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

1Key Laboratory of Photonics Technology for Information, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China 2Key Laboratory for Physical Electronics and Devices under Ministry of Education, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China 3Joint Laboratory of Functional Materials and Devices for Informatics, Xi’an Jiaotong University and Institute of Semiconductors, CAS, Xi’an, Shaanxi 710049, China


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 highfrequency and high-power device applications.So far, the formation of 2DEG at Zn polar Zn 1− Mg  O/ZnO interface has been observed by a few groups using molecular beam epitaxy (MBE) [4][5][6][7][8], pulse laser deposition (PLD) techniques [9], metal-organic vapor phase epitaxy (MOVPE) [10], and RF sputtering [11].However, in low Mg composition Zn 1− Mg  O/ZnO heterostructures ( < 0.1) high electron mobility can be observed but with very low 2DEG sheet density ( < 10 12 cm −2 ) [12], and in high Mg composition Zn 1− Mg  O/ZnO heterostructures ( > 0.1), 2DEG sheet density reached a considerable value (10 12 ∼10 13 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 cm 2 /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][15][16][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 twodimensional 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.

Materials and Methods
Zn polar ZnMgO/ZnO and ZnMgO/MgO/ZnO heterostructures were all grown on sapphire (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) substrates by radicalsource 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 radiofrequency (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 −3 Pa.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 K ( = 0.15406 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 Zn 1− Mg  O using the equation   () =   (0) + 2.145 [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.

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.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.C-V measurement.2DEG of Zn 0.95 Mg 0.05 O/ZnO structure with 20 nm thick Zn 0.95 Mg 0.05 O 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.

Electrical Property.
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 0.95 Mg 0.05 O (20 nm)/MgO/ZnO heterostructure showed very high Hall mobility of 332 cm 2 /Vs at RT and 3090 cm 2 /Vs at 10 K.

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 polarizationinduced 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 Zn 1− Mg  O/MgO/ZnO heterostructures yields the following expression for the 2DEG sheet density [11,15,[28][29][30]: where the effect of Zn  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 Δ  is equal to 0.9 × [  (MgO) −   (ZnO)] [31].We approximate the Fermi level   by the infinite triangular quantum well [23], which can be expressed as where the dielectric constant  is given as (8.75 + 1.08 * 1) for the very thin wurtzite MgO [10].The effective mass is taken to be  * ≈ 0.26  . is the polarization-induced charge density determined by the vector sum of the spontaneous polarization ( SP ) and the strain-induced piezoelectric polarization ( PE ) 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  = 0.029 (C/m 2 ) ( = 1) [12].Taking the tunneling of electrons from MgO/ZnO channel to Zn 1− Mg  O layer into account, we modified  by a coefficient V, and then it can be expressed as For V = 0.1, the calculated 2DEG density and experimental results were shown in Figure 6  structure with the layer thickness of 20 nm is 1.1 × 10 13 cm −2 , and it becomes 2.4 × 10 12 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.

Conclusion
In summary, formation of a 2DEG was confirmed for Zn polar ZnMgO/ZnO heterostructures with low Mg composition ( = 0.05).The enhancement of 2DEG concentration and mobility were realized in Zn 1− Mg  O/MgO/ZnO with low Mg composition by inserting of a thin (1 nm) MgO obviously.The sample shows a high Hall mobility of 3090 cm 2 /Vs at 10 K and 332 cm 2 /Vs at RT.The carrier concentration reached a value as high as 1.1 × 10 13 cm −2 .However, the study for higher Mg content Zn 1− Mg  O/MgO/ZnO 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.

Figure 1 :
Figure 1: Layer structure (a) and band diagram (b) for a Zn polar ZnMgO/MgO/ZnO heterostructure.The growth direction is <0001> for Zn polarity.

Figure 2 :
Figure 2: The evolution of RHEED pattern during growth of the MgO layer.The direction of MgO is <11-20>.(a) The thickness of the MgO buffer layer is 0.5 nm; (b) thickness of MgO = 1 nm; and (c) thickness of MgO = 1.5 nm.

Figure 3 : 3 )
Figure 3: The XRD spectra for the grown ZnMgO/MgO/ZnO structure; the inset is the cross-sectional TEM image of a ZnMgO/MgO/ZnO film.

Figure 4 :
Figure 4: C-V depth profiling of the 2DEG and net donor concentration in the Zn 0.95 Mg 0.05 O/ZnO and Zn 0.95 Mg 0.05 O/MgO/ZnO structures.

Figure 5
shows the results of temperature-dependent Hall measurements.The electron mobility of Zn 0.95 Mg 0.05 O (100 nm)/ZnO and Zn 0.95 Mg 0.05 O (100 nm and 20 nm)/MgO/ZnO structures increases with decreasing temperature, as shown in Figure5(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 0.95 Mg 0.05 O (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 0.95 Mg 0.05 O (100 nm)/ZnO and Zn 0.95 Mg 0.05 O (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.

2 )
. 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 0.95 Mg 0.05 O/MgO/ZnO Sheet carrier concentration (×10 13 cm −

Figure 6 :
Figure 6: The dependence of sheet carrier concentration as a function of ZnMgO layer thickness.