---
abstract: 'We study by photoacoustic spectroscopy the band–gap shift effect of CdS films. The CdS films were grown by chemical bath deposition and exposed to different annealing atmospheres over a range of temperature in which the sample structure is observed to change. We show the band–gap evolution as a function of temperature of thermal annealing and determine the process which produces the best combination of high band–gap energy and low resistivity. It allows us to know a possible procedure to obtain low–resistivity CdS/CdTe solar cells with high–quantum efficiency.'
author:
- |
    S.A. Tomás, O. Vigil$^*$, J.J. Alvarado-Gil, R. Lozada-Morales,\
    O. Zelaya-Angel\
    Departamento de Física\
    H. Vargas$^{\dag}$\
    Programa Multidisciplinario en Ciencias Aplicadas y\
    Tecnología Avanzada.\
    Centro de Investigación y de Estudios Avanzados del IPN\
    Apdo. Postal 14-740, México D.F. 07000, México\
    A. Ferreira da Silva\
    Instituto Nacional de Pesquisas Espaciais-INPE\
    Laboratorio Associado de Sensores e Materiais-LAS\
    CP 515,12201-970 Sao Jose dos Campos, S.P., Brazil
date:  
title: 'Study of the band–gap shift in CdS films: Influence of thermal annealing in different atmospheres.'
---

1em

$^*$ Permanent address: Physics Faculty, University of La Havana, La Havana, Cuba.\
$^{\dag}$ On leave of absence from the Universidade Estadual de Campinas, S.P., Brazil.\
PACS N. 43.85, 62.65, 78.20.D, 78.65, and 81.40.E

