A. B. acknowledgesЩnancial support from EU under the ICA1 -CT- 2002-7009 contra ct. R ef er en ces. [1] L. Jacak, P. Hawrylak, A.Wojs,QuantumDots,Springer, ...
V ol . 1 0 5 ( 2004)
A CT A P HY SIC A P O LO N IC A A
No . 6
P ro ceedi n gs of t h e XXX I I I I nt er n at io na l Sch ool of Sem i cond uc t in g Co m p ou n ds, Ja szo wi ec 200 4
L ocali zatio n of E x citon s in t h e W ettin g L ay er Ac com p an yin g Self -As sem b led In As / G aAs Q u antu m Dots A . B abin sk i a ;b , S. R ay mon d c , Z. W asil ews k i c , J. La po in te
c
and M. Po te msk i a a
G r eno bl e Hi gh M agneti c Fi el d La b orato r y, M PIF KF and CN R S 38042 G reno bl e, Fra nce b
In sti t ute of E xp eri m ental Physi cs, Wa rsaw Uni versi ty Ho âa 69, 00-681 W arszawa, Pol and
c
Insti tute for Mi cro structura l Sciences, NR C Cana da, Otta wa, Canada
A w ett i ng l ayer is a narr ow , highly straine d quantu m w ell, w hich accom panies quantum dots grow n in Strans ki { K rastan ow mo de. Its imp ortance for a full descripti on of the quantum dots prop erties has recently been pointed out. It has been show n for example that excitons can be locali zed by potential Ûuctuation s in the w etting layer. T his is equi valent to the formation of \natural " quantum dots in the W L. Excitonic emission from the single dots formed in the wetting layer accompanying the InA s/ GaA s self- assembled quantum dots has been investigated in a high magnetic Ùeld (up to 23 T ). Quadruplet splittin g of the investigated emission line has been observed. T he attribution of the emission line to the recombination of negatively charged exciton is discussed. PACS numb ers: 71.35.Pq, 71. 35.J i, 71. 70.Ej
1. I n t r o d u ct io n Semi conducto r qua ntum do ts ( Q D s) ha ve attra cted considera bl e attenti o n am ong researchers [1], thi s i nterest bei ng dri ven by both the funda m enta l physi cs and p otenti al for appl i cati ons. The QD s are som eti mes cal l ed \ arti Ùcial ato ms" [2], whi ch suggests tha t the conÙned sta tes i n QD s do no t i ntera ct wi th surro undi ng m edi a. Such a pi cture, ho wever, has recentl y b een questi oned, as experi m enta l evidence f or i ntera cti on b etween the self- assembl ed QD s and the surro undi ng wetti ng ( 547)
548
A . B abi n ski et al .
l ayer (W L) has b een presented. The W L i s a narro w, hi ghl y stra i ned qua ntum well (QW ), whi ch accompa ni es the QD s gro wn i n Stra nski { Kra sta now m ode [3]. It ha s b een shown tha t the l atera l coupl i ng b etween the neighb ori ng QD s i s m ediated by p otenti al Ûuctuati ons i n the W L [4]. The existence of several resonances i n the pho to lum inescence (PL) exci ta ti on o f a singl e QD ha s al so b een attri buted to \ mi xed" opti cal tra nsiti ons invol ving carri ers fro m the W L and the QD [5]. Hybri di zati on of the tri pl y- charg ed exci to ns i n the QD s wi th La nda u- li ke levels associ ated wi th the two -dim ensiona l density of sta tes i n the W L has also b een observed i n m agneti c Ùeld [6]. These observ ati o ns p oint out the i mp orta nce of the W L for the ful l descri pti on of the QD s physi cs. In thi s com muni cati on we rep ort on magneto l umi nescence study of the exci ton l ocal i zati on i n the W L, accom pa nying the self -assembl ed In As / G aAs QD s. W e di scuss the Ùeld- i nduced spl i tti ng of sharp emi ssion l i nes and we expl ai n the b ehavi or i n term s of recom bi nati on of negati vely charg ed exci to n. 2 . E x p er i m ent a l t ech n i qu e A sam pl e wi th a singl e l ayer of InAs QD s was gro wn on n - dop ed Ga As substra te by mo lecular b eam epi ta xy . The i ndi um Ûush techni que [7] has b een appl i ed to l im i t the height o f the do ts to 5 nm . Investi gated size o f the sam pl e was l i m ited using m esa-pa tterni ng . The PL measurem ents were p erform ed at l i qui d hel i um tem perature i n m agneti c Ùeld up to 23 T (F ara day conÙgura ti on) suppl i ed by a resisti ve m agnet at the G reno bl e Hi gh Ma gneti c Fi el d La b ora tory . La ser exci ta ti on (Ar + l aser, Ñ = 5 1 4 : 5 nm ) was del i vered usi ng a singl e-mo de Ùber and fo cused onto the sam pl e wi th two m icro l enses. An esti mated l aser spot size was 20 ñ m and i ts p ositi on over the sam pl e was contro ll ed by Atto cube pi ezo- stag es. The PL wa s col lected using a 60 0 ñ m core di ameter m ul tim ode Ùber pl aced ab ove the m esa. The PL spectra were di spersed by a 1 m do ubl e gra ti ng m onochro m ator and focused onto a CCD . 