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Si nanocone arrays are formed on Si(100) by Ar+ ion sputtering combined with metal ion co-deposition. The aspect ratio of Si cone is found to increases steadily with increasing sample temperature, but decreases slowly with increasing ion dose. Furthermore, the height and base diameter of Si cone increase monotonously with increasing dose at a constant temperature. The absorptivity increases in general with increasing aspect ratio and height. A close to unity and all-solar-spectrum absorption by the nanostructured Si is finally achieved, with the absorbance for λ = 350 to 1100 nm being higher than 96%, and that for λ = 1100 to 2000 nm higher than 92%. Photocurrents for different Si samples are also investigated.
©2012 Optical Society of America
1. Introduction
Nano- or micro-structured antireflecting Si has received much attention in the past decade due to its potential applications in supersensitive photo-detectors and high efficiency Si solar cells [1–7]. The low reflectivity of the textured Si (or TS) has been attributed to multiple light scattering [8–11], gradual variation in effective diffractive index [12], and/or substrate-coupled Mie resonance scattering [13]. Various approaches have been proposed to prepare various TS, such as electrochemical etching [4,14], nano-sphere lithography [7], plasma-enhanced chemical etching [15], reactive pulsed laser etching [1, 2], and heavy-metal incorporated low-energy Ar+ ion sputtering [12,16]. As a novel member of the TS family, the ion-sputtering induced nano-structured TS is featured by its cost effectiveness, toxicity-free preparation environment, and good epitaxial crystallinity [12,16–23]. Since the optical properties of TS are mainly determined by the size and composition of the nanostructure, tuning the size and composition so as to achieve optimal absorption becomes a natural adoption for future applications. In this paper, we report to tune the cone size and composition of Si nanocone arrays by adjusting ion-sputtering parameters. It is found that the aspect ratio and height of the nanocone, the surface composition, as well as the reflectance, transmittance and absorbance of the TS can be well tuned by merely changing the sample temperature and ion dose. With this flexible tuning approach, a close to unity and all-solar-spectrum absorption is achieved, with the maximal absorbance for λ = 350 to 1100 nm being higher than 96%, and that for λ = 1100 to 2000 nm higher than 92%. Combining the results of photocurrents, it is suggested that the ion-sputtering induced TS can be a promising material for Si solar cell and sensitive and broadband photo-detector.
2. Experimental
The Si (100) wafer (p-type, 0.5-1Ω cm, one side polish) was firstly degreased in a solution of H2SO4:H2O2 = 1:1 and rinsed with deionized water. It was then supersonically cleaned in acetone and alcohol for 15 min in sequence. The cleaned and dried sample was transferred into a high-vacuum chamber equipped with a Kaufmann-type ion source for ion bombardment. The ion beam faced normally to the polished side of the Si sample. The Si sample was fixed by a stainless steel mask consisting of Fe and Cr with a hole of diameter of 8 mm in the middle. Since the ion beam size was 50 mm in diameter, the mask and the Si sample were irradiated simultaneously. The edge of the mask hole was so tapered that Fe and Cr atoms sputtered out by irradiation could be partially re-deposited onto the Si sample. The base pressure was 1 × 10−6 Pa. During ion sputtering, it rose up to 1.5 × 10−2 Pa due to the back-filling of argon with purity of 99.999%. For a more detailed setup description, readers are referred to Ref [12]. The sample temperature was measured with a calibrated thermocouple, which would rise during ion sputtering due to ion energy dissipation. Additional sample heating was accomplished by electron-beam bombardment at the backside of sample. In this work, the sample temperature during ion sputtering was varied from 100 to 800°C with the help of electron-beam bombardment. The ion energy was maintained at 1500 eV, and the ion dose was varied from 0.56 to 1.7 × 1019 ions/cm2. The surface morphology was characterized by scanning electron microscope or SEM (Philips, XL30). Surface composition analysis was conducted on an X-ray photoelectron spectrometer or XPS (Kratos, Axis Ultra DLD). The reflectance (R) and transmittance (Tr) of the samples were measured on an UV-vis spectrometer (Perkin Elmer, Lambda 950) with an integrating sphere. The absorbance (A) was calculated in terms of A = 1-R-Tr. Photocurrents of Si with different absorptivities were investigated. Two Al ribbons 5 mm apart were evaporated onto Si surface, and the surface current was measured with a 2 V DC voltage across the Al electrodes. The photocurrent was characterized as I1-I0, where I1 is the surface current at the presence of light illumination and I0 the dark current. An AM1.5 solar simulator (Oriel, 94023A) and an IR semiconductor laser with λ = 1550 nm (Ando, 4321) were used as light sources, respectively.
