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The influence of chemisorption upon the electrical properties of germanium surfaces
SURFACE SCIENCE 2 (1964) 56-63; ~ ) North-Holland Publishing Co., Amsterdam
t
THE INFLUENCE OF CHEMISORPTION PROPERTIES
UPON THE ELECTRICAL
OF GERMANIUM
SURFACES
M. J. SPARNAAY, A. H. BOONSTRA and J. VAN RULER Philips Research Laboratories, N. V. Philips' Gloeilampenfabrieken, Eindhoven, The Netherlands
The resistance and the field effect of thin, pure Ge single crystal cylinders, pulled in the {111} direction, were measured and the influence of 02 and NHa was studied. The diameter of the Ge crystal could be reduced from about 1 mm to about 0.05 mm by means of a gas etching procedure. In separate experiments, using crushed Ge crystals, chemical reactions were studied, which might occur between the Ge surface and the same gaseous ambients as those applied in the electrical measurements. It appeared that at pressures below 10.9 Torr the surface space charge is weakly positive and that there are about 1015 surface states per cm 2. On addition of a number of (polar) gases (pressures 10-7-10 -5 Torr) the space charge becomes more positive. More gas addition led for a number of gases to a less positive space charge. Chemisorption usually leads to a decrease of the surface state density. In a number of cases, chemisorption reactions provide new gaseous products. Some of the first results of the study of these chemisorption reactions are communicated.
1. Introduction - E x p e r i m e n t s
The c o m b i n e d s t u d y o f electrical a n d chemical p r o p e r t i e s o f semicond u c t o r surfaces m a y lead to a better u n d e r s t a n d i n g t h a n the study o f these p r o p e r t i e s separately 1). This c o n s i d e r a t i o n p r o m p t e d the following experiments. The surface c o n d u c t a n c e 2) a n d the field effect 3) o f cleaned and o f p u r p o s e l y c o n t a m i n a t e d 4) G e surfaces were m e a s u r e d b y using wire-shaped, c o m p l e t e l y outgassed single crystals m o s t l y o f intrinsic conductivity type. The direction o f the core o f this " w i r e " was n o r m a l to the { l l l } - p l a n e s . The crystals were m o u n t e d vertically in a glass or d o u b l e - w a l l e d q u a r t z v a c u u m e q u i p m e n t (fig. 1). I n o r d e r to be able to c a r r y out electrical measurements, b o t h ends were p r o v i d e d with o h m i c contacts connected t h r o u g h sealed-in wires with the electronic e q u i p m e n t . The v a c u u m system contained a c o m p l e t e l y outgassed cylinder s h a p e d Pt-electrode (radius 0.5 cm) which was p l a c e d concentrically a r o u n d the central p a r t o f the G e single crystal. F i e l d effects were m e a s u r e d after a p p l y i n g a p o t e n t i a l difference varying f r o m a b o u t 1 k V to 10 k V o f either sign between the Pt-electrode and the G e crystal. The length o f the G e cylinder was 25 c m a n d the r a d i u s o f the 56
THEINFLUENCEOF CHEMISORPTION
57
central part (length l about 5 cm) could be made 25 microns by means of an oxygen gas etching technique: The central part was heated to 700°C by means of a cylindrical furnace placed concentrically outside the vacuum
l
f __~.
r-rconddc~ance r I
I_A_[
vacuum(~Egtorr) or gas
F- ---Tq
" ~ l ~l Ge sing/ecrystal
~@ ~ elec'~rode II II ti _~_'~2~Jfur~oce(zoo°c) 2Ge+0270 O°c 26e0 /
Fig. I. Apparatus used. Schematic representation. system around the crystal; oxygen (10-2 Torr) was then introduced resulting in the formation of volatile GeO. The initial value of the radius was 1 mm. Evacuation to a pressure of 10 .9 Torr and less, and cooling at r o o m temperature (at which all the electrical measurements were done) resulted in a surface, which we call a "clean" surface for the following reason: I f the same treatment was applied to Ge crystals, crushed in air, it appeared that their surface had the s a m e properties concerning oxygen uptake as those described by Green et al. s) for crystals crushed in high vacuum. The resistance of the whole crystal can be written as: f~ = ~ o + f 2 c
(1)
where f2 o is the contribution of the outer parts and ~2c that of the central part. Apart from surface effects, the values of ~2o and ~ are inversely proportional to the square of the radius r of the pertinent part of the Ge- "wire" and it is seen that, if the central part becomes thin with respect to the outer parts, we have ~2~ >> O o . In practical cases, n o was of the order of 50 k~2, and ~2~ was varied between 200 and 1000 k~. The measurements pertain only to the thin central part of the Ge crystal. The ohmic contacts were at about 10 cm distance away from this central part and were only outgassed together with the whole apparatus at the beginning of a set of experiments. Contaminations from this side are not to be expected. The field effect and the effects of added gases upon the conductance of the Ge crystal should increase with decreasing radius of the central part. The effect of the radius is given in the following equations, (2) to (12), which also served as a basis for the interpretations o f our results.
