Molecular Beam Deposition Method Research Articles

Characteristics of thin Sb2Se3 films obtained by the chemical molecular beam deposition method for thin-film solar cells

Sb2Se3 films were obtained by chemical molecular beam deposition on soda-lime glass substrates. As a source material, 99.999% purity semiconductor Sb2Se3 pieces were used. Their evaporation and the substrate temperature were maintained at 830 °C÷1000 °C and 500 °C, respectively. Using scanning electron microscopy, X-ray diffraction analysis, and Raman scattering, the influence of the temperature of the source of the binary compound Sb2Se3 on the chemical composition, morphology, and structure of the synthesized films of Sb2Se3 films was studied. It is observed that the films have a crystalline (orthorhombic) structure with compactly located crystallites having the form of rods with an average size: l = 5÷10 µm (length) and d = 1÷2 µm (diameter). An analysis of the dependency functions (αhν)2 = f(hν) showed that the Sb2Se3 films obtained at temperatures Тsource=900 °C and Тsource=840 °C have direct transitions with an optical band gap Еg=1.04 eV and Еg=1.12 eV, respectively. The electrical conductivity of the films, changed within 1.03·10−5÷4.13·10−5 (Om·cm)−1 depending on the ratio of Sb/Se atomic concentration.

Structural and optical properties of SbxSey thin films obtained by chemical molecular beam deposition method from Sb and Se precursors

SbxSey thin-films were obtained by chemical-molecular beam deposition (CMBD) on soda-lime glass from Sb and Se precursors. By the precise control of the Sb/Se ratio, Sb2Se3 thin films with stoichiometric composition were successfully obtained. The elemental and phase composition, as well as the crystal structure of SbxSey thin-films, were studied by energy-dispersive X-ray microanalysis, X-ray diffraction, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. The optical bandgap of the films was determined from the absorption spectra acquired by a spectrophotometer. The physical properties of SbxSey thin films with different compositions were investigated.

Optical and Recombination Parameters of CdS1−xTex Thin Films Obtained by the CMBD Method

This paper presents the results of the photoacoustic, SEM, and surface photovoltage experiments performed on the series of CdS1−xTex thin films. These CdS1−xTex (0 ≤ x ≤ 1) thin films were obtained on the glass substrate by the chemical molecular beam deposition (CMBD) method. The polycrystalline character of these films was revealed by SEM pictures. From the experimental optical characteristics, the optical absorption coefficient spectra of the samples and values of their energy gaps vs. their composition were determined. From the surface photovoltage characteristics, the diffusion lengths of the carriers were also determined.

Conductivity of SbxSey films grown by CMBD from Sb and Se precursors for use in solar cells

Antimony selenide (Sb2Se3) has been developed as attractive, non-toxic and earth-abundant solar absorber candidate among the thin-film photovoltaic devices. The growth of SbxSey thin films, by atmospheric pressure chemical molecular beam deposition (CMBD) method, from separate Sb and Se precursors has been reported. The conductivity of the films was investigated as a function of the vapor phase mixture of Sb and Se. By the precise control of the Sb/Se ratio we succeeded in obtaining stoichiometric Sb2Se3 films. It is also found out that we can control the conductivity by deliberately introducing the deviation from the stoichiometry. The conductivity was varied in the wide range of 10−5 ÷ 102 (Ohm × cm)−1 and samples had p- and n-type conductivity depending on Sb/Se ratio. The obtained results were explained by the formation of intrinsic point defects.

Optical Properties of CdS1 – xTex Thin Films Obtained by Chemical Molecular Beam Deposition Method

Optical Properties of CdS1 – xTex Thin Films Obtained by Chemical Molecular Beam Deposition Method

Surface Morphology and Phase Composition of (Zn,Sn)Se Thin Films, Obtained by Chemical-Molecular Beam Deposition

(Zn,Sn)Se films of different chemical composition ratio Zn/Sn were obtained by chemical molecular beam deposition (CMBD) method on borasilicate glass with variation the ratio of the partial pressure of ZnSe and SnSe in vapour phase during growth. Chemical and phase composition, as well as surface morphology of samples were investigated. The samples were found to be a thin films with a thickness of 2–3 μm with and evenly distribution of chemical elements. Grain structure, phase composition and surface roughness parameters dependencies on the elemental composition are determined.

