Morphological, compositional, structural, and optical properties of Si-nc embedded in SiO x films
© Luna López et al.; licensee Springer. 2012
Received: 30 April 2012
Accepted: 26 June 2012
Published: 30 October 2012
Structural, compositional, morphological, and optical properties of silicon nanocrystal (Si-nc) embedded in a matrix of non-stoichiometric silicon oxide (SiO x ) films were studied. SiO x films were prepared by hot filament chemical vapor deposition technique in the 900 to 1,400°C range. Different microscopic and spectroscopic characterization techniques were used. The film composition changes with the growth temperature as Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy reveal. High-resolution transmission electron microscopy supports the existence of Si-ncs with a diameter from 1 to 6.5 nm in the matrix of SiO x films. The films emit in a wide photoluminescent spectrum, and the maximum peak emission shows a blueshift as the growth temperature decreases. On the other hand, transmittance spectra showed a wavelength shift of the absorption border, indicating an increase in the energy optical bandgap, when the growth temperature decreases. A relationship between composition, Si-nc size, energy bandgap, PL, and surface morphology was obtained. According to these results, we have analyzed the dependence of PL on the composition, structure, and morphology of the Si-ncs embedded in a matrix of non-stoichiometric SiO x films.
Since the discovery of light emission from porous silicon , an intense investigation of materials compatible with silicon technology with excellent optical properties has been under development. Recently, materials containing silicon nanocrystal (Si-nc) have attracted the interest of researchers due to their optical properties. Therefore, a great variety of materials with these characteristics have been studied [2–5]. One of these materials is the non-stoichiometric silicon oxide (SiO x ); this material contains Si excess agglomerates to create Si nanoparticles embedded in an oxide matrix. SiO x shows some special compositional, structural, morphological, and optical properties that vary with the Si excess. In particular, the optical characteristics of SiO x films can be varied with the growth temperatures. For example, refractive index varies from 1.6 to 2.4 when the growth temperature is changed; also, SiO x emits visible light. These characteristics have given place to various types of applications such as waveguides, no volatile memories, light radiation, and detection devices [6–8]. Furthermore, the fabrication of SiO x films is completely compatible with complementary metal oxide semiconductor technology, providing an easy way for the optoelectronic integration on silicon. Several techniques have been employed to make thin SiO x films, such as high-dose Si ion implantation into SiO2 films [6, 9], low pressure chemical vapor deposition , sol–gel, hot filament chemical vapor deposition (HFCVD) [10, 11], and plasma enhanced chemical vapor deposition , in which an improvement of the optical and structural properties as the PL emission and Si excess has been reported. In this work, HFCVD technique was used to deposit SiO x films on silicon and quartz substrates. This technique allows us to obtain thin SiO x films with different properties just by varying the source-substrate distance during the deposit; this distance changes the growth temperature. Structural, compositional, morphological, and optical properties of SiO x films prepared by HFCVD and an analysis of the composition, morphology, Si-nc size, and their relation with the PL emission are presented.
Si-ncs embedded in thin SiO x films were deposited on quartz and n-type silicon (100) substrates, the silicon substrates with 2,000- to 5,000-Ω cm resistivity in a horizontal hot filament CVD reactor using quartz and porous silicon as the sources. A hot filament at approximately 2,000°C dissociates ultra-high purity molecular hydrogen which flows into the reactor at a 50 sccm rate and produces atomic hydrogen (H). Something worth mentioning in this process is the use of a hydrogen flux on top of the hot filament, which resulted in a remarkable improvement of the optical and structural properties of the SiO x films deposited. The substrates were carefully cleaned with a metal oxide semiconductor standard cleaning process, and the native oxide was removed with an HF buffer solution before being introduced into the reactor. The heating rate depends on the source-substrate distance (dss). The reactive species (H) forms a volatile precursor (SiO) deposited on the silicon substrate and produces Si-ncs embedded in thin SiO x films. The filament-source distance was kept constant (2 mm). The relationship between the filament temperature (approximately 2,000°C) and the variation of the dss of 2, 3, 4, 5, and 6 mm provides a change in the growth temperature (Tg) of 1,400°C, 