Ion Beam Induced Charge Imaging Research Articles

Effects of Te inclusions on charge-carrier transport properties in CdZnTe radiation detectors

The influence of tellurium (Te) inclusions on the charge collection efficiency in cadmium zinc telluride (CdZnTe or CZT) detectors has been investigated using ion beam induced charge (IBIC) technique. Combining the analysis of infrared transmittance image, most of the low charge collection areas in the IBIC images prove the existence of Te inclusions. To further clarify the role of Te inclusions on charge transport properties, bias dependent local IBIC scan was performed on Te inclusion related regions from 20V to 500V. The result shows that charge collection efficiencies in Te inclusion degraded regions experience fast ascent under low biases and slow descent at high applied biases, which deviates from Hecht rule. This behavior is attributed to the competitive influence of two mechanisms under different biases, namely charge carrier trapping due to uniformly distributed point defects and Te inclusion induced transient charge loss. A modified Hecht equation is further proposed to explain the effects of high-density localized defects, say Te inclusions, on the charge collection efficiency.

Evaluation of Silicon Detectors With Integrated JFET for Biomedical Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents initial results from electrical, spectroscopic and ion beam induced charge (IBIC) characterisation of a novel silicon PIN detector, featuring an on-chip <formula formulatype="inline"><tex Notation="TeX">$n$</tex> </formula>-channel JFET and matched feedback capacitor integrated on its <formula formulatype="inline"><tex Notation="TeX">$p$</tex></formula>-side (frontside). This structure reduces electronic noise by minimising stray capacitance and enables highly efficient optical coupling between the detector back-side and scintillator, providing a fill factor of close to 100%. The detector is specifically designed for use in high resolution gamma cameras, where a pixellated scintillator crystal is directly coupled to an array of silicon photodetectors. The on-chip JFET is matched with the photodiode capacitance and forms the input stage of an external charge sensitive preamplifier (CSA). The integrated monolithic feedback capacitor eliminates the need for an external feedback capacitor in the external electronic readout circuit, improving the system performance by eliminating uncontrolled parasitic capacitances. An optimised noise figure of 152 electrons RMS was obtained with a shaping time of 2 <formula formulatype="inline"> <tex Notation="TeX">$\mu$</tex></formula>s and a total detector capacitance of 2pF. The energy resolution obtained at room temperature (21<formula formulatype="inline"> <tex Notation="TeX">$^{\circ}$</tex></formula>C) at 27 keV (direct interaction of I-125 gamma rays) was 5.09%, measured at full width at half maximum (FWHM). The effectiveness of the guard ring in minimising the detector leakage current and its influence on the total charge collection volume is clearly demonstrated by the IBIC images. </para>

Ion beam induced charge (IBIC) irradiation damage study in synthetic single crystal diamond using 2.6 MeV protons

AbstractWe have studied the effects of irradiation induced damage on the detector response of a synthetic single crystal diamond radiation detector. Before introducing radiation damage, the spatial variation of the detector response was investigated using a highly focused 2.6 MeV proton beam. A very uniform response close to 100% charge collection efficiency (CCE) over the whole contact area was found at applied electric field strengths as low as 0.4 V μm–1. At lower biases, time dependent polarisation phenomena were observed and clearly reduced the average signal amplitude. Subsequently, the 2.6 MeV proton beam has been used to introduce radiation damage within selected areas. The ion beam induced charge imaging was repeated to probe the modified regions. Even at an applied electric field of 2.6 V μm–1, no signal above the analogue threshold of the system was observed in the areas which had received a dose larger then 5 × 1014 cm–2, whereas more than 90% CCE was reached in the area with 1012 protons cm–2. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ion beam induced charge imaging of charge transport in CdTe and CdZnTe

Ion beam induced charge (IBIC) imaging is a powerful technique for quantitative mapping of the charge transport performance of wide bandgap semiconductor materials. In this paper we present results from a study of electron and hole mobility–lifetime product and drift mobility in CdTe:Cl and CdZnTe, which are semiconductor materials used for radiation detector applications. IBIC imaging has been used to produce mobility–lifetime product maps in CdTe:Cl and CdZnTe, revealing the influence of extended defects and tellurium inclusions and assessing the large area response uniformity of the materials. The recent extension of this method in the form of digital time-resolved IBIC is also discussed and time of flight maps are presented which give quantitative images of electron and hole drift mobility.

