An anomalous feature on the DLTS spectrum of silicon

Solid State Communications, Vol. 62, No. 10, pp. 719-722, 1987. Printed in Great Britain.

0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd.

A N A N O M A L O U S F E A T U R E O N T H E DLTS S P E C T R U M O F SILICON C.A. Londos* Physics Department, University of Athens, 104 Solonos Str, Athens 106 80, Greece

(Received 30 April 1986; in revised form 11 September 1986 by M. Balkanski) The behaviour of a very curious feature arising on the DLTS spectrum of L N 2 electron-irradiated pulled silicon is studied. It is shown that this behaviour is not consistent with a normal DLTS peak caused by the trapping and emission of carriers from a deep level. However, it is almost certain that this feature is a result of the low-temperature irradiation of the pulled silicon. A model is proposed based on the oxygen trapping at abnormal sites in the silicon lattice. DEEP LEVEL T R A N S I E N T spectroscopy (DLTS) [1, 2] is a widely used technique for the characterization of defects found in the depletion region ofp-n junctions or Schottky diodes. An ideal DLTS spectrum is expected to display a single peak for each level present in the forbidden gap of the semiconductor. However the real DLTS spectrum may include contributions caused by a variety of reasons. Features due to free carrier tails [3, 4], strong lattice relaxations [5], the dissociation of a defect or to the reorientation of its dipole moment [6], may affect the DLIS output. On the other hand, semiconductor devices are never ideal capacitors. Leakage currents [7] or series resistance affect badly the accuracy of the capacitance measurement. The RC time constant of the sample itself obviously become important when approaching the angular frequency o f the capacitance measurement (namely when co ] = RC) or the width of the applied electrical pulses [2, 8]. The primary object of this work was to study the behaviour of an anomalous signal emerging on the DLTS spectrum of boron-doped silicon after 1.5 meV electron irradiation at 80 K. Typical beam currents were less than 0.3/~Acm -2 and the dose was about 2 x 1016 electrons cm -2. We used pre-polished wafers of hyper pure pulled material provided from Wacker. These wafers with a resistivity ranging from 3 to 5f~cm were divided by cleaving. The preparation procedure of aluminum-silicon Schottky structures has been described elsewhere [9]. We took great care to produce satisfactory samples. C-V analysis gave very linear C -2 vs I curves with voltage intercepts in the range of 0.6 to 0.7 V and net acceptor concentrations, calculated from their slopes, in the range of * The experiment has been carried out in Reading University (England), during leave of absence.

3.4 x 10JScm -3 to 3.8 x 10~Scm-3. These values are in excellent agreement with the reported data obtained from the bibliography and the resistivity quoted from the manufacturers. C-T test gave an almost ideal curve with a less than 10 % gradual variation of the capacitance in the temperature range from 80 to 220 K. DLTS scans showed no indication of any surface trap or contamination before irradiation. The low temperature irradiation followed by zero bias annealing for 30 min, at 220 K, a temperature at which the vacancy disappears. Afterwards the specimen was quenched at 80 K. Figure 1 shows two DLTS spectra, the first obtained with the temperature ramping upwards and the other with the temperature ramping downwards. In the latter spectrum there is one more feature which is labelled as H. Apart from the fact that this feature appears only with the decreasing temperature scanning mode it also exhibits some other interesting characteristics. At first, the temperature position of H is at least intriguing. Immediately after the irradiation and the migration of the vacancy H arises on the left side of the Ev + 0.13 eV level. After some days of measurements it suddenly begins to arise on the right side of the Ev + 0.13 eV level. Secondly the shape of peak H is not stable. Curves a, b and c show three of the most common different traces which H exhibits during different DLTS scannings. Curve a is a faint signal,, curve b looks like a spike and curve c has the appearance of a strong asymmetrical peak. One of the facilities of our operating system was the ability to put simultaneously two rate windows and hence get two spectra. For every pair of used rate windows H exhibits the same temperature for the maxima of the two corresponding peaks. Thus, H gives the impression of an unfinished peak that is a peak which suddenly fails to complete itself, as if a structural rearrangement happens during the thermal 719

