xN Alloys Research Articles

Si doping of high-Al-mole fraction AlxGa1−xN alloys with rf plasma-induced molecular-beam-epitaxy

Very high levels of n-type doping of AlxGa1−xN alloys were recently achieved by rf plasma-induced molecular-beam epitaxy on sapphire substrates and Si as a dopant. Electron concentrations were obtained up to 1.25×1020 cm−3 when the Al mole fraction was 50%, and 8.5×1019 cm−3 electrons were measured even when the Al mole fraction was 80%. Other material properties were determined by optical absorption, photoluminescence, cathodoluminescence, x-ray diffraction, and atomic force microscopy measurements and high optical and morphological qualities were shown.

Phase Separation, Gap Bowing, and Structural Properties of Cubic InxAl1—xN

We present first-principles calculations of thermodynamic, structural, and electronic properties of cubic InxAl1—xN alloys. They are based on the generalized quasi-chemical approach to disorder and composition effects and a pseudopotential plane-wave approximation for the total energy. The cluster treatment is generalized to study the influence of biaxial strain. We obtained the phase diagram T versus x of the alloys. The interplay of miscibility gap and strain influence is analysed. We predict bond lengths in the alloy. We found that gap fluctuations are very important, and provide an explanation for the discrepancies found in experimental values for the bowing parameter.

Phase diagram, chemical bonds, and gap bowing of cubic InxAl1−xN alloys: Ab initio calculations

Thermodynamic, structural, and electronic properties of cubic InxAl1−xN alloys are studied by combining first-principles total energy calculations and the generalized quasichemical approach. Results for bond-lengths, second-nearest-neighbors distances, and bond angles in the alloy are presented. The calculated phase diagram of the alloy shows a broad and asymmetric miscibility gap. The gap fluctuations in the alloy allow for the definition of a minimum gap and an average gap with different bowing parameters, that can provide an explanation for the discrepancies found in the experimental values for the bowing parameter. It is also found that lattice matched In0.2Al0.8N with GaN is suitable to form a barrier material for electronic and optoelectronic nitride based devices.

Probing the AlxGa1−xN spatial alloy fluctuation via UV-photoluminescence and Raman at submicron scale

We present a straightforward method for the study of alloy spatial compositional distribution at the submicron scale via photoluminescence and Raman spectroscopy. The spatial dependence of the band gap light-emission energy of AlxGa1−xN alloys at composition 0⩽x⩽1 was studied via deep UV-photoluminescence and Raman microscopy in order to address the issue of the spatial alloy fluctuation. The data were acquired in a random fashion from an area of ∼1 mm2 on the sample at steps of ∼1–200 μm utilizing the 244 nm laser line of probing spot size ∼300 nm radius. Our study indicates that the photoluminescence emission energy exhibits random type variations depending on locality: the alloys of composition x=0.12, x=0.22, x=50, and x=0.70 exhibit average variations of ∼10, 30, 45, and 25 meV, respectively. The photoluminescence of the pure GaN exhibits no significant spatial fluctuation. The stress contribution to the observed photoluminescence fluctuations was investigated via Raman analysis and was taken into account in order to estimate the local compositional fluctuation Δx. Our results indicate that for the higher Al composition alloys x=0.50 and 0.70 the stress and the compositional fluctuation can be resolved, resulting in average spatial fluctuations of Δx=0.004 and 0.002, respectively.

Comment on ?Band Gap of InN and In-Rich InxGa1?xN Alloys (0.36 < x < 1)?

physica status solidi (b)Volume 233, Issue 3 p. R8-R9 Rapid Research Note Comment on “Band Gap of InN and In-Rich InxGa1—xN Alloys (0.36 < x < 1)” B.R. Nag, B.R. Nag brn@cucc.ernet.in Institute of Radio Physics and Electronics, 92 Acharya Prafulla Chandra Road, Kolkata-700009, IndiaSearch for more papers by this author B.R. Nag, B.R. Nag brn@cucc.ernet.in Institute of Radio Physics and Electronics, 92 Acharya Prafulla Chandra Road, Kolkata-700009, IndiaSearch for more papers by this author First published: 26 September 2002 https://doi.org/10.1002/1521-3951(200210)233:3<R8::AID-PSSB99998>3.0.CO;2-ICitations: 8AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume233, Issue3October 2002Pages R8-R9 RelatedInformation

