The effects of microwave (2.45 GHz) treatment influence on the cross section for electron capture and the energy of the deep levels in the forbidden gap of GaAs monocrystals and n-n⁺ epitaxial structures have been investigated using acoustoelectric transient spectroscopy

Olikh, O.Ya.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Semiconductor Physics, Quantum Electronics & Optoelectronics. 2003. V. 6, N 4. P. 450-453.' 2003, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine450PACS: 72.50.+bAcoustoelectric transient spectroscopyof microwave treated GaAs-based structuresO.Ya. OlikhKyiv Taras Shevchenko National University, Physics Faculty, 6, pr. Glushkova, 03127 Kyiv, Ukraine,Phone: +380 (44) 2660510, ¯-mail: olikh@mail.univ.kiev.uaAbstract. The effects of microwave (2.45 GHz) treatment influence on the cross section forelectron capture and the energy of the deep levels in the forbidden gap of GaAs monocrystalsand n-n+ epitaxial structures have been investigated using acoustoelectric transient spectroscopy.Keywords: microwave treatment, deep level, gallium arsenide.Paper received 30.07.03; accepted for publication 11.12.03.1. IntroductionThe external factors influence on semiconductors andsemiconductor-based structures is widely studied, bothto understand degradation processes into semiconductordevices and to find new technological decisions of thesuch devices fabrication. The influence of some factors,such as radiation, is studied completely enough [1,2]. Inrecent years, a number of papers are devoted to the newinfluence methods, such as ultrasonic treatment [3,4] ormicrowave (MW) electromagnetic irradiation [57]. Forexample, it is shown that MW treatment leads to relaxa-tion of the internal mechanical strain of the Schottky-barrier GaAs structures [5], redistribution of the impuri-ties [5,6], gettering of the point defects [7]. However, in-formation about microwave influence on the structuredefect parameters is practically unknown.In this paper, we present results of investigation MWirradiation influence on the parameters of deep levelslocated into the under-surface region of n-GaAs mono-crystals and GaAs n-n+ epitaxial structures.2. Samples and experimental techniqueThe following three types of samples were investigated inthis work:1) The monocrystal GaAs wafer. The (100) wafer,300 µm thick, was doped with Sn. The electron concen-tration was (1.5÷2.5)×1018 cm3 (for the sample labeledas GA1) or (3÷5)×1016 cm3 (sample GA2). The (111) wa-fer, 300 µm thick, doped with Te, n = (1÷2)×1018 cm3,labeled GA3;2) The GaAs epitaxial n-n+ structures. The Te-dopedGaAs epitaxial layer (free carrier concentration was3.9×1015 cm3 for the sample GAE1, 3.5×1015 cm3 forGAE2 and 5×1015 cm 3 for GAE3) grown on monocrystalTe-doped GaAs substrate ( 318 æm102 −×=n ). The thick-nesses of the layer and the substrate were 6 and 300 µm,respectively.3) The GaAs:Te epitaxial n-n+ structures with a bufferlayer. The layer, 1 µm thick, 316 cm108 −×=n , and thelayer, 2 µm thick, 315 cm107 −×=n , grown consistentlyon (100) GaAs substrate ( 318 cm102 −×=n ). The GAB1and GAB2 samples are made from different wafers.All investigated epitaxial structures were fabricatedusing the industrial vapor phase epitaxy method and pos-sessed standard technical characteristics.Samples were irradiated in free space at the room tem-perature using magnetron radiation. The microwave fre-quency was 2.45 GHz, the specific power density was1.5 W/cm2. The total treatment time was 20 to 60 secondsfor different samples; the influence continuous durationwas 510 s to avoid sample heating.The parameters of deep centers  (the electron capturecross section nσ  and the energy depth of the electron traplevel counted from the edge of conductivity band)( tC EE − ) have been determined before and after themicrowave treatment. The method of acoustoelectric tran-O.Ya. Olikh: Acoustoelectric transient spectroscopy of microwave ...451SQO, 6(4), 2003sient spectroscopy is used [8,9]: the GaAs samples areplaced by the epitaxial layer on the piezoelectric plate( 3LiNbO   see Fig.1); during propagation of acousticwaves in the plate, the direct