INVESTIGATION OF Al2O3 AND TiO2 AS GATE INSULATORS FOR 4H-SiC PULSED POWER DEVICES

Q. Shuia, M. S. Mazzolab, X. Gua, C. W. Mylesc, and M. A. Gundersena, ζ

aDepartment of Electrical Engineering-Electrophysics, University of Southern California Los Angeles, CA 90089-0271

bDepartment of Electrical and Computer Engineering, Mississippi State UniversityMississippi State, MS 39762

cDepartment of Physics, Texas Tech UniversityLubbock, TX 79409-1051

Abstract

Achieving high quality and high electric breakdown of oxide on 4H-SiC substrates is still a major research challenge. 500Å Al2O3 and TiO2 were deposited on 4H-SiC by high vacuum and low-temperature e-beam evaporation. AFM (Atomic Force Microscopy) results show that a surface mean height of 4-7 Å of Al2O3 was found on Si substrate and 74 Å of Al2O3 was found on SiC substrate. MIS capacitors were fabricated using one mask to evaluate the quality of these gate materials. C-V measurements at 10kHz show that Al2O3 and TiO2 have dielectric constants of more than 9 and 20, respectively. The leakage current of an Al2O3 MIS capacitor is about 10-9A on a capacitor area of 150m 150m at a gate voltage of 10V. An Al2O3 MIS capacitor has a lower leakage current than a TiO2 MIS capacitor due to its large band gap offsets. Its breakdown electric field strength is more than 8MV/cm. This research indicates that e-beam deposited Al2O3 may be a promising dielectric material for the pulsed power SiC devices.

I. INTRODUCTION

It is important to develop solid-state-based pulsed power switches with hold off voltage considerably in excess of 10 kV, peak currents of kA to many kA, and turn-on in 10's of nanoseconds, for varying pulse lengths. 4H-SiC is an attractive candidate at this time because it has a large bandgap energy (3.26eV) relative to GaAs (1.43eV) and Si (1.12eV), breakdown electric field strength of 3-4 MV/cm, electron saturation velocity of 2.2 107cm/s and thermal conductivity of 4.9 W/cm.K [1-2]. Up to now, 5kV 4H-SiC MOSFETs with 100-m [3] N- drift region and several kilovolts of VJFETs have been proposed [4-8]. 4H-SiC-based switches are thus of interest as potentially efficient, low-cost, and compact alternatives to gas phase switches [9]. MOSFETs, although the most widely used power devices, have poor oxide quality and oxide breakdown which has thus far

limited the performance of 4H-SiC MOSFETs [10], and the high breakdown field strength of SiC has not been fully exploited. Utilizing a material with higher dielectric constant will reduce the electric field across the dielectrics [11], and provides a possible solution to the above-mentioned problems. The relatively large lattice mismatch between Al2O3 or TiO2 and SiC substrates restricts the use of these layers in SiC technology as SiO2 and AlN. So far, very limited research has been done in this field [12-13]. Nevertheless, Al2O3 ( =8-15, Eg=6-9eV) and TiO2 ( =20-110, Eg=3-5eV) have higher dielectric constants and high bandgap energies as well as the acceptance of Ti and Al in most modern CMOS fabrication facilities. A band alignment of Al2O3 on 4H-SiC with offsets of 2.2 and 1.5eV for the valence and conduction bands further makes it a potentially serious competitor to SiO2 [12]. In this paper, we study Al2O3 and TiO2 grown by e-beam evaporation. This method is a high vacuum and low-temperature process (T<200ºC), thus it is effective in limiting the formation of the interfacial SiO2 layer between the SiC and Al2O3 as well as the formation of high-temperature induced defects. Surface morphology of Al2O3 or TiO2 were investigated by AFM, and both Al2O3

and TiO2 MIS capacitors were fabricated to evaluate their potentials as gate dielectrics in pulsed power SiC devices.

