Electronic properties of Bi-doped GaAs(001) semiconductors∗

Y. Polyak, M. Vondracek, J. Kopecek, and J. HonolkaInstitute of Physics, Academy of Sciences of the Czech Republic,

Na Slovance 2, CZ-182 21 Praha 8, Czech Republic

F. Arciprete, C. Hogan, and E. PlacidiIstituto di Struttura della Materia, CNR-ISM, Via del Fosso del Cavaliere 100, 00133 Roma, Italy and

Universita di Roma ”Tor Vergata”, Dipartimento di Fisica,via della Ricerca Scientifica 1, 00133 Roma Italy

(Dated: February 1, 2018)

Despite its potential in the fields of optoelectronics and topological insulators, experimental elec-tronic band structure studies of Bi-doped GaAs are scarce. The reason is the complexity of growthwhich leaves bulk and in particular surface properties in an undefined state. Here we present anin depth investigation of structural and electronic properties of molecular beam grown GaAsBi epi-layers with high (001) crystalline order and well-defined surface structures evident from low-energyelectron diffraction (LEED). X-ray and ultraviolet photemission spectrocopy (XPS, and UPS) aswell angle-resolved photoemission (ARPES) data at variable photon energies allows to disentanglea Bi-rich surface layer with (1x3) LEED symmetry and the effects of Bi atoms incorporated in theGaAs bulk matrix. The influence of 1% Bi integrated in the bulk GaAs are visible in ARPES aftermild ion bombardment and subsequent annealing steps. Clear Bi-induced energy shifts are evidentin the dispersion of GaAs ∆1 and ∆3,4 bulk bands which are relevant for modulations in the opticalband gap and thus optoelectronic applications.

I. INTRODUCTION

GaAs1−xBix has enormous potentiality for a hugenumber of advanced applications, as a function of theBi content, in the fields of electronics, optoelectron-ics, nanophotonics, thermoelectricity, photovoltaics [1–3]. The incorporation of Bi into GaAs induces a largeband gap bowing even at Bi concentrations less than fewpercent [4], due to a large upward shift of the valenceband edge. This property offers a large freedom to en-gineer the band structure of the alloy for potential high-speed electronic and infrared optoelectronic applications,even since Bi alloying in GaAs1−xBix seems to improvethe hole mobility, while preserving the high electron mo-bility of GaAs [5]. Despite the strategic interest of theGaAs1−xBix alloy, a reliable phase of this material withwell controlled properties is still far from having beinggrown for several reasons. In fact, the existence of alarge miscibility gap requires low growth temperaturesleading to the formation of defects that degrade opticalproperties, while the surfactant action of Bi counteractsits incorporation and leads, at worst, to phase-separationphenomena (like surface Ga-Bi droplets). Finally, thelarge-radius Bi atom leads to a compressive lattice dis-tortion of the GaAs lattice during growth. To couplethe optical/electronic properties of the GaAs1−xBix al-loy and the aggregation properties of Bi in the alloy it isvery interesting to study the alloy with traditional andinnovative surface techniques. To this aim, in this paperwe challenge the matter of Bi inclusion in GaAs lattice bymeans of X-ray Photoemission spectroscopy (XPS) and

∗ A footnote to the article title

we study the electronic band structure by means of Ultra-violet Photoemission Spectroscopy (UPS) and k-resolvedPhotoemission Electron Microscopy (k-PEEM). Varyingthe Bi rate during MBE growth we are able to change theBi concentration in the bulk. Generally, our results showBi in two different states. One corresponds to a Bi-richsurface layer. The (1×3) LEED symmetry of the surfacesuggests a similar phase as the one found after surfacedeposited Bi onto (2× 4) GaAs(001) reported in the lit-erature [reference??]. The second Bi phase correspondsto Bi atoms integrated in the bulk with concentrationsup to 1 at. %, essential for bulk-related applications e.g.in optics. After removing the Bi-rich surface layer weare able to achieve well-ordered surfaces, which allows tostudy Bi-concentration dependent band structure prop-erties via UPS and k-PEEM. In k-PEEM we find that in-tegration of Bi into GaAs leads primarily to a blurring ofthe band structure, while changes in the band structureremain invisible. In highly-resolved UPS measurementsat synchrotron facilities, however, we observe a shift inthe dispersion of GaAs ∆1 and ∆3,4 bulk bands.

