Single crystal silicon carbide detector of emitted ions and soft x rays frompower laser-generated plasmasL. Torrisi, G. Foti, L. Giuffrida, D. Puglisi, J. Wolowski et al. Citation: J. Appl. Phys. 105, 123304 (2009); doi: 10.1063/1.3153160 View online: http://dx.doi.org/10.1063/1.3153160 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v105/i12 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Single crystal silicon carbide detector of emitted ions and soft x raysfrom power laser-generated plasmas

L. Torrisi,1,2,a� G. Foti,3 L. Giuffrida,1,2 D. Puglisi,3 J. Wolowski,4 J. Badziak,4 P. Parys,4

M. Rosinski,4 D. Margarone,5 J. Krasa,5 A. Velyhan,5 and U. Ullschmied5

1INFN-Laboratori Nazionali del Sud, V. S. Sofia 62, 95123 Catania, Italy2Dipartimento di Fisica, Università di Messina, Ctr. Papardo 31, 98166 S. Agata, Messina, Italy3Dipartimento di Fisica, Università di Catania, V. S. Sofia 64, 95123 Catania, Italy4Institute of Plasma Physics and Laser Microfusion, Henry Str., Warsaw, Poland5Institute of Physics, ASCR, Na Slovance 2, 182 21 Prague, Czech Republic

�Received 4 March 2009; accepted 14 May 2009; published online 22 June 2009�

A single-crystal silicon carbide �SiC� detector was used for measurements of soft x rays, electrons,and ion emission from laser-generated plasma obtained with the use of the Prague Asterix LaserSystem �PALS� at intensities of the order of 1016 W /cm2 and pulse duration of 300 ps.Measurements were performed by varying the laser intensity and the nature of the irradiated target.The spectra obtained by using the SiC detector show not only the photopeak due to UV and softx-ray detection, but also various peaks due to the detection of energetic charged particles.Time-of-flight technique was employed to determine the ion kinetic energy of particles emitted fromthe plasma and to perform a comparison between SiC and traditional ion collectors. The detectorwas also employed by inserting absorber films of different thickness in front of the SiC surface inorder to determine, as a first approximation, the mean energy of the soft x-ray emission from theplasma. © 2009 American Institute of Physics. �DOI: 10.1063/1.3153160�

I. INTRODUCTION

Semiconductor detectors have been mostly used in thelast year perhaps to begin to monitor the ionizing radiationemitted from laser-generated plasmas, permitting promptmeasurements of x rays emitted during the short life of thepulsed plasma �typically picosecond-nanosecond scale� andof fast electrons and slower ions emission.1 The possibility touse SiC detectors, producing signals proportional to the en-ergy deposited in its depletion layer by the incident photonsand particles, is very attractive because these detectors arenot sensitive to the visible and infrared light. Optical photonsare not able to produce electron-hole pairs because their en-ergy is below the 3.26 eV for 4H-SiC gap energy. So the SiCdevice can operate under visible light exposure at which it istransparent. The main advantages of 4H-SiC-based detectorsare radiation hardness, application at high temperature andvoltage levels, fast signal collection time, room temperatureoperation with low dark current, high signal-to-noise ratio,high detection efficiency, high energy gap, and controllabledepletion layer thickness.2 Photons, electrons, and ions ab-sorbed in the sensitive volume of the detector generateelectron-hole pairs, loosing 7.78 eV for a pair production,with results in arising voltage signal at the device electrodesthat is proportional to the deposited radiation energy. Thelack of sensitivity to the visible light permits the use of theSiC detector without any shielding filters to detect UV, xrays, electrons, and ion beams simultaneously. Table I reportsthe main properties of SiC compared to other semiconductordetector employed for diagnostics in laser-generatedplasmas.3

SiC Schottky diodes provide the evidence that a 100% ofcharge collection efficiency can be obtained. The high effi-ciency charge collection in a single-crystal detector makesprecise measurements of ionizing radiation dose and inten-sity. Due to these properties, SiC detectors can be employedwith success to monitor not only x-ray photons but also par-ticle emission from laser-generated plasmas and, by using atime-of-flight �TOF� configuration, they permit the measur-ing of the velocity of the particles emitted from the plasmasource, as demonstrated in this work.

