Abstract—Although electroless contacts are the most used contacts on II–VI semiconductor materials (CdTe family) to solve many problems, they are difficult to understand and to control.
For this paper, we have chosen two II–VI materials, CdTe and ZnTe, and three metals: Au, Pt, and Pd. For the characterization methods, we have chosen Rutherford backscattering (RBS), secondary ions mass spectroscopy (SIMS), and partially Auger for the contacts and interface structures, stoechiometry, and composition profiles. Electrical measurements, PICTS and TEES, are done to find out defect level concentrations and nature, as well as the possible correlation with the composition concentrations.
As first parameter change, we have chosen the solution dilution in the electroless deposition process, while the other parameters were frozen. This allows us to drive and to see a drastic variation in the profile of the metal, the two constituent materials and the oxygen in the metal layer. The same has been seen at the interface where theVCdconcentration profile shows sensible variation too.
We have found a clear connection with specific electrical defect concentrations. The validity of these concentration measurements in the bulk are then subject to questions. The comparison with evaporated contacts is done for bulk and surface property sep- arations. Contact induced stress is simulated by static pressure in longitudinal and transversal direction. We have driven out the induced defect natures and concentrations. The limit of operation under stress is also given.
Index Terms—CdTe, defect, electroless deposition thin films, nu- clear detectors, PICTS, RBS, semiconductors, SIMS, stress, ZnTe.
I. INTRODUCTION
CONTACTS on II–VI materials, especially CdTe and its parents, are a real deal for many applications like room temperature nuclear detectors used for medical, industrial, syn- chrotron, and spatial imaging systems, and also in IR detectors, optoelectronic, and photorefractivity [1]. All these applications need material of high electrical resistivity
to have low leakage current, extended electric field profile (de- pletion layer), and higher efficiency. This resistivity generally leads to the well-known space charge and polarization effects.
For nuclear detectors, these problems are solved by the deposi- tion of injecting contacts [2], the easiest method being the elec-
Manuscript received November 15, 2003. This work was supported in part by the CEDRE French-Lebanese Committee by Program 01TF12/L26 and by EURORAD SA of Strasbourg, France.
M. H. Ali, M. Ayoub, and F. Lmai are with the PHASE Laboratory, F67037 Strasbourg Cedex 2, France (e-mail: [email protected]).
M. Roumie, K. Zahraman, and B. Nsouli are with CLEA-CNRSL, Beyrouth, Lebanon.
M. Sowinska is with EURORAD II–VI, F67037, Strasbourg Cedex 2, France (e-mail: [email protected]).
Digital Object Identifier 10.1109/TNS.2004.832678
troless chemical deposition. The physical and electrical quali- ties of these contacts depend on many parameters, which need to be controlled: namely the metal thicknesses, the composition of the metal layer and interfaces, the mechanical-induced stress, and the corresponding electrical interactions. These parameters become of prime importance: as an example, the future pixi- lated monolithic detector matrices have pixels dimension as low as 50–150 nm and will be used for advanced imaging systems (medical, X, , and IR camera, ), where connections, at that scale, need the integrated circuits planar bonding technology re- quiring few in thickness of the deposited metal layer, while the thicknesses obtained until now have only 15–80 nm. The interface composition is far from the bulk stoechiometry, the el- ement lack acts then as defect. In addition, these layers are rich in other active elements (H, O, Cl, ) which act as doping, in- ducing electrical defect levels in the band gap. We must add that the bonding process and the presence at the surface of different layers with different natures and dilatation coefficients induce nonnegligible stress on the material and consequently electrical defect levels too.
In this paper, our goal was to study the behavior of few param- eters: on the one hand, to determine the metal layer thickness, the composition, and the defect level energy, and concentration as a function of a first parameter, the dilution of the reaction solution. On the other hand, the stress is studied in conjunc- tion with the electrical defect level energies and concentrations.
We have used few analysis methods, Rutherford backscattering (RBS), secondary ions mass spectroscopy (SIMS), auger, ther- moelectric effect spectroscopy (TEES), and photoinduced cur- rent transient spectroscopy (PICTS), for the process comprehen- sion and to find the optimal solution dilution degrees as a first step. Meanwhile, we have to validate the RBS and SIMS results by electrical measurements. For consistency, the presentation will be mainly focalized on CdTe results.
