Stephen M. Durbin

Zinc-blende MnTe: epilayers and quantum well structures

Epilayers of the previously hypothetical zinc‐blende MnTe have been grown by molecular beam epitaxy. Epitaxial layers (0.5 μm thick) of MnTe were characterized using x‐ray diffraction and transmission electron microscopy; optical reflectance measurements indicate a band gap of ∼3.2 eV. A series of strained single quantum well structures was fabricated with zinc‐blende MnTe forming the barrier to CdTe quantum well regions; photoluminescence spectra indicate optical transitions corresponding to strong electron and hole confinement.

Nuclear resonance vibrational spectroscopy of a protein active-site mimic

For many years, Mossbauer spectroscopy has been applied to measure recoilless absorption of x-ray photons by nuclei. Recently, synchrotron radiation sources have enabled the observation of weaker features separated from the recoilless resonance by the energy of vibrational quanta. This enables a form of vibrational spectroscopy with a unique sensitivity to the probe nucleus. Biological applications are particularly promising, because it is possible to selectively probe vibrations of a single atom at the active site of a complex biomolecule, while avoiding interference from the vibrations of thousands of other atoms. In contrast with traditional site-selective vibrational spectroscopies, nuclear resonance vibrational spectroscopy (NRVS) is not hampered by solvent interference and faces selection rule limitations only if the probe nucleus lies on a symmetry element. Here, we formulate a mathematical language appropriate for understanding NRVS measurements on molecular systems and apply it to analyse NRVS data recorded on ferrous nitrosyl tetraphenylporphyrin, Fe(TPP)(NO). This compound mimics the haem group found at the active site of many proteins involved in the biological usage of oxygen and nitric oxide. Measurements on such model compounds provide a baseline for evaluating the extent to which vibrations are localized at the active site of a protein, with the goal of elucidating the mechanisms of biological processes, such as intersite communication in allosteric proteins.

Iron normal mode dynamics in (nitrosyl)iron(II)tetraphenylporphyrin from X-ray nuclear resonance data.

The complete iron atom vibrational spectrum has been obtained by refinement of normal mode calculations to nuclear inelastic x-ray absorption data from (nitrosyl)iron(II)tetraphenylporphyrin, FeTPP(NO), a useful model for heme dynamics in myoglobin and other heme proteins. Nuclear resonance vibrational spectroscopy (NRVS) provides a direct measurement of the frequency and iron amplitude for all normal modes involving significant displacement of (57)Fe. The NRVS measurements on isotopically enriched single crystals permit determination of heme in-plane and out-of-plane modes. Excellent agreement between the calculated and experimental values of frequency and iron amplitude for each mode is achieved by a force-field refinement. Significantly, we find that the presence of the phenyl groups and the NO ligand leads to substantial mixing of the porphyrin core modes. This first picture of the entire iron vibrational density of states for a porphyrin compound provides an improved model for the role of iron atom dynamics in the biological functioning of heme proteins.

Ultraviolet and blue holographic lithography of ZnSe epilayers and heterostructures with feature size to 100 nm and below

We have employed short‐wavelength holographic laser lithography and reactive ion etching to define wire and dot‐like patterns in ZnSe thin epitaxial films and heterostructures with spatial feature size to better than 100 nm. Photoluminescence measurements suggest that surface damage from etching may be much less severe than in III‐V semiconductors.

Comparative optical investigations of ZnSe/GaAs epilayers grown by molecular beam and hot‐wall epitaxy

Comparative photoluminescence and excitation spectra of ZnSe/GaAs epilayers grown by molecular beam epitaxy (MBE) and hot‐wall epitaxy (HWE) show likewise features in the exciton energy regime. Two strain‐split components of the free exciton are observed, as well as characteristic sets of transitions from or into ground and excited states of acceptor‐ and donor‐bound excitons. However, all respective lines are shifted to lower energies in the HWE samples, due to the increased thermally induced strain as a consequence of the substrate temperatures being enhanced compared to the MBE growth. Whereas the dominant donors are of the same nature in both kinds of samples, specific acceptor centers are incorporated in the HWE films. Although the MBE‐grown layers are of superior quality, it is shown that HWE under optimized growth conditions is a cheap and useful alternative to obtain ZnSe epilayers of reliable properties.

