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PLENARY SESSIONPrimary processes in radiation damage
P. Townsend, F. Agullo-Lopez
To cite this version:
P. Townsend, F. Agullo-Lopez. PLENARY SESSIONPrimary processes in radiation damage. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-279-C6-283. �10.1051/jphyscol:1980671�. �jpa-00220109�
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-279
PLENARY SESSION.
Primary processes in radiation damage
P. D. Townsend and F. Agullo-Lopez (*) University of Sussex, Brighton, BNI 9QH, U.K.
Résumé. — Dans cet article, une revue des mécanismes les plus probables, produisant des défauts intrinsèques dans les isolants et semi-conducteurs par absorption des photons ou des électrons à basse énergie, est présentée. Des informations générales sont données pour ces solides, mais seul le cas des halogénures alcalins est examiné en détail, grâce aux résultats expérimentaux et théoriques.
Abstract. — This paper surveys the low energy processes which might lead to the generation of intrinsic defects in insulators and semiconductors. Several clear guidelines are apparent but only for the alkali halides has there been sufficient experimental and theoretical work to study the defect mechanisms in depth.
1. Introduction. — Our understanding of colour centres is biased towards those materials which are simple in structure and also are easily damaged.
Therefore it is not surprising that when we consider low energy mechanisms which can produce lattice defects we find our attention again focusses on the same materials [1-7]. It is possible to produce colour centres by charge redistribution among existing defects without causing intrinsic damage. These effects are the only ones observed in simple and mixed oxides. For this survey such effects are ignored and we shall concentrate on processes which lead to measurable defect production in regions of perfect material. We will try to emphasize models which are or might be applicable to a wide range of materials and point out the physical conditions apparently needed to induce efficient damage in ionic materials.
On the other hand the role of sputtering experi- ments [8-12] and fast pulse radiolysis techni- ques [13-18] to understand the basic mechanisms of damage will be stressed. However one should remark that the transient defects obtained through pulse radiolysis experiments need not be the same ones as are detected under steady state conditions. In particular one should be aware that when one com- pares the results of steady state and pulse irradiation experiments some differences are to be expected as energy deposition rates are often dramatically different for both types of experiments.
2. General considerations. — We can state some general guidelines for the operation of a low-energy displacement mechanism. These are :
1) There must be a mechanism for the localization of the excitation energy on a single lattice site;
(*) Permanent address : Facultad de Ciencias, Universidad Auto noma de Madrid, Madrid 34, Spain.
2) The lifetime of this localized excitation must be long enough for lattice relaxation and atomic motion;
3) The displacement energy (roughly the binding energy) should be less or comparable with the exci- tation energy which can be taken to be of the order of the band gap;
4) For stable defect formation the vacancy and interstitial need to move apart, perhaps over distances of several lattice units.
2.1 LOCALIZATION OF THE EXCITATION. — Trapping of free carriers is the simplest way to localize the excitation energy and could, in principle, initiate defect production. For example if one considers an insulator of the type A+ B~ then electron trapping at an A+ site would produce a neutral A0 atom which is not electrostatically bonded into the lattice.
This might then thermally diffuse into an interstitial site and separate from the vacancy. One early model of defect formation in Ag Br involved a similar step.
However later data suggests another version of the same basic process, that is the electrons are trapped at the thermally generated Ag+ interstitials after their migration to some special sites at the crystal surface [19, 20]. These processes are probably ope-"
rative in a wide range of photolytic materials including layered dihalide compounds [21] such as Pbl2 and are very likely involved in the photolytic decomposi- tion of such materials as the azides.
The situation can obviously be generalised to situations where irradiation induces additional charging of a lattice site. In our example of the system A+ B " we can imagine that charge capture or multiple ionization would lead to states such as A+ + , B ~ ~ , B+ + etc. There would then be a Cou- lombic repulsion between the ion and the normal neighbour sites which could induce relaxation of the lattice and ejection of the ion. This is essentially
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980671
C6-280 P. D. TOWNSEND AND F. AGIILLO-LOPEZ
the Varley mechanism [22] which was proposed to explain F centre formation in alkali halides. For the halide lattice the mechanism seems inappro- priate (e.g. [3]) firstly because the time for charge capture is less than that needed for lattice ion move- ment and secondly because a charged ion in an interstitial position would sense a large potential energy barrier for diffusion. This example einphasises the role of long-life excitation and an easy pathway for the escape of the primary defect if lattice damage is to be produced and stabilized.
