Metalorganic Molecular Beam Epitaxy Growth Research Articles

Growing clean crystals from dirty precursors: Solid-source metal-organic molecular beam epitaxy growth of superconducting Sr2RuO4 films

Ultra-high purity elemental sources have long been considered a prerequisite for obtaining low impurity concentrations in compound semiconductors in the world of molecular beam epitaxy (MBE) since its inception in 1968. However, we demonstrate that a “dirty” solid precursor, ruthenium(III) acetylacetonate [also known as Ru(acac)3], can yield single-phase, epitaxial, and superconducting Sr2RuO4 films with the same ease and control as III–V MBE. A superconducting transition was observed at ∼0.9 K, suggesting a low defect density and a high degree of crystallinity in these films. In contrast to the conventional MBE, which employs the ultra-pure Ru metal evaporated at ∼2000 °C as a Ru source, along with reactive ozone to obtain Ru → Ru4+ oxidation, the use of the Ru(acac)3 precursor significantly simplifies the MBE process by lowering the temperature for Ru sublimation (less than 200 °C) and by eliminating the need for ozone. Combining these results with the recent developments in hybrid MBE, we argue that leveraging the precursor chemistry will be necessary to realize next-generation breakthroughs in the synthesis of atomically precise quantum materials.

Growth of Catalyst-Free Hexagonal Pyramid-Like InN Nanocolumns on Nitrided Si(111) Substrates via Radio-Frequency Metal–Organic Molecular Beam Epitaxy

Hexagonal pyramid-like InN nanocolumns were grown on Si(111) substrates via radio-frequency (RF) metal–organic molecular beam epitaxy (MOMBE) together with a substrate nitridation process. The metal–organic precursor served as a group-III source for the growth of InN nanocolumns. The nitridation of Si(111) under flowing N2 RF plasma and the MOMBE growth of InN nanocolumns on the nitrided Si(111) substrates were investigated along with the effects of growth temperature on the structural, optical, and chemical properties of the InN nanocolumns. Based on X-ray diffraction analysis, highly <0001>-oriented, hexagonal InN nanocolumns were grown on the nitride Si(111) substrates. To evaluate the alignment of arrays, the deviation angles of the InN nanocolumns were measured using scanning electron microscopy. Transmission electron microscopy analysis indicated that the InN nanocolumns were single-phase wurtzite crystals having preferred orientations along the c-axis. Raman spectroscopy confirmed the hexagonal structures of the deposited InN nanocolumns.

Morphology evolution of nano-structured InN grown by MOMBE

In this paper, the evolution of surface morphology and electrical properties during metalorganic molecular beam epitaxy growth of InN on sapphire substrates was investigated. The growth parameters such as growth temperature, flow rate of trimethylindium (TMIn) and AlN buffer were observed to strongly correlated with InN growth. Structural property and surface morphology were analyzed by X-ray diffraction, scanning electron microscopy, atomic force microscopy and transmission electron microscopy, respectively. Electrical and transport properties were obtained by Hall-effect measurement. InN grown directly on sapphire substrate preferred two-dimensional rather than island growth at the growth temperature of 500 °C. Because of residual stress caused by lattice mismatch, however, the thickness of InN with smooth surface was limited at 50 nm. Moreover, In segregation was found under high TMIn flow rate condition. By inserting low-temperature-grown intermediate AlN buffer layer, the structural and electrical properties of InN can be effectively improved. These observations indicate that the growth parameters are essential for engineering the growth of indium nitride.

Superlattice-based quantum devices: from theory to practical applications

The concepts of resonant tunneling and superlattices were first developed by Esaki and Tsu. What started with the new physics of the Esaki tunnel diode has matured into nanoscale engineering of semiconductors superlattices to create whole synthetic band structures. While working at Thomson CSF in France, Manijeh Razeghi went on to develop the metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy growth of superlattice material as reported in my seminal volumes of The MOCVD Challenge. After years of considerable effort to bring this technology to maturity, we now see the results of this formidable new science in almost every electronic and photonic device that we encounter. Among the most successful triumphs are the type-II superlattice photodetectors and quantum cascade lasers – these technologies have demonstrated the beauty of turning fundamental concepts into practical devices, thanks to advanced growth technologies. This enables us to design and realize compact devices capable of mimicking or even exceeding nature. Using superlattice to pioneer the development of quantum systems is driving the research work at the Center for Quantum Devices.