Growth and characterization of ZnxSn1−xSe films for use in thin film solar cells

We have fabricated ZnxSn1−xSe (ZTSe) films for the first time. Samples were fabricated by chemical molecular beam deposition method at atmospheric pressure in hydrogen flow. ZnSe and SnSe powders with 99.999% purity were used as precursors. The temperature of precursors varied in the range of (850–950) °C. Films were deposited at substrate temperature of (500–600) °C. Borosilicate glass was used as a substrate. We have studied ZTSe films by EDS, XRD and SEM. The samples had orthorhombic and cubic structures depending on composition. Results of EDS have shown that stoichiometric composition of samples moved to ZnSe side by increasing with substrate temperature. SEM pictures have shown that samples had polycrystalline structure. The grain size varied in the range of (2–15) µm. The grain size of samples increased from (2–5) µm to (15–20) µm for substrate temperatures of 500 °C and 550 °C respectively. While, at a substrate temperature of 600 °C the grain size decreased up to (3–5) µm, possibly, because of increasing of ZnSe content. XRD analysis has shown that samples have ZnSe, SnSe, Se and Sn phases. The band gap of samples varied in the range of 1.0–2.0 eV depending on the film compositions. An inversion of the conductivity type was found: samples fabricated at 500 °C and 550 °C performed of p-type conductivity; while samples fabricated at 600 °C showed n-type conductivity.

Effect of Substrate Temperature on the Physical Properties of ZnхSn1 – хSe Films for Thin-Film Solar Cells

At the present time, 92% of the PV market is made using single crystal or polycrystalline wafer silicon. However, producing power with these cells remains expensive compared to conventional power generation. Thin film solar cells have been developed to reduce production costs, especially those based on cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Despite a number of successes in the development of thin film solar cells, some problems associated with the toxicity of cadmium and with the high cost of indium and gallium remain, which has prompted researchers to search for alternative materials for solar cells. Novel low cost and high efficiency zinc tin selenide (ZnxSn1 – xSe) thin film solar cells are without these drawbacks. However, there is no information in the literature about this new material. The samples of ZnxSn1 – xSe films were fabricated using the chemical molecular beam deposition (CMBD) method at atmospheric pressure in hydrogen flow. ZnSe and SnSe powders with 99.999% purity were used as precursors. The temperature of precursors varied in the range of 850–950°C. Films were deposited at substrate temperature of 500–600°C. Borosilicate glass was used as a substrate. The results showed that the composition of the samples changed toward ZnSe with the temperature of the substrate. The grain size of the samples increased from 2–5 μm to 15–17 μm at substrate temperatures of 500 and 550°C, respectively. At a substrate temperature of 600°C, the grain size decreased to 3–5 μm, which is possibly due to an increase in the ZnSe content. The X-ray diffraction pattern has shown that the samples have ZnSe, SnSe, Se, and Sn phases.

Growth and characterization of Sb2Se3 thin films for solar cells

The growth of antimony selenide (Sb2Se3) thin films the first time by atmospheric pressure chemical molecular beam deposition (CMBD) method has been reported. The morphological and structural properties of the films were studied as a function of the hydrogen flow rate at different substrate temperature. Experimental data indicate that Sb2Se3 films grown as Se-rich at low hydrogen flow rate and the samples have almost amorphous structure. In contrast, at higher hydrogen flow rate, the films have Sb-rich composition and polycrystalline structure. Interestingly, transition from amorphous to polycrystalline structure is depend on the flow rate of the transport gas, while surface morphology affected by the substrate temperature. Electrical and optical measurements revealed that polycrystalline films have p-type conductivity and optical bandgap of 1.1 eV with high absorption coefficient of 105 cm−1. These results showed that the CMBD grown films can be used as absorber layer for fabrication of thin film solar cells.

Influence of composition and heat treatment in CdCl2 solution on intrinsic point defects in CdTe films

CdTe films of different compositions were grown by the chemical molecular-beam deposition method. The activation energy and the nature of deep levels in relation with the composition of films based on the temperature dependence of electroconductivity are defined, and the influence on these levels of heat treatment process in CdCl2 solution has been studied. Deep levels with the following activation energies are defined: Ev +0.31 eV; Ev + 0.42 ± 0.03 eV; Ec–0.44 ± 0.01 eV; Ec–0.28 eV; Ev + 0.24 ± 0.01 eV.

Characterization of CdTe thin films with different compositions obtained by CMBD for thin film solar cells

CdTe thin films with different compositions, i.e. Cd/Te ratio of ∼0.79, 0.88, 0.94, 1.01, 1.05, 1.08, were fabricated by a novel and low cost chemical molecular beam deposition method (CMBD) under hydrogen flow in atmospheric pressure. For this purpose, Cd and Te granules were used as precursors. Thin films were deposited on glass and metal substrates at 600°C. The growth rate was varied in the range of (10–30)Å/s. The composition ratio in the samples was controlled by varying the molecular beam intensity (MBI) ratio. The morphological, structural, electrical and optical properties of the CdTe thin films were investigated by SEM, XRD, Hall-effect and photoluminescence (PL) measurements, respectively.

