Porous silicon (PS) exhibits numerous properties directly related to its microstructure, which in turn can be modified within a broad range of morphologies. Freshly etched PS offers a hydrogen-terminated surface. Due to the high surface area and the high reactivity, such as-etched PS oxidizes easily. It can be oxidized, e.g., by storing in air (native oxide layer) and via thermal or chemical treatment. Oxidation is the main aging aspect and therefore, knowledge about the oxidation state of the surface is of importance. Light illumination decreases the H-termination of as-etched samples. Photoirradiation in an oxygen ambient causes photo-oxidation at the surface and thus accelerates aging of the material. Chemical stability of PS is one of the preconditions rendering the material compatible with any application for which surface modification is desirable.
For practical applications of PS in solar cells, light-emitting diodes, chemical and gas sensors, etc., it is thus desirable to understand the behavior of PS in different ambients. The surface of PS is known to be sensitive to the surrounding environments [1–3]. For example, surface electronic states could be affected by gas species by physisorption, chemisorption, or desorption from the surface [4, 5]. On the other hand, filling of PS with magnetic metals [6, 7] is of interest due to both the distinct properties of the nanosized deposits and the employment of silicon as the base material, key for integration in microtechnology.
In this work, we employed transient surface photovoltage (SPV) to monitor the response of the surface electronic structure of PS to the change of ambience. SPV probes light-induced variations in the electric potential of a studied surface, mostly in semiconductors and insulators . Surface potential barrier in semiconductors is formed due to charges trapped in surface states. The illumination-induced changes of the surface barrier depend strongly on the surface/subsurface electronic structure, which, in turn, can be affected by the physisorbed and chemisorbed species. In transient SPV experiments, the surface potential is monitored as a function of illumination time which can provide information about the different transport mechanisms in semiconductors. SPV is a non-destructive and a highly surface-sensitive tool, which can be operated in different environments. A number of SPV studies on PS were reported in the literature, with most of them performed in ambient air [9–11]. Some authors addressed the influence of the surface chemistry on the SPV response in PS, revealing dependence on the microstructure and chemical environment of the surface [12–14]. However, there was insufficient experimental evidence of the influence of the surface environment (such as vacuum vs. gas) on the SPV response in PS. To address this, in our work, bare PS specimens as well as samples with embedded Ni deposits have been measured by SPV in vacuum and in different gaseous environments (O2, N2, Ar). It was revealed that the illumination-induced charge transport mechanisms were strongly influenced by the experimental ambiences. The behavior of the SPV transients obtained for gaseous environment was significantly different from that observed in high vacuum.