Mapping of polarization and detrapping effects in synthetic single crystal chemical vapor deposited diamond by ion beam induced charge imaging

Diamond has been regarded as a promising radiation detector material for use as a solid state ionizing chamber for decades. The parameters degrading the charge transport from what is expected from an ideal crystal are still not completely understood. Recently, synthetic chemical vapor deposited (CVD) single crystal diamond has become available, offering the opportunity to study the properties of synthesized material independent of grain boundaries. We have studied the charge transport of a synthetic single crystal diamond with α-particle induced charge transients as a function of temperature and established the presence of a shallow hole trap with an activation energy of 0.29±0.02eV in some parts of the detector. Ion beam induced charge imaging has been used to study the spatial variations of the charge transport in a synthetic single crystal diamond. Pulses influenced by the shallow hole trap had their origin close to the substrate∕CVD interface of the sample. They could be clearly distinguished from pulses affected by reduced charge carrier velocities due to polarization phenomena, which varied systematically with the growth direction of the CVD diamond material.

Effect of dislocations on charge carrier mobility–lifetime product in synthetic single crystal diamond

The authors report correlations between variations in charge transport of electrons and holes in synthetic single crystal diamond and the presence of nitrogen impurities and dislocations. The spatial distribution of these defects was imaged using their characteristic luminescence emission and compared with maps of carrier drift length measured by ion beam induced charge imaging. The images indicate a reduction of electron and hole mobility–lifetime product due to nitrogen impurities and dislocations. Very good charge transport is achieved in selected regions where the dislocation density is minimal.

Charge transport in polycrystalline and single crystal synthetic diamond using ion beam induced charge imaging

Ion beam induced charge (IBIC) imaging has been used to investigate charge transport in high quality polycrystalline CVD diamond and single crystal synthetic diamond. In polycrystalline material the intra-crystallite charge transport within single crystals was studied. Micrometer resolution images of charge signal amplitude in different regions of a diamond crystallite are obtained as a function of the electric field strength. The charge signal amplitude, which is directly related to charge carrier drift length, is correlated to the distribution of radiative centres in the diamond identified using cathodoluminescence imaging. By comparison, IBIC imaging of single crystal synthetic diamond was also carried out. The sample, which contained deliberately introduced nitrogen lines, showed electron and hole CCE values close to 100%, in the low nitrogen regions. By comparison with CL and UV luminescence images, direct evidence is observed for the role of nitrogen and dislocations as deep trapping centres and their reduction of both electron and hole lifetimes.

Ion beam induced charge imaging of epitaxial GaN detectors

We report the use of ion beam induced charge imaging to characterise the charge signal uniformity of epitaxial gallium nitride radiation detectors. The detectors were fabricated from 2μm thick semi-insulating gallium nitride, grown by MOCVD on a sapphire substrate. A carrier concentration of 1.4×1015cm−3 was measured using capacitance–voltage measurements. Ion beam induced charge imaging was carried out with a 2MeV alpha particle beam focussed to a 3μm diameter and raster scanned across the device. The resulting ion beam images show excellent charge signal uniformity in this material with no evidence of material defects or polycrystalline structure on the micrometer length scale. No evidence of charge signal trapping was observed in these devices.

IBIC imaging of intra-crystallite charge transport in CVD diamond at reduced temperature

We present high resolution maps of charge collection efficiency in polycrystalline CVD diamond film, acquired using ion beam induced charge imaging with a 2 MeV He 2+ microbeam. By measuring the charge collection efficiency in a coplanar electrode device, it is possible to observe the spatial variations in charge drift length within a single large-area diamond crystallite. The measured charge collection efficiency within the crystallite increases with bias voltage, with no saturation observed at voltages up to −250 V, corresponding to an average bulk electric field of 2.5 V/μm. When cooling the device from 297 K to 200 K, the maximum charge collection efficiency decreases from 70% to 13% due to a reduction in charge carrier lifetime. This data gives an insight into the role of relatively shallow traps present within diamond crystallites, which tend to limit the charge drift length in CVD diamond radiation detectors at room temperature.

