Effect of Growth Temperature on Crystallization of Ge_1−xSn_x Films by Magnetron Sputtering

Abstract

Ge_1−xSn_x film with Sn content (at%) as high as 13% was grown on Si (100) substrate with Ge buffer layer by magnetron sputtering epitaxy. According to the analysis of HRXRD and Raman spectrum, the quality of the Ge_1−xSn_x crystal was strongly dependent on the growth temperature. Among them, the GeSn (400) diffraction peak of the Ge_1−xSn_x film grown at 240 °C was the lowest, which is consistent with the Raman result. According to the transmission electron microscope image, some dislocations appeared at the interface between the Ge buffer layer and the Si substrate due to the large lattice mismatch, but a highly ordered atomic arrangement was observed at the interface between the Ge buffer layer and the Ge_1−xSn_x layer. The Ge_1−xSn_x film prepared by magnetron sputtering is expected to be a cost-effective fabrication method for Si-based infrared devices.

1. Introduction

Ge_1−xSn_x alloys have attracted considerable attention in Si-based optoelectronic device integration because of their enhanced carrier mobility, adjustable band gap structure and compatibility with the CMOS processes [1,2,3,4,5]. Efficient devices based on Ge_1−xSn_x have been fabricated, including thin film transistors [6,7,8,9], solar cells [10], photodetectors [11,12,13], LEDs [14,15,16,17], lasers [18,19], etc. By adjusting the content of Sn and the strain of Ge_1−xSn_x, the direct band gap can be extended to mid-infrared applications [20,21,22]. However, there are some difficulties in preparing Ge_1−xSn_x alloys with high Sn content and good crystal quality on a Si substrate [23,24]. At first, the equilibrium solid solubility of Sn in Ge is as low as about 0.5% [25]. Secondly, the phenomenon of Sn segregation easily appears during fabrication. Thirdly, the lattice mismatch between Ge and α-Sn is about 14.7% [26]. Thus, determining how to achieve a high Sn content and good quality Ge_1−xSn_x alloy is critical for fabricating high-performance Ge_1−xSn_x photoelectronic devices.

So far, researchers have successfully prepared Ge_1−xSn_x alloy films using molecular beam epitaxy (MBE) [27,28,29], chemical vapor deposition (CVD) [30,31,32], magnetron sputtering [33,34,35,36] and solid phase crystallization [37,38]. Among these methods, MBE is a method that can accurately control the thickness and structure of thin films. However, once the layer thickness exceeds the critical thickness [39,40], the screw dislocation begins to appear, which is caused by lattice mismatch, thus affecting the crystal quality of the material. CVD can release misfit dislocations at the interface rather than forming through dislocations, so the thin film materials epitaxy by this technology have perfect crystal quality [41,42,43]. Magnetron sputtering is a low-cost and feasible method for Ge_1−xSn_x mass production. Qian, L. et al. used Ge (100) and GaAs (100) substrates with high lattice matching to obtain a Ge_1−xSn_x layer with good crystal quality by sputtering [44,45,46]. Grant, J. et al. adopted a CMOS compatible Si (100) substrate, and the Ge_1−xSn_x layer obtained by sputtering was polycrystalline [34,47,48]. There are few published papers about obtaining single crystal Ge_1−xSn_x on silicon substrate by sputtering. Zheng Jun et al. [24] realized Ge_1−xSn_x single crystal film with low Sn content on Ge/Si (100) substrate, in which Ge was the buffer layer, the deposition temperature was 400 °C, the deposition temperature of Ge_1−xSn_x films was 150 °C, and the highest Sn content was 0.06. Tsukamoto, T. et al. [49] deposited single crystal Ge_1−xSn_x film with x of about 0.115 on Si (100) substrate by magnetron sputtering without a buffer layer, and the deposition temperature was 250 °C. At present, single crystal Ge_1−xSn_x alloy films deposited on Si (100) substrate by epitaxial sputtering have the problems of many dislocations and low Sn doping, and the highest Sn doping is 0.115 [26,49,50]. Generally, when x is greater than 0.1, the energy band of Ge_1−xSn_x will be transformed into a direct band gap, so it is necessary to further increase the Sn content of single crystal Ge_1−xSn_x grown on silicon substrate by sputtering.

