Growth Of CdTe Films Research Articles

Temperature difference in close-spaced sublimation (CSS) growth of CdTe thin film on ultra-thin glass substrate

In this study, cadmium telluride (CdTe) thin films were grown for the first time ever on ultra-thin (100 µm) Schott D263T glass substrates via high temperature deposition process namely close-spaced sublimation (CSS). CdTe thin films were deposited under different source and substrate temperatures ranging from 500 ℃ to 700 ℃ to investigate the influence of deposition temperature on the properties of CdTe thin films grown on ultra-thin Schott glass substrates. X-ray diffraction (XRD) analysis illustrated cubic CdTe structure with a sharp peak at (111) orientation. The scanning electron microscope (SEM) study elucidated that the grain growth and surface morphology were highly dependent on the deposition temperature. UV–Vis results showed approximate optical band gap range of 1.61–1.66 eV. Carrier concentration and resistivity were found in the order of 1013 cm−3 and 105 Ω-cm, respectively. Structural and opto-electronic characteristics were optimized for the best temperature range to be used for CdTe film deposition in CSS growth. Optimum conditions for CdTe film growth on ultra-thin (100 µm) substrate were found in the range of 500 ℃–600 ℃ for substrate and source temperature.

Structural and Optical Properties of RF-Sputtered CdTe Thin Films Grown on CdS:O/CdS Bilayers

In this work, we report the structural and optical properties of CdTe thin films on oxygenated Cadmium Sulfide (CdS:O)/Cadmium sulfide(CdS) bilayer using RF magnetron sputtering for different substrate temperatures ( 150, 200, 250, 300 and 350 °C) in Argon ambient. The XRD spectra reveal the polycrystalline nature of all the CdTe thin films with preferential cubic orientation along (111) direction. Raman Spectra show a dominant peak at 163.5 cm -1  and its overtone at 328.7 cm -1  which corresponded to the longitudinal optical (LO) phonon of CdTe. The SEM microscopy exhibits the uniform growth of CdTe films onto the entire glass substrate. The optical transmission of the CdTe films begin at the edge of 800 nm wavelength and showed interference fringe in the transmission spectra. The thickness of the deposited films and the optical constants were calculated from the fringe pattern. The thicknesses of the CdTe films are found to be increasing from 2.34 I¼m to 2.82 I¼m with increasing the substrate temperature i.e. from 150 to 250°C. Thereafter, the CdTe film thickness decreased to 2.24 I¼m while the temperature increased further up to 350°C. The optical bandgap of the deposited CdTe films follows an increasing trend of 1.46 eV to 152 eV with the increase of substrate temperature 150 to 250°C after that the bandgap decrease to 1.50 for 350°C. Hence the obtained structural and optical properties suggest that the deposited CdTe films can be used as a suitable absorber layer for the thin film-based solar cells.

Study of CdTe film growth by CSS on three different types of CdS coated substrates

CdTe (cadmium telluride) is a very useful material for various applications such as in solar cell fabrication, electro-optical modulators, radiation detectors etc.. It is one of the cost effective and suitable material for thin film solar cells production due to its ease of thin film fabrication, high absorption coefficient and ideal band gap. CSS (close spaced sublimation) is one of the suitable technologies of CdTe thin film fabrication for various applications. In this study, close spaced sublimation of CdTe for its deposition on CdS (cadmium sulfide) coated glass was performed using a custom-designed CSS system for solar cell application. The deposition characteristics of CdTe was studied on three types of CdS layer. First CdS layer was obtained by sputtering technique, and second CdS layer was obtained by CBD (chemical bath deposition) technique. And third CdS layer was CdS:O (oxygenated nanocrystalline CdS film). The surface morphology was studied using FESEM (field emission scanning electron microscope). The XRD was done to confirm the crystal details of the film. UV-VIS spectrophotometer was used for optical absorption and transmission characteristics of the film which was further used to calculate the band gap of the deposited film. The thickness of the film was measured using DektakXT, Bruker profilometer. Results of current study show that (a) the close spaced sublimation technique for CdTe deposition is very consistent irrespective of the quality of substrate used, (b) the pin hole free uniform films are obtained in all three types of CdS substrates used (c) little deviation in CdS:O substrate is seen as the transmission characteristic is concerned. The band gap gets closer to the theoretical value

Radiative and interfacial recombination in CdTe heterostructures

Double heterostructures (DH) were produced consisting of a CdTe film between two wide band gap barriers of CdMgTe alloy. A combined method was developed to quantify radiative and non-radiative recombination rates by examining the dependence of photoluminescence (PL) on both excitation intensity and time. The measured PL characteristics, and the interface state density extracted by modeling, indicate that the radiative efficiency of CdMgTe/CdTe DHs is comparable to that of AlGaAs/GaAs DHs, with interface state densities in the low 1010 cm−2 and carrier lifetimes as long as 240 ns. The radiative recombination coefficient of CdTe is found to be near 10−10 cm3s−1. CdTe film growth on bulk CdTe substrates resulted in a homoepitaxial interface layer with a high non-radiative recombination rate.

