3 FTFBSDI SUJDMF …downloads.hindawi.com/journals/jspec/2013/797232.pdf · 2019-07-31 · mpxfs...

Hindawi Publishing CorporationJournal of SpectroscopyVolume 2013 Article ID 797232 7 pageshttpdxdoiorg1011552013797232

Research ArticleStudy of Ultraviolet Emission Spectra in ZnO Thin Films

YM Lu1 X P Li1 P J Cao1 S C Su2 F Jia1 S Han1 W J Liu1 D L Zhu1 and X C Ma1

1 Shenzhen Key Laboratory of Special Functional Materials College of Materials Science and Engineering Shenzhen UniversityShenzhen 518060 China

2 Institute of Optoelectronic Materials and Technology South China Normal University Guangzhou 510631 China

Correspondence should be addressed to Y M Lu ymluszueducn

Received 7 June 2012 Accepted 21 August 2012

Academic Editor Vincenza Crupi

Copyright copy 2013 Y M Lu et alis is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Photoluminescence (PL) of ZnO thin lms prepared on c-Al2O3 substrates by pulsed laser deposition (PLD) are investigated Forall samples roomtemperature (RT) spectra show a strong band-edge ultraviolet (UV) emissionwith a pronounced low-energy bandtaile origin of this UV emission is analyzed by the temperature dependence of PL spectrae result shows that theUV emissionat RT contains different recombination processes At low temperature donor-bound exciton (D0X) emission plays a major role inPL spectra while the free exciton transition (FX) gradually dominates the spectrum with increasing temperatures It notes that atlow temperature an emission band (FA) appears in low energy side of D0X and FX and can survive up to RT Further conrmationshows that the origin of the band FA can be attributed to the transitions of conduction band electrons to acceptors (e A0) in whichthe acceptor binding energy is estimated to be approximately 121meV It is concluded that at room temperature UV emissionoriginates from the corporate contributions of the free exciton and free electrons-to-acceptor transitions

1 Introduction

ZnO with a direct band gap of 337 eV and a bindingenergy of exciton as high as 60meV at room temperature(RT) has been extensively studied as a candidate materialfor ultraviolet (UV) light emitting diodes (LEDs) and laserdiodes (LDs) [1ndash3] To realize the application of these devicesit is necessary to fabricate undoped ZnO thin lms whichavail to obtain a stable high-yield exciton emission at RTHowever it is well known that the fabrication of the highquality ZnO lms is rather difficult ecause it is commonto use Al2O3 as the substrate for the growth of ZnO thinlms 18 lattice mismatch between ZnO and Al2O3 resultsin the presence of various native defects in ZnO thin lms [4ndash7] ese defects oen control directly or indirectly dopingcompensation minority carrier lifetime and luminescenceefficiency Consequently it is very important to understandbehavior of these defects in ZnO-based materials Photolu-minescence (PL) emission spectroscopy is a useful method toexamine the quality of the grown ZnO thin lms which mayprovide important information on understanding the carrierrecombination processes and the role of defects in ZnO

In the reported PL spectra the origin of the roomtemperature UV emission was extensively studied Most ofthe authors suggested that the UV emission at RT originatesfrom free exciton recombination [8ndash12] However Ohashi etal reported that themost intense emission at RT for undopedcrystals was not free-exciton recombination but was relatedto an unspecied localized state [13] Zhao and Willanderfound that the room temperature UV emission contains twodifferent transitions in which one is related to the ZnO free-exciton and the other is related to the free-to-bound transi-tion [14] Up to now the room temperature UV emission isstill in debate In addition the controversies on PL propertiesalso present to someUV emission bands obtained at low tem-perature For example the 331 eV emission band observedin a great variety of ZnO materials has been interpreted con-troversially Many authors have assigned this UV emissionband to longitudinal-optical (LO) phonon replicas of theexcitons (FX-LO) [15 16] acceptor-bound excitons (A0X)[17] electron-hole recombination from donor acceptor pairs(DAP) [18] free electron-to-acceptor transition (e A0) [19]and so forth Noticeably the 331 eV emission band is alsofrequently observed in intentionally p-doped ZnO [20ndash25]

2 Journal of Spectroscopy

Most of theworks revealed that it is as a test criterion of p-typeconductivity formed by substitutional acceptors Recentlya remarkable work was reported in undoped ZnO epitaxiallayers grown on a-Al2O3 substrates in which the 331 eVemission band observed at low temperature originates froma (e A0) transition And they give clear evidence that thelocalized acceptor states causing the 331 eV luminescenceshould be associated with the stacking faults rather than thesubstitutional impurities [19]

On the other hand the measurements of the electricalproperties in undoped ZnO thin lms grown on Al2O3substrate show n-type conductivities with low electronmobilities of lt100 cm2Vminus1sminus1 which is quite smaller than300 cm2Vminus1sminus1 based on the reported value in ZnO lmsgrown on lattice-matched ScAlMgO4 substrates [2] Suchlow electron mobility implies the existence of the scatteringmechanisms due to unknown localized states In conclusionuntil now the impact of these defects on the optical andelectrical properties of ZnO is still a subject of much debatee clarifying of the PL origin not only can deepen and enrichthe research of the impurity and defect behaviors but alsois more advantageous for the development of ZnO-baseddevices

