Surface Morphology Of Epilayers Research Articles

Comparison of SiC sublimation epitaxial growth in graphite and TaC coated crucibles

The growth behaviour in graphite crucibles of different constructions and crucibles coated with TaC are studied with respect to growth performances such as growth rate, epilayer thickness variation and doping as well as deep levels. The variation of growth rate and the epilayer surface morphology with crucible position relative to the coil used for induction heating to applied growth temperature is studied. At low growth rate defects extended along the steps are observed.

Silicon effect on GaN surface morphology

Si-doped GaN epitaxial layers have been grown at 1050°C on optimized-AlN buffered (0001) sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy. In order to investigate the Si effect on the surface morphology of GaN epilayers, several samples were grown by varying the silane partial pressure. When the silane partial pressure increases above 1.7×10−8atm, the surface quality becomes rough. This shows the Si surfactant effect. A correlation between an in situ laser reflectometry and ex situ optical and atomic force microscopy characterizations on the one hand and between electrical properties and surface quality on the other hand were made. As the electron concentration increases, the surface becomes more and more rough and the mobility drops dramatically.

Surface Morphology of GaN Epilayer with Si<sub>x</sub>N<sub>1-x</sub> Buffer Layer Grown by Ammonia-source MBE

Surface Morphology of GaN Epilayer with Si<sub>x</sub>N<sub>1-x</sub> Buffer Layer Grown by Ammonia-source MBE

GaN epilayers and AlGaN/GaN multiple quantum wells grown on freestanding [1100] oriented GaN substrates

AbstractWe report on the homoepitaxial growth of GaN on freestanding [1100] oriented GaN substrates using metalorganic chemical vapor deposition. A proper pretreatment of the substrates was found to be essential for the GaN homoepitaxy. The influence of growth conditions such as V/III molar-ratio and temperature on the surface morphology and optical properties of epilayers was investigated. Optimized pretreatment and growth conditions led to high quality [1100] oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Based on these GaN epilayer, AlGaN/GaN multiple quantum wells have been grown on the freestanding M-plane GaN. Photoluminescence data confirm that built-in electric field for M-plane structures is very weak, and this situation results in a stronger PL intensity in comparison with C-plane multiple quantum wells in tests at low excitation level.

Improvement in the quality of ZnSe epilayers grown on (0 0 1) GaAs by the low temperature growth of a thin ZnSe buffer layer

The surface morphology, optical and structural properties of ZnSe epilayers grown on (0 0 1) GaAs at 450°C using metalorganic chemical vapor phase deposition are found to be greatly influenced by the initial growth of a thin ZnSe buffer layer at the low temperature of 360°C. Moreover, there is an optimum thickness for the buffer layer on which the quality of the epilayer grown is significantly better, as indicated by four independent quality gauges. In comparison to epilayers grown without a buffer layer, epilayers grown on a buffer layer of optimum thickness around 28 nm show: (1) about 3 orders of magnitude reduction in the density of growth hillocks, (2) narrow line width and about 3 orders of magnitude increase in the intensity of excitonic photoluminescence, (3) much narrower X- ray rocking curve peaks, and (4) mono-layer high surface steps with atomically smooth terraces in between.

Evolution of island–pit surface morphologies of InAs epilayers grown on GaAs (001) substrates

We report on a study of the morphological evolution of InAs layers grown on GaAs (001) substrates by molecular-beam epitaxy under In-rich conditions. The surface morphology of the InAs layers is characterized by a feature of island–pit combinations. We show that the vertical sizes of the islands and pits can grow simultaneously beyond the average layer thickness, up to several hundred nanometers. The composition of the islands is found to be ternary InxGa1−xAs rather than the expected binary InAs due to intermixing of the layer and substrate materials. We determine that this intermixing is caused by dissociation of the exposed GaAs at the pits, followed by migration of excess Ga atoms and their incorporation into the islands. The density of the island–pit combinations keeps nearly constant for different layer thicknesses. Eventually, as the layer grows beyond a certain thickness, the pits are filled up by the expanding islands, forming a nearly pure island morphology at the growth front.

AFM study and optical properties of GaAsN/GaAs epilayers grown by MOVPE

We report a study on surface morphology and optical properties of GaAsN epilayers grown on GaAs (1 0 0) vicinal substrates by metalorganic vapor-phase epitaxy (MOVPE), using dimethylhydrazine as the N source. The surface morphology was investigated by atomic force microscopy (AFM) as a function of N composition and growth temperature ( T g). At T g=530–550°C, we observed a transition from a bunched step/terrace structure towards a surface with indistinct step/terrace structure when increasing the N content. We attribute this evolution to a transition from step-bunching towards 2D-nucleation growth, related to decreased Ga surface-diffusion length. The optical properties were studied using room-temperature (RT) and low-temperature photoluminescence (PL), and photoreflectance (PR). Post-growth thermal annealing were carried out on GaAs-capped layers. A drastic increase of the RT PL intensity and a narrowing of the linewidth were observed under optimum annealing conditions. The influence of annealing temperature and duration were also examined.

