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OPTICAL TRANSMISSION UNDER MAGNETIC FIELDS IN CdTe/CdMnTe SUPERLATTICES
G. Couturier, J. Yoshino, L. Chang, S. von Molnar
To cite this version:
G. Couturier, J. Yoshino, L. Chang, S. von Molnar. OPTICAL TRANSMISSION UNDER MAG-
NETIC FIELDS IN CdTe/CdMnTe SUPERLATTICES. Journal de Physique Colloques, 1987, 48
(C5), pp.C5-341-C5-344. �10.1051/jphyscol:1987573�. �jpa-00226777�
Colloque C5, suppl6ment au noll, Tome 48, novembre 1987
OPTICAL TRANSMISSION UNDER MAGNETIC FIELDS IN CdTe/CdMnTe SUPERLATTICES
G. COUTURIER(^), J. Y O S H I N O ( ~ ) , L.L. CHANG and S. von MOLNAR IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598, U.S.A.
P.esume La trarisrriission optique des super-riseaux CdT?!(CdMn!Te a it6 mesurie % la teri..pirature de 1.6K et sous champ rfiqnetiq11e jusqll'a 50kOe. Les spectres prisentent des pahers d'absorption am?b116s %IS soi~s bandes d'energie dar~s les puits quar~tiql-ies de CdTe. L'iriteramion d'bchwge ie ratiriau Cf1-lnTe conduit h un iclatement des bandes de conduction et de valence et donc a m e ~lo6lfhnlon des specues de transmission au.
v o i s i n ~ e du seld d'absorption de CdMnTe, Due a la rbdulctiori cIu gap optique de CdMnTe, la transition asjocibe la sous hande d'enerzie la pbs elevbe dispardit a i m w e les sous barides lie basse Bnergie restent inchangbes.
Abstract We have measlnred the optical transmission in CdTeiCdMnTe superlattices at 1.GK arid under rnagnenc field up to 50kOe. The tiar~srfdssion spectra exhibit step-iike characteristics, which can be identified with subband trarisitions in the CdTe wells. In the presence of a rnagnenc field, conduction and valence haids of C d M n h are split because of the large exchnge iinteractitinn wluch rnoilifies the transrfdssion near ~t; absorption edge. The observed spectra show that the transition asjociated wifh the highest subband vanishes, as a resuit of the reduction of the effecti'v'e gap of CdMnTe, but lower energies remain essentially unchanged.
Dilute magnetic semiconductors (D.M.S.) are II-VI (or IV-VI) semicunductors such as CdTe (or PbTe) njth a fraction of the group I1 (or IV) element substituted by a transition metal such Mn These materials have generated a great deal of interest The free carriers in the conduction and valence bands interact strorgly, via an exchange interactionwith the localued 3d5 electrons of the magnetic Mnzt ions, giving rise to a large Zeeman splimg of the badj13.
The growth of good quality CdTdiCdMnjTe quantum wells has been realized by molecular beam epitaqa.5, and magnetic tunable laser emission has already been reported in Cdll,, MnfdTe,'Cd(l.,i Mnf,,Te superlatticesb. .. . More recently, the growth of In doped CdTd(CdMnjTe superlattices has also been successful?. However, problerns related to the valence band offset ,and the effects of a magnetic field on the energy levels remain to be answered J.A Brum and
3
suggested that magneto-lurrlinescence in a double quantum well would be suitable to yield information about the valence band offset Recently, luminescence excitation d a d performed on CdTeiCdfl- :~irYlnlxlTe ( ~ 0 . 4 5 ) have been successfuEy interpreted solely on the basis of strain vaience band offset lea- to a splittq of heavy and lght hole states. These results are consiItent with angle-resolved 113. photoemission m e a s ~ u e n e n t s ~ ~ which indicate a zero vdencs band nifset 1~1th an a&-macy off 50 meY.In luminescence, beside the free and defect bound excitons associated with the CdTe quantum weils, the spectra in CdTei(CdMnjTe superlattices exhbit lines at smaller snergies whch are also mqnetic field dependent These h e s sh& to iower energies in the presence of a magnetic iield in contrast to those associated with the Landau levels, which sWt to 1-#her energiesii. %s behavior suggests a penetration of hole and electron wave functions in the (CdMnjTe layers, resulting in interface locaked excitonsf213.
