Depth profiling and stoichiometry of constituents in platinum electroless contacts on CdTe(111) under different pH values

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Depth profiling and stoichiometry of constituents in platinum electroless contacts on CdTe(111) under different pH values

M. Roumié

^a,

^⁎ , F. Lmai

^b

, A. Awada

^a

, K. Zahraman

^a

, B. Nsouli

^a

, A. Zaiour

^c

, M. Hage-Ali

^b

, M. Ayoub

^b

, J. Faerber

^d

aIon Beam Analysis Laboratory, Lebanese Atomic Energy Commission, National Council for Scientific Research, Airport Road, P.O. Box 11-8281, Beirut, Lebanon

bLaboratoire PHASE-CNRS, 23 rue du Loess, BP 20, 67037 Strasbourg Cedex 2, France

cDepartment of Physics and Electronics, Lebanese University, P.O.Box 14-657, Hadeth, Lebanon

dIPCMS, 23 rue du Loess, P.O. Box 43, 67034 Strasbourg Cedex 2, France Received 7 April 2006; received in revised form 2 April 2007; accepted 5 April 2007

Available online 19 April 2007

Abstract

Rutherford backscattering spectrometry was performed using 2 MeV alpha-particles, to characterize electroless platinum contacts on cadmium telluride CdTe(111) crystals, aiming to improve and to understand the structure of the metal electroless chemically deposited. In this paper we have studied the platinum metal contact as well as the interface material-contact. The thickness, the stoichiometry and the concentration profile of platinum, cadmium, tellurium and oxygen present in the surface layers were determined as a function of many parameters, especially the variation trend as function of the chloride solution pH. This work showed the important effect of the crystallographic direction on the growth of Pt on CdTe II–VI semiconductors. Furthermore, the process was more pH dependence at the metalloid Te face than the Cd one.

Keywords:Cadmium telluride; Platinum; Electroless deposition process; Rutherford back-scattering spectroscopy; Depth profiling; pH; Crystalline direction

1. Introduction

II–VI materials such as CdTe are of great importance for room temperature X-and Gamma-ray detection used in medical, industrial and imaging systems. They also have applications in infrared detectors, optoelectronics and photo refractivity[1]. The most suitable contacts on these materials are made by noble metal electroless chemical deposition using as Au, Pd, and Pt chloride. Thus, the quality of these contacts influences the quality of the final devices. All these applications need material with high electrical resistivity (N10⁹–10¹¹Ωcm) in order to have low leakage current, extended electric field profile (depletion layer) and higher detection efficiency. However, this high resistivity leads to the well-known space charge and a polarization effect.

For nuclear detectors, these problems are solved by the deposition

of injecting contacts[2,3]. Electroless Pt deposition is one of the easiest and most convenient methods to form contact in CdTe detectors. The quality of the performed detectors depends on several factors, such as the thickness metal, the composition of the layer and the quality of the interfaces. These parameters become of prime importance when, for example, the future pixilated monolithic detector matrices that have pixels dimension as low as 50–150 nm, are used for advanced imaging systems (medical, X, γ and IR camera). At such dimensions, the integrated circuits planar bonding technology is needed but it requires metal film with a thickness of fewμm, while the typical thickness obtained till now is in the range of 15–80 nm.

The structural composition of interface layers, derived from the electroless process, is different from the ideal bulk CdTe stoichiometry, where the lack of Cd acts as vacancy defects. In addition other active elements like hydrogen, oxygen and chlorine can act as doping sources, inducing electrical defect levels in the band gap[4,5]. Moreover, the electroless process is highly non-uniform due to the action of several parameters

Thin Solid Films 515 (2007) 7843–7846

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⁎Corresponding author. Tel.: +961 1450811; fax: +961 1450810.

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

doi:10.1016/j.tsf.2007.04.038

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(temperature, dilution, pH, etc.). We have reported, in previous papers, on the variation of both the deposited metal thickness and the interfacial layer thickness, as a function of the solution dilution and the type of metal[4,5], as well as on the effect of the chemical pH action[6]. The non-uniformity of the results, obtained for some samples prepared under the same condi- tions (pH, temperature, dilution, etc.), leads us to consider the crystalline direction of CdTe, a significant parameter over- looked up to now. The present work targets the effect of the (111) plane direction of CdTe on the electroless deposition.

