Electrical and optical properties of ZnO films grown by molecular beam epitaxy

Zinc oxide (ZnO) films have been grown on sapphire by molecular beam epitaxy (MBE), and it is found that the grain size of the ZnO films increased with increasing the growth temperature. Photoluminescence (PL) study shows that the intensity ratio of near-band-edge emission to deep-level-related emission (NBE/DL) of the ZnO is significantly enhanced with increasing the growth temperature, and the dependence of the carrier mobility on the growth temperature shows very similar trend, which implies that there is a community factor that determines the optical and electrical properties of ZnO, and this factor is suggested to be the grain boundary. The results obtained in this paper reveal that by reducing the grain boundaries, ZnO films with high optical and electrical properties may be acquired.     

Anomalous electrical transport properties of a quenched iron film system

An iron film system, deposited on glass surfaces by thermal evaporation method and quenched with a floating oil layer immediately after the deposition, has been fabricated. The temperature dependence of the resistance and the transversal magnetoresistance (MR) of the iron films have been studied. The experiment shows that, as the temperature decreases, the sample resistance increases first and then drops monotonically, finally it increases again. Furthermore, a crossover of MR from positive to negative is observed as the magnetic field increases. It is proposed that these anomalous phenomena originate from the characteristic microstructure of the samples.     

InGaP/GaAs heterojunction bipolar transistor grown by solid-source molecular beam epitaxy with a GaP decomposition source

Lattice-matched InGaP epilayers on GaAs (001) and InGaP/GaAs heterojunction bipolar transistors (HBTs) were successfully grown by solid-source molecular beam epitaxy (SSMBE) with a GaP decomposition source. A 3 μm thick InGaP epilayer shows that low temperature photoluminescence (PL) peak energy is as large as 1.998 eV, full width at half maximum (FWHM) is 5.26 meV, which is the smallest ever reported, and X-ray diffraction (XRD) rocking curve linewidth is as narrow as that of GaAs substrate. The electron mobilities at room temperature of nominally undoped InGaP layers obtained by Hall measurements are comparable to similar InGaP epilayer grown by solid-source molecular beam epitaxy (SSMBE) with other sources or other growth techniques. The large area InGaP/GaAs HBTs show very good Dc characteristics.     

Well-aligned ZnO nanocolumns grown by reactive electron beam evaporation

We report on the reactive electron beam evaporative growth of well-aligned ZnO nanocolumns on Si (001) wafers in the environment of NH₃/H₂ gas mixture by using polycrystalline ZnO ceramic target as source material. The growth was carried out at low temperatures (400-450 °C) without employing any metal catalysts. Field emission scanning electron microscopy (FESEM) revealed that nanocolumns with uniform distributions in their diameters, lengths, and densities were grown vertically from the substrates and terminated by smooth hexagonal (0001) facets with no terrace-like steps emerged, which should render potential applications such as inherent resonance cavities in fabricating ultraviolet-laser arrays. X-ray diffraction measurements revealed that ZnO nanocolumns were highly c-axis oriented, which is well consistent with the FESEM observations. More importantly, photoluminescence investigations of the nanocolumns demonstrated the strong excitonic emission and extremely weak deep level emission, indicating the high crystalloid and optical quality of the nanocolumns.     

The high quality ZnO growth on c-Al2O3 substrate with Cr2O3 buffer layer using plasma-assisted molecular beam epitaxy

High quality epitaxial ZnO films were grown on c-Al₂O₃ substrates with Cr₂O₃ buffer layer by plasma-assisted molecular beam epitaxy (P-MBE). The hexagonal crystalline Cr₂O₃ layer was formed by oxidation of the Cr-metal layer deposited on the c-Al₂O₃ substrate using oxygen plasma. The epitaxial relationship was determined to be ZnO//Cr₂O₃//Cr//Al₂O₃ and ZnO//Cr₂O₃//[0 0 1]Cr//Al₂O_3. The Cr₂O₃ buffer layer was very effective in improving the surface morphology and crystal quality of the ZnO films. The photoluminescence spectrum showed the strong near band-edge emissions with the weak deep-level emission, which implies high optical quality of the ZnO films grown on the Cr₂O₃ buffer.     

