- Email: [email protected]
The influence of temperature treatment on the formation of Ni-based Schottky diodes and ohmic contacts to n-6H-SiC
ENGINEERING ELSEVIER
Materials Science and Engineering B46 (1997) 254-258
B
The influence of temperature treatment on the formation of Ni-based Schottky diodes and ohmic contacts to n-6ELSiC M.G. Rastegaeva *, A.N. Andreev, A.A. Petrov, A.I. Babanin, M.A. Yagovkina, I.P. Nikitina A.F. Ioffe Physico-Technical Institute, 26 Polytechnicheskoya St., 194021, St. Petemhurg, Rmia
Abstract Nickel-based Schottky contacts to n-6H-Sic, subjectedto various heat treatments, have been studied by the capacitancevoltage technique,X-ray diffractometry and AES sputter depth profile methods.Substraceheating during nickel depositionand additional
annealing
of Schottky contacts after deposition
of metal film lead to differences in structure and composition
between
contact layers formed on Si- and C-faces.As a result a distinctionin surfacebarrier heightsin Schottky diodesformed on the Cand Si-faceshas been observed.At annealingtemperatureshigher than 400-600°C formation of nickel silicidesin the contact layer is starting. Low-resistance( c 10V4 Q cm’) ohmic contacts to the SH-SiC polar faces were fabricated after annealing Ni-n-6H-SiC Schottky diodesat 1000°C.Q 1997Publishedby Elsevier ScienceS.A. Keywords: Schottky diodes; Nickel; Silicon carbide; Ohmic contact
under which nickel-based ohmic contacts can be fabricated.
1. Introduction
It is well known that heat treatment of a semiconductor just before depositing a metal, as well as heat treatment of metal-semiconductor (m-s) structures as a whole, has a strong influence on the properties of these structures. Therefore, the control over such heat treatments allows fabrication of m-s structures with
required parameters as has been done for the most investigated semiconductors (3, GaAs) [l]. However, the influence of thermal treatment on properties of metal-sic structures is scantily known at present. Nin-GH-SiC is one of the most interesting systems being studied now [Z-4]. The goal of the present work was to investigate the effect of thermal treatment of the initial n-6H-SiC substrate just before depositing Ni or additional annealing of the whole m-s structure in a wide temperature range on the surface barrier height, composition and structure of contact layer and contact layer/Sic
interface in Ni/n-GH-SiC structures formed on polar faces of GH-Sic. In addition, conditions were studied * Corresponding
author. E-mail: [email protected]
2. Experimental technique and objects of investigation
Two types of Ni-n&H-Sic structures were studied in the present work: (1) Surface-barrier structures formed after in situ heating of substrates in a vacuum at temperatures
T,=lOO-600°C; (2) Structures subjected to additional annealing Ni deposition
T, = 406 1100°C after preliminary T,= 100°C.
at at
All structures were produced by electron-beam deposition of Ni in a vacuum of 10 - 4 Pa onto Si- and C-faces of Lely substrates with a concentration of uncompensated donor impurity of (2 - 5) x lOi cm-3. Just before Ni deposition, the substrates were heated to T, by the resistive method in a vacuum. Additional annealing of structures (of type 2) was done in a vacuum in a special system with electron-gun heating. The surface barrier height in Schottky diodes (a,,) and uncompensated impurity concentration in substrates
0921-5107/97/517.00 0 1997 Published by Elsevier Science S.A. All rights reserved. PIISO921-S107(96)01989-7
M.G.
Rastegaeva
et al. /Materials
Science
loo
I 200
, 300
/ 400
I 500
( 693
Substrate temperature (Ts),(‘C Fig. 1. Dependence of Schottky barrier height on substrate temperature (Ts) before Ni deposition. 1 - C-face 6H-Sic; 2 - Si-face 6H-SiC
(2).
were measured by the capacitance-voltage method at 1 MHz. The composition and structure of contact layer were investigated both by X-ray diffractometry and by the AES sputtering protie method.
