2D Electron Systems in Undoped GaAs and InGaAs and Progress Towards Undoped GaAs Nano-Structures

Change log
Ramsay, Benjamin 

The MBE growth of high-quality GaAs/AlGaAs epilayer structures has enabled the study of novel physical phenomena, such as the Quantum Hall and Fractional Quantum Hall in a 2D electron system (2DES), 1D transport, and single-electron transport in 0D systems. The wide range of systems that can be studied all start with a 2DES from which 1D and 0D systems are formed by further confining the carrier gas. Undoped devices, which use an externally applied electric field to form a potential well for carriers, replicating the effect of dopants in a doped device, can have higher carrier mobilities and a lower charge impurity background than doped devices. This gives them advantages in specific applications such as nano-structures where charge impurities can prevent the device

functioning and examining the condition of the MBE system used to grow the material. Be- cause dopants are not needed in undoped devices, material systems were dopants are difficult

to work with due to contamination of growth system or causing significant disorder resulting in low carrier mobility can be studying using undoped devices, side stepping these difficulties. In this thesis, undoped AlGaAs/GaAs wafers allow the fabrication of 2D electron system (2DES) for n-type, p-type and ambipolar devices for studying the Quantum Hall effect in the Al0.33Ga0.67As and In0.1Ga0.9As material systems. The Quantum Hall effect for electrons and holes in a In0.1Ga0.9As quantum well showed remarkable different behaviour to GaAs quantum wells despite the low indium content. Undoped devices have their own fabrication challenges and needed optimisation to produce n-type, p-type, and ambipolar, heterostructures and quantum wells with high enough yields of 2DES that 1D and 0D systems can be fabricated with a reasonable success rate. Functioning 1D p-type channels demonstrate the successful fabrication of undoped nano-structures. The carrier mobility in undoped devices is limited by unintentional dopants included in the structure during growth. This means that the carrier mobility is a measure of the ‘cleanliness’ of the MBE growth system. From the carrier mobility-density curve shapes the dopant source of disorder in the structure can be determined allowing for feedback on the condition of MBE system, not possible with other device and techniques.

Ritchie, David
GaAs, InGaAs, Electron Systems, 2D Electron Gas
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge