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
Evidence that arsenic is incorporated as As4 molecules in the molecular beam epitaxial growth of Hg1−xCdxTe:As
Molecular arsenic, As4, is commonly used as the source for in situ p-type doping of Hg1−xCdxTe grown by molecular beam epitaxy. As incorporated, the arsenic is strongly self-compensated, requiring annealing for its p-type electrical activation. Here, a quasithermodynamic model is used to interpret the dependence of the arsenic concentration, cAs, as measured by secondary ion mass spectroscopy, on the incident As4 and Hg fluxes during growth. The results strongly suggest that the As4 is absorbed in its molecular form rather than being dissociated on the growth surface, as has previously been assumed. This clearly is relevant to the self-compensation of the arsenic in as-grown Hg1−xCdxTe.
Molecular beam epitaxy of II–VI compounds: CdTe
Abstract The Molecular Beam Epitaxy (MBE) of CdTe on CdTe(111) and (100) orientations has been investigated. Under thermodynamic conditions the growth of CdTe occurs at a temperature above 450°C. This temperature is too high when the epitaxy of CdTe is required in an infrared device where Cd x Hg 1 − x Te is involved. The substrates are polished and etched in a Br 2 - methanol solution before an in-situ Ar bombardment cleaning preceding a short annealing at 300°C. The evaporation ov CdTe is very nearly congruent and can be performed in a single effusion cell. For both orientations the RHEED diffraction patterns become smoother with growth which can be related to a two-dimensional growing mechanism. On the (111) orientation the epitaxy is well established up to 200°C and can be obtained at room temperature after a good preparation of the surface. The surface structure is a CdTe(111)2 × 2. On the (100) orientation the behaviour is rather different since below 80°C films become polycrystalline with a textured structure. Epitaxial films of good crystalline quality with thickness up to several microns present mirror-smooth and clean surfaces. These films are suitable for technological studies.