xGex Films Research Articles

Influence of germanium incorporation on the structural and electrical properties of boron-doped ultrathin poly-Si1−x Ge x films deposited by chemical vapour deposition

A few properties of polycrystalline silicon germanium (poly-Si1−x Ge x ) films can be tailored by modulating the germanium incorporation. In this paper, the structural, mechanical and electrical properties of heavily doped ultrathin (~100 nm) poly-Si1−x Ge x films (0.84 ≤ x ≤ 0.88) fabricated by low-pressure chemical vapour deposition were investigated. For a boron concentration of ~2.2 × 1021 atoms/cm3, a slight increase of germanium fraction significantly enhances the deposition rate, crystallinity and Hall mobility while having negligible influence on the Young’s modulus and hardness. The grain size increases from ~6 to ~12 nm while the grain structure becomes more columnar. In addition, the resistivity decreases from 7.4 to 1.1 m Ω cm with a corresponding increase in the Hall mobility from ~0.9 to ~4.2 cm2 V−1 s−1. However, the Young’s modulus (~101 GPa) and hardness (~8.8 GPa) are virtually unaffected within the range of germanium fraction explored. In practice, poly-SiGe layer having low resistivity, high modulus, high mobility and low surface roughness can be successfully applied for resonators, biosensors and nanoswitches among others.

Surfactant-mediated epitaxy of silicon germanium films on silicon (001) substrates

We report on the surfactant-mediated epitaxy (SME) of Si1-xGex films with x=0.23–1 on Si(001) using antimony as surfactant. We observe a transition in strain relaxation at a critical composition xT=0.58-0.66. Above this value full relaxation is achieved by a network of full edge dislocation confined to the interface in analogy to SME of pure germanium on Si(001). 100nm thick Si1−xGex films with surface roughness values less than 1nm and abrupt interfaces are obtained, as the surfactant reduces strain induced roughening and hinders interdiffusion.

Temperature dependence of resistance and crystallization in amorphous Si15Te85 − xGex thin films

Amorphous thin chalcogenide Si15Te85−xGex films (x: 5, 9, 10, 11, 12) are prepared by flash evaporation and the temperature dependence of resistance of these films has been studied in the temperature range 25–250°C. All the compositions show a linear variation of resistance in this temperature range. Apart from the linear variation, a sharp reduction in resistance at one or at two distinct temperatures (TTR1/TTR2) is seen. Thin films annealed at these temperatures, when subjected to X-ray diffraction studies suggest that the dominant crystalline phase at TTR1 and at TTR2 is the same and the two dips are associated with varying levels of crystallization. This is also reflected in the atomic force microscopic (AFM) study. Further, the resistance of these two phases shows no drift when the films are annealed for varying lengths of time (10min to 120min) suggesting the stability of the phases.

Epitaxial growth of germanium-rich silicon–germanium films on Si(0 0 1) substrate by reactive thermal chemical vapor deposition

In this paper, a study on the growth kinetics of Ge-rich Si1−xGex films on Si substrate through a reactive thermal chemical vapor deposition (RTCVD) is conducted using Si2H6 and GeF4 as the source gases. The growth temperature is lower than 400°C. The influence of substrate temperature and gas pressure on the microstructure and crystallinity of Si1−xGex epilayer is investigated. By optimizing the growth parameters, high quality epitaxial Si1−xGex layer is fabricated at 350°C, with a threading dislocation density of ∼7×105/cm2 and surface RMS roughness of 1.44nm. The results suggest that the epitaxial Si1−xGex films by RTCVD are preferable materials for low-cost electronic devices.

Low-temperature epitaxial growth of high quality Si1−xGex (x ≥ 0.99) films on Si(001) wafer by reactive thermal chemical vapor deposition

Epitaxial growth of silicon-germanium (Si1−xGex: 0 ≤ x ≤ 1) thin films on Si(001) substrate at low temperature has been investigated using reactive thermal CVD. Si2H6 and GeF4 are used as source gases. The results indicate that Si1−xGex (x ≥ 0.99) epilayer can be prepared directly on Si wafer at 350 °C with the degree of strain relaxation up to 98.5%. Etching reaction between GeF4 and the growth surface plays an important role in both improving the crystal quality and suppressing the propagation of dislocations. High quality epitaxial Si1−xGex has been fabricated with a threading dislocation density of ∼7.0 × 105 cm−2 and RMS roughness of 1.44 nm.