1,300°C, 1,150°C, 1,050°C, and 900°C, which was measured with a thermocouple in each position, respectively. These changes in the dss and Tg, consequently, have modified the silicon excess and defects in the non-stoichiometric SiO x films. The film refractive index and the film thickness were measured using a null Ellipsometer Gaertner L117 (Gaertner Scientific Co., Chicago, IL, USA) with a laser of He-Ne (632.8 nm); the film thickness was measured using a Dektak 150 profilometer (Veeco Instruments Inc., Plainview, NY, USA). FTIR spectroscopy measurements were done using a Bruker system model vector 22 (Bruker Instruments, Bellirica, MA, USA). XPS analysis was carried out using a Thermo Fisher spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a monochromatic Al radiation XR15 and energy of 15 eV. PL response was measured at room temperature using a Horiba Jobin Yvon spectrometer model FluroMax 3 (Edison, NJ, USA) with a pulsed xenon source whose detector has a multiplier tube, which is controlled by computer. The samples were excited using a 250-nm radiation, and the PL response was recorded between 400 and 900 nm with a resolution of 1 nm. Room-temperature transmittance of the SiO x films was measured using a UV–vis-NIR Cary 5000 system (Agilent Technologies Inc., Santa Clara, CA, USA). The transmittance signal was collected from 190 to 1,000 nm with a resolution of 0.5 nm. HRTEM measurements and XEDS were done using a Titan 80- to 300-kV model with an energy spread of 0.8 eV. HRTEM micrographs were analyzed using Gatan DigitalMicrograph software (Gatan Inc. Pleasanton, CA, USA) . The surface morphology of non-stoichiometric SiO x films was studied using a scanning probe microscopy of Ambios Technology (Santa Cruz, CA, USA), operated in non-contact mode. A 4 × 4-μm2 scanned area was used for each topographic image, and a 460-μm-long single-crystal Si n-type cantilever operated at 12 kHz (type MikroMash SPM Probes (San Jose, CA, USA)) was used. Four different scans were done for each sample, showing good reproducibility. AFM images were analyzed using scanning probe image processor software .
Results and discussion
Compositional results (atomic percentages of oxygen (O) and silicon (Si))
x = O/Si
Oxidation states of the SiO x films obtained by means of the convolution of the XPS curves
Peak position (eV)
From AFM images, the samples grown with lower Tg look more homogeneous than those grown with higher Tg. As shown in the HRTEM images, the silicon excess agglomerates to create Si-ncs; then, the roughness observed in AFM measurements can be associated with Si-ncs and compounds. In addition, FTIR spectra show a phase separation (Si and SiO2), which is deduced from the shift of the Si-O stretching vibration mode towards the SiO2 frequency value, and it is corroborated with both XPS and HRTEM. Therefore, elemental Si, SiO x , and SiO2 phases with Tg are present, and depending on Si excess, the roughness, size of Si-nc, oxidation states, and vibration modes of the Si-O-Si bonds, some of these phases could be dominant. This indicates that a direct correlation between the roughness, size of Si-nc, oxidation states, and vibration modes of the Si-O-Si bonds exists. In other words, the roughness is produced by the formation of Si-ncs and oxidation states. The diffusion of excess silicon at high Tg produces Si-ncs in the SiO x films, i.e., the silicon particles diffuse to create silicon agglomerates around the nucleation sites when the SiO x is grown at high Tg.
The optical properties such as the energy bandgap and the PL bands between (400 to 700 nm) are usually some of the important characteristics of these materials. The PL of SiO x films has been extensively studied in the literature [1–25]. Two major mechanisms for PL in this kind of materials are generally accepted: quantum confinement effects in the Si-ncs and defect-related effects, as defects at the Si/SiO x interface and defects associated with oxygen vacancies in the film. The first mechanism of light emission that we can consider in the SiO x material is related to some kinds of defects produced during the growth process, as shown in the EDS, XPS, and FTIR spectra, where we have bonding such as neutral charged oxygen vacancies (NOV) (Si-Si bonds), non-bridging oxygen hole center (NBOHC), positively charged oxygen vacancies (E’ centers), interstitial oxygen molecules and peroxide radicals [13, 14, 23, 29, 30], which can form Si-nps or E’ centers. Therefore, the increase of PL with the Tg is due to the activation of some of these radiative defects. In this study, the 550-nm PL band has been associated with silicon excess in the film in the NOV defects and E’ centers [2, 26] types. These bands appear well defined only if the film has been grown with temperatures within 900°C and 1,150°C. If the film was grown at a higher temperature, the band at 700 nm appears with its maximum PL emission.
