Optical investigation of micrometer and nanometer-size individual

Optical investigation of micrometer and nanometer-size individual GaN pillars fabricated by reactive ion etching F. Demangeot, J. Gleize, J. Frandon, M. A. Renucci, M. Kuball, D. Peyrade, L. Manin-Ferlazzo, Y. Chen, and N. Grandjean Citation: Journal of Applied Physics 91, 6520 (2002); doi: 10.1063/1.1468908 View online: http://dx.doi.org/10.1063/1.1468908 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/91/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical properties of GaN nanopillars fabricated using ICPRIE technique AIP Conf. Proc. 1447, 1089 (2012); 10.1063/1.4710386 Nanoscale structure fabrication of multiple Al Ga Sb In Ga Sb quantum wells by reactive ion etching with chlorine-based gases toward photonic crystals J. Vac. Sci. Technol. B 24, 2291 (2006); 10.1116/1.2348727 High optical quality GaN nanopillar arrays Appl. Phys. Lett. 86, 071917 (2005); 10.1063/1.1861984 Raman scattering in GaN pillar arrays J. Appl. Phys. 91, 2866 (2002); 10.1063/1.1445492 High resolution reactive ion etching of GaN and etch-induced effects J. Vac. Sci. Technol. B 17, 2759 (1999); 10.1116/1.591059

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 193.50.135.4 On: Sat, 26 Apr 2014 10:09:39

JOURNAL OF APPLIED PHYSICS

VOLUME 91, NUMBER 10

15 MAY 2002

Optical investigation of micrometer and nanometer-size individual GaN pillars fabricated by reactive ion etching F. Demangeot,a) J. Gleize, J. Frandon, and M. A. Renucci Laboratoire de Physique des Solides de Toulouse, UMR 5477 CNRS-IRSAMC-Universite´ Paul Sabatier, 31062 Toulouse Ce´dex 04, France

M. Kuball H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1 TL, United Kingdom

D. Peyrade, L. Manin-Ferlazzo, and Y. Chen LPN/CNRS, Route de Nozay, 91460 Marcoussis, France

N. Grandjean CRHEA/CNRS, rue B. Gre´gory, Sophia Antipolis, 06560 Valbonne, France

共Received 26 September 2001; accepted for publication 19 February 2002兲 We present an optical investigation of GaN pillars using both micro-Raman 共␮-Raman兲 and microphotoluminescence 共␮-PL兲 spectroscopy. GaN pillars of diameter ranging from 100 nm to 5 ␮m were fabricated by electron beam lithography and reactive ion etching 共RIE兲 with SiCl4 plasma. Optical measurements of both ␮-Raman and ␮-PL on individual pillars show consistent variations in the properties of the fabricated GaN structures as a function of GaN pillar size. ␮-PL mapping gives strong evidence for defect-induced donors and/or acceptors near the facets of the RIE etched pillars. RIE for the nanostructuration of GaN could be used in the future to allow spectroscopic studies of a few or single quantum objects such as GaN quantum dots. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1468908兴

INTRODUCTION

Low dimensional semiconductor structures are of current interest for many applications. Quantum wells and quantum dots are studied in order to develop high performance electronic and optoelectronic devices.1–3 Various nanofabrication technologies are used for device manufacturing with smaller and smaller dimensions.4 In the case of fine patterning of III–V nitrides high quality etching is still a processing challenge. Both dry and wet etching techniques have been explored.5 Focused ion beam etching has been successfully used for the fabrication of submicrometer III–nitride structures.6 Nevertheless, reactive ion etching 共RIE兲 appears to be a most useful technology because of its parallel processing capability.5,7 Dry etching such as RIE, however, is complicated by the inert chemical nature and strong bond energies of the group-III nitrides as compared to other compound semiconductors. For example, RIE techniques still need to be improved to reduce etch damage after systematic studies on etching performance by advanced optical characterization techniques. Indeed, only a few reports have been devoted to the assessment of etched nitrides structures, studying systematic defects induced by etching which could diminish the lifetime of future devices based on these nanostructures.8 –11 From a more fundamental point of view, the combination of surface structuration at the nanometer scale and high spatial resolution optical techniques opens up a detailed investigation into nano/micrometer-size individual objects. a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

