Using a monoenergetic positron beam, annealing study of the Al/n-GaSb system was performed by monitoring the Doppler broadening of the annihilation radiation as a function of the positron implanting energy. The S-parameter against positron energy data was successfully fitted by a three-layer model (Al/interface/GaSb). The annealing out of the open volume defects in the polycrystalline Al layer was revealed by the decrease in the S-parameter and the increase in the effective diffusion length of the Al layer. For the as-deposited samples, a∼5 nm interfacial region with S-parameter larger than those of the Al overlayer and the bulk was identified. After the 400^ºC annealing, this interfacial region extends to over 40 nm and its S-parameter dramatically drops. This is possibly due to the new phase formation at the interface. Annealing behaviors of SB and L+,B of the GaSb bulk showed the annealing out of positron traps (possibly the VGa-related defect) at 250ºC. However, a further annealing at 400ºC induces the formation of positron traps, which are possibly of another kind of VGa-related defect and the positron shallow trap GaSb antisite.published_or_final_versionProceedings of the 35th Polish Seminar on Positron Annihilation (PSPA), Turawa, Poland, 20-24 September 2004. In Acta Physica Polonica Series A: General Physics, Physics of Condensed Matter, Optics and Quantum Electronics, Atomic and Molecular Physics, Applied Physics, 2005, v. 107 n. 5, p. 874-87

