Bonde-Nielsen, K

Holzschuh, E

Kündig, W

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$21wzQlevels introduced by metallic impurities. OCR Outputscheme. Deep level transient spectroscopy will be used to identify the band gapThe data can be directly compared to theoretical calculations using the LMTOunique information on the local density of electron states around the probe atom.on the proper lattice sites. High resolution conversion electron spectroscopy yieldsspectroscopy will be used to investigate the incorporation of the implanted isotopesgroup IV, IV-IV, and III-V compound semiconductors will be studied. MossbauerThe electronic structure of isolated substitutional or interstitial impurities inSUMMARYSpokesman: J .W.PetersenInstitute of Physics, University of Ziirich, CH-8001 Ziirich, SwitzerlandInstitute of Physics, University of Aarhus, DK-8000 Aarhus C, DenmarkA.Svane‘, G.WeyerK.Bonde Nielsen], E.Holzschuh2, W.Kiindig2, A.Nylandsted Larsen], J .W.Petersen1,THE ELECTRONIC STRUCTURE OF IMPURITIES IN SEMICONDUCT ORS.PROPOSAL TO THE ISOLDE COMMITTEEC Q J I "°‘•~.» O Nov. 5, 1991CERN __ (SC ISC / P 18300004771IIII CERN/ISC 91-30_\_ MV OCERN LIBRARIES, GENEVApage 1Analogously, replacing a host atom with an impurity with a lower valence ( e.g. Al OCR Outputduction bands. The replacement also leads to modifications of the valence bands.states containing the so-called donor electrons are sources of electrons for the conof levels in the otherwise forbidden band gap, close to the conduction band. Thesewith a higher valence ( e.g. P in Si ), the most dramatic effect is the introductionstructure is in most cases confined to the valence bands. When replaced by an atomatom of the same valence ( e.g. Ge in Si ) the perturbations of the electronicband gap, and the empty conduction bands. When a host atom is replaced by anThe electronic structure can be described by the valence bands, which are filled, theInAs, and InSb ). The appropriate part of the periodic system is shown in Fig. 1.SnxGe1_x and SixGeySn,_x_y ) and the III-V compounds ( GaP, GaAs, GaSb, Ir1P,the group IV ( Si, Ge, alfa tin, diamond ), the IV-IV compounds ( SixC1_x, Si,Ge1_x,and are characterized by an average number of valence electrons per atom of four:The semiconductors discussed here all crystallize in the diamond structurescarcely available.been tested critically against experimental results, since these have only beenschemes in existence. However, many aspects of these theoretical schemes have notdeveloped over the last two decades are among the most sophisticated many-bodyelectronic properties - has profound basic interest. The theoretical methodsthe properties of semiconductors, the study of impurities - and particularly theirBesides the technologically stimulated interest of being able to manipulatebroad range of isotopes of one or more elements is available simultaneously.sources can be produced. This is essential for the CESVEC program. Thirdly, aessential feature for the DLTS measurements. Secondly, rather strong, short livedmost of the ion beams are free from contaminations by stable isotopes. This is theISOLDE is a unique tool for these investigations for several reasons: Firstly,tion and annealing processes.cases be used to prove the proper lattice site incorporation after the ion implantaElectron Spectroscopy ( CESVEC ). Mossbauer Spectroscopy ( MS ) can in severalelectronic density of states can be carried out using High Resolution Conversionsient Spectroscopy ( DLTS ). For others a more detailed investigation of theimpurities, producing deep levels in the band gap, this is done by Deep Level Transemiconductors well below their solid solubility. For certain - mainly metallicfor a thorough characterization of the electronic structure of impurities inthese shortcomings can be overcome by the techniques proposed here, which allowunable to cope with such low concentrations or are "chemically blind". Some of1°down to or below the 10cm? range. Most conventional techniques are eitheratoms. This dominance is manifest in very pure semiconductors for concentrationsgoverned by the concentration and chemical nature of wanted or unwanted impurityThe electrical - and partly the optical - properties of semiconductors areSe ) have become available at ISOLDE with good OCR Outputwe take advantage of the fact that isotopes of the 4sp elements ( Zn, Ga, Ge, As,Te incorporated in the group IV elements, in IV-IV and in III-HI compounds. Herean investigation of probe isotopes of the elements Zn, Ga, Ge, As, Se, Sn, Sb, andWe propose here to follow up on these first exploratory measurements and conductshould be the incorporation of lattice relaxation around the over-sized impurities.has also led to the conclusion that the next step in the theoretical developmentsystems ( amphoteric donors and acceptors in several III-V compounds ). This workLMTO scheme has thus been thoroughly tested for these rather complicatedshown here [2], and has been shown to be a more general phenomena [3]. The9 eV below the Fermi level. This is clearly reflected by the experimental resultsseveral III-V compounds leeds to the creation of a highly localized s-like state somemomentum dependent. The theory predicts that the incorporation of a Sn atom inweigthed according to the known conversion coefficients, which are strongly angularincorporated on As sites in GaAs. The s- and p·parts of the calculation have beenimental conversion electron spectrum of the 23874 eV M1 transition in Sn“°figure shows a comparison between the broadened theoretical results and the experlines represent the s-like states and the stippled lines the p-like states. Part b) of thehalf ), and around a Sn atom replacing an As host atom ( upper half ). The fullLMTO calculation of the local density of states around an As atom in GaAs ( lowerIS240 [1,2,3]. Fig.1 