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Electrical properties of surface states on silicon surfaces
SURFACE
SCIENCE
ELECTRICAL
4 (1966) 299-312 0 North-Holland
PROPERTIES SILICON SHUICHI
Department
OF SURFACE
STATES
ON
SURFACES
SATO, and SHINJI
of Physics, Faculty of Science,
Publishing Co., Amsterdam
KAWAJI
Gakushuin University, Mejiro, Tokyo, Japan
and AKIO KOBAYASHI Electrical
~omm~nicut~on Laboratory,
~~sashi~o-sh~,
Tokyo, Japan
Received 14 October 1965 The electrical properties of surface states on practical (“real”) silicon surfaces with various surface treatments were investigated by means of pulsed field effect techniques. On the basis of the experimental results which showed some systematic relations between the physical parameters of the observed surface states and surface treatments, possible physical models for surface states on practical silicon surfaces are discussed.
1. Introduction A numbers of papers have been published on the surface states on silicon single crystals”). The data of silicon surface states on, probably, the best defined surfaces so far available are those of clean surfaces in ultra high vacuum determined from the measurements of the work function and photoelectric threshold by Allen and Gobeli 2). On practical silicon surfaces, the energy position and the effective capture cross section of the surface states were studied in some details by means of the pulsed field effect technique by Rupprecht 3). The present paper is concerned with the observations of electrical properties of surface states of the foIlowing types of silicon (111) surfaces: 1) Surfaces etched in KOH solution and CP4-A; 2) Cleaved surfaces; 3) Surfaces covered with epoxy resin; 4) Thermally oxidized surfaces (high temperature); 5) Surfaces thermally oxidized (low temperature) after staining the surface in acid mixture; 6) Surfaces irradiated by I’-rays of 6oCo after etching in CP4-A and thermal oxidation. 2. Experimental procedure Samples
were rectangular
bars of 11 x 2 x 0.3 mm3 except for cleaving ex299
300
SHUICHI
SATO,
SHINJI
KAWAJI
TABLE
Surface treatments Surfaces
Type
Etched in p CW-A 7.-Etched in KOH
’
Cleaved
p
Covered with resin
P
Oxidation
’ n
Stain films
’
Rem 5 x 102 38 103
AND
AK10
KOBAYASHI
1
for various specimens Surface treatments
~
After lapping, specimens were etched in CP4-A for 6(r120 set at room temperature. -...______ _. ...~ After lapping, specimens were etched in 10% solution of KOH for 120-180 set at 70-100 “C.
After lapping, specimens were sandwiched between two 2 X 102 parallel glass plates and submitted to cleavage in air. The cleaving apparatus was shown in a previous paper 3. After etching in CP4-A, a tantalum field plate separated by thin epoxy resin was attached to the silicon surfaces. The sample was thermally hardened without the application of a DC field between the silicon and the field plate, Si(O), with 5 x 102 a DC field which was polarized as silicon positive, Si(+), and with a DC field polarized as silicon negative, Si(-). The temperature and time for heat treatment to harden resin are 150 “C and 7-10 hours, respectively. 30 50
After etching in CP4-A, samples were heated in wet oxygen gas (1 l/min) at 1 200 “C for 4 hours. Thickness of oxide films was about 5 700 A.
SO-70
After etching in CP4-A, stain films were grown by immersing the samples to 103: HF --I-1: HNOa mixture solution (20 “C) for a few minutes. Some samples were heated in oxygen gas for 10 hours at 300 “C, 500 “C or 800 “C.
periments in which the sample dimension was 8 x 2.5 x 0.8 mm3. The surface treatments are listed in table 1. A tantalum field plate with 2 x lo-‘mm mica spacer was attached on each specimen, then the sample was mounted in a cryostat which was evacuated immediately up to 2 x 10e4 Torr. At first, the surface conductivity type was measured by the large signal AC field effect measurement applying 1 SOOV,_ p at room temperature. The temperature dependence of the relaxation time (r,) in the pulsed field effects was measured under the applied step voltage of 500 V, positive or negative, depending on the surface conductivity type. Energy position below the conduction band edge (EC-ET) or above the valence band edge (E, - E,) and effective capture cross section for holes (a,/~) and electrons (onc() were determined from Rupprecht’s theory. Here, c1is the statistical weight factor of the surface states. The surface state density (NJ was estimated from the saturated conductance increment in the transient behavior of the pulsed field effect.
ELECTRICAL
The measurements
PROPERTIES
were performed
OF
SURFACE
in vacuum,
301
STATES
dry air, dry ammonia,
wet air,
ozone and oxygen gas. 3. Experimental
results
3.1. ETCHED SURFACES Typical results of log(T%,)-’ versus l/T obtained for the surfaces etched in KOH solution are shown in fig. 1. Two discrete levels of 0.35 eV and 0.12 eV above the top of the valence band with effective capture cross sections of 2 x lo-l6 cm’ and 2 x 10-20 cm2, respectively, for holes were determined from the data in vacuum. The densities of these surface states are almost the same and equal to 3 x lOlo cmw2. From the values of the effective capture cross section, the former level is neutral-like and the latter is donor-like. When the sample was exposed to wet air or dry ammonia, a discrete level whose concentration is about 10” cmm2 was observed at 0.72 eV above the top of the valence band with effective capture cross section of 9x lo-” cm2 for holes. This level is acceptor-like. On the surface of an-
x Etch 0 cu so, ~ Wet air or ammonia
103/T
Fig. 1.
