As-implanted Samples Research Articles

The migration behaviour of strontium co-implanted with helium into SiC at room temperature and annealed at temperatures above 1000 °C

The study investigated the migration behaviour of Sr implanted into SiC in the presence of helium (He). Sr ions were implanted into polycrystalline SiC samples (Sr-SiC) at room temperature (RT), and co-implanted with He ions also at RT (Sr + He-SiC). The samples were then annealed isochronally at 1100 °C, 1200 °C, and 1300 °C for 5 h. Transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS) were used to characterize both as-implanted and annealed annealed samples. Sr implantation induced amorphization of SiC, while co-implantation with He led to the formation of He nano-bubbles within the amorphous SiC matrix. During annealing, Sr migrated towards the surface, resulting in loss of Sr, cavity formation, and formation of Sr precipitates in the Sr-SiC samples. In Sr + He-SiC samples, He-induced cavities formed around the projected range of Sr, inhibiting epitaxial regrowth of SiC. As a result, the Sr distribution became concentrated around these He cavities, with Sr trapped both in front and behind them. The enhanced migration of Sr in annealed Sr + He-SiC is attributed to the slower recrystallization of the damaged SiC layer, the presence of larger He-induced cavities, and increased surface roughness. These findings provide insights into Sr migration the mechanisms in SiC, relevant for enhancing the safety of nuclear fuels.

The effect of ion implantation and annealing temperatures on the migration behavior of ruthenium in glassy carbon

Nuclear waste storage materials are inevitable in nuclear industry for preventing the release of radioactive waste products. Glassy carbon has been considered being beneficial to be used in the dry cask needed for nuclear waste storage. Thus, we studied the migration of ruthenium implanted in glassy carbon upon annealing. Our investigations show that ruthenium implantation caused defects in the glassy carbon structure, with more defects observed in the room temperature as-implanted samples compared to those implanted at 200 °C. Annealing the as-implanted samples from 500 to 800 °C showed no significant change in the ruthenium depth profiles, indicating the non-diffusivity of ruthenium in glassy carbon at these temperatures. However, annealing at higher temperatures (from 900 and 1300 °C) resulted in an increase in the maximum depth profile peaks, accompanied by a shift towards the surface, and a decrease in the full-width at half-maximum. These changes indicate the aggregation of ruthenium atoms in the near-surface region. Additionally, more ruthenium aggregation was observed in room temperature implanted samples compared to those implanted at 200 °C. This difference is attributed to the higher concentration of defects in room temperature implanted samples, which promotes ruthenium aggregation. Moreover, the migration and aggregation of ruthenium in the near-surface region contributed to an increase in the surface roughness of the glassy carbon.

Enhanced Activation in Phosphorous-Doped Silicon via Dual-Beam Laser Annealing.

In this study, we conduct a comparative analysis of single-beam laser annealing (SBLA) and dual-beam laser annealing (DBLA) techniques for semiconductor manufacturing. In the DBLA approach, two laser beams were precisely aligned to simultaneously heat a phosphorus-doped silicon (Si) wafer. The main objective was to investigate the impact of the two annealing techniques on the electrical properties, crystalline structure, and diffusion profile of the treated phosphorus-doped Si at equivalent laser powers. Both SBLA and DBLA improved the electrical properties of the phosphorus-doped Si, evidenced by increased carrier concentration and reduced carrier mobility. Additionally, the crystalline structure of the phosphorus-doped Si showed favorable modifications, with no defects and improved crystallinity. While both SBLA and DBLA produced similar phosphorus profiles with no significant redistribution of dopants compared to the as-implanted sample, DBLA achieved a higher activation ratio than SBLA. Although the results suggest improved dopant activation with minimal diffusion, further studies are needed to clearly confirm the effect of DBLA on dopant activation and diffusion.

Study of the effect of implantation temperature on the migration behaviour of Xe implanted into glassy carbon.

