Etch Pit Formation on Germanium Alloys by in situ Gaseous Hydrogen Chloride Etching

Etch pit formation is an effective method of crystal quality measurement, especially of heteroepitaxial films. Besides a few well-known wet etching methods, gaseous etch by hydrogen chloride has also been shown to reveal semiconductor crystal defects [1]. In this work we show that after etching in gaseous hydrogen chloride, etch pits can form on epitaxial germanium alloys and reveal threading dislocations in these alloys.

The deposition of the epitaxial germanium alloys was conducted on bare Si(001) substrate in a Centura® 300 mm RP Epi chamber from Applied Materials. Commercially available gas sources such as dichlorosilane (SiH₂Cl₂), germane (GeH₄), digermane (Ge₂H₆), and tin tetrachloride (SnCl₄) were used in the thermal chemical vapor deposition of epitaxial germanium, silicon germanium (Si_1-xGe_x), and germanium tin (Ge_1-xSn_x). Etching in hydrogen chloride was performed in the same chamber at a temperature below 400°C.

Before the deposition of germanium alloys, a layer of relaxed germanium was deposited on Si substrate. This layer provides a close lattice match to the germanium alloys to be deposited on top. This also provides a source of threading dislocations for etch pit study. An etch of this relaxed germanium layer in hydrogen chloride shows pitting as revealed by atomic force microscopy (AFM), illustrated in Fig. 1. The pit sizes depend on both etch temperature and time. It is further demonstrated that the locations of threading dislocations in this relaxed germanium layer coincide with etch pits. Fig. 2 shows a plan-view transmission electron microscopy (TEM) image on an etched relaxed germanium surface. The threading dislocations shown by diffraction contrast and the etch pits shown by thickness contrast have a clear overlap. This overlap can only be seen for the large pits with a fairly uniform size of approximately 0.5 μm in Fig. 2(a). Smaller pits, circled in Fig. 2(a), appear uncorrelated with threading dislocations.

On an as-deposit relaxed germanium layer, germanium alloyed with either silicon or tin was deposited. Germanium-rich silicon germanium was deposited at ≤ 500°C. A low temperature (≤ 350°C) process was used to deposit homogeneous and compositionally uniform germanium tin. The alloy film thickness is controlled below its critical thickness so that the film is fully strained with respect to the relaxed germanium layer underneath. Therefore the threading dislocation density in the germanium alloy layer is approximately the same as that on the surface of the relaxed germanium layer.

High-resolution X-ray diffraction (XRD) measurement shows that both silicon germanium and germanium tin are fully strained before and after etch on relaxed germanium. Fig. 3(a) and 3(b) show the ω/2θ scan and the AFM image, respectively, of a post-etch Si_0.14Ge_0.86 on relaxed germanium. The density of the large pits appears close to that on an etched relaxed germanium layer. The silicon germanium surface shows more smaller pits than the relaxed germanium surface.

Fig. 4(a) and 4(b) show the ω/2θ scan and the AFM image, respectively, of a post-etch Ge_0.92Sn_0.08 on relaxed germanium. The density of the large pits is again close to that on an etched relaxed germanium layer.

The authors thank Dr. Hong Zhang of Precision TEM for the TEM analysis.

References: [1] See, e. g. A. Abbadie et al., J. Electrochem. Soc., 154 (8) H713-H719 (2007).