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Ag and Cu deposits on HgXTe (X=Cd, Zn) alloys
Journal of Crystal Growth 101 (1990) 311—317 North-Holland
311
Ag AND Cu DEPOSITS ON HgXTe (X = Cd, Zn) ALLOYS C. NGUYEN VAN HUONG Laboratoire d’Electrochimie Interfaciale, CNRS, I Place A. Briand, F-92195 Meudon Cedex, France
R. TRIBOULET Laboratoire de Physique des Solides, CNRS, I Place A. Briand, F-92195 Meudon Cedex, France
and P. LEMASSON Laboratoire des Matériaux Moléculaires, CNRS, 2 Rue Henri Dunant, F-94320 Thiais, France
Electrodeposition of Ag and Cu in the dark at mercury cadmium tellunde (HCT) and mercury zinc telluride (HZT) alloys is studied by electrochemistry and electroreflectance (ER). Cu and Ag deposits are done in the dark and in a potential regime like that for metal substrates (underpotential deposition). Electroreflectance spectra recorded with metal-coated alloys exhibit an extremely specific behavior, the spectrum relative to the F 0 + 4~and the E1 transitions (HCT) or to E1 only (HZT) vanishing (Ag on HCT, Ag or Cu on HZT) or decreasing strongly (Cu on HCT) and new ER features being created. These features are located at a different energy for the HCT and HZT alloys; further, a good stability of the metal—alloy interface necessitates a metal deposit several monolayers thick for HCT while for HZT a metal sub-monolayer leads to stable results. Therefore, a specific chemical interaction between Ag and Cu and the alloy substrate is postulated which needs the presence of Hg but depends strongly on the other metal (Cd or Zn) included in the ternary compound.
1. Introduction Preparation of Schottky barriers with good rectifying properties is still a problem with HgCdTe (HCT) and HgZnTe (HZT). It is well established that the initial steps of metal—semiconductor interface formation are of particular importance in understanding the behavior of Schottky barriers [1]. For these investigations, growth of the metal layer is generally achieved using a vacuum deposition technique. However, the electrochemical method may appear as an attractive alternative to the previous one due to its particular versatility, Coverage, form and microstructure of the deposit can be easily monitored by controlling and changing solution composition, time and potential of deposition as well as mass transport conditions. While HCT has already been studied for several years [2], HZT alloys have been grown only re0022-0248/90/$03.50 © 1990
—
cently and they seem promising materials for production of infrared detectors and optoelectromc devices [3]. Hg1 ~X~Te (X = Cd, Zn) Il—VI alloys crystallize in the zinc-blende structure in the whole composition range (0 x 1) and the lattice parameter obeys Vegard’s law [2,4]. The E0 band gap and the E1 transition lie in the range 0 (— 0.14)—1.51 eV and 2.15—3.2 eV respectively for HCT, and 0—2.25 eV and 2.15—3.6 eV for HZT [4,6]. The case of metal deposits on HCT has received special attention [7] while that of deposits on HZT has been studied very recently by us [8]. In the case of HCT, it has been shown that Ag and Cu can be considered as metals of intermediate reactivity and experimental analysis of the interfaces by ultra-high-vacuum (UHV) techniques has led to the conclusion that deposits on cleaved and sputtered substrates lead to interfacial
Elsevier Science Publishers B.V. (North-Holland)
312
C. Nguyen Van Huong et al.
/ Ag and
Cu deposits on HgXTe (X = Cd, Zn) alloys
reactions, including formation of Ag and Cu tellurides as well as Hg losses. In the case of HZT, investigation of the interface formation was achieved using electroreflectance (ER) and the existence of a strong chemical interaction was postulated on the basis of the creation of a typical ER signal. In the present work, Cu and Ag deposits are prepared electrochemically in the dark. The electrolyte—semiconductor as well as electrolyte—
3. Experimental procedure
metal—semiconductor interface are studied using cyclic voltammetry, capacity and ER measurements. A comparison is done between results obtamed at HZT and HCT substrates for different alloy compositions and an attempt is made to correlate the experimental results with the structural properties of the metal layer and the chemical nature of the substrates.
alloy, a particular electrochemical treatment must be applied to the electrode which otherwise does not provide any featured spectrum. It consists mainly in an AC perturbation of large amplitude (1 V~~) for 1 to 5 mm while the DC potential is kept constant so that the mean resulting current crossing the interface remains 0 or slightly negative. Such a treatment as well as its consequences on the HCT surface are detailed in another publication [11]. Presently, it seems sufficient to mention that it does not have noticeable effect during the metal deposition, at least for the current-versus-potential curves. After this characterization, the HgXTe electrode is transferred to another electrochemical cell whose electrolyte contains the metal salt in order to achieve the metal deposition. The latter operation is done at fixed DC potential for HZT and by cyclic voltammetry for HCT. Last, after careful rinsing with ultrapure water, the coated electrode is dipped back in the first electrochemical cell which contains 0.1M HC1O4 alone, where it is characterized in a way similar to the bare electrode. The amount of metal deposited in metalsalt-containing solution is2 +calculated by~) coulomeCu°+ 2p and the try (Ag + —~Ag°+ p~ Cu coverage is estimated by assuming a close-packed arrangement of the metal atoms.
