Nb and F co-doped anatase TiO2 layers were deposited by low pressure chemical vapor deposition (LPCVD) at pressure of 3 mtorr using titanium-tetra-iso-propoxide (TTIP), O2 and NbF5 as precursor, oxidant and dopant respectively. Resistivity beyond 100 Ω cm for undoped layer was decreased with increasing supply of the dopant and dependent on the supply ratio of O2 to TTIP and decreased to 0.2 Ω cm by the optimization. X-ray fluorescent spectroscopy showed Nb-content in the layer was decreased with the O2-supply ratio. X-ray photo-spectroscopy indicated that F substituted O-site in TiO2 by O2-supply but carbon-contamination and F missing substitution in the O-site were significantly increased by excess O2-supply. Further, it was suggested that the substituted F played an important role to reduce resistivity without significant contribution of O-vacancies. XRD spectra showed F missing substitution in the O-site degraded the crystallinity.
Nowadays, Indium-Tin-Oxide (ITO) is widely used for transparent conducting oxide (TCO) to fabricate flat panel displays, solar-cells and so on. However, the amount of Indium on earth is significantly low as shown by Clarke number. Therefore, the alternative TCO without Indium has been attractively studied, for example, Ga-doped ZnO (GZO), F-doped SnO2 (FTO) etc. [
Conductivity controlled TiO2 layers are also expected to use for such as chemical sensors, solar-cells, and the other electronic devices in solutions because of the high resistance against acidand alkaline-electrolytes. For such device applications, chemical vapor deposition (CVD) is a useful process comparing to physical vapor deposition in the view of step-coverage on the three-dimensionally structured surfaces to enhance the sensitivity and the efficiency in addition to the micro-fabrication by reactive ion etching [
In this paper, low-pressure CVD (LPCVD) by TTIP and O2 mixed gas was applied for TiO2 deposition to control conductivity by Nb-F co-dope using NbF5 as the dopant with the studies by X-ray fluorescent spectroscopy, X-ray photo-spectroscopy and X-ray diffraction.
A bell-jar type reactor with the base pressure under 1 × 10−5 torr by a combination of diffusion pump (D.P.) and a rotary pump (R.P.) as shown in
troduced into the reactor. Niobium pentafluoride (NbF5: 98%-purity) powder was charged in a crucible consists of boron-nitride (BN) and then thermally evaporated. 1 mm-thick quartz plate with optically flat surface was used as substrate, which was mounted on a substrate holder after chemical cleaning. Temperature of the substrate holder and the crucible were monitored by K-type thermo-couples (T.C.) and controlled by resistive heating and PID-systems.
Thickness of the layer was checked by a surface profiler (DEKTAK150). Resistivity was evaluated by Van Der Pauw (VDP) method. Densities and chemical states of impurities were analyzed by X-ray fluorescent spectroscopy (XRF: Shimazu XRF-1700) and X-ray photo-spectroscopy (XPS: Thermo VG Scientific,UK). Crystallographic behavior was examined by θ-2θ X-ray diffraction (RIGAKU: RAD-C) using CuKα.
or
In such feature, the deposition above 360˚C can be recognized to be limited by TTIP-supply. In this work, all samples for the study of Nb-F doping were deposited at 380˚C in the TTIP-supply limiting region for removal residual impurities such as carbon and/or hydrocarbons and to enhance crystallization into the anatase-phase.
