Thermal atomic layer deposition of In2O3 thin films using a homoleptic indium triazenide precursor and water

Indium oxide (In2O3) is an important transparent conducting material widely used in optoelectronic applications. Herein, we study the deposition of In2O3 by thermal atomic layer deposition (ALD) using our recently reported indium(III) triazenide precursor and H2O. A temperature interval with self-limiting growth was found between ~270–385°C with a growth per cycle of ~1.0 Å. The deposited films were polycrystalline cubic In2O3 with In:O ratios of 1:1.20, and low levels of C and no detectable N impurities. The transmittance of the films was found to be >70% in visible light and the resistivity was found to be 0.2 mΩcm. The high growth rates, low impurities, high optical transmittance, and low resistivity of these films give promise to this process being used for ALD of In2O3 films that are good candidates for potential display and touch-screen applications.


I. INTRODUCTION
Indium oxide (In2O3) is a material of high interest due to its high electrical conductivity and optical transparency, making it a key material for transparent and optoelectronics 1 e.g. microelectronic displays on touch screens. The electrical conductivity of In2O3 2 is controlled by the stoichiometry of the film 3 and is typically enhanced by substituting the In atoms with 5-10 atomic % Sn 4,5 . All applications of In2O3 as transparent conducting layers require the deposition of high-quality thin films of In2O3. This requirement is the driving force for extensive studies by sputtering 6 , thermal evaporation 7 , chemical vapor deposition 8 , and atomic layer deposition (ALD) 9 . ALD of In2O3 is especially interesting as it can deposit high quality films with precise thickness, controlled composition, low impurity contents, and excellent conformality on complex substrates has been demonstrated. 10 ALD of In2O3 was first reported using InCl3 and either H2O or H2O2 at temperatures between 300-500 °C. 11 The low vapor pressure of InCl3, the generation of HCl as the reaction by-product and the possibility for InCl3 to etch the growing In2O3 layer 12 has motivated the use of other In precursors. Furthermore, there has been limitations on availability of efficient indium precursors that give processes with high growth rates, fully self-limiting reactions, low impurity contents and good film crystallinity. The indium compounds tested as precursors in combination with different oxidants, within different deposition temperature windows are shown in Table 1. to explain the faster kinetics of 1, which was thought to be due to its smaller endocyclic carbon substituent. This allowed the exocyclic N-isopropyl groups of the ligand to fold up more, leading to a surface species with less surface repulsion and a more exposed In centre for its subsequent reaction with H2O. 31 This surface chemical model was further confirmed when we used 1-3 for plasma ALD of InN. 37 Again, 1 was found to be superior to 2 and 3. To continue this trend, we replaced the substituted endocyclic carbon to a nitrogen atom. This led to our discovery of the indium(III) triazenide (4) (Fig. 1b), which we used with NH3 plasma to render high-quality InN. 38 Here, we investigate the use of 4 as In precursor in thermal ALD of In2O3 with water as oxidant. The ALD characteristics, optical and electrical properties of the deposited In2O3 films are investigated and a direct comparison between 4 and 1 is done.

A. Film deposition
Precursors 1 and 4 were synthesised using literature procedures 37,38 . Films were deposited using a homebuilt crossflow ALD reactor at 50 hPa. A flow of N2 (99.999%) was used as the carrier and purging gas. At all times during the deposition, the N2 pressure was kept constant. Precursors 1 or 4 and water were used as the In and O sources, respectively. The precursors were introduced separately into the reactor via stainless steel gas lines, which were extended into the reactor tube to ensure maximum substrate exposure. Precursors 1 and 4 were placed in a stainless-steel bubbler, which was heated to 120 °C, the gas delivery line was heated to 140 °C and flow of precursor into the chamber was aided by N2 carrier gas. The deionized H2O was kept at room temperature and delivered into the chamber without a carrier gas. Film deposition was undertaken on approximately 2×3 cm Si (100) and glass substrates, which were cleaned by dry N2 gas and used without further chemical cleaning. After loading the substrates, the chamber was baked at 155 °C overnight. The deposition zone of the reactor was defined by the heated zone of the tube furnace. Unless otherwise noted, a typical ALD cycle consisted of a 4 s pulse of 1 or 4 and 3 s pulse of H2O, with 10s N2 purges after the In precursor and the water pulses. The deposition process was studied between 150 and 520 °C.

B. Film characterization
The crystallographic phases of the films were characterized by a X-ray diffractometer (XRD) PANalytical X'pert Pro equipped with a Cu Kα X-ray source (λ = 1.54Å) in the θ-2θ mode. A Ni foil was used to filter the Kβ radiation. Film thickness was measured using the same PANalytical X'Pert Pro X-ray diffractometer in X-ray reflectivity (XRR) mode. The film thickness was obtained from the XRR data using X'Pert Reflectivity software and a two-layer model, In2O3/Si, to fit the data. The film morphology was analyzed by scanning electron microscopy (SEM) using a LEO 1550 scanning electron microscope with an acceleration energy of 2kV using the in-lens detector at a working distance of 3-6mm. SEM was also used to obtain film thickness on samples that were too thick or rough for XRR measurements. The composition and impurity levels of the deposited films were probed with high resolution X-ray photoelectron spectroscopy (XPS), Kratos AXIS Ultra DLD, equipped with an Ar (0.30 eV) sputtering source for 600 s, employing monochromatic Al Kα radiation (hν =1486.6 eV). Sputter-cleaning (sputter-etching) was undertaken to eliminate surface contamination, which resulted after exposure of the substrate to air.
Analysis of the photoelectron spectra was completed using the CasaXPS software package. 39 By quantitative analysis, the signals originating from the substrate and the thin film could be de-convoluted and the chemical composition was obtained. Gaussian-Laurentius functions and Shirley background were used to fit the experimental XPS data.
The optical, transmittance, absorbance, and bandgap properties of the films were obtained using a Shimadzu UV-2450 UV-VIS spectrophotometer in a wavelength range of 250-800 nm. The UV-VIS spectrophotometer was operated in the reflectance mode in analyzing films deposited on glass substrates. Determination of band gap was determined using the Tauc plot formalism from data obtained by UV-Vis spectrophotometry based on the Tauc relation: (αhν) 1/y = β (hν -Eg) where α is the absorption coefficient (absorbance/film thickness), β is the band tailing parameter, h is the Planck's constant, ν is the frequency of incident light, Eg is the energy of the optical band gap and y is the power factor, which depends upon the nature of the transition (semi-conducting materials, y =0.5 for direct allowed transitions). Therefore: (αhν) 2 = β(hν -Eg).
By drawing a tangential line from the plotted graph of hν versus (αhν) 2 , the value of the band gap was determined as the value of hν at the point where this line crosses the axis i.e the intercept of the extrapolation to near zero absorption with photon energy axis i.e.
(αhν) 2 → 0. Electrical characterization by resistivity measurements were performed using the 4-point probe technique. Measurements were done on a Jandel, Model RM3000 test unit or probing system, which was first calibrated using a standard glass sample before sample resistivities were determined.

