We have improved the feasibility of ZnMgSSe as cladding layers for blue and green laser diodes. Based on an ZnCdSe/ZnSSe/ZnMgSSe separate- confinement heterostructure (SCH), we have succeeded in attaining continuous-wave (CW) operation for blue-emitting laser diode (LD) with a wavelength of 489.9 nm and pulsed operation with a wavelength of 480.5 nm at room temperature. We also have achieved pulsed operation up to 834 mW at room temperature with a wavelength of 507 nm, and CW operation up to 80 degree(s)C. The device characteristics of II_VI wide band-gap LDs are virtually as good as the established III-V materials based LDs except the device lifetime. The life time of an MQW-SCH LD has been improved by using both a GaAs and a ZnSe buffer layers, and is as long as 9 min at room temperature under CW operation.

ZnMgSSe/ZnSSe/ZnCdSe laser structures were grown on n-type GaAs substrates by Molecular Beam Epitaxy (MBE). We have mounted lasers substrate up and substrate down and measured the thermal resistance. We have also fabricated index-guided and gain-guided laser devices enabling a direct comparison of their optical and electrical performance. Thermal measurements were made on gain-guided devices soldered substrate-up and substrate-down onto a copper heat sink. For devices with contact stripe width of 20 X 600 micrometers ², the thermal resistance is 48 Kelvin/Watt for substrate down mounting and 31 Kelvin/Watt for substrate-up mounting. CW lasing has been obtained for both mounting configurations. To allow for a direct comparison, 10 micrometers wide gain- guided structures were fabricated using an oxide as an insulating layer beside the contact stripe. The laser threshold voltage was 15 volts for this case. Both 2.5 micrometers and 10 micrometers width index-guided lasers were fabricated using reactive ion etching. The structure is a mesa etched to a depth to produce a real lateral refractive index step of approximately 3 X 10^-3. No facet coatings were used. When operated with 100 nsec pulses, the gain-guided devices have an external differential efficiency of .25 Watts/Amp and a strongly astigmatic beam. The 10 micrometers width index-guided devices show a clear reduction in astigmatism and an improvement of external differential efficiency to .33 Watts/Amp. The threshold current density and voltage remain the same.

We report the use of photoluminescence imaging as a quick and effective method for determining defect densities and giving insight into degradation mechanisms in II-VI Cd_xZn_1-xSe quantum well devices and heterostructures grown on GaAs. From our use of photoluminescence imaging we have observed that the device lifetimes are dependent on the stacking fault density. The stacking faults serve as nonradiative recombination centers that generate the dark line defects. In our studies, degradation rates were found to be independent of chlorine doping, barrier material, and the removal of the GaAs substrate.

The rapid degradation mechanism of current II-VI blue-green laser diodes based on the pseudomorphic ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructures (SCHs) grown by MBE has been studied by using electroluminescence (EL) and transmission electron microscopy (TEM). Nonluminescent dark defects observed in the laser diodes have been identified to be dislocation networks consisting of branches of dislocation dipoles developed at the quantum well region. Stacking fault-dislocation complexes introduced into the laser structure during the MBE growth have been found to be the origin for the nucleation of the dislocation networks. The experimental results indicate that the point defect density at the quantum well region can be reduced by optimizing the growth conditions of the region.

The growth of ZnMgSSe is a key component of long-lived II-VI laser diodes, however there are structural and electrical problems associated the compound that must be resolved. Precise lattice matching requires accurate growth temperature control due to the dependance of the sulfur sticking coefficient on the growth temperature. Electrically, p-type doping of ZnMgSSe is complicated by an apparent increase in the activation energy caused by a mechanism very much like that of the DX center in n-type III-V alloys. These factors must be addressed for II-VI light emitting devices in deep blue part of the spectrum.

