xN Alloys Research Articles

Investigating the electrical properties of Si donors in AlxGa1–xN alloys

In this paper a study of the electrical properties of Si-doped Al x Ga 1-x N is presented. The Al x Ga 1-x N layers, grown using the metalorganic chemical vapour deposition technique, were studied using variable temperature Hall effect and persistent photoconductivity measurements. High quality conducting Al x Ga 1-x N layers were obtained for 0≤x≤0.5, with the mobility decreasing from 215 cm 2 /V s (GaN) to 10 cm 2 /V s (Al 0.51 Ga 0.49 N), at 300 K. The carrier concentration of these layers was typically between 2.5 x 10 18 cm -3 and 7 × 10 17 cm -3 at 300 K. For Al x Ga 1-x N layers with 0 ≤x ≤ 0.41 the temperature dependence of the carrier concentration between 300 and 15 K could be studied, yielding the Si donor activation energy. The carrier concentrations of higher Al content samples (0.5 ≤ x ≤ 0.65) could not be accurately measured below 300 K, and thus the Si activation energy was obtained from the temperature dependence of the resistivity. It was found that the Si activation energy escalated with increasing Al content of the layers, and followed the trend predicted for a shallow hydrogen-like donor, up to x = 0.65. Illumination of the samples at low temperature with a blue LED showed that the layers exhibit little persistent photoconductivity, further confirmation that Si remains a shallow donor in Al x Ga 1-x N.

Aluminum incorporation into AlGaN grown by low-pressure metal organic vapor phase epitaxy

Aluminum (Al) incorporation in AlxGa1−xN films grown by low-pressure metal organic vapor phase epitaxy using trimethylaluminum (TMAl) and trimethylgallium as group III precursors has been systematically studied. The solid phase Al composition of the AlxGa1−xN films varied nonlinearly with the Al gas phase composition. The incorporation kinetics of AlxGa1−xN alloy has been analyzed by using an adsorption-trapping model. Two parameters were used to characterize the properties of Al incorporation, i.e., the capture radius and the adsorption time of Al atoms. An exponential function of the Al composition of the AlxGa1−xN films versus the TMAl gas flow rate was obtained. It was demonstrated that the adsorption time of the Al atom was larger than the growth time of one atomic layer. The effects of ammonia flow rate, crystal growth rate, and growth temperature on the adsorption parameters were also discussed.

Photoluminescence of energetic particle-irradiated InxGa1−xN alloys

A study of the photoluminescence characteristics of InxGa1−xN alloys in which the Fermi level is controlled by energetic particle irradiation is reported. In In-rich InxGa1−xN the photoluminescence intensity initially increases with irradiation dose before falling rapidly at high doses. This unusual trend is attributed to the high location of the average energy of the dangling-bond-type native defects (the Fermi level stabilization energy). Our calculations of the photoluminescence intensity based on the effect of the electron concentration and the minority carrier lifetime show good agreement with the experimental data. Finally the blueshift of the photoluminescence signal with increasing electron concentration is explained by the breakdown of momentum conservation due to the irradiation damage.

Exciton localization in AlGaN alloys

Deep ultraviolet (UV) photoluminescence emission spectroscopy has been employed to study the exciton localization effect in AlGaN alloys. The temperature dependence of the exciton emission peak energy in AlxGa1−xN alloys (0⩽x⩽1) was measured from 10to800K and fitted by the Varshni equation. Deviations of the measured data from the Varshni equation at low temperatures directly provide the exciton localization energies, ELoc. It was found that ELoc increases with x for x⩽0.7, and decreases with x for x⩾0.8. Our experimental results revealed that for AlGaN alloys, ELoc obtained by the above method has simple linear relations with the localized exciton thermal activation energy and the emission linewidth, thereby established three parallel methods for directly measuring the exciton localization energies in AlGaN alloys. The consequence of strong carrier and exciton localization in AlGaN alloys on the applications of nitride deep UV optoelectronic devices is also discussed.

