Electrochemical atomic layer epitaxy (ECALE)

543

J. Electroanal. Chem., 300 (1991) 543-561 Elsevier Sequoia S.A., Lausanne

Electrochemical Brian W. Gregory

atomic layer epitaxy (ECALE)

and John L. Stickney

School of Chemical Sciences, University

*

of Georgia, Athens, GA 30602 (USA)

(Received 5 July 1990)

Abstract

Electrodeposition holds promise as a low cost, flexible room temperature technique for the production of II-VI compound semiconductors. Previous studies, however, have resulted in the production of polycrystalline deposits in every case. This paper describes a new method, developed in this laboratory, for depositing these materials epitaxially. The method involves the alternate deposition of the component elements a monolayer at a time. To limit deposition to a monolayer, underpotential deposition (UPD) is employed. UPD occurs because of the enhanced stability provided by bond formation between the II and VI elements, relative to formation of bulk elemental deposits. This method is the electrochemical equivalent of atomic layer epitaxy (ALE), and is thus referred to as “electrochemical atomic layer epitaxy” (ECALE). This paper describes the first example of the ECALE method, involving the thin-layer electrodeposition of CdTe on a Au polycrystalline electrode.

INTRODUCTION

Work in this laboratory is directed toward the development of methods for the electrochemical formation of epitaxial, thin-film, single-crystal compound semiconductors. Presently, drawbacks to standard electrodeposition methods include the tendency to form amorphous or polycrystalline deposits, described by Tomkiewicz as “cauliflower” deposits because of their convoluted morphology [l]. The extensive grain boundary networks in polycrystalline materials increase their resistivity and provide recombination centers, and are thus detrimental to most applications [2]. Increasing the crystallinity of electrodeposited compound semiconductors is the next step in the evolution of electrodeposition as an important production technique for these compounds.

l

To whom correspondence

0022-0728/91/$03.50

should be addressed.

0 1991 - Elsevier Sequoia S.A.

1

II

Zn

Zn Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd

Cd Cd Cd Cd Cd Cd Cd Cd Cd

Cd Cd Cd Cd Cd Cd Cd

Ref.

18

19 20 21 22 23 24 25 26 21 28 29 30

31 32 33 34 35 36 31 38 39

40 41 42 22 43 44 45

S Se Se Se Se Se Se

S S S S S S S S S

Te S S S S S S S S S S Se

Te

VI

Electrodeposition

TABLE

1 M NaOH acid 1MKOH 5MKOH 10 acid acid

DMSO, DEG DMSO DMSO H,O+ DEG DMSO DEG DMSO, PC PC PC

base 9 1MKOH base base base 9 6.1, 2.8 2.3 2.3 1-2

4.5

(PH)

100

RT

120

110 110 120 RT, 60, 100

85-95

RT

100

T/OC

ox. red. ox. ox. red. red. red.

EC PEC, X, EM PEC PEC RRDE, PEC RRDE, PEC PEC

X, SEM, RBS RRDE, PEC SEM, X EC EC X, SEM EC, E EC

red. red. red. red. red. red. red. red. red.

OX.

OX.

ox. ox. ox. ox. red., ox. red. red. red. red.

X

Characterization

PEC RDE, PEC EC PEC EC, MS RRDE, EC, SEM EC, PEC RDE SEM, AC ED, TEM, X EL

_

OX.

Deposition mode

compound semiconductors a

Solvent

of II-VI T (anneal)

400, N,

500, Ar

200

120

/“C

86 76 17 77 80 80 81

80 81 82 83 84 84 87 88 88

84 76 77 17 78 78 80 81 81 84 86 63

64

Year

1

1-12 0.5

(2)

1

/pm

Thickness (crystal size)

Cd Ti Cd Fe, SS Au, Cd Au, Ag, Pt, Cu, C Ni, Ti

Pt, Au, Zn, Ni, SS, G Pt, Au, Cd AU MO Hg, Au, Pt Pt. Au, C SS, SnO, Ti Pt, Au

Te Cd Pt, Cd Fe, SS Cd Cd Cd Ti Pt MO, AI, Pt AI Cd

_

Substrate

z

Cd

Cd Cd Cd Cd Cd Cd Cd

Cd

Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd Cd

1

46 47 48 49 50 51 52

31

53 54 55 17 56 49 57 58 59 60 19 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te Te

