ALBERT

Notes: In order to understand the influence of various growth and processing procedures on the quality of silicon grown on sapphire (SOS), one needs a way to measure microtwin density as a function of distance from the interface. In previous studies, however, the technique best suited for determining such a density profile, transmission electron microscopy (TEM), has been applied to this problem in a relatively qualitative manner. We have chosen to apply TEM in a more quantitative fashion to this problem by keying our observations to microtwin morphology. These observations indicate that the microtwins occurring in SOS grown by chemical vapor deposition are faceted and grow contiguously from the interface. The volume fraction of silicon occurring as microtwins does not decrease away from the interface at as large a rate as indicated by previous TEM studies. Instead, the decrease in the number of twins away from the interface appears to be balanced by an increase in twin size. It also appears that the greatest volume fraction of twinned material occurs at least several hundred angstroms away from the interface rather that at the interface.

Notes: Channeled-substrate buried heterostructure (CSBH) lasers which were purged from populations undergoing high reliability qualification have been studied in detail. Gradual and rapid degradation mechanisms leading to accelerated aging failure modes have been analyzed by transmission electron microscopy, convergent beam electron diffraction, electroluminescence, energy dispersive x-ray analysis, and chemical etching. The gradual degradation mode of CSBH lasers is characterized by (1) a gradual increase in room-temperature threshold current; (2) a decrease in external quantum efficiency, typically a drop in peak value of dL/dI greater than 25%; (3) a drop in forward voltage at low current, indicating a change in junction characteristics; (4) a large peak inI(dV/dI) below threshold (at around 3 mA); and (5) an enhancement in the peak in I2(d2V/dI2) at laser threshold. A defect mechanism associated with the gradual degradation begins with a nucleation of extrinsic dislocation loops along the V-groove {111} p-n–type sidewall interfaces between the Cd-diffused p-InP and liquid-phase-epitaxial-grown n-InP buffer inside the groove. These dislocation loops subsequently grow out of the interfaces into the n-InP buffer region in the direction of minority-carrier injection, indicating a nonradiative recombination-assisted defect growth process. For those loops which enter the quaternary active region near the tip of the active crescent, the growth rate along the (001) and (010) planes is greatly enhanced and the loops eventually cut across the active stripe and become dark-line defects, as confirmed by electroluminescence. Nucleation of dislocation loops is not observed along the {111} p-p–type sidewall interfaces above the active stripe. The fact that the dislocation loops are all extrinsic in nature implies that the {111} sidewall interfaces as well as the quaternary active region contain a high density of interstitials. The possible causes for the generation and growth of the dislocation loops and the high density of point defects are discussed. The rapid degradation mode of the CSBH laser is characterized by a sudden drop in light intensity during the aging process. The associated defect mechanism starts with localized melting at the mirror facet or inside the lasing cavity. A metal-rich droplet subsequently forms which propagates along the center of the active stripe in the direction towards the cavity center via a meltback-regrowth process; i.e., material melts in front of the droplet and regrows after it propagates by. The nonideal condition of regrowth results in the formation of a wormlike defect composed of a cylinder of defective materials bounded by an off-stoichiometric interface. The wormlike defect is dark under electroluminescence. Complicated dislocation structures can also be grown from the wormlike defect under a nonradiative recombination-assisted defect growth process. These phenomena are presented and discussed.

Notes: Transmission electron microscope studies of Ti-doped, congruent lithium niobate (LiNbO3) have shown that extended structural faults and domain boundaries are only present within the Ti-diffused layer (i.e., the waveguiding region). Structural faults have also been observed in undoped Li-deficient LiNbO3, though not in undoped control crystals of congruent and stoichiometric LiNbO3. In both Ti-doped and Li-deficient LiNbO3, crystal chemistry predicts an increase in the concentration of the point defects V'''''Nb and Nb⋅⋅⋅⋅Li. Therefore, it appears that the coalescence of these defects is responsible for the creation of structural faults. Since the extent of these structural faults and domain boundaries is tens of microns, they are clearly potential scattering sites for photons. In this regard, a systematic understanding of their origin and thermal stability is crucial to integrated optical device technologies based on LiNbO3 and on the Ti-doped waveguide fabrication technique.

Notes: We derive an expression for the dislocation line tension in thin-film structures for double-kink (dislocation dipole) extension of a threading dislocation. From these calculations, we conclude, that for a double kink that is a distance of more than three times the vertical dipole separation from the free surface, the effect of the free surface on the double-kink line tension is insignificant. We consider the specific case of a double-kink bounding a 60-nm-thick Si0.7Ge0.3 strained layer grown on a silicon substrate and buried under a silicon cap. For this specific configuration, line tension is seen to increase with cap thickness (which is equivalent to distance of the double kink from the surface) only up to a cap thickness of 100 nm.

