The manipulation of fluids in channels with dimensions of tens of micrometres--microfluidics--has emerged as a distinct new field. Microfluidics has the potential to influence subject areas from chemical synthesis and biological analysis to optics and information technology. But the field is still at an early stage of development. Even as the basic science and technological demonstrations develop, other problems must be addressed: choosing and focusing on initial applications, and developing strategies to complete the cycle of development, including commercialization. The solutions to these problems will require imagination and ingenuity.

The origins and the future of microfluidics

We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

First-principles calculations for defects and impurities: Applications to III-nitrides

We present a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III–V semiconductors that have been investigated to date. The two main classes are: (1) “conventional” nitrides (wurtzite and zinc-blende GaN, InN, and AlN, along with their alloys) and (2) “dilute” nitrides (zinc-blende ternaries and quaternaries in which a relatively small fraction of N is added to a host III–V material, e.g., GaAsN and GaInAsN). As in our more general review of III–V semiconductor band parameters [I. Vurgaftman et al., J. Appl. Phys. 89, 5815 (2001)], complete and consistent parameter sets are recommended on the basis of a thorough and critical review of the existing literature. We tabulate the direct and indirect energy gaps, spin-orbit and crystal-field splittings, alloy bowing parameters, electron and hole effective masses, deformation potentials, elastic constants, piezoelectric and spontaneous polarization coefficients, as well as heterostructure band offsets. Temperature an...

Band parameters for nitrogen-containing semiconductors

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

Nanometer sized whiskers (nanowires) offer a vehicle for the study of size-dependent phenomena. While quantum-size effects are commonly expected and easily predicted, size reduction also causes more atoms to be closer to the surface. Here we show that intensity relations of below-band-gap and band-edge luminescence in ZnO nanowires depend on the wire radius. Assuming a surface layer wherein the surface-recombination probability is 1 (surface-recombination approximation), we explain this size effect in terms of bulk-related to surface-related material-volume ratio that varies almost linearly with the radius. This relation supports a surface-recombination origin for the deep-level luminescence we observe. The weight of this surface-luminescence increases as the wire radius decreases at the expense of the band-edge emission. Using this model, we obtain a radius of 30 nm, below which in our wires surface-recombination prevails. More generally, our results suggest that in quantum-size nanowires, surface-recombination may entirely quench band-to-band recombination, presenting an efficient sink for charge carriers that unless deactivated may be detrimental for electronic devices.

/pdf/size-dependent-surface-luminescence-in-zno-nanowires-mym2s9jf0i.pdf

Size-dependent surface luminescence in ZnO nanowires

Unique properties exist in nanofluidic channels. In this paper, we report a new phenomenon, ion enrichment/depletion, associated with nanochannel structures. As a voltage is applied across a nanochannel, ions are rapidly enriched at one end and depleted at the other end of the nanochannel. The degree of this enrichment and depletion is directly related to the extent of double-layer overlap. A simple model is presented to qualitatively interpret this phenomenon.

Ion-Enrichment and Ion-Depletion Effect of Nanochannel Structures

Porous silicon that strongly emits in the visible was analyzed using Raman scattering. The spectrum peaks near 508 cm−1, has a width of ∼40 cm−1, and is very asymmetric. Using a model of phonon confinement, this suggests that the local structure of porous silicon is more like a sphere than a rod and has a characteristic diameter of 2.5–3.0 nm. Polarization Raman measurements suggest that the structure does not consist of a series of parallel columns.

http://www.columbia.edu/~iph1/Download/Sui-PorousSiRaman-1992-ApplPhysLett_60_2086.pdf

Raman analysis of light‐emitting porous silicon

We report on the fabrication and characterization of p-π-n GaN ultraviolet detectors. The peak responsivity at ∼363  nm is measured to be 0.1 A/W in the photovoltaic mode, and 0.14 A/W with a bias of −15 V. Speed measurements have shown the photoresponse to be RC-limited with the response time decreasing from 17.4 ns at zero bias to 10.3 ns at −6 V bias. For a 200×200 μ m2 device, we measure the dark current to be 2.7 pA at −3 V bias, and a noise density of less than 10−25 A2/Hz, the noise floor of the measurement. Extrapolating the noise data taken at higher reverse biases, we estimate the noise equivalent power to be 6.6×10−15 W/Hz1/2.

Low noise p-π-n GaN ultraviolet photodetectors

We study the vibrational spectrum of AlN grown on Si(111). The AlN was deposited using gas-source molecular beam epitaxy. Raman backscattering along the growth c axis and from a cleaved surface perpendicular to the wurtzite c direction allows us to determine the ${E}_{2}^{1},$ ${E}_{2}^{2},$ ${A}_{1}(\mathrm{TO}),$ ${A}_{1}(\mathrm{LO}),$ and ${E}_{1}(\mathrm{TO})$ phonon energies. For a 0.8-\ensuremath{\mu}m-thick AlN layer under a biaxial tensile stress of 0.6 GPa, these are 249.0, 653.6, 607.3, 884.5, and 666.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. By combining the Raman and x-ray diffraction studies, the Raman stress factor of AlN is found to be $\ensuremath{-}6.3\ifmmode\pm\else\textpm\fi{}1.4{\mathrm{cm}}^{\ensuremath{-}1}/\mathrm{GPa}$ for the ${E}_{2}^{2}$ phonon. This factor depends on published values of the elastic constants of AlN, as discussed in the text. The zero-stress ${E}_{2}^{2}$ energy is determined to be $657.4\ifmmode\pm\else\textpm\fi{}0.2{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}.$ Fourier-transform infrared reflectance and absorption techniques allow us to measure the ${E}_{1}(\mathrm{TO})$ and ${A}_{1}(\mathrm{LO})$ phonon energies. The film thickness (from 0.06 to 1.0 \ensuremath{\mu}m) results in great differences in the reflectance spectra, which are well described by a model using damped Lorentzian oscillators taking into account the crystal anisotropy and the film thickness.

https://www.depts.ttu.edu/ntc/researchandpublications/publications/pdf/2001/PRB_Prokofyeva_2001.pdf

Henryk Temkin

Papers

Size-dependent surface luminescence in ZnO nanowires

Ion-Enrichment and Ion-Depletion Effect of Nanochannel Structures

Raman analysis of light‐emitting porous silicon

Low noise p-π-n GaN ultraviolet photodetectors

Vibrational properties of AlN grown on (111)-oriented silicon