Pressure MOVPE Research Articles

APMOVPE growth and characterisation of undoped GaAsN/GaAs heterostructures

Unusual properties of the diluted nitrides A IIIB V–N make them very attractive in the point of view of fundamental investigations and practical applications. This work presents the growth characteristics and properties of the undoped GaAs 1− x N x /GaAs heterostructures obtained by the atmospheric pressure metal organic vapour phase epitaxy (APMOVPE). The properties of the heterostructures were examined using high-resolution X-ray diffraction (HRXRD), photoreflectance spectroscopy (PR), photoluminescence (PL), photovoltage spectroscopy (PVS) and mercury probe C– V set-up with an HP 4192A impedance analyser (5 Hz–13 MHz). The maximum nitrogen content in GaAs 1− x N x epilayers, determined from PR spectra, did not exceed 1.74%. The influence of the growth temperature ( T g) and the nitrogen source concentration in gas phase ( X g) on the properties of the GaAs 1− x N x /GaAs heterostructures is presented and discussed.

The surface diffusion of Ga species on an AlGaN facet structure in low pressure MOVPE

The diffusion of Ga species in gas phase as well as on the surface are studied in selective area growth of AlGaN in low pressure metalorganic vapor phase epitaxy. The experiments were performed on a trapezoidal stripe with (0001) facet on the top and (1-101) facet on the sides. It was found that the ridge growth on the facets were sensitive to the growth pressure, in agreement with numerical results. At a low pressure of 100 Torr, we got a uniform thickness of AlGaN alloy, but the alloy composition was not uniform. Exponential variation of the composition gave the effective diffusion length of Ga on the order of 0.7 μm which was independent of the growth pressure. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth characteristics of ZnMgTe layer on ZnTe substrate by metalorganic vapor phase epitaxy

The ternary Zn 1− x Mg x Te epitaxial layers have been successfully grown on (1 0 0) ZnTe substrates by atmospheric pressure metalorganic vapor phase epitaxy using dimethylzinc, bis-methylcyclopentadienyl-magnesium ((MeCp) 2Mg) and diethyltelluride (DETe) as source materials. The effects of source transport rates and total gas flow rate upon the Mg composition in the Zn 1− x Mg x Te layers have been investigated. It was found that the Mg composition drastically increase with decreasing DETe transport rate or with increasing the total gas flow rate, which may be associated with the reduction of the premature reaction between DETe and (MeCp) 2Mg.

Analysis of GaN decomposition in an atmospheric MOVPE vertical reactor

The GaN decomposition versus the annealing ambient (H2, N2 + H2) have been investigated in atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) vertical reactor. A comparison between the low temperature GaN (LT-GaN) decomposition in a vertical reactor and a horizontal one was discussed in order to explain the failure of high temperature GaN (HT-GaN) growth on Al2O3 using 2D buffer layer under a mixture of H2 and N2 in our vertical reactor. Decomposition rates versus temperature and ambient gas were measured using real time laser reflectometry. The surfaces of these films were analyzed using a scanning electron microscopy (SEM). Hydrogen seems to considerably affect the LT-GaN surface morphology by a mass transport, diffusion on the substrate surface and by a decomposition process even in the presence of ammonia and nitrogen. The high decomposition rates of the LT-GaN measured in comparison to that determined for the HT-GaN one is attributed to the amorphous-like structure of the layer and the instability of Ga-N bonds. This favors the appearance of a mirror-like gallium surface in the beginning of this layer decomposition. The presence of liquid gallium catalyzes the decomposition process. HT-GaN SEM analysis revealed that the surface of the “totally” decomposed GaN surface is dominated by gallium droplets. These were removed. The region around and under these droplets is capped by a porous like GaN film. The coalesced gallium droplets may cover some areas of partially decomposed GaN and preserve them from H2 attack and total decomposition. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of dopant transport rate upon photoluminescence and electrical properties of ZnTe in atmospheric pressure MOVPE using tris-dimethylaminophosphorus

The P-doped ZnTe layers have been grown on ZnTe (1 0 0) substrates by atmospheric pressure metalorganic vapor phase epitaxy using tris-dimethylaminophosphorus (TDMAP). The effects of dopant transport rate upon the photoluminescence and electrical properties of ZnTe layers have been investigated. ZnTe layer with a carrier concentration of 1.3×10 18 cm −3 is obtained by using this dopant source. Most ZnTe epitaxial layers exhibit strong band-edge emission at room temperature. It is expected that TDMAP is a useful dopant source, compared to tertiary-butylphosphine.

