Gallium Zinc Oxide Research Articles

Steady-state and transient electron transport within the wide energy gap compound semiconductors gallium nitride and zinc oxide: an updated and critical review

The wide energy gap compound semiconductors, gallium nitride and zinc oxide, are widely recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these wide energy gap compound semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyzes of the electron transport within the wide energy gap compound semiconductors, gallium nitride and zinc oxide. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, zinc-blende gallium nitride, and wurtzite zinc oxide. The electron transport that occurs within bulk zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap compound semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials. The wurtzite gallium nitride and zinc-blende gallium arsenide results, being presented in a previous review article of ours (O’Leary et al. in J Mater Sci Mater Electron 17:87, 2006), are also presented herein for the sake of completeness.

Aerosol-Assisted Chemical Vapour Deposition of Transparent Zinc Gallate Films

Aerosol-assisted chemical vapour deposition (AACVD) reactions of GaMe3, ZnEt2 and the donor-functionalised alcohol HOCH2CH2OMe (6 equiv.) in toluene resulted in the deposition of amorphous transparent zinc gallate (ZnGa2O4) films at a range of temperatures (350–550 °C). The zinc–gallium oxide films were analyzed by scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray analysis, glancing-angle X-ray powder diffraction (XRD) and optical studies. The optimum growth temperature was found to be 450 °C, which produced transparent films with excellent coverage of the substrate. XPS confirmed the presence of zinc, gallium and oxygen in the films. Annealing these films at 1000 °C resulted in crystalline films and glancing-angle powder XRD showed that a zinc gallate spinel framework with a lattice parameter of a=8.336(5) A was adopted.

Analysis of Nitrides- and TCOs-Based Plasmonic Waveguides for Slow-Wave and Negative Index Sub-Wavelength Propagation

In this paper, a comparison between metal-insulator-metal (MIM) waveguides made of silver and newly emerging plasmonic materials is reported. In particular, titanium nitride (TiN) from the class of nitrides, and gallium zinc oxide (GZO) from the class of transparent conducting oxides, are proposed as alternatives to conventional metals for the more flexible exploitation of plasmonic properties. Depending on the specific application, the new choices of the plasmonic material allow for tuning of the surface plasmon resonance and may also reduce typical conductive losses. Moreover, compared to noble metals, these new plasmonic materials have the extremely important property of being compatible with the mature CMOS technology. In this paper, the specific application of MIM waveguides made of TiN and GZO for dispersion engineering (slow-wave propagation and negative effective index) is considered for the first time.

Influence of film thickness on laser ablation threshold of transparent conducting oxide thin-films

Abstract We report on a comprehensive study of the laser ablation threshold of transparent conductive oxide thin films. The ablation threshold is determined for both indium tin oxide and gallium zinc oxide as a function of film thickness and for different laser wavelengths. By using a pulsed diode pumped solid state laser at 1064 nm, 532 nm, 355 nm and 266 nm, respectively, the relationship between optical absorption length and film thickness is studied. We find that the ablation threshold decreases with increasing film thickness in a regime where the absorption length is larger than the film thickness. In turn, the ablation threshold increases in case the absorption length is smaller than the film thickness. In particular, we observe a minimum of the ablation threshold in a region where the film thickness is comparable to the absorption length. To the best of our knowledge, this behaviour previously predicted for thin metal films, has been unreported for all three regimes in case of transparent conductive oxides, yet. For industrial laser scribing processes, these results imply that the efficiency can be optimized by using a laser where the optical absorption length is close to the film thickness.

Study on Structural Properties of Gallium and Titanium Doped Zinc Oxide Films Deposited by Magnetron Sputtering

Thin films of transparent conducting gallium and titanium doped zinc oxide (GTZO) were prepared on glass substrates by magnetron sputtering technique using a sintered ceramic target. The microstructural properties of the deposited thin films were characterized with X-ray diffraction (XRD). The results demonstrated that the polycrystalline GTZO thin films consist of the hexagonal crystal structures with c-axis as the preferred growth orientation normal to the substrate, and that the working pressure significantly affects the crystal structures of the thin films. The GTZO thin film deposited at the working pressure of 0.4 Pa has the best crystallinity, the largest grain size and the lowest stress.

