We investigated temperature dependence of contact resistance of an
Au-Ti-Pd2Si ohmic contact to heavily doped n+-Si. The contact resistance
increases with temperature owing to conduction through the metal shunts. In
this case, the limiting process is diffusion input of electrons to the metal
shunts. The proposed mechanism of contact resistance formation seems to realize
also in the case of wide-gap semiconductors with high concentration of surface
states and dislocation density in the contact

Belyaev, A. E.

Boltovets, N. S.

Konakova, R. V.

Kudryk, Ya. Ya.

Sachenko, A. V.

Sheremet, V. N.

Semiconductor Physics Quantum Electronics & Optoelectronics

English

arXiv

We investigated temperature dependence of contact resistance of an

Au−Ti−Pd₂Si ohmic contact to heavily doped n⁺

-Si. The contact resistance increases with

temperature owing to conduction through the metal shunts. In this case, the limiting

process is diffusion input of electrons to the metal shunts. The proposed mechanism of

contact resistance formation seems to realize also in the case of wide-gap semiconductors

with high concentration of surface states and dislocation density in the contact

Belyaev, A.E.

Boltovets, N.S.

Konakova, R.V.

Kudryk, Ya.Ya.

Sachenko, A.V.

Sheremet, V.N.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

 Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 4. P. 436-438.  PACS 73.40.Cg, 73.40.Ns, 85.30.-z Temperature dependence of contact resistance  of Au−Ti−Pd2Si−n+-Si ohmic contacts A.E. Belyaev1, N.S. Boltovets2, R.V. Konakova1, Ya.Ya. Kudryk1, A.V. Sachenko1, V.N. Sheremet 1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine 41, prospect Nauky, 03028 Kyiv, Ukraine Phone: (380-44) 525-61-82; fax: (380-44) 525-83-42; e-mail: konakova@isp.kiev.ua 2State Enterprise Research Institute “Orion”, 8a Eugene Pottier St., Kyiv, 03057, Ukraine   Abstract. We investigated temperature dependence of contact resistance of an Au−Ti−Pd2Si ohmic contact to heavily doped n+-Si. The contact resistance increases with temperature owing to conduction through the metal shunts. In this case, the limiting process is diffusion input of electrons to the metal shunts. The proposed mechanism of contact resistance formation seems to realize also in the case of wide-gap semiconductors with high concentration of surface states and dislocation density in the contact.   Keywords: wide-gap semiconductor, ohmic contact, contact resistance.  Manuscript received 01.11.10; accepted for publication 02.12.10; published online 30.12.10.       1. Introduction  In the last few years, some reports have appeared on anomalous temperature dependence of contact resistance Rc (growth with temperature) of ohmic contacts to semiconductors with high dislocation density [1-3]. It should be noted that such behavior of Rc (T) curves cannot be explained by the classical mechanisms of current transport (thermionic, thermal-field, tunnel) in the Schottky contacts. The authors of the above papers related the obtained Rc (T) dependences to conduction through the metal shunts linked to dislocations. In the present work, we describe growing with temperature dependences of contact resistance (measured in the 80−380 K temperature range) that take place in  ohmic contacts to heavily doped silicon. To explain them, we also apply the mechanism of current transport through metal shunts. It is supposed that the limiting process is the mechanism of diffusion input of electrons to those metal shunts. The theoretical estimations showed that, in the case of heavily doped semiconductor material, such a limitation is possible only if accumulation band bending is realized in the semiconductor near the metal shunt ends. Such a situation seems rather realistic if one takes into account both the edge effect (leading to big increase of the electric field strength) and mirror image forces. In our case, not only a big decrease of barrier height near the shunt edge occurs but the band bending changes its sign as well. SiPdTiAu 2−−We calculated contact resistance, with current limiting mechanism of diffusion input of electrons, by assuming that a rather high barrier is realized in the regions far from dislocations, so that it is possible to neglect current transport through the above regions. In that case, the contact resistance Rc can be determined from the derivative of the current through all dislocations. The density of the thermionic current Jnc flowing through the contact at the site of dislocation outlet can be determined by solving the continuity equation for electrons. With allowance made for the diffusion limitation, the contact resistance related to a single dislocation, Rc0, is (in the case of a nondegenerate semiconductor) 004410cDcydTLynTceNqVeDVqkTR⎟⎟⎠⎞⎜⎜⎝⎛ +=α. (1) Here k is the Boltzmann constant, Т temperature, q electron charge, VT mean thermal velocity of electrons, Dn electron diffusion coefficient, yc0 dimensionless equilibrium band bending at the site of dislocation outlet, α numerical coefficient (of the order of unity), LD Debye shielding length, and Nd donor concentration in the semiconductor. If the semiconductor is degenerate and the current limitation due to the diffusion input takes place, one can  © 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  436  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 4. P. 436-438.  50 100 150 200 250 300 350 4000246 © 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  81012ρ c, ·10-6  ·cm2T,K12Ωalso use Eq. (1). In that case, however, the following relation between the diffusion coefficient Dn and electron mobility μn has to be used: 1)(ln−⎟⎟⎠⎞⎜⎜⎝⎛μ=zdndqkTD nn . (2) Here  is the dimensionless Fermi energy, and n is the electron concentration in the semiconductor bulk: kTEz f /=( )∫∞−+⎟⎠⎞⎜⎝⎛π= 05.1exp13002 dxzxxTNn c  , (3) where Nc is the effective density of states in the conduction band at Т = 300 K. The area from which the current flowing through a single dislocation is collected equals , where 2DLπFig. 