Se

Se Se Se Se Se Se Se

Se

2.2 3.5 2-3 1.5 l-2 acid acid 2.2

2 2.5-3

RT

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85-90 50-95

40-60 82

2.5-3 2.0-3.5

1

90

80 30

85

RT

RT-50 40

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base 1.4 1.4 1.4

4.5 2.5-3 1 2.2

DMSO, EG

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0.5 M H,SO,

red. red. red. electroless red. red. red. red. red. red. red. red., Pulse red. red. red.

red. red. red. red. red. red. red. red red.

OX.

red.

red. red. red. red. red. red. red.

red.

AA X PEC J-V, SEM RDE EC, PEC, X X, PEC PEC, X X, AES SIMS, X, SEM PEC PEC, SEM, E X J-V X, E, SEM RDE X AES, J-V RTD, AC X, AES, SEM X RDE SEM, RDE EC EC EC, X

AES, SEM, J-V, PEC SEM, X, PEC PEC, J-V EC, PEC EC, PEC, X PEC P, SEM, X, RBS SEM. E

300, Cd

200, air

155-650, 250, H,

350, air 590

600, Ar

200, He 200, He

250 300. air

100, v

600, N, 500,600

N,

600,0z, Ar 430, air 550, air

72 78 78 82 83 83 84 84 84 84 84 84 84 84 84 84 85 85 86 86 86 86 87 87 89 89

80

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1.8(4)

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1-3 1 (l-5) 1.5 (0.5-l) 0.15-2

(1)

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3.5

(conrimed on p. 546)

Ni, SnOz Ti Cu. Ni, SS Ti Ti Ti, Ni Ti, Ni Ti, Ni Ni Te Ti, G ITO, SS ITO, G Ti, G Pt MO ITO, MO Ni Cu, Ti, SS, ITO, SnO, Ni SnO, Cd C C SnO1

Pt, Au, Zn, SS, Ni, G

Ti Ti Ti Ti Ti Ti Ti Ti

“I

z

Cd Cd Cd Cd Cd

Hg

Hg

76 77 78 79 80

31

53

Neierojunctions 81 Cd Cd a2 Cd Cd 83 Cd Cd

Ii

Ref.

90 40-60

2.5-3

100 100 160 130 RT

T/QC

2

4.5

Te

S Te S Te S Te

DMSO, EC

PC PC EC DMSO 1 M NaOH

Solvent W)

s

Te Te Te Te Te

VI

TABLE 1 (continued)

red.

red.

red.

ox.

red.

red. red. red. red. ax.

Deposition mode

PEC, SEM, AES, J-V

5-700,Ar

85

84

J-V

72

80

a5 86 a9 a9 a9

Year

a2 400, air

100

400 200, Nz

T (anneal) /“C

J-V

AA

X. PEC. SEM PEC, SEM PEC, J-V, XPS, X, SEM SEM, ED, E RRDE, AES, XPS, PEG

Characterization

l-3

2

0.1-0s l-2

(2.5)

5.4 0.5

/pm

Thickness (crystal size)

ITO, MO

IT0

IT0

Pt, Au, Zn, Ni, SS, G

Ti Ti Ni SnOz Cd

Substrate

Cd Cd H&d H&d Cd Cd CdZn

88 89 85 90 91 92 93

1.6

1.6

2

SeTe SeTe Te SeTe SeTe SeTe Te

1 1 1.6 9.6 l-2.5 2.2 1

SeTe acid-base SeTe base SeTe DMSO

S Te S Te S Te

40-60 85

25 40 90

110

RT-50

90

90

90

red. red. red. red. red. red. red.

red. red. red.

red.

red.

red.

