Low Leakage Current Transport and High Breakdown Strength of Pulsed Laser Deposited HfO 2 /SiC MIS Device Structures

Transcription

1 Low Leakage Current Transport and High Breakdown Strength of Pulsed Laser Deposited HfO 2 /SiC MIS Device Structures S.S. Hullavarad, D.E. Pugel, E.B. Jones, R.D. Vispute, T. Venkatesan

2 Journal of ELECTRONIC MATERIALS 2007 DOI: /s Ó 2007 TMS Regular Issue Paper Low Leakage Current Transport and High Breakdown Strength of Pulsed Laser Deposited HfO 2 /SiC Metal-Insulator-Semiconductor Device Structures S. S. HULLAVARAD, 1,4 D. E. PUGEL, 2,3 E. B. JONES, 2 R. D. VISPUTE, 2 and T. VENKATESAN 2 1. Office of Electronic Miniaturization, University of Alaska Fairbanks, Fairbanks, AK99701, USA. 2. Center for Superconductivity Research, University of Maryland, College Park, MD20742, USA. 3. Detector Development Branch, Goddard Space Flight Center, NASA, Greenbelt, MD 20770, USA fnssh1@uaf.edu Hafnium oxide (HfO 2 ) thin films were deposited by the pulsed laser deposition (PLD) method on SiC substrates. The bandgap of HfO 2 thin films was observed to be 5.8 ev. The chemical nature and stoichiometry of the films were analyzed by x-ray photoelectron spectroscopy (XPS). Metal-insulator-semiconductor (MIS) structures with Ni as a top electrode and TiN as a bottom electrode were fabricated to study the leakage current properties. The devices exhibited leakage current density of 50 na/cm 2. The dielectric constant of these films is estimated to be in the range from capacitance-voltage (C-V) measurements. The frequency dependence of the interface trapped charges is studied. Key words: High k dielectric, passivation, pulsed laser deposition (PLD), x-ray photoelectron spectroscopy (XPS), Current-voltage (I-V), capacitance-voltage (C-V) INTRODUCTION High k dielectric metal oxide films have attracted much attention as promising candidates for the forthcoming gate dielectric materials of metal oxide field effect transistors, in place of SiO 2 films. 1 Silicon oxide and silicon nitride dielectrics are commonly used in conventional metal-insulatorsemiconductor (MIS) structures. These materials provide good voltage linearity and lower temperature coefficients, but are lacking due to low dielectric constants (k 3.8 for silicon oxide and 8 for silicon nitride). To increase the capacitance, while reducing the tunneling current, various kinds of high dielectric materials have been investigated as possible alternatives to SiO 2. 2 Owing to its ability to handle high critical electric field, electron saturation velocity, and thermal conductivity, SiC has advantages over Si (quantitatively 186 times better than Si). 3 In the case of SiC-based power devices (thyristors), one must find a way to apply an electric (Received January 20, 2006; accepted October 10, 2006) field with a strength close to the critical value for SiC. This puts enormous constraints on the field handling by the dielectric layer on the side walls of the device, and hence, high dielectric constant materials have been under study for the purpose of side wall passivation along with the planar dielectric applications. 4 The field strength in the insulator will be scaled by a factor of e SiC /e i, where e SiC = 9.66 and e i is the relative dielectric constant of an insulator. Hence, to reach a field strength in SiC of 2.5 MV/cm, the field in the SiO 2 (e = 3.9) must exceed 6 MV/cm, leading to severe reliability problems. In the past, we have investigated AlN as a viable gate dielectric passivation 5 and evolved different ways to deposit high quality, with lowest leakage current and uniform side wall coverage on vertical wall devices. In our quest to enhance the field handling capability, we have investigated HfO 2, in the present paper, because of its higher dielectric constant and high stability against thermal treatments. 6 HfO 2 has been in the forefront of dielectric applications in recent years employed in Si-based technology. However, recently, there have been