transverse acoustoelectricvoltage (TAV) arose in semiconductor samples. After anultrasonic impulse ending, TAV decreased to zero. Thisdecrease can be interpolated by the exponential func-tion; the characteristic relaxation time τ is known to bedetermined by parameters of the deep level, located inthe near-surface region [8,10]:( ) ( )( )kTEEN tCcTn −= − exp1υστ (1)where υT represents the free electron thermal velocity; Ncis the effective density of states at the bottom of the con-ductivity band. The experimental measuring of TAV ki-netic at different temperatures allowed to define depend-ence τ(T). The angle of the declination of a plot  ln(τ) on( ) 1−kT  allowed to determine )( tC EE −  and then to cal-culate σn by using the equation (1).Our measurements were carried out in the tempera-ture range 290 to 350 K except for GAB1 and GAB2; forthese samples TAV appeared after sample heating up to310 K only.3. Experimental results and discussionFig. 2 represents the typical τ temperature dependenciesfor the two samples before and after MW irradiation. Itis visible, that both the plot slopes (related to )( tC EE − )and the characteristic relaxation time value changed af-ter the microwave treatment. The other samples depend-encies are similar; all results are presented in the Table.The obtained results have some features. At first, theirradiation influence on the carrier trapping cross sec-tion is  considerably stronger than the influence on theenergy level position. It is confirmed by σn can changevalue by an order as well as, for example, by )( tC EE −did not change practically, while σn increased by a factorof 4 approximately at GAB1 after 20 s MW influence. Secondly, the irradiation dose for a substantial centerparameters changing for the epitaxial structures is higherthan for monocrystalline samples: it is possible to com-pare results for the samples of the GA-set and GAB-setafter 20 s irradiation. After the repeated 20 s irradiationof the GA sample, the TAV signal diminished consider-ably and exceed the measuring range. It correlates withdata obtained in [7]; this work reports of the decrease inthe concentration of the centers with energy level at theforbidden band top half due to microwave annealing.Thirdly, the changes character at the monocrystallinewafers and at the epitaxial structures is different:)( tC EE −  and σn decrease in monocrystals and increaseat structures with a buffer layer as well as without it.Finally, the MW stimulated changes degree depend onthe free carrier concentration. It is possible to compareresults for the GA1 and GA2; these samples differ by thefree carrier concentration value only.It should be noted that although the GaAs levels sub-jacent a conductivity band bottom on 0.25, 0.31, 0.33,t−tttexp (TAV)VGaAsLiNbO3Fig. 1. System for TAV signal measurements. Shown in the insertis the time dependence of the pulse high-frequency (2.6 MHz)voltage V for existing of with ultrasound in the piezoelectricplate and the resulting TAV signal.36 38 408.58.07.5a( )kT 1 1, eV 1 2lnt33 34 35 368.07.57.06.56.05.5b 1 2 3( )kT 1 1, eVlntFig. 2. Temperature dependence of the characteristic TAV re-laxation time for the samples GAE2 (a) and GAB1 (b). Thecurve 1 corresponds to the unirradiated samples, curves 23  tothe irradiated samples. The microwave treatment time is 20 s(curve 2, b), 40 s (curve 3, b), 60 s (curve 2, a).452SQO, 6(4), 2003O.Ya. Olikh: Acoustoelectric transient spectroscopy of microwave ...0.43 eV and having a small trapping cross section(1016÷1018 cm2) are known in literature (for example,see [11]), but the levels exact atomic configuration is notinvestigated; they are assumed to relate to the point de-fects containing an impurity atom. The level (EC  Et) == (0.40÷0.42) eV and σn = (1015÷1016) cm2 is usuallyassigned to the configuration VGa  VAs (EL5 center)[12, 13] .On our opinion, the changes of the deep level param-eters are related to the structural-impurity alteration of asemiconductor near-surface region, caused by microwavetreatment. Such alteration is earlier found out in [5-7].Namely, according to [5,7], defects are gettered in amaterial surface layer