Compound Semiconductor Thin Films Research Articles

화합물 반도체 기반의 유연 발광소자 및 디스플레이 제작

This article presents a review of research activities over past decades focused on the fabrication of flexible light-emitting diodes (LEDs) and micro-LED displays. LEDs exhibit excellent material characteristics, including high radiative recombination rates, high carrier mobility, and ultra-long-term stability. These features make LEDs promising candidates for not only the future metaverse display but flexible display applications. However, the brittleness of compound semiconductor thin films poses challenges for creating deformable LED devices. Consequently, significant efforts have been dedicated to imparting deformability to LED devices and displays. We initially discuss a display prepared using a nanowire-assembly process, followed by a strategy involving thin film LEDs for flexible device fabrication. Vertical nanowire LED arrays are presented, along with a discussion of their advantages for flexible devices and displays. Furthermore, we review the selective-area epitaxy of vertical nanowire LED arrays. Finally, we briefly introduce the assembly methods of LEDs onto backplane circuits, addressing several important issues, including the misalignment of LED transfers onto backplane circuits. We conclude with personal remarks on the challenges and future perspectives for research on flexible micro-LED displays.

Review of novel approach and scalability forecast of ZnSe and Perovskite/Graphene based thin film materials for high performance solar cell applications

Abstract In the recent years, the using of fossil energy source-based fuels are delivering to the predetermined nature, health and environmental exposure, there is a universal necessitate prepared to well improvement and consumption of renewable energy source and apparatus. With the rapid growth of human life, energy is more essential. The explosive growth of population and energy consumption demands are the exclusive issue of the present world. In response to the energy demands, the growth of highly efficient energy conversion and storage devices. With increasing energy demands and environmental pollution, there is a requirement of the world to great some novel conducting materials consist of Perovskite solar cells (PSCs) which is delivered that excellent photoconversion efficiencies (PCE) compare with the other silicon supported photovoltaics, and also semiconducting materials for the conversion of substitute energy sources and preparation of high high-performance semiconductor devices. Semiconducting thin films play an indispensable role in fashionable science and electronic technology. Among them, the II–VI compound semiconductor thin films are an important class of materials, and they are generally comprehensive wide-band gap materials. In addition, the un-doped and rare-earth metal ions doped zinc selenide (ZnSe) thin films are currently the most demanding and technologically important materials, which have the potential for optoelectronic devices (solar cells, photoelectrochemical cell and light emitting diodes) and are active throughout the entire visible spectrum extending into the infrared wavelengths. Various techniques for depositing thin coatings on these substances are utilized to a greater extent. In this review, the most recent advanced techniques in the application of semiconducting materials-based thin films were produced in various energy-generated fields, particularly solar cells, with a major focus on a review of recent progress in the development of various types of ZnSe thin film-based material for high-performance semiconducting thin film material for solar cell application. Lastly, the advantages and material challenges of semiconductor-based thin films for future sanitary energy device process are described.

Bilayer molybdenum contact prepared via high-power pulsed magnetron sputtering: Effects of substrate bias

Molybdenum (Mo) is widely used to fabricate the back contact of compound semiconductor thin film solar cells. Most of those contacts are deposited using direct current (DC) or radio frequency (RF) sputtering; however, those processes are ill-suited to achieving low resistance with good adhesion. In this study, we sought to overcome this issue using high-power pulsed magnetron sputtering (HiPIMS) with a focus on the effects of substrate bias on adhesion and resistance. When used with a HiPIMS power source, substrate bias was shown to promote the (110) preferred orientation of the top Mo contact, while reducing the (211) preferred orientation of the bottom Mo contact. This resulted in the lowest recorded resistivity (9.7 μΩ cm) when the bottom was deposited without bias and the top Mo contact was biased at −40 V.

