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Thin films of rare-earth metal silicides in microelectronics
Vacuum/volume 36/number Printed in Great Britain
0042-207X/86 $3.00+ .OO Pergamon Journals Ltd
1 O/pages 669 to 676/l 986
Thin films of rare-earth microelectronics V M Koleshko, V F Belitsky and A A Khodin, Ya Kolas St 6812, Minsk 220841, USSR
metal
silicides
in
Institute of Electronics, Byelorussian Academy of Sciences,
Structure, phase composition and physical properties of thin films of rare-earth metal (REM) silicides (of Y-subgroup) have been studied. The films were formed on (700) and (117) single-crystal Si substrates. X-ray diffraction analysis reveals the formation of a crystalline silicide phase of LnSi,_ x composition with hexagonal structure of the AIB,-type for all metals except SC, Gd, Lu. It is established that the crystalline silicide phase formation is determined by the crystallographic orientation of the Si substrate and that there is a relation between crystallographic parameters of REM silicide (REMS) and Si hexagonal lattices: the critical value of lattice parameter mismatch is f 1.3%, above which REMS has a disordered structure. The kinetics of the silicide phase growth was determined by measuring the conductance of thin-film structures. A model of REMS formation in thin-film structures on an Si substrate is proposed. Based on the model, conditions have been found for the formation of either quasi-amorphous, polycrystalline or monocrystalline REMS layers. The formation of a REMS amorphous film is most probable on a (100) substrate for such metals as Gd and Lu. Epitaxial growth of REMS films is typical of Tm, Er, Ho on (111) substrates. Also, investigation of the current transport in surface barrier diodes of the REM-p-Si and REMS-p-Si types shows that the potential barrier lowers as a result of silicide formation in REM-S/’ contacts and, accordingly, the silicide work function increases as compared with that of the initial metal. The photoelectric measurements indicate that the silicide-Si contact photosensitivity shifts to a longer optical wavelength as compared to that of the metal-Si contacts.
1. Introduction
Metal silicide thin films find an ever-growing use in microelectronics and semiconductor technology’. Thin films of silicides of many metals (mainly, those of IVa, Via, VIIa subgroups and platinoids) have been studied more or less thoroughly*. Formation methods have been developed, and the electrical and physical properties of metal silicides bulk and thin-film states have been investigated. Basic criteria for formation of stable electric contacts ‘silicidesilicon substrate’ and of epitaxial thin-film layered ‘silicide-silicon’ systems are as follows: high chemical activity of metal; relatively low silicide formation temperature; crystal lattice correspondence (crystallographic parameters and type+ubic, hexagonal, etc.) of silicide and semiconductor. Lattice mismatch between some transition metal silicides and silicon crystals does not exceed about 2%3.
formed at relatively low temperatures (about 600 K). Among other properties of the silicides5, their high microhardness (80.0-100.0 N mmm2) should be mentioned as this is nearly equal to that of silicon. Thermal expansion coefficients of REM disilicides are closer to silicon than those for Mn, Ti, Cr, but lower than those for Ta and MO silicides. REM silicides react with oxygen at temperatures higher than 773 K, and REM disilicides are stable up to 1273 K; unlike other refractory metal silicides (MO, W, Ta), stable oxide films form on their surface. However, thin-film REM disilicides have been most poorly studied. Thus, phase diagrams for these metals with silicon are lacking except for some metals. A complete diagram has been constructed only for Y, and, partially, for Dy, Er and Gd. There are no data for the enthalpy of silicide formation. Reliable electrical resistivity data are not available for all the REM silicides. Experimental values for the work function of silicides have not been obtained. As the work function is one of the crucial parameters for ‘silicide-Si’ contacts, it was calculated from the expression6:
2. Statement of the problem (psi,=((p”,(pa) l/(n+k) Analysis of chemical activity of various metals, formation temperature of their silicides, and comparison of lattice parameters and crystal structure of silicides and silicon (Tables 14) shows that rare-earth metal (REM) disilicides of Y-subgroup satisfy the above criteria to the greatest degree4. Indeed, REMs have a higher chemical activity than most other metals. Thus, shallow contacts in Si and other microcircuit components can be
(1)
where cp, is the work function of the metal, ps is the Si electron affinity, n and k are variable coefficients whose values correspond to various silicide types. The value for cpswas taken as 4.4 eV for low-doped Si and 4.2 eV for p-type Si. The calculated results are given in Figure 1. Calculated values of qpsi,were compared to the experimentally determined barrier heights in silicide on n- and 669
V M Koleshko
et al: Thin films of rare-earth metal silicides
Table 1. Comparison
of crystallographic
Sihcide
Lattice
