Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 681–684

Photoemission and inverse-photoemission studies of glassyAs Sex 12x

*Y. Hayashi, H. Sato , M. TaniguchiFaculty of Science, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 739-8526, Japan

Abstract

Valence-band and conduction-band electronic structure of glassy (g-) As Se with x50, 0.16, 0.34, 0.40, 0.44 and 0.55x 12x

has been investigated by means of in situ measurements of photoemission and inverse-photoemission spectra. The spectraexhibit changes at x50.4 in the energy position of Se 4p lone-pair states, the band gap energy and the widths of valencebands and the main peak in the inverse-photoemission spectra. Two peak structures derived from Se-chains also disappear atx50.4. These changes support the percolation phenomenon predicted for g-As Se . 1999 Elsevier Science B.V. Allx 12x

rights reserved.

Keywords: Glassy As Se ; Electronic structure; Percolationx 12x

1. Introduction material and g-As Se (x50.4) is assumed to be2 3

composed of the corner-sharing As(Se ) units1 / 2 3

Glassy semiconductors of the arsenic selenide without long range order [1]. A number of workssystem, g-As Se , have been of continued sci- have been performed, such as Raman scatteringx 12x

entific and practical interest, because of the real spectroscopy [2], low-temperature specific heat ex-opportunity of their technical uses such as photo- periments [3], far infrared reflection spectroscopyreceptor materials in electrographic applications. In [1], and so on.this system, the coordination number obeys the so- In this study, we have investigated valence-bandcalled 8-N rule, where N is the valency of an atom, and conduction-band electronic structure of g-and the material is the 3(As):2(Se) coordination As Se by means of the ultraviolet photoemissionx 12x

system. The local structure of g-Se (x50) consists of and inverse-photoemission spectroscopies (UPS andthe chains without order between them. Two 4p-like IPES). The x-dependence of the UPS and IPES

4electrons of (4p) configurations form covalent spectra is discussed in terms of the Phillips–Thorpebonds with the nearest neighbors, while the remain- rigidity percolation theory [4–6].ing two form lone-pair (LP) states. With an increaseof x, As(Se ) pyramidal units increase in the1 / 2 3

2. Experimental

*Corresponding author. Tel.: 181-824-24-7400; fax: 181-824-24-The apparatus for the UPS and IPES experiments0719.

are schematically shown in Fig. 2 of Ref. [7]. TheE-mail address: sato@hisor.material.sci.hiroshima-u.ac.jp (H.Sato) UPS spectrometer is composed of a He discharge

0368-2048/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 98 )00336-3

682 Y. Hayashi et al. / Journal of Electron Spectroscopy and Related Phenomena 101 –103 (1999) 681 –684

lamp (hn521.2 eV) and a double-stage cylindrical-mirror analyzer (DCMA). The kinetic energy ofphotoelectrons passing through the DCMA was set tobe 16.0 eV, corresponding to the energy resolution of0.2 eV. The base pressure of the UPS chamber was

2105310 Torr.The IPES spectrometer [8,9] connected to the UPS

chamber consists of a low-energy electron gun withErdmann–Zipf type, an Al reflection mirror coatedwith MgF film and the bandpass photon detector2

centered at 9.4 eV with a full width at a halfmaximum (FWHM) of 0.47 eV. The overall energyresolution including the thermal energy spread of0.25 eV of the electron gun is 0.56 eV. The base

210pressure of the IPES chamber was 1310 Torr.Energy calibration of the UPS and IPES spectra is

carried out using the spectra for polycrystalline Aufilm. The UPS and IPES spectra were in situmeasured for the same sample-surface and then thesespectra were connected at the Fermi level. Allexperiments were performed at room temperature.

The g-As Se films used for the present experi-x 12x

ments were prepared in the sample preparationchamber, which are connected to the UPS and IPESchambers, by thermal evaporation of the bulk materi-als onto Au substrates at room temperature. The bulkmaterials were grown by the standard melt-quench-ing method. During the evaporation, the pressure was

29of the order of 10 Torr. The thickness of the filmwas measured by the quartz monitor and the typical

Fig. 1. A series of the valence-band UPS and conduction-band˚value for the experiments is about 100 A. The UPS IPES spectra of g-As Se with x from 0 to 0.55. Energies arex 12xand IPES spectra were successfully collected with no referred to the valence-band maximum. Long and short verticalcharging effect for these enough thin films. bars indicate the positions of structures.

Non-crystalline forms of the films were confirmedfrom no peak in X-ray powder diffraction pattern andthe compositions were estimated by electron-probe energy is relative to the valence-band maximummicroanalysis for the thick films of |1 mm, which determined by a linear extrapolation of the leadingwere grown under the same condition as the film for edge to the base line. The present UPS spectra arethe UPS and IPES experiments. We also checked the consistent with the previous work by Kitahara andcompositions from the intensities of X-ray photo- Arai [10]. The UPS and IPES spectra at x50 (g-Se)emission spectra of As 3d and Se 3d core emissions consisting of the chain structure, exhibit structures atfor the thin films. 25.4 and 24.4, 21.6, 4.1 and |8 eV [7]. Their

structures are assigned to 4p-like bonding, 4p-likeLP, 4p-like antibonding and 4d and/or 5s states,

3. Results and discussion respectively. Here it should be noted that the twopeak structures in the 4p-like bonding state region is

Fig. 1 shows UPS and IPES spectra of g-As Se characteristic of the Se chain structure.x 12x

with x50, 0.16, 0.34, 0.40, 0.44 and 0.55. The As seen from Fig. 1, the UPS spectra with x50.16

