Study of the thickness of the dead layer below electrodes, deposited by electroless technique, in CdTe nuclear detectors

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Study of the thickness of the dead Layer below electrodes, deposited by electroless technique, in CdTe nuclear detectors

Article in IEEE Transactions on Nuclear Science · March 2006

DOI: 10.1109/TNS.2006.869846 · Source: IEEE Xplore

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Study of the Thickness of the Dead Layer Below Electrodes, Deposited by Electroless Technique, in

CdTe Nuclear Detectors

Khaled Zahraman, Mohamad Roumie, Adelaide Raulo, Natalia Auricchio, Mohamed Ayoub, Ariano Donati, Waldes Dusi, Makram Hage-Ali, Fatima Lmaï, Eugenio Perillo, Paul Siffert, Malgorzata Sowinska, and

Giulio Ventura

Abstract—The semiconductor nuclear detector structure is usu- ally a superposition of several different layers: the metallic elec- trode, the interface, the depleted zone, the nondepleted one, and fi- nally the second electrode. All inactive layers, and particularly the

“dead layer” region below the metallic electrodes, give rise to ab- sorption for photons (X, ) and to energy loss for charged particles.

As a consequence, the detectors measure lower count rates for pho- tons, particularly at low energies, and lower energies for charged particles. The amount of this effect can be only partially attributed to the inactive metallic electrode; the main part has to be attributed to the highly defected zone below the electrode in the interfacial region. This is a structural drawback which needs to be studied to reduce this effect, e.g., by developing new polishing and depo- sition techniques, or to perform the appropriate corrections. We have measured the thickness of this dead layer for various solution dilution electroless depositions of Pt electrodes on CdTe devices.

These measurements were carried out by using collimated (X, ) rays beams impinging at different angles, especially the grazing ones, and comparing the count rates registered at two selected an- gles, and by measuring Rutherford backscattering spectra and en- ergy loss of⁴He ions.

Index Terms—Author, please supply your own keywords or send a blank e-mail to keywords@ieee.org to receive a list of suggested keywords..

Manuscript received November 29, 2005; revised xxxx.au: please provide dateThis work was supported in part by the Italian National Institute for Nuclear Physics in the framework of the SINEC Program and in part by the CEDRE Program (CNRSL, Lebanon-CNRS, France).

K. Zahraman and M. Roumie are with the National Council for Scientific Research (CNRSL), Commission Libanaise de l’Energie Atomiqu,au: postal code?Beirut, Lebanon.

A. Raulo and E. Perillo are with Dipartimento di Scienze Fisiche, University Federico II, 80126 Napoli, Italy and also with the Italian National Institute for Nuclear Research (INFN), Sezione di Napoli, 80126 Napoli, Italy (e-mail: Ade- laide.raulo@na.infn.it).

N. Auricchio, A. Donati, and G. Ventura are with the Istituto Nazionale di As- trofisica (IASF), Italian National Research Center (CNR), Sezione di Bologna, 40129 Bologna, Italy.

M. Ayoub, M. Hage-Ali, and F. Lmaï are with the Laboratory Physique et Applications des Semiconducteurs (PHASE), French National Center for Sci- entific Research (CNRS), 67037 Strasbourg, France.

W. Dusi is with the Istituto Nazionale di Astrofisica (IASF), Italian National Research Center (CNR), Sezione di Bologna, 40129 Bologna, Italy and also with the Italian National Institute for Nuclear Research (INFN), Sezione di Bologna, 40129 Bologna, Italy.

P. Siffert is with the European Materials Research Society, 67037 Strasbourg, France.

M. Sowinska is with EURORAD, 67037 Strasbourg, France.

Digital Object Identifier 10.1109/TNS.2006.869846

I. INTRODUCTION

THE metallization process of a CdTe wafer surface needed to fabricate an X-ray detector can produce changes in the stoichiometry and crystal structure in the subsurface region of the material. These changes can modify the electrical behavior in the near-electrode region and introduce high concentration of electrically active defects. As a result, differently thick dead layers, that can strongly affect the counting rate and the energy resolution of the device, can be produced. Therefore, it is important, for detector users and manufacturers, to know this dead layer thickness, in order to choose suitable detectors for each specific application.