Energy band bowing parameter in AlxGa1−xN alloys

Molecular-beam epitaxy grown AlxGa1−xN alloys covering the entire range of alloy compositions, 0⩽x⩽1, have been used to determine the alloy band gap dependence on its composition. The Al chemical composition was deduced from secondary ion mass spectroscopy and Rutherford backscattering. The composition was also inferred from x-ray diffraction. The band gap of the alloy was extracted from low temperature optical reflectance measurements which are relatively more accurate than photoluminescence. Fitting of the band gap data resulted in a bowing parameter of b=1.0 eV over the entire composition range. The improved accuracy of the composition and band gap determination and the largest range of the Al composition over which our study has been conducted increase our confidence in this bowing parameter.

Achieving highly conductive AlGaN alloys with high Al contents

Si-doped n-type AlxGa1−xN alloys were grown by metalorganic chemical vapor deposition on sapphire substrates. We have achieved highly conductive n-type AlxGa1−xN alloys for x up to 0.7. A conductivity (resistivity) value of 6.7 Ω−1 cm−1 (0.15 Ω cm) (with free electron concentration 2.1×1018 cm−3 and mobility of 20 cm2/Vs at room temperature) has been achieved for Al0.65Ga0.35N, as confirmed by Hall-effect measurements. Our experimental results also revealed that (i) the conductivity of AlxGa1−xN alloys continuously increases with an increase of Si doping level for a fixed value of Al content and (ii) there exists a critical Si-dopant concentration of about 1×1018 cm−3 that is needed to convert insulating AlxGa1−xN with high Al content (x⩾0.4) to n-type.

Vibrational Properties of Cubic AlxGa1?xN and InxGa1?xN Ternary Alloys

The phonon dispersion curves and phonon density of states of the cubic AlxGa1—xN and InxGa1—xN alloys were calculated within the framework of a valence force field method and an effective charge approach for the long-range Coulomb interaction. The valence-force parameters and effective charges for the cubic GaN, AlN and InN binary compounds were fitted to the phonon frequencies at high-symmetry points in the Brillouin zone measured by Raman spectroscopy and calculated by ab initio methods. The scaling factor approximation was used to obtain the valence-force parameters and effective charges for the alloys. A good agreement between our calculations and recent Raman data for the alloys is achieved.

Interplay between GaN and AlN sublattices in wurtzite AlxGa1−xN alloys revealed by Raman spectroscopy

The large frequency shift displayed by the longitudinal optical (LO) phonons A1(LO) and E1(LO) when going from GaN to AlN promises an accurate determination of the composition in AlxGa1−xN bulk layers by Raman spectroscopy. However, this determination is affected by a large uncertainty for low Al mole fractions (x&amp;lt;0.20), due to the broadened spectral line shape exhibited by these modes. A detailed study of Raman spectra recorded on layers with x&amp;lt;0.27, grown either on sapphire or on silicon substrates, has been performed in order to elucidate the origin of that broadening. The influence on the A1(LO) line shape of the sapphire substrate modes, compositional inhomogeneities, residual strain, and those effects inherent to the lattice dynamics of ternary alloys, is analyzed. We conclude that the broadening is caused by intrinsic inhomogeneities of the microscopic polarization fields resulting from alloying. This effect is usually obscured in other III–V compounds, such as arsenides or phosphides, due to their considerably lower LO–transverse optical splitting.

