AV-AlSO 000 ANGLE-RESOLVED PHOTOEMISSION STUDY OF AUGA2 AND AUIN2 I/1NTERME ALL IC COMPO (UI CALIFORNIA UNIV LOS ANGELES

DEPT OF CHEMISTRY AND BIOCHEMISTR. G NELSON ET AL.

UNCEASS IF I ED DEC 84 IR-S NODO 4 83 K 0612 FIG 20/2 NL

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.§yncrtrnnr radliation - anqlIe-resolved jkiotoemissioni vaenc band structure

'The ((J01) surf.a t's of Au(-a) and Auln2 intermetallic compounds were studiedusing syn, hrotron radiation excited angle-resolved photoemission. SpectraL0i1le d for photoelectron emission normal to the sample surfaces wer# usedtt. map. the V- versus k dispersion relation of both compounds along the SX sym-metry line of the bulk Brillouin Zone. The results show that the Au 5d bindoof each compound are relatively flat, but the splittings of the bands at arnearly the same an for elemental Au. A surface state is also observed on each

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TMDONICAL RET10RT ?. 5

F~q.-REI)VE P JFIT ISSION STIDY OF AuCa AND Aun 2 ( Tc

INTRM?-ALIC OPLD vfI

by

. Nelson, W.J. Giqnac, S. Kim, J.R. Lince and R.S. Williams

Pretr-ed for Publication

in

Physical Review B

Departmet of (anistry and BiodemistryUniversity of California, Ito Angeles, CA 90024

Decerber, 1984

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im,-

V.

• .... q

Asle-Resolved ?ttouission Study

Of A~al sad AhIt latems.tall A. Compounds

Jeffrey 0. Nesa. 1.1. Uwe,. sehums Kim,

Jeffrey .LA.U.es ad I. Italey Wtillams

Departmeat of Cbmistry sad DAiehmstry

University of Cliforsis, Los Asgeles

Los Aageles, Californa 90024

Abet: set$

The (WI1) sreess of Am~a2 sad As153 iatemtallis sompeds were

studied waist sysehwettos radiation, eeitod esgle-rosolwod beoteemissios.

ftectre follocted fee photoeleetro.s misson. mgmal to the ample surfsess

were sed to mp the 3 Toes It dispeusion reletios. of both smpemds elena

the A mintry liss of the balk Suillosia Zem. The ressults @hew that the

An 56 beads of *ob mpowmd sze relatively (lets but the sulittiags of the

beside at F at* etly the smm as fee olmeatal. A. A meet se state is

also observed *a *sob surfase ia a ned gp regis. about 6 OT below the

PomA loiele of the Swe sampomde.

Au. A119% and AuIm2 forn an laterostiag series of notals for the study

of the Au 54 orbital* is solids. In elemental An. the stems reside ona£

faso-estezed subic (toe) latties with a mearest-moighbor diets... of

2.33 1. AWa2 ad An%5 have the fluorite structure. in whish the As at=*

form s fee sublsttism where eash As atom is at the "sar of a cube with

eight group III (as or Ia) atma situated at the cormors. In this

arrangement. the Au -Amsarest-mmighbor distasses are 4.29 1 a"d 4.60 1 for

A282 ad Wauil. respectively. The strength of the iateratiom, between the

Sd orbital* om neighboring Au stems is this group should decrease

dramatioally with iacreasiag atemic separation ad should be evident as a

narrowing is the &-band structure of these mestals. Euxsmiaiag the emeg

boads of the"e materials at the r Point of the 3rilleuis zorn (a). where

sach boad reduess to a siaglo type of stemio .f charaeter, will Provide

interesting iafcumatios about the An d-d isterastis as a fuoctioa of

primsrfil7 isteratesic distas.

As is an extremely wll-atudied material# bet most previous

iinestigtis of the oloctrosio structure, of the Au(groop 111) 2

istesmetallie sonmadeM have bees limited to optical receotivity

104a8urMDOStS. 1 Pomi sucfce detemintiesao. and total valoso band

deaity-se-state mossuraments. 4-6 eb results of vas Attelme. *t al.,

* she& that the total As Sd-baud width, as measured using a-reW photealectros

spectroeopy (MPB) of golyorystallim gnsploa, decresa is the caoeo As#

AM88# m Asa. lbre rooostly. the curiaso Wt ad oleatrosio structure

al the A54 (on) Sisle-erystsl surace have be** studied aging

a

low-oergy eloetron diffrsetion (IM). Auger elotron speetroscopy (ASS)

and oloestour-oer loe spsetroeopy (ELI) . is preparation for more

detailed iuetiptioss of the loetroioo baud struetureo.

ASgle-roeolvod photoolotron spectrosoepy (ARM) has provod to be an

effeetive toohique for studying the olootronie strueturo of siaglo-oryatal

meterisl by providiag information about the I voersu k dispersion relstion

along spoeific directions in the 1.0 16 This toohuique is wll suit*d to

measure the binding onorgies of the As $d-levels at variov points in the

W. whick is neesary to study the A Sd-Sd iterstious is the As. AWS2

A I 2 sersl. This type of detailed oloetrose atotro msurnat he

sot previously beau applied to those istemtallie empoumds.

