Substrate Miscut Research Articles

Effects of miscut of prestructured sapphire substrates and MOVPE growth conditions on [formula omitted] oriented GaN

In this work, the influence of sapphire mis-orientation on the quality of coalesced (112¯2) GaN layers grown on r-plane prestructured sapphire substrates (r-PSS) is investigated. It was found that the angle of the GaN (112¯2) plane towards the surface plane of the sapphire wafer can be adjusted by the mis-orientation of the substrate. Furthermore, we discovered that the c-direction of GaN is tilted by more than 1° towards the c-direction of the sapphire wafer.In a second experiment, the influence of the MOVPE growth temperature, V/III ratio and reactor pressure on the coalesced layer has been studied. While a high temperature and small V/III ratio are beneficial, the reactor pressure did not show any significant impact on the crystal quality and surface roughness.

Large Area Plan-View Transmission Electron Microscopy Sample Preparation for Direct-Bonded Interfaces

A focused ion beam (FIB) sample preparation technique is developed to produce large areas of electron transparent material for plan-view transmission electron microscopy measurements from direct-bonded interfaces without thinning the bonded wafers. A FIB cut allows plan-view sample creation from the bonded interface region, and therefore rapid characterization of the defect density as a function of bonding conditions, substrate miscut, or pre-bonding surface treatments. Uniform electron transparent plan-view areas are extracted from the interface region to facilitate observation of the interface reconfiguration. To demonstrate the technique, the electrically active interfaces of a series of bonded III-V structures is studied as a function of substrate miscut as well as in-plane misorientation. Distinct differences in the interface reconstruction are observed and the interface reconstruction produces screw and edge dislocation networks at the III-V interfaces. The degree of twist and tilt is confirmed with both cross section electron microscopy measurements as well as in-plane x-ray diffraction measurements.

Effects of Miscut Substrates on Electrical Conductivity Across InP and GaAs Wafer-Bonded Structures

Wafer bonding of GaAs and InP using an (NH4)2S treatment are investigated for the effect of the wafer offcut angle on the electrical conductivity of wafer-bonded III-V interfaces. HRTEM and STEM are used to confirm the misorientation of the bonded samples and to compare the interface morphology across the range of relative misorientations. A line ratio at the interface shows that the well-bonded crystalline regions to amorphous oxide inclusions is consistent across all bonded samples, indicating that the degree of misorientation does not affect the level of interface recrystallization at high temperatures. Fitting the zero-bias conductance over a range of temperatures reveals an increase in barrier heights for samples with greater than 4° misoriented bonded pairs. This demonstrates that the out-of-plane relative surface misorientation is a critical parameter to be monitored and thus any in-plane twist as well in order to achieve superior electrical conductivity in direct-bonded multijunction solar applications.

Effects of Miscut Substrates on Electrical Conductivity Across InP and GaAs Wafer-Bonded Structures

Direct wafer bonding of III-V materials is of great interest for the fabrication of next generation high efficiency solar cells. Traditional wafer bonding methods of III-V materials lead to highly unstable oxide interfaces and poor electrical properties. To resolve these issues, elevated annealing temperatures (>600 °C), long times, and large compressive forces are employed to break up this interfacial oxide layer, so that there is better contact between the III-V bonding interfaces. These harsh processing conditions ultimately limit the achievable efficiencies, due to dopant diffusion and residual thermal strains in processed devices. Sulfur surface treatment prior to bonding was found to reduce the inclusion unfavorable oxides at the interface, reducing the post-bonding thermal and compressive force requirements, and promoting the ability to bond large (100 mm diameter) wafers.. Various combinations of sulfur treated n-GaAs and n-InP directed bonded structures were measured to investigate the effects of offcut angles on the electrical conductivity. The impact of the relative offcut angle on the electrical properties is an new unexplored area which is of important issuece, as misorientated substrates are typically used for the growth of III-V epitaxial layers in the solar industry to avoid anti-phase boundaries. It is observed in both the GaAs and InP bonded structures, that out-of-plane relative surface orientations between the (001) planes that are greater than 4° exhibit non-ohmic behavior. A theoretical model that describes the electron tunneling across a grain boundary between semiconductor bicrystals is used to represent the bonded interface and estimate the barrier conduction height. Fitting the zero-bias conductance over a range of temperatures reveals an increase in barrier heights for samples with greater than 4° misoriented bonded pairs, correspondingly the interface resistance at room temperature increases. These results demonstrate that the out-of-plane relative surface misorientation is the critical parameter to be monitored in order to achieve superior electrical conductivity in direct-bonded multijunction solar applications. In addition, differences between GaAs-GaAs bonded structures and InP-InP bonded structures indicate that the barrier for GaAs-GaAs is significantly greater than for InP-InP. The impact of the miscut itself, however, is similar in both cases, showing that both the chemical nature and the structural interface properties play roles in trans-interface electrical conduction.

