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Haider Ali

Bio: Haider Ali is an academic researcher from University of Central Florida. The author has contributed to research in topics: Silicon & Crystalline silicon. The author has an hindex of 8, co-authored 26 publications receiving 400 citations.

Papers
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Journal ArticleDOI
TL;DR: Thin TiO2 films are demonstrated to be an excellent electron-selective contact for crystalline silicon solar cells featuring a full-areaTiO2 -based electron- selective contact.
Abstract: Thin TiO2 films are demonstrated to be an excellent electron-selective contact for crystalline silicon solar cells. An efficiency of 21.6% is achieved for crystalline silicon solar cells featuring a full-area TiO2 -based electron-selective contact.

286 citations

Journal ArticleDOI
TL;DR: In this article, the fabrication and characterization of solar blind photodetectors (SBPs) based on undoped β-Ga2O3 and Zn doped (∼5 × 1020 cm−3) epitaxial films with cutoff wavelength of ∼260 nm were reported.
Abstract: We report on the fabrication and characterization of solar blind photodetectors (SBPs) based on undoped β-Ga2O3 and Zn doped (∼5 × 1020 cm−3) β-Ga2O3 (ZnGaO) epitaxial films with cutoff wavelength of ∼260 nm. The epilayers were grown on c-sapphire by the metal organic chemical vapor deposition technique and their structural, electrical and optical properties were characterized using various methods. As grown films have a large number of defects, resulting in detectors with enhanced internal gain, hence, high spectral responsivity >103 A/W. Post growth annealing in oxygen improved the quality of the epilayers, leading to detectors with reduced dark current (∼nA to ∼pA) and increased out of band rejection ratio. At 20 V bias, a ZnGaO detector showed a peak responsivity of 210 A/W (at 232 nm) and an out of band rejection ratio (i.e., R232 nm/R320 nm) of 5 × 104. Alternatively, for a β-Ga2O3 detector these parameters were found to be five times and three times lower, respectively, suggesting that ZnGaO detectors have superior performance characteristics. These results provide a roadmap toward achieving high responsivity SBPs based on epitaxial ZnGaO films, laying a solid foundation for future applications.

76 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effectiveness of crystalline silicon surface cleaning by a simple UV-ozone process in comparison to the industry standard RCA clean for silicon photovoltaic applications.

21 citations

Journal ArticleDOI
TL;DR: The cross-section of electron-selective titanium oxide ( TiO2) contacts for n-type crystalline silicon solar cells were investigated by transmission electron microscopy and it was revealed that the excellent cell efficiency is due to strong surface passivation of SiO2/TiO2 stack as well as low contact resistivity at the Si/SiO 2/Ti O2 heterojunction.
Abstract: The authors would like to thank Eric Schneller for assistance with analysis of the quantum efficiency and reflectance data. The authors acknowledge financial support from the Australian Renewable Energy Agency (ARENA) under the Postdoctoral Fellowship. The authors would also like to acknowledge support for this work by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, in the Solar Energy Technologies Program, under Award Number DE-EE0004947. Finally, the Materials Characterization Facility at University of Central Florida (UCF) is acknowledged for usage of its facilities.

20 citations

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TL;DR: In this study, the thermal stability of a contact structure featuring hole-selective tungsten oxide and aluminum deposited onto p-type crystalline silicon was investigated using a combination of transmission line measurements (TLM) and in situ transmission electron microscopy (TEM) studies.
Abstract: In this study, the thermal stability of a contact structure featuring hole-selective tungsten oxide (WOx) and aluminum deposited onto p-type crystalline silicon (c-Si/WOx/Al) was investigated using a combination of transmission line measurements (TLM) and in situ transmission electron microscopy (TEM) studies. The TEM images provide insight into why the charge carrier transport and recombination characteristics change as a function of temperature, particularly as the samples are annealed at temperatures above 500 °C. In the as-deposited state, a ≈ 2 nm silicon oxide (SiOx) interlayer forms at the c-Si/WOx interface and a ≈ 2-3 nm aluminum oxide (AlOx) interlayer at the WOx/Al interface. When annealing above 500 °C, Al diffusion begins, and above 600 °C complete intermixing of the SiOx, WOx, AlOx and Al layers occurs. This results in a large drop in the contact resistivity, but is the likely reason surface recombination increases at these high temperatures, since a c-Si/Al contact is basically being formed. This work provides some fundamental insight that can help in the development of WOx films as hole-selective rear contacts for p-type solar cells. Furthermore, this study demonstrates that in situ TEM can provide valuable information about thermal stability of transition metal oxides functioning as carrier-selective contacts in silicon solar cells.

