Microstructure evolution and electrical properties of indium arsenide grown epitaxially on gallium phosphide

Vidyut Gopal, Purdue University

Abstract

The microstructural and electrical properties of InAs layers grown by molecular beam epitaxy on 11% lattice mismatched GaP substrates were investigated. Transmission electron microscopy images showed that the initial growth of InAs was in the form of islands. Edge misfit dislocations were introduced directly at island edges. In relatively undislocated islands, the large mismatch strain was partially relieved by elastic deformation. This was more effective far from the interface, so that islands grew vertically. Conversely, strain relief by dislocation introduction promoted lateral growth, and with increasing dislocation content the islands flattened out. Island coalescence left a continuous, partially-relaxed layer at a thickness of 5 nm. Subsequently, strain relaxation continued by threading dislocation loops gliding in pairs, which locked together to form sessile edge dislocations. Large numbers of threading dislocations were introduced during island coalescence. The InAs layer was fully relaxed at a nominal thickness of 30 nm, with an irregular, two-dimensional network of edge dislocations at an average spacing of 3.85 nm. Subsequent annealing equalized the inter-dislocation spacing. Electronic transport in InAs layers was studied by the Hall effect and correlated to the microstructure. Undoped, thin InAs layers had an invariant sheet carrier concentration of 1013 cm−2, indicative of electron accumulation at the InAs/GaP interface. Atomic modeling predicted an 18-atom defect complex at the intersection of 90° misfit dislocations, at a density of 1013 cm−2 . These defects could act as ordered structural donor sources. In thicker samples, the threading dislocation density decreased with epilayer thickness. Consequently, the carrier concentration decreased, and the electron mobility increased with thickness. The temperature invariance of carrier concentration suggested degenerate donor levels. The insensitivity of the mobility to the measurement temperature indicated the dominance of temperature-independent scattering over ionized defect/impurity and phonon scattering. Higher growth temperatures decreased the carrier concentration, enhanced the mobility, and increased its sensitivity to measurement temperature, all of which suggest fewer electrically active defects and temperature independent-scattering centers. Electrochemical capacitance-voltage depth profiles of carrier concentration confirmed the results obtained from Hall measurements. This technique was also demonstrated to be useful to study InAs layers that were doped both n- and p-type.

Degree

Ph.D.

Advisors

Kvam, Purdue University.

Subject Area

Materials science

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