Processing and charge state engineering of MoO_x

Molybdenum oxide (MoO_x) is a transition metal oxide showing extraordinary electrical, structural, chemical and optical properties, which depend on the oxidation state of Mo, on the degree of crystallinity, on the sample morphology and on environmental conditions. This material system, particularly in the form of thin and ultra-thin films, finds applications in a variety of technologically relevant fields, including catalysis,¹ gas sensors,^2,3 optically switchable coatings,^4,5 high-energy density solid-state microbatteries,^6,7 smart windows technology,^8,9 flexible supercapacitors,¹⁰ thin film transistors (TFTs)¹¹ and organic electronics.^12–22 Owing to its high work function – up to 6.9 eV [12] – and to the layered structure of $α$-$MoO3$⁠, MoO_x is also employed as a 2D material beyond graphene and as efficient hole contact on 2D transition metal dichalcogenides for p-type field effect transistors (p-FETs).^23–25 In view of a reliable device performance, the control over the chemical and physical properties of the MoO_x system is mandatory. It has been recently reported^14,26–32 that in MoO_x with $x<3$⁠, oxygen vacancies originated from partially populated d-states, give rise to occupied energy states within the forbidden gap – ∼ 3.0 eV at room temperature (RT)¹² – becoming bands above a critical concentration and driving the Fermi level close to the conduction band. The oxygen vacancies concentration, and consequently the averaged oxidation state of Mo, is a key parameter affecting the properties of the MoO_x system. When fully oxidized, i.e. for MoO₃ with the formal oxidation state 6+, MoO_x is a closed d⁰ oxide, transparent and devoided of oxygen vacancies. In this case, there are no occupied states within the wide band gap and the material follows an insulating behaviour. By reducing the metal cations, that is, by increasing the amount of oxygen vacancies, MoO₃ forms a series of stable and metastable suboxides (MoO_x, $2<x<3$⁠) showing semiconducting behaviour and a gradually increasing opacity. These sub-stoichiometric oxides present a complex x-ray photoelectron spectroscopy (XPS) spectrum, resulting from the convolution of three Mo valence states, namely +6, +5 and +4. Moreover, MoO₂, with oxidation state +4 and with the 4d² electron configuration, shows metallic conductivity and the highest opacity of the whole series. Lately, the effects of air exposure on MoO_x have attracted considerable attention.^33–35 For instance, recent developments in the field of organic solar cells have shown the critical impact of air exposure on the electronic structure of the MoO_x surface and, consequently, on the device efficiency and lifetime.^12,36–43 Surface hydration or hydroxylation are suggested as possible mechanisms for reduction of cations in the MoO_x system and, thus, for electronic structure changes. In particular, the hydroxylation mechanism is supported by recent density functional theory (DFT) calculations.⁴⁴ Moreover, several reports from the field of organic electronics^12,36,45,46 point at gap states formed not only by oxygen vacancies and cation reduction, but also by organic adsorbates at the interface between MoO_x and the organic layers. Process engineering for devices fabrication generally involves standardized steps in which thin layers are treated with photo-resists and solvents, but systematic studies on the effects of processing on the properties of MoO_x films are still wanted.

In this letter, we report on the effects of processing on the stability of ultra-thin MoO_x layers treated with conventional photo-resist (Shipley S1818) and solvents. The in-depth study is carried out on a series of ∼10 nm thick amorphous MoO_x layers fabricated by means of reactive sputtering and presenting different stoichiometry, spanning – over the series – from a metallic to a fully oxidized phase and including, in particular, Mo, $MoO2+ϵ$⁠, $MoO3−ϵ$⁠, and MoO₃. A combination of XPS, angle resolved XPS (ARXPS) and x-ray reflectivity (XRR) is employed to determine the charge state of the cations, the valence band spectra and the structural arrangement of the samples as-grown and upon treatment/exposure. A list of the as-grown samples under study is provided in Table I, while the labeling followed to identify the samples series according to the exposure/processing, is given in Table II.

