Plasma-assisted molecular beam epitaxy of ZnO on <i>in-situ</i> grown GaN/4H-SiC buffer layers

doi:10.1007/s11706-015-0292-x

Front. Mater. Sci.

2015, Vol. 9

Issue (2) : 185-191 https://doi.org/10.1007/s11706-015-0292-x

RESEARCH ARTICLE

Plasma-assisted molecular beam epitaxy of ZnO on in-situ grown GaN/4H-SiC buffer layers

David ADOLPH,Tobias TINGBERG,Thorvald ANDERSSON,Tommy IVE(

)

Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, G?teborg, Sweden

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Abstract

Plasma-assisted molecular beam epitaxy (MBE) was used to grow ZnO(0001) layers on GaN(0001)/4H-SiC buffer layers deposited in the same growth chamber equipped with both N- and O-plasma sources. The GaN buffer layers were grown immediately before initiating the growth of ZnO. Using a substrate temperature of 440°C–445°C and an O₂ flow rate of 2.0–2.5 sccm, we obtained ZnO layers with smooth surfaces having a root-mean-square roughness of 0.3 nm and a peak-to-valley distance of 3 nm shown by AFM. The FWHM for X-ray rocking curves recorded across the ZnO(0002) and ZnO(101ˉ5) reflections were 200 and 950 arcsec, respectively. These values showed that the mosaicity (tilt and twist) of the ZnO film was comparable to corresponding values of the underlying GaN buffer. It was found that a substrate temperature >450°C and a high Zn-flux always resulted in a rough ZnO surface morphology. Reciprocal space maps showed that the in-plane relaxation of the GaN and ZnO layers was 82.3% and 73.0%, respectively and the relaxation occurred abruptly during the growth. Room-temperature Hall-effect measurements showed that the layers were intrinsically n-type with an electron concentration of 10¹⁹ cm^–3 and a Hall mobility of 50 cm²·V^–1·s^–1.

Keywords ZnO molecular beam epitaxy (MBE) epitaxy

Corresponding Author(s): Tommy IVE

Online First Date: 21 April 2015 Issue Date: 23 July 2015

Cite this article:

David ADOLPH,Tobias TINGBERG,Thorvald ANDERSSON, et al. Plasma-assisted molecular beam epitaxy of ZnO on in-situ grown GaN/4H-SiC buffer layers[J]. Front. Mater. Sci., 2015, 9(2): 185-191.

URL:

https://academic.hep.com.cn/foms/EN/10.1007/s11706-015-0292-x
https://academic.hep.com.cn/foms/EN/Y2015/V9/I2/185

Tab.1 Summary of the growth conditions and properties for a representative set of samples presented in this work

Fig.1 (a) RHEED pattern for the [11 2 ˉ 0] azimuth from a ZnO/GaN/4H-SiC sample grown at T_Zn = 355°C and T_s = 440°C (sample S1 in Table 1). These conditions corresponded to the growth conditions within our optimum growth window. (b) RHEED pattern from a sample grown outside of the optimum window.

Fig.2 SEM micrographs of ZnO/GaN/4H-SiC layers grown with various Zn-fluxes corresponding to different T_Zn. (a) The surface of a ZnO sample (sample S3) grown with a high Zn-source temperature T_Zn = 420°C. (b) The surface of a sample (sample S4) grown with T_Zn = 380°C corresponding to an intermediate Zn-flux. (c) Micrograph of a ZnO surface for a sample (sample S1) grown with T_Zn = 355°C representing the Zn-flux inside the optimum window. (d) Cross-sectional SEM image of the sample in (c). The cross-section revealed a homogeneous and compact ZnO layer.

Fig.3 AFM micrograph of sample S2 grown on GaN/4H-SiC with a Zn-source temperature T_Zn = 355°C and a substrate temperature T_s = 445°C.

Tab.2 Selected properties of state-of-the-art ZnO layers found in the literature and grown with MOCVD, MBE on ZnO(0001), low temperature (LT) ZnO and MgO buffer layers, GaN-templates and 4H-SiC

Fig.4 RSM for the (10 1 ˉ 5) reflection of sample S1 grown on a 130 nm thick GaN buffer layer on 4H-SiC(0001) within the optimum growth window (T_Zn = 355°C and T_s = 440°C). (a) The contour peaks for the ZnO and GaN layer, respectively. Both layers showed a high degree of relaxation. (b) Contour peaks for the grown ZnO and GaN layers and the 4H-SiC substrate.

Fig.5 The growth rate of ZnO on GaN/4H-SiC with respect to (a) a variation of the substrate temperature for T_Zn = 390°C or T_Zn = 420°C and with Φ_O2 = 2.0 sccm and (b) a variation of the Zn-flux with T_s = 440°C–450°C and Φ_O2 = 2.0 sccm. The dashed lines serve as guides to the eye. The points within the dashed box correspond to samples grown within the optimum growth window.

Fig.6 The evolution of the RMS roughness and PV distance as a function of a varying Φ_O2 for samples grown at T_s = 440°C–445°C and T_Zn = 355°C. The dashed box contains the points corresponding to samples grown with the smoothest morphology within the optimum growth window.

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