Effect of Proton and Silicon Ion Irradiation on Defect Formation in GaAs

3016 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 6, DECEMBER 2008

Effect of Proton and Silicon Ion Irradiation on DefectFormation in GaAs

Jeffrey H. Warner, Christophe Inguimbert, Mark E. Twigg, Scott R. Messenger, Robert J. Walters,Manuel J. Romero, and Geoffrey P. Summers

Abstract—Electrical and structural changes in GaAs are moni-tored using electron beam induced current (EBIC) and transmis-sion electron microscopy (TEM) measurements after irradiationby protons and silicon ions. It has been determined that higher en-ergy protons (� 10 MeV) and silicon ions disordered regionsthat are electrically and structurally different than those producedby lower energy protons. The data suggest that these disorderedregions are responsible for causing the deviations between exper-imental data and NIEL. From analyses of the recoil spectra, highenergy recoils appear to be responsible for the formation of thesedisordered regions.

Index Terms—Defect formation, disordered regions, displace-ment damage, EBIC, GaAs, heavy ion, irradiation, NIEL, recoilspectrum, recombination centers, TEM.

I. INTRODUCTION

T HE concept of nonionizing energy loss (NIEL) and dis-placement damage dose have been used for at least two

decades to correlate displacement damage effects caused by dif-ferent particles [1]–[7]. The NIEL concept has proven to be asuseful in correlating displacement damage effects as the LETconcept has been in correlating the effects of ionization damageeffects. However, just as some deviations are observed in theapplication of ionizing dose to correlate ionization effects, dis-crepancies have emerged in the application of NIEL in corre-lating displacement damage effects. In particular, studies haveshown that for proton energies 10 MeV the damage coeffi-cients (DCs) for GaAs devices vary linearly with NIEL inde-pendent of the device type or device parameter studied. Thisresult means that the energy dependence of the DCs can be wellpredicted using NIEL and, hence, that the experimental charac-terization of radiation-induced hardness can be done at just one,conveniently chosen, proton energy [8].

Manuscript received July 11, 2008; revised September 05, 2008. Current ver-sion published December 31, 2008.

J. H. Warner is with the Naval Research Laboratory, Washington, DC 20375USA and also with the Department of Physics, University of Maryland, Balti-more, MD 21250 USA (e-mail: [email protected]; [email protected]).

C. Inguimbert is with the ONERA-DESP, 31055 Toulouse, France (e-mail:[email protected]).

M. E. Twigg, S. R. Messenger, and R. J. Walters are with the Naval ResearchLaboratory, Washington, DC 20375 USA.

M. J. Romero is with the National Renewable Energy Laboratory, Golden,CO 80401 USA (e-mail: [email protected]).

G. P. Summers is with the University of Maryland, Baltimore, MD 21253USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TNS.2008.2006266

In contrast, deviations from NIEL scaling have been observedusing protons having energies above 10 MeV. This high energyproton behavior has been observed in GaAs devices since someof the first correlation studies began in the 1980s [1]–[7]. Be-cause the experimentally determined deviations that occur withlinear NIEL scaling appear at proton energies where nuclearreactions start to dominate the displacement damage processand the corresponding cross sections are less well known, itwas initially suspected that NIEL calculations are not accuratein this energy region. However, other groups have calculatedNIEL using different approaches/models and the calculationsagree well, providing confidence in the high energy NIEL cal-culation [4], [9], [10]. This suggests the explanation for the dis-crepancy observed between experimental data and NIEL calcu-lations lies elsewhere. Another possibility lies in the applicationof the Lindhard partition factor in the NIEL calculation [11].The NIEL calculations by the groups described above all usethe Robinson approximation to partition out that portion of therecoil energy spectrum, which directly goes into displacements[12]. A new approach to obtaining an energy partition functionby using local and non-local potentials has been proposed byAkkerman and Barak [13] and demonstrated, for silicon, to givebetter agreement with experimental data over a larger recoil en-ergy range. Weller et al. [14] have used another method totallyignoring the Lindhard partition factor mathematically to calcu-late a quantity like NIEL, called nonionizing energy depositionrate (NIEDR), which incorporates the spatial nature of wherethe energy is deposited in the lattice. Messenger et al. [15] havealso treated this spatiality by introducing a set of parameterswhich determine the validity of the NIEL calculations in sev-eral interaction mechanisms. Although differences in the NIELcalculations have been observed when using the various modi-fied versions of the energy partition function, these differencesstill do not account for the variations between experimental datawith NIEL.

