P. S. Dobson

Oxidation, defects and vacancy diffusion in silicon

Abstract The mechanism causing extrinsic faults to grow into a silicon wafer during thermal oxidation has been investigated by annealing experiments on thin electron microscope foils. The defects grow on annealing in air at 1100°C and shrink on annealing in vacuo at the same temperature; this behaviour is explained in terms of the diffusion of vacancies between defect and surface. The sense of this flow is dependent on the vacancy concentration in equilibrium with the surface which is reduced to approximately 0·8 of the bulk equilibrium value because the vacancies are annihilated by the inward-growing oxide. During oxidation the faults emit vacancies to the surface, causing fault growth, whereas the vacancy flow is reversed on annealing in vacuo. The activation energy for fault shrinkage has been determined to be 2·1 ev, which is consistent with pipe diffusion along the core of the bonding Frank dislocation. The shrinkage rate of these faults at constant temperature is proportional to the equilibrium vaca...

Climb kinetics of dislocation loops in aluminium

Abstract The climb kinetics of dislocation loops in thin foils in terms of either the emission of vacancies from the loop or the diffusion of vacancies to the surface has been critically examined. Although both mechanisms give similar rate equations, predictions can be made which allow the rate controlling process to be determined experimentally. The annealing behaviour of faulted and prismatic loops in quenched aluminium has been studied using quantitative electron microscopy techniques and shown to support a theory based on vacancy diffusion. Hence by comparing the annealing rates of faulted loops with those of prismatic loops as a function of temperature the stacking-fault energy of aluminium has been determined to be 135±20 ergs/cm2. This value is shown to be consistent with the physical properties of aluminium in relation to other metals.

Defect structure of degraded GaAlAs-GaAs double heterojunction lasers

Abstract Detailed transmission electron microscopy contrast analysis is presented revealing the interstitial nature of the defect networks associated with the active region of a degraded double heterojunction GaAs laser. Three different dislocation networks with Burgers vectors of (a) a/2[011] inclined at 45° to the (001) junction plane, (b) a/2[110] lying in the junction plane and (c) a[001] normal to the junction plane have been observed and investigated. A model is proposed showing how restricted climb of dislocations threading the active region could explain the growth of dislocation networks of the types (a) and (b) and would account for the observed preferred ⟨100⟩ and ⟨110⟩ orientations of darkline defects.

The influence of a dilute magnesium addition on the growth ant shrinkage of dislocation loops in aluminium

Abstract The influence of a dilute magnesium addition to pure aluminium on the climb of dislocation loops has been studied by isothermal annealing of thin foils of Al-0·65% Mg. Faulted loops were observed to shrink at temperatures above 130°C, whereas prismatic loops above a critical size grew at temperature? ≳200°C; in pure aluminium they generally shrink. The various mechanisms previously proposed to account for excess vacancies have been critically examined and shown to be inconsistent with the experimental observations, and an alternative mechanism based on the growth of a surface oxide is proposed. Experimental observations consistent with the oxide growth mechanism are described. The variation of shrinkage rate with temperature for both prismatic and faulted loops over a wide temperature range has been determined. The results, which do not conform to emission controlled kinetics, have been analysed in terms of diffusion controlled behaviour to deduce an accurate value for the intrinsic stacking-faul...

The nature of defects in n+ gallium arsenide

Abstract Faulted defects and prismatic loops have been observed in gallium arsenide heavily doped with tellurium or selenium. It has been shown by contrast analysis that both types of defect are of interstitial character in contrast to previous conclusions that the prismatic loops were of vacancy type. The prismatic loops are composed of equal numbers of gallium and arsenic interstitials whereas the faulted defects are thought to be associated with layers of gallium telluride. The results of electron-irradiation experiments support the conclusion that the two types of defect are chemically different.

Formation of gallium interstitials during zinc diffusion into gallium arsenide

Abstract Transmission electron microscopy of transverse-section specimens has been employed to study the effect of zinc diffusion on the defect structure of tellurium-doped GaAs. Two diffusion sources were employed. Following diffusion from a Ga5A85oZn45 source at 720°, the diffused region was found to be defect-free, while the non-diffused region contained a high density of perfect and faulted loops typical of heat-treated Te-doped GaAs. Using aZn28AS72 source at 860°, however, it was found that several defect zones were produced, the extent and defect density of which were dependent on the source-weight to ampoule-volume (SW/AV) ratio. The first zone, extending in from the surface, was free from microdefects and was followed by a zone consisting of perfect dislocation loops whose size and density increased with increasing depth. In the third zone, which lies near the diffusion front, a high density of large perfect and faulted dislocation loops was found to be present. Beyond this zone the smaller loops...

The effect of oxidation on anomalous diffusion in silicon

Abstract The diffusion of impurities into silicon is usually carried out in the presence of an oxidizing environment and an oxide film is therefore formed on the surface of a silicon wafer during the diffusion process. The growth of this oxide takes place at the oxide-silicon interface and in order to accommodate the inward growing oxide, some of the material near the interface must be transported into the silicon matrix. This material flow will contain some impurity atoms and thus gives rise to an additional component in the impurity flux. The effect of this will be most apparent near the interface where the impurity concentration is greatest and gives rise to enhanced impurity diffusion. A model based on this effect has been analysed in detail and shown to account quantitatively for the anomalous profiles reported by other workers. For an interstitialcy diffusion mechanism an equation describing the profiles can be readily derived but it is necessary to treat the problem by irreversible thermodynamics a...

Oxidation-vacancy production in aluminium alloys

Abstract The climb of prismatic dislocation loops in dilute aluminium alloys has been investigated as a function of alloying addition by isothermal annealing of thin foils. It was observed that the loops annealed out by shrinking in all the alloys investigated except for aluminium-magnesium where loop growth occurred. It is proposed that the vacancy supersaturation necessary for the observed growth is due to the production of vacancies in the surface regions of the foil as magnesium atoms become absorbed by a growing surface oxide film. This model has been critically examined by investigating the rate of growth as a function of annealing time, annealing temperature and the oxygen pressure of the annealing environment. The results confirm this model of vacancy production and show that in the alloys containing 0·65% magnesium, diffusion of magnesium to the surface regions is the rate-controlling process whereas in the more concentrated alloys (3·3% and 6·6% Mg) surface oxidation is rate controlling.

The structure of rod defects in boron-implanted silicon

Abstract A transmission electron microscopy study of defects in boron-implanted silicon has demonstrated the existence of interstitial type defects, having a Burgers vector of 1/6⟨411⟩ and containing an intrinsic stacking fault. The defects have a rod-like shape and are elongated in the ⟨110⟩ direction which is perpendicular to the Burgers vector. This elongation is shown to be consistent with preferential growth arising from the different dissociation reactions undergone by the dislocation segments enclosing the loop. The generation mechanism of these defects is discussed.

Climb asymmetry in degraded gallium arsenide lasers

Abstract The dislocation networks associated with 〈100〉 ‘dark-line defects’ in GaAs consist of climb dipoles with an elongation E perpendicular to both the [001] junction-plane normal and the defect Burgers vector. Defining E as a unit vector parallel to the direction of ‘maximum’ elongation, the 〈100〉 networks have been fully analysed and have been found to exhibit an anisotropic growth which reflects the symmetry of the zinc-blende lattice. The polarity of the lattice has been determined and the crystallography of the dipoles identified. The anisotropic climb is accounted for by a difference in the dislocation core structures of opposite-sign parallel dislocations.