Low-pressure Metalorganic Vapor Phase Epitaxy Research Articles

Boron lattice site location in (BGa)As and (BGa)P thin films studied using RBS and NRA with a channeled He+ ion beam

Abstract In this study the boron lattice site location in ternary B x Ga 1− x As and B x Ga 1− x P thin films grown on (0 0 1) GaAs and (0 0 1) GaP, respectively, using low pressure metal-organic vapour-phase epitaxy (MOVPE) with boron concentrations between x = 0.8% and x = 3.2% was investigated with RBS and the 10 B(α,p) 13 C nuclear reaction using a 2.3 MeV He + ion beam. For this purpose, the ion beam was aligned with the [0 0 1], [0 1 1] and [1 1 1] axis and the RBS and proton yield from the nuclear reaction compared with random ion incidence. For comparison, theoretical proton yields which assume boron to be located on substitutional lattice sites only were calculated for each sample/axis combination and compared with the experimental yields. The RBS/channeling measurements show a very good crystal quality of the films with χ min being in the range of 3–5% for the [0 1 1] axis. The best crystal qualities, i.e. the lowest χ min values and dechanneling rates, are achieved for low boron concentrations. From NRA/channeling it can be deduced that in the B x Ga 1− x As films the fraction of interstitial boron is approximately 5% for low boron concentrations of x = 1% and 6–10% for concentrations up to x = 3.2%, whereas the fraction of interstitial boron is less than 3% in the B x Ga 1− x P film studied despite a concentration of x = 2.0%. This indicates that antisite effects of the boron incorporation are more likely in GaAs compared to GaP.

Substrate-free large gap InGaN solar cells with bottom reflector

In this study, we report on the realization of the In 0.085Ga 0.915N p– i– n solar cell by low-pressure metalorganic vapor phase epitaxy (MOVPE). The w–2θ scans of (0 0 0 2) reflection observation indicate good suppression of phase separation for the p– i– n solar cells with a 150-nm-thick i-In 0.085Ga 0.915N epilayer. The sharp and narrow signals in the photoluminescence spectra of In 0.085Ga 0.915N provided evidence for high material quality, even through the epilayer thickness exceeded its critical value. Furthermore, the laser lift-off (LLO) technique is used to fabricate the substrate-free thin-film solar cells (TF-SCs) with a bottom reflector. Although the samples have suffered from thermal-gradient-induced damage during LLO process, the fabricated TF-SCs still exhibit a low forward voltage of 3.3 V at 20 mA along with an ideality factor of 3.1. Finally, since unabsorbed photons can be reflected by the bottom reflector, the TF-SCs show a 13.6% increase in short-circuit current density as compared to their counterparts without a bottom reflector.

Structural and optical characteristics of GaN/ZnO coaxial nanotube heterostructure arrays for light-emitting device applications

We report on the structural and optical characteristics of position-controlled GaN/ZnO coaxial nanotube heterostructure and GaN/InxGa1−xN coaxial nanotube quantum structure arrays for light-emitting diode (LED) applications. The GaN/ZnO nanotube heterostructures were fabricated by growing a GaN layer on the entire surface of position-controlled ZnO nanotube arrays using low-pressure metal-organic vapour-phase epitaxy. As determined by transmission-electron microscopy (TEM), an abrupt and coherent interface between the core ZnO and the GaN overlayer was observed. The optical characteristics of heteroepitaxial GaN/ZnO nanotube heterostructures were also investigated using cathodoluminescence (CL) spectroscopy. This position-controlled growth of high-quality single crystalline GaN/ZnO coaxial nanotube heterostructures allowed the fabrication of artificial arrays of high-quality GaN-based coaxial quantum structures by the heteroepitaxial growth of GaN/InxGa1−xN multiple quantum wells along the circumference of the GaN/ZnO nanotubes. The optical and structural characteristics of the position-controlled GaN/InxGa1−xN coaxial nanotube quantum structures were investigated by using CL spectroscopy and TEM analysis, respectively. The green LED microarrays were successfully fabricated by the controlled heteroepitaxial coaxial coatings of GaN/InxGa1−xN coaxial nanotube quantum structures and the outermost Mg-doped p-type GaN layer onto the GaN/ZnO coaxial nanotube heterostructures, presumably implying that the position-controlled growth of high-quality GaN/ZnO coaxial nanotube heterostructure arrays provides a general and rational route of integrating vertical nanodevices for nanoscale electronics and optoelectronics.

