p-type doping by ion implantation into ZnSe epitaxial layers grown by metalorganic vapor phase epitaxy

Journal of Crystal Growth 101 (1990) 289—293 North-Holland

289

p-TYPE DOPING BY ION IMPLANTATION INTO ZnSe EPITAXIAL LAYERS GROWN BY METALORGANIC VAPOR PHASE EPITAXY Tokuo YODO, Kazuhiro UEDA and Ken YAMASHITA Tsukuba Research Laboratory, Nippon Sheet Glass Co., Ltd., 5-4 Tokodai, Tsukuba City, Ibaraki Prefecture 300-26, Japan

We discuss the optimum conditions (acceleration energy and dose density) of both Li~ and Na~ ion implantations and the problem of fast diffusion of alkali impurities in ZnSe heteroepitaxial layers (heteroepilayers) grown by atmospheric pressure metalorganic vapor phase epitaxy. Li is preferred to Na as a p-type dopant in ZnSe because Na easily generates self-activated (SA) centers. The optimum annealing temperature (1,) for activating Na acceptor levels without generating SA center levels is lower than that of Li, so that ion damage in Na-implanted epilayers cannot recover at the low Ta. Moreover, Na diffuses fast as well as Li in ZnSe heteroepilayers at Ta> 550°C, which is confirmed by SIMS and PL with a successive etching process. Na is redistributed almost uniformly in ZnSe as well as Li after optimum annealing. Particularly, Li in ZnSe forms high concentration of shallow acceptor levels without SA center levels after annealing.

1. Introduction It has been particularly required to develop practical blue light emitting devices in recent years for fabrication of flat panel color displays and to increase the density of optically recorded information by shortening the wavelength of light sources. For this purpose, ZnSe is the most suitable material because of the magnitude (2.7 eV) of the band-gap and the direct band structure, and has been researched very well expecting blue emission with high efficiency. However, practical blue light emitting devices have not been achieved yet because of the hard controllability of obtaining ptype ZnSe. Several researchers [1—5]have tried p-type doping from the vapor phase during epitaxial growth and others [6—10]have done it with ZnSe bulk crystals by ion implantation after growth. Although a few successful results were reported in the past [4,5], subsequent characterization has not been reported in detail. We reported p-type doping by Li~and Na~ion implantations into ZnSe epitaxial layers (epilayers) grown by atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE) in previous papers [11,12]. The purpose of this paper is to investigate the effects of ion implantation conditions on the formation of acceptor and self-activated (SA) center 0022-0248/90/$03.50 © 1990

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levels in ZnSe. Furthermore, it is to clarify problems of diffusion rate of Li and Na impurities in ZnSe epilayers. 2. Expenmental The AP-MOVPE system and procedures used here were reported elsewhere [13]. ZnSe heteroepilayers were grown at 250 °C and mole ratio ([VI]/[II]) of 40 for 1 h. Layer thicknesses of epilayers obtained were about 4 ~tm.After growth, Li ± or Na ± ions were implanted into the epilayers at acceleration energies between 10 and 200 keV, and the dose densities between 1011 and 1015 cm2. Thereafter the Li- and Na-implanted epilayers were annealed at 757°C for 1 mm and at 551°C for 5 mm, respectively, under nitrogen atmosphere as described previously [11,12]. Photoluminescence (PL) spectra were measured using the excitation line from a 6 mW He—Cd laser. In order to investigate the characteristics of inter-epilayers along the normal direction to the surface, the etching of epilayers by Br 2—CH3OH solution and PL measurement at 77 K were repeated many times. Also, the depth profile of Li or Na impurity was estimated by secondary ion mass spectroscopy (SIMS) analysis using a 7 keV O~ primary ion beam.

Elsevier Science Publishers B.V. (North-Holland)

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/ p-Type doping by ion implantation into ZnSe epitaxial layers grown by MOVPE

3. Results and discussion

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Fig. 1 shows PL spectra at 77 K of Li- and energy of 50 keV for various dose densities. Fig. 2 Na-implanted heteroepilayers at the acceleration shows PL at 300 K of as-grown, only annealed (at 757°C), and Li- and Na-implanted epilayers. Both as-grown and annealed epilayers do not generate SA centers. Li-implanted epilayers were annealed at 757°C in order to recover the crystalline quality and to activate the shallow acceptor level effectively. It is clearly shown that unknown Li acceptor-related emission is observed around 472 nm with comparable intensity even at 300 K except for band-edge (BE) emission at 460 nm (appearing at 445 nm at 77 K). This emission line observed at room temperature has never been reported. Also the SA emission is not observed, despite the high doping level. On the other hand, Na-implanted epilayers need to be annealed at 557°Cin order not to generate SA centers, so that the crystalline quality cannot be recovered, as seen easily from the weak PL intensity of BE emission [12]. Furthermore, comparing Na with Li, Na-implanted epilayers generate a low concentration of SA