58
M.J. SPARNAAY et al.
In the following we mainly consider f~o and regard fa o as a given constant. The conductance A~ = ~2c-1 of the central part can be written as: 2rcr 2 ( 1 + r 2 b ] A~ = --/- ~ 2 2~]
(2)
where 2~ and J~b a r e the specific conductivities of surface and bulk. Note, that they have different dimensions. For 2 b we write: (3)
2b = # q ( p + b n )
where p and n are the free electron and hole concentrations in numbers per unit volume, # is the hole mobility, #b the electron mobility and q the charge of a hole. For 2~ we write: 2 s = #~q(ps+b~n~)
(4)
where ps and n~ are the free electron- and hole concentrations in the surface region in numbers per unit area and #~ and #~b~ are the hole and electron mobilities respectively. Their values are, in practical cases, lower than the bulk values by an amount of 10 to 50~*. For a further interpretation we assume, that charges in t h e s u r f a c e states have zero mobility and that n s andps are wholly given by space charge parameters. For a pure crystal with intrinsic conductivity, this case will be mainly considered, one has n = p and space charge theory gives: n~ = A ( e q~'/2kr- 1)
(5)
p~ = A ( e - q o / 2 k r - 1)
where A = x/(enkT/2rc), e = 16 is the dielectric constant of Ge, k is Boltzmann's constant, T the absolute temperature and ~ is the electrostatic potential at the surface. The total amount of band bending is given by minus ~. The Debye-Hiickel length of Ge is less than 1 micron. Since r g 100 micron, the curvature of the surface can be neglected in the calculation of n, and Ps- Application of an electric potential (VR -- Vr) to the platinum cylinder and the Ge-wire causes them to behave as a condenser with charge + Q on the Pt and - Q on Ge. We have: VR -- Vr = 2Q In R .
l
(6)
r
The charge density on Ge is: =
(7)
-
2~rl * Schrieffer correction; see Semiconductor phia, 1957).
Surface Physics, Ed. K i n g s t o n (Philadel-
59
T H E I N F L U E N C E OF C H E M I S O R P T I O N
which we can also write as:
= %+%
(8)
where o-ssis the amount of charge per unit area, located in surface states, and a~c is that located in the space charge. Owing to the application of the transverse field, there is a change of the conductance which we write as: A,4c = 2rcj A2s. 1
(9)
The change A 2~, will be brought about by a change of ~, which we assume the only parameter affected by the transverse field. Since
a~c = q(ps-n~)
(10)
we have, for the high potential approximation [~1 >> kT/q
Aa~¢-
q- a~A~
2k T
(~9 > 0 and ~ <
0)
(11)
(O>0)-
(12)
and A2~ =2~T2sAO
(O< 0); A2 s = +q2~A0 2kT
The experiments confirmed the calculations on this point, and in particular the r-dependence was found as predicted. Activated or ionized gas atoms and molecules, as formed by ionization gauges, often lead to an anomalous increase of the conductance of the Ge sample, although the gas pressure may be 10 -s Torr or less. The conductance was measured as a function of the presence of various gaseous ambients: 02, NH3, H20, HzS, Clz, H2, N 2, CO, A, Hg, CHaOH and some other organic vapours. The field effect could only be one meosured after re-evacuation, which imposes a serious restriction. in ether vacuum systems, adsorption isotherms were measured and chemisorption properties of these ambients were studied. For this purpose use was made of crushed Ge crystals having a total surface area of about 10 a cm z. Prior to measurements, these samples were given the same thermal treatment in oxygen atmosphere followed by evacuation as given the single crystal. If, with the crushed Ge samples, it was found that a gaseous ambient showed a chemical reaction with the Ge surface at a temperature, different from room temperature, the single crystal wire was also heated to that temperature in the presence of that ambient. By this method data were obtained of an electrical nature and also of a chemical nature which applied to fairly similar surfaces. However, an unknown factor was still that crushed crystals
60
M.J. SPARNAAYet al.
have not only crystal planes normal to the (111) direction exposed to the influence of ambients but also any other crystal plane.