II. SAMPLE PREPARATION PROCEDUREAND EXPERIMENTAL METHODS

Clean and Bi-doped GaAs samples were grown byMBE on n(Si)-doped (1018 cm−3) GaAs(001) substrateswith a miscut angle of about 0.01◦. GaAs buffer layers ofapproximate thickness 500 nm were grown at 590 ◦C. Forthe growth of GaAsBi layers, the substrate temperaturewas decreased to 325◦C. Prior to GaAsBi deposition, thesample was exposed only to Bi flux for 30s. A layer of 150nm of GaAsBi was deposited at a flux rate of 0.5 ML/s.

2

At the end of both GaAs and GaAsBi epilayer deposi-tion, samples were cooled down to about -10◦C, and anamorphous As capping layer was deposited to prevent thesample from contamination during air exposure when exsitu shipped to XPS and k -PEEM facilities.Prior to surface sensitive photoemission electronmicroscopy (PEEM), low-energy electron diffraction(LEED), and X-ray photoemission spectroscopy (XPS,k-PEEM) measurements, the samples were decapped insitu under ultra-high vacuum (UHV) conditions at tem-peratures of 300◦C and then annealed at 400-440 ◦Cwhere a β2(2 × 4), for GaAs, and (1 × 3), for GaAsBi,LEED patterns were measured. The temperature wasdetected directly on the sample using a pyrometer at anemissivity of 0.37. We will in the following these samplesas decapped.As we discuss below we observe a Bi-rich phase at the sur-face of GaAs1−xBix in the decapped state. Since we areparticularly interested in the bulk properties we treatedthe samples by a short ion bombardment (up to 12min)and subsequent annealing to again 400-440 ◦C. Samplesin this state will be in the following be denoted as sputter-annealed.XPS and k -PEEM measurements were done using anOmicron NanoESCA instrument with laboratory lightsources. Highly resolved XPS was measured indepen-dently at the ELETTRA synchrotron (MSB beamline).The NanoESCA photoemission spectrometer is based ona PEEM column and an imaging double hemisphericalenergy filter [6]. A transfer lens in the electron opticsswitches between the real space and the angle-resolvedk -PEEM mode, which allows to detect classical x-rayphotoemission spectra with monochromatized Al Kα ra-diation, as well as an energy dependent mapping of theBrillouin zone using a helium discharge lamp at hν =21.2 eV with an energy resolution of ∆E = 0.2 eV.

III. X-RAY PHOTOEMISSIONSPECTROSCOPY

A. Referencing CLs to the valence band maximum

Before we discuss GaAs1−xBix XPS data we remindthe reader that in semiconductors the Fermi level (EF)with respect to valence band maximum (VBM) andconduction band minimum (CBM) is sensitive to defectsstates and possible surface band bending effects. Weexpect Bi dopants in GaAs to cause shifts of EF, whichin return will affect binding energy (BE) values detectedin photoemission experiments. Kraut et al. describedprocedures to quantify shifts in EF and band bendingin GaAs by referencing shallow core level bindingenergies EGa 3d to the VBM value EVBM. Fig. 1 showslow BE photoemission data of GaAs(001)(2 × 4) incomparison with decapped Bi-doped GaAs measuredat hν = 1.5 keV. The high photon energies correspondto a probing depth of several nanometers and thus

the data reflects mainly bulk properties. We estimatethe VBM EVBM using the leading edge method, whichapproximates the DOS by a linear line at the maximumsteepness of the VB edge as indicated in Fig. 1. Inthe case of our GaAs(001)(2 × 4) sample we derive avalue EGaAs

VBM = (0.83 ± 0.02). The relative BE of theGa 3d level with respect to VBM then amounts to(EGaAs

Ga 3d − EGaAsVBM ) = (18.82 ± 0.02), which is in good

agreement with the bulk value 18.80 eV reported onGaAs(110) [Kraut]. Comparing EGaAs

VBM with the case ofGaAsBi(3%), a shift to lower BEs is visible in Fig. 1,which we estimate to (0.23±0.03) eV. At the same time,however, (EGaAsBi

Ga 3d values shift in the same direction.Taking into account the limited energy resolution ofthe laboratory NanoESCA instrument we estimatebulk-related, Bi-induced changes in (EGaAsBi