High-resolution mapping of the mobility–lifetime product in CdZnTe using a nuclearmicroprobe

We present frontal ion beam induced charge imaging on a cadmium zinctelluride device using a 2.05 MeV He microbeam. Two sets of voltage-dependent3 mm × 3 mm scans over the device cathode were acquired at room temperature and 250 K respectivelyand the corresponding charge collection efficiency (CCE) images extracted. The reductionof CCE due to electron transport with reduced bias voltage can be described using a simplifiedHecht equation. This allows us to measure the mobility–lifetime product for electrons(μeτe) and so producehigh-resolution μeτe images at 296 and 250 K. At 296 K, CCE values up to 96% were observed for an electric field of3570 V cm−1. In general, theCCE values at 250 K are lower than the comparable values at 296 K withμeτe decreasingfrom 4.7 × 10−4 to 1.2 × 10−4 cm2 V−1,about 1/4 of its original value. Additionally, we observe an increase in CCE at 250 K due to previousirradiation of the material, caused by partial filling of electron traps by the ionbeam.

Digital IBIC—new spectroscopic modalities for ion-beam-induced charge imaging

The first implementation of digital ion-beam-induced charge (IBIC) imaging is presented, which permits spectroscopic time-resolved IBIC imaging of charge transport in semiconductors. A digital IBIC system has been developed which uses a high-speed waveform digitiser to capture the pulse shapes produced by interactions of a 1 μm resolution scanning microbeam in semiconductor samples. Using a variety of digital pulse shape analysis algorithms, quantitative images of charge signal amplitude, charge carrier lifetime, mobility and trapping phenomena can be acquired in real-time, with a time resolution of <20 ns. We report data from the commissioning experiment which shows digital IBIC images of electron and hole transport in CdZnTe radiation detectors, obtained by lateral scanning of the ion beam between the cathode and anode. The room temperature electron drift mobility in CdZnTe was measured with a value of 1.1×10 3 cm 2/Vs.

IBIC characterisation of novel detectors for single atom doping of quantum computer devices

Single ion implantation and online detection is highly desirable for the emerging application, in which single 31P ions need to be inserted in prefabricated silicon cells to construct solid-state quantum bits (qubits). In order to fabricate qubit arrays, we have developed novel detectors that employ detector electrodes adjacent to the prefabricated cells that can detect single keV ion strikes appropriate for the fabrication of shallow phosphorus arrays. The method utilises a high purity silicon substrate with very high resistivity, a thin SiO 2 surface layer, nanometer masks for the lateral positioning single phosphorus implantation, biased electrodes applied to the surface of the silicon and sensitive electronics that can detect the charge transient from single keV ion strikes. A TCAD (Technology Computer Aided Design) software package was applied in the optimisation of the device design and simulation of the detector performance. Here we show the characterisation of these detectors using ion beam induced charge (IBIC) with a focused 2 MeV He ions in a nuclear microprobe. The IBIC imaging method in a nuclear microprobe allowed us to measure the dead-layer thickness of the detector structure (required to be very thin for successful detection of keV ions), and the spatial distribution of the charge collection efficiency around the entire region of the detector. We show that our detectors have near 100% charge collection efficiency for MeV ions, extremely thin dead-layer thickness (about 7 nm) and a wide active region extending laterally from the electrodes (10–20 μm) where qubit arrays can be constructed. We demonstrate that the device can be successfully applied in the detection of keV ionisation energy from single events of keV X-rays and keV 31P ions.

Ion beam induced charge and cathodoluminescence imaging of response uniformity of CVD diamond radiation detectors

The uniformity of response of CVD diamond radiation detectors produced from high quality diamond film, with crystallite dimensions of >100 μm, has been studied using ion beam induced charge imaging. A micron-resolution scanning alpha particle beam was used to produce maps of pulse height response across the device. The detectors were fabricated with a single-sided coplanar electrode geometry to maximise their sensitivity to the surface region of the diamond film where the diamond crystallites are highly ordered. High resolution ion beam induced charge images of single crystallites were acquired that demonstrate variations in intra-crystallite charge transport and the termination of charge transport at the crystallite boundaries. Cathodoluminescence imaging of the same crystallites shows an inverse correlation between the density of radiative centres and regions of good charge transport.

