Reactive Epitaxy Research Articles

Demonstration of lateral epitaxial growth of AlN on Si (1 1 1) at low temperatures by pulsed reactive sputter epitaxy

We present a pulsed reactive magnetron sputter process for high quality AlN on Si (111) beneficially avoiding any high-temperature growth. Initially, metallic aluminium with a nominal thickness of about one monolayer is deposited at a substrate temperature around 850 °C in an Ar plasma followed by sputtering in an Ar/N plasma. For 250 nm thick AlN layers a surface roughness below 0.2 nm rms is obtained as determined by atomic force microscopy (AFM). Using an Al nucleation step prior to AlN growth substantially improves the crystalline properties of AlN. The FWHM values of the AlN (0 0 0 2) and the AlN (1 0 1 0) diffraction peaks with 0.45° and 0.86°, respectively, are comparable to state-of-the-art AlN on Si layers grown by metalorganic vapour phase epitaxy (MOVPE). Two different N-precursor gases, namely N2 and NH3, lead to distinct layer qualities as revealed by atomic force microscopy and transmission electron microscopy. Only with NH3 substantial lateral growth can be achieved at T = 850 °C which is mandatory to obtain smooth surface morphologies. In MOVPE such lateral AlN growth is typically only achieved at high growth temperatures (T > 1000 °C).

Electroluminescent 1.5-μm light-emitting diodes based on p +-Si/NC β-FeSi2/n-Si structures

The electroluminescence efficiency of silicon light-emitting diode structures with several layers of β-FeSi2 nanocrystallites embedded in the p-n junction is investigated. The nanocrystallites were formed by either solid-phase epitaxy or a combination of reactive and solid-phase epitaxy. For the structures in which the nanocrystallites were formed by the combined method, electroluminescence is observed only at low temperatures (below 70K). This is indicative of a high concentration of defects acting as nonradiative-recombination centers. For the structures with nanocrystallites formed by solid-phase epitaxy, intense electroluminescence is observed up to room temperature. The dependence of the electroluminescence intensity on the size of the nanocrystallites is studied.

Growth mode and atomic structure of MnSi thin films on Si(111)

Thin films of MnSi(111) in B20 structure formed by reactive epitaxy on Si(111) are studied using scanning tunneling microscopy (STM) and density functional theory calculations. Coexisting root 3 x root 3 structures with high or low corrugation are observed and assigned to different Mn coverage by using a detailed analysis of simulated STM images. Comparison with our interpretation of STM images of films previously grown by codeposition of Mn and Si provides us with evidence that the stacking sequence of Mn and Si lattice planes depends on the growth protocol.

Formation of Heusler alloy Co2FeSi thin films on the surface of single-crystal silicon

The initial stages of Heusler alloy (Co2FeSi) thin film growth by reactive epitaxy on the Si(100)2 × 1 surface are studied, and formation conditions for this alloy are found. At a substrate temperature of lower than, or equal to, 180°C, an island film of ternary Co-Fe-Si film grows on the surface. The silicon content in this film is lower than in the compound to be synthesized. The film becomes continuous when its thickness exceeds 1.2 nm. It is shown that post-growth annealing at 240°C can raise the silicon content in the film and be conducive to obtaining Heusler alloy of a desired composition. In situ measurements of the films show that ferromagnetic ordering in them has a threshold and shows up at the coalescence growth stage of the Co-Fe-Si island alloy.

Self-organized growth of higher manganese silicide nanowires on Si(1 1 1), (1 1 0) and (0 0 1) surfaces

Higher manganese silicide nanowires (NWs) with a large aspect ratio have been grown on three kinds of substrate, Si(111), (110) and (001), with the reactive epitaxy method at temperatures above ∼500°C. The self-assembly of the NWs is confined along the Si〈110〉 directions, resulting in the NWs orienting with the long axis along one, two and three particular directions on the Si(110), (001) and (111) surfaces, respectively. Scanning tunneling spectroscopy and transmission electron microscopy (TEM) studies indicate unambiguously that the NWs are tetragonal MnSi1.7. The width of the NWs has a maximum or well-defined value on each kind of substrate, though the length varies greatly. In the plan-view TEM micrographs, the NWs exhibit two sets of Moiré fringes: one is along the length direction and has a large periodicity, and the other is along the width direction and has a very small periodicity. These phenomena strongly indicate that the growth of NWs is governed by a strain-driven energetic mechanism and the NW shape results from anisotropic lattice mismatch between the silicide and the Si substrate. This research paves the way towards controllable growth of the silicide NWs on silicon substrates, which may help to advance the realization of practical applications of silicide NWs in nanoelectronics.

