Low-pressure Metalorganic Vapor Phase Epitaxy Research Articles

High Temperature Diffusion in AlxGa1-xN and P-Type AlGaN by Al4C3

The diffusion experiment of AlxGa1-xN (x = 0.00, 0.04, 0.45, 0.65, 0.86, 1.00) samples using a solid source of Al4C3 layer was performed by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The AlxGa1-xN (x≦0.45) samples were proven to be a p-type. In second ion mass spectroscopy (SIMS) analysis, the carbon profile is different from the simple complementary error function, but is the double of the complementary error function, meaning AlC or AlCO plus C. The diffusion length (L) was drastically decreased by increasing Al. The diffusion coefficient (D) was also calculated as a function of Al mole fraction.

Impacts of Si-doping and resultant cation vacancy formation on the luminescence dynamics for the near-band-edge emission of Al0.6Ga0.4N films grown on AlN templates by metalorganic vapor phase epitaxy

Luminescence dynamics for the near-band-edge (NBE) emission peak at around 250 nm of c-plane Si-doped Al0.6Ga0.4N films grown on AlN templates by low-pressure metalorganic vapor phase epitaxy were studied using deep ultraviolet time-resolved photoluminescence and time-resolved cathodoluminescence spectroscopies. For the films with the Si-doping concentration, [Si], lower than 1.9 × 1017 cm–3, the doping lessened the concentration of cation vacancies, [VIII], through the surfactant effect or the aid of the reactant doping in a form of H3SiNH2. However, the room-temperature nonradiative lifetime, and, consequently, the equivalent value of internal quantum efficiency in the weak excitation regime steeply decreased when [Si] exceeded 1018 cm−3. Simultaneously, the intensity ratio of the deep-state emission band to the NBE emission abruptly increased. Because the increase in [Si] essentially gives rise to the increase in [VIII] (for [Si]>1.9×1017 cm−3) and the overcompensation of Si is eventually observed for the film with [Si] = 4.0 × 1018 cm−3, the formation of acceptor-type native-defect complexes containing Si such as VIII-SiIII is suggested.

A study of the structural properties of GaN implanted by various rare-earth ions

GaN layers with 〈0001〉 crystallographic orientation, grown by low-pressure metal-organic vapour-phase epitaxy (MOVPE) on c-plane sapphire substrates, were implanted with 200 and 400keV Sm+, Tm+, Eu+, Tb+ and Ho+ ions at fluencies of 1×1015–1×1016cm−2. The composition of the ion-implanted layers and concentration profiles of the implanted atoms were studied by Rutherford Back-Scattering spectrometry (RBS). The profiles were compared to SRIM 2008 simulations. The structural properties of the ion-implanted layers were characterised by RBS-channelling and Raman spectroscopy. Changes in the surface morphology caused by the ion implantation were examined by Atomic Force Microscopy (AFM). A structural analysis showed a high disorder of the atoms close to the amorphised structure at the surface layer above an implantation fluence of 5×1015cm−2 while lower disorder density was observed in the bulk according to the projected range of 400keV ions. The post-implantation annealing induced significant changes only in the Sm and Eu depth profiles; a diffusion of rare-earths implanted at a fluence of 5×1015cm−2 to the surface was observed. The annealing caused the reconstruction of the surface layer accompanied by surface-roughness enhancement.

Implantation Studies on Silicon-Doped GaN

Silicon-doped GaN layers grown by low-pressure metalorganic vapor-phase epitaxy with Si concentrations ranging from 2 × 1017 Si/cm3 to 9.2 × 1018 Si/cm3 were investigated by means of the perturbed angular correlation (PAC) technique applied to implanted 111In(Cd). An undoped GaN film is used as a reference. The Si atoms replace Ga atoms in the lattice, and silicon, being a group IV element, acts as a donor on the Ga site and contributes one extra electron to the conduction band. Hall-effect measurements confirmed that the free charge carrier density is essentially increased and of the order of the silicon concentration. PAC investigations of the annealing behavior after implantation of the 111In probes show that best recovery is achieved after annealing at 1200 K and that high silicon concentrations make GaN films more stable at high temperatures. Further, it was found that the temperature dependence of the electric field gradient is reduced by increasing Si concentrations.

