ultrafast spectroscopy of zno/znmgo quantum wells

Laser & Photon. Rev. 3, No. 1–2, 85–96 (2009) / DOI 10.1002/lpor.200810017 85

Abstract We review recent work studying the dynamics in zinc

oxide quantum wells (QWs) using ultrafast optical spectroscopy.

These materials present exciting possibilities for optoelectronic

device applications. In order to develop these applications, it

is important to understand the mechanisms of the electronic

processes occurring in ZnO/ZnMgO QWs. In this review, we

discuss the excitonic lifetime and the impact of the internal

electric field, the potential profile, phonons, and defects. We also

discuss coherence dynamics and the observation of biexcitons

at room temperature.

The ZnO QW sample appears clear to visible light, but absorbs

and radiates in the near UV on laser excitation.

© 2009 by WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim

Ultrafast spectroscopy of ZnO/ZnMgO quantum wells

Jeffrey A. Davis1 and Chennupati Jagadish2,*

1 Centre for Atom Optics and Ultrafast Spectroscopy, Swinburne University of Technology, Melbourne 3122, Australia2 Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National

University, Canberra, ACT 0200, Australia

Received: 3 April 2008, Revised: 25 June 2008, Accepted: 30 June 2008

Published online: 12 August 2008

Key words: Zinc oxide, quantum wells, excitons, dynamics, ultrafast spectroscopy.

PACS: 78.47.-p, 78.55.Et, 78.67.Oe

1. Introduction

Zinc oxide has attracted great interest in recent times asan exciting ‘new’ material with many promising propertiesfor blue/UV optoelectronics, transparent electronics, andsensor applications. In actual fact, ZnO has been widelystudied since 1935 [1], and has been commonly used in itspolycrystalline form for over a hundred years, in applica-tions ranging from sunscreen to piezoelectric transducers,concrete to rubber, and many others.Many of the properties that make ZnO so interesting

revolve around the large direct band gap of 3.37 eV [2].This makes it transparent at visible wavelengths but with lu-minescence in the blue/UV spectral region, thereby making

it attractive for various optoelectronic applications, sim-ilar to GaN. The large exciton binding energy of ZnO(EB = 60meV, [3–5]) means that it has a large oscil-lator strength, which leads to strong absorption and thepotential for high efficiency in light emitting applications,and a significant advantage over GaN (EB = 25meV, [6])based devices. Zinc oxide normally exists with a wurtzitecrystal structure, and with very polar bonds between Znand O atoms [7]. The ionic nature of this crystal struc-ture means ZnO possesses a large piezoelectric coefficient,and the absence of any inversion symmetry means it has alarge spontaneous polarization. Other advantages of ZnOare its low optical power threshold for lasing [8], radiationhardness [9], and biocompatibility [10].

* Corresponding author: e-mail: [email protected], [email protected]

© 2009 by WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim

86 J. A. Davis and C. Jagadish: Ultrafast dynamics in ZnO QWs

The recently revived interest in ZnO has come aboutbecause of the development of growth technologies, andthe ability to grow high quality single crystals and epitaxiallayers of ZnO. This has made the way for the developmentof ZnO based electronic and optoelectronic devices [11,12].However, one of the major continuing challenges for therealization of future ZnO based devices is the developmentof a suitable p-type material.Meanwhile, much of the research into materials for

potential device applications in II-VI, and III-V semicon-ductor systems has revolved around the growth, charac-terization, optimization and exploitation of nanostructures.With the reduction in size, novel electrical, mechanical andoptical properties emerge due to surface and quantum con-finement effects. In ZnO, many different growth morpholo-gies can occur, including: nanobelts, nanowires, nanocages,nanocombs, nanorings, nanosprings, and nanohelices [13].However, it is only recently, with improved control overgrowth conditions, that high quality two-dimensional epi-taxial layers have been able to be grown, opening the doorto the growth of ZnO based quantum wells (QWs) [14–17].In most cases the barrier material is Zn1−xMgxO, wherethe band gap is given by 3.37+2.51x [18] with band offsetsrelative to the valence (Ev) and conduction (Ec) bands forZnO ofΔEc/ΔEv = 70/30 [19]. This development of theZnO QWs has provoked much interest, particularly in thesearch for efficient light emitting devices in the blue/UVspectral region, as many of the desirable properties are en-hanced, for example: the band-gap becomes tunable, theexciton binding energy may be increased, and the radiativeefficiency improved [20].In ZnO/ZnMgO QWs, the lattice mismatch at the in-

