Lateral epitaxial overgrowth of GaN films on SiO2 areas via metalorganic vapor phase epitaxy

Lateral epitaxial growth and coalescence of GaN regions over SiO₂ masks previously deposited on GaN/AlN/6H-SiC(0001) substrates and containing 3 μm wide rectangular windows spaced 7 μm apart have been achieved. The extent and microstructural characteristics of these regions of lateral overgrowth were a complex function of stripe orientation, growth temperature, and triethylgallium (TEG) flow rate. The most successful growths were obtained from stripes oriented along 〈1 00〉 at 1100°C and a TEG flow rate of 26 μmol/min. A density of ∼10⁹ cm⁻² threading dislocations, originating from the underlying GaN/AlN interface, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The surfaces of the coalesced layers had a terrace structure and an average root mean square roughness of 0.26 nm.     

Effects of substrate orientation and surface reconstruction on patterned substrate OMVPE of GaAs

Organometallic vapor phase epitaxial growth of GaAs on 320 nm high mesas was used to study the dependence of lateral growth upon the substrate misorientation from (100) and the mesa wall orientation on the substrate. GaAs (100) substrates were misoriented by 3° toward eight major crystallographic directions, consisting of the four nearest [111] and [110] directions. The mesa sidewalls were oriented either parallel to the 〈011〉 and 〈01 〉 directions or rotated by 45° to be parallel to the 〈001〉 and 〈010〉 directions. GaAs films were grown with TMGa and TBA at T=575°C. The lateral growth rates were up to 25 times higher than the vertical growth rate of 1.3 μm/hour. Optical microscopy and atomic force microscopy (AFM) showed that under the given growth conditions lateral growth off mesa sidewalls is most rapid in the 〈011〉 and/or 〈0 〉 directions and less in the perpendicular 〈01 〉 and 〈0 1〉 directions (lateral growth anisotropy). By raising the temperature to 625°C lateral growth in the 〈01 〉 -〈0 1〉 directions increased while it remained almost constant in the 〈011〉 -〈0 〉 directions. Published results show that the partial pressure of As also affects lateral growth. Differences in the lateral growth rates in the 〈011〉 and its opposite 〈0 〉 directions result from substrate misorientation but not from the orientation of the mesa walls on the substrate. Anisotropic lateral growth rates in different crystallographic directions appear to be caused by both, (1) 1-dimensional Ga diffusion defined by surface reconstruction, and (2) a relatively low energy barrier to atoms flowing over high-to-low terrace steps. A lateral growth model is proposed that describes anisotropic lateral growth at mesa sidewalls in terms of growth conditions and substrate misorientations. The model also explains the difference in the preferential lateral growth directions between MBE and OMVPE.     

Pendeo-epitaxial growth of gallium nitride on silicon substrates

Pendeo-epitaxy (PE)¹ from raised, [0001] oriented GaN stripes covered with silicon nitride masks has been employed for the growth of coalesced films of GaN(0001) with markedly reduced densities of line and planar defects on Si(111)-based substrates. Each substrate contained previously deposited 3C-SiC(111) and AlN(0001) transition layers and a GaN seed layer from which the stripes were etched. The 3C-SiC transition layer eliminated chemical reactions between the Si and the NH₃ and the Ga metal from the decomposition of triethylgallium. The 3C-SiC and the GaN seed layers, each 0.5 μm thick, were also used to minimize the cracking and warping of the GaN/SiC/silicon assembly caused primarily by the stresses generated on cooling due to the mismatches in the coefficients of thermal expansion. Tilting in the coalesced GaN epilayers of 0.2° was confined to areas of lateral overgrowth over the masks; no tilting was observed in the material suspended above the trenches. The strong, low-temperature PL band-edge peak at 3.456 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on AlN/6H-SiC(0001) substrates.     