Impact of the substrate and of the nucleation layer on the properties of AlGaN/GaN HEMTs on SiC

This work reports on the influence of the substrate properties and of the nucleation layer on the growth mode of AlGaN/GaN HEMT (high electron mobility transistor) structures, grown by low pressure metal organic vapour phase epitaxy (LP-MOVPE) on 3in. 4H-SiC and 6H-SiC semi-insulating substrates. In-situ normal incidence reflectance at 633nm and 950nm was used to monitor the evolution of the surface morphology during the growth. The analysis reveals that the growth of the GaN buffer layer on the AlN wetting layer is a critical key step of the process. Reflectance measurements are discussed in term of roughness-related light scattering and absorption. The influence of substrate surface properties has been studied extensively. Physical and electrical properties of the HEMT structures, as evaluated by a complete set of ex-situ characterisation techniques, were found to be strongly influenced by the quality of this interface.A significant improvement of the surface morphology during the growth was obtained by the insertion of an AlGaN/GaN multiquantum well (MQW) between the AlN wetting layer and the GaN layer. Smooth interfaces and an excellent coalescence of AlGaN on AlN and of GaN on the top of the MQW are reported. In addition the technique provides a powerful tool to manage the strain of the structure and to reduce the resulting curvature of the wafer.

Heteroepitaxial fabrication and structural characterizations of ultrathin GaN nanotube arrays

We report on the heteroepitaxial fabrication and the structural characterizations of ultrafine GaN nanotube arrays. The ultrathin GaN nanotube arrays were fabricated by using epitaxial growth of a ultrathin GaN layer on ZnO nanoneedles and etching core Zno nanoneedles. Epitaxial growth of GaN and precise etcing control of core ZnO nanoneedles were obtained by using low-pressure metalorganic vapor-phase epitaxy. ZnO nanoneedles were grown on Si and sapphire substrates by using catalyst-free metal-organic chemical vapor deposition. Furthermore, the structural properties of the nanostructures were investigated using both synchrotron-radiation X-ray diffraction and highresolution transmission electron microscopy. The strain induced during the GaN deposition on ZnO was nearly sustained even after the removal of core ZnO.

Position-controlled [100] InP nanowire arrays

We investigate the growth of vertically standing [100] zincblende InP nanowire (NW) arrays on InP (100) substrates in the vapor-liquid-solid growth mode using low-pressure metal-organic vapor-phase epitaxy. Precise positioning of these NWs is demonstrated by electron beam lithography. The vertical NW yield can be controlled by different parameters. A maximum yield of 56% is obtained and the tapering caused by lateral growth can be prevented by in situ HCl etching. Scanning electron microscopy, high-resolution transmission electron microscopy, and micro-photoluminescence have been used to investigate the NW properties.

Highly‐uniform 260 nm‐band AlGaN‐based deep‐ultraviolet light‐emitting diodes developed by 2‐inch×3 MOVPE system

AbstractHigh‐efficiency and high‐power deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) with emission wavelength between 200 and 350 nm have a wide range of potential applications. We have developed highly‐uniform 260 nm‐band AlGaN DUV LEDs fabricated on AlN templates on (0001) sapphire using a 2‐inch×3 reactor by low‐pressure metalorganic vapor phase epitaxy (LP‐MOVPE), for the purpose of realizing commercially‐available low‐cost DUV LEDs. An NH3 pulsed‐flow multilayer (ML) growth method was used for the fabrication of low threading dislocation density (TDD) AlN templates on sapphire substrates. 4 μm‐thick low TDD AlN templates were successfully grown on (0001) sapphire substrates. Abnormal hillocks due to polarity inversion were markedly decreased, when the growth temperature of the first NH3 pulsed‐flow AlN layer directly grown on sapphire was reduced to be 1100 ºC. DUV LEDs on the high quality AlN templates showed good uniformity in the emission wavelength and in the output power, in each 2‐inch wafer and among three wafers which were simultaneously grown. The DUV‐LEDs were flip‐chip bonded and demonstrated output power of 17.1 mW at 500 mA and 40 mW at 1500 mA measured under CW and pulsed operations, respectively. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strain control of GaN grown on 3C‐SiC/Si substrate using AlGaN buffer layer

