Thin Solid Films 576 (2015) 88–97

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r .com/ locate / ts f

Doped zinc oxide films grown by hot-wire chemical vapour deposition

A. Abrutis ⁎, L. Silimavicius, V. Kubilius, T. Murauskas, Z. Saltyte, V. PlausinaitieneVilnius University, Faculty of Chemistry, Naugarduko 24, LT-03225 Vilnius, Lithuania

⁎ Corresponding author. Tel.: 370 5 2193173; fax: 370E-mail address: adulfas.abrutis@chf.vu.lt (A. Abrutis).

http://dx.doi.org/10.1016/j.tsf.2015.01.0100040-6090/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 May 2014Received in revised form 22 December 2014Accepted 9 January 2015Available online 15 January 2015

Keywords:Transparent conducting oxidesThin filmsHot-wire CVDZinc oxideIn, Ga, and Al dopingElectrical propertiesOptical propertiesAnnealing

Hot-wire chemical vapour deposition (CVD)was applied to grow zinc oxide (ZnO)-based transparent conductingoxide films. Indium (In)-, gallium (Ga)-, and aluminium (Al)-doped ZnO films were deposited at 400 °C onsapphire-R, Si (100) and glass substrates using a cold wall pulsed liquid injection CVD system containing ni-chrome wires installed in front of the substrate holder. Zn, In, Al 2,2,6,6-tetramethyl-3,5-heptanedionates, andGa3,5-pentanedionate dissolved in 1,2-dimethoxyethanewere usedas precursors. Hallmeasurementswere per-formed to evaluate the resistivity, carrier concentration, and carrier mobility in doped ZnO films grown on sap-phire substrates at wire currents of 6 A and 9 A. The influence of the dopant type, doping level, substrate, andwire heating current on crystallinity and the electrical and optical properties of the films was investigated anddiscussed. The best electrical properties were obtained for Al- and Ga-doped films grown at 9 A wire current(resistivity ≈ 1 × 10−3 Ωcm, carrier mobility ≈ 50 cm2 V−1 s−1 and carrier concentration ≈ 1 × 1020 cm−3).The films exhibited a high transmittance in the mid-infrared region (≈90% at 2.5 μm). Additional annealing ofthe films at 400 °C in a mixture of Ar and hydrogen (10%) resulted in the increase in carrier concentration andmobility and in the reduction of film resistivity.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Zinc oxide (ZnO) doped with aluminium (Al), gallium (Ga), or indi-um (In) is now considered as the most promising transparentconducting oxide (TCO) to replace indium tin oxide (ITO) electrodesin optoelectronic applications [1–3]. The search for alternative TCOsfor industrial applications was motivated mainly by the scarcity andhigh price of In, the principal component of ITO. The utility of TCO thinfilms depends on both their optical and electrical characteristics. Fortechnological applications, TCO should have a low electrical resistivityclose to 1 × 10−4 Ωcm and optical transparency of ≥90% in the visiblespectral range. However, many other properties of TCO films (light scat-tering, high chemical durability, environmental and thermal stability,stability in plasmas, …) are also important and should be consideredin the development of alternative TCO for particular applications. Forsome applications, the high optical transparency of TCO films shouldbe extended into the ultraviolet (UV) and infrared (IR) spectral ranges.The cost of fabricationmethod is also a significant factor for the choice ofthe most appropriate TCO material.

Many techniques have been used to prepare In-, Ga- and Al-dopedZnO films: magnetron sputtering (In — [4], Ga — [5–8], Al — [9–12]),pulsed laser deposition (In— [13], Ga— [14,15], Al— [16,17]), molecularbeam epitaxy (Ga— [18], Al— [19]), sol–gel (In— [20,21], Ga— [22,23],Al— [24]), atomic layer deposition (In— [25], Ga— [26], Al— [27–29]),solution spray-pyrolysis (pneumatic nozzle or ultrasonic aerosol

5 2330987.

generator) using inorganic precursors (In — [30–33], Ga — [33], Al —[33–36]), atmospheric pressure chemical vapour deposition (CVD)from inorganic precursors (Ga — [37]), atmospheric pressure metal-organic chemical vapour deposition (MOCVD) that uses metal-organic(MO) precursors (Ga — [38]), aerosol-assisted atmospheric pressureMOCVD (In — [39], Al — [40]) and low pressure MOCVD (In — [41],Ga — [42–47], Al — [48,49]).

The quality of the deposited TCO films highly depends on the depo-sition method, which mainly influences the crystallinity and micro-structure of films, and as a consequence, their electrical and opticalproperties. Improved electrical properties are obtained in films grownby physical vapour deposition (PVD) techniques compared with thosegrown by chemical vapour deposition, mainly because of the improvedcrystalline quality. PVD techniques allows the growth of Ga- and Al-doped ZnO films with a resistivity very close to 1 × 10−4 Ωcm, whichis comparablewith the resistivity of ITOfilms. In some cases, a resistivitybelow 10−4 Ωcm was reported for Ga- and Al-doped ZnO films grownby pulsed laser deposition: 0.812 × 10−4 Ωcm for gallium doping [15]and 0.854× 10−4Ωcm for Al doping [17], with transmittance in the vis-ible spectral range higher than 85% and 88%, respectively. Although thecarrier mobility in these films is enhanced (31 cm2 V−1 s−1 [15] and47.6 cm2 V−1 s−1 [17]), their low resistivity is mainly determined by avery high carrier concentration (1021–1022 cm−3). Chemical depositionmethods still result in a higher resistivity of doped ZnO films (usually inthe range ≈ 10−3–10−1 Ωcm) compared to PVD films (≈10−4–

10−3 Ωcm). The highest carrier mobility (~40–60 cm2 V−1 s−1) wasmeasured in PVD-grown doped ZnO films [11,17–19]. Among thechemical deposition methods, the highest carrier mobility was

89A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

measured for doped ZnO films deposited by MOCVD techniques:29 cm2 V−1 s−1 [39], 30.4 cm2 V−1 s−1 [38], and 18.9 cm2 V−1 s−1

[29] for In, Ga, and Al doping, respectively. However, the dispersion ofthe reported values of carrier mobility for doped ZnO films is veryhigh in the case of both chemical and physical deposition methods,with the values varying over a large range from ~1 to 60 cm2 V−1 s−1.This dispersion demonstrates that the carrier mobility is very depen-dent not only on the deposition method but also on small variations inthe deposition conditions that influence the film composition, surfacequality, crystallinity, and microstructure. Although PVD techniquesstill allow the growth of doped ZnO films with better properties, theuse of CVD methods seems to be more desirable from a technologicalpoint of view because CVDmethods exhibit high growth rates, uniformand conformal coverage of large areas of uneven or structured surfaces,and are simpler and more flexible in the tuning of the film composition(e.g., doping level).

