Defect formation in semiconductor detectors by irradiation with focused ion beams of different properties

Zračenje poluvodičkih materijala brzim ionima nužno proizvodi defekte u kristalnoj strukturi u vidu vakancija i intersticija. Najveća gustoća defekata se opaža pri kraju dosega brzog iona u materijalu. Unutar tih lokaliziranih područja s visokim koncentracijama defekata može doći do rekombinacije ili formiranja kompleksnih defekata. Inicijalne kaskadne rekombinacije u pikosekundnim vremenskim prozorima su dobro proučene. Ipak, promatranje procesa smanjenja broja defekata nastalih ionizacijskim zračenjem u poluvodičima na duljim vremenskim skalama je još dosta otvoreno i manje razjašnjeno, te je upravo to bila tema ovog rada. Ispitivali smo dvije vrste detektora: Si PIN diode i 4H-SiC Schottky diode. Diode su ozračivane ionskim snopovima različitih karakteristika, a za proučavanje utjecaja nastalih električki aktivnih defekata na mobilnost nosioca naboja korištena je metoda IBIC mikroskopije. Kod silicija je uočeno značajno napuštanje nečistoća na sobnoj temperaturi u vremenskom periodu jednog dana, dok u SiC nema takvog efekta zbog šireg energijskog procijepa u ovom materijalu. Korištenjem pulsnog ionskog snopa pokušali smo proučiti vjerojatnosti rekombinacije defekata u milisekundnim vremenima. Rekombinacije koje se odvijaju za vrijeme pauza pulsnog snopa nisu proizvele statistički značajan utjecaj na učinkovitost skupljanja naboja, vjerojatno zbog nedovoljne gustoće ionskih oštećenja formiranih prilikom ozračivanja. Za SiC proučen je i utjecaj temperature na napuštanje oštećenja lokalno uvedenih ionizirajućim zračenjem. Rezultati su interpretirani na temelju postojećih DLTS mjerenja, kojima su se karakterizirali energijski nivoi u poluvodiču koji odgovaraju centrima uhvata nosioca naboja.Irradiating semiconductor material with fast ions creates defects in crystal lattice, namely vacancies and interstitials. The highest defect concentration is observed near the end of the range of high energy ions in the material. Within these localized volumes, where defect concentration is increased, defect recombination or complex defects formation is possible. Initial recombination cascades, which occur during the first picosecond time frames, are well studied. But, processes that are responsible for reducing the number of defects, produced during ionizing radiation of semiconductors, during longer time scales, are not so clarified, so these were the main subject of our study. Two detector materials were examined: Si PIN diodes and 4H-SiC Schottky diodes. Detectors were irradiated using ion beams of different properties. In order to study the influence of the created electrically active defects on the charge carrier’s mobility, IBIC method was utilized. For silicon, significant defect annealing was observed (at room temperature conditions) on time scales of one day, while no such effect was recorded in SiC due to the wide band-gap in this material. By using a pulsed ion beam, we tried to study defect recombination in the millisecond time frames. Recombinations which occur during pulsed beam pauses have not produced a statistically significant influence on the charge collection efficiency, probably because the density of the created damage events was insufficient. Also, for SiC detector, effect of the temperature on defect mobility (induced by locally damaging ions) was investigated. Results have been interpreted based on existing DLTS measurements, which have been used to characterize energy levels in the semiconductor that are responsible for trapping the charge carriers

Defect formation in semiconductor detectors by irradiation with focused ion beams of different properties

