~ Solid State Communications, Vol.40, ppo521-523. Pergamon Press Ltd. 1981. Printed in Great Britain.

0038-1098/81/410521-03502.00/0

ELECTRONIC STRUCTURE OF As~Ss AS A LOCAL STRUCTURAL MODEL FOR AMORPHOUS As2S3 FILM

T. Yamabe, K. Tanaka,* A. Tachlbana,** Y. Kobayashl, H. Teramae, and K. Fukul

Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan

(Received 25 August 1981 by H. Kawamura)

We have studied the electronic structure of As~Ss molecule as a model of the local pyramidal structure in amorphous As2Ss film. Emphasis has been put on the analysis of the behaviour of a trapped electron in this model concerning the consequent structural relaxation. It has been found that AsbSs species has affinity for an excess electron, and that the attachment of an excess electron promotes the cleavages of specific As-S bonds.

Local structure in amorphous As2S3 (a-As2 Ss) has been highly controversial in the last decade, 1'2 since the short-range order is con- sidered to be of central importance in deter- mining the electronic function in amorphous chalcogenide semiconductors. 3 The dominant geometrical models for the local structure of a-As2Ss are two-dlmensional layered crystalline As2Ss (orpiment), dense random packing of As~Ss molecules, and As~S~ (realgar) units deduced from various spectroscopic points of view. ~-11 One of the most decisive findings is the proba- ble existence of AsS3 pyramidal units in a-As2 $3 by infrared, Raman, and X-ray spectroscopy. 10,12-1s In particular, recent 7SAs nuclear quadrupole resonance measurement of the 300K- evaporated a-AszSs film 16 has strongly indi- cated the presence of AsSs pyramidal unit lack- ing in the longer-range order (orpiment-like correlation) which is present in the bulk a-As2 $3. Based on this result, the presence of a new kind of molecular cluster such as As~Ss has been suggested. 16 This molecule occurs natu- rally forming a van der Waals crystal, 17 and has a fused structure of two AsSs pyramidal units (As6-SIS2S~ and AsT-SIS3S5) as is shown in Figure i. Although there have been a good deal of investigations on the electronic struc- ture of the local structural models such as As2 $3 (orpiment), As~Se, or As~S~ for a-As2Ss, ~-e' s,9,1s,19 no studies on the electronic struc- ture of As~Ss have ever been performed. In this Communication, therefore, we attempt to study the electronic structure of As~Ss mole- cule as a model of the local pyramidal struc- ture in a-As2S3 film for the first time. In particular, we would like to examine in detail the behaviour of a trapped electron in this model concerning the possible tendency of the consequent structural relaxation. The calcu- lations are based on the seml-empirical INDO (Intermediate Neglect of Differential Overlap) level of the Hartree-Fock molecular orbital

*On leave from the Energy Conversion De- vices, Inc., Troy, Michigan 48084 U.S.A.

**Present address: Department of Chemistry, Shiga University of Medical Science, Otsu 520-21 Japan

js, / / .I" / ", s9 / s2 Z

X

>Y

Fig. i. The geometry of As~Ss being of approx- imate C2v sy~netry employed in the present cal- culation. Bond lengths and angles are the same with those obtained for the crystalline phase (see Ref. 17).

521

(MO) method for valence electrons. 2° The ge- ometry in Figure 1 is employed for the calcu- lation. The behaviour of an excess electron in As~Ss is studied by the analysis of the electronic structure of the monoanion As~Ss-, having the same geometry as As~Ss. For this anionic species with an open-shell structure, the unrestricted Hartree-Fock (D~HF) MO method- ology 21 is adopted. Spin contamination inher- ent in the UHF scheme is excluded by Lowdin's projection operator method. 22'2~ The method of calculations employed here has given fairly reasonable results for the,electrQnic structure of $2N2, S~N~, Ss~Se~, Sa 2~, Se82~, S~ 2~, Se~ 2~, and Te~2~, 2~-27 as well as those for As~S6 and As~S~ as local structural models for a-As2S3.19 The parameters adopted for As and S are the same as those in Refs. 25 and 20, re- spectively. In the present calculations, we neglect the d atomic orbltals (AO's) of sulfur and arsenic for the sake of simplicity, since

Table i. The atomic net charges of As~Ss and As~Ss-.

