Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH₃) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.     

Geometric modeling of homoepitaxial CVD diamond growth: I. The {1 0 0}{1 1 1}{1 1 0}{1 1 3} system

Plasma-assisted CVD homoepitaxial diamond growth is a process that must satisfy many stringent requirements to meet industrial applications, particularly in high-power electronics. Purity control and crystalline quality of the obtained samples are of paramount importance and their optimization is a subject of active research. In the process of such studies, we have obtained high purity CVD diamond monocrystals with unusual morphologies, namely with apparent {1 1 3} stable faces. This phenomenon has led us to examine the process of CVD diamond growth and build up a 3D geometrical model, presented here, describing the film growth as a function of time. The model has been able to successfully describe the morphology of our obtained crystals and can be used as a predictive tool to predetermine the shape and size of a diamond crystal grown in a given process configuration. This renders accessible control of desirable properties such as largest usable diamond surface area and/or film thickness, before the cutting and polishing manufacture steps take place. The two latter steps are more sensitive to the geometry of the growth sectors, which will be addressed in a companion paper.Our model, applicable to the growth of any cubic lattice material, establishes a complete mapping of the final morphology state of growing diamond, as a function of the growth rates of the crystalline planes considered, namely {1 0 0}, {1 1 1}, {1 1 0}, and {1 1 3} planes, all of which have been observed experimentally in diamond films. The model makes no claim as to the stability of the obtained faces, such as the occurrence of non-epitaxial crystallites or twinning. It is also possible to deduce transient behavior of the crystal morphology as growth time is increased. The model conclusions are presented in the form of a series of diagrams, which trace the existence (and dominance) boundaries of each face type, in presence of the others, and where each boundary crossing represent a topology change in terms of number of faces, edges and vertices. We validate the model by matching it against crystals published in the literature and illustrate its predictive value by suggesting ways to increase usable surface area of the diamond film.     

Beryllium doping and silicon amphotericity in (1 1 0) GaAs-based heterostructures: structural and optical properties

The use of beryllium as an acceptor at high doping levels in (1 1 0)GaAs-based heterostructures is found to be deleterious to the structural and optical properties of these epi-layers. This may limit the use of beryllium as a p-type dopant on the (1 1 0) surface. Because silicon is amphoteric on the (1 1 0), it can be used as an alternative p-type dopant, in addition to its traditional role as an n-type dopant. Transmission electron microscopy, optical absorption, and luminescence data indicate that high quality multiple quantum well structures with p-type GaAs buffer layers doped with silicon, rather than beryllium, can be grown.     

Heteroepitaxy of InSb films grown on a Si(0 0 1) substrate with AlSb buffer layer

The growth of InSb films on a Si(0 0 1) substrate with AlSb buffer layer was performed in an ultra high vacuum chamber (UHV) by a co-evaporation of elemental Indium (In) and antimony (Sb) sources. The samples were characterized by Auger electron spectroscopy (AES), X-ray diffraction (XRD) and atomic force microscopy (AFM). The surface morphology and the crystal quality of the grown films strongly depend on the flux ratio of Sb/In. It is found that the optimized flux ratio for the one-step growth procedure is about 2.9 to obtain the InSb films with smooth surface and good crystal quality, for the growth temperature of 300 °C. The two-step growth procedure was also used to further improve the crystal quality of the films.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Growth and characterization of GaSb/AlGaSb multi-quantum well structures on Si (0 0 1) substrates

GaSb/AlGaSb multi-quantum well (MQW) structures with an AlSb initiation layer and a relatively thick GaSb buffer layer grown on Si (0 0 1) substrates were prepared by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated definite MQW structures. The photoluminescence (PL) emission around 1.55 μm wavelength was observed for 10.34 nm GaSb/30 nm Al_0.6Ga_0.4Sb MQW structure at room temperature. Dependence of PL emission energy on GaSb well width was well explained by finite square well potential model.     

