Optical Characterization of Porous Silicon Multilayers
Uncontrolled fabrication errors for multilayer porous silicon structures could in some circumstances significantly and unexpectedly change their optical properties (reflectivity, refractive index, etc.). Therefore, optical characterization of these structures gains prominent importance before using these structures for various applications such as optoelectronics and sensing. It is the aim of this short review to discuss the importance of optical characterization of multilayer porous silicon structures, by way of some numerical modeling and experimental results. We will thereby illustrate some important aspects about how the optical performance of these structures can be increased by following some simple precautions in their fabrication. It is also our objective in this review to bring some of the recent studies and trends in the subject of optical characterization to the attention of readers.
KeywordsMeasurement Multilayered porous silicon Numerical analysis Optical characterization
Porous silicon structures have promising applications in photonics (Canham 1997; Escorcia-Garcia et al. 2009), luminescence (Cullis and Canham 1991), and sensors (Lorenzo et al. 2005) thanks to their high surface to volume ratio, integration with surface areas across a chip surface, and highly variable surface chemistry.
In addition to their structural, thermal, mechanical, and optical properties (see the handbook chapters “➔Microscopy of Porous Silicon,” “➔Thermal Properties of Porous Silicon,” “➔Mechanical Properties of Porous Silicon,” and “➔Refractive Index of Porous Silicon”), characterization of porous structures also plays a key role to fully investigate the behavior and response of porous silicon structures (for more details, see the handbook chapters “➔Characterization of Porous Silicon by Calorimetry,” “➔Magnetic Characterization Methods for Porous Silicon,” “➔Chemical Characterization of Porous Silicon,” and “➔Characterization of Porous Silicon by Infrared Spectroscopy”).
Porous silicon structures with miscellaneous properties (Rugate-filter, Fabry-Perot, or microcavity structure; multilayer or freestanding form) can be created by a simple and easy electrochemical anodic etching process. This process can in addition provide a broad refractive index contrast, a desirable feature in sensing applications. For more details, please refer to the book chapter “➔Porous Silicon Multilayers and Superlattices.” However, some uncontrolled fabrication errors, to be discussed in the following sections, alter the optical properties (refractive index, extinction coefficient, wavelength shift, reflectivity, and transmittivity) of porous structures, changing in turn their sensing performance. Therefore, optical porous silicon sensors have to be characterized to investigate their non-ideal characteristics before sensing applications such as organic vapor detection (Liu et al. 2009; Cho et al. 2013; Karacali et al. 2013; Hasar et al. 2015a) and inorganic gas vapor detection (Yan et al. 2014).
Optical characterization of porous silicon multilayer structures can be analyzed by different measurements such as photoluminescence, ellipsometry, and reflectometry. The photoluminescence measurements record spontaneous emission of light from porous silicon structures under optical excitation (Canham 1990; Pelant and Valenta 2012). Ellipsometry measurement is an optical characterization technique utilizing measurement of the polarization transformation that occurs after the reflection (or the transmission) of a polarized beam (Azzam and Bashara 1989; Tompkins and McGahan 1999; Pérez 2007). On the other hand, reflectometry or interferometry measurement , as its name specifies, is based on the response of interference phenomenon inside a material (Tompkins and McGahan 1999).
In the following short review, we first look at the nature of fabrication errors in preparation of multilayer porous silicon structures. Next, we present the basic theory for modeling the optical characterization of porous silicon structures and, then, demonstrate a typical measurement setup which could be used for optical characterization. Finally, we recapitulate some of the important findings of this review.
Modeling for Optical Characterization
In the electrochemical etching technique , generally controllable current densities are applied for some specific time over a p-type (or n-type) uniformly doped bulk silicon sample located within an etching cell (or tank) to fabricate multilayer porous silicon structures with desirable optical properties (Pavesi 1997; Agarwal et al. 2009; Ghulinyan et al. 2003a; Sailor 2012). Related book chapter is “➔Porous Silicon Multilayers and Superlattices.” Enlargement of current passage across first created layers due to long stay time of the sample in the electrolyte (Pavesi 1997; Ghulinyan et al. 2003a) and presence of some surface irregularities and internal impurities [doping inhomogeneities (Ghulinyan et al. 2003a)] can induce as a whole different optical properties in the axial dimension of multilayer porous silicon structures. Additionally, nonuniform surface current densities, natural drifts in the growth direction (Ghulinyan et al. 2003a), and dopant impurities can also result in different optical properties in the transverse dimension of these structures. Furthermore, higher variation of refractive index (a desirable property for increasing selectivity of sensors), which can be attained by low resistive substrates, large current variation, and a proper electrolyte concentration, is generally accompanied by an increased macroscopic roughness due to striations (Setzu et al. 2000). Therefore, effects of these fabrication errors on optical properties of porous multilayer structures need to be investigated for the analysis of their optical performance and sensing capability by performing a thorough optical characterization.
