Al gan nanocolumns and algan gan_algan nanostructures grown by molecular beam epitaxy
phys. stat. sol. (b) 234, No. 3, 717–721 (2002) AlGaN Nanocolumns and AlGaN/GaN/AlGaN Nanostructures Grown by Molecular Beam Epitaxy J. Ristic1) (a), M. A. Sanchez-Garcia (a), J. M. Ulloa (a), E. Calleja (a), ´ ´ ´ J. Sanchez-Paramo (b), J. M. Calleja (b), U. Jahn (c), A. Trampert (c), ´ and K. H. Ploog (c) (a) Departamento de Ingenierıa Electronica, ETSI Telecomunicacion, ´ ´ ´ Universidad Politecnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain ´ (b) Departamento de Fısica de Materiales, Universidad Autonoma de Madrid, ´ ´ Cantoblanco, 28049 Madrid, Spain (c) Paul-Drude-Institut fur Festkorperelektronik, Hausvogteiplatz 5–7, 10117 Berlin, ¨ ¨ Germany. (Received July 22, 2002; accepted October 1, 2002) PACS: 68.65.Fg; 68.70.þw; 78.55.Cr; 78.60.Hk; 81.15.HiThis work reports on the characterization of hexagonal, single crystal AlGaN nanocolumns withdiameters in the range of 30 to 100 nm grown by molecular beam epitaxy on Si(111) substrates.The change of the flux ratio between the Al and the total III-element controls the alloy composi-tion. The Al composition trend versus the Al flux is consistent both with the E2 phonon energyvalues measured by inelastic light scattering and the luminescence emission peaks position. Highquality low dimensional AlGaN/GaN/AlGaN heterostructures with five GaN quantum discs, 2 and4 nm thick, embedded into the AlGaN columns, were designed in order to study the quantumconfinement effects.1. Introduction III–nitride based Quantum Wells (QWs) have been extensively stu-died as active layers of different devices. The confinement effects under high electricfields in these structures are still a subject of debate. Localization effects versus elec-tric field, segregation, and efficiency are so far hot topics. Nanocavities and nanostruc-tures with lower dimensionality are envisaged as the next technological steps toachieve new generation devices. The technology and characterization of such nanos-tructures are still under development and their potential applications, as well as thebasics are quite appealing [1–3]. The inhibition of non-radiative recombination cen-ters, like dislocations, by localization of carriers, longer lifetimes, higher gains, andlower threshold currents as was the case for InGaAs-based laser devices , are to beexpected. The main advantages of the low dimensional GaN structures, like nanocolumns,already reported in [5, 6], are their high crystalline quality and their self-organizedgrowth process.2. Experimental AlGaN nanocolumns and AlGaN/GaN/AlGaN heterostructures weregrown by PAMBE on Si(111) buffered with a high temperature (830 C) AlN layer[7, 10]. AlGaN nanocolumns, as in the case of GaN nanocolumns, grow reproduciblyunder N-rich conditions . The reduction of the Ga surface diffusion, due to excess 1 ) Corresponding author; Phone: +34 91 549 57 00 (x420); Fax: +34 91 336 73 23;e-mail: firstname.lastname@example.org # 2002 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim 0370-1972/02/23412-0717 $ 17.50þ.50/0
718 J. Ristic et al.: AlGaN Nanocolumns and AlGaN/GaN/AlGaN Nanostructures ´ Ta b l e 1 Growth conditions for AlGaN/GaN/AlGaN nanocolumnssample buffer layer columns nitrogen FGa* FAl* FAl/ %Al o o (FAl þ FGa) struct. T( C) struct. T( C) FN2 OED PL CL Raman (sccm) m746 GaN 690 AlGaN 790 1.2 1.10 2.5 1.0 0.28 43 46 50m822 AlN 830 AlGaN 760 1.0 0.85 2.1 0.4 0.16 20 18 21m833 AlN 830 AlGaN 760 1.0 0.85 2.1 0.4 0.16 21 – – 26 1.6 mm AlGaN/5 Â [GaN(4 nm)/AlGaN(10 nm)]/AlGaN(10 nm)m834 AlN 830 AlGaN 760 1.0 0.85 2.1 0.4 0.16 20 – – 26 1.6 mm AlGaN/5 Â [GaN(2 nm)/AlGaN(10 nm)]/AlGaN(10 nm)*) FGa , FAl in 10À7 Torrnitrogen (“Ga-balling”) [8, 9] and a consequent growth by a “vapor–liquid–solid” epi-taxy are recognized as the kinetic models of the columnar growth. The dependence ofthe density of GaN columns and their average diameter on the III/V ratio has beenestablished . The AlGaN optimal growth temperature depends on the Al% and the Ga desorptionrate, which strongly depends on the growth temperature, and determines the Al incor-porated. At 760 C the best GaN columns were reproduced, thus it was chosen to growcolumnar AlGaN, allowing to grow columnar AlGaN/GaN/AlGaN heterostructures bysimple interruption of the Al flux. GaN Quantum Discs (QDs), spaced and capped byAlGaN, were incorporated into the AlGaN columns. Growth parameters and samplestructures are given in Table 1.2.1 Control of the Al incorporation The nanocolumns Al content was controlled bychanging the ratio between the III-element fluxes (Al to Ga), while the growth tem-perature and the total group III-element (Al þ Ga) flux were kept constant. The Alcontent of the samples, estimated by various characterization techniques (Table 1), fol-lows the trend established by Al flux to total III-element flux ratio. However, the ex-perimental values are generally higher than the nominal ones, most probably due tosignificant Ga desorption at a growth temperature (760 C). As