Carbon was implanted into GaAs at the energy of 1 MeV with doses between 131013 and 2 31015 cm22 at temperatures of 80 K, nominal room temperature (RT), and 300 °C. A markedly higher electrical activation was obtained in the samples implanted at 80 K compared to those implanted at RT or 300 °C, attaining a maximum hole concentration of 231019 cm23. The redistribution of the C profile during rapid thermal annealing at temperatures from 700 to 950 °C for 10 s was found negligible, independently of the implantation temperature. Similar improvements in the electrical properties were also verified in samples implanted at 80 K with a lower energy of 60 keV. We consider that despite the light mass of C ions, the reduced dynamic annealing at 80 K allows the accumulation of an abundance of As vacancies, which assist the C activation as a p-type dopant

Danilov, Iuri

Murel, A.V.

Pudenzi, Marcio Alberto Araujo

Souza, Joel Pereira de

Applied Physics Letters

English

Lume 5.8

Electrical activation of carbon in GaAs: Implantation temperature effectsI. Danilov, J. P. de Souza, A. V. Murel, and M. A. A. Pudenzi  Citation: Applied Physics Letters 78, 1700 (2001); doi: 10.1063/1.1356729 View online: http://dx.doi.org/10.1063/1.1356729 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/78/12?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Carbon reactivation kinetics in GaAs: Its dependence on dopant precursor, doping level, and layer thickness J. Appl. Phys. 91, 5923 (2002); 10.1063/1.1467953  Increased electrical activation in the near-surface region of sulfur and nitrogen coimplanted GaAs Appl. Phys. Lett. 77, 3607 (2000); 10.1063/1.1328766  Carrier distribution profiles in Si-doped layers in GaAs formed by focused ion beam implantation and successiveoverlayer growth J. Vac. Sci. Technol. B 18, 3158 (2000); 10.1116/1.1319683  Photoluminescence study of implantation dose and dose-rate dependence of Si doping of GaAs Appl. Phys. Lett. 70, 3597 (1997); 10.1063/1.119244  Activation modeling of Si implanted GaAs J. Appl. Phys. 81, 1131 (1997); 10.1063/1.363858    Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.54.44.137 On: Thu, 05 May 201613:42:53Electrical activation of carbon in GaAs: Implantation temperature effectsI. Danilov and J. P. de SouzaInstituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, R.S., BrazilA. V. MurelInstitute of Physics for Microstructures, RAN, 603600, Nizhny Novgorod, RussiaM. A. A. Pudenzia)Instituto de Fisica ‘‘Gleb Wataghin,’’ Universidade Estadual de Campinas, 13081-970 Campinas,S.P., Brazil~Received 13 October 2000; accepted for publication 26 January 2001!Carbon was implanted into GaAs at the energy of 1 MeV with doses between 131013 and 231015cm22 at temperatures of 80 K, nominal room temperature~RT!, and 300 °C. A markedlyhigher electrical activation was obtained in the samples implanted at 80 K compared to thoseimplanted at RT or 300 °C, attaining a maximum hole concentration of 231019cm23. Theredistribution of the C profile during rapid thermal annealing at temperatures from 700 to 950 °C for10 s was found negligible, independently of the implantation temperature. Similar improvements inthe electrical properties were also verified in samples implanted at 80 K with a lower energy of 60keV. We consider that despite the light mass of C ions, the reduced dynamic annealing at 80 Kallows the accumulation of an abundance of As vacancies, which assist the C activation as ap-typedopant. © 2001 American Institute of Physics.@DOI: 10.1063/1.1356729#Be, Mg, Zn, and Cd ions are commonly implanted intoGaAs to producep-type doped layers. These column-II ele-ments can be highly activated, yielding a hole concentrationthat reaches the 1019cm23 range.1 However, besides the ad-vantages associated with the high electrical activation, thelarge diffusivity of column-II atoms at concentrations.1018cm23 in GaAs~Ref. 2! constitutes a severe drawbackwhen a shallow or abrupt profile is required. Therefore, theas-implanted profile redistributes significantly by indiffusionand outdiffusion, even during rapid thermal annealing~RTA!. In particular, the up-hill diffusion of column-II ele-ments in GaAs,3,4 i.e., the movement of the implanted atomsin the direction of the concentration gradient during the ini-tial stage of annealing, introduces additional contribution tothe profile redistribution.Carbon is an amphoteric dopant in GaAs, which is pref-erentially incorporated in the As sublattice where it acts as asingle-electron acceptor. In contrast with the column-II ele-ments, C has a quite low diffusion coefficient, of less than1310216cm2/s at 950 °C.5 