The effect of frequency and illumination intensity on the main electrical characteristics of Al-TiW-Pd 2 Si/n-Si structures at room temperature

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1 JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vl. 12, N. 2, February 2010, p The effect f frequency and illuminatin intensity n the main electrical characteristics f Al-TiW-Pd 2 Si/n-Si structures at rm temperature H. USLU *, Y. ŞAFAK, İ. TAŞÇIOĞLU, Ş. ALTINDAL Physics Department, Gazi University, Ankara, Turkey The effect f frequency and illuminatin intensity n the main electrical parameters such as ideality factr (n), zer bias barrier height (Φ B), depletin layer width (W D), dping cncentratin (N D) and interface state densities (N ss) f Al-TiW- Pd 2Si/n-Si structures have been investigated by using current-vltage (I-V) and admittance spectrscpy (C-V and G/w-V) techniques at rm temperature. In additin, the dielectric cnstant (ε ) and dielectric lss (ε ), lss tangent (tanδ) and ac electrical cnductivity (σ ac) have been investigated using C-V and G/w-V measurements at varius frequencies and illuminatin intensities. Experimental results shw that bth the value f capacitance (C) and cnductance (G/w) increase with increasing illuminatin intensity and decreasing frequency. On the ther hand the value f R s decreases with increasing illuminatin densities. Als, the ε, ε, tanδ and σ ac values were fund strngly frequency, bias vltage and illuminatin intensity. The results can be cncluded t imply that the interfacial plarizatin can mre easily ccur at lw frequencies and high illuminatin intensities cnsequently cntributing t the deviatin f electrical and dielectric prperties f Al-TiW-Pd 2Si/n-Si structures. (Received January 15, 2009; accepted February 18, 2010) Keywrds: Al-TiW-Pd 2 Si/n-Si structures, Illuminatin effect, I-V and C-V measurement, Electrical characteristics, Dielectric prperties, AC cnductivity 1. Intrductin In general, platinum silicide (PtSi) cntacts suffer frm a number f prcessing difficulties assciated with the Pt due t its high melting temperature. Therefre, palladium silicide (Pd 2 Si) and Al-TiW-Pd 2 Si ffer a useful alternative t Al and PtSi as a rectifier cntact material. Thus, palladium silicide films are cmmnly used in the fabricatin f metal-semicnductr (MS) and metalinsulatr-semicnductr (MIS) type Schttky barrier dides (SBDs) n silicn devices [1-9]. Because f the imprtance f metal silicide in integrate circuit technlgy Pd 2 Si films n Si have received a lt f attentin in the past. The first studies n Pd 2 Si films were made by Kircher [1], wh investigated the metallurgical prperties and electrical characteristics f Pd 2 Si cntacts n n-si. Wittmer et al. [6] and Shepela [7] perfrmed the measurement f resistivity f Pd 2 Si film grwn n Si substrate. Hwever, the illuminatin effect n main these parameters f metal-pd 2 Si/Si structures at varius frequencies have been nt investigated in detail yet. Bth electrical and dielectric prperties f the SBDs especially depend n the native r depsited interfacial insulatr layer, the N ss and barrier frmatin at M/S interface. Palladium silicide (Pd 2 Si) ffers a useful alternative t Al-PtSi as a cntact material. In this study, the frequency and applied bias vltage dependent dielectric parameters such as dielectric cnstant (ε ) and dielectric lss (ε ), lss tangent (tanδ) and ac electrical cnductivity (σ ac ) f Al-TiW-Pd 2 Si/n-Si structures have been investigated by using C-V and G/w-V measurements at varius frequencies and illuminatin intensities at rm temperature. 2. Experimental prcedure The Al-TiW-Pd 2 Si structures were fabricated n 2 inch diameter n-type (P dped) single crystal silicn wafer with (111) surface rientatin, 0.07 Ω-cm resistivity, 3.5 μm thickness and the area f 8 x 10-6 m 2. After the cleaning prcess, high purity Al with a thickness f abut 2000 Å were thermally evaprated nt whle back side f Si wafer at a pressure abut 10-6 Trr in high vacuum system. The hmic cntacts were frmed by annealing them fr a few minutes at 723 K. Al was als depsited nt TiW- Pd 2 Si/n-Si structure by he same methd until the thickness f Al film n Ti 10 W 90 -Pd 2 Si-nSi layer was abut 1 μm. The Al-TiW-Pd 2 Si/n-Si structures and details f the fabricated prcedures were given in ur previus study [4]. I-V and cnductance measurements were perfrmed by use f a Keithley 2400 I-V surce-meter and an HP4192A LF impedance analyzer, respectively.

