Atomic layer deposition of copper sulfide thin films

    Atomic layer deposition of copper sulfide thin films Nathanaelle Schneider, Daniel Lincot, Fr´ed´erique Donsanti PII: DOI: Reference:

S0040-6090(16)00028-6 doi: 10.1016/j.tsf.2016.01.015 TSF 34954

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

8 September 2015 27 November 2015 8 January 2016

Please cite this article as: Nathanaelle Schneider, Daniel Lincot, Fr´ed´erique Donsanti, Atomic layer deposition of copper sulfide thin films, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.01.015

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Atomic Layer Deposition of copper sulfide thin films

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Nathanaelle Schneider1 ([email protected]), Daniel Lincot1, Frédérique Donsanti1

Institut de Recherche et Développement sur l’Energie Photovoltaïque (IRDEP,

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1

*Corresponding

author:

Tel.:

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UMR 7174 CNRS-EDF-Chimie Paristech), 6 quai Watier, 78401 Chatou, France +33

130878548 ;

E-mail

address:

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[email protected] (Nathanaelle Schneider)

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ABSTRACT:

Atomic Layer Deposition (ALD) of copper sulfide (CuxS) thin films from Cu(acac)2

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(acac = acetylacetonate = 2,4-pentanedionate) and H2S as Cu and S precursors is reported. Typical self-saturated reactions (“ALD window“) are obtained in the temperature range Tdep = 130 - 200°C for an average growth per cycle (GR) = 0.25 Å/cycle. The morphology, crystallographic structure, chemical composition, electrical properties and optical band gap of thin films were investigated using scanning electronic microscopy (SEM), X-ray diffraction under Grazing Incidence conditions (GI-XRD), X-ray reflectivity (XRR), energy dispersive spectrometry (EDS), Hall effect measurements, and UV–vis spectroscopy. The obtained copper sulfide films are heavily p-doped (charge carrier concentration ~ 1021 - 1022 cm-3) with optical band gaps in the range of 2.2 – 2.5 eV for direct and 1.6 – 1.8 eV for indirect band gaps. Depending on the number of ALD cycles, multiphase compounds (made of digenite

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Cu1.8S, chalcocite Cu2S, djurleite Cu31S16 and covellite CuS) or single-phase digenite

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Cu1.8S films are obtained via a growth mechanism that involves in-situ copper

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reduction and loss of sulfur by evaporation.

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Keywords: ALD, Atomic Layer Deposition, CuxS, copper sulphide, acac metal

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precursor, thin-film solar cells

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1 Introduction

Different strategies of thin-film solar cells have been elaborated to develop

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inexpensive and efficient systems for large scale production, such as the use of innovative materials or the device manufacturing via low-cost methods [1,2]. CIGS

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([Cu(In,Ga)(S,Se)2] absorber) solar cells are a well-established technology, with record

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cell efficiencies up to 21.7% [3]. A major challenge in this field is the reduction of their indium content [4]. A solution could be the “economy of atoms” by the development of ultra-thin solar devices (absorber thickness as low as 0.1 μm). In such architectures, as a back contact reengineering with optical confinement techniques is necessary [5], the process conditions have to be soft to allow the direct deposition of the absorber without damaging engineered substrates. Atomic Layer Deposition (ALD) is a method of choice to fill all these requirements (thickness and composition control, soft deposition conditions). Indeed, ALD is a thinfilm deposition method based on sequential, self-limited surface chemical reactions in the gaseous phase that allows the synthesis of films with an excellent control of the thickness and of the atomic composition at relatively low deposition temperature [6,7].

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These unique features make ALD a very attractive technique for many solar cell

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designs that require thin layers [8–11]. The deposition of quaternary materials by ALD is very challenging, and only few examples have been reported in the literature [12–

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16] and are usually based on the combination of binary growth cycles that are

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independently understood and controlled. Thus, CuxS is a required binary material to achieve the ALD of a quaternary CIGS absorber. Furthermore, CuxS is a very

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interesting material per se, which has been widely considered for Li-batteries [17], and as absorber in Cu2S-In2S3 heterostructure [18], Cu2S/CdS [19] and Cu2S/Cd1-xZnxS

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[20] solar cells [21]. It is known to form five solid phases at RT: chalcocite (x = 2),

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djurleite (x = 1.96), digenite (x = 1.8), anilite (x = 1.75), and covellite (x = 1) [22], and its synthesis by ALD has been previously reported with two different sets of

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precursors, Cu(thd)2/H2S (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) by Johansson et al. [23] and Cu2(dba)2/H2S (dba = N,N’-disec-butylacetamidinato) by Martinson et al. [24]. Depending on the deposition conditions (precursor, temperature), different stoichiometries and compositions were obtained. Indeed, a key parameter in an ALD growth process is the nature of the precursors as those have consequences not only on the deposition conditions, but also on the surface-reaction involved [25] and eventually on the film properties [26,27]. As we recently reported the suitability of acac-metal sources to grow CuInS2 films by ALD at temperature as low as 150°C [28], we report herein a new ALD process for the synthesis of copper sulfide films, based on Cu(acac)2 as Cu source, and explore its influence on the film growth and properties.

