Close-spaced Vapor Transport Technique Research Articles

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.

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.

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.

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.

CuInS2 thin films obtained through an innovative CSVT deposition method from solvothermal-generated precursors

Copper indium disulfide (CuInS2) nanoparticles with sizes 20 ± 5 nm were synthesized by solvothermal techniques. Subsequently these were deposited for the first time on glass via a close-spaced vapor transport technique (CSVT) using iodine as transport agent. Optimization of the temperature and time was carried out during the deposition process. The surface properties were investigated in details by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis–NIR and Raman spectrophotometer, energy dispersive spectroscopy and electron diffraction (EDS). The CuInS2 films presented polycrystalline morphology, preferentially oriented along the (112) crystallographic plane and have band-gap values between 1.44 and 1.60 eV. The development of these films is relevant for photovoltaic applications.

Effect of iodine utilisation conditions on CSVT deposited CuInSe2 films

In this work, we investigate the effect of the carrier gas (iodine) utilisation conditions on the growth of CuInSe2 (CIS) thin films deposited by the close spaced vapour transport technique. This study shows that the CIS films deposited at 430°C, with heated iodine and open reactor, are of high quality in structural, optical and electrical properties. Analysis by X-ray diffraction showed that these films are polycrystalline and have a chalcopyrite structure. The preferential orientation according to the plane (112) was obtained. The morphological study of these films showed that they present a compact aspect with crystallites of uniform size. The energy dispersive spectrometer analysis showed that these films are slightly copper rich. The optical and electrical studies at room temperature gave the following results: an energy gap of 1 eV, an absorption coefficient higher than 105 cm− 1, p type conductivity and a low resistivity of 0·12 Ω cm.

Doping and electronic properties of GaAs grown by close-spaced vapor transport from powder sources for scalable III–V photovoltaics

We report the use of a simple close-spaced vapor transport technique for the growth of high-quality epitaxial GaAs films using potentially inexpensive GaAs powders as precursors.

Synthesis of In–In2O3 microstructures by close-spaced vapor transport (CSVT) and their transformation to In2O3 nanobelts at low temperature

Indium oxide (In2O3) microstructures were synthesized through carbothermal reaction by the close-spaced vapor transport (CSVT) technique at low temperatures and without the use of catalysts. Morphological characterization by scanning electron microscopy (SEM) showed that the CSVT technique provides high-efficiency for the mass transport at low temperatures, as low as 650 °C, considering that in the synthesis indium oxide powders were used as a source. In addition, it is observed that the microstructures are formed mainly of microcubes with microspheres attached on their faces. Measurements of X-ray (XRD), Raman, and Energy-dispersive X-ray spectroscopy (EDS) showed that the microcubes tend to the stoichiometry of In2O3 and the microspheres have an excess of indium. Based on the above results, the microstructures were converted to In2O3 nanobelts by a simple post thermal annealing for 16 h in a nitrogen environment, according to additional morphological and structural characterizations. The results revealed that the In2O3 nanobelts have several micrometers in length with rectangular cross sections in the range from 50 to 250 nm and correspond to body-centered cubic (bcc) In2O3 single crystal. The growth mechanisms of the microstructures and nanobelts are discussed in detail. Finally, Photoluminescence spectrum showed a broad emission around 600 nm. Refined studies showed three peaks at 420, 592, and 673 nm, which were attributed to oxygen vacancies.

Effects of temperature, pressure and pure copper added to source material on the CuGaTe2 deposition using close spaced vapor transport technique

The quality of CuGaTe2 (CGT) thin films elaborated by close spaced vapor transport technique has been studied as a function of the source temperature (TS), iodine pressure (PI2) and the amount (XCu) of pure copper added to the stoichiometric starting material. A thermodynamic model was developed for the Cu–Ga–Te–I system to describe the CGT deposition. The model predicts the solid phase composition with possible impurities for the operating conditions previously mentioned. The conditions of stoichiometric and near-stoichiometric deposition were determined. The value of TS must range from 450 to 550°C for PI2 varying between 0.2 and 7kPa. Adding an amount up to 10% of pure copper to the starting material improves the quality of the deposit layers and lowers the operating interval temperature to 325–550°C. These optimal conditions were tested experimentally at 480°C and 500°C. The X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy have proved that the addition of pure copper to the stoichiometric source material can be considered as a supplementary operating parameter to improve the quality of CGT thin films.