2. Experimental Both HCT and HZT alloys were grown using the traveffing-heater method. HCT with 70% Hg composition was indium doped and showed n-type conductivity while 24.5% Hg HCT and HZT in the whole composition range were p-type. Sample preparation and mounting for electrochemistry measurements have been already reported [9]. Surface treatment includes mechanical polishing with diamond paste (0.5 ~Lm)and chemical etching in 1% bromine in methanol solution. The electrochemical cell is of the classical three-electrode type with a glassy carbon counterelectrode and aElectrolytes mercurous sulfate electrode (MSE) as a reference. (0.1M HC1O 4) are prepared from Suprapur perchioric acid and ultrapure water (CNRS patent). For metal deposition, a concentration AgNO3 or CuSO4 ranging be5Mofand 5 x 104M is used. In each tweensolutions 5 X 10 are deaerated by nitrogen bubbling. case, Electroreflectance measurements are performed with a 0.4—0.5 V~,15 Hz AC perturbation, in the energy range 2—4 eV with a light beam impinging the surface at near-normal incidence. The cornplete experimental setup has been described previously [10].
After chemical etching, the bare HgXTe electrode is dipped in the supporting electrolyte avoiding direct contact with air. The fundamental properties of the interface that is then formed are determined by measurement of the current- and admittance-versus-potential characteristics and of ER spectra at fixed DC potential. It is noteworthy to mention that in the case of the Hg0 3Cd07Te
—,
4. Results In this presentation, we will focus especially on results obtained with HZT (x 0.53) and HCT (x 0.70) alloys. It is worth mentioning that these results are representative of those obtained with the whole set of alloys. =
=
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Ag and Cu deposits on HgXTe (X= Cd, Zn) alloys
~
-
313
~1_J-i~
____________________________
—~
-6OxrO7\ -~
~
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Fig.
Potential (Vmse) Fig. 1. Current—potential charactenstics of the Hg0 3Cd04M 7Te AgNO 4M HC1O4, CuSO and (b) 5 X 10 alloy in contact with (a) 0.1M 3 ( ) and 5x10 4 (_ . - . -) containing solution. Ag- and Cu-coated electrodes in O.1M HC1O4 give a current-potential curve similar to that presented in (a). Sweep rate 10 mY s .
4.1. Electrochemistry The current-versus-potential characteristics in the dark (fig. 1) of the HCT alloys are very similar to those obtained with HZT (fig. 2). 2Both for place HCT ± takes andpotentials HZT electrodes, reduction of Cu for Ag~,this at lower than 0 V while reduction starts at 0 V; further, the current density measured during reduction of Ag + is noticeably larger than for Cu2t The voltammograms of ___________________________ x~0.53 E °
~
°
0 ~
b ~1O
~
i,~’ c -
-1
-
0 5
3.
Scanning electron micrograph of a Ag-coated
Hg
---
9
Potent a I (Vm ~ Fig. 2. Current—potential characteristics of the Hg 5M 047Zn0 AgNO 53Te 5M0.1M CuSOHC1O4, (b) 5 x 10 alloy in contact with (a) 3 and 5 x 10 4 (— — —) containing solution, and (c) Ag ( ) and Cu (— — —) coated alloy in 0.1M HC1O4. Sweep rate 10 mV s~.
047Zn053Te electrode. The experimental conditions are given in the text (X 20 000).
Ag- and Cu-coated electrodes are very different for HZT and HCT alloys (figs. 1 and 2). While in the former case (coated FIZT), anodic and cathodic currents take place which correspond to an oxidation of the metal deposit and to the reduction of its oxide, respectively, in the latter case, the characteristics are not very different from those obtained with a bare electrode thus indicating that the metal deposit is probably not modified during the The potential scan. capacitance-versus-potential behavior of coated electrodes does not correspond to a classical Mott—Schottky plot. We can, however, state that the capacitance increases with metal coverage and tends to values obtained with metal electrodes. Presented in figs. 3 and 4 are scanning electron micrographs of Ag-coated electrodes, HZT (x 0.53) and HCT (x 0.70), respectively. In the first case, the deposit is made in 5 X 105M Ag~electrolyte at fixed potential (—0.7 V); in the second case, the coating is achieved by a linear potential sweep between 0 and —0.4 V in 5 x 104M Ag~ electrolyte. For HZT, the coverage is less than a monolayer whereas for HCT, it corresponds to several layers. In both cases, coalescence and nucleation phenomena are noticeable. =
=
4.2. Electroreflectance The influence of metal deposit on ER spectra can be observed for the whole series of HCT and
314
C. Nguyen Van Huong etaL
/ Ag and
Cu deposits on HgXTe (X = Cd, Zn) alloys ENEROY(eVl I
I
I
I
2.25
2.5
2.75
3
X 0.53
-
~
\\\\
__i.
~
,,,i/’~\
I
-
r~
-
Fig. 4. Scanning electron micrograph of a Ag-coated Hg 0 3Cd0 7Te electrode. The experimental conditions for the metal deposit are detailed in the text (X 18,000). 600
HZT alloys. Fig. 5 presents the spectra obtained with electrolyte—M—HCT (M Ag, Cu) structures for x 0.7 as well as spectra recorded with bare HCT. In fig. 6 is presented the whole set of results (metal-coated, bare electrode) obtained with HZT (x 0.53), an alloy whose energy gap (— 1 eV) is nearly the same as that of HCT (x 0.7). Like for HCT, new ER features appear with the coated electrodes, It is worth mentioning that for Ag-coated HCT and Ag- and Cu-coated HZT, the magnitude of =
=
I
500
400 I
WAVELENGTN (nm) Fig. 6. Electroreflectance spectrum of a Hg0 47Zn0 53Te surface without
(-
— —)
and with Ag ( ) and Cu monolayer coating.
