Ion Beam Induced Charge Technique Research Articles

A multi-electrode two-dimensional position sensitive diamond detector

In multi-electrode devices, charge pulses at all the electrodes are induced concurrently by the motion of the excess charge carriers generated by a single ion. This charge-sharing effect is such that the pulse amplitude at each sensitive electrode depends on the device geometry, its overall electrostatic configuration, and the charge transport properties of the detecting material. Therefore, the cross-analysis of the charge pulses induced at each electrode offers implicit information on the position of the ion impact. In this work, we investigate the two-dimensional position sensitivity of a diamond detector fabricated by deep ion beam lithography. By exploiting the ion beam induced charge technique, the device was exposed to a 2 MeV Li+ ion micro-beam to map the spatial dependence of the charge collection efficiency (CCE) on the nominal micro-beam scanning position. The combination of the CCE maps revealed a two-dimensional position sensitivity of the device with micrometric resolution at the center of the active region.

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.

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 holes.

Characterization of ion beam induced polarization in scCVD diamond detectors using a microbeam probe

Diamond detectors are increasingly being used in many multidisciplinary areas due to their good spectroscopy properties. However, in certain cases of diamond employment as a nuclear detector, a significant deterioration of the signal properties due to the trapping of free charge carriers by defects can be observed. This phenomenon is known as polarization. Experimentally, a good understanding of the free carrier transport mechanisms can be obtained from direct measurements of the Charge Collection Efficiency (CCE) using the ion beam induced charge technique (IBIC). In this work IBIC was performed by focused proton and carbon ion beams of different energies in the MeV range, to probe the polarization induced changes of the CCE. The detectors being used in this study include a thin scCVD membrane detector (thickness 6 μm) and a scCVD crystal of 65 μm thickness. Furthermore, the detectors have been damaged with transmitted protons to study the influence of defects on polarization. The results that are presented show that the ion microprobe technique IBIC can be successfully used to fully characterize the polarization development in terms of time evolution in scCVD diamonds. More specifically, we show for the thin membrane detectors at high count rates and longer time irradiation a clear dependence of the pulse height signal decrease, induced by the polarization effect, with levels of radiation damage. A significant deterioration of the signal was observed in regions damaged with fluences between 1012 and 1014 ions/cm2 for both types of charge carriers. The results have shown that the energies used for the probing ion beams allow us to explore the electronic features from shallow to deeply penetration range and that has an effective impact on the temporal evolution of the build-up space charge. The experiments carried out have also shown that light can be used to suppress or amplify the polarization phenomena in damaged regions depending on the dominant type of charge carriers.

Diamond Detector With Laser-Formed Buried Graphitic Electrodes: Micron-Scale Mapping of Stress and Charge Collection Efficiency

The paper reports the micron-scale investigation of an all-carbon detector based on synthetic single crystal CVD-diamond having an array of cylindrical graphitic buried-contacts, about $20~\mu \text{m}$ in diameter each, connected at the front side by superficial graphitic strips. To induce diamond-to-graphite transformation on both detector surface and bulk volume, direct-laser-writing technique was used. Laser-treatment parameters and cell shape have been chosen to minimize the overlapping of laser-induced stressed volumes. Optical microscopy with crossed polarizers highlighted the presence of an optical anisotropy of the treated material surrounding the embedded graphitized columns, and non-uniform stress in the buried zones being confirmed with a confocal Raman spectroscopy mapping. Dark current-voltage characterization highlights the presence of a field-assisted detrapping transport mainly related to highly-stresses regions surrounding buried columns, as well as superficial graphitized strips edges, where electric field strength is more intense, too. Notwithstanding the strain and electronic-active defects, the detector demonstrated a good charge collection produced by 3.0 and 4.5 MeV protons impinging the diamond, as well as those generated by MeV $\beta $ -particles emitted by 90Sr source. Indeed, the mapping of charge collection efficiency with Ion Beam Induced Charge technique displayed that only a few micrometers thick radial region surrounding graphitic electrodes has a reduced efficiency, while most of the device volume preserves good detection properties with a charge collection efficiency around 90% at 60 V of biasing. Moreover, a charge collection efficiency of 96% was estimated under MeV electrons irradiation, indicating the good detection activity along the buried columns depth.