In this paper, we report the growth of Ge_1−xSn_x crystalline films on Si (100) substrates by sputtering. We have studied the possibility of growing single crystal Ge_1−xSn_x with Sn content more than 10% on Ge buffer layer. The Ge buffer layer was deposited at 300 °C, with a thickness of 572 nm. By adjusting the deposition temperature of Ge_1−xSn_x at 180~300 °C, single crystal Ge_1−xSn_x alloy films were obtained in which the highest Sn content (at%) reached 13%.

2. Materials and Methods

Ge_1−xSn_x epitaxial layers were deposited in a physical vapor deposition (PVD) system manufactured by ULVAC Corporation. Figure 1 is a schematic diagram of Ge_1−xSn_x deposition, in which Ge target and Sn target are installed at different target positions and sputtered at the same time. The substrate was placed on the bottom sample table, which can rotate horizontallyand move up and down and heat The sputtering rate of thin film was determined by sputtering power, sputtering distance, etc. The film thickness was obtained by scanning electron microscope profile test. Before deposition, we cleaned and activated the surface of the silicon substrate. Firstly, it was washed with sulfuric acid (H₂SO₄:H₂O₂ = 7:3) solution for 10 min and washed with deionized water 10 times to remove organic and inorganic pollution. Then, it was soaked in diluted HF (HF:H₂O = 1:10) solution at room temperature for 30 s to remove the oxide layer on the surface. Finally, it was washed with deionized water 10 times and dried with nitrogen. The base pressure of the sputtering chamber was less than 4.0 × 10⁻⁴ Pa. Then, the substrate was quickly loaded into the growth chamber. After the vacuum degree reached 4.0 × 10⁻⁴ Pa, the substrate was heated at 300 °C for 1 h, and then the Ge buffer layer was deposited with a thickness of about 572 nm. Then, different Ge_1−xSn_x alloy films were deposited on the Ge buffer layer by changing the temperature of the substrate to 180 °C, 200 °C, 225 °C, 240 °C, 250 °C, 260 °C, 275 °C and 300 °C, respectively. The Ge_1−xSn_x film was deposited by Ge and Sn co-sputtering, and its thickness was about 350 nm. In the process of sputtering deposition, argon was filled to make the chamber pressure reach 0.4 Pa. The composition of the Ge_1−xSn_x films was achieved by maintaining a constant DC power of 130 W for the Ge target, and RF power of 40 W for the Sn target. The deposition rate of Ge and Sn were controlled about 1.2 Å/s, 0.22 Å/s, respectively. For comparison, the sample with only a Ge buffer layer was deposited under the same conditions as the samples with Ge_1−xSn_x alloy film.

The properties of the Ge buffer layer and the GeSn thin film were tested and analyzed by high-resolution X-ray diffraction (HR-XRD, X’pert PRO, PANalytical, Suzhou, China), Raman scattering (Raman, Labram HR 800,HORIBA Jobin Yvon, Suzhou, China), atomic force microscope (AFM, Dimension ICON, Bruker, Suzhou, China) and high-resolution transmission electron microscope (HRTEM, Talos F200X, Thermo Fisher Scientific, Suzhou, China). Among them, HR-XRD measured the 2 θ-ω diffraction peaks of (004) and (224) planes, with a step size of 0.05°, a time per size of 1s, and a scanning range of 60–72° and 78–92°, respectively. It was used to study the crystallization quality of single crystal films and calculate the Sn content according to the diffraction peak position and other information. A Raman scattering experiment was carried out at room temperature, the spectral line was 532 nm, exposure time was 5 s, the accumulation numbers were twice, and the scanning range was 200–350 cm⁻¹. It was used to characterize the crystal quality and Sn composition of Ge_1−xSn_x film on the surface. AFM was used to characterize the surface roughness and morphology of Ge_1−xSn_x thin films in tapping mode with a scanning range of 5 μm × 5 μm, and to analyze the crystal particle size and morphology of the films. The working voltage of HRTEM is 200 kV. By observing the (110) surface of the sample, the atomic-scale material properties such as the lattice matching between materials, the crystal quality of single-layer materials and the composition ratio of alloy films could be analyzed.