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2 Experimental

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2.1 Thin-film fabrication

The depositions were carried out in a F-120 ALD reactor (ASM Microchemistry Ltd.),

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where the two 5 cm  5 cm substrates are located face to face within a distance of 1

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mm, on borosilicate glass and Si wafer substrates. The source materials for copper and sulfur were copper(II) acetylacetonate (Cu(acac)2, 98%, Alfa Aesar) and H2S (99.5 %,

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Air Liquide). Nitrogen (N2, 99.9999%, Air Liquide) was used as both carrier and purging gas. The Cu source was placed in a quartz boat inside the furnace and heated

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at TCu(acac)2 = 130°C while other reactants were kept at room temperature. CuxS films

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were deposited with the ALD program: n . { Cu(acac)2 pulse / N2 purge / H2S pulse / N2 purge}, with n = number of cycles = 200 - 8000 at deposition temperature Tdep =

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130 - 220°C. The pressure in the reaction chamber was kept in the range 1 – 5 mbar.

2.2 Thin-film characterization Transmittance and reflectance spectra were obtained using a Perkin Elmer Lambda 900 spectrophotometer with a PELA-1000 integrating sphere. X-ray diffraction (XRD) studies were performed under Grazing Incidence X-ray Diffraction conditions with a PanAnalytical Empyrean diffractometer using Cu-Kα radiations for crystallinity determination and XRR for thickness measurements of films < 50 nm. Samples deposited on Si wafer substrates were used to determine their thickness. Thin-film compositions and morphologies as well as thickness of films > 50 nm were determined with a Magellan 400L Scanning Electron Microscope provided by FEI, equipped with

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an Energy Dispersive X-ray Spectroscopy detector. All EDS measurements were

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carried out using Si (100) substrates and the reported values are atomic percentages (at.%). The SEM accelerating voltage was kept at 5 keV for all EDS measurements in

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order to suit the integration volume to the thickness of the films and measure atomic

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values with small errors. Electrical measurements were performed at room temperature using an ECOPIA HMS-3000 Hall effect measurements system with a permanent

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magnet of 0.5 T. Borosilicate glass samples of 6 cm2 were used to determine the electrical properties. Values of resistivity, carrier concentration, and electron mobility

3 Results

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chosen were the average of three measurements.

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3.1 ALD timing sequence and deposition temperature To ensure self-limiting surface reactions, the ALD timing sequence t1 – t2 – t3 – t4, where t1 is the exposure time for the Cu(acac)2 pulse, t2 the N2 purge time following the Cu(acac)2 exposure, t3 the exposure time for H2S, and t4 the N2 purge time following the H2S exposure, was evaluated. At Tdep = 150°C, for a constant number of cycles n = 400, each t was varied while other time values were kept constant, and plotted versus the growth rate (GR) of the resulting films (Figure 1). The GR is mostly independent of the precursor pulse duration for 0.1 s < t < 0.3 s, which is characteristic for a surface-limited process. Some deviation occurs for longer Cu precursor pulse time that could indicate an accumulation or a moderate decomposition of the precursor species [29]. With regards to the purge pulse time effect, the GR is almost independent

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of its duration for 0.25 s < t < 6 s. However, the GR decreases for longer purge times

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after H2S pulses (> 12 s). From these results, the base line ALD program 0.3 – 0.5 – 0.3 – 0.5 (s) was chosen and has been used in the rest of the study. The dependence of

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the growth rate on the deposition temperature is presented in Figure 2[a].

