J.L. Brebner

Evidence for bimolecular recombination in the luminescence spectra of SrTiO3

We present time-resolved measurements of the visible luminescence band seen in pure SrTiO3 at low temperatures. Under low excitation intensity conditions, the luminescence decay is well described by a simple model characterized by bimolecular processes. The variation of the decay with excitation intensity can be related to a change in the density of recombining species. Saturation of the bimolecular processes occurs when the density of electron-hole pairs created reaches 1018cm−3. Monomolecular processes then become important.

Neutron scattering measurements of the interlayer interaction in GaSe

Abstract Phonon dispersion curves along the c-axis in GaSe derived from inelastic neutron scattering experiments are presented. A model for the rigid layer and conjugate modes is used to obtain the interlayer interaction which is shown to be of short range. A discussion of the identification of polytypes in GaSe in also given.

Acoustic phonons in SnSe2

SummaryWe present the results of a measurement of the acoustic phonon dispersion curves in the semiconducting layer structure SnSe2. The measurements were made at the NRU reactor, Chalk River, by the technique of coherent inelastic neutron scattering. For modes propagating along thec*-direction both the LA and the doubly degenerate TA branches were measured. At the zone boundary the observed frequencies are (1.09±0.04) THz (LA) and (0.71±0.03) THz (TA). For modes propagating along [100] and [110], the LA branch and the TA branch with modes polarized predominantly alongc were measured. A group-theoretical analysis of the phonon symmetries is given for the major symmetry points in the zone, and the corresponding atomic displacements for the atoms composing a «layer» are derived. By assuming rigid layers the results are analysed to obtain a value for the interlayer interaction. This is compared with corresponding values obtained from the acoustic dispersion curves of other layered structures.RiassuntoSi presentano i risultati di una misura delle curve di dispersione dei fononi acustici nella struttura a strati semiconducente SnSe2. Si sono effettuate le misurazioni presso il reattore NRU, Chalk River, usando la tecnica dello scattering coerente anelastico dei neutroni. Si sono misurate, per modi che si propagano lungo la direzionec*, sia le branche LA che quelle doppiamente degeneri TA. Al confine di zona le frequenze osservate sono (1.09±0.04) THz (LA) e (0.71±0.03) THz (TA). Si sono misurate per modi che si propagano lungo [100] e [110] la branca LA e la branca TA con modi polarizzati prevalentemente lungoc. Si fà un’analisi teorica gruppale delle simmetrie fononiche per i punti principali disimmetria nella zona e si deducono gli spostamenti atomici corrispondenti per gli atomi che compongono uno «strato». Si analizzano, assumendo strati rigidi, i risultati per ottenere un valore per l’interazione fra strati. Si confronta questo valore con i valori corrispondenti ottenuti dalle curve di dispersione acustica di altre strutture lamellari.РезюмеМы предлагаем результаты измерения кривых дисперсии акустических фононов в полупроводящей слоистой структуре SnSe2. Измерения проводились на NRU реакторе, в Чок Ривере, с помощью техники когерентного неупругого рассеяния нейтронов. Для мод, распространяющихся вдольc*-направления, измеряются LA и дважды вырожденные TA ветви. На границе зон наблюдаемые частоты составляют (1.09±0.04) Тгц (LA) и (0.71±0.03) Тгц (TA). Для мод, распространяющихся вдоль [100] и [110], измеряются ветвь LA и ветвь TA с модами, преимущественно поляризованными вдольc. Проводится анализ симметрий фононов на основе теории групп для главных точек симметрии в зоне. Выводятся соответствующие атомные смещения для атомов, составляющих «слой». Предполагая жесткие слои, анализируются полученные результаты, чтобы определить величину, характеризующую взаимодействие между слоями. Эта величина сравнивается с соответствующими величинами, полученными из акустических дисперсионных кривых для других слоистых структур.

