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Isotopically modulated silicon and germanium multilayers are analyzed by means of femtosecond spectroscopy and pulsed X‐ray scattering for determining thermal conductivity and phonon modes. Isotopic modulation decreases thermal conductivity stronger than expected from a band bending model in the coherent phonon transport regime, in particular for silicon. Femtosecond spectroscopy and X‐ray scattering resolve zone‐folded vibration modes, which are located at the edge of the new, smaller Brillouin zone due to the multilayer periodicity. These modes can contribute to the reduction of thermal conductivity by Umklappprocesses within the zone‐folded mini‐bands. Color‐coded increase in ultrafast X‐ray scattering in vicinity to the mini‐zone boundary of a germanium multilayer.

No single material or materials system today is a clear choice for photoelectrochemical electrode applications. Generally, materials with narrow, well-aligned band gaps are unstable in solution and stable materials have band gaps that are too wide to efficiently absorb sunlight. Here, we demonstrate the narrowing of the ZnO band gap and combine a variety of electrical, spectroscopic, and photoelectrochemical methods to explore the opportunities for this so-called highly mismatched alloy in photoelectrochemical water splitting applications. We find that the conduction band edge of ZnO1–xSexis located at 4.95 eV below the vacuum level (0.5 V below the hydrogen evolution potential). Soft X-ray emission and absorption spectroscopies confirm that the previously observed ∼1 eV reduction in the ZnO band gap with the addition of selenium result from the formation of a narrow Se-derived band. We observe that this narrow band contributes to photocurrent production using applied bias incident photon to current efficiency measurements at an electrochemical junction. Electrical measurements, electrochemical flat band, and photocurrent measurements as a function of xin ZnO1–xSexalloys indicate that this alloy is a good candidate for an oxide/silicon tandem photoelectrochemical device because of the natural band alignment between the silicon valence band and the ZnO1–xSexconduction band. We observe that the photocurrent onset in preliminary ZnO1–xSex/silicon diode tandem devices is shifted toward spontaneous hydrogen production compared to ZnO1–xSexfilms grown on sapphire. With these findings, we hope that our method of band gap engineering oxides for photoelectrodes can be extended to devise better materials systems for spontaneous solar water splitting.

Dynamic nuclear polarization (DNP) of 29Si nuclear spins induced by saturation of electron paramagnetic resonance (EPR) transitions of lithium-related centers in float zone (FZ) grown silicon is reported. Both isolated Li and Li--O complex centers showed strong EPR absorption lines in the temperature range 3.4--10 K and led to very efficient orientation of 29Si nuclear spins. The temperature dependence and time constant of 29Si DNP are investigated in detail. The 29Si DNP of 0.72% was achieved at 3.4 K by excitation of the Li--O forbidden EPR transition under illumination, corresponding to a ${\sim}352$ fold increase with respect to the thermal equilibrium polarization.

In order to probe the fundamental behaviors of point defects in silicon and germanium, we studied the self-diffusion using isotope superlattices. In ion-implanted germanium, vacancies are in thermal equilibrium and transient enhanced diffusion is not present under the experimental conditions employed in this study. In contrast, silicon self-interstitials are supersaturated in ion-implanted silicon and the self-interstitial concentration is going down to the thermal equilibrium value toward the surface.

6H-SiC was doped with phosphorus (P) donors by neutron transmutation. IR transmission spectra were taken in the temperature range from 6 K to 300 K. A great number of absorption lines is observed at temperatures below 140 K; these lines are attributed to transitions of the donor electron between ground states and bound excited states of P-related donors. Based on Faulkner's theory, three series of effective-mass-like states (P1, P2, P3) could be identified. The corresponding ground state energies are: E(P1) = EC - 91.5 meV, E(P2) = EC - 81.8 meV, E(P3) = EC - 73.5 meV.

The absolute calibration and characterization of the Multiband Imaging Photometer for Spitzer(MIPS) 70 ?m coarse- and fine-scale imaging modes are presented based on over 2.5 yr of observations. Accurate photometry (especially for faint sources) requires two simple processing steps beyond the standard data reduction to remove long-term detector transients. Point-spread function (PSF) fitting photometry is found to give more accurate flux densities than aperture photometry. Based on the PSF fitting photometry, the calibration factor shows no strong trend with flux density, background, spectral type, exposure time, or time since anneals. The coarse-scale calibration sample includes observations of stars with flux densities from 22 mJy to 17 Jy, on backgrounds from 4 to 26 MJy sr?1, and with spectral types from B to M. The coarse-scale calibration is 702 ± 35 MJy sr?1MIPS70?1(5% uncertainty) and is based on measurements of 66 stars. The instrumental units of the MIPS 70 ?m coarse- and fine-scale imaging modes are called MIPS70 and MIPS70F, respectively. The photometric repeatability is calculated to be 4.5% from two stars measured during every MIPS campaign and includes variations on all timescales probed. The preliminary fine-scale calibration factor is 2894 ± 294 MJy sr?1MIPS70F?1(10% uncertainty) based on 10 stars. The uncertainties in the coarse- and fine-scale calibration factors are dominated by the 4.5% photometric repeatability and the small sample size, respectively. The 5 ?, 500 s sensitivity of the coarse-scale observations is 6-8 mJy. This work shows that the MIPS 70 ?m array produces accurate, well-calibrated photometry and validates the MIPS 70 ?m operating strategy, especially the use of frequent stimulator flashes to track the changing responsivities of the Ge:Ga detectors.