24.1pt [Introduction]{}\
The study of heterojunctions and their use in a large variety of technological applications has become one of the principal subjects in solid state research. Among other applications, we mention the use of heterojunctions in transistors and diodes, as well as in other optoelectronic devices. Likewise, it is well known the importance represented by heterojunctions in solar cells development. In particular, CdS/CdTe heterojunctions, represent one of the principal candidates for low cost and high conversion-efficiency solar cells [@Ferekides]. In this type of solar cells, the obtention of CdS films with low resistivity plays an important role because it helps to diminish the dipositive sheet resistance and obtain the space charge region in the active zone, i.e., in the CdTe film.\
It is also known that CdS samples present two different structural phases, namely, the highly stable hexagonal phase and the metastable cubic phase. Even though it is possible to know by means of reflectivity measurements at room–temperature that these two phases have a band–gap energy differing less than 0.1eV, it is not possible, however, to infer any other conclusion about them [@Cardona].\
CdS films present a band–gap shift (BGS) effect when exposed to certain experimental conditions. This effect was reported by Balkanski [*et al.*]{} [@Balkanski] for Si films as a function of temperature. Later on, because of its technological importance in optoelectronic design, many researchers have investigated the BGS of intrinsic and extrinsic semiconductors [@Berggren]-[@Wagner]. Recently, Zelaya–Angel [*et al.*]{} [@Zelaya] have reported the first investigation of BGS as a function of temperature of thermal annealing (TTA) for CdS films. Nevertheless, it is worth saying that such a BGS effect, as well as the growth procedures to obtain CdS films in either of the phases, are not well understood up to now.\
In this work we investigate by photoacoustic spectroscopy (PAS) the band–gap shift effect of CdS films as a function of TTA in different annealing atmospheres. By extension, we show the interesting and technologically important combination of low resistivity CdS/CdTe solar cells with high energy-gap. The annealing atmospheres investigated are Ar, Ar+S$_2$, H$_2$ + In, H$_2$, and air, each of them fixed at certain temperatures within a range from 200$^o$C to 450$^o$C. The purpose of studying it by PAS is that this technique allows us to obtain spectra which clearly show the BGS effect [@Zelaya]-[@Vargas].\
[Experimental.]{}\
The approximately 0.25$\mu$m thickness cadmium sulfide films were deposited by the chemical bath deposition method (CBD). This deposition occurs due to an ionic precipitation of salts dissolved in an aqueous solution. The salts used were CdCl$_2$, K0H, NH$_4$NO$_3$, and CS\[NH$_2$\]$_2$ (thiourea). The solution was kept at (80$\pm$3)$^o$C and continuously stirred to guarantee an homogeneous distribution of the chemical compounds, among other effects [@Kaur].\
The postdeposition thermal treatments given to the samples were performed in a hot wall furnace with a quartz tube. Taking advantage of the furnace temperature profile, CdS samples were simultaneously placed within a range of temperature from 200$^o$C to 450$^o$C. For Ar gas and Ar gas + S$_2$ vapor flux annealings, CdS samples were placed at 208, 260, 353, 398, and 447$^o$C (419$^o$C the last for Ar+S$_2$) during 28h. The S$_2$ vapor flux was obtained by placing a boat containing sulfur at 190$^o$C. For H$_2$ gas, H$_2$ gas + In vapor flux, and air annealings, CdS samples were placed at 200, 250, 300, 350, 400, and 450$^o$C during 1h. In the H$_2$+In case, a boat containing In was placed at 630$^o$C, whereas for the H$_2$ annealing, an H$_2$ flow was kept for 30 min. before placing the samples. The stimated pressure for all the treatments was about 1 atm.\
The photoacoustic absorption spectra of the films were obtained in the region 400–700nm by use of a standard photoacoustic spectrometer. This spectrometer is fitted with a 1000W Xenon lamp (Oriel), whose radiation is focused onto a variable–frequency light chopper set at 17 Hz. The photoacoustic signal is preamplified before providing the input to the signal channel lock–in amplifier. The resultant photoacoustic spectra are recorded in a computer, which simultaneously display the wavelength–dependent signal intensity.\
[Results and discussion.]{}\
In Fig.1 we illustrate the PA spectra corresponding to both the as–grown CdS sample and the whole set of CdS films annealed in Ar+S$_2$ atmosphere. We assume that the value of the band–gap energy is given by the energy which matches the spectrum inflexion point. With this assumption, we observe the BGS effect as a function of TTA. The values of the band–gap energy (BGE) for the samples annealed in Ar and Ar+S$_2$ atmosphere are shown in Table I, whereas the ones for H$_2$, H$_2$+In, and air are shown in Table II. Focusing us on the Ar+S$_2$ annealing we obtain the BGE, as shown in Fig.2. We observe that the BGE starts decreasing its value as a consequence of the thermal annealing. This effect carries down the BGE from its 2.42 eV initial value at 80$^o$C to 2.28 eV at roughly 350$^o$C, where a minimum is reached. Above this temperature the BGE presents an opposite behavior, increasing its value until 2.35eV at 418$^o$C. This narrowing–widening–like behavior presented by CBD/CdS films annealed in Ar+S$_2$ atmosphere has been interpreted as a cubic to hexagonal phase transition, which occurs at the temperature corresponding to the minimum BGE [@Zelaya]. The TTA coefficients of the BGE have also been approximately found in the linear part of the data [@Burger] with values of about $\mp 10^{-4}$ eV/TTA($^oK$), corresponding respectively to the narrowing and widening region. The magnitude of this coefficient agrees generally with those of other reported materials, e.g., Si, Ge, GaAs, and MgI$_2$ [@Burger],[@Sze].\
With regard to the other annealing atmospheres, we show in Tables I and II that all of them start with the same behavior as the Ar+S$_2$ case, that is, the samples present a narrowing–like behavior. Some of these annealings decrease the BGS faster of slower and reach a higher or lower minimum value than Ar+S$_2$. Nevertheless, it is very important to point out that none presents a clear widening–like behavior as does Ar+S$_2$. On the contrary, all of them show a stable tendency above the temperature corresponding to the minimum BGE. These results are illustrated in Fig.3 where the BGS, given by

