D. V. Yurasov

Optical monitoring of technological parameters during molecular-beam epitaxy

It is shown that one can use low-coherence tandem interferometry to measure the substrate temperature during the course of molecular-beam epitaxy in the case of oblique incidence of the probing light onto the surface. The temperature conditions in the Ob’-M installation for growing heteroepitaxial structures of cadmium and mercury tellurides and in the RIBER SIVA-21 installation for the growth of silicon-germanium structures are investigated. Calibration curves relating the readings of the standard thermocouple fixed within the heater to the true substrate temperature in the range 0–500°C are created.

Critical thickness for the Stranski-Krastanov transition treated with the effect of segregation

A new estimation of the critical thickness for the 2D–3D transition during epitaxial growth of the Ge x Si1 − x /Si(001) and In x Ga1 − x As/GaAs(001) systems is suggested. In the estimation, the segregation of atoms in the process of growth is taken into account. For the criterion of such a Stranski-Krastanov transition, the balance between the gain in the energy of elastic strains in the system due to relaxation of the island and the loss in surface energy because of the increase in the surface area is used. In contrast to calculations for previously known criteria, the energy of elastic strains is calculated taking into consideration all deposited layers rather than only one upper layer. The segregation is described in the model of thermally activated exchange of atoms between the surface and the upper layer. A comparison of the critical thickness, calculated for different compositions of the deposited alloy at different temperatures and rates of growth, with the experimental data shows rather good agreement for both systems. The transition mechanisms corresponding to the criterion suggested in the study are discussed.

Features of two-dimensional to three-dimensional growth mode transition of Ge in SiGe/Si(001) heterostructures with strained layers

The results of the study of the effect of strained SiGe layers on the critical thickness of two-dimensional growth of Ge layer in different SiGe/Si(001) structures are presented. A significant influence of buried strained SiGe layer on the growth of Ge has been found out, which remains considerable even for SiGe layers capped by unstrained Si layer of thickness up to 3.5 nm. The experimental results are well described by the proposed model, where obtained features are explained by means of introducing a phenomenological parameter called “effective decay length” of the strain energy accumulated in the structure.

Usage of antimony segregation for selective doping of Si in molecular beam epitaxy

An original approach to selective doping of Si by antimony (Sb) in molecular beam epitaxy (MBE) is proposed and verified experimentally. This approach is based on controllable utilization of the effect of Sb segregation. In particular, the sharp dependence of Sb segregation on growth temperature in the range of 300–550 °C is exploited. The growth temperature variations between the kinetically limited and maximum segregation regimes are suggested to be utilized in order to obtain selectively doped structures with abrupt doping profiles. It is demonstrated that the proposed technique allows formation of selectively doped Si:Sb layers, including delta (δ-)doped layers in which Sb concentrations can be varied from 5 × 1015 to 1020 cm−3. The obtained doped structures are shown to have a high crystalline quality and the short-term growth interruptions, which are needed to change the substrate temperature, do not lead to any significant accumulation of background impurities in grown samples. Realization of the p...

Monolithically integrated InGaAs/GaAs/AlGaAs quantum well laser grown by MOCVD on exact Ge/Si(001) substrate

We report on realization of the InGaAs/GaAs/AlGaAs quantum well laser grown by metallorganic chemical vapor deposition on a virtual Ge-on-Si(001) substrate. The Ge buffer layer has been grown on a nominal Si(001) substrate by solid-source molecular beam epitaxy. Such Ge buffer possessed rather good crystalline quality and smooth surface and so provided the subsequent growth of the high-quality A3B5 laser structure. The laser operation has been demonstrated under electrical pumping at 77 K in the continuous wave mode and at room temperature in the pulsed mode. The emission wavelengths of 941 nm and 992 nm have been obtained at 77 K and 300 K, respectively. The corresponding threshold current densities were estimated as 463 A/cm2 at 77 K and 5.5 kA/cm2 at 300 K.

TOF-SIMS 5 instrument sensitivity to matrix elements in GeSi Layers: Analysis based on recording of complex secondary ions

GexSi1 − x layers are investigated by means of secondary ion mass spectrometry (SIMS). Experimental results obtained with the use of a TOF-SIMS 5 instrument are presented. To surmount the so-called matrix effect, SIMS analysis is performed by using complex secondary ions: Ge2−, CsGe+, and Cs2Ge+.

