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appl ications in opti cal and electroni c devices such as UV LE Ds and HEMT tra n-. К corr esp on din g au t h o r; e-m ail : Ba rb ar a.Ch w ali [email protected] edu .pl.

V ol . 1 0 3 ( 2003)

A CTA P HY SIC A P O LO NIC A A

No . 6

P ro ceed i ng s o f t h e XXX I I I n t ern at io n al Sch oo l o f Sem icond uct i ng Co m p ou n ds, Ja szo wi ec 200 3

L o cal izatio n E ˜ ect s in G aN / AlG aN Q u antu m W ell | P h otolu m in escen ce Stu d ie s B . C hwal i sz a ; , A . W y smo¤ek a , R. Bo âek a , K. P. Ko r on a a , Ê

R. St ≤pn ie ws k i a , W. Kn ap b , K. Pak u¤a a , J. M. Ba ra no wsk i a , N . Gr an dj ea n c , J. Ma ssie sc , P. Pr y st aw k o d and I. Gr zego r y d a

In st it ut e of E xp eri ment al Ph ysi cs, W arsaw Uni versi ty Hoâa 69, 00- 681 Wa rsaw, Po l and

b

G ro up e d ' Etude des Semiconducteurs, CN RS-U MR 5650 Uni versi t Ç e Mo ntp ell i er2 Mo ntp ell i er, Fra nce c

CNR S | d

CR HE A, rue Berna rd G rÇ egory , V al b onne- Sophia -An ti p oli s, Fra nce Hi gh Pressure R esearch Center, Pol i sh Aca dem y of Sciences Soko ¤owska 29/ 37, W arsaw, Po l and

Ex cit on locali zati on in Ga N / A lGaN quantu m w ell st ructures is stu died by photolumin escence. A n anomalous temp erature behavior of the photolu minescence from the quantum w ell is observed. With increasi ng temp erature the energy p ositio n of the excitonic emission line Ùrst decreases up to 20 K , then increases, reaching a maximum around 90 K , and then decreases again in the higher temp erature range. T he observed behavior is discussed in terms of lo cali zati on at the interf ace p otential Ûuctuations. It is argued that the temp erature activ ated migration and subsequent release of the excitons from traps that occurs betw een 20 K and 90 K are resp onsibl e for the observed S-li ke shape of the energy dep endence. T he obtained results allow a direct characterization of the energy Ûuctuations present in GaN /A lGaN quantum w ells grow n by di˜erent techniques. PACS numb ers: 78.55. C r, 73. 20.{r, 73. 21.{b, 74.40. + k

1. I n t r o d u ct io n The I I I{ ni tri de system s are no w i ntensi vely studi ed due to thei r p ossibl e appl i cati ons i n opti cal and electro ni c devi ces such as UV LE Ds and HEMT tra nÊ

corr esp on din g au t h o r; e-m ail : Ba rb ar a.Ch w ali [email protected] edu .pl

(5 73)

574

B . Chwal i sz et al .

sisto rs. In pa rti cul ar, due to i ts l arg e energy gap and hi gh therm al stabi l ity , the G aN/ Al G aN system i s a pro m i sing candi date for the fabri cati on of hi gh- p ower devi ces. The opti cal qual i ty of G aN/ Al G aN qua ntum well s (QW s), as determ ined fro m the emi ssion e£ ci ency, i s sti l l no t as hi gh as tha t of G aN/ InG aN structures. The pho to l um inescence (PL) of two - dim ensional hetero systems can pro vi de im p orta nt i nfo rm ati on ab out the mechani sms i nv olv ed in the recom bi na ti on pro cessesas wel l as i ndi cati ons abo ut the energy di stri buti on of carri ers. T empera ture m odi Ùes contri buti ons to the emi ssion fro m free and l o cal i zed exci to ns. One can exp ect tha t at l ow tem p eratures the exci to ns occupy lo wer, lo cal i zed, energy sta tes i n the p otenti al Ûuctua ti ons, whi ch contri bute signi Ùcantl y to the l um i nescence. At hi gher tem p eratures emi ssion due to the recom bi na tio n o f free exci to ns do m ina tes the spectra . Thus, the study of the tem pera ture dependent l um inescence spectra can give qua nti ta ti ve i nf orm ati on ab out exci ton l ocali zati on energy and chara cter of p otenti al Ûuctua ti ons. Opti cal i nvesti gati ons of l o cal i zati on m echani sms perf orm ed on ZnSe/ ZnS [1, 2], InG a/ G aN [3] and InG a N/ Al G aN [4] l ow-dim ensional structures showed sim i lar tem p erature dep endent b ehavi or. In these m ateri al s interf ace ro ug hness and structura l di sorder pl ays a m aj or ro l e l eadi ng to the form ati o n of obj ects such as quantum slabs and/ or qua ntum do ts l ocali zing the exci to ns. In thi s pa per the i nÛuence of l ocal i zati on on the emi ssion pro p erti es of G aN/ Al G aN QW i s presented.

