William A. Jesser

The effect of frictional stress on the calculation of critical thickness in epitaxy

A critical‐thickness model is proposed for lattice‐mismatched epitaxial layers which incorporates for the first time both the equilibrium and kinetic components of misfit accommodation. The model is an attempt to provide a physical basis for the discrepancy between the observed and calculated critical thickness in zinc‐blende semiconductors. The equilibrium components of the model use a force balance and a Volterra description of the dislocation line tension with frictional forces includiing Peierls barriers and dislocation atmospheres. The effect of the frictional forces is to shift the critical thickness to a larger value and to produce a residual elastic strain which persists for large thicknesses. The frictional barriers are low in face‐centered‐cubic metals and produce minor changes from the previous theories; however, for semiconductors the frictional forces may be high and produce a substantial increase in the predicted critical thickness. The kinetic component of misfit accommodation has been addressed through a dynamic frictional force. The explicit form of the kinetic component of misfit accommodation depends on the operative mechanism of misfit‐dislocation generation. A simple criterion is proposed to estimate whether the generation of misfit dislocations will be controlled by equilibrium or kinetic factors.

Investigation of the asymmetric misfit dislocation morphology in epitaxial layers with the zinc-blende structure

Epitaxial growth of compound semiconductors on the (001) exhibits an asymmetry in the dislocation morphology in the two 〈110〉 directions for thicknesses near the critical thickness. The source of the asymmetry has been investigated by growth of a thickness wedge of p‐ and n‐type GaAs0.95 P0.05 on GaAs by metalorganic chemical vapor deposition. The effect of misorientation on the resolved shear stress for each slip system has been calculated and eliminated as the source of the asymmetry. The thickness gradient was also eliminated as the source. A definitive correlation can be made between the asymmetry and the differences in the Peierls barriers of the two types of dislocations. The asymmetry results in two different critical thicknesses, one for each type of dislocation. The Peierls barriers are more similar in p‐type material than in n‐type material. In agreement with this, a reduced amount of asymmetry was observed in the p‐type overgrowth as compared to the n‐type. The nucleation barrier differences be...

Low energy dislocation structures in epitaxy

Abstract The principle of minimum energy was applied to epitaxial interfaces to show the interrelationship between misfit, overgrowth thickness and misfit dislocation configurations were presented for selected interfacial geometries. A review of the interfacial energy calculations was made and a critical assessment of the agreement between theory and experiment was presented. Modes of misfit accomodation were presented with emphasis on the distinction between kinetic effects and equilibrium conditions. Two-dimensional and three-dimensional overgrowths were treated together with interdiffusion-modified interfaces, and several models of interfacial structure were treated including the classical and the current models. The paper is concluded by indicating areas of needed investigations into interfacial structure.

Improving yield and performance in pseudo-ternary thermoelectric alloys (Bi2Te3)(Sb2Te3)(Sb2Se3)

Abstract Thermoelectric alloys of Bi 2 Te 3 Sb 2 Te 3 Sb 2 Se 3 have typically been fabricated by Bridgman growth and zone growth [1, 2]. This study showed that the method of growth does not appear to produce a significant difference in the figure of merit. However, it was found that the zone techniques can produce a higher yield of material than Bridgman techniques in both the p- and n-type material without a degradation in the high figure of merit. The maximum figure of merit for p-type alloy (Bi 2 Te 3 ) 72 (Sb 2 Te 3 ) 25 (Sb 2 Se 3 ) 3 was found to be 3.6 × 10 −3 K −1 and for the n-type (Bi 2 Te 3 ) 90 (Sb 2 Te 3 ) 5 (Sb 2 Se 3 ) 5 , 3.2 × 10 −3 K −1 [1].

Optoelectronic device performance on reduced threading dislocation density GaAs/Si

A technique for the heteroepitaxy of GaAs/Si films having reduced threading dislocation density is presented. The important attribute of this technique is the suppression of three-dimensional Volmer–Weber island formation during initial deposition. This suppression is achieved by deposition of a stoichiometric GaAs buffer layer by a migration-enhanced epitaxy technique on silicon at 348 K to a thickness greater than the “monolithic thickness,” hm. Subsequent GaAs films deposited by conventional molecular beam epitaxy on buffer layers of thickness greater than hm possess structural and optical characteristics that exceed those for state-of-the-art GaAs/Si layers: an x-ray full width at half maximum (FWHM) of 110 arcsec with a dislocation density at the film surface of 3×106 cm−2 and a concomitant 4 K photoluminescence FWHM of 2.1 meV. The p-i-n structures suitable for use as light-emitting diodes (LEDs) that were grown on the reduced threading dislocation density GaAs/Si and by means of forward- and revers...

