Time-resolved cathodoluminescence assessment of deep-level transitions in hydride-vapor-phase-epitaxy GaN C. Díaz-Guerra, J. Piqueras, and A. Cavallini Citation: Appl. Phys. Lett. 82, 2050 (2003); doi: 10.1063/1.1565501 View online: http://dx.doi.org/10.1063/1.1565501 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v82/i13 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 11 Oct 2013 to 147.96.14.16. This article is copyrighted as indicated in the abstract. 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Cavallini&possible1zone=author&alias=&displayid=AIP&ver=pdfcov http://apl.aip.org/?ver=pdfcov http://link.aip.org/link/doi/10.1063/1.1565501?ver=pdfcov http://apl.aip.org/resource/1/APPLAB/v82/i13?ver=pdfcov http://www.aip.org/?ver=pdfcov http://apl.aip.org/?ver=pdfcov http://apl.aip.org/about/about_the_journal?ver=pdfcov http://apl.aip.org/features/most_downloaded?ver=pdfcov http://apl.aip.org/authors?ver=pdfcov APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 13 31 MARCH 2003 Time-resolved cathodoluminescence assessment of deep-level transitions in hydride-vapor-phase-epitaxy GaN C. Dı́az-Guerraa) and J. Piqueras Departimento Fı´sica de Materiales, Facultad de Fı´sicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain A. Cavallini INFM and Dipartimento di Fisica, Universita´ di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy ~Received 9 December 2002; accepted 7 February 2003! The temporal behavior of deep-level luminescence emissions in undoped hydride-vapor-phase-epitaxy GaN layers of different thicknesses has been investigated by time-resolved cathodoluminescence~TRCL!. The complex nature of the yellow luminescence is revealed in the TRCL spectra by the presence of two bands peaked at 2.22 and 2.03 eV. A red band with a decay time of 700ms, centered at about 1.85 eV, dominates spectra recorded for long delay times. Exponential transients with associated decay times of hundreds ofms were measured at 87 K for all the deep-level emissions found in the layers. ©2003 American Institute of Physics. @DOI: 10.1063/1.1565501# he gh ec ca ul a n en on rt g ed n le e o ly m on te n ce e ra s ax i ob ee n de- age rrier 00 ra- ac- a out elay y a ig- ith to ep- K in ig. 1 xci- at a ient both om- ked usly E- - tu- ism ble Gallium nitride has been intensively investigated in t last years due to its application in optoelectronics and hi temperature microelectronics. Nevertheless, it is widely r ognized that the role of deep levels controlling the electri and luminescence properties of this material should be f understood in order to achieve devices optimization. In p ticular, a broad emission centered at about 2.2 eV, know the yellow band, is commonly observed inn-type layers. The origin of this luminescence remains unclear and differ models1–3 have been proposed to explain the emissi Variations in peak position, shape and decay times repo in different studies3–5 suggest that several bands involvin different defects could contribute to the mention luminescence.6 On the contrary, deep-level-related emissio in the green and red ranges of the visible spectrum are frequently observed in undoped GaN.7–9 Cathodolumines- cence~CL! in the scanning electron microscope~SEM! has been used to investigate the spatial distribution of GaN de level emissions and their association to extended and p defects.6,10,11 Although electron beam excitation usual leads to light emission by all mechanisms of radiative reco bination present in a semiconductor, several investigati concerning the recombination kinetics of GaN defect cen have been carried out by time-resolved photoluminesce ~TRPL!,3–5,12 while time-resolved cathodoluminescen ~TRCL! studies of GaN deep levels have not yet been und taken. In this work, TRCL is used to investigate the tempo behavior of deep-level emissions in undoped GaN layer different thicknesses grown by hydride-vapor-phase epit ~HVPE!. Our results indicate that CL emissions observed the yellow and red ranges of the visible spectrum are pr ably related to transitions from the conduction band to d acceptor levels. Two GaN films with thickness of 2.6mm ~LH1232! and 55 mm ~LH1234! were investigated. Both layers were grow a!Electronic mail: cdiazgue@fis.ucm.es 2050003-6951/2003/82(13)/2050/3/$20.00 Downloaded 11 Oct 2013 to 147.96.14.16. This article is copyrighted as indicated in the abstract. R - - l ly r- as t . ed s ss p- int - s rs ce r- l of y n - p on sapphire substrates by HVPE following the procedure scribed in Ref. 13. Room-temperature capacitance–volt and Hall measurements, respectively, indicate a free-ca concentration ofn5831016 cm23 and a mobility of mn 5260 cm2/V s for the LH1232 sample, while values ofn 5231016 cm23 and