terfaces causes strain and an internal piezoelectric field.In all reported cases, the growth of ZnO QWs is c-axisoriented, which means that the electric field due to thespontanteous polarization is also large, as there is a sig-nificant difference between the values for spontaneous po-larization in the well and barrier materials. In such ZnO/Zn1−xMgxO QWs, the spontaneous and piezoelectric po-larizations are in opposite directions, with the spontaneouspolarization being roughly twice the piezoelectric contribu-tion (PPE = −34xmC/m2 and PSP = 66xmC/m2) [21,22].This leads to an internal electric field that varies with Mgconcentration, and has been measured to be as large as0.9MV/cm in ZnO/Zn0.78Mg0.22O QWs [23]. The effectsof such an internal electric field has previously been studiedextensively in CdSe/CdS Stark superlattices [24, 25]In the pursuit of ZnO based QWs for device applica-

tions, it becomes imperative to fully understand the pro-cesses and mechanisms occurring in these materials. Forexample, carrier transport dynamics and mechanisms; ra-diative and non-radiative recombination rates and mecha-nisms; the influence of phonons, defects, and surface stateson these processes; and the role of excitonic and biexci-tonic effects. In order to understand these processes, it isnecessary to study their dynamics. We present here a re-view of work looking at the ultrafast dynamical response ofZnO/ZnMgO QWs. For a review of ultrafast experimental

Figure 1 (online color at: www.lpr-journal.org) The potential

profiles for a well (a) without and (b) with internal electric field are

shown. The effects of the internal electric field can be seen in (b),

where the electron and hole energy levels are closer together, and

their wavefunctions (shown in red) become spatially separated.

techniques, some of which are used here, see for exam-ple [26].We begin by reviewing the details of exciton lifetime

studies, and the effect of QW size and shape. Followingthis we review the carrier capture, localisation and non-radiative recombination dynamics, and then discuss coher-ence dynamics and the details these resolve regarding theinteractions in the materials. Finally we discuss the role ofbiexcitons and the dynamics associated with them, beforegiving a conclusion and outlook to the future.

2. Exciton lifetime studies

At low temperature, excitonic effects dominate in ZnO,and even at room temperature they are expected to playa major role due to the large exciton binding energy(compared to the thermal energy at room temperature,EkT = 25meV) [27]. For this reason we ignore free carriereffects and concentrate only on excitons.

The property of ZnO that potentially exerts the greatestinfluence on the exciton lifetimes in ZnO/ZnMgO QWsis the internal electric field. The presence of the internalelectric field causes the spatial separation of the electronand hole wave functions, thereby reducing electron-holeoverlap and increasing the exciton lifetime. A correspond-ing effect is that there is also a strong dependence on wellwidth, as the larger a QW, the greater the separation ofelectron and hole wavefunctions can be, hence increasingthe lifetime. These two effects are shown schematically inFig. 1, where it can be seen that the electric field compressesthe electron and hole wavefunctions to opposite sides of thequantum well, thereby reducing the electron-hole overlapintegral. The results of time resolved photoluminescence(PL) experiments, from [29], for ZnO/ Zn0.7Mg0.3O QWsof different width (see [30] for growth details) are presented

© 2009 by WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim www.lpr-journal.org

Laser & Photon. Rev. 3, No. 1–2 (2009) 87

in the plot of PL lifetime as a function of well width shownin Fig. 2. These results show the lifetime increasing by sev-eral orders of magnitude as the well width is increased from1 nm to 7 nm. Other work by Morhain et al. has shown thatthe lifetimes of excitons in 9 nm QWs can be as long asseveral ms [23].

The results from the TRPLmeasurements in Fig. 2 showthat for QWs of width less than∼ 3 nm the lifetime does notchange much, whereas when the width is increased beyond4 nm the luminescence lifetime increases dramatically. Thistransition corresponds to the point where quantum confine-ment effects begin to be dominated by quantum confinedStark effects (QCSE). This transition can also be seen in thespectral measurements of the PL shown in Fig. 3 and thepeak energy plotted versus well width in Fig. 2, where forwell widths less than 3 nm the transitions lie above that ofZnO bulk, but for well widths greater than 3 nm the transi-tion is forced below the ZnO bulk transition energy (by upto 400meV) as the quantum-confined Stark effect beginsto dominate. The position of this change from quantumconfinement regime to the QCSE regime will depend onthe strength of the internal field, which in turn depends onthe Mg concentration in the barriers.A comparison of the PL lifetimes discussed above to

similar measurements in thin films, where lifetimes of sev-eral ns have been measured [31], show that for narrow wells,where the confinement causes greater electron-hole overlapthe lifetimes are shorter. Whereas for wide QWs, where theinternal field separates the electron and hole wavefunctions,the lifetimes are much longer. At room temperature, theeffects of the internal field in the wide QWs are reduced,and non-radiative effects dominate, and the lifetimes areagain reduced, as discussed in Sect. 3.The measurements of the exciton lifetimes provide a

direct insight into the oscillator strength of these opticaltransitions. The radiative lifetime is approximately propor-tional to the quantity:

f = |ϕλ(0)|−2

∣∣∣∣

∫ +∞

−∞dzfe(z)fh(x)

