[en] We investigate the polarization dependence of the absorption, reflectance, and photoreflectance spectra of a compressively strained, M-plane, wurtzite GaN(11(bar sign)00) film grown by molecular-beam epitaxy on a γ-LiAlO₂(100) substrate. The measurements are done with the electric-field vector (E) of the probe light being parallel ((parallel sign)) and perpendicular ((perpendicular sign)) to the c axis of GaN, which lies in the growth plane. We observe a significant increase in the effective optical band gap of the M-plane GaN film for E(parallel sign)c compared to its value for E(perpendicular sign)c. This result is explained by including the effect of the M-plane biaxial compressive strain on the electronic band structure of GaN. We also determine the extraordinary refractive index of GaN at energies below its band gap from the reflectance measurements

[en] Light emitting diodes for the visible spectral region based on planar (In,Ga)N/GaN heterostructures suffer from a high dislocation density and lack of suitable substrates. An alternative is the integration of these heterostructures into nanowires (NWs) grown on Si by molecular beam epitaxy. Indications for the absence of the quantum-confined Stark effect (QCSE) have been reported in the literature, when the piezoelectric polarization is reduced due to an efficient strain relaxation in the NW geometry. In order to elucidate the origin of the observed luminescence centered at 2.4 eV, we combine transmission electron microscopy, cathodoluminescence and micro-photoluminescence (μ-PL) spectroscopy on single NWs. The μ-PL spectra contain a combination of two types of transitions: (i) several sharp lines from localization centers, which are not affected by the excitation power and (ii) a broader band that blueshifts with higher excitation powers. The former are probably related to composition fluctuations in the (In,Ga)N, while the latter is attributed to an inter-well transition between the two 11 nm thick (In,Ga)N insertions separated by an only 2 to 3 nm thick barrier layer. The blueshift under high excitation evidences a screening of the polarization field. Thus, the QCSE appears to be present in these NW heterostructures in contrast to previous reports.

[en] We have directly measured the carrier diffusion length L_d of dilute nitride single quantum wells (QWs) by means of spatially resolved cathodoluminescence (CL) spectroscopy as a function of the CL detection energy (E) for different temperatures. While L_d (E) of a GaAs/(Al,Ga)As reference sample does not vary with E, L_d (E) of (In,Ga)(As,N)/GaAs QWs is significantly reduced with increasing E. The qualitative difference of L_d (E) between the binary and quaternary QWs is explained in terms of different transport mechanisms (hopping versus tunnelling, respectively) in conjunction with the presence of localized states. L_d of (In,Ga)(As,N) QWs decreases rapidly with increasing temperature. L_d can be significantly reduced by the presence of extended defects even at low temperatures. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] Static domain formation in doped, weakly coupled semiconductor superlattices results in several current branches separated by discontinuities that exhibit hysteresis. The transition from one branch to its adjacent higher and lower one is studied by time-resolved switching experiments. The distribution function of the relocation time for up and down jumps changes from a simple Gaussian to a first-passage time (FPT) form, when the final voltage on the adjacent branch approaches a discontinuity in the current–voltage characteristic. This observation indicates that for a smaller voltage distance from the discontinuity (FPT distribution) the system reaches the final state via an unstable intermediate state. In contrast, for a larger voltage distance from the discontinuity (Gaussian distribution), the system arrives directly at the final state, because the intermediate state does not exist.