Close-spaced Vapor Transport Research Articles

Impact of graphene as 2D interlayer on the growth of GaAs by CSVT on Si (100) and GaAs (100) substrates

GaAs layers were grown using the closed space vapor transport (CSVT) technique on GaAs (100) and Si (100) substrates, with and without Graphene (G) as 2D interlayer. We show that GaAs grown on G presents higher crystalline quality, compared to GaAs grown directly on the substrates without the G interlayer. X-ray diffraction ω−2θ scans suggest that the GaAs layers on G exhibited (111) and (220) preferred orientation, while GaAs grown directly on GaAs and Si substrates displayed three additional orientations, which were (100), (311), (331). Further investigation through pole figure measurements confirmed the preferred out-of-plane orientation and the presence of twin defects in GaAs layers. Remarkably, we observed a reduced presence of twins in GaAs grown on G compared to GaAs grown directly on the GaAs and Si substrates. Additionally, we found a significant impact on the morphology and on the growth rate of the GaAs on G, as determined using scanning electron microscopy and secondary ion mass spectrometry, respectively. These findings showed the importance of the G interlayer in enhancing the crystalline quality of GaAs growth by CSVT, demonstrating comparable results on both GaAs (100) and Si (100) substrates. This advancement holds significant promise for reducing the manufacturing cost of optoelectronic devices.

Femtosecond laser thinning for resistivity control of tungsten ditelluride thin-films synthesized from sol-gel deposited tungsten oxide

In this work we present a route for fabricating WTe2 thin-films together with femtosecond laser post processing, enabling to finely control the conductivity. First, we produce amorphous films of WO3 on Si by spin-coating a sol-gel precursor followed by a consolidating annealing and a reduction process in partial H2 atmosphere, leading to porous metallic tungsten cluster layers. To achieve WTe2, the films were exposed to the chalcogen vapours by isothermal closed space vapor transport. The formation of a tungsten ditelluride film composed of piled crystals could be confirmed and a gradient of surface rich Te identified through hard X-ray photoelectron spectroscopy. Finally, it is demonstrated that resistivity can be changed from 0.2 mΩ.m to 1 mΩ.m, while keeping the material characteristics. An anisotropic conductivity can be induced by direct selective thinning with fs laser writing (350 fs pulse duration, 515 nm laser wavelength) of 1D stripes. The obtained results, demonstrate that laser processing is a promising thin-film post-processing technique that can be applied to 2D transition metal dichalcogenide thin films.

Study of Nitridation Effect on Structural, Morphological, and Optical Properties of GaAs Film Growth on Silicon Substrates via Close Space Vapor Transport Technique

In this work, Gallium Arsenide (GaAs) films growth via Close Space Vapor Transport (CSVT) technique on n-type Silicon (Si) substrates (100) and its nitridation effect in the ammonia (NH3) environment is reported. The GaAs films were grown at 800, 900, and 1000 ∘C, and the nitridation process was carried out at 900 ∘C with an NH3:H2 gasses ratio. The GaAs films with and without nitridation process were analyzed using X-ray diffraction (XRD), Raman spectroscopy, Diffuse Reflectance Spectroscopy, and Scanning Electron Microscopy with Energy Dispersive X-ray analysis (SEM-EDX). Grazing incidence X-ray diffraction measurements of GaAs films nitrided confirm a polycrystalline GaN wurtzite structure with preferential orientation along (002), and additionality, a crystallographic plane (310) of low intensity is observed in 2θ=52.18∘ corresponding to Ga2O3. The average quantification results in weight (Wt. %) of GaAs films nitrided was determined by EDS; Ga∼79, N∼17.1, O∼2 and As∼1.8 Wt. %. The presence of GaN, GaxOy, Si, and GaAs modes were found by Raman measurements, demonstrating a partial nitriding. The band gap estimation by diffuse reflectance was between 3.2 and 3.38 eV such values are close to that reported for bulk GaN (3.4 eV). The presence of oxygen in the structure could be related to substrates or the GaAs source.