Plots of log(T2r&1
(OK-‘)
as a function of reciprocal temperature in KOH solution.
for surfaces etched
302
SHUICHI
other
sample
CuSO,, band
which
a discrete with
was dried
in air
level was observed
effective
its density
SHINJI KAWAJI AND AK10 KOBAYASHI
SATO,
capture
was about
10”
cross
after
immersing
at 0.58 eV above section
cm- 2. This
of 8 x lo-
in the
solution
of
the top of the valence I2 cm2 for
level is acceptor-like.
holes,
The
and
electrical
of these levels are summarized in table 2. Electrical properties of surface states on surfaces etched in CP4-A are summarized in table 3. The two energy levels observed in n-type material and two energy levels observed in p-type material are in good agreement with the values determined by Rupprecht. However, the agreement between properties
TABLE2 Electrical properties
of surface states on surfaces etched in the solution of KOH
Energy position ET ~ Ev in eV
Effective capture cross section 0,)/n in cm”
Surface state density N, m cm-’
0.35 0.12 0.72 0.58
2 x IO-‘” 2 x 10~“’
3 x 10”’
Vacuum
9 x lo-‘* 8 x 10 ‘2
- IO” - 10”
Wet air, ammonia CuSO4 solution
Ambient
TABLE3 Electrical properties Energy position in eV ET -
Ev
EC -
ET
0.54 0.71 0.53 0.66
of surface states on surfaces etched in CP4-A
Effective capture cross section in cm” cr,l/z 2 8 a*,n 2 1
x x x x
Surface states density in crnma 3 4 3 5
10-14 IO-‘1 10-l” IO-13
x x x x
lo”1 1011 10’” 10”’
Ambient
Vacuum Wet air, ammonia Vacuum Ozone
TABLE4 Electrical properties
Energy position in eV ET - Ev EC - ET Ec ~ ET
0.58 0.66 0.64 0.15
of surface states on surfaces etched in CP4-A after y-ray irradiation of 6 x 105 rijntgen Effective capture cross section in cm3 Q/n. 2 0nR 4 (Tna 8 4
x x x x
to-14 10-13 10-13 10-21
Surface state density in cm-’ 1 x 101” 2 x 10’2 2 x lo’?
Ambient
Vacuum Vacuum
ELECTRICAL
PROPERTIES
OF
SURFACE
303
STATES
Rupprecht’s data and ours in the magnitude of the effective capture cross sections of these levels is not as good. The samples etched in CP4-A were irradiated by y-rays of 6 x lo5 rijntgen from 6oCo. The results of pulsed field effect for these surfaces are shown in fig. 2. The electrical properties of surface states observed for these samples are summarized in table 4.
p-type specimen Negatrve pulse A Positive pulse
d
l
x
i:;\
I”
I
1
I
30
1
n-type
I
35 103/T;
specimen
I
I
,
I
4
OK-‘)
Fig. 2. Plots of log(T%&l as a function of reciprocal temperaturefor surfaces irradiated by y-rays.
3.2. CLEAVEDSURFACES The sample preparation, the cleaving apparatus and most of the results for surface states were reported in details elsewhere 4). The electrical properties of surface states on cleaved silicon surfaces after cleaving in air are summarized in table 5. 3.3. SURFACESCOVEREDWITH EPOXYRESIN AC field effect patterns corresponding to Si (+), Si( -) and Si(0) are shown in fig. 3. The Si( +) surface was p-type, the Si(-) surface was n-type and Si(0) surface showed the minimum of the conductance. The conductivity
SHUICHI SATO,
304
SHINJI
KAWAJI
AND
AKIO KOBAYASHI
TABLE 5 Electrical properties of surface states on cleaved silicon surfaces
Energy position ET -
Ev
in eV
Effective capture cross
&a
section in cn+
Surface state
density in cm-”
Ambient
0.38 0.16 0.58
1 X 10-l” 8 x lo-“” 3 x 10-13
2 - 6 X 10’1
Vacuum after cleaving
2 x 10’0
Dry ozone or
0.71 0.31
3 X lo-‘1 2 X 10-17
6 - 8 x 10”)
oxygen
Dry ammonia or wet air
b
Fig. 3. AC fieldeffect patterns for surfaces covered with epoxy resin; (a) Si(+); (b) Si( -); and (c) Si(0).
type of each surface was very stable for long exposures in various ambient gases, for example, wet air, dry ammonia, oxygen and ozone. The electrical properties of surface states for these samples are summarized in table 6. The surface states with the time constant from microseconds to milliseconds vary from specimen to specimen. However, the 0.8 eV level with effective capture cross section of 10 -I6 cm’ and with the time constant from seconds to minutes was observed in all samples. 3.4.
THERMALLY
OXIDIZED
AC field effect patterns
SURFACES
for thermaliy
oxidized
p- and n-type
samples
are
ELECTRICALPROPERTIESOF SURFACESTATES
305
TABLE6 Electrical properties Energy position Samples --
ET -_ Ev
_. ..
in eV ..__.
Si( -) .-
Sit-k)
Si(0)
of surface states on surfaces covered with resin Effective capture cross
section
&a
in cma
Surface state density Ns in cm-2
Short (S) or tong (L) time decay
0.88 0.79 0.73
2 X lo-‘” 5 x IO-‘3 2 X 10-13
* 101’
S
0.8
8 x 10-t’
9 X 10’0
L
0.k 0.79 0.58 0.57
2 5 4 4
N 10”
S
2 x 1011
L
N 10”
S
x IO-15 x lo-‘3 x 10-17 x IO-17
0.8
6 x 10-l”
0.48 0.44 0.51 0.60
j,
4 x 10-1s 2 x IO--‘7 3 X IO-‘6
0.8
9 x lo-‘7
._
-x ;o-_17--
2 x IO’1
L
b
C
d
Fig. 4. AC field effect patterns for surfaces thermally oxidized at high temperature; (a) p-type specimen; (b) n-type specimen, (c) p-type specimen after y-ray irradiation; and (d) n-type specimen after y-ray irradiation.
