Compound Semiconductor Films Research Articles

Advances in E-ALD By Slrr for the Growth of Ultra Thin Films of Metal and Alloys - in Recognition to John Stickney's Contribution to Electrodeposition

The idea realization progress of electrochemical atomic layer deposition by surface limited redox replacement (E-ALD by SLRR) is among the brightest and most significant contributions of Professor John Stickney. The approach proposed by him in early nineties of last century for epitaxial growth of compound semiconductor films was extended in the beginning of 21st century with his active participation and contribution to the deposition of ultra thin coatings of metals and alloys with a variety of applications thus establishing foundation for the development of one of the most useful and versatile thin film growth methods. In the beginning of this presentation an overview of existing approaches for epitaxial metal thin film growth along with E-ALD by SLRR will first be presented. A comparison of the kinetics manipulation approaches will be made with emphasis on the ability of those methods to produce epitaxial, flat and uniform deposits. In the course of this comparison illustrations will be provided with results, illustrating the advantages and limitations of these deposition approaches in the growth of Ag and Cu thin films on Au (111) substrates. To support the discussion, on the approaches employed, results of electrochemical, in-situ scanning tunneling microscopy (STM), X-ray Photoelectron Spectroscopy characterization experiments will be presented and compared. In the second part, the applicability of SLRR and electroless SLRR in the growth of Pt and Pd on Au (111) substrate will be discussed with special emphasis on the contribution and achievements of John Stickney’s research team. Experimentally demonstrated will be the SLRR growth of Pt and Pd layers with controllable thickness taking place in “shuttling” chamber, flow cell, and “one-pot” configuration. Open circuit potentiometry during the replacement reaction will be shown as a way to control the completion of each deposition event. Anodic stripping of the entire multilayer will be used for determining the overall film thickness. Cyclic Voltammetry and STM will be employed to characterize the growth of Pt and Pd layers with up to 10 nm thickness. High-resolution STM results and modelling concepts underlying the foundation of the proposed approaches will shed light on the mechanistic aspects associated with the nature of growth by redox exchange at a monolayer level. In the third component of this presentation an overview of recent work on the design of potent catalysts with application in energy production and storage will be presented. This will include the use of SLRR approach for coating nanoparticles and continuous nanoporous metal layers with ultra thin films of metal / alloy with specific functionality. Successful outcome of this coating renders the final product catalytically active. In this part, a brief description will be provided also on the direct use of SLRR based protocols for the smart design of bimetallic alloy and/or core shell clusters with application in the most commonly used reactions in the field of fuel-cell catalysis. The discussion on the synthetic component of all relevant activities will be concluded with demonstration of results of testing of accordingly developed catalysts. Finally, a glimpse into the future of this rapidly developing field will be also provided.

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.

Electrodeposition and Characterization of CuTe and Cu2Te Thin Films

An electrodeposition method for fabrication of CuTe and Cu2Te thin films is presented. The films’ growth is based on the epitaxial electrodeposition of Cu and Te alternately with different electrochemical parameter, respectively. The deposited thin films were characterized by X‐ray diffraction (XRD), field emission scanning electronic microscopy (FE‐SEM) with an energy dispersive X‐ray (EDX) analyzer, and FTIR studies. The results suggest that the epitaxial electrodeposition is an ideal method for deposition of compound semiconductor films for photoelectric applications.

Nanocrystallized Cu2Se grown on electroless Cu coated p-type Si using electrochemical atomic layer deposition

Abstract Cuprous selenide (Cu 2 Se) nanocrystalline thin films are grown onto electroless Cu coating on p-Si (100) substrates using electrochemical atomic layer deposition (EC-ALD), which includes alternate electrodeposition of Cu and Se atomic layers. The obtained films were characterized by X-ray diffraction (XRD), field emission scanning electronic microscopy (FE-SEM), FTIR, and open-circuit potential (OCP) studies. The results show the higher quality and good photoelectric properties of the Cu 2 Se film, suggesting that the combination of electroless coating and EC-ALD is an ideal method for deposition of compound semiconductor films on p-Si.

Analysis of selenization in supercritical ethanol for the production of compound semiconductor films

Selenization is an essential reaction for the production of the metal selenide absorption layer in compound semiconductor thin-film solar cells. Application of supercritical ethanol facilitates the conversion from a Cu–In film to a CuInSe2 film at a low temperature of 300°C. In this study, we investigate the contribution of supercritical alcohols to the selenization reaction by employing some kinds of supercritical alcohol media with different reduction powers. We found that the high reduction power of ethanol is necessary for selenization without excess selenium deposition at low temperature. Moreover, 1H NMR analysis revealed that at supercritical conditions, decomposition of diethyl selenide occurs and reactive selenium species with a short lifetime at process temperatures dictate the selenization reaction, whereas stable selenium species prevent the deposition of excess selenium.

