x-ray photoelectron spectroscopy depth profiling of aluminium nitride thin films

SURFACE AND INTERFACE ANALYSIS, VOL. 25, 99È104 (1997)

X-ray Photoelectron Spectroscopy Depth Proülingof Aluminium Nitride Thin Films¤

K. S. A. Butcher,* T. L. Tansley and Xin LiSemiconductor Science and Technology Laboratories, Physics Dept., Macquarie University, 2109 NSW, Australia

Aluminium nitride thin Ðlms grown at room temperature on degenerate silicon (conducting) substrates have beenstudied using XPS. The hydrolysis layer at the surface of the AlN was examined using valence band measurements,and the e†ect of 5 kV argon ion milling used to remove the hydrolysis layer was scrutinized using angle-resolvedXPS. The N/Al ratios found from the angle-resolved measurements indicate nitrogen depletion from the surface ofthe milled samples, whereas O/Al ratios indicate no such depletion of oxygen. After argon ion milling, carbonuptake from the ultrahigh vacuum analysis chamber was found to be signiÐcant. 1997 by John Wiley & Sons,(

Ltd.

Surf. Interface Anal. 25, 99È104 (1997)No. of Figures : 6 No. of Tables : 1 No. of Refs : 22

KEYWORDS: angle resolved XPS; aluminium nitride ; depth proÐling ; valence band

INTRODUCTION

Aluminium nitride has recently become of great interestto the semiconductor industry because of its dual roleas a bu†er layer and as an active constituent of theAlGaN alloy used in the construction of blue andpurple light-emitting diodes.1 The material has alsobeen widely studied for its insulating properties ; its highresistivity (1013È1015 ) É cm) large bandgap (D6.0 eV),low dielectric loss and its high transmission in the ultra-violet, visible and infrared sections of the spectrummake it an attractive material for passivation or as aninsulator in metalÈinsulatorÈsemiconductor (MIS)structures built on compound semiconductors.2 Manycompound semiconductors possess band structureproperties and have high carrier mobilities o†eringperformance superior to silicon in optoelectronic andhigh-speed applications. However, the processing tech-nology for most of these compounds has been con-strained by the lack of MIS structures of adequatequality. MIS structures based on silicon dioxide formthe basis of silicon CMOS circuitry, in comparison,the native oxides of compound semiconductors donot provide adequate insulation for the production ofMIS-based transistor devices. For many compounds,elevated temperatures promote rapid surface decompo-sition and are to be avoided. One approach has beenthe use of low-temperature growth methods for thedeposition of aluminium nitride ;2h6 for instance, Li andTansley2 have reported the growth of good qualitylayers at D180 ¡C on GaAs.

The low-temperature growth of AlN is therefore ofongoing interest, however impurity incorporationÈwhich can be quite signiÐcant for AlN at high growthtemperaturesÈis a more serious problem at reduced

* Correspondence to : K. S. A. Butcher.¤ Paper presented at the AustralasiaÈAsia XPS Symposium 1995,

Coogee Beach, Sydney, Australia, 14È17 November 1995.

temperatures. Oxygen and carbon are the main con-taminating species, oxygen contamination occursbecause of the propensity of the aluminium nitridelattice to getter oxygen during growth.7 Carbon canalso be incorporated in large amounts from the metalÈorganic sources commonly used for chemical vapourdeposition of aluminium nitride. Techniques that canaccurately determine the constituents of aluminiumnitride Ðlms are therefore needed to measure andcontrol the quality of the material. Unfortunately noone method seems to be particularly well suited to theanalysis of thin Ðlms of this low atomic number insulat-ing compound: Rutherford backscattering spectroscopy(RBS) requires the material to be grown on a substratewith a light element matrix so that glassy carbon sub-strates are necessary for accurate analysis ; charginge†ects are evident with SIMS and with x-ray micro-analysis in SEM systems ; x-ray Ñuorescence is generallyquantitative only for higher atomic number species ;x-ray di†raction can be used to identify some grossimpurities, however insufficient data are available toquantitatively determine the oxygen content of AlN bythis method.