1. The temperature dependences of contact resistivity  ρc for Au−Ti−Pd2Si−n+-Si ohmic contact (filled squares – experiment, curves – theory). Curve 1 (2) is plotted for Nd = 1019 cm-3 (3×1019 cm-3). The values of parameters used in calculation: VT = 107 cm/s; curve 1: yc0 = 2.7; ND1 = 2.8×109 cm-2; curve 2: yc0 = 1.5; ND1 = 5×109 cm-2. ( ) 2/1' 2/15.020 )(2−Φ⎟⎟⎠⎞⎜⎜⎝⎛ εε= zNqTkLcsD  (4) is the Debye shielding length at an arbitrary degree of semiconductor degeneracy,  vacuum (semiconductor) permittivity, and ( )sεε 0( )( )( )∫∞−+−π=Φ 0 2'2/1 exp1exp2)( dxzxzxxz . (5) The contact resistance for a contact of unit area determined by the mechanism of diffusion input is 120diffDDcNLRR π= ,  (6) where Nd1 is the density of dislocations that take part in current transport. In general, Nd1 is not equal to the density of dislocations that take part in electron scattering, Nd2. The current transport is related to the dislocations that are normal to the contact plane, while scattering occurs at those dislocations that are oriented at an angle to the contact plane. The quantity  (S is the contact area) has a meaning of the total area from which the current flowing through all dislocations is collected. As a rule, the value of  is much less than unity, even at maximal dislocation densities (about ), except the case of weakly doped semiconductors ( ). SNL DD 12π12DD NLπ21110 cm1010 −−315 cm10 −≤dNThe electron diffusion coefficient Dn was determined with fitting procedure using Eqs. (2) and (3), while the temperature dependence of electron mobility μn, when comparing the calculated and experimental dependences, was taken from [4] for the doping level of . The rated resistivity ρ319 cm108.2 −× Si of silicon samples used to make ohmic contacts was 0.002 Ω⋅cm. Taking into account its standard spread and the fact that predominant scattering at Т = 300 K is that on the charged impurities (as a result, the dependence of ρSi on the doping level is very weak), one can determine the limits for variation of donor concentration in the samples studied: ( ) 31919 cm10310 −×− . So we used both limiting values for donor concentration when plotting Fig. 1. Shown in Fig. 1 are the experimental temperature dependence of contact resistivity for Au−Ti−Pd2Si−n+-Si ohmic contact с ρSi = 0.002 Ω⋅cm and ρc (T) curves calculated for two donor concentrations:  and . The contact metallization was made using layer-by-layer vacuum thermal sputtering of metals onto an n319 cm10 −319 cm103 −×+-silicon substrate heated up to 330 °C. The ohmic contact was formed by the Pd2Si phase that appeared in a thin near-surface layer of  in the course of palladium sputtering. Because of misfit of PdSi−+n2Si and Si lattices and coefficients of thermal expansion, high density of structural defects (in particular, dislocations) is formed in the near-surface layer of  [5-7]. According to [1-3], metal shunts (that short-circuit the space charge region) may form at the above defects. One should note very good agreement between the theory and experiment. It was obtained by using the general Eqs. (2)–(5) that take into account degeneracy of semiconductor. Si−+nThe mechanism of contact resistance formation proposed in the present work for metal−semiconductor contacts has to realize, first of all, in the case of wide-gap semiconductors with high concentration of surface states in the contact. It looks rather paradoxical because current is transported through the regions accumulating electrons rather than the depletion regions. At the same time, the practically ideal agreement between the 437  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 4. P. 436-438.  calculated and experimental temperature dependences of contact resistivity is indicative of the validity of the above mechanism. One should note that, in the model proposed in [1-3], the temperature dependence of contact resistance was strictly linear. References 1. T.V. Blank, Yu.A. Gol’dberg, O.V. Konstantinov, V.G. Nikitin, E.A. Posse, Peculiarities in the mechanism of current flow through an ohmic contact to gallium phosphide // Technical Physics Letters 30(19), p. 806-809 (2004). 2. T.V. Blank, Yu.A. Gol’dberg, Mechanisms of current flow in metal-semiconductor ohmic contacts // Semiconductors 41(11), p. 1263-1292 (2007). 3. V.N. Bessolov, T.V. Blank, Yu.A. Gol’dberg, O.V. Konstantinov, E.A. Posse, Dependence of the   mechanism of current flow in the in n-GaN alloyed ohmic contact on the majority carrier concentration // Semiconductors 42(11), p. 1315-1317 (2008). 4. V.I. Fistul’, Heavily Doped Semiconductors. Nauka, Moscow (1967), in Russian. 5. A.E. Gershinsky, A.V. Rzhanov, E.I. Cherepov, Thin-film silicides in microelectronics // Mikro-elektronika 11(2), p. 83-94 (1982), in Russian. 6. P.E. Schmid, P.S. Ho, H. Foll, G.W. Rubloff, Electronic states and atomic structures at the Pd2Si−Si interface // J. Vac. Sci. Technol. 18(3), p. 937-943 (1981). 7. V.M. Ievlev, S.B. Kushev, A.V. Bugakov, S.A. Soldatenko, B.N. Markushev, I.G. Rudneva, Conjugation regularities and substructure of interphase boundaries in the Si−Me silicide systems (Me: Pt, Pd, Ni, Re, Ir, Mo, Ti) // Proc. ISFTE-12, Kharkov, Ukraine, 2002, p. 201-206.  © 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  438 

Temperature dependence of contact resistance of Au−Ti−Pd2Si−n⁺ -Si ohmic contacts

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Temperature dependence of contact resistance of Au-Ti-Pd2Si-n+-Si ohmic
  contacts

Temperature dependence of contact resistance of Au-Ti-Pd2Si-n+-Si ohmic contacts

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