Silicon luminescence

A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, E. Muller, R. Romestain, and J. C. Vial, Voltage-induced modifications of porous silicon luminescence. Thin Solid Films 255, 80, 1995. 

Koropecki R, Arce RD, Schmidt J (2004a) Photo-oxidation effects in porous silicon luminescence. Phys Rev B 69 205317 1-205317 6... 

Romestain R, Vial JC, Mihalcescu 1, Bsiesy A (1995) Saturation and voltage quenching of the porous silicon luminescence and importance of the auger effect. Phys Status Solidi B 190(l) 77-84... 

Littau KAefa/1993 A luminescent silicon nanocrystal colloid via a high temperature aerosol reaction J. Phys. Chem. 97 1224... 

The luminescence of macrocrystalline cadmium and zinc sulfides has been studied very thoroughly The colloidal solutions of these compounds also fluoresce, the intensity and wavelengths of emission depending on how the colloids were prepared. We will divide the description of the fluorescence phenomena into two parts. In this section we will discuss the fluorescence of larger colloidal particles, i.e. of CdS particles which are yellow as the macrocrystalline material, and of ZnS particles whose absorption spectrum also resembles that of the macrocrystals. These colloids are obtained by precipitating CdS or ZnS in the presence of the silicon dioxide stabilizer mentioned in Sect. 3.2, or in the presence of 10 M sodium polyphosphate , or surfactants such as sodium dodecyl sulfate and cetyldimethylbenzyl-ammonium... 

The first observations on the fluorescence of colloidal CdS were made using a colloid stabilized by colloidal silicon dioxide . The fluorescence spectrum consisted of a broad band with the maximum between 580 nm and 650 nm. The reproducibility of this red fluorescence was very poor. In the presence of excess Cd ions the intensity of the fluorescence was increased, which indicates that anion vacancies were centers of luminescence. Aging of the sol for a few weeks in the dark and in the absence of air was accompanied by an increase in fluorescence intensity by a factor of ten and a gradual red shift of the fluorescence band. However, even after this increase, the fluorescence quantum yield was still below 10 . ... 

In this sub-subsection, the Er doping of amorphous silicon is discussed. The problem of limited solubility of Er in crystalline silicon has been circumvented. However, the electrical properties of pure a-Si are poor compared to c-Si. Therefore, hydrogenated amorphous silicon is much more interesting. Besides, the possibility of depositing a-Si H directly on substrates, i.e., optical materials, would make integration possible. Both low-pressure chemical vapor deposition (LPCVD) [664] and PECVD [665, 666] have been used to make the a-Si H into which Er is implanted. In both methods oxygen is intentionally added to the material, to enhance the luminescence. 

The introduction of electronic deep levels is demonstrated in Fig. 9 with low-temperature photoluminescence spectra for n-type (P doped, 8 Cl cm) silicon before (control) and after hydrogenation (Johnson et al., 1987a). The spectrum for the control sample is dominated by luminescence peaks that arise from the well-documented annihilation of donor-bound excitons (Dean et al., 1967). After hydrogenation with a remote hydrogen plasma, the spectrum contains several new transitions with the most prominent peaks at approximately 0.95, 0.98, and 1.03 eV. These transitions identify... 

Fig. 9. Luminescence spectra for n-type (8 O cm) silicon before (control) and after hydrogenation (150°C, 30 min). The spectra are offset vertically to ease inspection (Johnson el al., 1987a).

Another open question is the relationship between the H-induced radiative recombination centers and the H-induced platelets. Controlled layer removal of the plasma-processed silicon surface reveals that the density of luminescence centers decays nearly exponentially with a decay length that is comparable to the depth over which the platelets form (Northrop and Oehrlein, 1986 Jeng et al., 1988 Johnson et al., 1987a). However, the defect luminescence has also been obtained from reactive-ion etched specimens in which platelets were undetectable (Wu et al., 1988). Finally, substantial changes in the luminescence spectra occur at anneal temperatures as low as 250°C (Singh et al., 1989), while higher temperatures... 

In this type of arrays, microbeads carrying different indicator or receptor molecules are randomly loaded into the wells of chemically etched optical fiber or silicon chips [89, 90, 100] (Fig. 6b). The beads are encoded with single or multiple luminescent indicators or labels embedded into the polymeric core and/or bound... 

Tails can be based on organic and silicon organic backbones. Four types of tails play a role, depending on the desired properties. (1) Nonreactive tails. (2) Tails that can undergo an isomerization after insertion under the influence of irradiation, heat, or a sufficiently small reactive. (3) Reactive tails that can bind to molecules inside of the channels. (4) Luminescent tails, that have the advantage of being protected by the zeolite framework. 

To understand the electrochemical behavior of silicon, however, the formation and the properties of anodic oxides are important The formation of an anodic oxide on silicon electrodes in HF and HF-free electrolytes will therefore be discussed in detail in this chapter. The formation of native and chemical oxides is closely related to the electrochemical formation process and will be reviewed briefly. The anodic oxidation of porous silicon layers is closely related to the morphology and the luminescent properties of this material and is therefore discussed in Section 7.6. 

Because of its indirect bandgap, bulk crystalline silicon shows only a very weak PL signal at 1100 nm, as shown for RT and 77 K in Fig. 7.9. Therefore optoelectronic applications of bulk silicon are so far limited to devices that convert light to electricity, such as solar cells or photodetectors. The observation of red PL from PS layers at room temperature in 1990 [Cal] initiated vigorous research in this field, because efficient EL, the conversion of electricity into light, seemed to be within reach. Soon it was found that in addition to the red band, luminescence in the IR as well as in the blue-green region can be observed from PS. 

Anodization of Si in HF under an applied magnetic field produces an enhancement of the PL efficiency at RT, accompanied by an enhanced porosity compared to PS samples prepared without an applied field. The degree of polarization of the emitted PL is reduced for field-assisted preparation [Na3]. At low temperatures (4.2 K), the Stokes shift and the decay time of the PL are found to be increased, if compared to PS formed under zero magnetic field. This has been interpreted as Zeeman splitting of the spin-triplet exciton states. It indicates that the ground state of the luminescing silicon crystallite is a triplet state [Kol3]. 

The decay on a picosecond time-scale, the so-called fast band, is understood as a quasi-direct recombination process in the silicon crystallites or as an oxide-related effect [Tr2, Mgl]. This fast part of the luminescence requires an intense excitation to become sizable it then competes with non-radiative channels like Auger recombination. The observed time dependence of the slow band is explained by carrier recombination through localized states that are distributed in energy, and dimensionally disordered [Gr7]. 

A visible red luminescence corresponds to a silicon cluster size between about 1 and 2.5 nm. 

With roughly 1000 atoms, the size of the silicon clusters that constitute the micro PS network is between the bulk crystal and a molecule. Hence models of the luminescence process based on size reduction of the crystal, as well as models based on molecular sttuctures, have been proposed, which are reviewed in detail in [Ca7, Ju3]. Generally the various models of the luminescence of PS can be classified into three major categories ... 

The molecular recombination model. Absorption of a photon and its radiative reemission occurs in chemical compounds such as siloxenes [Br6], polysilanes [Ta4] or silicon hydrides [Pr6]. This luminescence mechanism is independent of whether or not QC is present in PS. 

The quantum recombination model. The photogeneration of an exciton and its luminescent decay occur within a quantum-confined silicon crystallite [Ca6]. 

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