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Metallurgical boundary properties and electrophysical properties of molybdenum-silicon junctions
Thin SolM Films, 45 (1977) 87 93 Elsevier Sequoia S.A., Lausanne --Printcd in the Ncthcrlands
87
METALLURGICAL BOUNDARY PROPERTIES AND ELECTROPHYSICAL PROPERTIES OF MOLYBDENUM-SILICON JUNCTIONS* L. N. ALEKSANDROV, A. E. GERSHINSKII, R. N. LOVYAGIN, P. A. SIMONOV, B. I. FOMIN AND E. I. CHEREPOV
Institute of Semiconductor Physics, Siberian Branch of the Academy of Sciences of the U.S.S.R., Novosibirsk (U.S.S.R.) (Received April 12, 1977; accepted April 12, 1977)
An electrochemical investigation of the transition layer at the interface between condensed Mo and a silicon single crystal was carried out. The phase MoSi 2 was observed to be present and its thickness was determined from the amount of electricity necessary for its anodic dissolution. On heating n-type silicon at high temperature (1300 °C) in a vacuum of approximately 10 - 9 Torr and then coating with Mo a layer of p-type silicon was observed. We examined the electrical properties of the MoSi2-n-Si contact. Mesadiodes were shown to have current-voltage and capacitance-voltage characteristics close to ideal (within the Bethe theory of electron emission), the MoSi 2 thickness being not more than 500 A. The barrier height, determined using various methods, was 0.69-0.71 V for n-Si with p = 7.5 1) cm and 0.65 V for n-Si with p = 0.2 12 cm. The influence of treatments of the Si surface before MoSi 2 formation and of the method of mesa-structure preparation on the electrical characteristics of MoSi2-n-Si contacts is discussed.
1. INTRODUCTION In order to achieve an ohmic or blocking metal-semiconductor contact, materials should be selected so that the metal work function is, respectively, less than or greater than that of the semiconductor. This consideration is often inapplicable for several reasons, the screening effect of the surface being the major one. Furthermore, the condition is only valid when the metal work function and the semiconductor work function remain constant. This is in fact not the case and the end result after the formation of a metal-semiconductor contact cannot be predicted. In many cases calculation indicates a negative height for the barrier (which means an ohmic contact) but in practice the contact possesses rectifying properties. The metal work function ~M is known to be slightly sensitive to the state of the metal surface. For example, a value of 4.3 eV is quoted 1 for molybdenum and ref. 2 gives a value of 4.01 eV for the electron affinity qz of silicon. If the work function of molybdenum remains constant when the metal is placed in contact with silicon (and provided that the value of the electron affinity is also invariable) the *Paper presentedat the International Conferenceon Metallurgical Coatings, San Francisco, California, U.S.A., March 28 April I, 1977.
8{~
L.N. ALEKSANDROV e l
al.
maximum permissible height q@B, of the barrier is expected to be 0.29 eV. Experimental values 3 for the Mo-Si barrier range from 0.56 eV to 0.75 eV. This difference can be accounted for if the molybdenum work function increases to 4.57-4.76 eV. (According to Milnes and Foyht z the work function of Mo varies between 4.08 and 4.48 eV.) In addition, the formation of silicides MoSi z on the Mo-Si contact is possible at specific temperatures; these silicides may vary in composition and thickness depending on the experimental conditions. It is therefore necessary to study the properties of these interlayers : the nature of their conductivity, their work function, their structure and their stability.
2. EXPERIMENT
Mo-n-Si and Mo-p-Si structures were produced by evaporation of a molybdenum wire by passing an electric current through it or by sublimation of a molybdenum anode by heating with an electron beam. A ribbon-shaped tantalum cathode 15 lain thick that was heated by a high stability current served as the electron-emitting source. This circuit allowed the process to be easily controlled : it permitted the temperature of the anode to be measured and thus the rate of film growth to be monitored. In this way a good reproducibility of the results was ensured. The experiments were carried out in a dry vacuum system evacuated by an oil diffusion pump. A vacuum of l 0 - s Torr was attained after evacuation by heating the working volume, by cooling it to room temperature and by charging the traps with liquid nitrogen. Gettering of oxygen was also carried out. The partial pressures of the residual gases were approximately 5 x 10- ~1 Torr of oxygen and approximately l0 -1° Torr of hydrocarbon compounds. In order to measure the true
5 I.
3
2
,.~. ~...,,.