Structural and optical characterization of MOVPE grown InGaP/GaAs MQWs for advanced photovoltaic devices

The optimization of the electronic properties of InGaP/GaAs MQWs, to be inserted in multilayers heterostructure for novel photovoltaic devices, was performed by structural, optical and photoelectrical measurements. Different sequences of nominally undoped InGaP and GaAs alternated layers were grown by low-pressure metalorganic vapour phase epitaxy, employing tertiarybutylarsine and tertiarybutylphosphine as metalorganic precursors for the V-group elements. In order to minimize the As/P exchange effect, the interface In segregation, and to control the whole lattice matching, single and multi-quantum wells (MQWs) with different: (i) periods, (ii) well widths, (iii) growth temperatures, (iv) gas-switching sequences at the interfaces and (v) indium concentrations in the InGaP alloy, were prepared and investigated. The interface sharpness and the compositional fluctuation of thick MQW region containing up to 40 well-barrier sequences were investigated for the modelling, realization and evaluation of test structures based on low-dimensional systems for third generation solar cells.

Deep defects in GaN/AlGaN/SiC heterostructures

Deep level transient spectroscopy (DLTS) measurements were carried out on GaN/AlGaN/SiC heterostructures prepared by low-pressure metalorganic vapor phase epitaxy. Si-doped n-GaN layers were grown using an n-AlGaN nucleation layer (8% and 30% of aluminum) on two kinds of p-type 4H-SiC substrates. The DLTS spectra of on-axis (0001) grown samples exhibit a dominant peak of a majority carrier trap with apparent activation energy close to 0.80 eV and capture cross section of about 5×10−14 cm2 regardless of the AlGaN composition. The energy of this deep level decreases with increasing electrical field due to Poole–Frenkel effect. Carrier capture kinetics indicates interacting point defects arranged along a line, probably a threading dislocation. Two additional traps (0.52 and 0.83 eV) were found in on-axis samples with 8% AlGaN composition. For 30% Al content, only a 0.83 eV level was detected. Majority carrier trap with activation energy of 0.66 eV was observed in the off-axis grown samples. This level is probably related to an interface defect or to a defect lying near the heterojunction interface.

On an AlGaInP Light-Emitting Diode With a Modulation-Doped Multiquantum-Well (MD-MQW) Structure

An interesting AlGalnP multiquantum-well (MQW) light-emitting diode (LED) with an n-type modulation-doped (MD) structure, grown by low-pressure metal-organic vapor-phase epitaxy (LP-MOVPE), is fabricated and studied. This n-type MD-MQW LED exhibits lower dynamic resistance, higher luminescence, and higher luminous efficiency than those of a conventional undoped-MQW LED. Experimental results show a higher luminous efficiency of 16.05 lm/W and higher luminescence of 2.53 lm are obtained for the MD-MQW LED which are superior to the corresponding values of 14.49 lm/W and 2.04 lm for the undoped-MQW LED under dc operation. In addition, the n-type MD-MQW LED exhibits a higher quantum efficiency of 7.2% under dc operation as compared with the 6.7% of the undoped-MQW LED. The reduced junction temperature of 12degC at 200 mA is also acquired for the MD-MQW LED. Moreover, the brightness reliability of this new device is found to be comparable to the undoped-MQW LED. These positive results could be attributed to the presence of a higher electron concentration in the active region of the MD-MQW structure which causes the suppression of electron thermal velocity especially at a high level injection condition, and a reduced junction heating effect.

Free and bound exciton fine structures in AlN epilayers grown by low-pressure metalorganic vapor phase epitaxy

Exciton fine structures were observed in partially polarized optical reflectance and cathodoluminescence (CL) spectra of AlN epilayers grown by low-pressure metalorganic vapor phase epitaxy on (0001) Al2O3 substrates. A few free and four bound exciton lines were clearly resolved in the low-temperature CL spectra of the lowest threading dislocation density (∼1×108 cm−2) AlN film. From the energy difference between the ground-state and the first excited states, the hydrogenic A-exciton binding energy in the present compressively strained (Δa/a≈−1.68%) AlN was estimated to be approximately 51 meV.

One Step Nano-Selective Area Growth of Localized InAs/InP Quantum Dots For Single Photon Source Applications

AbstractWe demonstrate the feasibility of a new approach of Nano Selective Area Growth (Nano-SAG) to precisely localize InAs/InP QDs, by low-pressure Metalorganic Vapour Phase Epitaxy (MOVPE). This approach is based on a partial patterning with a dielectric mask containing nano-openings. The two main advantages of MOVPE are: the important diffusion length of the active species and the inhibition of growth on the dielectric mask. We demonstrate the synthesis of localized nanostructures with high structural properties and the precise control of their dimensions at the nanometer scale. This allows in principle the precise control of the tunability of the emission length.

