Epitaxial layers of ZnSe and ZnS have been grown on GaAs (100) substrates by low-pressure metal-organic vapor phase epitaxy (MOVPE) and etched out from their substrates, The optical qualify of the layers has been investigated at 2 K by low-intensity reflection, transmission and photoluminescence measurements.At high excitation intensities, the near-band-edge luminescence has been determined in both ZnS and ZnSe samples. Exciton and biexciton contributions have been studied. (C) 1997 Elsevier Science S.A.

We report on gain profile and spectral composition of stimulated emission from an optically excited MOVPE GRINSCH ZnCdSe-ZnSe quantum well.

The samples studied are thin GaN epilayers grown by metalorganic vapor phase epitaxy (MOVPE). At liquid helium temperature, the linear luminescence, transmission and reflection spectra of the films has been recorded, as well as the nonlinear emission spectra under high UV excitation. By using the variable strip length method, we have measured the gain coefficient as a function of the excitation intensity and the temperature. Pump and probe experiments have been performed in the femtosecond regime, showing crossed two-photon absorption without long lasting response. This allows to envisage the use of GaN for the characterization femtosecond pulses in the blue spectral range.

We have studied the stimulated emission of MOVPE-grown quantum wells of Zn0.78Cd0.22Se, sandwiched between two thicker, zinc-richer layers. The gain of these samples has been measured, as a function of temperature, using the variable strip-length method. Its mechanism has been shown to be the radiative recombination of an inhomogeneously broadened exciton system.

The variable stripe length method is used to study optical gain in MOVPE-grown ZnSGaAs epitaxial layers. Optical gain and spontaneous emission spectra are extracted from experimental results. A net optical gain of about 30 cm−1 under excitation by a XeCl laser having a power density of 100 kW cm−2 is observed at very low temperatures (T < 20 K) and it is rapidly quenched for higher temperatures. Gain is interpreted to be due to stimulated emission from the bound exciton state and from the biexciton ground state towards free exciton levels. The spectral shape of the gain spectrum is well fitted by the sum of these two contributions. Smaller gain at lower photon energies is obtained due to exciton-excition collisions.

The detailed characterization of metal organic vapour phase epitaxy grown ZnS layers on GaAs is the first step towards the study of their different non-linear optical properties performed with nanosecond lasers. Biexciton phenomena (with a binding energy of about 10 meV) are observed in photoluminescence-excitation and optical-gain spectra.

High quality MOVPE-grown ZnS/GaAs epilayers are characterized by reflection, transmission and linear photoluminescence (PL) spectroscopy at 2 K. The GaAs substrate of a part of each ZnS layer is removed and the optical properties of as-grown and etched surfaces are compared. The splitting (caused by the thermally induced strain) of the topmost Gamma(8) valence band into the heavy and light-hole subbands (Delta(hl)=4 meV) and the transitions involving exciton ground and excited states are better seen in PL excitation spectra than in ordinary PL spectra. From the analysis of the nonlinear PL and PL excitation spectra, we can conclude the existence of biexcitons with a binding energy of about 10 to 12 meV under an intermediate intensity of excitation (a density of about 2.5x10(16) to 2.5x10(17) e-h excited pairs per cm(3)).