Electronic structure of Indium-Tungsten-Oxide alloys and their energy band alignment at the heterojunction to crystalline silicon

The electronic structure of thermally co-evaporated indium-tungsten-oxide films is investigated. The stoichiometry is varied from pure tungsten oxide to pure indium oxide and the band alignment at the indiumtungsten-oxide/crystalline silicon heterointerface is monitored. Using in-system photoelectron spectroscopy, optical spectroscopy and surface photovoltage measurements we show that the work function of indiumtungsten-oxide continuously decreases from 6.3 eV for tungsten oxide to 4.3 eV for indium oxide, with a concomitant decrease of the band bending at the hetero interface to crystalline silicon than indium oxide.

The electronic structure of thermally co-evaporated indium-tungsten-oxide films is investigated.The stoichiometry is varied from pure tungsten oxide to pure indium oxide and the band alignment at the indiumtungsten-oxide/crystalline silicon heterointerface is monitored.Using in-system photoelectron spectroscopy, optical spectroscopy and surface photovoltage measurements we show that the work function of indiumtungsten-oxide continuously decreases from 6.3 eV for tungsten oxide to 4.3 eV for indium oxide, with a concomitant decrease of the band bending at the hetero interface to crystalline silicon than indium oxide.
Metal oxide layers have found a plethora of applications within different electronic devices.They are used as transparent conductive layers in optoelectronic devices such as flat-panel displays and solar cells, as electron and hole extracting contacts in many solar cell technologies such as perovskite 1 , silicon heterojunction 2,3 , chalcopyrite 4 and organic molecule based solar cells 5 , as well as in light emitting devices 6 .Additionally they are used as passivation layers and dielectrics in CMOS 7 and in electrochromic devices 8 .Two of the most commonly used metal oxides are indium(III)-oxide (In 2 O 3 ) and tungsten(VI)-oxide (WO 3 ).Indium oxide (InO x ) is most commonly doped with tin and used as n-type transparent conductive oxide in flat-panel displays and solar cells.Alternative dopants like hydrogen 9 , molybdenum 10 , zinc 9 and tungsten 11 have been applied and yielded high carrier mobilities, which in turn enabled higher conductivities, or lower free carrier absorption at similar conductivities compared to indium-tin-oxide (ITO).Indeed, the characteristic advantage of InO x is its comparatively high conductivity with a variety of different dopants.At typical doping levels InO x is degenerately doped and has a work function of about 4.7 eV 12 , which is below the level required to properly contact the valence band, or highest occupied molecular orbital, of typical semiconductors like crystalline silicon (c-Si) (at 5.12 eV 13 ), Cu(In,Ga)(S,Se) 2 (at about 5.5 eV 14 ), amorphous silicon (at 5.4 eV 15 ), CH 3 NH 3 PbI 3 (at 5.4 eV 16 ) and anthracene (at 5.75 eV 17 ).Indeed, for application in silicon heterojunction solar cells, this has been identified as limiting factor 18 .Tungsten oxide (WO x ) is most widely used as an electrochromic layer 8 .Additionally its valence band position is below the water oxidation energy, which has led to investigations of WO x as water oxidiser in water splitting devices 19,20 .Furthermore its high work function (about 5.5 to 6.5 eV 21 ) and good thermal stability 22 have led to applications as hole extraction layers in silicon heterojunction 18,23 , crystalline silicon 24 , perovskite 1 and organic molecule based solar cells 25 , a) Electronic mail: dorothee.menzel@helmholtz-berlin.de and in light-emitting devices 6 .
