Role of guard rings in improving the performance of silicon detectors

BARC has developed large-area silicon detectors in collaboration with BEL to be used in the pre-shower detector of the CMS experiment at CERN. The use of floating guard rings (FGR) in improving breakdown voltage and reducing leakage current of silicon detectors is well-known. In the present work, it has been demonstrated that FGRs can also be used to improve the spectroscopic response of silicon detectors. The results have been confirmed by carrying outα-particle (≈5 MeV) andγ-ray (60 keV) spectroscopies with the FGR floating or biased and the underlying physics aspect behind the change in spectra is explained. Although reduction in leakage current after biasing one of the guard rings has been reported earlier, the role of a guard ring in improving the spectroscopic response is reported for the first time. Results of TCAD simulations for silicon detectors with the guard ring under different biasing conditions have been presented. Low yield in producing large-area silicon detectors makes them very costly. However, with one of the FGRs biased even a detector having large surface leakage current can be used to give the same response as a very good detector. This makes the use of large-area silicon detectors very economical as the yield would be very high (>90%).     

Beam-induced radiation in the compact muon solenoid tracker at the Large Hadron Collider

The intense radiation environment at the Large Hadron Collider, CERN at a design energy of $ \sqrt s $ \sqrt s = 14 TeV and a luminosity of 10³⁴ cm⁻² s⁻¹ poses unprecedented challenges for safe operation and performance quality of the silicon tracker detectors in the CMS and ATLAS experiments. The silicon trackers are crucial for the physics at the LHC experiments, and the inner layers, being situated only a few centimeters from the interaction point, are most vulnerable to beam-induced radiation. We have recently carried out extensive Monte Carlo simulation studies using MARS program to estimate particle fluxes and radiation dose in the CMS silicon pixel and strip trackers from proton-proton collisions at $ \sqrt s $ \sqrt s = 14 TeV and from machine-induced background such as beam-gas interactions and beam halo. We will present results on radiation dose, particle fluxes and spectra from these studies and discuss implications for radiation damage and performance of the CMS silicon tracker detectors.     

A study of possibilities of development of GaAs detectors for high-power nanosecond X-ray pulses

It is proposed to use chromium-compensated semi-insulating GaAs detectors for detecting high-power nanosecond X-ray pulses. An X-ray facility based on a small direct-acting electron accelerator “Sinus-150” developed at the Institute of High-Current Electronics of the Siberian Branch of the Russian Academy of Sciences, Tomsk was used as a source of pulsed X-ray radiation. The detectors are shown to detect 5 ns pulses under exposure to 30 mR radiation during one pulse without distortions. In so doing, one-ampere currents flow through the active detector area 0.2 cm². It is found that the physical processes limiting the working capacity of GaAs detectors are due to capture of nonequilibrium electrons and holes by deep centers. At the nonequilibrium charge-carrier concentrations higher than 5·10¹³ cm³, nonuniform distributions of the electric field are formed in the active region and these can result in distortion of the output detector signal shape. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 14–18, September, 2008.     

Silicon heterojunction solar cell with amorphous silicon oxide buffer and microcrystalline silicon oxide contact layers

This Letter reports on the fabrication and characterization of silicon heterojunction solar cells with silicon oxide based buffer (intrinsic amorphous silicon oxide) and contact layers (doped microcrystalline silicon oxide) on flat p‐type wafers. The critical dependency of the cell performance on the front and rear buffer layer thickness reveals a trade‐off between the open circuit voltage V_oc and the fill factor FF. At the optimum, the highest efficiency of 18.5% (active area = 0.67 cm²) was achieved with V_oc = 664 mV, short circuit current J_sc = 35.7 mA/cm² and FF = 78.0%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Planar sulfur-doped silicon detectors for high-speed infrared thermography

Planar extrinsic sulfur-doped silicon detectors for infrared (IR) semiconductor-discharge gap image converters intended for use in high-speed thermography of remote objects have been developed. The detectors were fabricated by high-temperature diffusion of sulfur into silicon wafers from the vapor phase. The dependence of doping efficiency on the sulfur vapor pressure in the course of diffusion was analyzed. The detector fabrication technology was optimized to meet the specific requirements for their operation in the microdischarge devices considered. The detectors were tested in a laboratory setup comprising a blackbody source of IR light, an image converter, and a pulsed CCD camera for recording the converted images. The converter equipped with the detector can provide imaging of objects heated to a temperature, T_min ≈ 200 °C, with a temporal resolution on the order of 10^?6 s and spatial resolution of about 5 lines/mm.     

