Electrical characteristics of metal-dielectric-metal and metal-dielectric ...

JOURNAL OF APPLIED PHYSICS

VOLUME 84, NUMBER 12

15 DECEMBER 1998

Electrical characteristics of metal-dielectric-metal and metal-dielectricsemiconductor structures based on electron beam evaporated Y2O3, Ta2O5 and Al2O3 thin film V. Mikhaelashvili, Y. Betzer, I. Prudnikov, M. Orenstein,a) D. Ritter, and G. Eisenstein Electrical Engineering Department and Kidron Microelectronics Center, Technion, Haifa 32000 Israel

~Received 14 July 1998; accepted for publication 27 August 1998! This work examines the electrical properties of metal-dielectric-semiconductor (Au/Ti–D– pSi) and metal-dielectric-metal (Au/Ti–D–Pt/Ti– pSi) capacitors which incorporate as dielectrics Y2O3, Al2O3 and Ta2O5 films evaporated by an electron beam at room temperature. The emphasis of the results is twofold: the first is the high quality of the investigated films as evidenced by the small measured values of loss factor, flatband voltages, and surface states density as well as the low dispersion of the relative dielectric constants. The second is an analytical procedure for discrimination of current flow mechanisms, under different regimes of applied voltage. A detailed study of the power exponent parameter a 5d(Log I)/d(Log V) was found to be superior to conventional graphical representation of I – V data. The dominant mechanisms of charge transport through the metal-dielectric-metal structures was found to be the Schottky emission for Y2O3 and Al2O3 at low electrical fields. For structures with Y2O3 and Ta2O5 films operating in the high field regime, the charge transport mechanism is mainly space charge limited current. © 1998 American Institute of Physics. @S0021-8979~98!01723-X#

I. INTRODUCTION

significantly more informative than the usual exponential formalism of I – V characteristics,7 since it facilitates the determination of the bias voltage values where the dominant current flow mechanisms change.8 The different current flow mechanisms are clearly separable using the dependence of the a parameter on the bias and, consequently, the range of applied voltage relevant to each mechanism is easily identified.9,10

Thin silicon dioxide (SiO2) films are commonly used as the dielectric media for gates and capacitors in very largescale integrated circuits. Once the SiO2 thickness is reduced to the 7–10 nm range, the films may exhibit high leakage currents and relatively low breakdown voltages associated with high pinhole densities and enhanced tunneling. A possible solution is to replace the SiO2 with materials having larger dielectric constants, to enable the use of thicker films while maintaining the same capacitance density. Examples of such high dielectric constant materials include Al2O3, Ta2O5 and Y2O3. Deposition techniques and electrical characteristics of metal-dielectric-metal ~M-D-M! and metaldielectric-semiconductor ~M-D-S! structures using these dielectrics have been recently studied and extensively reported.1–6 In this article we report a detailed investigation of MD-M and M-D-S capacitors with Al2O3, Ta2O5 and Y2O3 thin films. We address various issues not dealt with in the literature such as voltage stability, loss factor, the dielectric constant dependence on frequency, and the detailed capacitance–voltage (C – V) and current–voltage (I – V) characteristics. In order to investigate the charge leakage mechanism through the dielectric films, we made use of an analytical procedure, which examines the parameter a

a5

d ~ log I ! , d ~ log V !

II. EXPERIMENTAL PROCEDURE

The dielectrics were deposited in an electron beam evaporation system. The system operates at room temperature and under relatively high vacuum allowing the deposition of high purity materials with stoichiometry, which is close to that of the sources. The vacuum system contains two chambers separated by a load lock arrangement. One chamber contains the sources and the second the sample holder 245 cm away from electron beam gun to prevent sample heating. Typical background pressures are 2 – 331028 Torr and the detailed deposition parameters for the three dielectrics are listed in Table I. No annealing was used following the electron beam depositions. The M-D-M devices were Au/Ti–D–Pt/Ti– pSi while the M-D-S devices were Au/Ti–D– pSi. For both device types we used Al2O3, Ta2O5 and Y2O3 films as a dielectric. The dielectric film thickness was 100 nm and the contact areas were 531025 – 331024 cm2 for the M-D-M devices and 2.531023 cm2 for the M-D-S devices. The electrical characterizations included current–voltage and capacitance– voltage as well as dielectric constant and loss factor measurements as a function of frequency.

~1!

where a describes the voltage dependence of the I – V characteristic slope in a Log–Log scale. This representation is a!