Han, RD

Ling, CC

Wang, HY

Weng, HM

Ye, BJ

Zhou, XY

Acta Physica Polonica A

HKU Scholars Hub

Title Annealing study of A1/GaSb contact with the use of dopplerbroadening techniqueAuthor(s) Wang, HY; Weng, HM; Ling, CC; Ye, BJ; Zhou, XY; Han, RDCitationProceedings of the 35th Polish Seminar on Positron Annihilation(PSPA), Turawa, Poland, 20-24 September 2004. In Acta PhysicaPolonica Series A: General Physics, Physics of CondensedMatter, Optics and Quantum Electronics, Atomic and MolecularPhysics, Applied Physics, 2005, v. 107 n. 5, p. 874-879Issued Date 2005URL http://hdl.handle.net/10722/80878Rights Creative Commons: Attribution 3.0 Hong Kong LicenseVol. 107 (2005) ACTA PHYSICA POLONICA A No. 5Proceedings of the 35th Polish Seminar on Positron Annihilation, Turawa, Poland 2004Annealing Study of Al/GaSb Contactwith the Use of Doppler BroadeningTechniqueH.Y. Wanga,b, H.M. Wengb, C.C. Linga, B.J. Yeb,X.Y. Zhoub and R.D. HanbaDepartment of Physics, The University of Hong KongPokfulam Road, Hong Kong, People’s Republic of ChinabDepartment of Modern Physics, University of Science and Technology of ChinaHefei 230026, People’s Republic of ChinaUsing a monoenergetic positron beam, annealing study of theAl/n-GaSb system was performed by monitoring the Doppler broadeningof the annihilation radiation as a function of the positron implanting en-ergy. The S-parameter against positron energy data was successfully fittedby a three-layer model (Al/interface/GaSb). The annealing out of the openvolume defects in the polycrystalline Al layer was revealed by the decreasein the S-parameter and the increase in the effective diffusion length of theAl layer. For the as-deposited samples, a ∼ 5 nm interfacial region withS-parameter larger than those of the Al overlayer and the bulk was iden-tified. After the 400◦C annealing, this interfacial region extends to over40 nm and its S-parameter dramatically drops. This is possibly due to thenew phase formation at the interface. Annealing behaviors of SB and L+,Bof the GaSb bulk showed the annealing out of positron traps (possibly theVGa-related defect) at 250◦C. However, a further annealing at 400◦C in-duces the formation of positron traps, which are possibly of another kind ofVGa-related defect and the positron shallow trap GaSb antisite.PACS numbers: 78.70.Bj, 68.35.Ct, 73.40.Sx1. IntroductionGallium antimonide is a narrow direct bandgap semiconductor suitable forfabricating high frequency electronic devices and is also the basic material forfabricating a variety of lattice matched optoelectronic materials operating in thewavelength range of 0.8−4.3 µm [1]. Although, making metal rectifying contacts to(874)Annealing Study of Al/GaSb Contact . . . 875the semiconductor material is crucial to device fabrication, studies on this issue onGaSb are relatively little as compared to those of GaAs or InP. Moreover, fabricat-ing reliable GaSb Schottky contact is not easy because of the Fermi level pinningclose to the valence band [2], and the insulating interfacial layer at the bound-ary [3]. Aluminum (Al) has been reported in some studies to be a good candidatefor fabricating rectifying contact with n-type melt grown GaSb (p ∼ 1018 cm−3)[3–8], although the rectifying properties were not consistent in all of the stud-ies. With the use of monoenergetic positron beam, Doppler broadening tech-nique has been used to study various metal–semiconductor interfaces (examples seeRefs. [9–13]). In the present study, Al contacts onto liquid encapsulated Czochral-ski grown Te-doped n-type GaSb samples (n ∼ 1018 cm−3) have been made byelectron beam evaporation. Depth profile of S-parameter was obtained by varyingpositron energies up to 15 keV and thus the interfacial properties of the Al/n-GaSbcontact, as well as its annealing behavior, were investigated.2. ExperimentalThe GaSb wafer used is Te-doped liquid encapsulated Czochralski (LEC)grown n-GaSb purchased from Wafer Technology, U.K. Samples (dimensions ∼1 cm× 1 cm) cut from the wafer were etched in HCl and then rinsed in deionizedwater. A 0.8 mm diameter Al circular disc was then deposited onto the samplesurface in an electron beam evaporator at the pressure of 10−5 Torr. Al/n-GaSbsamples with Al thicknesses of 100 nm and 200 nm were fabricated. The Alfilm thickness of the sample was measured by the quartz crystal film thicknessmonitor. Annealing of samples was performed in a forming gas (N2 : H2 = 80% :20%) atmosphere for a period of 30 minutes. The slow positron implantationmeasurements were carried out with the monoenergetic positron beam located inthe University of Science and Technology of China (USTC). Positrons with energiesup to 15 keV were implanted into the samples and the corresponding annihilationγ radiation spectra were collected with an HPGe detector system. Each spectrumcontains a total count of 1×105 under the 511 keV annihilation peak. The Dopplerbroadening of the annihilation γ ray radiation was monitored by the S-parameter,which is defined as the ratio of the central region area to the total area under theannihilation peak. The window width of the central region was chosen such thatthe obtained S-parameter is close to 0.5, and was thus set at a width of 1 keV onboth sides of the 511 keV peak. The data was then analyzed by the source codeVEPFIT [14].3. Results and discussionsThe S-parameter profiles of the as-fabricated Al/GaSb samples with 100 nmand 200 nm Al coverage are shown in Fig. 1a and b, respectively. In or-876 H.Y. Wang et al.Fig. 1. The S-parameter measured as a function of the positron implanting energy forthe Al/n-GaSb samples with 100 nm (a) and 200 nm (b) Al thicknesses. The solid linesrepresent the fitted curves using the Al/interface/GaSb model and the correspondingfitted parameters.der to analyze the S(E) data of the Al/GaSb system, we have employed theAl-interface-bulk three-layer model and thus the resultant S-parameter isgiven by: S(E) = fAl(E)SAl + fi(E)Si + fB(E)SB, where Sj andfj are the S-parameters and the fractions, respectively, of positron an-nihilating in the corresponding layers of Al, interface, and GaSb bulk.The fitted results of the S-parameters and the effective positron diffu-sion lengths of the different layers are shown in Fig. 2. For both theas-deposited samples with 100 nm and 200 nm Al coverage, their fitted boundarypositions agreed well with the expected values (i.e. the Al film thickness). Thefitted curves for the different samples are shown in Fig. 1 and it is noticed thatthey give good representations to the experimental data. From Fig. 2, it is clearlyshown that all the fitted S-parameters and L+ have the similar annealing trendsregardless of the samples’ Al film thickness. For the Al overlayer, SAl decreaseswith increasing temperature from 0.56–0.57 to ∼ 0.537, while L+,Al increases fromseveral nm to ∼ 50 nm. The Al film deposited by electron beam evaporation ispolycrystalline with a grain size of about 100 nm [15]. The decrease in SAl andthe increase in L+,Al with an annealing temperature increase are possibly due tothe annealing out of positron trapping vacancies in the Al film. From Fig. 2b,for the interfacial region, the Si has values of about ∼ 0.59 with annealing tem-perature lower than 250◦C. As the annealing temperature increases to 400◦C, Siabruptly drops to ∼ 0.47 and the positron diffusion length increases from ∼ 15 nmAnnealing Study of Al/GaSb Contact . . . 