Saitediek sad Narsth1 1 have oaleslsted se-rlstivoitio AW bead

otrustures for AvA12 0 Aa2, ad Aul 2 . Thse ealculatious show a mrked

irrowig is the - eads of As.a2 oempered to ASe2 . Both the 8ulttil8 of

the beads at r and the bead dispersion *a k varied outward to the sZ

boundaries wore muh maller for AuI%2 . wievero the total width of the

seleaited d-bad$ was NSA Mellor thum indiosted by the WS vooes bead

spectra$ of both empusds. 8iseo the ealelstious so8looted sumn-orbit

splitting, whisk should be large is As 9d orbitals, this lost ob ervatie

is Perhaps met surptisiss.

Is order to assist Is understadial the 6-b0ad **tsetse of Au% smd

A&Isa. a muid basis baud atrietro isteMpelatios ssbe imsludiu

apis-orbit splitting Wes dowolead. 1 Ts i*terpolstioa sbmO was I int

fit to the AN ealeslatioss. sed the the aranmetor we adjusted to

impeove the loremeat between the ARM sad the cloclaed beads at I. siSI _

this way. seei-mpicioel bead stractages for both A%9a2 and AIsi were

esstractod, which oeblod the symistrios of vales"c beads to be

detommiaed. &ad helped with the interpretation of all the ARMS spectra.

'The f...., of this paer. will be on meppiag the bead structure of Au~a2

mad Aula2 alas& A using orinl-missioa ARMS5 from (001) surfaces of sial*

crystals. From the results of the osperimiesael bulk baud structures at r

the toerystal field (A) end the sumc-orbit Q) parameters for the A.

A62 ad AuIa2 series were calculated Is order to leek at the An 5d-Sd

interactions as a fomtion of interatomic distance. A surface, state was

&Iso observed *a both surfaces isside a baud pp region of the 4-bads

alas& the A symistry exis of the 99. See. 11 will describe the

eaporiaestel, procedure followed is this work. while See. III presents the

results of the ARM3 eaporimasta on AsGA2 and AsIa2 . A discussios of the

fiadiags of this work and their reletiosehip to previous stuodies appears is

T..

4

'1

II. Iaxporinoetal Procedure

The experiments were performed on boom line 1-2 (the 80 port) at the

Stamford Syscbrotroa ladlatioa Laboratory (3L). All ARMS moeasurmeats

wets mas is a mUltra-hiab vacuu analysis chimbor provided by S53. with a

bes pressure of 6510- 10 tort. The Chamber vas equipped with a siale ails

sample manipulator. LED optics and a Vacuun Gonerators. Ltd. (TG) ADUS

400 augle-raolvmg photoelectron spectrometer that had as scoeptae aoem

half eagle of -30. Both samples were mounted semh that the plane of

ineidence of the photos bes eontained the polarization voctor of the

radiation (p-polarized) sad the 11101 axis of the crystals. Both normal

and off-moemal emissio spectra were collected with the photos bern 45o

frm the 10011 axls of the samples. A sinle spoetrum required

pomroalmstoly seven minutes to collect in order to insure that there wore a

wimiau of throe thousand eoats in the strongost feature of each speetrum.

The *toe&#* rinS (SPBMA) was typically operating with a barn enrgy of

3.0 GeV snd a eurrent of between 30 and 60 mdL Photos evorgies is the

range of 14 to 32 e were used, cnd the total amlyzer plus moaoehrmator

enrg resolution was better than 0.2 .V in all cases. All of the AIS

dats vete collected with the sample at ream tamperature.

The Iaerl Distribution Cuoree (aC) were collected as a function of

photooleetros kinetic esrl. In order to dotemine a referece binding

oemrgy for all the spectra, a Femi Level (Sy) was assiped to spectra that

were the eae of all MMRS speotra collected at each photon onargy. is

order to include lnitial states from a reasomably large portlo of the m.

This was eeumplished by establishing a baseline for each msed spootrum.

and then defining RF to be the energy where the E)C crossed a lie that was

half the distance from the baseline to a second line that was fit to the

flat a-p plateau of that KDC.

The procedure for the preparation of the AuGs 2 and Au!. 2 crystal

surfaces 1 3 used i. this study has been described in detail elsewhere. 7 , 1 3

3oth crystal surfaces were oriented to within I s of the (001) plane using

Lane x-ray diffractonctry. The earlier study showed that in the ase of

Am3a 2 . after alternating cycles of arson ion bombardoent at energies from

3 key to 500 eV and annealing to 575 K. a sharp LIE) pattern, which was

iaterprettsd is toms of two perpendicular domains with a (Vi zIS)34S

reconstruction, was seen. This sme surface reconstruction was also

observed for Auln2 (001) in this study. Although AU measurments were not

available duaing the ARM118 experiments. sharp LED spots with no streaking

or splitting and the absence of photoemiasion foatures in the valence bend

caused by oxygen or carbon indicated that both saimples wore free of

ot amin tion.

