Random lasing of ZnO thin films grown by pulsed-laser deposition

Accepted Manuscript Title: Random lasing of ZnO thin films grown by pulsed-laser deposition Author: C. Cachoncinlle C. Hebert J. Perri`ere M. Nistor A. Petit E. Millon PII: DOI: Reference:

S0169-4332(14)02188-6 http://dx.doi.org/doi:10.1016/j.apsusc.2014.09.186 APSUSC 28842

To appear in:

APSUSC

Received date: Revised date: Accepted date:

27-6-2014 25-9-2014 29-9-2014

Please cite this article as: C. Cachoncinlle, C. Hebert, J. Perri`ere, M. Nistor, A. Petit, E. Millon, Random lasing of ZnO thin films grown by pulsed-laser deposition, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.09.186 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Random lasing of ZnO thin films grown by pulsed-laser deposition C. Cachoncinlle (1)*, C. Hebert (2,3), J. Perrière (2,3), M. Nistor (4), A. Petit (1), E. Millon (1)

ip t

1) GREMI, UMR 7344 CNRS-Université Orléans, 45067 Orléans Cedex 2, France 2) Sorbonne Universités, UPMC Université Paris 06, UMR 7588, INSP, 75005, Paris, France 3) CNRS, UMR 7588, INSP, 75005, Paris, France 4) NILPRP, L 22 P.O. Box. MG-36, 77125 Bucharest-Magurele, Romania

Abstract:

Low-dimensional semiconductor structures on nanometer scale are of great interest because of their strong

cr

potential applications in nanotechnologies. We report here optical and structural properties on UV lasing in ZnO thin films. The ZnO films, 110 nm thick, were prepared using pulsed-laser deposition on c-cut sapphire

us

substrates at 500°C under 10-2 oxygen pressure. The ZnO films are nearly stoichiometric, dense and display the wurtzite phase. The films are highly textured along the ZnO c-axis and are constituted of nanocrystallites.

an

According to Hall measurements these films are conductive (0.11 cm). Photoluminescence measurements reveals a so-called random lasing in the range 390 to 410 nm, when illuminating at 355 nm with a tripled frequency pulsed Nd-YAG laser. Such random lasing is obtained at rather low optical pumping, 45 kW cm-2,

M

a value lower than those classically reported for pulsed-laser deposition thin films.

Keywords :

pt

* Corresponding author :

ed

Pulsed-laser deposition, ZnO film, photoluminescence, UV excitation, random lasing

Ac ce

C. Cachoncinlle : [email protected]

1 Page 1 of 16

Introduction ZnO is a natural n-type semiconductor, recognized as a highly potential material for electronic and optoelectronic nanometer devices. ZnO possesses a large exciton binding energy (60 meV) and three valence bands with respective energies at 3.37 eV, 3.39 eV and 3.43 eV [1], leading to exciting properties especially for solid-state UV laser. Published works report on both the amplifications of spontaneous emission (ASE) and of stimulated emission, i.e. lasing, under both optical and electrical excitations [2-4]. UV light emitting diodes (LEDs) and UV laser diodes require high quality films for which the composition and morpho-

ip t

structure have to be perfectly controlled [5]. Lasing below 400 nm has been observed in various ZnO nanostructures: thin films [6-9], nanorods, nanowires [10], nanobelts, microdisks [11], quantum wells [12]

cr

and powders [13]. Theoretical approaches have been proposed to explain the lasing effect: exciton-exciton scattering [6], exciton-carriers scattering [3], electron-hole plasma (EHP) recombination [14,15], and

us

recombination with emission of a plasmon-phonon mixed state quanta [3].

Generally the lasing effect occurs in particular conditions in which laser cavities may be formed in the

an

material network allowing an amplification of the light by multiple reflections. Various types of cavities have been envisaged: individual cavities formed inside micro- or nano- single structures such as microdisks [11, 16, 17], microspheres [18, 19], nanorods, nanowires and nanobelts [20-22]. In thin films, cavities are

M

not so well identified and one should explain the effect by random lasing: multi-scattering on the edge of crystallite interfaces or grain boundaries is known to form closed-loop path [23] which could be considered

ed

as random ring cavities for light [4, 13].

