DTIC ADA457814: Photoluminescence Studies of Rare Earth (Er, Eu, Tm) in situ Doped GaN

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MaterialsScienceandEngineeringB105(2003)91–96
Photoluminescence studies of rare earth (Er, Eu, Tm) in situ doped GaN
U. Hömmericha,∗, Ei Ei Nyeina, D.S. Leeb, J. Heikenfeldb, A.J. Stecklb, J.M. Zavadac
aDepartmentofPhysics,HamptonUniversity,Hampton,VA23668,USA
bNanoelectronicsLaboratory,UniversityofCincinnati,Cincinnati,OH45221,USA
cUSArmyResearchOffice,Durham,NC27709,USA
Abstract
Theemissionpropertiesofrareearth(RE)-dopedGaNareofsignificantcurrentinterestforapplicationsinfullcolordisplays,whitelighting
technology,andopticalcommunications.Wearecurrentlyinvestigatingthephotoluminescence(PL)propertiesofRE(Er,Eu,Tm)-doped
GaNthin-filmspreparedbysolid-sourcemolecularbeamepitaxy.ThemostintensevisiblePLunderabove-gapexcitationisobservedfrom
GaN:Eu(red:622nm)followedbyGaN:Er(green:537nm,558nm),andthenGaN:Tm(blue:479nm).Inthispaper,wepresentspectroscopic
resultsontheGa-fluxdependenceoftheEr3+ PLpropertiesfromGaN:ErandwereportontheidentificationofdifferentEu3+ centersin
GaN:Euthroughhigh-resolutionPLexcitation(PLE)studies.Inaddition,weobservedanenhancementoftheblueTm3+PLfromAlGaN:Tm
comparedtoGaN:Tm.IntensebluePLfromTm3+ionswasalsoobtainedfromAlN:Tmunderbelow-gappumping.
©2003ElsevierB.V.Allrightsreserved.
PACS:78.40.Fy;78.55.-m;78.55.Et
Keywords:Rareearth;GaN;AlGaN;Luminescence
1. Introduction In this paper, we present spectroscopic results of the PL
properties of GaN:Er as a function of Ga-flux employed
Rareearth(RE)-dopedIII-nitrideshaverecentlyemerged during molecular beam epitaxy (MBE) growth. As will be
asanewclassofphosphormaterialsforthin-andthick-film discussedinmoredetail,theGa-fluxduringgrowthsignifi-
electroluminescencedevices[1–4].Comparedtopreviously cantlyimpactstheEr3+latticelocationandtheconcentration
studied RE-doped semiconductors with relatively small ofopticallyactiveEr3+ ions.High-resolutionPLexcitation
band-gap (<∼1.5eV) like e.g. GaAs or Si [5], RE doping (PLE) studies were carried out on GaN:Eu, which allowed
of wide band-gap semiconductors such as GaN, AlN, and the identification of 5D ↔ 7F transitions and associated
0 0
SiC has led to the observation of intense RE emission at Eu3+ centers. Finally, we report on initial PL studies of
room temperature [6,7]. In addition, studies of GaN:Er and Tm-doped AlxGa1−xN (0 ≤ x ≤ 1) and the enhancement
GaN:Eu have shown that RE3+ ions can be incorporated oftheblueTm3+ emissionwithincreasingAlcontent.
into GaN at concentrations as high as 1–2at.% without
significantemissionconcentrationquenching[8,9].Current
research efforts on RE-doped nitrides are focused on the 2. Experimentaldetails
optimization of existing materials and EL devices [2,3,7],
evaluation of different doping techniques and dopant/host Rare earth (Er, Eu, Tm)-doped GaN and Tm-doped
combinations [4,7,10–12], as well as fundamental spec- AlxGa1−xN films with x = 0.13, 0.24, 0.29, 0.44, and 1
troscopic studies aimed towards a better understanding of (AlN)weregrownbysolid-sourceMBEonp-typeSi(111)
the RE incorporation, excitation schemes, and emission substrates. Elemental Ga, Al, and RE sources were used in
efficiency[7,13–16]. conjunction with a radio frequency (rf)-plasma source sup-
plying atomic nitrogen. The RE concentration in the GaN
∗ Correspondingauthor.Tel.:[phone]; and AlGaN films varied between ∼0.5 and ∼1at.%. More
fax:[phone]. detailsonthesamplepreparationwerepublishedpreviously
E-mailaddress:[email protected](U.Hömmerich). [2,17].