\I

III. bsults

Fig. 1 shows a set of normal emission AIVES spectra that were

collected from tke AsG&2 (001) (6- a C)1 4 5 reeonstructed surfae.. Various

sets of features have beem connected with lines and labeled A-G. Of these

features. only two (A and 3) show substeatial dispersion as the photon

energy is inereased. The peaks labeled A correspond to photoemission from

an s-p bead that rises steeply between r sad , erosesing the Fermi level

before reehing r • Feature 3 is only sees at low photon ene rgies

(16-20 @V). These two sets of features are broader then the others throsb

the entire ransge of photon emergies used in this experiment. The other

five features ahow relatively little dispersion. which Is indicative of

d-bads (is this case, mostly An 5d is character).

ARM spectra were also collected after the AuGa 2 sample was exposed

to -130 L of 02. The feature labeled F was much more seusitive to this

costamiation then say of the other features, is that a mall, mount of

adsorbed oxygen yes sufficient to reduce the intensity of feature F to

below a detectable limit. Off-morsa ARES spectra of the cleam surface

vere collected is order to observe the dispersion of any features with the

perallel ocupemat of mestm (k 11 ). Feature P was the only peak in the

d-ban relies to exhibit aotleeable dispersion of this type.

Pig. 2 oossists of a set of nrmat mission spectra from the (001)

(f axyl)&4S surface of A8I82 - Features are oee aPis someted and

labeled A-i1. These spectra are less eemplieated the& those of Ai~a2 ° sine

the features eorresposdia to 3 sad a is Fit. 1 are missis. boewer, the

Aws 2 spectra shew even more clearly the dispersion of the s-p bend

7

(feature A). The other four features (labeled 3-B) apparently correspond

to An 5d-like bands. Feature D is the Auln2 spectra dispersed in energy

with varying k11 in a manner very similar to that of feature F is the

Auga 2 spectra.

Fig. 3 contains a set of two spectra for sack compound, which clearly

show the sharp spin-orbit split d levels of Ga (3d) sad In (44). Table I

list@ the binding energies and the spin-orbit splittings for Ga 34 and In

44 for the An intormetallic eampounds compared with other materials. The

binding eneorgies for the Go 3d or Is 4d levels in each series of materials

agree with one another to within 0.1 sV. which demonstrates that chomical

shifts Is the"e systems are small. The core levels shown in Fig. 3 also

isdioate the total energy resolution of the ARMES spectra.

TV. Discussion

The data were analyzed using the direct-transition model. The

momentum component parallel to the surface Uk11 ) is conserved during the

alilt of the photoelectron through the surface. The normal momentum kL)

is changed during th. sxit since the photoelectron hoa to cross an energy

barrier. Since neither the conduction band structure nor the wavevector of

the photoom itted electron are known in advance. IL of the photoem itted

electron inside the solid must be estimated. Assuming a free-electron

conduction band structure (i.e. planewave final states). the normal

component of the photoemitted electron momentum inside the crystal is given

by:

k 2 2m rn (Ek 4 V 0)- Lksi 2O el(j *is ' i0 a

where Ekis the kinetic energy of the photoelectron in the vacuum, 0 is the

polar angle of mission with respect to the sample normal, V 0 is the inner

potential, which is sasmed to be independent of kinetic energy, and a is

the effective mass of the photoelectron. The values of the inner potential

used for Au3a2 and Auln2 vere estimated to be 11.16 eV and 11.41 eV

respect ively, which were taken to be the difference between the muffin-tin

zero of energy in the AMW calculations 1 1 and the vacuum level as determined

from the work functions of the two compounds. The experimental band

structure along the A line was then found by plotting the binding energy of

the photoelectron (relative to the Fermi level) versus k.L determined

using Eq. 1. for various values of m ImTese plots were then eampared to

the non-relativistic band structures of Switendick and Marath. 1 but the

alreament was poor for Ill1veilso of alp. especially with respect to C~

i-beads.

Whenever the direct transition model is used to interpret ARMS5

the limitations of the model must be considered is order to assess th

level of agreement between theory and erperiment. Move specifically.

situations that lead to the breakdown of moetm selection rules and

result in uncertainty is k muat be examined. The ef feet of the labor,

angular and emrl resolution of the eleetron analyzer. the crystal

Sameatm broadening of the phetoeleetron final states that results fv

finite mean free path lengths.1 and any broadening attributable to tC

Debye-Ualler factors of the @yet= must be erefully evalisted. 13

Due to the dispersion of the initial state beads. cheape* in the

momentu space region sampled in ARM3 can cause very large changes L

observed photoelectron energy distribution urve (900. The actual

width of features observed In ARM1 spectra deposits upon both the

resolution with which final momentu states are sampled ad the suinrg

dispersion of the initial state beads. this effieet is quite evident

Fig. 2. where the s-p bead at low binding enrgy is much breeder than

d-level bends at higher binding energ. In Pascal. "p band$ disper

much more rapidly than the almost flat d beads. thus, seeuting for t

relative width of the features seon in the ARM3 spectra.

The volume of the crystal momentu space sampled in an ARM1 ape

depends primarily "pon the angular resolution of the electron analyse

the crystal montum broadening in the photoemission final state. 2%

angular and enargy resolution discused in too. 1I result in as

10

isetruestal broadening of k.L of 0.23 1- 1 . or 11% of the U dimensions.

