Journal of the Korean Physical Society, Vol. 63, No. 10, November 2013, pp. 1980∼1983

Noise Properties of the Leakage Current Conduction in a ZrO2 Thin Film

Heejun Jeong∗

Department of Applied Physics, Hanyang University, Ansan 426-791, Korea

(Received 29 April 2013, in final form 16 July 2013)

In this paper, we report the leakage current conduction in a metal-oxide-semiconductor structurewith a ZrO2 gate dielectric and its current noise properties. The characteristics of the leakage currentin ZrO2 suggest that the leakage is caused by the trap-assisted tunneling in the low-bias region whilespace-charge-limited conduction is involved in the high-bias region. Lorentzian features disappearand the 1/f noise due to the carrier number fluctuation becomes dominant as the bias voltage isincreased. The correlation of the Lorentzian feature in the current noise power spectrum with thetime domain random telegraph signal fluctuation supports the trap-mediated carrier conductionmechanism. The transition from trap-assisted conduction to space-charge-limited conduction isdiscussed in terms of the noise response.

PACS numbers: 72.20.-i, 72.20.JvKeywords: ZrO2, Leakage current, Trap, RTS, Current noiseDOI: 10.3938/jkps.63.1980

I. INTRODUCTION

Insulators with high electrical permittivity arepresently investigated as possible replacements for a SiO2

gate dielectric to overcome the increased tunneling gateleakage current that arises as devices are scaled down.Among the several high-k materials currently being in-vestigated, zirconium oxide (ZrO2) is an attractive can-didate because it has a high dielectric constant, a highbreakdown field, a large band gap of more than 5 eV, asatisfactory conduction band offset (1.5 eV) with siliconsubstrates [1]. Even with these advantages, the leakagecurrent densities of ZrO2 are still larger than the cor-responding value for SiO2 gate dielectrics. Therefore,an understanding of the mechanisms that determine thecurrent flow through gate structures with high-k insu-lator layers is important. Several possible mechanismsfor the gate leakage current through an insulator, suchas Schottky emission, Poole-Frenkel emission, Fowler-Nordheim tunneling, trap-assisted tunneling and space-charge-limited conduction, have been proposed theoret-ically and experimentally based on standard current-voltage (I −V ) and capacitance-voltage (C −V ) charac-teristics. However, the presence of defects that cause aleakage current in an gate oxide is not directly observablein I−V and C−V results. The electronic noise has beenwidely recognized as an important source of informationon the transport mechanisms in nanoscale devices [2,3].In addition, noise, in particular 1/f noise and generation-recombination (GR) noise, is known to be sensitive to the

∗E-mail: hjeong@hanyang.ac.kr

presence of defects. Therefore, in the devices where car-rier transport is based on defects, noise provides usefulinsight into the leakage current conduction. To study thecarrier transport properties in an ZrO2 insulator layer,we measured two types of noise, a time-domain randomtelegraph signal (RTS) and a low-frequency noise. Inthis paper, we report our experimental investigations ofthe current-voltage (I − V ) characteristics and the cur-rent noise in a Pt/ZrO2/p-Si metal-oxide-semiconductor(MOS) structure.

II. EXPERIMENTS AND DISCUSSION

To fabricate a capacitor structure, we deposited aZrO2 layer on a cleaned silicon substrate with a p-type(100) orientation by using an atomic-layer deposition(ALD) technique at 220 ◦C. From the vacuum ultravioletellipsometer measurement and the capacitance-voltagemeasurement, the dielectric constant and the equivalentoxide thickness were estimated to be 17 and 3.7 nm, re-spectively. As a top electrode, platinum was depositedby using e-beam evaporation with a thickness of 30 nmand an area of 100 × 100 um2. We obtained the leak-age current characteristics of the fabricated Pt/ZrO2/p-Si capacitor structure by using a ground isolated DAC(digital-to-analog converter) voltage source and a low-noise current preamplifier (Ithaco 1211) at 300 K. Tomeasure the noise power spectrum and RTS, we used aSR 780 spectrum analyzer and a ground-isolated ADC(analog-to-digital converter).