Nanophotonics for efficient solution searching and decision making

At present, there is great demand for novel computing devices and architectures that can overcome the limitations of conventional technologies based solely on electron transfer, including the need to reduce energy consumption and solve computationally demanding problems. In particular, conventional digital computers have difficulty in dealing with intractable problems in which the number of possible solutions increases exponentially as a function of the problem size (referred to as ‘combinatorial explosion’). It is also challenging for conventional computers to make efficient and adaptive decisions in the uncertain, dynamically changing environments seen in real-world applications. A promising solution is near-field nanophotonics, which has been extensively studied with the aim of unveiling and exploiting light-matter interactions that occur at a scale below the wavelength of light. Recent progress made in experimental technologies—both in nanomaterial fabrication, such as quantum dots (QDs), and in characterization—is driving further advancements in the field. We have shown that the dynamics of optical energy transfer mediated by near-field interactions can be exploited to solve solution-searching and decision-making problems.1–4 This suggests that computing systems based on near-field nanophotonics may one day be able to exhibit intellectual abilities. When QDs share common resonant energy levels mediated by optical near-field interactions, optical energy is transferred from smaller QDs to larger ones. This process has been experimentally demonstrated in various quantum nanostructures, such as those fabricated in indium gallium arsenide, zinc oxide, and cadmium selenide. In addition, we have shown this transfer of optical Figure 1. (a) Quantum-dot-based decision maker (QDM) consisting of five quantum dots (denoted QDLL, QDML, QDS, QDMR, and QDLR) interacting via optical near-fields. The subscripts 1, 2, and 3 denote the (1,1,1), (2,1,1), and (2,2,2) energy levels, respectively, while U represents the various optical near-field interactions. (b) Quick adaptation of the QDM to a dynamically changing environment, which in this case is the change of reward probabilities between two slot machines (PA and PB). Softmax: Best conventional algorithm.

Device Performances of Organic Light-Emitting Diodes with Indium Tin Oxide, Gallium Zinc Oxide, and Indium Zinc Tin Oxide Anodes Deposited at Room Temperature

Thin films of Indium tin oxide (ITO), Gallium zinc oxide (GZO), and Indium zinc tin oxide (IZTO) were deposited on glass substrates by pulsed direct current magnetron sputtering at room temperature. The structural, optical, and electrical properties of the films were investigated towards evaluating their applications as flexible anodes. IZTO films exhibited the lowest resistivity (6.3 x 10(-4) Omega cm). Organic light-emitting diodes (OLEDs) were fabricated using the ITO, GZO, and IZTO films as anode layers. The turn-on voltages at a current density of 4.5 mA/cm2, 5.5 mA/cm2, 6.5 mA/cm2 were 5.5 V, 13.7 V, and 4.7 V for the devices with ITO, GZO, and IZTO anodes, respectively. The best performance was observed with the IZTO film, indicating its suitability as an alternative material for conventional ITO anodes used in OLEDs and flexible displays.

Resistive‐switching mechanism of transparent nonvolatile memory device based on gallium zinc oxide

AbstractWe report an indium‐free transparent resistive switching random access memory (TRRAM) device based on a gallium‐doped zinc oxide (GZO)‐Ga2O3‐ZnO‐Ga2O3‐GZO structure grown by metal‐organic chemical vapor deposition. The bipolar resistive‐switching behavior is investigated based on the filament model. ZnO insulator layer fabricated at different temperature are compared to further support the filament mechanism. The compliance current applied on the device is changed from low to high to study the properties of conductive filaments in ZnO. The Joule‐heating effect is also studied by adjusting the integrating time in voltage sweeping.

