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hetero - structures based on iii - nitrides @xcite and in particular on the combination al@xmath0ga@xmath1n / gan represent the basis of a variety of state - of - the - art ( opto)electronic devices like blue and white light - emitting diodes @xcite , laser diodes @xcite , blue lasers @xcite , high - power- @xcite , and high - electron - mobility - transistors @xcite . most of the above mentioned devices are commercially available and their performance continuously improved . furthermore , iii - nitrides doped with transition metals ( tm ) have also been the focus of considerable research efforts towards the demonstration of semiconductor spintronic functionalities @xcite . in this respect , while a remarkable number of reports on gan : mn provide an overview on the structural , optical , magnetic and electric properties of this material system @xcite , little is known about al@xmath0ga@xmath1n : mn @xcite and related nanostructures @xcite . recent findings @xcite indicate this alloy as particularly interesting for _ e.g.  _ the self - assembling of functional multilayers and for having revealed the decisive role of mn as surfactant during the epitaxial growth of al@xmath0ga@xmath1n : mn , considerably enhancing the critical thickness of al@xmath0ga@xmath1n : mn on gan , and opening new perspectives for the realization of _ e.g.  _ improved reflectors in gan - based laser structures . we report here on al@xmath0ga@xmath1n : mn grown by means of metalorganic vapor phase epitaxy ( movpe ) in a broad range of al concentrations and extensively investigated @xmath2 x - ray absorption spectroscopy ( xas ) , x - ray emission spectroscopy ( xes ) , energy - dispersive spectrometry ( eds ) , x - ray diffraction ( xrd ) , and high - resolution ( hr ) transmission electron microscopy ( tem ) , supported by density functional theory ( dft ) calculations . the results provide fundamental information on the microstructure and local environment in the layers and on the valence state of mn incorporated in the lattice over the whole range of al concentrations . the wurtzite ( wz ) al@xmath0ga@xmath1n : mn samples are grown in an aixtron 200rf horizontal - tube movpe reactor . all structures are deposited on @xmath3-plane sapphire substrates with trimethylgallium ( tmga ) , trimethylaluminum ( tmal ) , bis - methylcyclopentadienyl - manganese ( mecp@xmath4mn ) and ammonia ( nh@xmath5 ) as precursors for respectively ga , al , mn , n , and with h@xmath4 as carrier gas . the epitaxial process , developed from a well established procedure @xcite , consists of : ( i ) substrate nitridation ; ( ii ) low temperature ( 540 @xmath6c ) deposition of a gan nucleation layer ( nl ) ; ( iii ) its annealing under nh@xmath5 ; ( iv ) growth of a 1 @xmath7 m device - quality gan buffer deposited at 1020 @xmath6c ; ( v ) al@xmath0ga@xmath1n : mn layers at 850 @xmath6c , with the same tmga and mecp@xmath4mn flow rates and different - over the sample series - tmal flow rates ranging from 1 to 80 standard cubic centimeters per minute ( sccm ) . in order to have real time control over the entire fabrication process , the movpe system is equipped with an _ in situ _ isa jobin yvon ellipsometer that allows for both spectroscopic and kinetic measurements in the energy range 1.5 ev 5.5ev @xcite . the structures are routinely characterized by atomic force microscopy ( afm ) , secondary - ion mass spectroscopy ( sims ) and ( magneto)photoluminescence ( pl ) in order to get information on the surface roughness , chemical composition and magnetooptical response , respectively . measurements of squid magnetometry in the temperature range between 1.5 k and room temperature , confirm the samples to be paramagnetic . here , we focus on the effect of mn incorporation on the structural arrangement of al@xmath0ga@xmath1n : mn and on the local atomic environment of mn , with particular attention to the xrd and hrtem analysis as essential complement to the synchrotron xas and xes measurements . all considered al@xmath0ga@xmath1n : mn samples are listed together with their growth parameters in table  [ tab : growth ] . the mn concentration in all doped layers is @xmath81% cations , as established by sims analysis . +    .growth parameters for the al@xmath0ga@xmath1n : mn samples presented in this work . al concentration @xmath9 ( from xrd ) ; tmga and tmal flow rates and the pressure @xmath10 in the reactor during the process . the mecp@xmath4mn and nh@xmath5 flow rates are fixed at 490sccm and 1500sccm , respectively ; the substrate temperature during the growth of the gan buffer layer and during the deposition of the al@xmath0ga@xmath1n : mn layer are , respectively , 1020@xmath11c and 850@xmath11c . the nominal thickness is obtained from the kinetic ellipsometry spectra and confirmed by tem cross - sections . [ cols="^,^,^,^,^,^ " , ]     a quantitative analysis @xmath2 a least - squares fit of the exafs data