Freestanding Wurtzite Research Articles

Influencing the surface quality of free-standing wurtzite gallium nitride in ultra-high vacuum: Stoichiometry control by ammonia and bromine adsorption

Gallium nitride (GaN), a wide bandgap semiconductor with absorption and emission in the ultraviolet/visible range, is proposed as an alternative to metallic surfaces for assembling organic molecular structures aiming at optoelectronic applications. However, the formation of a persistent surface oxide layer in air considerably limits the use of GaN for well-defined interfaces. In this work, we have investigated, characterized and processed n-type free-standing c-plane hexagonal wurtzite GaN crystals grown by hydride vapor phase epitaxy and ammonothermal growth methods. Surface cleaning and full removal of the oxide layer on GaN surfaces could be reproducibly achieved via sputtering and annealing cycles, as evidenced by X-ray photoelectron spectroscopy and low-energy electron diffraction. Scanning tunneling microscopy, however, indicated substantial roughening of the GaN surface and the formation of unwanted Ga-rich islands and clusters. Although ammonia (NH3) and bromine (Br) treatments compensated the N/Ga atoms ratio reduced by sputtering, the surface morphology remained rough, exhibiting randomly shaped and distributed hillocks. In addition, we studied the effect of electron bombardment on the surface quality of GaN during NH3 annealing, on-surface debromination and polymerization of 1,3,5-tris(4-bromophenyl) benzene on GaN, and the removal of Ga atoms by Br atoms during the desorption.

Physics of high-efficiency 240–260 nm deep-ultraviolet lasers and light-emitting diodes on AlGaN substrate

High-efficiency III-nitride deep-ultraviolet (DUV) lasers and light-emitting diodes (LEDs) with emission wavelengths of 240–260 nm are extremely difficult to realize due to large defect density from III-nitride materials and existence of optical polarization crossover from conventional AlGaN-based quantum wells (QWs). Free-standing wurtzite AlGaN templates have been studied and developed recently; however, the physics and optical properties of AlGaN-based emitters on AlGaN templates are still relatively lacking. Therefore, this work theoretically investigates the optical properties and quantum efficiencies of the AlGaN-based QW on AlGaN substrates. The physics analysis based on a self-consistent 6-band k⋅p model shows the transverse electric (TE)-polarized optical gain increases from 558 cm−1 by using Al0.51Ga0.49N/AlN QW on the AlN substrate to 2875 cm−1 by using Al0.48Ga0.52N/Al0.72Ga0.28N QW on the Al0.72Ga0.28N substrate at 260 nm, which is attributed to the reduced strain effect and valence band rearrangement by using the AlGaN substrate. Correspondingly, the radiative recombination efficiency increases 1.66–4.43 times based on different Shockley–Read–Hall coefficients, indicating the promising potential of the use of the AlGaN substrate for high-efficiency DUV lasers and LEDs.

Au-catalysed free-standing wurtzite structured InAs nanosheets grown by molecular beam epitaxy

In this study, we report the growth of free-standing InAs nanosheets using Au catalysts in molecular beam epitaxy. Detailed structural characterizations suggest that wurtzite structured InAs nanosheets, with features of extensive $$\{11\overline{2}0\}$$ surfaces, grown along the $$ $$ direction and adopted {0001} nanosheet/catalyst interfaces, are initiated from wurtzite structured $$[000\overline{1}]$$ nanowires as the inclined epitaxial growth due to relatively higher In concentrations in Au catalysts, and grown from these inclined nanostructures through catalyst-induced axial growth and their enhanced lateral growth under the high growth temperature. Based on the facts that the nanosheets contain large low energy $$\{11\overline{2}0\}$$ surfaces and {0001} nanosheet/catalyst interfaces, the growth of our nanosheets is a thermodynamically driven process. This study provides new insights into fabricating free-standing III-V nanosheets for their applications in future nanoscale devices.

Contactless Optical Characterization of Carrier Dynamics in Free-Standing InAs-InAlAs Core-Shell Nanowires on Silicon.

