Electronic properties of the InP(100) surface

Surtacc Science 168 (1986) 68-7 ~, North-Holland Amsterdam

ELECTRONIC PROPERTIES OF THE lnP(100) SURFACE J M M O I S O N and M B E N S O U S S A N c entre National d'Etude~ de~ 7elecomrnum~attons, Laboratotre de Bagneu~, I~6, rue de Paris, k-92220 Bagneu~, Frame

Recmved 10 June 1985, accepted for pubhcatlon 12 Jub¢ 198s

We report a hrst mvestlgahon ot the structural and electromc propertms ot InP( 11101 Through an adequate preparation, a clean surface with a (4x2) reconstruction is obtained UPS and ELS studms ol this surface, with m-sltu reterence to the (11(1) cleaved surface reveal large densities of gap surface states both near the conduction and the valence bands A small denslt,~ o| m~d-gap surface states strongly pms the Ferm~level Measurements of the surface recombination velocity by photolummescence show that they also act as surlace traps It is concluded that the lnP(100) surface is very similar to perturbed (1101 surfaces and is rough on a mmroscoDc scale

1. Introduction T h e (100) surface of I n P is c o m m o n l y used as a substrate tor the growth of epltaxlal thin layers In s e m i c o n d u c t o r device t e c h n o l o g y H o w e v e r , up to now, only a few surface studies have b e e n p e r f o r m e d on this surface, p r o b a bly because of the p r o b l e m s raised by its p r e p a r a t i o n W e fred that a d e q u a t e t r e a t m e n t s p r o d u c e a clean a n d o r d e r e d surface T h e electronic p r o p e m e s of this surface are p r o b e d by U P S , ELS, a n d p h o t o l u m l n e s c e n c e with in-s~tu r e f e r e n c e to those of the (1101 cleaved surface t a k e n as a s t a n d a r d

2. Surface preparation and crystalline structure T h e (11(I) surfaces are o b t a i n e d by U H V cleavage of I n P rods d o p e d to 101:-10 is cm -~ with S (n-type) or Z n (p-type) T h e (1001 samples are wafers of similar materials polished m a b r o m i n e - m e t h a n o l bath a n d inserted in the analysis c h a m b e r t h r o u g h a lock c h a m b e r T h e i m p u r i t y level at this stage r e m a i n s low (fig 1) T h e y are t h e n c l e a n e d by successive ion etchings a n d a n n e a h n g s Both processes, which are k n o w n to r e m o v e p r e f e r e n t i a l l y P atoms, are kept at the lowest possible level the ion dose ( A r +, 1 keV) lies b e l o w 1013 cm 2 a n d a n n e a h n g s are short (5 m l n ) a n d r a t h e r l o w - t e m p e r a t u r e (300°C) S E M studms [1] have shown that such p r e p a r a t i o n c o n d i t i o n s ymld 0039-6028/86/$03 50 © Elsevier Publishers B V ( N o r t h - H o l l a n d Physics P u b h s h l n g Division)

J M Mozson, M Bensoussan / Electromc properttes o f lnP(lO0)

69

!. ;7

~ In

S?

C? In

In

f

Ill

0

In [~

0?

CLEANED ANNEALED ~ln

lnP 000) 16o

P

~I

260 36o 46o KINETIC ENERGY(eV)

560

Fig 1 AES spectrum of chemically pohshed and cleaned-annealed lnP(100) surfaces The primary beam energy is 3 keV and the p-p modulation as 2 5 eV (spherical analyzer VG CLAM100)

surfaces of high crystalhne quahty The small loss of P atoms frees the neighbourlng In atoms which aggregate to form small clusters covering less than one percent of the surface and leaving a renewed surface They are then invisible to surface analysis AES spectra are not significantly altered by them and neither UPS nor ELS reveals the features associated with metallic In, while those are clearly observed after a R T deposition of less than 0 2 monolayer of In [2] No contaminants can be detected by A E S (fig l) or XPS on the final surface Its P/In A E S peak ratio IS 0 95 + 0 l0 for normal incidence of the primary beam, and 1 74 + 0 15 for glancing incidence where shadowing effects enhance the relative contribution of surface atoms [3] The comparison of these values indicates a rather P-rich surface On the opposite, the comparison with the ratio of 1 32 obtained on P-rich surfaces annealed under P-pressure, also at normal incidence but with a different A E S configuration [4], could indicate that our surface is rather In-rich, as also deduced recently from secondary electron yield measurements [5] R H E E D and L E E D observations reveal a sharp (4× 2) reconstruction Similar patterns are observed on surfaces annealed under P-pressure It is however possible that, like in the case of G a A s , (2 x 4) or (4 x 2) reconstructions occur depending on the surface StOlChlometry A final conclusion about this Stolchlometry clearly requires extended experiments However, all results agree to point out that the crystal termination is not a perfect P or In plane, and that the surface is rough on a microscopic scale

