1. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Characterization of Single-Crystal CVD diamond using the Transient Current Technique (H. Pernegger, CERN RD42 collaboration meeting) zOverview uThe principle & Setup uRaw Measurements & Analysis uMeasurement of drift velocity uCharge lifetime uNet effective space charge

2. H. Pernegger, CERN, RD42 coll. Meeting May 2004 The principle  Use  -source (Am 241) to inject charge zmeasure charge carrier properties of electrons and holes separately zInjection uDepth about 14mm compared to 470mm sample thickness uUse positive or negative drift voltage to measure material parameters for electrons or holes separately uAmplify ionization current V  Electrons only Or Holes only

3. H. Pernegger, CERN, RD42 coll. Meeting May 2004 The readout zUse current amplifier to measure induced current uBandwidth 2 GHz uAmplification 11.5 uRise time 350ps zInputimpedance 45 Ohm zReadout with LeCroy 564A scope (1GHz 4Gsps) zCorrect in analysis for detector capacitance (integrating effect) zCross calibrated with Sintef 1mm silicon diode   e = 1520 cm2/Vs uI = 3.77 eV +/- 15%

4. H. Pernegger, CERN, RD42 coll. Meeting May 2004 The measured current curves zTwo effects uCharge trapping during drift uSpace charge : decrease of current for holes / increase for electons

5. H. Pernegger, CERN, RD42 coll. Meeting May 2004 The parameters zExtracted parameters zTransit time of charge cloud uSignal edges mark start and arrival time of drifting charge cloud uError-function fit to rising and falling edge zTotal signal charge uIntegral of curves uEventualy corrected for charge trapping t_c

6. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Measurement of velocity zAverage drift velocity for electrons and holes  Extract  0 and saturation velocity   0: uElectrons: 1714 cm2/Vs uHoles: 2064 cm2/Vs zSaturation velocity: uElectrons: 0.96 10 7 cm/s uHoles: 1.41 10 7 cm/s

7. H. Pernegger, CERN, RD42 coll. Meeting May 2004 … and “effective mobility” zDeduce a calculated mobility from the measured velocity (normaly mobility is defined only at low fields with linear relation between field and velocity) zTaking space charge into account: zNormal operation in region close to velocity saturation

8. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Carrier lifetime measurement zExtract carrier lifetimes from measurement of total charge zTotal ionization charge (from extrapolation) : 47.6 (e) & 47.5 (h) fC zLifetime: 34ns (+10/-6ns) for electrons & 36ns (+20/-9ns) holes

9. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Lifetime measurement by charge correction zCorrect the measured charge

10. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Extract the lifetime zFor the correct choice of the correction time, slope becomes zero zBoth measurements yield consistent results: uElectrons and holes of identical lifetime between 35 to 40ns zThe charge lifetime is much larger than the transit time (at typical detector operation voltes) zCharge trapping doesn’t seems to be a limiting issue for scCVD 40 +25 -10 ns

11. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Net effective space charge zShape of current pulses can be explained by net effective space charge in diamond bulk zSignal decrease due to decreasing electrical field zIn the simpliest model of a uniform space charge: linear field ->exponential current decrease zFurther considerations on field distribution: uSee Vladimir’s talk Voltage necessary to compensate Neff

12. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Linear Model Determination of N effective zNon-zero field region increases with V 1/2 zFor V=Vc : holes cloud arrive zNo electron signal below Vc (for this injection configuration) zSign of increase/decrease -> NEGATIVE space charge zVc = 96V zN eff = 2.8 x 10 11 cm -2 Vc

13. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Further considerations regaring space charge zLinear field maybe a good approximation for high fields but not at low fields (near Vc) uFlat region in current curve at end uExtrapolation for Q=0 yields 25V zAt V close to Vc the field may e.g. depend on combination of generation current and trapping center density which can lead to a non-uniform space charge zSpace charge may depend on detector bias voltage uElectron current increase stronger at higher voltages

14. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Comparion with Simulation: zSimulation uUses charge drift through detector uElectronics transfer function uMaterial parameters as measured (lifetime, velocity) zCan achieve good approximation of data zVary N eff See Vladimir’s simulation & talk

15. H. Pernegger, CERN, RD42 coll. Meeting May 2004 Conclusion zTCT allows to measure several charge transport properties in a single characterization and seems (to me) ideally suited for further additional studies of CVD properties. zIt allows to measure uDrift velocity uLifetime uSpace charge & characterize the field configuration inside the diamond zWe measure uLifetimes of approx. 40ns >> transit time at typical detector operation uSaturation velocity of 1 (e) to 1.4 (h) x 10 7 cm/s zPropose to continue measurements with uFurther scCVD samples in the next future (sample comparison) uStudy other dependence’s (e.g. surface and contact preparation)