However the above requirements could be ful- filled when one considers a more highly disturbed region of the lattice as will be produced during ion bombardment. Then, a multiple ionization mecha- nism for displacement may become possible and additionally the displacement energy for collision events may be reduced in the ionized or excited region. We should, therefore, look for enhanced damaged production rates in insulators and semi- conductors during ion bombardment 123, 24, 251.
In fact, ionization assisted damage has been claimed in Si and Ge even though neither material will directly form defects by ionization. Silica glass also shows a strong effect from the ionization component of the energy deposition during ion implantation [26,27,28].
Purely ionizing radiation generates defects in silica but with a greatly reduced efficiency [27].
There are many more examples of recombination enhanced defect reactions in semiconductor and insulators. One might mention the radiation-enhance- ment of silicon self diffusion [29] or the dislocation climb in 111-V compounds [30] activated by the injection stimulated motion of point defects. This is a technologicalIy relevant process since it leads to degradation of semiconductor laser devices. The basic physics underlying these effects which can be introduced either by local heating of the lattice or the localized electronic excitation has been recently reviewed by Stoneham [6].
Finally, one should mention that charge trapping could trigger intrinsic lattice damage in dirty crystals although it is not effective in pure samples. One clear example [31] of this is MgO which contains traces of hydrogen, substituting on to Mg sites and forming the V& centre. It has been shown that after electron capture hydrogen can be released to leave a magne- sium vacancy. In fact, the operation of this damage process at weak lattice sites is, very likely, a general phenomenon in inorganic crystals.
2.2 ENERGETICS. - The competition between the energy available from the excitation and the displace- ment energy necessary to create a primary Frenkel pair should roughly determine the feasability of damage production.. For example the high binding energy in oxides such: as MgO (321 is basically responsible for the absence of intrinsic lattice damage in these materials under low-energy excitation. The
energetics of the primary damage event appears to be very critical since even in alkali hali4es, where the colouring efficiency is very high, the energy available in the self-trapped exciton is scarcely sufficient to create F
+
H pairs.It appears that the threshold energy Ed for the lattice damage to many compound semiconduc- tors [33] can become very low and comparable to the band gap energy E,. One would expect that the damage efficiency is closely correlated with the ratio of these two energies Ed and E,. An illustrative example [34, 351 of this behaviour is provided by the series of compounds &Gal-,As because one is rapidly increasing the energy gap with decreasing values of x but the displacement energy is probably less sensitive to x. Light emitting diodes made from these compounds degrade during operation as defects are formed. These develop the dislocation networks and result in a reduction of the light emission effi- ciency [36] at the band gap energy. There is a clear correlation (Fig. 1) between the speed at which the diodes degrade and the value of E,.
10,000 hours
r
Fig. I. -- The graph indicates the operat~ng lifetime of 1 000 A cm-2 LEDs before the intensity degrades by 15 %. The data is based on dlodes of (In, Ga) P ; Ga(As, P) and In,Ga,
-as
(after ref. [34]).The stability of Ge tunnel diodes fits into this trend as the low value of E, (0.67 eV) is insufficient to provide displacement &ergy to the lattice. This view suggests that the more or less rapid failure of GaAs, Gap or ZnTe light emitting diodes is a direct consequence of a low energy defect production mechanism in the material. The details of the process are unknown but it is clear that the electric field enhances defect diffusion and can contribute to defect stabilization. For device manufacture using
PRIMARY PROCESSES IN RADIATION DAMAGE C6-28 1
these materials one must seek a solution where the light is emitted from a stable and large cross-section recombination centre and inhibit any non-radiative defect forming process. The problem is complicated by the electric field and the impurities needed to form the diodes.
