First principles calculation of the opto-electronic properties of (110) growth axis SiGe superlattices

Using two versions of the first principles full potential linear muffin-tin orbitals method (FPLMTO) which enable an accurate treatment of the interstitial regions, the electronic and optical properties of (110) growth axis Si/SiGe superlattices are investigated. A comparative study with (001) growth axis superlattices is made. In particular, it is found that the bottom of the conduction band (CB) is closer to Γ$\Gamma$ in the (110) system but the optical activity is not enhanced. Furthermore, the absorption spectra of the superlattices are calculated and are found to be quite different from those of bulk Si and Ge but fairly close to their average.     

Nanocrystal growth of single-phase Si1−xGex alloys

We present the formation of single-phase Si_1−xGe_x (x=0.2, 0.4, 0.6, and 0.8) alloy nanocrystals dispersed in a SiO₂ matrix. The studied samples were prepared by co-sputtering with excess Si_1−xGe_x in SiO₂ of approximately 33 at%. Upon heat treatment, crystallization of Si_1−xGe_x alloys was examined by using X-ray diffraction and high-resolution transmission electron microscopy measurements. Single structure of face-centered cubic nanocrystals in a space group Fd-3m was concluded. The average nanocrystal size (from 2 nm to 10 nm) and the lattice constant a of the single-phase Si_1−xGe_x nanocrystals were found to increase with the Ge composition parameter x. Density functional theory-generalized gradient approximation calculation showed the replacement of Ge into the Si sites and vice versa.     

Point-defect associated thermionic hole emissions from p-type Si/Si 1 − x Ge x /Si quantum well structures

The thermionic hole emissions from a p-type Si_0.67Ge_0.33 quantum well with a width of 7 nm and a point defect were investigated using deep level transient spectroscopy. An activation energy of 0.22 eV from the quantum well is consistent with the heavy hole level from the bottom of the well. The defect-related band with an energy of 0.30 eV originated from the space charge related to the point defect in the vicinity of the quantum well heterostructure. The origin of the point-defect-related band was confirmed by photoluminescence and the deep level was further clarified by using capacitance-voltage measurements and simulation by introducing a simple model of an interfacial hole trap center. The deep hole trap center apparently disappeared by an annealing effect, indicating that point defects are subject to thermal annealing. The microscopic measurement provides evidence on point defects in the quantum well structure and the thermal annealing also enhances the thermionic hole emission from the quantum well structure. Received: 22 August 1998 /Accepted: 15 February 1999     

Effect of annealing temperature for Si0.8Ge0.2 epitaxial thin films

This study investigates the effect of annealing temperature on the Si_0.8Ge_0.2 epitaxial layers. The Si_0.8Ge_0.2 epitaxial layers were deposited by using ultrahigh vacuum chemical vapor deposition (UHVCVD) with different annealing temperatures (400-1000 °C). Various measurement technologies, including high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and interfacial adhesion tester, were used to characterize the materials properties of the SiGe epilayers. The experimental results showed that the SiGe epilayers gradually reduced lattice-mismatch to the underlying substrate as annealing temperature increased (from 400 to 800 °C), which resulted from a high temperature enhancing interdiffusion between the epilayers and the underlying substrate. In addition, the average grain size of the SiGe films increased from 53.3 to 58 nm with increasing annealing temperature. The surface roughness in thin film annealed at 800 °C was 0.46 nm. Moreover, the interfacial adhesion strength increased from 476 ± 9 to 578 ± 12 kg/cm² with increasing the annealing temperature.     

Si/Si1−xGex/Si-based quantum wells infrared photodetector operating at 1.55 μm

A quantum well infrared photodetector consisting of self-assembled type II SiGe/Si based quantum wells operating around 1.55 μm at room temperature has been investigated. The Si_1−yGe_y/Si/Si_1−xGe_x/Si/Si_1−yGe_y stack results in a ‘W’ like profiles of the conduction and valence bands strain-compensated in the two low absorption windows of silica fibers infrared photodetectors have been proposed. Such computations have been used for the study of the p-i-n infrared photodetectors operating, around (1.3–1.55 μm) at room temperature. The quantum transport properties of electrons and holes were approved with Schrödinger and kinetic equations resolved self-consistently with the Poisson equation. The theoretical performances of the photodetector were carried out such as the dark current mechanisms, the temperature dependence of normalized dark current and the zero-bias resistance area product (R₀A).     

Comparison of total dose effects on SiGe heterojunction bipolar transistors induced by different swift heavy ion irradiation

The degradations in NPN silicon-germanium （SiGe） heterojunction bipolar transistors （HBTs） were fully studied in this work, by means of 25-MeV Si, 10-MeV C1, 20-MeV Br, and 10-MeV Br ion irradiation, respectively. Electrical parameters such as the base current （IB）, current gain （β）, neutral base recombination （NBR）, and Early voltage （VA） were investigated and used to evaluate the tolerance to heavy ion irradiation. Experimental results demonstrate that device degradations are indeed radiation-source-dependent, and the larger the ion nuclear energy loss is, the more the displacement damages are, and thereby the more serious the performance degradation is. The maximum degradation was observed in the transistors irradiated by 10-MeV Br. For 20-MeV and 10-MeV Br ion irradiation, an unexpected degradation in Ic was observed and Early voltage decreased with increasing ion fluence, and NBR appeared to slow down at high ion fluence. The degradations in SiGe HBTs were mainly attributed to the displacement damages created by heavy ion irradiation in the transistors. The underlying physical mechanisms are analyzed and investigated in detail.     