2 The effect f frequency and illuminatin intensity n the main electrical characteristics f Al-TiW-Pd 2 Si/n-Si Results and discussins 3.1. Electrical Characteristics f Al-TiW-Pd 2 Si/n-Si Fr SBDs with a series resistance Rs, the relatin between the applied frward bias vltage (V) and the current (I) can be written as [10,11]: I = I q( V IRs ) q( V IRs ) exp 1 exp (1) nkt kt where I is the reverse saturatin current and it can be described by 2 qφ I = AA* T exp - kt B (2) where the quantities IR s, A, A* (120A/cm 2 K 2 fr n-type Si) and T are the terms is the vltage drp acrss R s, the rectifier cntact area, the effective Richardsn cnstant and temperature in Kelvin, respectively. Fig. 2. The C -2 -V plts f Al-TiW-Pd 2 Si/n-Si structure under varius illuminatin levels at rm temperature. The N D, W D, Φ B and N ss values were btained frm the reverse bias C -2 - V plts (Fig. 2) as fllwing equatins [10] and was als given in Table 1. kt ΦB ( C V) = V0 + + E (3) F q where V 0 is the intercept vltage and E F values were btained accrding t, kt N C E = F ln (4) q N D Fig. 1. The semi-lgarithmic LnI-V characteristics f Al-TiW-Pd 2 Si/n-Si structure in and under varius illuminatin levels at rm temperature. Fig. 1. shws the Ln I-V characteristics f the Al- TiW-Pd 2 Si/n-Si structure. The increase in the frward can be attributed t prductin f electrn-hle pairs under illuminatin. The I, Φ B and n values were btained accrding t therminic emissin (TE) [10,11] were fund t be a strng functin f illuminatin level and are given in Table 1. where N C =4.82x10 15 T 3/2 (m e * /m 0 ) 3/2 is the effective density f states in Si cnductance band and m 0 =9.1x10-31 kg the rest mass f the electrn. As shwn in Table 1, the values f N D, W D and Φ B were fund t be a strng functin f illuminatin intensity. The values f W D and Φ B were fund t decrease, while the N D increases with increasing illuminatin level due t the shrinking f the depletin regin width (W D ) and restructure and rerdering f interface states under illuminatin effect. The N ss values were calculated frm Eq. 5 [12,13]and are given in Table 1. It is clear that the value f N ss decreases with increasing illuminatin density 2 ' εi 1 c2 = N 2 D ND = = (5) qεε s 0N D d( C )/ dv εi + qδnss n

3 264 H. Uslu, Y. Şafak, İ. Taşçiğlu, Ş. Altindal Table 1. Illuminatin dependent values f electrical parameters btained frm frward bias I-V, reverse bias C-V and C -2 -V characteristics f Al-TiW-Pd 2 Si/n-Si structure. Pwer (mw/cm 2 ) I ph (A) I (A) n Φ B0 (I-V) V D (V) N D (cm -3 ) E F W D (cm) Φ B N ss (ev -1 cm -2 ) x x x x x x x x x x x x x x x x x x x x x x Dielectric Prperties f Al-TiW-Pd 2 Si/n-Si Structure The real and imaginary part f dielectric cnstant (ε and ε ), tanδ and σ ac were btained frm C and G/w characteristics as fllwing equatins at varius illuminatin levels and frequency (in and 20 mw/cm 2 ) and are given Fig. 3 and Fig. 4, respectively. ε ' (a) ε '' (b) tan δ 0,4 0,3 0,2 (c) σ ac (Ω -1 cm -1 ) 7,E-07 6,E-07 5,E-07 4,E-07 3,E-07 (d) 0,1 2,E-07 1,E-07 0, ,E+00 Fig. 3. Vltage dependence f the (a) dielectric cnstant (ε ), (b) dielectric lss (ε ), (c) lss tangent (tan δ) and (d) ac cnductivity (σ ac ) in and under varius illuminatin levels fr Al-TiW-Pd 2 Si/n-Si structure.

