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Study of intrinsic defects in semiconductors with radioactive probes
Nuclear Instruments and Methods in Physics Research B 146 (1998) 329±340
Study of intrinsic defects in semiconductors with radioactive probes R. Sielemann
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Hahn-Meitner-Institut Berlin, Glienicker Str. 100, P.O. Box 390128, D-14109 Berlin, Germany
Abstract The use of radioactive probes to study defects in semiconductors is described. We present the application of Moessbauer Spectroscopy (MS) and Perturbed Angular Correlation Spectroscopy (PAC) with the emphasis on studying intrinsic defects (vacancies and self-interstitials). In order to obtain the simplest possible defect situation the defects are produced in such a way that mainly isolated single Frenkel pairs are created . This is done either by the neutrino recoil technique which utilizes the probe atom as the primary knock-on atom or by electron irradiation. Experiments performed with elemental and III±V semiconductors are described. It is shown that a wealth of microscopic defect information can be obtained. A clean doping of the samples with radioactive probes is fundamental to the experiments. The production of the probes by heavy-ion induced nuclear reactions and their incorporation by a recoil implantation technique is a versatile method giving access to a wide variety of probes and allows to utilize short lived species. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72 Vv; 61.72 Ji; 76.80 +y; 82.80.Ej Keywords: Semiconductor defects; Vacancies; Self-interstitials; PAC; Moessbauer eect
1. Introduction Point defects in semiconductors can be studied and characterized by a variety of experimental techniques. Of particular interest are methods capable of delivering information at the atomic scale with respect to geometric and electronic structure and at the same time providing the necessary in-
1
Fax: 49 30 8062 2293; e-mail: [email protected].
formation to identify a defect under study. Among these techniques nuclear probe methods like M ossbauer Spectroscopy (MS) and Perturbed Angular Correlation Spectroscopy (PAC) have increasingly made contributions in the last years. These methods are based on the measurement of the hyper®ne interaction of a probe nucleus with the charge distribution in its immediate environment yielding information on the probeÕs lattice site and defects which might be situated around it, for a recent review see [1]. In the present contribution we mainly focus on the intrinsic defect problem. An experimental
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 2 6 - 1
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approach is described which directly correlates the production of isolated Frenkel pairs (vacancies and interstitials) with MS and PAC, the neutrinorecoil technique. High energy neutrinos in the electron capture decay of radioactive precursors impart recoil energies to the the probe atoms just above the threshold for Frenkel pair production leading to the formation of isolated vacancy-interstitial pairs locally correlated to the probe atom. These experiments serve various purposes: on the one hand the simple defects produced facilitate the identi®cation with MS or PAC; on the other hand the mechanism of defect production can be studied microscopically if an identi®cation of the created defect has been successful. Experiments performed with Ge, Si, and the III±V compounds InSb and GaSb are described. In addition to the recoil experiments simple point defects were introduced by MeV electron irradiation. It is then investigated whether the produced defects can be trapped at the probes, i.e., whether an attractive interaction exists. Successful experiments of this kind performed with Ge show that a combination of the recoil with the trapping technique allows defect identi®cation and detailed microscopic information to be deduced. An essential prerequisite for the application of radioactive probes is their incorporation into the semiconductor. The most versatile technique is implantation which leads to a clean introduction in near-surface regions of the material. The method described in this article is a recoil implantation technique which combines production of the probes by a heavy-ion induced nuclear reaction with the implantation due to the high recoil energy imparted to the probe atom. Following the introduction we describe the radioactive probe methods PAC and MS with emphasis on a technique combining Frenkel pair production and analysis: the neutrino recoil method (Ch. 2.). Ch. 3. gives a survey of the recoil implanation technique performed at the heavy-ion accelerator of the HMI. In Ch. 4. experimental results on Frenkel pairs in elemental semiconductors (mostly Ge) are described (Sections 4.1 and 4.2) and in Sections 4.3 and 4.4 results on III±V semiconductors are presented (mostly InSb).
2. The nuclear probe method 2.1. Perturbed angular correlation spectroscopy (PAC) The defect property measured by PAC is the electric ®eld gradient (efg). This efg may be produced by a speci®c defect in close neighborhood to the probe atom leading to a quadrupolar hyper®ne interaction with the probeÕs nuclear quadrupole moment eQ resulting in a characteristic interaction frequency mQ eQVZZ =h: VZZ is the largest component of the diagonalized efg tensor. Not only a defect but also a noncubic lattice site of the probe atom may lead to an efg. The interaction frequency is obtained from a time-dierential measurement of the angular correlation of two successively emitted c-rays (Perturbed c±c Angular Correlation, PAC). The PAC probe with the best properties for the study of defects is 111 In with a hal¯ife of 2.8 d feeding in its decay an isomeric state of 111 Cd on which the quadrupole interaction is measured. Thus the notation 111 In/111 Cd is often used to characterize this circumstance. For this ``standard'' probe the time-dependent perturbation of the P3 c rays can be written as G2 t S0 n1 Sn cos xn t. If the probes are simultaneously situated in dierent microscopic environments with relative fractions fi , the total PAC P spectrum can be expressed as R t A2 i fi Gi2 t, where A2 only depends on the nuclear decay parameters. For axial symmetry the frequencies xn are related to the usual quadrupole interaction frequency mQ by xn n3p=10mQ . The amplitudes Sn contain information on the orientation of the efg with respect to the crystal axes. The expression R(t) is formed from the PAC spectra measured with several detectors at suitable angles which serves to eliminate the exponential factor due to the hal¯ife of the decaying intermediate state in 111 Cd and also corrects for dierent detector eciencies. Details can be found in [2]. 2.2. Moessbauer spectroscopy (MS) MS is, like PAC, a probe method measuring the hyper®ne interaction in close vicinity around the probe. In contrast to PAC, MS measures the
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hyper®ne interaction directly enabled by the high energy resolution of the Moessbauer eect. MS has two sensitive parameters, the efg (like PAC) leading to quadrupole splitting (QS) and the isomer shift (IS) which re¯ects the electron density at the probe nucleus. These interactions split the Moessbauer resonance line (QS) and/or shift the line to a dierent energy (IS). In this way the probeÕs lattice site and possibly associated defects are characterized by two hyper®ne interaction parameters. In addition, MS has a third parameter, the Debye±Waller Factor (DWF), which is a measure for the local dynamic situation of the probe. A MS probe with favorable properties for the study of defects in semiconductors is 119 Sb/ 119 Sn. Details may be found in [3]. 2.3. Trapping of radiation-induced defects Irradiation with electrons in the MeV energy range is the most clearcut way of producing simple point defects (Frenkel pairs). Typical defect concentrations used in experiments range from 1012 to 1018 cmÿ3 . Since a probe atom, on the other hand, is extremely short sighted and can recognize defects only within a nearest or next nearest neighbor distance, one has to ®nd a way to bring probe and defect together (a minimum concentration of stastistically distributed defects of about 0.1±1% is necessary to be detectable). This can be accomplished by a trapping technique: irradiation is performed at low temperature followed by annealing. If an interaction between probe and the (usually more mobile) defect exists, a closely bound pair may form. Empirically, it was found that Coulombic interaction between oppositely charged partners (e.g., donors and acceptors) are the main source of attraction in semiconductors; elastic forces, however, may also be responsible for trapping eects. Since the probe atom usually has well de®ned and known properties in the material under study, the conditions under which trapping of defects occurs can be studied. This in turn throws light on the defect properties. Two other types of irradiation should be mentioned. When heavy ions are used instead of electrons, not only simple Frenkel defects but also defect cascades with multiple defects are produced.
