Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study

Abstract

Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550–900 °C for 1 min) in N2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra. Beilstein J. Nanotechnol. 2019, 10, 1873–1882. 1874 Introduction Currently, there is considerable interest in the growth of selfassembled quantum dots their application in optoelectronics and nanosized structures. For instance, semiconducting Si, Ge and SiGe nanocrystals (NCs/NPs) embedded in a dielectric oxide matrix have been found to exhibit strong quantum confinement. These NCs present unique and interesting size-dependent physical properties for a wide range of application including lighting, non-volatile memories, and electronic and photovoltaic applications [1-3]. SiGe nanostructures exhibit a stronger quantum confinement effect than Si NCs [4] and have the advantage of a bandgap fine-tuning by varying the Ge atomic fraction [5,6]. These properties are useful for optoelectronic devices working in the visible to far-infrared region [4,7]. Issues commonly observed with the fabrication of such structures include inhomogeneity at the matrix/nanoparticle (NCs/ NPs) interfaces. Several studies have been devoted to the morphology of the interface between oxide matrices and NCs [8-10]. The interface of these structures has been a matter of concern in studying optical response as it may give rise to dangling bonds acting as electrically active interface traps (known as Pb-type defects). These interface traps produce scattering centers that can affect the mobility of charge carriers, thus altering the transport properties [11]. Moreover, sharp interfaces with an abrupt change in the dielectric constant or thermal expansion coefficients give rise to surface polarization effects due to local fields, which play a crucial role in systems characterized by strong charge inhomogeneity. Further, the development of strain in the structure influences the size and shape of the NCs, thus resulting in a change of the bandgap energy. A common method to obtain NCs embedded in an oxide matrix is by thermal annealing of multilayer structures. Several oxide matrices have been studied already [12-18], of which SiO2 is the most extensively studied as it remains amorphous up to high temperatures and due to its compatibility with Si-based technology [19-21]. Various fabrication methods have been utilized to fabricate structures with SiGe NCs embedded in an oxide matrix [13,17,22,23]. Magnetron sputtering is one of the most versatile methods and it allows for a good control over the NCs formation [24] by a addition of rapid thermal annealing. A rather recent variation of the magnetron sputtering technique, the so-called high-power impulse magnetron sputtering (HiPIMS), provides an alternative approach. It is an ionized physical vapor deposition method and has shown great promise in thin-film processing [25,26]. During HiPIMS the target is pulsed with short unipolar voltage pulses at low frequency and short duty cycle, achieving high discharge current densities leading to a high ionization fraction of the sputtered material [27,28]. This approach gives denser films [29] of higher crystallinity [30] than conventional direct current magnetron sputtering (dcMS) deposition technique. Thermal treatment, being one of the most common methods to obtain NCs embedded in an oxide matrix, improves the efficiency and stability of the devices by altering the size of the embedded NCs [31,32]. In the present study, a short (1 min) rapid thermal annealing is carried out over earlier investigated structures [22], where the use of HiPIMS to obtain Si1−xGex NCs in as-grown samples is demonstrated. Upon rapid thermal annealing, periodically arranged columnar self-assembled SiGe NCs are obtained. The NCs are characterized using grazing incidence X-ray diffraction (GIXRD) and high-resolution transmission electron microscopy (HRTEM). Strain relaxation and its effect on the formation of NCs and the resulting interface integrity was studied and compared with structures having a thicker (ca. 200 nm) SiGe layer [23], deposited by radio-frequency magnetron sputtering (rfMS). In another previous study [22] we demonstrated NCs in as-grown structures with broader spectral response and improved efficiency after exposure to hydrogen plasma. The effect of annealing of such structures is yet to be explored, in order to preserve the functionality of devices