0921-5107/$–seefrontmatter©2003ElsevierB.V.Allrightsreserved.
doi:10.1016/j.mseb.2003.08.022
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Photoluminescence studies of rare earth (Er, Eu, Tm) in situ doped GaN
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92 U.Hömmerichetal./MaterialsScienceandEngineeringB105(2003)91–96
PL spectra were measured using either the UV argon
a GaN:Er b
laser lines (336–363nm) or a visible argon laser line at
(2.2x10 -7 torr) (2.2x10 -7 torr)
496.5nm.PLexcitationspectrawererecordedusinganOp-
tical Parametric Oscillator system as the excitation source. (3.1x10 -7 torr) (3.1x10 -7 torr)
For low-temperature PL measurements the samples were u.) (3.8x10 -7 torr) (3.8x10 -7 torr)
a.
mounted on the cold-finger of a closed-cycle helium re- y (
frigerator. Infrared PL spectra were recorded using a 1m sit
n (4.5x10 -7 torr)
monochromator equipped with a liquid-nitrogen cooled Ge e
nt
detector.InvisiblePLstudiesathermo-electriccooledpho- L I
tomultiplier tube was employed for detection. The signal P (5.5x10 -7 torr) (4.5x10 -7 torr)
was processed using lock-in techniques or using a boxcar x10 (5.5x10 -7 torr)
averager.TheobtainedPLspectrawerenotcorrectedforthe (6.9x10 -7 torr) (6.9x10 -7 torr)
spectralresponseofthesetup.
500 600 700 800 900 1400 1500 1600 1700
Wavelength (nm)
3. Resultsanddiscussion
Fig. 1. Visible (a) and infrared (b) emission from Er-doped GaN as a
functionofGa-fluxunderbelow-gapexcitation(496.5nm).Forbothcases,
3.1. Er-dopedGaN the Er3+ emission intensity reached its maximum under slightly N-rich
fluxnearthestoichiometricgrowthcondition(Ga-flux:∼4.5×10−7Torr).
It was previously reported that the visible and infrared
emission properties of Er3+ ions in GaN films are strongly above-gap excitation (see Fig. 3). All investigated samples
dependent on the Ga-flux during growth [18]. Under above exhibited the onset of 1.54(cid:1)m PL saturation at relatively
band-gapexcitationtheEr3+ PLemissionreacheditsmax- lowpumpintensities(<2W/cm2),consistentwithahighex-
imumunderslightlyN-richgrowthconditions.Onthecon- citationefficiencyforabove-gappumping[16].Atthesame
trary, the band-edge emission from the GaN host reached time, it can be noticed that the Er3+ PL saturation level
its maximum under Ga-rich growth. This observation lead greatlyvariedforthedifferentsamples.Ithasbeendiscussed
to the conclusion that the Ga-flux strongly influences the in the literature before [16,22], that the Er3+ PL saturation
carrier-mediated excitation efficiency of Er3+ ions [18]. level is determined by the product of concentration of op-
In the following, more detailed spectroscopic study on the tically active Er ions (N ) and radiative decay rate (w ).
Er rad
Ga-flux dependence of the Er3+ PL properties are pre- PL lifetime studies revealed that w is approximately in-
rad
sented.Er-dopedGaNsamplesgrownunderGa-fluxeswith dependentofGa-flux.Therefore,thelargedifferenceinthe
beam equivalent pressures ranging from 2.2 × 10−7Torr PL saturation is attributed to a large difference in the con-
(“N-rich”)to6.9×10−7Torr(“Ga-rich”)wereinvestigated centration of optically active Er3+ ions. As can be derived
[18].Thestoichiometricgrowthconditionasdeterminedby from Fig. 3, the PL saturation level and hence N is ∼17
Er
filmthicknesssaturationwas∼4to5×10−7Torr. timeslargerforthesamplegrownunderslightlyN-richcon-
ThevisibleandIRPLspectraunderbelowband-gapexci-
tation(496.5nm)areshowninFig.1.Thisexcitationwave-
length overlaps an intra-4f Er3+ absorption line (4I15/2 →
4F9/2)[19,20].Similartoabove-gapexcitation[18],theEr3+ GaN: Er
PL strongly varied with Ga-flux under resonant excitation
u.) N-rich
and reached a maximum under slightly N-rich growth con- a.
