Substrate Misorientation Research Articles

III–Nitride Epitaxy on Atomically Controlled Surface of Sapphire Substrate with Slight Misorientation

The importance of the atomically well-controlled surface of sapphire substrate with slight misorientation and ideally minimized surface roughness for III–nitride epitaxy is discussed in detail. An atomically controlled surface of sapphire substrate with slight misorientation angle is modeled and an almost ideal level of atomic surface roughness of sapphire substrate is found to be obtained by a chemical mechanical polishing (CMP) with colloidal silica. Cathodoluminescence (CL) imaging indicated the complete absence of subsurface damage induced by mechanical polishing. GaN and AlN thin films are grown on misoriented sapphire substrate with an atomically controlled surface by the CMP to investigate the misorientation angle of both sapphire and grown GaN and AlN thin films. An interface model is proposed to explain the difference in misorientation angle between sapphire and III–nitride thin films, providing strong evidence of the necessity of atomically controlled surface of sapphire substrate for III–nitride epitaxy.

Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices

Growth of InGaN/GaN light-emitting devices on nonpolar or semipolar planes offers a viable approach to reducing or eliminating the issues associated with polarization-related electric fields present in c-plane III-nitride heterostructures. Although progress in device performance has been rapid since the introduction of high-quality free-standing nonpolar and semipolar GaN substrates, a full appreciation of the materials challenges unique to nonpolar and semipolar III-nitride semiconductors has been slower to emerge. Only recently have researchers begun to understand issues such as the origins of the pyramidal hillocks typically observed on nominally on-axis m-plane GaN films, the effects of m-plane substrate misorientation on surface morphology and device performance, the mechanics of anisotropic cracking in tensile strained m-plane AlGaN films, the formation of basal-plane stacking faults in long-wavelength m-plane InGaN quantum wells, and the mechanisms for stress relaxation in semipolar AlGaN and InGaN films. In this paper, we review the materials and growth issues unique to high-performance nonpolar and semipolar light-emitting devices grown on high-quality free-standing GaN substrates and provide an outlook for the opportunities and challenges that lie ahead.

III–Nitride Epitaxy on Atomically Controlled Surface of Sapphire Substrate with Slight Misorientation

The importance of the atomically well-controlled surface of sapphire substrate with slight misorientation and ideally minimized surface roughness for III–nitride epitaxy is discussed in detail. An atomically controlled surface of sapphire substrate with slight misorientation angle is modeled and an almost ideal level of atomic surface roughness of sapphire substrate is found to be obtained by a chemical mechanical polishing (CMP) with colloidal silica. Cathodoluminescence (CL) imaging indicated the complete absence of subsurface damage induced by mechanical polishing. GaN and AlN thin films are grown on misoriented sapphire substrate with an atomically controlled surface by the CMP to investigate the misorientation angle of both sapphire and grown GaN and AlN thin films. An interface model is proposed to explain the difference in misorientation angle between sapphire and III–nitride thin films, providing strong evidence of the necessity of atomically controlled surface of sapphire substrate for III–nitride epitaxy.

Mechanism of self-assembled growth of ordered GaAs nanowire arrays by metalorganic vapor phase epitaxy on GaAs vicinal substrates

We report the self-catalyzed growth of GaAs nanowire arrays by metalorganic vapor phase epitaxy (MOVPE) on GaAs vicinal substrates. The effect of substrate misorientation on the nanowire growth and the influence of growth parameters such as temperature and input V/III ratio have been studied in detail. Variation in the nanowire growth mechanism and consequential changes in the nanowire growth morphology were observed. A VLS growth mechanism with negligible effect of the vicinal surface gave rise to randomly distributed droplet-terminated GaAs nanowires at 400 °C and multiprong root-grown GaAs nanowire clusters at 500 °C with low V/III ratio. The substrate misorientation effect was dominant at 500 °C with higher V/III ratio, in which case the combined effect of the vicinal surface and the self-catalyzed Ga droplets assisted the realization of self-assembled and crystallographically oriented epitaxial nanowire arrays through the vapor–solid mechanism.