Optical absorption in PECVD deposited thin hydrogenated silicon in light of ordering effects

We report results obtained from measurements of optical transmittance spectra carried out on a series of silicon thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (μc-Si:H). Spectral refractive indices and absorption coefficients were determined from transmittance spectra. The spectral absorption coefficients were used to determine the Tauc optical band gap energy, the B factor of the Tauc plots, E ₀₄ (energy at which the absorption coefficient is equal to 10⁴ cm⁻¹), and the Urbach energy as a function of the hydrogen dilution. The results were correlated with microstructure, namely volume fractions of the amorphous and crystalline phase with voids, and with the grain size.      

Background neutron in the endcap and barrel regions of resistive plate chamber for compact muon solenoid/large hadron collider using GEANT4

In this study the performance of double gap RPC has been tested by GEANT4 Monte Carlo simulation code. The detector response calculations taken as a function of the neutron energy in the range of 0.01 eV-1 GeV have been simulated through RPC set-up. In order to evaluate the response of detector in the LHC background environment, the neutron spectrum expected in the CMS muon endcap and barrel region were taken into account. A hit rate of about 165.5 Hz cm⁻², 34 Hz cm⁻², 33.6 Hz cm⁻², and 27.0 Hz cm⁻² due to an isotropic neutron source is calculated using GEANT4 standard electromagnetic package for a 20 × 20 cm² RPC in the ME1, ME2, ME3 and ME4, respectively. While for the same neutron source and using GEANT4 package a hit rate of about 0.42 Hz cm⁻², 0.7182 Hz cm⁻² was measured for the MB1 and MB4 stations respectively. Similar characteristics of hit rates have been observed for GEANT4 low electromagnetic package.      

Development of high resolution silicon surface barrier detectors

Fabrication methods for silicon surface barrier detectors and their correlated properties which result in the production of high resolution (< 20 keV) devices have been studied. The techniques for fabrication and testing of the detectors currently employed at our Centre are presented. An FWHM of 14keV for 5·486 MeV²⁴¹Am α has been achieved. Our results are therefore comparable with the best in the world.     

Uncooled microbolometer detector: recent developments at ULIS

Uncooled infrared focal plane arrays are being developed for a wide range of thermal imaging applications. Fire-fighting, predictive maintenance, process control and thermography are a few of the industrial applications which could take benefit from uncooled infrared detector. Therefore, to answer these markets, a 35-μm pixel-pitch uncooled IR detector technology has been developed enabling high performance 160×120 and 384×288 arrays production. Besides a wide-band version from uncooled 320×240/45 μm array has been also developed in order to address process control and more precisely industrial furnaces control. The ULIS amorphous silicon technology is well adapted to manufacture low cost detector in mass production. After some brief microbolometer technological background, we present the characterization of 35 μm pixel-pitch detector as well as the wide-band 320×240 infrared focal plane arrays with a pixel pitch of 45 μm. The paper presented there appears in Infrared Photoelectronics, edited by Antoni Rogalski, Eustace L. Dereniak, Fiodor F. Sizov, Proc. SPIE Vol. 5957, 59570M (2005).     

High-mass dimuon resonances in Pb-Pb collisions at 5.5 TeV in CMS

The measurement of the charmonium (J/ψ, ψ′) and bottomonium (γ, γ′, γ″) resonances and Z ⁰ boson in nucleus-nucleus collisions provides crucial information on high density QCD matter. The observation of anomalous suppression of J/ψ at the CERN-SPS and RHIC is well established but the clarification of some important questions requires equivalent studies of the γ family, only possible at LHC energies. The Z ⁰ boson will be produced for the first time in heavy-ion collisions at the LHC and, since its dominant production channel is through q-q fusion, it is an excellent probe of the nuclear modification of quark distribution functions. This paper reports the capabilities of the CMS detector to study quarkonium and Z ⁰ production in Pb-Pb collisions at 5.5 TeV, through the dimuon decay channel.     

