Structural evolution of Ge-rich Si_1−xGe_x films deposited by jet-ICPCVD

Extensive works on nanocrystalline Si (nc-Si)^1,2 and Ge (nc-Ge)^3,4 film materials have been conducted for more than two decades. Increasing attentions have been given to nc-Si_1−xGe_x film,^5–7 which shows numerous potential applications, such as transistors,^8,9 photovoltaic¹⁰ and infrared detection,¹¹ due to its tunable optoelectronic properties. It is most significant to understand and control the microstructures of nc-Si_1−xGe_x films. Plasma enhanced chemical vapor deposition (PECVD) technology, where high density plasmas serves as an effective source to provide enough ions and radicals, has been most widely used in fabricating of semiconductor films. S. Oda et al. demonstrated the controllable nucleation and growth of nc-Si by adjusting time of pulsed plasma.^12,13 Kortshagen’s work suggested that the residence time (the time during which the source gas travel through the plasma region) is one of the most important factors which affect the size and property of the fabricated nanocrystals.¹⁴ Lots of works on the formations of nc-Si_1−xGe_x alloy suggested that it usually consists of hybrid crystalline structures, and more complex than that of nc-Si.^15–18 Y.M. Yang et al. adjusted the proportion of Ge and Si sources in magnetron sputtering and obtained the complex films containing various crystalline structures, namely, Ge nanocrystals, Ge/SiGe core/shell, and nc-Si_1−xGe_x alloy.¹⁹ Our previous work developed the deposition technique that combines very high-frequency inductively coupled plasma with gas-jet (jet-ICPCVD) in preparing amorphous and microcrystalline Si films.²⁰ In the present work, we modified the jet-ICPCVD system by separating the inlet of the source gases to adjust the residence time of Ge and Si precursors, respectively, then deposited Ge-rich Si_1−xGe_x films, and obtained Ge/SiGe core/shell nano-structure through further thermal annealing. On the basis of the experimental results of the microstructure characterizations, such as Raman, scanning electron microscope (SEM), x-ray diffraction (XRD), and energy disperse spectroscopy (EDS), we reveal the structural evolution of the Ge-rich Si_1−xGe_x films with a hybrid amorphous/microcrystalline structure in the annealing treatment process.

This work adopted dual-source jet-ICPCVD system to deposit Si_1−xGe_x films on silicon and quartz substrates (FIG. 1). Germane (20% diluted in hydrogen) accessed the upper road entrance, dissociated, and excited completely after going through the plasma region, thereby producing a large number of Ge precursors. Silane (20% diluted in hydrogen) accessed the lower road entrance. The fraction of Ge and Si was controlled by the flow ratio of germane, silane, and hydrogen. In this work, the flow rates of germane, silane, and hydrogen were varied in the range of 2 to 8, 2 to 6, and 60 to 90 sccm, respectively. The pressure in the chamber was approximately 100 Pa. The thickness of the deposited films was about 1 μm estimated from the SEM cross-sectional observation. The isochronal and isothermal annealing treatments under a vacuum condition were conducted. The isochronal annealing temperatures (Ta) ranged from 300 °C to 900 °C. Raman spectroscopy (Horiba HR800, with a laser wavelength of 514.5 nm) and XRD (RigakuD/max 2500VL/PC) were used to analyze the crystallization, and EDS (Horiba EX-250) to obtain the components in the films.

FIG. 2(a) shows the Raman spectrum of the deposited Ge-rich Si_1−xGe_x (x = 0.7) film through isochronal annealing treatments from 300 °C to 900 °C. No obvious peak is observed except for a broad band centered at ∼270 cm⁻¹ when Ta is below 500°C. This fact implies that the as-deposited film is mainly amorphous. When Ta is about 500°C, the peaks located at 285, 396.5 and 465 cm⁻¹ appear slightly. These peaks correspond to the optical phonon mode of Ge–Ge, Ge–Si, and Si–Si in the Si_1−xGe_x alloy,²¹ respectively, thus indicating that nc-Si_1−xGe_x begins to form. Heating the sample continuously, the related peak intensity is enhanced, and slight blue-shift occurs in all phonon modes. When Ta increases from 800 °C to 900 °C, the enhanced intensity and the less asymmetry of Ge-Ge mode indicate well crystallization of the film. The phenomena that the intensity of Ge-Ge mode increases whereas Si-Si mode nearly disappears suggest that the nanocrystals are locally Ge-rich, which is in agreement with similar reports.^22,23 Simultaneously, three optical phonon modes show obvious frequency shift (Table I), where the Ge–Ge mode is blue-shift, the Ge–Si and Si–Si modes are red-shift. FIG. 2(b) gives the Raman spectrum of film samples after isothermal annealing at Ta = 900 °C. As the annealing time increases, the peak intensities and frequencies of the peaks in the Raman spectrum change obviously, reflecting the similar trend annealed from 800 °C to 900 °C. During the deposition process, with longer residence time, Ge precursors have much more chance to nucleate. Furthermore, theoretical simulations have revealed that there is a tendency for clustering of like-atom bonds in amorphous Si_1−xGe_x alloy.²⁴ We may imagine that there are a large number of Ge-rich regions in the as-deposited films, and when Ta reaches 500 °C, these Ge-rich regions act as centers of crystallization to form Ge clusters. For a hybrid-structured SiGe film where Si and Ge clusters co-exist in amorphous phase, C. Tzoumanekas et al. demonstrated that these clusters mutually penetrate with the decreasing structural disorder during crystallization and then fuse into the Si and Ge core.²⁵ In the Ge-rich samples used in this work, Si atoms are much fewer and more easily spread. As a result, Si atoms diffuse preferentially and redistribute prior to forming the Ge/SiGe core/shell structure during the high-temperature annealing process. Obviously, the Ge core is affected by compress strain, meanwhile, the surrounding SiGe layer is affected by tensile stress. Consequently, the blue-shift in Ge-Ge mode where the red-shift in Ge-Si and Si-Si modes occurs, and the average frequency of red-shift is much larger than blue-shift considering the Ge-to-Si atomic ratio in the Ge-rich Si_1−xGe_x films.

