We have investigated the fundamental mechanism underlying the hydrogen-induced exfoliation of silicon, using a combination of spectroscopic and microscopic techniques. We have studied the evolution of the internal defect structure as a function of implanted hydrogen concentration and annealing temperature and found that the mechanism consists of a number of essential components in which hydrogen plays a key role. Specifically, we show that the chemical action of hydrogen leads to the formation of (100) and (111) internal surfaces above 400 °C via agglomeration of the initial defect structure. In addition, molecular hydrogen is evolved between 200 and 400 °C and subsequently traps in the microvoids bounded by the internal surfaces, resulting in the build-up of internal pressure. This, in turn, leads to the observed “blistering” of unconstrained silicon samples, or complete layer transfer for silicon wafers joined to a supporting (handle) wafer which acts as a mechanical “stiffener.”

Using scanning tunneling microscopy and thermal He diAraction we have studied the morphology of the vicinal Pt(997) surface, close to the hexagonal close-packed (111)face. While oxygen adsorption induces a step-doubling transition, the clean surface is thermally unstable towards faceting. Above T = 0.5T~the surface is found to undergo partial phase separation into large {II I i facets and regions of undisturbed step regions. The faceted phase is stabilized by a reduction of surface stress through reconstruction of the (111)faces. The faceting is found to proceed via a nucleation-and-growth mechanism.The structure of vicinal surfaces, i.e. , regularly stepped surfaces which are generated by a slight miscut with respect to a low-index plane, has been the subject of many recent studies [1][2][3][4]. They constitute ideal model systems to study the relation between surface structure and surface energy. Nominally, vicinal surfaces are composed of terraces of the low-index orientation separated by a superlattice of parallel steps accommodating the misorientation 8. The total surface energy y(8, T) of the vicinal is given byThe first term yo is the surface energy of the low-index terraces while the second term adds the contribution from each of the monatomic steps, with P being the energy per unit length to form an isolated monatomic step of height h. The last term B accounts for interactions between neighboring steps and aII is the unit vector along the step edge. The nominal structure is only stable if the vicinal orientation is a tangent in the Wulff plot (y versus 8) in polar form [5]. Unstable orientations phase separate into a "hill and valley structure" of coexisting regions of stable low-index orientations (facets).Because of the delicate balance between step and terrace energies also stable vicinals can be subject to morphological changes as a function of temperature or impurity concentration. The ordered superlattice of equally spaced steps is stabilized by the repulsive step-step interaction between neighboring steps. At elevated temperatures thermal disorder through excitation of kinks competes with the order established by the repulsive step-step interaction. This thermal kink proliferation eventually results in a roughening transition of the vicinal surface [6]. Changes in temperature might also modify the terrace and step-free energies, and thereby induce an orientational instability. So far only three examples of thermal faceting of clean surfaces are known: vicinal Si surfaces close to (111) and vicinal Pt surfaces close to (100) where the orientational instability is induced by reconstruction [2,7] and vicinal Pb surfaces close to (111) where faceting is driven by surface melting [8]. More frequently observed are faceting transitions induced by impurity adsorption [9]. In this Letter we report measurements of temperature and impurity induced changes in the surface morphology of the vicinal Pt(997) surface. Nominally this vicinal surface is composed of close-packed (111) terraces separated each 20.2 A b...