DLTS S P E C T R U M OF SILICON

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Fig. 1. The DLTS and the MCTS spectrum of borondoped pulled silicon after 1.5 MeV electron irradiation at 80 K. The solid curve was taken with increasing temperature and the dashed curve with decreasing temperature. Peaks a, b and c represent three different appearances of the feature H on the spectrum (rate window = 1 s- l ). emission of the trapped carriers. On the other hand H has not the expected behaviour of a real DLTS signal caused by the trapping and emission of carriers from a deep center. It always emerges in the temperature range of 105-120K irrespectively of the used rate windows which operate between 0.4 s-l and 2500 s -l. The position of its peak maximum does not shift to larger values when the used rate window increases speed, in sharp contrast with the DLTS theory. Thus, for the pair of the two fastest available rate windows of 1000s -~ and 2500s -1 the trace of H arises on the left side of the Ev + 0.13 eV level which as a normal peak has shifted towards larger temperatures. The third main trait of H is the fact that after its first detection it cannot be traced again in every subsequent scan with the temperature cycling in the range of 80-180 K. The feature arises again as the temperature of the specimen is getting larger than 180 K. Another specimen cut from the same wafer was irradiated under similar conditions as the previous one. Subsequent DLIS scans showed again peak H. We studied this peak by changing the filling pulse amplitudes in the range of 0 to 1 V. We also changed the ratio tp/T, where tp is the width of the filling pulse

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and T its period. There was no indication of any particular change in the general behaviour of H. Another scan was momentarily stopped on the H peak. Its height was seen to diminish strongly and the peak disappeared in about half a minute. We also applied the standard procedure which allows to detect, if there exist, metastable configurations of a defect [10]. It has been reported that the Ev + 0.34eV level exhibits charge-dependent peak amplitudes [11]. From Fig. 1 one may jump to the conclusion that a complementary behaviour exists between feature H and that of the Ev + 0.34eV level. However, this is not correct since if the temperature of the specimen is restricted below 180 K feature H fails to appear although the Ev + 0.34 eV level continues to display its charge-dependent characteristics. Furthermore, feature H is present in the spectrum after the annealing out of the Ev + 0.34 eV level. A careful study of the minority carrier trap spectrum (MCTS) has shown the presence of a signal labeled E (Fig. 1) in the same temperature region behaving in the same way as peak H in the DLTS spectrum. Annealing studies have shown that the feature disappears after a couple of days at temperatures larger than 315 K. Afterwards, the experiment was repeated for a third time. The emergence of feature H and its behaviour was identical with that of the previous experiments. By using the capacitance offset unit we compensated the quiscent capacitance of the diode to increase the sensitivity of the measurement and then the specimen was subjected to C-T tests. Figure 2 gives the obtained curves as the specimen has been cycled six times between 80 to 220 K. The small step on the C-T curve appears always under the same conditions as those which favour the appearance of H on the DLTS spectrum. The result proclaims that these two should be unquestionably related moreover since both of them are absent before the irradiation. On the other

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Fig. 2. The C-T curves for the pulled silicon after irradiation. The temperature was cycled six times in the temperature range of 80 to 220 K.

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DLTS SPECTRUM OF SILICON

hand there are many reasons to believe that peak H is a result of the low temperature iradiation damage of our pulled material. Firstly, it does not appear on the spectra of room temperature irradiated specimens cut from the same pulled silicon wafer. Secondly, it does not appear on the spectra of LN2 or room temperature irradiated specimens of similar resistivity borondoped, float-zone material. Finally, we exclude all possibility of bad instrumentation since feature H is always absent before irradiation. A precise interpretation of the bizarre behaviour of this new feature requires extensive sophisticated numerical work. Nevertheless, a qualitative interpretation can be attempted based upon the aforementioned experimental information. The fact that peak H is observed only in the pulled material plainly indicates that an "oxygen factor" is involved or at least affects the formation of this structure and even its intriguing manifestations on the DLTS spectra. It allows us to assume that its existence should be related to the excess interstitial oxygen and the energetics of its possible trapping sites in the crystal. We know that the silicon lattice is more distorted in the pulled material than in the float-zone due to the excess interstitial oxygen in the first. This means that the oxygen in the two silicon lattices has different "surroundings" which in turn may effect the general behaviour of the oxygen. In other words the content of the oxygen and its surroundings inside the lattice influences its dynamical behaviour and specifies certain differences between pulled and float-zone material. Let us consider now the pulled silicon. We assume that part of the oxygen which is weakly bound to its neighbours can diffuse to other sites of the lattice as the temperature increases. The increase of the temperature activates the oxygen which now can move to energetically more favoured sites; the old oxygen trapping positions may have become energetically unfavoured due to the creation of crystal defects by the radiation. The observed temperature of the sample Tdiff = 180 K over which the sample must be heated before decreasing the temperature in order to observe the new peak specifies the activation energy for the oxygen diffusion. On the other hand the oxygen diffusion should not be affected by the diffusion of the other defects induced by the irradiation. If this is satisfied, it can justify the experimental observation which specify the irradiation temperature of 80 K for the new peak to appear. Otherwise, if the activation energy of the oxygen were not smaller than the activation energy for diffusion of the other defects, one could expect the sample to come back to its standard form after arising the temperature at values greater than the corresponding to the activation energies for