Table 1. Out-of-plane (${\mathsf{\alpha }}_{\perp }$) lattice constants, in-plane (${\mathsf{\alpha }}_{\parallel }$) lattice constants, unstrained lattice constants (${\mathsf{\alpha }}_0$), measured Sn content (x_m), degree of strain relaxation (R) and in-plane strain (${\mathsf{\epsilon }}_{\parallel }$) of the Ge_1−xSn_x samples.

Growth Temperature (°C)	${\mathsf{\alpha }}_{\perp }$ (nm)	${\mathsf{\alpha }}_{\parallel }$ (nm)	${\mathsf{\alpha }}_0$ (nm)	xm (%)	R (%)	${\mathsf{\epsilon }}_{\parallel }$ (%)
180	0.57702	0.56998	0.57399	9.98	41.1	−0.007
200	0.57676	0.56990	0.57381	10.25	40.9	−0.007
225	0.57860	0.57688	0.57786	14.29	91	−0.002
240	0.57871	0.57738	0.57813	15.38	93.2	−0.001
250	0.57826	0.57616	0.57735	13.79	88.2	−0.002

shows the parameters of the Ge_1−xSn_x samples, such as out-of-plane (${\mathsf{\alpha }}_{\perp }$) lattice constants, in-plane (${\mathsf{\alpha }}_{\parallel }$) lattice constants, unstrained lattice constants (${\mathsf{\alpha }}_0$), measured Sn content (x_m), degree of strain relaxation (R) and in-plane strain (${\mathsf{\epsilon }}_{\parallel }$). These parameters were calculated according to the HRXRD test results. According to Bragg’s law, the out-of-plane and in-plane lattice constants of the Ge_1−xSn_x layers were extracted from the positions of the peaks of GeSn (004) and GeSn (224), which are given by:

$${\mathsf{\alpha }}_{\perp }=\frac{2\mathsf{\lambda }}{{\sin \mathsf{\theta }}_{004}}$$

(1)

$${\mathsf{\alpha }}_{\parallel }=\frac{\sqrt{2}\mathsf{\lambda }}{\sqrt{{\sin }^2{\mathsf{\theta }}_{224}{ - \sin }^2{\mathsf{\theta }}_{004}}}$$

(2)

Then, the Sn composition of Ge_1−xSn_x layers could be calculated by simultaneously solving the Vegard’s law and the Poisson’s relationship, in which the bowing parameter of 0.004 nm in the Vegard’s law was considered [51], and the elastic constants of Ge_1−xSn_x layers in Poisson’s relationship was obtained by the linear interpolation of Ge and Sn. The R and ${\mathsf{\epsilon }}_{\parallel }$ were calculated according to the following equation:

$${\mathsf{\alpha }}_0=\frac{{\mathsf{\alpha }}_{\perp }+\frac{{2\mathrm{C}}_{12}}{{\mathrm{C}}_{11}}{\mathsf{\alpha }}_{\parallel }}{1+\frac{{2\mathrm{C}}_{12}}{{\mathrm{C}}_{11}}}{=\mathsf{\alpha }}_{\mathrm{Ge}}\left(1-\mathrm{x}\right){+\mathsf{\alpha }}_{\mathrm{Sn}}\mathrm{x}+\mathrm{bx}(1-\mathrm{x})$$

(3)

$$R=\frac{{\mathsf{\alpha }}_{\parallel }-{\mathsf{\alpha }}_{\mathrm{sub}}}{{\mathsf{\alpha }}_0-{\mathsf{\alpha }}_{\mathrm{sub}}}$$

(4)

$${\mathsf{\epsilon }}_{\parallel }=\frac{{\mathsf{\alpha }}_{\parallel }-{\mathsf{\alpha }}_0}{{\mathsf{\alpha }}_0}$$

(5)

where ${\mathsf{\alpha }}_{\mathrm{sub}}$ and ${\mathsf{\alpha }}_0$ are the lattice constants of the Ge Buffer and unstrained Ge_1−xSn_x layer, respectively.

