Pulsed Laser-Induced Epitaxy / Gas Immersion Laser Doping is used to create borondoped heteroepitaxial p+/N Si1−xGex/Si layers and diodes. Borontriflouride is used as the gaseous dopant source. The mechanisms of heteroepitaxial layer growth of Si1−xGex using PLIE are described and impurity incorporation from the gas phase into the molten layer is investigated. Compared to other heteroepitaxial techniques, very different process parameters determine the growth. The energy fluence of the pulsed laser beam determines the melt depth and thus the layer thickness; Si and Ge intermix in the liquid phase, the diffusion of B dopant also depends on its diffusivity in the liquid phase. Boron incorporation is investigated as a function of laser energy fluence and number of laser pulses using SIMS and Hall-effect measurements. The dose of incorporated dopant is on the order of 1013cm−2 per pulse. The obtained boron profiles are flat except for a pile-up at the interface which is due to segregation. Boron and Germanium distribution are compared to turn-on voltage shifts obtained from p+/N Si1-xGex/Si heterojunction diodes fabricated with the technique. A two-step Laser process to independently control metallurgical and electrical junction depth of the diodes has been implemented. The selective nature of the epitaxial process is emphasized.

The study of the critical thickness/strain phenomenon inherent in metastable, layered heterostructures has led to the development of several models which describe elastic strain relaxation. Hitherto, the nucleation of misfit dislocations required for coherency breakdown is the least well understood aspect of strain relaxation, due to the paucity of experimental data. Moreover, existing theoretical calculations predict relatively large activation energy barriers (>10 eV) for misfit dislocation nucleation in relatively low misfit (<2%) systems. In this work it will be shown that the nucleation of misfit dislocations can occur spontaneously demonstrating a vanishingly small activation energy barrier. Specifically, experimental studies of a wide range of GexSi1−x/Si (x< 0.5) hetero-structures, grown by MBE and CVD techniques, have provided quantitative data from bulk specimens on the observed misfit dislocation nucleation rate and activation energy using large-area diagnostic techniques (eg. chemical etching/Nomarski microscopy). In parallel, the strained layer microstructure was studied in detail using crosssectional and plan-view electron microscopy in order to identify a new dislocation nucleation mechanism, the ‘double half-loop’ source. From the combined macroscopic and microscopic analyses, a theoretical treatment has been developed based on nucleation stress and energy criteria which predicts a “barrierless” nucleation process exists even at low misfits (< 1%). Accordingly, the observed misfit dislocation nucleation event has been found both experimentally and theoretically to be rate-controlled solely by Peierls barrier dependent, glide-activated processes with activation energies of ∼2 eV.

We describe a detailed quantitative theoretical and experimental analysis of strain relief mechanisms in GexSi1−x/Si(110). For this interface, both partial and total glide dislocations may effect strain relief. Detailed comparison between experimental measurement and theoretical prediction of the regimes in which the two types of dislocations are observed allows a very accurate determination of the stacking fault energy in GexSi1−x alloys.

The growth mode and the relaxation of MBE grown InxGa1-xiAs layers (0.13 ≦ x ≦ 1.0, nominal film thickness 5 nm) on GaAs(100) substrates with a lattice-parameter mismatch were investigated by transmission electron microscopy (TEM). The transition between two- and three-dimensional growth occurs at x ≈ 0.4. The three-dimensional growth mode for x ≥ 0.6 results in a wide spectrum of island sizes. In contrast to the two-dimensional growth mode, the strain state of a three-dimensionally growing layer is completely inhomogeneous because the relaxation of the strain is correlated with the island size. The reduction of elastic strain for islands is reasonably described by an energy balance model.

Narrow-gap PbSe and PbSe1−xTex films were grown by MBE on 3n-Si(111) wafers. The layers are used for IR-sensor array applications. Epitaxy was achieved by using an intermediate CaF2/BaF2 bilayer to overcome the large difference in lattice constants (14%) and thermal expansion coefficients (700%). While PbSe is rather soft, the hardness increases with x (x<0.5) in PbSe1−xTex. Thermal stresses are relieved by dislocation glide on (100)-planes, leading to slip lines on the surface. The surface morphology and step heights were studied with the scanning tunneling microscopy technique, and a model for the resulting (111)-surface structure is presented. The value of the remaining strain was determined with precision Rutherford backscattering spectrometry absolute angle measurements, X-ray diffraction and photoluminescence in the temperature range down to 10 K. It is found that no appreciable strain builds up even at cryogenic temperatures and after many temperature cycles. In contrast to PbSe, ternary PbSe1−xTex layers are strained elastically below room temperature owing to solid solution strengthening, and plastic deformation occurs only at higher temperatures.