"o
2
•
1
0
5
10
T5
20
t I~ (rnin I~ )
Fig. 1. Growth with time of the MoSi 2 phase at various temperatures: curve 1,525 C ; curve 2,550 C : curve 3, 575 C ; curve 4, 600 C. Curve 2' shows the rate of disappearance of Mo at the temperature of annealing (550 C ) .
Mo-Si JUNCTIONS
89
properties of the metal-semiconductor contact it is necessary to prepare the semiconductor surface thoroughly before metal spraying. Most chemical preparation methods do not allow removal of the layer of oxide 20 A thick that exists on the surface of the silicon. We used 20.5 mm × 0.5 mm silicon plates, oriented in the (111) plane, which had been previously degreased and then subjected to thermal etching in vacuum at 1300 °C by passage of an electric current. As a result of annealing an atomically smooth surface of silicon was obtained, with a low number of etching pits and a low content of carbide particles, ready for the Mo coating to be deposited. On heating n-Si a p layer was observed to form over the entire surface to a depth ranging from 0.5 to 1 lam, as found earlier by other authors (see, for example, ref. 4). We identified annealing conditions that preserved the conductivity type of the surface layer. After deposition of a molybdenum layer 0.1-0.2 lam thick on the silicon a further annealing was carried out at 525-600 °C to form silicides. The thickness of the molybdenum and silicide layers was determined electrochemically from the quantity of electricity consumed in anodic dissolution. The same method was employed to determine the chemical composition of the silicides s-v. The results of the electrochemical examination of the samples are presented in Fig. 1 8. One phase, MoSi 2, is seen to form in the Mo film-Si single crystal system in the temperature range 525-600 °C. This phase follows the parabolic law throughout the annealing. Thus to prepare samples with a layer of the MoSi 2 phase of a specific thickness we annealed the samples in vacuum under conditions chosen in agreement with the data in Fig. 1. The properties of the metal silicon transition layers were investigated using current-voltage and capacitance-voltage characteristics; the heights q~B, and q~Bpof the barrier were obtained from photoelectric measurements. Structures 0.4 mm 2 were prepared in two ways: (1) by selectively etching only the metal and silicide in a chemical etchant; (2) by mesa-etching 1.5-2 lam of silicon. Indium was employed as the lower contact. 3. RESULTS
We carried out an investigation of the properties of the molybdenum-silicon contact with silicides of various thicknesses. The resistivity of the silicon substrates was 7.5 and 0.2-0.3 f2 cm for the n-type layers and 0.1 and 10 ~ cm for the p-type layers. Figure 2 shows a straight portion on the characteristic for diodes prepared on the molybdenum-n-Si base (p = 7.5 f2 cm) by mesa-etching. This characteristic is practically the same as those for the diodes with silicide layers of various thicknesses (150 A, 200 A, 1600 A) as well as with no silicide layer at all. In addition, the measured characteristics obey perfectly the analytical dependence of the forward current on the applied bias given by the thermionic emission theory, in which the value of the current depends only on the height of the barrier. For Uv > 3kT/q the forward current is given by the expression
[
[qtl l
where A** is Richardson's constant (for our calculations its value was taken to be 120 A c m -2 K -2 (for free electrons)) and UF is the forward bias. The value of the current extrapolated to zero voltage corresponds to the saturation current Js, and
90
L . N . A L E K S A N D R O V t'l
al.
the height of the barrier is determined from the relation
kT
A**T 2
It is readily seen that the value of@sn is not very sensitive to the choice of the value of A**. The height of the barrier was estimated to be 0.69 eV. The theoretical dependence for the reverse branch of the current in the case of a bias UR > 3kT/q is described by the equation JR
[ q~Bol
= A**TZexpl-~-/exp~
fq(qo~/4ne~)"z~
kT
;
where
~ is the silicon permittivity and N D the donor concentration. The experimental IR(UR) dependences are shown in Fig. 3. I0 0
291K ,3
10-I -g3K
lo-1
,I
~ r~...~,4 ~
/
i
i0-2
u
.~
-,~ lo-~
ii
~ _
//I
~ ~o-J
;
u~
/ ~./.
