Ion beam analysis of CdTe nuclear detector contact grown by electroless process

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Ion beam analysis of CdTe nuclear detector contact grown by electroless process

M. Roumie´

^a,*

, K. Zahraman

^a

, B. Nsouli

^a

, F. Lmai

^b

, A. Zaiour

^c

, M. Hage-Ali

^b

, M. Ayoub

^b

, M. Sowinska

^d

aAccelerator Laboratory, Lebanese Atomic Energy Commission, CNRSL, P.O. Box 11-8281, Beirut, Lebanon

bLaboratoire PHASE, CNRS, 67037 Strasbourg cedex 2, France

cFaculty of Sciences I, Lebanese University, P.O. Box 14-657, Beirut, Lebanon

dEURORAD II-VI, 67037 Strasbourg, Cedex 2, France Available online 1 August 2005

Abstract

Ion beam analysis was performed on CdTe(II–VI) semiconductor materials in order to characterize metal contacts, namely Pt, deposited by electroless process. It is essential to perform such a material which can provide high detection quality that needs good deposited metal contact as thick as possible. Rutherford Backscattering Spectrometry was used to determine the thickness and the stoichiometry of the layers formed at the surface. The depth profiles of Pt, Cd, Te and O were determined as a function of the dilution solution and the pH parameters which seemed to be determinant factors in the Pt layer deposition process. The distribution of Cd deficiency at the interface layers was profiled using simulations and showed complex profiles in the samples, which can greatly influence the electrical quality of detectors.

PACS: 81.05.Dz; 34.50.Dy; 39.10.+j

Keywords: IBA; RBS; Electroless; Semiconductors; CdTe; SIMNRA

1. Introduction

High resistivity CdTe(II–VI) semiconductors are very strategic materials and are of great impor-

tance in many fields, such as room-temperature X- and gamma-ray detection, infrared detection, optronics, photo-refractivity for all optic switching and in many other sensor and device fields. How- ever, the high resistivity often induces a space charge carrier accumulation under barrier contacts, well known under the name of polarisation effect, which is common to all high resistivity

doi:10.1016/j.nimb.2005.06.135

* Corresponding author. Tel.: +961 1 450811; fax: +961 1 450810.

E-mail address:[email protected](M. Roumie´).

www.elsevier.com/locate/nimb

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materials [1]. To avoid problems related to this polarisation effect, the spontaneous chemical deposition, called ‘‘electroless’’ process[2], is used and leads to a useful injecting contact [3]. How- ever, this process needs to be improved and studied in more details, since it seems to suffer from irregularities in the metal thickness and contact qualities. The quality of the performed detector depends on the semiconductor substrate and especially on the deposited contact qualities. The quality of these contacts depends on the deposition time, the solution dilution, the pH and the temperature. In previous paper, we have studied the deposition time, the solution dilution and the deposited metal[4], while in a second one, we have additionally studied the effects of stress and of interfacial region on electrical characteristics [5].

In this work we restricted our study to the case of the Pt contact taking into account both the dilution and the pH as parameters, while the other parameters were ﬁxed, in order to obtain more coherent results. Under these experimental condi- tions, RBS measurements determined the thickness and the stoichiometry, of the layers formed at the surface, as well as the depth proﬁles of some present elements when possible.

2. Experimental

The RBS technique is still one of the best non- destructive and absolute methods for the estimation of the thickness and the composition of heavy metal layers of less than 500 nm of thickness. The 5SDH pelletron tandem accelerator of 1.7 MV located at the LAEC [6] was used to perform RBS measurements on the samples placed under normal incident beam. A partially depleted PIPS detector from Canberra, with 14 keV of energy resolution and 25 mm² of active area, detected the backscattered particles of 2 MeV⁴He²⁺beam, at a scattering angleh of 165.

The studied samples were prepared by electroless deposition of thin metal layer of Pt onto CdTe semiconductor substrate. One gram of chloro-pla- tinic acid salt was dissolved in 25 ml of de-ionized water yielding a ‘‘dilution 1’’ solution. By further dilutions, other solutions were obtained (5, 10,

15, 25 and 35 times). The dipping of the material wafers in the solution allowed the so-called electroless spontaneous deposition of a thin metal layer.

