Journal of Applied Spectroscopy, Vol. 76, No. 6, 2009
OPTICAL PROPERTIES OF SILICON-SILICIDE NANOHETEROSTRUCTURES GROWN BY CONSECUTIVE PLASMA-EPITAXY SYNTHESIS N. G. Galkin,a* V. M. Astashynski,b E. A. Chusovitin,a K. N. Galkin,a T. A. Dergacheva,c A. M. Kuzmitski,b and E. A. Kostyukevichb
UDC 533.924;539.23;53.082.53
Consecutive plasma-epitaxial synthesis on silicon wafers is used for the first time to fabricate monolithic nanoheterostructures with embedded nanocrystals (NC) of chromium disilicide (Si–NC CrSi2–Si). It is found that, initially, nanoislands form on the surface and within a coating layer of silicon, followed by the formation of small (10–15 nm) nanocrystals of semiconducting chromium disilicide (CrSi2) at a high occupation density ((2–3)⋅1011 cm–2). During formation of silicon-silicide-silicon heterostructures, CrSi2 nanocrystallites "float up" into the near surface area of the covering silicon layer. Keywords: silicon, chromium disilicide, compression plasma flow, nanoheterostructure synthesis, optical properties, interband transitions. Introduction. It has recently been shown [1,2] that when a compression plasma flow acts on silicon wafers, volume periodic structures develop in the form of nanosized wires with diameters of 200–2000 nm and lengths ≤500 μm. The morphological features of these structures and possible reasons for their formation have been analyzed [1-4]. The processing of silicon wafers by compression flows loaded with finely dispersed metallic particles provides for the simultaneous synthesis, within a single magnetoplasma compressor (MPC) discharge, of volume periodic structures and a continuous nanostructure coating formed by metallic clusters with diameters of 50–300 nm consisting of nanoparticles with sizes of 15–30 nm [5]. Later on, it was established experimentally [6, 7] that the formation of these quasi-one dimensional wires on the surface of a silicon wafer leads to substantial changes in its crystal structure and optical properties owing to the increase in the lattice constant as it is compressed by the compression plasma flow. This article is a study of the morphology and optical properties of hybrid silicon-silicide nanoheterostructures with embedded nanocrystals (NC) of CrSi2 first obtained on silicon wafers by a newly developed method for consecutive plasma-epitaxial synthesis. The proposed method involves preliminary synthesis of surface volume structures (nanowires) on silicon wafers by a compression plasma flow with consecutive epitaxial growth under ultrahigh vacuum conditions, first of chromium silicide nanoislands, and then of a nanosized coating of silicon over these nanostructures. Hybrid silicon-silicide nanostructures of this sort on silicon wafers may be of interest for the development of new opto-, micro-, and nanoelectronic components. Experiment. Compression plasma flows were generated by a compact gas-discharge quasistationary MPC plasma source [8] with energy storage in a 1200 μF capacitor bank. The MPC operated in a residual gas regime with the pumped-down vacuum chamber filled with the working gas (nitrogen) to a specified pressure of 100–1300 Pa. Varying the initial voltage on the capacitor bank from 2.8 to 4.0 kV increases the amplitude of the discharge current from 40 to 70 kA; this yields a compressive plasma flow of length 6–12 cm with a diameter of 1–2 cm for the maximally compressed zone at the outlet of the discharge device. The plasma flow is compressed as a result of the interaction of the longitudinal component of the discharge current carried by the plasma from the discharge structure of the ∗
To whom correspondence should be addressed. a
Institute for Automation and Control Processes, Far Eastern Branch of the Russian Academy of Sciences, 5 b Radio Str., 690041, Vladivostok, Russia; e-mail:
[email protected]; B. I. Stepanov Institute of Physics, National c Academy of Sciences of Belarus, Minsk, Belarus; Far Eastern State University, Vladivostok, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 6, pp. 891–897, November–December, 2009. Original article submitted August 24, 2009. 840
0021-9037/09/7606-0840 ©2009 Springer Science+Business Media, Inc.
