Research of Si and GaAs diode structures by Ion Beam Induced Charge (IBIC) collection

A Si pn junction diode and a GaAs Schottky diode were prepared for studying the basic mechanism of charge collection followed by high energy charged particle incidence in order to improve the resistance against single event upset. A 2 μm wide and 20 μm long rectangular Al electrode attached to a circular Al electrode with a 50 μm diameter was made on a 2.5 μm thick epilayer (minority carrier density 2 × 10¹⁵ /cm³). Both a Schottky electrode of Al (5 μm × 110 μm) and two ohmic electrodes of AuGe/Ni (110 μm × 110 μm) were made on a 2 μm thick epilayer (7.3 × 10¹⁵ /cm³) grown on a semi insulator GaAs substrate (1 × 10⁷ Ω cm). The internal device structure was examined by the IBIC (Ion Beam Induced Charge) method using a 2 MeV He⁺ ion microbeam. IBIC images clearly show an Al electrode, the SiO₂, and an epilayer. These results were then used to improve the qualities of the test diodes.     

Study of basic mechanisms of semiconductor devices using ion beam induced charge (IBIC) collection

A change of wave form of current transients induced by a single heavy ion was investigated around a pn junction with 8 μm width and 10 μm length as a function of the ion incident position. Three pn junctions were made on a 3 μm thick Si epilayer (1 × 10¹⁶/cm³) grown on Si substrate and were in a line along an aluminum electrode with 10 μm spacing between the adjacent junctions. The elements of a pn junction array were irradiated with a 1 μm diameter 15 MeV C⁺ heavy ion microbeam spacing steps by 3 μm. At a bias voltage of − 10 V, 148, 91, and 54 fC were collected at the pn junction center, and at 3 μm and 4 μm from the edge of the electrode, respectively. Internal device structure was examined by IBIC (ion beam induced current) method by using a 2 MeV He⁺ ion microbeam. From the IBIC spectrum and the IBIC image, the charge collected from the open space by the diffusion process was observed in addition to the charge collected from the depletion layer of the pn junction.     

RBS studies of the lattice damage caused by 1 Me V Si implantation into Al0.3Ga0.7As/GaAs superlattices at elevated temperature

The lattice damage accumulation in GaAs and Al_0.3Ga_0.7As/GaAs superlattices by 1 MeV Si⁺irradiation at room temperature and 350°C has been studied. For irradiations at 350°C, at lower doses the samples were almost defect-free after irradiation, while a large density of accumulated defects was induced at a higher dose. The critical dose above which the damage accumulation is more efficient is estimated to be 2 × 10¹⁵ + Si/cm² for GaAs, and is 5 × 10¹⁵ Si/cm² for Al_0.8Ga_0.7As/GaAs superlattice for implantation with 1.0 MeV Si ions at 350°C. The damage accumulation rate for 1 MeV Si ion implantation in Al_0.3Ga_0.7As/GaAs superlattice is less than that in GaAs.     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

Electron microscopy and Rutherford backscattering spectrometry characterisation of 6H SiC samples implanted with He

6H SiC single crystals were implanted at room temperature with 1 MeV He⁺ up to a fluence of 2 × 10¹⁷ at./cm². RBS-channeling analysis with a 2 MeV He⁺ beam indicated the formation of extended defects or the generation of point defects at a constant concentration over a depth of about 1 μm. Electron microscopy characterisation revealed the presence of two amorphous buried layers at depths of about 1.75 and 4.8 μm. They are due to the implantation and to the analysing RBS beam, respectively. No extended planar or linear faults were found in the region between the surface and the first amorphous layer. However, at the surface, a 50 nm thick amorphous layer was observed in which crystalline inclusions were embedded. Electron diffraction and HREM data of the inclusions were typical for diamond. These inclusions were even found in the crystalline SiC material below this layer, however at a reduced density.     

Helium retention of vanadium alloy after energetic helium ion irradiation

Helium irradiation experiments of V–4Ti alloy were conducted in an ECR ion irradiation apparatus by using helium ions with energy of 5 keV. The ion fluence was in the range from 1 × 10¹⁷ He/cm² to 8 × 10¹⁷ He/cm². After the helium ion irradiation, the helium retention was examined by using a technique of thermal desorption spectroscopy (TDS). After the irradiation, the blisters with a size of about 0.1 μm were observed at the surface, and the blister density increased with the ion fluence. Two desorption peaks were observed at approximately 500 and 1200 K in the thermal desorption spectrum. When the ion fluence was low, the retained helium desorbed mainly at the higher temperature regime. As increase of the ion fluence, the desorption at the lower temperature peak increased and the retained amount of helium saturated. The saturated amount was approximately 2.5 × 10¹⁷ He/cm². This value was comparable with those of the other plasma facing materials such as graphite.     

