Metalorganic Vapor Phase Epitaxy Growth Research Articles

First-principles and thermodynamic analysis of trimethylgallium (TMG) decomposition during MOVPE growth of GaN

We analyzed the decomposition mechanisms of trimethylgallium (TMG) used for the gallium source of GaN fabrication based on first-principles calculations and thermodynamic analysis. We considered two conditions. One condition is under the total pressure of 1atm and the other one is under metal organic vapor phase epitaxy (MOVPE) growth of GaN. Our calculated results show that H2 is indispensable for TMG decomposition under both conditions. In GaN MOVPE, TMG with H2 spontaneously decomposes into Ga(CH3) and Ga(CH3) decomposes into Ga atom gas when temperature is higher than 440K. From these calculations, we confirmed that TMG surely becomes Ga atom gas near the GaN substrate surfaces.

Improved crystalline quality of N-polar GaN epitaxial layers grown with reformed flow-rate-modulation technology

A reformed flow-rate-modulation technology was developed for the metalorganic vapor phase epitaxy (MOVPE) growth of the N-polar GaN epitaxial layers. To improve the crystalline quality of the N-polar GaN epitaxial layers, a GaN nucleation layer was grown at relatively low temperature with carefully-controlled pulsed supply of Ga source and showed diverse morphology with atomic force microscope (AFM). Furthermore, the electrical and optical properties of the grown N-polar GaN epitaxial layers were investigated extensively by means of Hall effect, photoluminescence (PL), and X-ray rocking curve (XRC) measurements. The characterization results revealed that as compared with the N-polar GaN epitaxial layer grown over the conventional GaN nucleation layer which was deposited with continuous supply of both N and Ga sources, the electrical and optical properties of the N-polar GaN epitaxial layer grown with optimized supply of Ga source for the GaN nucleation layer were significantly improved.

Influence of metal-organic vapor phase epitaxy parameters and Si(111) substrate type on the properties of AlGaN/GaN HEMTs with thin simple buffer

The present work is dedicated to the study of the influence of metal-organic vapor phase epitaxy (MOVPE) growth conditions on the properties of AlGaN/GaN high electron mobility transistor (HEMT) heterostructures with thin simple buffer layers on Si(111). In a first series of samples grown on high resistivity silicon, the conditions were varied within the GaN buffer layer while kept unchanged for the AlN nucleation layer and HEMT barrier and cap layers. XRD and AFM revealed some differences in epilayers structural quality especially in case of excessive growth pressure or V/III ratio. Capacitance–voltage (CV) measurements revealed differences in the pinch-off regime of the two-dimensional electron gas (2DEG) located at the AlN/GaN interface. Except in one case, the buffer leakage current between isolated devices correlates with the pinch-off behavior. The majority of these structures exhibited sheet carrier densities of 1 × 1013 cm−2 and electron mobility between 1100 and 1400 cm2 V −1 s−1 depending on the GaN channel growth conditions. The output and transfer characteristics (maximum drain currents and leakage currents) of the transistors are in agreement with the previous electrical characterizations. Thanks to the combination of structural and electrical characterizations we are then able to determine the optimized growth conditions for such HEMT structures. Compared with high resistivity silicon, we obtained better structural and electrical quality on conductive substrates. A 2DEG with an electron mobility of 1700 cm2 V−1 s−1 has been achieved within a structure with a 0.5 μm thick buffer producing an off-state breakdown voltage of 117 V. This reveals the crucial role of the substrate surface properties on the resulting quality of the heterostructures.

Effect of initial growth on the quality of GaN on patterned sapphire substrate with ex situ physical vapor deposition AlN seed layer

GaN epitaxy was explored on a cone-patterned sapphire substrate with an ex situ AlN seed layer prepared by physical vapor deposition (PVD). The effect of initial growth on the quality of the GaN epilayer was investigated using both ex situ PVD-AlN seed layers with various thicknesses and various deposition parameters such as temperature and reactor pressure in metal–organic vapor-phase epitaxy (MOVPE). It was found that the quality of GaN is insensitive to both the thickness of the ex situ PVD-AlN seed layer and the MOVPE growth conditions. A high-quality GaN film was realized, as indicated by room-temperature CL mapping (dark spot density of 1.6 × 108 cm−2), on a patterned sapphire substrate with a wide growth condition window by simply employing an ex situ PVD-AlN seed layer.

