Self-Assembled Epitaxial Growth of High Density β-FeSi2 Nanodots on Si (001) and Their Spatially Resolved Optical Absorption Properties Yoshiaki Nakamura,*,†,‡ Shogo Amari,§ Nobuyasu Naruse,‡,§ Yutaka Mera,‡,§ Koji Maeda,‡,§ and Masakazu Ichikawa†,‡
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 3019–3023
Quantum-Phase Electronics Center, Department of Applied Physics, School of Engineering, The UniVersity of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan, CREST, Japan Science and Technology Corporation, Japan, and Department of Applied Physics, School of Engineering, The UniVersity of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed February 5, 2008; ReVised Manuscript ReceiVed May 16, 2008
ABSTRACT: A self-assembly technique for high density (∼1 × 1011 cm-2) β-FeSi2 nanodots epitaxially grown on Si (001) substrates was developed using a codeposition method of Fe and Si on ultrathin SiO2 films with Si nuclei. Photoabsorption spectra of individual nanodots and their photoabsorption maps at the direct-transition photoabsorption edge were obtained using electric field modulation spectroscopy combined with scanning tunneling microscopy with a nanometer spatial resolution. Semiconducting β-FeSi2 films epitaxially grown on Si have attracted technological interest as Si-based light emitting materials with a wavelength of ∼1.5 µm (≈0.8 eV) suitable for optical fiber communication1–7 because strained β-FeSi2 epitaxially grown on Si can reportedly be a direct band gap semiconductor.8,9 The authors have paid much attention to β-FeSi2 nanodots owing to the peculiar properties of nanodots such as the lack of misfit dislocations, the enhancement of optical oscillator strength, and the quantum-size effect. In our previous study, we developed the epitaxial growth method of ultrahigh density (>1012 cm-2) coherent β-FeSi2 nanodots on Si (111)10 and observed the quantum-confinement effect of β-FeSi2 nanoislands on Si (111).11 As for the type of Si substrates, the use of Si (001) substrates is more suitable for device applications. Strong photoluminescence has also been observed from β-FeSi2 films epitaxially grown on Si (001) substrates.2–4,6 Despite this, the epitaxial growth of nanodots with high density on Si (001) substrates has never been successful presumably because of the preference of two-dimensional (2-D) growth over threedimensional (3-D) growth due to small lattice mismatch12 when compared with Si (111). It would be beneficial to characterize optical properties of individual nanodots, which can be different from those of bulk and films, not only for applications but also for scientific concerns. In addition to the quantum-confinement effect, as for the β-FeSi2 nanodots, large elastic straining can cause a drastic change in the optical properties, namely, those of being an indirect band gap semiconductor to those of being a direct one.8,9 Unfortunately, as far as photoabsorption spectroscopy is concerned, conventional techniques cannot spatially resolve the nanometer structures. Recently, electric field modulation spectroscopy combined with scanning tunneling microscopy (STMEFMS) has been developed as a method for accurate measurements of the optical absorption edge of semiconductor samples with spatial resolution at a nanometer scale,13–15 details of which were described elsewhere.13 The principle of the STM-EFMS is similar to that of electric field modulation reflectance * To whom correspondence should be addressed. E-mail: yoshiaki@ exp.t.u-tokyo.ac.jp. † Quantum-Phase Electronics Center, The University of Tokyo. ‡ CREST, Japan Science and Technology Corporation. § Department of Applied Physics, The University of Tokyo.
