Aimed at controlling the doping of nanowires to the level exceeding the thermodynamical equilibrium limit, the research objectives of this award are: 1) to demonstrate extreme doping levels in semiconductor nanowires comparable to those in the bulk, 2) to show uniform activated nanowire doping along the radial direction; and 3) to break the natural doping propensity of nanowire materials. Outreach activities are proposed to promote diversity in the nanoscience sector by the training and mentoring of scholars from diverse backgrounds. The approach will be to employ an extremely off-equilibrium technique combining ion implantation and pulsed-laser processing to control the doping kinetics and physics in nanowires synthesized by the vapor-liquid-solid method. The proposer?s preliminary results have shown superiority of this technique on all technical metrics in doping silicon nanowires compared to other equilibrium methods. Single-nanowire devices will be fabricated for evaluation of activated dopant concentration and distribution. Finite element modeling will be performed to simulate the electrostatics and electrodynamics of doped nanowires for analyzing experimental data.
If successful, the research will benefit society at large by laying a materials foundation for a new generation of microelectronic and optoelectronic technologies. A new strategy will be discovered to dope semiconductor nanowires at unprecedentedly high concentrations and uniform distribution. The natural doping propensity and disparity of many technologically important semiconductors will be overcome. The achievement would produce significant advancements in the understanding of defect physics and kinetics in nanoscale materials. Demonstration of such doping strategy will promote innovations in semiconductor nanostructure synthesis and device processing. New knowledge will be gained concerning the materials science and processing at the nanoscale.
Summary The research objective of this project is to control and understand the doping of nanowires to a level exceeding the thermodynamic equilibrium limit. At the same time, the project aims to train students and postdoctoral researchers as well as disseminate information on nanotechnology and science to a broader audience in collaboration with community groups and outreach programs. Intellectual Merits: (1) Advances in materials synthesis/processing (a) Group II-VI alloy nanostructures. We synthesized non-equilibrium alloy nanowires based on ZnSe and ZnTe. Electronic structure engineering is essential for producing materials suited for efficient solid-state devices. Mismatched semiconductors offer wide tunability of electronic structure with only a small change in composition. We demonstrated a combined compound-elemental source vapor transport method for synthesis of mismatched alloy nanowires of ZnSe1-xTex across the entire composition range (0 (b) Ultra-long oxide nanowires. We synthesized ultra-long, free standing vanadium oxide nanowires. We also achieved graded doped WxV1-xO2 nanowires. VO2 is a strongly correlated material with a metal-insulator phase transition at 68 degree C, promising tremendous opportunities in application ranging from thermochromics and non-volatile memory to sensor, thermometer, IR detector and bolometer. Appl. Phys. Lett., 100, 103111 (2012); ACS Nano, 5, 10102 (2011). (c) Hole doping and quantum confinement enhanced structural phase. It has been proposed that cubic zinc-blende (ZB) structure of wurtzite (WZ) semiconductors can be stabilized by introducing acceptors with high-lying d orbitals (e.g. Mn doping in GaN). This is because the p-d repulsion between the top of valance band state and the impurity d level is stronger in ZB structure than that in the wurtzite structure, consequently, creating holes costs less energy in ZB structure than in WZ structure, stabilizing the ZB phase. On the other hand, the p-d coupling is enhanced in nanostructures due to confinement of atomic orbital wavefunctions, therefore the stabilization of the cubic phase should be further enhanced in the nanocrystals at lower doping levels than the bulk. In this work, we successfully demonstrated controllable synthesis of InN nanocrystals, which, without Mn doping, has the WZ structure, but converts to the ZB structure when it is doped with Mn. This observation agrees with our first-principles total energy calculations, and lays an important foundation for defects control of crystal phases. To be submitted (2013) (2) Electrical behavior of doped nanostructures (a) Electrostatics in the presence of ionized dopants in nanowires. Using InN nanowires as a model system, we used high-energy alpha particle irradiation to vary the ionized donor concentration, and compared the decrease in mobility and increase in donor concentration to Hall effect results from high-quality thin films. Our results show that for nanowires with relatively high doping and large diameters, controlling Coulomb scattering from ionized dopants should be given precedence over surface engineering when seeking to maximize nanowire mobility. J. Appl. Phys., 110, 033705 (2011). (b) Electro-thermal effects within a nano-scaled thin film stack. Through the model developed, we discovered a fundamentally distinct electrothermal phenomenon that occurs in semiconductors where internal electrical fields interact with a temperature gradient. Steady current vortices and magnetic field develop in the system even without external carrier injection, causing Joule heating and reducing the thermal conductivity by an amount comparable to the electronic thermal conductivity. Phys. Rev. B, 84, 045205 (2011). Broader Impacts: In the past grant year, our group also hosted/participated the following outreach activities. Some photos of these activities can be found at www.mse.berkeley.edu/~jwu/courses.html. Our group, in collaboration with other graduate students in the department, developed interactive outreach demonstrations and projects relating to materials science appropriate for 8-12 year-olds. Our outreach team hosted a 4th grade class on their "college field trip", and implemented these activities as well as discussed our individual paths to science/engineering careers. On 3/26/2012 (and repeated annually thereafter), the PI and group hosted a one-day visit of Techbridge middle- school girls (www.techbridgegirls.org/) to our department and labs. During the visit, the PI’s students and the PI demonstrated experimentally to the visitors how materials behave differently at the nanoscale, how SEM works, how shape memory alloy work, how superconductors work, etc. We then had lunch discussion together followed by a guided campus tour. Each year there is a day in April reserved as the annual Berkeley open house day (the Cal Day, http://calday.berkeley.edu/). The PI gave public lectures to hundreds of visitors on the basic science of energy technologies, and demonstrated to them (with PI’s students) the mechanism of thermoelectrics, batteries, solar cells, etc.