In this paper we report near band edge UV luminescencein the ZnO thin lms grown on c-Al2O3 substrates byPLD method e mechanism of the UV emission band isinvestigated by the temperature dependence of PL spectrae 331 eV emission band observed at low temperatureis assigned to the transitions of conduction band electronsto acceptors It is suggested that at room temperature UVemission is composed of two recombination processes Oneis the free-exciton emission (FX) another is the free electron-to-acceptor emission (e A0)

2 Experiment

ZnO thin lms were fabricated on c-Al2O3 substrates byusing a KrF excimer laser (Lamda Physics Compexpro 205120582120582 120582 24120582 nm 120591120591 120582 20 ns pulse duration) e laser beam wasfocused onto a rotating target at a 45∘ angle of incidence andthe energy density of the laser beam at the target surface wasmaintained at about 2 Jcm2 A 9999 purity ZnO ceramictarget with the thickness of 4 mm and the diameter of 60mmwas used as source materials

Al2O3 substrates degreased in acetone and methanolfor 10min respectively and then etched in a hot (160∘C)solution of H2SO4 H3PO4 = 3 1 for 15min followed by arinse in deionized water and dried by the high-pure nitrogengas before being loaded into the growth chamber Priorto growth the chemical cleaned substrates were thermallytreated at 800∘C in high vacuum atmosphere (sim 6 times 10minus4 Pa)for about 30min to remove the surface contaminantsSequentially ZnO was deposited on this treated substrate at700∘C for 120min In the growth process O2 partial pressurein the growth chamber was varied from 02 to 5 Pa erepetition frequency of the laser was 5Hz and the target-substrate distance was 85 cm

Aer growth the sample quality was conrmed by aRigaku Omax-RA X-ray diffractometer with Cu 119870119870120572120572 radi-ation (120582120582 120582 0120582112058242 nm) Photoluminescence spectra weremeasured at different temperatures e sample was attachedto the cold nger of an optical cryostat in conjunction with acryogenic refrigerator and cooled down tosim10Ke 325 nmline of a He-Cd laser with a power of 20mW was used as theexcitation source e photoluminescence from the samplewas dispersed through a monochromator (ZLX-FS Omni120582120582-3005) and detected by a photomultiplier tube (HamamatsuR928) followed by a photon counter (Zolix DCS200PC) ecarrier concentration and Hall mobility were measured byET-9007 Hall measurement system through the Van de Pauwmethod

3 Results and Discussion

Figure 1 shows the patterns of X-ray diffraction (XRD) forfour samples of ZnO thin lms grown on c-Al2O3 substratesat different O2 partial pressures in the growth chamberwhich are 5 3 1 05 Pa for the samples A B C and Drespectively It is noted that besides the Al2O3 (006) peakonly ZnO (002) and (004) diffraction peaks can be observedfor all samples is indicates that the grown ZnO thin lmshave the wurtzite structure with a high c-axis orientation Tofurther conrm the crystal quality of the ZnO lms XRD(103) 120601120601-scan measurements were performed e inset ofFigure 1 shows the measured result for the sample A It isclearly seen the six peaks separated by 60∘ with almost sameintensities indicating the formation of a sixfold symmetricsingle-crystal ZnO

Figure 2 shows photoluminescence (PL) spectra in UVregion range for the above samples at RT excited by a He-Cdlaser with 325 nm line As seen in Figure 2 one UV emissionband with a central wavelength of 3775 nm (3284 eV) canbe observed for the four samples It is obvious that thisUV emission band has a large linewidth (gt100meV) andone shoulder (arrow in the gure) can be clearly seen atlower-energy side of this peak e ZnO RT UV emissionis extensive reported as a characteristic excitonic emissionin the literatures [8ndash12] In addition Most of the worksindicated that low-energy band tail of UV peak at RT isassociated to LO-phonon replica of free exciton [16 21]

In order to study the origin of the room temperatureUV emission band the temperature dependence of the PLspectrum from the ZnO thin lms grown on a sapphiresubstrate has been measured Figure 3 shows the PL spectraat various temperatures for the sample A At 10K thespectrummainly composed of a strong emission ofD0Xbandand a weak band labeled FA located at 3355 and 3309 eVrespectively As temperature increases to 50K the FA-LOband appears in low energy side of the band FA In additionone peak (FX) can be clearly observed in high energy side ofD0X band at 50K and becomes stronger and stronger withtemperature increasing to 130K At 90K the FX at 3370 eVis comparable in intensity to the remaining D0X emission at3350 eV As the temperature increases from 130 to 260K theintensity of D0X band decreases rapidly and the FX emission

Journal of Spectroscopy 3

20 30 40 50 60 70 80

0 100 200 300

ZnO (004)

ZnO (002)

ZnO

XRD

A

B

C

D

ZnO (103)

Inte

nsi

ty (

au

)

Inte

nsi

ty (

au

)Samples

Sample A

AL2O3 (006)

F 1 XD spectra of ZnO thin lms grown on c-Al2O3 sub-strate e inset is 120601120601-scan curve of the (103) reection of sample A

360 380 400 420 440

3775 nm

D

C

B

A

Samples

Wavelength (nm)

RT PL

Inte

nsi

ty (

au

)

ZnOAL2O3

F 2 UV PL spectra of the grown ZnO thin lms at roomtemperatures

band becomes increasingly important in spite of its intensitydecreases with increasing of temperature It notes that at110K an emission band labeled FA-2LO appears in lowenergy side of the FA-LO band and the bands of FA FA-LOand FA-2LO can survive up to 260K Signicantly a carefulstudy of these bands should be necessary to consider becauseof their contribution to the room temperature UV emission

