A new in-situ rapid thermal annealing (RTA) apparatus which can be used to anneal entire wafers in an ultra high vacuum environment has been designed to be used in conjunction with the epitaxial growth of heterostructures. Drastic improvement in the crystallinity of CaF2/Si(100) can be achieved with RTA, and our results suggest that RTA can be used as an on-line processing technique for novel epitaxial structures.

We have explored the possibility of fabricating a metal base transistor in the Si/CoSi2 material system. Utilizing recent advances in the growth of thin, pinhole free, CoSi2 layers on Si(111) we have measured the transistor characteristics of a Si/CoSi2/Si structure. The observed low common emitter current gain is attributed to an absence of current carrying states in the CoSi2 transistor base.

Silicon (001) substrates were implaynted with 350 keV Co at room temperature and 450°C with fluence of from l×l017 Co/cm2 to 6×1017 Co/cm . All samples were annealed at 1000°C in order to form the CoSi2 phase Concentration profiles were determined with Rutherford backscattering spectrometry (RBS), and the associated strain profiles were analyzed with double-crystal X-ray rocking curve measurements. Ion channeling was also used to characterize the silicide formation and crystal quality. An implantation of 6×1017 Co/cm2 at 450°C forms a single-crystal CoSi2 layer while lower fluences do not. A continuous, buried CoSi2 single-crystal layer is formed for the 3×1017 Co/cm2 sample implanted at 450°C and annealed at 1000°C. The continous CoSi2 layer is thicker than the critical layer thickness for a fully coherent film, and therefore the layer partially relaxes. A relaxation of 50% of the fully coherent value is observed for this buried silicide system.

Nucleation and growth of epitaxial CoSi2 on Si by the thermal reaction of vapour deposited Co on (001) and (111) Si have been studied by transmission electron microscopy (TEM). On (001)-Si the layer consists of CoSi2 grains. Apart from an aligned (a)-orientation, CoSi2 occurs in a number of orientations, including a (110) preferential (b)-orientation. On (111) Si, single-crystalline layers are obtained, predominantly in the B-type orientation, which is rotated through 180° relative to the aligned (111)-orientation (A-type). The interfacial defect structure consists of misfit dislocations of edge-type with Burgers vector b=a/6<112>, running in <110> directions. The observations for both (001) and (111) Si are related to geometrical lattice match between CoSi2 and Si. In addition to the experimental results, a computer program has been made which calculates the matching between various orientations of CoSi2 and Si. The nucleation of B-type CoSi2 for (111) Si and the different oriented grains for (001) Si are discussed in terms of a nucleation mechanism at steps at the interface in combination with a relatively large mismatch.

Nickel structures have been deposited on large (300 μm diameter), thin (1400 to 3000Å) single crystal (111) silicon membranes. On annealing, the nickel-silicon reaction generates strain which can be partially accommodated through buckling of the membrane, a mode of strain relief not available on bulk wafers. Examples of such buckling are presented in this paper. Features of the silicide structures observed include thin epitaxial Ni2 Si that grows on clean surfaces during deposition, vertical and lateral growth of NiSi2 into the membrane from nickel dots, and the absence of interfacial dislocations between NiSi2 and silicon, probably due to the stress relief.

We have studied the dependence of the structural and electrical properties of ultrathin cobalt silicide films on the annealing temperature and deposited Co thickness.

We recently reported that coherent, electrically continuous films of CoSi2 could be grown on Si(111) with thicknesses as small as 1 nm.1 The thinnest of these films had a high density of pinholes which covered about 20% of the area of the substrate. The resistivity of the films increased dramatically with decreasing thickness in a manner which was qualitatively consistent with quantum size effects. In order to investigate the dependence of both the pinhole density and the resistivity of ultrathin CoSi2 films on Si(111), we have deposited different thicknesses of Co onto near room temperature (<100°C) Si(111) substrates and have annealed the samples at different temperatures in order to study the effect of silicide formation conditions on the structural and electrical properties of the resulting films.

Co layers were deposited onto Si(111) wafers in a molecular beam epitaxy apparatus as described previously.1 In our earlier work, CoSi2 was formed by heating the sample to ∼600°C for a few seconds after Co deposition. Because higher temperature anneals are known to lead to pinhole formation in thicker films,2 annealing temperatures in the present experiments were constrained to be 600° or less. The reaction time was on the order of 5 minutes. The layers were examined using plan-view transmission electron microscopy (TEM) in order to avoid beam heating and ion beam mixing effects which might cause artefacts in cross-section TEM observation.

