Anal. Chem. 1990, 62,821-825
021
Thermal Analyses of Compound Semiconductors Using Differential Scanning Calorimetry. Application to Compositional Analyses of Cathodically Electrosynthesized Cadmium Telluride Wen-Yuan Lin, Kamal K. Mishra, Erik Mori, and Krishnan Rajeshwar* Department of Chemistry, T h e University of T e x a s a t Arlington, Arlington, T e x a s 76019-0065
Differential scanning calorimetry (DSC) was used, we believe for the first tlme, for compositional analyses of a compound semlconductor. The Cd-Te system electrosyntheslzed by a cathodlc route was used as a model to deflne the limits and power of thls analytical tool. Thus changes in the composltlon of the electrodeposited material were monltored as a function of deposition potential. Samples deposited at potentials 2-600 mV (vs SCE) contained excess Te in addition to CdTe and those synthesized at -696 and -698 mV contained both excess Cd and Te. The composition switched abruptly to a large excess of Cd near 700 mV. The excess Cd and Te were detected and quantitated by their characteristic DSC signals at 318 and 447 OC, respectively. Changes in compositional profiles as a function of solution hydrodynamics and TeO, concentration as well as chemical reactlons between the excess Cd and Te are described.
INTRODUCTION Compositional variability and nonstoichiometry are frequent problems associated with the growth of compound semiconductors. For example, formation of T e precipitates has been observed in thin films and single crystals of CdTe and Hg,_,Cd,Te grown by a variety of techniques including vacuum evaporation ( I ) , Bridgman (2, 3), and electrodeposition (4, 7). Tellurium precipitation is deleterious to many optical and optoelectronic device applications of these materials (2). Thus the development of analytical techniques for the compositional analyses of compound semiconductors has technological implications. Table I presents a summary of the state-of-the-science. Many candidates among the techniques listed (e.g. atomic spectroscopies, polarography) yield only the total amount of a given species, e.g. T e in CdTe. On the other hand, techniques such as XRD, TEM, and XPS which are sensitive to the chemical state of the semiconductor component (i.e., free T e vs T e in CdTe) are semiquantitative in nature and are severely limited by analysis and sample factors such as crystallinity (XRD, TEM) and peak resolution (XPS). The Raman microprobe has been effective for the detection of T e precipitates in CdTe crystals (2)and appears to be a technique of much promise, although its quantitative capabilities remain to be established. There is clearly a need for identifying and evaluating new techniques for the compositional analyses of compound semiconductors. This paper explores the applicability in this regard of thermal analysis techniques such as differential scanning calorimetry (DSC). Since its discovery in 1964, DSC has been successfully applied to the analysis of a wide variety of materials; reviews of these applications are available (18-20). The predecessor of this technique, namely differential thermal analysis (DTA),has also played a pivotal role in the elucidation of phase diagrams (including many involving components of importance t o semiconductor technology, cf. refs 21 and 22).
* Author for Correspondence
Reisman has described how DTA could be used to study the thermal synthesis of CdSe and other group 11-VI counterparts from the component elements (23). T o our knowledge, the utility of thermal analysis techniques such as DSC for semiconductor compositional analyses has not been previously explored. In this paper, we will describe the use of DSC as a semiconductor analysis tool with electrosynthesized CdTe as a model system. The electrosynthesis of CdTe was carried out via a cathodic route (24, 25).
