Keywords: sol-gel method; nanometer antireflection coating; Boehmite structure

Transcription

1 2016 International Conference on Power Engineering & Energy, Environment (PEEE 2016) ISBN: Preparation of a Novel Al 2 O 3 /SiO 2 /Nano-SiO 2 ARC on GaAs Wafer with Broadband Spectrum Wei Zhang 1,a, Jielei Tu 1,b, Xiangjiang Xiao 1,c, Dan Xue, Yuqing Sheng and Tengteng Guo 3,c 1 Yunnan Provincial Renewable Energy Engineering Key Laboratory, Kunming, China a @qq.com, b km-tjl@263.net, corresponding author Keywords: sol-gel method; nanometer antireflection coating; Boehmite structure Abstract. In the paper, a novel Al 2 O 3 /SiO 2 /Nano-SiO 2 triple-layer antireflection coating (ARC) on GaAs wafer with lower reflectance range on broadband spectrum was developed by sol-gel method. The experiments showed that: (1) to obtain stable and transparent liquid Al 2 O 3 sol, the three crucial factors the concentration of aluminum isopropoxide, hydrolysis temperature and reflux time should be controlled well, which could be 0.68~1.04mol/L, 90~95, and longer than 10h respectively. (2) The Al 2 O 3 film and SiO 2 film would be thicker with the acceleration of withdrawal speed, be thinner with the rising of annealing temperature; and the refractive index would be lower with the rising of annealing temperature also. (3) The Al 2 O 3 film would put forward Boehmite (γ-alooh) structure after 200 annealing; while when the annealing temperature was higher than 200, the film showed γ- Al 2 O 3 phase, with refractive index 1.71, and film thickness could be adjustable from 74.1 nm to 161 nm.(4) The amorphous SiO 2 thin-film could be adjustable also, which refractive index ranged from 1.21 to1.45, thickness varied from 132.8nm to 219nm. 1 Introduction The antireflection coating (ARC) could be necessary for solar cells which were effective devices of convert sunlight directly into electricity [1-4]. While, the traditional single layer or double layers ARC could not be satisfied with multiple-junction solar cells based on GaAs, in which the wave range should be 350nm to 1850nm, even from 300nm to 1900nm. So developing a novel ARC combined traditional multi-layer structure with nano-microstructure could be available. Sol-gel method [5-7] is widely applied in preparing thin film, in which the growth temperature is relatively low, and the refractive index of grown optical film could be controlled in a certain range. So in our work, a novel Al 2 O 3 /SiO 2 /Nano-SiO 2 three layers ARC based on GaAs wafer was investigated by sol-gel method (Czochralski method),that shows potential on GaAs-based multi-junction solar cells. 2 Experimental equipment and methods Our experiments were carried on by the dip-coater SYDC-1 produced by ShangHai Sanyan experimental instrument limited company. The Czochraliski method was adopted into growth of thin-films. The process was described briefly as following: putting cleaned substrate (GaAs single-crystal wafer) into the prepared Al 2 O 3 sol solution, lifting out with a certain speed of lift smoothly from sol, under the effect of viscosity and

2 gravity, a layer of uniform Al 2 O 3 films could be formed on surface, after suitable annealing, the other two-layer SiO 2 films could be coated in proper order, which the refractive index was controlled, and the layer with n=1.1 should be nano-structure. During the preparation of Al 2 O 3 thin films, aluminum isopropoxide (C 9 H 21 AlO 3 ) worked as precursors, deionized water as the solvent, and nitric acid as peptizes. The preparation described below: (1) Mixing a certain amount of C 9 H 21 AlO 3 and deionized water, and stirring 2 hours. (2) Adding nitric acid sol agent to obtain the PH value about 6~7. (3) Reflux condensing 5~15h under70~95, to obtain stable boehmite sol (AlOOH). (4) Putting the cleaned GaAs wafer on fixture of the dip-coater, and soaking into above AlOOH sol for 50 seconds, then lifting it up at a certain speed, drying the film in the air for 15 minutes. (5) The anneal temperature should be 400~800, and lifting rate should be 2.5~15cm/min. During the preparation of SiO 2 films, ethyl orthosilicate (TEOS) played as precursor, anhydrous ethanol as solvent, ammonia water and hydrochloric acid as catalyst. Meanwhile, catalysis of acid and of caustic was applied in fabrication of SiO 2 thin films. The brief process described below: TEOS, ethyl alcohol, ammonia-water, deionized water (1) Sol A by catalysis of caustic stirring agein backflow Sol A (2) Sol B by catalysis of acid TEOS, ethyl alcohol, Hydrochloric acid, deionized water stirring Sol B (3) Stirring the mixture of Sol A and sol B with a certain volume rate, and recirculating for a few hours. Then the SiO 2 sol with adjustable refractive index could be obtained. 3 The experimental results and discussing 3.1 Preparation of Al 2 O 3 thin films