Effect of Crystalline Quality on Photovoltaic Performance for ${\rm In}_{0.17}{\rm Ga}_{0.83}{\rm As}$ Solar Cell Using X-Ray Reciprocal Space Mapping

This paper presents the In0.17Ga0.83As solar cell grown on misoriented GaAs substrate (2°- and 15°-off) by metalorganic chemical vapor deposition. The crystalline quality of the In0.17Ga0.83As solar cell is determined by X-Ray reciprocal space mapping (RSM). RSM results show that the crystalline quality of In0.17Ga0.83As solar cell grown on 2°-off GaAs substrate is better than that of 15°-off GaAs substrate. Moreover, the photovoltaic performance of In0.17Ga0.83As solar cell grown on 2°-off GaAs substrate is found to be better than that of In0.17Ga0.83As solar cell grown on a 15°-off GaAs substrate, because the InxGa1-xAs epilayer grown on 15°-off GaAs substrate shows a large strain relaxation in the active layer of the solar cell. A large strain relaxation causes high dislocation density at the initial active layer/InxGa1-xAs graded layer interface for the solar cell grown on 15°-off GaAs substrate. The effect of dislocation defects on the solar cell performance can be alleviated using the p-i-n structure as the epilayer grown on 15°-off GaAs substrate.

Hug-like island growth of Ge on strained vicinal Si(111) surfaces

We examine the structure and the evolution of Ge islands epitaxially grown on vicinal Si(111) surfaces by scanning tunneling microscopy. Contrary to what is observed on the singular surface, three-dimensional Ge nanoislands form directly through the elastic relaxation of step-edge protrusions during the unstable step-flow growth. As the substrate misorientation is increased, the islands undergo a shape transformation which is driven by surface energy minimization and controlled by the miscut angle. Using finite element simulations, we show that the dynamics of islanding observed in the experiment results from the anisotropy of the strain relaxation.

Ge Growth on Vicinal Si(001) Surfaces: Island's Shape and Pair Interaction versus Miscut Angle

A complete description of Ge growth on vicinal Si(001) surfaces is provided. The distinctive mechanisms of the epitaxial growth process on vicinal surfaces are clarified from the very early stages of Ge deposition to the nucleation of 3D islands. By interpolating high-resolution scanning tunneling microscopy measurements with continuum elasticity modeling, we assess the dependence of island's shape and elastic interaction on the substrate misorientation. Our results confirm that vicinal surfaces offer an additional degree of control over the shape and symmetry of self-assembled nanostructures.

Effects accompanying GaN substrate misorientation in growth of InGaN and AlGaN layers

Effects accompanying GaN substrate misorientation in growth of InGaN and AlGaN layers

Correlation between morphology and electron spin relaxation time in GaAs/AlGaAs quantum wells on misoriented GaAs(110) substrates

We investigated the correlation between morphology and electron spin relaxation time in GaAs/AlGaAs multiple quantum wells (MQWs) on misoriented GaAs(110) substrates. For the MQW on a 2°-misoriented GaAs(110), formation of wide (110) step terraces due to step bunching was observed. Measurements of the temperature dependence of the electron spin relaxation time τs showed that the D’yakonov-Perel’ (DP) spin relaxation was suppressed even in the MQW on the 2°-misoriented GaAs(110) owing to the step terrace formation. In contrast, in the MQW on a 5°-misoriented GaAs(110), in which the surface showed ripple patterns, the τs became shorter than that on the on-axis GaAs(110), which we attribute to the enhanced electron spin relaxation originating from the bulk inversion asymmetry term in the DP mechanism that is induced by the substrate misorientation.

Realising epitaxial growth of GaN on (001) diamond

By an extensive investigation of the principal growth parameters on the deposition process, we realized the epitaxial growth of crystalline wurtzite GaN thin films on single crystal (001) diamond substrates by metal organic chemical vapor deposition. From the influence of pressure, V/III ratio, and temperature, it was deduced that the growth process is determined by the mass-transport of gallium precursor material toward the substrate. The highest temperature yielded an improved epitaxial relationship between grown layer and substrate. X ray diffraction (XRD) pole figure analysis established the presence of two domains of epitaxial layers, namely (0001) 〈101−0〉 GaN∥ (001)[110] diamond and (0001) 〈101−0〉 GaN∥ (001) [11−0] diamond, which are 90∘ rotated with respect to each other. The presence of these domains is explained by the occurrence of areas of (2×1) and (1×2) surface reconstruction of the diamond substrate. When applying highly misoriented diamond substrates toward the [110] diamond direction, one of the growth domains is suppressed and highly epitaxial GaN on (001) diamond is realized.