Flexible cadmium telluride thin films grown on electron-beam-irradiated graphene/thin glass substrates

We demonstrate the close-spaced sublimation growth of polycrystalline cadmium telluride (CdTe) thin films on a flexible graphene electrode/thin glass substrate structure. Prior to the growth of CdTe films, chemical-vapor-deposited graphene was transferred onto a flexible glass substrate and subjected to electron-beam irradiation at an energy of 0.2 MeV in order to intentionally introduce the defects into it in a controlled manner. Micro-Raman spectroscopy and sheet resistance measurements were employed to monitor the damage and disorder in the electron-beam irradiated graphene layers. The morphology and optical properties of the CdTe thin films deposited on a graphene/flexible glass substrate were systematically characterized. The integration of the defective graphene layers with a flexible glass substrate can be a useful platform to grow various thin-film structures for flexible electronic and optoelectronic devices.

Growth of CdTe films on Ni-coated microtextured silicon substrates

Based on modeling and calculation results and using appropriate photomasks, we produced microtextured Si substrates, which were then coated with a Ni layer. Next, polycrystalline CdTe films were produced on the Ni by quasi-closed space growth. Their surface morphology was examined by optical microscopy and scanning electron microscopy. We calculated the stress induced in such CdTe/Ni/Si structures by the lattice mismatch and thermal expansion mismatch between the materials in contact.

Dislocations in CdTe heteroepitaxial structures on GaAs(301) and Si(301) substrates

Initial stages of ZnTe growth on GaAs(301) and Si(301) substrates by the method of molecular beam epitaxy are investigated by means of in situ one-wave ellipsometry. Layer-by-layer growth of the ZnTe film is observed on GaAs(301) substrates, whereas 3D nucleation occurs during epitaxy on Si(301) substrates. Misfit dislocations (MDs) are inserted into the ZnTe film during the growth of the first monolayers. Owing to MDs, the film lattice turns with respect to the substrate lattice, which is confirmed by X-ray measurements. Threading segments of MDs in CdTe/ZnTe/GaAs(301) and CdTe/ZnTe/Si(301) heterostructures are subjected to etching. The etch dislocation pits are found to have different shapes, which testifies to different types of threading dislocations. In the case of layer-by-layer etching, the dislocation density is found to increase inward the CdTe film, which testifies to annihilation of dislocations in the course of growth of CdTe films. The dislocation annihilation rate is higher in films grown on GaAs(301) than in those grown on Si(301). A possible reason is the higher mobility of dislocations in CdTe films on GaAs(301) substrates.

Selective CdTe Nanoheteroepitaxial Growth on Si(100) Substrates Using the Close-Spaced Sublimation Technique Without the Use of a Mask

The development of HgCdTe detectors requires high sensitivity, small pixel size, low defect density, long-term thermal-cycling reliability, and large-area substrates. CdTe bulk substrates were initially used for epitaxial growth of HgCdTe films. However, CdTe has a lattice mismatch with long-wavelength infrared (LWIR) and middle-wavelength infrared (MWIR) HgCdTe that results in detrimental dislocation densities above mid-106 cm−2. This work explores the use of CdTe/Si as a possible substrate for HgCdTe detectors. Although there is a 19% lattice mismatch between CdTe and Si, the nanoheteroepitaxy (NHE) technique makes it possible to grow CdTe on Si substrates with fewer defects at the CdTe/Si interface. In this work, Si(100) was patterned using photolithography and dry etching to create 500-nm to 1-μm pillars. CdTe was selectively deposited on the pillar surfaces using the close-spaced sublimation (CSS) technique. Scanning electron microscopy (SEM) was used to characterize the CdTe selective growth and grain morphology, and transmission electron microscopy (TEM) was used to analyze the structure and quality of the grains. CdTe selectivity was achieved for most of the substrate and source temperatures used in this study. The ability to selectively deposit CdTe on patterned Si(100) substrates without the use of a mask or seed layer has not been observed before using the CSS technique. The results from this study confirm that CSS has the potential to be an effective and low-cost technique for selective nanoheteroepitaxial growth of CdTe films on Si(100) substrates for infrared detector applications.