dition.Sincebothexcitationschemes,carried-mediatedand y (
resonant intra-4f, exhibited the same Er3+ PL behavior it nsit
e
can be concluded that the carrier-mediated Er3+ excitation L Int near stoichiometry
isonlyweaklyaffectedbytheGa-fluxduringgrowth. P (slightly N-rich)
Fig. 2 shows the high-resolution 1.54(cid:1)m Er3+ PL spec- zed
tra of GaN:Er samples grown under different Ga-fluxes. A mali
significantdecreaseinthe1.54(cid:1)mPLlinewidthcanbeno- or
N
ticedaswellastheappearanceofnewspectralfeatureswith Ga-rich
increasing Ga-flux. The spectral narrowing of the emission
linesisconsistentwiththehighercrystallinequalityofGaN
1500 1550 1600 1650
grownunderGa-richconditions[18,21].Theobservationof
newspectrallinesindicatesthatEr3+ ionsareincorporated Wavelength (nm)
intodifferentlatticelocationsdependingontheGa-flux.
Fig.2.High-resolution,infraredemissionspectraat15KfromEr-doped
More information on the optical activation of Er3+ in
GaN grown under different Ga-fluxes. The emission was excited using
GaNcanbedrawnfrompowerdependentPLstudiesunder below-gapexcitation(496.5nm).
U.Hömmerichetal./MaterialsScienceandEngineeringB105(2003)91–96 93
1000 GaN:Er GaN: Eu
near stoichiometry 5D->7F
sity (a.u.) 680000 PL: 4I13/2 --> 4I15/2 (slightly N-rich) y (a.u.) 6202nm256D604->n7mF3 15K
L Inten 400 N-rich ntensit 5D600-0>n7Fm1
P L I
200 Ga-rich P 5D0->7F0 57D105->n7mF4 58D403->.47nFm5
0
0 2 4 6 8 10
Pump Intensity (W/cm2 ) 600 700 800
Wavelength (nm)
Fig. 3. Pump intensity dependence of the 1.54(cid:1)m PL from GaN:Er
samples grown with different Ga-flux during growth. The emission was Fig.4.EmissionspectrumofGaN:Euat15Kunderabove-gapexcitation.
excited using above-gap excitation (336–363nm). The solid lines are The main emission features arising from the 5D0–7FJ (J =0,1,2,3,4,5)
guidesfortheeye. transitions are indicated in the graph. More details on the identification
ofthe5D0–7F0 transitionsarepresentedinFig.5.
ditionscomparedtothatgrownunderGa-richconditions.In
contrast,previoussecondaryionmassspectroscopy(SIMS)
At room temperature, four PLE peaks were observed at
studieshaveshownthatthetotalErconcentrationinthein-
571, 585.2, 588.8, and 591.7nm. With decreasing temper-
vestigated GaN:Er films varied only by roughly a factor of
ature the peak at 591.7nm reduced in intensity and was
two[18].Therefore,thePLsaturationresultfromFig.3re-
notobservedat15K.Therefore,thislinedoesnotoriginate
veal that in GaN:Er grown under Ga-rich conditions only a
smallfraction(<10%)ofthetotalErconcentrationisopti- fromthe7F0 groundstate,butisduetothethermalpopula-
tionofhigherlying7F /7F manifolds.Thelinewidthofall
callyactiveunderabove-gappumping.Similarobservations 1 2
PLEpeaksobservedatroom-temperaturenarrowedslightly
were reported for Er-doped crystalline and amorphous Si
withdecreasingtemperatureandsomefinestructurewasre-
[23]. More comparative PL saturation studies employing a
well-characterizedEr3+sample(e.g.Er-dopedSiO )arere- solved. At low temperature (15K) five PLE lines at 571,
2
585.8,587.9,588.9,and589.4nmwereobservedandtenta-
quired to determine the absolute concentration of optically
activeEr3+ ionsinGaN:Er. tively assigned to 7F0 → 5D0 transitions. The 7F0 → 5D0
transition at ∼571nm is at an unusually low wavelength,
butsimilarcaseshavebeenreported[29,30].Therefore,we
3.2. Eu-dopedGaN
conclude that at least five different Eu3+ centers exist in
GaN:Eu.