Control of n-type doping of gallium oxide wide gap compound semiconductor thin films

Next-generation power electronics is a term that is used to refer to devices of the future that will need to process considerably more energy in order to function than is typical for current electronic devices. For instance, the amounts of power that electric cars will need to process in the future necessitates new methods and techniques, particularly those that permit a minimising of power loss and can dissipate heat efficiently. Professor Qixin Guo is focused on understanding more about next-generation power electronics. He explains that one means of achieving these ambitions is through the use of wide bandgap (WBG) semiconductors, which are preferred over narrow band semiconductors, such as Silicon. 'This is because the large energy separation between the conduction and the valance bands enables electronic devices to operate at elevated temperatures and higher voltages,' outlines Guo. Powering electronics necessitates pushing an electron into a conducting state and bandgaps measure how energy is required to do this. Therefore, the larger the bandgap, the more a material can withstand a stronger electric field. 'Ultimately, this means that components can be thinner, lighter and handle more power than components that are made up of materials with lower bandgaps.' With that in mind, researchers around the world are exploring materials that can be used as WBG semiconductors to usher in the next generation of power electronics. If this can be achieved, our lives will be affected in myriad ways, where new efficiencies enable power capabilities that would have been unthinkable even just a few years ago.

Investigation of reflectometry for in situ process monitoring and characterization of co-evaporated and stacked Cu-Zn-Sn-S based thin films

During preparation of compound semiconductor thin films undesired secondary phases and inhomogeneities may occur. Therefore, process control and monitoring are important aspects towards thin film optimization. In this study, white light reflectometry (WLR) was investigated as an in situ non-destructive optical technique to monitor thin films during thermal vacuum evaporation (PVD) of kesterite-type Cu-Zn-Sn-S thin films.The impact of composition on optical properties was studied ex situ by Raman spectroscopy and reflectometry to identify possibilities for in situ detection of secondary phases. A WLR setup was designed and integrated with the PVD system. Four Cu-Zn-Sn-S films were prepared by evaporation and direct reflection was monitored in situ. Reflection spectra were analyzed to identify the influence of process parameters and phases such as CuS and ZnS. Transfer matrix simulations were performed to further explain experimental observations.It is shown that the occurrence of different phases, growth rate/thickness are well related to reflection spectra. Time-dependent reflection spectra reveal specific properties that could serve as characteristic fingerprints for process control and monitoring. Therefore, this study extends the possibilities for reflectometry as a straightforward, low cost method for in situ process control of Cu-Zn-Sn-S based films.

Trade-Off in Fire-Retardant Solar Cell Materials and Environmental Issues

In recent years, several types of solar cells, such as polycrystalline silicon, compound semiconductor and organic thin films, have been and grown and developed as one of the promising renewable energy devices for low cost and safety compared with nuclear power generation. They were composed from many components and their materials generally. As for Dye Sensitized Solar Cell (DSSC), which may be high efficiency and easy to assemble but not be expensive so much, not only inorganic titanium oxide and organic or metal complex dyes but also organic solvents as electrolyte. Furthermore, a condensing lens made of plastic are also used to improve power generating efficiency.

Controlling Morphology in Polycrystalline Films by Nucleation and Growth from Metastable Nanocrystals.

Solution processingofpolycrystalline compound semiconductor thin film using nanocrystals as a precursor is considered one of the most promising and economically viable routes for future large-area manufacturing. However, in polycrystalline compound semiconductor films such as Cu2ZnSnS4 (CZTS), grain size, and the respective grain boundaries play a key role in dictating the optoelectronic properties. Various strategies have been employed previously in tailoring the grain size and boundaries (such as ligand exchange) but most require postdeposition thermal annealing at high temperature in the presence of grain growth directing agents (selenium or sulfur vapor with/without Na, K, etc.) to enlarge the grains through sintering. Here, we show a different strategy of controlling grain size by tuning the kinetics of nucleation and the subsequent grain growth in CZTS nanocrystal thin films during a crystalline phase transition. We demonstrate that the activation energy for the phase transition can be varied by utilizing different shapes (spherical and nanorod) of nanocrystals with similar size, composition, and surface chemistry leading to different densities of nucleation sites and, thereby, different grain sizes in the films. Additionally, exchanging the native organic ligands for inorganic surface ligands changes the activation energy for the phase change and substantially changes the grain growth dynamics, while also compositionally modifying the resulting film. This combined approach of using nucleation and growth dynamics and surface chemistry enables us to tune the grain size of polycrystalline CZTS films and customize their electronic properties by compositional engineering.