In t h ~ s letter, we report the first optical transmission measurements in CdTeiilZdMnjTe superlaticces rvith two different CdTe thic-hess layers. The eiiect of a magnetic field is also reported
The CdTe/Cdll,jMn!,lTe mulnlayers were grown by molecular beam epitaxy on a
iwn
CdTe buifer layerwhch~ 3grown on a [:GO) oriented 1.3.4s subbs?rste. The sitperlattices were (111) oriented Two superlattice s
coniiglxanons were invesngated 7T1e fir5 corifigzatiori #I consisted of CdIl,if4nfxlTe [x=11.07! lpers iath a thickness of 100A ,and CdTe Iayerz rslith a thckness iZ=90.& In the second coniiguration ~ $ 2 corresponding vaiues were x=0.013, 100A and ~ ~ = 2 5 0 . & The GaAs substrate thckness was reduced to atout 200q1 by mechanid polishmg and a window of 3 dwas produced by e t c ~ h g in H202-NH40H (20-1). The buffer layer of CdTe and a
(''present address : Universite de Bordeaux 1. Talence. France
( 2 ) Tokyo Institute of Technology. Tokyo. Japan
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987573
C5-342 JOURNAL DE PHYSIQUE
>art of the superlattice ia;pre etched off in a 0.13 solution of bromne methao! i e t z m wte 150bP~minj. The fmd thickness of the superlattice was about lr?0~.4-1500:% The optical transmission was measured at 1.6K linpoiaized light :??as u:ed in !he s,xperiments.
Figwe 1 iti:piz::z the opticd transmission sgenna for botk 31 3rd #?I jllperiuces. Both gpemtra 8:uilb:t step- like characteristics, with %:~j and 3is trafisitions observed in #! and #; sarnpies respecwet,:. Yrieze me hbeled n=1,2 ..., dernonstra*g the presence of energy subbands in ;he CtdTe quarrtunl rs.:e3s. T?e ground subban& hbeled n=l, moves to W i e r energf as iZ is reduced as a consequence of 1:mier coniinernent Vertical h e s , in m x e 1, indicate the transition ~nergies rlcldated by so1'mg the S c h ~ e T j y e r ' j egllationlf' in a quz,pm well a finite depth and ass- tra zero valence band offset.. Tn5 well depth was detersrlirled from fie f~qjh?lent,d abjorpfio~
edge, E,, ir tl? fCd!l.,iMnl,lTe k::ers, sr:hich ix>as rnea:lued in the hgh ezerzy region of the tlanincmission spem.i The Mn concentrations in the nvo si1perlatti;e coni&iuatlons were fso deduced fr~jrn E, i;&e.
ENERGY (aV)
1.6 1.7 1.8 1.9 2.0
I I I I I I 1
i??. 1: O?ti!A transmission spectra at
!.jF: of
30.5
ar~d 2.51jJ. thick CCdTe layers betvJeen jCdblnjTe k.&~isrs. "he CdTe krle show5 k e .&sorpnor. edge m PIX?'3'i"T..
m e cal~mlated results predimg respectively kvo and 5$ transition energies in #I and 8 2 samp!es are in good agreement r.~rith the e.qeriments. increasing discrepancies, however, tend to appear for zubbands rsith &her energy indices. may be due to both the sznpiicity of the theoretiui model and the eh~erimentd uncertainty in the well thckness. In addition, in the limit of our experiments the transmission spema do not e ~ h b i t sharp peak3 is!ated to excitcnic transitions, instead, they me rather broad As propos~d in Ref. ?, a small spli*~ of the hea1.n~ ad Qht holes in the L'dTe iayerj can be responsible for suchbroademg.
13ptical E.ansrnission v.as also rr?easl.ued m the presence of rnenehc field: up to 5OkCle in the Faraday geometlr;. Q l x e s ? and3 compare the zero and hgln field spema of #land #2 superlamce C O N ~ U ~ X ~ O ~ S . The &h field spema retain the jtep-!ke charact.ristics, and the transition cnergies below the ab:orption edge of Cd g.
,,k~ln(,lTa r e r n w ~ uncharged Near the absorption edge, however, the spema are sgnifira~tly mochfied; for e:mple, silbband transitions labe!ed n=2 and n=6 in #1 m d 8 2 jmples respectively are no longer observed. The v a n i s m of these subbands is a conseqiler?ce of the large exhang. iriteractioa in the Cdll-,,Mn~xlTe &luted mqnetic serniconducror la:/ers.Due to
~
interactiorb jUorig splimrigs in :he conduction and valence bands iicw leading to a decrease af the opticalgap. These splittiqs are believed to be also responsihle for the .weak ~s&tory behavior be!;ond the n=5 tranjition in the t 2 sample ?A Q m e 3.The reduction in the optical zap can be written:?
where E.(H=O] =1.6=1.57x in eV, xis the Mn rnoiar fraction, and N, and EN, are the exchaigr integrals for the conduction:md udenre bands which have been evalua!ed in the bull; ifiairrial by Ga! and dl?. These authors cjS;ain d14,=220 meV fiPi,=?YO melb. <SZ> is the ~ n ? + mean spin vdce in ~ ! e k i d direction and can 3e jirecTl:~
evaluated frorn magnetisahon rfieasursments. It is orixtlly :,witten a&:
Bolx magneton? T the temperature, arid Tg an effective temperature r.vhci! rellects the ariltlferrom.agnetic type of interaction -amerg the x n 2 + ions. For the same reasons, SM is smaller than the 5!2 ~Mdtspin value. T, and SM parmeters have a l ~ heenreported in Ref. 17.