In the present article, we demonstrate that an alternative chemicalmethod, hot-wire chemical vapour deposition (HW-CVD), can be usedto grow high-quality In-, Ga-, and Al-doped ZnO films at relatively lowtemperature. Currently, HW-CVD is largely used for device-qualitysilicon-related applications, mainly photovoltaic devices, for researchand industrial applications [50]. In HW-CVD, the source gases aredecomposed on a hot wire placed at some distance in front of the sub-strate, which leads to film deposition from highly active species. Typi-cally, high wire temperatures are used (1700–1800 °C, with atungsten, tantalum, or iridium filament) to obtain complete decomposi-tion of metal-organic precursors and evacuation of the formed radicalsfrom the hot wire towards the substrate. However, depending on thematerial deposited and the precursors used, such a high wire tempera-ture may not be required. For example, recently, we used HW-CVD todeposit Ge2S2Te5 and GeTe layers at a wire temperature of ≈600 °C[51,52]. Other applications of HW-CVD technique have appeared in re-cent years, including the deposition of metallic copper [53] and thegrowth of the oxides of TiO2 [54] and SnO2 [55]. To date, HW-CVD hasnot been used by other laboratories to grow ZnO and related TCOmate-rials. Recently, we reported [56] the results demonstrating the principalpossibility of using this technique for the deposition of films in the ZnO–In2O3 system. In this work, we present a much extended study on thedeposition of doped ZnO layers by the HW-CVD technique. This reportincludes three dopant elements (In, Ga, and Al) and more detailed in-vestigations of the electrical and optical properties of the film. A simul-taneous study of several doping elements (which is rarely presented inthe literature) allows direct comparison of the influence of the dopingelement on the film properties.

2. Experimental details

Our proposed deposition method is based on the combination ofHW-CVD with pulsed injection of the precursor solution [57], which al-lows for good control over the precursor dose and the film compositionand thickness.

A simplified diagram of the cold wall HW-CVD system used in thisresearch was presented previously [56]. Doses (several microlitres) ofprecursor solution are injected in pulses into a hot evaporator using acomputer-controlled electromagnetic injector. After flash evaporationof the injected doses under a vacuum, the resulting vapour mixture istransported by the carrier gas (Ar + O2) into the deposition chamber.Prior to reaching the substrate, the vapour passes through hot-wire spi-rals installed 2 cm in front of the substrate holder. The precursor decom-position on the hot wire and the simultaneous evacuation at lowpressure of the residual radicals from the wire yield an oxide film onthe cooler substrate. The use of a pulsed liquid injection system and asingle solution of mixed precursors provide great flexibility to changethe film composition and growth rate by altering the solution composi-tion/concentration and the time and frequency of the pulses.

Tungsten or tantalum wires, which are traditionally used in HW-CVD, are not suitable for oxide deposition because of their low resis-tance to prolonged heating in an oxygen-containing atmosphere. Ac-cording to the results of an investigation of the behaviour of variousmetals and alloys in oxidising environments [58], nichrome wire waschosen as the hot wire. Three spirals (45 turns, ∅ ≈ 3 mm, ≈35-mmlength) of nichromewire (∅≈ 0.25–0.3mm)were connected in paral-lel (with a gap of≈1 cmbetween each) in a single current block,≈2 cmfrom the substrate holder surface. The wires were heated by an AC cur-rent of I=0–9 A. A current of 9 Awas themaximumpossible prolongedcurrent because increasing it up to 10 A often caused thewires to break.The temperature of the wires was not measured; however, according totechnical data from the producers of nichromewire, the temperature ofthe wire at 9 A should be approximately 1000 °C.

2,2,6,6-Tetramethyl-3,5-heptanedionates of Zn, In, Al (Zn(thd)2,In(thd)3, Al(thd)3), and acetylacetonate of Ga (Ga(acac)3) synthesisedin our laboratory were used as precursors. These precursors are air sta-ble and do not require special conditions for handling andmanipulation,which facilitates the deposition procedure. However, their relativelyhigh thermal stabilities make it very difficult to achieve a sufficientgrowth rate of films at low temperature (≤400 °C) by simple thermalCVD. Mixtures of Zn(thd)2 and a precursor of the doping element withappropriate ratios were dissolved in 1,2-dimethoxyethane (total con-centration 0.05 M). The solution of precursors was then injected intothe evaporator, which was heated at 200 °C at a frequency of 2 Hz, apulse time of 3 ms, and a dose mass of ≈3 mg. The carrier gas flow(Ar+O2)was set at 250 sccm,with anO2 content of 20% and total pres-sure of 1.33 kPa. Films with a thickness of ≈160 nm were grown onsapphire-R, Si(100), and glass substrates. Sapphire-R was themain sub-strate for detailed study of the film properties.

Depositions were performed at the substrate temperature of≈400 °C, which was measured by an external thermocouple placedon the substrate holder surface near the substrate. An internal thermo-couple in the holder was used to adjust the temperature. Because thehot wire also heats the substrate, especially at higher currents (8–10 A), the substrate temperature cannot be fully stabilised attemperatures ≤ 400 °C. For example, in the case of a 9 A current, thesubstrate temperature increases from≈395 to≈405 °C during deposi-tion (mean temperature of 400 °C, with a reproducibility of ≈2 °C). Atlower currents (e.g., 6 A), the deposition temperature can be stabilised(within 2 °C) at lower temperatures (300–400 °C). A possible way tobetter stabilise the substrate temperature would be to increase the dis-tance between the hot wire and substrate (not tested here).

The crystallinity and phase composition of the filmswere studied byX-ray diffraction (XRD; Bruker D8 Advance, Cu-Kα radiation, Bragg–Brentano geometry), while their morphology and elemental composi-tion were examined using a scanning electron microscope (SEM;Hitachi SU-70) equipped with energy dispersive X-ray spectroscopy(EDS). SEM operating voltage of 2 kV was used in the study of film sur-face. EDS analysis offilm compositionwas performed at 15 kV (accumu-lation time of 100 s). Atomic force microscopy (AFM; VeecoMultimodeSPM) was used to evaluate the surface roughness of films. The filmthickness was measured by profilometry (Taylor–Hobson Talystep).The transparency of the films was investigated using UV–Vis and IRspectrometers (PerkinElmer Lambda 35, in the UV–Vis range and Fron-tier FTIR, in themid-IR range). The resistivity of the filmswas measuredby the standard four-point probe technique (Agilent 34410A). Hall Ef-fect measurements were performed in the van der Pauw configurationat room temperature in a 0.5 T magnetic field (ohmic In–Ga contacts).