Si Si TiSi, TiSi Ti,Si, HfSi, HfSi Hf,Si, VSi, V,Si, V,Si, V,Si V,Si TaSi, Ta,Si, CrSi, CrSi Cr,Si, MoSi,(a) MoSi,(B) Mo,Si, MosSi, WSi, W,Si, W,Si, Nisi,., Nisi CoSi, CoSi Ni,Si, Ni,Si(O) RhSi RhSi,,,
Hex fee Rhomb Rhomb Hex Rhomb Rhomb Hex Hex Hex Tetr Hex Cub Hex Hex Hex Cub Tetr Tetr Hex Tetr Hex Tetr Tetr Hex Cub Rhomb Cub Cub Rhomb Hex Cub Rhomb Cub Rhomb Rhomb Hex Rhomb Hex Rhomb Hex Rhomb Hex Cub Tetr
Rh,Si, Rh,Si IrSi, IrSi Ir,Si, PdSi Pd,Si PtSi Pt,Si Pt,Si Pt,Si
Table 2. Crystallographic
type
parameters
Lattice a (A)
and formation
Lattice
670
Silicide
Lattice
ScSi, _ I YSizmX YSi, -x GdSi,_, TbSi, _x DySi,_, HoSi, _ * ErSi, _x TmSi,_, YbSi, _x LuSi,-,
AlB, AlB, AlB, AlB, AlB, AIB, AlB, AlB, AlB, AIB, AIB,
parameters
3.80 5.428 8.252 6.531 1.456 3.611 6.855 7.890 4.571 11.851 9.429 7.135 4.721 4.783 1.414 4.43 1 4.629 9.170 3.203 4.642 9.642 7.28 3.211 9.605 7.19 5.4066 5.18 5.3627 4.4445 12.229 3.805 4.675 5.531 2.963 5.317 5.408 4.350 5.558 3.96 5.599 6.528 5.595 6.436 5.63 3.933
parameters
type
Hex Hex Hex Hex Hex Hex Hex Hex Hex Hex Hex
of metal silicides and silicon
c (A)
b (A)
Formation temperature (R)
6.28 8.540 4.897 5.162 3.649 3.191 5.558 6.312 3.623 4.157 4.842
4.783 3.631 14.550 3.753
873 713 lb23 8233973 873
6.565 5.224 6.364
873-923 123
4.636 7.855 6.529 4.905 5.00 7.868 4.964 4.85
800 800
923
5.62
3.34
6.924 4.890
10.805
6.362
3.063
3.895 7.383 6.610 6.213 5.126 6.133 3.437 5.932 3.569
10.131 3.930 3.211 3.381 3.603
1023 623-773 823 650-773 723 473-623 623-700
673 1273 673-713 1073 3733573 5w573 4733773 4733573
5.910
temperature
of REMS of yttrium
subgroup
Formation temperature
parameters
a (A)
c (A)
Y(%)
(R)
3.66 3.842 3.836 3.877 3.847 3.831 3.816 3.799 3.773 3.771 3.745
3.87 4.140 4.139 4.172 4.146 4.121 4.107 4.090 4.070 4.098 4.050
-3.7 +1.1 f0.9 f2.0 f1.2 +0.82 +0.42 -0.03 -0.71 -0.76 - 1.45
6733723 60&673 6OCk673 623-673 5733623 573-623 60&623 60&623 623673 4733623 6133723
V M Koleshko et a/: Thin films of rare-earth
metal silicides
Table 3. Crystallographic
parameters
and formation
Lattice a (A)
Lattice
LaSi, _-r LaSi, _ II LaSi La& La,%, or-CeSi, j3-CeSi, CeSi a-P&, B-PrSi, PrSi Pr,Si, a-NdSi,
Tetr Rhomb Rhomb Tetr Tetr Rhomb Tetr Rhomb Rhomb Tetr Rhomb Tetr Rhomb
4.322 4.270 8.404 7.87 7.95 4.19 4.175 8.302 4.17 4.291 8.240 7.93 4.13
13.86 14.05 6.059 4.50 14.04 13.92 13.848 5.694 13.85 13.76 5.920 13.97 13.97
kFP* Nd,Si, a-SmDi, /?-SmSi, SmSi Sm,Si, a-EuSi /I-EuSi,
Tetr Rhomb Hex Rhomb Tetr Rhomb Hex Rhomb Tetr
4.103 8.240 8.66 4.105 4.041 8.055 8.56 4.72 4.29
13.53 5.920 6.53 13.46 13.33 5.804 6.45 3.99 13.66
Silicide TiSi, VSi, CrSi, CoSi, Nisi Nisi, ZrSi, NbSi, MoSi, RhSi Pd,Si PdSi HfSi HfSi, TaSi,
WSi, Pt,Si PtSi
type
and physical
properties
of cerium subgroup Formation temperature
parameters c (A)
Silicide
Table 4. Electrical
b (A)
(K) 550
4.170 4.010
4.13
673-773 673-773
3.962 4.11 3.941 4.10 3.941 4.035 3.888 11.15
of silicides
Electric resistivity (~lO_~Qcm) 13-16.25 16.7-18 5&55 9.5-13.5 600 18-20.25 64.8 5&60 3540 50 4GlOO 18-75 3G35 20 45-50 3540 50-55 6@70 3&100 28-35
Si (Table 5). Such comparison enabled us to choose (psi, corresponding to experimental values of qb. It turned out, that the values of (psi, obtained correspond to the following set of coefficients: n = 1, k = 1-2, which are characteristic of the silicide phase of the type LnSi,_,, i.e. of REM disilicides. It should be noted that in this case equation (1) is closely approximated by a linear function: p-type
of REMS
Barrier height for n-type Si(eV)
Silicide work function (cV)
Metal work function (eV)
0.6
3.674.25
4.33 4.3
0.57 0.65
4.93 4.36
4.5 5.0
0.66 0.7 0.55
4.54.55 4.55 3.86
0.55 0.69 0.745
44.25 4.Gl.8 4.84.97 4.94-5.20
5.22 5.22 4.05 4.02-4.3 4.53
0.55 -
-
0.59
4.15
4.98 5.12 5.12 3.9 3.9 4.15425
0.65 0.78 0.87
4.55-4.8 5.54-5.71 5.15-5.75
4.554.63 5.65 5.65
Vpsil=
clVp,
+
B
(2)
where a=O.43-0.64, /I= 1.87-2.68. Analysis of the crystal lattices of the silicides of various metals as compared with silicon4 showed that REM disilicides of Y-subgroup provide, in some cases, almost perfect matching by the lattice parameter ‘a’ (Table 2). The crystal lattice of such 671
V M Koleshko ef al: Thin films of rare-earth
metal silicides
diffusion process and promote solid phase epitaxy of the silicide. In addition, such silicides (LnSi, _,) are characterized by a rather large homogeneity interval (x z o-0.4), that is x may vary without changing the disilicide crystal structure. This ensures good resistance of the silicide phase to variations in stoichiometry which is important for thin-film composition formation. Thus, it follows from theoretical discussion that disilicides of Y-subgroup REMs are most suitable for production of contacts to Si because of favourable chemical and thermodynamical properties of the ‘silicide forming metal-W system and due to matching of lattice types and parameters of the silicide and semiconductor.
3. Experimental
1.
.. 26
t
)
3.0
28
cp
m
3.2
(ev)
Figure 1. Relation between the work functions of silicide (Q) correspondingmetal(cp,).(l)n=l,k=4;(2)n=l,k=2;(3)~=l,k=4;(4) n=2,k=3;(5)n=l,k=2;(6)n=l,k=l;(7)n=2,k=1;(8)n=4,k=1;(3) and (5) (p,=4.8 eV; for the rest qo,=4.41 eV; (9) n= 1, k=2,
and
(p,=4.2eV.