Native defects in In xGa 1−xN alloys

To elucidate the role of native defects in determining the electronic and optical properties of In 1 −x Ga x N, energetic particle irradiation (electrons, protons, and 4He +) has been used to intentionally introduce point defects into In x Ga 1 −x N alloys. Optical absorption, Hall effect, and capacitance–voltage (CV) measurements are used to evaluate properties of these materials. Irradiation produces donor-like defects in In x Ga 1 −x N with x>0.34, while acceptor-like defects form in Ga-rich In x Ga 1 −x N ( x<0.34). A sufficiently high irradiation dose pins the Fermi level at the Fermi level stabilization energy ( E FS), as predicted by the amphoteric defect model. Pinning of the Fermi level at this energy is also responsible for the surface electron accumulation effect in unirradiated In-rich In 1 −x Ga x N.

Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys

We report the determination of the energy-band offsets between GaN and AlN using the linewidth (full width at half maximum) of an extremely sharp excitonic luminescence transition in AlxGa1−xN alloy with x=0.18 at 10K. Our sample was grown on C-plane sapphire substrate by metal-organic chemical-vapor deposition at 1050°C. The observed value of the excitonic linewidth of 17meV is the smallest ever reported in literature. On subtracting a typical value of the excitonic linewidth in high-quality GaN, namely, 4.0meV, we obtain a value of 13.0meV, which we attribute to compositional disorder. This value is considerably smaller than that calculated using a delocalized exciton model [S. M. Lee and K. K. Bajaj, J. Appl. Phys. 73, 1788 (1993)]. The excitons are known to be strongly localized by defects and/or the potential fluctuations in this alloy system. We have simulated this localization assuming that the hole, being much more massive than the electron, is completely immobile, i.e., the hole mass is treated as infinite. Assuming that the excitonic line broadening is caused entirely by the potential fluctuations experienced by the conduction electron, the value of the conduction-band offset between GaN and AlN is determined to be about 57% of the total-band-gap discontinuity. Using our model we have calculated the variation of the excitonic linewidth as a function of Al composition in our samples with higher Al content larger than 18% and have compared it with the experimental data. We also compare our value of the conduction-band offset with those recently proposed by several other groups using different techniques.

Temperature and compositional dependence of the energy band gap of AlGaN alloys

Deep-ultraviolet photoluminescence spectroscopy has been employed to study the temperature and compositional dependence of the band gap of AlxGa1−xN alloys in the temperature range between 10 and 800 K. Band-edge emission peaks in AlxGa1−xN alloys were fitted by the Varshni equation to obtain Varshni coefficients, which increase nonlinearly with x. The values of Varshni coefficients obtained for GaN and AlN binary compounds in the present study are in good agreement with the previously reported values. Based on the experimental data, the compositional and temperature dependence of the band gap of AlxGa1−xN alloy has been empirically determined for the entire alloy range.

Effect of native defects on optical properties of InxGa1−xN alloys

The energy position of the optical-absorption edge and the free-carrier populations in InxGa1−xN ternary alloys can be controlled using high-energy He+4 irradiation. The blueshift of the absorption edge after irradiation in In-rich material (x&amp;gt;0.34) is attributed to the band-filling effect (Burstein-Moss shift) due to the native donors introduced by the irradiation. In Ga-rich material, optical-absorption measurements show that the irradiation-introduced native defects are inside the band gap, where they are incorporated as acceptors. The observed irradiation-produced changes in the optical-absorption edge and the carrier populations in InxGa1−xN are in excellent agreement with the predictions of the amphoteric defect model.

Dielectric function and critical points of the band structure for AlGaN alloys

AbstractThe ordinary complex dielectric function (DF) of AlxGa1–xN alloys with 0 ≤ x ≤ 0.53 is determined by fitting spectroscopic ellipsometry data from the infrared to the vacuum ultraviolet spectral region (0.74 eV ≤ E ≤ 9.8 eV). The dispersion of the real part of the DF below the band gap is found to be in excellent agreement with previously published data. The obtained band gap energies are verified by photoluminescence and photoreflectance spectroscopy. In the high‐energy range, three critical points of the band structure are clearly resolved. By applying a third‐derivative based DF line shape analysis, the corresponding transition energies are determined. Their compositional dependences can be described on the basis of small bowing parameters. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ab initio study of structural parameters and gap bowing in zinc-blende AlxGa1−xN and AlxIn1−xN alloys