(_ . -
. —)
sub-
=
=
the metal coverage: for HCT, the new features are associated with metal layers which are several monolayers thick while for HZT the metal deposit
Energy (eV)
I
I
I
I
1.75
2.0
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2.5
~~ —0
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-
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is less than a monolayer. In the HCT case, the ER spectrum becomes featureless if the coverage becase, the same behavior is observed as soon as the comes greater than 10 monolayers and in the HZT coverage reaches 1 monolayer.
-
-
the E1 oscillation decreases drastically, whereas for Cu-coated HCT only a small dampening is observed. However, results reported for HCT and HZT do not correspond to the same magnitude of
X ~0.70
A
-
I
6~d I
550 I Wavetength(nm)
____
-
Fig. 5. Influence of Ag ( ) and Cu (_ . — . —) layers on the electroreflectance spectrum of the Hg0 3Cd07Te alloy ~— — —~.
Summing up, thedifferent. behaviorWhile of HCT that of of HZT seems rather in and the case Ag and Cu deposits on HZT and of Ag only on HCT, important new features take place which are characteristic of the new interface, in the case of Cu deposits on HCT, only a small deformation of the original ER spectrum is observed. Furthermore, in each case, the features obtained with metal-covered HZT and HCT are noticeably different in shape as well as in energy position.
C. Nguyen Van Huong et at
/ Ag and
Cu deposits on HgXTe (X = Cd, Zn) alloys
315
5. Discussion
ally different from those obtained at solid Cu [15] and Ag [16] electrodes as well as at semiconductor
The electrochemical Ag and Cu deposition at HgXTe electrodes in the dark occurs at potentials more positive than is predicted by the equilibrium Nernst potential (with a concentration of s x 105M, —0.108 V for Ag~/Ag°and —0.430 V for Cu2~/Cu°;with a concentration of 5 x 104M, —0.048 V for Ag~/Ag°and —0.40 V for Cu2~/Cu°)that is, before bulk deposition occurs. By analogy with what is observed at metal substrates [12] where this phenomenon is called “underpotential deposition”, we can assume that in this regime, a metal layer is formed whose coverage and thickness is dependent on the applied potential. Previous electrochemical observations [13] led to the supposition that the common anion rule [14] also holds true for the Te-based Il—VI cornpounds. Therefore, we can assume that the position of the valence band is almost the same for HZT and HCT alloys, thus leading to an electrochemical mechanism of reduction of the metal cations which is similar for both electrodes. Reduction of metal cation M~occurs by injection of holes from M~into the HgXTe valence band [8] according to
electrodes. It is worth mentioning that a stable and reproducible behavior is reached at HZT electrodes with sub-monolayer metal coverage whereas for HCT alloys a thick deposit (several layers) is necessary. In the case of thin (less than a monolayer) deposits on HCT, a modification (decomposition) of the electrode surface is noticeable. Therefore, it seems reasonable to assume that the electrochemical reaction for metal deposition occurs rapidly, leading to a total surface coverage. A phenomenon similar to a “freezing” of the metal—semiconductor interface is thus observed. However, when the metal deposition is slow (low cation concentration in the electrolyte), the reaction can be correlated with an increase of the reactivity of the HCT alloy. Such a behavior can be compared with previous observations of metal deposits on HCT, mainly by X-ray photoelectron spectroscopy, which indicate that Ag and Cu deposits lead to interfacial reactions, including formation of Ag and Cu tellurides as well as Hg losses [7]. Furthermore, the difference observed between the behavior of HCT and of HZT alloys can probably be related to the greater stability of HZT as compared to that of HCT [17].
M~, 1 M~d5+ P~B’ —~
where the subscripts sol, ads and VB indicate that the species are in the solution, adsorbed and in the semiconductor valence band, respectively, Once the Ag(Cu) deposition on the alloy surface is complete, the HgXTe—Ag(Cu) electrode exhibits a specific behavior when studied again in the supporting electrolyte and electrochemistry and ER provide convenient in situ tools for the investigation of the new interface which has been created. In electroreflectance, two features appear to be of particular importance: (i) the decrease (and vanishing, except for Cu-coated HCT electrodes) of the ER oscillations associated with the E0 + 4~ and E1 transitions for HCT and with the E1 transition for HZT; (ii) the creation of a new and generally broad ER spectrum in correlation with the electrochemical behavior of the metal-covered electrode, at least for HZT. Such spectra are tot-
The decrease in the E0 +40 and E1 ER signals can be treated most likely, as we have done preyously [8],in terms of a Fermi-level-pinning process induced by the metal deposition. On the other hand, a quantitative interpretation of the new ER features appears to be difficult, even if we were to assume that it is due to the specific behavior of the new interface formed by the metal deposit. In this direction, the chemical composition of the substrate as a whole seems to play a prominent role in the observed behavior (as indicated by the important differences in energy position) of the spectra obtained at metal-covered HCT and HZT electrodes. Nevertheless, the existence of new ER features observed with metal-covered HgXTe electrodes, together with their specific electrochemical behavior (indicating no electrochemical dissolution of the metal coating) tends to demonstrate the existence of a strong chemical interaction between the metal and the HgXTe surface.
