consequences of plasma oxidation and vacuum annealing on the …€¦ · reaction processes and...

Consequences of plasma oxidation and vacuum annealing on the chemical propertiesand electron accumulation of In2O3 surfaces

Theresa Berthold, Julius Rombach, Thomas Stauden, Vladimir Polyakov, Volker Cimalla, Stefan Krischok, OliverBierwagen, and Marcel Himmerlich

Citation: J. Appl. Phys. 120, 245301 (2016); doi: 10.1063/1.4972474View online: http://dx.doi.org/10.1063/1.4972474View Table of Contents: http://aip.scitation.org/toc/jap/120/24Published by the American Institute of Physics

http://aip.scitation.org/author/Berthold%2C+Theresahttp://aip.scitation.org/author/Rombach%2C+Juliushttp://aip.scitation.org/author/Stauden%2C+Thomashttp://aip.scitation.org/author/Polyakov%2C+Vladimirhttp://aip.scitation.org/author/Cimalla%2C+Volkerhttp://aip.scitation.org/author/Krischok%2C+Stefanhttp://aip.scitation.org/author/Bierwagen%2C+Oliverhttp://aip.scitation.org/author/Bierwagen%2C+Oliverhttp://aip.scitation.org/author/Himmerlich%2C+Marcel/loi/japhttp://dx.doi.org/10.1063/1.4972474http://aip.scitation.org/toc/jap/120/24http://aip.scitation.org/publisher/

Consequences of plasma oxidation and vacuum annealing on the chemicalproperties and electron accumulation of In2O3 surfaces

Theresa Berthold,1 Julius Rombach,2 Thomas Stauden,1 Vladimir Polyakov,3

Volker Cimalla,3 Stefan Krischok,1 Oliver Bierwagen,2 and Marcel Himmerlich1,a)1Institut f€ur Mikro- und Nanotechnologien MacroNano, Technische Universit€at Ilmenau, PF 100565,98684 Ilmenau, Germany2Paul-Drude-Institut f€ur Festk€orperelektronik, Hausvogteiplatz 5–7, 10117 Berlin, Germany3Fraunhofer-Institut f€ur Angewandte Festk€orperphysik, Tullastraße 72, 79108 Freiburg, Germany

(Received 23 August 2016; accepted 5 December 2016; published online 22 December 2016)

The influence of oxygen plasma treatments on the surface chemistry and electronic properties of

unintentionally doped and Mg-doped In2O3(111) films grown by plasma-assisted molecular beam

epitaxy or metal-organic chemical vapor deposition is studied by photoelectron spectroscopy. We

evaluate the impact of semiconductor processing technology relevant treatments by an inductively

coupled oxygen plasma on the electronic surface properties. In order to determine the underlying

reaction processes and chemical changes during film surface–oxygen plasma interaction and to

identify reasons for the induced electron depletion, in situ characterization was performed imple-menting a dielectric barrier discharge oxygen plasma as well as vacuum annealing. The strong

depletion of the initial surface electron accumulation layer is identified to be caused by adsorption

of reactive oxygen species, which induce an electron transfer from the semiconductor to localized

adsorbate states. The chemical modification is found to be restricted to the topmost surface and

adsorbate layers. The change in band bending mainly depends on the amount of attached oxygen

adatoms and the film bulk electron concentration as confirmed by calculations of the influence of

surface state density on the electron concentration and band edge profile using coupled

Schr€odinger-Poisson calculations. During plasma oxidation, hydrocarbon surface impurities areeffectively removed and surface defect states, attributed to oxygen vacancies, vanish. The recurring

surface electron accumulation after subsequent vacuum annealing can be consequently explained

by surface oxygen vacancies. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4972474]

I. INTRODUCTION

Indium oxide (In2O3) is traditionally known as active gas

sensor material or, if highly n-doped with Sn, as the transpar-

ent conducting oxide indium tin oxide (ITO) which has been

used as contact layer in displays and solar cells for decades.

Recent improvements in thin film deposition technology and

epitaxy with the possibility to produce high crystalline quality

In2O3 material with low impurity and defect densities stimu-

lated application of this large band gap material in transparent

semiconductor devices—see Ref. 1 and references therein.

The surface electron concentration and related processes for

its modification directly influence the characteristics of gas-

sensitive films based on In2O3 (Ref. 2) as well as the metal

contacts for electronic devices, either being of ohmic or recti-

fying nature.3 Based on improved material quality, the band

structure parameters and optical properties have been rede-

fined in recent years4–7 with consequences on the interpreta-

tion of the band edge profile at the surface, which was initially

interpreted in terms of an upward band bending.8 For In2O3films either grown by plasma-assisted molecular beam epitaxy

(PAMBE) or metal-organic chemical vapor deposition

(MOCVD), an enhanced electron concentration is found at

the surface compared to the bulk values,9–12 referred to as sur-

face electron accumulation layer (SEAL). In accordance,

the valence and conduction band (VB and CB) both bend

downward in the accumulation region at the surface and elec-

trons excited from the occupied CB states below the Fermi

level can be directly measured using photoelectron spectros-

copy (PES). The origin of this effect has been attributed to the

unique band structure and the related high charge neutrality

level in In2O3 (Ref. 9) which leads to donor like behavior of

many surface defects and impurities. For example surface oxy-

gen vacancies are predicted by density functional theory (DFT)

to be one major source for the SEAL13,14 and indications for

this aspect are also observed experimentally by PES.15

The bulk electron concentration nbulk in In2O3 is gener-ally adjustable by intentional doping, e.g., by Sn for high

nbulk values,16–18 while the incorporation of Mg acceptors is

not successful to generate p-type behavior,19,20 but signifi-

cantly lowers the film conductivity. Furthermore, implement-

ing a combination of Mg doping and subsequent annealing

in oxygen environment enables semi-insulating film charac-

teristics.20 The typically observed non-negligible electron

concentration in unintentionally doped (UID) epitaxial In2O3layers is caused by incorporated impurities and defects. The

related aspects and the extensive number of available studies

are reviewed in Refs. 1 and 21. Briefly, both incorporated

hydrogen atoms as well as oxygen vacancies are sources for

the unintentional n-type character of In2O3.