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Temperature-independent surface-controlled growth (“ALD window”) was obtained in the temperature range 130 – 200°C for an average GR of 0.25 Å/cycle, while higher

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GR values were obtained at higher temperatures, indicating CVD-like growth or a decomposition process. The dependence of CuxS film thickness on the number of

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growth cycles (n) at Tdep = 150°C is presented in Figure 2[b]. Typical film thickness

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values are 10.1 nm for 400 cycles and 275 nm for 8000 cycles. A linear dependence,

[a]

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typical for the ALD process, is observed. [b]

Figure 1 - Determination of the ALD timing sequence t1 – t2 – t3 – t4 (t1 = t pulse Cu(acac)2, t2 = t N2 purge, t3 = t pulse H2S and t4 = t N2 purge) at Tdep = 150°C with n = 400: growth rate (GR in Å/cycle) dependence on the [a] pulse times and [b] purge times

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Figure 2 - [a] GR (Å/cycle) as a function of the deposition temperature (Tdep) with n = 400, [b] Film thickness (nm) as a function of the number of growth cycles (n) at Tdep = 150°C

3.2 Compositional, structural, and morphological properties of the films

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EDX analyses have been carried out on films corresponding to the experiments

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presented in Figures 1 and 2. For CuxS films deposited at Tdep = 150°C, only a weak dependence of pulse parameters on the composition is observed, which corresponds to x values of 1.5 – 1.6. However, an increase of x toward the 1.9 – 1.95 range is observed when the thickness is increased (n = 4000, n = 8000). When films are deposited at different temperatures, for a constant number of cycles n = 400, the composition remains constant and corresponds to a value of 1.5 – 1.6 for x.

Figure 3 presents the XRD patterns under grazing-incidence conditions of CuxS films deposited at 150°C for different cycle numbers on borosilicate glass. For the highest cycle number (n = 8000), the films appear as highly crystalline digenite Cu1.8S. The diffraction peaks at 26.6, 29.7, 35.0, 42.3° indicate that it crystallizes in a

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rhombohedral phase (reference pattern 00-047-1748), rather than cubic phase as

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reported by others [30], in a favored (0 1 20) orientation. For thinner films (n = 800, 4000), films appear as multiphase compound comprising digenite, chalcocite (Cu2S,

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pattern reference 00-046-1195), djurleite (Cu31S16, pattern reference 00-034-0660), and

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covellite (CuS, reference pattern 020-006-0464) phases. The large FWMH and low intensity values of the peaks suggest a poor crystallinity. The mean size of the ordered

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crystalline domains of the films has been determined using the Debye-Scherrer

D=

K (1)  cos 

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formula, Eq. (1) [31]

where D is the mean size of the crystalline domain, K is the Scherrer constant and

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taken equal to 0.9,  is the wavelength of the Cu-Kα radiation,  is the Full Width at Half Maximum (FWHM) of the X-ray peak in radians and θ is the Bragg angle. The ordered crystalline domain varies from about 10 nm (n = 800) to 20 nm (n = 8000) along the (0 1 20) plane. Similar GI-XRD diffraction patterns were observed for films grown at different deposition temperatures for n = 400.

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Figure 3 - XRD patterns of CuxS films deposited at Tdep = 150°C : a) n = 800; b) n = 4000; c) n = 8000; and d) reference patterns of digenite (Cu1.8S, rhombohedral phase, 00-047-1748, in green), chalcocite (Cu2S, 00-046-1195, in red ), djurleite (Cu31S16, 00-034-0660, in black) and covellite (CuS, 020-006-0464, in blue)

Film morphologies of the CuxS films deposited on Si wafers were determined by cross-section and top view SEM observations. Thinner films (7 – 12 nm) are compact, uniform with a low surface roughness, typical of ALD deposited films, and is composed of small apparent grain sizes. The deposition temperature does not influence the film morphologies. However, increasing the thickness at constant deposition temperature (Tdep = 150°C), the apparent grain size increases from 30 - 35 nm (n = 200 – 800, film thickness = 7 – 14 nm) to 45 nm (n = 2000, film thickness = 37 nm), 60 nm (n = 4000, film thickness = 110 nm) and > 100 nm (n = 8000, film thickness = 220 nm) (Figure 4).

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Figure 4 - Cross-sectional and top view SEM images of CuxS films for different cycle numbers (n = 200, 800, 2000, 4000, 8000) at Tdep = 150°C

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3.3 Electrical properties of the CuxS films

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Figure 5 presents the evolution of the electrical properties of CuxS films deposited on borosilicate glass substrates as determined by the Hall effect as a function of the deposition temperature and the number of cycles. Both film deposition temperature (Figure 5[a]) and thickness (Figure 5[b]) have only a small influence on the electrical properties. All films are p-doped, with high charge carrier concentrations in the range of 1021 - 1022 cm-3, mobility of 0.2 – 3.5 cm²/Vs, and resistivity of 5 – 60.10-4 Ω.cm. These values are comparable to the ones reported in the literature [32].

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Figure 5 - Electrical properties of the CuxS films : dependence on the [a] deposition temperature (n = 400, thickness = 10 – 11 nm), [b] number of cycles (Tdep = 150°C, thickness = 7 – 220 nm).