3. R esu l t s It has been found tha t the l i neshap e of a W L- rel ated PL stro ng l y depends on the size of i nvesti gated m esa. A featurel ess PL peak due to recombi natio n of carri ers i n the W L can b e seen fro m m esas l arger tha n 10 ñ m , whereas sharp emi ssion l i nes can b e observed fro m the sub-m i cron size m esas (see Fi g. 1). In thi s comm unica ti o n we focus our attenti on on the spectrum shown i n the l owermost pa nel of Fi g. 1, ho wever our conclusi ons can al so b e appl ied to other observed features. It ha s b een f ound tha t the emi ssion l i ne m ark ed i n Fi g. 1 wi th an arro w spl its i n p erp endi cul ar magneti c Ùeld i nto f our com ponents (see Fi g. 2). Mo reover an addi ti ona l pai r of emi ssion l i nes emerges at hi gher energy wi th i ncreasing m agneti c Ùeld. The energy of observed emi ssion l i nes rel ati ve to thei r center of gra vi ty i s shown i n Fi g. 3. It ha s b een found tha t the exci toni c e˜ecti ve g - facto r of tho se Ê
L ocal i zat i on of E xci tons i n the Wet t i ng L ayer
Fig. 1.
Photolumines cence spectra from the w etting layer accompanying
self-assembled quantum dots measured on submicron- siz e mesas at
549
.. .
T
=
InA s/GaA s 4:2
K in zero
magnetic Ùeld. T he emission line marked w ith an arrow in the low ermost panel has b een investigated
in magnetic Ùeld.
three l i nes i s wi thi n experi menta l erro r the same and equal s 1.39 (see the i nset to Fi g. 3). 4 . D i scu ssio n The l ocal izati on of excito ns i n thi n QW s has b een studi ed i n several system s i .e. G aAs/ Al G aAs [8 ] or CdT e/ CdMg T e [9]. It has a lso been observed i n the W L accom panying the InAl As/ Al G aAs QD s [4]. It i s b eli eved tha t the l ocal i zati on of exci to ns can be due to mono ato mic i nterf ace Ûuctua ti ons. Charg ed excito ns may al so b e l ocal i zed by p otenti al Ûuctua ti ons due to residua l charg ed im puri ties i n the ba rri ers [10 ]. Fi na ll y the 2{ 3 ML thi ck bi - di m ensiona l pl atel ets, whi ch form on to p of the W L at the earl y sta ge of the Stra nski { Kra stano w tra nsf orm ati on m ay al so
550
Fig. 2.
A . Babi nski et al .
Evolution
of the emission line due to recombination
w etting layer in magnetic Ùeld ( T
=
4: 2
K ). Quadruplet
of exciton localized in the splittin g and the emerging
split- o˜ doublet can be seen.
b e respo nsibl e for excito n l ocal i zati on [11]. Al tho ug h no Ùnal statem ent can yet b e m ade on the ori gi n of the i nvesti gated l o cal i zati on, the pro p erti es of l o cal i zed exci to ns can b e anal yzed. In the fol l owi ng we focus on the emission l i ne m arked wi th an arro w i n Fi g. 1. W e pro pose to expl ai n i ts b ehavi or i n m agneti c Ùeld wi thi n the fol l owi ng mo del. Fi rstl y i t shoul d b e p oi nted out tha t al l four spl i t- o˜ l i nes and very l i kel y the hi gher- energy do ubl et app eari ng at hi gher magneti c Ùelds ha ve the same exci to ni c g -facto r. It has b een found tha t the g - facto rs of emi ssio n l i nes f rom other m esas Ûuctua te considera bl y b etween 1.1 and 1.6 . In our opi ni on thi s reÛects a com pl i cated chara cter of the val ence ba nd i n the W L, whi ch m ust b e very sensiti ve to stra i n Ûuctuati ons. The same val ues of the g - facto r observed for emi ssion l i nes f rom one subm i cron- size m esa (see the i nset to Fi g. 3) stro ng l y suggests thei r attri buti on to the same \ na tura l " dot. Our expl anati on of the observed qua drupl et spl i tti ng i nvolv es a negati vely charged exci to n in the do t, i n whi ch the excess carri er density bl ocks spi n rel axati on. Such m odel ha s b een pro p osed to expl ai n the qua drupl et spl i tti ng of emi ssion l i nes fro m a sing l e InAs/ Al Ga As QD [12] wi th a zero- Ùeld doubl et spli tti ng. If the do t i s popul ated by a ho l e and Ê
Ê
Ê
L ocal i zat i on of Exci tons i n the Wet t i ng L ayer
Fig. 3.
