Unstrained GaN Research Articles

The effect of biaxial tensile strain on structure and photoelectric properties of Fe-doped GaN monolayer

Under a biaxial tensile strain of 0% to 5%, the formation energy, band structure, dielectric function, absorption coefficient, reflectivity coefficient and loss function of Fe-doped two-dimensional (2D) GaN are systematically calculated based on the density functional theory, and the mechanism of bandgap variation is analyzed in detail. It is found that the bond lengths of Fe-N and Ga-N increase linearly and the bandgap changes quadratically with the increase of biaxial tensile strain. The results demonstrate that the 2D GaN: Fe is a direct bandgap semiconductor and its bandgap type would not be affected by biaxial tensile strain. The static dielectric constant of unstrained 2D GaN: Fe is 28.6, with the increase of biaxial tensile strain, the dielectric constant of 2D GaN: Fe increases, resulting in the charge storage capacity of 2D GaN: Fe enhancing and the parasitic capacitance increasing, the peak values of the absorption coefficient, refractive index and loss function increase with increasing strain, these results indicate that the biaxial tensile strain can seriously affect the optical properties of 2D GaN: Fe.

Study on AlN buffer layer for GaN on graphene/copper sheet grown by MBE at low growth temperature

Growth of III-nitrides on non-single-crystalline substrates at low growth temperature will greatly benefit the development of large-scale and flexible display/lighting technologies. Two-dimensional (2D) materials such as graphene and hexagonal boron nitride have the similar in-plane lattice arrangements to III-nitrides, hence III-nitrides can be grown on non-single-crystalline substrates by Van der Waals epitaxy through insertion of 2D material layers. In this paper, the growth of AlN buffer layer has been studied in order to obtain high-quality GaN film on graphene/copper sheet using molecular beam epitaxy at low growth temperature. The results reveal that the lateral coalescence of the AlN buffer layer is crucial in the initial stages of Van der Waals epitaxy. At low growth temperature, the migration enhanced epitaxy mode is an effect way to significantly enhance the lateral coalescence of AlN buffer layer on the surface of the graphene insertion layer. By adopting the AlN buffer layer as a template, unstrained GaN single-crystalline film is obtained on graphene/copper sheet at 530 °C. This result is believed to be applicable to Van der Waals epitaxy of III-nitride on other 2D material/non-single-crystalline substrates, which is a promising candidate for developing transferable and flexible devices.

Quality Improvement of GaN on Si Substrate for Ultraviolet Photodetector Application

GaN is grown on an Si substrate using metal-organic vapor-phase epitaxy. Compared with the full width at half maximum values from X-ray diffraction patterns and photoluminescence spectra of conventional GaN on the Si substrate, those of GaN on the Si substrate with the insertion of various-temperature AlN nucleation layers and Al0.3Ga0.7N/GaN superlattice intermediate layers are reduced by 34.9% and 25.6%, respectively. In addition, Raman spectra show that residual stress on the GaN epilayers decreased by 0.35 GPa. The c-lattice parameter of the GaN epilayer is 5.1844 Å, which is close to that of an unstrained GaN layer. Ultraviolet metal-semiconductor-metal photodetectors are fabricated on an almost-crack-free GaN surface. The dark current of a photodetector on the Si substrate is 2.4 ×10-11 A at a 9 V applied bias, which is one order of magnitude smaller than that of a photodetector on a conventional sapphire substrate. The maximum quantum efficiency value of a photodetector on the Si substrate is ~ 97% with an incident light wavelength of 360 nm and a 9 V applied bias.

Analytical and numerical studies on strain and crystal-orientation effects on the valence bands in hexagonal semiconductor layers and quantum wells

The effects of strain and crystal orientation on the effective masses of holes in bulk layers and in quantum wells (QWs) of hexagonal semiconductors are studied systematically within a six-band effective-mass theory. Analytical expressions of the effective masses in strained bulk layers are derived in some specific cases. The sum rule among the inverse effective masses in bulk layers as well as that among the inverse quantization masses in QWs is proved. Numerically calculated results on GaN bulk layers grown on non-polar planes reveal that strain modifies anisotropy in zone-center effective masses. The quantization energies of holes in unstrained GaN QWs are calculated as a function of the growth direction. The calculated results are also shown for quantization masses in (0 0 0 1) and ( 1 0 1 ¯ 0 ) strained QWs as functions of the well width and strain.