70

J M Motion, M Bensou~an I Electromc propertte~ o] lnP(lOOI

3. Occupied surface states T h e U P S s p e c t r a of l n P ( l O 0 ) a n d InP(110) that we o b t a i n are very s l m d a r to p u b h s h e d results on (110) [6-9] O n these s p e c t r a , the F e r m i level is p o s m o n e d using a m e t a l h c s a m p l e and the v a l e n c e - b a n d m a x i m u m ( V B M ) by a h g n m e n t of the b u l k f e a t u r e s o r by e x t r a p o l a U o n of the v a l e n c e - b a n d c o n t n b u t t o n O n the (100) surface, the s p e c t r u m r e v e a l s , b e s i d e s a p m n m g of the Ferret level which wtll be dtscussed f u r t h e r on, a p e a k l y m g m the b a n d gap, This f e a t u r e (fig 2) which IS n o t o b s e r v e d on (110) d i s a p p e a r s after o x y g e n a d s o r p u o n a n d its e n e r g y p o s l t t o n (0 05 + 0 05 e V a b o v e V B M ) d o e s not d e p e n d on the p h o t o n e n e r g y T h u s it c o r r e s p o n d s to a surface d e n s i t y

Reso~lul,on

~

• IO000L

•?5

C

/

",'"-T~

"¢' -I

"

~---~CLEAN

.';

0 Ev

o 1

INITIAL STATE ENERGY(oV) i

i

i

. I~212eV nvl--- z.O8eV

',I

~,

~

~,¥ ,, /v'~t '.¢kI ~_ii \i \\ ,

¢-

I~i i iv!

I

,~ \ \

:

X\

i- i

\\

,

,

-I0

-5

/ /1

,,,

I

;,,

~E,~Fc 0

VBM INITIAL STATE ENERGY (eV) Fig 2 UPS spectra ot lnP(100) taken at photon energies el 21 2 and 4(1 8 eV (bottom), enlargement of the 21 2 eV spectrum m.ar the VBM re~eahng a SDOS (top) The insert show,, the evolution of this SDOS under oxygen contammauon (spherical anal}zer CLAMI00 operated at a resolution el 0 2 eV)

J M Motion, M Bensoussan / Electromc properties of lnP(lO0)

71

of states (SDOS) which can be evaluated at 1 6 x 1014 cm -2 by comparison of its contribution to the UPS spectrum with the one of the valence band [10] The peak at 1 0 eV below VBM in the UPS spectrum is also sensitive to oxygen adsorption and may be associated wtth a SDOS, as already observed on the (110) surface [8]

4. Empty surface states The ELS spectra of (100) and (110) surfaces are shown together in fig 3 They are in fair agreement with previous measurements made separately [11-13] From their evolution under oxygen contamination or with prtmary energy, it is clear that the peaks at 2 5 and 9 0 eV and the doublet at 17 7-18 7

~12~np

(i00)

:~00 136

' ~ 57

9

7

IA~, ~'

189

~' InP Ep=120eV

I II A78A115 149179/1198222

(I '//g?

,V T

_11 I • I i/fl 0

or,,o0: v OXYGEN-COVEREC 10 20 ENERGY LOSS (eV)

Fig 3 ELS spectra of lnP(100) and l n P ( l l 0 ) and of these surfaces contaminated with oxygen at a primary beam energy of 120 eV (bottom), evolunon of the ELS spectrum of InP(100) with primary beam energy (top) All curves are n o r m a h z e d to give a constant elastic peak, the p - p modulation is 1 eV (spherical analyzer CLAM100)

72

J M Motson, M Bensoussan /Electrontc properttes o] InP(lO0)

( + 0 1) eV are surface-related This last leature is attributed to the transmon from the In 4d core doublet to an empty SDOS which creates a surface core exclton [12] From our UPS spectra at 40 8 eV, the initial states are found to lie at 16 6 and 17 8 eV below V B M The exclton binding energy is an ill-defined p a r a m e t e r - values from 0 2 to 0 5 eV have been proposed - but it should not depend on the face considered Hence the empty surlace state shifts () 2 eV downwards from (110) to (100) Assuming a doublet separation of 0 8 eV and a mean exclton energy of 0 35 eV, we may estimate the empty SDOS energy at 1 55 +_ 0 25 and 1 75 _+ 0 25 eV lor the (1011) and (110) surfaces This last value is slightly higher than results obtained on (110) by photoemission yield spectroscopy (1 7 eV) [14] and two-photon photoemlssion (1 45 eV) [15] The peak at 2 5 eV can be associated with a transition from the filled SDOS at - 1 0 eV to empty SDOS at 1 55 eV, with a low exclton binding energy, and the peak at 9 0 eV with a transmon between deep-lying filled SDOS (back bonds'~) and the same empty SDOS

