Raman study of ZnxBe1−xSe solid solutions

Optical Materials 17 (2001) 323±326

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Raman study of ZnxBe1 x Se solid solutions O. Pages a,*, M. Ajjoun a, J.P. Laurenti a, D. Bormann b, C. Chauvet c, E. Tournie c, J.P. Faurie c, O. Gorochov d a

Institut de Physique Groupe de Spectrometrie des Interfaces, Universit e de Metz, 1 Bd. Arago, Technopole 2000, 57078 Metz Cedex 3, France b Laboratoire de Physico-Chimie des Interfaces et Applications, Universit e d'Artois, Rue Jean Souvraz, 62037 Lens, France c Centre de Recherche sur l'H et ero epitaxie et ses Applications (CRHEA-CNRS), Sophia Antipolis, Rue Bernard Gregory, 06560 Valbonne, France d Laboratoire de Physique des Solides (CNRS), Place Aristide Briand, 92195 Meudon, France

Abstract Raman spectroscopy is used to identify the LO phonons of Znx Be1 x Se=GaAs systems in a wide composition range. On the layer side a two-mode behavior is evidenced. The eigenfrequencies of the BeSe- and ZnSe-like LO modes correspond to the maxima of Im‰ e…x; x† 1 Š. Excellent agreement is obtained with a model based upon the modi®ed random element isodisplacement (MREI) model. On the substrate side the LO mode couples with a plasmon (P) at the near-interface. The resulting LO±P mode is very sensitive to limit conditions at the junction. Below x  45% strong coupling is observed; above x  45% coupling is relaxed. The ®rst behavior appears to be a marker for clean interfaces while the second one goes with the deposition of a thin highly disordered layer before the nominal Znx Be1 x Se is grown up to the surface. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: ZnBeSe; GaAs; Raman; LO phonon±plasmon; Interface

1. Introduction There is a renewed interest in large gap II±VI materials for optoelectronic devices in the blue range in view of recent calculations which predicted covalent bonding for Be-chalcogenides [1]. The ®rst aim of this work is to study the polar phonons of Znx Be1 x Se alloys by Raman spectroscopy in a wide composition range with a special attention to the LO modes. The homogeneity in the composition of the alloys is ®rst investigated throughout the whole of the layer by using the microprobe along the slope

*

Corresponding author. E-mail address: [email protected] (O. Pages).

of beveled samples. Special attention is also awarded to the interfacial region, on both sides of the junction, in view of the numerous and puzzling problems currently encountered at more simple ZnSe/GaAs interfaces [2].

2. Experiment Four Znx Be1 x Se epilayers (x ˆ 97%, 86%, 69%, 38%) with thickness between 0.9 and 1.6 lm were grown by MBE onto semi-insulating (SI) (1 0 0) GaAs. For x ˆ 69% (A) and 38% (B) bevelededges were realized by chemical etching so as to obtain slopes as low as 1½ from the top of the layer down to the deep raw substrate. The Raman

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O. Pages et al. / Optical Materials 17 (2001) 323±326

(a)

(b)

Fig. 1. (a) and (b) display Raman spectra recorded along the (0 0 1) slope of beveled heterostructures A and B, respectively. The labeling of the spectra refers to the corresponding impact spots in the insert. In (b) geometries z…x0 x0 †z and z…x0 y 0 †z are considered both at the very near-interface and at the top of the layer. The bold line on the left side refers to an antiresonance.

spectra were obtained with the Dilor microprobe set-up at room temperature in backscattering along the growth axis, so that LO modes only can be observed. By using green excitation at 514.5 nm, from an Ar‡ laser, the samples consist in transparent (large gap)/absorbing (small gap) systems, so that the spectra bring the responses from both the whole of the layer and the interfacial substrate.

3. Results and discussion The raw spectra are shown in Fig. 1. The attention is ®rst focussed upon the spectra recorded from the top of the layers (refer to label J). We ®rst comment on the layer-related signal. The spectra of the alloys show a two-mode behavior, as expected from the MREI model since the mass of substituting element beryllium is smaller than the reduced mass of ZnSe [3]. The peaks labeled as LOZn Se and LOBe Se are, respectively, identi®ed as the ZnSe- and BeSe-like LO phonons since they merge within optical phonon bands [206; 252 cm 1 ] and [501; 579 cm 1 ] from the corresponding parent materials [4]. The frequency of the TOZn Se mode is also indicated although this mode was evidenced by using another geometry [4]. The antiresonance which shows up around 215 cm 1 and undergoes a red shift under x increase reminds of a Fano mode [5]. The latter would be involved with the ZnSe-like TO mode, since the

two modes merge systematically in the same frequency region (refer to Fig. 2). The attention is now focused on the substraterelated response. The two samples illustrate characteristic features on each side of x  55%. The expected LOGaAs mode at 292.3 cm 1 is replaced by either a rather narrow structure, labeled X, centered around the TOGaAs frequency at 268 cm 1 (x > 55%), or a broad band, labeled Y, covering the whole optical band, highly asymmetric on its high-energy side, and centered around the LOGaAs frequency (x < 55%). The assignment of both modes is discussed in the light of (i) their symme-

Fig. 2. Calculated and experimental frequencies of optical phonons. The circles and squares refer, respectively, to the BeSe- and ZnSe-like phonons, full and open symbols address, respectively, to the LO and TO modes.