2. Results and discussion The field effect is given by A2~[2~; from which Aa~/G~ocan be derived. It is seen from fig. 2, that this ratio is _+ 0.1 at most. Since aso is of the order cy
-0"001~J 0.005~-
K ,
0.010~ 0.02O[-
-o.,Io 0"1f 0.2 I
Time-'-~O
"I
2
- lOkV
3rnin 0
I
1
OkV I-20kV [ 2 3rain 0 1
I
OkV~- Potent/of 2 3m;n
Fig. 2. Field effect Aa/cr ( = A2s/ZB in the paper) versus applied potential: a) below 10 -s T o r t oxygen pressure (left part of fig. 2). b) exposure to 10 -6 Tort oxygen pressure (middle part o f fig. 2) followed by pumping, e) exposure to 10 -1 Torr oxygen pressure (right part of fig. 2) followed by pumping.
of 101° q per cm 2, Ao-sc is 10 9 q at most. If VR--V, = 0, then Q = 0 and -as~ = aso. If the value of VR- V, is of the order of 10 kV, then under the experimental conditions, a is of the order of 1012 q per c m 2. In other words, most of the induced charges are residing in the surface states. This is true both for negative and for positive values of V i i - Vr. Therefore it is possible to store either 1012 negative, or 10 t2 positive charges per cm 2 in the surface states. Assuming k types of accepter states, (k may be unity) state density of the i-th state Nal, energy EAI, and l types of donor states, (l may be unity) state density of the j-th state NDj and energy EDj, then:
61
THE INFLUENCE OF CHEMISORPTION
O'Ss ~
q
t~ k u=l i=k
= -- 2 i=1
- NA, -EF+(Em-qO ) 1 + exp kT
+2
j=l I +expEF--(EDj--qO)J kT
(13)
j=k tYAi "4- ~
O'Dj
j=l
where E F is the Fermi energy of the electrons. We have assumed that each state can only trap one electron (or hole). For Ao-ss, the change of the charge in the states as a result of the application of the transverse field, one has:
kT U = l l + e x p + E v - ( E a i - q~) kT
J=a 1 +exp - EF + (EDj - qO)J
kT (14)
where (q/kT)AO is immediately given by the field effect. The value of the energies (EF-Em) and (EF-EDi) is an unknown factor. We assume these energies, rather, arbitrarily, at a few times kT, but our lack of precise knowledge is the main reason, why the data given below are of a qualitative nature. The specific resistivity of our samples prior to any heat treatment was higher than 20 ~ cm (p-type or also n-type conductivity). The measured effects were in that case entirely similar for all the samples. Results with samples of lower specific resistivity will be given in a later publication. The results obtained with high resistivity samples lend support to the assumption that the heat treatment does not lead to an important reduction of the bulk resistivity. With Ao-~ = _ 0.3 x 1012 q per cm2; ( V k - V r = + i0 kV, r ~ 10 .3 cm) and - qAO/kT = _+ 1.5x 10 .3 , the bracketed part of eq. (14) is of the order of 1014 per cm 2 and the total state density is about 1015 per cm z. This is so for our "clean" surfaces. The absolute value of ~ was given by the conductance measurements alone and was deduced from the conductance pressure curves given in fig. 3. Handler 6) was the first to give curves of this shape for the influence of oxygen upon Ge surfaces cleaned by the Farnsworth 7) (ion bombardment) * technique. A conductance minimum (not shown here in fig. 3) can be obtained if much oxygen, or also mixtures of oxygen and ammonia, are added. This minimum was taken as indicating intrinsic conductivity up to the surface. Then, for other (higher) values of the conductance the analysis was carried out for a large * It should be stressed that an ion bombarded surface does not necessarily have the same properties as the surface dealt with in this paper. Thus, the space charge in the latter case appears to be less positive than in the former.
