Wide band gap GaN semiconductor has a variety of applications in optoelectronic devices such as lightemitting

diodes and laser diodes

Abdul Aziz, A.

Hassan, Z.

Hudeish, A.Y.

Tam, F.K.

Tan, C.K.

English

Repository@USM

,J | .-r\,r ny 1FrPstbi-r" iJ*^-^ /.r-, o-1 ( *l a'./t fti/r, -t,- $u,-^7^,-Lt^c,/s,tE8 fq- gh.,+ )trr.t?n / e_L/u)h r+-ut"n "" Qcvn'<n /phys. stat. sol. (z) zzz, No.z,zzz-zzz (2004) / DOI 10.1002/pssx.200400000I2456789l0ll1213l4l516t7l819202l2223242526272829303l32JJ343536aatt3839404l4243444546474849505152Electrical characteristics and thermal stabilitv of Ti contact top-GaNC. K. Tan, A. Abdul Ad'z,Z.Ifassan, tr'. K. Yam, A. Y. Iludeish.School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia.Received zzz, rcvised zzz, accepted zzzPublished onlinezzzPACS 73.30.+y, 7 3.6r.8y, 7 3.20.-rThe schottky barrier height (SBH), iD3 of the diodes were characterized by I-V-T method (en-ergy activation plot and modified Norde plot) in the range of 300K-373K ambient for variousannealing temperature (500-1000"C) in Nz gas flow. The ratio of kT/ Eoo=l,large A**andlatgedeviation of X from general value indicate that the mixed thermionic emission with thermionicfield emission are most possible to be occurred as current transport mechanism.@ 2004 WILEY-VCH Vulag GmbH & Co. KGaA, WeinheimI IntroductionWide band gap GaN semiconductor has a variety of applications in optoelectronic devices such as light-emitting diodes and laser diodes l1]. In addition, its high electron safuration velocity, large breakdownfield and thermally stable makes an attractive candidate for high temperature/trigh power devices. It hasbeen shown that devices such as field-effect transistors (FETs) 12,31 can be fabricated on GaN. For hightemperature operation of metal-semiconductor field-effect transistors (MESFETs), thermally stableSchottky contacts are necessary. The barrier height rFs is determined by the metal work funotion O,, theband-gap E, and the electron affinity 1of p-GaN and can be expressed as:Qn:6+E)-Q.The above expression is valid for an ideal Schottky diode [4]. Despite its decisive role for improvingdevice performance, electrical property of the p-type GaN layer, which differs much from the n-fypelayer due to the complicated behavior of Mg acceptors, has yet to be fully explored [5,6]. So far, only afew studies on this subject have been reported and the Schottky barrier heights (SBH) of various metalcontaots are not well defined yet. In addition, little has been reported in the literature on the thermalstability of Schottky barier contactl7,8l.In this work, the I-V-T characteristics of the contacts; as-deposited and annealed at 500-l000oC wereinvestigated in the consecutive elevation temperatures of 20oC from room temperature to 373K. TheSBHs of these contacts determined using current-voltage-temperature (I-V-T) measurements wsre calcu-lated using activation energy plot.2 ExperimentalO 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim(l)000000physica (x) xxx/x WordXP&R310; insgesamt 5 Seiten DisketleF:VCNS2005\FORMATedI(}{SII.DOC hn' nnwbArt.: W0000/Autor1 8.10.200512aJ456d8910llI2l3l41516t71819202l2223242526272829303132J_)3435363I3839404t4243444546474849505152S. Hildebrandt et al.: Manuscript preparation guidelines for arrthors of physica status solidiMg-doped p-GaN samples grown on sapphire (AlzO:) subshates were used in this study. The ohmicmetallization consists