1. Paul Sellin, Radiation Imaging Group Charge Drift in partially-depleted epitaxial GaAs detectors P.J. Sellin, H. El-Abbassi, S. Rath Department of Physics University of Surrey, Guildford, UK J.C. Bourgoin LMDH, Université Pierre et Marie Curie, Paris, France

2. Paul Sellin, Radiation Imaging Group Overview  Chemical reaction growth of thick epitaxial GaAs layers  Depletion thickness and residual impurity concentration  Performance of partially depleted detectors  C-V measurements of impurity concentration at low temperature  Optical probing of charge transport using a focussed laser

3. Paul Sellin, Radiation Imaging Group Potential challenges for epitaxial GaAs Strengths of epitaxial GaAs:  intermediate photon detection efficiency between Si and CZT/CdTe  metal-semiconductor contacts and device physics are well understood  epitaxial GaAs has low concentrations of native EL2 defect  source of highly uniform whole wafer material, compatible with flip-chip bonding and monolithic electronics Existing problems:  even high purity epitaxial is compensated due to residual impurities- does not exhibit intrinsic carrier concentrations  depletion thickness is severely limited  charge carrier lifetimes are reduced

4. Paul Sellin, Radiation Imaging Group Chemical Reaction growth of thick epitaxial GaAs Epitaxial GaAs material studied in this work was grown by a Chemical Reaction Method by Jacques Bourgoin (Paris). An undoped GaAs wafer is used as the material source, which is decomposed in the presence of high temperature high pressure water vapour to produce volatile species. Typically, growth rates of <10  m/hr are used to achieve EL2 concentrations of ~10 13 cm -3 L. El Mir, et al, “Compound semiconductor growth by chemical reaction”, Current Topics in Crystal Growth Research 5 (1999) 131-139.

5. Paul Sellin, Radiation Imaging Group Whole wafer photoluminescence mapping GaAs material uniformity is characterised using room temperature photo-luminescence mapping - a contact-less, whole wafer technique:  A 25 mW 633 nm HeNe laser is focussed to ~50  m on the wafer  the wafer is mounted on an XY stage, and scanned  PL intensity maps at peak the band edge emission wavelength (870 nm) are acquired

6. Paul Sellin, Radiation Imaging Group PL maps of GaAs Photoluminescence mapping clearly shows the uniformity of epitaxial GaAs compared to semi-insulating VGF material: H. Samic et al., NIM A 487 (2002) 107-112. Epitaxial GaAsBulk GaAs

7. Paul Sellin, Radiation Imaging Group Calculated depletion thickness This material is nominally 1-5 x 10 14 cm -3 - corresponds to a 10-20  m depletion thickness @ 30V, and 15-30  m @ 80V

8. Paul Sellin, Radiation Imaging Group V = 30V V = 80V Alpha particle spectra 5.48 MeV alpha particles are irradiated through the Schottky (cathode) contact - range in GaAs ~20  m. A peltier cooler controlled the device temperature in the range +25°C to -55°C. Shaping time = 0.5  s.

9. Paul Sellin, Radiation Imaging Group Alpha particle pulse shapes Alpha particle pulses at room temperature: preamplifier shaping amplifier time base = 1  s per division slow component fast component

10. Paul Sellin, Radiation Imaging Group Alpha particle tracks An un-collimated alpha particle source produces a characteristic ‘double peak’ pulse height spectrum if the depletion thickness is shallower than the particle range:

11. Paul Sellin, Radiation Imaging Group 59.5 keV gamma spectra Depth-dependent CCE produces poorly resolved gamma spectra: T = -50°C

12. Paul Sellin, Radiation Imaging Group Temperature dependent CV analysis Allows the doping density N D to be extracted from the gradient of 1/C 2 vs V :

13. Paul Sellin, Radiation Imaging Group Depletion Thickness vs Bias Voltage

14. Paul Sellin, Radiation Imaging Group Impurity Densities The CV analysis confirm the shallow depletion thicknesses achieved in these devices, and correspond to impurity densities of ~3 x 10 13 cm -3 in sample S16 at low temperature:

15. Paul Sellin, Radiation Imaging Group Focussed IR laser scans Probe the variation in pulse shape as a function of position from the Schottky contact, and temperature

16. Paul Sellin, Radiation Imaging Group Scanning optical bench 850nm laser 300ns pulse XY scanning table cryostat imaging camera

17. Paul Sellin, Radiation Imaging Group Laser pulse shapes T=273K, 20V At 60  m from cathode: no slow component to signal At 180  m from cathode: charge drift times are ~350  s IR laser spot appears to have significant beam waist

18. Paul Sellin, Radiation Imaging Group Laser pulse shapes (2) T=223K, V=90V At 60  m from cathode: no slow component to signal At 180  m from cathode: charge drift times are ~350  s IR laser spot appears to have significant beam waist

19. Paul Sellin, Radiation Imaging Group Pulse risetime and amplitude vs bias

20. Paul Sellin, Radiation Imaging Group Interaction close to the anode - inside depletion region

21. Paul Sellin, Radiation Imaging Group Interaction close to n+ substrate - in low field region

22. Paul Sellin, Radiation Imaging Group Temperature dependent pulse shapes (1)

23. Paul Sellin, Radiation Imaging Group Temperature dependent pulse shapes (2)

24. Paul Sellin, Radiation Imaging Group Conclusions  The epitaxial GaAs layers studied showed excellent uniformity, and a residual impurity concentration of 1-5 x 10 14 cm -3  Long electron lifetimes > 300  s were observed in the low field regions - confirms the very low EL2 concentration Lateral laser scans show:  good charge transport in the shallow depleted region  long-lived components to the pulse shapes when irradiated close to n+ substrate - consistent with slow electron diffusion towards the substrate  significant penetration of the depletion region when cooled to -50°C Future work:  further lateral scanning is required with focussed lasers and high resolution proton microbeams to quantify these phenomena  further modest reductions in impurity concentration will produce significant performance improvements

25. Paul Sellin, Radiation Imaging Group Acknowledgements This work was partially funded by the UK’s Engineering and Physics Science Research Council