Trimethylgallium Research Articles

Synthesis of nitrogen-doped crystalline Ga2O3 thin films via trimethylgallium doped NH3/H2/N2/O2 premixed stagnation flames

This study presents a novel and cost-effective technique for fabricating nitrogen-doped crystalline α-Ga2O3 and β-Ga2O3 thin films under atmospheric pressure conditions through flame synthesis for the first time. We employ metal–organic ammonia (NH3)-assisted flame synthesis (MO-NAFS) in a quasi-one-dimensional trimethylgallium (TMG) doped NH3/H2/N2/O2 flat flame to achieve a one-step deposition of Ga2O3 onto c-plane α-Al2O3 substrates. We found that hydrogen (H2) addition directly impacts the crystallinity of the Ga2O3 thin film. When only NH3 is employed as the fuel, a thin-film single-crystal nitrogen-doped β-Ga2O3 was observed. Blending the NH3 with 5 % of H2 yields a single-crystal nitrogen-doped α-Ga2O3, while a further increase of H2 fraction up to 10 % results in a mixture of both α-Ga2O3 and β-Ga2O3 crystals. Remarkably, the nitrogen doping through MO-NAFS was found to persistently result in an N/O atomic ratio as high as 1.12, as determined by the XPS analysis. This approach paves the way to a simple and scalable means for producing crystalline nitrogen-doped Ga2O3 thin films with potential control of crystal phase composition through parameter adjustments.

High growth rate metal organic chemical vapor deposition grown Ga2O3 (010) Schottky diodes

We report on the growth of Si-doped homoepitaxial β-Ga2O3 thin films on (010) Ga2O3 substrates via metal-organic chemical vapor deposition (MOCVD) utilizing triethylgallium (TEGa) and trimethylgallium (TMGa) precursors. The epitaxial growth achieved an impressive 9.5 μm thickness at 3 μm/h using TMGa, a significant advance in material growth for electronic device fabrication. This paper systematically studies the Schottky barrier diodes fabricated on the three MOCVD-grown films, each exhibiting variations in the epilayer thickness, doping levels, and growth rates. The diode from the 2 μm thick Ga2O3 epilayer with TEGa precursor demonstrates promising forward current densities, the lowest specific on-resistance, and the lowest ideality factor, endorsing TEGa’s potential for MOCVD growth. Conversely, the diode from the 9.5 μm thick Ga2O3 layer with TMGa precursor exhibits excellent characteristics in terms of lowest leakage current, highest on-off ratio, and highest reverse breakdown voltage of −510 V without any electric field management, emphasizing TMGa’s suitability for achieving high growth rates in Ga2O3 epilayers for vertical power electronic devices.

Effect of gas pre-decomposition device on the growth of GaN epitaxial layer

In previous studies, the influence of gas phase and surface reactions on the growth of GaN was mainly calculated through simulations. In this study, a novel gas pre-decomposition device (GPDD) was designed to experimentally investigate the effects of gas phase and surface reactions on GaN growth by changing the length and height of the isolation plates (IPs). By varying the structure of the GPDD, the effects on the growth rate and thickness uniformity of the GaN films were studied. The growth rate of the GaN sample slowed with the extension of the IPs because the longer partition plates led to insufficient gas mixing and premature consumption of the precursor trimethylgallium (TMG). The use of GPDD simultaneously achieves high crystal quality and smooth surface morphology of the GaN film. Owing to the use of GPDD, the decomposition of TMG in the pyrolysis pathway was promoted, which suppressed Ga vacancies and C impurities, resulting in weak yellow luminescence bands in the photoluminescence. This study provides a comprehensive understanding of the chemical reaction mechanism of GaN and plays an important role in promoting the development of metal-organic chemical vapor deposition equipment.

