Yong Zhang

University of North Carolina at Charlotte | UNC Charlotte · Department of Electrical & Computer Engineering

We attempt to clarify an ambiguity in the understanding of the electronic structure of an acceptor in a semiconductor. Instead of using only a single quantity acceptor binding energy E A as in the literature, there is in fact an impurity level E I that plays perhaps a more important role when dealing with the electronic transitions involving the ac...

Modern electronics are developing electronic-optical integrated circuits, while their electronic backbone, e.g. field-effect transistors (FETs), remains the same. However, further FET down scaling is facing physical and technical challenges. A light-effect transistor (LET) offers electronic-optical hybridization at the component level, which can co...

We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination and passivation, over multiple orders of photo-excitation density or carrier density appropriate for different applications. An analytic mo...

We show in ZnTe that the 2LO to 1LO resonant Raman intensity ratio, a measure of electron-phonon coupling (EPC), exhibits a much larger intrinsic value than that previously reported and minimal change from bulk to 30 nm nanowires. However, the ratio can be tuned extrinsically over one order in magnitude controllably either during or after growth, a...

Carrier diffusion is of paramount importance in many semiconductor devices, such as solar cells, photodetectors, and power electronics. Structural defects prevent such devices from reaching their full performance potential. Although a large carrier diffusion length indicates high material quality, it also implies increased carrier depletion by an i...

Chemical vapor deposition (CVD) has been proved to be the most useful method to produce two-dimensional (2D) materials, including tungsten disulfide (WS2). However, the existence of inhomogeneity of strain, doping, and defects in the CVD-grown WS2 monolayers may significantly influence the optical and electronic properties of the materials, thus af...

Although drying techniques are exciting alternatives to cryopreservation, it remains challenging to maintain tightly controlled temperatures and humidity levels during storage of dried products. The objective of this study was to determine if the addition of choline acetate to trehalose solution could enable a wider range of storage conditions for...

Vertically oriented Si microwires (Si MWs) of high aspect ratio were grown on degenerately doped n-Si (111) substrate in an atmospheric pressure chemical vapor deposition (APCVD) system by a vapor–liquid-solid (VLS) mechanism. A lithography-free de-wetting technique was used on as-deposited Cu thin film to form the catalyst seed layer for the Si MW...

Organic-inorganic hybrid materials often face a stability challenge. β-ZnTe(en)0.5 , which uniquely has over 15-year real-time degradation data, is taken as a prototype structure to demonstrate an accelerated thermal aging method for assessing the intrinsic and ambient-condition long-term stability of hybrid materials. Micro-Raman spectroscopy is u...

We demonstrate an all optical approach that can surprisingly offer the possibility of yielding much more information than one would expect, pertinent to the carrier recombination dynamics via both radiative and nonradiative processes when only one dominant deep defect level is present in a semiconductor material. By applying a band-defect state cou...

Contrary to the common belief that the light-induced halide ion segregation in a mixed halide alloy occurs within the illuminated area, we find that the Br ions released by light are expelled from the illuminated area, which generates a macro/mesoscopic size anion ring surrounding the illuminated area, exhibiting a photoluminescence ring. This intr...

Organic−inorganic hybrid perovskites display remarkable photovoltaic properties, but instability arises from the material’s surface and photoexcitation. The photodegradation process of methylammonium lead triiodide (MAPbI3) investigated with above bandgap excitation Raman spectroscopy displays two stages of structural change in polycrystalline film...

An ammonia (NH3) gas sensor based on ZnO/CuO (ZC) heterojunction was prepared by hydrothermal method and the sensing mechanism was studied by first principle calculations based on density function theory (DFT). Experiments show that NH3 sensor based on ZC heterojunction has low detection limit (0.39 ppm), fast response/recovery time (2.3 s/2.1 s),...

InAs/GaSb and InAs/InAs 1− x Sb x type II superlattices (T2SLs) have been demonstrated with great potential for next‐generation infrared (IR) detection. However, fundamental research and understanding of these superlattices (SLs) is much less than that of the much better known system GaAs/Al x Ga 1− x As, but they exhibit distinctly different chara...