However, the low conductivity of high work function WO x has proven problematic 18,26,27 .Furthermore, it is prone to the generation of oxygen vacancy defects, which reduce its work function 26 and transparency 1 .Overall these two materials are complementary regarding the applications as a hole contact on typical semiconductors and as an electrode for water oxidation.The work function of WO x is more than adequate to contact the valence bands of silicon, Cu(In,Ga)(S,Se) 2 , or perovskite and to oxidize water, but its conductivity needs to be improved.Contrary to this, InO x has a sufficiently high conductivity, but features a work function that is insufficient to contact the valence bands of the mentioned materials.The consequential idea is to combine both materials and search for a trade-off.There are reports on indiumtungsten-oxide (IWO x ) layers with up to 7 % WO x applied as transparent conductive oxides 11,28 .These layers are among the doped InO x layers with the highest mobility 11 .Furthermore WO x doped InO x films (1 wt%) have been applied in silicon heterojunction solar cells as double anti reflective coating leading to up to 23 % efficiency 29 .However, there is no systematic study investigating the full range of IWO x from pure WO x to pure InO x .The purpose of the present study is to investigate the electronic structure of IWO x alloys ranging in composition from pure In 2 O 3 to pure WO 3 using photoelectron spectroscopy and optical spectroscopy.200 µm thick, phosphorous doped n-type (1-2 Ωcm) crystalline silicon (c-Si) wafers were used as substrates.The wafers were cleaned using the RCA cleaning process.To remove the native silicon oxide on the surface, the wafers were dipped in 1 % HF for 2 min and transferred into vacuum.Samples were prepared and measured under ultra high vacuum conditions (10 −9 mbar base pressure) without breaking the vacuum inbetween deposition and photoelectron spectroscopy measurements.In 2 O 3 (99.994%) and WO 3 (99.99%) were deposited by thermal co-evaporation using an aluminium oxide boat and a tungsten crucible, respectively.During deposition the wafer was rotated with 1.3 rotations per second.The ratio between deposited In 2 O 3 and WO 3 was varied by changing the crucible temperatures from 920 • C to 1020 • C for WO 3 and from 1260 • C to 1350 • C for In 2 O 3 , yielding deposition rates from about 0.4 to 5 nm/min and about 10 nm thick layers.The substrate was not intentionally heated and is about 20 cm away from the crucibles.Therefore no significant heating of the substrate during evaporation is expected.However an impact on the oxygen vacancy density in the deposited metal oxides by the different crucible temperatures cannot be excluded.For this study the impact is neglected, since the crucible temperature range is small and therefore the effect should be negligible 30 .Thermal evaporation was applied in this study, since co-evaporation enables easy variations of the indium to tungsten ratio in the metal oxide alloy, without necessitating distinct changes of the deposition conditions.Apart from thermal evaporation 31,32 In 2 O 3 and WO 3 can be deposited using magnetron sputtering 10,33 , atomic layer deposition 34,35 , or chemical vapor deposition 36,37 .Depending on the applied deposition method the electronic structure i.e. the amount of oxygen vacancies 26 , the crystalline structure 33 and the density of precursor residues and contaminations 34 in the metal oxide can be affected.Instead of thermal co-evaporation, as performed in this study, also other co-deposition techniques, such as co-sputtering 28 are commonly used.X-ray photoelectron spectroscopy (XPS) with Al-K α excitation was used to investigate the In to W ratio of the metal oxide alloy.The XPS analysis was conducted using the W 4f, In 3d and O 1s core levels.The In and W contents of the metal oxides were calculated using sensitivity factors extracted from fully stoichiometric WO 3 and In 2 O 3 samples, which have been oxidized by an oxygen plasma at 10 −5 mbar for 10 min.The ratio of the metal oxide peak area to the O 1s peak area, corrected by the stoichiometry of the respective element was used as sensitivity factor.These sensitivity factors were used to obtain the fraction of InO x in the mixture of InO x and WO x .Afterwards the core levels were analysed in more detail.A Shirley background was assumed for all spectra.The In 3d 3/2 core