X PEC SEM, J-V, E PEC, X, RRDE X, AES, E, SEM, PEC X, SEM, E, PEC SEM, X, J-V

SEM, X, PEC EC, PEC, X, PEC X, SEM, RBS

J-V, X

SEM, E, J-V

AES, J-V

580, Ar

Cracks if heated 2-700, N, 630, Ar 400, air 3-400, N2 650, Ar

600, Oz +Ar

400, air

400, air

350, air

85 85 86 86 86 89 89

82 83 83

88

86

85

2

2 ( > 0.5)

Pt

2

2

1.5

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Ti Ti

IT0

IT0

a AA = atomic absorption; AC = ac impedance; DEG = diethylene glycol; E = EDAX; EC = cyclic voltammetry; ED = electron diffraction; EG = ethylene glycol; EL = electrical characterization; EM = electron microprobe; G = glass; J-V = current-voltage curves; MS = Mott-Schottky; P= polarography; PC = propylene carbonate; PEC = photoelectrochemical cell; RBS = Rutherford backscattering; RDE = rotating disk electrode; RRDE = rotating ring-disk electrode; RT = room temperature; SS = stainless steel; V = vacuum; X = X-ray diffraction.

Cd Cd Cd

Hga

Cd Cd Cd H&d Cd

Alloys 46 49 87

86

85

84

548

There is a wide variety of applications for compound semiconductors. The II-VI family of semiconductors, for example, are used in photovoltaics [3-7,105], luminescent displays [B-lo], radiation detectors [ll], lasers and laser windows [ll], infrared detectors [12], and Vidicon imaging devices [13]. These semiconductors will also be important components in the emerging electro-optical technologies based on layered structures [14]. They display a variety of band gaps and some form solid solutions. The ability to control stoichiometry in these solid solutions results in the ability to vary the bandgap as a function of its composition. The mercury cadmium telluride (MCT) system is an important example of band gap engineering [12,15,16]. Electrodeposition of semiconductors is a potential low cost, room temperature production technique [3,4,17]. The electrodeposition of II-VI compound semiconductors has been reported in a large number of articles (Table 1). The majority of these studies followed a procedure similar to that used by Panicker, Knaster and Kriiger [54,95] to electrodeposit CdTe. Cd and Te were codeposited from a concentrated CdSO, solution that was saturated with TeO,. The primary source of Te used in these aqueous studies, TeO,, has a low solubility; at pH 2, its solubility is about 0.2 mM [96-981. Deposition of Cd was carried out as an underpotential deposit (UPD) (described in the next section); the deposition potential was controlled so that Cd deposited only on Te, and not on itself [54,95]. The rate of deposition was therefore controlled by the transport of TeO, to the surface, since it was the limiting component and deposited rapidly at the surface. Variations in the deposition potential resulted in changes in deposit stoichiometry and allowed formation of p- or n-type CdTe [54,95]. Although the stoichiometry of the deposit was controllable, there was little control over the nucleation and growth kinetics of the deposition [99]. The result was formation of three-dimensional crystallites on epitaxial deposits, termed Stranski-Krastanov (SK) deposition [6]. All previous CdTe electrodeposition studies have resulted in polycrystalline deposits (Table 1). There are four primary reasons for the formation of polycrystalline deposits: nucleation and growth kinetics in the codeposition of a compound semiconductor; the absence of an ordered substrate structure; lattice match between substrate and deposit; and substrate, solvent, reactant and electrolyte contamination. To address these problems and to promote the epitaxial electrodeposition of compound semiconductors, the following methodologies are being developed in this laboratory: use of Electrochemical Atomic Layer Epitaxy (ECALE) to control nucleation and growth; careful selection, preparation and characterization of lattice-matched single-crystal substrates in order to avoid interfacial strain and promote ordered deposition; and use of ultrahigh purity solvents, gases, electrolytes and reactants in a low volume, high surface area thin-layer electrochemical cell (TLE) configuration in order to avoid contamination. This paper describes the new method for the electrochemical deposition of compound semiconductors, designed to result in epitaxial electrodeposits: ECALE. Vacuum-based methods for compound semiconductor growth (e.g., molecular beam epitaxy (MBE)) involve some of the same problems encountered in electrodeposition, such as the need for careful control of reactant fluxes in order to