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

ChemInform Abstract: Thin Films of Metals, Metal Chalcogenides and Oxides Deposited at the Water—Oil Interface Using Molecular Precursors

AbstractReview: 77 refs.

Thin films of metals, metal chalcogenides and oxides deposited at the water–oil interface using molecular precursors

The interface between two immiscible liquids is emerging as a powerful medium for the growth and deposition of nanostructured thin films of metals and compound semiconductors. Materials obtained through this exceedingly simple route seem to often adopt fascinating forms and morphologies. In this article, we outline features unique to the interface that shape the striking deposits and highlight the nature and properties of nanostructured films obtained using this technique.

ChemInform Abstract: New Metallo-Organic Precursors for Surface Processing

Abstract Metallo-organic compounds have been used for the deposition, by thermolysis or photolysis, of a wide range of inorganic materials as films on a variety of surfaces. This paper reviews recent work on metallo-organic precursors for the deposition of films of metals, metal oxides, metal nitrides, metal fluorides and compound semiconductors.

Supercritical Chemical Fluid Deposition of High Quality Compound Semiconductors

The main advantage of deposition from supercritical fluids over conventional CVD is the ability to fill high aspect ratio templates. Deposition of metals, indirect semiconductors and insulators into templates with dimensions down to 3 nm has been demonstrated by others, with no apparent pore blocking. In this paper we present a demonstration that it is also possible to deposit high quality, luminescent compound semiconductor films from supercritical CO2 or CO2/hexane mixtures. Depositions of CdS and III-V materials such as InP have been achieved through careful optimisation of precursor chemistry, reactor geometry and deposition conditions, supported by detailed measurements of the semiconductor film properties.

Electrochemical Synthesis of CdSe Nanoparticles Using a Se-modified RVC Electrode and Mercaptoacetic Acid as a Stabilizer

Semiconductor nanoparticles (NPs) are of fundamental and practical interest due to unique quantum size effects which result in strongly size dependent electronic, optical and electrochemical properties. Among them, CdSe nanoparticles are of much interest because of applications in solar photovoltaic cells, sensors and optoelectronic devices. CdSe NPs have been extensively studied following the development of chemical preparation procedures using hot trioctylphosphine oxide (TOPO) as a capping agent. Unlike the original method which employed toxic dimethyl cadmium as a cadmium source, other researchers have developed a “green synthesis” route and they synthesized CdSe NPs using CdO as a precursor. While hot TOPO-based approaches require rather severe experimental conditions such as toxic chemicals and high temperatures, electrochemical methods can provide much more “green” environments. Recently, CdTe NPs have been electrochemically synthesized using a Te disc electrode. In this method, a Te disc electrode was cathodically polarized in the electrolyte containing Cd and mercaptoacetic acid (MAA), which served as a stabilizer. We have developed a two-step method for the electrochemical synthesis of semiconductor films or particles. This approach is based on initial electrochemical modification of the substrate with the chalcogen. Subsequent electrochemical reduction of chalcogen in an aqueous electrolyte solution dosed with the desired metal generates compound semiconductor films on the substrate surface or particles in bulk solution with stirring. This approach has been successfully used for the synthesis of semiconductor NPs such as CdTe and films such as FeS2, In2S3, CdS, CdSe, CdTe, and ZnTe. Here we report the first example of electrochemical synthesis of CdSe NPs, in which particle size can be tuned by controlling heating time. In this study a RVC electrode was employed for the first time to synthesize CdSe NPs and this approach can be used for the synthesis of NPs in large quantities. Optical properties of NPs with different heating times were studied by absorption and photoluminescence spectroscopy.

Critical thickness of high structural quality SrTiO3 films grown on orthorhombic (101) DyScO3

Strained epitaxial SrTiO3 films were grown on orthorhombic (101) DyScO3 substrates by reactive molecular-beam epitaxy. The epitaxy of this substrate/film combination is cube on cube with a pseudocubic out-of-plane (001) orientation. The strain state and structural perfection of films with thicknesses ranging from 50 to 1000 Å were examined using x-ray scattering. The critical thickness at which misfit dislocations was introduced was between 350 and 500 Å. These films have the narrowest rocking curves (full width at half maximum) ever reported for any heteroepitaxial oxide film (0.0018°). Only a modest amount of relaxation is seen in films exceeding the critical thicknesses even after postdeposition annealing at 700 °C in 1 atm of oxygen. The dependence of strain relaxation on crystallographic direction is attributed to the anisotropy of the substrate. These SrTiO3 films show structural quality more typical of semiconductors such as GaAs and silicon than perovskite materials; their structural relaxation behavior also shows similarity to that of compound semiconductor films.

