Proton-irradiated Silicon Research Articles

Defect characterization studies on irradiated boron-doped silicon pad diodes and Low Gain Avalanche Detectors

High-energy physics detectors with internal charge multiplication, like Low Gain Avalanche Detectors (LGADs), that will be used for fast timing in the High Luminosity LHC experiments, have to exhibit a significant radiation tolerance. In this context, the impact of radiation on the highly boron-doped gain layer is of particular interest, since due to the so-called Acceptor Removal Effect (ARE) a radiation-induced deactivation of active boron dopants takes place, that is causing a progressive loss in the gain with increasing irradiation level. In this paper we present defect-spectroscopy measurements (Deep-Level Transient Spectroscopy and Thermally Stimulated Current technique) on neutron, proton and electron irradiated p-type silicon pad diodes of different resistivity as well as LGADs neutron irradiated at fluences up to 1×1015 neq/cm2. We show that compared to silicon pad diodes the determination of LGAD defect introduction rates is less straightforward as they are strongly influenced by the impact of the gain layer. The measured gain layer capacitance has a strong frequency and temperature dependence which makes DLTS measurements challenging to perform with results difficult to interpret. With the TSC technique the defects formed in the LGADs are nicely observed and can be compared to the defects formed in the silicon pad diodes. However, the exact assignment of defects to the gain layer or bulk region remains challenging and the charge amplification effect of the LGADs impacts the exact determination of defect concentrations. We also demonstrate that, depending on the TSC measurement conditions, defect induced internal electric fields are built up in the irradiated LGADs which impact the signal current.

Investigation of Energy Levels of Small Vacancy Clusters in Proton Irradiated Silicon by Laplace Photoinduced Transient Spectroscopy

Laplace photoinduced transient spectroscopy has been applied to determine the electronic properties and concentrations of deep traps in high purity n-type silicon irradiated with high fluences of 23-MeV protons. From the temperature dependence of thermal emission rates of excess charge carriers obtained by the analysis of the photocurrent relaxation waveforms measured at temperatures of 30–320 K, eight deep traps with activation energies ranging from 255 to 559 meV have been resolved. The dependence of these trap’s concentrations on the proton fluence are demonstrated for the fluence values ranging from 1 × 1014 to 5 × 1015 neq/cm2. In comparison to the previously reported results of theoretical and experimental studies on the electronic properties of small vacancy clusters in irradiated silicon, we tentatively attribute four detected traps with activation energies of 255, 367, 405, and 512 meV to the energy levels related to the 2−/− charge state changes of divacancy (V2), trivacancy (V3), tetravacancy (V4), and pentavacancy (V5), respectively. Simultaneously, we propose the attribution of four deep traps with higher activation energies of 415, 456, 526, and 559 meV to the energy levels related to the −/0 charge state changes of these small vacancy clusters, respectively.

Annealing-temperature-dependent evolution of hydrogen-related donor and its strong correlation with X-photoluminescence center in proton-irradiated silicon

We have investigated the formation and decay of hydrogen-related donors (HDs) and irradiation-induced intrinsic defects. N-type m:Cz and FZ silicon wafers, which were irradiated with 2 MeV protons and subsequently annealed at 100–600 °C, were analyzed using spreading resistance profiling and photoluminescence (PL). HDs formed at 260 °C and then disappeared in two stages at 400–440 and 500–540 °C. This decay behavior indicates the existence of two types of HDs with different thermal stabilities. PL measurements showed interstitial silicon clusters (W and X center), a carbon–oxygen complex (C center), and exciton lines bound to unknown shallow centers. The origin of the HDs was investigated based on the correlation of the formation and decay temperatures between HDs and irradiation-induced defects. The predominant defects at the early stage of annealing, such as the C and W centers, are ruled out as candidates for the core defects of HDs because annealing above 260 °C is indispensable for the HD formation. In contrast, the X center was found to be thermally generated above 200 °C and disappeared at 580 °C. The similarity of the formation and decay temperatures between the X and HD centers suggests that HDs are associated with the formation of the interstitial silicon-related defects attached to hydrogen. Our results suggest that controlling the formation of interstitial silicon-related defects is important for realizing desirable doping profiles with high accuracy and reproducibility for power devices. Annealing above 400 °C exclusively provides thermally more stable HDs, leading to the realization of more rugged power devices.