3. Experimental
results and discussion
3.1. Strzxtures of type 1
The dependence of surface barrier height on T, is shown in Fig. 1. As can be seen, there are essential differences between dependences for structures formed on 6H-SiC polar faces: at T, > 300°C the elevation of T, leads to an increase in surface barrier height Q)b for Schottky diodes obtained on the Cface, while for the Si-face the opposite tendency is observed. Note that in Fig. 1 the Q)b values are av-
a>
20
40
60
Sputtering time (min)
80
B46 (1997)
254-258
255
eraged over all sample series and account is taken of some scatter of the Qb value for m-s structures over the surface of each substrate (50-70 measurements on the whole). AES profiles of structures obtained at T, = lOO300°C on the Si and C-faces of Sic (Fig. 2(a)) show an abrupt Ni/6H-SiC interface. The fine structure of the Ni(LMM) Auger-line confirms that nickel does not interact with silicon carbide at the given temperatures. X-ray spectra (Fig. 2(b)) also testify to the presence of crystalline Ni phase with [ll l] orientation only. As can be seen from Fig. 1, all (& values obtained for Schottky diodes formed on the Si- and C-faces at T, = loo”-300°C are practically similar within the Q)b measurement error. We studied previously [5] the dependence of surface barrier height in meta&-6H-SiC Schottky diodes on the work function of the metal (Al, Cr, MO, Au). The Schottky diodes in [5] and in the present work were fabricated by the same technology. The @,, values for Ni/n-6H-SiC structures fabricated at T, = 100-300°C are in good agreement with data reported in Ref. [5] and, therefore, a high density of surface states at the interface (D, - 2 x 10’” cm - 2 eV - ‘) is typical of the m-so barriers obtained. For T, = 6OO”C, the process of Schottky barrier formation on the C- and Si-faces of BH-SiC is essentially different. The AES profiles (Fig. 3(a)) and X-ray spectra (which are similar to those in Fig. 2(b)) of a contact layer formed at T, = 600°C on the Si-face show that the applied experimental techniques fail to detect any interaction of nickel with silicon carbide. However, a tendency for the surface barrier height to decrease is observed in spite of the high scatter in experimental values of $,. The average @,, is 1.18 eV. This value corresponds to the theoretical curve @)b= @,(@A calculated within the Schottky-Motte limit, i.e. at D, = 0 (the electron affinity of n-GH-SiC was taken to be 4 eV) (see [5]). Hence, one of the possible reasons for the decrease
,’ .G s 1.4 _ -_ --- >;’ ,’ 4’ 9 1.3 ____---- ‘. ‘. ‘3 . \ $8 1.2 $I f 3 l.lm 1.0
and Engineering
20
b)
30
40
50
60
Angle (degrees)
Fig. 2. AES depth profile (a) and X-ray diffraction (b) of Ni-based contact covering deposited on Si- and C-face at Ts = lOO-300°C.
256
M,G.
Rastegaetia
et al. /Materials
Science
ad
Engineering
B46 (1997)
254-258
I SiCCOOO6)
ii s 4 9 +I .s 2 3 5
Ni$i(lOZ) t
20
a>
40
60
80
100
120
20
40
60
80
100
20
30
Sputtering time (min)
Sputtering time (min)
Ni$i(llO)
I
\ 40
50
60
Angle (degrees) c)
b)
Fig. 3. AES depth profile of Ni-based contact covering deposited on Si-face (a) and C-face (b) and X-ray diffraction for Ni-film deposited on C-face (c) at Ts = 600°C.
in aD, may be a reduction in surface states density at the Ni/n-6H-SiC interface. For structures formed at T, = 600°C on the C-face of Sic, the aggregate of AES and X-ray data (Fig. 3(b,c)) indicates that interactions are taking place in the contact layer. No abrupt NijSiC interface can be seen in the AES profile (Fig. 3(b)), the interface region is essentially extended. The Ni peak observed in X-ray spectra at T, = IOO-300°C disappears. This indicates a lack of preferred crystal orientation (except peaks of small intensity observed at 0 = 4748”, which is possibly due to the presence of the Ni2Si phase). The surface barrier height in such structures increases up to 1.57 eV. This is probably associated with changes in structure and properties of the conta~ct layer at a given T,. Changes in barrier height in metal-semiconductor structures, associated with chemical interaction, have been revealed previously, e.g. upon additional annealing of Al-Si or Pt-GaAs structures [6,7].