In this investigation a single-crystal 4H-SiC Schottkydiode detector is employed to monitor the energy of photonsand particles ejected from a high energetic plasma obtainedat PALS laboratory in Prague Asterix Laser System �PALS�laboratory, by using a laser pulse intensity of about1016 W /cm2.4 SiC measurements are compared with thosecoming from a TOF ion collector �IC�.

II. MATERIALS AND METHODS

The detector was built at the Epitaxial Technology Cen-ter �ETC� of Catania, in collaboration with the University ofCatania. Recently ETC has developed a process to grow4H-SiC n-epitaxial layers from a chemical vapor depositiontechnique, with thicknesses over 100 �m and purity under1014 /cm3, which are state-of-the-art worldwide.3

Nitrogen was used as n-type doping material at 2�1014 /cm3 concentration. Metal-semiconductor Schottkydiodes were employed depositing Ni2Si on the Si-n doped,generating a rectifying junction �Schottky barrier height=1.7 eV�. The thickness of the Ni2Si metallization is 200nm. Ni2Si top layer and Si-n+ high doped bottom substratewere contacted Ohmically with an Al bonding and covered

a�Author to whom correspondence should be addressed. Tel.: 39 095 54226.Electronic mail: torrisi@lns.infn.it.

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by a thin gold deposition layer. For such structures, the ap-plication of a reverse bias creates a depletion layer that liesalmost entirely in the lightly doped Si-material. Figure 1�a�indicates a cross-section scheme of the used SiC detectorreporting the ion depth penetration in the depletion layerzone. Figure 1�b� shows the depletion thickness incrementwith the inverse bias, which follows roughly the inverse biassquare root. Generally a bias voltage between 600 and 1000V, producing a depletion layer between 55 and 75 �m, wasused in these investigations. A used SiC diode detector, 1.4�1.4 mm2 surface and 300 �m thickness, glued onto ahigh temperature resistance sample holder, is shown in thephoto of Fig. 1�c�.

The SiC diode was employed with an electric circuit asthat reported in Fig. 1�d�. In such configuration, at 600 Vinverse bias, it has high linearity with the radiation energy,high energy resolution �FWHM=34 keV �FWHM denotesfull width at half maximum� at 5.8 MeV� and 100% chargecollection efficiency, as demonstrated detecting alpha par-ticles emitted from a 239Pu– 241Am– 244Cm source.2,3

Generally the SiC detector was placed at 150 cm dis-tance from the target and at 25° angle with respect to thenormal to the laser irradiated target surface. TOF technique,using a fast memory oscilloscope, was employed in thesemeasurements.

The laser of PALS Laboratory in Prague was operated atfundamental frequency ��0=1315 nm�. The laser pulse en-ergy EL ranges from 50 J up to 300 J with 300 ps duration.4

Thin polymers �PET �polyethylenetheraphtalate�, polysty-rene, and Kapton� and metals �Al, Cu, and Au�, 10–100 �mthickness, and polymers coupled to metals �metallic nanopar-ticles embedded into polymer and metalized polymer foils�were employed as targets and the laser incidence angle onthe target was 0° at which a spot size of 200 �m was mea-sured.

A flat IC was placed at a distance of 158 cm from thetarget and at 25° with respect to the target-normal direction.In another direction, at 30° detection angle with respect tothe target-normal, an ion energy analyzer �IEA� spectrometerpermitted to measure the ion energies and charge states. De-tails on the used IC and IEA are reported in literature.5 Fig-ure 2 shows a schematic view of the experimental setup. Thedetector distances from the target, D, were 151 cm for theSiC, 158 cm for the IC and 266 cm for the IEA. The x-raysignals were acquired by varying the laser pulse energy andthe mylar absorber thickness placed in front of the SiC de-tector. A full TOF spectrum was recorded in order to corre-late temporally the x-ray emission, the electron, and the ionemission, as it will be shown in the following.