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Another type of defect production is connected with the implantation of the probe atom (see below). This implantation causes defects in close vicinity of the probe atom (correlated damage) which, however, usually consists of a variety of simple and multiple defects. Though the study of these implantation cascade phenomena is interesting in itself, these more complicated defects make the study of the fundamental defect properties dicult. In the experiments presented in this paper the implantation damage is annealed before the more speci®c defect production takes place. 2.4. Combined Frenkel pair production and analysis by the neutrino recoil method The direct neighborhood between probe atom and defect is essential for the application of the probe methods. A full exploitation of this fact is made in another type of defect production which is centered on the probe atom. Instead of using a trapping technique of defects produced statistically distributed over the crystal, the neutrino recoil method unites the production and analysis process. This is accomplished by using a probe atom which at the same time serves as the primary knock-on atom (PKA): experimentally this was realized for the PAC method [4±6] and recently also for MS [7] by utilizing a radioactive decay preceeding the ordinary decay sequence. Figs. 1 and 2 show the principle. For PAC on the 111 In/ 111 Cd probe the experiment starts out with the precursor atom 111 Sn. In the electron capture decay to 111 In a high energy neutrino (2.5 MeV) is emitted which leads to a monoenergetic recoil energy of 29 eV on the 111 In probe atoms. This is an energy close to or above the threshold for single Frenkel pair production (one vacancy and one self-interstitial) in most materials in immediate vicinity of the analyzing 111 In/111 Cd probe. For MS the probe 119 Sb/119 Sn is used and the precursor activity is 119 Te. This time an even more versatile experiment is possible. The ground state decay (119g Te) leads to 12 eV monoenergetic recoil, a long-lived metastable state (119m Te) to 6 eV. Since both states can separately be utilized, experiments investigating the defect production threshold can be performed. Recoil energies associated with the
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Fig. 2. Neutrino recoil on the Moessbauer probe resulting from either the decay of 119g Te or 119m Te.
119
Sb/119 Sn
chemically dierent which is of utmost importance in semiconductors. Fig. 1. Partial decay scheme illustrating the neutrino recoil effect. The neutrino emitted in the EC decay of 111 Sn carries an energy of 2.5 MeV leading to 29 eV monoenergetic recoil on the PAC probe 111 In/111 Cd. 111
111
119
119
subsequent decay, In to Cd and Sb to Sn, respectively, are very small and cannot lead to defect production. Table 1 lists the probes used in the neutrino recoil experiments. In this context the following should be stressed: though PAC and MS are based on the same fundamental concept of radioactive probes measuring the nuclear hyper®ne interaction in close vicinity of the probes, the technical suppositions and physical details dier so strongly, that very rarely both methods have been focused on the same physical problem. In the present case both methods are directed at the same problem. This quite clearly brings out the similarities and dierences between both methods. Quite obviously the methods have dierent sensitivity in resolving hyper®ne interactions, they have dierent hyper®ne parameters and, very importantly, the probes are
3. Probe production and recoil implantation The described methods are based on the incorporation of radioactive probe atoms in the semiconductor material. The most versatile technique to accomplish this goal is ion implantation. An almost universial supply of these ions is available at the ISOLDE mass separator at CERN, a disadvantage in certain cases is, however, the low implantation energy (60 keV). In the Table 1 Probes of PAC and MS used for the Neutrino Recoil Technique. The recoil (ER ) is transferred to the probe by a neutrino emitted in the electron capture decay of the precursor Probe
Precursor
ER (eV)
Method
111
111
119
119g
29 12 6
PAC MS MS
In/111 Cd Sb/119 Sn 119 Sb/119 Sn
Sn Te 119m Te
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present case a heavy-ion accelerator is used to produce and implant the probes in a simultaneous process. The principle is shown in Fig. 3. A heavyion beam of typically 60±100 MeV bombards a thin target foil to produce the desired probes by a nuclear reaction. From the reaction mechanism the probes are emitted in a small cone centered on the beam direction receiving the forward momentum of the incoming beam which leads to recoil energies of typically several MeV. The probes are thus propelled out of the target (radioactive beam). Often used primary beams are C, O, and Ne. The beam atoms and the produced probes suer multiple scattering in the target foil. The large dierence in the atomic number Z and in energy allows a rather clean angular separation between beam and probes. A third major component emerging from the target foil is due to elastically (and inelastically) scattered target atoms. The whole process has been simulated by a combination of programs describing the nuclear reaction and the scattering of all three components (beam, probes, target atoms), quantitatively modelling the implantation process [8]. Some important implantation parameters are the following: Typical implantation depth is a few lm. Probe production typically is 106 cmÿ2 sÿ1 , radioactive contamination with undesired isotopes is low, typically within a factor of two of the desired isotopes. Target atoms are scattered at the same order of magnitude. The primary beam is strongly suppressed by selecting larger explantation angles. Due to the lm deep implantation probe concen-
Fig. 3. Sketch of the recoil implantation technique following heavy-ion induced nuclear reactions.
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trations necessary for the probe techniques are low: in the range 1013 ±1014 cmÿ3 . It follows that contaminations are of similar magnitude which obviously has little in¯uence on the electrical properties of semiconductors unless very lowly doped material is used. 4. Experimental results and discussion 4.1. Elemental semiconductors: germanium Ge was selected for the studies, as a large amount of experimental results on defect properties was obtained in the past with a variety of methods, but no convincing identi®cation of either the vacancy or the self-interstitial could be accomplished. Thus conclusions regarding the important defect properties have remained speculative. The neutrino-recoil technique was applied to p- and n-doped Ge in the following way [2,9,10]: 111 Sn was implanted and the radiation damage completely annealed. Following this treatment the probe was immediately cooled to 4.2 K so that the decay of 111 Sn to 111 In took place at low temperature. Subsequently PAC was measured (at 4.2 K) on the 111 Cd probe populated by the 111 In decay. The result for p-type Ge is shown in Fig. 4. A fraction of 11% of the probes displays a quadrupole interaction mQ1 52 MHz with axial symmetry (g 0), showing that a defect is induced at the probe. From additional experiments the orientation of the probe-defect complex can be inferred to be in a h1 1 1i crystal direction. This result is only obtained in p-type Ge. In intrinsic and ntype material there is no observable defect production. An isochronal annealing sequence (10 min), where after each annealing step PAC is measured (at 4.2 K) shows that the defect is stable up to a temperature between 200 and 293 K and then disappears. Since the threshold for Frenkel pair production in Ge is known to be between 15 and 19 eV [11], the neutrino recoil proceeding with 29 eV can produce only one Frenkel pair, one vacancy and one interstitial. The probe atom as the PKA can either knock a neighbor atom out of its site and take its position, leaving a vacancy behind (adjacent to the probe), or it may itself end up in an
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Fig. 4. PAC spectra of 111 In/111 Cd measured at various temperatures after neutrino recoil in p-Ge. The discrete signal (fraction (1)) is due to neutrino recoil.