Translational symmetry is the signature of a crystal. Organic-inorganic hybrid materials can be classified in four levels according to the degree of deviating from this strict definition of the crystal. Among the large number of organic-inorganic hybrid materials, only a small fraction can be considered as crystalline in a broad sense, whereas most...

Lead halide perovskites have emerged as promising photovoltaic (PV) materials owing to their superior optoelectronic properties. However, they suffer from poor stability and potential toxicity. Here, computational screening with experimental synthesis is combined to explore stable, lead‐free, and defect‐tolerant PV materials. Heavy cations with lon...

Spontaneous Lattice Inversion In article number 2200057, Zhiqiang Liu, Shenyuan Yang, Yong Zhang, Peng Gao, and co‐workers confirm the polarity of AlN on sapphire prepared by metal–organic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion. Furthe...

Despite the long history of research that has focused on the role of defects on device performance, the studies have not always been fruitful. A major reason is because these defect studies have typically been conducted in a parallel mode wherein the semiconductor wafer was divided into multiple pieces for separate optical and structural characteri...

The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metal–organic chemical vapor deposition is not inherited...

Photonic integration is typically limited to the communication between electronic sub-systems on a chip. A novel approach to achieve single-functional-unit level integration of electronic and photonic components is proposed, illustrated by FET-LET hybrid 6T SRAM.

A hexacoordinate organosilane with a SiC4N2 skeleton has been synthesized from the reaction of a tridentate, dianionic, CNC-diphenylpyridine (DPP) ligand with SiCl4. The complex has a distorted octahedral geometry with short S-N bond lengths averaging 1.9111(14) Å. The Si(DPP)2 complex is resistant to hydrolysis and thermally robust. Thermally evap...

Contrary to the common belief that light-induced halide ion segregation in a mixed halide al-loy occurs within the illuminated area, we find that the Br ions released by light diffuse away from the area, which generates a counter-balancing Coulombic force between the anion deficit and surplus region, together resulting in a macro/mesoscopic size an...

Van der Waals epitaxy provides a fertile playground for the monolithic integration of various materials for advanced electronics and optoelectronics. Here, a previously unidentified nanorod-assisted van der Waals epitaxy is developed and nearly single-crystalline GaN films are first grown on amorphous silica glass substrates using a graphene interf...

A broad range of technologies have been developed for the chip and wafer scale connections and integrations of photonic and electronic circuits, although major challenges remain for achieving the single functional-unit-level integration of electronic and photonic devices. Here we use field-effect transistor/light-effect transistor (FET– LET) hybrid...

This thesis describes an experimental and theoretical investigation of semiconductor quantum wires and dots. I have studied the formation and decay of excitons and related problems: luminescent efficiency, carrier relaxation, energy transfer, effects of localization, etc. in these quasi one-dimensional (1-D) and quasi zero-dimensional (0-D) semicon...

A light effect transistor (LET) is a photoconductive device with appropriate output and transfer characteristics, consisting of a nanowire (NW) conducting channel supported by a dielectric and has two metal contacts at two ends forming the drain and source [1]. Though working mechanism of a LET is different from conventional FETs, it can readily em...

A light effect transistor (LET) is a photoconductive device with appropriate output and transfer characteristics, consisting of a nanowire (NW) conducting channel supported by a dielectric and has two metal contacts at two ends forming the drain and source [1]. Though working mechanism of a LET is different from conventional FETs, it can readily em...

The objective of the present study was to investigate the effect of moisture content on DNA integrity in the germinal vesicle of feline oocytes following dehydration and 8 weeks of storage at 22-24°C (ambient conditions). Using microwave-assisted drying, the conditions that led to a predictable and stable moisture content in trehalose-Tris/EDTA buf...

Defects usually degrade device performance. Thus, many techniques and effort are devoted to studying semiconductor defects. However, it is rarely known: i) how individual defects affect device performance; ii) how the impact depends on the device operating conditions; iii) how the impact varies from one defect to another; and iv) how these variatio...

