Metalorganic Vapor Phase Epitaxy Reactor Research Articles

Highly Si‐Doped GaN Regrown by Metal–Organic Vapor‐Phase Epitaxy for Ohmic Contact Applied to Quaternary Barrier‐Based High‐Electron‐Mobility Transistors

The quaternary barrier InAlGaN is suitable for GaN high‐electron‐mobility transistors (HEMT) power microwave applications. High doping of the semiconductor under the drain and source is a known suitable solution to achieve low ohmic contact resistance. However, InAlGaN quaternary alloys require a low thermal budget to avoid indium desorption from the active layer during regrowth and thus deteriorating the barrier. Herein, a selective‐area growth technique at 850 °C by metal–organic vapor‐phase epitaxy (MOVPE) to achieve low contact resistance with respect to temperature constraint is presented. Regrowth temperature and mask geometry are investigated to achieve selectivity and control of the regrowth rate. The use of H2 as carrier gas decomposes the GaN buffer layer and damages the surface, creating material cluster during regrowth. Growth with N2 carrier gas shows nonselective epitaxy, as there are deposits on the entire surface of the dielectric mask. Switching from one carrier gas to another depending on the step in the MOVPE reactor helps to control both the morphology and selectivity. The resulting high doping levels of 8 × 1019 cm−3 lead to a low contact resistance of 0.26 Ω mm.

In-Situ Preparation of GaN Sacrificial Layers on Sapphire Substrate in MOVPE Reactor for Self-Separation of the Overgrown GaN Crystal

GaN layers on sapphire substrates were prepared by using metal organic vapor phase epitaxy (MOVPE) combined with an in-situ H2 etching process for the purpose of later self-separation of thick GaN crystals produced by hydride vapor phase epitaxy (HVPE) on such substrates. The etching process results in deep pits and long voids that formed on the surface and along the lower interface between GaN and sapphire, respectively. The pits, which were investigated by SEM analysis, can be modified in their aspect ratio and density by controlling the etching parameters. Using a proper set of in-situ etching parameters, a seed layer with internal voids can be prepared, which is suitable for HVPE overgrowth and the self-separation process. The quality of the in-situ-etched seed GaN layer and overgrown GaN crystal were characterized by X-ray diffraction (XRD) and defect selective etching (DSE). With the aid of atomic force microscopy (AFM) in tapping mode, the interface morphology of the separated GaN crystal was analyzed. The crystal quality of the separated HVPE-GaN crystal is comparable to the crystal grown on untreated GaN MOVPE-seed, which did not separate from the sapphire substrate. The introduced technique to promote the crystal separation during the HVPE process has no obvious drawback on the quality of the grown GaN crystals. Using this technique, the self-separation occurs more gently due to a weakened interface between GaN/sapphire. The conventional separation from an untreated seed by pure thermomechanical action results in higher mechanical forces on the crystal and consequently much higher risk of crystal breakage.

Why do electron traps at EC –0.6 eV have inverse correlation with carbon concentrations in n-type GaN layers?

We found that the graphite elements in a metalorganic vapor phase epitaxy reactor are a significant source of iron atoms, and lead to the formation of E3 electron traps at an energy EC –0.6 eV in n-type GaN layers. Thus, the previously reported inverse correlation between the E3 trap and carbon concentrations can be explained in terms of the correlation between the growth parameters. A high V/III ratio reduced carbon incorporation and simultaneously decreased the growth rate, resulting in increased iron incorporation in the layer grown. A high temperature also reduced carbon incorporation, while iron incorporation and the formation of E3 traps were both increased by enhanced etching of the graphite elements. By optimizing the growth conditions, we successfully suppressed both E3 trap formation and carbon incorporation in n-type GaN layers on 2 inch wafers.

InAs/InP Quantum Dots in Etched Pits by Droplet Epitaxy in Metalorganic Vapor Phase Epitaxy

The growth of InAs quantum dots (QDs) on InP(100) via droplet epitaxy in a metalorganic vapor phase epitaxy (MOVPE) reactor is studied. Formation of indium droplets is investigated with varying substrate temperature, and spontaneous formation of nanoholes is observed for the first time under MOVPE conditions. Indium droplets are crystallized into QDs under arsenic flow at different temperatures. For temperatures greater than 500 °C, a local etching takes place in the QD vicinity, showing an unexpected morphology which is found to be strongly dependent on the crystallization conditions. Such QDs are structurally and optically investigated and emission from single QDs in the telecom C‐band is detected via microphotoluminescence at low temperature.

