**Weight****in**g Potentials **in** **CdZnTe** y-**ray** Detectors **with** Segmented Electrodes0. Tousignant, L.A. Hamel, Menzbel; IEEE, D. VasilevskiGroupe de recherche en physique et technologie des couches m**in**ces (GCM),Universitk de Montrkal, MontrBal, H3C 3J7, CanadaAbstract**Weight****in**g **potentials** **in** the metallized and non-metallizedregions at the surface of CZT strip and coplanar **detectors**are measured by scann**in**g an a source or a laser spot acrossthe segmented face and record**in**g the charge **in**duced onvarious electrodes after the electron collection time, Thesemeasurements are found to he **in** good agrement **with** thesolution of Laplace equation **with** mixed boundary conditions.I. INTRODUCTIONRealistic model**in**g of semiconductor **detectors** is animportant issue **in** the design and optimization of new devices.Among the various processes to be accurately modeled are theelectron-hole pair generation mechanism, the carrief transportand trapp**in**g processes, the physical electric field F(Z) **in** thedetector and the weight**in**g field @w,i(Z) of each electrode. Inthe present paper we will concentrate on the last po**in**t anddiscuss how the weight**in**g fields or, alternatively, the weight**in**g**potentials** Vw,i(Z) can be properly calculated and how they canbe measured.11. WEIGHTING POTENTIALBy def**in**ition, the weight**in**g potential &(Z) of the ithelectrode is the potential that would he found at the position Z**in** the detector volume under the follow**in**g conditions: (1) theith electrode biased at 1 Volt, (2) all other electrodes kept at 0Volt, (3) all space charge removed, i.e. the detector materialbe**in**g replaced by vacuum [I, 21.In order to calculate Vu,(?'), the boudary conditions musthe specified everywhere on the detector surface, **in**clud**in**gthe non-metallized regions. In previous work [3, 4, 51 wehad used Dirichlet boundary conditions and, quite arbitrarily,assumed a l**in**ear variation of the surface weight**in**g potential**in** the non-metallized **in**terstrip regions. In order to improveour understand**in**g of signal shar**in**g between electrodes,we now calculate the weight**in**g **potentials** by us**in**g mixedDirichlet-Neumann boundary conditions. On the metallizedregions, Dirichlet conditions V,,i(Z) = 0 or 1 are still validhut on the non-metallized regions, a Neumann condition(cont**in**uity of the normal field component across the boundary)is imposed. Figure 1 shows the new calculated surfaceweight**in**g **potentials** 161 for 2 **detectors**, a strip detector **with**225 pm strips and 150 pm gaps and a coplanar detector **with**the width of both the strips and the gaps equal to 254 pm.Both **detectors** are 1.5 mm thick. Both calculations show anon-l**in**ear variation of Vwi **in** the gap and, **in** the case of thestrip detector, small satellite structures appear **in** the subsequentgaps even though gaps are bordered by strips kept at zeropotential. Due to the superposition pr**in**ciple, the weight**in**gpotential of a coplanar electrode is simply the sum of theweight**in**g **potentials** of its **in**dividual strips..A -0> >nv nv 1v nv nv;v,,, Detector0.1s 0> I0.8010.1a.2om4c4ssaaa1v ov 1v nv 1Figure 1: Calculated surface weight**in**g **potentials** for (top) as**in**gle strip of a CZT strip detector and (bottom) for one set ofelectrodes of a coplanar CZT detector.0-7803-5021-9/99/$10.000 1999 IEEE. 176