Until recently, there has been no direct experimental evidenceindicating that low and high energy protons produce differentdefect types. Warner et al. [16] showed that very active recom-bination volumes/centers were introduced for proton energies

53 MeV that are spatially inhomogeneous. They also showedthat the defects introduced after irradiation by silicon ions aresimilar to those produced by high energy protons using DLTS.They were able to conclude that nuclear interactions are not thesole cause for the formation of the active recombination vol-umes, because the energy of the silicon ions used in those studiesonly involve Coulombic interactions. Therefore, another mech-anism must be responsible. They concluded that the recombi-nation volumes were created as a result of high energy recoil

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WARNER et al.: EFFECT OF PROTON AND SILICON ION IRRADIATION ON DEFECT FORMATION IN GaAs 3017

effects and that a threshold displacement energy for the forma-tion of cluster type defects exists, but determining an accurateenergy range was difficult primarily because of a lack of in-elastic recoil data. At that time, the nuclear inelastic calcula-tions were simplified because differential scattering cross sec-tions were not available for the recoiling fragments producedduring spallation interactions. This paper continues the researchwith more detailed nuclear calculations and new experimentaldata involving electron beam induced current (EBIC) and trans-mission electron microscopy (TEM).

The primary purpose of this paper is to present new proton-in-duced experimental data for GaAs solar cells and presentnew recoil calculations to determine a possible cause of the for-mation of these recombination volumes. EBIC measurementsare used to monitor spatial variations corresponding to elec-trical changes of the device while TEM measurements are usedto monitor any radiation-induced structural changes in the ma-terial. TEM measurements were performed on the GaAs solarcells that were previously irradiated with silicon ions (22 MeV)and protons at different energies (2, 4, 53, 160, and 227 MeV)[16]. New EBIC data is presented after samples were irradiatedwith 10 MeV protons and compared with the existing data re-ported by Warner et al. [16].

II. EXPERIMENTAL DETAILS

The samples used in this study were 0.5 0.5 cm GaAssolar cells grown on n-type GaAs substrates by Molecular BeamEpitaxy (MBE). A detailed description of the solar cells is givenin [7]. The wafers were grown and processed at NRL.

EBIC measurements were performed on samples irradiatedwith 10 MeV protons having fluences ranging from to

cm at the University of Montreal radiation facility andalso performed on a sample previously irradiated with 227 MeVprotons to allow for a quantitative comparison. The new EBICdata in conjunction with the previous data in [16] provides afairly comprehensive data set showing electrical changes thatoccur in GaAs solar cells after irradiation with 2, 4, 10, 53,and 227 MeV protons and 22 MeV silicon ions. The structuralchanges were monitored using TEM. Results will be presentedfor the proton and silicon ion energies given above. A detaileddescription of the EBIC and TEM measurement setup and pa-rameters is given below.

EBIC measurements were performed using a modifiedsample holder of a JEOL 5800 scanning electron microscope(SEM) system. An electron beam energy of 10 keV with abeam current of 250 pA was used to generate the EBICmicrograph data. The EBIC method is also known as chargecollection scanning electron microscopy. EBIC images maybe considered as maps of the short circuit current density withbright and dark regions corresponding to regions of high andlow collection efficiency, respectively.

TEM measurements were performed using the HitachiH-9000UHR high-resolution TEM (HRTEM) operating at anaccelerating potential of 300 kV. The Scherzer resolution ofthe instrument is 0.19 nm and is more than sufficient to imageextended defects and inclusions in materials. Imaging wasperformed on cross sectional TEM (XTEM) samples preparedby mechanical lapping and Ar ion milling with an accelerating

Fig. 1. EBIC images for � � GaAs solar cells irradiated with 10 MeV protonsat different fluence levels: (a) �� , (b) �� , and (c) �� cm . The imageshown in (d) corresponds to 227 MeV protons at a fluence of 2.5�� cm .The image areas are 1156 �m .

voltage of 5 kV at liquid nitrogen temperature. XTEM sampleswere oriented for a beam direction perpendicular to thethinned sample. The zone axis includes two sets ofdiffracting planes. Because the spacing between planesin GaAs is 0.326 nm, and well within the 0.19 nm resolution ofthe instrument, both sets of planes are resolvable in theHitachi HRTEM.

III. EXPERIMENTAL RESULTS

A. EBIC Results

The EBIC micrographs generated from solar cells irradiatedwith 10 MeV protons at increasing fluences are shown inFigs. 1(a)–1(c). These images reveal regions of contrast inwhich dark regions are separated by brighter regions. The darkregions are active recombination volumes. These regions willbe referred to as recombination centers or localized defects. Itcan be seen that the density of dark spots increases with in-creasing proton fluence. This is in agreement with what wouldbe expected if these spots corresponded to radiation-induceddefects. Warner et al. [16] previously published EBIC data onnominally identical devices after irradiation with high energyprotons and silicon ions. In order to make a quantitative com-parison with those data, the sample irradiated with 227 MeVprotons was re-measured under identical EBIC conditions asthe sample irradiated with 10 MeV protons. The result is shownin Fig. 1(d). The active recombination centers are clearly ob-served in the image which agrees with the previous conclusionsgiven in [16]. However, one of the major differences is that thespots in the 227 MeV image are much darker compared with


Fig. 2. A weak beam g-3g image (dark field) measured on NRL � � GaAssolar cell irradiated with 227 MeV protons with a fluence of � � �� cm .The bright spots correspond to radiation-induced defects. The circle in the figuresurrounds one of these defects.

those observed in the 10 MeV proton images. This is relatedto the defect contrast or defect strength. The defects contrastis defined as where and is the signal atthe defect position and in the background, respectively [17].Although the spots in the image do typically vary in contrast,we can approximate the defect strength at the two protonenergies for comparison purposes. We find that the defectstrength is approximately 10% and 70% (corresponding to thedarkest spots) for the 10 MeV and 227 MeV proton images,respectively. This result implies that the defects generated by227 MeV protons are much more electrically active than thoseproduced by 10 MeV protons. This result will be discussedfurther in the discussion section.

B. TEM Results

Weak beam TEM imaging performed on samples prior to irra-diation and after irradiation with 2 and 4 MeV protons revealedno defect related contrast. However after irradiation with pro-tons having energies MeV, the situation is much different.An image corresponds to a sample irradiated with 227 MeV pro-tons with a fluence of cm is shown in Fig. 2. The g-3gweak beam image using g 004 revealed visibly damaged re-gions as represented by the bright spots in the image. One suchregion is surrounded by the circle. The direction of the irradi-ating beam is incident at the top of the image. The strained re-gions around the radiation-induced defects give rise to signifi-cant contrast allowing the defects to be observed using this tech-nique because it is sensitive to small regions of large strain gra-dient, i.e., variations in strain. These defect regions were plen-tiful from top to bottom of the sample, an observation in ac-cord with the calculated range for a 227 MeV proton in GaAs( 90 mm). Clearly, the 227 MeV protons are able to traversethe thickness of the wafer ( 300 m) uniformly with minimalenergy loss.

Fig. 3 corresponds to the weak beam TEM image for thesample irradiated with 22 MeV Si ions with a fluence of 2

ions/cm . The direction of the irradiating beam is incidentat the top of the image. This irradiation also resulted in the pres-ence of nanometer size inclusions similar to those observed inFig. 2. TEM images were recorded at different depths below thesurface, with the result that the inclusions were only observedover a distance of 7 m from the surface—an observation inagreement with the predicted range for a 22 MeV silicon ion in

Fig. 3. A weak beam g-3g image (dark field) measured on NRL � � GaAssolar cell irradiated with 22 MeV Si ions with a fluence of �� cm . Thebright spots correspond to radiation-induced defects. The circle in the figuresurrounds one of these defects. This image was recorded approximately 3 �mbelow the surface. The fringes in the figure correspond to thickness fringes. Theblack region to the right in the image corresponds to the edge of the sample(hole).

GaAs. These Si ion- induced inclusions appeared to be charac-terized by smaller strain gradients than in the high energy protonirradiated samples. For this case, g-3g weak beam imaging wasonly able to reveal the presence of inclusions when using thestronger g 220 reflection instead of the g 004 reflectionwhich was used for the proton irradiated samples.

The weak beam imaging results are important because theyshow that strain-related defects are produced only under cer-tain particle irradiation conditions. However, these results donot provide any information regarding the nature of the defectsintroduced. Therefore, high-resolution TEM was used to deter-mine the nature of the defects seen in the weak beam images.Fig. 4 is the lattice image from a GaAs solar cell after irra-diation by 227 MeV protons. The corresponding weak beamimage was presented in Fig. 2. The HRTEM image is essen-tially a cross hatch of lattice planes, and is referred to asa lattice image. The presence of the lattice image allows atomicscale features such as inclusions and extended defects to be rec-ognized. There is a significant contrast in the image with theradiation-induced damage regions being observed in the latticeimage as dark circular areas (spheroids) ranging from 1 to 3 nmin diameter. The variation of defect size is assumed to be re-lated to the statistical nature of the ion-target interactions. Theradiation-induced damage regions will be referred to as disor-dered regions. In most cases, the atomic rows in the disorderedregions in first approximation appear as near-perfect crystallineorder (i.e., the lattice fringes are easily seen) which is coherentwith the surrounding matrix. This is strong evidence that thedisordered regions are not amorphous. This is a very important


Fig. 4. HRTEM lattice image of NRL � � GaAs solar cell irradiated with227 MeV protons with a fluence of �� cm . The white circle surrounds aregion corresponding to a radiation-induced defect that is approximately 1–3 nmin size.

observation because it is well known that under certain irradi-ation conditions that GaAs can undergo a phase change fromcrystalline-to-amorphous [18] or by direct impact of energeticions [19].

