Nucleation and growth of gallium arsenide on silicon (111)

The nucleation and growth of undoped gallium arsenide (GaAs) epitaxial layers, grown by metalorganic vapour phase epitaxy (MOVPE) on Si (111) substrates were investigated by transmission electron microscopy (TEM). The initial stages of epitaxial growth are considered at high and low growth temperatures. The influence of growth time and thermal annealing on the initial stages of growth are also studied and reported.     

The epitaxial growth of germanium and silicon on an Ag(111) film on a mica substrate

With the aim of finding a method of obtaining self-supporting single-crystal films of silicon for solar cells we studied the epitaxial growth of silicon and germanium prepared by evaporation in ultrahigh vacuum onto an Ag(111) film evaporated in situ onto a mica substrate cleaved in air. The films were examined mainly by reflection high energy electron diffraction. Silicon and germanium films 50–200 Å thick were composed of crystallites with two main orientations relative to the substrate and unoriented crystallites in varying proportions depending on the substrate temperature T_s and the previous heat treatment temperature T_H of the mica. Nearly single-crystal films of silicon could be obtained for T_s = 350 °C and T_H = 250 °C. The sticking coefficient for silicon on silver was found to decrease almost to zero for T_s = 420 °C with T_H = 250 °C. No single-crystal films of germanium were obtained.     

Epitaxial growth of cadmium sulphide on (111) germanium substrates

Single-crystal expitaxial layers of CdS on (111) Ge substrates, 8 to 60 μm thick, have been grown from the vapour phase in a closed-tube system. Hydrogen was used as a transport agent. The experimental conditions (source and deposit temperatures, and initial pressure of hydrogen) have been defined where the growth of single-crystal expitaxial layers is feasible. Observations on the morphology of the layers are reported, which suggest that at least two different growth mechanisms should be active in the system. Finally, the composition of the gaseous phase was calculated by assuming a non-reactivity of the Ge substrate with the vapour phase.     

Epitaxial growth of relaxed germanium layers by reduced pressure chemical vapour deposition on (110) and (111) silicon substrates

The growth of Ge on (110) and (111) oriented Si substrates is of great interest to enhance the mobility of both holes and electrons in complementary metal oxide semiconductor transistors. However, the quality of thick, relaxed Ge layers grown epitaxially on these surfaces is usually much lower than similar layers grown on (100) Si, resulting in both higher defect densities (i.e. threading dislocations and stacking faults) and rougher surfaces. In this work we have investigated the growth of Ge layers on (110) and (111) Si substrates by reduced-pressure chemical vapour deposition using a two temperature process. We have found that the combination of suppressing the Ge seed layer roughness and high temperature post-growth annealing can reduce the rms surface roughness of (110) Ge layers to below 2 nm and the threading dislocation density to below 1 × 10⁷ cm^− 2. Thick (111) Ge layers were found to exhibit a very high density of stacking faults, that could not be reduced by post-growth annealing and a higher rms surface roughness of around 12 nm, which was limited by the Ge seed layer.     

Growth of germanium nanowires on silicon(111) substrates by molecular beam epitaxy

Heteroepitaxial growth of Ge nanowires was carried out on Si(111) substrates by MBE. Au seeds were used as precursor for the VLS growth of the nanowires. Even if the Au droplets do not act as catalyst for the dissociation of gas, they are local preferential areas where the energetic barrier of Ge nucleation is lowered compare to the remaining non activated surface. Two sets of Au seeds were used as precursors for the VLS process. The first set have an average diameter of 125 nm and the second of 25 nm. In-situ RHEED monitoring showed a Au wetting layer between these seeds before the nanowires growth as well as at the end of the Ge nanowires growth. It means that the wetting layer acted as a surfactant from the Si(111) surface to the Ge grown layer between the nanowires. Analysis of SEM images brought the fact that the diffusion of gold from the droplets on the surface and the sidewalls of the nanowires via the Ostwald ripening is a key parameter of the growth of the nanowires.     

Stranski-Krastanow growth of germanium on silicon nanowires

There have been extensive studies of germanium (Ge) grown on planar silicon (Si) substrates by the Stranski-Krastanow (S-K) mechanism. In this study, we present S-K growth of Ge on Si nanowires. The Si nanowires were grown at 500 degrees C by a vapor-liquid-solid (VLS) method, using silane (SiH4) as the gaseous precursor. By switching the gas source from SiH4 to germane (GeH4) during the growth and maintaining the growth conditions, epitaxial Ge islands deposited on the outer surface of the initially formed Si nanowires. Transmission electron microscopy (TEM), scanning TEM, and energy-dispersive X-ray spectroscopy techniques were utilized to identify the thin wetting layer and the three-dimensional Ge islands formed around the Si core nanowires. Cross-sectional TEM verified the surface faceting of the Si core nanowires as well as the Ge islands.     

