Deep level transient spectroscopy was used to investigate the thermal stability of electron traps in n-type GaN grown by metalorganic chemical vapor deposition. The concentration of traps at 160 and at 500K increased more than fivefold over the course of several 700Kanneal cycles, while a peak at 320K increased by a factor of only 1.19. The increase in the trap concentration with repeated annealing might be due to a mobile trap or loss of passivant. Hydrogen is very likely present in high concentration in the epilayer, and its passivating effects may be lost with annealing

Doğan, S.

Fu, Y.

Hu, Y.

Johnstone, D.

Leach, J.

Moon, Y.-T.

Morkoç, Hadis

Deep level transient spectroscopy was used to investigate the thermal stability of electron traps in n-type GaN grown by metalorganic chemical vapor deposition. The concentration of traps at 160 and at 500 K increased more than fivefold over the course of several 700 K anneal cycles, while a peak at 320 K increased by a factor of only 1.19. The increase in the trap concentration with repeated annealing might be due to a mobile trap or loss of passivant. Hydrogen is very likely present in high concentration in the epilayer, and its passivating effects may be lost with annealing. (C) 2004 American Institute of Physics

Fu, Y

Morkoc, H

Hu, Y

Moon, YT

Leach, J

Dogan, S

Johnstone, D

Atatürk University Research Information System

Thermal stability of electron traps in GaN grown by metalorganic chemical vapor deposition