Determination of radiation hardness of silicon diodes

In this paper, we describe an experiment aimed to measure the physical observables, which can be used for the assessment of the radiation hardness of commercially available silicon photo diodes commonly used as nuclear detectors in particle accelerator laboratories. The experiment adopted the methodology developed during the International Atomic Energy Agency (IAEA) Coordinated Research Project (CRP No. F11016) “Utilization of Ion Accelerators for Studying and Modelling Ion Induced Radiation Defects in Semiconductors and Insulators”.This methodology is based on the selective irradiation of micrometer-sized regions with different fluences of MeV ions using an ion microbeam and on the measurement of the charge collection efficiency (CCE) degradation by Ion Beam Induced Charge (IBIC) microscopy performed in full depletion condition, using different probing ions.The IBIC results are analyzed through a theoretical approach based on the Shockley-Read-Hall model for the free carrier recombination in the presence of ion-induced deep traps. This interpretative model allows the evaluation of the material radiation hardness in terms of recombination parameters for both electrons and holes.The device under study in this experiment was a commercial p-i-n photodiode, which was initially characterized by i) standard electronic characterization techniques to determine its doping and ii) the Angle-Resolved IBIC to evaluate its effective entrance window. Nine regions of (100 × 100) µm2 were irradiated with 11.25 MeV He ions up to a maximum fluence of 3·1012 ions/cm2. The CCE degradation was measured by the IBIC technique using 11.25 MeV He and 1.4 MeV He as probing ions.The model presented here proved to be effective for fitting the experimental data. The fitting parameters correspond to the recombination coefficients, which are the key parameters for the characterization of the effects of radiation damage in semiconductors.

Enhanced radiation hardness and signal recovery in thin diamond detectors

Using the advantage of the high spatial resolution of the Ruđer Bošković Institute (RBI) ion microprobe, small areas of a thin membrane single crystal chemical vapor deposition (scCVD) diamond detector were intentionally damaged with a high-intensity 26-MeV oxygen ion beam at various fluences, producing up to ∼1018 vacancies/cm3. The response of the detector was tested with the ion beam-induced charge technique (IBIC) using a 2-MeV proton beam as a probe. The signal amplitudes decreased down to approximately 50% of the original value at low electric fields (<10 V/μm) inside the detector. However, the increase of electric field to values of ∼100 V/μm completely recovers the signal amplitude. The results presented herein can facilitate the development of true radiation hard particle detectors.

The establishment and performance of IBIC microscopy at Fudan University

The ion beam induced charge (IBIC) microscopy was developed in the early 1990’s as a means of imaging device depletion regions and dislocations in semiconductor wafers. To better understand the operation performance of the micro-electronic devices and semiconductor radiation detectors, the IBIC microscopy was established on the microprobe system at Fudan University. A customized target holder was designed, on which the CdZnTe detector is fixed with adhesive tape.In this work, the device performance of a CdZnTe detector was evaluated using the established IBIC technique and we primarily choose the appropriate bias voltage and have successfully used the frontal IBIC to evaluate the quality of one CdZnTe detector. What’s more, the regions of the crystal defect and ion beam-induced radiation damage can be localized accurately.

Geiger mode mapping: A new imaging modality for focused ion microprobes

Geiger mode detectors fabricated in silicon are used to detect incident photons with high sensitivity. They are operated with large internal electric fields so that a single electron–hole pair can trigger an avalanche breakdown which generates a signal in an external circuit. We have applied a modified version of the ion beam induced charge technique in a nuclear microprobe system to investigate the application of Geiger mode detectors to detect discrete ion impacts. Our detectors are fabricated with an architecture based on the avalanche diode structure and operated with a transient bias voltage that activates the Geiger mode. In this mode avalanche breakdown is triggered by ion impact followed by diffusion of an electron–hole pair into the sensitive volume. The avalanche breakdown is quenched by removal of the transient bias voltage which is synchronized with a beam gate. An alternative operation mode is possible at lower bias voltages where the avalanche process self-quenches and the device exhibits linear charge gain as a consequence. Incorporation of such a device into a silicon substrate potentially allows the exceptional sensitivity of Geiger mode to register an electron–hole pair from sub-10keV donor atom implants for the deterministic construction of shallow arrays of single atoms in the substrate required for emerging quantum technologies. Our characterization system incorporates a fast electrostatic ion beam switcher gated by the transient device bias, duration 800ns, with a time delay, duration 500ns, that allows for both the ion time of flight and the diffusion of the electron–hole pairs in the substrate into the sensitive region of the device following ion impact of a scanned 1MeV H microbeam. We compare images at the micron scale mapping the response of the device to ion impact operated in both Geiger mode and avalanche (linear) mode for silicon devices engineered with this ultimate-sensitivity detector structure.