Metal-organic molecular beam epitaxy growth of InN films on highly orientated TCO/Si(1 0 0) substrates

This work reports on the heteroepitaxial growth of wurtzite indium nitride (InN) thin films on highly orientated n-ZnO buffer layer by RF metal-organic molecular beam epitaxy (RF-MOMBE) system. Highly orientated n-ZnO buffer layers were pre-sputtered using RF co-sputtering. We investigated the surface morphology and crystal structure properties by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), respectively. XRD pattern shows InN films were hexagonal wurtzite structure with preferential c-axis orientation. Electrical properties such as carrier concentration and mobility were performed by Hall measurement at room temperature. Particularly, the InN has high growth rate of 1.5 μm/h. SEM images also reveal the InN surface become rough as increasing the flow rate of trimethylindium (TMI) vapor. Meanwhile, the carrier mobility decrease at higher TMI flow rate. The phenomena probably originate from surface imperfection by excess In incorporation in the films. These results indicate that the high-quality buffer layer is essential for engineering the growth of InN on silicon wafer, and it might be also applicable for other lattice-mismatched III–V heteroepitaxial systems.

Influence of growth conditions and surface reaction byproducts on GaN grown via metal organic molecular beam epitaxy: Toward an understanding of surface reaction chemistry

The surface reaction byproducts during the growth of GaN films via metal organic molecular beam epitaxy (MOMBE) were investigated as a means to optimize material properties. Ethylene and ethane were identified as the dominant surface reaction hydrocarbon byproducts, averaging 27.63% and 7.15% of the total gas content present during growth. Intense ultraviolet (UV) photoexcitation during growth was found to significantly increase the abundance of ethylene and ethane while reducing the presence of H2 and N2. At 920°C, UV excitation was shown to enhance growth rate and crystalline quality while reducing carbon incorporation. Over a limited growth condition range, a 4.5×1019−3.4×1020 cm−3 variation in carbon incorporation was achieved at constant high vacuum. Coupled with growth rate gains, UV excitation yielded films with ∼58% less integrated carbon content. Structural material property variations are reported for various ammonia flows and growth temperatures. The results suggest that high carbon incorporation can be achieved and regulated during MOMBE growth and that in-situ optimization through hydrocarbon analysis may provide further enhancement in the allowable carbon concentration range.

Metalorganic molecular beam epitaxy of ZnO using DEZn and H 2O precursors

Metalorganic molecular beam epitaxy (MOMBE) growth of ZnO was carried out on the c-plane of sapphire substrate using diethylzinc (DEZn) and H 2O as source precursors. ZnO layers were grown at substrate temperature ( T s) between 200°C and 550°C. The c-axis oriented epitaxial ZnO layers were grown at T s between 375°C and 400°C. Photoluminescence (PL) spectra at 8 K exhibited the neutral donor bound exciton peak at 3.36 eV. The PL intensity was maximum and the half-width was minimum (7 meV) for T s of about 400°C. These results showed that the quality of the ZnO layer grown by this MOMBE system was best at T s of about 400°C.

Initial growth monitoring of GaN epitaxy on 6H-SiC by metal-organic molecular beam epitaxy

The lattice constants of GaN layers along the a-axis were estimated using reflection high-energy electron diffraction (RHEED) patterns during metal-organic molecular beam epitaxy (MO-MBE) growth. The lattice relaxation of GaN layers was observed during the initial growth. The crystal structure changed from the mixed (cubic and hexagonal) to the hexagonal phase at the high growth temperature of 814°C. On the other hand, the crystal structure changed from the hexagonal to the mixed phase at the low growth temperature of 772°C.

