Polycrystalline Si films, 0.5-mm thick, were obtained as a result of metal-induced growth by sputtering from a Si target on 25 nm thick Ni prelayers at 525 °C. Silicon grew heteroepitaxially on the NiSi2 layer formed due to the reaction between the sputtered Si atoms and Ni. Schottky diodes were fabricated on the Si films by deposition of a Schottky metal on the front surface of the film while Ni disilicide provided an intimate ohmic contact at the back. A Pd/n-Si diode using an n-Si film annealed for 2 h at 700 °C in forming gas demonstrated a rectification ratio of 106, while an as-deposited p-Si film provided an Al/p-Si diode with rectification of five orders of magnitude. Schottky barrier properties are briefly discussed

Anderson, Wayne A.

Guliants, Elena A.

Ji, Chunhai

Song, Young J.

English

University of Dayton

University of DaytoneCommonsElectrical and Computer Engineering FacultyPublicationsDepartment of Electrical and ComputerEngineering2002A 0.5 μm Thick Polysilicon Schottky Diode withRectification Ratio of 10^6Elena A. GuliantsUniversity of Dayton, eguliants1@udayton.eduChunhai JiState University of New York at BuffaloYoung J. SongState University of New York at BuffaloWayne A. AndersonState University of New York at BuffaloFollow this and additional works at: https://ecommons.udayton.edu/ece_fac_pubPart of the Computer Engineering Commons, Electrical and Electronics Commons,Electromagnetics and Photonics Commons, Optics Commons, Other Electrical and ComputerEngineering Commons, and the Systems and Communications CommonsThis Article is brought to you for free and open access by the Department of Electrical and Computer Engineering at eCommons. It has been acceptedfor inclusion in Electrical and Computer Engineering Faculty Publications by an authorized administrator of eCommons. For more information, pleasecontact frice1@udayton.edu, mschlangen1@udayton.edu.eCommons CitationGuliants, Elena A.; Ji, Chunhai; Song, Young J.; and Anderson, Wayne A., "A 0.5 μm Thick Polysilicon Schottky Diode withRectification Ratio of 10^6" (2002). Electrical and Computer Engineering Faculty Publications. 126.https://ecommons.udayton.edu/ece_fac_pub/126A 0.5-mm-thick polycrystalline silicon Schottky diode with rectificationratio of 106Elena A. Guliants,a) Chunhai Ji, Young J. Song, and Wayne A. AndersonDepartment of Electrical Engineering, State University of New York, Buffalo, New York 14260~Received 21 August 2001; accepted for publication 18 December 2001!Polycrystalline Si films, 0.5-mm thick, were obtained as a result of metal-induced growth bysputtering from a Si target on 25 nm thick Ni prelayers at 525 °C. Silicon grew heteroepitaxially onthe NiSi2 layer formed due to the reaction between the sputtered Si atoms and Ni. Schottky diodeswere fabricated on the Si films by deposition of a Schottky metal on the front surface of the filmwhile Ni disilicide provided an intimate ohmic contact at the back. A Pd/n-Si diode using an n-Sifilm annealed for 2 h at 700 °C in forming gas demonstrated a rectification ratio of 106, while anas-deposited p-Si film provided an Al/p-Si diode with rectification of five orders of magnitude.Schottky barrier properties are briefly discussed. © 2002 American Institute of Physics.@DOI: 10.1063/1.1454214#Despite the fact that the III–V and II–VI compounds forfast-switching and photonic devices have attracted consider-able interest in recent years, silicon remains one of the mostreliable materials in a mainstream semiconductor technology.Among Si-based microelectronic devices, Schottky diodesare particularly promising for the high-speed applicationsdue to the majority carrier transport mechanism and relatedto that virtual absence of minority carrier storage which lim-its the frequency response. The quality of a Schottky contactis extremely critical for the performance of high frequencyand microwave devices, such as diodes, metal–semiconductor field effect transistors, high electron mobilitytransistors, and static-induced transistors. Both the rectifica-tion ratio and the cut-off frequency depend on the interfacebetween the semiconductor surface and the depositedSchottky metal, interdiffusion between these materials, andquality of a semiconductor itself. There is a major trend toreduce the density of interface and semiconductor bulk stateswhich contribute to the parasitic capacitance and leakagecurrent. In order to obtain a low noise and high quality rec-tifier, a structurally perfect semiconductor material with nodeep levels in the energy gap is needed. To