ZnSe interfaces

,ouwuo~ CRYSTAL OROWTH Journal

of Crystal

Growth

184/M

Schottky barrier tunability

(1998) 193-198

in Al/ZnSe interfaces

M. Lazzarinoa**, G. Scarelb, S. Rubini”, G. Bratinaa*b, A. Bonannia*b, L. Sorba”, A. Franciosia>b bDepartment

aLaboratorio Nazionale TASC-INFM, Padriciano 99, I-34012 Trieste, Italy of Chemical Engineen’ng and Materials Science, University of Minnesota, Minneapolis,

MN 55455, USA

Abstract

The Schottky barrier for Al/ZnSe (0 0 1) junctions was determined in situ by X-ray photoemission spectroscopy following Al deposition on ZnSe (0 0 1) c(2 x 2), ZnSe (0 0 1) 2 x 1, or ZnSe (0 0 1) 1 x 1 surfaces fabricated by molecular beam epitaxy. While similar values of the barrier were found on the first two types of interfaces, the p-type Schottky barrier was 0.24 eV lower for the third type of interface. 0 1998 Elsevier Science B.V. All rights reserved. PACS:

73.4O.Lq; 73.30. + y; 79.60. - i

Keywords:

Schottky barrier; Photoemission;

Tunability

The high specific resistance of metal contacts to wide-gap II-VI materials is one of the major problems hindering the development of II-VI based blue lasers [l]. The poor contact performance stems from the combination of two factors. The Fermi level at metal/II-VI contacts is pinned deep in the wide band gap, yielding large values of the Schottky barrier (SB) on ZnSe, ZnSe, _,,S,,, and Zn, _XMg,Sel _,,S,,. Furthermore, the present limitations in doping technology for these materials hinder the exploitation of tunneling to reduce the resulting high contact resistance. Methods to change the actual value of the Schottky

*Corresponding author. Tel.: + 39 40 3756 4343; fax: + 39 40 226 767; e-mail: [email protected]. 0022-0248/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00642-S

barrier height could, therefore, have an important impact. Several studies have argued that the value of the Schottky barrier height in metal/semiconductor interfaces might be affected by the nature of the initial reconstruction of the semiconductor surface [24]. In this paper, we report large variations in the Schottky barrier for Al/ZnSe (0 0 1) junctions fabricated by metal deposition on c(2 x 2), 2 x 1, and 1 x 1 reconstructed surfaces. The ZnSe substrates were fabricated by molecular beam epitaxy (MBE) on GaAs (0 0 1) 2 x 4 buffer layers and GaAs (0 0 1) wafers following the procedures described in Refs. [5,6]. All of the semiconductor layers were n-doped (typically n - l-3 x lOi* cme3). The surface reconstruction was monitored by means of reflection high-energy

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M. Lazzarino et al. /Journal of Cytal

electron diffraction (RHEED). An amorphous Se cap layer was used to protect the samples during transfer in air to the photoelectron spectrometer connected with a preparation chamber. The Se cap layer was thermally desorbed in the preparation chamber using a thermocouple and an infrared optical pyrometer to monitor the sample temperature. Different surface reconstructions were obtained by varying the annealing conditions as described in the next section. Aluminum was deposited by thermal evaporation on the ZnSe substrate kept at room temperature, and the metal thickness was determined using a quartz thickness monitor. Layer thicknesses in the 2-3 nm range were typically used for in situ Schottky barrier determination. The barrier height was determined by photoemission spectroscopy (XPS) using a monochromatic X-ray photoemission spectrometer with Al K, radiation (1486.6 eV), an overall energy resolution of -0.8 eV, and an effective photoelectron escape depth of - 1.5 nm. Selected high-resolution results were also obtained using synchrotron radiation from the Aladdin 0.8 GeV electron storage ring at the Synchrotron Radiation Center of the University of Wisconsin-Madison. The radiation was monochromatized by means of a 6 m thoroidal grating monochromator, and the overall energy resolution (electrons + photons) was 0.14 eV. Fig. 1 shows 10 keV RHEED patterns from the four surface reconstruction sequentially observed (top to bottom) during desorption of the Se cap layer from the ZnSe surface. The patterns in the left column were obtained for a (1 0 0) sample azimuth, while those in the right column were recorded for a (1 10) azimuth. The two topmost patterns (Fig. la) were observed at 260°C and correspond to the Se-rich 1 x 1 reconstruction. Increasing the annealing temperature by 5-10°C leads to the emergence of a weak intermediate line in the RHEED pattern recorded in the (10 0) azimuth (see Fig. lb, left column), suggesting a Serich c(2 x 2) structure. The existence of a Se-rich c(2 x 2) surface reconstruction was recently predicted for 1.5 ML of selenium coverage [7] and a Te-rich c(2 x 2) reconstruction was, indeed, observed on the ZnTe (1 0 0) surface [S].