level was fitted with the following four signals: First oxidic indium (In 3+ ), second metallic indium (In(met.))with an energy separation of 1.2 eV towards lower binding energy 38 , third indium hydroxide (In(OH) 3 ) with a 0.9 eV higher binding energy 39 and a loss feature at about 2.7 eV higher binding energy compared to In 3+ .The model for the W 4f orbital consists of a doublet of the 7/2 and 5/2 spin states with an energy separation of 2.12 eV between the two components of the doublet, an area ratio of 3/4 and the same full width at half maximum for both peaks.In this experiment we observe W 6+ and W 5+ with a separation of 1.2 eV between the respective oxidation states 40,41 .Helium ultra-violet photoelectron spectroscopy (He-UPS) was applied to investigate the work function (WF) and valence band edge position (E V -E F ) of the metal oxides.The secondary electron cut-off was mea- sured and fitted using a Boltzmann-Sigmoid function to obtain the work function of the layers.Additionally (E V -E F ) was extracted from the He-UPS spectra using a linear fit to the leading edge of the spectrum.Layer thicknesses were determined from spectral ellipsometry data in a wavelength range of 190 to 850 nm using a model with a combination of a Drude-Lorentz and a Tauc-Lorentz 42 oscillator.For layers with more than 50 % indium oxide the Drude-Lorentz oscillator was mostly sufficient for the fitting and the area of the Tauc-Lorentz was negligible 43 , whereas for layers with more than 50 % WO x the amplitude of the Drude-Lorentz was negligible and the Tauc-Lorentz was sufficient for data fitting 44 .The band gaps of the layers were extracted using transmission and reflection measurements obtained from about 50 nm thick IWO x layers on glass in the wavelength range of 250-1500 nm.A Tauc-gap 45 with an energy dependence of (hνα) 1/2 was assumed to determine the band gap, where the exponent corresponds to an allowed indirect band transition 46 .Surface photo voltage (SPV) measurements with 905 nm laser excitation 47 were carried out on samples with IWO x on n-type c-Si.These measurements allow to quantify the c-Si band bending at the interface to the IWO x film.Fig. 1 shows a selection of XPS measurements of the W 4f (panel a) and In 3d 3/2 (panel b) core levels collected from samples with different InO x and WO x contents.It is obvious that samples with a high W 4f signal intensity show low intensities for In 3d 3/2 and vice versa.For both signals a shift towards lower binding energies with increasing InO x content is observed.This data in combination with the sensitivity factors is used to calculate the InO x and WO x content of each sample.In addition, the results of the peak fitting analysis of the W 4f and the In 3d 3/2 core levels are depicted in panels c and d, respectively.Evaluating the W 4f core level, we observe that the binding energy decreases linearly by about 700 meV with increasing InO x content from pure WO x up to 50 % InO x content.For an InO x content higher than 50 % the binding energy stays constant.Considering the relative intensity of the different oxidation states, we observe solely W 6+ for an InO x content higher than 50 %, which corresponds to stoichiometric tungsten trioxide, WO 3 .For an InO x content of less than 50 %, also W 5+ can be observed with a relative amount of 8 % to 32 %, which indicates the presence of oxygen deficient WO x in tungsten rich layers.For the In 3d core level (panel d) we observe almost the same shift in binding energy as in the W 4f core level.The binding energy decreases linearly with increasing InO x content up to 50 % InO x content and decreases only slightly with InO x content higher than 50 %.Up to 80 % InO x content the oxidic indium (In 3+ ) dominates the core level signal intensity, while indium hydroxide accounts for 10 % to 15 % relative intensity.For higher InO x content than 80 %, metallic indium gains a high relative proportion of the peak intensity, up to 45 % of the total signal intensity, and a loss feature at higher binding energy is observed.Since these two signals are loosely correlated, it is possible that the loss feature is related to plasmons of the metallic indium.We continue with analysing the electronic structure, especially work function (WF), (E V -E F ), band gap (E G ) and the band bending (φ) in the metal oxide/siliconjunction.The measured WF values are shown in Fig. 2a, the band bending