In this article, we report on the optical investigation of micrometer and nanometer GaN pillars etched on top of a AlN buffer layer. Atomic force microscopy 共AFM兲 measurements give details about the shape of these pillars while micro-Raman and microphotoluminescence 共␮-PL兲 experiments provide evidence for good crystalline quality even in 100 nm diameter pillars. Frequency shifts of the E 2 mode suggest a partial relaxation of the strain in the GaN pillars. Preferential localization of defects near the facet surface of the pillar was found by ␮-PL mapping data. EXPERIMENTS AND SAMPLES

300 nm thick GaN layers were grown along the 关0001兴 direction on top of a 400 nm thick AlN layer deposited on a sapphire substrate by molecular beam epitaxy.12 Electron beam lithography on a single layer poly共methylmethacrylate兲共PMMA兲 resist has been used to produce the etch pattern. After the development of the PMMA, a metallic mask 共Ni兲 was deposited and lifted off. Then, RIE was performed using SiCl4 gas at a pressure of 2 mT. Further details about the nanofabrication procedure were reported elsewhere.13 The etching depth was chosen as 400 nm. The fabricated series of pillars is composed by seven pillars of various sizes, ranging from 5 ␮m to 100 nm, spaced by 50 ␮m. Two unetched zones were used to compare the properties of the etched material with the as grown layer. A Park Scientific Instruments microscope 共CP model兲 was used in the AFM configuration in contact mode. A Dilor XY spectrometer equipped with optical microscope and charge coupled device was used for the micro-Raman experiments, and 488 nm laser excitation was chosen. A 1 ␮m2 laser spot

0021-8979/2002/91(10)/6520/4/$19.00 6520 © 2002 American Institute of Physics [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 193.50.135.4 On: Sat, 26 Apr 2014 10:09:39

J. Appl. Phys., Vol. 91, No. 10, 15 May 2002

Demangeot et al.

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FIG. 1. AFM images of GaN pillars of 共a兲 5 ␮m and 共b兲 200 nm diameter obtained by electron beam lithography and RIE techniques.

was focused on the sample with ⫻100 objective 关numerical aperture 共NA兲 0.95兴. PL spectra were recorded using the ultraviolet 共UV兲 Renishaw Raman microscope with HeCd laser excitation (␭⫽325 nm), and 2400 g/mm grating. ⫻40 UV objective 共NA 0.4兲 delivers an ⬃1.5–2 ␮m2 laser spot on the sample. A 0.5 ␮m step was employed in the micro-PL mapping experiments. RESULTS AND DISCUSSION

AFM images of a 5 ␮m and 200 nm diam pillar are shown in Figs. 1共a兲 and 1共b兲, respectively. Large surface roughness is evidenced 共few tens of nanometers兲. This is mainly due to the fact that the as-grown layer was very rough 共35 nm兲. Additional roughness induced by ion etching leads to a value of 45 nm. Pillar facet angles lie in the range of 15°–20° with respect to the perpendicular direction to the surface 共c axis兲. This was confirmed by scanning electron microscopy. We found a good homogeneity of the shapes of the pillars regularly spaced in the etched array. Raman results are presented in Fig. 2. In Fig. 2共a兲 we display a set of Raman spectra for the pillars of different sizes from 5 ␮m to 100 nm in diameter. Clear observation of the allowed modes in wurtzite GaN was achieved, i.e., the E 2 phonon at 568 cm⫺1 and the A 1 longitudinal optical 共LO兲 polar phonon at 735 cm⫺1. The Raman spectrum from the unetched area was identical to the one recorded on etched areas without pillars. Transverse optical branch scattering activation is observed in etched pillars together with an upward shift of the LO mode, which actually corresponds to the quasi-LO mode 共QLO兲.14 This can be explained in terms of angular dispersion of op-

