SiGe Layer Thickness Research Articles

High Resolution Determination of the Ge Depth Profile in SiGe Heterobipolar Transistor Structures by X-Ray Diffractometry

X-ray double crystal diffractometry is used to measure the rocking curve (RC) of SiGe heterobipolar transistor structures using CuKα 400 reflection. The depth profile of the Ge concentration is determined by simulation of RCs with a semi-kinematical theory and fitting the calculated RCs to the experimental one. The problem of achievable accuracy in the determination of the main structural parameters, Si cap layer thickness, total SiGe layer thickness, plateau layer thickness, and the maximum Ge content, is discussed in detail for three different samples by analyzing sets of independent simulations with slightly modified starting conditions. Using only one fitting procedure, it is possible to determine the thickness of the Si cap layer and the total thickness of the SiGe layer with an error of less than ±0.5 nm. The accuracy of the plateau thickness and of the maximum Ge content depends on the total SiGe layer thickness. Errors for the plateau layer thickness of less than ±2 nm and for the maximum Ge content of less than 0.5% are possible.

Observation of graded interfaces in strained Si/Si 1−xGe x layers (0.17 < x < 0.23) using photoreflectance

Photoreflectance has been used to study critical points along the (111) direction of Si/Si 1−xGe x (0.17 ≤ x ≤ 0.23) layers with SiGe layer widths between 30Åand 105Å. Transition energies of the SiGe E 1 and E 1 + Δ 1/E 0′ critic points, found from fits with third derivative functions, increase with decreasing well width. The fitted energies are compared with energies deduced from composition profiles of the SiGe layer showing graded SiGe interfaces. These energies agree well with experiment for SiGe layer thickness> 50Å, and so for these thicknesses the energy shifts can be explained by composition grading of the SiGe interfaces. For SiGe layers < 50Å, the energy shifts can be explained in terms of composition grading and quantum confinement.

Surface Morphology of LPE SiGe Layers Grown on (100) Si Substrates

AbstractWe have investigated the surface morphology of thick SiGe layers grown on Si(100) substrates. SiGe layers containing different Ge concentrations (from 0 to 16 at.%) and having thickness of about 15μm are prepared by liquid phase epitaxy (LPE) method using various growth conditions. The wavelength of undulation of SiGe layers is found to be increasing when we adopt low cooling rates during LPE process. The roughness of the layer does not show any significant change with cooling rate.

Tunable, long-wavelength PtSi/SiGe/Si Schottky diode infrared detectors

We have fabricated p-type PtSi/SiGe/Si Schottky diodes with barrier heights (from photoresponse) that are lowered (relative to PtSi/Si) and highly dependent on the applied bias. The variability in the barrier height is obtained by using the SiGe/Si valence band offset as an additional barrier. When placed in close proximity to the PtSi/SiGe Schottky barrier, the total effective barrier can be altered dramatically by adjusting the applied reverse bias. The voltage sensitivity of the total barrier height can be controlled by the SiGe layer thickness. The voltage-variable barrier heights range, for example, from 0.30 eV at zero bias to 0.12 eV at 2.4 V reverse bias for a 20%, 450 Å thick SiGe layer. This lowest barrier height corresponds to a cutoff wavelength of 10 μm, extending the detection range of PtSi infrared detectors to the long-wavelength range. The quantum efficiency coefficients C1 are normal at this long-wavelength end, but reduced over the rest of the tunable range, because hot carriers have to traverse the entire SiGe thickness in order to be detected. The hot carriers’ energy losses from quasielastic scattering in the SiGe are taken into account in a theoretical model that gives good agreement with data.

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

k-ratios of Ge-Lα and Si-Kα 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 μm. Moreover, a repulsion of holes due to the valence-band offset in a n-Si/SiGe heterostructure was detected.

New approach to the growth of low dislocation relaxed SiGe material

In this growth process a new strain relief mechanism operates, whereby the SiGe epitaxial layer relaxes without the generation of threading dislocations within the SiGe layer. This is achieved by depositing SiGe on an ultrathin silicon on insulator (SOI) substrate with a superficial silicon thickness less than the SiGe layer thickness. Initially, the thin Si layer is put under tension due to an equalization of the strain between the Si and SiGe layers. Thereafter, the strain created in the thin Si layer relaxes by plastic deformation. Since the dislocations are formed and glide in the thin Si layer, no threading dislocation is ever introduced in to the upper SiGe material, which appeared dislocation free to the limit of the cross sectional transmission electron microscopy analysis. We thus have a method for producing very low dislocation, relaxes SiGe films with the additional benefit of an SOI substrate.