Control of Initial Nucleation by Reducing the V/III Ratio during the Early Stages of GaN Growth

We investigated, by atomic force microscopy (AFM), the effect of the V/III ratio on the surface morphologies of GaN epilayers during the early stage of high-temperature (HT) GaN growth. The epilayers were grown on sapphire substrates by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). We observed that initial HT-GaN islands in the early stage of HT-GaN growth were larger when a lower V/III ratio was used during the low-temperature (LT) GaN-buffer-layer growth or during the early stage of HT-GaN growth (the HT-GaN-buffer growth). These larger islands were attributed to a reduced nuclei density when a lower V/III ratio was used during the LT-GaN-buffer-layer growth, or to an increased growth aspect ratio (the ratio of vertical and lateral growth rates) when a lower V/III ratio was used in the early stage of HT-GaN growth. The larger islands led to fewer defects when they coalesced, which demonstrated that the quality of GaN epilayers grown on sapphire substrates by LP-MOCVD can be improved by using a low V/III ratio during the LT or HT GaN-buffer-layer growth.

Characteristics of BeTe films grown by molecular beam epitaxy

Systematic investigations are performed on the surface morphology, structural properties, and p-type dopability of BeTe epilayers grown on GaAs (001) by molecular beam epitaxy under various growth temperatures and p(Te)/p(Be) flux ratios. A phase diagram for surface reconstruction is obtained with regard to the BEP (beam equilibrium pressure) ratio of p(Te)/p(Be) and growth temperature. Irrespective of growth temperature (300–500 °C), a surface phase transition from weak (4×1) to (2×1) reconstruction occurs near p(Te)/p(Be)∼10 as the p(Te)/p(Be) value increases. Epilayers grown at a low Te pressure of p(Te)/p(Be)⩽10 show Be droplets on the surface as observed by atomic force microscopy, which eventually leads to degraded crystallinity. It is noted that such growth features are different from conventional II–VI compounds. The differences in growth features are discussed in terms of the relationships between the equilibrium vapor pressures of the compound and constituent elements. By optimizing growth conditions, i.e., flux ratio of 15<p(Te)/p(Be)<20, epilayers with high structural quality and smooth surface are obtained. Good correlation between the surface morphology and crystallinity is found. p-type BeTe layers doped with N are grown using a radio-frequency nitrogen plasma source. The net acceptor concentration increases as the p(Te)/p(Be) flux ratio is decreased. It is shown that p-BeTe with high hole concentrations above 6×1018 cm−3 can be readily grown.

Atomic Force Microscopy Study of GaN Grown on Al2O3(0001) by LP-MOVPE

ABSTRACTThere is currently a high degree of interest in understanding the diverse mechanisms that determine the growth morphology of epitaxial films. To understand and control over these mechanisms, it is essential to know how the growth mechanisms are correlated with morphologies. In GaN MOVPE processes, growth temperature has a remarkable effect on the film morphologies and properties. In present work, the temperature dependency of surface morphology of GaN epilayers grown by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) was studied using atomic force microscopy. It was demonstrated that dislocations strongly influence the growth mechanisms and the morphology of the films. Three growth modes were identified at different growth temperature ranged from 1030°C to 1100°C, which are, (1) spiral growth dominated by dislocation with screw type, (2) monolayer step flow, and (3) nucleation-assisted step flow growth. The significant effect of dislocation on the surface morphologies and growth mechanisms may be attributed to the high dislocation density and higher activation energy for a step overcoming the dislocation pinning in GaN. Because few dislocations will be introduced under monolayer step flow growth mode, we can infer the useful information about GaN initial growth.

Surface morphology of InGaAs and InP materials grown with trimethylarsenic and arsine on vicinal InP substrates

In this study, we report on an investigation using atomic force microscope (AFM) to study lattice-matched InGaAs surface layers grown by MOVPE with two different sources of group V elements: trimethylarsenic (TMAs) and arsine. The growth was performed with a home made apparatus with a T shaped reactor. The substrate temperature ranged from 570 to 630°C under pure hydrogen or a mixture of hydrogen and nitrogen as carrier gas. After thermal annealing under PH 3 and growth of InP buffer layer, we could clearly observe the step/terrace like feature of the vicinal surface morphology for 0.2° misoriented substrates. For epilayers grown under nitrogen flow, step bunching was observed with terraces as large as 170 nm which is twice larger than the nominal value (80 nm). Lattice-matched InGaAs/InP epilayers were grown with arsine and TMAs. For TMAs-grown epilayers, we observed that the roughness depends on V/III ratio and growth temperature. The comparison of the surface morphology of TMAs and arsine-based epilayers assessed by AFM characterisation will be presented and discussed in terms of As-carbon adsorbed species.

