Density Of Structural Defects Research Articles

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on ‘‘ideal’’ seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)oriented diodes consistently exhibit a higher Voc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10 � 3 to 10 3 Suns. We determine 7mm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3mm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10mm. r 2002 Elsevier Science B.V. All rights reserved.

Preparation of InP-based semiconductor materials with low density of defects: effect of Nd, Ho and Tb addition

Specific features of some rare earth elements (REE=Nd, Ho or Tb) are exploited to improve structural, optical and electrical properties of InP-based layer compounds for applications in ionising radiation detector structures. InP layers were grown by liquid phase epitaxy on (100)-oriented single crystal InP substrates with individual REE addition to the melt. The dependence of the layer thickness, overall surface morphology and defect density on the growth conditions were monitored by employing optical and scanning electron microscopy. The evaluation of electrical properties was gained from C– V measurements performed with the mercury probe and from a free carrier concentration profile determined by the probe profiling method on bevelled samples. The low-temperature photoluminescence spectroscopy was used to study the changes in optical properties. Significant improvement of all the studied layer parameters with increasing amount of REE in the melt was observed up to certain critical value of REE concentration. The density of structural defects was reduced by more than a half order of magnitude, free carrier concentration was reduced effectively by up to four orders of magnitude, photoluminescence peaks were narrowed substantially and fine spectral features were resolved. The conductivity of layers prepared with Tb admixture changed from n to p type when Tb exceeded certain limit of concentration in the melt.

Stimulated Emission and Optical Gain in Homoepitaxial GaN

Stimulated emission in homoepitaxial GaN layers was studied by measuring luminescence spectra in backward and lateral configurations at room temperature. The dynamics of the spectra was investigated under excitation of carriers with different excess energies and by changing the excitation intensity. Two bands of stimulated emission caused by recombination in electron-hole plasma at the sample surface and in the dense exciton system located deeper in the layer were simultaneously observed. This is explained by large diffusion lengths for carriers (excitons) and low density of structural defects and cracks. Influence of carrier heating on the stimulated emission is demonstrated.

Growth of Si twinning superlattice

We review our attempts to grow a new type of single crystal Si epitaxially on Si(111). The new single crystal Si is a superlattice of layers that have twinned and untwinned orientations with respect to the substrate, and is called a twinning superlattice. The Si twinning superlattices are grown using the fact that Si layers grown epitaxially on Si(111)√3×√3-B have a twinned orientation with respect to the substrate. We investigate conditions for growing twinned epitaxial layers, and clarify that growth of twinned layers requires high surface B concentration and low densities of surface structural defects, such as steps and domain boundaries of the √3×√3 reconstruction. We also investigate the thermal stability of the twinned layers, and clarify that the temperature at which the twinned layers are transformed into untwinned layers strongly depends on the thickness. Additionally, we establish a technique for measuring crystallographic orientations in surface regions in situ during growth. Based on these results, we finally succeed in growing the Si twinning superlattice by repeated growth of Si with a unit thickness and post-growth annealing.

Domain-boundary structures in Ce-doped α-(Si-Al-O-N)

Abstract The incorporation of large Ce ions into the interstitial sites in an α-(Si–Al–O–N) crystal structure is found to have produced a high density of structural defects. The defects are of planar nature and connect to form domain boundaries. Analysis via transmission electron microscopy has revealed that only a single lattice translation of ⅓(1010) type is involved in the formation of domains which are enveloped by some specific faces in this strongly bonded covalent compound, that is (0001), {1011} and the common surface with the matrix crystal on {1010}. Development of the domain-boundary structures is discussed via the application of image simulation based on different structural models of defects.