Comparison of trimethylgallium decomposition models for epitaxy growth analysis in MOVPE reactors

Numerical analysis of the trimethylgallium (TMGa) decomposition during epitaxy growth process was analysed. The main aim is to compare previously published trimethylgallium decomposition models. Hence, thermo-fluidic and species transport simulations of the MOVPE (metal-organic vapour phase epitaxy) reactor were performed. Study is focused on a simplified, but most crucial mechanism of epitaxy growth: chemical model of a TMGa decomposition. Analysis of species densities and pyrolysis speed for different approaches were beneficial to understand emerging phenomena during epitaxy growth and to choose most applicable method for future consideration. Analysis have been conducted with the aid of Ansys Fluent software. Numerical simulations depict extreme results, indicating that selecting correct model as a base for the growth process modelling is crucial.

Unravelling the unwanted Ga incorporation effect on InGaN epilayers grown in CCS MOVPE reactors

We show that gallium pollution in an AIXTRON Close Coupled Showerhead (CCS) Metal-Organic Vapor Phase Epitaxy (MOVPE) reactor can strongly affect the growth process stability of InGaN layers. By comparing growth with and without gallium pollution from the showerhead, we find a decrease in the indium incorporation and an increase in layer thickness of up to three times when the pollution is present. This phenomenon varies with the quantity of gallium on the showerhead, with increased gallium due to thicker GaN growth causing a stronger effect. Higher TMIn flows also give a more pronounced effect. These data support our previous hypothesis of TMIn reacting with metallic gallium present on the showerhead surface to release TMGa, which is then incorporated as Ga into the grown layers. This study of the effect of gallium contamination on InGaN layers in CCS MOVPE reactors leads us to suggest that the growth in CCS reactors should be performed free from gallium pollution to give more stable and comprehensible results.

High-Speed MOVPE Growth of InGaP Solar Cells

Impacts of reactor pressure and growth temperature on the qualities and properties of InGaP epitaxial layers grown in a narrow-channel horizontal metalorganic vapor-phase epitaxy (MOVPE) reactor were investigated with a growth rate of 10 μm/h. The roughness of InGaP was improved remarkably when the pressure was increased from 6 to 10 and 15 kPa. The reason is most likely related to the absolute pressure of P and the migrations of In and Ga adatoms on the wafer surface. Owing to the gas-phase reactions and desorption of P from InGaP, the surface of InGaP became rough, and the pit dislocation was increased with an increase in growth temperature. Although a low temperature was beneficial in order to obtain a smooth InGaP surface, the carrier lifetime in InGaP was shortened as a drawback. Consequently, InGaP n-on-p solar cells grown at a low temperature suffered from the degradations of both short-circuit current density and open-circuit voltage. Background impurities and defect densities in low temperature grown InGaP were attributed to these deteriorations. At the optimized growth temperature of 650 °C and the reactor pressure of 15 kPa, the growth rate of InGaP was accelerated to 20 and 30 μm/h. InGaP solar cells were successfully fabricated with the rate of 30 μm/h that opened up the way for the fabrication of III–V multijunction solar cells in the high-speed MOVPE reactor.

Cp2Mg in-situ monitoring in a MOVPE reactor using a quantum cascade laser

We performed in-situ monitoring of bis-cyclopentadienyl magnesium (Cp2Mg) vapor pressure supplied into a metal-organic vapor phase epitaxy reactor. As a light source we adopted a quantum cascade laser (QCL) of 12.9 μm, which is the wavelength of Cp2Mg infrared absorption. We directly configured a laser path in the reactor to construct the Cp2Mg in-situ monitoring system. We streamed Cp2Mg and measured the vapor pressure in the reactor to evaluate it and its saturation and purge times in comparison with their calculated values which were determined from the measured values in the supply line. Next, we streamed Cp2Mg and other gas species used in gallium-nitride processes one by one to investigate their saturation times with the same QCL. Characteristic features of Cp2Mg such as the largest saturation time ratio and its large fluctuation were observed. These results indicate the importance of Cp2Mg in-situ monitoring in the reactor.