549

obtain epitaxial deposits. Atomic layer epitaxy (ALE) is a form of MBE currently under development which allows less stringent control of growth parameters [8,100]. Unique to ALE is compound growth of an atomic layer at a time. This technique relies on surface-specific reactions which result in only a monolayer of reactivity. If the reactant is an elemental vapor, the substrate temperature is adjusted so that bulk deposits sublime while the first monolayer remains due to an enhanced‘stability resulting from compound formation. After pumping (evacuation) of the first element, a similar procedure is performed with the second element [loo]. For a compound such as CdTe, a layer of Cd is formed followed by a layer of Te [loll. Thin film growth is achieved by repeating this cycle. This paper describes our application of this idea to the electrodeposition of thin film compound semiconductors in order to allow deposit structure and composition to be controlled by surface chemistry instead of nucleation and growth kinetics. In other words, deposition of a compound semiconductor is performed a monolayer at a time. Reactivity is limited to a monolayer by using underpotential deposition (UPD) of the elements [95,102-1041. In order to avoid problems associated with codeposition, alternate deposition of the elements is performed. We refer to this deposition method as ECALE. Preliminary results presented here demonstrate the ECALE method applied to the deposition of CdTe. Methodology

of electrochemical

atomic layer epitaxy (ECALE)

ECALE involves the alternated electrochemical deposition of elements to form a compound. Epitaxial deposition is achieved by using underpotential deposition (UPD) as the means to achieve surface chemistry-limited growth. The phenomenon of UPD is well-documented and reviewed [94,103]. It involves the deposition of one element on a second element at a potential prior to (under) that required for deposition of the first element on itself. Classically, UPD involves deposition of a less noble metal on a more noble metal [94,103,106]. The driving force for UPD is formation of a compound which is energetically favored relative to the bulk elements with a stoichiometry defined by the surface chemistry of the substrate. With metals, a two-dimensional bimetallic compound is formed. This driving force can also come from formation of other types of compounds. UPD can refer to processes such as the adsorption of hydrogen on a metal, as in the case of the hydrogen waves on Pt [107], or anodic processes such as the initial stages of oxide formation on metals [108]. In this section we will refer to deposition in terms of half monolayers (l/2 ML), where l/2 ML is either the IIB or VIA component of a full ML of a II-VI compound, for example. Compounds can be formed by depositing sequentially a VIA l/2 ML followed by a IIB l/2 ML. A thin-layer electrochemical (TLE) cell configuration is used for the deposition so that solutions are easily changed by blowing out one solution and allowing a second to flush in by capillary action [109]. This process is referred to as a “rinse.” The use of two different reactant solutions, and a pure electrolyte to rinse with in-between, avoids complications involved with

550

codeposition of the elements and allows a single reactant species to be deposited at a time. To limit the reactivity to l/2 ML at a time, deposition potentials are selected for UPD. This process avoids formation of three-dimensional crystallites and Stranski-Krastanov growth. Contamination is avoided by careful control of the purity of each solution component. Contamination prevention is also facilitated by the use of a TLE for the deposition. The very low solution volume-to-electrode surface area ratio (3 X 1O-3 ml/cm*) of the TLE results in a 103-lo4 decrease in the amount of solution being exposed to the deposit, compared with conventional deposition configurations (Table 1). Decreases in the solution volume result in decreases in contamination since any contaminants in the solution have an opportunity to adsorb on the deposit surface. All previous low temperature studies of compound semiconductor electrodeposition have been performed on polycrystalline substrates. The preliminary studies presented here also involve polycrystalline electrodes. It is clear that well-characterized single-crystal substrates should be employed and that epitaxial deposition will occur most readily on an ordered, lattice-matched substrate. These initial studies were carried out using polycrystalline electrodes in order to determine the potentials and coverages relating to the ECALE of CdTe. Studies of ECALE of CdTe on single-crystal substrates are the next step in the development of procedures for epitaxially electrodepositing single-crystalline thin film compound semiconductors.