Influence of composition and substrate miscut on the evolution of {105}-terminated in-plane Si1−xGex quantum wires on Si(001)

Isolated in-plane wires on Si(001) are promising nanostructures for quantum transport applications. They can be fabricated in a catalyst-free process by thermal annealing of self-organized Si1−xGex hut clusters. Here, we report on the influence of composition and small substrate miscuts on the unilateral wire growth during annealing at 570 °C. The addition of up to 20% of Si mainly affects the growth kinetics in the presence of energetically favorable sinks for diffusing Ge atoms, but does not significantly change the wire base width. For the investigated substrate miscuts of <0.12°, we find geometry-induced wire tapering, but no strong influence on the wire lengths. Miscuts <0.02° lead to almost perfect quantum wires terminated by virtually step-free {105} and {001} facets over lengths of several 100 nm. Generally, the investigated Si1−xGex wires are metastable: Annealing at ≥600 °C under otherwise identical conditions leads to the well-known coexistence of Si1−xGex pyramids and domes.

Step Terrace Tuned Anisotropic Transport Properties of Highly Epitaxial LaBaCo2O5.5+δ Thin Films on Vicinal SrTiO3 Substrates

Highly epitaxial LaBaCo2O5.5+δ (LBCO) thin films were grown on different miscut (001) SrTiO3 substrates (miscut angle of 0.5°, 3.0°, and 5.0°) to study the substrate surface step terrace effect on the in-plane electrical transport properties. The microstructure studies by X-ray diffraction and transmission electron microscopy indicate that the as-grown films are A-site disordered cubic perovskite structures with the c-axis highly oriented along the film growth direction. The four-probe scanning tunneling microscopy (STM) studies show that the LBCO thin films grown on the vicinal SrTiO3 substrates have a typical semiconductor behavior with the substrate surface terrace step inducing anisotropic electronic transport properties. These results indicate that in highly epitaxial thin films the surface terrace step induced local strains can play an important role in controlling the electronic transport properties and the anisotropic nature.

Influence of 3C–SiC/Si (111) template properties on the strain relaxation in thick GaN films

In this work, we study the influence of 3C–SiC/Si (111) template parameters (thickness, roughness and substrate miscut) on the GaN crystal quality and its strain state. For this, structures with an AlN nucleation layer and 1–4µm thick GaN layer have been grown by molecular beam epitaxy in order to select the best templates for the growth of thick GaN structures. Similar GaN structures have been grown directly on silicon for comparisons. The influence of the high silicon doping is confirmed on the enhancement of strain relaxation. Despite this limitation, a 5µm thick crack-free continuous GaN structure (with 1µm silicon doped) has been successfully grown on the best selected template. Furthermore, the growth by metal–organic chemical vapor deposition of structures with AlN and SiN inter-layers and thick continuous GaN layers on Si (111), 3C–SiC/Si (111) and SoPSiC (silicon on polycrystalline silicon carbide) has been achieved in order to show the relative benefit of each approach in terms of layer quality and strain state.

Effect of c-plane sapphire substrate miscut angle on indium content of MOVPE-grown N-polar InGaN

Nitrogen-polar (N-polar) InGaN films were grown on a GaN template/c-plane sapphire substrate by metal–organic vapor phase epitaxy (MOVPE). The effects of c-plane sapphire substrate miscut angle on the indium (In) content and crystal properties of N-polar InGaN films were investigated. The In content increased with increasing miscut angle in the vicinal region of less than 1.1°. This tendency is different from that of group-III-polar InGaN growth because of the difference in the atomic arrangement on the terraces and at step edges between these two inverted polar surfaces. In the case of N-polar growth, a spontaneous two-dimensional nucleation on terraces is difficult and the intentional introduction of steps is effective compared with group-III-polar growth. Furthermore, by observing the surface morphologies of GaN templates in view of both macroscopic and microscopic scales, a clear relationship between the macroscopic surface structure of GaN template and the In content of InGaN was revealed.

Influence of substrate miscut angle on surface morphology and luminescence properties of AlGaN

The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed.

Effects of dislocation strain on the epitaxy of lattice-mismatched AlGaInP layers

Strain fields arising from a non-uniform distribution of misfit dislocations in an underlying compositionally graded buffer are shown to be sufficiently strong to modify indium incorporation in III-phosphide light emitting layers. Composition fluctuations (xIn±0.02) in lattice-mismatched (AlyGa1−y)xIn1−xP thin films with length-scales of 5–10μm and a broadened light emission spectra are observed. Cathodoluminescence, photoluminescence and wavelength dispersive x-ray spectroscopies are used in this analysis to generate spatial maps of luminescence spectra and element distributions in metal-organic chemical vapor deposition (MOCVD) grown films. It is seen that these fluctuations due to misfit dislocations are hard to eliminate via growth-kinetics alone but can be lowered through the use of miscut substrates or spacer layers between the graded buffer layer and the active layer. A link between crosshatch surface-roughness and group-III atom distribution under group-V rich growth conditions in both AlInP and GaInP films is also demonstrated. In summary, the interaction of the dislocation strain field with the growth surface can affect the optical characteristics of lattice-mismatched LEDs even if the final threading dislocation density is low.