316
C. Nguyen Van Huong et at
/ Ag and Cu deposits on HgXTe
The particular case of Cu deposits at HgCdTe electrodes gives rise to small differences only, due to the fact that (i) the electro-optical behavior of the substrate is not totally screened by the coating and (ii) the new spectrum presents only a small deformation between 2.25 and 2.5 eV. We can thus reasonably conclude that in this case, the interaction between HCT and the Cu deposit is different from that for Ag deposits, in contradiction with previous observations by UHV techniques [7],which indicated that Ag and Cu reacted almost similarly with HCT surfaces. Furthermore, the Cu deposits are less adhesive than the Ag deposits on HCT. However, if the interaction between Cu-deposit and HCT is not strong like for low Ag-coverage, a surface decomposition of HCT is noticeable even when the Cu deposit, as deduced from coulometry, is thick. Therefore, we can presume that Cu weakens the bondings in HCT, while with Ag it is possible to stop this effect by increasing the coverage of the metal on the HCT.
(X = Cd, Zn) alloys
necessary for a stable behavior whereas in the case of HZT, a submonolayer deposit leads to a stable interface. This can be related reasonably to the greater reactivity of HCT which necessitates a complete metal coverage in order to stabilize the metal—HCT interface. Finally, the ER features connected with the metal—HgXTe interface are dependent on the alby composition as well as on the X component, as indicated by the energy positions of the features which are different for HCT and HZT. We can conclude therefore that if the presence of mercury in the alloy is necessary [8], it is the other metal cation which governs the global interfacial reactivity, in good agreement with previous statements on the different bondings in HCT and HZT [17].
References [1] See, e.g., L.J. Bnllson, Surface Sci. Rept. 2 (1982) 123; I. Lindau and T. Kendelewicz, CRC Critical Rev. Solid State Mater. Sci. 13 (1986) 27.
6.
Conclusion
The whole set of electrochemical and electrooptical results presented here tends to demonstrate that the surface of HgXTe (X Cd, Zn) alloys, covered with a deposit of Ag or Cu, behaves very specifically. For the first time, the creation of an electroreflectance signal is reported that is completely different from that of the substrate and linked to an interfacial phenomenon due to the metal coating. Therefore, it seems reasonable to postulate a strong chemical interaction between Ag and Cu atoms electrochemically deposited and the constituent atoms of the substrate. However, the case of Cu on HCT appears to be more particular and leads to a stronger reactivity of the alloy surface with the electrolyte. It is important to reiterate the fact that the similar electrochemical and ER behaviors observed with metal-coated HCT and HZT alloys are obtained with metal deposits whose thickness is not of the same order of magnitude. In the case of HCT, deposits several layers thick (Ag case) are =
[2] For general information about research on HCT alloys see, e.g., (a) Cadmium Mercury Telluride, Semiconductors and Semiinetals, Vol. 18, Eds. R.K. Willardson and A.C. Beer (Academic Press, New York, 1981); (b) Proc. 2nd Intern. Conf. on Il—VI Compounds, J. Crystal Growth 72 (1985); (c) Proc. 3rd Intern. Conf. on Il—VI Compounds, J. Crystal Growth 86 (1988).
13] R. Triboulet, J. Crystal Growth 86 (1988) 79. [4] B. Toulouse, R. Granger, S. Rolland and R. Triboulet, J. Physique 48 (1987) 247. [5] V.G. Sredin, V.G. Savitskii, Yu.V. Daniluk, M.V. Miliyanchuk and I.V. Petrovitch, Soviet Phys.-Semicond. 15 (1981) 249. 16] C. Nguyen Van Huong, R. Triboulet and P. Lemasson, Proc. SPIE 659 (1986) 249. [7] For a recent metal—HCT Junction formation, see e.g., G.D. review Davis, on WA. Beck, M.K. Kelly, D. Kilday, Y.W. Mo, N. Tache and G. Margaritondo, Phys. Rev. B38 (1988) 9694, and references therein. [8] C. Nguyen Van Huong and P. Lemasson, Phys. Rev. B40 (1989) 3021. [9] C. Nguyen Van Huong, Triboulet and P. Lemasson, J. Crystal Growth 86 (1988)R.570. [101 C. Hinnen, C. Nguyen Van Huong, A. Rousseau and J.P. Dalbéra, J. Electroanal. Chem. 95 (1979) 131. [11] P. Lemasson and C. Nguyen Van Huong, to be published.
C. Nguyen Van Huong et at
/ Ag and Cu deposits on HgXTe (X
[12] D.M. Kolb, in: Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, lids. H. Gerischer and C.W. Tobias (Wiley, New York, 1978) p. 125. [13] P. Lemasson, Solid State Commun. 48 (1983) 411. [14] JO. McCaldin, T.C. McGill and CA. Mead, Phys. Rev. Letters 36 (1976) 56.
=
Cd, Zn) alloys
317
[15] R. Kotz and H.J. Lewerenz, Surface Sci. 97 (1980) 319. [16] J.D. McIntyre, in: Advances in Electrochemistry and Electrochemical Engineering, Ed. R.H. Muller (Wiley, New York, 1973) p. 61. [17] A. Sher, M.A. Berding, M. Van Schilfgaarde, A.B. Chen and R. Patrick, J. Crystal Growth 86 (1988) 15.