measurements (photoreflectance and electroreflectance). The largest difference from the conventional methods is that the detected signal is not a change in the reflecting light intensity, but a change in the STM-tip current (not necessarily tunneling current) that is induced by a change in the surface photovoltage (SPV) caused by optical absorption. As demonstrated in previous papers,13,16 STM-EFMS measurements yield spatially resolved spectra with characteristic structures at singular interband optical transitions such as the absorption edge, which are similar to the macroscopic photo- (or electro-) reflectance spectra. In the present study, we developed an epitaxial growth technique for atomically flat and high density (∼1 × 1011 cm-2) β-FeSi2 nanodots with a lateral size of ∼20 nm and a height of ∼3 nm on Si (001) substrate using a method of codepositing Fe and Si at the disilicide stoichiometric deposition rates after Si predeposition onto the ultrathin SiO2 films on Si substrates. We characterized the optical absorption properties of the individual β-FeSi2 nanodots using STM-EFMS. The STMEFMS spectra show a large photoabsorption signal at the directtransition optical absorption edge, resulting in successful mapping of the optical absorption of individual β-FeSi2 nanodots on Si (001) at this photon energy. These spectra also reveal that the optical absorption properties of epitaxial β-FeSi2 nanodots strongly depend on the surface orientation of the Si substrates. Samples (8 × 1.5 × 0.4 mm) cut from an n-type Si (001) wafer were introduced into an ultrahigh-vacuum chamber at a base pressure of about 1 × 10-8 Pa. The chamber was equipped with a scanning tunneling microscope (STM), reflection highenergy electron diffraction (RHEED) apparatus, and two separate evaporators for Fe and Si deposition. Ultrathin SiO2 films with thicknesses of ∼0.3 nm were formed by oxidizing the Si surfaces at 600 °C for 10 min at an oxygen pressure of 2 × 10-4 Pa in the chamber after cleaning the Si(001)-(2 × 1) surfaces by flashing at 1250 °C. We deposited a small amount of Si (1-3 monolayers (ML) Si) on the ultrathin SiO2 films at 500-670 °C before codeposition of Fe and Si, a process that is referred to as Si predeposition.10 During Si predeposition, ∼1 nm voids (Si bare sites) were formed in SiO2 films and followed by Si nuclei formation. The Si nuclei are hemispherical and not faceted owing to their small size (600 °C)10,19 and bulk (>∼900 °C). This revealed that the formation energy of R-FeSi2 nanodots on Si (001) could be lower than that on Si (111). Assuming low formation energy of R-FeSi2, local structures of R-FeSi2 can exist in amorphous iron silicide formed by codeposition at 300-400 °C. In the presence of these R-FeSi2 seeds, amorphous iron silicide can crystallize to R-FeSi2 by postannealing despite β-FeSi2 being more stable at a temperature lower than ∼900 °C for bulk. Therefore, the formation of β-FeSi2 crystals on Si (001) covered with ultrathin SiO2 films requires the formation of completely
Crystal Growth & Design, Vol. 8, No. 8, 2008 3021
amorphous iron silicide with a stoichiometric composition by codeposition at RT, which does not include R-FeSi2 seeds, followed by annealing to give solid phase epitaxy (SPE) of β-FeSi2. The Si predeposition condition influenced the crystal orientation of formed β-FeSi2 nanodots, namely, epitaxial growth or nonepitaxial growth (polycrystalline). In our previous studies,17,18,24 we elucidated the Si or Ge deposition effect on the ultrathin SiO2 films. In the first stage of Si (Ge) predeposition, deposited Si (Ge) atoms react with the ultrathin SiO2 films and evaporate as SiO (GeO) molecules to form ultrahigh density voids in the ultrathin SiO2 films (Si bare sites).18,24 More deposited Si (Ge) atoms form ultrasmall Si (Ge) nuclei in contact with Si substrates via voids.18,24 In the present paper, during codeposition on the ultrathin SiO2 films with predeposited Si, deposited Fe and Si atoms were considered to diffuse to the Si nucleus sites and form iron silicide nanodots. As a result, ultrahigh density (1 × 1013 cm-2) nanodots were formed owing to the existence of ultrahigh density Si nuclei. In this framework, the epitaxial growth of nanodots depends on the contact with Si nuclei and Si substrates through voids. Nonepitaxial growth (polycrystalline) in the case of 1-ML Si predeposition can be explained by the insufficient void formation. Conversely, for the predeposition of a large amount of Si (3 ML), the voids became excessively large resulting in β-FeSi2 film growth on the Si bare areas, instead of dot formation, because film growth takes place on clean Si (001) surfaces. The predeposition condition for 2-ML Si at 670 °C is, therefore, one of the key factors in the epitaxial growth of β-FeSi2 nanodots on Si (001). β-FeSi2 nanodots on Si (001) were readily flattened by annealing. This can be explained by assuming that the small change in interface and surface energies due to β-FeSi2 dot formation on Si (001) resulted in the preference for a flat shape rather than a hemispherical shape. One of the reasons for this preference is the small lattice mismatch between β-FeSi2 and Si (001) of ∼2% (∼5% in the case of β-FeSi2 on Si (111)).12 This annealing effect also made the dot shape more homogeneous as shown in Figure 3c unlike the case for β-FeSi2 dots on Si (111) where Ostwald ripening occurs.19 The dot energy per atomic volume, E(L), in a lattice mismatched system is described as the product of the energy of a single dot with size L and ω/L3, where ω is the atomic volume, as follows.25