Ga 3d −EGaAsBiVBM )

to be smaller than 0.1 eV.Short ion bombardment as described in the above sectionintroduces surface-near defects. Under the influences ofsubsequent annealing cycles at temperatures of 400-440◦C surfaces partly reorder in 1x3 surface symmetries.Again we want to reference CLs to EVBM, which isshown in Fig. 2. During sputtering/annealing theVBM moves towards lower values EGaAsBi

VBM = 0.32 eVand Ga 3d and As 3d CLs become broadened, bothevidencing an enhanced disorder due to the sputteringprocedure. In the case of Bi 5d CLs a strong changeof the spectral shape is visible after sputtering andannealing, which corresponds to the suppression of ahigher BE component connected to metallic phase of Bias we will show in the following.

B. Bismuth 4f states

In the initial decapped state shown in Fig. 3(a), bot-tom panel, Bi 4f XPS can be decomposed into two Bidoublets with the main 4f 5/2 peak located at BEs (157.7± 0.2) eV and (157.0 ± 0.1) eV. The fit was derived us-ing the KolXPD program and respective parameters aregiven in Table ?. The BE position (157.7 ± 0.2) eVis close to literature values attributed to Bi metallicstate[7]. 12 minutes of Argon sputtering almost com-pletely removes the metallic Bi component at (157.7 ±0.2) eV (see middle panel) proving its surface-near char-acter, while the Ga 3s reference peak remains unchanged.The remaining Bi component at (157.0 ± 0.1) eV reflectsthe incorporation of single Bi atoms into the host GaAslattice, most likely BiAs substitutional sites as suggestedby Sales et al.[8]. Moreover, the component at (157.0± 0.1) eV corresponds to a bulk-like phase since vary-ing sputtering times between 1 min and 12 min does notlead to significant differences neither in Bi XPS spectralshapes nor in the ratio between Ga 3s and Bi 4f intensi-ties r = IBi4f / IGa3s (see Fig.? in the the supplementary).Fig. 3 (top panel) moreover shows that subsequent an-nealing up to 440◦ C after sputtering cycles does not trig-

3

2 1 2 0 1 9 1 8 9 8 7 6 5 4 3 2 1 00

1

2

3 E G a A sG a 3 d = ( 1 9 . 6 5 � 0 . 0 2 ) e V

( 0 . 2 5 � 0 . 0 2 ) e V

E G a A sV B M = ( 0 . 8 3 � 0 . 0 2 ) e V

XPS &

UPS in

tensity

(arbitr

ary un

its)

b i n d i n g e n e r g y ( e V )

G a A s ( 0 0 1 ) 2 x 4 B i 3 % - G a A s ( 0 0 1 )

h v = 1 . 5 k e V

( E G a A sG a 3 d - E G a A s

V B M ) = 1 8 . 8 2 e V

~ 0 . 2 e V

( 0 . 2 3 � 0 . 0 3 ) e V

FIG. 1. Valence band spectra and shallow Ga 3d CLs measured at 1.5 keV. The data of GaAs 2x4 (black) and Bi(3%)GaAs(blue) in the decapped state are plotted. Values EGaAs

VBM and EGaAsBiVBM were derived using the leading edge method.

2 1 2 0 1 9 1 8

s p u t t e r &a n n e a l e d V B M

B i 5 dA s 3 d

XPS i

ntensi

ty (arb

itrary

units)

b i n d i n g e n e r g y ( e V )

G a 3 dd e c a p p e d

4 3 4 2 4 1 4 0b i n d i n g e n e r g y ( e V )

8 6 4 2 0

( 0 . 5 5 � 0 . 0 2 )

b i n d i n g e n e r g y ( e V )

( 0 . 2 3 � 0 . 0 2 )

2 8 2 6 2 4

FIG. 2. Sputtering and Annealing effect on valence band spectra as well as (Ga 3d, Bi 5d, As 3d) CLs measured at 1.5 keV.Bi(3%)GaAs is shown in its decapped (blue) and sputtered-annealed state (red). The value EGaAsBi

VBM = 0.55 eV was derivedusing the leading edge method.

ger a reappearance of the metallic-like Bi phase in XPS,which proves the absence of major Bi diffusion processesat this temperature.

