E. Ho

Reduction of structural defects in II–VI blue green laser diodes

Early blue/green laser diodes based on ZnSe exhibited room temperature, continuous wave (cw) lifetimes of the order of a minute. Similar to the history of (Al,Ga)As lasers, the source of the degradation was the presence of extended crystalline defects. The dominant extended defects in the early room temperature cw lasers originated as stacking faults generated at the ZnSe/GaAs heterovalent nucleation event, and exhibited densities of the order of 106 cm−2. In this letter, a procedure is described which will ensure a consistent run to run reduction of the density of such extended defects to the mid to low 103 cm−2 over a 3 in. wafer.

Hydrogen passivation in nitrogen and chlorine‐doped ZnSe films grown by gas source molecular beam epitaxy

The incorporation of hydrogen in ZnSe:N and ZnSe:Cl films grown by gas source molecular beam epitaxy (GSMBE) using elemental Zn and H2Se as source material has been investigated. The hydrogenation behavior was found to be significantly enhanced when nitrogen was used as a dopant, and typically resulted in highly resistive films. On the other hand, Cl‐doped ZnSe films showed a hydrogen concentration at or near the background levels independent of the Cl concentration. ZnSe was also grown by conventional molecular beam epitaxy with intentionally introduced H2 in order to clarify the source of the hydrogen. Significant hydrogen incorporation was observed in the MBE‐grown ZnSe:N layers only when hydrogen gas was introduced. Injection of hydrogen in excess of the amount generated during typical GSMBE experiments was found to give rise to an unambiguous increase in the hydrogen concentration, but with a hydrogen:nitrogen ratio less than that measured in GSMBE films.

Photoassisted metalorganic molecular beam epitaxy of ZnSe

Photoassisted heteroepitaxy of ZnSe on GaAs by metalorganic molecular beam epitaxy has been performed using sources of diethylselenium and diethylzinc. Illuminating the substrate during growth with an Ar ion laser has been observed to significantly enhance the growth rate. Growth rate enhancement was found to be a function of substrate temperature, VI/II gas flow ratio, and laser wavelength and intensity. Photons having energies sufficient to generate electron/hole pairs in the growing ZnSe film resulted in growth rate enhancement. The photoassisted growth has application for (i) increasing the anomalously low growth rate which is observed, (ii) assisting in tuning the surface stoichiometry, and (iii) providing for selective area epitaxy.

Gas source molecular beam epitaxy of ZnSe and ZnSe:N

The use of gas source molecular beam epitaxy, using hydrogen selenide and elemental Zn as source materials, has resulted in the growth of high quality ZnSe on closely lattice-matched GaAs and (In,Ga)P. The undoped ZnSe epilayers are comparable in quality to material grown by molecular beam epitaxy, as indicated by narrow double-crystal x-ray diffraction rocking curves and intense photoluminescence dominated by a single donor-bound near-bandedge excitonic feature. Nitrogen species, derived from a radio frequency plasma source, are successfully used as acceptor impurities for ZnSe; photoluminescence spectra confirm the incorporation of nitrogen by the presence of the expected donor-to-acceptor pair recombination. Atomic concentrations of nitrogen as high as 5 x 1018 cm-3, are measured by secondary ion mass spectroscopy. Thus far, capacitance-voltage measurements indicate net acceptor concentrations (NA -ND) of approximately 1017 cm-3.

(In,Ga)P buffer layers for ZnSe-based visible emitters

Abstract The growth of ZnSe on pseudomorphic and partially relaxed (In,Ga)P epitaxial buffer layers has been investigated. The (In,Ga)P and ZnSe layers were grown in separate gas source molecular beam epitaxy systems using elemental sources for the cation species and cracked PH 3 and H 2 Se for the anion species. Surface morphology studies using scanning electron microscopy showed that the ZnSe layers were featureless at a magnification of 1.2x10 4 . Four crystal (400) X-ray rocking curves indicated that the ZnSe full width at half maximum (FWHM) was 130″, while the pseudomorphic (In,Ga)P buffer layer FWHM was 18″. The (511) reflections of X-ray rocking curves were used to measure the residual strain as well as the composition in the (In,Ga)P buffer layers. The low temperature photoluminescence spectra from the ZnSe films grown on partially relaxed (In,Ga)P exhibited intensities of the donor-bound and free exciton transitions of nearly equal magnitude, as well as transitions due to extended defects, suggesting highly pure material. The luminescence from the (In,Ga)P buffer layers was also detected.

