xN Superlattices Research Articles

Realizing crack-free high-aluminum-mole-fraction AlGaN on patterned GaN beyond the critical layer thickness

Wide-bandgap III-nitride heterostructures are required for a variety of device applications. However, this alloy system has a large lattice constant and thermal expansion coefficient mismatch that limits the alloy composition and layer thickness for many heteroepitaxial device structures. Consequently, various methods have been devised to allow the heteroepitaxial growth of AlInGaN heterostructures to accommodate this inherent strain. In this work, we describe a non-planar-growth approach that enables the deposition of crack-free high-Al-mole-fraction AlxGa1−xN on patterned GaN/sapphire templates and bulk GaN substrates with large-area mesas. We have studied the effects of the patterned mesa width, the mesa etch depth, and the gap between the mesas on the heteroepitaxy of AlxGa1−xN superlattices with an average Al molar fraction 0.11 < x¯ < 0.21 and non-planar overgrowth growth thicknesses up to 3.5 μm. Similar to the planar growth approach, increasing the thickness and Al mole fraction of the AlxGa1−xN superlattices leads to surface cracking when exceeding the critical layer thickness. However, limiting the mesa dimension in one direction enables strain mitigation and drastically increases the critical layer thickness. Additionally, larger etch depths of the mesas increase the Al alloy composition and thickness for crack-free AlGaN heteroepitaxy whereas the gap in between the mesas seems to have no crucial influence. We demonstrate that the Al alloy composition and layer thicknesses of such heterostructures can be increased far beyond the critical layer thickness for planar growth and demonstrate the growth of a crack-free full AlxGa1−xN/GaN quantum-well laser heterostructure designed for operation at ∼370 nm.

Investigation of coherency stress-induced phase separation in AlN/AlxGa1−xN superlattices grown on sapphire substrates

Suppression of compositional non-uniformity in AlxGa1−xN layers by using AlN/AlxGa1−xN superlattices through adjusting stresses in AlN/AlxGa1−xN layers is demonstrated.

Numerical Investigation of InGaN Light-Emitting Diode with Al/In-Graded p-AlGaN/InGaN Superlattice Electron-Blocking Layer

Three kinds of InGaN/GaN multiple-quantum-well light-emitting diode with different electron-blocking layers (EBLs) have been investigated numerically. The energy band diagrams, light–current curves, voltage–current curves, electrostatic fields, carrier concentrations in the quantum wells, electron current densities, radiative recombination rates in active region, and internal quantum efficiency (IQE) are reported. The results show that, when the conventional AlGaN EBL is replaced by a p-AlGaN/InGaN superlattice EBL or graded p-AlyGa1−yN/InxGa1−xN superlattice EBL, respectively, the light output power at 200 mA can be increased by 46.0% and 52.8%, the turn-on voltage can be decreased from 3.2 V to 3.0 V, the IQE can be increased by 45.6% and 53.8% at 200 mA, and the efficiency droop can be reduced from 31.2% to 21.6%. These improvements are mainly attributed to the properly modified energy band structures which favor carrier injection.

Growth and characterization of high quality AlN using combined structure of low temperature buffer and superlattices for applications in the deep ultraviolet

This study reports on the growth and characterization of AlN/AlxGa1−xN superlattices (SLs) for reduction of threading dislocations to grow high quality AlN layer. Insertion of optimized SLs is shown to effectively reduce the dislocation density, there by resulting in a high crystal quality 2.5-µm-thick AlN layer. It was found that full width at half-maximum (FWHM) of X-ray rocking curves (XRCs) around both (0002) and diffraction were decreased to 230 and 420, respectively. The results of XRC FWHM and dislocation densities from etch pit density (EPD) by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs are in a good agreement. This high quality and low dislocation density AlN layer is of key importance for high efficiency deep-UV light emitting diodes and power devices.

