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RAMAN SCATTERING AND EXCITATION SPECTROSCOPY IN CdTe/CdMnTe
SUPERLATTICES
L. Viña, L. Chang, J. Yoshino
To cite this version:
L. Viña, L. Chang, J. Yoshino. RAMAN SCATTERING AND EXCITATION SPECTROSCOPY IN CdTe/CdMnTe SUPERLATTICES. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-317-C5-320.
�10.1051/jphyscol:1987568�. �jpa-00226772�
Colloque C5, suppl6ment au n0ll, Tome 48, novembre 1987
RAMAN SCATTERING AND EXCITATION SPECTROSCOPY IN CdTe/CdMnTe SUPERLATTICES
L. V I G A ' ~ ) , L.L. CHANG and J. Y O S H I N O ( ~ )
I B M Thomas J. Watson Research Center, P.O. BOX 218, Yorktown H e i g h t s , BY 10598, U. S.A.
Abstract.- We have observed oscillatory struct,ure i n t h e excitation spectra of CdTe/
CdXMnl.,Te superlattices. A comparison of t h e s e spectra with conventional Raman spectra shows t h a t t h e s t r u c t u r e s correspond t o f i r s t and higher o r d e r LO-phonons of t h e CdTe wells and t h e CdTe/CdXMnl.,Te barriers, a s well a s combination of them. A strong enhancement in Resonance Raman scattering of both t h e CdTe and t h e CdMnTe phonons, a t t h e energy o f t h e heavy-hole exciton of t h e superlattice, suggests a small valence-band o f f s e t between t h e two materials.
INTRODUCTION
High quality superlattices (SL's) based on CdxMnl-,Te have been grown i n t h e last years using epitaxial techniquesl-3. Although t h e optlcal properties of these SL's have been profusely investigated with methods such us photoluminescence (PL) and excitation spectroscopy (PLE)", Raman studies a r e scarces-7. T h e phonon spectra of CdTe and Cd, Mnl.,Te h a v e been studied in g r e a t detail i n t h e literature=, and can be used a s a r e f e r e n c e t o compare new results in t h e SL's. Here we p r e s e n t d combined Resonance Raman scattering and PLE study of CdTe/CdMnTe SL's. A correlation between t h e Raman spectra and striking oscillatory s t r u c t u r e i n PLE is established. On t h e o t h e r hand, t h e resonant behavior of t h e CdTe and CdxMnl-,Te phonons indicates a small valence-band o f f s e t between t h e two materials.
EXPERIMENTAL RESULTS AND DISCUSSION
The samples used i n o u r study h a v e been grown by molecular beam epitaxy on (100)-oriented GaAs substrates. A thick (-200nm) (111)-CdTe b u f f e r l a y e r was followed by a CdTe/Cdo.87Mno.13Te superlattice. T h r e e d i f f e r e n t samples with constant well-width L,
=100.4 and barrier-widths Lb=100,50 and 25.4 were investigated. Details of t h e growth technique and characterization have been described i n a previous publication3.
The samples w e r e mounted on a variable t e m p e r a t u r e cryostat and kept i n a He-gas atmosphere a t 4.8K. An LD700 dye, pumped by a Kr+-ion laser, was used to excite t h e samples with power densities below lW/cm2 and 5W/cmz f o r t h e PLE and t h e Raman experiments, respectively. The scattered light was analyzed with a 3/4m-Spex double monochromator and detected with a GaAs-cathode photomultiplier.
