K. L. Kavanagh

Generation of misfit dislocations in semiconductors

The acting slip mechanism for the generation of misfit dislocations in diamond‐type–semiconductor heterostructures is investigated with transmission electron microscopy. It is shown that dissociation of the 60°‐mixed dislocations can lead to a difference in strain accommodation for tensile and compressive strain. A strain/thickness relation is obtained from the energy expression for nucleation of half‐loops. This relation is compared with other theoretical relations and with experimental strain data for Si/GaP(001) and In0.07Ga0.93As/GaAs(001) , measured with transmission electron microscopy and ion blocking.

Nonalloyed ohmic contacts to n‐GaAs by solid‐phase epitaxy of Ge

A low resistance nonalloyed ohmic contact to n‐GaAs is formed which utilizes the solid‐phase epitaxy of Ge through PdGe. Discussion focuses on the conditions necessary to attain low specific contact resistivity (∼10−6 Ω cm2 on 1018 cm−3 n‐GaAs) and on the interfacial morphology between the contact metallization and the GaAs substrate. MeV Rutherford backscattering spectrometry and channeling show the predominant reaction to be that of Pd with amorphous Ge to form PdGe followed by the solid‐phase transport and epitaxial growth of Ge on 〈100〉 GaAs. Cross‐sectional transmission electron microscopy and lattice imaging show a very limited initial Pd‐GaAs reaction and a final interface which is planar and structurally abrupt to within atomic dimensions. The presence of excess Ge over that necessary for PdGe formation and the placement of Pd initially in contact with GaAs are required to result in the lowest contact resistivity. The experimental data suggest a replacement mechanism in which an n+‐GaAs surface re...

Luminescent colloidal silicon suspensions from porous silicon.

A procedure for generating colloidal suspensions of Si that exhibit luminescence, attributed to quantum confinement effects, is described. Samples of n- or p-type Si that have been electrochemically etched to form porous Si can be ultrasonically dispersed into methylene chloride, acetonitrile, methanol, toluene, or water solvents, forming a suspension of fine Si particles that luminesce. Transmission electron microscopy analyses show that the Si particles have irregular shapes, with diameters ranging from many micrometers to nanometers. Luminescent, composite polystyrene/Si films can be made by the addition of polystyrene to a toluene suspension of the Si nanoparticles and casting of the resulting solution onto a glass slide.

Asymmetries in dislocation densities, surface morphology, and strain of GaInAs/GaAs single heterolayers

The dislocation densities, surface morphology, and strain of Ga1−xInxAs/GaAs epitaxial interfaces as a function of indium composition and layer thickness have been investigated by transmission electron microscopy, medium energy ion blocking, and double‐crystal x‐ray diffractometry. The electron microscopy shows that in the thinnest dislocated films (90 and 160 nm, x=0.07) 60° α dislocations form first in one 〈110〉 direction at the interface. Surprisingly, however, an asymmetry in residual layer strain is not detected in these samples, suggesting that the dislocations have the same Burgers vector or are evenly distributed between two Burgers vectors. Orthogonal arrays of dislocations are observed in films thicker than 300 nm (60° and edge‐type, x=0.07). In this case, dislocation densities in each 〈110〉 direction are equal to within experimental error while an asymmetry in in‐plane strain is measured (18% and 30% for x=0.07, 300, and 580 nm thick, respectively). An unequal distribution of Burgers vectors of...

Thin epitaxial Ge−Si(111) films: Study and control of morphology

The morphology and surface structure of Ge−Si(111) films during the initial stages of heteroepitaxial growth are studied with high-resolution RBS, RHEED, SEM and TEM. It is shown that the equilibrium growth mode of Ge on Si(111) is of the Stranski-Krastanov type. After the formation of a stable film with a thickness of four monolayers, island formation takes place. Substantial indiffusion does not occur. Si on Ge(111) is found to exhibit Volmer-Weber growth. Control of the morphology was obtained by applying SPE on a Si/Ge/Si(111) multilayer system. By capping the Ge film in this way, islanding is suppressed up to annealing temperatures of ~450°C.