Secondary cluster ions Ge2− and Ge3−for improving depth resolution of SIMS depth profiling of GeSi/Si heterostructures

New possibilities of improving depth resolution of SIMS depth profiling of GeSi/Si heterostructures using a TOF.SIMS-5 spectrometer are discussed. Contributions of ion sputtering artifacts and instrumental effects to depth resolution were analyzed in detail using a Talysurf CCI-2000 optical profilometer to control the shape and roughnesses of the sputtering crater bottom. It was found that the use of Cs+ ions for sputtering makes it possible to minimize roughness development during depth profiling of GeSi/Si structures to depths of 1–1.5 μm. It was shown that the use of secondary cluster ions Ge2− and Ge3− instead of Ge1− and Ge+ allows narrowing the transition regions in measured profiles.

Antimony segregation in Ge and formation of n-type selectively doped Ge films in molecular beam epitaxy

Antimony segregation in Ge(001) films grown by molecular beam epitaxy was studied. A quantitative dependence of the Sb segregation ratio in Ge on growth temperature was revealed experimentally and modeled theoretically taking into account both the terrace-mediated and step-edge-mediated segregation mechanisms. A nearly 5-orders-of-magnitude increase in the Sb segregation ratio in a relatively small temperature range of 180–350 °C was obtained, which allowed to form Ge:Sb doped layers with abrupt boundaries and high crystalline quality using the temperature switching method that was proposed earlier for Si-based structures. This technique was employed for fabrication of different kinds of n-type Ge structures which can be useful for practical applications like heavily doped n+-Ge films or δ-doped layers. Estimation of the doping profiles sharpness yielded the values of 2–5 nm per decade for the concentration gradient at the leading edge and 2–3 nm for the full-width-half-maximum of the Ge:Sb δ-layers. Electrical characterization of grown Ge:Sb structures revealed nearly full electrical activation of Sb atoms and the two-dimensional nature of charge carrier transport in δ-layers.

Influence of elastic strains in sublayers on the critical thickness of the Stranski-Krastanow transition for the GeSi/Si(001) system

A novel estimation of the critical thickness of the epitaxial layer in the Stranski-Krastanow (SK) transition is proposed. The transition criterion is based on accumulation of the energy of the effective strain up to a certain critical value. The calculation includes the elastic energy stored in elastically strained layers, takes into account the restriction on the depth of the strained layer still affecting the transition, and a segregation effect described by thermally activated atomic exchange. The equations of growth on the vicinal substrate surface that divide each monolayer into submonolayers parallel to the surface are used. Simulation results are compared with the corresponding experimental data for the heterostructures built of a strained sublayer covered by a spacer layer on which a wetting layer is deposited until the SK transition occurs. The Ge/Si structures of this type are grown by molecular beam epitaxy, and in addition, the experimental results for InAs/GaAs systems, published in the literature, are used. A comparison of experimental and calculated data on the dependence between the critical thickness of the wetting layer and the thickness of the spacer layer shows good agreement for both Ge/Si and InAs/GaAs systems.

Quantitative calibration and germanium SIMS depth profiling in GexSi1 − x/Si heterostructures

Methods for minimizing nonlinear matrix effects in the quantitative determination of germanium concentrations in GexSi1 − x layers by secondary ion mass spectrometry are discussed. The analysis conditions with positive SiCs+, GeCs+ and negative Ge−, Si− secondary ions produced during sputtering by cesium ions are used in the TOF.SIMS-5 setup with a time-of-flight mass analyzer. In contrast to published works for TOF.SIMS setups, the linear dependence of the ion-concentration ratio Ge−/Si− on x/(1 − x) is shown. Two new linear calibrations for the germanium concentration as a function of the cluster Ge2− secondary ion yield are proposed. The calibration factors are determined for all linear calibrations at various energies of sputtered cesium ions and Bi+ and probe Bi3+ ions. It is shown for the first time that the best depth resolution among the possible conditions of quantitative germanium depth profiling in GexSi1 − x/Si multilayer heterostructures is provided by the calibration mode using elemental Ge− and Si− negative secondary ions.