5. Fermi-level pinnning and surface recombination The position of the Fermi level at the surface of InP depends strongly on the face For (110), the band bending, if any, hes below the experimental uncertainty of 0 1 eV, in agreement with previous results [8] On the opposite, for the (100) surface, the Fermi level lies at 1 0 + 0 1 eV above VBM, whatever the bulk doping A calculation similar to those ot refs [16, 17] shows that such a strong pinning of the Fermi level is not caused by the SDOS we observe by UPS and ELS, but by smaller ( > 1012 cm -2) SDOS located much closer to the Fermi level Such levels have been assumed or observed on contaminated (110) surfaces [18-20] and associated with surface defects Their proximity to the mldgap makes them particularly efficient surface traps The surface trap density was evaluated from measurements of the surface recombination velocity by photoluminescence [21] It can be v a n e d by surface treatments and ItS variations correlate exactly with those of the filled SDOS observed by UPS, and quahtanvely with those of the empty SDOS observed by ELS [21] This would indicate a common origin ot all these states

6. Conclusion The above results yield a semi-quantitative picture of the electromc structure of the InP(100) surface They reveal important ( ~ 1014 cm -2) SDOS close to both edges of the band gap together with smaller SDOS (~1012 cm -2) near the mid-gap which pin the Fermi level and act as surface traps The variations of these three SDOS components with the surface treatments are parallel A

J M Motson, M Bensoussan / Electrontc ptopottes of lnP(lO0)

73

quite similar situation is observed on (110) perturbed by metal adsorption [14] the filled and empty S D O S which normally lie outside of the band gap m o v e towards it, while a slight defect-like S D O S spreads In the band gap and pins the Fermt level In view of this similarity and of the structural data, the InP(100) surface can be seen as a rough surface on a microscopic scale, lnvolvmg many local bond dtstortions, created for instance by adatoms or vacancies

Acknowledgements We wish to thank C Sdbenne for many frmtful dlscusslons and a critical readmg of the manuscript, M V a n R o m p a y , F Barthe and R Bertrand for technical assistance, and R Coquill6 and Y Toudlc ( C N E T Lannlon) for the cleavage rods

References [1] [2] [3] [4] [51 [6] [71 [8] [9] [1~] [11] [12] [13] [14] 115] [16] [171 [18] [19] 12[)1 [211

C Bablet, P Abraham and P Morro, to be pubhshed F Houzay, M Bensoussan and F Barthe. Surface So 168 (1986) 347 M Perdereau. Surlace Scl 24 (1971) 239 C W Tu. T T Sheng. M H Read. A R Schher, J G Johnson. W D Johnston. Jr and W A Bonnet, J Electrochem Soc 13(I (1983) 2081 B Gruzza. B Achard and C Panset. to be pubhshed P W Chyc, 1 Llndau, P Planetta. C M Gardner, C Y Su ,rod W E Splcer, Ph'~s Re~ BI8 (1978) 5545 P W Chye. C Y Su. I Lmdau, C M Gardner. P PIdnetta and W E Splcer Surlace ";el 88 (1979) 439 A McKmley G P Snvastava and R H Wdhams, J Phys C13 (1980) 1581 W Gudat and D E Eastman, J Vacuum ScJ Technol 13 (1976) 831 L F Wagner and W E Splcer, Phys Rev Letters 28 (1972) 1381 R H Wdhams and I F McGovern Surface Scl 51 (1975)14 J van Laar, A Hmjser and T L van Rooy. J Vacuum Sol Technol 14 (1977) 894 J Masslcs. P Etienne. F Dezaly and N T Lmh, Surtace Scl 99 (198(I) 121 P Malgnd. 12 Sebenne and A Taleb-lbrahlml, Surlace Scl 162 (1985) 663 R Halgt, J Bokor. J Stark. R H Storz R R Freeman a n d P H Bucksbaum. Phys Rev Letters 54 (1985) 1302 A Many, Y Goldstem and N B Grovcr Semiconductor Surfaces (North-Holland, Amsterdam, 1971 ) A lsm.nI, J M Palauand L Lassabatere, J PhysNue45 (1984) 1717 W E Splt.er 1 Lmdau, P Skeath C Y S u a n d P Chye, Phys Rcv Letters 44 (1980) 42(I R H W~lhams. A McKmlev. G J Hughes. V Montgomery and I F McGo~ern J V~lcuum Scl Technol 21 (1982) 594 Iv ( h e k l r . T Nc|latl. G N Lu, E Berber and C garret, Surlacc Scl 168 (1986) 82,8 J M Molson. M Van R o m p a v a n d M Bensoussan. Proc 1984Symp o n G a A s a n d R e l a t c d ( o m p o u n d s . Ed B de ( r e m o u x (Hllg~.r, Bristol, 1985)