Growth, branching, and kinking of molecular-beam epitaxial 〈110〉 GaAs nanowires

GaAs nanowires were grown on GaAs (100) substrates by vapor–liquid–solid growth. About 8% of these nanowires grew in 〈110〉 directions with straight, Y-branched or L-shaped morphologies. The role of strain-induced reduction in surface free energy is discussed as a possible factor contributing to the evolution of 〈110〉 nanowires. Kinking and branching is attributed to growth instabilities resulting from equivalent surface free energies for 〈110〉 growth directions. Transmission electron microscopy verified that 〈110〉 nanowires are defect free.GaAs nanowires were grown on GaAs (100) substrates by vapor–liquid–solid growth. About 8% of these nanowires grew in 〈110〉 directions with straight, Y-branched or L-shaped morphologies. The role of strain-induced reduction in surface free energy is discussed as a possible factor contributing to the evolution of 〈110〉 nanowires. Kinking and branching is attributed to growth instabilities resulting from equivalent surface free energies for 〈110〉 growth directions. Transmission electron microscopy verified that 〈110〉 nanowires are defect free.

Observation of quantum dot-like behavior of GaInNAs in GaInNAs/GaAs quantum wells

We report a quantum dot-like behavior of GaInNAs due to composition nonuniformity of N and In in GaInNAs/GaAs quantum wells (QWs). Images of cross-sectional transmission electron microscopy show that the wells of both Ga0.7In0.3As/GaAs and Ga0.7In0.3N0.02As0.98/GaAs are undulated due to lateral variations in strain. This effect is more pronounced in the N-containing QWs due to nonuniform In and N concentrations. Rapid thermal annealing causes a blueshift of the photoluminescence (PL) peak, and results in a splitting of the as-grown broad PL emission into two peaks. The In and N composition fluctuation after annealing becomes predominantly bimodal. The low-energy PL peak is attributed to excitons localized at deep levels, which are originated from In- and N-rich regions in the wells acting as quantum dots (QD). The high-energy peak PH is likely due to the excitons of the 2D QWs. To reduce the local strain, N atoms are preferentially localized in the In-rich regions, so the separation between these two peak...

Misfit dislocations in nanowire heterostructures

The development of strain relaxation in lattice-mismatched semiconductor nanowire heterostructures is reviewed. Theoretical predictions for critical geometries for axial and core–shell structures are summarized and compared to experimental reports. All agree that nanowires can accommodate a greater elastic strain than is commonly seen with planar interfaces. Large mismatch as high as 10% has been elastically accommodated consistent with theoretical predictions. Elastically strained nanowire examples predominate, likely since nucleation is otherwise inhibited. A (maximum) critical radius is observed for epitaxial growth directly onto lattice-mismatched substrates. The few examples where strain relaxation via dislocations or roughening has been observed have been reported for core–shell geometries.This paper is dedicated to the late Professor Ulrich Gosele whose innovative leadership and insights in this field will not be forgotten.

Strain relaxation of compositionally graded InxGa1−xAs buffer layers for modulation‐doped In0.3Ga0.7As/In0.29Al0.71As heterostructures

Modulation‐doped In0.3Ga0.7As/In0.29Al0.71As heterostructures have been grown on GaAs substrates by molecular beam epitaxy using a compositionally step‐graded InxGa1−xAs buffer layer. We found that the buffer layer produces essentially total relaxation with <2×106/cm2 dislocations present in the In0.3Ga0.7As layer. The structural perfection of this layer is reflected in the electrical and galvanomagnetic properties of its two‐dimensional electron‐gas channel which has a sheet‐electron density of 1.2×1012/cm2, peak mobilities of 9 300 cm2/V s at room temperature and 31 000 cm2/V s at 77 K, and a mobility anisotropy of ∼4% along orthogonal 〈110〉 directions.

Effects of GaAs substrate misorientation on strain relaxation in InxGa1−xAs films and multilayers

We have investigated the effects of GaAs substrate misorientation on strain relaxation in InxGa1−xAs films and multilayers. Our calculations of shear stresses due to misfit strain, resolved on the glide plane in the glide direction, reveal that the α and β 60° slip systems are influenced in a nearly identical fashion, for all substrate misorientation directions. Thus, classical models for nucleation and glide of 60° dislocations predict that a substrate misorientation will not influence the degree of 〈110〉 asymmetry in strain relaxation in lattice-mismatched zincblende semiconductor films. Contrary to these predictions, our experimental results reveal asymmetries in strain relaxation (for partially relaxed single layers) which favor those dislocations aligned with the offcut axis. These asymmetries depend on the substrate misorientation and growth temperature, and are not easily explained by differences in the intrinsic core properties of α and β dislocations. Furthermore, in fully relaxed multilayers (gr...