Novel single source precursors for MOCVD of AlN, GaN and InN

The MOCVD of AlN, GaN and InN thin films using the novel single source precursors (N₃)₂Ga[(CH₂)₃NMe₂] (1), (N₃)In[(CH₂)₃NMe₂]₂ (2), (N₃)Al[(CH₂)₃NMe₂]₂ (3) and (N₃)AlMe₂(H₂N^tBu) (4) is reported. The compounds are non-pyrophoric. Compound 3 is air stable. No additional N-sources were used for the growth of the nitrides. We achieved epitaxial (AlN, GaN) or polycrystalline (InN) growth at least 200°C below the decomposition temperature of the respective nitride. Some aspects of the reactivity of the precursors with ammonia and the resulting influence on the deposition process were investigated.     

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).     

Femtosecond transmission studies of n-type and p-type hydrogenated amorphous silicon pumped in the exponential band tails

The results of transient transmission studies utilizing femtosecond laser pulses on n-type and p-type hydrogenated amorphous silicon are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different materials are quite different, allowing the technique to clearly distinguish between them. It is shown that while recombination is dominant in intrinsic a-Si:H, trapping becomes important in the doped material. A model of the experimental results gives additional insight into the density of states in the band tail region.     

Ohmic contacts properties of Ni/Ag metallization scheme on p-type GaN

In the present work, we report on the characteristics of nickel(Ni)/silver(Ag) bi-layer contacts on p-type GaN. High quality doped GaN layers were grown on silicon (1 1 1) substrates using high temperature grown AlN (about 200 nm) as buffer layer by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PAMBE). Doping was done using high purity Mg as p-type dopant. In this work, a metal Ag layer was selected as the capping layer due to its reflective characteristics at visible wavelengths. The structural and electrical stability of the contacts at various annealing temperatures (400-700 °C) were investigated, as thermally stable metal semiconductor contacts are essential for high quality devices. Changes in the surface morphology of the contacts on annealing were examined using scanning electron microscopy (SEM). Specific contact resistivity was determined using transmission line method (TLM) and current-voltage (I-V) measurements. The low contact resistance is attributed to the reduction of the native oxide by Ni diffusion along with the formation of Ni and Ag related-gallide phases at the p-GaN surface region. We deduce that high Mg doping may lead to the creation of a large number of deep level defect in p-GaN, leading to the reduction of the depletion layer width in the p-GaN near the interface and an increase in the probability of thermionic filed emission.     

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.     

Tin as an n-type dopant in the molecular beam epitaxial growth of GaAs(111)A

We have made a systematic study of the tin doping of GaAs layers grown on GaAs(111)A substrates using molecular beam epitaxy (MBE). A series of samples were grown with a range of substrate temperatures (from 460 to 620°C), As:Ga flux ratios (5:1 to 25:1) and tin concentrations (10¹⁶ to 10²⁰ atoms cm⁻³). Layers grown on (111)A surfaces were n-type (in contrast to silicon doping) but with carrier concentrations dependent on growth conditions. We have used secondary ion mass spectrometry (SIMS) measurements to confirm the concentration of tin incorporated and its distribution within the layers.     

Efficiency optimization of p-type doping in GaN:Mg layers grown by molecular-beam epitaxy

The Mg-doping efficiency in GaN layers grown by molecular-beam epitaxy has been studied as a function of the growth temperature, the growth rate, and the Mg beam flux. The Mg cell temperature window for efficient p-type doping is rather narrow, being limited by the GaN n-type background doping density (lower limit) and by the Mg surface coverage that, beyond a threshold, induces a layer polarity inversion (N-polarity), leading to a reduction of the Mg incorporation (upper limit). An increase of the growth temperature avoids this polarity inversion, but the Mg flux must be increased to compensate the strong desorption rate. Thus, a trade-off between both temperatures has to be reached. A reduction of the growth rate has a strong effect on the p-type doping level, yielding up to 7×10¹⁷ holes/cm³ for a total Mg concentration of 1×10¹⁹ cm⁻³. This high Mg concentration does not seem to generate Mg-related defects or deep traps.     

New aluminium precursors for MOMBE (CBE): a comparative study

Recently different new Al precursors have been developed to improve the electrical and optical quality of AlGaAs layers grown by MOMBE (CBE), since AlGaAs layers still suffer from the high incorporation of oxygen and carbon. Three approaches are introduced and results obtained from Al_xGa_1−xAs layers (0 < x ≤ 1) are discussed. APAH, a double ring structure molecule, was found to yield AlGaAs layers with high contents of carbon and nitrogen. The use of an Alane-adduct decreases impurity concentrations and improves optical properties. However, TIBAl is superior and provides highest PL response together with carrier concentrations below p = 10¹⁶ cm^-3. Even though the concept of coordinative saturation is promising, results achieved by TIBAl showed that trialkyls could also be well suited for AlGaAs, assuming that they are properly synthesized.     

Crystal growth and dopant segregation of Ce:LiSrAlF6 and Eu:LiSrAlF6 crystals with high dopant concentrations

Ce:LiSrAlF₆ and Eu:LiSrAlF₆ crystals with different dopant concentrations were grown by the micro-pulling-down method. The crystals with high dopant crystal included the secondary phase as clusters with the plate shape in BSE images. The secondary phases were identified CeF₃ and EuF₂, respectively, by the EDS analysis and powder-XRD measurement. Eu concentration against the Sr sites in the Eu 2% doped LiSAF crystal were most uniform in the range 0.9–1.6 atm% using the EPMA.     

