Metalorganic Chemical Vapor Deposition Technology Research Articles

Multi-color emission based on InGaN/GaN micro-truncated pyramid arrays

3D micro-nano devices are expected to become the mainstay of multi-color solid-state lighting in the future because of their broad-band characteristic and the advantage of integrating the monolithic light-emitting diode on a single chip. In this work, InGaN/GaN micro-truncated pyramid arrays with six equivalent (101̄1) semi-polar facets and one (0001) polar facet were successfully prepared by the metal-organic chemical vapor deposition technology. The average diameter of the obtained uniform micro-truncated pyramids was 6.8 µm with a height of 2.4 µm. According to the results of micro-photoluminescence performed, the InGaN/GaN micro-truncated pyramid arrays can achieve multi-color emission from blue to red. The luminescent positions corresponding to different wavelengths were detected by the cathode luminescence spectrum. The multi-color emission was related to the quantum hybrid structures apart from the discrepancy of In composition in different positions. The developed microstructure can create multi-color emission by combining distinct luminescence modes, which can aid in the design of future optoelectronic devices.

Enhancing the quality of homoepitaxial (−201) β-Ga2O3 thin film by MOCVD with in situ pulsed indium

This article innovatively uses pulsed metal-organic chemical vapor deposition technology to optimize the quality of β-Ga2O3 thin films on (−201) β-Ga2O3 homo-substrate using indium pulse-assisted technology. The results demonstrate that the pulsed indium-assisted method, when compared with the traditional indium-assisted method, effectively suppresses the desorption of Ga2O, enhances the flatness of the β-Ga2O3 film, and reduces the surface roughness from 34.8 to 0.98 nm. The optimized single crystalline β-Ga2O3 film was grown with pulsed-indium, and the full width at half maximum of x-ray diffraction rocking curve was 30.42 arc sec, smaller than that of the continuous indium β-Ga2O3 (56.1 arc sec). In combination with the x-ray photoelectron spectroscopy O1s split-peak fitting analysis, the relative content of oxygen vacancies in the film was significantly reduced by pulsed indium-assisted method. The Hall mobility of films assisted by pulsed-indium is approximately 14 times higher than that of films assisted by traditional indium. The pulsed indium technology provides an idea for homoepitaxial growth of high-quality β-Ga2O3 films.

AlGaAs photocathode with enhanced response at 532 nm

The AlGaAs photocathode can be used in the field of underwater optical communication because of its fast response speed and adjustable spectral response range. In order to solve the problem that the low light absorption of the AlGaAs emission layer limits the improvement of its quantum efficiency, the distributed Bragg reflector (DBR) structure is used to reflect the light at a specific wavelength back to the emission layer to further increase the absorption rate, thus improving the response capability of the photocathode at 532 nm. The spectral response model of the AlGaAs photocathode with DBR structure is obtained by solving one-dimensional continuity equation. The optical model of the AlGaAs photocathode with enhanced response at 532 nm is established by the finite-difference time-domain method. The effects of the sublayer periodic pairs, the sublayer material and the thickness of emission layer and buffer layer on the absorption rate of emission layer are analyzed. The light absorption distributions of AlGaAs photocathode with and without DBR structure are compared, and the influence mechanism of DBR structure on the blue-green light absorption capacity of AlGaAs photocathode emission layer is clarified, which can provide a theoretical basis for designing its structural parameters. The results show that the DBR structure with a periodic pair of 20 and Al<sub>0.7</sub>Ga<sub>0.3</sub>As/AlAs has the best reflection effect on 532 nm light. Based on the DBR structure, when the thickness of the emission layer and buffer layer are 495 nm and 50 nm, respectively, the emission layer has the best absorption rate of 532 nm light. Furthermore, two kinds of AlGaAs photocathodes with and without DBR structure are prepared by the metal-organic chemical vapor deposition technology, and the reflectivity and profile structure of the grown samples are characterized. Then the Cs/O activation experiments are performed to compare the spectral response curves. It is found that the spectral response of the AlGaAs photocathode sample with DBR structure at 532 nm wavelength is about twice that of the sample without DBR structure.