C. Contribution of Recoil Spectra in NIEL Calculations

Warner et al. suggested in a previous paper that the localizeddefects observed in EBIC micrographs are produced by highenergy recoils [16]. In any given radiation-induced nuclear re-action, a number of different recoil fragments are created witheach having a distribution of energies. In high energy protonNIEL calculations, to estimate the nuclear interactions, a sim-plified approach was undertaken in the past based on the av-erage recoil energy and total cross section approximations [3].There was no consideration of the actual recoil species and en-ergy spectra. The purpose of this section is to explore certain as-pects of the displacement damage process that are important inthese analyses. This will include: 1) a comparison of the primaryrecoil spectra for Coulombic interactions at various particle en-ergies; 2) an example comparing the Coulombic and nuclear re-coil spectra as a function of recoil energy for a specific highenergy proton case; and 3) an investigation of the overall influ-ence of the recoil spectra on the cumulative damage induced fol-lowing particle irradiation as a function of recoil energy takinginto account both Coulombic and nuclear interactions. It shouldbe noted that throughout the literature the differential scatteringcross sections and recoil spectrum are used interchangeably (al-though in some textbooks the differential scattering cross sec-tion is defined as a function of scattering angle ).

A comparison of the primary recoil spectra for protons at dif-ferent energies and 22 MeV Si ions incident on GaAs is shownin Fig. 5. These data were calculated using the Ziegler, Biersack,Littmark (ZBL) screened Coulomb potential [20]. From Fig. 5,it is observed that: 1) each spectrum are weighted more toward

Fig. 5. Calculated recoil spectra for 2, 4, 10, and 227 MeV protons, and 22 MeVSi ions on GaAs for Coulombic interactions only. It is important to note that theending point of each curve in the figure corresponds to the maximum recoilenergy for that incident particle type and energy, and it is not merely the termi-nation point of the calculation.

lower energy recoils; 2) the 22 MeV Si ions produce more re-coil events at a given recoil energy throughout the recoil energyrange compared with protons; and 3) in some cases the recoilspectra contains energies MeV, which are considered to behigh energy recoils in this paper. It should be noted that the ter-mination of the lines in the figure corresponds to the maximumrecoil energy produced for the incident particle and is not arbi-trary.

The recoil spectra involved in nuclear interactions are usu-ally difficult to find in the literature. Some Monte Carlo codescan produce some of the data. MCNPX [21], for example, tal-lies many recoil species following high energy proton transportsimulations, but it only tallies the recoil energy spectra of el-ements up to an atomic number of 2. GEANT4 [22], on otherhand, does tally higher atomic number recoils and gives the as-sociated recoil energy spectra. Thus, for initial illustrative pur-poses, a comparison is made between the Coulombic and nu-clear recoil spectra in Fig. 6 for 227 MeV protons on GaAs.The symbols correspond to the various recoiling fragments pro-duced during the nuclear interaction. These recoil spectra werecalculated using a binary cascade model embedded in GEANT4.Although the recoil spectra are typically weighted toward lowerenergy recoils in both cases (Coulombic and nuclear inelastic in-teractions), the intensity of the Coulombic and nuclear spectraare approximately the same order of magnitude for recoils largerthan 10 keV. Initially, these recoil spectra may be misleadingbecause the recoiling fragments have different mass and charge,and therefore, the relative amount of energy that goes into dis-placements will be different for each fragment because of theRobinson energy partition function. Thus, each recoil spectrawill contribute a different amount of displacement damage en-ergy to the total NIEL. The amount of energy that goes intoproducing displacements decreases significantly as the atomicweight of the recoiling particle decreases so lighter fragmentscontribute less to the nuclear NIEL compared to heavier frag-ments (Note that the recoil spectra in Fig. 6 were integrated


Fig. 6. Recoil spectra for coulombic and nuclear interactions produced during227 MeV proton irradiation of GaAs. The solid line is the recoil spectrum forcoulombic interactions, while the symbols correspond to the recoil spectra ofany secondary ions (fragments) produced during the nuclear interaction.

using the appropriate NIEL equations and the total NIEL valuefor 227 MeV protons on GaAs was determined to agree wellwith the calculation performed by Jun et al. using MCNPX forthe nuclear interaction data [21] and the average recoil approx-imation described above.). So to investigate the influence of therecoil spectra produced from nuclear interactions on the totaldisplacement damage deposited, we consider the fraction of theNIEL or cumulative NIEL as a function of recoil energy.