'
7
/
/
10-z"
1
r!
IO-5
io-5 0
01
0.2
0.3
0./,
0
0.2
Forward ,*~ltage bias
Ferward ~,~tage bins U F I V )
(a)
01
0.3 UF
0 ¢, [V)
(b)
Fig. 2. F o r w a r d c u r r e n t d e n s i t y as a f u n c t i o n o f the a p p l i e d v o l t a g e in d i o d e s o f the f o l l o w i n g s t r u c t u r e s : c u r v e 1, 3000 A M o , 150 A M o S i 2 ; c u r v e 2, M o - S i ; c u r v e 3, M o , 200 A M o S i 2 : c u r v e 4, M o . 1600 A MoSi 2 : - - , theory.
Mo-Si JUNCTIONS
91
10-2__ 5~ 10-2
293K ~ - /
4
~]
~
I
~0-3~_
,9,K
}/ 3
.. ~0-3 10-~
= ~ 0 _ s ~ ====='=" 10-6 ~"
i0-~
I
Reverse voltage mas
I 10-1
102
101
!
101
102
Reverse voltage b~as UR (V)
U R (V)
(a)
(b)
Fig. 3. Reverse current density as a function of the applied voltage.
The mesa-diodes prepared from structures of Mo-n-Si or molybdenum, a thin (less than 50 nm) silicide interlayer and then silicon have reverse d.c. characteristics which obey the equation or are very close to the theoretical dependence until they break down. Structures with thick molybdenum silicide interlayers (less than 150 nm) have reverse d.c. characteristics with a rapid increase in current in the case of a reverse bias of more than 3 V. The dependence of I / C 2 on Uin Fig, 4 allows us to determine the height ~B, of 50 5Q ~
2g& K
f u
%
,o
o -4
k -3
-2
Reverse voltage
-1 UR
0 (V)
Fig. 4. The dependence of I/C 2 on the applied voltage.
the barrier and the donor concentration in silicon adjacent to the interface. The height of the barrier was estimated from the relation
qbB. = Ui-.I-uB w k T - A ~ q
L.N. ALEKSANDROVet al.
92
where Ui is the intersection on the voltage axis when the curve is extrapolated to values of C approaching infinity, while qU, is the distance between the Fermi level and the bottom of the conduction band. The expected decrease A¢,in the height of the barrier is comparable with the value ofkT/q; therefore in order to find qSan it is sufficient to measure U~ accurately and to calculate U,. The height of the barrier determined in this way was found to be close to 0.7 V. From photoelectric measurements we find that ¢~B. = 0.71 _+0.03 V. Figure 5 shows the dependence of
!
10-2
_
_
tO-3 -o
v
10-/"
10-5 10-6 0.01
03
10
Reverse voltage bias
10 UR
100
(V)
Fig. 5. The voltage dependence of the reverse current density for structures manufactured by selective etching and by mesa-etching.
the reverse current density on the applied voltage for the structures that were manufactured by mesa-etching (curve 1) and by selective etching (curve 2). For the diodes manufactured on silicon with resistivity 0.2 C) cm the height ~an of the barrier was found to be 0.65 eV, the saturation current density J~ being 10 - 4 A cm - 2 and the thickness of the MoSi 2 being 15 nm. The reverse current density with U R - - 10 - 2 V was 2.5 x 10 - 4 A c m - 2 for the diode with 15 nm of silicide and 5.5 × 10 -4 A cm -2 for the diode with 280 nm of silicide. Further increase in the thickness of the MoSi 2 led to a substantial increase in both the reverse current and the forward current. We investigated the molybdenum-silicon contact using alloyed boron with a resistivity of 0.1 and 10 fl cm. In all cases, whatever the temperature of the substrate during metal spraying and irrespective of the thickness of the MoSi 2, non-rectifying contacts were obtained. We recorded the current-voltage characteristics for the Mo-p-Si contact and we studied the noise parameters at room temperature and at the temperature of liquid nitrogen. There was no 1If noise. The equivalent noise power was less than 15 nV, which corresponds to the noise power of the amplifier. The resistance of a contact 1 cm z in area was 17.5 x 10 -2 ~. 4. DISCUSSION AND CONCLUSIONS The electrophysical properties of the contact are influenced decisively by the condition of the interface and the thickness of the MoSi 2 interlayer between the metal and the silicon above 0.2 ktm. This phenomenon may be explained as follows.