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Virginia Commonwealth UniversityVCU Scholars CompassElectrical and Computer Engineering Publications Dept. of Electrical and Computer Engineering2004Thermal stability of electron traps in GaN grown bymetalorganic chemical vapor depositionD. JohnstoneVirginia Commonwealth University, djohnstone@vcu.eduS. DoğanVirginia Commonwealth University, Atatürk UniversityJ. LeachVirginia Commonwealth UniversitySee next page for additional authorsFollow this and additional works at: http://scholarscompass.vcu.edu/egre_pubsPart of the Electrical and Computer Engineering CommonsJohnstone, D., Doğan, S., LEach, J., et al. Thermal stability of electron traps in GaN grown by metalorganic chemicalvapor deposition. Applied Physics Letters, 85, 4085 (2004). Copyright © 2004 AIP Publishing LLC.This Article is brought to you for free and open access by the Dept. of Electrical and Computer Engineering at VCU Scholars Compass. It has beenaccepted for inclusion in Electrical and Computer Engineering Publications by an authorized administrator of VCU Scholars Compass. For moreinformation, please contact libcompass@vcu.edu.Downloaded fromhttp://scholarscompass.vcu.edu/egre_pubs/132AuthorsD. Johnstone, S. Doğan, J. Leach, Y.-T. Moon, Y. Fu, Y. Hu, and Hadis MorkoçThis article is available at VCU Scholars Compass: http://scholarscompass.vcu.edu/egre_pubs/132Thermal stability of electron traps in GaN grown by metalorganic chemicalvapor depositionD. Johnstone,a) S. Doğan,b) J. Leach, Y. T. Moon, Y. Fu, Y. Hu, and H. MorkoçDepartment of Electrical Engineering and Department of Physics, Virginia Commonwealth University,Richmond, Virginia 23284(Received 7 June 2004; accepted 7 September 2004)Deep level transient spectroscopy was used to investigate the thermal stability of electron traps inn-type GaN grown by metalorganic chemical vapor deposition. The concentration of traps at 160and at 500 K increased more than fivefold over the course of several 700 K anneal cycles, while apeak at 320 K increased by a factor of only 1.19. The increase in the trap concentration withrepeated annealing might be due to a mobile trap or loss of passivant. Hydrogen is very likelypresent in high concentration in the epilayer, and its passivating effects may be lost withannealing. © 2004 American Institute of Physics. [DOI: 10.1063/1.1814801]The thermal stability of defects in GaN is a topic ofconcern in the reliability of many types of devices includinghigh power and high temperature transistors, ultraviolet sens-ing, and ultraviolet and visible emitters. The lack of nativesubstrate makes the fabrication of efficient and reliable de-vices particularly difficult, where growth on sapphire typi-cally results in a dislocation density of 1010/cm2 unless spe-cial precautions are taken.1 Propagation of threadingdislocations through active layers of electronic devices hasbeen reported to limit two-dimensional electron gas mobilityby providing charged scattering centers and boundaryscattering.2 Moreover, the dislocations provide parasitic cur-rent paths,3–5 decreasing the gain and increasing the noise inelectronic devices, increasing the threshold current and de-creasing the slope efficiency of lasers, and reducing the re-sponsivity of detectors, as well as introducing instabilitiesparticularly in devices relying on charge control and highelectric fields, such as field effect transistors. If the defectsthat act as traps, recombination or generation centers, andscattering centers change in character under device operatingconditions, they could have a detrimental impact on the reli-ability of the device. The thermal stability depends on theformation energy and diffusion energy.The energy of formation of native defects and their dif-fusion energies have been calculated, and depend on the typeand density of carriers at the growth temperature.6 Galliumvacancies are favored for n-type GaN, and nitrogen vacan-cies are strongly favored in p-type GaN. The VGa is expectedto be most stable in its 3− charge state, and acts as a tripleacceptor. Other native defects are unlikely due to muchhigher formation energies under equilibrium conditions,which may or may not be achieved. The diffusion energy ofthe VGa is 1.9 eV, and 2.6 eV for a VN, but only 1.2 eV for aGaI. Also, according to Ref. 6 there is a metastable positionfor the GaI with a barrier of only 0.9 eV. As another ex-ample, beryllium diffuses in the c-plane with diffusion en-ergy of 1.2 eV, compared to diffusion energy of 2.9 eValong the c axis.7 Another consideration is that dislocationsmay act as sources for native defects, e.g., growth of n-typeGaN under gallium rich conditions may encourage formationof either type of vacancy at dislocations,8 which can thendisperse within the crystal if the diffusion energy is lowenough. With potentially low and anisotropic diffusion ener-gies, we may expect that some of the defects in GaN aremobile at moderately high temperatures, within device op-erational range. Electron traps that have been characterizedby deep level transient spectroscopy are 0.14,9,100.18–0.30,9–20 0.49–0.61,9–12,15,16,18–21 0.65–0.67,9,12,16,18,200.72–0.78,10,13,15,18 0.89–0.96,10,13,15,18,20 and 1.63 eV,9some of which have been attributed to specific defects. Inthis letter we characterize and discuss the thermal stability ofelectron traps in n-GaN grown by metalorganic chemical va-por deposition (MOCVD).The GaN epilayers that were investigated were grown ina MOCVD system at a pressure of 200 Torr, on c-plane sap-phire sa-Al2O3d. Trimethylgallium (TMG) and ammoniasNH3d were used as sources of Ga and N, respectively. Thesubstrate was preheated in a stream of H2 at 1030 °C for3 min, on which a 30-nm-thick GaN buffer layer was grownat 550 °C. This was followed by the growth of a2.4-mm-thick GaN epilayer at 1010 °C. The layers weredoped with Si to 1.531017/cm3. The flow rates of the TMG,NH3, and H2 used in the growth of GaN were 78.58, 