interstitial position. So the defect connected with mQ1 must be the association of the probe atom with either a vacancy or represent the probe on some type of interstitial site. Theory has as yet not been able to calculate the efg pertaining to the dierent situations including possible relaxation eects with sucient accuracy. So it is a fact that even with the creation of the simplest possible defect situation there remains at ®rst sight an ambiguous situation with respect to the defect visible at the probe. However, combining the present result with results obtained from trapping experiments described in the following, it becomes clear that the 52 MHz defect represents the monovacancy adjacent to the probe (in h1 1 1i direction). The trapping experiments were performed in the following way: the 111 In probes were implanted (not starting from the 111 Sn activity as before) and the damage annealed. Subsequently 1.2 MeV electron irradiation was performed at 77 K and an annealing program was executed, monitored by PAC measurements. This kind of experiment was performed as a function of doping with both p(Ga) and n- (Sb) type material. The result is, that two types of defects are trapped at the probes: the already known defect from neutrino recoil with mQ1 52 MHz and a second type with mQ2 415 MHz (g 0 and orientation h1 1 1i), see Fig. 5. Since mQ1 occurs in both the recoil and the trapping experiment the probe has to occupy a position compatible with both types of defect production. This leaves only the substitutional site for the probe atom and identi®es mQ1 with a monovacancy as nearest neighbor to the probe. A possible recoil induced interstitial site of the probe might be invisible due to the ``blindness'' of PAC to cubic lattice positions. For mQ2 such an identi®cation is not possible since this defect is not produced after neutrino recoil. From an extended series of experiments [9,10] it could be excluded that an unintentional impurity trapping is observed. Thus by exclusion of this possibility one can rather con®dently associate mQ2 with a self-interstitial trapped close to the probe in a h1 1 1i direction. With this identi®cation several important properties can be assigned to the vacancy and the self-interstitial. Fig. 6 shows annealing experiments in p-Ge; rather surprisingly
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Fig. 5. PAC spectrum of perature.
111
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In/111 Cd and its Fourier transform after electron irradiation (at 77 K) in p-Ge measured at room tem-
one can deduce that both vacancy and interstitial perform long-range migration in the same temperature regime around 200 K meaning that both defects have almost the same migration energy, EM 0.5(1) eV. Dissolution of both the vacancy and the self -interstitial from 111 In occurs around 400 K, which gives a binding energy of similar magnitude, EB 0.5(1) eV. Fig. 7 shows the trap-
ping behavior as function of the carrier density (Fermi level). The vacancy is only observed in pGe, the interstitial in both n- and p-type but not in highly doped material. This whole trapping behavior can coherently be explained [9] when Fermi level dependent charge states are assigned to the defects, keeping in mind that the shallow acceptor 111 In is always negatively charged (or partly
Fig. 6. Normalized fractions of 111 In atoms forming complexes with vacancies (squares) and self-interstitials (circles) observed versus annealing temperature after electron irradiation at 77 K in p-Ge.
Fig. 7. Fractions of 111 In atoms forming complexes with vacancies (squares) and self-interstitials (circles) versus carrier concentration in n- and p-type Ge after electron irradiation at 77 K and subsequent warm-up to room temperature.
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which is a domain of the EPR technique [12] and, to a certain extent, of MS. 4.2. Elemental semiconductors: silicon
Fig. 8. Electrical levels of vacancy and self-interstitial in Ge as deduced from PAC.
neutral in heavily p-doped material). An acceptor state of the vacancy at EV + 0.2 eV (Fig. 8) leads to trapping at 111 In when the vacancy is neutral, whereas in the negative state no trapping occurs (Coulomb repulsion). Obviously, the trapping must be caused by elastic interaction between the vacancy and the strongly oversized 111 In atom in the Ge matrix. With similar considerations one can infer, that the Ga doping atoms in p-Ge, being shallow acceptors like the In probes, do not (or only very weakly) interact with the vacancies. The negative charge state of the vacancy in n-type and intrinsic Ge is certainly also the key for the negative result of the recoil experiments in these materials. For the self-interstitial the trapping behavior at the probe atoms can be explained by assuming a donor state close to the conduction band, see Fig. 8. In this case trapping occurs between the positive interstitial and 111 In (Coulomb attraction), loss of trapping in highly p-doped samples can be explained by competitive trapping of the interstitials at the Ga dopants. Loss of trapping in n-type samples is then due to neutralization of the interstitials in highly Sb doped material. From the various types of experiments a large amount of microscopic information is obtained: electric and kinetic properties of the elemental defects, their interaction with the probe atoms and structure information of these probe-defect pairs. No direct information can be obtained by PAC on the geometric structure of the isolated defects
Neutrino-recoil experiments as described above for Ge have also been performed with Si (utilizing the PAC probe 111 Sn). However, in contrast to the Ge results, no defect formation can be observed independent of the doping of the material. The threshold energy for Frenkel pair formation in Si is known to be around 19 eV [11]. Considering the recoil eect in a simple model as an elastic collision, due to the large mass dierence between 111 In and Si (mass 28) a fraction of maximal 65% of the recoil energy, i.e., 19 eV, can be transferred to a Si atom. This is around or below threshold and would explain the negative experimental result. The other possible process, a direct displacement of the probe atom to an interstitial site might be invisible to PAC as discussed before for Ge or might be hampered by the oversize of the probe. Electron irradiations have also been performed with the result that defects are trapped at the probe; identi®cation of the trapped defects, however, could so far not be given since the key experiment, the neutrino recoil technique, does not lead to observable Frenkel pair production. 4.3. III±V semiconductors: InSb A neutrino recoil experiment employing the PAC probe 111 Sn was also performed with InSb. This material was selected since the atoms of both sublattices, In and Sb are probe atoms in PAC and MS, respectively. Furthermore it is known, that Sn after implantation and annealing is incorporated on the In sublattice. Very surprisingly the experiment does not show any evidence for a recoil effect, there is no defect production observable. The experiment was performed with various doping concentrations (p and n) always yielding the same negative result. On the other hand, the defect threshold for III±V semiconductors is expected to be very low, between 6 and 9 eV [11]. Since the expected threshold energy is so low, neutrino recoil experiments employing the Moessbauer eect were performed, utilizing the probe
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Te decaying to 119 Sb and ®nally to 119 Sn, on which the Moessbauer eect is measured (recoil energies either 6 or 12 eV) [7]. An important difference to PAC is a consequence of the dierent probe atom: Te is incorporated on the Sb sublattice. The experiment is performed in a manner as described for the PAC recoil experiments. After implantation of 119g Te (12 eV recoil) the implantation damage is annealed and the sample cooled to 4.2 K. Fig. 9 (top) shows a Moessbauer spectrum thus obtained. In addition to a main line two smaller satellite lines can be observed at negative velocity, corresponding to higher electron density. Fig. 9 (bottom) shows a spectrum serving as a ``check'' experiment to ascertain the origin of the satellite lines. After four hal¯ives of 119g Te the remaining activity is almost completely 119 Sb. The sample is then annealed again at 450°C and
Fig. 9. Moessbauer spectrum of 119 Sb/119 Sn in InSb after neutrino recoil originating from the 119g Te decay measured at 4.2 K (top). Bottom: Spectrum of 119 Sb/119 Sn without neutrino recoil under the same experimental conditions.
