MOVPE of II–VI materials

The development of II–VI MOVPE is reviewed, contrasting the narrow bandgap materials with the wide bandgap. Common issues are the need to grow the layers at lower temperatures than their III–V cousins in order to avoid point defects. This means that II–VI MOVPE occurs in a surface kinetic regime for precursor decomposition and has stimulated a lot of research on alternative precursors. The narrow bandgap II–VI growers have settled on dimethyl cadmium (DMCd) combined with diisopropyl telluride (DIPTe) and a liquid Hg source but wide bandgap growers are split between pyrolytic and photo-assisted growth. Recent progress in p-type doping has enabled the demonstration of some new devices, including two colour infrared detectors and the first MOVPE grown green emitting laser structure. The common theme appears to be hydrogen passivation of the Group V dopant and some novel precursor solutions to this problem are discussed.     

Wide bandgap II–VI compounds grown by MOMBE

Recent developments of MOMBE for wide bandgap II–VI compounds are reviewed by considering both film properties and a novel growth technique such as atomic layer epitaxy (ALE). MOMBE using dimethylzinc and hydrogen has enabled growth of p-type ZnSe doped with nitrogen by using ammonia as a dopant source. Ideal ALE was obtained for ZnSe by using various gas source conditions. Furthermore, applications of MOMBE of II–VI compounds to the growth of strained layer superlattices are also reviewed.     

X-Ray Diffraction Studies on Point Defects in II–VI Compounds

The point defect concentration in Te-rich CdTe and Hg_1-xCd_xTe annealed at various temperatures has been estimated from precision lattice parameters using a simple continuum inclusion model and compared with densities of electrically charged defects determined by high-temperature Hall and conductivity measurements. The nonstoichiometry is realized by cation vacancies. Dependent on the CdTe content, the ratio of total to charged defect concentrations varies between about unity for HgTe-rich composition and 75 for CdTe. Therefore, it is necessary to distinct between “electrical” and “chemical” stability regions.     

Dynamic crystallization of II—VI compounds with a gaseous control of vapour concentrations

The method for dynamic crystallization of II—VI compounds with a gaseous control of vapour concentrations in the growth zone is developed. The predeterminated change of the transport gas flows which carry the vapours of the initial elements into the reaction zone together with the change of additional inert gas flows conducted directly to the crystallizer entries gives the posibility to displace the growth zone during the process of synthesis. It results in the considerable increase in crystal number and size and leads to best quality of grown crystals.     

Pyrolysis characteristics of iodine precursors for gas source n-type doping of II–VI compounds

The selection criteria and pyrolysis characteristics of iodine precursors were investigated to determine the best compounds for the n-type doping of CdTe during gas source molecular beam epitaxy. Ethyliodide and allyliodide were found to have the most suitable properties and to produce iodine dimers for pyrolysis temperatures above 650 and 600°C, respectively. The ethyliodide doping of CdTe was studied and produced highly conductive CdTe layers with room temperature electron concentrations as high as 3 × 10¹⁸ cmsu-3 with a mobility of ˜ 460 cm²/V·s.     

Dynamic crystallization of II–VI compounds from vapours of the component elements

Dynamic crystallization in II–VI compounds synthesis from the vapour phase has been discussed. The method consists in the displacement of the growth zone by the predetermined change of the evaporation temperatures of the initial elements at constant flows of the transport gas. It leads to uniform distribution of grown crystals in the crystallization region and to the increase of the size and of the yield of crystals. Dynamic crystallization makes it possible to control the photoelectrical properties of the crystals and to increase the yield of the crystals with the required photosensitivity by changing the rate and the direction of the displacement of the growth zone.     

Growth of InN films and nanostructures by MOVPE

Since a few years, a lot of research efforts have been devoted to InN, the least known of the semiconducting group-III nitrides. Most of the samples available today have been grown using the molecular beam epitaxy technique, and fewer using the metal organic vapor phase epitaxy (MOVPE) method. Whatever the method, the growth of InN is extremely challenging, in particular due to the fact that no lattice matched substrate is available.     