Stimulated emission has been evidenced in ZnO films grown by pulsed-laser deposition (PLD) on c-cut sapphire substrate [7, 8, 24], demonstrating their potential in solid-state nano-photonic devices. In these

pt

studies, the reported threshold for lasing was in the 0.4-2.5 MW cm-2 range. In our work, we observed random lasing on nanocrystallized PLD ZnO thin films grown onto c-cut sapphire substrates at 500°C under

Ac ce

a controlled oxygen pressure (10-2 mbar). A lasing threshold, around 45 kW cm-2, deduced from photoluminescence (PL) spectra under pulsed laser excitation is evidenced to be ten times lower than the previous reported values for PLD films (see Table I). This threshold value is fully compatible with the lowest value reported to our knowledge (around 40 kW cm-2) [5], which was obtained by atomic layer deposition (ALD) technique. The characteristics of the laser spectral components are in reasonable agreements with previous laser gain theories [14], and fully coherent with electron-hole plasma (EHP) calculations [15].

Experiments The ZnO films were grown onto c-cut (001) sapphire (both sides polished) substrates by PLD. A frequency quadrupled Nd:YAG laser working at 266 nm with a 10 ns pulse duration at a 10 Hz repetition rate was used to irradiate a ZnO ceramic target (purity 99.99%), in a PLD set-up previously described [25]. The laser fluence used to ablate the target is around 2 J cm-2. The film growth was carried out under an oxygen pressure of 10-2 mbar classically used to grow stoichiometric oxide films [26]. The substrate temperature was

2 Page 2 of 16

held at 500° C. After the growth, the films were cooled down to room temperature (RT) at same oxygen pressure. Rutherford backscattering spectrometry (RBS), using 2 MeV 4He+ ion beam was employed to determine the thickness and the precise composition of films. The quantitative value are determined by the use of RUMP simulation program, leading to a 4 % accuracy on the oxygen concentration mainly due to the low RBS yield for oxygen. The nature of the crystalline phase, lattice parameters and preferred orientation of films were determined by X-ray diffraction (XRD) analysis in θ/2θ Bragg-Brentano configuration with the Cu Kα

ip t

radiation (λ = 0.154 nm). The morphology of films was studied by scanning electron microscopy (SEM – Zeiss Supra 40). The resistivity, nature, concentration and mobility of carriers were determined with a MMR

cr

Hall measurement system (in the van der Pauw four-probe configuration) at RT and under a magnetic field of 0.3 T. The optical transmittance of films was measured with a spectrophotometer Cary UV 60 (Agilent

us

Technology) in the wavelength 200-1100 nm range.

The PL bench included a tripled frequency Nd-YAG laser emitting at 355 nm with pulse duration less than

an

10 ns at a 10 Hz repetition rate. No focusing optics was used but two dichroic mirrors were set on optical axis to reject both the green and infrared wavelength at 532 and 1064 nm, respectively. The beam spot size on the ZnO samples was 6 mm in diameter. The incident energy was measured and averaged over 100 shots.

M

To prevent any damages on films, we limit the energy of the laser pulse to 0.5 mJ. The corresponding fluence is 1.8 mJ cm-2. Under these experimental conditions the peak-power, i.e the energy of the laser pulse divided by the pulse duration, of the laser beam is typically less than 50 kW, which corresponds to a surface

ed

density peak power on films of 180 kW cm-2, the laser ablation threshold for ZnO being about 30-40 MW cm-2. The laser beam irradiated the samples close to normal incidence and the PL of films was observed at

pt

45°. The intensity of the PL was recorded using a compact CCD spectrometer (USB2000 from Ocean Optics) in the 200-800 nm range, with a resolution of 200 at 400 nm. The CCD operated at RT. A focusing

Ac ce

optic mounted on the optical fibre coupled the PL to the spectrometer. To improve the signal to noise ratio, the spectra were accumulated over 600 shots.