Mo-Si JUNCTIONS
93
The formation and movement of structural defects in the course of the annealing necessary to ensure the formation of a silicide layer of defined thickness causes a reduction in the charge carrier lifetime in the region of spraying (additional heating being unnecessary) which leads to better characteristics of the junctions. On an atomically smooth surface of p-Si molybdenum forms a non-rectifying, almost ohmic, contact, while on n-Si (p = 0.20 f~ cm) a barrier 0.7 V high is formed. The characteristics of the structures are affected by the etching method and the depth of etching. In selective etching of metal, high peripheral currents that result from the residual silicide remaining after etching and also from the stress fields and silicon dislocations in the vicinity of the metal electrode are observed. With mesaetching the properties of the contacts are improved since the silicides and the regions of deformation in the silicon are eliminated. REFERENCES 1 1. K. Kikoin (ed.), Critical Tables Reference Book, Atomizdat, Moscow, 1976. 2 A. Milns and D. Foyht, Heterojunctions and Metal-Semiconductor Junctions, Academic Press, New York, 1972; Russian transl. Mir, Moscow, 1976. 3 V.I. Strikha (ed.), Semiconductor Instruments with Schottky Barrier, Sovjetskoje Radio, Moscow, 1974. 4 F . G . Allen, T. M. Buck and J. T. Law, J. AppL Phys., 31 (1960) 979. 5 A.E. Gershinskii, A. A. Khoromenko and E. I. Cherepov. Phys. Status Solidi A, 31 (1975) 61. 6 B.I. Fomin, A. E. Gershinskii, E. I. Cherepov and F. L. Edelman, Phys. Status Solidi A, 36 (1976) 89. 7 B.I. Fomin, A. E. Gershinskii, E. I. Cherepov and F. L. Edelman, Phys. Chem. Metal Working, 6 (1976) 77. 8 B.I. Fomin, A. E. Gershinskii and E. I. Cherepov, Investigation of phase growth kinetics during interaction of Si single crystals and Mo thin films, Talanta, 24 (1977) 192.
87
METALLURGICAL BOUNDARY PROPERTIES AND ELECTROPHYSICAL PROPERTIES OF MOLYBDENUM-SILICON JUNCTIONS* L. N. ALEKSANDROV, A. E. GERSHINSKII, R. N. LOVYAGIN, P. A. SIMONOV, B. I. FOMIN AND E. I. CHEREPOV
Institute of Semiconductor Physics, Siberian Branch of the Academy of Sciences of the U.S.S.R., Novosibirsk (U.S.S.R.) (Received April 12, 1977; accepted April 12, 1977)
An electrochemical investigation of the transition layer at the interface between condensed Mo and a silicon single crystal was carried out. The phase MoSi 2 was observed to be present and its thickness was determined from the amount of electricity necessary for its anodic dissolution. On heating n-type silicon at high temperature (1300 °C) in a vacuum of approximately 10 - 9 Torr and then coating with Mo a layer of p-type silicon was observed. We examined the electrical properties of the MoSi2-n-Si contact. Mesadiodes were shown to have current-voltage and capacitance-voltage characteristics close to ideal (within the Bethe theory of electron emission), the MoSi 2 thickness being not more than 500 A. The barrier height, determined using various methods, was 0.69-0.71 V for n-Si with p = 7.5 1) cm and 0.65 V for n-Si with p = 0.2 12 cm. The influence of treatments of the Si surface before MoSi 2 formation and of the method of mesa-structure preparation on the electrical characteristics of MoSi2-n-Si contacts is discussed.