date, semicon-ductor layers thinner than 0.5 mm provided reliable Schottkycontacts only when grown by high-temperature epitaxy, suchas molecular-beam epitaxy, liquid phase epitaxy, and metal–organic vapor phase epitaxy.1–3 Accordingly, there is a trade-off between the Schottky quality and a common microelec-tronics trend to reduce the device cost by substituting expen-sive thick crystalline layers with microcrystalline thin filmsand the use of low-cost low-melting point substrates.Previously, we have demonstrated the properties of poly-crystalline Si ~polysilicon! films produced by metal-inducedgrowth ~MIG! on foreign substrates below 600 °C.4 Fullycrystallized Si films were obtained by Si deposition on a thinNi prelayer, where Si columnar grains were shown to growheteroepitaxially on the lattice-matched NiSi2 crystals nucle-ated at the Ni–Si interface. Moreover, NiSi2 on the backsurface of such a Si film provided an excellent ohmic contactto the film. This letter reports the performance of Schottkydiodes fabricated on 0.5 mm thick MIG-polysilicon films ofboth the n and p type.In our approach, the starting substrate was a single crys-tal Si wafer coated with a 300 nm thick SiO2 by plasma-enhanced chemical vapor deposition. This substrate was cho-sen for convenience and can be replaced by a thin metal,glass, plastic, or polymer substrate. A 25 nm thick Ni wasthermally evaporated on a SiO2 /Si substrate at a base pres-sure in the lower 1026 Torr range. Silicon was depositeddirectly on the Ni surface by dc magnetron sputtering from a99.99% Si target in a 5% H2 /Ar gas mixture at a pressure of1 mTorr ~after a vacuum level of 131027 Torr or lower wasachieved!. In all cases, the magnetron power and the sub-a!Electronic mail: elena.guliants@wpafb.af.milFIG. 1. The room-temperature performance of the best Schottky diode ob-tained on an n-type MIG-polysilicon film, demonstrating a rectifying ratio of;106. The diode area is 2 mm2.APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 8 25 FEBRUARY 200214740003-6951/2002/80(8)/1474/3/$19.00 © 2002 American Institute of Physics strate temperature were kept at 50 W and 525 °C, respec-tively, which provided a deposition rate of ;0.5 mm/h. Bothn-type ~P-doped! and p-type ~B-doped! Si targets with therespective resistivity values of 0.006 V cm, and 0.001 V cmwere used. Because of the intimate self-organized backohmic contact, Schottky diodes were produced by fabricationof rectifying contacts on the front surface of the Si filmsusing metals with the suited work function, i.e., Pd for n typeand Cr and Al for p-type Si films, respectively. The area ofall diodes was 2 mm2. The current–voltage (I – V) character-istics were measured in dark with an automated data acqui-sition system employing a Keithley 230 voltage source and aKeithley 617 multimeter.The attempt to fabricate a Schottky junction on an as-deposited n-type polysilicon film was not successful. Theannealing study was conducted on several samples, and theannealing variables were chosen to be the annealing tempera-ture and the annealing ambiance. In order to reduce the num-ber of variables, the annealing time was fixed at 2 h for allexperiments. At a temperature of 700 °C, annealing in eitherair or pure nitrogen failed to show the presence of a slightestbarrier after the Schottky metal was deposited. A 5% H2 /Armixture provided a weak rectification, and, finally, Forminggas ~10% H2 /N2! resulted in a well-defined junction. Theseresults may imply that the diode efficacy improves by in-creasing the hydrogen content in the annealing environmentwhich may be further explained by the saturation of danglingbonds at the grain boundaries closer to the surface. A mod-erate rectification result was achieved for the samples an-nealed at 650 °C in Forming gas, whereas a temperature of700 °C provided Si films for the Schottky diodes with a rec-tification ratio of 103 – 106 ~IR /IF taken at 21 V/1 V!. Thepotential of MIG to provide high electronic quality Si filmsis demonstrated in Fig. 1 showing the log(I)–V characteristicof a Schottky diode with the rectification of six orders ofmagnitude taken at room temperature.The carrier transport in Pd/n-polysilicon Schottky struc-tures was studied by taking temperature-dependent current–voltage (I – V – T) characteristics. The typical values for theideality factor n, saturation current density JS and Schottkybarrier height wB computed from the I – V – T characteristicsfor an average Pd/n-Si diode are shown in Table I. In thepure thermionic emission theory, both