Alternative slip system activation in lattice-mismatched InP/InGaAs interfaces

Relaxation of compressed InP and InGaAs at different strains and temperatures is examined in this study. For example, different compressive strain states of InP can be attained by depositing InP on an InxGa1−xAs graded buffer with different compositions, x. In slightly strained elemental and compound semiconductor films, including InP and InGaAs, relaxation occurs via the primary-slip system, a∕2⟨110⟩{111}, where glide takes place on {111}-type planes. At high lattice mismatches, Lomer edge dislocations are the predominant form of relaxation. In this work at intermediate lattice-mismatch strains, relaxation occurred via the secondary, a∕2⟨110⟩{110}, slip system in InP and InGaAs. For these strains above a critical strain of about 0.0085, both the primary- and secondary-slip systems are active evident by cross slip between these two slip systems. The extent of relaxation via the primary- or secondary-slip system was found to be dependant on growth temperature. Relaxation via the secondary-slip system, when it was active, increased as the temperature decreased and we found a 1.5 eV difference between the activation energy for glide via the primary- and secondary-slip systems.

Effects of substrate temperature on the microstructure and photoluminescence properties of ZnO thin films prepared by atomic layer deposition

Atomic layer deposition (ALD) is a very promising deposition technique for compound semiconductor films such as ZnO. However, there have been very few reports on ZnO-ALD. Effects of substrate temperature on the microstructure and photoluminescence (PL) properties of ZnO thin films were investigated using X-ray diffraction, PL and scanning electron microscopy analysis techniques. The ALD window is found to be 130–180 °C. The ZnO film growth rate in ALD increases as the substrate temperature increases in the temperature range except the temperature window. The crystal quality depends most strongly upon the substrate temperature among all the process parameters of ALD. The crystallinity of the ZnO thin film is improved by increasing the process time per ALD cycle or doing post-annealing treatment. The grain size of the ZnO film tends to increase and the grain shape tends to change from a worm-like longish shape to a round one as the annealing temperature increases from 600 to 1000 °C.

Current research topics and applications of gas cluster ion beam processes

Gas cluster ion beam technology employs accelerated ions consisting of clusters of a few hundreds to thousands of atoms (gas cluster ion beam technology, GCIB). This paper reviews recent novel GCIB process applications for (i) ultra-shallow junction (USJ) formation and doping in Si, (ii) surface smoothing of metals, (iii) selective smoothing of SOI, SiC and other compound semiconductor films, (iv) IC back end of the line (BEOL) processing and (v) high quality thin multi-layer film formation for reliable and durable optical filters.

Current research and development topics on gas cluster ion-beam processes

This report reviews the developing field of ion-beam technology that employs accelerated ions consisting of clusters of a few hundreds to thousands of atoms (gas cluster ion-beam technology, GCIB). Cluster ion-surface collisions have been found to produce low-energy bombardment effects at very high density, and GCIB processes exhibit unique nonlinear effects that are useful for surface processing applications. The effects include low-energy ion bombardment, lateral sputtering, and low-temperature thin-film formation. GCIB processing has been applied to shallow junction formation; high rate etching; surface smoothing of materials including metals, dielectrics, superconductors, and diamonds; and high-k oxide and diamond-like carbon thin-film deposition. We describe the unique characteristics of GCIB-surface interactions and GCIB process applications for (i) ultrashallow junction formation and doping in Si, (ii) surface smoothing of metals, (iii) selective smoothing of silicon-on-insulator SiC, and other compound semiconductor films, (iv) integrated circuit back-end-of-the-line processing, and (v) high-quality multilayer thin-film formation for reliable and durable optical filters.

