Local structure of heavily Zn+-implanted GaAs studied by photoluminescence and fluorescence EXAFS

398

Nuclear Instruments and Methods in Physics Research B19/20 (1987) 398-402 North-Holland, Amsterdam

L O C A L S T R U C T U R E O F H E A V I L Y Z n + - I M P L A N T E D G a A s S T U D I E D BY PHOTOLUMINESCENCE AND FLUORESCENCE EXAFS Kazuhiro KUDO*, Yunosuke MAKITA, Hiroyuki OYANAGI, Nobukazu OHNISHI**, Toshio NOMURA***, Katsuhiro IRIE**, Fumiya UEHARA*** and Yoshinobu MITSUHASHI E!ectrotechnical Laborato~. , 1-1-4 Umezono, Sakura-mura, Niihari-gun, Ibaraki 305, Japan

Low temperature photoluminescence measurements were performed for Zn+-implanted GaAs, where the concentration of Zn, [Zn], was widely varied from 3 × 1016 to 1 × 1021 cm -3. A newly discovered Zn-associated emission denoted by [g-g] was found to move towards lower energy levels with increasing [Zn]. [g-g] was theoretically suggested to be an acceptor-acceptor pair emission. [g-g] presents a strong optical compensation at high [Zn] and in order to study further properties of [g-g] we investigated the local atomic structure of Zn+-implanted GaAs by fluorescence extended X-ray absorption fine structure (F-EXAFS). The results show that the implanted Zn atoms occupy Ga sites and/or tetrahedral interstitial sites. It was also observed that the lattice in the region longer than the second-nearest neighbor is highly disordered, which brings about the selective self-optical compensation (SSOC) effect of [g-g] at high [Zn].

1. Introduction In the photoluminescence (PL) spectra of GaAs grown by molecular beam epitaxy (MBE), more than 30 sharp line emissions are frequently observed just below the emission of excitons bound to neutral acceptors (A °, X). Their origins have been explained in terms of solely defect-induced bound exciton states [1] or simply residual carbon (C) atoms [2]. Among many line emissions 'g' can be reproducibly formed by the C ion implantation [3,4]. Moreover, it was shown that 'g' is associated not solely with C but also with other acceptor atoms, Mg, Zn, and Cd [5]. However, 'g' cannot be obtained by pure donor atoms like S, Se, and Te, or by amphoteric atoms such as Si [6]. In detailed study of PL spectra as a function of C and Mg concentrations, we showed that another new emission, [g-g], is created commonly between 'g' and (e,A), an emission due to the transition from the conduction band to acceptor levels [3-5]. It was also revealed from the dual implantation of Zn + and Se + ions into GaAs that 'g' and [g-g] were selectively quenched by the simultaneous presence of donors and acceptors [7,8]. The purpose of the present paper is to investigate the manner in which the two emissions ('g' and [g-g]) behave for a rather heavy acceptor (Zn) as functions of its * Matsushita Electric Industrial Co., Ltd. 3-10-1 Higashimita, Tama-ku, Kawasaki-shi 214, Japan. ** Institute of Fundamental Analysis Inc., Ltd. 3-24-3 Yoyogi, Shibuya-ku, Tokyo 151, Japan. *** Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa 259-12, Japan. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

concentration, [Zn]. The atomic local structure around the implanted Zn is estimated by fluorescence extentecT X-ray absorption fine structure (F-EXAFS) technique for the further elucidation of [g-g] [9-12].

