Ga-doped MgZnO Research Articles

Synergistic effects of Ga doping and Mg alloying over the enhancement of the stress sensitivity of a Ga-doped MgZnO pressure sensor.

We demonstrate the synergistic effects of Ga doping and Mg alloying into ZnO on the large enhancement of the piezopotential and stress sensing performance of piezotronic pressure sensors made of Ga-doped MgZnO films. Piezopotential-induced pressure sensitivity was enhanced through the modulation of the Schottky barrier height. Doping with Ga (0.62 Å) of larger ionic radius and alloying with Mg (0.57 Å) of smaller ionic radius than Zn ions can synergistically affect the overall structural, optical and piezoelectric properties of the resulting thin films. The crystal quality of Ga-doped MgZnO films either improved (XGa ≦ 0.041) or deteriorated (XGa ≧ 0.041) depending on the Ga doping concentration. The band gap increased from 3.90 eV for pristine MgZnO to 3.93 eV at XGa = 0.076, and the piezoelectric coefficient (d33) improved from ∼23.25 pm V−1 to ∼33.17 pm V−1 at an optimum Ga concentration (XGa = 0.027) by ∼2.65 times. The change in the Schottky barrier height ΔΦb increased from −4.41 meV (MgZnO) to −4.81 meV (XGa = 0.027) and decreased to −3.99 meV at a high Ga doping concentration (XGa = 0.041). The stress sensitivity (0.2 kgf) enhanced from 28.50 MPa−1 for the pristine MgZnO to 31.36 MPa−1 (XGa = 0.027) and decreased to 25.56 MPa−1 at higher Ga doping concentrations, indicating the synergistic effects of Ga doping and Mg alloying over the pressure sensing performance of Ga-doped MgZnO films.

The demonstration of recessed anodes AlGaN/GaN Schottky barrier diodes using microwave cyclic plasma oxidation/wet etching techniques

This paper presents the recessed anodes AlGaN/GaN Schottky barrier diodes (SBDs) using cyclic microwave plasma oxidation and wet etching techniques which were investigated to reduce the damage and contamination on the recessed GaN-based surface. A wide bandgap Ga-doped MgZnO (GMZO) sidewall layer structure was also conducted to suppress the hard breakdown mechanism at high reserve voltage. The SBD with recessed anodes and GMZO sidewall structure had specific on-resistance (RON_SP) of 1.99 mΩ cm2, turn-on voltage (VON) of 0.40 V, and breakdown voltage of 465 V. The SBDs in this study realized higher RON_SP and lower VON than that of the conventional device. In addition, the reverse recovery and low frequency noise characteristics of a SBD with recessed anode and GMZO sidewall structure were lower than those in the conventional structure because the anode metals are close to a high-density 2DEG by using recessed structure and the leakage current from the etching damages is declined by using a GMZO sidewall structure.

Investigation of valence plasmon excitations in GMZO thin film and their suitability for plasmon-enhanced buffer-less solar cells

The approach of eliminating buffer layer in conjunction with plasmon-enhanced transparent conduction oxide (TCO) layer is an attractive methodology to realize low-cost ultrathin buffer-less solar cells (SCs) by introducing plasmon-enhanced absorption and reduced fabrication steps. Here, we report a novel method to generate wide-band sputter-stimulated plasmonic feature in Ga-doped-MgZnO (GMZO) thin-films, which are observed due to the different metallic and metal-oxide nanoclusters formation. Through an extensive analysis of photoelectron spectroscopy, spectroscopic ellipsometry, and field-emission scanning electron microscope measurements the evaluation of plasmonic features and correlation of them with various nanoclusters inside GMZO thin-film is performed. Additionally, the suitability and expected performance of plasmon-enhanced GMZO thin-film based buffer-less SCs are probed through; 1) band-offset analysis at the plasmon enhanced-GMZO/CIGSe heterojunction; 2) simulation studies to analyze the effect of conduction band-offset (CBO) on the performance of the buffer-less SCs; 3) predicting the performance of the buffer-less SC using the parameters of GMZO thin-films with varying CBO, and 4) envisaging the concept of ultrathin buffer-less SC with calculated CBO and absorber layer thickness (300 nm) for ultrathin SCs. Moreover, at the experimentally calculated band-offset with ultrathin absorber layer thickness (300 nm), theoretically calculated buffer-less SC performance parameters estimated to be open-circuit voltage (Voc): 0.75 V, short-circuit current density (Jsc): 17.29 mA/cm2, fill-factor (FF): 80.5%, and efficiency (Eff): 10.46%.