Solving the problem of gallium contamination problem in InAlN layers in close coupled showerhead reactors

Gallium contamination is a well-known problem for InAlN layers grown in close coupled showerhead metal-organic vapor phase epitaxy reactors, and we have previously suggested a model explaining this gallium incorporation and the associated reduction in indium [Mrad, J. Cryst. Growth 507, 139 (2019)]. Here we propose the hypothesis that increasing the showerhead face temperature during GaN growth should encourage the growth of more stable GaN rather than metallic gallium, and reduce the reactions between tri-methyl indium and gallium on the showerhead. Using a hot deposition shield on the showerhead, we have confirmed this, to grow for the first time almost entirely gallium free InAlN layers in a showerhead reactor.

Effect of gas phase temperature on InGaN grown by metalorganic vapor phase epitaxy

Wafer trays with different gaps (the distance between the top of the pocket and the bottom of the wafer) were used to grow InGaN/GaN multiple quantum wells (MQWs) in a horizontal metalorganic vapor phase epitaxy (MOVPE) reactor. The numerical reactor simulation revealed that at the similar surface temperature the gas phase temperature around the wafer increases due to an increased wafer tray temperature. This increased gas phase temperature is expected to increase the effective V/III ratio by enhanced NH3 decomposition. Photoluminescence (PL) intensities of long-wavelength MQWs increased at the same indium content by the enhanced gas phase temperature, while the emission became narrow. This is related to a smoother topography at higher gas phase temperatures.

Direct Growth of III–V/Silicon Triple-Junction Solar Cells With 19.7% Efficiency

Monolithic multi-junction solar cells made on active silicon substrates are a promising pathway for low-cost high-efficiency devices. We present results of GaInP/GaAs/Si triple-junction solar cells, fabricated by direct growth on silicon in a metal–organic vapor phase epitaxy reactor using a GaAs y P1- y buffer structure to overcome the lattice mismatch between Si and GaAs. A low-temperature (750 °C) Si surface preparation process and a SiN x diffusion barrier at the rear side have been implemented to maintain the minority carrier lifetime in the Si bottom cell. Conversion efficiencies up to 19.7% have been achieved under AM 1.5g spectral conditions. The cells are compared with identical GaInP/GaAs dual-junction solar cells grown on bulk GaP and GaAs substrates to identify loss mechanisms. Subcell electrical characterization using electroluminescence reveals a significant voltage loss of the III–V subcells on Si, compared with the same structures grown on GaP or GaAs. Electron channeling contrast imaging of the metamorphic GaAs y P1- y buffer shows a three times higher threading dislocation density on Si (1.4 × 108 cm−2) than on GaP substrates, and atomic force microscopy shows holes in the GaAs y P1- y buffer on Si that are not observed on GaP. Approaches to reach lower defect densities for the III–V layers on silicon are discussed.

Growth of InGaAs(P) in planetary metalorganic vapor phase epitaxy reactor using tertiarybutylarsine and tertiarybutylphosphine for photovoltaic applications

The effects of wafer (surface) temperature on the surface morphologies of InP, InGaAsP, and InGaAs grown in a planetary metalorganic vapor phase epitaxy (MOVPE) reactor using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) as group V precursors were investigated. Compared with small horizontal reactors, InP growths require a relatively low temperature to prevent TBP predecomposition and parasitic gas-phase reactions. However, such a low temperature can prevent the surface migration of In adatoms and degrade surface morphology, especially when a substantial amount of Ga precursors is present. Hence, growths of InGaAs require a higher temperature than that of InP and InGaAsP. In this work, 1.04 eV In0.79Ga0.21As0.46P0.54 and 0.74 eV In0.53Ga0.47As single-junction cells as well as InGaAsP/InGaAs dual-junction solar cells have been successfully fabricated. The cell performances reveal that InGaAs layers grown at 590 °C exhibit great crystal quality, while 550-°C-grown InGaAsP layers contain a noticeable extent of defect density and lattice imperfection.