AbstractGaN layers were grown on 3C‐SiC/Si substrates with AlGaN intermediate layers with various thickness and AlN mole fraction by low‐pressure metal‐organic vapor phase epitaxy (LP‐MOVPE). The effects of the interlayer on the crystal quality and strain‐status of GaN epilayer were investigated systematically by varing the thickness and the Al mole fraction of the AlGaN. Raman scattering and X‐ray diffraction results show that the AlGaN with thinner thickness and higher Al mole fractions can improve the crystalline quality of the GaN epilayer and the redue the strain at the same time. This phenomenon should be attributed to three dimensional (3D) growth at the initial stage of (the) GaN epitaxy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of ammonia flow in the AlN spacer on the electrical properties of InAlN/AlN/GaN HEMT structures

AbstractDuring the past few years it has been reported that a thin AlN spacer of few nanometers needs to be inserted in InAlN/GaN high electron mobility transistors (HEMTs) to obtain high 2DEG carrier mobility. This work presents a systematic study of the effects of varying the ammonia flow in the AlN spacer of InAlN/AlN/GaN HEMTs grown by low pressure metalorganic vapour phase epitaxy (LP‐MOVPE). The strain state, the surface roughness and the growth rate of AlN were found to be dependent on the V/III ratio. In addition the ammonia flow in the interlayer has a strong impact on the structural properties of the subsequent InAlN barrier layer and on the electrical properties of the structure. A sheet resistance as low as 327 Ω/□ with a sheet carrier density of 1.5 × 1013 cm−2 has been obtained at room temperature.

Growth of High-Quality Si-Doped AlGaN by Low-Pressure Metalorganic Vapor Phase Epitaxy

In this paper we report the growth of Si-doped AlGaN on an AlN/sapphire substrate by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) with an in situ monitoring system to fabricate a high-quality film with controlled thickness. The AlN mole fraction in AlGaN can be controlled by adjusting the growth temperature. We also discuss the quality and growth mechanism of AlGaN on AlN. GaN and AlGaN with an AlN mole fraction of 0.28 were free from stress because of the occurrence of three-dimensional (3D) growth. The lattice constant a of AlGaN with an AlN mole fraction of 0.40 was larger than that at an AlN mole fraction of 0.28 despite the two-dimensional (2D) growth mode. This indicates that the lattice mismatch between AlGaN and the underlying AlN adversely affects the crystal quality. AlGaN with an AlN mole fraction of over 0.60 was coherently grown on AlN owing to the fact that the AlN/sapphire template used in this work was subjected to compression. For this reason, in the samples with an AlN mole fraction of over 0.6, the crystal quality was high because of the coherent growth on the underlying AlN. Moreover, Si doping was performed for AlGaN. The carrier concentration increased linearly up to a Si concentration of 2 ×1018 cm-3, indicating that the activation rate was approximately 1.

Growth of High-Quality Si-Doped AlGaN by Low-Pressure Metalorganic Vapor Phase Epitaxy