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thereafter another spectrum taken at 4.2 K. The spectrum shows that only the main line has survived. From its isomer shift IS 1.99 mm/s it can be identi®ed with the probe residing on a substitutional Sb site. Thus clearly the lines with isomer shifts IS 2.98 mm/s and 3.77 mm/s represent defects connected with the Te activity originally present. A further check as to the origin of these lines was performed in an identical experiment, utilizing the second precursor state, 119m Te. In this case only 6 eV recoil energy is available. This time there is almost no Moessbauer spectral intensity except for the central line representing the substitutional site, no defect production occurs. Taking both experiments together the defect production threshold is seen to be con®ned to a ``window'' between 6 and 12 eV recoil energy (i.e., energy on the PKA, the probe atom). With 12 eV recoil Frenkel pair production is clearly visible, while with 6 eV the production probability is close to zero. This is in agreement with expectations from threshold determinations by electrical methods performed after electron irradiation [11]. A more detailed look at the thermal stability of the produced defects shows that one defect component disappears between 130 and 170 K and the second one at 400(20) K (annealing for 10 min is done when the probe is in the Sb state, i.e., when the probe is an eigenatom of the crystal). Since the defect production has taken place so close to the threshold Ed , only one Frenkel pair, i.e., one vacancy and one interstitial can have been produced. The fact that both defect components have higher electron density than the substitutional line can be qualitatively explained: an interstitial atom has little hybridization with its neighbors transforming the usual sp3 bonding into the direction of s2 p2 . This goes along with an increase of the s electron density which mainly determines the IS. But also a vacant lattice site adjacent to a probe atom would lead to a higher IS at the probe atom: a vacant site would lead to a dangling bond originating at the probe also reducing the sp3 hybridization [13] and therefore increases the electron density. In Ref. [7] a preliminary defect assignment was given. Since in this ®rst experiment the two defect lines shown in Fig. 9 were not resolved, the total spectral intensity at the high electron density was assigned to the
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probe on an interstitial site. A large amount of new data has since been measured and a strongly improved analysis has been performed [14]. These data suggest the possibility that one of the defect components (IS 2.98 mm/s) may be ®t as a quadrupole split doublet which still leaves the fact of a higher electron density (compared to the substitutional line) but to a much lesser extent (IS 2.3 mm/s). This value points to a vacancy adjacent to the probe which is expected if a replacement collision occurs. So very likely MS following the neutrino recoil with 12 eV detects both, the probeÕs direct displacement to an interstitial site and the replacement process. A detailed discussion of these data and their analysis will be presented in a forthcoming paper. If the ®nal assignment can be accomplished, the details of the Frenkel pair production process close to the threshold can be described in a quantitative way and the already measured kinetic parameters can be assigned to the respective defect partners. Fig. 10 shows a sketch of the neutrino recoil process in InSb. Little other experimental information exists
Fig. 10. Sketch of the neutrino recoil eect with the Moessbauer probe 119g Te decaying to 119 Sb/119 Sn in InSb. For details see text
on the microscopic details of the Frenkel pair production process in semiconductors with some notable exceptions [12]. Valuable information, however, may also come from theory: recently calculations applied to Si and GaAs have been published [15,16] which for the ®rst time elucidate this process with advanced theoretical methods. It should be pointed out here that trapping experiments performed in a way described above for Ge should also be possible. The 119m Te activity, which due to the small recoil (6 eV) does not produce defects by recoil eects, may be used as a trapping agent for electron irradiation produced defects. First experiments in InSb have not yet been successful but will be pursued further. Again it is anticipated that the combination of the neutrino recoil method with the trapping technique will give valuable additional information, in particular on the kinetic properties of the trapped defects. 4.4. III±V semiconductors: GaSb As a further material from the III±VÕs GaSb was selected and experiments performed along the lines described for InSb. Here a fundamental dif®culty was encountered preventing so far the observation of the neutrino recoil eect. After implantation of the radioactive probes the crystal has to be annealed to observe a possible Frenkel pair production in an unperturbed crystal. For 119 Te (both 119g Te and 119m Te) in GaSb strong defect components along with the substitutional line are always present up to the highest annealing temperatures of 750 K [14]. Comparison of MS data taken after 6 eV recoil and 12 eV show that these lines are not due to neutrino recoil but are directly associated with the act of implantation. This is a microscopic proof that the donor activity Te (on a Sb site) cannot be brought to an unperturbed site in GaSb by implantation. Similar observations have been made before by electrical measurements for Te implantations in GaAs [17]. A possible neutrino recoil eect which might occur in addition to the implantation induced defects is obscured by the presence of the intense spectral Moessbauer intensity due to implantation. A detailed discussion along with further experimental
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eorts to get rid of the implantation defects is described in [18].
5. Conclusion It is shown how the radioactive probe methods MS and PAC can be utilized to study intrinsic defects in semiconductors based on hyper®ne interaction measurements. The main emphasis is given to a method developed for both MS and PAC which correlates the production of Frenkel pairs with their analysis on a strictly local basis, the neutrino recoil technique. PAC and MS were used in these studies on the same footing, thus explicitely demonstrating similarities and dissimilarities in their capability of producing and detecting intrinsic defects. PACÕs sensitive parameter is the quadrupole interaction leaving cubic probe sites without a speci®c label. Therefore it has to rely on the formation of defect pairs in which the probe atom is one partner locally braking the cubic symmetry. MS on the other hand is in principle capable of identifying the probeÕs lattice site and its environment regardless of symmetry by measuring the IS (electron density). An example is the isolated self-interstitial Sb in InSb. Most detailed defect information has so far been obtained for Ge employing PAC on the probe 111 In/111 Cd along with its precursor 111 Sn. Both the monovacancy and the self-interstitial could be identi®ed and charge states, electrical levels in the bandgap as well as migration energies could be deduced from a combination of the neutrino recoil technique and defect trapping after electron irradiation. In the III±V semiconductor InSb Frenkel pair production by neutrino recoil was successful when MS on the probe 119 Sb/119 Sn with its precursor 119g Te is employed. Two types of Frenkel pair formation can be distinguished with strongly differing thermal stability of the produced defects. In contrast, application of the PAC recoil technique to InSb has so far not lead to observable defect production. This must be due to the details of the production process, which, however, are not yet fully understood.
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Another piece of information obtained by the recoil technique is the probability for the various production processes leading to Frenkel pairs at the recoil energies provided by MS and PAC (6, 12, 29 eV). This information may serve as a quantitative input for the calculations modelling these processes. We ®nally mention that ®rst successful experiments with CdTe show that the recoil technique can also be applied to II±VI semiconductors. So it may well develop in a rather universal instrument for the study of intrinsic defects. Complementary experiments may even broaden the scope of the available information, e.g., electrical and optical techniques may be performed on samples with radioactive probes incorporated. The element speci®c hal¯ife may then be used as an additional label to identify the defect under study. As an example the 111 In probe applied in the PAC work on Ge was also used in DLTS studies performed on the same samples. In this way a simultaneous measurement of the same defect centers (linked by the radioactive label) with two dierent microscopic methods was possible [19].
Acknowledgements It is a pleasure to thank H. Haesslein, L. Wende, G. Weyer and Ch. Zistl who strongly contributed to the work presented. Many enlightening discussions with V.V. Emtsev are also gratefully acknowledged. H. Haas and S. Klaumuenzer is thanked for critically reading the manuscript.
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[6] R. Sielemann, H. H aûlein, M. Br uûler, H. Metzner, Mater. Sci. Forum 83±87 (1991) 1109. [7] R. Sielemann, L. Wende, G. Weyer, Phys. Rev. Lett. 75 (1995) 1542. [8] K. Spohr, B. Schaal, R. Sielemann, to be published. [9] H. Haûlein, R. Sielemann, Ch. Zistl, Phys. Rev. Lett. 80 (1998) 2626. [10] H. Haûlein, Thesis, Freie Universit at Berlin, unpublished. [11] J.W. Corbett, J. Bourgoin, in: J.H. Crawford, L.M. Slifkin (Eds.), Point Defects in Solids, vol. 2, Plenum Press, New York, 1975. [12] G.D. Watkins, Mater. Sci. Forum 143 (1994) 9.
