xN Alloys Research Articles

Direct evidence for strain inhomogeneity in InxGa1−xN epilayers by Raman spectroscopy

This contribution is focused on Raman analysis of the InxGa1−xN alloy. It presents direct evidence that both strain and composition effects must be taken into account to interpret the Raman experimental results. Raman studies have been commonly discussed in view of composition inhomogeneity only, neglecting the possible existence of strain depth variations, recently shown to occur for layers grown above the critical layer thickness. The effects of this variation on the A1(LO) phonon frequency could only be investigated by combining both structural and Raman measurements. In this letter, a set of InxGa1−xN layers has been chemically etched during different periods, allowing the depth variation of the phonon frequency to be unambiguously evidenced. Comparing the Raman spectra before and after etching, two distinct InxGa1−xN regions, differing on their strain state, are identified: a relaxed one, found near the surface region; another one, grown coherently (i.e., pseudomorphic) to the GaN buffer layer. These results are in excellent agreement with an additional reciprocal space map analysis.

Ordering in cubic AlxGa1–xN and InxGa1–xN alloys due to biaxial strain

Chemical ordering in cubic epitaxial AlxGa1–xN and InxGa1–xN layers grown on GaN are investigated by combining first-principles pseudopotential plane-wave total energy calculations, a local concentrationdependent cluster-based method, and Monte Carlo simulations. It is found that for the unstrained or fully relaxed layers there are no stable ordered structures. The energetics of the AlxGa1–xN and InxGa1–xN layers pseudomorphycally grown on fully relaxed GaN (001) buffers show that biaxial strain acts as driving force for chemical ordering in both alloys. It is found that strained InxGa1–xN alloy comprises stable ordered structures while AlxGa1–xN is barely stable, or at most metastable possibly because of a slow kinetics of relaxation. Our results are used to shed light on the understanding of the observed ordered phases formation in both alloys. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Unique optical properties of AlGaN alloys and related ultraviolet emitters

Deep UV photoluminescence spectroscopy has been employed to study the optical properties of AlxGa1−xN alloys (0⩽x⩽1). The emission intensity with polarization of E⊥c and the degree of polarization were found to decrease with increasing x. This is a consequence of the fact that the dominant band edge emission in GaN (AlN) is with polarization of E⊥c(E∥c). Our experimental results suggest that the decreased emission efficiency in AlxGa1−xN alloys and related UV emitters could also be related with their unique polarization property, i.e., the intensity of light emission with polarization of E⊥c decreases with x. It is thus concluded that UV emitters with AlGaN alloys as active layers have very different properties from InGaN and other semiconductor emitters.

Phase separation and ordering in group-III nitride alloys

The electronic, structural, and thermodynamic properties of cubic (zinc blende) group-III nitride ternary InxGa1�xN and quaternary AlxInyGa1�xyN alloys are investigated by combining first-principles total energy calculations and cluster expansion methods. External biaxial strain on the alloys are taken into account in the calculations. While alloy fluctuations and strain effects play a minor role in the physical properties of AlGaN, due to the small lattice-mismatch, in the InGaN alloys we found a remarkable influence of strain on the phase separation. The effect of biaxial strain on the formation of ordered phases has been also investigated, and the results are used to clarify the origin of the light emission process in InxGa1�xN based optoelectronic devices. The dependence of the lattice constant and energy bandgap in quaternary AlxInyGa1�xyN alloys was obtained as function of the compositions x and y.