311
Ag AND Cu DEPOSITS ON HgXTe (X = Cd, Zn) ALLOYS C. NGUYEN VAN HUONG Laboratoire d’Electrochimie Interfaciale, CNRS, I Place A. Briand, F-92195 Meudon Cedex, France
R. TRIBOULET Laboratoire de Physique des Solides, CNRS, I Place A. Briand, F-92195 Meudon Cedex, France
and P. LEMASSON Laboratoire des Matériaux Moléculaires, CNRS, 2 Rue Henri Dunant, F-94320 Thiais, France
Electrodeposition of Ag and Cu in the dark at mercury cadmium tellunde (HCT) and mercury zinc telluride (HZT) alloys is studied by electrochemistry and electroreflectance (ER). Cu and Ag deposits are done in the dark and in a potential regime like that for metal substrates (underpotential deposition). Electroreflectance spectra recorded with metal-coated alloys exhibit an extremely specific behavior, the spectrum relative to the F 0 + 4~and the E1 transitions (HCT) or to E1 only (HZT) vanishing (Ag on HCT, Ag or Cu on HZT) or decreasing strongly (Cu on HCT) and new ER features being created. These features are located at a different energy for the HCT and HZT alloys; further, a good stability of the metal—alloy interface necessitates a metal deposit several monolayers thick for HCT while for HZT a metal sub-monolayer leads to stable results. Therefore, a specific chemical interaction between Ag and Cu and the alloy substrate is postulated which needs the presence of Hg but depends strongly on the other metal (Cd or Zn) included in the ternary compound.
1. Introduction Preparation of Schottky barriers with good rectifying properties is still a problem with HgCdTe (HCT) and HgZnTe (HZT). It is well established that the initial steps of metal—semiconductor interface formation are of particular importance in understanding the behavior of Schottky barriers [1]. For these investigations, growth of the metal layer is generally achieved using a vacuum deposition technique. However, the electrochemical method may appear as an attractive alternative to the previous one due to its particular versatility, Coverage, form and microstructure of the deposit can be easily monitored by controlling and changing solution composition, time and potential of deposition as well as mass transport conditions. While HCT has already been studied for several years [2], HZT alloys have been grown only re0022-0248/90/$03.50 © 1990
—
cently and they seem promising materials for production of infrared detectors and optoelectromc devices [3]. Hg1 ~X~Te (X = Cd, Zn) Il—VI alloys crystallize in the zinc-blende structure in the whole composition range (0 x 1) and the lattice parameter obeys Vegard’s law [2,4]. The E0 band gap and the E1 transition lie in the range 0 (— 0.14)—1.51 eV and 2.15—3.2 eV respectively for HCT, and 0—2.25 eV and 2.15—3.6 eV for HZT [4,6]. The case of metal deposits on HCT has received special attention [7] while that of deposits on HZT has been studied very recently by us [8]. In the case of HCT, it has been shown that Ag and Cu can be considered as metals of intermediate reactivity and experimental analysis of the interfaces by ultra-high-vacuum (UHV) techniques has led to the conclusion that deposits on cleaved and sputtered substrates lead to interfacial
Elsevier Science Publishers B.V. (North-Holland)
312
C. Nguyen Van Huong et al.
/ Ag and
Cu deposits on HgXTe (X = Cd, Zn) alloys
reactions, including formation of Ag and Cu tellurides as well as Hg losses. In the case of HZT, investigation of the interface formation was achieved using electroreflectance (ER) and the existence of a strong chemical interaction was postulated on the basis of the creation of a typical ER signal. In the present work, Cu and Ag deposits are prepared electrochemically in the dark. The electrolyte—semiconductor as well as electrolyte—
3. Experimental procedure
metal—semiconductor interface are studied using cyclic voltammetry, capacity and ER measurements. A comparison is done between results obtamed at HZT and HCT substrates for different alloy compositions and an attempt is made to correlate the experimental results with the structural properties of the metal layer and the chemical nature of the substrates.
alloy, a particular electrochemical treatment must be applied to the electrode which otherwise does not provide any featured spectrum. It consists mainly in an AC perturbation of large amplitude (1 V~~) for 1 to 5 mm while the DC potential is kept constant so that the mean resulting current crossing the interface remains 0 or slightly negative. Such a treatment as well as its consequences on the HCT surface are detailed in another publication [11]. Presently, it seems sufficient to mention that it does not have noticeable effect during the metal deposition, at least for the current-versus-potential curves. After this characterization, the HgXTe electrode is transferred to another electrochemical cell whose electrolyte contains the metal salt in order to achieve the metal deposition. The latter operation is done at fixed DC potential for HZT and by cyclic voltammetry for HCT. Last, after careful rinsing with ultrapure water, the coated electrode is dipped back in the first electrochemical cell which contains 0.1M HC1O4 alone, where it is characterized in a way similar to the bare electrode. The amount of metal deposited in metalsalt-containing solution is2 +calculated by~) coulomeCu°+ 2p and the try (Ag + —~Ag°+ p~ Cu coverage is estimated by assuming a close-packed arrangement of the metal atoms.
