Enhancement of Two-Dimensional Electron-Gas Properties by Zn Polar ZnMgO/MgO/ZnO Structure Grown by Radical-Source Laser Molecular Beam Epitaxy

Meng, Li; Zhang, Jingwen; Li, Qun; Hou, Xun

doi:https://doi.org/10.1155/2015/694234

Journal of Nanomaterials

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Wide-Bandgap Semiconductors: Nanostructures, Defects, and Applications

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Volume 2015 | Article ID 694234 | https://doi.org/10.1155/2015/694234

Enhancement of Two-Dimensional Electron-Gas Properties by Zn Polar ZnMgO/MgO/ZnO Structure Grown by Radical-Source Laser Molecular Beam Epitaxy

Li Meng,^1,2,3Jingwen Zhang,^1,2,3Qun Li,^1,2,3and Xun Hou¹

Academic Editor: Meiyong Liao

Received19 Jan 2015

Accepted09 Mar 2015

Published25 Oct 2015

Abstract

A Zn polar ZnMgO/MgO/ZnO structure with low Mg composition layer (x = 0.05) grown on a-plane (11–20) sapphire by radical-source laser molecular beam epitaxy was reported. The insertion of a thin (1 nm) MgO layer between ZnMgO and ZnO layers in the ZnMgO/ZnO 2DEG structures results in an increase of 2DEG sheet density and affects electron mobility slightly. The carrier concentration reached a value as high as 1.1 × 10¹³ cm⁻², which was confirmed by C-V measurements. A high Hall mobility of 3090 cm²/Vs at 10 K and 332 cm²/Vs at RT was observed from Zn_0.95Mg_0.05O/MgO/ZnO heterostructure. The choice of the thickness of MgO was discussed. The dependence of carrier sheet density of 2DEG on ZnMgO layer thickness was calculated in theory and the theoretical prediction and experimental results agreed well.

1. Introduction

ZnO and its heterostructures, which have several advantages including a high saturation velocity [1], a large conduction band offset for ZnMgO/ZnO heterostructures [2], and the possibility to form a high-density two-dimensional electron-gas (2DEG) [3], have great potential for high-frequency and high-power device applications. So far, the formation of 2DEG at Zn polar interface has been observed by a few groups using molecular beam epitaxy (MBE) [4–8], pulse laser deposition (PLD) techniques [9], metal-organic vapor phase epitaxy (MOVPE) [10], and RF sputtering [11]. However, in low Mg composition heterostructures () high electron mobility can be observed but with very low 2DEG sheet density ( cm⁻²) [12], and in high Mg composition heterostructures (), 2DEG sheet density reached a considerable value (10¹²~10¹³ cm⁻²) but electron mobility is still deeply affected by alloy disorder scattering, especially at low temperatures. In addition the obtained mobility in previous papers was lower than 250 cm²/Vs at RT [13]. It has been reported that modified AlGaN/AlN/GaN structures, which employ a thin AlN interfacial layer between AlGaN and GaN layers, show higher 2DEG properties than those of conventional AlGaN/GaN structures. This is reported to be a result of the reduction of alloy disorder scattering due to the suppression of carrier penetration from the GaN channel into the AlGaN layer [14–17]. However, the inserting of MgO into ZnMgO/ZnO structure has never been reported. In this work, we report a Zn polar ZnMgO/MgO/ZnO structure to enhance two-dimensional electron-gas properties and discuss the dependence of carrier sheet density of 2DEG on ZnMgO layer thickness which was calculated in theory and the theoretical prediction and experimental results agreed well.

2. Materials and Methods

Zn polar ZnMgO/ZnO and ZnMgO/MgO/ZnO heterostructures were all grown on sapphire (11–20) substrates by radical-source laser molecular beam epitaxy (RS-LMBE) system (Shenyang Scientific Instrument Co., Ltd., Chinese Academy of Sciences (SKY)). The 5 N purity of ZnO target was vaporized by KrF excimer laser (Lambda Physik, COMPex 102, 248 nm, 1–20 Hz, 100 mJ). At first, the substrates were treated by nitrogen plasma, which was ionized by radio-frequency (rf) plasma source (Oxford Applied Research, HD-25) at 700°C for 1 h to obtain nitrogen polarity surface and control the growth of single-domain Zn polar ZnO film [18]. The growth was conducted in an oxygen pressure of 10⁻³Pa. A 20 nm thick low temperature- (LT-) ZnO buffer layer was deposited at 250°C. Secondly, a 200 nm undoped ZnO layer was deposited at 700°C. Finally, an undoped ZnMgO layer was grown at 400°C. For ZnMgO/MgO/ZnO heterostructures (Figure 1(a)), the MgO was deposited before the growth of ZnMgO layer under same condition with ZnMgO layer. The crystalline qualities of the thin films were studied by Philips X’Pert PW3040 high resolution X-ray diffraction (XRD) system using Cu Kα ( nm). The growth evolution of MgO layer and the choice of its thickness were investigated by the streaky patterns of the reflection high-energy electron diffraction (RHEED). The Mg composition () was determined from the reflectance measurement of the exciton band gap energy of using the equation [19]. The crystal polarity was determined based on differences in etching-rate between Zn polar and O-polar samples. Chemical wet etching was carried out using 0.01 M hydrochloric acid solution at room temperature, for etching-rate measurements [20]. The capacitance-voltage (C-V) measurement was performed by using mercury contacts. The electrical properties were examined by Lake Shore 7707A Hall mobility system in a van der Pauw configuration with a magnetic-field of 1000 G.