Figure 4 shows the tted spectra by multipeaks ofLorentzian line shape at the four typical temperaturesAccording to their energy values the FX and D0X peaks wereattributed to the emission of free exciton and the recombi-nation of excitons bound to neutral donors respectively Asreported in [18] D0X assigned to donor-bound excitons with10ndash15meV binding energy dominates in low temperature

31 315 32 325 33 335 34 345 35

Photon energy (eV)

10 K

FA-2LO

FA

30 K

50 K

FA-1LO

70 K

90 K

110 K

FX

130 K

150 K

170 K200 K

260 K

230 K

Inte

nsi

ty (

au

)

D0X

F 3 Temperature-dependent PL spectra of sample A espectra were normalized and shied vertically for clarity

spectra and the free-exciton recombination plays amajor rolein high temperature By comparing with the peak positions ofthe FA and FX two peaks have the energy spacing of about47meV which is less than the energy of ZnO LO phonon[17] erefore it can be believed that the FA peak should beassociated with the impurity or defects rather than the rstLO phonon replica of the free exciton recombination (FX)In our previous work PL spectrum at 80K of undoped ZnOthin lm grown by plasma-assisted molecular beam epitaxy(P-B) clearly shows the rst and the second LO phononreplica of FX (FX-LO and FX-2LO) [21] In this paperthe fabrication of the samples was used by PLD methodon a deviation from the stoichiometric ratio condition emeasure of the lm thicknesses shows the increase of thegrowth ratewith increasingO2 partial pressureis indicatesthat the growth of the lms is on a rich-Zn conditionresulting in the observation of FA emission related to thedefects For FA-LO and FA-2LO bands we note that theenergy differences between the FA-LO and FA-2LO bands tothe FA band are close to one and two LO phonon energiesof ZnO is implies that FA-LO and FA-2LO bands shouldcorrespond to the rst and the second LO phonon replica ofthe FA band

Figure 5(a) exhibits the temperature (119879119879) dependence ofintegral PL intensities (119868119868) for FX band One can clearly seethat at high temperature the emission intensity representsthe decrease with temperature increasing due to the thermalquenching e dependence of 119868119868 on 119879119879 for FX band can betted by the following formula

119868119868 1198681198681198680

1 + 119860119860 119860119860119860 10076491007649minus119864119864119886119886 100764910076491198961198961198611198611198791198791007665100766510076651007665 (1)

where 1198681198680 is the peak intensity at temperature 119879119879 119868 0K 119860119860 is aparameter 119864119864119886119886 is the activation energy in the thermal quench-ing process and 119896119896119861119861 is the Boltzmann constant From the plots(solid line) the thermal activation energy is estimated to be59meV for FX band is value agrees well with the freeexciton binding energy of ZnO (sim60meV) [1ndash4]


3 31 32 33 34 35

FA

Fitting

10 K

Inte

nsi

ty (

au

)

D0X-LO

D0X

Photon energy (eV)

(a)

FA

FX

FA-LO

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Fitting

50 K

Photon energy (eV)

(b)

Fitting

130 K

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Photon energy (eV)

(c)

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

3 31 32 33 34 35

Fitting

260 K

Photon energy (eV)

(d)

F 4 e ts of PL spectra by Lorentian line shape at 10K (a) 50 K(b) 130K(c) and 260K (d)

Figure 5(b) shows the intensity ratio 1198771198770 of FA to FA-LO (FAFA-LO) as a function of temperature It can be seenthat the 1198771198770 value presents a downtrend as the temperatureincreases In the same temperature range the phonon replicascould be much stronger than the no-phonon recombinationdue to self-absorption but the intensity ratio of the rst tothe second LO-phonon replica should increase linearly withtemperature [26 27] As shown in Figure 5(b) it is found that1198771198770 is decreased with the temperature from 90 to 260KusFA-LO band is not the second LO replica of FX but rather isthe rst LO replica of FA that is FA band cannot be the rstLO replica of FX

Although we excluded that the FA band is from the rstLO replica of the free exciton the luminescence band stillexist many other controversial luminescent mechanisms [1415 17 18 28] e comparison with literature [14 19 28]strongly suggests that the observed FA band originates fromfree-to-acceptor transition that is the recombination of anelectron from the conduction band with a hole bound toan acceptor state labeled (e A0) A further conrmationof this assignment will be demonstrated below In Figure 3PL spectra exhibit that the FA band at low energy side ofD0X and FX can be clearly observed in whole temperaturerange from 11 to 260K At lower temperature (lt130K)


4 5 6 7 8 9 10 11 1250

100

150

200

250

300

FX

Fitting

Inte

nsi

ty (

au

)

(a)

50 100 150 200 250 300

1

2

3

4

FAFA-1LO

Inte

nsi

ty r

atio

Temperature (K)

(b)

F 5 (a) e integrated intensity of the FX emission as a function of the temperature for sample A e solid lines are the curves ttedby formula (1) (b) e intensity ratio 1198681198680 of FA emission to FA-LO emission at various temperatures for sample A

0 50 100 150 200 250

325

33

335

34

FX

FA

Ph

oto

n e

ner

gy

(eV

)

Temperature (K)