Ion beam induced charge characterisation of a silicon microdosimeter using a heavy ion microprobe

An ion beam induced charge (IBIC) facility has been added to the existing capabilities of the ANSTO heavy ion microprobe and the results of the first measurements are presented. Silicon on insulator (SOI) diode arrays with microscopic junction sizes have recently been proposed as microdosimeters for hadron therapy. A 20 MeV carbon beam was used to perform IBIC imaging of a 10 μm thick SOI device.

Imaging of charge transport in polycrystalline diamond using ion-beam-induced charge microscopy

Studies have been made using a 1 μm spatial resolution ion-beam-induced charge (IBIC) technique of the charge transport distribution in polycrystalline diamond produced by chemical vapor deposition. The devices tested used a coplanar electrode structure fabricated only on the growth side of the diamond film, and were predominantly sensitive to charge transport close to the growth surface of the diamond film where the diamond crystallites are largest. Irradiation with 5.48 MeV alpha particles gave a pulse height spectrum with a broad full energy peak and a mean charge collection efficiency of 15%. IBIC images obtained using microfocus proton and alpha particle beams showed spatially resolved regions of high charge collection efficiency correlating to individual diamond crystallites with a typical width of 20 μm, as observed by secondary electron microscopy.

Fluence dependence of IBIC collection efficiency of CMOS transistors

IBIC (Ion Beam Induced Charge) imaging of deep structures of semiconductor devices with a focused MeV light ion beam has been shown to offer significant advantages over the established EBIC (Electron Beam Induced Current) technique, because the large range and the small lateral straggling of the ion beam allows direct imaging of buried device structures. The technique is limited by the accumulation of radiation damage that reduces the charge collection efficiency. We report on measurements of the beam fluence dependence on IBIC collection efficiency for proton and alpha particle beams at 2000 keV energy. A HCF4007 CMOS transistor array was used in these measurements. The influence of surface passivation layers on charge collection efficiency and its evolution with ion dose is discussed.

The influence of ion induced damage on lateral charge collection and IBIC image contrast

High resolution, calibrated ion beam induced charge (IBIC) measurements from integrated circuit test structures have demonstrated that the measured charge collection in a device can exhibit significant change after only a few hundred ions/μm 2 exposure, which may easily be exceeded in the initial targeting of a structure. For the purposes of determining a circuit's upset immunity or undamaged charge collection characteristics, such behaviour must be accounted for in evaluating IBIC measurements. This paper examines the influence of low level, ion induced damage on the magnitude of the measured lateral charge collection and also its resulting impact on IBIC image contrast. The lateral charge collection process is first characterised by calculating the amount of charge which diffuses to a collecting junction as a function of carrier diffusion length and the distance between the ion strike and junction edge. The effect of accumulated ion induced damage on lateral charge collection is then incorporated as a decrease in the resultant diffusion length. Calibrated IBIC measurements from the drain of a test FET structure are then explained using this predicted behaviour.

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 × 10 15 /cm 3). 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 × 10 15 /cm 3) grown on a semi insulator GaAs substrate ( 1 × 10 7 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 SiO 2, 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 × 10 16/cm 3) 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.

Quantitative characterization of Si/SiO 2 interface traps induced by energetic ions by means of single ion microprobe and single ion beam induced charge imaging

The dependence of single ion induced interface traps on the oxide electric field during irradiation has been investigated quantitatively by using single ion microprobe (SIMP) and single ion beam induced charge (SIBIC) imaging. The result shows that the number of interface traps induced by 2 MeV He ions increases with the dependence of the E 0.2 ox on the oxide electric field. The dependence of interface traps on the oxide field during ion irradiation has been found to be very similar to that of oxide trapped holes. The cause of the field dependence of interface traps has been discussed quantitatively.