ZnO Heterostructures Grown by Molecular

Article pubs.acs.org/crystal

Study of Nonpolar GaN/ZnO Heterostructures Grown by Molecular Beam Epitaxy Chiao-Yun Chang,† Huei-Min Huang,† Yu-Pin Lan,† Tien-Chang Lu,*,† Li-Wei Tu,‡ and Wen-Feng Hsieh† †

Department of Photonics, National Chiao Tung University, Hsinchu 30050, Taiwan Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan

‡

ABSTRACT: The growth mechanism and characteristics of the nonpolar GaN/ZnO heterostructure grown on the r-plane sapphire substrate by using molecular beam epitaxy were studied. The crystal interaction between GaN and ZnO epitaxial layers was clarified by using transmission electron microscopy and X-ray diffraction. A new epitaxial relationship of ZnGa2O4 (220)//GaN (101̅3̅) in the normal surface direction was obtained in the GaN/ZnO heterostructure. It was believed that the formation of ZnGa2O4 (220) was due to the recrystallization of the ZnO layer with Ga atoms, which in turn resulted in the formation of semipolar-oriented GaN. In addition, the main optical transition in the GaN/ZnO heterostructure was attributed to the existence of the interface states and new ZnO:(Ga,N) alloys.

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INTRODUCTION GaN-based wide-bandgap semiconductors have attracted much attention in the application of optoelectronic devices.1−3 However, owing to the lattice mismatch between the epitaxial layer and substrate, the growth of GaN-based materials is usually accompanied by a high-defect density to limit the device performance. Therefore, selection of a suitable substrate for GaN-based optoelectronic devices will play a significant role. ZnO was regarded as a candidate for the substrate because of similar physical properties and amenability to conventional chemical wet etching. In particular, the lattice mismatches between the wurtzite GaN and ZnO are only 1.9% along the caxis direction and 0.4% along the a-axis direction, respectively. It indeed has the potential to achieve the high quality GaN epitaxial layer. The related growth method and characteristics have been widely studied, and the heterostructures consisted of GaN and ZnO were also extensively applied.4−7 However, the conventional GaN-based devices always suffered from the internal electric field effects along the c-axis direction and caused spatial separation of electron and hole wave functions that in turn gave rise to the restriction of carrier recombination efficiency. Thus, the semi/nonpolar-oriented GaN-based epitaxial layer and heterostructure grown on the r-plane sapphire substrate,8 γ-LiAlO2,9 and (100) MgAlO210 have been demonstrated to effectively improve the carrier recombination efficiency and to reduce the polarization effects. At present, the growth condition and the related optical and structural properties in the semi/nonpolar GaN/ZnO heterostructure were not yet clarified. In our previous work,11 we have demonstrated that the GaN/ZnO heterostructure grown on the © 2013 American Chemical Society

a-plane GaN template showed the intermediate phase at the interface, due to volatility issues at high temperatures. The intermediate phase appeared even if the growth temperature was relatively low. Therefore, in this work we made efforts to clarify the interaction between GaN and ZnO layers and its influences and characteristics.

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EXPERIMENTAL SECTION

The GaN/ZnO heterostructure was grown by the molecular beam epitaxy (MBE) system. First, the 2.0 μm thick a-plane GaN template was grown on the r-plane sapphire using the metal organic chemical vapor deposition (MOCVD). This was followed by deposition of an aplane ZnO film of about 400 nm thickness at 520 °C using pulsed laser deposition. Then, the GaN epitaxial layer of nominal 200 nm thickness was grown by MBE at 620 °C of the substrate temperature. After the growth, the structural variation and surface morphology for the GaN/ ZnO heterostructure were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), as shown in Figure 1. The thickness of the ZnO epitaxial layer was unexpectedly diminished from 400 to 150 nm. Some air−voids between the GaN and ZnO interface appeared, and the surface morphology variation of the GaN thin films was observed to be full of GaN grains. Since we have known that the growth temperature of the GaN epitaxial layer was higher than that of the ZnO epitaxial layer and the single-step growth was used without any buffer layer, the decomposition of the underneath ZnO epitaxial layer could be redecomposed and the Ga and N atoms could interact with the Zn and O atoms. Therefore, we used the high-resolution-X-ray diffraction (HR-XRD) and transReceived: April 5, 2013 Revised: June 11, 2013 Published: June 13, 2013 3098

