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Surface Review and Letters, Vol. 14, No. 1 (2007) 79–86c© World Scientific Publishing Company

ADSORPTION OF HYDROGEN ON β-Ga2O3(100):A THEORETICAL STUDY

ESTELA A. GONZALEZ, PAULA V. JASEN, CARLA R. LUNA and ALFREDO JUAN∗

Departamento de Fısica, Universidad Nacional del Sur, Av. Alem 1253,Bahıa Blanca (8000) Argentina

∗cajuan@criba.edu.ar

Received 7 August 2006

We have modeled the (100) surface of β-Ga2O3 with a clear-cut optimization from ab initiocalculations. The adsorption geometry of one hydrogen atom on this surface was determined bya semi-empirical quantum chemistry method. H is found bonded to a tetrahedral Ga atom andno bond with surface oxygen atoms is detected. As a consequence of this Ga–H bond, the Ga–Ooverlap population decreases. This result is in good agreement with the recent spectroscopicdetermination of Ga–H IR frequencies on supported catalysts. The orbital composition of theGa–H bond and density of states of tetrahedral and octahedral gallium ions, Ga(I), Ga(II), andthat of oxygen before and after H adsorption are also addressed.

Keywords:

1. Introduction

Gallium oxide has become an important materialin several processes involving breaking or formationof hydrogen-containing bond. Polycrystalline semi-conducting Ga2O3 thin films represent a promisingnew basic material for sensors used to detect hydro-gen, and catalysts containing supported gallium areknown to be active in light alkane dehydrogenationand aromatization (Cyclar process).1

Recently, gas sensor, especially oxygen sensor,has attracted the attention of many people. InEurope, the strict regulation for the exhaust gas hasimposed that every vehicle produced after year 2000should be equipped with two oxygen gas sensors.Hence, the demand for oxygen gas sensor is promptlyincreasing. Ogita et al. described a thin film gassensor device in its developing stage.2 It is stilldifficult to synthesize a perfect product in all thepoints of their complete sensing mechanism, stability,

reproducibility, lifetime and others. It will give agreat impact on the sensor field if the thin film gassensor can be realized on the commercial basis withsmall size, low cost and a simple production process.

Gallium oxide is one of the most promising mate-rials as a gas sensor. It exhibits an n-type semicon-ductor property over 873K due to the deficiency ofoxygen in the crystal. It is also well known that anelectrical conductivity appears owing to excess bond(electrons) instead of lattice bonds.3,4 Moreover, byadjusting the operating temperature, β-Ga2O3 canalso be used as an other gas sensor.

At ambient conditions, Ga2O3 occurs in themonoclinic (i.e. β) phase, although it can be trans-formed into four other high-pressure and tempera-ture polymorphs.3,5,6

Monoclinic gallium oxide (β-Ga2O3) has anumber of interesting bulk and surface proper-ties. This phase has one of the widest band gaps,

∗Corresponding author.

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80 E. A. Gonzalez et al.

about 4.4–4.8 eV among transparent conductingoxides (TCO). Because β-Ga2O3 has a unique trans-parency from the visible into the UV region, it isa very promising candidate for the optoelectronicdevices operating at short wavelengths, where stan-dard TCOs, including indium tin oxide, have poortransparency. Besides, β-Ga2O3 has a magneticmemory effect due to bistability of the conductionelectron spins. Very recently, an electron paramag-netic resonance (EPR) study for various magneticfield orientations was carried out and it was concludedthat the donor electrons are predominantly createdin one of the three different oxygen sites.7

The dc and ac conductivity measurements insingle crystals of β-Ga2O3 show that the dominantmechanism for migration below 900K is ionic due todiffusion of oxygen atoms in the lattice. Above 900K,the conductivity is predominantly electronic.8

