In situ characterization of formation and growth of high-pressure phases in single-crystal silicon during nanoindentation Hu Huang 1 • Jiwang Yan 1 Received: 20 December 2015 / Accepted: 5 March 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract Pressure-induced intermediate phases of silicon exhibit unique characteristics in mechanics, chemistry, optics, and electrics. Clarifying the formation and growth processes of these new phases is essential for the prepa- ration and application of them. For in situ characterization of the formation and growth of high-pressure phases in single-crystal silicon, a quantitative parameter, namely displacement change of indenter (Dh) during the unloading holding process in nanoindentation, was proposed. Nanoindentation experiments under various unloading holding loads and loading/unloading rates were performed to investigate their effects on Dh. Results indicate that Dh varies significantly before and after the occurrence of pop-out; for the same maximum indentation load, it tends to increase with the decrease in the holding load and to increase with the increase in the loading/unloading rate. Thus, the value of Dh can be regarded as an indicator that reflects the formation and growth processes of the high- pressure phases. Using Dh, the initial position for the nucleation of the high-pressure phases, their growth, and their correlation to the loading/unloading rate were predictable. 1 Introduction Single-crystal silicon, as an important semiconductor material in scientific research and industrial applications, has received intensive attention from multidisciplinary researchers [1–6]. Pressure-induced phase transformations of silicon are commonly observed during diamond anvil cell and indentation tests [7–15]. Kinds of intermediate phases induced by a contact load have been reported [16– 18], such as Si-II phase (b-tin structure), Si-III phase (body-centered cubic structure with 8 atoms per unit cell), and Si-XII phase (rhombohedral structure with 8 atoms per unit cell). Experimental and theoretical results indicated that these intermediate phases exhibit unique characteris- tics in mechanics [19], chemistry [6], optics, and electrics [20–22]. For example, the Si-II phase has better plasticity than the diamond cubic phase (Si-I phase), which provides the opportunity for ductile machining of single-crystal silicon [19]. The first-principle calculation showed that the Si-XII phase with a narrow band gap has greater overlap with the solar spectrum than other silicon phases [20, 21], which might exhibit improved absorption across the solar spectrum. The Si-III showed a feature of semimetal [22], which has potential applications in multiple exciton gen- eration and next-generation photovoltaics [23]. However, up to now, it is impossible to prepare a significant quantity of these intermediate phases of silicon at ambient pressure. To explore their potential applications in electronic products, photovoltaic cells, and microelectro mechanical system (MEMS), clarifying the formation and transforma- tion mechanisms of these intermediate phases under a contact load is undoubtedly very important. On loading to a pressure *11 GPa during nanoindentation, the Si-I phase transforms into a denser metallic Si-II phase [24, 25]. On unloading, the Si-II phase undergoes further transformation into a mixture of Si-XII/Si-III phases or a-Si phase depending on the unloading conditions [12, 14, 26–28]. Slow unloading is preferred to yield a mixture of Si-XII/Si- III phases whose density is *9 % less than the Si-II phase, and thus discontinuous displacement appears suddenly, & Jiwang Yan yan@mech.keio.ac.jp 1 Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan 123 Appl. Phys. A (2016)122:409 DOI 10.1007/s00339-016-9973-2