A. Nylandsted Larsen

Structural and electrical properties of silicon dioxide layers with embedded germanium nanocrystals grown by molecular beam epitaxy

A sheet of spherical, well-separated, crystalline Ge nanodots embedded in SiO2 on top of a p-(001)Si wafer was fabricated by molecular beam epitaxy (MBE) combined with rapid thermal processing and characterized structurally and electrically. The average size of the Ge nanodots was estimated to be 4.5 nm with an average aerial density of 3×1011 cm−2, situated at 4.4 nm in average away from the Si/SiO2 interface. Significant charge storage effects were observed through capacitance–voltage measurements of metal–oxide–semiconductor capacitors.

Boron and phosphorus diffusion in strained and relaxed Si and SiGe

The diffusion of boron and phosphorus has been investigated in SiGe grown by molecular beam epitaxy. The analysis was done in relaxed Si1−xGex for x=0, 0.01, 0.12, and 0.24 for B and x=0, 0.07, 0.12, 0.24, and 0.40 for P. B diffusion in relaxed samples shows little effect of changing the Ge content while for P diffusion, increasing Ge content increases the diffusion coefficient from Si up to Si0.76Ge0.24. This is explained by a pairing of B and Ge atoms retarding the diffusion. B diffusion in compressively strained Si0.88Ge0.12 and Si0.76Ge0.24 and tensile strained Si and Si0.88Ge0.12 was also investigated. Compressive strain was found to decrease the diffusion coefficient of B and tensile strain to increase it.

Heavy doping effects in the diffusion of group IV and V impurities in silicon

Studies of the diffusion of Ge, Sn, As, and Sb in Si at high donor concentrations provided by phosphorous doping have been performed. It is found that for donor concentrations, CD below ∼2×1020 cm−3, the diffusivity depends linearly on CD; for doping concentrations above ∼2×1020 cm−3, however, the diffusivity increases dramatically with increasing donor concentrations. This behavior has been successfully modeled within the vacancy–percolation model, and it is concluded that collective phenomena play a significant role at high donor concentrations.

The nature of electrically inactive antimony in silicon

Supersaturated solutions of substitutional, electrically active Sb in 〈100〉 silicon single crystals have been obtained by ion implantation, followed by short‐duration incoherent‐light annealing. Substitutional and nonsubstitutional fractions have been studied as functions of implanted dose and anneal temperature by Rutherford backscattering and channeling techniques, transmission‐electron microscopy, Hall‐effect and resistivity measurements (combined with layer removal), and Mossbauer spectroscopy. The maximum electrically active concentration, which can be incorporated on undisturbed substitutional sites, is found to be 4.5×1020 Sb/cm3 for 700 °C annealing. Upon further annealing, the supersaturated solution is reduced and approaches the Trumbore solubility value at temperatures of about 1100 °C. The Sb going out of solution is shown for the first time to be created in two different surroundings: Sb is predominantly found in Sb‐vacancy complexes for low doses and low annealing temperatures and in Sb prec...

Diffusion of silicon in crystalline germanium

We report the determination of the diffusion coefficient of Si in crystalline Ge over the temperature range of 550 to 900 C. A molecular beam epitaxy (MBE) grown buried Si layer in an epitaxial Ge layer on a crystalline Ge substrate was used as the source for the diffusion experiments. For samples annealed at temperatures above 700 C, a 50 nm thick SiO{sub 2} cap layer was deposited to prevent decomposition of the Ge surface. We found the temperature dependence of the diffusion coefficient to be described by a single activation energy (3.32 eV) and pre-factor (38 cm{sup 2}/s) over the entire temperature range studied. The diffusion of the isovalent Si in Ge is slower than Ge self-diffusion over the full temperature range and reveals an activation enthalpy which is higher than that of self-diffusion. This points to a reduced interaction potential between the Si atom and the native defect mediating the diffusion process. For Si, which is smaller in size than the Ge self-atom, a reduced interaction is expected for a Si-vacancy (Si-V{sub Ge}) pair. Therefore we conclude that Si diffuses in Ge via the vacancy mechanism.