3 Hullavarad, Pugel, Jones, Vispute, and Venkatesan reports by Afanasev et al. 5 and others on the application of HfO 2 stacks on SiC for low leakage current applications. HfO 2 has a dielectric constant varying from 15 to 26 with a bandgap of 5.8 ev, with a favorable conduction and valance band offsets with respect to SiC. 7 HfO 2 exhibits better thermodynamic stability, as would follow from the more negative Gibbs energy of formation 260 kcal mol )1. It has a refractive index of 1.92 at 0.6 lm and 2.1 at 250 nm. 8 HfO 2 has a higher density of gcm )3 compared to most of the oxide materials used in alternate dielectric applications. 9 HfO 2 -based dielectrics have been extensively evaluated and various deposition techniques were demonstrated to be capable of depositing HfO 2, including physical vapor deposition, 10 chemical vapor deposition, 11,12 and atomic layer deposition (ALD) 13,14 techniques. In these techniques, the resulting films though electrically stable might be optically inhomogeneous (nonsharp optical absorption) due to low kinetic energy (E 0.1 ev) 15 of the depositing atoms arriving at the substrate surface. However, the plasma-assisted processes such as pulsed laser deposition (PLD) and the reactive sputtering 16 techniques have advantages over these techniques in providing higher kinetic energy to the species. In this paper, we report the low leakage current transport from PLD deposited HfO 2 films sandwiched in the form of MIS. EXPERIMENTAL HfO 2 thin films (2000 A) were deposited by PLD using a laboratory prepared HfO 2 target (99.999% purity) at a oxygen pressure of 100 mtorr after obtaining a base vacuum of 2 10 )6 Torr in the chamber. The p-sic (Cree) substrates were cleaned in dilute HF for 2 minutes in an ultrasonic bath, leaving a hydrogen-terminated SiC surface. In order to avoid the formation of a SiO 2 interfacial layer, the films were deposited at a relatively low substrate temperature of 700 C. A KrF excimer laser (248 nm) with a laser fluence of 2 J/cm 2 and a repetition rate of 10 Hz was used. A thin nucleation (buffer) layer (thickness 20 A) of HfO 2 was deposited in vacuum before growing the film of thickness 2000 A. Thin films were characterized by UV-visible transmission spectroscopy (Shimadzu 2501 PC) to measure the optical band gap. The binding environments of HfO 2 with the SiC substrate and the chemical nature of different species were monitored by x-ray photoelectron spectroscopy (XPS) (Kratos) with Mg K a radiation. The chemical nature of constituent elements as a function of the film thickness was measured by Ar + etching in-situ XPS measurements. The MIS capacitors were fabricated by depositing Ni top electrodes using the PLD method through a shadow mask. An accurate electrode area of each capacitor was measured using scanning electron microscopy and found to be 300 lm 300 lm in size Fig. 1. Schematic of device configuration consisting of Ni/HfO 2 /p- SiC/TiN. Metal contacts were deposited by PLD. (Fig. 1). The back side of the SiC wafer was HF cleaned prior to metallization with TiN. A Hewlett- Packard 4194A impedance meter and Keithley 2700 source meter were used for the capacitance-voltage (C-V) and leakage-current density-voltage (J-V) measurements, respectively. The C-V measurement was carried out for the frequency range KHz. RESULTS AND DISCUSSION Figure 2 shows the optical transmission spectra for the HfO 2 films deposited on the Al 2 O 3 substrate. The films exhibited 80 85% optical transmission in the visible and UV range. The film showed a sharp drop in the optical transmission at 215 nm corresponding to a band gap of 5.8 ev, which is close to the bulk HfO 2 bandgap value. 17 Because the HfO 2 film will be studied for transport properties at higher temperatures, the as-deposited film was annealed at 500 C in oxygen. The optical transparency is enhanced by 3 4% when the film was annealed at Transmission (%) Annealed at 500 o C in O 2 E g = 5.8 ev HfO 2 /Al 2 O Wavelength (nm) Fig. 2. UV-visible transmission spectra of HfO 2 samples for as-deposited and annealed films in O 2 at 500 C.