dx.doi.org/10.1021/cg400497r | Cryst. Growth Des. 2013, 13, 3098−3102

Crystal Growth & Design

Article

Figure 1. Cross-sectional SEM images. (a) The epitaxial a-plane ZnO template before growing the GaN film. (b) The GaN epitaxial layer was regrown on the ZnO template to form the GaN/ZnO heterostructure. The thickness reduction of the ZnO template during the regrowth process could be clearly observed by comparing two SEM images of (a) and (b) with the same magnification rate. (c) The plane-view SEM image and (d) the AFM image of the GaN/ZnO heterostructure for surface morphology observation. mission electron microscopy (TEM) system to determine the related crystalline orientations and interface characteristics for the GaN/ZnO heterostructure. In addition, the optical properties of the GaN/ZnO heterostructure were discussed using the photoluminescence (PL) measurement and time-resolved photoluminescence (TRPL) measurement. The PL spectrum of samples was measured using the excitation source of the He−Cd laser (325 nm) with 5 mW, and the TRPL was performed using the Ti-sapphire pulse laser (266 nm) with 5 mW.

acceptor-pair (DAP) transitions or free-to-bound transitions (A°X) in the ZnO epitaxial layer. Moreover, the incorporation of elements of gallium (Ga) and nitrogen (N) could be treated as the external acceptor states to induce the p-type ZnO epitaxial layer, due to the heterovalent interface interaction. On the other hand, the GaN/ZnO heterointerface of the type-II band configuration generated some interface states, such as the excess or deficit charge at the heterovalent interface, which would affect the initial few-monolayer growth and optical transitions.16,17 In accordance with these research results, the emission peak at 3.22 eV could be assigned to the interface states formed with ZnO:(Ga,N) alloys. From the use of the TRPL at 10 K, the carrier lifetime of the emission peak energy of 3.22 eV was estimated to be approximately 440 ps and was longer than that of GaN NBE (∼270 ps), as shown in Figure 2b. It implied that the interface states of the type-II GaN/ZnO heterostructure could trap parts of carriers transitioning from higher to lower levels and elongate the carrier lifetime. In addition, the related carrier localization phenomenon due to the carriers trapped in the GaN/ZnO heterostructure was observed and discussed from the S-shaped energy shift in the temperature-dependent PL measurement previously.11 To clarify the interfacial phase formation in the GaN/ZnO heterostructure, the structural characteristics and the related variations during the growth of the GaN layer were further investigated. Figure 3 shows the on-axis and off-axis XRD scans for the GaN/ZnO heterostructure, confirming the surface orientations and the crystallographic relationship. The simple diffraction peaks of (112̅0) Al2O3 or sapphire, (112̅0) GaN, and

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RESULTS AND DISCUSSION We carried out the PL measurement after each growth procedure to study the related optical transitions. The PL spectra of the GaN/ZnO heterostructure, the ZnO layer on the GaN template, and the GaN template have been measured at T = 300 K and are shown in Figure 2a. It was previously demonstrated that the near-band-edge (NBE) emissions of GaN and ZnO were located at about 3.42 and 3.28 eV, respectively, which were identical to the results shown in our GaN template and the ZnO layer on the GaN template. After the GaN was grown on the ZnO layer, the GaN/ZnO heterostructure revealed two main emission peaks as shown in Figure 2a, and the emission energy at 3.42 eV was attributed to the GaN NBE emission. However, the other emission peak at 3.22 eV dominated the PL spectrum; the peak energy was smaller than the emission energy of ZnO by approximately 60 meV. In the previous literature, several studies were devoted to the discussion of the interaction between GaN and ZnO via the analysis of luminescence spectrum.12−15 The ultraviolet (UV) emission around 3.20 eV was related to the shallow donor− 3099



