Al-doped and undoped zinc oxide fi lms obtained by soft chemistry

Zinc oxide with a hexagonal wurzite type structure is an unique material that exhibits semiconducting, piezoelectric and pyroelectric properties. These properties play a key role for applications in optoelectronic devices. In the present work Al-doped and undoped ZnO fi lms were obtained by soft chemistry starting with zinc acetate dihydrate and Al(III) isopropoxide in absolute ethyl alcohol. Trietanolamine was used as chelating agent. The fi lms were deposited by dip coating technique on the silicon substrate and thermally treated at 500°C for one hour. The morphological characteristics of the fi lms were investigated by Atomic Force Microscopy (AFM). Optical constants, such as refractive index (n) and extinction coeffi cient (k), were established by Spectroellipsometry measurements. Electrical conductivity of the studied fi lms was determined in the 20–500°C temperature range by “the four point method”. The morphology of the fi lms is infl uenced by the starting sol composition, as found from AFM. According to the ellipsometric spectral data, more porous and thinner fi lms, with smaller refractive index were obtained in the case of Al-doped ZnO fi lms as compared with ZnO fi lms. Both ZnO and Al-doped ZnO fi lms presented high electrical resistivity.


I. Introduction
Pure zinc oxide, as an intrinsic n-type semiconductor with a band gap of about 3.3 eV, has attracted a great attention for obtaining transparent conductive fi lms used in solar cell technology, liquid crystal displays and energy-effi cient windows.Zinc oxide has a higher transmittance in the visible spectrum and is more stable in the presence of a hydrogen plasma than other conductive fi lms but its applications are limited by its low dc conductivity and low refl ectance in the infrared region [1].By doping with the Group VII or the Group III elements, the zinc oxide fi lms gain excellent and very stable electrical and optical properties [2].
The zinc oxide-based fi lms have been obtained both by physical methods including sputtering, evaporation, pulsed laser deposition [3][4][5][6] and chemical methods, such as chemical vapor deposition, sol-gel, chemical bath deposition [7][8][9][10].Interest has been increased during the last few years in the preparation of thin fi lms and oxide powders by the soft chemistry.With this type of method, multicomponent large scale oxide powders and fi lms can be obtained easily and with lower costs than with other methods, such as CVD, sputtering or vacuum evaporation.
In the present work Al-doped and undoped ZnO fi lms were obtained by soft chemistry starting with zinc acetate dihydrate and Al(III)-isopropoxide in absolute ethyl alcohol and triethanolamina (TEA) as chelating agent and catalyst.The morphological, electrical and optical characteristics of the obtained fi lms were determined.

Film deposition
The experimental conditions used for preparing the solutions needed for fi lms deposition were established on the previous paper published by Shuler and Aegerter [8].In our work the diethanolamine was replaced by tri-ethanolamine which allowed a signifi cant reduction in the amount of the required chelating agent used in order to obtain stable solutions of precursors (TEA/Zn = 1/5, as compared to DEA/Zn = 1/1).
The reagents used for obtaining ZnO and Aldoped ZnO thin fi lms were Zn(II)acetate dihydrate, Zn(CH 3 COO) 2 ×2H 2 O (ZAD, Reactivul) and Al(III)isopropoxide, Al(OC 3 H 7 i ) 3 (iPAl, Merck).Zinc and aluminium solutions of 0.1 M were obtained by dissolving zinc acetate dihydrate (ZAD) and Al(III)isopropoxide (iPAl) into absolute ethyl alcohol p.a reagent (Riedelde Haen).Zinc acetate solution was stirred at 50°C for 15 minutes then triethanolamine (TEA, Baker Analyzed) was slowly added drop wise in molar ratio of TEA/ZAD = 1/5 and continued with stirring at the same temperature for two hours for the obtaining Zn-sol.The Al-Zn-sol was obtained by adding the appropriate quantity of Al(III)-isopropoxide solution to the zinc acetate solution, so that fi nally aluminum atoms represented 5% in the Al-Zn mixture.In the Table 1 the experimental conditions for the solution preparation are presented.The clear and homogenous Zn and Al-Zn solutions were stored at room temperature for 24 hours before being used for the deposition.ZnO fi lms were deposited by dip-coating on thermally oxidized silicon wafers.
The thermal treatment of the deposited fi lms was realized at 500°C for 5 min with a heating rate of 5°C/ min, in the temperature programmable laboratory furnace.For multilayered coatings after each deposition the same thermal treatment was applied.All samples were additionally annealed at 500°C for 1 hour.The experimental conditions of the fi lms deposition and densifi cation are presented in Table 1.
The obtained fi lms were labelled as follows: ZO -ZnO fi lms; AZO -Al-doped ZnO fi lms.

