3 × 3 reconstruction along the (111) face of highly boron-doped Si upon vacuum annealing

586

Surface

6 x 6 RECONSTRUCTION OF HIGHLY BORON-DOPED S. BENSALAH, Lahorutoire

Science 211/212 (19X9) 5X6-592 North-Holland, Amsterdam

ALONG THE (111) FACE Si UPON VACUUM ANNEALING

J.P. LACHARME

and

C.A.

de Physrque des Solides, crsso& m CNRS

SJ?BENNE

154, Unwersrth P. et M. Curre,

75252 Pari.r Cedex 05, France

Received

11 July 1988; accepted

for publication

2X September

1988

Low energy electron diffraction, Auger electron spectroscopy (AES), and photoemisslon yield spectroscopy measurements were performed on vacuum cleaved Si(ll1) surfaces. The samples were p-type boron-doped at 4X 1Ol9 B cmm3; they were treated with successive annealings in the ultrahigh vacuum system up to 1150° C. After annealing, the usual reconstruction change from 2 X 1 to 7 X 7 in the 300 o C range is followed by the appearance of a blurred 1 x 1 pattern in the 900~1000°C range when traces of surface boron become detectable by AES. Then, in the 1050-1150 o C range. the surface displays a sharp fi X 6 R30 o diagram with a saturated surface concentration evaluated at about 0.1 monolayer (less than XX 1O’j B cmm2). The results arc discussed in terms of the relaxation of residual strains and lead to a new model for the boron-induced 43 surface structure.

1. Introduction High

temperature

clean

silicon

surfaces

[l-6]

which

has been

accumulation understood,

vacuum

annealing,

is known attributed

either

[6]. If the mechanism it is clear that the presence

to a diffusion surface region.

phenomenon

in the

to generate

which

1000” C range

a highly

to carbon

doped

enrichment

of this phenomenon of surface impurities

shows

up as a p-type

and

p-type

above.

surface

of

layer

[5] or to boron is not yet fully and defects leads overdoping

in the

Recently, in parallel with the present work, it has been found that a high concentration of boron as a p-type dopant in the bulk of silicon can lead to a higher surface concentration of B and induce a reconstruction change upon vacuum annealing under evaporation conditions [7] (1250-1350 o C). We have found that such boron surface 7 x 7 reconstruction into a fi temperatures, tions [g].

in the

1000-1100”

enrichment x fiR30” C range,

accompanied by a change of the structure occurs in fact at lower in agreement

with

other

observa-

In this paper the surface property changes of the clean cleaved (111) face of B-doped Si samples, occurring upon successive annealings at increasing tem-

0039-6028/89/$03.50 8 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

S. Bensalah el al. / Boron induced \ii

peratures, electron

are studied spectroscopy

results

using low energy (AES)

are compared

electron

and photoemission

to other

works

587

reconsrrucfion on Si(l I I)

diffraction

(LEED),

yield spectroscopy

[7,8] and a model

Auger

(PYS).

The

of reconstruction

is

discussed.

2. Experimental The multiple

technique

has been described

ultrahigh

elsewhere

vacuum

system used for the experiments

[9]. In the present

work, oriented

monocrystal-

line silicon bars were studied; they were boron doped, p-type, with 4 x 1019 B cm-3. Others, for reference in PYS, were n-type with 2.4 X 1014 P cm-‘. First a clean surface was obtained by cleavage at a base pressure slightly above lo-” Torr. Annealings were made by entering the end of the silicon bar into a cylindrical perature

furnace

powered

had been calibrated

by a stabilized against

current

supply.

the furnace current

bar into which a thermocouple was fixed. The actual 50” of the measured value. Even during the highest

The sample

tem-

using a drilled silicon

temperature temperature

was within annealings

the pressure remained in the lo- ” Torr range and Auger measurements showed that contamination by 0 and C was always negligible. Auger measurements were made with a cylindrical mirror analyzer with an axial electron gun. The photoemission

yield spectra

were recorded

between

4 and 6.6 eV photon

energy.