Impurity incorporation and the surface morphology of MOVPE grown GaAs

The impact of impurity incorporation on the development of the surface morphology of GaAs epilayers, grown by metalorganic vapor phase epitaxy (MOVPE), has been systematically investigated. A variety of different doping elements, including Mg, Zn, C, Si, O, and Se, were used to study the interaction between the impurity atoms and GaAs surface. Impurity atoms with smaller atomic weight, belonging to group II and VI, have a larger influence on the surface morphology than the other dopants. Different chemical sources for carbon doping were also used to explore the effect of surface growth chemistry on the formation of surface features. The epilayer surface morphology was affected by the combination of several physical and chemical factors. Factorsinfluencing the impact of an impurity on the growth front evolution are presented based on the interaction between the impurity atoms and the surface step structures.

An atomic force microscopy study of a temperature dependent morphology transition of GaN grown on sapphire by MOCVD

The dependence of the surface morphology of GaN epilayers grown by metal-organic chemical vapour deposition on the growth temperature at two different pressures has been studied using atomic force microscopy. First, the theoretical growth morphology for GaN is derived from the crystal structure. Connected nets were found for the {0 0 0 2}, {1 1 ̄ 0 1} and the {1 1 ̄ 0 0} faces. Experimental results show that on Ga terminated GaN films both the (0 0 0 1) and the {1 1 ̄ 0 1} faces are present for a considerable temperature range. At low deposition temperatures the {1 1 ̄ 0 1} faces dominate, whereas at higher growth temperatures the morphology is determined by growth in the [0 0 0 1] direction. It is shown that a growth rate parameter, α GaN, proportional to the relative growth rates of the (0 0 0 1) and the {1 1 ̄ 0 1} faces, can be used to describe the morphology of the films. The (0 0 0 1) surfaces show growth spirals emerging from screw dislocations. The single or double spirals are composed of monoatomic steps. The double steps tend to split at the spiral centres, which can be explained by entropic repulsion and a high surface diffusion. Most spiral centres are accompanied by hollow cores, the size of which is larger than predicted using Frank's theory. A possible explanation for the large diameter of the hollow cores is given by the precipitation of vacancies along the dislocations.

Temperature Dependent Morphology Transition of GaN Films

ABSTRACTThe temperature dependence of the surface morphology of GaN epilayers was studied with AFM. The layers were grown by low pressure MOCVD on (0001) sapphire substrates in the temperature range of 980°C-1085°C. In this range the (0001) and {1101} faces completely determine the morphology of 1.5 μm thick Ga-faced GaN films. For specimens grown at 20 mbar and temperatures below 1035°C the {1101} faces dominate the surface, which results in matt-white layers. At higher growth temperatures the morphology is completely determined by (0001) faces, which lead to smooth and transparent samples. For growth at 50 mbar, this transition takes place between 1000°C and 1015°C. It is shown that the morphology of the films can be described using a parameter αGaN, which is proportional to the relative growth rates of the (0001) and the {1101} faces.

Patterned regrowth of n-GaAs by molecular beam epitaxy using arsenic passivation

Patterned regrowth of n-GaAs through a Si3N4 mask prepared on molecular beam epitaxy (MBE) grown n-GaAs layers has been explored. An arsenic capping layer was deposited in the MBE growth chamber prior to mask definition, in order to protect the GaAs surface during ex situ processing. Removal of this cap from the lithographically defined mask area was accomplished by hydrogen plasma etching prior to Si3N4 deposition. After in situ thermal desorption of the As cap and phase selective regrowth, the samples were inspected with scanning electron and atomic force microscopy to establish the growth selectivity and examine the surface morphology. Electrolytic carrier profiling and secondary ion mass spectrometry measurements were carried out to characterize the growth-interrupted GaAs–GaAs interface. We find that the surface morphology and defect density of regrown GaAs epilayers are comparable to those of continuously grown epilayers of the same overall thickness. Structuring of the arsenic cap with hydrogen pla...