877Fig. 2. The fitted values of the S-parameters and the positron diffusion length of theAl overlayer (a), the interfacial region (b), and the GaSb bulk (c) as a function of theannealing temperature. The fitting of the S(E) data employed the three-layer model(i.e. the Al/interface/GaSb model).Fig. 3. The width of the interfacial region as a function of the annealing temperature.The triangles denote the 200 nm Al coverage, the circles — the 100 nm Al coverage.to ∼ 50 nm. From Fig. 3, the interfacial width increases from ∼ 5 nm to ∼ 45 nmafter the 400◦C annealing. At the first glance, the decrease in Si and the increasein L+,i with increasing annealing temperature can be explained by the removalof positron trapping center. It is noticed after the 400◦C annealing, the Si de-creases to the value of ∼ 0.47, which is significantly smaller than those of SAland SB at any annealing temperatures. However, at the same time, the value878 H.Y. Wang et al.of L+,i ∼ 50 nm after the 400◦C annealing is much smaller than the value ofL+,GaSb ∼ 400 nm after the 250◦C annealing. This implies, despite the very lowvalue of Si ∼ 0.47 after the 400◦C annealing, there are a significant amount ofpositron traps in the interfacial region. This leads us to speculate that the dra-matically drop of the Si and the increase in the interfacial width upon annealingis also related to the formation of new phase at the interface originated to theinter-diffusion of atoms across the boundary, for which the new formed phase hasa lower S-parameter. The Al/GaSb interface has been investigated by secondaryion mass spectroscopy (SIMS), Auger electron spectroscopy (AES) and soft X-rayphotoemission spectroscopy although the results are divergent. Oueini et al. [16],Rouillard et al. [17], and Walters and Williams [4] have observed a significantatomic inter-diffusion across the Al/GaSb interface, and Poole et al. [18] have ob-served an abrupt Al/GaSb boundary. For the GaSb bulk layer, the SB drops andthe L+,B increases from 100–200 nm to ∼ 400 nm after the 250◦C annealing. FromFig. 2, it is also noticed that after the 400◦C annealing, the SB increases despitethe magnitudes of increase for the two samples with different thicknesses are dif-ferent. Moreover, the L+,B significantly reduces from ∼ 400 nm to ∼ 30 nm afterthe 400◦C annealing. In our previous positron lifetime and temperature dependentHall (TDH) studies of the electron irradiated LEC grown p-type GaSb [19, 20], twodifferent kinds of VGa-related defects having lifetimes of 280 ps and 315 ps wereidentified. These two components were attributed to the two VGa-related defectshaving different microstructures rather than the same VGa-related defect havingdifferent charge states and thus different degree of relaxation. The VGa,315ps in-tensity decreases and the VGa,280ps intensity increases with increasing annealingtemperature while the annealing temperature is below 300◦C. As the annealingtemperature reaches 300◦C, the VGa,315ps component disappears. Moreover, theconcentration of the acceptor at EV + 34 meV increases from ∼ 2 × 1017 cm−3to 1.8× 1018 cm−3 after the annealing at 400◦C. This acceptor was attributed tothe GaSb antisite. The initial decrease in SB and increase in L+,B in the presentstudy are possibly the resultant effect of the annealing out of VGa,315ps despitethe creation of VGa,280ps. The subsequent increase in SB and the significant dropof L+,B from ∼ 400 nm to ∼ 30 nm are possibly due to the generations of theVGa,280ps and the 34 meV acceptor which acts as positron shallow trap.4. ConclusionAl/n-GaSb interface was studied by Doppler broadening technique with theuse of the monoenergetic positron beam. S-parameter profiles of the samples withdifferent Al thicknesses (100 nm and 200 nm) were obtained. The S(E) data werethen fitted with the assumption of the Al/interface/GaSb model and the fittedresults. The structural changes of the Al film, the interfacial region, and the GaSbbulk upon annealing were discussed.Annealing Study of Al/GaSb Contact . . . 879AcknowledgmentThis work was supported by the CERG, RGC, HKSAR (projectNo. 7107/02P) and the National Science Foundation of China (No. 10175061).References[1] P.S. Dutta, H.L. Bhat, J. Appl. Phys. 81, 5821 (1997).[2] P.W. Chye, T. Sukegawa, I.A. Babaola, N. Sunami, P. Gregory, W.E. Spicer,Phys. Rev. B 15, 2118 (1977).[3] I. Poole, M.E. Lee, M. Missous, K.E. Singer, J. Appl. Phys. 62, 3988 (1987).[4] S.A. Walters, R.H. Williams, J. Vac. Sci. Technol. B 6, 1421 (1988).[5] W. Mason, J.R. Waterman, J. Appl. Phys. 84, 1426 (1998).[6] A. Subekti, T.L. Tansley, E.M. Goldys, IEEE Trans. Elect. Dev. 45, 2247 (1998).[7] A.Y. Polyakov, M. Stam, A.G. Milnes, T.E. Schlesinger, Mater. Sci. Eng. B 12,337 (1992).[8] F.S. Juang, Y.K. Su, Solid-State Electron. 32, 661 (1989).[9] C.C. Ling, T.C. Lee, S. Fung, C.D. Beling, Huimin Weng, Jihua Xu, Shijun Sun,Rongdian Han, J. Phys., Condens. Matter 6, 1133 (1994).[10] C.C. Ling, T.C. Lee, S. Fung, C.D. Beling, Huimin Weng, Jihua Xu, Shijun Sun,Rongdian Han, Appl. Surf. Sci. 85, 305 (1995).[11] A. Uedono, S. Fujii, T. Moriya, T. Kawano, S. Tanigawa, R. Suzuki, T. Ohdaira,T. Mikado, J. Phys., Condens. Matter 9, 6827 (1997).[12] Ping Li, Chenglu Lin, Zuyao Zhou, Shichang Zou, Huimin Weng, Rongdian Han,Bingzong Li, Appl. Phys. Lett. 64, 2501 (1994).[13] J.L. Lee, J.K. Kim, M.H. Weber, K.G. Lynn, Appl. Phys. Lett. 78, 4142 (2001).[14] A. van Veen, H. Schut, J. de Vries, R.A. Hakvoort, M.R. Ijpma, in: PositronBeams for Solids and Surfaces, AIP Conf. Proc., Eds. P.J. Schultz, G.R. Mas-soumi, P.J. Simpson, Vol. 218, AIP, New York 1990, p. 171.[15] M. Missous, E.H. Rhoderick, K.E. Singer, J. Appl. Phys. 59, 3189 (1986).[16] W. Oueini, M. Rouanet, J. Bonnet, Surf. Sci. 409, 445 (1998).[17] Y. Rouillard, B. Jenichen, L. Da¨weritz, K. Ploog, C. Gerardi, C. Giannini,L. De Caro, L. Tapfer, J. Crystal Growth 204, 263 (1999).[18] I. Poole, M.E. Lee, K.E. Singer, Semicond. Sci. Technol. 6, 881 (1991).[19] C.C. Ling, M.K. Lui, S.K. Ma, X.D. Chen, S. Fung, C.D. Beling, Appl. Phys.Lett. 85, 384 (2004).[20] S.K. Ma, M.K. Lui, C.C. Ling, S. Fung, C.D. Beling, K.F. Li, K.W. Cheah,M. Gong, H.S. Hang, H.M. Weng, J. Phys., Condens. Matter 16, 6205 (2004).