17 citations


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TL;DR: The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed in this article.
Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

1,535 citations

Journal Article
TL;DR: The design and fabrication of a three-dimensional in vitro system to model vascular stenosis so that specific cellular interactions and responses to hemodynamic stimuli can be investigated and serve as an in vitro 3D culture system to investigate vascular pathogenesis.
Abstract: Vascular stenosis triggers adaptive cellular responses that induce adverse remodeling, which can progress to partial or complete vessel occlusion. Despite its severity, cellular interactions and biophysical cues that regulate pathological progression are poorly understood. We report the design and fabrication of a three-dimensional in vitro system to model vascular stenosis so that specific cellular interactions and responses to hemodynamic stimuli can be investigated. Tubular cellularized constructs (cytotubes) were produced using a collagen casting system to generate a stenotic arterial model. Fabrication methods were developed to create cytotubes containing co-cultured vascular cells, where cell viability, distribution, morphology, and contraction were examined (Figure). Fibroblasts, bone marrow primary cells, smooth muscle cells (SMCs), and endothelial cells (ECs) remained viable during culture and developed locationand time-dependent morphologies. We found cytotube contraction to depend on cellular composition, where SMC-EC co-cultures adopted intermediate contractile phenotypes between SMCand EC-only cytotubes. Our fabrication approach and resulting artery model can serve as an in vitro 3D culture system to investigate vascular pathogenesis.

570 citations

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TL;DR: De Wolf et al. as mentioned in this paper reviewed the fundamental physical processes governing contact formation in crystalline silicon (c-Si) and identified the role passivating contacts play in increasing c-Si solar cell efficiencies beyond the limitations imposed by heavy doping and direct metallization.
Abstract: The global photovoltaic (PV) market is dominated by crystalline silicon (c-Si) based technologies with heavily doped, directly metallized contacts. Recombination of photo-generated electrons and holes at the contact regions is increasingly constraining the power conversion efficiencies of these devices as other performance-limiting energy losses are overcome. To move forward, c-Si PV technologies must implement alternative contacting approaches. Passivating contacts, which incorporate thin films within the contact structure that simultaneously supress recombination and promote charge-carrier selectivity, are a promising next step for the mainstream c-Si PV industry. In this work, we review the fundamental physical processes governing contact formation in c-Si. In doing so we identify the role passivating contacts play in increasing c-Si solar cell efficiencies beyond the limitations imposed by heavy doping and direct metallization. Strategies towards the implementation of passivating contacts in industrial environments are discussed. The development of passivating contacts holds great potential for enhancing the power conversion efficiency of silicon photovoltaics. Here, De Wolf et al. review recent advances in material design and device architecture, and discuss technical challenges to industrial fabrication.

326 citations

Journal ArticleDOI
TL;DR: In this article, a comprehensive review of the applications of inorganic ultrawide-bandgap (UWBG) semiconductors for solar-blind DUV light detection in the past several decades is presented.
Abstract: Due to its significant applications in many relevant fields, light detection in the solar-blind deep-ultraviolet (DUV) wavelength region is a subject of great interest for both scientific and industrial communities. The rapid advances in preparing high-quality ultrawide-bandgap (UWBG) semiconductors have enabled the realization of various high-performance DUV photodetectors (DUVPDs) with different geometries, which provide an avenue for circumventing numerous disadvantages in traditional DUV detectors. This article presents a comprehensive review of the applications of inorganic UWBG semiconductors for solar-blind DUV light detection in the past several decades. Different kinds of DUVPDs, which are based on varied UWBG semiconductors including Ga2O3, MgxZn1−xO, III-nitride compounds (AlxGa1−xN/AlN and BN), diamond, etc., and operate on different working principles, are introduced and discussed systematically. Some emerging techniques to optimize device performance are addressed as well. Finally, the existing techniques are summarized and future challenges are proposed in order to shed light on development in this critical research field.

309 citations

Journal ArticleDOI
TL;DR: In this article, a comprehensive review on Ga2O3-based solar-blind UV photodetectors is provided, with a detailed introduction of the developmental process of material growth methods and device manufacturing in the past decade.
Abstract: In recent years, solar-blind ultraviolet (UV) photodetectors have attracted significant attention from researchers in the field of semiconductor devices due to their indispensable properties in the fields of high-temperature event monitoring, anti-terrorism, security and ad hoc network communication. As an important member of the third-generation semiconductors, β-Ga2O3 is considered to be one of the most promising candidates for solar-blind UV detectors due to its ultra-wide band gap (∼4.9 eV), economic efficiency, high radiation resistance and excellent chemical and thermal stability. Herein, we provide a comprehensive review on Ga2O3-based solar-blind UV photodetectors, with a detailed introduction of the developmental process of material growth methods and device manufacturing in the past decade. We classify the currently reported Ga2O3-based solar-blind UV photodetectors (mainly including photoconductive detectors, heterogeneous PN junction detectors and Schottky junction detectors) and summarize their respective superiorities and potentials for improvement. Finally, considering the actual application requirements, we put forward some meaningful suggestions, including energy band engineering and homogeneous epitaxy, for the future development of Ga2O3 material growth and device manufacturing.

293 citations