Critical Thickness For Pseudomorphic Growth Research Articles

Investigation of the Trap-Induced Power Conversion Limit for CdS/CdSe Cascade Quantum Dot Sensitized Solar Cells Fabricated by Using the Successive Ionic Layer Adsorption and Reaction Process

CdS/CdSe quantum dot sensitized solar cells (QDSSC) fabricated by successive ionic layer adsorption and reaction (SILAR) processes was investigated, and a rate-equation model for the trapinduced power conversion efficiency (PCE) limit was developed and used to explain the experimental results. The cascade structure with a CdS:CdSe (7:7) cycle ratio showed the highest PCE of 2.55%. However, excess cycles of CdSe beyond the optimum condition decrease the device performance. The current loss when exceeding the maximum PCE condition is attributed to the trap-induced charge field that impedes the carrier extraction from the absorber layer to the titanium dioxide (TiO2) and increases recombination due to dislocation generation when the critical thickness for pseudomorphic growth is exceeded. The simulation results show that the increase in the number of dislocations beyond the critical thickness increases the recombination rate and impedes charge transfer at the interface between TiO2 and the QDs.

Microstructure and transport properties of epitaxial topological insulator Bi2Se3 thin films grown on MgO (100), Cr2O3 (0001), and Al2O3 (0001) templates

We report the epitaxial integration of defect-induced room temperature ferromagnetic insulators, Cr2O3 and MgO, with topological insulators Bi2Se3 on c-sapphire substrate by pulsed laser deposition. The structural, magnetic, and magnetotransport properties of ∼15 nm Bi2Se3 thin films are investigated on each template. The lattice misfits of Cr2O3/Bi2Se3 and MgO/Bi2Se3 are ∼16% and ∼39%, respectively, where the critical thickness for pseudomorphic growth is less than one monolayer. The insulating behavior is more pronounced due to the additional scattering of the surface states of the Bi2Se3 layer by interfacing with MgO and Cr2O3. The weak antilocalization effect from the surface states is clearly suppressed, accounting for the presence of magnetic bottom layers. This work demonstrates an effective way to study the emergence of a ferromagnetic phase in topological insulators by the magnetic proximity effect in Bi2Se3, a step toward unveiling their exotic properties.

Structure and relaxation effects in thin semiconducting films and quantum dots

AbstractThe structural properties of two epitaxial model systems were investigated, NiMnSb thin films on a (In,Ga)As/InP(001) substrate and free‐standing CdSe quantum dots on a ZnSe(001) surface. For the first system the influence of surface‐oxidation on the relaxation behavior of the entire film was investigated by a comparison of capped and uncapped samples. We found that the critical thickness for pseudomorphic growth is more than doubled by the protective capping layers. For the second system under investigation a determination of the size and shape of the quantum dots was performed. They have a cylindrical shape with an almost constant diameter of ≈10 nm and a height of about 2 nm, whereby the lowermost region widens up to about 14 nm. The entire quantum dots are strained. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ characterization of lattice relaxation of the BaTiO 3/LaNiO 3 superlattices epitaxially grown on SrTiO 3 substrates

The onset of lattice strain relaxation against the number of bilayers ( N) in an epitaxial BaTiO 3 (3 nm)/LaNiO 3 (3 nm) superlattice grown on a SrTiO 3 (0 0 1) substrate was characterized in situ by X-ray diffraction and reflectivity measurements with synchrotron radiation. At the initial stage of superlattice growth, highly strained and smooth sublayers were found, indicating the repetition of two-dimensional nuclei and growth of the sublayer on the flat terrace of a SrTiO 3 substrate; on increasing the bilayer number to N = 10 , the growth front becomes rougher to relieve the lattice strain. Further increasing the bilayer number to N = 11 , an abrupt increase of interfacial roughness is also followed. It corresponds to the effective critical thickness for pseudomorphic growth of a superlattice, beyond which surface and interface undulation result from the generation of interfacial misfit dislocations.