The effect of implantation temperature on the migration behaviour of xenon (Xe) implanted into glassy carbon and the effect of annealing on radiation damage retained by ion implantation were investigated. Glassy carbon substrates were implanted with 320keV Xe+ to a fluence of 2×1016cm-2. The implantation process was performed at room temperature (RT) and 100°C Some of the as-implanted samples were isochronally annealed in vacuum at temperatures ranging from 300°C to 700°C in steps of 100°C for 10h. The as-implanted and annealed samples were characterized using Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. The RT implanted depth profiles indicated that the migration of Xe towards the surface of glassy carbon was accompanied by a loss of Xe ions. The samples implanted at 100°C indicated no diffusion or loss of Xe after annealing at 300°C. However, annealing at temperatures ranging from 400°C to 700°C resulted in a slight shift in the Xe profile tail-end towards the bulk of glassy carbon. The diffusion coefficients (D) in the temperature range of 300°C-700°C for the RT and 100°C implanted samples, activation energies (Ea), and pre-exponential factors (Do), were extracted. The values of D ranged from (9.72±0.48)×10-21 to (1.87±0.09)×10-20m2/s with an activation energy of (6.25±0.31)×10-5eV for RT implanted samples, and the samples implanted at 100°C, D ranged from (3.85±0.19)×10-21 to (6.96±0.34)×10-20m2/s with activation energy of (4.10±0.02)×10-5eV. The Raman analysis revealed that implantation at the RT amorphised the glassy carbon structure while the samples implanted at 100°C showed mild damage compared to RT implantation. Annealing of the RT-implanted sample resulted in some recovery of the damaged region as a function of increasing annealing temperature.

The influence of helium-induced defects on the migration of strontium implanted into SiC above critical amorphization temperature

The presence of radiation-induced defects and the high temperature of implantation are breeding grounds for helium (He) to accumulate and form He-induced defects (bubbles, blisters, craters, and cavities) in silicon carbide (SiC). In this work, the influence of He-induced defects on the migration of strontium (Sr) implanted into SiC was investigated. Sr-ions of 360 keV were implanted into polycrystalline SiC to a fluence of 2 × 1016 Sr-ions/cm2 at 600°C (Sr-SiC). Some of the Sr-SiC samples were then co-implanted with He-ions of 21.5 keV to a fluence of 1 × 1017 He-ions/cm2 at 350°C (Sr + He-SiC). The Sr-SiC and Sr + He-SiC samples were annealed for 5 h at 1,000°C. The as-implanted and annealed samples were characterized by Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Rutherford backscattered spectrometry (RBS). Implantation of Sr retained some defects in SiC, while co-implantation of He resulted in the formation of He-bubbles, blisters, and craters (exfoliated blisters). Blisters close to the critical height and size were the first to exfoliate after annealing. He-bubbles grew larger after annealing owing to the capture of more vacancies. In the co-implanted samples, Sr was located in three regions: the crystalline region (near the surface), the bubble region (where the projected range of Sr was located), and the damage region toward the bulk. Annealing the Sr + He-SiC caused the migration of Sr towards the bulk, while no migration was observed in the Sr-SiC samples. The migration was governed by “vacancy migration driven by strain fileds.”

Dopant profiling of ion-implanted GaAs by terahertz time-domain spectroscopy

We investigate terahertz time-domain spectroscopy (THz-TDS) as a non-destructive and non-contact technique for depth profiling of dopants in semiconductors. THz temporal waveforms transmitted through silicon-ion-implanted semi-insulating gallium arsenide substrates, as-implanted or post-annealed by rapid thermal annealing, were analyzed by assuming a multi-layered Gaussian refractive index profile in the ∼sub-micrometer-thick implantation region. The implantation energy and dosages in this work were 200 KeV, 1014, 5 × 1014, and 1015 ions/cm2, respectively. The average values of real (n) and imaginary (κ) parts of refractive indices of an as-implanted sample in the depth range of 0–800 nm are 5.8 and 0.7, respectively, at 0.5 THz and are 6.2 and 0.2, respectively, at 1 THz. On the other hand, the refractive index profile of the post-annealed samples displays a prominent Gaussian-like form, and peak refractive indices (n ∼ 25 and κ ∼ 32.7 at 0.5 THz and n, κ ∼17 at 1 THz) were found to be at the depth of 210 nm. Reconstructed dopant profiles in as-implanted, implanted, and post-annealed substrates were found to be in good agreement with measurements by secondary ion mass spectroscopy as well as simulation by the Monte Carlo method. We were also able to determine accurately the projected range (Rp), straggle (Rs), and concentration of dopants by the analysis of THz-TDS data. The spatial resolution, along the depth direction, of the THz-TDS technique for depth profiling of dopants was estimated to be as small as 8-nm. This work suggests the feasibility of using THz-TDS for nondestructive and non-contact diagnostics for profiling dopants in semiconductors.