barrier height and ide-ality factor decrease when temperature increases. An abnor-mal increase in barrier height with temperature has been ob-served before,5,6 and the phenomenon was attributed to theelectron states present at the metal–semiconductor interfacedue to inhomogeneities of the Schottky contact. In addition,the observed wB is smaller than the theoretical wB due to theimage force barrier lowering. Figure 2 shows the E05nkT/q values versus kT/q . Figure 2 indicates that the trans-port mechanism involved in the diode is dominated by ther-mionic emission ~TE! at higher temperatures with a smoothtransition to the start of thermionic field emission ~TFE! atlower temperatures. It is worth mentioning here that the datalisted in Table I were collected for a typical, or average,diode, and the performance of MIG-polysilicon films can befurther improved, as becomes evident from Fig. 1.The fabrication of Schottky diodes based on the p-typepolysilicon utilized two metals, Cr and Al. As compared tothe n-type Si films, both annealed and as-grown p-type Sifilms provided reasonable rectifying devices. The reason forFIG. 2. Experimental values of nkT/q as a function of temperature for atypical Pd/n-Si diode.FIG. 3. The room-temperature J – V characteristics of the Schottky diodesfabricated on the p-type MIG-polysilicon.TABLE I. Schottky parameters extracted from the I – V – T characteristics of a Pd/n-Si Schottky diode.Temperature~K!Rectifying ratio,I21 V /I1 V Ideality factorSaturation currentdensity ~A/cm2!Barrier height~eV!150 up to 106 1.92 2.2310214 0.596200 10000 1.61 3.3310212 0.720250 2500 1.43 9.2310210 0.786300 1300 1.33 2.731029 0.925350 125 1.24 3.331027 0.9441475Appl. Phys. Lett., Vol. 80, No. 8, 25 February 2002 Guliants et al. such a difference in the performance of n- and p-type poly-silicon films is currently under investigation. Figure 3 showsthe room-temperature current density–voltage (J – V) char-acteristics of three samples, fabricated on: an as-grown p-Sifilm using Al, an annealed film using Al, and an annealedfilm using Cr ~no rectification was achieved on as-grownp-Si film using Cr!. Annealing was done at 700 °C for 2 h informing gas. It is seen in Fig. 3 that the diode based on anas-grown film exhibited the best properties, with a rectifica-tion ratio of five orders of magnitude. The Schottky param-eters extracted from the I – V – T characteristics of this deviceare listed in Table II. However, the conduction mechanism inall three Schottky structures in Fig. 3 is similar to that in thePd/n-Si diode, with the TFE mechanism dominating at lowertemperatures and the TE mechanism dominating at highertemperatures.In summary, Schottky diodes were fabricated on 0.5 mmthick polysilicon films produced by Si sputtering on 25 nmthick Ni prelayers. The electrical characterization of the di-odes indicated the capability of the MIG technique to pro-duce high quality thin film silicon at low temperatures. APd/n-Si diode fabricated using the Si film deposited at525 °C and annealed for 2 h at 700 °C in forming gas dem-onstrated a rectification ratio of 106, while an Al/p-Si diodeon the as-deposited Si film provided rectification of five or-ders of magnitude. The performance of the Schottky diodesreported here is among the best results achieved onsub-1-mm-thick nonepitaxial Si films produced in a low-temperature process.1 T. L. Lin, and J. M. Iannelli, Appl. Phys. Lett. 56, 2013 ~1990!.2 J. R. Lothian, F. Ren, J. M. Kuo, J. S. Weiner, and Y. K. Chen, Solid-StateElectron. 41, 673 ~1997!.3 N. S. Peev, Microelectron. Eng. 43, 599 ~1998!.4 E. Guliants and W. A. Anderson, J. Appl. Phys. 87, 3532 ~2000!.5 H. Hong, W. A. Anderson, J. Haetty, A. Petrou, E. H. Lee, H. C. Chang,M. H. Na, H. Luo, J. Peck, and T. J. Mountziaris, J. Appl. Phys. 82, 4994~1997!.6 J. Osvald, J. Appl. Phys. 85, 1935 ~1999!.TABLE II. Schottky parameters extracted from the I – V – T characteristicsof an Al/p-Si Schottky diode.Temperature~K! Ideality factorSaturation currentdensity ~A/cm2!Barrier heighteV125 2.08 1.3310219 0.623150 1.84 1.2310216 0.665175 1.65 1.0310214 0.713200 1.54 2.0310213 0.768225 1.45 2.8310212 0.817250 1.41 2.0310211 0.870275 1.36 9.0310211 0.906300 1.33 7.0310210 0.9591476 Appl. Phys. Lett., Vol. 80, No. 8, 25 February 2002 Guliants et al.

A 0.5 μm Thick Polysilicon Schottky Diode with Rectification Ratio of 10^6

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A 0.5 μm Thick Polysilicon Schottky Diode with Rectification Ratio of 10^6

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