Contactless time-resolved optical pump-probe and external quantum efficiency measurements were performed in epitaxially grown free-standing wurtzite indium arsenide/indium aluminum arsenide (InAs-InAlAs) core-shell nanowires on Si (111) substrate from 77 to 293 K. The first independent investigation of Shockley-Read-Hall, radiative, and Auger recombination in InAs-based NWs is presented. Although the Shockley-Read-Hall recombination coefficient was found to be at least 2 orders of magnitude larger than the average experimental values of other reported InAs materials, the Auger recombination coefficient was reported to be 10-fold smaller. The very low Auger and high radiative rates result in an estimated peak internal quantum efficiency of the core-shell nanowires as high as 22% at 77 K, making these nanowires of potential interest for high-efficiency mid-infrared emitters. A greater than 2-fold enhancement in minority carrier lifetime was observed from capping nanowires with a thin InAlAs shell due to the passivation of surface defects.

Wurtzite GaP nanowire grown by using tertiarybutylchloride and used to fabricate solar cell

Growth of a freestanding wurtzite gallium phosphide (GaP) nanowire was investigated for solar-energy conversion applications. Tertiarybutylchloride (TBCl) was used to grow straight nanowires at high temperatures at which wurtzite GaP forms. By alternating the supply of triethylgallium with tertiarybutylphosphine and TBCl, we could form straight wurtzite GaP nanowires with few stacking faults. We also investigated p- and n-type dopants and found a concentration level of about 1018 cm−3 for each dopant. We fabricated a solar cell from an axially pin-type GaP nanowire diode sample and found that the nanowire-grown area had a high photocurrent density due to the light-concentrating property of the individual nanowires.

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require AlxGa1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of AlxGa1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite AlxGa1-xN wafers. Zinc-blende and wurtzite AlxGa1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and AlxGa1-xN wafers. At a thickness of ∼30µm, free-standing GaN and AlxGa1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and AlxGa1-xN wafers with thicknesses in the 30–100μm range may be used as substrates for further growth of GaN and AlxGa1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride AlxGa1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.

Luminescence characterizations of freestanding bulk single crystalline aluminum nitride towards optoelectronic application

Freestanding wurtzite aluminum nitride bulk single crystals were massively grown, exhibiting deep UV band edge and broad defect-related emissions.

Growth of free-standing bulk wurtzite AlxGa1−xN layers by molecular beam epitaxy using a highly efficient RF plasma source

The recent development of group III nitrides allows researchers world-wide to consider AlGaN based light emitting diodes as a possible new alternative deep ultra–violet light source for surface decontamination and water purification. In this paper we will describe our recent results on plasma-assisted molecular beam epitaxy (PA-MBE) growth of free-standing wurtzite AlxGa1−xN bulk crystals using the latest model of Riber's highly efficient nitrogen RF plasma source. We have achieved AlGaN growth rates up to 3µm/h. Wurtzite AlxGa1−xN layers with thicknesses up to 100μm were successfully grown by PA-MBE on 2-inch and 3-inch GaAs (111)B substrates. After growth the GaAs was subsequently removed using a chemical etch to achieve free-standing AlxGa1−xN wafers. Free-standing bulk AlxGa1−xN wafers with thicknesses in the range 30–100μm may be used as substrates for further growth of AlxGa1−xN-based structures and devices. High Resolution Scanning Transmission Electron Microscopy (HR-STEM) and Convergent Beam Electron Diffraction (CBED) were employed for detailed structural analysis of AlGaN/GaAs (111)B interface and allowed us to determine the N-polarity of AlGaN layers grown on GaAs (111)B substrates. The novel, high efficiency RF plasma source allowed us to achieve free-standing AlxGa1−xN layers in a single day's growth, making this a commercially viable process.

