Filter of SSI-LED Light Using Thin Films
Authors: Kuo Yue , Zhang Shumao 
Published: February 2020

Recently, Kuo and Lin presented a new-type of solid state incandescent light emitting device (SSI-LED) that was made from a MOS structure with a high-k dielectric thin film, such as the Zr-doped HfO2 (ZrHfO) [1-3]. This kind of device emits the warp white light from the thermal excitation of nano-sized conductive paths surrounded by the dielectric film during the passage of a large current [4]. The emitted light covers the visible to near-IR wavelength range similar to that emitted from an incandescent light bulb [4]. The device was fabricated using the IC compatible process. Possible applications of the SSI-LED include displays, sensors, and in-chip optical interconnects [5]. For certain applications, such as the short distance optical interconnect, the wavelength has to be around 850-900 nm [67], which is within the wavelength range of the light emitted from the SSI-LED. It is also suitable for the light sensing using the Si diode [7]. A proper thin film light filter may be used for this purpose. In this paper, PECVD SiNx and a-Si:H thin films were examined for the light filtration of the light emitted from the SSI-LED made of the ZrHfO high-k dielectric.

A 200 nm thick SiNx film was deposited on the dilute HF cleaned p-type Si (100) wafer and wet-etched into 250 µm diameter holes. A ZrHfO film was sputter-deposited on the substrate from a ZrHf (12/88 wt. %) target for 12 min in Ar/O2 (1:1) at 5 mTorr and 60 W. Then, an ITO film was sputter deposited on top of the ZrHfO film and wet etched into gate electrodes, which were connected to contact pads at the edge of the substrate through metal lines composed of Mo (200 nm)/Al (120 nm). The backside of the wafer was deposited with Mo to form the ohmic contact. The post metal annealing was done in a forming gas at 250°C for 5 min. The light emission spectrum of the SSI-LED was measured at the gate voltage (Vg
) of -20 V. Subsequently, the device was coated with a film of a 200 nm thick SiNx or a 6.5 nm thick a-Si:H deposited by PECVD. The spectrum of the light emitted from the same SSI-LED coated with the thin film was measured. Spectra of the same device with and without the filter were compared.

Figure 1 shows the normalized light emission spectra of the SSI-LED before and after coating with the SiNx film. Before the SiNx coating, the light emission spectrum covers the wavelength range of 480 nm to 900 nm with the peak located at 700 nm. It is similar to spectra of typical SSI-LEDs without any filter [1-4]. However, when coated with the SiNx film, the shape of the spectrum changed. None of the light below 500 nm wavelength is detected. The spectrum contains two peaks at around 550 nm and 720 nm. The light above 750 nm wavelength drops, which becomes more obvious with the increase of the wavelength. The spectrum of the SiNx filtered SSI-LED is similar to the light transmission spectrum of the SiNx film [8]. The strong light absorption of SiNx at 600-650 nm and 900-1000 nm wavelength ranges contributes to the dual-peak spectrum in Fig. 1.

Figure 2 shows the normalized light emission spectra of the SSI-LED before and after coating with an a-Si:H film. After coating with the a-Si:H film, the light emission spectrum shifted toward the long wavelength direction, i.e., from the peak of 700 nm to 750 nm. The intensity of the light in the 480 to 580 nm range dropped to zero. Previously, it was reported that the a-Si:H film strongly absorbed the short wavelength light in the 300 nm to 600 nm range [9]. Fig. 2 shows that the SSI-LED light below 650 nm is totally absorbed by the top a-Si:H film.

In summary, the broad band light emitted from the SSI-LED can be filtered to a narrow wavelength range with a deposited thin film.
[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] Y. Kuo, IEEE TED, 62, 3536 (2015).

[5] Y. Kuo, ECS Tran., 54, 273 (2013).

[6] D. A. B. Miller, IEEE Proc., 88(6), 728 (2000).

[7] S. Radovanovic et al, High-Speed Photodiodes in Standard CMOS Technology, p. 24 (2006).

[8] A. M. Perez et al, Opt. Eng., 45, 123801 (2006).

[9] X. Li et al, Opt. Express, 21, A677 (2013).

Figure 1