Chalcopyrite CuGaJn, -se2 semiconducting thin films produced by radio frequency sputtering

Transcription

1 Chalcopyrite CuGaJn, -se2 semiconucting thin films prouce by raio frequency sputtering J. L. HernAnez-Rojas, M. L. Lucfa, I. M&N, J. Santamaria, G. GonzAlez-Dfaaz, an F. Sgnchez-Quesaa Departamento e Electricia y Electrbnica, Faculta e Ciencias Fisicas, Umiversia Complutense, Mari, Spain (Receive 1 November 1991; accepte for publication 0 January 1992) CuGa,In, _ gez thin films have been eposite by rf sputtering from three targets with ifferent (Ga,In) content (x = 0.25,,Y = 0.5, an x = 0.75). A structural, compositional, optical, an electrical stuy has been carrie out for films grown at substrates temperatures higher than 50 C!. We have successfully obtaine chalcopyrite single phase stoichiometric films. Very sharp absorption eges are obtaine, with ban gaps of 1.12, 1.5, an 1.51 ev for x = 0, x = 0.5, an x = 0.75, respectively. P I-III-VI, chalcopyrite compoun semiconuctors have a great potential in photovoltaic evices, both in single heterojunction an in tanem structures. We have successfully prouce thin films of single phase chalcopyrite CuInSez (Refs. 2,) an CuGaSe, (Refs. 4,5) by raio frequency (if) sputtering. In this letter, we present first results on the preparation an characterization of rf sputtere CuGa,Ini --Sez. This compoun has been alreay grown by other techniques,6 but, to our knowlege, no reports on sputtere CuGa,Inr _ $e, have been publishe. The variation of the x value will allow us to tailor the properties of the films in orer to obtain high efficiency photovoltaic evices. Target preparation was one as follows: We synthesize 2% Se-rich CuGa,In~, --Se, of three ifferent Ga content (x = 0.25, 0.50, an 0.75) by a metho similar to the one alreay use for the preparation of CuInSez an CuGaSe, targets. 4 After the synthesis, we reuce the resulting ingots to power of grain size lower than 40,um an analyze it by means of an x-ray iffractometer. The power iffractometer ata for the three ifferent compositions (Table I) show, in all cases, single phase chalcopyrite structure. Then we col presse the power in the form of 2 iam pellets. We grew CuGa,In, -,Se, thin films from a commercial rf sputtering system (GCA Vacuum Inustries) on A120 an quartz substrates with an Ar pressure of 20 mtorr, a target voltage of 100 V an a target to substrate istance of 5 cm. Growth temperature was always higher than 50 C. We characterize the resulting thin films compositionally (energy ispersive spectrometry analysis), structurally, optically, an electrically. All the films were Cu poor with Ga, In, an Se contents near the stoichiometric composition. In Table II we show the x-ray iffraction (XRD) pattern results of a representative film from each target grown upon A120 substrates. In Fig. 1, we show the unit cell parameters, c an ~2 of the tetragonal structure, euce from the peak position, for both the targets an the thin films. In this figure we have also inclue the corresponing values for CuInSez an CuGaSe2. 5 We obtaine single phase chalcopyrite films only at very high growth temperatures. As it is known, the alloying of CuInSez with Ga has two main consequences from a structural point of view: (i) A lowering of the intensity in the superlattice chalcopyrite reflections [( lol), (10), (211), (105,21), (01)] ue to the similar value of the atomic scattering factor of Cu an Ga an (ii) a istortion of the tetragonal structure (c/a < 2) when the Ga/In ratio is higher than 1. Due to this istortion the (220)-(204) an (116)-(12) oublets are split. For x = 0.25 all the superlattice reflections of the orere chalcopyrite structure are clearly ientifie in both the target power (Table I) an the thin film (Table II). TABLE I. Power iffractometer ata for the three ifferent targets for 26 values ranging from 10 to 60. ilk I/r, 4 I/IO I/IO a 1875 Appl. Phys. Lett. 60 (15), 1 April /92/l 1992 American Institute of Physics 1875 Copyright All Rights Reserve.