High speed, low power, low leakage, and low noise circuits are extremely essential for modern VLSI chips. Since on-chip cache memories consume appreciable amount of the total chip area and energy, high performance and low power Static Random-Access Memories (SRAMs) are needed for high performance and low power electronic systems. A hybrid FET-LET 6...

Introduction—Maintaining a stable dry state is critical for long-term preservation of live biomaterials at suprazero temperatures. The objective of the study was to characterize the effect of moisture content on DNA integrity within the germinal vesicle (GV) of feline oocytes following dehydration and storage at 22–24 degree C. Methods—Using microw...

Over the last decades, great advances have been made in the development of nanoscale optoelectronics, such as solar cells, photodetectors, and light-emitting diodes. The design of heterojunctions with highly desirable interface properties and consequently controllable carrier transfer is of vital importance to construct high-performance optoelectro...

IntroductionMaintaining a stable dry state is critical for long-term preservation of live biomaterials at suprazero temperatures. The objective of the study was to characterize the effect of moisture content on DNA integrity within the germinal vesicle (GV) of feline oocytes following dehydration and storage at 22–24 °C.Methods Using microwave-assi...

High speed, low power, low leakage, and low noise circuits are extremely essential for modern VLSI chips. Since on-chip cache memories consume appreciable amount of the total chip area and energy, high performance and low power Static Random-Access Memories (SRAMs) are needed for high performance and low power electronic systems. A hybrid FET-LET 6...

There is an extremely high demand for a high speed, low power, low leakage, and low noise Static Random-Access Memory (SRAM) for high performance cache memories. The energy efficiency of SRAM is of paramount importance in both high performance and ultralow-power portable, battery operated electronic systems. In this article the factors affecting th...

This paper reports on the ohmic contact formed by the conductive glass layer found at the interface of Ag/Si contacts on lowly doped emitter silicon solar cells due to the presence of semimetal nanoparticles. The scanning electron microscopy and scanning transmission electron microscope analyses revealed an interface glass layer (IGL) thickness of...

Organic-inorganic hybrids may offer unique material properties not available from their components. However, they are typically less stable and disordered. Hybrid perovskites are the best examples. Long-term stability study of the hybrid materials, over the anticipated lifespan of a real device, is practically nonexistent. A family of highly ordere...

Oxygen-containing rare-earth metal hydride, YHxOy, is a newly found photochromic material showing fast photoresponse. While its preparation method, optical properties and structural features have been studied extensively, the photochromic mechanism in YHxOy remains unknown. Here, using excited-state molecular dynamics simulation based on the recent...

A semiconductor nanowire based photo-conductive device, referred to as light effect transistor (LET), is demonstrated for replicating the field effect transistor (FET) functions with potentially higher speed and lower switching energy, and offering novel, beyond FET functionalities, e.g., optical logic gates and optical amplification. In an integra...

Dye-sensitized solar cells are regarded as promising candidates to resolve the energy and environmental issues in recent years, arising from their solution-processable fabrication technology and high power conversion efficiency. However, there are still several problems regarding how to accelerate the development of this type of photovoltaics, incl...

In this work, HfO2 thin films were deposited on Si (100) wafer by using reactive atomic layer deposition at different temperatures. By characterizing the Raman spectroscopy and XRD patterns, we find that with the substrate temperature increases, the structure of the film transforms continuously, and a substrate with sufficiently high temperature ca...

In this work, we studied surface-enhanced Raman scattering (SERS) of MS2 (M=Mo, W) monolayers that were transferred onto Ag nanorod arrays. Compared to the suspended monolayers, the Raman intensity of monolayers on an Ag nanorod substrate was strongly enhanced for both in-plane and out-of-plane vibration modes: up to 8 (5) for E2g and 20 (23) for A...

A brief review of Huang–Rhys theory and Albrecht's theory is provided, and their connection and applications are discussed. The former is a first order perturbative theory on optical transitions intended for applications such as absorption and emission involving localized defect or impurity centers, emphasizing lattice relaxation or mixing of vibra...

We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination, over multiple orders of photo-excitation density. An analytic model is used to describe the excitation density dependence of PL intensity...