(Invited) Integration of III/V Hetero-Structures By Selective Area Growth on Si for Nano- and Optoelectronics

The integrated circuit (IC) technology has seen an impressive development over the last decades due to the superior interface properties between Silicon (Si) and Silicon dioxide. The main driving force for the constant improvement in microchip performance was an aggressive reduction in transistor device size. Currently this miniaturization of planar devices has reached physical limits and new device architectures are introduced and investigated such as FinFET, Gate-all-around nanowire devices and TFET. A further boost in device performance is expected from the implementation of new channel materials with higher electron mobilities than Si such as Germanium (Ge) or III/V compound materials. In addition, due to the excellent optoelectronic properties of III/V materials, the monolithic integration of III/V based hetero-structures on Si is a very promising route to clearly enhance Silicon Photonics. In particular the cost-efficient integration of a light source on Si substrate is still missing to fully profit from Silicon Photonics. Selective area growth (SEG) of lattice mismatched materials on patterned Si wafers is a novel integration approach with obvious advantages: Since most III/V candidates of interest have a larger lattice constant than Si, misfit and threading dislocations are formed to release the strain. SEG in very narrow oxide trenches allows for a pronounced defect reduction. The high aspect ratio of the oxide sidewalls leads to an efficient defect trapping in the bottom part of the trench so that the top region has a high crystal quality (aspect ratio trapping (ART)). Because thick metamorphic buffer growth is not required to adjust the lattice constant and to achieve a low defect density, SEG is also very cost-efficient and has a better compatibility with common CMOS integration schemes. The successful application of ART has already been demonstrated for numerous material systems in group IV as well as for III/V compound materials and opens several routes for implementing new device integration concepts. In this presentation we show the monolithic growth of GaAs nano ridges by metal organic vapor phase epitaxy (MOVPE) on 300mm patterned Si wafers. The deposition in narrow trenches ensures a clear reduction in defect density whereas the growth out of the trench is manipulated in such a way, that III/V ridges with increased volume and various shapes can be realized. The huge advantage of these nano ridges is that they provide a free standing lattice constant which can serve as a buffer for different pseudomorphically grown III/V hetero-structures. For example the growth of compressively strained InGaAs/GaAs multi quantum wells (MQWs) on a GaAs ridge offers an interesting approach for the monolithic integration of a laser diode, since the III/V nano ridge can also act as a wave guide. The impact of the chosen MOVPE growth parameters on the observed ridge evolution is discussed in detail based on scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction reciprocal space maps (XRD-RSM). The advantages and potentials of different hetero-structures on patterned wafers are assessed in view of new nano device applications in optoelectronics.

III/V nano ridge structures for optical applications on patterned 300 mm silicon substrate

We report on an integration approach of III/V nano ridges on patterned silicon (Si) wafers by metal organic vapor phase epitaxy (MOVPE). Trenches of different widths (≤500 nm) were processed in a silicon oxide (SiO2) layer on top of a 300 mm (001) Si substrate. The MOVPE growth conditions were chosen in a way to guarantee an efficient defect trapping within narrow trenches and to form a box shaped ridge with increased III/V volume when growing out of the trench. Compressively strained InGaAs/GaAs multi-quantum wells with 19% indium were deposited on top of the fully relaxed GaAs ridges as an active material for optical applications. Transmission electron microcopy investigation shows that very flat quantum well (QW) interfaces were realized. A clear defect trapping inside the trenches is observed whereas the ridge material is free of threading dislocations with only a very low density of planar defects. Pronounced QW photoluminescence (PL) is detected from different ridge sizes at room temperature. The potential of these III/V nano ridges for laser integration on Si substrates is emphasized by the achieved ridge volume which could enable wave guidance and by the high crystal quality in line with the distinct PL.

Low-temperature (≥400 °C) growth of InN by metalorganic vapor phase epitaxy using an NH3 decomposition catalyst

In this paper, we report the metalorganic vapor phase epitaxial (MOVPE) growth of InN using a NiO-based pellet-type NH3 decomposition catalyst. The use of the catalyst significantly changes the growth behavior of InN, which is dependent on the growth temperature (Tg). Continuous InN films without the incorporation of metallic In and a cubic phase are grown at Tg = 400–480 °C. An InN film grown at Tg ≈ 450 °C has a full-width at half maximum (FWHM) of 376 arcsec in the X-ray rocking curve for InN(0002) reflection. At Tg ≥ 500 °C, the deposition rate of InN rapidly decreases and the deposited films become discontinuous with large (ca. 1 µm) pyramidal grains of InN. Depositions are scarcely obtained at Tg ≥ 600 °C. Such changes in the growth behavior of InN are governed by the NH3 decomposition.