Current-Voltage Characteristics of 4 MeV Proton-Irradiated Silicon Diodes at Room Temperature

Current-Voltage Characteristics of 4 MeV Proton-Irradiated Silicon Diodes at Room Temperature

Electrical properties of 3 MeV proton irradiated silicon Schottky diodes

This work presents the effects of proton irradiations on the electrical properties of n-silicon-based Schottky diodes. A change in electrical properties of the diodes due to 3 MeV proton irradiation to the fluence of 1017 ion cm−2 was investigated by current-voltage (I-V) and capacitance-voltage (C-V) techniques. A drastic decrease in forward current by a factor of 103 indicates a reduction of free injected effective carrier density in the space charge region. This reduction in the density is observed by a decrease in rectification ratio after proton irradiation. The results in general indicate that proton induced defects are responsible for the recombination/compensation of charge carriers resulting in an increase in the resistivity of the material. An increase in the resistivity is confirmed by high series resistance and low doping density evaluated on the irradiated diode. A change in dominated diode conduction mechanism due to irradiation presented here is of technological importance. Furthermore, the results presented here would, certainly, contribute to an ongoing study on the induced damage by radiation in silicon.

Modelling of disordering regions in proton-irradiated silicon

A model of disordering regions generation as a result of interstitial atom-vacancy pairs separation in view of neutral and charged pair states was developed. The model allows to define the radius and the mean number of vacancies of a disordering region. Distribution profiles of interstitial silicon, vacancies, divacancies, disordering regions created by low-energy protons in silicon were calculated, as well as the disordering regions parameters dependencies on proton energies. It was shown that the disordering region distribution maximum is spatially separated from interstitial silicon, vacancies and divacancies distribution maximums. This allows to differentially modify superficial and volumetric properties of semiconductor structures.

Temperature scaling of reverse current generated in proton irradiated silicon bulk

The value of the scaling parameter Eeff of the temperature dependence for current generated in silicon bulk is investigated for highly irradiated devices. Measurements of devices irradiated to fluences above 1 × 1015 neqcm-2 have shown a different temperature scaling behaviour than devices irradiated to lower fluences. This paper presents the determination of the parameter Eeff for diodes irradiated with protons up to fluences of 3 × 1015 neqcm-2 in the bias range from 0 V to 1000 V at temperatures from −36° to 0° at different stages of annealing. It is shown that Eeff for highly irradiated devices depends on the applied electric field: below depletion voltage, Eeff is observed to have a lower value than above depletion voltage.

Comparison between proton irradiated triple gate SOI TFETS and finfets from a TID point of view

This paper compares, for the first time, the total ionizing dose degradation of 600 keV proton-irradiated silicon triple gate SOI tunnel FETs (TFETs) with SOI FinFETs fabricated with the same structure on the same wafer. This work is based on electrical measurements and TCAD simulations. Devices with different dimensions were exposed up to 10 Mrad(Si) dose of proton radiation. For transistors with narrow fin width, the radiation influence is negligible in both devices types. For WFIN = 250 nm and a proton radiation dose up to 10 Mrad(Si), on the other hand, a severe influence is observed in nFinFETs and pFinFETs present on the same die as the p-type TFETs. In the TFETs, no marked influence is observed. Only for a TFET of WFIN = 500 nm and 10 Mrad(Si) one can observe some radiation influence. The main degradation is caused by the buried oxide positive fixed charges and the interface traps which was also confirmed by TCAD simulations. The tunneling current of Tunnel-FETs presents a much better radiation hardness compared to the drift-diffusion current of SOI FinFETs.