4 The effect f frequency and illuminatin intensity n the main electrical characteristics f Al-TiW-Pd 2 Si/n-Si and C = ' m ε, C σ ac ε ' = G ωc ' m, '' ε tanδ = (6) ' ε '' = ωc tan δ ( d / A) = ε ωε (7) where C = ε 0 (A/d x ), d x is the interfacial insulatr layer thickness and ε is the permittivity f free space charge (ε = F/cm). As can be seen frm Figs. 4 and 5, the ε, ε and σ ac were btained strngly functin f illuminatin levels and frequency. These values increase with increasing illuminatin levels while decrease with increasing frequency due t space charge plarizatin caused by impurities r interstitials in the materials. Similar results have been reprted in literature [14-17]. Als, it is clear that bth the values ε' and ε'' are greater at lw frequencies due t the pssible interface plarizatin mechanisms [16-20] since interface states (N ss ) cannt fllw the ac signal at high frequencies [10-13]. These dispersins in ε' and ε'' with frequency can be attributed t Maxwell-Wagner [18] and space-charge plarizatin [19]. Fig. 4. Vltage dependence f the (a) dielectric cnstant (ε ), (b) dielectric lss (ε ), (c) lss tangent (tan δ) and (d) ac cnductivity (σ ac ) at different frequency (10, 50, 100, 500 khz) fr Al-TiW-Pd 2 Si/n-Si structure. 4. Cnclusins The electrical, dielectric and ac electrical cnductivity (σ ac ) f the Al-TiW-Pd 2 Si/n-Si structures have been investigated by using the frward and reverse bias I-V, C- V and G/ω-V measurements in and under illuminatin at rm temperature. Experimental results shw that the values f ε', ε'', tanδ and σ ac were fund t be a strng functin f frequency and illuminatin level. Als, the energy distributin prfile f Nss was btained frm the frward bias I-V characteristics by taking int accunt bias dependence f the effective barrier height (Φe). The values f Nss increase frm the midgap twards the tp f

5 266 H. Uslu, Y. Şafak, İ. Taşçiğlu, Ş. Altindal cnductance band and decrease with increasing illuminatin level. The ε, ε and σ ac increase with increasing illuminatin levels while decrease with increasing frequency due t space charge plarizatin caused by impurities r interstitials in the materials. The results shw that the interfacial plarizatin can mre easily ccur at lw frequencies and high illuminatin intensities cnsequently alter bth the electrical and dielectric prperties f Al-TiW-Pd 2 Si/n-Si structures. Interface plarizatin reaches a cnstant value due t the fact that beynd a certain frequency f external field the electrn hpping cannt fllw the alternative field. References [1] C. J. Kircher, Slid-State Electrn 14, 507 (1971). [2] S. Chand, J. Kumar, Semicnd. Sci. Technl. 10, 1680 (1995). [3] M. C. Petty, Slid States Electrnics 29, 89 (1986). [4] İ. Dökme, Ş. Altındal, İ. M. Afandiyeva, Semicnd. Sci. Technl. 23, (2008). [5] İ. M. Afandiyeva, İ. Dökme, Ş. Altındal, L. K. Abdullayeva, Sh. G. Askerv, Micrelectrn. Eng. 85, 365 (2008). [6] M. Wittmer, D. L. Smith, P. W. Lew, M. A. Niclet, Slid-State Electrn. 21, 573 (1978). [7] A. Shepela, Slid State Electrn. 16, 477. (1973). [8] B. Studer, Slid State Electrn. 23, (1980). [9] P. S. H, G.W. Rublff, J.E. Lewis, V.L. Mruzzi, A. R. Williams Phys. Rev. B 22, 4784 (1980). [10] S. M. Sze, Physics f Semicnductr Devices (secnd ed.), Wiley, New Yrk (1981). [11] E. H. Rhderick, R. H. Williams, Metal Semicnductr Cntacts (secnd ed.), Clarendn Press, Oxfrd, [12] A. Tatarğlu, Ş. Altındal, M. M. Bülbül, Micrelectrn. Eng. 81, (2005). [13] A. Singh, Slid State Electrn. 28, (1985) [14] G. C. Psarras, E. Manlakaki, G. M. Tsangaris, Cmpsites: Part A 34, 1187 (2003). [15] A. Laha, S. B. Krupanidhi, Materials Sci. And Engineer. B 98, 204 (2003). [16] A. A. Sattar, S. A. Rahman, Phys. Stat. Sl. (a) 200(2), 415 (2003). [17] K. Prabakar, S. K. Narayandass, D. Mangalaraj, Phys. Stat. Sl. (a) 199(3), 507 (2003). [18] O. Bidault, P. Gux, M. Kchikech, M. Belkaumi, M. Magline, Phys. Rev. B 49, 7868 (1994). [19] A. Kyritsis, P. Pissis, J. Grammatikakis, Plym. Sci. Ply. Phys. 33, 1737 (1995). [20] C. Fangga, G. A. Saunders, E. F. Lambsn, R. N. Hamptn, G. Carini, G. D. Marc, M. Lanza, J. Appl. Ply. Sci. 34, 425 (1996). * Crrespnding authr: h.uslu@gazi.edu.tr