CFD Modelling of Cadmium Telluride (CdTe) Thin Film Coating with Inline AP-MOCVD

A CFD numerical model has been established to study the growth of cadmium telluride (CdTe) on substrate using metalorganic chemical vapour deposition technique in atmospheric pressure (AP-MOCVD) and hydrodynamics in an inline MOCVD reactor. The numerical simulations have been conducted using the CFD code – ANSYS Fluent. Dimethylcadmium (DMCd) and diisopropyltelluride (DIPTe) have been employed as precursors while H2 is acting as the carrier gas. In order to assess the effect of various conditions on CdTe film growth and uniformity, heat transfer and mass transport behaviours of the chemical species in the reactor have been fully investigated. In addition, material utilization and fluid dynamics in the reactor are also discussed.

Epitaxial growth of CdTe films on GaAs-buffered (001) Si substrates by metal organic chemical vapor deposition

Epitaxial (001) CdTe thin film was successfully grown on a (001) Si substrate using an atomically smooth, thin (2nm) GaAs buffer layer deposited using the metal organic chemical vapor deposition method. High-resolution transmission electron microscopy studies revealed a periodic distribution of dislocations due to lattice mismatch along the interfaces between CdTe film, GaAs buffer layer, and Si substrate. Most of the dislocation cores were located inside the GaAs buffer layer or at the interface between the CdTe and GaAs layers, indicating that GaAs buffer layer originally coherent to Si, was fully relaxed during the CdTe layer growth. The thin GaAs buffer layer effectively absorbed any strain coming from the large lattice mismatch between a CdTe layer and a Si substrate by forming an array of structural defects in the GaAs layer, which permitted epitaxial growth of CdTe on the Si substrates.

High-fidelity simulations of CdTe vapor deposition from a bond-order potential-based molecular dynamics method

CdTe has been a special semiconductor for constructing the lowest-cost solar cells and the CdTe-based Cd1-xZnxTe alloy has been the leading semiconductor for radiation detection applications. The performance currently achieved for the materials, however, is still far below the theoretical expectations. This is because the property-limiting nanoscale defects that are easily formed during the growth of CdTe crystals are difficult to explore in experiments. Here we demonstrate the capability of a bond order potential-based molecular dynamics method for predicting the crystalline growth of CdTe films during vapor deposition simulations. Such a method may begin to enable defects generated during vapor deposition of CdTe crystals to be accurately explored.

Photoluminescence and Reflectance Spectra of CdTe films on GaAs(100) Substrates

Photoluminescence (PL) and reflectance spectra were measured on CdTe films grown on GaAs(100) substrates by metal organic chemical vapor deposition (MOCVD). By comparing the PL spectra with the reflectance spectra in a certain exciton energy range, we identified heavy- and light-hole exciton transitions in the CdTe films on the GaAs(100) substrates. The energy separation between the heavy- and light-hole exciton transitions was estimated to be 3.1 meV. This energy separation was caused by a compressive strain due to a difference in thermal expansion coefficient between the grown CdTe films and GaAs substrates. Bound exciton emissions and the 1.42 eV band in the PL spectra were also studied on the basis of their temperature dependence.

Growth of CdTe Films on Amorphous Substrates Using CaF2 Nanorods as a Buffer Layer

Continuous biaxially textured CdTe films were grown on biaxial CaF 2 buffer layers. The CaF 2 nanorods were grown by oblique angle vapor deposition and possessed a {111} biaxial texture. The CdTe film was deposited by metal organic chemical vapor deposition (MOCVD). Film morphology and the CdTe/ CaF 2 interface were studied by scanning electron microscopy and transmission electron microscopy. Characterization showed that small CdTe grains formed initially from the CaF 2 surfaces. These small grains then merged into large columnar grains during growth. Analysis revealed that the crystalline orientation of the CdTe film followed the biaxial texture of the CaF 2 nanorods.

Controlled growth of CdTe films by periodic voltammetry for solar cell applications

In the present investigation, polycrystalline n-CdTe films have been synthesized on SnO 2-coated glass substrate employing voltammetric technique under periodic cycles. Optical properties of the CdTe films were investigated and the band gap energy was found to lie within the range of 1.32–1.40 eV depending on the composition of the film matrix, as obtained by EDAX analysis. The films were subjected to SEM and AFM analyses in order to study the surface morphology, topographic structure of the semiconductor surface. X-ray diffraction studies indicated polycrystalline nature of deposits containing hexagonal and cubic phases of CdTe. The stability of the films was determined through anodic polarization studies using Tafel analysis. The amount of Cd 2+ dissolved during polarization was estimated by anodic stripping voltammetry. The Nyquist and Mott-Schottky plots derived from impedance spectroscopic measurements provided important information regarding electrical and semiconducting properties of the films. Cyclic voltammetry was found to be an effective method for developing potential thin films of CdTe, demonstrating photo-conversion efficiency up to the order of ∼4%.