Like in the crystals bulk, defects (oxygen vacancies)

and impurities (adsorbates) are also a source for changes ina)[email protected]

0021-8979/2016/120(24)/245301/10/$30.00 Published by AIP Publishing.120, 245301-1

JOURNAL OF APPLIED PHYSICS 120, 245301 (2016)

http://dx.doi.org/10.1063/1.4972474http://dx.doi.org/10.1063/1.4972474http://dx.doi.org/10.1063/1.4972474http://dx.doi.org/10.1063/1.4972474http://dx.doi.org/10.1063/1.4972474mailto:[email protected]://crossmark.crossref.org/dialog/?doi=10.1063/1.4972474&domain=pdf&date_stamp=2016-12-22

electron concentration at the surface. Both, oxidizing and

reducing surface treatments can therefore affect the nature of

the band alignment (downward/upward bending) and elec-

tron concentration Ns (accumulation/depletion) at the sur-face. Consequently, adsorption of impurities or thermally/

light induced desorption processes are expected to influence

these quantities. Within this context, the influence of O2 and

CO interaction at different temperatures was investigated for

(111) and (001) oriented In2O3 films indicating a slight

reduction of the SEAL electron density upon their adsorp-

tion.22 Furthermore, a reversible transition between a

reduced In-adatom structure formed during annealing at

500 �C in ultra-high vacuum (UHV) and an oxidized surfaceconfiguration generated by annealing at 500 �C in 10�7 mbarO2 was found in scanning tunneling microscopy experi-

ments.23 The existence of stable reduced and oxidized (111)

surfaces was also predicted by density functional theory

(DFT) including calculations of the corresponding surface

structures (in this case, an oxygen vacancy structure in the

reduced form).24 In agreement, Tambasov et al. found ametal-semiconductor transition at low temperatures (about

100 K) after UV illumination, which was absent after expo-

sure to an oxygen environment.25 They suggest that the

observed effect is due to a change of the degree of disorder

(mainly oxygen vacancies) through a photoreduction effect

after UV illumination. The surface can be furthermore chemi-

cally reduced by UV light or annealing in vacuum as observed

for nanocrystalline,26 polycrystalline, and textured12 In2O3films, where oxidizing/reducing surface treatments severely

influence the electron concentration of the material27 leading

to, e.g., highly sensitive ozone detectors.28,29 Furthermore, for

In2O3 single crystals with low electron concentration, the

absence of the SEAL was reported for the (111) surface after

in situ vacuum cleavage.30 This aspect indicates the impor-tance of the used surface preparation method and the influence

of surface defect states or adsorbates on the formation of the

surface electron accumulation layer.

Most significantly, for crystalline thin films, a drastic

reduction of Ns was realized for UID indium oxide films11 if

the films were modified by an oxygen plasma treatment,

severely depleting the surface from electrons. This effect was

directly monitored by a strong shift of the VB edge towards

the Fermi level EF at the surface and the depopulation of theCB electrons. Consequently, since the SEAL only allows the

production of ohmic metal contacts, a reactive (oxygen

enriched) plasma deposition process is required to produce rec-

tifying Schottky contacts on In2O3.3 This trend is a conse-

quence of the plasma-induced surface electron depletion,

which is a drawback if these layers are designated for sensing

applications of oxidizing gases,2 but might be beneficial for

the detection of reducing species. However, the origin of this

drastic reduction of Ns and the underlying compositional sur-face changes during oxygen plasma treatment are not fully

clarified so far. Possible reasons for this effect were sug-

gested11 such as the generation of crystal defects or the incor-

poration of interstitial oxygen atoms,31 which both could be

reversed by thermal treatments inducing SEAL re-occurrence.

Within this context, it has to be mentioned that for ITO

films, the influence of oxygen plasma processing on the

surface electronic properties has been reported much earlier,

demonstrating an increase of the work function by

0.1–0.5 eV,32–34 which is beneficial for display device tech-

nology, as well as changes in the surface plasmon energy,

band bending, and Fermi energy.32

In this study, In2O3 films were characterized before and

after two different oxygen plasma treatments using PES. The

influence of Mg-doping, plasma processing conditions, and

vacuum annealing on the chemical and electronic surface

properties are analyzed in order to determine the interrela-

tions between the presence of surface adsorbates and defects

and the value of band alignment and surface dipole energy

and to determine the reasons for the distinct surface electron

depletion after oxygen plasma processing of In2O3 films.

II. EXPERIMENTAL

About 350 nm thick UID as well as Mg-doped (Mg con-

centration up to 1021 cm�3) single crystalline In2O3(111) films

were grown on (111)-oriented Y-stabilized zirconia (YSZ)

substrates by PAMBE at �800 �C.2,35 The Mg-doped sampleswere not annealed in oxygen atmosphere thus exhibiting only

a slight reduction of the electron concentration due to the

effect of the compensating oxygen vacancy formation.

Oxygen-annealed Mg-doped samples typically approach a

semi-insulating character,20 which results in charging prob-

lems in PES measurements. In addition 330 nm thick textured

UID In2O3(111) films were deposited onto Al2O3(0001) sub-

strates by PAMBE (at �800 �C)2 or MOCVD. MOCVDgrowth was performed in a close-coupled showerhead system

(AIXTRON), using trimethylindium (In(CH3)3) and N2O as

precursors and N2 as carrier gas. For film growth, a �22 nmthick nucleation layer was deposited at 500 �C, followed bythe growth of a high-quality In2O3 epilayer of 300 nm thick-

ness at 1000 �C.12

After growth, the samples were exposed to ambient con-

ditions and cut into 6� 8 mm2 pieces. Room temperatureHall effect measurements in van-der-Pauw geometry as

described in Ref. 36 were performed to characterize the sheet

resistance Rsh and sheet electron concentration Nsh withinthe In2O3 epilayers after growth and after subsequent induc-

tively coupled plasma (ICP) oxygen plasma treatment.

Dividing Nsh by the film thickness yielded the apparent bulkelectron concentration nbulk.

Oxygen plasma modification was performed ex situusing a 13.56 MHz inductively coupled plasma (ICP) reac-

tive ion-etching (RIE) system (Samco – RIE-400iP, process

pressure 0.025 mbar, oxygen flow 10 sccm) operated at a

ICP power of 100 W and a RIE power of 50 W applied to the

sample for 5 min. The as-grown or plasma-treated samples

were mounted onto grounded Ta sample holders and inserted

into the ultra-high vacuum (UHV) surface preparation and

analysis system. A dielectric barrier discharge (DBD) was

generated by a high voltage (11 kV, 10 kHz), an oxygen pres-

sure of 200 mbar and quartz as dielectric barrier using a vac-

uum device on the basis of the design presented in Ref. 37.

