Development of semi-insulating GaAs detectors for digital radiography

UCLEAR PHYSIC5

PROCEEDINGS SUPPLEMENTS I-L,~;EVIER

Nuclear Physics B (Proc. Suppl.) 61B (1998) 633 637

Development of Semi-Insulating GaAs detectors for Digital Radiography E. Bertolucci a, U. Bottigli bt, M.A. Ciocci b, A. Cola c *, M. Conti a, M.E. Fantacci b, N. Romeob, P. Russo a, F. Quaranta c * and L. VasanelliC, d * a Dipartimento Scienze Fisiche, Universit~ di Napoli e Sezione INFN di Napoli, Mostra d'Oltremare Pad. 20, 80125, Napoli, Italy b Dipartimento di Fisica, Universit~ di Pisa e Sezione INFN di Pisa, Via Vecchia Livornese 582a, 56010, S.Piero a Grado (Pi), Italy c CNR, Istituto per lo studio di nuovi Materiali per rElettronica, Via per Arnesano, 73100, Lecce, Italy d Dipartimento di Scienza dei Materiali, Universit~ di Lecce, Via per Arnesano, 73100, Lecce, Italy

In this paper we present the results of an experimental study concerning different contact deposition processes on Semi-Insulating (S.I.) GaAs detectors aiming to study and optimize their performance in terms of leakage current, break-down voltage, charge collection efficiency and energy resolution when irradiated with 60 KeV photons; in particular the effect of the mesa etching treatment on the Schottky barrier side of the detectors has been studied. Such treatment certainly improves the detector performances related to the very important issue of the electric field uniformity. Detectors with satisfactory features in view of their possible application to Digital Radiography have been obtained. 1. INTRODUCTION GaAs is a good c a n d i d a t e for the detection of X-rays having typical energies for Digital Radiography (10-100 KeV). In fact, its Z value allows for a high detection efficiency and, as a consequence, for a reduction of the dose to the patient. However, in Semi-Insulating (SI) GaAs the charge collection properties are affected by the presence of traps. To partially overcome the charge loss due to the trapping mechanisms, bias voltage as high as possible should be applied thus increasing the average electric field. This is fundamental in the field of medical i m a g i n g where a high charge

t Now at CERN, Geneva Switzerland * This work has been partially supported by Sezione INFN di Lecce 0920-5632/98/$19.00 © 1998 Elsevier Science B.'~ All rights reserved. PlI S0920-5632(97)00630-0

collection efficiency (c.c.e.) and a good energy resolution are p r i m a r y r e q u i r e m e n t s . Previous studies resulted in detectors where a satisfactory compromise between c.c.e, and detection efficiency was achieved [1,2,3] and which showed good imaging capabilities [4]. In this work we present f u r t h e r results concerning detectors made by using different contact deposition process, in order to increase the break-down voltage and their performances, from the point of view of leakage current, c.c.e., energy resolution, and detection efficiency. Either single pixels of different sizes and matrices of pixels have been obtained. The influence of annealing and mesa etching process have been studied. The detectors have been prepared using SI GaAs substrates of different thicknesses. Their response to 60 keV photons has been studied in terms of c.c.e., energy resolution.

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E. Bertolucci et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 633-~637