Study the Structural/Microstructural and Electrical Properties of CdTe:Sb Thin Films Deposited by Flash Evaporation

in this research studied the structure and electrical properties of the Pure CdTe and metal Sb-doped CdTe thin films with 0.5, 1, and 1.5 wt.%, respectively. Theydeposited on glass substrates using flash evaporation technique. It was found that theprepared films had a cubic structure and a strong preferred <111> crystal orientation.Atomic force microscopy (AFM) results showed that the film surface doped Sb wasmore compact and uniform and also the root mean squared (RMS) roughnessincreased with Sb contents. The electrical properties were investigated as function ofSb wt.% doping. The results showed that the electrical conductivity (6) increasingtrend with increasing the wt.% of dopant , while the activation energies (E,i, E,2)showed an opposite trend, where the activation energies decreased with dopant.1. Introduction Thin films of II-VI semiconductors are widely used in many semiconductordevices such as photo electrochemical cells, field effect transistors, IR detectors,photodiodes, photo conductors and photovoltaic solar cells[1, 2]. CdTe has long beenidentified as a candidate for the absorber layer in low cost thin film photovoltaic solarcells because of its direct bandgap, high absorption coefficient and the possibility of avariety of preparation techniques such as close spaced sublimation (CSS)[3], vacuumdeposition [4], electrodeposition [5], and RF sputtering[6], close spaced vaportransport [7]. In this present work we focused attention on the prominent techniquesfor the deposition of thin films of multicomponent alloys whose constituents havedifferent vapor pressures. Thin films of CdTe have been prepared under suitablegrowth conditions by a simple flash evaporation (FE) technique. This techniquerequires only one boat maintained at sufficiently high temperature to evaporate theleast volatile component of the alloy. The main advantage of (FE) is that it does notrequire maintaining the critical vapor pressures of the components and temperatures ofthe boats unlike multisource thermal evaporation[8] . CdTe thin film still has many defects, such as it has extremely high resistivity. Tosolve this issue, the implying of the technique Sb-doped CdTe film to change electricalproperties [9]. In this work deposited Sb-doped CdTe films using the mixed powder of TO -AA suet Talal = - VN - Kya lal ayy cll 212 alae

Structural and optical properties of CuInSe2

Thin films of polycrystalline CuInSe2 were deposited on Pyrex substrates using a simple system of close spaced vapor transport (CSVT). The used CSVT system is an open horizontal reactor, this does not require vacuum, a gas flow is enough. During the growth phase, the CSVT system is continuously crossed by argon gas. A study on the influence of the source temperature and the deposition duration on the structural properties of the deposited films is re- ported. Analyses by X-ray diffraction have shown that these films are polycrystalline and have a chalcopyrite structure. The preferential orientation of the (112) plane was obtained for the films deposited at 550 ◦C. From the X-ray spectra we calculated the lattice parameters a and c, the ratio c/a was found to be close to 2. The characterization of the de- posited films by an energy dispersion spectrometer (EDS) has shown that their chemical composition is quasistoichiometric with a ratio Cu/In varying from 0.96 to 1.10. Analysis with a scanning electron microscope (SEM) of the de- posited films surface has shown that those slightly rich in indium present a more homogeneous morphology and smaller crystallites sizes than the films slightly rich in copper. The measurement of the photoconductivity of the prepared com- pound has allowed us to determine the value of its gap at room temperature. It was found to be close to 0.99 eV.

GaN obtained on quartz substrates through the nitridation of GaAs films deposited via CSVT

The use of gallium arsenide (GaAs) films to obtain gallium nitride (GaN) on a quartz substrate is reported herein. The GaAs films were deposited on quartz substrates via the close-spaced vapor transport technique (CSVT) with good crystalline quality. GaN was obtained via nitridation wherein the GaAs samples were subjected to ammonia and hydrogen flow at atmospheric pressure. Several experiments were conducted at 800 °C, 900 °C, and 1000 °C, suggesting that nitridation at 900 °C is suitable for GaN obtention. Another group of experiments was conducted to observe the effect of nitridation time on the morphology of GaN. The nitridation temperature was kept at 900 °C while the nitridation times were 10, 20, and 30 min. XRD and EDS characterizations revealed that the nitridation time determines the production of samples with both GaAs and GaN structures (10 min) or only the GaN structure (20 and 30 min). A GaN film exhibiting a wurtzite preferential structure was obtained subsequent to 30 min of nitridation.