2.z 5 5 2 2s .s 2
Typical AES profiles and X-ray spectra of structures annealed at T, = 1000°C are shown in Fig. 4(a,b) and Fig. 5(a,b). Data obtained indicate that at any Ta nickel interacts with silicon carbide in contact layers formed both on the Si-face and on the C-face. A contact layer of nickel siticide (Ni,Si) is mainly formed as a result. Nickel, carbon and silicon are uniformly distributed along the whole thickness of the contact layer (with the exception of the near-surface layer and contact layer/Sic interface region whose composition and structure varies with TJ (see Fig. 4(a,b)). Consider now changes near the contact layer/Sic interface. AES profiles of type 2 structures show the presence of a region enriched with carbon near the interface. The intensity ratio of C(KLL) and Si(LMM) Auger-lines smoothly varies in the interface region from the maximum value to a value corresponding to the stoichiometric Sic composition (in
‘;; .z 1 4 z 8 E .C g 100 200
a)
3.2. Structures of type 2
300
Sputtering time (min)
100
b)
200
300
Sputtering time (min)
Fig. 4. AES depth profile of Ni-based contact covering deposited on Si-face (a) and C-face (b) at Ts = 100°C after annealing at 1000°C.
251
20 4
30
40
50
60
Angle(degrees)
20
30
b)
40
50
60
Angle(degrees)
Fig. 5. X-ra]’ diffraction of Ni-based contact film deposited on S-face (a) and C-face (b) at Ts = 100°C after annealingat 1000°C
the direction of the substrate). Therefore, the presence of a Sic layer may be assumed with inclusions of carbon clusters as well as of a SIC layer with disrupted stoichiometry (i.e., with high concentration of silicon vacancies). Thus, a high density of defects appears at the interface upon annealing. This leads to an increase in leakage currents of surface-barrier structures which do not allow the surface barrier height to be measured by the C-l’ method. The maximum intensity ratio of C(KLL) and Si(LMM) Auger-lines in the interface region increases when T, is elevated, and leakage currents increase too. A nonrectifying type of contact (ohmic) with Y, = (8-9) x 1O-5 R cm2 is observed at temperatures near 1000°C (for both faces). The specific contact resistance (~3 was measured by TLM on ring contacts [8]. We surmise that the presence of a layer enriched with carbon is one of the key factors ensuring the formation of an ohmic contact. We observed the formation of a layer of this kind in all types of ohmic contacts to Iz-6H-SiC which have been formed by us previously (based on Ni [2], MO or W [9] as well as based on amorphous or polycrystalline Sic layers deposited by magnetron sputtering and located between substrate and metal [lo]). It can be noted that the investigation of type 1 and type 2 structures shows that the formation of nickel silicides is not sufficient for nonrectifying contacts to be produced. It should be noted that there are distinctions in how the contact layer and its interface with Sic are formed on the Si- and C-faces of GH-SIC. First of all, at a temperature of 1000°C (at which ohmic the contact is formed) the interface on the C-face is more enriched with carbon. Secondly, while the intensity of the Ni (LMM) Auger-line on the Si-face gradually decreases in moving through the contact layer in the direction of Sic, the profile of this line on the C-face, as well as the intensity profiles of
AES lines of Si (LMM) and C(KLL) are more complex. This indicates the presence of several interlayers with different compositions. Fig. 5(a,b) shows the presence of Ni,Si, and Ni,Si phases in the contact layer formed on the C-face, while in a film formed on the Si-face the X-ray spectra indicate only Ni,Si.
