ARTICLE IN PRESS Microelectronics Journal 40 (2009) 360–362
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Macroscopic defects in GaN/AlN multiple quantum well structures grown by MBE on GaN templates T.G. Andersson a,, X.Y. Liu a, T. Aggerstam b, P. Holmstro¨m b, S. Lourdudoss b, L. Thylen b, Y.L. Chen c, C.H. Hsieh c, I. Lo c a
¨teborg, Sweden Applied Semiconductor Physics, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Go Department of Microelectronics and Applied Physics, Royal Institute of Technology (KTH), SE-164 40 Kista, Sweden c Department of Physics, Institute of Materials Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan b
a r t i c l e in f o
a b s t r a c t
Available online 1 October 2008
We have used MBE to grow in AlN/GaN superlattices, with different number of periods, on 2.5-mm-thick MOVPE-GaN templates to study the development of defects such as surface deformation due to strain. After growth the samples were studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), XRD and Fourier transform infrared spectroscopy (FT-IR). The strain increased with the number of quantum wells (QWs) and eventually caused defects such as microcracks visible by optical microscopy at four or more QW periods. High-resolution TEM images showed shallow recessions on the surface (surface deformation) indicating formation of microcracks in the MQW region. The measured intersubband (IS) absorption linewidth from a four period structure was 97 meV, which is comparable with the spectrum from a 10 period structure at an absorption energy of 700 meV. This indicates that the interface quality of the MQW is not substantially affected by the presence of cracks. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Intersubband GaN MBE Surface cracks Sapphire substrate Template
1. Introduction GaN/AlN multiple quantum wells (MQWs) are promising candidates for ultrafast devices , owing to the large conduction band offset, 1.8 eV, and the very short relaxation time after intersubband (IS) absorption, 140 fs. Layer structures can be grown directly on sapphire  or on templates . Because of the large differences in lattice constant (2.5%) and the thermal expansion coefﬁcient (30% at room temperature) between GaN and AlN, large strain is generated inside GaN/AlN MQWs. To study the effect of the strain on the GaN/AlN MQW, we used MBE to grow multilayer structures with different number of periods on 2.5-mm-thick MOVPE-GaN templates. After growth the samples were studied by atomic force microscopy (AFM), SEM, transmission electron microscopy (TEM), XRD and Fourier transform spectroscopy.
2. Sample preparation Samples were grown on GaN templates in a Varian Gen II MBE machine. Gallium and aluminum were supplied from solid effusion cells, while the active nitrogen atoms were generated in Corresponding author.
E-mail address: [email protected]
(T.G. Andersson). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.07.065
a radio-frequency plasma source (model SVTA RF-4.5). The GaN template was grown by low-pressure MOVPE in a D-180 Emcore system on c-plane sapphire substrate. Trimethylgallium (TMG) and ammonia were used as precursors and silane for the Si-doping of the template to n ¼ 3 1018 cm3. Before growth the sapphire substrate was subjected to an in situ pretreatment (1200 1C) followed by deposition of a 100-nm-thick GaN nucleation layer at 550 1C. Then 2.5-mm-thick Si-doped GaN was grown at 1050 1C with the pressure of 200 mbar. Typically within a few days, the template was prepared for the MBE vacuum chamber by ultrasonically degreasing in three consecutive steps using trichloroethylene, acetone and methanol and thereafter rinse in de-ionized water. After drying in N2 ﬂow, the template was mounted on an In-free sample holder and outgassed at 250 1C for 12 h in the entry–exit chamber. The template was further outgassed for 1 h at 400 and 740 1C, in the buffer and growth chambers, respectively. Finally, the temperature was reduced to the growth temperature, 700 1C. Due to homoepitaxial growth, the Si-doped GaN layer was grown, with the optimum parameters, directly on the template. The Ga source temperature, nitrogen ﬂow rate and plasma power were kept constant at TGa ¼ 1080 1C, ﬂow ¼ 1.5 sccm and P ¼ 300 W, respectively. The Si-doping source temperature was 1310 1C, which provided the carrier concentration 3 1018 cm3, the same as the template. The GaN growth rate was 280 nm/h and during growth a bright and thin RHEED streaky pattern was observed, indicating a two-dimensional growth mode.
ARTICLE IN PRESS T.G. Andersson et al. / Microelectronics Journal 40 (2009) 360–362
On top of the 100-nm-thick MBE grown GaN:Si buffer layer, 10 periods AlN/GaN MQWs were grown with the nominal barrier and well thicknesses 2.1 and 4.8 nm, respectively, considering  AlN relaxation of approximative 40%. To improve the efﬁciency of the IS transition formed in the MQWs, Si d-doping was made in the middle of AlN barrier layers. On top of the MQWs, a 500-nm-thick GaN cap layer was grown, doped to the same level. A schematic structure is shown in Fig. 1. To further investigate the appearance of cracks three MQW samples were grown with 3, 4 and 5 QWs, respectively, but without the GaN cap layer.