FIG. 2. 共a兲 Raman spectra recorded in backscattering geometry parallel to the 共0001兲 direction from an individual pillar of 共a兲 5␮m, 共b兲 2 ␮m, 共c兲 1 ␮m, and 共d兲 600 nm diameter 共inset兲, and of 共e兲 400 nm, 共f兲 200 nm, and 共g兲 100 nm diameter for spectra in the main part. Asterisks indicate sapphire substrate Raman bands. 共b兲 Plot of the intensity ratio I E2 (GaN)/I E2 (AlN) 共squares兲 and of the pillar E 2 mode frequency 共circles兲 as a function of the pillar diameter. Long dashed line is a quadratic fit of the intensity ratio data. Short dashed line is a guide for the eye.

tical phonons in the wurtzite crystal.15 Light entering through the facets of the pillars confers to the phonon a tilted propagation direction with respect with the c axis 共0001兲. By calculating the tilt angle from the measurements to the frequency shift of the QLO, we found good agreement with the value of the facets angle 共25°兲. Additional spectral signatures such as the E 2 mode of AlN as at 656 cm⫺1 and two modes coming from the sapphire substrate at 577 and 750 cm⫺1 are also visible in the spectrum. A clear decrease of the intensity of the GaN E 2 phonon with respect to the AlN E 2 phonon is visible when decreasing the size of the pillar. Confocal geometry was used in these experiments to strongly reduce the depth of field and by this way to weaken the Raman signal of the substrate. We used an XY automated stage to accurately locate the subwavelength size pillars by entering their XY coordinates as imposed by the initial mask. In Fig. 2 we observed some frequency shifts of the E 2 mode as a function of the pillar size, indicating that the strain in the GaN pillar induced by lattice mismatch with the AlN buffer layer is partially reduced. The E 2 frequency as a function of pillar size is shown in Fig. 2共b兲. The small size of the pillars with its open surfaces allows efficient strain relaxation. For the 100– 400 nm size pillars nearly full strain relaxation was


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achieved with an E 2 phonon frequency of 567.1 cm⫺1 with respect to values reported for unstrained GaN of 566 –567 cm⫺1.16 Figure 2共b兲 displays the intensity ratio 关 I E2 (GaN)/I E2 (AlN) 兴 as a function of the size of the pillars. The variation of this quantity is practically a quadratic function of the size up to ⬃600 nm as expected. The intensity ratio evolves as ⬀ ␲ * (D/2) 2 , D being the pillar diameter, as far as the laser spot size is larger than the diameter of the pillar. The spatial resolution of the Raman experiments lies in the 600 nm–1 ␮m range, i.e., for pillar sizes above 600 nm the whole pillar is probed. From 600 nm to higher sizes, the intensity ratio therefore remains constant around a value of ⬃3, mainly representative of the difference of the Raman scattering cross sections between GaN and AlN. The good agreement with the quadratic relationship at small pillar sizes indicates that the crystalline quality is maintained in the nanometer-size GaN pillars. We performed 共not shown兲 a Raman mapping of a 5 ␮m pillar by carefully recording Raman spectrum each 200 nm along a line across the pillar. No changes related to the GaN modes were observed in the Raman spectra. To investigate defects induced by the RIE etching, room temperature PL spectra were recorded successively along a line across a 1 ␮m pillar, shown in Fig. 3. The main peak at 3.408 eV is attributed to band edge PL. The assignment of the second broad PL band, which is observed in the 3.1–3.25 eV range, is more difficult. The nature of the involved transitions has likely to be identified between band-to-acceptor 共or donor-to-band兲 transitions and donor–acceptor pairs 共DAPs兲, which could also be relevant in the hypothesis of higher carriers concentrations.17 On one hand, most of the previous works on PL measurements in GaN18 –20 pointed to the presence of PL signals related to DAP transitions in the 2.9–3.25 eV energetic range. On the other hand, deep acceptor states induced by dry etch damages, with an energy activation of 2.74 eV, have been reported in InGaN films by Pearton et al.21 These states can be involved in band-toacceptor transitions, which could be at the origin of the PL band centred at 3.15 eV in our experiments. It appears that further experiments need to be performed to clarify the exact nature of this PL, by low temperature measurements and the analysis of the dependence of PL peak intensity on the excitation power. Similarly, Cheung et al.22 reported the appearance of two lower energy transitions close to the band-edge luminescence in etched material, likely to be related to defects states. The integrated intensity profile of the PL band edge peak and of the donor and/or acceptor related PL band in our experiments, is presented in Fig. 3共b兲; there is a maximum of the intensity of the donors and/or acceptors related PL band centered on the facet. In addition, the main PL peak intensity vanishes faster than the 3.2–3.15 eV broadband when moving away from the pillar. Defects responsible for donor and/or acceptor density are therefore preferentially localized in the pillar boundary. Even slightly away from the pillar, significant 3.1 eV PL band is visible while the band edge PL signal is proportional to the pillar surface illuminated, indicating a possible redeposition of the RIE-etched GaN in the vicinity of the pillar. Alternately, we cannot com-

