Abrupt Ge-Si and GeSn-Si interfaces by solid phase crystallization

Abstract

Single crystalline germanium has exciting optical and electrical properties, which are promising for electronic and optical applications. Ge has higher carrier mobility for both electrons and holes than Si, which makes this material suitable as channel material for high-speed complementary metal-oxide semiconductor (CMOS) technology. GeSn has been predicted to exhibit carrier mobilities exceeding that of Ge. In addition to improved carrier mobilities, Ge and GeSn show increased optical absorption. This makes these materials much better suited for optoelectronic applications than Si. Epitaxial growth is mostly utilized to obtain a crystalline layer on top of another crystalline material. However, heteroepitaxial growth of Ge on Si is rather complicated because of the large mismatch of 4% between the two lattices. This difference in lattice dimensions leads to island growth, causing high surface roughness and high density of threading dislocations in the Ge layer. A surface roughness of ~25 nm has been reported for 200 nm of Ge on Si grown by chemical vapor deposition (CVD) at 400 °C. Furthermore epitaxial growth at elevated temperatures leads to mixing between the Ge layer and Si substrate which causes a rough Ge-Si interface and even an intermediate GexSi1-x layer. Obtaining high quality and smooth crystalline Ge, directly on Si, is therefore challenging. In this work we demonstrate atomically abrupt Ge-Si interface and smooth surface by using solid phase (hetero)epitaxy (SPE) of amorphous Ge, deposited at low temperature. The excellent structural properties lead to carrier mobilities which are 2 x higher than bulk Si for only 90 nm of Ge, see Table I. Previously we have demonstrated successful solid phase heteroepitaxial growth of amorphous Ge layers on Si substrates. For successful SPE it is important to deposit an amorphous layer without the presence of crystalline grains. Amorphous Ge layers on Si are obtained by limiting the adatom surface mobility and therefore using low temperatures (typically room temperature up to 150 ˚C). These low deposition temperatures are beneficial for the interface and surface roughness. Structural characterization has been carried out using high resolution X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). XRD ω/2ϑ scans shows the appearance of a Ge diffraction peak after thermal annealing, demonstrating solid phase epitaxial growth, see Figure 1. HRTEM of 20 nm Ge shows that the Ge layer is single crystalline, see Figure 2. The interface between the Ge and Si substrate is atomically abrupt and the surface is smooth. Ge twins are present. The low deposition temperature and the crystallization under N2 atmosphere or N plasma effectively keep the Si-Ge interface and Ge surface smooth. This is an important advantage of SPE of Ge in respect with heteroepitaxy, which is performed at much higher deposition temperatures. Both n-type as p-type Ge layers were fabricated by introducing dopants during deposition of the amorphous layer. Phosphine (PH3) was used to incorporate phosphor for n-type doping and diborane (B2H6) to incorporate boron for p-type doping. 90 nm doped germanium layers were deposited on insulating Si(111). After crystallization at 600 °C in N2 ambient, Hall measurements were performed. The excellent structural properties lead to carrier mobilities which are 2 x higher than bulk Si for only 90 nm of Ge. Besides pure Ge we have obtained SPE of Ge1-xSnx layers with about 4% Sn. Reciprocal space mapping (RSM) shows the (331) reflections of Si and GeSn, see Figure 3. The 37 nm GeSn layer has a relaxation of 107% in respect with the Si substrate. The GeSn layer is under small tensile strain as it is slightly offset from complete relaxation (tilted line). This strain can be explained by the thermal mismatch between Si and GeSn at the annealing temperature. The in plane strain is +0.31%. Finally we have obtained p-type GeSn with a hole concentration of 3 x 10^19 cm-3 by introducing Ga dopants.status: publishe