应变Si1-xGex层中p型杂质电离常温和低温特性

本文采用解析的方法计算了应变Si1-xGex层中p型杂质电离度与Ge组分x、温度T以及掺杂浓度N的关系.发现常温时,在同一Ge组分下,随着掺杂浓度的升高,杂质的电离度的先变小,而后又迅速上升到1.在同一掺杂浓度下,轻掺杂时,杂质的电离度随Ge组分的增加先变大,而后几乎不变;重掺杂时,杂质电离能变为0后,杂质电离度为1.低温下,轻掺杂时,载流子低温冻析效应较为明显,杂质的电离度普遍较小,当掺杂浓度大于Mott转换点时,载流子冻析效应不再明显,电离率迅速上升到1.在同一Ge组分下,随着掺杂浓度的升高,杂质的电离度先变小,后变大,而后又迅速上升到1.在同一掺杂浓度下,轻掺杂时,杂质的电离度随Ge组分的增加变大;重掺杂时,杂质电离能变为0后,杂质电离度为1.     

Analysis of excimer laser annealing of amorphous SiGe on La2O3//Si structures

The reduction of complementary metal oxide semiconductor dimensions through transistor scaling is in part limited by the SiO₂ dielectric layer thickness. Among the materials evaluated as alternative gate dielectrics one of the leading candidate is La₂O₃ due to its high permittivity and thermodynamic stability. However, during device processing, thermal annealing can promote deleterious interactions between the silicon substrate and the high-k dielectric degrading the desired oxide insulating properties.The possibility to grow poly-SiGe on top of La₂O₃//Si by laser assisted techniques therefore seems to be very attractive. Low thermal budget techniques such as pulsed laser deposition and crystallization can be a good choice to reduce possible interface modifications due to their localized and limited thermal effect.In this work the laser annealing by ArF excimer laser irradiation of amorphous SiGe grown on La₂O₃//Si has been analysed theoretically by a numerical model based on the heat conduction differential equation with the aim to control possible modifications at the La₂O₃//Si interface. Simulations have been carried out using different laser energy densities (0.26-0.58 J/cm²), different La₂O₃ film thickness (5-20 nm) and a 50 nm, 30 nm thick amorphous SiGe layer. The temperature distributions have been studied in both the two films and substrate, the melting depth and interfaces temperature have been evaluated. The fluences ranges for which the interfaces start to melt have been calculated for the different configurations.Thermal profiles and interfaces melting point have shown to be sensitive to the thickness of the La₂O₃ film, the thicker the film the lower the temperature at Si interface.Good agreement between theoretical and preliminary experimental data has been found.According to our results the oxide degradation is not expected during the laser crystallization of amorphous Si_0.7Ge_0.3 for the examined ranges of film thickness and fluences.     

Analytical,structural and electrical characterization of SiGe layers by electron microbeam techniques

k-ratios of Ge-L and Si-K measured at different beam energies allow to evaluate simultaneously composition and thickness of SiGe layers on a Si substrate. A simple technique applying backscattered electrons also enables estimation of composition of bulk SiGe and of composition and thickness of relatively thick (200 nm) SiGe layers on Si. Electron channeling patterns of pseudomorphic SiGe/Si structures and of pure Si substrate show no significant differences whereas in relaxed structures a smearing of the pattern with increasing density of misfit dislocations is observed. Under particular conditions the technique of the electron beam induced current permits imaging of recombination-active misfit dislocations with a spatial resolution around 0.2 m. Moreover, a repulsion of holes due to the valence-band offset in a n-Si/SiGe heterostructure was detected.     

Short period (Si6Ge4)p superlattices: photoluminescence and electron microscopy study

Si–Ge heterostructures made of 6 monolayers of Si and 4 monolayers of Ge repeated p times (Si₆Ge₄)_p, strained on Si (1 0 0) substrates, have been investigated by photoluminescence measurements and electron microscopy. The films were grown at 400°C by molecular beam epitaxy, using Sb as surfactant. The photoluminescence results of the whole set of samples show similar spectra, for both the single Ge quantum well (p=1) and the thicker heterostructure (p=30). The phonon assisted transverse optical line is measured at about 40 meV far from the no-phonon one, and this corresponds to the Ge–Ge vibration. Our results demonstrate that excitonic recombination occurs mainly in the Ge layers and it is indirect in nature, whatever the repetition number (p) is. Furthermore, we evidenced a high localization of the photoluminescence process excluding any superperiodicity effect.