306
SHUICHI
SATO,
SHINJI
KAWAJI
AND
AK10
KOBAYASHI
ELECTRICAL
PROPERTIES OF SURFACE TABLE
Electrical properties Specimens
of surface states on surfaces oxidized at high temperature Effective capture cross section in cm2
Surface state density in cm-”
ET - Ev
0.64 0.33 0.99
rTr/n 9 x lo-‘3 4 x IO-‘7 2 x 10-12
2 x 101’ 1 x 10’1 8 x 1012
Dry air
EC -
ET
0.46
0na 2 x 10-12
1 x 10”
Dry air
ET - Ev
0.58
cr,,/a 8 x IO-r3
4 x 10’1
Dry air after y-ray irradiation
EC - ET
0.52
0nLY2 x
2 x 10"
-
n
7
Energy position in eV
P
EC -
ET
ET - Ev
;:;;
0.47
307
STATES
lo-.'5
-
CT*&? 4 x IO-13
1 x to-19 0*/a 1 x 10-15
Ambients
Short (S) or long (L) time decay
Dry air
N 10'3
3 x 1011
Dry air after
1 x 10’2
y-ray irradiation
shown in fig. 4. The surface conductivity type of the p-type specimen was converted to n-type after the thermal oxidation, as shown in fig. 4a. After the y-ray irradiation, as shown in figs. 4c,d, the surface conductivity type changed remarkably and the n-type conversion on the surface disappeared, The results obtained in pulsed field effect are shown in fig. 5 and the data are summarized in table 7. 3.5.
SURFACES
WITH
STAIN
FILMS
AC field effect patterns of the samples with stain films are shown in fig. 6. The surface conductivity type (p-type) did not change until the 300°C heat treatment in oxygen gas. But, the surface potential started clearly to go down at temperatures higher than 500 “C and an n-type surface conduction appeared after heat treatments at 800 “C as shown in fig. 6d. Pulsed field effect data for these samples are shown in fig. 7 and are summarized in table 8. In spite of the different heat treatments, a discrete level located at 0.64 eV above the top of the valence band with the density of 4x 1011 cm-’ was always observed in the thermally oxidized surfaces. However, the effective capture cross section for holes of this level depended remarkably on the heat treatments as shown in table 8.
308
SHUICHI
SATO,
SHINJI
RAWAJi
AND
AK10
KOBAYASHI
b
c
Fig. 6. AC field effect patterns for surfaces with stain films heat treated at following temperatures; (a) no heat treatment; (b) heat treatment, 300 “C, 10 hours; (c) heat treatment, 500 “C, 10 hours; and (d) heat treatment, 800 “C, 10 hours.
I
I
30
I
I
I
33
I 3.6
103,/T (OK-‘)
Fig. 7. Plots of iog(T%,)-l as a function of reciprocal temperature for surfaces with stain films without h%at treatment and with heat treatment at 300, 500 and 800 “C.
ELECTRICAL PROPERTtES OF SURFACE STATXS TABLE Electrical Energy
position
ET - Ev in eV
properties
of surface
Effective
capture
cross section c& in cm2
0.66
1 x 10-12
0.64
4 x LO-14
0.64
1 x 10-13
0.64
7 x
309
8
states on surfaces
with srain films
Surface state density m cm_2
Heat treatments in Y
300 4 x 101’
10-l”
500
800
4. Summary and discussion
All data of surface states on silicon surfaces treated in various ways are summarized in table 9 according to their energy positions in the band gap. Here, the levels were classified to donor-like centers, acceptor-like centers or neutral centers according to their cross sections on the basis of that the cross section of a neutral center lie in the order of magnitude of 10-‘5-10- l6 cm2. In the pulsed field effect experiments, whether some surface states can be observed or not depends on the position of the Fermi level at the surface under the experimental conditions. It cannot be concluded only by the absence of the pulsed field effect information that some surface states do not exist. Taking into consideration the above, the surface states which are believed not to exist on some surfaces are denoted by x marks in table 9. Rupprecht explained his results by assuming that only two kinds of surface states, donor and acceptor states, exist on the CP-4 etched surfaces. However, the levels of Er-&==0.54-0.58 eV observed on the cleaved surfaces and the surfaces etched in KOH and immersed in CuSO, solution had the effective capture cross section of 3 x lo-l3 cm2 and 8 x lo-r2 cm’, respectively, for holes. These results show that at least one kind of acceptor levels exists at the energy position of ET-E, =0.54-0.58 eV in addition to the donor-like levels observed in the surfaces etched in CP4-A at the same energy position. From this viewpoint, the levels at I&.- Ev =0,54-0.58 eV observed in the surfaces etched in CP4-A are not identified to the donor-like states observed at EC-ET =0.64-0.66 eV in the surfaces of n-type material etched in CPCA. Similar donor-like or acceptor-like surface states on surfaces etched in CP4-A were also observed in KOH etched and cleaved surfaces under exposure in gases. In the surfaces etched in CP4-A, the surface states of 0.71 eV above the valence band edge could not be observed after y-ray irradiation
310
SHUICHI
SATO,
SHINJI
KAWAJI
TABLE
AND
AK10
KOBAYASHI
9
Summary of surface states on various silicon surfaces 1.2 ev 0 Surface treatments
c_-----
& - Ev EC - ET _______._.-__
0 v 1.2 ev
0.99
CP4-A CP4-A, y-ray irradiation
._,
n
X
Y /
a
X
A
x
D A
X
X
D
A
KOH Cleaved High temp. oxidation
A”)sbAx
X
---1-1High temp. oxidation y-ray irr.