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ⩽ 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.

Magnetic anisotropy in (Ga,Mn)As grown on vicinal GaAs: Effects of the orientation of microwave magnetic field

Ferromagnetic resonance (FMR) study of the magnetic anisotropy in epitaxial (Ga,Mn)As grown on vicinal GaAs is presented. The data collected in the growth plane reveal very limited dependence on the miscut angle η, with a negligible effect of η on the in-plane uniaxial anisotropy. In the out-of-plane configuration, the substrate misorientation has a pronounced influence on FMR, which features are partially explained with a phenomenological treatment of the atomic step-induced anisotropy. This simple model, however, does not account for the miscut-induced dependence of FMR on the orientation of microwave magnetic field, that is observed with the static field applied invariably along the growth direction.

Competition of compressive strain with substrate misorientation in CuPt-type ordered GaInP/AlGaInP quantum wells

Temperature-dependent photoluminescence (PL) measurements are carried out on lattice-matched and compressively strained GaxIn1−xP/(Al0.66Ga0.34)yIn1−yP quantum wells (QWs) with CuPt-type long-range (LR) ordering. Experimental data show that compressive strain and substrate misorientation of the QWs affect the degree of LR ordering. The compressive strain competes with the misorientation of 6° off toward [111]A (denoted as 6 °A) significantly. It not only affects the distribution of domains with different degree of LR ordering in the x−y plane but also introduces fluctuation of the degree of LR ordering along the z direction of the QWs, which in turn causes the splitting of PL peaks. A phenomenological model is proposed to account for the experimental phenomena based on the principle of minimum total free energy. The results suggest that 6 °A misorientation should not be preferable for compressively strained GaxIn1−xP QWs with LR ordering.

Optimization of growth conditions for InGaAs/InAlAs/InP quantum cascade lasers by metalorganic chemical vapor deposition

We investigate the growth conditions for lattice-matched InGaAs/InAlAs/InP quantum cascade lasers (QCLs) by metalorganic chemical vapor deposition (MOCVD). Effect of substrate misorientation, growth temperature, and V/III ratios of InGaAs and InAlAs layers on the surface morphology, optical quality, and impurity incorporation were systematically studied. It was found that epitaxial layers and multi-quantum-well structures grown at 720 °C with V/III ratios of 116 for InGaAs and 21 for InAlAs on InP substrates with an off-cut angle of ∼0.06° exhibit a stable step-flow growth and low oxygen and carbon contamination. Using these conditions, a ∼11.3-μm-thick QCL with an emission wavelength at ∼9.2 μm was grown and fabricated, which demonstrated excellent structural quality and operated at room temperature in pulsed mode with a threshold current density of 2.0 kA/cm 2 and a slope efficiency of 550 mW/A.

Effect of substrate misorientation on the material properties of GaAs/Al0.3Ga0.7As tunnel diodes

The effect of substrate misorientation on the material quality of the N++–GaAs/P++–AlGaAs tunnel diodes (TDs) grown on these substrates is investigated. It is found that the misorientation influences both surface roughness and interface properties of the N++–GaAs/P++–AlGaAs TDs. Smooth surface (rms roughness: 1.54 Å) and sharp interface for the GaAs/Al0.3Ga0.7As TDs were obtained when the (100) tilted 10° off toward [111] GaAs substrate was used. Besides, the oxygen content in N++–GaAs and P++–AlGaAs layers grown on the 10° off GaAs substrates was reduced due to the reduction of sticking coefficient and number of anisotropic sites.

Indium incorporation into InGaN and InAlN layers grown by metalorganic vapor phase epitaxy

Experimental data on indium incorporation in InGaN and InAlN layers grown by metalorganic chemical vapor epitaxy (MOVPE) on bulk GaN substrates are presented and discussed. For the step-flow growth mode, realized for InGaN layers grown at relatively high temperatures (around 800°C), incorporation of indium increases with growth rate, and similarly, with a decrease in GaN substrate misorientation. Both dependences are explained by a higher velocity of flowing steps incorporating the indium atoms. For InAlN layers, three-dimensional nucleation takes place, and thus no significant changes of indium incorporation versus either growth rate or GaN substrate misorientation were observed.