EXPERIMENTAL SECTION Differential scanning calorimetry was performed on a Du Pont 9900 thermal analysis system fitted with the Model 910 accessory module. The software supplied by the manufacturer was used for the analyses of DSC thermograms. Commercial samples of In and Zn (99.999% purity) were used as calibration standards. The melting transitions of In and Zn at 156.6 and 419.5 “C have enthalpies of 28.4 and 109 J/g, respectively (26). The melting endotherms were usually recorded after one or two initial “conditioning”heat-cool cycles through the transition. Nitrogen was flushed through the DSC cell at the rate of ca. 80 mL/min. Sealed Al sample pans were used in all the cases. Nominal sample mass was ca. 1 mg. A heating rate of 15 “C/min was employed. A standard three-electrode configurationwas employed for both stationary and hydrodynamic deposition of the CdTe thin films. The voltammetric data were displayed on a Houston Instrument Model 2000 X-Y recorder. The rotating disk electrode (RDE) assembly consisted of a Model RDE-4 Pine Instrument bipotentiostat and an AFMSRX rotator spun at a rate of 400 rpm. An EG&G Princeton Applied Research Model 273 potentiostat was used for the stationary deposition mode. A Pt disk electrode (1.1cm diameter) was used as the substrate for hydrodynamic deposition experiments. Its surface was pretreated using literature procedures (27). Titanium foil (Alfa) was used as the substrate for the stationary deposition experiments. Its surface was initially sanded on a 320-grit Carbimet disk (Buehler), subsequently polished with A1,03 and finally rinsed with distilled deionized water. The immersed foil area was ca. 6 cm2, and the amount of electrodeposited material on the foil was measured coulometrially (10-25 C). The electrodeposited material was scraped off from the substrate and then dried in an air oven at ca. 90 “C for 8 h to remove moisture. Electrodeposition of CdTe, unless otherwise noted, was carried out at 25 “C from a 0.5 M H,S04 matrix containing 0.5 M of CdS04 (Alfa)and 5.0 X 10”’ M of TeO, (Alfa, Puratronic Grade). All potentials herein are quoted with respect to a saturated calomel electrode (SCE) reference. RESULTS AND DISCUSSION Electrodeposition Chemistry. As discussed elsewhere (28), Pourbaix diagrams identify the major T e species a t pH 5 -2 to be HTeO,+. The important electrochemical and chemical reactions pertinent to this study are Cd2+ + 2e- = CdO; El0= -0.64 V (1) H T e 0 2 + + 3H+
+ 4e- = T e o + 2H20;Ez0 = 0.31 V
Cdo + Teo = CdTe; AGO = -9.97
X
lo4 J / m o l
(2)
(3)
As discussed by Kroger (25),the negative free energy of compound formation effectively shifts the reduction of Cd2+ 0003-2700/90/0362-0821$02.50/0 @ 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 8, APRIL 15, 1990
Table I. A Summary of Analytical Techniques Available for Compositional Analyses of Compound Semiconductors technique Auger electron spectroscopy (AES) X-ray photoelectron spectroscopy (XPS, ESCA) electron probe for microanalysis (EPMA) energy dispersive analysis of X-rays (EDAX) low-energy electron-induced X-ray spectrometry (LEEIXS) X-ray diffraction (XRD) transmission electron microscopy (TEM) neutron activation analysis atomic spectroscopics particle-induced X-ray emission polarography Raman microprobe a
refs"
comments
suffers from matrix effects; elemental oxidation state not readily available spectral resolution poor
5-8 34
requires rather thick (1 Fm) samples to avoid substrate interference; yields only total assays 6, 9, 10 matrix effects rather severe; yields only total species content
11,12
elemental oxidation state not available
13
films must be crystalline similar problems as above
4, 5, 7
yields only total species content same handicap as above yields only total species content; matrix effects yields only total species content able to discriminate species chemical state; quantitation capabilities as yet undefined
8 8 8
-
14-17 2
These e x a m d e studies have lareelv been drawn from literature on the CdTe svstem.
-
ions to potential regimes positive of the "free" Cd2+ Cdo reduction (reaction 1). Thus the underpotential deposition of Cd2+as CdTe may be represented by reaction 4. Cd2+ + TeO
+ 2e- = CdTe; E,"
= -0.10 V
(4)
The above thermodynamic expectations have been verified by experiment both by us (6, 10, 12, 28, 29) and by other authors (24,25),and CdTe formation has been detected by XRD (4,5, 7), electron spectroscopies (5-8), X-ray fluorescence (EDAX) (11, 121, (EPMA) (6,9, IO),and cyclic photovoltammetry (30). Panicker et al. (24) characterized the film composition in terms of their rest potentials, Erest, in the deposition bath. Thus films with Erest5-0.66 V contained almost exclusively CdO, those with E,,,, = --0.11 V were pure TeO, samples wtih E,,,, I-4.30 V consisted of n-type CdTe (Cd rich), and those with ErestI --0.30 V were comprised of p-type CdTe (Te rich). Such gradation of composition through the deposition potential range (see below) has subsequently been verified by other authors ( 4 , 31). Figure 1 contains a representative cyclic voltammogram for a T i electrode in contact with the electrodeposition bath containing Cd2+and HTe02+ions. On the basis of our previous work (28,29),we assign the initial wave at -0.30 V (which culminates in a plateau) to reaction 2. The discontinuity in this wave at --0.50 V is attributable to the underpotential assimilation of Cd2+ions into the preformed Te layer as CdTe (reaction 4). Finally, the sharp cathodic feature, the nucleation loop, and the anodic stripping wave on the return scan - 4 . 7 0 V originate from reaction 1. Note that reactions 2 and 4 are observed at potentials negative to the thermodynamically predicted values underlining the kinetics limitations associated with them. On the other hand, reaction 1 appears to be facile and occurs closer to equilibrium. Similar arguments may be advanced to account for the discrepancy between our observations from scanning experiments and the data from the rest potential measurements of Panicker et al. (24). To evaluate the utility of DSC as a compositional analysis tool, a series of samples were electrodeposited a t potentials ranging from 4 . 2 0 to - 4 . 7 5 V, and then analyzed by DSC. To define the limiting situations and for quantitative analyses, these data were also compared with those obtained for pure Te, Cd, and CdTe as reference standards. While the first two samples were electrodeposited from baths wherein Cd2+and T e 0 2 were omitted respectively, an authentic commercial sample of CdTe (in powder form) was used as the standard for DSC runs which are discussed next. DSC Thermograms of Cd, Te, and CdTe. In order that DSC proves to be efficacious for the detection and quantitation
t
T
O
'rA
00 00
-200
-400
POTENTIAL (mV, vs SCE)
Figure 1. Cyclic voltammogram (potential scan rate, 0.01 V s-') at a Ti foil electrode in 0.5 M H,S04 containing 5 X lo4 M TeO, and 0.5 M CdSO,.