The plasma was ignited for 5 min between the grounded

sample and quartz barrier having a distance of approximately

1 mm. The DBD plasma generator is part of a UHV surface

245301-2 Berthold et al. J. Appl. Phys. 120, 245301 (2016)

analysis system together with a sample heating stage and the

PES unit. Hence, in situ oxygen plasma preparation and anal-ysis could be realized to exclude the influence of gas adsorp-

tion under ambient conditions.

Thermal annealing of the samples in vacuum was per-

formed by radiative heating from the sample’s backside. The

substrates used for MBE film growth had a �1 lm thick Tibackside coating to provide good absorption of the heat radi-

ation. During vacuum annealing, a temperature of 500 �C, asdetermined by pyrometry from the sample front side, was

sufficient to efficiently remove the surface adsorbates as con-

firmed by the PES analyses. Since the substrates used for

MOCVD growth lacked a backside metallization, radiative

heating in the UHV system was less efficient, and pyrometry

was impossible due to the partial transparency of the sample

to infrared radiation. In order to compensate the effect of

lower heating efficiency, the heater filament current was

increased accordingly. In a power-dependent annealing

series, we determined that an increase of the heater power by

40% resulted in comparable effects of desired adsorbate

removal and surface cleaning and thus a comparable sample

temperature.

Photoelectron spectroscopy measurements have been

performed in normal emission using monochromated AlKa(h�¼ 1486.7 eV) as well as HeI (h�¼ 21.2 eV) radiation anda hemispherical electron analyzer operated with an electron

acceptance angle of 8� for standard measurements and 8� aswell as 1� for angle-dependent experiments. More detailsabout the setup and the used experimental conditions can be

found in Ref. 38. The binding energy (BE) scale and the

position of the Fermi level are regularly calibrated using a

silver reference sample.

III. RESULTS AND DISCUSSION

A. Initial surface characteristics after growth

Textured UID In2O3 films grown by MOCVD or

PAMBE on Al2O3(0001) substrates were analyzed in com-

parison to single crystalline, UID, and Mg-doped In2O3grown by PAMBE on YSZ(111) substrates in order to iden-

tify the influence of Mg concentration and crystallinity on

the electronic surface properties. Table I lists the general

characteristics of the investigated samples. After growth, all

samples were transported in ambient conditions before they

were introduced into the surface analysis system and subse-

quently characterized by PES. In this stage of surface

preparation (as loaded after growth), the In3d5=2 and O1s

core levels of the In2O3 structure are located at a binding

energy of 444.6 and 530.2 eV (Fig. 1(a)), respectively, and

an additional O1s component at higher BE (532.2–532.7 eV)

can be observed for all samples (see Fig. 2 for sample III).

This additional feature is caused by the existence of –OH

and –CO bonds,39 originating from the adsorption of surface

impurities during sample transport through air.

Consequently, also a C1s signal is detected including states

from the adsorbed hydrocarbon and carbon oxide molecules.

In Fig. 1 the influence of growth method, substrate as

well as Mg-doping of In2O3(111) films on the energy posi-

tion of core levels, the valence band maximum (VBM) and

on the work function U is shown together with the measuredVB spectra excited by X-ray radiation. A higher Mg

TABLE I. Growth method, Mg doping concentration NMg, substrate and the

crystal structure of the investigated In2O3(111) films. Textured films were

grown on Al2O3(0001) and single crystalline layers on YSZ(111)

substrates.2,12

No NMg (cm�3) Substrate/crystallinity Growth method

I … Al2O3(0001)/textured MOCVD

II … Al2O3(0001)/textured PAMBE

III … YSZ(111)/single crystalline PAMBE

IV 1020 YSZ(111)/single crystalline PAMBE

V 1021 YSZ(111)/single crystalline PAMBE

FIG. 1. (a) Influence of the growth method, substrate, and Mg concentration

on the position of core levels, valence band maximum (VBM), and work

function U after growth. (b) Illustration of the related VB edge emissionshift as measured by XPS (mon. AlKa radiation). The samples are numberedas described in Table I and were measured without any additional surface

preparation (as loaded).

FIG. 2. Surface cleaning effect of oxygen DBD plasma treatment and UHV

annealing at 500 �C demonstrated by the reduction of the C1s and O1sadssignals.


concentration results in a shift of the core levels and the

VBM to lower BE, while the work function is unaffected.

Mg acts as acceptor in In2O3, resulting in a movement of the

Fermi level (EF) from the CB edge deeper into the band gap.This aspect is reflected in the PES results that reveal a reduc-

tion of the distance between the occupied states and EF forincreasing Mg concentration. The observed effect is stronger,

if the binding energy of the considered core level peak is

lower (higher kinetic energy) and thus the information depth

is higher. In comparison, the results of textured UID

In2O3(111) films grown by MOCVD and PAMBE are also

shown in Fig. 1. In the initial state, no difference to single

crystalline UID In2O3 can be observed, except for the higher

work function in the case of MOCVD grown In2O3. This

effect might be due to a different composition of the surface

adsorbate layer for films that are removed from the MOCVD

growth reactor, where the reactive precursors can possibly

passivate the film surfaces.

Table II lists the quantitative results of the Hall measure-

ments considering the potential uncertainties of the apparent

bulk electron concentration and sheet resistance as discussed

next. The high electron concentration of the SEAL could theo-

retically lead to a slight overestimation of nbulk while for sam-ples that underwent an ICP oxygen plasma treatment, the

existing surface electron depletion layer would result in a

slight underestimation. The deviations in nbulk values for dif-ferent samples cut from one epitaxy wafer are below 20%.

For the UID In2O3 samples, the values are �2� 1018cm�3 orslightly below and were practically identical after additional

ICP oxygen plasma treatment, indicating that for these sam-

ples, the SEAL had no influence on the performed analysis.

The determined bulk electron concentration of the Mg-doped

sample IV with NMg¼ 1020cm�3 was almost one order ofmagnitude lower after growth (2.9� 1017 cm�3) and was fur-ther reduced to 2.2� 1017 cm�3 after the ICP oxygen plasmatreatment. Consequently, for the lower bulk concentration in

Mg-doped In2O3 films, the SEAL does have a slight influence

on the electron density analysis based on Hall experiments.

