MATERIALS MIEI~E & ENGINEERING ELSEVIER
Materials Science and Engineering B35 (1995) 117-159
B
Luminescence properties of p-type thin CdS films prepared by laser ablation B. Ullrich a, H. Ezumi b, S. Keitoku °, T. KobayashP aDepartment of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan bDepartmem of Electrical Engineering, Hiroshima-Denki Institute of Technology, Hiroshima 739-03, Japan ¢Hiroshima Women's University, Hiroshirna 734, Japan
Abstract Investigations of the luminescence of p-type CdS:Cu thin (less than or equal to 2 ~tm) films on glass substrate prepared by laser ablation were performed for the first time. The dependences of the luminescence on the Cu content in the thin films were studied at 300 K with argon laser lines at 457.9 nm, 488.0 nm and 514.5 nm. It is demonstrated that the luminescence excited with the 514.5 nm line corresponds to the donor-acceptor transition. Furthermore, it is shown that the intensity of the red emission of CdS:Cu films can be efficiently bleached by Cu doping.
Keywords: Laser processing; p-type CdS; Luminescence of p-type CdS; Cadmium sulphide; Doping effects; Thin films
I. Introduction The gap of the I I - V I compound semiconductor CdS lies in the technically attractive green .range of the spectrum. Hence, the material is predestinated to the production of photonic devices and solar cells. Extensive applications of CdS have, however, been hampered by difficulties in producing p-type CdS. In general, wide-gap I I - V I compounds exhibit an intrinsic n-type character and p-type materials are hard to fabricate owing to the lack of native defects [1]. Recently, Keitoku et al. [2] have demonstrated, a, novel way of preparing p-type CdS samples by laser ablation. In this paper the first study of the luminescence features of this new material is presented.
2. Sample preparation and experimental details The samples investigated were thin (less than or equal to 2 lam) CdS films prepared b y laser ablation using an N d : Y A G laser. The beam of the laser was guided on a target and the ablated material was deposited onto a glass substrate which was mounted 3 - 4 cm above the target [2,3]. The target was made of 0921-5107/95/$09.50 © 1995 - - Elsevier Science S.A. All rights reserved
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a cold-pressed mixture of pure (99.999%) powders of CdS and Cu. Doping with Cu determines the conduction type of the samples; in particular, if the Cu content in the sample exceeds approximately 1.0 at.%, the conduction type changes from n-type to p-type [2]. For the luminescence investigations, the samples were photoexcited by an argon laser using the lines at 457.9 nm, 488.0 nm, and 514.5 nm. In addition, the 632.8 nm line of an H e - N e laser was used. The luminescence was collected from the back side of the samples and detected by a photomultiplier tube attached to a spectrometer. All experiments were carried out at 300 K.
3. Results According to the fundamental absorption, radiative emissions from CdS films are expected in the vicinity of 500 nm at 300 K. However, in many cases CdS does not show green emission but only red and IR luminescence [4]. In fact, the luminescence intensities of CdS reveal two peaks around 740nm and 1020nm respectively [5,6]. Our investigations are restricted to the former peak. Fig. 1 shows the luminescence of differently doped CdS films. The measurements were carried out
B. Ullrich et al. / Materials Science and Engineering B35 (1995) 117-119
118
I
using the 457.9 nm line with an intensity of 79 W cm -2. Clearly, the red emission intensity decreases when the Cu content in the samples increases, as shown explicitly in Fig. 2. Furthermore, it was found that the peak wavelength of the red emissions depends on the Cu content and excitation energies. The results are summarized in Fig. 3.