Reactor-pressure dependence of growth of a-plane GaN on r-plane sapphire by MOVPE

a-plane GaN was grown directly on an r-plane sapphire (−0.45°) substrate by low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE), and the effects of the reactor pressure and growth temperature on the crystalline quality and surface morphology of a-plane GaN were studied. The reactor pressure and growth temperature were adjusted from 40 Torr (53 hPa) to 500 Torr (666 hPa) and from 1020 to 1100 °C, respectively. a-plane GaN with a smooth surface morphology was obtained under low-pressure conditions, and high-crystalline-quality a-plane GaN was obtained at a pressure of 500 Torr (666 hPa). By controlling the reactor pressure and growth temperature, high-quality a-plane GaN with a smooth surface was obtained.

Time-resolved characterization of InAsP∕InP quantum dots emitting in the C-band telecommunication window

The dynamic response of InAsP quantum dots, grown on InP(001) substrates by low-pressure metalorganic vapor phase epitaxy emitting around 1.55μm, is investigated by means of time-resolved microphotoluminescence as a function of temperature. Exciton lifetime steadily increases from 1ns at low temperature to reach 4ns at 300K while the integrated photoluminescence intensity decreases only by a factor of 23. These characteristics give evidence that such InAsP∕InP quantum dots provide a strong carrier confinement even at room temperature and that their dynamic response is not affected by thermally activated nonradiative recombination up to room temperature.

Wideband wavelength electroluminescence from InAs/InP QDs using double-cap procedure by MOVPE selective area growth

Wideband wavelength electroluminescence (EL) more than 400 nm was obtained in self-assembled Stranski–Krastanov (S–K) InAs/InP quantum dots (QDs) grown by selective area low-pressure metal-organic vapor-phase epitaxy (MOVPE). We used two techniques to control the EL peak spectrum in order to obtain the wideband emission spectrum of the device. One is selective area growth using a SiO 2 narrow 16-stripe mask array pattern, and the other is a double-cap procedure in the growth of the QDs layer to change the height of QDs layer by layer.

The influence of substrate surface preparation on LP MOVPE GaN epitaxy on differently oriented 4H-SiC substrates

The influence of surface preparation and off-cut of 4H-SiC substrates on morphological and structural properties of GaN grown by low-pressure metalorganic vapor phase epitaxy was studied. Substrate etching has an impact on the surface roughness of epilayers and improves its crystal quality. The GaN layers were characterized by atomic force microscopy (AFM) and high-resolution X-ray diffractometry (HRXRD) measurements. It was observed that on-axis 4H-SiC is most suitable for GaN epitaxy and that substrate etching improves the surface morphology of epilayer.

VLS growth of GaAs/(InGa)As/GaAs axial double-heterostructure nanowires by MOVPE

We report on the gold-based vapor–liquid–solid (VLS) growth of GaAs/(InGa)As/GaAs axial double-heterostructure nanowires using low-pressure metal-organic vapor phase epitaxy (MOVPE). Systematic growth investigations were performed to study the influence of the growth time, the temperature and the V/III ratio on the composition and the extent of the (InGa)As segments. A group-III precursor growth interrupt prior to and after the (InGa)As growth showed the highest solid indium fraction and the strongest composition gradients at the heterojunctions. The experimental results were discussed considering the droplet composition as well as MOVPE and VLS kinetics.

Micro-Raman for compositions characterization of selective area growth of Al xGa yIn 1−x−yAs materials by metal-organic vapor-phase epitaxy

Micro-Raman scattering measurements were used to study the phonon modes and to analyze the local variation of composition and strain in the micron range of Al x Ga y In 1− x − y As materials grown in the regime of selective area growth (SAG) at low-pressure metal-organic vapor-phase epitaxy (LP-MOVPE). For the planar Al x Ga y In 1− x − y As layers, the indium composition is almost constant at around 0.53 while the aluminum content covered the composition range from 0 to 0.47. The Raman spectra were recorded with a confocal micro-Raman spectrometer. Raman cartography at a lateral resolution of 1 μm was performed to study the spatial variation of the phonon peaks characteristics in the SAG Ga y In 1− y As and Al x Ga y In 1− x − y As materials. The three main Al x Ga y In 1− x − y As phonon modes were analyzed: the InAs-like, GaAs-like and AlAs-like LO modes. These modes characteristics depend strongly on composition, strain and position inside the mask aperture for the SAG materials, and this dependence gives the ability to analyze the local composition and strain variations in these systems, which is very important in order to understand the influence of MOVPE growth parameters on composition and strain and to optimize the SAG growth conditions.

