Diamond Detectors for Ionizing Radiation - HEPHY

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CHAPTER 5. DETECTOR MATERIAL COMPARISON 24 Collection Distance [µm] 400 350 300 250 200 150 100 10000 5000 Signal [e-] 50 Natural type IIa diamond 1990 1992 1994 1996 1998 Time [year] Figure 5.3: The history of the charge collection distance of CVD diamond. 250 Collection Distance (µm) 200 150 100 50 8000 6000 4000 2000 Mean Charge (e) 0 0 0.2 0.4 0.6 0.8 1 1.2 Electric Field (V/µm) 0 Figure 5.4: The collection distance vs. the electric eld. pairs. In order to remove the intrinsic charge from the bulk, a pn-junction is introduced. Usually starting with a weakly doped n-type silicon wafer, a thin layer on one side is heavily doped with boron acceptors, and a thin layer on the opposite side with arsenic donors, resulting in a p + nn + -diode. Alternatively, the bulk material can be of p-type, which makes no principal dierence. Finally, the surfaces are metallized to form Ohmic contacts. When the pn-junction is reverse-biased, all free carriers are drained from the bulk, and the detector is sensitive to ionizing radiation. Fig. 5.5 shows such a silicon detector with the applied bias voltage, which is above the depletion voltage 1 , and the resulting electric eld. The implant layers are much thinner in reality, thus a the electric 1 The depletion voltage is the minimum bias voltage required to establish a space charge zone across the whole bulk
CHAPTER 5. DETECTOR MATERIAL COMPARISON 25 eld is approximately constant throughout the bulk. Principally, the silicon detector is a wide-area diode. + p -implant y + n-type bulk n + -implant E Figure 5.5: Schematic cross-section of a silicon detector with implant thicknesses not to scale. The electric eld results from a bias voltage above the depletion voltage. Silicon detectors are made of very pure material, minimizing the number of charge traps and recombination centers. Nearly all charges excited in the bulk reach the electrodes, implying a charge collection eciency of (almost) 100%. According to the charge mobilities, the charge collection after a particle traversed the bulk takes a few nanoseconds. 5.3 Ge Detectors Germanium was the rst technically used semiconductor material. As the specic energy loss dE=dx is quite high in germanium compared to silicon, it better suits for calorimetry than for tracking purposes. For instance, lithium-drifted germanium detectors [26] with an active crystal volume of several cm 3 are used in nuclear spectroscopy. These detectors achieve an excellent energy resolution, however, they must be permanently cooled to liquid nitrogen temperature (T = 77 K). The low temperature not only conserves the arrangement of the lithium atoms inside the crystal, but also reduces the intrinsic carrier density dramatically. Only this fact permits the functioning of the device. Later, it became possible to produce extremely pure germanium material, which is more convenient to use. Still low temperature operation is essential, but an interruption of the cooling is no longer disastrous. 5.4 GaAs Detectors Gallium-arsenide is a III-V-type semiconductor. The semiconducting junction is introduced through a Schottky contact on the bulk material. Unlike silicon, the electric eld does not extend throughout the bulk [9], in fact, there is a passive layer with zero eld and the eld in the active layer is decreasing from a maximum at the Schottky contact to zero. Depending on the sample purity, there is a certain number of inter-band gap traps. Thus the charge collection eciency of the best samples is presently at the order of 50% to 80%.
Page 1 and 2: Diploma Thesis Diamond Detectors fo
Page 3 and 4: CONTENTS 3 7 Radiation Hardness 37
Page 5 and 6: CHAPTER 1. SYNOPSIS 5 primarily sil
Page 7 and 8: CHAPTER 2. INTRODUCTION 7 Figure 2.
Page 9 and 10: Chapter 3 Material Properties 3.1 G
Page 11 and 12: CHAPTER 3. MATERIAL PROPERTIES 11 Q
Page 13 and 14: CHAPTER 3. MATERIAL PROPERTIES 13 d
Page 15 and 16: Chapter 4 Solid State Detector Theo
Page 17 and 18: CHAPTER 4. SOLID STATE DETECTOR THE
Page 19 and 20: CHAPTER 4. SOLID STATE DETECTOR THE
Page 21 and 22: CHAPTER 5. DETECTOR MATERIAL COMPAR
Page 23: CHAPTER 5. DETECTOR MATERIAL COMPAR
Page 27 and 28: CHAPTER 6. CHARACTERIZATION 27 6.1.
Page 29 and 30: CHAPTER 6. CHARACTERIZATION 29 and
Page 31 and 32: CHAPTER 6. CHARACTERIZATION 31 Lid
Page 33 and 34: CHAPTER 6. CHARACTERIZATION 33 Figu
Page 35 and 36: CHAPTER 6. CHARACTERIZATION 35 300
Page 37 and 38: Chapter 7 Radiation Hardness 7.1 Ra
Page 39 and 40: CHAPTER 7. RADIATION HARDNESS 39 7.
Page 41 and 42: CHAPTER 7. RADIATION HARDNESS 41 Ca
Page 43 and 44: CHAPTER 7. RADIATION HARDNESS 43 ev
Page 45 and 46: CHAPTER 7. RADIATION HARDNESS 45 2
Page 47 and 48: CHAPTER 7. RADIATION HARDNESS 47 no
Page 49 and 50: CHAPTER 8. DETECTOR GEOMETRIES 49 F
Page 51 and 52: CHAPTER 8. DETECTOR GEOMETRIES 51 S
Page 53 and 54: CHAPTER 8. DETECTOR GEOMETRIES 53 i
Page 55 and 56: Chapter 9 Summary The possible appl
Page 57 and 58: Acknowledgements First of all, I am
Page 59 and 60: APPENDIX A. ABBREVIATIONS AND SYMBO
Page 61 and 62: APPENDIX A. ABBREVIATIONS AND SYMBO
Page 63 and 64: Bibliography [1] Austrian Academy o
Page 65: BIBLIOGRAPHY 65 [29] A. Hrisoho, Fr

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