silicides consists of alternating atomic layers of metal and silicon (Figure 2) the atomic arrangement being the same as that which occurs in separately taken metal and Si lattices. Such a lattice structure of silicide favours greatly its formation in thin-film structures. Layered arrangements of atoms should facilitate the (a)
Thin films of REM silicides were obtained by heat treating REM-Si film structures. Two types of structures were studied: a one-layer structure which, prior to annealing, represents a thin REM film deposited on an Si substrate, and a two-layer structure which differs in that an amorphous Si layer was deposited upon a substrate-supported metal film. Single-crystal (100) and (111) Si wafers doped with phosphorus up to 20 Q cm were used. Prior to the deposition of REM the wafers were chemically treated, i.e. thoroughly cleaned, degreased, ‘refreshed’ and etched in 1:lO HF: H,O and treated with an ammonium peroxide solution. Thin films of metals and silicon were obtained using vacuum evaporation at about 3 x 10m4 Pa from a tungsten evaporator. REM bi-distillates and milled single-crystal P-doped silicon were used as starting materials. The thickness of the REM and Si films was monitored by means of optical interference microscopy to within f5 nm. Before metal deposition, the substrates were heated to 423 K to remove impurity residues. The substrate temperature during film deposition did not exceed 373 K. The as-formed one- and two-layer thin-film structures were annealed under vacuum (3 x 10m4 Pa) by ir irradiation. The anneal temperature (T,) varied in the range 473-773 K, and the exposure ranged from 5-120 min. The phase composition of silicide films was studied using X-ray diffraction techniques and He+-Rutherford backscattering. The electrical resistivity was measured in air using a four-point probe technique with an accuracy of k 1%, after annealing and removing the sample from the vacuum chamber. The film morphology was studied using a scanning electron microscope.
4. Results and discussion
It was found that annealing
of most REMs on Si substrates at T, = 573-623 K (for Yb films already at T,, a473 K) resulted in the formation of REM disilicides with the general formula LnSi, _ x. Table 6 summarizes the results of the X-ray diffraction analysis of Si-REM-Si film structures subjected to thermal treatment. As shown, not all the metals analysed form a polycrystalline silicide phase of LnSi, _-xcomposition. The table compares X-ray data for lattice parameters of corresponding silicides with respect to the Si lattice. This analysis resulted in a correlation between the REM disilicide crystalline phase with A,B,-type lattice and the relative difference in CIparameter of silicide lattice (a,,J and silicon (asi):
I Figure 2. Crystallographic scheme of epitaxial contact of REM disilicide to Si(ll1) (a) metal sublattice, (b) silicon sublattice, (c) silicon lattice. 672
%i
I
For K-c 1.3% the formation of a crystalline phase is observed more intensively and at lower temperatures for smaller values of
V M Koleshko et al: Thin films of rare-earth metal silicides
Table 5. Electrical and
physical properties of REMS disilicides of yttrium subgroup
Silicide
Electric resistivity (lo-‘xncm)
cp; (EeV)
cp; (eV)*
‘psi1 (eV)
(“e;,
GdSi,
31
0.37
3.8Ck4.14
3.10
DySi,
30
0.37
3.8C4.21
3.25
HoSi, ErSi, _ x
28 11.4
0.37 0.39
3.844.19 3.8C4.21
3.22 3.25
YSi,_,
20.2
0.39
3.8C4.23
3.3
TbSi, YbSi,
17.4 9.8
0.4 -
0.71 (0.6wl.86) 0.73 0.807 0.640.85) 0.70-0.85 0.7cO.77 (0.640.85) 0.71 0.74-0.75t (0.640.85) (0.65M.86) (0.68X1.92)
3.814.16 3.58-3.91
3.15 2.60
* Authors’ calculations in brackets. t Data from ref 8. K. For K> 1.3% an REM silicide phase is not detected, at least in the range of annealing temperatures 573-673 K. REM silicides of A,B,-type are represented by alternating layers of Si and metal atoms (Figure 2). For small K, the lattice of such a silicide can be pictured as an extended Si lattice with builtin layers of metal atoms. In such a process of silicide formation the Si substrate seems to serve as a seed for oriented growth of REM silicide crystals in the film, the initial structure of which, formed by interdiffusion of REM and amorphous Si layers, is disordered. However, if the mismatch between REM silicide and Si lattices is large, the ordering of a crystalline structure due to a stimulating effect of the substrate does not occur within the temperature range studied. Recrystallization of this layer is probably possible at temperatures close to the REM melting point. Along the c axis of the hexagonal lattice, the difference between REM disilicide and Si is, as can be clearly seen from Table 4, 3439%, i.e. the elementary cell of the REM silicide is an Si cell deformed along the c axis (also, with the a parameter somewhat changed) by introduction of a REM atom between Si atoms. Such deformation causes a reduction in the interlayer atomic distance in the REM silicide lattice as compared with the Si lattice. With the above data taken into consideration, two variants of low-temperature (573-673 K) ordering of REM silicide crystalline structure can be envisaged in thin-film structures. In the first variant the atoms of metal penetrate into the semiconductor at random along one or several possible directions parallel to the Si layers. The lattice extends chaotically, thus leading to the formation of a fine crystal structure. When going from the substrate, prevailing grain orientation directions are notable which are characterized by a maximum growth rate, i.e. [ 1001 and [ 1101, while [l 1 l] directions are suppressed. In this case the REM silicide film experiences a build-up of stresses, with a component parallel to the substrate plane, thus leading to pit formation in the film. The equal probability in three directions of a maximum growth rate, (lOO), (OlO), (OOl), causes the formation of pits of trihedral and/or hexagonal shapes. In the second variant, atomic diffusion and reordering at the silicide crystal lattice occur normal to the substrate surface. As a result, the deformation stresses in REM silicide films reduce to ones which are purely normal to the substrate surface and the REM silicide-Si interface contains a minimum number of crystallographic imperfections. This mechanism is less probable
than the former because of a slower penetration ability of metal atoms normal to the Si layers, i.e. in the [ 11l] direction, as well as a higher sensitivity to the Si surface finish, metal film deposition and subsequent annealing conditions. Nevertheless, the REM silicide-Si epitaxial structure formation might be expected with a perfect interface, minimum stress and low contact resistance if vacuum ion-cleaning, magnetron sputtering of silicide targets or molecular-beam epitaxy techniques were employed. Judging by Tables 2 and 6, the best metal in this respect appears to be erbium. It should also be expected that silicides of those metals whose lattice parameter a is most close to that of Si, i.e. Er, Ho, Tm, would have the highest crystallization rate, while those with the greatest difference in this parameter, i.e. SC, Gd, Lu, would have the lowest rate. The kinetics of thin film REM silicide formation can be monitored by the change in electrical resistivity of the two-layer compositions during annealing (Figure 3). On the curve of p/pc=f(t) several characteristic portions can be noted (Figure 3(a)), curve 2). In the ‘ab’ portion, a rise in the film resistivity is observed due to formation of a disordered silicide phase. For a two-layer structure this phase forms both on the amorphous Si film side and on the single-crystal substrate side. On the amorphous film side a disordered phase is formed, and on the substrate side a crystalline phase builds up. When one-layer compositions are annealed the ‘ab’ portion is preceded by a smooth reduction in p associated with lowering of the film p under annealing. Silicide formation occurs at t, = b. At this point, upper and lower silicide fronts merge (on the film side and on the substrate side). A decay in p in the portion ‘bc’ can be explained by ordering of the silicide crystalline structure. A rise in p in the portion ‘cd’ is due to interaction of the silicide film with residual gases in the vacuum chamber and the film enrichment with Si from the substrate. These phenomena manifest themselves in the growth of pyramids on the film surface with the pyramid faces being oriented in the same way as the substrate. The silicide formation in the contact ‘REM-Si’ determines, in particular, the photoelectric properties of the contact. The potential barrier heights of a rectifying contact to p-type Si substantially decrease (by about 0.2-0.3 eV). The I-Vcharacteristics degrade insignificantly and the Z-V ideality factor increases from n= 1.01 to n= 1.08 (Figure 4). The latter is indicative of a planar character for the metal-Si substrate interaction front. 673
V M Koleshko et a/: Thin films of rare-earth
metal silicides
(0)
0.9
T
0.8
Q
0.7 0.6 0
20
40
60
0.5 0
80
20
to (min) Figure3. Variationin
resistivityof Tm; (8) Er. r, = 673 K.
