Optical and electrical studies of resonant tunneling heterostructures

In this thesis, a new measurement technique is used to study the elec­ tronic properties of resonant tunneling heterostructures. The measurement tech­ nique incorporates an active operational amplifier circuit to simulate negative resistance and capacitance, and is used to probe regions of intrinsic bistability. Intrinsic bistability is observed in the experimental current-voltage characteristics of some resonant tunneling heterostructures when measured using conventional techniques. The tunneling resonance characteristic of an asymmetric, double barrier resonant tunneling heterostructure which displays intrinsic bistability is shown to be smooth, continuous and multistable when measured using the new tech­ nique. It is the multistable region which is the main focus of this thesis. New features are measured within the multistable region, which remain inaccessible to conventional measurement techniques. The new features are shown to be as­ sociated with resonant tunneling via the excitation of a two dimensional plasmon in the central quantum well of the double barrier heterostructure. Such plas­ mon assisted tunneling is a previously unobserved tunneling mechanism in these heterostructures. The first three chapters are concerned with the electronic properties of group III-V semiconductors, a brief background on resonant tunneling, and the experimental techniques employed in the studies. The fourth chapter discusses the details of the new measurement technique, the negative output resistance voltage supply and total circuit stability. The final four chapters examine vertical transport, magnetotransport and optical properties of an asymmetric double barrier resonant tunneling het­ erostructure. Particular emphasis is placed on the bias direction which involves significant charge buildup in the central quantum well, and hence multistabil­ ity, in the tunneling resonance. Significant minority carrier buildup under high excitation conditions is also discussed. Sample temperature variation, magnetic fields applied parallel and perpendicular to the layer plane, as well as photolumi­ nescence spectroscopy are used in these studies. Acknowledgements I would like to express my thanks to the following: My wife, Mignon her continual support and motivation. My sons Aiden and more recently Callum for their added incentive to finish this thesis. My supervisor, Dr David Martin for his enthusiasm, encouragement and ingenious insight. Dr Phil Simmonds for his inspiration and many invaluable discussions. Dr Chao Zhang and Mr Duncan Fisher for their insight into the mysteries of resonant tunneling theory. Tania Silver for many valuable discussions and writing of the data collection programmes. Professor Laurence Eaves of Nottingham University, for providing samples. Dale Hughes, Peter Ihnat, Peter Anthony, Steve Cooper, John Bourke, Tony Powys, Geoff Hurt and Mick Weir for their expert technical advice. My other friends and colleagues in the Physics Department at Wollongong Uni­ versity: Bill Zealey, Xu Wen, Andrew Warner, Steve Wallace, Rodney Vickers, George Takacs, Dave Ryan, Anatoly Rozenfeld, Phil Randall, Paul Nulsen, Glen Moore, Jagdish Mathur, Deidre MacDonald, Roger Lewis, Greg Kaplan, Julie Gilbert, Carey Freeth, Merv Fletcher, Peter Fisher, Martin Carolan and Richard Baker. To the legends of M jrX, GLE and Xfig who made the writing of this thesis a much less daunting task.