Chapter 5 Results and Discussion
5.2 Effect of proportion HF, HNO 3 in porous formation
5.2.3 Effect of content water in etching solution
In this part, water is added into the acid mixture to control the average etching rate which is very important to achieve thin PS solar cell applications. The disadvantage of stain etching is hard to control the etching rate. In this study, the etchant of HF: HNO3 + H2O, with H2O varied from 10 ml to 70 ml were used. Following previous experiment, three ratios HF: HNO3 = 9:1, 4:1, 7:3 were used to test the effect of adding water. Figures 5.13 and 5.14 show the total reflectance and the AER for ratio 9:1 + H2O, respectively. For small amount of water, (20ml) AER was reduced from 4 µm/s to 1.9 µm/s but the still low reflectance (almost 0%) as sample without adding. For larger amount of adding water (plus 30, 40, 50 ml H2O) samples, the total reflectance are about 20% at visible wavelength. The surface morphology of these sample are shown in Figure 5.15. Porous shape changes from open-structure to nano particles. The change in surface morphology can be explained by the average etching rate. At lower AER ( with more adding water), etchant has less effect on the surface and the pore formation rate is lower than that of higher AER (with less adding water).
The similar tests were carried out for other acid ratio of (HF: HNO3 + H2O= 4:1 + 60, 7:3 + 70). The total reflectance for each ratio was shown in Figures 5.17, and 5.18. Figures 5.19 and 5.20 show the optimized results in AER for each etchant 4:1 + 50, 4:1 + 60, 7:3 + 50, and 7:3 + 70, respectively.
Consequently, it can be conclude that by adding wafer in the HNO3and HF can reduce the etching rate of stain etching. These three ratios: 9:1 +20, 4:1 + 60, 7:3 + 70 were chosen for further experiment in this study. For industrial application – large size devices, adding wafer promises the huge potential to control the etching rate.
5.3 Formation of PS on different types of silicon wafer 5.3.1 Different resistivity silicon wafer
The effect of acid etching the Si with different resistivity also investigated. The etchant of HF: HNO3 + H2O = 7:3 + 70 was chosen, and the resistivity of wafer vary from 1 Ωcm to 12 Ωcm.
The results showed that the etched wafer with higher resistivity (8-12 Ωm) have lower AER but higher total reflectance than that of Si (1-10 Ωm) and Si (1-5 Ωm). The etching rates of three samples are 0.169 µm/s, 0.288 µm/s, 0.348 µm/s, respectively. Figure 5.20 shows the reflectance spectrum for etched wafer with different resistivity. The etchant mixture acts more active in low resistivity substrate. After etching with the same duration, the low resistivity (5 Ωcm) wafer presented the lowest total reflectance (1%) as compared with 3% and 7% for 1-10Ωcm, 8-12Ωcm Si (100) wafer, respectively. The surface morphographies of these three samples are shown in Figure 5.21. At high magnification, low resistivity substrates (5Ωcm, 1-10Ωcm) have similar structure with random and open pore site.
5.3.2 Si with different orientations
In this part, we use the same previous etchant (HF: HNO3 + H2O= 7:3 + 70) for experiment. After 180s etching, both Si (100), Si (111) etched sample have total reflectance below 2%. However, Si (111) etched sample has total reflectance lower than that of Si (100) as can be seen in Figure 5.22. This result can be explained by the ARE, etchant reacts faster on Si (111) than on Si (100): 0.233 µm/s > 0.169 µm/s. The surface roughness increases with the increases of AER. Thus, the etched Si (111) sample resulted in the lower total reflectance.
Surface morphology of these two samples is quite similar which shown in Figure 5.23.
It can be concluded that both resistivity and orientation have effects on the AER and reflectance of the stain etching porous silicon. Starostina et al [61] noted that the doping concentration or resistivity of wafer strongly affect the etching rate. When the resistivity increases, the etching rate decreases. In this part, the same results with this conclusion. In addition, Si (111) is preferable for stain etching because it has the lower reflectance but higher
etching rate than Si (100). However, both the Si (111) and Si (100) wafer after stain etching process offered very low reflectance (below 2%)
5.4 Comparison between PS and Si
3N
4, SiO
2ARCs
In the final experiment part of this thesis, we tried to form porous layer on large size Si wafer (7.5x7.5cm). After that, the reflectance of the etched wafers were compared with 200 nm Si3N4 and 200 nm SiO2. The set up parameters for formation of PS are shown in Table 5.1
Figure 5.24 shows the total reflectance for each etched samples. PS3 sample gives the lowest reflectance (almost 0%). The PS1 and PS2 samples also provide the low reflectance (below 5%). Figure 5.26 explains why PS3 exhibits the lowest reflectance. The etched sample of PS3 has a very rough surface morphology. The root mean square (RMS) roughness observed from AFM image (Fig 5.27) shows PS3 sample has higher RMS value at 45.314 nm as compared to PS1, PS2 samples (29.464nm and 22.772 nm, respectively). Many researchers mentioned about the luminescence properties of PS [7, 62]. The PL spectra of three PS samples are shown in figure 5.27. PS1 and PS2 samples show the response in red peak, but PS3 sample does not present PL. The emission wavelength is located at around 650 nm. This correspond to a cluster size of 2-3nm in diameter as shown in Fig 5.25. This is consistent with previous conclusion as given by Kalem [62].
After the fabrication porous layer in large size wafer, we continue to compare these three PS samples with the conventional ARC material. These comparisons were shown in Figure 5.28, 5.29, 5.30, respectively.