Results and discussion

Fig. 1 shows the experimental RBS spectrum recorded for a ZnO film grown at 500°C under 10-2 mbar oxygen pressure and the simulated spectrum obtained by the RUMP program. A very good fit is obtained for an O/Zn ratio equal to unity and a 106 nm thickness. A typical XRD diagram is given on Fig 2, and it only shows a peak at 2= 34.44°, corresponding to the (002) planes of the ZnO hexagonal wurtzite structure. The deduced c-axis parameter is 0.5202 nm, a value slightly lower than the bulk one (0.5209 nm). Such a variation in the c-axis parameter of ZnO PLD films is generally attributed to the presence of strains in the films [27]. The main c-axis growth is the classical texturation of ZnO films observed whatever the deposition methods used [28]. By taking into account the full width at half maximum (FWHM) of the (002) diffraction line (0.57°), the size of crystalline domains along the film thickness deduced from the Scherrer law is around 15 nm. This small crystallite size may be related to the rather low growth temperature. Indeed, a substrate 3 Page 3 of 16

temperature of 700°C is required to improve the crystalline quality of ZnO films and consequently to rise the crystallite size [28]. Fig 3 depicts typical SEM views (cross section and top) of a ZnO film, 110 nm thick. The images reveal the classical dense and columnar-like structure (Fig. 3a) observed elsewhere [29]. It has to be noticed that the films are polycrystalline with small grain size about 100-150 nm (Fig. 3b), that induces the presence of many grain boundaries which can act as diffusion centres to scatter the light emitted under photonic excitation. The scattering mean free path is then reduced, leading to a strong scattering [13] and a significant lowering of the laser threshold. It appears therefore that the presence of small size nanocrystallites

ip t

in ZnO films is a priori favourable to random lasing by forming chaotic closed-loop path [23]. This could explain the higher laser threshold (900 kW cm-2) observed in better crystalline quality ZnO thin film [8].

cr

Our ZnO films are conductive as a resistivity of 0.11 cm is found by Hall measurements. This conductivity may be related to the composition of films, close to the stoichiometry, which limits the

us

formation of oxygen vacancies able to supply additional charge carriers. This is in agreement with the absence of the PL "green band" commonly attributed to the defects in films (see below). Such conductivity is also due to the presence of structural defects according to the low growth temperature. The concentration and

an

the mobility of charge carriers (electrons) are respectively around 3.1x1018 cm-3 and 18.7 cm2 V-1 s-1. This intrinsic carrier density is one order of magnitude lower than the Mott density for ZnO, which is reported in

M

the range 5.4 - 8.4 x1019 cm-3 [30, 31]

Optical transmittance of the ZnO films in the UV-Visible spectral domain has been recorded and a typical

ed

example is presented on Fig 4. The data are corrected from the raw substrate transmission. Fitting the transmission curves with a classical transmission model of the ZnO film on sapphire substrates leads to determine a film thickness (130 nm) in good agreement with the values measured by SEM (110 nm) and

pt

RBS (106 nm). The bump appearing below 370 nm is due to the PL of ZnO under UV excitation. From the transmission measurements, the Tauc's plot which represent (E)2 versus E, where E is the photon energy

Ac ce

and  the absorption coefficient, allows to estimate the optical band gap energy (insert on Fig. 4) assuming a direct band gap for ZnO. The band gap obtained by extrapolating the linear part of the absorption edge is 3.28 eV, a value measured at room temperature, commonly reported for ZnO thin film grown by PLD [27]. Typical PL spectra from 350 to 800 nm under UV excitation (355 nm) and different optical pump energies are shown on Fig 5. At low laser pump energy, the spectra are dominated by a broad UV band from 370 to 400 nm commonly attributed to the electron-hole recombination in ZnO [3]. It is noticeable that the so-called “green band”, around 450-550 nm, attributed to deep-levels emission from localized defects in the ZnO structure [32] is very weak and not observable. Only a plot on logarithm scale can reveal wide bump arising from these structural defects. Above an optical pump threshold (around 120 µJ), a sharp structure peaking around 393 nm, 6 to 8 nm wide, emerges on the red wing of the broad peak (centred around 384 nm and of 25 nm wide (FWHM)). A redshift from 393 nm to 398 nm of the peak maximum is also observed with increasing the pump energy. Recorded single-shot spectra (Fig. 5 insert) clearly show that the 6-8 nm wide structure is therefore the average of multiple spectral components. The spectrometer used cannot resolve 4 Page 4 of 16

