FABRICATION OF p-type ZnS WITH BLUE-Ag EMISSION BY TRIPLE-CODOPING METHOD. 185 Miyanokuchi, Tosayamada-cho, Kami-gun, Kochi , Japan

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1 FABRICATION OF p-type ZnS WITH BLUE-Ag EMISSION BY TRIPLE-CODOPING METHOD S. Kishimoto 1, T. Yamamoto 2 and S. Iida 3 1) Kochi National College of Technology, Monobe-otsu 200-1, Nankoku, Kochi , Japan 2) Department of Electronic and Photonic Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kami-gun, Kochi , Japan 3) Department of Electrical Engineering, Nagaoka University of Technology, Kamitomioka , Nagaoka , Japan ABSTRACT We have succeeded in the fabrication of low-resistivity p-type ZnS with blue-ag emission by triple-codoping using Ag, a Zn-substituting species, In, a Zn-substituting species, and N, a S-substituting species. For the realization of blue-ag emission at 436 nm, we use In species as co-activators with Ag activators. For the control of conduction type to obtain p-type ZnS thin films, we introduce N species as acceptors into ZnS codoped with the Ag and In. On the basis of the analysis of the experimental data and calculated results, we proposed a model for ZnS:(Ag, In, and N), in which some of the In species act as coactivators with Ag activators and other In species act as reactive codopants with N acceptors. INTRODUCTION ZnS with a direct bandgap of 3.68 ev at room temperature has attracted attention because of their possible application in highly efficient short-wavelength light-emitting devices (LED) and blue injection-laser diodes (LD). Ag impurities, which are Zn-substituting species, are well known to be the blue-ag center (usually called activator )[1,2]. In order to realize the blue emission of approximately 440 nm (2.81 ev), it is necessary to introduce donors as coactivators into ZnS films. On the other hand, we must solve a crucial doping problem: ZnS has proven to be difficult to be doped as p-type while it is very easy to obtain n-type doped ZnS. Very recently, p-type ZnS doped with N species (ZnS:N) was fabricated by Svob et al. [3] In order to solve the doping problem described above for wide-band-gap semiconductors such as GaN and ZnO, we have proposed a codoping method using acceptors and reactive donors. The theoretical predictions for the realization of p-type ZnO by codoping of Ga or Al donors with N acceptors (ZnO:(N, Ga or Al)) and of p-type GaN:(Mg, Si or O) were proposed by us [4,5]. Subsequent confirmation of the applicability of the codoping to fabricate low-resistivity p-type ZnO [6] and GaN [7] was sought by some experimental groups. We have succeeded in the realization of low-resistivity p-type ZnS with blue-ag emission [8-11]. Then, the aim of this paper is to give how to fabricate the material described above. For ZnS with zinc-blende structure, which is favored by covalent compounds, the codoping method using the overlap between donor orbitals with a large Bohr radius and N-acceptor ones is very effective for good p-type conductive to occur, as will be demonstrated below.

