Trimethylgallium Flow Research Articles

The carbon doping mechanism in GaAs using trimethylgallium and trimethylarsenic

A new thermodynamical model has been proposed for active carbon incorporation from undissociated monomethylgallium (GaCH3) radicals in GaAs. The model predicts that the carbon concentration increases with methyl (CH3) radical concentration and supplying CH3 radicals suppresses dissociation of GaCH3 radicals. The prediction is clearly verified under a higher trimethylarsenic (TMAs)/trimethylgallium (TMG) flow ratio than a critical ratio. For example, heavily carbon-doped GaAs (more than 1020 cm−3) has been achieved even at high temperatures around 600°C by controlling the TMAs/TMG flow ratio. In contrast, the carbon concentration decreases with increasing TMAs/TMG flow ratio at lower TMAs/TMG flow ratios which is not predicted from the model. In this regime, most carbon atoms form micro-inclusions which are electrically inactive. A mechanism for the inactive carbon incorporation remains unclear.The present model and experimental findings provide a new key parameter to grow heavily carbon-doped GaAs at high growth temperature.

Mass spectroscopy study of GaN metalorganic chemical vapor deposition

Metalorganic chemical vapor phase deposition of GaN on (100) GaAs has been studied using mass spectroscopy. With increasing substrate temperature, the amount of decomposed trimethylgallium (TMGa) was observed to increase exponentially with a characteristic energy of 1.5 eV. The presence of NH3 was found to suppress the production of CH3 in the gas phase. This implies that CH3 of TMGa reacts with the hydrogen atom of NH3, forming CH4 as a main gas product. Studies of nitrogen evaporation from the growth surface when TMGa flow was off lead to the conclusion that increased growth rate could result in decreased background electron concentration due to nitrogen vacancy. The presence of NH3 significantly promotes the decomposition of TMGa. Desorption of excess Ga atoms from the growth surface at low NH3 flow rates takes place as suggested by the increased ratio of peak intensity of Ga (m/e = 69) to that of DMGa ((CH3)2Ga, m/e- 99) with decreasing NH3 flow rate.

Growth mechanisms in excimer laser photolytic deposition of gallium nitride at 500°C

The mechanisms of controlling laser-induced chemical vapour deposition of GaN at substrate temperatures between 350 and 650°C have been investigated. Ultraviolet (193 nm) photolytic decomposition of trimethylgallium (TMGa) and ammonia (NH 3) precursors was examined in this range. Laser-induced fluorescence studies support the view that the dissociated intermediate fragments GaCH 3 and NH are the reacting species in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate temperatures below 500°C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500°C. The growth rate increases with both NH 3 TMGa ratio, and TMGa flow rate, while the temperature dependence shows a thermal activation energy of 0.2 eV which is smaller by a factor of five than that of films prepared by conventional thermal CVD. The large NH 3 TMGa ratios needed to achieve stoichiometry are interpreted in terms of the two-photon dissociation cross section of NH 3.

AlGaAs/GaAs light-emitting diode on a Si substrate with a self-formed GaAs islands active region grown by droplet epitaxy

We report the preliminary results on self-formed GaAs islands grown on the GaAs/Si substrate by metalorganic chemical vapor deposition using droplet epitaxy. Atomic force microscope observation shows that the GaAs islands exhibit a conical shape with heights of 90–170 nm, diameters of 600–750 nm, and densities of 107 cm−2, which are controlled by the trimethylgallium flow rate. In addition, an AlGaAs/GaAs light-emitting diode (LED) on Si with a self-formed GaAs island active region was fabricated by the use of this technique. The LED was operated up to 27 μW at 190 mA under direct current conditions at room temperature.

Self-limiting and step-propagating nature of GaAs atomic layer epitaxy revealed by atomic force microscopy

The self-limiting and step-propagating nature of GaAs atomic layer epitaxy (ALE) is verified by using atomic force microscopy (AFM). AFM measurements reveal that the GaAs surface having a monolayer-height step structure does not change regardless of the trimethylgallium (TMG) flow rate at the ALE growth condition. This indeed confirms the self-limiting nature of ALE. Moreover, it is found that lateral growths such as step-propagation and two-dimensional nucleation take place in ALE. The TMG migration lenght of Group-III species is evaluated to be as large as several hundred nanometers in spite of the low temperature growth at 440°C. The undecomposed TMG on As-stabilized surface is responsible for the large migration length in ALE.

Surface photo-absorption study of the laser-assisted atomic layer epitaxial growth process of GaAs

Thermal and laser-assisted decomposition of trimethylgallium (TMG) and triethylgallium (TEG) on GaAs surfaces under atomic layer epitaxy (ALE) processing conditions were monitored in real time using the (SPA) surface photo-absorption reflectivity technique to elucidate the mechanism of self-limiting deposition. When As-terminated (001) GaAs surfaces held between 340 and 390°C were exposed to TMG, the SPA reflectivity decreased to a steady state, indicative of alkylgallium on the surface. Upon laser irradiation, this surface changed to an ‘intermediate’ steady state surface that was found to be in equilibrium with gas-phase alkylgallium species. Alkyl species then desorb from this surface when the TMG flow is stopped, leaving a self-limited deposit of Ga on the surface. No self-limiting deposition of Ga could be detected using TEG in laser-assisted ALE or thermal ALE. In this case, the observation of self-limiting alkylgallum deposition could explain successful ALE growth using TEG.

