W. Taudt

Influence of compensation on the luminescence of nitrogen-doped ZnSe epilayers grown by MOVPE

Abstract The luminescence of ZnSe grown by metalorganic vapour phase epitaxy and doped by a DC nitrogen plasma is investigated. With increasing N 2 flux the donor-acceptor pair (DAP) band continuously develops into a structureless band peaking at 2.62 eV for highest doping levels. This broad band evolves back into a structured DAP band peaking at 2.698 eV with increasing excitation density. At high N concentrations and at large degree of compensation potential fluctuations become important for the spatially indirect DAP recombination. These fluctuations can easily be screened by optically excited carriers making the experimental conditions decisive for luminescence spectra of strongly doped ZnSe: N samples.

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.

Low-temperature growth and nitrogen doping of ZnSe using diethylzinc and ditertiarybutylselenide in a plasma-stimulated low-pressure metalorganic vapour phase epitaxy system

Ditertiarybutylselenide (DTBSe) and diethyliinc (DEZn) were used as precursors for the growth of ZnSe in a low-pressure metalorganic vapour phase epitaxy (MOVPE) process. To reduce the growth temperature further, we analysed both samples grown with and without plasma precracked DTBSe. ZnSe was doped with plasma activated nitrogen during the growth process. A planar doping scheme was developed to suppress the hydrogen and carbon incorporation into the layers. The diffusion controlled growth regime was observed down to 315°C. Below 315°C the kinetically controlled growth regime was found by using DTBSe without plasma enhancement. For these samples the emissions of bound excitons in 11 K PL spectra dominate, but in spectra of samples grown with plasma below 360°C deep center emissions appear. The homogeneously doped ZnSe layers contain carbon (4.0 × 10 16 cm −3 ) and hydrogen (1.8 × 10 18 cm −3 ) originating from the precursors. The separation of the doping and the growth process reduces the hydrogen incorporation down to 4.5 × 10 17 cm −3 . The nitrogen concentration was 3 × 10 16 cm −3 for both methods

Incorporation of nitrogen in ZnSe grown by metalorganic vapour phase epitaxy

Abstract The incorporation processes and efficiencies of nitrogen doping for p-type conductivity in metalorganic vapour phase epitaxy (MOVPE) grown ZnSe/GaAs epilayers are investigated by means of time-integrated and time resolved photoluminescence (PL) spectroscopy. Two nitrogen-doping methods are compared, plasma-enhanced doping during growth, and ion implantation of nitrogen with annealing after growth. Both types of doped layers exhibit the I N 1 transition from a neutral acceptor bound exciton complex (A 0 N , X), indicating an effective nitrogen embedding on selenium sites. With increasing nitrogen doping rates, a deeper bound exciton line I C 1 appears, lowering the intensity of the I N 1 . An observed reduction of the I N 1 and I C 1 lifetimes for higher nitrogen doping concentrations results from an enhanced overlap of the bound exciton wave functions with those of other impurity centres.

Time-resolved donor-acceptor pair recombination luminescence in highly n- and p-doped II–VI semiconductors

Abstract Time resolved luminescence measurements of the donor-acceptor pair (DAP) luminescence of n-doped CdS:In bulk crystals, undoped bulk ZnSe crystals grown with various VI/II ratios, and p-doped ZnSe:N epilayers in the near band gap region are presented. The decay of the donor-acceptor pair recombination luminescence is investigated for dopant concentrations below the Mott density. A discussion on the basis of a statistical theory by Thomas and Hopfield for DAP recombination yields, by knowledge of the Bohr radii of the impurities, the number of electronically active impurity centres. The reliability of this method to determine impurity concentrations is discussed.

Low temperature growth and plasma enhanced nitrogen doping of ZnSe by metalorganic vapour phase epitaxy

Abstract The combination of diisopropylselenide (DIPSe) and diethylzinc (DEZn) was used to grow ZnSe by metalorganic vapour phase epitaxy (MOVPE). The results are compared with those obtained from samples grown with diethylselenide (DESe) and DEZn. Furthermore, we analysed the influence of plasma stimulation of DIPSe and DESe, as well as plasma enhanced nitrogen doping. Plasma stimulated growth with DESe at 480°C results in ZnSe which shows resolved free and bound excitons and negligible deep centre emissions in 11 K PL spectra. By using DIPSe without plasma stimulation, the kinetically controlled growth regime was found in the temperature range from 380 to 480°C. In PL only broad distributed emissions from 2.4 to 2.78 eV were observed, arising from impurities in the Se precursor. With plasma enhancement the diffusion controlled growth regime was extended down to 330°C. In the PL spectra, excitonic emissions were found beside the emission band (2.4–2.78 eV). Deep centre emission (2.0 eV) appeared only for T D 2 flux causes an increasing nitrogen concentration. Samples grown in N 2 carrier gas at 380°C show a maximum measured nitrogen concentration of 1.7 × 10 18 cm -3 . The layers were highly compensated, due to the impurities in the Se precursor and to additional hydrogen and carbon incorporation.

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.

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.

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.