Compositionally graded layers of GaAs1−xPx grown on (100) planes of GaAs substrates were investigated as a function of the distance t from the substrate, using a 77 °K cathodoluminescence technique. It was found that luminescence intensity L of these layers was strongly dependent on the profile of the compositional gradient, dx/dt. A number of the nonradiative recombination centers are three−dimensionally distributed in the shape of a cross−grid. This is related to misfit dislocations associated with impurities in the graded layers. The gradient dependence of L demonstrates that the density of these nonradiative recombination centers, N, is proportional to (dx/dt)2. This quadratic relation shows that only grading profiles which satisfy the condition d2x/dt2<0 can successfully decrease N continuously with increasing t. Discontinuous variations of x and/or dx/dt cause a significant degradation in the subsequent layers. From the spatial variation in the ratio of the intensity of the 1.35−eV band to the near−gap band, it was found that an anomalous accumulation of Cu contaminant occurred at the substrate−epitaxial−layer interface region, where a relatively high density (∼4×108 cm−2) of these nonradiative recombination centers was present. This is because of a very steep compositional gradient (∼20% P/μ). Here, the concentration of Cu was optically estimated through cathodoluminescence data from Cu−diffused GaAs samples.
The structural quality of normalGaAs layers grown on Ge substrates has been optically characterized as a function of the distance d from normalGaAs‐normalGe interface along the growth axis, supported by electrical measurement and also by x‐ray measurement. It is found that the degradation of the structural quality due to interface alloying and autodoping of Ge can be detected in the grown layers only within ∼3μ from the interface, where the concentration of Ge involved would exceed 1×1017 normalatoms/cm3 . However, the degradation caused by misfit dislocations extends as far as 80μ from the interface, in spite of the fact that the dislocation density decreases exponentially with increasing d . These misfit dislocations introduce quite effective nonradiative recombination centers in normalGaAs , which quench the luminescence intensity, L , in a form of L=Alogfalse(1×106/Nfalse) , where N is the density of these nonradiative recombination centers and A is a constant. If the normalGaAs layers are contaminated with Cu during the growth process, it is found that L is reduced in such a way that the constant A in the above equation is decreased.
Die strukturelle Qualität einer auf Ge epitaktisch gewachsenen GaAs‐Schicht wird als Funktion des Abstandes von der GeAs‐Ge‐Grenzfläche durch Messung der Kathodo‐ und Photolumineszenzeigenschaften bei 77 K und der elektrischen Eigenschaften bei 350 K untersucht (Röntgentopographie, Transmissionselektronenmikroskopie).
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