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Optical Properties of Anti-Reflective Coatings on P-N Silicon and their Response to Gamma-Irradiation

Authored by: Henaish AMA

Abstract

The dc-pulsed plasma magnetron sputtering was used for depositing TiO2, ZrO2, and Ti50-Zr50 anti-reflection coating with thickness of 40 nm on p-n junction wafers and glass substrates. Furthermore, the antireflective properties of silicon nitride deposited elsewhere using plasma enhanced chemical vapor deposition (PECVD) were compared with those of metal oxide films. X-ray structural analysis reveals amorphous nature of the prepared films. The optical transmittance and reflectance measurements were investigated at room temperature in the wavelength range from 250 to 2500 nm. At long wavelengths, ZrO2 has a slightly higher transparency than that of TiO2 and Ti50-Zr50 oxide. On the other hand, the transparency at short wavelengths has different behavior. At about 300nm, a sharp decrease in the transparency of the three coatings was found. On the other hand, reflectance measurements show a decrease in reflectivity at 600 nm for ZrO2 and Ti50-Zr50 oxide as compared toTiO2. Samples irradiated with gamma rays at a dose of 5 kGray show different behavior in reflectivity due to displacement damage in Si-based cells.

Keywords: p-n Si cell; Dc-pulsed plasma magnetron sputtering; Anti-reflection coatings (ARC); TiO2; ZrO2; Ti50-Zr50 oxide

Abbreviations: PECVD: Plasma Enhanced Chemical Vapor Deposition; SCs: Solar Cells; ARC: Anti-Reflection Coatings

Introduction

Si-based solar cells (SCs) account for 90% of the current photovoltaic market. However, high production cost attitudes as an obstacle to their widespread performance [1]. Major industrial and researcher efforts are being made constantly to reduce the production cost while improving their power conversion efficiency. To do that, one of the main steps is to reduce reflection losses for solar cells, using anti-reflection coating (ARC) materials at the front surface of the cells, thereby coupling more sunlight into the cell to increase the photocurrent and efficiency [2].

Metal oxides nanostructure materials play an important role in the fields of optoelectronics and energy harvesting applications [3-5]. Oxides are a vital component of PV cells due to the various applications of oxides in PV cells. New oxide materials are being introduced for use in PV where numerous transition metal oxide nanostructures with various morphologies have been synthesized using numerous techniques to achieve the requirements of the desired application [6-9]. Several highly transparent and high refractive index materials have been investigated as ARC in Si solar cells such as ZnS [10], CeO [11] TiO2 [12] Ta2O5 [13], ZnO [14] and ZrO2 [15] either in a single layer or multiple layer coating. Besides, the choice of SiNx nowadays as a highly transparent ARC material is attributed to the large band gap energy (5.1 eV) with little absorption between 300 and 1150 nm where Si solar cells operate [16]. ARCs with uniform thickness and good optical properties are synthesized by different vacuum-based processes such as physical vapor deposition, reactive sputtering and chemical vapor deposition including plasma-enhanced chemical vapor deposition PECVD and atomic layer deposition [17].

Thin-film TiO2 has the potential to reduce the production cost of Si-based solar cells, depending on the cell design. This material can serve multiple purposes as anti-reflective coating (ARC) film, oxide passivation compatible film and doping source for emitter diffusion [18]. Zirconium as an abundant terrestrial metal (74 million metric) [19] is sufficient to produce Si solar cells on a terawatt scale [20]. Optical properties of ZrO2 are considered as a suitable ARC on Si solar cells [21] where it has a large band gap of 6.1 eV [22] for cubic ZrO2 and a refractive index value n=2 at 600 nm for dc-pulsed magnetron- sputtered ZrO2 films which similar to PECVD SiNx films [23]. Previously was also observed a similar trend in ZrO2 films prepared by spray deposition [24] and by pulsed laser deposition [25] and by Magnetron sputtering [26- 29]. In this work, the structural and optical properties of three ARC films, namely TiO2, ZrO2 and Ti50-Zr50 oxide, were prepared by dc-pulsed magnetron sputtering to investigate the suitability of the prepared films for ARCs on p-n Si cells.

Experimental Procedure

Thin films preparation

Dc pulsed magnetron sputtering was used to coat a fixed thickness of 40nmthin films of TiO2, ZrO2 and Ti50-Zr50 oxide on glass and Si (100) wafer p-n-junction. The films were grown by sputtering using ZrO2, TiO2 and Ti50-Zr50 Oxide targets with purity of 99.99%, 50mm diameter and 3mm thickness. The glass substrates were ultrasonically cleaned with, methanol, acetone and water. Si substrates were cleaned with diluted HF to remove native oxide from the surface. Sputtering is a type of physical vapor deposition technique that is widely used in both the laboratory and production. During sputtering an applied voltage is used to generate plasma that is confined close to a target material which ejects species that are then transferred to the substrate. Metal oxides can be grown directly from ceramic targets by sputter gas (argon), while metal targets can be sputtered using reactive oxygen together with the sputter gas. DC -pulsed magnetron sputtering can be used to deposit compact oxide coatings with relatively good properties at low temperatures. All details about magnetron sputtering can be found in separate reviews [30-32].