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3.4 Optical properties of the CuxS films

The optical properties of CuxS films deposited on borosilicate glass substrates

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have been investigated. Their optical absorptions were determined from transmittance

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(T) and reflectance (R) measurements at room temperature using the formula α = (1/t).ln(T/(1-R)), where α is the absorption coefficient and t is the film thickness. For all films, α is of the order 105 cm-1. By using the equation α(hν) = A.(hν-Eg)n where A is a constant, h is the Planck constant, ν the frequency of the incident beam and n is 0.5 for a direct gap and 2 for an indirect gap, and plotting (α(hν))2 and α(hν))1/2 versus hν, the direct and indirect band gaps of the films could be estimated [33]. All films have different band gap energies, in the range 2.2 – 2.5 eV for direct and 1.6 – 1.8 eV for indirect band gaps. The influence of the deposition temperature (at n = 400) and of the cycle number at Tdep = 150°C on the film transmittance spectra are presented in Figure 6. In all cases, the transmission strongly decreases from λ > 800 nm, which is even more pronounced for thicker films (higher n values) (Figure 6[b]). This phenomenon is

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well-known for degenerate semiconductors and corresponds to a high degree of free

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carrier absorption at photon energies less than the band gap [34].

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Figure 6 - Film transmittance spectra of CuxS films: dependence on [a] the deposition temperature (n = 400, thickness = 10 – 11 nm), [b] the number of cycles (Tdep = 150°C, thickness = 7 – 220 nm).

4 Discussion

Self-limited reactions were ensured for the ALD timing sequence {Cu(acac)2 pulse / N2 purge / H2S pulse / N2 purge} {0.3 / 0.5 / 0.3 / 0.5 (s)}, and an ALD window [130 – 200°C] with an average GR of 0.25 Å/cycle was determined. Due to the limited volatility of Cu(acac)2 (VP: 1,813 Torr at 150°C) [35,36], depositions at Tdep < 130°C could not be performed. At Tdep > 200°C, the higher GR might be due to a decomposition process or CVD-like growth. The GR value (0,25 Å/cycle) is much lower than one monolayer per cycle. This is most likely due to the bulkiness of the Cu(acac)2 and especially the steric hindrance of its ligand that limits their adsorption

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[37]. Similar GR (~ 0.3 Å/cycle) and surface-controlled temperature ([125-160°C])

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were reported by Johansson et al. who used another β-diketonate-based Cu precursor, Cu(thd)2, and the same S source, H2S [23].

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A linear dependency of the thickness vs the cycle number was observed, though two

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zones can be distinguished for n < 2000 and for n > 2000, which are accompanied with a change of the [Cu]/[S] ratio, from 1.5 to 1.9. Deposition conditions have already

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been reported to have a strong influence on the copper sulfide film stoichiometries. For instance, with the set of reactants Cu(thd)2/H2S, Reijnen et al. obtained CuS films at

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Tdep < 175°C, and Cu1.8S films at Tdep > 175°C [30]. They attributed their observations

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to the decomposition behavior of adsorbed Cu(thd)2 at Tdep > 175°C, i.e. its internal reduction from Cu2+ to Cu+ and the release of the thdH ligand. In our case, we do not

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observe such sharp transition from one compound to another when increasing the deposition temperature. Instead, a transition from of multiphase compound to a digenite compound is observed with increasing film thickness. However, similar reactivity (reduction of Cu2+ to Cu+, release of an acacH ligand) of adsorbed Cu(acac)2 is expected. Indeed, Cohen et al. have reported that when Cu(hfac)2 (hfac = hexafluoroacetyl acetonate) adsorbs on a metallic surface (Ag), one hfacH ligand is lost and adsorbs on the substrate while copper partly reduces to Cu+ [38]. A comprehensive study of the chemistry of Cu(acac)2 on Ni(110) and Cu(110) surfaces was reported by Zaera et al. and shows the generation of metallic Cu from 127°C [39]. If this was the only process, the GR should decrease when [Cu]/[S] increases as the number of Cu pulses vs the number of H2S pulses necessary to grow a single layer is

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higher, as observed by Reijnen et al. [30]. However, the GR appears almost constant in

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our case and there is a transition from a multiphase compound to a digenite compound. The conversion from Cu2S to a lower [Cu]/[S] ratio is often observed [32]. On the

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other side, converting CuS into Cu1.8S implies the reduction of copper and a loss of

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sulfur and can be attributed to the reaction: 2 CuS(s)  Cu2-xS(s) + S(s) and the subsequent evaporation of sulfur. The first reaction is slightly endothermic (ΔG = 17