4 Low Leakage Current Transport and High Breakdown Strength of Pulsed Laser Deposited HfO 2 /SiC Metal-Insulator-Semiconductor Device Structures 500 C in oxygen, and no degradation in the film quality was observed. Figure 3 shows the XPS spectra of Hf 4f, O 1s, and Si 2p core levels for the as-deposited and for Ar ion in-situ sputtering after 300 s and 600 s. The spectra were deconvoluted into Gaussian Lorentzian features corresponding to different chemical states. Table I lists the different species of Hf, O, and Si for the as-deposited and Ar ion-etched samples. One can see from the peak positions of Hf 4f, after successive depth profile from the top surface to the interface, that the peak positions have changed from ev for as-deposited to 17.8 ev and 18 ev for 300s and 600s Ar + ion-etched samples, respectively. Also, the presence of a peak at ev 19 in the 600 s etched sample indicates a metallic Hf. This could be due to nonstoichiometric HfO 2 film deposited at the beginning of the process. The rather high signal intensities in the BE region between the two Hf 4f peaks by spin-orbital splitting (4f 7/2 and 4f 5/2) suggests that the films were composed of HfO 2 and Hf silicate. 20 The low energy state for O1s in the as-deposited sample centered at ev, with a full-width at half-maximum (FWHM) of 1.3 ev, is attributed to O in HfO There is a small peak at ev, which might be attributed to the HfO x, where x <2. However, when the few monolayers have been sputtered for 300 s and 600 s, the O1s spectra has a nonstoichiometric HfO x with an additional peak around ev, which is found to be associated with HfSi x O y. 22 The absence of line Si2p at 99.5 ev commonly associated with the Si-Si bond and one at ev for Si-O bond suggest that the SiC surface does not have a much discussed SiO 2 layer. 23 The formation of SiO 2 seems to be dependent on the deposition process as observed in the ALD technique, where there is a distinct peak appearing at 103 ev. 24,25 The C-V measurements, as shown in Fig. 4, were carried out for the MIS devices in a frequency range of MHz. The dielectric constant of HfO 2 is evaluated in the accumulation mode and is found to be in the range of 17 24, which is close to the Hf 4f O 1s Si 2p 600 s Etched 4f 7/2 a 600 s Etched 600 s Etched Si 2p a O 1s a 4f 5/2 a 4f 7/2 b 4f 5/2 b O 1s b Si 2p b 300 s Etched 4f 7/2 a 300 s Etched O 1s a 300 s Etched Si 2p a 4f 5/2 a 4f 7/2 b 4f 5/2 b O 1s b Si 2p b 4f 7/2 4f 5/2 O 1s a Si 2p O 1s b Binding Energy (ev) Binding Energy (ev) Binding Energy (ev) Fig. 3. XPS spectra of HfO 2 thin films for the as-deposited, 300 s, and 600 s Ar ion sputter etched samples.

5 Hullavarad, Pugel, Jones, Vispute, and Venkatesan Table I. Binding Energy (ev) Values of Hf, Si, and O Species Obtained from the Gaussian Fitting of XPS Spectra. Hf 4f (ev) 7/2 5/2 O 1s (ev) Si 2p (ev) Conditions a b a b a b a b As-deposited s etched s etched C/C ox KHz 20KHz 40KHz 100KHz 200KHz 400KHz I g (A cm -2 ) o C 175 o C 150 o C 125 o C 100 o C 75 o C 50 o C RT RT 50C 75C 100C 125C 150C 175C 200C Voltage (V) Fig. 4. Capacitance-voltage plots as a function of frequency. A dielectric constant of was derived from these measurements V g (Volts) Fig. 5. Current-voltage characteristics for Ni/HfO 2 /p-sic/tin over the temperature range of RT 200 C. reported value of 25. The interesting point to note in C-V measurements is the observation of a hump while going from accumulation to depletion, which is a strong function of applied frequency. The origin of this hump lies in the trapped interface charges located at the interface between the dielectric and the SiC surface. It is well known that the presence of interfacial silicon dioxide degrades the performance of high k dielectric MIS devices. 26 Its low dielectric constant (k 3.9), in series with the high k material, lowers the overall capacitance of the gate dielectric stack. Low quality native oxide also results in a high concentration of dangling bonds (>10 12 /cm 2 ) at the SiC interface. 27 The interfacial oxide layer of the as-deposited samples is thicker than expected from the standard SiC oxidation rate at the deposition temperature, suggesting the presence of localized fixed charges. It has been proposed that the deposited metal oxide enhances oxygen dissociation, increasing the flux of oxidizing atoms at the interface and that Hf-SiC bonds form as temporary reactive intermediates during deposition, thereby enhancing the oxidation rate at the interface. 28 However, the interface-trapped charges related hump in the C-V curves slowly starts flattening after 40 khz. The strong dependence of this effect on the applied frequency confirms the dynamic trapping and detrapping of active sites during the measurement. Figure 5 gives the I-V characteristics of Ni/HfO 2 / SiC/TiN (MIS) devices for temperatures up to 200 C. The I-V measurements were carried out over the voltage range V, corresponding to a breakdown field of 0.5 MV/cm to. The I- V measurements were recorded as a function of temperature from room temperature to 200 C in steps of 25 C. The devices exhibited low leakage current density of 40 na cm )2 at room temperature and remained in the nanoampere region until they elevated to 75 C, and the leakage current density increased to 10 la/cm 2 for an applied voltage of 30 V corresponding to a breakdown field of 0.5 MV/ cm. The breakdown field mentioned in the study does not correspond to the actual breakdown voltage of the device; rather it is a safe value of field to handle the device. The actual field may be higher than this value. Figure 6 depicts the current