IV. ULTRAVIOLET PHOTOELECTRONSPECTROSCOPY STUDIES

Fig. 5 shows UPS data in normal emission taken atselected photon energies between 25 eV and 100 eV.Prior to the measurements Bi containing samples were

treated by a 12 min sputter and 440◦C annealing cycle.At high binding energies > 3 eV the spectra of allsamples are dominated by two weakly dispersive featuresat 7.6 eV and 12.3 eV (labeled d and e) and one stronglydispersing band c which joins d at a photon energy of40 eV. In addition pure GaAs shows a prominent featureb at 4.2 eV, which is strongly suppressed for 0.9% Biand fully disappears at 2.7%.

GaAs(001) spectra without Bi are in good agreementwith the literature e.g. for GaAs(001)-2 × 4 at hν =

4

1 5 41 5 61 5 81 6 01 6 21 6 41 6 6

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

( b )

d e c a p p e d s p u t t e r e d a n n e a l e d0

1 0

2 0

3 0

4 0

5 0

6 0

XPS a

mplitu

de (a

rbitra

ry un

its)

B i 4 f - 1 B i 4 f - 2 G a 3 s

a n n e a l e d4 0 0 C

s p u t t e r e d1 2 m i n

e x p f i t G a 3 s B i 4 f - 1 B i 4 f - 2 b c g

XPS i

ntensi

ty (arb

itray u

nits)

b i n d i n g e n e r g y ( e V )

d e c a p p e d

( a )

FIG. 3. (a) Bi 4f XPS spectra of Bi(3%)GaAs (sample L22)after decapping, subsequent 12 min sputtering, and final an-nealing at 440◦C (bottom to top). The fit shown to the dataconsists of one Ga 3s line and two Bi4f doublets. A broadplasmonic peak is present at about 155 eV, which originatesfrom the As 3p edges at lower BEs. Fit parameters are listedin Table?. In (b) the intensities of the 3 fit components areplotted for the three sample stages.

5 4 3 2 1 0

UPS i

ntens

ity (a

rbitra

ry un

its)

b i n d i n g e n e r g y ( e V )

0 . 3 4 e V

0 . 7 5 e V

p u r e G a A s , 2 x 4 0 . 7 % B i , 1 x 3 1 . 0 % B i , 1 x 3 0 . 4 4 e V

S S

FIG. 4. Normal emission UPS close to the Fermi edge for0%, 0.9% and 2.7% Bi concentrations. The data is measuredat hν = 25 eV. Grey shaded areas indicate SS contributionsassuming a Gaussian shape of 0.22 eV width (see text fordetails). The values for the EVBM were derived using theleading edge method as shown by vertical lines in the plot.

29 eV [9] according to which c and d are primary conebulk emission peaks and can be attributed to direct in-terband transitions from bulk valence bands to a free-electron like final state. The respective dispersive be-havior plotted in Fig. 6(a) versus photon energy showsa minimum of c at 43 eV, while d is flat. We have ref-erenced the band dispersion to respective VBM valuesEGaAs

VBM = 0.75 eV derived in Fig. 4 using the leading edgemethod. Both findings coincide very well with Cai etal [10] on GaAs(001)-1× 1, confirming that a and d areindependent of the particular surface reconstruction asexpected for bulk bands. The minimum of c correspondsto the X3 point in k⊥ along the Γ∆X direction (see labelin Fig. 6(a)) and is attributed to the ∆1 band. Closer tothe Fermi level the data of pure GaAs show two featuresa1 and a2, which join at low photon energies ≤ 23 eVto form one single band a (see label in Fig. 6(a)). Againthis is in good agreement with Cai et al. and suggeststhat a2 corresponds most likely to the ∆3,4 bulk bandtransition.

