J. Phys. Chem. C 2009, 113, 20589–20593
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InSb/Al-O Nanogranular Films Prepared by RF Sputtering Hiroyuki Usui,* Seishi Abe, and Shigehiro Ohnuma The Research Institute for Electric and Magnetic Materials (RIEMM), 2-1-1 Yagiyama-minami, Taihaku, Sendai 982-0807, Japan ReceiVed: June 29, 2009; ReVised Manuscript ReceiVed: September 26, 2009
Crystallized nanogranules of indium antimonide (InSb) embedded in an amorphous aluminum oxide (Al-O) matrix have been fabricated by a radio frequency sputtering. With increasing the composition of In and Sb, the nanogranule size is increased and the size distribution broadens. A calculation result using effective mass approximation with the Nosaka model shows that the band gap variation of the InSb nanogranules is attributed to the quantum size effect. The obtained InSb nanogranules in the Al-O matrix exhibit a narrower size distribution compared with previously reported InSb nanogranules embedded in a silicon oxide matrix. 1. Introduction Nanogranules of semiconductors with a direct band gap exhibit unique optical and electronic properties by the quantum size effect.1 In particular, phosphor nanogranules embedded in a dielectric oxide matrix2 can be applied to the inorganic electroluminescent (EL) materials because the emitting light wavelength is tunable by the size-dependent band gap of the phosphor nanogranules. In other words, the emissions of the three primary colors can be easily achieved by control of the nanogranule size. For a good reproducibility of the colors, a narrow size distribution is required for the phosphor nanogranules. Indium antimonide (InSb) is a groups III-V semiconductor of direct-transition type with a narrow band gap of 0.17 eV. An InSb film has been previously used for infrared (IR)-light detectors because the band gap corresponds to the wavelength of 7.3 µm in the IR region. On the other hand, in the case of nanosized InSb, the quantum size effect clearly appeared because its exciton Bohr radius of 66 nm is much larger compared with other groups III-V semiconductors.3 A drastic change in the band gap is expected for size-controlled InSb nanogranules. Thus, the size-controlled InSb nanogranules are very feasible for the inorganic EL material. InSb nanogranules embedded in an amorphous silica (SiO2) matrix have been previously fabricated by a radio frequency (RF) sputtering and a successive thermal annealing.3-5 Zhu et al. have reported that the optical band gap varied, depending on the size of obtained InSb nanogranules.3,4 The diameter of the nanogranules ranged from 5 to 25 nm. The size distribution is relatively broad in view of an application for the inorganic EL. Teˆtu et al. have obtained well-crystallized InSb nanogranules.5 However, the size distribution of the nanogranules was much broader than that prepared by Zhu et al., and In2O3 was also formed in the obtained samples. To fabricate the InSb nanogranules with a narrower size distribution, we consider that an optimization of matrix material is critically important because a property of the matrix will affect crystal growth of the nanogranules. We are suggesting here that aluminum oxide (Al-O) should be used as the matrix material because Al-O is thermodynamically stable compared with SiO2.6 In the previous study, we have demonstrated that germanium nan* To whom correspondence should be addressed. Phone: +81-857-315249. E-mail:
[email protected].