Polypropylene-high resistivity silicon composite for high frequency applications

Polypropylene-high energy proton irradiated silicon composite was prepared by hot pressing. The dielectric properties of the composite was measured at both low and microwave frequency range up to 45.9 GHz. The composite has a relative permittivity of 3.7 at 5 and 15 GHz but decreased to 2.82 at 45.9 GHz. The loss tangent decreased from 3.36 × 10−3 to 1.7 × 10−3 with increase in frequency in the same frequency range whereas the reported polymer-ceramic composites show an increasing trend in loss factor. The decreasing trend of the loss tangent with increasing frequency indicates the possibility of its use in high frequency applications such as IOT and 5G. The composite containing 30 vol% of high energy proton irradiated high resistivity Si has a relative permittivity of 2.82 and loss tangent 0.0017 at 45.9 GHz. The coefficient of thermal expansion (CTE) of polypropylene decreased from 140 to 60 ppm/°C and the composite has a thermal conductivity of 0.95 W/mK. The thermal conductivity of polypropylene (0.22) has increased by about 331% by the addition of 30 vol% of high resistivity silicon powder.

Cross-sectional investigation of radiation damage of 2 MeV proton-irradiated silicon carbide

Cross-sectional investigation is an important method to study ion irradiation effects in the depth direction. In this study, 2 MeV H+ was implanted in 6H-SiC single crystals to investigate the effects of light ion irradiation on SiC. Raman spectroscopy and scanning electronic microscopy (SEM) were carried out on cross-sectional samples to reveal the in-depth damage states and dopant behavior. The most damaged region is a little shallower than that predicted by the SRIM procedure, owing to the uncertainty in SRIM simulations. Layered structures representing zones of varying damage after 2 MeV H ion irradiation are clearly observed. Two bands are observed in SEM images, of which on band corresponds to the damage peak, while the other band at the end of the H ion-affected area is probably a result of H diffusion propelled by a hydrogen-rich layer during irradiation. A charge accumulation effect related with conductivity on the sample surfaces during SEM tests is observed in the H-implanted area. A model is proposed to explain these phenomena.

High resolution X-ray diffraction study of proton irradiated silicon crystals

Radiation-induced modification of semiconductors is achieved by controlled introduction of intrinsic structural and impurity defects. Conventionally, introduction of radiation-induced defects is used as an efficient tool for controlling the lifetime of metastable carriers in local areas of silicon based devices and supporting mechanisms of avalanche-like breakdown through radiation-induced defect levels. Desired parameters of damaged layers are typically achieved during post-implantation heat treatment. There are recent applications of proton irradiation in silicon technology. A significant growth of luminescence was observed in proton irradiated silicon and attributed to the formation of special rod-shaped clusters of interstitial type radiation defects.We have studied the transformation of radiation-induced defects forming as a result of proton implantation into n silicon crystals with a resistivity of 100Ωcm using high resolution X-ray diffraction and shown that sequential implantation of 100, 200 and 300keV protons with a fluence of 2.1016cm−2 causes the formation of a 2.4μm thick damaged layer with a greater lattice parameter. The layer forms simultaneously with intrinsic clusters of vacancy and interstitial type radiation-induced defects. Vacuum annealing of the irradiated crystals at 600°C increases the power of the radiation-induced defects of both types and reduces their quantity. Interstitial type defects dominate after annealing at 1100°C. We have assessed the power of the defects at every transformation stage.

