Surface Science 600 (2006) 116–123

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Photoemission study of sulfur and oxygen adsorptionon GaN(000�1)

Lukasz Plucinski a, Leyla Colakerol a, Sarah Bernardis a,Yufeng Zhang a, Shancai Wang a, Cian O�Donnell a, Kevin E. Smith a,*,

I. Friel b, T.D. Moustakas b

a Department of Physics, Boston University, 590 Commonwealth Ave., Boston, MA 02215, United Statesb Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215, United States

Received 25 March 2005; accepted for publication 10 October 2005Available online 28 November 2005

Abstract

The surface electronic structure of thin film n-type GaN(000�1) has been studied using high resolution angle resolvedphotoemission spectroscopy. Clean surfaces, sulfur-exposed surfaces, and oxygen-exposed surfaces were investigated.Spectra were recorded for the Ga 3d shallow core states and for the valence band states. The sulfur covered surfacewas found to be inert with respect to subsequent oxygen exposure. For the clean Ga-adlayer terminated (000�1) surface,the Fermi level is pinned by surface states at least 1.4 eV below the bottom of the conduction band. Sulfur and oxygencovered surfaces exhibit smaller values for surface band bending, with the Fermi edge lying 0.7–1.0 eV below theconduction band minimum. Finally, we discuss a possible new interpretation for the asymmetry in the shape of Ga3d photoemission feature.� 2005 Elsevier B.V. All rights reserved.

Keywords: Angle resolved photoemission; Gallium nitride; Sulfur; Surface electronic phenomena

1. Introduction

The wet chemical, ex situ passivation ofGaN surfaces with sulfur is the subject of much

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2005.10.021

* Corresponding author. Tel.: +1 6173536117; fax: +16173539393.

E-mail address: ksmith@bu.edu (K.E. Smith).

current research [1–8]. Among the importantfindings are improved photoluminescence andimproved ohmic contacts on sulfur treated sur-faces. While in situ studies of sulfur depositionon atomically clean semiconductor surfaces inultra-high vacuum (UHV) have been performedon several important substrates including GaAs[9], Si [10,11], Ge [12], and InP [13], no such studiesof GaN surfaces have been reported to date.

ed.

L. Plucinski et al. / Surface Science 600 (2006) 116–123 117

We present here the results of a high resolu-tion angle-resolved photoemission spectroscopy(ARPES) study of clean n-type GaN (000�1) sur-faces exposed to sulfur and oxygen under UHVconditions. The goal of this study was to establishwhether an atomically thin layer of sulfur on GaNprotects the surface from oxygen adsorption. Wealso discuss changes in the surface electronic struc-ture and surface band bending with sulfur andoxygen exposures, and report the observation onthe clean surface of a sharp surface state abovethe valence band maximum (VBM), similar to thatrecently observed in p-type GaN [14].

2. Experiment

The experiments were performed on undulatorbeamline U5UA [15] at the National SynchrotronLight Source (NSLS). This beamline is equippedwith a spherical grating monochromator with refo-cusing optics, providing a photon beam of approx-imately 100 lm diameter. The samples were 0.5 lmthick Si-doped wurtzite GaN thin films grown atBoston University by plasma-assisted molecularbeam epitaxy (MBE) on sapphire using a GaNbuffer layer [16]. The free carrier concentrationwas 3 · 1018 cm�3, resulting in slightly degeneraten-type material [17]. It is assumed that the sampleswere N-face, (000�1) polarity, as previously deter-mined for samples grown with the GaN bufferlayer. This polarity is also supported by our obser-vation (discussed below) of a surface state abovethe VBM from clean GaN surfaces; this surfacestate is related to the Ga-adlayer surface termina-tion [18,19]. Prior to introduction into the UHVchamber samples were rinsed in a 1:10 solutionof HCl, and then rinsed in de-ionized water. Thesamples were then mounted on an electron beamheated sample holder attached to a liquid nitrogencooled cryostat. They were outgassed at 850 �C forseveral hours, and cleaned by repeated cycles ofnitrogen ion sputtering and annealing to 850 �Cas described previously [14,20,21]. Spectra wererecorded from samples held at approximately180 K, and the base pressure in the spectrometersystem was 7 · 10�11 Torr. Surface cleanlinesswas monitored by measuring surface state emis-

sion, known to be highly sensitive to contamina-tion [20,21]. X-ray photoemission spectroscopy(XPS) measurements performed separately indi-cate that the surfaces cleaned in this fashion arefree of C contamination, and have a very smallO content.

