Properties of Bulk AlN grown by ...

Properties of Bulk AlN grown by thermodecomposition of AlCl3NH3 J. A. Freitas, G. C. B. Braga, E. Silveira, J. G. Tischler, and M. Fatemi Citation: Appl. Phys. Lett. 83, 2584 (2003); doi: 10.1063/1.1614418 View online: http://dx.doi.org/10.1063/1.1614418 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v83/i13 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS

VOLUME 83, NUMBER 13

29 SEPTEMBER 2003

Properties of Bulk AlN grown by thermodecomposition of AlCl3 "NH3 J. A. Freitas, Jr.,a) G. C. B. Braga,b) E. Silveira,c) J. G. Tischler, and M. Fatemi Naval Research Laboratory, ESTD, Washington, DC 20375-5347

共Received 21 May 2003; accepted 5 August 2003兲 Self-nucleated bulk AlN crystals were grown by thermodecomposition of AlCl3 •NH3 vaporized in the low-temperature zone of a two-zone furnace. X-ray diffraction of the AlN crystals show single lines with a small linewidth indicating high single-crystalline quality. Polarized Raman scattering experiments of these samples confirm the x-ray results based on the detection of a small linewidth for all allowed optical phonons. Low-temperature cathodoluminescence spectra show very sharp emission bands close to the optical band gap, which have been assigned to free-excitons A and B, and exciton-bound to shallow neutral impurity. The latter has a full width at half maximum smaller than 1.0 meV. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1614418兴

The III–V nitride semiconductor system combines extreme values of fundamental physical and chemical properties and has proven to be adequate for fabrication of a variety of devices capable of performing at extreme conditions of power, frequency, temperature, and in harsh environments. Despite the remarkable improvement in the quality of thin heteroepitaxial GaN and AlN films achieved in the last decade, their properties are still seriously limiting the performance of devices demanding higher material yields. Overcoming these limitations will require the use of native substrates to grow electronic-grade homoepitaxial layers. Only recently, large AlN crystals with improved physical properties have been fabricated and became commercially available.1 In the present work, we report on a detailed characterization of the structural, optical, and electronic properties of AlN crystals grown at a lower temperature by thermodecomposition of aluminum chloride monoammoniate (AlCl3 •NH3 ) in a two-zone furnace.2 The crystalline properties of our samples were verified by double-crystal x-ray diffraction 共XRD兲 and polarized micro-Raman scattering 共RS兲. Their electronic properties were investigated by variable temperature cathodoluminescence 共CL兲 spectroscopy. The growth of spontaneous nucleated AlN crystals from the single precursor AlCl3 •NH3 requires several successive steps, namely, the synthesis of AlCl3 •NH3 , the evaporation and transport to a reaction chamber, and the decomposition and growth of single-crystal AlN on graphite or quartz substrates in the temperature range between 1300 °C and 1400 °C. A detailed discussion of the growth technique has been reported elsewhere.2 Double-crystal XRD rocking curve experiments were performed on a high-resolution double-crystal diffractometer using Cu K ␣ 1 radiation with a Si 共100兲 crystal as a beam conditioner. Room-temperature RS measurements were carried out with the 514.5 nm line of an Ar⫹ laser. The CL spectrometer consists of a commercial electron gun and a

cold finger installed in an ultrahigh vacuum chamber, and an UV-visible double 0.85 m spectrometer fitted with an UVsensitive GaAs photomultiplier coupled to a computer controlled photon counter system. The sample temperature can be varied from 2 K to 320 K. AlN 共0002兲 XRD reflections obtained from various samples, using a beam width of 500 ␮m, yielded symmetric lines with a full width at half maximum 共FWHM兲 between 36 and 54 arcs. Figure 1 represents a XRD line with a FWHM of 36 arcs measured on one of our best samples with an area ⬃3 mm2 . The high intensity of the single line and its small FWHM value indicate the high single-crystalline quality of these samples. AlN crystal with a 2H structure belongs to the space group C6v and has two molecules per unit cell. Group theory predicts eight zone-center optical phonons, namely 1A1 关transverse optical 共TO兲兴, 1A1 关longitudinal optical 共LO兲兴, 2B1 , 1E1 (TO), 1E1 (LO), and 2E2 . The two B1 modes are silent or optically inactive, while all of the other six phonons are allowed and observed. The first-order RS spectra measured for two different sample orientations and light polarizations are shown in Fig. 2. The peak position and relatively small linewidth indicate that this low-temperature growth process can fabricate low defect and stress free crystals. A complete discussion of the RS study will be presented elsewhere. Figure 3 represents the low-temperature CL spectra of a selected sample in the spectral range between 2.0 eV and 6.2 eV. Spectral features introduced by the instrumental response