Self-assembled InN micro-mushrooms by upside-down pendeoepitaxy

Self-assembly of hexagonal InN micro-mushrooms on Si (111) substrates by molecular beam epitaxy is reported. Scanning electron microscopy (SEM) reveals hexagonal mushroom caps with smooth top surfaces and a step-like morphology at the bottom surface. A detailed growth study along with SEM measurements reveals that an upside-down pendeoepitaxy mechanism underlies the formation of these structures. Cryogenic temperature photoluminescence measurements on the InN disks show a dominant band-to-acceptor recombination peak at 0.68eV. Cross-section annular bright field (ABF-) scanning transmission electron microscopy (STEM) reveals that the growth of these structures occurs along the [0001¯] crystallographic orientation (N-face). Plan-view high angle annular dark field (HAADF) STEM in the center of the micro-disks reveals a hexagonal lattice indicative of stacking faults. However, at the outskirt of the micro-disk, surprisingly, a honeycomb lattice is observed in plan view STEM indicating a perfect freestanding Wurtzite InN disk that is free of stacking faults. This result opens a pathway for realizing strain-free, freestanding InN substrates.

Growth of free-standing wurtzite AlGaN by MBE using a highly efficient RF plasma source

Ultraviolet light emitting diodes (UV LEDs) are now being developed for various potential applications including water purification, surface decontamination, optical sensing, and solid-state lighting. The basis for this development is the successful production of AlxGa1−xN UV LEDs grown by either metal-organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). Initial studies used mainly sapphire as the substrate, but this result in a high density of defects in the epitaxial films and now bulk GaN or AlN substrates are being used to reduce this to acceptable values. However, the lattice parameters of GaN and AlN are significantly different, so any AlGaN alloy grown on either substrate will still be strained. If, however, AlGaN substrates were available, this problem could be avoided and an overall lattice match achieved. At present, the existing bulk GaN and AlN substrates are produced by MOVPE and physical vapor transport, but thick free-standing films of AlGaN are difficult to produce by either method. The authors have used plasma-assisted MBE to grow free-standing AlxGa1−xN up to 100 μm in thickness using both an HD25 source from Oxford Applied Research and a novel high efficiency source from Riber to provide active nitrogen. Films were grown on 2- and 3-in. diameter sapphire and GaAs (111)B substrates with growth rates ranging from 0.2 to 3 μm/h and with AlN contents of 0% and ∼20%. Secondary ion mass spectrometer studies show uniform incorporation of Al, Ga, and N throughout the films, and strong room temperature photoluminescence is observed in all cases. For films grown on GaAs, the authors obtained free-standing AlGaN substrates for subsequent growth by MOVPE or MBE by removing the GaAs using a standard chemical etchant. The use of high growth rates makes this a potentially viable commercial process since AlxGa1−xN free-standing films can be grown in a single day and potentially this method could be extended to a multiwafer system with a suitable plasma source.

Molecular beam epitaxy of free-standing wurtzite AlxGa1−xN layers

Recent developments with group III nitrides present AlxGa1−xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1−xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1−xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1−xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1−xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1−xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1−xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8µm/h on 2-in. diameter GaAs and sapphire wafers.

Polaron effect on linear and nonlinear intersubband optical absorption of a wurtzite GaN-based nanowire

Based on the density matrix approach and the perturbation treatment, the polaronic effect on the linear and nonlinear intersubband optical absorption coefficients in quasi-one-dimensional wurtzite nanowires (NWs) is investigated. The simple analytical formulas for polaronic states and the linear and nonlinear optical absorption coefficients in the NW systems are also deduced. Numerical calculation on a freestanding wurtzite GaN NW is performed. The polaronic effect and quantum size effect as well as the influence of incident optical intensity on the optical absorption properties are analyzed and discussed. The results show that, the polaronic effect results in significant increase of the absorption coefficients, and obvious decrease of saturation absorption intensity. In order to observe strong optical absorption, a nitride NW with relative small radius should be chosen. Moreover, a comparison with the situation of GaN-based quantum well systems is also done, and the profound physics is analyzed reasonably. • Polaronic states including the IO and QC phonon modes in wurtzite nitride NWs are obtained. • Polaronic effect results in significant increase of the linear and nonlinear absorption coefficients. • Taking the polaronic effect into account, saturation absorption intensity decreases obviously. • GaN-based NWs with smaller radius should be chosen for observing stronger optical absorption.

