Radiation effects in solids

NUCLEAR

INSTRUMENTS

AND

METHODS

109-I18;

28 (1964)

© NORTH-HOLLAND

PUBLISHING

CO.

R A D I A T I O N EFFECTS IN SOLIDS* J. J. L O F E R S K I

Division of Engineering, Brown University, Providence, R.I. Some of the special demands made on a Van de Graaff machine intended for solid state research are discussed. Voltage calibration, a voltage stabilizing circuit, a current ratio recorder, beam deflection and pulsing and vacuum problems are all described.

A brief description of some recent experiments performed with electron accelerators is offered. These include studies of damage thresholds, recombination radiation and solar cell radiation damage studies.

1. Introduction

ments of the minority carrier lifetime z in semiconductors with low values of z. Some transient effects, like cathodo-luminescence can be used to study the properties of semiconductors and insulators whether or not permanent radiation effects are involved. By far the largest amount of work on radiation effects in solids has been expended on the study of permanent changes in the bulk properties of irradiated materials. The earliest work on radiation effects was performed with nuclear reactors as radiation sources and was motivated by the need to determine the effect of reactor produced radiation on the mechanical properties of constituent materials. However, the radiation in a nuclear reactor is composed of neutrons (fast and thermal), gamma rays and beta rays so that the changes in the irradiated materials were extremely complex and not amenable to easy interpretation. Particle accelerators have obvious advantage as radiation sources for fundamental radiation damage experiments. By means of such accelerators it is possible to study radiation defects of gradually increasing complexity. Thus electrons of a few keV can transmit only a few tens of electronvolts energy to atoms in a solid and they produce the simplest kinds of defects, i.e., vacancy-interstitial pairs (Frenkel defects). As the electron energy increases, the energy transmitted to a lattice atom increases until the primary atom carries sufficient kinetic energy to produce secondary displacements, so that a number of displaced atoms result from a single collision between an electron and a lattice atom. Protons of only a few hundred electronvolts have sufficient energy to displace an atom from its equilibrium site so that proton beams with energies of a few hundred keV produce a considerably more complex damage than electrons of comparable energy. The average number of secondary displacements increases with proton energy so that even more complex defects are produced in the solid. One of the developments of the past few years has been an increased interest in the effects of protons on semiconductors because of the

This paper will be concerned with the use of charged particle accelerators in studying the effects of radiation on the properties of solids and solid state devices*. Accelerator beams make possible the introduction of relatively simple structural and, in some cases, chemical defects into solids. Studies of these defects and the defects resulting from their interaction with defects already present in the solid have made and continue to make important contributions to our understanding of the properties of solids. The paper begins with a brief discussion of the effects of radiation in solids which is followed by a discussion of the requirements imposed on an accelerator intended for solid state research. Specific solutions to some problems arising in this area are presented in this section. The final section offers a brief description of some recent work on radiation effects in semiconductors.

2. General remarks concerning radiation damage As is well known, high energy particles can produce both transient and permanent changes either at the surface or within the volume of a solid. In a sense practically all radiation produced changes in a solid are transient since they can be annealed by heat treatment. For the purpose of our discussion, however, a transient effect is one which continues only as long as the radiation persists. Thus, in a semiconductor, the increase in electrical conductivity which is associated with the ionizing properties of the radiation is a bulk transient effect. This particular effect is the basis of solid state particle counters and of solid state "ionization chamber" radiation detectors. The bulk transient effect is the basis of the use of electron accelerators for measure* This m a n u s c r i p t was prepared a n d s o m e of the w o r k described in this p a p e r was p e r f o r m e d with financial s u p p o r t f r o m the U.S. Air Force u n d e r C o n t r a c t No. A F 33(657)-8974. i Papers dealing with these topics have also been presented at the two previous Accelerator Conferences. See W. L. Brown, Nucl. Instr. a n d Meth. 5 (1959) 234 a n d P. Baruch, Nucl. Instr. a n d Meth. 11 (1961) 196.

109

iv. ELECTRON RESEARCH

110

J . J . LOFERSKI

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low energy protons cannot be fabricated. On the other hand, electron experiments often allow the option of irradiation via a window or inside the machine vacuum.