Anomalous electrical transport properties of Ag/Al bilayers grown on Si by molecular beam epitaxy

We have investigated the temperature dependent electrical resistivity, ρ(T), of Ag(100 nm)/Al(10 nm) bilayers grown on Si(111) and quartz substrates using molecular beam epitaxy (MBE). Bilayers grown on Si exhibited an anomalous negative temperature coefficient of resistivity (TCR) in the temperature range of 140-165 K of the ρ(T) plot. However, at temperatures below and above this negative TCR region, ρ(T) exhibited a characteristic positive TCR of metallic alloys. No such resistive anomaly was observed for the bilayers grown on quartz substrates. The observed resistive anomaly could be qualitatively explained by assuming two parallel conduction channels, that is, one at the interface having high Si content and obeying the polaronic behavior at <165 K and another far away from the interface having almost no Si impurity and thus exhibiting pure metallic behavior down to 4 K. In addition, bilayers exhibited a sharp resistive transition at ∼6.5 K, indicating a possibility of a new Ag-Al alloy being a superconducting material.     

Stability of MgO(1 1 1) films grown on 6H-SiC(0 0 0 1) by molecular beam epitaxy for two-step integration of functional oxides

Crystalline magnesium oxide (MgO) (1 1 1), 20 Å thick, was grown by molecular beam epitaxy (MBE) on hydrogen cleaned hexagonal silicon carbide (6H-SiC). The films were further heated to 740 °C and 650 °C under different oxygen environments in order to simulate processing conditions for subsequent functional oxide growth. The purpose of this study was to determine the effectiveness and stability of crystalline MgO films and the MgO/6H-SiC interface for subsequent heteroepitaxial deposition of multi-component, functional oxides by MBE or pulsed laser deposition processes. The stability of the MgO films and the MgO/6H-SiC interface was found to be dependent on substrate temperature and the presence of atomic oxygen. The MgO films and the MgO/6H-SiC interface are stable at temperatures up to 740 °C at 1.0 × 10⁻⁹ Torr for extended periods of time. While at temperatures below 400 °C exposure to the presence of active oxygen for extended periods of time has negligible impact, exposure to the presence of active oxygen for more than 5 min at 650 °C will degrade the MgO/6H-SiC interface. Concurrent etching and interface breakdown mechanisms are hypothesized to explain the observed effects. Further, barium titanate was deposited by MBE on bare 6H-SiC(0 0 0 1) and MgO(1 1 1)/6H-SiC(0 0 0 1) in order to evaluate the effectiveness of the MgO as a heteroepitaxial template layer for perovskite ferroelectrics.     

Growth process of β-FeSi2 epitaxial film on Si(1 1 1) by molecular beam epitaxy

We have reported a one step growth of a high quality β-FeSi₂ epitaxial film on hydrogen terminated Si(1 1 1) by using molecular beam epitaxy (MBE) without template layer or post-growth annealing. In the present work, the growth process was studied by analyzing X-ray diffraction (XRD) spectra, reflective high energy electron diffraction (RHEED) and atomic force microscopy (AFM) observations on the samples grown with different growth times from 10 s to 1 h. A phase transformation from γ-FeSi₂ to β-FeSi₂ was confirmed existing in the crystal film growth, as well as the growth mode changing from three-dimensional (3D) to two-dimensional (2D) mode.     

Surface morphology of highly mismatched InSb films grown on GaAs substrates by molecular beam epitaxy

InSb films on GaAs(001) substrates with and without GaAs buffer layer have been grown by molecular beam epitaxy. Rather than surface undulations, aligned ripples and pyramidal hillocks along the orthogonal 〈110〉 directions were observed on the surface of InSb films. Both the preferential growth and the termination of ripples were proved to be related to strain‐driven mass transport. A model was proposed to elucidate the formation of the hillocks, which are more efficient to relax strain than ripples. Due to the strain relaxation through hillocks with small bases predominantly, the surfaces of the InSb films grown without a GaAs buffer layer are smoother than those of films grown with a GaAs buffer layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First-principles study of arsenic impurity clusters in molecular beam epitaxy (MBE) grown HgCdTe

The understanding of the microstructures of the arsenic tetramer , dimer , and singlet of HgCdTe is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to p-type behavior. The stable configurations were obtained from the first-principles calculations for the arsenic cluster defects [ (n=1, 2, and 4)] in as-grown HgCdTe. According to the defect formation energies calculated under Te-rich conditions, the most probable configurations of , , and have been established. For the optimized and the energy is favorable to combine in a nearest neighboring mercury vacancy , and the corresponding configurations can be used to explain the self-compensated n-type characteristics in as-grown materials. is likely to be more abundant than in as-grown materials, but arsenic atoms are more strongly bounded in than in , thus more substantial activation energy is needed for than that for . The atomic relaxations as well as the structural stability of the arsenic defects have also been investigated.     