Phonon confinement is an important factor for Raman spectrum. According to the standard confinement model,²⁶ the first-order Raman spectrum I(ω) can be described as

where q is expressed in units of 2π/a, d is the mean size, a is the lattice constant, Γ is natural linewidth (for Ge, a_Ge = 0.565 nm, Γ ≈ 3.5 cm⁻¹ and ω₀ = 300.5 cm⁻¹). ω(q) is the dispersion relation for optical phonons in c-Ge which is given by

where A = 1.578 × 10⁵ cm⁻² and B = 1.000 × 10⁵ cm⁻². These parameters have been determined experimentally by neutron scattering.²⁷ By using the above equations, we calculate the average size d of Ge clusters from the full width at half maximum (FWHM) of Ge-Ge mode, as shown in FIG. 3. One can see that grain size increases with Ta, in agreement with the previous reports.²⁸ As a result, the Ge-Ge mode in Raman spectrum blue-shifts, which is a typical character of confinement effect.

Furthermore, we analyze the crystallization of the Si_1−xGe_x film and the variations of SiGe nanocrystal size during the annealing process by using XRD measurement. FIG. 4 shows the XRD spectrum of these samples. Three main peaks at 27.7°, 46.1°, and 54.5° are related to <111>, <220>, and <311> diffraction, respectively.²⁹ When Ta increases from 600 °C to 700 °C, the intensity of the <111> peak is enhanced, thus showing that the crystallization rate of the film grows. This result is consistent with the Raman measurement result in FIG. 2(a). Table II lists the <111> FWHM of samples in different Ta. The FWHM decreased at Ta = 700 °C, indicating that both the crystallization rate and grain size increase. However, it is noted that the FWHM does not change obviously with increasing Ta continuously, which is not fit with the Raman result. The discrepancy may be attributed to the different depth of detection between XRD and Raman measurements. In general, the XRD characterize the whole thickness of the film, whereas the Raman reflect only a certain thickness (tens nm) beneath the surface.

In the deposition and annealing process of Si_1−xGe_x alloy, Ge atoms partially migrate to the sample surface, because the covalent bond of Ge is longer and its surface energy is lower than those of Si.^30–32 To determine the atom concentrations of Si and Ge in the sub-surface of the films, we conducted EDS measurements on the samples. The EDS effective depth is approximately 100 nm on the basis of the electron accelerating voltage. FIG. 5 gives the dependence of the composition in the sub-surfaces of these samples on Ta. When Ta is below 900 °C, the atom concentrations of Ge and Si show no obvious changes. When Ta increases up to 900 °C, however, the Ge fraction increases from 67% to 73%. Measurement results suggest that the annealing at Ta ≥ 900 °C would induce the Ge atoms to segregate and partially diffuse toward the surface owing to the lower surface energy.³³ Consequently, higher fraction of Ge causes the increases for the frequency and intensity of the Ge–Ge mode, and decrease for those of the Si–Si mode.

To visually dipict the structural evolution of samples in annealing process, we give a schematic of the film section (FIG. 6). As-deposited SiGe films are mainly amorphous phase, simultaneously, and large amounts of Si and Ge-rich (mostly Ge-rich) regions are embedded in the amorphous SiGe matrix. As Ta increases up to 500 °C, the film begins to crystallize and form Si and Ge clusters (FIG. 6(a)). With raising Ta continually, the size of nanocrystals grows and the intensities of all phonon modes in the Raman spectrum increase (FIG. 6(b)). Due to the influence of confinement effect, the frequency blue-shifts slightly. When Ta = 900 °C, Ge clusters merge into Ge cores, whereas Si disperse in SiGe shells around the Ge cores (FIG. 6(c)). At the same time, the Ge atoms partially diffuse to the surface. These structural evolutions result in the increases for the frequency and intensity of Ge–Ge phonon mode, meanwhile, the decrease for the frequencies and intensities of both the Ge–Si and Si–Si phonon modes.

A dual-sources Jet-ICPCVD technique has been used to deposit Si_1−xGe_x films at low temperatures. By separating the inlets of germane and silane, different residence time of Ge and Si precursors is obtained. Ge precursors get longer residence time than that of Si precursors, and have more change to nucleate consequently. As a result, there are a large number of Ge-rich regions in the as-deposited films. When Ta reaches 500 °C, Si_1−xGe_x films crystallize and the Ge-rich regions act as crystallization centers to form Ge clusters. As raising Ta continually, the size of the nanocrystals increases and a transformation from clusters to composite nanocrystals with Ge core and SiGe shell is revealed. The formed core/shell structure induces the interact strain between the Ge core and the Ge-Si shell. When Ta increases up to 900 °C, part of Ge atoms diffuse to the film surface as a result of segregation. The confinement effect, strain and fraction change are discussed as the factors which cause the frequency shifts of the Raman phonon modes. The present work can help us understand the formation of nc-Si_1−xGe_x nanostructures and guide us in further exploring their applications.

The authors acknowledge the financial support from the National Basic Research Program of China under Grant Nos. 2013CB932900 and 2011CB922100, NSFC under Grant Nos. 61205057 and 61204050, and NSFJS under Grant Nos. BK2011011 and BK20130055.