Eu-dopedGaNisofgreatinterestfordisplayapplications
because of its bright red emission peaking around 622nm
[14,24–28].Eu3+ isalsoknownasa“spectroscopicprobe”
forthelocalenvironmentofEu3+ionsinsolids,becausethe GaN: Eu
mainemittinglevelisthenon-degenerate5D state[20].For 300K
0
iedxeanmtipfylet,hoebnsuermvbinegrothfeem5Dit0tin↔gE7uF30+trcaennstietirosninaallogwivsenonheostto. y (a.u.) 200K
Fig. 4 gives an overview of the low temperature emission sit 100K
n
ofGaN:Eu.Themainemissionlinescanbeassignedtothe nte
transitions:5D →7F (∼843nm),5D →7F (∼715nm), E I 75K
0 5 0 4 L
5D0 →7F3(∼664nm),5D0 →7F2(∼622nm),and5D0 → P 588.9 nm
7F (∼600nm). The weak emission features at ∼585 and 587.9 nm
1 589.4 nm
∼590nm are most likely due to 5D → 7F transitions 571 nm 585.8 nm
0 0 15K
suggestingmultipleEu3+ centersinGaN[14,27,28].
In order to identify the 5D ↔ 7F transitions and as- 565 570 575 580 585 590 595
0 0
sociated Eu3+ centers more clearly, we have performed Wavelength (nm)
high-resolutionPLexcitationstudiesintherangefrom560
Fig. 5. High-resolution PLE spectrum of Eu-doped GaN at 15K. The
to595nm.ThespectralresolutioninthePLEmeasurements
emission was monitored at ∼623nm. Absorption lines located at 571,
wsoausrcleim(∼ite0d.2bcym−th1e)(lFinigew.5id).th of the employed excitation 5tr8a5n.s5it,io5n8s7..9,588.9,and589.4nmaretentativelyassignedtothe7F0–5D0
94 U.Hömmerichetal./MaterialsScienceandEngineeringB105(2003)91–96
624.6 nm GaN: Eu
GaN: Tm
15K
nsity (a.u.) 558889..94nnmm y (a.u.) 3H48 0--3> n m3H6
PL Inte 558857..89nnmm L Intensit 1G4-->3H6 150K
621.6 nm P ~479 nm
571nm 300K
620 640 660 680 700
Wavelength (nm) 500 600 700 800 900
Wavelength (nm)
Fig. 6. Site-selective PL spectra at 15K excited at 571, 585.5, 587.9,
588.9,and589.4nm.TheEu3+ centerexcitedat571nmisdistinctfrom
Fig. 7. PL spectra of Tm-doped GaN under above-gap excitation at 15,
theotherEu3+ centersanddominatesunderabove-gapexcitation.
150,and300K.Theshoulderat479nmindicatesblueemissionfromthe
1G4→3H6 intra-4ftransitionofTm3+.
More support for the identification of the different Eu3+
centers was obtained from site-selective PL measurements. and GaN:Eu, the blue emission from GaN:Tm is orders
PumpingresonantlyintoeachofthePLElinerevealeddiffer- of magnitude weaker under above-gap pumping, which in-
entEu3+ emissionspectraandlifetimesasshowninFig.6. dicates that Tm3+ ions are not efficiently excited through
The spectral differences are especially pronounced for the carrierrecombinationprocesses.
