Avella, M.

Bosi, M.

Fornari, R.

Jimenez, J.

Martinez, O.

English

ESE - Salento University Publishing

Assessment of the short and long range homogeneity of MOVPE−grown InGaNepilayersR. Fornari 1) ,  M. Bosi 1) ,  M. Avella 2) ,  O. Martinez 2) ,  J. Jimenez 2)1) Istituto IMEM − CNR, Area delle Scienze 37/A, 43010 Fontanini, Parma, Italy ,  2) Dip. Fisica de la Materia Condensada, Universidad deValladolid, 47011 Valladolid, SpainAbstractInGaN epilayers grown by MOVPE were investigated by micro−Raman spectroscopy, photoluminescence andspectrally−resolved cathodo−luminescence in order to analyse the indium distribution at the micro and macroscale. It was found that the In molar fraction increases from centre to edge of the 2" wafers. The Raman spectra ofthe ternary alloys exhibit additional In−related modes, with respect to GaN spectra. One of these Raman bandsis tentatively ascribed to In clustering, further justified by CL spectral maps.IntroductionInGaN/GaN hetero−structures are at the basis of many opto−electronic applications. For such applications bothlong and short range composition homogeneity is required. In this work we shall present the results of an extensiveinvestigation on InGaN/GaN hetero−structures deposited by MOVPE on (0001) sapphire, using Ammonia, TMGand TMI precursors. The flow−dynamic conditions  were carefully optimized in order to achieve a uniform layerthickness, but it was found that this condition is not sufficient to obtain homogeneus In incorporation. Compositionprofiles along the wafer radius were indeed detected by micro−Raman spectroscopy, spectrally resolvedcathodo−luminescence (CL) and photoluminescence (PL). Generally the centre incorporates less indium than theperiphery, however the composition profile is seen to change according to growth parameters (probably inconnection with the MOVPE system flowdynamics). This information is a useful feedback for growthexperiments. Furthermore, CL maps of the emission wavelength maximum suggests that local In clustering occurs.ExperimentalThe growth of the InGaN layers was performed in a home−made MOVPE system using (0001) sapphire substratesand the standard Ammonia, TMG and TMI precursors. Typically the hetero−structure included a low−T GaNbuffer (80−100 nm), a GaN layer deposited at high temperature (500−600 nm thick) and the InGaN layer depositedat 800−820 °C. In molar fraction was controlled either by changing the TMG/TMI flow ratio or by addinghydrogen into the reaction chamber. Micro−Raman measurements were carried out in backscattering geometrywith a DILOR X−Y Micro−Raman spectrometer, using an Ar+ laser with line at 514.5 nm for excitation.Photoluminescence (PL) measurements were carried out both at room temperature and 12 K, using a He−Cd laser(line at 325 nm) for the excitation of the InGaN samples and a high−resolution monochromator for the analysis ofthe emitted light. Standard phototube and lock−in amplifier were used for signal detection. CL measurement wereperformed with an XiCLOne system from Gatan. The detection is done with a CCD camera, which allows afull spectral imaging, i.e. mapping of the spectral parameters of the selected luminescence band. Themeasurements were done at liquid nitrogen temperature (~80K). The spectra were typically obtained with anacceleration voltage of the e−beam of 5kV and beam current of 9nA, that give a spatial resolution around a fewhundred nm.Results and discussionFigure 1a shows typical Raman spectra of InGaN alloys with different nominal In molar fractions, and a GaNreference spectrum which exhibits the typical allowed modes for hexagonal GaN in backscattering geometry: E2 at567 cm−1 and A1(LO) at 734 cm−1. The InGaN layers present additional bands at about 522, 560 and 680 cm−1.The line at 560 cm−1  is ascribed to the E2 mode of the ternary alloy and is observed to shift linearly with the xmolar fraction of InxGa1−xN, within the range 0 − 0.18, as reported in Figure 1b. The broad band around 680 cm−1is seen to increase with the In content, suggesting that this Raman feature is probably generated by In10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.13R. Fornari et al clustering. These spectra agree with previous Raman studies on InGaN compunds [1].Fig 1: a) Raman spectra of GaN and InGaN layers, preset In content is 15 %, 6 % and 10% in samples 72, 68 and62, respectively; b) Raman shift of the E2 line in InGaN as a function of In content.Fig.2: Raman spectra at the centre (c), mid−radius (m) and periphery (p) of sample #62 (10% In content)In Figure 2 three Raman spectra collected at the centre, mid−radius and periphery of the layer # 62 are shown. Thealloy−related band at ~550 cm−1 shifts to lower frequencies when moving from centre to periphery of thewafer. Considering what said above about Figure 1b, this shift suggests that the wafer periphery contains more10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.13R. Fornari et al Indium than the centre.Fig.3: a) Band−band PL peak energy in different positions along the wafer diameter;  b) CL profile of emissionpeak for the three samplesThe composition gradient along wafer radius is confirmed also by PL and CL measurements. For