TABLE 1. Parameters of the initial silicon samples Sample
Substrate
U0, kV
Conductivity type
ρ, Ω⋅cm
A1–A6 B1–B4
Si(111) Si(111)
3.0–3.8 3.0–3.6
p p
1.0 10.0
C1–C4 D1–D4
Si(111) Si(100)
3.0–3.6 3.0–3.6
n p
0.3 4.5
MPC with the intrinsic azimuthal magnetic field; this provides the elevated parameters of the compression plasma flow. The compression plasma flow is stable for about ~100 μsec and then begins to decay. Depending on the initial 6 parameter of the MPC, the flow velocity of the compression plasma flow varies over (4–7)⋅10 cm/sec. The charged 17 particle density and temperature in the maximally compressed region of the compression plasma flow reach (1–10)⋅10 –3 cm and 1–3 eV, respectively [8]. A series of 18 samples were prepared during the experiments at the Institute of Physics of the National Academy of Sciences of Belarus. Samples of high purity n and p type single crystal silicon with (111) and (100) substrate orientations and different specific resistivities (0.3, 1.0, 4.5, and 10.0 Ω⋅cm) were processed by compression plasma flows (Table 1). The samples were mounted perpendicular to the compression plasma flow at a distance of 12 cm from the muzzle of the MPC discharge device. The initial voltage on the capacitor bank of the MPC was varied over 2 3.0–3.8 kV with a step size of 0.2 kV, which yielded a plasma energy fluence of 3–5 J/cm to the sample surface. These interactions of compression plasma flows with silicon wafers produce volume periodic structures (quasi-one dimensional wires) with different morphologies. Silicon was grown on the compression plasma flow processed samples using an ultrahigh vacuum chamber –10 (VARIAN) with a base pressure P = 2⋅10 Torr at the Institute for Automation and Control Processes of the Far Eastern Branch of the Russian Academy of Sciences. The chamber was equipped with a holder for three samples, chromium and silicon sublimation sources for performing reactive epitaxy of chromium disilicide and molecular beam epitaxy of silicon, and an Auger electron spectrometer (AES). Before they were loaded into the ultrahigh vacuum chamber, the compression plasma flow processed samples were cleaned of hydrocarbons in an ultrasonic bath with acetone and dried. The remaining contamination was removed from the sample surfaces in the ultrahigh vacuum o chamber by heating at 650 C by passing a dc current through the sample for 4–5 h followed by cooling for 12 h. Then the samples were subjected to low-temperature cleaning. The low-temperature cleaning process involved deposio tion of silicon at a rate of ~0.1 nm/min with the substrate at a temperature of 850 C for 20 min. This ensured reduction of the silicon dioxide to the monoxide in the silicon flow and its subsequent decomposition with formation of an atomically pure sample surface. The state of the surface after cleaning and after silicon growth was monitored by AES and electron energy loss spectroscopy (EELS). The source of silicon was a rectangular silicon wafer doped with boron 16 –3 to a concentration of 1⋅10 cm . The chromium source was a tantalum cell with a chromium hinge that was heated by passing a dc current directly through it. The deposition rates of silicon and chromium were monitored by a quartz thickness probe. For the subsequent molecular-beam epitaxy of the silicon coating layer the substrate temperature was o chosen to be 700 C, as a result of preliminary optimization of the temperature for epitaxial growth of silicon on nanosized islands of chromium disilicide grown on silicon by reactive epitaxy [9, 10]. The morphology of the sample surfaces after compression plasma flow processing, as well as after silicon coating layers were grown, were studied by atomic force microscopy (AFM) using a Solver P47 multimode scanning microscope. Optical transmission and reflection spectra of the samples with grown nanoheterostructures were obtained at room temperature using a Hitachi U-3010 automatic spectrophotometer with an integrating sphere and an MSDD1000 automated monochromator. Raman scattering spectra on a microscale were obtained using a NTEGRA SPECTRA scanning probe microscope with a laser excitation wavelength of 488 nm. Discussion. AFM studies of the entire series of samples after compression plasma flow processing confirmed the threshold character of the formation of the cylindrical surface structures (Fig. 1a) observed as "waves," regardless of the type of conductivity, specific resistivity, and orientation of the substrate. In the present studies the surface struc841
Fig. 1. AFM image of the surface of a silicon sample processed by a compression plasma flow with U0 = 3.0 kV (a) and transmission spectra of silicon samples processed by compression plasma flows with MPC voltages of U0 = 3.0 (2), 3.2 (3), 3.4 (4), and 3.6 kV (5) compared with the spectrum (1) of a sample of single crystal silicon without compression plasma flow processing (b).
