J . Phys. Chem. 1994, 98, 5338-5346
5338
Quantum Size Effects in the Study of Chemical Solution Deposition Mechanisms of Semiconductor Films Sasha Corer and Gary Hodes' Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Received: November 4, 1993; In Final Form: March 3, 1994"
Chemical solution deposited (CD) CdSe films possess a nanocrystalline structure and exhibit quantum size effects due to the small crystal size. This results in a blue shift of the optical spectra. It has been observed that there is a certain critical ratio between the complexing agent (nitrilotriacetate) and Cd concentrations used in preparing the films, denoted as &, above which there is a pronounced red shift of the optical spectra of the films. Using X-ray diffraction and electron microscopy, this red shift was correlated with an increase in crystal size. This sharp change suggested a changeover in the C D mechanism. Optical absorption spectra and laser scattering measurements of the deposition solutions in the absence of selenosulfate showed that Cd(OH)2 was present in solutions below R, (often only after an induction period during which the solution pH increased), but not above R,, although a visible Cd(OH)2 suspension was not apparent under normal deposition conditions, even below &. X-ray photoelectron spectroscopy indicated the presence of adsorbed colloidal Cd(OH)2 on the glass substrates only under conditions where Cd(OH)2 was also present in solution. It is proposed that, below R,,the C D mechanism is initiated on the Cd(OH)2 colloidal particles adsorbed on the substrate while, above R,, deposition occurs directly on the substrate by initial CdSe formation, without any mediation by Cd(OH)2. The change in crystal size at R, is explained by the change in mechanism. Similar behavior was obtained for CdS and PbSe, showing the generality of the conclusions.
Introduction Chemical solution deposition (CD) has been used for many decades to deposit films of chalcogenide semiconductors. In earlier years, emphasis was placed on PbS and PbSe for use as photoconductors with a gradual shift to CdS (mainly) and CdSe. Chopra et al.1 have reviewed the field up to the early 1980s. Recently, several groups have reported heterojunction photovoltaic cells based on CdTe or CuInSe2 with CD CdS as the window layer in both case^.^^^ The fact that these cells represent a significant improvement over previous cells using (normally) evaporated CdS has stimulated renewed interest in the CD method. One of the central issues in CD is whether the deposition proceeds by an ion-by-ion growth on the substrate (Le., growth by successive anion and cation adsorption on the growing crystal) or by a cluster or colloidal mechanism whereby colloids of the metal chalcogenide (or hydroxide) adsorb on the substrate and coagulate to form the film. Betenekov et al.*used a radiochemical technique based on radioactive Cd to show that Cd(0H)z was adsorbed on the glass substrates in their solutions for CdS deposition, and that CdS formation occurred by adsorption of thiourea on the Cd(OH)2 particles followed by decomposition of the Cd(OH)2-thiourea complex to CdS. This group also noted that, although CdS films could be formed under conditions where no Cd(OH)2 was present, they were much less adherent to the substrate compared to the films formed in the presence of hydroxide.5 Kaur et a1.6 compared the growth of CdS films from solutions containing Cd(OH)2 in the form of a visible suspension and from optically clear solutions. In general, the former gave adherent, specularly reflecting films while the latter gave poorly adherent, diffusely reflecting films. These results were explained by a cluster-by-clustergrowth from colloids in solution for the diffusely reflecting films and an ion-by-ion growth for the specularly reflecting films. The latter conclusion, which seems contrary to the conclusions of Betenekov et a1.,4 was based on two observae Abstract
published in Aduance ACS Abstracts, April 1, 1994.
0022-3654/94/2098-5338s04.50~0 0
tions: (1) that epitaxial growth could occur under certain conditions on single crystal substrates and (2) that a clusterby-cluster growth would not be expected to give adherent, specularly reflecting films. We have shown that adherent, specularly reflecting films of CdSe can be deposited with a granular structure.' This structure, together with the fact that the crystal size in the films is identical to that formed homogeneously in the deposition solution, strongly suggests a clustertype growth. The occurrence of epitaxial growth does present strong evidence for an ion-by-ion mechanism under the conditions of those experiments. In a recent, thorough investigation of the role of Cd(OH)2 in the formation of CdS films, Rieke and Bentjens found that there existed a certain range of conditions where Cd(OH)2 (measured by XPS) formed on Si substrates but not as a visible suspension in solution. Good (Le., adherent, specularly reflecting) films formed only in this range and not outside of it. At low values of pH ( Rc). The negative value of optical density as the wavelength is scanned toward shorter values is presumably due to thedecreased absorptionof the Cd-NTA complex at higher pH and does not concern us here. What is of importance for our purposes is the change in the spectra with pH in the 250-300-nm region where Cd(OH)z absorbs. Once the pH is high enough that the minimum in the spectra at 290 nm does not decrease further in optical density (pH 9.7). the spectrum does not change in this region up to pH 10, but the absorption, presumably of Cd(0H)z. does increase markedly at pH 10.3 and above. The upward-shifted spectrum at pH 11.2 (Figure 6b) is due to the presence of a visible cloudiness in the solution due to Cd(OH)r suspension. The resulting scattering causes a decrease in transmission and an apparent overall increase in optical density. However, the sharp increase in absorption below 300 nm due to Cd(OH)z is obvious here. For the solution where N T A C d > Rc (Figure 6b), there is no sign of Cd(OH)z. In this case, the presence of free NTA (in Figure 6a. most of the NTA was tied up by complexation with Cd) causes the sharp increase in optical density beginning at 290
5342 The Journal of Physical Chemistry. Vol. 98, No. 20. 1994
Gorer and H o d s
90 nm H Fimre 5. TEM image of a thin-film Au substrate immernsd in a deposition solution (NTACd = 1.63 [ Rc show very little Cd. The solution at pH = IO, which has very little Cd on the substrate, nevertheless behaves as a solution < & (from Figure 1 and also from Figure 6, although close to the borderline of & in this case). This apparent discrepancy will be explained later. That the results shown above are generally valid for CD, and notjustspificforCdSe, isseen fromqualitativelysimilarresults forCdSandPbSe. Figure7showstheXRDpatternfortwoCdS films: one deposited at NTACd < R. and the other > &. The crystallite sire of the latter is much larger than the former, even more than for CdSe. The CdS is (at least partly) hexagonal in contrast to the cubic CdSe, particularly below &. Similar results are found for PbSe (Figure 8) although, in this case, R. is less well defined and colloid formation w u n in the depositionsolutionunderallconditionsused by us,althoughmuch less when T S C P b > R. than when the ratio is less than &. For
The Journal of Physical Chemistry, Vol. 98, No. 20, 1994 5343
Chemical Solution Deposition Mechanisms
further and scattering by the Pb-OH suspension can be seen from the overall upward shift of the spectra. For the 2.33 ratio, the presence of a visible suspension is barely visible while, for the 2.17 ratio, it is already clearly visible.
NTACd-1.63 / D-5 M1
Discussion
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26. 2 8 (DEW
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Figure 7. XRD spectra for CdS films deposited at 40 OC with NTA:Cd = 2.25 (>R,) and 1.63 (