High resolution APRES experiments were per-formed using a Gammadata Scienta SES100 hemi-spherical electron energy analyzer in a geometrywhere the incoming photon beam was fixed at40 �C with respect to the analyzer. Different loca-tions in the two-dimensional surface Brillouin zonewere reached by changing the sample orientationwith respect to the analyzer. The incoming photonbeam, the surface normal, and the polarizationvector of the photons all lay in the same horizontalplane. The SES100 entrance slit was oriented ver-tically. All spectra presented here were recordedat the photon energy hm = 49.2 eV, at an overallinstrumental resolution of 50 meV or better, asdetermined from an atomically clean Cu sam-ple. The Fermi level (EF) was determined froman atomically clean Au foil in the electric contactwith the sample. Low-energy electron diffraction(LEED) patterns recorded from clean surfaceswere hexagonal (1 · 1). The patterns were verysharp with a low background, and no evidenceof a surface reconstruction was observed. Sampleswere oriented along high symmetry directions inreciprocal space for ARPES scans using the LEEDpatterns. The VBM was conventionally deter-mined by extrapolation of the leading edge of thenormal emission valence band spectrum to zero[21].

Sulfur was deposited from an UHV compatibleelectrochemical cell. The sulfur flux generated bysuch cells has been shown to be primarily molecu-lar in nature and has been used successfully in pre-vious experiments [9–13,22]. The deposition ratewas controlled by monitoring the pressure in theUHV chamber and mass spectrometer output. Asthe pressure in the UHV chamber increases signif-icantly during deposition, the exposure is reportedin Langmuirs (1 L = 1 · 10�6 Torr s). This methodlikely underestimates the deposition rates used inthis work, but since the sulfur exposed surfacesare subsequently annealed (see below), this is nota significant concern. Mass spectroscopy revealed

20 18 16 14 12

_

phot

ocur

rent

[arb

itrar

y un

its]

binding energy [eV ]relative to the VBM

Ga 3dshoulder

Ga 3dbulk

N 2s

hν = 49.2 eV

Κ_

Γ

Fig. 1. Ga 3d photoemission measured from clean GaN(000�1)surface in normal emission and at 22� off-normal (along C–K–M direction in Brillouin zone). Dots show the spectra andresiduals, solid lines show the fit, fitted components, andpolynomial background. See text for details.

118 L. Plucinski et al. / Surface Science 600 (2006) 116–123

no increase in the concentration of any other ele-ment or compound in the chamber during sulfurdeposition. Exposure to research grade oxygenwas performed via use of an UHV compatibleleak-valve.

The standard sequence of exposures used inthis experiment was as follows. First a cleanGa-adlayer (000�1) surface was prepared byrepeated sputtering and annealing, and ARPESmeasurements made. This surface was thenexposed to 40 L of sulfur and ARPES measure-ments repeated. This surface was then in turnexposed to a further 400 L of sulfur, leading tothe complete disappearance of any Ga 3d featuresin the photoemission spectra. The sample was thenannealed at 300 �C for 5 min, leading to the reap-pearance of the Ga 3d photoemission feature. Thissulfur exposed and annealed surface has a stablecoverage of S in the sense that S cannot be furtherremoved from this surface without sputtering andannealing. (The exact coverage is unknown.)Finally, this surface was exposed to 5000 L of oxy-gen. Repeated cycles of sputtering and annealinglead to the complete recovery of a clean Ga-adlayer surface. For studies of oxygen adsorptionon the clean surface, the sputtered and annealedsurface was exposed to 5000 L, and then an addi-tional 5000 L, of oxygen. Again, repeated sputter-ing and annealing cleaning cycles fully recoveredclean Ga-adlayer surface.

3. Results

Fig. 1 shows two spectra in the region of Ga 3dshallow-core level rom a clean n-type GaN surface.This surface exhibited a sharp surface state abovethe VBM, similar to that obtained previously onp-type GaN [14]. (The surface state will be dis-cussed further below.) The sample is oriented withthe CKM direction in the horizontal plane (i.e. inthe plane defined by the incoming beam and thefixed detector). The spectra are angle-integrated±5� in the CM direction, and recorded at differentemission angles and thus represent different posi-tions in the 3-dimensional Brillouin zone. Themain feature in the spectra at approximately18 eV is primarily due to emission from bulk Ga

3d states, while the weaker feature at approxi-mately 15 eV is primarily due to emission frombulk N 2s states. In agreement with previous work,spin orbit splitting of the Ga 3d state is notobserved [23]. However, the Ga 3d feature isclearly asymmetric, with a shoulder in the spectraat higher binding energies. We refer to the compo-nents of the Ga 3d emission as the bulk feature andthe shoulder, as indicated in Fig. 1.