First, Fowler-Nordheim (FN) tunneling is considered-1980-

Noise Properties of the Leakage Current Conduction in a ZrO2 Thin Film – Heejun Jeong -1981-

Fig. 1. Fowler-Nordheim tunneling plot of ln(J/E2) vs.1/E (squares) and space-charge-limited conduction plot oflnJ vs. lnV 2 (circles) under negative bias.

to analyze the current conduction properties [4]. The FNtunneling current density is given by

JFN =q2

8πhφE2exp

(8π√

2m∗qφ3

3hE

), (1)

where q is the electronic charge, h is the Plank constant,φ is the barrier height between Pt and ZrO2, m∗ is theelectron’s effective mass in the dielectric layer which ism∗ = 0.3 m0 (m0 being the free-electron mass), and E isthe electric field. Figure 1 shows a Fowler-Nordheim plotof ln(J/E2) vs. 1/E under a negative bias. From the lin-earity in the electric field region lower than 2.5 MV/cm,the conduction mechanism is Fowler-Nordheim tunnelingconduction. The barrier height can be calculated fromthe slope of the straight line, and the extracted value is0.36 eV. The junction barrier height of Pt/ZrO2 is notclearly known yet. Because Pt on ZrO2 has a large workfunction (5.05 eV) and the electron affinity of ZrO2 isabout 3.5 eV [5], we infer that the Pt/ZrO2 junctionbarrier height is more than 2 eV. The extracted barrierheight for our device is not in agreement with the ex-pected level of 2 eV. If the insulator layer contains acertain number of traps formed by both the film’s depo-sition process and the structural disorder, field ionizationof trapped electrons has an effect on the tunneling con-duction [6]. When the trapped electrons are a major partof conduction, the barrier will be a trap barrier, not aninterfacial one, so the experimentally extracted value of0.36 eV should be the barrier height of electron traps.

In the high electric field region where the ln(J/E2) vs.1/E plot is nonlinear, we found that lnJ vs. lnV 2 showeda linear region, which is a signature of space-charge-limited conduction [7]. A power-law dependence of thecurrent density on the voltage in insulators has been pro-posed as a strong indication of the space-charge-limited

Fig. 2. Leakage current noise spectra at different voltages.A clear 1/f behavior is observed at low frequencies. In thehigh-frequency region, a Lorentzian shape is observed, andthis feature is degraded as the bias voltage is increased.

(SCL) conduction mechanism. In our device, direct elec-tron or hole injection into the insulator conduction orvalence band is not plausible because the ZrO2 film haslarge band offsets with silicon for both the conductionand the valence band. When large negative voltages areapplied, the traps will be highly occupied, and a strongaccumulation of trapped charges results in an SCL cur-rent. Thus, the power-law behavior reflects SCL conduc-tion being a possible mechanism in the high-electric-fieldregion.

To get insight into the effects of the conduction prop-erties of traps on the noise signal, we performed noisemeasurements. The power spectral density (PSD) of thegate leakage current noise at different voltages is shownin Fig. 2. The acquired Fourier transformed signal is100-times averaged over the range of 1 - 800 Hz, andis 1000-times averaged over the range of 800 - 12.8 kHz.The PSD shows a line shape of 1/fγ with a spectral slopeγ = 1 ± 0.2 in the low-frequency range (<1 kHz) anda line shape of 1/f2 component in the high-frequencyrange (>1 kHz). The occurrence of a Lorentzian noisefeature is a signature of generation-recombination noise,and traps are responsible for its appearance [8]. TheLorentzian feature is degraded as the bias voltage be-comes more negative. This is another indication thatthere is a transition in the transport mechanism from atrap-assisted fluctuation to SCL conduction.

We also obtained the time traces of the current fluctu-ation at several different bias voltages, as shown in Fig.3. A RTS fluctuation with a strong non-Gaussian ampli-tude distribution is present at –2 V where the noise PSDshows a Lorentzian feature. Random telegraph noise inthe time domain is evidence of charge traps caused bydefects and has been observed in a variety of physicalsystems [9]. Andersson et al. reported the two-levelcurrent fluctuation phenomenon related to rap-assistedtunneling in a MOS tunnel diode [10]. The RTS fluctu-

-1982- Journal of the Korean Physical Society, Vol. 63, No. 10, November 2013

Fig. 3. Time domain traces of the leakage current at differ-ent fixed biases. Distinct two-level fluctuations arising fromtrapping and detrapping of carriers are observed at –2 V anddisappear as the bias voltage becomes stronger.

ation occurring in a MOS capacitor structure has beenthe subject of considerable theoretical and experimentalinvestigations [11, 12]. In general, a discrete temporalfluctuation of a current at a fixed bias voltage betweentwo levels is known to be caused by carrier trapping orde-trapping in the vicinity of the carrier transport chan-nel [13]. As the bias voltage becomes more negative,the signature of the random telegraph fluctuation dis-appears. The correlation of the Lorentzian feature innoise PSD with the random telegraph fluctuation is theresult of trap-assisted tunneling in the low-bias-voltageregion and supports the low barrier value extracted fromthe Fowler-Nordheim plot being, indeed, the trap barrierheight rather than the interfacial height.