Partial dislocations and their associated stacking faults are identified as the primary defects responsible for the initial relaxation of tensile-strained layers of fcc structure. The critical thickness for the formation of 90° partial dislocations at the strained interface is almost a factor of two smaller than that predicted for the formation of 60° perfect dislocation. Microstructures revealed by transmission electron microscopy from strained layers of various lattice mismatches and thicknesses agree with the prediction of the standard free-energy minimization model.

Reflection high energy electron diffraction (RHEED) has been used to detect strain relaxation in SiGe during growth on <001>- oriented Si for various layer compositions and substrate temperatures. The RHEED-technique permits the dynamic monitoring of the in-plane lattice constant of the growing layer by measuring the distance between diffraction features. The actual RHEED pattern is recorded by a CCD camera and subsequently processed in real time by a computer. This way, the layer relaxation can be followed conveniently; a detection limit for a variation in the lattice constant of Δa/a=5.10−4 has been obtained.

Molecular beam epitaxial (MBE) growth condition of SrF2 directly on Si(111) substrates has been optimized in terms of both X min and the surface morphology. Lattice distortion measurements were carried out with ion channeling along off-normal channeling directions in the strained layers grown at the optimal condition. The relationship of residual strain vs. film thickness for SrF2 on Si(111) was provided by the first time. The experimental data demonstrated a special thickness in this relation, at which the derivative of strain vs. film thickness changes its sign. This unique behavior was understood as the result of competition between the large lattice mismatch and the large thermal mismatch between SrF2 and Si.

In a recent paper[l] we presented the notion of a “critical composition” of InxGa1−xAs (x=0.18) which when exceeded results in threading dislocation evolution in InxGa1−xAs (x ≥ 0.18) material grown on GaAs. For InxGa1−xAs compositions below x=0.18, threading dislocation evolution occurs in the GaAs substrate material but not in the InxGa1−xAs epitaxial layer material, this phenomenon being attributed to a yield strength mismatch in favor of InxGa1−xAs at the epilayer growth temperature.

In this paper we describe recent results concerning an extension of this work to the study of strain relief in multiple layer systems. It was found that large x (x-0.5) InxGa1−xAs/GaAs layers can be grown having extremely low dislocation densities (<104/cm2 ) by step increasing the InxGa1−xAs composition at a series of InxGa1−xAs/InyGa1-yAs heterointerfaces such that a “critical composition difference” is not exceeded at each heterointerface. High resolution x-ray diffraction analysis was used to determine the extent of relaxation in each layer of these multilayer systems. As evidenced by cross-sectional TEM analysis, dislocations propagate only in the underlying material at each heterointerface for suitably selected compositional differences which results in the top InxGa1−xAs layer in such multiple layer schemes having a low dislocation density. It is to be noted that conventional methods for blocking dislocations such as strained layer superlattices and compositionally graded layers were not employed in our multilayer systems.

The density of misfit dislocations in GexSi1−x films has been measured as a function of deposition temperature and r-f plasma power in Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD). The misfit dislocation density decreases as the deposition temperature is lowered from 450°C to 410°C. As the plasma power is increased from 6.6 to 16W, the dislocation density peaks at lOW and then decreases with increasing power.

Fundamental bandgaps and Schottky barrier heights of strain-relaxed quaternary InxAlyGa1−x-yAs alloys with 0 < x < 0.35 and 0 < y < 0.30 were studied. The alloys were grown on GaAs substrates by molecular beam epitaxy. The lattice mismatch (up to 2.5%) and mismatch strain were accommodated by a compositionally-step-graded buffer. A residual compressive strain of less than 0.5% was determined by x-ray diffraction. Measured Schottky barrier heights v.s. bandgap deviate from the values predicted by the “commonanion” rule. This behavior is attributed to the compositional inhomogeneities and chemical reactivity of the air-exposed InAlGaAs surfaces.

Epilayer tilt about an in-plane axis has been measured for a number of InGaAs/GaAs interfaces grown by solid source molecular beam epitaxy on (001) substrates cut nominally flat or cut 2 degrees towards the <010> or <111> direction. It is found that a much larger tilt develops in epilayers grown on off oriented wafers although a comparable degree of strain was relaxed. The tilt is a function of epilayer thickness for thicknesses exceeding a second critical thickness which we associate with the onset of relaxation by predominantly dislocation multiplication processes.

The misfit dislocation structure of vicinal InP-on-GaAs heterostructures is studied by transmission electron microscopy (TEM). An island type growth is identified. The misfit stress is not fully relaxed at the interface. X-ray measurements on strain relaxation and epilayer misorientation are also reported, and the latter results are explained with the asymmetric introduction of 6Ø° dislocations at island edges. Comparing the results, it is concluded that x-ray data supply additional, although indirect, information on initial growth which is hardly detectable by TEM.