of a single layer of Ni (50nm) whereas; Ti (50nm) Schottlcy is used as single layerof Schottlcy contact. Prior to the metallization, conventional boiling aqua regia cleaning method was usedto chemically etch and clean the samples. Ni was fust formed on p-GaN to act as Ohmic contacts. Thiscontact was checked and confirmed to be Ohmic by I-V measurement prior to the deposition of Schottkycontact. Metallized dots, which consist of an array of dots with a diameter of 200 pm for Schottky con-tacts, were sputtered via a stainless steel metal mask onto the p-GaN samples. The annealed sampleswere conducted inversely to as-deposited sample, where Schottky contacts (Ti) were annealed in respectto selected temperature and duration of annealing prior to metallization of Ohmic contact. In this investi-gation, Schottky diodes were annealed at temperature ranging from 500-1000"C with the flow of N2 gas.Digital hot chuck system was used to heat up the samples from 27-100oC and the I-V-T characteristics ofTilp-GaN Sohottky diodes was analyzedusing standard semiconductor parameter analyzer.3 Result and discussionThe electrical parameters, i.e., SBHs, iF3, saturation current, Io, and ideality factor, 4 can be determinedby I-V measurements. For thermionic emission and V>3kT/q, the general diode equations are [9]:I: I"exp{qV/r7kT}I,: AA*f exp{-q A a/kr }(2)(3)[4f,o@a-gt'l', ""**tto*.'*t* -aaaatat''t'+RT' o 313K--+-- 333K- +- . 353K_{_ 373K0.0 0.1 0.2 0.3 0,4 0.5applied bias (V)Fig. 1 (a) Measured I-V-T curves of the Tilp-GaN diode annealed at 500"Crange 300-373K to obtain saturationcurrent, Io. (b) Activation energy plots method in extracting barrier heights and effective Richardson coefficients fromI-V-T data for different annealing ternperature of Schottky contact on p-GaN.As usual, A is the contact area, k is the Boltznann's constant, Z is the absolute temperature, 03 is thebarrier height and .4 * is the effective Richardson coefficient. The theoretical value of A* can be calcu-lated using a*:47v71*qF/h3, where h is Planck's constant and m*:0.80mo is the effective hole mass forGaN [0,] l]. As shown in Fig. 1(a), the plot of ln I vs V will give a straight line with a slope of q/qkT,and the intercept with y-axis will yield 1,. The banier heights of the Schofiky diodes were deterrrinedfrom the slope of ln (I/An vs l/T base on Eq. (3), as shown in Fig. 1(b).The SchottJcy barrier height and effective Richardson constants were fust extracted using modifiedNorde Plot [4] and activation energy plot of the measured I-V-T data. Use of either lpe of plot allowsfor the independent determination of barrier heights and effective Richardson coeffrcients. Determinationof these parameters with the modified Norde plot method involves plotting the forward-biased I-V-T dataas F/ vs Zwhere the function Fl is defined as:@ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim000000physica (x) xxxlx WordXP&R310; insgesamt 5 Seiten DisketteArt.: W0000/Autor18.10.2005F:VCNS2005\FORMATed ICNS II.DOCBearb.:I2J4568910llt213t4l516t718l9202l2223242526272829303l32JJ343536373839404l4243444546474849505l52phys. stat. sol. (z) zz, No. z (2004) / www.physica-status-solidi.cornFI:(qV/2kT) - tn (I/T) (4)Where q is the elementary charge, k is the Bollzmann's constant, and T is the measutement tenperatwe.A typical Fl vs Tplot for a Ti Schottky diode is shown in a Fig. 2(a) for measurement tomperature rang-ing from 300-373K. Each curve contains a minimum FI or Fl., which is recorded along with the corre-sponding current, I-. FL. and 1, are then used in the left-hand side of Eq. (2) given by2 Fl^+ (2-r) In Ut'):2 -n fln (At*x1+IJ + @\AilkT) (s)where iDso is the zero-bias Schottky barrier height, and 4 is the ideality factor. The values^ of 06, and ,4 **are extracted from the slope and y-intercept, respectively, of the 2 Fl* +.