2 . E x p er i m ent al r esu l t s an d d i scu ssi o n The resul ts of two sampl esconta i ni ng G aN/ Al G aN QW are presented. Sam pl e A was gro wn by l ow pressure metal - org ani c chem i cal vap or dep ositi on (MOCVD ). The sapphi re substra te was covered by a 40 nm G aN bu ˜er and a 3 ñ m G aN l ayer. Further on, the structure consists of a singl e 40 ¡A QW emb edded b etween 200 nm and 100 nm Al G aN barri ers wi th a n al um in um content of appro xi m atel y 9%. Sampl e B was gro wn by m ol ecular b eam epi ta xy (MBE) on a bul k G aN substra te. Fi rst, an 0 : 5 ñ m G aN l ayer, i ntenti onal ly n -typ e do p ed (a ppro xi m atel y 5 È 1 0 1 7 cm 3 ) wa s dep osited. Further on, the structure consists of a sing l e 20 ¡A QW emb edded between 500 ¡A Al G aN barri ers wi th an al um in um content of appro xi m atel y 9%. PL m easurem ents were p erform ed at tem p eratures fro m 4.2 K up to 220 K i n a conti n uous Ûow cryo stat. The sam pl es were exci ted by the Ñ = 3 2 5 nm l i ne o f a He{ Cd l aser (wi th a p ower of appro xi matel y 2 m W ). The PL signa l was di spersed by a SPE X 5 00M sing l e g rati ng m ono chro m ato r and detecte d by a ni tro gen cool ed pho to m ul ti pli er coupl ed to a pho to n counti ng system . The PL spectra of the sampl es A and B measured at 4.2 K are shown i n Fi g. 1. The l ow energy regi on of the spectra i s do mi na ted by sharp emi ssion l i nes whi ch ori gi na te fro m the bul k G aN l ayer. The li nes observed i n sam pl e B are À

L ocalizat i on E˜e ct s i n GaN /A l GaN Q uant um Wel l

Fig. 1.

.. .

575

PL spectra of the sample A and B at 4.2 K .

b etter resol ved, maki ng p ossibl e a di sti ncti on b etween the emi ssion due to recombi nati on of 3D exci tons b ound to neutra l do nors (l i ne D 0 X at 3.472 eV) and to neutra l accepto rs (l i ne A 0 X at 3.466 eV). Mo reo ver, i n thi s sampl e the l um i nescence rel ated to free excito ns (FE) associated wi th the A, B, and C val ence ba nds i s observed. In contra st, the sam pl e A exhi bi ts onl y the dono r- b ound- excito n l i ne (l i ne D 0 X at 3.484 eV) shifted to wards hi gher energy due to stra i n present i n thi s hetero epi ta xi al structure . A sma ll feature due to the free A exci to n i s al so observed. Em issions rel ated to the recom bi na tio n of exci to ns i n the QW app ear at 3.546 eV and 3.533 eV i n sampl es A and B, respecti vely. The di screpancy of the QW recom bi na tio n energi es observed i n b oth sampl es i s caused by the di ˜erence i n wel l wi dth, stra i n, and bui l d- i n electri c Ùeld. The ful l wi dth at hal f maxi mum (FW HM) of b oth QW emi ssions i s ab out 17 meV. Thi s val ue i s simi la r to the resul ts obta i ned for anal ogous G aN/ Al G aN QW s gro wn by MBE, exhi bi ti ng FW HM of 20{ 25 m eV [4, 5]. For the inv esti gated structure s, one m ono l ayer step- l i ke QW wi dth Ûuctua ti ons wo ul d resul t in an emi ssion spli tti ng of ab out 15 m eV [6 ]. Such b ehavi or is no t observed i n the lum i nescence of b oth sam pl es. Thus, we b eli eve tha t i n the i nvesti gated structures we are deal i ng wi th sm al l size and hi gh density i nterf ace ro ughness and/ or com p ositi ona l Ûuctuati ons i n the barri er. T o i nv estig ate the carri er l ocali zati on at the G aN/ Al G aN QW i nterf ace we studi ed the tem pera ture dep endence of the QW emi ssion. The PL spectra of sam pl es A and B were measured at di ˜erent tem p eratures, seeFi g. 2. For b oth sampl es an anom al ous tem p erature shi ft of the QW emi ssion energy positi on was observed.

