Non-linear solid solution strengthening of InGaAs alloy

Abstract

Solid solution strengthening of III–V semiconductors is one way to reduce dislocation density after crystal growth and to improve the mechanical behavior [1, 2]. Because of their minimal influence on the electrical behavior, isoelectronic dopings (atoms from columns III or V) have been used. Indium doping was reported to improve considerably GaAs mechanical strength [3, 4]. Nonetheless, the hardening mechanism is not yet well understood and depends on the temperature as well as on the doping type and concentration. The following hardening mechanisms have been mentioned: the electrical interaction between impurity clouds and dislocations, the enhanced bonding between the dopant and the Ga or As atoms, the effect of the size of the impurity, the stacking fault energy reduction which induces larger dislocation dissociation (Suzuki effect) and hardens their cross-slip [4]. Nonetheless, only a small range of impurity concentrations has been investigated as the dopants are usually added in concentrations less than 1019 cm−3. In this letter, we report measurements made on a InGaAs alloy with concentrations of In and Ga of the same order. We have compared the mechanical behaviors of this alloy with the behaviors of the binary compositions (InAs and GaAs). The latter were bulk materials. InGaAs alloys were as epitaxial layers. Therefore, we have used the nanoindentation test under low forces that is suited to the micron scale of the layers [5–7]. The plastic zone structure and size generated by nanoindentation were analyzed by transmission electron microscopy (TEM). This approach lets us get a better understanding of the alloy strengthening. 〈001〉 oriented Czochalski-grown single-crystals of undoped InAs and undoped GaAs were used as binary references. Heteroepitaxial InGaAs thick layers (thickness 9 μm) were grown on a (001) surface of InP by chemical beam epitaxy (CBE at 500◦C). The composition of the layer was Inx Ga1−x As with x = 0.516. This composition is very close to the lattice-matched composition (x = 0.532). The layer was just elastically relaxed (no dislocations were observed by TEM) and under a small tension (e=−0.107%). The samples were deformed by a Berkovitch diamond pyramid using a ND100 machine [7]. The tests were performed in the force-control mode of the machine. One of the sides of the pyramid was set parallel to one 〈110〉 crystallographic direction. Indentation arrays were made for subsequent relocalization in the TEM samples. Large indentations were used as markers. The maximum load was varied between 600 and 10 000 μN. For TEM, the undeformed side of the samples (back side) was mechanically and chemically thinned with a bromine-methanol solution until sufficiently thin to transmit the electron beam. Fig. 1 shows loading and unloading curves obtained for the three compositions under about 4500 μN maximum loads. During the indentor loading, both reversible and non reversible deformation occurred. During unloading, the reversible (elastic) deformation was recovered leaving a plastic deformation with a residual depth hr. InAs is the softest materials (largest hr) and is much softer than GaAs in good agreement with the hardness values reported in the literature (3.3 GPa compared to 7.5 GPa) [8]. However, the hardest material is InGaAs, extrapolation yields a Vickers hardness Hv of 14 GPa (Table I). We conclude that a linear dependence of Hv between InAs and GaAs compositions is not valid. In contrast, a much stronger solid solution hardening is obtained in the alloy. Sometimes, a discontinuity in the loading curve (popin) was observed in InAs and GaAs using light loads [9, 10]. Such pop-ins have been observed in several kinds of single-crystal materials [7, 11, 12]. This phenomenon has been attributed to the very poor defect density prior to the indenting test so that the onset of