Growth 1841185 (1998) 193-198

For an annealing temperature of 330°C the RHEED patterns (Fig. lc) clearly identify the Sestabilized 2 x 1 reconstruction. At higher annealing temperatures (2 43O”Q the well-known c(2 x 2) reconstruction was observed (Fig. Id). Such a reconstruction was found to be stable for annealing temperatures as high as 600°C. The three reconstructions depicted in Fig. la, Fig. lc and Fig. Id have all been reported before. The Zn stabilized c(2 x 2) reconstruction (Fig. Id) is usually observed during ZnSe MBE in Zn-rich growth conditions, and corresponds to a surface terminated by half a ML of Zn atoms on a complete ML of Se, i.e., to an ordered array of Zn vacancies on the outermost atomic layer [9]. The Se-stabilized 2 x 1 reconstruction is usually observed during ZnSe MBE in Se-rich growth conditions, and is characterized by a fully dimerized monolayer of Se at the surface [3,4]. Recent first principle total energy calculations have shown the c(2 x 2) and 2 x 1 reconstructions to be the lowest energy configurations among those examined for Zn-rich and Se-rich surfaces, respectively [lo]. The Se-rich 1 x 1 reconstruction in Fig. la is typically not observed during ZnSe MBE, and has only been reported during desorption of a Se cap layer. Although we are not aware of any published photographs of the corresponding RHEED patterns, Kahn and coworkers reported a Se-rich 1 x 1 reconstruction for an annealing temperature of - 200°C and tentatively associated this reconstruction with a presence of 2-3 ML of excess Se on the surface [3,4,10]. Lopinsky et al. studied the same surface and reported compelling similarities between the corresponding electronic states and those observed by electron energy loss spectroscopy upon deposition of a single amorphous monolayer of Se onto an unreconstructed, Se-terminated ZnSe surface [ll]. In our experimental conditions we could estimate the excess Se coverage corresponding to the 1 x 1 reconstruction as compared to the other reconstructions. Synchrotron radiation photoemission studies of the Se/Zn intensity ratio for the three surfaces at a photon energy of 120 eV were found to be consistent with an excess Se-coverage of 0.4 f 0.2 ML relative to the Se-terminated, 2 x 1 reconstruction [12].

M. Lazzarino et al. /Journal

of Crystal Growth 1841185 (1998) 193-198

195

Fig. 1. Reflection high-energy electron diffraction 10 keV patterns from the four surface reconstructions sequentially observed (top to bottom) during desorption of a Se cap layer from a ZnSe (0 0 1) surface. The patterns in the left column were obtained for a (10 0) sample azimuth, while those in the right column were recorded for a (1 1 0) azimuth. (a) Se-rich 1 x 1 reconstruction; (b) Se-rich c(2 x 2) reconstruction; (c) Se-stabilized 2 x 1 reconstruction; (d) c(2 x 2) reconstruction.

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M. Lazzarino et al. /Journal

of Crystal Growth 1841185 (1998) 193-198

Al deposition was performed in-situ on the 1 x 1, 2 x 1 and c(2 x 2) surface reconstructions illustrated in Fig. la, Fig. lc and Fig. Id. We did not attempt studies of Al deposition on the Se-stabilized c(2 x 2) reconstruction illustrated in Fig. lb because of the difficulty in consistently achieving such a reconstruction, which appears to be an intermediate stage of the 1 x 1 + 2 x 1 conversion. The Schottky barrier was determined in situ by XPS from the measured position of the Zn and Se 3d core levels before and after deposition of 1.5 and 3 nm thick Al overlayers. Prior to metal deposition, the position of the 3d centroids relative to the valence-band maximum was determined using a least-squares fit and a linear extrapolation of the leading edge of the valence band [13]. Upon deposition of 1.5 and 3 nm of Al, new measurements of the core level positions and the known position of the spectrometer Fermi level EF were used to obtain the p-type Schottky barrier c$, - E, - E,. The n-type barrier was estimated as c#J,,- E, - E, with E, - E, = 2.70 eV for ZnSe at room temperature. The procedure is illustrated in Fig. 2 using the emission from the Zn 3d core levels in Al/ZnSe junctions fabricated by depositing 3 nm thick Al overlayers on the c(2 x 2), 2 x 1 and 1 x 1 surface reconstructions (top to bottom, respectively). Prior to metal deposition, the centroid of the Zn 3d core doublet was found to lie 9.16 + 0.04 eV below the top of the valence band for all surfaces [14]. Upon metal deposition, the Al 2p core-level emission appears 72.87 f 0.04 eV below EF (leftmost section of Fig. 2) - i.e., at the position expected for elemental metallic Al [13] for all interfaces examined. The position of the Zn 3d centroid varies instead for the three interface, and can be used to infer the position of the valenceband maximum E, relative to EF (center and rightmost sections of Fig. 2). From the results in Fig. 2 we determined & = 0.55 f 0.06 eV (top), & = 0.59 + 0.06 (center) and 4” = 0.79 + 0.06 eV (bottom) for interfaces fabricated on c(2 x 2) 2 x 1 and 1 x 1 reconstructions, respectively. While the results for the c(2 x 2) and 2 x 1 reconstructions are consistent within the experimental uncertainty, a 0.24 eV reduction in the barrier is observed for interfaces fabricated on the Se-rich 1 x 1 surface reconstruction.