576

B. Chwal i sz et al .

Fig. 2.

PL spectra measured at di˜erent

temp eratures for the sample A (lef t part ) and

B (right part).

Fig. 3.

T emp erature dependence of the Q W excitonic emission energy for the sample

A (lef t part) and B (right part). T he solid curves corresp ond to the best Ùt of Eq. (1).

The energy positi on corresp ondi ng to the emi ssion m axi mum is pl otted i n Fi g. 3. The energy of the QW emi ssion decreases i n the ra nge of tem p eratures between 4.2 K and 20 K and then i ncreases, rea ching a m axi mum aro und 90 K. Ab ove thi s tem p erature the emi ssion energy decreases agai n. The sol i d curves i n Fi g. 3 correspond to the b est Ùt obta i ned at elevated tem p eratures using the fo rm ul a [7]: E (T ) = E

0

À

Ñ= [ exp ( Ù = T )

À

1 ];

(1 )

where Ñ; Ù , and E 0 (the tra nsiti on energy i n the 0 K l i mi t) are m ateri al dep endent consta nts and here are Ùtti ng para m eters. The curves match the da ta very wel l i n the hi gh tem p erature ( T > 9 0 K) regi m e. Thus, one can concl ude tha t sta rti ng fro m a certa i n chara cteri sti c tem p erature the energy shift of the QW emi ssio n fol l ows the changes of the QW m ateri al ba nd gap, i .e. tha t of G aN. The val ues of the Ùtted para m eters are Ñ sa m p l e A = ( 0: 4 Ï 0: 1) eV ; Ùsa mple A = ( 6 70 Ï 55) K and Ñ sa mple B = ( 0: 4 Ï 0: 2) eV ; Ùsa mple B = ( 670 Ï 11 0) K and a re di ˜erent tha n tho se rep orted for bul k G aN [7]: Ñ = (0 : 1 2 1 Ï 0 : 0 0 5 ) eV, Ù = (3 1 6 Ï 8 ) K. Such a di ˜erence can resul t fro m the stra i n present i n a QW structure.

L ocalizat i on E˜e ct s i n GaN /A l GaN Q uant um Wel l

Fig. 4.

Schematic diagrams illustrati ng occupation

.. .

577

of the exciton traps at di˜erent

temp eratures (see text).

