Visible-light amorphous silicon alloy thin film light emitting diodes and their applications in optoelectronics

Abstract

A novel Thin Film Light Emitting Diode (TFLED) has been developed for the first time. The TFLED has a basic structure of glass/ITO/p-i-n layers of amorphous silicon alloys/Al. The amorphous silicon alloys employed in this work are wide optical energy gap materials, so-called, hydrogenated amorphous silicon oxide (a-SiO:H). The TFLED can emit the visible light having the colors from red, orange, yellow to green and white-blue depending on the optical energy gap of the i-layer. A detailed study has been done on the basic properties including structural properties (IR absorption, ESR), optical properties (absorption coefficient, optical energy gap, photoluminescence) and electrical properties (conductivity) of the amorphous films.The TFLED is a carrier-injection type electroluminescent device. The light output comes from the radiative recombination of holes and electrons injected from the p- and n-layers, respectively, into the i-layer. The typical bias voltage is about 5-15 volt, the injection current density is about 100-1000 mA/cm², giving the brightness of about 0.1~ 1 cd/m². The theoretical and experimental results show that the optimal thickness of the i-layer is about 500. A study has been done on the effect of the material used in the i-layer on the performances of the TFLED. The result shows that the TFLED with a-SiC:H as the i-layer gives the highest brightness, while a-SiN:H gives the brightness better than a-SiO:H. A series of trials has also been done on the improvement of the brightness of the TFLED. The first attempt is to improve the radiative recombination efficiency by using a metal substrate instead of a glass substrate. The metal substrate is good thermal conductive material so that it can dissipate heat from the TFLED to the ambient with better efficiency than a glass substrate. The other attempt is to improve the injection efficiency of holes by using p-type highly-conductive and wide optical energy gap amorphous silicon oxide (p-a-SiO:H) and microcrystalline silicon oxide (p-µc-SiO:H) instead of conventional p-a-SiC:H. The result shows that the brightness was improved to the level of 10 cd/m². A series of experiments has been done on the fabrication of TFLED displays that can emit light with desired emitting patterns. A new type of dot matrix TFLED display has also been proposed and fabricated for the first time. The screen size for the demonstration is 8×8 cm². In this thesis a new type of amorphous photocoupler having the amorphous TFLED as a light source and the amorphous silicon solar cell as a detector has been developed for the first time. The advantages of the amorphous TFLED are as follows: 1) low-cost, because the TFLED is made from low-cost amorphous materials and uses a low temperature CVD process (190 °C), 2) it has the possibility to be produced as a large area display, 3) it can be deposited on various substrates, such as glass, metal, plastic, ceramic sheets; therefore various forms of displays can be realized, etc.