Growth Rate Of CdTe Research Articles

Fabrication of high efficiency sputtered CdS:O/CdTe thin film solar cells from window/absorber layer growth optimization in magnetron sputtering

In this study, CdTe (up to 2.0µm thick) and oxygenated CdS (CdS:O, up to 100nm thick) films were deposited by magnetron sputtering and optimum conditions of film growth were investigated for CdS:O/CdTe solar cells. Favourable TeO2 has been confirmed in XRD after the CdCl2 heat treatment of the CdTe films. Moreover, improved structural, optical and electrical properties are observed in the CdCl2 heat treated films. A detailed quantitative study has also been executed using XPS that finds sulfide, sulfate and an intermediate oxide as a function of oxygen content. In many cases, CdS contribution remains predominant, however, the CdSOx contribution increases with the increase of oxygen's partial pressure and decrease of growth rate. The complete solar cell device was fabricated of various CdTe thin films with different growth rates in sputtering. A highly resistive transparent (HRT) buffer layer ZnO:Sn was placed in between the FTO and CdS:O to avoid the forward leakage problem and screen printed C:Cu/Ag is used as the back contact for low cost fabrication. The J-V characteristics and external quantum efficiency (EQE) were measured for the solar cells under the illumination of AM 1.5G and the highest efficiency of 10.3% was achieved for the optimized CdTe growth rate of 5.4Å/s, while CdS:O growth rate was 0.25Å/s.

Growth of CdTe thin films on graphene by close-spaced sublimation method

CdTe thin films grown on bi-layer graphene were demonstrated by using the close-spaced sublimation method, where CdTe was selectively grown on the graphene. The density of the CdTe domains was increased with increasing the number of the defective sites in the graphene, which was controlled by the duration of UV exposure. The CdTe growth rate on the bi-layer graphene electrodes was 400 nm/min with a bandgap energy of 1.45–1.49 eV. Scanning electron microscopy, micro-Raman spectroscopy, micro-photoluminescence, and X-ray diffraction technique were used to confirm the high quality of the CdTe thin films grown on the graphene electrodes.

TEM analysis of the container effect of Au‐based catalyst droplets during vapour‐liquid‐solid growth of axial ZnTe/CdTe nanowires

AbstractAxial heterostructure nanowires (NWs) of ZnTe/CdTe were grown by vapour‐liquid‐solid growth realized in a molecular beam epitaxial chamber. By alternative supply of Zn or Cd and constant Te the heterostructure was generated. The liquid phase is provided by a Au‐based eutectic droplet which stays at the tip of the NW during the entire growth. For structural and chemical characterization by TEM the NWs were harvested from the substrate and transferred to a holey carbon film. The NWs exhibit an expansion of the diameter correlated with the interface region between ZnTe and CdTe. Idiomorphic growth of the CdTe is evident from electron diffraction experiments. The growth rate of CdTe appears to be smaller compared to that of ZnTe at the same temperature. Both, quantitative high‐resolution TEM and energy dispersive X‐ray spectroscopy line scans reveal a smeared ZnTe/CdTe interface along about 200 nm. The smearing is due to both, the liquid catalyst which buffers the supply of Cd instead of Zn at the liquid/solid interface and to the strain which is induced by the lattice mismatch. It forces the system to consume the remnant Zn for the NW growth in favour of Cd. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Real-time mass spectroscopy of reflected fluxes during molecular beam epitaxy growth of HgCdTe

The interaction between molecular beams present in the growth of HgCdTe was studied using reflection mass spectroscopy (REMS). The incident Hg flux was found to strongly influence the amount of Te and Cd reflected from the growth surface, and also, the incident Te flux influences the amount of Cd reflected from the surface. Based on these observations, it appears that the CdTe growth rate is influenced by the amount of excess Te and/or Hg available at the growth surface. A technique to control accurately the layer mole fraction by measuring the ratio of Cd to Cd+Te fluxes during growth has been investigated. The results of the postgrowth layer characterization by secondary ion mass spectroscopy and infrared transmission indicate a strong correlation between the REMS ratio and the mole fraction of the resulting layer. This technique has successfully been used to correct for long term and short term drifts in effusion cell output.

Selective growth of CdTe on patterned CdTe∕Si(211)

The authors have studied selective growth of cadmium telluride on Si(211) by molecular beam epitaxy (MBE). Patterned substrates were produced by optical lithography of MBE-grown CdTe∕As∕Si(211). Photoemission microscopy was used as the main tool to study selective growth. This is very powerful because Si or SiO2 can be very easily distinguished from areas covered with even small amounts of CdTe due to contrast from work function differences. It was found that CdTe grows on CdTe without sticking on bare Si areas if the temperature is sufficiently high. Based on the analysis of the temperature dependence of the growth rate of CdTe, we suggest that different physisorption energies on Si and CdTe are the main cause of this selective growth.