The Advantages of Ga-Graded Obtained by Growth Profile Modification and Na Incorporation on Cu(In,Ga)Se<sub>2</sub> Solar Cells

Cu (In,Ga)Se2 (CIGS) compound is a p-type semiconductor that has been used as light absorber layer in high efficiency thin film solar cell. The CIGS compound can be adjusted the band gap energy by varying the ratio of [Ga]/([In]+[Ga]) ratio (x). From theoretical and simulation, it was found that band gap grading in CIGS thin films showed the advantages to increase the efficiency of solar cells. Generally, the band gap grading can be done by the growth of non homogeneous x-ratio in depth of CIGS thin films. In this work, we develop two approaches to create band gap grading in CIGS thin films; (1) modifying the growth profile and (2) using Na incorporation in the growth process. The effects of Ga-graded would be revealed and compared with homogeneous CIGS thin films. CIGS thin films were grown on soda-lime glass and Al2O3 coated soda-lime glass substrates by molecular beam deposition method. The growth process was based on 2-stage and 3-stage growth profiles. The as grown films were characterized for their structural property, chemical composition and optical transmission as well as solar cell performance. The Auger electron spectroscopy in depth profiles revealed the variation of x-ratio increasing from the surface toward the back contact in CIGS films with our modified growth profile and Na incorporation. This result indicated Ga-graded in CIGS thin films. The structural property of Ga-graded CIGS films showed the (112) preferred orientation of the chalcopyrite structure with a broad asymmetric spectrum related to the inhomogeneous structure. The optical transmission measurements of the Ga-graded CIGS film showed the broad transition near the absorption edge indicating the effect of the band gap grading as a result of the variation in depth of the Ga-content. From I-V measurements, the solar cell efficiencies significantly increase due to the advantages of Ga-graded constitute.

Revolutionary novel and low cost CMBD method for fabrication of CdTe absorber layer for use in thin film solar cells

We developed revolutionary novel and low cost and non-vacuum chemical molecular beam deposition method for fabrication of thin film II–VI solar cells in the atmospheric pressure gas (He, Ar and H2) flow. High quality polycrystalline CdTe films with different compositions (stoichiometric and Cd/Te ⩽1·0 and Cd/Te ⩾1·0) and thickness of 2–3 μm were fabricated on ceramic (SiO2–Al2O3) substrates at a temperature of 600°C. Separate sources of Cd and Te with respective purities of 99·999% were used as precursors. The growth rate was varied in the range of 9–30 Å s−1. The effects of the composition and CdCl2 treatment on the structure, intrinsic point defects and electrical properties of CdTe films were investigated by X-ray diffraction, atomic force microscopy, Raman spectra, photoluminescence and Hall methods.

Effect of CdCl2 treatment on physical properties of CdTe films with different compositions fabricated by chemical molecular beam deposition

CdTe films with different compositions (Cd-rich, Te-rich and stoichiometric) were fabricated by revolutionary novel and low cost chemical molecular beam deposition (CMBD) method in the atmospheric pressure hydrogen flow. Cd and Te granules were used as precursors. The films were deposited on ceramic (SiO2: Al2O3) substrates at 600°C. The growth rate was varied in the range of 20–30 A/s. The composition of the samples was changed by controlling the molecular beam intensity (MBI) ratio Cd/Te. Effect of CdCl2 treatment on morphology, photoluminescence and electrical properties of CdTe films was investigated by AFM, Raman, photoluminescence (PL) and Hall methods.

Effect of the composition on physical properties of CdTe absorber layer fabricated by chemical molecular beam deposition for use in thin film solar cells

We developed revolutionary novel and low cost and nonvacuum chemical molecular beam deposition method for fabrication of thin film II-VI solar cells in the atmospheric pressure gas (He, Ar, H2) flow. High quality polycrystalline CdTe films with different compositions (stoichiometric and Cd/Te ≤1.0 and Cd/Te ≥ 1.0) and thickness of 2–3 μm were fabricated on ceramic (SiO2:Al2O3) substrates at temperature of 600 °C. Separate sources of Cd and Te with respective purities of 99.999% were used as precursors. The growth rate was varied in the range of 9–30 Å/s. The effect of the composition and CdCl2 treatment on the structure, intrinsic point defects, and electrical properties of CdTe films was investigated by XRD, AFM, Raman spectra, photoluminescence, and Hall methods.