[13] M. Lannoo, A. Svane, H. Overhof, H. Katayama-Yoshida, Hyper®ne Interactions of Defects in Semiconductors, Ref. [2]). [14] L. Wende, Thesis, Freie Universit at Berlin, unpublished. [15] T. Mattila, R.M. Nieminen, Phys. Rev. Lett. 74 (1995) 2721. [16] M. Sayed, J.H. Jeerson, A.B. Walker, A.G. Cullis, Nuclear Instr. and Meth B 102 (1995) 232. [17] S.J. Pearton et al., J. Appl. Phys. 65 (1989) 1089. [18] L. Wende, R. Sielemann, G. Weyer, Contribution to HFI11 Conference, Durban, South Africa, 1998. [19] Ch. Zistl et al., Mater. Sci. Forum 258±263 (1997) 53.
Study of intrinsic defects in semiconductors with radioactive probes R. Sielemann
1
Hahn-Meitner-Institut Berlin, Glienicker Str. 100, P.O. Box 390128, D-14109 Berlin, Germany
Abstract The use of radioactive probes to study defects in semiconductors is described. We present the application of Moessbauer Spectroscopy (MS) and Perturbed Angular Correlation Spectroscopy (PAC) with the emphasis on studying intrinsic defects (vacancies and self-interstitials). In order to obtain the simplest possible defect situation the defects are produced in such a way that mainly isolated single Frenkel pairs are created . This is done either by the neutrino recoil technique which utilizes the probe atom as the primary knock-on atom or by electron irradiation. Experiments performed with elemental and III±V semiconductors are described. It is shown that a wealth of microscopic defect information can be obtained. A clean doping of the samples with radioactive probes is fundamental to the experiments. The production of the probes by heavy-ion induced nuclear reactions and their incorporation by a recoil implantation technique is a versatile method giving access to a wide variety of probes and allows to utilize short lived species. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72 Vv; 61.72 Ji; 76.80 +y; 82.80.Ej Keywords: Semiconductor defects; Vacancies; Self-interstitials; PAC; Moessbauer eect
1. Introduction Point defects in semiconductors can be studied and characterized by a variety of experimental techniques. Of particular interest are methods capable of delivering information at the atomic scale with respect to geometric and electronic structure and at the same time providing the necessary in-
1
Fax: 49 30 8062 2293; e-mail: [email protected].
formation to identify a defect under study. Among these techniques nuclear probe methods like M ossbauer Spectroscopy (MS) and Perturbed Angular Correlation Spectroscopy (PAC) have increasingly made contributions in the last years. These methods are based on the measurement of the hyper®ne interaction of a probe nucleus with the charge distribution in its immediate environment yielding information on the probeÕs lattice site and defects which might be situated around it, for a recent review see [1]. In the present contribution we mainly focus on the intrinsic defect problem. An experimental
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 2 6 - 1
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approach is described which directly correlates the production of isolated Frenkel pairs (vacancies and interstitials) with MS and PAC, the neutrinorecoil technique. High energy neutrinos in the electron capture decay of radioactive precursors impart recoil energies to the the probe atoms just above the threshold for Frenkel pair production leading to the formation of isolated vacancy-interstitial pairs locally correlated to the probe atom. These experiments serve various purposes: on the one hand the simple defects produced facilitate the identi®cation with MS or PAC; on the other hand the mechanism of defect production can be studied microscopically if an identi®cation of the created defect has been successful. Experiments performed with Ge, Si, and the III±V compounds InSb and GaSb are described. In addition to the recoil experiments simple point defects were introduced by MeV electron irradiation. It is then investigated whether the produced defects can be trapped at the probes, i.e., whether an attractive interaction exists. Successful experiments of this kind performed with Ge show that a combination of the recoil with the trapping technique allows defect identi®cation and detailed microscopic information to be deduced. An essential prerequisite for the application of radioactive probes is their incorporation into the semiconductor. The most versatile technique is implantation which leads to a clean introduction in near-surface regions of the material. The method described in this article is a recoil implantation technique which combines production of the probes by a heavy-ion induced nuclear reaction with the implantation due to the high recoil energy imparted to the probe atom. Following the introduction we describe the radioactive probe methods PAC and MS with emphasis on a technique combining Frenkel pair production and analysis: the neutrino recoil method (Ch. 2.). Ch. 3. gives a survey of the recoil implanation technique performed at the heavy-ion accelerator of the HMI. In Ch. 4. experimental results on Frenkel pairs in elemental semiconductors (mostly Ge) are described (Sections 4.1 and 4.2) and in Sections 4.3 and 4.4 results on III±V semiconductors are presented (mostly InSb).
2. The nuclear probe method 2.1. Perturbed angular correlation spectroscopy (PAC) The defect property measured by PAC is the electric ®eld gradient (efg). This efg may be produced by a speci®c defect in close neighborhood to the probe atom leading to a quadrupolar hyper®ne interaction with the probeÕs nuclear quadrupole moment eQ resulting in a characteristic interaction frequency mQ eQVZZ =h: VZZ is the largest component of the diagonalized efg tensor. Not only a defect but also a noncubic lattice site of the probe atom may lead to an efg. The interaction frequency is obtained from a time-dierential measurement of the angular correlation of two successively emitted c-rays (Perturbed c±c Angular Correlation, PAC). The PAC probe with the best properties for the study of defects is 111 In with a hal¯ife of 2.8 d feeding in its decay an isomeric state of 111 Cd on which the quadrupole interaction is measured. Thus the notation 111 In/111 Cd is often used to characterize this circumstance. For this ``standard'' probe the time-dependent perturbation of the P3 c rays can be written as G2 t S0 n1 Sn cos xn t. If the probes are simultaneously situated in dierent microscopic environments with relative fractions fi , the total PAC P spectrum can be expressed as R t A2 i fi Gi2 t, where A2 only depends on the nuclear decay parameters. For axial symmetry the frequencies xn are related to the usual quadrupole interaction frequency mQ by xn n3p=10mQ . The amplitudes Sn contain information on the orientation of the efg with respect to the crystal axes. The expression R(t) is formed from the PAC spectra measured with several detectors at suitable angles which serves to eliminate the exponential factor due to the hal¯ife of the decaying intermediate state in 111 Cd and also corrects for dierent detector eciencies. Details can be found in [2]. 2.2. Moessbauer spectroscopy (MS) MS is, like PAC, a probe method measuring the hyper®ne interaction in close vicinity around the probe. In contrast to PAC, MS measures the
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hyper®ne interaction directly enabled by the high energy resolution of the Moessbauer eect. MS has two sensitive parameters, the efg (like PAC) leading to quadrupole splitting (QS) and the isomer shift (IS) which re¯ects the electron density at the probe nucleus. These interactions split the Moessbauer resonance line (QS) and/or shift the line to a dierent energy (IS). In this way the probeÕs lattice site and possibly associated defects are characterized by two hyper®ne interaction parameters. In addition, MS has a third parameter, the Debye±Waller Factor (DWF), which is a measure for the local dynamic situation of the probe. A MS probe with favorable properties for the study of defects in semiconductors is 119 Sb/ 119 Sn. Details may be found in [3]. 2.3. Trapping of radiation-induced defects Irradiation with electrons in the MeV energy range is the most clearcut way of producing simple point defects (Frenkel pairs). Typical defect concentrations used in experiments range from 1012 to 1018 cmÿ3 . Since a probe atom, on the other hand, is extremely short sighted and can recognize defects only within a nearest or next nearest neighbor distance, one has to ®nd a way to bring probe and defect together (a minimum concentration of stastistically distributed defects of about 0.1±1% is necessary to be detectable). This can be accomplished by a trapping technique: irradiation is performed at low temperature followed by annealing. If an interaction between probe and the (usually more mobile) defect exists, a closely bound pair may form. Empirically, it was found that Coulombic interaction between oppositely charged partners (e.g., donors and acceptors) are the main source of attraction in semiconductors; elastic forces, however, may also be responsible for trapping eects. Since the probe atom usually has well de®ned and known properties in the material under study, the conditions under which trapping of defects occurs can be studied. This in turn throws light on the defect properties. Two other types of irradiation should be mentioned. When heavy ions are used instead of electrons, not only simple Frenkel defects but also defect cascades with multiple defects are produced.