X-ray photoelectron spectroscopy is one method thathas been widely applied to study the chemistry of AlNÐlms. Recently, however, Kumar and Tansley8 haveshown that there is a discrepancy between the amountof oxygen observed for samples grown by reactive r.f.sputtering on Si, examined using XPS argon ion depthproÐling techniques, and samples grown on glassycarbon under similar conditions and analysed with RBStechniques. They ascribed this discrepancy to theuptake of oxygen from the XPS chamber and fromoxygen recycling in the sample as an artefact of the ionmilling process.8 However, no supporting evidence waso†ered to conÐrm this view. In this paper we examinemore closely the role of oxygen and carbon during XPSdepth proÐling measurements. We use angle-resolvedXPS, ion beam depth proÐling, valence band measure-ments, time-dependent measurements and quantiÐca-tion of the photoelectron peaks to examine the chemical

CCC 0142È2421/97/020099È06 $17.50 Received 24 November 1995( 1997 by John Wiley & Sons, Ltd. Accepted 2 October 1996

100 K. S. A. BUTCHER, T. L. TANSLEY AND X. LI

constitution of the AlN Ðlms, and also to assess thedetrimental e†ects of argon ion milling on the depthproÐling processes.

EXPERIMENTAL

The surface of the AlN samples was hydrolysed byatmospheric exposure prior to argon ion etching. Meth-odology was therefore required which could distinguishthe oxide contribution of the bulk material from thehydroxides and oxyhydroxides present on the samplesurface. The related oxides, hydroxides and oxyhydrox-ides of aluminium are indistinguishable using the linepositions of the Al core-level peaks found by XPS. Forthe Al KLL Auger line, the nitride can be distinguishedfrom these surface species by a greater energy shift thanexists for the XPS peaks ; however, again the oxides,hydroxides and oxyhydroxides are indistinguishable.9Davis, Ahearn and Venables10 were able to obtainaccurate stoichiometric O/Al ratios for oxide, hydroxideand oxyhydroxide standards from a quantiÐcation ofXPS core levels. However, for impure samples exposedto a series of surface treatments they found that therewas a considerable scatter in the O/Al ratios, whichthey attributed to physisorbed water and CÈO bonding.It therefore appears that non-ideal samples may not beeasily characterized by reference to the O/Al ratio.Some information may be obtained by examining the O1s core level if sufficient instrumental resolution is avail-able. This has was done by Klauber11 when examiningthe surface of It has been shown by Thomas andAl2O3 .Sherwood,12 however, that the valence band spectra ofthese materials have features that are readily distin-guishable. The valence band spectra were therefore usedhere to distinguish between oxides present in the bulkAlN Ðlm and the oxygen-bearing species present due tosurface hydrolysis. Valence band spectra were collectedin 0.1 eV steps and with a long dwell time of 5 s perpoint.

Angle-resolved measurements were performed toallow non-destructive depth proÐling of the AlN surfaceafter argon ion milling. According to the work ofKumar and Tansley,8 it may be suspected that argonion milling signiÐcantly alters the aluminium nitridesurface. The angle-resolved measurements were appliedto test this hypothesis. By varying the angle of thesample relative to the analysis optics of the XPS system,the e†ective analysis depth will be attenuated by afactor proportional to cos h because the photoelectronpathlengths through the sample can be regarded asbeing constant ; this allows angle-resolved depth pro-Ðling of the sample surface. The principles of angle-resolved XPS depth proÐling are dealt with in standardtexts on XPS, e.g. the monograph by Hofmann in thetext edited by Briggs and Seah.13

The C 1s photoelectron line was examined over timeafter milling the AlN samples. This was done toexamine whether quantiÐcation of the C 1s peak wasbeing a†ected by the growth of a hydrocarbon layer inthe ultrahigh vacuum environment used.