Electrical signatures of ferromagnetism in epitaxial FeSi2 nanowires

We perform electrical characterization of epitaxial FeSi2 nanowires (NWs) realized by reactive epitaxy. Typical resistance values exceed 100 kΩ, a high value that is attributed to the combined influence of interfacial scattering and process-related damage. Negative magnetoresistance due to weak localization, as well as hysteresis and anisotropic magnetoresistance, are also observed at low temperatures, confirming the ferromagnetic nature of these NWs.

10.1007/s11451-008-2023-y

Growth and the optical properties of epitaxial heterostructures Si(111)/(CrSi2 nanocrystallites)/Si(111) based on nanosized islands of chromium disilicide (CrSi2) on Si(111) were studied using low-energy electron diffraction, atomic-force microscopy, and optical reflection and transmission spectroscopy. The heterostructures with thicknesses of 0.1, 0.3, 0.6, 1.0, and 1.5 nm were formed by reactive epitaxy at a temperature of 500°C followed by the epitaxial growth of silicon at 750°C. The specific features of changes in the density and sizes of CrSi2 islands on the silicon surface were determined at T = 750°C as the chromium layer thickness was increased. It was established that, in the heterostructures with chromium layer thicknesses exceeding 0.6 nm, a small part of faceted Cr2Si2 nanocrystallites (NCs) emerge into near-surface region of the silicon, which is confirmed by the data from optical reflectance spectroscopy and an analysis of the spectral dependence of the absorption coefficient. A critical size of NCs is shown to exist above which their shift to the silicon surface is hampered. The decreased density of emerging NCs at chromium layer thicknesses of 1.0–1.5 nm is associated with the formation of coarser NCs within a silicon layer, which is confirmed by the data from differential reflection spectroscopy.

Formation of manganese silicide nanowires on Si(111) surfaces by the reactive epitaxymethod

Manganese silicide nanowires (NWs) with a largelength/width ratio have been predominantly formed on Si(111)-7 × 7 surfaces with the reactive epitaxy method by a delicate control of growth parameters. Thesupply of free Si atoms per unit time plays a crucial role in the formation of the NWs. High growthtemperature and low Mn deposition rate are favorable for the growth of long NWs with a largelength/width ratio and the formation of islands with other shapes can be greatly restrained under theseconditions. The formation of NWs is driven by the minimization of the strain energy caused bythe lattice mismatch between the silicide and substrate. Scanning tunneling spectroscopymeasurements show that the NWs exhibit a semiconducting character with a band gap of∼0.8 eV, which is consistentwith that of bulk MnSi1.7.

Tuning reactive epitaxy of silicides with surface steps: Silicide quantum dot arrays on Si(111)

We have studied the nucleation and growth of cobalt silicides on vicinal silicon surfaces. The latter exhibit one-dimensional arrays of steps with narrow terrace widths ranging from 20 Å to 40 Å (miscut angle between 9 ∘ and 4.6 ∘, respectively). Submonolayer amounts of Co were deposited by thermal evaporation on such vicinal Si(111) surfaces by reactive thermal deposition, and the resulting silicide structures were analysed by scanning tunnelling microscopy. The presence of narrow terraces in the system notably alters the usual nucleation process of cobalt silicide on flat Si(111). It is observed that by varying the Co coverage and vicinal angle, the size and amount of the hexagonal silicide clusters can be controlled producing a dense array of quantum dots. This behaviour contrasts to that observed on flat substrates, where nanocluster structures are notably scarce.

Effect of the chromium layer thickness on the morphology and optical properties of heterostructures Si(111)/(CrSi2 nanocrystallites)/Si(111)

Growth and the optical properties of epitaxial heterostructures Si(111)/(CrSi2 nanocrystallites)/Si(111) based on nanosized islands of chromium disilicide (CrSi2) on Si(111) were studied using low-energy electron diffraction, atomic-force microscopy, and optical reflection and transmission spectroscopy. The heterostructures with thicknesses of 0.1, 0.3, 0.6, 1.0, and 1.5 nm were formed by reactive epitaxy at a temperature of 500°C followed by the epitaxial growth of silicon at 750°C. The specific features of changes in the density and sizes of CrSi2 islands on the silicon surface were determined at T = 750°C as the chromium layer thickness was increased. It was established that, in the heterostructures with chromium layer thicknesses exceeding 0.6 nm, a small part of faceted Cr2Si2 nanocrystallites (NCs) emerge into near-surface region of the silicon, which is confirmed by the data from optical reflectance spectroscopy and an analysis of the spectral dependence of the absorption coefficient. A critical size of NCs is shown to exist above which their shift to the silicon surface is hampered. The decreased density of emerging NCs at chromium layer thicknesses of 1.0–1.5 nm is associated with the formation of coarser NCs within a silicon layer, which is confirmed by the data from differential reflection spectroscopy.

