Growth and Etch Forms of Germanium Microcrystals on a Silicon Oxide Substrate
Authors: Huang Yi-Chiau , Cai Man-Ping , Zhou Hongwen , Chung Hua 
Published: February 2020

In the fabrication of electronic and photonic devices, it is often necessary to control the geometric forms of single-crystal semiconductors—such as silicon or germanium—to optimize the device performance. This work presents a series of geometric forms of germanium microcrystals after either germanium growth or etch, which can change the size and shape of an existing crystal and bring it to a distinctive polyhedral form. This form is usually different from the equilibrium form, which minimizes the sum of surface free energy and strain energy for a given crystal.
Germanium crystals are first formed on a substrate of thermal silicon oxide in a Centura® 300 mm RP Epi chamber from Applied Materials. Germane (GeH4) is used as the germanium source. These germanium crystals are then thermally annealed. After anneal, the crystals are approximately 5 to 10 μm in diameter. Fig. 1 shows the scanning electron micrograph of (a) a small and (b) a large germanium crystal after a 850°C anneal. After anneal the crystals are near equilibrium, and certain low-index faces can be seen. The largest appear to be {113} faces, while {111} and {110} faces can also be resolved. The surface between these faces is rounded. The orientation of the visible faces with respect to the substrate appears to be random. Only the part of the crystal surface not in contact with the substrate, as shown in Fig. 1, will be discussed.
Further growth of germanium on a crystal in Fig. 1(a) makes certain faces larger and the rounded surface smaller. Fig. 2 shows the scanning electron micrograph of a Fig. 1(a) crystal with more germanium growth at 700°C. The majority of the surface is covered by {113}, {111}, {110}, and {100} faces. The crystal in Fig. 2 may experience more change in shape and size if the germanium growth were to be continued.
Fig. 3(a) shows a germanium crystal in Fig. 1(b) after a 30 sec. etch in chlorine at 330°C. {113} faces are dominant as in Fig. 2. {111} and {110} faces have mostly disappeared. Between neighboring (113), (131), and (311) faces are sharp edges. These edges can still be seen, as in Fig. 3(b), after a 180 sec. etch. This etch form is clearly different from both the equilibrium form in Fig. 1 and the growth form in Fig. 2.
The etch form of a germanium crystal can depend on the etch conditions. A 560°C etch of a crystal in Fig. 1(b) in hydrogen chloride is shown in Fig. 4. After a 15 sec. etch, the surface is mostly covered by faces that are close to {103} (Fig. 4(a)). These faces are generally rough, somewhat rounded, and partly covered by shallow triangular pyramids. The surface becomes even rougher after a 30 sec. etch (Fig. 4(b)), although the nearly {103} oriented faces and the edges between them can still be resolved.
The crystal forms shown in this work are determined by the atomic processes of growth and etch as well as the diamond structure of germanium. These forms therefore provide important information about the site-specific reactivity on germanium surfaces in growth and etch.

Figure 1