O. Pages et al. / Optical Materials 17 (2001) 323±326

try, and (ii) the location of the related scattering volumes. Geometries z…x0 ; y 0 †z and z…x0 ; x0 †z, where z// [0 0 1], x0 ==‰1 1 0Š and y 0 ==‰ 1 1 0Š, correspond, respectively, to LO-forbidden and LO-allowed setups. Y obeys the LO symmetry (refer to impact J in Fig. 1 (b)). It was checked that X exhibits the same symmetry. Point (ii) is investigated by probing directly the substrate side of beveled-edge B. The uncoupled LOGaAs from the deep SI substrate is progressively replaced by X when the microprobe approaches the junction. Y in Be-rich samples also stems from the near-interface. Besides, the activation of the theoretically forbidden TOGaAs is attributed to TOallowed scattering on (1 1 0) surfacial facets resulting from the etching procedure. The whole of this is consistent with the assignment of X and Y as LO phonon±plasmon (LO±P) modes which result from coupling between the LO phonon and the plasmon (P) from a carrier gas located at the near-interfacial GaAs. The carrier accumulation is attributed to combined e€ects of hole transfer from the alloy to GaAs and p-doping of GaAs by Zn-di€usion, as observed with ZnSe/ GaAs [6]. Y is involved with a low density carrier gas since the full screening of the electric ®eld from the LO phonon, corresponding to a full shift down to the TO frequency as observed with X, is not achieved. The reason for the discrepancy between the LO±P characteristics on both sides of x  55% is investigated by driving the study on the layer side. Above x  55% the near-interfacial response (refer to impact D) corresponds straight to the bulk one (refer to impact J), which is a guarantee for both high homogeneity in the composition of the layer from the very beginning of the deposition, and clean interfaces. X appears as a marker for latter ideal interfacial conditions. Below x  55% a parasitical thin bu€er layer, with a speci®c Raman response labeled Z, is evidenced at the ®rst stage of the deposition. Z merges within the optical band of ZnSe which suggests that this bu€er layer is mostly ZnSe-like. Moreover, it appears as highly disordered in character in view of the anomalous persistence of Z in the z…x0 ; y 0 †z geometry. This disordered layer brings degraded limit conditions

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at surfacial GaAs. It is suggested that the interfacial defects act as carrier traps and greatly a€ect the interfacial hole gas. This ends with the relaxation of LO±P coupling testi®ed by the substitution of Y for X. It is worth to notice that the nominal composition of the alloy is recovered abruptly just above the thin disordered layer and then does not deviate; as evidenced by the progressive growth of LOBe Se mode at the ®xed bulklike frequency. We address now the frequency-versus-x dependencies of the ZnSe- and BeSe-like LO modes. Within the dielectric approach [7] the LO Raman lineshapes can be approximated by I / Im‰ e…x; x† 1 Š. In this work, the dielectric constant is expressed from the set of equations given by the MREI model [3]. By neglecting the mechanical coupling between the oscillators in a ®rst approximation the dielectric constant can be expressed as: e…x; x† ˆ …1

x†e1 …x; x† ‡ xe2 …x; x†

with ei …x; x† ˆ ei1 ‰1 ‡ X2i L0i Š. Labels i ˆ 1, 2 refer, respectively, to the BeSeand ZnSe-like oscillators. X2i ˆ x2Li x2Ti is related to the LO±TO splitting of the polar modes in endcrystal i. L0i corresponds to Ti Li , where Li ˆ 1 x2Ti …x2Ti …x† x2 † ; Ti ˆ x2Ti …x†xTi2 and xTi …x† describe the transverse modes of the system. The full treatment of I on the basis of the MREI model will be detailed elsewhere. The additional parameters used in the calculations express as follows in the MREI notation [3]. For ZnSe, x(ZnSe:Be) ˆ 448 cm 1 , xL2 ˆ 254:4 cm 1 , xT 2 ˆ 207:2 cm 1 , e12 ˆ 5:75; for BeSe, x(BeSe:Zn) ˆ 239 cm 1 , xL1 ˆ 579 cm 1 , xT 1 ˆ 501:3 cm 1 , e11 ˆ 5:32 [4]. The resulting force constants and h parameter are FZnSe ˆ 2:653  106 , FBeSe ˆ 2:032  106 , FBeZn ˆ 1:093  106 amu=cm2 ; and h ˆ 0:421396. x(BeSe:Zn) is estimated by assuming a linear variation of the square of the ZnSe-like TO frequency versus x, as expected from the MREI model [3]. The derived frequencies of the LO modes are reported in Fig. 2 together with the experimental data. The evolutions of the TO modes, obtained directly from the MREI model [3], are also shown. Both sets of curves provide a good agreement with the data.

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References [1] C. Verie, in: B. Gil, R.L. Aulombard (Eds.), Semiconductors Heteroepitaxy, World Scienti®c, Singapore, 1995, p. 73. [2] D.J. Olego, Phys. Rev. B 39 (1989) 12743. [3] I.F. Chang, S.S. Mitra, Phys. Rev. B 172 (1968) 924.

[4] O. Pages, M. Ajjoun, J.P. Laurenti, D. Bormann, C. Chauvet, E. Tournie, J.P. Faurie, Appl. Phys. Lett. 77 (2000) 519. [5] U. Fano, Phys. Rev. B 124 (1961) 1866. [6] O. Pages, M.A. Renucci, O. Briot, R.L. Aulombard, J. Appl. Phys. 77 (1995) 1241. [7] D.T. Hon, W.L. Faust, Appl. Phys. 1 (1973) 241.