Conservative indentation flow throughout thin (011) InP foils

Abstract

Development of III–V semiconductor heterostructures for optoelectronic applications has been limited, as plastic relaxation deleteriously affects the performance of the devices [1]. As the sizes of the structures decrease progressively, the plasticity of thin films has to be taken into account especially when considering temperatures around 400–500 ◦C necessary for the devices fabrication. Indentation tools allow to test small volumes (for a review refer to [1]), unfortunately nanoindentation at elevated temperature is very difficult to be performed [2] and most studies were reported at intermediate temperatures [3]. Here, we performed microindentation tests at 400 ◦C. Furthermore, contact mechanics was developed for semi-infinite isotropic half space [4]. It has been shown that deviations to the ‘cavity’ model happen when the size of the plastic zone becomes of the order of one dimension of the object [5, 6]. In [6], we showed that when thin foils of (011) InP were indented under increasing loads, material flow in normal {111} slip planes throughout the sample is detected while pile-ups progressively vanished at the indented surface. Here, we report an optical interferential study on indented thin foils of (011) InP, which permits a better understanding of the plastic flow events (pile-up and back side deformation) and demonstrates plastic flow conservation. We used a Czochalski grown single crystal of (011) InP, which was not intentionally doped (n ∼1016 cm−3 Hall effect measurement). One of the faces was epiready, the other one was mechanically and chemically polished using a 1% bromine-methanol solution in order to obtain a 250 μm thick sample with smooth surfaces. The thinned sample was deformed by a Vickers diamond pyramid at 400 ◦C in air atmosphere using five different loads ranging from 0.4 to 2.9 N. For each load the indentation test was repeated twice. The sample was set on a home-designed holder in which a 1.3 mm-wide trench was machined so that the bottom face of the sample underneath the indent site was not in contact with the holder. The load was applied for 60 s on the sample before the indenter was raised. One side of the indentation was aligned along the [011] direction, the other one being aligned along the [100] orthogonal direction. We used a white-light interferometer (Micromap 570 ATOS, Pfungstadt Germany) operating at 580 nm to obtain quantitative images of the surface topography of the indentation sites and of the opposite bottom surface. Fig. 1 shows the surface topography of the indented side (Fig. 1a) and of the back side (Fig. 1b) obtained under the highest load used for this study (2.9 N). These topographical images can be usefully compared to observations made by optical microscopy and in the cathodoluminescent mode of the scanning electron microscope (Figs 2 and 3 in [3], respectively). Interferometry provides 2-dimensional (2D) topography of the plastically deformed zones and allows precise determining of the deformed volumes as discussed below. On the topographic images obtained at the indented surface, we can observe the mark of the indentation that corresponds to levels below the original surface while pile-ups observed around the indent site correspond to levels above the original surface. As discussed below, under large loads pile-ups progressively vanish, however 2D optical interferometry allows detection of them even under the highest loads used here (2.9 N). In order to image the pile-ups, the resolution and the scale have been choosen accordingly. As a result, the indentation center is not properly resolved in the Fig. 1a. In fact the indentation center was detected at −11.5 μm below the original surface. This saturated scale permits enhancement of the contrast because of slight differences in height and detection under a 2.9 N load of four different pile-ups in a twofold symmetry. Along both 〈100〉 directions, two pile-ups form a V -shaped zone with an angular aperture of about 70 ◦. Interestingly, only one V -shaped zone (along [100]) was observed on GaAs (011) surfaces indented at elevated temperature [7]. Pile-ups reveal reverse flow of matter in {111} planes inclined to the indented surface. At the [100] side they correspond to glide in (111)B planes while at the [100] side they correspond to gliding in (111)A planes. Hence, the anisotropy observed in GaAs may be attributed to the stronger polar character of GaAs as compared to InP. In fact, the difference in mobility between α and β dislocations is more important in GaAs than in InP [8]. It should be noted that a slight anisotropy is detected in InP, as pile-ups at the [100] side are less pronounced than at the [100] side.