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kJ.mol-1 at 100°C for x = 0 [40]), but the conversion may occur due to the subsequent non-reversible evaporation of sulfur. Nair et al. reported such reactivities, i.e. the

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conversion of CuS thin films to Cu1.8S and Cu1.96S after 1 hour annealing at 300°C and

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400°C, respectively, under reduced pressure (100 mTorr) [34]. In our case, a higher number of cycles leading to longer deposition time, such annealing processes can

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explain the composition difference between thinner (n < 2000) and thicker films (n > 2000). To explore more those aspects, three different post-deposition treatments were performed on a CuxS thin film (n = 800, 13.8 nm) deposited at Tdep = 150°C inside the ALD reactor (pressure = 1 – 5 mbar) at 150°C: (1) {N2 purge} {120 (min)}; (2) {H2S pulse / N2 purge} {15 /105 (min)}; (3) (x7200) {H2S pulse / N2 purge} {0.3 /1.3 (s)}. A change from a multiphase compound into a digenite compound was observed only in the latter case. This indicates that the ALD conditions, and in particular its pulsing nature leads to specific conditions corresponding to a dynamic annealing process. As other reported systems, which involve different copper sources (Cu(I) complex, other reacting ligands), led to different mechanistic pathways, this also evidences the critical role of the precursors in ALD growth.

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The atomic compositions are in good agreement with the crystalline properties

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as determined from X-ray diffraction studies. Indeed, thinner films (low cycle number) appear as multiphase compounds made of digenite (Cu1.8S), chalcocite (Cu2S),

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djurleite (Cu31S16), and covellite (CuS), which is coherent with the [Cu]/[S] = 1.5 ratio

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that does not correspond to any specific copper sulfide compound. Films of higher thickness (n = 8000) crystallize as digenite (Cu1.8S), and that corresponds roughly to a

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[Cu]/[S] = 1.9 ratio. The mean size of the ordered crystalline domains, as determined from the Debye-Scherrer formula, increases with the cycle number, as well as the

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particle sizes as observed by SEM. Electrical properties of the CuxS films on the other

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side are only slightly affected by the thickness of the films and their deposition temperature and are comparable to the literature values [32]. All films appear as

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heavily intrinsically doped p-materials, and due to their very high carrier concentration (up to 1022 cm-3), the CuxS films can be considered as degenerated semi-conductors. This p-doping is due to Cu vacancies in the films that result in free positive charges and such heavy p-doping indicates a low participation of Cu2S phase. The high carrier concentration of the films leads to low transmittance spectra for λ > 800 nm in all films. The evolution of the transmittance spectra when varying the cycle number is due to both the increase of thickness and the change in composition. The electrical properties of the films make an accurate estimation of the band gap difficult. Indeed, due to free carrier absorption at photon energies less than the band gap energy, only a small region in the high energy side of the optical transmittance curve for the optical

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band gap analysis is left. Thus, discussing the influence of x on the band gap values,

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though it is known to increase for lower x values, is not possible here [34].

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5 Conclusions

Copper sulfide (CuxS) thin films were deposited by ALD from Cu(acac)2 and H2S.

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Typical self-saturated reactions (“ALD window“) are obtained in the temperature range Tdep = 130 - 200°C for an average growth GR of 0.25 Å/cycle. Copper sulfide

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films are heavily p-doped (charge carrier concentration ~ 1021 - 1022 cm-3) with optical

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band gaps in the range of 2.2 – 2.5 eV for direct and 1.6 – 1.8 eV for indirect band gaps. Depending on the number of ALD cycles, multiphase compounds (made of

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digenite Cu1.8S, chalcocite Cu2S, djurleite Cu31S16 and covellite CuS) or single-phase digenite Cu1.8S films are obtained, via a growth mechanism that involves in-situ copper reduction and loss of sulfur by evaporation. This study confirms the importance of the role of precursors in ALD processes and the possibility to control the composition of copper sulfide films by adjusting the deposition conditions. Besides the use of this binary material as cathode in Li-batteries or absorber in solar cells, its synthesis is also a necessary step toward the elaboration of ultra-thin solar cells with a quaternary CIGS absorber synthesized by ALD.

6 Acknowledgements The authors would like to thank the ANR’s ULTRACISM project for their financial

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support.

7 References

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CuxS films were synthesized by atomic layer deposition from Cu(acac)2 and H2S Self-saturated reactions at Tdep = 130 - 200°C for growth = 0.25 Å/cycle Multi- or single- phase films are obtained depending on the number of cycles Growth mechanism involves copper reduction and loss of sulfur by evaporation

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