2. E X P E R I M E N T A L All detectors have been made on Semi I n s u l a t i n g GaAs (100) wafers. The wafers belong to different c o m p a n i e s a n d have different thicknesses: AXT wafers are 100 and 200~m thick, whereas the 400~m thick ones belong to J a p a n Inc.. Two kinds of Schottky barriers have been tested: (100A Cr / 800A Au) deposited by thermal evaporation with a deposition pressure of 10 -6 m b a r and a deposition rate of 10 A/s and (800A Ti / 500A Au) d e p o s i t e d by e l e c t r o n b e a m e v a p o r a t i o n in t h e s a m e d e p o s i t i o n conditions. Pixels of different areas ranging from 200x200 ~m 2 to 2x2 mm 2 and matrices of p i x e l s have been obtained by photolithography using the lift-off technique. After the, barrier deposition the back contact h a s been obtained depositing a multilayer (500A eutectic Au-Ge / 50A Ni / 700A Au) on the whole surface by t h e r m a l evaporation. This alloy c o n t a c t p r e s e n t s an ohmic b e h a v i o u r if a n n e a l e d in the a p p r o p r i a t e conditions or a Schottky barrier behaviour if not [5]. Such annealing has been carried out in hydrogen atmosphere at 380 °C for 180s with fast h e a t i n g and cooling in order to obtain the required interdiffusion for ohmic contact fbrmation [6].We have analysed the effect of the annealing and the effect of a m e s a e t c h i n g p r o c e d u r e on the typical properties of the detectors. The mesa etching procedure consists on a wet etching of the upper face (pixel side) by a H2SO4:H202:H20 (4:2:1) solution at 40°C in order to obtain m e s a s t r u c t u r e for the pixels~ The etching r a t e w a s a p p r o x i m a t e l y 6A/s and t h e thickness of the etched material was about 2 ~m. In some sample we have repeated the etching procedure reaching a etched thickness of 20~m. The current-voltage characteristics were measured by a Keithley 617 picoammeter/dcvoltage source. All d e t e c t o r s w e r e i r r a d i a t e d with a 241Am alpha screened source and the full set of m e a s u r e m e n t s was p e r f o r m e d at the corresponding photon energy (60 keV). The signals from the irradiated detectors were collected by a low noise charge-sensitive p r e a m p l i f i e r ( O R T E C 142A), a s h a p e r amplifier (ORTEC 673) with a shaping time

of 1.0 ps and a multichannel analyser. All m e a s u r e m e n t s were p e r f o r m e d at room temperature. 3. R E S U L T S A N D D I S C U S S I O N 3.1. I-V m e a s u r e m e n t s

Before annealing, no a p p r e c i a b l e difference have been noticed between s a m p l e s with Cr or Ti contacts. This is p a r t l y e x p e c t e d since S c h o t t k y b a r r i e r s values on GaAs are i n d e p e n d e n t of which metal is deposited. All samples behave as a double blocking structure because the ohmic contact is still not formed. The a n n e a l i n g procedure is effective in m a k i n g the back c o n t a c t ohmic: u n d e r f o r w a r d bias the current density is linearly proportional to the applied v o l t a g e up to m o d e r a t e v a l u e s becoming then superlinear at a voltage that depends on the detector thickness. We focus o u r a n a l y s i s on t h e r e v e r s e v o l t a g e characteristics because these are the conditions in which this device isoperated as radiation detector. Ti b a r r i e r s do not change appreciably their reverse current characteristics after the a n n e a l i n g w h e r e a s we noticed a clear worsening in the Cr barriers. This different b e h a v i o u r can be a t t r i b u t e d to the goldgallium interdiffusion in SI GaAs on the Schottky contact side during the annealing which is p r o b a b l y not b l o c k e d by the C h r o m i u m layer as efficiently as by the T i t a n i u m l a y e r [7]. M o r e o v e r , it is worthwhile to mention that we have used a much t h i c k e r layer of T i t a n i u m than of Chromium. The effect of the mesa etching procedure can be seen in Fig.1 where the c u r r e n t densities of 200x200 ~m 2 and 2x2 mm 2 diodes before and after m e s a etching are reported. A detailed discussion of the current characteristics, together with a description of the various transport mechanism, is reported in [8]. For the small mesa diode we notice a clear reduction of the current density respect to the not mesa, especially at low voltage. This effect is significant of a strong non areal c u r r e n t c o n t r i b u t i o n , t h a t is p r o b a b l y e n h a n c e d by the geometrical a s y m m e t r y between front and back contacts, and which becomes less evident at high voltage, when

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B I A S V O L T A G E (V) Figure 1. C u r r e n t density vs. reverse bias voltage for 2001xm thick detectors with Cr barriers of different areas with and without mesa structure the lines of force of the electric field become more parallel to each other. As the pixel area increase, the differences between mesa and not m e s a diodes reduces and the c u r r e n t density values converge towards a common value. In fact, as can be seen in Fig. 1, in the case of bigger area, there is no appreciable difference between the values of the current densities for mesa and not-mesa diodes and these v a l u e s are lower t h a n those of the smaller diodes. These results suggest t h a t there is a contribution to the leakage current t h a t depends on the diode perimeter and so it is more effective for small dimensions; the creation of a mesa structure by wet etching is an efficient tool to r e m a r k a b l y reduce, but not completely eliminate, such currents. If f u r t h e r e t c h i n g s are p e r f o r m e d on a p r e v i o u s l y e t c h e d s a m p l e no o t h e r appreciable m o d i f i c a t i o n s of the l e a k a g e c u r r e n t are observed. Even if a s t r o n g p e r i m e t r a l contribution is due to surface currents, a n o t h e r p e r i m e t r a l contribution which can not be suppressed by the m e s a etching is predominant at high voltage. In Fig 2 are reported the current densities vs. r e v e r s e voltage for m e s a diodes of d i f f e r e n t a r e a s . It is e v i d e n t t h a t J-V