Tin Disulfide-Oxide (SnS2-xOx) as n-type Heterojunction Layer Processed by Chemical Bath Technique for Cd Free Fabrication of Compound Semiconductor Thin Film Solar Cells

ABSTRACTThe chemical bath deposition of wide bandgap n-type SnS2 films and role of sulphur precursor in suppression of p-type SnS phase is described. The as-deposited films are highly polycrystalline in hexagonal crystal structure. Inclusion of oxygen in the film phase is shown by the x-ray photoelectron spectroscopy (XPS) and Raman scattering methods which suggests the CBD films are better described as tin disulfide-oxide (SnS2-xOx) x=0.1. A possible mechanism of film formation is presented. Optical analysis showed energy bandgap 2.76 eV for SnS2-xOx film which decreases to 2.62 eV with the inclusion of the secondary SnS phases.

Erratum: Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation [Phys. Rev. B 92, 155310 (2015)

Erratum: Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation [Phys. Rev. B 92, 155310 (2015)

Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation

In polycrystalline compound semiconductor thin films, structural defects such as grain boundaries as well as lateral stress can form during film growth, which may deteriorate their electronic performance and mechanical stability. In Cu-based chalcogenide semiconductors such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ or ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$, temporary Cu excess during film growth leads to improved microstructure such as a reduced grain boundary density, a strategy that has been used for decades for high-efficiency chalcopyrite thin film solar cells. However, the mechanisms responsible for the beneficial effect of Cu excess are yet not fully clarified. Here, we investigate the evolution of lateral stress, grain growth, and Cu-Se segregation during Cu-Se deposition onto Cu-poor ${\mathrm{CuInSe}}_{2}$. Real-time x-ray diffraction and fluorescence analysis with a double-detector setup reveals that sudden stress relaxation occurs shortly prior to Cu-Se segregation at the surface and precisely coincides with domain growth and change of texture. Numerical reaction-diffusion modeling provides an explanation for the observed delay of Cu-Se segregation. Our results show that partial recrystallization of the film can be already reached without the necessity of an overall Cu-rich film composition and thus suggest a new synthesis route for the fabrication of high-quality chalcopyrite absorber films.

Effects of deposition temperature and CdCl2 annealing on the CdS thin films prepared by pulsed laser deposition

Pulsed laser deposition (PLD) technique is suitable for the deposition of high-quality compound semiconductor thin films, and has been widely developed in recent years. However, pulsed laser deposition of CdS films has rarely been reported. In this work, we prepared CdS thin films using PLD. The effects of growth temperature on the PLD-CdS thin films were studied towards high-performance CdS/CdTe thin film solar cells. Results showed that the CdS film prepared at 400 °C has the best crystallinity and optical transmittance, while 200 °C is more suitable for the window layer of CdTe solar cells due to the highest energy conversion efficiency and the best short-wavelength response. CdCl2 annealing treatment was also employed on the 200 °C-deposited and 400 °C-deposited PLD-CdS layer. Annealing treatment further enhanced crystallinity, and obviously enlarged the grain size. Optical transmittance spectra showed that the band gap of the CdS films increased after annealing. Fermi level of CdS films shifted closer to the conduction band from XPS analysis. CdTe solar cells with annealed windows obtained further improved performance, including higher short-circuit current, open-circuit voltage and energy conversion efficiency.

Effect of Changed Structure As Well As Composition on the Behaviour of Sn(Se,Te) Compound Semiconductor Thin Films and Schottky Diodes for Solar Cell Applications

