DSpace at VNU: Comparative optical study of Eu3+ ions doping in InGaN GaN quantum dots and GaN layer grown by molecular beam epitaxy

Analysis of the5D0!7F2transition as a function of the excitation wavelength shows that Eu3+ions in InGaN:Eu QDs are located inside InGaN QDs and also in the GaN barrier layer.. For InGaN

Comparative optical study of Eu 3+ ions doping in InGaN/GaN quantum dots and GaN layer grown by

molecular beam epitaxy Thomas Andreev a,*, Nguyen Quang Liem a,b,c, Yuji Hori a,d, Mitsuhiro Tanaka d,

Osamu Oda d, Bruno Daudin a, Daniel Le Si Dang e

a CEA/CNRS/UJF Research Group Nanophysique et Semiconducteurs, DRFMC/SP2M/PSC CEA-Grenoble,

17 rue des Martyrs, 38054-Grenoble Cedex 9, France

b Institute of Materials Science (IMS), Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam

c College of Technology, Hanoi National University, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam

d NGK Insulators, LTD 2-24 Sudacho, Mizuhoku, Nagoya, Japan

e CEA/CNRS/UJF Research Group Nanophysique et Semiconducteurs, Laboratoire Spectrome´trie Physique (CNRS UMR5588),

Universite´ J Fourier, BP 87, 38402 Saint Martin dÕHe`res, France

Available online 2 November 2005

Abstract

We report on a comparative optical study of InGaN:Eu quantum dots (QDs) and GaN:Eu layer grown by molecular beam epitaxy (MBE) Analysis of the5D0!7F2transition as a function of the excitation wavelength shows that Eu3+ions in InGaN:Eu QDs are located inside InGaN QDs and also in the GaN barrier layer The existence of Eu3+ions in the GaN barrier layer is explained by

Eu segregation/diffusion during growth For Eu3+ions located inside InGaN QDs the photoluminescence (PL) shows only a slight decrease with temperature from 5 K to 300 K In contrast, the PL from Eu3+ions in the GaN barrier layer or in GaN thick layer shows

a much more pronounced thermal quenching

 2005 Elsevier B.V All rights reserved

1 Introduction

The combination of rare earth (RE) luminescence with

the wide band gap of (In)GaN is a promising solution

for full color devices, since efficient energy transfers can

occur from carriers of the semiconductor host to the RE

excited states Successful RE doping of GaN films was

(MBE) growth

To reduce the nonradiative recombination channels

derived from the high dislocation densities in GaN layers

we propose to combine the confinement properties of

quantum dots (QDs) with the RE luminescence to achieve

high luminescence even at room temperature We have already demonstrated Eu- (red), and Tm- (blue) doped GaN QDs embedded in an AlN matrix, which showed stable luminescence in the temperature range of 5–300 K Furthermore a strong enhancement of the radiative quan-tum efficiency, by about one to two orders of magnitude

as compared to rare earth doped films, was observed at

However the injection of carriers into AlN is hindered

by the difficulties in p-type and n-type doping of AlN, so that one solution could be to use RE doped InGaN QDs grown on GaN for current injection devices, since p- and n-doping of GaN are well controlled by MBE Eu was found to act as a surfactant in MBE growth of GaN layers, leading to drastic changes in adatom kinetics, as we

AlN, we found that Tm is located inside GaN QDs, but 0925-3467/$ - see front matter  2005 Elsevier B.V All rights reserved.

doi:10.1016/j.optmat.2005.09.061

*

Corresponding author Tel.: +33 438 78 5416; fax: +33 438 78 5797.