Atom probe tomography evidence for uniform incorporation of Bi across the growth front in GaAs1−xBix/GaAs superlattice

The three-dimensional distribution of Bi atoms in a GaAs1−xBix/GaAs superlattice grown by metalorganic vapor phase epitaxy (MOVPE) was studied using atom probe tomography (APT). The Bi distribution in the growth direction deduced from APT agreed quantitatively with the complex Bi concentration profile that was discovered using high-angle annular dark-field scanning transmission electron microscopy in a previous study. More importantly, APT revealed the Bi atom distribution in the growth planes at near atomic resolution. Bi nearest neighbor distribution and concentration frequency distribution analysis of the APT data indicated a statistically random distribution of Bi atoms in 1–2nm thick layers oriented perpendicular to the growth direction. These results provide evidence that Bi is incorporated homogeneously across the growth front even when the concentration profile in the growth direction is complex. They also suggest that MOVPE growth conditions can promote uniform Bi distribution within GaAs1−xBix layers, opening a path for application of these materials in the optoelectronic devices for which they show much promise.

Evaluation of excess In during metal organic vapor-phase epitaxy growth of InGaN by monitoring via in situ laser scattering

Using an in situ laser absorption and scattering method, the surface roughness and incorporation of In in InGaN layers grown by metal organic vapor-phase epitaxy (MOVPE) were monitored. We observed that the laser light with energy higher than the GaN bandgap was fully absorbed in a GaN layer with a smooth film surface. On the other hand, we observed that the scattering laser light from the surface when the roughness of the InGaN surface increased owing to the formation of In droplets. Laser light with energy lower than the GaN bandgap was weakly absorbed by the GaN layer and was scattered at the back surface of the wafer. Furthermore, laser light intensity decreased during InGaN growth because of In incorporation. The threshold of trimethyl-In (TMIn) for the formation of In droplets as a function of growth temperature was determined using our in situ system. Moreover, we observed that the In droplets were removed by thermal or H2 treatment. The results indicate that multiwavelength laser absorption and scattering enable the optimization of the growth conditions for In-rich InGaN.

Theoretical investigations of initial growth processes on semipolar AlN surfaces under metal–organic vapor-phase epitaxy growth condition

The initial growth processes on semipolar AlN surfaces, such as adsorption behavior of Al adatoms, are investigated on the basis of ab initio calculations and kinetic Monte Carlo (MC) simulations. By using surface phase diagrams, which are obtained by comparing the adsorption energy from ab initio calculations with gas-phase chemical potentials, we find that the adsorption of Al adatoms under H-poor condition is much easier than that under H-rich condition. Furthermore, our kinetic MC simulations demonstrate that the surface lifetime (diffusion length) of Al adatoms under H-poor condition is found to be four (two) orders of magnitude larger than that under H-rich condition. These differences implie that the growth under H-poor condition is much faster than that under H-rich condition, qualitatively consistent with the experimental results.

Metalorganic chemical vapor phase epitaxy of narrow-band distributed Bragg reflectors realized by GaN:Ge modulation doping

We report on metalorganic vapor phase epitaxy (MOVPE) of distributed Bragg reflectors (DBR) applying a periodic modulation of the GaN doping concentration only. The doping modulation changes the refractive index of GaN via the Burstein-Moss-effect. MOVPE growth of highly doped GaN:Ge and modulation of the dopant concentration by at least two orders of magnitude within few nanometers is required to achieve a refractive index contrast of 2–3%. Such modulation characteristic is achieved despite the presence of Ge memory effects and incorporation delay. We realized DBRs with up to 100 layer pairs by combining GaN:Ge with a nominal doping concentration of 1.6×1020cm−3 as low-refractive index material with unintentionally doped GaN as high-refractive index layer. Scanning transmission electron microscope images reveal DBR structures with abrupt interfaces and homogenous layer thicknesses in lateral and vertical direction. Reflectance measurements of DBRs designed for the blue and near UV-spectral region show a narrow stopband with a maximum reflectivity of 85% at 418nm and even 95% at 370nm. InGaN/GaN multi-quantum well structures grown on top of such DBRs exhibit narrow emission spectra with linewidths below 3nm and significantly increased emission intensity.

Influence of trench period and depth on MOVPE grown [formula omitted] GaN on patterned r-plane sapphire substrates.