Optimization quality for indium pulse-assisted of β-Ga2O3 thin film on sapphire surface

In the field of wide-bandgap semiconductors, fabricating high quality β-Ga2O3 films on heterogeneous substrates remains a tremendous challenge. This article innovatively uses pulsed metal-organic chemical vapor deposition (MOCVD) technology to optimize the quality of β-Ga2O3 thin film on sapphire substrates using indium pulse-assisted technology. The findings indicate that full width at half maximum of the (−201) β-Ga2O3 crystal plane orientation is reduced by 2700 arcsec after pulsed indium-assisted. The addition of pulsed indium atoms effectively suppresses the desorption of low oxide Ga2O, improving the flatness of β-Ga2O3 films surface and reduced the surface roughness from 30.1 nm to 5.4 nm. Moreover, the UV–Visible transmittance of the film exceeds 80%, accompanied by an increase in bandwidth. In combination with the XPS O1s split-peak fitting analysis, the quality of the films was optimized by using pulsed indium-assisted, resulting in a reduction in the relative content of oxygen vacancies in the film. The pulsed indium technology provides a new idea for heteroepitaxy growth of high quality β-Ga2O3 films.

Enhanced structural and optical properties of semipolar (112¯2) AlGaN film with insertion of AlN/AlGaN superlattice

The high-quality semipolar (112¯2) AlGaN films with high Al contents were successfully deposited on (101¯0) m-plane sapphire substrates with the insertion of AlN/AlGaN superlattice (SL) by metal-organic chemical vapor deposition technology. The dependence of structural and optical properties of the (112¯2) AlGaN film on the deposition parameters for the inserted AlN/AlGaN SL was investigated extensively premised on the characterization results of the optical microscope, atomic force microscopy, relative optical transmittance spectroscopy, high-resolution x-ray diffraction, and photoluminescence spectroscopy. It was discovered that the insertion of the AlN/AlGaN SL grown under an optimized stabilization time of 10 s between the deposition of AlN and AlGaN sublayers could be used to make significant enhancements in surface morphological characteristics, crystal quality, and optical properties of the (112¯2) AlGaN film. The mechanism for the defects reduction in the (112¯2) AlGaN film was revealed to be owing to the increase in bending and annihilating probability of the threading dislocations, basal-plane stacking faults, and other structural defects promoted by introducing sufficiently high desorbed Ga atom-induced vacancy concentration in the optimized thermal treatment process.

High-power and low-threshold 1200 nm InGaAs/GaAs quantum-wells VECSEL grown by MOCVD

Here, we demonstrate an InGaAs/GaAs quantum-wells (QWs) based vertical-external cavity surface emitting laser (VECSEL) with multi-watt emitting beyond 1200 nm. The development is motivated by the need for high-performance 1200 nm laser sources, which are located in the C–H bond photoacoustic tomography window and atmospheric transmission band. In the exploration, strain compensation, epitaxy growth, and package strategy are all optimized for effective performance improvement. Highly strained InGaAs/GaAs QWs epitaxy structures are grown on GaAs substrate with high crystalline quality by metal-organic chemical vapor deposition (MOCVD) technology. With effective thermal management, the thermal impedance of the chip is as low as about 0.92 K/W. As a result, the 1200 nm VECSEL can emit a power of 9.4 W under continuous wave (CW) operation, and the threshold pump power density is as low as about 1.61 kW/cm2. The VECSEL can also operate at a quasi-continuous wave (QCW) emission with tunable pulse width and repetition rates. Under a duty cycle of ∼9.5%, the VCSEL is with a peak power of 16.6 W and a slope efficiency of about 15.5%, and the corresponding pulse energy is as high as about 1.57 mJ. No thermal rollover is observed. This exploration significantly broadens the VECSEL performance and can be regarded as an extension and enrichment of the earlier works on this topic.