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centers through ion implantation even at a low Ta, as seen in fig. 1. Also the increase of dose density weakens the total PL intensity drastically. This indicates that Na~ion implantation worsens the crystalline quality and generates SA centers probably because the mass of Na~ion is larger than that of Li~ion and Na is not better than Li as a shallow acceptor in ZnSe. The notable fact is that the absolute PL intensity of free-to-acceptor (FA) emission2with peaktoatdecrease 460.8 nm saturates at and the begins gradually over 1013 1013 cm cm2 in Li~ion implantation. This shows that all the implanted Li impurities do not always form shallow acceptors levels and there is still a high density of Li interstitial impurity, considering the dose density of implanted Li + ion. The formation rate of acceptor levels would saturate at 1013 cm2 and decrease over 1013 cm2 after annealing at 757°Cfor 1 mm. Therefore it would be necessary to raise 7~or to lengthen the annealing period in order to increase the acceptor concentration in epilayers at 1015 cm2 if the annealing does not generate SA centers. Actually, we confirmed the hypothesis. Fig. 3 shows the PL at 4.2 K of epilayer-implanted Li~ion at 50 keV under vanous dose densities. The acceptor bound exciton

T. Yodo et al.

/ p-Type doping by ion implantation into ZnSe epitaxial

layers grown by MOVPE

line (I1(Li)) due to the Li acceptor is observed at 2.7904 eV only for 1015 cm2. However, even below 1014 cm2, if Ta is lowered, a sharp I 1(Li) line was observed. Also, when Ta was increased or

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concentration is unexpectedly the weakest of the three. The reason is thought to be that Li impurity implanted at 10 keV is distributed in a very shallow region from the surface and therefore would be transported not only in an epilayer but also out of the surface in the vapor through the annealing. Therefore it is desirable to implant the Li± ion at more than 50 keY. Fig. 5 shows the depth profile (after annealing) of Li- and Na-implanted (at 50 keV) epilayers measured by SIMS It is confirmed that Li diffuses at 757°C and Na at 557°C almost uniformly in ZnSe, despite the high density of Li and Na impurities localized near the surface in as-implanted epilayers. The concentration of Li impurity is estimated from SIMS to be of the order of 1017 cm3, which is about 1/10 as low as that (2.5 x 1018 cm3) calculated from the dose density and layer thickness. It is probably because most of implanted Li~ions evaporate from the surface of

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the epilayers during the annealing even at 50 keV. Fig. 6 shows PL at 77 K of Li-doped ZnSe interepilayer (1014 cm2 200 keY) at the etching depth x. As expected from the SIMS analysis, the depth profile of FA PL intensity is almost uniform ex-

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T. Yodo et a!.

/ p-Type doping by ion

implantation into ZnSe epitaxial layers grown by MOVPE

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cept at the surface and interface. The FWHM of FA emission observed in these regions is broader than that in others. This indicates that diffusing Li impurities would trend to pile up a little near the surface and interface. It is concluded that the

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tion conditions. Annealing above 757°Cwould be necessary to activate more acceptor levels at the high dose density if the annealing does not generate SA center levels. The results clearly show that the high level doping of Li acceptor can be suc-

T. Yodo et a!.

/ p-Type doping by ion implantation into ZnSe

cessfully achieved while maintaining the crystalline quality of the ZnSe epilayers.

References [1] W. Stutius, J. Crystal Growth 59 (1982) 1 [2] T. Mitsuyu, K. Ohkawa and 0. Yamazaki, Appl. Letters 49 (1986) 1348. [3] A. Ohki, N. Shibata and S. Zembutu, Japan. J. Phys. 27 (1988) L909. [4] J. Nishizawa, K. Itoh, Y. Okuno and F. Sakurai, J. Phys. 57 (1985) 2210. [5] T. Yasuda, I. Mitsuishi and H. Kukimoto, Appl. Letters 52 (1988) 57.

Phys. Appl. Appl. Phys.

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[6] Y.S. Park and C.H. Chung, Appl. Phys. Letters 18 (1971)

[71Y.S. Park

and B.K. Shin, J. Appl. Phys. 45 (1974) 1444. [8] Z.L. Wu, J.L. Merz, C.J. Werkhoven, B.J. Fitzpatrick and R.N. Bhargava, Appi. Phys. Letters 40 (1982) 345. [9] A.J. Rosa and B.G. Streetman, J. Luminescence 16 (1978) 191. [10] J.S. Yermaak and J. Petruzzello, J. Appl. Phys. 55 (1984) 1215. [11] T. Yodo and K. Yamashita, Appl. Phys. Letters 53 (1988) 2403. [12] T. Yodo and K. Yamashita, Appl. Phys. Letters 54 (1989) 1778. [13] T. Yodo, H. Oka, T. Koyama and K. Yamashita, Japan. J. Appl. Phys. 26 (1987) L561.