Growth and Characterization of Unintentionally Doped GaSb Nanowires

GaSb nanowires were synthesized on c-plane sapphire substrates by gold-mediated vapor–liquid–solid (VLS) growth using a metalorganic chemical vapor deposition process. A narrow process window for GaSb nanowire growth was identified. Chemical analysis revealed variations in the catalyst composition which were explained in terms of the Au-Ga-Sb ternary phase diagram and suggest that the VLS growth mechanism was responsible for the nanowire growth. The nominally undoped GaSb nanowires were determined to be p-type with resistivity on the order of 0.23 Ω cm. The photoluminescence was found to be highly dependent on the V/III ratio, with an optimal ratio of unity.     

Characteristics of GaN stripes grown by selective-area metalorganic chemical vapor deposition

We report on the selective-area metalorganic chemical vapor deposition of GaN stripes in the size range of 50 to 125 urn and the characterization of the morphology, topography, and optical properties of these stripes. GaN films (∼1–3 μm) grown on (0001) sapphire are used as the substrates. Excellent surface morphology is achieved under optimized growth conditions which include a higher V/III ratio than broad area growth. It is found that, under certain growth conditions, (0001) terraces of ~5 μm in width develop at the edges of all stripes, independent of stripe size and orientation. The selectively grown GaN yields stronger band-edge emission than the “substrate” GaN which indicates an improvement in optical quality. However, the donor-acceptor pair recombination (or conduction band to acceptor transition) and yellow emission are also enhanced in certain areas of the stripes. The spatial correlation of these emission bands is established by cathodoluminescence wavelength imaging, and the origin of these emissions is speculated.     

Growth of high quality of ZnSe epilayers on (001) vicinally oriented GaAs substrate by molecular beam epitaxy

We have investigated the structural properties of ZnSe epilayers that were grown by molecular beam epitaxy on (001) GaAs substrate with different tilt angles. Two-dimensional growth mode increased with increasing tilt of (001) GaAs toward [010] direction. This was confirmed by atomic force microscopy (AFM) analysis and double crystal X-ray diffraction. We have obtained ZnSe of high crystalline quality layers on (001) GaAs off oriented toward [010] direction. Light emitting devices that were fabricated by ZnSe wafers grown on 4° off toward [010] GaAs substrate have longer lifetimes than the ones that were grown on nominal (001) GaAs substrate, which provide an evidence that crystalline quality improvement effects may become apparent by using (001) vicinal substrate surfaces.     

Formation of InAs quantum dots in a GaAs matrix during growth on misoriented substrates

The formation of InAs quantum dots grown by submonolayer migration-enhanced molecular-beam epitaxy on GaAs(100) surfaces with various misorientation angles and directions is investigated. It is shown for the deposition of 2 monolayers (ML) of InAs that increasing the misorientation angle above 3° along the [010], , and [011] directions leads to the formation of several groups of quantum dots differing in geometric dimensions and electronic structure. Fiz. Tekh. Poluprovodn. 32, 95–100 (January 1998)     

Contrast modulations in InAIAs/InP

This work reports the transmission electron microscopy characterization of matched and mismatched In_xAl_1−xAs layers grown in a range of high temperatures (570-590°C) on (100) InP substrates. We have observed two different coarse contrast modulations both oriented along the 〈010〉 directions. The appearance of such structures in cross-section and plan view orientations is described. We also show the evolution of these contrast modulations as approaching the substrate. The origin of one of these coarse patterns has been related to composition inhomogeneities driven by segregation and desorption phenomena at high temperature, and the other to composition modulations.     

A study of chemical beam epitaxy of GaAs using tris-dimethylaminoarsenic

The growth of GaAs by chemical beam epitaxy using triethylgallium and trisdimethylaminoarsenic has been studied. Reflection high-energy electron diffraction (RHEED) measurements were used to investigate the growth behavior of GaAs over a wide temperature range of 300–550°C. Both group III- and group Vinduced RHEED intensity oscillations were observed, and actual V/III incorporation ratios on the substrate surface were established. Thick GaAs epitaxial layers (2–3 μm) were grown at different substrate temperatures and V/III ratios, and were characterized by the standard van der Pauw-Hall effect measurement and secondary ion mass spectroscopy analysis. The samples grown at substrate temperatures above 490°C showed n-type conduction, while those grown at substrate temperatures below 480°C showed p-type conduction. At a substrate temperature between 490 and 510°C and a V/III ratio of about 1.6, the unintentional doping concentration is n ∼2 × 10¹⁵ cm⁻³ with an electron mobility of 5700 cm²/V·s at 300K and 40000 cm²/V·s at 77K.     

