H. Hamadeh

Metalorganic chemical vapor epitaxy and doping of ZnMgSSe heterostructures for blue emitting devices

Blue-green semiconductor laser diodes operating at room temperature are still the domain of wide bandgap II-VI compound semiconductors. CW operation at room temperature and hours of lifetime were reported. However, the conductivity control, defect generation and the ohmic contacts still need improvement. Therefore we focused our work on the MOVPE growth and the optimization of ZnMgSSe/ZnSSe/ZnSe heterostructures as well as on nitrogen doping of ZnSe. To verify the layer quality characterization was carried out by X-ray diffraction, electron probe microanalysis, electrical measurements and photoluminescence. ZnMgSSe/ZnSSe/ZnSe and ZnSSe/ZnSe quantum wells and superlattices were grown to investigate structural as well as interface properties. Electron beam and optical pumping was used to clarify the laser mechanism and to clarify the suitability of a MOVPE process to grow laser quality material. The electrical compensation of ZnSe doped with nitrogen is still controversially discussed whereas high n-type doping with chlorine was reproducible achieved. ZnSe:N doped at different growth conditions (II/VI ratio, growth temperature, nitrogen supply) using N 2 excited in a plasma source or by the use of nitrogen containing precursors was investigated to study the compensation mechanisms.

Characterization of Zn(1 − y)MgySxSe(1 − x) epilayers grown by low-pressure metalorganic vapour phase epitaxy

Abstract Optical properties of Zn 1 − y Mg y S x Se 1 − x epilayers grown on GaAs substrates by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) were investigated with photoluminescence spectroscopy, reflection spectroscopy and X-ray diffraction. The sulphur ( x ) and magnesium ( y ) content in the samples measured by EPMA (electron probe micro analysis) was varied in the range 0 ≤ x , y ≤ 0.25, respectively. The photoluminescence study of these quaternary epilayers was performed in the temperature range between 14 and 300 K. Low and negligible concentration of deep levels was achieved. Comparison of the measured halfwidths of the band edge emissions with calculated minimum alloy broadening values allowed the estimation of the composition fluctuation of the majority of the epilayers to be less than 1%. However, at large x or large y ( x , y ≥ 0.15) broadening in the band edge emission indicates an increased composition deviation from randomness. Bandgap reduction up to 75 meV with respect to the theoretically expected bandgap in the case of total randomness was observed. Enhanced phase separation and ordering effects especially at large x or large y seem to be the reason for the emission broadening as well as for the bandgap reduction.

Optical, structural and lattice dynamical properties of epitaxial Zn1 − yMgySxSe1 − x layers grown by metalorganic vapour phase epitaxy

The optical, structural and lattice dynamical properties of Zn 1-y Mg y S x Se 1-x epilayers grown by low pressure metalorganic vapour phase epitaxy were investigated. The composition of these layers, determined by electron probe microanalysis, was varied in the range of 0≤x, y ≤ 0.2. Photoluminescence measurements in the temperature range between 14 and 300 K were performed. The alloy broadening of the bandedge emission depending on the composition of the layer was calculated on the basis of a binomial statistical model. The comparison between the measured bandedge photoluminescence halfwidth and the calculated alloy broadening showed high composition homogeneity for samples with low sulphur and magnesium contents (x, y < 0.16) and poor homogeneity for samples with large x and y. Scanning transmission electron microscopy investigations showed a low density of structural defects in samples with narrow bandedge emission and low x and y, whereas a high defect density was detected in samples with large bandedge broadening. These results are correlated to the observation of symmetric and asymmetric profile shapes of the broadened X-ray rocking curves, respectively. Four-mode behaviour of the ZnMgSSe lattice vibrations was testified bv Raman measurements.

Nitrogen doping of ZnSe with trimethylsilylazide, triallylamine or bisditrimethylsilylamidozinc during metalorganic vapour phase epitaxy

Abstract Several nitrogen precursors without nitrogen hydrogen bonds were tested to dope ZnSe in a MOVPE process. Bisditrimethylsilylamidozinc (ZnBTM), trimethylsilylazide (TMSiN) and triallylamine (TAN) were used to grow ZnSe:N with ditertiarybutylselenide (DTBSe), diisopropylselenide (DIPSe) and dimethylzinc-triethylamine (DMZnTEN) at growth temperatures between 340 and 420°C. The samples were analysed by photoluminescence (PL), current-voltage ( I  V ), capacitance-voltage ( C  V ) and Hall measurements. The dependence of nitrogen incorporation and electrical activation on growth temperature, VI/II ratio and dopant flow were investigated. In the PL spectra of the layers grown with TMSiN an intense bound-exciton emission, correlated to Zn vacancies is observed, besides the I N 1 emission line for high dopant flows. The samples grown with ZnBTM and TAN as precursors exhibit a broadening of the excitonic emissions and a shift towards the energetic position of the I N 1 emission line for increasing nitrogen incorporation in the layers. The intensity of the excitonic emissions in comparison to the donor-acceptor pair emissions remains dominating. Although Hall measurements of several samples doped with ZnBTM and TAN indicate p-type conductivity, the analysis of the C  V and I  V measurements shows the n-type or semi-insulating character of the samples.