Overall, these findings indicate that, although a surface elec-

tron channel exists after growth, the Hall measurements are

generally suitable to determine the bulk electron concentration

in the samples examined in this study, which is an essential

parameter for further analysis of the PES measurements and

the surface band alignment.

The determined bulk electron concentration nbulk wasused to calculate the associated position of the Fermi energy

EF with respect to the conduction band minimum (CBM) inthe bulk of the material implementing recent band structure

parameters by Feneberg et al.7 that include the nonparabolic-ity of the conduction band and hence, the electron concentra-

tion dependence of the effective electron mass. The

determined position of the bulk Fermi energy is shown of

Table II. For all considered samples, EF is located close tothe CBM, i.e., slightly above the CBM for the UID samples

and 49 meV below the CBM after growth and 56 meV after

ICP oxygen plasma treatment for the Mg-doped sample IV.

Consequently, the changes in the bulk Fermi energy in the

investigated samples are relatively small and have no consid-

erable influence on the band bending analysis discussed

below and conclusions drawn from it.

In combination with the determined position of the

VBM at the surface via linear extrapolation of the measured

XPS-VB spectra and assuming a band gap of Eg¼ 2.7 eV,6we have calculated the band bending Vbb at the surface usingVbb ¼ Eg þ ðEF � CBMÞbulk � ðEF � VBMÞsurface, wherenegative Vbb values account for downward band bending andelectron accumulation. All experimentally determined quan-

tities are compiled in Table II, negative values of EF�CBMindicate that the Fermi level is below the CBM. In the

preparation state directly after insertion into the vacuum sys-

tem (as loaded), all samples exhibit a slight downward bend-

ing of the bands at the surface of Vbb¼�0.2 eV for UIDIn2O3 and �0.1 eV if Mg acceptors are incorporated.Consequently, for the Mg-doped In2O3 films, the larger shift

of the VBM at the surface than in the bulk indicates a reduc-

tion of the downward band bending compared to the UID

films. In this context, it has to be mentioned that band gap

renormalization effects as discussed in Ref. 15 can be

neglected for the fairly low electron concentration of the

samples under investigation,7 since they are in the range of

the experimental accuracy of determining absolute BE val-

ues in PES (100 meV).

B. Chemical changes and adsorbates

Changes of the chemical surface composition are ana-

lyzed in in sections B and C for different sample treatments.

As already mentioned, all samples initially exhibit a surface

adsorbate layer due to their transport in ambient conditions.

After both investigated in situ processes (vacuum annealingas well as DBD oxygen plasma treatment), changes in the

surface adsorbate layer are observed. Fig. 2 exemplarily

TABLE II. Electrical characteristics (sheet resistance Rsh and electron concentration nbulk) of the UID and Mg-doped In2O3 films after growth and the calcu-

lated location of the Fermi energy EF in the bulk based on recent band structure parameters.7 Location of the surface valence band maximum (VBM) and the

work function U of the In2O3 films as determined by PES measurements after different surface preparation processes: AL; as loaded, DBD; DBD oxygenplasma treatment, ICP; ICP oxygen plasma treatment, T; vacuum annealing (500 �C), T! DBD – DBD oxygen plasma treatment after vacuum annealing.

No Rsh (X) nbulk (cm�3) (EF – CBM)bulk (meV)

(EF – VBM)surface (eV) U (eV)

AL DBD ICP T T! DBD AL DBD ICP T T! DBD

I (1.2 6 0.2)� 103 (2.1 6 0.2)� 1018 10 2.9 2.5 2.0 2.9 2.5 4.1 5.1 4.2 4.2 5.1II (1.8 6 0.6)� 103 (1.6 6 0.5)� 1018 1 2.9 2.2 2.0 2.9 2.3 3.9 5.0 4.3 4.2 5.1III (1.3 6 0.1)� 103 (1.8 6 0.3)� 1018 5 2.9 2.2 2.0 2.9 2.2 3.9 4.9 4.2 4.2 5.3IV (3.6 6 2.3)� 104 (2.9 6 0.9)� 1017 �49 2.8 2.2 2.0 2.8 2.2 3.9 5.1 4.1 4.2 4.7


documents the changes of the C1s and O1s spectra for an

UID In2O3/YSZ(111) film (sample III), that were also recog-

nized for all other samples. Noticeably, the carbon content is

drastically reduced after the vacuum annealing or the DBD

oxygen plasma process. The reactive oxygen species, which

are generated in the plasma region right in front of the sur-

face efficiently oxidize the carbon impurities leading to vola-

tile species that desorb from the surface. At the same time,

reactive oxygen species attach at the In2O3 surface as

directly identified by the side feature O1sads (2.0 eV higher

BE compared to the oxygen atoms in the crystal lattice) of

the O1s state. The shift of absolute binding energy indicated

by vertical dashes in Fig. 2 is due to band bending effects

that will be discussed below. Hence, the treatment of the

indium oxide films by oxygen plasma effectively cleans the

surface from hydrocarbons with the effect of replacing the

species that saturate free surface sites by oxygen.

The increased binding energy of O1sads compared to the

chemical state of the bulk O atoms indicates that the addi-

tional oxygen provided by the plasma treatment is not

attached at crystal sites, but in a different configuration.

Possible bonding sites could be saturated In surface bonds or

interstitial positions. The (111) orientation of In2O3 is the

most stable surface configuration with fivefold and sixfold

coordinated In sites24 and negligible surface relaxation com-

pared to the bulk crystal structure. It is expected that the

reactive oxygen atoms can attach to free bonds at the surface

without significant structural reconfiguration as for example,

possible for the (100) surface where at high oxygen partial

pressures, formation of a peroxide phase is suggested24,40

corresponding to an oxygen-rich surface configuration.

