Cross-section 022 bright-field TEM of In0.66Al0.34P on a lattice-matched In0.2Ga0.8As graded buffer on GaAs. The complete structure grown at 650 °C on an exact (100) substrate is shown in (a). Samples (b)–(d) are grown on substrates miscut 6° towards (−1−11)A. InAlP initiated on InGaAs at (b) 725 °C has a large number of structural defects, which quench photoluminescence. Samples initiated at (c) 650 °C and (d) 620 °C show room temperature photoluminescence.
Cross-section two-beam bright-field TEM of InAlP layers grown at 650 °C and 725 °C on an exact (001) substrate. The samples are viewed along the  and [−110] directions. Figures (a) and (c) were imaged using the diffraction vector g = 022 and (b) and (d) using g = 004. Lateral composition modulation is visible in layer grown at 650 °C, but disappears at 725 °C. The length scale of the modulation is larger in the [−110] direction. Very fine speckle contrast is visible in both the g = 004 images.
Cross-section two-beam bright-field TEM of InAlP layers grown at 650 °C and 725 °C on an (001) substrate miscut 6° towards the (−1−11)A plane. The samples are viewed along the  and [−110] directions. Figures (a) and (c) were imaged using the diffraction vector g = 022 and (b) and (d) using g = 004. Vertical composition modulation is visible as striations in layer grown at 650 °C. The angle of the striations in (d) is roughly 9°–10° from the substrate surface, a few degrees higher than the angle of miscut.
Cross-section two-beam dark-field (002) TEM of InAlP layers grown at 650 °C and 725 °C on (a) an (001) substrate miscut 6° towards the (−1−11)A plane and (b) exact (001) surface. The samples are viewed along the . The intensity of the 002 diffracted beam is proportional to the difference in atomic number of the group III and group V elements.
(a) Schematic of the surface of a miscut wafer during InAlP growth showing step-bunches and terraces. CuPt type ordering occurs only along the [−111] and [1–11] directions and is visible in TEM images prepared along the  direction, but not the [−110] direction. The phase-separated regions form planes in the direction of the miscut but at a slightly larger angle. The creation of additional tilt Δθ is shown. (b) Cross-section TEM along the [−110] axis showing step-bunching on the InAlP surface grown at 650 °C.
ADF-STEM images of 0.08 atomic fraction In-rich InAlP quantum wells embedded in a matrix of InAlP with a lower indium fraction. The thick layers grown at 650 °C and 725 °C have the same composition to within 0.01 atomic fraction. Sensitive to composition, the In-rich quantum wells appear brighter than the rest of the layers. The degree of tilted striations due to vertical composition modulation increases with layer thickness and even appears to disrupt the uniformity of the quantum well (indicated by the arrow). Image intensity profile along the dotted rectangle is plotted alongside the image. An estimate of the maximum degree of phase-separation is determined to be about 0.02–0.03 atomic fraction. Large scale contrast variation is due to non-uniformity in sample thickness.
Cross-section HRTEM of InAlP grown at 650 °C along the  direction grown on substrates (a) 6° miscut towards (−1−11A) and (b) exact (001) showing double-variant CuPt-B type ordered regions as well as phase separation. The arrows indicate what we interpret as regions of higher indium composition. Well defined regions of lower order parameter are seen in the miscut sample. The images are Bragg-filtered with the (000) spot included to observe large-scale contrast variation.
(a) Cross-section 022 two-beam bright-field TEM of InAlP grown on InGaAs at 650 °C showing compositional inhomogeneity as a function of silicon doping from 1 × 1019/cm3 to 8 × 1019/cm3 on an exact (100) substrate. Increasing silicon doping lowers compositional inhomogeneity. It was not possible to achieve similar levels of doping with Zn. (b) Cross-section bright-field TEM image along the  zone-axis of p-i-n layers of InAlP grown on a miscut 6 A substrate. Selected area diffraction was used to collect  zone TED patterns from the three layers. Doping with Si and Zn removes ordering and very high levels of Si doping also removed phase-separation.
Room-temperature photoluminescence spectra of InAlP films grown at 620 °C, 650 °C, and 725 °C on exact (001) and 6° miscut substrates towards (−1−11)A. The composition and order parameter vary between the samples. The peak-widths of the spectra are greater on exact (001) substrates and in samples grown at lower temperatures.
Article metrics loading...
Full text loading...