(')permanent address : Instituto de Ciencia de Materiales, Zaragoza, Spain
( 2 ) Tokyo Institute of Technology, Tokyo, Japan
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987568
C5-318 JOURNAL DE PHYSIQUE
Figure 1 depicts t h e PL (dashed line) and t h e PLE (solid line) spectra of a 25-periods superlattice with IOOA-wide wells and barriers. A Stokes s h i f t of 8meV between t h e PL and t h e PLE, which in GaAs/GaAlAs SL's has been attributed t o well-width fluctuations9 o r t o impuritieslo, is clearly observed in t h e figure. The PLE spectrum shows t h r e e broad s t r u c t u r e s (1.636, 1.653 and 1.695eV) t o g e t h e r with s h a r p peaks, which correspond t o f i r s t and higher o r d e r LO-CdTe phonons. T h e vertical-downward arrows show t h e results of a calculation, in t h e envelope-function approximation, f o r t h e f i r s t heavy- (hi) and light-hole (11) excitons, a s well as f o r excitons associated with t h e second conduction- and valence-band levels [he, hie, hzi, w h e r e t h e f i r s t (second) subindex indicates conduction- (valence-) band level]. Effective massesil me*=0.096, mh*=0.4 and m~*=O.l, a free-exciton dependence on Mn concentrationl2, x, E(x)=1.5976+1.564x(eV), and a somehow arbitrarily chosen 90-10% r u l e f o r t h e conduction- and valence-band discontinuities were used f o r t h e calculation. Taking into account t h e uncertainties in t h e p a r a m e t e r s used f o r t h e calculation and s t r a i n e f f e c t s in these lattice-mismatched superlatticesl2, t h e agreement with t h e experiment is satisfactory and h i n t s t o a small valence-band o f f s e t between t h e two materiais of t h e SL.
Excitation spectra of a SL with 501%-wide barriers a r e shown in Fig.2. As t h e detection energy (arrow) is moved toward higher values, s h a r p periodic structures a r e resolved.
These dominate completely t h e spectrum obtained a t t h e high-energy tail of t h e PL (dashed line). The energy distance of t h e f i r s t two peaks t o t h e detection energy is Zl.3meV (LO) and 24.3meV (LOz), which corresponds t o t h e CdTe (well) and MnTe-like (in CdMnTe) LO-phononss. The additional structures, with a period between t h e main peaks of 21.3meV, correspond t o higher o r d e r LO phonons of t h e CdTe wells and t h e CdTe/Cdo.s7 Mno.13 barriers and combinations of them. Oscillatory s t r u c t u r e i n t h e PLE spectra of
bulk CdTe has been observed previously
ENERGY (ev) 13. Since phonon lines can b e observed 1.70 1.65 1.60 below and above t h e h i exciton, and a
I I I I I I second periodicity, slightly higher t h a n t h e LO frequency, is absent in our 100h CdTe
-
100h C~,,,MII,,~T~ spectra, hot-exciton f o r m a t i o n and 25 perlods hot-electron e f f e c t s can b e ruled out, leaving Resonance Raman scattering as t h e only mechanism responsible f o r t h e observed periodic structureis.n Notice t h a t t h e phonon peaks a r e also
+ V)
.-
T = 4 . 8 ~ observed in t h e PL spectrum, howeverC a much higher sensitivity is obtained in
3 t h e PLE spectra.
$
I m p o r t a n t i n f o r m a t i o n aboutw 0 electronic excitations in a
semiconductor can b e obtained f r o m
4
t h e resonant behavior of t h e RamanV) intensities, which occur whenever t h e
Z
W energies of t h e incident (incoming
I- channel) o r scattered photons (outgoing
z
channel) coincide with t h e energy oft h e excitationl4. We have investigated f i r s t - and higher-orders Resonant
! Raman scattering in t h e s e SL's. Figure 3 shows t h e results f o r t h e first-order .'L scattering a t 4.8K f o r t h e SL with 1'
-'.
100A-wide barriers. T h e spectra were,-.-
I I I I I I ..5 recorded in t h e zJ(x'x')2', w h e r e x' is in 7200 7400 7600 7800 t h e plane of t h e layers and z' along t h e
WAVELENGTH
(A)
SL axis ( t h e energy of t h e incident photon is indicated f o r each spectrum). Two well-defined peaks a t Figure 1: Photoluminescence (PL) and 1 7 2 c m - 1 ( ~ ~ - ~ d ~ e ) and 196cm-i(L02 excitation spectra (PLE) of a CdTe/CdMnTe MnTe-like), together with a small superlattice with 100A-wide wells. shoulder a t 168cm-%(LO1 CdTe-like)WAVELENGTH
(A)
Raman shlft(cm-')Figure 2: PL (dashed line) and PLE Figure 3: Raman spectra f o r t h e s a m e spectra t a k e n a t d i f f e r e n t energies sample as in Fig.1 f o r d i f f e r e n t laser (arrows) of a SL with SOA-wide wells. energies.