Characterization of (Al)GaAs/AlAs distributed Bragg mirrors grown by MBE and LP MOVPE techniques

Optical reflectance (R) and high-resolution X-ray diffraction (HR XRD) have been used for characterization and verification of the distributed Bragg reflectors (DBR) grown by two competing and complementary techniques: molecular beam epitaxy (MBE) and low-pressure metalorganic vapor phase epitaxy (LP MOVPE). The DBRs under study consisted of the stack of (Al)GaAs and AlAs quarter wavelength layers deposited on (1 0 0) oriented GaAs substrates. We demonstrate experimentally that employment of the above-mentioned methods allows for comprehensive characterization of the Bragg mirror properties and optimization of their fabrication procedure for application in highly demanding optical devices.

Optimization of the properties of MOVPE-grown GaP epitaxial layers on GaP (1 1 1)B substrates

Gallium phosphide (GaP) homoepitaxial layers are optimized by varying the growth temperature, growth rate and V/III ratio in a low-pressure metal organic vapour phase epitaxy (MOVPE) process. It is observed that we need high growth temperature, low growth rate and an optimum value of the V/III ratio in order to obtain a mirror finish surface morphology. The surface morphology is mirror finish for an epitaxial layer grown ⩾820 °C when viewed under a stereo-zoom microscope for a V/III ratio of about 100 or more. A sharp and intense excitonic photoluminescence (PL) feature and a fine structure associated with donor–acceptor pair recombination confirm a high optical quality of the grown layer, which is supported by high-resolution x-ray diffraction measurements. For silicon doping of GaP using silane in low-pressure MOVPE, our results confirm theoretical predictions available in the literature and the values of a distribution coefficient lie in a similar range known for other conventional semiconductors. This optimization provides high-quality GaP (1 1 1)B epilayers needed for the development of either nanostructure-based optoelectronic devices on transparent substrates or polarization-sensitive infrared photodetectors based on two-photon absorption.

Heteroepitaxial growth of CdTe films on (1 0 0) GaAs treated with Sb by low-pressure metalorganic vapor phase epitaxy

CdTe films were deposited on antimony-treated (1 0 0) GaAs substrates by low-pressure metalorganic vapor phase epitaxy. Prior to growth of CdTe layer, GaAs substrates were treated by triethylantimony for various treatment times. The effect of antimony treatment on crystal orientation of CdTe layer was discussed. At the optimum treatment time, CdTe films were oriented to (1 0 0) and their in-plane X-ray measurements show fourfold symmetries, that is, it is found that CdTe film was epitaxially grown on antimony-treated (1 0 0) GaAs substrate.

A terahertz quantum cascade laser grown by low-pressure metalorganic vapor phase epitaxy

Demonstration of a terahertz quantum cascade laser grown by a low-pressure metalorganic vapor phase epitaxy is reported. The structural analysis of the grown structure shows a very high degree of vertical uniformity and sharp interfaces. Lasing emission at λ=90μm up to 93K with a threshold current density Jth=330A∕cm2 at 7K was obtained in a structure incorporating a single plasmon waveguide.

One-step nano-selective area growth (nano-SAG) of localized InAs/InP quantum dots: First step towards single-photon source applications

We demonstrate the feasibility of a new approach based on the nano selective area growth (nano-SAG), which allows the precise spacial localization, in the nanometer scale, of InAs/InP quantum dots (QDs) grown by low-pressure metal organic vapor-phase epitaxy. By using the hydrogen silsesquioxane-negative resist, we partially pattern the substrate in only one step (e-beam lithography) with dielectric masks containing nano-openings. We demonstrate that the one-step nano-SAG technique leads to the formation of site-controlled InAs/InP QDs with good structural properties, and allows the good control of the growth rate and thus of the QD size into the nano-openings.

The surface potential of GaN:Si

The surface potential of GaN:Si, of fundamental interest for knowledge of the electrostatic potential and electric field strength, is determined for Si doping in the device relevant range from 6×1017 cm−3 to 2.3×1019 cm−3, in layers grown by low-pressure metal-organic vapor-phase epitaxy. Comparing the Si doping concentration, measured by secondary ion mass spectrometry, to the free electron concentration, measured by Hall effect, shows that all Si doping atoms are activated. We used the sheet resistance of the samples with different thicknesses measured by eddy current, a nondestructive, contactless method, to determine the depleted region. From the width of the depletion layer, which is dependent on the doping concentration, we obtained the GaN:Si surface potential on the basis of the depletion approximation. The surface potential decreases with increasing carrier concentration from about 1.5 eV to 0.2 eV. A qualitative explanation based on the theoretical description of two surface states, an occupied surface state at 1.7 eV and an empty state 0.6 eV below the conduction band edge, is given.