$$\Delta Eg(T) = Eg(80^oC) - Eg(T),$$

is plotted versus the TTA. Here, $Eg(80^o$C) is the band–gap energy of the as–grown samples. We notice that, among the investigated annealings, the highest $\Delta Eg$ values are produced by the air, the maximum $\Delta Eg$ = 0.17 eV taking place at roughly 350$^o$C. Moreover, only the Ar+S$_2$ annealing produces a considerable diminution of the BGE after having reached its maximum value.\
Owing to our interest in finding the thermal process which produces CBD/CdS films with the best combination of high band-gap energy and low resistivity, we deal now with the sample electrical resistivity as a function of TTA. As all the annealing atmospheres produce similar resistivity effects in the samples, we take advantage of it and describe qualitatively their general behavior. Electrical measurements were performed by the two probes method for high resistivity films and the four probes method for low resistivity films at room temperature. The four probes technique was not satisfactory for high resistivity films because of the same order of magnitude of the film resistance and the electrometer input impedance. Resistivity of the as–grown samples was measured to be $\rho = 2.3\times 10^7 \Omega$ cm. Subsequently, as the annealings were performed, the resistivity decreased with increasing TTA until reaching a minimum value, then increasing again. As can be seen in Table III, $\rho(T)$ takes different minimum values for each different annealing atmosphere.\
The different minimum values of $\rho$ are analysed below. The Ar+S$_2$ annealing diminishes the resistivity by increasing the grain size, therefore decreasing the number of grain boundaries in the films. On the other hand, the Ar annealing, besides of in creasing the grain size, originates a high amount of sulfur vacancies as a consequence of non-equilibrium conditions. In the air annealing, the presence of oxygen creates both CdO and SO$_4$Cd layers on the surface, increasing slightly the resistivity of the films respect to the Ar case [@Kolhe]. The thermal treatments in H$_2$ and H$_2$+In present the advantage of containing H$_2$, which is a strong agent for grain boundary passivation by oxygen chemiabsorption [@Melo],[@Chopra], decreasing the resistivity by 4 and 5 or ders of maagnitude, respectively, respect to the former cases.\
Now, in order to consider simultaneously the band–gap energy and the resistivity, we define

$$\xi(T) = \frac{\Delta\rho (T)}{\Delta Eg(T)} \ \ = \ \ \frac{\rho (80^oC)-
\rho(T)}{Eg(80^oC)-Eg(T)} \quad .$$

