EFFECT OF SURFACE Si-Si DIMERS ON PHOTOLUMINESCENCE OF SILICON NANOCRYSTALS IN THE SILICON DIOXIDE MATRIX
O. B. Guseva* A. V. Ershovb, D. A. Grachevh, B. A. Andreev€, A. N. Yablonskiyc
"Ioffe Physical-Technical Institute, Russian Academy of Sciences 194021, Saint-Petersburg, Russia
b Lobachevsky State University of Nizhni Novgorod 603950, Nizhny Novgorod, Russia
'"Institute for Physics of Microstructures, Russian Academy of Sciences 603950, Nizhny Novgorod, Russia
Received November 27, 2013
The effect of surface states of silicon nanocrystals embedded in silicon dioxide on the photoluminescent properties of the nanocrystals is reported. We have investigated the time-resolved and stationary photoluminescence of silicon nanocrystals in the matrix of silicon dioxide in the visible and infrared spectral ranges at 77 and 300 K. The structures containing silicon nanocrystals were prepared by the high-temperature annealing of multilayer SiO^/SiOa films. The understanding of the experimental results on photoluminescence is underlain by a model of autolocalized states arising on surface Si Si dimers. The emission of autocatalized excitons is found for the first time, and the energy level of the autolocalized states is determined. The effect of these states on the mechanism of the excitation and the photoluminescence properties of nanocrystals is discussed for a wide range of their dimensions. It is reliably shown that the cause of the known blue boundary of photoluminescence of silicon nanocrystals in the silicon dioxide matrix is the capture of free excitons on autolocalized surface states.
DOI: 10.7868/S0044451014050061
1. INTRODUCTION
A11 intense photolumincsccncc (PL) of porous silicon in the visible spectral range at room temperature was first observed by Canliam in 1990 [1]. This PL, not inherent to silicon (which is an indirect gap semiconductor), was explained as the result of the size quantization effect of electron states in silicon nanocrystals. The possibility to create optoelectronic devices based thereon and the prospects of their use in photovoltaics promoted research 011 the optical and electric properties of silicon nanocrystals in various matrices [2,3].
Currently, much attention is attracted to silicon nanocrystals in the matrix of silicon dioxide (SiO-2). This is due to the high thermal and chemical stability of the material and its complete compatibility with the traditional silicon microelectronic technology.
To obtain an efficient emission in the visible and near-IR spectral ranges, the nanocrystals should be of the size not exceeding several nanometers. At this size,
* E-mail: oleg.gusevfflmail.ioffe.ru
the ratio of the nanocrystal surface to its volume is large, and the surface states may significantly affect the excitation and emission mechanisms of the nanocrystals.
It is known that with a decrease in the size of silicon nanocrystals in the SiO-2 matrix, the maximum of the PL band at the room temperature shifts from the IR region only to some "blue boundary" of the wavelength about 0.6 //111 (corresponding to the emission energy 2.1 eV), which has not been predicted by theoretical calculations based 011 the size quantization effect [4]. Yet the PL band of silicon nanocrystals in the silicon nitride matrix (Sis^) undergoes the blueshift up to the ultraviolet region of 0.41 //.111 (approximately 3 eV) as the size of nanocrystals decreases to 2.6 11111 [5]. The blueshift up to 3.5 eV is also observed for the PL of porous silicon when the surface of the crystals is covered with hydrogen [6]. Therefore, the existence of the "blue boundary" of 2.1 2.2 eV for PL of silicon nanocrystals in the silicon dioxide matrix demonstrates a strong effect of the nanocrystal surface 011 the PL characteristics.
The most widely spread preparation method of silicon nanocrystals in the matrix of silicon dioxide is the high-temperature annealing of initial films of silicon suboxide Si03. (x < 2), which can be carried out by various procedures [3]. The annealing process induces the separation of the Si and SiO-2 phases with the formation of silicon nanocrystals embedded in SiO-2- The size of silicon nanocrystals depends on the amount of excess Si, as well as on the annealing temperature. The results of the studies of the fine structure of X-ray absorption spectra near the edge structure (XANES) indicate that the nanoparticles obtained by the high-temperature annealing of Si03. films are silicon nanocrystals surrounded with a thin layer of 0.C 0.8 mil of amorphous SiOy of variable composition x [7]. This way, several types of structural defects Si=0, Si O Si, Si 0,and surface Si Si dimers form on the surface of silicon nanocrystals.