Self-assembly of silicide quantum dot arrays on stepped silicon surfaces by reactive epitaxy

Reactive epitaxy of Co on vicinal Si(111) surfaces is found to be a flexible and a convenient method for the preparation of dense arrays of Co silicide quantum dots. In the present work, submonolayer amounts of Co were deposited at 800K on vicinal and flat Si surfaces, analyzing the resulting structures by scanning tunneling microscopy. On vicinal Si(111) surfaces with relatively narrow (∼40–100Å) terrace width, such reactive epitaxy leads to self-assembled arrays of CoSi2 quantum dots with a sharp size distribution function. In contrast, the growth of Co on flat Si(111) results in an inhomogeneous array of dots mixed with a variety of silicide phases.

Obtaining films of CoSi2/Si (100) and the analysis of their morphology and stoichiometry through molecular-beam, solid-phase and reactive epitaxy methods

The results of studying the processes of thin CoSi2 film formation on Si (100) surfaces by MBE, SPE, and RE methods are presented. The regularities of the initial stage of CoSi2 film growth are studied. Stoichiometry of CoSi2 film is analyzed by the Auger electron spectroscopy method. The results of investigation show strong dependence of the morphological and electrophysical properties of CoSi2/Si (100) structures on the growth conditions. The measurements of the surface resistance show that the resistance of the CoSi2 film grown at T > 600°C changes under different growth conditions.

Growth process and structure of Fe/Si(1 1 1) ultrathin film: Transition from single-domain Fe(1 1 1)/Si(1 1 1) to β-FeSi 2

The growth processes and structures of Fe/Si(1 1 1) ultrathin films grown by solid-phase reactive epitaxy were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS). It has been revealed that the Fe(1 1 1) thin films with a bcc-type structure were epitaxially grown on a Si(1 1 1) crystal, even at room temperature, and formed a single-domain structure: Fe(1 1 1) 〈 1 ¯ 1 ¯ 2 〉 ∥Si(1 1 1) 〈 1 1 2 ¯ 〉 . After annealing at above 600 °C, the Fe(1 1 1) films were transformed into β-FeSi 2 via the collapse of the bcc-type structure to an amorphous or polycrystalline structure. On the basis of the thickness dependences of the growth processes, this phenomenon was discussed in terms of the diffusion of Si into Fe thin films.

Influence of domain boundaries on polarity of GaN grown on sapphire

GaN films were grown on sapphire substrates by laser-induced reactive epitaxy. The domains in the films were determined to be the Ga-polarity by the convergent beam electron diffraction (CBED) technique, while the adjacent matrices had the N-polarity. The domain boundaries were characterized as inversion domain boundaries (IDBs). An atomic structure of the IDB is proposed based on high-resolution transmission electron microscopy (HRTEM) investigations. Control of the polarity of GaN/sapphire films was achieved by suppressing the formation of IDBs with an interlayer of AlGaN and a low-temperature GaN buffer layer.

Size- and shape-controlled GaN nanocrystals grown on Si(111) substrate by reactive epitaxy

We report the investigation of coherent GaN nanocrystals spontaneously formed by nitrogen-plasma-assisted reactive epitaxy with Ga droplets supported on the single crystalline Si3N4(0001)∕Si(111) surface. The distribution of grown GaN nanocrystals, as revealed by scanning electron microscopy and cross-sectional transmission electron microscopy, is very uniform in size (∼16nm) and shape and the distribution width is significantly narrower than that of Ga droplets deposited in the Volmer–Weber mode. By using high-resolution electron microscopy, the shape and crystalline structure of the self-assembled GaN nanocrystals can be determined to be truncated triangular pyramids formed by the facets of the GaN wurtzite lattice.