{
L ( 2πa ) + DL} Lω
E(L) ) -AL3 + BL2 - CL ln
3
(1)
where A is the energy of elastic relaxation and chemical potential change per unit volume due to dot formation, B is the change in the surface and interface energies per unit surface area due to dot formation, the third term is the contribution of dot edges to the elastic relaxation energy with a lattice spacing, a, and the fourth term is the short-range energy of the edges. In this equation, the dipole-dipole elastic repulsion energy among dots is neglected owing to the small lattice mismatch. When the control parameter k, defined by BL0e0.5/C with the characteristic size L0 ) 2πa/exp(D/C+1/2), is larger than 2e-1/2 ≈ 1.2, E(L) does not have the minimum value for ripening to occur. On the other hand, for a k value less than 1.2, the most stable state in E(L) exists near L0. In the present study, the preference of a flat shape in β-FeSi2 nanodot growth indicates a small B value as mentioned above, which makes the k value small resulting in uniform dot formation with a size of approximately L0 (∼20 nm). The density of annealed β-FeSi2 nanodots was reduced to 1 × 1011 cm-2 owing to coalescence, and this value is still high compared with that of Stranski-Krastanov islands (∼1010 cm-2).
3022 Crystal Growth & Design, Vol. 8, No. 8, 2008
Figure 4. (a) STM-EFMS spectra of a hydrogen-terminated β-FeSi2 nanodot on Si (001) measured at 96 K in a bias voltage modulation scheme (upper curve) and optical modulation scheme (lower curve). Both spectra were acquired from the tunneling regime at a modulation frequency of 5 kHz. (b) STM-EFMS spectrum obtained from a hydrogen-terminated β-FeSi2 nanodot epitaxially grown on Si (111) under the same experimental conditions as those for the bias voltage modulation scheme in (a). (c) STM image of hydrogen-terminated β-FeSi2 nanodots epitaxially grown on Si (001) acquired at a sample bias voltage of +4.0 V and a tunneling current of 100 pA. These nanodots were formed by codeposition of 4-ML Fe and 8-ML Si at RT on ultrathin SiO2 films with 2-ML Si predeposited at 670 °C and by successive postannealing at 650 °C. (d) Two dimensional map for the same area as (c) of the absolute difference of STM-EFMS signals detected at 0.89 and 0.85 eV in 230 mV bias voltage modulation. The photon energies of 0.89 and 0.85 eV correspond to the peak and offpeak energies in (a).
Figure 4a shows the typical STM-EFMS spectra for a β-FeSi2 nanodot on Si (001) acquired in the two modulation schemes. For reference, the spectrum of a β-FeSi2 nanodot on Si (111) in the bias voltage modulation scheme is also shown in Figure 4b. The inclination of the baseline in Figure 4b may be due to long-period fluctuations of displacement current caused by the tip-sample distance altering with a long acquisition time in the tip retracted condition, or by a positional drift of the tip.16 The formation method for β-FeSi2 nanodots on Si (111) was previously reported.11 In these specific measurements, the STM tip was retracted from the sample by a distance of ∼1-5 nm out of the tunneling regime after the STM tip was positioned over the target nanodot. In such cases, the tip current is dominated by the displacement current16,22 that is generated by stray capacitance between the tip and sample.16,22,26,27 This measurement sacrifices spatial resolution but only to a limited extent.16 The two spectra for the nanodot on Si (001) in Figure 4a exhibit a common peak at ∼0.89 eV, which agrees well with the direct-transition optical absorption edge of β-FeSi2 films28–30 and bulk.31 Beside the 0.89 eV signal, the two EFMS spectra for the nanodot on Si (001) in Figure 4a also share a common small dip near 0.72 eV. According to photoabsorption measurements of a bulk sample of β-FeSi2,31 this energy of 0.72-0.74 eV was assigned to the indirect-transition optical absorption edge with phonon absorption for β-FeSi2 crystal at 100 K. It should be noted that the spectra of β-FeSi2 nanodots on Si (001) with the clear peak at 0.89 eV, which are similar to those for films and bulk, are different from those obtained from the epitaxial β-FeSi2 nanodots on Si (111) in Figure 4b, which shows a quite
Nakamura et al.