Nevertheless, it has been shown that the oxygen content at

the (111) oriented In2O3 surface can be modified by thermal

treatments switching between an oxidized and a reduced sur-

face configuration.23 These experiments demonstrate the

capability of the surface to adsorb oxygen atoms at the sur-

face. In addition, the bixbyte crystal structure might allow

incorporation of oxygen at interstitial sites and possibly

enable implantation in subsurface layers.41

In order to identify whether the DBD oxygen plasma

treatment enables a penetration of activated species into

deeper regions beneath the top surface layer, we have per-

formed angular-dependent XPS measurements. Fig. 3 includes

a series of O1s spectra in dependence of emission angle H.Generally, if H is increased from normal emission (0�) tograzing emission, the depth of information is reduced by a fac-

tor of cos(H) within some experimental limitations. As a con-sequence, for a possible surface/adsorbate layer, the relative

contribution of adatom signals would be drastically increased

while any substrate signal would be even stronger attenuated,

resulting in a typical H-dependence of adsorbate and filmintensity. As other extremum, a uniform intermixing (constant

depth profile) would result in no angular dependence.

The chemical component of the In2O3 oxygen lattice

atoms (O1sbulk) and the oxygen adsorbate signal (O1sads)

have been fitted using Gaussian-Lorentzian profiles and their

area ratio is plotted in the right part of Fig. 3. The angular

dependence was furthermore modeled using different

assumptions to account for a possible distribution of

incorporated oxygen species from the plasma. All samples

exhibit ordered and smooth surface topographies with a low

rms roughness (0.5 nm for MBE-grown In2O3/YSZ and

MOCVD-grown In2O3/Al2O3 and 1.2 nm for MBE-grown

In2O3/Al2O3 (Refs. 2 and 12)). Therefore, the surface of the

films has been regarded as ideally sharp without any struc-

tural deviations. The isolines in Fig. 3 (right) correspond to

the scenario that oxygen is only adsorbed at the free bonds

of the surface without any penetration into deeper layers. A

fairly good agreement to the experiment is found for an

effective surface coverage c of 0.7 ML. We have also consid-ered different implantation profiles of oxygen atoms that

might be incorporated into deeper layers (exponential decay,

Gaussian decay profiles, linear decay, and step function) and

fitted these profiles to the measured data. Although these pro-

files include one further free parameter, none of the extracted

angle-dependencies was able to better describe the experi-

mental data compared to the model of a simple surface

adsorbate. For comparison, the isolines and experimental

values for these four model distributions are shown in the

supplementary material exemplarily assuming a width dads(width at half maximum of the individual decay profile) of

0.5 nm. When fitting these profiles in dependence of the

parameter dads and surface coverage c, dads always convergedto the value of a single In–O bond length. This result

matches the situation of a classic adsorbate contribution.

As a consequence, these results show strong evidence

that no significant incorporation of larger amounts of oxygen

into deeper layers occurs for the DBD oxygen plasma treat-

ment. Only surface O adatoms and incorporation of O atoms

in the top In2O3 surface layer accounts for the angular depen-

dence of the O1s spectra in Fig. 3. In this context, it has to

be mentioned that the discussion is only valid within the

FIG. 3. Left: angular-dependent measurements (variation of emission angle

H from 0� to 70�) of the O1s state after DBD oxygen plasma treatment of anUID In2O3 film (sample II). Right: comparison of the experimentally deter-

mined signal ratio between oxygen adsorbate component (O1sads) and the

oxygen bulk signal (O1sbulk) from oxygen lattice sites (dots) with calcula-

tions based on a two-layer model with In2O3 film and oxygen adatoms

(isolines–variation of surface coverage from 0.1 to 1.0 ML). In this model,

one monolayer (1 ML) is equivalent to an effective oxygen surface coverage

of 1.8� 1015 atoms/cm2. The datapoints represent the experimental valuesof the DBD treatment in red, a single ICP treatment in blue and the average

value for several ICP treated samples measured in normal emission as the

orange dot.


ftp://ftp.aip.org/epaps/journ_appl_phys/E-JAPIAU-120-020648

sensitivity limit of the used experimental method (�0.1 at. %for XPS). Considering the experimental and modeling

aspects of the performed analysis, it needs to be noted that

this approach is not capable to exclude the existence of rela-

tively low oxygen concentrations, which might diffuse into

deeper layers and occupy vacancies or interstitial sites.

Nevertheless, the measurements demonstrate that after the

DBD plasma treatment, the majority of attached oxygen spe-

cies are directly located at the In2O3 surface. Thus, the

resulting variations in the electronic properties, that will be

discussed in the following, are a result of the adsorbed oxy-

gen species that saturate the free bonds at the outermost sur-

face. The binding energy of the O1s adsorbate component is

reproducibly �2.0 eV higher than the O1s state of the O lat-tice atoms. Previous experiments on gas interaction of poly-

crystalline In2O3 have shown that the O� species are

adsorbed at the surface after molecule dissociation.26 Since

the spectral fingerprints are comparable for the DBD treat-

ment, a similar reaction mechanism could be possible for the

reactive oxygen species that are generated in the plasma

excitation.

The oxygen adsorbate component O1sads is also found

for samples that were processed in an ICP plasma etching

reactor, but broadened and not as well defined as after

the DBD oxygen plasma treatment. In this case, the subse-

quent removal from the vacuum recipient allows readsorp-

tion of surface contaminants. Hence, the O1sads signal could

also contain contributions from other species that typically

adsorb in air such as water, hydroxyl groups or oxygen con-

taining carbon-based molecules. In consistence, a consider-

able uptake of carbon is also detected, comparable to the

amount for samples after growth. Such additional surface

adsorbates certainly influence the angular dependence of the

XPS signal. Nevertheless, we have characterized the angular

distribution of the O1sads/O1sbulk ratio also for ICP oxygen

plasma treated samples. The results are included in Fig. 3 as

blue datapoints for a MBE grown UID In2O3 sample on

Al2O3 (equivalent to sample II) and the orange dot represents

the mean value for several UID In2O3 films grown by either

MOCVD or MBE on different substrates. This large set of

samples was only characterized in normal emission. As for

the DBD oxygen plasma treatment, the experimental data of

the ICP treatment follow the h-dependence of the model iso-lines for a simple oxygen adatom layer; in this case for a sur-

face coverage of �1 ML. One might expect a strongerphysical impact of the high density ICP plasma, since the

reactive oxygen species reach kinetic energies of up to a few

hundred eV in this case compared to the gentle �10 eV ionsin a DBD discharge that typically only act at the surface. In

consistence, the total amount of adsorbed oxygen is higher,

which agrees with the fact that the ICP treatment leads to a

stronger electron depletion at the surface (see Section III D).