ogranules with a narrow size distribution can be fabricated in a matrix of an amorphous aluminum oxide.7 We fabricated nanogranular films consisting of InSb nanogranules and a Al-O matrix by RF sputtering for the first time and investigated the crystal structure of the films. The size and optical property of the InSb nanogranules were investigated, and then we discussed the size distribution and the quantum size effect of the InSb nanogranules. 2. Experimental Section A composite film of InSb nanogranules embedded in Al-O (InSb/Al-O) was prepared by RF sputtering using a composite target. The composite target consisted of an amorphous Al2O3 disk (10 cm diameter) and InSb chips (5 × 5 mm2) placed on the disk. The sputtering was also performed using an Sb chip (5 × 5 mm2) with the InSb chips to obtain Sb-rich films. By changing the number of the InSb chips, we obtained InSb/Al-O composite films with different compositions of In and Sb. The chamber was first evacuated to a vacuum of 2.0 × 10-5 Pa before introducing argon (Ar) gas. The InSb/Al-O films were deposited on a Corning no. 7059 substrate cooled by water. The distance between the target and the substrate was kept constant at 93 mm. The Ar gas pressure was fixed to 2.7 × 10-1 Pa. The RF power and deposition time were kept constant at 300 W and 60 min, and no RF bias was applied to the substrate. A further detailed procedure of the sputtering has been described in our previous papers.6,8,9 The thickness of the obtained films was about 1.0 µm. After the deposition, the InSb/Al-O films were successively annealed for 60 min in vacuum to crystallize InSb nanogranules. The annealing temperatures were 573, 723, 823, and 923 K. The crystal structure of the films was studied by X-ray diffraction (XRD) using an X-ray diffractometer (Rigaku RAD-X). The nanostructure of the films was observed using a high-resolution transmission electron microscope (TEM, Jeol JEM-4000EX). The TEM specimens were prepared by sequentially cleaving into small pieces from the films by a sharp needle, dispersing the pieces in ethanol, adding dropwise the dispersed solution on an amorphous carbon film mounted on a copper mesh, and supporting the pieces on the amorphous carbon film. The optical absorption spectrum of the films was measured by ultraviolet (UV)-visible near-infrared (NIR) spectroscopy (UV 3500 Shimadzu), and the composition of the films was analyzed by energy dispersion spectroscopy (EDS, Phoenix EDAX).
10.1021/jp906051b CCC: $40.75 2009 American Chemical Society Published on Web 11/05/2009
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Figure 1. XRD patterns of InSb/Al-O composite film prepared without the Sb chip. The composition ratio of In and Sb was 11 and 8 atom %, respectively.
3. Results 3.1. Structure Analysis of InSb/Al-O Films Prepared without Sb Chip. In the case of InSb/Al-O films prepared without the Sb chip, a small amount of excessive In was found as a result of the EDS analysis. On the other hand, the composition ratio of O to Al was about 1.5, which indicates that the composition ratio of the film does not differ considerably from that of the Al2O3 disk target. Figure 1 depicts XRD patterns of the InSb/Al-O film with the composition ratio of 11 atom % In and 8 atom % Sb, which was prepared without the Sb chip. No diffraction peak was observed for the as-deposited film and the annealed film at 573 K. Above the temperature of 723 K, we can recognize diffraction peaks of cubic InSb (JCPDS No. 06-0208). This result suggests that a crystallization of InSb occurs at annealing over 723 K in the experiments. At 923 K, both diffraction peaks of the InSb and those of cubic In2O3 (JCPDS No. 06-0416) were clearly observed. It is suggested that the excessive In was oxidized by the high-temperature annealing and that the crystallization progressed by a thermal diffusion and a coalescence of the InSb nanogranules in the film. We consider that a suitable annealing temperature is 723 K for crystallization of InSb nanogranules with a smaller size embedded in the film. Figure 2a shows a bright-field image of the InSb/Al-O film with 11 atom % In and 8 atom % Sb after the annealing at 723 K. We can see crystallized nanogranules densely embedded in an amorphous matrix. The TEM observation revealed that the obtained InSb/Al-O film has a nanogranular structure. The averaged particle diameter was 7.0 nm. Figure 2b is a corresponding selected area electron diffraction. Debye-Scherrer rings appeared, and those were indexed as the InSb and the In2O3. Figure 2c presents an enlarged TEM image of a typical nanogranule shown in Figure 2a. Lattice fringes along two different directions can be seen in the nanogranule. The fringe spacings of d1 and d2 were estimated to be approximately 0.32 and 0.37 nm, and the two fringe directions make an angle of 55°. The d1 and d2 are nearly equivalent with lattice spacings of d200 ) 0.324 nm and d111 ) 0.374 nm in the bulk InSb (JCPDS No. 06-0208). The lattice alignment of the nanogranule corresponds to that of bulk InSb observed in the direction along the [011] zone axis. Therefore, the nanogranule is a crystallized InSb. In other areas, many crystallized InSb nanogranules were similarly observed. The InSb/Al-O composite film basically consists of crystallized InSb nanogranules and amorphous Al-O intergranules, and however, the film slightly contains In2O3. From the viewpoint of application for the inorganic EL material, the In2O3 formation should be avoided. We thus performed the