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Another type of defect production is connected with the implantation of the probe atom (see below). This implantation causes defects in close vicinity of the probe atom (correlated damage) which, however, usually consists of a variety of simple and multiple defects. Though the study of these implantation cascade phenomena is interesting in itself, these more complicated defects make the study of the fundamental defect properties dicult. In the experiments presented in this paper the implantation damage is annealed before the more speci®c defect production takes place. 2.4. Combined Frenkel pair production and analysis by the neutrino recoil method The direct neighborhood between probe atom and defect is essential for the application of the probe methods. A full exploitation of this fact is made in another type of defect production which is centered on the probe atom. Instead of using a trapping technique of defects produced statistically distributed over the crystal, the neutrino recoil method unites the production and analysis process. This is accomplished by using a probe atom which at the same time serves as the primary knock-on atom (PKA): experimentally this was realized for the PAC method [4±6] and recently also for MS [7] by utilizing a radioactive decay preceeding the ordinary decay sequence. Figs. 1 and 2 show the principle. For PAC on the 111 In/ 111 Cd probe the experiment starts out with the precursor atom 111 Sn. In the electron capture decay to 111 In a high energy neutrino (2.5 MeV) is emitted which leads to a monoenergetic recoil energy of 29 eV on the 111 In probe atoms. This is an energy close to or above the threshold for single Frenkel pair production (one vacancy and one self-interstitial) in most materials in immediate vicinity of the analyzing 111 In/111 Cd probe. For MS the probe 119 Sb/119 Sn is used and the precursor activity is 119 Te. This time an even more versatile experiment is possible. The ground state decay (119g Te) leads to 12 eV monoenergetic recoil, a long-lived metastable state (119m Te) to 6 eV. Since both states can separately be utilized, experiments investigating the defect production threshold can be performed. Recoil energies associated with the
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Fig. 2. Neutrino recoil on the Moessbauer probe resulting from either the decay of 119g Te or 119m Te.
119
Sb/119 Sn
chemically dierent which is of utmost importance in semiconductors. Fig. 1. Partial decay scheme illustrating the neutrino recoil effect. The neutrino emitted in the EC decay of 111 Sn carries an energy of 2.5 MeV leading to 29 eV monoenergetic recoil on the PAC probe 111 In/111 Cd. 111
111
119
119
subsequent decay, In to Cd and Sb to Sn, respectively, are very small and cannot lead to defect production. Table 1 lists the probes used in the neutrino recoil experiments. In this context the following should be stressed: though PAC and MS are based on the same fundamental concept of radioactive probes measuring the nuclear hyper®ne interaction in close vicinity of the probes, the technical suppositions and physical details dier so strongly, that very rarely both methods have been focused on the same physical problem. In the present case both methods are directed at the same problem. This quite clearly brings out the similarities and dierences between both methods. Quite obviously the methods have dierent sensitivity in resolving hyper®ne interactions, they have dierent hyper®ne parameters and, very importantly, the probes are
3. Probe production and recoil implantation The described methods are based on the incorporation of radioactive probe atoms in the semiconductor material. The most versatile technique to accomplish this goal is ion implantation. An almost universial supply of these ions is available at the ISOLDE mass separator at CERN, a disadvantage in certain cases is, however, the low implantation energy (60 keV). In the Table 1 Probes of PAC and MS used for the Neutrino Recoil Technique. The recoil (ER ) is transferred to the probe by a neutrino emitted in the electron capture decay of the precursor Probe
Precursor
ER (eV)
Method
111
111
119
119g
29 12 6
PAC MS MS
In/111 Cd Sb/119 Sn 119 Sb/119 Sn
Sn Te 119m Te
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present case a heavy-ion accelerator is used to produce and implant the probes in a simultaneous process. The principle is shown in Fig. 3. A heavyion beam of typically 60±100 MeV bombards a thin target foil to produce the desired probes by a nuclear reaction. From the reaction mechanism the probes are emitted in a small cone centered on the beam direction receiving the forward momentum of the incoming beam which leads to recoil energies of typically several MeV. The probes are thus propelled out of the target (radioactive beam). Often used primary beams are C, O, and Ne. The beam atoms and the produced probes suer multiple scattering in the target foil. The large dierence in the atomic number Z and in energy allows a rather clean angular separation between beam and probes. A third major component emerging from the target foil is due to elastically (and inelastically) scattered target atoms. The whole process has been simulated by a combination of programs describing the nuclear reaction and the scattering of all three components (beam, probes, target atoms), quantitatively modelling the implantation process [8]. Some important implantation parameters are the following: Typical implantation depth is a few lm. Probe production typically is 106 cmÿ2 sÿ1 , radioactive contamination with undesired isotopes is low, typically within a factor of two of the desired isotopes. Target atoms are scattered at the same order of magnitude. The primary beam is strongly suppressed by selecting larger explantation angles. Due to the lm deep implantation probe concen-
Fig. 3. Sketch of the recoil implantation technique following heavy-ion induced nuclear reactions.
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trations necessary for the probe techniques are low: in the range 1013 ±1014 cmÿ3 . It follows that contaminations are of similar magnitude which obviously has little in¯uence on the electrical properties of semiconductors unless very lowly doped material is used. 4. Experimental results and discussion 4.1. Elemental semiconductors: germanium Ge was selected for the studies, as a large amount of experimental results on defect properties was obtained in the past with a variety of methods, but no convincing identi®cation of either the vacancy or the self-interstitial could be accomplished. Thus conclusions regarding the important defect properties have remained speculative. The neutrino-recoil technique was applied to p- and n-doped Ge in the following way [2,9,10]: 111 Sn was implanted and the radiation damage completely annealed. Following this treatment the probe was immediately cooled to 4.2 K so that the decay of 111 Sn to 111 In took place at low temperature. Subsequently PAC was measured (at 4.2 K) on the 111 Cd probe populated by the 111 In decay. The result for p-type Ge is shown in Fig. 4. A fraction of 11% of the probes displays a quadrupole interaction mQ1 52 MHz with axial symmetry (g 0), showing that a defect is induced at the probe. From additional experiments the orientation of the probe-defect complex can be inferred to be in a h1 1 1i crystal direction. This result is only obtained in p-type Ge. In intrinsic and ntype material there is no observable defect production. An isochronal annealing sequence (10 min), where after each annealing step PAC is measured (at 4.2 K) shows that the defect is stable up to a temperature between 200 and 293 K and then disappears. Since the threshold for Frenkel pair production in Ge is known to be between 15 and 19 eV [11], the neutrino recoil proceeding with 29 eV can produce only one Frenkel pair, one vacancy and one interstitial. The probe atom as the PKA can either knock a neighbor atom out of its site and take its position, leaving a vacancy behind (adjacent to the probe), or it may itself end up in an
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Fig. 4. PAC spectra of 111 In/111 Cd measured at various temperatures after neutrino recoil in p-Ge. The discrete signal (fraction (1)) is due to neutrino recoil.