The results of our experiments are summarized in Figure 1. The figure indicates the silicide phases found in a film as a function of deposited Co thickness and annealing temperature. The numbers in each square correspond to the residual resistivity, ρo, measured for each film. Perhaps the most striking finding of this study is the abrupt change in the structure of the films between 0.7 and 1.4 nm of deposited Co. For all temperatures investigated (including near room temperature), films formed by depositing less than 1 nm of Co form CoSi2 immediately upon deposition. (For films approaching this thickness, there is also a small amount of Co2Si which persists at all temperatures

The performance of future microelectronic circuits will be strongly enhanced by the monolithic integration of superlattice devices with convenventional integrated circuits ontop of a silicon substrate. With this material concept the mismatch between the superlattice materials and the silicon substrate has to be accommodated. SiGe/Si is a model system for the study of mismatch effects because of similar chemistry and well pronounced strain effects.

Growth of SiGe/Si strained layer superlattices (SLS) by molecular beam epitaxy is reported. Adjustment of strain within the superlattice influences strongly the stability and band ordering of the SLS. SiGe/Si superlattices with strain symmetrization exhibit good thermodynamic stability and band ordering of type II. The concept of strain symmetrization by a thin homogeneous buffer layer is explained. Design rules for a virtual substrate consisting of the actual substrate with the thin buffer layer on it are given.

Strained layer epitaxy is a process for the formation of new materials with a strain and composition modulation in the one to one hundred monolayer range. Two aspects of strained epitaxial growth are discussed in this paper. We first consider the clustering process, a basic limitation in strained layer epitaxy. A second experiment examines the strain in few monolayer epitaxial films of Ge embedded in Si(100). We show that the strain in these monolayer films is comparable to that expected from bulk elastic constants and discuss the new properties of these films.

Systematic trends for the minimum gap and low-lying direct optical transitions for superlattices alternating 2, 4 or 6 atomic layers of Si and Ge grown on Si(001) and Ge(001) substrates are calculated using the local density functional approach. Results of a many-body self energy calculation for the optical transition energies of the Si4Ge4/Si(001) case are compared to photocurrent and electroreflectance data. Emphasis is placed on the narrow type II heterostructure formed by the Si4Ge4 region between the Si(001) substrate and cap layer.

Si1−zGez single crystal layers were grown on Si(iii) by liquid phase epitaxy (LPE) over the entire composition range (0 ≤ x ( 1). Using Sn as solvent all compositions were produced; alloys with compositions 0 ≤ x ≤ 0.75 were obtained using In as solvent; the range 0.75 ≤ x ≤ 1 was covered with Bi solvent. The solvents used, the composition of the solution, the saturation temperature, and the cooling rate, were found to strongly influence the growth morphology.

Surface reconstruction during the molecular beam epitaxy (MBE) growth of GexSi1−x ( x = 0.2 - 1.0 ) film on Si(111) was studied using reflection high energy electron diffraction (RHEED). A series of reconstruction pattern transitions was observed due to the formation of strain layer and its relaxation. The critical thickness obtained using the thickness of the GexSil-x film at the transition of the reconstruction pattern agrees well with the previously reported values. The strain dependence of RHEED patterns for GexSi1−x film was substantiated by Raman scattering.

We report ion scattering and TEM investigations of the first layers of GaAs grown on clean Si(111). Based on observations of pure Ga cluster growth on As terminated Si(111) we propose a model for GaAs growth on Si, where Ga atoms are mobile and tend to cluster, but are immobilized by arriving As. These results establish conditions necessary for uniform film growth of GaAs/Si at the important film/substrate interface.

In this paper we introduce the technique of graded thickness sample deposition to study the heteroepitaxial growth mechanisms of GaAs on Si. We can observe the continuous evolution from the initial clean surface, through nucleation, growth and coalescence of the deposited material.

High-quality GaAs epitaxial layers have been grown by molecular beam epitaxy on nominally (100) oriented silicon substrates annealed in a hydrogen-ambient at 1250 ? C. Growth procedure involves an in-situ thermal annealing step after the initial deposition of a thin GaAs buffer layer. Rutherford backscattering and channeling of 2.1 MeV He+ ions, interference optical and transmission electron microscopy have been used to characterize these layers. Comparative studies indicate that the epitaxial layers grown on hydrogen-ambient annealed substrates have a superior surface morphology and a lower interface disorder than those on chemically cleaned silicon (100) substrates. Microtwins, dislocations and antiphase domain boundaries are the predominant defects observed in these layers. A lower incidence of microtwins were observed in the hydrogen annealed substrates.

