Semiconductor Device Fabrication
last updated Sep 10, 2020 tc
Semiconductors are of great importance for applications and industry (transistors, integrated circuits etc.) as well as for fundamental research and future emerging technologies (2D electron gases, quantum Hall effect, spin transistors, qubits, quantum computers). GaAs materials and 2D electron gases in particular are of great interest for fundamental nano electronics and quantum physics research. Hallmark mesoscopic physics such as conductance quantization and the (fractional) quantum Hall effect, for which the Nobel prize has been awarded, have been discovered in GaAs 2D gas materials. Applications in quantum computation and spintronics with GaAs materials are actively pursued around the world. Further, GaAs is in use in various commercial applications, for example high frequency amplifiers, as well as optical applications such as lasers and LEDs. Therefore, fabrication of GaAs nano scale structures is of great importance.
The goal of this course is to provide insight into semiconductor device fabrication and physics with a hands-on experience: with the guidance of a teaching assistant, you will fabricate a gated GaAs Hall bar sample -- from the bare wafer to the completed sample -- in our in-house clean room, and perform some electrical sample characterizations at room temperature and in liquid Helium (4.2 K).
Quantum Transport Experiments
last updated November 26, 2018, tc
Quantum transport is the study of quantum phenomena probed with electrical means, typically using nanoscale devices at low, cryogenic temperatures. Conductance quantization and the quantum Hall effect are some of the most striking and beautiful phenomena discovered in quantum transport experiments. Further, spin qubits for quantum computation realized in quantum dots are currently being developed with quantum transport techniques.
The goal of this course is to provide a first impression of the field of quantum transport by studying one of the phenomena listed below (or a similar quantum effect) in a hands-on experiment in our cryo lab.
The Quantum Hall Effect
Take a gas of electrons confined in a nano scale 2D layer in an appropriate material and apply a (strong) magnetic field perpendicular to the 2D plane. Classically, due to the Lorentz Force, the electrons move in circles in the 2D plane. Quantum mechanically, however, in clean enough samples at low enough temperatures, the integer and fractional quantum Hall effects result, which are manifest in measurable electrical properties such as plateaus at integer or fractional multiples of the conductance quantum e2/h (see Nobel prize 1985 (integer quantum Hall effect, wiki) and Nobel prize 1998 (fractional quantum Hall effect, wiki)).
Apart from fundamental scientific interest in a variety of different integer and fractional quantum Hall states, which are still subject of ongoing studies, the Hall resistance has the additional property of being incredibly precise, allowing a definition of a new practical standard for electrical resistance which is used in resistance calibrations worldwide. The quantum Hall effect also provides an extremely precise independent determination of the fine structure constant, a quantity of fundamental importance in quantum electrodynamics.
Conductance Quantization in 1D Channels
What happens to the electrical conductance of a wire if its diameter is reduced from macroscopic dimensions to nanoscale sizes? The conductance of a macroscopic wire is proportional to its cross section. However, if the diameter becomes small enough, quantum physics will become dominant (at sufficiently low temperature) and the conductance is quantized in multiples of 2e2/h, where e2/h is the quantum of conductance. The factor of 2 arises because of spin-degeneracy. In a magnetic field, spin degeneracy can be lifted, and conductance is then quantized in units of e2/h (rather than 2 e2/h), acting as a spin-filter -- one of the basic building blocks of spintronics.
Quantum Dots and Spin Qubits
A quantum dot is a nanoscale object on which a small number of electrons (down to a single electron) can be confined in all three spatial dimensions, with the possibility of attaching leads for measuring an electron current through the dot. Because of the nanoscale size, the behavior of the dot is radically different from macroscopic objects, displaying quantum mechanical effects such as discrete energy levels (size quantization) and the spin of the electron, as well as classical charging effects (Coulomb Blockade). The electron spin can also be important in transport through quantum dots (e.g. Kondo effect). Further, electron spins in quantum dots are among the leading candidates for future quantum information processors.