Fig. 6(b) shows the band dispersions of Bi-dopedsamples with 0.9% and 2.7% Bi concentrations.Respective VBM values for Bi-doped samples are

EGaAs(0.9% Bi)VBM = 0.44 eV and E

GaAs(2.7% Bi)VBM = 0.34 eV

shown in Fig. 4. Data of pure GaAs from (a) is plottedas lines for direct comparison.Comparing the two Bi-doped samples it is striking thatthe two data sets coincide except of the weak feature b,as discussed above. In comparison with pure GaAs(001)-2 × 4, significant differences are visible towards Γ aswell as (a1 , a2) bands close to the Fermi level. In theenergy range 40-45 eV close to X3 we observe a perfectoverlap in the dispersion of c for all samples. Whenapproaching Γ, however, Bi-doped samples semm tosettle at lower BEs, which suggests an downwards shiftof ∆1 by 0.3 eV. Also d is slightly shifted downwardsfor Bi-doped samples. A respective trend in the bulkband ∆3,4 is hard to identify, since a distinction betweena1 and a2 is not possible as shown on a larger scale inFig. 4. Nevertheless, the data points lay about 0.4 eVabove a2 for pure GaAs, and thus our data would notbe against a shift of ∆3,4 in the same direction of ∆1

when GaAs is doped with Bi. Again we want to stressthat these differences with respect to pure GaAs do notreflect a rigid shift of the band structure. This is evidentfrom the perfect overlap of c in the photon energy range40-45 eV in Fig. 6(b) or Fig. 4 but also the fact thatband e shows an opposite trend with Bi doping andis shifted downwards with respect to pure GaAs (seesupplementary figure ?).So far we have neglected surface states (SSs), whichcontribute to the density of states close to the Fermi edgeand should be most affected by reconstruction effects. Itis known from the literature on various GaAs surfacesthat SSs become visible as typical shoulders [10, 11] inthe UPS intensity close to the VBM. In Fig. 5 such ashoulder is labeled SS as an example. The introductionof Bi in the GaAs matrix affects the VB lineshape,

5

binding energy (eV)binding energy (eV)binding energy (eV)

UP

S in

ten

sity (

arb

it.

un

its)

c c c

(a) (b) (c)

e e e

d d d

b b

25262728293032343638404244455060708090100

hv [eV]25262728293032343638404244455060708090100

hv [eV]25262728293032343638404244455060708090100

hv [eV]

a2

a1

SS

FIG. 5. Photon energy dependent normal emission UPS spectra for 0%, 0.9% and 2.7% Bi concentrations (a)-(c), respectively.Prior to the measurements the samples were sputtered (12 min) and then annealed at 440◦C. Indications a-e and SS areexplained in the text.

which appears steeper in Fig. 4 for Bi concentration0.9% and 2.7% measured at with hν = 25 eV. Onlyfor pure GaAs the shoulder is clearly separated fromthe intensities a1 and a2 and we approximate therespective SS contributions by a Gaussian function withan approximated width of 0.22 eV depicted in grey.Upon Bi doping this shoulder partly overlaps with thebroadened contribution a and it is shifted to lowerBEs. In contrast, intensities c representing bulk con-tributions ∆1 remain at the same BE as discussed earlier.

V. ANGLE-RESOLVED k-PEEM STUDIES

A. Pure GaAs(001)

A prerequisite for momentum resolved k -PEEM mea-surements is a crystalline surface order of samples understudy, which we monitor by LEED. We first want to dis-cuss the case of pure GaAs materials. After decapping ofpure GaAs(001) (0% Bi) surfaces we observe the (2× 4)reconstruction as shown in Fig. 7(a), which correspondsto the well-known surface Brillouin zone drawn in (b).In the 2 × 4 reconstruction the bulk symmetry between[110] and [110] directions is broken and respective sur-face electronic properties should differ. In Fig 7(c)-(e)high-symmetry (E, k) cuts along the Γ-J’1× 1, Γ-J1× 1,Γ-2× 1-K directions are shown, respectively, which werederived from 3-dimensional (E, kx, ky) k-PEEM datataken at photon energies hν = 21.2 eV. For comparison

FIG. 6. Dispersion of bands derived from normal emissionUPS data in Fig. 5. (a) shows GaAs(001)-2x4 and (b) a com-parison with Bi doped samples. In (b) the data of (a) isincluded as lines. For better comparison with the literatureall BEs in this plot are given with respect to EB = 1.0 eV,which is about the VBM of GaAs(001)-2x4 (see Fig. 5(a)).