-4

3

d

L

hv= 1486.6

eV

Al 2p

Zn 3d

I

I

E,

E,E,

I I I

.z? 3 c” -z .-K c2

.-0

.-rJ E 3 % 2

Binding

energy

(eV)

Fig. 2. Al 2p (left) and Zn 3d (center) core-level emission, together with the corresponding position of the Fermi level EF in the gap for Al/ZnSe (0 0 1) junctions fabricated by depositing 3 nm thick Al overlayers on ~(2 x 2) 2 x 1 and 1 x 1 surface reconstructions (top to bottom, respectively). All results were obtained by means of X-ray photoemission spectroscopy at a photon energy of 1486.6 eV.

We emphasize that the core-level shifts observed in Fig. 2 reflect a true change in the Schottky barrier as opposed to surface photovoltage variations or chemical shifts. First, in the presence of continuous Al layers 1.5 and 3 nm thick, no surface photo[12,13], as voltage effects were observed demonstrated by the constant 3d core-level position as a function of Al coverage, and by the Al 2p core-level position. Second, Zn 3d chemical shifts due to Zn-Al alloying as a result of Se-Al reactions give rise to a low binding energy doublet clearly resolved in Fig. 2, but do not affect the line shape of the main Zn 3d feature, which reflects the ZnSe bulk below the overlayer. Selected high-resolution results for junctions fabricated on the 1 x 1 reconstruction are summarized in Fig. 3. In the topmost section we show the Zn 3d emission - 9.13 + 0.03 eV below the leading edge of the valence band - prior to metal deposition. In the lower section we show the Al 2p and Zn 3d emission after deposition of a 3 nm thick Al overlayer.

M. Lazzarino et al. /Journal

1 xl

of Crystal Growth 184j18.Y (1998) 193-198

reconstruction

hv=120

eV

Al 2p

Binding

Zn

I

3d

energy

(eV)

Fig. 3. High-resolution, synchrotron radiation photoemission studies of Al/&Se (0 0 1) junctions fabricated on the Se-rich 1 x 1 reconstruction. Top: Zn 3d emission and leading valence band edge prior to metal deposition. Bottom: Al 2p (left) and Zn 3d (center) emission after deposition of a 3 nm thick Al overlayer, together with the corresponding position of E, in the gap. The low binding energy 3d doublet (dashed line) reflects Zn atoms segregated in the metallic overlayer. The high binding energy 3d doublet (solid line) derives from the semiconductor substrate.

At the selected Al coverage the overlayer is metallic, as shown by the characteristic metallic line shape of the Al 2p doublet. The Zn 3d line shape (solid circles) shows two well-defined contributions (solid and dashed lines). The low binding energy doublet (dashed line) develops during the early stages of interface formation, and reflects Zn atoms displaced from the semiconductor following an Al-Se interface reaction, and segregated in the metallic environment of the overlayer. This reacted component is visible as a shoulder in the XPS results of Fig. 2, while it is emphasized in the synchrotron radiation results of Fig. 3 due to the higher surface sensitivity [ 151. The high binding energy doublet in Fig. 3 (solid line) derives from the semiconductor substrate, and can be used to monitor band bending at the semiconductor surface. The results in the lower section of Fig. 3 are consistent with