Effects of increased growth rate of well layer on 2.1% compressive strained InAsP-MQWs grown by metalorganic molecular beam epitaxy (MOMBE)

Effects of growth rate on metalorganic molecular beam epitaxy (MOMBE) growth of 2.1% compressive strained InAsP-multi-quantum wells (MQWs) was investigated using photoluminescence (PL), double-crystal X-ray diffraction (DCXRD) analysis and cross-sectional transmission electron microscopy (XTEM). As the growth rate of a well layer increases from 0.17 to 0.33 nm/s, the PL intensity increases remarkably and the width of satellite peaks in the DCXRD spectra becomes narrower. The amplitude of a well layer surface undulation, ascertained by XTEM, decreases as the growth rate increases, simultaneously. This indicates that increasing of the PL intensity and narrowing of the satellite peak width are attributed to decreasing of amplitude of the well layer surface undulation caused by increasing the growth rate. Utilizing this finding, we can increase the number of strained InAsP wells up to 20 without any evidence of material degradation due to well layer surface undulation. In addition, we fabricated buried heterostructure MQW lasers emitting a 1.56 μm wavelength light with a low threshold current of 7.0 mA and a large differential gain of 9.0×10 −16 cm 2.

A study of void defects in metalorganic molecular-beam epitaxy grown HgCdTe

Void defects that occur under Hg deficient conditions during the metalorganic molecular beam epitaxy (MOMBE) growth of HgCdTe have been characterized using secondary electron microscopy (SEM) and energy dispersion spectrometry (EDS) mapping as well as by EDS quantitative analysis. For a set of HgCdTe samples grown under a range of Hg fluxes, it was found that the surface morphology had a significant dependence on the Hg flux. An optimum growth window defined by a narrow range of Hg fluxes was identified in which there exists a smooth surface with few voids, whereas at either side of the Hg window surfaces were rough. This surface morphology correlated very well with a minimum in the x-ray line widths and maximum hole concentration and mobility values. This correlation is important for the growth of HgCdTe materials and subsequent device fabrication. Several types of void morphologies have been observed with different correlation to Te and Hg. It was found that there is a pronounced Te enrichment and Hg deficiency associated with most of the developed voids, as compared to the composition of the HgCdTe films. It was also found that most of the voids originated within the HgCdTe film. A mechanism for void formation and growth is proposed. In addition, it was found that annealing caused the voids to separate from the HgCdTe film.

Growth of InP using TBP and DTBP in metalorganic molecular beam epitaxy

The growth of high purity InP layers using trimethylindium (TMIn) and different commercial tertiarybutylphosphine (TBP) batches as well as ditertiarybutylphosphine (DTBP) as a new phosphorous precursor was studied in a production type metalorganic molecular beam epitaxy (MOMBE) growth chamber. The cracking behaviour of TBP was investigated by mass spectrometry. Increasing the beam equivalent pressure from 10 −6 Torr to growth relevant pressures in the 10 −5 Torr range, a transition of the pyrolysis regime was observed, accompanied with a reduced H 2+P 2 production and a strong formation of the hydride PH 3 and dihydride P 2H 4. This behaviour depends on the conditions on the cracker surface and indicates that the decomposition of TBP occurs by a radical formation process. At 77 K the deposited InP layers exhibited Hall mobilities better than 100 000 cm 2/V s for the use of three TBP batches from two different suppliers with a best value of 167 000 cm 2/V s at n=1.4×10 14 cm −3. The linewidth of the donor bound exciton in the 1.8 K photoluminescence spectrum of the sample with the highest electron mobility was about 0.09 meV. The first use of DTBP in MOMBE for the growth of high purity InP was also encouraging. The best value of the Hall mobility at 77 K was 77 300 cm 2/V s with n=5.1×10 14 cm −3.

Differential gain and threshold current of 1.3 μm tensile-strained InGaAsP multi quantum well buried-heterostructure lasers grown by metalorganic molecular beam epitaxial growth

The laser characteristics of 1.3 μm tensile-strained InGaAsP/InP multi quantum well (MQW) lasers were studied. The tensile strain of well layers ranged from 0.5% to 1.8%. The level of the strain dependence of the threshold current shows that the threshold current decreases drastically with increase in the tensile strain from 0.5% to 1.3%, while it stays almost constant in the 1.45%–1.8% range. Among all the samples, the 1.3% tensile-strained MQW laser shows superior performance in terms of minimum threshold current and maximum differential gain (∂g/∂N). The 1.3% tensile strain is the optimum level for the tensile-strained MQW buried-heterostructure lasers.