Development of epitaxial AlxSc1−xN for artificially structured metal/semiconductor superlattice metamaterials

Epitaxial nitride rocksalt metal/semiconductor superlattices are emerging as a novel class of artificially structured materials that have generated significant interest in recent years for their potential application in plasmonic and thermoelectric devices. Though most nitride metals are rocksalt, nitride semiconductors in general have hexagonal crystal structure. We report rocksalt aluminum scandium nitride (Al,Sc)N alloys as the semiconducting component in epitaxial rocksalt metal/semiconductor superlattices. The AlxSc1−xN alloys when deposited directly on MgO substrates are stabilized in a homogeneous rocksalt (single) phase when x < 0.51. Employing 20 nm TiN as a seed layer on MgO substrates, the homogeneity range for stabilizing the rocksalt phase has been extended to x < 0.82 for a 120 nm film. The rocksalt AlxSc1−xN alloys show moderate direct bandgap bowing with a bowing parameter, B = 1.41 ± 0.19 eV. The direct bandgap of metastable rocksalt-AlN is extrapolated to be 4.70 ± 0.20 eV. The tunable lattice parameter, bandgap, dielectric permittivity, and electronic properties of rocksalt AlxSc1−xN alloys enable high quality epitaxial rocksalt metal/AlxSc1−xN superlattices with a wide range of accessible metamaterials properties.

Growth and characterization of Cr-doped semi-insulating GaN templates prepared by radio-frequency plasma-assisted molecular beam epitaxy

Cr-doped semi-insulating GaN templates were grown using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). The samples were characterized by high resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The Cr incorporation was strongly dependent on the growth condition, which was primarily influenced by Cr cell temperature. The activation energy of dark conductivity was about 0.48eV which corresponds to the depth of the dominant electron traps pinning the Fermi level. The experimental results indicated that Cr doping did not affect the crystalline orientation of the GaN film but introduced more threading dislocations, and step-graded GaN/AlxGa1−xN superlattices (SLs) played an important role in hindering the penetration of the threading dislocations.

Full optical phonon states and their dispersive spectra of a wurtzite GaN/AlGaN superlattice: Quantum size effect

AbstractThe polar optical phonon states of four types of phonon modes including the interface optical (IO) modes, the propagating (PR) modes, the quasi‐confined (QC) phonon modes in well‐layer material (QCw), and the QC phonon modes in barrier‐layer material (QCB) in a wurtzite GaN/AlxGa1–xN superlattice (SLs) are investigated within the dielectric continuum model framework. The analytical phonon states of the four types of modes and their dispersive equations in the wurtzite GaN/AlxGa1–xN SL structures are obtained. Numerical calculations on the dispersive spectra of phonon modes and the quantum size effect are performed for a wurtzite GaN/Al0.15Ga0.85N SL. Results reveal that dispersive curves of phonon modes in SLs form a series of frequency bands. The behaviors of QCw modes reducing to PR and IO modes are observed clearly. With the increase of well‐layer GaN width d1 of the SLs, the dispersion of the QCB modes become weaker and weaker, and their frequency bands become narrower and narrower. But the PR, IO, and the QCw phonon modes become more dispersive, and the frequency bands of these modes become wider and wider as d1 increases. The present theoretical scheme and numerical results are quite useful for analyzing the dispersive spectra of full phonon modes and their polaronic effect in wurtzite GaN/AlGaN SL structures.

Study on threading dislocations blocking mechanism of GaN∕AlxGa1−xN superlattices

Ga N ∕ Al x Ga 1 − x N superlattices (SLs) with different period thicknesses tp were grown as interlayers between GaN and AlyGa1−yN epilayers. The effect of threading dislocations (TDs) blocking became more evident with increasing tp. Transmission electron microscopy analysis shows that TDs are inclined to be bended in SLs and terminated in GaN wells as a result of strain. X-ray diffraction measurement also validated that GaN wells played a more important role as a TDs filter. The blocking of TDs in SLs resulted in an abnormal decrease in relaxation factors R(SLs) with increasing tp.

Enhanced Pockels effect in GaN∕AlxGa1−xN superlattice measured by polarization-maintaining fiber Mach-Zehnder interferometer

Six-period 4nm GaN∕10nm AlxGa1−xN superlattices with different Al mole fractions x were prepared on (0001) sapphire substrates by low-temperature metal-organic chemical vapor deposition. The linear electro-optic (Pockels) effect was studied by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55μm. The measured electro-optic coefficients, γ13=5.60±0.18pm∕V, γ33=19.24±1.21pm∕V (for sample 1, x=0.3), and γ13=3.09±0.48pm∕V, γ33=8.94±0.36pm∕V (for sample 2, x=0.1), respectively, are about ten times larger than those of GaN bulk material. The enhancement effect in GaN∕AlxGa1−xN superlattice can be attributed to the large built-in field at the interfaces, depending on the mole fraction of Al.