Optical properties of bulk‐like GaN nanorods grown on Si(111) substrates by rf‐plasma assisted molecular beam epitaxy

AbstractWe have investigated the optical properties of bulk‐like GaN nanorods by cathodoluminescence measurements at room temperature. The bulk‐like nanorods were grown on a Si (111) substrate by rf‐plasma assisted molecular beam epitaxy and are characterized be being surrounded by a compact region. A weak and broad emission line near 3.36 eV was observed at an accelerating voltage of 5 kV. This emission occurs at a lower energy than the emission from unstrained GaN nanorods that protrude from the compact region. Upon increasing the electron beam energy, and hence investigating regions deeper in the sample, the peak emission energy redshifts due to the strain at the substrate interface. The behaviour of the emission line is further investigated with cathodoluminescence measurement on single nanorod's (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strain states in semipolar III-nitride semiconductor quantum wells

Strain states in wurtzite III-nitride semiconductor quantum wells (QWs) are investigated. X-ray diffraction (XRD) reciprocal space mapping using semipolar (112¯2) and (11¯01) InGaN/GaN QWs as test samples demonstrates that the projections of reciprocal lattice vectors of unstrained GaN and pseudomorphically strained InGaN on the interface agrees, indicating continuity of layers across the interface. High resolution transmission electron microscopy supports the XRD analysis. Based on the experimental results, strain tensor elements are extracted for arbitrary crystalline orientation. Furthermore, expansion of the model to arbitrary crystal structures is suggested.

Time-resolved spectroscopy on GaN nanocolumns grown by plasma assisted molecular beam epitaxy on Si substrates

A detailed study of excitons in unstrained GaN nanocolumns grown by plasma assisted molecular beam epitaxy on silicon substrates is presented. The time-integrated and time-resolved photoluminescence spectra do not depend significantly on the (111) or (001) Si surface used. However, an unusually high relative intensity of the two-electron satellite peak of the dominant donor-bound exciton line is systematically observed. We correlate this observation with the nanocolumn morphology determined by scanning electron microscopy, and therefore propose an interpretation based on the alteration of wave functions of excitonic complexes and of donor states by the proximity of the semiconductor surface. This explanation is supported by a model that qualitatively accounts for both relative intensities and time decays of the photoluminescence lines.

Complete in-plane polarization anisotropy of the A exciton in unstrained A-plane GaN films

Using reflectance spectroscopy, the in-plane polarization behavior of unstrained C- and A-plane GaN films is experimentally investigated. While no in-plane polarization anisotropy is observed for all three band-gap related excitons (A, B, and C) in unstrained C-plane GaN films, the A exciton is completely linearly polarized perpendicular to the c axis in unstrained A-plane GaN films. However, the B and C excitons are only partially polarized. This observation is in excellent agreement with results based on band-structure calculations using the Bir-Pikus Hamiltonian for the wurtzite crystal structure.

Fully unstrained GaN on sacrificial AlN layers by nano‐heteroepitaxy

AbstractUsually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer, since compressive strain causes instabilities due to buckling effects. For group III‐nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield fully unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence and optical reflectance investigation of semipolar and nonpolar GaN

AbstractOptical properties of semipolar and nonpolar unstrained GaN are studied employing photoluminescence, optical reflectance and the k·p perturbation theory in this article. We find that donor bound and free A excitons show a strong polarization for photoluminescence in {11$ \bar 2 $2} and {10$ \bar 1 $0} GaN, while that of B excitons has only a little dependence on crystal orientations. Optical reflectance measurements are also performed to investigate resonance energies and transition probabilities of free excitons. The oscillator strength of A and C excitons has a strong dependency on the angle between c‐axis and growth direction, while that of B is nearly unchanged in every unstrained GaN plane. The experimental results can be well explained by our k·p calculations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical polarization anisotropy in strained A‐plane GaN films on R‐plane sapphire