We report the growth of Si nanostructures, either as thin films or nanoparticles, on graphene substrates. The Si nanostructures are shown to be single crystalline, air stable and oxidation resistive, as indicated by the observation of a single crystalline Si Raman mode at around 520 cm –1 , a STM image of an ordered surface structure under ambient...

We investigate the temperature and injection dependence of the electroluminescence from an InGaN/GaN LED to characterize the defect-related recombination mechanism in this system. In contrast to the standard ABC recombination model, we show that the defect-related recombination rate varies superlinearly with carrier density. The elevated loss rate...

In order to understand the impact of nano-crystallites on current transport mechanisms in screen-printed c-Si solar cells with lowly-doped emitter, Te-glass based Ag pastes with different transition temperatures (T g ) were used. The Te-glass with lower T g showed lower R c than the one with higher T g due to the formation of nano-crystallites in t...

Laser-induced-modification Raman spectroscopy coupled with high spatial resolution is demonstrated to obtain additional structure information beyond what the conventional techniques offer, revealing microscopic scale variation between nominally similar alloys. CZTSe films are used as examples.

A neutral hexacoordinate silicon complex containing two 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) ligands has been synthesized and explored as a potential electron transport layer and electroluminescent layer in organic electronic devices. The air and water stable complex is fluorescent in solution with a λmax = 510 nm and a QY = 57%. Thin films...

A brief history of the impurity theories in semiconductors is provided. A bound exciton model is proposed for both donor- and acceptor-like impurities and point defects, which offers a unified understanding for "shallow" and "deep" impurities and point defects. The underlying physics of computational results using different density-functional theor...

Confocal micro-Raman microscopy performed in the transparent spectral region of a semiconductor can, in principle, be used for operando three-dimensional (3D) thermometry with optical diffraction-limit spatial resolution. However, when applied to high-power GaN-based light-emitting diodes (LEDs), the applicability is hindered by the often strong se...

Impurity resonant state p-doping was recently proposed as an alternative to the commonly used p-GaN for high-efficiency nitride-based light-emitting diodes (LEDs) in order to address the issue of electron leakage and efficiency droop. We demonstrated that due to the effective acceptor ionization and reduction of the potential barrier for hole injec...

Correlative Characterization of Defect Clusters in GaAs Solar Cells by High-Resolution TEM and Spatially Resolved Optical Techniques - Volume 24 Supplement - B.S. McKeon, Q. Chen, S. Zhang, C.-K. Hu, T.H. Gfroerer, M.W. Wanlass, Y. Zhang, David J. Smith

We report the growth of Si nanostructures, either as thin films or nanoparticles, on graphene substrates. The Si nanostructures are shown to be single crystalline, air stable and oxidation resistive, as indicated by the observation of a single crystalline Si Raman mode at around 520 cm-1, a STM image of an ordered surface structure under ambient co...

In this paper, the influence of Te-glass frit on silver screen-printed contact formation in crystalline Si solar cell is reported. During fire through dielectric (FTD) contact process, the Te-glass frit enhances the formation of Ag2Te and PbTe in the reformed glass underlying the front Ag gridline. The presence of Ag2Te, which is a semi-metal, heig...

The InAs/InAs1−xSbx superlattice system distinctly differs from two well-studied superlattice systems GaAs/AlAs and InAs/GaSb in terms of electronic band alignment, common elements at the interface, and phonon spectrum overlapping of the constituents. This fact leads to the unique electronic and vibrational properties of the InAs/InAs1−xSbx system...

While producing comparable efficiencies and showing similar properties when probed by conventional techniques, such as Raman spectroscopy, photoluminescence, and x-ray diffraction, two thin-film solar-cell materials with complex structures, such as quaternary compound Cu2ZnSnSe4 (CZTSe), may, in fact, differ significantly in their microscopic struc...

While producing comparable efficiencies and showing similar properties when probed by conventional techniques, such as Raman, photoluminescence and X-ray diffraction, two thin film solar cell materials with complex structures, such as quaternary compound CZTSe, may in fact differ significantly in their microscopic structures. In this work, laser in...