4. Summary The main results obtained in the present work may be summarized as follows: (1) Preliminary heating of a substrate before Ni deposition to T, > 300°C results in a difference in surface barrier height of m-s structures formed on the Si- and C-faces of GH-Sic. The difference may be as high as -0.4 eV at T,=6OO"C. (2) Processes of interaction between Ni and SIC start on the C-face at T, = 600°C. No interaction is observed on both faces up to T, = 300°C. (3) Beginning with T, = 400-6OO”C, annealing of Ni-n-6H-SiC strucfures leads to the formation of a contact layer consisting predominantly of nickel silitide Ni,Si. (4) A low resistivity ohmic contact (rc = (8 - 9) x 10 - ’ R cm’) is formed on both faces of GH-SiC beginning with the temperature T, = 1000°C. The appearance of a SIC layer enriched with carbon on the contact layer-sic interface plays a decisive role in the formation of ohmic contacts.
Acknowledgements The authors wish to thank Dr. A.M. Strel’chuk for setting up the problem of investigation of Ni/M6H-SiC Schottky diodes. The work was supported in part by Arizona University.
258
M.G.
Rastegaeua
et al. /Materials
Science
References [l] S.M. Sze, Phys. Semicond. Dee., 111981). [2] M.G. Rastegaeva et al., Tech. Digest Inf. Con!
Silicon Carbide Materials, Kyoto, Japan, 1995, pp. 152-153. J. Crofton et al., J. Appl. Phys,, 77 (3) (1995) 1317-1319. J.R. Waldrop et al., Appl. Phys. Lett., 62 (21) (1993) 2685-2687. and Related
[3] [4]
and Engineering
346
(1997)
254-258
[5] A.N. Andreev et al., Semiconductors, 29 (10) (1995) 957-962. [6] A.K. Sinha et al., Solid State Electron., 19 (1976) 489. [7] K. Chino, Solid State Electron., 16 (1973) 119. [8] L. Boberg et al., Phpsica Scripta, 24 (1981) 405-407. [9] M.M. Anikin et al.? in G.L. Harris, M.G. Spencer and C.Y.-W. Yang (eds.), Amorphous aud Crystalline Sikcon Carbide Iii, Springer Verlag, New York, 1992. [IO] A.N. Andreev et al., Pisma v ZhTF, 20 (18) (1994) 11-15.
Materials Science and Engineering B46 (1997) 254-258
B
The influence of temperature treatment on the formation of Ni-based Schottky diodes and ohmic contacts to n-6ELSiC M.G. Rastegaeva *, A.N. Andreev, A.A. Petrov, A.I. Babanin, M.A. Yagovkina, I.P. Nikitina A.F. Ioffe Physico-Technical Institute, 26 Polytechnicheskoya St., 194021, St. Petemhurg, Rmia
Abstract Nickel-based Schottky contacts to n-6H-Sic, subjectedto various heat treatments, have been studied by the capacitancevoltage technique,X-ray diffractometry and AES sputter depth profile methods.Substraceheating during nickel depositionand additional
annealing
of Schottky contacts after deposition
of metal film lead to differences in structure and composition
between
contact layers formed on Si- and C-faces.As a result a distinctionin surfacebarrier heightsin Schottky diodesformed on the Cand Si-faceshas been observed.At annealingtemperatureshigher than 400-600°C formation of nickel silicidesin the contact layer is starting. Low-resistance( c 10V4 Q cm’) ohmic contacts to the SH-SiC polar faces were fabricated after annealing Ni-n-6H-SiC Schottky diodesat 1000°C.Q 1997Publishedby Elsevier ScienceS.A. Keywords: Schottky diodes; Nickel; Silicon carbide; Ohmic contact
under which nickel-based ohmic contacts can be fabricated.