∣∣∣∣

−2

where∫ +∞−∞ dzfe(z)fh(x) is the overlap integral of the

electron (fe(z)) and hole (fh(z)) envelope wavefunctions,and ϕλ(ρ) = (

√

2/π/λ) exp(−ρ/λ)) is the in plane exci-tonic contribution, with ρ the in-plane relative co-ordinateand λ the pseudo Bohr radius [23].It can therefore be identified from the measured radia-

tive lifetimes that the oscilator strength of ZnO/ZnMgOQWs decreases as the well width increases, due largelyto the internal electric field. So, whilst the internal fieldincreases the tuning range of the QW transitions, which isbeneficial for potential devices, it also causes a reductionin the efficiency for QWs greater than ∼ 3 nm wide, whichis detrimental for potential devices.

Measurements of the lifetime and transition energy as afunction of well width thus also allow a quantitative mea-surement of the strength of the internal electric field. Using

an envelope function model that includes the variationalcalculation of the exciton binding energy, and by com-parison with TRPL experiments, the internal electric fieldin ZnO/Zn0.78Mg0.22O QWs has been determined to be0.9MV/cm [23]. The magnitude of the internal electricfield is highly dependent on the Mg concentration, and forZnO/Zn1−xMgxO QWs has been extrapolated to be 4.1x(in MV/cm) . Other measurements for QWs with differ-ent Mg concentration in the barriers agree with this value,within experimental uncertainties [29, 32, 33].

It is clear that the internal electric field can play an im-portant role in the dynamics and mechanisms of electronicprocesses in ZnO QWs. It is therefore important to considerany possible shielding of this field by excess free carriersin the well and/or barrier layers. These carriers could bepresent due to a residual concentration as a result of un-intentional doping, or on a transient basis following highexcitation densities. Shielding of the electric field can alsooccur due to nearby QWs in multiple QW samples, par-ticularly where the barriers are narrow relative to the wellwidth. The effect of shielding the internal field is to quenchthe QCSEs, in other words, the transitions are blue-shiftedand lifetimes reduced. Indeed, many studies [34–36] havemeasured much shorter lifetimes than those discussed here,and have not observed any great effects of the electric field,in some cases due to shielding of the internal field by suchmeans. Ignoring the presence of the internal electric fieldhas also led to possible misinterpretation of results. Forexample, Wei et al. observe a multi-exponential decay intime-resolved PL experiments from 20 nm wide QWs, andspectral red-shifts as a function of delay after the initialexcitation [37]. They interpret these results as being due toemission initially from free excitons and later due to boundexcitons. However, given that the spectrum shifts graduallyas a function of delay, an alternative interpretation could bethat the extent of shielding of the electric field decreasesas the excess carrier density decreases over time, therebyinducing a red-shift and increasing the radiative lifetime.Morhain et al. [23] demonstrated shielding of the electricfield due to high excitation density through observations ofa blue-shift of the PL peak as the intensity of the excitinglaser was increased.On the other hand, Makino et al. [38] have shown that

where the barriers are sufficiently small (x ≤ 0.12), andfor QWs less than 4.5 nm wide, the effects of the internalelectric field are small and can largely be neglected. In thiscase they have shown that confinement effects dominateand the luminescence lifetime becomes much more sampledependent as the role of defects and well width fluctuationsin localising carriers becomes important.

2.1. Evidence of enhanced electron-holeexchange interaction

Further work by the group of Gil [39–42] has shown thatfor narrow wells, where quantum confinement effects domi-nate, the PL decay is actually a combination of two different

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Figure 2 (online color at: www.lpr-journal.org) Time-resolved PL (red

dots) and PL peak energy (black squares) are shown for ZnO/Zn0.7Mg0.3O

QWs as a function of well width. Little change is observed in the lifetimes

for the narrow wells, but for wells greater than 3 nm wide, the lifetimes

increase rapidly with well width. Adapted from [29].

Figure 3 The PL spectra for ZnO/Zn0.7Mg0.3O QWs

with varying widths show increasing red-shifts as well

width is increased, beyond the ZnO bulk band gap, due

to the QCSE.

decay rates that differ by one to two orders of magnitude,one sub-nanosecond and the other 10’s of nanoseconds.In bulk ZnO, five transitions are typically observed, corre-sponding to 3Γ5 excitons (which couple to light polarizedparallel to c-axis oriented QWs) and 2Γ1 excitons (whichcouple to light polarized perpendicular to c-axis orientedQWs). Thus, in the QWs discussed here, it is only possi-ble to observe emission from the Γ5 excitons, which arelabeled A, B, and C in bulk ZnO. The dispersion relation ofthese different excitons is determined by the crystal field,the spin-orbit interaction, and the electron-hole exchangeinteraction.