Intense luminescence emission and lattice damage of the implanted Eu ions in KTiOPO4 crystals

The doping of rare earth elements revolutionized applications of optoelectronic devices. Europium (Eu) ions acting as activators can provide the fundamental effect on luminescence of host materials. We investigate photoluminescence, lattice damage formation of Eu-implanted potassium titanyl phosphate (KTiOPO4) crystals using 400-keV Eu ions with a fluence of 5 × 1015 ions / cm2 at room temperature. Effective doping is achieved in the depth range of about 50 to 200 nm from the surface of KTiOPO4 crystal, and the luminescent activity of Eu ions is inhibited due to significant implantation damage. However, the samples showed intense luminescence emission at room temperature through the lattice recovery after annealing at 600°C and 700°C. The lattice damage of as-implanted and annealed samples was investigated by Rutherford backscattering spectrometry in channeling configuration. Furthermore, the relative strain depth curves of Eu implanted KTiOPO4 samples at different annealing temperatures were studied using the high resolution x-ray diffraction and Raman spectra. The present research reveals that Eu implanted KTiOPO4 samples have great prospects in the field of optical applications and waveguide quantum reservoirs.

Interface state density distribution near conduction band edge at Al2O3/Mg-ion-implanted GaN interface formed after activation annealing using AlN cap layer

An interface state density (Dit) distribution near the conduction band edge (EC) at the Al2O3/Mg-ion-implanted GaN interface was measured after ion implantation, annealing with an AlN protective cap, and cap layer removal. Mg ions were implanted into n-GaN with a Si concentration of 6 × 1017 cm−3 at a maximum Mg concentration of 2 × 1017 cm−3, resulting in the maintenance of the n-type conduction in GaN even after the activation of Mg dopants. Activation annealing was carried out at 1250 °C for 1 min using an AlN cap layer. The complete removal of the AlN cap layer was accomplished by wet etching, which was confirmed by x-ray photoelectron spectroscopy. The photoluminescence spectrum showed donor–acceptor-pair emission after annealing, indicating the activation of Mg acceptors. By applying the capacitance–voltage method to a completed metal–oxide–semiconductor diode, we derived a continuous distribution of relatively low Dit below 5 × 1012 cm−2 eV−1, which increased monotonically toward EC in the range from EC − 0.15 to EC − 0.45 eV. Compared with the Dit distribution of the as-implanted sample, the density of the discrete level at EC − 0.25 eV generated by divacancies markedly decreased upon 1250 °C annealing.

Ruthenium ion modification of glassy carbon: Implication on the structural evolution and migration behaviour of implanted Ru atoms

Glassy carbon samples were implanted with ruthenium ions to a fluence of 1 × 1016 cm−2 at room temperature (at 150 keV). The implanted samples were annealed isochronally in vacuum from 1000 to 1300 °C for 5 h. The resulting microstructural changes were investigated using X-ray diffraction (XRD), Raman spectroscopy and atomic force microscopy (AFM). The diffusion behaviour of ruthenium in glassy carbon was investigated using secondary-ion mass spectrometry (SIMS). Raman results showed that the implantation of ruthenium into glassy carbon caused amorphization and increase the tensile stress in the implanted region. XRD showed that the amount of tensile stress in virgin glassy carbon increased from 0.016 GPa to 0.19 GPa after ion implantation which is in qualitative agreement with the Raman results. Annealing of the samples exhibited more recrystallization and changed the tensile stress to compressive stress. SIMS results showed that annealing of the as-implanted samples at 1000 °C caused aggregation of the ruthenium atoms, while annealing at higher temperatures led to some segregation of ruthenium atoms at a depth of 155 nm below the glassy carbon surface. The aggregation of ruthenium atoms after annealing (as observed by SIMS) played a role in the surface roughness as observed by AFM.