2 METHODOLOGY Experimental procedure Materials Research Society 2002 Spring Meeting, San Francisco, The vapor-phase epitaxial growth apparatus is similar to the one reported in our earlier paper [12,13]. ZnS layers were grown under H 2 +NH 3 gas flow on semi-insulating GaAs(100) substrates using a luminescence-grade unactivated ZnS powder source and the impurity source in the form of elements: Ag (99.99%) and In (99.99%). The total flow rate of H 2 +NH 3 gas was ccm and the amount of NH 3 flow was ccm. The ZnS source temperature was kept at 920 ºC, while impurity source temperatures were changed depending on the source used: Ag;920 ºC and In; ºC. The substrate temperature was 590 ºC. The growth period was 2-4 hours. Layer thickness grown under these conditions were 1-4 µm. Further details are given elsewhere [8]. Photoluminescence (PL) was performed using a He-Cd laser (Omnichrome 374X 7mW) with a wavelength of 313 nm as the excitation source. PL spectra were obtained for the samples contained in a liquid-nitrogen filled optical Dewar. PL was dispersed with a monochromator (NARUMI RM-23), and detected by a multiplier (HAMAMATSU R943-02) and photocounter (HAMAMATSU C767). For excitation intensity dependence measurements, different kinds of optical neutral-density filters were used to vary the intensity by setting them in the optical path. Hall-effect measurements in the temperature region from 100 K to room temperature were performed by the Van der Pauws method on the grown layers using a Hall-effect measurement system (KEITHLEY Package 80). Electrical contacts were made on the surface of the layer grown on the semi-insulating substrate by evaporation of gold, followed by heat treatment in nitrogen atmosphere at 273 K for 3 min. Calculation methodology The results of our band structure calculations for ZnS crystals were based on the local-density approximation (LDA) treatment of electronic exchange and correlation [14-16] and on the augmented spherical wave (ASW) formalism for the solution of effective single-particle equations [17]. For undoped ZnS crystals, Brillouin zone integration was carried out for 84-k points in an irreducible wedge and for 24-k points for doped and codoped crystals. For valence electrons, we employs outermost s, p and d orbitals for Zn atoms and outermost s and p orbitals for the other atoms. We studied the crystal structures of ZnS with periodic boundary conditions by generating supercells that contain the object of interest. For ZnS:N, we replaced one of the 32 sites of S atoms with a N site. For ZnS:(In, N), we replaced one of the 32 sites of S atoms with a N site and one of the 32 sites of Zn atoms with an In site. We determined the crystal structure of the material by minimizing the total energy. We find that an In Zn -N S pair that occupies nearest-neighbor sites in the crystal is formed. For ZnS:(In, 2N), we replaced one of the remaining 31 sites of S with another N site for ZnS:(In, N), as described above. The total energy calculations show that the formation of the trimer, N S -In Zn -N S, that occupies the nearest-neighbor sites, is energetically favorable. The difference in the total energy between the crystal structure having the minimum total energy and the one having the second-minimum total energy, such as the In Zn -N S pair and another N which occupy the second-nearest neighbor sites from the In sites, is 832 mev.

3 RESULTS AND DISCUUSION Experimental results We show PL spectrum of undoped ZnS as a reference and ZnS:(In,Ag,N) layer in figure 1. For undoped ZnS, an emission was seen at 430 nm. This originates in the self-activated (SA) center which is an associated center consisting from a Zn vacancy and a donor impurity. For ZnS doped with Ag alone, a free-to-bound emission at 420 nm related to the Ag acceptors has been observed [8]. On the other hand, for the N-incorporated ZnS with In donors and Ag acceptors, which were grown under different flow rates of NH 3 gas, an emission at 436 nm, which has a half-width of 0.32 ev, was dominant in common. From the measurement of the dependence of the PL emission intensity for the emission peak on the excitation power, we find that the shifts in terms of the excitation can be considered to be the characteristic criteria of donor-acceptor (DA) pair transitions; we interpret that the corresponding donors and acceptors are In and Ag species, respectively. For the emission peak almost at 510 nm, we have little information concerning the nature of defects. Hall-effect measurements at room temperature revealed that the conduction type of the three samples described above is p-type, and free hole concentration and mobility values are ( ) cm -3 and cm 2 /Vs, respectively [8]. From ab initio electronic band structure Ex. 313 [nm] 77[K] PL Intensity [a.u.] (a) undoped ZnS (010517) (b) ZnS:In,Ag,N (TH971012) NH 3 : 500 ccm (c) ZnS:In,Ag,N (TH971013) NH 3 : 300 ccm (d) ZnS:In,Ag,N (TH971014) NH 3 : 200 ccm Hole Concentration [cm 3 ] ZnS:In,Ag,N (TH971013) Wavelength [ ] /T [K 1 ] Figure 1. Low-temperature (77 K) photoluminescence spectra of (a) undoped ZnS and (b)-(d) ZnS:(In, Ag, N). Figure 2. Dependence of hole concentration on temperature for ZnS:(In,Ag, N)..

