Superlattices and Microstructures 41 (2007) 347–351www.elsevier.com/locate/superlattices

A zinc oxide microwire laser

C. Czekalla∗, J. Lenzner, A. Rahm, T. Nobis, M. Grundmann

Universitat Leipzig, Fakultat fur Physik und Geowissenschaften, Institut fur Experimentalphysik II, Linnestr. 5,04103 Leipzig, Germany

Available online 10 April 2007

Abstract

In this paper, we report stimulated emission from a zinc oxide (ZnO) microcrystal grown bycarbothermal evaporation observed by spatially resolved photoluminescence (PL) and high excitationspectroscopy (HES).c© 2007 Elsevier Ltd. All rights reserved.

Keywords: ZnO; Microwire; Photoluminescence spectroscopy; High excitation spectroscopy; Laser

1. Introduction

Currently there is an intense interest in developing small sized semiconductor devices likeLEDs and lasers. The wurtzite semiconductor ZnO is a promising material for such futureoptoelectronic devices due to its band gap in the ultraviolet range and its large exciton bindingenergy of about 60 meV. In contrast to GaN, this allows the observation of excitonic effectsat or even above room temperature (RT). In the past few years optically pumped lasing fromvarious ZnO structures has been reported [1,2]. Micrometer sized ZnO structures are promisingcandidates for laser devices and should allow fundamental investigations of light–mattercoupling. Hexagonally shaped ZnO microwires with diameters in a range from a few hundrednanometers to several hundred micrometers can be achieved by carbothermal evaporation [3].Those structures show a high photoluminescence (PL) signal under low excitation conditions.

If ZnO is optically excited under high intensities, typically an additional peak arises in theZnO PL spectrum. This peak is most probably caused by an inelastic exciton–exciton scattering

∗ Corresponding author.E-mail address: czekalla@physik.uni-leipzig.de (C. Czekalla).

0749-6036/$ - see front matter c© 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.spmi.2007.03.027

348 C. Czekalla et al. / Superlattices and Microstructures 41 (2007) 347–351

Fig. 1. SEM images of a ZnO microwire on a carbon gluepad.

process and exhibits a fine structure. The emitted photons have the energies [4]

En = Eex − Ebx

(1 −

1n2

)−

32

kBT, (1)

where Eex is the energy of the free exciton luminescence, Ebx is the binding energy of the

excitons and kBT the thermal energy. The parameter n is the quantum number of the problem.The respective peak is called P-band and in many cases stimulated emission related to this bandcan be observed. Under even higher excitation, the carrier density in the sample increases and theelectron hole plasma (EHP) peak at even lower energies than the P-band becomes observable.

A laser is a device emitting stimulated optical radiation. For practical purposes, one uses acavity for optical feedback. A ZnO microcrystal per se cannot be expected to act as such a cavitydue to the high losses caused by the short distance of the mirrors. The mirror loss for reflectivitiesR1 and R2 in a cavity of the length L is given by [5]

α =1

2Lln

(1

R1 R2

). (2)

The reflectivity of the ZnO/air interface is approximately 0.2. For a 10 µm cavity, Eq. (2)gives a loss of 1600 cm−1. In a real structure, other losses due to scattering and absorption willadditionally play a role. Typical gain reported for ZnO (150 cm−1 in Ref. [6], 600 cm−1 inRef. [7]) cannot overcome these losses. The observation of Fabry–Perot modes in lasing thusseems unlikely in our structures. However, we will demonstrate stimulated emission as result ofa single pass gain.

2. Experiments

The ZnO microwires were synthesized by thermal evaporation of a pressed ZnO–graphite(mass-ratio 1:1) target at ambient pressure. The growth temperature was 1100 ◦C. An argondownstream was applied as transport gas. Further details can be found in Ref. [3]. The ZnOmicrowires were isolated on a carbon gluepad. SEM images of a microwire are shown in Fig. 1.

Fig. 2 shows a scheme of the setup used for the PL and high excitation spectroscopy (HES)measurements. It allows spatially resolved PL measurements and the observation of emittedlight from single microstructures. For PL, the excitation source was a continuous-wave He–CdKimmon laser with a wavelength of 325 nm and a power of 20 mW. For HES measurements, apulsed Thales DIVA II Nd:YAG laser beam (266 nm, 10 ns, 20 Hz) is focused on the sample. The

C. Czekalla et al. / Superlattices and Microstructures 41 (2007) 347–351 349

Fig. 2. Setup for spatially resolved PL measurements. The inset shows the orientation of the wire with respect to thelaser beam and the objective.

energy per pulse is 2 mJ. Using these two lasers, the excitation intensity has been varied in a rangefrom 1 to 2500 kW/cm2. The angle of incidence of the pump beam is approximately 60 deg withrespect to the surface normal as depicted in the inset of Fig. 2. Excitation spots smaller than thediameter of the pump laser beam are possible using a focusing lens. Furthermore, the excitationintensity can be controlled by a reflective Newport attenuator for high power lasers.

