PHYSICAL REVIEW B VOLUME 50, NUMBER 7 15 AUGUST 1994-I

One- and two-phonon scattering processes in Znse/Zns, set, superlattices studied

by micro-Raman spectroscopy

G. Scamarcio, V. Spagnolo, C. Corvasce, and M. LugaraDipartimento di Fisica dell Universita degli Studi di Bari, via Orabona 4, I-70126 Bari, Italy

I. SuemuneResearch Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan

(Received 22 April 1994)

We report a study of Raman scattering at resonance with the band gap in (ZnSe)d(ZnSO tssee s2)d superlat-

tices, 20 A~d(150 L Taking advantage of a microprobe, all the independent geometries have been excited in

backscattering either from the sample surface or the superlattice edge. The energies of optical phonons with

their wave vectors both parallel and normal to the growth axis have been measured, thus showing the expected

anisotropic behavior. The comparison between 6rst- and second-order spectra allows us to assess the nature ofthe phonons dominating the iterated electron-phonon scattering as a function of the well width.

I. IIVTRODUCTION

The nature of phonons in semiconductor nanostrueturesand their interaction with electrons are currently subjects ofextensive investigation. This information is fundamental be-cause the electron-phonon interaction plays a major role in

determining energy-loss rates and mobilities of excited car-riers in optoelectronic devices.

The lattice dynamics of the prototypical system, theGaAs/AIIAs superlattice (SL), has been explained in detail bymicroscopic calculations. Dispersionless confinedphonons with wave vector q along the growth axis z trans-

form into highly dispersive modes when q has a nonzerocomponent in the interface plane [x, y]. The anisotropic Cou-lomb interaction and the lack of cubic symmetry in two-dimensional systems give rise to the anisotropic behavior ofthe optical phonons at the I' point (q~0). Mixing of modeswith the same symmetry further complicates the angular dis-

persion at 1 . Analytical expression of the phonon dispersioncurves of binary GaAs/AlAs SL's have been recently derivedin the framework of macroscopic dieletric continuum model

by properly considering the effect of confinement and

mixing.The vast majority of experimental investigations of SL

phonons have been performed by means of conventional Ra-man spectroscopy in the accessible backscattering geometryalong the z axis, where only confined longitudinal optical(LO) phonons with q ll z are allowed. By fully exploiting theadvantages of microprobe techniques, Raman experimentshave been carried out in backscattering from in-plane direc-tions, thereby allowing the investigation of optical phononswith in-plane q in the allowed scattering geometry. ' Thedetails of mixing and angular dispersion predicted by themicroscopic models have been recently investigated bymicro-Raman experiments in a (GaAs) (AIAs)„ultrathinlayer SL. '

Wide band gap II-VI ZnSe-based heterostructures aresuitable for optoelectronic devices and particularly for blue-emitting diodes and injection lasers. ' The vibrational

properties of ZnSe/ZnS, Se, , quantum wells have been in-

vestigated by means of conventional Raman scattering. ' Atpresent, information on their vibrational properties and theelectron-phonon interaction is poor.

In this paper we present an investigation of a series ofZnSe/ZnS, Se, „SL's by means of micro-Raman spectros-copy. To the best of our knowledge, this is the first Ramanstudy by microprobe techniques reported on II-VI superlat-tices. In addition to the zone-center anisotropy of the ZnSe-like optical phonons, the comparison between first- andsecond-order polarized spectra gives information on phononsdominating electron-phonon interaction.

II. EXPERIMENT

(ZnSe)d(ZnSo, sSeus2)d symmetric SL's 20 A~d~150 A,have been grown at 515 'C on a (001)-GaAs substrate bymetal organic chemical vapor deposition under atmosphericpressure. In the following the samples will be labeled d/d.The number of periods is varied in the different samples inorder to have a total epilayer thickness of 0.8 p, m.

Raman spectra were measured at room temperature usinga Jobin Yvon T64000 triple spectrometer with a focal lengthof 0.64 m. The slit widths were set in order to obtain aspectral resolution of 0.6 cm . A very high signal-to-noiseratio was obtained by using a liquid N2 cooled CCD detector.The 476.5 nm (2.60 eV) line of an Ar+ laser was used toexcite the scattering slightly below resonance with the Epgap of ZnSe (2.7 at room temperature). The incident powerwas kept below 10 mW (=5X 10 W/cm ) in order to avoidheating effects. The spectra were recorded in backscatteringgeometries from the sample surface (qllz) and the sampleedge (q ll

y') by using a 100X microscope objective bothfor focusing and collecting light. Typical laser spot diameterless than 1 p, m was achieved. The incident photon polariza-tion was rotated by means of a half-wavelength plate. Ananalyzer was placed in front of the entrance slits to select thepolarization of the scattered light.