In this paper we report the growth of Si-doped AlGaN on an AlN/sapphire substrate by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) with an in situ monitoring system to fabricate a high-quality film with controlled thickness. The AlN mole fraction in AlGaN can be controlled by adjusting the growth temperature. We also discuss the quality and growth mechanism of AlGaN on AlN. GaN and AlGaN with an AlN mole fraction of 0.28 were free from stress because of the occurrence of three-dimensional (3D) growth. The lattice constant a of AlGaN with an AlN mole fraction of 0.40 was larger than that at an AlN mole fraction of 0.28 despite the two-dimensional (2D) growth mode. This indicates that the lattice mismatch between AlGaN and the underlying AlN adversely affects the crystal quality. AlGaN with an AlN mole fraction of over 0.60 was coherently grown on AlN owing to the fact that the AlN/sapphire template used in this work was subjected to compression. For this reason, in the samples with an AlN mole fraction of over 0.6, the crystal quality was high because of the coherent growth on the underlying AlN. Moreover, Si doping was performed for AlGaN. The carrier concentration increased linearly up to a Si concentration of 2 ×1018 cm-3, indicating that the activation rate was approximately 1.

TEM analysis of InMnAs layers and dots prepared by low pressure MOVPE

The properties of InMnAs epitaxial layers and quantum dots prepared on (100) GaAs substrates using Metal-Organic Vapor Phase Epitaxy (MOVPE) are reported. Transmission Electron Microscopy (TEM) observation confirmed that the quantum dot structures were plastically relaxed. The degree of relaxation depends on the amount of Mn incorporated in the structure. Moiré fringes at TEM plan view images were used to evaluate strain in the InMnAs quantum dot structures. TEM observation confirmed that the InMnAs dots were uniform and they grew in the zinc-blende structure. The bright field TEM micrographs (cross-sectional) of the epitaxial layers showed the presence of InMnAs precipitates located under the interface between the InMnAs and substrate.

Electrical properties of AlxGa1-xN/GaN heterostructures with low Al content

Electrical properties of AlxGa1−xN/GaN heterostructures with an Al content below 15% and carrier concentrations as low as 1.0 × 1012 cm−2 were investigated by Hall effect measurements and capacitance–voltage profiling. The nominally undoped GaN capped structures were grown by low-pressure metal-organic vapor-phase epitaxy. The threshold voltage of transistor devices follows the trend already found for high Al-containing structures, which are described by a model indicating a surface potential independent of Al content. Photoreflectance spectroscopy confirms the results for as-grown heterostructures. The Hall effect measured on the as-grown samples, however, shows a stronger decrease in carrier concentration than expected from the effect of polarization and constant surface potential. In contrast, Hall effect data determined on samples with Ni Schottky contacts and capacitance-voltage profiling on as-grown samples yield the expected behavior, with surface potentials of 0.86 eV and 0.94 eV, respectively. The inconsistency is eliminated by describing the results of the Hall effect on as-grown samples by a two-carrier model. Self-consistent Schrödinger–Poisson calculations support these considerations if we take into account a transition range at the AlxGa1−xN/GaN interface.

Reactor dependent starting transients of doping profiles in MOVPE grown GaN

The quality of heterostructures greatly depends on the accurate placement of dopants. However, doping transients remain as drawback in vapor-phase epitaxy. In this work starting transients of doping profiles of Si and Mg in GaN layers grown by low pressure metal-organic vapor-phase epitaxy (MOVPE) on sapphire have been investigated. Here we restrict the investigation to concentrations typically used for electrical or electro-optical devices which are in the range of 2×10 17 cm −3 to 1×10 19 cm −3 for Si and 3×10 19 cm −3 to 1×10 20 cm −3 for Mg. In the case of Mg possible diffusion was excluded by growing the doped layers at a lower growth temperature, far too low for device structures, where the profiles show essentially no sign of Mg back-diffusion. In the case of lower Si concentrations we did not observe diffusion at usual growth temperatures. The profiles were measured by secondary ion mass spectrometry. By comparing the transients from two different reactor types we show and quantitatively explain the influence of adsorption at the internal surfaces of the reactor on the dopant incorporation.

Optimization of low density InP/GaInP quantum dots for single-dot studies

We achieve well-controlled and reproducible growth by low-pressure metalorganic vapor phase epitaxy (MOVPE) of low density InP/GaInP quantum dots optimized for single-dot physics and applications. We overcome the common occurrence of multi-modal distributions of quantum dot sizes by optimizing the growth on (100) GaAs substrates with a 3° misorientation towards ⟨111⟩. In contrast to other epitaxial techniques for quantum dot growth, very controllable dependence of the quantum dot sizes and densities on the nominal thickness of the InP layer is observed, enabling highly reproducible growth.