However, one would also expect a penetration of additional

oxygen into deeper layers for the ICP plasma. A clear indica-

tion for an implantation scenario cannot be found from

the performed measurements but we want to point out that

these results cannot provide proof that these effects do not

happen due to the mentioned uncertainties caused by the

additional adsorbates and due to the limited sensitivity of

XPS (�0.1 at. %).

C. Effects of vacuum annealing

As already mentioned, after vacuum annealing, both

adsorbate signals (C1s and O1sads) have vanished, indicating a

successful removal of surface impurities (Fig. 2). In a temper-

ature dependent series, we have determined that heating to

500 �C is sufficient, in consistence with the comparable resultsof bulk In2O3 samples and sputtered thin films.

41 Therefore,

this routine is effective to prepare clean In2O3(111) surfaces.

In addition, if such a vacuum annealing is performed after

oxygen DBD plasma modification, a complete removal of the

oxygen adatoms (O1sads signal) is also achieved. This allows

to study the repeated cycles of oxidation and reduction experi-

ments at this specific surface.

Figures 4(a) and 5 include the O1s and VB spectra of

films I–IV after annealing the samples in vacuum. In com-

parison with the measurements of the samples in their initial

FIG. 4. O1s core level spectra of (a) different vacuum-annealed In2O3 films

(clean surface) and (b) after subsequent oxygen DBD plasma treatment. The

binding energies of the O1s components of the In2O3 crystal structure as

well as the plasma-induced oxygen adsorbate component are indicated.

FIG. 5. Comparison of the XPS-VB spectra of UID In2O3(111) films after

vacuum annealing (solid lines) and after subsequent DBD oxygen plasma

treatment (dotted lines): (a) sample I; In2O3(111)/Al2O3(0001) by MOCVD,

(b) sample II; In2O3(111)/Al2O3(0001) by PAMBE, (c) sample III;

In2O3(111)/YSZ(111) by PAMBE and (d) sample IV; Mg-doped

(NMg¼ 1020 cm�3) In2O3(111)/YSZ(111) by PAMBE.


condition after introduction into the UHV chamber (as

loaded), the core level BE and VBM values do not change

after vacuum annealing although the desorption of oxygen

adatoms and other surface impurities occurs, and the emis-

sion of photoelectrons of the SEAL near the Fermi energy

clearly remains. In addition, solely for the In2O3 films on

YSZ(111) (samples III and IV), a broad emission close to the

VB edge extending up to �1 eV below EF is observed afterannealing. This feature is assigned to the thermally induced

formation of surface defects (oxygen vacancies42,43) caused

by desorption of lattice oxygen atoms. Interestingly, the oxy-

gen DBD plasma treatment completely saturates this defect

state emission, indicating that the reactive oxygen species in

the plasma can heal intrinsic surface defects in the material.

We assign these changes for films on YSZ to the greater

differences in thermal expansion coefficients and lattice mis-

match between the substrate and the epilayer in the case of

YSZ compared to Al2O3. Considering the room temperature

lattice constants and thermal expansion coefficients of

In2O3,44–46 Al2O3 (Refs. 47 and 48) and YSZ,

44,49 we expect

a slight compressive strain of �0.3% at room temperatureand �0.4% at 500 �C for In2O3 films on sapphire which iseasily accommodated within the film without causing struc-

tural damage. However, In2O3 epilayers on YSZ are esti-

mated to be tensile strained (1.6% at room temperature and

1.8% at 500 �C). We anticipate that the increasing tensilestrain during heating up in vacuum could cause a deteriora-

tion of the structural integrity of the crystal and possibly a

defect formation at higher temperatures.

Furthermore, annealing indium oxide thin films in vac-

uum is known to increase the electron concentration of the

layers due to formation of oxygen vacancies.20 In order to

quantify the influence of the annealing/surface cleaning pro-

cedure on the electron transport properties, UID and Mg-

doped In2O3 films on Al2O3(0001) and YSZ(111) substrate

have been characterized in air by Hall measurements after

performing the vacuum heating procedure. Afterwards, the

samples exhibited a bulk electron concentration slightly

above 1019 cm�3. As a consequence, the position of the bulk

Fermi energy is shifted to �200 meV above the CBM. It isremarkable that these changes in (EF – CBM)bulk do not havean influence on the measured BE position of the core levels

and the VBM at the surface if one compares the situation

before and after annealing or the oxygen DBD plasma treat-

ment of an untreated vs. an annealed sample. This observa-

tion might be explained by a pinning of the surface Fermi

level for both sample conditions, (i) in case of electron accu-

mulation and (ii) in case of the oxygen plasma treated sur-

face which exhibits a surface electron depletion layer. The

origin of this effect is not clarified yet and is the subject of

ongoing studies. Nevertheless, these variations of the bulk

Fermi level do not significantly influence the observed

changes in band bending after oxygen plasma treatments that

will be analyzed in the successive section.

D. Changes in electronic properties

The O1s and VB edge spectra of samples I–IV after vac-

uum annealing and after a subsequent oxygen DBD plasma

treatment are compared in Figs. 4 and 5, respectively.

Comparable measurements of the samples after growth (as

loaded) and after direct oxygen DBD processing, which

exhibit very similar characteristics, can be found in the sup-

plementary material. After the DBD oxygen plasma process,

a reproducible shift of all core level states as well as the

VBM by 0.2–0.4 eV towards lower BE is observed for sam-

ple I grown by MOCVD, while all samples prepared by

MBE exhibit an even stronger shift of 0.6–0.8 eV after the

DBD treatment. In addition, we have also analyzed samples

that were prepared ex situ by an oxygen plasma in an ICPreactor. In this case, the occupied states shift up to 0.9 eV

towards the Fermi level for all samples under investigation

(spectra not shown, compare to Fig. 4 in Ref. 2 and the num-

bers given in Table II). Furthermore, the emission close to

the Fermi energy of electrons from the initial surface elec-

tron accumulation layer10 is completely vanished after the

oxygen DBD plasma (Fig. 5) or the oxygen ICP plasma treat-

ment, indicating a removal of the surface electron channel

by these plasma treatments. Similar spectral changes have

been observed in an earlier PES study.11

The shifts in core level and the VB binding energy are a

result of variations of the surface band alignment, i.e., the

surface band bending Vbb for the different surface treatments,since the bulk Fermi level is not expected to be strongly

affected by oxygen plasma modifications.11 We have ana-

lyzed the surface band structure based on the information

from PES characterization and Hall measurements. As

already discussed, the samples initially exhibit a slight down-

ward band bending and a related emission from electrons of

the SEAL. Based on the shifts of the core level BE and

VBM, we have calculated the variation of Vbb and comparethese quantities with the work function U values as deter-mined from UPS measurements that are included in Table II.