3. Results and discussion

The preliminary study of the growth rate dependences of ZnO, In-doped ZnOand In2O3films on the current passing through the nichromewires was reported in our previous work [56]. That work indicated thatthe growth rates are very low(1–2.5 nm/min at a substrate temperature

90 A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

of 400 °C) when the wires are not heated at all (pure CVD growth) orheated using a low current (3 A). The heating of thewires starts to influ-ence thefilm growthwhen the current reaches 4 A. A further increase inthe current results in a sharp increase in the film growth rate, with apartial saturation found for currents above 6 A. In general, the hotwire had a marked effect: the growth rates increased up to 15–20 nm/min, i.e., by a factor of ≥10 compared with that of pure CVD. The in-crease in current from 4 to 7 A facilitated the incorporation of In intothe In-doped ZnO films, with saturation observed at higher currents.

In the present extended study, a series of In-, Ga-, and Al-doped ZnOfilms with variable doping levels were grown at 400 °C. Two heatingcurrents of nichromewires (6 A and 9 A)were chosen for thefilm depo-sitions to investigate the possible influence of the current on the filmproperties. In total, six series of doped ZnO films were prepared andinvestigated.

3.1. Composition, structure, and surface morphology of the films

Relationships between the content of a doping element in solutionand that in the film for all prepared series of films are presented inFig. 1. Near linear dependences were observed at both studied currents,allowing for a rather good control of thefilm composition. Incorporationof the doping element into the ZnO film is improved at higher wire cur-rent. It ismore difficult to incorporate Al compared to In and Ga. This ef-fect may be partially related to a clearly higher thermal stability of theAl(thd)3 precursor compared to the precursors of In(thd)3 andGa(acac)3.

According to XRD data, the un-doped and doped ZnO films deposit-ed at both 6 A and 9 A current on Si(100) and glass substrates werepolycrystalline, with a variable ratio of (100)/(002)/(101) peaks intheir XRDpatterns. Films on sapphire-R substrates had a pure (110) tex-ture (only a peak at 2θ≈ 56.5° is visible in the XRD patterns). A similarresult was reported in [56], where a (110) texture was observed for In-doped ZnO films on sapphire-R and LaAlO3 (001) substrates. A shift in

Fig. 1.Relationship between the quantity of doping element (In, Ga, or Al) in solutions andin ZnO films grown at wire currents of 6 A (a) and 9 A (b).

the diffraction peaks to lower 2θ values was observed for films on allof the substrates as the content of In increased. This shift correspondsto the increase in the lattice parameter because some of the Zn ions(0.074 nm) are replaced by larger In ions (0.084 nm) in the ZnO crystal-line lattice (a similar effect was reported in [21,39] and an opposite ef-fect was reported in [31]). This replacement is exemplified in Fig. 2a,taking the In–ZnOfilms grown on sapphire as an example. In this figure,the change in distance between (110) planes (d(110)) is presented in re-lation to the In content in films and with the current passing throughthe wire. Fig. 2b and c shows similar data for the films of ZnO–Ga andZnO–Al, respectively. Ga ions have a smaller radius (0.062 nm) thanZn ions; however, the replacement of Zn ions by Ga ions almost doesnot change the lattice parameters. In contrast, doping by Al decreasesthe lattice parameters of ZnO due to the change in Zn ions by the clearlysmaller Al ions (0.051 nm). A similar effect of Al dopingwas observed in[27,29]. It is interesting that the doping level is not the only factorinfluencing the lattice parameter, as it is alsodependent on thewire cur-rent. This effect of the wire current is especially visible in the case ofdoping by large In ions, while it is less expressed when ZnO is dopedby Ga and Al. Fig. 2d, e, and f represents the change in the full width athalf maximum (FWHM) of the ZnO XRD (110) peak in relation to thedoping element, the quantity of dopants in the film, and the wire cur-rent. The smaller width of the diffraction peaks indicates better crystal-line quality and larger crystallites of the film. One can see that theFWHM decreases with the doping level and reaches the minimumvalues at an intermediate concentration of the doping element in thefilm. Lower FWHM values and accordingly better crystalline qualityare obtained for films grown at higher (9 A) wire current; moreover,the FWHM values slightly decrease in the order of In–Ga–Al. A similarXRD study of the annealed films revealed that annealing does notchange the crystalline quality of doped ZnO films, that is, the values ofd(110) and FWHM(110) remain almost the same.

Roughness average (Ra) values of the surfaces of the films onsapphire-R are presented in Fig. 3 in relation to the quantity of dopingelement. Lower Ra values are observed for films containing intermedi-ate quantities of doping elements. The values of Ra for these films varybetween 1 nm and 2 nm, i.e., the films exhibit quite smooth surfaces.The nature of the doping element had no significant influence on thesurface roughness of films grown at 9 A current. However, films of In-doped ZnO grown at lower (6 A) current were rougher than thosedoped by Ga and Al. The SEM images presented in Fig. 4 show the sur-face morphologies of the smoothest films containing different dopingelements and grown at two different currents of wires.

3.2. Electrical properties of the films

The results of Hall measurements for all six series of films depositedon sapphire-R substrates are presented in Figs. 5 and 6, where the de-pendences of carrier mobility (μ) and concentration (n) as well as filmresistivity (ρ) on the doping level and wire current are shown. Disper-sion of the measured μ, n and ρ values was within 10% for samples ob-tained in different deposition runs. The figures also contain similardata for films annealed for 30 min at 400 °C in an argon mixture withhydrogen (10%). A common feature of these dependencies is the pres-ence of maxima or minima at intermediate contents of a doping ele-ment. The carrier concentration increases up to a certain doping leveland starts to decrease in heavily doped films. The rise in the carrier con-centration and conductivity with the doping may be explained by thefact that the doping element in the Zn centre is a more effective donorthan the intrinsic donor centres in ZnO, usually identified as oxygen va-cancies or metallic interstitials [1,2]. Similarly, the evolution of the car-rier mobility with the doping level reaches a maximum, whichcorrelates with the best crystalline quality and the smoothest surfaceof thedopedfilms (Fig. 2 and 3). The further reduction in the carriermo-bility may be related to the significant increase in carrier scattering byionised dopant atoms and with the lower crystalline quality of heavily

Fig. 2.Distance between crystallographic planes (100) (a, b, c) and FWHM values of XRD (110) reflection (d, e, f) in relation to the quantity of doping element (In, Ga, or Al) in ZnO filmsgrown at 6 A and 9 A wire currents.

91A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

doped ZnO films [1,2]. A high dopant concentration could also result inclustering of dopant ions, which may increase the scattering rate [1].