1. Introduction
It is well known that heat treatment of a semiconductor just before depositing a metal, as well as heat treatment of metal-semiconductor (m-s) structures as a whole, has a strong influence on the properties of these structures. Therefore, the control over such heat treatments allows fabrication of m-s structures with
required parameters as has been done for the most investigated semiconductors (3, GaAs) [l]. However, the influence of thermal treatment on properties of metal-sic structures is scantily known at present. Nin-GH-SiC is one of the most interesting systems being studied now [Z-4]. The goal of the present work was to investigate the effect of thermal treatment of the initial n-6H-SiC substrate just before depositing Ni or additional annealing of the whole m-s structure in a wide temperature range on the surface barrier height, composition and structure of contact layer and contact layer/Sic
interface in Ni/n-GH-SiC structures formed on polar faces of GH-Sic. In addition, conditions were studied * Corresponding
author. E-mail: [email protected]
2. Experimental technique and objects of investigation
Two types of Ni-n&H-Sic structures were studied in the present work: (1) Surface-barrier structures formed after in situ heating of substrates in a vacuum at temperatures
T,=lOO-600°C; (2) Structures subjected to additional annealing Ni deposition
T, = 406 1100°C after preliminary T,= 100°C.
at at
All structures were produced by electron-beam deposition of Ni in a vacuum of 10 - 4 Pa onto Si- and C-faces of Lely substrates with a concentration of uncompensated donor impurity of (2 - 5) x lOi cm-3. Just before Ni deposition, the substrates were heated to T, by the resistive method in a vacuum. Additional annealing of structures (of type 2) was done in a vacuum in a special system with electron-gun heating. The surface barrier height in Schottky diodes (a,,) and uncompensated impurity concentration in substrates
0921-5107/97/517.00 0 1997 Published by Elsevier Science S.A. All rights reserved. PIISO921-S107(96)01989-7
M.G.
Rastegaeva
et al. /Materials
Science
loo
I 200
, 300
/ 400
I 500
( 693
Substrate temperature (Ts),(‘C Fig. 1. Dependence of Schottky barrier height on substrate temperature (Ts) before Ni deposition. 1 - C-face 6H-Sic; 2 - Si-face 6H-SiC
(2).
were measured by the capacitance-voltage method at 1 MHz. The composition and structure of contact layer were investigated both by X-ray diffractometry and by the AES sputtering protie method.
3. Experimental
results and discussion
3.1. Strzxtures of type 1
The dependence of surface barrier height on T, is shown in Fig. 1. As can be seen, there are essential differences between dependences for structures formed on 6H-SiC polar faces: at T, > 300°C the elevation of T, leads to an increase in surface barrier height Q)b for Schottky diodes obtained on the Cface, while for the Si-face the opposite tendency is observed. Note that in Fig. 1 the Q)b values are av-
a>
20
40
60
Sputtering time (min)
80
B46 (1997)
254-258
255
eraged over all sample series and account is taken of some scatter of the Qb value for m-s structures over the surface of each substrate (50-70 measurements on the whole). AES profiles of structures obtained at T, = lOO300°C on the Si and C-faces of Sic (Fig. 2(a)) show an abrupt Ni/6H-SiC interface. The fine structure of the Ni(LMM) Auger-line confirms that nickel does not interact with silicon carbide at the given temperatures. X-ray spectra (Fig. 2(b)) also testify to the presence of crystalline Ni phase with [ll l] orientation only. As can be seen from Fig. 1, all (& values obtained for Schottky diodes formed on the Si- and C-faces at T, = loo”-300°C are practically similar within the Q)b measurement error. We studied previously [5] the dependence of surface barrier height in meta&-6H-SiC Schottky diodes on the work function of the metal (Al, Cr, MO, Au). The Schottky diodes in [5] and in the present work were fabricated by the same technology. The @,, values for Ni/n-6H-SiC structures fabricated at T, = 100-300°C are in good agreement with data reported in Ref. [5] and, therefore, a high density of surface states at the interface (D, - 2 x 10’” cm - 2 eV - ‘) is typical of the m-so barriers obtained. For T, = 6OO”C, the process of Schottky barrier formation on the C- and Si-faces of BH-SiC is essentially different. The AES profiles (Fig. 3(a)) and X-ray spectra (which are similar to those in Fig. 2(b)) of a contact layer formed at T, = 600°C on the Si-face show that the applied experimental techniques fail to detect any interaction of nickel with silicon carbide. However, a tendency for the surface barrier height to decrease is observed in spite of the high scatter in experimental values of $,. The average @,, is 1.18 eV. This value corresponds to the theoretical curve @)b= @,(@A calculated within the Schottky-Motte limit, i.e. at D, = 0 (the electron affinity of n-GH-SiC was taken to be 4 eV) (see [5]). Hence, one of the possible reasons for the decrease
,’ .G s 1.4 _ -_ --- >;’ ,’ 4’ 9 1.3 ____---- ‘. ‘. ‘3 . \ $8 1.2 $I f 3 l.lm 1.0
and Engineering
20
b)
30
40
50
60
Angle (degrees)
Fig. 2. AES depth profile (a) and X-ray diffraction (b) of Ni-based contact covering deposited on Si- and C-face at Ts = lOO-300°C.
