C. Levade

Vickers indentation on the {001} faces of GaAs under infrared illumination and in darkness

Abstract Vickers indentation tests have been performed on the (001) faces of GaAs single crystals, in darkness and under laser light illumination with a wavelength close to the band absorption edge. When low loads (0.196 N or less) are applied to the indenter, illumination results in a decrease in the Vickers hardness. This confirms the softening effect of photonic excitation (negative photoplastic effect), as previously reported by Mdivanyan and Shikhsaidov (1988, Phys. Stat. sol., (a), 107, 131) from compression experiments. However, the effect of illumination is less marked on microhardness than on plastic flow. The spectral dependence of the negative photoplastic effect has been investigated; it is shown that the mechanism responsible for the illumination-induced softening is operative on both sides of the band absorption edge. The defect structure around the microindents has been studied by transmission electron microscopy (200 kV and 1 MV), with particular attention to indentation rosettes. Rosette arms (which expand along perpendicular (110) directions) contain perfect dislocations with Burgers vector parallel to the surface, but microtwins are formed only in {111} planes in zone with [110]. Perfect dislocations nucleate in the bulk as elongated half-loops; in contrast, twinning dislocations nucleate on the indented surface. In darkness, perpendicular rosette arms have approximately the same length; the α—β asymmetry is not observed in the experimental conditions (light applied load, room temperature). Under infrared illumination the rosette pattern presents a well marked twofold symmetry; the movement of α dislocations is enhanced under photonic excitation whereas the movement of β dislocations appears rather insensitive to illumination. These results are discussed in connection with the radiation-enhanced dislocation glide mechanisms.

Transmission electron microscopy in situ investigation of dislocation mobility in semiconductors

TEM in situ straining experiments provide a unique way to investigate in real time the behaviour of individual dislocations under applied stress. The results obtained on a variety of semiconductors are presented: numerous dislocation sources are observed which makes it possible to measure the dislocation velocity as a function of different physical parameters (local shear stress, temperature, dislocation character, length of the moving dislocation, ...). The experimental results are consistent with a dislocation glide governed by the Peierls mechanism, even for II-VI compounds which have a significant degree of ionic character. For compounds, a linear dependence of the dislocation velocity on the length of the moving segment is noticed, whereas for elemental semiconductors a transition between a length-dependent and a length-independent velocity regime is observed. Analysed in the framework of the kink diffusion model (Hirth and Lothe theory), these results allow an estimation of the kink formation and migration energies. For a variety of semiconductors, the dislocation behaviour is sensitive to electronic excitations. A strong increase of dislocation mobility with increasing electron beam intensity is observed (radiation-enhanced dislocation glide). It is attributed to a lowering of the lattice friction, due to non-radiative recombinations of electronic carriers at dislocation sites.

Photoplastic effect and Vickers microhardness in III-V and II-VI semiconductor compounds

Abstract The influence of light illumination on the dislocation behaviour in GaAs and ZnS has been investigated by room temperature indentation tests in darkness and under illumination. It is shown that the photoplastic effect (PPE) can be evidenced by this technique providing: (i) small loads are applied to the indentor; and (ii) the illumination wavelength is close to the absorption edge of the semiconductor. Transmission electron microscopy studies indicate that: (i) in GaAs the negative PPE originates in an illumination induced increase of the mobility of α dislocations, due to non radiative recombination of excited carriers at dislocation sites; and (ii) in ZnS the positive PPE originates in an illumination induced increase of the Peierls stress.

Vickers indentation on the {001} faces of ZnS sphalerite under UV illumination and in darkness. Crack patterns and rosette microstructure

Abstract Vickers microhardness indentations are performed on the (001) surface of ZnS sphalerite single crystals in darkness and under UV illumination, close to the band absorption edge. For low applied loads, the crack length and the rosette size confirm the hardening effect of illumination (positive photoplastic effect) that causes a lowering of the dislocation mobility. For high applied loads the positive photoplastic effect is masked by a strong workhardening in the highly strained zone. In darkness as well as under irradiation, perpendicular rosette arms have the same length, showing that α and β perfect dislocations have similar mobilities at variance with III-V compounds. However, the observed asymmetry in the dissociation behaviour suggests that partial dislocations have different mobilities.