3. Results and discussion
Figure 1(a) shows a bird’s eye view SEM image of self-organized Si nanocone arrays induced by Ar+ ion sputtering with ion dose of 0.56 × 1019 ions/cm2 at 800°C. In Fig. 1(b), a corresponding cross sectional SEM image is given. The average height (H) and base diameter (D) of Si cone were calculated by measuring H’s and D’s in terms of cross sectional images followed by statistical processing.
Fig. 1 A bird-eye view SEM image (a) and a side view one (b) of Si nanocone arrays induced by Ar+ ion sputtering with ϕ = 0.56 × 1019 ions/cm2 at 800°C. H and D indicate height and base diameter of a nanocone, respectively.
Figure 2 plots the H and D of Si cone as functions of sample temperature (T). The ion density is 333.3 μA/cm2, and the ion dose (ϕ) remains at 0.56 × 1019 ions /cm2. For ion sputtering at such an ion density, T rises to 100°C during ion sputtering, and only a few cones are formed sparsely. When T approaches 200°C with the help of additional electron-beam heating, self-organized Si nanocone arrays form. Therefore, in the following, the data at 100°C will not be analyzed. It is seen from Fig. 2 that H increases monotonously with the increasing T, while D increases slightly. Let’s define aspect ratio (AR) of Si cone as AR = H/D, then AR increases with the increasing T from 0.6 to 3.1 as shown in the inset of Fig. 2.
Based on the last sample in Fig. 2, we further examine the evolution of H and D with the change of ϕ while keeping T at 800°C. In Fig. 3 , it is seen that H and D increase with the increasing ϕ. With the growth of the cone size, AR decreases from 3.1 to 2.3 as shown in the inset of Fig. 3. The results of Figs. 2 and 3 indicate that the cone size tuning by ion sputtering is effective and flexible. It has been found [12] that the middle and base regions of cone possess good epitaxial crystallinity, while Fe and Cr atoms are enriched at the apex region, which makes the crystallinity a little poorer there.
In Fig. 4 , XPS intensities of main surface elements (the others are O and C) of the TS are depicted as a function of ϕ for T = 800°C. The surface contents of Fe and Cr increase at first and then approach saturated values.
The formation process of Si nanocone arrays is proposed as follows. Si nanodot arrays are firstly formed on Si(100) [18–23] with Fe and Cr enriched at the tops of dots [21]. Due to the catalytic effect of Fe, which could be similar to that of Ni in the growth of catalyst-sphere-leading ZnSe nanowires [24], cones then grow. For a constant ϕ, the higher the T is, the more effective the catalysis is, therefore the larger the H becomes. This is the case of Fig. 2. Once the ion sputtering and the sample heating are terminated, the metal clusters atop cool down and metal silicides are formed. The slight change of D versus T (Fig. 2) is not clear at the moment, but it should be more related to a ballistic effect than a thermal one. On the other hand, with the proceeding of ion sputtering, smaller cones among bigger and taller ones would diminish till disappear, since more sputtering energies are deposited in the valley regions among bigger cones where smaller cones may exist [25]. Hence, bigger cones would continuously grow at the expense of smaller ones. For a fixed T, the larger the ϕ is, the more the smaller cones are merged into bigger ones, and the larger the average H and D become. This is the case of Fig. 3.
