Porous silicon: a quantum sponge structure for silicon based optoelectronics
Introduction
The end of the last century has seen a progressive move down in dimensionality of semiconductors. Quantum wells, quantum wires and quantum dots were exotic terms a decade ago, while now they are at the very basis of many devices, e.g. lasers or HEMT, and are the key to the development of the technology of the future, e.g. nanoelectronics. At the same time, new applications for Si have been found either in the form of an alloy with Ge for high frequency applications, or as a promising material for photonic applications. This last aspect is the focus of this review paper.
The promotion of Si from being the key materials for micro-electronics to an interesting material for photonic applications is a consequence of the possibility to reduce its dimensionality by a cheap and easy technique. In fact, electrochemical etching of Si under controlled conditions leads to the formation of nanocrystalline Si where quantum confinement of photoexcited carriers yields to a band gap opening and an increased radiative transition rate. Efficient light emission is the product. The resulting material is named porous silicon (PS) due to its morphology composed by a disordered web of pores entering into Si. Its structure is like a sponge where quantum effects play a fundamental role, i.e. PS could be viewed as a quantum sponge, and as a sponge it can be permeated by a variety of chemicals and its enormous internal surface rules its properties. These features (being a quantum system and a sponge) are keys both to the success and to the failure of PS. In fact, many possible applications exploit the quantum confinement (e.g. in light emitting diodes) or the high reactivity of its surface (e.g. sensor applications) but to promote PS to real and commercial devices one has to master its quantum sponge nature. The disordered distribution of nanocrystal sizes, interconnectivities, and surface compositions hampers a real engineering of PS properties. Its enormous and active inner surface causes time and ambient dependent properties, aging effects and uncontrolled deterioration of device performances. It is an interesting piece of scientific work in the understanding of the properties of this material and the mastering of some of its properties to obtain devices which work in a predictable way. Here we try to present an overview of this work.
It should be pointed out that in the last few years various conference proceedings [1], [441], [442], [443], review articles [2], [444], [445], [446], [447], [448], [449], [450], [451], [452], [453], [454], [455], [456] and edited books [3], [457], [458], [459], [460], [461] have been published on PS. We complement these papers or books by presenting a unified and up-to-date picture of this rapidly evolving field.
Section snippets
How porous silicon is made?
PS is formed by an electrochemical etching of Si in an HF solution. Following an electrochemical reaction occurring at the Si surface a partial dissolution of Si settles in. Let us concentrate on the various factors which rule this process.
Electrolyte. Usually, HF is sold in an aqueous solution with up to 50% of HF. Thus, the first attempts to form PS were performed using only HF diluted in deionized and ultra-pure water. Due to the hydrophobic character of the clean Si surface, absolute
Structural properties
The microstructure of PS has been extensively studied. Owing the large variety of the PS structures that can be produced (depending on substrate doping types, doping concentrations and fabrication parameters, see Section 2), the experimental results are often referred to very different materials. Therefore, it is worth to discuss the experimental results considering the various techniques rather than the material type.
Theoretical calculations
Low-dimensional semiconductors exist in three flavors: two-dimensional (2D) systems like quantum wells or quantum slabs, one-dimensional (1D) systems like quantum wires, and zero dimensional (0D) systems like quantum dots or nanoclusters. Thus, the calculations about PS have mainly focused on the investigation of Si quantum wells [200], [201], [202], [203], [204], [205], [206], [207], [208], [209], [210], [211], [212], [213], [214], Si quantum wire [185], [200], [207], [208], [209], [211], [215]
Photoluminescence
PS based structures have been reported to luminesce efficiently in the near infrared (0.8 eV), in the whole visible range and in the near UV (Fig. 76) [289]. Such a broad range of emission energies arises from a number of clearly distinct luminescent bands, which are listed in Table 8 [21]. In addition, PS has been used as an active host for rare earth impurity, e.g. Er, or dye solutions. Direct energy transfer between PS and the impurity or dye is demonstrated.
Electrical properties
Despite the numerous papers published on the optical properties of PS, less attention has been paid on its electrical properties. A review of them appeared in [459]. Therefore, here we will concentrate on more recent results which mostly concern the problem of the AC conductivity on the measurement of the carrier mobilities and on the transport models. In the study of the electrical properties of PS care should be paid to separate the contribution of contacts to that of the bulk of the samples
Device applications
Even though the interest in PS renewed after the observation of its emission properties, the potential application areas of PS are much wider than simple light emission. In Table 11, a list is presented. A review of applications of PS in optoelectronics is presented in [371]. In this section we will review some of them.
Conclusion
In this review we have tried to present the state-of-the-art of the research on PS. A question, however, has not been addressed yet: is PS still competitive towards a silicon-based photonic? Here we try to present our view.
After 10 years of research no efficient and fast LEDs have been demonstrated yet, nor any amplification of light which could be used in laser applications. On the other hand, the PS formation is fully CMOS compatible, the external quantum efficiency of PS based LED has been
Acknowledgements
The work on PS has been carried out within the POESIA (Porosity Effects in Silicon for Applications) collaboration between the experimental group of the University of Trento and the theoretical group of the University of Modena–Reggio Emilia. We acknowledge fruitful discussions and exchanges with G. Mariotto, P. Bellutti, L.C. Andreani, F. Arnaud D’Avitaya, D. Bensahel, A. Borghesi, M. Caldas, L.T. Canham, N. Capuj, J.P. Chalzaviel, H. Cruz, A. Fasolino, A. Halimaoui, N. Koshida, D. Luis, E.
References (478)
- et al.
J. Crystal Growth
(1985) J. Electroanal. Chem.
(1990)- et al.
Thin Solid Films
(1997) - et al.
Thin Solid Films
(1995) - et al.
Mater. Lett.
(1984) - et al.
Microelectron. Eng.
(1988) - et al.
Mater. Lett.
(1996) - et al.
Thin Solid Films
(1997) - et al.
Thin Solid Films
(1997) Appl. Surf. Sci.
(1990)
Thin Solid Films
Thin Solid Films
J. Lumin.
Thin Solid Films
J. Crystal Growth
J. Crystal Growth
Thin Solid Films
J. Crystal Growth
J. Lumin.
Appl. Phys. Lett.
Appl. Phys. A
J. Phys. C
J. Electrochem. Soc.
Appl. Phys. Lett.
J. Electrochem. Soc.
Phys. Rev. Lett.
J. Electron. Mater.
Appl. Phys. Lett.
Jpn. J. Appl. Phys.
J. Appl. Phys.
J. Appl. Phys.
Jpn. J. Appl. Phys.
J. Electrochem. Soc.
J. Electrochem. Soc. Lett.
J. Electrochem. Soc.
Nature
J. Electrochem. Soc.
La Rivista del Nuovo Cimento
J. Lumin.
Mater. Res. Soc. Symp. Proc.
J. Phys. D
Appl. Phys. Lett.
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