\
The above parameter contains the information we are interested in. In effect, high band–gap energy $Eg(T)$ is equivalent to low $\Delta Eg(T)$, and similarly, low resistivity $\rho(T)$ implies high $\Delta\rho(T)$. Thus, the higher $\xi(T)$ value, the better combination of high $Eg(T)$ and low $\rho(T)$. According with it, we have found a procedure to find out the TTA which produces samples with most favorable characteristics. We should observe that $\xi(T)$ is basically governed by $\Delta\rho(T)$ because of its high values compared to the $\Delta Eg(T)$ ones. It gives us an idea of what TTA should be in advance investigated, namely, those with high $\Delta\rho(T)$ values.\
We have calculated $\xi(T)$ for all the annealing atmospheres. With their highest values, which are written in Table IV, we have plotted the figure of merit shown in Fig.4. This figure allows us to know that, among the analysed annealings, the best effects are produced by H$_2$ and H$_2$+In atmospheres. In particular, H$_2$+In annealing is lightly better than H$_2$ annealing, its best value $\xi= 4.6 \times 10^6 \Omega$ cm/eV taking place at 250$^o$C. It is worth to point out that the measured resistivity of this sample is $\rho= 5 \times 10^{-2} \Omega$ cm. A similar value has been reported [@Toyotomi] but for the crystal grown by the melt, using the Tamman method in Ar gas of about 110 atm pressure. Moreover, for the Ar+S$_2$ atmosphere, the minimum resistivity is $\rho= 1.9 \times 10^3 \Omega$ cm. This resistivity is roughly reproduced $(2.3 \times 10^3 \Omega$ cm) for a normal CdS crystal but at 4.2$^oK$[@Toyotomi].\
The X-Ray diffraction patterns show that all the annealed samples present a light interplanar–distance increase compared to the as–grown samples, but only the samples exposed to Ar and Ar+S$_2$ annealings undergo a cubic to hexagonal phase transition. On the contrary, the samples annealed in H$_2$, H$_2$+In and air atmospheres preserve the same structural phase along the complete thermal process [@Melo]. The Mechanism that explains this microestructural behavior is under study at present.\
Photoluminiscence spectra of CdS samples annealed in Ar and Ar+S$_2$ atmospheres present a shift of the luminescent peaks as a function of TTA. This shift has been interpreted as an evidence of evolution from S$_2$ vacancies to interstitial S$_2$ formation. For these two annealings, where a cubic to hexagonal phase transition is completed, interstitial S$_2$ becomes part of the hexagonal structure itself. PL spectra of CBD/CdS samples annealed in H$_2$, H$_2$+In, and air atmospheres has not been performed up to now.\
In summary we have found that CBD/CdS films present optimum conditions of low resistivity and high band–gap energy when exposed to H$_2$+In annealing atmosphere at 250$^o$C. This result allows us to know a possible procedure to obtain low resistivity CdS/CdTe solar cells with high quantum efficiency, therefore a real candidate for high conversion-efficiency solar cells.\
[Acknowledgements]{}\
This work was partially supported by the Mexican agency CONACYT. H. V. and A. F. S. acknowledge the Brazilian agency CNPq/MCT for financial support. A. F. S. acknowledges CINVESTAV-IPN for its hospitality.

[99]{} J. Britt and C. Ferekides, Appl. Phys. Lett. [**62**]{}, 2851 (1993) M. Cardona, M. Weinstein, and G. A. Wolff, Phys. Rev. [**140**]{}, A633 (1965) M. Balkanski, A. Aziza and E. Amzallag, Phys. Status Solidi [**31**]{}, 323 (1969) K.-F. Berggren and B.E. Sernelius, Phys. Rev. B[**24**]{}, 1971 (1981) W.P. Dumke, Appl. Phys. Lett. [**42**]{}, 196 (1983) K.E. Newman, A. Lastras-Martínez, B. Kramer, S.A. Barnett, M.A. Ray, J.D. Dow, J.E. Green, and P.M. Raccach, Phys. Rev. Lett. [**50**]{}, 1466 (1983) B.E. Sernelius, K.-F. Berggren, Z.C. Jin, I. Hamberg, and C.G. Granquist, Phys. Rev. B[**37**]{}, 10244 (1988) J. Wagner and J.A. del Alamo, J. Appl. Phys. [**63**]{}, 425 (1988) O. Zelaya–Angel, J.J. Alvarado-Gil, R. Lozada-Morales, H. Vargas, and A. Ferreira da Silva, Appl. Phys. Lett [**64**]{}, 291 (1994) L. Eaves, H. Vargas, and P.J. Williams, Appl. Phys. Lett. [**38**]{}, 768 (1981) A. Mandelis, [*Photoacoustic and Thermal Wave Phenomena in Semiconductores*]{}, (North Holland, New York, 1987) H. Vargas and L.C.M. Miranda, Phys. Rep. [**161**]{}, 43 (1988) I. Kaur, D.K. Pandya, and K.L. Chopra, J. Electrochem. Soc. [**127**]{}, 943 (1980) A. Burger and D. Nason, J. Appl. Phys. [**71**]{}, 2717 (1992) S.M. Sze, [*Physics of Semiconductor Devices*]{}, (New York, John Wiley, 1981), Pg. 15 S. Kolhe, S.K. Kulkarni, A.S. Nigavekar, and S.K. Sharma, Sol. Energy Mat. [**10**]{}, 47 (1984) O. de Melo, L. Hernández, O. Zelaya-Angel, R. Lozada-Morales, M. Becerril, and E. Vasco, Appl. Phys. Lett [**65**]{}, 1278 (1994) K.L. Chopra, R.C. Kaaintla, D.K. Pandya, and A.P. Thakoor, Phys. Thin Films [**12**]{}, 167 (1987) S. Toyotomi and K. Morigaki, J. Phys. Soc. Jpn. [**25**]{}, 807 (1968)

