Mechanism of Radiative Mn 2+ Intra-Shell Recombination in Bulk ZnMnS

Abstract

Origin of a fast component of the photoluminescence decay of Mn2+ intra-shell 4T1 → 6A1 transition is discussed based on the results of photoluminescence, photoluminescence kinetics and optically detected magnetic resonance experiments performed for bulk ZnMnS samples with about 1% Mn fraction. It is demonstrated that a fast component of the photoluminescence decay, reported previously for quantum dot structure and related to quantum confinement effects, is also observed in bulk samples and is related by us to very efficient spin cross-relaxation effects.

Full text

Vol. 100 ( 2001) ACT A P HY SIC A POLON IC A A No . 3
Pr oceedi ngs of th e XXX Internatio nal Sch oo l of Sem icond ucti ng Com pounds, Jaszowi ec 2001
M echanism of Radiati ve Mn 2+
Int ra -Shell R ecom bination
in B ulk Zn Mn S
V.Y u. I vanov, M. Go dl ewski, S. Yat sunenko,
A . K hac ha purid ze, M.S. Li and Z. Go ¤ac ki
Instit ute of Physics, Polish Academ y of Sciences
Al . Lo tni k§w 32/ 46, 02-668 Warsaw, Poland
Origin of a fast component of t he photo lumi nescence decay of Mn 2 +
intra -shell 4T1!6A1transition is discussed based on the results of pho-
toluminesce nce, photolumi nescence kinetics and opticall y detected magnetic
resonance experiments performed for bulk ZnMn S samples with about 1%
Mn fraction . I t is demonstrated that a f ast comp onent of the photolumi nes-
cence decay , reported pre viousl y for quantum dot structure and related to
quantum con Ùnement e˜ects, is also observed in bulk samples and is related
by us to very e£cient spin cross-relaxati on e˜ects.
PACS numb ers: 71.55.Gs, 76.30.Fc, 76.70.Hb, 78.55.Et
1. I nt rod uct io n
It was pro posed [1] that quantum conÙnement e˜ects result in a dramati c
reducti on of a lifetime of Mn2 + intra -shell 4T1!6A1photo lum inescence (PL).
Mn 2 + PL decay ti me is shortened from ms ti mescale, observed in lightly doped
bul k crysta ls, to ns ti mescale, Ùrst repo rted by Bhargava and Gal lagher [2] for Mn
doped ZnS quantum dots. Thi sobservati on was intui ti vely expl ained by Bha rgava
[1] by quantum -conÙnement-enhanced sÀphybri dizati on wi th the dstates of Mn
ions. The validit y of the model was recentl y argued by Bol and Mei jerink [3].
These autho rs reported the observati on of a norm al ms-range PL decay of the
Mn 2 + PL in ZnMnS quantum dots and rejected the idea of pronounced inÛuence
of quantum conÙnement e˜ects on the rate of radi ati ve recombi natio n. Bol and
Mei jerink related the observed fast component of the Mn2 + PL decay to a fast
decay of an underl yi ng low-energy wing of the blue band ZnS PL emission.
(351)