A method to simulate the observed surface properties of proton irradiated silicon strip sensors

During the scheduled high luminosity upgrade of the LHC, the world's largest particle physics accelerator at CERN, the position sensitive silicon detectors installed in the vertex and tracking part of the CMS experiment will face a more intense radiation environment than the present system was designed for. To upgrade the tracker to the required performance level, extensive measurements and simulation studies have already been carried out. A defect model for the bulk properties of proton irradiated silicon has been created with the Synopsys Sentaurus TCAD simulation package which produces simulated results that closely match the measured properties of silicon strip detectors. However, the expected behaviour due to increased surface damage is not so well predicted by the model. The solution requires an approach that does not affect the accurate bulk properties produced by the proton model, but only adds to it the required radiation induced properties close to the surface. These include the observed position dependence of the strip detector's charge collection efficiency (CCE). In this paper a procedure to find a defect model that reproduces the correct CCE loss, along with other surface properties of a strip detector up to a fluence 1.5×1015 1 MeV neq cm−2 (Φeq), will be presented. When applied to CCE loss measurements at different fluences, this method may provide a means for the parametrization of the accumulation of oxide charge at the SiO2/Si interface as a function of dose.

Measurement of thermal conductivity in proton irradiated silicon

We investigate the influence of proton irradiation on thermal conductivity in single crystal silicon. We apply a laser based modulated thermoreflectance technique to measure the change in conductivity of the thin layer damaged by proton irradiation. Unlike time domain thermoreflectance techniques that require application of a metal film, we perform our spatial domain measurement on uncoated samples. This provides greater sensitivity to the change in conductivity of the thin damaged layer. Using sample temperature as a parameter provides a means to deduce the primary defect structures that limit thermal transport. We find that under high temperature irradiation the degradation of thermal conductivity is caused primarily by extended defects.

EXPERIMENTAL CHARACTERIZATION OF PROTON RADIATED SiGe POWER HBTs AT EXTREME TEMPERATURES

The performances of proton irradiated silicon–germanium (SiGe) power heterojunction bipolar transistors (HBTs) at extreme temperatures (liquid nitrogen temperature and high stage-temperature of 120°C with junction temperature over 160°C) are reported in this work. SiGe power HBTs with total emitter area of ~ 1460 μm2 are fabricated in a commercial BiCMOS process, and irradiated with proton at different fluences from 1 × 1012 p/cm2 to 5 × 1013 p/cm2. Experimental characterizations are conducted for pre- and post-radiation devices at room temperature, cryogenic temperature and high temperature. The results demonstrate that the proton-irradiated SiGe power HBTs are naturally suitable for electronic operations at extreme temperatures. Specifically, investigation of proton radiation on SiGe power HBTs at liquid nitrogen temperature (77 K) indicates a significant potential for space applications. In addition, SiGe power HBTs show better tolerance of proton radiation at high temperature of 120°C (junction temperature over 160°C). SiGe power HBTs demonstrate great potential in power amplification for wireless communication systems under severe radiation and extreme temperature environment (cryogenic and high temperatures) even without any intentional radiation hardening.

RF characteristics of proton radiated large-area SiGe HBTs at extreme temperatures

The radio frequency (RF) characteristics of proton irradiated large-area silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs) at extreme temperatures (liquid nitrogen temperature of 77K and high temperature of ∼430K) are reported in this work. Large emitter area high-power SiGe HBTs fabricated in a commercial BiCMOS process were irradiated with proton, at various fluences from 1×1012p/cm2 to 5×1013p/cm2. Small-signal ac experimental characterizations were achieved for pre- and post-radiation devices at room temperature, cryogenic and high temperatures. We show that proton radiated large-area SiGe HBTs are naturally suitable for electronic operations at extreme temperatures. Investigation of proton radiation on large-area SiGe HBTs at liquid nitrogen temperature (77K) demonstrates great potential of SiGe power HBTs for space applications. Moreover, performance of proton radiated SiGe power HBTs at high temperature of ∼430K indicates a better tolerance of proton radiation than room temperature. Possible underlying mechanisms for the improvement of radiation robustness at cryogenic and high temperatures are discussed. The results demonstrate the possibility of using large-area SiGe HBTs in power amplification for wireless applications under severe radiation and extreme temperature environment without any intentional radiation hardening.