2. Experimental Both HCT and HZT alloys were grown using the traveffing-heater method. HCT with 70% Hg composition was indium doped and showed n-type conductivity while 24.5% Hg HCT and HZT in the whole composition range were p-type. Sample preparation and mounting for electrochemistry measurements have been already reported [9]. Surface treatment includes mechanical polishing with diamond paste (0.5 ~Lm)and chemical etching in 1% bromine in methanol solution. The electrochemical cell is of the classical three-electrode type with a glassy carbon counterelectrode and aElectrolytes mercurous sulfate electrode (MSE) as a reference. (0.1M HC1O 4) are prepared from Suprapur perchioric acid and ultrapure water (CNRS patent). For metal deposition, a concentration AgNO3 or CuSO4 ranging be5Mofand 5 x 104M is used. In each tweensolutions 5 X 10 are deaerated by nitrogen bubbling. case, Electroreflectance measurements are performed with a 0.4—0.5 V~,15 Hz AC perturbation, in the energy range 2—4 eV with a light beam impinging the surface at near-normal incidence. The cornplete experimental setup has been described previously [10].
After chemical etching, the bare HgXTe electrode is dipped in the supporting electrolyte avoiding direct contact with air. The fundamental properties of the interface that is then formed are determined by measurement of the current- and admittance-versus-potential characteristics and of ER spectra at fixed DC potential. It is noteworthy to mention that in the case of the Hg0 3Cd07Te
—,
4. Results In this presentation, we will focus especially on results obtained with HZT (x 0.53) and HCT (x 0.70) alloys. It is worth mentioning that these results are representative of those obtained with the whole set of alloys. =
=
C. Nguyen Van Huong et al.
0
:;::::
.~.‘
/
/( /
/ ~\
! /
~-20
~ -40
1
/
Ag and Cu deposits on HgXTe (X= Cd, Zn) alloys
~
-
313
~1_J-i~
____________________________
—~
-6OxrO7\ -~
~
-05
Fig.
Potential (Vmse) Fig. 1. Current—potential charactenstics of the Hg0 3Cd04M 7Te AgNO 4M HC1O4, CuSO and (b) 5 X 10 alloy in contact with (a) 0.1M 3 ( ) and 5x10 4 (_ . - . -) containing solution. Ag- and Cu-coated electrodes in O.1M HC1O4 give a current-potential curve similar to that presented in (a). Sweep rate 10 mY s .
4.1. Electrochemistry The current-versus-potential characteristics in the dark (fig. 1) of the HCT alloys are very similar to those obtained with HZT (fig. 2). 2Both for place HCT ± takes andpotentials HZT electrodes, reduction of Cu for Ag~,this at lower than 0 V while reduction starts at 0 V; further, the current density measured during reduction of Ag + is noticeably larger than for Cu2t The voltammograms of ___________________________ x~0.53 E °
~
°
0 ~
b ~1O
~
i,~’ c -
-1
-
0 5
3.
Scanning electron micrograph of a Ag-coated
Hg
---
9
Potent a I (Vm ~ Fig. 2. Current—potential characteristics of the Hg 5M 047Zn0 AgNO 53Te 5M0.1M CuSOHC1O4, (b) 5 x 10 alloy in contact with (a) 3 and 5 x 10 4 (— — —) containing solution, and (c) Ag ( ) and Cu (— — —) coated alloy in 0.1M HC1O4. Sweep rate 10 mV s~.
047Zn053Te electrode. The experimental conditions are given in the text (X 20 000).
Ag- and Cu-coated electrodes are very different for HZT and HCT alloys (figs. 1 and 2). While in the former case (coated FIZT), anodic and cathodic currents take place which correspond to an oxidation of the metal deposit and to the reduction of its oxide, respectively, in the latter case, the characteristics are not very different from those obtained with a bare electrode thus indicating that the metal deposit is probably not modified during the The potential scan. capacitance-versus-potential behavior of coated electrodes does not correspond to a classical Mott—Schottky plot. We can, however, state that the capacitance increases with metal coverage and tends to values obtained with metal electrodes. Presented in figs. 3 and 4 are scanning electron micrographs of Ag-coated electrodes, HZT (x 0.53) and HCT (x 0.70), respectively. In the first case, the deposit is made in 5 X 105M Ag~electrolyte at fixed potential (—0.7 V); in the second case, the coating is achieved by a linear potential sweep between 0 and —0.4 V in 5 x 104M Ag~ electrolyte. For HZT, the coverage is less than a monolayer whereas for HCT, it corresponds to several layers. In both cases, coalescence and nucleation phenomena are noticeable. =
=
4.2. Electroreflectance The influence of metal deposit on ER spectra can be observed for the whole series of HCT and
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/ Ag and
Cu deposits on HgXTe (X = Cd, Zn) alloys ENEROY(eVl I
I
I
I
2.25
2.5
2.75
3
X 0.53
-
~
\\\\
__i.
~
,,,i/’~\
I
-
r~
-
Fig. 4. Scanning electron micrograph of a Ag-coated Hg 0 3Cd0 7Te electrode. The experimental conditions for the metal deposit are detailed in the text (X 18,000). 600
HZT alloys. Fig. 5 presents the spectra obtained with electrolyte—M—HCT (M Ag, Cu) structures for x 0.7 as well as spectra recorded with bare HCT. In fig. 6 is presented the whole set of results (metal-coated, bare electrode) obtained with HZT (x 0.53), an alloy whose energy gap (— 1 eV) is nearly the same as that of HCT (x 0.7). Like for HCT, new ER features appear with the coated electrodes, It is worth mentioning that for Ag-coated HCT and Ag- and Cu-coated HZT, the magnitude of =
=
I
500
400 I
WAVELENGTN (nm) Fig. 6. Electroreflectance spectrum of a Hg0 47Zn0 53Te surface without
(-
— —)
and with Ag ( ) and Cu monolayer coating.