352 V.Yu. Ivanov et al.
Werej ect the latter explanati on, taki ng into account the results of the present
PL ki netics and opti cally detecte d magneti c resonance (ODM R) experiments per-
form ed by us for above and f or below band-gap excitati on conditi ons. For both
excitati on pro cesseswe observed the appearance of a fast component of the PL
decay, which is enhanced in the case of the above band-gap excita ti on. W e wi ll
argue tha t the observed shorteni ng of the PL decay ti me, observed upon above
band-gap excitati on, i s of a di˜erent ori gin tha n that proposed by Bha rgava [1, 2].
2. E xper i m en tal
OD MR and electro n spin resonance (ESR ) i nvestigatio ns were perform ed on
aQ-band (36 GHz) system developed by the authors, with a microwave cavity
mounted in a split- coil m agnet of the Oxford Instrum ents, using argon laser and
a second and fourth harm onics of YAG:Nd pul sed laser for PL excitati on. In the
OD MR experiment we measured PL changes (intensi ty or polari zati on rate) at
magneti c resonance condi ti ons, whi ch were measured synchronously with on-o˜
m odul ated m icrowa ve p ower. The exp eriments were perform ed on bul k ZnMnS
crysta ls wi th ab out 1% Mn fracti on gro wn by chemical tra nsport metho d.
3. Exp er i m ental r esul t s an d di scu ssion
In Fi g. 1 we show the PL ki neti cs observed at 2 K tem perature at two di f-
ferent excitati on conditi ons, for excita ti on withi n energy levels of Mn 2 + ions and
for the above band- gap excita tion. These data cl earl y show tha t a fast component
of the PL decay is observed also i n bul k sam ples. Thus, we agree wi th the conclu-
sion of Bol and Meijerink [3] that thi s component of the PL decay is not related
to quantum conÙnement e˜ects, as was proposed in Refs. [1, 2]. In addition to a
\ normal" m s-range component of the PL kineti cs we observe a faster decay, wi th
100À150 ñ s PL decay time, and also the one in a sub-ñs ti m erange, better seen
in the inset of Fi g. 1.
We relate the former component, based on the resul ts of our calculati ons, to a
faster decay rate of adjacent Mn ions. In these calculati ons wem odi Ùed the pertur-
bati on scheme intro duced in Refs. [4, 5] and we estimated the exp ected shorteni ng
of the PL decay ti m e i n the case of adj acent Mn i ons coupl ed by spin{ spin ex-
change interacti on. These calculati ons indi cate that the pair mechanism leads to
radi ative relaxati on ti mes faster by maximum 100 ti m es.W ecan thus account for
¤100 ñ s component of the PL decay, but we cannot expl ain the fast component
of the PL decay reported by Bhargava and the one observed by us in a sub-ñs
ti m e range.
The fastest component of the PL decay ispronounced for the above band-gap
excita ti on. Thus, i t can b e due to spin- dependent intera cti on between free carri ers
and Mn ions. To veri fy thi spoint we perf ormed the OD MR inv estigati ons of ZnMnS
bul k crysta ls wi th ab out 1 % Mn fracti on.

Mechani sm of Radi ati ve Mn 2 + Int ra-Shell Recombinati on . . . 353
Fig. 1. PL kinetics of Mn 2+ emission observed at two excitation condition s | for
the excitation into the second excited state of Mn2+ ion and at the above band- gap
excitation . In the inset we show the fastest comp onent of the PL kinetics.
In Fi g. 2 we show the OD MR signal detected by us via an increase in the
PL intensity or by a changein the polari zati on rate of the 4T1!6A1intra -shell
emission of Mn 2+ ions. The same resonance signal is detected vi a green and blue
color donor{ acceptor pair (D AP) PL emissions observed by us together wi th the
Mn 2 + PL emission. By performing OD MR and ESR experiments in the same
system and at the same conditi ons we could identi fy the OD MR signal (see Fi g. 2).
Fig. 2. The ESR and ODMR signals of Mn 2+ ions in ZnMnS bulk crystal detected in
aQ-band magnetic resonance setup at the same experimental conditio ns.