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Since the first report of the Unibond process,l there has been much interest in reproducing Si exfoliation by H implantation and in understanding the mechanism leading to such a remarkably uniform shearing. We have previously demonstrated that, contrary to the initial speculation? there are in fact three distinct aspects to the process3 i) The generation of damage to the crystalline material by the implantation; ii) The unique surface chemistry of hydrogen and silicon that drives the thermal evolution of this damage region and; iii) The creation of internal pressure that ultimately causes exfoliation of the overlying Si layer. Therefore, a detailed understanding of the exfoliation mechanism involves the study of initial damage, of H-passivation of various internal structures and of the mechanical forces exerted by trapped gases as a function of hydrogen implantation doseldepth and annealing temperature. In this work, we have used different hydrogen implantation conditions (ion energies ranging from 1 V to 1 MeV and substrate crystallographic orientations) as well as co-implantation of a variety of other elemental species, in combination with novel spectroscopic configurations, to further explore these different mechanistic aspects.Infrared spectroscopy has played a key role in elucidating the microscopic details of the process, due to its high sensitivity and selectivity and inherent non-destructiveness. However, the frequency range accessible was limited to above 1500 cm-1 so that only the Si-H stretching vibrations could be observed. Recently, we have developed novel optical configurations that allow probing of the Si-H bending modes (at -600-650 cm-1) and scissor modes (850-910 cm-I), allowing definitive identification of the different defect modes. Using this approach, in combination with a variety of other techniques, we have been able to definitively show that exfoliation consists of the following distinct mechanistic steps: Above the critical dose of 6 x 1016/cm2, the IR spectrum shows evidence for monohydride-terminated, multi-vacancy defects that are typically found in hydrogenated amorphous silicon. The formation of such a "multivancy" defect region is critical to exfoliation, because it allows both formation of agglomerated defects and the evolution of molecular H2. These defects, in turn, develop into (100) and (1 11) internal cracks which act as traps for the H2 leading to the build-up of internal pressure and subsequent shearing. It is the synergetic combination of H-passivation of internal surfaces and H2 pressure within these intemal cracks that leads to the shearing in the presence of the joined wafer, that acts as a mechanical stiffener. Importantly, in the absence of the stiffener, the surface 'blisters'; in the absence of sufficient damage (below the critical dose), the hydrogen diffuses away from the implanted region, preventing exfoliation.Recent experiments4 have isolated the physical and chemical contributions to exfoliation by co-implanting He, Li and Si along with H and demonstrated that...

Discrete row growth during the initial stage of molecular beam epitaxy of rare gases and metals on the vicinal Pt(997) surface has been observed. The row-by-row growth is revealed by intensity oscillations in thermal-energy atom scattering at grazing incidence. Thermodynamic modeling provides an estimate of the excess binding energy close to the step edges. [S0031-9007(96)01999-0] PACS numbers: 79.20.Rf, 82.65.Dp Manipulating the morphology of epitaxial films through detailed control of the growth kinetics has attracted much interest recently. A primary goal is the layer-by-layer growth of smooth films with abrupt interfaces. The most widely used techniques for monitoring the growth mode are diffraction techniques [1][2][3][4], where the occurrence of intensity oscillations provides unique evidence for the desired two-dimensional (2D) growth. These oscillations in the diffracted or specularly reflected intensity reflect the periodically varying step density of homogeneously nucleating and successively coalescing 2D islands.The presence of substrate steps can suppress the homogeneous nucleation on terraces in favor of heterogeneous step nucleation still permitting smooth two-dimensional growth. Binding energies for adatoms at step sites are in general larger than on terrace sites due to the increased coordination. As a consequence 2D islands preferentially nucleate at steps if the average adatom diffusion length is larger than the terrace width [5][6][7]. In the submonolayer range this can be exploited to grow quasi-one-dimensional systems like quantum wires using substrate step arrays as a template [8,9]. Similar to the growth mode classification in thin film epitaxy different step decoration modes can be distinguished, the occurrence of which depend on the detailed interaction of the adsorbate with the substrate step [10].In the present Letter we demonstrate that the step decoration modes can be studied by specular thermal energy helium scattering at grazing incidence. For a regularly stepped Pt(997) surface we find during the adsorption of rare gases and during the deposition of metals in the submonolayer range oscillations in the reflected helium intensity. These intensity oscillations reflect the sequential growth of rows at the steps (named here discrete row growth or row-by-row growth) during deposition.The vicinal Pt(997) surface, consisting of about 20 Å wide (111) terraces separated by ͑111͒ monatomic steps, has been chosen as substrate because it is known to exhibit a regular step-terrace ordering with a narrow terrace width distribution [11,12]. The initial growth of the rare gas Xe and the metal Ag on this surface has been studied with a novel triple axis He-surface spectrometer [13] in grazing incidence scattering geometry. While the rare gases are adsorbed on the Pt(997) surface from the ambient gas phase, silver is deposited with a molecular beam effusion cell.Xe films on Pt(997) grow in a 2D growth mode. This can be seen from the oscillations in specularly reflected He intensity shown in Fig. ...