E-mail address: andreev@drfmc.ceng.cea.fr (T Andreev).

www.elsevier.com/locate/optmat

also at the GaN/AlN interface[5] Such a situation could

happen also in the case of Eu doping in InGaN/GaN

QDs due to the complex interaction of RE atoms with

the formation of QDs

Hence the aim of this article is to address the locations,

in InGaN/GaN QDs and to compare to those in GaN

layer

2 Experimental

The growth was performed on 1 lm thick AlN

pseudo-substrates deposited by metal organic chemical vapor

degreasing procedure and acid cleaning with HF, the

pseudo-substrate was fixed with an indium bonding on a

molybdenum sample holder, and introduced in a MBE

chamber equipped with Al, Ga, In and Eu effusion cells

and a radio-frequency plasma cell to produce monatomic

nitrogen The growth conditions were controlled with

reflection high-energy electron diffraction, which allows

in situ and real time monitoring the 2D–3D transition

corresponding to the formation of self-organized QDs

Stranski-Krastanow (SK) growth mode, i.e the QDs

appear after the deposition of a wetting layer of typically

mode of InGaN QDs on GaN, a minimal In concentration

of around 20% is needed Before the growth of InGaN

QDs, a 150 nm thick GaN buffer layer was deposited at

was opened to dope the material Then, the QDs were capped with about 8 nm of non-intentionally doped (n.i.d.) GaN This process was repeated 165 times to achieve stacks of QD planes sandwiched in GaN barriers From chosen growth conditions we estimate an Eu content

of 1%

For reference measurements, a GaN:Eu layer was grown

on a n.i.d GaN buffer The Eu concentration of the GaN film reference sample was measured by RBS to be about 0.2% Morphology of the grown sample has been studied

at room temperature in air with an AFM Dimension

3100 microscope

PL spectra were measured using a tunable excitation source consisting of a 500 W high-pressure Xe lamp equipped with a Jobin-Yvon high-resolution double-grat-ing monochromator (Gemini 180) The excitation power

analyzed by another Jobin-Yvon grating monochromator (Triax 550) and detected by a CCD camera operating at liquid nitrogen temperature The sample was mounted on the cold finger of a micro-cryostat which enabled us to record PL spectra at various temperatures from 5 K to

380 K

3 Results and discussion

Fig 1 shows AFM images of InGaN:Eu QDs in

The sur-face morphology is characterised by the typical spiral

Fig 1 AFM images of InGaN:Eu QDs Areas of images: (a) 4000 · 4000 nm 2 , (b) 2000 · 2000 nm 2 , (c) 1000 · 1000 nm 2 and (d) 500 · 500 nm 2

hillocks of GaN (Fig 1a and b) InGaN:Eu QDs are found

aligned on the atomic terraces around the hillocks as visible

inFig 1b–c The higher resolution image ofFig 1d shows

the InGaN:Eu QDs, which present diameters between

15 nm and 40 nm and rather small height, between

0.4 nm and 1 nm The quantum dot density was found to

Fig 2a shows room temperature PL spectra of

InGa-N:Eu QDs and a GaInGa-N:Eu layer excited at 360 nm Stark

spectra are located at 620, 622 and 633.5 nm According

width is slightly broader in the case of InGaN QDs as

resulting from internal electric field and strain inside InGaN QDs or poorer crystalline quality A remarkable difference between the two spectra is the larger intensity

of the 622 nm line in doped QDs, whereas the 620 and 633.5 nm lines display the same intensity ratios This

might occupy different sites in the two structures (InGaN and GaN), which can explain the modification of the

exhibit a tendency to segregate on the surface during the growth of GaN and AlN, so that it is quite possible that

a small amount of Eu content can be found also in the

in the PL of InGaN:Eu QDs, both contributions from

GaN barrier layer are superimposed Notice that for Tm doping of GaN QDs grown on AlN the situation is even

To assess further the origin of emission lines, PLE measurement was carried out at 5 K as shown in

Fig 2b The GaN:Eu layer exhibits rather similar PLE spectra for emissions at 620, 622, and 633.5 nm (not shown) The PLE spectra are characterised by band-to-band absorption at the GaN band-to-band gap and a weak absorption tail below band gap down to 400 nm, which could be due to defect states However, no well-resolved absorption due to a trap level at 400 nm as previously

which illustrates the complexity of carrier-mediated energy transfer processes in semiconductors doped with

