Three independent but complementary methods (OSL photoetching combined with etch rate profiling, spatially resolved PL and LST) were employed to Study the distribution of microdefects and electrically active centres in commercially available SI undoped. LEc-grown GaAs after different ingot-annealing treatments. A one-to-one correlation was obtained on comparing the microscale distribution of decoration precipitates (OPS), matrix precipitates (MPI) and microdefects (MMS) by the DSL and LST methods. Clustering of MPS is also revealed by PL intensity profiling and photoetch rate measurements. Two types of matrix microdefects are distinguishable by OSL photoetchlng In Samples after two-stage annealing. A high degree of homogeneity across the dislocation cells (after low-temperature and multiple annealing) was clearly revealed by a drastic decrease in the uniformity parameter (U) after DSL photoetching and by a significant decrease in the x intensity measured at the cell walls with respect to the cell interiors. Using the present results and recent data from the literature, an explanation of phase transitions during ingot annealing is proposed. This is based on the assumption that the final properties and structures after annealing are the result of two competitive processes: (i) formation of decoration and matrix microdefects; (ii) generation of electrically active point defects (clusters), presumably EL2
We have studied undoped GaAs of shallow acceptor concentrations close to and below 5·1014cm-3. After ingot annealing with slow cooling rate this material shows resistivity values below 107 Ωcm, spanning several orders of magnitude. By an additional annealing procedure these values can be raised to some 107 Ωcm. DLTS results are presented indicating that the electrical characteristics in the ingot annealed state are governed by uncompensated deep donors shallower than EL2, mainly EL6 near E
c -0.35 eV. After the additional annealing a drastic decrease in EL6 concentration is observed, which explains the rise in resistivity. This is confirmed by theoretical calculation of the resistivity in dependence of the EL2 and EL6 to net acceptor compensation ratio, which agrees quantitatively with the experimental values.
The contrast formation of localized non-radiative semiconductor defects in cathodoluminescence (CL) micrographs is analyzed using a linear relationship between the CL intensity and the excess carrier density 8p. The latter is evaluated on the basis of Donolato's Born approximation and for generation by a point source. Numerical calculations for a threading dislocation a t right angle to a surface of infinite recombination velocity yield an exponential decay of the CL contrast a t sufficient distance from the dislocation with a decay constant of 0.63 minority carrier diffusion lengths. The contrast mechanism is seen to be based primarily on the reduction of the total number of excess carriers due to enhanced recombination a t the dislocation rather than on the associated reduction of the luminescence quantum efficiency.Die Kontrastbildung lokalisierter nicht-strahlender Halbleiterdefekte in Kathodolumineszenz (KL)-Mikrobildern wird unter Zugrundelegung einer linearen Beziehung zwischen der KL-Intensitiit und der tfberschuBladungstrigerdichte Sp analysiert. Sp wird auf der Grundlage von Donolatos Bornscher NiCherung und unter der Annahme punktformiger Generation ermittelt. Numerische Berechnungen fur eine Versetzung senkrecht zur Oberfliiche, fur die eine unendliche Oberfliichenrekombinationsgeschwindigkeit zugrunde gelegt wird, ergeben fur genugend grol3e Abstiinde von der Versetzung einen exponentiellen Abfall des KL-Kontrasts mit einer Abfallkonstante von 0,63 Minoritiitstriigerdiffusionsliingen. Es zeigt sich, daD der Kontrastmecbanismus primiir auf der Reduktion der gesamten OberschuDtriigerzehl durch verstiirkte Rekombination an der Versetzung beruht und nicht so sehr auf der damit verbundenen Reduktion der Lumineszenzquantenausbeute.
The electron beam induced conductivity (EBIC) of zinc oxide varistor ceramics is studied in the scanning electron microscope (SEM). It is found that only particular grain boundaries give rise to an EBIC signal and that the signal strength and its linescan profile show strong variation with bias voltage. The experimental results are discussed in terms of Schottky emission of majority carriers across the grain boundary potential barrier
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