354 V.Yu. Ivanov et al.
Identi cal signals are observed in both cases. Thus, we detect magneti c resonance
of Mn2 + ions in the 6A1ground state in the OD MR study .
W e observed the ESR signal at 15 dB dum pi ng of the m icrowave p ower. The
ESR signal satura tes and is not observed at an increased microwave power. In
contra st, the OD MR signal increasesin the intensi ty wi th an increasing microwave
power, whi ch indi cates a very fast T1ti me for the Mn ions studi ed in the OD MR .
Thi s must be due to very e£ cient spin-Ûip processesfor Mn ions studied with the
OD MR and suggests the possible explanati on of the ODMR detecti on mechanism.
To identi fy the relevant mechanism of the OD MR detecti on we should Ùrst
answer the questi on why the m agneti c resonance of Mn2 + in the ground state
a˜ects the rate of radiati ve recom bi natio n of either Mn 2 + or DAP PL emissions.
The relevant m echanisms were proposed by Zink and co-workers [6] and by Kl uge
and Donecker [7]. Zi nk et al. [6] proposed a cross-relaxati on pro cessof a spin Ûip
between excited D AP and Mn ions or between a pair of two adjacent Mn ions, of
whi ch one is in the ground state and one isin the excited state. In turn, Kl uge and
D onecker [7] pro posed spin- dependent tra nsfer. In the l atter pro cess D APs and
Mn ions tra nsfer their excitati on energy and the process depends on thei r spin
ori enta ti on.
Our OD MR experiments indi cate that e£ cient relaxati on of spin selection
rul esof Mn 2 + intra -shell PL can be related to e£ cient spin cross-relaxation e˜ects
for adj acent Mn i ons. W e propose tha t thi s e˜ect i s enti rel y responsible for hi ghl y
e£ cient Mn 2 + PL observed by us for ZnMnS bul k crysta ls and also for ZnMnS
quantum dots studi ed by Bha rgava [1, 2]. For close associates of Mn ions spin
cross-relaxati on process is e£ cient and the PL decay ti me is short. Such Mn ions
contri bute to a faster component of the PL observed at a higher energy wi ng of
the PL, as detected by us from the ODMR -PL experiment shown in Fi g. 3. In the
latter experiment we studi ed the range of the PL from which the OD MR signal
comes, i.e., Fig. 3 shows the dependence of the intensi ty of the OD MR signal on
the detecti on energy. We conclude that a slow decay component of the PL decay is
related to the PL decay of isolated Mn ions, when spin selection rul es are a˜ecti ng
the rate of the Mn 2 + PL recom binati on and its contri buti on can be opti mized
in the OD MR experi ment by appl yi ng slow m odul ati on ra tes of m icrowaves and
detecti ng signal at low- energy wi ng of the PL.
The PL investigati ons perform ed by us under polari zed light excitati on and
at detecti on set at di˜erent polari zati on components of the PL emission indi cate
relati vely long spi n memory e˜ects for these isolated Mn ions. In tha t case the
spin selection rul es for radiati ve tra nsiti ons are not relaxed and the Mn2 + PL
emission is fairly ine£ cient and occurs in m s ti me range. In turn, we relate the
100 ñs ti m e range to radi ati ve recombi natio n of adj acent Mn i ons coupl ed by an
exchange interacti on.
As already menti oned, the latter pro cess cannot account for the sub- ñs com-
ponent of the PL decay. Our experim ental results suggest tha t the fastest compo-

Mechani sm of Radi ati ve Mn 2 + Int ra- Shell Recombi nati on . . . 355
Fig. 3. Comparison of the PL emission and ODMR- PL spectrum. The latter spectrum
was measured at detection set at the Mn2+ resonance conditi ons with signal intensity
detected at di˜erent energies within the PL band.
nent of the PL decay, the one in a sub-ñs ti me range, can be related to spin-Ûip
pro cesses between Mn ions and free carri ers. Thi s is why we observe the shortest
PL decay rates at the above band-gap excita tion. Our new experimental results,
perform ed for CdMnT e quantum dots, which are not discussed here, conÙrm thi s
expl anati on of the PL kineti cs da ta.
Thi s work was partl y supp orted by grants no. 5 P03B 007 20 (OD MR and
PL investigati on) and 7 T08A 006 20 (crysta l growth) of the State Comm ittee for
Scienti Ùc Research. M. S.L. was supported by the Sta te Committee for Scienti Ùc
Research grant numb er 2P03B-146-18.
R efer en ces
[1] R.N . Bharga va, J. Lumin. 70 , 85 (1996).
[2] R.N. Bharga va, D. Gallagh er, Phys. Rev. Lett. 72, 416 (1994).
[3] A.A . Bol, A . Mei jerink, Phys. Rev. B 5 8, R15997 (1998).
[4] D. Boulanger, D. Curie, R. Parrot, J. Lumi n . 680 (1991).
[5] D. Boulanger, R. Parrot, 5500 (1989).
[6] K. Zink, A. K rost, H. Nelkow ski, J. Sahm, H . Stutenb ecker,
603 (1990).
[7] J. Kluge, J. Donecker, 675 (1984).