Notes: Single-crystal germanium on (11¯02) sapphire films are grown after a substrate preanneal of 1400 °C and at growth temperatures above 700 °C. At a growth temperature of 800 °C, the nucleation site density was ∼1011 cm−2. For thin germanium films, the isolated islands were singly oriented, with single-crystal films obtained for thicker grown films. A 400 °C growth temperature on sapphire was insufficiently high to get epitaxial growth and produced polycrystallites. Growth at 400 °C on an 800 °C grown germanium template did result in epitaxy. However, a high fraction of the twinned orientation was produced, resulting in a bicrystalline film.

Notes: Buried Si1−xGex layers grown on Si at elevated temperatures of 700 to 800 °C generally exhibit x-ray rocking curves which are significantly broader than those predicted for perfect crystals, suggesting that the layers are strain-relieved. However, calculations using these rocking curves show the materials to be either nearly- or fully-strained. The source of x-ray broadening accompanied by high strain is found to be an abrupt, thermally-induced fragmentation of the layer into small, slightly misoriented regions during the cool-down, such that the as-grown strain remains unchanged. The fragments are typically rectangles a few micrometers wide, with well-defined boundaries along [110]-type directions.

Notes: The interface surfaces of short-period GaSb/InAs superlattices grown by molecular beam epitaxy have been studied in situ with scanning tunneling microscopy. Migration enhanced epitaxy was used at the interfaces in order to control bond type. Interfaces on GaSb(001) are found to be smoother than those on strained InAs(001), and the InSb-like interfaces are smoother than GaAs-like ones. The primary source of disorder at these interfaces appears to be the kinetically determined topography of the growth surfaces, with intermixing playing a secondary role. © 1995 American Institute of Physics.

Notes: Using image processing algorithms based on nonlinear imaging theory, we have analyzed high-resolution transmission electron microscopy images of InAs/GaSb superlattices (SLs) grown by molecular beam epitaxy. Our analysis indicates that InSb-like interfaces have a roughness of 1 monolayer (ML), for a SL grown on a GaSb buffer layer. For GaAs-like interfaces, however, the interface roughness is found to be 2 MLs when the SL is grown on a GaSb buffer. For SLs grown on an InAs buffer, the roughness of GaAs-like interfaces (3 MLs) is also greater than that of InSb-like interfaces (2MLs). These results suggest two general observations. The first is that GaAs-like interfaces are rougher than InSb-like interfaces. This difference may be due to the high surface energy of GaAs as compared to InSb. The second observation is that interface roughness is greater for an InAs/GaSb SL grown on an InAs buffer layer than for the same SL grown on a GaSb buffer layer. This difference in interface roughness may be due to InAs SL layers being in tension when grown on a GaSb buffer layer, whereas GaSb SL layers are under compression when grown on an InAs buffer layer. © 1995 American Institute of Physics.

Notes: For heteroepitaxial systems, such as silicon on sapphire, microtwins can usually be observed in the epitaxial layer. It has also been suggested that microtwins play a significant role in strain relief in these systems. From a knowledge of the differential volume fraction of microtwins occurring in a heteroepitaxial systems, it is possible to estimate the greatest possible strain relief due to microtwins. Measurements of the differential volume fraction of microtwins in silicon-on-sapphire, however, indicate that the strain relief due to microtwins cannot be greater than 0.7%, even though the lattice mismatch between silicon and sapphire is an order of magnitude larger. Therefore, if the silicon/sapphire interface is coherent, the misfit strain must be relieved by another mechanism.

Notes: With x-ray diffraction techniques, it is possible to routinely measure lattice parameters to several parts in 104 for thin-film samples. However, measurements of lattice parameter changes for quaternary device structures several microns in width are not usually feasible with x-ray diffraction techniques. For this reason, transmission electron microscopy has been used to determine the position of higher-order Laue zone lines within convergent-beam electron diffraction patterns from thin foil cross sections of planar quaternary layers grown on InP substrates. A calibration curve has been generated which describes the position of higher-order Laue zone lines as a function of the lattice parameter determined from x-ray diffraction measurements. For the active quaternary region of an elecro-optical device structure, it is shown that ths calibration procedure may be sensitive to a relative change in lattice parameter as small as ±2 parts in 104.