3C-SiC nanocrystal growth on 10° miscut Si(001) surface

The growth of 3C-SiC nano-crystal (NC) on 10° miscut Si(001) substrate by CO2 thermal treatment is investigated by scanning and high resolution transmission electron microscopies. The vicinal Si(001) surface was thermally oxidized prior to the annealing at 1100°C under CO2 atmosphere. The influence of the atomic steps at the vicinal SiO2/Si interface on the SiC NC growth is studied by comparison with the results obtained for fundamental Si(001) substrates in the same conditions. For Si miscut substrate, a substantial enhancement in the density of the SiC NCs and a tendency of preferential alignment of them along the atomic step edges is observed. The SiC/Si interface is abrupt, without any steps and epitaxial growth with full relaxation of 3C-SiC occurs by domain matching epitaxy. The CO2 pressure and annealing time effect on NC growth is analyzed. The as-prepared SiC NCs can be engineered further for potential application in optoelectronic devices and/or as a seed for homoepitaxial SiC or heteroepitaxial GaN film growth.

The anisotropic distribution of dislocations and tilts in metamorphic GaInAs/AlInAs buffers grown on GaAs substrates with miscut angles toward (1 1 1)A

The metamorphic GaInAs/AlInAs buffers grown by metal organic chemical vapor deposition (MOCVD) on GaAs substrates with miscut angles toward (111)A exhibit anisotropic properties in the two 〈110〉 directions. A higher dislocation density is observed along [110] direction for samples with substrate miscuts of 2° and 7°, while along [1−10] direction for samples with a substrate miscut of 15°. The different nucleation energies and glide velocities of α and β dislocations contribute to the anisotropic dislocation distribution for the 2° and 7° samples, however, the unequal resolved shear stress (RSS) distribution between different slip systems introduced by the miscut is responsible for the 15° sample. A modified model is proposed to determine the tilt angle of epilayer with respect to the substrates, and the results show that different tilt angles in the two 〈110〉 directions is mainly attributed to the different strain relaxation in two 〈110〉 directions caused by the miscut toward (111)A.

Dramatically enhanced self-assembly of GeSi quantum dots with superior photoluminescence induced by the substrate misorientation

A dramatically enhanced self-assembly of GeSi quantum dots (QDs) is disclosed on slightly miscut Si (001) substrates, leading to extremely dense QDs and even a growth mode transition. The inherent mechanism is addressed in combination of the thermodynamics and the growth kinetics both affected by steps on the vicinal surface. Moreover, temperature-dependent photoluminescence spectra from dense GeSi QDs on the miscut substrate demonstrate a rather strong peak persistent up to 300 K, which is attributed to the well confinement of excitons in the dense GeSi QDs due to the absence of the wetting layer on the miscut substrate.

Effect of substrate miscut on the electron mobility in InSb(001) structures on Ge and Ge-on-insulator substrates

InSb epilayers and InSb/Al0.20In0.80Sb quantum wells were grown on Ge(001) substrates and Ge-on-insulator (GeOI)-on-Si(001) substrates by molecular beam epitaxy. Growth on both on-axis and 4°-off-axis substrate orientations was studied. Anti-phase domains were formed when InSb films were grown on on-axis substrates, but suppressed significantly by the use of 4°-off-axis substrates. Such off-axis substrates also reduced the densities of micro-twin defects and threading dislocations. The defect reduction resulted in an increase in the room-temperature electron mobility from 37,000 to 59,000 cm2/Vs in 4.0-μm-thick InSb epilayers and from 10,000 to 20,000 cm2/Vs in 25-nm-thick InSb quantum wells on Ge(001) and GeOI-on-Si(001) substrates.

Effects of substrate miscut on threading dislocation distribution in metamorphic GaInAs/AlInAs buffers

Metamorphic GaInAs/AlInAs buffers with a total mismatch of 2 % with respect to GaAs have been grown by metal organic chemical vapor deposition on GaAs substrates with miscuts of 2°, 7° and 15° toward (111)A. The buffer grown on 2° miscut substrate exhibits a dislocation pile-up formation which increases the threading dislocation density greatly. However, using 7° and 15° off substrates eliminates the dislocation pile-up and decreases the dislocation density by an order of magnitude compared with that on the 2° substrate miscut, and the best crystalline quality of buffer is obtained on the 15° substrate miscut. Experimental results show that the buffers grown on the 7° and 15° miscut substrate exhibit shallower trenches on the surface and a weaker strain field in the InGaAs cap layers, and these help to suppress the occurrence of dislocation pile-up, leading to a lower threading dislocation density.

