High Quality Ge Growth on Si (111) and Si (110) by Using Reduced Pressure Chemical Vapor Deposition
Authors: Yamamoto Yuji , Wen Wei-Chen , Schubert Markus Andreas , Corley-Wiciak Cedic , Tillack Bernd 
Published: November 2022

Heteroepitaxial growth of Ge on Si has great interest for various optoelectronic applications such as Ge photodiodes(1). However 4.2% of lattice mismatch causes dislocation formation and island growth. High quality Ge(001) growth techniques are reported in ref.(2-4). Moreover, Ge(111) surface is also interesting because of higher carrier mobility(5). Furthermore, Ge(110) is preferred orientation of virtual substrates for epitaxial graphene growth(6). In the case of the Ge deposition on Si(111) and Si(110) substrates, it seems that the process conditions used for Ge growth on Si(001) are not suitable to realize high crystallinity and smooth surface (7). In this paper, we present a method of high quality and smooth Ge layer growth on Si(111) and Si(110), which is the same level as the Ge growth on Si(001).
Epitaxial growth of Ge on Si(111) and Si(110) is carried out using a reduced pressure chemical vapor deposition system. After HF last clean, a wafer is baked at 1000°C and cooled down to 600°C in H2 and further to 300-550°C in N2 to form a hydrogen-free Si surface. Then a 100 nm thick Ge layer is deposited as a seed layer using GeH4 with N2 carrier gas. Afterward the wafer is heated up to 450-650°C in H2 and the main part of Ge is deposited using a H2-GeH4 gas mixture. For threading dislocation density (TDD) reduction, annealing at 800°C in H2 is performed for several times (cyclic annealing) by interrupting the Ge growth. Atomic-force microscopy (AFM) is used for surface roughness analysis. Scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) are used for structural characterization of the Ge layer. Secco defect etching combined with angle view scanning electron microscopy (SEM) or optical microscope is used for TDD evaluation.
Figure 1(a,b) summarize the root mean square (RMS) roughness of Ge(111) and Ge(110) seed layers grown at 300-550°C before and after postannealing at 600-800°C. If the growth temperature is lower than 350°C for Ge(111) and 400°C for Ge(110), a significant increase of the surface roughness is observed after postannealing at 700°C and 800°C, respectively. For both crystal orientations, the lowest RMS roughness is observed by depositing at 450°C for as deposited and postannealed samples. The maintained RMS roughness even after postannealing at 800oC may be indicating good crystal quality even at as deposited condition.
To confirm the influence of the growth temperature on the crystallinity, cross section TEM images of the Ge(111) and the Ge(110) seed layers deposited at 300°C and 450°C are shown in Fig. 2(a-d). In the case of Ge growth at 300°C (Fig. 2(a,b)), a very high density of stacking faults (SF) and high surface roughness are observed for both crystal orientations. In contrast, by depositing at 450°C (Fig. 2(c,d)), lower SF density in the Ge layer is observed compared to that at 300°C. By postannealing, an improvement of crystallinity is observed for the Ge seed layers deposited at 450°C. However, in the case of 300°C, the crystallinity cannot be improved by the postannealing, because a too high density of dislocations and SF may cause irregular Ge atom migration. As the result, surface roughening occurs.
Figure 3(a,b) show AFM surface roughness images after 5 μm-thick Ge(111) and Ge(110) deposited with cyclic annealing at 800°C, respectively. Clear terraces of ~0.3 and ~0.2 nm, whose heights are close to those of Ge(111) bilayer and Ge(110) monolayer, are observed, respectively. RMS roughness of the Ge(111) and the Ge(110) are 0.51 and 0.35 nm, respectively. These RMS roughnesses are comparable to a level reported for Ge (001) in ref.(1).
Figure 4 shows TDD of Ge(111) and Ge(110) surfaces as a function of the Ge thickness deposited with cyclic annealing on Si(111) and Si(110) substrates. For both orientations, TDD of ~4×108 cm-2 is obtained for 500 nm-thick samples. With increasing the Ge thickness, the TDD is reduced and levels below TDD of ~5×106 cm-2 are achieved for both Ge (111) and Ge(110) for 5 μm-thick Ge.
These methods enable high quality virtual substrate fabrication not only for (001) surfaces but also for (111) and (110) orientation without a chemical mechanical polishing process.

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Figure 1