The fraction of the NIEL or normalized cumulative NIEL(damage) produced by recoils with energies below recoil en-ergy following irradiation with protons and silicon ions isshown in Fig. 7. The curves reveal that the recoil energy at agiven normalized cumulative NIEL (e.g., 0.5) is shifted towardhigher energy recoils with increasing incident proton energy.This is because higher energy protons produce higher energyrecoils which contribute more to the overall damage produced.At a recoil energy of approximately 0.2 MeV, there is an inflec-tion point for the proton curves having incident proton energies

30 MeV. This is because the nuclear component of the NIELis comparable to the Coulombic interactions generate high en-ergy recoils having energies above 0.2 MeV, which contributesignificantly to the overall displacement damage.

IV. DISCUSSION

In this paper, we have monitored the electrical and struc-tural changes in GaAs solar cells under certain particle irradi-ation conditions using electron beam induced current (EBIC)and transmission electron microscopy (TEM) measurements.We can gain further knowledge as to when the localized de-fects are produced if the previous published EBIC data in [16]is included with the recent 10 MeV proton data presented inSection III-A.

A comparison of the of the EBIC images in Fig. 1 with theEBIC images in [16] reveals some important information aboutthe contrast in the EBIC images. Although not all the EBIC datahas been published, it should be mentioned that EBIC and TEMimages were generated prior to and after irradiation with 2, 4,

Fig. 7. Fraction of the total NIEL (cumulative damage) produced by recoilswith energies below the recoil energy � following irradiation with protonsof various energies and with 22 MeV Si ions. These calculations include bothCoulombic and nuclear interactions.

10, 53, and 227 MeV protons and 22 MeV Si ions. Prior to ir-radiation, no contrast was observed in the EBIC image (imagenot published) [16]. The 2 MeV proton image in [16, Fig. 6]had more contrast in comparison to the pre-irradiation imageso the contrast seen in that image obviously stems from theproton irradiation but the nebulous features observed do not in-crease with increasing proton energy. One may argue that thecontrast is similar to that observed in the 10 MeV proton imagebut since those images were generated under nominally identicalEBIC conditions only a qualitative comparison can be made andnot a quantitative contrast comparison. The 10 MeV proton im-ages have very distinct features and the density increases withincreasing proton fluence in accord with the production of ra-diation-induced defects. Thus, although there is some contrastin the 2 MeV images individual defects cannot be resolved asin the high-energy proton cases. These observations lend addi-tional support to the view that localized defects are not presentafter irradiation with 2 MeV protons. If by chance the local-ized defects are introduced for proton energies 10 MeV, thenthe concentration is so low that other contributions control thevariation of the EBIC signal such as surface roughness resultingfrom the irradiation. This may explain the source of contrast andthe nebulous features seen in the 2 MeV proton EBIC image. Itshould be mentioned that the features in the 4 MeV proton EBICimage were very similar to the 2 MeV image. The TEM obser-vations are in agreement with all the EBIC measurements pre-sented herein and in conjunction with those in [16]. Therefore,it has been determined that higher energy protons and siliconions produce defects that are electrically and structurally dif-ferent than those produced by lower energy protons. From thispoint forward, low energy protons will be consider to be thosehaving energies 10 MeV while high energy protons will beconsider to be those having incident energies 10 MeV. There-fore, it can be concluded that the localized defects observed inthe EBIC images are related to the disordered regions in theTEM images since they are only observed under the same ir-radiation conditions.


The HRTEM results in Fig. 4 show that high energy pro-tons and Si ions produce defect regions that are localized. Thelattice fringes are quite evident within the center of the disor-dered regions which are surrounded by a perfectly crystallinelattice structure. These observations strongly suggest the disor-dered regions are not amorphous. Our samples were preparedfor TEM post-irradiation so there was initially some concernas to whether the sample preparation (Ar ion milling) couldhave caused crystallization of the disordered region because ofion-induced annealing. Previous work in which samples wereprepared prior to irradiation without Ar milling has shown that50 keV Ar ions in GaAs can cause the formation of amorphousregions by direct impact [19]. Therefore, the formation of amor-phous regions in their work is only associated with the irradi-ation itself and not from sample preparation. Other work hasshown that 7 keV Ar ion milling can also cause amorphous-likeregions but the effects are not severe where these layers are thin( not to exceed 15 angstroms for both milled surfaces) with re-spect to the crystal film [23]. Therefore, we would expect the Arion milling to produce amorphous regions and not cause crys-tallization. One particular study, however, has shown that elec-tron-beam-induced crystallization of some isolated amorphousregions can occur in GaAs within a few minutes [24]. How-ever, not all of the amorphous regions had crystallized duringthat time interval. We had never observed any such amorphousregions so it seems plausible that the disordered regions werenever amorphous from the beginning.