Metal-Organic Chemical Vapor Deposition of Hg1−x Cd x Te Fully Doped Heterostructures Without Postgrowth Anneal for Uncooled MWIR and LWIR Detectors

Uncertainty in precursor pulse delays and shapes has been found to be an important factor in Hg1−x Cd x Te metal-organic chemical vapor deposition (MOCVD) growth using the interdiffused multilayer process (IMP). Herein, metal-organic concentration changes in the growth zone are examined using an in␣situ infrared (IR) absorption gas monitoring system, and modifications to the interdiffused multilayer process are applied for in␣situ control of stoichiometry, improved morphology, minimized process length, and consumption of precursors. Dimethylcadmium (DMCd) introduction during IMP flush stages in HgTe was used for stoichiometry control. The final stage of heterostructure formation was optimized to prevent Hg outdiffusion. As a result, vacancy concentration was reduced far below the equilibrium level at the growth conditions so the background of n-type doping was revealed. Acceptor doping with arsine (AsH3) and trisdimethylaminoarsenic (TDMAAs) was examined over a wide range of compositions, and doping levels of 5 × 1015 cm−3 to 5 × 1017 cm−3 were obtained. The presence of both arsenic dopants significantly increased the CdTe growth rate. This caused an increase of Cd mole fraction in the grown material. Doped heterostructures can be grown without any postgrowth anneal and used for mid- and long-wavelength infrared (MWIR and LWIR) devices operating at near-ambient temperatures.

The growth of CdTe thin film by close space sublimation system

The effects of source and substrate temperature, ambient gas pressure and the separation between source and substrate on the growth rate of CdTe using the close space sublimation (CSS) system have been investigated. The growth rate increased as the source temperature increased with an activation energy of 1.9 eV and it was constant and independent of the substrate temperature, up to some breakpoint temperature, above which the rate decreased rapidly to zero. Free sublimation and transport is involved at low pressures such as 7.5 × 10−5 mbar, whereas diffusion-limited transport was involved at pressures of 2 and 6 mbar of N2. The growth rate increased as the separation between the source and the substrate decreased. The film's grain size increased from <1 μm at 335 °C to more than 2.5 μm at above 445 °C. Analysis of the XRD traces indicated that the films grown at 335 °C were a highly preferred (111) orientation and the (111) texture coefficient reduced when the substrate temperature increased. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of a compressive strain on the stoichiometry of the (0 0 1)CdTe surface during molecular beam epitaxy

We present here a reflection high-energy electron diffraction study of the (0 0 1)CdTe growing surface during molecular beam epitaxy. The presence of additional weakly bound Te atoms adsorbed on top of the Te dimers terminating the surface during the growth is demonstrated. The coverage of these weakly bound Te atoms is strongly dependent on the growth conditions (temperature and strain). The resulting surface stoichiometry variations have a significant effect on the CdTe growth rate and on the incorporation mechanisms of Cd and Te atoms.

Deposition and growth with desorption for CdTe molecular beam epitaxy

It is well known that during epitaxial growth of thin films of almost all II–VI semiconductors, the growth rates show a pronounced temperature dependence which is due to desorption of one or both components from the growing surface. The measured desorption rate appears to be thermally activated with a strikingly small value: a few tenths of an eV. The explanation generally put forward is that the desorption of a weak-binding state acts as a “precursor” to chemisorption. According to this point of view, the small measured activation energy is a real energy corresponding to a well-defined microscopic process. We argue that no weak-binding precursor state is needed for reproducing the experimental growth rate of CdTe. Using Burton, Cabrera and Frank's theory and by performing Monte Carlo simulations of a one-particle model for deposition, diffusion, aggregation and desorption, we have found that the macroscopic desorption rate appears to be thermally activated over a large range of temperatures. This rate is a combination of all the microscopic energies — diffusion barrier and desorption barrier — and it can take values of a few tenths of an electronvolt, even though all microscopic energies are much larger. A very simplified model of CdTe growth is thus proposed and tested against experimental measurements of growth rates for various temperatures and deposition fluxes

Real-time monitoring and control during MOVPE growth of CdTe using multiwavelength ellipsometry