Recombination
.. .
551
energies for the exciton localized in the w etting layer plotted
against magnetic Ùeld. Diamagnetic shif t has been remo ved for more clarity . T he Zeeman splitti ng of the three pairs of lines is show n in the inset.
two electro ns of opp osite spi n, al l carri ers can rel ax to thei r gro und states b efore recombi nati on. Theref ore the recombi natio n o f such negati vel y charg ed exci to n l eaves an extra electro n i n i ts gro und sta te. Ho wever, i f the two electro ns ha ve the sam e spins and the spin rel axa ti on is bl o cked by excess carri ers, the exci to n recombi nati on ta kes pl ace l eavi ng the extra electro n i n the exci ted state. Al tho ugh i n both cases the exci to ns recom bi ne fro m the gro und state, thei r energy di ˜ers due to a di ˜erence i n Coul omb intera cti on between carri ers i n tho se conÙgura ti ons. In the i nv esti gated case the spectra l wi dth of the emi ssion l i ne (3 00 ñ eV) at B = 0 T prevents the observati on of i ts Ùne structure . Ho wever, the interp ol ati on of the energi es of the qua drupl et-spl it l ines suggests thei r zero- Ùeld spli tti ng of 400 ñ eV (see the i nset to Fi g. 3), whi ch i s simi la r to the value observed for the self- assembl ed InAs/ Al G aAs QD [12]. Fi na l l y the ori gi n of the spi l t- o˜ do ubl et observed at hi gher energy (see Fi g. 3) can be addressed. In our opi ni on thi s m ust b e due to the recombi nati on of a neutra l exci ton i n the same \ natura l " dot. No n- resonant exci ta ti on a nd rel ati vely l ong col l ecti on ti m e (usua l l y 2 mi nutes) m ay fa vor subsequent charg i ng- di scharg ing pro cesses whi ch shoul d b e reÛected i n the simul ta neous observati on of b oth the neutra l and the charg ed exci to n. Thi s ha s previ ously b een observed i n self-assembl ed QD s [13] and qua ntum ri ngs [14]. Ê
Ê
Th e e˜ ect of t h e em issi on -lin e b ro ad enin g h as b een o bserved i n t he si n gle-d ot spect r oscop y
o f self -assembl ed Q Ds in m esa-p at t er n ed sam pl es. I t i s very li kely t h at cha rge Ûuct ua t i ons on l at er al sur fa ces o f m esa-st ru ctu res in Ûuen ce r eco mb i na t io n en ergy o f ex cit ons in QDs t h rou g h t h e St a rk e˜ ect . Th erefo re an em issio n b a nd ra t h er t h a n a h om og eneou sl y b roa dene d l in e is ob served d u ri ng t he CCD m ea surem ent (see Re f . [12 ]).
552
A . Babi nski et al . 5 . Co n cl u si o n s
Exci to ni c emi ssion fro m the sing l e \ na tura l " dots form ed i n the W L accom pa nyi ng the InAs/ G aAs self- assembl ed QD s has b een i nv estigated i n m agneti c Ùeld. Qua drupl et spl i tti ng of the i nv esti gated emi ssion l i ne ha s b een observed. The attri buti on of the emi ssion l i ne to the recom bi nati on of negati vely charg ed exci to n has been di scussed. Ac kn owl ed gm en t s A. B. ackno wledg esÙnanci al support f rom EU under the ICA1 - CT- 2002-7009 contra ct. R ef er en ces [1 ] L. J acak, P. H awrylak, A . W oj s,Quant um D ots, Springer, Berlin 1998; D. Bimb erg, M. Grundmann, N .N . Ledentso v, Qu antum Do t H et erostru ctur es, John Wiley & Sons Ltd. , N ew York 1998. [2 ] R. C . A sho ori, N atur e 379 , 413 (199 6). [3] D. Leonard, K . Pond, P.M. Petro˜,
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[12] M. Bayer, G. Ortner, O. Stern, A . K uther, A .A . Gorbuno v, A . F orchel, P. H aw rylak, S. Faf ard, K . H inzer, J.P. Reithmai er, F. K lopf , F. Schaf er, 195315 (2002). [ 13] J.J . Finley , P.W. F ry , A .D. A shmore, A . Lema ^tre, A .I. T artako vskii, R. Oulton, D. J . Mow bray , M. S. Skolnick, M. H opkinson, P.D. Buckle, P.A . Maksym, 161305 (2001). [14] R. J . Warburton, C. SchaÛein, D. H af t, F. Bickel, A . Lorke, K . K arrai, J .M. Garcia, W. Schoenf eld, P.M. Petro˜, 124 (2001).