a consequence, theequation: FAl/(FAl þ FGa) underestimates the actual Al composition. This assumptionis further confirmed when increasing the growth temperature by 30 C (m746). Detailsabout the growth procedure, basic aspects, and the system can be found elsewhere [5, 7]. The samples were studied by Scanning Electron Microscopy (SEM) with a JEOL5800 microscope. Catodoluminescence (CL) was measured in a SEM equipped with anOxford Mono-CL system with electron beam energies ranging from 5 to 25 keV. Low-temperature Photoluminescence (PL) was excited with a He–Cd laser (325 nm) andwith a second-harmonic generator pumped by Arþ laser (257 nm). Raman spectra weretaken at room temperature using the 514.5 nm Arþ laser line and a double spectro-meter with a charge coupling device detector. The phonon frequencies were determinedwithin 0.1 cm – using the spectral lines of a Ne lamp for calibration. Transmission elec- –1tron microscopy (TEM) analysis was carried out in a Jeol JEM 3010 microscope operat-
phys. stat. sol. (b) 234, No. 3 (2002) 719ing at 300 kV. Free-standing columns were glued for mechanical stabilization and thenprepared by standard procedures.3. Results and Discussion3.1 Structural and morphological characterization As previously reported for GaNnanocolumns [5–7], the decrease of the III/V ratio changes the AlGaN morphologyfrom compact to columnar. High-density, small-diameter (30 to 100 nm) columns wereobtained under highly N-rich conditions at a growth rate of 0.35 mm/h, as shown inFig. 1a. The samples show a compact AlGaN portion near the substrate, resulting from thecoalesced individual columns, and an upper region with isolated nanocolumns alignedalong the (0001) direction (Fig. 1a). Cross section TEM images of sample m834 show the GaN QDs inside the AlGaNcolumns (Fig. 2b), having thicknesses between 2.1 and 2.5 nm. There is no indication ofdefects of any type in the columns. Extended defects are present only at the bottomcompact layer. Presumably, this fact points to the GaN QDs being fully strained.3.2 Raman spectroscopy Raman spectra reveal the “GaN-like” E2 mode around570 cm – for increasing Al% in the AlGaN columns. For higher Al% the correspond- –1ing “AlN-like” E2 mode is also observed at 640 cm – . From 568 cm – of pure GaN, the –1 –1GaN-like E2 energy increase with the total Al flux is observed, indicating a progressiveincrease in the Al% of the columns. The Al% is estimated using the reported depen-dence of the E2 phonon frequency for relaxed AlGaN thick films . Raman signalsfrom GaN QDs were not seen due to the low Al% that results in a small shift betweenGaN and AlGaN E2 modes.3.3 Cathodoluminescence Low temperature CL scans along the AlGaN columns showa step-like energy increase moving the probe from the column top to the substrate. Thismay be either due to a strain change when the columns get isolated (strain-free), or toa different Al incorporation. The first assumption leads to a huge compressive biaxialstrain (8 Â 10 – ), opposite to what should be expected. Similar CL scans along GaN –3 m822 m822 100 nm 10 nma) b)Fig. 1. Cross-sectional TEM images (11– –20) for the sample m822
720 J. Ristic et al.: AlGaN Nanocolumns and AlGaN/GaN/AlGaN Nanostructures ´ HRTEM  M834 M834 [11-20] glue [1-100] GaN AlGaN AlGaN 50nm b) 0.5 µm Si a) a)Fig. 2. Cross-sectional TEM images (11– –20) in m834columns show an opposite shift pointing towards strain relaxation. The second possibi-lity implies Al% variations of 2.5–4.5% originating from the different growth kineticsbetween compact and columnar AlGaN.3.4 Photoluminescence Figure 3 shows spectra from nanocolumns with and withoutQDs. The AlGaN signature is observed at 3.792 to 3.813 eV, while the emission at3.622 eV is attributed to the QDs. An estimation based on the envelope function ap-proximation of the one dimensional Schrodinger equation considering spontaneous and ¨piezoelectric fields, but not band mixing effects, yields transitions at 3.49–3.54 eV and3.63–3.70 eV for 4 and 2 nm thick QDs. Further PL blue shifts are expected due to thelateral confinement. PL spectra versus power showed no peak shifts, while the low energy emission inten-sity increased. The low energy emission remained stronger when increasing the tem-perature. Both features point to this emission as originating from the GaN QDs.4. Conclusions High quality AlGaN nanocolumns have been obtained by self-orga-nized MBE growth under a highly N-rich regime. The growth conditions were tuneduntil single cristal and strain-free columns were obtained. AlGaN/GaN/AlGaN hetero- 0.30 0.5 m822 3.792eV 1.6µm AlGaN m833 3.813eV 0.25 8.5K 3.751eV 0.4 8.5KPL Intensity (a.u.) PL Intensity (a.u.) 0.20 3.622eV 0.3 0.15 0.10 0.2 Si(111) Si(111) 3.510eV 0.05 3.507eV 0.1 Si(111) 0.00 a) 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 0.0 Energy (eV) 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8a) b) Energy (eV)Fig. 3 (online colour). a) LT PL spectrum of m822; b) LT PL spectrum of m833
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