S. P. Guo

Quaternary Znx′Cdy′Mg1−x′−y′Se/ZnxCdyMg1−x−ySe quantum wells grown on InP substrates for blue emission

Quaternary Znx′Cdy′Mg1−x′−y′Se/ZnxCdyMg1−x−ySe quantum well (QW) structures in which both the well and the barrier layers are composed of quaternary alloys lattice matched to InP and having various well thicknesses have been grown and investigated. A blue emission with narrow linewidth was achieved by using a relatively thick well thickness (40–60 A). The quaternary QW emission exhibits excitonic recombination behavior and it has higher quantum efficiency than the ternary ZnxCd1−xSe/ZnxCdyMg1−x−ySe QW of comparable thickness. The temperature dependence of the photoluminescence measurements shows a high activation energy of 68 meV, indicating a strong quantum confinement by introduction of Mg in the QW region.

Ion-beam-produced damage and its stability in AlN films

Structural characteristics of single-crystal wurtzite AlN epilayers (grown on sapphire substrates) bombarded with 300 keV Au-197(+) ions at room and liquid-nitrogen temperatures (RT and LN2) are studied by a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy. Results reveal extremely strong dynamic annealing of ion-beam-generated defects in AlN. Lattice amorphization is not observed even for very large doses of keV heavy ions at LN2. An increase in irradiation temperature from LN2 to RT has a relatively small effect on the production of stable structural damage in AlN. In contrast to the case of AlxGa1-xN with xless than or equal to0.6, neither damage saturation in the crystal bulk (below the random level) nor preferential surface disordering is revealed for AlN. Results also show that structural lattice disorder produced in AlN by high-dose keV heavy-ion bombardment is stable to rapid thermal annealing at temperatures as high as 1000 degreesC

Structural disorder in ion-implanted AlxGa1−xN

The accumulation of structural damage in AlxGa1-xN films (with x=0.05-0.60) under heavy-ion bombardment at room temperature is studied by a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy (XTEM). Results show that an increase in Al concentration strongly enhances dynamic annealing processes in AlGaN and suppresses ion-beam-induced amorphization. All AlGaN wafers studied show damage saturation in the bulk for high ion doses. Interestingly, the disorder level in the saturation regime is essentially independent of Al content. In contrast to the case of GaN, no preferential surface disordering is observed in AlGaN during heavy-ion bombardment. XTEM reveals similar implantation-produced defect structures in both GaN and AlGaN

Patterned three-color ZnCdSe/ZnCdMgSe quantum-well structures for integrated full-color and white light emitters

We report the growth and characterization of patterned ZnCdSe/ZnCdMgSe quantum-well (QW) structures grown adjacent to each other on a single InP substrate. Each structure emits at a different wavelength range spanning the visible range. Stripe and square-shaped QW structures of different emission wavelengths, with lateral dimensions between 15 and 60 μm, were deposited sequentially by shadow mask selective area epitaxy (SAE) steps. Conventional and microphotoluminescence measurements were used to characterize the patterned QWs. They exhibit well-defined excitonic emission in the red, yellow, and green regions of the visible spectrum. This result demonstrates the feasibility of fabricating integrated full-color light emitting diode and laser-based display elements and white light sources using the ZnCdMgSe material system and shadow mask SAE.

Molecular-beam epitaxy growth and nitrogen doping of ZnSe1−xTex alloys grown on InP substrates

High-quality ZnSe1−xTex alloys spanning the entire compositional range have been grown by molecular-beam epitaxy. Good compositional control can be obtained by adjusting the beam equivalent pressure ratio of Se to Zn during growth. Double-crystal x-ray rocking curves with full widths at half maximum as narrow as 70 arcsec were obtained suggesting excellent crystalline quality. The p-type doping with nitrogen of ZnSe1−xTex alloys having Te contents from 0% to 100% has been systematically studied. The free hole concentration increases from 1017 to 1019 cm−3 as the Te content increases from 12% to 40%. The N-doped ZnSeTe lattice matched to InP has a free hole concentration of 2×1019 cm−3. This highly doped material was used as the p-type ohmic contact layer in light-emitting diodes made from ZnCdMgSe/ZnCdSe quantum-well structures grown on InP substrates that emit in the red, green and blue regions.

