Post Deposition Annealing Temperature Effect on White-Light Emitting of WOx Thin Film Stack on Si

Replacing the existing lighting technology with the efficient, environmental friendly, and high quality white light emitting device (LED) can reduce ~38% of the total lighting energy usage in the United States [1]. White LEDs have been reported in the literature, e.g., the combination of a blue InGaN LED with the yellow YAG:Ce phosphor, adding a blue-orange ZnSe layer on a doped ZnSe substrate, or mixing of the blue, green, red LEDs [2]. However, since they all involve multiple devices, the operation can be complicated and the cost can be high. Recently, the authors invented a new type single-chip, white-light emitting device (W-LED) that  had a MOS capacitor structure with a metal oxide high-k gate dielectric stack [2-5]. The light emission is due to the thermal excitation of the conductive paths formed within the high-quality Zr-doped HfO₂ or the HfO_x dielectric film. This kind of LED can be easily fabricated using the IC compatible process, i.e., sputtering and rapid thermal annealing, without toxic chemicals. Light can be emitted continuously for more than 1,000 hours with failure except minor degradation of the intensity. WO_x is a high-k material used in the thin film transistor [6]. The light emission phenomenon was observed in the WO_x based LED [7]. Since the post deposition annealing (PDA) temperature is critical to the bulk and interface layers’ material properties, it is necessary to understand how the PDA temperature influences the light emission characteristics .

                LEDs of the MOS capacitor structure with the WO_x gate dielectric layer were fabricated on the p-type (10¹⁵ cm^-3) (100) Si wafer. The WO_x layer was sputter deposited from the W target under Ar/O₂ (1:1) atmosphere, 5 mTorr, and 60 W conditions for 2 min. The PDA condition was 800^oC, 900^oC, or 1,000^oC under the N₂ atmosphere for 3 min. The 300 µm diameter and 80 nm thick ITO electrode was prepared by sputter deposition followed by wet etching. After that the deposition of the aluminum film on the back of the wafer, the complete sample was treated with a post metal annealing (PMA) step 400°C under H₂/N₂ (1:9) for 5 min. The current density-voltage (J-V) curve was measured with the Agilent 4155C semiconductor parameter analyzer. The light emission spectrum was recorded with an optical emission spectrometer (StellarNet BLK-C-SR-TEC) through an optical fiber.

                Figure 1 (a) are the high-magnification photos of light emission from WO_x LEDs with PDA temperatures of 800^oC, 900^oC, and 1,000^oC, separately, at V_g = -20 V. The light  was emitted from discrete bright dots, i.e., from the thermal excitation of the conductive paths in the WO_x dielectric film[7]. The conductive paths were formed while the applied V_g is larger than the magnitude of the breakdown voltage V_BD [7]. The number of the bright dots decreased but the brightness of each dot increased with the increase of the PDA temperature. The reduction of the number of bright dots with the increase of the PDA temperature is due to the improvement of the bulk and interface layers dielectric properties, e.g., less defects and larger V_BD. The brightness of each dot increased with the  increase of the PDA temperature, which can be explained by the higher local current density in each conductive path. Figure 1(b) shows the emission spectra of the three devices in Fig. 1(a). They all show the same broad band spectra including the visible wavelength to the near IR range. With the increase of the PDA temperature,  the light intensity increased and the peak (λ_peak) location shifted to the red wavelength direction. For example, the λ_peak locations and intensities are 665 nm and 4.06 × 10^-5 W/m²,  674 nm and 5.81 × 10^-5 W/m², and 694 nm and 7.92 × 10^-5 W/m² for the 800^oC, 900^oC, and 1,000^oC PDA samples, separately. The size, length, composition, chemical bonds, and morphology of the conductive path are all affected by the PDA temperature and therefore, the electric resistivity and the light emission characteristics.

                Figure 2 shows the J-V curves of the same three samples as those in Fig. 1. The magnitude of the V_BD increased with the increase of the PDA temperature, i.e., -4.5 V, -5.1 V, and -6.1 V for 800^oC, 900^oC and 1,000^oC PDA samples, separately. The leakage current density J decreased with the PDA temperature. The change of the electrical properties with the PDA temperature is consistent with the reduction of the defect density in the bulk and interface films as discussed in the previous section [5].  

                The emitted lights of these three samples were investigated for their chromaticity coordinates in the 1931 CIE chart, correlated color temperatures (CCT’s), and color rendering indices (CRI’s), as shown in Table 1. They are all located in the white region of the CIE chart. In addition, the CRI values are all above samples are > 90, which is much higher than that of the commercial YAG:Ce-based white LED, i.e., 79. The thermal excitation generated the broad band light while the semiconductor based LED emits the narrow band light.  

[1] M. A. Schreuder, et. al., Nano Lett. 10, 573 (2010).

[2] H. S. Chen et. al., Appl. Phys. Lett. 86, 131905 (2005).

[3] Y. Kuo and C. -C. Lin, Appl. Phys. Lett. 102, 031117 (2013).

[4] Y. Kuo and C. -C. Lin, Electrochem. Solid-State Lett. 2, Q59 (2013).

[5] Y. Kuo and C. -C. Lin, Solid-State Electron. 89, 120 (2013)

[6] M. Lorenz, H. v. Wenckstern, and M. Grundmann, Adv. Mater. 23, 5383 (2011).

[7] C. -C. Lin and Y. Kuo, under publication consideration.