Demangeot et al.

FIG. 3. 共a兲 Micro-PL spectra recorded successively along a line across a 1-␮m-diam pillar and spaced by a 0.5 ␮m step. The spectrum at the bottom corresponds to an illumination at the center of the pillar while the top spectrum was obtained 2.5 ␮m away from the center of the pillar. 共b兲 Plot of the integrated intensity of PL peak at 3.408 eV 共filled circles兲 and of PL DAP band 共intensity increased by a factor ⫻10 for clarity兲 centered at 3.15 eV 共open diamond兲 as a function of the position of the laser spot measured from the center of the pillar.

pletely exclude that the PL signal related to donors and/or acceptors recorded slightly outside the pillar originates from carriers diffusing from the pillar, and/or from carriers within the pillar excited under the laser beam. In view of the determination of the carriers diffusion length in GaN found as low as 0.25␮m,23 this effect is not believed to play a key role in determining the spatial resolution of our PL. Based on some qualitative tests, we estimated the spatial resolution in our experiments to be ⬃2 ␮m, larger than the theoretical spatial resolution which takes into account both the wavelength of the light 共325 nm兲 and the NA of the microscope objective 共0.4兲. One finds a theoretical value around ⬃500 nm. This is not surprising when keeping in mind the difficulties encountered in the manufacturing of UV optics. As a consequence, when looking at the position scale of the Fig. 3共b兲 and keeping in mind that the position labeled ‘‘0’’ corresponds to the middle of the pillar, there shouldn’t be any illumination of the pillar boundary, when the laser spot is centered at the 2.5 ␮m position. Nevertheless, cumulative effects 共diffusion length carriers exceeding the value previously reported and spreading of the laser spot on the sample over estimated size兲


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could also explain our results, ruling out any redeposition of the material hypothesis. N vacancies and/or Ga interstitial defects have been reported on etched surfaces which have been found to be no longer stoichiometric because of a preferential removal of the lighter atom, i.e., nitrogen, during the etching process8,11 and are therefore a likely candidate for the RIE-induced 3.1 eV PL. Also C impurities have been found near the surface of the investigated sample in Auger spectroscopy measurements and are likely to contribute to the 3.1 eV PL. C atoms on nitrogen sites (CN) could act as acceptors.24 Donor or/and acceptor related PL signals could also come from redeposited GaN material spreading out over the facet pillar and slightly out of the pillar. CONCLUSION

We reported on the optical investigation of individual nano/micrometer-size RIE-etched GaN pillars, as small as 100 nm in diameter, complemented by AFM measurements. Raman shifts of the GaN E 2 mode indicate increased strain relaxation in the pillars with decreasing pillar size. For the 100– 400 nm size pillars nearly full strain relaxation was achieved. PL mapping illustrates that defects due to the RIE process lie preferably near the facet’s surface of the pillar. Possible indications for redeposition of GaN in the vicinity of the pillars were observed. 1

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