Protoplastic effect and vickers microhardness in ZnS sphalerite

A remarkable feature of the plasticity of a number of semiconductors, both elemental and compound, is the photoplastic effect (PPE). The PPE is the phenomenon in which the flow stress in straining tests is affected by illumination by a light near the fundamental absorption edge of the material. Since microhardness is closely related to the plastic behavior of the material, an influence of photon illumination on microhardness is expected in these compounds. In order to get more reliable information on this question, the authors have performed Vickers indentations on (001) faces of sphalerite ZnS, under illumination and in darkness. ZnS was chosen because it shows a very strong positive PPE at room temperature: the compression yield stress increases by approximately 70% under UV illumination. In this paper the authors report on the influence of light illumination on Vickers microhardness.

Dislocation mobility and electronic effects in semiconductor compounds

In situ transmission electron microscopy experiments provide a unique way to investigate in real time the dislocation behaviour at a microscopic scale and to decide which elementary process controls the dislocation glide in semiconductors. In this review the experimental results obtained on different semiconductors are presented and discussed. Particular attention is devoted to the radiation‐enhanced glide process.

TEM in-situ observation of recombination-enhanced mobility of dislocations in II–VI compounds

Abstract The effect of electron-beam irradiation on the motion of dislocations in II–VI compounds has been studied by TEM in-situ experiments. Straining experiments on ZnS samples demonstrate that the dislocation mobility is proportional to the electron-beam intensity. In CdTe, screw dislocation vibrate under the electron beam and finally acquire a serrated form. These observations are discussed in terms of enhancement of dislocation motion due to non-radiative recombination of electron-hole pairs at the dislocation.

⟨310⟩ misfit dislocations in ZnSe/GaAs(001) heterostructure

Strain relaxation in ZnSe/GaAs(001) heterostructure grown by molecular beam epitaxy is studied by transmission electron microscopy. In as-grown samples, an array of perfect misfit dislocations, lying along 310 directions, with Burgers vector (1/2)011 inclined to the interface is observed. The corresponding threading segments propagate by glide in {331} planes, leaving misfit segments in the interface. From a mechanical equilibrium analysis, it is concluded that, in the case of low misfit (0.27%), the critical thickness for {331} planes is less than for {111} glide. Dislocations with the (1/2)011 Burgers vector lying along 310 directions are more efficient at relaxing the misfit strain than dislocations lying along 110 directions.

Transmission electron microscopy study of crystal defects in ZnSe/GaAs(001) epilayers

ZnSe thin films grown on GaAs(001) substrate by molecular beam epitaxy to a thickness of 2500 A have been studied by transmission electron microscopy (TEM). Three types of structural defect have been observed: (i) Triangle-shaped stacking faults, with the apex close to the interface, either isolated or paired. They are bounded by two different Shockley dislocations. (ii) Stacking faults generated from the surface of the ZnSe epilayer by movement of a Shockley half-loop. (iii) An array of perfect misfit dislocations. Their Burgers vectors are inclined to the interface. Most of them lie along 310 directions; only a few are parallel to 110.

Transmission electron microscopy in situ investigation of dislocation behaviour in semiconductors and the influence of electronic excitation

In situ straining experiments in a transmission electron microscope provide a unique way to investigate in real time the influence of various parameters (temperature, electron beam intensity, etc.) on the dislocation behaviour in semiconductors. A systematic study of the influence of electronic excitation on the dislocation behaviour in single phase ZnS crystals is reported. The observed radiation enhanced dislocation motion is attributed to a lowering of the lattice friction, due to non-radiative recombination of carriers at electronic levels associated with dislocations. Analysis of the results makes it possible to determine which elementary mechanism of dislocation glide is affected by this effect. The defect dynamics in a ZnSe/GaAs heterostructure in the course of in situ heating experiments is investigated. Different dislocation mechanisms are analyzed, which emphasize the influence of electronic excitation on the dislocation behaviour. The contribution of these mechanisms to the strain relaxation is discussed.