Elimination of surface site blockage due to ethyl species in MOMBE of ZnSe

The metalorganic molecular beam epitaxial growth of ZnSe using diethylzinc and/or diethylselenium gas sources results in an abnormally low growth rate of several hundred angstroms per hour. Experiments with dimethylzinc or elemental zinc with elemental selenium confirm that adsorbed ethyl-based radicals contribute to the low growth rate. Surface sites for incorporation of the metal atom are saturated by the chemisorbed ethyl radicals or by an ethyl molecule that is postulated to cause the growth rate limitation. It is observed that laser and electron beam irradiation overcome the site blockage phenomenon under appropriate growth conditions. For beam-assisted growth, significant increases over the unilluminated growth rate are measured. Experiments designed to investigate the wavelength dependence of the photon-enhanced growth rate provide evidence that photo-generated electron/hole pairs are necessary to assist in releasing the ethyl radical from the unpyrolyzed DEZn molecule that is adsorbed to the surface.

Comparison of hydrogen passivation of ZnSe:N using gas source and conventional molecular beam epitaxy

Abstract The use of various gaseous source epitaxial techniques for the growth of II–VI compounds, such as metalorganic vapor phase epitaxy (MOVPE) or gas source molecular beam epitaxy (GSMBE), offers the promise of improved flux control and source flexibility. However, passivation of the nitrogen acceptors by hydrogen (which is usually present in abundance in these growth environments) has proven to be a major obstacle in the successful application of MOVPE and GSMBE for the growth of highly conductive p-type ZnSe. Although there have been a few reports of successful nitrogen doping of ZnSe:N by GSMBE and MOVPE, the mechanisms of hydrogen incorporation, and its dependence on the growth conditions as well as post-growth treatments, are still unclear. In this paper, we will compare the hydrogen passivation behavior in ZnSe:N grown by: (1) GSMBE using H 2 Se and elemental Zn as sources; and (2) conventional molecular beam epitaxy (MBE) with intentional introduction of hydrogen. A comparison of the electrical properties existing between the GSMBE-grown ZnSe:N films with those grown by MBE with an intentional flux of hydrogen will be presented.

Gas source molecular beam epitaxy growth of ZnSe on novel buffer layers

Thin films of ZnSe on pseudomorphic and partially relaxed (In,Ga)P buffer layers (on GaAs) have been examined; both epilayers were grown by gas source molecular beam epitaxy. The ZnSe layers were grown using elemental Zn and thermally decomposed H2Se. The II–VI nucleation occurred following an ex situ transfer in air of the As‐passivated (In,Ga)P buffer layers, grown using In, Ga, and cracked PH3. Microstructural characterization of the II–VI/III–V heterostructures was performed with high‐resolution x‐ray diffraction and transmission electron microscopy. The (511) reflection of the x‐ray rocking curves was used to measure the residual strain in the ZnSe/(In,Ga)P/GaAs structures, and to determine the alloy composition of the (In,Ga)P. The (400) reflection of the x‐ray diffraction rocking curves indicated peaks having full width at half‐maximum of 130 and 18 arcsec for the relaxed ZnSe on thin (1 μm) pseudomorphic (In,Ga)P buffer layers, respectively. Transmission electron microscopy confirmed the relaxed o...

Effect of laser on MOMBE of ZnSe using gaseous and solid sources

Abstract The effect of laser irradiation on the growth rate of ZnSe grown by metalorganic molecular beam epitaxy (MOMBE) using diethylzinc (DEZn) and diethylselenium (DESe), and by molecular beam epitaxy (MBE), has been studied. An argon ion laser and a Ti:sapphire laser were used. It was observed that laser irradiation significantly enhanced the growth rate for MOMBE using uncracked DEZn and either cracked DESe or solid Se. A suppression of the growth rate was found for MBE or “MBE-like” MOMBE growth. The enhancement or suppression was only observed when the photon energy of the selected laser emission line was sufficient to generate electron-hole pairs, implying that photo-generated carriers were responsible for both phenomena. Electron beam-induced growth rate enhancement was also observed under the same conditions which produced growth rate enhancement with laser irradiation. Secondary ion mass spectroscopy (SIMS) revealed that carbon incorporation was very low in the ZnSe films grown with these metalorganic sources.

p-Type and n-type doping of ZnSe: Effects of hydrogen incorporation

The hydrogenation behavior of p- and n-type ZnSe grown on GaAs by gas source molecular beam epitaxy (GSMBE) is presented. Recent advances in p-type doping, using a radio frequency (RF) plasma source with nitrogen, have led to the successful fabrication of blue /green light emitters based on the (Zn,Mg)(S,Se) material system grown by molecular beam epitaxy (MBE). GSMBE replaces the high vapor pressure group VI elements with hydride gases which are amenable to regulation using precision mass flow controllers, and has the potential to deliver improved compositional control and reproducibility. We have found that the presence of hydrogen does not affect the electrical conductivity of ZnSe:Cl grown by GSMBE. In contrast, nitrogen-doped ZnSe is speculated to be electrically passivated by hydrogen for certain growth conditions as evidenced by: (1) coherent tracking of the hydrogen concentration with variations in the nitrogen concentration, which is measured by secondary ion mass spectrometry (SIMS), and (2) indications of high resistivity determined by capacitance-voltage (C-V) measurements. Conventional and rapid thermal annealing (RTE) have been investigated to modify the degree of hydrogen passivation