The broadensg due to the final state width is inversely proportional to

the photolectron mees free path (R) and the angle (0) between the

monestn vector and the surfae normal. 1 6 Usia& the inelastic meas free

path (l10P) formal* of eak and Doenh. 1 7 end seeming an average

photoelectron kinetic energy of 20 9T. the electron seas free path of AsGa2

and A*Iu2 is calculated to be 6.3 1. Thus. the expectod k1 -bradoaig is

0.30 1-1 In both AvGa2 and Auin2 . which agrees well with the observed

uncertainty in the neoatem (0.31 1e1) estimated (nm the full-width at

balf-mazinne (FIU) of the -p photoemission peak A of Aalo 2 in Pit. 2 and

the 9 versus k dispersion of the slcalssted A, valence bead. 1 1 eio value

oorrespods to a final state IM mmeatus broadening which is 19 of the

3Z dimnsions. The total volueo of k-sIce sampled In eeah spectrum of the

present ARM masurements caused by the angular resolution of the aunlyser

snd the uncertainty in k1L May be estimated as a Cylinder with a volum of

0.015 1-3. which corresponds to 0.3% of the volue of the U. Although

this may appear to be a rather nall sampling voluma of moentun space, it

is responaible for the broad pbotemission peaks from s-p like bauds.

he last important broadening mehkansi to be sensidered is thermal

broadening due to isdirect er phomer-aseisted transitions. Suh

eontributions to the meertainty is mant= ea best be est~tated by

looking at the Dbye-aller faters for seh syetem. The bulk Debyo

temperatures of AV962 and AsIn are 1% 1 and 187 1. respetivol,. 1i A

tough estimate of the the mea-emuered vibrationl amplitude ad eas atom

in the ompounds wes obtained using average atmel masses of 111.1 (AuG 2 )

@ad 142.2 (Au/sy) and the bul Dsbvo tempratures. The resuting values of

11

the Debye-Taller faeter for A5642 sad Au Ia are 0.84 and 0.88.

respectively. Thse" ubers represest a modest .oatributiom (40) of

iadiroet trasitious to the spectra sad shoh that the direct tramnitioa

model la still Justified Is those systems. To rodue the offset of lattie.

vibustions e the epeetrao the smples seul& be •••led to liquid mitr•SOn

temperatures. at which the bobye-Taller feters wold be approzimetely

0.96 (AIGe2) ead 0.97 (Aula 2 ). To appreclably impeeve the spectra. the

samplia8 voleme of mote ssee would also have to be deareased

sobsteatially by uia eo ARMS eulyser with mue bettor msinular

resolutios sad higher photom emergies to increase the moes free peths of

the photooloetroas.

Although aet seglisible. the metu broedeiag, effeots discussed

above are mot serious eaough to imiel idate the direct trasmitioa model so

applied to the photeemissieo spectre of AIe 2 sad Aua. Thus. the

disegaoemeat between the thoeetieal LM beads1 1 ead the eperimestally

detemined beads is largely the result of the imneeurieoms in the

calculatio. Gas major misoios in the AM sleulatien was neglect of

spi-erbit oesuliasg11 whish should be relatively largo for As 5d orbits.

Also& the bindag omrNP pesitioms with respoet to 1p of the estreide of

the d-beands were wer-estimted by s"out I iY. ad the d-bend widths (even

dissousting the meglOOt of epit•-ebit effects) were severely

underest imstod. 1 2

Is order to ebteia a better theetisal estimate of the d-beads is

Aie 1 sad Aul*a1 a mised besis istepolstios aseme imoludig pi- Pobit

splitting Ve developed fog flaoeito atruetare eempudso. 1 2 The

12

me-clativistis AN bands of Aa. mini Aula2 vere fit using this

prooedure. Them mating as estimate of the biaing energies of the i-bads

at r obtained from the direct tisasitim model with amm 1.6 ad the most

istease photeemaissiom features, the parmoters of the Interpolatiom scheme

more adjuseted to predso the three ibais at r that agreed with these fran

the AILl masucamnts. Most. the energy bans ealeulated from the MS

spectra for various so values veto ompared with the interpolated basis. 4

sad reasonable agremant was feusd for so- 1.25. Is priseiple. as

iterative proeedure ould hae been used is which a amsot of binding

energies for the i-basic at r would be determined for the no 80 Vallee

Moovovr. for Agai and Weia1 the i-basise were so flat that this was set

necesary.

Figure 4(a) shors the energy bans of As esleulated by the

isterpolatios shee, uaimg parometora from Nof. 12. as well as the

iaterpolated sai euperimatally detomiseod basic of Aa 1e sand AuX&2. By

sompariag Figs. 4(a). 4(b). sas 4(e). omne a see that the aplitting, of the

An Sd-levels is almost idestisal is each sase, though the absolute binding

energies are somewhat different. The spoacig of the Am "E banis at r mo

be gsed to detemime the 'c rystal, field' (A) ad the *ptsbit (t)

parmetoe% is the Masur or leher, et al 1 0 Uses prmoters. ihis& are

msay mioatmgf at r, where the crystal iomeatum is zere, son, be used to

sovpare the relative strengths of i-orbital intercetioe ad the apis-orbit

eupliug. Patag the energy cigeualne soclculated by 94llhosema 19 4 and

A ca be eatrated frem the AMBl data and compared to the come parameters,

for elemental As. Table 11 saists of e'orimmtal sad thooretieal

detormintimas of A sad 4 for the series As. A082 sad A*192.

