Thin Si Buffer Research Articles

Formation of poly-crystalline BaSi2 thin films by pulsed laser deposition for solar cell applications

A new growth method for BaSi2 thin films by using pulsed laser deposition (PLD) on transparent SiO2 and CaF2 substrates has been developed. X-ray diffraction and Raman spectroscopy revealed the poly-crystalline property of the deposited films. By introducing a thin Si buffer layer on the SiO2 substrate, the crystalline quality of BaSi2 thin films were improved. BaSi2 thin films exhibited a Ba/Si ratio very close to 0.5, indicating the good stoichiometry control of PLD growth. The absorption coefficient of the poly-BaSi2 thin film reached 105 cm−1 and its band gap was deduced to be 1.32 eV, which are similar to those grown by molecular beam epitaxy or sputtering. A maximum photoresponsivity of 12.5 mA/W was achieved in the BaSi2 thin film, which implies the potential of PLD-deposited BaSi2 for thin film solar cell applications.

Structural properties of templated Ge quantum dot arrays: impact of growth and pre-pattern parameters

In this study we analyze the impact of process and growth parameters on the structural properties of germanium (Ge) quantum dot (QD) arrays. The arrays were deposited by molecular-beam epitaxy on pre-patterned silicon (Si) substrates. Periodic arrays of pits with diameters between 120 and 20 nm and pitches ranging from 200 nm down to 40 nm were etched into the substrate prior to growth. The structural perfection of the two-dimensional QD arrays was evaluated based on SEM images. The impact of two processing steps on the directed self-assembly of Ge QD arrays is investigated. First, a thin Si buffer layer grown on a pre-patterned substrate reshapes the pre-pattern pits and determines the nucleation and initial shape of the QDs. Subsequently, the deposition parameters of the Ge define the overall shape and uniformity of the QDs. In particular, the growth temperature and the deposition rate are relevant and need to be optimized according to the design of the pre-pattern. Applying this knowledge, we are able to fabricate regular arrays of pyramid shaped QDs with dot densities up to 7.2 × 1010 cm−2.

Resistive switching behavior of a CeO2 based ReRAM cell incorporated with Si buffer layer

We propose a novel resistive switching device with a W/CeO2/Si/TiN structure by incorporating a very thin Si buffer layer in the interface, the memory performance of this device such as forming voltage, operation power, and window and endurance characteristics were found to be remarkably improved compared with the performance of the device without the Si layer. This improvement was attributed to the formation of Ce-silicate and thus proper introduction of oxygen vacancies at the interface. The gradual reset process of the W/CeO2/Si/TiN device under sweeping voltage was quantitatively analyzed by parallel conductive filaments model. Our results provide a guideline for the operation voltage control for further optimizing device performance and give new insights into the gradual reset process.

Low temperature boron and phosphorous doped SiGe for recessed and raised sources and drains

We have first of all studied the growth kinetics of boron and phosphorous doped, 20–30 nm thick Si 0.65Ge 0.35 layers, the aim being their integration in recessed and raised sources and drains. Those layers have been grown at low pressure (20 Torr) and low temperature (650 °C) with a heavily chlorinated chemistry (in order to be selective versus SiO 2 (isolation) and Si 3N 4 (sidewall spacers)). We have quantified the impact of the diborane and phosphine flow on (i) the SiGe crystalline quality and strain state, (ii) the amount of boron or phosphorous atoms incorporated in it and (iii) the SiGe:B or SiGe:P growth kinetics. As the diborane flow increases, the SiGe:B growth rate definitely increases, while the real Ge concentration (from Time-Of-Flight Secondary Ions Mass Spectrometry) is more or less steady. B atoms, being much smaller than Si or Ge, partially compensate the compressive strain in the SiGe:B layers, leading to a decrease in the apparent Ge concentration (from X-ray diffraction). Opposite trends are observed for SiGe:P. There is indeed a definite decrease in the SiGe:P growth rate together with a definite increase in both the apparent and real Ge concentration as the phosphine flow increases. The Boron atomic concentration increases linearly with the diborane flow, with however a practical limit around 3.5×10 20 cm −3 in order to stay monocrystalline. Meanwhile, the phosphorous atomic concentration saturates quite rapidly at values close to 8–9×10 19 cm −3 as the phosphine flow increases (most likely due to surface segregation). In the second part we have highlighted some of the difficulties encountered when trying to selectively grow intrinsic SiGe raised S/Ds at 650 °C on patterned, extra-thin Silicon-On-Insulator wafers and some of the solutions identified in order to obtain 2D thick films. Amongst those, we can mention the 750 °C deposition of a thin Si buffer or the in-situ high HCl partial pressure etch at 650 °C of a few nm of Si prior to SiGe Selective Epitaxial Growth (SEG), the drastic reduction of the incoming HCl flow during SiGe SEG, etc.