mn5810 cm2/V s were obtained for the LH1234 layer. CL observations were performed in a Hitachi S-25 SEM at accelerating voltages from 5 to 20 kV and tempe tures between 85 and 295 K. Steady-state spectra were quired using a Hamamatsu PMA-11CCD camera with built-in spectrograph. TRCL measurements were carried using a pulsed electron beam. To record CL spectra at d times ranging from 500 ns to 3 ms, the signal detected b photomultiplier was collected trough a boxcar integrator tr gered by a pulse generator and then fed to a computer.14 The decay transients of the CL emissions were monitored w the aid of a digital oscilloscope. CL spectra of the samples investigated were found depend on beam excitation conditions. In particular, de level-related emissions could be better appreciated at 85 spectra obtained with a defocused electron beam, as F shows. Near-band-gap emission related to donor-bound e tons is centered at 3.472 eV in the LH1234 sample and 3.492 eV in the LH1232 sample. This shift is caused by compressive strain due to the thermal expansion coeffic mismatch between GaN and sapphire.15 Other peaks corre- sponding to shallow donor–acceptor pair~DAP! transitions appear centered between 3.4 and 3.2 eV in spectra of layers. Some of the deep-level-related emissions are c mon to the samples investigated, as the blue CL band pea at 2.92 eV. A band centered at 2.88 eV has been previo observed in cross-sectional CL investigations of HVP grown GaN layers11 and associated with point defects or im purities decorating grain boundaries and dislocations. Ac ally, oxygen donors seem to be involved in the mechan responsible for this emission.16 In addition, CL emission is also observed in the yellow and red ranges of the visi 0 © 2003 American Institute of Physics euse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions nd a 3 2 e e l e ck e a n le e en g r d su f le en he gu e .8 3 lts elay y ted en- in- ies ted V the the for K, or- her . in that to re the per- ned ble lec- 2051Appl. Phys. Lett., Vol. 82, No. 13, 31 March 2003 Dı́az-Guerra, Piqueras, and Cavallini spectrum. Gaussian deconvolution of spectra recorded u different excitation conditions in the thick layer indicate th the bands observed in Fig. 1~a! are centered at 1.81 and 2.2 eV. These bands are also observed peaked at 1.85 and eV in CL spectra from the thin layer@Fig. 1~b!#, but decon- volution reveals the existence of an additional emission, c tered at 2.02 eV, in this sample. Such observation sugg that the latter peak could be related to structural defects cated at the GaN–sapphire interface. In fact, an increas the threading dislocation density with decreasing layer thi ness has been found17 in HVPE layers similar to those her investigated, while an enhanced yellow emission w observed11 near the substrate interface in CL investigatio of thin HVPE GaN films. In order to evidence a possib thickness dependence of the distribution of the radiative c ters involved in the CL bands, depth-resolved measurem were performed by varying the beam voltage while keepin constant injection rate~i.e., keeping a constant beam powe!. The intensities of the 1.81 and 2.23 eV bands were foun decrease by increasing the accelerating voltage, which gests that the concentration of deep levels responsible such emissions is higher near the surface of the samp This result supports previous PL works1,18 revealing a sig- nificant concentration of yellow-luminescence-related c ters at the surface of different GaN layers. The temporal behavior of the deep-level bands and t decay times were assessed by TRCL spectroscopy. Fi 2~a! shows TRCL spectra from the LH1234 sample record at 85 K for different delay times after excitation with a 20ms pulse. A progressive delay time increase favors the red 1 eV emission, while the relative intensity of the yellow 2.2 eV band decreases in comparison. The 2.92 eV CL can FIG. 1. CL spectra of the LH1234 layer~a! and the LH1232 film~b! re- corded with a defocused electron beam~85 K, 15 kV!. Downloaded 11 Oct 2013 to 147.96.14.16. This article is copyrighted as indicated in the abstract. R er t .22 n- sts o- of - s s n- ts a to g- or s. - ir re d 1 be still observed for delays longer than 1 ms. Similar resu were obtained in the LH1232 layer, as Fig. 2~b! shows. The 2.22 eV band dominates the spectrum recorded for a d time of 1ms, but the 2.03 eV CL band—previously found b deconvolution procedures—can be now clearly apprecia as a shoulder of the main emission. Present day views g erally agree in the acceptor character of the deep levels volved in the GaN yellow luminescence, being Ga vacanc and its complexes with oxygen or carbon often sugges candidates.1,2 The relative weights of the 2.03 and 1.85 