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A two-dimensional electron gas was observed in Zn polar ZnMgO∕ZnO (ZnMgO on ZnO) heterostructures grown by radical source molecular beam epitaxy. The electron mobility of the ZnMgO∕ZnO heterostructures dramatically increased with increasing Mg composition and the electron mobility (μ∼250cm2∕Vs) at RT reached a value more than twice that of an undoped ZnO layer (μ∼100cm2∕Vs). The carrier concentration in turn reached values as high as ∼1×1013cm−2 and remained nearly constant regardless of Mg composition. Strong confinement of electrons at the ZnMgO∕ZnO interface was confirmed by C-V measurements with a concentration of over 4×1019cm−3. Temperature-dependent Hall measurements of ZnMgO∕ZnO heterostructures also exhibited properties associated with well defined heterostructures. The Hall mobility increased monotonically with decreasing temperature, reaching a value of 2750cm2∕Vs at 4K. Zn polar “ZnMgO on ZnO” structures are easy to adapt to a top-gate device. These results open new possibilities for high electron mobility transistors based upon ZnO-based materials.

The crystallographic polarity of ZnO epilayers was determined by x-ray diffraction (XRD) using anomalous dispersion near the Zn K edge. The method is not destructive and is straightforward to carry out using a typical XRD measurement system. The polarity difference between the Zn (0001) and O (0001¯) surfaces could be easily determined using a {0002} diffraction peak and the Bremstrahlung radiation from a Cu rotating anode source. By using the normalized pre- and post-Zn K-edge diffraction intensity ratios of the (0002) diffraction peak, Zn polar and O polar ZnO layers could always be distinguished but, the absolute value of the ratio was found to change with layer thickness. The absolute value of the ratio with layer thickness was found to have a linear dependence on layer thickness allowing determination of the polarity of (0001) ZnO epilayers with a single x-ray measurement and the known layer thickness in conjunction with standard data. Acid etching results confirmed the veracity of the polarity determination of the XRD measurement. To test the technique, Zn and O polar ZnO layers were grown by radical source molecular beam epitaxy (RS MBE) on MgO buffer layers on c-sapphire substrate and O polar ZnO layers were grown on a-plane substrates and measured using the x-ray technique with excellent agreement.

A strong excitonic optical transition in a Zn1−xMgxO alloy grown by radical source molecular beam epitaxy was observed using both optical reflectivity measurements and photoluminescence (PL) measurements. Clear and strong reflectance peaks at room temperature (RT) were observed from 3.42eV (x=0.05)to4.62eV (x=0.61) from ZnMgO layers at RT. Distinct clear PL spectra at RT were also observed for energies up to 4.06eV (x=0.44). The peak intensity of the reflected signal increased for x values up to x∼0.2 simultaneously with an increase in PL intensity; however, a Stokes shift between the reflectance peak and the PL peak was not observed for x values below 0.2. These facts suggest that the oscillator strength of ZnMgO is enhanced by alloying, and the underlying mechanism is discussed. Furthermore, we demonstrate that the strong reflectance properties even at RT provide an easy method to determine the Mg composition of a thin ZnMgO layer in a ZnMgO∕ZnO heterostructure.

The band profiles of ZnMgO/ZnO heterostructures were confirmed through surface potential measurements by Kelvin probe force microscopy. A simple model for the band profile was proposed and the various band parameters were evaluated experimentally and theoretically based on the band model. The band profile was calculated and validated with experimental results using the Schrödinger–Poisson equation. The energy level of the ZnMgO surface donor state, which serves as the source of the two-dimensional electron gas in ZnMgO/ZnO heterostructures, was estimated from the band parameters; nearly identical energy levels around 0.8 eV were obtained for Zn1−xMgxO layers with Mg compositions x ranging from 0.12 to 0.42 and the corresponding charge densities were estimated to be 8×1012 cm−2.