2. Experimental details

A 5SDH pelletron tandem accelerator of 1.7 MV located at the Lebanese Atomic Energy Commission [7] was used to perform Rutherford Backscattering Spectrometry measurements (RBS) on the samples under normal incident beam and in a random direction to avoid channelling. A partially depleted PIPS (Passivated Implanted Planar Silicon) detector from Canberra, with 14 keV energy resolution and 25 mm²active area, detected the backscattered particles of the MeV ⁴He⁺ beam, at a scattering angleθof 165° where the solid angle of the detector was 5.45 × 10⁻³sr.

Crystalline directions and planes were set up in CdTe ingots using the X-ray Laue method [8]. Then, the ingots were cut along the desired planes with a wire-SiC powder saw in order to reduce the inclusions and defects present at the surface with alumina powders. Samples were polished mechanically and then chemically by 5% Br-Methanol solution. The studied samples were prepared by electroless deposition of thin Pt layer on CdTe wafers which were immersed into dihydrogen hexachloroplatinate (IV) hexahydrate [H2PtCl6⁎(H2O)6] solution. The basic solution consisted of the dissolution of 1 g of this compound in 25 mL water, which leads to a concentration of 77.3 mmol/L. Consequently, by diluting this basic solution 5 times, one can obtain the wanted concentration of 15.4 mmol/L.

Indeed, in previous studies [4–6], we have reported that the thickest Pt layer was found at a concentration of 15.4 mmol/L and at a pH of 1.5, but without taking into account the effect of the direction plane of CdTe. However, a significant difference in the growth of the Pt layer thickness was clearly seen on one sample presenting two crystals with different direction planes.

Hence, the direction plane has been considered as a potential parameter. In the present work, each sample was produced in duplicate for a fixed concentration of 15.4 mmol/L, a fixed plane (111) and different pH-values (1.5, 1.8, 2 and 2.6). Unlike the other crystallographic directions, the two sides of the samples cut along the crystallographic plane (111) were not symmetrical. This plane presents a special characteristic: at one side there is mainly a metallic Cd row (face A) while at the other, there is a metalloid Te row (face B).

3. Results and discussion

The different RBS spectra were processed with the SIMNRA simulation code [9] where the depth profile of the present elements was extracted. In an earlier paper[4], Auger and SIMS

measurements showed that oxygen is the main element in the surface layer, due to the electrolysis process and its affinity with Te. This validates the choice of oxygen in the simulations even if it is not well distinguished in the RBS spectra. In fact, the determination of the low Z oxygen element in a high Z matrix (CdTe) is a difficult task for RBS technique. Even, with higher fluence than 2μC (6, 13 and 25μC) the oxygen signal in the spectra was not improved and, therefore, its profile determination. Though, by simulation, if we introduce the oxygen to make up for the lack of stoichiometry in the different layers, the oxygen amount can be determined. The other undetected elements, like H and Cl, are probably present but in relatively small amounts (less than 2–3%) and should not alter signi- ficantly the composition and the thickness of the surface layers deduced by the SIMNRA simulations [4,5]. Nevertheless, H and Cl act as doping impurities and induce electrical defect levels in the band gap. Therefore, their determination (by Elastic Recoil Detection for H and by Proton Induced X-ray Emission for Cl) could help explain the electrical behavior and the char- acteristics of the detector.

The RBS results presented the average values of the duplicated CdTe (111) samples and have shown the following behaviors:

–For the Cd face (A), a clear thick deposited Pt layer con- taining was observed a low mixture of Te and O at all pH values (see e.g.Fig. 1).

–For the Te face (B), the results of the RBS spectra were more dispersed.Fig. 2shows that in a first group (pH 1.5 and pH 1.8), the thickness layer and the related stoichiometry of Pt were less significant than the second group (pH 2 and pH 2.6). For these last two samples, there was a similar behavior for the Pt layer formation (composition and thickness) with the Cd face samples as it can be confirmed inFig. 3a.