Composition dependence of Si and Ge diffusion in relaxed Si1−xGex alloys

Diffusion of silicon (Si) and germanium (Ge) in silicon-germanium Si1−xGex-isotope heterostructures with Ge contents x=0, 0.05, 0.25, 0.45, and 0.70 was investigated in a temperature range between 690 and 1270 °C. The concentration profiles of the stable Si-isotopes and Ge-isotopes were recorded by means of time-of-flight secondary ion mass spectrometry. Analysis of the experimental profiles shows that the Si and Ge diffusion coefficients in elemental Si agree within experimental accuracy. However with increasing Ge content the diffusion of Ge gets increasingly faster compared to that of Si. An Arrhenius type temperature dependence of diffusion is observed for all compositions with slightly lower values for the activation enthalpy of Ge compared to Si. The more pronounced Ge diffusion indicates that with increasing Ge concentration the diffusional jumps of Ge atoms become more successful compared to those of Si. This trend is explained with an increasing contribution of vacancies to self-diffusion in Si1−...

The antimony-vacancy defect in p-type germanium

Ge-n+p mesa diodes have been produced in 2-Ωcm single crystals using a molecular-beam epitaxy (MBE) process to grow the Sb-doped epitaxial Ge n+-top layer. The diodes are characterized by a leakage current at room temperature of 8×10−4A∕cm2 at a reverse bias of 3 V. The diodes have been used to study irradiation-induced defects in p-type Ge, in particular Sb-related defects, where Sb stems from in-diffusion during the MBE growth. Two lines in the deep level transient spectroscopy (DLTS) spectra are related to the presence of Sb. One of these lines originates from the single-acceptor state of the SbV pair with an enthalpy of ionization of ΔHp=(0.309±0.007)eV, the other from a state with an enthalpy of ionization of ΔHp=(0.095±0.006)eV, which is concluded to be the single donor-charge state of the SbV pair.

Strain-relaxed SiGe/Si heteroepitaxial structures of low threading-dislocation density

Abstract A new method of stepwise equilibration for molecular beam epitaxy (MBE) growth of relaxed, low dislocation-density Si 1− x Ge x alloy layers on a (001)-Si substrate is presented. The stepwise buffer is prepared in a layer-by-layer manner. The growth of each layer includes two main stages, a low temperature stage and an average-temperature stage, which allows to separate the processes of nucleation and multiplication of misfit dislocations and the propagation of the threading dislocations. An in situ equilibration annealing at considerably high temperature is implemented before the next growth step to remove the threading dislocations from the layer. The dislocation morphology in these stepwise graded Si 1− x Ge x buffers is investigated by transmission electron and atomic force microscopy. The ability to grow fully strain relaxed, almost dislocation-free, virtual substrates of different compositions is demonstrated.

Self-assembled Al nanoparticles on Si and fused silica, and their application for Si solar cells.

This paper presents a novel method for the self-assembly of aluminum nanoparticles on Si and fused silica. Due to high reactivity with oxygen, ex-vacuo annealing of thin deposited metal films, a method used extensively with other metals, does not work with aluminum. In the present experiment this problem was overcome by annealing the samples in-vacuo in the deposition chamber. Aluminum was thermally evaporated onto substrates at elevated temperatures (200-400 ° C) and annealed for 60 min without breaking the vacuum. It is shown that at 300 and 400 ° C the average particle size can be controlled by adjusting the amount of evaporated aluminum. Particle diameters ranging from 20 to 130 nm are demonstrated. These particles support localized surface plasmon resonances, a property that can be utilized for enhancing the efficiency of thin Si solar cells. This is explored here, and an increase in external quantum efficiency of up to 15% in a thin-film Si solar cell is demonstrated.

Pseudomorphic Si1−xSnx alloy films grown by molecular beam epitaxy on Si

Strained Si1−xSnx (0.001≤x≤0.052) alloys were synthesized on (001) Si substrates by molecular beam epitaxy at 220 and 280 °C. The as‐grown alloys were found to be pseudomorphic to Si with no indication of extended defects and tin precipitates. Within the accuracy of our studies the compressive strain in the alloys corresponds to that deduced from Vegard’s linear interpolation between the lattice parameters of Si and α‐Sn. The annealing experiments show that the alloys are thermally unstable at a higher temperature (1000 °C) and that the transition of the Si1−xSnx/Si system to a lower energy state occurs through two channels: (i) alloy decomposition through precipitation of tin atoms into metallic β‐Sn, and (ii) introduction of 60° misfit dislocations.