New multi-wavelength in-situ ellipsometer hardware and data analysis software are described. The hardware can simultaneously acquire accurate ellipsometric data at 12 wavelengths in less than 1 s, is simple and compact and is well suited for in-situ monitoring. The data analysis software implements a “virtual interface” approach to determine in real time the characteristics (growth rate and composition) of the near-surface region of the film. These new tools were used to study the metal-organic vapor phase epitaxy (MOVPE) growth of CdTe on GaAs. From a post-deposition analysis of the in-situ data, dielectric constants of CdTe at growth temperature were obtained. Non-uniformity in the CdTe film thickness, and film nucleation during the initial stages of growth, were also observed in the post-deposition analysis. The determined CdTe dielectric constants were utilized in subsequent depositions to determine the growth rate of a CdTe film in real time. Feedback control of the CdTe growth rate was effected by connecting an analog control voltage line from the data acquisition/analysis computer to the Cd mass flow controller.

Fast vapor growth of cadmium telluride single crystals

Cadmium telluride single crystals were grown at growth rates of 35 mm per day by the physical vapor transport (PVT) method under high temperature gradient conditions. This is believed to be the highest PVT growth rate of CdTe reported to date. Lamellar twins are the only ones present in the CdTe crystals grown under optimal conditions in this work. At growth rates up to 15 mm per day, the crystals have a dislocation density of ∼104 cm−2. The etch pit density increases to ∼105 cm−2 with an increase of the growth rate up to 35 mm per day. Based on a uniform thermal field and high interface stability, which are established by large temperature gradients up to 40°C/cm at the growth interface, spurious nucleation and lateral twins were effectively eliminated, and the density of the lamellar twins remained low at crystal growth rates up to 35 mm per day. The major contributions of the high temperature gradients to the single crystalline growth and the apparent origin of polycrystalline grains are also discussed in this paper.

RHEED studies of MBE growth mechanisms of CdTe and CdMnTe

We report on reflection high energy electron diffraction (RHEED) studies of molecular beam epitaxy (MBE) growth of CdTe and CdMnTe on (100) oriented CdTe substrates. RHEED oscillations were measured for both the growth and desorption of CdTe and CdMnTe as a function of flux and temperature. For the first time, the influence of laser and electron irradiation on the growth rate, as well as desorption, of CdTe is studied in detail using RHEED oscillations. We found a very small effect on the growth rate as well as on the CdTe desorption rate. The growth rate of CdTe was determined for different temperatures and CdTe flux ratios. The obtained experimental results are compared with a kinetic growth model to get information on the underlying growth processes, taking into account the influence of a precursor by including surface diffusion. From the comparison between model and experimental results the sticking coefficients of Cd and Te are determined. The growth rate of CdMnTe increases with Mn flux. This dependence can be used to calibrate the Mn content during growth by comparing the growth rate of CdTe with the growth rate of CdMnTe. The change in growth rate has been correlated with Mn content via photoluminescence measurements. In addition, the sticking coefficient of Mn is derived by comparing experimental results with a kinetic growth model. For high manganese content a transition to three-dimensional growth occurs.

Growth mechanisms of CdTe during molecular beam epitaxy

We report here on studies of the growth mechanism of CdTe during molecular beam epitaxy on (100) oriented CdTe substrates by reflection high energy electron diffraction (RHEED). The growth rate of CdTe as a function of the Cd/Te ratio in the impinging molecular beam was investigated in detail. The growth rates were determined by RHEED intensity oscillations. Fluxes were calibrated by film thickness measurements. From the growth rates dependence on the Cd/Te flux ratio we determined the Cd and Te sticking coefficients by comparing the results with kinetic models of molecular beam epitaxial growth. Both Cd and Te sticking coefficients are dependent on the surface concentration of free Cd and Te sites, respectively. The main result is that the influence of a precursor state has to be taken into account to describe the experimental results.

The effect of growth orientation on the morphology, composition, and growth rate of mercury cadmium telluride layers grown by metalorganic vapor phase epitaxy

We have measured the growth rate dependence of CdTe and HgTe layers in the growth surface direction. The growth mechanism of HgTe and CdTe were concluded to be heterogeneous, surface kinetic limited. The morphology of CdTe and mercury cadmium telluride interdiffusion multilayer process layers is found to be strongly effected by the large anisotropy of the CdTe growth rate. The best morphologies were found to be on surfaces having directions of maximum CdTe growth rates, with respect to surfaces of other crystallographic directions. A model for hillocks formation is given, based on the CdTe growth rate anisotropy. The different nature of twins stacking faults in (111)A and (111)B layers is explained by the different morphologies of these layers.

