The Effect of Tensile Strain on Optical Anisotropy and Exciton of m-Plane ZnO

Anisotropy and Exciton of

m-Plane ZnO

Volume 7, Number 2, April 2015

H. H. Wang

J. S. Tian

C. Y. Chen

H. H. Huang

Y. C. Yeh

P. Y. Deng

L. Chang

Y. H. Chu

Y. R. Wu

J. H. He

DOI: 10.1109/JPHOT.2015.2415672

1943-0655

Ó 2015 IEEE

The Effect of Tensile Strain on Optical

Anisotropy and Exciton of

m-Plane ZnO

L. Chang,2 Y. H. Chu,2Y. R. Wu,3and J. H. He1

1_{Computer, Electrical and Mathematical Sciences and Engineering Division, King Abdullah University}

of Science and Technology, Thuwal 23955-6900, Saudi Arabia

2_{Department of Materials Science and Engineering, National Chiao Tung University,}

Hsinchu 300, Taiwan

3_{Institute of Photonics and Optoelectronics and Department of Electrical Engineering,}

National Taiwan University, Taipei 10617, Taiwan

DOI: 10.1109/JPHOT.2015.2415672

1943-0655Ó 2015 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received February 25, 2015; revised March 12, 2015; accepted March 15, 2015. Date of publication March 20, 2015; date of current version April 1, 2015. Corresponding author: J. H. He (e-mail: [email protected]).

Abstract:The near band edge emission of the tensile-strained m-plane ZnO film grown on (112)LaAlO3 substrates shows abnormal low polarization degree (
¼ 0:1). The

tem-perature dependency of polarization degree clarifies the origins of different emission peaks. In tensile-strained m-plane ZnO, the [0001] polarized state is upper shifted and is overlapping with the [1120] polarized state. This phenomenon causes the abnormal low polarization degree and reveals the effect of strain on the emission anisotropy of m-plane ZnO.

Index Terms:Optical films, photoluminescence, optical polarization, strain, II-VI semicon-ductor materials.

1. Introduction

ZnO, which is known to be a wide band gap material with high exciton binding energy, is a promising material for optoelectronic devices [1], [2]. For light-emitting applications, quantum wells (QWs) in light emission devices can effectively increase the emission efficiency. Unfortu-nately, similar to c-plane GaN QWs, c-plane ZnO QW structures also suffers from the intrinsic internal electric field that leads to the spatial separation of the wave functions of electron and hole, which causes the decrease in transition efficiency [3]. To boost the emission efficiency via eliminating the internal field in QWs, the non-polar ZnO QW, i.e., m-plane or a-plane, has at-tracted considerable attention [4], [5].

The polarization anisotropy in light emission is another feature of the non-polar ZnO-based light emitting device, which originates from the band splitting of the valence bands (VBs) in wurtzite ZnO. However, a polarized light source is sometimes highly undesirable in a certain optical system, including an optical communication system, an interferometer, and sensors [6], [7]. Therefore, the management of polarization anisotropy is critical to achieve light source with desired properties. Previously, it has been demonstrated that under a strong compressive strain, the polarization anisotropy of ZnO films is distinct from bulk ZnO and the reordering of VBs is observed [8], [9]. Nevertheless, the optical characteristics of non-polar ZnO films under tensile strain remain unclear since the sapphire, which is the most commonly used substrate to grow

non-polar ZnO films, usually results in compressive strain. Recently, LaAlO3 (LAO) has been

newly developed as a substrate to grow non-polar ZnO films because of its small lattice mis-match comparing to sapphire, thermal stability and potential to integrate ZnO with Si8. According to the lattice mismatch of 2.8%ðkcÞ and 1.0% ð? cÞ between m-plane ZnO and (112)LAO, ten-sile strain is expected. Accordingly, the optical characterization on m-plane ZnO films grown on (112)LAO under tensile strain can not only help us evaluate the feasibility of this new substrate, but also expand our general understanding of the optical property of tensile-strained ZnO. In this work, the tensile strain-induced reordering of VBs and the related strain relaxation are found to result in weakened optical anisotropy in m-plane ZnO on (112)LAO by measuring temperature- and polarization-dependent photoluminescence (PL) spectra and simulation, which employs the k p method and is performed to confirm the experimental result.