Effects of Si doping on the strain relaxation of metamorphic (Al)GaInP buffers grown on GaAs substrates

We investigate the effects of Si doping on the strain relaxation of the compositionally step-graded (Al)GaInP buffers grown by metal-organic chemical vapor deposition on (001) GaAs substrates with different miscuts toward (111)A. It is found that in the 2° samples, high Si doping can reduce both the α and β dislocation densities by delaying and suppressing the formation of phase separation in the buffer. In contrast, in the 7° samples, Si dopants deteriorate the buffer quality through increasing the dislocation density accompanying with the tilt reduction along the [110] direction, and a striking feature, bunches of β dislocations away from the interfaces, is observed in the [110] cross-sectional transmission electron microscopy images. A cross-slip mechanism closely associated with the pinning effect of Si on α dislocation motion is proposed to explain the multiplication of β dislocations. These results indicate that selecting a moderate Si doping density and substrate miscut are critical for the design and fabrication of metamorphic optoelectronic devices.

The impact of substrate miscut on the microstructure and photoluminescence efficiency of (0001) InGaN quantum wells grown by a two-temperature method

The impact of the miscut of a (0001) c-plane substrate on the structural and optical properties of InGaN/GaN quantum wells grown by metal-organic vapour phase epitaxy using a two-temperature method has been investigated. The two-temperature growth method involves exposure of the uncapped InGaN quantum well to a temperature ramp in an ammonia atmosphere before growth of the GaN barrier at a higher temperature. The resulting quantum well, consists of interlinking InGaN strips containing gaps which may impede carrier diffusion to dislocations. By increasing the substrate misorientation from 0° to 0.5° we show that the density of InGaN strips increases while the strip width reduces. Our data show that the PL efficiency increases with miscut and that the peak efficiency occurs at a lower excitation power density.

Triclinic deformation of InGaN layers grown on vicinal surface of GaN (00.1) substrates

We report on a triclinic unit cell deformation of fully strained InGaN layers grown on vicinal GaN (00.1) substrates. The samples were examined using the high-resolution X-ray diffraction (HR XRD) using a set of asymmetrical reflections and one symmetrical reflection of 00.2. The substrate miscut induced triclinic deformation of the layer unit cells, breaking the hexagonal symmetry. The experimental results are compared with predictions of the theory of elasticity. We formulate equations for unit cell parameters of layers grown on substrates cut in any direction, based on the equations given by Romanov et al. [J. Appl. Phys. 100, 023522 (2006)]. Additionally, the paper provides a recipe of the XRD measurements necessary to establish unit cell parameters (useful for composition determination of ternary compounds) of the hexagonal mismatched layers grown on off-axis substrates.

Effects of temperature and substrate miscut on the crystalline quality of metamorphic AlInAs layers grown on GaAs substrate

We have investigated the effects of temperature and substrate miscut on the crystalline quality of metamorphic AlInAs layers grown on a GaAs substrate by metal organic chemical vapor deposition (MOCVD). The AlInAs layers grown at 700°C show a much better crystalline quality than the ones grown at 675°C, because a high temperature can suppress phase separation and enhance the glide velocity of dislocations during the strain relaxation process. Compared with the miscuts of 2° and 7°, the 15° miscut is more effective in suppressing phase separation during the growth of the AlInAs buffers. Theoretical calculations show that the miscut will facilitate the strain relaxation by dislocation gliding. These results indicate that a high growth temperature and a large miscut will improve the quality of the metamorphic AlInAs layers grown on the GaAs substrate by MOCVD.

Surface morphology evolution of m-plane (11¯00) GaN during molecular beam epitaxy growth: Impact of Ga/N ratio, miscut direction, and growth temperature

We present a systematic study of morphology evolution of [11¯00] m-plane GaN grown by plasma-assisted molecular beam epitaxy on free-standing m-plane substrates with small miscut angles towards the –c [0001¯] and +c [0001] directions under various gallium to nitrogen (Ga/N) ratios at substrate temperatures T = 720 °C and T = 740 °C. The miscut direction, Ga/N ratio, and growth temperature are all shown to have a dramatic impact on morphology. The observed dependence on miscut direction supports the notion of strong anisotropy in the gallium adatom diffusion barrier and growth kinetics. We demonstrate that precise control of Ga/N ratio and substrate temperature yields atomically smooth morphology on substrates oriented towards +c [0001] as well as the more commonly studied –c [0001¯] miscut substrates.