316,and 286 mmol/min, respectively. The resulting GaN filmsare Ga polar.The samples were cleaned in acetone, methanol, and de-ionized water. HC1 was used to remove the oxide layer.Ohmic contacts were formed by depositingTi/Al/Ti/Aus300 Å/300 Å/150 Åd in a vacuum of betterthan 4310−7 Torr and annealed at 900 °C for 1 min byrapid thermal annealing in nitrogen ambient. Schottky con-tacts were from Ni/Aus300 Å/750 Åd, 250 mm in diameter.The diodes had series resistances in the range of 20–40 V.The deep level transient spectroscopy (DLTS) system isbased on a SULA capacitance meter, with the transients digi-tized for subsequent processing to generate the spectra, andfitting for emission rate and amplitude. The signal to noiseratio is significantly improved by averaging several thousandtransients at each temperature step. The temperature is incre-mented by 3 K from 80 up to 700 K. Compared to typicalDLTS systems that use ratewindow analysis, at temperaturesa)Author to whom correspondence should be addressed; electronic mail:djohnstone@vcu.edub)Also at: Atatürk University, Faculty of Art and Science, Department ofPhysics, 25240 Erzurum, Turkey.APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 18 1 NOVEMBER 20040003-6951/2004/85(18)/4058/3/$22.00 © 2004 American Institute of Physics4058 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:128.172.48.59 On: Fri, 17 Apr 2015 18:05:23up to only 425 K, this systems has better sensitivity to over-lapping trap signals, and deeper traps.The DLTS spectra taken under various bias conditionsduring cooling from 700 K are shown in Fig. 1. Standardratewindow analysis for the peak at 150 K gave a trap sig-nature of 0.25 eV activation energy and, 5.5310−16 cm2capture cross section. However, spectra simulated from thetrap signature obtained from the ratewindow analysis did notcompare well to the experimental data, being at the correcttemperature but too narrow. From a fit of the transients,which was best for two components, the peak at 150 K con-sists of traps at 0.10 and 0.14 eV, with capture cross sectionsof 7310−21, and 4.5310−19 cm2, respectively. The peak at320 K, in the spectrum measured at −1 V, had an energy of0.55 eV and capture cross section of 6310−16 cm2 using ei-ther method. At −3 V measurement bias, the ratewindowanalysis gave an energy of 0.48 eV, and exponential fittinggave a slightly better fit for two traps at 0.43 eV, 2310−17 cm2, and 0.34 eV, 3310−19 cm2. The second trapmay have been more easily detected at the higher measure-ment bias due to the larger volume scanned. The reduction inenergy, from 0.55 down to 0.48, or 0.43 eV, at larger reversebias indicates a Poole–Frenkel barrier lowering which ischaracteristic of a donor trap potential profile, subject to fur-ther detailed measurements given later. A ratewindow analy-sis of the broad peak at 500 K gave an energy of 0.87 eV,4310−16 cm2 at high emission rates (higher temperatures).Two peaks were distinguishable at smaller emission ratess,200/sd, resulting in a measured trap energy of 0.62 eV,and capture cross section of 2310−18 cm2, and a trap ataround 1.2 eV, 3310−14 cm2. However, fitting of the tran-sients for two components did not result in linear Arrheniusplots, indicating that there may be more than two compo-nents, or an emission mechanism other than thermionic.Measurements were also made to determine if the trapsshowed a metastable behavior similar to the results of Auretet al. (Ref. 17) in hydride vapor phase epitaxy (HVPE) GaN.However, as the measurements were repeated on the samediode, all of the traps appeared to increase in concentration,especially the peaks centered at 150 and 500 K. The ratio ofthe final to initial peak height is 5.65 for the peak at 150 K,1.19 for the peak at 320 K, and 5.43 for the peak at 500 K.One possibility for this behavior is a change in the con-tact or epilayer resistivity that would simply change the sen-sitivity of measurements. The I–V relationship was mea-sured prior to each DLTS scan to obtain the series resistance.The series resistance did not change appreciably, varyingfrom 35, 27, and 31 V, respectively.Further DLTS investigations characterized the defectspatial and potential profiles, and the capture kinetics. Pairsof filling and measurement biases were used to characterizethe change in trap concentration with depletion width for thethree peaks. Figure 2 is a plot of the transient amplitudesnormalized to the scanned volume versus square root of ap-plied bias, which is proportional to the depletion width. Thedefects all showed uniform concentration, within the confi-dence of the capacitance–voltage profiles that were used tonormalize the capacitance transient amplitudes.The emission rate was also measured as a function ofmeasurement bias to indicate the charge state after emission.There was an increase in emission rate with increasing re-verse bias indicating a donor trap nature for the traps at320 K, but not for the peaks at 150 or 500 K. The increase inemission rate with electric field was less than expected for atrap with a 1+ charge after emission, but the electric fieldwas averaged over a wide range in the depletion region.To determine if there is a capture barrier, the filling pulsewas increased from 1 to 100 ms at temperatures of 140, 330and 500 K. The temperatures were chosen to obtain lowemission rates in order to increase the likelihood of seeing achange in concentration of filled traps within the range offilling pulse durations used. However, the transient ampli-tudes did not change, indicating there are no significant cap-ture barriers related to lattice relaxation or line defects. Sev-eral of the native defects are predicted to have a significantlattice relaxation as a result of a change of charge state.6The increase in the trap concentration with repeated an-nealing at 700 K might be due to a mobile species presenteither as a trap or as a passivant. Mobile defects could bediffusing from the substrate-epilayer interface, from the con-tact metalization, or from defect clusters or dislocations. Then-type GaN was grown under gallium rich conditions, whichFIG. 