1. INTRODUCTION In order to achieve an ohmic or blocking metal-semiconductor contact, materials should be selected so that the metal work function is, respectively, less than or greater than that of the semiconductor. This consideration is often inapplicable for several reasons, the screening effect of the surface being the major one. Furthermore, the condition is only valid when the metal work function and the semiconductor work function remain constant. This is in fact not the case and the end result after the formation of a metal-semiconductor contact cannot be predicted. In many cases calculation indicates a negative height for the barrier (which means an ohmic contact) but in practice the contact possesses rectifying properties. The metal work function ~M is known to be slightly sensitive to the state of the metal surface. For example, a value of 4.3 eV is quoted 1 for molybdenum and ref. 2 gives a value of 4.01 eV for the electron affinity qz of silicon. If the work function of molybdenum remains constant when the metal is placed in contact with silicon (and provided that the value of the electron affinity is also invariable) the *Paper presentedat the International Conferenceon Metallurgical Coatings, San Francisco, California, U.S.A., March 28 April I, 1977.
8{~
L.N. ALEKSANDROV e l
al.
maximum permissible height q@B, of the barrier is expected to be 0.29 eV. Experimental values 3 for the Mo-Si barrier range from 0.56 eV to 0.75 eV. This difference can be accounted for if the molybdenum work function increases to 4.57-4.76 eV. (According to Milnes and Foyht z the work function of Mo varies between 4.08 and 4.48 eV.) In addition, the formation of silicides MoSi z on the Mo-Si contact is possible at specific temperatures; these silicides may vary in composition and thickness depending on the experimental conditions. It is therefore necessary to study the properties of these interlayers : the nature of their conductivity, their work function, their structure and their stability.
2. EXPERIMENT
Mo-n-Si and Mo-p-Si structures were produced by evaporation of a molybdenum wire by passing an electric current through it or by sublimation of a molybdenum anode by heating with an electron beam. A ribbon-shaped tantalum cathode 15 lain thick that was heated by a high stability current served as the electron-emitting source. This circuit allowed the process to be easily controlled : it permitted the temperature of the anode to be measured and thus the rate of film growth to be monitored. In this way a good reproducibility of the results was ensured. The experiments were carried out in a dry vacuum system evacuated by an oil diffusion pump. A vacuum of l 0 - s Torr was attained after evacuation by heating the working volume, by cooling it to room temperature and by charging the traps with liquid nitrogen. Gettering of oxygen was also carried out. The partial pressures of the residual gases were approximately 5 x 10- ~1 Torr of oxygen and approximately l0 -1° Torr of hydrocarbon compounds. In order to measure the true
5 I.
3
2
,.~. ~...,,.
"o
2
•
1
0
5
10
T5
20
t I~ (rnin I~ )
Fig. 1. Growth with time of the MoSi 2 phase at various temperatures: curve 1,525 C ; curve 2,550 C : curve 3, 575 C ; curve 4, 600 C. Curve 2' shows the rate of disappearance of Mo at the temperature of annealing (550 C ) .
Mo-Si JUNCTIONS
89
properties of the metal-semiconductor contact it is necessary to prepare the semiconductor surface thoroughly before metal spraying. Most chemical preparation methods do not allow removal of the layer of oxide 20 A thick that exists on the surface of the silicon. We used 20.5 mm × 0.5 mm silicon plates, oriented in the (111) plane, which had been previously degreased and then subjected to thermal etching in vacuum at 1300 °C by passage of an electric current. As a result of annealing an atomically smooth surface of silicon was obtained, with a low number of etching pits and a low content of carbide particles, ready for the Mo coating to be deposited. On heating n-Si a p layer was observed to form over the entire surface to a depth ranging from 0.5 to 1 lam, as found earlier by other authors (see, for example, ref. 4). We identified annealing conditions that preserved the conductivity type of the surface layer. After deposition of a molybdenum layer 0.1-0.2 lam thick on the silicon a further annealing was carried out at 525-600 °C to form silicides. The thickness of the molybdenum