TABLE I. Principal properties at room temperature of SiC compared to other semiconductors/insulators interesting for radiation detection.

Property 4H-SiC 3C-SiC 6H-SiC Si GaAs Diamond

Crystal structure Hexagonal Zinc-blende Hexagonal Diamond Zinc-blende DiamondBand structure Indirect Indirect Indirect Indirect Direct IndirectEnergy gap Eg �eV� 3.26 2.20 2.86–3.03 1.12 1.43 5.45Electron mobility �n �cm2 /V s� 800–1000 1000 370–600 1400–1500 8500 1800–2200Hole mobility �p �cm2 /V s� 100–115 50 50 450–600 400 1200–1600Breakdown electric field Ec �MV/cm� 2.2–4.0 1.2 2.4–3.8 0.2–0.3 0.3–0.6 10Thermal conductivity �th �W /cm °C� 3.0–5.0 3.0–5.0 3.0–5.0 1.5 0.5 20Saturation velocity �s �cm /s��107 0.8–2.2 2.0–2.7 2.0 0.8–1.0 1.0–2.0 2.2–2.7Relative dielectric constant �r 9.7 9.7 9.7 11.8 12.8 5.5Max working temperature �°C� 1240 1240 1240 300 460 1100Melting point �°C� 1800 1800 1800 1420 1240 3500e-h pair energy �eV� 7.78 3.62 4.21 13Hole lifetime �p �s� 6�10−7 2.5�10−3 10−9

Density ��g /cm3� 3.21 3.21 3.24 2.33 5.32 3.52Physical stability Excellent Excellent Excellent Good Fair Very goodAtomic weight 44 76 46 28 144.6 12Lattice constant �� a=3.07 c=10.05 4.36 a=3.07 c=15.12 5.43 5.65 3.567Electron affinity �V� 3.08 3.83 3.34 4.07 4.07 �1.07

FIG. 1. �Color online� Cross-section scheme of the ion SiC detector and ofthe ion trajectories �a�, depletion layer thickness vs reverse bias polarization�b�, photo of the SiC detector on its holder �c�, and electrical scheme usedfor the SiC detector �d�.

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III. RESULTS AND DISCUSSION

Figures 3 and 4 show a comparison between four typicalSiC-TOF spectra for ion detection from laser-generatedplasma and, for comparison, the corresponding IC-TOFspectra. In these measurements the solid angles subtended bySiC and IC are 8.8�10−7 and 7�10−5, respectively. Spectraindicate the presence of a very fast photopeak followed by atail, probably due to fast electrons detection and/or to de-layed x-ray emission. and a series of peaks due to slowerions. In particular Figs. 3�a�–3�d� are referred to Cu-bulkablation at 128 J and Au-PET �Au nanoparticles embeddedinto 15 �m PET,� ablation at 119 J, respectively. Figures4�a�–4�d� are referred to 10 �m pure PET ablation at 256 Jand 8 �m Al-Kapton �8 �m Kapton film covered with 25nm gold film� ablation at 126 J, respectively. These spectraconsist of various peaks due to different ion species �hydro-gen, carbon, aluminum, copper, and gold� and different ion

groups �fast and slow�. Many peaks are represented by wideTOF signals, as a result of the high number of ion chargestates, while the peak due to protons is single and narrower.The measured mean proton energy was 50 keV for Cu, Au-PET, and Al-Kapton targets irradiated at about 120 J, and 47keV for PET pure polymer irradiated at 256 J. This resultconfirms that the high electron density in the plasma, due tothe presence of metallic species in the ablated target, in-creases the electric field generated inside the plasma andconsequently increases the ion acceleration along the normalto the target surface, as reported in our previous articles.6

The pure PET, in fact, shows a lower proton and carbonkinetic energies with respect to the case of irradiation ofpolymers enriched with gold nanoparticles or covered bythin metallic films.