An extremely high hole concen-tration can be established~up to 5.831020cm23! when C isintroduced into GaAs during metalorganic molecular-beamepitaxial growth.6 In contrast, the electrical activation of Cimplanted in GaAs is considerably low,7 with the hole con-centration typically lower than 131018cm23. Co implanta-tion of C with inert gas ions or with column-II or -III ele-ment ions has been employed to enhance the activation.8–12The damage introduced by the coimplanted ions and/or thecreation of As vacancies as a consequence of the incorpora-tion of column-II or -III atoms in the Ga sublattice areclaimed to be the causes which lead to activation enhance-ment. Among the applications of C doping in GaAs are thehigh-concentration doping of a transistor base in epitaxial ofheterobipolar structures13 and the formation of buriedp-typelayers in field-effect transistors.14Previous studies10–12 have demonstrated that the as-implanted damage is beneficial forp-type activation of C. Itis well known that implantation damage is, to a large extent,determined by the intensity of the dynamic annealing. There-fore, one expects that during implantation of C ions at a verylow temperature~like at 80 K!, in spite of the light ion mass,the necessary damage concentration to favor the activation ofC would be accumulated. The correctness of this statement isdemonstrated in the present investigation.The samples used in this work are from semi-insulatingliquid-encapsulated Czochralski-grown GaAs wafers of~100! orientation. C-ion implantation was performed at theenergy of 1 MeV using the ‘‘Tandetron’’ ion accelerator ofthe Physics Institute at Porto Alegre. The implantation wasperformed with the substrate at temperatures of 80 K, nomi-nal room temperature~RT!, or 300 °C, with doses rangingfrom 131013 to 231015cm22. In order to minimize chan-neling effects, the samples were tilted 10° and rotated 25°with respect to thê110& direction. The as-implanted sampleswere submitted to capless RTA~Si proximity! for 10 s in Aratmosphere in the temperature range from 700 to 950 °C.Hall-effect measurements in Van der Pauw devices15provided the sheet resistance (Rs), sheet carrier density (ps),and effective carrier mobility (meff) values. The Ohmic con-tacts in the devices were prepared by manually applying Into the GaAs surface and then sintering at'300 °C during 3min. The depth distribution of the C atoms was obtained bysecondary ion mass spectrometry~SIMS! measurements witha 10 keV Cs1 beam at 45° incidence on an MIQ-256 quad-rupole microprobe. The crater depth was measured using aDektak-3ST. ElectrochemicalC–V (EC–V) profiling wasa!Electronic mail: pudenzi@ifi.unicamp.brAPPLIED PHYSICS LETTERS VOLUME 78, NUMBER 12 19 MARCH 200117000003-6951/2001/78(12)/1700/3/$18.00 © 2001 American Institute of Physics Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.54.44.137 On: Thu, 05 May 201613:42:53performed on annealed samples using equipment describedelsewhere.16The lowest annealing temperature that provided measur-able electrical activation was 700 °C and the doped layerswere always ofp-type conductivity. In the RT-implantedsamples,ps increased with the RTA temperature reaching amaximum at 850 °C and then decreased slightly for highertemperatures.Figure 1 shows theps values versus the C dose afterRTA conducted at 850 °C. Curves~a!, ~b!, and ~c! corre-spond to implantation at RT, 300 °C, and 80 K, respectively.In the RT-implanted samples the electrical activation yield~EAY!, i.e., the ratio ofps and the implanted dose, is prac-tically constant~>20%! for doses,131014cm22 @see curve~a!#. However, for doses.131014cm22, ps saturates at'331013cm22. This means that the EAY decreases essentiallyfor doses.131014cm22. Since C is an amphoteric dopant inGaAs,17 one could suppose that such decrease in thep-typedoping efficiency results from donor compensation, i.e., fromC incorporated in the Ga sublattice. However, this hypoth-esis cannot be supported by the mobility data from thepresent work. For example, in samples implanted at RT withthe doses of 231014cm22 ~sample A! and 131015cm22~sample B!, the measuredps and meff values after RTA at900 °C are, respectively, 2.831013cm22 and 208 cm2/V s~sample A! and 3.3 1013cm22 and 185 cm2/V s ~sample B!.One should note that the respective values ofps andmeff insamples A and B are not too different. Considering the am-photeric activation of C,17 the concentration of ionized scat-tering centers (Ni) in sample B should be>5 times higherthan in sample A. Assuming scattering of holes by phononsand ionized centers,18 the calculated hole mobility results in71 cm2/V s for sample