A study by Transmission Electron microscopy (TEM) of strained InGaAs/InAlAs systems on InP substrates is presented. The influence of the lattice mismatch, epilayer thickness and modulation of the lattice parameter on the morphology of the system is analyzed. A discussion of the strain relaxation mechanisms occurring for each growth morphology is also presented.

Two series of samples consisting in a strained layer of InAs (InP) of different thickness, InAs N monolayers (ML) with N=1,2,3, and 4, and, InP M ML with M=2,3,4,5,6 and 7, covered by a GaAs cap layer of 200 nm were grown by Atomic Layer Molecular Beam Epitaxy (ALMBE).

The samples have been characterized by X-ray diffraction in order to measure the critical thickness of InAs and InP on GaAs.

Computer simulation using dynamical theory of X-ray diffraction is used to fit the experimental patterns. In this way we determine the composition, thickness, and strain both in the strained layer of InAs or InP and in the cap layer of GaAs.

A disagreement between simulated and experimental curves is reached at a thickness where the beginning of relaxation takes place (i.e. critical thickness). We have found that the critical thickness of InAs on GaAs(001) is 2.3 ML (0.75 nm) and the critical thickness of InP on GaAs(001) is 5.6–5.7 ML (1.71-1.74 nm), both grown by ALMBE.

The strain relaxation mechanism via the homogeneous nucleation of misfit dislocations from interface during interdiffusion in lattice-matched semiconductor heterostructures has been investigated. Transmission electron microscopy studies in intermixed GaInAsP/InP heterostructures revealed that the critical interdiffusion depth for the nucleation of 90° 1/6<112> partial dislocations from a tensile interface is much shallower than that of 60° 1/2<110> perfect dislocations from a compressive interface. A critical thickness model for the interface nucleation of these dislocations is developed as a modification of the classical surface nucleation'model.

It is generally observed that strain relaxation, which occurs by misfit dislocation formation, in lattice-mismatched heteroepitaxial layers is accompanied by the formation of threading dislocations. However, our group and others have observed that strain-relaxed epitaxial layers of In1−xGaxAs on GaAs substrates can be grown without the formation of threading dislocations in the epitaxial layer. We have been able to grow strain-relaxed layers up to 13% In concentration without observable densities of threading dislocations in the epilayer but do observe a large number of dislocations pushed into the GaAs substrate. The ability to grow strain-relaxed, lattice-mismatched heteroepitaxial layers has important practical applications. We have succeeded in growing dislocation-free layers of ZnSe on appropriately lattice-matched layers of In1−xGaxAs.

Heteroepitaxial Ge layers were grown on vicinal Si(100) by molecular beam epitaxy (MBE) at substrate temperatures of 500°C and 900°C. The layers grown at 500°C were topographically smooth with planar interfaces and a typically high threading dislocation density at the surface (> 108cm−2). Layers grown at 900°C (nucleation at 500°C) showed smooth topography, an interface highly facetted on (111) planes and a dense network of dislocations that is well confined to the interfacial region. Etch pit density measurements indicated that defect densities at the surface were as low as 1 × 105cm−2. Biaxial tensile strains of 1.2 × l0−3 and 2.5 × l0−3 were measured by Raman spectroscopy for the 500°C and 900°C layers, respectively. RBS/channeling spectra supported crosssection TEM results indicating interfacial defect confinement in the layers grown at 900°C. A model describing thermal radiation absorption suggests that interfacial Ge melting occurred during the high temperature growths.

A new mechanism which describes how misfit dislocations in epitaxial layers multiply is presented. This work demonstrates how a single threading dislocation can give rise to an array of dislocation sources, where each source generates a single dislocation loop perpendicular to the primary misfit dislocation. As a threading dislocation with pure screw character glides through an epilayer, certain processes occur which lead to the production of a single dislocation half-loop, and the regeneration of the original threading dislocation. The regenerated threading dislocation continues to propagate on its primary glide plane, which allows the process to repeat itself at some later time. The result of this sequential process is an array of half-loops perpendicular to the primary misfit dislocation. The shape and symmetry of the arrays also contains information regarding how the mechanism operates. The proposed mechanism is related to misfit dislocation arrays in a single Si0.87Ge0.13 layer on Si(001).

A Transmission Electron Microscopy (TEM) study of In0.53Ga0.47As Molecular Beam Epitaxy films grown at different temperatures onto misoriented Si (100) substrates is presented. The evolution of the density of the different kind of defects is discussed as a function of the growth temperature in the range between 200 and 500° C. The results are compared with the characterization techniques of Double Crystal X-Ray Diffraction and Hall effect.