The IEA spectrometer permits to determine the ioncharge state because, by fixing the electrostatic deflectionbias, it detects, with a high energy resolution, the energy-to-charge state of the particles.5 In order to measure the ionenergy distributions, depending on the charge states, manymeasurements should be done by changing the deflectionbias. A typical TOF-IEA spectrum obtained irradiating theAu-PET target with 119 J is reported in Fig. 5�a�. This spec-trum shows 18 charge states for the Au ions and, from adetail of the faster ions reported in Fig. 5�b�, six charge statesfor the C ions and a single fast H+ peak. The six charge statesof C ions are well time-resolved in the TOF-IEA spectrumbut they appear convolved in a single structured large peak inthe TOF-SiC and TOF-IC spectra. The ion energy distribu-tions follow Coulomb–Boltzmann-shifted distributions, asdemonstrated in previous articles.7

The higher energy, due to the C6+ ions, is about 45 keVfor Al-Kapton and Au-PET targets, while it decreases toabout 15 keV for pure PET target, i.e., for a lower electrondensity plasma, according to previous measurements.6 Themean energy measurements with SiC detector are in wellagreement with the IC detector.

Probably different CxHy groups and/or slow C ions are

FIG. 2. Scheme of the experimental setup for the target irradiation in thevacuum chamber at PALS laboratory.

FIG. 3. Comparison of the SiC and IC spectra for irradiation of Cu-bulktarget at 128 J ��a� and �b�� and for Au- PET film target at 119 J ��c� and �d��,respectively.

FIG. 4. Comparison of the SiC and IC spectra for irradiation of pure PETfilm target at 256 J ��a� and �b�� and for Al-Kapton film target at 126 J ��c�and �d��, respectively.

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generated from the ablated polymers, as shown in the SiCand IC spectra at 5–20 �s TOF from PET ablation �Figs.4�a� and 4�b��. The CH2 monomers would have a kineticenergy of about10 keV and the Cn+ slower ions about 8.5keV.

The Cu and Au ions are detected for TOF higher than1 �s �Figs. 3�a� and 3�b� for Cu and Figs. 3�c� and 3�d� forAu�, and the Al ions from Kapton are detected for TOFhigher than 4 �s �Figs. 4�c� and 3�d��. Their maximum ki-netic energies, calculable from SiC and IC spectra, corre-spond to about 15 keV.

Ion spectra from SiC and IC have a different shapewhich is due to the different detection method. SiC signalsare proportional to the particle and photon energy depositedin the depletion layer. IC signals are due to the ion chargedeposited on the Faraday cup surface. Thus, photons and lowcharge state particles, such as protons, at high energy inducehigh signal in the SiC, while high charge states ions inducehigh signal in the IC. In both cases the signal is proportionalto the ion and photon flux. Thus observing the spectra com-parison reported in Figs. 3�c� and 3�d�, for example, it ispossible to observe that the first Au peak in the SiC detectoris practically absent for the IC detector, demonstrating that itis due to high energetic Au and H ions �well detected by SiC�but with a low current �not well detected by IC�. On the otherhand, the Au tail at high TOF times, practically absent in theSiC spectra and significantly present in IC ones, corresponds

to the slower Au ions having low energy �not well detectedby SiC� but high current �well detected by IC�.

The SiC proton peaks in the spectra can be fitted with aCoulomb–Boltzmann-shifted distribution, according to theapproach of Torrisi et al.7 reported in previous papers

f�v� = A� m

2kT�3/2

v3 exp�−m�v − vk − vc�2

2kT� , �1�

where v represents the total velocity along the normal to thetarget surface �v=vt+vk+vc� in which the components arethe thermal velocity vt, the adiabatic expansion velocity invacuum vk, and the Coulombian velocity vc, given, respec-tively, by the following relations:

vt = �3kT/m�1/2; vk = �kT/m�1/2; vc = �2ezVo/m�1/2,

�2�

where m is the ion mass, the adiabatic coefficient, e theelectron charge, z the charge state, and Vo the equivalentvoltage developed inside the plasma.