interstitial position. So the defect connected with mQ1 must be the association of the probe atom with either a vacancy or represent the probe on some type of interstitial site. Theory has as yet not been able to calculate the efg pertaining to the dierent situations including possible relaxation eects with sucient accuracy. So it is a fact that even with the creation of the simplest possible defect situation there remains at ®rst sight an ambiguous situation with respect to the defect visible at the probe. However, combining the present result with results obtained from trapping experiments described in the following, it becomes clear that the 52 MHz defect represents the monovacancy adjacent to the probe (in h1 1 1i direction). The trapping experiments were performed in the following way: the 111 In probes were implanted (not starting from the 111 Sn activity as before) and the damage annealed. Subsequently 1.2 MeV electron irradiation was performed at 77 K and an annealing program was executed, monitored by PAC measurements. This kind of experiment was performed as a function of doping with both p(Ga) and n- (Sb) type material. The result is, that two types of defects are trapped at the probes: the already known defect from neutrino recoil with mQ1 52 MHz and a second type with mQ2 415 MHz (g 0 and orientation h1 1 1i), see Fig. 5. Since mQ1 occurs in both the recoil and the trapping experiment the probe has to occupy a position compatible with both types of defect production. This leaves only the substitutional site for the probe atom and identi®es mQ1 with a monovacancy as nearest neighbor to the probe. A possible recoil induced interstitial site of the probe might be invisible due to the ``blindness'' of PAC to cubic lattice positions. For mQ2 such an identi®cation is not possible since this defect is not produced after neutrino recoil. From an extended series of experiments [9,10] it could be excluded that an unintentional impurity trapping is observed. Thus by exclusion of this possibility one can rather con®dently associate mQ2 with a self-interstitial trapped close to the probe in a h1 1 1i direction. With this identi®cation several important properties can be assigned to the vacancy and the self-interstitial. Fig. 6 shows annealing experiments in p-Ge; rather surprisingly
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Fig. 5. PAC spectrum of perature.
111
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In/111 Cd and its Fourier transform after electron irradiation (at 77 K) in p-Ge measured at room tem-
one can deduce that both vacancy and interstitial perform long-range migration in the same temperature regime around 200 K meaning that both defects have almost the same migration energy, EM 0.5(1) eV. Dissolution of both the vacancy and the self -interstitial from 111 In occurs around 400 K, which gives a binding energy of similar magnitude, EB 0.5(1) eV. Fig. 7 shows the trap-
ping behavior as function of the carrier density (Fermi level). The vacancy is only observed in pGe, the interstitial in both n- and p-type but not in highly doped material. This whole trapping behavior can coherently be explained [9] when Fermi level dependent charge states are assigned to the defects, keeping in mind that the shallow acceptor 111 In is always negatively charged (or partly
Fig. 6. Normalized fractions of 111 In atoms forming complexes with vacancies (squares) and self-interstitials (circles) observed versus annealing temperature after electron irradiation at 77 K in p-Ge.
Fig. 7. Fractions of 111 In atoms forming complexes with vacancies (squares) and self-interstitials (circles) versus carrier concentration in n- and p-type Ge after electron irradiation at 77 K and subsequent warm-up to room temperature.
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which is a domain of the EPR technique [12] and, to a certain extent, of MS. 4.2. Elemental semiconductors: silicon
Fig. 8. Electrical levels of vacancy and self-interstitial in Ge as deduced from PAC.
neutral in heavily p-doped material). An acceptor state of the vacancy at EV + 0.2 eV (Fig. 8) leads to trapping at 111 In when the vacancy is neutral, whereas in the negative state no trapping occurs (Coulomb repulsion). Obviously, the trapping must be caused by elastic interaction between the vacancy and the strongly oversized 111 In atom in the Ge matrix. With similar considerations one can infer, that the Ga doping atoms in p-Ge, being shallow acceptors like the In probes, do not (or only very weakly) interact with the vacancies. The negative charge state of the vacancy in n-type and intrinsic Ge is certainly also the key for the negative result of the recoil experiments in these materials. For the self-interstitial the trapping behavior at the probe atoms can be explained by assuming a donor state close to the conduction band, see Fig. 8. In this case trapping occurs between the positive interstitial and 111 In (Coulomb attraction), loss of trapping in highly p-doped samples can be explained by competitive trapping of the interstitials at the Ga dopants. Loss of trapping in n-type samples is then due to neutralization of the interstitials in highly Sb doped material. From the various types of experiments a large amount of microscopic information is obtained: electric and kinetic properties of the elemental defects, their interaction with the probe atoms and structure information of these probe-defect pairs. No direct information can be obtained by PAC on the geometric structure of the isolated defects
Neutrino-recoil experiments as described above for Ge have also been performed with Si (utilizing the PAC probe 111 Sn). However, in contrast to the Ge results, no defect formation can be observed independent of the doping of the material. The threshold energy for Frenkel pair formation in Si is known to be around 19 eV [11]. Considering the recoil eect in a simple model as an elastic collision, due to the large mass dierence between 111 In and Si (mass 28) a fraction of maximal 65% of the recoil energy, i.e., 19 eV, can be transferred to a Si atom. This is around or below threshold and would explain the negative experimental result. The other possible process, a direct displacement of the probe atom to an interstitial site might be invisible to PAC as discussed before for Ge or might be hampered by the oversize of the probe. Electron irradiations have also been performed with the result that defects are trapped at the probe; identi®cation of the trapped defects, however, could so far not be given since the key experiment, the neutrino recoil technique, does not lead to observable Frenkel pair production. 4.3. III±V semiconductors: InSb A neutrino recoil experiment employing the PAC probe 111 Sn was also performed with InSb. This material was selected since the atoms of both sublattices, In and Sb are probe atoms in PAC and MS, respectively. Furthermore it is known, that Sn after implantation and annealing is incorporated on the In sublattice. Very surprisingly the experiment does not show any evidence for a recoil effect, there is no defect production observable. The experiment was performed with various doping concentrations (p and n) always yielding the same negative result. On the other hand, the defect threshold for III±V semiconductors is expected to be very low, between 6 and 9 eV [11]. Since the expected threshold energy is so low, neutrino recoil experiments employing the Moessbauer eect were performed, utilizing the probe
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Te decaying to 119 Sb and ®nally to 119 Sn, on which the Moessbauer eect is measured (recoil energies either 6 or 12 eV) [7]. An important difference to PAC is a consequence of the dierent probe atom: Te is incorporated on the Sb sublattice. The experiment is performed in a manner as described for the PAC recoil experiments. After implantation of 119g Te (12 eV recoil) the implantation damage is annealed and the sample cooled to 4.2 K. Fig. 9 (top) shows a Moessbauer spectrum thus obtained. In addition to a main line two smaller satellite lines can be observed at negative velocity, corresponding to higher electron density. Fig. 9 (bottom) shows a spectrum serving as a ``check'' experiment to ascertain the origin of the satellite lines. After four hal¯ives of 119g Te the remaining activity is almost completely 119 Sb. The sample is then annealed again at 450°C and
Fig. 9. Moessbauer spectrum of 119 Sb/119 Sn in InSb after neutrino recoil originating from the 119g Te decay measured at 4.2 K (top). Bottom: Spectrum of 119 Sb/119 Sn without neutrino recoil under the same experimental conditions.