Fig. 3. Roughness average (Ra) values in relation to the quantity of the doping element (In,Ga, or Al) in ZnO films grown at 6 A (a) and 9 A (b) wire currents (Ra calculated from1 × 1 μm2 AFM scans).

Comparison of Figs. 5 and 6 reveals a pronounced difference be-tween the electrical properties of doped ZnO films grown at two differ-ent currents (6 A and 9 A) of the wire. As-deposited films grown at 6 Acurrent exhibit a low carrier mobility (μ ≈ 11–13 cm2 V−1 s−1) and ahigh carrier concentration (n≈ 2–2.5 × 1020 cm−3). In contrast, the car-rier concentration is significantly reduced (up to ≈1 × 1020 cm−3) infilms grown at 9 A current, while the carrier mobility is markedly in-creased (the best values are ≈16, 45 and 52 cm2 V−1 s−1 for In, Ga,and Al doping, respectively). Such a high mobility of carriers is a ratherunusual property of doped ZnO films grown by chemical depositiontechniques; a much lower mobility of ≤10 cm2 V−1 s−1 is typically ex-hibited for such films.

The electrical properties of deposited films exhibiting the lowest re-sistivity are presented in Table 1. One can see that films grown at 9 A ofcurrent exhibit lower resistivity than those grown at 6 A (except filmsdoped with In). In general, a rather low resistivity of films close to10−3 Ωcm may be obtained by HW-CVD; however, the factors respon-sible for film resistivity may be different. The low resistivity of filmsgrown at a current of 6 A ismainly caused by the high carrier concentra-tion, while in the case of films grown at a current of 9 A, the low resis-tivity is determined by the high mobility of the carriers. The carriermobility increases in the order of In–Ga–Al doping, which is more visi-ble for films grown at 9 A of current. This result suggests that the defectsin a crystalline lattice formed by the doping element with the smallerionic radius have less detrimental effects for carrier mobility in filmsgrown by HW-CVD.

It is difficult to explain such a large difference betweenmobility andconcentration of carriers in films grown at different wire currents; amore extended study is necessary to ascertain the reasons for such adifference. However, this effect indicates that the electrical propertiesmay be effectively varied in films by changing the wire current(temperature).

The resistivity of polycrystalline doped ZnO films grown on Si andglass substrates (Table 2) is clearly higher than those for highly texturedfilms on sapphire-R substrate, especially in the case of a higher wire

Fig. 4. SEM images of the surfaces of In-, Ga-, and Al-doped ZnO films grown at 6 A (left) and 9 A (right) wire currents. The quantity of the doping element in the film and the roughnessaverage Ra values are indicated below each image.

92 A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

current (9 A). The lowest resistivity in polycrystalline films was obtain-ed at a slightly higher concentration of doping element compared to theepitaxial films (compare Tables 1 and 2). Moreover, an inverse order ofresistivity was observed for polycrystalline doped ZnO films comparedto epitaxial ones — the resistivity of polycrystalline films decreases inthe order of Al–Ga–In doping. This difference demonstrates a significanteffect of film orientation on the electrical properties of TCO films.

The results of the annealing study presented in Figs. 5 and 6; Table 1indicate that even a short annealing time (30 min) of films at 400 °Ctemperature in an Ar + H2 (10%) mixture can markedly improve theelectrical properties of films. Significant improvement in the electricalproperties by annealing in Ar–H2 mixture was also observed for Al–ZnO films grown by the atomic layer deposition method [27]. Themain effect of annealing is the increase in the carrier concentration,which may be related to an incorporation of hydrogen atoms as addi-tional donor centres [59,60]. The effect of annealing on carrier mobilityis less expressed, except for the films doped by Al and grown at 9 A,where some larger increase in mobility was observed. It is difficult to

explain the increase in carriermobility by annealing because the changein lattice parameters and in crystalline quality (FWHM) of annealedfilms is negligible. In general, the simultaneous increase in the carrierconcentration and mobility resulted in a significant reduction of resis-tivity of annealed films, especially for Al-doped films grown at a currentof 9 A (decrease in the resistivity of up to ≈4 × 10−4 Ωcm).

3.3. Optical properties of the films

Effective TCO films have a high transmittance in the visible spectralrange, which is limited by the edge in the near-UV spectral range andthe edge in the mid-IR spectral range. The position of the transmittanceedge in the near-UV range is determined by the optical band gap (Eg),while the transmittance edge in the mid-IR region exists mainly dueto reflection at the plasma frequency [1,2]. Both edges usually have aclear correlation with the carrier concentration. The high carrier con-centration increases the value of Eg as well as the reflection at the plas-ma frequency, so both optical transparency edges become shifted

Fig. 5.Carriermobility (μ), carrier concentration (n), and resistivity (ρ) of as-deposited and annealed doped ZnOfilms in relation to the quantity of doping element (In, Ga, orAl) in thefilm.The films are grown at 6 A wire current.

93A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

towards higher photon energies (lowerwavelengths). The results of thestudy of the optical properties of the deposited films are presentedbelow, and they are found to completely agree with the known theoret-ical statements [1,2].

Fig. 6.Carriermobility (μ), carrier concentration (n), and resistivity (ρ) of as-deposited and anneThe films are grown at 9 A wire current.

UV–Vis spectra of the best doped ZnO films on sapphire substrates(of the films considered in Table 1) are shown in Fig. 7a, b, and c. Thespectra are presented for as-deposited and annealed films, and they in-dicate a high transparency of films in the visible spectral range. The

aled doped ZnOfilms in relation to the quantity of doping element (In, Ga, orAl) in thefilm.

Table 1Electrical properties of as-deposited and annealed In-, Ga-, and Al-doped ZnO films grown on sapphire-R substrates.

Wire current,A

Doping element Doping level in film,at. %

As-deposited films Annealed films (400 °C, 30 min, Ar + 10% H2)

ρ,Ωcm

n,cm−3

μ, cm2 V−1 s−1 ρ,Ωcm(change, %)

n,cm−3

(change, %)

μ,cm2 V−1 s−1

(change, %)

6 In 3.0 2.70 × 10−3 2.10 × 1020 11.0 1.93 × 10−3 (−28.5) 2.57 × 1020 (22.4) 12.6 (14.5)Ga 4.3 1.94 × 10−3 2.60 × 1020 12.4 1.15 × 10−3 (−37.8) 3.53 × 1020 (40.1) 15.4 (18.5)Al 4.0 2.23 × 10−3 2.19 × 1020 12.8 1.06 × 10−3 (−52.5) 3.50 × 1020 (59.8) 16.8 (31.3)