One of the main problems for the fabrication of optoelectronic devices in the blue spectral range is the limited p-dopability and the resulting high contact resistivity of p-ZnSe. We show that the admixture of even small Te fractions to ZnSe has a strong beneficial effect on doping levels and contacts of MBE grown material doped with DC nitrogen plasma. Since the energy gap decreases through the admixture of Te, we additionally introduced Mg in order to remain in the blue range of the spectrum. P-doping levels above 10¹⁸ cm^-3 and Ohmic contacts to ZnMgSeTe could be obtained for Te contents of more than 20%, the highest levels being 10²⁰ cm^-3 for Zn_(1-x)Mg_xSe_(1-y)Te_y with x < 0.2 and y > 0.4. On the other hand, the admixture of Te to ZnSe strongly reduced the obtained n-doping levels. As a viable alternative we used n-type layers of Chlorine doped ZnMgSe. This allowed the growth of pn-junctions of n-ZnMgSe and p- ZnMgSeTe. The Mg content in the n- and p-layer was adjusted in a way that both layers had the same lattice constant and the same doping level of 10¹⁸ cm^-3. The diodes emit light in the blue-green spectral range at 77 K. Although the optical properties have to be improved for device applications, ZnMgSeTe could be an interesting alternative for blue light emitters since high doping levels, ohmic p- contacts and low operation voltages are obtainable in a relatively easy way.

Gas source molecular beam epitaxy (GSMBE) of ZnSe has been performed with emphasis in understanding the incorporation of substitutional donor and acceptor impurity species. Elemental Zn and hydrogen selenide provide the constituent species, whereas ZnCl₂ and nitrogen gas, combined with the use of a radio frequency plasma cell, provide the dopant species for n- and p-type ZnSe, respectively. The hydrogenation behavior of ZnSe:Cl and ZnSe:N are compared, and we have found that the presence of hydrogen does not significantly affect the electrical behavior of n-type layers. However, the presence of hydrogen very effectively passivates acceptor species of nitrogen to prohibit p-type conductivity. Conventional and rapid thermal annealing have been investigated to modify the degree of hydrogen passivation.

We have grown high quality lattice-matched ZnCdSe and ZnSeTe on InP. To optimize the interfaces, the initial growth temperature was lowered and an As flux was used during the thermal treatment of InP substrates prior to epitaxial growth. Under optimized condition, 2D nucleation was observed by reflection high energy electron diffraction (RHEED) throughout the entire growth. Photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM) were used to carry out the sample characterization. Low temperature PL spectra for ZnCdSe show a narrow excitonic emission. PR spectra from ZnCdSe samples also suggest very high quality layers. The ZnSeTe exhibits a strong defect level emission at energy close to band gap and very weak deep level emission. TEM study suggest that the interfaces are comparable to those obtained between ZnSe and GaAs. These results, combined with the new possibilities from these materials, make InP an attractive substrate for II-VI epitaxy.

Stimulated blue light emission has now been produced via electrical injection through a p-n junction using MBE grown II-VI wide bandgap semiconductors. This is not yet possible using the MOVPE technique, due to the somewhat unsuccessful realization of p-type doping of ZnSe. Microgun pumped lasers (where injection is achieved using an array of microtips as an electron gun) could be an alternative solution, but requires structures of high crystalline quality based on ZnSe/ZnCdSe. In this perspective, we have studied the MOVPE growth of ZnSe layers and ZnCdSe alloys and ZnCdSe based heterostructures using two different zinc precursor adducts: triethylamine-dimethylzinc (TEA:DMZ) and tetramethylmethylenediamine dimethylzinc (TMMD:DMZ). The growth conditions have been investigated and the solid composition versus gas phase composition has been studied for both adducts. The structural properties of ZnSe layers below the critical thickness have been studied versus the initial growth conditions and optimal growth conditions are proposed. Particular attention has been devoted to the investigation of the spectroscopic properties of the heterostuctures in view of the characterization of the abruptness of the heterointerfaces. Photoluminescence, reflectance and photoreflectance experiments will be presented.

Spectroscopic work is reviewed which focuses on the microscopic mechanism of gain in ZnSe-based quantum well (QW) lasers, under optical and electrical injection, respectively. Excitonic processes are rather distinct at cryogenic temperatures in the strongly quasi-2 dimensional case of a ZnCdSe QW. More strikingly, recent studies on the room temperature diode lasers show that electron-hole pairwise Coulomb correlations remain relevant in this case as well.

Two important requirements for improving blue-green laser devices towards room temperature, cw operation are to decrease barrier resistance at the ZnSe-GaAs heterointerface for p-type materials, and, in general, reduce contact resistance in p-n junction devices. In both areas, application of newly developed interfacial engineering methods holds substantial promise. For ZnSe-GaAs heterojunctions, we have exploited molecular beam epitaxy growth kinetics to achieve different interface configurations and change the band alignment. Our results indicate a strong correlation between the Zn/Se beam pressure ratios employed during the early growth stage, the interface composition and the band offsets. Interface stability, however, also depends on interface composition. As far as ohmic contacts are concerned, the fabrication of suitable local interface dipoles at metal/II-VI interfaces should be explored as a new method to lower the Schottky barrier.