a region with different contrast that extends to the MQW structure. These layers are divided in two sections by a 10-nmwide area with different contrast, which starts about 30 nm below the MQW. The surface recession is identical with the features identiﬁed as microcracks in the AFM image and the sizes measured by the two techniques are in agreement with each other. It is obvious that the recession is a surface feature but is related to a defect at the MQW. It is not clear what kind of defect that is propagating between the MQW-defect and the recession. Two features in the TEM image also indicates that the microcracks
3. Experimental results and discussion After MBE-growth of the MQW periods, the surface morphology of the sample was studied by tapping mode AFM. The 10 10 mm2 area scan in Fig. 2 shows many lines formed on the surface, interpreted as cracks . These are about 250 nm widths and 50 nm deep (with the limitation of AFM tip). The angle between the lines are 601 or 1201 providing single crystalline areas from 1 1 to 5 10 mm2 in size. Fig. 3 shows a TEM cross-section image of an area around a crack. The 10 period SL is clearly shown below a 340-nm-thick GaN cap layer. At the surface there is a 150-nm-deep and 235nm-wide recession. In the layer just below this recession, there is
Cap (GaN:Si 500 nm) 10 x GaN/AlN QWs Buffer (GaN:Si 100 nm) Template (GaN:Si 2.5 µm)
Fig. 3. Cross-section view of the area around a crack imaged by transmission electron microscopy. As shown there is a defect in the MQW stack under the surface groove shown in Fig. 2.
Fig. 1. Schematic drawing of the multiple quantum well structures grown by MBE on MOVPE-grown template.
10 QW, ×0.2 5 QW 4 QW 3 QW
0.06 0.04 0.02 0 500
Fig. 2. An AFM image of a sample with 5 AlN/GaN QWs and without cap layer. The larger features seen on the surface are Ga droplets.
700 1000 800 900 Photon Energy [meV]
Fig. 4. Intersubband absorption spectra measured in the multipass waveguide geometry shown in the inset. In addition to the intersubband absorption peaks, smaller periodic interference peaks due to the thick GaN template are seen. The absorbance A is determined from the ratio of the transmission spectra of the MQW samples (TMQW) and a reference sapphire sample without epitaxial layers (Tref), for TM polarized light, as A ¼ log10(TMQW/Tref). A background absorption with linear slope was subtracted.
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T.G. Andersson et al. / Microelectronics Journal 40 (2009) 360–362
are related to the growth rather than the ﬁnal cool down procedure. The darker region at the bottom of the microcrack may be interpreted as AlN deposited during growth after the microcrack formed. Also, from the ﬁfth grown QW, the layers bend close to the defect in a V-shape indicating that the cracks are formed during the growth. Absorption in the QWs was characterized in a multipass geometry using Fourier transform infrared spectroscopy (FT-IR). Fig. 4 shows the IS absorption spectra for a 10 period QW sample as well as for three samples with 3, 4 and 5 QWs, respectively. Using AFM and optical microscopy we saw that cracks appeared at four or more QWs in our samples. This coincides with the number of QWs grown without bent layers in the TEM image. Samples with three or less QWs appeared smooth with no visible surface defects. The measured IS absorption linewidth from a four period structure was 97 meV, which is comparable with the spectrum from a 10 period structure at an absorption peak energy of 700 meV. This indicates that the interface quality of the MQW is not substantially affected when the MQW relaxes by the surface formation. The absorption strength per QW is smaller in the 3–5 QWs samples than in the 10 QWs sample. This is explained by a lower light intensity at the sample surface in the
multipass characterization geometry and probably also by depletion of the electrons in the QWs at the semiconductor surface in the 3–5 QWs samples. In the 10 QWs sample the n-GaN cap layer thickness was designed with the MQW near an intensity maximum of the standing wave pattern formed upon internal reﬂection at the top surface of the multipass waveguide.
References  P. Holmstro¨m, X.Y. Liu, H. Uchida, T. Aggerstam, A. Kikuchi, K. Kishino, S. Lourdudoss, T.G. Andersson, L. Thyle´n, Intersubband photonic devices by group-III nitrides, Proceedings of SPIE, vol. 6782, 2007, p. 67821N1-1-14.  X.Y. Liu, P. Holmstro¨m, P. Ja¨nes, L. Thyle´n, T.G. Andersson, Intersubband absorption at 1.5–3.5 mm in GaN/AlN multiple quantum wells grown by molecular beam epitaxy on sapphire, Phys. Status Solidi b244 (2007) 2892.  T. Aggerstam, T.G. Andersson, P. Holmstro¨m, P. Ja¨nes, X.Y. Liu, S. Lourdudoss, L. Thyle´n, GaN/AlN multiple quantum well structures grown by MBE on GaN templates for 1.55 mm intersubband absorption, in: Quantum Sensing and Nanophotonic Devices IV, Proceedings of the SPIE, vol. 6479, 2007, pp. 64791E–647911.  X.Y. Liu, T. Aggerstam, P. Holmstro¨m, S. Lourdudoss, T.G. Andersson, Cracks in GaN/AlN multiple quantum well structures grown by MBE, J. Phys.: Conf. Ser. 100 (2008) 042026.