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0021-8979/98/84(12)/6747/6/$15.00

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© 1998 American Institute of Physics

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Mikhaelashvili et al.

J. Appl. Phys., Vol. 84, No. 12, 15 December 1998

III. RESULTS AND DISCUSSION A. M-D-M and M-D-S parameters

A predictable linear capacitance dependence on contact area for an Al2O3 capacitor is shown in Fig. 1 while Fig. 2 demonstrates the stability of the capacitors. The constant capacitance density observed for applied bias levels in the range of 240 to 140 V shows that the capacitor stability holds up well to an electric field of E F 5106 V/cm for all three dielectrics. The deviation from constant values for Ta2O5, Al2O3 and Y2O3 starts at field levels of 1.23106 , 2.53106 and 3.53106 V/cm, respectively. Capacitance versus voltage measurements in M-D-S devices for all three dielectrics show a change from depletion to accumulation modes when the bias varied from positive to negative values, as seen in Fig. 3. In all cases, the flatband voltage (V fb) is negative indicating that the films contain predominantly positive charges. The small value of V fb indicates a small density of surface states (N SS). The detailed results are listed in Table II. For the Si–Y2O3 boundary, the value of N SS is close to that obtained for Si-thermal SiO2, 7 denoting the high quality of the Y2O3 dielectric films. The values of the V fb and N SS for the not-annealed Y2O3 films are close to the values obtained1 by electron beam evaporation followed by annealing at 625 °C and are lower than values obtained for annealing at lower temperatures.2,3 The frequency dependencies of the relative dielectric constant ~e! and loss factor (tan d) are depicted in Figs. 4 and 5, respectively. The dispersion of e is rather small in the frequency range of 100 Hz–7 MHz. At high frequencies, the measurements become less reliable since effects not exclusively related to the insulator start to play a role. For ex-

FIG. 1. Capacitance vs contact area for an Au/Ti–Al2O3 –Pt/Ti/Si capacitor.

ample, the influence of the dielectric permittivities of transition layers near the metallic electrodes become important.8 It has been well established in the literature11 that the rise of tan d at low frequencies is associated with the presence of a parallel leakage resistance, while the increase of the loss factor at high frequencies is connected to the presence of a ~smaller! series resistance. In accordance with Fig. 5 we concluded that both types of resistance are present for the structure with Ta2O5 as a dielectric while for structure based on Al2O3 and especially on Y2O3 their effect on the loss factor is minimal. This is further demonstrated in the current–voltage characteristics shown in Fig. 6~a!, where the resistance in the low voltage regime for the Ta2O5 structure is lower than the corresponding values for the Al2O3 and Y2O3 based devices indicating larger leakage. Detailed parameters extracted for the capacitors and dielectric films are given in Table III. B. Current transport mechanism

We have studied the leakage current mechanisms of the Ta2O5, Y2O3 and Al2O3 layers by a comprehensive analysis of their I – V characteristics. The measured I – V curves of the M-D-M structures, shown in Fig. 6~a!, were translated to the a -V characteristics shown in Fig. 6~b!. Examination of Fig. 6 reveals that the Ta2O5 film has different characteristics than the Al2O3 and Y2O3 films. For structure with Ta2O5 as a dielectric the linear part ~I! of the I – V curve with slope one ~the low bias range for which a 51! becomes superlinear ~II! under large bias. In this regime, a rises monotonically and eventually reaches a

FIG. 2. Voltage stability of the capacitance for the Au/Ti–D–Pt/Ti/Si capacitors. The contact area is 331024 cm2. The thickness of the dielectric layers is 100 nm. 1—Ta2O5; 2—Y2O3; 3—Al2O3.




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FIG. 3. High frequency capacitance vs voltage characteristics for Au/Ti–D– pSi structures with Ta2O5, Y2O3, and Al2O3 as dielectrics. The contact area is 2.531023 cm2. The thickness of the dielectric layers is 100 nm. 1—Ta2O5; 2—Y2O3; 3—Al2O3.

