Growth and Characterizations of GeSn Films with High Sn Composition by Chemical Vapor Deposition (CVD) Using Ge2H6 and SnCl4 for Mid-IR Applications
Authors: Zhang Lin , Chen Qimiao , Wu Shaoteng , Son BongKwon , Lee Kwang Hong , Chong Gang Yih , Tan Chuan Seng 
Published: December 2020

Epitaxial GeSn films present several advantages which can provide the semiconductor devices with higher frequency operation [1-2]. GeSn films have been theoretically and experimentally shown to become direct bandgap materials with about 6-10% of Sn composition [3-7]. This opens the room for potential optoelectronic applications such as amplifiers, lasers, photodetectors [8-13]. The red shift in the absorption due to the reduction of band-gap of the GeSn film enables the optoelectronic devices to operate in the mid-infrared (MIR) region [14-16]. However, it is very challenging to incorporate Sn into Ge crystal lattice due to the extremely limited solid solubility of Sn [17]. Therefore, low temperature growth techniques were developed to create non-equilibrium growth conditions so as to incorporate higher Sn content in high quality GeSn layer. In this work, the epitaxial growth of GeSn alloys with Sn content varying from 3.4 to 11% using Ge2H6 and SnCl4 as precursors in a CVD system under reduced pressure condition is reported in detail. Characterisation results are also presented. High quality GeSn film with Sn content as high as 11% has been achieved.
The growth process consists of Ge buffer layer growth followed by GeSn epitaxial growth: 1 µm of Ge buffer layer was grown on 6“ Si wafers in the CVD reactor. Prior to actual growth of Ge, the wafer was baked in H2 at 1000°C for 2 min to remove the native oxide. 10% Ge2H6 diluted in H2 was used as Ge precursor, and H2 worked as the carrier gas. The Ge growth was then carried out at 400°C for ~10 min to target a thickness of 1 µm. The wafers were annealed soon after growth at ~850°C for another 20 min in H2 environment to reduce the threading dislocation density (TDD). Subsequently, SnCl4 was introduced into the chamber. The chamber temperature was kept below 350°C. Table I summarizes the growth conditions for some of the grown GeSn films. GeSn film with Sn content as high as 11% has been achieved successfully.
Atomic force microscope (AFM) images in Figure 1(a)-(d) show that the RMS value of the surface roughness of the GeSn films is 0.52, 1.09, 0.95 and 0.91 nm for sample 1 to 4, respectively. The comprehensive studies showed that higher SnCl4flow rate, lower Ge2H6flow rate or higher growth temperature resulted in higher surface roughness (Figure 1(e)-(g)). Secondary ion mass spectrometry (SIMS) results shown in Figure 2 confirm the existence of Sn element and the Sn content. For sample 1 to 4, the Sn content is 3.4, 8, 10 and 11%, respectively.
The X-Ray Diffraction (XRD) results of GeSn films are shown in Figure 3(a). It revealed that the growth temperature played the most important role in varying the Sn content in the GeSn film. The Sn content value extracted from rocking curve aligned well with the SIMS data. From the RSM mapping shown in Figure 3(b), sample 3 has a film relaxation of ~49% and compressive strain of ~0.7% based on models reported in the literatures [18-20].
Etching pits density (EPD) measurements were carried out to study the threading dislocation density (TDD) of the GeSn films. Figure 4(a) shows the scanning electron microscope (SEM) image of sample 3 (Ge0.9Sn0.1) after EPD. The TDD of Ge0.9Sn0.1 is ~3.33 x 107cm-2. Figure 4(b) summarised that the TDD increases as Sn content increases which is due to the increased lattice mismatch, moreover, rapid thermal annealing (RTA) does not effectively reduce the TDD of the GeSn film.
Raman analysis results are shown in Figure 5. The peak position of sample 3 (Ge0.9Sn0.1) is at 293.7 cm-1. Sn content and compressive strain were calculated to be 10 and 0.7%, respectively, by using the models in literature [21]. The values are consistent with those from XRD RSM and SIMS results. Figure 6 shows the absorption coefficient of GeSn films from ellipsometry analysis. Sn-induced red shift is observed. The cut off wavelength of the absorption extends from ~1,500 nm for bulk Ge to ~2,175 nm for sample 2 (Ge0.92Sn0.08), and it enables the devices fabricated to work in the MIR region.
High resolution transmission electron microscopy (HRTEM) images in Figure 7(a)-(b) confirm that Ge buffer layer has a thickness of ~916.4 nm, while the thickness of GeSn film in sample 3 is ~175.9 nm. Figure 7(c) shows the selected area electron diffraction (SAED) pattern of the GeSn film and it concludes that the film is single crystalline.
We would like to acknowledge support from the National Research Foundation Singapore under the Competitive Research Program (NRF-CRP19-2017-01).

Figure 1