CRYSTAL GROWTH & DESIGN
Phase Stabilization and Phonon Properties of Single Crystalline Rhombohedral Indium Oxide Ch. Y. Wang,*,† Y. Dai,‡ J. Pezoldt,† B. Lu,§ Th. Kups,† V. Cimalla,† and O. Ambacher†,4 Institute of Micro- and Nanotechnologies, Technical UniVersity Ilmenau, P.O. Box 100565, 98684 Ilmenau, Germany, Department of Physics, and Instrumental Analysis and Research Center, Shanghai UniVersity, Shanghai 200444, China, and Fraunhofer-Institut für Angewandte Festkörperphysik (IAF), Tullastrasse 72, 79108 Freiburg, Germany
2008 VOL. 8, NO. 4 1257–1260
ReceiVed September 20, 2007; ReVised Manuscript ReceiVed NoVember 14, 2007
ABSTRACT: We report on the phase stabilization of rhombohedral (rh-) In2O3 films on sapphire substrate deposited by metal organic chemical vapor deposition. With the help of a high-temperature nucleation layer and evolutionary structural selection of rhombohedral phase during the growth process, stable epitaxial growth of single crystalline rh-In2O3 is achieved. The mechanism of phase selective epitaxial growth is studied by means of high-resolution X-ray diffraction and transmission electron microscopy measurements. Furthermore, Raman spectroscopy measurements are carried out to investigate the phonon properties of rh-In2O3. Raman-active phonon modes of rh-In2O3 are first identified. Introduction Two important crystal structures have been reported for indium oxide (In2O3): body-centered cubic (bcc-, Ia3, a ) 10.118 Å) and rhombohedral (rh-, R3jc, a ) 5.478 Å and c ) 14.51 Å). The physical and optical properties of bcc-In2O3 have been well investigated due to its novel applications as a material for transparent electrodes,1 for ultra sensitive gas detection,2 and as a barrier layer in tunnel junctions.3 The optical band gap of bcc-In2O3 was determined to be ∼3.7 eV,4–6 and the electrical properties of bcc-In2O3 can be tuned, for example, by tin doping (ITO). Furthermore, bcc-In2O3 can be obtained by means of different deposition methods, such as evaporation,4 magnetron sputtering,3 sol–gel processing,7 and molecular beam epitaxy.8 In the case of rh-In2O3, there are only a few reports concerning the growth and characterization of rh-In2O3 in the literature, because rh-In2O3 can only be synthesized through high pressure and high temperature processes.9,10 Sorescu et al. have grown nanocrystalline rh-In2O3 particles at moderate temperatures and pressures starting with In(NO3)3 · H2O using coupled hydrothermal and post annealing methods.11 However, the growth of single crystalline rh-In2O3 thin films has not been reported in the literature. Thus, further research on the properties and applications of this material is hindered due to lack of appropriate deposition methods. In contrast to common deposition techniques, metal organic chemical vapor deposition (MOCVD) offers the possibility to adjust the structural (single12 or polycrystalline,13 cubic or rhombohedral14) and electrical properties (metallic, semiconducting, or insulating) of In2O3.15 In our previous work, rhIn2O3 films mixed with bcc-In2O3 were grown by MOCVD.14 It was found that rh-In2O3 exhibits different structural, optical, and electrical properties compared to bcc-In2O3.16 An optical absorption edge of ∼3.0 eV for rh-In2O3 was found, which is much lower in comparison to the optical band gap of bcc-In2O3. Furthermore, probably due to the epitaxial growth of rh-In2O3 * To whom correspondence should be addressed. E-mail: chunyu.wang@ hotmail.com. † Technical University Ilmenau. ‡ Department of Physics, Shanghai University. § Instrumental Analysis and Research Center, Shanghai University. 4 Fraunhofer-Institut für Angewandte Festkörperphysik (IAF).
on sapphire substrates, the concentration of oxygen vacancies is lower than that in bcc-In2O3. As a result, rh-In2O3 is found to be insulating.15 However, in our previous work, a mixture of cubic and rhombohedral phases was deposited.14 Pure rhIn2O3 has not been obtained by MOCVD. Moreover, to the best of our knowledge, further optical characterization of single crystalline rh-In2O3, in particular phonon properties, has not been carried out or reported yet. In this paper, we report on the phase stabilization of single crystalline rh-In2O3 films on sapphire substrates by MOCVD. With the help of a high-temperature nucleation layer the epitaxial growth of rh-In2O3 is realized. The structural properties of the epitaxially grown rh-In2O3 films are characterized. Furthermore, Raman studies are carried out on both cubic and rhombohedral In2O3 films to determine the phonon vibration modes. Experimental Section The epitaxy of rh-In2O3 is carried out in a horizontal MOCVD reactor (AIXTRON 200) on c plane sapphire substrates. Trimethylindium (TMIn) and H2O vapor with flow rates of 3.6 and 1160 µmol/min are used as the indium and oxygen sources, respectively. Nitrogen serves as the carrier gas. To achieve phase stabilization, a two-step growth process has been designed, consisting of the formation of a nucleation layer at high substrate temperatures with the desired crystalline structure, followed by an epitaxial growth at low substrate temperatures, at which the selectively grown polymorphic modification of In2O3 is stable. The nucleation layer with an effective thickness of ∼150 nm was deposited at a substrate temperature of 600 °C with the TMIn flow rate of 3.6 µmol/min for 2 h by MOCVD, followed by a low temperature (300 °C) deposition of rh-In2O3 with 9 µmol/min TMIn flow rate for 5 h. The flow rate of water vapor was held at ∼1100 µmol/min during the growth process. The growth conditions of the nucleation layer were chosen according to the phase diagram described in our previous work.14 In contrast, bcc-In2O3 was grown at 600 °C on a low-temperature nucleation layer (300 °C). The crystal structure of rh-In2O3 is characterized ex situ by high-resolution X-ray diffraction (HRXRD), highresolution transmission electron microscopy (HRTEM), and atom force microscopy (AFM). The average layer thickness of the rh-In2O3 layer was determined to be ∼1 µm. Micro-Raman spectra measurements were carried out by a Raman spectrometer (Renishaw inVia) and a laser microscopy system with 514 nm Ar+ laser excitation.