The aluminium nitride thin Ðlms used for theseexperiments were grown at room temperature on 0.01) É cm silicon by remote plasma-enhanced chemical

vapour deposition techniques described elsewhere.14,15The Ðlm characteristics have also been reported,14,15however, in brief these Ðlms have excellent electricalproperties with resistivities as high as 1015 ) É cm andbreakdown Ðelds typically greater than 1 MV cm~1.X-ray di†raction has revealed the Ðlms to be amorp-hous. As pointed out by Griess,16 electron di†ractione†ects can occur during angle-resolved XPS measure-ments, thereby a†ecting the validity of elemental quanti-Ðcation. It was therefore important to use amorphousmaterial for this study because the randomized matrixprevents electron di†raction from occurring. Infraredtransmission spectra indicate that the Ðlm compositionis predominantly AlN with large amounts of oxygenpresent. The Ðlms also have a direct bandgap varyingfrom 5.5 eV to 6.0 eV, commensurate with reportedvalues for AlN2 and remote from which has aAl2O3 ,bandgap of 6.7 eV.17 The sample properties may there-fore be summarized as being AlN-like despite signiÐcantoxygen incorporation.

The samples were examined in a Kratos XSAM 800XPS system capable of argon ion milling between thecollection of spectra. The C 1s photoelectron peak,resulting from the adventitious physisorption of hydro-carbons and known to have a value of 284.8 eV,18,19was used to calibrate the energy axis of the spectra, asper standard procedures. QuantiÐcation was achievedusing the core-level photoelectron spectra for the partic-ular elements analysed (i.e. Al 2p, O 1s, N 1s, C 1s, Ar2p) and the standard peak quantiÐcation software avail-able with the Kratos XPS system.

X-ray photoelectron spectroscopy core-level photo-electron spectra and valence band spectra wereobtained prior to and after argon ion milling. Details ofthe operating conditions for the milling and for the XPSoperation are given in Table 1. The milling conditionsused throughout these experiments gave a milling rateof D0.2 s~1. Rastered ion milling was used to obtainÓa wide crater area of 7] 5 mm while the analysis areawas 450 lm in diameter at the centre of the crater. Forthe angle-resolved measurements, the analysis angle hwas deÐned as being at 0¡ when the sample was normalto the analysis optics used to collect the photoelectrons.The analysis angle h was varied from 0¡ to 65¡ duringthe measurements. For the milled samples great carehad to be taken in ensuring that at such large presen-tation angles the analysis area remained within themilling crater. The analysis area spreads on the samplewith increasing angle to a width of D1.1 mm at 65¡,and this area may be further enlarged by the samplebeing tilted into unfocused areas of the electron collec-tion optics. Any misalignment between the analysis area

Table 1. Operating conditions for XPS and XPS depth pro-Ðling using the Kratos XSAM 800

Background pressure 2 Ã10É10 Torr

Argon pressure 4 Ã10É8 Torr

Mg Ka x-ray source power level 120 W (12 keV, 10 mA)

Hemispherical energy analyser

Pass energy 80 eV, 40 eV

Spot size 450 lm

Ion beam accelerating voltage 5 kV

Ion rastor 7 mm Ã5 mm

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 99È104 (1997)

XPS DEPTH PROFILING OF AlN THIN FILMS 101

and the crater was eliminated here by checking that nodi†erential charging e†ects were apparent in the C 1sspectra, because as a result of hydrocarbon and watervapour contamination on the original surface, theunmilled AlN surface was observed to have signiÐcantlyless charging (by as much as 2 eV) than the milledsurface of this insulator.