Optimized metalorganic vapour phase epitaxy of ZnMgSSe heterostructures

Abstract ZnMgSSe single layers and heterostructures have been grown on GaAs substrates in a low-pressure MOVPE system at 400 hPa and 330°C. Optimization of the growth process using the precursor combination of dimethylzinc(triethylamine adduct) (DMZn(TEN)), ditertiarybutylselenium (DTBSe), ditertiarybutylsulphur (DTBS) and bismethylcyclopentadienylmagnesium ((MeCp) 2 Mg) allowed the reproducible adjustment of the S and Mg contents in a wide range and thus to control the band gap and the lattice constant simultaneously. Pseudomorphic layer structures (lattice-matched or strain-balanced) have been deposited that exhibit near-band edge emissions of up to 3.35 eV. An inhomogeneous broadening of photoluminescence (PL) emissions and high-resolution X-ray diffraction (HRXRD) peaks due to phase separation can be observed especially for lattice-matched samples. ZnMgSSe/ZnSe multi quantum well (MQW) structures have been grown and characterized in order to study the effects on the luminescence properties.

Low temperature growth and planar doping of ZnSe in a plasma-stimulated LP-MOVPE system

In a low-pressure metalorganic vapor phase epitaxy process, we used dc-plasma activated nitrogen to dope ZnSe, grown with ditertiarybutylselenide and dimethylzinc-triethylamine. The nitrogen concentration of up to 2 × 1018 cm−3 in the doped layers can be adjusted by the growth temperature, the dc-plasma power, and the N2 dopant flow. Due to the high n-type background carrier concentration of the order of 1017 cm−3 in undoped samples, the doped layers show n-type conductivity or were semi-insulating because of an additional compensation by hydrogen incorporated with a concentration of the order of 1018 cm−3. A planar doping scheme was applied to reduce this hydrogen incorporation by one order of magnitude, although H2 was used as carrier gas.

Near-band-edge photoluminescence of MOVPE-grown undoped and nitrogen-doped ZnSe

Abstract Optical properties of undoped and nitrogen-doped ZnSe GaAs epilayers grown by MOVPE were investigated by using CW HeCd laser excitation or by pulsed N2-laser radiation in the temperature range between 10 and 300 K and at excitation intensities in the range of Iexc = 10−1−106 W/cm2. The presence of deep donor states with an ionization energy of ED ≈ 60 meV and acceptor states with EA ≈ 80 meV as well as the existence of trap levels with a concentration comparable to the amount of incorporated nitrogen were detected. The concentration of the electron-hole plasma (EHP) is saturated at Iexc > 200 kW/cm2 in undoped and moderately doped layers due to plasma expansion through the layer into the substrate. High doping levels as well as the presence of a barrier layer between ZnSe and the substrate leads to a further increase of the EHP concentration. The overheat of the near-surface layer which is much higher compared to the average temperature of the irradiated area leads to a high temperature gradient (ΔT ∼ 150 K) in the layer.

HIGH-TEMPERATURE OPTICALLY PUMPED LASING IN ZNMGSSE/ZNSE HETEROSTRUCTURES GROWN BY METALORGANIC VAPOR PHASE EPITAXY

Lasing and optical properties of ZnMgSSe/ZnSe-, ZnMgSSe/ZnSSe/ZnSe-, and ZnMgSSe/ZnMgSSe/ZnSe-based single- and multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy were studied, and the characteristics were found to depend on the excitation intensity Iexc, temperature, and well width. Laser action under transverse optical pumping was achieved only for well widths Lz⩾4 nm and optical confinement factors Γ>0.04. In separate confinement heterostructures, lasing with the lowest threshold (Ithr=10–30 kW/cm2 at T=78 K) was achieved and device characteristics were studied up to T=577 K.

Optical and electrical properties of MOVPE-grown ZnSe:N using triallylamine as a nitrogen precursor

Photoluminescence (PL), C-V and I-V characteristics of MOVPE-grown ZnSe :N doped using triallylamine (TAN) have been investigated. In our experiments, the use of TAN as dopant source does not lead to the incorporation of nitrogen into the lattice at the VI/II ratio optimized for the growth of undoped ZnSe. At smaller VI/II ratios a high concentration of donor states (10 17 -10 18 cm -3 ) appears leading to the formation of a broad PL band at 2.791-2.792 eV caused by a transition from the states formed by an overlap of the conduction band and donor states to the valence band. The acceptor states with an activation energy of 80 meV are most likely formed by oxygen contamination of TAN.

Nitrogen as carrier gas for the growth of ZnSe and ZnSe:N in plasma enhanced metalorganic vapor phase epitaxy

For the precursor combination dimethylzinc-triethylamine and ditertiarybutylselenide the use of nitrogen carrier gas was investigated for the metalorganic vapor phase epitaxy (MOVPE) of ZnSe and ZnSe:N. The nitrogen doping was carried out with a separate nitrogen flow which was activated by a dc-plasma. In the photoluminescence spectra of undoped layers grown at 340°C with a VI/II ratio of 0.18 only excitonic emissions, separated into free and donor bound excitons, can be observed. The background carrier concentration was of the order of 1 x 10 16 cm -3 . The growth rate of epilayers grown in nitrogen is reduced by about 75% in comparison to the value obtained by using hydrogen as carrier gas. The doping can be regulated by the dopant flow and by the total pressure in the reactor. With increasing flow of plasma activated nitrogen and a reduced total pressure, the PL spectra showed broadened DAP emission without excitonic emissions. The electrical and optical properties obtained with nitrogen carrier gas are comparable with the results obtained with hydrogen carrier gas. So far, no p-type conductivity was measured. Therefore, the problem of compensation of p-type conductivity of ZnSe:N doped by dc-plasma enhanced N 2 was not solved by the use of N 2 carrier gas.