a兲

Electronic mail: [email protected] Also at: Instituto de Fı´sica, Universidade de Brasilia, 70000, Brasilia-DF, Brazil. c兲 Also at: Departmento de Fı´sica, Universidade Federal do Parana´, 81531990, Curitiba-PR, Brazil. b兲

0003-6951/2003/83(13)/2584/3/$20.00

FIG. 1. X-ray rocking curve measured on a c-plane AlN sample with beam width of 500 ␮m. 2584

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FIG. 4. High-resolution CL spectra measured at different temperatures. The inset represents the CL intensity as a function of reciprocal temperature for the line initially at 6.0089 eV (XD ⴰ ).

FIG. 2. Raman scattering spectra measured, on the sample represented in Fig. 1, at two different sample orientations and light polarizations. The letters outside of the bracket represent the direction of the incident and scattered light, while the letters inside the brackets represent the incident and scattered light polarizations.

were removed using a calibrated light source in the spectral range between 2.0 eV and 4.1 eV. The spectrum measured with a 0.5 ␮A electron-beam 共e-beam兲 current shows two broad overlapping bands with peaks at 3.6 eV and 4.3 eV. The first has been attributed to the presence of oxygen impurities.3 Although, a peak around 4.4 eV has been previously observed in carbon-doped AlN films,4 we cannot rule out that the 4.3 eV band is related to O complexes. The spectra measured with 1.0 ␮A and 5.0 ␮A currents shown an additional band at 5.3 eV. Additional experiments will be carried out to further investigate the nature of these three broadbands. A relatively intense sharp emission line around 6.1 eV was also observed in all three spectra, which has been previously tentatively attributed to recombination processes associated with the annihilation of excitons.4 –7 Similar spectra, as represented in Fig. 3, have been recently observed on samples fabricated by sublimation–recondensation method.8 Since excitonic recombination is associated with intrinsic

FIG. 3. 6 K CL spectra measured at three different e-beam currents. The spectra were corrected in the spectral range between 2.0 eV to 4.1 eV to remove spectral features introduced by the instrumental response.

material properties, it is important to study this sharp emission band in detail. In direct gap wurtzite semiconductors, the valence-band maximum is split by both noncubic crystal-field (⌬ CF :⌫ 6 →⌫ 6 ,⌫ 1 ) and spin–orbit interaction (⌬ 0 :⌫ 1 →⌫ u7 ;⌫ 6 →⌫ 9 ,⌫ ᐉ7 ), resulting in three valence bands at the center of the Brillouin zone. Reported calculations generally agree that ⌬ CF is negative and promotes the ⌫ u7 band to higher energy than the ⌫ 9 band. However, there is considerable disagreement in regard to the calculated values for effective masses (m e* between 0.27 and 0.35m o , and m h* between 1.54 and 1.62m o ) crystal field, and spin-orbit splitting.9–11 Despite the considerable discrepancy in the measured values of the dielectric constant 共7.54 to 9.14兲,12,13 we can establish lower and upper bound values for the free-exciton energy 关based on the hydrogenic expression E x ⫽( ␮ * / ␬ 2 )•13.61, where ␮* is the reduced effective mass and ␬ is the dielectric constant兴 of 37 meV and 70 meV, respectively. Assuming that the band gap at low temperature is 6.12 eV,6 we expect to observe excitons in the spectral range between 6.08 eV and 6.05 eV. Figure 4 represents four higher-resolution CL spectra of the sharp line observed near 6.1 eV in Fig. 3, measured on one of our c-plane samples at various temperatures. The spectra show the expected systematic reduction of the CL intensity upon increasing temperature from 5 K to 90 K. Note that the intensity of the line at 6.0089 eV reduces at a faster rate than that of the lines at 6.0250 eV and 6.0363 eV. This behavior is commonly observed in recombination processes involving complexes, such as excitons bound to neutral impurities, where one of the components 共i.e., the exciton兲 binds with lower binding energy. Based on this observation and the similarity of these spectra with that of GaN, we assign the line at 6.0089 eV to the recombination process associated with the annihilation of exciton bound to a shallow neutral impurity 共probably a donor, XD ⴰ , as discussed later兲, while the lines at 6.0250 eV and 6.0363 eV we assign to recombination processes associated with the annihilation of free-excitons A and B (FX A and FX B), respectively. The value of the FX A binding energy to the shallow donor is given by the slope, E X /kT, of the exponential quenching curve at the highest temperatures 共i.e., 17.8 meV兲, as represented in the inset of Fig. 4, and is consistent with


