C. Jordan

Optical measurement of the ambipolar diffusion length in a ZnCdSe-ZnSe single quantum well

We describe a straightforward technique for the measurement of carrier diffusion in semiconductors. Using an optical microscope we can spatially image luminescence with a resolution of ∼500 nm. We measured the ambipolar diffusion length in a Zn0.75Cd0.25Se–ZnSe single quantum well by fitting the spatially resolved luminescence profile with the solution of the two-dimensional diffusion equation. The ambipolar diffusion length was found to be 498 nm at a carrier density of ∼1×1018 cm−3 and we deduce an ambipolar diffusion constant of 1.7 cm2 s−1.

Carrier-density dependence of the photoluminescence lifetimes in ZnCdSe/ZnSSe quantum wells at room temperature

Photoluminescence lifetimes have been measured at room temperature as a function of carrier density in ZnCdSe/ZnSSe quantum wells. We show that, at low carrier density (5×109–5×1010 cm−2), nonradiative recombination dominates, while radiative recombination becomes more dominant as the carrier density is increased from 5×1010 to 5×1011 cm−2. Above ∼5×1011 cm−2, band filling effects are shown to produce a saturation of the lifetimes. A simple rate equation model approach can be used to describe the carrier density dependence of the photoluminescence decay data obtained on a wide range of samples. A representative band-to-band recombination coefficient of 8×10−4 cm2 s−1 and a Shockley–Read–Hall rate of 7.3×107 s−1 were determined for one of the better samples studied. We believe that the excellent quality of our samples has allowed for the radiative recombination coefficient to be characterized accurately at room temperature.

Optical gain in (Zn, Cd)Se-Zn(S, Se) quantum wells

We have investigated the mechanism of stimulated emission in ZnCdSe–ZnSSe quantum wells through optically pumped measurements of the gain spectrum in a variety of structures from 270 to 77 K. We also calculated the optical gain, using a model that includes many-body effects, and found excellent agreement between the calculated gain line shapes and our measurements. Under the conditions studied, which are close to those found in an operating laser diode, we conclude that the stimulated emission arises from an electron–hole plasma in our samples, even down to 77 K. Although our measurements do not rule out exciton gain mechanisms at other temperatures or operating conditions, sensitive line-shape fitting does not require them in our case. However, our line-shape analysis does show that Coulomb enhancement is significant, even at room temperature.

Calculation of gain-current characteristics in ZnCdSe-ZnSe quantum well structures including many body effects

The gain‐spontaneous recombination characteristics have been calculated for a 40 A Zn0.8Cd0.2Se‐ZnSe quantum well including many body effects. We examine the effect of the inclusion of the Coulomb enhancement on the gain spectra and the gain‐current relationship. We show that, in the absence of the Coulomb enhancement, the threshold current density of a 340 μm 40 A Zn0.8Cd0.2Se‐ZnSe quantum well laser is underestimated by approximately 40% and the lasing wavelength overestimated by 4 nm. Our calculation of the scattering lifetime for the first electron‐heavy hole transition gives a lifetime varying between 29 and 37 fs, and shows that the carrier‐phonon scattering mechanism in II‐VI quantum wells is more dominant than in III‐V materials. We also comment on the effect the neglect of Coulomb enhancement has on the calculation of leakage currents in a laser at threshold.

Optical gain in ZnCdSe-ZnSe quantum well structures

We have measured the gain spectrum of an optically pumped 40 angstrom ZnCdSe-ZnSe multiple quantum well. Our calculation, which includes many body effects such as Coulomb enhancement and spectral broadening due to carrier scattering, gives excellent agreement with the experimental gain measurements. We then show the importance of the inclusion of the Coulomb enhancement for the calculation of optical gain when predicting laser threshold currents. This is emphasized by using our gain calculation as a basis to theoretically optimize a simple ZnCdSe-ZnSe quantum well laser structure incorporating the leakage current over the p-type cladding.

Confocal photoluminescense microscopy in II-VI materials: annealing and degradation dynamics

Confocal photoluminescence imaging is an important tool in the investigation of recombination in semiconductors and in the characterization of material growth. This characterization is particularly important for II-VI wide band-gap semiconductors where the potential for blue-green lasers is being explored currently. To achieve room-temperature cw operation of these lasers over the multi-thousand hours necessary for commercialization, extremely low defect densities are required. The confocal microscope is used in this work to image photoluminescence from II-VI materials to characterize the defect formation and propagation within the quantum well region of the material. This imaging approach permits the degradation to be monitored in real time and over a large area in samples with low defect densities. The additional advantages of this set-up over a conventional microscope are, of course, the higher lateral resolution and narrow depth of field associated with a confocal microscope. While considerable effort has been focused on the degradation in these II-VI semiconductors, we have recently observed that annealing can occur simultaneously in the same sample when the material is exposed to intense optical excitation. Images of annealing and degradation of a range of II-VI samples will be presented to highlight these observations.

Recombination lifetimes in undoped and doped ZnCdSe laser structures

The carrier recombination dynamics in doped and undoped ZnCdSe quantum well structures are presented using time-resolved photoluminescence spectroscopy. Measurements were performed at room temperature as a function of excitation intensity upto lasing densities of 10 19 cm -3 . A simple rate equation model enabled us to determine the relative contributions from the radiative and non-radiative recombination mechanisms to the photoluminescence lifetime. This model showed good agreement with results obtained for undoped structures. In contrast, the interpretation of results obtained for pn doped laser structures was found to be more difficult due to the presence of carrier trapping at defects. A model developed to take account of carrier trapping gives a good fit to the lifetimes for the doped structures.

Defect annealing in a II–VI laser diode structure under intense optical excitation

Defect annealing under intense pulsed optical excitation has been observed in a II‐VI laser diode structure at room temperature. More than one order of magnitude increase in photoluminescence intensity has been obtained when the annealed area is probed at low excitation intensity. High-resolution confocal photoluminescence images of the annealed region do not show any sign of degradation. Together, these results suggest that an initial density of intrinsic point defects present within the active region can be removed by the optical annealing. Recombination-enhanced defect reactions in the vicinity of the point defects are responsible for this nonthermal annealing effect.

Optical gain in (Zn,Cd)Se-Zn(S,Se) quantum wells as a function of temperature

Abstract We have investigated the mechanism of stimulated emission in ZnCdSeZnSSe quantum wells through measurements of the optical gain spectrum between 77 and 270 K. We also calculated the optical gain using a model which included many-body effects and found excellent agreement with our measurements. Our results are inconsistent with an excitonic gain mechanism and we conclude that the stimulated emission arises from an electron—hole plasma in our samples. However, we find that the electron-hole Coulomb interaction is still significant at room temperature in II–VI heterostructures.

Optical Gain in ZnCdSe-ZnSe Quantum Well Structures

We have measured the gain spectrum of an optically pumped 40A ZnCdSe-ZnSe multiple quantum well. Our calculation, which includes many body effects such as Coulomb enhancement and spectral broadening due to carrier scattering, gives excellent agreement with the experimental gain measurements. We then show the importance of the inclusion of the Coulomb enhancement for the calculation of optical gain when predicting laser threshold currents. This is emphasised by using our gain calculation as a basis to theoretically optimise a simple ZnCdSe-ZnSe quantum well laser structure incorporating the leakage current over the p-type cladding.