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thereafter another spectrum taken at 4.2 K. The spectrum shows that only the main line has survived. From its isomer shift IS 1.99 mm/s it can be identi®ed with the probe residing on a substitutional Sb site. Thus clearly the lines with isomer shifts IS 2.98 mm/s and 3.77 mm/s represent defects connected with the Te activity originally present. A further check as to the origin of these lines was performed in an identical experiment, utilizing the second precursor state, 119m Te. In this case only 6 eV recoil energy is available. This time there is almost no Moessbauer spectral intensity except for the central line representing the substitutional site, no defect production occurs. Taking both experiments together the defect production threshold is seen to be con®ned to a ``window'' between 6 and 12 eV recoil energy (i.e., energy on the PKA, the probe atom). With 12 eV recoil Frenkel pair production is clearly visible, while with 6 eV the production probability is close to zero. This is in agreement with expectations from threshold determinations by electrical methods performed after electron irradiation [11]. A more detailed look at the thermal stability of the produced defects shows that one defect component disappears between 130 and 170 K and the second one at 400(20) K (annealing for 10 min is done when the probe is in the Sb state, i.e., when the probe is an eigenatom of the crystal). Since the defect production has taken place so close to the threshold Ed , only one Frenkel pair, i.e., one vacancy and one interstitial can have been produced. The fact that both defect components have higher electron density than the substitutional line can be qualitatively explained: an interstitial atom has little hybridization with its neighbors transforming the usual sp3 bonding into the direction of s2 p2 . This goes along with an increase of the s electron density which mainly determines the IS. But also a vacant lattice site adjacent to a probe atom would lead to a higher IS at the probe atom: a vacant site would lead to a dangling bond originating at the probe also reducing the sp3 hybridization [13] and therefore increases the electron density. In Ref. [7] a preliminary defect assignment was given. Since in this ®rst experiment the two defect lines shown in Fig. 9 were not resolved, the total spectral intensity at the high electron density was assigned to the
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probe on an interstitial site. A large amount of new data has since been measured and a strongly improved analysis has been performed [14]. These data suggest the possibility that one of the defect components (IS 2.98 mm/s) may be ®t as a quadrupole split doublet which still leaves the fact of a higher electron density (compared to the substitutional line) but to a much lesser extent (IS 2.3 mm/s). This value points to a vacancy adjacent to the probe which is expected if a replacement collision occurs. So very likely MS following the neutrino recoil with 12 eV detects both, the probeÕs direct displacement to an interstitial site and the replacement process. A detailed discussion of these data and their analysis will be presented in a forthcoming paper. If the ®nal assignment can be accomplished, the details of the Frenkel pair production process close to the threshold can be described in a quantitative way and the already measured kinetic parameters can be assigned to the respective defect partners. Fig. 10 shows a sketch of the neutrino recoil process in InSb. Little other experimental information exists
Fig. 10. Sketch of the neutrino recoil eect with the Moessbauer probe 119g Te decaying to 119 Sb/119 Sn in InSb. For details see text
on the microscopic details of the Frenkel pair production process in semiconductors with some notable exceptions [12]. Valuable information, however, may also come from theory: recently calculations applied to Si and GaAs have been published [15,16] which for the ®rst time elucidate this process with advanced theoretical methods. It should be pointed out here that trapping experiments performed in a way described above for Ge should also be possible. The 119m Te activity, which due to the small recoil (6 eV) does not produce defects by recoil eects, may be used as a trapping agent for electron irradiation produced defects. First experiments in InSb have not yet been successful but will be pursued further. Again it is anticipated that the combination of the neutrino recoil method with the trapping technique will give valuable additional information, in particular on the kinetic properties of the trapped defects. 4.4. III±V semiconductors: GaSb As a further material from the III±VÕs GaSb was selected and experiments performed along the lines described for InSb. Here a fundamental dif®culty was encountered preventing so far the observation of the neutrino recoil eect. After implantation of the radioactive probes the crystal has to be annealed to observe a possible Frenkel pair production in an unperturbed crystal. For 119 Te (both 119g Te and 119m Te) in GaSb strong defect components along with the substitutional line are always present up to the highest annealing temperatures of 750 K [14]. Comparison of MS data taken after 6 eV recoil and 12 eV show that these lines are not due to neutrino recoil but are directly associated with the act of implantation. This is a microscopic proof that the donor activity Te (on a Sb site) cannot be brought to an unperturbed site in GaSb by implantation. Similar observations have been made before by electrical measurements for Te implantations in GaAs [17]. A possible neutrino recoil eect which might occur in addition to the implantation induced defects is obscured by the presence of the intense spectral Moessbauer intensity due to implantation. A detailed discussion along with further experimental
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eorts to get rid of the implantation defects is described in [18].
5. Conclusion It is shown how the radioactive probe methods MS and PAC can be utilized to study intrinsic defects in semiconductors based on hyper®ne interaction measurements. The main emphasis is given to a method developed for both MS and PAC which correlates the production of Frenkel pairs with their analysis on a strictly local basis, the neutrino recoil technique. PAC and MS were used in these studies on the same footing, thus explicitely demonstrating similarities and dissimilarities in their capability of producing and detecting intrinsic defects. PACÕs sensitive parameter is the quadrupole interaction leaving cubic probe sites without a speci®c label. Therefore it has to rely on the formation of defect pairs in which the probe atom is one partner locally braking the cubic symmetry. MS on the other hand is in principle capable of identifying the probeÕs lattice site and its environment regardless of symmetry by measuring the IS (electron density). An example is the isolated self-interstitial Sb in InSb. Most detailed defect information has so far been obtained for Ge employing PAC on the probe 111 In/111 Cd along with its precursor 111 Sn. Both the monovacancy and the self-interstitial could be identi®ed and charge states, electrical levels in the bandgap as well as migration energies could be deduced from a combination of the neutrino recoil technique and defect trapping after electron irradiation. In the III±V semiconductor InSb Frenkel pair production by neutrino recoil was successful when MS on the probe 119 Sb/119 Sn with its precursor 119g Te is employed. Two types of Frenkel pair formation can be distinguished with strongly differing thermal stability of the produced defects. In contrast, application of the PAC recoil technique to InSb has so far not lead to observable defect production. This must be due to the details of the production process, which, however, are not yet fully understood.
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Another piece of information obtained by the recoil technique is the probability for the various production processes leading to Frenkel pairs at the recoil energies provided by MS and PAC (6, 12, 29 eV). This information may serve as a quantitative input for the calculations modelling these processes. We ®nally mention that ®rst successful experiments with CdTe show that the recoil technique can also be applied to II±VI semiconductors. So it may well develop in a rather universal instrument for the study of intrinsic defects. Complementary experiments may even broaden the scope of the available information, e.g., electrical and optical techniques may be performed on samples with radioactive probes incorporated. The element speci®c hal¯ife may then be used as an additional label to identify the defect under study. As an example the 111 In probe applied in the PAC work on Ge was also used in DLTS studies performed on the same samples. In this way a simultaneous measurement of the same defect centers (linked by the radioactive label) with two dierent microscopic methods was possible [19].
Acknowledgements It is a pleasure to thank H. Haesslein, L. Wende, G. Weyer and Ch. Zistl who strongly contributed to the work presented. Many enlightening discussions with V.V. Emtsev are also gratefully acknowledged. H. Haas and S. Klaumuenzer is thanked for critically reading the manuscript.
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