Stripe Mesas Research Articles

Non-planar growth of high Al-mole-fraction AlGaN on patterned GaN platforms for mitigating strain-induced cracks beyond the critical layer thickness

Non-planar growth of AlxGa1-xN epitaxial layers with an average alloy composition up to Al-mole-fraction of x ∼ 0.21 was performed on patterned c-plane GaN on (0001) sapphire substrates with stripe-shaped mesa structures. This approach successfully realized the growth of crack-free AlGaN layers on the top of mesas with layer thicknesses greater than the expected critical layer thickness by relaxing the in-plane stress on the top of the mesa. The effectiveness of the relaxation strongly depends on the width and depth of the stripe mesa. The relaxation of in-plane stress and resultant suppression of cracks in AlGaN layer are qualitatively discussed.

Formation Mechanism of AlGaAs/GaAs Stripe Structure Made of (11n)A Facets in Selective Molecular-Beam Epitaxy

The evolution of the shape of a stripe mesa during molecular-beam epitaxy of GaAs is studied under different growth conditions (e.g., growth temperature, As pressure). If the stripe mesa is along the <110> axis on a (001) GaAs substrate, {11n}A facets (n=1, 3, 4, 5) are formed. We found that {111}A and {113}A predominate if the stripe width is less than 1.5 µm. If the stripe width is wider than 2.0 µm, however, we have a shallow groove made of {115}A facets on the top of the mesa. Thus, the cross-sectional shape (appearance/disappearance of each facet) depends on the growth time. The overall evolution is well explained numerically assuming diffusion of adatoms on the facets. We found that the mass transport is the dominant mechanism to determine the cross-sectional shape of the stripe. Slight discrepancy between the experimental and numerical results is attributed to the presence of a potential barrier for the diffusion across the boundary between different facets.

MBE growth of one-dimensionally ordered AlInAs quantum dots on a patterned GaAs substrate

The growth of AlxIn1−xAs self-assembled quantum dots (QDs) is studied on a stripe mesa of GaAs by molecular beam epitaxy. The stripe is along the [1 −1 0] direction on a (0 0 1) GaAs substrate and formed with {1 1 1}A and {1 1 4}A facets on the side, and {1 1 5}A facets on the top. The {1 1 5}A facets give a shallow V-groove on the top of the mesa. The dots are grown selectively at the bottom of the V-groove via an AlGaAs interlayer to form one-dimensionally aligned AlInAs QDs. We find that the diameter of an individual dot is elongated along the stripe direction, and the size as well as the density is varied by the sharpness of the V-groove structure. The manner of the QD formation is investigated as a function of the growth temperature and the Al composition x.

Distribution of self-assembled InAs dots on patterned GaAs (1 0 0) substrates

We have used pre-growth patterning of a (1 0 0) GaAs substrate into mesas to modify the nucleation of InAs quantum dots (QDs) grown by molecular beam epitaxy. The distribution of InAs quantum dots has been assessed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). On stripe mesas patterned with their long axis in the [ 0 1 1 ¯ ] and [0 1 1] directions, we observe a narrowing of the mesa width during GaAs overgrowth for the growth conditions used. We have observed several different distributions of dots such as; a single line of dots, dots clustered along the edges of the mesa or the top of the mesa being totally covered with dots. These distributions depend on the net migration of indium adatoms on the sidewall facets, the underlying GaAs surface morphology and width of the mesa top (1 0 0) plane.

Polytype homogeneity and doping distribution in homoepitaxial 4H SiC grown on nonplanar substrates

SiC was grown around stripe mesas, oriented along the 〈112̄0〉 and 〈11̄00〉 lattice directions on 4H SiC substrates. The grown layers were investigated with respect to polytype homogeneity by using cathodoluminescence. It was found that 3C inclusions rarely occur at normal growth conditions. However, 3C inclusions were prevalent at low growth temperatures and high C:Si ratios at the mesa tops, where the (0001) plane is revealed during growth. The doping distribution was recorded on cleaved mesa cross sections by using scanning capacitance spectroscopy. It was found that the p-type doping (using Al as the dopant) was considerably lower at the mesa walls than on the mesa tops or between the mesas, while the n-type doping (using N as the dopant) was independent of the substrate geometry.