Effects of Dual Moderate-Temperature-Grown AIN Interlayers on Structural and Optical Properties of Semipolar (1122) AIN Film

High quality semipolar (1122) AlN films have been grown on (1010) m-plane sapphire substrates with the help of dual moderate-temperature-grown (MTG) AlN interlayers by using metal-organic chemical vapor deposition technology. The layer thickness of the semipolar (1122) AlN film was determined by employing relative optical transmittance spectrum measured with ultraviolet-visible spectrophotometer. The effect of the insertion of 80 nm-thick MTG AlN interlayer on structural and optical properties was investigated in detail based on the characterization results of the atomic force microscopy, high-resolution X-ray diffraction, and Raman spectroscopy. Comparing with the semipolar (1122) AlN film grown without the MTG AlN interlayer, both the surface morphology and crystalline quality of the semipolar (1122) AlN film grown with the insertion of dual 80 nm-thick MTG AlN interlayers have been improved significantly. In fact, the root mean square value of the surface roughness decreased from 3.5 to 1.4 nm, and the full width at half maximum value of X-ray rocking curve decreased from 1667 to 1174 arcsec, respectively. These facts reveal that the insertion of the dual MTG AlN interlayers is a powerful method to improve the surface morphology and crystalline quality of the semipolar (1122) AlN films owing to the formation of nanoscale patterned substrate-like structure and its blocking effect on the propagation of the dislocations.

Improvements in characteristics of N-polar Si-doped AlGaN epi-layer grown on mis-oriented c-plane sapphire substrate

The N-polar Si-doped n-type AlGaN epi-layer with an Al composition up to 54% was successfully grown on vicinal c-plane sapphire substrates by metal-organic chemical vapor deposition technology. High-resolution X-ray diffraction, atomic force microscopy, and Hall effect measurement were used to evaluate the influence of the use of vicinal sapphire substrate on the crystalline quality, the surface morphology, and the electrical property of the N-polar Si-doped AlGaN epi-layer, respectively. The characterization results reveal that the use of an optimized vicinal sapphire substrate plays a vital role in the improvements in the crystalline quality, the surface morphology, and the electrical property for the N-polar Si-doped n-type AlGaN epi-layers. In fact, a relatively smooth surface morphology with a root-mean-square value as small as 17.3 nm, an electron concentration of 1.9 × 1019 cm−3, and an electron mobility of 23.5 cm2 V−1 S−1 were achieved with the N-polar Si-doped n-type AlGaN epi-layer sample grown on a 1° mis-oriented c-plane sapphire substrate in this study.

Metal-Organic Chemical Vapor Deposition Precursors: Diagnostic Check for Volatilization Thermodynamics of Scandium(III) β-Diketonates

Scandium complexes with β-diketonate ligands are valuable precursors for the metal–organic chemical vapor deposition (MOCVD) of scandia based materials, but data on their volatilization thermodynamics crucial to MOCVD technology are in a huge disarray. We have addressed this issue with a diagnostic tool based on the principles of group additivity and structure–property relationships, which had been developed by us specifically for metal–organic objects. For this purpose, a mass of experimental data on the vapor pressures and enthalpies of sublimation, vaporization and fusion available in the literature for scandium(III) β-diketonates has been compiled and analyzed. Additionally, saturated vapor pressures and thermodynamic sublimation characteristics have been obtained for scandium(III) complexes with acetylacetone, hexafluoroacetylacetone, and 3-methyl-2,4-pentanedione by transpiration and thermogravimetric methods. New data have allowed us to arbitrate the conflict of literature data. As a result, a consistent set of enthalpies of the three discussed processes has been obtained for eight scandium complexes. Dispersion interactions and non-additive effects have been shown to be typical for metal tris-β-diketonates. They have been taken into account to improve the diagnostic check. It is now possible to quite easily assess the thermodynamics of tris-β-diketonate complexes with different metals which are in demand as precursors in gas-phase technology.

Conductometric nitrogen dioxide gas sensor based on gallium nitride quantum dots film

Gallium nitride (GaN) has some challenges for gas sensing, such as high detection lower limit (LOD) or complicated epitaxy of nanostructures. To address these issues, we exclusively designed a GaN quantum dots (QDs) film to enhance the gas interaction capability which fabricated by metal-organic chemical vapor deposition (MOCVD). The GaN QDs film gas sensor exhibits a low LOD (5 ppb) to nitrogen dioxide (NO2) at room temperature (RT), which has superior gas performance than other GaN-based sensors. The results of morphology and structure characterization manifest that many ultra-small QDs around 2 nm in diameter were stacked to form QDs of larger size, and confirm the existence of vacancies for the GaN QDs film. And the GaN QDs film sensor displays a ultra-high electron mobility of 1237.37 cm2 V−1s−1 at RT, which is greatly beneficial for gas sensing. The sensing mechanisms were analyzed based on its abundant active sites, ultrahigh carrier mobility, high surface energy and chemical activity, grain boundary barrier and good electrical conductivity. Moreover, the sensor processes high selectivity, repeatability, stability and reusability based on MOCVD technology which can realize precise control of structure and morphology, thus this sensor is expected to achieve rapid industrialization.