A

Is the ease of Ass previous euperimstP12 1 determisd a A value of

1.22 6V and a apis-orbit parmeter of 0.71 OV. whish are is poed agremeast

with the cae* parameters extracted from theoretical band structures. 2

*cwaver. the experimental valse of A for AO 2 sad As%. are much larger

than the the a..-gelativistis dbsod splittiag calculated by biteadiek sad

11Narath aad are easeatially idestiosl to ass csothes, whereas the AM

saleulations predicted a substaatial decrease is A for Au!.1 with respect

to A810 2 . This observatiou is very surprising. sime the An "- overlap

iategrals which give rime to the splittiags at r (is the LC cases of

$later ami Kosts.2 3 ) should decros dramatically Is the series As, Ax~m1

NAd Au!.2 (as the AMW results predict1 1 ). The large crystal field effet

is the iatermetalliecmpounds may arise from interactions of the As 5d

orbiteels aith the d-crbital a of the group III notelsa (OS 8d or Is 44). Use

As Sd-bads of Am~a2 should be napped is detail to teat this hypothesis.

Si... Al has so occupied d-rwbitals, eas might expect the *wystal field

splittiag to be much smaller for AsU 2 then for Am% or AuI.2. Is fast,

the total width of the Auhl 2 &-heads Is =allot thas for A@2 as shewn is

the valesce bead 31 speetra.4 The *ai-orbit paameter. which should

sesostially be a property ouly of the As 5d orbital&, is very similar for

all three systms.

Soe width of the d-bands of An Is broademod eecidorably by mixing

with the lowest oemrg plaae-were band, as shows Is Fig. 4(a). ses the

lattice osetasta of Am@% and AmIa2 are sow mu lersor than for Am, the I

dimossia are mu" mallow. Ia the 0ase of the two iatemetallie

cmpounds, the pleas-wr head roehes the U bomaeay at I before, it sea

wes the &-basic, as @m ia Pip. 4(b) sad (a). Thus. the d-beade of

14L

Asa2 ad A%1* 2 essentially reside within a banigpP is th. plano-way.

bead*. and do net six with the highlY dispersing state. This allows the

&-bead width of the As density of states to be mash larger than for Aga.2

ad Aui 12 even though the d-band splitting at r' is nearly the 90116 for all

three materials.

In addition to the eoserws-tioa of immotum senditions. there exist

uniquely solid state selection, rules is ARM that deal with final states

obsoryod sleng syssmtry directions.2 3imes, an ARMS omporimat chooses a

pertioulas final state (whisk has psrtioular syinmtry properties), the

initial states that may be sampled are detemined by the radiation

pelarizatien with respect to the crystalline as$ of the enple. In the

ease of semual photeemmission from a (Mi) sorfase of a crystal with Td

Symetry ad isnoring rolatiyistie effects1 the final state symistry simat

be A, is order for the pheteenitted electron to res the detecter. Ibis

requires the initial state to have either A 5 or A, eymtry 4 both of

which are allowed by the cqporimntal gemetry showen for this eqierinent.

These symarly selection rules are only ripros for detection systes with

infinitely poed angular resolution, but they provide a basis for analysis

of ARNS with goed angalar resolution

These selsetiet ruaes Will be sed, in analyning pbotmissien frum the

a-p Vale=*e bag"e that Reside to the Io*e binding emra side at the

d-bemdo. sim spin-Sebit effects were mot included is the interpolation

sabane feor s -p beside, Peature A is the AR% spectre (Fit. 1) ist

soused by trasitiesa trams as spbead beteenom p and L. Oortaolye its

msitiom with respect to the calculated bane WPis, 4(b)) is *a& that It

Is ot possible to detomie if the trasition origintoes fm a A or As

initial states since the oorimental polnts oessontialy fall beomeos two

0eallatod bands. Photomilsio frm the flat A baud is not allowed by

the seleotion rule, and in fast no peak coresponding to this basd is

obseoved in or of the ARM Socotra. This obsewatioe is actually

Smeewhat surprisg. aim" the solotim rules are met eapected to be

oemplotely rip roes. and the flat bond should yield an ontrmely high

density of initial states to be osmplod.

The A642 features labeled 3, whieh only appears at photon energies

boeoos 1f and 20 ST. are the result of a surfo nk8lapp procss frm a

roegin of the a with the fon: I(1J81IS8o). Thee ozits a arfao

rooipreoal lattlc eetw (-1/).-1IO) arisin8 frn the (VI 4D)"1

reeestrooted sourfto whie has the soroot mapituds and diretis to

diffysot photoolotreom emitted tim Initial states Is this rogion of the

§* to the dirootio (0,0.1). which mea roeh thel olotreu oomr analyzor

is the "Mal mission omety. Peaks labeled a are also a result of this

cme lspp geses, 1ooo sasipents have boo oo e omed by 0emratiag

the voaeee basds on1o f-(ISl,1II) is the uin the imte"Platioa

oekhmo aud oboese ag that valomee bands exist with the serrost biudiag

energioes cd valme of 1. Features C. P. ud 0 is the d% spoetra have

boon assiped to the thee d-lovels that have fqotries (going

relativistic notation) of rO N ad O ospootively. The d-uds are

fairly flat. disoeroing 081y mall movot belooo r Ou 1. oegoially

hen empatod to the soroeudial 4-baude of As. so Teemeant beoos

the dispersion of the ictorpolated aud thee ooperimeata -bede hat vote

mpped out semiag a P l see fimi state with 6*0 I.S is qmlti atvOly

14

correct. but difference$ of a few tooth. of as SV smust as the beads

approach 1.