containing such structures [32]. A comparison is made to present the effect of SiGe thickness on strain accumulation in NCs and demonstrate the effectiveness of mild thermal exposure, applicable to structures prone to decomposition at elevated temperatures. Results and Discussion The multilayer structures (MLs) deposited in this study are similar to structures studied in our recent work [22] regarding stacking order (i.e., SiO2/SiGe/SiO2) and individual layer thicknesses. The difference in the fabrication is that during co-sputtering of the SiGe layer, we apply a lower cathode voltage for the Ge deposition, i.e., 445 V instead of 470 V, at a repetition frequency of 300 Hz, with an average power of 103 W. For Si (co-deposited via dcMS) the power is kept constant at 180 W. Structural analysis Earlier we demonstrated that for structures with a pure Ge-film sandwiched between SiO2 layers, the Ge films were crystalline when sputtered by the HiPIMS method due to the high electron density in the plasma (high power density). The higher electron density increases the ionization of Ge sputtered off the target, leading to a better quality of the film through ion bombardment. As described later in the Experimental section and also in our earlier study [22], the Si1−xGex layer was co-deposited via combined dcMS and HiPIMS from Si and Ge targets, respectively. Figure 1a shows the GIXRD diffractograms for the Beilstein J. Nanotechnol. 2019, 10, 1873–1882. 1875 Figure 1: (a) GiXRD diffractograms of MLs annealed from 550–900 °C along with the as-grown MLs. The SiGe crystallographic peaks (111), (220) and (311) are positioned between the tabulated peaks of Si and Ge presented by the dotted lines (for cubic Ge (2θ = 27.45°, 45.59° and 54.04°; ASTM 01-079-0001) and cubic Si (28.45°, 47.31° and 56.13°; ASTM 01-070-5680)). (b) Deconvoluted GIXRD diffractogram for SiO2/SiGe/SiO2 MLs, as-deposited (black circles) with the Gaussian fits shown by the red line. as-grown and annealed MLs (550–900 °C). Two broad reflections are evident for the as-grown structure. The first one corresponds to the (111) planes of SiGe and the second one to the (220) and (311) planes, which overlap indicating the presence of (nano)crystallites [22]. In Figure 1b, a deconvolution of diffractogram for the as-grown MLs was achieved using Origin software (ver. 10.0) (checked using X’Pert HighScore Plus software from PANalytical, ver. 2.2). The size of the crystallites was calculated from the (111) peak using the Scherrer equation [33,34] with a shape factor (k) of 0.9 and an instrumental error, i.e., beam broadening of 0.12. Although this is an indecisive approach [22,23], the parameters used to calculate the crystallites size are mentioned in Figure 1b and it was found to be 2.1 ± 0.8 nm. This reduction in crystallite size, compared to previously investigated structures is due to variation in deposition parameters such as cathode voltage. After annealing, three separate and distinctive peaks are evident (Figure 1a). An increase in the XRD peak intensity was observed along with a decrease in full width at half maximum (FWHM), indicating an increased crystallinity. The size of the NCs was determined, using the (111) peak using the multiple peak feature of Origin (ver. 10.0). It varies from 7.3 to 13.4 ± 0.8 nm in the annealing temperature range from 550 to 900 °C. Another feature is that, for samples annealed at 550 and 600 °C (Figure 1a), a sharp peak over a broad hump (extending from 25° to 31°) is seen, indicating that the SiGe layer is mainly amorphous but with crystalline regions (nanoparticles) (as seen in TEM images later in Figure 5a and Figure 5c). With inFigure 2: GIXRD diffractogram (upper part) with zoomed-in view (lower part) of crystallographic plane (111) of MLs annealed at 800 °C for 1 min. creased annealing temperature, peaks corresponding to the (111), (220) and (311) planes get sharper and narrower as a sign of increased crystallinity of the SiGe layer. Moreover, a small peak at a standard Si position (28.45°) is observed at annealing temperatures above 600 °C (Figure 2, selected zoomed view of peak (111) for MLs annealed at 800 °C), along with a shoulder positioned at a standard Ge position (27.45°). Based on these observations, it can be concluded that the structure consists of core–shell NCs/NPs with the core being Ge-rich Si1−xGex NCs (crystallographic peak (111) position, shifts from 27.87° to 27.75° for MLs in as-grown and annealed at