Growth mode of epitaxial superlattices[BaCuO2+x]2/[CaCuO2]3on vicinal (001)SrTiO3substrates studied by x-ray diffraction

Structural and morphological properties of $[{\mathrm{BaCuO}}_{2+x}{]}_{2}/[{\mathrm{CaCuO}}_{2}{]}_{3}$ superlattices with different thicknesses, grown on specially prepared vicinal (001) ${\mathrm{SrTiO}}_{3}$ substrates, are investigated. Using undulator radiation from a third-generation synchrotron we have performed x-ray-diffraction and diffuse scattering measurements, both in conventional and grazing-incidence geometries. We obtain detailed information about the role of the miscut of the substrate on the growth process of these superlattices. The critical thickness for pseudomorphic growth is found to depend on the miscut angle and, to a lesser extent on the film thickness. Thus the miscut of the substrate surprisingly influences the phase transition from the pseudomorphic, pseudotetragonal state to the orthorhombic state. Our results show that the interface morphology depends on the specific miscut of the substrate. Isotropic ripples, anisotropic ripples, and a fractal structure are formed upon slightly increasing the miscut angle from about 0.15\ifmmode^\circ\else\textdegree\fi{} up to angles less than 1\ifmmode^\circ\else\textdegree\fi{}. The ripples accompany the two-dimensional, layer-by-layer growth mode, which prevails for thin layers of the superlattices on well-oriented surfaces.

The effect of epitaxial strain on RBa 2Cu 3O 7 thin films and the perovskite substrate

Originating from the lattice mismatch and the difference in the thermal expansion coefficients between substrate crystal and the epitaxial film, a strain field is formed at the interface. The superconducting transition temperature of epitaxial layer of high-temperature superconductors (HTS) can be tuned by the strain field. In our study, the HTS films were deposited by pulsed laser ablation on SrTiO 3 (STO) and NdGaO 3 (001) substrates with thickness below and above the critical thickness for pseudomorphic growth. Results of X-ray structural investigations will be shown. Our observations fit within the frame of the available data on the effect of high pressure on HTS materials. We will present evidence that the strain field is affecting the substrate, namely the cubic to tetragonal transition of STO, too.

Luminescence from erbium implanted silicon–germanium quantum wells

We have investigated the luminescence emitted at 1.54 μm from erbium-implanted strained ultrahigh vacuum chemical vapor deposition-grown (UHVCVD-grown) Si1−xGex quantum wells. Germanium fractions of up to 13% were used, and all well widths were below the critical thickness for pseudomorphic growth. A preliminary study was carried out on Si1−xGex quantum wells implanted with amorphizing doses of silicon at 77 K in order to study the regrowth across the interfaces, and subsequent structural and optical recovery. After amorphization and regrowth by a two stage anneal process, transmission electron microscopy (TEM) clearly showed the presence of the quantum wells, with sharp contrast. X-ray diffraction (XRD) studies showed that good regrowth has been achieved, with line widths very similar to the original material. However, the photoluminescence (PL) was found to be dependent upon the duration of the first anneal. Increasing the anneal time resulted in PL spectra being dominated by broad signals between 0.9 a...

Reactive deposition epitaxy of CrSi 2

CrSi 2 layers were synthesized by deposition of Cr onto a Si(111) substrate kept at 450°C, allowing full reaction of the metal with silicon. In situ RHEED patterns confirmed the formation of the CrSi 2 phase with matching faces CrSi 2(0001)/Si(111) and of the type A azimuthal orientation CrSi 2 <10 1 0> ‖ Si<110>. The epitaxy of the initially textured films is enhanced by post-deposition annealing up to 1200°C. With high-temperature treatment, the initially nearly continuous silicide layers break up into CrSi 2 islands, the height and diameter of which increases with the annealing temperature. Backscattering spectra confirm the presence of exposed silicon, by indicating that the apparent stoichiometry of the film is Si rich CrSi x ( x > 2). In the best case, a minimum channeling yield lower than 30% was found in the Cr signal of the backscattering spectrum. As far as we know, this is the first evidence of channeling in CrSi 2 films grown by any technique, other than ion beam synthesis. Both the areal fraction of exposed Si and the crystalline quality of the silicide islands depend strongly on the annealing temperature and the amount of Cr deposited. From perpendicular strain measurements, the critical thickness for pseudomorphic growth is estimated to be the equivalent of 20 Å of deposited Cr.

Electron-spin polarization in photoemission from strained GaAs grown on GaAs1-xPx.

Spin-polarized electron photoemission has been investigated for strained GaAs epitaxially grown on a GaAs{sub 1{minus}{ital x}}P{sub {ital x}} buffer. The lattice-mismatched heterostructure results in a highly strained epitaxial layer and significant enhancement of electron-spin polarization is observed. The effect of epitaxial layer strain is studied for a variety of samples with epitaxial-layer thicknesses varying from 0.1 to 0.3 {mu}m and the phosphorus concentration {ital x} varying from 0.21 to 0.28. Electron-spin polarization as high as 90% has been observed. The 0.3-{mu}m-thick sample, well in excess of theoretical estimates for the critical thickness for pseudomorphic growth, reaches an electron-spin polarization of 80%, demonstrating a significant persistence of lattice strain.