6

GG¢ G¢

GG¢ G¢

G

G

J`1x1 J`1x1 K2x1

J1x1 J1x1 K1x1 K1x1

bin

din

g e

ne

rgy (

eV

)b

ind

ing

en

erg

y (

eV

)

K2x1

-1k (Ǻ ) ║ [110]

-1k (Ǻ ) ║ [240]

-1k (Ǻ ) ║ [110] -1k (Ǻ ) ║ [100]

(a) (b)

(c) (e)

(d) (f)

CCE

B

A

CE

B

A

D

D

FIG. 7. k-PEEM data of 2× 4 GaAs(001). (a) and (b) showsa typical LEED pattern and respective surface Brillouin zonescheme. In (c)-(e) (k, E) cuts along the high-symmetry sur-face Brillouin zone directions Γ - J‘2 × 1 - X, Γ - J2 × 1 -X, andΓ - K2 × 1 are shown respectively. For comparison, (f) showsthe bulk GaAs(001) high-symmetry Γ - K1 × 1 cut along [100].

the cuts along the bulk high symmetry direction Γ-K1× 1

is shown in (f). Exemplary energy cuts at E = 0.75 eV,2.0 eV, 3.4 eV and 4.5 eV are shown in Fig. 8, whichconfirm asymmetries between [110] and [110] directions.Cuts along Γ-J‘1× 1 and Γ-J1× 1 directions in Fig. 7 (c)and (d) clearly show the 2nd gamma points Γ’ at around1.6 A−1, which we use as a reference for k-space calibra-tion. Both cuts show a similar band structure with threedistinct band features, which we label as A, B, and C,following the notation of S. Souma et al. [12]. Band Bshows a maximum at Γ and continuously disperses downto EB = 3.0 eV (indicated in Fig. 8 by a dashed line). Itis observed in all cuts including the bulk high symmetrydirection Γ-K1× 1. It is worth to note that in the energycut at EB = 0.75 eV in Fig. 8 the (kx, ky) intensity re-veals a cloverleaf structure which is elongated along the[110] direction with respect to [110]. The bulk GaAs elec-tronic structure close to EF is known to have As 4p char-

FIG. 8. k-PEEM data of 2x4 GaAs(001). Left imagesshow (kx, ky) intensity distributions at four binding energies0.75 eV, 2.0 eV, 3.4 eV, and 4.5 eV. The energies are indicatedas horizontal lines in the (E, k) cut along Γ - J‘2 × 1 X.

acter with four-fold cubic symmetry in the bulk, while forGaAs 2×4 surfaces we expect such a symmetry breaking.Band C we assign to the light-hole bands supported bythe rather spherical shape in the (kx, ky) distribution atEB = 2.0 eV (see cut #2 Fig. 8). Heavy hole bands on theother hand were shown to become visible only at higherphoton energies as shown e.g. Kanski et al. [13]. In con-trast to the literature our data in Fig. 8) on GaAs(001)2x4 has an additional feature E, which crosses B at EB =1.8 eV and k = ±0.4 A−1, reminiscent to a backfoldingeffect to the 1st Brillouin zone. Finally, at higher BE wecan assign the strongly dispersing feature A to the bulkband ∆3,4. At Γ and a photon energy 21.2 eV, A appearsat BE = (3.1± 0.2) eV and thus compares well with thenormal emission data (k‖ = 0, hν = 25 eV) in Fig. 6(a)in particular if the difference in photon energy is takeninto account. Our data is well in line with off-normalARPES data (k ‖ [100] and hν = 23 eV) by Cai et al.[10] who are able to model the dispersion of A assumingdirect transitions from GaAs ∆1 bulk bands into primarycone free electron states. We comment here that the ∆1

band minimum X3 at the BZ border K1x1 is expected atBEs of about 6-7 eV and thus is beyond the BE range inFig. 7. The visible upwards dispersing parabolic featureD with a minimum at BE = 4 eV at K1×1 (see Fig. 7(f))instead fits rather well to the expected ∆3,4 bulk banddispersion along [100] with a minimum X5 at about BE= 3.8 eV [10, 14].Please note that when comparing our k-PEEM datato the literature, we took into account that e.g. inRef. [10, 14] BEs are given with respect to the GaAsVBM, while our BEs refer to the NanoESCA analyser‘sFermi level at BE = 0. In k-PEEM data of pure GaAs wefind the VBM located at about BE = (1.1± 0.2) eV (seesupplementary Fig. 11(b) or Fig. 9(e)), which is consis-tent with our UPS data from synchrotron photon sourcesin Fig. 4.

