GaAs radiation imaging detectors with an active layer thickness up to 1 mm. D.L.Budnitsky, O.B.Koretskaya, V.A. Novikov, L.S.Okaevich A.I.Potapov, O.P.Tolbanov, A.V.Tyazhev. Siberian Physical Technical Institute, Russia, Tomsk Fax: ,

2 Outline Introduction Experimental data Electric field distribution in GaAs detectors based on GaAs:EL2 Resistivity distribution in GaAs:Cr slices Electrophysical characteristics of high resistivity GaAs I-V characteristics of GaAs:Cr CCE dependencies on bias voltage of detectors based on GaAs:Cr Conclusion

3 The electric field profile of a LEC SI-GaAs as shown in [1, 2] Distance from p + contact,  m Electric field, V/  m Electric field, kV/cm depth,  m [1] – k. Berwick et al., Proc. Semiconductors for room-temperature radiation detector applications,San Francisco, CA, USA, April 1993, MRS Symp. Proc., vol. 302 [2] - A. Cola et al./ Nuc.Instr. And Meth. In Phys. A395 (1997)

4 The calculated electric field and electrostatic potential profiles as shown in [2] depth,  m potential, V electric field, kV/cm Reverse bias: 1- 20V, 2- 40V, V, 4 – 80V The active layer thickness has penetration rate  1  m/V active thickness,  m applied voltage, V

5 The amplitude spectrum of the LEC SI-GaAs detector with current oscillations The presence of current oscillations makes difficult the detection of the desired signal in the amplitude spectrum 1 – with  -radiation 2 - without  -radiation U= 300V, 241 Am source

6 Experimental setup for electric field distribution profiling based on Franz-Keldysh effect I R  nm 1 – DLM (diffraction lattice monochromator) 2- detector sample, 3 – optical system, 4 – IR-camera (charge-coupled device) U bias

7 Electric field strength oscillations in LEC SI-GaAS We have measured samples made in different firms. The analysis of the results shows that in all structures fabricated by means of LEC SI-GaAs technology, a non- uniform  (х) distribution and electric field strength oscillations are observed.

8 Electric field distribution (1) (EL2 COMPENSATED GaAs LAYERS ) GaAs:EL2 t1t1 t2t2 t3t3 d Spatial distribution F function and light transmission (T) through detector thickness under bias voltage 250 V in various time instants t 1, t 2 t3 (t 1  t 2 < t 3 ) F=1-T

The main disadvantages of the LEC SI-GaAs The low value of the electron life time  n (0.2-1 ns). It results in the decrease of the electron drift length and, consequently, in low values of the electron component of the charge collection efficiency. The maximum value of the electric field penetration depth up to 500  m that limits the sensitive layer thickness of the LEC SI-GaAs structures and provide non-uniform electric field distribution,  (х), through the detector thickness. Current oscillations are formed in the external circuit at a rather low average value of the electric field strength in the detector  1kV/cm. Electric field increase of the capture cross section of EL2 + centers.

10 Advantages of Cr impurity as compared to the EL2 centers for detector material production Deep acceptor Deep donor - + small value of the electron capture cross section and absence of the field increase of the capture cross section on the electric field intensity absence of current oscillations possibility to reach uniform high electric field distribution through whole the detector with the thickness up to 1 mm

11 Technological cycle of manufacturing GaAs compensated with Cr n-GaAs GaAs:Cr with  up to 10 9  *cm

12 Resistivity distribution in the slice thickness The experimental values of the resistivity are (0.2-1)  10 9  cm, which are more than an order higher as compared to the resistivity of structures on the basis of LEC SI-GaAs.

13 Electrophysical characteristics of high resistivity GaAs Material  о (10 -9 /  *cм) n o (10 5 cm –3 ) p o (10 5 cm –3 ) L n (cm) GaAs EL GaAs Cr – 0.2 The hole concentration in GaAs:Cr exceeds the concentration of electrons. The difference changes from 10 to 100 times depending on conditions of the diffusion process and the initial material characteristics.

14 Current-voltage characteristics (DIFFUSION CROMIUM COMPENSATED GaAs LAYERS ) High resistivity causes a transformation of the structure of a barrier type to the structures of a resistor type. The structure current-voltage characteristic is linear. The current density value at operating voltage does not exceed A/cm 2.

15 Electric field distribution (2) (DIFFUSION CROMIUM COMPENSATED GaAs LAYERS ) The most important distinction of our structures, as compared to the traditional LEC SI- GaAs, is the uniform electric field distribution and the absence of current oscillations. Spatial distribution of the F function and light transmission (T) in detector thickness d, F=1-T GaAs:Cr 500 V 1000 V d

16 Amplitude spectra of the GaAs:Cr detector for various energies of the gamma radiation There is the % of CCE in a wide range of gamma-quantum energies (E  = 60, 122, 140 keV ) for the bias about 600V and the detector thickness of 780  m.

17 Amplitude spectra for GaAs:Cr detectors with different active layer thicknesses. E   60 keV ( 241 Am source) It should be noted that value of CCE declines with increase of the detector thickness. Nevertheless, CCE is about 70 % for the detector with an active layer thickness d=1.2 mm when bias voltage is 1000V.

18 Conclusion The technology of high temperature Cr doping of n-type GaAs allows to produce the high resistive GaAs layers with resistivity up to 10 9  *cm and thickness up to 1 mm. The detector structures based on GaAs:Cr have more uniform electric field distribution as compared to the detector structures based on GaAs:EL2 in a wide range of the applied bias voltage. The detector structures based on GaAs:Cr have applicable values of the CCE in a wide range of the gamma quantum energies (E  = 60, 122, 140 keV) and can be used in the production of pixel detector. We suppose to apply the 3” wafers to produce detector material in the nearest future.