Figures 5(a) -5(c) show the R, Tr and A spectra for the samples in Fig. 2. It is seen that with the increasing AR or H, both R and Tr decrease steadily in the whole wavelength range, leading to a steady increase in A. “Kinks” appearing at λ = ~850 nm are systematic errors due to grating exchange during measurements. Beyond λ = 1100 nm, which corresponds to the bandgap width of bulk Si (1.12 eV), the absorptivity of planar Si is greatly reduced. However, for the TS’s, the absorbance in the infrared (IR) range of λ = 1100 to 2000 nm become significantly enhanced. The increase in A for λ = 350 to 1100 nm can be well explained in terms of multiple light scattering [8–11], gradual variation in effective diffractive index [12], and substrate-coupled Mie resonance scattering [13], but the strong below-bandgap absorption for λ = 1100 to 2000 nm by the TS cannot be explained by the three models. This IR absorption could be attributed to the modification of Si bandgap by incorporations of Fe and Cr [26–29]. In Fig. 5(c), the maximal A achieved at AR = 3.1 is >92% for λ = 350 to 1100 nm, and >85% for λ = 1100 to 2000 nm. Based on the last sample in Fig. 2 or Fig. 5(c), i.e. T = 800°C and ϕ = 0.56 × 1019 ions/cm2, further ion sputtering has been performed, and a maximal A is finally achieved in this work at T = 800°C and ϕ = 1.7 × 1019 ions/cm2, which is >96% for λ = 350 to 1100 nm, and >92% for λ = 1100 to 2000 nm, as shown in Fig. 5(d).
Fig. 5 Reflectance, transmittance and absorbance of the TS’s for T = 200-800°C (ϕ = 0.56 × 1019 ions/cm2) (a-c). Absorbance of the TS’s for ϕ = 1.1-1.7 × 1019 ions/cm2 (T = 800°C) (d).
To examine the enhanced photo-response due to strong absorptivity of TS, in Fig. 6(a) , the photocurrent enhancement, defined as ΔI/ I0 = (I1- I0)/ I0, versus three different Si samples are plotted. The light source is the AM1.5 solar simulator. The three sample are planar Si (A = 64.8% at 600 nm, and 9.15% at 2000 nm), TS1 with ϕ = 0.56 × 1019 ions/cm2 and T = 200°C (A = 78.1% at 600 nm, and 69.3% at 2000 nm), and TS2 with ϕ = 1.7 × 1019 ions/cm2 and T = 800°C (A = 97.6% at 600 nm, and 92.4% at 2000 nm). It is seen that under AM1.5 illumination, ΔI/ I0 increases linearly with the increasing A. This could be mainly attributed to the enhancement of visible light absorption. In Fig. 6(b), the corresponding IR response is plotted; the light source is the IR laser (λ = 1550 nm). As compared with planar Si, the IR response for TS1 is still weak although the IR absorbance is not low. It is noted that TS1 is prepared at a relatively low temperature (T = 200°C), so the density of radiative surface defect might still be high, which suppresses the carrier transport. However, for TS2 where T = 800°C, the IR response is quite enhanced. This could be attributed to even higher IR absorbance, and less density of radiative defect arising from high temperature annealing.
Fig. 6 Photocurrent enhancement for planar Si, TS1 and TS2 under AM1.5 solar simulated illumination (a) and 1550 nm IR one (b)
4. Conclusion
In this work, a close to unity and all-solar-spectrum photoabsorption by ion-sputtering induced Si nanocone arrays is reported. The cone size and composition of Si nanocone arrays can be flexibly tuned by ion sputtering under well-defined conditions, and the reflectance, transmittance and absorbance of Si nanocone arrays can be readily optimized. Fe and Cr incorporations help below-bandgap absorption due to the formation of silicides. A close to unity and all-solar-spectrum absorption has been achieved, with the maximal absorbance for λ = 350 to 1100 nm higher than 96%, and that for λ = 1100 to 2000 nm higher than 92%. The strong and broadband absorptivity suggests that the ion-sputtering induced textured Si can be used as a promising material for Si solar cell and sensitive and broadband photo-detector.
Acknowledgments
This work was supported by the National Basic Research Program of China (973 Program) under the grant number of 2012CB934303, and by the National Natural Science Foundation of China under the grant numbers of 10974034 and 61275178.
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