Photoluminescence associated with basal stacking faults in c-plane ZnO epitaxial film
grown by atomic layer deposition
S. Yang, C. C. Kuo, W.-R. Liu, B. H. Lin, H.-C. Hsu, C.-H. Hsu, and W. F. Hsieh
Citation: Applied Physics Letters 100, 101907 (2012); doi: 10.1063/1.3692730 View online: http://dx.doi.org/10.1063/1.3692730
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/10?ver=pdfcov Published by the AIP Publishing
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Photoluminescence associated with basal stacking faults in c-plane ZnO
epitaxial film grown by atomic layer deposition
S. Yang,1C. C. Kuo,1W.-R. Liu,2B. H. Lin,1,2H.-C. Hsu,3C.-H. Hsu,1,2,a)and W. F. Hsieh1,a)
1
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
2
Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
3
Institute of Electro-Optical Science and Engineering and Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan
(Received 21 January 2012; accepted 20 February 2012; published online 8 March 2012)
Basal plane stacking faults (BSFs) with density of1 106cm1 are identified as the dominant
defect in the annealed ZnO thin films grown onc-plane sapphire by atomic layer deposition. The dominant peak centered at 3.321 eV in low-temperature photoluminescence measurements is attributed to the emission from the BSFs. The emission mechanism is considered to be the confined indirect excitons in the region of quantum-well-like structure formed by the BSFs. The observed energy shift of 19 meV with respect to the BSF-bounded exciton at low temperature may be caused by the localization effect associated with the coupling between BSF quantum wells. VC 2012
American Institute of Physics. [http://dx.doi.org/10.1063/1.3692730]
ZnO, a wide direct band gap material, has attracted much attention for its applications in ultraviolet photonic devices of which the performance is strongly influenced by the structural defects such as impurities, dislocations, and stacking faults. Photoluminescence (PL) has been widely used to study the influence of the defects on the optical prop-erties of ZnO epitaxial films. Many defects associated emis-sions, including donor bound exciton (D0X), acceptor bound exciton (A0X), donor acceptor pairs (DAPs), and emission originated from edge dislocations have been identified.1–6 Various deposition methods, such as metal-organic chemical vapor deposition,1 pulsed laser deposition (PLD),6–10 and molecular beam expitaxy,3,4have been employed to fabricate high quality ZnO epi-films. Recently, atomic layer deposi-tion (ALD), which possesses the advantages of atomic-level thickness control, high uniformity, and low growth tempera-ture, has also been employed to grow ZnO epitaxial films.11–15In our previous work,11c-plane ZnO films grown at 200C by ALD on c-plane sapphire exhibited a non-twisted in-plane orientation, i.e.,f1120gZnOjjf1120gsapphire,
with a large lattice mismatch of31.8%.7Basal-plane stack-ing faults (BSF) are identified as the dominant defects in the ALD-grownc-plane ZnO layers by transmission electron mi-croscopy (TEM) analysis. Its density (1.0 106cm1) is comparable to what observed in many non-polar ZnO epi-films and significantly higher than what is typically found in PLD-grownc-plane ZnO films.16–18BSFs in the materials of wurtzite structure, such as GaN and ZnO, can be considered as a sheet of zinc blend structure embedded in the wurtzite structure and is expected to strongly affect the PL spectrum.17–21 However, the exact influence of the BSFs to ZnO luminescence and the associated emission mechanism have rarely been reported. In this work, we performed power-dependent PL at 10 K and temperature-dependent PL
on the annealed c-plane ZnO films deposited by ALD on c-plane sapphire to examine the nature of the BSF emissions. Thec-plane ZnO epitaxial films were grown on c-plane sapphire substrates by ALD. Diethylzinc (DEZn) with chem-ical formula of Zn(C2H5)2and de-ionized (DI) water of 18
MX cm were adopted as the zinc and oxygen precursors, respectively. Each growth cycle consists of precursor expo-sures and N2 purge following the sequence of DEZn/N2/
H2O/N2with corresponding duration of 5 s/15 s/5 s/15 s. The
substrate temperature was maintained at 200C under the vacuum of 1–2 Torr during the deposition. This procedure was repeated 200 cycles, yielding a ZnO layer of 100 unit cells along the growth direction, i.e., about 52 nm thick. The as-deposited samples were then annealed at 800C for 1.5 h in pure oxygen gas at 1 atm.
