Characterization of charge collection in CdTe and CZT using the transient current technique

The charge collection properties in different particle sensor materials with respect to the shape of the generated signals, the electric field within the detector, the charge carrier mobility and the carrier lifetime are studied with the transient current technique (TCT). Using the well-known properties of Si as a reference, the focus is laid on Cadmium-Telluride (CdTe) and Cadmium-Zinc-Telluride (CZT), which are currently considered as promising candidates for the efficient detection of X-rays. All measurements are based on a transient-current technique (TCT) setup, which allows the recording of current pulses generated by an 241Am alpha-source. These signals will be interpreted with respect to the build-up of space-charges inside the detector material and the subsequent deformation of the electric field. Additionally the influence of different electrode materials (i.e. ohmic or Schottky contacts) on the current pulse shapes will be treated in the case of CdTe. Finally, the effects of polarization, i.e. the time-dependent degradation of the detector signals due to the accumulation of fixed charges within the sensor, are presented.Comment: 20 pages, 17 figure

Three-dimensional charge transport mapping by two-photon absorption edge transient-current technique in synthetic single-crystalline diamond

We demonstrate the application of two-photon absorption transient current technique to wide bandgap semiconductors. We utilize it to probe charge transport properties of single-crystal Chemical Vapor Deposition (scCVD) diamond. The charge carriers, inside the scCVD diamond sample, are excited by a femtosecond laser through simultaneous absorption of two photons. Due to the nature of two-photon absorption, the generation of charge carriers is confined in space (3-D) around the focal point of the laser. Such localized charge injection allows to probe the charge transport properties of the semiconductor bulk with a fine-grained 3-D resolution. Exploiting spatial confinement of the generated charge, the electrical field of the diamond bulk was mapped at different depths and compared to an X-ray diffraction topograph of the sample. Measurements utilizing this method provide a unique way of exploring spatial variations of charge transport properties in transparent wide-bandgap semiconductors.Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Applied Physics Letters and may be found at https://doi.org/10.1063/1.509085

Study of interpad-gap of HPK 3.1 production LGADs with Transient Current Technique

The Phase-2 upgrade of the Large Hadron Collider (LHC) to High-Luminosity LHC (HL-LHC) allows an increase in the operational luminosity value by a factor of 5-7 that will result in delivering 3000 fb(-1) or more integrated luminosity. Due to high luminosity, the number of interactions per bunch crossings (pileup) will increase up to a value of 140-200. To cope with high pileup rates, a precision minimum ionising particles (MIPs) timing detector (MTD) with a time resolution of similar to 30-40 ps and hermetic coverage up to a pseudo-rapidity of vertical bar eta vertical bar = 3 is proposed by the Compact Muon Solenoid (CMS) experiment. An endcap part (1.6 <vertical bar eta vertical bar <3) of the MTD, called the endcap timing layer, will be based on low-gain avalanche detector (LGAD) technology. LGADs provide a good timing resolution due to a combination of a fast signal rise time and high signal-to-noise ratio. The performance of the ETL depends on optimising the crucial features of the sensors, namely; gain, signal homogeneity, fill factor, leakage current, uniformity of multiple-pad sensors and long term stability. The paper mainly focuses on the study of the fill factor of LGADs with varying temperature and irradiation at varying proton fluences as these sensors will be operated at low temperatures and are subjected to a high radiation environment. The 3.1 production of LGADs from Hamamatsu Photonics K.K. (HPK) includes 2x2 sensors with different structures, in particular, different values of narrower inactive region widths between the pads, called the no-gain region. In this paper, the term interpad-gap is used instead of no-gain region in order to follow the conventional terminology. These sensors have been designed to study their fill factor, which is the ratio of the area within the active region (gain region) to the total sensor area. A comparative study on the dependence of breakdown voltage with the interpad-gap width for the sensors has been carried out. Using infrared light (as the electron-hole pair creation by IR laser mimics closely to the traversing of MIPs) from the Scanning-Transient Current Technique (Scanning-TCT) set-up shows that the fill factor does not vary significantly with a variation in temperature and irradiation at high proton fluences.Peer reviewe

Electric Field Measurement by Edge Transient Current Technique on Silicon Low Gain Avalanche Detector