3. Exciton models, alkali halides. - Although charge trapping and subsequent relaxation can be an appropriate process to start damage, as typified in a number of photolytic materials, exciton localization and decay, when operative, might lead to a more efficient energy transfer to the lattice.
This is the case in many halide compounds, including alkali halides, divalent [37] halides, and mixed halides (perovskites) [38, 391, where the exciton appears t o b e rather easily localized as some excited state of the molecular halide ion (e.g. Cl;). The self-trapping of the exciton directly stems from the self-trapping of the hole at an anion site. This behaviour, which has an essential bearing on the efficient damage sensitivity of halide compounds appears to be related to the stability of the halogen molecular species X;
inside the crystal matrix. Moreover, exciton self- trapping has been recently shown to apply to a completely different class of materials, the chalco- genide glasses [40, 411, where exciton self-trapping leads to bond breaking and ionic rearrangement.
However, it is in the alkali halides where the details of the process are better known as a consequence of a vast amount of effort over the last 50 years. Although Pooley [42] (as well as Hersh [43] and Vitol [44]) proposed a non-radiative exciton decay mechanism for the primary process of damage, some of the pre- dictions have not been experimentally supported and only recently have some of the subtleties of the process become apparent (e.g. [7]). We shall now comment on the major developments.
First of all, production of Frenkel pairs follows from free-exciton relaxation, there is also some evidence that electron capture by self-trapped holes (V, centres) is an effective alternative [45]. In both cases the system should have enough energy to form a vacancy-interstitial pair (either as F
+
H centres or a+
I centres). At variance with naive predictions based on simple energetics [46], the primary pair has been shown to be the F, H (energy of formation-
7.0 eV for KCl) and not the a. I (energy of formation-
3.7 eV for the same crystal). This has been mainly inferred from the short pulse radiolysis experiments and one can now advance the idea that it probably results as the best choice for the primary pair to become well separated and stabilized.Anyhow the energetics of the system is complex, particularly when one takes into account the multi- coordinate space corresponding to the various possible dynamical modes for relaxation. Figure 2 [7] typifies the energy level scheme as inferred from some of the proposals advanced for defect production. From
e-h
/
v. b. A B C
-
X nn nnnQl
@aC
Q2@Q [1101
SELF TRAPPING DEFECT FORMATION
Fig. 2. - A possible energy scheme for two halide ions whlch trap an exciton, relax and form a defect (after ref. [T).
these diagrams one basically tries to obtain the evolution of the various electronic states during relaxation, the possible crossings of levels and the potential barriers to be overcome on taking the system from the initial excitation state to the final defect state (F f H). There is no reason to suppose that defects are only formed by a single set of states, indeed depending on the mode of relaxation alter- natives would seem reasonable.
Recent calculations [6, 71 indicate that the self- trapped exciton (STE) which is the precursor of the F
+
H pair is neither of the luminescent states (n: or o), but the defect precursor state should lie between them. Support for this result comes essen- tially from experiments where the n emission is observed during the anneaIing of F+
H pairs.However, one should be rather cautious in accepting these experiments as some controversial data have also been reported [47]. Anyhow it can be mentioned here that one well-defined excited state of the STE, obtained by optical excitation from the ground state has been shown to yield F
+
H pairs [48].Some additional interesting data on the energetics of exciton relaxation have been obtained by Itoh et al. [49] using computer simulation on a small crystallite (57 atoms). The main conclusion is that relaxation of the exciton into the F
+
H pair has to surmount a high potential barrier if the hole is in its ground state, whereas the adiabatic potential is essentially flat if it is in an excited state. This result is in accordance with a previous suggestion made by Itoh and Saidoh [50] in order to understand efficient momentum transfer along the ( 110 ) collision chains.At this point one should comment on the very short delay times (-- ps) for F centre production as measured by pulse radiolysis techniques [7].
These experiments have stressed the importance of the very first moments following excitation in deter-
C6-282 P. D. TOWNSEND AND F. AGULLO-LOPEZ
mining the nature of the damage, before any secon- dary reaction might take place. This emphasises the difficulty of direct comparisons between the results of pulsed and steady irradiation experiments so that one should put much care in distinguishing transient and permanent damage. Moreover one has to be aware that the irradiation conditions (energy depo- sition rates) are dramatically different in both types of experiments.