Wesch et al. studied the effect of ionization, i.e., the electronicenergy loss per unit path length (electronic energy deposition)and found that heavy ion irradiation in some semiconductorssuch as InP, InSb, InAs, GaSb and Ge [25] may cause amor-phous tracks if a certain value of the electronic energy deposi-tion is exceeded. If enough electronic energy is deposited, thiscauses melting around the ion trajectories followed by re-solid-ification leaving behind amorphous tracks which was explainedin the framework of the thermal spike model [26]. However,the authors found that no amorphous regions were formed inGaAs when the incident ion had an electronic energy deposi-tion up to 33 keV/nm [27]. It was determined that the lowdamage concentration measured by Rutherford backscatteringspectroscopy (RBS) was attributed to the nuclear energy depo-sition. It has been suggested that an ion having an electronic en-ergy deposition in GaAs must exceed a value of 36 keV/nmto form amorphous tracks [30]. The maximum electronic energydeposition in our studies was produced by 22 MeV Si ion irra-diation having a value of 4.8 keV/nm. The electronic energydeposition for a 2 MeV proton and 10 MeV proton is 0.04 and0.014 keV/nm, respectively. The electronic energy deposition inour studies is well below the suggested threshold value to formamorphous regions not to mention the electronic energy loss fora 10 MeV proton is less than that produced by a 2 MeV proton.Thus, the formation of the disordered regions observed in ourstudy cannot be explained by the electronic energy depositionor the defect regions would have been observed following irra-diation with 2 MeV protons.

Our analysis of the electronic energy and nuclear energy de-position as calculated using SRIM has shown that in all caseswhen heavy ions produced amorphous tracks or regions in semi-

conductors by high electronic energy deposition [28]–[31], theelectronic energy deposition profile as a function of depth in thematerial was much greater than the displacement energy depo-sition. Only for the cases when the displacement energy depo-sition was greater than the electronic energy deposition wereamorphous regions formed by the direct impact model [19].The 50 keV Kr ion that created amorphous regions in GaAsby direct impact had a maximum nuclear energy deposition of

2000 keV/nm while the maximum nuclear energy depositionin our case was 100 keV/nm produced by the 22 MeV Si ions.Therefore, it is not expected that the incident particles used inour study would deposit enough nuclear energy to create amor-phous regions.

There have been a few TEM studies on GaAs over the yearswhere radiation-induced defect regions have been observedwhich were not considered to be associated with amorphousregions. The defects were either considered as interstitialdislocations, dislocation loops of vacancy and interstitial type,edge dislocations, and even damage clusters [23], [32], [33].It is very difficult to determine from the HRTEM images ifsimilarities exist between those images and Fig. 4 because thequality of the images is not very good. The disordered regionsobserved in the HRTEM image are best described as beingassociated with a cluster of vacancies and displaced atoms.That is, the spatial separation of interstitials atoms created fromvacancies results in a core region that is rich in vacancies anda surrounding region rich in interstitials. These types of disor-dered regions are typically produced when a large number ofneighboring atoms are displaced in collision cascades known asthe displacement spike [34]. High density collisional cascadeswould be produced by high energy recoils.

It had previously been suggested that the deviations withNIEL at high proton energy were observed because someelectrical properties of the device are more or less sensitive todefects produced by nuclear interactions [7], which implied thatnuclear interactions produced a defect type that was differentthan those produced by Coulombic interactions. However,this hypothesis does not hold when the high energy protonand the 22 MeV Si ion results are considered because thedisordered regions or localized defects have been observed fortwo irradiating particle types where the dominate interactionmechanisms are completely different. The data presented herehas shown that localized defects produced by high energyprotons can also be produced by heavier ions under certainirradiation conditions. The conclusions derived from the TEMresults imply the disordered regions are created because ofcollision cascades produced by high energy recoils. Therefore,a closer look at the recoil spectrum is required.

The average recoil energies and cross sections are very im-portant parameters for estimating the creation of defects sincethe cross section determines the frequency of recoil events andthe recoil energy defines the local dimensions of the PKA. Wehave employed the approximation of using the average recoil en-ergy for calculation of the nuclear component to the NIEL, andour results are comparable to those attained using different ap-proaches [35], [36]. The average recoil energies of 2 MeV pro-tons and 22 MeV Si ions is 93 eV and 144 eV, respectively, whilehigh energy protons ( 10 MeV) generate recoils having av-


erage energies above 1 MeV [37]. Although the average recoilenergies have been used as a figure of merit, it does not explainthe formation of localized defects. This is because the localizeddefects were observed for 22 MeV Si ions and not for 4 MeVprotons, which produce similar average recoil energies. The in-teraction of an incident particle with a target material generatesa recoil energy spectrum having energies ranging from the dis-placement threshold up to the maximum recoil energy. There-fore, it needs to be determined if 22 MeV Si ions produce recoilshaving energies comparable to those generated by high energyprotons.