Y. Hayashi et al. / Journal of Electron Spectroscopy and Related Phenomena 101 –103 (1999) 681 –684 683

and 0.34 are similar to that of g-Se. The energy proposed that the covalent bonding may be opti-position of the structure due to the Se 4p-like LP mized when the number of constraints equals thestate is almost unchanged at 21.6 eV. In addition, the dimensionality or number of degrees of freedom intwo peak structures around 25 eV are also observed the system [4]. Thorpe predicted that a rigiditywith a slight blurring. At x50.4, the energy position percolation takes place as an average coordinationof the LP-derived structure shifts to shallower energy number krl of the system passes through the thres-side by 0.2 eV. The two peak structures completely hold value of 2.4 [5].disappear and become just one peak at 24.9 eV (A). According to the Phillips–Thorpe rigidity percola-The weak structure also shows up at 22.9 eV (B). tion theory, krl52.4 is achieved at x50.4 for g-The UPS spectra with x$0.4, the energy positions of As Se . Below x50.4, an As atom is bonded tox 12x

structures A, B and the LP-derived one, are un- three Se atoms and As(Se ) pyramidal units are1 / 2 3

changed, and the structure B becomes prominent partly formed in the material. In this stage, Se chainswith the increase of x. We also notice that the width still exist between the clusters, which is supported byof the valence bands becomes somewhat narrow the fact that we notice the two peak structures due toaround and above x50.4. The structure around 210 Se chains in the spectra at x50.16 and 0.34. Just ateV is due to the As 4s state. x50.4, the rigid cluster units percolate over the

On the other hand, the IPES spectra are found to material. The extreme values of the properties, suchfollow the behaviors of UPS spectra. The FWHM of as the optical band gap energy [11], activationthe main peak around 4 eV increases with x from 0 energy of the electrical conductivity [13], transverseto 0.40; 2.4 (x50), 2.6 (x50.16), 2.8 (x50.34) and optical phonon frequency [1], atomic volume [14],3.0 eV (x50.40). For x$0.40, the FWHM is almost and so on, appear around x50.4. The changes of theunchanged at 3.0 eV. Furthermore, the conduction- spectral features at x50.4, such as the energyband minimum determined by a linear extrapolation position of the LP-derived structure, disappearanceof the leading edge to the base line shifts toward of the two peak structures around 25 eV, the widthlower energy side from x50 to 0.40, which means of the valence bands, the band gap energy derivedthat the band gap energy becomes narrow with x. from the UPS and IPES spectra and the FWHM ofThis trend of the band gap energy is consistent with the main peak in the IPES spectra, are assumed to beresults of the optical measurements [11]. The optical characterized by the percolation of the As(Se )1 / 2 3

band gap energy takes a minimum value around pyramidal units over the material.x50.4 and again increases with x gradually. This Recently, we have investigated valence-band andcomposition dependence above x50.40 is not ob- conduction-band electronic structure of g-Ge Sex 12x

served clearly due to the experimental resolution. (0#x#0.33) by means of UPS and IPES [15]. In theFrom these composition dependence of the spec- case of g-Ge Se , which is a 4(Ge):2(Se) coordi-x 12x

tral features, it is reasonable to assume that x50.4 is nation system, krl52.4 takes place at x50.2. Thea critical point of the electronic structure of g- UPS and IPES spectra for x#0.18 do not show anyAs Se . The UPS and IPES spectra at x50.4 noticeable change in comparison with that of g-Se.x 12x

(g-As Se ) are in good agreement with the theoret- Just at the rigidity percolation threshold (x50.2), the2 3

ical density of states derived from the band-structure spectra exhibit a drastic change with respect to theircalculation for the crystalline As Se composed of spectral shapes and energy positions of the structures2 3

the corner-sharing As(Se ) pyramidal units [12]. and become close to that of g-GeSe . In particular, a1 / 2 3 2

The structures at 24.9 (A) and 22.9 (B), 21.4, 3.6 main peak of the IPES spectrum of g-Se splits intoand |8 eV are attributed to the As–Se bonding, Se two peaks at x50.20 due to Ge–Se antibonding4p-like LP, As–Se antibonding states and 4d and/or states. Above x50.20, the spectral feature gradually5s states of As and Se atoms, respectively. For and continuously approaches that of g-GeSe (x52

x.0.4, As–As bonding states also contribute to the 0.33).structure B. The changes of the spectral features of g-As Sex 12x

For non-crystalline covalent networks constrained near the percolation threshold (x50.4), in particular,by bond-stretching and bond-bending forces, Phillips those of the IPES spectra, are less remarkable in

684 Y. Hayashi et al. / Journal of Electron Spectroscopy and Related Phenomena 101 –103 (1999) 681 –684

[4] J.C. Phillips, J. Non-Cryst. Solids 34 (1979) 153.comparison with the results of g-Ge Se [15]. Thisx 12x[5] M.F. Thorpe, J. Non-Cryst. Solids 57 (1983) 355.would be due to the fact that both the two energy[6] J.C. Phillips, M.F. Thorpe, Solid State Commun. 53 (1985)

levels due to As–Se antibonding states just overlap 699.with the main peak in the IPES spectrum of g-Se, in [7] I. Ono, P.C. Grekos, T. Kouchi, M. Nakatake, M. Tamura, S.contrast to g-GeSe . This is supported by assign- Hosokawa, H. Namatame, M. Taniguchi, J. Phys.: Condens.2

Matter 8 (1996) 7249.ments of structures in the IPES spectrum of g-[8] K. Yokoyama, K. Nishihara, K. Mimura, Y. Hari, M.As Se [16] using As 3d [17] and 2p [18] core-2 3 3 / 2 Taniguchi, Y. Ueda, M. Fujisawa, Rev. Sci. Instrum. 64

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