W.S. Kuhn

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.

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.

Thermal dissociation of ditertiarybutylselenide and methylallylselenide and its impact on the growth of ZnSe by metalorganic vapour phase epitaxy

Abstract The kinetic growth limitation in atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) of ZnSe was investigated using the selenium precursors methylallylselenide (MASe) and ditertiarybutylselenide (DTBSe). The onset of mass transfer limited growth was found at ∼ 480 and ∼ 290°C for MASe and DTBSe, respectively. The activation energies for the kinetic growth were determined to be 11 kcal/mol for MASe and 14 kcal/mol for DTBSe. The decomposition of the alkyls was studied in an isothermal quartz tube by mass spectroscopy. For both precursors, an increase of the decomposition is found between 300–500°C. For DTBSe, the catalysis by a ZnSe surface lowers this temperature range to 140–260°C. Isobutane and isobutene were found as decomposition products of DTBSe. The intensities of these species depended on the catalytic or homogeneous nature of the pyrolysis. The co-pyrolysis of DTBSe with DMZn took place between 180–310°C, thus matching with the kinetic limitation observed in the growth.

Investigation of hydrogen, carbon and further impurities in the metalorganic vapour phase epitaxy of ZnSe with ditertiarybutylselenide and methylallylselenide

Abstract The impurities in ZnSe layers grown by metalorganic vapour phase epitaxy (MOVPE) on (001) GaAs have been investigated by photoluminescence (PL) and secondary ion mass spectrometry (SIMS) measurements. The layers grown with the alkyl combination methylallylselenide/diethylzinc (MASe/DEZn) exhibit the incorporation of C and H detected by SIMS. The use of helium instead of the hydrogen carrier gas increases the incorporation of C and H. At very high concentrations (H ≈ 10 20 cm -3 ) a new PL peak at 2.786 eV appeared. The mass spectroscopic investigation of the pyrolysis of MASe revealed a simple bound cleavage as the dominant mechanism which generates the intermediate species SeCH 3 . The following extrinsic impurities were found in the layers: (1) Cu from the growth system (PL, SIMS); (2) halogen (Br and I) from the Se source (PL, synthesis, SIMS); (3) oxygen from the system or carrier gas (SIMS); (4) Te from former ZnTe growth (SIMS); (5) As and Ga from the substrate (SIMS). The layers grown with the alkyl combination ditertiarybutylselenide/dimethylzinc-triethylamine (DTBSe/DMZn-TEN) show weak contaminations by C and H as detected by SIMS. The mass-spectroscopic investigation of the pyrolysis of DTBSe revealed H 2 Se and elemental Se as products from parallel mechanisms. The volatile alkyls isobutane and isobutene are found as reaction products. The role of the Zn alkyl as the source of the C and H incorporation is not yet clarified. The following extrinsic impurities were found in the layers: (1) Cu from the growth system (SIMS); (2) either Al or Cl as a donor (PL); (3) O from the system or carrier gas (SIMS); (4) S possibly from substrate preparation (SIMS); (5) As and Ga from the substrate (PL, SIMS). However, the layer purity is already sufficient for first doping experiments. With P doping, a hole concentration of 10 15 cm -3 is achieved.

The metal organic vapour phase epitaxy of ZnTe: II. Analysis of growth conditions

Abstract We report on the MOVPE of ZnTe in two typical reactor geometries. Horizontal flow in rectangular tubes with even and tilted susceptors are compared with axisymetric vertical downward flow arrangements on even (horizontal) and tilted susceptors. Growth parameters such as temperature and input partial pressures of the Zn and Te precursors are studied. The growth with the precursors diethylzinc (DEZn) and dimethylzinc-triethylamin (DMZn-TEN) in various combinations with the Te-alkyls diethyltelluride (DETe), diisopropyltelluride (DIPTe) and methylallyltelluride (MATe) is discussed. The hydrodynamics of the reactors are characterised by dimensionless numbers. Analytical and numerical simulation of the mass transport supports the interpretation of growth results. Furthermore, a simple reaction boundary layer model has been developed. The model is especially useful for a first estimation of the contribution of gas phase dissociation to the growth. Catalytic dissociation of precursors was found to play an essential role in the growth process. Analytical models of the surface processes are discussed. The kinetic entrance zone in horizontal reactors has a strong influence on the mass transport limited growth rates. Gas phase nucleation and particle growth cause severe problems under certain circumstances. Finally, a complete picture of ZnTe-MOVPE is developed including all the main aspects of hydrodynamics and mass transport, chemical kinetics and the thermodynamic description of the growth process. We will point out that the correct interpretation of growth phenomena often presumes an understanding of all main aspects of MOVPE growth as flow, temperature and concentration fields, mechanisms and kinetics of the source decomposition, surface processes, supersaturation and nucleation phenomena, which are often not well enough known.