3. Results 3.1. LEED After single 7

x

cleavage,

the usual 2

or multidomain

7 reconstruction

x

depending

1 reconstructed

diagram

on the cleave.

upon annealings

is observed,

It transforms

at any temperature

into

either a sharp

from a few hundred

to less than 900 o C (fig. la). Beyond 900 o C, clear 1 x 1 spots can be observed against an increased background together with traces of the vanishing 7 x 7 diagram (fig. lb). The 1 x 1 pattern stays on up to about 1050°C when faint and wide spots of a fi x &R30” pattern start to come out of a strong background (fig. lc). Long annealing at 1150 o C (1 hour, as compared to the usual 10 or 15 minutes) leads to a sharp fi background as in fig. Id. This LEED pattern further

annealing,

the temperature

diagram remains

with a much smaller unchanged upon any

being always kept below any sublimation.

3.2. AES The two Auger peaks of interest in this work are Si(LW) at 90 eV and B(KLL) at 177 eV since no other element is observed. On the cleaved surface

Fig. 1. LEED diagrams of a cleaved (111) face of a highly boron-doped Si crystal: (a) after vacuum annealing for 10 min at 640° C. E = 63 eV: (h) after 10 min at 920 o C. E = 66 eV: (c) 11 min at 1080 o C, E = 76 eV; (d) after one hour at 1150 o C. E = X0 2V.

as well as on the surfaces

annealed up to at most 900 o Ct no boron is detected at the sensitivity of our system. Since the bulk concentration is equal to 4 x lOI9 5 cm -3, the boron content of a double (111) plane of silicon atoms (3.135 A thick with 2.57 X lOI Si cm-.‘) is 1.25 X 10” B cm-‘: it means that a surface coverage of less than lo-’ monolayer of boron is not detected in AES. A trace of boron becomes observable after annealing beyond 900°C when the 7 x 7 LEED diagram has almost vanished. The boron surface content increases significantly upon annealings at and beyond 1050°C and reaches a saturation after prolonged annealings at 11 SO’ C. The Auger peak of silicon decreases only slightly and this attenuation is not clearly associated with the boron signal increase. The numerical results are given in table 1. A

S. Bensalah et al. / Boron induced 6

reconstruction

on Si(l I I)

589

Table 1 Evolution changes, induced by thermal treatments, of some properties of highly boron-doped silicon cleaved samples: LEED diagram, Si(90 eV) Auger peak height in arbitrary units, B(177 eV) Auger peak height referred to its saturation value, work function changes A+ = 9 -4.70 as deduced from PYS Thermal

treatment

LEED

Si (au)

B/B sat

A+ (eV)

640 o C, 10 min 920 o C, 10 min 1080°C. 11 mm

7x7 Traces 7 x 7 1x1

1 - 0.95 - 0.90

0 0.16 0.45

0 + 0.08

1150 o C, 15 min

J7xfiR30°

- 0.95

1

0

1150”C,60min

&xfiR30°

- 0.95

1

- 0.03

tentative evaluation of the surface boron content at saturation will be discussed below. Both the Si and B Auger signals at saturation are shown in the insert of fig. 2.

Y(E))

I.....,..

5

!...

6

E(ev,

Fig. 2. Photoemission yield (number of emitted electrons per incident photon) as a function of photon energy for a cleaved (111) face of silicon after annealing: (A) 7x 7 reconstructed low doped sample; (B) 7 X 7 reconstructed highly boron doped sample, after 10 min at 640 o C; (C) the same sample as for B, after 10 min at 92O’C; (D) the same after one hour at llSO”C, boron saturated surface. Insert: the Si(90 eV) and B(177 eV) Auger lines at boron saturation (EP = 2 keV; modulations: 1 and 2 V peak to peak for Si and B, respectively).

590

S. Bensulah

3.3 Photoemission

et al. / Boron mduced

J-3 reconstructionon .%(II I)

yield spectroscopy

Yield curves are displayed in fig. 2. Curve A, as a reference, is from the 7 X 7 reconstructed (111) surface of a lightly doped Si sample. It is markedly different from the other three spectra. all relative to highly boron-doped samples. It shows how much the yield curve is sensitive to the bulk doping in the space charge region. The degenerate character of the valence band edge due to the high boron doping gives a metal-like threshold at low photon energy (curves B-D). The slight changes in the threshold position reflect the work function changes A+ upon annealing; the + values are given in table 1. It is clear that the general shape of the yield curve remains almost unchanged upon annealing. This is an important result since it means that, altogether. the doping in the space charge region stays at about 4 x 10” cmP3 and the band bending remains about the same throughout the heat treatments. Therefore the ionization energy changes follow those of the work function and the density of surface states is not strongly affected by the presence of surface boron.

