InAs/GaSb
interface investigation by high resolution
HAADF
‐
STEM
Authors: Favia Paola , Richard Olivier , Kazzi Salim El , Marcke Patricia Van , Bender Hugo 
Published: December 2016

Heterojunctions with broken band alignment are of significant interest for tunnel field effect transistors (TFETs) because they decrease the tunneling distance for band‐to‐band tunneling and thereby increase the tunnel current [1]. InAs and GaSb crystallize in the familiar zincblende structure which consists of two interleaved face‐centred cubic sublattices. The lattice constant of these two materials is about 0.61nm so that lattice‐matched heterostructures can be grown [2]. At such a heterojunction between two semiconductors that do not share a common species, two distinct bond configurations are possible with either In–Sb or Ga–As bonds. Which kind of bond is formed depends on the details of the growth process and presents different characteristics in terms of defects and stress.

Broken gap InAs/GaSb heterojunctions are grown by molecular beam epitaxy. In order to characterize the nature of the interface two cross‐section transmission electron microscopy (TEM) samples are prepared for observation along the [110] and [‐110] zone axes respectively. The TEM specimens are prepared by FIB lift‐out. HAADF‐STEM is performed using a double corrected TEM (FEI Titan
3
60‐300) at 300 kV.

In Figure 1, a schematic of the two possible bonds (In‐Sb type in Figure 1 a,b and Ga‐As type in Figure 1 c,d) interface configurations is shown along the [110] (a,c) and [‐110] (b,d) zone axes. For a perfectly flat interface, adjacent intensity profiles across the interface along the [001] growth direction will show:

‐ in Figure 1a vs b, and in 1c vs d, a reversal of the brightness of the dumbbells in the bulk of the InAs and GaSb for the 2 zone axes.

‐ that the brightness reversal is present at the interface for all traces in case of Figure 1b (low‐high high‐low) and 1c (high‐low low‐high) while, in Figure 1a and 1d, alternating traces show either reversed brightness or dumbbells with nearly equal brightness at the interface (high‐high on Figure 1a or low‐low on Figure 1d).

Based on these models a unique interpretation of the nature of the interface can be made if the interface is free of roughness over the thickness of the TEM specimen. In Figure 2, HAADF‐STEM micrographs along the [110] (a) and [‐110] (b) zone axes are shown. Three adjacent intensity profiles across the interfaces, corresponding to traces 1‐3 in the micrographs, are shown in Figures 2c and d. In Figure 2c, traces 1 and 3 show unequal intensities ‐ high‐low dumbbells ‐ whereas trace 2 reveals an “equal” intensity dumbbells at the interface consistently with the case in Figure 1a. The brightness of the “equal” dumbbells is not as high as expected for an In‐Sb bond. This is probably caused by stress at the interface which also results in the dark band contrast as can be seen on Figure 2a,b. Figure 2d shows intensity profiles with unequal intensity dumbbells (low‐high) which for all traces invert at the interface. This is consistent with Figure 1b. The observations along both [110] and [‐110] zone axes indicate that the interface is In‐Sb type, which is consistent with the RHEED patterns acquired during the MBE growth of the stack.