AP-MOVPE of thin GaAs 1−xBi x alloys

GaAs 1− x Bi x alloy was grown by atmospheric-pressure metalorganic vapour-phase epitaxy using a horizontal reactor. GaAs 1− x Bi x epilayers were elaborated on exactly (1 0 0)-oriented p-GaAs substrates. Trimethyl-gallium, trimethyl bismuth (TMBi), and arsine were used as precursor sources at a growth temperature of 420 °C within a very narrow range of V/III ratios and molar flow rates of TMBi. The lattice mismatch between the layer and the substrate was examined by using high-resolution X-ray diffraction technique. The measurements were performed on (0 0 4) and (1 1 5) planes. The solid composition of GaBi content in the GaAs 1− x Bi x alloy reaches a maximum value of about 3.7%. In analyzing the surface morphology, scanning electron microscopy (SEM) and SEM-energy dispersive X-ray spectrometer were used to qualify films properties.

Laser reflectometry in situ monitoring of InGaAs grown by atmospheric pressure metalorganic vapour phase epitaxy

InGaAs layers on undoped GaAs (0 0 1) substrates were grown by atmospheric pressure metalorganic vapour phase epitaxy (AP-MOVPE). In order to obtain films with different indium composition ( x In), the growth temperature as a growth parameter, was varied from 420 to 680 °C. Furthermore, high-resolution X-ray diffraction (HRXRD) measurements were used to quantify the change of x In. Crystal quality has been also studied as a function of growth conditions. On the other hand, laser reflectometry (LR) at 632.8 nm wavelength, was employed to in situ monitor epitaxy. Reflectivity–time signal was enabled to evaluate structural and optical properties of samples. We have fitted experimental data to determine optical constants and growth rate of InGaAs at 632.8 nm. In addition, the fitting provided InGaAs thickness as a function of growth time. Based on ex situ characterization by scanning electronic microscopy (SEM) and HRXRD, we propose a practical method, relating the contrast of first reflectivity maximum with the X-ray diffraction peak angular difference between the substrate and epitaxial layer, to determine in situ the In solid composition in InGaAs alloys.

High temperature refractive indices of GaN

AbstractUndoped GaN (u‐GaN) films were grown by low pressure metalorganic vapour phase epitaxy (LP‐MOVPE) on sapphire substrates. In situ optical monitoring was applied to the growth process either using a LayTec EpiR‐DA TT spectroscopic reflectometer or Filmetrics F30. Refractive indices of u‐GaN films at 1060 °C were obtained in a spectral range from 370‐900 nm. A peak at 412±5 nm in refractive index spectra was observed, which most likely corresponds to the band‐gap of hexagonal GaN at a temperature of 1060 °C. Refractive indices below this band‐gap are fitted well to the first‐order Sellmeier formula. As an example of the applications of the refractive indices, the effective film thicknesses of GaN during the resumption from 3 dimensional (3D) to 2 dimensional (2D) growth have been calculated from the spectra recorded by a LayTec system using the optical constants obtained. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Towards a better understanding of nano‐islands formed during atmospheric pressure MOVPE

AbstractWe report on transmission electron microscopy (TEM) characterisation of nano‐islands formed during atmospheric pressure growth of InGaN on GaN by metal‐organic vapour phase epitaxy (MOVPE). Although initial results seemed to indicate that three‐dimensional InGaN nano‐islands had formed, detailed TEM characterisation revealed that the islands do not have a hexagonal crystal structure, and electron energy loss spectroscopy suggests that the observed structures are metallic indium droplets formed during growth, which may be partially nitrided, rather than the expected strain‐induced epitaxial InGaN nano‐islands. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strain and microstructure in Fe‐doped GaN layers grown by low pressure metalorganic vapour phase epitaxy

AbstractIn order to get semi‐insulating GaN layers with a low dislocation density on sapphire, two kinds of Fe doping were explored by low pressure metalorganic vapour phase epitaxy (LP‐MOVPE): the modulation doping (MD) and the continuous doping (CD). The high crystalline quality and the semi‐insulating character are obtained in the case of the Fe‐MD layers. The effect of Fe doping on the strain was investigated in both kind of layers (MD and CD) by X‐ray diffraction, photoluminescence and reflectivity. The lattice parameters are functions of the iron doping level, whatever the doping mode. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ monitoring of GaN epitaxial lateral overgrowth by spectroscopic reflectometry

The application of spectroscopic reflectometry to the monitoring of epitaxial lateral overgrowth of GaN in low pressure metalorganic vapor phase epitaxy has been investigated. Real-time vertical and lateral growth rates and hence thickness and wing width of the growing GaN are extracted. A vertical growth enhancement was clearly observed at an early stage, followed by vertical growth suppression until full coalescence was achieved. The lateral to vertical growth ratio was obtained showing clear time dependent characteristics. The observations were explained by considering the mass transport between the growing (0001) facets and the {112¯0} sidewall facets.