Resistivity of the undoped-layer deposited at 380˚C was higher than 100 Ωcm because the resistivity was over the evaluation-limit in our VDP-system which was able to evaluate the resistivity below 100 Ωcm. On the other, the resistivity of Nb-F doped TiO2 layer was significantly reduced with the crucible temperature for NbF5 evaporation (TNbF5) and dependent on O2/(TTIP + O2) gas supply ratio as shown in
the TNbF5 above 160˚C, but increased in the range from 70 to 100˚C. The increase of resistivity with the TNbF5 was due to melt of NbF5 at the temperature above 70˚C, that is, the NbF5 was sublimated below 70˚C and evaporated above 100˚C after melting. It is noted that the carrier density and Hall mobility could not be determined by the VDP-system because of trap-induced conductions such as hopping, but conventional check by Zeebeck effect showed n-type conductivity for the doped layers. In addition to the decrease of resistivity, the deposition rate was decreased with NbF5-supply as shown by open circles in
spectra of Nb-F doped TiO2 layers deposited by the O2/(TTIP + O2) supply ratio of 0.27 (green-line), 0.38 (black-line) and 0.61 (red-line) respectively, with the C1s spectrum of undoped layer by the O2-supply ratio of 0.50 (blue-line). It is noted that the spectrum originated from Nb was under detectable limit. Extremely weak C1s spectrum peak at 284.3 eV was appeared on the undoped layer, which indicated TTIP was successfully dissociated to TiO2. On the other, significantly increased spectra were observed in the doped layers depending on the O2-supply ratio, where the peak energy was shifted to 284.7 eV. Although the peak at 284.3 could be assigned to physisorbed carbons, the shift of peak-energy and the obvious increase of intensity indicated that the origin was not only adventitious carbons but also carbon-compounds included in the layers. Recently, Karlsson et al. studied TTIP-dissociation in ultra-high vacuum by XPS and showed C1s spectra peak at 285 eV is attributed to adsorbed methyl-group originated from TTIP [
The role of oxygen has been in progress by first-principles molecular-orbital calculations, but it can be expected as follows.
NbF5 is adsorbed on the deposition surface (probably to titanium) where H2O formed during Ti-O-Ti bridging from Ti-OH---OH-Ti is simultaneously supplied to the NbF5, then the NbF5 is oxidized and HF is desorbed from the surface as the below scheme.
In this scheme, F-content in the layer is decreased with H2O-density on the surface.
NbF5 adsorbed on the deposition surface is dissociated by oxygen as follow scheme, and then the dissociated F with the high reactivity adsorbs to titanium on the surface with high sticking probability or fluorinates the TTIP.
The adsorbed F decreases the sticking probability of the next coming NbF5 and TTIP, which resultantly decreases Nb-content in the layer and the deposition rate. Further, the fluorination disturbs thermal dissociation of TTIP, which is resulted in decreased of the deposition rate and increase of residual carbons in the layer. Of course, the dissociation of NbF5 by H2O as shown in the Scheme 1 is also including during the deposition but the oxygen-contribution becomes dominant by the high O2-supply ratio according to the increased O2-partial pressure comparing to H2O on the surface.
Oxygen chemical state in the layer was also influenced by the O2/(TTIP + O2) supply ratio.
luted spectra using Gaussian-function were also shown by dot-lines. Single spectrum peak at 529.5 eV were observed for the undoped and the doped layers deposited by the gas ratio of 0.27 and 0.38, but the other spectrum peak at 531.9 eV was also included for the doped layer by high O2-supply ratio of 0.61. Previously, the double spectra was discussed for ITO layers, in which the spectrum at low high energy is concluded to be originated from oxygen neighboring cations coordinated six oxygens but the spectrum is shifted toward higher energy-side by contribution of oxygen-vacancies [
peak angle should be shifted toward high angle-side (decrease of the d-spacing) comparing to that of the layers deposited in Region I and Region II. It is therefore difficult to recognize that the shift was due to the substituted Nb and F and expected to be caused by the fluorine missing the substitution in the oxygen site without crystallization into TiOF2. As a result, it could be mentioned that removal the insufficient fluorine is required to improve crystallinity for further reduction of the resistivity.
Nb and F were simultaneously doped in anatase-TiO2 by using NbF5 on low pressure chemical vapor deposition using TTIP and O2. Nb and F-content in the layer were dependent on the O2-supply ratio, in which the Nb-content was decreased with increasing the gas supply ratio but F was increased in the high gas-supply ratio. XPS studies indicated F substitutes O-site in TiO2 by the O2-supply but carbon contamination is also increased by the excess O2-supply. It was suggested by comparison between the resistivity and the XPS results that F in the O-site plays an important role to reduce the resistivity without oxygen-vacancies. XRD spectra speculated F missing substitution in the O-site degrades the crystallinity.