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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: +7-3822-233034, E-mail: [email protected] 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 2 Electric field, V/m Electric field, kV/cm The electric field profile of a LEC SI-GaAs as shown in [1, 2] Distance from p+ contact, m depth, m [1] – k. Berwick et al., Proc. Semiconductors for room-temperature radiation detector applications,San Francisco, CA, USA, 12-16 April 1993, MRS Symp. Proc., vol. 302 [2] - A. Cola et al./ Nuc.Instr. And Meth. In Phys. A395 (1997) 98-100 3 The calculated electric field and electrostatic potential profiles as shown in [2] Reverse bias: 1- 20V, 2- 40V, 3 - 60V, 4 – 80V potential, V active thickness, m applied voltage, V 3 1 2 electric field, kV/cm 4 depth, m The active layer thickness has penetration rate 1 m/V 4 The amplitude spectrum of the LEC SI-GaAs detector with current oscillations 70 1 – with -radiation 2 - without -radiation 60 Counts 50 U= 300V, 241Am source 40 30 1 20 10 2 0 0 100 200 300 400 Channel The presence of current oscillations makes difficult the detection of the desired signal in the amplitude spectrum 5 Experimental setup for electric field distribution profiling based on Franz-Keldysh effect. 1 Ubias 2 3 4 IR 890-910 nm 1 – DLM (diffraction lattice monochromator) 2- detector sample, 3 – optical system, 4 – IR-camera (charge-coupled device) 6 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 nonuniform (х) distribution and electric field strength oscillations are observed. 7 Electric field distribution (1) (EL2 COMPENSATED GaAs LAYERS ) GaAs:EL2 t1 t2 t3 d Spatial distribution F function and light transmission (T) through detector thickness under bias voltage 250 V in various time instants t1, t2 t3 (t1 t2 < t3) F=1-T 8 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 500m 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. Advantages of Cr impurity as compared to the EL2 centers for detector material production - Deep acceptor • absence of current oscillations • small value of the electron capture cross section and absence of the field increase of the capture cross section on the electric field intensity • possibility to reach uniform high electric field distribution through whole the detector with the thickness up to 1 mm + Deep donor 10 Technological cycle of manufacturing GaAs compensated with Cr n-GaAs GaAs:Cr with up to 109 *cm 11 10 9 10 8 10 7 10 6 10 5 detector thickness, d , ohm*cm , ohm*cm Resistivity distribution in the slice thickness surface 0 200 400 600 slice thickness, m 800 1000 10 9 10 8 10 7 10 6 10 5 0 200 400 600 800 slice thickness, m 1000 1200 The experimental values of the resistivity are (0.2-1)109cm, which are more than an order higher as compared to the resistivity of structures on the basis of LEC SI-GaAs. 12 Electrophysical characteristics of high resistivity GaAs о no po Ln (10 -9/*cм) (10 5 cm –3) (10 5 cm –3) (cm) GaAs EL2 6-9 70-100 4-6 0.03 - 0.05 GaAs Cr 0.6-1.1 2-3 120-200 0.07 – 0.2 Material 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. 13 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 10-6 A/cm2. 14 Electric field distribution (2) (DIFFUSION CROMIUM COMPENSATED GaAs LAYERS ) The most important distinction of our structures, as compared to the traditional LEC SIGaAs, is the uniform electric field distribution and the absence of current oscillations. 500 V GaAs:Cr d 1000 V Spatial distribution of the F function and light transmission (T) in detector thickness d , F=1-T 15 Amplitude spectra of the GaAs:Cr detector for various energies of the gamma radiation 160 100 Detector thickness d=780 m 241 1 - Ey=60 keV, Am source 120 3 2 - Ey=122 keV, Co source 3 - Ey=140 keV, 80 Tc source Bias voltage U= 600 V 1 2 99m CCE, % 1 Counts 80 57 60 Detector thickness d=780 m 241 1 - Ey=60 keV, Am source 40 57 40 2 - Ey=122 keV, Co source 2 3 - Ey=140 keV, 20 99m Tc source 3 0 0 200 400 Channel number 600 800 100 200 300 400 500 600 700 U, V There is the 70-80 % 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 780m. 16 Amplitude spectra for GaAs:Cr detectors with different active layer thicknesses. E 60 keV (241Am source) 800 241 E = 60 keV (Am ) 500 V 1 - d = 1200 m, U = 800 V 2 - d = 830 m, U = 600 V 3 - d = 715 m, U = 500 V 400 CCE, % Counts 600 90 1 200 80 600 V 70 2 800 V 3 0 60 0 100 200 channel number 300 400 800 1000 1200 detector thickness, d 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. 17 Conclusion • The technology of high temperature Cr doping of n-type GaAs allows to produce the high resistive GaAs layers with resistivity up to 109 *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. 18