For analyzing the optical response and characterization of multilayer porous silicon structures, macroscopic electromagnetic properties (e.g., refractive index and extinction coefficient) are used for describing electromagnetic responses (reflectivity and transmittivity) of these microscopically inhomogeneous but homogenized structures. These properties will be effective ones if the value of the operating wavelength is much higher than the diameter of the pores in each fabricated layer (effective medium theory) (Saarinen et al. 2008; Suarez et al. 2011). In this circumstance, different effective medium approximations can be used such as Maxwell-Garnett, Bruggeman, or their mixture (Jylha and Sihvola 2007). Such approximations relate the optical properties of each constituent (e.g., pore and bulk silicon) of the composite porous silicon layer to effective optical properties.
Selected example studies related to optical characterization of porous silicon multilayers by different techniques and with different objectives
1. Effects of optical loss coefficient of Si substrate on transmittivity of freestanding single and coupled microcavities were analyzed (Ghulinyan et al. 2003a)
2. Wavelength shift of Bragg mirrors was analyzed by considering the capillary condensation effect after exposure of the mirrors to organic vapors (Snow et al. 1999)
3. Effects of optical loss coefficient of Si substrate on transmittivity of freestanding single and coupled microcavities were analyzed (Ghulinyan et al. 2003a)
4. Influence of linear drifts in porosity and thickness of the layers on reflectivity and transmittivity of freestanding coupled microcavities was investigated (Ghulinyan et al. 2003b)
5. Optical constants (complex refractive index and thickness) of each layer of multilayer periodic Bragg mirrors were determined by using genetic algorithms (Torres-Costa et al. 2004)
6. Photonic band gap properties of quasi-periodic Thue-Morse and Fibonacci porous silicon multilayer structures were investigated from oblique incidence case (Moretti et al. 2006; Palavicini and Wang 2013)
7. Quantitative analysis of the effect of pore diameter on the refractive index and thus sensitivity of porous silicon optical biosensors was performed (Ouyang et al. 2006)
8. Analysis for reflectance spectrum of multilayer one-dimensional microcavity porous silicon structure for off-normal incidence (s- and p-polarizations) considering a uniaxially anisotropic dielectric tensor was carried out (Saarinen et al. 2008)
9. Effects of surface modification (formation of silicon dioxide, occurrence of volume expansion due to oxidation, and application of sensed analyte) on the effective refractive index of periodic porous silicon Bragg mirrors and microcavities for biosensing applications were examined (Charrier and Dribek 2010; Suarez et al. 2011)
10. Effects of variations in thickness, refractive index, and extinction coefficient of each individual layer on reflectivity and transmittivity of multilayer Fabry-Perot porous silicon structures were evaluated (Hasar et al. 2012; 2015b)
11. Optical response of porous silicon Bragg reflectors was measured by variable angle-of-incidence infrared spectroscopic ellipsometry technique (Zangooie et al. 2001)
The overall interference response (M h ) of a multilayer porous silicon structure with N layer by reflectometry measurements can be found by application of the TMM [for the exp(+iωt) time reference] (Born and Wolf 1999).
