Autodoping during the deposition of epitaxial Silicon layers from the gas phase (I). Autodoping from the gas phase

The doping profile within the autodoping range of epitaxial layers deposited on lowresistance substrates, is considered the result of redistributing a substrate doping fraction through the substrate layer boundary, of incorporating a limited amount of dopants from the gas phase, originating from the preepitaxial process, and the result of incorporating dopants, externally admitted to the gas flow during the deposition. Assuming a special incorporation equilibrium to exist for the gas phase fraction of autodoping an analytic expression is derived, correlating the autodoping profile characteristic to the limited amount of dopants in the gas phase. The extent is discussed, to which the gas phase fraction of autodoping may be influenced by varying the deposition process.     

Study of the structural and photoluminescence properties of CdTe polycrystalline films deposited by close-spaced vacuum sublimation

The polycrystalline CdTe films were deposited by the close-spaced vacuum evaporation at the different substrate temperatures (150–550 °C). The X-ray diffraction measurements of structural and substructural properties of these films were carried out to study their phase composition and texture. The films’ parameters such as the coherent scattering domain size, microdeformation level and mean density of dislocations were determined based on the broadening of diffraction peaks. In this case the Hall and three-fold convolution approximations were used. Surface morphology, grain size and growth mechanism of the films were determined by the scanning electron microscopy. The low temperature photoluminescence measurements allowed us to establish the correlation between the point and extended defect structure on the one hand and the growth conditions on the other. As a result, the growth conditions of CdTe polycrystalline films with fairly good crystal and optical quality were determined.     

Characterization of silicon layers epitaxially grown on metallurgical-grade polycrystalline substrates

The structural and electrical properties of silicon layers epitaxially grown on metallurgical-grade polycrystalline silicon substrates are examined to clarify the effect of grain boundaries, crystal defects and impurities in the substrates. Chemical etching of the epitaxial layer reveals that all the grain boundaries continue from the substrate into the epitaxial layer, whereas lines of high density etch pits do not always continue. The polycrystalline thin film solar cells are fabricated on the metallurgical-grade silicon substrates by successive deposition of p and n⁺ layers. These cells show short circuit current densities around 70% of that of the conventional single crystal cell. This reduction of the short circuit current is caused mainly by the short minority carrier diffusion length in the grains probably due to impurities involved in the epitaxial layers. The origins of such impurities are discussed by considering autodoping and solid-state diffusion from the substrate during growth of epitaxial layers.     

Surface structure of obliquely deposited layers and liquid crystal alignment

The dependence of the orientational action of obliquely deposited SiO_x layers on the deposition angle and the residual gas pressure has been investigated. The variation of the pressure gave rise to a modification of the general dependence of the pretilt angle of liquid crystals on the deposition angle. A model has been developed which explains the action of all deposition parameters on the orientational behaviour of liquid crystals. In a first step, some tendencies of the layer formation are stated. Thereby, in a second step, it is possible to explain the change in the orientation at variations of deposition parameters using the knowledge of these tendencies and experimentally stated informations on the relative change of layer parameters.     

Growth and real structure of diamond films synthesized on polycrystalline tungsten substrates

X-ray diffraction studies of diamond films obtained by chemical transport reactions at the concentrations of 2% methane and 98% hydrogen under pressures of 10.7 and 21.3 kPa showed that they are textured. The character of the texture depends on the substrate temperature. The films have {110}, {311}, or double {110} + {311} textures. It is established that the dependence of the growth rate of diamond films on the substrate temperature has maxima. The diamond films are finely dispersed and are characterized by pronounced micro-deformations and high dislocation densities. The temperature dependence of the growth rate correlates with the parameters of the real structure of the films.     

Thin layers deposited from V2O5 gels: I. A conductivity study

The conductivity ? of V₂O₅ layers deposited from gels of various V⁴⁺ content C was measured over a very large temperature range. The observed variations agree with Mott predictions on polaron hopping. Indeed below ≈ 50 K we reached a new conductivity regime with a well-defined activation energy. The fit of the experimental data to the Schnakenberg model was excellent and thus we could determine the polaron energy W_p, the disorder energy W_D and the mean optical phonon frequency v₀. All these values decrease with increasing C. In contrast whatever C the coupling constant γ was ≈ 10. The variation of W_D with C agrees quantitatively with the values calculated by means of Miller and Abrahams' theory and this led us to the conclusion that in those amorphous V₂O₅ layers disorder is mainly a potential disorder related to non-stoichiometry.     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