Raman Spectroscopy as a Tool for Characterization of Strained Hexagonal GaN/AlxGa1?xN Superlattices

It is shown that such parameters of GaN/Al x Ga 1-x N superlattice as the period, built-in strain, composition of the alloy, and individual layer thicknesses can be extracted from the energy positions, intensities, and line shapes of various optical and acoustic modes detected in Raman scattering.

Recent Progress in GaN-Based Superlattices for Near-Infrared Intersubband Transitions

A review of the recent progress in intersubband transitions in GaN-based superlattice structures grown by plasma-assisted molecular beam epitaxy (MBE) is presented. Careful control of the growth parameters resulted in reduced threading dislocation density as well as optimized interfaces for the superlattices. Intersubband absorption has been observed from a variety of configurations involving GaN single or asymmetric double quantum wells with either thick AlxGa1—xN barriers or short-period GaN/AlxGa1—xN superlattice barriers. The peak wavelength of absorption can be varied between 1.4–4.2 μm by changing the quantum well thickness. Electron scattering times were measured by the pump–probe technique and were found to be 240–330 fs at 1.55 μm. In addition, intersubband transitions have also been observed for GaN/AlN superlattices grown with a non-polar (110) orientation.

Fabrication of a low-threading-dislocation-density AlxGa1−xN buffer on SiC using highly Si-doped AlxGa1−xN superlattices

An in situ technique to reduce the threading dislocation density (TDD) of an AlxGa1−xN buffer within submicron-thick growth is demonstrated using metalorganic vapor-phase epitaxy. We achieved a marked reduction in the TDD of the AlGaN buffer on a SiC substrate by inserting superlattices (SLs) consisting of highly Si-doped AlGaN and undoped AlGaN layers. The TDD of AlGaN decreased from 2×1010 to 7×107 cm−2 by inserting SLs with total growth thickness of 0.8 μm. The Si incorporation in the highly Si-doped AlGaN layers of the SLs was estimated to be approximately 1.2×1020 cm−3 (0.24%). This is strictly an in situ technique that does not require complicated fabrication processes, and the surface is kept flat throughout the entire growth. This method is especially useful on SiC wafers to prevent cracks in a thin growth layer.

Growth and Characterization of GaN/AlGaN Superlattices for Near-Infrared Intersubband Transitions

We report the results of the growth by molecular beam epitaxy of GaN/AlxGa1—xN superlattice (SL) structures with single or double quantum wells and thick AlxGa1—xN or short-period SL barriers. The dependence of intersubband absorption on the well width for single well SL structures was systematically investigated. For GaN quantum wells interleaved with SL barriers, the upper state electron confinement was maintained by Bragg reflection from the minigaps formed within the barriers. In addition, evidence of electron transfer from doped SL barriers to undoped GaN quantum wells was obtained. Abrupt interfaces and coherent periodicities of the SL structures were confirmed by transmission electron microscopy and X-ray diffraction measurements. Intersubband absorption at peak wavelengths between 1.4 and 4.2 μm has been observed.

Characteristics of Ti/Pt/Au Ohmic Contacts on p-type GaN/AlxGa1−xN Superlattices

Ti/Pt/Au metallization on p-type GaN/AlxGa1−xN (x=0.10 and 0.20) superlattices (SL) were investigated as ohmic contacts. Current-voltage and specific contact resistance measurements indicate enhanced p-type doping in the superlattice structures compared to that in GaN. Ti/Pt/Au is shown to be an effective ohmic metallization scheme on p-type GaN/AlxGa1−xN superlattices. A specific contact resistance of Rc = 4.6×10−4 ω-cm2 is achieved for unalloyed Ti/Pt/Au on GaN/Al0.2Ga0.8N SL. This is reduced to 1.3×10−4 ω-cm2 after annealing for 5 minutes at 300 °C.

Confined acoustic and propagating optical phonons in GaN/Ga 1-xAl xN superlattices

The lattice vibrations in GaN/Ga 1-xAl xN(001) superlattices are studied within a rigid-ion model. The elastic forces are characterized by a generalized three-parameter Keating model, whereas the microscopic electric field is fully included via an Ewald summation. The composition and the disorder in the barriers are described within a generalized REI model. Cubic symmetry is assumed for each single material layer in the superlattice. In contrast to the GaAs/Ga 1-xAl xAs case we observe a confinement of the acoustic phonons in the frequency region between the zone-boundary phonons of GaN and AlN. In the region of optical phonons both propagating and confined modes are found. The results are explained in terms of the light common anion of the compounds.