these individual components. They present a strong chaotic behaviour. More peaks emerge with increasing the pump energy. The global redshift observed on the average spectrum (Fig. 5) when increasing pump energy results precisely from the apparition of new peaks mainly on the red wings of the structure. These spectral components are classically attributed to random lasing. We present in Fig. 6 the increase of integrated PL in the 393-398 nm range versus laser pump energy. Two secant straight lines may fit this data, leading to a threshold in the 120-130 µJ range, corresponding to a laser threshold of about 45 kW cm-2. It seems to be now accepted that this lasing effect cannot be attributed to the simple exciton recombination,

ip t

since exciton does not exist any more as the carrier density induced by optical pumping (3.6 x 1019 cm-3 [15]) is of the order of the Mott density (5.4-8.4 x1019 cm-3 [31]). From the experimental study described in this

cr

work, it is clearly shown that the gain media have to be interpreted as EHP in which the carrier density screens electron–hole pairs [14]. According to this theory, a negative absorption coefficient, i.e. a positive

us

optical gain [3], is generated close to the absorption edge. For example, at a carrier concentration of 3x1019 cm-3, the gain region spreads from 3.15 to 3.35 eV [14]. This energetic domain corresponds to the wavelength range from 370 to 393 nm where lasing is commonly reported. Since the precise mechanism clarify the origin of the observed spectral structures.

M

Conclusion

an

involved in random lasing is still under matter of discussion, forthcoming work will be performed in order to

Nearly stoichiometric and nanocrystallised ZnO thin films, 110 nm thick, are obtained by PLD at 500°C

ed

under 10-2 mbar oxygen pressure. A classical columnar-like microstructure with a (002) texturation of films is observed. The crystallite size is about 15 nm along the c-axis and the grain size parallel to surface substrate is in the 100-150 nm range. The presence of many grain boundaries, which can act as diffusion

pt

centres to scatter the light emitted under UV photon excitation (355 nm), may originate the random lasing observed on these ZnO films in the 393 – 398 nm spectral domain. It is worth noticing that optical pump

Ac ce

threshold needed to evidence the lasing is around 45 kW cm-2, which is largely lower than the values already reported in PLD experiments. The carrier density induced by optical pumping being in the order of the Mott density, the random lasing can be interpreted as electron-hole recombination in an electron-hole plasma. Further works are in progress to correlate the random lasing to the composition and microstructural properties of ZnO PLD films and to deep insight the observed PL fingerprints.

Acknowledgements

The authors are grateful to the cooperative structure between the University Pierre et Marie Curie of Paris and the University of Namur under the Convention SAFIR@ALTAÏS. The authors would like to thank W. Seiler from the PIMM-Arts et Metiers ParisTech for XRD measurements, and O. Aubry from GREMI (CNRS - Université d'Orléans) for UV-Visible absorption spectroscopy.

5 Page 5 of 16

References [1] M.R. Wagner, G. Callsen, J.S. Reparaz, R.Kirste, A. Hoffmann, A.V. Rodina, A. Schleife, F. Bechstedt, and M. R. Phillips, Phys. Rev. B 88 (2013) 235210 [2] L. Banyai, S.W. Koch, Z. Phys. B - Condensed Matter 63 (1986) 283-291 [3] C. Klingshirn, R. Hauschild, J. Fallert, H. Kalt, Phys. Rev. B 75 (2007) 115203 [4] Y. Tian, X. ma, L. Xiang, M.V. Ryzhkov, A.A. Borodkin, S.I. Rumyantsev, D. Yang, Optics Comm. 285 [5] M-J Chan, J-R Yang, M. Shiojiri, Semicond. Sci. Technol. 27 (2012) 074005