To aid in the analysis of the spectra, we have per-formed least-square fitting of the data. Combina-tions of Voigt doublets for the Ga 3d feature,Gaussian singlets for the N 2s feature, and a thirdorder polynomial background delivered a success-ful fit. The parameters of the Gaussians were alwaysreleased during the fitting, whereas the parametersof the Voigt spin–orbit doublets were partly con-strained: amplitude ratio fixed at 0.6 eV, separation0.46 eV, Lorentzian width 0.2 eV, but Gaussianwidth released. The amplitudes and bindingenergies of the doublets were always released dur-

22 20 18 16 14 12 22 20 18 16 14 12 22 20 18 16 14 12

(d)

(c)

(b)

phot

ocur

rent

[arb

itrar

y un

its]

(a)

binding energy [eV] relative to the VBM

(f)

(e)

Fig. 2. Series of Ga 3d photoemission spectra recorded atdifferent coverages of sulfur and oxygen: (a) clean Ga-adlayersurface, (b) clean surface with 40 L of sulfur, (c) clean surfacewith 440 L of sulfur annealed to 300�C, (d) previous surfaceexposed to 5000 L O2, (e) clean surface exposed to 5000 L O2

and (f) and additional 5000 L O2. Dots show the spectra andresiduals, solid lines show the fit, fitted components, andpolynomial background.

L. Plucinski et al. / Surface Science 600 (2006) 116–123 119

ing the fitting iterations. The amplitude and ratioof the Voigt doublets are in agreement with theliterature [24] and the width of the Lorentziancomponent was found from an optimal fit ofclean surface spectra. For all presented fits thevalue of v2 was higher than 0.9996. All presentedspectra were aligned such that the Ga 3d5/2bulk feature lies at 17.69 eV with respect to theVBM.

In Fig. 1 the Ga 3d feature was fitted with twoVoigt spin–orbit doublets, each one for the bulkand shoulder features. The N 2s feature was fittedwith one Gaussian for the 22� off-normal spec-trum, but required two for the normal emissionspectrum. It is clear from the spectra in Fig. 1 thatthe feature related to the N 2s changes with photo-emission angle. Although this change could bepartly related to the selection rules in our photo-emission geometry, it can also be related to thedispersion of the N 2s bands, as predicted byfirst-principles band structure calculations [25].Furthermore, the amplitude ratio of the two Voigtdoublets also changes slightly with emission angle.We have found a separation between the twoVoigt doublets of 0.80 eV for the 22� off-normalemission and 0.84 eV for normal emission.

Fig. 2 shows a series of spectra in the Ga 3dregion recorded at an emission angle of 22� forclean, sulfur, and oxygen exposed surfaces. As inFig. 1, the spectra are angle integrated ±5� in thevertical plane. Widths and relative positions ofthe features used to fit the spectra in Fig. 2 aregiven in Table 1. Both the total full-width-half-maximum (FWHM) of the Ga 3d features andthe separation between the two Voigt doublets is

Table 1Parameters of the fits presented in Fig. 2

Surface Total FWHM ofGa 3d feature

Separation (eV) of band shoulder feature

(a) Clean 1.51 0.80(b) S#1 1.56 0.88(c) S#2 1.47 0.82(d) S+O2 1.52 0.84(e) 5000 L O2 1.62 0.95(f) 10,000 L O2 1.64 0.94

The values of FWHM are given for the spectra with the background

significantly larger in clean surface exposed to oxy-gen than in surface previously exposed to sulfur.Although it is not reflected in residuals this mayindicate an additional oxygen-related componentin the shoulder peak.

The valence band spectra associated with thesame sequence of surface treatments are presentedin Figs. 3 and 4. For these spectra the angular res-olution was set to 0.4�. Fig. 3 shows normal emis-sion spectra which reflect the electronic structureat the surface Brillouin zone center, C. Fig. 4shows spectra recorded at 22� off normal, reflect-ing the electronic structure at the K point [14].The spectrum from the clean surface in Fig. 4

ulks

Gaussian component width of the Voigt doublet

Ga 3d shoulder [eV] Ga 3d bulk [eV]

0.30 0.320.24 0.400.23 0.350.29 0.380.39 0.420.36 0.42

extracted.

10 8 6 4 2 0 -2

(f)

(e)

(d)

(c)

(b)

(a)

phot

ocur

rent

[ar

b.un

its]

binding energy [eV]relative to the VBM

normal emission

hν=49.2 eV

Fig. 3. Normal emission ARPES spectra at different sulfur andoxygen coverages. Spectra labeled (a)–(f) correspond to theexposures listed in Fig. 2. The surface-related feature is markedwith a triangle. See text for details.