Vacuum annealing causes no shift of core levels and the

VBM with respect to the initial state (as loaded), resulting in

the preservation of the surface electron accumulation as can

be directly monitored by the emission of electrons close to

EF. This aspect points out that the existence of the surfaceaccumulation layer cannot solely be caused by surface adsor-

bates. We anticipate that the existence of surface states of

unsaturated bonds and surface defects (e.g., oxygen vacan-

cies) play an important role for the SEAL formation, since

the plasma-induced surface saturation by O adsorbates leads

to a complete depletion of electrons at the surface. This sce-

nario might be different for thin films grown in a vacuum

chamber or samples that have been annealed in UHV, where

the formation of surface defects is expected at higher temper-

atures, compared to clean (111) surfaces that have been pre-

pared by crystal cleavage at room temperature,30 where no

SEAL layer was found.

In addition, the desorption of surface impurities affects

the work function which is found to be 4.2 eV for a clean

In2O3(111) surface. The DBD oxygen plasma treatment sig-

nificantly changes the electronic surface properties: after-

wards an upward band bending (formation of a surface

depletion layer) is observed with Vbb¼�0.2 eV for theMOCVD sample and �0.5 eV for the samples prepared byPAMBE (for the previously annealed samples according to


ftp://ftp.aip.org/epaps/journ_appl_phys/E-JAPIAU-120-020648ftp://ftp.aip.org/epaps/journ_appl_phys/E-JAPIAU-120-020648

the change in bulk EF, the values are �0.2 eV larger). It isinteresting to note, that for the Mg-doped sample, the ener-

getic shifts are slightly smaller, resulting in the same final

Vbb value of 0.5 eV.A clear explanation of the differences measured for

In2O3 films prepared by the two different growth methods

cannot be given, but from several repetitions of the oxygen

plasma treatments, we have found a correlation between the

adsorbate coverage and change in Vbb. Fig. 6(a) comparesthe change in surface band bending DVbb after oxygen DBDplasma treatment of an initially clean surface in correlation

with the calculated oxygen adatom coverage c based on theanalysis of the O1sads/O1sbulk ratio for each measurement. A

slight scattering in the dependence of cðDVbbÞ is found, but ageneral trend of monotonic increase is observed. For the

MOCVD samples, the values are located in the lower left

corner and the reduced change in band bending correlates

with a lower surface adatom density. For the MBE samples,

DVbb scatters around 0.7 6 0.2 eV for c¼ 0.7 6 0.1 ML.Hence, the degree of upward band bending is increasing with

the uptake of reactive oxygen from the plasma, and the

experimental data-points can be fitted by an empirical power

function resulting in DVbb [eV]¼ 1.17 � (c [ML])1:38. In con-clusion, while Mg incorporation or the film crystallinity

(grain boundaries) seem to have a minor influence on oxygen

adsorption, a strong effect of the preparation method was

observed. For the MOCVD samples, only values slightly

above flat band conditions are found, but the SEAL is effec-

tively depleted in these cases as well.

For the ICP oxygen plasma treatment, the surface

upward band bending is even higher and found to be inde-

pendent of the sample and growth method at Vbb¼ 0.7 eV.This indicates a broadening of the depletion layer width in

this case. We anticipate that due to the higher plasma power

and density of this ICP reactor, a stronger oxidation of the

In2O3 surface occurs, possibly leading to a higher surface

coverage and a stronger degree of surface passivation.

Unfortunately, due to additional surface adsorbates originat-

ing from the exposure to air, a direct analysis of the correla-

tion between adsorbed oxygen species from the plasma and

upward band bending is not possible for these samples.

To get a deeper insight into the correlations between

bulk/surface electron concentrations induced by different

oxygen plasma treatments of as grown In2O3 samples with-

out vacuum annealing, we calculated the depth profiles of

the conduction/valence band edges as well as of the electron

density at various surface potentials and bulk electron con-

centrations. To obtain these depth distributions, the

Schr€odinger and Poisson equations are self-consistentlysolved. For the Poisson equation, the Dirichlet boundary con-

ditions (i.e., fixed surface potential relative to the bulk Fermi

level position) are used, and we assume the full ionization of

donors present in the samples. The effect of the conduction

band nonparabolicity is also accounted for and the band

structure parameters as well as the dielectric parameters

have been taken from Ref. 7. The net surface charge is calcu-

lated from the charge neutrality condition applied to the

whole sample. Fig. 6(b) compares the variation of band

bending in dependence of the density of surface states Nssfor different bulk electron concentrations nbulk between1� 1017 cm�3 and 1� 1019 cm�3. Generally, if the localizednegative charges exist at the surface, they induce a depletion

of electrons (upward band bending) in the subsurface region

for charge compensation. In contrast, a positive surface net

charge is compensated by a delocalized electron accumula-

tion layer (downward band bending) at the In2O3 surface.

Such localized surface charges can either be caused by filled

or empty electron states of unsaturated bonds at the outer-

most surface atoms, by missing atoms (vacancies) in the near

surface layers or by adatoms that bond to these free electrons

and extract or donate charge from/towards the semiconduc-

tor. Obviously, the surface Fermi level and the surface band

bending Vbb depend on the surface donor/acceptor concentra-tion Nss as well as the bulk electron concentration nbulk in anonlinear manner. If we consider the effect of the oxygen

DBD plasma treatment, we start with an initial slight down-

ward band bending followed by adsorption of oxygen species

that extract electron density from the semiconductor surface

and represent a negatively charged adatom at the surface.