MOVPE growth optimisation of CdTe epitaxial layers for p‐i ‐n diode X‐ray detector fabrication

AbstractWe report on the atmospheric pressure metalorganic vapour phase epitaxy of both nominally undoped and iodine (I) donor doped CdTe layers on high resistivity (111)B CdTe substrates, as a preliminary step towards the fabrication of p‐i‐n diode X‐ray detectors. CdTe epilayers were grown at 330 °C after in‐situ H2 heat treatment of the substrate for surface oxide removal. We show that the homoepitaxy of CdTe critically depends on in‐situ heat treatment temperature (TA), best epilayer morphology and reduced surface roughness being obtained for TA = 350 °C. Secondary ion mass spectrometry analysis of I‐doped samples shows good dopant incorporation, which increases for Te:Cd precursor molar flow ratios in the vapour below unity. I‐doped samples grown under the latter conditions have room temperature resistivity three orders of magnitude lower than for undoped layers and electron concentrations ∼1016 cm–3. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of boron‐doped ZnTe homoepitaxial layer by metalorganic vapor phase epitaxy

AbstractWe have investigated boron (B) as a n‐type dopant for ZnTe homoepitaxial layer by atmospheric pressure metalorganic vapor phase epitaxy using triethylboron (TEB). The introduction of TEB induces considerable change in the photoluminescence spectrum of the layer. Two distinctive luminescence peaks associated with excitons bound to neutral dononor and donor‐acceptor pair (DAP) transition are emerged in the spectrum, implying that the incorporated B acts as donor impurity in ZnTe. From the temperature dependence of the PL intensity of DAP recombination, the value of donor level was estimated to be 18 meV. This value seems to be reasonable, comparing with the previous studies on Cl and Al doping of ZnTe. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of substrate temperature upon photoluminescence and electrical properties of ZnTe in atmospheric pressure MOVPE using tris‐dimethylaminophosphorus

The effect of substrate temperature upon the photoluminescence (PL) properties of phosphorus-doped ZnTe layers grown by atmospheric pressure metalorganic vapor phase epitaxy using tris-dimethylaminophosphorus as a dopant source has been investigated together with the growth rate behaviour. At a substrate temperature of 400 °C corresponding to the growth condition close to the mass transport to surface kinetic transition region, the PL spectrum at 4 K is characterized by a strong Ia and a weak free-to-bound transition emission, implying a P-doped ZnTe layer of good crystalline quality. For the layers grown at the substrate temperatures away from this temperature, on the other hand, a donor-acceptor pair recombination emission appears in the spectra. The P-doped layers exhibit only the edge emission at ∼549 nm in the room temperature PL specrum, independent of substrate temperature. The relationship between the electrical properties of P-doped layers and the substrate temperature has also clarified. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InAs pixel matrix detectors fabricated by diffusion of Zn in a metal-organic vapour-phase epitaxy reactor

We introduce a zinc diffusion process to fabricate an InAs-based detector matrix using an atmospheric pressure metal-organic vapourphase epitaxy reactor. Current–voltage characteristics are measured and different diffusion parameters are experimented. Spectral alpha radiation response of the diode is reported. To our knowledge, this is the first time that InAs was used as an alpha particle detector.

Inhomogeneities in MOVPE InN

This paper reports the in-depth and in-plane inhomogeneities in InN films. Samples of InN are grown on sapphire substrates without or with a buffer using the atmospheric-pressure MOVPE. For the in-depth inhomogeneity analysis, the conventional PL measurement using an excitation source with a different wavelength is made from both the front surface and the film/substrate interface sides of samples. The measurement reveals that samples grown for a long time without buffer contain a deteriorated layer, which shows a higher PL peak energy, near the film/substrate interface. Such a layer is never found on the surface side. The deterioration is markedly enhanced when the sample is annealed in an NH 3 flow. Since the coexistence of TMI and NH 3 (growth environment) suppresses the deterioration and the InN samples used in this experiment have an In-polarity, such a deterioration is hydrogen-related degradation on the N-face of InN film. In-plane inhomogeneity of InN grown with a different buffer is studied by analyzing PL and absorption data. A wider PL spectrum and a larger discrepancy between PL peak energy and absorption edge, those are observed for films grown without buffer, are well explained by taking account of a large in-plane inhomogeneity in the samples. The existence of the large in-plane inhomogeneity is confirmed by the near-field PL mapping obtained by the SNOM. The SNOM analysis also shows that, although the use of the buffer improves the apparent in-plane uniformity, such a film has a fine inhomogeneity due to small grains of the buffer.