Electro- and photoluminescence of InAs/GaAs quantum dot structures

In this communication, we study the effect of composition of the low-temperature grown quantum dot (QD) capping layer on electro- (EL) and photoluminescence (PL) from self assembled InAs QDs grown by metal organic vapour phase epitaxy (MOVPE). QD structures were prepared by low-pressure MOVPE in a reflectance anisotropy spectroscopy LayTec equipped AIXTRON 200 machine, using Stranski–Krastanow growth mode on n-type Si doped and on semi-insulating GaAs substrates. To enable EL characterization, QD structures grown on the conducting substrate were embedded into a p-i-n structure. We focused on the shift of the emission maximum towards longer wavelengths by using InGaAs strain reducing layer (SRL). Three concentrations of In were used 0%, 13% and 23%. The shift of the emission maximum towards the low energy side of spectrum was observed only for the p-i-n sample with 13% In in SRL, while further increasing of In content to 23% caused the blue shift of the position of both PL and EL maxima.

A ‐plane AlN and AlGaN growth on r ‐plane sapphire by MOVPE

AbstractThe growth of a‐plane (11‐20) AlN and AlGaN on r‐plane (1‐102) sapphire has been investigated with the aim of application to LEDs and LDs in the deep ultra‐violet region. a‐plane AlN and AlGaN were grown on r‐plane sapphire by low‐pressure metalorganic vapor phase epitaxy (LP‐MOVPE). The crystalline quality and surface morphology of a‐plane AlN were optimized at a growth temperature of 1250 °C with the off‐angle of r‐plane sapphire between ‐1° and +1°, and a‐plane AlGaN grown on the AlN layer using ‐0.45°‐off r‐plane sapphire as a substrate exhibited high crystalline quality. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Inhomogeneities of InGaN/GaN MOVPE multi quantum wells grown with a two temperatures process studied by transmission electron microscopy

AbstractThe structural investigation of InGaN/GaN:Si multiple quantum well (MQW) samples grown by low‐pressure metal‐organic vapor phase epitaxy (LP‐MOVPE) in a two temperatures (2T) process on high‐pressure GaN mono‐crystalline substrates is performed by transmission electron microscopy (TEM). A sample in which barriers and wells were grown at 780 °C is compared with another in which the barriers were deposited at 900 °C and the wells at 730 °C. For both samples the indium composition in the QWs reaches the level of about 20 at.%. The local indium composition was measured through strain measurements by digital processing from the lattice fringes images taken by TEM. Cross‐sectional investigations are performed in two zone axes – [$1\bar {1}00$] and [$11\bar {2}0$] – with the use of axial and off‐axis illumination. During TEM investigations the formation of “false indium clusters ” of the size of 2–4 nm was observed for both samples just after 2 min of LaB6 electron beam illumination at 200 kV. Large lateral fluctuations of indium content in the QWs of the length of 30–130 nm were detected in 2T sample. The plan view analysis was carried out to characterize the anisotropy of the indium fluctuation inside the QWs.

Study of High-Quality and Crack-Free GaN Growth on 3C-SiC/Separation by Implanted Oxygen (111)

We investigated the growth of GaN on 3C-SiC fabricated by the deposition and carbonization of a separation by implanted oxygen (SIMOX) surface. The dependences of AlN buffer thickness on the crystalline quality and surface morphology of the GaN grown on 3C-SiC/SIMOX (111) by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) were examined. We studied the stress of GaN by the in situ monitoring of the reflectance and substrate curvature. High-quality GaN with a smooth surface was obtained using an AlN buffer of 10 nm thickness. Moreover, the fabrication of crack-free GaN was achieved using an AlN/GaN superlattice (SL) structure on an AlGaN/AlN buffer.