Usui et al. sputtering using the InSb chips and the Sb chip in order to compensate for a deficiency of Sb. 3.2. Size and Optical Band Gap of InSb/Al-O films Prepared Using Sb Chip. In the case of InSb/Al-O films prepared using the Sb chip, the composition ratio of In was slightly smaller than that of Sb. The Sb deficiency was compensated. The composition ratio of O to Al was found to be about 1.5. After the annealing at 723 K, no diffraction of In2O3 and Al2O3 was observed in the experiments of selected area electron diffraction. Many crystallized nanogranules of InSb were found by the TEM observations. It was thus confirmed that the InSb/Al-O films consisted of crystallized InSb nanogranules and amorphous Al-O intergranules. These results suggest that we successfully fabricated InSb/Al-O films without In2O3 crystal by the compensation of the Sb deficiency. The particle diameters of InSb nanogranules were measured by using the TEM images. A statistical analysis for the diameters gives an averaged particle diameter and a standard deviation of diameter distribution, σ. The particle diameter distribution broadened with increasing the composition of In and Sb. Figure 3 shows a histogram of particle diameter distribution of the InSb/ Al-O film containing 9.5 atom % In and 13.5 atom % Sb. The diameters are mainly ranging from 4 to 15 nm. In contrast, in the case of InSb nanoparticles with a comparable averaged diameter in a SiO2 matrix fabricated by Zhu et al., the diameters are mainly ranging from 5 to 25 nm.3 It should be noted that InSb/Al-O film prepared in this study exhibits a much narrower size distribution compared with the InSb/SiO2 film. Figure 4 depicts a relationship between the averaged particle diameter and the summed composition of In and Sb. The error bars indicate the standard deviation σ. The averaged particle diameter was increased with increasing the summed composition of In and Sb. Figure 5 shows optical transmittance spectra of the InSb/ Al-O films with different averaged particle diameters. As seen in the figure, the onset of absorption is shifted from the UVlight region to the NIR-light region with increasing the averaged particle diameter. In our preliminary experiments for Al-O films and Sb/Al-O composite films, we have confirmed that there is no absorption band in these regions. The spectrum variation is consequently attributed to a difference in optical band gaps of InSb nanogranules with the different averaged particle diameters. Regarding a semiconductor with a direct band gap, the optical band gap can be approximately obtained by extrapolations in plotting of photon energy versus squared absorption coefficient. Figure 6 shows the optical band gap of the InSb/Al-O films as a function of the InSb particle diameter. The optical band gap was markedly increased with decreasing the particle diameter. The behavior of the InSb/Al-O films is very similar to that of InSb nanoparticles embedded in SiO2 films,3 which is shown as open circle plotted in the figure. In this study, we have successfully fabricated four samples whose optical band gaps are in the visible-light region between 1.6 and 3.2 eV. These results obviously suggest that the combination of InSb nanogranules and Al-O matrix is much more effective, compared with the previously studied InSb/SiO2 system,3-5 for controlling the composition and size in order to fabricate the inorganic EL materials emitting visible light. 4. Discussion Effective mass approximation (EMA) using a single band model has been widely used to discuss the quantum size effect of the energy band gap in nanosized semiconductors. Assuming an infinite potential energy barrier at the interface between the
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Figure 2. (a) Bright-field image of the InSb/Al-O film with the composition of 11 atom % In and 8 atom % Sb after the thermal annealing at 723 K. (b) Corresponding selected area electron diffraction. The observed Debye-Scherrer rings can be indexed as InSb and In2O3. (c) Enlarged TEM image of a typical nanogranule shown in Figure 2a.