62
M.J. SPARNAAYet al.
number of values of the diameter of the thin central part of the crystal. This analysis was described by two of us some years ago z) and will not be i 3.0
,umho 2.5
Fig. 3. Conductance against oxygen pressure (drawn curve) and against ammonia pressure (dotted curve) of a single crystal Ge (111) wire. The arrows indicate that the pressure is first increased and then, at 10-1 Torr, decreased by pumping. The part of the curve marked "pumping" points to the irreversibility of the effect. For ammonia the adsorption is somewhat more reversible than for oxygen.
f
2.0
1.5
pump!ng -7
-6
-5
-~
-3
% -2
-1
repeated here. The results were, that the space charge near the "clean" surface was weakly p-type (0 = - 7 5 mV). If, at r o o m temperature, oxygen was added, the p-type conductivity increased (0 ~ - 120 mV) and decreased after further gas addition to almost intrinsic at 10 -1 Tort. Subsequent pumping did not lead to a conductivity change (see curve marked "pumping"). After 24 hours the conductivity was still unaltered. This is in agreement with the well-known fact that oxygen is chemisorbed and forms a surface oxide which is not volatile at r o o m temperature. A rather similar conductance versus pressure curve was found for NH3 additions. Only the part marked " p u m p i n g " was notably different, indicating that now at least some of the adsorbed N H 3 molecules could be removed by pumping. A number of other gaseous ambients showed the same behavior as N H 3 . Exceptions were Nz, Hz and A. These gases were inacti'~e (apart from a false ionization gauge effect, see above). The addition of CO and of Hg vapor at a pressure of 10 - s Torr or more led to an increased p-type conductivity, no decrease being observed up to pressures of 10 - 4 Tort. Fig. 2 also gives the field effect for Ge surfaces contaminated with Oz at 10 -6 Torr (2b) and 10 - t T o r t (2c). It is inferred from the given curves that the fast state density decreases to about 10 t2 per cmz and that slow states are formed after further addition. Other gases also lead to slow state formation, for example C12 at r o o m temperature. On the whole the fast state density appeared to decrease upon chemisorption. This statement must be con-
THE INFLUENCE OF CHEMISORPTION
63
sidered with some reserve, all the necessary data not yet being available. Although notably for NH3, H20, H2S and CH3OH more or less the same conductance vs pressure curves were found, the adsorptive properties s) of these ambients were rather different especially at elevated temperatures. For NH3 about one molecule per 3 Ge surface atoms remained present after pumping. At elevated temperatures ( > 100°C) hydrogen is released, the activation energy being of the order of 20 kcal per mole. An "activated complex" { G % - ( N H y ) } may have formed (3 < x < 10 and 0 < y < 3). The disintegration reaction consists of various stages as revealed by the rate of H 2 formation at various temperatures. The water case was investigated previously 9). Judging from the H2 formation rate, the activation energy is about 20 kcal per mole and a surface oxide, similar to that obtained with 02, is formed. The situation for CH3OH (adsorption at room temp. about: CH3OH per 2 Ge surface atoms) was found to be very complicated because the composition of the gases formed by the disintegration reaction at elevated temperatures depends strongly on this temperature. High temperatures promote the formation of a greater variety of gaseous products. For H/S which has a tendency to be adsorbed to the amount of 1 molecule per 2 Geatoms the activation energy of the disintegration reaction (H2 is again released at 100°C or higher) is only about 3 kcal/mole. All these compounds may, at room temperature, form "activated complexes". The velocity with which they adsorb is decreased if the surface is oxidized at an oxygen pressure of 10 .2 Torr, but at room temperature the final amount adsorbed is nearly independent of the state of oxidation of the surface. The "activated complexes" may involve not only the outermost Ge atoms, but the second or the third layer inside, it is possible that only the effective dielectric constant is important for the state energies. This would explain their rather non-specific role with respect to their influence upon the band bending.