2. Experimental Detail

The m-plane ZnO film was grown on (112)LAO using laser molecular beam epitaxy with a KrF excimer laser ð ¼ 248 nmÞ. The microstructure characteristics and strain condition were in-vestigated by X-ray diffraction (XRD) and the Raman spectroscopy, respectively. The Raman spectrum was measured by a Jobin Yvon T64000 triple spectrometer equipped with a charged-coupled device cooled at 160 K. The temperature dependent PL spectra were obtained by using an He-Cd laser ð ¼ 325 nmÞ with the laser power density of 0.8 W/cm2 as the excitation source. For polarization-dependent PL, we placed a polarizer in front of the monochromator and calibrated the system in order to reduce the polarization dependency caused by the instrument itself.

3. Material Growth and Structural Analysis

For the growth of non-polar m-plane ZnO, we choose a (112)LAO substrate which can be con-sidered as a 35.26 miscut of (001) along [110]LAO. The (112) surface of LAO with rectangular template of 5.36 Å x 6.57 Å gives the lattice mismatch of 2.8%ð==cÞ and 1.0% ð? cÞ with ZnO [8]. To investigate the crystallinity of the as-grown ZnO films on (112)LAO, we performed the

Fig. 1. X-ray diffraction pattern of (a)–2 and (b) in-plane  scan of ZnO film grown on (112)LAO. (c) Raman spectra of m-plane ZnO film grown on (112)LAO (red) and strain free ZnO nanorods (black).

XRD –2 scan and in-plane ’ scan. Fig. 1(a) shows the –2 scan of ZnO films grown on (112)LAO substrates, showing (1100), (2200) peaks of ZnO and (112) peak of LAO, thus con-firming that the m-plane ZnO was successfully grown on the (112)LAO substrate [10]. Based on the (2200) peak at 66.55, the corresponding strain "yy along out of plane direction can be

evaluated as 0.21%. Fig. 1(b) shows the in-plane ’ scan of the ZnO film, indicating two-fold symmetry, which suggests the epitaxial growth of m-plane ZnO films on (112)LAO [10]. More detailed materials characterizations for epitaxial m-plane ZnO film on (112)LAO can be found elsewhere [8].

To further confirm on the strain condition in the ZnO film on (112)LAO, the Raman spectrum was measured. In Fig. 1(c), the Raman shift of ZnO E_2high mode, the mode for accurate deter-mination of the strain components, [11] is red-shifted for 2 cm1 as compared with that of the strain-free ZnO nanorods. By using the stiffness constant and phonon deformation potential of ZnO reported in previous literatures, the strain along [1120] and [0001] is calculated [12], [13]. The strain "xx ð? cÞ and "zz ðkcÞ are 0.12% and 0.27%, respectively. This result indicates the

tensile strain along c-axis is larger, in accordance with the lattice match of 2.8%ðkcÞ and 1.0% ð? cÞ.

4. Temperature and Polarization Dependent Photoluminescence Spectra

To investigate the optical emission properties of the tensile-strained m-plane ZnO, we measured the PL spectrum of the m-plane ZnO film on (112)LAO, as shown in Fig. 2. We identified the ori-gins of the emission peaks observed in PL spectra by varying the temperature and polarization. The optical anisotropy in non-polar ZnO can be attributed to the splitting of VBs. For wurtzite structure, the p-like VB splits into three subbands due to crystal field interaction and spin orbital interaction. These three subbands with the symmetry of
9,
7, and
7 has their own selection

rule of the emission polarization parallel to [1120](a-axis), [1100] (m-axis) and [0001] (c-axis), in

Fig. 2. Temperature dependent PL spectrum of our m-plane ZnO film for emission polarized in E

only 5 meV, which are mainly mixed with [1100] and [1120] polarized subbandsðjX  iYiÞ. The energy separation between [0001] polarized subbandðjZiÞ and the first top subband jX  iYi is about 43 meV [16]. As a result, in c-plane ZnO, [1100] and [1120] polarized subbands have similar electron-hole pair recombination probabilities and thus low anisotropy in light emission, while the emission from [0001] polarized state is forbidden for surface emitting [8]. On the other hand, for non-polar m-plane ZnO, the [1100] polarized state is forbidden for surface emitting and since the energy separation between [0001] and [1120] polarized state is large, the [1120] polarized state dominates in light emission, leading to the anisotropy in light emission for non-polar m-plane ZnO.