Direct growth of highly oriented GaN thin films on silicon by remote plasma CVD

We report on low-temperature (500 °C) and low-pressure (0.3 mbar) direct growth of GaN thin films on silicon (100) substrates using remote plasma chemical vapour deposition (RP-CVD). In the custom-designed reactor, an RF inductively coupled plasma is generated remotely from the substrate’s area to facilitate the decomposition of group-V precursor, N2 with added H2, while group-III precursor trimethylgallium (TMGa), is directly injected into the growth chamber mixed with H2 carrier gas. Growth parameters such as RF power, process pressure and gas flow rates have been optimized to achieve a film growth rate of about 0.6 µm h−1. Several characterization techniques were used to investigate the plasma and the properties of the grown thin films in terms of their crystallinity, morphology, topography, and composition. The films are highly textured with a preferential orientation along the c-axis of the wurtzite structure. They present a small roughness in the nanometer range and a columnar microstructure with a grain size of one hundred nanometer, and a gallium polarity (+c plane oriented). Rutherford backscattering spectrometry and nuclear reaction analysis show that the chemical composition is homogeneous through the depth of the layer, with a III/V ratio close to 1, a very low content of oxygen (below the detection limit ∼1%) and a carbon content up to 11%. It was shown that the increase of plasma power helps to reduce this carbon contamination down to 8%. This research paves the way for a growth method compatible with cost reduction of III–V thin film production achieved through reduced gas consumption facilitated by RP-CVD operation at low pressure.

Epitaxial growth of high-quality GaN with a high growth rate at low temperatures by radical-enhanced metalorganic chemical vapor deposition

Using our recently developed radical-enhanced metalorganic chemical vapor deposition (REMOCVD) technique, we have grown gallium nitride (GaN) on bulk GaN and GaN on Si templates. Three features make up this system: (1) applying very high-frequency power (60 MHz) to increase the plasma density; (2) introducing H2 and N2 gas in the plasma discharge region to produce active NHx radical species in addition to nitrogen radicals; and (3) supplying radicals under remote plasma arrangement with a Faraday cage to suppress charged ions and photons. Using this new REMOCVD system, it was found that high-quality crystals can be grown at lower temperatures than that of MOCVD but the disadvantage was that the growth rate was smaller as 0.2–0.8 μm/h than that by MOCVD. In the present work, we have used a pBN inner shield to prevent the deactivation of radicals to increase the growth rate. The growth conditions such as the plasma power, trimethylgallium (TMG) source flow rate, N2 + H2 gas mixture flow rate, and the ratio of N2/H2 were optimized and it was found that the growth rate could be increased up to 3.4 μm/h with remarkably high crystalline quality comparable to that of MOCVD. The XRD-FWHM of GaN grown on the GaN/Si template and the bulk GaN substrate were 977 arcsec and 72 arcsec respectively. This work may be very promising to achieve high-power GaN/GaN devices.

Gas-phase study of the behavior of trimethyl gallium and triethyl gallium by optical emission spectroscopy and quadrupole mass spectroscopy for the growth of GaN by REMOCVD (radical-enhanced metalorganic chemical vapor deposition)

Metal-organic CVD (MOCVD) is a well-established means of epitaxial growth of III-nitrides in terms of production. To overcome the drawbacks of MOCVD, we have developed a radical-enhanced MOVCVD (REMOCVD) technique which is promising to grow group-III nitride materials at lower temperatures without ammonia gas. The gas phase behavior of trimethyl gallium (TMG) and triethyl gallium (TEG) in the chamber is studied with optical emission spectroscopy (OES) and quadrupole mass spectroscopy. From OES results, it is found that the parasitic reactions due to activated Ga and CN could be avoided by introducing hydrogen as a source gas together with nitrogen gas. The TEG is completely decomposed in REMOCVD at 550 °C which is a much lower temperature compared to TMG in a hydrogen atmosphere. Also, it is found that due to the low decomposition temperature of TEG, TEG contaminated the gas line which needs to be cleaned often.

Adsorption mechanism of dimeric Ga precursors in metalorganic chemical vapor deposition of gallium nitride

Gallium nitride (GaN) has attracted significant interest as a next-generation semiconductor material with various potential applications. During metalorganic chemical vapor deposition (MOCVD) of GaN using trimethyl gallium (TMG) and NH3, dimeric precursors are produced by gas-phase reactions such as adduct formation or thermal decomposition. In this work, the surface adsorption reactions of monomeric and dimeric Ga molecules including TMG, [(CH3)2Ga(NH2)]2, and [(CH3)GaNH]2 on the GaN surface are investigated using density functional theory calculations. It is found that [(CH3)2Ga(NH2)]2 is the most predominant form among the various dimeric precursors under typical GaN MOCVD process conditions. Our results indicate that the dimeric [(CH3)GaNH]2 precursor, which is generated through the thermal decomposition of [(CH3)2Ga(NH2)]2, would have higher reactivity on the GaN surface. Our work provides critical insights that can inform the optimization of GaN MOCVD processes, leading to advancements in GaN-based high-performance semiconductors.