RESULTS AND DISCUSSION

Aluminium nitride is known20 to undergo hydrolysis inthe atmosphere by the reaction equation

AlN] 2H2O ] AlOOH] NH3 ,

it has also been shown20 that a further reaction canoccur after long exposures to a humid environment,yielding i.e.Al(OH)3 ,

AlOOH] H2O % Al(OH)3The unmilled surface of an AlN Ðlm might thereforecontain AlOOH or a mixture of the two,Al(OH)3 ,depending on the age of the sample. Below this surfacelayer there is a layer of AlOOH with trapped or physi-sorbed the thickness of this layer depends on theNH3 ;age and quality of the Ðlm. Finally, below this hydro-lysed layer is the bulk AlN with oxygen that has beenincorporated during growth.

By comparing the valence band spectra of unmilledAlN Ðlms shown in Fig. 1 (which are representative of alarge number of samples) with those of Thomas andSherwood,12 we can conÐrm the presence of Al(OH)3and AlOOH in di†erent mix ratios dependent on thesample age. Also indicated in Fig. 1 are the O/Al ratiosmeasured from the core-level spectra for the samesamples ; interestingly the ratio for the bayerite orb-phase of was consistently measured at highAl(OH)3values of D4. As discussed below, we believe this couldpossibly be due to the presence of what may be a car-bonate species (D5 at.%) present in these Ðlms,although the carbonate could not be bound to alu-minium if it were to increase the O/Al ratio. Physi-sorbed water vapour is another possible source ofexcess oxygen. It might be expected that this excessoxygen would be visible in the valence band spectra ;however, given the inherent noise levels and complexityof valence band spectra, it may not necessarily be recog-nized or resolved. The ratios given in Fig. 1(b) corre-spond quite well to the gibbsite or a-phase of Al(OH)3 ,while the ratio for the sample of Fig. 1(c) corresponds toa mixed gibbsiteÈboehmite (boehmite is the c-phase ofAlOOH) surface again in agreement with the features ofthe spectra. It may be that for the samples with thelower levels of hydration, represented by the measure-ments shown for the samples of Figs 1(b) and 1(c), thecarbonate was not greatly interferring with the mea-sured O/Al ratios because it was only present at a 2È3at.% concentration.

During this symposium Klauber presented data indi-cating that the hydroxides and oxyhydroxides of Alwere reduced to resulting from the long-termAl2O3([2 h) exposure of his hydrolysed samples to the MgKa x-ray source of an XPS machine.11 Our valenceband spectra were collected over a much shorter time

Figure 1. Valence band spectra of hydrolysed AlN samples after :(a) 15 weeks of aging in air ; (b) 5 weeks of aging in air ; (c) 1week of aging in air.

span so that no change was observed during the mea-surements. However, angle-resolved measurements ofthe hydrolysed surface of some of our samples taken at60¡ gave a smaller O/Al ratio than the 0¡ position(D10% lower for x-ray exposure periods of D15 min).This seems to indicate the early stages of the reductionprocess identiÐed by Klauber.

It might be noted that Graziani and Bellosi20 identi-Ðed the constituent of hydrolysed AlN as beingAl(OH)3the bayerite, although clearly, as shown here, the gibbs-ite phase can be present for samples that are hydrolysedfor shorter periods.

After milling away the hydrolysed surface, an AlOOHunderlayer region can be identiÐed by an O/Al ratiothat is greater than the steady state achieved in the bulkregion. The transition to the bulk AlN layer may not bea sharp transition, therefore the new region may not beeasily identiÐed by a change in the N concentrationbecause nitrogen in the form of trapped is presentNH3in the hydrolysed layers as a hydrolysis product. Figure2 shows an XPS depth proÐle, collected using ionmilling. The valence band spectra for the unmilledsurface is shown in Fig. 1(c) ; this can be compared toFig. 3, which shows the spectra for the same sample



Figure 2. X-ray photoelectron spectroscopy depth profile of AlNsample collected with argon ion milling.

