Materials Science Forum, ISSN: 1662-9752, Vol. 1159, pp 81-86

doi: 10.4028/p-7NEtpc

© 2025 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2024-09-13

Revised:
2025-05-07

Accepted:
2025-05-08

Online:
2025-09-09

Effect of Inserting an Intervening Layer on Reduction in TiN Schottky

The original version of this paper is available on https://www.scientific.net/MSF.1159.81.pdf

Shigeaki Takagi , Hideo Yoshihashi, Katsuhisa Tanaka, Tomomi Imamura and Hiroshi Kono

Semiconductor Div., Toshiba Electronic Devices & Storage Corporation, 300, Ikaruga, Taisi-cho, Ibo-gun, Hyogo 671-1595, Japan

shigeaki.takagi@toshiba.co.jp, hideo1.yoshihashi@toshiba.co.jp, katsuhisa1.tanaka@toshiba.co.jp, tomomi.imamura@toshiba.co.jp, hiroshi.kono@toshiba.co.jp

Abstract

This report describes the application of titanium nitride (TiN) with a silicon nitride (SiN) intervening layer as a Schottky electrode in a Schottky barrier diode (SBD) made of 4 H -silicon carbide (SiC). This reduced the Schottky barrier height ( ) to 0.74 eV at room temperature, and it was confirmed that the reduction in was due not only to the application of TiN but also to the intervening layer containing SiN at the interface. Furthermore, TiN with SiN was applied to a device as a Schottky electrode, and the electric field reduction effect was verified by changing the high energy implantation and JBS width. As a result, the forward voltage ( ) was found to be reduced by a maximum of 0.23 V while suppressing leakage current. The reason for describing the interlayer as "intervening layer containing SiN" is that there may be other substances besides SiN.

Introduction

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Silicon carbide (SiC) devices have the advantages of high breakdown voltage, low on-resistance, high-speed switching and high-temperature operation compared with silicon ( Si ) devices, and are expected to further expand the market to photovoltaic inverters (PVs), electric vehicle (EV) charging stations and on-board chargers (OBCs). For these applications, SiC Schottky barrier diodes are useful devices that take advantage of the excellent properties of SiC, such as low , and various materials have been evaluated as a Schottky electrode [1,2]. However, it is necessary to select a material with a low work function and lower Schottky barrier height . Metal nitrides are known to have a lower work function than pure metals [3,4], and it has also been reported that, for the case of Si, can be reduced by inserting an extremely thin insulating film between the Schottky electrode and the substrate [5]. In order to reduce in SiC devices, we adopted TiN as the Schottky electrode. In addition, we formed intervening layers containing SiN of different thicknesses as ultrathin films and investigated their impact on . Furthermore, we applied TiN with SiN as a Schottky electrode in a device and verified the electric field reduction effect by changing the high energy injection and JBS width.

Sample Structure

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Fig. 1 shows a sample structure used to confirm the effect of reducing . We prepared (0001) wafers and formed an oxide film, and then exposed part of the SiC surface by performing a contact opening. Intervening layers of different thicknesses were then formed at the interface, after which a back electrode was formed. "Intervening layer of different thickness" mean that the thickness of the SiN layer has been varied.

Fig. 1. Sample structure used to confirm the .

Fig. 1. Sample structure used to confirm the .

Experimental Methods

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Cross-sectional observation was performed using high-resolution scanning transmission electron microscopy (STEM) to confirm the details of the SiC/Schottky electrode interface. STEM is a technique in which a thin sliced sample is irradiated with an electron beam, and the electrons that pass through or are scattered by the sample are imaged and observed at high magnification, making it possible to observe and analyze the microstructure of and lattice defects in the sample at nano-level high resolution. The results are shown in Fig. 2. In both Fig. 2(a) and 2(b), it can be seen that an intervening layer was formed between the SiC and the Schottky electrode. Moreover, the thickness of the intervening layer differed between Fig. 2(a) and 2(b), with that in Fig. 2(b) being thicker. The thickness was at the atomic layer level under all conditions. In addition, as the intervening layer thickened, the TiN crystal orientation changed from a uniform azimuth to polycrystalline with strong anisotropy. Uniform azimuth means that TiN is in the same direction as SiC, polycrystalline with strong anisotropy means that TiN has different orientations than SiC.

Fig. 2. High-resolution STEM images of the SiC/Schottky electrode interface.

Fig. 2. High-resolution STEM images of the SiC/Schottky electrode interface.

Next, in order to observe the bonding state of atoms in the intervening layer, we performed electron energy-loss spectroscopy (EELS) analysis using the samples observed by STEM. EELS is a technique that measures the energy lost by electrons as they pass through a thin film, allowing the analysis of atoms and electronic structure. The results are shown in Fig. 3. Regardless of the thickness of the intervening layer, peaks were observed near 405 eV . It was found that these peaks originated from SiN . The intensity of the peak near 405 eV increased as the intervening layer became thicker.