High-speed growth of thick high-purity β-Ga2O3 layers by low-pressure hot-wall metalorganic vapor phase epitaxy

High-speed growth of thick, high-purity β-Ga2O3 homoepitaxial layers on (010) β-Ga2O3 substrates by low-pressure hot-wall metalorganic vapor phase epitaxy was investigated using trimethylgallium (TMGa) as the Ga precursor. When the reactor pressure was 2.4–3.4 kPa, the growth temperature was 1000 °C, and a high input VI/III (O/Ga) ratio was used, the growth rate of β-Ga2O3 could be increased linearly by increasing the TMGa supply rate. A thick layer was grown at a growth rate of 16.2 μm h−1 without twinning. Incorporated impurities were not detected, irrespective of the growth rate, demonstrating the promising nature of β-Ga2O3 growth using TMGa.

Al Incorporation up to 99% in Metalorganic Chemical Vapor Deposition‐Grown Monoclinic (AlxGa1–x)2O3 Films Using Trimethylgallium

Growths of monoclinic (AlxGa1−x)2O3 thin films up to 99% Al contents are demonstrated via metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) as the Ga precursor. The utilization of TMGa, rather than triethylgallium, enables a significant improvement of the growth rates (>2.5 μm h−1) of β‐(AlxGa1−x)2O3 thin films on (010), (100), and (01) β‐Ga2O3 substrates. By systematically tuning the precursor molar flow rates, growth of coherently strained phase pure β‐(AlxGa1−x)2O3 films is demonstrated by comprehensive material characterizations via high‐resolution X‐ray diffraction (XRD) and atomic‐resolution scanning transmission electron microscopy (STEM) imaging. Monoclinic (AlxGa1−x)2O3 films with Al contents up to 99, 29, and 16% are achieved on (100), (010), and (01) β‐Ga2O3 substrates, respectively. Beyond 29% of Al incorporation, the (010) (AlxGa1−x)2O3 films exhibit β‐ to γ‐phase segregation. β‐(AlxGa1−x)2O3 films grown on (01) β‐Ga2O3 show local segregation of Al along (100) plane. Record‐high Al incorporations up to 99% in monoclinic (AlxGa1−x)2O3 grown on (100) Ga2O3 are confirmed from XRD, STEM, electron nanodiffraction, and X‐ray photoelectron spectroscopy measurements. These results indicate great promises of MOCVD development of β‐(AlxGa1−x)2O3 films and heterostructures with high Al content and growth rates using TMGa for next‐generation high‐power and high‐frequency electronic devices.

Calculation of the Second Virial Coefficient of The TMGa Molecule

In this work, interaction energy values were calculated using ab initio method for Trimethylgallium (TMGa) molecule. For the Trimethylgallium molecules, the minimum energy values obtained by the ab initio method have been used to calculate second virial coefficient by placing in Yukawa potential. The obtained second virial coefficient results are compared to theoretical data reported in the literature. Furthermore, the obtained results for Yukawa potential compared to with other potentials such as Lennard Jones (12-6) potential and Morse potential in the literature. For most of the varied temperatures of the investigated TMGa molecule, the second virial coefficient predicted by theoretical result computations showed reasonable agreement with theoretical second virial coefficient values.

Understanding the influence of physical properties on the mechanical characteristics of Mg-doped GaN thin films

