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energiesArticleApplication Properties of ZnO and AZO Thin Films Obtainedby the ALD MethodBarbara Swatowska 1, * , Wiesław Powroźnik 1 , Halina Czternastek 2 , Gabriela Lewińska 1 , Tomasz Stapiński 1 ,Rafał Pietruszka 3 , Bartłomiej S. Witkowski 3 and Marek Godlewski 3123* Department of Electronics, Faculty of Computer Science, Electronics and Telecommunications,AGH University of Science and Technology, 30-059 Krakow, Poland; [email protected] (W.P.);[email protected] (G.L.); [email protected] (T.S.)Institute of Physics, Pedagogical University of Cracow, ul. Podchora żych 2, 30-084 Krakow, Poland;[email protected] of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland;[email protected] (R.P.); [email protected] (B.S.W.); [email protected] (M.G.)Correspondence: [email protected]; Tel.: 48-126-173-039; Fax: 48-126-332-398Abstract: The thin layers of ZnO and ZnO: Al (Al doped zinc oxide—AZO) were deposited bythe atomic deposition layer (ALD) method on silicon and glass substrates. The structures weredeposited using diethylzinc (DEZ) and deionized water as zinc and oxygen precursors. A precursorof trimethylaluminum (TMA) was used to introduce the aluminum dopant. The present studyof ALD-deposited ZnO and AZO films was motivated by their applications in photovoltaics. Weattempted to expose several properties of such films. Structural, optical (including ellipsometricmeasurements) and electrical investigations were performed. We discussed the relations betweensamples doped with different Al fractions and their properties.Citation: Swatowska, B.; Powroźnik,W.; Czternastek, H.; Lewińska, G.;Stapiński, T.; Pietruszka, R.;Keywords: AZO thin films; ALD technique; optical properties; structural properties; electrical parametersWitkowski, B.S.; Godlewski, M.Application Properties of ZnO andAZO Thin Films Obtained by theALD Method. Energies 2021, 14, 6271.https://doi.org/10.3390/en14196271Academic Editor: Cuma TyszkiewiczReceived: 13 August 2021Accepted: 27 September 2021Published: 1 October 2021Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affiliations.Copyright: 2021 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).1. IntroductionZinc oxide (ZnO) is one of the most popular semiconductor materials with a wideband gap equal to 3.27 eV at room temperature, with an exciton binding energy of60 meV, but most of all, with a free electron concentration regulated in a wide range,up to 1020 cm 3 [1–3]. Due to such properties, ZnO-based thin layers can be used as thetransparent conducting oxide (TCO) in solar cells and in ultraviolet light emitters [1–3].ZnO films when used for TCO applications [4] can replace the widely used indium tinoxide (ITO). Even though ITO layers are still superior—they have high conductivity andoptical transparency in the wide range of the solar spectrum [4,5]—their price becomes toohigh due to the increasing price of indium. This fact started an active search for alternativeTCO materials [4,6]. One of the most promising alternative TCO materials is ZnO, inparticular when doped with Al (AZO), Ga (GZO), In (IZO) or F (FZO) [4].In the present study we concentrate on AZO films deposited by the atomic layerdeposition (ALD) technique. Such films can be deposited using a variety of techniques,including most of all magnetron sputtering [7], pulsed laser deposition [8], chemical vapordeposition [9] and sol–gel deposition [10,11].To select the growth method of ZnO (AZO) films, several parameters must be takeninto account, especially the required TCO parameters [12], but also price, the possibilityof low temperature growth and scalability to large substrates as well. This is why PLDis not used in industrial processes despite the best electrical parameters of the PLD films.Magnetron sputtering is the most commonly used deposition technique. The so-obtainedlayers show resistivity on the order of 10 4 Ω cm, with optical transparency 80% in thevisible spectrum [13]. The reported AZO film resistivity and transparency (above 80% inEnergies 2021, 14, 6271. .com/journal/energies

