Studies of Alternating-Current and Voltage-Tunable Amorphous Thin-Film Light-Emitting Diodes

• Main Authors: Rong-Hwei Yeh, 葉榮輝
• Other Authors: Jyh-Wong Hong
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/77vf5y

博士 === 國立中央大學 === 電機工程研究所 === 96 === In this dissertation, first, the feasibility of developing visible light-emitting devices on n-type c-Si substrate with the SiO2-isolated structure had been demonstrated. This i-a-SiC:H based thin-film LED (TFLED) revealed a brightness of 855 cd/m2 at an injection current density of 490 mA/cm2, a broad electroluminescence (EL) peak with wavelength ranging from 610 to 680 nm, and a full-width at half maximum (FWHM) of 205 nm at an applied voltage of 15 V. Then, the voltage-tunable i-a-SiC:H/i-a-SiN:H p-i-n TFLEDs with SiO2-isolation on c-Si has been proposed and fabricated. Its EL peak wavelength exhibited blue-shift from 655 to 565 nm with applied voltage increasing from 15 to 19 V, but the EL peak wavelength was red-shifted from 565 to 670 nm with further increase of voltage from 19 to 23 V. By comparing with the EL spectra of TFLEDs having an i-a-SiC:H or an i-a-SiN:H luminescent layer only, this voltage-tunable characteristic could be due to voltage-dependent EL contributions from radiative transitions in the i-a-SiC:H layer, i-a-SiN:H layer and i-SiC:H/p-SiC:H junction, respectively. Also, by discarding the traditional n- and p- layers of a dc-operated p-i-n TFLED, the nearly symmetrical a-SiC:H TFLEDs fabricated on ITO(indium-tin-oxide)-coated glass substrate and exhibiting EL under either a DC (positive or negative) bias or an ac voltage have been demonstrated. The EL intensity of this alternating-current TFLED (ACTFLED) would vary with the frequency of applied ac bias. The EL intensity of this ACTFLED increased with the frequency up to 500 kHz and then decreased rapidly and became very weak as the frequency increased to about 1 MHz. A model based on the equivalent circuit has been proposed to explain this frequency-dependent EL behavior. At the same time, the contact behavior between the employed metal electrode and amorphous film was also investigated in this study. Furthermore, by employing the very thin i-a-C:H or i-a-SiN:H film as the luminescent layers, the ACTFLEDs could emit white light. The EL spectra of the alternating-current white TFLEDs (ACW-TFLEDs) had peak wavelength ranging from 505 to 530 nm and broad FWHM ranging from 240 to 260 nm under either DC forward or reverse bias, or the sinusoidal AC voltage. These devices revealed the brightnesses about 800 (500) cd/m2 under DC forward (reverse) bias at an injection current density of 600 mA/cm2 with the i-a-C:H film as the luminescent layer, and about 170 (168) cd/m2 at an injection current density of 100 mA/cm2 with the i-a-SiN:H film as the luminescent layer, respectively. In addition, it was found that in-situ hydrogen plasma treatment was a very effective way to improve the optoelectronic characteristics of these devices, such as increasing the EL intensity, reducing threshold voltage and broadening the FWHM of the EL spectrum. However, its EL spectrum would be red-shifted with the increased AC frequency. This phenomenon could be due to the carrier recombination occur mainly in the composition-graded (CG) i-a-SiC:H or i-a-SiN:H layer and emit light of longer wavelength when the AC frequency was increased.