Eu3+ center excited at 571nm. This Eu3+ center exhib- An energy transfer model based on a RE-related trap
ited a relatively narrow 5D0 → 7F2 PL at ∼622nm with level was proposed by Takahei et al. for InP:Yb and later
a nearly exponential lifetime of ∼235(cid:1)s. The 5D0 → 7F2 extendedtootherRE-dopedsemiconductors[34,35].Inthis
PLspectraoftheotherEu3+centersarebroaderandshifted model,REdopingofasemiconductorleadstotheformation
tolongerwavelength(∼624nm).Inaddition,thePLdecay ofaRErelatedlevelintheband-gapofthehost.Thislevel
transients of these Eu3+ centers are non-exponential with can trap photo-excited free carriers, which subsequently
averagelifetimesoflessthan200(cid:1)s.Thesmallerlinewidth recombine and transfer their energy to intra-4f RE transi-
of the 571nm Eu3+ center indicates less inhomogeneous tions. Our recent PL excitation measurements of GaN:Eu
broadening compared to the other Eu3+ centers. It can be provided experimental evidence for an Eu trap-level at
speculated that the 571nm center is associated with Eu3+ ∼0.3eV below the conduction band of GaN [27]. A simi-
ions in substitutional Ga3+ lattice positions, whereas the lar level was reported by Li et al. using Fourier transform
other Eu3+ centers are due to Eu3+ ions incorporated into infrared (FTIR) measurements [9] and by Vantomme et al.
interstitial sites with close vicinity to other defects and im- usingdeepleveltransientspectroscopy(DLTS)[36].AnEr
purities.Interestingly,thePLspectrumexcitedat571nmis related trap-level at ∼0.3eV was also reported for GaN:Er
verysimilartoPLspectraobtainedunderabove-gappump- [37]. The recombination energy of carriers trapped at the
ing, which shows that this Eu3+ center dominates under Eu/ErrelatedtrapsinGaNisthenestimatedtobe∼3.1eV,
carrier-mediatedexcitation.Moresite-selectivePLandPLE which energetically matches intra-4f transitions of Eu3+
studiesofthedifferentEu3+centersinGaNareinprogress. andEr3+,respectively[19].Therefore,thecarrier-mediated
energytransfertoEr3+ andEu3+ seemstooccurwithhigh
3.3. Tm-dopedAlxGa1−xN(0≤x≤1) efficiency. On the contrary, the energy level structure of
Tm3+ does not exhibit an energy level at ∼3.1eV. Con-
PLspectraofTm-dopedGaNunderabove-gapexcitation sequently, a carrier-mediated energy transfer process to
at 15, 150, and 300K are shown in Fig. 7. The emission Tm3+ ions is less likely to occur in GaN:Tm. A similar
is dominated by a broad band centered at ∼530nm, which explanation was recently proposed for the poor above-gap
extends from roughly 450 to 750nm and resembles the excitationefficiencyofTb3+ ionsinGaN:Tb[9].
well-known yellow band emission from GaN [31,32]. An In an effort to optimize the carrier-mediated excitation
infrared emission line at ∼803nm can be assigned to the ofTm3+ ionsinIII-nitrides,wearecurrentlyexploringthe
intra-4f transition 3H → 3H of Tm3+ ions [33]. A weak energy transfer process to Tm3+ as a function of band-gap
4 6
shoulder at ∼479nm indicates the blue emission line from energy. Fig. 8 shows preliminary emission spectra for
the 1G4 → 3H6 transition of Tm3+. The PL intensity of Tm-doped AlxGa1−xN with x = 0 (GaN), 0.13,0.24, 0.29,
479nm line increases slightly when cooling the sample 0.44, and 1 (AlN). The corresponding band-gap energies
down to 15K. Compared to the visible PL from GaN:Er are 3.4eV (GaN), 3.64, 3.88, 3.99, 4.89, and 6.2eV (AlN)
U.Hömmerichetal./MaterialsScienceandEngineeringB105(2003)91–96 95
is still in progress and will be discussed in a forthcoming
paper.