Let us consider the recoil spectra for 22 MeV Si ions, 2 MeVand 10 MeV protons as shown in Fig. 5. These energies havebeen chosen because the dominant interaction mechanism inall cases is Coulombic. Although nuclear interactions are ap-parent for 10 MeV protons, they are minimal compared to thatfor Coulombic interactions. Analysis of these recoil spectra donot explain why the localized defects are produced. The reasonis 2 MeV protons produce a much larger number of recoil eventsat a given recoil energy throughout the recoil energy range incomparison to 10 MeV protons. These spectra are equivalent toirradiation with a particle fluence of unity so to gain further in-sight into the possible reason the localized defects are produced,we shall also consider the fluence dependence for the three casesdiscussed above.

This can be accomplished by considering the following func-tion, referred to as the fingerprint of the displacement process,

, given by [38]

(1)

where is the particle fluence, is the recoil energy, andis the primary recoil spectrum (e.g., those given in

Fig. 5). The fingerprint of the displacement process is a mea-sure of the number of atoms that could possibly be displacedper unit energy from their lattice positions. This function isplotted as a function of recoil energy for a few incident particlesin Fig. 8. It is important to mention that when localized defectswere observed at a given particle irradiation energy, thesedefects were observed over the entire fluence range studied asgiven in [16]. The 10 MeV proton and 22 MeV Si ion fluencelevels listed in Fig. 8 correspond to the lowest fluence levelstudied while the 2 MeV proton fluence corresponds to thehighest irradiation fluence studied. These calculations showthat localized defects must be independent of the total damageenergy deposited since these defects were not observed inthe2 MeV proton EBIC image at this fluence level, which sohappens to produce the largest energy spectrum for recoils upto about 0.2 MeV. So, the formation of the localized defectscannot depend on the total damage energy deposited. Onemajor difference between the three spectra in Fig. 8 is that 10MeV protons and 22 MeV Si ions produce high energy recoils.Therefore, the localized defects must be produced by recoilshaving a minimum threshold energy, which will be referred toas the cluster formation energy . Srour et al. [39] havediscussed this quantity in great detail in correlating displace-ment damage effects in Si. Since localized defects were also notobserved in samples irradiated with 4 MeV protons, this implies

Fig. 8. Energy spectra of primary collisions between incident particles andatoms of the solid known as the fingerprint of the displacement process. Theenergy spectrum on GaAs are plotted for 2 MeV protons with a fluence of�� cm , 10 MeV protons with a fluence of �� cm , and 22 MeV Si ionwith a fluence of � � �� cm .

that a threshold energy range for cluster formation in GaAscan be determined using the 4 and 10 MeV proton maximumrecoil energy. That is, the cluster formation energy must lie inthe recoil energy range between 0.22 0.54 MeV.Although we cannot definitively conclude that the localizeddefects are not produced for intermediate proton energies, thelocalized defects seen in the EBIC images for the 10 MeVirradiated samples showed a significant decrease in electricalactivity compared to those produced for higher proton energies.This result suggests that, if the localized defects are introducedfor proton energies below 10 MeV, then the localized defectsmust have minimal influence on the device parameters, whichwould explain why the experimental damage coefficients arealways seen to agree with NIEL for proton energies below10 MeV.

The general behavior of the normalized cumulative damagefor recoil energies above the threshold energy for cluster for-mation can be easily seen in Fig. 9. This plot represents theenergy dependence of the cumulative damage that goes intoproducing localized defects. It can be seen that the fraction ofdamage for recoils above this threshold increases significantlyas the incident proton energy increases because nuclear inelasticinteractions generate high energy recoils that contribute to theoverall displacement damage. For example, approximately 50%of the damage is produced by recoils having energies above0.22 MeV for 30 MeV protons. The ratio of the fraction ofthe cumulative damage for recoil energies above 0.22 MeV for227 MeV to 10 MeV protons is approximately a factor of 6.5,which is consistent with the ratio of the contrast for the local-ized defects (darks spots) in the 10 and 227 MeV EBIC images(Figs. 1(a)–1(d)). The contrast in the EBIC images is represen-tative of the defect strength, which appears to follows a sim-ilar trend as the fraction of the damage for recoils above 0.22MeV. The fraction of the damage for recoils energies above 0.22MeV following irradiation with 22 MeV Si ions is 0.22 repre-sented by the inverted triangles in Fig. 9. This fraction is larger


Fig. 9. Fraction of the cumulative damage as a function of particle energy forrecoil energies above the maximum recoil energy of a 4 MeV proton on GaAs,which is 0.216 MeV. The square symbols are for different proton energies andthe inverted triangle corresponds to the 22 MeV silicon ions.

than that for 10 MeV protons by a factor of 1.6. Given thisresult, it may not be surprising that the localized defects wereintroduced following 22 MeV Si ion irradiation. Weak beamTEM imaging revealed that Si ion-induced inclusions seen inFig. 3 were characterized by smaller strain gradients comparedwith those produced by 227 MeV protons (see Fig. 2) becausea stronger reflection (g 220) was required. This observationmight be consistent with the fact that the fraction of the damageproduced above the threshold energy for 227 MeV protons com-pared with 22 MeV Si ions is about 60% more (as determinedfrom Fig. 9). Therefore, much more energy is deposited in pro-ducing the defects seen in the TEM images.

