Voltage-Controlled Anodic Oxidation of Porous Fluorescent SiC for Effective Surface Passivation.

Kosuke Yanai,Satoshi Kamiyama,Motoaki Iwaya,Tetsuya Takeuchi,Dong-Pyo Han,Weifang Lu,Haiyan Ou,Isamu Akasaki,Yoma Yamane

doi:10.3390/nano10102075

Abstract

This study investigated the fabrication of porous fluorescent SiC using a constant voltage-controlled anodic oxidation process. The application of a high, constant voltage resulted in a spatial distinction between the porous structures formed inside the fluorescent SiC substrates, due to the different etching rates at the terrace and the large step bunches. Large, dendritic porous structures were formed as the etching process continued and the porous layer thickened. Under the conditions of low hydrofluoric acid (HF) concentration, the uniformity of the dendritic porous structures through the entire porous layer was considerably improved compared with the conditions of high HF concentration. The resulting large uniform structure offered a sizable surface area, and promoted the penetration of atomic layer-deposited (ALD) Al2O3 films (ALD–Al2O3). The emission intensity in the porous fluorescent SiC was confirmed via photoluminescence (PL) measurements to be significantly improved by a factor of 128 after ALD passivation. With surface passivation, there was a clear blueshift in the emission wavelength, owing to the effective suppression of the non-radiative recombination rate in the porous structures. Furthermore, the spatial uniformity of emitted light was examined via PL mapping using three different excitation lasers, which resulted in the observation of uniform and distinctive emissions in the fluorescent SiC bulk and porous areas.

Highlights

Commercialized white light-emitting diodes (LEDs) comprise a blue LED and a yellowish phosphor layer, such as cerium-doped yttrium aluminum garnets [1,2,3,4,5,6]
High emission intensity can be achieved in these types of LED chips; the absence of a highly efficient red-light component means that the color-rendering index is relatively low
The planar scanning electron microscope (SEM) and AFM images of the as-grown fluorescent substrate used in this experiment were characterized before anodic oxidation

Summary

Introduction

Commercialized white light-emitting diodes (LEDs) comprise a blue LED and a yellowish phosphor layer, such as cerium-doped yttrium aluminum garnets [1,2,3,4,5,6]. The current-controlled anodic oxidation method is mainly used to fabricate porous structures on common SiC substrates, despite the difficulty in realizing uniform porous structures throughout the entire layer. The present study conducted anodic etching on highly doped fluorescent 6H–SiC using a constant voltage-controlled method. The application of a high constant voltage can concentrate a large number of electrons/holes on the surface of fluorescent 6H–SiC substrates to provide stability for the anodic oxidation reaction process. This is expected to result in large and uniform porous structures for effective surface passivation using ALD–Al2O3 films.

Materials and Methods

Rcesults and D2i0s0cussion 30

Results and Discussion