EXPERIMENTAL

The thin-layer electrochemical cells (TLE’s) utilized in this study were similar in design to those previously published [109]. The Au electrode was a polycrystalline rod, finished with 600 emery paper. It was initially flame-annealed for lo-15 s in a gas/O, flame, resulting in a dim red glow, then immediately quenched in triply-distilled water. Afterwards, it was immersed in concentrated nitric acid (Baker Analyzed Reagent) for approximately 5 s. Prior to each electrochemical experiment, a standard cleaning procedure was performed which consisted of cycles of oxidation and reduction in 1 M H,SO,. During each cleaning cycle, the electrode cavity was flushed with 10 rinses of fresh H,SO, solution at both the oxidizing and reducing potentials. Following the cleaning cycles, a clean current-potential curve for Au was obtained in 1 M H,SO,, equivalent to that shown in Fig. 1. All potentials are reported using a Ag/AgCl (1 M NaCl) reference electrode. Cadmium solutions were prepared using CdSO, (Morton Thiokol, 99.6%), NaClO,. H,O (Fisher Scientific, Purified, Cat. no. S360-WO), and HClO, (Baker Analyzed Reagent, 60-628). Tellurium solutions were prepared with Puratronic TeO, (Johnson Matthey, 99.9995%) and HCIO,. The concentrations of the actual solutions used were: 1.0 mM CdSO, + 1 M NaClO, (pH = 2.9); 0.505 mM TeO, + 0.10 M HClO, (pH = 1.3); and 1 M NaClO, (pH = 2.5). All were pH adjusted with

551

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0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

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(vs.A~/A~CI)

AU

Fig. 1. Current-potential mV/s.

curve for clean, polycrystalline Au substrate in 1 M H,SO,.

Scan rate = 5

solutions made from concentrated HClO,. Pyrolytically triply-distilled water [110] was used to prepare all solutions. Experimental coverages are reported as the ratio of deposited atoms/substrate surface atoms. The number of Au substrate surface atoms was obtained from measurements of the “real” surface area. This was accomplished by integration of the charge required to reduce IO;, which was formed by oxidation of a close-packed monolayer of adsorbed I atoms [ill]. This method yielded a value of 1.29 cm2. The volume of the Au TLE cavity was determined using the charge for reduction of a 4.81 mM FeCl, + 1.0 M H,SO, solution. The calculated volume was 2.90 ~1.

RESULTS AND DISCUSSION

A previous thin-layer electrochemical study of Cd and Te deposition on polycrystalline Pt, Cu, and Au electrodes revealed the advantages of using Au electrodes for investigating Cd and Te voltammetry [112]. UPD Te deposition and stripping on Pt coincided with Pt surface oxide features, and Cd UPD resulted in three peaks which occurred in the same potential range as the hydrogen waves. The coverages and potentials for Cd and Te UPD were therefore ill-defined. Cu electrodes dissolve at potentials where TeO, is normally reduced; exposure of a Cu electrode to TeO, resulted consequently in the spontaneous deposition of Te, and led to ambiguity as to the amount of Te present on the copper surface. Au electrodes were thus selected