The results presented above are generally suggestive that thetotal energy deposited is partitioned into creating different typesof defects. A portion of the energy goes into creating primary de-fects such as vacancy-interstitial pairs for recoil energies below0.22 MeV and the remaining portion goes into creating defectclusters (localized defects) such as those observed in the EBICand TEM images. It is believed that the minimum formation en-ergy is around 0.22 MeV but it is most likely not a step func-tion. The best estimate for the cluster formation energy rangethat can be determined from the data presented in this work isprobably between 0.22 to 0.54 MeV. It is believed that, as theenergy density within the cascades increases (as illustrated inFig. 9), the density of the defects and hence the defect strengthwould increase, which is generally consistent with EBIC andTEM observations. However, the displacement damage energydeposited into the production of localized defects will eventu-ally saturate. This is because the energy that goes into displace-ments is restricted by the energy partition function. For protonson GaAs, less than 10% of the energy goes into producing dis-placements for recoils having energies above 10 MeV becausemost of the energy goes into ionization. If the total fraction ofthe damage energy shown in Fig. 9 is channeled into producinglocalized defects then it would be expected that all damage co-efficients would follow the total NIEL but in some cases theydo not [40]. This suggests that only a fraction of the total de-

fects produced survive recombination, resulting in stable defectsand the concentration of defects surviving recombination de-pends on the PKA spectrum. The partitioning of the defect typesinto different categories has also been suggested by others [39],[41]–[43].

The fact that the production of these localized defects corre-sponds to the same proton energy range where proton-induceddamage coefficients (DCs) are not always seen to vary linearlywith NIEL strongly suggests that these localized defects are theprimary reason for the non-linearity. Weller et al. used the con-cept of nonionizing energy deposition rate (NIEDR) to explainwhy device parameters did not agree with NIEL [14]. They pos-tulated that “NIEL blurs the distinction between a beam and adevice property. Its value quantifies the energy surrendered bythe beam at a given point, without regard to where that energy ul-timately manifests itself in a device. For practical applications, itis the disorder in the three-dimensional sensitive volume of a de-vice, and more specifically the disorder that leads to electricallyactive defects, that is important. NIEL does not attempt to dothis spatial bookkeeping.” Although the calculation for NIEDRtracks all the necessary particles in order to compute the de-posited energy density as a function of position from first prin-ciples, the data, when plotted along with the GaAs NIEL, liesdirectly between the total and Coulombic NIEL for proton ener-gies above 10 MeV. Although NIEDR maybe used as an alter-native approach for predicting displacement damage effects, itclearly does not tell the full story nor explain why multiple elec-trical parameters associated with a single device would followdifferent energy dependences with NIEL [40]. Thus, it can beconcluded that in certain instances the localized defects or de-fect clusters that are produced by high energy protons affect var-ious device electrical parameters differently causing the DCs tonot always follow the same NIEL curve at high proton energies.

V. CONCLUSION

The TEM and EBIC results clearly show that high energy pro-tons MeV and 22 MeV silicon ions produce defects thatare electrically and structurally different than those produced bylow energy protons. HRTEM imaging shows that the disorderedregions are not amorphous but probably most likely a cluster ofvacancies and a surrounding region rich in interstitials, whichis produced when a large number of neighboring atoms are dis-placed in collision cascades known as the displacement spike.Currently this is the most plausible explanation and descriptionof the disordered region without further extensive TEM imagingand analysis. Analysis of the recoil spectra have shown that thetype of disordered regions observed in the EBIC and TEM im-ages are produced for recoils having energies above a minimumthreshold energy of 0.22 MeV. It was found that the forma-tion of the disordered regions is independent of the total dis-placement damage energy deposited, but created when the en-ergy density within the cascade exceeds a specific value. It wasdetermined that if the fraction of the damage produced for re-coils above 0.22 MeV has a value approximately greater than0.15 than these disordered regions are formed. These results alsodemonstrate why NIEL scaling is sometimes violated for protonenergies above 10 MeV because the efficiency of these defectsinfluencing various device electrical parameters is different.


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Documents

Effect of Proton and Silicon Ion Irradiation on Defect Formation in GaAs