A novel methodology, named the diffusion profile method, is proposed in this research to measure the electric field of a low gain avalanche detector (LGAD).The proposed methodology utilizes the maximum of the time derivative of the edge transient current technique (edge-TCT) test waveform to quantify the dispersion of the light-induced carriers. This method introduces the estimation of the elongation of the carrier cluster caused by diffusion and the divergence of the electric field force during its drift along the detector. The effectiveness of the diffusion profile method is demonstrated through the analysis of both simulated and measured edge-TCT waveforms. Experimental data was collected from a laser scan performed on an LGAD detector along its thickness direction.A simulation procedure has been developed in RASER (RAdiation SEmiconductoR) to generate signals from LGAD.An assumption of immediate one-step carrier multiplication is introduced to simplify the avalanche process.Simulation results were compared with transient current data at the waveform level and showed a favorable match. Both simulation and experimental results have shown that the diffusion profile method could be applied to certain edge-TCT facilities as an alternative of electric field measurement

Transient Current Technique (TCT) Characterization of Silicon Particle Detectors

Hiukkasilmaisimet altistuvat huomattavalle säteilylle tulevaisuudessa korkean energian hiukkasfysiikkakokeissa, esimerkiksi kun CERN:n hiukkaskiihdytin LHC ajanmukaistetaan. Näin ollen pii-ilmaisimien säteilynkestävyyden parantamista eli erilaisten säteilyn aiheuttamien kidevirheiden sekä ilmaisinsovellusten muokkaamista tutkitaan laajasti. Magneettisella Czochralski-menetelmällä (MCZ) valmistetuista kiekoista prosessoituja p-tyypin ilmaisimia on kuitenkin tarkasteltu melko vähän. Tässä työssä verrattiin sekä n- että p-tyypin MCZpii-ilmaisimia sekä float zone -menetelmällä (FZ) valmistetuista kiekoista prosessoituja n-tyypin ilmaisimia suorittamalla virtatransienttimittauksia (TCT). Tulokset osoittavat, että MCZ-piin sähköiset ominaisuudet ovat huomattavasti säteilynkestävämpiä kuin FZ-piin. Tutkittujen parametrien perusteella voidaan myös päätellä, että p-tyypin MCZ-pii saattaa olla säteilynkestävämpi ilmaisinmateriaali kuin n-tyypin MCZ-pii. Tämän työn tulokset tukevat siis oletusta, että LHC-kokeiden nykyisten FZ-pii-ilmaisimien vaihtaminen n- tai p-tyypin MCZ-ilmaisimiin olisi varteenotettava vaihtoehto.In future high energy physics experiments, such as in the foreseen upgrade of the particle accelerator LHC at CERN, particle detectors will be exposed to significantly high particle radiation doses. Thus, extensive research is being conducted on improving the radiation hardness of silicon particle detectors and various defect and device engineering methods are examined. However, little data has been gathered especially from p-type magnetic Czochralski silicon (MCZ-Si) devices. This thesis presents the results of comparative transient current technique (TCT) measurements of both n-type and p-type MCZ silicon detectors as well as n-type Float Zone (FZ) silicon detectors. The results demonstrate that the MCZ silicon devices have superior radiation hard properties in comparison with the FZ silicon devices. Various examined parameters also suggest that p-type may be a more radiation hard material solution for silicon detectors than n-type MCZ silicon. Thus, replacing the currently installed FZ silicon detectors in LHC experiments with either n-type or p-type MCZ-Si devices is a viable option

Simulation of Heavily Irradiated Silicon Pixel Sensors and Comparison with Test Beam Measurements

Charge collection measurements performed on heavily irradiated p-spray DOFZ pixel sensors with a grazing angle hadron beam provide a sensitive determination of the electric field within the detectors. The data are compared with a complete charge transport simulation of the sensor which includes signal trapping and charge induction effects. A linearly varying electric field based upon the standard picture of a constant type-inverted effective doping density is inconsistent with the data. A two-trap double junction model implemented in the ISE TCAD software can be tuned to produce a doubly-peaked electric field which describes the data reasonably well. The modeled field differs somewhat from previous determinations based upon the transient current technique. The model can also account for the level of charge trapping observed in the data.Comment: 8 pages, 11 figures. Talk presented at the 2004 IEEE Nuclear Science Symposium, October 18-21, Rome, Italy. Submitted to IEEE Transactions on Nuclear Scienc