References 1) w. H. Brattain, Introductory Remarks to the lind Conf. of Semiconductor Surfaces, J. Phys. Chem. Solids 14 (1960) VIII. 2) M. J. Sparnaay and J. van Ruler, Physica 27 (1962) 153. 3) A. H. Boonstra, J. van Ruler and M. J. Sparnaay, Proc. Kon. Ned. Akd. Wetenschap B66 (1963) 70. 4) A. H. Boonstra, J. van Ruler and M. J. Sparnaay, ibid B66 (1963) 64. 5) M. Green, A. Kafalas and P. H. Robinson, Semiconductor Surface Physics, Ed. Kingston (Philadelphia, 1957) p. 349. 6) P. Handler, Bull. Am. Phys. Soc. 1 No. 3 (1956) 144. 7) P. Handler, Semiconductor Surface Physics, Ed. Kingston (Philadelphia, 1957) p. 23. R. Missman and P. Handler, J. Phys. Chem. Solids 8 (1958) 109. R. E. Schlier and H. E. Farnsworth, Semiconductor Surface Physics, Ed. Kingston (Philadelphia, 1957) p. 3. 8) M. Green, J. Phys. Chem. Solids 14 (1960) 77. A. H. Boonstra and J. van Ruler, to be published. 9) M. J. Sparnaay, Ann. N. Y. Acad. Sci. I01 (1963) 973,
t
THE INFLUENCE OF CHEMISORPTION PROPERTIES
UPON THE ELECTRICAL
OF GERMANIUM
SURFACES
M. J. SPARNAAY, A. H. BOONSTRA and J. VAN RULER Philips Research Laboratories, N. V. Philips' Gloeilampenfabrieken, Eindhoven, The Netherlands
The resistance and the field effect of thin, pure Ge single crystal cylinders, pulled in the {111} direction, were measured and the influence of 02 and NHa was studied. The diameter of the Ge crystal could be reduced from about 1 mm to about 0.05 mm by means of a gas etching procedure. In separate experiments, using crushed Ge crystals, chemical reactions were studied, which might occur between the Ge surface and the same gaseous ambients as those applied in the electrical measurements. It appeared that at pressures below 10.9 Torr the surface space charge is weakly positive and that there are about 1015 surface states per cm 2. On addition of a number of (polar) gases (pressures 10-7-10 -5 Torr) the space charge becomes more positive. More gas addition led for a number of gases to a less positive space charge. Chemisorption usually leads to a decrease of the surface state density. In a number of cases, chemisorption reactions provide new gaseous products. Some of the first results of the study of these chemisorption reactions are communicated.
1. Introduction - E x p e r i m e n t s
The c o m b i n e d s t u d y o f electrical a n d chemical p r o p e r t i e s o f semicond u c t o r surfaces m a y lead to a better u n d e r s t a n d i n g t h a n the study o f these p r o p e r t i e s separately 1). This c o n s i d e r a t i o n p r o m p t e d the following experiments. The surface c o n d u c t a n c e 2) a n d the field effect 3) o f cleaned and o f p u r p o s e l y c o n t a m i n a t e d 4) G e surfaces were m e a s u r e d b y using wire-shaped, c o m p l e t e l y outgassed single crystals m o s t l y o f intrinsic conductivity type. The direction o f the core o f this " w i r e " was n o r m a l to the { l l l } - p l a n e s . The crystals were m o u n t e d vertically in a glass or d o u b l e - w a l l e d q u a r t z v a c u u m e q u i p m e n t (fig. 1). I n o r d e r to be able to c a r r y out electrical measurements, b o t h ends were p r o v i d e d with o h m i c contacts connected t h r o u g h sealed-in wires with the electronic e q u i p m e n t . The v a c u u m system contained a c o m p l e t e l y outgassed cylinder s h a p e d Pt-electrode (radius 0.5 cm) which was p l a c e d concentrically a r o u n d the central p a r t o f the G e single crystal. F i e l d effects were m e a s u r e d after a p p l y i n g a p o t e n t i a l difference varying f r o m a b o u t 1 k V to 10 k V o f either sign between the Pt-electrode and the G e crystal. The length o f the G e cylinder was 25 c m a n d the r a d i u s o f the 56
THEINFLUENCEOF CHEMISORPTION
57
central part (length l about 5 cm) could be made 25 microns by means of an oxygen gas etching technique: The central part was heated to 700°C by means of a cylindrical furnace placed concentrically outside the vacuum
l
f __~.
r-rconddc~ance r I
I_A_[
vacuum(~Egtorr) or gas
F- ---Tq
" ~ l ~l Ge sing/ecrystal
~@ ~ elec'~rode II II ti _~_'~2~Jfur~oce(zoo°c) 2Ge+0270 O°c 26e0 /
Fig. I. Apparatus used. Schematic representation. system around the crystal; oxygen (10-2 Torr) was then introduced resulting in the formation of volatile GeO. The initial value of the radius was 1 mm. Evacuation to a pressure of 10 .9 Torr and less, and cooling at r o o m temperature (at which all the electrical measurements were done) resulted in a surface, which we call a "clean" surface for the following reason: I f the same treatment was applied to Ge crystals, crushed in air, it appeared that their surface had the s a m e properties concerning oxygen uptake as those described by Green et al. s) for crystals crushed in high vacuum. The resistance of the whole crystal can be written as: f~ = ~ o + f 2 c
(1)
where f2 o is the contribution of the outer parts and ~2c that of the central part. Apart from surface effects, the values of ~2o and ~ are inversely proportional to the square of the radius r of the pertinent part of the Ge- "wire" and it is seen that, if the central part becomes thin with respect to the outer parts, we have ~2~ >> O o . In practical cases, n o was of the order of 50 k~2, and ~2~ was varied between 200 and 1000 k~. The measurements pertain only to the thin central part of the Ge crystal. The ohmic contacts were at about 10 cm distance away from this central part and were only outgassed together with the whole apparatus at the beginning of a set of experiments. Contaminations from this side are not to be expected. The field effect and the effects of added gases upon the conductance of the Ge crystal should increase with decreasing radius of the central part. The effect of the radius is given in the following equations, (2) to (12), which also served as a basis for the interpretations o f our results.