Influence of channel‐deposition conditions and gate insulators on performance and stability of top‐gate IGZO transparent thin‐film transistors

Abstract— Amorphous‐oxide‐semiconductor thin‐film transistors (TFTs) have gained wide attention in recent years due to their many merits. In this paper, a series of top‐gate transparent thin‐film transistors (TFTs) based on amorphous‐indium—gallium—zinc—oxide (a‐IGZO) semiconductors have been fabricated and investigated. Specifically, low‐temperature SiNx and SiOx were used as the gate insulator and different Ar/O2 gas‐flow ratios were used for a‐IGZO channel deposition to study the influences of gate insulators and channel‐deposition conditions. In addition to the investigation of device performance, the stability of these TFTs was also examined by applying constant‐current stressing. It was found that a high mobility of 30‐45 cm2/V‐sec and small threshold‐voltage shift in constant‐current stressing can be achieved using SiNx with suitable hydrogen‐content stoichiometry as the gate insulator and the carefully adjusted Ar/O2 flow ratio for channel deposition. These results may be associated with hydrogen incorporation into the channel, the lower defect trap density, and the better water/oxygen barrier properties (impermeability) of the low‐temperature SiNx.

Superspace description of the homologous series Ga2O3(ZnO)m

A unified description for the structures of the homologous series Ga(2)O(3)(ZnO)(m), gallium zinc oxide, is presented using the superspace formalism. The structures were treated as a compositely modulated structure consisting of two subsystems. One is constructed with metal ions and the other with O ions. The ideal model is given, in which the displacive modulations of ions are well described by the zigzag function with large amplitudes. Alternative settings are also proposed which are analogous to the so-called modular structures. The validity of the model has been confirmed by refinements for phases with m = 6 and m = 9 in the homologous series. A few complex phenomena in real structures are taken into account by modifying the ideal model.

Instability dependent upon bias and temperature stress in amorphous‐indium gallium zinc oxide (a‐IGZO) thin‐film transistors

Abstract— The effects of gate‐bias stress, drain‐bias stress, and temperature on the electrical parameters of amorphous‐indium gallium zinc oxide (a‐IGZO) thin‐film transistors have been investigated. Results demonstrate that the devices suffer from threshold‐voltage instabilities that are recovered at room temperature without any treatments. It is suggested that these instabilities result from the bias field and temperature‐assisted charging and discharging phenomenon of preexisting traps at the near‐interface and the a‐IGZO channel region. The experimental results show that applying a drain‐bias stress obviously impacts the instability of a‐IGZO TFTs; however, the instability caused by drain bias is not caused by hot‐electron generation as in conventional MOSFETs. And the degradation trend is affected by thermally activated carriers at high temperature.

Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF sputtering at room temperature. A lowest resistivity of ∼2.8×10 −4 Ω cm was achieved for a film thickness of 1100 nm (sheet resistance ∼2.5 Ω/□), with a Hall mobility of 18 cm 2/V s and a carrier concentration of 1.3×10 21 cm −3. The films are polycrystalline with a hexagonal structure having a strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained results are discussed.

High mobility indium free amorphous oxide thin film transistors

High mobility bottom gate thin film transistors (TFTs) with an amorphous gallium tin zinc oxide (a-GSZO) channel layer have been produced by rf magnetron cosputtering using a gallium zinc oxide (GZO) and tin (Sn) targets. The effect of postannealing temperatures (200, 250, and 300°C) was evaluated and compared with two series of TFTs produced at room temperature (S1) and 150°C (S2) during the channel deposition. From the results, it was observed that the effect of postannealing is crucial for both series of TFTs either for stability as well as for improving the electrical characteristics. The a-GSZO TFTs (W∕L=50∕50μm) operate in the enhancement mode (n-type), present a high saturation mobility of 24.6cm2∕Vs, a subthreshold gate swing voltage of 0.38V/decade, a turn-on voltage of −0.5V, a threshold voltage of 4.6V, and an Ion∕Ioff ratio of 8×107, satisfying all the requirements to be used as active-matrix backplane.