Structural, electrical and optical properties of CuIn0.8Ga0.2S2 thin film deposited by close spaced vapor transport technique

Abstract High quality CuIn0.8Ga0.2S2(CIGS) absorber material for thin film solar cells applications was deposited on soda lime glass substrate through the close spaced vapor transport (CSVT) technique. Deposition conditions were chosen to obtain high quality films. CIGS thin layer was investigated by scanning electron microscope (SEM), X-Ray diffraction (XRD), Vis-NIR spectroscopy and Hall Effect measurements. The temperature dependent Hall Effect measurements in the range 80–400 K have been performed. Activation energies were estimated from the Arrhenius plot of the conductivity versus 1/T and were42 meV and 15 meV at high and low temperature regimes, which coincide with the activation energies of the Cu vacancy (VCu) and the Ga vacancy (VGa), respectively. The mobility's temperature dependence of the charge was discussed and the predominant scattering mechanisms were assigned to the ionized impurity and non-polar optical phonon scattering at low and high temperature regimes, respectively.

Growth of out-of-plane standing MoTe2(1-x)Se2x/MoSe2 composite flake films by sol–gel nucleation of MoOy and isothermal closed space telluro-selenization

This study describes the sol–gel processing of MoOy on Si (100) to subsequently achieve out-of-plane MoTe2/MoSe2 flake composite films by an isothermal closed space vapor transformation. The oxide precursor films have been prepared from a Mo isopropoxide solution in isopropanol and acid catalysis induced by HCl. Thermal annealing at 200, 400 and 600 °C enhanced the condensation after xerogel formation. An x-ray absorption analysis demonstrates that films condensed at 200 °C are at an intermediate chemical state between MoO3 and MoO2. To achieve MoTe2/MoSe2 composite films, the precursor oxide films were reduced in H2 and exposed to the chalcogenides by isothermal closed space vapor transport at 600 °C. The multilayered nanocomposite films grow with an out-of-plane flake-like structure and an evident integration of Se in the MoTe2 phase according to a MoTe2(1-x)Se2x alloy, with an estimation of x of 0.25. The alloy and the orientation of the flakes are consistent with the bands present in the Raman spectrum. These films are attractive for applications requiring high surface area interfaces favoring gas or ion exchange reactions with transition metal dichalcogenides.

Argon vs. air atmosphere in close spaced vapor transport deposited tin sulfide thin films

The fundamental understanding and tailoring of material properties play a fundamental role on the performance of tin (II) sulfide (SnS)-based photovoltaic devices. In this regard, this work reports on the deposition of single-phase, p-type SnS thin films synthesized by close spaced vapor transport (CSVT) and the impact of employing argon (Ar) and bare vacuum (air) atmospheres on CSVT-SnS thin film properties is presented and compared for the first time. The analysis of film properties was performed by structural, directional, morphological, topographical, along with thermoelectric and optoelectronic characterizations of the samples. It is demonstrated that by changing the CSVT annealing atmosphere, the SnS grains change their orientation from perpendicular (respect to substrate) in Ar to parallel in a bare vacuumed (air) conditions. Furthermore, Hall measurements show that the hole concentration are mostly the same but that hole mobility depends, in turn, on the direction of preferred orientation of the SnS crystals. This translates into a twice higher lateral conductivity on the air- respect to the argon-SnS thin films. In this way, the different atmospheric exhibit a profound impact on the physical, morphological and structurally orientated properties of the SnS grains. These results demonstrate that photovoltaic grade p-type SnS films can be deposited in air without the need of an inert gas atmosphere which opens the way to a cost reduction in the fabrication of SnS-based photovoltaic devices.

Interfacial strain defines the self-organization of epitaxial MoO2 flakes and porous films on sapphire: experiments and modelling

The epitaxy of MoO2 on c-plane sapphire substrates is examined. A theoretical approach, based on density functional theory calculations of the strain energy, allowed to predict the preferred layer/substrate epitaxial relationships. To test the results of these calculations, MoO2/(0 0 1) Al2O3 heterostructures were grown using the chemically-driven isothermal close space vapour transport technique. At the early stages of the growth, two kinds of morphologies were obtained, using the same growth parameters: lying and standing flakes. The composition and morphology, as well as the layer/substrate epitaxial relationships were determined for both kind of morphologies. Experimental epitaxial relationships coincide with those predicted by DFT calculation as the most favourable ones in terms of strain energy. For thicker films, the standing flakes evolve to form an epitaxial porous layer composed by coalesced epitaxial flakes. The interfacial strain between the sapphire substrate and MoO2 enables a self-organization from nanometer to micron scales between separated or coalesced flakes, depending on deposition condition.