(2- fl ln (I*tT) vs qkTplotormodified Norde plot. The modified Norde plots for typical diode of each deposited metal ne shown inFig.2(b).0.0 0.2 0.4 0.6 0.4 1.030329$$&qftT (1N)Fig. 2 Plots used in the modified Norde plot method in extracting barrier heights and effective Richardson coef-ficients from data. (a) Fl vs applied bias (V) plot ofa typical Ti diode for a series oftemperatures annealed at500'C, (b) 2F1-(VJ+(2- D ln QJt') vs q/kT plots for Ti on p-GaN, as-deposited and annealed at 500-1A00"C.The built-in potential isVi: iFslq - VoWhereV,:(En-Ev)/q:(kT/q)/n(NyA{j (7)The effective intensity of states in the valence band for Wurtzite crystals is given by [12]Ny:8.9xlo1sxT3/21cm') (8)At T:300K, Nv : 4.62 x l01e cn3 and Vn =0.1143 eV. Thus, the built-in potential could be calculated.The measured value of the SBHs were stated in Table I for chosen arcrcaling temperatures on Ti/p-GaN.To predict the behavior of p-GaN, we try to obtain the electron affin|y 1 of the semiconductor for aSchottky barrier, which is related to the work function of the metal, O6 and band-gap, as state in Eq. (1).This relationship is not so simple in practice because of charges that often exist at the junction caused bysurface states, metal-induced gap states or chemical reaction at the interface. Taking O, : 4.33eV for Tif13l and Ee:3.39eY [l2], as-deposited O, as standard, the electron affinity is determined tobe.Y--1.8leV. This value is factor of 2 lower than estimatedX: 4.leY [12]. This may indicates that the naturebehavior of p-GaN which usually exhibit leaky I-V characteristics, making the measurement of SBHsdifficult using I-V and C-V methods [4].@ 20A WLEY-VCH Verlag GmbH & Co. KGaq Weinheim40@1to1@1-s-3@K@5040(6)000000physica(x)xxx/x WordXP&R31; insgpsamt 5 Seiten Diskette4ft.: W0000/Autor18.10.2005F:\ICNS2O05\FORMATed ICNS II.DOCBearb.:+Rt'' o 313K--+-- 3(-+-- sq-{- 373K; E#"I2J4567891011t2l3t415l6t71819202l2223242526272829303132aaJJ343536JI3839404t4243444546474849505152S. Hildebrandt et al.: Manuscript prepaxation guidelines for authors of physica status solidiThe value of A** was found to vary between each metal. Ignoring optical phonon scattering andquantum mechanical reflection/tunneling, the theoretical value of A** is detennined tobe (103.8 Acm-'K') [10] based on an effective hole mass of m*:0.80mo. However, an ideal A** of 26.4 Acm-"I{' for n-GaN, and experiments have shown that A*xdoes not agree with this ideal value U5-17]. As a result, therange of measured A** for various annealing temperatures to Tilp-GaN, which varies over 2 order ofmagnitude and even more from the theoretical value, are then also expected cannot be explained in termsof quantum or phonon scattering [18]. The differences in A** between metals suggests that the charac-teristics of the metal, such as metal work function or room-temperature difftrsion/intermixing of metalinto GaN, may affect the nature of the metaVGaN interface [18]. Error in the determination of A** canalso arise from the