Properties of N-polar GaN films and AlGaN/GaN heterostructures grown on (111) silicon by metal organic chemical vapor deposition

Smooth N-polar GaN films and GaN/AlGaN/GaN heterostructures with properties comparable to Ga-polar GaN-on-silicon films were grown by metal organic chemical vapor deposition on (111) silicon substrates with a misorientation angle of 3.5° toward the [112¯] direction. For heterostructures with a sheet electron density of 9×1012 cm−2 an electron mobility of 1500 cm2/V s was derived for transport parallel to the surface steps which formed as a result of the substrate misorientation.

Effect of carrier gas and substrate misorientation on the structural and optical properties of m-plane InGaN/GaN light-emitting diodes

The properties of Si-doped GaN (GaN:Si) thin films and InGaN/GaN light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition on free-standing {1 0 1̄ 0} m-plane GaN substrates were investigated with regard to carrier gas and substrate misorientation toward the [0 0 0 1̄] c − direction. The surface morphology of the GaN:Si thin films and the LEDs was found to be strongly dependent on the choice of carrier gas and substrate misorientation. Growth of GaN:Si thin films on nominally on-axis substrates produced surfaces with a high density of four-sided pyramidal hillocks, regardless of the composition of the carrier gas. In contrast, growth of GaN:Si thin films on substrates with misorientation angles greater than 0.7° yielded moderately to severely faceted surfaces when grown with H 2 as the carrier gas and atomically smooth surfaces when grown with N 2 as the carrier gas. Although the surface morphology varied significantly with substrate misorientation, the average indium mole fraction in the InGaN-based quantum wells, the output power of the LEDs, the electroluminescence (EL) peak wavelength, and the EL linewidth showed little dependence on substrate misorientation. Substrates with misorientation angles of about 1° were successfully used to grow LEDs with improved surface morphology but similar optical properties, compared to LEDs grown on nominally on-axis substrates.

Comparative study of InGaAs integration on bulk Ge and virtual Ge/Si(1 0 0) substrates for low-cost photovoltaic applications

Ge virtual substrates have been developed via low-temperature CVD based on new hydride molecular chemistry routes fully compatible with conventional CMOS. These are designed to enable integration of III–V compounds directly on Si and therefore have the potential to replace expensive Ge wafers in multijunction photovoltaics grown on the conventional 4–6″ format. Here we first describe in detail the protocols needed to produce defect-free and atomically flat Ge buffers with ∼0.25–3 μm thicknesses directly on vicinal (5°, 8°) and on-axis Si (1 0 0) substrates with up to 4″ diameters. Industrial MOCVD is then used to grow Ge-matched InGaAs films with thicknesses of 0.8–2.5 μm, both on our virtual substrates and on vicinal (6°) Ge wafers. A thorough characterization of the films’ morphological, structural, and optical properties allows a meaningful comparison of our best “virtual-substrate-grown” films with a mature and commercially available InGaAs technology on bulk Ge. Our studies confirm that the InGaAs films grown on Ge wafers exhibit the highest quality, followed closely by those grown on miscut Ge buffered Si. The latter films, in contrast to their on-axis counterparts, are devoid of antiphase-boundary defects and exhibit smoother surfaces and superior crystallinity, indicating the need for misoriented substrates to successfully integrate InGaAs on large area Si platforms. Collectively our work demonstrates the promise for transitioning our virtual substrate technology to the industrial scale production of photovoltaic III–IV films on Si(1 0 0) platforms.

N-Polar InAlN/AlN/GaN MIS-HEMTs

N-polar metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) were fabricated from a GaN/AlN/InAlN/GaN heterostructure grown by metalorganic chemical vapor deposition on a vicinal sapphire substrate, using Si3N4 as the gate insulator. Hall measurements in van der Pauw geometry on the heterostructure showed a sheet charge density and a mobility of 2.15 × 1013 cm-2 and 1135 cm2·V-1·s-1, respectively. Resistance measurements revealed anisotropic conductivity with respect to the surface steps induced by the substrate misorientation, and the sheet resistance of the 2-D electron gas was as low as 226 Ω/□ in the parallel direction. MIS-HEMTs with a gate length of 0.7 μm and a source-drain spacing of 2.2 μm had a peak drain current of 1.47 A/mm and an on-resistance of 1.45 Ω·mm. At a drain bias of 8 V, the current- and power-gain cutoff frequencies were 14 and 25 GHz, respectively.