(_ . -
. —)
sub-
=
=
the metal coverage: for HCT, the new features are associated with metal layers which are several monolayers thick while for HZT the metal deposit
Energy (eV)
I
I
I
I
1.75
2.0
2.25
2.5
~~ —0
I 2.75
-
~ ,~~~-;:~:J .ii~ —.
‘~
~ -2
/ ‘
750 I
I’
is less than a monolayer. In the HCT case, the ER spectrum becomes featureless if the coverage becase, the same behavior is observed as soon as the comes greater than 10 monolayers and in the HZT coverage reaches 1 monolayer.
-
-
the E1 oscillation decreases drastically, whereas for Cu-coated HCT only a small dampening is observed. However, results reported for HCT and HZT do not correspond to the same magnitude of
X ~0.70
A
-
I
6~d I
550 I Wavetength(nm)
____
-
Fig. 5. Influence of Ag ( ) and Cu (_ . — . —) layers on the electroreflectance spectrum of the Hg0 3Cd07Te alloy ~— — —~.
Summing up, thedifferent. behaviorWhile of HCT that of of HZT seems rather in and the case Ag and Cu deposits on HZT and of Ag only on HCT, important new features take place which are characteristic of the new interface, in the case of Cu deposits on HCT, only a small deformation of the original ER spectrum is observed. Furthermore, in each case, the features obtained with metal-covered HZT and HCT are noticeably different in shape as well as in energy position.
C. Nguyen Van Huong et at
/ Ag and
Cu deposits on HgXTe (X = Cd, Zn) alloys
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5. Discussion
ally different from those obtained at solid Cu [15] and Ag [16] electrodes as well as at semiconductor
The electrochemical Ag and Cu deposition at HgXTe electrodes in the dark occurs at potentials more positive than is predicted by the equilibrium Nernst potential (with a concentration of s x 105M, —0.108 V for Ag~/Ag°and —0.430 V for Cu2~/Cu°;with a concentration of 5 x 104M, —0.048 V for Ag~/Ag°and —0.40 V for Cu2~/Cu°)that is, before bulk deposition occurs. By analogy with what is observed at metal substrates [12] where this phenomenon is called “underpotential deposition”, we can assume that in this regime, a metal layer is formed whose coverage and thickness is dependent on the applied potential. Previous electrochemical observations [13] led to the supposition that the common anion rule [14] also holds true for the Te-based Il—VI cornpounds. Therefore, we can assume that the position of the valence band is almost the same for HZT and HCT alloys, thus leading to an electrochemical mechanism of reduction of the metal cations which is similar for both electrodes. Reduction of metal cation M~occurs by injection of holes from M~into the HgXTe valence band [8] according to
electrodes. It is worth mentioning that a stable and reproducible behavior is reached at HZT electrodes with sub-monolayer metal coverage whereas for HCT alloys a thick deposit (several layers) is necessary. In the case of thin (less than a monolayer) deposits on HCT, a modification (decomposition) of the electrode surface is noticeable. Therefore, it seems reasonable to assume that the electrochemical reaction for metal deposition occurs rapidly, leading to a total surface coverage. A phenomenon similar to a “freezing” of the metal—semiconductor interface is thus observed. However, when the metal deposition is slow (low cation concentration in the electrolyte), the reaction can be correlated with an increase of the reactivity of the HCT alloy. Such a behavior can be compared with previous observations of metal deposits on HCT, mainly by X-ray photoelectron spectroscopy, which indicate that Ag and Cu deposits lead to interfacial reactions, including formation of Ag and Cu tellurides as well as Hg losses [7]. Furthermore, the difference observed between the behavior of HCT and of HZT alloys can probably be related to the greater stability of HZT as compared to that of HCT [17].
M~, 1 M~d5+ P~B’ —~
where the subscripts sol, ads and VB indicate that the species are in the solution, adsorbed and in the semiconductor valence band, respectively, Once the Ag(Cu) deposition on the alloy surface is complete, the HgXTe—Ag(Cu) electrode exhibits a specific behavior when studied again in the supporting electrolyte and electrochemistry and ER provide convenient in situ tools for the investigation of the new interface which has been created. In electroreflectance, two features appear to be of particular importance: (i) the decrease (and vanishing, except for Cu-coated HCT electrodes) of the ER oscillations associated with the E0 + 4~ and E1 transitions for HCT and with the E1 transition for HZT; (ii) the creation of a new and generally broad ER spectrum in correlation with the electrochemical behavior of the metal-covered electrode, at least for HZT. Such spectra are tot-
The decrease in the E0 +40 and E1 ER signals can be treated most likely, as we have done preyously [8],in terms of a Fermi-level-pinning process induced by the metal deposition. On the other hand, a quantitative interpretation of the new ER features appears to be difficult, even if we were to assume that it is due to the specific behavior of the new interface formed by the metal deposit. In this direction, the chemical composition of the substrate as a whole seems to play a prominent role in the observed behavior (as indicated by the important differences in energy position) of the spectra obtained at metal-covered HCT and HZT electrodes. Nevertheless, the existence of new ER features observed with metal-covered HgXTe electrodes, together with their specific electrochemical behavior (indicating no electrochemical dissolution of the metal coating) tends to demonstrate the existence of a strong chemical interaction between the metal and the HgXTe surface.