to
REM films under annealingon(a)(lOO)Si
substrateand
v Figure 4. I-Vcharacteristics 623 K for 30 min.
of Er-p-Si(ll1)
contact
in case of silicide formation
Examination of the surface of the contact structure by scanning electron microscopy showed that it remains uniform over the entire contact area. The photosensitivity of silicide structures shifts to the longwave part of the spectrum as compared to that of a REM-Si contact (Figure 5, curves 3,6 and 1,4). The sensitivity of the contacts increases in the near ir-range (l.Gl.2 pm). Initial REM films not subject to heat treatment have enhanced photosensitivity in the 500-850 nm range (Figure 5, curves 1,4, portion ab). This is associated with the presence of a potential barrier in the REM-Si contact whose height exceeds the bandgap of Si’. As the annealing temperature is increased and the silicide, whose work function is much larger (Figure l), is formed, the barrier height is reduced and the portion nb disappears (Figure 5, curves 2,3, 5,6). 674
40
60
80
(min)
(b)(ll l)Si substrate. (l)Tb;(2) Dy; (3)Yb(4)Y;(5) Lu;(6)Gd;
(7)
(VI
under thermal
annealing.
(1) Initial structure,
(2) after annealing
at
The observed shift of the photosensitivity peak may also be associated with a high doping of the near-surface Si layer arising from metal diffusion during annealing of the REM film on the Si substrate.
5. Conclusions (1) When selecting metal silicides for preparation of stable electrical contacts to silicon the following fundamental criteria should be used: the silicide-forming metal should have a high chemical activity; the thin-film silicide should be formed at a relatively low temperature (about 473673 K), and the silicide lattice (crystallographic parameters and structure) should corre-
V M Koleshko et al: Thin films of rare-earth metal silicides
Table 6. Formation of REM disilicides of Y-subgroup during thermal treatment of two-layer compositions Silicide composition
Lattice parameters a (A) c (A)
Si
3.80
6.28
ScSi, _ x
3.66
3.87
YSi,_,
3.842
GdSi,_,
Disilicide
Thermal treatment conditions
-3.68
No
673 K, 30 min
4.140
+1.11
Yes
673 K, 30 min
3.877
4.172
+ 2.03
No
673 K, 30 min
TbSi,_,
3.847
4.146
+ 1.24
Yes
623 K, 30 min
DySi, --x
3.831
4.121
+0.82
Yes
573 K, 30 min
HoSi,_,
3.816
4.107
+0.42
Yes
573 K, 30 min
ErSi, _-x
3.799
4.090
-0.03
Yes
673 K, 30 min
TmSi, -x
3.773
4.070
-0.71
Yes
673 K, 30 min
YbSi,_,
3.771
4.098
-0.76
Yes
623 K, 30 min
LuSi, --r
3.745
4.050
- 1.45
No
673 K, 30 min
K (%)
Thin film phase composition (X-ray diffraction dash patterns)
spond as much as possible to that of the Si-crystal lattice. Based on these criteria, films of the rare-earth metal silicides hold particular promise as compared with silicides of other metals. (2) It is found that the crystalline REM silicide phase is formed in the case of a mismatch between lattice parameters of silicide (4
and Si (4
I I %il ~
‘Si
%i
=
K<1.3%.
For K> 1.3% crystalline silicide phase does not form. (3) The formation of silicide in a REM contact determines its electrical and physical properties: the potential barrier height of a rectifying contact to p-type Si substantially decreases (by 0.2-0.3 eV); the Z--Videality factor n rises from 1.01 for a REM to 1.08 for a REM silicide. The photosensitivity of the silicide-Si structure shifts to a longer optical wavelength as compared to that of a metal film. The photosensitivity of contacts in the near ir range (1.0-1.2 pm) becomes higher. 675
V M Koleshko et al: Thin films of rare-earth
metal silicides
IO
06
Xlnm) Figure 5. Photovoltage spectra of thin-film contacts ofY (l-3), Tb (46) annealed on Si substrates: (1,4) non-annealed (3) T0 = 450°C t,40 min; (5,6) T,3OO”C, t, = 30 min. (5) Metal film was not coated with amorphous Si.
References r A E Gershinsky, A V Rzhanov and E I Cherepov, Microelectronika, 11, 83 (1982). ’ E E Kachurina, V I Mjakinenkov and V V Shcheglova, Obzory po Electronnoy tekhnike-Poluprovodn. Pribory, Vol &7 (1982). 3 A Zur, T C McGill and M-A Nicolet, J appl Phys, 57, 600 (1985). 4 V M Koleshko, V F Belitsky and A A Khodin, Deux Semin des Sciences
676
samples; (2) T,,=4X%
to = 20 min;
des Materiaux Res des Commun Univ de Constantine, 13-15 Mai 1985, Constantine, p 70 (1985). s G V Samsonov, L A Dvorina and R M Rud, Silitsidy Metallurgija, Moskva (1979). ’ G W Rubloff, Liltramicroscopy, 14, 107 (1984). ’ V M Koleshko and A A Khodin, Fiz Techn Poluprouodn, 13,1890(1979). * H Norde, J de Sousa Pires, F D’Meurle, F Pesavento, S Petersson and P A Tove, Appl Phys Letts, 38, 865 (1981).
