Ultra Thin Silicon Nitride Interface Engineering
Fischer, Sergej; Knoch, Joachim (Thesis advisor); Vescan, Andrei (Thesis advisor)
Dissertation / PhD Thesis
Metal-semiconductor junctions are playing a crucial role for almost all semiconductor technology based devices. Here, the main drawback is the formation of a Schottky-barrier at the silicon surface which reduces the transmission of carriers. Conventional measures for a Schottky-barrier reduction are approaching their economical and physical limits. While the most common approach is based on doping by minimizing the effect oft he Schottky-barrier the here in this thesis studied concept focuses on the root of the Schottky-barrier. The incorporation of an ultra thin silicon nitride layer with a thickness of a few atoms (0.8nm) resolves the Fermi-level pinning at the silicon surface. These thermally grown silicon nitride layers offer an excellent and passivated tunneling barrier to suppress the metal induced silicon gap states . Both qualities lead to a decoupling of the Fermi-level and the huge interface state density so that the metal free carrier concentration dominates almost the silicon surface.Ultra thin silicon nitride layers with thicknesses between 0.6nm up to 3.7nm were thermally grown in an ammona atmosphere. These layers are characterized by their excellent thickness conformity on wafer scale, huge etching resistance and high density. The main pillars of this work are dopant-free: metal-oxide-silicon-field effect transistors (MOSFET), diodes, metal-insulator-silicon-solar celss and resonant tunneling diodes realized by ultra thin silicon nitride layers. The ambipolar characteristics of Schottky-barrier MOSFETs is suppressed by the incorporation of an ultra thin silicon nitride layer. Hence, for the first time unipolar N-type and P-type MOSFETs without contact doping are successfully fabricated. Furthermore, the combination of low and high work function metals with silicon nitride enable Schottky-Mott diodes which exhibit forward voltages of approximately 1V, reverse capacitance below 5pF and a temperature coefficient close to 2mV/K proving the existence of a Schottky-Mott diodes at the silicon-silicon nitride-metal. Increasing the insulator thickness reduces the forward voltage. Highly doped substrates lead to an one-side Schottky-Mott junction which exhibits Schottky diode behavior owing to the small junction depth. In addition, it could be demonstrated for the first time that holes can propagate on a long distance in a low doped substrate despite the fact of the carrier freeze-out state.Micro- and nanostencil masks are the keys to satisfy the claim of a highly pure fabrication procedure for Schottky-Mott diodes and increasing the overall yield. For the first time, metal-insulator-silicon solar cells made of ultra thin silicon nitride layer were fabricated and take advantage of the ohmic tunneling junction which demonstrated a fill factor of 0.74. Thinking of energy selective contacts, first-time resonant tunneling diodes consisting of silicon nitride-silicon nanocrystals-silicon nitride sandwiches were realized by crystallization prior to the second nitridation.In summary, this thesis confirmed on various domains of application the cost-effective, dopant-free and successful ultra thin silicon nitride interface engineering.