Study of the permeability of thin films of a-Si:H using MeV ion beams

MeV heavy ion irradiation of hydrogenated plasma-deposited silicon nitride induces formation of the volatile molecules H₂ and N₂ inside the material. This type of nitride appears permeable for these molecules and they effuse at relatively low temperature. These effusing molecules are used to study the low temperature permeation in a 100 nm hydrogenated amorphous silicon layer, deposited onto the nitride. Upon irradiation of the double layer stack with 43.3 MeV Ag ions, appearance of D₂ and N₂ from the bottom deuterated silicon nitride layer in the vacuum does not take place up to an ion fluence of 3×10¹² ions/cm². This shows that the 100 nm plasma-deposited hydrogenated amorphous silicon top layer is initially not permeable for D₂ and N₂ molecules.     

Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system

In the present study, a 500 Å thin Ag film was deposited by thermal evaporation on 5% HF etched Si(1 1 1) substrate at a chamber pressure of 8×10⁻⁶ mbar. The films were irradiated with 100 keV Ar⁺ ions at room temperature (RT) and at elevated temperatures to a fluence of 1×10¹⁶ cm⁻² at a flux of 5.55×10¹² ions/cm²/s. Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system was investigated using scanning electron microscopy (SEM). A percolation network pattern was observed when the film was irradiated at 200°C and 400°C. The fractal dimension of the percolated pattern was higher in the sample irradiated at 400°C compared to the one irradiated at 200°C. The percolation network is still observed in the film thermally annealed at 600°C with and without prior ion irradiation. The fractal dimension of the percolated pattern in the sample annealed at 600°C was lower than in the sample post-annealed (irradiated and then annealed) at 600°C. All these observations are explained in terms of self-diffusion of Ag atoms on the Si(1 1 1) substrate, inter-diffusion of Ag and Si and phase formations in Ag and Si due to Ar ion irradiation.     

Heavy-ion-induced luminescence of amorphous SiO2 during nanoparticle formation

Silica glass was implanted with negative 60 keV Cu ions at an ion flux from 5 to 75 μA/cm² up to a fluence of 1 × 10¹⁷ ions/cm² at initial sample temperatures of 300, 573 and 773 K. Spectra of ion-induced photon emission (IIPE) were collected in situ in the range from 250 to 850 nm. Optical absorption spectra of implanted specimens were ex situ measured in the range from 190 to 2500 nm.

IIPE spectra showed a broad band centered around 560 nm (2.2 eV) that was assigned to Cu⁺ solutes. The band appeared at the onset of irradiation, increased in intensity up to a fluence of about 5 × 10¹⁵ ions/cm² and then gradually decreased indicating three stage of the ion beam synthesis of nanoclusters: accumulation of implants, nucleation and growth nanoclusters. The IIPE intensity normalized on the ion flux is independent on the ion flux below 20 μA/cm²at higher fluences. The intensity of the band increased with increasing samples temperature, when optical absorption spectra reveal the increase of Cu nanoparticles size.     

Effect of electric charge accumulation on low energy ion implantation in insulators

Charge accumulation at the surface of insulators during low energy ion implantation is related to two processes: ion impinging on the sample and secondary electron emission. Samples composed of a piece of Si (having the size of the ion beam) fixed on the centre of polyethylene (PE) coupons have been implanted with 2.2 keV H₂ ions to a fluence of 2 × 10¹⁶ H/cm². ERD (Elastic Recoil Detection) depth profiles of the implanted ions are shallower with an increase of the PE coupon size. The relative critical Si/PE size to repel all the incident ions is around 1.1 × 1.1 cm²/2.5 × 2.5 cm². The potential of the secondary electron suppressor has been varied from −500 V to +500 V. It changes the secondary electron distribution around the implanted area and, consequently, affects the accumulation of charges at the sample surface. When the potential is 0 V, a uniform ion implantation with little effect of charge accumulation for all sizes of PE coupons is obtained. A two-dimension model has been performed and gives a good explanation for the mechanism of the electric charge neutralisation.     