10 8 6 4 2 0 -2

(f)

(e)

(d)

(c)

(b)

22 deg off- normal

phot

ocur

rent

[ar

b.un

its]

binding energy [eV]relative to the VBM

(a)

Fig. 4. Off-normal emission ARPES spectra recorded at 22�emission angle. Spectra labeled (a)–(f) correspond to theexposures listed in Fig. 2. The surface-state is marked with atriangle. See text for details.

120 L. Plucinski et al. / Surface Science 600 (2006) 116–123

shows a well defined emission feature above theVBM, which is related to Ga dangling bonds,while the spectrum from the clean surface inFig. 3 shows a feature at the bottom of the valenceband that was predicted due to a surface staterelated to Ga s-orbitals [19]. Detailed analysis ofthe dispersion of these surface states will be pre-sented elsewhere. As noted above, these surfacestates are related to the Ga-adlayer structure on(000�1) N-face surface [18,19].

It is clear from the data presented in Figs. 2–4and Table 1 that the GaN surface exposed to Sis far less reactive to subsequent O2 exposure thanis the clean surface, i.e. the presence of S on thesurface inhibits the chemisorption of O on GaN.Only minor changes are observed in both the Ga3d spectra and the valence band spectra uponexposure of the S-exposed surface to up to5000 L of oxygen. This contrasts to the clear oxy-gen adsorption effects observed when the cleansurface is exposed to O2, as also indicated by thedata in Figs. 3 and 4. From Table 1 the totalFWHM, separation, and widths of Gaussian com-ponents are largest on clean surface exposed to theoxygen. We note that while exposure of the cleansurface to S (and subsequent annealing) has a min-imal effect on the states at zone center, markedchanges are observed at the K point (Fig. 4), par-ticularly for states near the top of the valenceband, 2.3 eV below the VBM. The sulfur overlayeron GaN(000�1) passivates the surface against oxy-gen adsorption.

Conventionally, the Ga 3d photoemission fea-ture from clean GaN has been fit to at least twocomponents, one associated with the Ga–N bondin bulk material and the other to a Ga–O bondfrom surface or bulk contamination. The featureattributed to the Ga–O bond is typically observedto lie about 1 eV towards higher binding energies[24,26,27]. The photoemission feature associatedwith the N 2s state is observed to lie at approxi-mately 2–3 eV lower binding energy relative tothe main Ga 3d emission feature and overlaps withthe Ga 3d emission. Density functional theory cal-culations in the local density approximation(DFT-LDA) do not agree with experiment in therelative binding energy of the N 2s and Ga 3dderived bands [28,29]. However, the application

L. Plucinski et al. / Surface Science 600 (2006) 116–123 121

of appropriate corrections results in a calculationin good agreement with experiment [25]. It hasbeen routine practice to compare the Ga 3d regionin photoemission spectra taken from GaN withthose taken from GaAs and to infer that there isa similar oxygen induced chemical shift of Ga 3din GaN and GaAs. However, the plane-wavepseudopotential study of Matilla and Nieminen[30] reveals that the nature of oxygen point defectsin GaN and GaAs is very different, and smallerchemical shift could be expected in GaN [30]. Todate there are no published calculations that sup-port a Ga–O chemical shift of order 1 eV com-pared to the bulk Ga 3d position.

Our spectra from clean GaN(000�1) show thesurface state above the VBM which is clearlyrelated to the Ga-adlayer structure [18]. Auger-electron spectroscopy (AES) measurements indi-cate less than 1 at.% oxygen contamination onsuch surface [14]. From Fig. 1 the intensity ratioof the doublets used fit Ga 3d peak is 0.24 for nor-mal emission and 0.38 for 22� off-normal emission.If the shoulder at the high binding energy side ofthe Ga 3d photoemission feature from a cleanGa-adlayer surface is to be assigned to Ga–Obonds, these intensity ratios would indicate amuch higher O concentration on the surface thanrevealed by AES. Thus the identification of theshoulder in the Ga 3d emission as being due toGa–O bonds is problematic. An alternative expla-nation is that the shoulder is due to N 2s (and to alesser extent, 2p [31]) states hybridized with theGa 3d core level, and thus intrinsic to the mate-rial [29]. LDA calculations indicate that in theenergy region of the Ga 3d state, the DOS containsthree sets of bands: one related to the spin–orbitsplit Ga 3d levels and two related to the Ga–Nd-to-s bonding [29,32]. It is possible that the shoul-der in Ga 3d levels, visible in Figs. 1 and 2 for theclean surface, is related to this latter density ofstates.