We report the results of pump and probe transmission and magneto- stimulated emission experiments performed in ZnCdSe/ZnSe multiple quantum wells of different well width and composition. The pumping conditions for the excitonic bleaching and the stimulated emission are directly correlated to the exciton stability in different samples. A transition from excitonic to free carrier lasing is observed by finely tuning the injection rate around the exciton ionization threshold.

This paper addresses the growth of short period ZnS-ZnSe superlattices by low pressure Metal-Organic Vapor Phase Epitaxy. We have correlated the photoluminescence line shape to the interface roughness within the context of a phenomenological interface disorder. Using additional reflectivity experiments we could develop envelope function calculation and find the band offset. Finally we have calculated the exciton binding energy in ZnSe-ZnS quantum wells in the context a the variational approach using models of varying sophistication.

A model to calculate the optical gain due to excitonic transitions is developed and used to calculate unstrained as well as strained ZnCdSe/ZnMgSSe multiple quantum well (MQW) lasers. Strain induced changes in energy band gap and effective masses of light and heavy holes are included in the gain coefficient and threshold current density calculations. The theoretical simulations are matched with the experimental data on compressively strained ZnCdSe-ZnSSe devices grown on GaAs substrates. Our calculations predict lower threshold current density for the tensile strained Zn_.8Cd_.2Se-Zn_.2Mg_.8S_.03Se_.97 quantum well lasers grown on InP substrates. Unlike the III-V strained layer quantum well lasers, the contribution to gain coefficient due to excitonic transitions is predominant in II-VI systems as the exciton binding energy is larger by a factor of 5. This results in a primary role for excitons in lasing, which has been verified experimentally.

In this paper we describe Scanning Tunnelling Microscopy (STM) studies of the initial stages of the growth of ZnSe on GasAs(001). The starting GaAs(001)-(2 X 4) surface on GaAs epilayers and on oxide desorbed surfaces has been imaged. The GaAs epilayer surface is found to be much smoother than the GaAs oxide desorbed surface. ZnSe growth on a Se- terminated GaAs-(2 X 1) surface has been found to result in 3D nucleation and island growth and results in a high density of stacking faults in the ZnSe film. ZnSe growth started directly on the GaAs-(2 X 4) surface is more 2D, although rather disordered, and results in a lower stacking fault density in the ZnSe film.

New sources for heterogeneous nucleation of both <$110> and <110> extended screw-type misfit dislocations to relax the lattice shear stress on the ZnS_xSe_1-x/GaAs interface has been observed for the first time. These are identified as Shockley partial dislocations originating in areas close to the ZnS_xSe_1-x/GaAs interface. The Shockley partials form to accommodate the stacking errors produced upon island coalescence. In-situ electron beam- induced heating studies were carried out to observe the dislocation generation mechanism in the films. Our results show that the stress (sigma) approximately equals 1 X 10⁹ dyne/cm² stored in the ZnS_xSe_1-x films gives rise to bowing of the threading segments of the Shockley partials. The bowing result from the fact that the dislocations are pinned at the film/substrate interface and at the film surface. This process involves an increase in the length of the treading segments under the stress. With further accommodation of the lattice strain, the line tension of the bowing threading segments is relieved by the movement of the pinning points at the film surface accompanied by gliding of the threading segments toward the film/substrate interface. This process takes place by the movement of either one or both of the pinned points at the film surface. Finally, a segment of extended screw- type dislocation is generated when the segments of the Shockley partials reach the ZnS_xSe_1-x/GaAs interface. A <110> extended screw-type interfacial dislocation is generated by gliding of the threading segments of the Shockley partials on (111)-type planes with Burger vectors b equals a/6<121>-type and a/6<211>-type toward the interface. On the other hand, a <110> extended screw-type interfacial dislocation is generated by gliding of the threading segments of the Shockley partials on (111)- type planes with Burger vectors b equals a/6<121>-type and a/6<211>-type toward the interface. Finally, the shear stress between film and substrate is relaxed by the generation of a grid of extended interfacial dislocations with screw components along the <110> and <110> directions.