552

for the present study because of their large double-layer region (i.e., the potential region between -0.20 and 1.05 V vs. Ag/AgCl (Fig. la) where no surface-specific faradaic prosesses occur). It should not be concluded from these studies that Au is the best substrate for forming these deposits; Au is simply the best for observing the electrochemistry of Cd and Te. Epitaxy will be much better facilitated if well-ordered lattice-matched single-crystal electrodes are used as substrates. The Au(100) single-crystal surface does offer a close lattice-matching possibility for CdTe; formation of a Au(lOO)(& x fi)R26.6% unit cell results in only a 0.6% mismatch with CdTe. Figure 1 displays the voltammetry of the clean Au TLE. This surface was obtained reproducibly before each experiment by application of cleaning cycles, which consisted of alternate rinsing with pure 1 M H,SO, at -0.5 V and + 1.4 V. No assumptions are made here about the order or crystallography of the polycrystalline Au surface, but the electrode was thermally annealed in a gas/oxygen flame after polishing and prior to these studies. The diagram at the bottom of Fig. 1 represents a side view of the Au substrate surface. Measurements of the electrode surface area, including roughness factor, were made by quantitating the amount of iodine adsorbed upon exposure of the clean electrode to three aliquots of mM KI [ill]. Recent studies in this laboratory of I adsorption on Au(ll1) using low energy electron diffraction and Auger electron spectroscopy showed that iodine formed a (5 x 6) unit cell with a coverage of 0.4 (i.e., 4 iodine atoms per 10 Au substrate atoms). Thus, there are 5.4 x lOI I atoms/cm2 for a close-packed I atom layer on the Au(ll1) surface. Using this number and the integrated charge for adsorbed iodine oxidation, the electrode surface area was determined to be 1.29 cm2, corresponding to a roughness factor of 1.04. The first step in the deposition process was to rinse in an aliquot (2.90 ~1) of 0.5 mM TeO, at 0.8 V (Fig. 2). Initial Te deposition is considerably different from subsequent layers since Te is depositing on Au. The scan proceeded from right to left and resulted in a reduction peak at 0.25 V which corresponded to the UPD of Te onto Au. A coverage of 0.49 for the Te UPD peak was calculated using the electrode surface area and the charge associated with the peak. Again, no assumptions were made about the structure of the polycrystalline Au substrate surface, so none will be made about the surface structure resulting from Te UPD. It is clear from previous studies of UPD that the cessation of Te deposition current indicates that either the surface is covered or all available surface sites have been filled [104]. Continuation of the reductive scan resulted in a second peak for deposition of bulk Te (i.e., deposition of Te on itself). The amount of bulk deposition was limited due to the finite amount of TeO, originally present in the TLE cavity, and thus appears as a peak [109]. The charge under these first two peaks corresponds to deposition of the total TeO, aliquot by a 4-electron reduction to Te(0). By stopping the scan after the first peak and rinsing in pure supporting electrolyte, the excess TeO, is removed. This process leaves a single l/2 ML of Te on the substrate surface, which is diagrammed at the bottom of Fig. 2. The solid stripping curve in Fig. 2 is observed in the reverse scan. Stripping curves are shown in each

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Fig. 2. Current-potential curve for Te deposition and stripping on a polycrystalline Au substrate. Deposition and stripping Te UPD layer. (- - - - - -) Deposition and stripping of bulk Te deposit. ( -) (. . .) Reduction of bulk Te and excess H+ to H,Te and H,, respectively, followed by reoxidation of HsTe to Te at - 0.75 V during positive sweep. (- .- .-) reverse negative sweep showing re-reduction of Te to H,Te without the presence of H+ reduction. Scan rate = 5 mV/s.

figure as a way of monitoring deposited amounts of Cd and Te. The dashed curve in Fig. 2 corresponds to deposition and stripping of bulk Te. The dotted curves shown in Fig. 2, at negative potentials, were obtained after deposition of both UPD and bulk Te. Deposition was complete, such that no HTeO: remained in the cell. The first cycle involves two processes: reduction of H+ to H, and reduction of Te to H,Te. On the subsequent oxidative scan, a single small peak is observed for the oxidation of H,Te back to Te. Hydrogen is not reoxidized at these potentials because the electrode is still well below the reversible hydrogen potential. The fact that the protons were not reduced until -0.8 V in the first reduction scan demonstrates a larger overpotential for hydrogen formation on Te-covered Au as compared to clean Au. The second cycle (Fig. 2) shows considerably less hydrogen formation since the pH in the thin-layer cavity was substantially increased by the first reduction scan. The reduction of Te to H,Te is clearly visible at -0.82 V. If the TLE is rinsed with pure electrolyte at -0.85 V, the H,Te is removed. A subsequent positive scan shows no H,Te oxidation at -0.75 V, but a peak for Te UPD stripping is observed at 0.55 V, equivalent to the solid curve in Fig. 2. This indicates that Te is UPD-stabilized relative to H,Te formation on the Au surface and only the excess, or bulk, Te is converted to H,Te. Thus, there are two ways to