Design of Ultrathin InP Solar Cell Using Carrier Selective Contacts

Most recently, III–V based ultrathin solar cells have attracted considerable attention for their inherent advantages, such as increased tolerance to defect recombination, efficient charge carrier separation, photon recycling, flexibility, and reduced material consumption. However, so far, almost all reported devices make use of conventional doped p-i-n kind of structures with a wide-bandgap III–V lattice-matched epitaxial window layer, for passivation and reduced contact recombination. Here, we show that a high-efficiency device can be obtained utilizing an InP thin film of thickness as low as 280 nm, without the requirement of a conventional p-n homojunction or epitaxial window layer. This is achieved by utilizing a wide-bandgap electron and hole selective contacts for electrons and holes transport, respectively. Under ideal conditions [assuming interface recombination velocity (IRV) = 103 cm/s and bulk lifetime = 1 μs], the proposed solar cell structure can achieve efficiency as high as 28%. Although, in the presence of bulk and interface Shockley–Read–Hall recombination, the efficiency reduces, still for bulk minority carrier lifetime as low as 2 ns and an IRV as high as 105 cm/s, an efficiency of ∼22% can be achieved with InP thickness as low as 280 nm. The proposed device structure will be beneficial in cases where the growth of controlled p-n homojunction and window layer can be tedious as in case of low-cost deposition techniques, such as thin-film vapour–liquid–solid and close-spaced vapour transport.

The growth of methylammonium lead iodide perovskites by close space vapor transport.

Vapor deposition processes have shown promise for high-quality perovskite solar cells with potential pathways for scale-up to large area manufacturing. Here, we present a sequential close space vapor transport process to deposit CH3NH3PbI3 (MAPI) perovskite thin films by depositing a layer of PbI2 then reacting it with CH3NH3I (MAI) vapor. We find that, at T = 100 °C and pressure = 9 torr, a ∼225 nm-thick PbI2 film requires ≥125 minutes in MAI vapor to form a fully-reacted MAPI film. Raising the temperature to 160 °C increases the rate of reaction, such that MAPI forms within 15 minutes, but with reduced surface coverage. The reaction kinetics can be approximated as roughly first-order with respect to PbI2, though there is evidence for a more complicated functional relation. Perovskite films reacted at 100 °C for 150 minutes were fabricated into solar cells with an SLG/ITO/CdS/MAPI/Spiro-OMeTAD/Au structure, and a device efficiency of 12.1% was achieved. These results validate the close space vapor transport process and serve as an advance toward scaled-up, vapor-phase perovskite manufacturing through continuous vapor transport deposition.

Non-epitaxial carrier selective contacts for III-V solar cells: A review

In the last few years, carrier selective contacts have emerged as a means to reduce the complexities and losses associated with conventional doped p-n junction solar cells. Still, this topic of research is only at its infancy for III-V solar cells, in comparison to other solar cell materials such as silicon, perovskites, chalcogenides, etc. This could be because high quality III-V solar cell materials can be achieved relatively easily using epitaxial growth techniques such as MOCVD (metal organic chemical vapor deposition) and MBE (molecular-beam epitaxy). However, current epitaxial III-V solar cells are very expensive and cannot compete for the terrestrial market, and therefore, researchers are developing alternative growth methods such as thin-film vapor–liquid–solid (TF-VLS), hydride vapor phase epitaxy (HVPE) and closed space vapor transport (CSVT), which are significantly lower in cost compared to epitaxial III-V solar cells. However, at present, these relatively nascent low cost growth methods, face severe optimization issues when it comes to growth of controlled p-n junction, along with heavily doped window and back surface field layers. In such cases, carrier selective contacts can be hugely beneficial. In this review, we cover some of the most recent research on the use of carrier selective contacts for III-V solar cells. Future prospects, challenges, and new device concepts using carrier selective contacts will also be discussed.