B, which is well below the experi-mental value~185 cm2/V s!. However, a mobility of 192cm2/V s is estimated assuming scattering byNi5Ps , in closeagreement with the experimental value. Based on this rea-soning, one can infer that the substantial concentration of Catoms nonactivated during RTA in samples implanted at RTto high doses remains electrically neutral. Very likely, theneutral C atoms form precipitates in the GaAs matrix likethose observed by electron microscopy analysis.19Curve~b! in Fig. 1 shows the dose dependence of the Celectrical activation in samples implanted at 300 °C afterRTA at 850 °C. There is only a marginal improvement in theelectrical activation in the hot-implanted samples comparedto those implanted at RT. This result clearly indicates thatthe implantation of C ions at high temperature is not effec-tive in increasing the activation efficiency.The dependence of electrical activation on the dose im-planted at 80 K is depicted in curve~c! of Fig. 1. The data incurve ~c! show remarkable electrical activation for doses.531013cm22. One notices thatps increases with the dose,with no propensity for saturation. The EAY reached a maxi-mum value of 42%~for the dose of 531014cm22!, which is1.8 times higher than the corresponding value in the RT-implanted samples~22% for doses of 3 – 531013cm22!.Curves~1! and~2! in Fig. 2 are the SIMS concentrationdepth profiles of C after RTA at 950 °C in samples implantedto a dose of 531014cm22 at RT~sample I! and 80 K~sampleII !, respectively. The concentration depth profile estimatedby TRIM ~Ref. 20! code simulation is included in curve~0!,for comparison. The SIMS profiles from sample I beforeannealing~not shown! and after annealing@curve~1!# closelysuperimpose, clearly confirming the low diffusivity of C. Anear coincidence between the SIMS profiles and theTRIM-simulated one occurs for depths from 0.7 to 1.4mm. The tailin the measured profiles for depths.1.4mm probably resultsfrom imperfect channeling suppression. In the cold-implanted sample the tail is less pronounced, very likely be-cause the higher accumulated implantation damage contrib-utes to dechanneling. Close to the surface, from depths of0.1–0.7mm, the measured C concentrations are 2–3 timeshigher than theTRIM-predicted ones. The causes for this dis-crepancy are unclear at present.The EC–V depth profile for the acceptors obtained afterRTA at 950 °C is shown in Fig. 2 in curves~3! and ~4! forsamples I and II, respectively. One can observe that the ac-ceptor concentrations in sample I are well below those of Catoms along the entire profile@compare curves~3! and~1! inFig. 2#. This result correlates with the low values ofps~'7% of implanted dose! measured in sample I. On the otherhand, the acceptor profile in sample II reproduces approxi-mately the C atom profile@compare curves~4! and~2! in Fig.FIG. 1. Sheet hole concentration after annealing at 850 °C for 10 s as afunction of implanted C-ion dose. The implantation temperatures were~a!nominal room temperature,~b! 300 °C and~c! 80 K.FIG. 2. Curve~0! is the C-atom distribution estimated by theTRIM codesimulation. Curves~1! and ~2! are profiles of C atoms measured by SIMSafter annealing at 950 °C for 10 s in samples implanted with a dose of 531014 cm22 at nominal room temperature~sample I! and at 80 K~sampleII !, respectively. Curves~3! and ~4! are acceptor profiles measured by theelectrochemicalC–V method in samples I and II, respectively.1701Appl. Phys. Lett., Vol. 78, No. 12, 19 March 2001 Danilov et al. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.54.44.137 On: Thu, 05 May 201613:42:532#, indicating that a substantial fraction of the implanted dosewas activated.Figure 3 shows, in curves~a!–~d!, the EC–V acceptorprofiles after RTA at 850 °C measured in samples implantedat 80 K with doses in the range from 231014 to 231015cm22. It is interesting to notice that the shape of pro-files ~a!–~d! closely resembles theTRIM distribution @curve~0! in Fig. 2#. In addition, the concentration of acceptorsincreases approximately proportional to the implantationdose. For the implanted dose of 231015cm22, the acceptorconcentration at the peak reached 231019cm23, which isabout 20 times higher than the maximum value normallyobtained in GaAs implanted with C at RT. The correspond-ing Rs andmeff values are 150V/h and 80 cm2/V s, respec-tively. This activated concentration is comparable to thehighest obtained by Ga11C1 coimplantation.9Themeff in the samples implanted at 80 K decrease withthe increase ofps , according to the common concentrationdependence model,18 meaning that the same scatteringmechanisms take place