after 900 s of ion milling. The spectra for the milledsample shows features that are typical of andAl2O3 ,the O/Al ratio of 1.58 conÐrms that this compound ispredominant in what appears to be a low nitrogencontent compound. It should, however, be noted thatRBS measurements made on a sample grown undersimilar conditions on a glassy carbon Ðlm revealed a20% nitrogen content for that Ðlm. The low nitrogencontent seen for the sample of Fig. 2 therefore seems toindicate the same descrepancy noted by Kumar andTansley.8

The high carbon content shown in the depth proÐleof Fig. 2 was examined more closely. Core-level spectraof the C 1s peak were taken with a pass energy of 40 eVdirectly after argon ion milling. Figure 4 shows some ofthe collected spectra with the time indicated after argonion milling at which the spectra collection began (eachspectra taking 1 min to collect). These spectra were notcorrected for charging e†ects, however, it can be seenthat there are two carbon peaks and that the peak atD288 eV was growing with time. This peak was due to

Figure 3. Valence band spectra of the sample depth profiled inFig. 2, after 1200 s of argon ion milling.

Figure 4. Carbon 1s core-level spectra of AlN sample after argonion milling. The spectra show the growth with time of a hydrocar-bon peak at Á288 eV.

adventitious hydrocarbons that were depositing fromthe vacuum chamber during the measurements. Thegrowth of the hydrocarbon layer was occurring in avery short time span, which could interfere with themeasurement of carbon in the bulk if it was not recog-nized that this peak was due to hydrocarbons. Thesecond carbon peak at D293 eV (uncorrected) wasobserved at approximately the same concentrationregardless of the milling depth and was not seen tochange with time, so that it is believed to have been dueto a bulk carbon. The metalÈorganics used for thegrowth of the aluminium component are a major sourceof bulk carbon contamination and a likely candidate forthe origin of this second peak. The peak actually pre-sents itself at the corrected C 1s position of acarbonate21 and, although other carbon compounds areknown to have similar binding energies,22 a bulk car-bonate may be a possible non-volatile product of one ofmany possible reactions between undissociated metalÈorganic and oxygen in a limited oxygen environment.

Having distinguished the rapid growth of a hydrocar-bon peak on the milled surface of the material from thepresence of bulk carbon, the hydrocarbon peak couldsubsequently be used for the calibration of the binding


XPS DEPTH PROFILING OF AlN THIN FILMS 103

energy axis for the milled samples. A steel sample exam-ined in the same analysis chamber over a similar mea-surement period showed no indication of hydrocarbongrowth after argon ion milling, so that the AlN surfaceseems to have a relatively high sticking probability (andperhaps an attraction potential) for hydrocarbons.

A sample with a higher nitrogen content than thatshown in Fig. 2 was examined with angle-resolved XPSafter argon ion milling. A depth proÐle of this sample isshown in Fig. 5 (again, the high carbon content islargely due to the absorption of hydrocarbons duringthe measurement). This sample was used in order toobtain a lower noise value for N/Al ratios. The ratios ofO/Al, N/Al and Ar/Al were measured at 5¡ intervalsand these data are shown in Fig. 6(a). Figure 6(b) showsthe same data replotted as cos h, because this value isapproximately proportional to the e†ective analysisdepth of the photoelectrons. Also for Fig. 6(b), the com-positional ratio values have been normalized to theirmaximum value to facilitate comparison. The noisepresent in the Ar/Al ratios results from the low levels ofAr present (1È3%) ; some noise is also present in theO/Al and N/Al ratios, which is partly due to the lowcollection statistics for small-area analysis and partlydue to the choice of baseline for quantiÐcation. The Arsignal was present because of Ar implantation from theion beam. After a considerable period of milling, anequilibrium Ar concentration should be reached so thatthe concentration of Ar observed by angle-resolvedXPSÈover considerably thinner sections than thatremoved by the millingÈwould be expected to beuniform (otherwise, the implantation proÐle of the Arwould be so broad as to again be unobservable withangle-resolved XPS). However, in Fig. 6(b) it can beseen that the Ar/Al ratio drops, with smaller cos hvalues indicating that the surface is depleted of Ar. Thiscan be simply interpreted as a preferential milling ofimplanted Ar compared to Al ; the Ar component nor-mally occurring as an inert gas will only be weaklybound within the Ðlm and can easily be removed as agaseous product from the near-surface area comparedto Al. Similarly it can be seen in Fig. 6(b) that the nitro-

Figure 5. Depth profile of high nitrogen content AlN sample usedfor angle-resolved measurements (see Fig. 6).