1. Deep level spectra for MOCVD GaN showing an increase in trapconcentrations with repeated cycling to 700 K. The effects of bias duringcooling from 700 K were secondary to the effects of heating.FIG. 2. Deep level profiles for each of the peaks shown in Fig. 1. Uniformdepth profile for each trap indicates that they do not result from diffusionfrom either the contact surface or the substrate side of the epilayer duringannealing.Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004 Johnstone et al. 4059 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:128.172.48.59 On: Fri, 17 Apr 2015 18:05:23could encourage formation of either gallium or nitrogen va-cancies at dislocations, although the formation energy is highenough that the concentration should be negligible unlessgrowth conditions are far from equilibrium.8 Of the two, onlythe nitrogen vacancy would behave as an electron trap. How-ever, the diffusion energy of the nitrogen vacancy is too highto have an appreciable effect at 700 K. Hydrogen is knownto have a high concentration in molecular beam epitaxy(MBE) and MOCVD GaN, and to passivate defects.22,23 H+implanted into GaN creates infrared absorption bands, all ofwhich anneal out by 650 °C, although the frequencies ob-served were attributed to VGa-H complexes.24 Ammonia thatis not fully dissociated can be a source of hydrogen, with aconcentration in the range of 931020/cm3 as measured bynuclear reaction analysis in MBE GaN.23 GaN exposed tohydrogen plasma showed deep level concentrations reducedby a factor of 4.7, measured by thermally stimulated currentat temperatures below 350 K.22 The passivation of deep lev-els by hydrogen remained until an anneal temperature of900 °C was reached. In other work, the concentration of oneof three traps found in HVPE GaN, corresponding to the trapat 320 K in Fig. 1, was found to increase when annealedfrom 370 K under reverse bias, and decrease when annealedfrom 370 K, under zero bias.16 The bias annealing effect wasattributed to hydrogen dissociation and reversible motion ofhydrogen into and out of the bulk, controlled by the electricfield. For the MOCVD sample characterized in the presentstudy, the change was of the same size as in Ref. 16 for thetrap at 320 K but the effect is not reversible in our case.Furthermore, the increase in concentration of the other twopeaks with annealing at 700 K is a much larger effect, over500%, compared to 20% for the trap at 320 K. The differ-ences in annealing behavior may be due to the differences ingrowth method and growth temperature, where HVPE wasused in Ref. 16.From the DLTS measurements presented here, trap pas-sivation is lost at a temperature below 425 °C. This workfurther investigates previous bias annealing measurements tohigher temperatures for deeper traps.16 Also, annealing athigher temperatures than previously used, identified a moresignificant effect on trap concerntration than bias annealing.The concentration of traps at 150 and 500 K increased by afector greater than 5 over the course of several 700 K an-neals. The deep level profiles showed a uniform concentra-tion of each of the deep levels, indicating that the source isnot an impurity diffusing either from the substrate or fromthe contact surface. None of the traps showed capture behav-ior related to threading dislocations of lattice relaxation.Only the trap at 300 K exhibited an increase in emission rateat higher electric fields, indicating a donor nature. Hydrogenis very likely present in high concentration in the epilayer,and may be responsible for the change in defect concentra-tion with annealing. Previous works have shown various hy-drogen effects at various threshold temperatures, suggestingseveral mechanisms such as loss from the surface region, ordissociation of hydrogen from complexes with a variety ofspecies.This work was funded by AFOSR (Dr. G. L. Witt), NSF(Dr. L. Hess and Dr. U. Varshney), and ONR (Dr. C. E. C.Wood).1H. Morkoç, Nitride Semiconductors and Devices (Springer, Berlin, 1999).2D. C. Look and J. R. Sizelove, Phys. Rev. Lett. 82, 1237 (1999).3J. W. P. Hsu, M. J. Manfra, D. V Lang, S. Richter, S. N. G. Chu, A. M.Sergent, R. N. Kleiman, L. N. Pfeiffer, and R. J. Molnar, Appl. Phys. Lett.78, 1685 (2001).4B. S. Simpkins, E. T. Yu, P. Waltereit, and J. S. Speck, J. Appl. Phys. 94,1448 (2003).5J. Spradlin, S. Dogan, J. Xie, R. J. Molnar, A. A. Baski, and H. Morkoç,Appl. Phys. Lett. 84, 4150 (2004).6S. Limpijumnong and C. Van de Walle, Phys. Rev. B 69, 035207 (2004).7S. Limpijumnong, C. Van de Walle, and J. Neugebauer, Mater. Res. Soc.Symp. 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Auret, S. A. Goodman, W. E. Meyer, F. K. Koschnick, J.-M. Spaeth,B. Beaumont, and P. Gibart, Mater. Sci. Eng., B 71, 77 (2000).18Z.-Q. Fang, D. C. Look, J. Jasinski, M. Benamara, Z. Liliental-Weber, andR. J. Molnar, Appl. Phys. Lett. 78, 332 (2001).19H. K. Cho, C. S. Kim, and C.-H. Hong, J. Appl. Phys. 94, 1485 (2003).20C. D. Wang, L. S. Yu, S. S. Lau, E. T. Yu, W. Kim, A. E. Botchkarev, andH. Morkoç, Appl. Phys. Lett. 72, 1211 (1998).21W. I. Lee, T. C. Huang, J. D. Guo, and M. S. Fen, Appl. Phys. Lett. 67,1721 (1995).22Y. Shon, N. H. Kim, Y. H. Kwon, D. Y. Kim, S. U. Yuldashev, W. H. Jung,and T. W. Kang, Curr. Appl. Phys. 1, 191 (2001).23J.-P. Zhang, X.-L. Wang, D.-Z. Sun, X.-B. Li, and M.-Y. Kong, J. Cryst.Growth 189/190, 566 (1998).24S. J. Pearton, C. R. Abernathy, R. G. Wilson, J. M. Zavada, C. Y. Song, M.G. Weinstein, M. Stavola, J. Han, and R. J. Shul, Nucl. Instrum. MethodsPhys. Res. B 147, 171 (1999).4060 Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004 Johnstone et al. 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Thermal stability of electron traps in GaN grown by metalorganic chemical vapor deposition

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