A thermodynamical model of molecular beam epitaxy, application to the growth of II VI semiconductors
We present a model of molecular beam epitaxy based on thermodynamics. The equilibrium between a compound or a ternary alloy with the gas phase is illustrated in the case of HgTe and CdHgTe. The meaning of equilibrium pressure is analysed and we make the assumption that the fluxes leaving the actual growing compound are linked by the mass action law where the pressures are transformed into the equivalent fluxes. A detailed balance of the species leaving or arriving at the surface is then possible and leads to a set of equations which can be solved to predict the growth rate, the x value of a ternary alloy, the excess of one of the species, the incorporation coefficient. Examples of calculation are shown for CdTe and Cd x Hg 1−x Te and are compared with experimental results and show a good agreement. The main interest of this model resides in its ability to predict the effects of the modification of one of the growth parameter. Nevertheless it predicts only the limit conditions which are not necessary reach due to the kinetic of the reactions Presentation d'un modele thermodynamique de l'epitaxie par jets moleculaires. Illustration par les cas de HgTe et CdHgTe. La comparaison avec les resultats experimentaux montre la validite des previsions du modele, dont l'interet principal est de permettre d'evaluer l'influence de la modification d'un parametre de croissance. La validite de ce modele est cependant limitee si la cinetique des reactions ne permet pas d'atteindre l'equilibre; ce n'est pas le cas pour les alliages II-VI etudies ici
CdTe-HgTe multilayers grown by molecular beam epitaxy
Monocrystalline multilayers CdTe‐HgTe have been grown for the first time using the molecular beam epitaxy (MBE) technique. A multilayer consisting alternately of CdTe (44 A thick) and HgTe (180 A thick), repeated 100 times, has been grown at 200 °C with a good crystal quality. As for epilayers grown by MBE the crystallinity of the multilayers is improved by a raise in substrate temperature; moreover, the crystal quality is higher for HgTe than for CdTe up to 200 °C. An upper limit of 40 A for the interdiffusion depth between HgTe and CdTe layers has been determined from Auger electron spectroscopy and ion microprobe profiling measurements carried out on a 13 1/2 ‐period multilayer, each period consisting of 150 A for the CdTe layer and 400 A for HgTe layer. We have also observed that no decrease occurs in the peak‐to‐valley ratio Auger signals if the focus of the electron gun is maintained at the center of the ion crater, during all the analysis, using the secondary electron image.
ROOM- AND HIGH-TEMPERATURE MAGNETIC PROPERTIES OF Sm(CobalFexCu0.088Zr0.025)7.5 (x=0.30) SINTERED MAGNETS
The effect of Fe content on room- and high-temperature magnetic properties of Sm(CobalFexCu0.088Zr0.025)7.5 (x=0-0.30) sintered magnets, prepared by the conventional powder metallurgy method, has been systematically studied. With increasing Fe content, the remanences BT and maximum energy products (BH)max at room temperature gradually increase and reach optimal values of 1.055 T with x=0.27 for BT, and 205 kJ/m3 with x=0.21 for (BH)max, respectively, and then drop sharply with further Fe increase. An excessive addition of Fe (x≥0.24) results in rapid deterioration of permanent magnetic properties due to the formation of a considerable amount of FeCo soft magnetic phase. It was found that the effect of Fe content on high temperature stability is huge. The lower the Fe content, the smaller the temperature coefficient of coercivity. When x=0.07, a temperature coefficient of coercivity of -0.14%/K (293-723 K) can be achieved, as compared with -0.23%/K for x=0.21. BT of 0.725 T, bHc of 517 kA/m, iHc of 764 kA/m, and (BH)max of 95 kJ/m3 at 723 K were obtained, and the B-H demagnetization curve remains a straight line.