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reflection angle corresponding to 2θ, ∼64.5° was observed, which could have originated from two possibilities, such as the Ga2O3-related reflections and the wurtzite GaN (101̅3̅) reflection.19−21 Due to the possible diffusion effects of the ZnO layer during the regrowth of the GaN epitaxial layer, the formation of additional alloys at the GaN/ZnO interface led to the other reflection generation. Combining the results of 2θ scans and the observation of SEM images, it could be suggested that the ionized Zn and O diffused from the ZnO template may react with Ga and N to form the intermediate phases at the GaN/ZnO heterointerface. The crystalline ZnGa2O4 spinel structure was the cubic symmetry, which changes the hexagonal stacking ···ABAB··· in ZnO to the cubic stacking ···ABCABC··· in ZnGa2O4; then, the hexagonal stacking of GaN was deposited on the cubic stacking of ZnGa2O4. Interestingly, the coexistance of the oxides of the spinel structure and nitrides of the semipolar orientation was not accidental. In accordance with the investigations in previous literature, the crystallographic relationship between wurtzite semipolar nitrides and spinel oxides have been proposed.20,22 Baker et al. indicated that the planar (101̅3̅) GaN film could be grown on the (110) spinel structure, and its XRD 2θ scan for this orientation revealed that the GaN (101̅3̅) plane was normal to the surface and parallel to the spinel (220) and (440) planes. The angle between the (1013̅ )̅ and the (0001) planes in GaN is 32.0° along the [11̅00] m axis. The in-plane epitaxial relationship was [303̅2̅]GaN∥[001]spinel and [12̅10]GaN∥[1̅10]spinel. Therefore, it could be believed that the reflection angle corresponding to 2θ ∼ 64.5° should be associated with the wurtzite (101̅3̅) GaN as a result of the existence of the crystalline ZnGa2O4 spinel structure. However, the PL spectrum of the GaN/ZnO heterostructure did not clearly exhibit the emission peak of ZnGa2O4. It could be due to the fact that the energy of excitation source was lower than the absorption edge of spinel ZnGa2O4 at the 4.7 eV.23,24 Besides, the off-axis φ scans of the GaN/ZnO heterostructure exhibited the in-plane epitaxial relationship, as shown in Figure 3b and the formation of the wurtzite (101̅3̅) GaN in the GaN/ZnO heterostructure could be further identified. The spinel ZnGa2O4 layers at the GaN/

Figure 2. (a) Room temperature photoluminescence spectra of the epitaxial GaN template, the ZnO layer on the GaN template, and the GaN/ZnO heterostructure with blue triangle, red square, and black circle points, respectively. (b) Time-resolved photoluminescence spectra for two main emission peaks appearing in the epitaxial GaN/ZnO heterostructure were measured at a low temperature of 10 K. The blue solid line exhibited the exponential fitted curve to indicate the carrier decay time.

(112̅0) ZnO can be observed from the ZnO layer on the GaN template sample from the bottom XRD curve of Figure 3a. In contrast, from the top XRD curve of Figure 3a, it was clearly seen that some XRD diffraction peaks were affected after the regrowth process. The reflection angles of 2θ detected at approximately 30.3° and 63.1° could be attributed to the crystalline (220) and (440) ZnGa2O4 spinel structure.18 The

Figure 3. (a) XRD 2θ−ω scans of the ZnO layer on the GaN template (black line) and the GaN/ZnO heterostructure (red line) to identify the surface orientation and the crystalline quality. (b) XRD φ scans of the GaN/ZnO heterostructure revealed the in-plane epitaxial relationship. 3100



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between GaN and ZnO not only influenced the optical transition but also resulted in the additional crystalline structures, mainly due to the growth condition difference between GaN and ZnO.