small peak around 0.89 eV. Although similar small STM-EFMS signals were also observed near the indirect-transition optical absorption edge (0.72-0.74 eV) for both nanodots on Si (001) and (111), photon energy positions are slightly different as shown in Figure 4a,b. To prove the signal origin, we also performed mapping of STM-EFMS signals obtained in the bias voltage modulation scheme in the tunneling regime with spatial resolution of a few nanometers.16 In this measurement at a modulation frequency of 5 kHz, displacement current was considered to be the major component in the STM-tip current rather than tunneling current.16 The STM topographic image of the investigated sample area is shown in Figure 4c. Bright contrasts are hydrogenterminated β-FeSi2 nanodots with an average height of 3 nm. Figure 4d shows the 2-D map of the same area for the absolute value of the STM-EFMS signal difference between 0.89 and 0.85 eV, which correspond to the peak and off-peak energies, respectively, in the STM-EFMS spectra (Figure 4a). The bright contrast in Figure 4d is localized in the β-FeSi2 nanodots imaged in Figure 4c, which clearly indicates that the 0.89 eV signal arises from the β-FeSi2 nanodots. This result supports that the 0.89 eV signal originates from the optical absorption at the direct-transition optical absorption edge of β-FeSi2 nanodots. A large STM-EFMS signal at 0.89 eV for β-FeSi2 nanodots on Si (001) enables us to obtain photoabsorption maps of individual nanodots at the direct-transition optical absorption edge. At this photon energy, we failed to map the photoabsorption of nanodots on Si (111) with a weak signal at ∼0.89 eV. The two EFMS spectra for nanodots on Si (001) in Figure 4a also share a common small dip near 0.72 eV, which coincides with the indirect-transition optical absorption edge of β-FeSi2 bulk at 100 K.31 A 2-D map similar to Figure 4c for the signal difference between 0.72 and 0.70 eV (not shown) reveals the bright contrasts are, though less clear, certainly localized at the nanodots. These experimental results suggest the β-FeSi2 nanodots are semiconductors having an indirect-transition optical absorption edge at a lower energy position than the directtransition optical absorption edge, indicating an indirect band gap semiconductor. The strong dependence on the face orientation of Si substrates is unique to epitaxial nanodots and not found for films or bulk. We can only speculate the optical properties change owing to the strains from the lattice mismatch between nanodots and Si substrates. In this framework, the difference in optical properties between the substrates of Si (111) and Si (001) could be due to a difference in the magnitudes of interfacial strains. Single β-FeSi2 nanodots on Si (111) retain an average strain of approximately 1% owing to nanodot structures according to high resolution cross sectional transmission electron microscope (HRTEM) observations.10,19 However, the interface between nanodots and substrates can be completely strained owing to the absence of misfit dislocations in spite of the lattice mismatch,10,11,19 leading to larger strains in the interface than those in the epitaxial films with some misfit dislocations. Owing to the large volume fraction of the interface in nanodots, the electronic structure of β-FeSi2 nanodots on Si (111) can be affected by the strain effect. On the other hand, the strains in β-FeSi2 nanodots on Si (001) are expected to be much smaller because of the small lattice mismatch compared with the case for nanodots on Si (111), resulting in optical properties similar to those of bulk and films. In conclusion, we developed a formation method for β-FeSi2 nanodots with a high density of 1.0 × 1011 cm-2 epitaxially grown on Si (001) substrates using ultrathin SiO2 films after Si
Epitaxial Growth of High Density β-FeSi2 Nanodots
predeposition. The formed β-FeSi2 nanodots were ∼20 nm in lateral size and ∼3 nm in height. For the SPE growth of β-FeSi2 nanodots, the Si predeposition condition, namely, 2-ML Si predeposition at ∼670 °C, is a key factor. STM-EFMS measurements showed the optical absorption edges of indirect and direct transitions of β-FeSi2 nanodots near ∼0.72 and 0.89 eV, respectively. We measured the photoabsorption spectra of individual β-FeSi2 nanodots on Si (001) and successfully mapped the photoabsorption at the direct-transition optical absorption edge. The optical absorption properties of nanodots demonstrated a strong dependence on the surface orientation of Si substrates, which is unique to epitaxial nanodots. Acknowledgment. This work was partly supported by a Grant in Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, JSPS.KAKENHI (19710088).
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