Zračenje poluvodičkih materijala brzim ionima nužno proizvodi defekte u kristalnoj strukturi u vidu vakancija i intersticija. Najveća gustoća defekata se opaža pri kraju dosega brzog iona u materijalu. Unutar tih lokaliziranih područja s visokim koncentracijama defekata može doći do rekombinacije ili formiranja kompleksnih defekata. Inicijalne kaskadne rekombinacije u pikosekundnim vremenskim prozorima su dobro proučene. Ipak, promatranje procesa smanjenja broja defekata nastalih ionizacijskim zračenjem u poluvodičima na duljim vremenskim skalama je još dosta otvoreno i manje razjašnjeno, te je upravo to bila tema ovog rada. Ispitivali smo dvije vrste detektora: Si PIN diode i 4H-SiC Schottky diode. Diode su ozračivane ionskim snopovima različitih karakteristika, a za proučavanje utjecaja nastalih električki aktivnih defekata na mobilnost nosioca naboja korištena je metoda IBIC mikroskopije. Kod silicija je uočeno značajno napuštanje nečistoća na sobnoj temperaturi u vremenskom periodu jednog dana, dok u SiC nema takvog efekta zbog šireg energijskog procijepa u ovom materijalu. Korištenjem pulsnog ionskog snopa pokušali smo proučiti vjerojatnosti rekombinacije defekata u milisekundnim vremenima. Rekombinacije koje se odvijaju za vrijeme pauza pulsnog snopa nisu proizvele statistički značajan utjecaj na učinkovitost skupljanja naboja, vjerojatno zbog nedovoljne gustoće ionskih oštećenja formiranih prilikom ozračivanja. Za SiC proučen je i utjecaj temperature na napuštanje oštećenja lokalno uvedenih ionizirajućim zračenjem. Rezultati su interpretirani na temelju postojećih DLTS mjerenja, kojima su se karakterizirali energijski nivoi u poluvodiču koji odgovaraju centrima uhvata nosioca naboja.Irradiating semiconductor material with fast ions creates defects in crystal lattice, namely vacancies and interstitials. The highest defect concentration is observed near the end of the range of high energy ions in the material. Within these localized volumes, where defect concentration is increased, defect recombination or complex defects formation is possible. Initial recombination cascades, which occur during the first picosecond time frames, are well studied. But, processes that are responsible for reducing the number of defects, produced during ionizing radiation of semiconductors, during longer time scales, are not so clarified, so these were the main subject of our study. Two detector materials were examined: Si PIN diodes and 4H-SiC Schottky diodes. Detectors were irradiated using ion beams of different properties. In order to study the influence of the created electrically active defects on the charge carrier’s mobility, IBIC method was utilized. For silicon, significant defect annealing was observed (at room temperature conditions) on time scales of one day, while no such effect was recorded in SiC due to the wide band-gap in this material. By using a pulsed ion beam, we tried to study defect recombination in the millisecond time frames. Recombinations which occur during pulsed beam pauses have not produced a statistically significant influence on the charge collection efficiency, probably because the density of the created damage events was insufficient. Also, for SiC detector, effect of the temperature on defect mobility (induced by locally damaging ions) was investigated. Results have been interpreted based on existing DLTS measurements, which have been used to characterize energy levels in the semiconductor that are responsible for trapping the charge carriers

Electronic Properties of a Synthetic Single-Crystal Diamond Exposed to High Temperature and High Radiation

Diamond, as a wide band-gap semiconductor material, has the potential to be exploited under a wide range of extreme operating conditions, including those used for radiation detectors. The radiation tolerance of a single- crystal chemical vapor deposition (scCVD) diamond detector was therefore investigated while heating the device to elevated temperatures. In this way, operation under both high-temperature and high-radiation conditions could be tested simultaneously. To selectively introduce damage in small areas of the detector material, a 5 MeV scanning proton microbeam was used as damaging radiation. The charge collection efficiency (CCE) in the damaged areas was monitored using 2 MeV protons and the ion beam induced charge (IBIC) technique, indicating that the CCE decreases with increasing temperature. This decreasing trend saturates in the temperature range of approximately 660 K, after which CCE recovery is observed. These results suggest that the radiation hardness of diamond detectors deteriorates at elevated temperatures, despite the annealing effects that are also observed. It should be noted that the diamond detector investigated herein retained its very good spectroscopic properties even at an operation temperature of 725 K (≈2% for 2 MeV protons)

Ion microprobe study of the polarization quenching techniques in single crystal diamond radiation detectors