Self-Assembly of Quantum-Dot Molecules: Heterogeneous Nucleation of SiGe Islands on Si(100)

We report on the formation of clusters of self-assembled quantum dots (termed quantum-dot molecules). Each cluster, typically consisting of four closely spaced SiGe islands, is formed by preferential nucleation around the edges of square pits. Uniform-sized pits are directly formed by controlled deposition of Si and C on the initial Si(100) surface, followed by the growth of a thin Si buffer layer. Formation of ${105}$ pit walls as precursors to island formation and elastic relaxation effects near the pits appear to influence island nucleation. Quantum-dot molecules may have potential applications in nanoelectronic devices and may exhibit novel electronic and optical properties.

Preferential nucleation of Ge islands at self-organized pits formed during the growth of thin Si buffer layers on Si(110)

The epitaxial growth of thin (∼20–40 nm) Si buffer layer on Si(110) leads to the formation of ∼100-nm-wide, uniformly sized faceted pits. The cause of these rhombohedral pits is revealed to be the overgrowth of a homoepitaxial layer over clusters of coherent contaminant particles, possibly SiC. Deposition of Ge on such “pitted” surfaces shows highly selective nucleation of pairs of coherent islands at the opposite corners of the pits along the 〈110〉 direction. Continued deposition leads to strain relaxation of one or both of the islands within the pit which then rapidly coarsen to form a single Ge island within the pit. Our observations offer insight into heterogeneous nucleation mechanisms important for producing controlled arrays of self-assembled quantum dots.

Nucleation and Pattern Formation of Coherent Islands During Epitaxial Growth of Ge on Si(110) Surfaces

AbstractMolecular beam epitaxial growth of Ge on Si(110) surfaces reveals interesting aspects of the heterogeneous nucleation of coherent Ge islands. Cleaning of the Si substrate by desorption of a passivating oxide layer at high temperature creates surface pits. Two sets of experiments, including deposition of Ge on as-cleaned substrates, and surfaces with a thin Si buffer layer are compared to illustrate the nucleation behavior of Ge. Typical Ge deposition temperatures range from 600°C to 725°C.For Ge deposition on as-cleaned surfaces, the faceted edges of pits serve as preferential sites for the heterogeneous nucleation of coherent Ge islands. Experiments were also performed on surfaces with thin (˜20nm) Si buffer layers grown on the as-cleaned surface. Though the faceted pits have not been completely covered by the Si buffer layer, they have decreased in lateral size. In addition, the Si(110) surface shows ledges that are formed along specific crystallographic directions. Ge deposited on the Si buffer nucleates first at the corners of the pits, in an interesting dipole orientation, as well as along the ledges on the surface.

Control of initial surface configuration for GaAs-on-Si MBE using a Si buffer layer?

Gallium arsenide layers were grown by molecular beam epitaxy on (100) Si substrates misoriented by 1.7° and 4° toward [011], with a thin Si buffer layer deposited before GaAs growth for some samples. Reflection high energy electron diffraction observations showed that the substrates misoriented by 1.7° had mostly single-layer steps, while those misoriented by 4° had mostly double-layer steps, regardless of the conditions for the Si buffer layer growth. For 3 μm thick GaAs layers, the measured full widths at half-maximum of the GaAs (400) X-ray reflection ranged from 680 to 870 μrad (about 140 to 180 arc sec), with linewidths about 15% lower for the layers grown on the 4° misoriented substrates. Photoluminescence spectra for the GaAs layers were dominated by carbon-related luminescence, possibly caused by contamination during in situ annealing cycles. However, a GaAs/AlAs superlattice grown on a GaAs-on-Si buffer layer exhibited a single luminescence peak with a linewidth of 8 meV.

Studies of molecular-beam epitaxy growth of GaAs on porous Si substrates

GaAs has been grown on porous Si directly and on Si buffer layer–porous Si substrates by molecular-beam epitaxy. In the case of GaAs growth on porous Si, transmission electron microscopy (TEM) reveals that the dominant defects in GaAs layers grown on porous Si are microtwins and stacking faults, which originate from the GaAs/porous Si interface. GaAs is found to penetrate into the porous Si layers. By using a thin Si buffer layer (50 nm), GaAs penetration diminishes and the density of microtwins and stacking faults is largely reduced and localized at the GaAs/Si buffer interface. However, there is a high density of threading dislocations remaining. Both Si (100) aligned and four tilted substrates have been examined in this study. TEM results show no observable effect of the tilted substrates on the quality of the GaAs epitaxial layer.