J. Gutowski

Impurity and defect distribution in ZnTe/GaAs epilayers of different thickness

Abstract For the understanding of impurity incorporation and self-compensation effects, which prevent an efficient n doping of ZnTe, the knowledge of the main acceptor types and their distribution in the layer is imperative. To make progress concerning these problems we investigate systematically ZnTe/GaAs epilayers of different thickness grown by hot-wall epitaxy (HWE) by using optical and X-ray rocking-curve characterization methods. Additionally, ZnTe samples were used which are annealed in vacuum or As atmosphere, as well as free-standing ZnTe samples with the GaAs substrates removed. Based on the experimental results, a model of the acceptor and strain distribution in ZnTe/GaAs epilayers is developed, involving three differently distributed acceptor types originating from source contaminations, As atoms diffused from the GaAs substrate, and dislocation-related centers. Further, we observe a strong relaxation of the thermally induced strain with increasing layer thicknesses.

Comparative optical investigations of ZnSe/GaAs epilayers grown by molecular beam and hot‐wall epitaxy

Comparative photoluminescence and excitation spectra of ZnSe/GaAs epilayers grown by molecular beam epitaxy (MBE) and hot‐wall epitaxy (HWE) show likewise features in the exciton energy regime. Two strain‐split components of the free exciton are observed, as well as characteristic sets of transitions from or into ground and excited states of acceptor‐ and donor‐bound excitons. However, all respective lines are shifted to lower energies in the HWE samples, due to the increased thermally induced strain as a consequence of the substrate temperatures being enhanced compared to the MBE growth. Whereas the dominant donors are of the same nature in both kinds of samples, specific acceptor centers are incorporated in the HWE films. Although the MBE‐grown layers are of superior quality, it is shown that HWE under optimized growth conditions is a cheap and useful alternative to obtain ZnSe epilayers of reliable properties.

Excitonic processes in highly excited ZnSe epilayers

Generally, luminescence spectra of ZnSe layers grown by molecular-beam epitaxy (MBE) show strong high-density P bands. Supported by excitation measurements, we assigned these P bands to exciton-exciton collision processes. Actually, the situation changes drastically for free-standing ZnSe layers obtained after removal of their GaAs substrates. For Iexc⩾150 kW/cm2, a strong band appears at 2.7835 eV, here called N band. For increasing Iexc, it shifts stronger towards longer wavelengths than the P bands, which vanish at highest excitation densities. We discuss the N band in terms of electron-hole plasma (EHP) formation in differently treated ZnSe epilayers, taking into account results from photoluminescence, excitation, reflection and gain spectroscopy.

Thermally induced optical bistability in ZnSe epilayers grown by molecular‐beam epitaxy

For the first time, ZnSe epitaxial layers grown by molecular‐beam epitaxy are shown to exhibit large contrast, low power, and extremely long‐term stable thermally induced absorptive optical bistability. It is observed over a wide temperature range between 169 K and RT, with a strongly variable loop width and a switch‐back adjustable to be less than 50% of the respective switch‐down value. Critical slowing down as well as switch‐down times are studied in dependence of the excess over the switch‐down intensity values.

Optical properties of molecular beam epitaxy grown ZnTe epilayers

Systematic comparative studies of the optical properties in the excitonic energy regime of MBE-grown ZnTe/GaAs and ZnTe/GaSb epilayers are presented. For these different substrate materials we investigate the influence of strain between layer and substrate the possible incorporation of impurities the electronic structure of the impurity-related exciton complexes and biexciton recombination processes at high-density excitation.

Growth and characterization of ZnSe and ZnTe grown on GaAs by hot-wall epitaxy

Two different versions of hotwall--epitaxy reactors were used to grow ZnSe and ZnTe layers. In the first type of hotwall epitaxy reactor Zn and Se were evaporated seperately from elemental sources. By changing the Zn/Se ratio in the gas phase we could alter the growth direction of the ZnSe layers from 111 to 100 Raman spectroscopy proved the existence of a Ga2Se3 layer at the GaAs/ZnSe interface. In the other HWE reactor stoichiometric ZnSe and ZnTe were used as source materials. Photoluminescence and Xray rocking curves proved the high quality of the epilayers. The rocking curves showed a full width at half maximum of 75 arcseconds for ZnTe.

Long-term stable thermally induced absorptive optical bistability in MBE-grown ZnSe epilayers

Abstract MBE-grown ZnSe epilayers released from their GaAs substrates exhibit thermally induced absorptive optical bistability of high contrast ratios and low switching powers in a wide temperature range up to room temperature. For increasing sample thickness or for enhanced reflection realized by inserting the samples into a resonator, counteracting dispersive switching becomes visible. The interplay of both the absorptive and dispersive effects is studied for different samples and varied experimental parameters. Long-term stability of the samples with regard to all switching effects makes them promising candidates for application studies.

Dispersive and absorptive thermally induced optical bistability in ZnSe epilayers

Abstract Using expitaxial ZnSe layers of various crystalline quality, grown by molecular-beam (MBE) and hot-wall epitaxy (HWE), we investigated thermally induced absorptive and dispersive optical bistability in dependence on layer thickness and starting temperature. To optimize absorptive switching for samples of minor quality and to obtain dispersive switching ZnSe epilayers are embedded in external Fabry-Perot resonators, fabricated by evaporated Au and Ag mirrors of different reflectivities.

Nonlinear transmission of MBE-grown ZnSe epilayers in the near-band-edge region

Abstract The nonlinear optical transmission of ZnSe epilayers grown by molecular beam epitaxy (MBE) is studied in the near-band-edge region. In single-beam transmission as well as in pump-and-probe experiments at 1.8 K, induced absorption is observed in the excitonic regime for increasing (pump) laser intensity. The diminishing of the excitonic absorption features, the failure to observe specific pump-laser resonances of the induced absorption in pump-and-probe experiments, and the fact that the ‘efficiency edge’ of the pump process is shifted to lower energies for increasing excitation intensity show that electron-hole plasma is created in the thin layer which prevents longitudinal carrier diffusion. However, at 77 K, induced transmission is observed in the excitonic regime indicating exciton bleaching, most probably caused by exciton screening and collision broadening for excitation densities not yielding excess of the Mott density of the created carriers.