Synthetic single crystal diamond grown using the chemical vapor deposition technique constitutes an extraordinary candidate material for monitoring radiation in extreme environments. However, under certain conditions, a progressive creation of space charge regions within the crystal can lead to the deterioration of charge collection efficiency. This phenomenon is called polarization and represents one of the major drawbacks associated with using this type of device. In this study, we explore different techniques to mitigate the degradation of signal due to polarization. For this purpose, two different diamond detectors are characterized by the ion beam-induced charge technique using a nuclear microprobe, which utilizes MeV energy ions of different penetration depths to probe charge transport in the detectors. The effect of polarization is analyzed by turning off the bias applied to the detector during continuous or discontinuous irradiation, and also by alternating bias polarity. In addition, the beneficial influence of temperature for reducing the effect of polarization is also observed. Finally, the effect of illuminating the detector with light is also measured. Our experimental results indicate that heating a detector or turning off the bias, and then applying it during continuous irradiation can be used as satisfactory methods for recovering the CCE value close to that of a prepolarized state. In damaged regions, illumination with white light can be used as a standard method to suppress the strength of polarization induced by hole

Charge transport in single crystal CVD diamond studied at high temperatures

The capability of single crystal diamonds to maintain their unique electronic properties even at high temperatures is, in particular, relevant for its applications as a radiation detector. In order to explore characteristics of charge transport at high temperatures (up to 450 °C), diamond was exposed to MeV energy ions, both, to induce radiation damage and to probe subsequent influence on detector’s properties. Dependence of mobility-lifetime product with temperature has been obtained for electrons and holes. For holes, mu-tau displays a linear degradation with rising temperature, while for electrons, change with temperature is less evident. Furthermore, deep trapping levels induced in the material by radiation damage, were studied through time-resolved charge signals. Detrapping time was extracted from this data. Hole trap level, with the activation energy of 0.53 ± 0.01 eV has been detected in the regions of the diamond detector previously irradiated by 5 MeV damaging proton beam, but not in the pristine regions. This indicates that the trap was formed due to defect induction during radiation damage exposure. Activation of this deep level is important for charge transport performance in diamond detectors operating at high temperatures and high radiation conditions

Energy loss of MeV protons in diamond: Stopping power and mean ionization energy

The energy loss of protons, in the range between 1.6 MeV and 6 MeV, in a 3.5 μm thick single-crystal diamond membrane was determined by the transmission method. The thickness and surface uniformity of the target were checked by two independent techniques before ion beam irradiation. The stopping power of diamond was evaluated from these data and compared with SRIM Monte Carlo simulations of ion transport, showing a slight overestimate of the simulated values over the experimental stopping powers. In addition, a comparison was made with theoretical calculations based on the Bethe formula to extract the mean ionization potential, I, of carbon atoms in diamond. The obtained I-value was 81 ± 4 eV. A discussion and comparison with results of other authors is given

An ion beam spot size monitor based on a nano-machined Si photodiode probed by means of the ion beam induced charge technique

In this work the utilization of the Ion Beam Induced Charge (IBIC) technique is explored to assess the resolution a 2 MeV Li + ion microbeam raster scanning a micrometer-sized FIB-machined hollows in a silicon photodiode. The analysis of the maps crossing the FIB machined structures evidenced a drop in charge collection efficiency across the perimeter of the hollows combined with a significant recovery of the signal amplitude at the center of the microstructures, thus forming a micrometer-sized feature which can be exploited to estimate the resolution of the probing beam. The results were interpreted according to numerical simulations based on the Shockley-Ramo-Gunn as originating from a FIB-induced surface space charge density. These results offered additional information with respect to what achievable by a confocal photocurrent microscopy analysis of the same device, due to the significantly shorter focal depth of the latter with respect to the probing ion beam. This study suggests the viability of an effective method to evaluate of the resolution of ion microbeams in processes and experiments, which could be beneficial in emerging fields (deterministic implantation, micro-radiobiology, ion lithography) demanding beam spot sizes below the micrometer scale

Utjecaj zračenja i visokih temperatura na elektronička svojstva dijamantnih detektora