58
M.J. SPARNAAY et al.
In the following we mainly consider f~o and regard fa o as a given constant. The conductance A~ = ~2c-1 of the central part can be written as: 2rcr 2 ( 1 + r 2 b ] A~ = --/- ~ 2 2~]
(2)
where 2~ and J~b a r e the specific conductivities of surface and bulk. Note, that they have different dimensions. For 2 b we write: (3)
2b = # q ( p + b n )
where p and n are the free electron and hole concentrations in numbers per unit volume, # is the hole mobility, #b the electron mobility and q the charge of a hole. For 2~ we write: 2 s = #~q(ps+b~n~)
(4)
where ps and n~ are the free electron- and hole concentrations in the surface region in numbers per unit area and #~ and #~b~ are the hole and electron mobilities respectively. Their values are, in practical cases, lower than the bulk values by an amount of 10 to 50~*. For a further interpretation we assume, that charges in t h e s u r f a c e states have zero mobility and that n s andps are wholly given by space charge parameters. For a pure crystal with intrinsic conductivity, this case will be mainly considered, one has n = p and space charge theory gives: n~ = A ( e q~'/2kr- 1)
(5)
p~ = A ( e - q o / 2 k r - 1)
where A = x/(enkT/2rc), e = 16 is the dielectric constant of Ge, k is Boltzmann's constant, T the absolute temperature and ~ is the electrostatic potential at the surface. The total amount of band bending is given by minus ~. The Debye-Hiickel length of Ge is less than 1 micron. Since r g 100 micron, the curvature of the surface can be neglected in the calculation of n, and Ps- Application of an electric potential (VR -- Vr) to the platinum cylinder and the Ge-wire causes them to behave as a condenser with charge + Q on the Pt and - Q on Ge. We have: VR -- Vr = 2Q In R .
l
(6)
r
The charge density on Ge is: =
(7)
-
2~rl * Schrieffer correction; see Semiconductor phia, 1957).
Surface Physics, Ed. K i n g s t o n (Philadel-
59
T H E I N F L U E N C E OF C H E M I S O R P T I O N
which we can also write as:
= %+%
(8)
where o-ssis the amount of charge per unit area, located in surface states, and a~c is that located in the space charge. Owing to the application of the transverse field, there is a change of the conductance which we write as: A,4c = 2rcj A2s. 1
(9)
The change A 2~, will be brought about by a change of ~, which we assume the only parameter affected by the transverse field. Since
a~c = q(ps-n~)
(10)
we have, for the high potential approximation [~1 >> kT/q
Aa~¢-
q- a~A~
2k T
(~9 > 0 and ~ <
0)
(11)
(O>0)-
(12)
and A2~ =2~T2sAO
(O< 0); A2 s = +q2~A0 2kT
The experiments confirmed the calculations on this point, and in particular the r-dependence was found as predicted. Activated or ionized gas atoms and molecules, as formed by ionization gauges, often lead to an anomalous increase of the conductance of the Ge sample, although the gas pressure may be 10 -s Torr or less. The conductance was measured as a function of the presence of various gaseous ambients: 02, NH3, H20, HzS, Clz, H2, N 2, CO, A, Hg, CHaOH and some other organic vapours. The field effect could only be one meosured after re-evacuation, which imposes a serious restriction. in ether vacuum systems, adsorption isotherms were measured and chemisorption properties of these ambients were studied. For this purpose use was made of crushed Ge crystals having a total surface area of about 10 a cm z. Prior to measurements, these samples were given the same thermal treatment in oxygen atmosphere followed by evacuation as given the single crystal. If, with the crushed Ge samples, it was found that a gaseous ambient showed a chemical reaction with the Ge surface at a temperature, different from room temperature, the single crystal wire was also heated to that temperature in the presence of that ambient. By this method data were obtained of an electrical nature and also of a chemical nature which applied to fairly similar surfaces. However, an unknown factor was still that crushed crystals
60
M.J. SPARNAAYet al.
have not only crystal planes normal to the (111) direction exposed to the influence of ambients but also any other crystal plane.