Room-temperature deposition of transparent conducting Al-doped ZnO films by RF magnetron sputtering method

Transparent conductive Al-doped zinc oxide (AZO) films with highly (0 0 2)-preferred orientation were deposited on quartz substrates at room temperature by RF magnetron sputtering. Optimization of deposition parameters was based on RF power, Ar pressure in the vacuum chamber, and distance between the target and substrate. The structural, electrical, and optical properties of the AZO thin films were investigated by X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The 250 nm thickness AZO films with an electrical resistivity as low as 4.62 × 10⁻⁴ Ω cm and an average optical transmission of 93.7% in the visible range were obtained at RF power of 300 W, Ar flow rate of 30 sccm, and target distance of 7 cm. The optical bandgap depends on the deposition condition, and was in the range of 3.75-3.86 eV. These results make the possibility for light emitting diodes (LEDs) and solar cells with AZO films as transparent electrodes, especially using lift-off process to achieve the transparent electrode pattern transfer.     

Dependence of photoluminescence of ZnO/Zn0.85Mg0.15O multi-quantum wells on barrier width

Temperature-dependent photoluminescence (PL) from two multi-quantum well (MQW) structures with different barrier widths has been systematically investigated. The PL band in the well layers is dominated by localized excitons (LE), D⁰X, and D⁰X-1LO. As the temperature increases, luminescence from the excitons localized in the well layers shows an ‘S’-shaped shift in the thin barrier MQW whereas a monotonic redshift is observed from the thick barrier MQW. Quenching of well-related emission is associated with delocalization of the excitons in the potential minima induced by interface fluctuations or alloy disorder. The activation energies correlated with depths of the local potential are deduced to be 7 and 17 meV for the thick and thin barrier MQWs, respectively.     

Effect of thickness on the stability of transparent conducting impurity‐doped ZnO thin films in a high humidity environment

The resistivity of transparent conducting Al‐ and Ga‐doped ZnO (AZO and GZO) thin films prepared with a thickness in the range from 20 to 200 nm on glass substrates at a temperature below 200 °C was found to increase with exposure time when tested in a high humidity environment (air at 90% relative humidity and 60 °C). The resistivity stability (resistivity increase) was considerably affected by the thin film thickness. In particular, thin films with a thickness below about 50 nm were very unstable. The increase in resistivity is interpreted as carrier transport being dominated by grain boundary scattering resulting from the trapping of free electrons due to oxygen adsorption on the grain boundary surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-ray photoelectron spectroscopy study on the CrN surface grown on sapphire substrate to control the polarity of ZnO by plasma-assisted molecular beam epitaxy

We have studied on the polarity selection procedure of ZnO grown on CrN buffer by using X-ray photoelectron spectroscopy (XPS). XPS studies have been performed on the O- and Zn-treated CrN/Al₂O₃surfaces and revealed that Cr₂O₃ and Zn-chromate-like structures are formed on O- and Zn-treated CrN surfaces, respectively. The polarity selection procedure is explained in terms of the variation of bonding coordination by the formation of ZnO on the topmost O- and Zn-atoms of each surface.     

Structure and electrical transport properties of bismuth thin films prepared by RF magnetron sputtering

Bismuth thin films were prepared on glass substrates with RF magnetron sputtering and the effects of deposition temperature on surface morphology and their electrical transport properties were investigated. Grain growth of bismuth and the coalescence of grains were observed above 393 K with field emission secondary electron microscopy. Continuous thin films could not be obtained above 448 K because of the segregation of grains. Hall effect measurements showed that substrate heating yields the decrease of carrier density and the increase of mobility in exponential ways until 403 K. Resistivity of sputter deposited bismuth films has its minimum (about 0.7 × 10⁻³ Ω cm) in range of 403-433 K. Annealing of bismuth films deposited at room temperature was carried out in a radiation furnace with flowing hydrogen gas. The change of resistivity was not significant due to the cancellation of the decrease of carrier density and the increase of mobility. However, the abrupt change of electrical properties of film annealed above 523 K was observed, which is caused by the oxidation of bismuth layer.     