Suppression of charge carrier collection in diode with retrograde well and epitaxial layers for soft-error immunity

One of the solutions to the reduction of soft errors is suppression of carrier collection generated by an energetic particle emitted from typical contaminants of packaging or wiring materials. Suppression of carrier collection by a retrograde well or an epitaxial layer has been investigated by nuclear microprobes to clarify the validity of various well structures against soft errors in dynamic random access memories (DRAMs). The carrier collection efficiency of a retrograde well, a retrograde well with a buried layer on a Si wafer, and on an epitaxial wafer has been investigated for DRAM application using the ion-beam-induced-current (IBIC) measurement. The collection of carriers induced by proton microprobe irradiation could be reduced by the retrograde well formed at different boron doses. n⁺p diodes with retrograde well structures were fabricated by B⁺ implantation at 0.4–1.3 μm and to doses of 3.5 × 10¹⁶–3.4 × 10¹⁷ ions/cm³. It should be noted that an increase in the B⁺ implantation dose effetively reduces carrier collection efficiency, though a buried layer implanted with a higher dose degrades the characteristics of devices by the increase in leakage current. The carrier collection efficiency of the n⁺p diode with the higher dose retrograde well can be reduced by more than half compared to the diode with a retrograde well in an epitaxial layer.     

Ion implantation doping and electrical conductivity enhancement of C60 films

Pristine C₆₀ films sublimed onto sheet mica were implanted with 20 keV K⁺ ions and I⁺ ions at doses of 1.0 × 10¹⁶/cm², 3.0 × 10¹⁶/cm² and 5.0 × 10¹⁶/cm², and with 20 keV Ar⁺ ions at a dose of 5.0 × 10¹⁶/cm². The distributions of dopants were studied using Rutherford backscattering spectrometry (RBS). The temperature dependence of sheet resistivity of the films was investigated applying a four-probe system. It was proposed that the conductivity enhancement of K⁺ implanted C₆₀ films was due to the implanted ions in the films, while for I⁺ implanted C₆₀ films, both implanted I⁺ ions and irradiation effects of the ions contributed to the enhancement of conductivity.     

Study of 160 MeV Ni ion irradiation effects on electrodeposited polypyrrole films

Conducting polymer polypyrrole thin films doped with LiCF₃SO₃, [CH₃(CH₂)₃]₄NBF₄ and [CH₃(CH₂)₃]₄NPF₆ have been electrodeposited potentiodynamically on ITO coated glass substrate. The polymer films are irradiated with 160 MeV Ni¹²⁺ ions at three different fluences of 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². An increase in dc conductivity of polypyrrole films from 100 S/cm to 170 S/cm after irradiation with highest fluence is observed in four-probe measurement. X-ray diffractogram shows increase in the crystallinity of the polypyrrole films upon SHI irradiation, which goes on increasing with the increase in fluence. Absorption intensity increase in the higher wavelength region is observed in the UV–Vis spectra. The SEM studies show that the cauliflower like flaky microstructure of the surface of polypyrrole films turns globular upon SHI irradiation at fluence 5 × 10¹¹ ions cm⁻² and becomes smooth and dense at the highest fluence used. The cyclic voltammetry studies exhibit that the redox properties of the polypyrrole films do not change much on SHI irradiation.     

Release of nitrogen from SiOxNy films during RBS measurement

We have found that nitrogen atoms are released very rapidly from ultrathin SiO_xN_y films (2.6 nm) during RBS measurement with 500 keV He⁺ ions. The release behavior strongly depends on the preparation technique of the SiO_xN_y films. There is no release from the film prepared by thermal nitridation of SiO₂, while 80% of the nitrogen atoms are released from the film prepared by plasma nitridation at a fluence of 1×10¹⁶ cm⁻². The release cross-section for plasma SiO_xN_y films is of the order of 10⁻¹⁶ cm². This large cross-section cannot be explained by a simple recoil mechanism. The nitrogen release is also observed under irradiation with 5–10 keV electrons though the cross-section is of the order of 10⁻¹⁹ cm². These findings suggest that the observed nitrogen release is an electronic excitation induced process.     