From the RTS data at –2 V, the average emission (τe)and the capture time (τc) are obtained to be 4.8 ms and1.3 ms, respectively. We also obtained a characteristictime τ0 and a corner frequency fc from the relationship1/τ0 = 1/τe + 1/τc = 2πfc [14]. The calculated value ofthe corner frequency is 6.1 kHz and is indicated by anarrow in Fig. 2. The value from the RTS data is consis-tent with the position in the noise PSD, which shows that

Fig. 4. Noise power as a function of the leakage current. SI

is proportional to I2. The inset shows the normalized noisepower SI/I2 as a function of the bias voltage.

traps involving carrier transport in the low-bias-voltageregion is a valid picture. With larger negative bias volt-ages (< –3 V), the line shape of the noise becomes morestraight while the RTS fluctuation is degraded; in otherwords, the distribution of the current amplitude has aGaussian nature.

The 1/f -type noise can be described by using the em-pirical formula introduced by Hooge [11]:

SI

I2=

αH

Nf, (2)

where αH is the Hooge parameter and N and f are thetotal carrier number and the frequency, respectively. αH

is a few times 10−3 for ordinary metals. Figure 4 showsthe leakage current noise PSD, SI , at 20 Hz as a functionof the current derived from the noise data. The currentnoise power scales as I2, which is typical for 1/f noisein many systems [15,16]. From Eq. (2), we found thatthe Hooge parameter was αH gg 1 in our case. Largevalues of Hooge’s parameters are also frequently observedin oxide materials like CrO2, Fe3O4 and some magneticoxides, where the origin of the 1/f noise is attributedto a change in the carrier number via traps [17]. Thenormalized noise power SI/I2 vs. negative bias voltageis shown in the inset of Fig. 4. The results in Fig. 4 showthat the voltage scaling of SI/I2 is almost constant overa wide voltage range, consistent with the trap-assistedorigin of 1/f noise in the stress-induced leakage currentin SiO2 [18,19].

Finally, we examine the noise characteristics in theSCL region. In the I − V analysis, our device startsto show a space-charge-limited behavior around –3.5 V.As previously reported [18, 19], the noise power SI in-creases sharply as the bias voltage is increased in thetrap-assisted-tunneling region. In Fig. 2, the rapid in-creasing tendency of SI is reduced significantly in thehigh-voltage SCL region. Generally noise is known to be

Noise Properties of the Leakage Current Conduction in a ZrO2 Thin Film – Heejun Jeong -1983-

reduced in space-charge-limited field emitters (like vac-uum tubes) [11]. However, we can infer that the noise be-havior will not simply show a decreasing feature in solid-state insulators. As the applied voltage is increased,traps are filling appreciably (entering the SCL region),and competition may exist between the trap-filled SCLcurrent fluctuation and the current fluctuation due to theremaining non-filled traps. If this aspect is consideredalong with the decreasing noise property caused by theSCL effect, the increasing rate of the noise power willbe reduced as the bias voltage is increased, as in Fig.2. Accompanied by a diminishing Lorentzian feature,this suggests that trap-mediated tunneling conduction isgradually replaced by SCL conduction. After a certainthreshold voltage (forming a full-filled steady-state SCLregion), a decreasing noise power may exist for higherbias, but observation of this feature is limited by a finitebreakdown voltage, which introduces permanent damageto the device. Samples with a thicker, high-quality ZrO2

insulation layer may be a possible option for observingthe crossover from increasing to decreasing noise power.In Fig.4, a constant voltage scaling of the normalizednoise power is still maintained in the high-bias region,but no distinctive feature of an SCL conduction transi-tion is present in these data. Because of the complicatednature of the origin of fluctuations in this intermediateconduction transition region, further experimental andtheoretical studies are necessary to understand the nor-malized noise behavior.

III. CONCLUSION

A metal-oxide-semiconductor capacitor structure de-vice with a ZrO2 gate dielectric is fabricated and theleakage current characteristics are studied. The corre-lation between the Lorentzian feature and the randomtelegraph fluctuation, and the consistent values extractedfrom the noise data suggest that trap-assisted tunnel-ing may be a major conduction mechanism in the low-bias region. In the high-bias region, the generation-recombination noise is degraded and the increasing rateof SI is greatly reduced as the SCL conduction mecha-nism plays a role.

ACKNOWLEDGMENTS

This research was supported by the Basic Science Re-search Program through the National Research Founda-tion of Korea (NRF) funded by the Ministry of Educa-tion, Science and Technology (Grant No. 2010-0012069).

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