The typical dimensions of the samples were 1·1 cm²in surface and about 1 mm in thickness.

The other experimental parameters were fixed, such as the temperature (RT), the pH (acid 1–2), the luminosity (hood lamp) and the deposition time (4 min). It was observed, in a preliminary study, that the deposition time reaction seemed to be without a significant effect while the solution dilution degree was more efficient at increasing the thickness of the metal layer. In the current study, the first part aimed at determining the characteristics of deposited layers as a function of the dilution degree. The second part concerned a set of samples prepared at a fixed ‘‘dilution 5’’ with variable pH values (0.5, 1, 1.5, 2 and 2.4).

3. Results and discussion

The different RBS spectra were processed with the SIMNRA simulation code[7], in order to characterize the newly formed layers. The complexity of the RBS spectra required fitting with several successive layers of different thickness. Consider- ing the nature of the chemical reaction (water electrolysis), our early results with others methods showed that the present elements are mainly Pt, Cd, Te, O, Cl and H. In fact, O has been clearly detected by Auger and SIMS methods [4]. How- ever, in the current study, the Elastic Scattering technique at 5 MeV alpha-particle beam showed a weak resonance signal, due to the overlapping of Cd and Te (Fig. 3(b)). Nevertheless, the O con- tent was determined successfully by completing the stoichiometry of the different layers. Concerning H, a preliminary study by ERDA technique showed the presence of H in relatively small amounts. More extended and specific studies are foreseen in the future to determine H by ERDA, Cl by PIXE and O by NRA using the resonance at 3.04 MeV. Indeed, in RBS spectra, the low masse of H should not alter significantly the composition and the thickness of the heavy element layers deduced by SIMNRA simulations. How- ever, H and Cl act as doping impurities and induce

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electrical defect levels in the band gap. The deter- mination of these impurities as well as the formed TeOx interface layers could help explain the electrical behaviour and the characteristics of the devices [5]. Two main parts of this study, related to the formed layers, are discussed as a function of the ‘‘dilution’’ and the ‘‘pH’’ of the solution.

3.1. Dilution variation

Two sets of samples were prepared at different dilutions: (1, 5, 15, 25 and 35) and (1, 5 and 35), respectively. As observed in a previous work [4], which has been realized without pH control, Fig. 1 showed that the maximum deposited Pt occurred at dilution 5 and the variation was respected however, a difference in thickness was observed, for the same dilution, and was most probably due to a different pH value in each set.

Hence, it appears, that the pH parameter is also a determinant factor for a more coherent process.

It has been noticed[5]that at the beginning of the electroless process there was dissolution of a part of the substrate (Cd and Te), followed by a re-deposition of elements mainly O and Te.

The Cd proﬁle or concentration was extracted from the simulated spectra and showed a Cd leak or deﬁciency in the newly formed interfacial layers.

In fact, a rough estimation of the atomic concentration for both Cd and Te, within the interface layers, showed that the stoichiometric ratio between Cd and Te (usually Cd/Te = 1) fluctuated around 0.5 with a minimum observed at dilution 5 (Fig. 2). In the same way, the stoichiometric ratio between O and Te (Fig. 2), at different dilution levels, showed the formation of TeOx layers with xP1 except for dilution 1 whenx< 1. In general, Cd deficiencies, even in the micron range thickness, have a considerably important role in the device behaviour. Similarly, the complete and proper formation of TeO2layer has an important function in the mechanical and electrical behaviour of the device contacts. When this layer is well formed, the mechanical and electrical characteristics are more reliable and stable.

More specifically, if we consider the case of dilution 5, the depth profiles of Pt, Cd, Te and O (Fig. 3(a)) were well extracted from the SIMNRA simulated spectrum where five different successive layers were necessary to fit the experimental spectrum (Fig. 3(b)). In most cases, there is formation of admixture or separated TeO2and TeOxlayers, between the deposited metal and the substrate.