This study investigates the impact of Mg doping concentration in GaN thin films on their physical and mechanical properties, specifically on their conductivity transition, luminescence, hardness, Young's modulus, and creep behavior. Mg-doped GaN layers were grown on sapphire substrates using the MOCVD technique, with trimethylgallium (TMG) and bis(cyclopentadienyl) magnesium (Cp2Mg) as precursors for Ga and Mg, respectively. The TMG flow rate was set at 20 μmol/min, while the Mg flow rate was adjusted by tuning the temperature of the thermostatic bath of Cp2Mg. Three different Cp2Mg flow rates: 2, 7 and 9 μmol/min were utilized to explore their effects on the physical and mechanical properties of the grown epilayers. The results demonstrate that an increase in the Mg concentration leads to a conductivity transition from n-type to p-type, with the highest hole concentration (4.5 ± 0.2) × 1017 cm−3 and blue luminescence attained with Mg concentration equal to 2.1 × 1019 atoms/cm3. Moreover, the hardness, Young's modulus and the intensity of compressive stress increase with the Cp2Mg flow rate due to the evolution of pre-existing dislocation density and point defects incorporation. Additionally, this study evaluates the creep behavior of the Mg-doped GaN thin films. It shows that both the creep stress exponent and the maximal creep depth decrease with the increase of the Cp2Mg flow rate. This is attributed to dislocation glides and climbs governing the creep mechanism. Accordingly, this investigation highlights the importance of understanding the relationship between physical properties and mechanical characteristics of Mg-doped GaN thin films, which have potential implications for various technological applications.

Mechanical Properties and Creep Behavior of Undoped and Mg-Doped GaN Thin Films Grown by Metal–Organic Chemical Vapor Deposition

This paper investigates the mechanical properties and creep behavior of undoped and Mg-doped GaN thin films grown on sapphire substrates using metal–organic chemical vapor deposition (MOCVD) with trimethylgallium (TMG) and bis(cyclopentadienyl)magnesium (Cp2Mg) as the precursors for Ga and Mg, respectively. The Mg-doped GaN layer, with a [Mg]/[TMG] ratio of 0.33, was systematically analyzed to compare its mechanical properties and creep behavior to those of the undoped GaN thin film, marking the first investigation into the creep behavior of both GaN and Mg-doped GaN thin films. The results show that the incorporated [Mg]/[TMG] ratio was sufficient for the transition from n-type to p-type conductivity with higher hole concentration around 4.6×1017 cm−3. Additionally, it was observed that Mg doping impacted the hardness and Young’s modulus, leading to an approximately 20% increase in these mechanical properties. The creep exponent is also affected due to the introduction of Mg atoms. This, in turn, contributes to an increase in pre-existing dislocation density from 2 × 108 cm−2 for undoped GaN to 5 × 109 cm−2 for the Mg-doped GaN layer. The assessment of the creep behavior of GaN and Mg-doped GaN thin films reveals an inherent creep mechanism governed by dislocation glides and climbs, highlighting the significance of Mg doping concentration in GaN thin films and its potential impact on various technological applications.

The role of carbon and C-H neutralization in MOCVD β-Ga2O3 using TMGa as precursor

In this Letter, the role of background carbon in metalorganic chemical vapor deposition (MOCVD) β-Ga2O3 growth using trimethylgallium (TMGa) as the Ga precursor was investigated. The quantitative C and H incorporations in MOCVD β-Ga2O3 thin films grown at different growth rates and temperatures were measured via quantitative secondary ion mass spectroscopy (SIMS). The SIMS results revealed both [C] and [H] increase as the TMGa molar flow rate/growth rate increases or growth temperature decreases. The intentional Si incorporation in MOCVD β-Ga2O3 thin films decreases as the growth rate increases or the growth temperature decreases. For films grown at relatively fast growth rates (GRs) (TMGa > 58 μmol/min, GR > 2.8 μm/h) or relatively low temperature (<950 °C), the [C] increases faster than that of the [H]. The experimental results from this study demonstrate the previously predicted theory—H can effectively passivate the compensation effect of C in n-type β-Ga2O3. The extracted net doping concentration from quantitative SIMS {[Si]-([C]-[H])} agrees well with the free carrier concentration measured from Hall measurement. The revealing of the role of C compensation in MOCVD β-Ga2O3 and the effect of H incorporation will provide guidance on designing material synthesis for targeted device applications.

Kinking of GaP Nanowires Grown in an In Situ (S)TEM Gas Cell Holder

AbstractNanowires are a promising structure to create new defect‐free heterostructures and optoelectronic devices. GaP nanowires grown via the VLS mechanism using tertiary‐butyl phosphine (TBP) and trimethylgallium (TMGa) as precursors in an in situ closed gas cell heating holder are shown. This holder is a model system to investigate the processes in metal‐organic vapour phase epitaxy (MOVPE). GaP nanowires change their growth direction after random distances by producing kinks. Statistics of these kink angles show dominant values of around 70.5°, 109.5°, and 123.7°. A custom holder tip capable of holding a single heating chip is used to perform scanning precession electron diffraction (SPED) measurements on the nanowire kinks. The results show that the predominant kink angles result from micro twins of first and second order. Understanding the defect formation and resulting geometry changes in GaP nanowires can lead to increased control over their shape during growth and mark a huge step toward applicable nanowire devices.