All-sputtered CdTe solar cell activated with a novel method

CdTe thin film solar cells are usually fabricated depositing the absorber layer by Close Space Sublimation (CSS) and Vapor Transport Deposition (VTD), mainly due to the high deposition rate allowed by these techniques. However, most of the solar cell films can be deposited by sputtering, making to deposit also the CdTe by this technique attractive. Indeed, an all-sputtered solar cell could be deposited in one sequential chamber at a relatively low temperature and without breaking the vacuum, resulting in a sensible decrease in the production cost at the industrial level. Usually, treatments based on CdCl2, also known as activations, are applied to the CdTe solar cells to achieve high power conversion efficiency (PCE). However, it was observed that the usual activation methods could be aggressive for all-sputtered CdTe solar cells, damaging the films and resulting in low performance. In this work, we propose a novel activation method based on Nitrogen/Oxygen/CHCIF2 gas mixture, which is more suitable for this kind of device. Morphological, structural, optical and electrical properties were analyzed and compared to our CSS-CdCl2 baseline. A PCE of 12% was achieved for all-sputtered CdTe solar cells.

CuIn0.7Ga0.3Se2 thin films\u2019 properties grown by close-spaced vapor transport technique for second-generation solar cells

In this paper, CuIn0.7Ga0.3Se2 (CIGS) thin films are deposited on both glass (SLG) and glass/SnO2:F (SLG/FTO) substrates, by close-spaced vapor transport technique. The Hall effect measurements are performed in the temperature range (300–438 K) for the two SLG/CIGS samples namely CIGS1 and CIGS2, grown at substrate temperature (Ts) of 470 °C and 510 °C, respectively, to investigate the temperature effect on the electrical parameters such as hole concentration (p), conductivity (σ) and mobility (µ). As results, from Arrhenius diagram of (p) and (σ), bandgap energy (Eg) of about 1.38 eV and 1.24 eV are extracted for CIGS1 and CIGS2, respectively. Besides, activation energies (Ea) at 563.9 meV and 239.4 meV are determined for CIGS1 whereas values at 584.2 meV and 72.7 meV are obtained for CIGS2. Furthermore, average mobilities of 1.83 cm2/V s and 1.77 cm2/V s are achieved for CIGS1 and CIGS2 thin films, respectively. Pure aluminum (Al) Schottky contacts are deposited on the front side of FTO/CIGS thin film-devices by physical vapor deposition. Current–voltage (I–V) characteristics are measured and used to extract the electrical parameters of FTO/CIGS/Al Schottky diode using the one diode model. The electrical parameters including series resistance (Rs) of about 93.7 Ω and an ideality factor (n) around 3.47 indicate that the generation–recombination mechanism is predominant.

Optical and electrical properties of MoO2 and MoO3 thin films prepared from the chemically driven isothermal close space vapor transport technique

Chemically—driven isothermal close space vapour transport was used to prepare pure MoO2 thin films which were eventually converted to MoO3 by annealing in air. According to temperature-dependent Raman measurements, the MoO2/MoO3 phase transformation was found to occur in the 225 °C–350 °C range while no other phases were detected during the transition. A clear change in composition as well as noticeable modifications of the band gap and the absorption coefficient confirmed the conversion from MoO2 to MoO3. An extensive characterization of these two pure phases was carried out. In particular, a procedure was developed to determine the dispersion relation of the refractive index of MoO2 from the shift of the interference fringes of the used SiO2/Si substrate. The obtained data of the refractive index was corrected taking into account the porosity of the samples calculated from elastic backscattering spectrometry. The Debye temperature and the residual resistivity were extracted from the electrical resistivity temperature dependence using the Bloch–Grüneisen equation. MoO3 converted samples presented a very high resistivity and a typical semiconducting behavior. They also showed intense and broad luminescence spectra composed by several contributions whose temperature behavior was examined. Furthermore, surface photovoltage spectra were taken and their relation with the photoluminescence is discussed.