Film characterization
FT-IR spectroscopy measurements were realized with a Nicolet 6700 apparatus in 400-4000 cm -1 domain.
XRD analysis of the fi lms was performed with Bruker D8 Advance diffractometer in the Bragg-Brentano con-fi guration.The scanning was made at room temperature in the range of 5-90°, with steps of 0.01 and 4 °/second.
The AFM experiments were carried out in the dynamic (non-contact) mode using an EasyScan 2 apparatus (Nanosurf AG, Switzerland) by means of a 10 μm × 10 μm scanner with vertical range of 2 μm and z-axis resolution of 0.027 nm.The scan rate was in the range of 1-2 Hz.The cantilever was with spring constants of about 34 N/m and the shape of the SiN tips was square pyramidal with radius of curvature of less than 10 nm and half angle 350.Scanning Probe Image Processor (SPIP™) software package (version 4.6.0.0) was used for image processing in terms of roughness and grain analysis.
The thickness of the layers (d) and volume fractions of the components were calculated from Spectroellipsometric data obtained with a null type ellipsometer.Experimental SE spectra have been simulated using the multilayer and multicomponent Bruggemann's Effective Medium Approximation (BEMA) model [11].
Electrical resistance measurements were performed by "the four point method".In order to obtain low specific resistance Ohmic contacts were deposited on the fi lm surface [12] (Ti (20 nm) / Au (30 nm) ohmic electrodes) ex-situ, by UHV e-beam evaporation.Copper leads were attached to the electrode with silver paste and were connected to an automatic polarization system.The electric voltage (V) was then measured with applied currents (I).The fi lms were put in an atmosphere controlled tubular furnace and the heating rate was 5°C/min.

Structure and morphology of prepared fi lms
Stable, clear and homogenous Zn and Al-Zn solutions were obtained in the conditions presented in the Table 1.Annealing temperature of the fi lms was established according to the data obtained from thermal analysis of the ZnO and Al-doped ZnO dried gels, formed after removal of the solvent at room temperature from the sol-gel solutions.The as-prepared fi lms deposited on silicon supports are amorphous and no characteristic peaks appear in the X-ray diffraction patterns.By thermal treatment at 500°C (Fig. 1), the crystallization of ZnO was observed by the presence of the diffraction lines at (100) (002) ( 101) and ( 110).
The very low intensity of the diffraction lines could be assigned to the low amount of oxide material investigated, taking into account greatly reduced thickness of the obtained fi lms (see Table 3).
FT-IR spectra of the ZO and AZO thermally treated fi lms present only characteristic bands assigned to Zn-O bonding at 430 cm -1 .
The AFM images of the as-prepared ZO and AZO fi lms with one deposition on silicon wafers are shown in Fig. 2. The surface root mean square roughness (RMS) of the as-prepared ZnO fi lm was determined to be about 4 nm (Fig. 2a).The smaller RMS roughness value was calculated for the as prepared Al-doped ZnO fi lm, being of about 1.4 nm (Fig. 2b).
The AFM images of the surface of the thermally treated ZO and AZO fi lms obtained after fi ve layer depositions are presented in Fig. 3.It can be noticed that the fi lms develop a distinct morphology.The surface roughness is higher than in the case of the as-prepared samples, being of about 6.8 nm for ZnO fi lm and 20.6 nm for the alumina-doped ZnO fi lm.The decreasing of the thickness of the fi lms by thermal treatment could be connected to the elimination of the residual organics from the solution used for fi lms deposition.
In the Table 2, values of median and maximum superfi cial grain length, width and height calculated by processing AFM images are presented.The superfi cial grains have been analyzed using SPIP software package based on the so-called watershed (with or without  gradient) and threshold methods [SPIP manual -available on www.imagemet.com]depending on the particular topography of each surface (e.g. the surface of AZO fi lm with 5 depositions exhibits a combination of grains and pores which could be separately identifi ed and analyzed).
From Table 2, an increase of the grain length can be seen, showing values of 2-5 times higher than the grain width.Also the height of the granules is about two orders of magnitude smaller than the length.This tendency suggests a two-dimensional development of the granules formed on the surface of the fi lm deposited on the silicon substrate and it is more pronounced in the case of Al-doped ZnO (AZO) fi lms.