Effect of hydrogenation on the luminescence of strained Si1−xGex alloy layers grown by molecular beam epitaxy

Thick Si1−xGex strained alloy layers grown by molecular beam epitaxy (MBE) are investigated using photoluminescence (PL) spectroscopy. Near-band-edge luminescence with well resolved phonon structures is observed for both as-grown and deuterated samples. The low energy broad band frequently encountered in MBE-grown alloy layers is shown to be annihilated by deuteration, giving rise to the no-phonon and phonon-assisted near-band-edge PL peaks. The broad band recovers by annealing at T≥360 °C while the intensity of the near-band-edge luminescence vanishes. Secondary ion mass spectroscopy and the effect of deuterium passivation are used to help locate and assign the defects responsible for the low PL efficiency of MBE-grown thick SiGe layers.

A Novel Approach for The Production of Low Dislocation Relaxed Sige Material.

AbstractIn this growth process a new strain relief mechanism operates, whereby the SiGe epitaxial layer relaxes without the generation of threading dislocations within the SiGe layer. This is achieved by depositing SiGe on an ultrathin Silicon On Insulator, SOl, substrate with a superficial silicon thickness less than the SiGe layer thickness. Initially, the thin Si layer is put under tension due to an equalization of the strain between the Si and SiGe layers. Thereafter, the strain created in the thin Si layer relaxes by plastic deformation. Since the dislocations are formed and glide in the thin Si layer, no threading dislocation is ever introduced into the upper SiGe material, which appeared dislocation free to the limit of the cross sectional Transmission Electron Microscopy (TEM) analysis. We thus have a method for producing very low dislocation, relaxed SiGe films with the additional benefit of an SO substrate. This buffer structure is significantly less than a micrometer in thickness and offers distinct advantages over the thick SiGe buffer layers presently in use.

Strain relaxation and ordering in SiGe layers grown on (100), (111), and (110) Si surfaces by molecular-beam epitaxy

Si0.5Ge0.5/Si superlattices and thick Si0.5Ge0.5 layers grown on (100), (111), and (110) Si surfaces by molecular-beam epitaxy (MBE) exhibit different growth morphologies and defect structures. The best morphology is achieved on (100) surfaces at low temperatures (∼400 °C), while thin and defect-free SiGe layers grown at higher temperatures (∼600 °C) tend to exhibit undulated surfaces due to the mismatch strain. Strained SiGe layers grown on (111) and (110) surfaces are much more susceptible to twin formation. SiGe layers grown on (100) surfaces at low temperatures exhibit a long-range order along the 〈111〉 directions. Our results indicate that such ordering occurs only in thick and relaxed SiGe layers but not in thin SiGe layers strained in a SiGe/Si superlattice structure. No ordering was observed in SiGe layers grown on (111) and (110) surfaces.

Mechanisms controlling growth quality of Si–Ge superlattices in different growth directions

We apply an empirical potential-based approach to the study of growth quality of Si–Ge/Si superlattices in various directions. The particular emphasis is on understanding quantitatively the strain energetics at the interface. Recent experimental work by Kuan and Iyer show that the best growth morphology is obtained in the [100] directions, while twin formation substantially affects quality on [110] and [111] surfaces. We show that twins do not relax misfit strain, as originally thought, but that twin surfaces allow growth to switch over to energetically more favorable twin directions. We obtain the relative stability of strained Si–Ge superlattices for different orientations which are consistent with the predictions of harmonic elasticity theory. The results should also be applicable to III–V semiconductor superlattices. KI also showed that long-range ordering ocurred only on thick relaxed SiGe layers grown on [100] surfaces and not on thin strained-layer superlattices. We present some numerical results and discuss the mechanism for this phenomenon.

The Mechanism of Oxidation of SiGe.

AbstractWe have studied the oxidation mechanism of SiGe alloys, prepared by UHV/CVD. Ge is found to be completely rejected from the oxide and to pile up at the oxide/substrate interface. In the case of thick SiGe layers, this can give rise to very heavily concentrated layers of SiGe (up to 80% Ge) several hundreds of angstroms thick between the oxide and the substrate. The oxidation of Si then proceeds by Si diffusing though the Ge-rich layer. When only a thin layer of Ge is present on top of Si at the start, it acts as a marker, moving, unchanged, with the SiO2/Si interface. The rate of oxidation is enhanced in this case. The atomic motion and chemical reactions involved in the process are discussed and possible mechanisms are described to explain the data. Oxidation of a SiGeB sample is also discussed and compared with the previous case.