RE ions For InGaN:Eu QDs the PLE spectra measured for the 620 and 633.5 nm emission lines (not shown) are similar, but they differ from that measured for the emis-sion at 622 nm In the later case, the peak absorption is

at lower energy and the low energy absorption tail below

380 nm is much stronger This different behaviour is con-sistent with our assumption that the 622 nm emission observed in InGaN:Eu QDs is to be associated to the

and 633.5 nm emissions More precisely we assign the

located in InGaN QDs, and the 620 nm and 633.5 nm

In fact, a recent PL study of Eu doping in GaN/AlN QDs showed that only the 622 nm line could be observed

GaN layer which exhibit three emission lines at 620, 622

at these two types of locations However, it is reasonable

to assign them to the different site-symmetry/ligand of

For InGaN:Eu QDs, the bump marking the beginning

of band-to-band absorption in PLE spectra is at lower

618 621 6 24 627 630 633 636

Wavelength(nm)

5

D0→7

InGaN:Eu QDs

GaN:Eu layer

(a)

@ 620 nm

@ 622 nm

GaN

@ 620 nm

GaN

Wavelength (nm)

5 K @ 622 nm

InGaN

InGaN

(b)

Fig 2 (a) PL spectra of InGaN:Eu QDs (upper curve) and GaN:Eu layer

(lower curve) taken at room temperature with the 360 nm excitation (b)

PLE spectra for Eu emissions at 622 nm and 620 nm in InGaN:Eu QDs

(upper) and GaN:Eu layer at 622 nm and 620 nm (lower) measured at 5 K.

The horizontally dotted lines are PLE base lines The dashed part

illustrates Eu emission by exciting InGaN QDs with certain size/shape

distribution.

energy than the GaN gap This can be explained by In

dif-fusion of about 1% into the GaN spacing layer or strain

effect of the GaN spacer as it was grown onto the InGaN

QDs On the other hand the PLE spectrum of the 622 nm

line of InGaN QDs shows a remarkable strong absorption

at 365 nm with respect to band-to-band absorption and

pronounced low energy tail absorption below the band

gap All these features can be assigned to absorption in

an inhomogeneous ensemble of QDs with a distribution

An excitation wavelength of 400 nm (below the band

gap of GaN) was used to obtain the 5 K PL spectra from

The PL integration time was longer by two orders of

mag-nitude in the case of the GaN layer as compared to InGaN

QDs This shows that defect related excitation mechanism

is very weak in our GaN layer and that a significant part of

the InGaN QD distribution can be still excited at 400 nm

622 nm consisting of two sharp lines with FWHM smaller

than 0.3 nm A small blueshift by about 0.1 nm as well as a

spectral broadening are observed for emission from

In-GaN:Eu QDs, which could be induced by electric field

effects or the different environment of Eu in the InGaN

InGaN:Eu QDs at T = 300 K, using different excitations

above (360 nm) and below (390 nm and 471.5 nm) the

GaN band gap Using 390 nm as excitation wavelength,

hardly any emission at 620 nm or 633.5 nm can be detected,

since Eu atoms in the GaN spacing layer are not excited, in

agreement with the PLE results The total emission

inten-sity is higher in the case of excitation above the barriers

(360 nm) due to stronger absorption into the GaN barrier

and carrier diffusion from the barriers into the QDs

We now discuss the different thermal quenching

Fig 4b the temperature dependence between 5 K and

380 K of the 622 nm emission is shown with exciting at

360 nm for InGaN:Eu QDs (above the corresponding InGaN band gap) and 390 nm for the GaN spacing layer (below its band gap) A reduced thermal quenching can

be found for excitation below GaN gap This is due to the fact that electron and hole pairs are directly injected and confined in QDs, which should strongly reduce their capture by non-radiative recombination centres at elevated temperatures Furthermore the weak thermal quenching suggests that the carrier mediated energy transfer to RE ions in QDs should be faster than non-radiative recombi-nation channels experienced by carriers in QDs, which

GaN:Eu layer

Wavelength (nm)

InGaN:Eu QDs

Fig 3 PL spectra of InGaN:Eu QDs (upper spectrum) and GaN:Eu layer

(lower spectrum) with the 400 nm excitation at 5 K The vertical dotted

lines guide to the eye.