2. Results and discussion The field effect is given by A2~[2~; from which Aa~/G~ocan be derived. It is seen from fig. 2, that this ratio is _+ 0.1 at most. Since aso is of the order cy
-0"001~J 0.005~-
K ,
0.010~ 0.02O[-
-o.,Io 0"1f 0.2 I
Time-'-~O
"I
2
- lOkV
3rnin 0
I
1
OkV I-20kV [ 2 3rain 0 1
I
OkV~- Potent/of 2 3m;n
Fig. 2. Field effect Aa/cr ( = A2s/ZB in the paper) versus applied potential: a) below 10 -s T o r t oxygen pressure (left part of fig. 2). b) exposure to 10 -6 Tort oxygen pressure (middle part o f fig. 2) followed by pumping, e) exposure to 10 -1 Torr oxygen pressure (right part of fig. 2) followed by pumping.
of 101° q per cm 2, Ao-sc is 10 9 q at most. If VR--V, = 0, then Q = 0 and -as~ = aso. If the value of VR- V, is of the order of 10 kV, then under the experimental conditions, a is of the order of 1012 q per c m 2. In other words, most of the induced charges are residing in the surface states. This is true both for negative and for positive values of V i i - Vr. Therefore it is possible to store either 1012 negative, or 10 t2 positive charges per cm 2 in the surface states. Assuming k types of accepter states, (k may be unity) state density of the i-th state Nal, energy EAI, and l types of donor states, (l may be unity) state density of the j-th state NDj and energy EDj, then:
61
THE INFLUENCE OF CHEMISORPTION
O'Ss ~
q
t~ k u=l i=k
= -- 2 i=1
- NA, -EF+(Em-qO ) 1 + exp kT
+2
j=l I +expEF--(EDj--qO)J kT
(13)
j=k tYAi "4- ~
O'Dj
j=l
where E F is the Fermi energy of the electrons. We have assumed that each state can only trap one electron (or hole). For Ao-ss, the change of the charge in the states as a result of the application of the transverse field, one has:
kT U = l l + e x p + E v - ( E a i - q~) kT
J=a 1 +exp - EF + (EDj - qO)J
kT (14)
where (q/kT)AO is immediately given by the field effect. The value of the energies (EF-Em) and (EF-EDi) is an unknown factor. We assume these energies, rather, arbitrarily, at a few times kT, but our lack of precise knowledge is the main reason, why the data given below are of a qualitative nature. The specific resistivity of our samples prior to any heat treatment was higher than 20 ~ cm (p-type or also n-type conductivity). The measured effects were in that case entirely similar for all the samples. Results with samples of lower specific resistivity will be given in a later publication. The results obtained with high resistivity samples lend support to the assumption that the heat treatment does not lead to an important reduction of the bulk resistivity. With Ao-~ = _ 0.3 x 1012 q per cm2; ( V k - V r = + i0 kV, r ~ 10 .3 cm) and - qAO/kT = _+ 1.5x 10 .3 , the bracketed part of eq. (14) is of the order of 1014 per cm 2 and the total state density is about 1015 per cm z. This is so for our "clean" surfaces. The absolute value of ~ was given by the conductance measurements alone and was deduced from the conductance pressure curves given in fig. 3. Handler 6) was the first to give curves of this shape for the influence of oxygen upon Ge surfaces cleaned by the Farnsworth 7) (ion bombardment) * technique. A conductance minimum (not shown here in fig. 3) can be obtained if much oxygen, or also mixtures of oxygen and ammonia, are added. This minimum was taken as indicating intrinsic conductivity up to the surface. Then, for other (higher) values of the conductance the analysis was carried out for a large * It should be stressed that an ion bombarded surface does not necessarily have the same properties as the surface dealt with in this paper. Thus, the space charge in the latter case appears to be less positive than in the former.
62
M.J. SPARNAAYet al.
number of values of the diameter of the thin central part of the crystal. This analysis was described by two of us some years ago z) and will not be i 3.0
,umho 2.5
Fig. 3. Conductance against oxygen pressure (drawn curve) and against ammonia pressure (dotted curve) of a single crystal Ge (111) wire. The arrows indicate that the pressure is first increased and then, at 10-1 Torr, decreased by pumping. The part of the curve marked "pumping" points to the irreversibility of the effect. For ammonia the adsorption is somewhat more reversible than for oxygen.