First principles molecular dynamics study of amorphous AlxGa1−xN alloys

We have systematically investigated atomic structures, electronic and dynamical properties of amorphous aluminum–gallium–nitride alloys (a-AlGaN) by performing first principles local basis molecular dynamics simulations. The network topology and defects of the amorphous 216-atom model system have been analyzed with the radial distribution function, the angular distribution function, the ring statistics, and the local coordination. It was found that the models have mixed threefold and fourfold coordinations, and the number of threefold (fourfold) coordinated atoms in alloys decreased (increased) with increasing Al composition. No odd rings are found, indicating that no wrong bonds (homonuclear bonds) appear in the a-AlxGa1−xN alloys. The Ga–N and Al–N bond lengths show a small variation with the Al composition, which is in agreement with recent extended x-ray absorption fine structure experimental measurements. The electronic properties examined by the electronic density of states and local bonding character demonstrate that no mid-band-gap states exist. The band-gap dependence on Al fraction x in a-AlxGa1−xN alloys shows a nearly linear variation with Al composition, and exhibits a small downward bowing behavior. It was also shown that valence band tail states are mostly localized on the threefold coordinated N sites, while the conduction band tail states are mostly localized on the threefold coordinated Ga and Al sites, and the electronic localization tends to become weaker with the addition of Al. We find a mixture of sp3 and sp2 bonds present in the network and their interaction plays a key role in the dynamical properties of a-AlxGa1−xN alloys.

Raman study of collective plasmon-longitudinal optical phonon excitations in cubic GaN and AlxGa1−xN epitaxial layers

The plasmon-longitudinal optical (LO) phonon collective excitations were studied by Raman scattering in cubic GaN doped with Si and in intrinsically doped cubic AlxGa1−xN alloys. The specific asymmetry of the corresponding Raman lines, associated with the dispersion of the coupled excitations was clearly detected in GaN. The spatial coherence of the relevant coupled oscillations was determined from the comparison of the experimental Raman spectra with the calculated ones. A broad line, which was previously attributed to the overdamped plasmon-LO phonon oscillations, was also observed in the GaN and AlxGa1−xN samples in the range between the transversal optical and LO phonon frequencies.

Gap bowing and Stokes shift in InxGa1−xN alloys: First-principles studies

First-principles pseudopotential plane-wave calculations are used to investigate the electronic properties of InxGa1−xN alloys. The alloys are described within a cluster-expansion method considering configurations in large 64-atom supercells. The unusual behavior of the gap bowing is discussed as a function of composition. We show that the strongest contribution to the gap bowing is due to a structural effect, i.e., the composition-induced disorder in the bond lengths. We explain the observed huge Stokes shifts by band gap variations due to composition fluctuations and phase separation.

Optical and electrical properties of Mg-doped p-type AlxGa1−xN

Mg-doped AlxGa1−xN epilayers with Al content up to 0.27 were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). p-type conduction in these alloys has been achieved, as confirmed by variable temperature Hall-effect measurements. Emission lines of band-to-impurity transitions of free electrons with neutral Mg acceptors as well as localized excitons have been observed in the p-type AlxGa1−xN alloys. The Mg acceptor activation energies EA were deduced from photoluminescence spectra and were found to increase with Al content and agreed very well with those obtained by Hall measurements. From the measured activation energy as a function of the Al content, EA versus x the resistivity of AlxGa1−xN alloys with high Al contents can be deduced. Our results thus indicated that alternative methods for acceptor activation in AlGaN alloys with high Al contents must be developed. Our results have also shown that PL measurements provide direct means of obtaining EA, especially where this cannot be obtained accurately by electrical methods due to high resistance of Mg-doped AlxGa1−xN alloys with high Al content.

Spinodal decomposition in BxGa1−xN and BxAl1−xN alloys

We present first-principles calculations of the structural and thermodynamic properties of cubic BxGa1−xN and BxAl1−xN alloys. The calculations are based on the generalized quasichemical approach to disorder and composition effects and a pseudopotential-plane-wave approximation. The bulk moduli and lattice constants are found to vary linearly with the alloy composition. Due to the large lattice mismatch between BN and binaries GaN and AlN, the phase diagrams display large miscibility gaps in the temperature range usually adopted to grow the corresponding alloys. This explains the difficulties reported in growing these alloys with boron content higher than 0.1.

