(Invited) Status Review of SSI-LEDs
Authors: Kuo Yue 
Published: February 2020

After several years of confirmation of the light emission phenomena and theory development, the author’s group first reported the solid-state incandescent light emitting device (SSI-LED) in 2013 [1]. The status of this kind of device is reviewed with respect to the structure, composition materials, fabrication process, physics, and reliability.

The principle of light emission from the SSI-LED is the thermal excitation of the high-k dielectric surrounded nano-resistors during the passage of a current. Figure 1 shows (a) the cross-sectional view of the structure and (b) a top view of light emitted from a device. These nano-resistors are formed from the hard breakdown of the MOS capacitor. Different from the conventional wisdom that the whole dielectric film would be broken from a single location, the film breaks at a large number of spots evenly distributed across the gate electrode covered area. This phenomenon can be contributed to the good initial gate dielectric thin film, i.e., low leakage current, homogeneous physical and chemical properties, and uniform thickness. SSI-LEDs have also been fabricated from MOS capacitors with various kinds of high-k dielectrics, such as HfOx, Zr-doped HfOx (ZrHfO), WOx, AlOx, TaOx, TiOx, and NiOx. In addition, SSI-LEDs with gate dielectrics composed of the high-k/nanocrystals (nc-Si, -ZnO, -ITO, -CdSe, -MoOx, etc.)/high-k thin film stacks have also been successfully fabricated [2,3].

The emission spectrum of the SSI-LED convers the whole visible and part of the IR wavelength range independent of the original dielectric material or the including of nanocrystals in the dielectric structure. The shape of the emission spectrum is similar to that of the solar light in the visible wavelength. Other light characteristics, such as the CCT, CRI, and chromaticity, are similar to those of the incandescent light. Therefore, light is emitted from the black body radiation following the Planck’s law.

The light emission efficiency of a SSI-LED is determined by the efficiency of the thermal excitation of the nano-resistor, which can be estimated from the emission spectrum energy and the input power. Since the range and shape of the emitted light is fixed, the former is dependent on its intensity. In our studies, it was observed that the intensity of the emitted light from a SSI-LED was directly related the current density, which is related to the resistance along the current path. The resistance along the current path is mainly composed of resistances of the nano-resistor, the nano-resistor/Si contact, the Si substrate, and the gate electrode. The first two items are dependent on the material of the original high-k dielectric and the bond structure of the conductive path. The electrical properties of the nano-resistor is between those of a metal and a semiconductor [4].

The SSI-LED made of the nc-CdSe embedded ZrHfO gate dielectric has been operated for a long period, e.g., > 20,000 hours, under atmosphere without failure [5]. The surrounding dielectric film prevents the exposure of the nano-resistor the atmosphere. The failure of the device is gradual, which shows as the decrease of the leakage current and the red shift of the light spectrum with the increase of the time.

In summary, the principle of light emission, composition materials, efficiency, and lifetime of the SSI-LED have been examined and discussed. The warm white light emitted from this kind of device can be used in a wide range of lighting, on-chip interconnect, and other applications.

The author acknowledge his students, postdoctoral and visiting scholars Chi-Chou Lin, Shumao Zhang, and Xiaoning Zhang for dedicated work in the fabrication and testing of SSI-LEDs.

[1] Y. Kuo et al., APL,
102, 031117 (2013).

[2] Y. Kuo, IEEE TED, 62, 3536 (2015).

[3] C.-C. Lin et al., Appl. Phys. Lett., 106, 121107 (2015).

[4] S. Zhang et al., IOP J. Phys. D, accepted (2017).

[5] Y. Kuo et al., AVS 63rd Intl. Symp. Abst. #1844 (2016).

Figure 1