Thickness dependent surface microstructure evolution of bismuth thin film prepared by molecular beam deposition method

The evolution of surface microstructure on bismuth thin film deposited by molecular beam deposition method is investigated. Morphological, topographical, structural, and electrical property changes of the film with various thicknesses are studied by means of AFM, XRD, XRR, and 4-point probe. Drastic change of surface grain in shape, which transforms from round shape to polyhedral shape, is detected around 13–18 nm film thickness. Abrupt horizontal profile change of surface grain is verified with power spectral density (PSD) function. At this threshold thickness, the film shows very low roughness value and surface area ratio. Then both increase steeply as the film thickness surpasses the thickness. As the bismuth film is deposited thicker, it has textured structure and high roughness on surface. With increment of the thickness, the electrical sheet resistance of the films is significantly decreased. We explain this surface microstructure evolution on the bismuth film with the evolutionary selection model.

Structural, photoluminescent and electrical properties of CdTe films with different compositions fabricated by CMBD

CdTe films with different compositions (Cd-rich, Te-rich and stoichiometric) were fabricated by a novel and low cost chemical molecular beam deposition method (CMBD) in atmospheric pressure hydrogen flow. Cd and Te granules were used as precursors. The films were deposited on ceramic (SiO 2:Al 2O 3) substrates at 580 °C and 600 °C. The growth rate was varied in the range of 9–30 Å/s. The composition (Cd/Te) of the samples was changed by controlling the molecular beam intensity (MBI) ratio. Three samples fabricated at MBI ratios Cd/Te = 0.5, 1.0 and 1.16 were investigated by XRD, AFM, EDX, SEM, photoluminescence (PL) and Hall methods.

Continuous-wave laser annealing of free-standing Si∕SiO2 superlattice: Modification of optical, structural, and light-emitting properties

Raman, optical, and photoluminescence studies of a free-standing Si∕SiO2 superlattice (SL) are described with emphasis on laser-induced thermal effects (laser annealing). The Si∕SiO2 SL (500 repeats of 2-nm-thick Si and SiO2 layers) on a Si substrate was grown by a molecular beam deposition method and annealed at 1100°C for 1h in an oven, which promoted Si nanocrystals. Then the Si substrate was partially chemically etched producing free-standing areas. Continuous-wave laser annealing (∼104W∕cm2) of the as-prepared free-standing SL strongly increases the Raman band of Si nanocrystals at ∼515cm−1, which features laser-induced crystallization and presumably originates from melting of Si nanostructures. The obtained results show that thermal annealing at 1100°C does not finish structural reorganization of the Si∕SiO2 SL material and a large proportion of Si excess can be undetectable by Raman spectroscopy in related thermally annealed materials. For the laser-crystallized samples, various material characteristics (Raman spectra, light emission, and absorption) have been measured as a function of laser-induced temperature and period of laser annealing. The light emission is found to be a straightforward function of the temperature whereas the absorption coefficient depends on the laser-annealing period suggesting additional laser-induced structural reorganization.

Unusual hydrogen distribution and its change in hydrogenated amorphous silicon prepared using bias electric-field molecular beam deposition

Hydrogenated amorphous silicon (a-Si:H) films prepared using a molecular beam deposition (MBD) method show an unusually sharp, narrow infrared absorption peak at 2080–2090cm−1, which is thought to result from surface SiH species in the a-Si:H. The sharp, narrow peak is eliminated by a bias electric field of several kilovolts during deposition. The absence or presence of incoming high-energy silicon atoms or clusters accelerated by the field seems to be crucial for the resulting network structure and hydrogen distribution in a-Si:H prepared using MBD.

Effect of annealing on the mobility and morphology of thermally activated pentacene thin film transistors

Pentacene thin film transistors (TFTs) were fabricated by the organic molecular beam deposition method. The TFTs were characterized in order to study the effect of thermal annealing on the morphology and carrier mobility of the transistors. For all the TFT samples the mobility exhibited an Arrhenius relationship with temperature, indicating a thermally activated transport that could be explained by the carrier trap and thermal release transport mechanism. Therefore, in order to investigate the annealing effect, we tested the data for a significant period of time after annealing until the temperature recovered to room temperature, so that the thermal activation effect was screened and possible effects of thermal expansion and stress were also ruled out. As a result, we found that only with a temperature below a critical temperature of approximately 45°C could annealing improve the mobility, while annealing with T>50°C would decrease the mobility compared to the value before annealing. Atomic force microscopy observation and x-ray diffraction (XRD) data indicated that annealing caused decreased grain size and decreased XRD peak intensity for all samples. Increasing the annealing temperature to 70°C caused obvious desorption because of the low van der Waals intermolecular forces in the organic film. The mobility deterioration after high temperature annealing may be ascribed to the deteriorated microstructure, while the improved mobility may result from the increased crystallinity in the bottom several layers near the substrate film interface. The results also suggested that the influence of possible structure evolution should be distinguished when investigating temperature dependent transport properties.