Selective epitaxial growth of Si1−xGex films via the alternating gas supply of Si2H6, GeH4, and Cl2: Effects of Cl2 exposure

Si1−xGex films were epitaxially grown on Si(001) substrates at various ratios of flow rates of Si2H6 and GeH4. Si1−xGex films were also selectively grown on Si windows through modified alternating gas supply manner, repeating the unit cycle which consists of the Si1−xGex growth step and Cl2 exposure step. Injection of Cl2 enhanced the selectivity of the selective growth of Si1−xGex by etching away the spurious nuclei or deposits formed on oxide area. The separate addition of Cl2 resulted in decrease of the growth rate and Ge concentration of Si1−xGex film. Meanwhile, Ge concentration in Si1−xGex films was insignificantly affected by the variation of the flow rates and exposure durations of Cl2.

Facile fabrication of ordered Si1−xGex nanostructures via hybrid process of selective epitaxial growth (SEG) and self-assembled nanotemplates

In this study, a facile fabrication method of the ordered Si1−xGex nanodots (NDs) and nanowires (NWs) was successfully developed via hybrid process of selective epitaxial growth (SEG) of Si1−xGex and self-assembled nanotemplates, i.e., anodic aluminum oxide (AAO) and diblock copolymer (DBC) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA). Si1−xGex films were selectively grown on the Si windows against the oxide area at a growth temperature of 550°C by repeating the unit cycle consisting of two consecutive steps; Si1−xGex deposition step using Si2H6 and GeH4 and Cl2 exposure step for removing the nuclei or deposits formed on the oxide area during the preceding Si1−xGex deposition step. The chemical composition of the Si1−xGex films was readily controlled by adjusting the flow rate of GeH4 from 20sccm to 45sccm while that of Si2H6 gas was fixed at 10sccm, giving rise to the variation of Ge composition in Si1−xGex from 22.2% to 34.0%.In order to fabricate well-ordered Si1−xGex nanostructures, Si windows with hexagonal arrangement were fabricated using AAO and PS-b-PMMA. AAO was prepared through multi-step anodization of the Al films of Al/Si(001) substrate under suitable anodizing conditions. Subsequently, ordered Si windows were fabricated by removing the barrier layer at the bottom of the AAO membrane by reactive ion etching (RIE). In case of PS-b-PMMA, SiO2 templates with ordered Si windows were fabricated through replication of nano-cylindrical pattern of PS-b-PMMA to the 20-nm thick SiO2 layers of SiO2/Si. By utilizing the ordered Si windows obtained from both AAO and PS-b-PMMA, Si1−xGex was selectively grown on Si windows against the oxide area, viz., aluminum oxide in AAO and SiO2 templates. Hexagonally ordered NDs and freestanding NWs were facilely fabricated on the Si substrates after removing the AAO and SiO2 templates.

Enhanced Interfacial-Nucleation in Al-Induced Crystallization for (111) Oriented Si1–xGex (0 ≤ x ≤ 1) Films on Insulating Substrates

Orientation-controlled Si1–xGex (0 ≤ x ≤ 1) films on insulating substrates are essential to achieve high-efficiency solar-cells and high-speed thin-film-transistors. We investigate Al-induced crystallization of amorphous-Si1–xGex/Al-oxide/Al/quartz stacked-structures as a function of Ge-fraction, air-exposure time of Al surfaces, and thicknesses of Al and SiGe films. By tuning interfacial Al-oxide layer thickness, which is controlled by air-exposure time, (100) or (111) Si1–xGex (x < 0.2) layers are selectively formed. However, increase of Ge-fraction significantly enhances the random bulk-nucleation, which results in poly-crystalline Si1–xGex (x ≥ 0.2) with random-orientations. To enhance interfacial-nucleation at Al/quartz over bulk-nucleation, film-thickness of Al and SiGe are thinned to 50 nm. This achieves (111)-oriented Si1–xGex films with the whole Ge fractions. This oriented-growth can be explained by the theoretical calculation that (111) SiGe has the minimum surface energy on quartz substrates.