is then performed . due to the complexity of the system under investigation and in order to keep the correlation between the fitted variables as low as possible , a model with a minimum set of parameters to describe the whole al concentration range is found . this corresponds to the best fitting model and consists of a mn@xmath12 defect in al@xmath0ga@xmath1n  expanded in three sets of single scattering paths : mn - n , mn - al and mn - ga , corresponding to the first three coordination shells . for each sample , the fit is performed in r - space , limited to the [ 13.5 ]   range . both vgi and hgi data sets ( weighted by the noise level ) are included in a single fit in order to correctly account for the polarization effects . this permits to report the average bond distances for the out - of - plane ( vgi , parallel to @xmath3 ) and in - plane ( hgi , perpendicular to @xmath3 ) atomic configurations . the results are shown in table  [ tab : exafs - fits ] and in supplementary fig . [ figs : exafs - fits ] . the model is built as follows : the passive electron reduction factor @xcite , @xmath13 , is fixed to the calculated value of 0.935 ; the coordination numbers for mn - n and mn - al are fitted , respectively , @xmath2 the variables @xmath14 and @xmath15 , while the coordination number of the second cation shell is constrained to sum to 12 ; a common debye - waller factor , which accounts for both the structural and thermal disorder , is fitted to @xmath16 for all single scattering paths ; three variables are employed for the mn - n , mn - al and mn - ga average distances , @xmath17 , @xmath18 and @xmath19 , respectively , with a common expansion / contraction factor in the two orthogonal directions ( vgi and hgi ) ; a common variable is fitted also for the shift of the edge energy , @xmath20 . this model permits to keep the numerical correlation between the variables below a 50% level . the r - factor of the fits ranges from 0.009 to 0.04 , affecting the propagated error bars , as reported in table  [ tab : exafs - fits ] . several additional fitting models have been tested , either increasing the number of fitted variables or introducing additional scattering paths from other defects , as mn interstitials ( mn@xmath21 and mn@xmath22 ) . in all cases those models do not pass a f - test @xcite , meaning that the improvement in the fit quality is not statistically relevant . +    , @xmath23 , @xmath18 and @xmath19 variables ( as reported in table [ tab : exafs - fits ] ) . the horizontal lines are the corresponding average bond distances for gan ( dashed ) and aln ( dotted ) . ] the exafs quantitative analysis indicates that the majority of mn atoms is in a mn@xmath12 configuration . on the other hand , the fitted percentage of @xmath24 does not correspond exactly to the percentage of mn@xmath12 in the samples . in fact , the absolute value of this variable , which represents the coordination of the first coordination shell ( mn-4n tetrahedron ) , is affected by the numerical correlation with @xmath16 and by the presence of nitrogen vacancies , as found in similar samples @xcite . for this reason , we rely on the results of the xld analysis , which is much more sensitive to the symmetry of the crystal , for determining the level of mn@xmath25 in the samples . nevertheless , a strong @xmath26-independent amplitude reduction of the exafs signal is obtained for the aln : mn sample ( # h ) . as shown by the tem micrographs , this sample has a columnar structure , thus the amplitude reduction is attributed to an increased local disorder , as it was demonstrated by exafs simulations combined with molecular dynamics calculations for mn nano - columns in ge : mn @xcite . the second percentage parameter,@xmath15 , is extracted from the fitted coordination number of the second coordination shell , keeping the constraint of 12 total neighbors ( ga / al ) dictated by the wurtzite structure . the results follow a linear dependence and match , within the error bars , with the al concentration found by xrd . furthermore , it is found that the average mn - n bond distance is larger than those of ga - n or al - n and is not affected by the al doping , while mn - al and mn - ga show a contraction going from gan : mn to aln : mn , as expected by the reduction of the lattice parameters . this implies that the lattice distortion introduced by the mn incorporation is local and mainly limited to the first coordination shell . +     herfd - xanes spectra ( bottom ) for the al@xmath0ga@xmath1n : mn series with the corresponding simulations ( top ) for hgi ( left panel ) and vgi ( right panel ) geometries . the vertical dashed lines are guides to the eye of the main spectral features . ]    in order to further confirm the local structural description obtained @xmath2 exafs analysis , the xanes region is investigated through _ ab initio _ simulations . in fig . [ fig : xanes ] the normalized k@xmath27 herfd - xanes spectra are shown together with their relative simulations ( using the fdmnes @xcite code ) for the hgi and vgi geometries . the herfd - xanes spectra correspond to a diagonal cut in the 2d resonant inelastic x - ray scattering ( rixs ) plane @xcite and can be approximated to a standard xanes spectra only in the region above the main absorption edge , where the spectral features arise from electric dipole transitions from 1@xmath28 to 4@xmath29 empty states of the absorbing atoms ( mn ) . on a first order approximation , this energy range can be described by multiple scattering theory employing simple muffin - tin potentials @xcite within a one - electron approach , _ i.e.  _ the level of theory employed for the simulated spectra shown in this study . the spectral features present in the pre - edge region of the herfd - xanes spectra can not be fully described by the level of theory employed here and a quantitative analysis requires to account for the full rixs plane , not only for line cuts @xcite . nevertheless , the presence of an intense pre - edge peak in the k - edge xas spectra of 3@xmath30 transition metals is the fingerprint of tetrahedral ( t@xmath31 ) symmetry @xcite , due to allowed electric dipole transitions to the @xmath29-character of the @xmath32 spin - polarized 3@xmath30 states . the spectral features present in the xanes region do not correlate straightforward with a given coordination shell or scattering species , but are the result of full multiple scattering configurations . this induces an enhanced sensitivity to the geometry around the absorber . on the other hand , this also makes challenging to quantitatively model the xanes via _ ab initio _ methods . as shown in fig . [ fig : xanes ] , all the spectral features and the trend with increasing al concentration are reproduced by the simulations using a substitutional model based on the dft - relaxed supercells , rescaled to the experimental lattice parameters . to better evaluate the quality of each simulation , the supplementary figs . [ figs : xanes - subs ] and [ figs : xanes - ints ] show the comparison with experimental spectra for the nominal wyckoff sites and the dft - relaxed positions . the defects investigated are mn@xmath12 , mn@xmath22 and mn@xmath33 in al@xmath0ga@xmath1n . in order to get more quantitative results , a linear combination fit ( lcf ) analysis of the xanes spectra is performed . the constraints imposed are : the presence of the mn@xmath12 phase ; the number of components is limited to two ( one substitutional and one interstitial ) ; an energy shift for the interstitial phase is allowed ( fitted ) . all combinations among the four interstitial cells are performed and the fits are ranked by @xmath34 . in all samples / geometries it is found that the mn@xmath12 phase is @xmath3580% and the complementary phase is the non relaxed mn@xmath22 defect . on the other hand , the @xmath34 values of the best fits do not pass a statistical test ( f - test ) , meaning that the increase in the fit quality is not relevant . this confirms what previously found by exafs and the formation energies results of the dft , that is , that mn@xmath22 defect in al@xmath0ga@xmath1n is not stable and has a high formation energy . + a more quantitative analysis to establish the percentage of mn atoms incorporating as substitutional defects in the host matrix , is obtained by studying the xld spectra . it is established that xld is extremely sensitive to the symmetry of non - cubic sites @xcite and it was shown to be a powerful tool to determine the quality of substutional inclusions in dilute magnetic semiconductors @xcite . the xld spectra for the studied samples are reported in fig . [ fig : xld ] and are obtained from the difference between the herfd - xanes spectra in vgi and hgi geometries . the amplitude of the xld main oscillation at the edge position highlighted in fig . [ fig : xld ] is taken as a figure of merit for mn@xmath12 . in fact , the maximum xld amplitude would be obtained for 100% mn@xmath12 dilute in a perfect al@xmath0ga@xmath1n lattice . the mn@xmath22 interstitial shows a xld signal too , however it is not in phase with the mn@xmath12 xld signal and the resulting xld amplitude in the region of interest is reduced . as a reference for the 100% case , we arbitrarily rescale the experimental xld amplitudes to the xld amplitude at the ga k - edge of a gan : mn layer from ref . the results are reported in the inset to fig . [ fig : xld ] . the increasing values of the mn@xmath12 percentage for al @xmath3682% are due to the accuracy of the normalization procedure employed . in fact , a more accurate procedure would require to rescale the mn k - edge xld amplitudes to the ga k - edge ( or al k - edge ) xld amplitude measured for each sample in the same experimental conditions . on the other hand , the systematic errors are estimated to be within a @xmath3710% bandwidth . the dramatically low mn@xmath12 value for aln : mn can be