White-Light Emission from Amorphous ZrHfO Thin Film Dielectrics with and without Embedded Nanocrystalline CdSe Dots

Solid-state light emitting device (LED) has been used to replace the incandescent light and fluorescent technology for lighting purpose since the solid-state LEDs have the potential to significantly reduce the energy consumption [1]. White-light LED is needed for the general lighting and many other applications. However, the white light needs to be generated by mixing red, green, and blue LEDs with different band-gap energies or using an UV or blue LED in combination with a phosphor material [2,3]. The former has the color balance issue while the latter has the Stokes energy loss while converting the wavelength [3]. Recently, Kuo and Lin proposed a new type solid state LED made from the amorphous Zr-doped HfO₂ (ZrHfO) or HfO_x high-k dielectric thin film, which emits the broad band white light [4-6]. The intensity and the wavelength range of the emitted light were enhanced by embedding the nanocrystalline ZnO (nc-ZnO) dots in the ZrHfO film, which creates extra stress within the film for easy formation of the conductive paths. [4] Previously, authors have proved that the nanocrystalline CdSe (nc-CdSe) dots could be embedded in a high-k dielectric layer to serve as the charge trapping medium for the nonvolatile memory application [7]. Defects were generated in the ZrHfO film after embedding the nc-CdSe layer, which caused a larger leakage current density (J) than that of the ZrHfO film. The magnitude of the current passing through the conductive path is a key parameter for the light emission process in this new type LED, which is based on the thermal excitation principle.  In this paper, authors investigated the optical, electrical, and material characteristics of the nc-CdSe embedded ZrHfO high-kdielectric stack.        

                The new LED has a MOS capacitor structure fabricated on a p-type (100)  Si wafer. For the control sample, the 12 min sputter deposited ZrHfO is used as the gate dielectric layer. For the nc-CdSe embedded sample, the (2 min ZrHfO/3 min CdSe/10 min ZrHfO) trilayer structure was sputter deposited in one pump down. The post deposition annealing (PDA) step was carried out in the N₂ atmosphere under 800^oC for 3 min. The ITO electrode was fabricated by sputter deposition and wet etched with a lithography masking step. The rest device fabrication process can be found in Ref. 7. For the measurement of the emitted light, the sample was loaded on a probe station (Signatone S-1060R) and the ITO electrode was stressed with a gate voltage (V_g). The emission spectrum was recorded with an optical emission spectrometer (StellarNet BLK-C-SR-TEC) with a 1,000 µm optical fiber.

                Figure 1 (a) shows the low- and high- magnification photos for the control and nc-CdSe embedded ZrHfO samples at V_g = -50 V. The light was emitted from many discrete bright dots in both samples. For the conventional band gap energy based LEDs, the light is emitted evenly across the electrode surface. Therefore, their light emitting principles may be different. Also, both the light intensity and the number of the bright dots in new LED increased with the inclusion of the nc-CdSe layer in the ZrHfO film. Figure 1 (b) shows the current density-voltage (J-V) curves of the control and nc-CdSe embedded samples in the hole-injection region, i.e., from V_g = 0 V to -20 V. The breakdown voltages (V_BD's) of the two samples are -11.2 V and -9.7 V, separately. The control sample has a larger V_BD than the nc-CdSe embedded sample has. Defects were generated in the nc-CdSe sample because of its high stress mismatch with the bulk ZrHfO, which caused its larger J compared to that of the control sample [7]. Different from the conventional LEDs, the light was emitted from the thermal excitation principle, e.g., blackbody emission, similar to that of the incandescent light bulb. After the device is broken, many small leakage paths were formed inside the high-kdielectric, which were excited by the passage of a large current to emit the light.  

                Figure 2 shows the emission spectra of the control and nc-CdSe embedded samples at V_g= -50 V. Both samples emit the same broad band spectrum including the visible and near IR wavelengths. The normalized emission spectra are the same, which represents the same light emitting principle, as shown in the inset. However, the light intensity increased with the inclusion of the nc-CdSe layer. This phenomenon is consistent with the results of Fig. 1 (a). The high leakage current of the nc-CdSe embedded sample corresponds to either the high temperature in the conductive path or the large number of conductive path. 

                The emitted lights of these two samples were investigated for their chromaticity coordinates in the CIE chart
(1931 standard), correlated color temperature (CCT), and color rendering index (CRI). The result is shown in Table 1. Both lights are within the white light region of the CIE chart. The CCTs and CRIs do not change with the embedding of the nc-CdSe layer in the ZrHfO film. The large CRI values of  98.4 are close to that of the conventional incandescent light bulb, i.e., 100. The commercial YAG:Ce-based white LED has a lower CRI of 79. This result also confirms that the increase of the emission intensity in the nc-CdSe embedded sample is contributed by the increase of the number of the conductive paths not the increase of the temperature within the path.

[1] J. M. Phillips et.al., Laser and Photon. Rev. 1, 307 (2007).

[2] J. Kundu, et. al.,, Nano Lett.12, 3031 (2012).

[3] E.F. Schuber and J. K. Kim, Science 308, 1274 (2005).

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

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

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

[7] C. C. Lin and Y. Kuo, to be published.