9 In 3.5 3.69 × 10−3 1.05 × 1020 16.1 2.14 × 10−3 (−42.0) 1.60 × 1020 (52.4) 18.2 (13.0)Ga 3.5 1.36 × 10−3 1.01 × 1020 45.5 7.29 × 10−4 (−46.4) 1.72 × 1020 (70.3) 49.8 (9.5)Al 3.0 1.22 × 10−3 0.99 × 1020 52.0 4.34 × 10−4 (−64.4) 2.19 × 1020 (121.7) 65.7 (26.3)

94 A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

insets in these figures correspond to the transformations of these spec-tra into the plots using the coordinates (αhν)2–hν (α— absorption co-efficient, hν — photon energy), from which the optical band gaps maybe determined [28]. Fig. 7d, e, and f shows the relations between theband gap and the quantity of the doping element for as-deposited andannealed films grown at 6 A and 9 A wire currents. The Eg value of as-deposited films increases with the quantity of the doping element andreaches a maximum at a certain doping level. The increase in thevalue of Eg with the doping level exhibits a rather good correlationwith the increase in the carrier concentration in films (Figs. 5 and 6)and may be explained in terms of the blue shift due to the Burstein–Moss effect [1]. The blue shift is more significant for doped filmsgrown at 6 A current because the carrier concentration achieved by dop-ing in these films is significantly higher compared to that of the filmsgrown at 9 A current (compare Figs. 5 and 6). The increase in the Egvalues of the annealed films may be explained similarly because themain result of annealing is the increase in the carrier concentration inthe films (Figs. 5 and 6).

The results of the spectroscopic study of the as-deposited andannealed films in the mid-IR region are presented in Fig. 8. Fig. 8a, b,and c shows spectra for the best In-, Ga-, and Al-doped ZnO films,respectively, grown at 6 A and 9 A (films included in Table 1). A clearshift of the IR-absorption edge towards lower wavelengths is observedwhen the wire current is changed from 9 A to 6 A and when the filmsare annealed. This effect is related to a higher carrier concentration inthe films grown at 6 A and to the increase in the carrier concentrationafter annealing.

Fig. 8d, e, and f represents the change in transmittance of films at awavelength of 2.5 μm in relation to the dopant element, doping level,andwire current. These dependences correlate ratherwell with the var-iations in the carrier concentration in the films (Figs. 5 and 6). The car-rier concentration reaches a maximum value at a certain intermediatedoping level, which also determines the minimum transmittance inthe mid-IR region. A higher carrier concentration in films grown at 6 Aand in all annealed films results in a lower transmittance at λ=2.5 μm.

Table 2Resistivity of In-, Ga-, and Al-doped ZnO films grown on silicon and glass substrates.

Substrate Wire current,A

Doping element Doping level in film,at.%

ρa

Ωcm

Si (100) 6 In 5.8 7.0 × 10−3

Ga 5.1 1.1 × 10−2

Al 6.1 1.4 × 10−2

9 In 7.0 8.2 × 10−3

Ga 6.7 1.5 × 10−2

Al 4.4 1.5 × 10−2

Glass 6 In 5.8 8.0 × 10−3

Ga 5.1 7.5 × 10−2

Al 6.1 5.5 × 10−1

9 In 7.0 2.5 × 10−2

Ga 6.7 1.7 × 10−1

Al 4.4 1.6 × 100

a Measured by the four-point probe technique (Hall measurements not performed).

An important result of this work is that the doped ZnO films grownby HW-CVD at 9 A exhibit simultaneously low resistance and anextended range of high transmittance in themid-IR region. The resistiv-ity of the best films deposited at 9 A is close to 10−3 Ωcm, while theirtransmittance at λ = 2.5 μm is close to 90% (89% — In, 92% — Ga,88% — Al). This simultaneous reduced resistivity and enhanced trans-mittance are possible because the low resistivity offilms is causedmain-ly by a high carrier mobility, so the reduced carrier concentration inthese films results in a lower absorption in the mid-IR region. Thesefilms are promising for applications as TCO in the extended IR range(e.g., solar cells and IR-lasers). On the contrary, the deposition of filmsat 6 A and film annealing in Ar – H2 mixture allows the significant in-crease in the carrier concentration and the Eg values and the shift inthe high transmittance range into the UV region. Low resistivity dopedZnO films prepared in such a way exhibited significantly enhancedband gap values (Eg = 3.52 eV — In, 3.63 eV — Ga, 3.69 eV — Al) com-pared to the un-doped ZnO films (Eg ≈ 3.3 eV).

There remains a broad scope to improve these initial results by fur-ther optimisation of the HW-CVD process, including the use of otherprecursors, testing of various wire materials, variation of the wire dis-tance from the substrate, etc. The main attempts should be performedto improve the crystalline quality of the doped ZnO films, which mayallow a further increase in the carrier mobility, decrease in the resistiv-ity, and extension of the high transmittance spectral range. The presentwork revealed a significant influence of the wire current (wire temper-ature) on thefilmproperties, so this study should be extended to exploitthis possibility of the tuning of the film properties. For this purpose,other metals and alloys should be investigated on the role of a hotwire, especiallymaterials allowing a significant increase inwire temper-ature in an oxygen-rich environment.

A promising way to improve the properties of as-deposited TCOfilms is to anneal films under different conditions. In the present work,the applied annealing of films at 400 °C in an inert gas and hydrogenmixture led to a significant increase in the carrier concentration,conductivity, and optical band gap. The lowest resistivity of≈4×10−4Ωcmwas obtained for annealed Al-doped ZnOfilms. This re-sistivity is among the best results obtained by chemical deposition butremains far from the target resistivity of ≤10−4 Ωcm and from thevalues measured for the best doped ZnO films (≈1 × 10−4 Ωcm)grown by physical deposition techniques. We expect that further de-tailed optimisation of the annealing conditions (temperature, gas com-position, pressure, and time) might be an alternative means to improvethe properties of doped ZnO films.

4. Conclusions

The presented results demonstrate that high-quality TCO films of In-, Ga-, and Al-doped ZnO can be grown byHW-CVD. The use of a hotwirein the CVD process makes it possible to grow such films at lower tem-perature and with a higher growth rate compared with those of simplethermal CVD. Films grown on Si (100) and glass substrates were poly-crystalline, while those grown on sapphire-R substrates exhibitedhigher crystalline quality and had a pure (110) texture. Better electrical

Fig. 7. UV–Vis spectra (a, b, c) of as-deposited and annealed (dashed lines) In-, Ga-, and Al-doped ZnO films grown at 6 A and 9 A wire currents (corresponds to the films presented inTable 1). The insets in the figures show the Eg values determined for these films. Figures d, e, and f demonstrate the change in the Eg values of as-deposited and annealed films withthe quantity of doping element in the films grown at 6 A and 9 A currents.