6 Low Leakage Current Transport and High Breakdown Strength of Pulsed Laser Deposited HfO 2 /SiC Metal-Insulator-Semiconductor Device Structures J (A/cm 2 ) MV/cm 0.5 MV/cm 1.0 MV/cm 1.5 MV/cm 2.0 MV/cm 2.5 MV/cm 3.0 MV/cm ln I g MV/cm 2.5 MV/cm 2.0 MV/cm 1.5 MV/cm 1.0 MV/cm 0.5 MV/cm E a =1.31 ev /T (1/K) Fig. 6. Ni/HfO 2 /p-sic/tin capacitor leakage current density as a function of inverse temperature from RT to 200 C for several gate fields /T (1/K) Fig. 7. Arrhenius plots for Ni/HfO 2 /p-sic/tin capacitor for gate fields of e i 0.5. densities derived from I-V characteristics for dielectric fields of 0.5 MV/cm and as a function of inverse temperature. From the measurements, it is clear that the maximum leakage current densities at maximum device temperature of 200 C are 5 10 )7 A/cm 2 and 5 10 )5 A/cm 2 for dielectric field strengths of 0.5 MV/cm and 3.5 MV/ cm, respectively. As noted by Scozzie et al., the rate limiting conduction process, as evident in the temperature-activated region of the spectrum, is because of the Frenkel Poole (FP) or Schottky emission from the SiC conduction band into the HfO 2 dielectric insulator. However, below 100 C, the leakage current is independent of temperature and is due to the tunneling of carriers across the bands. This tunneling mechanism might be due to emission of trapped holes in the lower filled band in the dielectric insulator. The holes in the SiC valance band drift tunnel into the localized charged regions at the interface of HfO 2 and SiC. The Arrhenius plots for the I-V measurements shown in Fig. 5 are plotted in Fig. 7 for dielectric fields from 0.5 MV/cm to in a temperature range of C. From the plots, the activation energy is calculated to be 1.31 ev. Figure 8 gives Schottky plots 29 for the leakage current densities of Fig. 5 for e i of 1.0 MV/cm and. All curves give exceedingly linear relationships, although the slope is slightly greater for the 3.5 MV/ cm case. Values for the Richardson constant or current density prefactor for Schottky emission can be determined from the intercepts of the plots in Fig. 8. Current prefactors were extracted from the data. These prefactors are in remarkable agreement with the theoretical value of 43 A/cm 2 /K 2 for the Richardson constant for 4H-SiC, which is calculated using an effective hole mass from the formula ln (J/T 2 ) MV/cm A ¼ 4pqk2 m h /T (1/K) Fig. 8. Schottky plots for Ni/HfO 2 /p-sic/tin structures for gate fields of e i 0.5 and. where m * is the hole mass, q is the electronic charge, k is BoltzmannÕs constant, and h is PlankÕs constant. The hole effective mass, m * /m, for 4H-SiC is calculated to be 0.21, which is in close agreement with the values reported in the range of CONCLUSIONS The PLD deposited HfO 2 samples on SiC show excellent leakage properties (40 na/cm 2 ) with dielectric constant values of close to the ideal

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