We present first-principles calculations of the structural and electronic properties of zinc-blende AlxGa1−xN and AlxIn1−xN alloys by application of the all-electron full-potential linearized augmented plane-wave method within density-functional theory and the local-density approximation. When the parameter x varies, both the lattice constant a and the bulk modulus B are found to vary linearly for AlxGa1−xN, while for AlxIn1−xN the lattice parameters show an upward bowing. The calculated band-gap variation for the two alloys varies nonlinearly as a function of composition x, with a strong downward bowing for AlxIn1−xN.

Thermoelectric effects in wurtzite GaN and AlxGa1−xN alloys

We have investigated theoretically the thermoelectric effects in wurtzite GaN crystals and AlxGa1−xN alloys. The electron-transport model includes all dominant energy-dependent electron-scattering mechanisms, such as charged dislocation and ionized impurity scattering, polar optical phonon, deformation potential, and piezoelectric acoustic-phonon scattering. The results of the calculation show that GaN-based alloys may have some potential as thermoelectric materials at high temperature. It was found that the thermoelectric figure-of-merit for bulk GaN at T=300K is about 0.0017 while it can reach 0.2 in the thermally resistive Al0.4Ga0.6N alloy at T=1000K. The obtained results agree well with available experimental data. The developed calculation procedure can be used for the optimization of the thermoelectric properties of GaN alloys. The proposed integration of the GaN high-power microwave and optoelectronic devices with the active thermoelectric cooling implemented on the same material system can improve the device performance.

Deep impurity transitions involving cation vacancies and complexes in AlGaN alloys

Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to study deep impurity transitions in AlxGa1−xN (0⩽x⩽1) epilayers. Two groups of deep impurity transitions were observed, which are assigned to the recombination between shallow donors and two different deep level acceptors involving cation vacancies (Vcation) and Vcation complexes in AlxGa1−xN alloys. These acceptor levels are pinned to two different energy levels common to AlxGa1−xN alloys (0⩽x⩽1). The deep impurity transitions related with Vcation complexes were observed in AlxGa1−xN alloys between x=0 and 1, while those related with Vcation were only observed in AlxGa1−xN alloys between x=0.58 and 1. This points out to the fact that the formation of Vcation is more favorable in Al-rich AlGaN alloys, while Vcation complexes can be formed in the whole range of x between 0 and 1. The implications of our findings to the UV optoelectronic devices using AlGaN alloys are also discussed.

The investigation of mole fraction dependence of mobility for In xGa 1−xN alloy

We have calculated the mobility of In x Ga 1− x N alloy by the iterative method interval x = 0 - 1 in low field. We have investigated x mole fraction dependence of calculated mobility. The Boltzmann transport equation involved in relevant scattering mechanisms has been solved. These scattering mechanisms are polar optic phonon scattering, ionized impurity scattering, acoustic phonon deformation potential scattering, acoustic phonon piezoelectric scattering and alloy scattering. Vegard's law was used for the lattice constants of these compounds. Furthermore, Phillip's electronegative difference theorem and the values of alloy potential in Ref. [11] were used to calculate the alloy scattering potential for alloy scattering and for comparison.

Thermal conduction in AlxGa1−xN alloys and thin films

We report on experimental and theoretical investigation of thermal conduction in AlxGa1−xN alloys. A focus of this study is on understanding the effect of the Al mass fraction x and temperature on thermal conductivity in AlxGa1−xN thin films. The thermal conductivity of a set of AlxGa1−xN thin films as well as a pure GaN sample was measured using the differential 3ω technique in the temperature range from 80 to 400 K. Application of the virtual-crystal model allowed us to elucidate the strength of the mass-difference and strain-field-difference phonon scattering in AlxGa1−xN alloy system. The obtained thermal-conductivity temperature dependence indicates the high degree of disorder in the system. The measured variation of the thermal conductivity with the Al fraction x is in good agreement with the theory predictions. The measured data and calculation procedure are useful for evaluating the self-heating effect in AlxGa1−xN/GaN heterostructure field-effect transistors and for the device structure optimization.