a r e resolved in a broad background due t o t h e luminescence emission of t h e sample. The frequencies a r e in good agreement with t h e results of Ref. 8 f o r a Mn concentration
~ ~ 0 . 1 3 . A s t h e energy of t h e scattered photons approaches h i (1.636eV). a strong enhancement of t h e LO and LO2 phonons is observed.
The intensity of t h e LO phonon, uncorrected f o r t h e absorption in t h e sample, is plotted in Fig. 4 a s a function of laser wavelength. T h e PLE spectrum is also shown in t h e f i g u r e f o r comparison. T h e Raman points have been s h i f t e d by 1LO=21.3meV towards lower energies, so t h a t t h e resonance of t h e outgoing channel can b e directly compared with t h e h l exciton seen in t h e PLE spectrum. A strong enhancement by "2 orders of magnitude is observed a t t h e energy of hi. A second s t r u c t u r e a t 1.61SeV, 1LO phonon frequency below hi, corresponding t o t h e incoming channel is also observed. The asymmetry in t h e lineshape of t h e outgoing resonance, as well as i t s stronger enhancement, have been a t t r i b u t e d t o impurity effectsis. A much weaker e n h a n c e m e n t f o r t h e 11 exciton can b e observed in t h e figure; i n t h i s case t h e incoming and outgoing channels show a comparable behavior, in agreement with r e c e n t results of Chang e t al.6
An enhancement of t h e LO2 phonon a t t h e e n e r g y of h i h a s also been observed, although in t h i s case t h e resonance could n o t be traced below t h e PLE peak d u e t o t h e luminescence emission of t h e sample and t o t h e lower intensity of LO2 compared t o t h a t of LO. I n t h e energy range of 1.75-1.80eV, corresponding t o t h e band gap of CdTe/Cdo.wMn o.ioTe, t h e phonons of t h e barriers and t h e wells a r e again resonantly enhanced. These two f a c t s indicate a large penetration of t h e hole wavefunctions i n t o t h e b a r r i e r material, and t h e r e f o r e a a small valence-band o f f s e t between t h e two materials. Our results contradict recent Raman studies of Suh e t al.7, who have found a selective resonance enhancement of t h e LO phonons in t h e barriers and i n t h e wells. The d i f f e r e n c e could probably lie i n t h e larger Mn concentration used i n t h e i r samples (x>0.25). Furthermore, we have not observed any f r e q u e n c y dependence on t h e incident photon energy of t h e LOX and LO2 phonons, reported f o r [Ill] c d T e / c d ~ n T e SL's in Ref. 7.
C5-320 JOURNAL DE PHYSIQUE
ENERGY (eV)
1.68 1.66 1.64 1.62 1.60 1
1 1 1 1 1 1 1 1 I I
1001 CdTe / 100h Cdom7Mna.tsTe 25 periods
Finally, we should mention t h a t , since no gappreciable s h i f t has been measured in t h e 1.0 phonon, t h e CdTe layers a r e strain-free. The s t r a m in t h e CdxMnl-xTe layers i s m o r e difficult t o assess f r o m o u r Raman measurements, since t h e Mn composition has been estimated only f r o m growth parameters.
ACKNOWLEDGMENTS
We want t o t h a n k H. Munekata and M. ]long f o r t h e i r help during t h e sample growths, and A.V. Nurmikko f o r t h e availability of t h e d a t a prior t o publication.
Figure 4: Excitation spectrum of t h e s a m e sample as i n Fig. I. The points show t h e Resonance Raman profile of t h e CdTe-LO phonon, s h i f t e d by a phonon frequency.
-
7400 7500 7600 7700 7800 WAVELENGTH
(A)
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