Low pressure MOVPE of GaN and GaInN/GaN heterostructures

GaN single layers and GaInN/GaN heterostructures have been grown by low pressure metalorganic vapor phase epitaxy on sapphire substrates. We found best growth conditions and the highest growth rate for GaN to be at about 1000°C, whereas the growth rate decreased for both, higher and lower temperatures. In contrast, GaInN with a significantly high In content could only be grown at lower temperatures around 700°C. Besides growth temperature and reactor pressure, the composition of the carrier gas was found to play an important role: the In incorporation rate is about doubled when reducing the hydrogen/nitrogen ratio. GaInN/GaN quantum wells show even higher In contents compared to bulk layers.     

Approach for dislocation free GaN epitaxy

The characteristics of confined epitaxial growth are investigated with the goal of determining the contributing effects of mask attributes (spacing, feature size) and growth conditions (V/III ratio, pressure, temperature) on the efficiency of the approach for dislocation density reduction of GaN. In addition to standard (secondary electron and atomic force) microscopy, electron channeling contrast imaging (ECCI) is employed to identify extended defects over large (tens of microns) areas. Using this method, it is illustrated that by confining the epitaxial growth, high quality GaN can be grown with dislocation densities approaching zero.     

Synthesis and properties of zinc-nitrogen compounds for the MOVPE of p-type ZnSe

Novel nitrogen-based compounds for p-type doping of ZnSe have been studied. Photoluminescence spectra of epilayers doped with synthesized zinc amides Zn(NRR')₂ and with corresponding amines HNRR' (R and R' are organic ligands) indicate an insufficient stability of the Zn---N bond preventing effective doping of the tested zinc amides. Doping efficiency is improved by replacing t-butyl groups (---CMe₃) of the ligands by trimethylsilyl groups (---SiMe₃). Nitrogen incorporation under usual growth conditions remained, however, too low for device applications.     

Metalorganic vapour phase epitaxy of GaN and GaInN/GaN heterostructures and quantum wells

This paper reviews the growth and some characteristics of group III-nitrides by metalorganic vapour phase epitaxy with a particular focus on GaInN layers and heterostructures. We discuss the problems encountered with the low In incorporation efficiency. This can be partly compensated by larger growth rates and higher nitrogen-hydrogen ratios in the carrier gas. However, the grown layers with larger In content show evidence of composition fluctuations and even surface roughening due to problems probably arising from the large lattice mismatch to GaN and from the miscibility gap predicted for essentially the whole composition range. This influences strongly the spectroscopic properties. Consequences on the functionality of optoelectronic devices are also shortly discussed.     

Mechanical activation of precursors for nanocrystalline materials

Nanostructured materials win big scientific interest and increasingly economic meaning through their specific exceptional properties. Precursors that were compacted by pressing and sintering are normally used preparation of materials. In present work, the influence of mechanical activation by grinding on the structure as well as on compacting and sintering behavior of oxides from magnesium, aluminium and silicon has been investigated. Starting materials for each metal oxide differ in microstructure, dispersity, and porosity. The influence of mechanical activation on the destruction of crystalline structure to nanocrystalline, as well as to the amorphous stage and the compaction of powders with nano‐particles, as well as structures with nanoscale pores have been compared. The possibilities of the consolidation of nanostructured materials were investigated. The mechanical activation took place in a disc vibration mill. The mechanical activated materials as well as their pressing and their sintering products were characterized by density, particle‐sizedistribution, specific surface, pore‐structure, microstructure, and crystallite size by X‐ray powder diffraction (XRD). The mechanical activation of the model‐substances led, in most cases, to an improvement of the compaction properties; thus, this improvement can be achieved with subsequent sintering densities up to 98% of the theoretical density. From these experiments, generalizations transferable to other materials can be made.     

Material optimisation for AlGaN/GaN HFET applications

An optimisation of some growth parameters for the epitaxy of AlGaN–GaN based heterostructure field effect transistors (HFET) at low pressure in a new 3 _* 2″ MOVPE reactor is presented. Some possible processes for the growth of semi-insulating buffers have been identified and are described. TEM analysis shows that the insulating character is not due to a high density of dislocations, whereas SIMS analysis shows that classical impurity (Si, O and C) concentrations are in the same range as in conductive undoped layers. Further studies are needed to identify the traps responsible for the compensation of the GaN layers. The properties of the two-dimensional electron gas (2DEG) located at the AlGaN–GaN interface can be tuned by modifying the characteristics of the AlGaN layer and of the insulating buffer. The best mobility (1500 cm² V⁻¹ s⁻¹ for n6×10¹² cm⁻²) is obtained when using a thick buffer layer, whereas the sheet carrier density is found to increase with the Al content in the undoped supply layer and reaches 1.1×10¹³ cm⁻² for a composition of 24%.     

Cathodoluminescence of epitaxial GaN and ZnO thin films for scintillator applications

A comparative study of cathodoluminescence ultraviolet photon yields and decay times of large area GaN and zinc oxide (ZnO) layers grown for scintillator applications by metalorganic vapor phase epitaxy is presented. Silicon-doped GaN and non-intentionally-doped ZnO yield up to 1.4±0.2 photons/kVe⁻ and 1.3±0.2 photons/kVe^– at room-temperature, respectively. For GaN the decay times scatter between 0.4 and 0.9 ns, and for ZnO between 2.5 and 3.0 ns. The GaN and the ZnO absorption coefficients, α, internal efficiencies, η_i, and radiative constants, B, are determined. The characteristics of thin-film scintillators based on these materials are compared with commercially available granular scintillators.