A c c ~ i W to Ep 1, at 1.W. and in a SOkOe nagfie"; field the opard gap in Cdl;,:)MnhlTa deceases by about 4SmeV and 52meY in #1 and #2 sarr:ples r e s p e ~ v e l y , the hole qumtbtion in the Cd!l.,jl.lni,iTe layers has been neglejcted in this C%lci11atior~ in the presence of raagnetic fields it rnsy be posd'ole that E,(H) becomes s m d r than Uie tr:msition energies asso ciatell with the highest s1:bbands. The ce Zansitions, consequently, o;~erlap the continuum of absc.rption in the Cd~l,iP.Inl,lTe rfiaterif For hstance? in f t 2 sarfiple, the & d a t e d ,ansiQon energy associated i.uiU1 the n=6 subband is I.?bBeV, ie. hgher now than the ::due of E,(H=5DkCe)=1.?52e\r. Sirdarly, n=2 suhband oanjition in #1 sample is 1.S43eir, whereas ES,(H=iOkOei=1.66ie:;i.
ENERGY (eV)
Cd Te/Cd Mn Te e2 I-X X x=0.13 T=1.6K
-
+ S P " ' I I
I 1
\I
n.1121 31 41 51 61
+SPINIII 1
I I I
C d l - x M n I H = \ O kOeH=O Te
I
18 0 0 7 5 0 7 0 0 6 5 0 6 0 0
FQ.2: :?ptlca! trarlsmission spectnim at 1.6K Q.3: ~ p t i d - n m s r m s c i o n s p e ~ m m at 1:iK
!:i 85.4 S9.k CdT? layers bep;:ten of 300.S thick TdTe layers between L'dil.xlMn!,iTe barriers in a SOkOe Cd(t,,MnI>:j'Fe bamers b a OkOe
nagnetic field magnstic field.
So f.3 we have neglected the Zeernan splirtings m d the Landau !eve15 of tiie energy subbands. These effects are very. small for the coniiinons of olur experiment The anergy sMt%w& due to the Landau levels is about 2meV
, 9).
. _ _ _
a;>umiriZ the same eifective mass of electrons as rhat in Cd'i'e, ie. q r = O . 1 q v a d the classid Zeeman splitmg g p g B 15 about Il.Sme'%' for each subband L'arrrBions, due ?a :be est:h.zge inter.mion m :he Cil,l,lbln,,iTe iayer: .- - . ~..,which result in diifeyent well depth: for spin up md down xbbands of the CdT? layers, moafy L!!5 ?.w.ber. T'r:us, the totd Zeeman splitting is lwzar than gp@ and depend5 on the indes n of the 5:lbbsanl larger for hgher n values.
Sir'ple calculations show that the splimng resultmg from the exchange interaction is :bout Ime12 for the n = l subband in 81 sample. In 82 sample, the jpli!ting of me n=5 subband is 2meV but iess t9an lmeTI for the n=l subband As expzcted, of these effects are sm.al: and :ewe the optic& ?iarljmissiun essanti31:y ~ m c k a r ~ e d below 'he gap. We should menuon tkat there is no evidence of spatially indirect tranjitions bemeen the valence band of Cdilal t':nIxjTe a d rhe wnr',uction barid of C u e . Lruninescence measurements dso do not exhibit jllch transition:
but onlj: a small W! to lower energy in :he pesence of ifi?~g?stic field
in smunar!?. optical transmission h CdTelC~d~i,~Mn~,~Te s~lperlattices has been measured -2. ncmber oi subband Z,minsinonj have teen &ser;ed .rid a e y are cofislster~t with a oi npLer :m& vileace hkid offset. In the presence oi rfiagnetic field. optical trmsmitlijnon is sriorgiy moMied near ?iie ah;opom edge of the L'djl.
,lMnixjTe laysrs. This demonsoates tbe large hlnence o! *he caz%r-!o~A mnoment ::<c.h.xge i!teraztion on ~e magneto optical properties of &lute mqnetic semiconductor sl.iperBttices.
We ackxnwledge G-Giiritherodt for tdj help ir, the iniad s:ages of m&irg rneas:xerfients m d t:e L ~ n y Research Office fur the partial suppen of this work
C5-344 JOURNAL DE PHYSIQUE
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