Large-Area Direct Hetero-Epitaxial Growth of 1550-nm InGaAsP Multi-Quantum-Well Structures on Patterned Exact-Oriented (001) Silicon Substrates by Metal Organic Chemical Vapor Deposition

We employ a simple two-step growth technique to grow large-area 1550-nm laser structures by direct hetero-epitaxy of III–V compounds on patterned exact-oriented (001) silicon (Si) substrates by metal organic chemical vapor deposition. Densely-packed, highly uniform, flat and millimeter-long indium phosphide (InP) nanowires were grown from Si v-grooves separated by silicon dioxide (SiO₂) stripes with various widths and pitches. Following removal of the SiO₂ patterns, the InP nanowires were coalesced and, subsequently, 1550-nm laser structures were grown in a single overgrowth without performing any polishing for planarization. X-ray diffraction, photoluminescence, atomic force microscopy and transmission electron microscopy analyses were used to characterize the epitaxial material. PIN diodes were fabricated and diode-rectifying behavior was observed.     

High-Dose Phosphorus-Implanted 4H-SiC: Microwave and Conventional Post-Implantation Annealing at Temperatures ≥1700°C

Semi-insulating 4H-SiC ⟨0001⟩ wafers have been phosphorus ion implanted at 500°C to obtain phosphorus box depth profiles with dopant concentration from 5 × 10¹⁹ cm⁻³ to 8 × 10²⁰ cm⁻³. These samples have been annealed by microwave and conventional inductively heated systems in the temperature range 1700°C to 2050°C. Resistivity, Hall electron density, and Hall mobility of the phosphorus-implanted and annealed 4H-SiC layers have been measured in the temperature range from room temperature to 450°C. The high-resolution x-ray diffraction and rocking curve of both virgin and processed 4H-SiC samples have been analyzed to obtain the sample crystal quality up to about 3 μm depth from the wafer surface. For both increasing implanted phosphorus concentration and increasing post-implantation annealing temperature the implanted material resistivity decreases to an asymptotic value of about 1.5 × 10⁻³ Ω cm. Increasing the implanted phosphorus concentration and post-implantation annealing temperature beyond 4 × 10²⁰ cm⁻³ and 2000°C, respectively, does not bring any apparent benefit with respect to the minimum obtainable resistivity. Sheet resistance and sheet electron density increase with increasing measurement temperature. Electron density saturates at 1.5 × 10²⁰ cm⁻³ for implanted phosphorus plateau values ≥4 × 10²⁰ cm⁻³, irrespective of the post-implantation annealing method. Implantation produces an increase of the lattice parameter in the bulk 4H-SiC underneath the phosphorus-implanted layer. Microwave and conventional annealing produce a further increase of the lattice parameter in such a depth region and an equivalent recovered lattice in the phosphorus-implanted layers.     

Growth and characterization of Ga1−xInxAs by low pressure metalorganic chemical vapor deposition

Conditions for growth at 550°C of high structural quality GaInAs by LPMOCVD are presented. The sensitivity of compositional grading to changes in the V/III molar ratio, growth rate and inclusion of InP buffer layers is discussed. Crystalline uniformity is indicated by double crystal x-ray rocking curves with FWHM (GaInAs) = 0.017°. By careful control of the V/III molar ratio, epitaxial GaInAs/InP heterostructures with δa/a ≤ 10⁻⁴ can be grown. Quantitative data for the TEIn-AsH₃ elimination reaction rate is presented. The composition of Ga_1−xIn_xAs which is expected in the presence of this reaction is calculated; evaluation of the corresponding rate constant shows that the adduct formation reaction proceeds at a modest but detectable rate. The problems associated with the purity of electronic grade triethylindium (TEIn) are addressed. Impurities in commercial TEIn have been determined by low resolution mass spectroscopy.     