ZnO heterointerface created two crystalline domains rather than the single crystalline distribution. The [100] direction of spinel ZnGa2O4 layers differed from the [11̅00] of the GaN template, approximately 55° in the in-plane epitaxial relationship, which could also affect the following GaN epitaxial layer growth on top of it and cause the rough surface morphology shown in Figure 1. Figure 4 shows the TEM images, the energy-dispersive spectrometer (EDS), and the selected area diffraction (SAD) to

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CONCLUSION In summary, the optical and structural characteristics of the GaN/ZnO heterostructures were influenced by the interaction between GaN and ZnO due to the existence of the interface states and additional ZnO:GaN alloys. The new spinel ZnGa2O4 and semipolar orientation GaN formed at the interface of the GaN/ZnO were verified by the XRD and TEM measurements. The formation of the interface states induced the lower transition energy and longer carrier lifetime in the GaN/ZnO heterostructure. Our experiment results clearly described the growth mechanism and the optical properties of nonpolar ZnO/GaN heterostructures, which provides useful information for future heteroepitaxial growth of oxide and nitride materials.

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AUTHOR INFORMATION

Corresponding Author

*Author to whom correspondence should be addressed; Tel: 886-3-571-2121 ext.: 31234; fax: 886-3-571-6631; E-mail: [email protected]. Figure 4. (a) Bright-field TEM images of the GaN/ZnO heterostructure. (b) The EDS mapping image for ZnGa2O4. (c−f) The SAD patterns for different regions of the GaN/ZnO heterointerface.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thanks Professor H. C. Kuo and Professor S. C. Wang in the Department of Photonics of National Chiao Tung University for their great technical support. This work was supported by the MOE ATU program and in part by the National Science Council of the Republic of China (ROC) in Taiwan under Grants NSC-99-2120-M-009-007 and NSC 992221-E-009-035-MY3.

clarify the structural variation of the GaN/ZnO heterointerface. Bright-field TEM images were taken along the [112̅0] zone axis of the sapphire substrate, as shown in Figure 4a, and the elemental mapping of ZnGa2O4 was measured using EDS with the TEM image as shown in Figure 4b. The SAD patterns taken from the different regions in the GaN/ZnO heterointerface were shown in Figure 4 (panels c−f), respectively. On the basis of the diffraction patterns, the orientation relationship between the GaN template and the sapphire substrate is usually seen in the case of the a-plane GaN epitaxial layer grown on an r-plane sapphire.25,26 The original wurtzite a-plane ZnO epitaxial layer indeed has been transformed into the spinel ZnGa2O4 to induce the emergence of the semipolar orientation GaN, and the crystallographic relationship between GaN and ZnGa2O4 was determined as (110)znGa2o4∥(101̅3̅)GaN. Because the formation of intermediate spinel ZnGa 2 O 4 was partly distributed in the ZnO film, the semipolar orientation GaN (1013̅ )̅ and original nonpolar orientation GaN (1120̅ ) were hybridized among the GaN/ZnO film interface. Although the intermediate spinel ZnGa2O4 twisted the partial crystal orientation of GaN, the original nonpolar orientation GaN (1120̅ ) still dominated on the topmost GaN film. These results well-confirmed the XRD measurements shown in the previous figures. However, we found that the topmost epitaxial GaN layer revealed multiple crystalline orientations, which consisted of the (1120̅ ) GaN near the sample surface and (1013̅ )̅ GaN near the interface of the GaN/ZnO heterostructure. It indicated that the a-plane ZnO epitaxial layer should partly be transformed into the spinel ZnGa2O4. The new spinel ZnGa2O4 and semipolar orientation GaN were formed around the interface between GaN and ZnO. However, this result indeed seriously affected the optical and structural characteristics of the GaN/ZnO heterostructure. Such interaction

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REFERENCES

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ZnO Heterostructures Grown by Molecular

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