Comparison of Temperature and Light Distributions Around Nano-Resistors in SSI-LEDs

Abstract

A recent optoelectronic device termed the Solid-State Incandescent LED (SSI-LED) was discovered by Kuo’s group in 2013 [1]. The device is made by subjecting a MOS Capacitor of transparent gate material and appropriate high-k properties to dielectric breakdown, which results in the formation of numerous nano-scale conduction paths called nano-resistors (NRs). Due to its extremely small cross-sectional area, NRs display high current densities for relatively small voltages applied across the device. Consequently, the high current densities yield high local temperatures that approach black body radiation and allow NRs to undergo incandescence, where white light is emitted from the top surface through the transparent gate electrode by thermal excitation of electrons. Since the heat generated from Joule heating is contained within the NRs, a large temperature drop occurs over the surrounding volume as the heat flux transitions from nanoscale to microscale, keeping the whole device at a much lower temperature. The light emission from the SSI-LED is similar to that of an incandescent lamp where a spectrum of a wide range of wavelengths is produced in contrast to the narrow band light emitted from the conventional pn junction-structured LED. Therefore, SSI-LEDs are suitable for natural light or white light applications [2].It is desirable to have a direct comparison of emitted light intensity, temperature, and current density profiles of not just single NRs, but of NRs with various layouts to quantify the interaction of these variables. Earlier simulations using COMSOL Multiphysics have produced temperature and current density distributions without any accompanying light intensity study [3]. Using the python simulation framework developed earlier, these variables were studied in COMSOL Multiphysics for 3 NRs arranged as a right-angled triangle, 4 NRs as a square, and 5 NRs in the shape of a '+' sign. The effect of mesh quality on the temperature and current density distributions was also quantified. Since the current density profile is highly dependent on the relative positions of the NRs, the mesh quality has a noticeable effect on the temperature surrounding the NRs. In accordance with theoretical calculations, there is a direct relationship between the current density as well as the temperature and light intensity. The resultant light intensity profiles qualitatively match experimental observations and offers a degree of validation at near screen distances. Near the circumference at the surfaces of the NRs, the current density is high [4].Figure 1(a) shows the arrangement of NRs which are all of 10 nm radius with NR centers separated from adjacent NR centers by 40 nm. Fig. 1(b) shows the light intensity (in log10 scale) of the same patterns at a distance of 1 nm above the gate electrode surface, where the light is emitted from the blackbody radiation with corresponding the CCT to the NR at the top surface. Fig. 1(c) is the temperature of the gate electrode surface for the same set of NR patterns. Blue regions have higher temperature and white regions are close to room temperature. The temperature on the gate electrode surface blends to form a distribution that resembles individual NRs, when in fact, the observed temperature is for the pattern as a whole. The possible reason for this is that temperature is distributed in terms of sinusoidal waves according to the solution of the heat equation and is largely diffused when measured at the gate electrode surface, while the light intensity distribution obtained from the Stefan-Boltzmann law and inverse square law, varies with the 4th power of temperature close to the gate and shows a significantly steeper curve with individual NRs still distinct.More detailed comparisons of the temperature and light distributions will be shown in the presentation.1. Y. Kuo and C.-C. Lin, Appl. Phys. Lett., 102, 031117 (2013).2. Y. Kuo, 2014 IEEE International Electron Devices Meeting, 4.7.1-4.7.4 (2014).3. A. Shukla and Y. Kuo, ECS J. Solid State Sci. Technol., 9, 065017 (2020).4. K. Natarajan and Y. Kuo (To be published). Figure 1