Diluted magnetic Ga1-xMn xN alloys: a first-principles study

The utilization of the quantum properties of the electron spin wave function will allow the development of a new class of devices. The problem is still controversial and unsettled, even qualitatively, especially for concentrated spin systems such as 3d metals and their alloys. The variety of crystal structures of 3d metals makes difficult the direct comparison between the experimental results and the theoretical conclusions. On the account of this difficulty, substitutional alloys with the same crystal structure, especially face-centered cubic alloys, have been investigated extensively. In this work the properties of diluted Ga1-xMnxN (x = 0.0630; 0.0315) alloys are calculated in the zinc-blende phase, within the framework of the density functional theory, using the fullpotential linearized augmented plane wave (FLAPW) method and the local density approximation (LDA). The alloys are simulated using 32-atom and 64-atom large unit cells, containing one substitutional Mn atom for a Ga atom. The calculations are spin-polarized and we analyze band structures, density of states and total magnetizations. A half-metallic state is predicted at a0 ~ 4:45A. The majority-spin band has a rather sharp peak, characteristic of a narrow band, while the minority-spin has a gap. The total magnetization of the cell is 4.00mB which does not change with the Mn concentration. The valence band is ferromagnetically coupled with the Mn atoms, and the spin splitting is not linearly dependent on the Mn concentration.

Modulus characteristics during the γ→ε martensitic transformation in Fe–Mn–Si–Cr–N alloys

The temperature dependences of internal friction and modulus were measured for the Fe–Mn–Si–Cr– xN alloys. A stable modulus softening of parent was newly observed to associate with the γ→ε martensitic transformation in the alloys containing certain amount (e.g. >0.086 mass%) of nitrogen, although the rather strong softening has been always recognized to occur during its reverse transformation. The enhanced strength and the increased stacking fault energy of γ matrix by nitrogen alloying are suggested to make contribution to the soft mode.

Type I to type II transition at the interface between random and ordered domains of AlxGa1−xN alloys

We analyze the optical and transport consequences of the existence of ordered and random domains in partially ordered samples of AlxGa1−xN alloys. Using atomistic empirical pseudopotential simulations, we find that the band alignment between random and ordered domains changes from type I to type II at x≃0.4. This leads to an increase by two to three orders of magnitude in the radiative lifetime of the electron–hole recombination. This can explain the experimentally observed mobility-lifetime product behaviors with changing Al concentration. The type I to type II transition results from a competition between the ordering-induced band folding effect and hole confinement on Ga-rich monolayers within the ordered structure.

Radiative and nonradiative processes in strain-free AlxGa1−xN films studied by time-resolved photoluminescence and positron annihilation techniques

Radiative and nonradiative processes in nearly strain-free AlxGa1−xN alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately −0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100–250 meV and both SS and FWHM of the PL increased with the increase in x for x⩽0.7. Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for x=0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signals showed a biexponential decay at low temperature, and the slower component became longer with the increase in x (over 40 ns for x=0.49). Simultaneously, density or size of cation vacancies (VIII) and relative intensity of the deep-level emission over that of the near-band-edge one at 300 K increased as x increased to x=0.7. Consequently, certain trapping mechanisms associated with VIII where suggested, and excitons were then detrapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire TRPL signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the TRPL signal became single exponential due to the increasing dominance of nonradiative recombination processes in the free states, resulting in lower internal quantum efficiency (ηint) with increasing x for x⩽0.7. Therefore, realization of AlGaN-based efficient deep-UV light emitters requires further reduction of the nonradiative defect density as well as the VIII-related trap density.

TaN-TiN binary alloys and superlattices as diffusion barriers for copper interconnections

Binary alloys and superlattices of TaN-TiN thin films were grown on Si(100) substrates with a TiN buffer layer using pulsed laser deposition. A special target assembly was used to manipulate the concentrations of these binary component films. The 60% TaN resulted in a TaN (3 nm)/TiN (2 nm) superlattice, while 30% and 70% TaN generated uniform TaxTi1−xN alloys. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) confirmed the single-crystalline nature of these films. Four-point probe resistivity measurements suggest that these alloy and superlattice films have a lower resistivity than pure single-crystalline TaN films. The Cu-diffusion characteristic studies showed that these materials would have the potential as high-temperature diffusion barriers for Cu in ultra-large-scale integration technology.