256
M,G.
Rastegaetia
et al. /Materials
Science
ad
Engineering
B46 (1997)
254-258
I SiCCOOO6)
ii s 4 9 +I .s 2 3 5
Ni$i(lOZ) t
20
a>
40
60
80
100
120
20
40
60
80
100
20
30
Sputtering time (min)
Sputtering time (min)
Ni$i(llO)
I
\ 40
50
60
Angle (degrees) c)
b)
Fig. 3. AES depth profile of Ni-based contact covering deposited on Si-face (a) and C-face (b) and X-ray diffraction for Ni-film deposited on C-face (c) at Ts = 600°C.
in aD, may be a reduction in surface states density at the Ni/n-6H-SiC interface. For structures formed at T, = 600°C on the C-face of Sic, the aggregate of AES and X-ray data (Fig. 3(b,c)) indicates that interactions are taking place in the contact layer. No abrupt NijSiC interface can be seen in the AES profile (Fig. 3(b)), the interface region is essentially extended. The Ni peak observed in X-ray spectra at T, = IOO-300°C disappears. This indicates a lack of preferred crystal orientation (except peaks of small intensity observed at 0 = 4748”, which is possibly due to the presence of the Ni2Si phase). The surface barrier height in such structures increases up to 1.57 eV. This is probably associated with changes in structure and properties of the conta~ct layer at a given T,. Changes in barrier height in metal-semiconductor structures, associated with chemical interaction, have been revealed previously, e.g. upon additional annealing of Al-Si or Pt-GaAs structures [6,7].
2.z 5 5 2 2s .s 2
Typical AES profiles and X-ray spectra of structures annealed at T, = 1000°C are shown in Fig. 4(a,b) and Fig. 5(a,b). Data obtained indicate that at any Ta nickel interacts with silicon carbide in contact layers formed both on the Si-face and on the C-face. A contact layer of nickel siticide (Ni,Si) is mainly formed as a result. Nickel, carbon and silicon are uniformly distributed along the whole thickness of the contact layer (with the exception of the near-surface layer and contact layer/Sic interface region whose composition and structure varies with TJ (see Fig. 4(a,b)). Consider now changes near the contact layer/Sic interface. AES profiles of type 2 structures show the presence of a region enriched with carbon near the interface. The intensity ratio of C(KLL) and Si(LMM) Auger-lines smoothly varies in the interface region from the maximum value to a value corresponding to the stoichiometric Sic composition (in
‘;; .z 1 4 z 8 E .C g 100 200
a)
3.2. Structures of type 2
300
Sputtering time (min)
100
b)
200
300
Sputtering time (min)
Fig. 4. AES depth profile of Ni-based contact covering deposited on Si-face (a) and C-face (b) at Ts = 100°C after annealing at 1000°C.
