Solid State Incandescent Light Emitting Device Made of WOx Embedded Zr-Doped HfO2 High-k Stack on Si

The conventional semiconductor based LED emits the narrow band light corresponding to the band gap energy of the composing film. For the white light application, a group of red, green, and blue LEDs or a combination of a blue or UV LED with a phosphor layer has to be used [1].  Recently, Kuo and Lin reported a new type of solid state incandescent LED (SSI-LED) that emitted the warm white light similar to that of the conventional incandescent bulb [2-6]. The single chip SSI-LED has a simple MOS structure made on a p-type Si wafer with a gate dielectric composed of a metal oxide high-k layer such as the Zr-doped HfO₂ (ZrHfO) or HfO_x film with or without an embedded nano dots or a continuous film [6]. It can be used in lightning or the on-chip optical interconnect. Previously, it was proved that the SSI-LED can be fabricated using the WO_x gate dielectric layer [4]. In this study, authors investigated the electrical and optical characteristics of the SSI-LED made of the WO_x embedded ZrHfO high-k film.

                The ZrHfO/WO_x/ZrHfO tri-layer stack was sputter deposited on a p-type Si (100) (10¹⁵ cm^-3) wafer by one pump down without breaking the vacuum. The ZrHfO films were sputter deposited from a ZrHf (12/88 wt. %) target under the Ar/O₂ (1:1) atmosphere at 5 mTorr and 60 W, i.e., 2 min for the bottom layer and 10 min for the top layer. The WO_x layer was deposited from a W target under the Ar/O₂ atmosphere at 5 mTorr and 60 W for 3 min. The sample was annealed at 800°C in N₂ for 3 min. The ITO film was deposited on the high-k stack and etched into gate electrodes. After the Al was deposited on the back of the wafer and annealed at 400°C for 5 min under the H₂/N₂ atmosphere, the sample was characterized.   

                Figure 1 shows the high magnification photos of the control and the WO_x embedded SSI-LEDs stressed at V_g = -60 V. For both samples, the light is emitted from discrete bright dots uniformly distributed across the gate electrode. The control sample contains more smaller bright dots than the WO_xembedded sample does. The light emitted from the former visually appears to be stronger than that of the latter. It was reported that the nano bright dots in the SSI-LED were generated from the thermal excitation of the conductive paths formed in the dielectric film [1-4]. The excitation efficiency is related to the geometry of the conductive path, such as the cross-sectional area and the length, and the composing material. The visual observation of the brightness is related to the spectrum of the light, i.e., the wavelength distribution.

                Figure 2 shows the I-V curves of the control and WO_x embedded SSI-LEDs with V_g swept from 0 to -10 V. The abrupt breakdown of the dielectric film occurs at -6.1 V for the control sample and -7.9 V for the WO_x embedded sample. The large breakdown of the former is probably due to the addition of the embedded layer that increased the physical thickness. The formation of the conductive path in the high-k stack can also be related to the defect density, e.g., in the bulk film or at the Si/high-k interface [5]. The oxide trapping density (Q_ot) and the interface density of states (D_it) of the WO_x embedded layer are higher than those of the control sample. The Fig. 2 result shows that defect densities are less important than the thickness in determining the high-k stack’s dielectric breakdown voltage. 

                Figure 3 shows the emission spectra of the control and WO_x embedded samples at V_g = -60 V. Both samples emit broad band lights covering the whole visible and part of the near IR wavelengths. The light emitted from the control sample has a higher intensity at below 540 nm wavelength, e.g., UV to blue and part of the green range, than that of the WO_xembedded sample. However, the latter has a higher intensity at the above 540 nm range. The higher visible brightness of the control sample in Fig. 1 is contributed by the blue-green portion of the light.  

[1]   Y. Kuo and C.-C. Lin, APL, 102, 031117 (2013).

[2]   Y. Kuo and C.-C. Lin, ESSL, 2, Q59 (2013).

[3]   Y. Kuo and C.-C. Lin, SSE, 89, 120 (2013).

[4]   C.-C. Lin and Y. Kuo, JVSTB, 32, 011208-1 (2014).

[5]   C.-C. Lin and Y. Kuo, EJSSST, 3, Q182 (2014).

[6]   Y. Kuo and C.-C. Lin, IEDM, Abst., 4.7  (2014).