2 TABLE II. Ditl ractometer ata for single phase chalcopyrite thin films of the three compositions for 26 values ranging from 10 to 60. hkl I/IQ I/I I For x = 0.50 the two most intense superlattice chalcopyrite reflections [ ( 101 ), (211)] are ientifie in the target power (Table I), meanwhile in the thin film pattern only the (101) reflection is present (Table II). It has been shown that the XRD pattern of CuGac,=Jnc$Sez power only exhibits the most intense superlattice reflections with very low intensities. Furthermore, it is easily prove that the structure factor of the superlattice reflections ecreases with the content in Ga ue to the very similar values of the atomic scattering factor of Cu an Ga an the ifference of this factor between Cu an In. Then the high Ga content an the strong preferential orientation of the film seem to be responsible for the absence of the superlattice reflections when x>o.5. The presence of the (101) reflection is the only evience of the chalcopyrite structure for this film, provie that the tetragonal istortion in CuGa&nc,Se, (c/a = 1.995) oes not allow the splitting of the (220) (204) an ( 116) ( 12) oublets. This nonsplitting is also evient in the XRD patterns of the target power (Table I). For the highest Ga content (x = 0.75) we o not observe superlattice reflections. However, a tetragonal istortion of c/u = allows the oublets splitting in both the power (Table I) an the film (Table II), thus asserting the presence of the chalcopyrite structure for this composition of the alloy.g Films grown at temperatures lower than 400 C reveal, for the three targets, the presence of the isorere sphalerite structure. Similar results have been observe in evaporate thin films.6 By hot probe experiments we prove that all the films exhibit p-type conuctivity. We measure the resistivity of the films by the Van er Pauw metho with a square geometry. For the three compositions we observe an increase in the resistivity from 1 n cm for films grown at 70 C to lo5!a cm for films grown at 450 C. This increase of the resistivity takes place in spite of the following: The nonvariation in the composition of the films with growth temperature an the fact that the scanning electron microscopy observations we performe showe a monotonous increase in the grain size of the films with growth temperature up to 0.5 pm. To clarify the meaning of this variation of the resistivity, we are now performing measurements of the electrical properties of the films at ifferent specimen temperatures. We performe the optical characterization measuring the transmittance an reflectance of films grown on quartz substrates with a Per-kin-Elmer Lamba 9 Spectrophotom- - _ -~~ I ~~ 11.8 i * i II.6 t I-,._ 4.< -5 u ~,, n u r., II a IT.0..- I, 01, 025, 0.50, 0 75 i 54 I.oo X BIG. 1. Chalcopyrite unit cell parameters, u ( + : target power; *: thin film) an c (0: target power; Cl: thin fm) vs Ga content euce from the reflections in the XRD patterns. We also show these values for the corresponing ternary compouns. lo;*&y~-----~ 2 00.,: Energy (ev) BIG. 2. Absorption coefficient vs photon energy for three ifferent films of CuGa,In, _ $e, (continuous line for x = 0.25, ashe line for x = 0.50, otte line for x = 0.75 ) Appl. Phys. Lett., Vol. 60, No. 15, 1 April 1992 Hernanez-Rojas et al Copyright All Rights Reserve.

3 eter. We obtaine the values of the absorption coefficient by means of a calculation that takes into account the multiple reflection processes at the air-film, film-substrate, an substrate-air interfaces, an the ifferent nonuniformities of the film. In Fig. 2 we show the absorption coefficient for one representative film grown from each target at very high substrate temperatures. Well efine, sharp absorption eges are visible in the figure. By fitting (ah~)~ vs hv we euce that the funamental ban gap is allowe irect withvaluesof1.12, 1.5,an 1.51 evforx=0.25,0.5,an 0.75, respectively. These values are in goo agreement with previous results for evaporate films.6 The low absorption coefficient values at energies lower than the ban gap are inicative of goo stoichiometry an absence of seconary phases.6 In the high absorption zone we can see a shouler that inicates the presence of an aitional optical transition, as it is usual in chalcopyrite compouns.2*597 In summary, we have mae targets of single phase chalcopyrite CuGa,In, _ $e2 of three ifferent Ga contents (x = 0.25, 0.5, 0.75). From these targets we have grown thin films by rf sputtering on A120 an quartz substrates. For growth temperatures higher than 400 C! we obtaine single phase chalcopyrite films from all three targets. We euce the unit cell parameters from the XRD patterns. The optical characterization has shown a well efine ban ege with ban gaps of 1.12, 1.5, an 1.51 ev for x = 0.25, 0.5, an 0.75, respectively. All these characteristics are similar to those obtaine on evaporate quaternary CuGa,Inl _.Se2 thin films. The authors woul like to express their acknowlegments to S. Garcia-Martin (XRD facilities) an J. Carabe (Optical Measurement facilities). W. E. Devaney, W. S. Chen, J. M. Stewart, an R. A. Mickelsen, IEEE Trans. Electron. Devices 7, 428 (1990). I. M&l, J. Santamarfa, E. Iborra, G. Gonzalez-Diaz, an F. Sanchez- Quesaa, J. Appl. Phys. 62, 416 ( 1987). F. Sanchez-Quesa, C. Case, G. Gonzalez-Diaz, I. Mart& J. Santamaria, E. Iborra, R. Beaulieu, an J. J. Loferski, Appl. Surf. Sci. /4, 844 (1988). I. Mbrtil, G. Gonzalez-Diaz, J. Santamaria, M. L. Lucia, J. L. Hernanez-Rojas, an F. Sanchez-Quesaa, J. Mater. Sci. Lett. 19,27 (1990). s I. M&%1, J. Santamaria, G. Gonzalez-Dlaz, an F. Sanchez-Quesaa, J. Appl. Phys. 68, 189 (1990). 6D. S. Albin, J. R. Tuttle, G. D. Mooney, J. J. Carapella, A. Dua, A. Mason, an R. Nou6, Proceeings of the 2lst IEEE Photovoltaic Solar Energy Conference (IEEE, New York, 1990), D. Albin, R. Noufi, J. Tuttle, J. Goral, an S. H. Risbu, J. Appl. Phys. 64, 490 (1988). D. K. Suri, K. C. Nagpal, an G. K. Chaha, J. Appl. Crystallogr. 22, 578 (1989). 9T. Tinoco, C. Rincon, M. Quintero, an G. Sanchez Perez, Phys. Status Solii A 124, 427 (1991). OJ. L. Hernanez-Rojas, M. L. Lucia, I. Mart& G. Gonzalez-Diaz, J. Santamaa, an F. Sanchez-Quesaa. Appl. Opt. (in press) Appl. Phys. Lett., Vol. 60, No. 15, 1 April 1992 Hernanez-Rojas et al Copyright All Rights Reserve.

4

5 Applie Physics Letters is copyrighte by the American Institute of Physics (AIP). Reistribution of journal material is subject to the AIP online journal license an/or AIP copyright. For more information, see