Investigation of diffractive processes with the CMS detector: New results

An experiment aimed at studying leading neutrons at LHC with the aid of the CMS detectors is proposed. Data of this experiments can be used to extract cross sections for π ⁺ p and π ⁺ π ⁺ scattering. Numerical estimates are presented for the proposed measurements.     

CALICE silicon-tungsten electromagnetic calorimeter

A highly granular electromagnetic calorimeter prototype based on tungsten absorber and sampling units equipped with silicon pads as sensitive devices for signal collection is under construction. The full prototype will have in total 30 layers and be read out by about 10000 Si cells of 1 × 1 cm². A first module consisting of 14 layers and depth of 7.2X ₀ at normal incidence, having in total 3024 channels of 1 cm², was tested recently with e ⁻ beam. We describe the prototype and discuss some preliminary testbeam results on its performance with respect to position resolution, response inhomogeneity and transverse containment.      

Realistic limits to computation III. Climbing the third dimension

As far as ultra-dense crossbars are related to correspondingly dense wire arrays, the crossbar route to tera-scale integration depends on the availability of preparation techniques for wire arrays with density of 10⁶ cm⁻¹ or more. This linear density implies, for a planar arrangement, a pitch of 10 nm or less, which not only is at the limits of the current technical possibilities, but also can modify appreciably the band structure of silicon. A dramatic increase of density could only be achieved if it were possible to organize the nanowires in a three-dimensional fashion still exploiting the planar technology. In this work processes are described for the fabrication of out-of-plane, vertically arranged, polycrystalline silicon nanowires via a rigorously top-down batch process. These techniques are consistent with the production of wire arrays with linear density (projected on the surface) larger than that achievable with any other proposed top-down process. Used for the fabrication of the bottom wire arrays of crossbars, these processes should eventually allow a cross-point amount per unit area in excess of 10¹² cm⁻², thus providing candidate technologies for ultra tera scale integration. The technique developed for such out-of-plane crossbars can be used to implement new functions like coils, solenoids and transformers.     

Silicon nitride/silicon oxide interlayers for solar cell passivating contacts based on PECVD amorphous silicon

This Letter demonstrates improved passivating contacts for silicon solar cells consisting of doped silicon films together with tunnelling dielectric layers. An improvement is demonstrated by replacing the commonly used silicon oxide interfacial layer with a silicon nitride/silicon oxide double interfacial layer. The paper describes the optimization of such contacts, including doping of a PECVD intrinsic a‐Si:H film by means of a thermal POCl₃ diffusion process and an exploration of the effect of the refractive index of the SiN_x. The n⁺ silicon passivating contact with SiN_x /SiO_x double layer achieves a better result than a single SiN_x or SiO_x layer, giving a recombination current parameter of ～7 fA/cm² and a contact resistivity of ～0.005 Ω cm², respectively. These self‐passivating electron‐selective contacts open the way to high efficiency silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Defect removal after low temperature annealing of boron implantations by emitter etch‐back for silicon solar cells

Ion implantation offers new possibilities for silicon solar cell production, e.g. single side doping that can be structured in‐situ with shadow masks. While phosphorus implantations can easily be annealed at low temperature, the annealing of boron implantations is challenging. In this study, we use low energy implantations of boron (1 keV and 5 keV) with a projected range of 5.6 nm and 21.2 nm that form defects causing charge carrier recombination after a low temperature anneal (950 °C, 30 min). An ozone‐based wet chemical etching step is applied to remove this near surface damage. With increasing chemical etch‐back the electrical quality (i.e. emitter saturation current density, J_0e) improves continuously. The calculated limit for J_0e was reached with an abrasion of 35 nm for 1 keV and 85 nm for 5 keV implantations, showing that the relevant defects causing charge carrier recombination are located very close to the surface, corresponding to the as‐implanted profile depth. This emitter etch‐back allows for the fabrication of defect free boron doping profiles with good sheet resistance uniformity (standard deviation <2%). With the resulting characteristics (sheet resistance <100 Ω/sq, surface doping concentration >5 × 10¹⁹ cm^–3, J_0e < 30 fA/cm²), these boron profiles are well suited for silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Time-of-flight system for the multipurpose detector (MPD)

