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Title: Charge controlled electron transport in nanometer scale semiconductor pillar arrays
Author: Durrani, Z. A. R.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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This dissertation considers the physics of electron transport in novel, nanometer scale semiconductor pillar arrays, where conduction can be controlled by strong surface charging and single electron charging effects. Densely packed arrays were fabricated with a novel natural lithography process where a thin granular metal film acts as a mask for dry etching of double barrier heterostructure (DBHS) and single barrier heterostructure (SBHS) GaAs/AlAs material. A 3nm-7nm thick Au, AuGe, AuPd or Sn film thermally evaporated on GaAs consists of discrete grains less than 50nm in lateral size. Reactive ion etching of the film creates a GaAs pillar array where the individual pillars are 10nm-50nm in diameter. The array packing density is ˜60% over the entire area of the thin film. The pillars were individually characterised with a scanning tunnelling microscope (STM). Single pillar contact measurements demonstrate that each pillar acts as a novel transistor with strong gating of resonant tunnelling peaks in the I-V characteristics. This behaviour can be explained by considering the charge trapped in surface states along the pillar. The surface charge creates an asymmetric potential barrier in the conduction band which gates resonant tunnelling through the energy levels of the DBHS and the energy levels of a bias dependent potential well created by the barrier itself. The pillars were collectively characterised using 'multiple pillar' devices. In DBHS devices, single electron charging effects were observed up to 60K and clear Coulomb staircases up to 35K. Unusual bistable switching behaviour was also observed up to room temperature, where the device could be cycled between a high conductance and low conductance state. The bistability can be explained by considering the device conductivity when the surface states are mostly empty or filled. Empty states trap electrons injected from the contacts and the conductivity is low. With mostly filled states, electrons can move in the conduction band without trapping and the conductivity is high. The DBHS resonant level may switch the device between the bistable states.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available