The as-grown virgin samples, labeled as A_v, are stored under cleanroom conditions until they are stabilized after chemisorption of oxygen and moisture on the surface. Once stabilized, that is, once the samples show stable XPS spectra measured a week apart, the reference samples A_ref are coated with photo-resist which is then removed following the different standard procedures considered here below and summarized in Table II. When treated according to a conventional cleaning protocol of 15 min in acetone ultrasonic bath followed by 15 min in 2-propanol, and ending with 1 min rinse in DI water, the previously stabilized A_ref samples are referred to as samples B. When subjected to the previous procedure, but avoiding the final rinse in DI water, the samples are labelled as series C. Subscripts 0 and 1 (B₀, B₁, C₀, and C₁) correspond to samples measured one hour and four days after processing, respectively.

The series of amorphous MoO_x ultra-thin films are reactively sputtered in DC mode from a 100 mm diameter metallic Mo target in Ar/O₂ atmosphere on glass substrates (Corning Eagle XG) at RT. For all the samples the Ar flow rate and the target power are kept constant at 22 sccm and 400 W, respectively. The oxygen partial pressure is controlled by a lambda probe (Zirox vacuum probe). The base pressure is kept at 1x10⁻⁶ mbar. Additional growth parameters for the different MoO_x stoichiometries considered in this work, are provided in Table I. The high resolution XPS and ARXPS spectra, with probing depth of 5-10 nm, are collected using Al $Kα$ radiation (1486.6 eV), with a constant pass energy of 50 eV (1 eV full-width-at-half-maximum on Ag3d_5/2) and energy step size of 0.05 eV. The binding energies (BEs) are calibrated with respect to the conventional C1s peak at 284.8 eV. For the XRR measurements, a wavelength of 1.54 Å in a Seifert XRD 3003 PTS-HR is employed and the GenX reflectivity fitting package⁴⁷ supports the data analysis. Details on XPS data analysis and XRR measurements are provided in the supplementary material.

In Fig. 1 detailed ARXPS depth profiles, as well as the high resolution Mo3d core level XPS spectra, for the virgin, reference (upon stabilization in cleanroom atmosphere), and processed B and C series, are represented. For each valence state, the core level is split into the 3d_5/2 and 3d_3/2 doublets, separated by 3.1 eV, and with binding energies assigned according to established reference values.⁴⁸ A significant surface oxidation is observed for all the samples exposed to atmosphere (⁠$Av→Aref$⁠), as evidenced in the top panels of Fig. 1(a). As expected, the oxidation effect is particularly pronounced in virgin samples with a more metallic character. As illustrated in the left panel of Fig. 1(a), upon stabilization in cleanroom atmosphere, the metallic component Mo⁰ of the Mo film vanishes from the first atomic layers, while the fully oxidized valence state Mo⁶⁺ becomes predominant over the whole profile, as expected from oxidation of a metal. On the other hand, for $MoO2+ϵ$⁠, and as reported in the middle panel of Fig. 1(a), Mo⁴⁺ and Mo⁵⁺ are significantly quenched upon exposure to atmosphere, while the intensity of the Mo⁵⁺ component decreases for the two oxidation states with more oxygen content, $MoO3−ϵ$ and MoO₃, but it is not completely suppressed, as evidenced in the right panel of Fig. 1(a). For all the initial stoichiometries, the oxidation process interests the whole probing depth.

By considering the samples B₀, i.e. the A_ref upon conventional treatment involving DI water, it is inferred from Fig. 1(a) that in the sub-stoichiometric samples $MoO2+ϵ$ and $MoO3−ϵ$⁠, the Mo3d emission does not diminish significantly, but shifts to lower BE, pointing to a significant chemical reduction enhanced for the bulk angle, that is, for deep atomic layers. After four days of atmospheric exposure, i.e. $B0→B1$⁠, further oxidation leads to a lowering of the intensity of the Mo⁴⁺ emission, while the one of Mo⁶⁺ is augmented, recovering a spectrum resembling the one obtained from the virgin samples A_v before stabilization.

Upon processing without DI water, i.e. $Aref→C0$⁠, in both sub-stoichiometric compounds $MoO2+ϵ$ and $MoO3−ϵ$ the reduction effect is less pronounced than upon processing with DI water. The features of the virgin samples A_v one hour after treatment (C₀) are recovered, and no remarkable changes are detected after four days of atmospheric exposure (C₁). In the case of the pure metallic layer, processing with DI water (⁠$Aref→B0$⁠) fosters the full removal of the superficial oxide layer, while upon treatment without DI water, the oxide layer is reduced in thickness. The particular case of the fully oxidized sample after processing, MoO₃, is shown in Fig. 1(b). No remarkable changes are detectable after each phase of the cleaning protocol, until the last step with DI water is applied. Only upon water exposure the emission from the Mo3d core level is completely quenched, pointing to a complete removal of Mo from the layer.