_ L!!
X
X
Stain film
--s
x
--_.
II “l.~.-
A D
n
A
Ir. Acceptor-like states on p-type specimens. C, Acceptor-like states on n-type specimens. ) Donor-like states on p-type specimens. D Donor-like states on n-type specimens. 1 Neutral-like states on p-type specimens.
of 6 x IO5 riintgen. But, the surface states of 0.54 eV above the valence band edge were observed. This can be explained on the basis that the quasi Fermi level at the surface is fixed by the increased number of surface states at 0.54 eV after y-ray irradiation. Similar phenomena were confirmed by y-ray irradiation of the thermally oxidized surfaces. Simultaneously, acceptor levelsof 0.15 eV below the conduction band edge which are very close to the bulk levels536) produced by y-ray irradiation were observed. On thermally oxidized surfaces at high temperature, surface states of 0.71 eV were observed. Accordingly, it is plausible that surface states of 0.54-0.58 eV above the valence band edge be ascribed to surface defects or dislocations. This explanation is supported by the fact that the surface state density in CP4-A etched surfaces was increased one hundred times after y-ray irradjation and
ELECTRICAL
PROPERTIES
OF
SURFACE
STATES
311
that the surface state density in thermally oxidized surfaces was increased about ten times after y-ray irradiation. Moreover, the results on the surface state density show that thermally oxidized films have protective effect for y-ray irradiation. The surface states of 0.38 eV and 0.16 eV above the valence band edge on cleaved silicon surfaces are in good agreement with the data of Allen and Gobeli 2) on the cleaved surfaces in vacuum. The surface state density for these states was about ten times larger than that of etched surfaces. Moreover, the surface state of 0.38 eV above the valence band edge was not observed in all other specimens. Therefore, this level is probably an electronic state due to the surface dangling bonds. When the cleaved surfaces were exposed to various ambient gases as stated before, the surface states of 0.58 eV and 0.71 eV above the valence band edge, which were similar to those found on etched surfaces, were also observed. The properties of surface states show that the structure of the surface states in the surfaces etched in the solution of KOH is similar to that in the cleaved surfaces. Acceptor-like surface states of 0.64 eV above the valence band edge were observed on p-type silicon surfaces which were oxidized at high temperature, on p-type silicon surfaces which were covered with stain films and on p-type silicon surfaces which were thermally oxidized after staining in acid mixture. These donor surface states are new levels which were not observed in any other surfaces etched, cleaved or covered with resin. Moreover, on thermally oxidized surfaces of p-type specimens, donor-like surfaces states of 0.46 eV below the conduction band edge were observed. The magnitude of the decay time constant shows that these surface states exist in silicon-silicon dioxide interface. The n-type conversion due to oxidation at high temperature cannot be explained by only the creation of these surface states. Thus, the positive charges in silicon dioxide is necessary to explain this phenomenons). Surface conductivity type of thermally oxidized p-type samples measured by AC field effect converted from n-type to p-type after y-ray irradiation. Then, acceptor-like surface states of 0.58 eV above the valence band edge and of 0.25 eV, 0.46 eV below the conduction band were observed. The cause of the p-type shift can be speculated on the basis, that negative charges in these new acceptor-like surface states formed in silicon dioxide interface or free Compton electrons produced by interaction between y-rays and the valence electrons neutralize the positive space charges in silicon dioxides. Surface states of 0.25 eV below the conduction band edge are close to bulk levels 5-7) created by y-ray irradiation. In the process of heat treatments of p-type specimens covered with stain films in oxygen gas, the surface conductivity type observed by AC field effect was initially p-type. Then, the conductivity type started to convert into
312
SHUICHI
SATO,
SHINJI
KAWAJI
n-type at about 500 “C and converted high temperature oxidation. However,
AND
AK10
KOBAYASHI
into n-type at 800 “C the same as in the position of surface states did not
change during the heat treatments and it was 0.64 eV above the valence band edge the same as specimens oxidized at high temperature. These results show also the creation of positive chargess) in the silicon dioxide other than the charges in the surface states at the silicon-silicon dioxide interface which convert the surface conductive type from p-type to n-type by thermal oxidation. In the surfaces covered with epoxy resin, the data was scattered from specimen to specimen except the 0.8 eV levels above the valence band edge. The physical nature of these surface states is not well known.
Acknowledgements The authors would like to express their sincere thanks to Prof. 1. Nakata of The Institute for Solid State Physics, The University of Tokyo, for the y-ray irradiation, to Mr. H. Arata of Electrical Communication Laboratory for the oxidation of silicon single crystals.
References 1) See, e.g., P. F. Schmidt and J. E. Sandor, Trans. AIME, 233 (1965) 517. 2) F. G. Allen and G. W. Gobeli, Phys. Rev. 127 (1962) 150; J. Appl. Phys. 35 (1964) 597. 3) G. Rupprecht, J. Phys. Chem. Solids 14 (1960) 208; Ann. N. Y. Acad. Sci. 101 (1963) 960. 4) S. Sato, S. Kawaji and A. Kobayashi, Surface Sci. 3 (1965) 98. 5) G. Bemski, J. Appl. Phys. 30 (1959) 1195. 6) E. Sonder and L. C. Templeton, J. Appl. Phys. 31 (1960) 1279. 7) H. Y. Fann and A. K. Randas, J. Appl. Phys. 30 (1959) 1127. 8) T. E. Thomas, Jr. and D. R. Young, IBM J. Res. Develop. 8 (1964) 369.
