Photoluminescence Study of Piezoelectric Polarizationin Strained AlxGa1—xN/GaN Single Quantum Wells

V. Kirilyuk1) (a), P. R. Hageman (a), P. C. M. Christianen (a),

F. D. Tichelaar (b), and P. K. Larsen (a)

(a) Research Institute for Materials, University of Nijmegen, Toernooiveld 1,NL-6525 ED Nijmegen, The Netherlands

(b) National Center for HREM, Laboratory of Materials Science,Delft University of Technology, Rotterdamseweg 137, NL-2628 AL Delft, The Netherlands

(Received June 25, 2001; accepted July 19, 2001)

Subject classification: 77.65.Ly; 78.55.Cr; 78.67.De; S7.14

We report a low temperature photoluminescence study on two identical Al0.13Ga0.87N/GaN singlequantum wells (QWs), which are pseudomorphically grown on either a GaN or an AlGaN bufferlayer. The red shift of the QW emission due to the quantum confined Stark effect, is found to bestrongest in the QW deposited on AlGaN, in contrast to what is to be expected form the estimatedbuilt-in electric fields due to spontaneous and piezoelectric polarization fields. Screening of thebuilt-in electric field by a relatively high sheet charge is one of the possible reasons for the ob-served discrepancy.

Introduction The optical properties of AlxGa1––xN/GaN quantum wells (QWs) stronglydepend on the built-in electric field that originates from both spontaneous polarizationand strain-induced piezoelectric fields. In particular, QW transition energies vary in anon-trivial manner with well and barrier width, excitation intensity, doping profile andbuilt-in strain. Theoretically [1, 2] and experimentally [3–8] obtained values of thepiezoelectric and spontaneous polarization show considerable discrepancies, which areprimarily due to uncertainties in the material constants, the high sensitivity of the QWstructures to the growth conditions and the large variety in the layer compositions ofthe heterostructures used. Most of the reported studies have been performed on multi-ple AlxGa1––xN/GaN QWs with a relaxed QW region and strained barriers. In order todetermine the relative importance of strain-induced piezoelectric effects, we have inves-tigated two identical single AlxGa1––xN/GaN QWs deposited by metal-organic chemicalvapor deposition (MOCVD) on either a GaN (sample I) or an AlxGa1––xN (sample II)buffer layer. Assuming that the QW structures are grown coherently on the buffer,either the barrier (I) or the QW (II) is strained. Low temperature photoluminescence(PL) measurements reveal a significantly larger red shift of the ground state energy forthe QW deposited on AlGaN as compared to that on GaN. The change in the transitionenergies is discussed in terms of the differences in piezoelectric and spontaneous polariza-tion fields and screening due to the presence of an excess background charge density.

Experimental Details The two single QW structures, with a (2.8 � 0.3) nm well widthand a (55 � 1) nm barrier thickness, were grown in a horizontal MOCVD reactor on

1) Corresponding author; Phone: +31 24 365 30 75; Fax: +31 24 365 26 20;e-mail: vika@sci.kun.nl

phys. stat. sol. (b) 228, No. 2, 563–566 (2001)

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(0001) sapphire substrates [9]. The QWs were deposited either on a 2.7 mm thick GaNor on a 1.9 mm thick Al0.09Ga0.91N buffer layer. In the latter case, a 1 mm GaN inter-mediate layer was used between the Al0.09Ga0.91N layer and the sapphire substrate. Thegrowth of the GaN and Al0.09Ga0.91N buffers was performed at 1170 and 1190 �C, re-spectively, resulting in a growth rate of �1.7 mm/h. Ammoniac, trimethylgallium andtrimethylaluminium were used as precursors. The Al0.13Ga0.87N cladding layers weregrown at conditions identical to those of the GaN deposition, i.e. a growth temperatureof 1170 �C, to avoid long growth stops at the QW interfaces. Both the active and clad-ding layers were grown using a 50% reduced growth rate in order to increase the inter-face sharpness. Transmission electron microscope (TEM) cross-sectional images wereobtained on a CM30T Philips instrument using an accelerating voltage of 300 kV. Thesamples were mechanically ground, polished to a �15 mm thickness and subsequentlythinned to electron transparency by a Gatan PIPS 691 ion mill, using Ar at 3.5–4.5 kV.The TEM images (Fig. 1) were used to determine the thickness of the layers and tomonitor the continuity of the QWs, the sharpness of the interfaces, and the dislocationdensities.Low temperature photoluminescence (PL) spectra were measured in a cold-finger

helium-flow cryostat using the 325 nm line of a 50 mW CW HeCd laser. The PL signalwas dispersed by a 0.6 m single grating monochromator and detected by a cooled GaAsphotomultiplier, resulting in a spectral resolution of about 0.25 meV.

