K. Wolf

Growth and doping of ZnTe and ZnSe epilayers with metalorganic vapour phase epitaxy

Abstract In this work we report on the growth and doping of ZnTe and ZnSe layers with metalorganic vapour phase epitaxy (MOVPE). Low restistive p-type ZnTe was grown by doping with arsenic and phosphorus. Acceptor concentrations of up to 3.5 × 1017 cm-3 were achieved in the case of phosphorus-doped samples. For the first time nitrogen acceptors in ZnTe were investigated with photoluminescence at various temperatures and under resonant conditions. ZnSe layers were grown with DTBSe and DMZnTEN. The crystalline quality was investigated by high resolution X-ray diffraction. The halfwidths (150–500 arc sec) depend on the degree of relaxation and therefore on the layer thickness. Undoped ZnSe layers show at 2 K a strong recombination of free and bound excitons and a weak donor-acceptor pair luminescence.

Low temperature MOVPE growth of ZnSe with ditertiarybutylselenide

Abstract The results on the synthesis of the selenium alkyl ditertiarybutylselenide and its application in atmospheric pressure MOVPE are presented. In combination with dimethylzinc-triethylamine, single crystalline ZnSe layers were grown on GaAs at temperatures lower than 350°C. Good morphology, crystalline and interface quality are demonstrated by optical and electron microscopy, X-ray diffraction and Raman spectroscopy. Photoluminescence at 2 K reveals chlorine as an impurity. The electron mobility of 500 cm 2 /V·s at room temperature supports a fairly low compensation.

Resonant excitation of intrinsic and shallow trap luminescence in MOVPE grown ZnTe layers

Abstract We present high resolution spectra of excitons, shallow donors and acceptors in ZnTe epilayers grown on GaAs and GaSb by atmospheric pressure metal-organic vapor-phase epitaxy (MOVPE). Resonant excitation made the observation of selective-pair luminescence (SPL), two-electron transitions (TET) and two-hole transitions (THT) possible. The investigation of donor states in I-doped layers yields m ∗ e = 0.117 m 0 and a static dielectric constant ϵ st = 9.4. The Luttinger parameter γ 1 = 3.8 was obtained from 1s-and 2s-free exciton transitions. As-acceptor states were observed in strain-free layers. A fit to calculations of Baldareschi and Lipari leads to γ 2 = 0.72 and γ 3 = 1.3. Level shift and splitting in magnetic fields corroborated the present assignments. The magnetic parameters K A = -0.27 and q A = -0.015 were obtained from As-acceptor bound excitons and the first excited acceptor state.

Low-pressure metalorganic vapor phase epitaxy of ZnSe-based light emitting diodes

Abstract We report on the growth and p-and n-doping of ZnSe with low-pressure metalorganic vapor phase epitaxy. The epitaxy of ZnSe was performed on semi-insulated and highly doped (001) GaAs substrates at a growth temperature of 340°C. The reactor pressure was varied between 100 and 1000 hPa to improve the lateral homogeneity of the layer thickness. Optical and structural properties of the samples were determined with photoluminescence at 2 K and X-ray diffraction. For the doping experiments triallylamine and n-butylchlorine were applied. The achieved free carrier concentration was determined with C-V and Hall effect measurements. Both doping precursors were used for the fabrication of light emitting p / n junctions. Different types of light emitting diode (LED) structures were grown. Electroluminescence was excited with DC and pulsed current in the temperature range of 25 to 300 K.

Luminescence caused by extended lattice defects in epitaxially grown ZnTe layers

Two strong emission bands (Y1 and Y2), 210 and 250 meV lower than Egap of ZnTe, have been studied in heteroepitaxially grown ZnTe layers. The phonon coupling and the thermalization energy of these emissions are usually small, similar to those of the Y band in ZnSe and CdTe layers. The Y luminescence in ZnTe is emitted above all near the ZnTe/GaAs interface which contains a high density of lattice defects (mainly misfit dislocations). An increasing concentration of point defects leads to a strong decrease of the Y emission. The observation of this luminescence in ZnTe bulk material or in homoepitaxially grown ZnTe is not possible because of a strong dependence of the intensity on the defect concentration and the dislocation density. Both bands are excited only by laser light resonant to or higher than the free exciton energy. The strain dependent energy shift behaves similar to that of bound excitons. The intensity of the Y bands observed at liquid helium temperature decreases under strong laser excitation. A recovery effect of both lines is observed when the sample is heated to liquid nitrogen or room temperature.