4. Discussion When a trivalent element starts to form a fi structure along the (111) face of silicon, the model which comes immediately to the mind is the one now generally adopted in the case of Al, Ga or In adsorption on the same surface [lo]: the trivalent atom is bonded to three Si surface atoms, nearest neighbours in the first atomic plane along the ideal (111) surface, just above a silicon atom of the second plane. This is the so-called T4 adsorption site. Of course, the structure is complete at l/3 of a monolayer coverage. Taking into account the various covalent radii and the tendency to approach the tetrahedral symmetry around Si atoms, the local structure stabilizes with typical distortions, as shown quantitatively by grazing incidence X-ray scattering on a similar system with heavier atoms [ll]. In the case of boron, the usual model may be questioned because, contrary to the other cases. the sum of the boron and silicon convalent radii, 0.88 and 1.17 A, respectively, gives a bond length which is smaller than 2.21 A, the distance between the Si surface atoms and the center of the equilateral triangle they form. If the usual model must be questioned, one should get evidences from the adsorption of B on Si(ll1). This process has been studied recently [12] and two puzzling effects were observed which had not been seen with the trivalent metals. First, the boron Auger signal does not increase linearly at the beginning of the deposition; on the contrary, it is very faint up to a deposit of 0.1 monolayer (as if the boron were “lost” into the silicon). Second, less than 0.1 ML of boron (in any case, at most 0.17 ML) seems to be effectively present at the surface when the fi structure is established. It shows that a surface

concentration

S. Bensalah et al. / Boron mduced fi

reconstruction

of boron

ML is needed

structure. It is important experiments annealing

much less than l/3

to determine

and

therefore

was evaluated.

if such

the

surface

on Si(l1

a situation boron

I)

to complete

occurs

content

We have used two approaches.

591

the v%

in the present

at

saturation

The Auger

after lines in

the insert of fig. 2 have first been compared to standard spectra from ref. [13]. Assuming an escape length of 5 A, we find a maximum boron content lower than 0.1 ML. Second,

assuming

that boron and carbon

as substitutional

atoms

in crystalline silicon have similar Auger efficiencies, we have used Auger results on SIC crystals (141 to calibrate the two lines: we find a boron surface content

of 0.03 ML. Both evaluations

are much lower than l/3 ML. Consider-

ing the differences in experimental conditions, largest boron surface concentration of 0.15 evaporation

in ultrahigh

Considering

vacuum

they agree quite well with the ML observed upon thermal

[7].

now the scanning

tunnel microscope

results [S], the images are

very regular along the 6 reconstructed surface: this is not compatible with an uncompleted layer of B atoms in T4 sites. Another model would be that Si atoms

occupy

the T4 sites,

centers

in the double

because

it would explain

or C to accumulate reconstructed similar

to T4 sites

additional

adatoms

the presence

atoms Such

serving a model

In order

as stress-relaxing is very tempting

of small atoms like B

[5,6]. It is well known

contains

are needed

underneath

below.

the surface

already

[15].

plane

on a general basis the tendency

along

surface

the small boron

atomic

12 adatoms

to switch

to a fi

and the process

of a few boron

that the 7 x 7

per unit cell in positions structure,

only

a few

may very well be triggered

by

per 7 x 7 unit cell. Since

the

atoms

7 X 7 is not fully relaxed [16] but is the complicated

result of the contradictory

tendencies to minimize the number of dangling bonds and to keep bond angles and lengths as close as possible to bulk values, the small size of boron must be of great help in finding a simpler surface structure. It is clear that the fi reconstruction is the simplest one minimizing the number of surface dangling bonds. A good check for this explanation dangling bond states after completion

would be to determine the density of of the fi structure. This is not easy to

do because strong band bending effects are unavoidable. However the conservation of the yield spectra upon annealing (fig. 2) shows that the value of the band bending

is essentially

constant,

meaning

that the filled surface

states

in the gap are at least conserved. In summary,

the boron

induced

fi

reconstruction

on Si(ll1)

surfaces

may

be imposed by stress relaxation effect and occurs at less than 0.1 ML boron surface concentration. The actual reconstruction is different from the accepted one in the case of adsorption of larger trivalent elements. The main lines of a new model are drawn but the details of this possible structure remain to be determined.

The critical edged.

reading

of the manuscript

by Dr. F. Proix is gratefully

acknowl-

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