of these three species, the analytical signals (i.e. endotherms) originating from them must be preferably located at distinctly different temperature regimes. A rough analogy here is the analytical situation involving the spectroscopic analysis of a mixture using analytical peaks of the components a t characteristic wavelengths. Parts a and b of Figure 2 contain DSC traces for Cd and Te, respectively. The melting transitions of Cd and Te are clearly well separated. The transition temperatures of 318 and 447 "C for Cd and Te (with an associated estimation uncertainty of f l "C) are in reasonable agreement with previously published (23)DTA values of 321 and 450 "C, respectively. By use of Zn as calibration standard, fusion enthalpies of 59 and 140 J / g were determined for Cd and Te, respectively. These may be compared with previously quoted values of 58.4 and 137.4 J / g (32). A DSC scan of pure CdTe revealed no features up to 500 "C-an expected behavior since this compound melts at 1041 "C (32),and no solid-state (crystallographic) transformations are known in the range from 25 to -500 OC. Thus, in the Cd-Te system, the expectation is that excess Cd and Te may be detected by the presence of characteristic DSC features at 318 and 447 "C. A quantitative estimate of these components is available from the areas encompassed by the observed DSC peaks, and the (known) fusion enthalpy of the pure component (cf. Figure 2a,b).
ANALYTICAL CHEMISTRY, VOL. 62, NO. 8, APRIL 15, 1990
'
823
~ T 2w/g
T
2Wlg I
L
EX0 EX0
t
(b)
c
3
0
END0
i i
G 8 0)
iii0)
I
I
300
340
380
Temperature,
420
I
"C
Figure 2. DSC thermograms (heating rate, 15 'C/min) for electrodeposited samples of (a) Te, (b) Cd, and (c) a mixture of Cd and Te in 5050 weight ratio.
To assess the quantitative efficacy of DSC for excess Te and Cd determinations, a series of authentic mixtures of CdTe with Te and Cd were respectively prepared and then analyzed by this technique. From these tests, we estimate that 5% excess (by weight) of Te may be quantitated with a nominal error of *5%, 2% excess of this species at &7% and 1% detected with an uncertainty of *ll%.Similar performance levels were observed for Cd. The fact that the detection uncertainties are comparable in the two cases in spite of the rather large difference in the magnitude of the transition enthalpies (see above) suggests that a major portion of the analytical error resides with weighing and sample preparation. At this stage of the work we have not further optimized the variables in the quantitation procedure (e.g. sample mass) such that the sensitivity quoted above should be regarded as conservative; i.e. further improvements ought to be possible. The data in Figure 2 on the Cd-Te system underline the sensitivity of DSC to the chemical state of a particular species, i.e. both Cd and T e manifest distinctly different signals depending on whether they are present in the elemental state or as a compound (CdTe). This is a feature that only four (XRD, TEM, XPS, Raman microprobe) of the many techniques listed in Table I share. (Auger electron spectra are sensitive in many instances to the chemical bonding situation, for e.g. Si vs SiO,; cf. ref 33. However, the consequent alterations in the line shape profiles are subtle and not readily amenable to quantitative analyses.) Techniques such as XRD and TEM will not work if the samples are highly amorphous. In XPS, the shifts in the binding energies with oxidation state are certainly not very large (for example, 572.7 eV for Te in the elemental state and 572.1 eV for the -2 oxidation state, cf. ref 34), and quantitation requires peak deconvolution
I
340
380
Temperature,
420
'
'
0
"C
Figure 3. DSC thermograms (heating rate, 15 'C/min) for electrodeposited films on Ti substrate containing Te, Cd, and CdTe. The deposition potential (in mV) was (a) -600, (b) -696, (c) -698, and (d)
-700.