Mechanism of irradiation assisted stress corrosion crack initiation in thermally sensitized 304 stainless steel

Thermally sensitized 304 stainless steels, irradiated up to 1.2 × 10²¹ n/cm² (E > 1 MeV), were slow-strain-rate-tensile tested in 290 °C water containing 0.2 ppm dissolved oxygen (DO), followed by scanning and transmission electron microscopic examinations, to study mechanism of irradiation-assisted-stress-corrosion-crack (IASCC) initiation. Intergranular (IG) cracking behaviors changed at a border fluence (around 1 × 10²⁰ n/cm²), above which deformation twinning were predominant and deformation localization occurred earlier with increasing fluence. The crack initiation sites tended to link to the deformation bands, indicating that the crack initiation may be brought about by the deformation bands interacted with grain boundaries. Thus the border fluence is equivalent to the IASCC threshold fluence for the sensitized material, although the terminology of IASCC is originally given to the non-sensitized materials without microstructural definition. The IASCC threshold fluence was found to change with irradiation conditions. Changes in IASCC susceptibility and IASCC threshold fluence with fluence and DO were further discussed.     

Study of localised radiation damage to PIPS detectors by a scanning ion microprobe: Measured effects and the consequences for STIM analysis

Radiation damage effects in a PIPS particle detector have been studied by direct irradiation with a 4 MeV He⁺ focused ion beam, using the Eindhoven scanning ion microprobe set-up. This set-up enables extreme sensitivity and accuracy in pulse-height measurement. Pulse-height dependence on ion dose and the lateral variations of pulse-height response after damaging were studied, for different bias voltages. Pulse height was found to decrease linearly with ion dose and this could be observed after only a few ions per scan position, corresponding to a dose of 10⁸/cm². Pulse-height loss was found to be restricted to damaged detector areas. Consequences of this pulse-height dependence on ion dose and scan position for the accuracy of STIM analyses are discussed.     

Ion beam enhanced etching of LiNbO3

Single crystals of z- and x-cut LiNbO₃ were irradiated at room temperature and 15 K using He⁺- and Ar⁺-ions with energies of 40 and 350 keV and ion fluences between 5 × 10¹² and 5 × 10¹⁶ cm⁻². The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He⁺-irradiation of z-cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO₃ in the case of Ar⁺-irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He⁺- and Ar⁺-ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min⁻¹. The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO₃ are discussed.     

Effects of 160 MeV Ni ion irradiation on polypyrrole conducting polymer electrode materials for all polymer redox supercapacitor

Electronically conducting polymers are suitable electrode materials for high performance supercapacitors, for their high specific capacitance and high dc conductivity in the charged state. Supercapacitors and batteries are energy storage and conversion systems which satisfies the requirements of high specific power and energy in a complementary way. Ion beam {energy > 1 MeV} irradiation on the polymer is a novel technique to enhance or alter the properties like conductivity, density, chain length and solubility.

Conducting polymer polypyrrole thin films doped with LiClO₄ are synthesized electrochemically on ITO coated glass substrate and are irradiated with 160 MeV Ni¹²⁺ ions at different fluence 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². Dc conductivity measurement of the irradiated films showed 50–60% increase in conductivity which is may be due to increase of carrier concentration in the polymer film as observed in UV–Vis spectroscopy and other effects like cross-linking of polymer chain, bond breaking and creation of defects sites. X-ray diffractogram study shows that the degree of crystallinity of polypyrrole increases in SHI irradiation and is proportionate to ion fluence. The capacitance of the irradiated films is lowered but the capacitance of the supercapacitors with irradiated films showed enhanced stability compared to the devices with unirradiated films while characterized for cycle life up to 10,000 cycles.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

Sputtering yield and residual vacuum influence during titanium implantation into iron

Ti implantation into iron-based alloys is known to improve tribological properties (low friction and wear) by formation of an amorphous Fe-Ti-C surface layer due to the interaction of the carbonaceous molecules in the residual vacuum with the surface during implantation. To state precisely the conditions of this amorphous-layer formation, thin evaporated iron targets were implanted with 110 keV Ti ions at room temperature with fluences ranging from 6 × 10¹⁶ to 3 × 10¹⁷ Ti/cm² and at different residual pressures. Samples were analysed using backscattering spectrometry with 5.7 MeV ⁴He ions to obtain titanium profiles and to follow the evolution of sputtering yield versus fluence and residual vacuum pressure, and also for determining the amount of carbon and oxygen incorporated on the Ti-implanted surface as a function of fluence and pressure. Theoretical calculations of sputtering and high-fluence Ti distributions were performed and compared to experimental data. The importance of the reactions that occur between the implanted surface and the residual gases in the vacuum during implantation (C and O competition) is discussed.