Energies 2021, 14, 62712 of 17the visible light range) are very close to those of ITO. Thus, AZO films are successfullyused as transparent electrodes in silicon-based (see e.g., [14]) or copper indium galliumdiselenide (CIGS) solar cells [15].Recently, ALD technology has attracted a lot of attention. Increasing interest in ALD isrelated to the unique capability with the possibility of deposition of pinhole free conformallayers on flat or textured surface, low temperature deposition and scalability to largesurfaces [16–18]. Properties of ZnO films grown by ALD at low temperature (LT) werediscussed by us in detail in our previous works [19,20]. Due to the self-limited growthrate, the layer thickness can be controlled with a precision of 1 nm by controlling thenumber of ALD cycles. The combination of these features allows for the production of new,unique and nano-scale optoelectronic devices [21]. However, despite the first industrialapplications, basic research is still needed to understand the dependencies between growthparameters and structural, electrical and optical properties of the films. The latter is welldocumented by our investigations of ALD-grown ZnO films in which we observed a rangeof puzzling film properties both for ZnO [22–24], as well as AZO films [25,26]. We list hereonly a few puzzling properties of these films. For example, why does the growth mode ofAZO films change with an increase of Al fraction in the films [25]? Is the growth epitaxialor it is depositional [23]? Why do we observe a large shift of an optical band gap withincreasing Al fraction in the films [25]?This work attempts to clear up some of these questions. ZnO layers and Al doped ZnOlayers with different concentrations of Al are grown using ALD and doping proceduresdescribed in the literature [25,26]. These layers were tested by means of X-ray diffraction(XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS),optical spectrophotometry and spectroscopic ellipsometry (SE). Electrical measurements(Hall effect studies) were also performed to obtain data on electron concentration andtheir mobility.2. Materials and Methods2.1. Sample PreparationALD layers were deposited on silicon and glass substrates. A high-resistance 4”(100) Czochralski silicon wafer (Cz-Si) was cut into pieces of size 1 1 cm2 for SEMmeasurement and 2.5 2.5 cm2 for structural measurements (XRD and ellipsometry). Theglass substrates (bought from Carl Rothe, Karlsruhe, Germany, id. 0656.1) were cut intopieces of size 2.5 2.5 cm2 for optical study. Prior to deposition processes, the substrateswere cleaned using our standard procedure, first in acetone, and then in isopropanoland deionized water for 5 min in an ultrasonic cleaner. After cleaning, the samples wereblow-dried using 5N purity nitrogen.ALD Layer DepositionThe layers were deposited on silicon and glass substrates using a Beneq TSF 200 commercialALD reactor. A rotary vane pump allowed us to keep the pressure in the growth chamberat 3 10 1 Torr. The temperature during the growth of all samples was kept at 160 C.Our earlier investigations indicated that films deposited at this temperature are of a relatively good crystallographic quality and show relatively high free electron concentrationsand low deep defect concentrations [23,24]. A higher temperature (200 C) was used in aprevious study [25,26]. The zinc oxide and aluminum doped zinc oxide layers were grownusing diethylzinc (DEZ; CAS: 557-20-0) and trimethylaluminum (TMA; CAS: 75-24-1) aszinc and aluminum precursors, respectively. Deionized water was used as the precursor ofoxygen [25,26]. During the growth process, diethylzinc, trimethylaluminum and deionizedwater were kept at room temperature (22 2 C). These precursors were sequentiallyintroduced into the growth chamber using pure nitrogen (5N purity) as a carrier gas. EachALD cycle included four steps: (i) pulse of the first reagent (precursor), (ii) purge of thegrowth chamber by an inert gas, (iii) pulse of the second reagent, (iv) purge of the growthchamber by an inert gas. We used 1 s or 2 s purging times. This may have affected the