Structural properties of heteroepitaxial germanium-carbon alloys grown on Si (100)

Abstract Heteroepitaxial films of Ge1-x C x (x < 7%) alloys have been grown on Si (100) substrates using ultrahigh-vacuum chemical vapour deposition reactions involving GeH4 and several different germylmethane precursors (GeH3)4-x CH x (x = 1-3), at temperatures in the range 470-540°C. The layer composition and crystallinity were assessed using Rutherford back-scattering spectroscopy, including C-resonance analysis, and the film microstructure was characterized using cross-sectional transmission electron microscopy. Irrespective of the particular germylmethane precursor used for deposition, alloys with low C concentrations had high crystallinity with low defect density, whereas those with high C content had an increased density of structural defects. Greater C incorporation generally led to films with flatter and smoother surfaces, implying that the addition of C had compensated strain due to lattice mismatch and promoted two-dimensional growth.

Advances in crystal growth and characterizations of gallium orthophosphate, GaPO 4

Gallium orthophosphate, GaPO 4, is a piezoelectric material isostructural with α-quartz, SiO 2, with better piezoelectric characteristics: higher coupling coefficient for its temperature compensated cut and wider thermal stability (up to 933°C). A new investigation of the solubility of GaPO 4 in different acids and their mixtures allows to determine the metastable zone of which the knowledge is primordial for a good growth restart. The comparison of experimental growth results obtained in horizontal glass vessels ( T c<170°C) and vertical autoclaves ( T c>170°C) leads to specify a necessary minimum value of the solute supply at the interface crystal/solution. On the other hand, natural large seeds being not available, the seed lengthening is carried out with the splicing technique followed by crossed crystal growths to decrease the density of structural defects. At the same time, characterizations of crystals have been undertaken to check: the OH content by infrared spectroscopy; the crystalline quality (specially crystals after splicing) and the growth restart by X-ray topography; the piezoelectric properties.

Near-unity below-band-gap absorption by microstructured silicon

We increased the absorptance of light by silicon to approximately 90% from the near ultraviolet (0.25 μm) to the near infrared (2.5 μm) by surface microstructuring using laser-chemical etching. The remarkable absorptance most likely comes from a high density of impurities and structural defects in the silicon lattice, enhanced by surface texturing. Microstructured avalanche photodiodes show significant enhancement of below-band-gap photocurrent generation at 1.06 and 1.31 μm, indicating promise for use in infrared photodetectors.

Corrosion resistance of sapphire surface as a function of its crystallographic characteristics

Corrosion resistance of a sapphire surface has been studied as a function of various technological factors (growth method, presence of impurities, growth medium), the density of structural defects (pores, vacancies, dislocations), and the crystallographic characteristics. It is shown that the main contribution to the corrosion resistance depends on the type of the mechanical treatment and the crystallographic characteristics of the surfaces and that the resistance of different crystallographic planes to aggressive media can differ up to seven times.

Ion channeling studies of hydrogen in homoepitaxially grown CVD diamond

Abstract Using ion channeling measurements in conjunction with the resonant 1 H( 15 N,αγ) 12 C nuclear reaction or with proton Rutherford backscattering spectrometry we studied the incorporation of hydrogen from the plasma into homoepitaxial (100) and (111) CVD diamond films. The measured hydrogen concentration and the structural defect density of the diamond lattice derived from proton channeling yields were observed to be correlated. Hydrogen lattice location studies for different incident ion channeling directions showed no dominant fraction of H occupying either one of the two theoretically predicted sites. These results suggest that in CVD films with quite high dislocation densities and relative H concentrations above 10 −3 , hydrogen is predominantly incorporated at structural defect sites uncorrelated with the lattice symmetry.

Reduction of Defect Density in Structures With InAs-GaAs Quantum Dots Grown at Low Temperature for 1.55 [.proportional]m Range.

ABSTRACTTransmission electron microscopy (TEM), and photoluminescence (PL) have been used to evaluate defects and the efficiency of defect-reduction techniques in structures with InAs quantum dots (QDs) for the 1.55 µm range grown at low substrate temperature (LT) using molecular beam epitaxy (MBE). We show that capping of the QDs with thin GaAs layer accompanied by growth interruption at 600oC (flash) allows to eliminate large islands, containing dislocations, while the smaller islands containing local defects (e.g. dislocation dipoles) still remain. If the flash procedure is accompanied with further depositing of thin AlAs cap layer, and followed by high temperature (~700oC) annealing (HTA), an almost complete elimination of defects is observed. The structures emit in the range of 1.55 µm due to lateral agglomerates of LTQDs. Simultaneously bright luminescence due to isolated QDs and GaAs matrix are detected at high excitation densities.