II. EXPERIMENTAL PROCEDURE4-8m-thick n- layer (~11016cm-3) and 0.3 m-thick

buffer layer, grown on the (0001) Si-faced 4H-SiC substrate (~11019cm-3), were provided by Mississippi State University. Fig.1 illustrates the schematic structure of the MIS capacitors. Only one mask was used to fabricate these capacitors. Before the deposition of nickel and gate dielectric materials, the samples were cleaned and dipped in buffered oxide etch (7:1) for 1min. 2000 Å nickel was e- beam evaporated and annealed at 900 ºC for 1min to form backside ohmic contacts. 500 Å Al2O3 and TiO2 were also e-beam deposited at temperatures less than 200ºC, respectively. Ti (200 Å) /Au (1500Å) were chosen

This work supported by the AFOSR MURI Program and the ARO. email: mag@usc.edu

as gate electrodes. They were patterned by standard photolithography techniques, and annealed at 450ºC for 30s by rapid thermal annealing (RTA).

Figure 1. The schematic structure of investigated 4H-SiC MIS capacitors.

The capacitance-voltage (C-V) measurements of these MIS capacitors were done by a HP 4263 LCR meter at a frequency of 10kHz. The small ac voltage is 1V on an applied dc gate voltage. Dielectric constants of Al2O3 and TiO2 grown by e-beam evaporation were calculated. Current-voltage (I-V) measurements were performed by using a HP 4145B semiconductor parameter analyzer on 150m150m gate patterns.

III. RESULTS AND DISCUSSION

A. Film morphology Fig. 2 (a) shows the surface morphology of the n- layer (~11016cm-3) grown on heavily doped 4H-SiC substrate. The surface shows a regular terrace structure with width of about 1m. The surface mean height is 84Å. An AFM image was also taken after the deposition of Al2O3 or TiO2

(Fig. 2(b)). We found that the surface morphology improves and the surface mean height is about 74 Å. We also measured Al2O3 (500 Å) deposited by the same e-beam evaporator on Si substrate, and the surface mean height is 4-7 Å (Fig.2(c)). It is obvious that the surface morphology not only depends on the growth conditions, but also depends on the substrate. We conclude that from the point of surface morphology, high vacuum and low-temperature e-beam evaporation proves to be a simple and reliable way for the growth of good surface morphology of Al2O3 and TiO2 gate dielectrics.

B. Electrical Characterization Figures 3(a) and 3(b) show the C-V characteristics of metal-Al2O3-SiC and metal-TiO2-SiC capacitors measured on different contact areas, 120m 120m and 100m 100m, respectively. Measurements were performed at a frequency of 10kHz at room temperature. Since an n- epilayer is below the gate insulators, at positive voltages,

Figure 2(a). An AFM image (10m10m) of the starting material (4H-SiC n- epilayer).

Figure 2(b). An AFM image (10m10m) of 500 Å Al2O3 deposited on 4H-SiC n- epilayer.

Figure 2(c). An AFM image (10m10m) of 500Å Al2O3 deposited on Si substrate.

majority carrier electrons would experience a force toward the oxide-semiconductor interface. At large positive voltages, the capacitance saturates, indicating that an accumulation layer is formed below the gate materials. The capacitance for the MIS capacitor in the accumulation mode is just oxide effective capacitance. The oxide effective capacitance may also include the contribution of a very thin SiO2 formed in the subsequent

Figure 3(a) Capacitance-voltage characteristics of Metal-Al2O3 (500 Å)-SiC capacitors measured at 10kHz.

Figure 3(b). Capacitance-voltage characteristics of Metal-TiO2 (500 Å)-SiC capacitors measured at 10kHz.

