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 .
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 . because of the quantum confinement effect
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 .
unfortunately , the thermal treatment is inevitable for current complementary metal - oxide semiconductor ( cmos ) technique . in this regard
, the electrical and optical properties of the tixal1xoy films with thermal treatment might differ largely from the amorphous films in the case of the existence of the nanocrystals .
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 ti0.25al0.75ox thin films grown on n type silicon ( 100 ) substrates by a pulsed laser deposition procedure . the dense ti0.25al0.75ox 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 ti0.25al0.75ox thin films were deposited on silicon substrates with  = 23  cm at 400 c in a chamber of a low oxygen partial pressure 6.0  10 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 1.62.0 j / cm per pulse was employed .
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 .
the 50-nm - thick pseudobinary ti0.25al0.75oxfilms were post - annealed at 800 and 900 c after deposition , respectively . the cross - sectional hrtem image of the s-1 is shown in fig .
1 . a representative image displays a fairly smooth interface layer between the film and the silicon substrate .
some changes have appeared in the bulk of the s-1 after post - annealing treatment .
there are several observable bright / dark contrast fluctuations in the film . the electron diffraction pattern of the s-1 as inset of fig .
1has shown a typical amorphous halo , which indicates the film is still in amorphous state .
therefore , it could be deduced that these locations are some composition - rich regions , even a few small nanocrystals .
when the films have been annealed approaching to the crystallization temperature , the grain coarsening has occurred , and the increase in grain size has been observed .
hrtem cross - section image of s-1.insetelectron diffraction image of s-1 in comparison , several crystal regions have been observed in the hrtem image of the s-2 and are shown in fig .
the fast fourier transformation ( fft ) measurement has been carried out on these regions to obtain the complex situations of these nanocrystals , and the relevant image is shown in the right as inset figure . from the figure one can observe that it is a mixed nanocrystal region , because the diffraction pattern is a superposition of the patterns from two pieces of nanocrystals .
both of interplanes spacings , whose values are about 0.237 nm and lie at an angle of near to 60 , are of regular parallelogram with a center and corresponding to the   and   planes of the hexagonal al2o3 , respectively . as for the other dots , the evaluated two interplanes spacings are equivalent to 0.242 nm .
it is presumed that the two spots correspond to the ( 004 ) and   planes of orthorhombic tial2o5 , respectively .
theinseton therightshows the magnified image and the fft image of the selected nanocrystals as indicated above , the hrtem cross - section and electron diffraction patterns of the ti0.25al0.75ox 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 ti0.25al0.75ox film is anisotropic . because of the nonstoichiometric composition , no evidence of the presence of tio2 nanocrystals was detected in this sample .
3a , b. the s-1 has very good insulating properties , as apparent from the substantial current of about 10 a / cmat an electric field of 2 mv / cm applied between the silicon substrate and the metal contact .
comparably , the s-2 exhibits a significantly increased leakage current of 10 a / cmat the same electric field , which is almost as much as 4 orders of magnitude derived from s-1 .
considering the hrtem results , this confirms the crucial role of the amorphous al2o3 in the insulating properties of the dielectric stack , despite its small amount and thickness .
current density versus bias electric field foras-1 andbs-2 at room temperature however , the sweep loop characteristics of the investigated samples disclose the hysteresis .
it is ascribed to traps located within the bulk ti0.25al0.75oxfilm or near the ti0.25al0.75oxfilm / 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 ,
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 10/cm .
as we all known , raman spectrum provides a fast and convenient method to detect the small structural changes .
4 that show the same peaks at about 618 cm and 814 cm , which are usually detected in amorphous ti  o materials , and ascribed to ti  o stretching and ti  o  ti stretching , respectively .
the latter stretching may also have contributions from a ti  o stretch assigned to a short ti 
therefore , the large intensity of this peak indicates the a two dimensional connectivity and provides the evidence of the presence of ti  o  ti chainlike structure with a shortening ti 
its full width at half maximum ( fwhm ) of 50 cm is in contrast to the s-1 of fwhm = 40 cm at the similar peak region .
the raman linewidth broadening , primarily caused by phonons confinement in nanocrystals , is inversely proportional to the size of the nanocrystals .
optical raman spectra for s-1 and s-2 in order to further understand the nature of the charge carrier trapping , migration and transfer in ti0.25al0.75ox films with small nanocrystals and the pl excitation spectroscopy with the emission wavelength fixed at 255 nm were performed for the sample s-2 . in general , it is difficult to observe the photoluminescence phenomenon at room temperature for bulk tio2 due to its indirect transition nature .
however , some nano - sized tio2 particles and mesoporous - structured powders have been reported to exhibit room temperature photoluminescence .
figure 5 shows the pl excitation spectroscopy of a broad excitation peak centered at ~361 nm . the samples exhibit the very small stokes shift between the absorption and the emission , which characterizes the energy relaxation resulting from interfacial roughness , defects , and other structural imperfection .
herein , the main probability lies in the defects of nanoclusters and/or nanocrystals in the bulk film .
generally , the electrons are trapped by oxygen vacancies or confined within quantum dots in nanocrystals region . on the other hand
, the excited electrons can transfer from the valance band to the new levels that exist upper of the conduction band introduced by the dopant .
nevertheless , such a meaningful value has not been previously reported for nanostructural films comprising titania and alumina , and its realization within the present films is notable consideration that no attempts were made to control the size of the nanocrystals .
photoluminescence emission spectra for s-2 , excitation wavelength 255 nm.insetthe energy band structure of the sample
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 .
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 ) .
 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 . 
Introduction
Materials and Methods
Results and Discussion
Conclusions
Acknowledgments