We note that we have fitted the Ga 3d regionwith Voigt doublets. If the shoulder in the Ga 3dpeak does not result from a chemical shift of themain Ga 3d state, then we should not necessarilyhave used a Voigt doublet of the same separation,intensity ratio, and Lorentzian width as mainpeak. The practical advantage of our fit is that it

allows comparison to previously published work[24,26].

We turn now to consideration of surface bandbending. In our experiments on the clean surfacewe observed that the intensity of surface stateabove the VBM decreased with time. Typically,this state was undetectable approximately 2 hafter the end of a cleaning cycle, even with thepressure in the spectrometer chamber in therange 8 · 10�11–1 · 10�10 Torr during the mea-surements. Simultaneously, a continuous shift ofthe spectra with time towards higher binding ener-gies with respect to the Fermi edge was observed.This shift is due to surface band bending inGaN. (Surface photovoltage effects can be ruledout since the Ga-adlayer surface is metallic andthe bulk material is not insulating.) The energy dif-ference between EF and the VBM was calculatedfrom the measured binding energy of the Ga 3dstate. The lowest measured difference from theclean sample was EVBM � EF = 2.03 eV, and thisincreased by 0.3 eV within an hour. EVBM � EF

was between 2.42 and 2.48 eV for the S/GaN sur-face, 2.64 eV for the S/GaN surface exposed to5000 L of oxygen, 2.6 eV for the GaN surfaceexposed to 5000 L O2, and 2.71 eV for the GaNsurface exposed to 10,000 L O2. The precision ofthese values is not better than 0.1 eV due to thefact that the spectra were shifting with time tohigher binding energies as a result of residual gas(primarily H2) contamination and due to inevita-ble inaccuracy in experimental determination ofthe VBM [19]. Fig. 5 presents graphically ourdetermination of surface band bending on theclean and oxygen exposed surfaces. Assuming abulk band gap of 3.4 eV, and EF pinned at the bot-tom of the bulk conduction band, we obtain anupward surface band bending in clean Ga-adlayerGaN(000�1) surface of approximately 1.4 eV.This is reduced by 0.7 eV when this surface isexposed to 10,000 L O2. Sulfur exposure reducedthe band bending by 0.4–0.5 eV. Ex situ processed[33], or non-processed [34] (000�1) N-face GaNsamples show very little or no surface band bend-ing if not cleaned under UHV. Our values for sur-face band bending for S/GaN and O/GaN arewithin the range reported for sputtered andannealed samples [34]. The upward bending of

O/GaN10000 L

0.7 eV

2.7 eV

1.4 eV

EF

3.

2 eV

Ga-addl ayer

4 eV

Fig. 5. Schematic representation of the surface band bendingdeduced from the photoemission results.

122 L. Plucinski et al. / Surface Science 600 (2006) 116–123

1.4 eV for clean Ga-adlayer surface is larger thanthe value of 1 eV for nitrogen-ion sputtered sam-ples [34], but lower than the value of 1.6 eV mea-sured on ex situ processed Ga-face GaN samples[33].

4. Summary

We have performed a high-resolution valenceband and shallow core level photoemission studyof clean Ga-adlayer GaN(000�1) surfaces, andthese surfaces exposed to sulfur and oxygen. Theidentification of the surface as Ga-adlayer termi-nated was made by comparison of our data to the-ory. The clean surface was found to be veryreactive and sensitive to the residual gas contami-nation even under UHV conditions. Exposure ofthe GaN surface to sulfur inhibits oxygen adsorp-tion, as determined from valence band spectra.Comparison of the spectra from clean and oxygenexposed surfaces led us to propose a new interpre-tation for the asymmetric line shape of the Ga 3dfeature in photoemission. We postulate that theshoulder on the high binding energy side of theGa 3d feature is intrinsic, and related to bulkGa–N d-to-s bonding. We find that surface stateson the clean surface pin the Fermi edge at least1.4 eV below the CBM, while sulfur and oxygencovered surfaces exhibit smaller surface bandbending values of 0.7–1.0 eV.

Acknowledgments

We would like to thank G. Hughes for numer-ous helpful discussions and for providing us theelectrochemical sulfur cell. Fruitful discussionswith E. Vescovo are also acknowledged. We thankthe personnel of NSLS for the technical support.This work was supported in part by the NSFunder grant numbers DMR-0311792, by the USARO under grant 40126-PH, and by the US AirForce Office of Scientific Research.

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