(a)

(b)

3. Results and Discussion

3.1. The Thickness of the MgO Insert Layer

To optimize the MgO thickness and make sure that the MgO has grown as wurtzite structure, the growth evolution of MgO layer was investigated by RHEED pattern, shown as in Figure 2. Initially, the MgO layer grew 2-dimensionally on c-ZnO as the thickness was 0.5 and 1 nm in Figures 2(a) and 2(b). Then the growth mode of MgO layer changes from 2-dimensional to 3-dimensional. When the MgO thickness was 1.5 nm, the RHEED spots appeared (Figure 2(c)), which suggests that the crystal structure of the MgO layer changes with increasing layer thickness. Therefore, in this paper, a 1 nm MgO layer was chosen.

(a)

(b)

(c)

3.2. Structural Property

Figure 3 shows the XRD spectra for the grown ZnMgO/MgO/ZnO structure on sapphire substrate. Because of the low concentration of Mg in the ZnMgO layer, the ZnO (0002) peak and ZnMgO (0002) are overlapped. The crystal quality of the Zn_0.95Mg_0.05O/MgO/ZnO structure was characterized by a cross-sectional TEM image (JEM-2100F). A typical image recorded near the ZnMgO/MgO/ZnO interface is shown in the inset of Figure 3. The interface could not be detected in the TEM micrograph clearly, which suggests that the structure has a high degree of crystalline quality. The MgO has no phase transition from wurtzite structure to rock salt structure. High electron mobility will benefit from the enhanced structure and crystal quality. It also can be observed that the MgO interfacial layer is grown with a thickness of approximately 1 nm, which corresponds to the designed thickness.

3.3. Electrical Property

Figure 4 displays carrier density derived from capacitance as a function of distance from the top surface for the Zn_0.95Mg_0.05O/ZnO and Zn_0.95Mg_0.05O/MgO/ZnO structures, and both of the thicknesses of Zn_0.95Mg_0.05O layers are 100 nm. It shows that high concentration 2DEG was confined at the interface. Formation of 2DEG at the interface was demonstrated by the features. Comparing with Zn_0.95Mg_0.05O/ZnO structure, a sharp 2DEG peak is shown around interface in Zn_0.95Mg_0.05O/MgO/ZnO structure without penetration into the layer, which behaves similarly with previous report about AlGaN/AlN/GaN [14]. From these results, it can be concluded that the thin MgO interfacial layer effectively suppresses carrier penetration into the ZnMgO layer and enhances the confinement of 2DEG in ZnO channel. In addition, a sheet carrier concentration of 1.1 × 10¹³ cm⁻² was observed in the Zn_0.95Mg_0.05O/MgO/ZnO structure with 20 nm thick Zn_0.95Mg_0.05O layer confirmed by C-V measurement. 2DEG of Zn_0.95Mg_0.05O/ZnO structure with 20 nm thick Zn_0.95Mg_0.05O layer was not observed by C-V. Owing to the low Mg content and thickness of ZnMgO barrier layer, the conduction band offset was small and the providing of electron was not enough to format 2DEG.