Uniform and ordered self-assembled Ge dots on patterned Si substrates with selectively epitaxial growth technique

In this work, we report the controlled growth of one-dimensional (1D) and two-dimensional uniform, well-arranged self-assembled Ge dots grown on patterned Si (0 0 1) substrates. Selectively epitaxial growth (SEG) of Si mesas was first performed in an MBE system equipped with a gas source of Si 2H 6 and a Knudsen cell of Ge. Self-assembled Ge dots on one-dimensional Si SEG stripe mesas demonstrate the periodic arrangement with uniform size, which is different from the bi-modal distribution as normally seen. This cooperative arrangement is attributed to the nature of self-regulation of the self-assembled system with the assistance of the spatial confinement. A systematic study of the one-dimensional arrangement will be discussed. The atomic force microscopic results of two-dimensional arrays show that there are several kinds of the arrangement configurations, indicating the possibility of the controlling the placement of self-assembled Ge dots. Finally, we will discuss the mechanisms of the cooperative arrangements and the possibility to control freely spontaneous growth of Ge dots on pre-grown Si mesas.

Control of the arrangement of self-organized Ge dots on patterned Si(001) substrates

We report the controlled arrangement of self-organized Ge dots on patterned Si(001) substrates. Selective epitaxial growth (SEG) of Si was first carried out in a molecular-beam epitaxy system with a Si2H6 gas source and a Ge Knudsen cell. Si SEG mesas were formed in 〈110〉-oriented Si windows followed by subsequent Ge growth. The atomic force microscopic (AFM) results showed that Ge dots with a regular spacing were perfectly aligned on the ridges of Si stripe mesas. Only one row of dots was seen on each ridge of a stripe mesa with a mesa base width ranging from 0.5 to 0.9 μm. The formation of regularly spaced Ge dots may be attributed to the balance between the strain energy of the dots and the repulsive interaction of the neighboring dots. Using preferential nucleation scheme, we demonstrate the placement of the dots at the predetermined sites. We also discuss the mechanism of the self-registration of the self-organized Ge dots.

Perfect alignment of self-organized Ge islands on pre-grown Si stripe mesas

Self-organized Ge islands grown on patterned Si(001) substrates have been investigated. Selective epitaxial growth (SEG) of Si is carried out with gas-source molecular beam epitaxy to form Si stripe mesas followed by subsequent Ge island growth. Self-aligned Ge islands with regular spacing are formed on the -oriented ridges of the Si mesas. The regular spacing is driven by the repulsive interaction between the neighbor islands through the substrates. A mono-modal distribution of the islands has been observed on the ridges of the Si mesas. The spatial confinement as well as the preferential nucleation is believed to be the mechanism of this alignment of the self-organized Ge islands.

Application of CCl 2F 2- and CCl 4-based plasmas for RIE of GaSb and related materials

We report on the RIE etching of GaSb, AlGaAsSb and InGaAsSb materials in CCl 2F 2 and CCl 4 plasmas diluted with either H 2 or N 2. The highest etch rate (∼2 μm/min for GaSb) was achieved with CCl 4/N 2 chemistry, however, with an important degree of selectivity in etching of GaSb and quaternary alloys. By contrast, CCl 4/H 2 plasma enables anisotropic etching of heterostructure materials at an etch rate of ∼0.2 μm/min. CCl 2F 2-based plasma exhibited very slow etch rates (≤100 nm/min) and high selectivity in etching of GaSb over AlGaAsSb. The obtained results were applied in patterning of micrometer-size circular and stripe mesas in photonic GaSb/InGaAsSb/AlGaAsSb device structures, as well as in the fabrication of submicron circular gratings in GaSb.

Controlled arrangement of self-organized Ge islands on patterned Si (001) substrates

We report the ability to arrange self-organized Ge islands on patterned Si (001) substrates. Selective epitaxial growth of Si is carried out with gas-source molecular beam epitaxy to form Si mesas followed by subsequent Ge growth. Self-aligned and regularly spaced Ge islands are formed on the 〈110〉-oriented ridges of the Si stripe mesas. A mono-modal size distribution of the islands has been observed on the ridges. Using preferential nucleation sites allows us to place Ge islands at predetermined positions. The controlled arrangement of self-organized nanostructures offers the potential applications of island arrays for the implementation in nanoelectronics and quantum computation.

Equilibrium crystal shapes for 6H AND 4H SiC grown on non-planar substrates

Growth of 6H and 4H SiC has been performed on Si-face substrates with etched stripe mesas under low supersaturation conditions (which is obtained by keeping the C:Si ratio in the gas phase low) at the two different growth temperatures of 1480 and 1620°C. The growth around the mesas was investigated by scanning electron microscopy and the facets which appear around the mesas could be identified as low index lattice planes. The difference in growth rate between these lattice planes was found to be less than ±25%. Modified polar Wulff plots are used to illustrate the growth rate in the different lattice directions, and could also be used to predict the growth habit for different geometries. As expected, the average growth rate is larger on the {11 2 n} than on the {1 1 0n} planes, but slow and fast growing planes exist for both sets of planes. Generally the growth rate is larger for planes with high n, and for the {1 1 0n} planes it is found that a slow growth rate could be correlated with a larger percentage of C ad-atoms to the plane. At the lower growth temperature a larger difference in growth rate between the fast and slow growing planes could be observed, probably due to a larger influence of the surface energies under these conditions.