As can be seen from Table 1, the content of Sn in the Ge_1−xSn_x film deposited at 240 °C is the highest, with the calculated value of 15.38%, which is 0.02 larger than the EDX test result of 13%, and the strain relaxation degree is also the highest, with the value of 93.2. The in-plane strain results show that all Ge_1−xSn_x films have small compressive stress, and the strain of Ge_1−xSn_x deposited at 240 °C is the largest. The results show that the unstrained lattice constant and stress increase with the increase of Sn content.

The Ge–Ge LO Raman peak (${\mathsf{\omega }}_{\mathrm{Ge}–\mathrm{Ge}}$) of Ge_1−xSn_x can be influenced by the Sn composition (x) and in-plane strain (${\mathsf{\epsilon }}_{\parallel }$) as:

$${\mathsf{\omega }}_{\mathrm{Ge}–\mathrm{Ge}}{=\mathsf{\omega }}_{0 }{+\mathrm{ax}+\mathrm{b}\mathsf{\epsilon }}_{\parallel }$$

(6)

where ${\mathsf{\omega }}_0$ is the bulk Ge Raman frequency at 301 cm⁻¹, and the coefficients $\mathrm{a}$ and $\mathrm{b}$ are chosen as −95.1 and −435.6 cm⁻¹, respectively [52,53]. According to the EDX test, the Sn content of the sample grown at 240 °C is about 13%, and the calculated Ge–Ge is 288.2 cm⁻¹, which is close to the Raman measurement value of 287.5 cm⁻¹.

3. Results and Discussion

Figure 2 shows the X-ray diffractograms of the Ge_1−xSn_x films deposited on buffered Ge layers in different conditions. The (004) and (224) plane XRD diffraction peaks of Si, Ge and GeSn are obvious in Figure 2. When the deposition temperature is between 180–250 °C, there is a diffraction peak of GeSn. This shows that the Ge_1−xSn_x alloy films under these process conditions are single crystal films. With the increase of deposition temperature, the diffraction peak intensity of Ge_1−xSn_x increases, and the position of diffraction peak decreases. At 240 °C, the intensity of the diffraction peak is the highest, and the position of the diffraction peak is the lowest, which indicates that the preferred orientation of Ge_1−xSn_x thin film crystal grown under this condition is obvious, and the Sn content is the highest. When the temperature rises to 260–300 °C, the diffraction peak of Ge widens, which indicates that the diffraction peak position of Ge_1−xSn_x is close to Ge; that is, the content of Sn drops sharply.

As we all know, Raman measurement is a common surface detection technology. By analyzing the intensity, peak position change and full width at half maximum of the Raman peak, the information of material composition, strain and crystal quality of semiconductor materials can be determined. Figure 3 shows the Raman spectrum of the Ge buffer layer and the Ge_1−xSn_x thin films prepared at different deposition temperatures. The spectrum of the Ge buffer can be seen to consist of one strong Ge–Ge LO Raman peak at 301.87 cm⁻¹. Similarly, the spectra of the Ge_1−xSn_x films consists of a strong Ge–Ge LO Raman peak between 287.5 cm⁻¹ and 290.6 cm⁻¹. However, with the decrease of the deposition temperature of Ge_1−xSn_x, the Ge–Ge LO Raman peak in Ge_1−xSn_x alloys gradually widens and moves to a higher wavenumber. The Ge–Ge LO Raman peak of the thin film deposited at 240 °C is the lowest, which is in consistent with the results of HRXRD.