First-order Raman scattering in cylindrical wurtzite nanowire

Abstract We have presented a theoretical study on electron resonant Raman scattering (ERRS) process associated with the bulk longitudinal optical (LO), surface optical (SO) and quasi-confined (QC) phonon modes in a free-standing wurtzite nanowire (NW). We consider the Frohlich electron–phonon interaction in the framework of the dielectric continuum model. Numerical calculations on the GaN material reveal that differential cross-section (DCS) is sensitive to the wire size. The bulk LO and high-frequency quasi-confined (QC+) phonons make main contributions to the DCS and the impact of the SO phonon can be negligible in the ERRS process. Moreover, scattering intensity of the bulk LO phonon is strongly enhanced as the incident photon energy approaches the energy band-gap of the GaN.

Single-step growth dynamics of core–shell GaN on Ga2O3 freestanding nanoprotruded microbelts

Freestanding wurtzite GaN nanoprotruded microbelts with Ga2O3 core, with typical thickness 1–10 μm, and length of few millimeters are synthesized by thermal annealing of Ga metal and subsequent reaction with ammonia at a low flow rate. They are of distinctive rectangular shape with a typical width of 10–100 μm. Thickness of the belt is about 1/10th of the width and length up to a few millimeters. The GaN, Ga2O3 layers and the GaN–Ga2O3 interface are characterized by high-resolution transmission electron microscopy after focused ion beam sectioning of the belt. Initially, Ga2O3 nucleates after reaction with the O2 available in the environment, and subsequent reaction with NH3 results in the formation of core–shell structure in the catalyst-free vapor–solid growth process. Having a low-symmetry phase, Ga2O3 can grow only in certain preferred directions thus controlling the final morphology of the belt. Nanoscale protrusions ~50–100 nm found on the surface of the belts could be an ideal system for building functional devices.

Zinc-blende and wurtzite AlxGa1−xN bulk crystals grown by molecular beam epitaxy

There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1−xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1−xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (∼10μm) zinc-blende and wurtzite AlxGa1−xN films were grown by PA-MBE on 2-in. GaAs (001) and GaAs (111)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1−xN wafers can be achieved by PA-MBE for a wide range of Al compositions.

Characterization of freestanding semi-insulating Fe-doped GaN by photoluminescence and electromodulation spectroscopy

A detailed optical characterization of a freestanding wurtzite semi-insulating Fe-doped GaN (SI-GaN:Fe) grown by hydride-vapor-phase-epitaxy was carried out by photoluminescence (PL) and contactless electroreflectance (CER) at 10 and 300 K, respectively, and photoreflectance at 300 K. Low-temperature PL spectrum of the Ga-face consisted of the Fe3+ impurity characteristic series of IR peaks with a sharp zero-phonon line (ZPL) at 1.299 eV, yellow and blue broad emission bands, and near-band-edge (NBE) emission in the ultraviolet region. The narrow linewidth of 135 μeV of the ZPL and the measured energy difference of ∼70 meV between the ZPL and E2 (high) phonon replica, which is sensitive to the lattice strain, shows good crystal quality and a strain-free incorporation of iron. The obtained transition energies of A, B, and C excitonic features in the CER spectra and the n = 2 excited states (2s) of the A and B excitons enable the estimation of the exciton binding energies. In addition to the free excitonic recombination, the PL spectrum of the Ga-face exhibited clear donor and acceptor related features in the NBE region, while the N-face exhibited a broad emission band related to the free-to-bound recombination only. The differences were explained by the presence of impurity-induced band-tail states in the N-face SI-GaN:Fe due to an increased impurity density and the incorporation of large volume vacancy-type defects.

