Crystal growth related twins and point defects in semiconductors and dielectrics

Resume : In this work, we investigate the epitaxial lateral overgrowth of micrometer scale GaAs crystals on a 0.6 nm thick SiO2 layer from nanoscale Si seeds in order to develop GaAs monolithic hetero-epitaxy onto (001)Si. The nucleation from small width openings enables to avoid the emission of misfit dislocations and the formation of antiphase domains. The only defects found by transmission electron microscopy in each island are pairs of twins, and a simple model based on the anisotropy of zinc blende crystal is proposed to explain the mechanism of twinning formation during crystal growth. One can expect that the twin formation proceeds as soon as the growing crystal reaches the Si to SiO2 boundary, what alters locally the crystalline order. So, if we consider the lateral growth of GaAs crystal along the [-110] direction, the As atom at the edge of the SiO2 layer cannot bind with a fourth bottom Ga atom, and thus exhibits a dangling bond. In this case, this As atom may "attract" a neighbour Ga atom situated on the SiO2 layer in order to insure the neutrality along the As [110] column at the edge of the SiO2 layer. The resulting bond distortion provokes the anticlockwise rotation of the GaAs lattice by 109.47° around the [110] direction which leads to this {111} twin plane formation [C. Renard, APL 102 (2013) 191915]. Finally, micro-photoluminescence measurements performed at room temperature show that these twins are not detrimental for the quality of microscale GaAs crystals.

Resume : The heterogeneous epitaxy of III-V compounds onto Si substrate has been widely studied for decades in the context of low cost monolithic integration of III-V photonics and PVs on silicon. However, most III-V semiconductors on Si lead to a still very high dislocation density (due to a large lattice mismatch). GaP quasi-lattice matched on Si substrate provides an alternative route for the realization of III-V/Si dislocation-free pseudosubstrates. However, Antiphase boundaries (APB) and microtwins (MT) are still difficult to avoid. In this paper, thorough XRD analyses (lab set-up and synchrotron) of 20-nm and 40 nm-thin GaP layers MBE-grown onto Si substrates are presented and combined with AFM and different TEM techniques in order to quantitatively study these defects originating at the GaP/Si interface. The MT XRD and darkfield TEM analyses allowed the improvement of the growth procedure, with the use of a migration enhanced epitaxy technique combined with a two-step growth procedure, leading to a fractional MT volume down to 0.5% of the total GaP layer volume and a lowering of the APB density. We believe here that APB boundaries are vertically oriented in the first step and kink in the second growth step to favor self-annihilation. GaP growth on an optimized Si buffer allowed STEM-HAADF observations of a sharp GaP/Si with double Si steps. This result is very promising for a APB limitation. Support: ANR projects MENHIRS 2011-PRGE-007-01, SINPHONIC N° 2011 JS03 0.

Resume : This will be the roundtable on twins

Resume : AIIBVI semiconducting materials have a number of unique properties which significantly depend on nano-scale defect type and concentration. Native point defects generating as a result of deviation from stoichiometric composition influence on luminescence, conductivity, and other structure sensitive properties similar to dopants. In the research nonstoichiometry of ultrapure polycrystalline AIIBVI compounds were studied under both S(AB)LV and S(AB)V equilibria. The deviation from stoichiometry was determined by the ?extraction? technique[1]. In the case of CdS, CdSe, CdTe, ZnTe we found out a high-temperature polymorphic modification at pressures less than 106 Pa contrary to low-temperature zinc blende based modification. For the first time the crystal lattice parameters for powdered hexagonal wurtzite-based ZnTe was obtained using experimental XRD data. We suggested the polymorphous transition accompanied by peritectic reactions from both Chalc- and Me-rich sides. We observed a significant widening of homogeneity limits in a high-temperature polymorph which indicated strong disordering in crystal structure. The vacancy mechanism of disordering was discussed. 1. I. Avetissov et al. Nonstoichiometry and luminescent properties of ZnSe crystals grown from melt and vapor. J. Cryst. Growth 2014.DOI: 10.1016/j.jcrysgro.2014.01.003

Resume : The growth of Pb doped LiF crystals is not straightforward and has been reported only very recently in the literature. The present study aims to describe the process of LiF doping with Pb using ab initio methods for structural modeling compared to experimental data from X-ray absorption spectroscopy. The values of formation energy of several complexes involving Pb ions show that the presence of charge-balancing vacancies ease the metal incorporation in the crystal [1]. The metal successively captures the vacancies to form a final stable complex that contains a Pb substitutional atom in a Li site and a Li vacancy. Experimental data from X-ray absorption spectroscopy confirm the presence of Pb in LiF crystals in the latter site revealing both Pb-F bonds and collinear Pb-F-Li atomic configurations. A maximum solubility of Pb in LiF of the order of 3 10^20 cm-3 can be estimated from these calculations [1] F. D'Acapito, P. Pochet, F. Somma, P. Aloe, R. M. Montereali, M. A. Vincenti, and S. Polosan Appl. Phys. Lett. 102 081107 (2013) ; http://dx.doi.org/10.1063/1.4793751

Resume : Solar cells based on the ternary compound CuInSe2 and its related alloys Cu(In,Ga)Se2 are attracting increasing attention with reported conversion efficiency above 20%. The interaction of oxygen in CuInSe2 (CIS) is of practical importance. It has already been established that annealing in air or oxygen of the as-fabricated device based on CIS results in an improvement in its performance. The aim of this paper is to present an analysis of non-radiative defect states in oxygen ion implanted CuInSe2 single crystals using a high resolution near-infrared photoacoustic spectrometer of the gas-microphone type. CIS samples, as-grown n-type, have been implanted with O+ at 40 keV with doses of 1015 and 1016 ions/cm2. PAS spectra have been measured before and after implantation. Oxygen has produced significant changes in PA spectra. A theoretical model based on a two-layer configuration has been developed in order to extract the absorption coefficient spectral dependence of the implanted layer from that of the substrate. The latter has been used to establish ionization energies for several intrinsic and extrinsic defect-related states (shallow and deep) that have been observed in the absorption tail of the spectra.The results obtained here are discussed in the light of previously published data.