In order to determine the surface morphology of the Ge_1−xSn_x layer, atomic force microscopy (AFM) measurement was performed. Figure 4a–i shows the typical 5 µm × 5 µm AFM images of the Ge Buffer layer and Ge_1−xSn_x layers, and the RMS (root mean square roughness) value of the Ge_1−xSn_x samples was extracted from AFM scans. It was found that the root mean square roughness (Rq) value of Ge Buffer was 1.21 nm and that of Ge_1−xSn_x samples was 4.42–41.9 nm, respectively. Figure 5 shows the comparison results of Rq values of Ge and Ge_1−xSn_x samples. With the increase of temperature, the roughness increases at first, decreases at 200 °C and then increases continuously. When the temperature is higher than 260 °C, the roughness increases by an order of magnitude, and the surface of the sample is silvery white and evenly distributed. It can be explained that the increase of deposition temperature intensifies the segregation of Sn; the diameter of meta-Sn particles is about 2 μm.

In order to further characterize the crystallinity, XTEM measurement was carried out on the Ge_1−xSn_x alloy film deposited at 240 °C. As shown in Figure 6a, the interfaces between Ge_1−xSn_x/Ge buffer/Si substrate were clear and recognizable. Figure 6b confirms the relaxation characteristics of the Ge buffer layer and clearly proves that the threading dislocations appear in the Ge buffer layer due to the mismatch between Ge and Si. In Figure 6c, the interface between the Ge_1−xSn_x/Ge buffer layer, atomic observation shows a highly ordered atomic arrangement in the interface. In Figure 6d–f is fast Fourier transformation (FFT) patterns of Ge_1−xSn_x, Ge_1−xSn_x/Ge buffer layer interface and Ge buffer layer. Diffraction spots are typical face-centered cubic [011] patterns, and clear lattice fringes of Ge and the Ge_1−xSn_x layer indicate that single-crystal films are grown by sputtering and the Ge_1−xSn_x layer is coherent with the Ge crystal.

Electron dispersive X-ray (EDX) spectroscopy was performed to identify the elemental composition of the Ge and Ge_1−xSn_x films, and the results are shown in Figure 7. Figure 7a shows a spectral scan of the concentration values of uniformly distributed Sn, Ge and Si. Figure 7b shows the line scan of the concentration values of Sn, Ge and Si, where the Sn content (at%) is about 12.6–13.06%. According to the content distribution of Sn, there is no Sn segregation on the surface of the Ge_1−xSn_x film. Compared with references of single crystal Ge_1−xSn_x deposited by magnetron sputtering on silicon substrate, the tin content is the highest in this work, and the specific data are shown in

Table 2. Comparison of the Sn content of crystalline Ge_1−xSn_x with those found in other references.

Reference	Sn Content (at%)	Buffer
[26]	6	Ge
[49]	11.5	None
[50]	3	Ge
This work	12.6–13.06	Ge

.

In this work, Ge_1−xSn_x alloy thin films were prepared by a Ge–Sn co-sputtering principle, and the Ge target and Sn target were mounted on two targets, respectively. The sputtering power of Ge and Sn was controlled to achieve different atomic ratios of Sn. The temperature of the substrate can be adjusted. The higher the temperature, the higher the mobility energy of Sn atoms, and the easier it is to move to the appropriate position. Therefore, with the increase of temperature, Sn atoms occupied the position of Ge atoms forming a Ge_1−xSn_x alloy, and with the increase of temperature to 240 °C, the Sn doping peak was reached. When the temperature continued to rise, segregation occurred. When the temperature rose to 260 °C, segregation intensified, Sn atoms migrated to the surface of the sample, and the Sn content of Ge_1−xSn_x alloy decreased.

4. Conclusions

In this paper, the structural characteristics of Ge_1−xSn_x thin films at different substrate temperatures were studied in detail. XRD, Raman, TEM and AFM analysis showed that the coherent growth of Ge_1−xSn_x and Ge can be achieved by adjusting the substrate temperature, and the Sn content in the Ge_1−xSn_x film deposited at 240 °C was the highest, with the Sn content (at%) reaching 13%. These results show that single crystal Ge_1−xSn_x films can be grown on Si (001) wafer by sputtering at a temperature near the melting point of Sn, and its Sn content is the highest among GeSn prepared by sputtering on Si (001) substrate reported at present.