3 The practical results showed that, the stability of Boehmit sol seemed be sensitive with aluminum isopropoxide concentration, hydrolysis temperature and time, and the amount of nitric acid gel solvent as well. Seen from table 1, it was obviously that, to obtain clear and stable sol, the concentration of aluminum isopropoxide should be ranged 0.68~1.04mol/l. Otherwise, there was white in it. Table 1. Influence of concentration of aluminum isopropoxide on Boehmit sol. aluminum isopropoxide concentration (mol/l) Clear status of sol A few white A few white white Clear and stable and A few white stable Setting 0.44mol/l as concentration of aluminum isopropoxide, we varied hydrolysis temperature and investigated the status of Boehmit sol, in which mass of aluminum isopropoxide was 18g, capacity of water and nitric acid were 200ml and 1.1ml respectively. Table 1. Influence of hydrolysis temperature and time on Boehmit sol. sample hydrolysis temperature hydrolysis temperature backflow time (h) status of sol (joining deionized water)( ) (joining acid)( ) nitric A few white A few white Very few white, semitransparent very thin layer, semitransparent Clear, stable The five contrast tests, as shown in table 2, showed that when the hydrolysis temperature less than 80, reflux time of 5h, sol state did not change, and still had a few white. And when hydrolysis temperature was ranged between 80~90, and reflux time was 10h, precipitation would be fewer further and the sol exhibited semitransparent. Then if increasing the temperature to 90~95

4 and reflux time to 15h further, the clear and stable sole could be gotten. Therefore, a conclusion could be drawn that higher hydrolysis temperature and longer reflux time would be benefit for sufficient hydrolysis, which should bring about fewer precipitation, stability and transparency. As shown in figure 1(a), different withdrawal rate would lead to variety of the thickness of Al 2 O 3 thin films. Here the set annealing temperature was 400, and the volume ratio of Sol B and sol A was Obviously, the faster lift speed could cause the thicker film. So, we could obtain suitable thickness ranging from 74.1nm to 161nm by adjusting lift speed. Meanwhile, we could use a simple simulation to describe above as d=46.91v There, d is the thickness (nm) of Al 2 O 3 thin films, v is the lift speed (cm/min). Furthermore, the thickness and refractive index of Al 2 O 3 thin films prepared under different annealing temperature were investigated, as shown in figure 1(b). There, the annealing temperature were 200, 400, 600 and 800, and annealing time was the same 30min. The comparison revealed that there was little variety when temperature changed from 400 to 600 ; and refractive index would decrease slowly from 1.71, when temperature was 200 ; while from 600, the value would be depress rapidly then; finally, when temperature was 800, the value of refractive index was It could be explained that higher temperature would lead to volatilization of solvent and decomposition of unstable material, which induced interspace manifold or cracking. (a) (b) Figure 1. (a) Influence of lift speed on thickness of Al 2 O 3 films; (b)influence of annealing temperature on thickness and refractive index of Al 2 O 3 films. Correspondingly, X-ray diffraction of Al 2 O 3 thin films was carried out, as shown in figure 2. After annealing under 200, there were two more apparent characteristic peak at 27.3 and And comparing with PDF standard card, they should be diffraction peaks corresponding crystal plane (120) and (200) of Boehmite (γ-alooh) structure. When annealing temperature was greater than 200, additional peak would appear at 56.2, which is the characteristic peak of γ- Al 2 O 3 phase.

