Strain relaxation defects in
Ge
crystals grown on
Si
pillars
Authors: Rojas Dasilva Yadira Arroyo , Rossell Marta D. , Erni Rolf , Isa Fabio , Isella Giovanni , von Känel Hans , Gröning Pierangelo 
Published: December 2016

Due to the differences in lattice parameters and thermal expansion coefficients, Ge films grown on Si substrates suffer from a high density of threading dislocations, cracks and wafer bowing. One method to avoid these problems is to grow Ge crystals on (001)‐Si pillars by low energy plasma enhanced chemical vapor deposition (LEPECVD). With this strategy, threading dislocations of 60° type with Burgers vector (
b
)
b
=1/2 can be avoided in the bulk as they end at the sidewalls of the Ge crystals. However, the misfit strain is not totally relaxed; it is accommodated by misfit dislocations (MDs) of 60° and 90° types, while planar defects have not been reported in these Ge crystals.

This work presents a detailed analysis of defects formed to relax the misfit strain between the Ge crystals and Si pillars using high angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM).

In Ge/Si interface, the misfit strain is accommodated by 60° and 90° MDs. The 90° MDs are Lomer MDs lying on (001) planes which are formed by interaction of two 60° MDs. These pairs of MDs form steps at the interface leading to an atomic roughness.

Most interestingly, besides the MDs, there is a high density of planar defects which has not been reported before. These 2D defects are formed at the Ge/Si interface and extend between 3 to 40 nm along the {111} planes. We observed coherent twin boundaries (CTB), incoherent twin boundaries (ITBs) and stacking faults (SF) bounded by partial dislocations (PDs). Figure 1a shows CTBs of the ∑3{111}‐type (red arrows), the distance between them is 2.95 nm. The inset of Fig. 1a is its Fourier transform (FFT), the twin plane correspond to GA(1‐1‐1)=GB(1‐11) (GA and GB are grains A and B) and the corresponding planes for GA and GB are (1‐11) and (1‐1‐1) respectively. The CTBs ends inside the crystal with an ∑3{112}‐ITB (Fig 2b) having 6, 7, 5 rings along the boundary. Figure 1c shows an image of the CTB at the interface, the yellow arrows are the PDs accommodating the mismatch in the CTB. They are 30° Shockley PDs since the Ge grows in compressive films. The CTB are formed nearby the steps formed by the pairs of MDs.

Figure 1d shows a misfit partial dislocation (MPD) at the end of a SF. The Burgers circuit around the dislocation gives
b
=1/6[1‐12] which corresponds to a Shockley PD. The stacking sequence of the SF changes from AaBbCcAaBbCc… to AaBbCc
Bb
AaBbCc (Fig. 2e). The SF is of extrinsic type since there is an extra plane in the stacking sequence. For the compressive Ge, the Shockley MPD with an extrinsic SF corresponds to 90° at the interface which in addition has a perfect 60° dislocation close to the PD. Figure 2f shows two SFs in (1‐11) and (1‐1‐1) planes, they annihilate each other forming a stair rod dislocation (SRD). The stacking sequence of the SFs changes from AaBbCcAaBbCc… to AaBbCcBbCcAaBb… The SFs are of intrinsic type since there is a missing plane in the stacking sequence. The configuration of the MPDs at the interface in Ge with intrinsic SFs is 30° MPD at the interface and 90° MPD in the Ge. The two 90° PDs in the Ge interact with each other and form the SRD with
b
=1/3[‐110].

SFs are also found in the Ge crystal (Fig. 1g). The Burgers circuit around the PDs gives
b
=1/6[1‐12] which corresponds to Shockley PDs. These intrinsic SFs (Fig. 1h) are formed by the dissociation of perfect 60° dislocations.

We conclude that the misfit strain between Si and Ge is accommodated by 60° and 90° MDs, the 60° MDs splitting to form MPDs and CTB. The MPDs form extrinsic or intrinsic SFs. Intrinsic SFs are interacting with each other forming SRD. Overall, it can be summarized that the defect chemistry in Ge pillars is more complex than reported in earlier studies. This might have significant impact on the electronic properties of these binary semiconductor heterostructures.