Gap EN of Si-nc (eV)
Diameter of Si-nc (nm)
Peak position obtained by deconvolution from PL spectra and defect types relationated with the peak position
Peak positions (nm)
NOV defects (O3 ≡ Si-Si ≡ O3)
Centers of defects E’δ
E’δ center or oxygen deficiency
Defect vacancies of oxygen (O ≡ Si-Si ≡ O)
Oxide relationated in the interface of Si/SiO x
The existence of Si-ncs in the SiO x films was corroborated with the HRTEM measurements. The diffusion of Si excess due to the deposit at high temperature, i.e., when the SiO x films are being deposited, could produce Si-ncs. The silicon particles diffuse themselves to create silicon agglomerates around the nucleation sites. If the Si excess is high enough, the Si agglomerates will be crystallized to form Si-ncs. A decrease in the Si-nc diameter has been detected when the growth temperature reduces. The high growth temperature induces the formation of crystals as the statistical analysis of the crystal size distribution, obtained from the HRTEM images, shown. Therefore, the mean diameter of Si-ncs depends on the growth temperature.
Then, two transition mechanisms are possible as the above results and discussion showed widespread bandgap transitions induced by quantum confinement and interface state transitions associated with defects in the oxide. The widespread transitions in Si-nc may bring about high energy peaks (blueshifted PL peaks), and if this energy decays between defects (NOV, NBOHC, and E’ center-related interface states), it can give place to low energy peaks (redshifted PL peaks). All these data indicate that light emission from the films is due to the Si-ncs embedded in the amorphous SiO x matrix and defects. Accordingly, we have proposed a combination of mechanisms to explain the photoluminescence in the films.
SiO x films deposited by HFCVD at different growth temperatures were analyzed. These films exhibit an intense PL with a main peak at 550 nm. The strongest PL was obtained for SiO x films deposited at 1,150°C. Ellipsometry, XEDS, XPS, FTIR, AFM, PL, transmittance, and HRTEM techniques were used to obtain the structural, compositional, and optical properties of the SiO x films, and they were studied as a function of the growth temperature. Thicker samples were obtained when the growth temperature was increased from 900°C to 1,400°C. A variation in the refractive index from 1.4 to 2.2 was obtained when the growth temperature was increased from 1,150°C to 1,400°C. From AFM images, the samples grown with the lower growth temperatures look more homogeneous than those grown with the higher Tg. As shown in the HRTEM images, the silicon excess agglomerates to create Si-ncs; in this way, the roughness observed through AFM measurements can be associated with Si-ncs and compounds. In addition, FTIR spectra show a phase separation (Si and SiO2), which is deduced by the shift of the Si-O stretching vibration mode towards the SiO2 frequency value, which is corroborated with XPS and HRTEM. A clear relation between the surface roughness, size of Si-nc, oxidation states, composition, and the PL property was obtained. Therefore, PL depends strongly on the Tg and properties of the SiO x films.
JALL is currently a researcher and professor in the Science Institute - Center of Investigation in Semiconductors Devices (IC-CIDS) from Autonomous University of Puebla, Mexico. He started to work on electrical and optical characterization of the MOS structures. His research interest is the physics and technology of materials and silicon devices. Moreover, his research interests are, too, the nanotechnology, material characterization, and optoelectronic devices such as sensor, LEDs, and solar cells.
GGS received his PhD in the Electronic and Solid State Department from the Center of Research and Advanced Studies, National Polytechnic Institute, Mexico City in 2003. He started to work on the growth and characterization of non-stoichiometric silicon oxide. His current research interests include metallic oxides obtained by the HFCVD technique, GaN obtained by the metal organic CVD technique and porous silicon gas sensor devices.
FJFG is a coordinator and researcher at the Posgrado en Dispositivos Semiconductores in Benemérita Universidad Autónoma de Puebla, México and was a participant in many important international conferences. Professor FJFG has published many journal articles. His research interests include experiments and models in photoluminescence and quantum confinement in off stoichiometry silicon oxides.
This work has been partially supported by CONACyT-154725 and VIEP-BUAP-2012. The authors acknowledge INAOE and CIQA laboratories for their help in the sample measurements.
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