and silicide layers was determined electrochemically from the quantity of electricity consumed in anodic dissolution. The same method was employed to determine the chemical composition of the silicides s-v. The results of the electrochemical examination of the samples are presented in Fig. 1 8. One phase, MoSi 2, is seen to form in the Mo film-Si single crystal system in the temperature range 525-600 °C. This phase follows the parabolic law throughout the annealing. Thus to prepare samples with a layer of the MoSi 2 phase of a specific thickness we annealed the samples in vacuum under conditions chosen in agreement with the data in Fig. 1. The properties of the metal silicon transition layers were investigated using current-voltage and capacitance-voltage characteristics; the heights q~B, and q~Bpof the barrier were obtained from photoelectric measurements. Structures 0.4 mm 2 were prepared in two ways: (1) by selectively etching only the metal and silicide in a chemical etchant; (2) by mesa-etching 1.5-2 lam of silicon. Indium was employed as the lower contact. 3. RESULTS
We carried out an investigation of the properties of the molybdenum-silicon contact with silicides of various thicknesses. The resistivity of the silicon substrates was 7.5 and 0.2-0.3 f2 cm for the n-type layers and 0.1 and 10 ~ cm for the p-type layers. Figure 2 shows a straight portion on the characteristic for diodes prepared on the molybdenum-n-Si base (p = 7.5 f2 cm) by mesa-etching. This characteristic is practically the same as those for the diodes with silicide layers of various thicknesses (150 A, 200 A, 1600 A) as well as with no silicide layer at all. In addition, the measured characteristics obey perfectly the analytical dependence of the forward current on the applied bias given by the thermionic emission theory, in which the value of the current depends only on the height of the barrier. For Uv > 3kT/q the forward current is given by the expression
[
[qtl l
where A** is Richardson's constant (for our calculations its value was taken to be 120 A c m -2 K -2 (for free electrons)) and UF is the forward bias. The value of the current extrapolated to zero voltage corresponds to the saturation current Js, and
90
L . N . A L E K S A N D R O V t'l
al.
the height of the barrier is determined from the relation
kT
A**T 2
It is readily seen that the value of@sn is not very sensitive to the choice of the value of A**. The height of the barrier was estimated to be 0.69 eV. The theoretical dependence for the reverse branch of the current in the case of a bias UR > 3kT/q is described by the equation JR
[ q~Bol
= A**TZexpl-~-/exp~
fq(qo~/4ne~)"z~
kT
;
where
~ is the silicon permittivity and N D the donor concentration. The experimental IR(UR) dependences are shown in Fig. 3. I0 0
291K ,3
10-I -g3K
lo-1
,I
~ r~...~,4 ~
/
i
i0-2
u
.~
-,~ lo-~
ii
~ _
//I
~ ~o-J
;
u~
/ ~./.
'
7
/
/
10-z"
1
r!
IO-5
io-5 0
01
0.2
0.3
0./,
0
0.2
Forward ,*~ltage bias
Ferward ~,~tage bins U F I V )
(a)
01
0.3 UF
0 ¢, [V)
(b)
Fig. 2. F o r w a r d c u r r e n t d e n s i t y as a f u n c t i o n o f the a p p l i e d v o l t a g e in d i o d e s o f the f o l l o w i n g s t r u c t u r e s : c u r v e 1, 3000 A M o , 150 A M o S i 2 ; c u r v e 2, M o - S i ; c u r v e 3, M o , 200 A M o S i 2 : c u r v e 4, M o . 1600 A MoSi 2 : - - , theory.
Mo-Si JUNCTIONS
91
10-2__ 5~ 10-2
293K ~ - /
4
~]
~
I
~0-3~_
,9,K
}/ 3
.. ~0-3 10-~
= ~ 0 _ s ~ ====='=" 10-6 ~"
i0-~
I
Reverse voltage mas
I 10-1
102
101
!
101
102
Reverse voltage b~as UR (V)
U R (V)
(a)
(b)
Fig. 3. Reverse current density as a function of the applied voltage.
The mesa-diodes prepared from structures of Mo-n-Si or molybdenum, a thin (less than 50 nm) silicide interlayer and then silicon have reverse d.c. characteristics which obey the equation or are very close to the theoretical dependence until they break down. Structures with thick molybdenum silicide interlayers (less than 150 nm) have reverse d.c. characteristics with a rapid increase in current in the case of a reverse bias of more than 3 V. The dependence of I / C 2 on Uin Fig, 4 allows us to determine the height ~B, of 50 5Q ~
2g& K
f u
%
,o
o -4
k -3
-2
Reverse voltage
-1 UR
0 (V)
Fig. 4. The dependence of I/C 2 on the applied voltage.
the barrier and the donor concentration in silicon adjacent to the interface. The height of the barrier was estimated from the relation
qbB. = Ui-.I-uB w k T - A ~ q
L.N. ALEKSANDROVet al.
