Growth of GeSi nanoislands on nanotip-patterned Si (100) substrates with a stress-induced self-limiting interdiffusion
© Tang et al.; licensee Springer. 2012
Received: 15 March 2012
Accepted: 17 June 2012
Published: 26 June 2012
GeSi nanoislands grown on nanotip pre-patterned Si substrates at various temperatures are investigated. Nanoislands with a high density and narrow size distribution can be obtained within an intermediate temperature range, and the Ge atom diffusion length is comparable to half of the average distance of the Si nanotips. The Ge concentration distributions at the center and edge of the GeSi nanoislands are measured by scanning transmission electron microscopy. The results reveal that there is a Si core at the center of the GeSi nanoisland, but the Ge concentration presents a layered distribution above the Si nanotips. The radial component of the stress field in Ge layer near the Ge/Si interface on the planar, and the nanotip regions is qualitatively discussed. The difference of the stress field reveals that the experimentally observed concentration profile can be ascribed to the stress-induced interdiffusion self-limiting effect of the Si nanotips.
KeywordsGeSi nanoislands Nanotip pre-patterned Si substrates Ge concentration distribution Stress-induced interdiffusion self-limiting effect PACS 68.35.Gy 61.46.-w 66.30.Pa
The self-organized growth of semiconductor nanodots has attracted considerable interest. Fundamental investigations on their structural, electronic, and optical properties, as well as on their potential use in electronic and optoelectronic devices, have been extensively conducted [1–5]. Among the broad range of semiconductor material systems, group IV semiconductor (Si and Ge) nano/heterostructures are distinctive in that they offer material compatibility and facilitate integration with well-established Si-based technology [6–13]. Ge possesses a number of properties superior to those of Si in device applications, e.g., higher carrier mobility and larger exciton radius, which result in a stronger quantum confinement within the nanostructure and the possibility of low processing temperatures. Thus, Ge can be easily integrated with conventional devices. Among the fabrication methods of Ge nanostructures, the strain-driven formation of Ge nanoislands on Si during epitaxial growth has been wide studied for both plain and patterned substrates. This process is a promising way of fabricating self-assembled nanoislands via the Stranski–Krastanov growth mode [2, 14–18]. On plain substrates, islands nucleate on randomly distributed sites. The morphological island evolution depends on a subtle interplay between kinetics and thermodynamics set by the growth conditions . The random character of the formation process results in a broad distribution of island size and concentration .
To narrow down the statistical size distribution of Ge nanoislands, several methods of predefining the nucleation sites have been reported, including buried stressors [21, 22], pre-patterned SiO2 layers , as well as pit- [8, 12] and stripe-patterned substrates . However, for most of the pre-patterned methods, a sub-100 nm distance of nucleation sites is difficult to reach. Thus, the growth temperature of Ge nanoislands with a narrow size distribution must be sufficiently high to match the diffusion length of Ge ad-atoms on Si and the distance of nucleation sites.
In the present study, nanotip pre-patterned Si substrates with an average intertip distance of 50 nm are fabricated by anodic aluminum oxide (AAO) assistant etching and subsequent thermal desorption in an ultra-high vacuum chemical vapor deposition (UHVCVD) system. Using these nanotip pre-patterned Si substrates, GeSi nanoislands with a high density and narrow size distribution are obtained when the growth temperature is 500°C. The different Ge concentration distributions at the center and edge of the GeSi nanoislands are also investigated. The results reveal that there is a Si core at the center of the GeSi nanoisland, but the Ge concentration presents a layered distribution above the Si nanotips. The mechanism for determining the Ge concentration profile on the Si nanotips is discussed.
The samples were prepared from n-type (100) Si wafers with 0.1 to 1.2 Ωcm resistivity. The fabrication methods of the Si-based nanotips on Si substrates were the same as those detailed in our previous publication . To reduce further the size of the nanotips on Si substrates, the samples were treated with a standard Radio Corporation of America (RCA) cleaning process and thermal desorption procedure at 850°C in an ultra-high vacuum. The nanotip pre-patterned Si substrates were then formed. The surface morphology of AAO was observed by a scanning electron microscopy (SEM) system (LEO 1530, Carl Zeiss AG, Oberkochen, Germany). The surface morphologies of Si-based nanotips on Si substrates and the nanotip pre-patterned Si substrates were examined by an atomic force microscopy (AFM) system (Nanoscope IIIa, Veeco Instruments Inc., Plainview, NY, USA.
The GeSi nanoislands were grown by UHVCVD epitaxy using the nanotip pre-patterned Si substrates described above directly, i.e., there is not Si buffer layer deposited prior to the GeSi nanoislands growth. GeSi islands were grown using pure germane (GeH4) as the gas source with a flow rate of 1 sccm at 450, 500, and 550 °C for 4 min for samples A, B, and C, respectively. The growth rates of samples A, B, and C are approximately 5.5, 6.3 and 7.0 ML/min, respectively. The surface morphologies of the GeSi nanoislands were investigated by AFM. For sample B, the cross-sectional structure of the GeSi islands and concentration distribution at the center and edge position of the GeSi islands were investigated by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM).