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discovery of intense proton radiation in the regions of outer space traversed by earth satellites. As for surface effects, very little work has been done in this area. It is known that in the case of semiconductors radiation whose energy quanta are too low to produce bulk damage can induce changes in surface properties. The fact that the Telstar satellite experienced a failure because of the effect of radiation on the surface of a transistor has excited interest in this area. It is interesting to point out that it is possible to pursue significant investigations of the effects of radiation on solids even with accelerators whose maximum energy is only 400 keV. Electrons of energy < 400 keV produce permanent defects in Si, Ge, GaAs, lnP, lnAs, ZnSe, CdS and many other materials. Protons with energy < 400 keV can of course produce damage in any material but the use of such protons is complicated by their low range in solids. To illustrate this point, fig. 1 shows range-energy curves for electrons in a group of interesting materials while fig. 2 and 3 show such data for protons. Thus, 400 keV protons have a range of 4 to 5 microns in Si, A1, Ge and Pb, whereas electrons of this same energy have a range of hundreds of microns in these same materials. This means that all proton experiments with energies below 1 MeV must be performed inside the machine because windows thin enough to transmit such

If the accelerator is simply being used to introduce defects into a solid, it is often not necessary to have precise information about the energy or current of the particles. This is especially true for both proton and electron beams of higher energies where 10% changes in energy will not produce any significant changes in the nature or concentration of the defects. If, however, one is interested in using electrons to study the fundamental defect production process which involves measurements as a function of bombarding energies, then it is of crucial importance that the machine energy be known accurately (within a few per cent) and that the machine energy remain constant during the course of a bombardment which must often continue for periods of a few hours. First, consider the problem of calibration. An electron accelerator whose maximum energy is less than 1.65 MeV is incapable of causing nuclear reactions and it is therefore usually calibrated by some method of range measurement. Sandwiches consisting of aluminum foils 500

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R A D I A T I O N EFFECTS IN SOLIDS

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with some sort of sensitive paper in between are commonly used for this purpose, but such a measurement is usually good to only -+-10~o on a 1 MeV machine and it is even worse in the case of a 400 keV machine. The use of a rocksalt single crystal to provide a "photograph" of the penetration profile is an improvement over AI foils. In this case, a piece of rocksalt is irradiated at some specified energy in a machine which was calibrated by a nuclear reaction or an analyzer. A second piece is irradiated in the beam of the machine

which is being calibrated. The penetration of the beam in the two pieces is compared with the help of a traveling-stage microscope and in this way the energy of the unknown beam is deduced. Of course, the best way to calibrate a machine is to have the machine produce a nuclear reaction whose threshold energy has been established by electrostatic or magnetic analyzers. For energies below 1.65 MeV, this means that the machine should be capable of delivering protons. There are many (PT) reactions which occur for energies below 1 MeV1). A number of reactions occurring at energies below 500 keV have also been reported2). For example, Bll(pT)Cl2 occurs at 163.8 ___ 0.3 keV, F'9(pT)Ne 2° at 224.4 + 0.4 keV and LiT(pjBe 8 442.4 _+ 1.5 keV. These reactions are easily detected with a simple particle counter. It is especially useful to point out that a LiF target irradiated by protons yields two points for calibration of, say, a generating voltmeter which can then be used as a secondary standard. Now let us consider the problem of energy control. If the controls of a Van de Graaff machine are set for a certain energy, it is usually found that the energy drifts in a random fashion around the desired energy. Such drift may be of the order of +_ 10~. A way of circumventing this problem is to design a feedback circuit which receives a signal from the generating voltmeter, compares it to a fixed reference and then adjusts the charge sprayed on the belt to correct the energy. There are a number of ways of accomplishing this goal and fig. 4 shows a scheme devised by Mr. David Linhares* to stabilize the output voltage of a 400 keV Van de Graaff machine at Brown University. In this circuit the input from the generating voltmeter is fed

* Presentaddress: Electrical Engineering Department, Rensselaer Polytechnic Institute, Troy, New York.