Electroless Pt deposition is one of the easiest and more conve- nient methods for contact deposition on CdTe detectors. There is no need for vacuum and sophisticated systems and, above all, this method allows us to solve the polarization problem.

However, this process is highly nonuniform and many pa- rameters dependent; thus the study requires systematic measurements and many preparation precautions.

We have shown in previous papers the variation of both the deposited metal thickness and the interfacial layer thickness, as a function of the solution dilution and the metal nature [1], [2]. Furthermore, we have shown that these contacts produce important dead layers affecting the performance of these devices when used as photon detectors [3].

In this work we use two different methods to measure this dead layer thickness, for several CdTe devices with Pt electrodes fabricated starting from various dilutions Pt solutions. In particular, the decrease of photon count rates and the light ion energy loss due to the presence of the dead layer are investigated. Also, we try to find a correlation of the dead layer thickness with the interfacial layer defects, or, at least, to understand how this layer is produced.

The radiation-matter interaction is strongly dependent on the radiation nature. In the case of photons, three main kinds of interaction are possible:photoelectric effect, compton effect, and pair creation. In the photoelectric case (the most probable at low X-ray energies) only the output photon number is decreased be- yond an absorber, while the photon energy remains the same.

In the case of particles, an opposite behavior occurs. After passing through a thin matter layer the number of particles remains the same, while the particle energy is decreased, owing to the energy loss in the absorber.

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Now, the inactive layer can be divided in two parts: the metal contact and the strictly speaking dead layer. Both the methods used here allow the measurement of the total absorbing layer; we have measured independently and accurately the metal contact thickness by Rutherford Backscattering Spectrometry (RBS), thus the detector dead layer thickness can be obtained.

Photons have been used in the past to measure the dead layer thickness in Si (Li) X-ray detectors [4]; also, low-energy (150 keV) protons have been used to investigate Si surface barrier detectors [5], [6].

II. EXPERIMENTALPROCEDURES

A. Sample Preparation

The detectors were fabricated by EURORAD (Strasbourg, France) starting from CdTe:Cl crystals grown by the THM technique, with resistivity of the order of cm. The dimensions of the tested detectors are (3 10 2) mm ; two platinum electrodes were deposited (platinum hexachloride solution, 4 min, room temperature, random direction crystals), by means of the electroless technique, on the two opposite (3 10) mm faces.

Prior to the metallization the detector surfaces were lapped and 1 m polished, and then etched for 2 min (100 m dissolution) with 5% Brome-Methanol solution; subsequently, the surface defected layer is thinner than 5 nm (verified by channeling).

In order to investigate possible effects due to changes of Pt concentration in the electroless solutions, we have used four Pt solution dilutions: 1, 5, 15, and 25, where 1 is the original solution (1g salt diluted in 25 cm water). Two wafers were used for each dilution. The wafers, 15 mm in diameter, were Pt con- tacted and from the central region of each wafer two detectors were fabricated, A and B, while the remains were used for the RBS measurements. Two different series of samples were used for the two types of measurements, because the RBS ones re- quire the use of 1–3 MeV He ions, that can damage the detector contacts and affect their electrical properties. The total number of samples at our disposal consisted of 16 detectors and 16 RBS samples.

B. Photon Measurements

The method used here is based on irradiating a detector through the cathode, by means of a narrow 22-keV energy photon beam, at two incident angles (0 and 82 ) with respect to the normal to the cathode and on recording the photopeak areas versus the angle of incidence.

The detectors are mounted on a mechanical system that facil- itates the modification of the value of the angle between the direction of the impinging photon beam and the normal to the cathode surface, from 0 to 90 , as shown in Fig. 1.

The value of the angle can be read on a graduated scale and set with a precision of half a degree.