Realization of waveguiding epitaxial GaN layers on Si by low-pressure metalorganic vapor phase epitaxy

Waveguiding GaN epitaxial layers have been grown by low-pressure metalorganic vapor phase epitaxy on Si(111) substrates using AlN/GaN short period-superlattice (SPS) buffer layer systems. The AlN/GaN SPS has been studied by x-ray diffraction where it appears as pseudoternary AlxGa1−xN alloy. Using elastic theory an effective Al content of 44% is calculated. This value is confirmed by the average Al content calculated from the AlN:GaN layer thickness ratio measured in cross-section transmission electron microscopy. The GaN waveguiding properties have been assessed using the prism coupling method. They sensibly improve with the total thickness of the underlying AlN/GaN superlattice as well as if an additional AlN/GaN SPS is grown atop the GaN waveguiding layer.

Isoelectronic doping of AlGaN alloys

AbstractIsoelectronic doping of AlxGa1−xN alloys with arsenic in films grown by molecular beam epitaxy has been investigated. In photoluminescence spectra of these AlxGa1−xN layers, there is a gradual increase of the band gap and a smaller progressive shift of the position of blue emission band to the higher energies as the Al content increases. To explain the change in the energy of the blue band in As‐doped AlxGa1−xN alloys a model has been proposed, which predicts the shift in the blue band energy to be equal to the valence band offset from AlGaN to GaN. This prediction is consistent with the experimental results. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence and Raman study of hexagonal InN and In‐rich InGaN alloys

AbstractWe present results of a detailed study of X‐ray, photoluminescence, and Raman measurements of hexagonal InN, In‐rich InxGa1−xN (0.36 < x < 1) alloys, and InN samples annealed in oxygen. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principles investigation of lattice constants and bowing parameters in wurtzite AlxGa1−xN, InxGa1−xN and InxAl1−xN alloys

First-principles calculations, by means of the full-potential augmented plane wave method using the local density approximation, were carried out for the structural and electronic properties of the AlxGa1−xN, InxGa1−xN and InxAl1−xN alloys in the wurtzite structure. We have investigated the lattice parameters and band gap energies. The lattice constants a and c are found to change linearly for the AlxGa1−xN alloy, while for both InxGa1−xN and InxAl1−xN alloys the lattice parameters, a, exhibit an upward bowing. The calculated band gap variation for the three alloys exhibit a downward bowing of 0.71 eV, 1.7 eV and 4.09 eV for AlxGa1−xN, InxGa1−xN and InxAl1−xN, respectively.

Isoelectronic doping of AlGaN alloys with As and estimates of AlGaN/GaN band offsets

The isoelectronic doping of AlxGa1−xN alloys with arsenic in films grown by molecular-beam epitaxy has been investigated. In photoluminescence spectra of AlxGa1−xN layers, with an increase in Al mole faction, there is a progressive shift of the position of the blue band emission towards higher energies. The observed energy shift for blue band emission is less than the corresponding increase in the band gap of AlxGa1−xN. A model is presented, which can explain the observed shift in the energy of the blue band emission. This model also allows the AlxGa1−xN/GaN valence band offset to be estimated.

Strain-induced ordering in InxGa1−xN alloys

The energetics and thermodynamic properties of cubic (c-)InxGa1−xN alloys are investigated by combining first-principles total energy calculations, a concentration-dependent cluster-based model, and Monte Carlo simulations. The search for the ground-state energies leads to the conclusion that biaxial strain suppresses phase separation, and acts as a driving force for chemical ordering in c-InxGa1−xN alloys. Ordered superlattice structures, with composition x≅0.5 and stable up to T=1000 K, arises as the relevant thermodynamic property of the strained alloy. We suggest that the In-rich phases recently observed by us in c-GaN/InxGa1−xN/GaN double heterostructures are ordered domains formed in the alloy layers due to biaxial strain.