In this paper, ion channeling techniques are used to show that epitaxial GaAs layers grown on vicinal Si(001) wafers do not have their [001] axis precisely aligned with that of the Si substrate. Instead, the [001] axis of the GaAs layer is found to be tilted toward the surface normal of the Si substrate. This tilt was found to be ∼0.2° on vicinal Si(001) substrates which have their [001] axis tilted 4° toward the [110] azimuth. It is speculated that this misalignment is reponsible for the residual density of threading dislocations in the GaAs on Si layer. An approach described here, which can be used to avoid strain in the GaAs layer, is to grow a CaF2 buffer layer between the Si substrate and the epitaxial GaAs layer. High quality epitaxial GaAs layers have been obtained on both CaF 2 /Si(001) and CaF 2 /Si(111) substrates. Strain measurements of the epitaxial GaAs on the CaF 2 buffer layers indicate that these layers have strains below our detection limits.

Surface structure and heteronucleation of GaAs, In(Ga)As and Ge onto Si surfaces by Molecular Beam Epitaxy are studied by transmission electron microscopy as functions of substrate orientation and cleaning. We find that Shiraki cleaning of vicinal ( 4 degree misorientation ) (100) Si surfaces can produce significant surface facetting at temperatures ∼ 900°C. Subsequent nucleation of GaAs is then influenced by the facet distribution. Nucleation of InAs onto vicinal (100) Si surfaces exhibits a highly bimodal distribution of nuclei dimensions and anomalously high surface diffusion lengths. Sputter and annealing of Si surfaces produces microroughness of an amplitude a few monolayers high on (211) and vicinal and exact (100) surfaces. Subsequent growth of Si buffer layers increases the surface roughness amplitude slightly on the exact (100) and (211) surfaces and significantly on the vicinal (100) surface. Nucleation densities and surface diffusion lengths of Ge on the Si buffer layers surfaces correlate to the surface roughness amplitude. We also describe techniques for experimental quantification of surface roughness amplitudes and periodicities from electron microscope lattice structure images.

This paper reviews briefly some of the recent progress in chemical beam epitaxy (CBE) for the preparation of GaInAs(P)/InP and GaAs/AlGaAs quantum wells, superlattices, and heterostructure devices. Chemical beam epitaxy can be viewed as a chemical vapor deposition process but with the pressure inside the growth chamber sufficient, ow (< 10-4 torr) so that the transport of the gaseous reactants becomes molecular beam instead of via viscous flow. This not only eliminates the complicated gas phase reactions and the stagnant boundary layer above the substrate through which the reactants have to diffuse, but also allows for quick transitions of material compositions and dopings as those achievable by molecular beam epitaxy (MBE). For the growth of HI-V semiconductors, the group Inl elements are derived by the pyrolysis of organometallics (or inorganometallics such as dopant gases) on the heated substrate surface, while the group V elements are derived by the thermal decomposition of hydrides using a high temperature cracker. For the growth of group IV semiconductors, beams of inorganometallic compounds are used. Thus, both organometallic and inorganometallic compounds can be used as starting sources. There are two other alternatives: the gas source MBE (GSMBE), which uses group III elements evaporated from solid sources as in MBE and thermally decomposed hydrides, and the metalorganic MBE (MOMBE), which uses metalorganics as group III sources and group V elements evaporated from solid sources as in MBE. These other processes will not be reviewed here. Introd

High quality GaAs layers have been grown by chemical beam epitaxy using triethylgallium and arsine. Undoped GaAs epilayer with net acceptor concentration NA - ND = 3}10 14cm-3 has been obtained at a low growth temperature of 500°C. Si dopant diffusion at such low temperature during growth is negligible. Using monolayer doping technique epilayers with Si impurities localized in a 2-dimensional plane were prepared. Capacitance-voltage profiling showed a high sheet electron concentration of lx1013cm-2 and peak widths of 22Å and 18Å at 300K and 77K, respectively, which are the narrowest ever reported. For samples grown or annealed at higher temperatures, significant impurity diffusion was observed.

.We report the growth and magnetic properties of epitaxial Fe films, prepared by MBE on films of GaAs and In0.2Ga0.8As, on GaAs (100) substrates. We find that the magnetic properties of the Fe films, which are in the thickness range 300-900Å have virtually no dependence on the nature of the buffer film between the GaAs substrate and the Fe film. In terms of FMR linewidth, 4πMs and anisotropy, the Fe films are of the highest quality yet reported. Values of exchange constant and surface anisotropies were derived from an analysis of spinwaves observed in the thickest films.

A new method has been developed for the growth of graded band-gap AlxGal-xAs alloys by molecular beam epitaxy which is based upon electron beam evaporation of the Group III elements. The metal evaporation rates are measured real-time and feedback controlled using beam flux sensors. The system is computer controlled which allows precise programming of the Ga and Al evaporation rates. The large dynamic response of the metal sources enables for the first time the synthesis of variable band-gap AlxGal-.xAs with arbitrary composition profiles. This new technique has been demonstrated in the growth of unipolar hot electron transistors, graded base bipolar transistors, and Mshaped barrier superlattices.