Growth and characterization of BeCdSe alloys and BeCdSe/ZnCdMgSe quantum wells on InP substrates

We report the molecular-beam-epitaxy growth and characterization of BexCd1−xSe (0.06<x<0.2) epilayers and Be0.08Cd0.92Se/Zn0.32Cd0.25Mg0.43Se quantum-well (QW) structures on InP substrates. Good optical properties and high crystalline quality were established using photoluminescence and x-ray diffraction measurements. Narrow x-ray rocking curves with line widths down to 49 arc sec were obtained for Be0.2Cd0.8Se, closely lattice-matched to InP. A strong luminescence emission with energy of 2.072 eV and a full width at half maximum of 27 meV at 77 K was obtained from a QW structure with a 48-A-thick QW. Strong room-temperature luminescence was also observed from the QW. A linear dependence of the QW photoluminescence intensity on the excitation laser density and an absence of shift in the emission energy indicates that the QW emission has an excitonic behavior. Based on these results and on the expected lattice-hardening properties of BeSe, we propose that BeCdSe is an attractive quantum-well material for v...

Distributed Bragg reflectors based on (Zn, Cd, Mg)Se for use in the visible spectral range

Distributed Bragg reflectors (DBRs) based on ZnCdSe and ZnCdMgSe quarter wave layers have been grown on InP (001) substrates by molecular beam epitaxy. Their reflectivity maxima were controlled by the individual thicknesses of constituent layers, and were adjusted in the range of 615–500 nm, covering the red, green, and blue-green regions of the visible spectrum. The crystal quality of the structures was assessed by double crystal x-ray diffraction measurements. Reflectivity as high as 95% was achieved for the DBR structures that consisted of 16 periods. Theoretical calculations, based on the transfer matrix method, predict that a reflectivity above 99% can be achieved by increasing the number of periods to between 20 and 26.

Effects of Be on the II–VI/GaAs interface and on CdSe quantum dot formation

The effects of Be on the II–VI/GaAs interface and on CdSe quantum dot (QD) formation were investigated. A (1×2) surface reconstruction was observed after a Be–Zn coirradiation of the (001) GaAs (2×4) surface. ZnBeSe epilayers grown after the Be–Zn coirradiation show very high crystalline quality with x-ray rocking curve linewidths down to 23 arcsec and a low etch pit density of 4×104 cm−2, and good optical quality with a band-edge photoluminescence (PL) emission peak linewidth of 2.5 meV at 13 K. However, ZnBeSe epilayers grown after Zn irradiation alone have poor crystalline quality and poor optical properties. Atomic force microscopy measurements show that CdSe QDs grown on ZnBeSe have higher density and smaller size than those grown on ZnSe. A narrower PL emission peak with higher emission energy was observed for the CdSe QDs sandwiched by ZnBeSe. These results indicate that the formation of CdSe QDs as well as the II–VI/GaAs interface are modified by the presence of Be.

p-type doping of (Zn,Mg,Cd)Se alloys using a radio frequency discharge nitrogen plasma source

(Zn,Mg,Cd)Se quaternary alloys lattice matched to InP, a novel material for the fabrication of light emitting devices with light emission spanning the visible range, are difficult to dope p-type while their n-type doping is relatively easy. To address this issue, the p-type doping of the Zn1−xMgxSe and Zn1−yCdySe ternaries was investigated as a function of composition using a radio frequency discharge nitrogen plasma source. Besides conventional (uniform) doping, a type of δ doping that involves codoping with N and Te [(N+Te)δ doping] was performed. The p-type doping level was increased by the (N+Te)δ doping, however, the net acceptor concentration (NA–ND) of both ternaries decreased with the addition of either Mg or Cd, indicating that both Mg and Cd atoms hinder the p-type doping of the alloys. To overcome these doping difficulties, we propose the growth of a Zn1−x′−y′Mgx′Cdy′Se/ZnSe:N superlattice structure, in which the p-type doping is selectively performed only in the ZnSe region, as a substitute fo...

Distributed Bragg reflectors for visible range applications based on (Zn,Cd,Mg)Se lattice matched to InP

Lattice-matched (Zn,Cd,Mg)Se epilayers were grown by molecular-beam epitaxy on InP substrates. X-ray diffraction and photoluminescence measurements show the high crystalline quality of the epilayers. Using a prism coupler technique, the index of refraction of the epilayers was investigated at four discrete wavelengths. With these results, (Zn,Cd,Mg)Se-based distributed Bragg reflector structures have been grown, covering nearly the entire visible spectral range. A maximum reflectivity of 95% has been achieved for structures having 16 periods.