GaAsSb alloys lattice-matched to InP substrate have been used in various electronic and optoelectronic applications due to their highly desirable band alignment for high-speed double heterojunction bipolar transistors. There is however an issue with GaAsSb alloys, composed approximately of 50% As and 50% Sb, lattice-matched to an InP substrate; it exhibits a miscibility gap, which is a significant problem for crystal growth. This paper addresses the effect of substrate tilting on the material properties of GaAsSb alloys closely lattice-matched to InP substrates by molecular beam epitaxy (MBE). InP(100) substrates tilted 0°off-(on-axis), 2°off-, 3°off-, and 4°off-axis were used for MBE growth, then the material qualities of GaAsSb epitaxial layers were compared using various techniques, including high resolution X-ray diffraction, photoluminescence (PL), Raman scattering, and transmission-line measurements (TLM). Substrate tilting improved the GaAsSb alloys with crystalline quality, shown by a narrower x-ray linewidth and enhanced optical quality as evidenced by a strong PL peak. The results of TLM show that the lowest sheet resistance was achieved at a 2° off-axis tilt. The results are expected to be applicable in devices that incorporate GaAsSb in the active layer grown by MBE.

Low resistance ohmic contacts are important for high frequency applications of InP-based heterojunction bipolar transistors and high electron mobility transistors. In this paper, the authors investigate the use of an InAs layer as the low-resistance ohmic contact to these heterostructure devices. Selective area crystal growth of InAs on a dielectric [benzocyclobutene (BCB) polymer] covered InP(100) was carried out by molecular beam epitaxy (MBE). Additionally, direct growth of InAs on InP substrates was performed to allow comparisons with InAs grown on BCB-covered InP. Different growth temperatures were tested to optimize the crystal quality of the InAs layer, which was characterized by scanning electron microscope and x-ray diffraction. Heavy doping of InAs using Te was carried out to determine the lowest sheet resistance. As the substrate temperature was increased from 210 to 350 °C, the crystallinity improved from a polycrystal layer to a single crystal layer with corresponding improvement of surface morphology. Moreover, sharp x-ray diffraction indicated the 3.3% lattice-mismatch was fully relaxed without misorientation. However, a trade off was reached around 290 °C between crystallinity and optimized dopant incorporation of Te into InAs for lowest sheet resistance. The results indicate the potential of Te-doped InAs as emitter contact for high frequency devices by MBE.