The final feature i. the ARM spectra is Is a "-and gap along A in

Aua Pseatrs F. which has a bindiag emergy of -4.2 OT, lies betwees, the

lower sad d+ -lseela Thee peaks so, so dispersion, as the photo*

owergy is inceased from 16 to 20 OT. Is addition. the"e features are note

seasitiue to surface ontamimstion than pbotoemission peaks assigned to

bulk beads. Lastly. sme dispersion weesaeon is this feature as the

parallel esupomut of the Vwvector was inezeaaed frm zeoe by moving the

detector of f-normal. A the polar eagle of miasiou (0) was inereased. the

direction of totatiou was is the plas sontaining the fool, and 11101 a"e$.

This is equivalent to siuluseonnl totatiag from r to 3 &Md T to so in

the reetangulat surace UZ. shows in the isset of Fig. S. since the Surface

roconstruetion is ssemed to contain to psupeadioulat domain* in analogy

with the Ge (001) 221 roseatrustion. The 44=2 symbol& in Fig. 5

illustrate the dispersion of feature F with k11 . Is vimw of the"e

observations. feature F was assigned as a sao state.

The ARM *ssetra (Pig. 2) of aL*2 are similor than. fm~ A%. Few

features. which oriac from Wkl Valences band tranaitions. wte coon in the

LUl2 se". ?be teatureo labeled A cworrspood to transitions trm ansp

bend whicesence the Fermi level about halfway beforeen r ad L The

agiemont beteso this soerimetal bend ad the L, band seleslated by the

interpolation scheme (Wis. 4(s)) is good, bo the A feature is &ssigood, to

the Valene bend with A, symtry. Within the mneortainty in, the montims.

the ewerimat ad the theory agree quite well as to the binding energy of

17

the bead at the I point. but the dispersion of the bead is set is as pood

agreement.

Feature$ 3. C, end 9 in the £1132 spectra result free transitions frm

d-bsads with the se symatry as thoe" observed fer Ao2 'hese bands

show owes loe dispersion between r and I then AWei. presumably because

the Au-As seperetios is latter in £13 sad the orbital interactions of the

neighboring at=&s is smaller. The areemeat of the three expeuimental

d-bas with the interpolation seheme, calculation is emsellext over the

entire region between r ad 1. £1132 exhibits the .. e (62 z1Th)3sm

reconstruction as Asa 2 . so it should sot be surprising to find a similar

suface state on the (00) surface of AM%.. Feature 3 is the MAm%

spectra (Wis. 2) shows the mass behavior as feet=*e MiS. 1) is the case

of AGS2. it io mwoe sensitive to Sucr am atmimetien than any of the

bell features. It shows a "esion for mal shotoesisaios, is the

gease of Photos, emarties good is this experiment. Uinwewer, feature S does

dispers, with ko, . as shown by the circles is Fit. S. Therefore# this

feature has elso boes assigned as a srfae state.

It would be possible to masmbiuosly mp out 'both the Valance and

easduotion bands of these two eampeuads Saint the tniaagulstien technique

of collecting AIMS spectra frem two different werfooes. 2 5 V If two

features are sees at the ma. binding escrow from two different surfaces.

then the Val"e of I sea be detemined absolutely. This Would Provide the

mossmary isfesmatios for plotting the 3 Versus I diapersien relations of

both isitiel ma fimil states withot iavekimg a Plae-Wave fri state

approximetiom. 2he roel ting eqerimostal oserW bands could theu be need

Lasi Us eamet the Astplautts ebm to detemiss a trly

oveiuA..l bead stucture.

Ir

V. Conclusions

Ie ARM spectra of AsGA2 and Afs% are reasonably simple,, sad all

features in both s"to of spectra ea be assigned to either bulk or surfase

trarnsitions. going a Plane-mee final state approximation. the i-bads of

both cmpounds ore is good agreement with as interpolation scheme

calculation. which was fit to a first principles AMV caleulation and

adjusted to agree with the experimental results on1y at the r point of the

DZ. The experimental s-p beads do sot agree with the isterpolatiouseme

as well as the 4 levels. Nowever, the spectral features of the a-p bands

are smoeker broader, **%&is& a larger uncertainty in the crystal momentum.

The AuGei spoctra are Smewhat wore eomplleated thus in £3532. is that they

contain features due to sarfeec Duklapp peoesse. sesatially idestleal

surface sta tes that reside is a bead gap between r and the lower ri emrw

positions is the valeame beads have bees foed 0s the (001) surfaces of

A642 and Aslsi

Surprisingly. the splittaga of the As 3d heads at r for As6#2 sad

Aasa2 &ro nearly idestioal with eash other ad with elemental As. despite

the large difference in the Lw-Au distemss in the three systms.* A

setisfeetery explanation, of this observation requires further esporimnste

ad/or detailed ab initie calculatios. T1he difference observed is the

total "-and width of the two esupovuds aries becaus the d-besds of Asa92

isperca loe than those of AuS S2 whereas the d-baadwidth of As is also

hroadoed by mixing of the, d-bend with a plame-weve state.