As shown in Fig. 2, the orange line presents the component of emission light polarized perpendicularly to the c-axis of ZnOðE!? cÞ and the green line presents the component of the emission light polarized parallel to the c-axis of ZnOðE!kcÞ. Before gaining an insight into opti-cal anisotropy of m-plane ZnO films on (112)LAO, the origin of the emission peaks is discussed first. For the temperatures below 130 K, the near band edge emission is dominated by a peak at 3.362 eV, which is assigned to a donor bound exciton ðDX1Þ due to thermal quenching

characteristics [17]. Note that the suffix “1” in DX1 indicates an emission originates from VBs

with specific band structure. The first and second phonon replica (DX0₁ and DX00₁) and the two-electron satellites transition related to DX1 ðDX1 TESÞ can be observed for E!kc at 3.291 eV,

3.218 eV, and 3.325 eV, respectively [16], [17]. These assignments are made according to both the difference in photon energy that equals to the energy of longitudinal optical phonon in ZnO and the similar temperature-dependent behavior associated with DX1. The latter inference

is based on the fact that as the donor bound excitons are thermally dissociated, the related emission, DX1, DX01, DX001 and DX1 TES, are weakened simultaneously [16], [17]. In other

words, as the temperature increases, the donor bound excitons gradually gain enough energy to overcome the binding energy of the donor defect state and thus become free excitons (FX). In our case, when the temperature becomes larger than 130 K, the peak at 3.377 eV begins to dominate; we assign this peak and another two peaks at 3.315 eV and 3.241 eV to be free exciton, FX1, and its phonon replicas, FX01and FX001 [17]. This assignment of emission peaks is

based on the fact that optical anisotropy in the polarization of FX1 and DX1 is similar, which

implies that these two transitions originate from excitons recombining at the VBs with similar band structures.

In our m-plane ZnO film on (112)LAO, the expected optical anisotropy is not observed for DX1, FX1 and their relevant emissions. In contrast to DX1 and FX1 emissions, the peak at

3.310 eV and the shoulder band at the high-energy side of DX1 at 14 K exhibit high optical

anisotropy while the origins of these two peaks remain unknown. To extract more information and the possible VBs reordering, we define the polarization degree
, which can basically be interpreted as the polarization anisotropy, by

¼I? Ik

I_?þ I_k (1)

where I_? and I_k are the peak intensities of E!kc and E!? c. Difference in polarization degree can be ascribed to the different energy separation of VBs in which the excitons recombine. In previous studies, the peak at 3.31 eV has been assigned to be the free electron-acceptor (e-A) recombination originated from the stacking fault [13], [15]; however, in our work, the peak at 3.31 eV shows thermal quenching behavior so that it is almost unable to observe for tempera-ture above 50 K, while in previous studies, the e-A transition remains observable at room temperature [18], [19]. To clarify the origins of the peak at 3.31 eV and its shoulder band at the right of DX1, polarization degree is plotted as a function of temperature in Fig. 3. Two

3.310 eV and 3.380 eV and decrease simultaneously as the temperature increases. Based on the similar behavior of polarization degree as a function of temperature, one can see that these two peaks originate from exciton recombination at similar VBs, which is different from DX1.

Since these two peaks are both thermally quenched at 50 K and the photon energy difference of these two peaks is about 72 meV, it leads us to conclude that these two peaks are donor bound excitons and that the peak at 3.31 eV is the first phonon replica of the peak at 3.38 eV, which we name as DX2and DX02.