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The particular case of Cu deposits at HgCdTe electrodes gives rise to small differences only, due to the fact that (i) the electro-optical behavior of the substrate is not totally screened by the coating and (ii) the new spectrum presents only a small deformation between 2.25 and 2.5 eV. We can thus reasonably conclude that in this case, the interaction between HCT and the Cu deposit is different from that for Ag deposits, in contradiction with previous observations by UHV techniques [7],which indicated that Ag and Cu reacted almost similarly with HCT surfaces. Furthermore, the Cu deposits are less adhesive than the Ag deposits on HCT. However, if the interaction between Cu-deposit and HCT is not strong like for low Ag-coverage, a surface decomposition of HCT is noticeable even when the Cu deposit, as deduced from coulometry, is thick. Therefore, we can presume that Cu weakens the bondings in HCT, while with Ag it is possible to stop this effect by increasing the coverage of the metal on the HCT.
(X = Cd, Zn) alloys
necessary for a stable behavior whereas in the case of HZT, a submonolayer deposit leads to a stable interface. This can be related reasonably to the greater reactivity of HCT which necessitates a complete metal coverage in order to stabilize the metal—HCT interface. Finally, the ER features connected with the metal—HgXTe interface are dependent on the alby composition as well as on the X component, as indicated by the energy positions of the features which are different for HCT and HZT. We can conclude therefore that if the presence of mercury in the alloy is necessary [8], it is the other metal cation which governs the global interfacial reactivity, in good agreement with previous statements on the different bondings in HCT and HZT [17].
References [1] See, e.g., L.J. Bnllson, Surface Sci. Rept. 2 (1982) 123; I. Lindau and T. Kendelewicz, CRC Critical Rev. Solid State Mater. Sci. 13 (1986) 27.
6.
Conclusion
The whole set of electrochemical and electrooptical results presented here tends to demonstrate that the surface of HgXTe (X Cd, Zn) alloys, covered with a deposit of Ag or Cu, behaves very specifically. For the first time, the creation of an electroreflectance signal is reported that is completely different from that of the substrate and linked to an interfacial phenomenon due to the metal coating. Therefore, it seems reasonable to postulate a strong chemical interaction between Ag and Cu atoms electrochemically deposited and the constituent atoms of the substrate. However, the case of Cu on HCT appears to be more particular and leads to a stronger reactivity of the alloy surface with the electrolyte. It is important to reiterate the fact that the similar electrochemical and ER behaviors observed with metal-coated HCT and HZT alloys are obtained with metal deposits whose thickness is not of the same order of magnitude. In the case of HCT, deposits several layers thick (Ag case) are =
[2] For general information about research on HCT alloys see, e.g., (a) Cadmium Mercury Telluride, Semiconductors and Semiinetals, Vol. 18, Eds. R.K. Willardson and A.C. Beer (Academic Press, New York, 1981); (b) Proc. 2nd Intern. Conf. on Il—VI Compounds, J. Crystal Growth 72 (1985); (c) Proc. 3rd Intern. Conf. on Il—VI Compounds, J. Crystal Growth 86 (1988).
13] R. Triboulet, J. Crystal Growth 86 (1988) 79. [4] B. Toulouse, R. Granger, S. Rolland and R. Triboulet, J. Physique 48 (1987) 247. [5] V.G. Sredin, V.G. Savitskii, Yu.V. Daniluk, M.V. Miliyanchuk and I.V. Petrovitch, Soviet Phys.-Semicond. 15 (1981) 249. 16] C. Nguyen Van Huong, R. Triboulet and P. Lemasson, Proc. SPIE 659 (1986) 249. [7] For a recent metal—HCT Junction formation, see e.g., G.D. review Davis, on WA. Beck, M.K. Kelly, D. Kilday, Y.W. Mo, N. Tache and G. Margaritondo, Phys. Rev. B38 (1988) 9694, and references therein. [8] C. Nguyen Van Huong and P. Lemasson, Phys. Rev. B40 (1989) 3021. [9] C. Nguyen Van Huong, Triboulet and P. Lemasson, J. Crystal Growth 86 (1988)R.570. [101 C. Hinnen, C. Nguyen Van Huong, A. Rousseau and J.P. Dalbéra, J. Electroanal. Chem. 95 (1979) 131. [11] P. Lemasson and C. Nguyen Van Huong, to be published.
C. Nguyen Van Huong et at
/ Ag and Cu deposits on HgXTe (X
[12] D.M. Kolb, in: Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, lids. H. Gerischer and C.W. Tobias (Wiley, New York, 1978) p. 125. [13] P. Lemasson, Solid State Commun. 48 (1983) 411. [14] JO. McCaldin, T.C. McGill and CA. Mead, Phys. Rev. Letters 36 (1976) 56.
=
Cd, Zn) alloys
317
[15] R. Kotz and H.J. Lewerenz, Surface Sci. 97 (1980) 319. [16] J.D. McIntyre, in: Advances in Electrochemistry and Electrochemical Engineering, Ed. R.H. Muller (Wiley, New York, 1973) p. 61. [17] A. Sher, M.A. Berding, M. Van Schilfgaarde, A.B. Chen and R. Patrick, J. Crystal Growth 86 (1988) 15.