from SPV is shown in Fig. 2b and  E V -E F and E G values are shown in Fig. 2c.We can observe a strong decrease of the WF from 6.27 eV for pure WO x to about 4.5 eV for an InO x content of about 50 %.The work function remains almost constant for an InO x content between 50 % and 80 %.We can observe a second, much less pronounced decrease in WF by about 0.2 eV for IWO x with more than 80 % InO x content.Finally, the WF of pure InO x is about 4.24 eV.All in all the WF of the metal oxide mixture and the binding energies of the W 4f and In 3d 3/2 core levels follow the same trend, but the change in WF is a lot more pronounced than the one seen for the core levels.Additionally the minor WF decrease for high InO x contents coincides with the increasing signal of metallic indium and the rise of the loss feature in the In 3d 3/2 orbital.Overall the work function changes by almost 2 eV for varying InO x and WO x content in the alloy.Furthermore, a small amount of InO x in WO x leads to a significant decrease in WF, whereas a small amount of WO x in InO x only slightly increases WF.The band bending data (2b) shows pronounced fluctuations, but the same overall trend as the WF data.The total change in band bending from pure WO x to pure InO x is about 200 meV.This will be evaluated in more detail later in the discussion of the IWO x /c-Si interface.Interestingly, the variations in the valence band position relative to E F (Fig. 2c) are only 320 meV, i.e. of the same order of magnitude as the E G variation, but much less pronounced than the variation in WF.
Combining the above mentioned data leads to the band energy alignment vs. varying InO x content as shown in Fig. 3, where all energy values are plotted relative to the Fermi level (E F ).The most pronounced change in the band alignment is the change in distance of E F from the vacuum energy, i.e. the work function.As discussed above (Fig. 2a) the valence band edge as well as the band gap follow the same trend and decrease slightly by about 0.4 eV towards higher InO x content.Hence, the energetic position of the conduction band stays constant at about +0.3 eV relative to E F , whereas the valence band approaches the Fermi level from -3.5 eV for pure WO x to -3.18 eV for pure InO x .The finding of a fixed E F -E C ∼0.3 eV is in line with previous findings 48 .Klein et al. 12 measured the work function of common transparent conductive oxides like ITO, aluminium doped zinc oxide and antimony doped tin oxide and related them to the valence band positions of these materials.Comparing these results to the work functions and valence band positions measured in this study, we find that WO x offers higher work functions at the same valence band positions achievable for ITO and tin-oxide.Indium doping of WO x could thus enhance the electrical conductivity of WO x based water oxidizers in photo-electrical water splitting devices 19 , without elevating the valence band above the water oxidation level.
Using the additional information on c-Si band bending from 2b, we now discuss the heterojunctions of InO x /c-Si (black line) and WO x /c-Si (grey line) in Fig. 4. Crystalline silicon is assumed with a band gap of 1.12 eV, an electron affinity of 4 eV 13 .With the present dopant concentration this yields a WF of 4.3 eV.The band alignment at the IWO x /c-Si contact is defined by the band bending in the c-Si as measured using SPV and the valence-and conduction band positions of the metal oxide layer as discussed above.Note, that we assume no band bending in the IWO x film, i.e. a strong E F pinning by the states at 0.3 eV below E C Due to the conduction band position of WO x at 0.37 eV above E F and the band bending at the interface of -0.59 eV there is an offset of 58 eV between the conduction band of WO x and the valence band of c-Si.It has been surmised that the conduction of carriers in these high work function metal oxides is facilitated by defect states close to E F and the metal oxide conduction band 2,24 .The offset between the Fermi-level in WO x and the valence band of c-Si is only 0.21 eV.This offset can easily be overcome by thermionic emission 49 and therefore enables a suitable recombination contact.However, a good selectivity for transport is only obtained, if a strong band bending is preventing electrons from reaching the interface 13,50 .The band bending of -0.59 eV should suffice to achieve this carrier selectivity.