tures are produced when the bank voltage on the MPC U0 = 3.0–3.4 kV. The diameter of the quasiuniform wires increases as the voltage is raised. The minimum observed diameter of these structures was ~400 nm and the maximum, ≤2 μm. Note that the transmission spectra of the objects with synthesized surface structures on silicon substrates are essentially all the same (up to U0 = 3.2 kV) as the transmission spectrum of single crystal silicon (Fig. 1b). Thus, the changes in the electronic structure of the samples are insignificant, as is the scattering of light on the surface structures. When U0 is raised to 3.4–3.6 kV, the transmission coefficient of the compression plasma flow processed silicon wafers decreases by a factor of ~4. The surface structures are completely destroyed for U0 = 3.5–3.8 kV. Then the sample surface becomes highly disordered (root mean roughness 150–250 nm) and porous (Fig. 2b); this causes a sharp reduction in the transmission (by up to a factor of 20) in the 0.7–1.0 eV transparency region of silicon (Fig. 1b) owing to scattering and irreversible loss of light in multiple reflections within the porous regions. Data on the reflection spectra (Fig. 2b) for photon energies of 1.5–6.2 eV show that as U0 is increased the reflectivity decreases, while broadening and a shift of the peaks (3.35 and 4.50 eV) can be seen. In the region where surface structures exist, the peaks shift toward lower energies by 0.1–0.2 eV. On the other hand, when the MPC voltage is 3.8 kV, the peak at 4.5 eV is observed to shift toward higher energies. In the former case this corresponds to changes in the energy structure of the transitions owing to formation of a modified silicon crystalline structure in the quasiuniform wires [6, 7]. For the highest MPC voltages, when the substrate has a developed relief (Fig. 2a), this corresponds to amorphization of the silicon after plasma processing of the sample surface. Silicon wafers with (111) and (100) orientations that had been processed in advance by a plasma flux from the MPC with an initial bank voltage of no more than 3.4 kV were used for growing Si–NC CrSi2–Si (with embedded NC CrSi2) heterostructures. Three samples were placed in a single load of the ultrahigh vacuum chamber to conduct experiments under identical conditions. After compression plasma flow processing of the silicon, a sample was cleaned at low temperature in the vacuum chamber. AES studies show (Fig. 3) that all the intrinsic silicon oxide was completely removed, and compounds with nitrogen were also not observed, but the oxygen peak from the surface remained. The surface of the samples with periodic structures, however, because free of oxides, so that the formation of nanosized islands of CrSi2 on the surface could be studied. Islands of chromium disilicide were formed on a cleaned silicon surface with surface structures by reactive o epitaxy with the deposition of a chromium layer of thickness 0.3 nm with a substrate temperature of 550 C. One sample was removed from the vessel before a silicon coating layer was deposited. AFM studies showed (Fig .3) that small 842
Fig. 2. The morphology of a silicon surface after compression plasma flow processing with a voltage on the MPC of U0 = 3.8 kV (a) and reflection spectra of silicon samples processed with MPC voltages of U0 = 3.0 (2), 3.2 (3), 3.4 (4) and 3.8 kV (5) compared with the spectrum (1) of a single crystal silicon sample without compression plasma flow processing (b).