safely attributed to an actual reduction of mn@xmath12 in this sample . +    ga@xmath1n : mn series . the amplitude in the highlighted region is taken as a figure of merit for mn@xmath38 . inset : quantitative analysis of the results . ]    as final point we discuss the mn valence state inferred from the integral of the absolute difference of the k@xmath39 xes data ( integrated absolute difference  iad analysis @xcite ) . this method is preferred over the one employing the position of the main absorption edge for the possibility it gives to quantitatively follow the evolution of the effective spin moment on mn ( @xmath40 ) as a function of a given parameter and to directly compare the results with dft calculations @xcite . the total magnetic moment per unit cell calculated with dft is in all cases 4@xmath41 and corresponds to @xmath40@xmath82.0 , as found in the frame of a bader partitioning scheme @xcite . this result is confirmed experimentally , as reported in fig . [ fig : xes ] . the mn valence state is constant within the error bar for the whole series , with the exception of the aln : mn sample , as expected and supporting all previous results . xes data over the whole series of samples . inset : relative iad analysis . the vertical dashed line is a guide to the eye . ] we have carried out an extensive study of epitaxial al@xmath0ga@xmath1n : mn on a series of samples with al concentration up to 100% . by xrd we have found that the al content in the layers matches  over the sample series  the one expected from growth conditions . the lattice parameters as a function of the al concentration are also obtained by xrd . the dft computations on the formation energy for the incorporation of al in a gan matrix let us to conclude that al and ga are randomly distributed into the lattice , and in al@xmath0ga@xmath1n : mn the mn ions have the tendency to preferentially substitute for ga . the formation of mn interstitial defects is not favored . a coherent growth without local aggregation or precipitation is obtained for al@xmath0ga@xmath1n : mn with al concentrations up to 82% , confirming the surfactant role of mn already reported @xcite . synchrotron radiation xas has been employed to probe the local atomic and electronic structure of mn . from exafs , xanes and xld it is found that the majority of the mn ions is dilute , _ i.e.  _ homogeneously distributed over the doped layers . an iad analysis of the xes data allows to determine the valence state of mn as constant up to an al concentration of 82% . due to the reduced lattice parameters with respect to _ e.g.  _ gan : mn , enhanced hybridization of the orbitals can be expected in al@xmath0ga@xmath1n : mn , making it a material system worth to be investigated in view of spintronic functionalities . moreover , this work paves the way to the understanding and control of the role played by mn in particular and transition metals in general on the structure and properties of the alloys al@xmath0ga@xmath1n : tm . significantly , the incorporation of mn has been found to promote the growth of al@xmath0ga@xmath1n on gan , to defer the relaxation of the layers and to increase the critical thickness also for al concentrations up to 82% , with remarkable potential effects on the fabrication of _ e.g.  _ distributed bragg mirrors for iii - nitride - based optoelectronic devices . the authors gratefully acknowledge the european synchrotron radiation facility ( esrf ) for providing synchrotron radiation beam - time ( proposal he-3825 ) . this work was supported by the esrf trainee program , by the austrian science fundation ( fwf projects 24471 and 26830 ) , by the nato science for peace programme ( project 984735 ) , by the eu 7@xmath42 framework programme through the capacities project regpot - ct-2013 - 316014 and by the european research council ( advanced grant 22790 ) . +    * supplemental information * in the plots of fig .  [ figs : exafs - fits ] , the quality of the fits performed on the exafs data for both vgi and hgi geometries is reported . +     +     + the quality of the simulated xanes spectra for mn@xmath12 is shown in fig .  [ figs : xanes - subs ] , while the quality of the simulated xanes for the mn@xmath22 and mn@xmath21 interstitials in al@xmath0ga@xmath1n is given in fig . [ figs : xanes - ints ] .
synchrotron radiation x - ray absorption and emission spectroscopy techniques , complemented by high - resolution transmission electron microscopy methods and density functional theory calculations are employed to investigate the effect of mn in al@xmath0ga@xmath1n : mn samples with an al content up to 100% . 
 the atomic and electronic structure of mn is established together with its local environment and valence state . 
 a dilute alloy without precipitation is obtained for al@xmath0ga@xmath1n : mn with al concentrations up to 82% , and the surfactant role of mn in the epitaxial process is confirmed . 
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