L. Vanzetti

Microgun‐pumped blue lasers

Microgun‐pumped lasers, in which electron beam pumping is achieved via an array of low‐voltage, field‐emission microtips, were demonstrated in the blue and blue‐green regions of the spectrum. The devices exploit graded index, separate confinement Zn1−xCdxSe/ZnSe heterostructures grown by molecular beam epitaxy on In1−xGaxAs(100) substrates. Lasing thresholds were in the 4–20 kW/cm2 range for temperatures between 83 and 225 K, and the device lifetime exceed several hours at a 12.5% duty cycle for a laser operating at 83 K and a wavelength of 478.4 nm.

Optimization of interface parameters and bulk properties in ZnSe‐GaAs heterostructures

The flux parameters employed during molecular beam epitaxy of ZnSe on GaAs control both the heterojunction band alignment and the optical quality of the II–VI wide‐gap layer. Independent optimization of the band discontinuities and ZnSe optical properties was achieved by fabricating a 2‐nm‐thick interface layer in nonstoichiometric conditions to control the band alignment, followed by the optically active layer in near‐stoichiometric conditions.

Structural properties of ZnSeGaAs(100) heterostructures with engineered band offsets

Abstract The band discontinuities in ZnSeue5f8GaAs(100) heterostructures grown by molecular beam epitaxy depend on the local interface composition. We used a thin (20 A thick) composition control layer fabricated at the interface in Zn-rich or Se-rich conditions to control the band offsets for heterostructures in which the bulk of the II–VI epilayer was fabricated in stoichiometric growth conditions. Here we focus on the resulting variations in atomic interdiffusion across the interface, and on the local and long-range structural properties of the II–VI epilayer.

Band-offset determination in Zn1−xCdxSeZnSe multiple quantum wells

We report the experimental determination of conduction-band discontinuities in Zn 1-x Cd x Se/ZnSe multiple quantum well structures grown by molecular beam epitaxy and pseudomorphically strained to ZnSe. The values have been obtained from electrical and intersubband absorption measurements for 0.23 ≤ x ≤ 0.33. The offset is found to vary from 165 ± 14 to 253 ± 14 meV with increasing cadmium concentration in the examined range. The resulting valence-band discontinuity yields rather poor light-hole confinement. This will be shown to affect the linear optical properties of quantum well heterostructures.

Low resistance graded contacts to n‐type ZnSe

The specific resistance of metal contacts to Cl‐doped, n+‐type ZnSe can be lowered by over one order of magnitude for comparable doping by fabricating a Zn1−xCdxSe ternary layer of the interface. Photoemission studies of the Al/Zn1−xCdxSe Schottky barriers, and current–voltage measurements indicate that the decrease in the specific contact resistivity reflects the grading of the conduction band discontinuity across the heterostructure.

Influence of growth parameters on the properties of ZnSe-GaAs(001) heterostructures

Abstract ZnSe-GaAs(001) heterostructures were fabricated by molecular beam epitaxy at 290°C employing different Zn/Se flux ratios. Se-rich growth conditions resulted in a Se-rich interface composition and lower valence band offsets (as low as 0.6 eV). Zn-rich conditions yielded a Zn-rich interface composition, and higher valence band offsets (as high as 1.2 eV). Improved optical properties of the ZnSe overlayers were achieved by employing non-stoichiometric growth conditions only in the early stages of growth (∼ 2 nm).

Degradation of microgun-pumped blue lasers

Microgun-pumped blue lasers with lasing thresholds in the 4-20 kW/cm(2) range for temperatures between 83 and 225 K were fabricated by molecular beam epitaxy. The devices exploit graded index, separate confinement Zn1-xCdxSe/ZnSe heterostructures and use an array of low-voltage field-emission microtips for electron beam pumping. Degradation occurs through the formation of characteristic (100) dark-line defects, as in diode lasers. The device lifetime exceeded several hours at 83 K for a laser operating at 478.4 nm with a 12.5% duty-cycle.

Interfacial engineering in blue laser structures

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.

Continuous wave half-gap second-harmonic generation in ZnCdSeZnSe asymmetric coupled quantum wells

We report continuous-wave interband second-harmonic generation (SHG) at 460-480 nm in strained Zn 1-x Cd x Se/ZnSe asymmetric quantum wells grown by molecular beam epitaxy. The dependence of the SHG intensity on the fundamental frequency allowed us to identify the individual quantum-well exciton resonances in the second-harmonic signal. The strongest contribution to SHG associated with the light-hole exciton is discussed and related to confinement properties of the light-hole. Numerical calculations of the SHG intensity that take into account exciton and continuum states have been performed as a function of the valence-band contribution Q v to the band-gap difference in the heterostructures. Best agreement with experiment is found for Q v = 0.33.

Improved contact resistance to n-type wide gap II–VI semiconductors

The Schottky barrier at Al/Zn 1-x Cd x Se junctions was determined through high resolution synchrotron radiation photoemission studies of interfaces fabricated in situ on atomically clean, Se-rich surfaces. A gradual decrease in the n-type barrier with increasing x reflects the bandgap variation in the ternary alloy and the band alignment in Zn 1-x Cd x Se/ZnSe heterojunctions. Correspondingly, transport studies of Al/n-Zn 1-x Cd x Se contacts fabricated ex situ show a dramatic decrease of the specific contact resistance with increasing x. Improved performance of Al/n-ZnSe contacts can therefore be achieved through fabrication of graded Zn 1-x Cd x Se layers at the interface.