extrapolation over a narrow range of measurement data to the y-axis in the activationenergy plot. Two aforementioned approaches involving I-V-T measurements were thus utilized in deter-6ining SBH because the value of Axx is not required to obtain SBH.Table I The measured Schottky parameter for Tilp-GaN at different annealing temperatures.AnnealingtemperatureIdealitvfactor,rlActivation energyplots ModifiedNorde plotTunnelingprobability, E* (eV)Builrinpotential,Vi(eV)oB(eV)A*x(Acm-2K-2) iDs (eV)Ax*(Acm-2K-2)As- 0.87 t.t92xt0+ 0.82 1599.56 0.0524 0.76Deposited500"C,5min 2.85 1.00 2.79x10"600"C, Smin 2.23 0.89 310.04700oC,Smin 2.7I 0.40 0.0228800'C,2min 1.69 0.96 59.22x103900"C, lmin 2.78 0.79 775.961000'C.Imin 3.28 0.51 0.0901.020.850.380.830.770.373.18x1062704.r0,00562494.37275.530.0002050.08200.05780.073r0.0s460.07210.10570.890.780.290.850.680.40Note: The values shown are averages of at least 8 diodes of each metal.The standard deviation of SBHs and ideality factor for each annealing temperature rospectively are 0.05eV and 0.06.The thermionic emission model in Eq. (3) predicts a l/kT dependence of a linear region af the lnlversus Zcurves, in contast to the parallel slopes of the lnI-V qrves shown in Fig. I (a). This parallelbehavior of the lnI-T curves is commonly observed for carrier transport with a dominant tunnel compo-nentfL9,20l.It is appropriate to analyze the I-V characteristics using a tunneling model [21]:I - AA*B e\p {-q Qu/E*} exp {qV/ E,,} (e)(10)Where Na is the acceptor concentration, €, the dielectric constant of semiconductor, B a paftlmeter re-lated to the temperature and the Fermi level in the semiconductor. Tlte Eoo is a characteristic energy re-lated to the tunneling probability. The defect-assisted tunneling across the barrier would lead to an Eoovalue larger than that calculated by Eq. (10) based on acceptor concentration alone 1201. According toEq. (9), the slope of lnI-V curves is independent of temperature. The parallel shiffs of I-V curves weredue to the temperature-dependent pre-exponential factor, B. Furthermore, the slope of the lnl versus ,/linear region yields the value of E o. Based on the tunneling model and the experimental results, we de-dvced Eoo values of each annealing temperature as stated in table I. According to Morkoc et al. 122f, forkT/ Eoo=l a mixture of thermionic, thermionic field emission and tunneling mechanisms is observed.Therefore, the deduced values is likely to contain a tunneling component caused by defect stateg locatedin the near surface region of the semiconductor in addition to the acceptor concentration (Nd expressedin Eq. 10.An ideality factor, 4 >1 could be ascribed to interface states at a thin oxide between the metal and semi-conductor 123f, or generation-recombination currents within the space region [13].@ 20et WLEY-VCH Verlae GmbH & Co. KGa,\ Weinheim000000physica(x)xxx/x WordXP&R310; insgesamt 5 Seiten DiskefteArt.: W0000iAutorI 8.10.2005F:\ICNS2005\IORMATed ICNS II.DOCBearb,:(Nulm*e,)12phys. stat. sol. (z) zzz, No. z (2004) / www.physica-status-solidi.comI2J4)689t01lt213t41516I7l8t9202l2223242526272829303l32JJ3435363839404l42434445464748495051524 ConclusionFrom experimental results such as the ratio of kT/ Eoo=I, large A**and large deviation of ,ffrom generalvalue indicate that the mixed thermionic emission with thermionic field emission are most possible to beoccurred as current transport mechanism. Leaky