The ion spectra represents a particle distribution as afunction of their time of flight dN /dt. This distribution canbe transformed in velocity distribution dN /dv through thefollowing relationship:

dN

dv=

dN

dt

dt

dv=

dN

dt

tTOF2

L=

Y

2ezR

tTOF2

L, �3�

where tTOF is the time-of-flight, L is the target-detector flightdistance, Y is the yield �millivolt� measured at the oscillo-scope, and R is the input resistance of the oscilloscope�50 ��.

Typical SiC ion velocity distributions relative to the pro-ton peak TOF position are reported in Fig. 6 �points� fordifferent laser-target irradiations. The distribution shows anexponential increment at low velocity due to the signal com-ing from heavy ions. Typical data fits of proton peaks arehere reported, indicating that the equivalent plasma tempera-ture kT is about 8 keV for the Au-PET �a� irradiated at 119 J,8.9 keV for Al-Kapton �b� irradiated at 126 J, and �c� about3.6 keV for the pure PET polymer irradiated at 256 J. Thefits of proton velocity distributions indicate that the equiva-lent voltage acceleration developed inside the plasma isabout 8 kV for Au-PET and Al-Kapton polymers and about7.2 kV for the pure PET polymer. This result confirms thatthe acceleration voltage, and consequently the electric fielddeveloped inside the plasma and due to the different electronand ion mobility, increases with the electron density as aconsequence of the presence of metal vapors in the plasmagases.

These results confirm that, due to the six charge states,the maximum carbon ion energies E=ezVo should be of theorder of 48 keV, in good agreement with the experimentaldata. Thus the equivalent ion plasma temperature in the corezone, for Au-PET and Al-Kapton, should be of about kT=2E /3�5.3 keV.

The TOF spectra show a fast photopeak in its beginningzone generated by UV and soft x-ray detection. The SiCphotopeak intensity is very high in absence of any absorber

FIG. 5. Typical IEA spectrum relative to the irradiation of Au nanoparticlesembedded in 15 �m PET polymer at 119 J �a� and particular of the IEA fastpeaks relative to the detection of C and H ions �b�.

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placed in front of the detector. It decreases exponentiallywith the increase in the mylar film thickness used as an ab-sorber.

A simple and approximated consideration was devotedto determine the soft x-ray energy spectrum obtained usingSiC detectors. This approach was performed assuming neg-ligible the contribution of possible x-ray lines in comparisonto the continuum x-ray spectrum; the emission of x rays dueto electron bremsstrahlung, in fact, is higher with respect tothe that coming from fluorescence characteristic lineemission.8 Our measurements demonstrated that the photo-peak intensity increases proportionally to the laser pulse en-ergy and decreases proportionally to the detection angle, asreported in previous measurements.9 Increasing the thicknessof thin mylar films placed in front of the SiC detector it ispossible to measure the exponential decay of the transmittedand detected x-ray yields and to calculate the mean absorp-tion coefficient value of the mylar, as reported in Fig. 7�a� for

Au-PET laser ablation at 119 J pulse energy. The dependenceof x-ray yield on the mylar thickness permitted to determinethe experimental absorption coefficient of mylar � whichwas evaluated to be 690 cm−1. By comparing the literaturedata concerning the x-ray absorption coefficient versus thephoton energy in mylar,10 the mean x-ray energy correspond-ing to the experimentally measured absorption coefficientwas determined, as reported in Fig. 7�b�. The x-ray energyvalue ET=2.0 keV represents the mean value of the x-rayenergies measured by using the SiC detector. The x-ray ab-sorption in the 200 nm Ni2Si metallic film is negligible. Thisresult indicates that the plasma core electron temperatureshould be lower than 8 keV; it should be approximatelykTe=2Ee /3�1.3 keV. The SiC and IC detections of thefast electrons ejected from the plasma is not very simple,because at low TOF values, there is a limitation due to thehigh photopeak signal and at higher TOF values there is alimitation due to the fast ion detection.

SiC detector may measure the energy released by elec-trons in the depletion layer but, because the Ni2Si metalliclayer is 200 nm thick, only electrons with energy higher than8 keV can be transmitted. Thus the used SiC detector, due to

FIG. 6. �Color online� SiC proton peak fits by using a Coulomb–Boltzmann-shifted distribution for irradiation of Au-PET film at 119 J �a�, Al-Kaptonfilm at 128 J �b�, and pure PET film at 256 J �c�.