Kinetic lattice Monte Carlo simulations of silicon and germanium epitaxial growth on the silicon (100) surface

We present kinetic lattice Monte Carlo simulations of epitaxial growth of Si and Ge films on the Si (100) surface. Our simulations take into account surface reconstruction, in particular, how it makes the diffusion properties of ad-dimers and adatoms on the surface depend on the direction of motion and whether they are moving over a row or a trough. In the case of Ge expitaxial growth, when dealing with growth of Ge films, we incorporated the effect of Ge-Si exchange through a mechanism involving the ad-dimers. This results in a significant fraction of the first epitaxial layer containing Si, with an abrupt increase at one monolayer of coverage.     

Nucleation and growth of GaN nanowires on Si(111) performed by molecular beam epitaxy

GaN nanowires (NWs) have been grown on Si(111) substrates by plasma-assisted molecular beam epitaxy (PAMBE). The nucleation process of GaN-NWs has been investigated in terms of nucleation density and wire evolution with time for a given set of growth parameters. The wire density increases rapidly with time and then saturates. The growth period until the nucleation of new nanowires is terminated can be defined as the nucleation stage. Coalescence of closely spaced nanowires reduces the density for long deposition times. The average size of the well-nucleated NWs shows linear time dependence in the nucleation stage. High-resolution transmission electron microscopy measurements of alternating GaN and AlN layers give valuable information about the length and radial growth rates for GaN and AlN in NWs.     

Epitaxy of noble metals and (111) surface superstructures of silicon and germanium II: Study after annealing

We have studied by LEED, RHEED and REM the growth mode, after annealing, of Ag and Au on the (111) cleavage faces of Si and Ge and the surface superstructures induced by these deposits. It is generally admitted that three-dimensional growth occurs after completion of an ordered adsorption phase, which would account for the superstructure patterns observed. Having defined the domain of existence of each superstructure as a function of coverage ratio and temperature we are led to question the classical models previously proposed.     

Structural phase control in self-catalyzed growth of GaAs nanowires on silicon (111)

Au free GaAs nanowires with zinc blende structure, free of twin planes and with remarkable aspect ratios, have been grown on (111) Si substrates by molecular beam epitaxy. Nanowires with diameters down to 20 nm are obtained using a thin native oxide layer on the Si substrates. We discuss how the structural phase distribution along the wire length is controlled by the effective V/III ratio and temperature at the growth interface and explain how to obtain a pure twin plane free zinc blende structure.     

Epitaxy of noble metals and (111) surface superstructures of silicon and germanium part I: Study at room temperature

We have studied the growth mode and orientation at room temperature of Ag and Au deposits on the (111) cleavage faces of Si and Ge. Two-dimensional epitaxial layers are formed. In addition, we have observed by LEED and RHEED new superstructure patterns which depend strongly on the initial structural state of the substrates.     

Initial epitaxial growth of (111) Au/(111) Cu and (111) Cu/(111) Au

The room temperature modes of growth of Au/(111) Cu and Cu/(111) Au are described. For the former growth mode initial deposits (2.4 Å) of gold on copper form smooth flat islands delineated by coincidence lattice misfit dislocations. For 6.0 Å of gold deposit, both thick and thin gold areas were observed with almost complete substrate coverage. For a 10 Å deposit, surface coverage was complete. Strain measurements and dislocation densities obtained on the (111) Au/(111) Cu films suggest the presence of two separate misfit dislocation networks at the interface. The coincidence lattice networks were large enough for transmission electron microscopy observation but contributed little to total overlayer strain. The (van der Merwe) natural lattice misfit dislocations were too closely spaced for direct observation but their presence was inferred because of the strain measurements. The initial epitaxy of Cu/(111) Au was similar to the Stranski-Krastanov model: the initial monolayer of copper (also delineated by coincidence misfit dislocations) grew smoothly on the gold; additional copper formed essentially stress-free “nuclei” on top of the initial copper layer.     

Nickel diffusion and silicide island formation on silicon (111)

Crystalline nickel disilicide islands have been observed on the Si(111) surface by atomic force microscopy (AFM). The nickel disilicide islands coalesce following a high temperature anneal (≈1260K). The islands differ from those formed at lower temperature in both shape and orientation. To explain the differences, we discuss kinetically limited growth accompanying phase and surface segregation of Ni from the bulk silicon wafer, and condensation of a Ni-rich NiSi_2−x liquid phase at the surface. Condensation from the liquid phase to NiSi₂ is concluded to be responsible for the structure of the crystallites. High temperature growth conditions lead preferentially to A-type (non-twinned) silicide structures.     

Nucleation and growth of aluminium oxide on silicon in the CVD process

Films of aluminium oxide have been formed on single crystal silicon substrates using AlCl₃-CO₂-H₂ gas mixtures in a cold-walled chemical vapour deposition (CVD) reactor. The nucleation and subsequent growth of the deposit have been observed under the varying process parameters. It is found that the nucleation and growth of the Al₂O₃ are dependent on the H₂O flux and H₂O supersaturation. An activation energy of 34.8 Kcal mol^–1 is obtained for the growth rate indicating that the CVD of Al₂O₃ on silicon is a thermally activated process and limited by surface reaction. Scanning electron micrographs (SEM) show that the deposited films are amorphous at low temperature, 850° C, but change to fine grained polycrystalline structure at high temperature, 1000° C.     