Research performed with semiconductors with controlled isotopic composition is evolving from the measurement of fundamental properties in the bulk to those measured in superlattices and nanostructures. This is driven in part by interests associated with the fields of “spintronics” and quantum computing. In this topical review, which is dedicated to Prof. Abstreiter, we introduce the subject by reviewing classic and recent measurements of phonon frequencies, thermal conductivity, bandgap renormalizations, and spin coherence lifetimes in isotopically controlled bulk group IV semiconductors. Next, we review phonon properties measured in isotope heterostructures, including pioneering work made possible by superlattices grown by the group of Prof. Abstreiter. We close the review with an outlook on the exciting future possibilities offered through isotope control in 1, 2, and 3 dimensions that will be possible due to advances in nanoscience. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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We study electronic Raman scattering of phosphorus and nitrogen doped silicon carbide (SiC) as a function of temperature in the range 7K < T < 300K. We observe a series of peaks in the Raman spectra which we assign to electronic transitions at nitrogen and phosphorus donors on different lattice sites. These transitions are identified as valley orbit transitions of the 1s donor ground state. From the polarization dependence of the observed peaks, we find that all electronic Raman signals have E2-symmetry of C6v for the hexagonal polytypes (6H-SiC and 4H-SiC) and E-symmetry of C3v for 15R-SiC. We find a reduction of the intensities of all valley-orbit Raman signals with increasing temperature and ascribe this reduction to the decreasing occupation of donor states.

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μLCR spectra from single-crystal Si∶Al have been taken at 30 K with the field along the 〈111〉 direction. The spectra show strong resonances near 12 and 50 mT, and weaker resonances near 38 and 42 mT. There is also indication of lines near 15 and 21 mT. The strong signals are due to the well-known zero crossings for Mu* in Si, and the weak signals may be associated with the quadrupole interaction of27Al, as predicted byab initio electronic structure calculations.

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We have studied the electron-phonon decoupling in Neutron-Transmutation Doped (NTD) Germanium thermistors below 50 mK, and measured a characteristic time constant of this phenomenon. We will discuss how our decoupling model accounts for observed non-linearities in I–V characteristics of NTD Ge and for the time structure of phonon pulses detected in a Ge crystal operated at 20 mK.

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Fourier transform spectroscopy (FTS) is one of the most important tools in the study of shallow level donors and acceptors in semiconductors. When combined with a two-step photothermal ionization process detected photoconductively, FTS allows measurement of optical transitions of donor-bound electrons (and acceptor-bound holes) in ultra-pure germanium samples with impurity concentrations <109 cm-3 (i.e. one electrically active impurity in 4×1013 host atoms). The experimental high resolution study of the hydrogen-like excited state series of shallow levels has yielded as many as 19 lines of width as small as 10µeV for some centers. These results have stimulated theoretical work which has led to the unambiguous assignment of quantum states to many bound excited states. Extensive studies of ultra-pure Ge crystals grown under different well-controlled conditions have led to the discovery of a large number of novel shallow impurity complexes. Study of the multiplicities and symmetries of the associated electronic states has led to a detailed understanding of the unusual static and dynamic structures of these novel centers. The chemical composition has been deduced from correlations between the concentration of a particular center and the materials involved in crystal gowth. Isotopic substitution of hydrogen with deuterium has led to the unambiguous proof of the presence of hydrogen in several of the novel centers. In addition to the high resolution spectra of shallow electronic levels, vibrational spectra of bond-centered interstitial oxygen in ultra-pure Ge are noteworthy for their extraordinarily sharp lines.

Hydrogen has been shown to activate the neutral impurities carbon, silicon and oxygen in ultra-pure germanium and form shallow level complexes. The double acceptors beryllium and zinc in silicon and germanium, as well as the triple acceptor copper in germanium, can be partially passivated, leading to single hole acceptors. The study of the electronic level spectrum of the single carrier bound to these centers at low temperatures has provided much information on symmetry and composition. Most centers reveal a symmetry axis along [1 1 1] and are static. In some cases hydrogen has been found to tunnel between equivalent real space positions. Photothermal ionization spectroscopy (PTIS) has been the most important tool for the study of the optical transitions of the hole (electron) in these hydrogen containing complexes. This photoconductivity technique combines high sensitivity with high resolution and permits the study of shallow acceptors or donors present at concentrations as low as 108cm-3. Even lower limits may be attained under favorable circumstances.