The electron-hole exchange interaction is particularlystrong in ZnO, and, as it is dependent on electron-holeoverlap, is enhanced in narrow QWs. Gil et al. [39] presentsimulated data showing the electron-hole exchange energy,and the energy separation between the A and B excitons, asa function of well width. This shows that for well widthsgreater than 2–3 nm the exchange energy becomes less thanthe bulk value due to the separation of the electron andhole wavefunctions by the internal electric field. Whilst fornarrow QWs, the exchange interaction is enhanced com-pared to the bulk as a result of the quantum confinement,giving a splitting of up to 12meV. It thus becomes evident,that for narrow QWs the electron-hole exchange interactioncauses significant mixing between the A and B type exci-tons, which leads to splitting in energy and redistributionof oscillator strengths, which otherwise are approximatelyequal for pure A and B exciton states [12]. The difference inoscillator strengths is reflected in the TRPL measurements,where the two different decay rates correspond to emissionfrom the two mixed A-B exciton states. This provides thefirst suggestion of the role played by the electron-hole ex-change interaction in ZnO QWs. Direct observation of the

energy splitting of 12meV has thus far not been possible asit is obscured by inhomogeneous broadening arising fromfluctuations in the well width and barrier composition.

2.2. Lifetime dependence on potential profile

Studies of the luminescence lifetime as a function of theshape of the potential profile have recently been performedon a series of ZnO/Zn0.7Mg0.3O QWs [29]. The shape ofthe wells was altered by post growth O− ion implantationand subsequent rapid thermal annealing. The low energyion-implantation introduces defects in the form of vacanciesand interstitials and their complexes in the QW material,and the thermal anneal then induces Zn/Mg intermixing.This leads to a slowly varying Mg content across the bar-riers of the well, and hence a smooth potential profile asindicated in Fig. 4. The interdiffusion can be modeled, asdescribed in [43], giving the Mg distribution across the sam-ple, and, since the band gap is directly dependent on the Mgconcentration, the potential profile accross the well. The in-ternal electric field in such a sample will also be altered, asit too depends on the Mg concentration and the differencebetween the spontaneous polarizations of adjacent materi-als, and can be calculated based on these parameters [29].The extent of the intermixing and hence the amount of‘smoothing’ of the barriers depends on the ion implanta-tion dose received by the sample, with greater implantationdoses leading to greater smoothing of the barrier [43].Figs. 5 and 6 show the spectra and time-resolved

PL measurements, respectively, for a 4 nm QW follow-ing different ion-implantation doses. For an ion dose of5 × 1014 cm−2 the calculated potential profile and wave-functions are shown in Fig. 4b. A major blue-shift of the



Figure 4 (online color at: www.lpr-journal.org) The

calculated band structure for the 4 nm QW as-grown

(left) and following ion implantation at a dose of 5 ×1014 cm−2 with an internal electric field as described

in the text. The envelopes of the electron and hole

wavefunctions are shown in red, and their overlap

function shaded in green. The electron-hole overlap

increases by 70% following intermixing.

Figure 5 (online color at: www.lpr-journal.org) The

PL for a 4 nm QW is shown before and after Zn/Mg

intermixing. The large-blue shift observed is due to

the altered potential profile and reduced effect of the

internal electric field. Adapted from [58].

transition energy is seen in Fig. 5, and is attributed to thechanging QW profile as a result of the Zn/Mg intermixingand in part to a quenching of the quantum confined Starkeffect in [43] and [29]. An alternative mechanism for theblue shift could be that the introduction of defects createsmany more localisation sites, which would be blue-shiftedrelative to the QW states. This is discarded as a majorcontribution, as the luminescence efficiency would also beexpected to decrease with the number of defects, and indeedprior to annealing this was the case, however, after anneal-

ing the luminescence intensity was fully recovered, therebysuggesting that the contribution from localised states is un-changed following ion implantation and thermal annealing.

Fig. 6 shows the effect of the change in potential pro-file on the luminescence lifetime, and consistent with thequenching of the QCSE, and increased electron-hole over-lap, the lifetimes decrease dramatically. Both of these ef-fects are predicted by the calculated wavefunctions andenergy levels depicted in Fig. 4, which show significantchanges in the energy levels and electron-hole overlap. In-



Figure 6 (online color at: www.lpr-journal.org) The time-

resolved PL for a 4 nm QW is shown before and after Zn/Mg

intermixing due to ion implantation doses from 5 × 1014 and

5 × 1015 cm−2. The major decrease in the lifetime is attributed

to the altered potential profile and quenching of the QCSE. Little

difference is observed between the different implantation doses

as the electron-hole overlap is close to maximised even for the

weakest dose. Adapted from [29].