There has been a great motivation to achieve new material types and fabricate electronic devices which show unusual properties in the field of energy generation and transformation. The recent investigations in this area are directed towards the development of cost-effective, non-toxic and abundant materials, the chalcogeide materials have emerged as a convenient choice in the regard. IV-VI chalcogenide semiconductors have exhibited flexibility to possess varied bandgap energies (bandgap tailoring) which allowed the materials to tap different regions of the solar spectrum. These Devices have achieved highest energy conversion efficiencies among the thin film technologies (above 20%). Further, Sn(S,Se,Te) chalcogenide thin films have been reported to possess the optical and electrical properties suitable for applications for solar harvesting. These have also been reported to be used in infrared LEDs/Detectors, fast switches and optoelectronic Devices. Comparative analysis of some Sn(Se, Te) compound semiconductor thin films and schottky diodes have been made. SnSe used as primary material has transformed to SnSe2 on changing its structure from orthorhombic to hexagonal. It, however, attained SnTeSe (Orthorhombic) composition on replacement of some of the Se atoms with Te. Films of all the three types of materials have been deposited at different substrate temperatures and changed thickness. These were then characterized on the basis of their structural, morphological, electrical and optical properties. The grain size of SnSe films increased on its transformation to SnTeSe as well as SnSe2. The electrical as well as optical analysis of the films showed that the bandgap and resistivity decrease for SnSeTe, the same however increase for SnSe2. Further, the conductivity type remained p-type for SnSeTe but changed to n-type for SnSe2 films. The Schottky Barrier Diodes of the deposited films were formed and their characteristics studied for the temperature dependent current-voltage (I-V) and capacitance-voltage(C-V) behaviour. The current voltage characteristics of Al/n-SnSe2 Schottky diodes demonstrated better diode parameters (ideality factors, barrier heights and breakdown voltage) in comparison Ag/p-SnSe and Ag/p-SnSeTe Schottky Diodes. Further, the ideality factors decreased and barrier heights increased with increase in temperature in all types. The temperature dependence of the barrier heights and ideality factors has been explained on the basis of “barrier inhomogenities” existing over the metal-semiconductor interface. Further, there have been variations between the theoretically generated data (on the basis Gaussian Distribution Function) to that from the experimental results which indicated the possibility of the existence of tunneling current component existing in the current transport of the Schottky Diodes. The breakdown voltages of all the three types of undertaken diodes were increased with decrease in temperature and provided negative temperature coefficient and depicted soft breakdown due to ‘Defect Assisted Tunneling’ phenomenon existing over the Schottky interface.

Non-destructive Evaluation of Compound Semiconductor Thin-Film Solar Cells by Photothermal Beam Deflection Technique

In this paper, it is demonstrated that the photothermal beam deflection technique can be used for measuring the series resistance, optimum load resistance, and conversion efficiency of thin-film solar cells. This technique is also used for determining the carrier transport properties of an absorber and window layer of \(\hbox {CuInS}_{2}/\hbox {In}_{2}\hbox {S}_{3}/\hbox {Ag}\)-based solar cells during different stages of cell fabrication. Transport properties such as the carrier mobility, lifetime, and surface recombination velocity of the individual absorber and window layer are shown to influence the open-circuit voltage and short-circuit current of the final photovoltaic device. The cell parameters measured using the photothermal technique agree well with the electrical measurements. The principle of the technique is explained on the basis of the “mirage effect” and maximum power transfer theorem.

Silicon solar cells: Past, present and the future

There has been a great demand for renewable energy for the last few years. However, the solar cell industry is currently experiencing a temporary plateau due to a sluggish economy and an oversupply of low-quality cells. The current situation can be overcome by reducing the production cost and by improving the cell is conversion efficiency. New materials such as compound semiconductor thin films have been explored to reduce the fabrication cost, and structural changes have been explored to improve the cell’s efficiency. Although a record efficiency of 24.7% is held by a PERL — structured silicon solar cell and 13.44% has been realized using a thin silicon film, the mass production of these cells is still too expensive. Crystalline and amorphous silicon — based solar cells have led the solar industry and have occupied more than half of the market so far. They will remain so in the future photovoltaic (PV) market by playing a pivotal role in the solar industry. In this paper, we discuss two primary approaches that may boost the silicon — based solar cell market; one is a high efficiency approach and the other is a low cost approach. We also discuss the future prospects of various solar cells.

Microreactor-Assisted Solution Deposition for Compound Semiconductor Thin Films

State-of-the-art techniques for the fabrication of compound semiconductors are mostly vacuum-based physical vapor or chemical vapor deposition processes. These vacuum-based techniques typically operate at high temperatures and normally require higher capital costs. Solution-based techniques offer opportunities to fabricate compound semiconductors at lower temperatures and lower capital costs. Among many solution-based deposition processes, chemical bath deposition is an attractive technique for depositing semiconductor films, owing to its low temperature, low cost and large area deposition capability. Chemical bath deposition processes are mainly performed using batch reactors, where all reactants are fed into the reactor simultaneously and products are removed after the processing is finished. Consequently, reaction selectivity is difficult, which can lead to unwanted secondary reactions. Microreactor-assisted solution deposition processes can overcome this limitation by producing short-life molecular intermediates used for heterogeneous thin film synthesis and quenching the reaction prior to homogeneous reactions. In this paper, we present progress in the synthesis and deposition of semiconductor thin films with a focus on CdS using microreactor-assisted solution deposition and provide an overview of its prospect for scale-up.