20

VI. Acknowledgements

The suthors wish to thank R.I. Baughmam of Sandia National

Laboratories for supplying the AuGa 2 and AwImn single crystals, and

J.A. YeTuroff. T.C. Tsai and R. luentkal for assistance with the

experiental measurements. This work was performed at the Stanford

Synchrotron Radiation Laboratory. which is supported by the United States

Deparment of nergy under grant No. W-AC03- m t3000. in cooperation witl

the Stanford Linear Accelerator Center. Support for this project was

provided by the Office of Naval Research. R5 acknowledges the Camillo ani

lonry Dreyfus Foundation for providing a Teacher-Scholar Grant and the

Alfred P. Slosn Foundation for a Fellowship.

21 __

IRef ereace s

1. S.8. YViesubhlts &ad J.P. Jan. Philos. Oha. 16. 45 (1967).

2. J.T. Longo. P.A. Schroder and D.J. Sellmeyer. Phys. Rev.12. 656 (1949).

3. J.C. Abele, J.1. Brewer and E.L Nallorsa. Solid StateComan. 9. 977 (1971).

4. P.D. Chs sod D.A. Shirley, tLjtrqa"s Dtal y 91 IILtJedited by L.L Beaaet (U.S. GFO. Washington D.C..1971). p.323.

S. S. hfser, J.L Weorisk and 1.1. West. lid State Cemmas.10, 1013 (1972).

6. P.E.TU.N. van Attek.m. G.K. Wertheim. G. Ceealis sadJ.1. Weraick. Phys. Rev. 122. 3998 (1980).

7. J.G. Nelson. J.L Liaoe. 1.3. Gipse sad 3.5. Willims,

J. Vsc. Sci. Tech. A2. 534 (1934).

3. G.V. l-asso &ad S.A. Floodstram. Phys. IRe. r17. 473 (1973).

.. Stchr, P.S. Wehaer, LS. Willims, 0. Apsi and D.A. Shirley.Pbys. Rov. I7. 587 (197).

10. P.S. lehmer, L. Williams. S.D. Coves. D. Denley andD.A. Shirley. Phys. Rey. 219. 6164 (1979).

11. A.C. SIteoadiek end A. Naritho Phys. Rev. Lott. 22, 1423 (1969).

12. 5. in.. I.e. Nels sad I.. Williams. Phys. Rev. 3, aeeeptedfor pIblisatios.

13. ]he AVO, sad AulIs esrystels used in this eperihat werepises ftan lazer ingets which were Stes by LJ. Reaghuamof Saudis National Laboetories. Alblverque. New Nailo.Por details of the growth of the erystals see: 1.3. Beauhma.Mhter. Res. 3all. 7, 505 (1972).

14. S.D. Ikhu. Phys. Ism. 3. 4334 (1970).

15. L. Williams, P.L Webmr, J. Stebr mad D.A. Shirley.Ptys. Rev. Lott. 59, 302 (1977).

16. F.J. Peibelma, $wt. 3.i. 46. 5qS (1974).

17. I.P. Soak and V.A. leash, Serf. sod laterfase JAuly. 1. 2 (1979).

18. I.A. Mayns, Phs. Lott. 7, 114 (1963).

22

19. C.J. Ballhausen. hixngsctijs tU9 Liasd Fiei0 Th±ory(McGraw-Bll, 1962). v.118.

20. K.A. Nille, LF. Davis, S.D. Kevan, G. Thorton and D.A. Shirley.Phys. Rev. 822, 581 (1930).

21. Richard F. Davis. Ph.D. Thesis. University of California, 1931.upubl ished.

22. B. Eckhardt. L. Fritsche and J. Noffe. J. Phys. P. 14. 97 (1984).

23. J.C. Slater and G.F. Koster. Phys. Rev. 94. 1498 (1954).

24. J. leruason. Solid State Commua. 22. 9 (1977).

23. E.O. Ko, Pbys. Rev. Lett. 12. 97 (1964).

26. S. Nedderuseyr. Sol. State Cemmas. 40. 809. (1981).

27. M. Pease K. Lidrosa. 3. Asons. end N.Y. Smith. Pkys. Rev. Ms.738 (1932).

23

Table I. Binding -mrlioe of a 3d and Is 44 levelo.

spi-otbi tas 3de5/2 as Ud3/2 splitting

GaAsb 11.60 19.04 0.44

Gasbb 18.70 19.13 0.43

A%9a 2 e 18.60 19.19 0.39

pis-orbitIs 445/2 Is 4d3/2 spl Ittiat

In metal 4 16.74 17.64 0.90

lash e 14.71 17.65 0.84

Aulat 16.83 17.74 0.0

(a) All value in V. Estimated uee.rtaisty is 0.1 *Y.

(b) D.3. Sasta, T.C. hais. P. bolm.. nd P.1. EItpsel.