The results of these high-resolution STEM and EELS analyses revealed that intervening layers of different thicknesses were formed at the SiC/TiN interface, and that the amount of SiN was greater when the intervening layer was thicker.

Fig. 3. EELS spectrum at the SiC/Schottky electrode interface.

Fig. 3. EELS spectrum at the SiC/Schottky electrode interface.

In order to confirm the effect of using TiN with SiN, the current-voltage (IV) curve and were investigated for the above sample and conventional sample. "Conventional sample" refers to SBDs that use TiN electrodes without an intervening layer. The IV curve was measured using a semiconductor parameter analyzer after assembling individual chips onto a direct bonded copper (DBC) substrate after forming the back electrode. was calculated by calculating the saturation current density ( ) from the IV curve measured at -1.0 to 1.0 V using the Shockley diode equation shown in equation (1), and then calculating from using equation (2), where J is the current density, V is the applied forward voltage, q is the elementary electric charge, k is the Boltzmann constant, T is the measurement temperature and n is an ideality factor. A Richardson constant of was assumed [6]. The results are shown in Fig. 4.

was 0.74 eV for a thick intervening layer and 0.89 eV for a thin intervening layer. Compared with the 1.25 eV of a conventional sample, was therefore reduced by 0.51 eV by the thick intervening layer. In all conditions, the current of SBD is constant at , but this is due to the measurement conditions.

Fig. 4. IV curve and .

Fig. 4. IV curve and .

It was found that the anisotropy of TiN changed as the intervening layer became thicker and the amount of SiN increased. Even for metals with the same work function, the value of the work function changes depending on the crystal orientation. When the crystal orientation is aligned, electrons easily jump out of the crystal, and the crystal has a positive charge. Therefore, a lot of energy is required to produce residual electrons in the crystal, and the work function becomes larger. However, if the crystal orientation is not aligned, electrons are attracted between the crystals, and the crystals becomes close to neutral. Therefore, energy is not required for the emission of residual electrons, and it is thought that the work function decreases. This suggests that when the intervening layer was thick, may have been significantly reduced.

Application to Devices

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Although it was confirmed that it is possible to reduce by using TiN with SiN, it is important to understand the degree of impact on and when applied to a device. While the device created in Fig. 4 has a simplified SBD structure, the device in this chapter is a full device. Fig. 5 shows the curve and the curve when a TiN with SiN electrode is applied to a conventional device structure. and were checked at 12 A and 650 V . As shown in Fig. 5(a), it can be seen that is 0.23 V lower for TiN with SiN. This is believed to be due to the reduction of by TiN with SiN. However, as shown in Fig.5(b), in the TiN with SiN sample, is on the order of , which is two orders of magnitude larger than that in the conventional sample. This is due to low , and it is clear that suppressing leakage is an issue.

Fig. 5. (a) curve and (b) curve when using a conventional device structure.

Fig. 5. (a) curve and (b) curve when using a conventional device structure.

The device structure was therefore changed to suppress leakage. Fig. 6 shows a schematic diagram of the device model. This device has a JBS structure in which a p-layer is embedded in a part of the n -layer on the semiconductor surface, with the length of the n -layer being and the length of the p -layer being . High energy implantation was performed to change the JBS width. During this process, the half-width of the n -region was changed while keeping the cell pitch constant.

Fig. 6. Schematic diagram of a device model with modified element structure.

Fig. 6. Schematic diagram of a device model with modified element structure.

Fig. 7 shows the correlation between and when and are changed. Ir was less than regardless of the ratio of the n -region. This shows that it is possible to suppress leakage current by more than two orders of magnitude by changing the device structure. Moreover, as the proportion of the n -region increased, decreased and increased. This shows that by adjusting the widths in a device with a modified element structure, it is possible to optimize while suppressing leakage.

Fig. 7. Relationship between and .

Fig. 7. Relationship between and .

Finally, Fig. 8 shows the relationship between and compared with a conventional sample when a modified device structure was applied using a TiN with SiN Schottky electrode. Compared with the conventional samples, at 12 A was reduced by a maximum of 0.23 V . at 650 V was on the order of , which was equal to or lower than that of conventional sample. From the above, it was possible to achieve both low and suppression even when applying TiN with SiN that can reduce to a device.

Fig. 8. Relationship between at 12 A and at 650 V .

Fig. 8. Relationship between at 12 A and at 650 V .

Summary

The original version of this paper is available on https://www.scientific.net/MSF.1159.81.pdf

It was found that a significant reduction in in can be achieved by selecting TiN with a low work function and by forming an intervening layer containing SiN. This is believed to be because the crystal orientation changes as the intervening layer becomes thicker, causing a change in the Schottky work function. Furthermore, TiN containing an intervening layer was applied to a device as a Schottky electrode, and the electric field reduction impact was verified by changing the energy implantation and JBS width. As a result, it was confirmed that the was reduced by up to 0.23 V while suppressing leakage, demonstrating that it is possible to achieve both a low and suppression.

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The original version of this paper is available on https://www.scientific.net/MSF.1159.81.pdf

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