2. Experimental The undoped GaAs layers used for this study were grown by MBE at 550°C. The ion implantation of z n was carried out at room temperature with three energies, 180, 320 and 400 keV which ensured a flat distribution of [Zn] between 0.07/~m and 0.17/~m from the surface [13]. After ion implantation, samples were annealed at 800°C for 20 min. PL measurements were performed at 2 K by using a 1-m monochromator and a GaAs photomultiplier. The resolution of the PL spectra was less than 0.06 meV. Samples of EXAFS was also fabricated by MBE and ion implantation with a single energy of 400 keV. A silicon (111) bent crystal monochromator was used to provide an energy resolution of 1 eV at 9 keV [12,13]. A typical photon flux at 109-10 l° photons/s was achieved when the storage ring was operated at 2.5 GeV and at about 100 mA. The undoped and Zn+-implanted sampies were measured at room temperature.

3. Results PL spectra of Zn+-implanted GaAs for [Zn] from 4 × 1016 to 1 × 1019 cm -3 are presented in fig. 1, together with that of as-gown GaAs. For the as-grown

399

K. Kudo et al. / Local structure of heavily Zn +-implanted GaAs

PHOTON ENERGY (eV) 1.52 1.50 1.48 1.46 1.52 1.50 1.48 146 1

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Fig. 2. The peak energies of 'g', [g-g], (e, Zn), (D, Zn), g' and [g-g]' emissions as a function of [Zn]. On the upper scale of this figure the average separation of an acceptor-acceptor pair, (rA), is also indicated.

I

810 820 830 840 850 810 820 830 840 850 WAVELENGTH(nm) Fig. 1. Photoluminescence (PL) spectra of as-grown and Zn ÷implanted GaAs samples at 2 K as a function of Zn concentration, [Zn]. material, the free-exciton emission, (FE), the emissions of excitons bound to the neutral donors, (D °, X), to the ionized donors, (D+,X), and to residual neutral C acceptors, (C°,X), are easily identified. The emissions (e, C) and (D, C) correspond to the transition from the conduction band to the C acceptor level and to the emission from pairs between unidentified donors and residual C acceptors, respectively. As for Zn +implanted samples, it can be easily noticed that [g-g] exhibits a very complicated behavior against [Zn] compared with other Zn-related emissions like (Zn°,X), 'g' and (e, Zn). Here, (Zn°,X) is the emission of excitons bound to neutral Zn acceptors and (e, Zn) is the transition from the conduction band to the Zn acceptor level. 'g' is observed for the entire [Zn] region examined, while [g-g] appears for [Zn] greater than 4 X 1016 cm-3. On increasing [Zn], the peak position of [g-g] shifts towards lower energy levels and finally seems to be locked at an energy which is a little bit higher than (e, Zn). For a critical [Zn] region from 7.5 × 1017 to 1 x 10 t8 cm -3 two new peaks are obtained at 1.511 eV ( ' g " ) and at 1.509 eV ([g-g]') [14]. In the samples with

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[Zn ] (cm -3) Fig. 3. The PL intensity and the energy half-width of [g-g] as a function of [Zn] and (rA).

[Zn] greater than 2 x 10is cm -3, [g-g] is simply a low intensity hump on the higher energy side of (e, Zn). In order to examine the above behavior of [g-g] along [Zn], the peak energy of [g-g] is plotted as a function of [Zn]. The results are shown in fig. 2 together with other Zn-related emissions. From fig. 2 it is noted that the peak energies of Zn-related emissions excluding [g-g] are fixed over the entire [Zn] range. The PL intensifies and the energy half-width of [g-g] as a function of

III. SEMICONDUCTORS

K. Kudo et al. / Local structure of heavily Zn +-implanted OaAs

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Zn*imp. GaAs

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Fig. 6. (a) Zn K-edge absorption spectra for Zn+-implanted GaAs with Zn dose of 5 x 1016 cm-2; (b) the normalized Zn K-edge F-EXAFS oscillation x ( k ) as a function of the photoelectron wave number, k.