ip t

(2012) 5323-5326 [6] D.B. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997)

cr

2230-2232 [7] A. Mitra, R.K. Thareja, J. Appl. Phys. 89 (2001) 2025-2028

us

[8] P.H. Dupont, C. Couteau, D.J. Rogers, F. Hosseini Teherani, G. Lerondel, Appl. Phys. Lett. 97 (2010) 261109

[9] R.P. Wang, H. Muto, X. Gang, P. Jin, M. Tazawa, J. Crystal Growth 282 (2005) 359-364

an

[10] H.Y. Yang, S.F. Yu, G.P. Li, T. Wu, Optics Express 18 (2010) 13647-13654

[11] K. Nomenyo, A-S Gadallah, S. Lostcheev, D.J. Rogers, G. Lerondel, Appl. Phys. Lett. 104 (2014) 181104

M

[12] H.D. Sun, T. Makino, N.T. Tuan, Y. Segawa, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Appl. Phys. Lett. 77 (2000) 4250-4252

ed

[13] H. Cao, Y.G. Zhao, S.T. Ho, E.W. Seelig, Q.H. Wang, R.P.H. Chang, Phys. Rev. Lett. 82 (1999) 22782281

[14] M. Versteegh, T. Kuis, H.H. Stoof, J.I. Dijkhuis, Phys. Rev. B 84,035207 (2011)

pt

[15] T. Nakamura, K. Firdaus, S. Adachi, Phys. Rev. B 86 (2012) 205103 [16] R. Chen, B. Ling, X.W. Sun, H.D. Sun, Adv. Mater. 23 (2011) 2199-2204

Ac ce

[17] J. Dai, C.X. Xu, X.Y. Xu, J.T. Li, J.Y. Guo, Y. Lin, APL Materials 1 (2013) 032105 [18] R.S. Moirangthem, P-J. Chang, P.C-H. Chien, B.T.H. Ngo, S-W. Chang, C-H. Tien, Y-C. Chang, Optics Express 21 (2013) 180785

[19] P.D. Garcia, C. Lopez, J. Mater. Chem. C 1 (2013) 7357-7362 [20] M.A.M. Versteegh, D. Vanmaekelbergh, J.I. Dijkhuis, Phys. Rev. Lett. 108 (2012) 157042 [21] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897-1899

[22] B. Zou, R. Liu, F. Wang, A. pan, L. Cao, Z.L. Wang, J. Phys. Chem. B 110 (2006) 12865-12873 [23] H. Cao, Y.G. Zhao, H.C. Ong, S.T. Ho, J.Y. Dai, J.Y. Wu, R.P.H. Chang, Appl. Phys. Lett. 73 (1998) 3656-3658 [24] R. Sharma, B. Kim, C. Cho, K. Kyhm, J. Phys. D: Appl. Phys. 42 (2009) 135421 [25] O. Pons Y Moll, J. Perrière, E. Millon, R.M. Defourneau, D. Defourneau, B. Vincent, A. Essahlaoui, A. Boudrioua, W. Seiler, J. Appl. Phys. 92 (2002) 4885-4890 [26] N. Sbaï, J. Perrière, W. Seiler, E. Millon, Surf. Sci. 601 (2007) 5649-5658 6 Page 6 of 16

[27] R. Triboulet, J. Perrière, Progress in Crystal Growth and Characterization of Materials 47 (2003) 65-138 [28] S. Tricot, M. Nistor, E. Millon, C. Boulmer-Leborgne, N.B. Mandache, J. Perrière, W. Seiler, Surf. Sci. 604 (2010) 2024-2030 [29] V. Craciun, J. Perrière, N. Bassim, R.K. Singh, D. Craciun, J. Spear, Appl. Phys. A 69 (1999) S531S533 [30] N.F. Mott, Philos. Mag. 6 (1961) 287-309 [31] J.G. Lu, S. Fujita, T. Kawaharamura, Y. Kamada, T. Ohshima, Z.Z. Le, Y.J. Zeng, Y.Z. Zhang, L.P.