The photoluminescence studies on single crys-tals of β-Ga2O3 reveal three dominant bands in theultraviolet (UV), blue and green regions of emis-sion spectrum.9 The UV emission is an intrinsicphenomenon attributed to the recombination of anelectron and a self-trapped hole.10 However, the blueand green emissions are found to occur only in thepresence of specific impurities, such as Be, Ge, Zrand Si, in the lattice.11

Blanco et al. have reported studies on energet-ics and migration of point defects in Ga2O3. Theoxygen interstitials are predicted to be dominantcarriers in the intrinsic and oxygen-rich conditions,while it is suggested that the oxygen vacancy mecha-nism dominates the conductivity in oxygen-deficientconditions.12

Small clusters of gallium oxide can be taken as aprototype to understand the physics and chemistryof the surfaces and nanostructures.13,14

Through the calculations of the extended Huckelmethod, together with the measurements of the EPRand electron nuclear double resonance, Binet et al.concluded that the electrical anisotropy should comefrom the anisotropy distribution along the octahedralchains of the structure.15 Based on semi-empiricalquantum-chemical calculations, Hajnal et al. alsoconcluded that the optical and electrical anisotropiesare attributed to the anisotropy of the electronicstructure of the perfect lattice.16

In a previous work, we found that H is notadsorbed in the (100) plane of the β-Ga2O3 surface,

which exposes oxygen atoms. To be active, it is neces-sary to introduce oxygen vacancies on the surface.After that, the exposed Ga adsorbs H that is moststable in the tetra-coordinated sites. In this presentstudy we will model the β-Ga2O3(100) surface fromab initio optimization. We have used a clear-cut opti-mization of the oxide surface followed by the adsorp-tion of H.

2. Crystal Structure andComputational Model

β-Ga2O3 has a monoclinic structure, with spacegroup C 2/m.3 The lattice parameters at roomtemperature are a = 12.23 A, b = 3.040 A, c =5.807 A and β = 103.7◦. Eight Ga atoms and twelveO atoms are evenly distributed into two Ga (I, II)and three O (I, II, III) non-equivalent positions. TheGa(I) sites form slightly distorted tetrahedral withfour O, while Ga(II) form highly distorted octahe-dral with six O ions. The Ga and O coordination isshown in Fig. 1.

We have modeled the (100) surface because thesolid cleaves easily on this plane.17,18 The optimizedcoordinates for the (100) plane were taken fromab initio calculations of Bermudez.19 This plane isterminated in the nearest-neighbor rows of Ga(II)and O(III) atoms, each singly-unsaturated. Ga(I)and O(I) atoms at the surface are fully coordinated,and there are no O(II) atoms in the surface plane.Figure 2 shows the top and side view of that plane.

We have optimized the geometry by the AtomSuperposition and Electron Delocalization (ASED-TB).20,21 The ASED-TB method is a modification ofthe extended Huckel method that includes a repul-sive term for the electrostatic interaction betweennucleus.20 The energy was computed as the differ-ence (∆E) between the Ga–O–H composite systemwhen the H atom is adsorbed at a specified geometryand when it is far away from the Ga–O surface. Itcan be expressed as

∆ETotal = E(H − Ga2O3) − E(Ga2O3)

−E(H) + Erepulsion,

where E is the electronic energy, Erepulsion is therepulsive energy for nucleus j in the presence of afixed atom i:

Erepulsion =12

∑

i

∑

j �=i

Eij ,

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Adsorption of Hydrogen on β-Ga2O3(100): A Theoretical Study 81

(a)

(b)

Fig. 1. (a) Three-dimensional crystal structure of β-Ga2O3 with a unit cell. Gallium atoms are in tetrahedral oroctahedral coordination denoted by Ga(I) (light gray) and Ga(II) (dark gray) respectively. Black spheres represent theoxygen atoms. (b) Left — tetrahedral Ga(I) and right — octahedral Ga(II) symmetry sites.

where Eij is a pairwise electrostatic term. Thesummation extends over all Ga–Ga, O–O, Ga–H andO–H pairs.