behavior also expected for our diodes. Beside, iD3 couldbe improved by heat treatment with appropriate annealing temperatures and duration.Acknowledgements This work was conducted under an IRPA RMK-8, Strategic Research grant (MO-STI). The support from Universiti Sains Malaysia is gratefully acknowledged.Referencestll F. A. Ponce, and D. P. Bour, Nature (London) 386, 351 (1997).l2l M. S. Shur and M. A. Khan, MRS Bull, 22,44 (1997).t3l M. A. Khan, J. N., Kuzma, A. R. Bhattarai, and D. T. Olson, Appl. Phys. Lett.62,1786 (1993).l4l D. K. Schroder, Semiconductor material and devices characterization (I.{ew York John Wiley &Sons, 1998) p207.tsl T. Mori, T.Kozavta, T. Ohwaki, Y.Taga, S. Nagai, S. Yamasaki, S. Asami, N. Shibata, IVI. Koike,Appl. Phys. Letl. 69,3537 , (1996).16l K. A. Rickert, A.B. Ellis, J. K. Kim, J. -L. Lee, F. J. Himpsel, F. Dwikusuma, T. F. Kuech, J.Appl. Phys. 92, 667 l, (2002).t7l I. Adesida, A C. Schmitz, A. T.Ping, and M. A. Khan, presented at the Spring MRS Meeting, SanFrancisco. C. A. April, (1996).t8] K. N. Lee, X. A. Cao, C. R. Abernathy, S.J. Pearton, A. P. Zhang, F. Ren, R. Hiclman, J. M. VanHove, Solid-State Electronics 44, 1203-1208, (2000).t9l S. M. Sze, Physics of Semiconductor Devices, 2od edition (John Wiley & Sons, New York), 245,(1e89).t10l D. Donoval, V. Kulikov, P. Bofio, f. Racko,ASDAM2002.[ 1] J. I. Pankove, S. Bloom, and G. Harbeke, RCA Rev.36, 163 (1975).l12l M. E. Levinshtein, S. L. Rumyantsev, M. S. Shur, Properties of advanced semiconductor materials,( A Wiley-Interscience publication ), 2001.t13l E. H. Rhoderick anA n. ff. Williams, Metal-Semiconductor Contacts (Clarendon Press, Oxford) 2"dedition, 1988.U4l H. Ishikawa, S. Kobayashi, Y. Koide, S. Yamasaki, S. Nagai, J. Umezaki, M. Koike, and M.Murakami, J. Appl. Phys., 81, pp. 1315-1322,1997.t l5l P. Hacke, T. Detchprohm, K. Hiramatsu and N. Sawaki, Appl. Phys. Lett., 63,2676, 1993.[16] A. T. Ping, A.C. Schmitz, M. A. Khan and I. Adesida, Electron. Lett.32,68,1996.|71 J. D. Guo, M. S. Feng, R. J. Guo, F. M. Pan and C. Y. Chang, Appl. Phys. Lett.,67,2657,1995.U8l C. Schmitz, A.T. Ping, M. Asif Khan, Q. Chen, J. W. Yang. And I. Adesida, Journal of ElectronicMaterials, 27 , 4,7998.U9l J. M. Andrews, J. Vac. Sci. Technol. ,11,972, 7974.[20] S.M.Sze,PhysicsofsemiconductorDevices,2noed.(Wiley,NewYork, 1981),chapter5.[21] L. S. Yu, Q. Z. Liu, D. J. Qiao, S. S. Lau, and J. M. Redwing, J. Appl. Phys., 84, pp2099-2104,1998.[22] H. Morkoc, Nitride Semiconductors and Devices (Berlin: Springer, I999),p203.I23l J. H. Werner, A. F. J. Levi, R. T. Tung, M. Anzlower and M. Pinto, Phys. Rev. Lett. 60 (1988) 53.@ 2004 WLEY-VCH Valag GmbH & Co. KGaA, Weinhein000000 physica (x) xxxlx WordXP&R3l; insgesamt 5 Seiten DisketteArt.: W0000/Autor1 8.10.2005F:VCNS20OS\FORMATed ICNS II.DOCBeaft.:

And I.

Metal-Semiconductor Contacts (Clarendon Press, Oxford) 2&quot;d edition,

Nitride Semiconductors and Devices

Physics of Semiconductor Devices, 2od edition

presented at the Spring MRS Meeting,

Properties of advanced semiconductor materials, ( A Wiley-Interscience publication ),

Semiconductor material and devices characterization (I.{ew York

Electrical Characteristics And Thermal Stability Of Ti Contact To p-Gan.

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Electrical Characteristics And Thermal Stability Of Ti Contact To p-Gan.

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