Growth and microstructural characterizations of GaN films grown by laser induced reactive epitaxy

Hexagonal GaN thin films have been grown by laser induced reactive epitaxy (LIRE) and characterized by various techniques. The films were deposited on sapphire (0001) and SiC (0001) without and with a buffer layer. Dislocations with predominant edge type and inversion domains were observed in the films. Dislocation density measured by x-ray diffraction is in fair agreement with that measured by transmission electron microscopy. Studies on the polarity of films indicate that Ga polarity was obtained for the films grown on SiC, while the films grown directly on sapphire were of N polarity. The atomic structure with a displacement of c/8 across the inversion domain boundary was deduced from the fringe contrast analyses and high resolution transmission electron microscopy studies. For the films grown on sapphire, the Ga polarity was achieved by using an AlGaN prelayer coupled with the introduction of low-temperature GaN buffer layer, which led to a clear improvement of the film quality. The typical cathodoluminescence spectra of such GaN films are comparable to those reported in the literature. The present work provides insight into the crystal growth and microstructure of GaN films and indicates that LIRE is a promising method to grow high quality GaN films.

Reactive epitaxy of cobalt disilicide on Si(100)

The growth of cobalt disilicide on the Si(100) surface by reactive epitaxy at T=350°C was studied within the 10–40 ML cobalt coverage range. A new method of mapping the atomic structure of the surface layer by inelastically scattered medium-energy electrons was employed. The films thus formed were shown to consist of CoSi2(221) grains of four azimuthal orientations turned by 90° with respect to one another. This domain structure originates from substrate surface faceting by (111) planes, a process occurring during silicide formation. B-oriented CoSi2(111) layers grow epitaxially on (111) facets.

Investigation on growth modes of GaN layers grown by LIRE

GaN layers are usually grown by metalorganic chemical vapour deposition (MOCVD), MBE and related processes. In this work we compare the growth of GaN layers by laser induced reactive epitaxy (LIRE) with the growth by conventional methods with special interest in growth modes of the different processes. The influence of deposition parameters as temperature and Ga/N-ratio, for LIRE represented in the energy of the laser beam, is discussed. MOCVD produces smooth films with growth steps on the surface, indication of one-dimensional growth. MBE films usually exhibit island-like growth, changing with Ga/N ratio from a meander structure over a closed surface with pits to a surface with etching-features due to desorption. The best luminescence intensities are achieved with surfaces of meander structure and with surfaces showing slight etching features. LIRE layers are found to be similar to MBE layers in surface structure and in the influence of the Ga/N-ratio.

Polarity Selection in GaN Films Grown by Laser Induced Reactive Epitaxy

Gallium nitride (GaN) films have been grown on sapphire(0001) and 6H-SiC(0001) substrates without a buffer layer. Micro-structural analysis was performed by TEM investigations. Under identical experimental conditions, the GaN films showed different polar characteristics when deposited onto different substrates. Various techniques, such as convergent beam electron diffraction and chemical etching were employed to determine the polarity of the GaN films. The present paper implies that films grown by laser induced reactive epitaxy (LIRE) show the same dependence of polarity on substrates and growth conditions as layers grown by molecular beam epitaxy.

Laser-induced reactive epitaxy of binary and ternary group III nitride heterostructures

Group III nitride films are deposited on sapphire (0 0 0 1) substrates using the laser-induced reactive epitaxy (LIRE) method. Liquid group III metal targets are ablated by a focused nanosecond Nd:YAG laser beam in a nitrogen atmosphere of 0.1 mbar. In the plasma plume, the molecular nitrogen is cracked to atomic nitrogen. The cracking process is investigated by optical plasma spectroscopy. By varying the laser pulse energy, the III/V ratio can be controlled. The investigated heterostructures consist of a AlGaN buffer layer, a low temperature (LT) GaN layer and finally a GaN epilayer. By optimising the III/V ratio and the growth temperature of both LT-GaN buffer and epilayer, the luminescent and electronic properties are strongly enhanced. As best results, we achieved residual carrier concentrations of 2×10 17 cm −3 and mobilities of 70 cm 2/Vs. The FWHM value for the near band-edge luminescence was 76 meV and for the rocking curve at the (0 0 0 2) reflex it was 720 arcsec. InN and InGaN films with In contents up to 13% are grown using molten In and InGa alloy as target materials. The InN films commonly have high electron mobilities. We found values up to 350 cm 2/Vs. For the Mg-doping of GaN, a liquid GaMg alloy is used. The results show that we are able to incorporate the required amount of Mg into the nitride.