Effects of swift heavy ion irradiation and annealing on the microstructure and recrystallizationof SiC pre-implanted with Sr ions

Polycrystalline SiC wafers were implanted with 360 keV strontium (Sr) ions at room temperature (RT)to a fluence of 2 × 1016 cm−2. Some of the implanted samples were irradiated with xenon (Xe) ions of 167 MeV to a fluence of 3.4 × 1014 cm−2 and 8.4 × 1014 cm−2at RT. The as-implanted and implanted then irradiated samples were vacuum annealed (isochronally) at temperatures ranging from 1,100 to 1,400°C in steps of 100°C for 5 h. Annealing induced modification of the microstructure of the implanted and swift heavy ions (SHIs) irradiated SiC was studied by Raman spectroscopy, scanning electron microscopy (SEM) and backscattering spectrometry (RBS). Sr ions bombardment caused formation of an amorphous layer in SiC, while irradiation by Xe ions led to partial recrystallization of the amorphized layer. After annealing at 1,100°C, the samples with low Sr retained ratio showed full recrystallization, while the samples with high Sr retained ratio showed poor recrystallization. This suggests that the presence of Sr within the implanted region inhibited the recrystallization of SiC. Annealing of the as-implanted samples at temperatures from 1,100°C and 1,200°Cresulted in larger average crystal size compared to the SHIsirradiated samples annealed in the same temperature range. The difference in the average crystal sizes between the as-implanted and SHIs irradiated samples was due to the differences in the nucleation rate per amorphous area in the two samples. Ramanspectroscopy results showedthat the intensity of the LO mode of SiC increases with increasing crystal size. However, several factors such as pores and defects in SiC play a role in the decrease of the LO mode intensity of SiC (even if the average crystal size is large).

Comprehensive Analysis of Phosphorus-Doped Silicon Annealed by Continuous-Wave Laser Beam at High Scan Speed.

We report an in-depth analysis of phosphorus (P)-doped silicon (Si) with a continuous-wave laser source using a high scan speed to increase the performance of semiconductor devices. We systematically characterized the P-doped Si annealed at different laser powers using four-point probe resistance measurement, transmission electron microscopy (TEM), secondary-ion mass spectroscopy, X-ray diffractometry (XRD), and atomic force microscopy (AFM). Notably, a significant reduction in sheet resistance was observed after laser annealing, which indicated the improved electrical properties of Si. TEM images confirmed the epitaxial growth of Si in an upward direction without a polycrystalline structure. Furthermore, we observed the activation of P without diffusion, irrespective of the laser power in the secondary-ion mass-spectrometry characterization. We detected negligible changes in lattice spacing for the main (400) XRD peak, showing an insignificant effect of the laser annealing on the strain. AFM images of the annealed samples in comparison with those of the as-implanted sample showed that the laser annealing did not significantly change the surface roughness. This study provides an excellent heating method with high potential to achieve an extremely low sheet resistance without diffusion of the dopant under a very high scan speed for industrial applications.

Crystalline Orientation-Dependent Ferromagnetism in N+-Implanted MgO Single Crystal.

Samples of (110), (100), and (111) MgO single crystals were implanted with 70 keV N ions at room temperature. All as-implanted samples showed room temperature hysteresis in magnetization loops. The observed saturation magnetization (Ms) was 0.79 × 10−4 emu/g, 1.28 × 10−4 emu/g, and 1.5 × 10−4 emu/g for (110), (100) and (111) orientation implanted-MgO and follows the relation Ms(111) > Ms(100) > Ms(110), indicative of crystalline orientation-dependent ferromagnetism in N-implanted MgO. The samples were characterized by X-ray photoelectron spectroscopy (XPS), high resolution X-ray diffraction (HRXRD), reciprocal space mapping (RSM), and photoluminescence (PL). The results indicated that the amount of N-substitute-O and N-interstitial defects in these three N-implanted MgO samples showed the same changing tendency as compared with Ms data. Thus, we conclude that the N-substitute-O and N-interstitial defects may play a crucial role in controlling the N+-implanted-induced ferromagnetism.