Table 1 shows the typical values of Hall mobility and 2DEG density () measured at 300 K and 10 K for ZnMgO/ZnO structures with or without MgO interfacial layers. From Table 1, it is clearly seen that the Hall mobility was increased by the insertion of MgO interfacial layers. In particular, the Zn_0.95Mg_0.05O (20 nm)/MgO/ZnO heterostructure showed very high Hall mobility of 332 cm²/Vs at RT and 3090 cm²/Vs at 10 K. Figure 5 shows the results of temperature-dependent Hall measurements. The electron mobility of Zn_0.95Mg_0.05O (100 nm)/ZnO and Zn_0.95Mg_0.05O (100 nm and 20 nm)/MgO/ZnO structures increases with decreasing temperature, as shown in Figure 5(a). This trend is nearly identical to that reported for AlGaAs/GaAs [21–23], AlGaN/GaN [24, 25], and ZnMgO/ZnO heterostructure [5, 13], which is consistent with the existence of a 2DEG at the heterointerface. Compared with reported values for both Zn polar and O-polar ZnMg(Mn)O/ZnO heterostructures, the high mobility is obvious [11, 13, 26, 27]. The mobility of Zn_0.95Mg_0.05O (20 nm)/ZnO structure changed similarly to a single ZnO thin film as the temperature was changing, which indicates no 2DEG was formatted because of thin barrier layer. By the insertion of MgO layer, a 2DEG was observed. The results agree well with the results of C-V measurement. The high electron mobility of the ZnMgO/MgO/ZnO heterostructure was mainly attributed to the reduction of alloy disorder scattering. In Figure 5(b), the sheet carrier concentration of Zn_0.95Mg_0.05O (100 nm)/ZnO and Zn_0.95Mg_0.05O (100 nm and 20 nm)/MgO/ZnO structures changes little with increasing temperatures, indicating the good confinement of channel electrons. The insert of a thin (1 nm) MgO layer between ZnMgO and ZnO enhanced the sheet carrier concentration almost one order of magnitude. It also confirms the thin MgO enhanced the confinement effectively.

(a)

(b)

3.4. The Calculation and Experimental Results of the Dependence of Carrier Sheet Density of 2DEG on ZnMgO Layer Thickness

It is noted that the presence of strong polarization-induced fields in both MgO and ZnMgO cap layers leads to a very interesting dependence of the 2DEG sheet density in ZnMgO/MgO/ZnO structures on ZnMgO cap thickness. The density of 2DEG decreases with increasing ZnMgO thickness. To discuss the behavior of 2DEG density changing with the thickness of ZnMgO layer in ZnMgO/MgO/ZnO structures, simple electrostatic analysis of the heterostructures yields the following expression for the 2DEG sheet density [11, 15, 28–30]:where the effect of cap was approximately equal to a ZnO cap layer because of the low Mg composition and where (1 nm here) and are the thicknesses of the MgO and top layer barrier layers, correspondingly, as shown in Figure 1(b); the surface potential is assumed to be pinned at surface with a level of 0.8 eV below ZnO conduction band edge. The conduction band offset is equal to 0.9 × [] [31]. We approximate the Fermi level by the infinite triangular quantum well [23], which can be expressed aswhere the dielectric constant is given as () for the very thin wurtzite MgO [10]. The effective mass is taken to be . is the polarization-induced charge density determined by the vector sum of the spontaneous polarization () and the strain-induced piezoelectric polarization () while there is no external field. We assume that the thin MgO layer is fully strained on ZnO and polarization constants vary linearly with composition. Thus, the dependence of total polarizations-induced charges in MgO layer can be expressed as (C/m²) () [12]. Taking the tunneling of electrons from MgO/ZnO channel to layer into account, we modified by a coefficient , and then it can be expressed as

For , the calculated 2DEG density and experimental results were shown in Figure 6. The solid line is the calculated plot extracted from (1). The sheet carrier concentration decreases rapidly as the thickness of ZnMgO layer increases. The 2DEG density of the Zn_0.95Mg_0.05O/MgO/ZnO structure with the layer thickness of 20 nm is 1.1 × 10¹³ cm⁻², and it becomes 2.4 × 10¹² cm⁻² when the thickness is 120 nm. The theoretical prediction and experimental results agreed well below 80 nm, which confirmed the ZnMgO layers in ZnMgO/MgO/ZnO structure, to a certain extent, behaved similar to a ZnO cap layer. After the thickness became larger than 80, the 2DEG density became lower than the calculated line. That is because the stress of the ZnMgO strain layer increases as the thickness increases, which results in the deterioration of crystal quality. There might be other reasons which need to be studied in the future.

We note that there is somewhat a discrepancy in the sheet carrier concentration values obtained by C-V and Hall measurement. Since the data observed from Hall measurement includes the contribution of bulk carrier, the value was higher than the true value.

4. Conclusion

In summary, formation of a 2DEG was confirmed for Zn polar ZnMgO/ZnO heterostructures with low Mg composition (). The enhancement of 2DEG concentration and mobility were realized in with low Mg composition by inserting of a thin (1 nm) MgO obviously. The sample shows a high Hall mobility of 3090 cm²/Vs at 10 K and 332 cm²/Vs at RT. The carrier concentration reached a value as high as 1.1 × 10¹³ cm⁻². However, the study for higher Mg content structures will be done in the future. The results demonstrate a well defined heterostructure and the possibility for fabrication of ZnMgO/MgO/ZnO HEMT devices.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by Project 863 of Xi’an Jiaotong University (2013AA03A101) and National Natural Science Foundation of China (no. 60876042 and no. 61176018).

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Copyright © 2015 Li Meng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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