Optical characteristics of prepared fi lms
The results concerning the optical constants and thickness of the ZnO-based fi lms obtained using the spectroscopic ellipsometry (SE) method (previously presented in paper [13]) are shown in the Table 3.The experimental SE spectra have been fi tted by taking as fi t parameters the thickness of the fi lm (d) and the volume fractions of the components: ZnO, Al 2 O 3 and voids (related to the fi lm's porosity).For the as prepared fi lms, besides the above mentioned components, there have been taken into account the dielectric constants of the solution used for deposition (denoted as "sol" in Table 3), previously measured with a Pulfrich-type refractometer.
The thickness of the as-deposited ZnO fi lm (d = 17 nm) (one deposition) is smaller than for the similar Al-doped ZnO fi lm (d = 43 nm).In the case of thermally treated fi lms, the thickness of Al-doped ZnO fi lms (d = 29 nm) is smaller than the thickness of the ZnO fi lms (d = 34 nm) (see Tabel 3).In the literature, it is mentioned that on SiO x /Si substrate the thickness of the ZnO fi lm obtained by aqueous sol-gel route does not exceed 20 nm, pointing out the weak adherence of the solution to the SiO x /Si substrate [14].The calculated quantity of voids, about 62% for ZnO fi lms and about 70% for Al-doped ZnO fi lms, indicates obtaining of porous fi lms in our experimental conditions.

Sample
SiO 2 buffer Film Optical constants (refractive index and extinction coeffi cient) for the studied fi lms are presented in Fig. 4.
Refractive indexes with 1.50 < n < 1.55 values and extinction coeffi cient k < 0.06 were determined for as-deposited ZnO fi lm (ZO as-dep ).Smaller values were obtained for as-deposited Al-doped ZnO fi lm (AZO as-dep ).The thermally treated ZO and AZO fi lms have small refractive indices due to the large volume of voids (≈62% and ≈70% respectively) in the bulk of the fi lm (see Table 3).

Electrical behaviour
The resistance of the ZnO and Al-doped ZnO fi lms obtained after fi ve layers deposition was measured by a standard four-probe technique.Measurements were made very carefully and repeated fi ve times to make sure that it is the true behaviour of the studied fi lms.
Fig. 5 shows the temperature dependence of the electrical resistance of the ZnO fi lm on silicon wafer.The ZnO fi lms have electrical resistance in the range of 10 2 -10 5 Ohm, higher values than those mentioned in the literature data for the fi lm obtained by sol-gel process [8].
A decrease of the electrical resistivity versus temperature indicates n-type semiconducting behaviour in the 300-600 K temperature range.
The electrical behaviour of the Al-doped ZnO fi lm is presented in the Fig. 6.
Higher electrical resistance values in the 300-800 K temperature range were obtained for Al-doped ZnO fi lm in comparison with ZnO fi lm.It can be noticed that in the 300-550 K temperature interval the electrical resistivity of the AZO fi lms remains unchanged and then decreases with the increase of the temperature up to 700 K when the resistivity increases.
This behaviour can be assigned to the high surface roughness and large volume of voids of the Al-doped ZnO fi lm.

IV. Conclusions
Thin ZnO and Al-doped ZnO fi lms deposited by dip-coating technique on SiO x /Si wafers were obtained by chemical route using zinc acetate dihydrate and al- uminium isopropoxide as zinc and aluminium precursors respectively, absolute ethanol as solvent and triethanolamine as chelating agent.Morphology of the fi lms is infl uenced by the starting sol composition, as found from AFM.
According to the ellipsometric spectral data, more porous and thinner fi lms, with smaller refractive index were obtained in the case of Al-doped ZnO fi lms as compared with ZnO fi lms.
Both ZnO and Al-doped ZnO fi lms present high electrical resistivity.
The high electrical resistance of fi lms deposited on silicon wafers obtained by chemical route recommend the ZnO and Al-doped ZnO fi lms for bulk acoustic resonator (FBAR).

Concentration
Figure 1.X-ray diffraction lines of the thermally treatedZnO fi lm Table1.Experimental conditions of the solution preparation, fi lms deposition and post-deposition thermal treatment

Aknowledgements:
The fi nancial support of the Romanian National Management Program, PN II type Project under the contract no.11061/18.09-2007 is gratefully acknowledged.Dr. S. Mihaiu acknowledges the COST 539 Action-Elena for the support in attending the 4 th Workshop "Fabrication, Properties & Applications of Electroceramic Nanostructures", Genoa, June 26-28, 2008.

Table 3 . Thickness (d) and volume fractions of the components of the studied fi lms
ZO as-dep -as-prepared ZO fi lm; AZO as-dep -as-prepared AZO fi lm *