554

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obtain the l/2 coverage Te UPD on Au: reduction of TeO, with removal of excess TeO, at 0.15 V; or deposition of the complete TeO, aliquot at -0.3 V with subsequent reduction of the excess Te to H,Te at -0.85 V, followed by rinsing away the HzTe with pure electrolyte. A variation on the latter procedure is to rinse in the TeO, solution initially with the electrode potential at -0.85 V. The result is reduction of all the TeO, to Te with further reduction of the excess, or bulk, Te to H,Te. It has been shown in the present study that H,Te formation from TeO, is at least a two-step mechanism. TeO, is initially reduced to Te(0) on the surface, which is then converted to H,Te in a slower step. This was determined by rinsing in TeOz at - 0.85 V, followed by rinsing rapidly with pure electrolyte at - 0.4 V to flush out any H,Te that had formed. Stripping of the remaining Te revealed that nearly all the Te remained. Allowing the deposited Te to sit for several seconds at -0.85 V was required for all the bulk Te to be converted to H,Te. The choice of depositing Te first, instead of Cd, was made based on the tendency of Te to displace Cd from the Au surface. At potentials positive of -0.35 V, initially deposited Cd is displaced spontaneously by the depositing Te, forming soluble Cd2+. Cadmium may be held on if a sufficiently negative potential is applied while rinsing in Te02, but this does not appear to be the most energetically favorable configuration. Deposition of Cd on Te-covered Au is shown in Fig. 3. A broad UPD process occurs which manifests itself as two peaks when using this low mM Cd concentra-

555

tion. Stopping the scan after the first peak, at -0.50 V, resulted in a coverage of 0.45, which is approximately equivalent to the Te coverage. Excess Cd2+ was then removed by flushing with pure electrolyte at - 0.68 V. Thistmore negative rinsing potential was required so that the deposited Cd would not dissolve spontaneously in the rinsing solution where the activity of Cd2+ was minimal. Also evident in Fig. 3 is the variability in the amount of deposited Cd accompanying variations in the deposition potential. Studies have shown that multiple UPD peaks can result from a polycrystalline substrate where different peaks correspond to deposition on different planes [104]. Multiple peaks can also correspond to changes in deposit structure if the packing density is increased as the potential is decreased. During this initial study on a polycrystalline substrate, it was decided to end the deposition at -0.5 V since it resulted in nearly equivalent coverages of Cd and Te. The origins of the Cd UPD voltammetry on Te-covered Au would be best investigated using single-crystal substrates and surface-sensitive spectroscopies. The solid line in Fig. 3 shows the stripping of the Cd layer. The subsequent stripping of the Te UPD is shown as the dot-dashed line. UPD Cd deposition is

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Fig. 4. Current-potential stripping curve following deposition of excess Te on top of previously deposited Cd. Note that size of bulk Te stripping peak at 0.4 V is considerably larger than that of stripping peak for UPD Te on Au at 0.5 V, indicating that there is a larger amount of Te on top of the Cd, compared to the original amount of UPD Te on the Au. Also shown at - 0.2 V is the stripping peak belonging to the inner Cd layer. Scan rate = 5 mV/s.

556

diagrammed as a l/2 monolayer on top of the previously deposited Te in the diagram at the bottom of Fig. 3. The most important factor in determining the potential at which to deposit Cd is the prevention of bulk Cd formation, and thus the prevention of three-dimensional nucleation. The next layer of Te cannot be deposited by scanning reductively from 0.8 V (Fig. 2) since both the previously deposited Cd and Te would be stripped at that potential. To keep these two components on the surface, the potential must be kept below -0.45 V. One method for forming the next Te layer involves first depositing a full aliquot of TeOz at -0.5 V so that UPD and bulk Te result, followed by reduction of the bulk Te to soluble H,Te. Figure 4 shows the stripping scan which results after a single rinse of the CdTe-covered Au electrode in TeO, solution at - 0.5 V following Fig. 3. The oxidative peak at -0.20 V corresponds to Cd stripping, and the two peaks at 0.35 V and 0.55 V correspond to bulk Te and UPD Te stripping, respectively. UPD Cd stripping occurs before the overlying layer of Te is stripped, and suggests that Cd2+ diffuses rather easily through these overlayers. The bulk Te peak is much larger than the UPD peak, indicating that this deposition results in more than a doubling of the original UPD peak (Fig. 2). The amount of Te deposited was a function of the original TeO, concentration and was sufficient to deposit an amount equivalent to the Te l/2 ML and some bulk Te. A negative scan