251
20 4
30
40
50
60
Angle(degrees)
20
30
b)
40
50
60
Angle(degrees)
Fig. 5. X-ra]’ diffraction of Ni-based contact film deposited on S-face (a) and C-face (b) at Ts = 100°C after annealingat 1000°C
the direction of the substrate). Therefore, the presence of a Sic layer may be assumed with inclusions of carbon clusters as well as of a SIC layer with disrupted stoichiometry (i.e., with high concentration of silicon vacancies). Thus, a high density of defects appears at the interface upon annealing. This leads to an increase in leakage currents of surface-barrier structures which do not allow the surface barrier height to be measured by the C-l’ method. The maximum intensity ratio of C(KLL) and Si(LMM) Auger-lines in the interface region increases when T, is elevated, and leakage currents increase too. A nonrectifying type of contact (ohmic) with Y, = (8-9) x 1O-5 R cm2 is observed at temperatures near 1000°C (for both faces). The specific contact resistance (~3 was measured by TLM on ring contacts [8]. We surmise that the presence of a layer enriched with carbon is one of the key factors ensuring the formation of an ohmic contact. We observed the formation of a layer of this kind in all types of ohmic contacts to Iz-6H-SiC which have been formed by us previously (based on Ni [2], MO or W [9] as well as based on amorphous or polycrystalline Sic layers deposited by magnetron sputtering and located between substrate and metal [lo]). It can be noted that the investigation of type 1 and type 2 structures shows that the formation of nickel silicides is not sufficient for nonrectifying contacts to be produced. It should be noted that there are distinctions in how the contact layer and its interface with Sic are formed on the Si- and C-faces of GH-SIC. First of all, at a temperature of 1000°C (at which ohmic the contact is formed) the interface on the C-face is more enriched with carbon. Secondly, while the intensity of the Ni (LMM) Auger-line on the Si-face gradually decreases in moving through the contact layer in the direction of Sic, the profile of this line on the C-face, as well as the intensity profiles of
AES lines of Si (LMM) and C(KLL) are more complex. This indicates the presence of several interlayers with different compositions. Fig. 5(a,b) shows the presence of Ni,Si, and Ni,Si phases in the contact layer formed on the C-face, while in a film formed on the Si-face the X-ray spectra indicate only Ni,Si.
4. Summary The main results obtained in the present work may be summarized as follows: (1) Preliminary heating of a substrate before Ni deposition to T, > 300°C results in a difference in surface barrier height of m-s structures formed on the Si- and C-faces of GH-Sic. The difference may be as high as -0.4 eV at T,=6OO"C. (2) Processes of interaction between Ni and SIC start on the C-face at T, = 600°C. No interaction is observed on both faces up to T, = 300°C. (3) Beginning with T, = 400-6OO”C, annealing of Ni-n-6H-SiC strucfures leads to the formation of a contact layer consisting predominantly of nickel silitide Ni,Si. (4) A low resistivity ohmic contact (rc = (8 - 9) x 10 - ’ R cm’) is formed on both faces of GH-SiC beginning with the temperature T, = 1000°C. The appearance of a SIC layer enriched with carbon on the contact layer-sic interface plays a decisive role in the formation of ohmic contacts.
Acknowledgements The authors wish to thank Dr. A.M. Strel’chuk for setting up the problem of investigation of Ni/M6H-SiC Schottky diodes. The work was supported in part by Arizona University.
258
M.G.
Rastegaeua
et al. /Materials
Science
References [l] S.M. Sze, Phys. Semicond. Dee., 111981). [2] M.G. Rastegaeva et al., Tech. Digest Inf. Con!
Silicon Carbide Materials, Kyoto, Japan, 1995, pp. 152-153. J. Crofton et al., J. Appl. Phys,, 77 (3) (1995) 1317-1319. J.R. Waldrop et al., Appl. Phys. Lett., 62 (21) (1993) 2685-2687. and Related
[3] [4]
and Engineering
346
(1997)
254-258
[5] A.N. Andreev et al., Semiconductors, 29 (10) (1995) 957-962. [6] A.K. Sinha et al., Solid State Electron., 19 (1976) 489. [7] K. Chino, Solid State Electron., 16 (1973) 119. [8] L. Boberg et al., Phpsica Scripta, 24 (1981) 405-407. [9] M.M. Anikin et al.? in G.L. Harris, M.G. Spencer and C.Y.-W. Yang (eds.), Amorphous aud Crystalline Sikcon Carbide Iii, Springer Verlag, New York, 1992. [IO] A.N. Andreev et al., Pisma v ZhTF, 20 (18) (1994) 11-15.