Estimation of Ge nanocrystals size by Raman, X-rays, and HRTEM techniques

Abstract

Ge NCs have attracted considerable attention because of their potential applications in nonvolatile memory and integrated optoelectronics. A number of groups have already proposed integrate flash memories based on Ge NCs embedded SiO 2 matrix [1]. Since Al 2 O 3 presents a high dielectric constant comparatively to SiO 2 , it is a good candidate to replace silica in flash memory systems, and therefore improve their performances [2]. Moreover, Al 2 O 3 presents good mechanical properties, and supports high temperature, which leads it to be an ideal material for Si processing conditions. However, a few studies have been reported on Ge NCs embedded in Al 2 O 3 matrix [3].In this work, Ge NCs embedded in Al 2 O 3 were grown on a commercial RF magnetron Alcatel SCM 650 apparatus using a conventional co-sputtering method. Two materials, Al 2 O 3 (99,99%) and polycrystalline Ge (99,99%), were simultaneously used as target to produce the doped films. Low electrical resistivity (3-6 Ω cm) n-type Si(111) 2 inches wafers were used as substrates. Prior to sputtering, a pressure of at least 1x10 -6 mbar was reached inside the chamber and in situ argon plasma treatment of target and substrates was performed in order to clean the surface and remove any impurities. More details of the samples preparation can be found in pervious works [4,5]. The as grown films were annealed at 800 ºC and 900 ºC, during one hour, under air pressure at 1,0x10 -6 and 1,0x10 -3 mbar, in order to improve the cristallinity of the Ge phase and to achieve control over the NCs size. With the aim to study the cristallinity and distribution of the Ge NCs size Raman spectroscopy, X-ray, and high resolution transmission electron microscopy (HRTEM) techniques were used. X-ray diffraction in conventional θ−2θ geometry (Philips PW1710) was performed towards the crystallographic structure investigation, using Cu K α radiation. The identification of the crystalline phases was made using the JCPDS (Joint Committee of Powder Diffraction) data base. Raman scattering spectra were obtained using a Jobin-Yvon T64000 system with an optical microanalysis system and a CCD detector, in a backscattering geometry. Raman spectroscopy was performed at room temperature using 514.5 nm and 488.0 nm line of an argon laser at a power of 50 μW focus on an area of the sample ~1 μm 2 . After preparing cross-section specimens by standard procedures the structure of the samples was examined using HRTEM.XRD diffraction shows the characteristic peaks of Ge diamond structure, which confirms the high cristallinity of our samples. Figure 1 illustrates the XRD diffraction of an as grown sample and after annealing. It is clear that the post grown annealing treatment leads to sharp peaks and a therefore better cristallinity. The average size of Ge NCs was estimated using the Debye-Scherrer equation [6]. Using Lorentzian functions to fit each XRD peak of the samples, we obtained mean diameter values of 3.0 nm and 5.3 nm for sample B (see table) before and after annealing, respectively. Figure 2 ...

Microstructure and dielectric functions of Ge nanocrystals embedded between amorphous Al2O3films: study of confinement and disorder

“…The Raman peaks are also blue shifted with compare to bulk Ge (300.7 cm À1 ) (Figure 3). Similar blue shift of Raman spectrum, which is not in agreement with the phonon confinement theory, has been reported in the literature for sapphire embedded [8,19,20] and silica-embedded [10,21] Ge-NCs grown by RF magnetron sputtering technique followed by high temperature (800 C-900 C) processing. The behaviour has been attributed [10] to the matrix-induced compressive stress on embedded NCs.…”

“…The effective Bohr radius for Wannier excitons in Ge is about 25.3 nm, which is much larger than that of bulk Si crystal (4.9 nm) implying more prominent quantum size effects in Ge-NCs than in Si-NCs [2][3][4][5]. A number of groups have already demonstrated charge storage properties based on Ge-NCs embedded in SiO 2 matrix [1,6,7]; however, few reports are available on Ge-NCs embedded in Al 2 O 3 matrix [8]. Since Al 2 O 3 has higher dielectric constant than SiO 2 , it may be a potentially more effective dielectric material in flash memory device applications [8,9].…”

“…A number of groups have already demonstrated charge storage properties based on Ge-NCs embedded in SiO 2 matrix [1,6,7]; however, few reports are available on Ge-NCs embedded in Al 2 O 3 matrix [8]. Since Al 2 O 3 has higher dielectric constant than SiO 2 , it may be a potentially more effective dielectric material in flash memory device applications [8,9]. Furthermore, Al 2 O 3 possesses good mechanical strength and can withstand high processing temperature.…”

“…Furthermore, Al 2 O 3 possesses good mechanical strength and can withstand high processing temperature. These advantages of Al 2 O 3 which is compatible to normal Si processing technologies can be an ideal dielectric material for NC based memory devices [8,9]. However, the dielectric spectra, which provide a wealth of information regarding the average microstructure and confinement effects, have not yet been explored for such a system.…”

Effect of Ge Content on the Formation of Ge Nanoclusters in Magnetron-Sputtered GeZrOx-Based Structures

“…Most of the research were performed on the Ge-ncs embedded in SiO 2 [1–5], but a few studies of the Ge-ncs embedded in Al 2 O 3 [6, 7] and HfO 2 [8, 9] were done. Recently, the Ge-ncs embedded in ZrO 2 [10, 11] and TaZrO x [12] were investigated.…”