FIG. 4. The dependencies of the relative dielectric constant ~e! on frequency. The contact area is 331024 cm2. The thickness of the dielectric layers is 100 nm. 1—Ta2O5; 2—Y2O3; 3—Al2O3.

maximum. For the Al2O3 and Y2O3 films, the I – V curve ~I! has no linear regime. The a -V curves show that in the small bias region a is smaller than 1 and changes slowly with bias. At large bias levels ~II!, a exhibits a maximum for the Y2O3 film while the a -V characteristic is discontinuous in the Al2O3 case. Several conduction mechanisms contribute, in principle, to the I – V characteristic of M-D-M structures. These include7 double injection, barrier emission of carriers, which is known as the Schottky mechanism, field ionization of trapping levels in dielectric in accordance with Poole–Frenkel, Fowler–Nordheim tunneling through insulator, and space charge limited current. The real I – V characteristic usually represents simultaneous contributions of all these mechanisms. However, only one usually dominates at a given bias range and its determination is an important issue. The Schottky and Poole–Frenkel mechanisms are commonly used to explain current transport under moderate and high fields. The current due to Schottky emission for an assumed ideal contact ~no gap and no surface states on the metal-dielectric or semiconductor boundary! is

and ionization energy of the levels, respectively. A * is Richardson constant and C is a constant related to the density of ionized traps. S is the contact area. The coefficient b depends7 on the dielectric constant and the interelectrode distance L according to

F

I Sch5SA * T 2 exp 2

G

qfb exp@ 21 b AV # kT

~2!

b5

q kT

Ae

q . p L

~4!

The distance L equals the dielectric thickness for an ideal M-D-M contact. If the Schottky model dominates the carrier transport at moderate fields, the logarithm of the current depends linearly on the square root of the applied voltage @Eq. ~2!#. Similarly, if the current is determined by the Poole–Frenkel model, we see from Eq. ~3! that Ln(I/V) is a linear function of the square root of the voltage. In addition, the slope of the curve in the linear regime is a measure of the relative dielectric constant. An examination of the I – V curve is not always sufficient either to determine the actual conduction mechanism at high bias levels or to extract the relevant range of voltages where current flow is limited by either carrier emission over a Schottky barrier or by ionization of the trapping levels.

and the predicted current due to the Poole–Frenkel model is I PF5S

F G

CV qfi exp 2 exp@ b AV # , L kT

~3!

where q, T, k, F b , and F i denote the elementary charge, absolute temperature, Boltzmann constant, barrier height,

FIG. 5. The dependencies of the dielectric loss factor (tan d) on the frequency. The contact area is 331024 cm2. The thickness of the dielectric layers is 100 nm. 1—Ta2O5; 2—Y2O3; 3—Al2O3.


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FIG. 6. The current density ~a! and a ~b! vs applied voltage. The Au electrode is negatively biased. The area of the capacitor is 331024 cm2. The thickness of dielectric is 100 nm. Regions I and II denote, respectively, low and high-voltage regimes. 1—Ta2O5; 2—Al2O3; 3—Y2O3.

Here, the a -V presentation becomes extremely useful. The a -V dependencies derived from Eqs. ~2! and ~3! are

a Sch5 41 b AV

~5!

a PF511 21 b AV.

~6!

and

In the case of carrier emission over a Schottky barrier, the value of a Sch is zero at zero bias @see Eq. ~5!#, while for field ionization of trapping levels, in accordance with the Poole– Frenkel law, the a -V curve intersects the axis at a 51 @see Eq. ~6!#. Indeed, it follows from Eqs. ~3! and ~6! that for extremely low bias level, the behavior is ohmic ( a PF'1). Thus, in both cases the parameter b can be ex-

FIG. 7. The Ln(I), Ln(I/V) ~a! and power exponent and relative dielectric constant ~b! vs V 1/2 characteristics for Au/Ti–Y2O3 –Pt/Ti/Si capacitor. The dashed lines indicate Schottky ~curve 1! and Poole–Frenkel ~curve 2! conduction mechanisms in accordance with Eqs. ~2! and ~3!. The Au electrode is negatively biased. The capacitor area is 331024 cm2. The thickness of dielectric is 100 nm. In ~b! curves 1 and 4 are experimental ~a! and ~e!; 2 and 3 are calculated values in accordance with Eqs. ~5! and ~6! for e 514.

tracted from Eqs. ~5! or ~6! and then the relative dielectric constant is easily found using Eq. ~4!. Knowing b, we may extract the barrier height or ionization energy using Eqs. ~2! and ~3! if the values of coefficients A * and C are known. This systematic procedure avoids the errors usually encountered while determining these parameters by conventional graphical methods from the linearized regions of I – V characteristics, which yield their averaged values. Figures 7–9 describe the dependencies of Ln(I), Ln(I/V) and a on the square root of the voltage for the M-D-M structures with the three dielectrics. For all dielectric films, the curves of Figs. 7–9~a! display at least one linear