Pulse radiolysis experiments are valuable in indicat- ing the fastest path by which intrinsic defects can be generated. Alternative mechanisms involving longer lived states are still possible but experimentally these may be difficult to separate from secondary processes from the fast components.
It is easily conceivable that some materials can exhibit transient damage, for example in a pico second time scale but not present permanent damage.
This situation appears to be exemplified in the case of oxide glasses where the formation and rapid decay of E' centres under 3 ns. pulses of 500 keV electrons has been reported [51].
Another subject still showing much controversy is that concerning the mechanisms responsible for the separation of the primary Frenkel pair [7, 521. A variety of ways can be envisaged to achieve separation of the F and H partners but the occurrence of efficient collision chains along the ( 110 ) close-packed halogen rows seems now well established [3, 8, 91. At the end of the chain, which also involves charge exchange between the colliding species (XO and X - ) an X0 is left in a faraway interstitial position. However, one should remark that the experiments showing direc- tional sputtering of halogens along ( 110 ) rows are the only direct proof of such a mechanism [53].
Therefore special attention has to be paid to the developments in this closely-related field of low-
Question. - A. E. HUGHES.
Can you please explain why the sputtering experi- ments are done at relatively high temperatures (above RT). Is it possible to carry out similar work below RT, where most other studies of the primary processes of damage have been done ?
Reply. - P. D. TOWNSEND.
Only because at higher temperatures the excess metal evaporates and one thus generates new crystal and produces more sputtering. At low temperature more sensitive analysis (e.g. Auger spectroscopy) confirms the same processes occur.
Question. - H . J . STOCKMANN.
Are there similar experiments or calculations, as you have shown for the alkali halides, available for the alkaline earths ?
energy sputtering. For example since it has been shown that only a fraction of the halogen emission is energetic one may suspect that one of the proposed thermally activated diffusion processes is also operat- ing [lo, 11, 121. On the other hand, the occurrence of directional sputtering along the ( 21 1 ) (and probably
<
31 1 )) directions as well as the regular variation ofthe ratio of molecular to atomic sputtering with crystal matrix adds some new elements to this puzzle.
4. Final remarks. - In this survey we have listed some of the key steps for low energy mechanisms of defect formation (section 2) and in the case of the alkali halides continued to discuss more detailed possibilities. One would like to cite other examples but for intrinsic damage in pure crystals we can only specifL systems in which damage is known to occur, not the details of the process. In very many cases we may be misled by the presence of trace impurities or thermally generated defects which provide an easy route for permanent damage formation.
Even for the alkali halides we still need more information on the mechanism of damage stabilisation at low temperatures ; temperature dependence of the damage process, the kinetics of hot and relaxed exciton motion and their relationships with decay paths via luminescence and displacement. Impurity effects are not yet excluded and certainly more information about the sputtered particles should provide data on the mechanisms of interstitial transport.
Clearly if we are still at this intermediate stage with alkali halides then our understanding of defect processes in other materials needs much more progress. The question is sufficiently intriguing that no doubt some of the answers will be forthcoming as pulse techniques etc., are applied to pure materials.
JSSION
Reply. - P. D. TOWNSEND.
Calculations exist but the F-H pairs tend to be very close, so are thermally unstable. However such' transient defects are detectable by pulse measurements.
Question. - L. W . HOBIT$.
Have you been able to ascertain that, in the sputter- ing experiments, you are looking at the major fraction of radiolysis events in the near surface region, and not some minor component ? To convince me that your directional sputtering observations prove the existence of replacement collision sequences, you would have to demonstrate a v&y high sputtering quantum efficiency.
Reply. - P. D. TOWNSEND.
The sputtering yield is high if one realises that not all the damage occurs in the surface layer. Then for
PRIMARY PROCESSES IN RADIATION DAMAGE C6-283
both electron and photon induced sputtering the E,,,it,,. The FOM experiments show that up to efficiency estimate is between 0.1 and 0.5 particles per 50
%
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