95A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

properties were found for films grown on sapphire-R, which demon-strated the importance of film epitaxy on the electrical properties. The

Fig. 8. IR spectra (a, b, and c) of as-deposited and annealed (dashed lines) In-, Ga-, and Al-dopTable 1). Figures d, e, and f show the change in transmittance at λ = 2.5 μm of as-deposited acurrents.

resistivity close to 10−3 Ωcmwas obtained in the case of all three dop-ing elements (1.2 × 10−3, 1.4 × 10−3, and 3.7 × 10−3 Ωcm for Al, Ga,

ed ZnO films grown at 6 A and 9 A wire currents (corresponds to the films presented innd annealed films with the quantity of doping element in the films grown at 6 A and 9 A

96 A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

and In, respectively). A marked influence of wire current on the electri-cal and optical properties of films was demonstrated, which allows foran effective control of film properties by changing the wire current. Al-though the resistivity of the films grown at different wire currents wasrather similar, the factors determining the low resistivity of films werethe opposite. The best films grown at 6 A exhibited a high carrier con-centration (≈2–2.5 × 1020 cm−3) and low mobility (≈11–13 cm2 V−1 s−1), while films prepared at 9 A current had a reducedcarrier concentration (≈1 × 1020, similar for all three dopants) andenhanced mobility (≈16, 45, and 52 cm2 V−1 s−1 for In, Ga, and Aldoping, respectively). Different carrier concentrations and mobilityvalues caused different optical properties of films grown at 6 A and9 A currents. Due to a higher carrier concentration, the blue sift of theUV absorption edge with doping is larger in the case of 6 A current(Eg = 3.47–3.57 eV at 6 A and 3.33–3.46 at 9 A). In contrast, the highercarrier mobility and lower carrier concentration in the films grown at9 A were the main reason for the shift in the IR-transmittance edgetowards higher wavelengths (compared to that of the films grown at6 A). These films exhibited high transmittance close to 90% at λ =2.5 μm and are promising for applications in IR optoelectronic devices.

It was found that an effective way to improve the electrical prop-erties of doped ZnO films grown by HW-CVD is an annealing in Ar–H2 (10%) atmosphere at 400 °C. The main effect of the annealing isa significant increase in the carrier concentration, while the increasein the carrier mobility is less expressed. The greatest effect of theannealing was observed for Al-doped ZnO films grown at 9 A: theincreases in the carrier concentration and the mobility resulted inthe reduction of the resistivity down to≈4 × 10−4 Ωcm. The furtherimprovement in the electrical and optical properties of doped ZnOfilms should be possible by more detailed optimisation of the HW-CVD process (use of other precursors, testing of various wire mate-rials, variation of the wire distance from the substrate, etc.) andannealing conditions (time, temperature, gas atmosphere, andpressure).

Acknowledgements

This work was supported by the Research Council of Lithuania(Grant No. MIP-023/2013).

References

[1] L. Castaneda, Present status of the development and application of transparent con-ductors oxide thin solid films, Mater. Sci. Appl. 2 (2011) 1233.

[2] H. Liu, V. Avrutin, N. Izyumskaya, U. Ozgur, H. Morkoc, Transparent conducting ox-ides for electrode applications in light emitting and absorbing devices, Superlattice.Microst. 48 (2010) 458.

[3] U. Ozgur, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho,H. Morkoc, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98(2005) 041301.

[4] K. Djessas, I. Bouchama, J.L. Gauffier, Z. Ben Ayadi, Effects of indium concentration onthe properties of In-doped ZnO films: applications to silicon wafer solar cells, ThinSolid Films 555 (2014) 28.

[5] D.L. Zhu, Q. Wang, S. Han, P.J. Cao, W.J. Liu, F. Jia, Y.X. Zeng, X.C. Ma, Y.M. Lu, Optimi-zation of process parameters for the electrical properties in Ga-doped ZnO thin filmsprepared by r.f. magnetron sputtering, Appl. Surf. Sci. 298 (2014) 208.

[6] Yoon-Duk Ko, Ki-Chul Kim, Young-Sung Kim, Effects of substrate temperature onthe Ga-doped ZnO films as an anode material of organic light emitting diodes,Superlattice. Microst. 51 (2012) 933.

[7] Kuang-Chieh Lai, Chien-Chih Liu, Chih-HungYeh Chun-Hsiung Lu, Mau-PhonHoung, Characterization of ZnO:Ga transparent contact electrodes for microcrystal-line silicon thin film solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 397.

[8] Hisao Makino, Naoki Yamamoto, Aki Miyake, Takahiro Yamada, YoshinoriHirashima, Hiroaki Iwaoka, Takahiro Itoh, Hitoshi Hokari, Hisashi Aoki, TetsuyaYamamoto, Influence of thermal annealing on electrical and optical properties ofGa-doped ZnO thin films, Thin Solid Films 518 (2009) 1386.

[9] Z. Ben Ayadi, H. Mahdhi, K. Djessas, J.L. Gauffier, L. ElMir, S. Alaya, Sputtered Al-doped ZnO transparent conducting thin films suitable for silicon solar cells, ThinSolid Films 553 (2014) 123.

[10] Jinsu Yoo, Jeonghul Lee, Seokki Kim, Kyunghoon Yoon, I. Jun Park, S.K. Dhungel, B.Karunagaran, D. Mangalaraj, Junsin Yi, High transmittance and low resistive ZnO:Al films for thin film solar cells, Thin Solid Films 480–481 (2005) 213.

[11] F. Ruske, M. Roczen, K. Lee, M. Wimmer, S. Gall, J. Hüpkes, D. Hrunski, B. Rech, Im-proved electrical transport in Al-doped zinc oxide by thermal treatment, J. Appl.Phys. 107 (2010) 013708.

[12] B. Stannowski, F. Ruske, S. Neubert, S. Schönau, S. Ring, S. Calnan, M. Wimmer, O.Gabriel, B. Szyszka, B. Rech, R. Schlatmann, Potential of high-mobility sputteredzinc oxide as front contact for high efficiency thin film silicon solar cells, ThinSolid Films 555 (2014) 138.

[13] R.K. Gupta, K. Ghosh, R. Patel, S.R. Mishra, P.K. Kahol, Band gap engineering of ZnOthin films by In2O3 incorporation, J. Cryst. Growth 310 (2008) 3019.

[14] V. Bhosle, J. Narayan, Microstructure and electrical property correlations in Ga:ZnOtransparent conducting thin films, J. Appl. Phys. 100 (2006) 093519.

[15] Sang-Moo Park, Tomoaki Ikegami, Kenji Ebihara, Effects of substrate temperature onthe properties of Ga-doped ZnO by pulsed laser deposition, Thin Solid Films 513(2006) 90.