The b ehavi o r observed in the l ow tem p erature regi m e (T < 9 0 K) can be expl ai ned i n term s of the l ocal i zati on of the excito ns i n the QW . At l ow tem p erature ( T ¤ 4 : 2 K), the exci to ns are tra pp ed by ra ndo m p otenti al Ûuctua ti ons. These l ocal i zed excito ns mai nl y contri bute to the l um inescence since they do not ha ve enoug h therm al energy to b e acti vated (Fi g. 4a). A sli ght i ncrease in the tem p erature (up to 2 0 K) al l ows the excito ns to m i gra te. Thus, they can Ùnd l ocal m i nim a of the Ûuctua ti ng p otenti al (Fi g. 4b), whi ch l eads to a decrease i n the emi ssio n energy. Further increase i n the tem p erature p erm its the rel ease of the excito ns fro m these tra ps (Fi g. 4c). Thi s resul ts i n an i ncrease i n the emission energy b etween 20 K and 90 K. Ab o ve thi s tem p erature, the exci to ns are no l ong er l o cal i zed (Fi g. 4d) and the emi ssion fro m the QW fol l ows the tem p erature dependence of the G aN ba nd gap. The di ˜erence b etween the experi m enta l ly determ i ned PL energies and tho se obta i ned fro m the Ùt of Eq. (1 ) al l ows to estim ate the exci to n lo cal i zati on energy. It i s found i n thi s way tha t the deepest tra ps shoul d b e up to 5{ 7 m eV b elow the free excito n l evel for b oth sam pl es. The obta i ned val uesof the chara cteri sti c tem p erature and l o cal i zati on energy seem to b e i ndep endent of the gro wth m etho d. Si nce the sampl es were deposited ei ther on bul k G aN or on sapphi re, i t can b e speculated tha t the l ocal i zati on of exci tons canno t b e attri buted to structura l defects. Thi s suggests tha t the observed l ocal i zati on e˜ects ori gi na te fro m the G aN/ Al G aN i nterf ace i tself . The observed di sorder m ay b e due to the materi al comp ositi on Ûuctua ti on and/ or smal l size wel l wi dth Ûuctua ti ons. Al so, the residua l stra in tha t i nduces an i nho mogeneous el ectri c Ùeld di stri buti on i n the structure can i nÛuence the l ocal i zati on of excito ns. It i s b eli eved tha t the l ocal i zati on of hol es pl ays a m ajo r ro l e, since the spati al di stri buti on of the electro n wa ve functi on i s to o bro ad to p enetra te sm al l Ûuctuati ons of the potenti al [6]. The electro n experi ences an a verage p otenti al ori g ina ti ng f rom the i nterf ace di sorder whi l e the ho l e can be easy l ocal i zed i n p otenti al tra ps due to the smal l extent of its wa ve functi on.

578

B. Chwal i sz et al . 3 . Co n cl u si o n s

In sum mary , we ha ve studi ed the l ocali zati on of f ree exci tons i n the G aN/ Al G aN QW s. In the l ow tem p erature regi m e the carri ers are conÙned i n tra ps up to 5{ 7 meV deep. By increa sing the tem pera ture ab ove 90 K, when k T i s i n the ra nge of the l ocal i zati on energy, the exci to ns becom e del ocal ized and the l um i nescence energy dep ends on tem pera ture sim i l arly to the b ehavi or i n bul k G aN. The observed S-shap ed dep endence i s most pro ba bl y caused by ro ug hness and di sorder of the G aN/ Al G aN i nterf ace. Ac kn owl ed gm en t s Thi s wo rk was parti al l y supp orted by the State Co mm ittee for Scienti Ùc R esearch gra nt No. 2P0 3B 011 22 and NA TO Science Pro gra m . R ef er en ces [1] H .C . K o, D. -C . Park, Y . K awakami, S. F uj ita, S. F uj ita, Y .-S. K im, A ppl . Phys. Let t. 7 3, 1388 (1998). [2 ] P. T omasini, K . A rai, F. Lu, Z. Q. Zhu, T . Sekiguchi, T . Y ao, M. Y . Shen, T . Goto, J. A ppl . Ph y s. 83, 6028 (1998). [3] Y .H . C ho, G. H . Gainer, A .J . Fisher, J .J. Song, S. K eller, S.P. Den- Baars, A ppl . Ph y s. Lett . 73, 1370 (1998).

U .K . Mishra,

[4] M. Leroux, N . Grandj ean, M. La ugt, J . Massies, B. Gil, P. Lef eb vre, P. Bigenw ald, Ph ys. R ev. B 58, R13371 (1998). [5] H . H aratizadeh, P.P. Pasko v, G. Pozina, P.O. H oltz, B. Monemar, S. K amiyama, M. Iw ana, H . A mano, I. K asaki, A ppl . Ph y s. Lett. 8 0, 1373 (2002). [6] M. Gallart, P. Lef eb vre, A . Morel, T . T aliercio, B. Gil, J. A llegre, H . Mathieu, B. Damilano , N . Grandj ean, J . Massies, Ph y s. Status Soli di A 61 (2001). [7 ] K .P. K orona, A . Wysmolek, K . Pakula, R. Stepniew ski, J .M. Baranow ski, I . Grzegory , B. Lucznik, M. W roblew ski, S. Porow ski, 788 (1995).