Evaluating the InO x /c-Si heterojunction, a lower band bending of about -0.44 eV is measured.This leads to an offset of about 0.73 eV between the InO x conduction band and the c-Si valence band, or 0.36 eV between E F in InO x and the c-Si valence band.These bigger offsets should lead to a comparatively worse recombination contact at this interface, as compared to WO x /c-Si, and the lower band bending should lead to less pronounced carrier selectivity.
Comparing the WF difference from pure WO x to pure InO x , we find that the overall change of the SPV band bending amounts to only 10 % of the overall change of the work function.SPV measurements are known to slightly underestimate the actual band bending 51 .Both, a higher band bending as well as a downwards band bending in the IWO x film would reduce the band offset, so that the above mentioned values of the band offsets have to be seen as upper limits.Nevertheless, the significant deviation of the difference in WF and the measured band bending suggests that the non-passivated interface between c-Si and IWO x leads to a high defect density and dipoles at the interface, which can also be seen in Fig. 4 with band offsets of more than 1.3 eV in vacuum energy.Comparing the offset in the vacuum energy levels at the InO x /c-Si and WO x /c-Si interface, even an inversion of the surface dipole charge is observed.
The factor of about 0.1 between the actual band bending and the difference in work functions indicates that the naive Anderson model of calculating band offsets from WFs does not hold, in accordance with similar studies on other heterointerfaces.Similar factors have been reported due to Fermi-level pinning 52 in semiconductor-heterojunctions, dipoles on silicon surfaces 53 and the defective interface in Schottky junctions 54 .Nontheless, indium-tungsten-oxide's higher work function could prove useful for hole contacts to typical device relevant semiconductors used in microelectronics and photovoltaics, such as silicon and Cu(In,Ga)(Se,S) 2 , or perovskites.However, the presence of interface defects and dipoles leads to a much less pronounced change of the band line up between c-Si and IWO x with changing stoichiometry than expected.Further studies on the influence of SiO x 55 and/or additional passivating interlayers at the interface, direct measurement of the band offset by surface inversion layer conductance measurements 56 or XPS valence band spectra 57 are suitable next steps towards a full understanding of this material system

ACKNOWLEDGEMENT
The authors would like to thank Thomas Lußky for vacuum system maintenance, Kerstin Jacob for wafer cleaning and Daniel Meza for the deposition of thick layers on glass.This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 727523 (NextBase).
FIG. 1. Background-corrected XPS spectra of the (a) W 4f and (b) In 3d 3/2 core levels for 8 samples starting from pure InOx (grey, 100 % indium) to pure WOx (black, 0 % In).The legend shows the fraction of InOx in the IWOx.The results of the analysis of the (c) W 4f 7/2 and (d) In 3d 3/2 core levels are shown, where the InOx fraction corresponds to the vertical and the binding energy to the horizontal axis.The symbol area represents the relative area of the individual peaks compared to the entire core level intensity.

FIG. 2 .
FIG. 2. (a) work function (WF) of IWOx, (b) band bending (ϕ) in IWOx/c-Si junctions and (c) valence band edge position (EV-EF) and band gap (EG) of IWOx layers plotted versus the fraction of InOx in the IWOx.The line in panel (b) is a guide to the eye.

FIG. 3 .
FIG. 3. Band alignment of IWOx thin films on silicon plotted versus the InOx content in IWOx.The vacuum energy (Evac), the valence (EV) and conduction band edge (EC) position are reported relative to the Fermi-level (EF) and plotted versus the fraction of InOx in the IWOx alloy.

3 FIG. 4 .
FIG. 4. Sketch of the heterojunction between WOx (grey line)/ InOx (black line) and n-type c-Si (dark grey line) with work function (WF), valence band edge (VB) and band gap (EG).Values for WOx and InOx values obtained using He-UPS, optical spectroscopy and surface photovoltage and values for c-Si were taken from literature with a donor density of 10 15 cm −3 .