Amorphous gallium indium zinc oxide thin film transistors: Sensitive to oxygen molecules

In this study, the authors report characteristic of indium gallium zinc oxides (GIZOs) which is strongly associated with the film surface. In ambient air, turn-on voltage of GIZO thin film transistors is approximately −7V. However, at the pressure of 8×10−6Torr, the turn-on voltage dramatically shifts to nearly −47V of the negative gate bias direction. When the oxygen is introduced in the chamber, the turn-on voltage returns to the normal value, that of air. It is believed that the adsorbed oxygen forms depletion layer below the surface, resulting in Von shifts. The carrier concentration of the channel varies from 1×1019to1×1020cm−3 due to oxygen adsorption.

Role of order and disorder on the electronic performances of oxide semiconductor thin film transistors

The role of order and disorder on the electronic performances of n-type ionic oxides such as zinc oxide, gallium zinc oxide, and indium zinc oxide used as active (channel) or passive (drain/source) layers in thin film transistors (TFTs) processed at room temperature are discussed, taking as reference the known behavior observed in conventional covalent semiconductors such as silicon. The work performed shows that while in the oxide semiconductors the Fermi level can be pinned up within the conduction band, independent of the state of order, the same does not happen with silicon. Besides, in the oxide semiconductors the carrier mobility is not bandtail limited and so disorder does not affect so strongly the mobility as it happens in covalent semiconductors. The electrical properties of the oxide films (resistivity, carrier concentration, and mobility) are highly dependent on the oxygen vacancies (source of free carriers), which can be controlled by changing the oxygen partial pressure during the deposition process and/or by adding other metal ions to the matrix. In this case, we make the oxide matrix less sensitive to the presence of oxygen, widening the range of oxygen partial pressures that can be used and thus improving the process control of the film resistivity. The results obtained in fully transparent TFT using polycrystalline ZnO or amorphous indium zinc oxide (IZO) as channel layers and highly conductive poly/nanocrystalline ZGO films or amorphous IZO as drain/source layers show that both devices work in the enhancement mode, but the TFT with the highest electronic saturation mobility and on/off ratio 49.9cm2∕Vs and 4.3×108, respectively, are the ones in which the active and passive layers are amorphous. The ZnO TFT whose channel is based on polycrystalline ZnO, the mobility and on/off ratio are, respectively, 26cm2∕Vs and 3×106. This behavior is attributed to the fact that the electronic transport is governed by the s-like metal cation conduction bands, not significantly affected by any type of angular disorder promoted by the 2p O states related to the valence band, or small amounts of incorporated metal impurities that lead to a better control of vacancies and of the TFT off current.

An interrogation of the zinc oxide–gallium oxide phase space by plasma enhanced chemical vapor deposition

Polycrystalline metal oxide thin films were deposited by mixing combinations of diethylzinc and trimethylgallium into an oxygen plasma. Plasma-enhanced chemical vapor deposition was shown to be a flexible tool for materials exploration, as the entire zinc–gallium-oxide phase space was explored by simply altering precursor flow rates. Film identification was performed using measurements of intrinsic optical properties as well as X-ray diffraction. The compounds synthesized included zinc oxide, gallium-doped zinc oxide (ZnO:Ga), the spinel ZnGa2O4, and amorphous gallium oxide. A phase diagram was established for PECVD synthesis as a function of the organometallic precursor composition. It was found that gallium addition had a profound impact on both the deposition rate and resistivity of the films. Small levels the gallium addition produced an order of magnitude improvement in both deposition rate and electrical properties. When the gallium fraction was >50% the deposition rate saturated and the films were insulating. Optical emission spectroscopy was used to probe the plasma chemistry of the system. It was shown to be quite complex, typified by the example that decreasing the diethylzinc fraction in the feedstream dramatically increased the density of atomic zinc in the plasma.