AlN:Tm
4. Conclusions
Spectroscopic results on the PL properties of GaN:Er,
u.) x=0.44 GaN:Eu, GaN:Tm and AlGaN:Tm were presented.
a. High-resolution PL and pump-power dependent PL studies
y ( x=0.29 of GaN:Er samples revealed that the Er3+ incorporation
sit andtheconcentrationofopticallyactiveErionsisstrongly
n
nte dependent on the Ga-flux during MBE growth. Based on
L I x=0.24 PL saturation experiments, we concluded that only a small
P fraction (<10%) of the total Er ions in GaN are optically
active for samples grown under Ga-rich conditions. More
x=0.13 PLsaturationexperimentsarestillinprogresstodetermine
the absolute concentration of optically active Er3+ ions in
GaN:Tm GaN:Er films. The low optical activation of Er3+ ions in
GaN is similar to recent observations made for Er-doped
Si/amorphous Si [23]. This issue needs to be further ad-
400 500 600 700 800 900
dressed in future investigations to explore the full potential
Wavelength (nm) of RE-doped GaN for device applications. High-resolution
PL excitation studies were performed on GaN:Eu and al-
Fig. 8. PL spectra of Tm-doped AlxGa1−xN with x = 0 (GaN), 0.13, lowed the identification of at least five Eu3+ centers. An
0.24,0.29,0.44,and1(AlN)under250nmexcitation.
absorptionlineatanunusuallylowwavelengthof∼571nm
was also tentatively assigned to the 5D ↔ 7F transition.
respectively. The emission was excited at 250nm, which The Eu3+ center selectively excited th0rough t0his 571nm
corresponds to above-gap pumping for all investigated absorption line is distinct from the other Eu3+ centers and
AlGaN samples, but results in below-gap excitation for
seems to dominate the above-gap pumped PL emission
AlN:Tm. As discussed before, hardly any blue emission
spectrum. GaN:Tm exhibited only a weak blue emission
was observed from GaN:Tm (lowest trace in Fig. 8). With fromthe1G →3H transitionofTm3+ ions.Asignificant
ifnrocmreaTsimng3+Ailscoobnsteernvtedan. Iennhadandciteiomne,nat onfewtheblu4e78enmmissliionne enhancemen4t of the6blue Tm3+ emission was obtained un-
lineappearsat∼466nm.Basedonacomparisontoexisting der above-gap pumping of Tm-doped AlxGa1−xN samples.
Besidesemissionfromthe1G →3H transition,asecond
literature [38], the 466nm line is tentatively attributed to 4 6
emissionfromthe1D2 →3F4 transitionofTm3+ ions.The bwlhuiechemwiasssiaosnsilginneedaptopethareed1Daro→und3F466trannmsitfioornxof>Tm0.31+3.,
overall blue emission from both lines at 466 and 479nm Strongblueemissionfromthe21D and4 1G levelsofTm3+
reached a maximum for an Al content of x = 0.29, with 2 4
was also observed from Tm-doped AlN under below-gap
the 466nm line having the highest PL intensity. At higher
excitation. The large sensitivity of the blue emission from
Alcontentthetotalblueemissionstartedtodecreaseandat Tm3+ ontheband-gapofAlGaNsuggeststhepossibilityto
x=0.44onlyweakblueemissionat466nmwasobserved.
optimizetheREexcitationandemissionpropertiesthrough
Onthecontrary,below-gappumpingat250nmofAlN:Tm
carefulband-gapengineeringofthehost.
resulted into strong blue emission with intense lines at
∼466 and 479nm. For AlN:Tm, the blue emission is most
likelyexcitedthroughsomebroaddefectlevelinthehost. Acknowledgements
Fig. 8 clearly demonstrates the sensitivity of the blue
Tm3+emissionontheAlcontentandhencetheband-gapof TheauthorsfromH.U.acknowledgefinancialsupportby
AlGaN. It cannot be excluded, however, that also chemical AROthroughgrantDAAD19-02-1-0316.TheworkatU.C.
effectsrelatedtothepresenceofAlchangetheTm3+incor- wassupportedbyAROgrantDAAD19-99-1-0348.Helpful
porationandexcitationmechanisms,similartoobservations discussions with F. Pelle and F. Auzel are also acknowl-
made for Er-doped AlGaAs [39,40]. The initial spectro- edged.
scopic data on AlxGa1−xN:Tm indicate that the excitation
efficiencyofthe1G stateofTm3+increasesuptox∼0.24.
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