0042-207X/86 $3.00+ .OO Pergamon Journals Ltd
1 O/pages 669 to 676/l 986
Thin films of rare-earth microelectronics V M Koleshko, V F Belitsky and A A Khodin, Ya Kolas St 6812, Minsk 220841, USSR
metal
silicides
in
Institute of Electronics, Byelorussian Academy of Sciences,
Structure, phase composition and physical properties of thin films of rare-earth metal (REM) silicides (of Y-subgroup) have been studied. The films were formed on (700) and (117) single-crystal Si substrates. X-ray diffraction analysis reveals the formation of a crystalline silicide phase of LnSi,_ x composition with hexagonal structure of the AIB,-type for all metals except SC, Gd, Lu. It is established that the crystalline silicide phase formation is determined by the crystallographic orientation of the Si substrate and that there is a relation between crystallographic parameters of REM silicide (REMS) and Si hexagonal lattices: the critical value of lattice parameter mismatch is f 1.3%, above which REMS has a disordered structure. The kinetics of the silicide phase growth was determined by measuring the conductance of thin-film structures. A model of REMS formation in thin-film structures on an Si substrate is proposed. Based on the model, conditions have been found for the formation of either quasi-amorphous, polycrystalline or monocrystalline REMS layers. The formation of a REMS amorphous film is most probable on a (100) substrate for such metals as Gd and Lu. Epitaxial growth of REMS films is typical of Tm, Er, Ho on (111) substrates. Also, investigation of the current transport in surface barrier diodes of the REM-p-Si and REMS-p-Si types shows that the potential barrier lowers as a result of silicide formation in REM-S/’ contacts and, accordingly, the silicide work function increases as compared with that of the initial metal. The photoelectric measurements indicate that the silicide-Si contact photosensitivity shifts to a longer optical wavelength as compared to that of the metal-Si contacts.
1. Introduction
Metal silicide thin films find an ever-growing use in microelectronics and semiconductor technology’. Thin films of silicides of many metals (mainly, those of IVa, Via, VIIa subgroups and platinoids) have been studied more or less thoroughly*. Formation methods have been developed, and the electrical and physical properties of metal silicides bulk and thin-film states have been investigated. Basic criteria for formation of stable electric contacts ‘silicidesilicon substrate’ and of epitaxial thin-film layered ‘silicide-silicon’ systems are as follows: high chemical activity of metal; relatively low silicide formation temperature; crystal lattice correspondence (crystallographic parameters and type+ubic, hexagonal, etc.) of silicide and semiconductor. Lattice mismatch between some transition metal silicides and silicon crystals does not exceed about 2%3.
formed at relatively low temperatures (about 600 K). Among other properties of the silicides5, their high microhardness (80.0-100.0 N mmm2) should be mentioned as this is nearly equal to that of silicon. Thermal expansion coefficients of REM disilicides are closer to silicon than those for Mn, Ti, Cr, but lower than those for Ta and MO silicides. REM silicides react with oxygen at temperatures higher than 773 K, and REM disilicides are stable up to 1273 K; unlike other refractory metal silicides (MO, W, Ta), stable oxide films form on their surface. However, thin-film REM disilicides have been most poorly studied. Thus, phase diagrams for these metals with silicon are lacking except for some metals. A complete diagram has been constructed only for Y, and, partially, for Dy, Er and Gd. There are no data for the enthalpy of silicide formation. Reliable electrical resistivity data are not available for all the REM silicides. Experimental values for the work function of silicides have not been obtained. As the work function is one of the crucial parameters for ‘silicide-Si’ contacts, it was calculated from the expression6:
2. Statement of the problem (psi,=((p”,(pa) l/(n+k) Analysis of chemical activity of various metals, formation temperature of their silicides, and comparison of lattice parameters and crystal structure of silicides and silicon (Tables 14) shows that rare-earth metal (REM) disilicides of Y-subgroup satisfy the above criteria to the greatest degree4. Indeed, REMs have a higher chemical activity than most other metals. Thus, shallow contacts in Si and other microcircuit components can be
(1)
where cp, is the work function of the metal, ps is the Si electron affinity, n and k are variable coefficients whose values correspond to various silicide types. The value for cpswas taken as 4.4 eV for low-doped Si and 4.2 eV for p-type Si. The calculated results are given in Figure 1. Calculated values of qpsi,were compared to the experimentally determined barrier heights in silicide on n- and 669
V M Koleshko
et al: Thin films of rare-earth metal silicides
Table 1. Comparison
of crystallographic
Sihcide
Lattice