The X-ray sources are strongly collimated by a 20 mm thick tungsten collimator with a (0.1 0.5) mm slit. At the photon beam spot on the cathode surface is about (0.18 0.90) mm with the wider dimension orthogonal to the long side of the detector, in order to allow us to use large angles without losing incoming photons. Both tungsten collimator and source holder are mounted on a manual two-dimensional micrometric

Fig. 1. Schematic drawing of the X-ray irradiation configuration.

positioning system, capable of an accuracy of 5 m, to allow the best positioning of the photon beam on the detector.

The detectors were systematically biased at 100 V. The value of 50 V/mm is typical for CdTe detectors; in any case, we tried different bias voltages, observing that at V V the charge collection time takes already its minimum value, that is lower than 300 ns. Anode pulses were derived, through a decou- pling capacitor, via commercial charge-sensitive preamplifiers (Clear Pulse mod. 515-2); the opposite side was connected to the ground. A 0.5 s shaping time was selected at the main ampli- fier, as that gave the best energy resolution without affecting the counting rate. The pulses were shaped, stretched, and recorded by a standard nuclear electronic chain.

Spectra at and were recorded for a Cd

source (22 keV peak). Background radiation measurements were performed in order to correct the spectra.

All the recorded spectra were analyzed using the PeakFit software package [7] and the Aptec MCA Application Version 6.31 in order to extract the photopeak areas. A simple Gaussian dis- tribution was used, due to the good symmetry of the analyzed peaks.

The dead layer evaluation was performed taking into account that the photons to be detected have to pass through both the cathode and the detector dead layer. Fig. 1 shows the dead layer thickness and the electrode thickness at an irradiation angle , where their effective thickness for absorption of the incident X-ray beam can be increased as 1/cos . In particular, at , it becomes more than 7 times thicker than at 0 . As a consequence, the peak areas decrease of about 15 % at 22 keV (see Table I).

The linear (1) allows one to obtain the dead layer thickness, , as a function of the electrode thickness, , the linear absorption coefficients in the electrode material (Pt) and in the dead layer material (assumed here as CdTe; different assump- tions about the dead layer composition, e.g., CdTe TeO , and so on, do not affect significantly the results), at the used energies, the two irradiation angles, and , and the two peak areas obtained at these two angles for a definite energy

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TABLE I

EXPERIMENTALRESULTS ANDCALCULATEDDATAOBTAINEDFROM THEPRESENTMEASUREMENTS

where in our case, is the peak

area measured at is the peak area at 82 is the linear absorption coefficient for the platinum electrode

( cm ), is the electrode thickness and

is the linear absorption coefficient for the CdTe material

( cm ).

If the electrode thickness is known (as already indicated we have taken here the values coming out from the RBS measurements-see in the following) the application of (1) gives straight- forwardly the value of . The obtained values are reported in Table I.

A more detailed description of the used method can be found in [3].

C. Particle Measurements

The second method is based on the particle energy losses and uses light ions for the irradiation of the devices, while -rays are used for energy calibration, as the detector energy response to this kind of radiation is not affected by the dead layer thickness.

Thus, the photon peak positions allow one to set the forecast positions of the peaks in the particle spectra, independently of the dead layer window thickness. On the contrary, the energy response to an incoming particle is directly related and actually decreased by this dead layer. The recorded spectra give the experimental energy values, i.e., as affected by this window. The observed energy differences allow one to measure the dead layer thickness, making use of the well-known Energy Loss tabula- tions [8].

For these measurements we have used the He beam of the 5 MeV accelerator facility at the CLEA–CNRSL–Beirut- Lebanon, which allows us to perform RBS measurements as well as detection of the scattered beam by the CdTe detectors, for dead layer thickness determination. For the RBS measurements, a 2 MeV He ion beam and a Si surface barrier detector positioned at an angle of 165 to the beam direction were used.