SURFACE POLARON SELF-ENERGY AND EFFECTIVE MASS IN A WURTZITE GaN NANOWIRE

The ground-state self-trapping energy and effective mass of surface polarons in a freestanding wurtzite GaN nanowire (NW) are studied using the second-order perturbation approach. Based on the dielectric continuum and Loudon's uniaxial crystal models, the polar optical phonon modes in the one-dimensional (1D) systems are analyzed, and the vibrating spectra of surface optical (SO) modes and electron–SO phonon coupling functions are discussed and analyzed. The calculations of the ground-state polaron self-trapping energy and the correction of effective mass due to the SO phonon modes in the 1D GaN NWs reveal that the polaron self-trapping energy and the correction of effective mass is far larger than those in 1D GaAs NW systems. The reasons for this obvious difference in the two 1D structures can be attributed to the different electron–phonon coupling constants and electron effective masses of bulk material constituting the two types of 1D confined systems. Finally, the polaronic properties of the wurtzite 1D GaN NWs are compared with those of wurtzite GaN -based two-dimensional quantum wells. The physical origins leading to these characteristics and their distinction in the different-dimensionality systems is carefully analyzed.

Polaronic effects due to surface optical vibration modes in a freestanding cylindrical wurtzite nanowire

The ground-state polaron self-trapped energy and effective mass due to the surface optical (SO) phonon modes in a freestanding wurtzite GaN nanowire (NW) were studied by means of the Lee–Low–Pines variational approach. Based on the dielectric continuum and Loudon’s uniaxial crystal models, the polar optical phonon modes in the one-dimensional (1D) systems are analyzed, and the vibrating spectra of SO modes and electron–SO phonon coupling functions are discussed and analyzed. The calculations on the ground-state polaron self-trapped energy and correction of effective mass due to the SO phonon modes in the 1D GaN NWs reveal that the polaron self-trapped energy and correction of effective mass are far larger than those in 1D GaAs NW systems. The reasons resulting in this obvious difference in the two 1D structures are mainly due to the different electron–phonon coupling constants and electron effective masses of bulk materials constituting the two types of 1D confined system. Finally, the polaronic properties of the wurtzite 1D GaN NWs have been compared with those of the wurtzite GaN-based two-dimensional quantum wells. The physical origination of these characteristics and their distinction in the different-dimensionality systems has been analyzed in depth.

Polaronic self-trapped energy and effective mass of a cylindrical wurtzite GaN nanowire

The ground-state self-trapped energy and effective mass of the polaron in a freestanding wurtzite GaN nanowire (NW) are studied by employing the perturbation approach. Based on the dielectric continuum model and the Loudon uniaxial crystal model, the full polar optical phonon modes, i.e. quasi-confined (QC) phonon modes and surface optical (SO) phonon modes, in the one-dimensional (1D) wurtzite systems have been analyzed. The vibrating spectra of the two types of phonon modes and their electron–phonon coupling properties have been discussed and analyzed. The numerical results on the ground-state polaron self-trapped energy and correction of effective mass in the 1D GaN NWs reveal that these are far larger than those in 1D GaAs NW systems. The reasons resulting in this obvious difference in the two 1D structures can be attributed to the different electron–phonon coupling constants and electron effective masses of bulk materials constituted of the two types of 1D confined systems. Moreover, our calculated results regarding the polaronic self-trapped energy are consistent with the recent experimental observation in GaN NW systems.

ZnO nanorod light-emitting diodes fabricated by electrochemical approaches

We report simple electrochemical methods for rapid growth of freestanding wurtzite ZnO nanorods in an aqueous solution and the following fabrication of a ZnO nanorod/poly(3-methylthiophene) heterojunction light-emitting diode. The typical electroluminescence spectra obtained from this heterostructure device under forward bias show a strong ultraviolet peak and a weak visible emission. The emission of the heterostructure device is much stronger than that of the ZnO-only structure and the turn-on voltage is about 7 V.