II. EXPERIMENTALPROCEDURE
A. Sample Preparation
We have chosen two semiconductor substrates (generally, two sets of CdTe and one set of ZnTe) and three deposited metals:
Au, Pt, or Pd. Materials are THM ZnTe and CdTe:Cl from Eu- rorad SA (Strasbourg—France). The samples are mechanically polished (Br: Methanol), without chosen crystalline direction;
this part will be studied later. The first chosen parameter was the dilution. The studied samples were prepared by electroless deposition of a thin metal layer. Indeed, metal acid chloride
0018-9499/04$20.00 © 2004 IEEE
Fig. 1. RBS spectra for three solution dilutions of CdTe: Pt electroless deposited contact. (a) First set with apparent codeposition. (b) Second set without apparent codeposition. (Pt1.3 nm/channel, CdTe3 nm/channel).
salts of Au, Pt, and Pd were dissolved in determined amount of deionized water as a “dilution 1” solution. By diluting it again, two others solutions “dilution 5” and “dilution 25” were ob- tained. The dipping of the material wafers in these solutions al- lows the so-called electroless spontaneous deposition of a com- plex thin metal layer. The typical dimensions of the samples are in surface and about 1 mm in thickness. In a first step, the others different parameters of the deposition reaction were roughly fixed, like the temperature (RT), pH (acid 1–2), lumi- nosity (hood lamp), and deposition time (1 for Au, 2 for Pd, and 4 min for Pt). In fact, it was observed in previous studies that the deposition time reaction seemed to be without significant effect (Pt case) [1], while the solution dilution degree shows more ef- ficiency in the variation of the metal layer thickness.
B. Physical Studies
1) RBS Measurements, Results and Discussion: The RBS technique is still one of the best nondestructive and absolute methods to estimate the thickness and composition of heavy metal layers less than 500 nm. RBS measurements were per- formed at the accelerator facility of the LAEC [3], having a pel- letron tandem of 1.7 MV, with alpha particle beam ener- gies up to 5 MeV. Backscattered particles are detected by surface barrier detector (SBD), with an energy resolution of 16 keV and 50 of active area, located at 6 cm far from the target and at a scattered angle of 165 referring to the beam direction.
A charge integrator and a digital counter measured the total de- posited charge of the beam on the target. Alpha particles with energy of 2 MeV were used to perform the RBS analysis of the
Fig. 2. RBS thickness evolution of the deposited metals layers Au, Pt, and Pd at different dilution degrees on: CdTe substrate(—), ZnTe substrate(- - -).
samples [4], [5] at room temperature and vacuum.
The obtained spectra, e.g., for the Pt-CdTe case (Fig. 1) showed some complexity of the formed layers that derived from the elec- troless reaction. Spectra were processed with the SIMNRA sim- ulation code [6] to drive out quantitative thickness and composi- tion [Fig. 1(b)]. Three layers were used to simulate the profiles.
From the careful study of obtained spectra, we can conclude that there is generally a similar behavior for the thickness evolution of each deposited metal layer, in both CdTe and ZnTe substrates.
We have only changed the dilution while the others parameters are roughly still the same (excepted the crystalline direction).
However, greet dispersion between different sample set can be observed, only coherent results are shown, the other parameter action will be started later.
The first observation concerns the thickness of the formed layers. Two parts can be distinguished.
— The first part concerns the deposited metal itself Au, Pt, and Pd (Fig. 2): i) In case of Au, for each material, the layer thickness showed a minimum at “dilution 5”
and increased slightly again at “dilution 25,” with com- plex concentration profile simulated by at least four different layers in composition. ii) In the Pt case, the thickness showed a maximum at dilution 5 and de- creased again at dilution 25. iii) For Pd, the thick- ness decreased at dilution 5 without any significant in- creasing at dilution 25.
— The second part concerns the codeposited elements (Fig. 3), mainly oxygen (O) and tellurium (Te) (other elements like H and Cl are also present in smaller amount. In fact, there is a redeposition of the dis- solved Te from the bulk material while the Cd is still remaining in the solution ([1 p. 274]). These codeposited elements appear first as a lower height and abnormal profile of the deposited metallic layers, in comparison with a full height layer, for example, (Fig. 1). Second, we found their finger prints in the element spectrum profiles under the layer and they can be extracted roughly qualitatively and quantitatively by the SIMNRA simulation code. Presented results are those finally injected in SIMNRA. The codeposited element concentrations follow the same behavior than the metal layer ones: i) in the Au case, both O and Te show a minimum at dilution 5. ii) In the Pt case, both O and Te show a maximum at dilution 5, with a huge
Fig. 3. Concentration evolution of the codeposited elements Te (—) and oxygen (- - -), with the dilution degree, in the case of CdTe substrate.
value for Te concentration. iii) In the Pd case, both O and Te decrease continuously, the maximum could be at a lower dilution.
The second observation concerns the profile composition of the interface layer under the metallic deposited layer [Fig. 1(b)].