[16] O. Bamiduro, H. Mustafa, R. Mundle, R.B. Konda, A.K. Pradhan, Metal-like conductiv-ity in transparent Al:ZnO films, Appl. Phys. Lett. 90 (2007) 252108.

[17] H. Agura, A. Suzuki, T. Matsushita, T. Aoki, M. Okuda, Low resistivity transparentconducting Al-doped ZnO films prepared by pulsed laser deposition, Thin SolidFilms 445 (2003) 263.

[18] Ken Nakahara, Kentaro Tamura, Mitsuhiko Sakai, Daisuke Nakagawa, Norikazu Ito,Masayuki Sonobe, Hidemi Takasu, Hitoshi Tampo, Paul Fons, Koji Matsubara,Kakuya Iwata, Akimasa Yamada1, Shigeru Niki, Improved external efficiencyInGaN-based light-emitting diodes with transparent conductive Ga-doped ZnO asp-electrodes, Jpn. J. Appl. Phys. 43 (2004) L180.

[19] T. Ohgaki, Y. Kawamura, T. Kuroda, N. Ohashi, Y. Adachi, T. Tsurumi, F. Minami, H.Haneda, Optical properties of heavily aluminum-doped zinc oxide thin films pre-pared by molecular beam epitaxy, Key Eng. Mater. 248 (2003) 91.

[20] M. Thambidurai, Jun Young Kim, Chan-mo Kang, N. Muthukumarasamy, Hyung-JunSong, Jiyun Song, Youngjun Ko, Dhayalan Velauthapillai, Changhee Lee, Enhancedphotovoltaic performance of inverted organic solar cells with In-doped ZnO as anelectron extraction layer, Renew. Energy 66 (2014) 433.

[21] K.J. Chen, F.Y. Hung, S.J. Chang, Z.S. Hu, Microstructures, optical and electrical prop-erties of In-doped ZnO thin films prepared by sol–gel method, Appl. Surf. Sci. 255(2009) 6308.

[22] Chien-Yie Tsay, Yu. Shih-Hsun, Optoelectronic characteristics of UV photodetectorsbased on sol–gel synthesized GZO semiconductor thin films, J. Alloys Compd. 596(2014) 145.

[23] Amal AlKahlout, A wet chemical preparation of transparent conducting thin films ofGa-doped ZnO nanoparticles, J. Sol–Gel Sci. Technol. 67 (2013) 331.

[24] Zhanchang Pan, Junming Luo, Xinlong Tian, Wu. Shoukun, Chun Chen, JianfengDeng, Chumin Xiao, Hu. Guanghui, ZhigangWei, Highly transparent and conductiveSn/F and Al co-doped ZnO thin films prepared by sol–gel method, J. Alloys Compd.583 (2014) 32.

[25] Andrea Illiberi, Robert Scherpenborg, Mirjam Theelen, Paul Poodt, Fred Roozeboom,On the environmental stability of ZnO thin films by spatial atomic layer deposition, J.Vac. Sci. Technol. A 31 (2013) 061504.

[26] Paul R. Chalker, Paul A.Marshall, Simon Romani, JosephW. Roberts, Stuart J.C. Irvine,Daniel A. Lamb, Andrew J. Clayton, Paul A. Williams, Atomic layer deposition of Ga-doped ZnO transparent conducting oxide substrates for CdTe-based photovoltaics, J.Vac. Sci. Technol. A 31 (2013) 01A120.

[27] Su Cheol Gong, Yong-June Choi, Hyuncheol Kim, Chang-Sun Park, Hyung-Ho Park, JiGeun Jang, Ho Jung Chang, Geun Young Yeom, Aluminum-doped zinc oxide formedby atomic layer deposition for use as anodes in organic light emitting diodes, J. Vac.Sci. Technol. A 31 (2013) 01A101.

[28] Tara Dhakal, Daniel Vanhart, Rachel Christian, Abhishek Nandur, Anju Sharma,Charles R. Westgate, Growth morphology and electrical/optical properties of Al-doped ZnO thin films grown by atomic layer deposition, J. Vac. Sci. Technol. A 30(2012) 021202.

[29] Hai Yuan, Bing Luo, Dan Yu, An-jen Cheng, Stephen A. Campbell, Wayne L.Gladfelter, Atomic layer deposition of Al-doped ZnO films using ozone as the oxy-gen source: a comparison of two methods to deliver aluminum, J. Vac. Sci. Technol.A 30 (2012) 01A138.

[30] Merike Kriisa, Malle Krunks, Erki Kärber, ValdekMikli Mart Kukk, Arvo Mere, Effectof solution spray rate on the properties of chemically sprayed ZnO:In thin films, J.Nanomater. 2013 (2013) 423632.

[31] C.E. Benouis, M. Benhaliliba, A. Sanchez Juarez, M.S. Aida, F. Chami, F. Yakuphanoglu,The effect of indium doping on structural, electrical conductivity, photoconductivityand density of states properties of ZnO films, J. Alloys Compd. 490 (2010) 62.

[32] J. Wienke, A.S. Booij, ZnO:In deposition by spray pyrolysis—influence of the growthconditions on the electrical and optical properties, Thin Solid Films 516 (2008)4508.

[33] M. de la, L. Olvera, H. Gomez, A. Maldonado, Doping, vacuum annealing, and thick-ness effect on the physical properties of zinc oxide films deposited by spray pyroly-sis, Sol. Energy Mater. Sol. Cells 91 (2007) 1449.

[34] G. Kenanakis, N. Katsarakis, E. Koudoumas, Influence of precursor type, depositiontime and doping concentration on the morphological, electrical and optical proper-ties of ZnO and ZnO:Al thin films grown by ultrasonic spray pyrolysis, Thin SolidFilms 555 (2014) 62.

[35] Sophie Gledhill, Alexander Grimm, Dieter Greiner, Wolfgang Bohne, Martha Lux-Steiner, Christian-Herbert Fischer, Doping induced structural and compositionalchanges in ZnO spray pyrolysed films and the effects on optical and electrical prop-erties, Thin Solid Films 519 (2011) 4293.

[36] S.M. Rozati, Sh. Akesteh, Characterization of ZnO:Al thin films obtained by spray py-rolysis technique, Mater. Charact. 58 (2007) 319.

[37] Tomoaki Terasako, Yoshinori Ogura, Ken Ohmae, Shohei Fujimoto, Masakazu Yagi,Sho Shirakata, Morphological, electrical and optical properties of highly oriented

97A. Abrutis et al. / Thin Solid Films 576 (2015) 88–97

undoped and doped zinc oxide and cadmium oxide films grown by atmospheric–pressure chemical vapor deposition, Surf. Coat. Technol. 230 (2013) 245.