deed, the calculated 70% change in the electron-hole over-lap integral is, by Eq. 1, expected to lead to a reduction inthe radiative lifetime by a factor of three, which matchesthe observed change in Fig. 6. Increased implantation dosesdon’t alter the lifetimes much more as the electron holeoverlap is close to maximised by the weakest implanta-tion dose. The calculations also show that the electric fieldstrength at the middle of the well is unchanged by theZn/Mg intermixing. However, the two easily observableeffects of the internal field, namely the large red-shift andlong lifetimes, are significantly quenched. Thus, it becomesclear that by altering the potential profile of the QWs, therecombination efficiency can be greatly enhanced, allowingthe development of more efficient light emitting devices.One problem with this method of altering the QW shapeby post growth techniques is that it is difficult to accuratelycontrol the shape of the resulting QWs, and so work intogrowing QWs with graded barriers is progressing, in orderto reduce the effects of the internal electric field.

3. Carrier capture, localisation andnon-radiative recombination

To this point, we have only considered exciton dynamics atlow temperature, however, for device applications we needto consider the dynamics at room temperature, where non-radiative recombination dynamics are expected to play a

Figure 7 (online color at: www.lpr-journal.org) (a) The PL spec-

tra at different temperatures, and (b) the decay time as a function

of emission energy at RT show that relaxation from high energy

states is very rapid, whilst the exciton lifetime increases as the

depth of localisation increases, i.e. emission energy decreases.

Reprinted with permission from [46]. Copyright 2007 IOP Pub-

lishing Ltd.

greater role, and reduce radiative efficiency [28](this can beseen in Fig. 7a). It is also important to note at this point thatthe quality of the QW structures will play a very importantpart in carrier localisation and non-radiative processes.

Makino et al. [27, 34, 44] have shown that where thebarriers are sufficiently small (x ≤ 0.12), and for QWsless than 4.5 nm wide, the effects of the internal electricfield are small and can largely be neglected. In this casethey have shown that for a series of QWs with well widthvarying from 4.5 nm to 0.7 nm the PL lifetime at 5K in-creases as the well width decreases. This is in contradictionto the expected behaviour resulting from increased electron-hole overlap, and is instead interpreted as emission fromlocalised states, which have longer lifetimes and becomedeeper for narrower wells. This is confirmed by lifetime



measurements as a function of emission energy for severalQW widths. These show that the low energy tail of theemission, which corresponds to localised states, has a longlived decay that decreases monotonically from the centre ofthe peak as the emission energy increases, a behaviour thatis well known [45]. From these measurements the lifetimesof the localised excitons can be determined (assuming thisis not dependent on emission energy), as can the potentialdepth of the localised states, and the absorption edge. In theresults presented by Makino [27, 34, 44], the PL lifetimeand depth of localised states follow very similar trends as afunction of well width, thereby supporting the conclusionthat the observed increasing lifetime with decreasing wellwidth is due to emission from localised states.

Similar work byWen et al. used two-colour pump probespectroscopy to directly probe the dynamics at room temper-ature for different energy levels [46]. In these experiments,the pump beam was set to 3.65 eV (around 200meV abovethe PL peak) and the probe energy was varied from 3.55 eVto 3.25 eV, thereby probing high-energy QW states, groundfree-exciton states, and localised exciton states. Fig. 7bshows the pump-probe decay time at room temperature as afunction of probe energy. These results show that when theprobe beam is on the high energy side of the PL spectrum,the decay time of the transient absorption is very short,indicating rapid cooling of the excitons from high energystates. As the detection energy is decreased, the decay timeof the transient absorption, and hence the lifetime of thestates being probed increases. This is consistent with thework of Makino et al. [27, 34, 44] discussed above and isdue to emission from localised states. There is, however,an important difference: in the results shown in Fig. 7 thelifetimes are an order of magnitude smaller, and continue toincrease with decreasing probe energy rather than reachinga constant value. This suggests that the localisation depthincreases as the energy of the states decreases.

Other measurements of PL lifetimes at room tempera-ture have given values from sub-picosecond up to severalhundred ps [35, 46, 47], which is several orders of magni-tude smaller than that at low temperatures. This is due tonon-radiative recombination processes, and phonon-carrierinteractions and is reflected in Fig. 7a where the radiativeefficiency decreases as temperature is increased from 20K to 100 K. The large variations in measured lifetimesare due to differences between samples, as the role playedby defects and interface fluctuations in localising excitonsand as scattering points is highly dependent on the qual-ity of the sample and the growth conditions. For example,Makino et al. showed that by adding just a few percent ofCd to the ZnO QWs, the temperature dependence of thePL changed dramatically due to the role played by the Cdatoms in localising excitons. In that case, they were ableto model the predominant dynamics using carrier hoppingmodels [48–50]. Hence, rather than try to discuss the roleof defects and localisation effects for all possible cases, werefer the reader to the existing literature on this work: seee.g. [27, 34, 44, 46, 48–54].