4 calculations, it is expected that metallic conduction occurs due to the enhancement of the radius of N-impurity orbital in the In and N simultaneously codoped ZnS:Ag, which will be discussed below. We verified that hole concentrations for p-type ZnS:(N, Ag, In) did not show temperature dependence, indicating the formation of the impurity band, as shown in figure 2. In order to investigate what happens by codoping of In donors with N acceptor, we will discuss the effects of the codoping on the electronic structure on the basis of the analysis of the calculated results obtained by ab initio electronic band structure calculations as below. Theoretical results Figure 3 shows the total density of states (DOS) for (a) undoped ZnS crystals as a reference, for (b) ZnS:N, and (c) N-site-decomposed DOS for ZnS:N. The S 3s states are included in the calculation as valence states, but those that are located between ev and ev are omitted in figure 3. Energy is measured relative to the Fermi level (E F ). For undoped ZnS, zero energy indicates the top of the valence band. Figure 3(a) shows two groups in the valence band: (1) the first group is bands from 7.01 ev to 6.16eV with strong d characteristics originating mostly from d states at Zn sites; (2) the second group, located in the upper valence band above approximately 5.61 ev, which corresponds to a high-symmetry point L 1, mainly originates from the S 3p states. The letter A at 4.97 ev refers to a strong interaction between S p states and Zn s states. The lowest conduction bands, anti-bonding states of the interaction stated above, have a strong Zn 4s contribution; there are charge transfers from Zn 4s to S 3s and 3p due to the mixing between the s and p states at S sites and the s states at Zn sites. As a result, the s and p states of the surrounding Zn shift the center of gravity of the local DOS at the S sites towards the lower energy region. Figure 3(b) shows the formation of a N-impurity band just above the valence band. The arrow in this figure at 0.1 ev refers to a sharp DOS peak induced by N doping. Figure 3(c) indicates that the peak mentioned above is due to narrow N-impurity band where electrons are very localized by repulsion effects: The localized N-impurity state of which the DOS (1/eV) A at N sites p states E F Energy ev ev (a) (b) (c) Figure 3. Total DOS for (a) undoped ZnS (b) Total and (c) site-decomposed DOS for ZnS:N.

5 center of gravity of the DOS at N sites are pushed up to a higher energy region due to the strong repulsive potential. This suggests that N doping generates a slightly deeper acceptor level above the valence band, which explains the experimental data well [3]. Thus, the postulation to produce the required reduced ionization energy of N acceptors is the enhancement of the incorporation of the acceptors and of their bandwidth, leading to a larger Bohr radius. Figure 4 shows (a) total, (b) and (c) site-decomposed DOSs for ZnS:(2N, In). The sharp DOS peak around the E F to be discussed in ZnS:N cannot be observed, as shown in figure 4(a). The arrow at 3.67 ev in figure 4(a) indicates a DOS peak resulting from the interaction between p states at N sites close to the In site and s states at the In site. The new state described above from the valence band maximum due to a strong coupling between N acceptors and In donors was verified by X-ray Photoelectron Spectroscopy (XPS) measurement [18]. From figures 4(b) and 4(c), we find a strong hybridization between p states at In sites and p states at N sites close to the In sites. This causes a drastically change from narrow N-impurity bands, as shown in figure 3(c), to broad ones in figure 4(b). On the basis of the analysis of the DOS at the E F for ZnS:N in figure 3(b), we determined that the ratio of the partial DOS of p-states at the N sites to the total DOS at the E F is 22%. The other DOS at the E F is the sum of those at the S sites in the vicinity of the N sites. This indicates the small radius of the N-acceptor orbital. On the other hand, for ZnS:(In, 2N), we determined that the ratio of the sum of partial DOSs of p-states at the two N sites to the total DOS at the E F is 9.2%; we must note that the partial DOS not only at the S sites in the vicinity of the N sites but also at the more distant S sites contribute to the total DOS at the E F for ZnS:(In, 2N). This means an enhancement of the Bohr radius of the N acceptors, causing the reduced acceptor ionization energy based on the Bohr theory, due to the codoping of the In donors. DOS (1/eV) (a) (2N, In) (b) at N-sites p states (c) at In sites p states Energy (ev) E F Figure 4. (a) Total, (b) N-site-decomposed and (c) In-site-decomposed DOSs for ZnS:(2N,In).

