STRONG PHOTOLUMINESCENCE OF THE N-TYPE DOPED AMORPHOUS SiC MATERIAL AFTER ANODIC ETCHING IN LOW HF-CONCENTRATION SOLUTION

STRONG PHOTOLUMINESCENCE OF THE N-TYPE DOPED AMORPHOUS SiC MATERIAL AFTER ANODIC ETCHING IN LOW HF-CONCENTRATION SOLUTION

Dao Tran Cao¹, Cao Tuan Anh^2*, Luong Truc Quynh Ngan¹, Pham Thi Lien² and Pham Thi Mai Hoa³

Usually for making the n-doped amorphous SiC (n-aSiC) thin film porous, we etch it anodically in the HF/H₂O solution. In this way, the largest pore size and the most dense pore density were achieved when the HF concentration was about 1%. However, the photoluminescence (PL) measurements showed that the PL yield of the porous layer above, even in the case of best porosity, is relatively low. Surprisingly, when we reduce the concentration of HF in the etching solution to just about 0.1%, the intensity of photoluminescence emitted from the n-aSiC sample after anodic etching increased hundreds of times. This report presents the results of initial research on said interesting phenomenon.

1. 
   INTRODUCTION
   

Up to now, in the literature there is a large number of reports about the PL of PSiC [1-13]. Concerning the mechanism of PL, because until now it has been not observed the PL light with the energy higher than the width of the bulk SiC band gap, most authors agree with the point of view that the PL from PSiC arises from the surface or defect states, which are generated during anodization. This is a difference with PSi, because in PSi the luminescence originates not only from surface states but also from quantum confinement effect. Because the luminescence from PSiC occurs primarily from surface states, it is easy to understand why the PSiC luminescence spectra of different authors (using different surface treatment methods) vary widely.

In 2004, Se-Young Seo et al. [14] reported an intense (visible to the naked eyes under daylight conditions) blue-white PL from a carbon-doped silicon-rich silicon oxide film, which was fabricated by the electron cyclotron resonance PE-CVD method using SiH4, O2 and CH4 as the source gases followed by a high-temperature annealing. They have shown that the conditions for observation of this intense blue-white PL were a nearly equal amount of C and excess Si in the film and after an annealing at 950oC. In our opinion, it is probably that this PL is originated from SiC nano-clusters embedded inside a SiO2 matrix. Therefore, if we could fabricate such a structure, we will observe the intense PL like which has been observed by the authors of [14].

In recent years, at the Institute of Materials Science, we have established a SiC research group. One of the first task of the group is to find out a way to make a PSiC layer on thin SiC films by electrochemical etching (anodization). In this direction we have found that for amorphous SiC (aSiC) in general, if the electrochemical solution used is an aqueous HF solution, than with the HF concentration higher than 1%, the porous layer structure is composing of the separate holes deep-rooted in the aSiC film. In this region, smaller HF concentration will result in higher porosity of the porous aSiC layer, which is expressed in the larger average pore diameter and the thicker pore density. The best porosity has been achieved with the 1% HF solution [15]. In the process of searching for the best porosity HF concentration mentioned above, we have tried to use a solution with 0.1% HF and to our great surprise, the morphology of the porous layer obtained was totally different. After that, we were even more surprised when we have seen that this sample showed the PL intensity of hundred times higher than the PL intensity of the sample etched in the 1% HF solution. Therefore we have decided to study systematically the etching of the aSiC in the dilute HF solutions (with the HF concentrations less than 1%) and the PL properties of porous layers obtained after etching in such solutions. The research results in this direction are presented in this report. Due to the limitations of a paper, in this report we focus only to the results obtained with the n-type aSiC (n-aSiC).

2. 
   EXPERIMENTAL
   

We have made n-aSiC thin films porous by electrochemical etching (anodization). All samples have been anodized by using a voltage constant source (type 051 TYP, a product of Hungary), which output voltage (we used 260V for all samples) is applied to the load consisting of a sample connected in series with a 180 kΩ resistor. All the anodization processes were carried out in 30 minutes. The HF/H2O electrolyte was kept in a Teflon etching tank. The n-aSiC/Si sample serves as the anode (positive voltage was applied on the Al layer which has been evaporated on the backside of the n-aSiC/Si sample), and a platinum plate is used as a cathode. During the etching process, the voltage and current were monitored and recorded. The HF concentration in electrolytes was varied between 0 and 1%. The used HF concentrations were 0.05; 0.1; 0.2; 0.3; 0.4; 0.5 and 1 % (HF percentage is the volume percentage of concentrated (48%) HF in water). After the etching process, the samples were washed in the deionization water and dried in the air.

3. 
   RESULTS AND DISCUSSION
   

a) 0,05% HF

b) 0,1% HF

c) 0,2%HF