Si/SiGe heterostructures are used in novel device architectures such as stacked nanowires [1] or neuromorphic memories [2]. Epitaxial growth of SiGe on Si is not lattice matched (4.2% difference between Si and Ge), so controlling strain relaxation in thick layers is important. We have recently shown that nano-heteroepitaxy yielded smooth and fully relaxed SiGe layers with a thickness of only a few hundreds of nm [3]. These could then be used as templates for tensile strained Si layers. However, the generation of defects such as stacking faults or twins was inherent to the process itself regardless of the nature of the nano-pillars (Si or SiGe). It is therefore likely that these defects occur during the coalescence process itself, during which adjacent pillars merge in order to form a 2D layer. In this work, we show the characteristics of our SiO₂ based nano-template and investigate SiGe nano-pillars at different stages of the coalescence process.

We have studied the coalescence of Si_0.75Ge_0.25 nano-pillars in a 300 mm industrial Reduced Pressure-Chemical Vapour Deposition tool. Dichlorosilane and germane were used to grow these structures at 700°C and a pressure of 20 Torr. The mean growth rate was around 47 nm/min. An integration scheme based on diblock copolymer patterning was used in order to fabricate a nano-template with a honeycomb pattern, with a pitch of 35 nm (see Figure 1). Selective epitaxy using a chlorinated chemistry of SiGe nano-pillars was investigated for different thickness, with a surface preparation based on a Siconi NH₃/NF₃-based remote plasma treatment [4].

SiGe nano-pillar merging was first examined with Atomic Force Microscopy (see Figure 2). Starting at 30 nm, the shape of the grains takes various forms depending on the number of merging nano-pillars, making the coalescence process heterogeneous in terms of shape evolution. As expected, an increase in average grain diameter and a decrease in the number of grains per unit surface area occurs as the deposited thickness increases (see Figure 3). The coalescence degree (i.e. the inverse of the number of grains) allowed us to predict a full coalescence thickness at ~80 nm.

X-Ray Diffraction was performed, with omega-2theta scans performed around in-plane and out-of-plane orders to investigate the structural properties of the nano-pillars at various degrees of coalescence. This showed an increasing macroscopic degree of strain relaxation with thickness, confirming that growth from nano-cavities induces a fast relaxation.

Scanning Spreading Resistance Microscopy showed a good sensitivity concerning the electrical activity of the nano-pillars (see Figure 4). At 20 nm, the electrical distinction between nano-pillars and the oxide mask was straightforward. The resistance of individual pillars was otherwise rather uniform. At 35 nm, we detect local resistivity variations within the coalescing nano-pillars, which are likely due to electrically active defects.

We used cross-sectional Transmission Electron Microscopy to better understand the generation of structural defects during coalescence. Imaging of the 20 nm thick sample showed that individual nano-pillars were facetted and defect free (see Figure 5). The analysis of the 35 nm thick sample showed various cases ranging from defect free nano-pillar merging to the generation of stacking faults and twinning at the early stages of coalescence (see Figure 6).

In conclusion, SiGe nano-pillars coalescence was investigated using a 300 mm industrial Reduced Pressure-Chemical Vapour Deposition tool. An integration scheme based on diblock copolymer patterning was used to provide nanometer-scaled templates for the epitaxy of SiGe nano-pillars. In order to study their merging, growth of nano-pillars with thicknesses ranging from 20 to 35 nm was performed and samples characterized by AFM, XRD, SSRM and TEM. Results showed evolution of grain shape, size and number, and high degrees of macroscopic strain relaxation were obtained even in the emerging nano-pillars. Defects such as stacking faults and twins were identified at the early stages of nano-pillars coalescence.

References

[1] T. David et al., Nano Lett. 17, 7299 (2017).
[2] S. Choi et al., Nat. Mater. 1 (2018).
[3] M. Mastari et al., Nanotechnology (to be published).
[4] P.E. Raynal et al., Microelectron. Eng. 187-188, 84 (2018).