relations still strongly depend on the pixel area. In Fig.2 each curve has been plotted up to the m a x i m u m voltage before the breakdown. It can be seen t h a t the breakdown voltage increase for decreasing diode dimension. This effect should be related to the non uniformity of the electric field inside the detector and it is e n h a n c e d by the geometrical difference between Schottky and ohmic contacts. The greater importance of the perimetral currents and t h e r e f o r e the non u n i f o r m i t y of the electric field for the s m a l l e r diodes with r e s p e c t to t h e l a r g e r ones r e d u c e the extension of the active t h i c k n e s s in the former case. As the break-down mechanism is r e l a t e d to the back c o n t a c t c a r r i e r injection, which takes place when the active region reaches such contact, the dependence of the break-down voltage on the diode size is easily explained. Some recent results [9] on a new k i n d of ohmic c o n t a c t which s u s t a i n s very high electric field inside the detectors confirms such explanation for the break-down mechanism. However, we have found t h a t the m e s a procedure increases sistematically and significantly the breakdown voltage, expecially for small diodes. Such a f i n d i n g suggests a more complex '

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Figure4. E n e r g y resolution vs. reverse bias voltage for 200~m thick detectors with Cr barrier with and w i t h o u t mesa etching. Diodes area is 0.25 mm 2

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the energy resolution for T i t a n i u m detectors 100 and 440 ~m thick are reported. A m a t r i x of 25 s q u a r e pixel (200x200 ~m 2) h a s been obtained on a 100 pm thick, mesa detector; for each pixel c.c.e, and energy resolution have been individually m e a s u r e d to h a v e a check a b o u t t h e d e t e c t i o n uniformity, a very i m p o r t a n t feature in view of a n y i m a g i n g a p p l i c a t i o n . D e f i n i n g uniformity ~ simply as:

3.2 Detection properties As in the case of the reverse current, only the a n n e a l e d detectors with a Cr b a r r i e r have poor detection properties. In all other cases the detection properties are good and there are not appreciable differences between Cr and Ti barriers. In Fig.3 and in Fig.4 are shown, versus the bias voltage, t h e collection c h a r g e efficiency (c.c.e.) and the energy resolution for a 200~m thick Cr barrier with and without the m e s a e t c h i n g t r e a t m e n t . The etching t r e a t m e n t to create the m e s a s t r u c t u r e always improves the detection performances: it slightly increases the charge collection efficiency, and it strongly improves the energy resolution. These results are clearly related to the better uniformity of the electric field inside the detectors, together with the lower values of the current density. The improvement due to mesa procedure is even more evident for thick detectors but it is still appreciable for thin detectors as it is show in Fig.5 and Fig.6 where the c.c.e, and

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4. CONCLUSIONS

encouraging from the point of view of any imaging application.

The response to irradiation with low e n e r g y photons, such those used in Radiographic applications has been studied for a sample of m a n y S.I. GaAs pixel detectors. This includes detectors of various thicknesses and pixel sizes and different types of layer for the Schottky barriers. Their performance has been studied in terms of leakage current, break-down bias voltage, charge collection efficiency and energy resolution. The mesa etching procedure performed on the Schottky barrier side of the detectors remarkably improves thier overall performance, reducing surface currents, allowing much higher values for the breakdown voltage, and significantly reducing the energy resolution down to about 5%. Also the c.c.e, improves, especially in the case of the thicker diodes. The values obtained for 100pm thick detectors (>87%) are quite satisfactory in view of a possible Digital Radiography application. The uniformity of behaviour shown by the 25 pixels of a 1 0 0 ~ m thick pixel matrix is also very

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