Recent progress in GaInAsSb thermophotovoltaics grown by organometallic vapor-phase epitaxy

Studies on the materials development of Ga 1− x In x As y Sb 1− y alloys for thermophotovoltaic (TPV) devices are reported. Ga 1− x In x As y Sb 1− y epilayers were grown lattice-matched to GaSb substrates by organometallic vapor-phase epitaxy (OMVPE) using all organometallic precursors including triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony with diethyltellurium and dimethylzinc as the n- and p-type dopants, respectively. The epilayer characteristics of these alloys depend on growth temperature, In and As content, V/III ratio, and substrate misorientation. A mirror-like surface morphology and room-temperature photoluminescence (PL) are obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.5 μm. The material properties of the metastable alloys improve for growth temperature decreasing from 575°C to 525°C and with decreasing In and As content, as based on epilayer surface morphology, X-ray diffraction, and low-temperature PL spectra. In general, GaInAsSb layers grown on (1 0 0)GaSb substrates with a 6° toward (1 1 1)B misorientation exhibit overall better material quality than layers grown on the more standard substrate (1 0 0) 2° toward (1 1 0). Consistent growth of high-performance lattice-matched GaInAsSb TPV devices is also demonstrated.

OMVPE growth of GaInAsSb in the 2–2.4 μm range

Ga 1− x In x As ySb 1− y epilayers were grown lattice matched to GaSb substrates by organometallic vapour phase epitaxy using all organometallic precursors, which include triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony. Layers were grown over a temperature range between 525 and 575°C, a V/III ratio range between 0.9 and 1.7, x<0.2 and y<0.2, and on (1 0 0)GaSb substrates with 2° toward (1 1 0) or 6° toward (1 1 1)B misorientation. The overall material quality of these alloys depends on growth temperature, In content, V/III ratio, and substrate misorientation. A mirror-like surface morphology and room temperature photoluminescence (PL) are obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.4 μm. Based on epilayer surface morphology and low temperature PL spectra, the crystal quality improves for growth temperature decreasing from 575 to 525°C, and with decreasing In content. In general, GaInAsSb layers grown on (1 0 0) GaSb substrates with a 6° toward (1 1 1)B misorientation exhibited smoother surfaces and narrower full width at half-maximum values of 4 K PL spectra than layers grown on the more standard substrate (1 0 0) 2° toward (1 1 0). Nominally undoped GaInAsSb layers grown at 550°C are p-type with 300 K hole concentration of ∼5×10 15 cm −3 and hole mobility of ∼430–560 cm 2/(V s). The n- and p-type doping of GaInAsSb with diethyltellurium and dimethylzinc, respectively, are also reported.

Potential use of the tendency of III—V alloys to separate for fabrication of low dimensionality structures

Improved understanding and control of compositional modulations in epitaxial III-V alloys can result to efficient fabrication methods for 1-D and 0-D quantum device structures. We have made TEM, SEM and AFM observations on In xAl 1-xAs and In yGa 1-yAs alloy thin films, grown by MBE on (100) and (111) InP substrates, to reveal the correlation between substrate surface topography (i.e., vicinal orientations), epilayer surface morphology, and the evolution of compositional modulations. The spontaneous growth of vertical superlattice-like structures and quantum wire-like structures is shown. A good control for (100) InAlAs compositional modulations has been achieved and a process enabling the direct growth of InGaAs quantum-wire structures is described.

MBE grown InAlAs/InGaAs lattice mismatched layers for HEMT application on GaAs substrate

Lattice mismatched high electron mobility transistors with InGaAs and InAlAs buffer layers have been grown by molecular beam epitaxy on GaAs substrate. Surface morphology of epilayers has been investigated using atomic force microscopy. An inverse step layer at the exit of the linear grade has been used to enhance relaxation of strain in heterostructures with indium contents ranging from 30% to 50% and impact of surface roughness on electrical properties has been investigated.

Process-Induced Morphological Defects in Epitaxial CVD Silicon Carbide

Silicon carbide (SiC) semiconductor technology has been advancing rapidly, but there are numerous crystal growth problems that need to be solved before SiC can reach its full potential. Among these problems is a need for an improvement in the surface morphology of epitaxial films that are grown to produce device structures. Various processes before and during epilayer growth lead to the formation of morphological defects observed in SiC epilayers grown on SiC substrates. In studies of both 6H and 4H-SiC epilayers, atomic force microscopy (AFM) and other techniques have been used to characterize SiC epilayer surface morphology. In addition to the well-known micropipe defect, SiC epilayers contain growth pits, triangular features (primarily) in 4H-SiC, and macro step due to step bunching. In work at NASA Lewis, it has been found that factors contributing to the formation of some morphological defects include: defects in the substrate bulk, defects in the substrate surface caused by cutting and polishing the wafer, the tilt angle of the wafer surface relative to the basal plane, and growth conditions. Some of these findings confirm results of other research groups. This paper presents a review of published and unpublished investigations into processes that are relevant to epitaxial film morphology.