Pseudomorphic ZnTe/AlSb/GaSb heterostructures by molecular beam epitaxy

A series of pseudomorphic ZnTe/AlSb/GaSb epilayer/epilayer heterostructures, aimed at the realization of novel wide band‐gap light‐emitting devices, was grown by molecular beam epitaxy. The structures were evaluated by several techniques including transmission electron microscopy (TEM), x‐ray rocking curves, photoluminescence (PL), and Raman spectroscopy. Reflection high‐energy electron diffraction intensity oscillations were observed during nucleation of ZnTe. The presence of dislocation‐free pseudomorphic structures was confirmed by TEM. The PL spectra of ZnTe epilayers showed dominant near‐band‐edge features composed of free and shallow impurity bound excitons.

Quantum wells with zincblende MnTe barriers

In this paper we describe a series of MnTe/CdTe/MnTe and MnTe/InSb/MnTe single quantum well structures. For the CdTe quantum wells we report the observation of luminescence covering the entire visible range from red to blue; a quantized state in the InSb well is used to implement resonant tunneling. X-ray diffraction and transmission electron microscopy (TEM) were used to evaluate the microstructural quality of the structures. Dark-field TEM showed that, in spite of the 2.3% lattice mismatch, the MnTe layers remained pseudomorphic and dislocation-free. High resolution images (also used to determine dimensional details) indicated that the interfaces were atomically abrupt, and that the CdTe and InSb wells were essentially unstrained in each of the structures; most of the strain was contained in the MnTe barrier layers. Optical properties of the single quantum well structures have been studied using photoluminescence and photoluminescence excitation spectroscopy. Blue luminescence at 2.59 eV (n = 1 transition) has been observed from a structure with a 10 A CdTe well. The negative differential resistance observed from MnTe/InSb resonant tunneling structures represents, to our knowledge, the first report of a dimensionally quantized state in InSb.

Thermally induced optical bistability in ZnSe epilayers grown by molecular‐beam epitaxy

For the first time, ZnSe epitaxial layers grown by molecular‐beam epitaxy are shown to exhibit large contrast, low power, and extremely long‐term stable thermally induced absorptive optical bistability. It is observed over a wide temperature range between 169 K and RT, with a strongly variable loop width and a switch‐back adjustable to be less than 50% of the respective switch‐down value. Critical slowing down as well as switch‐down times are studied in dependence of the excess over the switch‐down intensity values.

Intermolecular Dynamics in Crystalline Iron Octaethylporphyrin (FeOEP)

The new technique of nuclear resonance vibrational spectroscopy (NRVS) has increased the range and quality of dynamical data from Fe-containing molecules that when combined with Raman and infrared spectroscopies impose stricter constraints on normal mode simulations, especially at lower frequencies. Going beyond the usual single molecule approximation, a classical normal-mode analysis that includes intermolecular coupling and the full crystalline symmetry is found to produce a better fit with fewer free parameters for the heme compound iron octaethylporphyrin (FeOEP), using NRVS data from polycrystalline material. Off-diagonal force constants were completely unnecessary, indicating that their role in previous single molecule fits was just to emulate intermolecular coupling. Sound velocities deduced from the calculated phonon dispersion curves are compared to NRVS measurements to further constrain the intermolecular force constants. The NRVS data by themselves are insufficient to rigorously determine all unknown force constants for molecules of this size, but the improved crystal model fit indicates the necessity of including intermolecular interactions for normal-mode analyses.

Spectroscopy in CdTe/MnTe and ZnTe/MnTe single quantum wells : new binary wide gap II-VI heterostructures

Abstract With the incorporation of cubic zincblende MnTe, a range of optical studies have been carried out on single quantum wells of ZnTe/MnTe and CdTe/MnTe. By using thin MnTe barrier layers the structures appear to be nearly pseudomorphic and show evidence for good electron-hole confinement.