Fabrication of visible P-I-N type thin-film light-emitting diode using a hydrogenated amorphous carbon thin-film for active layer and its performance improvement

Abstract

This thesis can be summarized in three parts. The first one is a simple optical property modeling of microcrystalline silicon (μc-Si) by the effective medium approximation (EMA) method. This material is adapted to a window layer of thin-film light-emitting diode (TFLED). The second part is concerned with fabrication and performance improvement of TFLED using an amorphous carbon (a-C:H) thin film as a luminescent active layer. The last part comprises some applications of UST technique to the characterizations of TFLED for improving the material and device performance. This UST technique has recently been employed for defects engineering. At first, form the EMA method, it is possible to explain the optical behavior of μc-Si easily by simply assuming that μc-Si is composed of two-phase mixture of amorphous silicon (a-Si) and crystalline silicon (c-Si) with volume fractions of $f_a$ and $f_c$, respectively. From this simple assumption, the optical absorption behavior of μc-Si could be understood. It can be found from EMA method that the higher crystalline volume fraction of μc-Si results in the lower absorption characteristics, and also that a-Si embedded in the c-Si contains little hydrogen. When the doping concentration increases, the crystalline volume fraction decreases and embedded a-Si begins to contain hydrogen. It is thought that the embedded a-Si reveals a high optical bandgap and that p type μc-Si has a lower absorption spectra than n type μc-Si over all visible spectrum. Whin using normal doping ratios, the optical bandgap of p type is greater than that of n type μc-Si which is nearly the same as undoped μc-Si. Accordingly, p type μc-Si can be used as a window layer for TFLED. Secondly, when we used an intrinsic a-C:H thin film with a thickness 25 nm as an active layer of a p-i-n type TFLED, we could observe the blue-shifted light emission from the device without using any hydrogenation technique or p/i or i/n interface band grading for enhancing elect...