f
2.0
1.5
pump!ng -7
-6
-5
-~
-3
% -2
-1
repeated here. The results were, that the space charge near the "clean" surface was weakly p-type (0 = - 7 5 mV). If, at r o o m temperature, oxygen was added, the p-type conductivity increased (0 ~ - 120 mV) and decreased after further gas addition to almost intrinsic at 10 -1 Tort. Subsequent pumping did not lead to a conductivity change (see curve marked "pumping"). After 24 hours the conductivity was still unaltered. This is in agreement with the well-known fact that oxygen is chemisorbed and forms a surface oxide which is not volatile at r o o m temperature. A rather similar conductance versus pressure curve was found for NH3 additions. Only the part marked " p u m p i n g " was notably different, indicating that now at least some of the adsorbed N H 3 molecules could be removed by pumping. A number of other gaseous ambients showed the same behavior as N H 3 . Exceptions were Nz, Hz and A. These gases were inacti'~e (apart from a false ionization gauge effect, see above). The addition of CO and of Hg vapor at a pressure of 10 - s Torr or more led to an increased p-type conductivity, no decrease being observed up to pressures of 10 - 4 Tort. Fig. 2 also gives the field effect for Ge surfaces contaminated with Oz at 10 -6 Torr (2b) and 10 - t T o r t (2c). It is inferred from the given curves that the fast state density decreases to about 10 t2 per cmz and that slow states are formed after further addition. Other gases also lead to slow state formation, for example C12 at r o o m temperature. On the whole the fast state density appeared to decrease upon chemisorption. This statement must be con-
THE INFLUENCE OF CHEMISORPTION
63
sidered with some reserve, all the necessary data not yet being available. Although notably for NH3, H20, H2S and CH3OH more or less the same conductance vs pressure curves were found, the adsorptive properties s) of these ambients were rather different especially at elevated temperatures. For NH3 about one molecule per 3 Ge surface atoms remained present after pumping. At elevated temperatures ( > 100°C) hydrogen is released, the activation energy being of the order of 20 kcal per mole. An "activated complex" { G % - ( N H y ) } may have formed (3 < x < 10 and 0 < y < 3). The disintegration reaction consists of various stages as revealed by the rate of H 2 formation at various temperatures. The water case was investigated previously 9). Judging from the H2 formation rate, the activation energy is about 20 kcal per mole and a surface oxide, similar to that obtained with 02, is formed. The situation for CH3OH (adsorption at room temp. about: CH3OH per 2 Ge surface atoms) was found to be very complicated because the composition of the gases formed by the disintegration reaction at elevated temperatures depends strongly on this temperature. High temperatures promote the formation of a greater variety of gaseous products. For H/S which has a tendency to be adsorbed to the amount of 1 molecule per 2 Geatoms the activation energy of the disintegration reaction (H2 is again released at 100°C or higher) is only about 3 kcal/mole. All these compounds may, at room temperature, form "activated complexes". The velocity with which they adsorb is decreased if the surface is oxidized at an oxygen pressure of 10 .2 Torr, but at room temperature the final amount adsorbed is nearly independent of the state of oxidation of the surface. The "activated complexes" may involve not only the outermost Ge atoms, but the second or the third layer inside, it is possible that only the effective dielectric constant is important for the state energies. This would explain their rather non-specific role with respect to their influence upon the band bending.
References 1) w. H. Brattain, Introductory Remarks to the lind Conf. of Semiconductor Surfaces, J. Phys. Chem. Solids 14 (1960) VIII. 2) M. J. Sparnaay and J. van Ruler, Physica 27 (1962) 153. 3) A. H. Boonstra, J. van Ruler and M. J. Sparnaay, Proc. Kon. Ned. Akd. Wetenschap B66 (1963) 70. 4) A. H. Boonstra, J. van Ruler and M. J. Sparnaay, ibid B66 (1963) 64. 5) M. Green, A. Kafalas and P. H. Robinson, Semiconductor Surface Physics, Ed. Kingston (Philadelphia, 1957) p. 349. 6) P. Handler, Bull. Am. Phys. Soc. 1 No. 3 (1956) 144. 7) P. Handler, Semiconductor Surface Physics, Ed. Kingston (Philadelphia, 1957) p. 23. R. Missman and P. Handler, J. Phys. Chem. Solids 8 (1958) 109. R. E. Schlier and H. E. Farnsworth, Semiconductor Surface Physics, Ed. Kingston (Philadelphia, 1957) p. 3. 8) M. Green, J. Phys. Chem. Solids 14 (1960) 77. A. H. Boonstra and J. van Ruler, to be published. 9) M. J. Sparnaay, Ann. N. Y. Acad. Sci. I01 (1963) 973,