0 J - B. Marion, Rev. Mod. Phys. 33 (1961) 139. 2) S. E. Hunt, Proc. Phys. Soc. (London) A65 (1952) 982.

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into a collector modulated d.c. amplifier which produces a 60 cps output. The modulated carrier is compared to a 60 cps reference which is 180 ° out of phase with the modulator output. The sum o f these two voltages is added to another 60 cps signal which is in phase with the reference 60 cps signal. If the modulator voltage exceeds the reference, the third signal is decreased and vice versa. This third, corrected voltage is fed through a power amplifier whose output controls the charging screen power supply o f the Van de Graaff. The effectiveness of this stabilizing network is shown in fig. 5 which shows the generating voltmeter output with and without the stabilizing circuit. At 100 keV the voltage with stabilization varies by _+ 2% as compared to ___ 6% without the circuit. At energies in excess o f 2 0 0 k e V , long term stability o f _+ 1% has been achieved with this simple circuit. 3.2. CURRENT VARIATIONS

Beam current variations are not troublesome if one is interested in measuring the effect of an integrated flux of electrons. For instance, if it is required to determine the change in conductivity resulting from a certain a m o u n t o f radiation, one can use a commercially available current integrator to measure the flux. It should be pointed out that semiconductor and insulator irradiations usually involve low current d e n s i t i e s (10 - 9 A / c m 2 for protons, 10 - 6 A / c m 2 for electrons) so that the integrator must be able to handle

such low values of beam current. Usually a Faraday cup is used to measure the flux because measurement of the current stopped in a metal plate can be complicated by reflection of incident particles, secondary emission, etc. It is especially difficult to measure beam currents if the F a r a d a y cup is not in an evacuated chamber because of ionization effects in the surrounding gas. It is also quite difficult to measure the proton beams o f 10 - 9 A and less, unless extreme precautions are taken with shielding the F a r a d a y cup. If, however, the experiment involves the transient effects associated with charged particle induced ionization, then it is extremely important that the beam current be constant or that some scheme be devised for measuring the ratio of beam current to the effect of ionization. Fig. 6 shows such a circuit which has been used in the study of the electron-voltaic effect in a semiconductor p-n junction. In this circuit, the ionization produced by the electrons results in a current I s flowing t h r o u g h the low resistance Re while the beam current IB passes to ground through the larger resistances Ral and RB2 and the ammeter A. Usually Is is three to four orders o f magnitude greater than IR. A null detector G measures the voltage between X and ground and changes the value o f R,2 to re-establish the null. The value of RB2 c a n then be used to measure the ratio of l s / I a which changes as radiation defects are introduced into the specimen. A variation on this system can be used in cathodo-luminescence experi-

RADIATION

EFFECTS

ments where the total luminescent output plays the role of Is. In summary, it appears that the beam energy can be stabilized with relative ease. This usually leads to a stabilization of beam current. The remaining variations of beam current cannot be eliminated so easily and one must design the data recording to circumvent the changes in I B. 3.3. BEAM POSITIONING AND FOCUSING

Usually the specimens involved in solid state radiation experiments are small so that efficient use of the beam makes it necessary to build deflection and focusing circuits. Electrostatic deflection plates are

113

IN S O L I D S

can be lowered into the beam or raised while beam current and energy are being adjusted. This is accomplished by evacuating or pressurizing the space between a pair of bellows at the top of the apparatus. The apparatus can be used for irradiation at liquid helium temperature. It includes a shutter which can be positioned to interrupt the electron beam without shutting off the machine; a Faraday cup for measuring the beam current and provision for placing a phosphor screen in the region to be occupied by the sample during the beam positioning procedure. A magnet with holes in its pole pieces provides a magnetic field at the sample position so that Hall coefficient measurements can be made without moving the sample. The sample

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Fig. 6. Schematic diagram of circuit for recording ratio probably easiest to construct. If one uses 10 cm long plates separated by about 2 cm, the voltage needed to deflect the beam through about 10 cm at a distance of about 1 meter is of the order of a few kV. Some sort of system for observing the beam is also very helpful. A phosphor can be applied in the target area and viewed by an arrangement of mirrors and telescopes or else by a closed circuit television system. For some experiments, the electron beam needs to be chopped or else beam pulses are required. Electrostatic deflection plates provide a simple and convenient way of performing both of these tasks but it should be noted that the amplitude of the pulse or of chopping voltage must be of the order of 1 kV if one is to produce a useful beam deflection. 3.4. REPRESENTATIVE IRRADIATION CHAMBERS