We have used a secondary target in order to reduce the beam intensity and energy, as the CdTe detectors cannot be hit directly

by the beam. For this purpose we have used a 20 Å thick Cr layer on SiO substrate; the CdTe detectors were positioned at 165 to the beam, the He ion beam energy was chosen to be 1805 keV. In such a way the He ions backscattered by the Cr atoms and impinging the CdTe detector have an energy of 1332.5 keV;

the corresponding peak is observed at a given channel X in the spectrum. In order to obtain an absolute energy calibration, two -ray sources were used, Cs (peak at 661.66 keV) and Co (peaks at 1173.2 and 1332.5 keV). In this way, the channel Y corresponding to the full energy peak of the 1332.5 keV gamma-ray was determined. The difference Y-X represents the total energy lost by the He ions in the entire absorption region before the sensitive detector volume. The thickness of this total dead layer was estimated by means of the SIMNRA code [9].

Taking the Pt thickness as obtained by RBS, we were able to determine the strictly speaking dead layer for every sample.

III. RESULTS

A. Photon Measurements

The spectra obtained for Cd at 0 and 82 are shown in Fig. 2 The numerical values of the corresponding 22 keV photopeak areas are reported in Table I.

From the analysis of these values we can infer that they clearly decrease, about 15% in the average, changing the impinging angle from 0 to 82 , owing to the increased effective dead layer thickness. The dead layer thicknesses, derived from the X-ray attenuation measurements, were calculated by (1) using the corresponding Pt electrode thickness measured by RBS method, and are reported in Table I.

B. Particle Measurements

A typical spectrum of the scattered He beam, recorded with a CdTe detector, is shown in Fig. 3, together with the forecast position of the 1332.5 keV peak. The significant (Y-X) difference (see Section II-C), due to the total dead layer absorption,

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Fig. 2. Typical spectra for Cd source at 0 and 82 .

Fig. 3. Typical RBS spectrum of the He ions at incident energy 1.805 MeV, scattered by a Cr/SiO sample and recorded with a CdTe detector (1405-66A).

1 channel= 1:28keV.

can be clearly pointed out. The corresponding values of the energy loss are reported in Table I for all the investigated samples.

The values, , of the Pt electrode thickness were extracted from RBS spectra (one example is reported in Fig. 4) by means of the SIMNRA simulation code and are reported in Table I for all the investigated samples.

IV. DISCUSSION ANDCONCLUSION

It should be noticed that the two methods used here are quite different in their nature, as follows.

—The particles beam has a rather small range in the detector, therefore the surface zone gives a significant contribution to the absorbed energy.

—The low-energy X-ray photons have an exponential absorption, thus penetrating deeper into the semiconductor crystal. In this way photons experience also all the defects present in the bulk, mostly the metal precipitates (Te, for example, see [10]), which can act as individual dead vol- umes.

Despite the differences in the absorption mechanism, the measured values are very close for the window thicknesses metallic contact dead layer , indicating that the studied detectors are roughly of equal quality as, for all of them, bulk defects do not affect significantly the effective dead layer thickness. Another important question is to learn more about

Fig. 4. Typical RBS spectrum of He ions at incident energy 2.0 MeV, scattered by a CdTe detector -like sample (1405-66), recorded with a standard silicon detector.

Fig. 5. Cd vacancies versus depth, samples 1383-15b and 1405-66b.

Fig. 6. Dead layer thickness versus Pt electrode thickness.

the origin of the dead layer and the related thickness (values ranging from 1.5 to 2.3 m), much larger than the metallic contact (values ranging from 279–680 Å), and essentially independent of the solution dilution. This goes back to the mechanism of deposition of the Pt contact, i.e., a solid-state chemical exchange, allowing the diffusion of the heavy metal into the semiconductor. In other words, between the metallic contact and the bulk CdTe an interfacial zone exists, where the

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Cd/Te ratio, as well as the chemical composition, is largely perturbed.

Actually, SIMS measurements have shown that platinum as well as oxygen is present in the surface and the interfacial zone, and also that the Cd/Te ratio is completely changed [1].