Si Si TiSi, TiSi Ti,Si, HfSi, HfSi Hf,Si, VSi, V,Si, V,Si, V,Si V,Si TaSi, Ta,Si, CrSi, CrSi Cr,Si, MoSi,(a) MoSi,(B) Mo,Si, MosSi, WSi, W,Si, W,Si, Nisi,., Nisi CoSi, CoSi Ni,Si, Ni,Si(O) RhSi RhSi,,,
Hex fee Rhomb Rhomb Hex Rhomb Rhomb Hex Hex Hex Tetr Hex Cub Hex Hex Hex Cub Tetr Tetr Hex Tetr Hex Tetr Tetr Hex Cub Rhomb Cub Cub Rhomb Hex Cub Rhomb Cub Rhomb Rhomb Hex Rhomb Hex Rhomb Hex Rhomb Hex Cub Tetr
Rh,Si, Rh,Si IrSi, IrSi Ir,Si, PdSi Pd,Si PtSi Pt,Si Pt,Si Pt,Si
Table 2. Crystallographic
type
parameters
Lattice a (A)
and formation
Lattice
670
Silicide
Lattice
ScSi, _ I YSizmX YSi, -x GdSi,_, TbSi, _x DySi,_, HoSi, _ * ErSi, _x TmSi,_, YbSi, _x LuSi,-,
AlB, AlB, AlB, AlB, AlB, AIB, AlB, AlB, AlB, AIB, AIB,
parameters
3.80 5.428 8.252 6.531 1.456 3.611 6.855 7.890 4.571 11.851 9.429 7.135 4.721 4.783 1.414 4.43 1 4.629 9.170 3.203 4.642 9.642 7.28 3.211 9.605 7.19 5.4066 5.18 5.3627 4.4445 12.229 3.805 4.675 5.531 2.963 5.317 5.408 4.350 5.558 3.96 5.599 6.528 5.595 6.436 5.63 3.933
parameters
type
Hex Hex Hex Hex Hex Hex Hex Hex Hex Hex Hex
of metal silicides and silicon
c (A)
b (A)
Formation temperature (R)
6.28 8.540 4.897 5.162 3.649 3.191 5.558 6.312 3.623 4.157 4.842
4.783 3.631 14.550 3.753
873 713 lb23 8233973 873
6.565 5.224 6.364
873-923 123
4.636 7.855 6.529 4.905 5.00 7.868 4.964 4.85
800 800
923
5.62
3.34
6.924 4.890
10.805
6.362
3.063
3.895 7.383 6.610 6.213 5.126 6.133 3.437 5.932 3.569
10.131 3.930 3.211 3.381 3.603
1023 623-773 823 650-773 723 473-623 623-700
673 1273 673-713 1073 3733573 5w573 4733773 4733573
5.910
temperature
of REMS of yttrium
subgroup
Formation temperature
parameters
a (A)
c (A)
Y(%)
(R)
3.66 3.842 3.836 3.877 3.847 3.831 3.816 3.799 3.773 3.771 3.745
3.87 4.140 4.139 4.172 4.146 4.121 4.107 4.090 4.070 4.098 4.050
-3.7 +1.1 f0.9 f2.0 f1.2 +0.82 +0.42 -0.03 -0.71 -0.76 - 1.45
6733723 60&673 6OCk673 623-673 5733623 573-623 60&623 60&623 623673 4733623 6133723
V M Koleshko et a/: Thin films of rare-earth
metal silicides
Table 3. Crystallographic
parameters
and formation
Lattice a (A)
Lattice
LaSi, _-r LaSi, _ II LaSi La& La,%, or-CeSi, j3-CeSi, CeSi a-P&, B-PrSi, PrSi Pr,Si, a-NdSi,
Tetr Rhomb Rhomb Tetr Tetr Rhomb Tetr Rhomb Rhomb Tetr Rhomb Tetr Rhomb
4.322 4.270 8.404 7.87 7.95 4.19 4.175 8.302 4.17 4.291 8.240 7.93 4.13
13.86 14.05 6.059 4.50 14.04 13.92 13.848 5.694 13.85 13.76 5.920 13.97 13.97
kFP* Nd,Si, a-SmDi, /?-SmSi, SmSi Sm,Si, a-EuSi /I-EuSi,
Tetr Rhomb Hex Rhomb Tetr Rhomb Hex Rhomb Tetr
4.103 8.240 8.66 4.105 4.041 8.055 8.56 4.72 4.29
13.53 5.920 6.53 13.46 13.33 5.804 6.45 3.99 13.66
Silicide TiSi, VSi, CrSi, CoSi, Nisi Nisi, ZrSi, NbSi, MoSi, RhSi Pd,Si PdSi HfSi HfSi, TaSi,
WSi, Pt,Si PtSi
type
and physical
properties
of cerium subgroup Formation temperature
parameters c (A)
Silicide
Table 4. Electrical
b (A)
(K) 550
4.170 4.010
4.13
673-773 673-773
3.962 4.11 3.941 4.10 3.941 4.035 3.888 11.15
of silicides
Electric resistivity (~lO_~Qcm) 13-16.25 16.7-18 5&55 9.5-13.5 600 18-20.25 64.8 5&60 3540 50 4GlOO 18-75 3G35 20 45-50 3540 50-55 6@70 3&100 28-35
Si (Table 5). Such comparison enabled us to choose (psi, corresponding to experimental values of qb. It turned out, that the values of (psi, obtained correspond to the following set of coefficients: n = 1, k = 1-2, which are characteristic of the silicide phase of the type LnSi,_,, i.e. of REM disilicides. It should be noted that in this case equation (1) is closely approximated by a linear function: p-type
of REMS
Barrier height for n-type Si(eV)
Silicide work function (cV)
Metal work function (eV)
0.6
3.674.25
4.33 4.3
0.57 0.65
4.93 4.36
4.5 5.0
0.66 0.7 0.55
4.54.55 4.55 3.86
0.55 0.69 0.745
44.25 4.Gl.8 4.84.97 4.94-5.20
5.22 5.22 4.05 4.02-4.3 4.53
0.55 -
-
0.59
4.15
4.98 5.12 5.12 3.9 3.9 4.15425
0.65 0.78 0.87
4.55-4.8 5.54-5.71 5.15-5.75
4.554.63 5.65 5.65
Vpsil=
clVp,
+
B
(2)
where a=O.43-0.64, /I= 1.87-2.68. Analysis of the crystal lattices of the silicides of various metals as compared with silicon4 showed that REM disilicides of Y-subgroup provide, in some cases, almost perfect matching by the lattice parameter ‘a’ (Table 2). The crystal lattice of such 671
V M Koleshko ef al: Thin films of rare-earth
metal silicides
diffusion process and promote solid phase epitaxy of the silicide. In addition, such silicides (LnSi, _,) are characterized by a rather large homogeneity interval (x z o-0.4), that is x may vary without changing the disilicide crystal structure. This ensures good resistance of the silicide phase to variations in stoichiometry which is important for thin-film composition formation. Thus, it follows from theoretical discussion that disilicides of Y-subgroup REMs are most suitable for production of contacts to Si because of favourable chemical and thermodynamical properties of the ‘silicide forming metal-W system and due to matching of lattice types and parameters of the silicide and semiconductor.
3. Experimental
1.
.. 26
t
)
3.0
28
cp
m
3.2
(ev)
Figure 1. Relation between the work functions of silicide (Q) correspondingmetal(cp,).(l)n=l,k=4;(2)n=l,k=2;(3)~=l,k=4;(4) n=2,k=3;(5)n=l,k=2;(6)n=l,k=l;(7)n=2,k=1;(8)n=4,k=1;(3) and (5) (p,=4.8 eV; for the rest qo,=4.41 eV; (9) n= 1, k=2,
and
(p,=4.2eV.
