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Excitons and Phonons in GaN. Magnetooptical and Spatially Resolved Investigations

100%

Hoffmann A., Eckey L., Siegle H., Kaschner A., Christen J., Bertram F., Liu Q.

Acta Physica Polonica A

1998

vol. 94

issue 2

125-137

A comprehensive study of the direct photoluminescence from free-exciton states in GaN using polarization-dependent and magnetooptical measurements is presented. We measured and identified fine-structure splittings in the n=1 state of the A-exciton. From the magnetoluminescence data obtained in fields up to 15 T we determined the g-values of the conduction and valence bands parallel and perpendicular to the c-axis. Self-organized hexagonal GaN pyramids of 5 μm width and covered by six {11̲01} side facets were investigated by spatially resolved cathodoluminescence and micro-Raman spectroscopy. Beside a narrow luminescence peak at 355 nm, originating from the 2 μm thick GaN layer, an additional broad luminescence band was observed from the GaN pyramids around a wavelength of 357 nm. A strong energy shift is found along the {11̲01} pyramidal facets and directly visualized by monochromatic cathodoluminescence images and line scans. In GaN epilayers grown on GaAs substrates a series of sharp modes in the range between 60 cm^{-1} and 250 cm^{-1} for temperatures below 100 K was found. The intensities of these modes increased drastically with decreasing temperature. Raman excitation spectra showed a maximum between 514.5 nm and 568 nm. A comparison of spatially resolved investigations with that of intentionally doped GaN epilayers showed that the in-diffusion of As from the substrate plays an important role. Raman spectra as a function of external fields, like magnetic field and hydrostatic pressure, gave additional information about the defect type and the underlying scattering mechanism.

Calorimetric Absorption Spectroscopy of Deep Defects and Quantum Dots

100%

Heitz R., Podlowski L., Böhrer J., Hoffmann A., Broser I., Bimberg D.

Acta Physica Polonica A

1995

vol. 88

issue 4

619-630

In recent years calorimetric absorption spectroscopy has been developed to a powerful tool of semiconductor spectroscopy based on the detection of nonradiative relaxation processes. Calorimetric absorption spectroscopy is an ultrasensitive quantitative absorption technique. Recent investigations of Fe in III-V semiconductors and of InAs/GaAs quantum dots are presented here to illustrate the potential of the method. Sharp absorption lines are observed at the low energy onset of the Fe^{3+/2+} charge transfer band in III-V semiconductors. Calorimetric absorption spectroscopy measurements in the mK range reveal a strong temperature dependence of their absorption strength identifying unambiguously Fe^{3+}(^{6}A_{1}(S)) as a ground state. The excited state is attributed to (Fe^{2+},h). The importance of exchange interaction for the observed fine structure is pointed out and binding energies are determined. The quantum yield of the intracenter ^{5}Τ_{2}-^{5}E transition of F^{2+} is determined to be below 50% at 2 K. A correlation between the nonradiative relaxation rate and the isotope splitting of the ^{5}Τ_{2}-^{5}E transition is observed, demonstrating the crucial role of the dynamical Jahn-Teller coupling of the ^{5}Τ_{2} state to local Τ_{2} modes for the multiphonon relaxation process. Quantum dots having a d-function density of states should exhibit no Stokes shift between absorption and emission as observed for one- and two-dimensional systems. Calorimetric absorption spectroscopy demonstrates ground state absorption coinciding in energy with the luminescence for self- organized InAs/GaAs quantum dot structures grown by MBE. Transitions into excited hole states are resolved and a comparison to photoluminescence excitation spectroscopy is presented.

Photoluminescence of Co-Doped ZnCdSe and ZnSSe Alloys

84%

Surkova T., Born H., Thurian P., Hoffmann A., Busse W., Gumlich H., Broser I., Giriat W.

Acta Physica Polonica A

1997

vol. 92

issue 5

1013-1016

Photoluminescence measurements have been carried out for Zn_{1-x} Cd_{x} Se:Co and ZnS_{x}Se_{1-x}:Co mixed crystals. Changes of recombination channels are observed in mixed crystals as compared to the emission of host binary compounds. Character of changes is slightly different for the alloys with cation and anion substitution. Photoluminescence kinetics of the L-line and of two other Co^{2+} intra-shell emission bands was measured to determine radiative decay rates.

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3 Acta Physica Polonica A

1 1998

1 1997

1 1995

3 Hoffmann A.

2 Broser I.

1 Bertram F.

1 Bimberg D.

1 Born H.

1 Busse W.

1 Böhrer J.

1 Christen J.

1 Eckey L.

1 Giriat W.

1 Gumlich H.

1 Heitz R.

1 Kaschner A.

1 Liu Q.

1 Podlowski L.

1 Siegle H.

1 Surkova T.

1 Thurian P.

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Excitons and Phonons in GaN. Magnetooptical and Spatially Resolved Investigations

Calorimetric Absorption Spectroscopy of Deep Defects and Quantum Dots

Photoluminescence of Co-Doped ZnCdSe and ZnSSe Alloys