STRUCTURAL CHARACTERIZATION OF HIGHLY STRAINED INAS N-MONOLAYER LASERS AND QUANTUM-WELL STRUCTURES BY X-RAY-DIFFRACTION AND TRANSMISSION ELECTRON-MICROSCOPY

X-ray interference effect and transmission electron microscopy are used to study the relaxation process in a series of laser structures as a function of InAs content in the quantum well. It is shown that the X-ray interfrence effect is a powerful, fast and non-destructive method to assess the strain status in samples of this kind. A set of strained layer laser structures containing N monolayers of InAs (N X (InAs)1(GaAs)3 with N = 1, 3, 5, 7) in an 8 nm quantum well active region and a set of strained layer quantum wells consisting of P monolayers of InAs (P x (InAs)1(GaAs)Q with P = 2, 4 and Q = 2, 4) were grown [Dotor et al., J. Crystal Growth 127 (1993) 46] by atomic layer molecular beam epitaxy. X-ray interference effect and cross-section transmission electron microscopy analysis of the samples show that in the series of lasers with N monolayers of InAs the whole laser structure is coherent with the substrate (and consequently dislocation free) for 1 and 3 monolayers of InAs, while a sample with 5 monolayers of InAs is in a certain stage of relaxation (dislocation density n(d) congruent-to 10(7) cm-2) and a sample with 7 monolayers of InAs is almost completely relaxed (n(d) congruent-to 10(8) cm-2). In strained layer quantum well samples, the influence of the InAs/GaAs thickness ratio (P/Q) on the critical thickness has also been studied. These results are compared with those predicted by theoretical critical thickness models. Optical characterization as well as threshold current measurements of the lasers are correlated with X-ray diffraction and transmission electron microscopy relaxation status results.