Defect-engineered Si1−xGex alloy under electron beam irradiation for thermoelectrics

We report the development of a defect-engineered thermoelectric material using Si1−xGex alloys grown on a c-plane sapphire substrate via electron beam (E-beam) irradiation. This paper outlines the idea of growing the Si1−xGex film at relatively high temperatures to obtain good crystalline properties, then controlling the amount of twins or dislocations through ex situ electron-beam irradiation. The current work suggests that structure reconstruction by bond rearrangement through E-beam irradiation may be used for tailoring thermoelectric properties.

Low temperature (~ 250 °C) layer exchange crystallization of Si1 − xGex (x = 1–0) on insulator for advanced flexible devices

Low-temperature (~250°C) layer exchange crystallization of poly-Si1−xGex (x=1–0) films on insulators has been investigated for realization of advanced flexible devices. We propose utilization of Au as catalyst to enhance the crystallization at low temperatures. By annealing (~250°C, 20h) of the a-Si1−xGex (x=1–0)/Au stacked structures formed on insulating substrates, the SiGe and Au layers exchange their positions, and Au/poly-SiGe stacked structures are obtained. The Ge fractions of the obtained poly-SiGe layers are identical to that of the initial a-SiGe layers, and there is no Si or Ge segregation. This low temperature crystallization technique enables poly-SiGe films on plastic substrates, which are essential to realize advanced flexible devices.

Etch characteristics of Si1−x Ge x films in HNO3:H2O:HF

The etch characteristics of Si1−xGex films in HNO3:H2O:HF were examined. The etch rate ratio (etch selectivity) between Si1−xGex and Si escalated with the growth of HNO3 concentration at low concentration level, and when the HNO3 concentration exceeded a critical level the etch selectivity descended with higher HNO3 concentration. The dependence of etch selectivity on the HNO3 concentration was due to the higher critical HNO3 concentration for etching Si than that for etching Si1−xGex. Since the Ge-Ge bond energy was weaker than that of Si-Si and Si-Ge, the Ge atoms were oxidized preferentially once the HNO3 composition exceeded the critical concentration of etching Si1−xGex, which was manifested by the XPS spectra of Si1−xGex etched in HNO3:H2O:HF. When the HNO3 volume rose to another critical value, the significant growth of the Si etch rate lowered the etch selectivity. Although both the etch rates of Si1−xGex and Si dropped with lower HF concentration, the etch rate ratio of Si1−xGex to Si boosted remarkably due to the water-soluble characteristics of GeO2.

Noncontact, in-line measurement of boron concentration from ultrathin boron-doped epitaxial Si1–xGex layers on Si(100) by multiwavelength micro-Raman spectroscopy

Abstract

SIMS quantification of SiGe composition with low‐energy ion beams

AbstractSystematic SIMS analyses with low‐energy (250 eV ∼1 keV) oxygen, cesium and krypton primary beams have been carried out on a set of fully strained uniform epitaxial Si1−xGex films (x = 5 ∼ 60%), as well as a germanium ion‐implanted silicon standard to investigate the matrix effect under various conditions. It is shown that preferential ion yield enhancement of one matrix component over the other can occur as the result of primary ion incorporation. Through defining a matrix yield factor, this work demonstrated that constant secondary ion yield ratios between Si ion and Ge ion over a large concentration range are only valid under some very specific analysis conditions. Emphases were placed on oxygen beam analyses with regard to steady‐state ion yields and surface transients. Both show some unique features only accessible under low‐energy conditions. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Relaxation of misfit strain in silicon-germanium (Si1−xGex) films during dry oxidation

The effects of oxidation on strain relaxation in Si1−xGex layers on silicon substrates were investigated. Si1−xGex layers, with different Ge fractions (x=0.15 and 0.3), were grown on chemically cleaned silicon substrates by an ultrahigh vacuum chemical vapor deposition process. Oxidation at 800 and 900 °C under O2 ambient in a tube furnace resulted in the production of silicon oxide layers on top and a Ge-rich region in the Si1−xGex films. It was observed that the oxidation of Si0.85Ge0.15 films at 900 °C produced the relaxation of the misfit strain in the remnant Si1−xGex layer and the increase in strain in the Ge pile-up layer with increasing oxidation time, while the oxidation at 800 °C produced no changes in the misfit strain in the Si1−xGex layers. The oxidation of Si0.70Ge0.30 films at 800 °C showed the relaxation of the misfit strain in the remnant Si1−xGex layer with the accumulation of stain in the Ge pile-up layer. However, the oxidation of Si0.70Ge0.30 layers at 900 °C exhibited the strain relaxation in the Ge pile-up layer after the Ge pile-up was extended into the remaining Si1−xGex layer, with the formation of misfit dislocations at the interface between the remaining Si1−xGex and Si substrate.

