The structural and optical characterizations of titania incorporated with alumina nanocrystals have been presented in this paper and the films exhibit excellent properties like low current density, small hysteresis as well as high photoluminescence quantum yields of about 361 nm. These properties are promising for the applications in future electronic devices.
KeywordsNanocrystal Electrical hysteresis Photoluminescence Pulsed laser deposition
During the past few years, many metal-oxide nanocrystals have attracted much attention because of their interesting electronic and optical properties for a wide range of applications. For example, SnO2 nanocrystals by doping with various additives have shown perfect detection of analytes in ppm concentration and long-term stability as metal-oxide gas sensors [1–3]. Similarly, ZnO2 nanocrystals have demonstrated the efficient blue-green emission for fluorescence-based applications [4, 5]. The research on new oxide materials with homogeneous nanocrystals is of key importance in order to achieve optimum performance in different electronic devices.
The amazing potential for these nano-size materials arise from the fact that it is possible to fabricate structures of radius smaller than the electron hole pair (exciton) Bohr radius [6, 7]. Because of the quantum confinement effect, the charge carriers can strongly be confined in nanocrystals. Therefore, the band gap will increase obviously as compared with the bulk material. Furthermore, in the confinement region, the band gap is conveniently tuned by virtue of adjusting the nanocrystal diameter to achieve some special electrical or optical properties. This particular property of nanocrystals supplies with the prime motivation to further investigate and optimize the new oxide materials.
Recently, it has been found that titania-incorporated alumina pseudobinary films as the next generation gate dielectrics can enlarge the band gap and restrain the exceeding leakage current . Although these properties are very attractive for the alternative gate dielectrics, it has also been reported that during high temperature (approach to the crystallization temperature) annealing of the amorphous films, the composition may decompose into some nanocrystals, and this may degrade the electrical characteristics of the gate dielectric, especially, for the pseudobinary system [9, 10]. Unfortunately, the thermal treatment is inevitable for current complementary metal-oxide semiconductor (CMOS) technique. In this regard, the electrical and optical properties of the Ti x Al1−xO y films with thermal treatment might differ largely from the amorphous films in the case of the existence of the nanocrystals.
Materials and Methods
Through a large number of experiments of the pseudobinary titania/alumina system, the deposition conditions and the film composition have been optimized. Here, we describe the characterization of the Ti0.25Al0.75O x thin films grown on n type silicon (100) substrates by a pulsed laser deposition procedure. The dense Ti0.25Al0.75O x target used in the experiment was prepared by a solid-state reaction process with pure starting materials of Al2O3 and TiO2 in a mole ratio of 1.5:1. The mixed powder in this ratio was ball-milled for 24 h, and then sintered at 1,500 °C for 7 h to form a dense ceramic target. The Ti0.25Al0.75O x thin films were deposited on silicon substrates with ρ = 2–3 Ω cm at 400 °C in a chamber of a low oxygen partial pressure 6.0 × 10−5 Pa. A KrF excimer laser (COMPex, Lambda Physik, 248 nm in wavelength, 30 ns in pulse width) running at 5 Hz with an average energy density of about 1.6–2.0 J/cm2 per pulse was employed. The distance between the substrate and the target was about 8 cm. The silicon substrates were ultrasonically cleaned by acetone and de-ionized water. Afterward the silicon substrates were immersed in the diluted hydrofluoric acid solution to remove the native silicon dioxide, thus leaving a hydrogen-terminated silicon surface. After the deposition, the amorphous films were in situ annealed at 400 °C in the chamber for 20 min to reduce the defects in the films. Based on the earlier research, the crystallization temperature of the film is a bit higher than 800 °C . Therefore, the deposited films were then annealed at 800 and 900 °C in the hermetic quartzose tubes full of argon for 1 h, respectively (named as S-1 and S-2 below). The samples were characterized by high-resolution transmission electron microscopy (HRTEM), current–voltage (I–V) measurement, and photoluminescence (PL) excitation spectroscopy. The PL excitation measurement was carried out using excitation source of 255 nm of xenon lamp at room temperature. Samples with different thicknesses according to the different measurements were prepared in the same procedure.
Results and Discussion
As indicated above, the HRTEM cross-section and electron diffraction patterns of the Ti0.25Al0.75O x films demonstrated the formation of nano-sized crystals. Only one-dimensional diffraction patterns of orthorhombic TiAl2O5 phase in the images indicate that the nanocrystals of the film preferably promote epitaxial c-axis-oriented growth. This promotion has been demonstrated by the results derived from the X-ray diffraction . With the X-ray diffraction results, only a small crystal peak attributed to the orthorhombic TiAl2O5 phase could be observed. From the macroscopical aspect, the preferable orientation is obvious, and the crystallization of the Ti0.25Al0.75O x film is anisotropic. Because of the nonstoichiometric composition, no evidence of the presence of TiO2 nanocrystals was detected in this sample.
However, the sweep loop characteristics of the investigated samples disclose the hysteresis. It is ascribed to traps located within the bulk Ti0.25Al0.75O x film or near the Ti0.25Al0.75O x film/silicon interface, such as oxygen vacancies and the other defects, which get filled with electrons from the applied electrical field upon sweeping to more positive gate voltages. At room temperature, the hysteresis of S-1 is larger than that of S-2. Such a decrease in hysteresis with annealing temperature reveals the presence of trap charging upon the temperature factor. Moreover, in the absence of applied electrical field, the negative shift (~0.2 MV/cm) of S-1 proves the existence of positive charges in the bulk film as well. By virtue of its capacitance–voltage curves (not shown here), it is calculated that the oxide trapped charges density is about as much as 1012/cm2.
In conclusion, we have performed a systematical analysis of titania-incorporated alumina nanocrystals. The present experiments demonstrate that the nanocrystals exhibit excellent properties like low current density and small hysteresis. Moreover, they offer high photoluminescence quantum yields at room temperature. This approach can be extended to other conditions such as low temperature, anion doping, and crystal size controlling.
This work was sponsored by National Natural Science Foundation of China (Grant number of 60576023 and 60636010), the State Key Program for Basic Research of China (2004CB619004), the State Key Program for Science and Technology of China (2009ZX02101-4) and Jiangsu Province Planned Projects for Postdoctoral Research Funds (0204003426).