Thin layers deposited from V2O5 gels: II. An optical absorption study

Absorption spectra of V₂O₅ layers deposited from gels of various V⁴⁺ contents C were studied from near UV down to near IR. The high absorption region due to the charge transfer transition extends above ≈ 2 eV. Whatever C the optical gap is ≈ 2.25 eV. On the low energy side an Urbach tail is observed whose slope increases with C. In the near infrared absorption is related to the V⁴⁺ ions and increases linearly with C. The absorption band due to the optically induced polaron hopping is detected. The corresponding maximum E_max suffers a red shift when C increases. The experimental E_max data are very close to the values predicted from the polaron and disorder energies as deduced from the conductivity study in an earlier paper. The slight difference between the two sets of data allowed us to estimate the transfer integral J ≈ 0.015 eV.     

Effect of electrical field on growth of gallium arsenide layers from vapour phase

The effect of electrical fields on the growth rate and morphology of gallium arsenide layers in the GaAs AsCl₃ H₂ system is investigated. The fields both parallel and perpendicular to the substrate surface are used. It is found that application of the parallel field results n increase of growth rate of (111)A face but does not change that of 2° (001). The morphology of layers grown in such a field is better than that grown without any. The application of perpendicular field results in decrease of growth rate for both crystallographic orientations, the layer morphology worthening. In both cases the effect is greater when the substrate has a negative potential. Possible mechanisms of the field influence on the growth rates of gallium arsenide layers are discussed such as: supersaturation decrease because of nucleation in vapour phase and the change in the rates of surface processes.     

Vapour phase growth of epitaxial silicon carbide layers

After a brief overview of different epitaxial layer growth techniques, the homoepitaxial chemical vapour deposition (CVD) of SiC with a focus on hot-wall CVD is reviewed. Step-controlled epitaxy and site competition epitaxy have been utilized to grow polytype stable layers more than 50 μm in thickness and of high purity and crystalline perfection for power devices. The influence of growth parameters including gas flow, C/Si ratio, growth temperature and pressure on growth rate and layer uniformity in thickness and doping are discussed. Background doping levels as low as 10¹⁴ cm⁻³ have been achieved as well as layers doped over a wide n-type (nitrogen) and p-type (aluminium) range.

Furthermore the status of numerical process simulation is mentioned and SiC substrate preparation is described. In order to get flat and damage free epi-ready surfaces, they are prepared by different methods and characterised by atomic force microscopy and by scanning electron microscope using channelling patterns. For the investigation of defects in SiC high purity CVD layers are grown. The improvement of the quality of bulk crystal substrates by micropipe healing and so-called dislocation stop layers can further decrease the defect density and thus increase the yield and performance of devices. Due to its high growth rate functionality and scope for the use of multi-wafer equipment hot-wall CVD has become a well-established method in SiC-technology and has therefore great industrial potential.     

Morphological and X-ray study of SiC crystals grown from gas phase

SiC crystals obtained from the gas phase were investigated in order to characterize their morphology and polytypic modifications. Specific X-ray patterns showed the occurrence of one-dimensional disorder in some of α-SiC crystals.     

Influence of gas flow direction on DC glow-discharge deposited a-Si films

Influence of the electrical and of the aerodynamical parameters of a DC glow discharge in silane on the properties of a-Si films deposited in different sites of the glow-discharge has been investigated. It has been found that the electric as well as aerodynamic asymmetry of the DC discharge influence considerably the film properties. Under certain conditions it is possible to obtain semimetallic films without photoeffect.     

A windows application: Program for qualitative phase analysis of polycrystalline mixtures

The use of modern methods of programming (the Delphi system) for the further development or adaptation of the programs designed earlier and providing their functioning as 32-bit applications controlled by the operation system Windows-95 has been considered. New possibilities of the user dialog with a computer are described on an example of the developed FAZAN-Windows program for the phase analysis of polycrystalline mixtures.     