A conceptual design for a multipurpose detector (MPD) [1] is proposed for the study of hot and dense barony matter in collisions of heavy ions over the atomic mass range A = 1–197 at center-of-mass energies of up to 11 GeV (for Au⁷⁹⁺). The MPD experiment is scheduled to be performed at a future JINR accelerator complex facility for heavy ions, the Nuclotron-based Ion Collider Facility (NICA), which is designed to reach the required parameters with an average luminosity of L = 10²⁹ cm⁻² s⁻¹. Identification of charged hadrons (PIDs) at intermediate momenta (0.1–2 GeV/c) is achieved via time-of-flight (TOF) measurements. As a base element of the TOF detector, we consider a 10 gap MRPC with a strip or pad readout. Results from an MRCP prototype test are presented.     

Atmospheric gamma rays in the energy region 40–1000 GeV

A large stack of lead-emulsion sandwich detector assembly was flown over Hyderabad, India. High energy gamma rays at the float altitude were unambiguously identified from the cascades they induced, and their energies reliably determined by improved methods. From an analysis of 163 gamma rays of energy ≳ 30 GeV, it is found that the differential energy spectrum is represented by the power lawJ _r (E)= 129·4E ^{−2·62±0·12} photons m⁻² sr⁻¹sec⁻¹ GeV⁻¹ at an effective atmospheric depth of 14·3 g cm⁻²; this is the first reliable balloon measurement of atmospheric gamma rays in the energy range 40–1000 GeV. After correcting for the gamma rays radiated by the primary cosmic ray electrons, the production spectrum of gamma rays, resulting from the collisions of cosmic ray nuclei with air nuclei, at the top of the atmosphere isP _r (E, 0)=8·2 × 10⁻⁴ E^2.60±0.09 photons g⁻¹sr⁻¹sec⁻¹ GeV⁻¹. The atmospheric propagation of the electromagnetic component due to the cascade process is also derived from the gamma ray production spectrum.     

Fabrication of lithium-drifted silicon detectors by constant temperature method

A new approach for lithium drifting in silicon is described where the silicon devices under drift are held at constant temperature and bias at normal air ambient, and the drift process is terminated at the end of an estimated time depending upon the thickness of wafers. A 4-channel lithium drifting unit with electronically controlled oven has been constructed for this purpose. Full details of the fabrication procedure are given. A sizable number of Si(Li) detectors have been fabricated using this approach. The quality of the detectors is tested with²⁴¹Am alphas and conversion electrons from²⁰⁹Bi and¹³⁷Cs sources. The detectors are regularly used for nuclear physics experiments at this Centre.     

Decay studies of N ≈ Z nuclei from 75Sr to 102Sn

Neutron deficient nuclei near ¹⁰⁰Sn have been produced by fragmentation of a 1 ^. AGeV ¹¹²Sn beam. The fragments were separated, identified and stopped in a highly segmented silicon strip detector stack. This detector measured the total energy of emitted β⁺-particles. γ-radiation was measured with surrounding detectors. The half-lives for many nuclides have been determined for the first time and give important information for the following topics: For the heaviest particle-stable odd-odd nuclei ⁹⁰Rh, ⁹⁴Ag and ⁹⁸In we observed for the first time fast β-decays, compatible with superallowed Fermi transitions and confirmed such decays for ⁷⁸Y, ⁸²Nb and ⁸⁶Tc. We have also observed long-lived T = 0 states in some of these nuclei. We measured the half-lives of all rp-process waiting-point nuclei from ⁸⁰Zr up to ^{92, 93}Pd. In addition we find the proton drip line nucleus ⁷⁷Y to decay dominantly via β-decay. To study the Gamov-Teller strength in the β-decay near the doubly magic ¹⁰⁰Sn we measured the half-life, β- and γ-spectrum of ¹⁰²Sn. We propose a level scheme for the daughter nuclide ¹⁰²In and deduce the Gamov-Teller strength (B _GT = 4.0±0.6). This is one of the largest values known. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: thomas.faestermann@ph.tum.de