These profiles of Pt, Te, Cd, and O can probably be modified, depending on the deposition conditions, i.e., on the presence of defects, vacancies, precipitates, etc. [10]. A closer analysis of the RBS spectra indicates that Cd vacancies are present in large concentration in the shallow layer between the electrode and the bulk (Fig. 4 compares a simulated and an experimental spectrum). Fig. 5 gives explicitly the concentration of the Cd vacancies, calculated as percentage reduction with respect to the simulated Cd spectrum, for two investigated samples.

As one can see, the region where the effect is most significant corresponds to a thickness ranging from 0.3–1 m. However, the Cd vacancies amount cannot explain completely the total extension of the dead zone.

A possible interpretation of this feature, besides the Cd va- cancy concentration, can be that for a given amount of Pt atoms deposited on the surface, a small fraction of the Pt diffuses inside the bulk. This diffusion depends on the physical and chemical deposition conditions, as well as on the concentration of the ex- isting vacancies.

This Pt diffusion, even at really low concentration, will reduce considerably the transport properties of the charge carriers in this zone, acting in some way as a dramatic lifetime reducer, owing to the recombination effect.

We can notice that a correlation exists between the dead layer thickness and the Pt contact thickness (see Fig. 6); it seems that the dead layer decreases when the Pt thickness increases. This can be interpreted taking in mind that a more extended dead layer means a higher number of Cd vacancies; this, in turn, leads to an increasing concentration of Cd atoms and a decreasing platinum concentration in the solution, which decreases the Pt deposition reaction velocity.

A practical conclusion can also be drawn: if very thin win- dows are needed, like for low-energy X-ray detectors, the electroless contact deposition may not be the best choice.

If a different deposition method is used, care has to be taken in choosing a technique not inducing polarization effects in the material, like, for example, when indium sputtering is used.

This polarization would induce a progressive decrease in both counting efficiency and pulse amplitude.

ACKNOWLEDGMENT

Thanks are due to EURORAD, Strasbourg, France, for preparing and supplying the detectors used in this work.

REFERENCES

[1] M. Roumie, M. Hage Ali, K. Zahraman, B. Nsouli, and G. Younes,

“Characterization of electroless Au, Pt, and Pd contacts on CdTe and ZnTe by RBS and SIMS techniques,”Nucl. Instrum. Meth. Phys. Res.

B., vol. 219, pp. 871–874, 2004.

[2] M. Hage Ali, M. Ayoub, M. Roumie, F. Lmaï, K. Zahraman, B. Nsouli, and M. Sowinska, “CdTe nuclear detector electroless contact studies, new results on contact structure, interfaces, and stress,”IEEE Trans.

Nucl. Sci., vol. 51, no. 4, pp. 1875–1880, Aug. 2004.

[3] W. Dusi, A. Donati, G. Landini, E. Perillo, A. Raulo, G. Ventura, and S.

Vitulli, “An experimental method to evaluate the dead layer thickness of X- and Gamma-ray semiconductor detectors,”IEEE Trans. Nucl. Sci., vol. 51, no. 6, pp. 3090–3093, Dec. 2004.

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[6] P. Siffert, G. Forcinal, and A. Coche, “Pulse height defects due to nuclear collisions measured with thin window silicon surface barrier detector,”

IEEE Trans. Nucl. Sci., vol. NS-14, pp. 532–536, 1967.

[7] Peak Separation and Analysis Software: User’s Manual, 1995. Jandel Scientific Software, PeakFit v.4.0.

[8] W. K. Chu, J. W. Meyer, and M. A. Nicolet, Eds.,Backscattering Spec- trometry. Orlando, FL: Academic, 1978.

[9] M. Mayer, “SIMNRA user’s guide,” Max-Planck-Inst. Plasmaphys, Garching, Germany, Rep. IPP 9/113, 1997.

[10] M. Hirasawa, “Dead region and electric thickness in the CdTe (Cl) radiation detector,” inKEK Proc., vol. 20, 2001, pp. 106–121.

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