Important research targets to be explored for β-FeSi 2 device making

To place β-FeSi 2 as a 3rd generation Kankyo (Environmentally Friendly) Semiconductor after GaAs, one should demonstrate its superior features by fabricating practical devices. One has to prepare high-quality β-FeSi 2 films with (1) precisely controlled Fe/Si ratio (1:2), (2) flat and cracks-free surface, (3) pinhole-free surface and interfaces, (4) flat interfaces, (5) well-controlled electron or hole concentration with residual carrier concentration as low as ∼10 16 cm −3. One should accordingly explore novel thin film manufacturing technologies by considering specific properties of constituent Fe and Si atoms. Conventional growth methods used for III–V and II–VI compound semiconductor films are not suited for β-FeSi 2. Here we summarize the current status of film preparation technologies and describe their advantages and drawbacks. To explore the possibility of β-FeSi 2 for low cost and high conversion efficiency solar cells, high quality β-FeSi 2 films have been formed on Si substrates by molecular beam epitaxy (MBE) and sputtering methods. The critical feature about the device structure is an optimized thin β-FeSi 2 template buffer layer on Si(111) substrate. The template served as a substrate for epitaxial growth of single crystal β-FeSi 2 film and restrains the Fe diffusion into Si at β-FeSi 2/Si interface. For n-β-FeSi 2/p-Si structure under air mass 1.5, an energy conversion efficiency of 3.7% was obtained, showing that β-FeSi 2 is practically a promising semiconductor for making solar cells.

Important research targets to be explored for $beta;-FeSi2 device making

To place β-FeSi2 as a 3rd generation Kankyo (Environmentally Friendly) Semiconductor after GaAs, one should demonstrate its superior features by fabricating practical devices. One has to prepare high-quality β-FeSi2 films with (1) precisely controlled Fe/Si ratio (1:2), (2) flat and cracks-free surface, (3) pinhole-free surface and interfaces, (4) flat interfaces, (5) well-controlled electron or hole concentration with residual carrier concentration as low as ∼10 16 cm−3. One should accordingly explore novel thin film manufacturing technologies by considering specific properties of constituent Fe and Si atoms. Conventional growth methods used for III–V and II–VI compound semiconductor films are not suited for β-FeSi2. Here we summarize the current status of film preparation technologies and describe their advantages and drawbacks. To explore the possibility of β-FeSi2 for low cost and high conversion efficiency solar cells, high quality β-FeSi2 films have been formed on Si substrates by molecular beam epitaxy (MBE) and sputtering methods. The critical feature about the device structure is an optimized thin β-FeSi2 template buffer layer on Si(111) substrate. The template served as a substrate for epitaxial growth of single crystal β-FeSi2 film and restrains the Fe diffusion into Si at β-FeSi2/Si interface. For n-β-FeSi2/p-Si structure under air mass 1.5, an energy conversion efficiency of 3.7% was obtained, showing that β-FeSi2 is practically a promising semiconductor for making solar cells.

Band gap engineering in polycrystalline Cu(In,Ga)(Se,S) 2 chalcopyrite thin films

The reaction of metallic alloys to selenium and/or sulfur is a promising two-step growth process to produce chalcopyrite absorber films for application in thin film photovoltaic devices. However, the success of these classical two-step growth processes is limited by insufficient control over the lattice parameters and the band gap of the compound semiconductor films. This problem, which is perceived to be insurmountable, is directly related to the poor control over the diffusion properties of gallium and sulfur in the thin films under typical growth conditions. In this contribution, it is indicated for the first time that single-phase Cu(In 1- x Ga x )(Se 1- y S y ) 2 pentenary alloys can be produced by the selenization/sulfurization of copper–indium–gallium metallic alloys. Even more significantly, it will be demonstrated that the reaction parameters (i.e. gas concentrations, reaction temperature, and time) during the selenization process can be manipulated to control the lattice parameters and band gap values of the pentenary semiconductor alloys.

An accurate continuous level indication for precursor bubblers

Precursors for use in metal organic vapour phase epitaxy (MOVPE) of compound semiconductor films are supplied in high integrity stainless steel containers to ensure the highest levels of purity and safety are maintained. However, this means that an accurate level determination is difficult to obtain in situ and growers must rely on usage calculations based on source vapour pressure values quoted in the literature. A number of level sensing systems have been investigated with some degree of success to address this problem, but all have suffered from excessive complexity, being limited to a fixed level indication, fragility and other issues. The most suited technology has been identified as capacitance related and data is reported here obtained from a simple, compact, robust probe fitted to a standard MOVPE bubbler. Results from standard source materials trimethylaluminium (TMA), trimethylgallium (TMG), dimethylzinc (DMZ) and diethylzinc (DEZ) under static and simulated chemical vapour deposition (CVD) operating conditions reported here clearly demonstrate that an accurate continuous level can be measured. Information on the repeatability, material compatibility and temperature dependence of the probe is also presented.