Firstly, the total reflectance between PS samples and bare Si wafer with covered Si3N4 or SiO2 are compared. After covered Si3N4 or SiO2, these PS samples present interference reflectance. It can be seen that etched wafer show the remarkable low total reflectance compared with the wafer covered by Si3N4 or SiO2. Secondly, the effect of Si3N4, or SiO2 deposited onto porous surface is also investigated. It is shown that reflectance of three original PS samples was not affected by covered Si3N4, SiO2 layers.
Finally, EDS was carried on to see the composition in surface of etched samples. As many researchers mentioned, Si very easy oxidized in air ambient to form SiO2. EDS analysis shows that on top of porous layer presented oxygen and silicon elements (Fig 5.32). The top surface includes 7.48% oxygen and 92.52% silicon in weight which are shown in table 5.2
Fig. 5.1 Overall process for porous silicon formation
Fig. 5.2 Total reflectance for porous silicon layers etched in a solution HF: HNO3 + H2O (7:3 + 70) with varying etching time etching.
Fig. 5.3 SEM image : a) Before etching. b) After etching. c) Cross-section d) high magnification top-view of porous layer
a b
c d
Fig 5.4 Reflectance spectrum for HF dominant group
Fig 5.5 The average etching rate as the function of HNO3 proportion
Fig 5.6 Top-view SEM image for HF dominant group
Fig 5.7 PL spectrum for HF dominant group
Fig 5.8 Reflectance spectrum for HNO3 dominant group
Fig 5.9 The average etching rate as the function of HF proportion
Fig 5.10 Top-view SEM image for HNO3 dominant group
Fig 5.11 Transition from polishing etching to porous formation
Fig 5.12 PL spectrum for porous etching and polishing etching
Fig 5.13 Reflectance spectrum for etchant HF: HNO3 + H2O = 9:1 + H2O
Fig 5.14 Average etching rate for etchant HF: HNO3 + H2O = 9:1 + H2O
Fig 5.15 Top-View SEM image for etchant HF:HNO3 +H2O= 9:1 + H2O
Fig 5.16 Reflectance spectrum for etchant HF: HNO3 + H2O = 7:3 + 70
Fig 5.17 Reflectance spectrum for etchant HF: HNO3 + H2O = 4:1 + 60
Fig 5.18 Average etching rate for etchant HF: HNO3 + H2O = 4:1 + H2O
Fig 5.19 Average etching rate for etchant HF: HNO3 + H2O = 7:3 + H2O
Fig 5.20 Reflectance spectrum for different resistivity etched wafer
Fig 5.21 Top-view SEM image of different resistivity etched wafer
Fig 5.22 Reflectance spectrum for different orientation etched wafer
Fig 5.23 Top-view SEM image of different orientation etched wafer
Table 5.1 Parameter set up for formation of PS in big size wafer
Fig 5.24 Reflectance spectrum for PS1, PS2, PS3 sample.
Sample PS1 PS2 PS3
Etchant HF: HNO3 + H2O
4:1 + 60 9:1 + 20 7:3 + 70
AER (µµµm/s) µ 0.414 2.147 2.467
RMS (nm) 29,464 22,772 45,314
Fig 5.25 Top-view SEM image of PS1, PS2, PS3 sample
Fig 5.26 AFM image for PS1, PS2, PS3 sample
Fig 5.27 PL spectrum for PS1, PS2, PS3 sample
Fig 5.28 Reflectance spectrum for PS1, Si3N4, SiO2
Fig 5.29 Reflectance spectrum for PS2, Si3N4, SiO2
Fig 5.30 Reflectance spectrum for PS3, Si3N4, SiO2
Fig 5.31 Top-view SEM image of PS sample covered Si3N4, SiO2
Table 5.2 EDS analysis for porous sample
Fig 5.32 EDS analysis for porous silicon
Element Weight% Atomic%
O K 7.48 12.43
Si K 92.52 87.57
Totals 100.00
Chapter 6
Conclusions and future works
6.1 Conclusions
We have investigated the chemical formation of PS in HF: HNO3, HF: HNO3 + H2O etchants and determined the ranges in which the ratio of HNO3 should be varied in order to change from etch polishing to porous formation. The etching transition was applied to p-type Si (100) with ρ=8-10Ωcm. When the proportion of HNO3 is lower than 40% in etching solution, porous etching occurs. We successfully fabricated porous layer on top of Si substrate with lower total reflectance (0% in visible light) as compared with bare silicon wafer or conventional AR within Si3N4, or SiO2. We observed that after covering 200nm Si3N4, or SiO2 on top of etched wafer, these AR coating do not have effect on reflectance of porous silicon layer. For controlling the etching rate which is the disadvantage of PS stain etching, adding water is the solution.
Adding water into the acid etchant not only reduce the etching rate but also skips the total reflectance of etched wafer at acceptable level for ARC applications. We successfully optimized the stain etching process from small size Si wafer (2x2cm) to large size Si (7.5x7.5cm). We obtained that three etchant solutions HF: HNO3 + H2O = 9:1 + 20, 4:1 + 60, 7:3 + 70 are suitable for industrial purpose. In addition, in this study, we also observed the PL properties of PS layer.
Comparing with bare Si which does not have PL response, porous layer presents PL signal at red peak (630nm). Consequently, PS layer not only offers low reflectance in visible light but also presents the PL properties which open the huge potential for ARCs application and light emitting based silicon devices.
6.2 Future works
The effect of resistivity on stain etching process not clearly understood, our future works will be figured out this effect. Thus, we have to consider about life time of carrier in PS layer which is very important for AR applications. After clarifying these problems, we will carry on fabricate complete PS anti reflection solar cell module.
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