D0X

F 6 Temperature dependences on the peak energies of FX(∎)1198631198630119883119883119883119883119883 and FA() emissions for ZnO thin lms grown on c-Al2O3substrate e solid lines are the curves tted by formula (2)

the intensity of the FA and FA-LO bands gradually becomesstrong with increase temperature At 130K four evidentemission peaks labeled FXD0X FA and FA-LOare located at3360 eV 3346 eV 3301 eV and 3227 eV respectively Withfurther increases in ZnO sample temperature the FA banddeceases in relative intensity and becomes more pronouncedat the high-energy tail is is a typical feature of the free-to-bound transition [19] In undoped ZnO typical donors

have binding energies in the range of 46ndash63meV [29] whileacceptor binding energy is larger (gt100meV) [30] Due tothe release of the electrons from donors with smaller bindingenergy the electron concentration in the conduction bandincreases with increasing temperature (lt130K) resulting inthe FA emission intensity increases For gt130K the observedthermal quenching of the FA band is related to hole releasefrom acceptors

In order to verify that the recombination of free-to-acceptor is responsible for the observed FA band the tem-perature dependent peak position is analyzed as shown inFigure 6 e open circles in Figure 6 are the data generatedfrom the FA band Because the temperature dependenceof the free-to-bound transition energy differs from thebandgap energy by 1198961198961198611198611198791198791198792 a curve-tting analysis of thetemperature dependence of the FA transition energy by usingthe following formula [31]

119864119864FA 119883119879119879119883 = 119864119864119892119892 119883119879119879119883 minus 119864119864119886119886 +12119896119896119861119861119879119879119879

119864119864119892119892 119883119879119879119883 = 119864119864119892119892 1198830119883 minus1205721205721198791198792

10076491007649119879119879 + 11987911987910076651007665119879

(2)

where 119864119864119892119892119883119879119879119883 and 119864119864FA119883119879119879119883 are the temperature-dependentband gap energy and FA band energy respectively 119864119864119886119886 is theacceptor binding energy 119896119896119861119861 is the Boltzmann constant 120572120572 and119879119879 are constants 1198641198641198921198921198830119883 is the band gap energy at 119879119879 = 0119879119879e blue curves in Figure 6 represent the results of the best taccording to (2) e energy 119864119864119886119886 is obtained to be 121meVe tted values of 1198641198641198921198921198830119883 120572120572 and 119879119879 are equal to 3440 eV86 times 10minus4 eVK and 800K respectively which are in goodagreement with those reported by Wang and Giles [31]isfact provides evidence that the observed FA transition has


the characteristic of the free-to-bound transition In Figure6 the FX and D0X peak energies from PL spectra are alsoplotted with solid square and solid circle symbols As canbe seen although the FX the D0X and the FA transitionenergies are reduced with increasing temperature the changeof their transition energies is different e transition ener-gies of the free and bound excitons show similar temperaturedependence as the band-gap energy (red lines) Hence theredshi of the FA emission peak is signicantly smaller thanthe FX and D0Xis further identies the FA band as a free-to-bound transition

ough we identify the FA band as a free-to-boundtransition a further investigation is required to clarify thisbound state is donor-like or acceptor-like Because this bandaround 3310 eV always appears in p-type ZnO not onlyin N-doped [20 21] but also in P-doped [22 23] and As-doped [24 25] samples it has been assigned to (e A0)transition related to these substitutional acceptors Howeverthis luminescence band has indeed frequently been observedin undoped ZnO samples especially in ZnO nanostructurematerials [14 15 17 18] In our previous works the 331 eVluminescence assigned (e A0) transition is observed in N-doped p-type ZnO thin lms [21] and ZnO nanowalls [28]If these reported results in undoped ZnO are consistent withp-type ZnO that is the observed luminescence band around331 eV is ralated to acceptors it is necessary to discuss theorigin of the acceptors in undoped n-type ZnO

Recently some research results conrm the existence ofcertain acceptor states in n-type ZnO with relatively highconcentrations [19 32] Janotti and Van De Walle giventhe acceptordonor concentration ratio of 041 in ZnO Gasamples and suggested the dominant acceptors are likely zincvacancies (119881119881Zn) andor neutral complexes related to 119881119881ZnIndeed the theoretical study shows that119881119881Zn is deep acceptorwith a low formation energy and it can act as compensatingcenters in n-type ZnO [5] Simultaneously on the experimen-tal119881119881Zn has been directly identied as the dominant acceptorin as-grown ZnO [7] On the other hand Schirra andSchneider reported that the acceptor states related to stackingfault in ZnOlms grown on a-Al2O3 substrates [19] inwhichthe 331-eV luminescence assigned to (e A0) transition isfound to be related to a high local density of acceptors inconjunction with crystallographic defects e existence ofthese acceptors with the estimated concentration of 1018 sim1020 cmminus3 which might exceed the donor concentrationwill play a vital role for the electrical properties [19 32]From this consideration the room temperature electricalproperties of ZnO lmsweremeasured by the four-probe vander Pauw method Based on these measurements the grownsamples show n-type characteristics with a resistivity of theorder of 10Ωsdotcm In addition obtaining such high resistivitycorresponds to a mobility of 10 cm2 (V s)minus1 and a carrierconcentration of 1015 cmminus3 It is well known that undopedZnO lms has a nature of the residual n-type conductivitydue to donor-like intrinsic defects such as oxygen vacancies(119881119881119874119874) and interstitial zinc atoms (Zn119894119894) In the majority of thepertinent works [2 3] the obtained carrier concentration

in undoped ZnO lms is the order of 1016ndash1018 cmminus3Obviously these values are much higher than that of oursample us high resistivity in our work is suggested to bedue to the compensating effect formed by large numbers ofacceptor states Here the acceptors likely arising from thenative defects will cause the electrical properties degradationNot only such these acceptor states will bring importantinuence on room temperature UV emission To identify theacceptor origin need further investigation in detail