In this article, the influence of the trench period and depth of pre-structured r-plane sapphire substrates on the metalorganic vapor phase epitaxy (MOVPE) growth of (112¯2) GaN is investigated. We found that a larger trench period is beneficial for a small basal plane stacking fault (BSF) and threading dislocation (TD) density on the wafer surface, because it facilitates a better formation of a coalescence gap, which effectively blocks defects from penetrating to the surface. Further, the amount of BSFs emerging from the −c-wings of the uncoalesced GaN stripes is directly related to the trench period. With the help of in situ deposited marker layers we studied the development of the individual GaN stripes and observed that the trench depth and hence the ratio of the sapphire c-plane area relative to the total surface area heavily influences the coalescence process and defect development. Moreover, it is observed that the parasitic donor concentration increases for samples with smoother wafer surface.

MOVPE growth studies of Ga(NAsP)/(BGa)(AsP) multi quantum well heterostructures (MQWH) for the monolithic integration of laser structures on (001) Si-substrates

High-quality, pseudomorphically strained Ga(NAsP)/(BGa)(AsP)-multiple quantum well heterostructures (MQWH) have been deposited on exactly oriented (001) Si-substrate by metal organic vapour phase epitaxy (MOVPE) in a wide temperature range between 525°C and 700°C. The individual atomic incorporation efficiencies, growth rates as well as nanoscale material properties have been clarified by applying detailed high-resolution X-ray diffraction (HR-XRD), photoluminescence (PL) spectroscopy and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) studies. An almost constant N-incorporation efficiency is obtained for a wide growth temperature range from 550°C up to 650°C. The P-incorporation is steadily increasing with increasing growth temperature reaching values at high temperatures in excess of the applied gas phase ratio. While the lower interface from the binary GaP- to the quaternary Ga(NAsP)-material system is very sharp, the upper interface is significantly rougher with a roughness scale of ±0.43nm in quantum well thickness variation at a growth temperature of 525°C. This roughness scale increases steadily with increasing growth temperature. No indication of any phase separation effects is detected in the Ga(NAsP)-material system even at the highest growth temperature of 700°C. The obtained experimental results are briefly discussed with respect to the anticipated metastable character of the novel dilute-nitride Ga(NAsP)-material system grown lattice-matched to (001) Si-substrate.

Chemical Reaction Pathways for MOVPE Growth of Aluminum Nitride

The chemical reaction pathways for metal-organic vapor phase epitaxial growth of aluminum nitride (AlN) were investigated using elementary reaction simulations and density functional theory calculations. We found that alkyl-aluminum amide (DMA-NH2), which initiates parasitic chemical reactions such as oligomerization, is one of the major reactive species involved in AlN growth. The simulation results indicated that DMA-NH2 is adsorbed on the surface, followed by the elimination of methane with no energy barrier. The results clarify the role of parasitic reactions and their products in the growth of AlN.

Polar and semipolar (112) InAlN layers grown on AlN templates using MOVPE

AbstractWe report on metalorganic vapor phase epitaxial growth of InAlN layers on (0001) and (112) AlN templates at different temperatures (725–800 °C). The indium content (8.0–15.2%) of the (112) InAlN layers was found to be lower compared to that for the (0001) layers (9.0–23.0%). The higher indium content of the (0001) layers was attributed to a high density of small hillocks and a larger degree of relaxation. The small hillocks on the (0001) layer surface acted as quantum‐dot‐like structures that caused a stronger emission at longer wavelength compared to weaker emission at shorter wavelength from a “bulk” layer underneath. A large Stokes‐shift of about 300 and 480 meV was observed for the (0001) and () layers, respectively. The larger shift of the (112) layers was attributed to a higher degree of localization. The larger degree of localization and higher unintentional oxygen incorporation enhanced the luminescence intensity of the (112) layers compared to that for the (0001) layers.