SCIENCE
ELECTRICAL
4 (1966) 299-312 0 North-Holland
PROPERTIES SILICON SHUICHI
Department
OF SURFACE
STATES
ON
SURFACES
SATO, and SHINJI
of Physics, Faculty of Science,
Publishing Co., Amsterdam
KAWAJI
Gakushuin University, Mejiro, Tokyo, Japan
and AKIO KOBAYASHI Electrical
~omm~nicut~on Laboratory,
~~sashi~o-sh~,
Tokyo, Japan
Received 14 October 1965 The electrical properties of surface states on practical (“real”) silicon surfaces with various surface treatments were investigated by means of pulsed field effect techniques. On the basis of the experimental results which showed some systematic relations between the physical parameters of the observed surface states and surface treatments, possible physical models for surface states on practical silicon surfaces are discussed.
1. Introduction A numbers of papers have been published on the surface states on silicon single crystals”). The data of silicon surface states on, probably, the best defined surfaces so far available are those of clean surfaces in ultra high vacuum determined from the measurements of the work function and photoelectric threshold by Allen and Gobeli 2). On practical silicon surfaces, the energy position and the effective capture cross section of the surface states were studied in some details by means of the pulsed field effect technique by Rupprecht 3). The present paper is concerned with the observations of electrical properties of surface states of the foIlowing types of silicon (111) surfaces: 1) Surfaces etched in KOH solution and CP4-A; 2) Cleaved surfaces; 3) Surfaces covered with epoxy resin; 4) Thermally oxidized surfaces (high temperature); 5) Surfaces thermally oxidized (low temperature) after staining the surface in acid mixture; 6) Surfaces irradiated by I’-rays of 6oCo after etching in CP4-A and thermal oxidation. 2. Experimental procedure Samples
were rectangular
bars of 11 x 2 x 0.3 mm3 except for cleaving ex299
300
SHUICHI
SATO,
SHINJI
KAWAJI
TABLE
Surface treatments Surfaces
Type
Etched in p CW-A 7.-Etched in KOH
’
Cleaved
p
Covered with resin
P
Oxidation
’ n
Stain films
’
Rem 5 x 102 38 103
AND
AK10
KOBAYASHI
1
for various specimens Surface treatments
~
After lapping, specimens were etched in CP4-A for 6(r120 set at room temperature. -...______ _. ...~ After lapping, specimens were etched in 10% solution of KOH for 120-180 set at 70-100 “C.
After lapping, specimens were sandwiched between two 2 X 102 parallel glass plates and submitted to cleavage in air. The cleaving apparatus was shown in a previous paper 3. After etching in CP4-A, a tantalum field plate separated by thin epoxy resin was attached to the silicon surfaces. The sample was thermally hardened without the application of a DC field between the silicon and the field plate, Si(O), with 5 x 102 a DC field which was polarized as silicon positive, Si(+), and with a DC field polarized as silicon negative, Si(-). The temperature and time for heat treatment to harden resin are 150 “C and 7-10 hours, respectively. 30 50
After etching in CP4-A, samples were heated in wet oxygen gas (1 l/min) at 1 200 “C for 4 hours. Thickness of oxide films was about 5 700 A.
SO-70
After etching in CP4-A, stain films were grown by immersing the samples to 103: HF --I-1: HNOa mixture solution (20 “C) for a few minutes. Some samples were heated in oxygen gas for 10 hours at 300 “C, 500 “C or 800 “C.
periments in which the sample dimension was 8 x 2.5 x 0.8 mm3. The surface treatments are listed in table 1. A tantalum field plate with 2 x lo-‘mm mica spacer was attached on each specimen, then the sample was mounted in a cryostat which was evacuated immediately up to 2 x 10e4 Torr. At first, the surface conductivity type was measured by the large signal AC field effect measurement applying 1 SOOV,_ p at room temperature. The temperature dependence of the relaxation time (r,) in the pulsed field effects was measured under the applied step voltage of 500 V, positive or negative, depending on the surface conductivity type. Energy position below the conduction band edge (EC-ET) or above the valence band edge (E, - E,) and effective capture cross section for holes (a,/~) and electrons (onc() were determined from Rupprecht’s theory. Here, c1is the statistical weight factor of the surface states. The surface state density (NJ was estimated from the saturated conductance increment in the transient behavior of the pulsed field effect.
ELECTRICAL
The measurements
PROPERTIES
were performed
OF
SURFACE
in vacuum,
301
STATES
dry air, dry ammonia,
wet air,
ozone and oxygen gas. 3. Experimental
results
3.1. ETCHED SURFACES Typical results of log(T%,)-’ versus l/T obtained for the surfaces etched in KOH solution are shown in fig. 1. Two discrete levels of 0.35 eV and 0.12 eV above the top of the valence band with effective capture cross sections of 2 x lo-l6 cm’ and 2 x 10-20 cm2, respectively, for holes were determined from the data in vacuum. The densities of these surface states are almost the same and equal to 3 x lOlo cmw2. From the values of the effective capture cross section, the former level is neutral-like and the latter is donor-like. When the sample was exposed to wet air or dry ammonia, a discrete level whose concentration is about 10” cmm2 was observed at 0.72 eV above the top of the valence band with effective capture cross section of 9x lo-” cm2 for holes. This level is acceptor-like. On the surface of an-
x Etch 0 cu so, ~ Wet air or ammonia
103/T
Fig. 1.