197

$,, = 0.78 + 0.04 eV, i.e., in good agreement with the XPS result 4” = 0.79 ) 0.06 eV from Fig. 2. Previous determinations of the n-type Schottky barrier for Al/ZnSe junctions focused on the c(2 x 2) reconstruction and yielded values of 0.55 + 0.10 eV) [16] and 0.58 + 0.10 eV [17], consistent with those reported here for the same reconstruction. The 0.24 eV increase in the n-type barrier (decrease in the p-type barrier) observed here for interfaces fabricated on the Se-rich 1 x 1 reconstruction is among the highest reported barrier changes versus semiconductor reconstruction [2]. Kahn and coworkers recently examined Au/ZnSe (0 0 1) interfaces and observed a similar decrease in the p-type barrier in comparing junctions fabricated on c(2 x 2) and 1 x 1 reconstructions [3,4]. An analogous interpretation of this effect in terms of metal-Se charge transfer at the interface [3,4], however, seems precluded by the vastly different variation at the interface electronegativity (Ax = 0.94 for Al-Se versus 0.01 for Au-Se in Pauling’s scale) in the presence of similar barrier changes (0.24-0.25 eV). A true microscopic understanding of these effects is likely to require first principle calculations of the local interface dipole as a function of the atomic interface termination such as those which have recently been clarifying the role of heterovalent interface layers in tunable Schottky barriers [18]. From a purely phenomenological point of view, the trend toward a lower p-type Schottky barrier with increasingly Se-rich surface compositions might have important technological implications in view of the current need for lower specific resistance contacts to p-type wide-gap II-VI materials. In conclusion, Al/ZnSe (0 0 1) contacts were fabricated by Al deposition on ZnSe surfaces with different atomic terminations and long-range order. The value of the Schottky barrier was determined in situ through photoemission studies. On the Se-rich ZnSe (0 0 1) 1 x 1 reconstruction, we found an n-type Schottky barrier of 0.79 f 0.06 eV. On the 2 x 1 and on the c(2 x 2) reconstructions the corresponding values were 0.59 + 0.06 and 0.55 f 0.06 eV, respectively. The resulting 30% change in barrier height with surface reconstruction is among the highest ever reported for a metal/semiconductor junction.

198

M. Lazzarino et al. 1 Journal of Crystal Growth 184/185 (1998) 193-198

The work in Trieste was supported in part by INFM under the TUSBAR Advanced Research Project. The work in Minneapolis was supported in part by the National Science Foundation under grant DMR-9116436. Useful discussions with N. Binggeli and A. Kahn are gratefully acknowledged.

References [l] [2]

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[4] [S] [6] [7] [S]

See, for example, R.L. Gunshor, A.V. Nurmikko (Eds.), Proc. SPIE 2346 (1994) 1. See, for example, L.J. Brillson, in: P.T. Landsberg (Ed.), Handbook on Semiconductors, vol. 1, Elsevier, Amsterdam, 1992, p. 281, and references therein. W. Chen, A. Kahn, P. Soukiassan, P.S. Mangat, J. Gaines, C. Ponzoni, D. Olego, J. Vat. Sci. Technol. B 12 (1994) 2639. W. Chen, J. Gaines, C. Ponzoni, D. Olego, P.S. Mangat, P. Soukiassian, A. Kahn, J. Crystal Growth 138 (1994) 1078. G. Bratina, L. Vanzetti, R. Nicolini, L. Sorba, X. Yu, A. Franciosi, G. Mula, A. Mura, Physica B 185 (1993) 557. G. Bratina, R. Nicolini, L. Sorba, L. Vanzetti, G. Mula, X. Yu, A. Franciosi, J. Crystal Growth 127 (1993) 387. A. Garcia, J.E. Northrup, Appl. Phys. Lett. 65 (1994) 708. B. Daudin, S. Tatarenko, D. Brun-Le Cunff, Phys. Rev. B (1995) 7822.

[9] H.H. Farrel, M.C. Tamargo, S.M. Shibli, J. Vat. Sci. Technol. B 8 (1991) 884. [lo] C.H. Park, D.J. Chadi, Phys. Rev. B 49 (1994) 16467. [11] G.P. Lopinsky, J.R. Fox, J.S. Lannin, F.S. Flack, N. Samarth, Surf. Sci. Lett. 355 (1996) L355. [12] M. Lazzarino, G. Scarel, S. Rubini, G. Bratina, A. Bonanni, L. Sorba, A. Franciosi, C. Berthod, N. Binggeli, A. Baldereschi, unpublished. [13] M. Cantile, L. Sorba, P. Faraci, S. Yildirim, G. Biasiol, G. Bratina, A. Franciosi, T.J. Miller, M.I. Nathan, L. Tapfer, J. Vat. Sci. Technol. B 12 (1994) 2653. [14] In the case of the c(2 x 2) surface, a Zn surface contribution to the 3d line shape is detected using highly surface sensitive, soft-X-ray synchrotron radiation photoemission. In the XPS results, however, the reduced surface sensitivity makes this contribution negligible, and the Zn 3d line shape and position prior to metallization were found to be independent of the surface reconstruction. [15] M. Lazzarino, T. Ozzello, G. Bratina, J.J. Paggel, L. Vanzetti, L. Sorba, A. Franciosi, Appl. Phys. Lett. 68 (1996) 370. [16] W. Chen, A. Kahn, P. Soukiassan, P.S. Mangat, J. Gaines, C. Ponzoni, D. Olego, Phys. Rev. B 49 (1994) 10790. [17] M. Vos, F. Xu, S.G. Anderson, J.H. Weaver, H. Cheng, Phys. Rev. B 39 (1989) 10744. [18] C. Berthod, N. Binggeli, A. Baldereschi, Europhys. Lett. 36 (1996) 67.