Cross sectional TEM specimens were prepared by focused ion beam (FIB), and the TEM images were taken with a Philips TECNAI-20 field emission gun type TEM. The PL measurements were carried out in a closed cycle cryogenic system using a He-Cd laser at 325 nm as the exci-tation source. The emission was conducted into a spectrome-ter (TRIAX 320) equipped with a photo-multiplier tube.
A cross sectional TEM image recorded along the ½1210ZnO zone axis is shown in Fig. 1(a). The associated
selected area electron diffraction (SAED) pattern displayed in Fig. 1(b) confirms the single crystalline of the ZnO thin film. Many lateral lines are observed in the cross-sectional image in Fig. 1(a). These lines are also visible in the dark-field (DF) image with diffraction vector g equal toð1012ÞZnO
in Fig. 1(c), but are invisible in the DF image with g¼ ð0002ÞZnO in Fig.1(d). Based on the extinction rules11 and the visibility of these lateral contrast lines with various diffraction vectors, these lines were identified to be intrinsic type BSF (I1or I2) (Ref.22) whose density is estimated to be
about 1 106cm1.
To examine the optical properties of the ZnO layers, we performed low temperature (10 K) PL measurements. The PL spectra are illustrated in Fig. 2(a). For comparison, a
a)Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected].
spectrum taken from a 200 nm thick PLD-grown c-plane ZnO film with the BSF density about 4 105cm1is plotted in Fig.2(b). The most intense peak of the PLD-grown sam-ple is the near band edge (NBE) emission originating from D0X and free-exciton (FX). The other pronounced peaks centered at3.330 eV is attributed to the two-electron satel-lite (TES) and/or the electrons bound by the stacking faults,23and at the lower energy side, a weak peak marked as “FX-1LO” is the typical longitudinal optical phonon replicas of the FX emission.8 The two peaks centered at 3.23 eV were assigned to DAP and free-electron to acceptor (eA0) emissions, respectively. In contrast, the spectrum of the ALD grown ZnO shows distinct features. Three peaks, (1) NBE emission at3.37 eV, (2) BSF emission at 3.321 eV,
and (3) “LO” emission at 3.28–3.29 eV, were observed. Because the much larger density of BSFs is the dominant dif-ference in structural properties between the ALD- and PLD-grown ZnO films, the observed spectral difference in Fig. 2
may be attributed to the high density of BSFs. To verify this argument, we conducted the following studies.
Figure3(a)displays the PL spectra of ALD-grown ZnO taken at temperatures between 10 and 280 K. The NBE emis-sion has a rather large line width (FWHM 22 meV) as compared with those of the PLD-grown ZnO films (FWHM 10 meV and typical 9–15 meV).7,8The broad spectra could be caused by defects which reduce the exciton lifetime and thus broaden the FWHM of the exciton transition.10 Such broadening increases the difficulty to distinguish D0X from FX emission in the NBE emission. Peak energies of NBE (including both D0X and FX), BSF, and LO emissions are plotted as a function of temperature in Fig. 3(b). These peak energies decrease monotonically with increasing tem-perature that can be fitted by the Varshni’s formula, E(T)¼ E(0) aT2/(Tþ HD), where E(0) is the energy at
T¼ 0 K, a is a fitting parameter and HDdenotes the Debye
temperature, which is set to 920 K according to Refs.24and
25. The best fit of the NBE emission energy for temperatures above 100 K, depicted by the dashed curve in Fig. 3(a), yields E(0)¼ 3.374 eV and a ¼ 1.4 meV/K, which agree well
FIG. 1. A cross-section TEM image (a) and a selected area electron diffrac-tion pattern (b) taken along the½1210ZnOzone axis of the ALD grown ZnO
layer. The dark field images with diffraction vector g set toð1012ÞZnO(c) andð0002ÞZnO(d), respectively.
FIG. 2. Low temperature PL spectra taken at 10 K of the ZnO films grown by (a) ALD and (b) PLD methods. The ALD grown ZnO with 50 nm thick-ness was deposited at 200C and annealed at 800C in oxygen for 1.5 h.
The PLD grown ZnO with 200 nm thickness was deposited onc-plane sap-phire at growth temperature of 600C.