Microstructural characterization of InGaAs/GaAs quantum wells grown at low substrate temperatures

The composition and thickness of InxGa1-xAs/GaAs quantum wells play an important role in the determination of their optical and electronic properties. InxGa1-xAs/GaAs single quantum wells (SQWs) grown at low substrate temperatures with various In compositions (x) and well thicknesses were investigated by reflected high energy electron diffraction (RHEED) and photoluminescence (PL) techniques to determine when the critical thickness for pseudomorphic growth was exceeded. Cross-sectional TEM methods were employed to directly determine the critical layer thickness of InGaAs on GaAs and the presence of the dislocations generated in these layers.The samples were grown by MBE on semi-insulating (100) GaAs substrates at substrate temperatures of 410°C and 460°C. They consisted of undoped SQWs and barriers in which the well width was varied from 1 nm to 22 nm. The In composition in the well layers was varied from 0.3 to 0.5. The growth rates and In compositions were deduced from RHEED intensity oscillation measurements . Cross-sectional specimens for TEM studies were prepared in the usual manner, by mechanical thinning followed by Ar+ ion milling. A Philips EM400T operated at 120 keV was used to observe the specimens.

Temperature Dependence of Critical Thickness for Two Dimensional Growth of GexSi1−x on Si Substrate

ABSTRACTFor obtaining good structural perfection, the molecular beam epitaxial (MBE) growth of GexSi1−x on Si substrate should not only be kept in the pseudomorphic form but also in layer-by-layer growth stage. We found that the two dimensional layer-by-layer growth of GexSi1−x on Si could persist to a certain deposition thickness, beyond that the transition to islanding growth occurs. The transition thickness is significantly dependent on the growth temperature and germanium content, and is always smaller than the critical thickness of pseudomorphic growth. In order to obtain good crystalline quality in growing GexSi1−x superlattices on Si substrates, the thickness of GexSi1−x layers should be controlled below the transition thickness and lower growth temperature is favorable.

RHEED intensity oscillations of mbe grown Ge/Si ultra-thin multi-layered structures

The behavior of RHEED intensity oscillations during molecular beam epitaxy of Ge/Si ultra-thin multi-layers on Si(100) and Si(111) substrates has been studied. During the growth of alternating Ge and Si layers on a Si(100) substrate, a single atomic layer mode RHEED intensity oscillation is observed along either 〈011̄〉 or 〈010〉 azimuthal direction. The RHEED intensity oscillations for continuously growing Ge on Si(100) could last up to 6 periods, which correspond roughly to the critical thickness of pseudomorphic growth of Ge on Si. The RHEED intensity oscillations for growing Ge and Ge/Si multi-layers on Si(111) seem more complicated than that on Si(100). Along the 〈011̄〉 azimuth, the RHEED intensity oscillations show a non-uniform periodicity, which is caused by a change in surface reconstructions. The phase-locked behavior and the intensity recovery of RHEED oscillations after growth interruption have been observed, which reveal the different surface diffusion constants of Ge and Si atoms on Si(100) and Si(111).

RHEED INTENSITY OSCILLATIONS IN THE PROCESS OF MOLECULAR BEAM EPITAXY GROWTH OF Ge AND Si ON Si SUBSTRATES

RHEED intensity oscillations in the process of MBE growth of Ge and Si on Si (100) and Si (111) substrates were observed. It is revealed that a finite-thick buffer layer can improve the surface flatness of Si substrates, and the employment of lower growth rate or growth interruption procedure can improve tke crystal quality of epilayers. During the growth of Ge and Si on Si (100), a single atomic layer mode RHEED intensity oscillation was observed along either [100] or [110] azimuthal direction, which is due to the existance of double domain (2×1)+(1×2) reconstruction on the surface. During the growth of Ge on Si (111) substrates, RHEED intensity oscillation show a bilayer mode observed along [112] azimuth, but a non-uniform periodicity along [110] azimuth. The oscillations for continuosly growing Ge-on either Si (100) or Si (111) could exist up to an epilayer thickness of 6ML, which corresponds to the critical thickness of pseudomorphic growth of Ge on Si.