Due to its ultra-wide bandgap, diamond is a material that offers a unique combination of excellent electrical, mechanical and thermal properties. Contrary to silicon, the operation of diamond-based radiation detectors should thus be possible under specific harsh conditions. In this work, hardness against temperature and radiation damage was investigated with two custom-designed thermally resilient diamond detectors. High purity single crystal diamond samples were used, with tungsten electrodes deposited on the opposing crystal faces. The operation of diamond as a radiation detector was investigated by exposure to fast ions in the MeV energy range, focused on a micrometer spot with the ion microprobe setup. By changing the ion energy and mass, the penetration depth or ionization density can be modified. These capabilities provide information about the interaction volume between the radiation particles and the device, which was exploited in two experimental scenarios: to probe the electronic properties by inducing ionization in the detector (probing ions), or to deposit the radiation damage by exposure to a higher ion dose (damaging ions). The analysis of the induced signal gives us the possibility to extract important parameters related to both the macroscopic detector performance, and the fundamental semiconductor properties of the diamond material used. Charge transport at elevated temperatures was characterized by measuring charge collection efficiency, mobility-lifetime product and drift time of electrons and holes, using both charge- and currentsensitive pulse-processing electronics. Testing the detector for nuclear spectroscopy operation revealed a highest operating temperature of 720 K, at which the energy resolution and collection efficiency of the detector remained virtually unaffected by thermal effects. It was also found that the radiation hardness, after deposition of the radiation damage with 5 MeV protons, deteriorates with elevating temperature. However, the decrease is stopped at temperatures above 660 K, which can be attributed to the beneficial mechanism of thermally induced detrapping of charge carriers. Analysis of the time evolution of the transient charge signal in the detector provided a framework to extract the energy levels of the responsible deep traps. These results are particularly important for the development and new applications of diamond radiation detectors in high-temperature and high-radiation conditions. Finally, an additional investigation was performed to understand the influence of the space-charge-limited regime on the charge carrier dynamics. It was demonstrated that exposure to elevated temperatures led to depolarization of the detector, whereupon the adverse effects on the charge transport were mitigated and collection efficiency was restored.Dijamant je poluvodički materijal koji, zbog ultra-širokog zabranjenog pojasa, nudi jedinstvenu kombinaciju izvrsnih električnih, mehaničkih i termalnih svojstava. Dijamantni kristali visoke čistoće se mogu dobiti procesom umjetne sinteze u laboratorijskim reaktorima, ponajprije procesom kemijskog taloženja iz parne faze (engl. Chemical Vapour Deposition - CVD). Tehnološki razvoj ove metode u zadnjih 20 godina je doveo do široke dostupnosti i dobre kontrole kvalitete uzoraka. To je otvorilo vrata za masovniju uporabu dijamanta u elektroničkim uređajima, od kojeg su za ovaj rad zanimljivi dijamantni detektori zračenja. Dijamant bi trebao zadržati svoja intrinzična elektronička svojstva u specifičnim surovim uvjetima gdje bi korištenje silicija, kao poluvodiča uskog zabranjenog pojasa, bilo nemoguće. U ovom radu je testiran utjecaj visokih temperatura i ionizirajućeg zračenja na dijamant, korištenjem dva detektora s prilagođenim dizajnom razvijenim za toplinsku-otpornost pri radu. Detektori su bazirani na dijamantnim kristalima različite debljine (500 μm i 65 μm) gdje su na suprotne stranice kristala deponirane elektrode od volframa. Na spoju dijamanta i volframa je stvoren omski kontakt