We report the comparison studies of molecular beam epitaxial growth of AlGaAs/Ge/GaAs heterostructures on differently oriented GaAs substrates. We have investigated the molecular beam epitaxial growth of Ge on GaAs, and GaAs and AlGaAs on Ge epitaxial layers with the aim of obtaining device-quality interfaces of AlGaAs and GaAs epilayers on Ge which are free of antiphase domains. Our results show that the junctions grown on (311)B oriented substrates have better electrical characteristics than those grown on (100) oriented substrates. This is due to the absence of antiphase domains and less interface charge in heterostructures grown on (311)B substrates.

The feasibility of peeling the active AlGaAs/GaAs heterojunction bipolar transistor (HBT) epitaxial thin films from the original GaAs substrate and the subsequent van der Waals bonding of the films to AlN, InP and diamond on silicon substrates is discussed. HBTs were first fabricated on epitaxial layers containing an AlAs separating layer. The wafer was divided, and a portion of the wafer was then peeled and van der Waals bonded to various substrates. The van der Waals bond demonstrated excellent thermal and mechanical stability by withstanding subsequent processing steps up to 250°C. The lifted devices bonded to the higher thermal conductivity substrates show excellent DC and RF performance. The average device junction temperature was also characterized. Further improvements can be accomplished by utilizing substrates with a smooth surface morphology having a high thermal conductivity

Short period AlAs/GaAs superlattices, or pseudoalloys, are investigated as insulators in n+ GaAs–insulator–n GaAs structures, or superlattice barrier capacitors (SLBC’s), and as tunneling barriers in superlattice double barrier diodes (SLDBD’s). The energies of the lowest superlattice states in the SLBC’s and the energies of the quantum well quasibound states in the SLDBD’s are determined using thermionic emission analysis. Depending on the GaAs and AlAs layer thicknesses, the lowest states in the SLBC’s may be Γ states in the GaAs layers or X states in the AlAs layers. It is also possible for the barrier to behave as a random alloy. We observe all three cases. The SLDBD’s exhibit strong negative resistance at low temperatures and we find evidence that the superlattice barriers (SLB’s) behave as ideal Γ band structures, regardless of the layer thicknesses. Second current peaks which are attributed to resonant states in the SLB’s appear in the SLDBD I–V curves when the GaAs layers are thicker than 5 monolayers.

A new technique is described to study heterostructure field-effect transistors. This technique is based on the application of an electric field perpendicular to the channel via a p-doped substrate. The measurements demonstrate that the centroid of the two-dimensional electron gas resides approximately 10 nm below the heterointerface and that application of a negative substrate potential increases the confinement of these charges towards the interface. A negative substrate voltage decreases the K′ factor of the transistor and also reduces the well capacity. Capacitance voltage measurements confirm that a parasitic channel in the donor layer can be formed and that it is shielded from the substrate by the two-dimensional electron gas.

Electrical barrier height measurements on n+-GaAs–insulator–n-GaAs structures with short-period AlAs/GaAs superlattices forming the insulator show the effective conduction-band discontinuity (ΔEC) of a superlattice barrier (SLB) to be defined by the lowest superlattice energy state. Five structures with different AlAs and GaAs SLB layer thicknesses are investigated. A SLB with GaAs layers greater than 10 monolayers is found to have a ΔEC defined by Γ-valley states in the GaAs layers, while a SLB with GaAs and AlAs layers less than 10 monolayers and with thicker AlAs layers than GaAs layers is found to have a ΔEC defined by X-valley states in the AlAs layers. The SLB with GaAs and AlAs layers less than 10 monolayers and thicker GaAs layers than AlAs layers behaves as a random alloy. Negative differential resistance is observed in the current-voltage characteristic of the sample whose barrier height is defined by Γ-valley states in the GaAs layers.

Quantum well diodes with barriers formed by thin, short-period binary AlAs/GaAs superlattices were fabricated and found to have very high peak-to-valley current ratios. The effects of varying the AlAs and GaAs layers in the barriers are studied. The peak current density is found to decrease by orders of magnitude for monolayer increases in the AlAs layer thicknesses. Tunneling current peaks due to both resonance levels in the quantum well and resonance levels in the superlattice barriers are observed.

We report the largest peak-to-valley current (PVC) ratios to date from AlGaAs/GaAs double barrier (either alloy barrier or superlattice barrier) diodes. PVC ratios as high as 3.6 and 21.7 were obtained from an AlAs/GaAs superlattice barrier structure at 300 and 77 K, respectively. In an alloy barrier structure with x=0.42 (x=0.3), PVC ratios of 3.9 (2.2) and 14.3 (7.0) were observed at 300 and 77 K, respectively. We attribute these excellent results to a ‘‘two-step’’ spacer layer incorporated in the devices studied which facilitated the growth of high material quality.