ELECTRONIC STRUCTURE OF As~S 5 Vol. 40, No. 5

this omission does not seem to be critical in most cases of the molecules including chalcogen

2 8 - 3 1 atoms. The calculated AO densities of S1 indicate

the presence of two lone-palrs each consisting of 38 and 3p AO components. The other sulfur atoms (S2%SsT also have two lone-palrs, but the components of them are each 38 and 3pZ-3p, mixed AO's. All the arsenic atoms have ~ne lone-palr mainly occurring from 48 A0 compo- nents. The atomic net charges listed in Table 1 show that all the sulfur atoms in As~Ss are

As~Ss As~Ss-

Z

X

Si -0.558 -0.618 S2, S3 -0.500 -0.603 S~, Ss -0.492 -0.592 As6, As7 +0.794 +0.707

Ase +0.480 +0.275 As9 +0.474 +0.319

negatively charged reflecting the order of the electronegativity: As<S. 32 The pattern of the highest occupied molecular orbital (HOMO) of As~Ss is of the ppo and the pp~ (4Pa-4Pa) bond- ing character between Ase and As9. The pat- terns of the (HO-2)~(HO-4)MO's are assigned to be the p lone-pairs of sulfurs. The energy level of the HOMO is -9.94eV and those of p lone-palrs of sulfurs are -ll.05~-12.00eV. This trend seems to be co,~on to As-rich chal- cogenides such as As~S~ species, le'19'33

The vertical electron affinity of As~Ss is calculated to be 0.923eV. This quantity is defined as the difference between the total en- ergy of As~Ss and that of As~Ss- without the structural relaxation. The present positive value signifies that As~Ss local unit is apt to attract another electron which might be itiner- ating in the a-As2S3 matrix as a charge carri- er. As As~Ss captures an excess electron, the lowest unoccupied MO (LUMO) of As~Ss becomes to be half occupied. In this respect, it would be interesting to analyze the pattern of the LUMO of As~Ss, which is illustrated in Figure 2. Since the 4p. AO's of Ass and As9 are the largest component~ in the LUMO, one can expect that the nature of another pp~ (4p -4p.) bond between As0 and As9 will come out ~n t~e case of As~Ss-. As a matter of fact, 4py AO den- sities of Ase and As9 are calculated to be in- creased considerably according as As~Ss changes into As~Ss • This also influences the atomic net charges of Ass and Ass in As~Ss-, reducing the positive net charges from e~. 0.5 to 0.3 as shown in Table i. Another important feature is that the four As-S bonds (AsÙ-S2, Ass-S3, As9-S~, and As9-Ss) connecting the top portion (fused structure of two AsS3 pyramidal units consisting of SI~Ss, Ass, and As7) and the bot- tom portion (Ass-As9 unit) should become weaker according to the accommodation of an excess electron, because of the antlbonding combina- tions between the 3F -3pz coupled AO's of S2~ Ss and the 4p~ AO's ~f As8 and As9 as shown in the pattern o~ the LUMO of As~Ss. Simulta- neously, these four sulfur atoms become more

522

Fig. 2. The pattern of the LUMO of As~Ss. atomic positions are the same with those in Figure i.

The

negatively charged showing a tendency to have the third lone-pair occurring from these 3p - 3p~ coupled AO components. The intera~omi~ interaction energies in As~Ss and As~Ss can be analyzed by the partition of the total energy. 3~ The calculated results show that all the As-S bonds between the top and the bottom por- tion become weaker by 1.527eV in average, and that the Ass-As9 bond stronger by 1.515eV due to the presence of an excess electron. These results indicate that the cleavages of the As- S bonds are promoted leading to the separation between the top and the bottom portions in the As~Ss unit as an excess electron is captured.

The minimum singlet excitation energy of As~Ss is calculated to be 4.683eV assigned as the electronic transition from the HOMO to the LUMO. This signifies the transparency of As~Ss toward the photon with the energy of 2.41 eV corresponding to the absorption edge of a- As2S3. 3s This is in contrast with the minimum singlet excitation energy, 2.503eV, of As~S6 previously calculated. 19

In conclusion, we have shown (1)As~Ss spe- cies has a tendency to accommodate an excess electron which might be itineratlng around this unit in a-As2S3 matrix, (2)when As~S5 captures an electron, a consequent structural relaxation is likely to occur, in which the fused struc- ture of two AsS3 pyramidal units separates the counter moiety, and (3)As~Ss is transparent toward the photon with the energy less than 4.683eV.

Vol. 40, No. 5 ELECTRONIC STRUCTURE OF As~S 5

Acknowledgement - We are grateful to the Data Processing Center of Kyoto University for its permission to use the FACOMM200 Computer.

This work was partly supported by a Grant-ln- Aid for Scientific Research from the Ministry of Education of Japan (Grant No. 255315).

523

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