Effect of annealing ambient flux on InAs islands grown on GaAs(0 0 1) substrates by metal–organic vapor phase epitaxy

Effects of post-growth annealing under dissimilar ambient fluxes on InAs islands grown on GaAs(0 0 1) substrates by low pressure metal–organic vapor phase epitaxy (LP-MOVPE) have been investigated. It is found that InAs islands annealed under different ambient fluxes exhibit different surface morphology evolution. Islands annealed under DMHy flux exhibit dissolving of the 3D island morphology. In contrast, upon annealing under TMSb flux, the 3D island morphology is enhanced with increased island density. The result suggests that we can fine tune the size and density of InAs islands by post-growth annealing under different ambient fluxes and this method is expected to be applicable to the growth of QDs of other material systems. Our findings are discussed in terms of the existence of critical nucleus in heterogeneous growth processes.

LP MOVPE growth and characterization of high Al content Al xGa 1−xN epilayers

High quality Al x Ga 1− x N layers have been grown on sapphire (0 0 0 1) by low pressure metalorganic vapour phase epitaxy (LP MOVPE) at 1180 °C. The aluminum content covers a wide composition range varying from 0 to 65%. In order to ameliore the quality of these epilayers especially for high aluminum content ( X s Al >0.35), growth conditions are carefully optimized. The growth rate of AlGaN increases linearly with increasing group III elements partial pressure. The reduction of the growth pressure from 50 to 20×10 −3 atm induces an increase of the growth rate and raises the solid Al content from 19.6 to 53.3%. Additionally, a linear relationship (slope about 1) is observed between gas and solid aluminum content in the whole range of Al content in AlGaN alloys grown at 20×10 −3 atm. The structural properties of Al x Ga 1− x N layers, with different aluminum composition, are characterized by double X-ray diffraction (DDX) showing a high quality of our epilayers. A degradation of the crystalline quality of Al x Ga 1− x N films is observed with increasing aluminum content. The optical properties of these layers are investigated by photoluminescence measurements at low temperature (12 K). With increasing the Al content, the principal peak (neutral donor bound exciton) shifts to higher energies and the full width at half maximum (FWHM) increases significantly.

Control of Zn composition (0 < x < 1) in Cd 1− xZn xTe epitaxial layers on GaAs substrates grown by MOVPE

Cd 1− x Zn x Te epitaxial layers over the entire composition range ( x from 0 to 1) were grown on (1 0 0) GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy. A growth temperature of 560 °C, and the group VI/II precursor flow rate ratio of 2 or larger enabled us to control the Zn composition strictly on the grown epilayers. Epitaxial layers with high crystal quality were obtained in a wide range of Zn composition. The double crystal rocking curves (DCRC) full-width at half maximum (FWHM) values were between 260 and 670 arcsec at the end points of alloy range. The low-temperature PL measurements showed distinct bound-exciton emissions and weak deep-level emissions.

Structural and optical characterisation of CuGaS 2 thin films grown by MOVPE

Epitaxial growth of CuGaS 2 using atmospheric pressure metalorganic (MO) vapour phase epitaxy (MOVPE) was carried out using two different copper metalorganic precursors, namely, cyclopentadienyl copper triethylphosphine (CpCuTEP) and hexafluoroacetylacetate copper triethylamine (Cu(hfac) 2·NEt 3). A change in the Cu content (most significant for epilayers grown using the former precursor) was found to occur along the flow direction. A study using X-ray diffractometry could not conclusively determine which of the possible structure types were present in the epilayer. Analysis of the transmission electron diffraction (TED) patterns using the <211> zone axes indicates that a mixture of disordered zinc blende and CuPt-type ordering occurs in both types of epilayers, whereas a tetragonal crystal structure (chalcopyrite or CuAu-type ordering) likely occurs within surface crystallites found in epilayers grown using Cu(hfac) 2·NEt 3. The photoluminescence (PL) for both epilayers consisted of a broad band at 580–620 nm, typical of Ga-rich material.