This aspect is also confirmed by the strong rise in work func-

tion from 4.2 eV to (5.0 6 0.3) eV (see Table II). The changein work function is a sum of both effects: a possible variation

of surface band bending DVbb and the modification or emer-gence of a surface dipole DUdip (DU ¼ DUdip þ DVbb). Afterthe oxygen DBD plasma process, the increase in work func-

tion is larger than the variation of Vbb which can beexplained by an effective electric dipole (DUdip) formedbetween the In2O3 surface and the attached negatively

FIG. 6. (a) Influence of the surface O adsorbate coverage on band bending

after oxygen DBD plasma treatment for different In2O3 samples with a fit of

the experimental data by a power function in grey and (b) dependence of the

density of localized surface states Nss on the bulk electron and band bendingVbb from the accumulation (Vbb< 0 eV) to the depletion range (Vbb> 0 eV)as determined from Schr€odinger-Poisson calculations.


charged O adatoms, which increases the barrier for electron

emission. The fact that the absolute work function is rather

independent of O coverage, while Vbb is gradually increasingfor higher adsorbate densities, indicates that the effective

surface dipole induced by the attached O species is decreas-

ing for a higher coverage, maybe due to collective phenom-

ena or coulomb interactions between the negatively charged

oxygen adatoms. The consequence of the attachment of these

electronegative adsorbates is a cumulative saturation of posi-

tive surface states resulting in a change from electron accu-

mulation towards depletion at the In2O3(111) surface. In

qualitative agreement with the results from Schr€odinger-Poisson calculations, the experimentally determined change

in band bending follows this trend in dependence of cover-

age (compare Figs. 6(a) and 6(b)).

Besides the experimental evaluation of surface band

bending and surface states, we have used the Schr€odinger-Poisson calculations to determine the distribution of electrons

close to the surface as well as the corresponding band profiles.

We have used the bulk electron concentration and the value

of Vbb, determined as described above, to calculate n(z) andCBM(z)/VBM(z) with respect to EF at 0 eV. In Fig. 7, theresults for one of the UID films (sample III), which all exhibit

an nbulk of �2� 1018cm�3, as well as the Mg-doped sampleIV with an almost one order of magnitude lower bulk electron

concentration are presented. For the samples after growth (as

loaded), the electron accumulation layer is found to be only a

few nm wide and the SEAL sheet electron concentration

NSEAL is found to be as high as 2.9� 1012cm�2 for the UIDsample, while expectedly, the value is significantly reduced

for the Mg-doped sample, since both nbulk and Vbb are lower

in this case. After the two different oxygen plasma treatments,

the near surface region is depleted from electrons with a space

charge layer width of several 10 nm, hence significantly

broader than that in the case of electron accumulation. In both

cases of electron depletion or accumulation, the net surface

charge has to be screened by sub-surface charges. In case of

the positive surface charge (downward band bending),

because of the high accumulation electron density, only a nar-

row sub-surface region is required to fully screen the surface

charge. In the opposite case of upward band bending, the neg-

ative surface charge is screened by fixed positively charged

donors. Due to the much lower donor concentration, the

screening length is substantially larger in this case. Hence, for

the Mg-doped sample with a lower bulk electron concentra-

tion, the depletion layer is almost three times as broad com-

pared to the UID samples, while only slight variations in the

electron and band profiles are found if the two oxygen plasma

treatments are compared.

IV. CONCLUSIONS

The oxygen plasma induced changes of chemical and

electronic surface properties of In2O3(111) thin films are

identified. The reactive oxygen species of the plasma excita-

tion remove the adsorbed carbon impurities and saturate free

surface bonds as well as oxygen vacancies of the outermost

surface layer. The chemical changes and incorporation of

oxygen are restricted to the adsorbate and topmost layer for

an oxygen DBD plasma treatment. A penetration/implanta-

tion effect of O species into subsurface layers was not

observed and hence, the changes in electronic properties are

solely induced by surface adsorbates in the case of the DBD

oxygen plasma modification. The attached electronegative

oxygen adsorbates extract electrons from the substrate form-

ing a surface dipole that enlarges the barrier for electron

emission (work function). These charge transfer mechanisms

induce a strong depletion of the initially accumulated surface

electrons that manifests itself by a strong upward bending of

the CBM and VBM at the surface. A direct correlation

between the O adsorbate coverage and quantity of surface

band bending and depletion layer width is observed, which

highlights a tunability of the surface electronic properties of

In2O3 semiconductor thin films. Surface oxygen plasma

treatments in an ICP reactor induce a slightly stronger elec-

tron depletion and a higher amount of incorporated surface

oxygen species. The SEAL depletion was confirmed for

unintentionally n-doped and Mg-doped In2O3 layers. The

effects of oxygen plasma modification can be fully reversed

by a vacuum annealing procedure above 500 �C resulting inthe desorption of the attached oxygen species and an enrich-

ment of the surface by electrons, i.e., recovery of the SEAL.

This result confirms that surface oxygen vacancies are one

possible origin of the SEAL. The observed effects of surface

electron depletion and enhancement of work function

induced by oxygen plasma processing are of great relevance

for any subsequent device processing for applications of

In2O3 in modern semiconductor devices or of ITO films in

solid-state lighting devices and displays as well as solar

cells.

FIG. 7. Electron density distribution n(z) (left) and corresponding VBM/CBM profiles (right) based on Schr€odinger-Poisson calculations using theexperimental bulk electron concentration and band bending values after dif-

ferent surface preparation processes as input parameter for the simulations.

The related Vbb values are indicated by the corresponding color. Top: UIDIn2O3/YSZ(111) (sample III) and bottom: Mg-doped (NMg¼ 1020 cm�3)In2O3/YSZ(111) (sample IV). In the latter case, the profiles for a virtual

band bending of �0.2 eV are also included. The gray shaded areas representthe sheet electron density NSEAL of the electron accumulation region, beingthe difference between the n(z) profiles for the samples with downward bandbending (Vbb< 0 eV) and the hypothetical case of flat band conditions(Vbb¼ 0 eV).


SUPPLEMENTARY MATERIAL

See supplementary material for the XPS data of samples

after growth (as loaded) and after direct oxygen DBD plasma

treatment as well as for detailed results of modeling the oxy-

gen depth distribution from angular dependent XPS

measurements.

ACKNOWLEDGMENTS

We are grateful for the financial support by Deutsche

Forschungsgemeinschaft (DFG) within the project “Seebeck

Gas Sensors” (Grant Nos. HI 1800/1-1, AM 105/31-1 and BI

1754/1-1). We thank O. H€offt and M. Marschewski(Technische Universit€at Clausthal) for support and fruitfuldiscussions on the design of DBD plasma systems.

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