Impacts of hydrogen flow and growth pressure on characteristics of semipolar (11 [formula omitted] 2) plane AlInGaN quaternary epilayers

The semipolar (11 2‾ 2) plane AlInGaN quaternary epilayer was successfully grown by metal-organic chemical vapor deposition (MOCVD) technology. The impacts of hydrogen flow and growth pressure on indium incorporation, growth rate, surface morphology, and crystalline quality of the semipolar AlInGaN epilayers were investigated extensively. It was found that the indium incorporation, growth rate, surface morphology, and crystalline quality of the semipolar AlInGaN epilayers were strongly dependent on the constituent of reaction gas and growth pressure in MOCVD growth process. In fact, an evident decrease in the number of hillock-like features and a slight increase in the size of hillock-like features were observed on the surfaces of the semipolar AlInGaN epilayers as the proportion of hydrogen in reaction gas was gradually increased. A root mean square value as small as 2.7 nm and an indium composition of 3.2% were achieved by adding extra 15% hydrogen in reaction gas. On the other hand, an enhanced indium composition of 3.9% was obtained together with a 64% reduction in growth rate and dramatic degradation in surface morphology and crystalline quality by increasing the growth pressure from 40 to 100 Torr for the semipolar AlInGaN epilayer due to aggravated parasitic reactions. This fact indicates that the conventional method, namely simply increasing growth pressure is not a good option to obtain high indium composition in the semipolar AlInGaN quaternary alloy.

Guest Editorial Introduction to the Special Issue on Semiconductor Optoelectronic Materials and Devices

This Special Issue of the IEEE Journal of Quantum Electronics on Semiconductor Optoelectronic Materials and Devices is dedicated to Prof. P. Daniel Dapkus and the impact he has had on the development of metal-organic chemical vapor deposition (MOCVD) for the growth and manufacture of advanced photonic devices. Prof. Dapkus has focused his career activities on the research and development of photonic materials and devices including semiconductor lasers, LEDs, solar cells, and detectors. While at Rockwell International, he led the group that demonstrated the potential and viability of MOCVD as the preferred technology for creating and manufacturing photonic devices. Prof. Dapkus, along with his colleagues Prof. Dupuis, Dr. H. Manasevit, and Prof. J. J. Coleman are widely considered to be the leading pioneers in the development of the MOCVD technique. Dapkus’ group at Rockwell International was the first to report lasing in AlGaAs/GaAs double heterostructures at room temperature in 1977. This work along with their demonstration of multiple quantum well laser diodes in 1978 has led to MOCVD becoming the dominant technique for the production of laser diodes and LEDs, along with other key photonic devices. As a Professor at USC, Dapkus’ lab investigated MOCVD fundamental growth mechanisms and selective area growth employing MOCVD. His lab made pioneering contributions to the technology of long-distance fiber-optic lasers, ultra-low-threshold datacom lasers, vertical cavity surface-emitting lasers, and photonic integrated circuits. In particular this work led to strained quaternary quantum wells became the preferred design in 1.3- and 1.55- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \textrm {m}$ </tex-math></inline-formula> laser diodes. The MOCVD technology and the quantum well material designs that were demonstrated with it now are dominant in the fabrication of devices for fiber-optic systems, data communications, facial recognition, space solar cells, solid-state lighting, and cell phone amplifiers.

