymer residues. The unique strategies used in this process include leveraging the penetration of water between hydrophobic MoS2 films and hydrophilic growth substrates to lift off the films and dry transferring the film after the lift off. This is in stark contrast with the previous transfer process for synthesized MoS2 films, which explores the etching of the growth substrate by hot base solutions to lift off the films. Our transfer process can effectively eliminate the mechanical force caused by bubble generations, the attacks from chemical etchants, and the capillary force induced when transferring the film outside solutions as in the previous transfer process, which consists of the major causes for the previous unsatisfactory transfer. Our transfer process also benefits from using polystyrene (PS), instead of poly(methyl methacrylate) (PMMA) that was widely used previously, as the carrier polymer. PS can form more intimate interaction with MoS2 films than PMMA and is important for maintaining the integrity of the film during the transfer process. This surface-energy-assisted approach can be generally applied to the transfer of other 2D materials, such as WS2.

Effects of substrate type and material-substrate bonding on high-temperature behavior of monolayer WS2

This study reveals that the interaction between a 2D material and its substrate can significantly modify its electronic and optical properties, and thus can be used as a means to optimize these properties. High-temperature (25–500 °C) optical spectroscopy, which combines Raman and photoluminescence spectroscopies, is highly effective for investigating the interaction and material properties that are not accessible at the commonly used cryogenic temperature (e.g., a thermal activation process with an activation of a major fraction of the bandgap). This study investigates a set of monolayer WS2 films, either directly grown on sapphire and SiO2 substrates by CVD or transferred onto SiO2 substrate. The coupling with the substrate is shown to depend on the substrate type, the materialsubstrate bonding (even for the same substrate), and the excitation wavelength. The inherent difference in the states of strain between the as-grown and the transferred films has a significant impact on the material properties.

Spatially resolved study of quantum efficiency droop in InGaN light-emitting diodes

We investigate the spatial variation of the external quantum efficiency (EQE) of InGaN light-emitting diodes. Two different types of EQE droop are examined in one single device, offering unambiguous analyses on the underlying material physics without the complications of the processing variation. The interplays of microscopic defects, extended defects, and energy fluctuation dictate the mechanisms of the droop, which represents a common theme in various optoelectronic devices. The two droop types correspond to the two extreme situations of energy fluctuation that affects the carrier diffusion and recombination. The finding suggests ways for improving the device performance, depending on operation conditions.

Temperature coefficients of phonon frequencies and thermal conductivity in thin black phosphorus layers

We investigate the temperature dependence of three major Raman modes of black phosphorus (BP) prepared by mechanical exfoliation from room temperature to 325 °C. With increasing temperature, all the Raman peaks show redshift in peak position and broadening in linewidth, but they depend on the film thickness. The first-order temperature coefficients of Ag1, B2g, and Ag2 are measured to be −0.0199, −0.0304, and −0.0321 cm−1/K, respectively, in a ∼20-layer film. With decreasing thickness, the temperature coefficient decreases. The average thermal conductivity of a 70-nm thick BP film at room temperature is determined to be 15.8 W/mK when suspended, and 29.2 W/mK when supported on a SiO2/Si substrate. Thermal decomposition temperature is found to be around 350 °C in N2 environment.

Interplay of point defects, extended defects, and carrier localization in the efficiency droop of InGaN quantum wells light-emitting diodes investigated using spatially resolved electroluminescence and photoluminescence

We perform both spatially resolved electroluminescence (SREL) as a function of injection current and spatially resolved photoluminescence (SRPL) as a function of excitation power on InGaN quantum well blue light-emitting diodes to investigate the underlying physics for the phenomenon of the external quantum efficiency (EQE) droop. SREL allows us to study two most commonly observed but distinctly different droop behaviors on a single device, minimizing the ambiguity trying to compare independently fabricated devices. Two representative devices are studied: one with macroscopic scale material non-uniformity, the other being macroscopically uniform, but both with microscopic scale fluctuations. We suggest that the EQE–current curve reflects the interplay of three effects: nonradiative recombination through point defects, carrier localization due to either In composition or well width fluctuation, and nonradiative recombination of the extended defects, which is common to various optoelectronic devices. By comparing SREL and SRPL, two very different excitation/detection modes, we show that individual singular sites exhibiting either particularly strong or weak emission in SRPL do not usually play any significant and direct role in the EQE droop. We introduce a two-level model that can capture the basic physical processes that dictate the EQE–current dependence and describe the whole operating range of the device from 0.01 to 100 A/cm2.

In Situ Monitoring of the Thermal-Annealing Effect in a Monolayer of MoS2

We perform in-situ two-cycle thermal cycling and annealing studies for a transferred CVD-grown monolayer MoS2 on a SiO2/Si substrate, using spatially resolved micro-Raman and PL spectroscopy. After the thermal cycling and being annealed at 305 deg C twice, the film morphology and film-substrate bonding are significantly modified, which together with the removal of polymer residues cause major changes in the strain and doping distribution over the film, and thus the optical properties. Before annealing, the strain associated with ripples in the transferred film dominates the spatial distributions of the PL peak position and intensity over the film; after annealing, the variation in film-substrate bonding, affecting both strain and doping, becomes the leading factor. This work reveals that the film-substrate bonding, and thus the strain and doping, is unstable under thermal stress, which is important for understanding the substrate effects on the optical and transport properties of the 2D material and their impact on device applications.

Erratum: “Spatially resolved study of quantum efficiency droop in InGaN light-emitting diodes,” [Appl. Phys. Lett. 101, 252103 (2012)]

Yue Lin, Yong Zhang, Zhiqiang Liu, Liqin Su, Jihong Zhang, Tongbo Wei, and Zhong Chen Citation: Applied Physics Letters 103, 119902 (2013); doi: 10.1063/1.4821196 View online: http://dx.doi.org/10.1063/1.4821196 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Interplay of point defects, extended defects, and carrier localization in the efficiency droop of InGaN quantumwells light-emitting diodes investigated using spatially resolved electroluminescence and photoluminescence J. Appl. Phys. 115, 023103 (2014); 10.1063/1.4861150 Spatially resolved study of quantum efficiency droop in InGaN light-emitting diodes Appl. Phys. Lett. 101, 252103 (2012); 10.1063/1.4772549 Rate equation analysis of efficiency droop in InGaN light-emitting diodes Appl. Phys. Lett. 95, 081114 (2009); 10.1063/1.3216578 Reduction of efficiency droop in InGaN light emitting diodes by coupled quantum wells Appl. Phys. Lett. 93, 171113 (2008); 10.1063/1.3012388 Erratum: “Evidence of localization effects in InGaN single-quantum-well ultraviolet light-emitting diodes” [Appl.Phys. Lett. 76, 1671 (2000)] Appl. Phys. Lett. 78, 679 (2001); 10.1063/1.1343504