On the development of single and multijunction solar cells with hot-wire CVD deposited active layers

We present an overview of the scientific challenges and achievements during the development of thin film silicon based single and multijunction solar cells with hot-wire chemical vapor deposition (HWCVD) of the active silicon layers. The highlights discussed include the development of Ag/ZnO coatings with a proper roughness and morphology for optimal light trapping in single and multijunction thin film silicon solar cells, studies of the structural defects created by a rough substrate surface and their influence on the performance of nc-Si:H n–i–p single junction solar cells, and studies of the phase change during the growth of nc-Si:H by HWCVD and the use of a ‘reverse’ H₂ profiling technique to achieve nc-Si:H single junction n–i–p cells with high performance. Thus far, the best AM1.5 efficiency reached for n–i–p cells on stainless-steel with HWCVD i-layers is 8.6% for single junction nc-Si:H solar cells and 10.9% for triple junction solar cells. The opportunities for further improvement of cell efficiency are also discussed. We conclude that the uniqueness of HWCVD and of the i-layers deposited with this technique require some adjustments in the strategy for optimization of single or multijunction solar cells, such as using a reverse H₂ profiling technique for the deposition of nc-Si:H i-layers. However, the output performance of solar cells with HWCVD deposited i-layers is close to those with i-layers deposited by other techniques. The difference between the best nc-Si:H n–i–p cells obtained so far in our lab and the reported best n–i–p cells with PECVD i-layers can be mainly attributed to the differences in the rough substrates and to the use of rather thin i-layers.     

Influence of hydrogen on structural and optical properties of low temperature polycrystalline Ge films deposited by RF magnetron sputtering

To improve the properties of polycrystalline Ge thin films, which are a candidate material for the bottom cells of low cost monolithic tandem solar cells, ∼300 nm in situ hydrogenated Ge (Ge:H) thin films were deposited on silicon nitride coated glass by radio-frequency magnetron sputtering. The films were sputtered in a mixture of 15 sccm argon and 10 sccm hydrogen at a variety of low substrate temperatures (T_s)≤450 °C. Structural and optical properties of the Ge:H thin films were measured and compared to those of non-hydrogenated Ge thin films deduced in our previous work. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increase in T_s. It is found that the introduction of hydrogen gas benefits the structural properties of the polycrystalline Ge film, sputtered at 450 °C, although the onset crystallization temperature is ∼90 °C higher than in those sputtered without hydrogen. Compared with non-hydrogenated Ge thin films, hydrogen incorporated in the films leads to broadened band gaps of the films sputtered at different T_s.     

Incorporation of phosphorus during gas phase epitaxy

The PH₃ desorption rate can be reduced and the decomposition rate increased, thereby increasing the P incorporation efficiency by replacing TMIn with InCl created by the pyrolysis of DEIn. InCl generated by cracking DEInCl and uncracked PH₃ could be used for CBE growth of InP provided that H on the PH₃ can be used to remove Cl from the InCl. Evidence is presented that this is possible. Evidence is also provided that P is more readily incorporated during OMVPE growth using TBP than PH₃ because PH₂ is a primary pyrolysis product, and it is less likely to desorb and more likely to decompose than PH₃.     

Electro-optical properties of InGaAs layers grown by hydride vapour phase epitaxy

A series of epitaxial layers of the InGaAs alloy were deposited on (001) oriented InP substrates by using hydride VPE technique. The layers were characterized by Double Crystal Diffractometry (DCD), Photoluminescence (PL), Hall effect and Capacitance-Voltage (C-V) measurements. The growth parameters and the quality of the grown layers are discussed on the basis of electrical and structural data analysis.     

2 dimensional electron gas uniformity of GaN HFET layers on SiC

As GaN power transistor technology matures it is increasingly important to understand any links between substrate “quality”, epi-layer growth and electrical characteristics of the 2-dimensional electron gas (2DEG), which forms the active part of devices. We present a study, which makes use of full wafer mapping techniques to examine these relationships. Substrate off-cut is shown to be an important parameter in controlling the uniformity of GaN HFET device layers on SiC substrates.     

Low-temperature Si epitaxy on large-grained polycrystalline seed layers by electron–cyclotron resonance chemical vapor deposition

The epitaxial thickening of polycrystalline Si films on glass substrates is of great interest for the realization of crystalline Si thin film solar cells and other large-area thin film devices. In this paper we report on the epitaxial growth of Si at temperatures below on polycrystalline seed layers using electron–cyclotron resonance chemical vapor deposition. The Si seed layers were prepared by aluminum-induced crystallization. The quality of the ECRCVD-grown films strongly depends on the orientation of the underlying seed layer grains. Due to a mainly favorable orientation of the seed layers more than 73% of the substrate area were epitaxially thickened. It turned out that a (1 0 0) preferential orientation is favorable for epitaxial thickening. This, however, is not the only requirement for successful low-temperature epitaxial growth of Si.