7

B. Bi-doped GaAs(001)

We now turn to the effect of Bi doping on the GaAsband structure. Fig. summarizes ARPES data of sput-ter/annealed GaAs(2.7% Bi) in comparison with de-capped (unsputtered) GaAs(0% Bi) in (b). RespectiveLEED images in (a) and (b) shown on the top panels ev-idence a Bi-induced change of surface reconstruction froma 2x4 to a 2x3 symmetry. The band structure shown in(c) and (d) are cuts along Γ-K1x1 (top panel) and Γ-J’1× 1

(bottom panel). The ARPES intensities exhibit similarfeatures within our resolution, although Bi doping leadsto blurring of ARPES.

VI. SUMMARY

- Generally, results of XPS and ARPES are aboutthe same for 0.7% and 3% GaAsBi after sputtering andannealing 440C. Bulk properties after sputter/annealingthus reflect the solubility limit of GaAsBi. Excess Biforms Bi-rich surfaces with respect to bulk.

- Close to the Brillouin zone borders, UPS and k-PEEM data indicate a Bi-induced shift of the bulk bands∆1 to lower BEs by about 0.2eV. Approaching the Γpoint ∆1 shifts increase, while at X3 the dispersion over-laps. Since the direct band gap in GaAs is located at Γ,the Bi induced change is interesting for photonic proper-ties such as luminescence.

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ACKNOWLEDGMENTS

VII. SUPPLEMENTARY INFORMATION

Fig.(a) shows ARPES data of decapped GaAs(0.7%Bi) and for comparison in (b) again GaAs(0% Bi). Re-spective LEED images shown on the top panels evidencethat the integration of Bi into GaAs changes the surfacereconstruction from a (2 × 4) to a (1 × 3) surface sym-metry. The band structure has similar features withinour resolution, although Bi doping leads to blurring ofARPES.

8

(b) pure GaAs

-1k (Ǻ )║

[100]

(a) GaAsBibin

din

g e

nerg

y (e

V)

-1k (Ǻ )║

bin

din

g e

nerg

y (e

V) [-110]

integratedk-space intensity

(e)hv = 21.2eV

[100]

[-110]

(d)(c)

FIG. 9. k-PEEM data of sputter/annealed 2.7% Bi (left) incomparison with pure GaAs (right) measured with a labora-tory Helium lamp at hν = 21.2 eV. (a) and (b) show respec-tive in situ LEED images revealing a 2x3 reconstruction ofGaAsBi with respect to the GaAs(001) 2 × 4 symmetry. k-PEEM intensities along the [100] (top panel) and [-110] (bot-tom panel) directions are shown in (c) and (d). Horizontaldotted lines mark BE levels of 1 eV and 3 eV for compari-son. In (e) the k-space integrated intensity close to the VBMis plotted, which is equivalent to UPS data at low photonenergies hν = 21.2 eV.

9

FIG. 10. k-PEEM data of decapped 0.7% Bi (right) in com-parison with decapped GaAs (left) measured with a labora-tory Helium lamp at hν = 21.2 eV. (a) and (b) show respec-tive insitu LEED images revealing a 1x3 reconstruction ofGaAsBi with respect to the GaAs(001) 2 × 4 symmetry. k-PEEM intensities along the directions [100], [0-10], [110] and[-110].

10

FIG. 11. Calibration of the GaAs(001) 2×4 valence band top.(a) and (b) show full UPS scans in the binding energy range[-2.5, +18.5] eV and [-2.5, +5.3] eV, respectively, measuredat a photon energy 21.2 eV in the initial state and after Fedeposition (black and red lines). In (c) an ARPES image ofGaAs [100] cut is shown for direct comparison in the bindingenergy range.

1 5 41 5 61 5 81 6 01 6 21 6 41 6 60 . 0

0 . 1

0 . 2

0 . 3

0 . 4

0 . 5

0 . 6

1 2 m i n I B

6 m i n I B

1 m i n I B

XPS i

ntens

ity (ar

bitrar

y unit

s)

b i n d i n g e n e r g y ( e V )

d e c a p p e d

FIG. 12. Effect of sputtering on the XPS spectra of the Bi 4flines. Bottom to top show .....