Because of the much reduced luminescence efficiencyat high temperature, direct studies of the dynamics as afunction of temperature are largely missing. This makes itdifficult to attribute precisely the role of phonons and othermechanisms in the non-radiative recombination. However,the role that phonons play in the recombination dynamicsin ZnO QWs is predictably temperature dependent, andstudies of time integrated PL spectra as a function of tem-perature have gone some way to explaining the role theyplay. Measurements of the linewidth of the QW excitontransitions as a function of temperature provide detail ofthe exciton phonon coupling strength. The full width at halfmaximum (FWHM) is approximately described by:

Γ (T ) = Γinh + γphT + ΓLO/[exp(�ωLO/kBT )− 1]

where Γinh is the temperature independent inhomogeneouslinewidth, γphT is due to acoustic phonon scattering, withγph the acoustic phonon coupling strength. The final term isdue to LO phonon scattering, where ΓLO is the exciton-LOphonon coupling strength, and �ωLO is the LO phonon en-ergy, which has been shown to be 72meV in bulk ZnO [19].At low temperatures it is expected that acoustic phononinteractions will dominate the homogeneous linewidth asthe LO phonons are frozen out, however, depending onthe sample, the inhomogeneous contribution may be themain contributor to the total linewidth up to 100 K. As thetemperature increases, the effect of LO phonon scatteringincreases, leading to the broadening of the transition, and athigh temperature LO phonon Frolich scattering dominates.For a 4.7 nm wide QW with low barriers (i.e., low Mg

concentration in the barriers) and hence minimal internalelectric field effects, the terms representing the couplingstrengths of the acoustic and LO phonons have been calcu-lated to be γph = 31 μeV/K and ΓLO = 341.5meV, respec-tively [27]. These values differ greatly from values obtainedin bulk, where γph = 16 μeV/K and ΓLO = 47meV, respec-tively [12,55]. The much higher value of ΓLO reported forthe quantum well would suggest an unrealistic homoge-neous linewidth of 1.7 eV for A excitons if an effectiveoptical phonon energy is used, as in [55]. However, in [27],they assume the LO phonon is the dominant optical phonon,with energy 72meV. Taking this into account, the contri-butions of each component to the total linewidth is similarfor the QW and bulk reports. This serves to highlight thatdifferent measurements of phonon coupling strengths cangive significantly different values based on the assumptionsmade, thus, care needs to be taken in making comparisonsbetween different measurements. Nonetheless, it is clearthat the phonon coupling in ZnO and ZnO QWs is verystrong and much stonger than, e.g., in GaAs QWs [55].Furthermore results have shown that as the well width de-creases, the LO phonon coupling strength decreases due tothe increasing exciton binding energy [27] [56]. Whilst inQWs with high Mg content, where the internal electric fieldspatially separates the electron and hole wavefunctions,the exciton-LO phonon coupling is shown to be furtherenhanced [27].



4. Coherence dynamics

The previous discussion on phonon interactions addressestheir impact on the linewidth, however, in the presenceof inhomogeneous broadening it becomes impossible tomeasure directly the inhomogeneous linewidth at low tem-perature. In these cases, the‘ homogeneous linewidth, andhence phonon interaction strength may be determined bymeasurements of the decoherence time (T2) by transientfour-wave mixing (FWM) techniques, [26], where T2 isinversely proportional to the homogeneous linewidth. Co-herence effects in semiconductor nanostructures have alsobeen of significant interest over the past decade for quan-tum information applications and studies of fundamentalquantum mechanical interactions (such as polaritons, BECs,and entanglement).

To date there has been very little reported assessing thecoherence properties of ZnO QWs, however recent workby Davis et al. [57, 58] has measured the dephasing timein 4 nm QWs with high barriers (and hence strong electricfield) to be several hundred fs at room temperature. Thisvalue is similar to values obtained for GaAs based QWsat room temperature, despite the increased exciton-phononcoupling in ZnO. This is possibly because of the reducedphonon density in ZnO at room temperature due to thehigher phonon energy in ZnO.

Fig. 8 shows the spectrally resolved transient four-wavemixing signal for a 4 nm ZnO/Zn0.7Mg0.3O QW as grown,(a), and following Zn/Mg intermixing, (b), and the corre-sponding spectrally integrated signal in (c). Recall from pre-vious discussions that Zn/Mg intermixing causes a changeto the potential profile that reduces the effects of the inter-nal electric field, and that the exciton-LO phonon couplingstrength increases with increasing electric field. Thus, wemay expect the dephasing time to increase as a result of theZn/Mg intermixing. Fig. 8 shows that the dephasing timedoes not change due to the reduced electric field, whichsuggests that internal field does not play a major role indephasing at room temperature despite altering the LO-phonon coupling strength, probably due to the abundanceof acoustic phonons.