Growth and Characterization of Direct-connecting AlGaAs/GaAs TJS Light Emitting Device on SI GaAs Substrate by LPE

High output power (above 3 mW/facet) AlGaAs/GaAs Transverse-Junction Stripe light emitting diodes have been grown on Semi-Insulating (100) GaAs substrates by Liquid Phase Epitaxy. these light emitting diodes utilize a “Direct-connecting” transverse-junction stripe structure, which can confine the transverse-current and reduce the series resistance. By thinning the thickness of the “effective active-layer” of this structure, a room-temperature pulsed lasing operation is also achieved with a threshold current as low as 35 mA and a peak wavelength around 904 nm. This “Direct-connecting” transverse-Junction Stripe light emitting device with a Metal-Semiconductor Field Effect Transistor on an electrical isolated semi-insulating substrate in the future.     

Synthesis and Characterization of Violet Light Emitting Polymer based on Fluorene and Dimethylsilane

New fluorene based light emitting polymer, poly[(4-(9,9-didecyl-9H-fluoren-2-yl) phenyl)dimethyl(phenyl)silane] (PFDPS), was synthesized by palladium-catalzed Suzuki coupling reaction. The obtained copolymer was characterized by ¹H-NMR, and IR-spectroscopy. The polymer showed good solubility in common organic solvents and weight average molecular weight of 16,300 with polydispersity index of 1.4. The maximum photoluminescence of the solution and film of the polymer was observed at 392 nm and 410 nm, respectively.

The double-layered device with the configuration, ITO/PEDOT/PFDPS/LiF/Al structure has a turn-on voltage at about 5.5 V and maximum brightness of 9.40 cd/m², and emitted violet light at 414 nm.     

Growth of heavily C-doped GaAs/AlGaAs MQW structures by MOVPE for 2–3 μm normal incidence photodetectors

The growth and intersubband optical properties of high quality heavily doped p-type GaAs/AlGaAs multiple quantum well (MQW) structures are reported. The MQWs were fabricated by the atmospheric pressure metalorganic vapor phase epitaxy process using liquid CCl₄ to dope the wells with C acceptors (N_a ≈ 2 × 10¹⁹ cm⁻³). A constant growth temperature was maintained for the entire structure while different V/III ratios were used for the well and barrier regions. By this process it is possible to achieve both high C doping densities in the wells and to simultaneously obtain good quality AlGaAs barriers. Fourier transform infrared spectroscopy measurements on heavily doped 10-period MQW structures reveal a new absorption peak at 2 μm with an effective normal incidence absorption coefficient of 4000 cm⁻¹. Photocurrent measurements on mesa-shaped diodes show a corresponding peak at 2.1 μm. The photodiodes exhibit a symmetrical current-voltage characteristic and a low dark current, which are indicative of a high quality MQW structure and a well-controlled C doping profile. The 2 μm absorption represents the shortest wavelength ever reported for any GaAs/AlGaAs or InGaAs/AlGaAs MQW structure and should be very useful for implementing multicolor infrared photodetectors.     

MOVPE Growth and Characterisation of Zn‐Doped (GaIn)P Layers with Respect to Surface Structure and Ordering

(GaIn)P grown on (001)GaAs substrates by metal‐organic vapour phase epitaxy (MOVPE) is highly ordered material. In this work the effect of Zn doping on the epitaxial crystal growth, the ordering behaviour and the surface morphology is investigated. Zn‐doped (GaIn)P layers, grown with phosphine (PH₃), tertiarybutylphosphine (TBP) and ditertiarybutylphosphine (DitBuPH) as phosphorous precursors, reveal a strong drop of the binary growth rate of InP r_InP with increasing Zn/III ratio, whereas r_GaP remains nearly constant. The Zn doping efficiency and the ordering behaviour are observed to be dependent on the misorientation of the substrates. Finally, the surface morphology as a function of the different parameters was analysed by atomic force microscopy (AFM), and no considerable change of the growth mechanism was found for Zn‐doped layers in comparison to undoped layers.     

Synthesis and Structure of New Glutarate-Containing U(VI), Np(V), and Pu(VI) Compounds

Crystallography Reports - The structures of three new glutarate-containing compounds have been characterized. These compounds are (CN3H6)2[(UO2)2(C5H6O4)3] · 4H2O (I),...     

