AVS2013 Session EM+AS+NS+SS-WeA: Semiconductor Heterostructures/Heusler Alloys

The past two decades have witnessed phenomenal progress in optoelectronic display and illumination devices enabled by AlGaInN (III-N). As the technology of conventional devices enters commercial maturity, innovation is called for continual advances in device applications.

In this talk we will discuss the possibility of extending III-N devices in the directions of flexible and large-area applications. Our recent work in using electrochemical etching to achieve layer slicing will be presented with preliminary device demonstrations. We envision new possibilities in the manufacture of ultrathin and flexible GaN devices. To circumvent the difficulty in the growth of GaN on silicon, we investigated the concept of evolutionary growth combining modern fabrication techniques with epitaxy to provide new freedoms in tackling this grand challenge. The result of preparing high quality GaN on SiO2 will be reported.

In previous work an approximate phase invariant dynamical diffraction model (PIDDM) has been developed for the analysis of high-resolution x-ray rocking curves from metamorphic semiconductor heterostructures containing threading dislocations. In principle, use of the PIDDM allows depth profiling of composition, strain, and dislocation density in device structures. It accounts for the broadening of diffraction linewidths through the use of an effective deviation parameter, which includes the angular mosaic spread and lattice spacing mosaic spread of a crystal distorted by dislocations. Although the PIDDM correctly predicts the linewidth broadening of diffraction profiles, it is unable to account for some features of the rocking curves from metamorphic structures, such as the suppression of the Pendellosung fringes and the extinction behavior of layers with high dislocation densities. To address these issues, we have developed a refined model for dynamical diffraction from dislocated semiconductor heterostructures, the Mosaic Crystal Dynamical Diffraction Model (MCDDM), which includes phase variations between blocks of the mosaic crystal. In this paper, we compare the PIDDM and MCDDM for the case of metamorphic InGaAs/GaAs (001) heterostructures, and illustrate the situations in which the more complex MCDDM needs to be applied.

Metamorphic devices such as high electron mobility transistors (HEMTs), heterojunction bipolar transistors (HBTs), light-emitting diodes (LEDs), laser diodes, and solar cells have been fabricated on GaAs substrates using graded buffer layers. These metamorphic buffer layers usually employ linear grading of composition, and materials including In_xGa_1-xAs and InAs_1-yP_y have been used. The most important function of the linearly- graded buffer layer is to minimize the threading defect density by enhancing the mobility and glide velocity of dislocations, thereby promoting long misfit segments with relatively few threading arms. In general, there are three features of graded buffers which reduce the thread density: (i) a misfit dislocation free zone (MDFZ) near the substrate interface which reduces pinning interactions with substrate defects, (ii) a second MDFZ near the surface which reduces pinning interaction near the device layer; and (iii) a large built-in strain in the top MDFZ which enhances glide of dislocations to sweep out threading arms. However, non-linear grading may be beneficial for better control of the widths of the MDFZs and for higher built-in strain in the surface MDFZ. In this work, we present minimum energy calculations for sublinearly graded heterostructures, with logarithmic and power law grading, and compare the cases of cation (Group III) and anion (Group V) grading. We show that differences in the elastic stiffness constants give rise to significantly different behavior in these two commonly-used buffer layer systems. Moreover, the use of different grading profiles for sublinear buffer layers allows for correlation of a sublinearity factor to the width of dislocated region and peak misfit density.

Heusler alloys have received a lot of attention because of their large range of properties, which include electronic, piezoelectric, magnetic, thermoelectric and shape memory. Their properties depend on the number of valence electrons per formula unit and have been predicted to be semiconductors, metals, ferromagnets, antiferromagnets, half metals, superconductors and topological insulators. Similar to compound semiconductors, the band structure and lattice parameters of Heusler alloys can also be tuned through alloying but over a much larger range of properties. Magnetic tunnel junctions using Heusler alloys that are predicted to be half metals have shown record tunneling magnetoresistance. They can be lattice matched to most compound semiconductors and have also been used for spin injecting contacts. This presentation will emphasize the molecular beam epitaxial growth on III-V semiconductors and tuning of their properties with examples for spintronic, semiconductor, topological insulator and shape memory applications.

High spin polarization ferromagnets, including half-metallic ferromagnets, are attractive choices for use as spin injectors into semiconductors as well as other spintronic devices. Co₂MnSi is predicted to be half-metallic [1], and with a lattice constant of 5.65Å, is almost perfectly lattice-matched to GaAs (0.06% mismatch), and a strong candidate for use as a spin-injector in the ferromagnet/GaAs system. We demonstrate the growth of epitaxial Co₂MnSi films directly on GaAs (001) by molecular beam epitaxy (MBE) for use as a spin injector, as well as successful spin injection and detection. In addition, we observe a growth temperature dependence of spin injection/detection in three-terminal lateral transport devices on GaAs. X-ray diffraction studies verify near lattice matching of Co₂MnSi films to GaAs. We employ the use of a shuttered growth technique to initiate Co₂MnSi growth on GaAs. This technique allows for the initial deposition of either a full Co monolayer or a full MnSi monolayer, influencing the interfacial ordering of the ferromagnet/semiconductor interface. In order to study the effects of interfacial ordering,scanning transmission electron microscopy (STEM) and x-ray diffraction techniques, as well as electrical measurements of spin injection and transport are used. Magnetic hysteresis measurements show extremely low coercivities of grown films, as well as auniaxial anisotropy with an easy axis in the [110] direction. Co₂MnSi has been grown at growth temperatures varying from 180⁰C to 320⁰C, and a layer-by-layer growth mode has been confirmed for this temperature range by the presence of intensity oscillations in reflection high energy electron diffraction (RHEED) patterns. The Co₂MnSi layers have also been studied by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). The STM and STS studies have been carried out at room temperature, and at 4.5K at which atomic resolution was obtained. STS has been performed with both a Nb tip and a W tip at room temperature and 4.5K to study the local density of states. In addition, we demonstrate a growth temperature dependence of spin injection/detection in three-terminal lateral transport devices on GaAs. These methods were used to better understand the growth mechanisms and electronic properties of the films.

This work was supported by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885 and by the Semiconductor Research Corporation under award number 2011-IN-2153.

[1] S. Picozzi, A. Continenza, and A. J. Freeman. Phys. Rev. B 69 (9), 094423 (2004)