Transmission electron microscopy of short-period Si/Ge strained-layer superlattices on Ge substrates

Structural investigations of high quality Si/Ge strained-layer superlattices (SLSs) on [001] oriented Ge substrates prepared by molecular beam epitaxy are presented. Cross-sectional transmission electron microscopy reveals that a defect-free superlattice is achieved for a structure composed of a 20-period sequence of 3 monolayers (ML) Si and 9 ML Ge. High-resolution lattice images and electron diffraction patterns show that the whole structure is matched to the Ge substrate. Experimental values for the tetragonal deformation of the Si layers within the SLS are in good agreement with theory. An equivalent sample containing 120 periods exceeds the critical thickness for pseudomorphic growth of the SLS and shows the formation of twin lamellae.

GaAs/GaSb strained-layer heterostructures deposited by metalorganic vapor phase epitaxy

The growth of strained GaSb/GaAs quantum wells has been attempted for the first time (7% lattice mismatch), with the antimonide layers being constrained to take on the GaAs lattice parameter in the interface plane. The critical thickness for pseudomorphic growth of the strained layer was about 15 Å, with further growth resulting in islands of GaSb crystallites over the wafer surface. Photoluminescence spectra and photoconductivity from both single and double wells showed a strong signal at approximately 1.3 eV, identified as a Γ point transition. This was not consistent with band structure calculations for a GaSb/GaAs well, suggesting an error in the estimation of the band offsets and/or As incorporation in the strained layer.

X-ray diffraction of strain relaxation in Si-Si1−xGex heterostructures

We report a double-crystal x-ray diffraction study of the relaxation of molecular beam epitaxy grown Si-Si1−xGex strained single layers and superlattices on (100) Si. The thermal stability of the heterostructures was investigated by annealing between 600 and 900 °C. Measurement of (400) rocking curves demonstrated that all the heterostructures were initially coherently strained and had excellent crystallinity. Upon annealing deterioration of the crystal quality and progressive relaxation was observed on some of the samples while on others no relaxation or loss of crystalline quality was detected. These observations are consistent with the mechanical equilibrium theory predicting the critical thickness for pseudomorphic growth of lattice mismatch materials. However, the concept of critical stress needs to be invoked to account for the stability of dilute Si1−xGex alloy layers on Si.

Critical thickness for pseudomorphic growth of Si/Ge alloys and superlattices

A model, based on energy considerations, to determine the critical thickness for pseudomorphic growth of Si/Ge alloys and superlattices is introduced. Comparison with experiments shows a good agreement between the theory and experiment. It is also shown that in films which are initially dislocation-free, the energy barrier for the nucleation of dislocation loops can be crossed by thermal activation if the film is thicker than the critical value provided by the model.

Study of CoSi2/Si strained layers grown by molecular beam epitaxy

In this paper, we report a study of the strain and its relaxation of epitaxial CoSi2 grown on Si by molecular beam epitaxy (MBE). The strain is measured by a x-ray rocking curve technique and misfit dislocations are determined by transmission electron microscopy (TEM) as the strain relaxes. Results show that the critical thickness of pseudomorphic growth is about 30 nm for growth temperature of 550 °C although there is still a residual strain remained for the thicker films. No apparent complete relaxation of the strain is obtained. Thermal annealing of CoSi2 films is performed and the results of the strain relaxation are discussed.

The effect of anharmonicity in epitaxial interfaces: II. Equilibrium structure of thin epitaxial films

The influence-of anharmonicity on the equilibrium structure of thin epitaxial films is studied in the framework of the theoretical model, introduced in Part I. Using the conventional Frank and van der Merwe model as a reference basis, it is shown that allowing for anharmonicity of the interatomic forces leads to qualitatively new effects, such as: (i) a significant asymmetry of dislocation length, strains in the deposit, activation energy for surface diffusion of epitaxial islands with respect to the sign of natural misfit between substrate and deposit; (ii) a split of the critical stability limits for pseudomorphic growth as well as for spontaneous generation of misfit dislocations whereby the limits corresponding to negative misfit rapidly increase whereas the positive misfit limits decrease (in absolute terms) with growing degree of anharmonicity. Experimental evidence, concerning the pseudomorphic growth of metals, semiconductors and ferromagnetic garnets, is shown to agree favourably with these theoretical results. A dependence of structural properties of the overlayer, such as critical thickness of pseudomorphic growth, density of dislocations beyond the critical thickness and preferred epitaxial orientation, not only on absolute value but also on sign of the natural misfit is predicted. It appears important as a guideline for material research in the field of epitaxial growth.