Plots of log(T2r&1
(OK-‘)
as a function of reciprocal temperature in KOH solution.
for surfaces etched
302
SHUICHI
other
sample
CuSO,, band
which
a discrete with
was dried
in air
level was observed
effective
its density
SHINJI KAWAJI AND AK10 KOBAYASHI
SATO,
capture
was about
10”
cross
after
immersing
at 0.58 eV above section
cm- 2. This
of 8 x lo-
in the
solution
of
the top of the valence I2 cm2 for
level is acceptor-like.
holes,
The
and
electrical
of these levels are summarized in table 2. Electrical properties of surface states on surfaces etched in CP4-A are summarized in table 3. The two energy levels observed in n-type material and two energy levels observed in p-type material are in good agreement with the values determined by Rupprecht. However, the agreement between properties
TABLE2 Electrical properties
of surface states on surfaces etched in the solution of KOH
Energy position ET ~ Ev in eV
Effective capture cross section 0,)/n in cm”
Surface state density N, m cm-’
0.35 0.12 0.72 0.58
2 x IO-‘” 2 x 10~“’
3 x 10”’
Vacuum
9 x lo-‘* 8 x 10 ‘2
- IO” - 10”
Wet air, ammonia CuSO4 solution
Ambient
TABLE3 Electrical properties Energy position in eV ET -
Ev
EC -
ET
0.54 0.71 0.53 0.66
of surface states on surfaces etched in CP4-A
Effective capture cross section in cm” cr,l/z 2 8 a*,n 2 1
x x x x
Surface states density in crnma 3 4 3 5
10-14 IO-‘1 10-l” IO-13
x x x x
lo”1 1011 10’” 10”’
Ambient
Vacuum Wet air, ammonia Vacuum Ozone
TABLE4 Electrical properties
Energy position in eV ET - Ev EC - ET Ec ~ ET
0.58 0.66 0.64 0.15
of surface states on surfaces etched in CP4-A after y-ray irradiation of 6 x 105 rijntgen Effective capture cross section in cm3 Q/n. 2 0nR 4 (Tna 8 4
x x x x
to-14 10-13 10-13 10-21
Surface state density in cm-’ 1 x 101” 2 x 10’2 2 x lo’?
Ambient
Vacuum Vacuum
ELECTRICAL
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OF
SURFACE
303
STATES
Rupprecht’s data and ours in the magnitude of the effective capture cross sections of these levels is not as good. The samples etched in CP4-A were irradiated by y-rays of 6 x lo5 rijntgen from 6oCo. The results of pulsed field effect for these surfaces are shown in fig. 2. The electrical properties of surface states observed for these samples are summarized in table 4.
p-type specimen Negatrve pulse A Positive pulse
d
l
x
i:;\
I”
I
1
I
30
1
n-type
I
35 103/T;
specimen
I
I
,
I
4
OK-‘)
Fig. 2. Plots of log(T%&l as a function of reciprocal temperaturefor surfaces irradiated by y-rays.
3.2. CLEAVEDSURFACES The sample preparation, the cleaving apparatus and most of the results for surface states were reported in details elsewhere 4). The electrical properties of surface states on cleaved silicon surfaces after cleaving in air are summarized in table 5. 3.3. SURFACESCOVEREDWITH EPOXYRESIN AC field effect patterns corresponding to Si (+), Si( -) and Si(0) are shown in fig. 3. The Si( +) surface was p-type, the Si(-) surface was n-type and Si(0) surface showed the minimum of the conductance. The conductivity
SHUICHI SATO,
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TABLE 5 Electrical properties of surface states on cleaved silicon surfaces
Energy position ET -
Ev
in eV
Effective capture cross
&a
section in cn+
Surface state
density in cm-”
Ambient
0.38 0.16 0.58
1 X 10-l” 8 x lo-“” 3 x 10-13
2 - 6 X 10’1
Vacuum after cleaving
2 x 10’0
Dry ozone or
0.71 0.31
3 X lo-‘1 2 X 10-17
6 - 8 x 10”)
oxygen
Dry ammonia or wet air
b
Fig. 3. AC fieldeffect patterns for surfaces covered with epoxy resin; (a) Si(+); (b) Si( -); and (c) Si(0).
type of each surface was very stable for long exposures in various ambient gases, for example, wet air, dry ammonia, oxygen and ozone. The electrical properties of surface states for these samples are summarized in table 6. The surface states with the time constant from microseconds to milliseconds vary from specimen to specimen. However, the 0.8 eV level with effective capture cross section of 10 -I6 cm’ and with the time constant from seconds to minutes was observed in all samples. 3.4.
THERMALLY
OXIDIZED
AC field effect patterns
SURFACES
for thermaliy
oxidized
p- and n-type
samples
are
ELECTRICALPROPERTIESOF SURFACESTATES
305
TABLE6 Electrical properties Energy position Samples --
ET -_ Ev
_. ..
in eV ..__.
Si( -) .-
Sit-k)
Si(0)
of surface states on surfaces covered with resin Effective capture cross
section
&a
in cma
Surface state density Ns in cm-2
Short (S) or tong (L) time decay
0.88 0.79 0.73
2 X lo-‘” 5 x IO-‘3 2 X 10-13
* 101’
S
0.8
8 x 10-t’
9 X 10’0
L
0.k 0.79 0.58 0.57
2 5 4 4
N 10”
S
2 x 1011
L
N 10”
S
x IO-15 x lo-‘3 x 10-17 x IO-17
0.8
6 x 10-l”
0.48 0.44 0.51 0.60
j,
4 x 10-1s 2 x IO--‘7 3 X IO-‘6
0.8
9 x lo-‘7
._
-x ;o-_17--
2 x IO’1
L
b
C
d
Fig. 4. AC field effect patterns for surfaces thermally oxidized at high temperature; (a) p-type specimen; (b) n-type specimen, (c) p-type specimen after y-ray irradiation; and (d) n-type specimen after y-ray irradiation.