The metal organic vapour phase epitaxy of ZnTe: I. Properties and decomposition kinetics of organometallics

Abstract The growth of high quality II-VI devices by MOVPE is dependent upon the progress achieved in certain key research activities. Here, the precursors are the crucial point for a successful epitaxy. The metallorganics Et 2 Te, i Pr 2 Te, MeTe-allyl, Et 2 Zn, Me 2 Zn and the Me 2 Zn-Et 3 N adduct are used for the growth of ZnTe epilayers. The relevant properties of these alkyls such as purity, vapour pressures, RT-stability and bond disruption energies are discussed. Special emphasis is put on the thermal decomposition. Reaction mechanisms and kinetics were studied in an isothermal ersatz reactor and by mass spectroscopy. Literature results are quoted to complete the interpretation. The evaluation of the measurements is often complicated due to the condensation and autocatalysis. Homogeneous and heterogeneous pyrolysis compete with each other ( i Pr 2 Te). Homolysis followed by radical reactions is the dominant reaction mechanism for Me 2 Zn, Et 2 Zn and MeTe-allyl. β-hydrogen elimination competes with homolysis in certain cases (Et 2 Te, i Pr 2 Te). For most precursor combinations, strong interactions in pyrolysis are observed. For the relatively stable Te-alkyls Me 2 Te and Et 2 Te, the decomposition is enhanced by co-pyrolysis with Et 2 Zn. For the less stable Te-alkyls i Pr 2 Te and MeTe-allyl, the decomposition kinetics remain essentially unchanged with Et 2 Zn. The activation energies and prefactors for the elemental reaction steps are discussed. Basic considerations lead to an expression which allows the evaluation of E a for the surface reaction in a flow tube. Under similar experimental conditions, activation energies for the surface pyrolysis were found to be comparable to the growth activation energies.

Photoassisted metal organic vapour-phase epitaxy of ZnTe on GaAs

Abstract Photoassisted epitaxy of ZnTe on GaAs by a metal organic vapour-phase process has been performed using sources of diethylzinc and diethyltellurium or diisopropyltellurium. Illuminating the layer during growth with a xenon lamp increases the growth rate. Growth rate enhancement was found to be a function of light wavelength and intensity. Only photons having energy higher than the bandgap of ZnTe increase the growth rate. The characterization of the layers by X-ray diffraction and of the surface morphology by Nomarski microscopy is reported. We observed the degradation of the surface morphology of irradiated layers in comparison with unilluminated layers. We tentatively explain the growth rate enhancement by a photocatalytic surface process involving electron - hole pairs created in the ZnTe layers.

Metalorganic vapour phase epitaxy growth of Zn1-xMgxTe layers

We report on metalorganic vapour phase epitaxy (MOVPE) growth of the IIa-VI compound MgTe and the variable band gap ternary semiconductor Zn 1−x Mg x Te. Bulk MgTe shows wurtzite type structure and is a direct band gap material (∼3.5 eV). The precursors used in this study were diisopropyltellurium (DIPTe), diethylzinc (DEZn) and bis-methylcyclopentadienylmagnesium (MCP 2 Mg). The growth of MgTe on (100) GaAs substrates was found to be polycrystalline. The films were very quickly degraded by hydratation, even if they were covered by a ZnTe film. Zn 1−x Mg x Te epitaxial layers on (100) GaAs were grown with a Mg concentration of 0 < x < 0.6. In that range of composition they appeared stable in air. For 0 < x < 0.5, the thin films showed zincblende structure and followed Vegards law. The crystalline parameters were determined by the measurement of the relative position of the (004) X-ray reflection of the substrate and of the layer. The crystalline quality of the layers of Zn 1−x Mg x Te alloys was comparable to pure ZnTe. Photoluminescence measurements exhibited a deep emission in the green region

Photoassisted metalorganic vapor-phase epitaxy of ZnSe on GaAs

Abstract Photoassisted growth of ZnSe on (100) GaAs substrates by a metalorganic vapor-phase epitaxy process was carried out using dimethylzinc and ditertiarybutylselenide as precursors. Illuminating the surface of the layer during the growth with a high intensity xenon arc lamp, the growth rate was strongly enhanced, only when the photon energy selected by the narrowband interference filters (10 nm bandwidth) was higher than the ZnSe bandgap energy. X-ray diffraction patterns as well as energy-dispersive X-ray spectra were used to determine the crystalline quality and the chemical composition of the ZnSe layers grown under different conditions. The photo-induced charge transfer (photocatalysis) and the supersaturated Se film mechanism are discussed to explain the large enhancement of the growth rate (factor ∼ 3) observed under irradiation.