Nucleation and Grain Boundaries in Thin Film CdTe/CdS Solar Cells

AbstractThe early stage formation mechanisms operating during the sublimation growth of CdTe films on CdS has been evaluated using a growth interrupt methodology for deposition under 100 Torr of N2. Key stages of the growth were identified and are discussed in terms of the processes of island nucleation, island growth/coalescence, channel formation and secondary nucleation that have been reported for other materials systems. It was demonstrated that the grain size could be manipulated by means of controlling the gas pressure in the range 2 – 200 Torr, with the grain diameter increasing with pressure linearly as D (μm) = 0.027(± 0.011) × P (Torr) + 0.90(± 0.31). For a series of solar cells made using such material, the performance parameters were seen to increase with grain size up to a plateau corresponding to grains of ∼4 μm in this case. Equivalent circuit parameters for resistive components arising from grain boundaries, and the contact to the CdTe, were measured. It is considered that grain boundary barriers in CdTe are harmful to PV performance, and that the plateau in performance occurs when the grain size is increased to the level where the contact resistance is greater than that due to grain boundaries.

Properties and mechanism of solar absorber CdTe thin film synthesis by unipolar galvanic pulsed electrodeposition

Electrochemical deposition of CdTe semiconductor thin films over transparent conducting glass substrates by sequential unipolar current pulses is described. The magnitude of pulsed current and pulse periodicity affects the crystalline structure, morphology, optical absorbance and composition of CdTe films. CdTe films formed under high magnitude pulsed current density ~5–15 mA cm−2 are crystalline with dominant cubic structure having (111) plane oriented parallel to the substrate. Stoichiometric CdTe film growth occurs with current pulses of short 25–300 ms periodicity and 3–50 ms duration. A mechanism of the CdTe growth involving in situ cathodic tellurization process step involving H2Te formation and reaction with electrochemically deposited Cd monolayer is described. CdTe film growth in the pulsed electrodeposition occurs under mass transport conditions under strong influence of high magnitude pulsed current. This results in much higher growth rates ~5–8 μm h−1 for CdTe films which is attractive for CdTe solar cells in a production environment.

Heteroepitaxial growth of CdTe films on (1 0 0) GaAs treated with Sb by low-pressure metalorganic vapor phase epitaxy

CdTe films were deposited on antimony-treated (1 0 0) GaAs substrates by low-pressure metalorganic vapor phase epitaxy. Prior to growth of CdTe layer, GaAs substrates were treated by triethylantimony for various treatment times. The effect of antimony treatment on crystal orientation of CdTe layer was discussed. At the optimum treatment time, CdTe films were oriented to (1 0 0) and their in-plane X-ray measurements show fourfold symmetries, that is, it is found that CdTe film was epitaxially grown on antimony-treated (1 0 0) GaAs substrate.

Growth of CdTe Films by ML-ALD

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Nucleation and growth mechanism of CdTe cluster grown on CdS films

The growth of CdTe films on CdS/glass was investigated. Using atomic force microscopy (AFM), we have obtained images of the coverage of CdTe deposited by radio-frequency magnetron sputtering (RF sputtering) on a CdS substrate. A nucleation kinetics model has been derived from the analysis of the particle size distribution observed by AFM. The reconstructed nucleation process was characterized by a systematic behaviour. The nucleation rate, negligible at the beginning, reaches a maximum after a certain time and then tends to zero for a longer deposition time. The nucleation process shows a temperature-dependent incubation time. The incubation time is related to the time of formation of a “critical CdTe cluster”. The size of the critical cluster is explained by the thermodynamic properties of the system and by the amount of interface misfit strain energy of CdTe lattice with respect to the substrate. The films were deposited at a substrate temperature of 250, 300, 350 and 400 °C, the incubation time calculated values are 817, 503, 283 and 68 s for each temperature, respectively, and the activation energy that the CdTe molecules have to overcome to pass from the gas phase to the solid phase have been found to be 0.332±0.003 eV.

Growth of ultrathin films of cadmium telluride and tellurium as studied by electrochemical atomic force microscopy

Time dependent, cathodic electrodeposition of ultrathin CdTe and Te films has been studied in 50 mM H 2SO 4 + 1 mM CdSO 4 + 0.1 mM TeO 2 solutions at room temperature under potential control using electrochemical atomic force microscopy (EC-AFM). The films were also characterized electrochemically and with X-ray diffraction. The growth mechanism and the composition of the films depends on the applied potentials. Island-like growth mode was observed for CdTe films when the deposition potential was − 0.35 V (SHE). At a more positive deposition potential of 0.138 V (SHE), Cd was not codeposited into the film but affected the dynamic growth mode of the deposit. At this voltage smooth Te films were obtained. Depending on the applied potential, Cd acts either as a codeposition element for CdTe film growth, or as a mediator for layer-by-layer growth of Te films.