e bands increase by further increasing delay time, shifting CL emission maximum towards higher energies. As in case of the thick layer, the 1.85 eV band is dominant delay times above 100ms. The intensity of this CL band is almost independent of temperature between 85 and 295 which differs from the intensity change of more than an der of magnitude observed in the same interval for ot GaN red PL bands8,19 attributed to deep DAP transitions Moreover, no peak shift of the 1.85 eV band is observed our spectra by increasing delay time, which suggests deep donors are not involved in the mechanism giving rise this emission.20 It should be mentioned that the temperatu behavior of our red band is similar to that observed for PL band peaked at 1.92 eV associated by Reshchikovet al.21 to Ga vacancies—related defects bound to structural im fections in HVPE GaN layers. The decay times of the observed emissions obtai from CL transients recorded at 87 K are summarized in Ta FIG. 2. Normalized TRCL spectra of the LH1234 layer~a! and the LH1232 layer ~b! recorded at 85 K for different delay times using a defocused e tron beam. euse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions de ou d th P m e m n u re nt lar gh 87 t a on y d tu d e po 7 rk t if co ne nds bout the elay s of he re- ep- . . B h- t. l. - and th , J. nd nd d J. . . th 2052 Appl. Phys. Lett., Vol. 82, No. 13, 31 March 2003 Dı́az-Guerra, Piqueras, and Cavallini I. Such transients are well fitted by single exponential cays. The shorter decay time~;250ms! corresponds in both layers to the 2.23 eV band, while long decay times of ab 430 and 700ms were respectively found for the 2.92 an 1.81 eV emissions. An intermediate decay time of 340ms was measured for the 2.03 eV band observed in the layer, in good agreement with TRCL spectra. Previous TR investigations show a clear controversy between decay ti measured for the yellow emission.3–5 In particular, nonexpo- nential transients with decay times in the 1021– 103 ms range have been reported3,4 and explained in the frame of th Thomas–Hopfield model20 for DAP recombination. In prin- ciple, our exponential transients with associated decay ti of hundreds ofms do not agree with the DAP recombinatio theory, which predicts a wide distribution of instantaneo decay times extending from the ns to the ms range.3,4 This discrepancy can be explained considering both the diffe nature of the excitation source and the different experime conditions used in PL and CL experiments. In particu electron pulses of 20ms were used in this work while in PL experiments the sample is commonly excited with ns li pulses.3,4,7 Moreover, our CL transients were recorded at K, while the mentioned PL investigations were carried ou liquid He temperature. A change in the recombinati mechanism by increasing temperature has been recentl ported for GaN deep-level PL.7 While a DAP-type recombi- nation was observed at 15 K, PL decay curves were foun approach an exponential behavior by increasing tempera due to an increase of the free-electron concentration an decrease of the number of neutral donors. A similar proc could account for the present CL results. Actually, the ex nential character of our transients suggests that above 8 GaN yellow and red CL emissions observed in this wo could be related to transitions from the conduction band deep acceptors. In summary, deep-level transitions in GaN layers of d ferent thicknesses have been assessed by TRCL. The plex nature of the yellow emission observed in the thin TABLE I. Deep-level CL emissions decay times measured at 87 K in GaN layers investigated after excitation with a 20ms beam pulse. Peak energy~eV! Sample LH1234~ms! Sample LH1232~ms! 2.92 430610 420610 2.23 2606 5 2506 5 2.02 ¯ 340610 1.81–1.85 700620 670620 Downloaded 11 Oct 2013 to 147.96.14.16. This article is copyrighted as indicated in the abstract. R - t in L es es s nt al , t t re- to re a ss - K, o - m- r sample is evidenced by TRCL spectra revealing two ba peaked at 2.22 and 2.03 eV. A red emission, centered at a 1.85 eV, shows a higher intensity near the surface of samples and dominates spectra recorded for long d times. Exponential transients with associated decay time hundreds ofms suggest that, above 87 K, emissions in t yellow and red ranges of the visible spectrum could be lated to transitions from the conduction band to deep acc tor levels. This work was supported by MCYT~Project No. MAT2000-2119!. R. Molnar and D. C. Look are gratefully acknowledged for providing the samples. 1H. Morkoc, Mater. Sci. Eng., R.33, 135 ~2001!, and references therein. 2J. Neugebauer and C. G. Van de Walle, Appl. Phys. Lett.69, 503 ~1996!. 3D. M. 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