450C gate electrode annealing process. A minimum capacitance at large negative voltages in Fig. 3(a) and 3(b) mean that a maximum depletion width is reached. We first assume that there is no thin interfacial SiO2 layer formed, the dielectric constants for Al2O3 and TiO2 on two different areas are consistent and they are 9 and 20, respectively, based on the accumulation capacitance . The theoretical flat-band capacitance then can be determined from the following equation [14]:

(1)

We note that the calculated flat-band capacitance for Al2O3 (A=120m 120m) is 13.5pF. This value is even below , indicating that a thin interfacial layer SiO2 is in series with the Al2O3 and that it reduces the effective dielectric constant of the insulating systems. Similarly, the theoretical value of for TiO2 (A=120m 120m)

is determined to be 20 pF. This value is also below our expected value deduced from the experimental data in Fig 3 (b). Since the dielectric constant TiO2 is several times larger than that of Al2O3, the thin SiO2 layer doesn’t reduce the dielectric constant of TiO2/SiO2 system as much as it does for Al2O3/SiO2 system. There are two approaches to determine the thicknesses of SiO2 and Al2O3 or TiO2. The first is to fabricate another set of MIS capacitors with different thicknesses of Al2O3 or TiO2

under the same fabrication conditions. The second is to employ material characterization equipments, such as AES (Auger Electron Spectroscopy), SIMS (Secondary Ion Mass Spectrometry) and TEM (Transmission Electron Microscope). That is part of our future work.

Figure 4. Current-voltage characteristics of MIS capacitors. The capacitor area is 150m 150m and the thicknesses of Al2O3 and TiO2 are all 500 Å.

Leakage currents were measured on MIS capacitors with area of 150m 150m. From Fig. 4 it is found that the leakage current is dependent on the polarity of the gate bias. The dielectric of Metal-Al2O3-SiC capacitors even doesn’t break down at a gate voltage of 40V, which corresponds to an electric field strength of 8MV/cm. Al2O3 has band offsets of 2.2eV for the valence band, and 1.5eV for the conduction band [12]. These band offsets promise higher barriers for both electrons and holes. Fig.4 also shows that Al2O3 is highly resistive, and that its resistivity is more than 11011cm. The experiments demonstrate that Al2O3 is a low-leakage current gate material. TiO2 has a higher leakage current at negative gate voltages than Al2O3. This can be explained by the observations [15-17] that materials with higher dielectric constants tend to suffer from higher leakage currents due to smaller bandgaps. It is known that the bandgap of TiO2

is 3-5eV, the smaller bandgaps lead to lower band offsets with respect to 4H-SiC. Nitridation is widely used in silicon technology and it is usually achieved by annealing the sample in nitric oxide (NO), nitrous oxide (N2O) or NH3 atmosphere. Nitridation is not only very effective in limiting the formation of an unnecessary interfacial SiO2, but also reduce the Si

dangling bond and Si-O-Si strained bonds at the interface, hence decreasing the leakage current. To gain insight into the role that NO plays at the interface between the insulator Al2O3 and 4H-SiC, after depositing Ni and alloying of the backside Ni ohmic contacts at 900ºC, the samples were subjected to annealing in NO at 1000ºC for 10min. then dipped in buffered oxide etch (7:1) for 1min. The subsequent fabrication conditions are the same as described above. We found that decreases to 16.8pF for a 120m 120m capacitor. We think that a thicker SiO2 is formed and nitric oxide doesn’t seem to limit the formation of SiO2 in this case. I-V tests show that the leakage currents remain almost the same.

IV. CONCLUSIONS AND FUTURE WORK

We have performed e-beam deposition of Al2O3 and TiO2 on 4H-SiC for the purpose of pulsed power application. High-vacuum and low-temperature e-beam evaporation shows a mean height of about 4-7 Å on Si substrate. The MIS C-V characteristics show that the dielectric constant of Al2O3 is more than 9, and TiO2 is more than 20. I-V curves show that Al2O3 is highly resistive and has much lower leakage current than TiO2

due to its large band gap offsets. Therefore, Al2O3

deposited by e-beam evaporation can be considered as a promising material among the gate insulators for high power SiC devices which will be used in pulsed power in the future.

Further research is needed to identify detailed information of gate materials by X-Ray Diffraction (XRD), TEM, etc. Dielectric constants, flat-band voltage, fixed oxide charges, and interface trapped charge densities need to be determined.

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