H. P. Wagner

Photoluminescence properties of MOVPE grown ZnTe layers on (100) GaAs and (100) GaSb

Single-crystalline ZnTe layers have been successfully grown on (100) oriented GaAs and GaSb substrates by atmospheric-pressure MOVPE. The surfaces are mirrorlike and show a microstructure smaller than 1 μm when grown at temperatures between 330 and 370°C. These low growth temperatures have been achieved using diethylzinc and diisopropyltelluride as metalorganic precursors. Temperature-dependent photoluminescence (PL) measurements have been carried out to identify incorporated impurities and to optimize the growth conditions. Several luminescence bands can be assigned to shallow traps formed by specific impurities as Ga (binding energy EB = 18.6 meV), As (EB = 78.5 meV) and possibly Si. Best growth results have been achieved at a growth temperature of 350°C and a ratio R (DiPTe/DEZn) = 2.0. In ZnTe layers thus obtained, free exciton luminescence was observed at 2.3795 eV (Mj = ±32 valence band) and 2.3806 eV (Mj = ±12) at 10 K. PL spectra of ZnTe grown on GaSb at 350°C and R = 2.0 show the Si line but not the As line; instead, a line corresponding to EB = 73.8 meV was tentatively assigned to Sb. Likewise the optical quality had strongly increased, due to the much smaller lattice mismatch of 0.1% between ZnTe and GaSb compared with 8% for GaAs. Also the free exciton Mj = ±12 transition (2.3808 eV at 10 K) is clearly separated from the one with Mj = ±32 (2.3794 ev) by an energy of 1.4 meV.

Investigation of strain in metalorganic vapor‐phase epitaxy grown ZnTe layers by optical methods

We have studied the strain in ZnTe epilayers grown by atmospheric‐pressure metalorganic vapor‐phase epitaxy on (001) GaAs and GaSb substrates. Reflectivity and absorption measurements are performed at 2 K using single‐crystalline layers with thicknesses of 0.2–2 μm. The biaxial strain in the samples caused by the lattice mismatch of layer and substrate is deduced from the splitting of the degenerate heavy‐ and light‐hole exciton. A polariton model is used to describe the reflectivity structure at the E0 gap and to determine the transverse exciton energies. The deformation potentials obtained from an analysis of the absorption structures are a=−5.5 eV and b=−1.4 eV. The critical thickness for ZnTe/GaSb is lower than 0.8 μm near thermodynamic equilibrium. It also slightly depends on growth temperature which has its optimum at 345 °C.

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.

Photoluminescence and photothermal effect of Fe3O4 nanoparticles for medical imaging and therapy

Photoluminescence (PL) of Fe3O4 nanoparticle was observed from the visible to near-infrared (NIR) range by laser irradiation at 407 nm. PL spectra of ∼10 nm diameter Fe3O4 nanoparticles organized in different spatial configuration, showed characteristic emissions with a major peak near 560 nm, and two weak peaks near 690 nm and 840 nm. Different band gap energies were determined for these Fe3O4 nanoparticle samples corresponding to, respectively, the electron band structures of the octahedral site (2.2 eV) and the tetrahedral site (0.9 eV). Photothermal effect of Fe3O4 nanoparticles was found to be associated with the photoluminescence emissions in the NIR range. Also discussed is the mechanism responsible for the photothermal effect of Fe3O4 nanoparticles in medical therapy.

The MOVPE growth and doping of ZnTe

Zinc telluride layers have been grown on (100) GaAs, GaSb and ZnTe substrates at 350 degrees C with atmospheric pressure MOVPE. Diisopropyl-telluride, dimethylzinc-triethylamine and diethylzinc were chosen as metallorganic precursors. The samples were characterized by photoluminescence at 2 K and the Hall effect. In the PL spectra the light hole, heavy hole and bound exciton transitions are well resolved. Most of the transitions have been assigned. The partial pressure ratios of the alkyls were optimized. The influence of composition and stoichiometry of the substrates on the purity of the ZnTe is revealed. Various alkyls as sources of As, Bi, Ga, In and I were investigated for their suitability as p- or n-type dopants. It is shown that tetraethylbiarsine and ethyliodide are promising alkyls for p- and n-type doping of ZnTe.

Phase matched second harmonic generation using thin film ZnTe optical waveguides

Wide gap II‐VI semiconductors have strong second order susceptibilities χ(2) and are therefore promising materials for efficient second harmonic generation. We have grown high quality single crystalline ZnSe/ZnTe/ZnSe/GaAs (001) waveguides by metalorganic‐vapor‐phase‐epitaxy. Using end‐fire coupling we observe a phase matched signal of the 1170.5 nm fundamental wave. The fundamental beam is generated by a tuneable KTP optical‐parametric‐oscillator pumped by a ps‐Ti:sapphire laser system. Phase matching is achieved by coupling the TE0 fundamental mode with the TM2 second harmonic mode within the symmetric ZnTe waveguide.

Investigation of the hydrostatic pressure dependence of the E0 gap, the excitonic binding energy and the refractive index of MOCVD-grown ZnTe layers

The authors have determined the pressure dependence of the excitonic transition energy in thin MOVPE ZnTe layers by measuring the absorption coefficient and the refractive index. Pressures up to 12 GPa were applied in a diamond anvil cell. A special gasket technique was used to suppress the scattered light. The absorption coefficient was measured in 1 mu m thick films at T=115 K and at T=300 K up to 50000 cm-1. The spectra are dominated by a sharp excitonic structure. Its pressure dependence is evaluated using a model which allowed them to determine the rate of pressure-induced shift of the binding energy dEx/dP and the rate of change of the Rydberg energy dRx*/dP. The refractive index n was measured on 2.4 mu m-thick films at T=300 K and at T=115 K by observing the pressure shift of the interference extrema in the transparency region. The resulting refractive index was extrapolated to the frequency of the longitudinal optical phonon to obtain the constant in infinity . The static dielectric constant in st was obtained using the Lyddane-Sachs-Teller relation. Using this value and the determined effective masses at the applied pressures, the authors calculated the exciton binding energy. At the pressure P=9.4 GPa, a phase transition was observed. The obtained spectra for this phase were compared with a model describing indirect transitions in semiconductors. A further phase transition was observed at P=10.9 GPa.

Investigation of strained ZnTe epilayers by high resolution electron microscopy

Abstract Structural properties and strain relaxation at the ZnTe/GaAs (001) interface are investigated by high resolution electron microscopy (HREM). Images taken from ZnTe MOVPE-grown epilayers with a resolution of 0.2 nm show an array of misfit dislocations at the interface. Their analysis is facilitated by using a digital image processing method. The most common line defects at the ZnTe/GaAs interface are Lomer and 60° dislocations. Also a small number of stacking faults, limited by partial dislocations, can be observed at the interface. The spacing between the misfit dislocations allows an estimate of the strain relaxation.