Polymeric Materials for Microelectronic Applications - American

Chapter 34

Optical Properties of Silicon-Based Polymers with Different Backbone Structures 1

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Katsunori Suzuki , Yoshihiko Kanemitsu , Soichiro Kyushin , and Hideyuki Matsumoto Downloaded by UNIV OF NEW SOUTH WALES on August 11, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch034

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Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan

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We have studied optical properties of quasi-one-dimensional Si backbone polymers and small Si clusters with eight Si atoms. The branch and ladder Si polymers whose backbones are constructed of the organosilicon units having three Si-Si bonds exhibit broad photoluminescence (PL) spectra. These PL characteristics are entirely different from those of the chain structure constructed of the organosilicon units having two Si-Si bonds only. Weak visible PL in Si based materials is caused by the introduction of Si atoms with three Si-Si bonds into the Si backbone. Moreover, even in small Si clusters, the PL spectra strongly depend on the shape of the clusters. In the cubic structure, the weak and temperature -sensitive PL originatesfromthe radiative recombination of triplet excitons. Silicon-based polymers and clusters exhibit a wealth of unique optical phenomena. 8

Optical and electronic properties of low-dimensional semiconductor nanostructures have attracted much attention, because they exhibit new quantum phenomena and have potentials for becoming novel and future optoelectronic devices (2,2). In exploring new optoelectronic materials and devices, a great deal of research effort is focused on reducing the dimensionality of the electronic structures. In this sense, chemically synthesized semiconducting polymers are regarded as natural quantum wires whose unique properties are primarily attributed to the quantum confinement effect on the conjugated electrons delocalized in the onedimensional (ID) polymer backbone chains. The polysilanes, σ-conjugated polymers, are well-known as ID silicon-based materials that have alkyl or aryl groups in their side chains (3). Although the highly hydrogenated amorphous silicon (a-Si:H) also contains Si chains, there are many differences in optical properties between a-

0097-6156/94/0579-0425$08.00/0 © 1994 American Chemical Society

In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF NEW SOUTH WALES on August 11, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch034

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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

Si:H and chain-like Si polymers. For example, the broad photoluminescence (PL) spectrum due to the band tail emission is observed in a-Si:H but a sharp PL band with a large quantum efficiency is observed in various chain-like Si polymers. The broad PL in a-Si:H rather resembles that in network Si polymers (4). Because the local Si structures can be controlled in chemically synthesized Si polymers, studies of optical properties of Si polymers with different backbone structures help to understand the electronic structures and the microscopic PL mechanisms in a variety of Si-based materials. Moreover, studies of optical properties of small Si clusters help to understand the dimension effects on electronic properties of Si-based nanostructure, and the mechanism of visible PL in nanometer-size Si crystallites such as porous Si (5). In this paper, we report the optical properties of quasi-ID Si polymers with different backbone structures and small Si clusters. We discuss the luminescence properties in a variety of Si materials. Experiment Three types of quasi-ID Si polymers used in this work are (a) poly(methylphenylsilane), (b) poly[(dimethylphenyisilyl)phenylsilane] and (c) dodecaisopropyltetracyciodecasiiane. The Si backbone structures are illustrated in Figure 1 and hereafter, we call these structures as the chain [Figure 1(a)], branch [Figure 1(b)], and ladder structures [Figure 1(c)]. In the chain structure, the organosilicon units on the polymer backbone have two Si-Si bonds only. In the branch and ladder structures, the polymer backbones are constructed of the organosilicon units having three Si-Si bonds. Chain, ladder, and cubic structures of S i clusters are also illustrated in Figure 2. Synthetic and purification methods were described in the literature (6). It is theoretically reported that the small clusters of pure Si form crystal structures that are neither diamond-like nor tetrahedrai. This complication makes it difficult to understand the size dependence of electronic properties in pure Si clusters systematically, because the changes of size are accompanied by the changes of symmetry and the formation of dangling bonds. Therefore, saturatediy bonded Si clusters terminated by an organic substituent have no dangling bonds and become a new model system for small Si clusters. Absorption spectra of Si polymers dissolved in tetrahydrofuran (THF) were measured. For PL spectrum measurements, thin solid films were prepared on a quartz substrate from THF solution. The PL spectra were measured by using 325 nm excitation light from a He-Cd laser. Picosecond PL decay under a 1-ps and 300-nm laser excitation was measured using a monochromator of subtractive dispersion and a synchroscan streak camera. The calibration of the spectral sensitivity of the whole measuring systems was performed by using a tungsten standard lamp. g

Results and Discussion Quasi-One-Dimensional Si Polymers. Figure 3 shows PL and absorption spectra in quasi-ID Si polymers. In the chain structure [Figure 3(a)], sharp PL


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SUZUKI ET AL.

Optical Properties of Silicon-Based Polymers All

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(a)

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Si atom Si-Si bond Figure 1. Illustration of quasi-lD Si polymers with different backbones: (a) chain, (b) branch, and (c) ladder structures.

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(a)

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(c)

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Si atom Si-Si bond

Figure 2. S i clusters: (a) chain, (b) ladder, and (c) cubic structures. 8



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and sharp absorption bands are observed. The structures of chain-like Si polymers take a variety of conformations such as trans-planar, trans-gauche, 7/3 helical, and disordered forms, depending on organic substituents attached to the polymer backbones (7). However, the PL band is very sharp in all conformations including disordered forms. Therefore, the sharp PL band is the most important feature of the optical properties of the chain structure. In the branch and ladder structures, broad PL spectra are observed. Moreover, the intensities of the lowest absorption peak per organosilicon unit in the branch and ladder structures are very small compared with that in the chain structure. In the ladder structure, the peak of the weak PL appears near the edge of the low energy tail of the lowest absorption band. This feature is often observed in an indirect gap semiconductors. These broad visible PL spectra were not observed in the chain structure at room temperature. The weak and sharp phosphorescence spectrum is observed near the strong fluorescence at low temperatures below 100 K. This phosphorescence spectrum is completely quenched at room temperature. The characteristics of the board PL spectra in the branch and ladder structures are different from those of phosphorescence in the chain structure. Figure 4 shows the initial PL decay dynamics at peak energies in the chain, branch and ladder structures. The initial PL decay in the chain and ladder structures approximately exhibit a single exponential decay but the initial PL decay in the branch structure shows nonexponential decay. The nonexponential PL decay and the broad PL spectrum in the branch structure resemble the bandtail emission in a-Si:H. To evaluate the radiative decay rate of excitons, we approximately determine the time constant x of the initial decay by using the single exponential function indicated by broken lines. In the branch and ladder structures, the quantum efficiency of PL, η, is very low (

Polymeric Materials for Microelectronic Applications - American

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