semiconductor and a surrounding material, Brus has calculated an optical band gap Eopt broadened by the quantum size effect.10-12 The dependence of Eopt on the particle radius R can be described by the following equation
Eopt(R) ) Ebulk + h2 /(8m0R2)(1/me*+1/mh*) 1.8e2 /(4πε0εrR) (1) where Ebulk is the energy band gap of a bulk semiconductor, h is Planck’s constant, m0 is the electron mass, while me* and mh* are the effective masses of electrons and holes, e is the electron charge, ε0 is the vacuum permittivity, and εr is the relative dielectric constant of the semiconductor. By using these
parameters of the bulk InSb and the particle diameter obtained in this study, the optical band gap of InSb nanogranules was calculated by EMA with the Brus model. The EMA calculation result was plotted by a dotted line in Figure 6. A remarkable discrepancy between experimental and calculated values was observed. It is well-known that the Brus model calculation often overestimates the optical band gap for extremely small-sized semiconductors.3,13 Nosaka has suggested that the Brus model calculation essentially overestimates Eopt because a penetration of wave function into the surrounding material has been not considered due to the infinite potential energy barrier. Thus, Nosaka has assumed a finite potential energy barrier that allows the wave function penetration and has developed the Brus model to predict the quantized optical band gap more accurately.14 In fact, many
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Figure 3. Histogram of the particle diameter distribution of the InSb nanogranules in the InSb/Al-O composite film. The film contained 9.5 atom % In and 13.5 atom % Sb.
Figure 4. Relationship between the averaged particle diameter and the summed composition of In and Sb. The composition of In and Sb of the films is described on the plots. The error bars indicate the standard deviation of diameter distribution σ.
researchers have reported that there is an excellent agreement of predicted values to experimental data.13,15,16 In the Nosaka model, the following formula has been derived
Eopt(R) ) Ebulk + E0[a + b/{(E0me*/m0)1/2R + c}2] + E0[a + b/{(E0mh*/m0)1/2R + c}2] - 1.786e2 /(4πε0εrR) (2) where E0 is the finite potential energy barrier. The energy gaps of amorphous Al2O317 and bulk InSb are 6.24 and 0.17 eV, respectively. In this study, the E0 was estimated to be about 3.0 eV by assuming a conduction to valence band offset ratio of 50:50. The constant parameters a, b, and c depend on the charge carriers’ effective masses. In Nosaka’s paper,14 these parameters are given by plots of the relationship between the parameters and the effective mass ratios me*/m0 and mh*/m0. The Eopt calculated using the Nosaka model is plotted as a solid line in Figure 6. The calculated Eopt basically agreed with the experimental results in our study (filled triangles) and Zhu’s study (open circles).3 Accordingly, we arrived at the conclusion that the Eopt variation in this study is attributed to the quantized wave function confined by the finite potential barrier in the InSb/ Al-O nanogranular structure. The crystal growth of nanogranules embedded in a matrix by postannealing generally depends on the annealing conditions and thermodynamic parameters of the nanogranules and the
Usui et al.
Figure 5. Optical transmittance spectra of the InSb/Al-O films with different averaged particle diameters.
