Surface characterisation of Germanium detectorsSurface characterisation of Germanium detectors T.EngertT.Engert1,21,2, I.Kojouharov, I.Kojouharov11, J.Gerl, J.Gerl11, P.Nolan, P.Nolan22, Th.Krings, Th.Krings33

11GSI, 64291 Darmstadt, GermanyGSI, 64291 Darmstadt, Germany22 University of Liverpool, UK University of Liverpool, UK

33SEMIKON, Detector GmbH, GermanySEMIKON, Detector GmbH, Germany

AbstractAbstract 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 γγ-ray detector. -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 Detector surface scanned with AFM

Lapped-Ge Grinded-Ge Lapped-Ge Grinded-Ge

Ra [nm] 285,83 nm 660,08 nmRa [nm] 285,83 nm 660,08 nm

Rq [nm] 358,88 nm 820,37 nmRq [nm] 358,88 nm 820,37 nm

Rz [µm] 4,41 µm 9,82 µmRz [µm] 4,41 µm 9,82 µm

Rt [µm] 3,1 µm 8,61 µmRt [µm] 3,1 µm 8,61 µm

Depletion Depletion voltage voltage

Voltage [V]Voltage [V]

Cu

rren

t [A

]C

urr

ent [

A]

Operational Operational voltage voltage

γγ -ray -ray en

ergy resolution

energy resolu

tion

Edge chips

Dimples

Scratches

Mechanical defects define the voltage-current characteristics

Ge-detector characteristicsGe-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.

PreparationPreparation 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.

CharacteristicsCharacteristics 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 Roughness of Ge-surface in x- and y-direction scanned with a profilery-direction scanned with a profiler

Difficult Difficult geometrical geometrical shapes!!!!!shapes!!!!!

Excellent surface Excellent surface quality!!!!!quality!!!!!

Ø 3,5mm

Ø 2mm

Ge-crystal shaped with the Ge-crystal shaped with the ultrasonic-grinding machineultrasonic-grinding machine

ResultsResults Mechanical surface parameters Mechanical surface parameters (mean values) recorded with AFM (mean values) recorded with AFM and Profiler:and Profiler:

Better cost-efficiency ratio!!!!!

DetectorsDetectors 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 proposedGe detectors are proposed

Spectroscopy tests Spectroscopy tests planned with radioactive planned with radioactive sources sources

241241 Am and Am and 137 137 CsCs

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 ameasurements with a Scanning Electron Microscope are in Scanning Electron Microscope are in preparation. preparation.

pp++ contact contact

nn- - contactcontact

[l] length= 8 mm[l] length= 8 mm

[b] width= 8 mm[b] width= 8 mm

[h] height= 10 mm[h] height= 10 mm

This kind of This kind of sophisticated sophisticated mechanical mechanical treatment treatment

allows:allows:Ultrasonic grinding Ultrasonic grinding methodmethod

> micro structure with a > micro structure with a tolerances of 2 µmtolerances of 2 µm

> degree of roughness is a > degree of roughness is a factor of 4 times better than factor of 4 times better than with standard grinding techniqueswith standard grinding techniques

> and a factor of 2 times > 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 Plateau voltage voltage

Measurement principle: A diamond

needle (Profiler & AFM) scans

the intrinsic surface

Safe treatment of Safe treatment of the crystals!!!!!the crystals!!!!!

AFMAFM

ProfilerProfiler

Dimensions are 32,5x32,5x15 mm3

with rounded corners and a

special guard ring