92
where Ui is the intersection on the voltage axis when the curve is extrapolated to values of C approaching infinity, while qU, is the distance between the Fermi level and the bottom of the conduction band. The expected decrease A¢,in the height of the barrier is comparable with the value ofkT/q; therefore in order to find qSan it is sufficient to measure U~ accurately and to calculate U,. The height of the barrier determined in this way was found to be close to 0.7 V. From photoelectric measurements we find that ¢~B. = 0.71 _+0.03 V. Figure 5 shows the dependence of
!
10-2
_
_
tO-3 -o
v
10-/"
10-5 10-6 0.01
03
10
Reverse voltage bias
10 UR
100
(V)
Fig. 5. The voltage dependence of the reverse current density for structures manufactured by selective etching and by mesa-etching.
the reverse current density on the applied voltage for the structures that were manufactured by mesa-etching (curve 1) and by selective etching (curve 2). For the diodes manufactured on silicon with resistivity 0.2 C) cm the height ~an of the barrier was found to be 0.65 eV, the saturation current density J~ being 10 - 4 A cm - 2 and the thickness of the MoSi 2 being 15 nm. The reverse current density with U R - - 10 - 2 V was 2.5 x 10 - 4 A c m - 2 for the diode with 15 nm of silicide and 5.5 × 10 -4 A cm -2 for the diode with 280 nm of silicide. Further increase in the thickness of the MoSi 2 led to a substantial increase in both the reverse current and the forward current. We investigated the molybdenum-silicon contact using alloyed boron with a resistivity of 0.1 and 10 fl cm. In all cases, whatever the temperature of the substrate during metal spraying and irrespective of the thickness of the MoSi 2, non-rectifying contacts were obtained. We recorded the current-voltage characteristics for the Mo-p-Si contact and we studied the noise parameters at room temperature and at the temperature of liquid nitrogen. There was no 1If noise. The equivalent noise power was less than 15 nV, which corresponds to the noise power of the amplifier. The resistance of a contact 1 cm z in area was 17.5 x 10 -2 ~. 4. DISCUSSION AND CONCLUSIONS The electrophysical properties of the contact are influenced decisively by the condition of the interface and the thickness of the MoSi 2 interlayer between the metal and the silicon above 0.2 ktm. This phenomenon may be explained as follows.
Mo-Si JUNCTIONS
93
The formation and movement of structural defects in the course of the annealing necessary to ensure the formation of a silicide layer of defined thickness causes a reduction in the charge carrier lifetime in the region of spraying (additional heating being unnecessary) which leads to better characteristics of the junctions. On an atomically smooth surface of p-Si molybdenum forms a non-rectifying, almost ohmic, contact, while on n-Si (p = 0.20 f~ cm) a barrier 0.7 V high is formed. The characteristics of the structures are affected by the etching method and the depth of etching. In selective etching of metal, high peripheral currents that result from the residual silicide remaining after etching and also from the stress fields and silicon dislocations in the vicinity of the metal electrode are observed. With mesaetching the properties of the contacts are improved since the silicides and the regions of deformation in the silicon are eliminated. REFERENCES 1 1. K. Kikoin (ed.), Critical Tables Reference Book, Atomizdat, Moscow, 1976. 2 A. Milns and D. Foyht, Heterojunctions and Metal-Semiconductor Junctions, Academic Press, New York, 1972; Russian transl. Mir, Moscow, 1976. 3 V.I. Strikha (ed.), Semiconductor Instruments with Schottky Barrier, Sovjetskoje Radio, Moscow, 1974. 4 F . G . Allen, T. M. Buck and J. T. Law, J. AppL Phys., 31 (1960) 979. 5 A.E. Gershinskii, A. A. Khoromenko and E. I. Cherepov. Phys. Status Solidi A, 31 (1975) 61. 6 B.I. Fomin, A. E. Gershinskii, E. I. Cherepov and F. L. Edelman, Phys. Status Solidi A, 36 (1976) 89. 7 B.I. Fomin, A. E. Gershinskii, E. I. Cherepov and F. L. Edelman, Phys. Chem. Metal Working, 6 (1976) 77. 8 B.I. Fomin, A. E. Gershinskii and E. I. Cherepov, Investigation of phase growth kinetics during interaction of Si single crystals and Mo thin films, Talanta, 24 (1977) 192.