procedures. Methods based on optical spectroscopies (such as Raman scattering) do have the required molecular specificity for qualitative identification. As mentioned before, however, the quantitative capabilities of the Raman microprobe remain to be established. It must also be recognized that while complete resolution of the analytical signals due to component species (the case with the model Cd-Te system here, cf. Figure 2) does prove to be an analytical convenience, it is conceivable that thermal signals could be overlapped in other systems. Even in these cases, DSC could be effectively used for compositional analyses, for example by appropriate control of variables such as purge gas composition, temperature scan rate, etc. (35). A further analogy may be drawn here with the matrix solution techniques employed for the analysis of multicomponent mixtures by optical spectroscopics. Again, a t this (relatively infant) stage of development of the semiconductor analysis protocol by DSC, we have chosen not to consider such complications. DSC Analyses of Electrodeposited Samples. Figure 3 contains DSC thermograms for samples electrodeposited a t four (increasingly negative) potentials starting at -600 mV. Comparison with Figure 2 reveals that the sample electrodeposited at -600 mV contains excess Te (in addition to CdTe), those electrosynthesized at -696 and -698 mV contain both Cd and Te in the (free) elemental state, and the sample formed at -700 mV contains a large excess of Cd. Further quantitative estimates by DSC are contained in Figure 4 which
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 8, APRIL 15, 1990
'oor===-i
100
90
EO
t
70 60
s
#
Te
h
v
2
50 40
I
30
-200
20
,\
10
I
-600
-680
-690 -692
I
-400
I
-500
I
-600
DEPOSITION POTENTIAL ( mV, Figure 5.
I
-700
-I
0
SCE)
Same as Figure 4 but on a Pt rotating disk cathode
.
0
-500
I
-300
-700
DEPOSITION POTENTIAL (mV, vs SCE) Effect of deposition potential on the electrodeosit composition. The material was electrodeposited on Ti substrate from a stationary bath. The CdTe content was determined by difference. Figure 4.
also includes data on other specimens not shown in Figure 3. It is quite remarkable how the composition of the electrodeposited material switches dramatically over a potential range of 4-5 mV near -700 mV! While models had previously been advanced which predicted such abrupt changes in composition for CdTe (36), experimental verification was hitherto not available. Influence of T e Concentration a n d Solution Hydrodynamics on Electrodeposit Composition. The Kroger model (24,25)emphasizes the need to maintain a low (I-lo4 M)concentration of the Te species and a high concentration of Cd2+ ions in the electrolyte in order to deposit equal amounts of Cd and Te. The interfacial concentration of HTe02+thus is maintained close to zero during the deposition process. The influence of stirring rate on the current density was briefly studied by Panicker et al. (24),although no corresponding film analyses were made. We thought it would be of interest to examine the sensitivity of the electrodeposit composition to solution hydrodynamics and TeO, concentration. To do this, the data described in the preceding section were compared with the results from experiments employing a 2-fold increase in the T e 0 2 concentration and a rotating Pt cathode. The results from DSC analyses of such samples are contained in Figure 5 . Contrary to the previous case (cf. Figure 4),the electrodeposit composition changes from 100% Te to 80% excess Cd within a range of -50 mV near -700 mV. In other words, there is negligible formation of CdTe when the solution is stirred and a relatively high concentration of TeOz is employed! The electrodeposition of Teo is enhanced by 1-2 orders magnitude by a combination of solution stirring and the high T e 0 2 concentration. However, the underpotential deposition of Cd as CdTe (reaction 4) is kinetically sluggish and cannot effectively compete under these conditions. When potentials close to where free CdO deposition is thermodynamically possible (reaction 1) are accessed, this kinetically
