Nanocone SiGe antireflective thin films fabricated by ultrahigh-vacuum chemical vapor deposition with in situ annealing
Introduction
SiGe thin films have attracted considerable attention because of their high electron and hole mobilities [1], [2], [3]. The preparation of these films is compatible with Si-based technologies, which have been used to fabricate high-performance optoelectronic SiGe devices, such as optical modulators [4] and field effect transistors [5]. Although SiGe thin films also have potential as components of highly efficient solar cells [6], [7], reflection loss, which occurs when the cell surface reflects much of the incident light, is a serious issue that severely reduces their efficiency. Thus, an effective approach for the fabrication of antireflective thin films is required if we are to manufacture high-efficiency solar cells and optoelectronic devices.
Many methods have been developed for manufacturing SiGe thin films featuring antireflective patterns. For instance, Brammer et al. [8], and Forniés et al. [9] obtained thin films with antireflective patterns by wet etching using hydrochloric acid and nitric acid, respectively. Nositschka et al. [10] used reactive ion etching to produce multicrystalline Si displaying an antireflective pattern on a solar cell. Hattori [11] introduced polymer particles to cause destructive interference and, thereby, reduce reflectance. Nevertheless, such schemes suffer either from chemical contamination or the need for an excessive fabrication period.
In this study, we fabricated nanocone-presenting SiGe antireflective thin films using in situ thermal annealing. This approach has several significant advantages. First, contamination of the SiGe thin films is entirely prevented because the films do not come into contact with any impurities during the fabrication and annealing processes; therefore, accurate data on the films can be obtained (e.g., variations in structural properties following thermal treatment). Second, heat treatment is a simple procedure that takes only a small fraction of the fabrication time required for batch-produced devices. Finally, thermal annealing is fully compatible with Si-based technologies. Nanocone SiGe thin films themselves have several attractive features that make them suitable for numerous applications. For example, they have ultraviolet (UV)-antireflective properties, which can increase the efficiency of SiGe-based solar cells. In this study, we used transmission electron microscopy (TEM), high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and spectrophotometry to measure the properties of SiGe thin films. We found that the mean reflectance of the SiGe thin films for UV rays reduced from 61.7 to 28.5% after annealing at 900 °C.
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Experimental details
SiGe epitaxial thin films were grown onto 6-inch p-type Si (100) in an ultrahigh-vacuum chemical vapor deposition (UHVCVD) system (ANELVA SRE-612, Japan) [12], [13]. The Si wafers had undergone standard cleaning according to the guidelines of the Radio Corporation of America (RCA) [14]; they were dipped in dilute hydrofluoric acid to passivate their surfaces. As a result, when the wafers were transported through air and introduced into the loadlock chamber of the UHVCVD system, their surfaces
Results and discussion
Fig. 2(a) presents a cross-sectional TEM image of the as-deposited SiGe epitaxial thin film. The SiGe layer was 102 nm thick with a Si (100) orientation. The EDS analysis revealed that the thin film was composed of Si and Ge elements. The Ge content of the SiGe epilayers was ca. 25% prior to annealing. The high-resolution TEM image of the as-grown SiGe epitaxial thin film indicates that it had a highly textured structure, produced by the UHVCVD system. The diffraction pattern from within the
Conclusion
We have used UHVCVD and in situ thermal annealing to fabricate nanocone-presenting SiGe antireflective thin films as a result of SiGe clustering on SiGe surfaces. The mean reflectance of the as-grown SiGe film toward UV rays was as high as 61.7%. The mean reflectance of the film that had been annealed at 900 °C had decreased to 28.5% as a result of the presence of the nanocone array on its surface. Thus, the nanostructures had antireflective properties superior to those of the as-grown sample;
Acknowledgements
The authors would like to thank Dr. Jiann Shieh and Shih-Chiang Chuang for useful discussions, Fu-Kuo Hsueh for UHVCVD, Chiung-Chih Hsu for TEM and Jie-Yi Yao for XRD, and Mei-Yi Liao for spectrophotometric in National Nano Device Laboratories (NDL) and National Science Council of the Republic of China for financially supporting this research under Contract no. NSC 95-2221-E-005-043-MY2.
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