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4. Discussion
- 13.6m*
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where m 0 is the free electron mass, m* is the effective mass of the electrons (m*=0.2mo) [7] or holes ( m * = 5.0m0) [7], e ( e = 8.9) [7] is the static dielectric constant and n is an integer. We find ED = 34 meV and EA = 858 meV from Eq. (1). We point out that Eq. (1) leads to reasonable agreement with the binding energies found in the literature. In particular, the position of Cu is reported to be 1.1 eV above the top of the valence band [8]. Neglecting Coulombic interactions and van der Waals polarizations, the energy hvtr for d o n o r - a c ceptor transitions is expressed by hvtr = Eg - (EA + ED), where Eg (Eg = 2.43 eV) is the magnitude of the fundamental gap. According to curve (c) in Fig. 3, the value hvtr = 1.54 eV agrees quite well with the transition positions of the p-type samples which have Cu contents of 3.0 at.% and 3.8 at.%. The decrease in photon energies shown by curve (c) because of the increase in the Cu content refers to the so-called band-gap renormalization of p-type semiconductors [9]. For p-type CdS, the I
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m0
\ X \
The binding energies of donors and acceptors ED.A are given by the relation E D , A / e V --
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00
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0
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I
3 (at.°/,)
hole mass is large so that most of the holes are in the valence band tails. Therefore, electrons recombine into hole states which are near or even above the parabolic valence band edge. As a consequence, the emission peaks are shifted towards lower energies. Curves (a) and (b) in Fig. 3 show that the peaks of the red emissions shifted towards higher energies when the samples were excited with blue lines. It is worthwhile stressing at this point that luminescence responses caused with blue light in CdS depend on specific surface
1.8
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if) E
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Fig. 2. Intensity dependence of the red emission in Fig. 1 on the Cu content in the CdS film.
(o)
•~O 1.0
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1 2 Cu content in film
I
600
,
I
700 Wavelength
,
I
800 (nm)
,
900
Fig. 1. Red luminescence of differently prepared CdS films• The sample of curve (a) is undoped. The thin CdS films of curves (b)-(d) contain 1.5at.%, 3.0at.% and 3.8at.% Cu. The emissions were excited by the 457.9 n m line with an intensity o f 79 W cm -2. The measurements were taken at 300 K.
I
I
I
I
0
1
2
3
Cu c o n t e n t
I
in fJJm ( o t . % )
Fig. 3. Dependences of the photon energy of the red emission peak on the Cu content and excitation energy. The following laser lines and intensities were used: (a) 457.9 nm, 79 W cm -2, (b) 488.0 nm, 119 W cm - 2 and (c) 514.5 nm, 160 W cm -2. All experiments were carried out at 300 K.
B. Ullrich et al. / Materials Science and Engineering B35 (1995) 117-119
properties of the samples, which are probably responsible for those energy enhancements. According to Figs. 1 and 2 an increase in the Cu content leads to a decrea,;e in luminescence intensities. This is caused by the emergence of shallow acceptor states making the oscillator strength of the radiative donor-acceptor transitions according to Eq. (1) weaker. Indeed, investigations with the He-Ne line have shown that doping with Cu creates acceptor states 0.57 eV above the valence; band.
5. Summary The luminescence results here give clear evidence that the laser ablation of CdS films from a CdS:Cu target creates non-compensated acceptor states in the material resulting in p-type CdS, Hence, further research is highly justified to explore applications of the laser
119
ablation of CdS in the preparation of solar cells and light emitting devices. References [1] D.B. Laks and C.G. Van De Walle, Physica B, 185 (1993) 118. [2] S. Keitoku, H. Ezumi, H. Osono and M. Ohta, Jpn. J. Appl. Phys., 34 (1995) L138. [3] H. Ezumi and S. Keitoku, Jpn. J. Appl. Phys., 32 (1993) L1783. 14] C. Bouehenaki, B. Ullrich and J.P. Zielinger, J. Lumines., 48-49 (1991) 649. [5] B.J. Feldman and J.A. Duisman, Appl. Phys. Lett., 37 (1980) 1092. [6] G.F.J. Garlick, in S. Fliigge (ed.), Handbueh der Physik, Band XXVI, Licht und Materie II, Springer, Heidelberg, 1958, p. 37. [7] M. Bleicher, Halbleiter-Optoelektronik, Hiithig, Heidelberg, 1986, p. 240. [8] R.H. Bube, in M. Aven and J.S. Prener (eds.), Physics and Chemistry of H - V I Compounds, North-Holland, Amsterdam 1967, p. 657. [9] H.C. Casey, Jr., D.D. Sell and K.W. Wecht, J. Appl. Phys., 46 (1975) 250.