silicides consists of alternating atomic layers of metal and silicon (Figure 2) the atomic arrangement being the same as that which occurs in separately taken metal and Si lattices. Such a lattice structure of silicide favours greatly its formation in thin-film structures. Layered arrangements of atoms should facilitate the (a)
Thin films of REM silicides were obtained by heat treating REM-Si film structures. Two types of structures were studied: a one-layer structure which, prior to annealing, represents a thin REM film deposited on an Si substrate, and a two-layer structure which differs in that an amorphous Si layer was deposited upon a substrate-supported metal film. Single-crystal (100) and (111) Si wafers doped with phosphorus up to 20 Q cm were used. Prior to the deposition of REM the wafers were chemically treated, i.e. thoroughly cleaned, degreased, ‘refreshed’ and etched in 1:lO HF: H,O and treated with an ammonium peroxide solution. Thin films of metals and silicon were obtained using vacuum evaporation at about 3 x 10m4 Pa from a tungsten evaporator. REM bi-distillates and milled single-crystal P-doped silicon were used as starting materials. The thickness of the REM and Si films was monitored by means of optical interference microscopy to within f5 nm. Before metal deposition, the substrates were heated to 423 K to remove impurity residues. The substrate temperature during film deposition did not exceed 373 K. The as-formed one- and two-layer thin-film structures were annealed under vacuum (3 x 10m4 Pa) by ir irradiation. The anneal temperature (T,) varied in the range 473-773 K, and the exposure ranged from 5-120 min. The phase composition of silicide films was studied using X-ray diffraction techniques and He+-Rutherford backscattering. The electrical resistivity was measured in air using a four-point probe technique with an accuracy of k 1%, after annealing and removing the sample from the vacuum chamber. The film morphology was studied using a scanning electron microscope.
4. Results and discussion
It was found that annealing
of most REMs on Si substrates at T, = 573-623 K (for Yb films already at T,, a473 K) resulted in the formation of REM disilicides with the general formula LnSi, _ x. Table 6 summarizes the results of the X-ray diffraction analysis of Si-REM-Si film structures subjected to thermal treatment. As shown, not all the metals analysed form a polycrystalline silicide phase of LnSi, _-xcomposition. The table compares X-ray data for lattice parameters of corresponding silicides with respect to the Si lattice. This analysis resulted in a correlation between the REM disilicide crystalline phase with A,B,-type lattice and the relative difference in CIparameter of silicide lattice (a,,J and silicon (asi):
I Figure 2. Crystallographic scheme of epitaxial contact of REM disilicide to Si(ll1) (a) metal sublattice, (b) silicon sublattice, (c) silicon lattice. 672
%i
I
For K-c 1.3% the formation of a crystalline phase is observed more intensively and at lower temperatures for smaller values of
V M Koleshko et al: Thin films of rare-earth metal silicides
Table 5. Electrical and
physical properties of REMS disilicides of yttrium subgroup
Silicide
Electric resistivity (lo-‘xncm)
cp; (EeV)
cp; (eV)*
‘psi1 (eV)
(“e;,
GdSi,
31
0.37
3.8Ck4.14
3.10
DySi,
30
0.37
3.8C4.21
3.25
HoSi, ErSi, _ x
28 11.4
0.37 0.39
3.844.19 3.8C4.21
3.22 3.25
YSi,_,
20.2
0.39
3.8C4.23
3.3
TbSi, YbSi,
17.4 9.8
0.4 -
0.71 (0.6wl.86) 0.73 0.807 0.640.85) 0.70-0.85 0.7cO.77 (0.640.85) 0.71 0.74-0.75t (0.640.85) (0.65M.86) (0.68X1.92)
3.814.16 3.58-3.91
3.15 2.60
* Authors’ calculations in brackets. t Data from ref 8. K. For K> 1.3% an REM silicide phase is not detected, at least in the range of annealing temperatures 573-673 K. REM silicides of A,B,-type are represented by alternating layers of Si and metal atoms (Figure 2). For small K, the lattice of such a silicide can be pictured as an extended Si lattice with builtin layers of metal atoms. In such a process of silicide formation the Si substrate seems to serve as a seed for oriented growth of REM silicide crystals in the film, the initial structure of which, formed by interdiffusion of REM and amorphous Si layers, is disordered. However, if the mismatch between REM silicide and Si lattices is large, the ordering of a crystalline structure due to a stimulating effect of the substrate does not occur within the temperature range studied. Recrystallization of this layer is probably possible at temperatures close to the REM melting point. Along the c axis of the hexagonal lattice, the difference between REM disilicide and Si is, as can be clearly seen from Table 4, 3439%, i.e. the elementary cell of the REM silicide is an Si cell deformed along the c axis (also, with the a parameter somewhat changed) by introduction of a REM atom between Si atoms. Such deformation causes a reduction in the interlayer atomic distance in the REM silicide lattice as compared with the Si lattice. With the above data taken into consideration, two variants of low-temperature (573-673 K) ordering of REM silicide crystalline structure can be envisaged in thin-film structures. In the first variant the atoms of metal penetrate into the semiconductor at random along one or several possible directions parallel to the Si layers. The lattice extends chaotically, thus leading to the formation of a fine crystal structure. When going from the substrate, prevailing grain orientation directions are notable which are characterized by a maximum growth rate, i.e. [ 1001 and [ 1101, while [l 1 l] directions are suppressed. In this case the REM silicide film experiences a build-up of stresses, with a component parallel to the substrate plane, thus leading to pit formation in the film. The equal probability in three directions of a maximum growth rate, (lOO), (OlO), (OOl), causes the formation of pits of trihedral and/or hexagonal shapes. In the second variant, atomic diffusion and reordering at the silicide crystal lattice occur normal to the substrate surface. As a result, the deformation stresses in REM silicide films reduce to ones which are purely normal to the substrate surface and the REM silicide-Si interface contains a minimum number of crystallographic imperfections. This mechanism is less probable
than the former because of a slower penetration ability of metal atoms normal to the Si layers, i.e. in the [ 11l] direction, as well as a higher sensitivity to the Si surface finish, metal film deposition and subsequent annealing conditions. Nevertheless, the REM silicide-Si epitaxial structure formation might be expected with a perfect interface, minimum stress and low contact resistance if vacuum ion-cleaning, magnetron sputtering of silicide targets or molecular-beam epitaxy techniques were employed. Judging by Tables 2 and 6, the best metal in this respect appears to be erbium. It should also be expected that silicides of those metals whose lattice parameter a is most close to that of Si, i.e. Er, Ho, Tm, would have the highest crystallization rate, while those with the greatest difference in this parameter, i.e. SC, Gd, Lu, would have the lowest rate. The kinetics of thin film REM silicide formation can be monitored by the change in electrical resistivity of the two-layer compositions during annealing (Figure 3). On the curve of p/pc=f(t) several characteristic portions can be noted (Figure 3(a)), curve 2). In the ‘ab’ portion, a rise in the film resistivity is observed due to formation of a disordered silicide phase. For a two-layer structure this phase forms both on the amorphous Si film side and on the single-crystal substrate side. On the amorphous film side a disordered phase is formed, and on the substrate side a crystalline phase builds up. When one-layer compositions are annealed the ‘ab’ portion is preceded by a smooth reduction in p associated with lowering of the film p under annealing. Silicide formation occurs at t, = b. At this point, upper and lower silicide fronts merge (on the film side and on the substrate side). A decay in p in the portion ‘bc’ can be explained by ordering of the silicide crystalline structure. A rise in p in the portion ‘cd’ is due to interaction of the silicide film with residual gases in the vacuum chamber and the film enrichment with Si from the substrate. These phenomena manifest themselves in the growth of pyramids on the film surface with the pyramid faces being oriented in the same way as the substrate. The silicide formation in the contact ‘REM-Si’ determines, in particular, the photoelectric properties of the contact. The potential barrier heights of a rectifying contact to p-type Si substantially decrease (by about 0.2-0.3 eV). The I-Vcharacteristics degrade insignificantly and the Z-V ideality factor increases from n= 1.01 to n= 1.08 (Figure 4). The latter is indicative of a planar character for the metal-Si substrate interaction front. 673
V M Koleshko et a/: Thin films of rare-earth
metal silicides
(0)
0.9
T
0.8
Q
0.7 0.6 0
20
40
60
0.5 0
80
20
to (min) Figure3. Variationin
resistivityof Tm; (8) Er. r, = 673 K.