The DOS (density of states) and COOP (crystalorbital overlap population) were computed using theYAeHMOP code.22 The DOS curve is a plot of thenumber of orbital per unit energy. The COOP curveis a plot of the overlap population weighted DOS vs.energy. The integration of the COOP curve up to theFermi level (EF) gives the total overlap populationof the bond specified and it is a measure of the bondstrength.

All parameters necessary for calculations arelisted in Table 1.

3. Results and Discussion

After energy minimization, H is finally bonded toGa(I) surface species at a bond distance of 1.165 A.

This Ga(I)–H bond has been recently characterizedby IR spectroscopy by Collins et al.1 Using B3LYPcalculations, Wang and Andrews23 have determinedthat a GaH diatomic molecule is generated from theendothermic reaction of Ga with H2 with a differ-ence in energy close to that obtained by Collinset al.1 for H2 on Gallia surfaces. Himmel et al.24

have reported Ga–H distances of about 1.52 A fordigallane Ga2H6.25,26

In Fig. 3, we show the calculation of the total andprojected DOS. After H adsorption, a small addi-tional peak can be observed at –17.80 eV. The Hprojected DOS match the position of this peak.Considering the gallium oxide alone, our DOS resultsare in qualitative agreement with that of Hajnalet al.16 and Binet et al.15

The conduction band edge has 60% contribu-tion from Ga 4s states (see Fig. 4(a)). Binet et al.found a splitting between the 4s tetrahedral (Ga(I))

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82 E. A. Gonzalez et al.

[010]

[001]

(a)

(b)

Fig. 2. (a) Top and (b) lateral view of β-Ga2O3 (100) plane after optimization, dark gray Ga(II), light gray Ga(I),black O. The small white circle indicates H location.

Table 1. Parameters for ASED-TB calculations.

Ionization Slater exponent Electronegativity

Atom Orbital potential (eV) (au−1) (Pauling)

H 1s 13.60 1.000 2.1

Ga 4s 6.00 2.200 1.84p 15.00 2.590

O 2p 28.48 2.163 3.42s 13.62 2.174

and 4s octahedral (Ga(II)) states in the conductionband.15

We find a different contribution from Ga(I) andGa(II). A peak at −10.85eV disappears when H is

bonded to Ga and it is not present in the case ofGa(II) (see Fig. 4(a)–4(d)).

The DOS curves for oxygen show that whateverthe energy range, the O ions contribute equally to

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Adsorption of Hydrogen on β-Ga2O3(100): A Theoretical Study 83

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-10

Ene

rgy

(eV

)

DOS

Total Total + H H

EF

-17.8 eV

Fig. 3. Total DOS of β-Ga2O3 (in arbitrary units) before H adsorption (a); after H adsorption (b) and H projectedDOS (c).

-35

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rgy

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)

DOS

EF

Ga(I) Ga(I) + H Ga(II) Ga(II) + H

Fig. 4. Ga(I) and Ga(II) projected DOS before (a,c) and after H adsorption (b,d).

the total DOS though their coordination numbersare different (see Fig. 5). A small H contribution tothe O DOS can be observed at −17.8 eV. The O sstates are located at (−28.4,−30.5)eV and p statesat (−13,−18) eV for both O(I) and O(II). O(III) hasno changes after H adsorption.

Table 2 presents the electronic structure andbonding data for Ga2O3 before and after H adsorp-tion. Ga(I)–H bond has orbital by orbital composi-tion of 4s–1s (73%) and 4p–1s of 27%. The Ga–O OP

values follow the same tendency as that reported byBermudez. We found no Ga–Ga bonds. The Ga(I)–O(I) and Ga(II)–O(II) OP decrease from 0.390 to0.348 (11%) and from 0.399 to 0.339 (15%), respec-tively. The bonds with O(III) show almost no change.