5. Biexciton dynamics

It was mentioned previously that the large exciton bindingenergy ( 60meV) in ZnO should allow efficient excitoniclasing at room temperature (although this has been seri-ously questioned [55, 59]). It is also predicted, and hasbeen shown in other semiconductor systems, that biexci-tonic transitions play an important role in stimulated emis-sion and can operate with an even lower threshold [60].The biexciton binding energy in bulk ZnO has been mea-sured to be in the range 12–16meV [61–63], and in narrowZnO/ZnMgO quantum wells (QWs) it is expected to beenhanced due to quantum confinement. However, in com-petition to this, the strain induced electric field reducesthe exciton and biexciton binding energies in wider QWs.

Figure 8 (online color at: www.lpr-journal.org) The spec-

trally resolved transient FWM signal is shown for a 3 nm

ZnO/Zn0.7Mg0.3O QW as grown, (a), and following Zn/Mg in-

termixing, (b). (c) shows the integrated signal for the as-grown

(black squares) and intermixed (red dots) samples, from which

dephasing times of 280±40fs are obtained. The dephasing time isunchanged despite the quenching of the QCSE, which suggests the

electric field does not play a major role exciton-phonon coupling

at room temperature . Adapted from [58].

Hence, provided the wells are sufficiently narrow and con-finement effects dominate, biexciton binding energies largerthan 26meV for QWs narrower than 2.5 nm have been ob-served [63–66]. This is greater than thermal energy at 300K, and opens the door to the prolonged existence of biex-citons and possibly biexciton lasing with only minimalcooling required.

Until recently, there had been no experimental obser-vation of biexcitonic emission from such samples at roomtemperature, largely because the increased broadening ofthe transitions prevents spectral resolution of the excitonand biexciton peaks. In the work of Davis et al. they useda combination of one- and two-colour spectrally resolved



Figure 9 (online color at: www.lpr-journal.org) The spec-

trally resolved (a), integrated (b) and transient FWM sig-

nal for resonant degenerate excitation (wavelength 350 nm

(3.55 eV)) is shown for a 3 nm ZnO/Zn0.7Mg0.3O QW for

intensities varying from 1mJcm−2 per pulse to 10mJcm−2

per pulse. The increasing signal at negative delays provides

evidence of many-body effects, possibly including biexci-

tons, contributing to the signal. Reprinted with permission

from [57]. Copyright 2006, American Institute of Physics.

Figure 10 (online color at: www.lpr-journal.org) The spectrally re-

solved transient FWM signal following two-colour excitation (wave-

lengths 360 nm (3.45 eV) and 370 nm (3.35 eV)) is shown for a 3 nm

ZnO/Zn0.7Mg0.3O QW for high intensity excitation (10mJcm−2 perpulse). The presence of a signal at negative delays, in the absence of

any extended signal at positive delays, provides clear evidence for the

presence of a two-photon coherence, most likely from a biexciton. A

decoherence time of 100 fs is determined from the integrated signal.

Adapted from [57].

transient FWM experiments to identify biexciton transitionsin ZnO/ZnMgO quantum wells at room temperature [57].

The signature of biexcitons is their quadratic depen-dence on excitation intensity, and in transient FWM experi-ments the presence of a signal at negative delays. However,a signal can also be generated at negative delays as a resultof other many-body effects, such as local-field effects, andexcitation-induced dephasing [26]. In the case of biexci-tons, the signal at negative delays arises when two photonsestablish a two-photon coherence (e.g. a biexciton), fromwhich a photon of another pulse can diffract, giving a signalthat decays at a rate determined by the dephasing time ofthe two-photon coherence [67].

In the case of two-colour FWM experiments, the ‘nor-mal’ signal typically exists only where there is coherenttransfer of the polarisation between the states probed bythe two different colours. The signal from a two-photoncoherence, however, can still exist where the two pulses areof a different colour, since pulse-1 simply needs to diffractfrom the coherent biexciton. It is by the identification of thissignal, that the presence of biexcitons at room temperatureis confirmed in [57].

The results for the one and two-colour transient FWMexperiments are reproduced here in Figs 9 and 10, respec-tively. In these two beam experiments, the third order signalis detected in the phase matched direction given by 2k2−k1,



where k1 and k2 are the wave vectors of the two incidentbeams. The expected frequency of the signal in this ge-ometry is given by ω = 2ω2 − ω1, where ω1 and ω2 arethe frequencies of the beams with wavevector k1 and k2

respectively.

Fig. 9 shows the spectrally resolved FWM signal andthe corresponding spectrally integrated data with �ω1 =�ω2 = 3.55 eV for three different intensities as labeled,with I0 = 1mJcm−2 per pulse. For each of the excitationintensities in Fig. 9, the decay of the signal for positivedelays remains the same; however, for negative delays, thedecay time is very short for low excitation density, butincreases with excitation intensity. This signal at negativedelays, beyond the temporal overlap of the laser pulses, andwhich is only present at high excitation density indicatesthe presence of many body effects, although their precisenature cannot be determined.