6 CONCUSIONS Materials Research Society 2002 Spring Meeting, San Francisco, The present investigation leads to the following conclusions. (1) codoping of In coactivators with Ag activators is very effective to realize blue-ag emission with 436 nm. (2) codoping of In-reactive donors with N acceptors causes broad N-impurity bands around the top of the valence band with the enhancement of the incorporation of the N acceptors. We have succeeded in the control of the compatibility between the two kinds of the codoping described above and realized low-resistivity p-type ZnS with blue-ag emission. ACKNOWLEDGMENTS The authors thank to Dr. Sadao Ibe, general manager, and Dr. Yasuhiro Ueshima, general manager, of Asahi Kasei Corporation for their support of this work. One of the authors, T.Y., would like to express sincere thanks to Dr. Jürgen Sticht for his technical support. We have ESOCS code by accelrys. This work is a part of Dr. Thesis by one of the authors, S. Kishimoto [19]. REFERENCES 1. K. Era, S. Shionoya, and Y. Washizawa, J. Phys. Chem. Solids 29, 1827 (1968). 2. K. Era, S. Shionoya, Y. Washizawa, and H. Ohmatsu, J. Phys. Chem. Solids 29, 1843(1968). 3. L Svob, C Thiandoume, A Lusson, M. Bouanani, Y. Marfaing, and O. Gorochov, Appl. Phys. Lett. 76, 1695 (2000). 4. T. Yamamoto and H. Katayama-Yoshida, Jpn. J. Appl. Phys. 38, L166 (1999). 5. T. Yamamoto and H. Katayama-Yoshida, Jpn. J. Appl. Phys. 36, L180 (1997). 6. M. Joseph, H. Tabata and T. Kawai, Jpn. J. Appl. Phys. 38, L1205 (1999). 7. R. Y. Korotkov, J. M. Gregie, and B. W. Wessels, Appl. Phys. Lett. 78, 222 (2001) 8. S. Kishimoto, T. Hasegawa, H. Kinto, O. Matsumoto, S. Iida, J. Crystal Growth 214/215, 556 (2000). 9. S. Kishimoto, A. Kato, A. Naito, Y. Sakamoto and S. Iida, physica status solidi (b) 229, 391 (2002). 10. T. Yamamoto, S. Kishimoto and S. Iida, Physica B: Condensed Matter, , 916 (2001). 11. T. Yamamoto, S. Kishimoto and S. Iida, physica status solidi (b) 229, 371 (2002). 12. S. Iida, T. Yatabe and H. Kinto, Jpn. J. Appl. Phys., Part 2 28, L535 (1989). 13. S. Iida, T. Yatabe, H. Kinto and M. Shinohara, J. Crystal Growth 101, 141 (1990). 14. W. Kohn and L. J. Sham, Phys. Rev., 140, A1133 (1965). 15. L. Hedin and B. I. Lundquist, J. Phys., C4, 3107 (1971). 16. U. von Barth and L. Hedin, J. Phys., C5, 1629 (1972). 17. A. R. Williams, J. Kübler and C. D. Gelatt, Phys. Rev., B19, 6094 (1979). 18. S. Kohiki, T. Suzuka, M. Oku, T. Yamamoto, S. Kishimoto and S. Iida, J. Appl. Phys., 91, 760 (2002). 19. S. Kishmoto, Dr. Thesis, Dept. of Electrical Engineering, Nagaoka University of Technology, Niigata, 2001.

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