In normal RBS or SIMS spectra of a stoechiometric com- pound, the height of each element is still constant from the sur- face toward the depth. However, in the majority of our observed RBS or SIMS spectra, profiles of complex Cd lack or vacan- cies are clearly seen, depending on the dilution degree, but cer- tainly on other parameters to be determined too. When it hap- pens, these vacancies behave as the reverse of the metal or the Te amount in the metal. The total amount of these vacancies in free or complex forms are localized up to a depth of 1.5 and can be estimated by SIMNRA simulation, for the highest seen value, to be around (especially in the Pt case di- lution 25). Cadmium vacancies are of great importance in the behavior of the devices as we will see in the Section II-C.
In our samples, both metals and codeposited elements show strictly the same behavior, as a function of the dilution degree.
But it should be mentioned that conclusions are derived from only three dilution points and with only one parameter variation.
Exact maxima and minima positions could be at intermediate or other dilutions, in more refined analysis. However, general tendencies should be anyway respected.
2) SIMS Measurements, Results and Discussion: The well-known SIMS is used for elementary mass and profile analysis. Element separations and resolution are better than in standard RBS. However, the depth profiles are only done by the ion milling time, and the method suffers from the ionization en- hancement phenomena from coupling elements. Nevertheless, the SIMS was valuable to confirm our RBS results. We found similar behavior for the metal thicknesses and the interface layer with the lack of Cd, as a function of the dilution degree, which validate both measurements (Fig. 4). The same behavior for is also shown. In addition, the comparison of the two techniques allowed us to drive the scaling of SIMS results, in real thickness , for each metal instead of milling time.
We have deduced, with our SIMS conditions (oxygen ions gun, 12 KeV and 0.2 current), that the time unit (seconds)
Fig. 4. SIMS thickness evolution of the deposited metals layers Au, Pt, and Pd at different dilution degrees on: CdTe substrate (—), ZnTe substrate (- - -).
corresponds to 2–3 for the Au case, 1–2 for Pt and 3–5 for Pd.
From RBS and SIMS spectra studies, one can see that the general behavior of the contact and interface could be roughly illustrated in a general manner like in [1] page 272: first a layer of metal including Te and O, then a layer of , followed by layer of , including some Cd, vanishing to reach the stoechiometric composition of CdTe but that occurs with large shape variations depending on the conditions.
C. Electrical Studies
1) PICTS Measurements, Results, and Discussion: In both RBS and SIMS, we have often seen the occurrence of nonstoe- chiometric behavior under the contact in the interfacial layers, with clear presence of high amount of Cadmium vacancy in this region, up to 1.2 in depth and a total concentration up to . One must remind then that in their both free or complex forms, when ionized, act as active elec- trical defects. Such a concentration should affect greatly both resistivity and the electrical properties of materials and devices even if these vacancies are situated only at the surface and es- pecially they are more effective at the active side.
Electroless contacts are widely used in device elaboration and especially on samples dedicated to the analysis, in the estimation of the electrical defects energy and concentration. However, if the measured defects are essentially due to the interface and to the contact process, rather than to the real bulk defects, that affects greatly the rightness of the material measured defects on the one hand, and on the other hand, the contact process can affect also the performance of the processed devices.
To verify and to quantify this effect and especially the par- tition of defects between interface and bulk, we have used the PICTS [7] which is an efficient electrical spectroscopic method for defects level identification (at vacuum).
We have seen that defects in the electroless process can reach a depth as deep as 1 and the consequent concentration by RBS. While evaporated contacts on etched surfaces have at least 40–60 due to the Br-Methanol etching process and to the in- terfacial oxidation [8], and polished samples present extended defects up to few . We have then used evaporated contacts on etched samples as reference to deduce the bulk defects in a given material instead of the electroless contacts.
Electroless contact solution dilution was used as a mean to in- duce a variation of the . For better coherency, in each run the
Fig. 5. PICTS spectra of the same sample with different contacts: 1) Evaporated contact. 2) Electroless Pt contacts with dilution 1, 5, and 25.
Fig. 6. Defects concentration in the same sample with different contacts: (A) Evaporated contact (D1, D5, and D25). Electroless Pt contacts with dilution 1, 5, and 25. (Ea) is the activation energy.
same sample was analyzed first with the evaporated contact and then consecutively with the three electroless solution dilution deposited contacts (1, 5, and 25) always on the same sample. In Fig. 5 we can see the PICTS spectra with these four contacts.
Five main defect energy bands were identified and have to be considered: the levels at 0.21, 0.30, 0.92 eV remain roughly constant as a function of the dilution (Fig. 6), the 0.14 eV band decreases with dilution, while the 0.55 eV band increases clearly with the dilution. Indeed, this last band is often and widely at- tributed to the [9]. The defect concentration in this band is really low with evaporated layer (within experimental errors for nonwell-resolved peaks band). The meaning is that in the bulk of this sample are in minority, but they increase with dilu- tion to reach high amount at dilution 25. This behavior is indeed similar to the evolution of Cd lack in RBS and SIMS measure- ments.