facile reaction (see above) takes over and the composition abruptly switches to a large excess of Cd. The dramatic difference in the compositional profiles in Figures 4 and 5 provides rather conclusive experimental verification for the essential correctness of the Kroger model (24, 25). While substrate differences (Ti in the stationary deposition case vs Pt in the experiment described in the preceding paragraphs) could possibly also play a role, we certainly do not believe it to be the overriding factor. Chemical Reaction between Free Cd and Te. A possible complication that must be considered is chemical reaction between the excess component species in the free (elemental) state; i.e. in the model system considered here, it is conceivable that excess Cd could react with excess Te to yield CdTe in situ. The DSC thermogram in Figure 2c presents such a situation wherein a mixture of Cd and T e was utilized as the starting material. A chemical reaction between Cd and Te is signaled by an exothermic signal in the vicinity of the Te fusion. The heat released in the chemical reaction (reaction 3) swamps the endothermic effect from T e fusion. Thus a closer examination of the Te DSC signal reveals a complex shape with an endothermic tendency preceding the exothermic peak and a further endothermic deflection at the trailing edge of the exotherm. Apparently the chemical reaction between Cd and T e is promoted by Te fusion. A similar exotherm a t 429 "C (heating rate, 2.5 "C/min) was noted by Reisman and Berkenblit in their DTA study (23). Reheat cycles of DSC scans similar to those contained in Figure 2c reveal attenuation of the signals from the free Cd and Te resulting from their conversion to CdTe. In fact, quantitative analyses of such trends permit elucidation of the chemical reaction kinetics between Cd and Te as a function of time and temperature. Such experiments are in progress and will be described in a future report. It is interesting to note the absence of exothermic effects (due to CdTe formation) under the experimental conditions pertaining to Figure 3b,c. Particularly noteworthy is the situation in Figure 3c, wherein although both Cd and Te are present in noticeable amounts in the elemental state, the topochemistry is presumably such that in situ formation of CdTe is hindered. We believe that the CdTe, which is also present as the third component in this case, precludes effective contact of the Cd and Te phases. Finally, many chalcogens such as Te are known to undergo amorphous to cyrstalline transitions. For example, Okuyama
ANALYTICAL CHEMISTRY, VOL. 62, NO. 8, APRIL 15, 1990
and Kumagai (37) noted a "rapid and drastic" decrease in electrical resistivity of vacuum-evaporated Te thin films on a glass substrate at temperatures ranging from 10 to 40 "C-an effect that these authors attributed to crystallization of the amorphous samples. Amorphous-to-glass and glass-to-crystalline transitions have been observed by DTA a t -50 and -110 "c for Se prior to melting at ca. 220 O C (38-40). Exothermic effects from crystallization of T e were not noted in the DSC scans for any of our T e samples. It is thus likely that crystallization occurred in our samples close to ambient temperatures. By the same token, the occurrence of crystallization concomitant with fusion (an effect that would introduce significant error into the quantitation of Te, see above) is also considered extremely unlikely. Even for electrodeposited CdTe, annealing a t 200 "C appears to be sufficient to induce crystallinity as monitored by XRD (41).
ACKNOWLEDGMENT Preliminary experiments were conducted by Ranjana Segal. We also thank the Du Pont Co. for the donation of some of the thermal analysis equipment used in this work. LITERATURE CITED (1) Fujii, M.; Kawai, T. Sol. Energy Mater. 1988, 78, 23-35. (2) Shin, S. H.; Bajaj, J.; Moudy, L. A.; Cheung, D. T. Appl. Phys. Lett. 1983, 4 3 , 68-70. ( 3 ) Wada, M.; Suzuki, J. Jpn. J . Appl. Phys. 1988, 2 7 , L972-975. (4) Takahashi. M.; Uosaki, K.; Kita, H. J . Appl. Phys. 1984, 55, 3879-388 1. (5) Takahashi. M.; Uosaki, K.; Kita, H.; Suzuki, Y. J . Appl. Phys. 1985, 58, 4292-4295. (6) Bhattacharya, R N.; Rajeshwar, K. J . Appl. Phys 1985, 58, 3590-3593. (7) Llabr6s. J.; Deimas, V. J . Electrochem. Soc. 1986, 733, 2580-2585. (8) Lyons, L. E.; Morris, G. C.; Horton, D. H.; Keyes, J. G. J . Electroanal. Chem. Interfacial Electrochem. 1984, 768, 101-116. (9) Shih, I.; Qui, C.-X. Mater. Lett. 1985, 3 , 446-448. (10) Bhattacharya, R. N.; Rajeshwar, K.; Noufi, R. N. J . Electrochem. SOC. 1985, 132, 732-734. (11) Fatas, E.; Herrasti, P.;Arjona, F.; Camarero, E. G.; Medina, J. A. Electrochlm. Acta 1987, 3 2 , 139-148. (12) Bhattacharya, R. N.; Rajeshwar, K.; Noufi, R. N. J . Electrochem. SOC.
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RECEIVED for review November 17, 1989. Accepted January 16, 1990. This work was supported in part by the National Science Foundation (Grant MSM-86-17850).
