This thesis presents an experimental and theoretical study of some of the electronic properties and device applications of GaAs/AlxGa1-xAs single and double barrier tunnel structures. In Chapter 2, energy band diagrams are calculated for heterostuctures in which tunneling occurs between two degenerately doped electrodes separated by a single quantum barrier. When a bias voltage is applied to a structure, the energy band profile gives the voltage drop distribution in the cladding layers as well as in the barrier. This distribution may differ significantly from that based on the commonly made assumption that the entire applied voltage drops linearly across the barrier layer. It is shown that band bending effects become more important for larger applied voltages, thicker barriers, smaller electrode doping densities and larger barrier doping concentrations. Energy band diagrams are found to be useful for calculating tunneling currents and determining what the dominant low temperature current transport mechanisms occurring in these structures are. In some cases, they reveal that these mechanisms are different from those predicted when band bending is neglected.
In Chapter 3, elastic and inelastic tunneling processes are investigated in GaAs-AlAs-GaAs single barrier heterostructures grown on [100]-oriented substrates. The GaAs electrodes are degenerately doped n-type with Se, and the AlAs quantum barriers are doped either p-type with Mg or n-type with Se. In p-type barrier structures, low temperature current transport is found to be dominated by elastic and inelastic electron tunneling through the AlAs band gap at the Γ-point and at the X-point. Anomalous zero-bias conductances obtained from several of the samples are also discussed. A theoretical model, which treats trap levels in the AlAs barrier as intermediate states for two-step tunneling processes shows that impurity-assisted tunneling becomes more important as the tunnel barrier is made thicker. In heterostructures in which the n-type barrier layers are thick enough and/or sufficiently doped, the AlAs conduction band at the X-point is not totally depleted of electrons. The dominant low temperature current transport mechanism is then tunneling through two reduced AlAs X-point barriers separated by a bulk region of AlAs. When the n-type AlAs barrier layer is sufficiently thin, the AlAs conduction band remains fully depleted of carriers. As a result, electrons tunnel through the AlAs band gap at the X-point and/or at the Γ-point in a one-step process. In these structures, it is found that plasmons located near the GaAs/AlAs interfaces interact with GaAs and AlAs longitudinal optical (LO) phonons when the doping density in the n-type GaAs electrodes is such that the plasma frequency becomes comparable to the LO phonon frequencies.
Chapter 4 presents a study of resonant tunneling in GaAs/AlxGa1-xAs double barrier heterostructures grown epitaxially in the [100]-direction. In these structures, electrons tunnel through two AlxGa1-xAs quantum barriers separated by a thin GaAs layer forming a quantum well. The resonant energy levels in the GaAs well which produce negative differential resistances in the experimental I-V characteristics are identified by calculating the energy band diagrams of the structures. In samples having pure AlAs barrier layers, tunneling via resonant states confined in the well by the AlAs Γ-point potential energy barriers is often inconsistent with experimental results. However, the experimental data can usually be explained by tunneling via quasi-stationary levels confined in the well by the AlAs X-point potential energy barriers as well as the AlAs Γ-point barriers. The relative contributions of tunneling via resonant Γ- and X-states in the well are found to depend upon the samples studied and sometimes upon the sign of the applied bias. Resonant tunneling is also investigated in double barrier heterostructures in which a low doped GaAs buffer layer is grown before the first AlxGa1-xAs barrier. As a result of this structural asymmetry, the peaks in current corresponding to a given resonant state in the quantum well may be observed in the experimental I-V characteristics at very different applied voltages in reverse bias than in forward bias.
In Chapter 5, we propose and analyze two types of three-terminal devices based upon resonant tunneling through quantum well and quantum barrier heterostructures. The first type includes two configurations in which a base voltage controls the emitter-collector tunneling current by shifting the resonances in a quantum well. In the proposed devices, the relative positions of the base and collector are interchanged with respect to the conventional emitter-base-collector sequence as a means for obtaining negligible base currents and large current transfer ratios. The second type of three-terminal devices includes three configurations in which the current through a double barrier structure is modulated by a Schottky barrier gate placed along the path of the electrons. These devices feature, in their output current-voltage (ID-VD) curves, negative differential resistances controlled by a gate voltage.
Chapter 6 presents a growth uniformity study performed on several of the heterostructures discussed in the thesis. First, the reproducibility and uniformity of the electrical characteristics of GaAs/AlAs tunnel structures are used to show that the doping concentrations and layer thicknesses are uniform across the samples under test. Secondly, discrete fluctuations in layer thicknesses are discussed in GaAs/Al0.35Ga0.65As double barrier heterostructures. These fluctuations are manifested by non-uniform experimental results and by sequences of negative differential resistances in the I-V characteristics of many devices.
