Burgers Vector Analysis of Vertical Dislocations in Ge Crystals by Large-Angle Convergent Beam Electron Diffraction

Abstract

By transmission electron microscopy with extended Burgers vector analyses, we demonstrate the edge and screw character of vertical dislocations (VDs) in novel SiGe heterostructures. The investigated pillar-shaped Ge epilayers on prepatterned Si(001) substrates are an attempt to avoid the high defect densities of lattice mismatched heteroepitaxy. The Ge pillars are almost completely strain-relaxed and essentially defect-free, except for the rather unexpected VDs. We investigated both pillar-shaped and unstructured Ge epilayers grown either by molecular beam epitaxy or by chemical vapor deposition to derive a general picture of the underlying dislocation mechanisms. For the Burgers vector analysis we used a combination of dark field imaging and large-angle convergent beam electron diffraction (LACBED). With LACBED simulations we identify ideally suited zeroth and second order Laue zone Bragg lines for an unambiguous determination of the three-dimensional Burgers vectors. By analyzing dislocation reactions we confirm the origin of the observed types of VDs, which can be efficiently distinguished by LACBED. The screw type VDs are formed by a reaction of perfect 60° dislocations, whereas the edge types are sessile dislocations that can be formed by cross-slips and climbing processes. The understanding of these origins allows us to suggest strategies to avoid VDs.

Dislocation Analysis in SiGe Heterostructures by Large-Angle Convergent Beam Electron Diffraction

“…One complication of this geometry was that the commonly employed plan-view specimens were not applicable due to the pillar structure. Thus, next to HRTEM dislocation characterization [31,32] on cross-sectional specimens, the LACBED method became the method of choice for clarifying these types of VDs [55,56].…”

Anisotropic extended misfit dislocations in overcritical SiGe films by local substrate patterning

“…Strain engineering and dislocation formation mechanisms in epitaxial SiGe/Si heterostructures are long standing topics of both fundamental and practical interest [1,2]; SiGe alloys are highly compatible with Si technology and allow for strain and quantum confinement effects, in order to modify the physical properties of the semiconductor for optical, electrical, or thermoelectric applications [3][4][5]. The development of SiGe heterojunction bipolar transistors and strained Si/SiGe CMOS technology have promoted a large research effort aimed at controlling the growth of the SiGe layer and tailoring the desired material properties [6,7].…”

“…In (a), the patterns with a pitch of 4 μm and a flat area are reported showing the propagation of the defect outside the pre-patterned region along one preferential direction. Scan areas are 20×20 μm2 . In (b), a sketch of the TEM's lamellas (1 and 2) is shown: the first lamella (line n°1) was prepared outside the patterned area at about 1.5 μm outside the pit-patterned region, while the second TEM lamella (line n°2) was realized between the pits and flat region.…”

Strain relaxation in epitaxial Ge crystals grown on patterned Si(001) substrates

“…Thus, the 60 • TD density is reduced or even eliminated, depending on the height of the Ge crystals. Instead, new TDs are formed in the Ge which are identified as edge and screw dislocations with Burgers vectors b= 1 2 < 110 > and b = [001], respectively [11,6,12]. So far, planar defects have not been reported in Ge crystals grown on Si pillars.…”