From this close correlation, one can conclude that the 0.55 eV visible band in the PICTS spectra represents mainly the defects present at the interface seen in RBS and SIMS, rather than those present in the bulk.
We must then be really careful about the choice of deposi- tion and contact process for sample analysis, to obtain the real amount of defects in the material.
Fig. 7. Leakage current at 300 V applied voltage and count rate under Co ray irradiation as a function of longitudinal pressure.
Fig. 8. Leakage current at 300 V applied voltage and count rate under Co ray irradiation as a function of transversal pressure.
2) Stress Measurements, Results, and Discussion: II–VI semiconductors in general and CdTe in particular are ionic and piezoelectric materials. They are then really sensitive to:
i) the pressure or all kind of stress, especially those due to the difference of dilatation coefficients between the material and intimate metal contacts; ii) to the applied pressure during bonding process; iii) to the necessary pressure for contacting by Indium balls process or silver glue for example.
Difference of the dilatation coefficients induces transversal stress under the contact, while pressed contacts induce longi- tudinal ones. There is no practical mean to measure the stress pressure of the contact in real conditions. So we decided to sim- ulate this effect by applying a known static pressure. For the longitudinal part, the pressure is applied directly on the con- tact, and on the side of the contact for the transversal part. Sam- ples are , contacts are standard Pt deposition.
The pressure was applied throughout a silicon rubber for the pressure uniformity and on a well known surface, and a pres- sure gauge for the pressure measurement. Pressures of more than 21 were used, leakage current at 300 V applied voltage, count rate under ray irradiation, TEES and PICTS spectra were used as characterization parameters under the pressure.
Fig. 7 shows the case of longitudinal pressure and Fig. 8 the case of the transversal ones. In both cases we can see a decrease in the count rate by more than 20% under few hundred of
Fig. 9. PICTS spectra of defect intensities under different transversal pressures.
Fig. 10. TEES spectra of defect intensities under different transversal pressures.
and a constant decrease down to the vanishing point and noise at 18 . At the same time the leakage current is still roughly constant up to this last pressure, then an abrupt increase by 10 time, and remains constant and noisy at this value even if we return to lower pressure (hysterezis). That means we have then reached the breakdown point of the elastic deformation, which gives the limit of operation pressure, not to be reached.
In Fig. 9, we can see the PICTS spectra under different static transversal pressures. At 0 , we can see the electrical compensation equilibrium between the 0.14 and the 0.96 eV bands [10]. At 1 , the situation is reversed in favor of the 0.14 eV associated to an increase of the 0.4–0.5 eV band and a decrease of the resistivity. This last band is generally seen with mechanical stress like ion implantation or thermal quenching [9], it remains after 21 while the 0.14 and 0.92 eV levels disappear in favor of a new one at 0.7–0.86 eV. In this case, the material is completely destroyed. In TEES spectra (Fig. 10) levels at 0.14, 0.20, 0.3, 0.5 eV are still small at zero , but with pressure as low as 100 an increase of these levels is clearly seen, especially the 0.5 eV which is (n) type level, while the others are (p) type. That certainly changes the compensation
materials is a three-dimension matrix related to the main crystal directions. For more precise measurements that must be taken into consideration, in addition to the displacement of the Fermi level in the band gap.
III. CONCLUSION
This study showed the complex and delicate microchemistry of the surface and interface structures derived from the electro- less deposition process. A general structure is illustrated in [1, p.
272 ] where five different layers can be seen. As a consequence, the heterojunctions between these layers determine the quality of the contact behavior.
The RBS and SIMS techniques allowed seeing the evolution of the deposited metal thickness and the interface as a function of the dilution solution. It appeared that this has an important effect later on the metal thickness growth, which is required to make possible the use of planar technology bonding technique.
On the other hand, an enhancement of the defect level peak in the electrical characteristics is related to a lack of Cd, which is observed, in RBS and SIMS spectra, at the interfaces between the metal and the bulk material. The maximum thickness could be reached between dilution 5 and dilution 25, or in other cases at lower dilutions. The electrical importance of theses was clearly demonstrated in the PICTS measurements. The behavior of device contacts under stress was studied for the two possible longitudinal or transversal actions. We found a first step at 0.1 with 20% count rate decrease, and a limit pressure oper- ation around 15–17 . Further studies will be undertaken to study other parameters (temperature, H, oxide layers, etc.) and to find better dilution solution degrees for each metal.
ACKNOWLEDGMENT
The authors are grateful to J. M. Koebel, C. Rit, F. Klotz, M.
Schott, and M. Brutt for their hard effort in the progress of this paper.
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