Compositional Analysis of Electrodeposited Cu-Se Compound Semiconductor Thin Films Using Combined Voltammetry and Flow-Electrochemical Quartz Crystal Microgravimetry

Compositional Analysis of Electrodeposited Cu-Se Compound Semiconductor Thin Films Using Combined Voltammetry and Flow-Electrochemical Quartz Crystal Microgravimetry

Onset of flow recirculation in vertical rotating‐disc chemical vapor deposition reactors

Preventing flow recirculation during the epitaxial deposition of thin films of compound semiconductors is essential for growing multilayer films with atomically abrupt interfaces that form a basis for modern optoelectronic devices. A mathematical model describing mass continuity, flow and heat transfer in a vertical rotating‐disc chemical vapor deposition reactor is used to investigate the onset of buoyancy‐ and inertia‐driven flow recirculation. Numerical simulations of axisymmetric flow patterns are performed and different regimes of operation are identified in the parameter space defined by the Reynolds number, the rotational Reynolds number, and the Grashof number. Design criteria for establishing recirculation‐free flows in reactors used for metalorganic vapor‐phase epitaxy (MOVPE) of compound semiconductors are presented. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3530–3538, 2013

Soft X-rays shedding light on thin-film solar cell surfaces and interfaces

Thin-film solar cells based on compound semiconductors consist of a multilayer structure with various interfaces and contain a multitude of elements and impurities, etc. A rapid progress of these photovoltaic technologies can only be achieved by an insight-driven optimization/development. Hence it is crucial to characterize and understand the relationship between the chemical and electronic properties of these components. This paper reviews some examples of our recent work characterizing compound semiconductor thin films using laboratory- and synchrotron-based electron and soft X-ray spectroscopic characterization methods. It is demonstrated how these different analytical techniques are extraordinarily powerful to reveal the material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of the related electronic and chemical properties. As examples, the paper will discuss the electronic surface structure of chalcopyrite thin-film solar cell absorbers, the chemical structure of the CdS/chalcopyrite interface, present the band alignment at the CdS/kesterite interface, and report on how post-deposition treatments cause chemical interaction/interdiffusion processes in CdTe/CdS thin-film solar cell structures.

Wide bandgap Cu(In,Ga)Se2 solar cells with improved energy conversion efficiency

ABSTRACTWe report on improvements to the energy conversion efficiency of wide bandgap (Eg > 1.2 eV) solar cells on the basis of CuIn1−xGaxSe2. Historically, attaining high efficiency (>16%) from these types of compound semiconductor thin films has been difficult. Nevertheless, by using (a) the alkaline‐containing high‐temperature EtaMax glass substrates from Schott AG, (b) elevated substrate temperatures of 600–650 °C, and (c) high vacuum evaporation from elemental sources following National Renewable Energy Laboratory's three‐stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2 < Eg < 1.45 eV. The current density–voltage (J–V) data we present includes efficiencies >18% for absorber bandgaps of ~1.30 eV and efficiencies of ~16% for bandgaps up to ~1.45 eV. In comparing J–V parameters in similar materials, we establish gains in the open‐circuit voltage and, to a lesser degree, the fill factor value, as the reason for the improved performance. The higher voltages seen in these wide gap materials grown at high substrate temperatures are due to reduced recombination. We establish the existence of random and discrete grains within the CIGS absorbers that yield limited or no generation/collection of minority carriers. We also show that interfacial recombination is the main mechanism limiting additional enhancements to open‐circuit voltage and therefore performance. Solar cell results, absorber materials characterization, and experimental details and discussion are presented. Copyright © 2012 John Wiley & Sons, Ltd.

The complex material properties of chalcopyrite and kesterite thin‐film solar cell absorbers tackled by synchrotron‐based analytics

ABSTRACTIn view of the complexity of compound semiconductor based thin‐film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the structural, chemical, and electronic properties of these components. Hence, this paper gives a review of our recent progress in the characterization of compound semiconductor thin films using synchrotron‐based characterization methods. It is demonstrated how different analytical techniques are extraordinarily powerful to reveal the material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. Light will be shed on structural phase transitions, reactive thin‐film formation mechanisms, surface off‐stoichiometries, and the electronic structure of chalcopyrite and kesterite compound semiconductors. Copyright © 2012 John Wiley & Sons, Ltd.