A weak peak around 267 cm, due to the scattering by

0163-1829/94/50(7)/4988(4)/$06. 00 50 4988 1994 The American Physical Society

50 ONE- AND TWO-PHONON SCATTERING PROCESSES IN. . . 4989

I I

(ZnSe) (ZnS „ Se 253I

M

LIJI—

z(vx)z

z(xx)z

TO, IF

(zx )v

LO0

252

~ 251O

iF g)

200

v(x xI I 1

220 240 260RAMAN SHIFT (cm )

280 25030 60 90

d(A)120 150

FIG. 1. Polarized Raman spectra of sample 40/40 recorded in

backscattering along the growth axis x or a cleavage axis y'. Thearrows mark the frequencies of highest energy modes calculated as

explained in the text.

FIG. 2. %ell width dependence of the experimental phonon fre-

quencies. The symbols have the following meaning: (~), LO&,

(V), LO2, (~), IF; (0), overtone peak frequency divided by afactor 2.

bulk GaAs phonons in the substrate, can be observed in sev-eral spectra also in backscattering along x. This scatteringprocess is observable due to the transparency of the SL struc-ture at the used laser frequency.

Due to the quasiresonant condition the scattering byZnSe-like phonons is enhanced so that in this paper we willfocus on them.

III. RESULTS AND DISCUSSION

Figure 1 shows the polarized Raman spectra of sample40/40, representative of the investigated series, in the range180—280 cm ', where bands due to first-order scatteringwith ZnSe-like phonons fall. The topmost traces have beenrecorded in backscattering from the (001) surface. They aredominated by two bands peaked at 252.6 crn ' and 251.6cm ', in crossed (yx) and parallel (xx) polarizations, re-

spectively, which are ascribed to LO1 and LO2 ZnSe phononsconfined in the ZnSe well, according to energy position andpolarization selection rules. ' The two traces recorded in

backscattering from the edge, along the y' direction, show a

richer structure. A peak redshifted to 250.7 cm dominatesthe spectrum in the (zx') polarization, where the scattering

by interface (IF) modes is allowed. ' We ascribe the abovepeak to the IF mode with in-plane q, evolving from LO1 dueto the angular dispersion.

The described behavior is a clear display of the aniso-tropic behavior of SL optical phonons at the zone center. Adetailed description of this effect in GaAs/AlAs SL's can befound in Ref. 9. For the sake of the present discussion herewe just summarize a few features of general validity. Due totheir large dipole moment, the n=1 principal mode sufferthe strongest angular dispersion. Only mixing with otherodd-numbered confined phonons, sharing the same symme-try, can take place. Hence, when the angle 8 formed by qwith the z direction increases from 0 to m/2, the LO~ phononlowers it frequency and eventually merges into an IF modewith energy close to that of LO3.

Unfortunately, no calculation of phonon dispersion curvesin ZnSe/ZnS„Se~, SL are available, a lack shared by allII-VI heterostructures. Therefore, keeping in mind the above

arguments, it seems worthwhile to compare the experimental

phonon frequencies with those of the three highest energyconfined modes calculated in the framework of a simple lin-

ear chain model, as shown by the lines in Fig. 1. A goodagreement can be observed.

It should be said that a second IF mode is expected tomove towards higher energies from the ZnS, Se1, alloy LO(247 cm '). However, no significant feature is observable onthe low energy side of the ZnSe bands. Scattering by the

ZnS„Se1 -like IF mode is expected to become more impor-tant under resonant excitation close to the alloy band gap.

'The band at 205.3 cm in the y'(zx')y' spectrum of

Fig. 1 is due to a superposition of quasidegenerate ZnSe and

ZnS, Se1, transverse optical modes as well as IF modes oftransverse character. Weaker signals around 205 cm ' can bedetected in backscattering from the surface, due to partialrelaxation of momentum selection rules induced by disorder.