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ELECTRICAL
PROPERTIES OF SURFACE TABLE
Electrical properties Specimens
of surface states on surfaces oxidized at high temperature Effective capture cross section in cm2
Surface state density in cm-”
ET - Ev
0.64 0.33 0.99
rTr/n 9 x lo-‘3 4 x IO-‘7 2 x 10-12
2 x 101’ 1 x 10’1 8 x 1012
Dry air
EC -
ET
0.46
0na 2 x 10-12
1 x 10”
Dry air
ET - Ev
0.58
cr,,/a 8 x IO-r3
4 x 10’1
Dry air after y-ray irradiation
EC - ET
0.52
0nLY2 x
2 x 10"
-
n
7
Energy position in eV
P
EC -
ET
ET - Ev
;:;;
0.47
307
STATES
lo-.'5
-
CT*&? 4 x IO-13
1 x to-19 0*/a 1 x 10-15
Ambients
Short (S) or long (L) time decay
Dry air
N 10'3
3 x 1011
Dry air after
1 x 10’2
y-ray irradiation
shown in fig. 4. The surface conductivity type of the p-type specimen was converted to n-type after the thermal oxidation, as shown in fig. 4a. After the y-ray irradiation, as shown in figs. 4c,d, the surface conductivity type changed remarkably and the n-type conversion on the surface disappeared, The results obtained in pulsed field effect are shown in fig. 5 and the data are summarized in table 7. 3.5.
SURFACES
WITH
STAIN
FILMS
AC field effect patterns of the samples with stain films are shown in fig. 6. The surface conductivity type (p-type) did not change until the 300°C heat treatment in oxygen gas. But, the surface potential started clearly to go down at temperatures higher than 500 “C and an n-type surface conduction appeared after heat treatments at 800 “C as shown in fig. 6d. Pulsed field effect data for these samples are shown in fig. 7 and are summarized in table 8. In spite of the different heat treatments, a discrete level located at 0.64 eV above the top of the valence band with the density of 4x 1011 cm-’ was always observed in the thermally oxidized surfaces. However, the effective capture cross section for holes of this level depended remarkably on the heat treatments as shown in table 8.
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b
c
Fig. 6. AC field effect patterns for surfaces with stain films heat treated at following temperatures; (a) no heat treatment; (b) heat treatment, 300 “C, 10 hours; (c) heat treatment, 500 “C, 10 hours; and (d) heat treatment, 800 “C, 10 hours.
I
I
30
I
I
I
33
I 3.6
103,/T (OK-‘)
Fig. 7. Plots of iog(T%,)-l as a function of reciprocal temperature for surfaces with stain films without h%at treatment and with heat treatment at 300, 500 and 800 “C.
ELECTRICAL PROPERTtES OF SURFACE STATXS TABLE Electrical Energy
position
ET - Ev in eV
properties
of surface
Effective
capture
cross section c& in cm2
0.66
1 x 10-12
0.64
4 x LO-14
0.64
1 x 10-13
0.64
7 x
309
8
states on surfaces
with srain films
Surface state density m cm_2
Heat treatments in Y
300 4 x 101’
10-l”
500
800
4. Summary and discussion
All data of surface states on silicon surfaces treated in various ways are summarized in table 9 according to their energy positions in the band gap. Here, the levels were classified to donor-like centers, acceptor-like centers or neutral centers according to their cross sections on the basis of that the cross section of a neutral center lie in the order of magnitude of 10-‘5-10- l6 cm2. In the pulsed field effect experiments, whether some surface states can be observed or not depends on the position of the Fermi level at the surface under the experimental conditions. It cannot be concluded only by the absence of the pulsed field effect information that some surface states do not exist. Taking into consideration the above, the surface states which are believed not to exist on some surfaces are denoted by x marks in table 9. Rupprecht explained his results by assuming that only two kinds of surface states, donor and acceptor states, exist on the CP-4 etched surfaces. However, the levels of Er-&==0.54-0.58 eV observed on the cleaved surfaces and the surfaces etched in KOH and immersed in CuSO, solution had the effective capture cross section of 3 x lo-l3 cm2 and 8 x lo-r2 cm’, respectively, for holes. These results show that at least one kind of acceptor levels exists at the energy position of ET-E, =0.54-0.58 eV in addition to the donor-like levels observed in the surfaces etched in CP4-A at the same energy position. From this viewpoint, the levels at I&.- Ev =0,54-0.58 eV observed in the surfaces etched in CP4-A are not identified to the donor-like states observed at EC-ET =0.64-0.66 eV in the surfaces of n-type material etched in CPCA. Similar donor-like or acceptor-like surface states on surfaces etched in CP4-A were also observed in KOH etched and cleaved surfaces under exposure in gases. In the surfaces etched in CP4-A, the surface states of 0.71 eV above the valence band edge could not be observed after y-ray irradiation
310
SHUICHI
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9
Summary of surface states on various silicon surfaces 1.2 ev 0 Surface treatments
c_-----
& - Ev EC - ET _______._.-__
0 v 1.2 ev
0.99
CP4-A CP4-A, y-ray irradiation
._,
n
X
Y /
a
X
A
x
D A
X
X
D
A
KOH Cleaved High temp. oxidation
A”)sbAx
X
---1-1High temp. oxidation y-ray irr.