AbstractWe have used polarized photoreflectance (PR) spectroscopy to study [11$ \bar 2 $0]‐oriented A‐plane GaN films on R‐plane sapphire substrates. For this type of films, the c‐axis of GaN lies in the film plane. When the probe beam is polarized perpendicular to the c‐axis, we observe a single feature in the PR spectrum, while two features at higher energies are observed for parallel polarization. Comparing the transition energies with the ones obtained by electronic band‐structure calculations, we identify the observed PR spectral features as the three possible transitions around the fundamental band gap of GaN. However in comparison to unstrained GaN, their polarization properties and energies have been significantly modified by the in‐plane anisotropic strain in the film. Finally, we can also determine the individual in‐plane anisotropic strain components. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Lattice Constants of GaN and InGaN on npn-Type GaN/InGaN Heterojunction Bipolar Transistors

We investigated the a- and c-axis lattice constants of GaN buffer and 180-nm-thick p-InGaN layers grown on SiC and sapphire substrates using reciprocal space mapping of the X-ray diffraction intensity. It was found that the a-axis lattice constant of the GaN buffer layer on a SiC substrate is larger than those of unstrained GaN and a GaN buffer layer on a sapphire substrate. As a result, the p-InGaN layer on GaN/SiC is fully strained even at the In mole fraction of 9.0% where that on GaN/sapphire is relaxed. This result means that fewer defects are generated in p-InGaN on GaN/SiC at higher In mole fractions. This is another advantage of SiC substrate for npn-type GaN/InGaN heterojunction bipolar transistors, in addition to its high thermal conductivity. The collector current density dependence of current gain shows the ideality factor of 2 for GaN/InGaN HBTs on both SiC and sapphire substrates. This is ascribed to the recombination current at the emitter-base interface, which arises from the threading dislocations generated at the interface between the substrate and nitride buffer layer.

Fully Unstrained GaN on Thick AlN Layers for MEMS Application

ABSTRACTUsually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer. Compressive strain would lead to instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure.

Anomalous behaviour of the photoluminescence from GaN/AlGaN quantum wells

Temperature and excitation power dependencies of photoluminescence from GaN/AlGaN quantum well structures are studied. We show that depending on the well width, the increase of the excitation power results in the blue or red shift of the emission energy. Temperature affects the observed emission energy shifts in a way which is entirely different from that for unstrained GaN bulk. Surprisingly, the observed differences are larger for wider wells. These results can be understood assuming that not only potential fluctuations but also built-in electric field and strain effects must be taken into account. The role of the particular mechanisms is discussed on the basis of performed calculations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the AlGaN buffer layer on the biaxial strain of GaN epilayers grown on 6H-SiC (0001) by molecular-beam epitaxy

The role of AlxGa1−xN buffers in the control of residual strain in GaN epitaxial layers grown on 6H-SiC (0001) substrates by molecular-beam epitaxy was investigated. The initial GaN layer on the AlxGa1−xN (x=0%–10%) buffer nucleates in two-dimensional form, which promotes a step-flow growth mode in subsequently grown GaN layers. As the Al content in the AlxGa1−xN (x∼20%) buffer was increased, GaN began to nucleate as incoherent polygonal islands, coalescing as the thickness increased to 15nm. The overall residual strain induced in GaN is tensile (εxx) for GaN grown directly on SiC and compressive (−εxx) for GaN grown using an AlGaN buffer. The stiffness coefficient (−2C13∕C33) of GaN on the AlxGa1−xN (x∼10%) buffer was estimated to be −0.53 assuming that the defect-induced hydrostatic stress of unstrained GaN (c0∕a0) was constant.