Built-in Electric Fields Figure 2 shows a schematic representation of the spontaneousand piezoelectric polarizations in the QWs under investigation. Spontaneous polariza-tion depends only on the material composition and has a fixed direction in the crystal.Since the structural parameters of both QWs are the same, the electric fields inducedby the spontaneous polarization in the active layer must be identical in samples I and II[1]. A tensile strained barrier with negative piezoelectric polarization has basically thesame effect as a compressively strained well with positive piezoelectric polarization.Moreover, because the thickness of both QWs is the same, the difference in the in-duced electric fields does only depend on the piezoelectric polarization. The piezoelec-tric constants of AlGaN are larger than those of GaN [2], while the change in thelattice parameters is identical. This implies that the polarization induced electric field

564 V. Kirilyuk et al.: PL Study of Piezoelectric Polarization in AlxGa1––xN/GaN

Fig. 1. TEM images of samples a) I and b) II

and, therefore, the red-shift of the QW-related PL peak due to the quantum confinedStark effect is expected to be larger in sample I, which is grown on GaN.

Photoluminescence Results Figure 3, curves a and b display PL spectra measured atT = 4 K using an excitation power of 30 W/cm2, on samples I and II, respectively.Pronounced PL peaks are observed from the QW layers (FWHM �30 meV), the bar-riers and the buffer layers. From the energy positions of the barrier and buffer relatedpeaks we can accurately determine the Al composition in the different layers and theamount of stress in the thick GaN layer of sample I. The energy position of the free Aexciton in the bulk-like GaN buffer shows a slight compressive strain in comparisonwith homoepitaxial GaN layers [9]. The resulting strain in the active layer of sample Ican, however, be neglected because of its much smaller value as compared to that ofsample II. The Al compositions in the barriers and the thick AlGaN buffer of sample IIare estimated to be �(13 � 1)% and �(9 � 1)%, respectively (Ref. [7]). Secondary IonMass Spectroscopy (SIMS) measurements have confirmed this slightly lower Al fractionin the buffer layer, which was caused by the difference in the growth rates. The moststriking result in Fig. 3 is the fact that the red shift of the QW emission appears to belargest in sample II, grown on the Al0.09Ga0.91N buffer, in contradiction with the predic-

phys. stat. sol. (b) 228, No. 2 (2001) 565

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Fig. 2. Spontaneous PSP and piezoelectric PPZ polarizations in pseudomorphic Al0.13Ga0.87N/GaNsingle QWs grown on a) thick GaN or b) AlGaN buffer. In case a) the barrier is tensile strainedand the active layer is relaxed, in b) the barrier is relaxed and the active layer is compressivelystrained. The spontaneous polarization is identical in both structures

Fig. 3. Photoluminescence spectra(T = 4 K) of the Al0.13Ga0.87N/GaNsingle QWs grown on a (a) thickGaN or (b) AlGaN buffer layer

tions mentioned above. It should be mentioned that the PL peaks of both QWs do notshift with varying the excitation power by two orders of magnitude (not shown), whichproves that the experiments correspond to the low power regime, meaning that screen-ing of the internal electric fields by photo-excited carriers is absent.

Discussion Following the reasoning above the electric field induced in the GaN singleQW grown on a GaN buffer (sample I), should be slightly larger as compared to thatof the identical QW grown on AlGaN (sample II). However, the PL data (Fig. 3) showthe largest red-shift (meaning larger built-in electric field) for sample II. From our esti-mations, the possible experimental errors and uncertainties in the determined param-eters of the QWs cannot account for the observed discrepancy. The only reason for theunexpected small red shift of the QW peak in sample I may be the high sheet chargedensity in this sample. The sheet charge densities found in our HEMTs structures,grown under similar conditions are varying in the range (3–6) � 1012 cm––2, which ishigh enough to partially screen the polarization-induced electric fields in the QWs.Indeed, the position of the PL peak in QW I is considerably blue-shifted as comparedto similar QWs grown by MBE, suggesting a high carrier concentration [5]. The reasonwhy both samples, grown under similar conditions but on different buffer layers, wouldcontain such different background concentrations is not clear at present, and furtherinvestigations are necessary.

Conclusions Low temperature experiments of Al0.13Ga0.87N/GaN single QWs grownon either a GaN or an AlGaN buffer are presented. Both structures show pronouncedPL emission of the QW, the peak position of which reflects the presence of polariza-tion-induced electric fields. The PL peak in the QW grown on AlGaN is strongershifted to lower energy as compared to that of the QW grown on GaN. This effect isattributed to screening of the built-in electric field in the latter sample by the presenceof sheet charges at the well/barrier interfaces.

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566 V. Kirilyuk et al.: PL Study of Piezoelectric Polarization in AlxGa1––xN/GaN