Mapping ion beam induced current changes in a commercial MOSFET

We demonstrate a novel nuclear microprobe imaging and analysis modality for micrometre-scale field effect transistor devices probed with focused beams of MeV ions. By recording the drain current as a function of time during ion irradiation it is possible to identify current transients induced by the passage of single ions through the sensitive structures of the device. This modality takes advantage of the fact that the ionization produced by the passage of a single ion acts in an equivalent way to a transient change in the gate bias which therefore modulates the drain current as a function of time. This differs from the traditional ion beam induced charge technique where the ionization drifts in an internal electric field and induces a single charge pulse in an electrode applied to the device. Instead a richer variety of phenomena are observed, with different time constants which depend on the proximity of the ion strike to the channel of the device. The signals may be used to examine device function, radiation sensitivity or to count ion impacts within the channel.

Characterization of silicon polycrystalline solar cells at cryogenic temperatures with ion beam-induced charge

We present a spatial analysis of biased polycrystalline silicon solar cell samples at cryogenic temperatures by the ion beam-induced charge technique. Performing the measurements at cryogenic temperatures leads to freezing out of deep trapping centers, a reduction of the specimen’s capacitance, an increase in the mobility of carriers, and an improvement in the signal to noise ratio and charge collection efficiency. At this low temperature, it permits a higher sensitivity analysis when compared to room temperature measurements, which are limited to zero bias due to leakage current that occurs via surface carrier trapping and detrapping along the grain boundaries. As a result we find the images are significantly more sensitive to subtle differences in charge collection efficiency arising from intra-grain defects.

Characterisation of SiC by IBIC and other IBA techniques

Several new technological applications of silicon carbide have attracted significant attention in recent years. As a wide gap semiconductor it has the capability to be used as a room temperature radiation detector. For most applications, material properties like homogeneity of charge transport, presence of defects, resistance to radiation damage, influence of light impurities (such as hydrogen) are of prime importance. Two different kinds of samples, crystalline (4H–SiC) and thin, amorphous (a-Si 1− x C x :H) films, were studied using ion beam techniques. In the case of SiC single crystal radiation detectors, Li and proton beams with a wide range of energies were used to probe the charge collection efficiency at different device depths using the ion beam induced charge technique. Thin, amorphous and microcrystalline Si 1− x C x :H films with a different stoichiometry and different degrees of structural ordering were examined using RBS and ERDA.

Investigation of 4H–SiC Schottky diodes by ion beam induced charge (IBIC) technique

Ion beam induced charge technique has been used in order to investigate the charge collection properties of 4H–SiC epitaxial Schottky barrier nuclear detectors. In this work, 2 MeV He ions microbeam about 2 μm spot size scanned the total surface area of the detector in order to obtain maps of the charge collection efficiency (CCE) at different bias voltages. The maps turned out to be very homogeneous, with the conclusion that energy resolution of the detectors is not affected by non-homogeneous broadening due to fluctuations of CCE. Fitting the experimental data obtained for low bias voltages—corresponding to depletion layer widths narrower than He ions range—was possible only by including the contribution of diffusion. By this way, holes diffusion length was determined to be 2 μm.

Testing of radiation detectors by IBIC imaging

The nuclear microprobe technique IBIC (Ion Beam Induced Charge) was used for tests of different radiation detectors. By using a 2–6 MeV proton microbeam with a current of less than 1000 protons per second, images and profiles of charge collection efficiency in radiation detectors can be produced. This application of the IBIC technique provides results that can be used to study mobility of charge carriers or inhomogeneities of the electric field inside the depletion region. Presented are examples of IBIC tests performed on CVD diamond, CdTe and Si detectors.

A theory of ion beam induced charge collection

The ion beam induced charge technique can image the depletion regions of microelectronic devices through their thick metallization and passivation layers, and buried dislocation networks in semiconductor material by measuring the number of charge carriers created by a focused MeV light ion beam scanning over the sample surface. In this paper it is shown how the charge pulse height can be calculated in terms of the ion type and energy, and the minority carrier diffusion length. The effect of surface layers, depletion layers, ion induced damage, and ion channeling on the measured charge pulse height are also considered. It is shown how this approach can be used to simulate the charge pulse height reduction due to ion induced damage.

Dislocation imaging using ion beam induced charge

The recently developed ion beam induced charge technique has been used to image bands of misfit dislocations in a 4 μm thick epitaxial layer of Si0.875Ge0.125 grown on a Si substrate. The smallest resolvable bandwidth is 0.8 μm under present conditions. The factors which presently limit this value, and methods for improving it are discussed.