On the other hand, the ellipsometric parameters in ellipsometry measurements can be related to the reflection coefficients of the light polarized parallel and perpendicular to the plane of incidence r p and r s , respectively. The measured complex ratio ρ (modulus and phase) of the these two reflection coefficients is defined as (Azzam and Bashara 1989; Tompkins and McGahan 1999; Pérez 2007; Zangooie et al. 2001)
It is seen from Fig. 1a that a change in refractive index of middle layers drastically affects the resonance characteristics of Fabry-Perot cavities . This is an important issue for fabrication of thick multilayer porous silicon cavities since variation of electrolyte concentration could pose a porosity gradient along the depth of the cavity as a consequence of diffusion issue (Thonissen and Berger 1997). In addition, the dependence in Fig. 1a shows that for the fabrication of Fabry-Perot cavities, low resistive p++ type substrates [assuming appropriate electrolyte solution, viscosity level, and pulsed anodic etching are used (James et al. 2009)] could be preferred to high resistive ones to increase the interface quality (decrease interface roughness) between the layers (Escorcia-Garcia et al. 2009). On the other hand, as seen from Fig. 1b that a similar change of refractive index of almost all layers (from first to last) alters the reflectivity value of Fabry-Perot cavities, demonstrating as well a caution on fabrication of Fabry-Perot cavities considering above fabrication recipes.
Measurement Setup for Optical Calibration of Porous Multilayer Structures
Numerical analysis for optical characterization can provide insight regarding the effect of fabrication errors of porous multilayer structures. However, for a complete optical characterization, in addition to numerical analysis, measurements of reflectivity and/or transmittivity of empty porous multilayer structures should be carried out.
Before starting measurements for optical characterization, the setup should be calibrated. Toward this end, firstly apertures of fiber optic cables should be properly oriented so that maximum reflection is possible. Second, reflected signal from a flat highly reflective (alumina) plate (around 99.7%), which will be used as a reference signal for reflectivity measurements, should be measured. Then, for each wavelength, reflectivity measurements over the whole band could be achieved by dividing the reflected signal from the porous structure to the signal from the plate used. If needed, the effect of dark signal (no lamp signal) could also be included into reflectivity measurements to improve the measurement accuracy.
It is seen from Fig. 5 that the whole spectrum of reflectivity of both cavities changes from point to point. This is because reflectivity is associated with changes in optical (refractive index) and physical (thickness) properties of any layer (or layers). Besides, variation of resonance wavelength indicates that either effective refractive index and/or thickness of layers (or a layer) near microcavity region changes (Hasar et al. 2015b). Resonance wavelength of the lossless cavity changes more than that of the lossy cavity for the same amount of surface point variation (25 μm), necessitating more careful fabrication of lossless multilayer porous silicon structures than their lossy counterparts.
This review has collated information about the current trends and stage of optical characterization of multilayer porous silicon structures. It is noted that fabrication errors such as nonuniform surface current densities and doping inhomogeneities, along with the effect of surface modification, could significantly alter the optical properties, via a change in refractive index and thickness, of especially multilayer porous silicon structures, if proper attention is not exercised in the fabrication process. Thus, considering the ever-increasing demand for application of porous silicon sensors in biology, medicine, and food and beverage control quality, optical characterization of these sensors plays a key role and thus an indispensable part in detection and identification of unknown agents (organic or inorganic gas vapor, biological or chemical molecule, etc.).
- Azzam RMA, Bashara NM (1989) Ellipsometry and polarized light. North Holland, AmsterdamGoogle Scholar
- Canham LT (ed) (1997) Properties of porous silicon. IEE Inspec, LondonGoogle Scholar
- Hasar UC et al (2015b) Characterization of porous silicon Fabry-Perot optical sensors for reflectivity and transmittivity measurements. IEEE J Sel Topics Quantum Electron 21:2900110Google Scholar
- Pérez EX (2007) Design, fabrication and characterization of porous silicon multilayer optical devices. Ph. D. Thesis, Universitat Rovira I VirgiliGoogle Scholar
- Sailor MJ (2012) Porous silicon in practice: preparation, characterization and applications. Wiley, WeinheimGoogle Scholar
- Suarez I, Chirvony V, Hill D, Martinez-Pastor J (2011) Simulation of surface-modified porous silicon photonic crystals for biosensing applications. Photon Nanostruct: Fundam Appl 9:304–311Google Scholar
- Thonissen M, Berger MG (1997) Multilayer structures of porous silicon. In: Canham LT (ed) Properties of porous silicon. INSPEC Publications, London, p 35Google Scholar
- Tompkins HG, McGahan WA (1999) Spectroscopic ellipsometry and reflectometry: a user’s guide. Wiley, LondonGoogle Scholar