H. L. Tsai

Defect reduction by thermal annealing of GaAs layers grown by molecular beam epitaxy on Si substrates

Post growth thermal annealing has been used to reduce the defect density of GaAs layers grown on Si substrates by molecular beam epitaxy. Transmission electron microscopy indicates a 100× reduction of the true defect density. Twins and stacking faults were eliminated entirely. Most misfit dislocations were confined within the first ∼150 A GaAs layer and formed a regular and narrow network along the Si/GaAs interface. Similar results were obtained from an ion implanted and annealed specimen.

Defect structures at the GaAs/Si interface after annealing

High‐resolution electron microscopy was applied to investigate the effect of post‐annealing on the defect structure at the GaAs/Si interface. This study indicates that annealing results in dislocation rearrangement at the interface to form the majority of Lomer’s dislocations with their Burgers vectors parallel to the interface. Dislocations with inclined Burgers vectors (type 2) at the interface after annealing are often observed at steps introduced by the substrate surface roughness. This observation is discussed in terms of the shrinking of stacking faults and microtwins and the preferential nucleation of both stacking faults (or microtwins) and type 2 dislocations at surface steps.

Resonant tunneling through a double GaAs/AlAs superlattice barrier, single quantum well heterostructure

Resonant tunneling has been demonstrated through a double barrier, single quantum well heterostructure in which the barriers have been replaced by thin, short period binary superlattices. The superlattice structure does not exhibit the asymmetry around zero bias in the electrical characteristics normally observed in the conventional AlxGa1−xAs barrier structures, suggestive of reduced roughness at the inverted interface by superlattice smoothing. The structure exhibits an anomalously low barrier height and a peak to valley tunnel current ratio of 1.8:1 at 300 K.

Generation of misfit dislocations in GaAs grown on Si

The misfit dislocation configurations in initial GaAs islands grown on silicon were studied by high‐resolution electron microscopy. Misfit dislocations, especially 60° type and stacking faults, were observed to generate from near the edges of the islands. Large steps on the substrate surface were observed to help the nucleation of these dislocations. The presence of threading dislocations in thicker films is attributed to the misfit dislocation segments in initial GaAs islands. A mechanism is proposed to explain the propagation and multiplication of misfit dislocations during the coalescence of islands and their subsequent growth.

Microstructural characterization of patterned gallium arsenide grown on 〈001〉 silicon substrates

The microstructure of patterned GaAs grown on Si substrates by molecular beam epitaxy has been examined with both transmission and scanning electron microscopies. The GaAs was found to be single crystal with excellent morphology to the limit of the plasma oxide defining mask. In samples where the native oxide was not completely desorbed from the silicon substrate, the GaAs surface morphology was observed to degrade significantly within 20 μm of the single crystal to polycrystalline transition. Even in the region exhibiting a high density of surface defects, the underlying GaAs remained single crystal. Transmission electron microscopy showed a very low defect density in the center of the patterned growth region. The transition from polycrystalline to single‐crystal growth occurred directly above the termination point of the oxide mask.

Patterned growth of gallium arsenide on silicon

The molecular‐beam epitaxial (MBE) growth of GaAs on silicon through openings in an oxide or nitride mask is an attractive alternative to the conventional full wafer deposition of GaAs on silicon for the monolithic integration of GaAs and Si devices. We have investigated techniques for the patterned growth of GaAs on Si and have found that single‐crystal GaAs regions of high‐crystalline quality can be successfully grown on silicon substrates. When the MBE growth is performed through holes in a mask onto the original planar silicon surface, the GaAs transformed from polycrystalline to single‐crystal growth directly above the termination point of the wet etched oxide mask. Postgrowth annealing was found to drastically change the defect structure by causing single‐crystal overgrowth of the mask, an elimination of the transition region defects, and significant grain growth in the remaining polycrystalline GaAs. Growth within etched trenches in the Si substrate also produced GaAs of good crystalline quality, a...

Quantitative Resonant Tunneling Spectroscopy: Current-Voltage Characteristics of Precisely Characterized Resonant Tunneling Diodes

A systematic comparison of precisely characterized resonant tunneling structures is presented. A self‐consistent band bending calculation is used to model the experimentally observed resonant peak positions. lt is found that the peak positions can be accurately modeled if the nominal characterization parameters are allowed to vary within the measurement accuracy of the characterization. As a result, it is found that the asymmetries in the current‐voltage characteristics are solely explainable by tunnel barrier thickness fluctuations.

Effect of post‐growth annealing on patterned GaAs on silicon

We have investigated the effect of post‐growth thermal annealing on the defect surface of patterned GaAs on silicon. Significant improvements in defect structure of the patterned GaAs were observed by transmission electron microscopy (TEM) after short anneal times (15 min) at temperatures as low as 750 °C. At anneal temperatures of 950 °C, long‐range single‐crystal lateral regrowth of the original polycrystalline GaAs deposited over the amorphous patterning mask was observed. Micro‐Raman spectroscopic characterization of the GaAs grown on the oxide/nitride mask corroborated the TEM results by showing a significant improvement in the crystalline quality of patterned GaAs on silicon following post‐growth annealing.

Epitaxial growth of an Al/CaF2/Al/Si(111) structure

Despite their large lattice mismatch (∼25%), epitaxial CaF2 films have been grown on single crystal Al(111) on Si(111) by low temperature molecular beam epitaxy. X‐ray diffraction shows that the orientations of the CaF2 are the same as those of the Al films, whether the orientations of the Al are the same rotated 180° or with respect to the underlying Si substrate. Furthermore, our successful fabrication of an epitaxial Al/CaF2/Al/Si(111) structure suggests that Al can be a useful conductor material in three‐dimensional device integration.

Generation mechanisms of crystal defects in arsenic and phosphorus implanted silicon devices

Arsenic and phosphorus are often implanted into silicon for localized doping during the processing of integrated circuits. The implant damages the silicon lattice, resulting in crystal defects during annealing. Crystal defects observed by transmission electron microscopy (TEM) appear at three different locations in patterned devices. The mechanisms underlying the generation of these defects are presented and discussed, based on an extensive study under various process conditions. These mechanisms form the basis for eliminating/reducing defects in arsenic or phosphorus implanted silicon devices.