Annealing study of A1/GaSb contact with the use of doppler broadening technique

Using a monoenergetic positron beam, annealing study of the Al/n-GaSb system was performed by monitoring the Doppler broadening of the annihilation radiation as a function of the positron implanting energy. The S-parameter against positron energy data was successfully fitted by a three-layer model (Al/interface/GaSb). The annealing out of the open volume defects in the polycrystalline Al layer was revealed by the decrease in the S-parameter and the increase in the effective diffusion length of the Al layer. For the as-deposited samples, a 5 nm interfacial region with S-parameter larger than those of the Al overlayer and the bulk was identified. After the 400ºC annealing, this interfacial region extends to over 40 nm and its S-parameter dramatically drops. This is possibly due to the new phase formation at the interface. Annealing behaviors of S$\text{}_{B}$ and L$\text{}_{+,B}$ of the GaSb bulk showed the annealing out of positron traps (possibly the V$\text{}_{Ga}$-related defect) at 250ºC. However, a further annealing at 400ºC induces the formation of positron traps, which are possibly of another kind of V$\text{}_{Ga}$-related defect and the positron shallow trap GaSb antisite

Annealing study of A1/GaSb contact with the use of doppler broadening technique

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Annealing study of A1/GaSb contact with the use of doppler broadening technique

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