S. Niki

Uniaxial locked epitaxy of ZnO on the a face of sapphire

High-quality, c-oriented ZnO epitaxial films have been grown on the a surface using molecular-beam epitaxy. The use of a-oriented sapphire eliminates rotational domains and related structural defects which have limited the use of ZnO in electronic applications. The ZnO epitaxial layers are uniquely oriented with the ZnO/sapphire orientational relationship [0001]‖[1120] and 〈1120〉‖[0001]. This unique orientation is a consequence of the anisotropy of the a-sapphire surface in conjunction with a strong correlation along a single direction leading to the term uniaxial locked epitaxy. High-resolution x-ray diffraction measurements show an increase in x-ray lateral coherence length from several tens of nanometers to >0.7 μm for growth of c-oriented ZnO on the a surface as opposed to the c surface of sapphire.

Interactions between gallium and nitrogen dopants in ZnO films grown by radical-source molecular-beam epitaxy

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.

Band-gap modified Al-doped Zn1−xMgxO transparent conducting films deposited by pulsed laser deposition

Al-doped Zn1−xMgxO films have been deposited on glass substrates at a substrate temperature of 200°C by a pulsed laser deposition system. A resistivity of 3×10−4Ωcm was obtained at x=0.06. Film resistivity was found to increase with further increases in Mg composition. The maximum band gap of films with a resistivity ρ⩽1×10−3Ωcm was found to be 3.97eV, demonstrating band-gap engineering possibilities in the range of Eg=3.5–3.97eV with a resistivity ρ⩽1×10−3Ωcm. The average transmittance of the films was higher than 90% in the wavelength region λ=400–800nm, a range suitable for transparent conducting film applications.

Polarization-induced two-dimensional electron gases in ZnMgO/ZnO heterostructures

Both the formation mechanism and the origin of the two-dimensional electron gas (2DEG) in ZnMgO/ZnO heterostructures have been investigated. The 2DEG in the heterostructures was confirmed to originate from polarization-induced charge and was found to be dominant for transport at low temperatures as well as room temperature (RT) by transport measurements. The origin of 2DEG was concluded to be the surface of the ZnMgO layer based on both capacitance-voltage measurements and the dependence of the carrier concentration on the ZnMgO layer thickness. The largest sheet carrier concentration was 1.1×1013 cm−2 and the highest mobility for the heterostructure was obtained for a Mg composition of 0.61 at RT.

Two-dimensional electron gas in Zn polar ZnMgO∕ZnO heterostructures grown by radical source molecular beam epitaxy

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 elec...

Degenerate layers in epitaxial ZnO films grown on sapphire substrates

ZnO films were grown on low-temperature (LT) buffer layers on sapphire a-plane (11–20) substrates by radical source molecular-beam epitaxy. The LT buffer layers were found to effect the electrical properties of subsequently grown undoped ZnO films, and their presence was found to be indispensable for the growth of films with low carrier concentrations and high mobilities. Temperature-dependent Hall measurements showed the existence of a degenerate region related to the LT buffer layers. It was found that the effects of degenerate layers could be reduced by using annealing treatments and nitrogen doping of the LT buffer layers. The dominant residual donor energy of 110 meV was found to be different than previously reported. The carrier concentration of a ZnO film fabricated using a nitrogen-doped buffer layer was 7.5×1016 cm−3 with a mobility of 132 cm2/V s at room temperature.

Effects of the surface Cu2−xSe phase on the growth and properties of CuInSe2 films

Drastic changes in average molecularities (m=Cu/In) from m≫1 to m=0.92–0.93 and in hole concentrations from p≫1019 cm−3 to as low as p=7.5×1016 cm−3 have been observed in molecular beam epitaxy grown CuInSe2 after selective etching of the Cu–Se phase by a KCN aqueous solution; high hole concentrations and Cu-excess compositions of the as-grown films were attributed to the Cu–Se phase. On the other hand, well-defined photoluminescence emissions were found characteristic of intrinsic CuInSe2. The presence of the Cu–Se phase made possible the growth of high-quality CuInSe2 epitaxial films at a temperature well below the melting point of any Cu–Se compound. Surface topology measurements showed that the surface of the as-grown films was not fully covered by Cu–Se grains, leaving holes with depths of 200–300 nm after KCN etching. The enhanced two-dimensional growth and the reduced defect concentration imply that a very thin Cu-excess surface layer controls the growth of CuInSe2 when grown under Cu-excess condit...

Determination of crystallographic polarity of ZnO layers

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 deter...

Strong excitonic transition of Zn1−xMgxO alloy

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.

Photoluminescence characterization of Zn1−xMgxO epitaxial thin films grown on ZnO by radical source molecular beam epitaxy

The authors report that high-quality Zn1−xMgxO alloys are very brilliant light emitters, even more brilliant than ZnO, particularly in the high-temperature region; both the emission bandwidth and the oscillator strength of the photoluminescence from Zn1−xMgxO alloys increase remarkably with increasing Mg composition ratio x. The authors have revealed that the increase in the oscillator strength is mainly due to the increase in the activation energy required for the nonradiative recombination processes. Therefore, it is suggested that the localization of excitons, because of the compositional fluctuation, takes place in Zn1−xMgxO alloys and that the degree of the localization increases with increasing x.