The role of the V/III ratio in the growth and structural properties of metalorganic vapor phase epitaxy GaAs/Ge heterostructures

GaAs epitaxial layers have been grown on (001) 6† off-oriented toward (110) Ge substrates by metalorganic vapor phase epitaxy. In order to study the influence of V/III ratio on the growth mechanisms and the structural properties of the layers, the input flow of arsine was changed over a wide range of values, while keeping constant all other experimental settings. Optical microscopy in the Nomarski contrast mode, x-ray topography and high resolution diffractometry, transmission electron microscopy and Rutherford backscattering have been used to investigate the epilayers. It has been found that the growth rate increases and the surface morphology worsens with increasing V/III ratio. The abruptness of the layer-substrate interface has also been found to strongly depend on the V/III ratio, the best results being obtained under Ga-rich conditions. The main structural defects within the layers are stacking faults and misfit dislocations. Layers grown under As-rich conditions only contain stacking faults, probably originated by a growth island coalescence mechanism, whereas layers grown under Ga-rich conditions contain both misfit dislocations and stacking faults generated by dissociation of threading segments of interfacial dislocations. In spite of the different defects, the strain relaxation has been found to follow the same trend irrespective of the V/III ratio. Finally, the relaxation has been found to start at a thickness exceeding the theoretical critical value.     

A novel processing technique to fabricate planar InGaAsP/InP electroabsorption waveguide modulators

A novel processing technique has been developed to fabricate planar electroabsorption waveguide modulators in compound semiconductor heterostructures. The lateral confinement of light is achieved by introducing controllable, reproducible, and stable stresses into semiconductor heterostructures using WNi surface stressor stripes, which also serve as electrodes for the waveguide modulators. Self-aligned helium implantation is employed to achieve electrical isolation using the Stressors as the templates for the ion masks. An increase as large as 33000 times has been obtained in the dc resistance between the neighboring waveguide modulators 25 μm apart. Propagation loss of 1.7 dB/cm is observed in the photoelastic waveguides at a wavelength of 1.53 μm following the He implantation. A post-implant thermal annealing at 310°C for 40 min increases the dc resistance between the neighboring devices to the maximum value, and at the same time reduces the optical loss to its value before ion implantation (less than 1 dB/cm). Using a combination of the photoelastic effect and helium implantation, planar InGaAsP/InP Franz-Keldysh-effect waveguide modulators 430 μm long with a 10 dB extinction ratio at 3 V for the TM mode have been fabricated. Planar electroabsorption quantum-confined Stark effect waveguide modulators have also been demonstrated. This planar device processing technique may prove valuable in future photonic integrated circuit technology.     

Characterization of InGaAs/GaAs(001) films grown by metalorganic vapor phase epitaxy using alternative sources

Thin films of In_xGa_1−xAs (0<x<0.012) on GaAs (001) were grown by metalorganic vapor phase epitaxy using triisopropylindium, triisobutylgallium, and tertiarybutylarsine. The effect of the process conditions, temperature, and V/III ratio on the film quality was studied using high resolution x-ray diffraction, scanning tunneling microscopy, and Hall measurements. High quality films were grown at temperatures as low as 475 °C and at a V/III ratio of 100. However, under these conditions, a change in growth mode from step flow to two-dimensional nucleation was observed.     

A RHEED study of the transition from two-dimensional to three-dimensional growth in the InAs/GaAs system

A specially developed detection system and an analysis of RHEED diffraction patterns were used to investigate the dynamics of transition from two-dimensional to three-dimensional growth mechanism in the heteroepitaxial InAs/GaAs system. An analysis of the dynamics of the diffraction patterns was used for the first time to investigate the dynamics of formation of quantum dots. A time shift in the dynamic behavior of the diffracted intensity for diffraction patterns recorded at different angles was found. This shift is explained in terms of the size differences in the three-dimensional islands at the initial stage of decay of the pseudomorphic layer. InAs/GaAs quantum dots grown under certain conditions produce reflections at 45° relative to the principal reflections. This is evidence for the ordering of islands in the [001] and [010] crystallographic directions. Fiz. Tekh. Poluprovodn. 31, 1230–1232 (October 1997)     