Orientation-Dependence of Low Temperature Epitaxial Silicon Growth

ABSTRACTElectronic properties of thin epitaxial silicon films deposited at temperatures below 650°C by means of ion-assisted deposition strongly depend on substrate orientation as well as on deposition temperature: In (100)-oriented epitaxial films we find a low density of structural defects, and the minority carrier diffusion length is only limited by the presence of point defects or point defect complexes. These investigations also show an improvement of the electronic quality with increasing deposition temperature. Epitaxy on non-(100)-oriented substrates results in a significantly higher density of structural defects. The electronic properties of films deposited on stable flat surfaces, such as (111)- and (113)-oriented substrates are inferior as compared to (100)-oriented films, but are still superior to those of films deposited on faceted surfaces, as shown by light beam induced current and electron back-scattering diffraction measurements of polycrystalline thin films.

Morphology and microstructure of the Ar +-ion sputtered (0001) α-Al 2O 3 surface

The morphology and microstructure of Ar +-ion bombarded (0001) α-Al 2O 3 surfaces were studied by employing analytical electron microscopy (AEM) and high-resolution transmission electron microscopy (HRTEM). Surface bombardment with 1 keV Ar +-ions resulted in the formation of a ca. 3-nm thick γ-Al 2O 3 layer with a high density of structural defects. A well-defined epitaxial orientation relationship between the γ-Al 2O 3 layer and the substrate was observed: (0001) α ∥(111) γ, [10 1 0] α ∥[110] γ, and [11 2 0] α ∥±[11 2 ]γ.

CdS/CdTe interface analysis by transmission electron microscopy

CdTe-based polycrystalline solar cells are leading candidates for terrestrial photovoltaic applications. High efficiency devices have been obtained despite large lattice mismatch between hexagonal CdS and cubic CdTe. In the present work, CdS was deposited by chemical bath deposition on Si substrates, and CdTe was deposited by close spaced sublimation. The chemical nature of the CdS/CdTe interface, structural properties, and their dependence on the fabrication parameters, e.g., substrate temperature (475–600 °C) and postdeposition CdCl2 heat treatment, were analyzed. In addition, the effects of crystallinity of CdS on the interface structural defects were examined. Small spot energy dispersive spectroscopy of the interface revealed a considerable amount of sulfur in CdTe. The concentration of sulfur, in general, was higher in the grains with higher density of structural defects and at the grain boundaries. The planar defect density in CdTe films increased with substrate temperature, whereas the threading dislocation density decreased. Interface analysis showed that the majority of the crystalline defects in the CdTe films deposited on CdS were generated at the interface. The crystallinity of CdS did not have a major influence on the interface defect generation.

High-resolution transmission electron microscopy study on strained epitaxial manganite thin films and heterostructures

Pulsed laser deposition has been used to grow epitaxial La 2/3Sr 1/3MnO 3 and La 2/3Ba 1/3MnO 3 thin films as well as La 2/3Ba 1/3MnO 3/SrTiO 3 heterostructures. The microstructure was studied by four-circle X-ray diffraction and high-resolution transmission electron microscopy. Strain effects due to lattice mismatch between substrate and film, and between different layers in the heterostructures were found to play a key role in determining the thin film properties. La 2/3Ba 1/3MnO 3 films on SrTiO 3 substrates show perfect epitaxy and grow coherently strained over the whole film thickness. In contrast, La 2/3Sr 1/3MnO 3 films on SrTiO 3 substrates were found to form two layers with different lattice parameters separated by an intrinsic interface region containing a high density of defects. La 2/3Ba 1/3MnO 3/SrTiO 3 superlattices are coherently strained and show a very low density of structural defects and sharp interfaces. The observed interface roughness was below 1 nm allowing the controlled growth of continuous two unit-cell thick La 2/3Ba 1/3MnO 3 layers. We also discuss the influence of strain on the electrical transport properties.