First-principles study of cubic Al xGa 1− xN alloys

We present first-principles calculations of the structural and electronic properties of cubic Al x Ga 1− x N ordered alloys in the chalcopyrite and luzonite structures. We have investigated the lattice parameters and band gap energies. The lattice constant a exhibits a small downward bowing. The calculated band gap variation gives a small bowing in good agreement with the experimental reports. We notice a direct to indirect band gap crossover at x (Al) = 0.51.

Enhanced room-temperature luminescence efficiency through carrier localization in AlxGa1−xN alloys

AlGaN samples grown by plasma-assisted molecular-beam epitaxy on sapphire (0001) substrates, with 20%–50% Al content and without the use of indium, show intense room-temperature photoluminescence that is significantly redshifted, 200–400meV, from band edge. This intense emission is characterized by a long room-temperature lifetime (∼375ps) comparable to that seen in low defect density (∼108cm−2) GaN. Room-temperature monochromatic cathodoluminescence images at the redshifted peak reveal spatially nonuniform emission similar to that observed in In(Al)GaN alloys and attributed to compositional inhomogeneity. These observations suggest that spatial localization enhances the luminescence efficiency despite the high defect density (&amp;gt;1010cm−2) of the films by inhibiting movement of carriers to nonradiative sites.

A study of InxGa1−xN growth by reflection high-energy electron diffraction

Epitaxial growth of InxGa1−xN alloys on GaN(0001) by plasma-assisted molecular-beam epitaxy is investigated using the in situ reflection high-energy electron-diffraction (RHEED) technique. Based on RHEED pattern changes over time, the transition of growth mode from two-dimensional (2D) nucleation to three-dimensional islanding is studied for various indium compositions. RHEED specular-beam intensity oscillations are recorded during the 2D wetting-layer growth, and the dependences of the oscillation period/frequency on the substrate temperature and source flux are established. By measuring the spacing between diffraction spots in RHEED, we also estimated indium composition, x, in alloys grown under different flux combinations. Incorporation coefficients of both gallium and indium are derived. Possible surface segregation of indium atoms is finally examined.

Misfit dislocation formation in the AlGaN∕GaN heterointerface

Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3⟨11̱23⟩∕{112̱2} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mechanisms for relaxation of strained III-nitride heterostructures that can contribute significantly to the overall defect density.

Transport properties of highly conductive n-type Al-rich AlxGa1−xN(x⩾0.7)

We report here the growth and transport studies of conductive n-type AlxGa1−xN alloys with high Al contents (x⩾0.7). Si-doped AlxGa1−xN alloys were grown by metalorganic chemical vapor deposition on AlN-epilayer∕sapphire substrates with very smooth surface. Low n-type resistivities have been obtained for Al-rich AlxGa1−xN alloys. The resistivity was observed to increase rapidly with increasing x due to the deepening of the Si donor energy level. Transport measurements have indicated that we have achieved n-type conduction in pure AlN. From the temperature dependence of the resistivity, the donor activation energy was estimated to vary from 23to180meV as x was increased from 0.7 to 1.0.

Spatial variation of luminescence from AlGaN grown by facet controlled epitaxial lateral overgrowth

Interesting phenomena have been observed in the epitaxial lateral overgrowth of AlxGa1−xN alloys using facet control on serrated GaN templates. A complex microstructure is observed that involves misfit dislocation arrays that are closely related to regions with significantly large variations in composition. The dislocations are on inclined planar boundaries and result from basal-plane slip, which is allowed in this inclined facet geometry. The spatial variation of the aluminum composition in the overgrowth region is determined by cathodoluminescence spectroscopy and ranges from x=0.06to0.27, for constant growth conditions that after planarization result in a uniform composition at x=0.16. These results indicate that aluminum incorporation depends significantly on the growth direction with marked preference for facets parallel to the basal plane.