The response of the Mo layers summarized in the left panel of Fig. 1(a) is confirmed by the evolution of the XPS valence band energy distribution curves of Mo depicted in Fig. 2(a), and characterized by a broad overall band with a maximum intensity at 2 eV below the Fermi level⁴⁹ E_F. The broad emission centered at ∼6 eV is assigned to an O2p photoemission signal of adsorbed oxygen, with contributions from various oxygen species. Its intensity is correlated with the presence of Mo⁶⁺ and Mo⁵⁺ oxidation states in the Mo3d core level spectra in Fig. 1(a). As evidenced in Figs. 2(b–d), in the case of the oxide films, the valence band region shows the conventional transition metal oxide two band structure³⁰ resulting in: (i) one peak mainly due to O2p orbitals centered at ∼6 eV, and which identifies the valence band maximum (VBM) at ∼3 eV, (ii) and a second peak emerging between the VBM and the E_F. In the case of the intermediate oxides, this latter band of gap states originates from oxygen vacancies partially filling the empty Mo d levels.

For the as-grown samples before processing, there is a systematic correlation between the Mo oxidation state as in Fig. 1 and the relative intensity of the emission related to the gap states and of the one from the O2p levels, as evidenced in Figs. 2(a–d). The width of the emission from the gap states is assigned to the presence of different types of vacancies,⁵⁰ whose complex geometry is beyond the resolution of the XPS system.

Processing involving DI water promotes substantially the development of gap states. This effect is paticularly pronounced for the less oxidized samples $MoO2+ϵ$⁠, as summarized in Fig. 2(b). When $MoO2+ϵ$ is treated in the absence of DI water (⁠$Aref→C0$⁠), the intensity of the peak related to the gap states assumes a value intermediate between the one for A_v and the one for A_ref, and it keeps stable (⁠$C0→C1$⁠). As shown in Fig. 2(d), for the fully oxidized MoO₃, the intensity of the initial band states is minimized upon stabilization (⁠$Av→Av$⁠) and does not significantly change after processing without DI water (⁠$Aref→C0,C1$⁠).

In all cases, the correlation between the reduction process emerging from the data shown in Fig. 1 and the evolution of the gap states is preserved, hence we attribute the enhancement of the gap state density to the presence of MoO_x reduced states, in accordance with previous reports by other authors.^12,37,50,51 Moreover, we can state that not only surface states are affected, but also deeper layers within the material, as deduced from the ARXPS measurements in Fig. 1.

In summary, reactively sputtered MoO_x ultra-thin film layers with different oxidation states (Mo, $MoO2+ϵ$⁠, $MoO3−ϵ$⁠, and MoO₃) have been exposed to different conventional cleaning protocols and investigated by XPS, ARXPS and XRR. Water strongly reduces the Mo oxidation state for all the samples, and causes the complete dissolution of MoO₃ after 1 min of exposure. The high etching selectivity between Mo and MoO₃ opens wide perspectives for structuring of MoO_x by means of simple DI water chemical etching.^52,53 The reduction effect, not confined only to the surface layers, is correlated with the development of gap states – likely due to chemisorbed species – which lead to an electron transfer to the transition metal oxide system.^12,37 Independently of the processing protocol, reduction and gap state intensities are stable after four days of air exposure. We have found by means of XRR measurements that fully metallic and fully oxidized phases have the highest stability against the various cleaning protocols. Our findings indicate that the optimization of the protocols for processing MoO_x ultra-thin films are crucial for the engineering of band states, fundamental for e.g. charge transport applications.^45,54

See supplementary material for additional information on XPS data analysis and XRR measurements.

This work was supported by the $O¨$sterreichische Forschungsf$o¨$rderungsgesellschaft FFG (P841482), by the Austrian Science Foundation – FWF (P24471 and P26830), by the NATO Science for Peace Programme (P984735), and by the European Research Council (ERC Advanced grant 227690). The authors acknowledge J. Duchoslav and R. Steinberger for support in the XPS and ARXPS measurements.