Manufacturing of Gallium Nitride Thin Films in a Multi-Wafer MOCVD Reactor

Abstract Gallium nitride (GaN) thin films have attracted considerable attention for manufacturing optical and electronic devices. They have wide bandgap and superb performance in these applications. The reliability and durability of optoelectronic devices depend on the quality of the GaN thin films. The metal-organic chemical vapor deposition (MOCVD) process is a common manufacturing technique for fabricating high-quality thin films. By manipulating the operating conditions and the reactor design, one can control the deposition rate and the uniformity of the thin film. In this paper, the manufacturing process for GaN thin films in a multi-wafer MOCVD reactor is simulated based on the three-dimensional computational model of an experimental system which provides data for validation as well as realistic design parameters. The reactor pressure and the flow rate of the precursor, trimethyl-gallium (TMG), significantly affect the deposition rate and film uniformity. The incursion of impurities in the deposition can be reduced by increasing the volumetric ratio of NH3 to TMG (V/III) and reducing the reactor pressure. The deposition rate and quality of the thin film are enhanced using an appropriate mixture of H2 and N2 as the carrier gas. The design of the inlet can also be varied to improve the utilization of metal-organic precursors and increase the deposition rate. This paper presents and discusses results on these aspects for this important manufacturing process. Thus, it leads to a better understanding of the basic mechanisms involved and provides guidelines for obtaining high deposition rates with high film quality in practical chemical vapor deposition reactors.

Challenges in atomic layer etching of gallium nitride using surface oxidation and ligand-exchange

Two atomic layer etching (ALE) methods were studied for crystalline GaN, based on oxidation, fluorination, and ligand exchange. Etching was performed on unintentionally doped GaN grown by hydride vapor phase epitaxy. For the first step, the GaN surfaces were oxidized using either water vapor or remote O2-plasma exposure to produce a thin oxide layer. Removal of the surface oxide was addressed using alternating exposures of hydrogen fluoride (HF) and trimethylgallium (TMG) via fluorination and ligand exchange, respectively. Several HF and TMG super cycles were implemented to remove the surface oxide. Each ALE process was monitored in situ using multiwavelength ellipsometry. X-ray photoelectron spectroscopy was employed for the characterization of surface composition and impurity states. Additionally, the thermal and plasma-enhanced ALE methods were performed on patterned wafers and transmission electron microscopy (TEM) was used to measure the surface change. The x-ray photoelectron spectroscopy measurements indicated that F and O impurities remained on etched surfaces for both ALE processes. Ellipsometry indicated a slight reduction in thickness. TEM indicated a removal rate that was less than predicted. We suggest that the etch rates were reduced due to the ordered structure of the oxide formed on crystalline GaN surfaces.

Deposition Mechanism and Properties of Plasma-Enhanced Atomic Layer Deposited Gallium Nitride Films with Different Substrate Temperatures

Gallium nitride (GaN) is a wide bandgap semiconductor with remarkable chemical and thermal stability, making it a competitive candidate for a variety of optoelectronic applications. In this study, GaN films are grown using a plasma-enhanced atomic layer deposition (PEALD) with trimethylgallium (TMG) and NH3 plasma. The effect of substrate temperature on growth mechanism and properties of the PEALD GaN films is systematically studied. The experimental results show that the self-limiting surface chemical reactions occur in the substrate temperature range of 250-350 °C. The substrate temperature strongly affects the crystalline structure, which is nearly amorphous at below 250 °C, with (100) as the major phase at below 400 °C, and (002) dominated at higher temperatures. The X-ray photoelectron spectroscopy spectra reveals the unintentional oxygen incorporation into the films in the forms of Ga2O3 and Ga-OH. The amount of Ga-O component decreases, whereas the Ga-Ga component rapidly increases at 400 and 450 °C, due to the decomposition of TMG. The substrate temperature of 350 °C with the highest amount of Ga-N bonds is, therefore, considered the optimum substrate temperature. This study is helpful for improving the quality of PEALD GaN films.