Quantification of Bi distribution in MOVPE-grown Ga(AsBi) via HAADF STEM

The importance of dilute bismide III/V semiconductors increases and their physical properties open up a wide range for applications. Therefore, high quality layers are required, what is difficult to achieve, as these alloys are highly metastable and phase separation can occur. We use HAADF (high angle annular dark field) imaging in aberration-corrected STEM (scanning transmission electron microscopy) to quantify the Bi distribution in MOVPE (metal organic vapor phase epitaxy) grown material at large length scales as well as down to the nanoscale. This is done for different Bi fractions in the solid, which are achieved by changing the MOVPE growth conditions. The composition of the Ga(AsBi) was determined by comparing frozen lattice annular dark field simulations of different Ga(AsBi) supercells with the experimental HAADF STEM images. The derived compositions are in quantitative agreement with results of HR-XRD (high resolution X-ray diffraction) and SIMS (secondary ion mass spectroscopy) of the same samples. We furthermore show a homogeneous Bi distribution for the investigated samples, which contain up to 5% Bi. By separating the group III intensities from the group V intensities in high resolution HAADF STEM images, we can investigate the group V intensity distribution only. Moreover from the statistical evaluation, we conclude that the Bi is distributed homogeneously across the group V lattice positions, confirming the excellent structural quality of the layers. This result is also important for device applications of dilute bismide alloys, as homogeneous layers are a prerequisite for optimizing optoelectronic applications.

Comprehensive study of Cp2Mg p-type doping of InP with MOVPE growth technique

Bis-cyclopentadienyl magnesium (Cp2Mg) is used as a p-type dopant in Indium Phosphide (InP) which is grown by metalorganic vapor phase epitaxy (MOVPE). The holes carriers concentration and magnesium (Mg) incorporation efficiency are studied using Hall effect measurements, high-resolution X-ray diffraction (HR-XRD), Secondary ion mass (SIMS), Nikon interference contrast Normarski microscope and photoluminescence (PL) spectroscopy-type InP layers with holes concentration up to 2.3 × 1018 cm−3 are obtained. From PL measurements, the doped samples have a near emission band to band (B–B) transition, an Mg-related emission band–acceptor (B–A) and a donor–acceptor (D–A) transition. We demonstrated that an increase of the Mg flow was effective to control the intrinsic n-type doping coming from carbon impurities of the precursors but at the cost of crystallinity degradation originating from too high Mg atoms concentration in the InP lattice.

Crystal phase control in GaAs nanowires: opposing trends in the Ga- and As-limited growth regimes

Here we demonstrate the existence of two distinct regimes for tuning crystal structure in GaAs nanowires from zinc blende to wurtzite using a single process parameter: V/III-ratio, or variation of the group V precursor flow. Extensive previous studies have shown that crystal structure is sensitive to V/III-ratio, and even that it is possible to change structure entirely using this single parameter. However, an open question has remained about whether the observed dependencies are related to growth technique or types of precursors used. Specifically, opposite trends have been reported for molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE): while wurtzite GaAs growth is reported for high nominal V/III-ratio in MBE, zinc blende GaAs is formed in MOVPE under apparently the same parameter change (increasing precursor V/III-ratio). Here we show that these observations are not necessarily contradictory, as it may first appear, by providing a consolidated picture covering all regimes in one MOVPE growth machine only. More precisely, we observe wurtzite formation for medium nominal V/III-ratios with a critical sensitivity to the balance between Ga and As supply. Slight deviations from wurtzite conditions will result in zinc blende formation for either low V/III-ratio in the As-limited regime or high V/III-ratio in the Ga-limited regime. Our observations strongly indicate that the applied growth conditions are the crucial ingredients for crystal structure control in GaAs nanowires rather than the growth technique or precursors used.

Ab initio-based study for surface reconstructions and adsorption behavior on semipolar AlN() surfaces during metal–organic vapor-phase epitaxy growth

The reconstructions and adsorption behavior of Al atoms on semipolar AlN() surfaces under various growth conditions are investigated by first-principles pseudopotential calculations. Our calculations reveal that there are several reconstructions depending on the growth conditions: the surface with Al adatoms is stabilized under H-poor conditions while that with Al–H and N–H bonds is favorable under H-rich conditions. We also find that the adsorption of Al atoms strongly depends on the surface reconstruction. The adsorption of Al atoms on the reconstructed surface under H-poor conditions is found to be much easier than that under H-rich conditions. These results indicate that similarly to the case of a polar AlN(0001) surface the growth of AlN on a semipolar () surface during metal–organic vapor-phase epitaxy is prominent under H-poor conditions.

Electrical compensation by Ga vacancies in Ga2O3 thin films

The authors have applied positron annihilation spectroscopy to study the vacancy defects in undoped and Si-doped Ga2O3 thin films. The results show that Ga vacancies are formed efficiently during metal-organic vapor phase epitaxy growth of Ga2O3 thin films. Their concentrations are high enough to fully account for the electrical compensation of Si doping. This is in clear contrast to another n-type transparent semiconducting oxide In2O3, where recent results show that n-type conductivity is not limited by cation vacancies but by other intrinsic defects such as Oi.