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[Zn] are presented in fig. 3. The intensity of [g-g] is strongly enhanced in the range of [Zn] from 1 x 1017 to 3 x 1017 cm -3 and is gradually reduced when [Zn] exceeds 3 x 1017 cm -3. The energy half-width of [g-g] is moderately increased with increasing [Zn] in the region from 6 x 1016 to 2 X 1017 cm -3. It is worth noting that in the [Zn] region where a steep shift of [g-g] towards lower energy levels occurs, a drastic decrease of the [g-g] emission intensity and the broadening of the half-width take place. These observations suggest that there is a selective self-optical compensation (SSOC) effect in high

[Zn] region. The dependence of the peak energy of [g-g] upon the excitation density is shown in fig. 4 for the various [Zn] values. At [Zn] lower than 8 x 1016 cm-3, [g_g] is not sensitive to the excitation density. When [Zn] increases further until [g-g] begins to be suppressed through the SSOC effect, [g-g] is shifted towards the higher energy

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8.00

10.00

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Fig. 7. The radial distribution of the atoms around the implanted Zn, IF(r)}. The solid and dashed lines show IF(r)l for the Zn+-implanted and the undoped GaAs, respectively. In the R region the behavior of IF(r)l for Zn+-implanted GaAs is markedly different from that of the undoped one.

levels with increasing excitation density. From this figure, the energy shift of [g-g], A E~.s, can be expressed experimentally by zlEg.g = k log ( I / I o ) , where I / I o is relative excitation density. The coefficients k obtained from fig. 4 are shown in fig. 5 as a function of [Zn]. The maximum k is established at [Zn] = 2 x 1017 cm -3, where a maximum emission intensity of [g-g] is obtained.

K. Kudo et al. / Local structure of heavily Zn +-implanted GaAs The results of F-EXAFS are presented in figs. 6 and 7. The K-edge absorption spectra for Zn in Zn +implanted GaAs with a dose of 5 × 1016 cm -2 is represented in fig. 6a. The normalized Zn K-edge F-EXAFS oscillation, X ( k ) , is plotted in fig. 6b as a function of the photoelectron wave number, k, where x ( k ) is defined in refs. [9-12]. The Fourier transformation of x ( k ) , [F(r)[, is shown in fig. 7, where the solid and the dashed lines indicate the results for the Zn+-implanted and undoped samples, respectively. The distance from Zn to the first-nearest neighbor atom almost coincides with the first-nearest G a - A s distance. The region designated by R from 2.8 to 5.0 A corresponds to the atomic position of the second- and the third-nearest neighbor atoms, where the behavior of [F(r) [ of Zn +-implanted sample is appreciably different from that of undoped sample.

4. Discussion Although [g-g] is commonly observed in acceptorimplanted GaAs, the behavior of Zn+-implanted sample presents a marked contrast with the case of Mg + or C+-implanted GaAs [3-5], where the intensity of [g-g] is sufficiently large until a higher acceptor concentration gives rise to a complex emission between [g-g] and (e, A). We now turn our interest to the theoretical aspect of [g-g] and we consider the formation of the acceptoracceptor pairs as an origin of [g-g]. The average separation of this pair, ( r A), is given by ( r A ) = (4 ~rNA/3 ) - ,/3,

(1)

where NA is acceptor concentration, e.g., for NA = 1 × 10XTcm-3, (rA) is 133.7 ~k. (rA) from eq. (1) is plotted in fig. 2 as a function of peak energy. If hydrogen-type wave-functions can be assumed for the corresponding orbitals of holes bound to acceptor ion~, the mean radius, ( r ) , of the orbital with principal number, n, and orbital-angular momentum number, l, is given by (r) =~[1

+ 1(1

1 ( l + 1)

/]a0"

where the charge of the acceptor ion acting on the hole is described as Z e and ao* is the effective BoN radius. The value of ao* is estimated to be 0.53(mo/mh)c A, where m0//mh is the free electron to hole mass ratio and c is the relative dielectric constant. When m h / m o = 0.45 [15], c = 13.1 [161 and Z = 1, ( r ) of the ls, 2s and 2p orbitals are obtained as ( r ) l s = 15.4, (r)2 s = 92.5 and (r)2 p = 77.2 .A., respectively. It should be again noted that [g-g] is largely enhanced for [Zn] at 3 x