Optical and Structural Characteristics of Virtually Unstrained Bulk-Like GaN

Bulk-like GaN with high structural and optical quality has been attained by hydride vapor-phase epitaxy (HVPE). The as-grown 330 µm-thick GaN layer was separated from the sapphire substrate by a laser-induced lift-off process. The full width at half maximum values of the X-ray diffraction (XRD) ω-scans of the free-standing material are 96 and 129 arcsec for the (1 0 -1 4) and (0 0 0 2) reflection, respectively, which rank among the smallest values published so far for free-standing HVPE-GaN. The dislocation density determined by plan-view TEM images is 1–2×107 cm-2. Positron annihilation spectroscopy studies show that the concentration of Ga vacancy related defects is about 1.5×1016 cm-3. The high-resolution XRD, photoluminescence, µ-Raman, and infrared spectroscopic ellipsometry measurements consistently prove that the free-standing material is of high crystalline quality and virtually strain-free. Therefore it is suitable to serve as a substrate for stress-free growth of high-quality III–nitrides based device heterostructures.

Phase separation suppression in InGaN epitaxial layers due to biaxial strain

Phase separation suppression due to external biaxial strain is observed in InxGa1−xN alloy layers by Raman scattering spectroscopy. The effect is taking place in thin epitaxial layers pseudomorphically grown by molecular-beam epitaxy on unstrained GaN(001) buffers. Ab initio calculations carried out for the alloy free energy predict and Raman measurements confirm that biaxial strain suppress the formation of phase-separated In-rich quantum dots in the InxGa1−xN layers. Since quantum dots are effective radiative recombination centers in InGaN, we conclude that strain quenches an important channel of light emission in optoelectronic devices based on pseudobinary group-III nitride semiconductors.

Optical spectroscopic study of electric field sharing effects in piezoelectric GaN/Al0.65Ga0.35N multi-quantum well structures

We have studied a series of GaN/Al0.65Ga0.35N samples of varying well and barrier widths by cathodoluminescence and photoluminescence spectroscopy. These samples consist of unstrained GaN quantum wells and strained Al0.65Ga0.35N barriers. Some of the samples exhibit micro-cracks on their surface around which the barriers are partially relaxed and the quantum wells strained. The observation of the quantum well emission under two different strain conditions allows the relative effects of spontaneous polarization and piezoelectric polarization on the emission energies to be compared. We are able to accurately model the emission energies for both strain configurations if we consider the large electric fields present within the quantum wells to be due almost entirely to piezoelectric and field sharing effects.

MOVPE homoepitaxy of high-quality GaN: Crystal growth and devices

Epitaxial growth on GaN bulk single crystal substrates sets new standards in GaN material quality. The outstanding properties provide insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) with a precision not obtainable from heteroepitaxial growth on sapphire or SiC. With metalorganic vapor phase epitaxy (MOVPE) we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. By the use of dry etching techniques for surface preparation, an important improvement of crystal quality is achieved. Those layers reveal an exceptional optical quality as determined by a reduction of the low-temperature photoluminescence (PL) linewidth from 5 meV to 0.1 meV and a reduced X-ray diffraction (XRD) rocking curve width from 400 to 20 arcsec. As a consequence of the narrow PL linewidths, new features as, e. g. a fivefold fine structure of the donor-bound exciton line at 3.471 eV was detected. Additionally, all three free excitons as well as their excited states are visible in PL at 2 K. Dry etching techniques for surface preparation allow morphologies of the layers suitable for device applications. We report on InGaN/GaN multi-quantum-well (MQW)_ structures as well as GaN pn- and InGaN/GaN double heterostructure light emitting diodes (LEDs) on GaN bulk single crystal substrates. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping.

Determination of Band Structure Parameters in Nitride Alloys for Use in Quantum Well Calculations

The empirical pseudopotential method has been used to conduct band structure calculations on gallium and indium nitride and their alloys. Initially, the pseudopotential form factors required for the semi-empirical calculations were obtained using a Monte Carlo fitting approach to obtain good agreement of the predicted band structure with experimental and first-principles band structure results for bulk, unstrained GaN and InN. Empirical band structures were then found using a model based on the virtual crystal approximation for a series of compositions of the alloy GaInN. A complete set of K · p parameters have been extracted from our empirical models using a method which obtains the parameters directly from k · p formulae for them in terms of the wavefunctions and energies at the zone centre.