Thermal etch pits and surface morphology of vacuum sublimated (111) and (001) CdTe crystals

Sublimation of (111) and (001) CdTe was carried out at 350°C in a vacuum of 10 -8 Torr. Distinctive thermal etch pits were observed on (111)Te and (111)Cd faces allowing us to unequivocally determine the crystallographic polarity of (111) CdTe. Microcrystals in the form of truncated prisms grew on the sublimated (001) CdTe. The bases of these microcrystals were aligned in the [110] and [1 1 0] directions. Their concentration and alignement were similar to those observed for chemical etch pits which were developed on (001) CdTe with the EAg-1 solution. The Auger electron spectroscopy (AES) studies showed that the (111)Te face became slightly richer in Cd whereas those of (111)Cd and (001) remained unchanged. These results are consistent with the measured vacuum sublimation rates, and they verify the differences in the effective growth rate of CdTe and possibly of other A II−B VI materials on the (111) and (001) CdTe substrates.

Excimer laser‐assisted metalorganic vapor phase epitaxy of CdTe and HgTe on (100) GaAs

The growth of CdTe and HgTe by excimer laser‐assisted metalorganic vapor phase epitaxy on (100) GaAs at 165 °C is reported. HgTe layers have been grown on a 4000‐Å buffer layer of CdTe on GaAs with the use of dimethylmercury and a new Hg‐containing source material, divinylmercury. A linear dependence of CdTe growth rate on laser power and reactant partial pressure has been observed using in situ time‐resolved reflectivity. The photodissociation mechanism for diethyltellurium (DETe) has been investigated using laser induced fluorescence. Results indicate that irradiation of DETe with 248‐nm photons produces ground‐state Te atoms by a single‐photon process.

Temperature-independent unassisted pyrolytic MOCVD growth of cadmium telluride at 250°C using 2,5-dihydrotellurophene

The molecule 2,5-dihydrotellurophene (DHTe) was identified, synthesized and tested to determine its potential as a Te source for unassisted pyrolytic MOCVD growth of HgxCd1-xTe at substrate temperatures ⋍ 300°C. Experiments were concentrated on unassisted pyrolytic growth of CdTe using DHTe and dimethylcadmium (DMCd) as a Cd source. The long-term stability of DHTe far exceeded theoretical expectations. Cadmium telluride growth rates were a linear function of the DHTe flow rate and independent of substrate temperature at or above 250°C. The maximum CdTe growth rate measured, 4 μm/h, was obtained by flowing 800 SCCM H2 through the DHTe sublimer (sublimer temperature 10°C). All CdTe layers grown on high-resistivity CdTe substrates were p-type, with carrier concentrations between 6.2×1015 and 4.2×1016 cm-3. The best electrical properties were obtained at a growth temperature of 250°C, where the p-type carrier concentration was 6.2×1015 cm-3 and the Hall mobility was 80 cm2/V·s.

Analysis of the effect of mass transfer and thermal diffusion on the growth rate of CdTe by MOCVD

Abstract The supply of reacting molecules by mass transfer is an important aspect for MOCVD processes. In this paper, we have analyzed the effects of mass transfer on growth rates of CdTe, with special consideration to thermal diffusion. Our results indicate that the calculated growth rates will be reduced by 15% at a temperature gradient of 100°K/cm to about 53% at 300°K/cm, if compared with the growth rates for isothermal conditions. However, at 900°K or higher, the equilibrium conditions at the surface will give rise to significant increase in backflux and become eventually the dominant factor in growth rates.

ChemInform Abstract: Mechanism of Growth of CdTe by Organometallic Vapor‐Phase Epitaxy.

AbstractPyrolysis of dimethylcadmium (DMCd) has been studied in the temp. range 230‐400 °C, and the growth rate of CdTe has been measured as a function of the partial pressure of DMCd and diethyltelluride (DETe) and of temperature.

Excimer Laser-Assisted Movpe of Cdte on Gaas (100): Crystal Growth and Mechanisms

ABSTRACTExcimer laser-assisted MOVPE has been used to deposit thin films of CdTe (111) on GaAs (100) substrates. Auger spectroscopy indicates that the films are stoichiometric and that carbon and oxygen contamination are below the levels of detectability. Good crystalline quality of the layers is confirmed by x-ray diffractometry. Growth rates up to 2 /μm/hr have been recorded in real time using time-resolved reflectivity. The CdTe growth rate is found to have a linear dependence on reactant partial pressure and laser average power, but was independent of substrate temperature in the range of 150° C to 300* C. Laser-induced fluorescence spectroscopy has been used to determine that photolysis of diethyltellurium at 248 nm is a one photon process which produces ground state Te atoms. The flux of reactive species to the substrate surface has been probed by mass spectrometry. Evidence for the generation of monoethyltelluride, either by recombination or direct photolysis, is presented.