SHORT COMMUNICATION

The Raman Spectrum of Cd3Asz

S. Jandl,* S. Desgreniers, C. Carlone and M. 1. Aubin Groupe de Recherche sur les Semiconducteurs et les DiClectriques, Dipartement de Physique, UniversitC de Sherbrooke, Sherbrooke, QuCbec, Canada, J1K 2R1

The first Raman data for Cd3As, are reported, showing nine peaks at room temperature with a strong fluorescent background.

Cadmium arsenide has been the subject of many publica- tions concerning transport measurements because of its unusual electrical proper tie^,'-^ name1 an unavoidable large electron concentration (ca. 10 cm 3, coupled with a high mobility (> lo 000 cm2 V-' s-' at room temperature). Several optical and magneto-optical experiment^^-^ have also been reported, but these were often hampered by the large Burstein shift forbidding transitions to or near the bottom of the conduction band. Several models of the electronic band structure have been proposed over the years, starting with an isotropic picture and going over to a slightly anisotropic pic- ture'-" which reflects the crystal structure. The thermal energy gap in these band models is essentially zero.

The crystal structure itself will, of course, affect the phonon spectrum, which was completely unknown until recently. The only data available so far are the infrared reflection data of Gelten and van Es12 and those of Thielmann et a l l3 , who reported three and five different phonons, respectively. One would expect the phonon spectrum to be involved because of its crystal structure. The a phase, which exists at room temperature and below, has 160 atoms per unit ce1114 in a body-centered tetragonal structure (C:',). However, one in every six sites is a Cd vacancy. If these vacancies were filled, the unit cell would then contain 10 atoms in a fluorite structure.

Using the real structure we analyzed by group theory the 240 normal modes at the center r of the Brillouin zone and deduced that these can be described by the

7 -

I 1cm-l) I I I I

10 100 200 300 400

Figure 1. Room-temperature unpolarized Raman spectrum of CdaAs,.

* Author to whom correspondence should be addressed.

irreducible representations of the C4, point group as

r = 64E +28(A1+A2,+B1+ B2).

According to the group analysis, one expects the 63E and 27A1 modes to be infrared active and the 63E, 27A1 and 28B2 modes to be Raman active. These num- bers are in sharp contrast with the three or five modes observed in the infrared reflectivity and with the absence of any Raman data in the literature.

In this paper we report on the first measurements of the Raman spectrum of Cd3As2. The single crystals that were used had been obtained by vapour deposition as described in Ref. 15. Both argon ion and krypton ion laser lines were used at 300 and 20 K. With the 6471 8, krypton ion line and 200 mW power on the sample, we observed nine phonons at 300 K with a strong fluores- cent background. At 20 K the fluorescent background was enhanced by a factor of 8, masking the observed room-temperature phonons. In Fig. 1 the Raman spec- trum is shown and the observed phonon frequencies are given with the infrared ones in Table 1.

No Raman peaks were observed with the argon ion lines. Exciting Cd3As2 with the A = 6471 8, line enhances the Raman scattering process since it corresponds to a pre-resonance condition as may be seen from thermo- reflectance measurements.8

Table 1. Observed room-temperature Raman frequencies of Cd3As, and the infrared frequencies from Refs 12 and 13

Infrared frequencies Raman frequencies (cm-')

(cm-') -

Ref 12 Ref 13 (this work)

26 35 45 47 57 62 65 95

114 158 155 189 183

194 196 219 220

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JOURNAL OF RAMAN SPECTROSCOPY, VOL. 15, NO. 2, 1984 137

S. JANDL, S. DESGRENIERS, C. CARLONE AND M. J. AUBIN

Two remarks follow from this work: expected seem to contribute to the Raman scattering. Such a situation has been observed in allogermanium,*6 which also has a large unit cell (128 atoms) and where very few phonons were detected in Raman scattering.

1. The small number of observed pre-resonant Raman phonons points to very weak polarizability tensors in the Cd3As2 structure. These tensors are even weaker in the case of the infrared and Raman-active modes since only one of them, namely the 194cm-' mode, is observed. Acknowledgement 2. The large number of vacancies inherent in the crystal structure should relax the group theory selection rules and allow even more phonons, but fewer phonons than

The authors acknowledge the valuable help of J. Y. Harbec in the determination of the symmetry characteristics of CdgAs, phonons.

REFERENCES

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(1 983).

Received 23 August 1983

138 JOURNAL OF RAMAN SPECTROSCOPY, VOL 15, NO. 2.1984