The schematic diagram of the dc–pulsed magnetron sputtering system is presented in Figure 1. Briefly, the system consists of a stainless-steel chamber with a diameter of 30 cm and was evacuated by turbo and rotary pumps to obtain a base pressure of 9× 10−6 mbar. Argon gas (Ar) was introduced into the chamber to launch a total working gas pressure of 7×10−3 mbar which was measured by means of a manometer. The dc-pulsed power supply model: Pinnacle Plus pulsed was conducted to initiate the discharge. A powerful magnet was employed to ionize the target materials and direct it to settle on the substrate in the form of thin film. The selected substrates were fixed on a sample holder at a distance of 7cm from the target. The film thickness was adjusted to be around 40nm. The operating parameters of ZrO2, TiO2 and Ti50-Zr50 oxide of the coatings are listed in Table 1.

Thin-film characterization

The optical properties of deposited thin films were investigated from transmittance and reflectance measurements at room temperature in the wavelength range of 200-1000nm using Jasco V-670 UV/VIS spectrophotometer. The structural of the deposited films were determined using Bruker D8 Advance diffract meter operated at low angle incidence using 40 keV and 40 mA with Cu-Kα1 radiation (λ = 1.54056 Å).

Gamma rays irradiation effect

The output power of solar cells can be increased by improving the efficiency (η) and radiation resistance. In such a situation, radiation resistance is an important factor, because the continuous impact of high-energy particles damages the semiconductor lattice, degrades the performance of the cell, and thus limits its life. The MEGA gamma I irradiation facility in Egypt as type J-6500 was supplied by the Canadian Atomic Energy Corporation Ltd. at the National Center for Radiation Research and Technology in Cairo, with a source of cobalt-60 used to irradiate samples with 5k grey [33,34].

Result and Discussion

X-ray diffraction patterns of various antireflection coatings TiO2, ZrO2 and Ti50-Zr50 oxide deposited on a glass substrate with a constant thickness t=40 nm demonstrated in Figure 2. It is obvious from the figure that the small diffraction peaks at 24˚ and 62˚ correspond to (1 0 1) and (2 0 0) diffraction planes for the anatase phase and another very small peak at around 45˚ corresponds to the (211) of the rutile phase for TiO2 antireflection coating. The X-ray diffraction patterns of ZrO2 displays very small diffraction peaks at 25.5˚ and 45.4˚ related to monoclinic structure oriented in (110) and (202) planes, respectively. The antireflection coating of Ti50-Zr50 oxide has a completely amorphous structure.

Optical properties

The optical transmittance and reflectance of TiO2, ZrO2 and Ti50-Zr50 oxide on p-n silicon as a function of wavelength are shown in Figure 3. The appearance of interference fringes, which indicates that the films are homogeneous and flat, has been reported elsewhere [31]. The optical transmittance and reflectance depend on the type of ARCS and the category of deposition technique. In our case, the optical reflectance for long wavelengths (>600nm) of different ARCs reduces close to zero. Moreover, the optical reflectance of ZrO2 is the lowest with respect to that of anti-reflectors TiO2 and Ti50-Zr50 oxide. At long wavelengths, ZrO2 has slightly higher transmission compared to the others of anti-reflectors and p-n silicon. On the other hand, the transmission at short wavelengths has different behavior. At 300 nm it finds a decrease in transmission and starts to increase until the maximum transmission is at about 600 nm for Ty50-Zr50 oxide.

Where β is a constant that depends on the transition probability, h is the plank’s constant, υ is the photon frequency, Eg is the energy band gap. The optical energy gap is the minimum energy required to excite an electron from the valence band to the conduction band by an allowed optical transition. The value of the optical energy gap is usually determined by measuring the optical absorption coefficient as a function of the photon energy. The exponent (m) determines the type of direct and indirect transition that have values 1/2, 2, respectively.

Figure 4a illustrates the absorption coefficient depending on wavelength for the entire study spectra. This figure reflects the absorption edge in the visible region. It can be seen that at the wavelength up to 1000 nm, all metal oxides precipitating at the pn junction have almost the same values of absorption coefficient except for Ti50-Zr50 oxide at 300 to 500 nm and TiO2 at 400 to 650 nm. Figure 4b shows the variations of (αhυ)0.5 versus the photon energy (hυ) for the prepared samples. The values of the energy band gap (Eg) and refractive indices are listed in Table 2. The values of Eg are higher than those of SiN3 at thickness of 75 nm. The increase in the energy band gap in the present work may be due to the up shift in the conduction band and downshift in the valence band leading to the band gap widening at 600nm. The results show that the band gap energy of Ti50-Zr50 oxide is the highest values than other antireflective coating TiO2 and ZrO2. Moreover, the minimum energy gap value for ZrO2 was obtained.

Where (λ) is the wavelength of the normal-incidence light in a vacuum. For solar cell applications, the refractive index and thickness of the ARC are designed to minimize the reflection at 600nm. This wavelength is close to the maximum power point of the solar spectrum.