In the y'(x'x')y' spectrum the band at 205 cm ' is dueto IF modes of transverse character, according to energy and

symmetry selection rules. ' Both IF modes of longitudinalcharacter and LO2 phonons can contribute to the strong bandat 250.7 cm ', although, due to the sharp coincidence in

energy, IF modes appear to dominate.It is worth noting that the scattering selection rules pre-

dicted by microscopic calculations are well followed in thepresent experiments, performed below resonance. Instead,substantial deviations are reported in previous micro-Ramanmeasurements on GaAs/AIAs SL's, carried out aboveresonance.

Following the above arguments we have analyzed the po-larized Raman spectra of all the investigated samples. Figure2 summarizes the results for ZnSe-like phonons showing thewell width dependence of the experimental LO&, LO2, andIF mode frequencies. We note that the LO& frequency is al-rnost well width independent and stands very close to thebulk ZnSe value in all the samples. This trend is consistentwith the almost negligible effect of confinement on phonons,expected in the considered range of thickness. However,ZnSe wells in pseudomorphic ZnSe/ZnS, Se~ „SL's are sub-

4990 SCAMARCIO, SPAGNOLO, CORVASCE, LUGARA, AND SUEMUNE 50

b)

R

150

a)

I

300 450 600 750

RAMAN SHIFT (cm )

FIG. 3. Raman spectra of the 62/62 sample recorded in parallel

(a) and (b) or crossed (c) polarization excited with 2.41 eV (a) and

2.60 eV (b) and (c) laser lines.

ject to a moderate compressive strain which, in turn, mayinduce a blueshift of the ZnSe phonon energies. The mea-sured LO& energies suggest a diffuse strain relaxationthroughout the SL structure.

Figures 3(a), 3(b), and 3(c) show the Raman spectra ofsample 62/62 recorded in an extended energy range includ-

ing higher-order scattering processes. The spectrum in Fig.3(a) has been measured in parallel polarization, where over-tones are allowed, ' with photon energies far belo~ the E0resonance (Et = 2.41 eV). Several bulklike features relatedto the two-phonon density of states can be observed, due toovertones of acoustic and optic branches. ' When the excit-ing phonon energy approaches the ZnSe band gap [Fig.3(b)], a sharp band peaked at 502 cm rises. The linewidthis 16 cm ' (full width at half maximum), only two timeslarger than that of first-order bands. No signal can be ob-served at the above frequency in crossed polarization [Fig3(c)], where combinations of modes are allowed. On thebasis of energy and polarization selection rules and takinginto account both the linewidth and the characteristic reso-nance behavior' we ascribe the peak at 502 cm ' to aniterated scattering process by I -point LO phonons of ZnSe.Analogously, an additional weak band at 756 cm ', observ-able in Fig. 3(b), can be ascribed to iterated third-order scat-tering processes.

The above assignment opens up the possibility to directlycompare the results of first- and second-order scattering,thereby giving access to information on the nature ofphonons dominating the electron-phonon interaction in theconsidered excitation regime. In Fig. 2 the peak values of theovertone bands recorded in z(x'x')z scattering geometry, di-vided by a factor 2 for the sake of comparison, are plotted.The scattering geometry has been chosen because mul-

tiphonon processes involving optical modes of any symmetryare allowed. The comparison in Fig. 2 clearly show that forwell width thinner than about 50 A the dominant phononsare IF modes evolving from LO& with in-plane q. For largerwells LO2 phonons give a stronger contribution to the inter-action.

A scaling of the ratio of IF to LO2 phonon scattering as afunction of the well widths in polar semiconductor superlat-tices is predicted by calculations performed in the frameworkof the dielectric continuum model. ' This is due to the de-creasing overlap between the electron-hole wave functionand the IF phonon potential increasing the heterointerfaceseparation. A similar crossover behavior has been reported inGaAs/A1As superlattices with excitation energies well abovethe fundamental optical transition where IF modes dominatefor layers thinner than about 6 ML.

IV. CONCLUSION

The anisotropic behavior of I'-point optical phonons, with

q parallel to the growth axis or along an in-plane direction,have been demonstrated in ZnSe/ZnS„Set „SL's by meansof micro-Raman spectroscopy. A comparison between one-and two-phonon scattering processes with photon energyclose to the ZnSe gap show that the electron-phonon inter-action at I' is dominated by in-plane propagating ZnSe IFphonons evolving from LO& for well widths smaller than 50A, whereas the interaction with LO2 modes plays a majorrole for larger wells.

ACKNOWLEDGMENTS

Fruitful discussions with E. Molinari and J. Ryan are ac-knowledged. This work was supported in part by ItalianCNR and MURST.

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