401

I0 '7 cm -3 with (rA) = 92.7 /k and it is of great interest to note that this (rA) value almost coincides with (r)2 s or (r)2 p. Since the binding energy of the pair is supposed to increase with decreasing pair separation due to the overlapping of the hole wave-functions, the energy of the emission ascribed to this pair should be shifted towards lower energy levels. In the same time the various energy levels are expected to appear corresponding to the various pair separation with increasing [Zn]. Therefore, the envelope energy half-width of [g-g] corresponding to the increase of various energy levels becomes significantly broadened. Due to this reason the peak energy of [g-g] presents a blue shift with increasing excitation density. These considerations bring about the conjecture that [g-g] is formed by the overlapping of the excited states (2s or 2p) of hole orbitals. Moreover, it is assumed that at the [Zn] producing a maximum k, the maximum number of the pairs is established. The fact that [g-g] becomes not sensitive to the excitation density for greater than 7.5 x 1017 cm -3 shows that the SSOC effect begins to occur and the pairs start to be destroyed beyond this concentration. It can be considered that as the pair separation becomes small, the impurity band begins to be formed rather than the formation of the pair. This situation makes [g-g] insensitive to the excitation density, since the energy band width of the impurity band is supposedly narrower than that of the pair emission. The results of F-EXAFS show that the distance from Zn to the first-nearest neighbor atoms almost coincides with the first-nearest G a - A s distance. However, when a Zn atom occupies the tetrahedral interestitial site, the distance from this Zn atom to its first nearest-neighbor atoms coincides with that from a substituted Zn atom at Ga site to its first-nearest atoms. The coordination number for the As atom in both cases is identically the same. Therefore, it should be reasonably considered that the implanted Zn atoms occupy the Ga sites a n d / o r the tetrahedral interstitial sites. As for the difference of [F(r)[ between the Zn+-implanted sample and undoped one observed at the R region in fig. 7, the following two facts are supposed to be primary origins. One is that the lattice arrangement in this region is highly disturbed by the radiation due to ion implantation. The other one is the large difference of the atomic distance from a Zn atom to the second-nearest neighbor atoms between the interstitial Zn atom ( a / 2 ) and the substitutional one ( v ~ a / 2 , where a is the lattice constant of GaAs). Hence, from the results of F-EXAFS, it is suggested that the crystalline disorder observed in the [F(r)] spectrum is one of the presumed origins for the destruction of acceptor-acceptor pairs which is responsible for the SSOC effect of [g-g].

III. SEMICONDUCTORS

402

K. Kudo et al. / Local structure of heavily Zn +-implanted GaAs

In conclusion, it was found that the newly discovered emission denoted by [g-g] in Zn+-implanted GaAs shifts pronouncedly towards lower energy levels with increasing [Zn]. The intensity of [g-g] is considerably suppressed at high [Zn]. This selective self-optical compensation effect (SSOC) was found to be formed by ion implantation of rather heavy acceptor atoms like Zn. It was suggested theoretically that [g-g] is due to the formation of acceptor-acceptor pairs where the overlapping of excited orbitals of holes occurs. The F-EXAFS spectra concerning the implanted impurities in GaAs are for the first time measured. The difference of the radial atomic distribution between Zn ÷implanted and undoped GaAs suggests that the lattice is disturbed by the radiation due to ion implantation. It was conclusively proposed that implanted Zn atoms are situated either at Ga sites or at the tetrahedral interstitial sites. It was assumed that this crystalline disorder is also closely related with the aforementioned SSOC effect of [g-g]. The authors would like to express their gratitude to J. Shimada, Y. Takeuchi, H. Tanaka, T. Izumi, T. Kobayhashi, H. Tanoue for their encouragement and support.

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