koji omogućuje uspostavu napona i očitanje signala kroz električne kontakte. Nakon primjene električnog polja, volumen između elektroda postaje osjetljiv na zračenje. Čestice zračenja ioniziraju dijamant i stvaraju nosioce naboja koji nadalje svojim kretanjem u polju induciraju izlazni signal na elektrodama. Proučavanjem svojstava tog signala mogu se ekstrahirati razni parametri vezani za detekciju zračenja (primjerice energija deponirana prilikom interakcije) kao i fundamentalna poluvodička svojstva samog dijamanta. Glavnina eksperimentalnih tehnika korištenih u ovom radu je bazirana na ionskim mikrosnopovima MeV-skih energija, koji su prostorno fokusirani magnetskim poljima, uporabom tzv. ionske mikroprobe u akceleratorskom postrojenju Instituta Ruđer Bošković. Korištenje fokusiranog ionskog snopa, zajedno s mogućnošću promjene dubine prodora korištenjem iona različite mase i energije, pruža preciznu informaciju o interakciji iona s poluvodičem na mikrometarskoj skali koja se može kontrolirati prilikom eksperimenta. Ove funkcionalnosti su iskorištene u dva različita scenarija: za karakterizaciju svojstava samog detektora različitim tehnikama analize ionskim snopom, kao i za deponiranje defekata u kristalnu strukturu ozračivanjem velikim dozama iona. U posljednjem slučaju ione ne smatramo česticama za ispitivanje, već za oštećivanje poluvodiča. Detektori su bili izloženi ionskim snopovima i visokim temperaturama u vakumskoj komori ionske mikroprobe, što je zahtijevalo tehničku nadogradnju komore, kao i razvoj specifičnih eksperimentalnih procedura za potrebe ovog rada. Signal induciran u detektoru je procesiran korištenjem nabojno- i strujno-osjetljive elektronike da bi se iz njegovih svojstava odredili različiti parametri, kao što su: učinkovitost prikupljanja naboja, produkt vremena života i mobilnosti (μτ) nosioca naboja, driftno vrijeme itd. Ova svojstva su izmjerena i za elektrone i za šupljine u širokom rasponu povišenih temperatura. Pokazalo se da mobilnost šupljina opada brže nego za elektrone s porastom temperature. Uzrok pada mobilnosti leži u povećanoj vjerojatnosti raspršenja na kristalnoj rešetci ili defektima u rešetci. Dobiveni rezultati su uspoređeni s ranijim mjerenjima dostupnim u literaturi. Nadalje, izmjerena je ovisnost μτ parametra o temperaturi, te je uočena znatna razlika u ponašanju dvije vrste nosioca. Uzrok ovakvog rezultata nije bilo moguće jednoznačno povezati s jednostavnom fizikalnom zakonitosti, što indicira da je potrebno dublje razumijevanje pozadinskih mehanizama koji određuju svojstva transporta naboja u dijamantu. Ovdje znatnu važnost imaju defekti u kristalnoj strukturi, koji pri skupljanju naboja postaju centri uhvata (zamke) nosioca naboja te utječu na svojstva nabojnog transporta. Čak i umjetni dijamanti visoke kvalitete, kakvi su korišteni u ovom radu, imaju veće razine nečistoća u odnosu na one koje su standardno prisutne u siliciju. Neželjeni efekt koji nastaje kao posljedica uhvata veće količine nosioca u zamkama je stvaranje prostorne raspodjele naboja između elektroda (polarizacija), koja dovodi do zasjenjenja vanjskog električnog polja i narušavanja svojstava nabojnog transporta. Snažan utjecaj polarizacije je zamijećen pri radu debljeg (500 μm) detektora, gdje, zbog veće udaljenosti među elektrodama, efekti zasjenjenja lakše dolaze do izražaja. Proučavanjem prijelazne (tranzijentne) strukture strujnog signala zaključeno je da se polarizacija javila zbog nakupljanja šupljina, i to uniformno kroz cijeli volumen dijamanta. No, grijanje detektora na blago povišene temperature (oko 370 K) je dovelo do termalno inducirane depolarizacije, što se moglo kvantificirati iz promjena induciranog signala koje su upućivale na oporavak transportnih svojstava šupljina. Također, tranzijentni signali su pokazali da i pri povišenim temperaturama i stvaranju polarizacije, brzina drifta elektrona i šupljina nije znatno narušena i ostaje velika (u usporedbi sa silicijem). Ovo je osobito korisno u primjenama za brzo brojanje ili vremensko