610 615 620 625 630 635 640

390 nm

Wavelength (nm)

5

D

F

360 nm

(a)

0.01 0.1 1

1000/T (1/K)

(b)

Fig 4 (a) PL spectra from InGaN:Eu QDs measured with different excitation wavelengths (indicated at the curves) at 300 K The spectra are normalized by the excitation power density and the accumulation time The 471.5 nm spectrum is multiplied by a factor 100 for clarity (b) Temperature-dependent PL of InGaN:Eu QDs for emissions at 622 nm between 5 K and 380 K in double logarithmic scale The excitation wavelengths were 360 nm (filled dots) and 390 nm (filled squares) The dashed lines are used simulations to get thermal activation energies The corresponding activation energy is also indicated in the figure The vertical line marks 300 K for clarity.

are found to be on a time scale of 1 ns for carriers in GaN/

4 Conclusion

In conclusion, InGaN:Eu QDs imbedded in GaN

barri-ers have been studied by PL and PLE at various

function of the excitation wavelength has shown that

InGaN QDs showed only a slight decrease from 5 K up

in the GaN barrier layer or in GaN thick layer shows a

much more pronounced thermal quenching

Acknowledgements

We acknowledge Marle`ne Terrier, Yann Genuist and

Yoann Cure´ for their technical assistance One of the

authors (NQL) thanks the National Programme for Basic

Research (Vietnam) and CNRS (France) for financial

supports

References [1] H.J Lozykowski, W.M Jadwisienczak, J Han, I.G Brown, Appl Phys Lett 77 (2000) 767.

[2] Ei Ei Nyein, U Ho¨mmerich, J Heikenfeld, D.S Lee, A.J Steckl, J.M Zavada, Appl Phys Lett 82 (2003) 1655.

[3] H Bang, S Morishima, J Sawahata, J Seo, M Takiguchi, M Tsunemi, K Akimoto, M Nomura, Appl Phys Lett 85 (2004) 227 [4] Y Hori, X Biquard, E Monroy, F Enjalbert, Le Si Dang,

M Tanaka, O Oda, B Daudin, Appl Phys Lett 84 (2004) 206 [5] T Andreev, Y Hori, X Biquard, E Monroy, D Jalabert, A Farchi,

M Tanaka, O Oda, Le Si Dang, B Daudin, Phys Rev B 71 (2005) 115310.

[6] T Andreev, Y Hori, X Biquard, E Monroy, D Jalabert, A Farchi,

M Tanaka, O Oda, Le Si Dang, B Daudin, Superlattices Micro-struct 36 (2004) 707.

[7] Y Hori, D Jalabert, T Andreev, E Monroy, M Tanaka, O Oda,

B Daudin, Appl Phys Lett 84 (2004) 2247.

[8] T Shibata, K Asai, T Nagai, S Sumiya, M Tanaka, O Oda, H Miyake, K Hiramatsu, Mater Res Soc Symp Proc 693 (2002) 541 [9] C Adelmann, J Simon, G Feuillet, N.T Pelekanos, B Daudin, G Fishman, Appl Phys Lett 76 (2000) 1570.

[10] C Brecher, H Samelson, A Lempicki, R Riley, T Peters, Phys Rev.

155 (1967) 178.

[11] K.P ÕDonnell et al., Mater Res Soc Symp Proc 831 (2004) 96 [12] J Simon, N.T Pelekanos, C Adelmann, E Martinez-Guerrero,

R Andre´, B Daudin, Le Si Dang, Phys Rev B 68 (2003) 035312.