The structure and melting character of sub-micron In-Sn and Bi-Sn particles

Abstract Vapor deposits of In-Sn and Bi-Sn were made, primarily in-situ, for analytical transmission electron microscopy studies which provide information on composition, phase, and melting character. Particles of the two alloys with diameters of 5 to 50 nm exhibit several properties that differ from the bulk alloys. In relation to the bulk phase diagrams, small particles have a lower liquidus line and extended terminal solubilities. For In-Sn particles, dark-field TEM and diffraction reveal stable amorphous phases for some ranges of composition. These amorphous phases nearly coincide with the two-phase regions of the bulk phase diagram at room temperature. In contrast, particles of Bi-Sn exhibit crystalline structures for all of the compositions that were studied. Dark-field TEM revealed the presence of internal boundaries over the full range of particle sizes. Convergent beam microdiffraction shows that phase boundaries are present. Upon heating, the coexistence of a solid core and a liquid outer layer was observed for particles of certain composition ranges before the two-phase particle completely transformed abruptly to liquid.

Room-temperature anisotropic, thermoelectric, and electrical properties of n-type (Bi2Te3)90 (Sb2Te3)5 (Sb2Se3)5 and compensated p-type (Sb2Te3)72 (Bi2Te3)25 (Sb2Se3)3 semiconductor alloys

The anisotropy ratios (parallel to the c axis versus perpendicular to the c axis) for the electrical resistivity, Seebeck coefficient, and thermal conductivity of horizontal Bridgman-grown n-type (Bi2Te3)90 (Sb2Te3)5 (Sb2Se3)5 and p-type (Sb2Te3)72 (Bi2Te3)25 (Sb2Se3)3 were measured at 300 K. The orientation having the largest thermoelectric figure of merit was perpendicular to the zone axis of the c planes (along the natural growth direction). For the n-type alloy, the maximum thermoelectric figure of merit was determined to be 3.2×10−3/K. For the p-type alloy, the maximum thermoelectric figure of merit was determined to be 3.6×10−3/K when compensated with tellurium only, and 3.8×10−3/K when compensated with both tellurium and iodine. These values represent a significant increase over previously reported p-type thermoelectric figure of merit values. Hall coefficient data coupled with Seebeck coefficient measurements suggest a difference in carrier transport phenomena caused by an increased compensation of holes in the p-type alloy without the deleterious precipitation of the tellurium-rich second phase.

A theory of pseudomorphism in thin films

Abstract It is shown that the large interface-to-volume ratio found in thin bicrystal films can stabilize the overgrowth in the structure of the substrate rather than its normal bulk structure. Van der Merwes expression for the misfit energy of an interface was employed in conjunction with elastic strain energy calculations by Mott and Nabarro and by Cabrera to derive the critical deposit size beyond which a transformation from the pseudomorphic structure to the normal structure is favored. Continuous films of uniform thickness and hemispherical nuclei are treated.

Numerical study of the influence of reactor design on MOCVD with a comparison to experimental data

Abstract Comparisons were made between experimental data and a two-dimensional model for the MOCVD of GaAs from trimethylgallium and arsine in two horizontal air cooled reactor geometries. A unique feature of this work was that comparison was made, not only on the wafer, but over the entire deposition regime. Excellent agreement was achieved for growth at low system pressures. Experimental deposition profiles under atmospheric pressure were much less uniform than those predicted by the model. This difference could not be attributed to a pressure dependence of heterogeneous reactions. Inclusion of thermal diffusion had little effect on the uniformity of the calculated deposition profile but decreased the magnitude of the growth rate by up to 12%. Through model calculations and experiments, it was determined that a channel with a tilted upper wall and a horizontal susceptor has the same growth rate profile as does a standard horizontal upper wall channel with a tilted susceptor. Substrate rotation was predicted to cause growth rate uniformity within less than 4.3% in horizontal channel flow and 3.8% in a channel with tilted walls.

Reduced carbon contaminant, low-temperature silicon substrate preparation for “defect-free” homoepitaxy

A low-temperature cleaning technique incorporating an ultraviolet light–ozone treatment with conventional hydrogen-passivation techniques is shown to yield a low concentration of surface contaminants: 0.01 monolayer. An additional advantage is that no ultrahigh vacuum prebaking treatments are required for substrates receiving this treatment prior to epitaxy. Epitaxial silicon layers deposited onto substrates receiving the present technique are characterized by a threading dislocation density on the order of 104/cm2, which is considerably lower than the 105/cm2–106/cm2 order measured for films receiving more conventional preparation techniques. The results of this study suggest that aqueous solutions are a significant source of surface carbon and that residual carbon on the silicon surface before epitaxy contributes to the generation of threading dislocations in Si homoepitaxial films.