Atomic layer epitaxy of GaP and elucidation for self-limiting mechanism

Atomic layer epitaxy (ALE) of GaP was performed for the first time in a low-pressure metalorganic vapor phase epitaxial (MOVPE) reactor using trimethylgallium (TMG) and phosphine (PH3) as sources. Growth was self-limiting for the exposure time of each reactant. X-ray photoelectron spectroscopy (XPS) analyses were carried out to identify the adsorbates on the growth surface. There were no methyl groups on the surface Ga and the self-limiting mechanism is due to the selective adsorption of TMG by the surface P atoms. When the substrate was exposed to a sufficient TMG flow after a submonolayer Ga was deposited by triethylgallium (TEG), growth was still self-limiting. This supports the selective adsorption model.

Ultrahigh doping of GaAs by carbon during metalorganic molecular beam epitaxy

Recent advances in heterostructure bipolar transistor technology have created a need for p-type doping at levels ≥1020 cm−3. Furthermore, such levels may eliminate the need for alloying during ohmic contact formation. We have achieved p-type doping levels as high as 5×1020 cm−3 using an unconventional dopant, C, derived from the gaseous source chemical, trimethylgallium (TMG), during metalorganic molecular beam epitaxial (MOMBE) growth of GaAs. We have controllably achieved doping levels between 1019 and 5×1020 cm−3 by diluting the TMG flow with another metalorganic, triethylgallium (TEG). By utilizing the so-called δ-doping or atomic planar doping method we have also been able to grow C-doped spikes with hole concentrations as high as 7×1019 cm−3, with a full width at half maximum of ∼50 Å at 300 K. This doping level is the highest yet reported for planar doping, and the narrow width indicates that the C atoms are restricted to one or two atomic planes. By switching out the TMG, and switching in the TEG to continue the growth of C-free GaAs we have grown sandwich-type structures with C levels of 1020 cm−3, which fall off within 210 Å to C levels of <1017 cm−3. High-temperature annealing of such structures reveals a C diffusion coefficient of ≤10−16 cm2 s−1 at 950 °C, in agreement with other reports. This is at least three orders of magnitude less than for the other conventional p-type dopants, Be and Zn. Finally, we report the presence of strain in the highly C-doped layers, detected by x-ray diffraction. The lattice constant obtained corresponds roughly to that calculated by assuming a Vegard’s law mixture of GaAs and 0.7% GaC. This distortion of the GaAs lattice has not been previously measured.

Lateral Growth Process of GaAs over Tungsten Gratings by Metalorganic Chemical Vapor Deposition

Lateral epitaxial growth of over tungsten gratings with 5 μm wide lines and spaces on (001) substrates is performed using metalorganic chemical vapor deposition. A study is made of the dependence of facet shapes and growth rates of the overgrown layers on grating direction, growth temperature, and arsine and trimethylgallium (TMG) flow rates. From the perspective of crystallography, all the facets observed in the overgrown layers were found to be classified into four individual groups: {110}, {111}As, {1̅12}As, and {113}Ga faces. The opening direction on the (001) substrate surface is found to be the most essential parameter for determining the crystallographic planes of the facets. Other important controlling parameters for the facet formation are the growth temperature and partial pressure of. The partial pressure of TMG has no influence on the faceting growth. On the other hand, the overall growth rate of the overgrown layer is limited only by the TMG flow rate. These results can be qualitatively explained by the Langmuir‐Rideal model.

GaAs Growth Using TMG and AsCl3

A new vapor‐phase epitaxial (VPE) technique using trimethylgallium (TMG) and arsenic trichloride has been developed. This VPE is free from the source instability in chloride VPE and fatally toxic arsine in hydride and metalorganic VPE. The growth reaction is based on the gallium chloride transport method. Growth rates of 300–4000 Å/min were obtained at growth temperatures between 650° and 780°C. At III/V ratios below 2.3, the growth rate increased as the TMG flow rate increased and the flow rate decreased. The growth rate was independent of the temperature in the gas mixing region of the reactor. High purity with a 77 K mobility of 36,000 cm2/Vs was obtained.

Low Temperature Plasma‐Enhanced Epitaxy of GaAs

Low temperature (⩽450°C) deposition of single‐crystal using a new plasma‐enhanced MO‐CVD technique is reported. In this technique, plasma is created by a dc potential and the substrate is not directly exposed to the plasma. Deposition of was achieved at extremely low plasma power (0.3–0.5 W/cm2) using trimethylgallium (TMGa) and arsine (or trimethylarsenic) reactants. The resulting epitaxial films show excellent surface morphology and thickness uniformity over a large area substrate. A linear dependence of growth rate upon TMGa concentration was observed with a typical growth rate of 0.1 μm/min for a TMGa flow rate of 15 ml/min. Undoped films were found to be n‐type with a room temperature mobility in the range of 5200 cm2 V−1 · s−1. Measurements on Schottky barrier devices fabricated on n/n+ layers show uniform impurity doping profiles. Temperature dependence of the diode capacitance indicates a density of deep trapping centers as low as .