ip t

Zhu, H.P. He, B.H. Zhao, J. Appl. Phys. 101 (2007) 083705

Ac ce

pt

ed

M

an

us

cr

[32] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285-2287

7 Page 7 of 16

Tables Table I: Reported values for laser threshold in ZnO micro devices and thin films. The data obtained by PLD technique are underlined. TF: thin film. ALD: atomic layer deposition, MBE: molecular beam epitaxy, MQW: multiple quantum wells. Observed Threshold (kW.cm-2)

Reference

TF ALD

≈38

[5]

Nanowire

≈40

[22]

TF PLD

≈45

This work

Nanowire

≈200

[23]

MQW

≈200

[12]

Disk

≈280

[16]

TF MBE

≈300

Nanowire

400-800

TF PLD

≈400

Disk

500-2700

[11]

Powders

≈600

[13]

cr

ip t

Technology & Elaboration

[6]

an

us

[10]

TF PLD Powders

≈900

[8]

1000-3000

[15]

≈2500

[7]

Ac ce

pt

ed

M

TF PLD

[24]

8 Page 8 of 16

Figures

Fig. 1: RBS spectrum of PLD ZnO film grown on c-cut sapphire substrate at 500°C under 10-2 mbar oxygen pressure : the experimental data (dots) are fitted with RUMP simulation program (black line) using O/Zn=1.

ip t

Fig. 2 : Typical /2 XRD pattern of a PLD ZnO film grown on c-cut sapphire substrate at 500°C under 10-2 mbar oxygen pressure.

cr

Fig. 3 : SEM images in (a) cross section and (b) top view of a PLD ZnO film grown on c-cut sapphire substrate at 500°C under 10-2 mbar oxygen pressure. The cross section image has been recorded on a ZnO

us

film grown onto Si substrate in the same conditions.

Fig. 4 : UV-vis-near IR transmittance for a ZnO film grown under 10-2 mbar at 500 ° C. The insert shows the

an

corresponding Tauc's plot.

M

Fig. 5 : Room temperature PL spectra under excitation at 355 nm with different laser pump energies of a ZnO PLD films grown on c-cut sapphire substrate at 500°C under 10-2 mbar oxygen pressure. The spectra are averaged over 600 shots. The pump energies are from the upper curve to the lower: 350, 300, 230, 170, 140,

ed

120, 70 and 60 µJ. The dashed straight-line evidences the global redshit discussed in the text. The insert shows 3 examples of single-shot PL spectra in the 380-420 nm range.

Ac ce

pt

Fig. 6: Room temperature PL intensity versus optical pump energy integrated in the range 393-398 nm.

9 Page 9 of 16

Ac ce

pt

ed

M

an

us

cr

ip t

Fig 1

10 Page 10 of 16

Fig 2

140000

(002) 120000

ip t

80000

cr

60000

20000

30

32

34

36

38

an

0 28

us

40000

pt

ed

M

2 (degrees)

Ac ce

Intensity (a.u.)

100000

11 Page 11 of 16

Ac ce

pt

ed

M

an

us

cr

ip t

Fig. 3

12 Page 12 of 16

Ac ce

pt

ed

M

an

us

cr

ip t

Fig 4

13 Page 13 of 16

Ac ce

pt

ed

M

an

us

cr

ip t

Fig 5

14 Page 14 of 16

Ac ce

pt

ed

M

an

us

cr

ip t

Fig 6:

15 Page 15 of 16

Highlights :

ip t

ed

M

an

us

cr

Random lasing interpreted by the electron-hole plasma (EHP) model

pt

-

Random lasing at RT in nanocrystalline ZnO PLD thin film (< 100 nm) Low optical pumping threshold (<30 kW cm-2) for UV random lasing

Ac ce

-

16 Page 16 of 16