_ L!!
X
X
Stain film
--s
x
--_.
II “l.~.-
A D
n
A
Ir. Acceptor-like states on p-type specimens. C, Acceptor-like states on n-type specimens. ) Donor-like states on p-type specimens. D Donor-like states on n-type specimens. 1 Neutral-like states on p-type specimens.
of 6 x IO5 riintgen. But, the surface states of 0.54 eV above the valence band edge were observed. This can be explained on the basis that the quasi Fermi level at the surface is fixed by the increased number of surface states at 0.54 eV after y-ray irradiation. Similar phenomena were confirmed by y-ray irradiation of the thermally oxidized surfaces. Simultaneously, acceptor levelsof 0.15 eV below the conduction band edge which are very close to the bulk levels536) produced by y-ray irradiation were observed. On thermally oxidized surfaces at high temperature, surface states of 0.71 eV were observed. Accordingly, it is plausible that surface states of 0.54-0.58 eV above the valence band edge be ascribed to surface defects or dislocations. This explanation is supported by the fact that the surface state density in CP4-A etched surfaces was increased one hundred times after y-ray irradjation and
ELECTRICAL
PROPERTIES
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SURFACE
STATES
311
that the surface state density in thermally oxidized surfaces was increased about ten times after y-ray irradiation. Moreover, the results on the surface state density show that thermally oxidized films have protective effect for y-ray irradiation. The surface states of 0.38 eV and 0.16 eV above the valence band edge on cleaved silicon surfaces are in good agreement with the data of Allen and Gobeli 2) on the cleaved surfaces in vacuum. The surface state density for these states was about ten times larger than that of etched surfaces. Moreover, the surface state of 0.38 eV above the valence band edge was not observed in all other specimens. Therefore, this level is probably an electronic state due to the surface dangling bonds. When the cleaved surfaces were exposed to various ambient gases as stated before, the surface states of 0.58 eV and 0.71 eV above the valence band edge, which were similar to those found on etched surfaces, were also observed. The properties of surface states show that the structure of the surface states in the surfaces etched in the solution of KOH is similar to that in the cleaved surfaces. Acceptor-like surface states of 0.64 eV above the valence band edge were observed on p-type silicon surfaces which were oxidized at high temperature, on p-type silicon surfaces which were covered with stain films and on p-type silicon surfaces which were thermally oxidized after staining in acid mixture. These donor surface states are new levels which were not observed in any other surfaces etched, cleaved or covered with resin. Moreover, on thermally oxidized surfaces of p-type specimens, donor-like surfaces states of 0.46 eV below the conduction band edge were observed. The magnitude of the decay time constant shows that these surface states exist in silicon-silicon dioxide interface. The n-type conversion due to oxidation at high temperature cannot be explained by only the creation of these surface states. Thus, the positive charges in silicon dioxide is necessary to explain this phenomenons). Surface conductivity type of thermally oxidized p-type samples measured by AC field effect converted from n-type to p-type after y-ray irradiation. Then, acceptor-like surface states of 0.58 eV above the valence band edge and of 0.25 eV, 0.46 eV below the conduction band were observed. The cause of the p-type shift can be speculated on the basis, that negative charges in these new acceptor-like surface states formed in silicon dioxide interface or free Compton electrons produced by interaction between y-rays and the valence electrons neutralize the positive space charges in silicon dioxides. Surface states of 0.25 eV below the conduction band edge are close to bulk levels 5-7) created by y-ray irradiation. In the process of heat treatments of p-type specimens covered with stain films in oxygen gas, the surface conductivity type observed by AC field effect was initially p-type. Then, the conductivity type started to convert into
312
SHUICHI
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n-type at about 500 “C and converted high temperature oxidation. However,
AND
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into n-type at 800 “C the same as in the position of surface states did not
change during the heat treatments and it was 0.64 eV above the valence band edge the same as specimens oxidized at high temperature. These results show also the creation of positive chargess) in the silicon dioxide other than the charges in the surface states at the silicon-silicon dioxide interface which convert the surface conductive type from p-type to n-type by thermal oxidation. In the surfaces covered with epoxy resin, the data was scattered from specimen to specimen except the 0.8 eV levels above the valence band edge. The physical nature of these surface states is not well known.
Acknowledgements The authors would like to express their sincere thanks to Prof. 1. Nakata of The Institute for Solid State Physics, The University of Tokyo, for the y-ray irradiation, to Mr. H. Arata of Electrical Communication Laboratory for the oxidation of silicon single crystals.
References 1) See, e.g., P. F. Schmidt and J. E. Sandor, Trans. AIME, 233 (1965) 517. 2) F. G. Allen and G. W. Gobeli, Phys. Rev. 127 (1962) 150; J. Appl. Phys. 35 (1964) 597. 3) G. Rupprecht, J. Phys. Chem. Solids 14 (1960) 208; Ann. N. Y. Acad. Sci. 101 (1963) 960. 4) S. Sato, S. Kawaji and A. Kobayashi, Surface Sci. 3 (1965) 98. 5) G. Bemski, J. Appl. Phys. 30 (1959) 1195. 6) E. Sonder and L. C. Templeton, J. Appl. Phys. 31 (1960) 1279. 7) H. Y. Fann and A. K. Randas, J. Appl. Phys. 30 (1959) 1127. 8) T. E. Thomas, Jr. and D. R. Young, IBM J. Res. Develop. 8 (1964) 369.