4 Conclusion

In summary we have performed a detailed study aboutphotoluminescence properties of ZnO thin lms grown on c-Al2O3 substrates by pulsed laser depositione origin of UVemissions at RT is studied carefully by measuring differenttemperature spectra of ZnO thin lms e result showsat low temperature donor-bound exciton emission plays amajor role in PL spectra while the free-exciton transitiongradually dominates the spectrum with increasing temper-atures e room temperature UV emission contains twodifferent transitions One is related to the ZnO free-excitonand the other is related to the free-to-bound transitione focus is put on the conrmation of this the free-to-bound transition observed at 3309 eV at low temperatureIt is strongly suggested that the 3309 eV band originatesfrom free-electrons-to-acceptor recombinatione acceptorbinding energy is estimated to be about 121meV

Acknowledgments

is work was supported by the National Natural ScienceFoundation of China under Grant no 60976036 the Scienceand Technology Research Items of Shenzhen and the Itemsof Shenzhen Key Laboratory of Special Functional Materials(Grant nos T201101 and T201109)

References

[1] Z K Tang G K L Wong P Yu et al ldquoRoom-temperatureultraviolet laser emission from self-assembled ZnO microcrys-tallite thin lmsrdquo Applied Physics Letters vol 72 no 25 pp3270ndash3272 1998

[2] A Tsukazaki A Ohtomo T Onuma et al ldquoRepeated temper-ature modulation epitaxy for p-type doping and light-emittingdiode based on ZnOrdquo Nature Materials vol 4 no 1 pp 42ndash462005

[3] Y R Ryu J A Lubguban T S Lee et al ldquoExcitonic ultravioletlasing in ZnO-based light emitting devicesrdquo Applied PhysicsLetters vol 90 no 13 Article ID 131115 2007

[4] A F Kohan G Ceder D Morgan and C G Van De WalleldquoFirst-principles study of native point defects in ZnOrdquo PhysicalReview B vol 61 no 22 pp 15019ndash15027 2000

[5] A Janotti andCGVanDeWalle ldquoNative point defects inZnOrdquoPhysical Review B vol 76 no 16 Article ID 165202 2007

[6] N Y Garces N C Giles L E Halliburton et al ldquoProductionof nitrogen acceptors in ZnO by thermal annealingrdquo AppliedPhysics Letters vol 80 no 8 pp 1334ndash1336 2002


[7] F Tuomisto V Ranki K Saarinen andD C Look ldquoEvidence ofthe Zn vacancy acting as the dominant acceptor in n-type ZnOrdquoPhysical Review Letters vol 91 no 20 Article ID 205502 2003

[8] D M Bagnall Y F Chen M Y Shen Z Zhu T Goto andT Yao ldquoRoom temperature excitonic stimulated emission fromzinc oxide epilayers grown by plasma-assisted MBErdquo Journal ofCrystal Growth vol 184-185 pp 605ndash609 1998

[9] Y Chen S K Hong H J Ko M Nakajima T Yao andY Segawa ldquoPlasma-assisted molecular-beam epitaxy of ZnOepilayers on atomically at MgAl2O4(111) substratesrdquo AppliedPhysics Letters vol 76 no 2 pp 245ndash247 2000

[10] D A Lucca D W Hamby M J Klopfstein and G CantwellldquoChemomechanical polishing effects on the room temperaturephotoluminescence of bulk ZnO exciton-LO phonon interac-tionrdquo Physica Status Solidi B vol 229 pp 845ndash848 2002

[11] L Wang and N C Giles ldquoTemperature dependence of the free-exciton transition energy in zinc oxide by photoluminescenceexcitation spectroscopyrdquo Journal of Applied Physics vol 94 no2 pp 973ndash978 2003

[12] Z K Tang M Kawasaki A Ohtomo H Koinuma and YSegawa ldquoSelf-assembled ZnO nano-crystals and exciton lasingat room temperaturerdquo Journal of Crystal Growth vol 287 no 1pp 169ndash179 2006

[13] N Ohashi T Ishigaki N Okada T Sekiguchi I Sakaguchi andH Haneda ldquoEffect of hydrogen doping on ultraviolet emissionspectra of various types of ZnOrdquoApplied Physics Letters vol 80no 16 pp 2869ndash2871 2002

[14] Q X Zhao M Willander R E Morjan Q-H Hu and E E BCampbell ldquoOptical recombination of ZnO nanowires grown onsapphire and Si substratesrdquo Applied Physics Letters vol 83 no1 pp 165ndash167 2003

[15] X Wang H Iwaki M Murakami X Du Y Ishitani and AYoshikawa ldquoMolecular beam epitaxy growth of single-domainand high-quality ZnO layers on nitrided (0001) sapphiresurfacerdquo Japanese Journal of Applied Physics vol 42 no 2 ppL99ndashL101 2003

[16] J Jie G Wang Y Chen et al ldquoSynthesis and optical propertiesof well-aligned ZnO nanorod array on an undoped ZnO lmrdquoApplied Physics Letters vol 86 no 3 Article ID 031909 pp 1ndash32005

