Electrical and Optical Characteristics of WOx, ZrHfO, and Tri-Layer WOx Embedded ZrHfO High-k Based SSI-LEDs
Authors: Lin Wen-Shan , Kuo Yue 
Published: July 2021

The solid state incandescent light emission device (SSI-LED), which can emit the broadband warm white light, is made from the high-k MOS capacitor after the dielectric breakdown [1-4]. Electrical properties of the capacitor, such as interface defect density (Dit
), oxide trapped charge density (Qot
), equivalent oxide thickness (EOT), and leakage current density (Jleakage
) are important to the dielectric breakdown process and eventually the optical properties of the emitted light. Since the zirconium hafnium oxide (ZrHfO) high-k material has superb gate dielectric properties, e.g., high crystallization temperature, low interface state density, oxide trapped charges, and high effective k value, it has been used as the gate dielectric layer of the MOS capacitor [5,6]. On the other hand, the WOx dielectric-based MOS capacitor and SSI-LED has been studied. The latter emitted light with relative high intensity due to the high emissivity coefficient of the W component [7]. However, the capacitor has a high leakage current which can be a reliability concern. In this paper, three types of MOS capacitors, i.e., with gate dielectrics made of ZrHfO, WOx, and ZrHfO/WOx/ZrHfO/WOx/ ZrHfO/WOx/ZrHfO, separately, have been fabricated into SSI-LEDs and studied for electrical and optical properties.
MOS capacitors were prepared on a DHF pre-cleaned p-type <100> silicon (1015 cm-3) wafer. For the ZrHfO capacitor, the gate dielectric layer was sputter-deposited from the Zr/Hf (12/88 wt%) target in Ar/O2 (100:100 sccm) at 5 mTorr and 60 W for 12 min. The WOx layer was sputter-deposited from the W target (99.999 %) in Ar/O2 (40:100 sccm) at 5 mTorr and 60 W for 12 min. For the WOx embedded capacitor, the ZrHfO (2 min)/WOx (2 min)/ZrHfO (2 min)/WOx (2 min)/ ZrHfO (2 min)/WOx (2 min)/ZrHfO (2 min) stack was deposited in one pump down under the same conditions as each individual layer. The post-deposition annealing (PDA) was conducted at 900 ̊C under the O2 atmosphere for 5 min in furnace. An 80 nm thick ITO was sputter deposited and wet etched into round gate electrodes using an aqua regia solution. Then, the backside of the wafer was deposited with an aluminum thin film and annealed at 400 ̊C under H2/N2 (10/90) for 5 min. The dielectric breakdown voltage of the capacitor was determined from the current density-gate voltage (J-V) curve. The light emission characteristics the SSI-LED were measured from an optical emission spectrometer (OES, StellarNet).
Electrical characteristics of the capacitors before and after the dielectric breakdown were discussed with respect to the gate dielectric material properties. Figure 1 shows the emission spectra of the 3 SSI-LEDs. They all emit broad band light in the range of UV to near IR. They are similar to the visible wavelength region of the solar light. The peak lengths λpeak’s of the ZrHfO, WOx, and WOx embedded devices are 638 nm, 687.15 nm, and 663 nm, respectively. The WOx SSI-LED has the highest emission intensity. It was reported that the emissivities of the pure (99.99 %) tungsten and the hafnium-3 wt% zirconium materials were 0.434 [8] and 0.30 [9] at 2000 K, respectively. Therefore, the inclusion of W component in conductive path may increase the light emission intensity of SSI-LED. According to the emission spectra, the CIE chromaticity chart has been prepared. The color correlation temperatures (CCT’s) and color rendering indices (CRI’s) were calculated.

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Figure 1