It has been shown in the literature that heavy N-doping of ZnSe leads to the introduction of deep donors, which are evidenced by new luminescence bands, deeper than the 'standard' donor-acceptor pair band. We suggest, based on reported shifts of the peaks, with excitation intensity, of the new bands, as well as on differences in their position among different samples, that these bands may involve 'strong' preferential pairing between the deep donors and the N acceptors. This type of pairing would indicate a high ion mobility of the deep donors. We have calculated the peak shifts, as a function of excitation intensity, for various reasonable parameter values. Based on these results, we conclude that strong preferential pairing cannot, at this time, be proven, but is nevertheless strongly indicated. We suggest that a proof should be possible by studying the intensity dependence over a wider range.

This paper describes the usage of a novel asymmetric resonant tunneling structure (RTS) to obtain low voltage drop contacts to pZnSe and other wide energy gap semiconductors. Generally, the contact to p-ZnSe is achieved by the formation of a Schottky barrier or by forming a graded layer interface to pZnSe. These techniques have been employed in fabricating ZnSe based blue-green lasers, reported during the past few years using structures grown by Molecular Beam Epitaxy. Most of these approaches result in contact voltage drops ranging from 6-30 volts for a typical current above laser threshold. In the case of pGaAs-pZnSe (or pZnCdSe) interface, the presence of a large valence energy band offset results in high voltage drop due to a rectifying interface. The use of asymmetric resonant tunneling structure(s) at these heterointerfaces is shown to result in a significantly low (0.4 volts) voltage drops at current densities above threshold (approximately equals 600 A/Cm²). The incorporation of asymmetric resonant tunneling structures is also proposed for lasers on InP substrates. It is a generic technique which can be used for realizing low resistance contacts in material systems where standard contact techniques produce poor results. This technique is applicable for both p- and n-type heterojunction interfaces.

Strained layer Zn_1-xCd_xSe/ZnSe multiple quantum well structures are presently at the core of most blue-green lasers. Laser degradation has focused attention on such issues as residual strain, stress concentration, and dislocation nucleation and propagation in these materials. We fabricated Fabry-Perot cavities through cleavage in Zn_1-xCd_xSe/ZnSe multiple quantum well structures grown by molecular beam epitaxy with x equals 0.24, 0.3 and x equals 0.50 and a variety of quantum well parameters. The cavity facets were examined by means of atomic force microscopy and lateral force microscopy. We found previously unreported, cleavage-induced nanosize defects related to the discontinuity in the mechanical properties of the strained layer quantum wells. In most cases, the defects can be assimilated to surface delamination features preferentially located at the boundaries between the strained active ternary material and the relatively unstrained binary buffer or barrier layers. Such features run parallel to the interface, exhibit typical depths and surface widths in the 10 nm range, and may act as important stress concentrators during laser processing and operation.

Rapid degradation is a severe problem for the new blue-green laser diodes, even leading to speculation that the cause may be a fundamentally unstable lattice. Recent work has shown, however, that the mechanisms of degradation appear to be similar to those seen in GaAlAs systems. Twelve reasons, encompassing electronic, mechanical, thermodynamic and chemical parameters, are given that demonstrate the ruggedness of II-VI materials. Factors that could lead to enhanced degradation are also discussed. Several reasons for the rapid degradation of these diodes are postulated: strain, recombination- enhanced diffusion and phase instability. Approaches to improvements in lifetime, such as using material with hexagonal crystal structure and magnesium doping, are suggested.

Gas-source molecular beam epitaxy (GSMBE) was applied for the growth of ZnMgSSe layers and quantum well (QW) structures. The source materials were elemental Zn and Se, as well as gas sources of bis- methylcyclopentadienyl-magnesium ((MeCp)₂Mg) and H₂S. Mg and S compositions were well controlled by the flow rate of (MeCp)₂Mg and H₂S, respectively. ZnSe/ZnMgSSe QWs with abrupt heterointerface have successfully been grown on [100]-oriented GaAs substrates under in-situ monitoring of specular beam intensity oscillation in reflection high energy electron diffraction (RHEED). Photoluminescence (PL) at 4.2 K revealed sharp and intense emission from single QWs, which is attributed to n equals 1 heavy-hole free exciton. The photopumped lasing of a double heterostructure was achieved at room temperature with low threshold excitation intensity (110 kW/cm²), suggesting formation of well-defined heterostructures and promising potential of GSMBE for device applications.