11
–2
(10–15 nm) islands with a very high occupation density ((2–3)⋅10 cm ) were present on the surface. Note that the synthesized islands cover the entire wafer surface uniformly with periodic structures. Our data show that the occupation density of islands on the surface of the wafer samples was highest for the Si(111)–Cr system. An analysis of our results shows that the most interesting result is the establishment of the composition of the synthesized islands, their properties, and the reasons for their high occupation density. The spectra of the reflections from the grown structures (Fig. 4a) show reveal something about the composition of the grown islands. Spectra obtained after island formation (Fig. 4a) indicate an increased reflection at photon energies of 1.3–3.3 eV and a decrease for energies exceeding 4.5 eV. The fraction of reflected light increases owing to the high density of states in the layer of islands, despite their small effective thickness. The reduction is caused mainly by a reduction in the density of states in the islands, which is typical for the narrow band of states in chromium disilicides [11]. The contribution of UV scattering to the nonuniform relief of the surface is taken into account since the spectra have been recorded using an integrating sphere. The absence of maxima in the reflection spectrum, which is typical for semiconducting chromium disilicide [11, 12] at energies of 1.5–2.5 eV, is indicative of the metallic character of light absorption in a disilicide island layer and, apparently, of their stressed state. Information on the crystalline state of the substrate with islands was obtained from the Raman scattering –1 spectra (Fig. 4b). It is known that the major Raman scattering peaks for CrSi2 lie within the region of 295–305 cm –1 [11] which overlaps with the observed 2TA(X) peak of silicon at 305 cm . Following formation of the CrSi2 islands, the amplitude and position of the detected signal does not change in any noticeable way. Since the deposited islands of chromium disilicide are very thin and have metallic absorption, they cannot be responsible for a Raman scattering peak in this region. In addition, when the crystalline quality of the highly dense layer of nanosized disilicide islands is poor, the signal from the underlying silicon should be reduced in amplitude owing to additional scattering. This did not happen, which means that both observations indicate a good crystalline quality of both the substrate and the disilicide islands on it. The high density of disilicide islands formed on periodic structures of silicon is evidently caused by stresses in the crystal lattice of the silicon under the islands [7], which lead to an increased density of seed centers and a reo duction in their critical size. It is known that, for this substrate temperature (T = 550 C) and chromium deposition rate, islands of semiconducting chromium disilicide form on single crystal silicon [10]. Thus, the composition of the islands deposited on surface structures formed after plasma processing is close to chromium disilicide, but their stressed crystalline structure leads to metallic reflection and absorption in a layer of islands with nanometer dimensions.
843
Fig. 3. AFM images of silicon after processing by a compression plasma flow, cleaning in ultra-high vacuum, and growth of chromium by reactive epitaxy: (a) 2D image, (b) phase contrast. o
Molecular-beam epitaxy at 700 C was used to grow 50 and 100-nm thick coating layers of silicon on nanosized islands of chromium disilicide. AFM data show that this layer covers all the surface structures and islands of disilicide and is made up of grains of sizes 100–200 nm which have grown together without preferential orientation. The grown silicon films do not yield the diffraction pattern of slow Si(111)–(1 × 1) electrons, which indicates that the grains are disordered in the film region near the surface. The film on a silicon surface with (100) orientation consists of dense grains that have grown together with less disorder than in the films grown on (111) silicon. As the silicon grows, the disilicide islands interact with the silicon atoms, and this may lead to elastic relaxation of the crystal lattice of the chromium disilicide. However, the large thickness of the silicon coating layer does not allow detection of the contribution to the spectra (Fig. 4b) from chromium disilicide nanocrystals at depths below 10 –1 –1 nm. No shifts in the main (LO + TA)(X) Raman scattering peak at 520 cm and the 2TA(X) peak at 305 cm were observed in the Raman spectra of samples with silicon coating layers. Only a slight broadening of these peaks is noticeable; that is an indication of a major contribution to the Raman spectra from single crystal grains of silicon which are slightly disoriented relative to one another. Thus, when