Approaches to improving the accuracy in X-ray fluorescence analysis of Si1 − x Ge x films

This work examines approaches to improving the accuracy in the X-ray fluorescence determination of the composition and thickness of thin Si1 − xGex films on Polikor alumina substrates. We calculate correction coefficients that take into account the interelement effects in the films and demonstrate the possibility of reaching the required accuracy level in determining film composition and thickness. The results are used to optimize conditions for the preparation of films with tailored electrical properties.

Valence band energy in confined Si1−xGex (0.28&amp;lt;x&amp;lt;0.93) layers

Using internal photoemission of electrons we monitored the composition dependent shift of the valence band in Si1−xGex (0.28&amp;lt;x&amp;lt;0.93) films confined between two layers of SiO2, as prepared by the condensation technique. As referenced to the bottom of the oxide conduction band, the increase in Ge content results in upshift in the valence band top with respect to that of pure silicon. This shift occurs nearly linearly with Ge concentration x and can be approximated as δEv(eV)=(0.55±0.05)x.

Low-temperature solid-phase crystallization of amorphous SiGe films on glass by imprint technique

Low-temperature (<500°C) solid-phase crystallization (SPC) of amorphous Si1−xGex (x: 0–0.7) films was examined on insulating substrates by using Ni-imprint technique. Incubation time for SPC was remarkably reduced by catalytic effects without changing growth velocity. As a result, Ni-free large SiGe grains (∼4μm) were obtained at controlled positions. The crystallinity of the grown regions was almost the same as that of poly-SiGe formed by the conventional high temperature SPC at 600°C.

Correlation of structural and magnetic properties of ferromagnetic Mn-implanted Si1−xGex films

We present a comprehensive study relating the magnetic properties to structural properties of Mn+-implanted Si1−xGex films as a function of Ge content (x=0–0.5). Ferromagnetic ordering with three critical temperatures, TB∼10–16K, TC1∼650–780K, and TC2∼825–860K, are reported in this material system. Element specific x-ray absorption fine structure results show that the majority of the Mn ions are nonsubstitutional in all samples. The transmission-electron microscopy coupled with z contrast and chemical analysis reveals the presence of Mn-rich nanosized clusters including Mn4Si7 in Si-rich samples and Mn7Ge3 phases in Ge-rich samples. A composition transition occurred at x∼0.2–0.3, where we observe a change in bond lengths and defect structures. Additionally, an enhancement in magnetizations with an increase in both TB and TC1 as well as a conversion from n-type to p-type conduction are also detected.

Characterization of electrical and structural properties of strained-Si-on-insulator layers

The electrical and structural properties of strained-Si-on-insulator (sSOI) wafers were investigated. The strain, calculated from two-dimensional reciprocal space mapping, was found to be 0.78%, which is comparable to that of fully relaxed Si1−xGex film with Ge concentration of 20.6at.%. Based on the Raman peak shift combined with measured value of strain, the strain shift coefficient is extracted to be −736cm−1. The pseudo-metal-oxide-semiconductor field-effect transistor measurements, employed to characterize the electrical properties of sSOI wafers, showed that both electron and hole mobilities are enhanced by strain. The enhancement factor of electron mobility is larger than that of hole mobility.

Ni-imprint induced solid-phase crystallization in Si1−xGex (x: 0–1) on insulator

Position control of solid-phase crystallization in the amorphous Si1−xGex (x: 0–1) films on insulating substrates was investigated by using Ni-imprint technique. Crystal nucleation at the imprinted positions proceeded approximately 2–20 times, depending on Ge fraction, faster than the conventional solid-phase crystallization, which was due to the catalytic effect of Ni. As a result, large SiGe crystal regions (∼2μm) were obtained at controlled positions. On the other hand, the growth velocity did not changed, which suggested that grown regions contained few residual Ni atoms.