Fig. 7 is a cross section diagram of an irradiation chamber used at RCA Laboratories for Hall coefficient and conductivity measurements*. The sample mount

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mount is provided with a heater so that observations can be made as a function of temperature. Fig. 8 shows a simple kind of irradiation chamber which consists of a 4" i.d. Pyrex cross fitted with plates which perform various functions. One of them has a thin A1 window to admit the high energy particles; another, a glass window to admit light; a third provides sample support and cooling and the fourth permits evacuation of the apparatus. This leads us to the question of the kind of vacuum required in irradiation experiments. 3.5. CONCERNING VACUUM SYSTEMS

Mercury or oil diffusion pumps coupled with mechanical forepumps have been the standard building blocks of vacuum systems in Van de Graaff accelerators. For most applications, especially those involving studies of bulk effects such vacuum systems are * This c h a m b e r h a s been used by R. L. N o v a k in a study o f d a m a g e in p- a n d n-silicon. IV. E L E C T R O N

RESEARCH

114

J. J. L O F E R S K I

adequate. However, if one is studying the effect o f r a d i a t i o n on surfaces or if one is studying the effects o f low p e n e t r a t i o n r a d i a t i o n , then c o n v e n t i o n a l v a c u u m systems generate certain problems. Oil v a p o r s can condense on the surface o f the specimen a n d a c a r b o -

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range, which can lead to e r r o n e o u s i n t e r p r e t a t i o n o f the data. These v a c u u m p r o b l e m s can be eliminated by the use o f an ion p u m p i n g system coupled with s o r p t i o n forepumps. Such a p u m p i n g system can be used to

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naceous deposit can b u i l d - u p as i r r a d i a t i o n progresses. The presence o f such a layer o f foreign m a t e r i a l will lead to spurious results as far as surface studies are concerned a n d since low energy p r o t o n s only penetrate a few microns into the material, the thickness o f the deposit can become an a p p r e c i a b l e fraction o f the

evacuate an i r r a d i a t i o n c h a m b e r which is s e p a r a t e d from the accelerator v a c u u m by a thin a l u m i n u m window. Such a scheme works very well with lowenergy electrons used in surface studies since suitable windows are available for them. It is preferable to have all o f the accelerator tube evacuated by such a p u m p

RADIATION

EFFECTS

but effective use of ion pumps would require replacement of the neoprene rubber O-ring seals with metal gaskets. This in turn would require re-design of the accelerator tube. 4. Review of some recent work on the effects o f radiation on semiconductors

4.1. GENERALREMARKS Probably the most important conclusions which have been reached in the past few years concerning radiation defects in semiconductors is that they are not usually simple vacancy--interstitital pairs, but rather that they

115

1N S O L I D S

vicinity of the radiation damage threshold and from studies of the energy levels of stable defects in Si, both of which will be described in some detail below. Evidence for such complexity of defects in germanium has been extracted from studies of introduction and annealing rates of defects which control conductivity and minority carrier lifetime in that materialS-7). It should therefore not be surprising if some defects in materials other than semiconductors should also consist of something more complex than vacancy-interstitial pairs, especially when one remembers that none of these materials is obtainable with the same degree of structural perfection and freedom from chemical impurities as germanium and silicon.

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Fig. 8. Experimental arrangement for irradiation of solar cells. are complex centers involving interaction between chemical impurities already present in the lattice and the vacancy or interstitial atom produced by irradiation. The strongest evidence for this picture is derived from the studies of electron spin resonance spectra of electron irradiated silicon 3 .4). These studies have shown that the dominant defect in n-type silicon consists of an oxygen atom associated with a vacancy, or if the oxygen concentration is too low, then the defect involves a complex consisting of a phosphorous atom and a vacancy. The electron spin resonance spectra of irradiated silicon silicon are quite complex. They contain evidence for other as yet unidentified complex defects consisting of vacancies, interstitial atoms and chemical impurities. Other evidence that stable radiation defects consist of something more than vacancyinterstitial pairs has been extracted from studies of the introduction rates of defects in n- and p-type Si in the

