GaN layers were grown by molecular beam epitaxy and doped with carbon of nominal concentrations ranging from 1016 cm−3 to 1020 cm−3. The incorporation of carbon leads to a reduction of the background electron concentration by one order of magnitude but the material remains n-type. For high carbon concentrations a re-increase of the carrier concentration is observed which is related to selfcompensation. Investigations of the donor-acceptor-pair luminescence show that doping with carbon is accompanied by the generation of a new donor exhibiting a thermal activation energy of about 55 meV. Layers grown by atomic layer epitaxy are marked by an increased intensity of the donor-acceptor-pair band luminescence which is attributed to the enforced incorporation of carbon onto the nitrogen sublattice. The yellow luminescence is found to be a typical feature of all carbon doped layers in contrast to nominally undoped samples.
The realization of a monolithic all II-VI-based vertical cavity surface emitting laser (VCSEL) for the green spectral region is reported. Optically pumped lasing operation was achieved up to room temperature using a planar VCSEL structure. Taking advantage of distributed Bragg-reflectors based on MgS/Zn(Cd)Se superlattices as the low-refractive index material and ZnS 0.06 Se 0.94 layers as the high-index material with a refractive index contrast of ∆n = 0.6, a quality factor exceeding Q = 2000 is reached by using only 18 Bragg periods for the bottom DBR and 15 Bragg periods for the top DBR. The threshold power density is 0.32 MW/cm 2 at a temperature of 10 K (emission wavelength 498.5 nm) and 1.9 MW/cm 2 at room temperature (emission wavelength 502.3 nm).1 Introduction Up to now, only II-VI-based laser diodes are able to provide laser emission in the blue-green spectral region. Edge-emitters using ZnCdSSe quantum wells as the active part cover the whole wavelength range between 500 nm and 560 nm just by varying the Cd content [1], while a recently demonstrated laser containing CdSe quantum dots shows emission at 560 nm [2]. However, an electrically pumped monolithic vertical-cavity surface-emitting laser (VCSEL) emitting in the blue-green spectral region has not been realized yet. This kind of devices is of high interest concerning optical data transmission using plastic optical fibers (POFs) because of their circular shaped low-divergence beam profile and the fact that they can easily be integrated into optical systems. POFs have their absorption minimum in the green and therefore II-VI-based surface emitters will increase the transmission length compared to the presently used red emitters where the damping is higher. Because of the small resonator length of a few hundred nanometers, the cavity of a VCSEL has to be surrounded by mirrors with reflectivities exceeding 99%. This can be achieved only by distributed Bragg reflectors (DBRs) consisting of a sufficient number of Bragg pairs. A Bragg-pair is made of a two layers with different refractive index and quarterwave optical thickness each. The higher the refractive index contrast between these layers, the lower is the overall number of Bragg-pairs needed for a certain reflectivity. Furthermore, DBRs with high index contrast provide a comparatively high field amplitude inside the cavity and therefore an improved overlap of optical wave and active region.In this paper we report on the growth of high-reflectivity DBRs with high index contrast using MgS/Zn(Cd)Se superlattices (SLs) for the low refractive index material and ZnSSe for the high index material. The structural properties and emission characteristics under optical pumping for a VCSEL realized by using these DBRs will be discussed.
We show that energy position and line shape of donor-acceptor-pair luminescence bands in ZnSe:N/ GaAs epilayers depend very sensitively on excitation density and compensation. A continuous development from structureless red-shifted broad to well structured donor-acceptor-pair ͑DAP͒ bands is observed for increasing excitation density. The red shift is explained by the fluctuating potential affecting the bands and impurity levels and is caused by random distribution of charged impurities in highly compensated samples. The shift is reduced when these charge fluctuations are diminished due to an increasing number of impurities being neutralized via light-induced carrier excitation. These effects have not been taken into account in previous work concerning doped II-VI materials; however, they have to be considered when evaluating the frequently used hypothesis of a deep donor in ZnSe:N as an explanation of low-energy broadband DAP emission. The influence of band fluctuations on the behavior of the DAP luminescence and excitation spectra is qualitatively discussed.
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