shows an example of the results obtained: Part a) shows anprobe atom. First results have been obtained and partly published within the projectexcept for the fact that the conversion electron emission process is localized to thesimilar to other electron spectroscopies, e.g. X-ray photo electron spectroscopy,states at the probe atom. Conversion electron spectroscopy is thus principallysufficiently high resolution, from theer intensity a mapping of the occupied electronfrom the valence shells of the probe atom. When their energy is analyzed with-`. conversion electrons emerging with the highest energies ( close to EN ) originateelectron with quantum numbers n,l, and WCC is the spectrometer work function. Thewhere EN is the nuclear transition energy, EB(n,l) is the binding energy of the(1) ECC = EN - EB(n,l) - Wenergy of an outgoing conversion electron is given by:can take place also in the valence shells — albeit with a very low probability. Themonitored spectroscopically. The method relies on the fact that intemal conversionUsing CESVEC the modifications of the valence band structures can bemodifications of the valence band structure.greater than one, complex level schemes in the band gap can occur together withas a source of holes. When the valence difference between host and impurity isin Si ) introduces levels in the band gap, close to the valence band, which can actpage 3Fig.2 OCR Output Fig.3·nm6 [ days]TEMPERATURE [K]70 120 170 @0 270 320 6 1b gb 3b 4b 5b gb°·°°°1§ 0.001§ .1% ¤| an ·0.01 \E\.__.f ‘~,lan;-gygggglll0.1spectrum during the decay of mlr.of 13 days to 1890s. Fig.3 shows the disappearence of the two main peaks in thisthe DLTS signal from an n-type Si sample doped with mlr decaying with a halflifeleading to an unambigous identification of the involved impurities [4,5]. Fig.2 shows). This results in time dependent DLTS signals as the radionuclei transmute thuspartly overcome by doping the semiconductors with radioactive impurities ( IS01-39"chemical blindness" of these methods. We have shown that this problem can belevels. However, the identification has proven very cumbersome due to theDLTS ) have been developed to detect their presence and the associated band gapidentified. These impurities have very low solubilities so sensitive methods ( e.g.these problems, the levels introduced by a given metallic impurity have to beal ) depends critically on their valence in a non-understood way. However, to solve___interesting since their choice of lattice site ( generally interstitial versus substitutionbut in most cases it creates severe problems. Theoretically, these impurities arethis can be beneficial in some cases - the growing field of band gap engineeringdonor and acceptor levels in the band gap of most semiconductors. Technologically,gap. Especially metallic impurities constitute a source of wanted and unwantedstructure of the system is the introduction of complicated level schemes in the bandbecomes sufficiently large, the most important modification of the electronicWhen the valence difference between an impurity and the host lattice atomsimprovements implemented as justified by the experimental results.state-of-the-art LMTO schemeyxill be carried out on the same systems and possibleoriginally built for neutrino mass experiments. Theoretical calculations using thebecome possible by an upgrade to higher energies of the Zurich beta spectrometer,yields. Furthermore in the case of the 5sp elements Sb and Te, experiments haveOCR Outputpage 5International Conference on Defects in Semiconductors, Bethlehem, PA, July 1991.5.J.W.Petersen, J.Nielsen, To be published in the proceedings of the 16 th.4.J.W.Petersen, J.Nielsen, Appl.Phys.Lett. 56, 1122 (1990)3.J.W.Petersen, A.Svane, E.Holzschuh, to be published.Methods.B.MRS spring meeting, Strassbourg, 1991, to be published shortly in Nucl.Ir1strum.2.J .W.Petersen, G.Weyer, A.Svane, E.Holzschuh, W.Kiindig, Invited talk at the E1.J.W.Petersen, G.Weyer, E.Holzschuh, W.Kiindig, Phys. Letters A A6, 226 (1990).References.performed in the solid state physics laboratory.user groups. Likewise sample treatment prior to and after the implantation can becarried out on the stationary collection positions requested by the solid state physicscurrently avaliable. With respect to space requirements, the implantations can beout is outstanding and cannot be obtained by any other experimental methodshifts of beam time per measurement. On the other hand, the information comingthat the CESVEC experiments are very beam time consuming; often requiring threetarget / ion source combinations. The rather large request is mainly due to the factFor the period of 1992 to 1994 we request 50 shifts of beam time on existingtime of the start of the new ISOLDE facility.ty epitaxial films of these new materials. The materials should be available by theAstronomy in Aarhus has just aquired an MBE sytem to be able to grow high qualifast integrated optical and electronic components. The Institute of Physics andphysics but even more in electronids providing the basis for the next generation ofThe IV-IV compounds are likely to be the materials of the future, both in basiclevels introduced by the 3d, 4d and 5d elements in Si, Ge, and Si,Ge,_x compounds.a dispute which has gone on for more than thirty years [4]. We propose to study thethese levels all originate from undisturbed, substitutional impurities. This has settledthe levels introduced by Pt a¤d.Au impurities in Si and to prove indirectly thatwhich we believe is substitutional Ir. In a similar manner we were able to identifylevels [5]. Likewise it was shown that both levels originate from the same defect,This proves beyond any doubt that Ir is involved in the center giving rise to thesepage 6

The electronic structure of impurities in semiconductors

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