Maskless Lateral Epitaxial Overgrowth of GaN on Sapphire

ABSTRACTWe demonstrate a technique of lateral epitaxial overgrowth (LEO) of GaN, termed 'maskless' LEO, in which no mask is deposited prior to LEO regrowth. Instead, a bulk (&gt; 2 μm) GaN layer on sapphire is selectively dry etched, leaving ∼5 μm-wide stripe mesas oriented in the &lt;1010&gt;GaN direction, with a 20 μm period. These stripes serve as seeds for LEO GaN growth, which proceeds from the tops of the stripes and expands laterally, resulting in a ‘T’, or overhang, morphology. As for LEO over an SiO2 mask, significant defect reduction (from ∼109 cm−2 to ∼106 cm−2 ) is observed in cross-sectional transmission electron microscopy (TEM). Atomic force microscopy of the top surface of the LEO GaN reveals that no threading dislocations with screw component terminate at the surfaces of laterally overgrown regions. X-ray diffraction measurements reveal that the wings exhibit a crystallographic tilt away from the seed regions in an azimuth perpendicular to the stripe direction; the tilt angle (∼0.4 – 0.5°) is relatively independent of growth temperature and wing aspect ratio.

Polarized cathodoluminescence study of selectively grown self-assembled InAs/GaAs quantum dots

We have examined the optical properties of self-assembled InAs quantum dots (QDs) grown on prepatterned GaAs(001) substrates with polarization sensitive and time-resolved cathodoluminescence (CL) imaging and spectroscopy techniques. The InAs QDs were formed using a novel application in self-assembled molecular beam epitaxial growth, which entailed the growth of InAs on preformed [11̄0]-oriented stripe mesas. Interfacet In adatom migration occurred along the stripe side-walls during growth, enabling the selective formation of linear arrays of InAs QDs on the stripe mesas. The total InAs deposition needed to induce the two-dimensional to three-dimensional morphology change on the stripes is less than that required to initiate QD formation on the unpatterned substrates. The QDs formed on the mesa top were found with a luminescence distribution redshifted relative to QDs in the valley region, indicating that QDs with a larger average size were formed on the mesa top. The lower density of QDs in the valley region led to a weaker emission and sharper δ-like transitions at lower beam currents, relative to emission from QDs on the mesa. CL imaging was employed to study the spatial distribution of luminescence and identified the presence of relatively small QDs situated near the edges of the valley region along the lower surface of the stripe edges. An excitation- and energy-dependent polarization anisotropy relative to the 〈110〉 directions was observed in CL emission. A polarization anisotropy reversal was found between CL from QDs on the mesa and in the valley regions, revealing the importance of the stress anisotropy in both the formation of QDs and their subsequent optical properties. The CL was examined as a function of temperature to evaluate the thermal re-emission of carriers and the associated activation energies. The carrier relaxation kinetics were studied with time-resolved CL to measure differences in the CL onset and decay rates for QDs in the valley and mesa regions.

Fabrication of strained InAs island ensembles on nonplanar patterned GaAs(001) substrates

We report on the behavior of InAs deposited on nonplanar GaAs(001) substrates patterned with ≲0.5 μm wide stripe mesas oriented along the [11̄0] and 〈100〉 directions and with square mesas with a lateral size of ≳0.5 μm oriented along the 〈100〉 directions. Interfacet migration of In from the sidewalls to the mesa top leads to an enhanced InAs island density on the stripe as well as square mesa tops compared to that on the planar unpatterned region. Using such interfacet migration and InAs deposition amount less than needed for island formation on planar GaAs(001), we demonstrate complete selectivity in the positioning of InAs islands on the [11̄0] oriented stripe mesas of widths ≲100 nm, with islands forming exclusively on the mesa tops. These islands arrange in mesa-width-dependent parallel chains. They show photoluminescence (PL) comparable to that from the islands on the planar substrates. The polarization dependence of the PL suggests the presence of anisotropy in strain fields and potential elongation of islands in the [11̄0] direction. The significance of the stripe mesa edge orientation to the island formation is revealed by vastly different island densities and InAs morphology on the [11̄0] oriented versus 〈100〉 oriented stripe mesas. These differences reflect difference in migration from the different sidefacets that surround the (001) mesa top.

