Trevor Clark

Growth of Magneto-optically Active (Zn,Mn)Se Nanowires

We describe the growth of Zn(1-x)Mn(x)Se nanowires in ultrahigh vacuum seeded by Au nanodroplets. Electron microscopy reveals the formation of single-crystal c-axis wurtzite nanowires (typically 1-3 microm long) with Mn concentrations up to x approximately 0.6, accompanied by a dense horizontal undergrowth of shorter, crooked nanowires. Magnetophotoluminescence measurements show evidence for sp-d exchange effects in a reduced symmetry environment. We find that the optical emission is surprisingly dominated by the undergrowth of crooked nanowires.

Theory and New Applications of Ex Situ Lift Out

The ex situ lift out (EXLO) adhesion forces are reviewed and new applications of EXLO for focused ion beam (FIB)-prepared specimens are described. EXLO is used to manipulate electron transparent specimens on microelectromechanical systems carrier devices designed for in situ electron microscope analysis. A new patented grid design without a support film is described for EXLO. This new slotted grid design provides a surface for holding the specimen in place and also allows for post lift out processing. Specimens may be easily manipulated into a backside orientation to reduce FIB curtaining artifacts with this slotted grid. Large EXLO specimens can be manipulated from Xe+ plasma FIB prepared specimens. Finally, applications of EXLO and manipulation of FIB specimens using a vacuum probe lift out method are shown. The vacuum probe provides more control for placing specimens on the new slotted grids and also allows for easy manipulation into a backside configuration.

Reliability of aluminum-bearing ohmic contacts to SiC under high current density

The degradation produced by high current density stressing of a contact to p-SiC consisting of an Al-bearing ohmic contact, a TiW diffusion barrier, and a thick Au overlayer was studied. The test structure allowed for vertical current stressing and the measurement of the specific contact resistance before and after stressing. A threshold current for contact failure was established for the Ti/Al and W/Al contacts, at which a large increase in specific contact resistance was measured and extensive voiding occurred in the ohmic contact region. The high current stressing generated a flux of Al from the ohmic contact layer, through the TiW barrier, to the surface to be oxidized, along with a flux of Au into the ohmic contact layer. The voiding in the ohmic contact layer, caused by the unequal fluxes of Al and Au, decreased the active area of the contact, consequently increasing the current density and the associated effects from electromigration and Joule heating, initiating a runaway event.

3D Imaging of Biological Cells Using a CryoFIB/SEM and a CryoTEM

The emerging discipline of cryo-electron tomography provides unique opportunities to determine 3dimensional (3D) cellular architectures in their native conditions. However, one major limitation, specimen thickness, has hindered its broader application in cellular structural biology, since it’s performed in TEM by tilting the specimen through a series of angles. Thinning a biological sample using cryo-ultramicrotomy and a diamond knife has yielded limited success due to technical difficulties and artifacts associated with this mechanical sectioning method [1]. It has been shown that a dual beam system consisting of a scanning electron microscope (SEM) and focused ion beam (FIB) used under cryogenic conditions can successfully mill biological samples for use in cryoTEM. This technique eliminates the cutting artifacts that are associated with ultramicrotomy, while thinning specimens enough for electron tomography [2, 3].

Optimizing Van der Waals Forces For FIB ex situ Lift Out

Van der Waals forces are 10-12 orders of magnitude larger than the force of gravity acting on focused ion beam (FIB) prepared specimens and other small particles and fibers, and are the primary adhesion forces responsible for successful, reliable, and rapid ex situ lift out (EXLO) and micromanipulation to suitable specimen carriers [1]. Electrostatic forces which were previously believed to be the operating mechanism for EXLO micromanipulation should be avoided whenever possible [1]. A glass probe is preferred for EXLO since it may be easily drawn to a pointed tip (i.e., ~ 1 μm or less) and remains strong with some flexibility during the manipulation process. Conversely, metal probe tips may easily curl and deform during the manipulation process and therefore are not ideal.

Vacuum Assisted ex situ Lift Out of FIB Prepared Specimens

Conventional ex situ lift out (EXLO) relies on adhesion forces to pick up a specimen with a solid probe tip and place it on a suitable carrier [1-3]. The primary adhesion forces at work are Van der Waals, capillary, and electrostatic forces [3]. New developments in ex situ lift out include a new grid carrier design and methods which allow fast and easy manipulation of specimens outside of the costly FIB [38]. Once manipulated to this new grid design, the specimen may be further processed via FIB, broad beam ion milling, or plasma cleaning. Specimens can be easily positioned into a backside orientation and then FIB polished, reducing curtaining artifacts [5,6]. EXLO is also amenable to very large specimens routinely available from plasma FIB instruments [7,8]. A single ex situ lift out system can support multiple FIB instruments, and when coupled with its speed and ease of use, reinforces its cost effectiveness.