Accelerated GaAs growth through MOVPE for low-cost PV applications

The high growth rate of epitaxial GaAs was investigated using a novel horizontal metalorganic vapor phase epitaxy (MOVPE) reactor, from the point of view of realizing low-cost photovoltaic (PV) solar cells. The GaAs growth rate exhibited an approximately linear relationship with the amount of trimethylgalium (TMGa) supplied, up to a rate of 90 μm/h. The distribution of growth rate was observed for a two-inch wafer, along the flow direction, and the normalized profile of the distribution was found to be independent of the precursor input, from 20 to 70 μm/h. These tendencies indicated that significant parasitic prereaction did not occur in the gaseous phase, for this range of growth rate. GaAs p-n single-junction solar cells were successfully fabricated at growth rates of 20, 60, and 80 µm/h. The conversion efficiency of the cell grown at 80 µm/h was comparable to that of the 20 µm/h cell, indicating the good quality and properties of GaAs. The epitaxial growth exhibited good uniformity, as evidenced by the uniformity of the cell performance across the wafer, from the center to the edge. The result indicated the potential of high-throughput MOVPE for low-cost production, not only for PV devices but also for other semiconductor applications.

Fast Growth of Smooth AlN in a 3 × 2″ Showerhead‐Type Vertical Flow MOVPE Reactor

The conditions required for a high growth rate of AlN in a 3 × 2″ showerhead‐type vertical flow metalorganic vapor phase epitaxy (MOVPE) reactor are studied. It is found that at the standard growth conditions (low V/III, 50 mbar, 1110 °C, H2), the growth rate linearly increases with the trimethylaluminium (TMAl) flow rate until about 280 μmol min−1 with some drop of precursor utilization efficiency at higher pressures. While the pre‐reaction of TMAl with NH3 at 140 μmol min−1 of TMAl is still not a major issue, it is not possible, however, to maintain a smooth AlN surface morphology during this “fast” growth. To suppress the surface morphology deterioration, the growth pressure requires optimization. An increase of the growth pressure, to 75 mbar, is found to be critical to grow 20+ μm of smooth AlN at a rate of about 3.6 μm h−1 on bulk AlN substrates.

Extremely High-Speed GaAs Growth by MOVPE for Low-Cost PV Application

Epitaxial growth of GaAs in a horizontal metalorganic vapor phase epitaxy reactor was investigated to realize ultrafast growth rate, which has been demonstrated to be 90 μm/h at maximum. The GaAs growth rate had more or less a linear relationship with the amount of trimethylgallium supply. With extremely fast deposition >70 μm/h, slight saturation of growth rate was observed, which is most likely due to parasitic gas-phase reactions and/or limitation in bubbling. The impact of growth rates was examined using GaAs p-n solar cells, with the 2-μm-thick n-GaAs base layers grown at 20, 60, and 80 μm/h. The studies have not evidenced a significant change in performance as a function of growth rate. The base thickness was expanded to 3.5 μm with the growth rate of 80 μm/h to increase light absorption. The growth rate of 80 μm/h did not induce significant degradation in the efficiency of the cells as compared with the ones grown at 20 and 60 μm/h. The average efficiency of 23.6% (16.3% on the basis of a mesa area) under AM1.5G was realized in the cells with 3.5-μm-thick bases grown at 80 μm/h.

High-quality AlN film grown on a nanosized concave–convex surface sapphire substrate by metalorganic vapor phase epitaxy

We developed a method for fabricating high-crystal-quality AlN films by combining a randomly distributed nanosized concavo–convex sapphire substrate (NCC-SS) and a three-step growth method optimized for NCC-SS, i.e., a 3-nm-thick nucleation layer (870 °C), a 150-nm-thick high-temperature layer (1250 °C), and a 3.2-μm-thick medium-temperature layer (1110 °C). The NCC-SS is easily fabricated using a conventional metalorganic vapor phase epitaxy reactor equipped with a showerhead plate. The resultant AlN film has a crack-free and single-step surface with a root-mean-square roughness of 0.5 nm. The full-widths at half-maxima of the X-ray rocking curve were 50/250 arcsec for the (0002)/(10–12) planes, revealing that the NCC surface is critical for achieving such a high-quality film. Hexagonal-pyramid-shaped voids at the AlN/NCC-SS interface and confinement of dislocations within the 150-nm-thick high-temperature layer were confirmed. The NCC surface feature and resultant faceted voids play an important role in the growth of high-crystal-quality AlN films, likely via localized and/or disordered growth of AlN at the initial stage, contributing to the alignment of high-crystal-quality nuclei and dislocations.