after this MW irradiation. Thecharged defects concentration increase must result in theelectric field strength increase in this region. Simultane-ously, the capture cross section of the levels EC  0.33 eV,EC  0.31 eV and σn = (1017÷1018) cm2 is known [11] todecrease when the electric field increase. The cross sec-tion decrease is observed for the samples GA1-3. If thefree carrier concentration is higher, than the chargeddefect screening is more effective and, consequently, thechange of GA1 (n = 1018 cm3) is less significant than thatof GA2 (n = 1016 cm3). In addition, the samples withhigh resistance have the larger skin-depth of the mi-crowave penetration [14]. The microwave induced in-crease of the surface curvature radius and the internalmechanical strain relaxation of n-n+-GaAs structure near-surface layers were observed in [5]. It takes place due tothe separate dislocation origin and their diffusion alongsliding planes toward structure depth. Thus, the chang-ing of value of the both electric and mechanical field in-tensity take place in the near-surface region, and it, onour opinion, results in the deep level alteration and pa-rameter changing. And, if the changes take place due tothe linear and point defects spatial moving, then the mi-crowave influence must depend on crystal orientation (seeGA1 and GA3, for example).It is considered in [8,9] that the trap TAV appearancein the epitaxial structures relates to centers at the inter-nal epilayer-substrate boundary (internal surface). Themonocrystal TAV is caused by the defects at the externalsurface. On our point of view, it is this spatial locationdifference that is the reason of the MW induced changescharacter difference (point defects move from sampledepth toward external surface, dislocation to oppositeside) as well as dose the dependence difference for theepitaxial and monocrystalline wafers.4. ConclusionsThe microwave irradiation influence on the electron cap-ture cross section and the energy level of centers in theforbidden gap of n-GaAs monocrystal and n-GaAs-basedepitaxial structures has been investigated for the first time.It was found that the deep level parameters are modified,and the character of changes is different for monocrystalsand epitaxial structures. The observed changes is likelyto be due to gettering the defects  in the material surfacelayer and internal stress relaxation induced by the mi-crowave treatment applied.AcknowledgementThe author would like to thank Prof. R.V. Konakova forhelpful discussions.References1. F.P. Korshunov, Yu.V. Bogatirev, V.À. Vavilov, The radia-tion influence on integrated circuits, Nauka, Minsk (1986)(in Russian).2. V.V. Kozlovskii, V.A. Kozlov, V.N. Lomasov, Modificationof Semiconductors with Proton Beams. A Review // Semicon-ductors, 34 (2), p. 123-144 (2000).3. B.N. Zaveryukhin, N.N. Zaveryukhina, R.A. Muminov, O.M.Tursunkulov, Alternating-Strain-Induced Drift of Nonequi-librium Charge Carriers in GaAs Photodetectors // TechnicalPhysics Letters, 28 (3), p. 207-210 (2002).Table. Parameters of deep levels in GaAs structures before andafter the microwave treatment.Sample Total time (EC  Et), eV σn*, cm2of microwavetreatment, s GA1 0 0.31 3×1017 20 0.30 2×1017 GA2 0 0.33 4×1017 20 0.28 6×1018 GA3 0 0.49 5×1014 20  0.4 2×1015 GAE1 0 0.24 2×1018 60 0.29 1×1017 GAE2 0 0.25 2×1018 60 0.30 2×1017 GAE3 0 0.43 8×1017 60 0.46 7×1016 GAB1 0 0.39 1×1017 20 0.39 4×1017 40 0.43 1×1016 GAB2 0 0.40 1×1016 20 0.41 1×1016 40 0.45 4×1016 *T=300 K for samples GA1GA3, GAE1GAE3, andÒ=340 ˚  for GAB1 and GAB2O.Ya. Olikh: Acoustoelectric transient spectroscopy of microwave ...453SQO, 6(4), 20034. ˛.Ya. Olikh, I.V. Ostrovskii Ultrasound-Stimulated Increasein the Electron Diffusion Length in p-Si Crystals // Physics ofthe Solid State, 44(7), p.1249-1253 (2002).5. N.S. Boltovets, A.B. 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Acoustoelectric transient spectroscopy of microwave treated GaAs-based structures

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Acoustoelectric transient spectroscopy of microwave treated GaAs-based structures

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)