[17] F X Xiu Z Yang L J Mandalapu et al ldquoDonor and acceptorcompetitions in phosphorus-doped ZnOrdquo Applied Physics Let-ters vol 88 pp 152116ndash152118 2007

[18] B P Zhang N T Binh Y Segawa KWakatsuki and N UsamildquoOptical properties of ZnO rods formed by metalorganicchemical vapor depositionrdquo Applied Physics Letters vol 83 no8 pp 1635ndash1637 2003

[19] M Schirra R Schneider A Reiser et al ldquoStacking fault related331-eV luminescence at 130-meV acceptors in zinc oxiderdquoPhysical Review B vol 77 no 12 Article ID 125215 2008

[20] D C Look D C Reynolds C W Litton R L Jones D BEason and G Cantwell ldquoCharacterization of homoepitaxial p-type ZnO grown by molecular beam epitaxyrdquo Applied PhysicsLetters vol 81 no 10 pp 1830ndash1832 2002

[21] J W Sun Y M Lu Y C Liu et al ldquoNitrogen-related recom-bination mechanisms in p -type ZnO lms grown by plasma-assistedmolecular beam epitaxyrdquo Journal of Applied Physics vol102 no 4 Article ID 043522 2007

[22] J D Ye S L Gu F Li et al ldquoCorrelation between carrierrecombination and p-type doping in P monodoped and In-Pcodoped ZnO epilayersrdquo Applied Physics Letters vol 90 no 15Article ID 152108 2007

[23] T Nobis E M Kaidashev A Rahm M Lorenz J Lenznerand M Grundmann ldquoSpatially inhomogeneous impurity dis-tribution in ZnO micropillarsrdquo Nano Letters vol 4 no 5 pp797ndash800 2004

[24] Y R Ryu S Zhu D C Look J M Wrobel H M Jeong andHWWhite ldquoSynthesis of p-type ZnO lmsrdquo Journal of CrystalGrowth vol 216 no 1 pp 330ndash334 2000

[25] H S Kang G H Kim D L Kim H W Chang B D Ahn andS Y Lee ldquoInvestigation on the p -type formation mechanism ofarsenic doped p -type ZnO thin lmrdquo Applied Physics Lettersvol 89 no 18 Article ID 181103 2006

[26] CKlingshirn ldquoLuminescence of ZnOunder high one- and two-quantum excitationrdquo Physica Status Solidi B vol 71 no 2 pp547ndash556 1975

[27] B Segall and G D Mahan ldquoPhonon-assisted recombination offree excitons in compound semiconductorsrdquo Physical Reviewvol 171 no 3 pp 935ndash948 1968

[28] S C Su Y M Lu Z Z Zhang et al ldquoOxygen ux inuenceon the morphological structural and optical properties ofZn1minus119909119909Mg119909119909O thin lms grown by plasma-assisted molecularbeam epitaxyrdquo Applied Surface Science vol 254 no 15 pp4886ndash4890 2008

[29] B K Meyer H Alves D M Hofmann et al ldquoBound excitonand donor-acceptor pair recombinations in ZnOrdquo PhysicaStatus Solidi B vol 241 no 2 pp 231ndash260 2004

[30] D C Look ldquoElectrical and optical properties of p-type ZnOrdquoSemiconductor Science and Technology vol 20 no 4 ppS55ndashS61 2005

[31] L Wang and N C Giles ldquoDetermination of the ionizationenergy of nitrogen acceptors in zinc oxide using photolumines-cence spectroscopyrdquo Applied Physics Letters vol 84 no 16 pp3049ndash3051 2004

[32] D C Look K D Leedy D H Tomich and B BayraktarogluldquoMobility analysis of highly conducting thin lms applicationtoZnOrdquoApplied Physics Letters vol 96 no 6 Article ID0621022010

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


Most of theworks revealed that it is as a test criterion of p-typeconductivity formed by substitutional acceptors Recentlya remarkable work was reported in undoped ZnO epitaxiallayers grown on a-Al2O3 substrates in which the 331 eVemission band observed at low temperature originates froma (e A0) transition And they give clear evidence that thelocalized acceptor states causing the 331 eV luminescenceshould be associated with the stacking faults rather than thesubstitutional impurities [19]

On the other hand the measurements of the electricalproperties in undoped ZnO thin lms grown on Al2O3substrate show n-type conductivities with low electronmobilities of lt100 cm2Vminus1sminus1 which is quite smaller than300 cm2Vminus1sminus1 based on the reported value in ZnO lmsgrown on lattice-matched ScAlMgO4 substrates [2] Suchlow electron mobility implies the existence of the scatteringmechanisms due to unknown localized states In conclusionuntil now the impact of these defects on the optical andelectrical properties of ZnO is still a subject of much debatee clarifying of the PL origin not only can deepen and enrichthe research of the impurity and defect behaviors but alsois more advantageous for the development of ZnO-baseddevices

In this paper we report near band edge UV luminescencein the ZnO thin lms grown on c-Al2O3 substrates byPLD method e mechanism of the UV emission band isinvestigated by the temperature dependence of PL spectrae 331 eV emission band observed at low temperatureis assigned to the transitions of conduction band electronsto acceptors It is suggested that at room temperature UVemission is composed of two recombination processes Oneis the free-exciton emission (FX) another is the free electron-to-acceptor emission (e A0)