The origin of this signal was clarified by the results fromthe two-colour experiments presented in Fig. 10 which showthe spectrally resolved and integrated FWM signal from thesame 3 nm wide QW with high excitation energy, and wave-lengths 360 nm and 370 nm (energies 3.45 eV and 3.35 eV)for pulse-1 and pulse-2 respectively. In these experiments,the geometry is such that the signal detected is expectedto have energy equal to �ω = 2�ω2 − �ω1 = 3.25 eV.This is indeed where the detected signal is observed, andit can be seen in Fig. 10 that there is only a rapid decay atpositive delays, but an extended signal at negative delays,well beyond the time when the two pulses are temporallycoincident. This is clear evidence that the extended signalarises from the generation of a two-photon coherence, mostlikely in the form of a biexciton. A closer inspection ofthe intensity dependence of this signal, and the signal atnegative delays in the one-colour experiments reveals aquadratic dependence of the signal intensity on the excita-tion intensity, thereby providing further evidence for thepresence of biexcitons at room temperature.

An exponential fit of the decay of this signal gives a de-coherence time for the two-photon coherence of 100±10 fs.Other sources of a two-photon coherence, such as an un-bound exciton pair, would be expected to dephase evenmore rapidly, and within the laser pulse, due to increasedinteraction with the environment. The observation of a de-coherence time sufficiently long to be measured is a goodindication that the two-photon coherence observed here isindeed a biexciton. These results give great hope for theuse of biexciton transitions in ZnO/ZnMgO quantum wellsin blue-ultraviolet laser applications, however, further workis needed to confirm and provide further detail of these ob-servations, including measurements of biexciton lifetimesat room temperature.

6. Conclusions and outlook

Progress towards devices based on ZnO QWs has beenrapid, with several observations of stimulated emission and

lasing. In this work we have primarily focussed on exci-tonic effects due to the large binding energy and predomi-nance up to room temperature. Indeed, it has been assumedthat observations of lasing in ZnO and its nanostructuresis due to exciton scattering. However, this has been se-riously questioned recently, and alternative mechanismsincluding recombination from an electron hole plasma pro-posed [55, 59]. What is clear however, is that the dynamicsand mechanisms occurring in these QWs is still not fullyunderstood, and requires further clarification in order to ac-curately determine the mechanisms of the various processesoccurring. It has been shown that under most circumstancesexcitons dominate the dynamics, particularly at low temper-ature, and because of the potentially large internal electricfield, the properties vary significantly with changing Mgconcentration in the barriers and quantum well width. Theexact role and nature of defects for different growth param-eters still requires a systematic study to assess their specificeffects on radiative and non-radiative recombination. TheQW potential profile has been shown to reduce the quantumconfined Stark effects, and again further systematic workto fully assess the role of QW profile is required. Furtherwork is necessary to fully assess the role played by biexci-tons in ZnO QWs, however the initial observations stronglysuggest the presence of biexcitons at room temperature andmay open the door to the possibility of biexcitonic lasing.

In general, the results from optical experiments on ZnOQWs provides great hope for their future use in blue/UVoptoelectronic devices, and significant progress is beingmade towards understanding the important processes thatoccur within them. However, much work is still required tofully understand the radiative and nonradiative dynamicsand the mechanisms responsible in ZnO based QWs.

Acknowledgements We thank many of our co-workers and collab-orators for their contribution to the research reviewed here. The

authors gratefully acknowledge the Australian Research Council

for financial support.

Jeffrey Davis received a BSc degreefrom Monash University, Australiain 2001, and a PhD degree from TheUniversity of Cambridge in 2005.Since then he has been employed inthe Centre for Atom Optics and Ultra-fast Spectroscopy at Swinburne Uni-versity of Technology in Melbourne,Australia. His current research inter-

ests include ultrafast dynamics and non-linear spec-troscopy in semiconductor nanostructures.



Chennupati Jagadish was born andeducated (BSc, MSc(Tech), MPhil,PhD) in India and worked in In-dia and Canada prior to moving toAustralia in 1990 where he estab-lished a major research program inthe field of semiconductor optoelec-tronics and nanotechnology. Profes-sor Jagadish is currently a Federation

Fellow, Professor and Head of Semiconductor Optoelec-tronics and Nanotechnology Group in the Department ofElectronic Materials Engineering, Research School ofPhysical Sciences and Engineering, the Australian Na-tional University (ANU). His research interests includecompound semiconductor optoelectronics and nanotech-nology including quantum dots, nanowires, lasers, pho-todetectors, photonic integrated circuits, photonic crys-tals, THz photonics.

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ultrafast spectroscopy of zno/znmgo quantum wells

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