A. Film deposition
The ALD process with 4 and water was studied by varying their pulse lengths 290 °C. The resulting saturation curves (Fig. 2a)

B. Film properties
The XRD analysis, acquired in the θ-2θ mode, of films deposited at 315 °C and 430 °C (Fig.   3) showed cubic In2O3 as the only crystalline phase except the Si substrate. In addition, the films are polycrystalline with a preferred (222) orientation.
XPS analysis of the films showed the C impurities range from 0.8 to 3.5 at. % and N impurities are below the detection limit. As summarized in Table 2, XPS analysis also shows the films are oxygen deficient, which is comparable to literature. 10 The elemental composition of the films displayed in Table 2 (Fig. 4) show smooth films at lower temperatures (Fig. 4a), and larger, flattened, and faceted grains at higher temperatures (Fig. 4b). Cross-section micrographs further indicate smooth and uniform films grown at 315 °C (Fig. 4c). The results above shows that In2O3 can be deposited from 4 and water in an ALD process.
We speculate that the primary reaction mechanism for the film deposition is a surface stoichiometry is not ideal since it has no oxygen vacancies, which results in increased resistivity. 43 In2O3 typically exhibits n-type semiconducting properties from the oxygen vacancies 19 , which increase carrier density and hence act as donors. 1 The deposited films in this study show a general decrease in resistivity, i.e., an increase in conductivity, with longer water pulse time. The improved conductivity for films deposited with increased H2O dosage may be attributed to incorporation of hydroxyl groups which cause selfdoping. 44 The resistivities of different film samples, all with approximately 50 nm thicknesses, deposited with 6 s, 9 s and 12 s of water pulse were 1.2; 2.6 and 0.16 mΩcm, respectively (Table 3). Upon postdeposition annealing at 520 °C for 2 hours in air, the In:O ratio in the film changed from 1:1.2 to 1:1.3 but this lead to an increase in the resistivity from e.g., 1.2 to 2.0 mΩ.cm (shown in Table 3). This result is in line with previous reports on the importance of the oxygen vacancies for the electrical conductivity of the In2O3 films.  5 shows that the transmittance increases with longer water pulse, which from Table   2, renders higher oxygen content in the film.
The results from the resistivity and transmittance measurements are in line with the literature on In2O3 where the oxygen stoichiometry in the films which has been shown to impact both optical transmission and electrical conductivity of the films. 19 The high concentration of oxygen vacancies causes our films to be more conducting than those reported in previous studies 42,47 .

C. Comparison to the In(famd)3 precursor (1).
From the results above, ALD using 4 and water renders In2O3 films with properties comparable to the previously reported In2O3 films deposited using 1, albeit 4 seems to require higher deposition temperatures. A temperature window of 150 -275 °C for In2O3 films using 1 and H2O was reported, but slow surface kinetics of 1 were observed. 31 This was then circumvented using a stop-flow process to give 1 more time to react with the surface. We cannot do stop-flow ALD in our ALD reactor, and therefore undertook a direct comparison between 1 and 4 by studying ALD of In2O3 using 1 and water in our reactor.
We found a temperature interval of 245 -315 °C where the GPC is constant with the temperature when using a 5 s pulse of 1 and 4 s pulse of H2O (Fig. 7). This is higher than the 150 -275 °C reported for 1, again suggesting a slow surface kinetics for 1. A stop flow mode was used to allow for a lower deposition temperature of In2O3 using 1, whilst we employed a multiple-pulse approach using 1 for depositing InN by ALD. 37 The higher temperature interval found for ALD of In2O3 using 1 in our ALD reactor suggest that the temperature window obtained for 4 could possibly also be lowered by using stop flow mode in the ALD process. What perhaps speaks against this is that ALD of InN using

IV. SUMMARY AND CONCLUSIONS
In summary, we used our recently reported In(III) triazenide precursor in combination with H2O to deposit In2O3 films in a thermal ALD process. Polycrystalline In2O3 films with a cubic structure were successfully deposited. The GPC, impurities, and In/O ratios are in line with the literature for other In precursors. The transmittance was found to be >70% in visible light and the resistivity was found to be 0.2 mΩcm. A temperature interval with self-limiting growth was found between 270 -385°C. This is higher than that for ALD of In2O3 using the previously reported In(famd)3 precursor. After a direct comparison between the two In precursors used in this study, we suggest the temperature interval for 4 could be lowered by using a stop flow mode ALD process.