Joanna Millunchick

Engineering of Strain Relaxation Using Ion Beam Modified Substrates

Sponsor: Army Research Office (ARO)
Lattice mismatch in heteroepitaxial semiconductor systems remains one of the greatest barriers to the use of otherwise promising film combinations. Many III-V semiconductor systems have bandgaps that make the desirable for high efficiency and high output optical applications, but their use thus far has been limited by the availability of lattice matched substrates. Growth upon commercially available GaAs and InP wafer substrates can produce a large mismatch strain, which results in defects and surface roughening that degrade film properties. One proposed method for reducing strain and defect density in mismatched heteroepitaxial systems is growth upon patterned substrates. The goal of this work is to achieve strain relaxation and decreased defect densities in lattice mismatched films grown on patterned or modified substrates. A variety of substrate modification approaches are being examined, included direct 3-D substrate patterning, 2-D patterning, and uniform blanket irradiation. As part of this effort, focused ion beam (FIB) patterning is being employed as a flexible method for 2-D and 3-D patterning of substrates. The FIB approach allows for many different patterned areas to be placed on a single III-V semiconductor substrate, allowing a wide range of patterning types and parameters to be easily examined. FIB patterning is being carried out using the Millunchick group UHV FEI Magnum FIB column integrated in vacuo with an molecular beam epitaxy (MBE) growth system. This integrated system allows multiple film patterning and growth steps to be carried out without the need to ever remove a semiconductor sample from high vacuum conditions. The FIB system may also be used to blanket irradiate substrates with Ga ions. Additional blanket ion implantation of samples has also been carried out using ex-situ ion implantation facilities at the Michigan Ion Beam Laboratory (MIBL), and the effect this has on film growth will also be examined. Once grown on modified substrates, lattice mismatched III-V films will be characterized by SEM, TEM, high-resolution XRD, and AFM. Because this project relies so heavily on FIB modification of III-V substrates, the FIB response of common III-V materials is also being studied. GaAs, InAs, InP, GaSb, and AlAs have all been examined and their FIB response characterized by SEM, XEDS, and AFM. The results of these studies are being used to guide the ongoing examination of FIB patterning for strain relaxed film growth described above.
Highlights (Click an image for more information)
  • SE image of FIB milled pattern of raised mesas in InP

    Secondary electron image of a pattern of raised square InP mesas.  The pattern was created by milling away the surrounding InP substrate material using a FIB.  III-V semiconductor film growth on patterns such as this one may help to lower strain and the number of crystalline defects in those films. 

  • InAs islands on FIB patterned InP substrate

    Secondary electron image of InAs islands grown on raised mesas patterned into an InP substrate using a 30 kV focused ion beam (FIB).  The substrate was patterned using a UHV FIB column connect in vacuo to the molecular beam epitaxy (MBE) system used to grow the InAs film.  Film growth on patterned III-V substrates may alter the morphology and degree of strain relaxation of the film in the patterned regions.