Study of the growth of CuAlS2 thin films on oriented silicon (111)

Within the chalcopyrite family the sulphur based compounds CuMS₂ (M = In, Ga, Al) have attracted much interest in recent years because they show a direct wide band-gap covering from E_gap = 1.53 eV (CuInS₂) over E_gap = 2.43 eV (CuGaS₂) to E_gap = 3.49 eV (CuAlS₂). Therefore they are particularly suitable for optoelectronic as well as photovoltaic applications. The CuAlS₂ semiconductor is one of these compounds and has good luminescent properties and a wide direct gap of 3.5 eV making it suitable for the use as material for light-emitting devices in the blue region of the spectrum. To dig up fully its potential a better understanding of the fundamental properties of the CuAlS₂ film itself is essential, which could be achieved from high-quality single-crystalline materials. So, the aim of this work has been to study the growth of multilayer CuAlS₂ thin films on Si(111) substrates at a substrate temperature of 723 K. One, two and three layers with 60, 120 and 180 nm thicknesses, respectively, were deposited on Si(111) substrate. The effect of the CuAlS₂ layer numbers on the structure, morphology and optical properties of the samples was investigated. The X-ray diffraction studies revealed that all the samples are polycrystalline in nature, single CuAlS₂ phase and exhibiting chalcopyrite structure with a preferred orientation along the (112) direction. However, the sample with three CuAlS₂ layers exhibit the highly oriented (112) plane with grain sizes of 80 nm. So we show that this experimental process affects significantly the structural properties of the CuAlS₂ films. Raman spectroscopic measurements indicated five prominent peaks at 193, 205, 325, 335 and 370 cm^− 1. The possible origin of the 370 cm^− 1 peak was investigated and was found to be some local vibration in the structure. The peaks at 193-205 and 335 cm^− 1 were ascribed to A₁ and B₂ modes, respectively.     

Si(111)衬底上生长GaN晶绳的研究

利用热壁化学气相沉积在Si(111)衬底上获得GaN品绳，采用傅里叶红外吸收谱(FTIR)、扫描电子显微镜(SEM)、选区电子衍射(SAED)、X射线衍射(XRD)和光致发光谱(PL)对晶绳进行组成、结构、形貌和光学特性分析。初步结果证明：在Si(111)衬底上获得择优生长的六方纤锌矿结构的GaN晶绳。SEM显示在均匀的薄膜上出现φ6μm的晶绳，FTIR显示GaN薄膜的主要成分为GaN同时含有少量的C污染，由XRD和SAED的综合分析得出晶绳呈六方纤锌矿单晶结构，PL测试表明晶绳呈现不同于GaN薄膜的发光特性。     

Electrochemical deposition of Prussian blue on hydrogen terminated silicon(111)

Electrochemical deposition of Prussian blue (PB) was performed by cyclic voltammetry on hydrogen terminated n-type Si(111) surface. The characterization of the samples based on atomic force microscopy and X-ray diffraction spectroscopy showed a nanocrystal form of the PB films on the silicon surface. The thickness of PB films as a function of the potential cycling number was monitored simultaneously by Raman spectroscopy, proving that the growth of the films is in a good controllable manner.     

Strain accommodation in Ga-assisted GaAs nanowires grown on silicon (111)

We study the mechanism of lattice parameter accommodation and the structure of GaAs nanowires (NWs) grown on Si(111) substrates using the Ga-assisted growth mode in molecular beam epitaxy. These nanowires grow preferentially in the zincblende structure, but contain inclusions of wurtzite at the base. By means of grazing incidence x-ray diffraction and high-resolution transmission electron microscopy of the NW-substrate interface, we show that the lattice mismatch between the NW and the substrate is released immediately after the beginning of NW growth through the inclusion of misfit dislocations, and no pseudomorphic growth is obtained for NW diameters down to 10 nm. NWs with a diameter above 100 nm exhibit a rough interface towards the substrate, preventing complete plastic relaxation. Consequently, these NWs exhibit a residual compressive strain at their bottom. In contrast, NWs with a diameter of 50 nm and below are completely relaxed because the interface is smooth.     

Evidence of silicene in honeycomb structures of silicon on Ag(111)

In the search for evidence of silicene, a two-dimensional honeycomb lattice of silicon, it is important to obtain a complete picture for the evolution of Si structures on Ag(111), which is believed to be the most suitable substrate for growth of silicene so far. In this work we report the finding and evolution of several monolayer superstructures of silicon on Ag(111), depending on the coverage and temperature. Combined with first-principles calculations, the detailed structures of these phases have been illuminated. These structures were found to share common building blocks of silicon rings, and they evolve from a fragment of silicene to a complete monolayer silicene and multilayer silicene. Our results elucidate how silicene forms on Ag(111) surface and provides methods to synthesize high-quality and large-scale silicene.