A two-dimensional electron gas (2DEG) was observed in Zn polar ZnMgO/ZnO (ZnMgO on ZnO) heterostructures
grown by radical source molecular beam epitaxy. Reflection high energy electron diffraction patterns taken during the
growth of the ZnMgO layer remained streaky; x-ray diffraction measurements showed no evidence of phase separation
for up 44 % Mg composition. These results shows that the high quality ZnMgO layers up to 44 % Mg composition were
obtained without phase separation. The electron mobility of the ZnMgO/ZnO heterostructures dramatically increased
with increasing Mg composition and the electron mobility (&mgr;~250 cm<sup>2</sup>/Vs) at RT reached a value more than twice that
of an undoped ZnO layer (&mgr;~100 cm<sup>2</sup>/Vs) due to the 2DEG formation. The carrier concentration in turn reached values
as high as ~1x10<sup>13</sup> cm<sup>-2</sup> 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 4x10<sup>19</sup> cm<sup>-3</sup>.
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 2750 cm<sup>2</sup>/Vs at 4 K. 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 (HEMTs) based upon ZnO based materials.
We have used molecular beam epitaxy (MBE) to deposit gallium (Ga) doped ZnO (ZnO:Ga) films. The as-deposited ZnO:Ga films have worked as ohmic contacts for the p-type GaN layers without any kinds of post annealing process. The as-deposited ZnO:Ga films on <i>a</i>-plane sapphire substrates have resistivities of 2-4×10<sup>-4</sup> Ωcm, and over 80 % transparency in the near-UV and visible wavelength regions. The brightness of InGaN light-emitting diodes (LEDs) with ZnO:Ga <i>p</i>-contacts has doubled compared to LEDs with conventional Ni/Au semi-transparent p-contacts when measuring the brightness from right above the device surfaces. In addition, using MBE, we have grown homoepitaxial polar ZnO films on (000+1)-plane (+c-plane) ZnO substrates, and also grown non-polar ZnO films on (1-100)-plane (<i>m</i>-plane) and (11-20)-plane (<i>a</i>-plane) ZnO substrates. Growth temperatures have not affected nitrogen-doping levels for +c-axis oriented (Zn-polar) nitrogen doped ZnO (ZnO:N) films. The phenomena were quite different from that for (000-1)-axis (-<i>c</i>-axis) oriented (oxygen-polar) growth, where nitrogen concentrations in ZnO decrease with increasing growth temperatures. We have observed <i>c</i>-axis direction growth for both of <i>m</i>-axis and <i>a</i>-axis oriented films. Oxygen-rich growth conditions flatten surfaces for both <i>m</i>-axis and <i>a</i>-axis oriented films, and the surfaces of <i>m</i>-axis oriented ZnO films flatten with increasing growth temperatures. Nitrogen concentrations in <i>m</i>-axis oriented ZnO:N films have been independent on growth temperatures.