Substrate temperature influence on the properties of nanostructured ZnO transparent ultrathin films grown by PLD

Zinc oxide films of 40 nm thickness have been deposited on glass substrates by pulsed laser deposition using an excimer XeCl laser (308 nm) at different substrate temperatures ranging from room temperature to 650 °C. Surface investigations carried out by using atomic force microscopy have shown a strong influence of temperature on the films surface topography. UV-VIS transmittance measurements have shown that our ZnO films are highly transparent in the visible wavelength region, having an average transmittance of ∼90%. The optical band gap of the films was found to be 3.26 eV, which is lower than the theoretical value of 3.37 eV. Besides the normal absorption edge related to the transition between the valence and the conduction band, an additional absorption band was also recorded in the wavelength region around 364 nm (∼3.4 eV). This additional absorption band may be due to excitonic, impurity, and/or quantum size effects. Photoreduction/oxidation in ozone of the ZnO films lead to larger conductivity changes for higher deposition temperature. In conclusion, the ozone sensing characteristics as well as the optical properties of the ZnO thin films deposited by pulsed laser deposition are strongly influenced by the substrate temperature during growth. The sensitivity of the films towards ozone might be enhanced significantly by the control of the films deposition parameters and surface characteristics.     

Optical spectroscopy of a-plane-oriented ZnO epilayers grown by plasma-assisted molecular beam epitaxy

a-plane ZnO layers were successfully grown, by plasma-assisted molecular beam epitaxy, on r-plane (011–2) sapphire substrates. Several features attributed to the A, B and C free excitonic transitions are identified through temperature-dependent photoluminescence (PL) and reflectivity measurements. The temperature dependence of the peak energy positions of these transitions was studied from 8 K to 300 K. So, the PL peak energy of the A free exciton was plotted and fitted with a Varshni empirical equation. In the σ polarization (E⊥c), for which A and B are allowed, the reflectivity spectrum measured at 8 K was fitted by using a program based on the theory of the spatial resonance dispersion Hopfield model. Our results indicate that the A and B free excitonic features are at a higher energy than those in ZnO on c-oriented sapphire and show a good PL even at 250 K. These results also show that the new and intense emission peak observed in the region of the first phonon replica (3.33–3.28 eV) and identified as a stacking fault in the epilayer disappears at high temperature above 150 K.     

Growth and properties of ZnO nanorod and nanonails by thermal evaporation

ZnO nanorods and nanonails have been synthesized on silicon wafers by a three-step catalyst-free thermal evaporation method in oxygen atmosphere. All the samples were hexagonal phase ZnO with highly c-axis preferential orientation. Different morphologies of ZnO nanostructures, i.e. ZnO nanorods and two kinds of nanonails, were observed at various temperature regions. Photoluminescence, transmission electron microscopy, and energy-dispersive X-ray spectroscope were employed to elucidate the reason for the formation of such different rod-like structures. The analysis results demonstrated that the caps of nanonails possess a large number of oxygen vacancies, which may play a key role in determining the formation of nanonails and the high intensity of green emission.     

Fabrication of high hole-carrier density p-type ZnO thin films by N-Al co-doping

In order to obtain p-type ZnO thin films, effect of atomic ratio of Zn:N:Al on the electronic and structural characteristic of ZnO thin films was investigated. Hall measurement indicated that with the increase of Al doping, conductive type of as-grown ZnO thin films changed from n-type to p-type and then to n-type again, reasons are discussed in details. Results of X-ray diffraction revealed that co-doped ZnO thin films have similar crystallization characteristic (0 0 2 preferential orientation) like that of un-doping. However, SEM measurement indicated that co-doped ZnO thin films have different surface morphology compared with un-doped ZnO thin films. p-type ZnO thin films with high hole concentration were obtained on glass (4.6 × 10¹⁸ cm⁻³) and n-type silicon (7.51 × 10¹⁹ cm⁻³), respectively.     

Stabilization in electrical characteristics of hydrogen-annealed ZnO:Al films

Al-doped ZnO (ZnO:Al) films prepared by RF magnetron co-sputtering at room temperature were thermally treated in hydrogen ambient at 300 °C to enhance the films’ characteristics for transparent conductive oxide applications. The electrical properties of the hydrogen-annealed films were improved and preserved in air ambient, even though the crystal structures of the films were not changed by the thermal treatment. The optical and oxygen bonding characteristics of ZnO:Al films manifested that absorbed oxygen species on the films were removed by the hydrogen-annealing process. These results supported that the development of the electrically reliable ZnO:Al films could be realized using the hydrogen-annealing process.