in samples implanted at 80 K andRT.The improved activation of C in samples implanted at 80K should be attributed to the higher as-implanted damageconcentration. During the initial stage of RTA, an oversatu-ration in the point-defect concentrations should be estab-lished, by virtue of the damage dissolution. The presence ofa high concentration of As vacancies should facilitate theincorporation of C atoms in the As sublattice, leading to theenhancement of the EAY of C. In contrast, during the Cimplantation at RT or 300 °C, intense recombination of pointdefects takes place in the collision cascades. During anneal-ing, a lower concentration of As vacancies is present and,hence, a lower EAY of C results.In a separate experiment, C ions were implanted to thedose of 531014cm22 and energy of 60 keV at RT and at 80K. The EAY values measured after RTA at 850 °C for 10 sare 19% and 0.4%, for the 80 K and RT-implanted samples,respectively. These results demonstrate that the implantationat 80 K is also highly effective in improving the activation ofC implanted at a low energy.In summary, the influence of the substrate temperatureduring C-ion implantation on the electrical activation wasinvestigated. A significantly higher electrical activation yieldwas obtained in samples implanted at 80 K compared tothose implanted at RT or 300 °C. The acceptor concentrationin the 80 K implanted samples reached a value of 231019cm23 at the profile peak, which is' 20 times higherthan in the best activated RT-implanted samples. From thedependence of the effective hole mobility on the sheet holeconcentration, it was inferred that C atoms nonactivated asacceptors remain electrically neutral. We have demonstratedthat p-type layers in GaAs of low sheet resistance and neg-ligible profile redistribution during rapid thermal annealingcan be formed via low-temperature~80 K! C-ion implanta-tion.This work was partially supported by Conselho Nacionalde Pesquisas~CNPq!, Fundac¸ão de Amparo a` Pesquisa doEstado do Rio Grande do Sul~FAPERGS!, and Fundac¸ão deAmparo àPesquisa do Estado de Sa˜o Paulo~FAPESP!.1J. P. de Souza and D. K. Sadana, inHandbook on Semiconductors, editedby S. Mahajan~North Holland, Amsterdan, 1994!, Vol. 3b, p. 2036.2Y. M. Haddara, C. C. Lee, J. C. Hu, M. D. Deal, and J. C. Bravman,Mater. Res. Bull.20, 41 ~1995!.3H. Robinson, M. D. Deal, and D. A. Stevenson, Appl. Phys. Lett.56, 554~1990!.4H. G. Robinson, M. D. Deal, G. Amaratunga, P. B. Griffin, D. A. Steven-son, and J. D. Plummer, J. Appl. Phys.71, 2615~1992!.5C. R. Albernathy, S. J. Pearton, F. Ren, W. S. Hobson, T. R. Fullowan, A.S. Katz, A. S. Jordan, and J. Kovalchik, J. Cryst. Growth105, 375~1990!.6K. Saito, E. Tokumitsu, T. Akatsuka, M. Miyauchi, T. Yamada, M. Kona-gai, and K. Takahashi, J. Appl. Phys.64, 3975~1988!.7Y. Danilov, V. T. Karmanov, P. V. Pavlov, and E. A. Pitirimova, Sov.Phys. Semicond.15, 971 ~1993!.8B. K. Shin, J. E. Ehret, Y. S. Park, and M. Stefiniw, J. Appl. Phys.49,2988 ~1978!.9S. J. Pearton and C. R. Albernathy, Appl. Phys. Lett.55, 678 ~1989!.10A. J. Moll, K. M. Yu, W. Walukiewics, W. L. Hansen, and E. E. Haller,Appl. Phys. Lett.60, 2383~1992!.11R. Morton, B. Shu, S. S. Lau, and D. B. Poker, Appl. Phys. Lett.66, 1132~1995!.12R. Morton, S. S. Lau, D. B. Poker, P. K. Chu, F. F. Fung, and N. Wang,J. Appl. Phys.84, 4929~1998!.13J. Shirakashi, T. Azuma, F. Fukuchi, M. Konagai, and K. Takahashi, J.Cryst. Growth150, 585 ~1995!.14J. C. Zolper, M. E. Sherwin, A. G. Baca, R. J. Shul, J. F. Klem, and V. M.Hietala, IEEE Electron Device Lett.15, 493 ~1994!.15L. J. Van der Pauw, Philips Res. Rep.13, 1 ~1958!.16I. V. Irin and A. V. Murel, Instrum. Exp. Tech.36, 918 ~1993!.17W. H. van Berlo, J. Appl. Phys.73, 2765~1993!.18S. Blakemore, J. Appl. Phys.53, R123~1982!.19D. I. Piskunov, Y. A. Danilov, S. K. Maksimov, and E. A. Pitirimova,Phys. Status Solidi A96, 141 ~1986!.20J. F. Ziegler, J. P. Biersack, and U. Littmark, inThe Stopping and Rangeof Ions in Solids~Pergamon, New York, 1985!.FIG. 3. Profiles of acceptors obtained by the electrochemicalC–V methodin samples implanted with C ions at 80 K and annealed at 850 °C for 10 s todoses of~a! 231014 cm22, ~b! 531014 cm22, ~c! 131015 cm22, and~d! 231015 cm22.1702 Appl. Phys. Lett., Vol. 78, No. 12, 19 March 2001 Danilov et al. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  143.54.44.137 On: Thu, 05 May 201613:42:53

Electrical activation of carbon in GaAs : implantation temperature effects

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Electrical activation of carbon in GaAs : implantation temperature effects

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