Ga/N flux ratio influence on Mn incorporation, surface morphology, and lattice polarity during radio frequency molecular beam epitaxy of (Ga,Mn)N

The effect of the Ga/N flux ratio on the Mn incorporation, surface morphology, and lattice polarity during growth by rf molecular beam epitaxy of (Ga,Mn)N at a sample temperature of 550 °C is presented. Three regimes of growth, N-rich, metal-rich, and Ga-rich, are clearly distinguished by reflection high-energy electron diffraction and atomic force microscopy. Using energy dispersive x-ray spectroscopy, it is found that Mn incorporation occurs only for N-rich and metal-rich conditions. For these conditions, although x-ray diffraction in third order does not reveal any significant peak splitting or broadening, Rutherford backscattering clearly shows that Mn is not only incorporated but also substitutional on the Ga sites. Hence, we conclude that a MnxGa1−xN alloy is formed (in this case x∼5%), but there is no observable change in the c-axis lattice constant. We also find that the surface morphology is dramatically improved when growth is just slightly metal rich. When growth is highly metal-rich, but not Ga-rich, we find that Ga polarity flips to N polarity. It is concluded that the optimal growth of Ga-polar MnGaN by rf N-plasma molecular beam epitaxy occurs in the slightly metal-rich regime.

Determination of the refractive indices of AlN, GaN, and AlxGa1−xN grown on (111)Si substrates

The refractive indices of several AlxGa1−xN alloys deposited on silicon are determined by ellipsometry and reflectivity experiments at room temperature. The AlGaN layers are grown on (111)Si substrate by molecular-beam epitaxy on top of an AlN/GaN/AlN buffer in order to reduce the strain of the alloy. The Al composition is deduced from energy dispersive x-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k are determined in the 300–600 nm range. For the transparent region of AlxGa1−xN, the refractive index is given in form of a Sellmeier law.

Effect of composition on the band gap of strained InxGa1−xN alloys

The band gap of pseudomorphically strained InxGa1−xN alloys has been measured using optical absorption spectroscopy. X-ray diffraction measurements indicated that the in-plane lattice parameter of the InxGa1−xN film equaled that of the underlying GaN layer. For strained InxGa1−xN, it was determined that the band gap shift versus composition is given by dEg/dx=−4.1 eV for x<0.12. Our results contradict some recent reports that InxGa1−xN has a relatively small bowing parameter. Possible reasons for the discrepancies are discussed.

Determination of AlxGa1xN refractive indexes at low and room temperature, in the 300–600 nm range, for the optimisation of GaN-based microcavities

AlxGa1–xN alloys are deposited on (111)Si substrate by molecular beam epitaxy. The Al composition is deduced from energy dispersive X-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k, at room temperature, in the 300–600 nm range, are extracted from a two-stage procedure based on spectroscopic ellipsometry and reflectivity measurements. In the transparent region, the refractive index is determined as a function of wavelength, using a Sellmeier law. The accurate knowledge of the later is essential for the fabrication of optimised GaN-based microcavities. For this purpose, two simulations of microcavities with nitride-based distributed Bragg reflectors are presented from the refractive indexes determined in this work at 5 K. The first one is constituted by a (λ/2) GaN cavity embedded between two Al0.19Ga0.81N/Al0.675Ga0.325N distributed Bragg reflectors. In the second one, a lower aluminium composition (0.47) is used in place of (0.675) in order to reduce the strain effects due to the difference between the lattice parameters

Thermoelectric Properties of Hot-Pressed GaN and InN

ABSTRACTAn attempt was made to obtain bulk III-nitride semiconductors such as InN, GaN and InxGa1−xN alloy using hot-press method in order to test their high temperature thermoelectric properties. The Seebeck coefficient and the resistivity were –10μV/K and 1.8×10−6Ωm for InN, and –50μV/K and 1.9×10−4Ωm for GaN at 300K, respectively. Thermal conductivity determined by laser flash method with porosity correction was 17W/mK for InN and 2.6W/mK for GaN. For InN the Seebeck coefficient and the resistivity increased monotonously with increasing temperature, which indicates that InN is a metal or a degenerately doped semiconductor. The power factor and the figure of merit were 2.1 × 10−4W/mK2and 1.5×10−5K−1for InN and 6.9 × 10−5W/mK2and 2.6×10−5K−1for GaN at 650K, respectively.