Effects of implantation temperature and annealing on structural evolution and migration of Se into glassy carbon

The use of glassy carbon (GC) as a future nuclear waste storage material depends on its capability to retain all radioactive fission products found in spent nuclear fuels. Selenium (79Se) is found in trace amounts in uranium ores, spent, and reprocessed nuclear fuel. This work investigates the effects of implantation temperature and annealing on the structural evolution and migration of Se implanted GC. To achieve these objectives, 150 keV Se+ was implanted into GC samples separately at room temperature (RT) and 200 °C to a fluence of 1 × 1016 cm−2. Some of the as-implanted samples were annealed at 1000, 1100 and 1200 °C for 5h and characterised by transmission electron microscopy (TEM), Raman spectroscopy, and secondary ion mass spectrometry (SIMS). Both TEM and Raman spectroscopy showed that implantation caused defects in the GC structures, with more defects in the RT as-implanted sample. Annealing caused the healing of both sample types but retained some radiation damage. No migration of Se atoms was observed in the RT and 200 °C as-implanted samples. However, a different migration behaviour was seen after annealing the RT and 200 °C samples up to 1200 °C, attributed to the trapping and de-trapping of Se atoms in different amounts of defect induced by implantation.

Investigating structural changes and surface modification in glassy carbon induced by xenon ion implantation and heat treatment

In order to ascertain the suitability of glassy carbon as a material for encapsulation of nuclear waste, glassy carbon was implanted with Xe and the structural changes and surface modification were investigated before and after annealing. This was performed using Raman spectroscopy analysis, high-resolution transmission electron microscopy (HRTEM) measurements, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The Raman spectrum of the implanted sample showed that ion bombardment amorphised the glassy carbon structure. The HRTEM analysis of the virgin glassy carbon exhibited some features which are similar to those of fullerenes. One of the features is the appearance of closed onion-like nanoparticles and several graphitic fringes of varying sizes and orientations embedded in the glassy carbon structure. The presence of the onion-like features, as well as the graphitic fringes within the glassy carbon structure, suggest that glassy carbon is a disordered form of carbon. The HRTEM analysis of the as-implanted sample also shows some dark spots within the implanted region which are likely xenon bubbles. The SEM and AFM analysis showed that the grain size becomes larger and more prominent with increasing annealing temperature, leading to an increase in the surface roughness of glassy carbon.

Raman and RBS Analysis of Silicon Ion Implanted Gallium Arsenide

This paper analyzes the effect of 100[Formula: see text]keV silicon negative ion implantation in semi-insulating gallium arsenide sample for the fluences varying between [Formula: see text] and [Formula: see text][Formula: see text]ion[Formula: see text]cm[Formula: see text] using Raman spectroscopy, Rutherford backscattering spectroscopy and Electron spin resonance spectroscopy. The gallium arsenide sample implanted with silicon negative ion for different fluences showed shift in the TO peak position with respect to unimplanted gallium arsenide sample. Increase in the broadening of TO peak was observed in the as-implanted samples, indicating development of stress and phonon confinement due to the incorporation of silicon in gallium arsenide crystal lattice. Annealing of as-implanted samples showed stress relaxation. Increase in RBS backscattering yield was observed in the as-implanted samples. Annealing of as-implanted (with high fluence) sample showed flat RBS yield response. ESR measurement study revealed restructuring of defects in the gallium arsenide sample implanted with fluence of [Formula: see text][Formula: see text]ion[Formula: see text]cm[Formula: see text] after annealing to the temperature of [Formula: see text]C.

Effect of SHI irradiation and high temperature annealing on the microstructure of SiC implanted with Ag

Scanning electron microscopy (SEM), Raman spectroscopy and Rutherford backscattering spectrometry (RBS) were used to study the influence of swift heavy ion (SHI) irradiation and annealing on the microstructure of polycrystalline SiC implanted with silver (Ag). Polycrystalline SiC specimens were first implanted with 360 keV Ag + ions at room temperature (RT) to a fluence of 2 × 1016cm−2. Thereafter, some of the implanted samples were irradiated with Xe ions of 167 MeV (SHI) at room temperature to a fluence of 3.4 × 1014 cm−2 and 8.4 × 1014 cm−2. Both the as-implanted and implanted then irradiated samples were annealed in vacuum at temperatures ranging from 1100 to 1400 °C in steps of 100 °C for 5 h. Raman and SEM results showed that implantation of silver (Ag) resulted in complete amorphization of the near surface region of the SiC substrates. However, SHI irradiation of the as-implanted SiC resulted in partial recrystallization of the initially amorphized layer. The as-implanted samples exhibited more crystallinity after annealing at 1100 °C as compared to SHI irradiated samples annealed at same conditions. This poor recrystallization of the SHI irradiated SiC samples was due to the amount of impurities (i.e. concentration of Ag atoms) retained after annealing at 1100 °C. Raman and SEM results showed that annealing of the as-implanted samples at 1100 °C resulted in larger average crystal size compared to the SHI irradiated samples annealed in the same conditions. The intensity of the longitudinal optical (LO) phonon in Raman spectra increases with the increasing the average crystal sizes of SiC.