4.2. RADIATION DAMAGE THRESHOLD 1N n - AND p - TYPE Si The elementary radiation damage event involves the elastic scattering of charged particle by a nucleus. If the energy delivered to the nucleus during this encounter is great enough, the atom can break the bonds which constrain it to remain in its normal position and can then move to some interstitial site. Because the diamond lattice of germanium and silicon is quite open, the energy needed to displace a Ge or Si atom is of the order of 15 eV in contrast to the 22 eV required by a Cu atom in its face-centered cubic lattice. Recently, radiation damage thresholds have been measured in both n- and p-type silicon using the electron-voltaic method in one investigation s ) and the Hall coefficient and conductivity as a function of temperature in another9). For such experiments, machine calibration and energy stability are exceedingly important. In addition, the electron voltaic technique requires, either beam current stability or recourse to a ratio recording circuit like that shown in fig. 6. The change in the ratio IS/IB was measured at various electron energies which resulted in curves like those shown in fig. 9. The slopes of these lines are proportional to the number of stable defects introduced per incident particle. Such slopes are plotted in fig. 10 for both n- and p-type Si. The important conclusions of this work is that there is a difference in thresholds for producing stable defects in 3) G. D. W a t k i n s a nd J. W. C orbe t t , Phys. Rev. 121 (1961) 101. 4) G. Bemski a nd B. S z yma ns ki , J. Phys. Chem. Solids 24 (1963) I.

5) W. L. Brown and W. M. Augustyniak, J. Appl. Phys. 30 (1959) 1300. o) O. L. Curtis Jr. and J. H. Crawford Jr., Phys. Rev. 124 (1961) 1731. 7) j. W. McKay and E. E. Klontz, Proc. IAEA Conf. on Radiation Damage in Solids, (Venice 1962). s) H. Flicker, J. J. Loferski and J. Scott-Monck, Phys. Rev. 128 (1962) 2557. 9) R. L. Novak (Ph. D. Dissertation, Univ. of Pennsylvania, Philadelphia, Pa., 1963). IV, E L E C T R O N

RESEARCH

J.J. LOFERSKI

116

the two conductivity types. The experiments on conductivity changes confirm the difference in thresholds a n d also demonstrate that the defects have a different structure in p- and n-type silicon. This conclusion is based on observed differences in p r o d u c t i o n a n d SILICON N ON P BASE DIODE 321 - I D ^ EXPOSED AREA: 0 3 2 crn~ ,,215 keV Wo: 10.8 eV/PAIR o =

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a n n e a l i n g rates at various temperatures. Both sets o f experiments d e m o n s t r a t e that the shape of the curve relating the probability of producing a defect a n d electron energy is entirely different than that predicted on the a s s u m p t i o n of Rutherford scattering and the p r o d u c t i o n of vacancy-interstitial The electron voltaic effect has been used to study radiation damage thresholds in a n u m b e r of other d i a m o n d lattice semiconductors. The defect p r o d u c t i o n cross section of G a A s 1°) is plotted as a function of electron energy in fig. 11. I n a recent publication, Bauerlein 1~) has reviewed threshold determinations in other semiconductors of similar structure. In general, the energy needed to displace a n atom is even lower in these materials than in Ge or Si. In certain cases it is possible to observe the displacement of one of the

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species of atoms which compose the semiconductor and the displacement of the other at a higher energy. Thus in lnP, 6.7 eV are needed to displace In and 8.7 to displace P; in InAs, 6.7 eV for In a n d 8.3 for As; in InSb, 5.7 eV for In, 6.6 for Sb; in CdS 8.7 eV for S and 10 eV for Cd. In view of the possible differences in nand p-type material and in studies based on conductivity rather t h a n electron-voltaic effect, there is a large a m o u n t of work still to be done on threshold deterruination in semiconductor. 4.3.

RECOMBINATION AND

RADIATION

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If a semiconductor or insulator is b o m b a r d e d by electrons, it emits light (cathodo-luminescence) whose

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10) H. Flicker, J. J. Loferski and T. Elleman, Trans. Prof. Group on Electron Devices, IEEE, ED-11 (1964) 2. 11) R. Bauerlein in Radiation damage in solids, (Academic Press, New York, 1962) p. 358.