Structural and Morphological Investigation of Pendeo-Epitaxy 3C-SiC on Si Substrates

Successful pendeo-epitaxy growth of cubic silicon carbide (3C-SiC) on off-axis Si(001) substrates was achieved. The structural and morphological characteristics of pendeo-epitaxy 3C-SiC were strongly affected by underlying stripes and seed layer thickness. Stripes perpendicular to the Si substrate off-axis provide about three times faster lateral growth rate compared with parallel oriented stripes. Root-mean-square (RMS) measurements using atomic force microscope (AFM) indicate that the surface morphology of Pendeo-epitaxy 3C-SiC films remarkably improves with increasing seed layer thickness: from 9.8 nm for 3 μm thickness to 0.5 nm for 10 μm thickness. These effects on pendeo-epitaxy 3C-SiC are discussed with scanning electron microscopy (SEM) and AFM investigation.     

Growth of InAs quantum dots on vicinal GaAs(001) surfaces misoriented in the [010] direction

Atomic-force microscopy (AFM) is used to study InAs quantum-dot structures grown by molecular-beam epitaxy (MBE) on vicinal GaAs(001) surfaces misoriented in the [010] direction by 1, 2, 4, and 6°. It is shown for a chosen misorientation direction that a vicinal GaAs(010) surface is covered with a network of stepped terraces. The thickening of the network of terraces with increasing misorientation angle leads to the suppression of adatom surface diffusion and makes it possible to achieve higher densities and more uniform ensembles of quantum dots, while simultaneously decreasing the probability of their coalescence. Fiz. Tekh. Poluprovodn. 32, 860–865 (July 1998)     

Basic studies of gallium nitride growth on sapphire by metalorganic chemical vapor deposition and optical properties of deposited layers

We have studied the growth of gallium nitride on c-plane sapphire substrates. The layers were grown in a horizontal metalorganic chemical vapor deposition reactor at atmospheric pressure using trimethylgallium (TMG) and ammonia (NH₃). Variation of the V/III ratio (150–2500) shows a distinct effect on the growth rate. With decreasing V/III ratio, we find an increasing growth rate. Variation of the growth temperature (700–1000°C) shows a weak increase in growth rate with temperature. Furthermore, we performed secondary ion mass spectroscopy measurements and find an increasing carbon incorporation in the GaN films with decreasing ammonia partial pressure and a growing accumulation of carbon at the substrate interface. Photoluminescence measurements show that samples with high carbon content show a strong yellow luminescence peaking at 2.2 eV and a near band gap emission at 3.31 eV. With increasing carbon content, the intensity of the 3.31 eV line increases suggesting that a carbon related center is involved.     

Crystallographic texture of C54 titanium disilicide as a function of deep submicron structure geometry

Detailed analysis of the crystallographic texture of C54 TiSi₂ was performed and showed a strong correlation between the geometry of the silicide structures and preferential crystallographic orientation. The study was undertaken on blanket and patterned TiSi₂ films formed in the reaction between 32 nm of Ti and undoped polycrystalline silicon using both in situ x-ray diffraction during heating and post-anneal four-circle pole figure reflection geometry measurements. Full pole figures were taken to determine the distribution of C54 TiSi₂ grain orientations in narrow (0.2 to 1.1 μm) lines which was compared with similar results obtained from unpatterned (blanket) films. While in blanket films we found the presence of weak <311> C54 TiSi₂ crystallographic orientation perpendicular to the sample surface, the <040> preferential orientation dominated in patterned submicron line structures and increased with decreasing linewidth. Using pole figure analysis, we observed strong <040> fiber texture in narrow lines with a slight variation in the tilt of the (040) planes normal in the direction perpendicular to the line (full width at half maximum [FWHM] ≈6°), but very little along the length of the line (FWHM ≈2°). In addition, a preferred in-plane (azimuthal) orientation of <040> crystals was found which showed that most of the <040> grains had their (004) plane normals oriented parallel with the line direction. These findings support a model of the C49 to C54 TiSi₂ transformation involving rapid growth of certain orientations favored by the one-dimensional geometry imposed by narrow lines.