MOCVD growth and thermal stability analysis of 1.2 µm InGaAs/GaAs multi quantum well structure

InGaAs/GaAs multiple quantum well (MQW) is desirable for many optoelectronic devices with the wavelength around 1.2 µm. However, the high defect density and instability caused by high strain have always been the problems. In this paper, an InGaAs/GaAs MQW structure was grown on GaAs substrate at low temperature by metal-organic chemical vapor deposition (MOCVD) technology. The InGaAs layers were grown with triethylgallium (TEGa) and trimethylgallium (TMGa), respectively. We studied the stability of InGaAs/GaAs MQW by evaluating the photoluminescence (PL) peak shift after annealing. It was found that the peaks of MQW grown by TEGa and TMGa were both blue-shifted after annealing. A smaller shift (<4 nm) in the MQW grown with TEGa exhibits better thermal stability compared to that of MQW grown with TMGa. This could be due to the weaker indium (In) and gallium (Ga) interdiffusion at the InGaAs/GaAs interface for the sample grown with TEGa. Furthermore, we fabricated a vertical external cavity surface emitting laser (VECSEL) based on the 6-period InGaAs/GaAs MQW grown with TEGa with an emission wavelength of ~1.2 µm.

Single-step synthesis of TiC mesocrystals on the MWCNTs surface by the pyrolysis of Cp2TiCl2

Titanium carbide mesocrystals have been manufactured by the one-stage metalorganic chemical vapor deposition (MOCVD) technology using the pyrolysis of titanocene dichloride, Cp2TiCl2 (Cp = η5-C5H5) on the surfaces of multi-walled carbon nanotubes (MWCNTs). The initial stages of the Cp2TiCl2 gas-phase dissociation were modeled with density functional theory (DFT) calculations. A series of syntheses were carried out with a step-by-step increase of the Cp2TiCl2 amount. These experiments allows to observe intermediate stages of self-assembly of TiC mesocrystals. Thus, it was possible to identify that the decomposition of Cp2TiCl2 at 900 °C leads to the formation of TiC crystals nanoparticles with their accompanying self-assembly. As a final stage of this process, perfect mesocrystals of a cubic habit with a size of approximately 50 μm were found. The usage of powdered MWCNTs as a substrate in the chemical vapor deposition (CVD) processes provides heterogeneous conditions, which can be promising for the search for new forms of nanostructured materials.

Reduction of hexagonal defects in N-polar AlGaN epitaxial layers grown with reformed pulsed-flow technology

The N-polar AlGaN epi-layers were successfully grown on (0001) c-plane sapphire substrates by using the reformed metal-organic chemical vapor deposition technology featured with an optimized pulsed NH3-flow supplying scheme. In order to reduce the hexagonal defects in the N-polar AlGaN epi-layers, the conventional NH3 pulsed-flow technology was reformed. Particular attention should be paid on the impacts of the variation in the duty ratio and the NH3-purge term within each growth cycle on the surface morphology and crystalline quality of the grown N-polar AlGaN epi-layers. A smooth surface morphology with a root-mean-square value as small as 5.7 nm was obtained. On the other hand, the significantly decreased density of the hexagonal defects confirmed with microscope and X-ray diffraction measurements implies a remarkable improvement in crystalline quality of the N-polar AlGaN epi-layers. These results indicate that the reformed pulsed-flow technology with an optimized source supplying scheme plays a crucial role in improving the surface morphology and crystalline quality of the N-polar AlGaN epi-layers.

Integrated fabrication of a high strain InGaAs/GaAs quantum well structure under variable temperature and improvement of properties using MOCVD technology

An InGaAs/GaAs quantum well (QW) structure was prepared by metal-organic chemical vapor deposition (MOCVD) via a new growth method, where the InGaAs well layer and the GaAs barrier layer were grown under a variable temperature. A GaAs protection layer was employed to avoid indium atom evaporation on the InGaAs surface during the temperature change. Room and low-temperature photoluminescence (RT/LT-PL), atomic force microscopy (AFM), and high-resolution X-ray diffraction (HRXRD) were carried out to investigate the effect of the variable temperature growth method. The theoretical and experimental results indicated that it could erase the 2D islands and rebuild the surface morphology to a step-flow mode surface. The quality of the InGaAs crystal layer was also improved because of the annealing-like treatment. In addition, the study found that when the thickness of the GaAs protection layer was 2 nm and the growth temperature of the InGaAs layer was 560°C, the maximal properties of the InGaAs/GaAs QW were achieved. Moreover, high-quality multiple QWs with five periods were grown with the tailored structure. The growth method will improve the properties of strained InGaAs/GaAs QW materials and provide technical support for a semiconductor laser's performance optimization.

