Andrew M. Hoff

Evidence that blue luminescence of oxidized porous silicon originates from SiO2

We have analyzed red and blue luminescence from porous silicon as a function of oxidation parameters and feature dimension determined with an atomic force microscope. We have found correlation between blue luminescence intensity and the increase in feature size caused by oxidation. We have further shown that blue luminescence, is identical, with respect to spectrum and fast decay, to that of high microelectronic quality SiO2 grown on crystalline silicon using dry oxygen plus an organic chlorine compound. Thus, we conclude that blue luminescence originates from SiO2 film rather than from the silicon nanocrystals in the porous material. Intensity enhancement, as compared to SiO2 on crystalline wafers, comes from the gigantic surface area of porous silicon.

Iron detection in the part per quadrillion range in silicon using surface photovoltage and photodissociation of iron-boron pairs

The photodissociation of iron‐boron pairs in p‐type silicon produces lifetime killing interstitial iron and may be combined with noncontact surface photovoltage (SPV) measurement of the minority carrier diffusion length to achieve fast detection of iron. We found that, for iron concentrations ranging from 8×108 to 1×1013 atoms/cm3, the pair dissociation using a white light (10 W/cm2) was completed within 15 s. Surface recombination was a major rate limiting factor. Passivation of the surface enhanced the rate by as much as a factor of 20. The photodissociation rate increased with increasing temperature, however, the increase was smaller than that of the thermal dissociation rate. These characteristics are consistent with a previously proposed recombination enhanced dissociation mechanism. For practical iron detection, it is important that the detection limit of the approach is close to one part per quadrillion.

Biocompatibility and wettability of crystalline SiC and Si surfaces

Crystalline silicon carbide (SiC) and silicon (Si) biocompatibility was evaluated by directly culturing three mammalian cell lines on these semiconducting substrates. Cell proliferation and adhesion quality were studied using MTT [3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays and fluorescent microscopy. The reported results show that SiC is indeed a more biocompatible substrate than Si. The surface wettability of SiC and Si samples was evaluated through static contact angle measurements, which provided interesting information regarding the influence of different cleaning procedures on the SiC surfaces. The cell proliferation data are discussed in light of the contact angle measurements results. This joint analysis leads to interesting conclusions that may help to uncover the main factors that define a semiconductors biocompatibility.

Plasma facilitated delivery of DNA to skin

Non‐viral delivery of cell‐impermeant drugs and DNA in vivo has traditionally relied upon either chemical or physical stress applied directly to target tissues. Physical methods typically use contact between an applicator, or electrode, and the target tissue and may involve patient discomfort. To overcome contact‐dependent limitations of such delivery methodologies, an atmospheric helium plasma source was developed to deposit plasma products onto localized treatment sites. Experiments performed in murine skin showed that samples injected with plasmid DNA encoding luciferase and treated with plasma demonstrated increased levels of expression relative to skin samples that received injections of DNA alone. Increased response relative to injection alone was observed when either positive or negative voltage was used to generate the helium plasma. Quantitative results over a 26‐day follow‐up period showed that luciferase levels as high as 19‐fold greater than the levels obtained by DNA injection alone could be achieved. These findings indicate that plasmas may compete with other physical delivery methodologies when skin is the target tissue. Biotechnol. Bioeng. 2009; 104: 1034–1040.

Culture of Mammalian Cells on Single Crystal SiC Substrates

Crystalline silicon carbide (SiC) has the potential to become an important biomaterial and a versatile interface between the electronic and biological world. In this work, single crystal SiC biocompatibility is investigated by culturing mammalian cells directly on SiC substrates. The cell morphology and the quality of the cell adhesion have been studied using fluorescence microscopy, while MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays have been performed to quantify cell viability and number. Standard culture-wells and silicon (Si) substrates were used as controls in the final assessment of crystalline SiC biocompatibility.

Characterization of plasma mediated molecular delivery to cells in vitro

Ion-based strategies have recently emerged as a method to facilitate molecular delivery. These methods are attractive as they separate the applicator from the treatment site avoiding some issues encountered with other electrically driven methods. Current literature on plasma delivery has shown utility in vitro and in vivo for both drugs and genes. To advance this technology more information must become available on the mechanism responsible for delivery and the effects of ion exposure on eukaryotic cells. This in vitro investigation found that molecular delivery facilitated by a DC-based plasma follows a dose-response behavior, with optimum uptake of Sytox Green occurring in two cell lines after 600 s of exposure. In both cell lines exposure to the discharge caused no adverse effects in viability for exposure times up to 600 s. It was also found that membranes treated with ions remained permeabilized for several minutes following plasma treatment and that membrane resealing exhibited first order kinetics.

New approach to measuring oxide charge and mobile ion concentration

We discuss the determination of oxide charge from simultaneous noncontact measurement of the surface potential barrier, Vs, (via surface photovoltage) and the voltage drop across the oxide, Vox, (via contact potential vibrating probe). These two measurements enable us to separate the contributions from total charge and oxide charge. In combination with corona charging and low temperature stress, this approach can be used for wafer-scale determination of the mobile Na+ concentration. The principles of the approach are presented and typical results are given which contrast the effects of ion drift and charge injection in the oxide. Experimental results also illustrate the noncontact, wafer-scale mapping of the mobile ion distribution.

Electronic Passivation of 3C-SiC(001) Via Hydrogen Treatment

Electronic passivation of single-crystal, atomically flat (001) surfaces of cubic silicon carbide (3C-SiC) was attempted via hydrogen annealing and HF exposure and investigated by monitoring the surface potentials of the treated samples. It was found that HF treatment causes a negative charging of the surface and that only hydrogen annealing is effective in producing well-passivated 3C-SiC. The degree and stability of the surface electronic passivation was dependent on the final hydrogen cooling temperature of the annealing process. Surface charge densities of the hydrogen-treated surfaces were calculated from the measured surface potentials and were found to be in the 1010 cm -2 range.

Molecular Delivery to Cells Facilitated by Corona Ion Deposition

A novel method of inducing the delivery of nonpermeant molecules to the cytosol of cells is presented in this paper. Corona discharge in air was utilized to produce ions that in turn were deposited onto the liquid surface of media containing cultured cells. Murine B16 melanoma cells were used to demonstrate the molecular delivery of fluorescent dye calcein, the drug bleomycin, and a nucleic acid stain SYTOX-green. None of these molecules penetrate cells with intact membranes. Following the corona treatment, cells were observed to admit significant quantities of these molecules from the culture media, relative to control samples. Further, greater than 95% viability of treated cells was observed by Trypan Blue assay. This method may provide an attractive alternative to electroporation where a physical contact between electrodes and cells is needed to deliver molecules to the cytosol.

A novel approach to monitoring of plasma processing equipment and plasma damage without test structures

Routine monitoring of oxide charging in IC manufacture requires real-time evaluation of the results of plasma processes. Whole-wafer images of dielectric charging produced by plasma exposure and generated by a new diagnostic tool using reusable oxidized wafers, are shown to be effective tools in the correlation of plasma and equipment characteristics to charging.