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FIG. 8. The Ln(I), Ln(I/V) ~a! and power exponent ~b! vs V 1/2 characteristics of Au/Ti–Al2O3 –Pt/Ti/Si capacitor. The dashed lines indicate Schottky ~curve 1! and Poole–Frenkel ~curve 2! conduction mechanisms in accordance with Eqs. ~2! and ~3!. The Au electrode is biased negative. The area of the capacitor is 331024 cm2. The thickness of the dielectric is 100 nm. In ~b! curve 1 is experimental ~a!; 2 and 3 are calculated values in accordance with Eqs. ~5! and ~6! for e 59.

FIG. 9. The Ln(I), Ln(I/V) ~a! and power exponent ~b! vs V 1/2 characteristics of Au/Ti–Ta2O5 –Pt/Ti/Si capacitor. The dashed lines indicate Schottky ~curve 1! and Poole–Frenkel ~curve 2! conduction mechanisms in accordance with Eqs. ~2! and ~3!. The Au electrode is biased negative. The area of the capacitor is 331024 cm2. The thickness of the dielectric is 100 nm. In ~b! curve 1 is experimental ~a!; 2 and 3 are calculated values in accordance with Eqs. ~5! and ~6! for e 524.

portion which results from either Schottky or Poole–Frenkel mechanisms in accordance with Eqs. ~2! and ~3!. The actual dominant mechanism is further established from the a plots, which for the Y2O3 and Al2O3 films fit very well the prediction of Eq. ~5!. This indicates that Schottky emission dominates in the low voltage regime. Moreover, the voltage range for which this mechanism dominates is easily extracted from the a plots. In both cases it covers the range of V50 to about 10 V. For the Ta2O5 film, Fig. 9, for bias levels of 0.7 V–2.5 V, a '1, which is an ohmic regime. The a plot reveals that neither a Schottky nor a Poole–Frenkel type mechanism takes place at any bias regime, although linear regions in Ln(I) and Ln(I/V) vs V 1/2 plot are observed @see Fig. 9~a!#. The value of e for Y2O3 and Al2O3 dielectrics, determined from the first part of the a -V characteristics and Eqs. ~4! and ~5!, are the same as those obtained from the low frequency capacitance versus frequency measurements ~Fig. 4, curves 2 and 3!. It is also seen from curve 4 of Fig. 7~b! that only in the linear range of the a -V 1/2 plot, the values of the dielectric constant are approximately unchanged, while the values of e, calculated from the slopes of the first linear part for Ta2O5 ~see Fig. 9~a!, curve 1! and the second linear parts for all films @curves 1, Figs. 7–9~a!#, differ from the values determined from capacitance measurement. Extract-

ing the dielectric constant from the slope of the straight line region of the Ln(I/V) vs V 1/2 dependencies @Figs. 7–9~a!, curves 2# based on Poole–Frenkel emission also leads to the value of e which is different from that obtained from the capacitance characteristics. The value of zero bias barrier height at the M–Y2O3 and M–Al2O3 boundaries, estimated in accordance with Eq. ~1! using an assumed value of A * 550 A cm22 K 22 for the Richardson coefficient, is 0.79 and 0.77 eV, respectively. To determine the conduction mechanism at bias levels beyond the Schottky regions in the Al2O3, Y2O3 films and the ohmic region in the Ta2O5 case, we use a discrimination criterion8,9 based on a coefficient, Q m which is determined from ratio of the space charge density trapped in the levels of the dielectric ~or semiconductor! band gap to the free carrier concentration. It is given by Q m5

SV 2m u r m u ~ 2 a m 21 ! 2 ~ a m 21 ! 5 em , qn m 4 p I mL 3 a 3m

~7!

where r m and n m are space charge and free carrier concentration, respectively, q is the electron charge, m is the free carrier mobility, V m and I m are the voltage and total current corresponding to the maximum value of a ( a m ). In the case of any type of field ionization, the discrimination coefficient is9


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Q m ~ FI! 5

~ 2 a m 21 ! 2 ~ a m 21 ! . ~ a m 11 ! 2

~8!

In accordance with Eq. ~7! and previous publications,8,9 double carrier injection takes place when Q m !1 ~the charge accumulation on the trapping levels is absent!. Monopolar injection-space charge limited current occurs if Q m @1 and any field ionization processes are characterized by Q m