MOMBE growth of InGaAs using trisdimethylaminoarsenic

Abstract InGaAs layers were grown by metalorganic molecular beam epitaxy (MOMBE) using trimethylindium (TMIn), triethylgallium (TEGa) and unprecracked trisdimethylaminoarsenic (TDMAAs). Substrate temperature dependence of the growth rate and In composition of InGaAs for the fixed TMIn, TEGa and TDMAAs flow rates are similar to those for the use of TMIn, TEGa and thermally cracked arsine (AsH3). Mirror-like surface In0.53Ga0.47As layers lattice matched to InP are successfully grown, for the first time, using TDMAAs as an As precursor. Growth rate is 0.3 μm/h and electron concentration is 3 × 1016cm−3. 77 K photoluminescence spectrum has a band-to-band transition peak with a full width at half-maximum as narrow as 9.8 meV.

Growth rate enhancement of III–V antimonides on high-index substrates in metalorganic molecular beam epitaxy

Metalorganic molecular beam epitaxy growth of GaSb, InSb and AlSb on ( N 1 1) high-index substrates is studied. The growth rates of antimonides on (1 0 0) substrates are generally low because the decomposition of group III species is suppressed by the excessively adsorbed antimony atoms on the surface, i.e. the site-blocking effect. However, in the growth of GaSb using triethylgallium (TEGa) and elemental antimony, growth rates are enhanced on (3 1 1), (4 1 1) and (5 1 1) substrates. This substrate orientation dependence is also observed in the InSb growth using trimethylindium (TMIn) at low substrate temperatures, although with increasing substrate temperature, the growth rate becomes independent of substrate orientation due to the complete thermal decomposition of TMIn. These results imply that the decomposition of TEGa and TMIn is enhanced at atomic steps consisting of (1 0 0) and (1 1 1) planes and that the growth on high-index substrates is effective for the low-temperature growth of GaSb and InSb. On the other hand, the growth rate of AlSb using trimethylamine alane (TMAAl) is independent of substrate orientation because of low-temperature (∼ 100°C) decomposition of TMAAl.

Optimization of the structural properties of Hg1−x CdxTe (x = 0.18−0.30) alloys: Growth and modeling

The conditions for the metalorganic molecular beam epitaxial growth of Hg1−xCdxTe (x = 0.18−0.32) alloys at very low growth temperatures (T ≤150°C) have been optimized by correlating the surface properties and crystalline perfection with the incident Hg flux. A window for growth has been defined for x = 0.18, 0.23, and 0.32. A thermodynamic model has been developed to account for void formation. A neural net model has been used for the first time to model the dependence of void density on the Hg flux and the x-ray rocking curve widths on growth parameters. The combination of these two complementary modeling techniques allows for a flexible process optimization to be carried out with a minimum effort spent in calibration runs.

MOMBE growth of highly tensile-strained InGaAsP MQWs and their applications to 1.3-μm wavelength low threshold current lasers

We have studied metalorganic molecular beam epitaxy (MOMBE) of InGaAsP multi-quantum wells (MQWs) with 0.5-1.8% tensile-strained well layers. The MQW photoluminescence intensity depends significantly on the barrier layer strain to compensate well layer strain. Optimizing the strain compensation conditions, i.e., keeping the net strain less than 0.3%, makes it possible to grow high optical quality MQWs with 1.8%-strained, 10-well layers. A critical thickness-net strain (t c -e*) curve for the tensile-strained MQWs has been experimentally determined by analyzing more than 50 samples with various strains and total thicknesses. The t c -e* curve fairly agrees with the one calculated by the Matthews formula for more 0.5% net strain, but the t c values are much higher than the expected ones below that range. For 6-well MQW lasers, the threshold current densities, J th 's, decrease with increasing tensile strain; the minimum J th obtained is 0.6 kA/cm 2 at 1.3% strain.