In-situ photoluminescence measurements during MOVPE of GaN and InGaN in a CCS reactor

Abstract Gallium Nitride (GaN) and Indium Gallium Nitride (InGaN) have become important semiconductor materials for the LED lighting industry. Recently, a photoluminescence (PL) technique for direct in-situ characterization of GaN and InGaN layers during epitaxial growth in a planetary metalorganic vapor phase epitaxy (MOVPE) reactor was reported. The PL signals reveal – at the earliest possible stage – information about current layer thickness, temperature, composition, surface roughness, and self-absorption. Thus, the PL data is valuable for both controlling and optimizing the growth parameters, thereby promising both better devices and a better yield for the LED industry. This technical report describes an extension of this PL technique to close coupled showerhead (CCS) reactors with narrow optical viewports. In contrast to the wide aperture optics in previous investigations, a compact and all-fiber optical probe without voluminous lens optics, filter elements or beam splitters was used.

Effects of various dopants on properties of GaAs tunneling junctions and p–i–n solar cells

Several GaAs tunneling junctions (TJs) and p–i–n single junction solar cells grown using a planetary metalorganic vapor phase epitaxy (MOVPE) reactor utilizing various dopant species including Zn, C, S, and Te were investigated. The incorporation of Te atoms into GaAs was approximately two orders larger than that of S atoms. Although only 30% of Te atoms could be electrically activated, a carrier concentration of 1019 cm−3 was achieved. Highly C-doped GaAs was successfully obtained by decreasing the growth temperature and increasing the amount of H2 carrier gas in order to prevent the predecomposition of CBr4 dopant gas. A hole concentration of about 1020 cm−3 was realized with a growth temperature of 450 °C. The C–Te-doped GaAs TJ exhibited the best ohmic tunneling behavior with a resistivity of 12.5 mΩ·cm2, while the others had diode characteristics. The GaAs solar cell grown with the Zn–S dopant showed the highest conversion efficiency ascribed to a longer minority carrier lifetime.

Decomposition of trimethylgallium and adduct formation in a metalorganic vapor phase epitaxy reactor analyzed by high‐resolution gas monitoring system

In this study, by using time‐of‐flight (TOF) high‐resolution and high‐sensitivity mass spectrometry measurements, the decomposition of trimethylgallium into dimethylgallium and monomethylgallium along with its adduct formation with NHx in a commercially available horizontal metalorganic vapor‐phase epitaxy (MOVPE) reactor with a resistive heater was investigated and observed in more detail than previously reported. The results confirmed the use of the TOF monitoring system in analyzing the elementary reaction process in actual MOVPE systems.

Synthesis and properties of ultra-long InP nanowires on glass

We report on the synthesis of Au-catalyzed InP nanowires (NWs) on low-cost glass substrates. Ultra-dense and ultra-long (up to ∼250 μm) InP NWs, with an exceptionally high growth rate of ∼25 μm min−1, were grown directly on glass using metal organic vapor phase epitaxy (MOVPE). Structural properties of InP NWs grown on glass were similar to the ones grown typically on Si substrates showing many structural twin faults but the NWs on glass always exhibited a stronger photoluminescence (PL) intensity at room temperature. The PL measurements of NWs grown on glass reveal two additional prominent impurity related emission peaks at low temperature (10 K). In particular, the strongest unusual emission peak with an activation energy of 23.8 ± 2 meV was observed at 928 nm. Different possibilities including the role of native defects (phosphorus and/or indium vacancies) are discussed but most likely the origin of this PL peak is related to the impurity incorporation from the glass substrate. Furthermore, despite the presence of suspected impurities, the NWs on glass show outstanding light absorption in a wide spectral range (60%–95% for λ = 300–1600 nm). The optical properties and the NW growth mechanism on glass is discussed qualitatively. We attribute the exceptionally high growth rate mostly to the atmospheric pressure growth conditions of our MOVPE reactor and stronger PL intensity on glass due to the impurity doping. Overall, the III–V NWs grown on glass are similar to the ones grown on semiconductor substrates but offer additional advantages such as low-cost and light transparency.