Phys. Row. Lett. 43, 6M (190). (Bindia enrg relative to

(e) this work (Siadis8 oNr. relative to Pemi level)

(d) .A. Pollack. S.F. eowalovk . L aoy and D.L Sirley.Phys. Ro. Lott. 29. 274 (1972). (Binding oner relativeto Petal level)

(9) L. Ley. LA. Vllok. P.R. Nooly. S.F. Kwalosyk andD.A. Shirley. Phys. Nw. W. 600 (1974). (Biading enerPrelative to wetl level)

24

Table II. Disdis8 *aergiOss of As $d ly•ls at r

lattle1.o...tmt (X) z(r3) a(r) BP() A

As 4.08 xp.b -3.55 -4.45 -S.90 1.23 0.71

theo. O -3.38 -4.33 -5.75 1.28 0.70

AvG 2 6.04 02p. d -4.92 -5.68 -7.31 1.07 0.78

thee. -6.88 -7.60 -7.60 0.72

Aum 2 6.30 ep. a -4.72 -5.48 -7.05 1.06 0.75

the•.. -4.47 -6.94 -4.94 0.47

(e Ail values in OV. Matmated Usertasty in the

spetismatal binding enmtler is 0.1 ev.

(b) ref. 20

(a) ret. 22

(d) this work

(e) ret. 11

25

Figure Captios

(Fig. 1) ARM speetra of slesm Aug W)(~ ~F)3Stk.a aia

mission with the Masple at roan tinperatue. The lim labeled A @here the

peaks that hae. been assigned to transitioas frM the eeooad 0-0 bead of

AuGa 2* Poeaue@ 3 ad B have bees asaigmed to a surfase %us"ap Pee..

Lines C. D sad 6 iadieto bell transitions fme the An 5d a*is-ebit solit

levels. Finally. the peaks labeled P. ubish lie in a bead gap Is Av9@2

along A. have been assagned to a surnase state.

(Fig. 2) ARMS wsectra of dOss AsI% (001) (T2' ua1S)3S takea at sernal

mission with the nmple at rem tesperature. The line labeled A shovs

peaks that howe bees assigned to trasitions tree the seed e-p bae of

Asl. Features 3. C. ad a eorrespemd to the 56 bas* am& ar'se free

surf aee state mission.

(Pig. 3) ARME spectra of ea AzGe sad Auli that extend to higher

biadiag onergies to reveal the Gs Sd sad Im 44 levels.

(Fig. 4(a)) The bead structure of elemstal As aso& A of the fee-lattiee

3rilloin Zes.. Ike bead$ were calculated, using as interpolation asksai

that will be doesuibed is detail oleeWhre? 2 The permeters used is this

eIl uation were detornised fres of the position of the An Sd beads at r as

estimated free the smtal mission ARM dats of We. 20 sad 21.

(Fig. 4(b)) BaMW atreturO Of Aa6 alln A. The dotted lime were

seloslatod by the me Interpolation $@&me that was sed fee elmestal As.

26

The initial state asasmeats of tof pbotmessisoa tuassitiessag beys.. as

sques of eireles seruespeudial to etc*"g or week feetres in the spetra

of Fit. 1. teOssetiYOly. per symbol where there ageos " ertie lines

tadicetins the energ a uetaisty is lestis a peak is the isepeetrum.the height of the sybo oorrospeniu to ee *Bsed$i the ineartaisty in the

measurement.

(Pig. 4(s)) Same as Fig. 43. eseept for AfsI. Ike esinrese irsAl*$

represat peaks Is the seetra of Fig. 2.

(Fig. SI The iieversiom of the fater** assigned as sursees States is

Auge (squares) sod Aumhi feirelee) with kgj. Moeesse the serfas

reoetruetios seassts, of fte perpemditelar dmmaiss, k, varies

siultaneously fron F to I end r to so (is the surfaee DZ) as the

pbtoeletroa mission aftle A. varied is the (1101 asimsth. The two

perpemdilular surf see U age @her& Am the inset.

AuG 2

*r4

3 3

C . 28

A 26

z

IFI

-10-8-6-4-2 0 2

ENERGY (eV)

-M 1- - L -4

AuIn2 B

hv

33c" 28

A ' 26 "

L

21

z

z

-10 -8 -6 -4 -2 0 2

ENERGY (eV)I FIG. 2

[AuhIr2 hv

424

F-

z

1-14

-10 -8 46 OM4 -2 0 2ENERGY (eV)

FIG. 2

D4

p '1

AulM 2

* fU

00

VPO

-22 -18 -14 -10 -6 -2 2

ENERGY (eV)

. .. 3

(a)

-10 Au

re x

0 oo-2o****oo ooo° |

*.

***oooO°O 0 °

00

oooe O OO0. oooooooo0 0:8:: oo..(U . 0 . 0 0 000 00 0 0

00o0O 0 0000

F xgq °°°°°F°°°.4A

00

-22

Ces

0lJ

CDD

-12

''so

z .13000BL ___as*

.. .. .

> -4-

.w~IG 3E m9sm J; q ........ 0.0 e....e

-6 ro .m -e- -e-jDLD E

S 00000LU- Ow,% SG.*SS 0*00G*e

z 08 S

10- Au~h- 2

-12

Fx

FIG, 4C

-5. 0 ato(J)r (K

> .5. 5 J"

Z -6. 5 *-uo

-7.~* _____ -AuIn 2j

FIG. 5

I.