Energies 2021, 14, x FOR PEER REVIEW3 of 18Energies 2021, 14, 62713 of 17reagent (precursor), (ii) purge of the growth chamber by an inert gas, (iii) pulse of thesecond reagent, (iv) purge of the growth chamber by an inert gas. We used 1 s or 2 spurging times. This may have affected the growth mode of the layers since we found thatgrowth mode of the layers since we found that for longer purging times (we tested forfor longer purging times (we tested for times up to 12 s) the growth on the c-axistimes up to 12 s) the growth on the c-axis perpendicular to the surface was preferential [23].perpendicular to the surface was preferential [23]. The technological details of layer growthThe technological details of layer growth are summarized in Table 1.are summarized in Table 1.Table 1. The ALD growth parameters of ZnO and AZO layers.Table 1. The ALD growth parameters of ZnO and AZO 2 OH2OTMATMAH2 OH2ODEZDEZH2 OH2OPulse (s)PulsePurge(s) 2111211Ratio of RatioALD CyclesPurge (s)of ALD Cycles1111:m1:m(1 TMA H2O2(1 TMA H2 O m DEZ H2 O)1 1m DEZ H2O)AAzincoxidelayerwas depositedintroducingthe DEZtheandDEZH2 O precursorszincoxidelayerwas depositedintroducingand H2O alternately.precursorsForthe growthAZOlayers,the followingsolutionwassolutionused: hof AZOlayers, thefollowingused:obtainthem(DEZ HO ZnO)cycleswereseparatedbyone(TMA HO AlO)cycle.In3)AZO layer, 2m (DEZ H2O ZnO) cycles were separated by one2(TMA 2H23O edustoobtainaZnOlayerwithcycle. In this case, m equaled 7, 9, 19 and 29. This approach allowed us to obtain a ZnOdifferentAldifferentdopant concentrations.If the numberm is small,aluminumcontentlayer withAl dopant concentrations.If theofnumberof mtheis small,the aluminuminthe ZnOvalueincreases,concentrationof dopant decreases.contentin isthehigh.ZnOAsis thehigh.As ofthemvalueof m theincreases,the concentrationof dopantToinitiate growth,wegrowth,startedwefirststartedwith afirstTMApulse,as inour aspreviousstudy [25,26].decreases.To initiatewitha TMApulse,in our ngintheALDtechnology.[25,26]. Figure 1 shown below shows the method of doping in the ALD cs,Inc.,Inc.,Rochester,NY, ceanOptics,Rochester,NY, nSi.Additionally,thethicknesswas used to measure the thickness of the layers deposited on Si. Additionally, theofeach filmfrom SEMcross-sectionimages.images.These valueswere thenthicknessof waseach determinedfilm was determinedfromSEM cross-sectionThese valueswereconfirmedby ellipsometricmeasurements.The crosstop viewand EDSthen confirmedby ellipsometricmeasurements.The sections,cross sections,top imagesview imagesandspectrawere measuredusing usinga HitachiSU-70 SU-70scanningelectronmicroscope(SEM),(SEM),at anEDS spectrawere measureda geof 5 kVandand15 kV(images).Fortheseat an acceleratingvoltageof 5(EDS)kV (EDS)15 sited on silicon substrates were ytical)X-rayX-raydiffractometerdiffractometer .k.,Sp.k.,Warszawa,Poland)withCuK Warszawa, Poland) with CuKαα 1.5425 Å radiation was used to investigate the 5115degrees.degrees.The electrical parameters of zinc oxide (ZnO) and aluminum doped zinc oxide (AZO)were measured by RT Hall in the van der Pauw configuration. A permanent magnet fromRH2035PhysTech system provided a magnetic field of 0.4 T. Hall measurements were

Energies 2021, 14, 62714 of 17carried out on layers deposited onto glass substrates (1 1 cm2 ) to avoid the influence ofSi substrate on electrical parameters.The variable angle spectroscopic ellipsometry measurements for four incident angles,namely 60 , 65 , 70 and 75 , were carried out using a spectroscopic ellipsometer (M-2000,J.A. Woollam Co. Inc., Lincoln, NE, USA) following the method described in [27–29].The measurements of Ψ—amplitude component, and —phase shift as a function ofwavelength λ, in the range from 300 to 1700 nm provided refractive index n and extinction coefficient k dispersion relations. The ellipsometric modeling was performed usingComplete EASE Data Acquisition and Analysis Software for Spectroscopic Ellipsometers(J.A. Woolam Co. Inc., Lincoln, NE, USA) [30].Optical transmission and reflectance of ZnO and AZO layers were measured bymeans of a Perkin-Elmer Lambda 19 spectrophotometer (PerkinElmer Polska sp.