Epitaxial growth of semi-polar (11–22) plane AlInGaN quaternary alloys on m-plane (10-10) sapphire substrates

The semi-polar (11–22) plane AlInGaN quaternary alloys were grown successfully on the m-plane (10-10) sapphire substrates with metalorganic chemical vapor deposition technology. Optical microscope, atomic force microscope, energy-dispersive X-ray spectroscopy, X-ray diffraction, and room temperature photoluminescence spectroscopy were used to study the effects of V/III ratio and trimethyl-indium flow rate on the characteristics of the grown semi-polar (11–22) plane AlInGaN quaternary alloys. It was found that the In composition of the semi-polar (11–22) plane AlInGaN quaternary alloys decreased as the V/III ratio increased. It was also revealed that the surface morphology, crystalline quality, and the optical property could be improved, and the phase separation in the quaternary alloy could be suppressed significantly by optimizing the V/III ratio and the trimethyl-indium flow rates.

Higher Operating Temperature IR Detectors of the MOCVD Grown HgCdTe Heterostructures

This paper summarizes progress in metal organic chemical vapour deposition (MOCVD) technology achieved in recent years at the Institute of Applied Physics, Military University of Technology and VIGO System S.A. MOCVD with a wide range of composition and donor/acceptor doping and without post-growth annealing is a very convenient tool for the deposition of HgCdTe epilayers used for uncooled infrared detectors. Particular attention is focused on: surface morphology improvement, doping issues, diffusion processes during growth study, substrate issues, crystallographic orientation selection. In this respect, MOCVD technology improvement influencing IR detector parameters is shown. CdTe buffer layer deposition allows HgCdTe heterostructure growth on GaAs substrates. Theoretical modelling using APSYS platform supports designing and better understanding of the carrier transport mechanism in detector’s structures. Secondary ion mass spectrometry profiles allows to compare projected and obtained structures and revealed diffusion processes of the elements. A wide range of different types of infrared detectors operating at high operating temperature conditions has been constructed: photoresistors, non-equilibrium photodiodes, dual-band photodiodes, barrier and multiple detectors. The methodical research efforts contributed to the time constant reduction are important in many detector applications. Future challenges and prospects are also discussed.

Preparation and properties of heteroepitaxial β-Ga2O3 films on KTaO3 (100) substrates by MOCVD

Ga2O3 films were deposited on (100) oriented KTaO3 substrates by the metal-organic chemical vapor deposition technology. Structural and chemical composition analyses confirmed that the as-deposited film at 850 °C was β phase Ga2O3 with high crystalline quality. The epitaxial relationships between the film and substrate were determined as β-Ga2O3 (100) || KTaO3 (100) with β-Ga2O3 [001] || KTaO3 〈011〉 by X-ray diffraction, high resolution transmission electron microscopy and selected area electron diffraction results. The average transmittance in the visible range of the sample prepared at 850 °C was 73.6%, and the optical band gap of the β-Ga2O3 film was approximately 4.79 eV with a resistivity of 5.01 × 103 Ω·cm.

Crucial influential factor on background electron concentration in semi-polar (11[formula omitted]2) plane AlGaN epi-layers

The semi-polar (112¯2) plane Al0.3Ga0.7N epi-layers with different background electron concentrations were deposited on (101¯0) m-plane sapphire substrates with metal-organic chemical vapor deposition technology, and characterized with Hall effect measurement, photoluminescence spectroscopy, high-resolution X-ray diffraction, and atomic force microscopy. It was found that there is a strong positive correlation between the background electron concentrations and the anisotropy in two-dimensional (2D)-growth rate of the triangular structures on the surface of semi-polar Al0.3Ga0.7N epi-layers. That is to say, the anisotropy in the 2D-growth rate is indeed the crucial influential factor on the background electron concentration. In fact, by carefully suppressing the anisotropy in 2D-growth rate, the background electron concentration in semi-polar Al0.3Ga0.7N epi-layers was reduced successfully from 1.38×1019 cm−3 to 4.20×1015 cm−3. This work should open a new way to realize the p-type semi-polar AlGaN epi-layers, which is of great significance for the manufacturing of semi-polar AlGaN-based DUV-LEDs.