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the defects. As a result of the above assumptions the appearance of the new DLTS peak during decreasing the temperature of the sample from values above 180 K can be attributed to the trapping of the oxygen at other than the usual positions of the crystal. These positions may be referred to as "decorated vacancies" (vacancies with other interstitials next by), unrelaxed vacancies etc., which depend on annealing conditions, radiation damage etc. During the increasing temperature scanning mode peak H cannot be traced since the sample needs to get to at least 180 K in order the oxygen to become again energetic. The amplitude of the peak depends on the amount of oxygen and the available abnormal sites for its trapping, created in every temperature cycle. Another thing: as the decreasing temperature of the sample approaches the region of 105-120K a structual change should take place in the defect causing its rapid disappearance from the spectrum. The activation energy of the corresponding electrical level introduced by this defect, it happens to specify a region in the temperature axis of the DLTS spectra which coincide with the temperature where this structural change occurs. Assuming this to be true we have a rationale to explain the appearance of the peaks in the same positions irrespectively of the used rate windows. Simply in every spectrum the obtained peak only verify the phenomenon which partly represents. Thus the standard DLTS theory and specifically the Arrhenius relation cannot be applied here since the obtained temperatures for the peaks maxima are inaccurate. This is verified by our experimental findings where different scans carried out with the same rate window gave peak H at different positions and with different shape. Another thing: we have observed that feature H, the first days after the irradiation, appears at a temperature range which is lower than that where it is stabilised later. We may attribute this to the change of the oxygen surroundings in the crystal lattice due to the migration of some of the primary defects and the creation of new ones after some temperature cyclings in the region 80-220 K. Thinking in the same line we can also explain the lack of appearance of feature H after the room temperature irradiation. Firstly, it is obvious that the crystal lattice and more specifically the oxygen surrounding after the room temperature irradiation is different than that after the 80 K irradiation due to the different kind of created defects. Secondly, the condition for the oxygen activation energy for diffusion in comparison with the other present defects may not be valid in this case, hindering the creation of the phenomenon. In concluding, we can say that the new signal

DLTS SPECTRUM OF SILICON

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refers to additional levels in the gap which are associated with the various possible configurations of a site which is offered for oxygen trapping.

Acknowledgements - - I am obliged to the Physics Department of Reading University for providing the research facilities and particularly to Dr. P.C. Banbury for valuable guidance and cooperation during the experiments. I also thank Dr. A.N. Andriotis for discussions related to the interpretation of the data.

3. 4. 5. 6. 7. 8. 9.

REFERENCES 1. 2.

D.V. Lang, J. Appl. Phys. 45, 3023 (1974). G.L. Miller, D.V. Lang & L.C. Kimerling, Ann. Rev. Mat. Sci. 7, 377, (1977).

10. 11.

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E. Meijer L.A. Ledebo & Z.-G. Wang, Solid State Commun. 46, 255 (1983). S.K. Brierley, J. Appl. Phys. 59, 168 (1986). A. Sibile & A. Mircea, Phys. Rev. Lett. 47, 142 (1981). J.A. Van Vechten, C.M. Ransom & T.I. Chappell, J. Phys, Soc. Japan 49, Suppl. A. p. 251 (1980). M.C. Chen, D.V. Lang, W.C. DautremontSmith, A.M. Sergent & J.P. Hardison, Appl. Phys. Lett. 44, 791 (1984). D.V. Lang, J.D. Cohen & J.P. Horbison, Phys. Rev. B25, 5285 (1982). C.A. Londos, Phys. Status Solidi (a) 92, 609 (1985). J.L. Benton & M. Levinson, Defects in Semiconductors II, p. 95 (Edited by Mahajan and J.W. Corbett), North-Holland, New York, (1983). C.A. Londos & P.C. Banbury, J. Phys. C. 20, 645 (1987).