The origin of the Ga–H bond can be related tothe decrease in Ga–O bonding (see Fig. 6(b)). Asmall peak at −17.8 eV (also present in the DOS,see Fig. 3) reveals the interaction for all Ga(I)–Hbondings (see Fig. 6(c)).

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84 E. A. Gonzalez et al.

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-10E

nerg

y(e

V)

DOS

EF

O(I) O(I) + H O(II) O(II) + H

s

p

s

p-17 .8 eV

Fig. 5. O(I) and O(II) projected DOS before (a,c) and after H adsorption (b,d).

Table 2. Electron density, overlap population, charge and distances for the β-Ga2O3(100)surface before and after H adsorption.

Electron density

Structure s p d Charge OP Distance (A)

β-Ga2Oa3

Ga(I)–Ga(II) Ga(I) 0.94 0.28 0.00 1.779 0.000 3.215Ga(II) 0.85 0.57 0.00 1.575

Ga(I)–Ga(I) Ga(I) 0.85 0.47 0.00 1.671 0.000 3.036Ga(I)–O(I) O(I) 1.83 5.13 0.00 −0.964 0.390 1.798Ga(I)–O(II) O(II) 1.78 4.81 0.00 −0.597 0.399 1.790Ga(II)–O(I) O(I) 1.88 5.47 0.00 −1.348 0.263 1.932Ga(II)–O(II) O(II) 1.78 4.81 0.00 −0.597 0.278 1.896Ga(II)–O(III) O(III) 1.83 5.20 0.00 −1.037 0.244 1.969

β−Ga2O3 + Hb

Ga(I)–Ga(II) Ga(I) 0.95 0.58 0.00 1.467 0.000 3.215Ga(II) 0.86 0.57 0.00 1.569

Ga(I)–Ga(I) Ga(I) 0.85 0.47 0.00 1.671 0.000 3.036Ga(I)–O(I) O(I) 1.84 5.34 0.00 −1.203 0.348 1.798Ga(I)–O(II) O(II) 1.79 5.09 0.00 −0.889 0.339 1.790Ga(I)–H H 1.09 0.00 0.00 −0.095 0.799 1.165Ga(II)–H 0.000O(II)–H 0.000Ga(II)–O(I) O(I) 1.88 5.46 0.00 −1.348 0.262 1.932Ga(II)–O(II) O(II) 1.79 5.09 0.00 −0.889 0.285 1.896Ga(II)–O(III) O(III) 1.83 5.17 0.00 −1.006 0.243 1.969

Note: aEF = −13.31 eV; bEF = −13.30 eV.

4. Conclusions

The electronic structure (DOS) and bonding of thehydrogen on optimized β-Ga2O3(100) surface hasbeen determined at a semi-empirical level.

H is adsorbed on Ga atoms and no O–H bond isdeveloped. The projected DOS on O does not changeafter adsorption. A Ga–H bond is formed at thetetra-coordinated Ga(I) sites at the expense of theGa–O bonds.

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Adsorption of Hydrogen on β-Ga2O3(100): A Theoretical Study 85

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Ene

rgy

(eV

)

- COOP +

Ga(I)-O(I) Ga(I)-H

EF

Ga(I)-O(I) + H

Fig. 6. COOP curves for Ga(I)–O(I) before (a); and after hydrogen adsorption (b); and Ga(I)–H (c).

The Ga–H bond includes mainly Ga 4s–H 1s andGa 4p–H 1s interactions. The DOS curves show thecontribution of adsorbed H at –17.80 eV, exhibitingbonding characters with Ga(I).

Acknowledgments

Our work was supported by UNS-Fısica, PIP-2007 CONICET and J. S. Guggenheim Foundation.A. Juan is a member of CONICET, E. Gonzalez andP. Jasen are fellows of that institution.

We thank Dr. Vic Bermudez from the USNavy who kindly supplied us with the β-Ga2O3

coordinates.

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