Substrate Effect on Light Emission of SSI-LEDs

Abstract

A new type of solid-state incandescent light emitting device (SSI-LED) that emitted broad-band warm white light has been reported recently [1-4]. This kind of device is made from a simple MOS capacitor structure composed a high-k gate dielectric on p-type Si substrate [1-4]. The light emission principle is the thermal excitation of nano-sized conductive paths, i.e., nano-resistors, formed from the dielectric breakdown of the MOS capacitor upon the pass of a large current [4]. Since the whole device is made with IC-compatible materials and processes, one of the possible applications is the on-chip optical interconnect [5]. It also shows antifuse- and diode-like characteristics which are suitable for IC and other applications [6,7]. Since the nano-resistors are formed from the breakdown of the MOS structure, all materials in the current path, i.e., including the dopant concentration of the substrate, can contribute to the light emission process [8]. In this paper, authors investigated the Si substrate effect on light emission of the SSI-LED. SSI-LEDs made of p-type <100> Si wafers of two different dopant concentrations, i.e., p (~1015 cm-3) and p + (~1017 cm-3), were fabricated. The Zr-doped HfO2 (ZrHfO) dielectric film was sputtered from a Zr/Hf (12/88 wt. %) target on the substrate in Ar/O2 (1:1) at 5 mTorr and 60 W for 12 min. Then, the sample was annealed at 800°C in N2 for 3 min. Subsequently, an ITO film was deposited on top of the high-k stack and wet-etched into 300 µm-diameter dots as the gate electrodes. An Al film was deposited on the backside of the wafer to form the ohmic contact. Finally, the sample was annealed at 400°C in forming gas for 5 min. The complete SSI-LED was stressed at a gate voltage (Vg ) of -25 V for the light emission characteristics. Figure 1 are top-view photos of light emission from SSI-LEDs made on (a) p and (b) p+ Si wafers. Both photos showed that the light was emitted from many discrete bright dots evenly distributed across the gate electrode. Each dot corresponds to a nano-resistor. The p+ sample contains many more dots than the p sample does. The bright dots were almost distributed everywhere across gate electrode of the p+ substrate device. This is because the breakdown strength decreases markedly with the increase of the substrate doping concentration [8]. Therefore, under the same Vg stress, it is easier to form more conductive paths in the gate dielectric thin film in the p+ sample than in the p sample. The light emission photo of the former looks brighter than that of the latter. Figure 2 shows the light emission spectra of the p and p+ samples. Both spectra cover the same wavelength range including the visible and some of the near IR lights. The p+ sample has a much higher light intensity than the p sample has, which is consistent with the observation of Fig. 1. The more nano-resistors are formed, the stronger the light is emitted from the SSI-LED. The peak wavelengths of both devices are very close, i.e., 700 nm for the p substrate device and 707 nm for the p+ substrate device. The CIE color coordinates, color correlated temperature (CCT), and color rendering index (CRI) values were calculated and listed in Table 1. Both samples have similar CIE characteristics, which indicates the similar conductive path properties. In summary, the light emission characteristics of the SSI-LED is influenced by the dopant concentration of the Si wafer, which affects the breakdown of the MOS capacitor and the formation of nano-resistors. [1] Y. Kuo et al, APL, 102, 031117 (2013). [2] Y. Kuo et al, ESSL, 2, Q59 (2013). [3] Y. Kuo et al, SSE, 89, 120 (2013). [4] C.-C. Lin et al, APL, 106, 121107 (2015). [5] S. Zhang et al, ECS JSS, 6, Q39 (2017). [6] Y. Kuo, ECST, 69, 23 (2015). [7] Y. Kuo, ECST, 67, 183 (2015). [8] C. Osburn et al, JES, 119, 597 (1972). Figure 1