Thermodynamic prediction and experimental verification of optimal conditions for the growth of CuGa0,3In0,7Se2 thin films using close spaced vapor transport technique

The aim of this work is to predict optimal conditions for growing good quality crystalline thin films using close spaced vapor transport (CSVT) technique. A thermodynamic model was developed and tested for the Cu–Ga–In–Se–I system to describe the deposition of CuGa0,3In0,7Se2 (CIGS). We considered a multiphase mixture containing 48 chemical compounds derived from the combination of the simple elements Cu, Ga, In, Se and I. The minimization of the Gibbs energy of the system was performed to calculate the composition of the mixture at the equilibrium state. In this way, the solid phase composition with possible impurities was predicted for various source temperature (TS) and iodine pressure (PI2). The conditions of stoichiometric and quasi-stoichiometric deposition are from 400 to 600°C for TS and from 30Pa to 14kPa for PI2. These optimal conditions were tested experimentally at 475°C, 500°C, 525°C and 550°C. The elaborated CIGS thin films were of good quality as revealed by X-ray diffraction and scanning electron microscopy. The experimental results proved that the thermodynamic model is a helpful tool for the prediction of the optimal conditions in the CSVT process.

Properties of low-refractive-index films obtained by the close-spaced vapor transport technique under the sublimation of graphite in a quasi-closed volume

The properties of low-refractive-index carbon films obtained by close-spaced vapor transport at graphite sublimation are studied. The optical properties of the films are investigated by monochromatic multiple-angle ellipsometry, and their morphology is examined by AFM. It is found that the films have a columnar structure with a background surface roughness of about 1 nm. In addition, the surface of the film contains islands up to 50 nm in height with a footprint of ≈200 nm. A low-refractive-index carbon film deposited by close-spaced vapor transport on silicon tips is found to decrease the field emission threshold and drastically raise the current.

CuGa xSe y chalcopyrite-related thin films grown by chemical close-spaced vapor transport (CCSVT) for photovoltaic application: Surface- and bulk material properties, oxidation and surface Ge-doping

Device-grade ternary Cu–Ga–Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10×10 cm 2. A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe 2-related defect compound structures (DC) with stoichiometries of CuGa 3Se 5 and CuGa 5Se 8. In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe 2-related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa 3Se 5 formation in gallium-rich CCSVT-grown CuGa x Se y films. The chemical and electronic surface and interface structure of CuGaSe 2 thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced “cliff-like” conduction band offset at the respective interface. Furthermore, the CuGaSe 2 thin film degradation under ambient as well as under thermal conditions of CuGaSe 2 thin films has been studied by XPS. During thermal oxidation, the formation of predominantly Ga 2O 3 and some amount of SeO 2 were observed, but no copper oxides could be detected in the near-surface region of the thin films. The same oxides are found after native oxidation in air under ambient conditions. An additional sodium oxide compound formed at the thin film surface, Na x O and Na 2CO 3 after thermal and native oxidation, respectively. Germanium ion implantation technique of the near-surface region of CuGaSe 2 thin films has been used in order to prove the feasibility of n-type doping. In photoluminescence (PL) studies, the occurrence of a new emission line is identified as Ge related and explained as a donor–acceptor-pair (DAP) recombination. The precise role the Ge is playing in this doping of CuGaSe 2 is revealed by X-ray absorption spectroscopy (XANES and EXAFS) and ab initio calculations based on the density functional theory. The studies indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe 2 originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted anti-bonding Ge–Se4sp 3 defect band appearing well above the Fermi level for the Ge 1+ Ga point defect system can be directly linked to a Ge-dopant-related donor–acceptor-pair transition as observed in our photoluminescence spectra.