to
REM films under annealingon(a)(lOO)Si
substrateand
v Figure 4. I-Vcharacteristics 623 K for 30 min.
of Er-p-Si(ll1)
contact
in case of silicide formation
Examination of the surface of the contact structure by scanning electron microscopy showed that it remains uniform over the entire contact area. The photosensitivity of silicide structures shifts to the longwave part of the spectrum as compared to that of a REM-Si contact (Figure 5, curves 3,6 and 1,4). The sensitivity of the contacts increases in the near ir-range (l.Gl.2 pm). Initial REM films not subject to heat treatment have enhanced photosensitivity in the 500-850 nm range (Figure 5, curves 1,4, portion ab). This is associated with the presence of a potential barrier in the REM-Si contact whose height exceeds the bandgap of Si’. As the annealing temperature is increased and the silicide, whose work function is much larger (Figure l), is formed, the barrier height is reduced and the portion nb disappears (Figure 5, curves 2,3, 5,6). 674
40
60
80
(min)
(b)(ll l)Si substrate. (l)Tb;(2) Dy; (3)Yb(4)Y;(5) Lu;(6)Gd;
(7)
(VI
under thermal
annealing.
(1) Initial structure,
(2) after annealing
at
The observed shift of the photosensitivity peak may also be associated with a high doping of the near-surface Si layer arising from metal diffusion during annealing of the REM film on the Si substrate.
5. Conclusions (1) When selecting metal silicides for preparation of stable electrical contacts to silicon the following fundamental criteria should be used: the silicide-forming metal should have a high chemical activity; the thin-film silicide should be formed at a relatively low temperature (about 473673 K), and the silicide lattice (crystallographic parameters and structure) should corre-
V M Koleshko et al: Thin films of rare-earth metal silicides
Table 6. Formation of REM disilicides of Y-subgroup during thermal treatment of two-layer compositions Silicide composition
Lattice parameters a (A) c (A)
Si
3.80
6.28
ScSi, _ x
3.66
3.87
YSi,_,
3.842
GdSi,_,
Disilicide
Thermal treatment conditions
-3.68
No
673 K, 30 min
4.140
+1.11
Yes
673 K, 30 min
3.877
4.172
+ 2.03
No
673 K, 30 min
TbSi,_,
3.847
4.146
+ 1.24
Yes
623 K, 30 min
DySi, --x
3.831
4.121
+0.82
Yes
573 K, 30 min
HoSi,_,
3.816
4.107
+0.42
Yes
573 K, 30 min
ErSi, _-x
3.799
4.090
-0.03
Yes
673 K, 30 min
TmSi, -x
3.773
4.070
-0.71
Yes
673 K, 30 min
YbSi,_,
3.771
4.098
-0.76
Yes
623 K, 30 min
LuSi, --r
3.745
4.050
- 1.45
No
673 K, 30 min
K (%)
Thin film phase composition (X-ray diffraction dash patterns)
spond as much as possible to that of the Si-crystal lattice. Based on these criteria, films of the rare-earth metal silicides hold particular promise as compared with silicides of other metals. (2) It is found that the crystalline REM silicide phase is formed in the case of a mismatch between lattice parameters of silicide (4
and Si (4
I I %il ~
‘Si
%i
=
K<1.3%.
For K> 1.3% crystalline silicide phase does not form. (3) The formation of silicide in a REM contact determines its electrical and physical properties: the potential barrier height of a rectifying contact to p-type Si substantially decreases (by 0.2-0.3 eV); the Z--Videality factor n rises from 1.01 for a REM to 1.08 for a REM silicide. The photosensitivity of the silicide-Si structure shifts to a longer optical wavelength as compared to that of a metal film. The photosensitivity of contacts in the near ir range (1.0-1.2 pm) becomes higher. 675
V M Koleshko et al: Thin films of rare-earth
metal silicides
IO
06
Xlnm) Figure 5. Photovoltage spectra of thin-film contacts ofY (l-3), Tb (46) annealed on Si substrates: (1,4) non-annealed (3) T0 = 450°C t,40 min; (5,6) T,3OO”C, t, = 30 min. (5) Metal film was not coated with amorphous Si.
References r A E Gershinsky, A V Rzhanov and E I Cherepov, Microelectronika, 11, 83 (1982). ’ E E Kachurina, V I Mjakinenkov and V V Shcheglova, Obzory po Electronnoy tekhnike-Poluprovodn. Pribory, Vol &7 (1982). 3 A Zur, T C McGill and M-A Nicolet, J appl Phys, 57, 600 (1985). 4 V M Koleshko, V F Belitsky and A A Khodin, Deux Semin des Sciences
676
samples; (2) T,,=4X%
to = 20 min;
des Materiaux Res des Commun Univ de Constantine, 13-15 Mai 1985, Constantine, p 70 (1985). s G V Samsonov, L A Dvorina and R M Rud, Silitsidy Metallurgija, Moskva (1979). ’ G W Rubloff, Liltramicroscopy, 14, 107 (1984). ’ V M Koleshko and A A Khodin, Fiz Techn Poluprouodn, 13,1890(1979). * H Norde, J de Sousa Pires, F D’Meurle, F Pesavento, S Petersson and P A Tove, Appl Phys Letts, 38, 865 (1981).