Results and Discussion Figure 1a,b presents the HRXRD θ-2θ scans of the nucleation layer and the rh-In2O3 layer grown on c plane sapphire substrates
10.1021/cg700910n CCC: $40.75 2008 American Chemical Society Published on Web 02/21/2008
1258 Crystal Growth & Design, Vol. 8, No. 4, 2008
Wang et al.
Figure 2. (a) XTEM image shows cross sections of In2O3/Al2O3 structure; (b) HRTEM micrograph presents the rh-In2O3 structure near the surface area; (c) SAED pattern of In2O3/ Al2O3 interface: ∆ (Al2O3), 0 (bcc-In2O3), and n (rh-In2O3); (d) SAED pattern of bcc-In2O3 near the In2O3/Al2O3 interface taken along the direction; and (e) SAED pattern of rh-In2O3 near the rh-In2O3 surface taken along the direction.
Figure 1. (a) HRXRD θ-2θ scans of the nucleation layer and the rhIn2O3 layer grown on c plane sapphire substrates. (b) HRXRD Φ scans, which are taken for asymmetric rh-In2O3 (116) and Al2O3 (116) planes, respectively. The inset shows the detailed peak shape for rh-In2O3. (c) AFM image of the rh-In2O3 layer.
as well as Φ scans taken for asymmetric rh-In2O3 (116) and Al2O3 (116) planes, respectively. For the rh-In2O3 layer, four peaks are observed at 2θ ) 30.570, 35.506, 37.174, and 41.706°, corresponding to the diffractions from bcc-In2O3 (222), bccIn2O3 (400), rh-In2O3 (006), and Al2O3 (006), respectively. The lattice constant of rh-In2O3 was estimated to be 14.50 Å from the peak positions of the θ-2θ measurements by applying a kinematic diffraction model. The full width at half-maximum (fwhm) of the rh-In2O3 (006) peak is determined to be 0.06°. Furthermore, as one can see in Figure 1a, the intensity ratios between the rh-In2O3 (006) and bcc-In2O3 (222) diffraction peaks (Irh(006)/Ibcc(222)) are calculated to be ∼6 and ∼1200 for the nucleation layer and the rh-In2O3 layer, respectively, demonstrating that with the help of the nucleation layer, the growth of rhombohedral phase dominates, while the growth of cubic phase is suppressed. The In2O3 nucleation layer is deposited at high substrate temperature (600 °C) with a low TMIn flow rate, since at low substrate temperatures only the cubic In2O3 phase can be obtained.14 The role of the nucleation layer is to supply rh-In2O3 nucleation centers as well as to promote lateral growth of rh-In2O3 terraces.17 This growth
mechanism of rh-In2O3 is proven by the atomic force microscopy (AFM) measurements. As shown in Figure 1c, hexagonal structures of the rh-In2O3 terraces originating from rh-In2O3 with a mean diameter of ∼2 µm can be seen on the surface, proving that the rhombohedral phase dominates in the near surface region of the epilayer. The root square mean (rms) roughness of the surface layer (10 × 10 µm) is determined to be ∼48 nm, while the rms of the rh-In2O3 terraces (1 × 1 µm) is found to be only ∼3 nm. Furthermore, the HRXRD Φ scans (Figure 1b) taken for asymmetric planes of the substrate and the layer show that both rh-In2O3 and Al2O3 have a 6-fold symmetry. The six diffraction peaks originating from the {116} rh-In2O3 planes show very good agreement with those of the {116} planes of Al2O3, as shown in Figure 1b. Therefore, the in-plane epitaxial relation is determined to be rh-In2O3 |Al2O3. In addition, the inset in Figure 1b shows that a double peak for rh-In2O3 is observed due to tilted In2O3 terraces, which is in good agreement with the previous observations.14 These results are confirmed by cross-sectional transmission electron microscopy (XTEM) measurements, as shown in Figure 2a-e. Figure 2a shows a XTEM micrograph of an In2O3 terrace. A smooth surface of ∼1080 nm thick rh-In2O3 terraces can be obviously seen. A typical HRTEM image of the surface area is presented in Figure 2b. The arrow in the figure displays the (204) atomic plane of rh-In2O3. The surface roughness is determined to be