5 Figure 2. X-ray diffraction of Al 2 O 3 thin films under different annealing temperature. 3.2 Preparation of SiO 2 thin films Similarly, we would discuss the fabrication of SiO 2 thin films. Firstly, our experiments resulted in, as shown in fig.3(a), that controlling lift speed could obtain the expected thickness. There, the volume ratio of sol B and sol A was 0.83, annealing temperature was 100. Obviously, the faster lift speed was, the thicker films grew, which could be simulated as d 1 =89.72v And the thickness was ranged from 132.8nm to 219nm. In the equation, d and v have the same meaning as above. Furthermore, the thickness and refractive index of SiO 2 thin films prepared under different annealing temperature were investigated, as shown in figure 3(b). Here, the annealing temperature were 100, 200, and 300, and annealing time was the same 30min. As can be seen, the thickness of SiO 2 thin films decrease rapidly with the increase of annealing temperature, it could be explained that higher temperature would cause thinner film. And when the temperature was lower than 200, refractive index performed almost same; while it higher than 300, the value would decrease rapidly to (a) (b) Figure 3. (a) Influence of lift speed on thickness of SiO films. (b) Influence of annealing temperature on thickness and refractive index of SiO 2 films.

6 Correspondingly, X-ray diffraction of SiO 2 thin film was carried out, as shown in figure 4. As can be seen, there were no apparent characteristic peak after annealing, and SiO 2 thin film belong to amorphous. Figure 4. X-ray diffraction of SiO 2 thin films under different annealing temperature. 3.3 The regulation of SiO 2 thin films refractive index. The sol B was added to the sol A according to different volume ratio, and refluxed a few hours, then the SiO 2 layer refractive index of which is adjustable would be obtained, the results are presented in Fig.5. The refractive index of SiO 2 layer is 1.21 before adding the sol B, and then the refractive index increases rapidly with the increase of sol B. After the volume ratio of sol B to sol A is more than 0.176, the index increases slowly. The refractive index of SiO 2 is 1.45, after 3 day s aging of sol B. So the refractive index of SiO 2 could be controlled at 1.21~1.45by changing the volume ratio of the sol B to sol A. Figure 5. Influence of volume ratio on refractive index of SiO 2 films. 4 Conclusion (1) To obtain stable and transparent liquid Al 2 O 3 sol, the three crucial factors the concentration of aluminum isopropoxide, hydrolysis temperature and reflux time should be controlled well, which could be 0.68~1.04mol/L, 90~95, and longer than 10h respectively.

7 (2) The Al 2 O 3 film and SiO 2 film would be thicker with the acceleration of withdrawal speed, be thinner with the rising of annealing temperature; and the refractive index would be lower with the rising of annealing temperature also. The Al 2 O 3 film and SiO 2 film would be thicker with the acceleration of withdrawal speed, be thinner with the rising of annealing temperature; and the refractive index would be lower with the rising of annealing temperature also. (3) The Al2O3 film would put forward Boehmite (γ-alooh) structure after 200 annealing; while when the annealing temperature was higher than 200, the film showed γ- Al2O3 phase, with refractive index 1.71, and film thickness could be adjustable from 74.1 nm to 161 nm. (4) The amorphous SiO 2 thin-film could be adjustable also, which refractive index ranged from 1.21 to1.45, thickness varied from 132.8nm to 219nm. Acknowledge This work is supported by Collaborative Innovation Center of Research and Development of Renewable Energy in the Southwest Area ( ). References [1] Sung-Mok Jung, et al. Design and fabrication of multi-layer antireflection coating for III-V solar cell[j].current Applied Physics,2011,11(3): [2] Daniel J. Aiken. High performance anti-reflection coatings for broadband multi-junction solar cells [J]. Solar Energy Materials & Solar Cells, 2000, 64(2): [3] Silke L. Diedenhofen. Broadband and omnidirectional anti-reflection layer for III-V multi-junction solar cells[j]. Solar Energy Materials & Solar Cells, 2012, 195(101): [4] K.M.A. Sobahan. Nanostructured porous SiO2 films for antireflection coatings[j]. Optics Communications, 2011, 284(3): [5]Hyung-Jun J, Sung-Chul Y, Seong-Geun O. Preparation and antibacterial effects of Ag-SiO2 thin films by sol-gel method.[j]. Biomaterials, 2003, 24(27): [6] Dingguo Chen. Anti-reflection (AR) coatings made by sol-gel processes: A review. Solar Energy Materials & Solar Cells 2001, 68: [7]Kim J Y, Lee J S, Hwang J, et al. Fabrication of Hydrophobic Anti-Reflection Coating Film by Using Sol-gel Method[J]. Korean Journal of Materials Research, 2014, 24(12):