Implanted gallium impurity detection in silicon by impedance spectroscopy

The results of determining the energy levels of boron-doped silicon implanted with gallium ions by impedance spectroscopy are reported. In the as-implanted sample the boron level remains the same and a second level appears close to the Ga-level reported in literature. In the sample annealed at 1000 °C, two levels are observed neither of which corresponds to the literature values for boron and gallium. It is assumed that in the as-implanted sample this method detects levels of gallium atoms located at a depth where ions penetrate due to the channeling effect, since a large concentration of defects at shallower depths does not allow detection of energy levels due to the Fermi level pinning. Explaining the results for the sample annealed after implantation requires additional research. The main result of this work is to establish the possibility of detecting impurity levels in ion-implanted silicon by impedance spectroscopy even in the absence of subsequent annealing.

Characterization of 4H-SiC Lattice Damage After Novel High Energy Ion Implantation

A multi-energy implantation system has been developed for deep implantation of dopant atoms (Al, B, N, P) in 4H-SiC wafers to fabricate deep junctions for medium and high voltage devices. Energies used range from 13MeV to 66MeV, far higher than those used in conventional implantations. Therefore, lattice damage induced by the implantation process and the recovery by annealing must be characterized in detail for understanding the nature of damage, extent of recovery and its possible effect on device properties. To this end, 4H-SiC wafers with 12 um epilayer were implanted by 13.8 MeV to 65.7 MeV Al and N ions using the multi-energy implantation system. Samples were implanted in the form of alternative Al and N pillars or blanket co-implanted with Al and N. The nature of strains induced by the implantation process in as-implanted and post-annealed samples were investigated using Synchrotron X-ray Rocking Curve Topography (SXRCT) and Reciprocal Space Map (RSM). As-implanted samples are characterized by tensile strains. By comparing samples implanted with different fluences, we have confirmed that implantation with higher total fluence introduces higher levels of strains in the 4H-SiC epilayer.

Microstructure Evolution in He-Implanted Si at 600 °C Followed by 1000 °C Annealing.

Si single crystal was implanted with 230 keV He+ ions to a fluence of 5 × 1016/cm2 at 600 °C. The structural defects in Si implanted with He at 600 °C and then annealed at 1000 °C were investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The microstructure of an as-implanted sample is provided for comparison. After annealing, rod-like defects were diminished, while tangled dislocations and large dislocation loops appeared. Dislocation lines trapped by cavities were directly observed. The cavities remained stable except for a transition of shape, from octahedron to tetrakaidecahedron. Stacking-fault tetrahedrons were found simultaneously. Cavity growth was independent of dislocations. The evolution of observed lattice defects is discussed.

Effect of thermal annealing on SHI irradiated indium implanted glassy carbon

The effects of swift heavy ion irradiation on implanted glassy carbon, the modification and migration of indium after vacuum annealing have been investigated. Radiation damage was introduced to the glassy carbon substrates after room temperature implantation by 360 keV indium ions to a fluence of 2.0 × 1016 ions/cm 2. The implanted samples were subsequently irradiated at room temperature by swift heavy ions (167 MeV Xe 26+ ions). Isochronal annealing of both sets of samples from 200 to 600 °C for 1 h was performed in vacuum. TheSHI irradiation, induced some restructuring in the damaged glassy carbon substrates. Vacuum annealing of the SHI irradiated samples gave rise to recovery and the diffusion of implanted In which was different from that of the as-implanted sample. Higher retention was observed at each temperature for SHI irradiated samples compared to as-implanted samples. The diffusion coefficients of the SHI irradiated samples were lower than for un-irradiated sample.