FIG. 3. (Color online) (a) Temperature dependent PL spectra of the ZnO film taken between 10 and 280 K. (b) The energy versus temperature plot of the BSF and NBE emissions. The dashed lines depict the fitting results to the Varshni’s law.
with the FX emission of wurtizte ZnO.24–26The deviation of the experimental data from the fitting curve for temperatures below 100 K is attributed to the contribution of D0X, which dominates over FX at low temperatures. The weak LO band emission at 3.28–3.29 eV, whose peak position varies com-plying with the Varshni’s formula, is the typical longitudinal optical phonon replicas of the D0X emission with the phonon energy about 72 meV. The energy of the BSF emission exhibits a different behavior as a function of temperature; it progressively blue shifts from 3.321 to 3.331 eV as the tem-perature increases from 10 to 88 K and then red shifts with further temperature increase. Fitting the peak energy of the BSF emission above 88 K by the Varshni’s formula yields E(0)¼ 3.34 eV, which gives a blue shift of 19 meV with respect to the measured 3.321 eV at 10 K. This phenomenon indicates that the BSF emission could be associated with an exciton transition with energy of 3.34 eV and coupled with a local trap with 19 meV trapping energy.
The emission at3.321 eV has also been assigned to the DAP transition in ZnO with intentionally doped acceptors, such as N, P, As, and Sb.2–5To clarify the nature of the BSF emission in our ZnO epitaxial films, we performed power de-pendent PL measurements at 10 K; the spectra are plotted in Fig.4. The intensities of the NBE and BSF emissions as a function of excitation powerP are depicted in the upper-left inset. The peak energy of the BSF emission remains un-shifted even though its intensity rises with increasing excita-tion power. For both the NBE and BSF emissions, their curves of emission intensity versus excitation power nicely follow a power law:I ! Pawith the exponents a¼ 1.31 and 1.37, respectively. For a free exciton or bound-exciton emis-sions, the value of the exponent a should fall in the range of 1 5 a 5 2, but for the DAP transitions, a should be less than 1.9,27,28 The obtained exponent 1.37 of the BSF emission excludes the mechanism of DAP transition. Together with the absence of peak energy shift with excitation power, the results reasonably infer the BSF emission is associated with excitonic transition.
From the point of atomic stacking sequence, a I1and I2
type BSF in wurtzite structure can be considered as a thin layer of zinc blend structure with a thickness of, respectively, 1.5c (0.78 nm) and 2c (1.04 nm), where c is the lattice param-eter along the c-axis, sandwiched by the wurtzite barriers. According to the ab initio calculation,16 the BSF forms a quantum-well (QW)-like region with negative band offsets in the conduction band minimum (CBM) and the valance band maximum (VBM) with respect to those of the wurtzite barriers. This means that the BSF structure would act as a potential well in the conduction band and a potential barrier in the valence band, and the simple band model is schema-tized in the upper-right inset of Fig. 4.21 Therefore, the obtained E(0) of 3.34 eV by fitting the BSF emission could result from the recombination of the confined indirect exci-tons in BSF (BSF-EX), which were composed of the elec-trons captured in the potential wells of the BSF and the holes confined at the interface of BSF and wurtzite structure via attractive Coulomb interaction.29 The 19 meV red shift of the BSF emission from 3.34 eV at temperature about 10 K could be ascribed to the localization effect applied onto the BSF-EX. The localization effect could be attributed to the extrinsic donors (which would bind the BSF-EX) formed by the point defects in or in the vicinity of the QW structures. Moreover, the multiple BSFs could be considered as a coupled QWs structure in which the coupling effect of the electron wave function may be responsible for the localization effect. To consider the reasonableness, the probability density of the electron wave function in single QW (BSF) is estimated about 10% at a penetration length of 3 nm by using a single QW structure with the dimension and potential heights derived from the ab initio calculation.16As illustrated in Fig.1, the BSF are distributed in the entire ZnO film with a separation of 2–10 nm along the growth direction (c-axis). They could form coupled QWs structure and lead to the localization effect. Similar phenom-enon of the localization of excitons have been reported in the BSF in GaN epi-layers17–20 and also observed in the alloy materials such as InGaN,30AlGaN,31GaInNAs/GaAs single QW,32and InGaN/GaN multiple QWs.33
High density (1.0 106cm1) of BSF is identified to be the dominant structure defect in the annealed ZnO epitax-ial films grown by ALD on c-plane sapphire as verified by TEM and XRD measurements. Each BSF in ZnO is com-posed of a thin layer of zinc blend structure sandwiched by the wurtzite barriers and forms a type-II QW. The dominant emission centered at 3.321 eV in PL spectra is ascribed to the transition associated with the BSF bounded indirect exci-tons (3.34 eV) which could be trapped by the local defects and/or the potential induced bundled BSF QWs at low temperatures.
This work is partly supported by National Science Council of Taiwan under Grant Nos. NSC-99-2112-M-006-017-MY3, 2112-M-213-002-MY3, and NSC-100-2112-M-006-002-MY3.
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