Thermoreflectance of GaTe near room temperature

Abstract Thermoreflectance measurements on GaTe near 300 K are reported between 1.6–4.75 eV. Strong polarisation effects have been found to depend on the orientation of the electric vector with respect to the b-axis. Correlations between the present results and previously reported work are given in addition to the enumeration of hitherto unobserved features.

Photoreflectance of InSb at the E1 and E1 + Δ1 transitions

Abstract The Photoreflectance at high interband transitions ( E 1 and E 1 + Δ 1 ) is obtained at 8 K for InSb bulk samples. For a p -type sample, the lineshape is well reproduced by Aspens third derivative model, while for an undoped sample, the photoreflectance spectra is better reproduced by a first derivative of an exciton lineshape, suggesting the presence of a saddle-point exciton at low temperature. The transition energies determined from the spectra indicate an excitonic resonance at about 12 meV below the interband transitions in agreement with the 2D exciton model. The transition broadenings obtained from the first derivative function agree with the rigid pseudo-ion model predictions.

The optical properties of SrTiO3 above 0.14 eV

Abstract The narrow absorption peak at 163 meV has been studied as a function of temperature. It is proposed that the peak is due to an electronic transition at a localised defect and that it and its satellites at higher energy are typical of a general class of absorption spectra in ionic crystals.

Band-to-band transitions and free exciton states in low-symmetry crystalline GaTe

In this paper we derived the selection rules for band-to-band transitions in low-symmetry crystalline GaTe and explained the low-temperature (2 K) absorption spectra at two different polarization configurations for this material. It is found that the top two valence bands of crystalline GaTe at the high symmetry Γ-point have one-dimensional degeneracy and are weakly separated (<0.5 meV). From this band structure, we re-analyzed the free exciton states in crystalline GaTe and found that the X1 peak is composed of the heavy-hole (HH) and light-hole (LH) singlet excitons, while the X2 peak is composed of the HH and LH triplet excitons.

Hot electron induced negative photoconductivity in n-InSb/GaAs with above gap illumination at low temperature

Abstract The low temperature photoconductivity under pulsed illumination has been studied experimentally in n-InSb/GaAs heterostructures grown by metalorganic magnetron sputtering (MOMS). At photon energies slightly greater than the band gap of InSb (up to ≈ 0.4 eV), we observe the usual positive photoconductivity effect due to the increase in the density of carriers. However, illumination with photons of still higher energies (≥ 0.4 eV) leads to the coexistence of a positive and a slow negative photoconductivity. The latter, which increases with increasing photon energy and decreasing film thickness, is tentatively associated with diffusion of hot electrons to the interface and with a reduction of the electron mobility in the layer.

Thermoreflectance study of direct and indirect excitons near the fundamental edge of GaSe

Abstract Thermoreflectance spectra have been obtained on GaSe single crystals between 50 and 300 K and a theoretical model is developed to fit the experimental curves. The main feature of the spectra is due to the disappearance of the back reflection of the sample caused by the small absorption below the direct exciton. The direct excitonic structure is observed at higher energy.

Effect of rapid thermal annealing on the optical and structural properties of highly strained InAsInP quantum well structures

Abstract The effect of rapid thermal annealing on the photoluminescence and x-ray diffraction characteristics of a highly strained InAs InP single quantum well structure grown by low-pressure metalorganic chemical vapor deposition has been investigated. Rapid thermal annealing has been found to significantly improve the crystalline quality of the strained InAs InP single quantum well structure. The luminescence intensity of the quantum well at 8 K was increased by a factor of 4 after annealing at 700 °C for 5 s. The results obtained by high-resolution x-ray diffractometry have demonstrated no noticeable change of the structure in samples annealed at lower temperatures (below 750 °C). At higher temperatures, rapid thermal annealing deteriorated the x-ray diffraction characteristics, and also reduced the quantum well luminescence efficiency. Furthermore, we found that the luminescence efficiency of the deep radiative levels in the samples were also affected by rapid thermal annealing.