They could be thick or thin and could include Cd. These layers can electrically act as hetero-junc-

0 40 80 120 160 200

0 10 20 30 40

Dilution degree

Thickness (10E15 atoms /cm2) Pt1

Pt2

Fig. 1. Variation of the Pt thickness or areal density (·10¹⁵atoms/cm²) as function of the dilution degree for 2 independent series of samples: Pt1 and Pt2 (the pH value of each set was not controlled).

0 0.5

1 1.5

0 10 20 30 40

Dilution degree

Stoichiometric ratio

O/Te Cd/Te

Fig. 2. Variation of the stoichiometric ratio for both Cd/Te and O/Te as function of the dilution degree. These ratios have an important role for a better mechanical and electric behaviour of the obtained material.

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tions rather than the classical Schottky metal–

semiconductor barriers.

3.2. pH variation

If the dilution showed a clear eﬀect on the process, the study of the deposited Pt layer as a function of pH at dilution 5 revealed many inter- esting results:

• The deposited Pt was negligible from pH = 0.5 to 1, then it increased exponentially and reached its maximum at pH 1.5, then it decreased slowly even if it was still high enough till pH = 2.4 (Fig. 4(a)). This is the signature of two contra-

dictory chemical behaviours before and after pH = 1.5. It is to be noted that the obtained Pt layer at pH = 1.5 is the thickest that has been observed until now (Fig. 4(b)).

• Concerning Te and O behaviours in the Pt layer and at the interface, the general tendency follows roughly the same rules as the Pt behaviour. As seen in the dilution variation approach, here again the stoichiometric ratio Cd/Te was around 0.5, which conﬁrmed the Cd leak in the formed surface layers, caused by the electroless process. Moreover, at all pH values, the stoichiometric ratio O/Te showed a formation of one or two TeOx layers at the interface, as it was observed for the analyzed samples as function of the dilution. The sample of pH = 1.5, which had the thickest Pt layer, con- tained the thickest and well-separated TeO2

layer.

0 10 20 30 40 50 60 70

0 200 400 600 800 1000

Depth (10E15 at/cm²)

Atomic %

Pt O Te Pt Cd

Te O

Cd

Channel

200 300 400 500 600 700 800

Counts

0 1000 2000 3000

Experimental Simulated

Pt O

Te Cd (b)

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Fig. 3. (a) Depth proﬁles extracted from SIMNRA simulated spectrum of Pt, Cd, Te and O, at dilution 5, till reaching the stoichiometric composition of CdTe. (b) Experimental and simulated RBS spectra of the same sample, bombarded with 2 MeV alpha-particles beam and a ﬂuence of 2lC.

0 200 400 600 800

0 0.5 1 1.5 2 2.5 3

pH Thickness (10E15 at/cm2)

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Channel

200 300 400 500 600 700 800

Counts

0 1000 2000 3000

Experimental Simulated

(b) Pt

Te Cd O

Fig. 4. (a) Variation of the Pt thickness (·10¹⁵atoms/cm²) as function of the pH, with a ﬁxed dilution degree (dilution 5). (b) Experimental and simulated RBS spectra of the pH 1.5 sample, bombarded with 2 MeV alpha-particles beam and a ﬂuence of 2lC.

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4. Conclusion

The main aim of this study was to characterize, by the RBS technique, the newly formed layers resulting from electroless contact deposition of Pt. The thickest layers were obtained at dilution 5 and pH = 1.5. However, the general trend of the obtained curves (Figs. 1 and 4(a)) suggests undertaking further studies in the dilution range between 5 and 15 or between 1 and 5. In the same way, for the pH parameter, a better growth of Pt layer could be obtained between 1 and 2.4. In all samples, the stoichiometric ratio Cd/Te appeared to have a fixed value, thus indicating a Cd deficiency. Finally, the well-formed TeO2 layers seemed to follow the same behaviour as the deposited Pt layer. In this study, we have shown that the pH parameter played also a major role as the dilution one. However, other parameters, such as the temperature of the solution and the crystalline direction of the sample, which were fixed during these experiments, have also determinant action in this process. Their action needs to be further studied for a better knowledge of the electroless process.

Acknowledgement

The authors would like to thank the Lebanese–

French committee CEDRE for their ﬁnancial sup- port of this work.

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