Defect states due to grown-in dislocations in ultrapure p-type germanium have been observed using Deep Level Transient Spectroscopy (DLTS) and Hall effect. These states are found to be composed of two bands of acceptor type levels whose energy and half width are influenced by both the presence of hydrogen and the crystal growth direction. At a net acceptor concentration of 1010cm−3a threshold dislocation density of ∼ 104cm−2is required for the observation of dislocation bands.

We report on the superconducting characteristics of the filled skutterudites LaPt4Ge12($T_{\text{c}}=8.3$ K) and PrPt4Ge12($T_{\text{c}}=7.9$ K) by 73Ge-nuclear quadrupole resonance (NQR) at a zero field. In PrPt4Ge12, the nuclear spin--lattice relaxation rate 73has revealed a coherence peak just below $T_{\text{c}}$, indicating that the superconductivity of PrPt4Ge12is accounted for in terms of the conventional BCS regime. The result was consistently reproduced by not only an isotropic $s$-wave gap model but also an anisotropic $s$-wave gap model with a point node. In the conventional superconductor LaPt4Ge12, the coherence peak in 73is markedly suppressed owing to some damping effect of quasiparticles. We note that the $T_{\text{c}}$s of LaPt4Ge12and PrPt4Ge12being higher than that of other $R$Pt4Ge12may be attributed to the larger density of states, not to some local anharmonic phonon modes derived from the possible rattling of $R$ ions.

A fabrication procedure for electrically pumping photonic crystal membrane devices using a lateral p-i-n junction has been developed and is described in this work. The lateral junction is optimized to efficiently inject current into a photonic crystal nanocavity. We have demonstrated electrically pumped lasing by using the lateral junction to pump a quantum dot photonic crystal nanocavity laser. Continuous wave lasing is observed at temperatures up to 150K, and a threshold of 181nA at 50K is demonstrated - the lowest threshold ever demonstrated in an electrically pumped laser. At room temperature we find that our devices do not lase, but behave as single-mode light-emitting diodes (LEDs). When directly modulated, we find that our LEDs have an ultrafast electrical response up to 10 GHz corresponding to less than 1 fJ/bit energy operation. In addition, we have demonstrated electrical pumping of photonic crystal nanobeam LEDs, and have built fiber taper coupled electro-optic modulators in the same lateral junction platform.

We evaluated quantitatively the germanium (Ge) displacement induced by arsenic (As) implantation as a function of the depth from the sample surface both in the amorphous and single-crystalline regions using 70Ge/natGe isotope superlattices (SLs). The profiles of 74Ge in the Ge isotope SLs were measured by secondary ion mass spectrometry and the sample structure along the depth was observed by cross-sectional transmission electron microscopy. The critical Ge displacement for amorphization induced by As implantation is found to be 0.75 nm, which is independent of the implantation doses. This value is 50% larger than 0.5 nm for Si.

In order to probe the fundamental behaviors of point defects in silicon and germanium, we studied the self-diffusion using isotope superlattices. In ion-implanted germanium, vacancies are in thermal equilibrium and transient enhanced diffusion is not present under the experimental conditions employed in this study. In contrast, silicon self-interstitials are supersaturated in ion-implanted silicon and the self-interstitial concentration is going down to the thermal equilibrium value toward the surface.

Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) have been used to study compositional modulation in InxGa1−x N layers grown with compositions close to miscibility gap. The samples (0.34 < x < 0.8) were deposited by molecular beam epitaxy using either a 200 nm thick AlN or GaN buffer layer grown on a sapphire substrate. Periodic compositional modulation leads to extra electron diffraction spots and satellite reflections in XRD in the θ–2θ coupled geometry. The ordering period Δ measured along c-axis was about Δ = 45 Å for x = 0.5 and Δ = 66 Å for x = 0.78 for samples grown on AlN buffer layer. TEM and XRD determinations of Δ were in good agreement. Compositional modulation was not observed for the sample with x = 0.34 grown on a GaN buffer layer. Larger values of Δ were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. The possibility that the roughness of the AlN growth surface promotes strong In segregation on particular crystallographic planes leading to compositional modulation is considered.

A bound exciton is observed in far-infrared magnetoabsorption measurements of photo-excited beryllium doped germanium below 4.2 K. The absorption corresponds to the 1s to 2p±like transition of the electron in the bound exciton. Fifteen branches of peaks converge to about 3.4 meV at zero magnetic field. The 1s like ground state splits into more than two levels at zero magnetic field. The 2p±like excited state with an ionization energy of 1.7 meV is well described by the effective mass approximation.