Effect of the Gaseous Atmosphere in GaAs Films Grown by Close-Spaced Vapor Transport Technique

The effect of the gaseous atmosphere in the growth of gallium arsenide (GaAs) films was studied. The films have been grown by close-spaced vapor transport (CSVT) technique in a home-made hot filament chemical vapor deposition (HFCVD) reactor using molecular hydrogen and molecular nitrogen as the transport agent. An important point about the gaseous atmosphere is the ease in creating volatile compounds when it makes contact with the GaAs source, this favors the transport of material in a CSVT system. Chemical reactions are proposed in order to understand the significant difference produced from the gaseous atmosphere. The films grown with hydrogen are (almost) continuous and have homogeneous layers with preferential orientation (111). The films grown with nitrogen are granular and rough layers with the coexistence of the orientations (111), (220) and (311) in the crystals. The incorporation of impurities in the films was corroborated by energy dispersive spectroscopy (EDS) showing traces of oxygen and nitrogen for the case of the samples obtained with nitrogen. Films grown in a hydrogen atmosphere show a higher band gap than those grown in a nitrogen atmosphere. With the results of XRD and micro-Raman we observe a displacement and broadening of the peaks, characteristic of a structural disorder. The calculations of the FWHM allow us to observe the crystallinity degree and determine an approximate crystallite size using the Scherrer’s equation.

Close-spaced vapor transport reactor for III-V growth using HCl as the transport agent

Low-cost methods of III-V deposition are an important component of making high-efficiency III-V solar cells cost-competitive for terrestrial applications. Here, we report the design and testing of a close-spaced vapor transport system for the growth of epitaxial GaAs films using solid precursors and HCl as the transport agent. Previous work on a related system demonstrated growth of high-quality GaAs using H2O as the transport agent, but the use of H2O generates oxide-related defects and limits material compatibility. The new design also enables independent handling of source and substrate material. The effect of process conditions on growth rate, surface morphology, and substrate-orientation-dependent growth is discussed. We demonstrate successful doping of Si and Zn with average transport efficiencies of approximately 30% and 20%, respectively, based on secondary-ion-mass-spectrometry data. Room-temperature hall mobilities approached those achieved for GaAs grown by metal-organic vapor phase epitaxy and water-based close-spaced vapor transport, 2210–3400 cm2 V−1 s−1 for n-GaAs and 70–110 cm2 V−1 s−1 for p-GaAs depending on dopant concentration. Initial results on doping and cross-contamination suggest this system should be capable of homoepitaxial growth of p-n junctions.

WTe2 Synthesis by Tellurization of W Precursors Using Isothermal Close Space Vapor Transport Annealing

The preparation of WTe2 by isothermal tellurium annealing of sputtered W or WO3 precursors is reported. X‐ray diffraction, energy dispersive, and Raman spectroscopies confirm the presence of 1T́ WTe2 phase in the annealed films which are preferentially oriented with the [001] crystallographic direction normal to the surface. Atomic force microscopy images indicate noticeable morphology differences between thin samples tellurized from different precursors. Rounded small grains or small platelets are obtained when W and WO3 precursor are used, respectively.

Structural and photoluminescent properties of CuGaxIn1−xSe2 thin films prepared by close-spaced vapor transport technique

CuGaxIn1−xSe2 (CIGS) (x = 0, x = 0.3 and x = 1) absorber layers were deposited onto transparent conducting oxide (SnO2) substrates using close-spaced vapor transport technique (CSVT). The aim of this work was to assess the quality of CIGS absorber films with various gallium contents. Composition found by Energy Dispersive Spectroscopy (EDS) showed that the investigated samples were Cu-poor and CuGa0.3In0.7Se2 thin film had a composition close to 0.3 as planned. The structural properties were investigated by X-ray diffraction analysis (XRD) and Raman spectroscopy, while optical properties were investigated by measuring transmittance and photoluminescence spectroscopy. It was found that all investigated samples crystallize with (112) as a preferential crystallographic direction. The shift of the XRD peaks towards a higher value of 2θ with the increase of Ga composition was observed. The grain size determined from the width of 112 XRD peak was within 40–75 nm range with no clear correlation with Ga content. Raman spectra of samples with x = 0 and 0.3 featured mainly a peak assigned to the A1 mode at about 175 cm−1. In CuGaSe2 the secondary Cu2Se phase was detected both by XRD and Raman. The band gap as evaluated by optical transmission measurement is shifted towards lower than expected values due to substantial defect-related bands broadening. A large concentration of defects in the samples was also confirmed by low or absent PL signal in the vicinity of the bandgap, while the emission due to deep defects was prevailing in the spectrum. The optoelectronic properties as indicated by photoluminescence have to be improved if CSVT method of absorber preparation is to be used more widely in photovoltaics.