Growth of Pb on Si(3 3 5)–Au surface

The growth, crystallographic structure and electronic properties of ultrathin Pb films grown on a vicinal silicon surface are investigated with reflection high energy electron diffraction (RHEED) and specific resistivity measurement techniques. A Si(3 3 5) surface with a perfect distribution of monoatomic steps separated with (1 1 1) terraces induced by a submonolayer amount of Au is used as a substrate. In the early stage, Pb growth is anisotropic. Apparently, the presence of steps forces the growth of short crystalline Pb chains along the steps. The layer is amorphous in the perpendicular direction. With the increasing thickness, a phase transition takes place between 3 and 4 monolayers (ML) that makes crystalline order also across the terraces. A further increase in thickness causes the layer surface to repeat the substrate morphology. It consists of regularly distributed monoatomic steps and narrow (1 1 1) terraces.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Effects of vicinal angles from (0 0 1) surface on the Boron-doping features of high-quality homoepitaxial diamond films grown by the high-power microwave plasma chemical-vapor-deposition method

Vicinal surface effects on homoepitaxial growth and boron-doping processes have been studied in case of single-crystalline diamond (0 0 1) surfaces grown using the high-power microwave plasma chemical-vapor-deposition (MWPCVD) method. The off-angles inclined from the on-orientation (0 0 1) surfaces ranged to 5° along the [1 1 0] or [1 0 0] direction, while the concentration of doping B(CH₃)₃ gas was kept constant with a B/C ratio of 50 ppm. Although a number of square-like growth hillocks often appeared, depending substantially on the crystalline quality of the high-pressure/high-temperature-synthesized (HPHT) Ib diamond substrates employed, the number and shape of the hillocks changed significantly with the increasing off-angle. For the vicinal surfaces with off-angles of ≈3° inclined along the [1 1 0] direction, macroscopically flat surfaces were obtained, compared with the other off-angle cases examined. Furthermore, the growth rate and acceptor density of substitutional boron atoms in the homoepitaxial layers were found to substantially increase with the increasing off-angle. These indicate that the step density can play important roles not only in the homoepitaxial growth but also in the boron-incorporation process during the high-power MWPCVD growth.     

The growth of epitaxial aluminium on As containing compound semiconductors

The growth of epitaxial aluminium on different (1 0 0) oriented compound semiconductors grown using the molecular beam epitaxy technique have been studied. After deposition of the first complete adlayer between the aluminium and the GaAs surface as evidenced by in situ reflection high electron energy diffraction (RHEED), ex situ atomic force microscopy (AFM) images agree that subsequent aluminium deposition is via a 3D nucleated growth mode. RHEED observations during continued deposition of epitaxial aluminium indicate a 2D growth mode dominated by the (1 0 0) orientation. AFM images of the surface of the aluminium reveal that the surface morphology consists of a plateau–valley structure, while transmission electron microscopy characterisation reveals that the aluminium is a (1 0 0) oriented single crystal. For growth of epitaxial aluminium on different (1 0 0) compound semiconductors the resultant hillock–valley morphology of the aluminium is remarkably similar regardless of the underlying semiconductor. There is no apparent difference between the aluminium growth on GaAs and Al_0.6Ga_0.4As indicating that the aluminium content of the semiconductor is having no effect on the growth of the aluminium, whereas there can be a difference in the hillock widths for aluminium grown on In_0.53Al_0.47As and In_0.55Ga_0.45As. The dominant orientation that the aluminium recrystallises to, appears to be determined by the strain between the aluminium 3D nucleates and the underlying semiconductor with (1 0 0) oriented aluminium for tensile strain (growth on GaAs and AlGaAs) and (1 1 0) oriented aluminium for compressive strain (growth on InAlAs and InGaAs).     

Microstructure of interfacial HgTe/CdTe superlattice layers for growth of HgCdTe on CdZnTe (2 1 1)B substrates

Transmission electron microscopy has been used to characterize the microstructure of HgTe/CdTe superlattices (SLs) grown by molecular beam epitaxy on CdZnTe(2 1 1) B substrates. The purpose of these intermediate layers was to improve the quality of subsequent HgCdTe (MCT) epilayers intended for infrared detectors. The observations confirmed that the SLs smoothed out the surface roughness of the substrate, and showed that threading dislocations were prevented from reaching the MCT epilayers. High-quality growth of MCT on CdZnTe using the HgTe/CdTe interfacial layers has been demonstrated.     