FIG. 7. Normalized photopeak intensity vs mylar absorber thickness �a� andmylar absorption coefficient vs photon energy indicating a mean x-ray en-ergy of about 2.0 keV �b�.

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the surface metallization thickness, is not adapt in detectingthe electron emission from the investigated plasma, whichenergy is below 8 keV.

IC was also not apt in detecting fast electrons, because itshould have a negative peak for electron detection but thisvalue is influenced by the positive tails of the photopeak �dueto electron emission from the collector� and to fast ion peaks.Generally in these measurements a minimum IC negativepeak is observed for TOF values of about 50–100–150 ns,corresponding to possible electron detection with a kineticenergy in the range of about 2800–700–315 eV, respectively.However this peak is a little intense and it indicates that theelectron contribution is not so high such as for photons andions. Moreover in this region the SiC and IC signals can bedue not only to fast electrons ejected from the plasma butalso to delayed x-ray emission from the walls of the vacuumchamber due to the impact of charged particles, and possibleion recombination processes occurring within 150 ns fromthe laser shot.

IV. CONCLUSIONS

This article shows that single-crystal silicon carbide de-tectors can be used as an active detector of ions and soft xrays emitted from a laser-generated plasma. Thanks to theTOF technique employed in this experiment it is possible todistinguish the different particles and photon contributions tothe detected spectra.

Because the laser-plasma duration is comparable withthe laser pulse duration, that is about 300 ps, and because theplasma core temperature and density are high �of the order of8 keV and 1020 /cm3,11 respectively�, the number of photonsand particles emitted from the plasma is very high. However,due to the high target-SiC distance and to the low sensitivesurface size of the detector, the solid angle subtended by theSiC is low in comparison to the IC one. Mainly for thisreason the SiC spectra show signal-to-noise ratio comparableor lower than the IC ones. A pile-up signal effect, due to thesimultaneously absorption of particles and photons with dif-ferent energies, may occur in the SiC detector, but measure-ments demonstrated that the proportionality with the full de-posited energy is maintained using our experimental setup.

Moreover, the comparison of SiC and IC spectra indi-cates that, although the solid angle subtended by SiC is about80 times lower than the IC one, the signal yield of the twodetectors are comparable. This demonstrates that the SiCsensitivity is higher with respect to the IC one. However, theSiC response YSiC is different than the IC one YIC becausethe first is mainly proportional to the energy deposited by theimpinging radiation while the second mainly to the particlecharge states, according to the following relations:

YSiC � � �

�

dN

dtE , �4�

YICD � � �

��i=1

Z

idNi

dt, �5�

where N is the number of particles �or photons� arriving inthe sensitive region of the detector, E is the mean energy of

such particles �or photons�, � is the detector efficiency�100%�, � /� the solid angle subtended by the detector, zthe maximum charge state, and Ni the number of particleswith i charge state. The mean particle �or photon� energy isdefined by the following relation:

E = Ef�E�dE

f�E�dE

, �6�

where f�E� is the particle �or photon� distribution, which,according to literature, can be well approximated by a Bolt-zmann energy distribution.

Measurements demonstrated that SiC shows importantadvantages with respect to other semiconductor detectors.They consist in the absence of a response to visible light andto the ability to detect the ions contemporary to photons andto separate the two contributions thanks to the differentTOFs.

The SiC detectors, as others semiconductors, make itpossible to observe fast changes in the ionizing radiationemitted from laser-produced plasmas. An important aspect ofthe SiC detector application of laser-generated plasma con-cerns the possibility to correlate the x-ray emission to the ionand electron emission, as discussed here and in our previouspapers.9

ACKNOWLEDGMENTS

Many thanks to ASCR, that has supported this work inthe ambit of the project number IAA 100100715, and toINFN-Commission V, for the useful support given to PLE-IADI project. Thanks also to Mr. S. Urso of the PhysicsDepartment of Catania University for the technical assistancegiven for the SiC electronics equipment.

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