RADIATION

117

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spectrum is related to the defects present in the lattice. Such beam bombardment induced luminescence has attractive features when it is considered as a method for studying radiation defects. For one thing, it does not require either ohmic or rectifying contacts to the specimen, which means that many materials are possible objects of study by this technique. Furthermore, the emitted spectrum may yield information about both the concentration and energy position of radiation induced defects. The technique involves irradiation of the specimen at some energy below the threshold for bulk damage production and recording of the emitted spectrum. The beam energy is then increased to a higher value for some specified time and the energy is then dropped to the reference voltage. The beam current and the spectrum are recorded again and examined for changes. An example of irradiation induced changes in GaAs is shown in fig. 12, which is from unpublished work by M. H. Wu of Brown University. This technique 10

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has been used by Kulp and Detweiler 12) t o measure the thresholds for motion of the Cd atom in CdS and by Kulp 13) to measure the damage threshold in ZnSe. In this case, he was able to show that certain lines in the luminescence spectrum of ZnSe increased in intensity as linear functions of the integrated electron flux provided the energy of the incident electrons exceeded a threshold value. 4.4.

IRRADIATION

OF S O L A R CELLS

A review of recent work on the use of accelerators in radiation damage studies m u s t necessarily include some reference to the work on irradiation of solar cells which has been motivated by the desire to continue the use of photovoltaic solar cells as the basic unit of space vehicle power supplies. The silicon solar cell has to reside on the surface of the vehicle if it is to be exposed to sunlight but its position on the skin makes it a vulnerable x2) B. A, Kulp and R. M. Detweiler, Phys. Rev. 129 (1963) 2422. 13) B. A. Kulp, Phys. Rev. 125 (1962) 1865. IV. E L E C T R O N

RESEARCH

118

J. J. LOFERSKI

target for the Van Allen belt electrons and protons. Measurements have confirmed that the particles in the belt consist of electrons with energies up to a few MeV and protons with energies up to the GeV range. The proton intensity drops off exponentially with energy so that it is not necessary to consider the effect of protons with energy in excess of about 20 MeV. It turns out that the Si photovoltaic cell is admirably suited to making fundamental studies of some radiation damage phenomena so that the work on such cells has proven to be of considerable fundamental as well as practical interest. Fig. 13 shows a set of i-V characteristics of a silicon solar cell as a function of integrated proton flux incident on the cell. By analyzing such curves, it is possible to determine the number of defects per cm travel of a particle and to compare these values with existing theories14). One important practical result of such studies with electron accelerators was the observation that n/p Si solar cells exhibit considerably less damage in a given flux than do p/n cells. This is shown in fig. 14. An example of more fundamental work based on Si cells is the comparison between defect introduction rates calculated by assuming that below 10 MeV, the damage production can be attributed to Rutherford scattering while the optical model describes elastic scattering from 10 to 100 MeV~5). Experimental data on silicon cells cover irradiations by protons with energies ranging from a few keV to a few hundred MeV so that comparisons with theory are possible over this wide energy range.

5. Summary and conclusions 1. A particle accelerator intended for studies of radiation defects in solids is required to have its energy calibrated to + 1% or better; long term beam energy stability of a few percent; beam current stability; beam focusing and positioning circuits and, in some instances, specially clean vacua. 2. Circuit for achieving beam energy stability and for circumventing beam current changes have been described. 3. A few recent experiments involving particle accelerators were discussed briefly. There is strong evidence for complex defects resulting from interaction between radiation defects and other defects in the solid. This means that elucidation of the damage mechanism in each material will be an even greater challenge than had previously been imagined. Acknowledgements It is a pleasure to acknowledge contributions to various phases of this work with the accelerator at Brown University by undergraduate students, N.Koren, D. Linhares, and graduate students R. Santopietro and M. H. Wu. I also wish to thank Mr. R. L. Novak, RCA Laboratories, Princeton, N.J., for permission to refer to unpublished work. J4) j. A. Baicker and B. W. Faughnan, J. Appl. Phys. 33 (1962) 3271. 15) j. A. Baicker, H. Flicker and J. Vilms, Appl. Phys. Letters 2 (1963) 104. 16) G. W. Simon, J. Denney and R. G. Downing, Phys. Rev. 129 (1963) 2454.