Band-Gap tuning through mechanical semiconductor heterostructures

Abstract

Band-gap engineering of semiconductor heterostructures has become commonplace for laser diodes and photodetectors. However, the quantum states of these devices are largely fixed during crystal growth. This thesis presents a novel method to control the energy of electron states of surface wells. In essence the method adjusts the thickness of a surface quantum well through controlled interaction with a second well. A cantilever with a quantum heterostructure on its underside collapses on top of an identical heterostructure. The air gap between the well serves as a potential barrier and its width determines the interaction between the wells. At the tip the electron gases of both wells overlap to form a well of their combined width. Along the cantilever, the varying air gap dictates the interaction between the wells. A transition zone forms where the electronic configuration changes from the fully coupled case to the case of two individual wells. After successfully releasing cantilevers, interferometric measurements showed that the shape of the collapsed cantilevers matches with theoretical calculations. Van-der-Waals forces across the 125 nm wide air gap do not affect its shape. An actuator to adjust the well separation is demonstrated. It provides vertical deflections from 17 nm towards to 5 nm away from the surface with atomic resolution. Photoluminescence experiments at 4.2 K investigate the energy of electron states. Quantum coupling is demonstrated in 200 Å wide surface quantum wells. The energy shift of up to 12 meV matches well with theoretical calculations.