To investigate the origins of the donors of DX1and DX2, we then fit the donor binding energy

of DX1and DX2using

I¼ I0 Iþ a  eEakT

(2)

where I is the emission intensity at the temperature T ; I0is the emission intensity at 0 K, which

is approximated by the emission intensity at 14 K; a is a constant; Ea is the binding energy of

donor; and k is the Boltzmann constant [20]. The binding energy of DX1 and DX2 are found to

be 7.0 meV and 9.0 meV, which are both close to the reported activation energy of neutral donor-bound exciton [21]–[23].

In the above study, by investigating how the polarization degree changes with the tempera-ture, an analysis technique to study the origin of PL peaks is demonstrated. In our case, we found that FX0₁ and DX0₂has the identical energy near 3.31 eV. Consequently, if the intensity of DX0₂wasn't thermally quenched strong enough to be differentiated from FX0₁under temperature variation, one may conclude these two peaks to be the same, which is wrong as one takes the difference in polarization degree of these two emission into consideration. The polarization degree reveals the information hidden in the PL spectra, as in our case, DX2in Fig. 2.

To realize the emission property with unexpected optical anisotropy in tensile-strained m-plane, we propose a strain-modified band structure based on the PL spectra. In the PL spec-tra, main emission peaks DX1 and FX1 have low polarization degree (10%) at 240 K. The low

polarization degree of emission is mainly due to overlapping of two VBs; therefore, we suggest that DX1 originates from exciton recombination at the VBs whose two subbands polarized in

[0001] and [1120] overlapped. DX2 with high polarization degree up to 70%, on the other

hand, originates from exciton recombination at the VBs whose two bands are separated. The two different VBs result from the thickness variation of the film. As the thickness increases, the strain is relaxed and therefore changes the band structure of VBs. DX1 with stronger emission

intensity originates from the surface of the film where the strain is partially relaxed, and DX2

originates from the bottom part of the film near the interface between ZnO films and LAO sub-strates where the strain is not relaxed. The intensity of DX1is stronger than the intensity of DX2

because most incident light is absorbed at the surface. Since the emission of DX2 has stronger

Fig. 3. Temperature dependency of polarization degree where the lower part is the PL spectrum at 14 K for comparison.

value for E!? c, we conclude that the upper subband, which has higher electron hole density, has the selection rule of E!? c. The inferred band structure reordering under tensile strain is shown in Fig. 4. The energy difference of the two subbands polarized in [0001] and [1120] is around 40 meV under strain-free condition [16]; however, the band polarized in [0001] seems to upper shift to the position of the [1120] polarized state in the tensile-strained ZnO film. The influ-ence of tensile strain on the band structure of m-plane ZnO film is thus investigated.

Fig. 5. Calculated valance band of (a) unstrained, (b) fully strained film, and (c) strain relaxed in [1120] direction. (d) Simulated photoluminescence spectrum of film with strain relaxed in [1120] direction. The inlet is the experimental PL spectrum at 14 K.

5. Band Simulation by

k  p Method

We further applied the k p method to confirm the effect of tensile strain on m-plane ZnO, as shown in Fig. 5 [24], [25]. In Fig. 5(a) and (b), under the condition that the lattice mismatch are 2.8% and 1.0% along [0001] and [1120], respectively, the [0001] polarized subband has shifted closer to the [1120] polarized subband. However, as shown in Fig. 5(c), the two subbands would only be closely overlapped if the strain along [1120] is partially released, in agreement with the model we propose. Accordingly, the simulated PL spectrum in Fig. 5(d) has shown overlapped near band edge emission in the two polarization correspond to the DX1 peaks in experimental

spectrum. This result indicates that there are some strain relaxation mechanisms existing along the direction of [1120], which may be related to the defects [26].

6. Conclusion

In conclusion, through temperature- and polarization-dependent PL spectra with polarization degree analysis, we investigated the origins of each emission and related the emission prop-erties to the VBs reordering of m-plane ZnO grown on (112)LAO under tensile strain. The subband polarized in [0001] in the tensile-strained ZnO m-plane film is found to upper shift to the position of the subband polarized in [1120], leading to the abnormal low optical anisotropy ð
¼ 10%Þ of near band edge emission of the m-plane ZnO film grown on (112)LAO substrate. This study shows that to retain the desired optical anisotropy of emission, the strain in film must be taken into serious consideration.

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