2 Experiment

ZnO thin lms were fabricated on c-Al2O3 substrates byusing a KrF excimer laser (Lamda Physics Compexpro 205120582120582 120582 24120582 nm 120591120591 120582 20 ns pulse duration) e laser beam wasfocused onto a rotating target at a 45∘ angle of incidence andthe energy density of the laser beam at the target surface wasmaintained at about 2 Jcm2 A 9999 purity ZnO ceramictarget with the thickness of 4 mm and the diameter of 60mmwas used as source materials

Al2O3 substrates degreased in acetone and methanolfor 10min respectively and then etched in a hot (160∘C)solution of H2SO4 H3PO4 = 3 1 for 15min followed by arinse in deionized water and dried by the high-pure nitrogengas before being loaded into the growth chamber Priorto growth the chemical cleaned substrates were thermallytreated at 800∘C in high vacuum atmosphere (sim 6 times 10minus4 Pa)for about 30min to remove the surface contaminantsSequentially ZnO was deposited on this treated substrate at700∘C for 120min In the growth process O2 partial pressurein the growth chamber was varied from 02 to 5 Pa erepetition frequency of the laser was 5Hz and the target-substrate distance was 85 cm

Aer growth the sample quality was conrmed by aRigaku Omax-RA X-ray diffractometer with Cu 119870119870120572120572 radi-ation (120582120582 120582 0120582112058242 nm) Photoluminescence spectra weremeasured at different temperatures e sample was attachedto the cold nger of an optical cryostat in conjunction with acryogenic refrigerator and cooled down tosim10Ke 325 nmline of a He-Cd laser with a power of 20mW was used as theexcitation source e photoluminescence from the samplewas dispersed through a monochromator (ZLX-FS Omni120582120582-3005) and detected by a photomultiplier tube (HamamatsuR928) followed by a photon counter (Zolix DCS200PC) ecarrier concentration and Hall mobility were measured byET-9007 Hall measurement system through the Van de Pauwmethod

3 Results and Discussion

Figure 1 shows the patterns of X-ray diffraction (XRD) forfour samples of ZnO thin lms grown on c-Al2O3 substratesat different O2 partial pressures in the growth chamberwhich are 5 3 1 05 Pa for the samples A B C and Drespectively It is noted that besides the Al2O3 (006) peakonly ZnO (002) and (004) diffraction peaks can be observedfor all samples is indicates that the grown ZnO thin lmshave the wurtzite structure with a high c-axis orientation Tofurther conrm the crystal quality of the ZnO lms XRD(103) 120601120601-scan measurements were performed e inset ofFigure 1 shows the measured result for the sample A It isclearly seen the six peaks separated by 60∘ with almost sameintensities indicating the formation of a sixfold symmetricsingle-crystal ZnO

Figure 2 shows photoluminescence (PL) spectra in UVregion range for the above samples at RT excited by a He-Cdlaser with 325 nm line As seen in Figure 2 one UV emissionband with a central wavelength of 3775 nm (3284 eV) canbe observed for the four samples It is obvious that thisUV emission band has a large linewidth (gt100meV) andone shoulder (arrow in the gure) can be clearly seen atlower-energy side of this peak e ZnO RT UV emissionis extensive reported as a characteristic excitonic emissionin the literatures [8ndash12] In addition Most of the worksindicated that low-energy band tail of UV peak at RT isassociated to LO-phonon replica of free exciton [16 21]

In order to study the origin of the room temperatureUV emission band the temperature dependence of the PLspectrum from the ZnO thin lms grown on a sapphiresubstrate has been measured Figure 3 shows the PL spectraat various temperatures for the sample A At 10K thespectrummainly composed of a strong emission ofD0Xbandand a weak band labeled FA located at 3355 and 3309 eVrespectively As temperature increases to 50K the FA-LOband appears in low energy side of the band FA In additionone peak (FX) can be clearly observed in high energy side ofD0X band at 50K and becomes stronger and stronger withtemperature increasing to 130K At 90K the FX at 3370 eVis comparable in intensity to the remaining D0X emission at3350 eV As the temperature increases from 130 to 260K theintensity of D0X band decreases rapidly and the FX emission


20 30 40 50 60 70 80

0 100 200 300

ZnO (004)

ZnO (002)

ZnO

XRD

A

B

C

D

ZnO (103)

Inte

nsi

ty (

au

)

Inte

nsi

ty (

au

)Samples

Sample A

AL2O3 (006)


360 380 400 420 440

3775 nm

D

C

B

A

Samples

Wavelength (nm)

RT PL

Inte

nsi

ty (

au

)

ZnOAL2O3




31 315 32 325 33 335 34 345 35

Photon energy (eV)

10 K

FA-2LO

FA

30 K

50 K

FA-1LO

70 K

90 K

110 K

FX

130 K

150 K

170 K200 K

260 K

230 K

Inte

nsi

ty (

au

)

D0X




119868119868 1198681198681198680

1 + 119860119860 119860119860119860 10076491007649minus119864119864119886119886 100764910076491198961198961198611198611198791198791007665100766510076651007665 (1)



3 31 32 33 34 35

FA

Fitting

10 K

Inte

nsi

ty (

au

)

D0X-LO

D0X

Photon energy (eV)

(a)

FA

FX

FA-LO

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Fitting

50 K

Photon energy (eV)

(b)

Fitting

130 K

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Photon energy (eV)

(c)

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

3 31 32 33 34 35

Fitting

260 K

Photon energy (eV)

(d)





4 5 6 7 8 9 10 11 1250

100

150

200

250

300

FX

Fitting

Inte

nsi

ty (

au

)

(a)