Synthesis of molybdenum oxide microsheets via close-spaced vapor transport

Growth of molybdenum oxide microsheets on silicon (1 0 0) substrates using the close-spaced vapor transport (CSVT) technique is proposed. Molybdenum oxide powder is employed as source, the synthesis is carried out at atmospheric pressure with a nitrogen ambient by employing short times (a few minutes), water as reactant and moderate temperatures. The growth process is efficient, fast, and without the use of catalysts. Changes in morphology and structure of products when temperature varies are reported. The produced molybdenum oxide microsheets are analyzed with SEM, XRD and micro-Raman techniques.

Chemically deposited CdS by an ammonia-free process for solar cells window layers

Chemically deposited CdS window layers were studied on two different transparent conductive substrates, namely indium tin oxide (ITO) and fluorine doped tin oxide (FTO), to determine the influence of their properties on CdS/CdTe solar cells performance. Three types of CdS films obtained from different chemical bath deposition (CBD) processes were studied. The three CBD processes employed sodium citrate as the complexing agent in partial or full substitution of ammonia. The CdS films were studied by X-ray diffraction, optical transmission spectroscopy and atomic force microscopy. CdS/CdTe devices were completed by depositing 3 μm thick CdTe absorbent layers by means of the close-spaced vapor transport technique (CSVT). Evaporated Cu–Au was used as the back contact in all the solar cells. Dark and under illumination J–V characteristic and quantum efficiency measurements were done on the CdS/CdTe devices to determine their conversion efficiency and spectral response. The efficiency of the cells depended on the window layer and on the transparent contact with values between 5.7% and 8.7%.

Structural and optical properties of Cu(In,Ga)Se 2 grown by close-spaced vapor transport technique

Polycrystalline thin films CuGa 0.3In 0.7Se 2 (CIGS) were deposited onto coated soda-lime glass and transparent conducting oxide (SnO 2) substrates at 480 °C substrate temperature with a low cost and potentially large area deposition method by using close-spaced vapor transport technique (CSVT) designed in our laboratory (LAMPS). Morphological, structural and compositional properties were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. Optical transmission spectra were used to determine absorption coefficient and gap energy of the sample. Using photoacoustic spectroscopy, we report the activation energies of shallow and deep levels. Finally, we discuss the results obtained.

Chemically-deposited Te layers improving the parameters of back contacts for CdTe solar cells

A chemical bath process was carried out for the deposition of Te layers on CdTe films grown by the close-space vapor transport technique (CSVT) on conducting SnO 2:F substrates. The Te layers were chemically-deposited on as-grown CdTe films and on previously CdCl 2-treated CdTe ones. After Te deposition, the CdTe films were annealed at temperatures from 200 to 400 °C. The Te layer on top of the CdTe films was studied by Raman Spectroscopy and by Scanning Electron Microscopy. The electrical resistivity of the annealed CdTe films was determined from current versus voltage measurements in a sandwich configuration, employing gold contacts on top of the CdTe modified surface. The results show that the combined effect of the Te layer on CdTe together with previous CdCl 2 treatment improves the electrical properties of CSVT-CdTe films. These results are quite promising for increasing performance of CdS/CdTe solar cells.

Morphology of nanostructured GaP on GaAs: Synthesis by the close-spaced vapor transport technique

Nanostructures of gallium phosphide (GaP) were fabricated through the close-spaced vapor transport technique (CSVT). Such nanostructures were grown on crystalline gallium arsenide (GaAs) by using a GaP powder source in the absence of any catalyst. Nanostructured GaP products were structurally and morphologically characterized by means of physicochemical and spectroscopic techniques, such as SEM, EDS, XRD, and NMR. Results showed that GaP structures present a nanoflower-like morphology. Indeed, these nanoflowers are constituted by numerous nanowires. The diameters of the GaP nanowires were in the interval of 80–300 nm, with lengths varying from several to tens of micrometers.