Determination of radii of nanoobjects in porous systems and some defective materials by positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) is shown to be one of the effective modern methods to determine sizes of nano-objects (vacancies and vacancy clusters), free volumes of pores, cavities, voids, and their concentrations and chemical compositions in nanomaterials and porous systems. A brief review is given of experimental studies on nanodefects in proton irradiated porous silicon, silicon, and quartz single crystals.

On the feasibility of investigation of some defect and porous systems by means of positron annihilation spectroscopy

It was shown that positron annihilation spectroscopy is an effective technique for the determination of the size of nanodefects (vacancies, vacancy clusters); the free volumes of pores, cavities, and voids; and their concentration and chemical composition at the site of annihilation in some defect and porous systems. Experimental studies of nanodefects in porous silicon, proton-irradiated monolithic silicon and quartz single crystals, and quartz powders are briefly surveyed.

Low-temperature TCT characterization of heavily proton irradiated p-type magnetic Czochralski silicon detectors

n +/p −/p + pad detectors processed at the Microelectronics Center of Helsinki University of Technology on boron-doped p-type high-resistivity magnetic Czochralski (MCz-Si) silicon substrates have been investigated by the transient current technique (TCT) measurements between 100 and 240 K. The detectors were irradiated by 9 MeV protons at the Accelerator Laboratory of University of Helsinki up to 1 MeV neutron equivalent fluence of 2×10 15 n/cm 2. In some of the detectors the thermal donors (TD) were introduced by intentional heat treatment at 430 °C. Hole trapping time constants and full depletion voltage values were extracted from the TCT data. We observed that hole trapping times in the order of 10 ns were found in heavily (above 1×10 15 n eq/cm 2) irradiated samples. These detectors could be fully depleted below 500 V in the temperature range of 140–180 K.

Test beam results of a proton irradiated Czochralski silicon strip detector

We report on test beam results obtained with a 32.5 cm 2 microstrip detector processed on an n-type 380 μm thick magnetic Czochralski (MCZ) grown silicon substrate with 1200 Ωcm effective resistivity. The full depletion voltage of the as-processed detector was 420 V with a leakage current of 2 μA. The AC coupled detector had 1024 p + strips, 10 μm by width and 6.154 cm by length with a pitch of 50 μm. The detector was irradiated with 10 MeV protons to 1.6×10 14 1 MeV neutron equivalent fluence and annealed for 345 days at room temperature. The post-irradiation full depletion voltage of the detector was 225 V. The leakage current at the full depletion measured at −10 °C was 261 μA. The beam tests were carried out at the CERN H 2 area using a Silicon Beam Telescope, which consists of pairs of horizontal and vertical position sensitive silicon detectors. This telescope determines the tracks of incoming particles and hence provides a reference measurement for the detector characterization. In the beam test an average signal to noise ratio of 3 with a spatial resolution of 20 μm and a particle detection efficiency of 36% were measured. These results show that the MCZ device detected particles, which encourages further investigations of MCZ silicon as a detector material. The poor performance of the MCZ detector may be explained by the problems observed in the reference telescope.

Study of carrier recombination and trapping processes in γ-ray- and proton-irradiated silicon

Carrier lifetime variations dependent on proton irradiation at fluences in the range from 5×10 12 to 10 15 cm −2 and γ-ray doses ranging from 50 to 400 Mrad were investigated in high-resistivity oxygenated silicon wafers and pad detectors. Fast recombination and slow trapping constituents within recombination transients have been distinguished by combining analyses of excess carrier decays dependent on excitation intensity and temperature, measured using the technique of microwave absorption by free carriers. Difference in defect formation rate and type of defects in the ranges of moderate and highest proton irradiation fluences as well as between γ-ray- and proton-irradiated material have been revealed from the inverse lifetime dependence on irradiation fluence and on temperature. The activation factors of the capture centers have been evaluated from carrier lifetime variations in the range of low and elevated temperatures.