New Ge substrate cleaning method for Si 1−x−yGe xC y MOMBE growth

A new method of cleaning a Ge wafer based on a wet chemical treatment and Si 1−x−y Ge x C y /Ge(001) metalorganic molecular beam epitaxial (MOMBE) growth has been performed. It consists of removing the native oxide by ammonia aqueous followed by a diluted sulfuric acid rinse and hydroperoxide treatment to form a passive oxide layer. An extremely uniform surface was obtained by 300 s ammonia etching of the native oxide. Thickness of the oxide layer grown in hydroperoxide is estimated to be ≈1.87 nm using angle-resolved X-ray photoelectron spectroscopy measurement. A well-ordered 2×1 reflection high-energy electron diffraction pattern appeared after 400 °C, 30 min annealing in ultra high vacuum. Si 1−x−y Ge x C y MOMBE growth has been performed on cleaned Ge(001) using disilane (Si 2H 6) and monomethylgermane (CH 6Ge). The 〈001〉 axis of the ternary alloy layers is oriented toward Ge(001). No peak corresponding to SiC was detected in X-ray diffraction data. © 1997 Elsevier Science S.A. All rights reserved.

Growth of 1.55 μm DH laserstructures using TBAs and TBP in MOMBE

In a comparative study we have chosen TBAs and TBP as well as AsH 3 and PH 3 for the growth of InP/GaInAs(P) heterostructures for laser applications in a production metalorganic molecular beam epitaxy (MOMBE) system. The n-type doping was performed with Si from an effusion cell, whereas for the p-type doping Be and DEZn were utilized. InP layers using TBP under optimized cracking conditions exhibit excellent surface morphology with good electrical properties in the low 10 15 cm -3 range of carrier concentrations. The MOMBE growth mechanism is not disturbed by the hydride replacement compound. This allows for a convenient replacement without losing calibration data from the hydride process. Broad-area DH laserstructures with GaInAsP (λ = 1.55 μm) active regions were grown with AsH 3 /PH 3 and TBAs/TBP. Comparable threshold current densities in the range of 1.6-2.3 kA/cm 2 are achieved for the lasers, grown with both sets of precursors combined with DEZn source doping. These results are in good agreement with the standard set by the hydride MOVPE process.

Metalorganic molecular beam epitaxy growth and etching of GaSb on flat and high-index surfaces using trisdimethylaminoantimony

Metalorganic molecular beam epitaxy growth and etching of GaSb using triethylgallium (TEGa), trisdimethylaminoantimony (TDMASb) and elemental antimony (Sb 4) on flat and high-index surfaces are investigated. A higher growth rate is obtained on the ( n11) surface ( n = 5, 4, 3) using TEGa and Sb 4, indicating that the incorporation efficiency of Ga atoms is enhanced on these surfaces. This is explained by taking into account the steps where the decomposition of TEGa is enhanced. When only TDMASb is supplied, GaSb surfaces are etched for all misoriented GaSb substrates studied. On the other hand, when TEGa is simultaneously supplied in addition to TDMASb, the surface reaction is changed from etching to growth on the n = 5, 4, 3 surfaces. This is because the growth of GaSb is increased more than the etching by TDMASb on these surfaces. 77 K photoluminescence spectra show a sharp band-to-band transition peak at about 804 meV with a full width at half maximum of less than 9 meV indicating good optical quality. Unintentionally doped GaSb epilayers show p-type conduction with a 77 K hole mobility of 820 cm 2/V s and a hole concentration as 1.2 × 10 16cm −3.

Kinetics of chemical beam epitaxy for high quality ZnS film growth

A comprehensive study is reported of the chemical and metalorganic molecular beam epitaxy ( CBE MOMBE ) growth of ZnS on Si and GaAs(100) substrates for applications in advanced electroluminescent displays and for optoelectronic device integration on Si. Growth kinetics studies of conventional and photoassisted MOMBE and CBE using diethylzinc (DeZn) with H 2S, di-isopropylsulfide (DipS) and a novel precursor, t-butyl mercaptan (t-BuSH), are reported. The results obtained indicated that the low temperature growth of ZnS using cracked DeZn and DipS was hindered. This was attributed to the reattachment of the alkyl radicals to surface growth sites, which resulted in a lower growth rate. However, by using uncracked DeZn and H 2S or cracked DeZn and t-BuSH, this growth hindrance was removed by the surface reaction of hydrogen with the alkyl radicals. The addition of a thin CaF 2 buffer layer was found to remarkably improve the growth of ZnS on Si.