silicon is grown on nanosized chromium disilicide islands, three dimensional epitaxial nucleation of silicon takes place between them, followed by lateral growing together with formation of intergrain boundaries, whose density is greater for an Si(111) surface. The formation of NCs of semiconducting chromium disilicide at depth was reliably detected in reflection spectra taken within the semitransparency range of the silicon covering layer [10] in the form of additional peaks in the region 2.0–2.8 eV of the maximum density of states in chromium disilicide [9]. The reflection spectra of two grown heterostructures (Fig. 4a) contain the main peaks of single crystal silicon (3.35 and 4.50 eV), but with reduced amplitudes. The reduction in amplitude of the main silicon peaks indicates a lowering of this density of states in the silicon owing to formation of a high density of interface boundaries between single crystal grains, as well as a deterioration in the crystal quality of the silicon coating layer. In nanostructures formed on Si(111) surfaces, a shift in the 4.5 eV peak toward lower energies and a drop in the reflectivity over the entire range of photon energies can be seen; this confirms a deterioration in the crystal quality of the silicon grains, an increase in the density of intergrain boundaries, and a reduction in the density of states. Besides the main peaks, the reflection spectra for the nanoheterostructures include faint peaks at 2.0 and 2.8 eV (Fig. 4a). Peaks in the reflection spectra have been observed at these energies for a single crystal and epitaxial film of chromium disilicide [11, 12], in association with interband transitions in the band structure of CrSi2. Thus, the idea that CrSi2 may be formed in the interior of a silicon coating layer is uniquely confirmed by the reflection spectra. The observed drop in the spectrum at 2.2–2.3 eV corresponds to differences in the way the reflection signal is formed for nanoheterostructures on Si(111) and Si(100) substrates at energies of 1.5–3.0 eV and indicates that the structure of the coating layer (intergrain boundaries) and embedded chromium dis844
Fig. 4. Reflection (a) and Raman scattering (b) spectra of: (1) a sample of single crystal silicon without processing by a compression plasma flow, (2) with growth of islands without a silicon coating layer, (3, 4) with nanoheterostructures with embedded nanocrystals of chromium disilicide on samples of Si(100) and Si(111).
ilicide NCs (point defects at the NC-silicon interface) are of different quality, as well as indicating differences in the depth of deposition of the chromium disilicide NCs. The appearance of intergrain boundaries in the silicon coating layer leads to enhanced diffusive transport of chromium disilicide nanocrystals along it and to their emergence into the surface region. This latter effect increases the contribution of low- and high-energy interband transitions in CrSi2 at photon energies of 2.0–2.8 eV in the reflection spectra of Si(111)–NC CrSi2–Si heterostructures and distorts the reflection spectra from silicon in this energy range (Fig. 4a). o Conclusions. Ultrahigh vacuum high-temperature (T = 850 C) annealing in a weak flux of silicon has been used for the first time to clean samples of Si(100) and Si(111) following initial processing by a compressed plasma flow. Reactive epitaxy with deposition of 0.3 nm Cr on samples of silicon with plasma-synthesized periodic structures (quasi-one dimensional wires) has been used for the first time to create small sized (10–15 nm) islands of chromium 11 –2 disilicide with a high occupation density (2–3)⋅10 cm ) and metallic light absorption owing to compression of the crystal lattice. Continuous silicon films consisting of single crystal grains with slight relative disorientation are formed o during molecular-beam epitaxy of silicon on nanosized islands of chromium disilicide at a temperature of 700 C. It was found that during growth of the silicon the lattice relaxes in the chromium disilicide nanocrystals and they enter a semiconducting state. When silicon is grown on nanosized chromium disilicide islands on an Si(111) wafer, a continuous layer of silicon is observed to form which consists of grains of size 100–200 nm that have grown together. During growth of the silicon layer the islands of chromium disilicide move because of diffusion along intergrain boundaries and they emerge at the surface; this leads to distortions in the reflection spectra from a sample owing to changes in the spatial distribution with depth of the CrSi2 nanocrystals. Acknowledgements. These studies were supported by the Russian Foundation for Basic Research (grant No. 08-02-90000_Bel_a) and the Foundation for Basic Research of the Republic of Belarus (grant No. F08R-198).
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