razlučivanje u eksperimentima visokog protoka čestica. Dio mjerenja je izveden s namjerom karakteriziranja parametara bitnih za primjene detektora u eksperimentima nuklearne fizike, kao što su nuklearna spektroskopija. Skupljanje zadovoljavajuće statistike događaja induciranih zračenjem može zahtijevati dugo vrijeme rada detektora. Stoga se za rad detektora koji je izložen povišenim temperaturama treba testirati stabilnost performansi na dužim vremenskim skalama. Protoni energije 2 MeV su korišteni kao ulazne čestice koje se u potpunosti zaustavljaju i deponiraju cjelokupnu energiju unutar detektora. Testirani su parametri učinkovitosti skupljanja naboja kao i energijska razlučivost. Postignuta temperatura od 720 K, do koje su neželjeni termalni efekti na svojstva detekcije ostali gotovo zanemarivi, je bila najviša prijavljena u znanstvenoj literaturi u trenutku pisanja ovog rada. Razlučivost od 2% (40 keV) i učinkovitost sakupljanja naboja od oko 95% se mogu smatrati iznimno dobrim za mnoge primjene. Detektor je također bio izložen zračenju α-česticama (iz kalibriranog α-izvora s tri radionuklida poznatih energija). Mjerenja energiv jske spektroskopije s alfa česticama na visokim temperaturama su već objavljena u nekoliko radova prethodnih godina pa je dana direktna usporedba s tim rezultatima. Pri usporedbama se pokazalo da je važno uzeti u obzir različitu debljinu korištenog dijamanta, kao i karakteristike tehničkih izvedbi samog detektora koje se znatno razlikuju u različitim radovima. Posebice je zanimljiv rezultat da debljina dijamanta ima znatan utjecaj na svojstva transport naboja, s obzirom da postoji sistematska tendencija da deblji kristali imaju niže maksimalne temperature prije sloma elektroničkih svojstava. Isto je uočeno u ovom radu. Za testiranje otpornosti na zračenje detektor je prvo izložen velikim dozama protona energije 5 MeV-a, koji u potpunosti penetriraju volumen među elektrodama, i ostavljaju pritom uniformnu raspodjelu primarnih defekata (intersticija i vakancija) u ozračenom dijelu. Svojstva induciranog signala u oštećenim i neoštećenim područjima su direktno uspoređena, iz čega se uočilo da količina sakupljenog naboja u oštećenom dijamantu opada sa temperaturom. Ipak ovaj trend gdje temperatura negativno utječe na otpornost na zračenje je zaustavljen pri oko 660 K, zbog efekta otpuštanja naboja iz zamki. Te zamke su nastale zbog oštećivanja (unosa defekata) ionskim snopom. Nosioci naboja inducirani u područjima ispunjenim tim zamkama će prilikom kretanja prema elektrodama imati veliku vjerojatnost uhvata. Nakon uhvata elektroni ili šupljine mogu biti otpušteni ukoliko prime energiju grijanjem (termalno inducirani otpust nosioca naboja). Stoga će vremenska struktura nabojnog signala znatno ovisiti o temperaturi, posebice za one temperature oko temperature aktivacije zamke. Iz induciranog signala evaluirana je vremenska konstanta koja odgovara prosječnom vremenu uhvata nosioca naboja na različitim temperaturama. Korištenjem Shockley-Read-Hall statistike, iz temperaturne ovisnosti vremenske konstante se pronašao energijski položaj duboke zamke odgovorne za uhvat naboja. Takve zamke se nalaze daleko od rubova vodljivog i valentnog pojasa i teško otpuštaju zarobljene nosioce naboja. Rezultat je uspoređen sa sličnim rezultatima u prijašnjim znanstvenim radovima, no nije bilo moguće jednoznačno odrediti kristalnu strukturu defekta koji bi odgovarao tom dubokom nivou. Ipak moglo se zaključiti da je defekt bio prisutan samo u dijelovima izloženim visokim dozama zračenja, ali ne i u neoštećenom dijamantu, te da je zarobljavao i otpuštao šupljine ali ne i elektrone. Njegova temperaturna aktivacija omogućava bolji rad detektora na visokim temperaturama. Svi ovi rezultati mogu biti značajni za razvoj i nove primjene dijamantnih detektora namijenjenih radu o uvjetima povišene temperature i zračenja, kao i za bolje razumijevanje ograničenja koje kvaliteta kristala nameće na poluvodička svojstva dijamanta kao materijala za nove generacije elektroničkih uređaja