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.

Theoretical study of the Al xGa 1− xN alloys

In this work we use a first-principles method based on the density functional theory, the full-potential linear augmented plane-wave method (FPLAPW), in order to calculate the electronic structures of the Al x Ga 1− x N alloys in the cubic modification. We adopt a model which allows the simulation of the composition x=0.0, 0.25, 0.50, 0.75 and 1.0. We obtain the equilibrium lattice parameters, the bulk moduli, the formation energies, the miscibility curves and the effective masses of the conduction and valence bands in the [100], [111] and [110] directions. The results can be used in the parameterization of theories based on effective hamiltonians. To our knowledge, this is the first time such a systematic ab initio study of effective masses of these semiconductor alloys is accomplished.

Band gap bowing and exciton localization in strained cubic InxGa1−xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy

Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1−xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1−xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1−xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material’s inhomogeneity is much enhanced compared to that of the hexagonal polytype.

Influence of low-temperature buffer layers on the optical waveguiding properties of AlxGa1−xN alloys

The influence of buffer layers on the optical waveguide properties of nitride related semiconductors films grown on sapphire by metalorganic vapor phase epitaxy has been investigated. Using the prism coupling technique, we measured the ordinary (nTE) and extraordinary (nTM) refractive indices of AlxGa1−xN/AlN and AlxGa1−xN/GaN heterostructures, where AlN, GaN are low temperature buffer layers. We investigated the relationship between the refractive indices of AlxGa1−xN alloys and the Al content (x) at a wavelength of 632.8 nm: nTE=2.3240–0.33x. This investigation demonstrates optical waveguiding in gallium nitride materials. It is shown that the indices of thick AlxGa1−xN films are independent of the buffer layer (AlN, GaN). However, the optical propagation losses are highly sensitive to the crystalline quality of the layers, particularly to the density of dislocations in the film. The best optical result of 1.2 dB/cm is obtained when the GaN buffer layer was employed, in comparison to 1.8 dB/cm with the AlN buffer layer. We interpret these results in relation with the transmission electron microscopy analysis.

Al x Ga 1−x N/GaN band offsets determined by deep-level emission

We present studies of the compositional dependence of the optical properties of AlxGa1−xN(0&amp;lt;x&amp;lt;0.22) alloys by modulation spectroscopy and photoluminescence. The yellow luminescence, which is well known in GaN and is generally assigned to shallow donor–deep acceptor pair recombination has also been observed in AlxGa1−xN. As aluminum concentration increases, the color of the band changes from yellow (2.2 eV) to blue (2.6 eV). The shift was less than that of the band gap. Together with previously published studies, it implies that the deep acceptor level is pinned to a common reference level to both materials, thus the deep level responsible for the yellow emission is used as a common reference level to determine the band alignment in AlxGa1−xN/GaN heterojunctions. Combining with the near-band-edge modulation spectra, the estimated ratio of conduction-to-valence band discontinuity is 65:35. Our results are close to the values obtained from PL measurements on Al0.14Ga0.86N/GaN quantum wells and those calculated by linear muffin-tin orbital method and linearized augmented plane wave method.

Composition dependence of the optical phonon energies in hexagonal AlxGa1−xN

We report optical studies on AlxGa1−xN alloy layers across the full composition range. The series of alloy layers was grown on (111)-oriented silicon substrates using gas-source molecular beam epitaxy. From reflectance measurements, we determine the composition dependence of the energy gap to be Eg=3.42+1.18x+1.56x2, in good agreement with previous work. By combining Fourier transform infrared and Raman spectroscopy studies, we determine the composition dependence of phonons having A1(TO), E1(TO), E22, A1(LO), and E1(LO) symmetry. The longitudinal optic phonons exhibit one-mode alloy behavior. Two-mode alloy behavior is observed for all transverse optic and the E22 phonons (i.e., each mode has AlN-like and GaN-like branches). All phonons are seen to blueshift with increasing x. The influence of stress on the phonon energies is discussed.