50 100 150 200 250 300

1

2

3

4

FAFA-1LO

Inte

nsi

ty r

atio

Temperature (K)

(b)


0 50 100 150 200 250

325

33

335

34

FX

FA

Ph

oto

n e

ner

gy

(eV

)

Temperature (K)

D0X





119864119864FA 119883119879119879119883 = 119864119864119892119892 119883119879119879119883 minus 119864119864119886119886 +12119896119896119861119861119879119879119879

119864119864119892119892 119883119879119879119883 = 119864119864119892119892 1198830119883 minus1205721205721198791198792

10076491007649119879119879 + 11987911987910076651007665119879

(2)







4 Conclusion


Acknowledgments


References











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


20 30 40 50 60 70 80

0 100 200 300

ZnO (004)

ZnO (002)

ZnO

XRD

A

B

C

D

ZnO (103)

Inte

nsi

ty (

au

)

Inte

nsi

ty (

au

)Samples

Sample A

AL2O3 (006)


360 380 400 420 440

3775 nm

D

C

B

A

Samples

Wavelength (nm)

RT PL

Inte

nsi

ty (

au

)

ZnOAL2O3




31 315 32 325 33 335 34 345 35

Photon energy (eV)

10 K

FA-2LO

FA

30 K

50 K

FA-1LO

70 K

90 K

110 K

FX

130 K

150 K

170 K200 K

260 K

230 K

Inte

nsi

ty (

au

)

D0X




119868119868 1198681198681198680

1 + 119860119860 119860119860119860 10076491007649minus119864119864119886119886 100764910076491198961198961198611198611198791198791007665100766510076651007665 (1)



3 31 32 33 34 35

FA

Fitting

10 K

Inte

nsi

ty (

au

)

D0X-LO

D0X

Photon energy (eV)

(a)

FA

FX

FA-LO

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Fitting

50 K

Photon energy (eV)

(b)

Fitting

130 K

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Photon energy (eV)

(c)

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

3 31 32 33 34 35

Fitting

260 K

Photon energy (eV)

(d)





4 5 6 7 8 9 10 11 1250

100

150

200

250

300

FX

Fitting

Inte

nsi

ty (

au

)

(a)

50 100 150 200 250 300

1

2

3

4

FAFA-1LO

Inte

nsi

ty r

atio

Temperature (K)

(b)


0 50 100 150 200 250

325

33

335

34

FX

FA

Ph

oto

n e

ner

gy

(eV

)

Temperature (K)

D0X





119864119864FA 119883119879119879119883 = 119864119864119892119892 119883119879119879119883 minus 119864119864119886119886 +12119896119896119861119861119879119879119879

119864119864119892119892 119883119879119879119883 = 119864119864119892119892 1198830119883 minus1205721205721198791198792

10076491007649119879119879 + 11987911987910076651007665119879

(2)







4 Conclusion


Acknowledgments


References











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


3 31 32 33 34 35

FA

Fitting

10 K

Inte

nsi

ty (

au

)

D0X-LO

D0X

Photon energy (eV)

(a)

FA

FX

FA-LO

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Fitting

50 K

Photon energy (eV)

(b)

Fitting

130 K

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

D0X

3 31 32 33 34 35

Photon energy (eV)

(c)

FA-LO

FA-2LO

FA

FX

Inte

nsi

ty (

au

)

3 31 32 33 34 35

Fitting

260 K

Photon energy (eV)

(d)





4 5 6 7 8 9 10 11 1250

100

150

200

250

300

FX

Fitting

Inte

nsi

ty (

au

)

(a)

50 100 150 200 250 300

1

2

3

4

FAFA-1LO

Inte

nsi

ty r

atio

Temperature (K)

(b)


0 50 100 150 200 250

325

33

335

34

FX

FA

Ph

oto

n e

ner

gy

(eV

)

Temperature (K)

D0X





119864119864FA 119883119879119879119883 = 119864119864119892119892 119883119879119879119883 minus 119864119864119886119886 +12119896119896119861119861119879119879119879

119864119864119892119892 119883119879119879119883 = 119864119864119892119892 1198830119883 minus1205721205721198791198792

10076491007649119879119879 + 11987911987910076651007665119879

(2)







4 Conclusion


Acknowledgments


References











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


4 5 6 7 8 9 10 11 1250

100

150

200

250

300

FX

Fitting

Inte

nsi

ty (

au

)

(a)

50 100 150 200 250 300

1

2

3

4

FAFA-1LO

Inte

nsi

ty r

atio

Temperature (K)

(b)


0 50 100 150 200 250

325

33

335

34

FX

FA

Ph

oto

n e

ner

gy

(eV

)

Temperature (K)

D0X





119864119864FA 119883119879119879119883 = 119864119864119892119892 119883119879119879119883 minus 119864119864119886119886 +12119896119896119861119861119879119879119879

119864119864119892119892 119883119879119879119883 = 119864119864119892119892 1198830119883 minus1205721205721198791198792

10076491007649119879119879 + 11987911987910076651007665119879

(2)







4 Conclusion


Acknowledgments


References











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of






4 Conclusion


Acknowledgments


References











































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

3 FTFBSDI SUJDMF …downloads.hindawi.com/journals/jspec/2013/797232.pdf · 2019-07-31 · mpxfs...

Documents

Transcript of 3 FTFBSDI SUJDMF …downloads.hindawi.com/journals/jspec/2013/797232.pdf · 2019-07-31 · mpxfs...