[38] Yen-Chin Huang, Zhen-Yu. Li, Hung-hsin Chen, Wu-Yih Uen, Shan-Ming Lan, Sen-Mao Liao, Yu-Hsiang Huang, Ku. Chien-Te, Meng-Chu Chen, Tsun-Neng Yang,Chin-Chen Chiang, Characterizations of gallium-doped ZnO films on glass substrateprepared by atmospheric pressure metal-organic chemical vapor deposition, ThinSolid Films 517 (2009) 5537.

[39] C.H. Lee, D.W. Kim, Preparation of In doped ZnO thin films by MOCVD using ultra-sonic nebulization, J. Ceram. Proc. Res. 14 (2013) 145.

[40] Davinder S. Bhachu, Gopinathan Sankar, Ivan P. Parkin, Aerosol assisted chemicalvapor deposition of transparent conductive zinc oxide films, Chem. Mater. 24(2012) 4704.

[41] Tammy Ben-Yaacov, Tommy Ive, Chris G. Van de Walle, Umesh K. Mishra, James S.Speck, Steven P. Denbaars, Properties of In-doped ZnO films grown bymetalorganicchemical vapor deposition on GaN(0001) templates, J. Electron. Mater. 39 (2010)608.

[42] Hanearl Jung, Doyoung Kim, Hyungjun Kim, The electrical properties of low pres-sure chemical vapor deposition Ga doped ZnO thin films depending on chemicalbonding configuration, Appl. Surf. Sci. 297 (2014) 125.

[43] Ray-Hua Horng, Kun-Ching Shen, Chen-Yang Yin, Chiung-Yi Huang, Dong-SingWuu, High performance of Ga-doped ZnO transparent conductive layers usingMOCVD for GaN LED applications, Opt. Express 21 (2013) 14452.

[44] Arne Behrends, Alexander Wagner, Mohamed Aid Mansur Al-Suleiman, Hans-Jurgen Lugauer, Martin Strassburg, Robert Walter, Andreas Weimar, AndreasWaag, Andrey Bakin, Transparent conductive Ga-doped ZnO films fabricated byMOCVD, Phys. Status Solidi A 209 (2012) 708.

[45] J.L. Zhao, X.W. Sun, H. Ryu, Y.B. Moon, Thermally stable transparent conducting andhighly infrared reflective Ga-doped ZnO thin films by metal organic chemical vapordeposition, Opt. Mater. 33 (2011) 768.

[46] Naoki Nishimoto, Takahiro Yamamae, Takashi Kaku, Yuki Matsuo, KasilingamSenthilkumar, Obuliraj Senthilkumar, Jun Okamoto, Yasuji Yamada, Shugo Kubo,Yasuhisa Fujita, Growth of Ga-doped ZnO by MOVPE using diisopropylzinc and ter-tiary butanol, J. Cryst. Growth 310 (2008) 5003.

[47] B.M. Ataev, A.M. Bagamadova, V.V. Mamedov, A.K. Omaev, M.R. Rabadanov, Highlyconductive and transparent thin ZnO films prepared in situ in a low pressure sys-tem, J. Cryst. Growth 198 (1999) 1222.

[48] Su. Jianfeng, Chunjuan Tang, Qiang Niu, Chunhe Zang, Yongsheng Zhang, Fu. Zhuxi,Microstructure, optical and electrical properties of Al-doped ZnO films grown byMOCVD, Appl. Surf. Sci. 258 (2012) 8595.

[49] Maria Elena Fragala, Graziella Malandrino, Maria Michela Giangregorio, MariaLosurdo, Giovanni Bruno, Stefano Lettieri, Luigi Santamaria Amato, PasqualeMaddalena, Structural, optical, and electrical characterization of ZnO and Al-dopedZnO thin films deposited by MOCVD, Chem. Vap. Depos. 15 (2009) 327.

[50] Papers of the 6-th international conference on hot-wire CVD (Cat-CVD) process,Thin Solid Films 519 (2011) 4409.

[51] A. Abrutis, V. Plausinaitiene, M. Skapas, C. Wiemer, O. Salicio, A. Pirovano, E. Varesi,S. Rushworth, W. Gawelda, J. Siegel, Hot-wire chemical vapor deposition ofchalcogenide materials for phase change memory applications, Chem. Mater. 20(2008) 3557.

[52] A. Abrutis, V. Plausinaitiene, M. Skapas, C. Wiemer, W. Gawelda, J. Siegel, S.Rushworth, Hot-wire chemical vapor growth and characterization of crystallineGeTe films, J. Cryst. Growth 311 (2009) 362.

[53] G. Papadimitropoulos, D. Davazoglou, Investigation of thermal and hot-wirechemical vapor deposition copper thin films on TiN substrates using CupraSelect®as precursor, J. Nanosci. Nanotechnol. 11 (2011) 8169.

[54] T. Iida, Y. Takamido, T. Kunii, S. Ogawa, K. Mizuno, T. Narita, N. Yoshida, T. Itoh, S.Nonomura, TiO2 thin films using organic liquid materials prepared by hot-wireCVD method, Thin Solid Films 516 (2008) 807.

[55] H. Natsuhara, T. Tatsuyama, M. Ushiro, M. Furuhashi, T. Fujii, F. Ohashi, N. Yoshida, S.Nonomura, Preparation of SnO2 thin films at low temperatures with H2 gas by thehot-wire CVD method, Thin Solid Films 519 (2011) 4538.

[56] A. Abrutis, L. Silimavicus, V. Kubilius, T. Murauskas, Z. Saltyte, S. Kuprenaite, V.Plausinaitiene, On the possibility to grow zinc oxide-based transparent conductingoxide films by hot-wire chemical vapor deposition, J. Vac. Sci. Technol. A 32(2014) 020602.

[57] J.P. Senateur, C. Dubourdieu, V. Galindo, F. Weiss, A. Abrutis, Application of pulsedinjection MOCVD for the deposition of oxide single layers and superlattices, in: K.-L. Choy (Ed.), Innovative Processing of Films and Nanocrystalline Powders, ImperialCollege Press, London, 2002, p. 71.

[58] T. Kudoh, Y.-I. Ogita, Resistance characteristics to oxidation of the metal-catalyzerfor Cat-CVD, Thin Solid Films 517 (2009) 3443.

[59] Gregory J. Exarhos, Xiao-Dong Zhou, Discovery-based design of transparentconducting oxide films, Thin Solid Films 515 (2007) 7025.

[60] Matthew A. Thomas, Johnathan C. Armstrong, Jingbiao Cui, New approach towardtransparent and conductive ZnO by atomic layer deposition: hydrogen plasmadoping, J. Vac. Sci. Technol. A 31 (2013) 01A130.