It has been shown in theoretical investigations [8,9] that the blue boundary of PL for silicon nanocrystals in the SiO-2 matrix can be described with a model of au-tolocalized exciton states forming on the surface Si Si dimers. We refer to these states as self-trapped exciton state, STE(Si Si). We note that according to the calculations, radiative recombination of autolocalized excitons on STE(Si Si) should result in the appearance of a PL band in the range 1.2 1.5 eV for small silicon nanocrystals in the matrix of silicon dioxide. The observation of this band can become the proof of the suggested model. However, no experimental studies that detected PL of autolocalized excitons on surface Si Si dimers have appeared until now.
Recently, results were published of an experimental investigation of induced light absorption in silicon nanocrystals, which were explained based on a model of the capture of hot excitons into the autolocalized surface states on the Si O bond [10]. It was shown that in the time interval about 0.1 lis, a hot exciton from the nanocrystal is captured on the surface metastable state, from which it returns into the nanocrystal. This process was described using a phcnonicnological model of autolocalized excitons constructed in the framework of a single-mode approximation of Huang Rhys. The Si O vibration was taken as the vibration mode where the exciton was localized. We refer to these autolocalized surface excitons as STE(Si O). Based on this model, the processes of "hot" exciton capture from the nanocrystal into the autolocalized state, the backward return into the nanocrystal by thcrnioactivc tunneling, and the processes of the optical ionization of the exciton and its radiative and nonradiativc recombination were theoretically discussed in [10,11]. The STE(Si O)
model may also explain the blue boundary of the PL in silicon nanocrystals. In [10], no attempts were made to detect PL of the autolocalized exciton. It should also be noted that the study of induced light absorption in a silicon nanocrystal cannot be regarded as a direct method of the investigation of autolocalized states.
The purpose of this paper is to provide experimental evidence for the existence of autolocalized states on the surface of silicon nanocrystals in the matrix of silicon dioxide and to reveal their nature and their influence on PL of free and autolocalized excitons. We investigated the stationary and time-resolved PL of silicon nanocrystals in the matrix of SiC>2 in the visible and IR spectral ranges. The obtained experimental results are well understandable based on the model of autolocalized excitons on surface Si Si dimers.
2. PREPARATION OF SAMPLES AND MEASUREMENT PROCEDURES
In this work, we studied structures composed of thin films of silicon dioxide containing silicon nanocrystals and separated by layers of pure silicon dioxide. This allowed decreasing a probable effect of energy transfer from a silicon nanocrystal of a smaller size to the nearest-neighbor of a larger size. In thin (close to the size of the silicon nanocrystal) layers of the Si(>2 matrix, the number of nearest neighbors is several times smaller than in the case where the thickness of the Si02 layer with silicon nanocrystals is larger than the nanocrystal size.
Multilayer nanostructurcs u-Si03./u-SiCh were obtained by alternate evaporation in a vacuum of appropriate initial materials from two separate sources, with the layer thickness monitored photometrically as described in [12,13]. The u-Si03. layers were deposited by evaporation of SiO powder from an effusion tantalum source, and the u-SiCh layers, by electron beam evaporation of fused silica. The temperature of the silicon substrate during the deposition was maintained at 200 °C. The thickness of u-SiCh layers was set at a constant value of 2.8 mil, and the structures were distinguished only by the thickness of u-Si03. layers. In all cases, the first deposited layer was that of silicon suboxide, followed by Si(>2, u-Si03., etc.; the upper layer consisted of SiC>2. After deposition, structures were annealed at high temperature (about 1100 °C) for two hours in an N2 atmosphere. As a result, structures were obtained with silicon nanocrystals of the mean dimensions 2.5, 3.5, and 4.5 mil, approximately in accordance with the thickness of the deposited layer of
u-Si03.. The size control of the nanocrystals was carried out with high-resolution transmission electron microscopy as described in [12,13].
The stationary PL was exited with a semiconductor laser emitting at A = 405 11111 and was registered by a photomultiplier or by a liquid nitrogen-cooled germanium detector. The time- resolved PL spectra were obtained at the excitation with a pulse nitrogen laser emitting at A = 337 11111, with a pulse duration of 7 lis and the repetition frequency 45 Hz. The PL measurements were performed with the use of grating nionochromator, a stroboscopic voltmeter with the strobe pulse duration of 4 lis, and a digital oscillograph. The time resolution of the registration system with the use of a photomultiplier tube in the spectral range 400 850 mil was 20 ns, and in the region 700 160011111, when the cooled germanium detector was used, 20 //s. All PL spectra are c
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