The light emitted from the sample can be deflected into a digital camera by a movable mirrormaking the setup work as an optical microscope. If the mirror is removed, the emitted light isfocused onto an optical fused silica fibre and directed to the monochromator. It is then spectrallydispersed with a 2400 lines/mm grating (blaze 330 nm) and detected by a liquid nitrogen cooledback-illuminated CCD camera.

The setup is diffraction-limited and confocal and allows a spatial resolution of 1 µm.

3. Results and discussion

Fig. 3 shows the low and high excitation spectra of a ZnO microcrystal in a semilogarithmicplot at room temperature. The dashed cw spectrum was shifted for clarity and does not fit to thesame scale. The excitation intensities were chosen between 45 and 250 kW/cm2.

Two peaks at 3.20 and 3.28 eV are observed in the cw spectrum. The lower energy peak ismost probably related to the first LO-phonon replica of the 3.28 eV maximum. Under higherexcitation only the 3.20 eV peak is visible. It can be seen from Fig. 3 that a sharp peak at3.154 eV is predominant in the spectrum for higher excitation intensities. The FWHM is 88 meVfor 85 kW/cm2 and 29 meV for 250 kW/cm2, indicating spectral narrowing as depicted in Fig. 4.

The peak intensity increases superlinearly with increasing excitation intensity as depictedin Fig. 5. The spectral narrowing (Fig. 4) and the superlinear characteristic curve in Fig. 5implies the presence of stimulated emission from the microcrystal. The threshold pump intensityis approximately 170 kW/cm2.

350 C. Czekalla et al. / Superlattices and Microstructures 41 (2007) 347–351

Fig. 3. Room temperature PL spectra of a ZnO microcrystal arranged on a carbon gluepad under low (dashed curve) andhigh (solid curves) excitation intensities varied between 45 and 250 kW/cm2. At higher excitation intensities, the P-bandarises from the free exciton emission band. The dashed curve of the cw spectrum was shifted for clarity.

Fig. 4. FWHM of the PL peak as a function of the excitation intensity. Dashed lines are guides to the eye.

For T = 300 K, Eq. (1) gives values between 3.234 eV (n = 2) and 3.219 eV (n = ∞). Theobserved values are smaller than those expected from Eq. (1). We tentatively attribute the laserpeak to the P-band and the energy shift of about 70 meV from the expected room temperatureposition to a local increase of the sample temperature by approximately 100 K, taking intoaccount the temperature dependence of the band gap energy [8]. Further excitation density andtemperature dependence measurements will clarify this point.

4. Conclusions

In conclusion, we have found RT stimulated emission in a ZnO microcrystal grown bycarbothermal evaporation. Further work will be directed towards enhancing the reflectivity ofthe end facets of the microcrystals.

C. Czekalla et al. / Superlattices and Microstructures 41 (2007) 347–351 351

Fig. 5. Dependence of the peak-intensity on the excitation intensity. The superlinear dependency is clearly visible. Thethreshold intensity for the stimulated emission is approximately 170 kW/cm2.

Acknowledgment

We gratefully acknowledge the support by the DFG within FOR522.

References

[1] M.H. Huang, S. Mao, H. Jeick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897.[2] J.C. Johnson, H. Yan, P. Yang, R.J. Saykally, J. Phys. Chem. B 107 (2003) 8816.[3] M. Lorenz, J. Lenzner, E.M. Kaidashev, H. Hochmuth, M. Grundmann, Ann. Phys. 13 (2004) 39.[4] C. Klingshirn, Phys. Status Solidi B 71 (1975) 547.[5] M. Grundmann, The Physics of Semiconductors, Springer Verlag, 2006.[6] J. Cui, S. Sadofev, S. Blumstengel, J. Puls, F. Henneberger, Appl. Phys. Lett. 89 (2006) 051108.[7] Y. Chen, Y. Segawa, N.T. Tuan, S.-K. Hong, T. Yao, Appl. Phys. Lett. 78 (2001) 11.[8] R. Schmidt-Grund, N. Ashkenov, M. Schubert, W. Czakai, D. Faltermeier, G. Benndorf, H. Hochmuth, M. Lorenz,

J. Blasig, M. Grundmann, ICPS Poster (2006).