Where (λ) is the coefficient of absorption, λ is the wavelength of the incident beam, A is the absorbance and d is the thickness (Figure 6).

It is observed that the k values increase with increasing wavelength up to 600 nm for TiO2 and up to 350 nm for ZrO2 and Ti50-Zr50 oxide and thus the k values are very sensitive to the antireflection coating (ARC) type in the visible region. However, at the longer wavelengths, all (ARC) TiO2, ZrO2 and Ti50-Zr50 oxide are close. In this work, it was found that TiO2 has a refractive index of about 2.7 at t = 40 nm and ZrO2 and Ti50-Zr50 oxide have a refractive index of about 1.95 and 2.19, respectively. Elsewhere, SiNx has a refractive index of 2.05 at t = 75 with different deposition techniques as shown in Table 2 [15].

Effect of radiation on optical properties of (p-n) junction

In order to understand the behavior of samples with gammairradiation, one has to investigate the effect of gamma-radiation on solid material. Gamma-irradiation produces mainly ionization and excitation in the constituent atoms of the material. Moreover, it may also cause a small amount of atomic displacement depending on its energy and the atomic number of the target atoms [33,35]. The effect of ϒ – radiation on optical properties of Si p-n junction cells coated with TiO2, ZrO2 and T50-Zr50 oxide antireflection films with 40nm has been investigated at an irradiation dose of 5K Grey. The Optical reflectivity of the studied antireflection coatings before and after irradiation with dose 5K Grey is shown in Figure 7.

It can be observed that there is a noticeable change in reflectivity of the samples after irradiation as compared with un-irradiated ones. For irradiated TiO2 films, the decrease in reflectivity lies in the wavelength range from 400-700nm while the relative increase in its value from 700-1000 nm occurs for irradiated lies in the wavelength range from 300 to700 films. For irradiated ZrO2, the decrease in reflectivity lies only in the wavelength range from 275- 475 nm and for irradiated Ti50-Zr50 oxide has slightly difference with that of un-irradiated one. Based on the former notion, one has to discuss this effect on the evaporated films and the Si base as well. For anti-reflection coating films, ϒ-radiation may have a minor effect on their optical properties because of the following reasons

a) The film thickness is ultra-thin and therefore radiation damage is minimal.

b) The amorphous nature of the evaporating films may cause the radiation effect to be temporary with no permanent damage.

Therefore, the major effect of gamma-irradiation on samples reflectivity comes from its impact on the Si base. In other words, the change of reflectivity with irradiation may be attributed to active vacancies produced in the Si base by gamma radiation, where electrons are trapped at these vacancies. Figure 8 show the variation of absorption coefficient with wavelength for various (ARC) TiO2, ZrO2 and Ti50-Zr50 oxide on p-n silicon after gamma irradiation with 5KGrey. It can be observed that the intensity of the absorption decreases after irradiation by the ratio 2-4% at 600nm for TiO2. In addition, a noticeable change in the optical transitions between the bound states of the trapped electrons coupled with the vibration of the host Si crystal leads to vibrational absorption and emission as compared non-irradiated ones [34] and hence reduction in reflectivity over the certain optical range is expected to occur and vice versa. The values of energy band gap Eg increase after irradiation due to the increase of structural defects and the decrease at 600nm due to the damage of the substrate under the effect of gamma irradiation. The gamma irradiation induces changes in the refractive index over a wide range of wavelengths. The refractive index tends to be greater in the visible range (600nm) compared to the values measured in the other wavelengths. The induced refractive index increases after irradiation when moving toward the infrared range of the spectra. The gamma irradiation induces disordering of p-n junction, which can lead to broadening and spectral shift in the fundamental IR absorption band. The absorption modification results change in the refractive index of the materials. The values of energy band gap refractive indices before and after gamma irradiation with 5KGray are listed in Table 3.

Conclusion

The possibility of using various metal oxides prepared by dc pulsed magnetron sputtering such as TiO2, ZrO2 and Ti50-Zr50 oxide as a coating for n-p Si junction has been studied. The structural and optical properties of dc magnetron sputtering TiO2, ZrO2 and Ti50-Zr50 oxide were studied and compared with commercial PECVD SiNx. The X-ray diffraction patterns of the antireflection TiO2, ZrO2 and Ti50-Zr50 oxide show a completely amorphous structure. The optical reflectance for ZrO2 and Ti50-Zr50 oxide drops to nearly zero in the range from 600 to 1000 and even lower than n-p Si junction alone. The decrease in reflectivity (10%) of the various coatings at 600 nm is similar to that of SiNx. The results showed that the TiO2, ZrO2 and Ti50-Zr50 oxide has a refractive index of about 2.00 at 600 nm which is similar to that of SiNx. The energy gap value and refractive indices of coated n-p Si increase slightly with that of uncoated substrate. Samples irradiated with gamma rays at a dose of 5 k Gray show different behavior in reflectivity due to displacement damage in Si-based cells. The results show that ZrO2 thin films prepared by dc pulsed magnetron sputtering can be used as anti-reflective coatings for solar cells and other siliconbased photosensitive devices.

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