Results and discussion
Analogous to growth on planar substrates, the surface diffusion length L of ad-atoms moving on patterned substrates is given by . D is the diffusion constant, τ is the time interval for ad-atom diffusion, a is the lateral motion (3.84 A) corresponding to each hop of an ad-atom diffusion, v is a prefactor (about 1013), E is the effective barrier for ad-atom diffusion on patterned substrates, K B is the Boltzmann constant, and T is the growth temperature. E was assumed to be 1.5 eV, and τ was adopted to be one quarter of the growth time of 1 ML (when, on the average, no nearest neighboring site is occupied by other atoms around each ad-atom to avoid the interaction between ad-atoms during diffusion). The estimated diffusion lengths (L) can be calculated as 11, 23 and 45 nm at the growth temperatures of 450, 500 and 550°C, respectively.
The AFM image in Figure 2b illustrates the surface morphology of sample B grown at 500°C, and GeSi islands with uniform size and distribution can be observed. The GeSi islands are rectangular with widths of approximately 30 ± 2 nm and a density of approximately 5 × 1010 cm−2, which was larger than that of the Si nanotips shown in Figure 1c. This finding may be related to the influence of the Si nanotips. At an intermediate temperature of 500°C, L can be calculated to be 23 nm, which was comparable to d/2. Most deposited Ge ad-atoms can diffuse into the corresponding nanotips on the Si substrate , where the strain energy is relatively low, and then form GeSi islands . This mechanism was also supported by the cross-sectional high-resolution TEM image of sample B in Figure 2d. A GeSi island can be observed at the top of the Si nanotip. The height of the GeSi islands is approximately 5 nm, which well agrees with the AFM image. Given that d is the average distance between Si nanotips, the distance between part of the Si nanotips can be assumed to be larger than d. Thus, part of the Ge ad-atoms cannot diffuse onto the nanotips, and they form GeSi islands on the planar region of the Si substrate. Consequently, the density of the GeSi islands was larger than that of the Si nanotips. The morphology of the GeSi islands is very similar to the hut clusters described in previous reports . According to the reference, we believe that the island edges are oriented along <100 > directions.
Figure 2c shows the AFM image of sample C, which was grown at the relatively high temperature of 550°C. The average diameter increased to approximately 70 ± 20 nm, and the density decreased to approximately 0.5 × 1010 cm−2. These results can be attributed to the larger L (45 nm) at the relatively high growth temperature of 550°C, which is very close to d. In the previous studies, ordered islands will grow when L is comparable to d. We think that the difference of our experiment and the previous studies might be due to the pre-pattern methods. In our case, the height of the nanotips is as small as 1.3 nm, thus the difference of the strain energy on the planar and the nanotip regions is not sufficient for influencing the nucleation of the GeSi nanoislands obviously at a relatively high temperature.
The EDX data also indicated that the Ge concentrations at the center and edge of the GeSi nanoislands were almost the same at the region between 5 and 9 nm. Accordingly, the Ge concentration presented a layered distribution above the Si nanotips. This finding can be explained by the lateral diffusion of Si atoms into the GeSi layer. For the Si nanotip region, the stress-induced interdiffusion self-limiting effect prevented the Ge atoms from diffusing into the Si nanotips. At the same time, the Si atoms were prevented from diffusing into the GeSi layer. Thus, the amount of Si atoms diffusing from the Si nanotips into the GeSi layer were much lower than the amount of Si atoms diffusing from the planar Si substrate into the GeSi layer. There may be a Si concentration gradient in the GeSi layer between the planar Si and nanotip Si regions. Accordingly, the lateral diffusion of Si atoms occurred above the Si nanotips, and the Ge concentrations were uniform on the planar Si and nanotip Si regions.
Uniform self-assembling GeSi nanoislands were fabricated using nanotip pre-patterned Si substrates. The unique features of the GeSi nanoislands were investigated as a function of the growth temperature. These findings were discussed in terms of the surface diffusion length L compared with half of the average distance of Si nanotips on the substrate. The different Ge concentration distributions at the center and edge of the GeSi nanoislands were also investigated by EDX spectra. The results revealed a Si core at the center of the GeSi nanoisland, but the Ge concentration presented a layered distribution above the Si nanotips. The discussion of the radial component of the stress field in Ge layer near the Ge/Si interface on the planar and the nanotip regions can qualitatively explain the experimentally observed Ge concentration distributions at different regions.
RT is a postgraduate student on physics. He researches on the growth of GeSi materials at Xiamen University. Dr. KH works as an associate professor at the Xiamen University. He works on fabrication and the optics behaviors of nanomaterials. Professors HL, CL, ZW, and JK are experts of physics at Xiamen University.
anodic aluminum oxide
ultra-high vacuum chemical vapor deposition
Radio Corporation of America
scanning electron microscopy
atomic force microscopy
transmission electron microscopy
scanning transmission electron microscopy
This work was supported by the National Basic Research Program of China under grant Nos. 2011CB301905, 2012CB933503, National Natural Science Foundation of China under grant Nos. 61108064, 61036003, 61176092, the Fundamental Research Funds for the Central Universities (2011120143), and Ph.D. Programs Foundation of Ministry of Education of China (20110121110025).
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