Growth of highly oriented crystalline α-Fe/AlN/Fe3N trilayer structures on Si(1 1 1) substrates by molecular beam epitaxy

We have realized highly oriented nitride-based α-Fe/AlN/Fe₃N ferromagnetic hybrid structures on Si(1 1 1) substrates by molecular beam epitaxy using AlN/SiC intermediate layers. A two-step hysteresis loop, typically observed in magnetic tunneling junctions, was clearly observed in magnetization versus magnetic field measurements. This result indicates the formation of ferromagnetic α-Fe and Fe₃N layers separated by 8-nm-thick AlN layers over approximately 1 cm² area, and also shows the difference in coercive field between the two ferromagnetic layers.     

Transmission electron microscopy investigation of AlN growth on Si(111)

AlN layers with a thickness of 250 nm were grown by plasma-assisted gas source molecular-beam epitaxy on Si(111) at substrate temperatures between 600 °C and 900 °C. The surface morphology and microstructure of the AlN layers were analyzed by scanning and transmission electron microscopy. Different defect types are observed in the AlN layers and at the AlN/Si(111) interfaces as a function of the temperature: inclusions of pure Al in the Si-substrate, crystallites of the cubic AlN phase, dislocations, stacking faults and inversion domain boundaries. The formation and concentration of the defects depends strongly on the substrate temperature during the growth. X-ray diffraction rocking curves for the (0002) reflection yield minimum full width at half maximum values for the sample grown at the 900 °C under Al-rich conditions indicating optimum structural quality. However, the discussion of the entity of defects will show that a more differentiated view is required to assess the overall quality of the AlN layers.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

Growth mechanisms of GaAs nanowires by gas source molecular beam epitaxy

GaAs nanowires were grown on GaAs (1 1 1)B substrates in a gas source molecular beam epitaxy system, using self-assembled Au particles with diameters between 20 and 800 nm as catalytic agents. The growth kinetics of the wires was investigated for substrate temperatures between 500 and 600 °C, and V/III flux ratios of 1.5 and 2.3. The broad distribution of Au particles enabled the first observation of two distinct growth regimes related to the size of the catalyst. The origins of this transition are discussed in terms of the various mass transport mechanisms that drive the wire growth. Diffusion of the growth species on the 2-D surface and up the wire sidewalls dominates for catalyst diameters smaller than ∼130 nm on average, while direct impingement on the catalyst followed by bulk diffusion through the Au particle appears to sustain the wire growth for larger catalyst diameters. A change in wire sidewall facets, indicating a probable transition in the crystal structure, is found to be primarily dependent on the V/III flux ratio.     

Formation of microtwins in TmP/GaAs heterostructures

Microtwins in semi-metal thulium phosphide (TmP) epilayers grown on (0 0 1) GaAs substrates by molecular beam epitaxy have been studied by transmission electron microscopy. Selected area diffraction patterns show extra spots along 〈1 1 1〉 and 〈2 2 4〉 corresponding to three times the normal rock-salt periodicity. Only one or two twin variants are found in the crystal. The occurrence of the observed microtwins in the TmP-GaAs heterostructure can be accounted for by the growth-accident mechanism, i.e., the formation of microtwins is via growth accidents in the stacking sequence on {1 1 1} and {1 1 2} planes. The growth accidents appear to occur due to rapid growth rates and/or contamination.     

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1? 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1? 1? 2 0] || Si[1 1? 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.     

Low-temperature atomic assembly of stoichiometric gallium arsenide from equiatomic vapor

The low-temperature atomic assembly of homoepitaxial GaAs thin films on the (0 0 1) surface has been investigated using molecular dynamics with a Stillinger–Weber potential energy function. During equiatomic vapor deposition, crystalline growth was observed for substrate temperatures above 35% of the melting temperature. Below this temperature, the critical epitaxial thickness began to rapidly decrease as defects were increasingly incorporated and eventually nucleated an entirely amorphous structure. The atomic assembly mechanisms of arsenic dimer incorporation, as well as gallium vacancy formation, were studied just above the amorphous/crystalline growth transition temperature. The adsorption of arsenic dimers was found to show dependence upon the orientation of the deposited molecule. Atomic processes responsible for the formation of the gallium vacancy defects were observed, and the influence of growth temperature on defect formation was also identified.