Figure 6. Angle-resolved data for AlN sample after 600 s ofargon ion milling at 5 kV.

gen component has also been preferentially removedfrom the lattice, while the level of oxygen in the near-surface region appears to be reasonably constant to thedepth probed. It is believed that the preferentialremoval of nitrogen from the surface region results fromthe breaking of Al bonds in the areas subjected to thecollisional cascade, and that reformation of AlÈO bondstructures occurs in preference to AlÈN bonds due tothe strong chemical attraction between Al and O atoms.The decomposition of compounds under ion bombard-ment has been reported for a number of systems.13 Themeasurements made here using angle-resolved XPSmeasurements show that nitrogen is lost from the near-surface region so that a greater oxygen content isobserved for AlN Ðlms containing some oxygen. Itshould be mentioned that the Al itself has little propen-sity to leave the lattice because it does not easily formvolatile species. Qualitatively identical results wereobtained for a number of samples using initial millingperiods of up to 3000 s.

The constant value of O/Al seen at all values of cos hgives no indication of excess oxygen at the samplesurface, which might otherwise indicate the uptake ofoxygen from the analysis chamber or from redepositedspecies from the sample surface as suggested by Kumar



and Tansley.8 The results cannot, however, discreditsuch a hypothesis.

CONCLUSIONS

X-ray photoelectron spectroscopy measurements areoften used to quantify bulk materials after milling thesample surface with an ion beam. In this paper we haveshown that using XPS to examine aluminium nitrideafter milling with a 5 kV argon ion beam results in sub-stantial errors in the quantiÐcation of oxygen andcarbon impurities present in this material. Angle-resolved XPS indicates that AlN samples containingsome oxygen are depleted of nitrogen as a result of themilling procedure. No indication of the redeposition ofoxygen species was observed because the O/Al ratio wasconstant throughout the depth probed by the measure-ments ; only a loss of nitrogen was observed, so that alower than expected nitrogen content and consequentlyhigher than expected oxygen content is observed forthese Ðlms.

It was further shown that in the present UHV system,with a background pressure of 2 ] 10~10 Torr, surfacehydrocarbon Ðlm growth on the milled aluminiumnitride surface occurred on a time scale which a†ected

the atomic carbon percentage measured for the AlN. Inthe same system, with the same background pressure,no hydrocarbon growth was observed on milled steelsurfaces examined over a similar measurement period.It is believed that the ion milled AlN surface has a highsticking probability for hydrocarbons, which contrib-utes to the rapid accumulation of a hydrocarbon layer.

Aluminium nitride surfaces exposed to air undergo ahydrolysis reaction. To ensure that the milling used hereextended beyond the hydrolysed surface and into thebulk material, valence band spectra were used to indi-cate the depth of hydrolysis for the AlN samples. Thevalence band spectra were also used to investigate thedegree of surface hydrolysis for a number of agedsamples.

Acknowledgements

The XPS measurements were carried out at the Joint UniversitiesSurface Analytical Facility, housed at the University of New SouthWales. The authors would like to acknowledge Dr R. Lamb and DrP. Pigram of the University of New South Wales for useful dis-cussions on the identiÐcation of aluminium oxides and hydroxides. K.S. A. Butcher would like to acknowledge the support of a MacquarieUniversity Postgraduate Research Award during the execution of thiswork.

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x-ray photoelectron spectroscopy depth profiling of aluminium nitride thin films

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