[TABLE I]{}. Values of the band–gap energy, $E_g$, and band–gap shift, $\Delta E_g$, as a function of temperature of thermal annealing for Ar and Ar+S$_2$ annealings.

  --------------- ---------- ----------------- ---------- -----------------
                                                          
                   $Eg$(eV)   $\Delta Eg$(eV)   $Eg$(eV)   $\Delta Eg$(eV)
   80 (as–grown)     2.42            –            2.42            –
        208          2.40          0.02           2.40          0.02
        260          2.36          0.06           2.37          0.05
        296          2.35          0.07           2.29          0.13
        353          2.30          0.12           2.28          0.14
        398          2.29          0.13           2.33          0.09
        418                                       2.34          0.08
        447          2.31          0.11                   
  --------------- ---------- ----------------- ---------- -----------------

[TABLE II.]{} Values of the band–gap energy, $E_g$, and band–gap shift, $\Delta E_g$, as a function of temperature of thermal annealing for H$_2$, H$_2$+In and air annealings.

  -------------- ---------- ----------------- ---------- ----------------- ---------- ---------
                                                                                      

                  $Eg$(eV)   $\Delta Eg$(eV)   $Eg$(eV)   $\Delta Eg$(eV)   $Eg$(eV)   $\Delta
                                                                                       Eg$(eV)

   80(as–grown)     2.42            –            2.42            –            2.42        –

       200          2.40          0.02           2.40          0.02           2.37      0.05

       250          2.39          0.03           2.37          0.05           2.34      0.08

       300          2.37          0.05           2.34          0.08           2.27      0.15

       350          2.31          0.11           2.28          0.14           2.25      0.17

       400          2.27          0.15           2.27          0.15           2.26      0.16

       450          2.27          0.15           2.28          0.14           2.26      0.16
  -------------- ---------- ----------------- ---------- ----------------- ---------- ---------

[TABLE III.]{} Resistivity minimum values and corresponding $E_g$ as a function of temperature in different annealing atmospheres.

  ----------- ------------------ ------- -------------
   annealing    minimum $\rho$    $E_g$   temperature
                ($\Omega$ cm)     (eV)      ($^o$C)
      Ar       2.5$\times 10^2$   2.30        353
   Ar+S$_2$    1.9$\times 10^3$   2.29        296
     H$_2$           0.15         2.37        300
   H$_2$+In          0.05         2.37        250
      air      1.0$\times 10^3$   2.27        300
  ----------- ------------------ ------- -------------

[TABLE IV.]{} $\xi$(T) highest values in different annealing atmospheres.

  ----------- ----------------------- -------------
   annealing       highest $\xi$       temperature
               (10$^8 \Omega$ cm/eV)     ($^o$C)
      air              1.53                300
   Ar+S$_2$            1.76                296
      Ar               2.09                447
     H$_2$             4.59                300
   H$_2$+In            4.60                250
  ----------- ----------------------- -------------

[Figure Captions.]{}\

1.  PA Normal Intensity of CdS films annealed in Ar+S$_2$ atmosphere for different TTA as a function of energy.

2.  Band–gap energy dependence on TTA for CdS annealed in Ar+S$_2$ atmosphere.

3.  Band–gap shift dependence on TTA for CdS annealed in different atmospheres.

4.  $\xi(T)$ highest value for each different annealing atmosphere.