Relaxation process and luminescence of lattice defects in epitaxially grown ZnSeGaAs layers

Abstract In this work, we report on the dependence of strain relaxation in epitaxially grown ZnSe GaAs layers on the growth process. New investigations about luminescence signals are presented, which are caused by lattice defects in these layers. The strain is due to different lattice constants of layer and substrate material and is relaxed by the nucleation of misfit dislocations during growth. This relaxation process depends on layer thickness and growth conditions. We determine the residual strain for different layer thicknesses and growth processes with X-ray diffractometry. Furthermore, the created misfit dislocations give rise to a luminescence at 2.6 eV, the so-called Y line. We found that its behaviour in ZnSe is analogous to that of Y lines in epitaxially grown ZnTe GaAs layers. An excitation of the Y luminescence is only possible with photon energies higher than the recombination energy of free excitons. Its intensity decreases with increasing impurity concentration. It also decreases strongly for a steady state excitation at 2 K. A recovery effect of luminescence intensity is observed, if the sample is heated to room temperature. Furthermore, the strain dependence of the Y luminescence in ZnSe is compared with the maxima of bound exciton luminescence from shallow donors and acceptors.

Spectroscopic investigations of donor and acceptor states in n- and p-doped ZnTe epilayers

ZnTe epilayers were grown on (001) GaAs and (001) GaSb substrates by metalorganic vapour phase epitaxy (MOVPE) and hot wall epitaxy (HWE). An extensive study of near gap photoluminescence (PL) has been made using selective-pair luminescence (SPL), two-electron transitions (TETs) and two-hole transitions (THTs). As grown and intentionally doped layers have been used. Excited donor states were investigated in I-doped ZnTe/GaAs layers. The magnetic field splitting of p-states leads to an effective conduction band mass of m∗e = 0.117m0. The hydrogenic model implies a static dielectric constant ϵst = 9.4. A small biaxial strain splits the excitonic transitions. We obtained the Luttinger parameter γ1 = 3.8 from the free exciton 1s and 2s levels. The excited states of an As acceptor were investigated by resonant excitation of free standing As-doped ZnTe layers and the Luttinger parameters γ2 = 0.72 and γ3 = 1.3 were derived. Magnetic field splitting yields the parameters κA = −0.27 and qA = −0.015.

Luminescence from structural defects in heteroepitaxial MOVPE-grown ZnTe

Abstract Two new strong deep emission bands with a weak phonon coupling were observed in heteroepitaxial ZnTe layers. The study of PL, PLE spectra in the layers with different thicknesses has shown that these emissions are produced in the ZnTe/GaAs interface region. It is suggested that the observed luminescence is related to the recombination of excitons bound to some extended structural defects, probably misfit dislocations. An unusual behavior of these bands - PL ‘fatigue’ effect - was observed under the strong laser excitation.

Growth and photoluminescence investigations of phosphorus- and iodine-doped MOVPE ZnTe layers

Abstract ZnTe layers doped with phosphorus and iodine were grown on (001)-GaAs substrates by atmospheric-pressure metalorganic vapour phase epitaxy (MOVPE). Temperature-dependent near-band-gap photoluminescence allowed the ground state of phosphorus acceptors ( E 1 S 3 2 P = 62 meV ) to be determined. Resonant excitation methods as selective-pair luminescence (SPL) and two-hole transitions (THTs) enabled the investigation of excited acceptor states in epitactically grown ZnTe layers. The energy levels of these excited states of phosphorus acceptors investigated in free-standing layers have led to the Luttinger parameters γ 1 =0.72 and γ 3 =1.3. Iodine-doped ZnTe layers showed strong emission bands at 2.24 and 2.22 eV. Their intensities were found to be dependent on the partial pressure ratios of the iodine organyl to the tellurium bands during growth. The first emission band is due ro the incorporation of impurity atoms with a binding energy of about 145 meV, which is in a good agreement with the binding energy of copper in ZnTe. The emission band at 2.22 eV is assigned to zinc-vacancy-iodine complexes (V Zn -I). A binding energy of 170 meV of these luminescence centres was derived from temperature-dependent measurements.

Exciton recombination in ZnSexTe1−x/ZnTe QWs and ZnSexTe1−x epilayers grown by metalorganic vapour phase epitaxy

We present the optical characterization of MOVPE grown ZnSe x Te 1-x epilayers with 0<x<0.35 and ZnSe x Te 1-x single quantum wells (SQWs) with L z =2.0-8.5 nm. The structures were investigated using photoreflectance and photoluminescence spectroscopy. Transmission electron microscope analysis was used to determine quantum well thickness. The studied ZnSe 0.3 Te 0.7 SWQs show a bright emission band with a halfwidth ∼10 meV in the spectral region of 100-250 meV below the band gap of the corresponding mixed crystal. We suggest a type II bands alignment in ZnSe x Te 1-x /ZnTe single quantum wells