Figure 6. Optical band gap of the InSb/Al-O films as a function of the InSb particle diameter. The optical band gap obtained in this study was plotted as filled triangles. The open circles denote the values of InSb nanoparticles embedded in the SiO2 matrix, which has been reported by Zhu et al.3 The dotted line shows the EMA calculation result using the Brus model.10-12 The solid line presents the EMA calculation result using the Nosaka model.14
matrix. The annealing temperature in this study is nearly equal to that in Zhu’s study.3 Nevertheless, the size distribution of the InSb/Al-O film was much narrower compared with that of the InSb/SiO2 film. We consider that one possible reason is a difference in the thermodynamic stabilities. Because Al2O3 has lower values compared to those of SiO2 in the heat of formation and standard Gibbs energy of reaction,7 the Al-O matrix would be thermodynamically more stable than the SiO2 matrix. A thermal diffusion of In and Sb is relatively suppressed in the Al-O matrix, which results in less coalescence of InSb nanogranules. Therefore, the narrower size distribution was achieved in the InSb/Al-O film. On the other hand, partial decomposition and oxidation of InSb presumably occurs in the thermodynamically unstable SiO2 matrix. The InSb/SiO2 films exhibit a broader Eopt compared with that of the InSb/Al-O films, as shown in Figure 6. This implies that actual diameters of pure InSb in SiO2 are smaller than the observed diameters shown in Figure 6 because of the oxidation of the outer boundary of InSb nanogranules. 5. Conclusion We fabricated InSb/Al-O nanogranular films by RF sputtering. It was experimentally confirmed that the crystallized InSb nanogranules were embedded in an amorphous Al-O matrix. With increasing the composition of In and Sb, the averaged particle diameter was increased and the size distribution of the
InSb/Al-O Films Prepared by RF Sputtering nanogranules broadened. The optical band gap of the InSb nanogranules was significantly increased with decreasing the averaged particle diameter. The EMA calculation result using the Nosaka model showed that the variation of the optical band gap is attributed to the quantum size effect. The obtained InSb nanogranules exhibit a narrower size distribution compared with that of previously reported InSb/SiO2 film. The reason is possibly that InSb nanogranules embedded in the Al-O matrix would be thermodynamically more stable than those embedded in SiO2. We believe, therefore, that novel nanogranular films of InSb/Al-O were successfully fabricated for the application of the inorganic EL. Acknowledgment. The present work was supported by a Grant-in-Aid for Scientific Research (B) (No. 18360338), from the Japan Society for the Promotion of Science (JSPS). A part of this work (TEM observations) was supported by the Center for Integrated Nanotechnology Support at Tohoku University and also by the “Nanotechnology Network Project” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government. We gratefully acknowledge the valuable comments and continuous encouragement of Prof. K. Masumoto and Prof. T. Masumoto.
J. Phys. Chem. C, Vol. 113, No. 48, 2009 20593 References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Zhou, J.; Li, L.; Gui, Z.; Buddhudu, S.; Zhou, Y. Appl. Phys. Lett. 2000, 76, 1540. (3) Zhu, K.; Shi, J.; Zhang, L. Solid State Commun. 1998, 107, 79. (4) Zhu, K.; Shi, J.; Wei, Y.; Zhang, L. Chin. Sci. Bull. 1998, 43, 1610. (5) Teˆtu, A.; Chevallier, J.; Nielsen, B. B. Mater. Sci. Eng., B. 2008, 147, 141. (6) Abe, S.; Ohnuma, M.; Ping, D. H.; Ohnuma, S. J. Appl. Phys. 2008, 104, 104305. (7) Kubaschewski, O.; Alock, C. B. Metallurgical Thermochemistry, 5th ed.; Pergamon: Oxford, U.K., 1979. (8) Abe, S.; Ohnuma, S. Appl. Phys. Express 2008, 1, 111304. (9) Abe, S.; Ohnuma, M.; Ping, D. H.; Ohnuma, S. Appl. Phys. Express 2008, 1, 095001. (10) Brus, L. E. J. Chem. Phys. 1983, 79, 5566. (11) Brus, L. E. J. Chem. Phys. 1983, 80, 4403. (12) Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (13) Pejova, B.; Grozdanov, I. Mater. Chem. Phys. 2005, 90, 35. (14) Nosaka, Y. J. Phys. Chem. 1991, 95, 5054. ¨ .; Hartikainen, J.; Marlow, F.; Linde´n, M. (15) Tiemann, M.; Wieβ, O ChemPhysChem. 2005, 6, 2113. (16) Tiemann, M.; Marlow, F.; Brieler, F.; Linde´n, M. J. Phys. Chem. B 2006, 110, 23142. (17) Mo, S.-D.; Ching, W. Y. Phys. ReV. B 1998, 57, 15219.
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