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Fig. 5. Current-potential stripping curve following removal of bulk Te via the reductive formation of soluble H,Te. () Negative scan to potential where HzTe formation occurs, followed by rinsing H,Te out of TLE cell. (.-. - .) Stripping of remaining deposit, showing decrease in size of bulk Te stripping peak (compared to Fig. 4). Bulk Te and UPD Te peaks now equivalent in size. Scan rate = 5 mV/s.

557

-l.V

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Fig. 6. Current-potential stripping curve following underpotential deposition of Cd onto previously deposited Te. Note that the size of Cd stripping peak has doubled compared to the previous figure. Scan rate = 5 mV/s.

from -0.45 V reduces the bulk Te to H,Te, leaving only the Te in contact with the previously deposited Cd (Fig. 5). Flushing the electrode with pure electrolyte removed all H,Te. The subsequent stripping scan demonstrates the decrease in the bulk Te peak at 0.35 V. The total Te deposited is now double the amount initially deposited (Fig. 2). An alternative to depositing Te and then scanning reductively to reduce off the excess Te (Figs. 4 and 5) is to perform the Te deposition at -0.85 V. At this potential, as discussed previously, the TeO, will reduce first to Te and then excess Te will reduce further to H,Te, which can be rinsed out of the TLE cavity leaving only the Te in contact with Cd. The second layer of Cd must be deposited at a potential sufficiently negative to prevent Cd stripping, yet sufficiently positive to prevent bulk Cd deposition. Figure 6 shows the stripping curve after Cd UPD at -0.60 V. The Cd peak area has doubled while the Te peaks remain unchanged from Fig. 5. The diagram at the bottom of Fig. 6 shows the two completed layers of CdTe on the Au substrate. SUMMARY

In the previous section, the deposition of two monolayers of CdTe was described using the ECALE method. These studies demonstrate how the natural tendency to

558

form surface compounds electrochemically through UPD can be used to produce a compound semiconductor. This is the first study where compound materials are synthesized electrochemically one atomic layer at a time. It appears that this method of deposition avoids the formation of three-dimensional nuclei, and the resulting cauliflower deposits, by never exceeding the monolayer regime. The deposition potentials are controlled such that only deposition sufficient to cover the deposit occurs and elemental growth is prevented. Each successive element combines only with the previous element deposited, which therefore prevents the build-up of any one element. As a result, CdTe nucleation phenomena encountered with the codeposition of Cd and Te (Table 1) are prevented. Alternate deposition allows the choice of both deposition potentials and solution compositions which are optimal for each element being deposited. Codeposition of the elements leads to compromises in the choice of deposition potentials and solution compositions versus those that are optimal for each individual element. The thin-layer configuration for the deposition reduces the volume of solution exposed to the electrode at a given time, which thus results in significantly less contamination. The adsorption of surface-active contaminants present in the solution can easily result in a monolayer of contamination, which may subsequently disrupt epitaxial deposition if a sufficient amount of the contaminant is present. Use of high purity solutions and the TLE prevents this. Other factors which will result in the disruption of epitaxial deposition involves the grain boundaries which are present when polycrystalline substrates are used. Studies are currently under way using single-crystal substrates. It is our opinion that single-crystal epitaxial deposits of compound semiconductors can be formed by electrodeposition. The UPD-assisted alternated electrodeposition of the component elements, referred to here as ECALE, will probably facilitate the formation of these deposits. It is probable that deposition in a TLE configuration and use of single-crystal substrates will be necessary for high quality deposits. Presently it appears that compounds such as CdTe, CdS, and CdSe, as well as alloys of these materials, should be producible by the ECALE method. Other materials such as the III-V semiconductors and II-VI compounds of Zn and Hg are also possible, but the initial step in their deposition by ECALE is the investigation of the UPD chemistry of the individual elements on each other. REFERENCES

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