Growth of 4H and 6H SiC Trenches and Around Stripe Mesas

Growth of 4H and 6H SiC Trenches and Around Stripe Mesas

Stress-engineered spatially selective self-assembly of strained InAs quantum dots on nonplanar patterned GaAs(001) substrates

The lattice-mismatch stress-induced two-dimensional-to-three-dimensional morphology change is combined with interfacet adatom migration to selectively assemble parallel chains of InAs islands on top of [11̄0] oriented stripe mesas of sub-100-nm widths on GaAs(001) substrates. On such mesa stripes, prepared in situ via size-reducing epitaxy, deposition of InAs amounts subcritical for island formation on planar GaAs (001) is shown to allow self-assembly of three, two, and single chains of InAs three-dimensional island quantum dots selectively on the stripe mesa tops for widths decreasing from 100 nm down to 30 nm.

Homoepitaxy of 6H and 4H SiC on nonplanar substrates

Growth by vapor phase epitaxy around stripe mesas and in trenches formed by reactive ion etch on 6H and 4H SiC substrates has been investigated. The mesas were aligned with the low index 〈112̄0〉 and 〈11̄00〉 directions, as well as with the high index 〈1,1+∛,2+∛¯,0〉 directions, in order to reveal and study the growth habit. It was found that a low C:Si ratio gave a smooth growth and small differences in growth rate between lattice planes. A larger C:Si ratio gave more faceted growth, both limited by surface kinetics and surface diffusion, and the growth rate was 10% lower in the [11̄00] direction and 10% higher in the [112̄0] direction, than on the substrate. Growth on mesas oriented parallel to the substrate off-orientation shows clear step-flow growth, while growth on mesas oriented perpendicular to the off-orientation reveals the singular (0001) plane, where islands are observed, which might indicate Stranski–Krastanov growth.

On the atomistic and kinetic nature of strained epitaxy and formation of coherent 3D island quantum ☐es

Some remarkable recent results of the in situ scanning tunneling microscope (STM) and atomic force microscope (AFM) studies of InAs coherent 3D island initiation and evolution on GaAs(001) are presented in the larger context of the field of strained epitaxy. The role of nano and meso scale mesas in manipulating stress/strain is illustrated through examples of growths on in situ, purely growth control prepared stripe and square mesas with linear dimensions as small as ∼ 40 nm. A set of basic kinetic processes and their strain dependence is identified and suggested to form a good core that has the potential for providing a unified framework for understanding strained epitaxy ranging from low misfits to high misfits.

Threading dislocations in GaAs on pre-patterned Si and in post-patterned GaAs on Si

A detailed correlation has been made between the threading dislocation density, the residual stress and the growth area for striped and square mesas of molecular beam epitaxially grown ∼ 3 μm thick GaAs on pre-patterned Si, and of post-patterned ∼ 3 μm thick GaAs on Si. The morphology and distribution of the dislocations are also compared for samples of various sizes between 1 and 100 μm in width, which are observed before and after ex situ annealing, using cross-sectional transmission electron microscopy. Micro-photoluminescence measurements reveal that the residual stress of the patterned GaAs decreases with a decrease in the mesa size, especially for sizes of less than 25 μm, independent of the patterning method and the shape of the samples. However, no clear reduction in the dislocation density is observed in as-grown layers on pre-patterned stripe and square Si mesa smaller than 10 μm. On the contrary, the defect density in the layers having pattern sizes less than 4 μm is higher than that in layers having sizes larger than 10 μm, due to the additional generation of microtwins/stacking faults from the {111} faceted regions which are formed at the sides along the 〈110〉 directions of the grown films. Annealing treatment at 900°C for 10 s effectively reduces the density of these latter defects in patterned films by both pre- and post-patterning procedures, irrespective of the pattern sizes. However, no distinct decrease in the dislocation density is detected in small size patterns of less than 10 μm. This feature is equally observed for post-patterned samples which include thin Si insertion layers in GaAs films. The obtained results strongly suggest that thermal-expansion mismatch-induced stress does not cause any further generation of dislocations during cooling from the growth temperature to room temperature. The dislocations are generated during growth due to lattice mismatch-induced stress.

Morphological modifications during selective growth of InP around cylindrical and parallelepiped mesas

Morphological modifications during selective epitaxy of InP by hybride vapour phase epitaxy around reactive ion etched stripe mesas as in the case of edge emitting lasers were studied by growth of alternating layers of InP and InP:S. The growth profiles were analysed and compared with the selective growth behaviour around reactive ion etched cylindrical and parallelepiped mesas as in the vertical cavity surface emitting lasers. The very initial growth pattern is found to be responsible for the emerging features. The growth along the [110] direction is faster than that along its orthogonal [ 1 10] direction, providing a better planarization along the former. Earlier planarization however can cause mask encroachment, especially in the case of cylindrical mesas, but can be suppressed by using wet etched mesas.