Carbon and silicon background impurity control in undoped GaN layers grown with trimethylgallium and triethylgallium via metalorganic chemical vapor deposition

The unintentional impurity incorporation in GaN epitaxial layers impacts the electrical conductivity and optical properties of the films grown by metalorganic chemical vapor deposition (MOCVD). It is critical to control impurity-related states for device structure development. The aim of this work is to contribute to the understanding of the reasons for the presence of certain background impurities. In this paper, the unintentional impurity incorporation of carbon, hydrogen, oxygen, and silicon in undoped (u-) GaN films grown by low-pressure MOCVD were studied. Background impurity concentrations were evaluated by secondary ion mass spectroscopy (SIMS) to optimize growth parameters including V/III ratio, growth temperature (Tg), growth pressure (Pg), and gallium (Ga) precursors of trimethylgallium (TMG) or triethylgallium (TEG). Unintentional [C] and [Si] incorporations are found to be highly dependent on the growth parameters. For the same growth conditions, u-GaN films grown with a TEG precursor exhibited a lower background concentration of C compared to that of GaN grown with TMG. However, the lowest background [C] achieved was grown with TMG using optimized conditions and exhibited [C] < 4 × 1015 cm−3 which was at the detection limit of the SIMS measurement. These results were measured for a u-GaN layer grown with TMG at 200 Torr, 1030 °C, and a V/III of 3700. The lowest background [Si] of 4 × 1015 cm−3 was achieved by growing with TMG at 200 Torr, 1000 °C, and V/III of 650.

Relation between Ga Vacancies, Photoluminescence, and Growth Conditions of MOVPE-Prepared GaN Layers.

A set of GaN layers prepared by metalorganic vapor phase epitaxy under different technological conditions (growth temperature carrier gas type and Ga precursor) were investigated using variable energy positron annihilation spectroscopy (VEPAS) to find a link between technological conditions, GaN layer properties, and the concentration of gallium vacancies (VGa). Different correlations between technological parameters and VGa concentration were observed for layers grown from triethyl gallium (TEGa) and trimethyl gallium (TMGa) precursors. In case of TEGa, the formation of VGa was significantly influenced by the type of reactor atmosphere (N2 or H2), while no similar behaviour was observed for growth from TMGa. VGa formation was suppressed with increasing temperature for growth from TEGa. On the contrary, enhancement of VGa concentration was observed for growth from TMGa, with cluster formation for the highest temperature of 1100 °C. From the correlation of photoluminescence results with VGa concentration determined by VEPAS, it can be concluded that yellow band luminescence in GaN is likely not connected with VGa; additionally, increased VGa concentration enhances excitonic luminescence. The probable explanation is that VGa prevent the formation of some other highly efficient nonradiative defects. Possible types of such defects are suggested.

Thickness-dependent physical and nanomechanical properties of [formula omitted] thin films

A set of undoped AlxGa1−xN epilayers with different thicknesses were grown on (0001) sapphire substrates using metal-organic chemical vapor deposition (MOCVD) technique, using the same gas-phase composition of trimethylaluminum (TMAl) and trimethylgallium (TMGa). Giving that the first growth stage is crucial for the successful fabrication of AlxGa1-xN epilayers; thus, SiN nano-mask, as an alternative method to nanoimprint lithography (NIL) which is used for patterning substrates, was deposited on sapphire substrate. Indeed, SiN treatment initiates the three-dimensional (3D) growth mode to efficiently decrease the threading dislocation density. It was found that depending on the film surface coalescence degree, the physical and nanomechanical properties are thoroughly dependent. As the non-coalescent surface is present, the porosity effects are dominant. As soon as the film thickness attains the surface coalescence, the Al incorporation is enhanced and convoyed with it point defects. Despite the decrease of the threading dislocations along with the film thickness evolution, it was observed that the physical properties are slightly decreased and the nanomechanical properties are improved. This is likely related to point defect interaction with nucleated dislocations. Moreover, the defects did not severally affect the plastic deformation capacity (ductility) of AlxGa1-xN films, where high indentation force was applied and no failure was revealed. Therefore, the balance between AlxGa1−xN physical and nanomechanical performances devotes to manufacture a wider range of efficient UV devices.