example, themain band−band emission peak of PL spectra reveals that the material bandgap is generally smaller at the waferperiphery than in the centre, i.e.larger In fraction in the periphery (Figure 3a). The CL radial profiles of the peakwavelength for the same three samples show that the wavelength increase towards the wafer edge (Figure 3b),again indicating that In is more effectively incorporated in this area.Fig. 4: Panchromatic CL image taken on sample #62 and corresponding wavelength distribution along the markedline10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.13R. Fornari et al The homogeneity at the micro scale was studied by CL mapping. The panchromatic image, shown in Figure 4together with the λmax distribution, exhibits the granular−like contrast typical of nitride layers [2,3], i.e. grains withdimensions of less than 1 µm. From the wavelenght profile a spread of peak wavelength of +/− 2 nm is found,which can be interpreted in terms of In clustering (small incorporation fluctuations or post−growth getteringeffects) in agreement with the granular structure of the map.  The idea of post−growth gettering is supported byprevious TEM investigations which showed in our samples an average threading dislocation density around 1010cm−2, with a strong tendency for the dislocations to group together in small loops of 100 – 150 nm diameter, ratherthan distributing randomly. Considering the fact that the CL contrast does not reflect the exact dimensions ofdefects but, due to carrier diffusion, it covers a broader area, the dimensions of clusters seen in CL map lead tothink that the In accumulation is somewhat driven by these dislocation clusters. A further hint in this sense isprovided by the intensity of the broad disorder Raman band centred at about 680 cm−1 which increases remarkablyfor increasing In molar fractions (see Figure 1).It should also be noted that the wavelength change of +/−2nm (i.e. about +/−10 meV) over micrometric distancescorresponds to variation of the In fraction of +/− 0.005. In other words, the granular structure of sample 62 (Figure4) is produced by a InxGa1−xN alloy where x changes by +/−0.005 with respect to his average value. If bunches ofdislocations rather than individual dislocations really act as getters for In, it is to be expected that the Influctuations can be minimized after a substantial reduction of the dislocation density.The employed micro−Raman, PL and CL techniques detected a general tendency for In molar fraction to increasefrom the wafer center to the periphery. The slope and shape of In content are however very different in the threesamples: for example the composition profile of sample 62 was quite flat up to about 13 mm from centre (see Fig.3b) then it increased in the final 10 mm rim. On the other hand, sample 72 exhibited a nearly linear behaviour overthe distance between centre and a radius of about 20 mm, where the peak wavelength passed from 415 nm to 435nm and a steep increase occurred only at the extreme periphery where the wavelength reached 456 nm. It isdifficult to give a correct interpretation of the observed profiles, because the samples 62 and 72 were grown underdifferent conditions, such as precursor partial pressures, temperature (820 °C for sample 62 and 800 °C for sample72), total reactor flow and substrate rotation (675 rpm for sample 62 and 200 rpm for sample 72). Moreover, thethickness of sample 72 was uniform while sample 62 was 1.5 µm in the centre and 1.41 µm at the periphery. It isstrange that the layer with uniform thickness exhibits the most pronounced gradient. At the present stage ofinvestigation we suspect that the rotation speed and the total flow in the reactor play a dominant role in the speciestransport and in shaping the boundary layer, which ultimately determines the different In profiles.ConclusionsInGaN epilayers grown by MOVPE were studied by Raman spectroscopy, PL and spectrally−resolved CL. It wasfound that the lattice disorder increases for increasing In fractions and that In incorporation is non−uniform at themicroscopic scale. The In molar fraction is seen to change by +/− 0.005 respect to the average value. Dislocationsmay be responsible for this clustering effect. Finally, the In molar fraction was seen to increase along the radius,between centre and periphery, probably because of the boundary layer shape imposed by reactor flowdynamicconditions.Spanish group was funded by MCYT (Project CICYT 2002−04 MAT 2001−1878)References[1] H.Harima, E.Kurimoto, Y.Sone, S.Nakashima, S.Chu, A.Ishida, H.Fujiyasu, Phys.Stat.Sol. B 216 (1999) 785[2] A.Cremades, J.Piqueras, J.Mater.Sci.Eng. B91−92 (2002) 341[3] F.Bertram, S.Srinivasan, L.Geng, F.Ponce, T.Riemann, J.Christen, S.Tanaka, H.Omiya, Y.Nakagawa,Phys.Stat.Sol. B 228 (2001) 3510th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.13R. Fornari et al 

Assessment of the short and long range homogeneity of MOVPE-grown InGaN epilayers

Università del Salento: ESE - Salento University Publishing

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Assessment of the short and long range homogeneity of MOVPE-grown InGaN epilayers

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