Surface characterisation of Germanium detectors T.Engert 1,2, I.Kojouharov 1, J.Gerl 1, P.Nolan 2, Th.Krings 3 1 GSI, Darmstadt, Germany 2 University of Liverpool, UK 3 SEMIKON, Detector GmbH, Germany Abstract Abstract The project deals with the surface characterisation of The project deals with the surface characterisation of Germanium (Ge) detectors through all necessary Germanium (Ge) detectors through all necessary processing steps, from a raw crystal to a final processing steps, from a raw crystal to a final detector diode. The aim is to improve the surface detector diode. The aim is to improve the surface quality in order to obtain the best performance as a quality in order to obtain the best performance as a γ-ray detector. γ-ray detector. It is motivated by the need for lower contaminations It is motivated by the need for lower contaminations and reduced mechanical defects. This is achieved by and reduced mechanical defects. This is achieved by applying new mechanical treatment procedures at an applying new mechanical treatment procedures at an better cost-efficiency ratio. better cost-efficiency ratio. Detector surface scanned with AFM Lapped-Ge Grinded-Ge Lapped-Ge Grinded-Ge Ra [nm] 285,83 nm 660,08 nm Rq [nm] 358,88 nm 820,37 nm Rz [µm] 4,41 µm 9,82 µm Rt [µm] 3,1 µm 8,61 µm Depletion voltage Voltage [V] Current [A] Operational voltage γ-ray energy resolution Edge chips Dimples Scratches Mechanical defects define the voltage-current characteristics Ge-detector characteristics Ge-detector characteristics The operational region (depletion voltage to plateau voltage) The operational region (depletion voltage to plateau voltage) has to be reached for the required energy resolution. has to be reached for the required energy resolution. Preparation Preparation For the characterisation For the characterisation three different machined three different machined germanium crystals with germanium crystals with different surface qualities different surface qualities were prepared. were prepared. Characteristics Characteristics The surface analysis was The surface analysis was realized by an Atomic realized by an Atomic Force Microscope AFM Force Microscope AFM (right) and a Profiler (left). (right) and a Profiler (left). Roughness of Ge-surface in x- and y-direction scanned with a profiler Difficult geometrical shapes!!!!! Difficult geometrical shapes!!!!! Excellent surface quality!!!!! Excellent surface quality!!!!! Ø 3,5mm Ø 2mm Ge-crystal shaped with the ultrasonic-grinding machine Results Results Mechanical surface parameters Mechanical surface parameters (mean values) recorded with AFM (mean values) recorded with AFM and Profiler: and Profiler: Better cost-efficiency ratio!!!!! Detectors Detectors On the basis of these mechanical On the basis of these mechanical results obtained with ultrasonic results obtained with ultrasonic grinding, novel types of planar grinding, novel types of planar Ge detectors are proposed Ge detectors are proposed Spectroscopy tests Spectroscopy tests planned with radioactive planned with radioactive sources sources 241 Am and 137 Cs Further investigations Further investigations To get more information about the To get more information about the surface structure and contaminations surface structure and contaminations from these Germanium detectors (left) from these Germanium detectors (left) and from the raw crystals, and from the raw crystals, measurements with a measurements with a Scanning Electron Microscope are in Scanning Electron Microscope are in preparation. preparation. p + contact n - contact [l] length= 8 mm [b] width= 8 mm [h] height= 10 mm This kind of sophisticated mechanical treatment allows: Ultrasonic grinding Ultrasonic grinding method method > micro structure with a tolerances of 2 µm tolerances of 2 µm > degree of roughness is a factor of 4 times better than factor of 4 times better than with standard grinding techniques with standard grinding techniques > and a factor of 2 times better compared to standard better compared to standard lapping techniques. lapping techniques. Germanium surface machined with a different feed parameter Germanium surface machined with the best mechanical parameters Plateau voltage Measurement principle: A diamond needle (Profiler & AFM) scans the intrinsic surface Safe treatment of the crystals!!!!! Safe treatment of the crystals!!!!! AFM Profiler Dimensions are 32,5x32,5x15 mm 3 with rounded corners and a special guard ring