Modeling of misfit induced defects in Co/Cr recording media

A 2D model to study the effect of crystal orientation on interfacial structural misfit and Co grain defect density for hcp Co/bcc Cr thin film system has been introduced. The magnitude of misfit depends strongly on the Co/Cr orientation, their respective lattice parameters, the grain size and to a lesser extend, the relative lattice orientation across the interface. However, the modeling results showed that by introducing lattice defects such as fcc stacking faults and misfit dislocations in the Co layer, one can greatly reduce this misfit and relax the interface.

Electrical properties of epitaxial indium arsenide and narrow band solid solutions based on it

The electrical properties of epitaxial InAs and solid solutions based on it (InGaAsSb, InAsSbP, InAsGa, InAsP) have been investigated. It is shown that intentionally undoped crystals have n-type conductivity, which is determined by shallow donor impurities (E 1=0.002–0.003 eV) and structural defects (E 2=0.02–0.03 eV and E 3=0.09–0.10 eV). It is shown that growth of epitaxial InAs using the neutral solvent Pb and also rare-earth elements makes it possible to reduce the electron density by almost an order of magnitude (to levels as low as 3×1015 cm−3) due to due to a decrease in the density of structural defects.

Effects of homo-epitaxial LaAlO3 layer on microstructural properties of SrTiO3 films grown on LaAlO3 substrates

Effects of homo-epitaxial LaAlO3 (LAO) layer on microstructural properties of SrTiO3 (STO) films grown on LAO substrates have been investigated. The STO films were deposited by pulsed laser deposition (PLD) on LAO (100) substrate with and without a homo-epitaxial LAO (HE–LAO) layer. The microstructure of the STO films was examined by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). TEM and HREM studies indicate the HE–LAO layer contains a much higher density of structural defects than the LAO substrate and the STO films grown on its top. In addition, the STO films deposited with the HE–LAO layer have a reduced planar defect (boundary) density relative to those films deposited directly on the LAO substrate. The reduced planar defect density in the STO film is strongly correlated with the previously observed improvement in microwave properties of devices based on these films.

Quality of Selective Silicon Epitaxial Films Deposited Using Disilane and Chlorine

We have previously reported on the selectivity and growth of a silicon epitaxy process using and in an ultrahigh‐vacuum rapid thermal chemical vapor deposition reactor. In this report, we have extended the previous work and provide information regarding the structural and electrical quality of thick (3000 Å) selective silicon epitaxial layers deposited under a variety of growth conditions. Electrical test structures, including enclosed n‐channel metal oxide semiconductor field effect transistors (MOSFETs) and large‐area gated diodes, were fabricated within the epitaxial layers. We demonstrate that variations in the chlorine to silicon ratio (Cl/Si) and the process temperature can lead to structural defects and low generation lifetimes. The defects, however, had a benign effect over the MOSFET drive current and channel transconductance. Overall, the results in this study indicate that high levels of chlorine, as well as low growth temperatures, can potentially inhibit the structural and/or electrical quality of selectively deposited silicon films. However, for growth at or above 800°C with a Cl/Si ratio of 0.23, excellent selectivity as well as extremely high bulk generation lifetimes can be obtained for films with structural defect densities well below the detection limits used within this study. © 1999 The Electrochemical Society. All rights reserved.

Synthesis and micro-structural study of one-dimensional nano-materials

Silicon nano-wires (SiNWs) and boron nitride nano-tubules (BN-NTs) were successfully synthesized by excimer laser ablation at high temperature. These one-dimensional nano-materials synthesized by this method have a very high yield, a uniform diameter distribution, and a high purity. Micro-structures of these nano-materials were investigated by transmission electron microscopy (TEM). The SiNWs have a high density of structural defects of microtwin, stacking faults, and low-angle boundary, which are closely related to the formation of SiNWs and the determination of morphology of the nano-wires. BN-NTs are mainly single atomic-layered and the outer surface of tubules is clean without any attachment. The formation of single atomic-layered tubule is attributed to the catalyst effect which makes the axial rate of BN-NTs much higher than the radial growth