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the 16.1 meV separation between the XD ⴰ and FX A lines. The asymmetry of the XD ⴰ line may result from the presence of an additional line at 6.0058 meV, resolved with spectral fitting assuming Lorentzian line shapes, which may be associated with the presence of a second shallow donor. We also observe, in the spectra of Fig. 4, two additional weak lines at 5.9726 eV and 5.9583 eV, which show a rather strong temperature dependence considering their spectral position. Note that the 5.9726 eV line intensity is readily reduced at 40 K. Similar behavior was observed in GaN where lines with similar behavior were associated with recombination processes leaving the donors in the excited state 共i.e., two electron satellite or 2ES lines兲.14 Spectral separation between the 2ES lines and the ground-state XD ⴰ lines yield the intracenter transition energies of the impurities. If the shallow donor at 6.0089 eV has a 2ES at 5.9726 eV and we assume the simple hydrogen approximation, we obtain 48 meV for its higher binding energy. This value is within the lower and upper bounds of 44 meV and 84 meV estimated for the effective Rydberg energy of AlN. Although this model is attractive, it must be verified with additional experiments. In spectra covering larger energy ranges, we also observed an emission line at 5.8947 eV, which is about 114 meV redshifted from XD ⴰ . This line, observed in Fig. 3, is assigned to the 1LO phonon replica of XD ⴰ , which is in good agreement with the 110.2 meV value measured from our RS spectra, shown in Fig. 2. We have performed measurements at a higher spectral resolution than that represented in Fig. 4 to measure the typical XD ⴰ linewidth of our samples. The spectrometer band pass, fit with 1200 grooves/mm gratings blazed at 250 nm, at 253.65 nm 共Hg calibration lamp兲, was found to be ⬃0.65 meV, while the XD ⴰ linewidth was typically ⬃0.97 meV. Therefore, we can conservatively estimate a XD ⴰ linewidth of ⭐0.7 meV. This value is more than one order of magnitude smaller than the sharpest linewidth previously reported for AlN.7 In summary, our XRD and RS studies demonstrated that

high-quality bulk AlN crystals can be grown by low temperature using the thermodecomposition of AlCl3 •NH3 in a twozone furnace. Analyses of our high-resolution CL results yield an exciton-donor binding energy of 16.1–17.8 meV and 11.3 meV split between FX A and FX B . In addition, we observed an extremely sharp XD ⴰ line at 6.0089 eV with a linewidth of only ⭐0.7 meV. Assuming that this shallow donor behaves as an effective mass and the line at 5.9726 eV is its 2ES, we estimate the shallow donor binding energy as 48 meV. This work was in part supported by the Department of the Navy 共Grant No. N00014-02-1-4087 issued by the ONRIFO兲 and by ONR 共Contract No. N00014WR20015兲共Dr. C. E. C. Wood兲. Dr. B. V. Shanabrook is gratefully acknowledged for helpful discussions. The authors would like to thank Dr. L. M. Ivanova for providing the samples.

1

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