Bridgman Growth and Characterization of LuAlO3—Ce3+ Scintillator Crystals

Chemical and structural effects in LuAlO₃-Ce³⁺ single crystals grown by the Bridgman method were studied using spectral emission, x-ray, etching and optical techniques. The Ce concentration distribution was found to exhibit the normal freeze behavior with partition coefficient of 0,17. The expansion of the unit cell volume due to incorporation of Ce was measured. Dislocation etch pits were revealed on (010), (100) and (001) crystal faces. Thermal expansion behavior of LuAlO₃ was studied along the major crystallographic axes in between the room temperature and 1000 °C. The scintillation performance of 5 × 5 × 10 mm³ samples was measured using a 662 keV gamma source with a shaping time of 1.2 μs. The light yield was increasing from 40% BGO to 70% BGO with increasing of the Ce³⁺ content from 0.13 at.% to 0.9 at.%.     

Synthesis and Characterisation of Bulk Textured Phases in the Bi(Pb)—Sr—Ca—Cu—O System

Growth of lead-doped textured Bi-cuprate, using an immersed heater floating zone (TSFZ) apparatus, from a narrow supercooled melt created an essentially dense structure consisting of locally aligned long platelet crystal grains. The extended grains have their long axes parallel to the growth axis with a low angle misorientation. The effect of gas bubbles and the pulling rates on the microstructure and superconducting transition temperature of the textured boules have been studied. The decomposing nature of the 2223 phase on melting leads to form the energetically favourable low T_c 2212 phase.     

Alternative precursors for MOVPE growth of InN and GaN at low temperature

We report on the use of dimethylhydrazine (DMHy) and tertiarybuthylhydrazine (TBHy), as alternative nitrogen precursor for GaN low-temperature growth, as well as to improve the InN growth rate. Lowering the GaN growth temperature, would allow growing InN/GaN heterostructures by MOVPE, without damaging the InN layers. Increasing the low InN MOVPE growth rate is of major importance to grow reasonably thick InN layers. In this respect, triethylindium (TEIn) was also used as an alternative to trimethylindium (TMIn).     

Crystal Growth of High‐melting Multi‐component Rare Earth‐Transition Metal Intermetallic Compounds from the Melt

The crystal growth of different classes of high melting multi‐component Rare Earth‐Transition Metal‐Compounds by vertical floating zone melting with inductive heating has been investigated. Phase diagram features of the multi‐component systems relevant for the crystallization process have been revealed. The critical zone travelling rate for RENi₂B₂C crystal growth is one order of magnitude smaller than for RE₂TMSi₃ compounds. This is attributed to different solidification modes of both classes of compounds, peritectic and congruent melting, respectively. The crystal perfection, element segregation and selected properties of the bulk crystals such as critical temperature of superconductivity were studied as function of the axis co‐ordinate. In the case of RE₂TMSi₂ plate‐like RESi or RESi₂ precipitates were detected in the single crystalline matrix. They were partially dissolved by annealing and subsequent quenching. The anisotropy of various superconducting and magnetic properties was determined at YNi₂B₂C and TbNi₂B₂C single crystalline specimens.     

AgGaSe2 on {100} and {110} GaAs—Special Features of Epitaxial Growth

AgGaSe₂ thin epitaxial layers onto {100} and {110} oriented GaAs substrates were prepared by flash evaporation technique and investigated by the RHEED method (reflection high energy electron diffraction). Special epitaxial relationships were found and the results will be discussed.     

MOVPE growth of ternary and quaternary tensile strained MQW structures for polarization insensitive devices

Devices for future all-optical network systems will have a strong requirement for polarization independent operation, which can be achieved by introducing biaxially tensile strain into MQW active regions. We report on InP based low pressure MOVPE growth of MQW structures for 1.55 μm wavelength with tensile strained InGaAs and InGaAsP wells and lattice matched InGaAsP barrier layers. Taking into account measurement results from X-ray and PL together with theoretical evaluations we were able to grow polarization insensitive device structures. For all-optical wavelength converters we used MQW stacks with 5 tensile strained ternary wells and achieved λ conversion with a polarization dependence of less than 1 dB. Using quaternary wells a polarization insensitive electroabsorption modulator was realized. The residual polarization dependence is < 0.4 dB for 1550 nm and < 1 dB in the wavelength range from 1540 to 1560 nm (extinction ratio 10 dB).