–With Cd face A, the thickness of Pt layer reached its maximum at pH 1.5 with 7 × 10¹⁷ at/cm² and decreased at higher pH-values to 4 × 10¹⁷at/cm², while with Te face B the thickness trend showed different behavior (Fig. 3a). The total amounts of O and Te were averaged in both Pt and interface

Fig. 1. Experimental and simulated RBS spectra of the (111) sample at pH 1.5 and concentration of 15.4 mmol/L. The measurement was done on the Cd face, using 2 MeV alpha-particle beam and a fluence of 2μC. The dashed vertical lines indicate the edge position of Te, Cd and O at the junction area between the Pt Layer and the CdTe substrate.

7844 M. Roumié et al. / Thin Solid Films 515 (2007) 7843–7846

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layers (Fig. 3b, c). On average, higher amounts of Te and O were present in both the Pt contact and the interface of the Te face samples than the Cd face ones.

– In most spectra, a Cd lack was detected behind the Pt layer, at the interface, suspected to be of Cd vacancy nature (Fig. 1).

The ratio between the total amount of Cd and Te was less than one in most cases (Fig. 4).

– The presence of more O than Te could lead probably to the formation of TeO2phases. As an example,Fig. 5shows the concentration profile of Pt, O, Cd and Te, at pH 2, where a formation of TeOxlayers was observed.

Indeed, during the reaction with the metal chloride solution, a part of the substrate was dissolved: (i) the lack of Cd acts as p type (VCd− ); (ii) Te precipitated on the CdTe surface as TeOxlayers with some concentration of Cd. Then, roughly two hetero-junction layers are formed between the Pt contact and the substrate: TeOx-substrate and Pt-TeOx. This feature can explain the electrical behavior of such metal contacts and, therefore, the quality of the performed detector[10].

If we compare the results of the plane directions (111) to the electrical data in the literature[11], it was found that each side had a different diode behavior, though the same contact deposition process was applied on both faces. In fact, the barrier heightΦb

measured on the Cd face was 0.72 eV while the barrier heightΦb

measured on the Te face was 0.92 eV. Since on both sides, we have the same materials, Pt and CdTe, the results obtained by RBS are more suitable to explain this anomaly rather than the Schottky surface barrier height theory. Indeed, in the hetero-junction layers, between Pt and CdTe, a significant amount of O and Te was found in the TeOxform most likely as TeO2. Both Te oxides are

Fig. 3. Total amount, present in the layers above the CdTe substrate of the (111) series samples, expressed as the areal density in at/cm²as function of the pH for both Cd and Te face: (a) for Pt, (b) for O and (c) for Te.

Fig. 4. Stoichiometric ratio between Cd and Te, for all samples, which indicates a Cd lack and thus a cadmium vacancy.

Fig. 5. Depth profiles of Pt, Cd, Te and O till reaching the stoichiometric composition of CdTe. These concentrations were extracted from the SIMNRA simulated spectrum of the sample: Cd face of (111), pH 2 and concentration of 15.4 mmol/L.

Fig. 2. RBS experimental spectra of the Te face of the (111) samples at different pH, showing obviously pH dependence of the Pt electroless deposition process.

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semiconductors, so the measured barrier is a junction between Pt and Te oxides as well as between Te oxides and CdTe. In addition, the Te face was marked by the large amounts of Te and O, which were few times more than those in the Cd face. So, it is normal to have a higher barrier heightΦb(0.92 eV) on the Te side than on the Cd side (0.72 eV).

4. Conclusion

This work showed the important effect of the crystallographic direction on the process of electroless deposition of Pt on CdTe II–VI semiconductors. At the metallic Cd face, the Pt deposition was well realized with a low mixing of Te and O for the entire pH values. For the metalloid Te face, the process was more pH dependent, in particular for low pH values where high amounts of Te and O were present in both the Pt contact and the interface, leading to a much higher barrier heightΦbthan the Cd side (0.92 eV versus 0.72 eV). The RBS measurements were very helpful in understanding the electrical behavior, which is correlated to the deposited layers and their composition.

Acknowledgments

This work was partially supported by the Lebanese-French committee CEDRE and Eurorad Co. The main author would

like to thank Malek Tabbal, from the American University of Beirut, for the linguistic revision.

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