Impact of sputter-instigated plasmonic features in TCO films: for ultrathin photovoltaic applications

The structural and optical properties of Ga-doped ZnO (GZO) and Ga-doped MgZnO (GMZO) individual films are analyzed. Sputter-instigated plasmonic features are observed in individual GZO and GMZO films due to the formation of metal and metal oxide nanoclusters. The plasmon generation is verified by electron energy loss spectra obtained by ultraviolet-photoelectron spectroscopy, spectroscopic ellipsometry, and field-emission scanning-electron microscopy measurements. This is promising in terms of increasing the efficiency of the solar cell by increasing the optical path length in the absorbing layer while keeping the same physical length by light scattering and trapping mechanism.

Plasmon generation in sputtered Ga-doped MgZnO thin films for solar cell applications

The crystalline, electrical, morphological, optical properties and plasmonic behaviour of Ga doped MgZnO (GMZO) thin films grown at different substrate temperatures (200–600 °C) by a dual ion beam sputtering (DIBS) system are investigated. Transmittance value of more than ∼94% in 400–1000 nm region is observed for all GMZO films. The particle plasmon features can be detected in the absorption coefficient spectra of GMZO grown at 500 and 600 °C in the form of a peak at ∼4.37 eV, which corresponds to a plasmon resonance peak of nanoclusters formed in GMZO. The presence of such plasmonic features is confirmed by ultraviolet photoelectron spectroscopy measurements. The values of particle plasmon resonance energy of various nanoclusters are in the range of solar spectrum, and these can easily be tuned and excited at the desirable wavelengths while optimizing the efficiency of solar cells (SCs) by simple alteration of DIBS growth temperature. These nanoclusters are extremely promising to enhance the optical scattering and trapping of the incident light, which increases the optical path length in the absorber layer of cost-effective SCs and eventually increases its efficiency.

Evaluation of the band alignment and valence plasmonic features of a DIBS grown Ga-doped Mg0.05Zn0.95O/CIGSe heterojunction by photoelectron spectroscopy

The bandgap alignment of a Ga-doped MgZnO (GMZO)/CIGSe heterojunction exposed to short duration Ar+ ion beam sputtering has been investigated by ultraviolet photoelectron spectroscopy measurement. The offset values at the valence and conduction band of the GMZO/CIGSe hetrojunction are calculated to be 2.69 and −0.63 eV, respectively. Moreover, the valence band onsets of GMZO and CIGSe thin films before and after few minutes Ar+ ion sputtering have been investigated. The presented study demonstrates the photoelectron-induced generation of resonant valence bulk and surface plasmonic features of various metal and metal oxide nanoclusters embedded within a GMZO matrix. The presence of such nanoclusters is proven to be beneficial in realizing cost-effective, ultra-thin, and high-performance photovoltaics based on the heterojunction.

Erratum to: Formation mechanism of thermally optimized Ga-doped MgZnO transparent conducting electrodes for GaN-based light-emitting diodes

Erratum to: Formation mechanism of thermally optimized Ga-doped MgZnO transparent conducting electrodes for GaN-based light-emitting diodes

Formation mechanism of thermally optimized Ga-doped MgZnO transparent conducting electrodes for GaN-based light-emitting diodes

We report a highly transparent conducting electrode (TCE) scheme of MgxZn1-xO:Ga/Au/NiOx which was deposited on p-GaN by e-beam for GaN-based light emitting diodes (LEDs). The optical and electrical properties of the electrode were optimized by thermal annealing at 500°C for 1 minute in N2 + O2 (5:3) ambient. The light transmittance at the optimal condition increased up to 84–97% from the UV-A to yellow region. The specific contact resistance decreased to 4.3(±0.3) × 10-5 Ωcm2. The improved properties of the electrode were attributed to the directionally elongated crystalline nanostructures formed in the MgxZn1-xO:Ga layer which is compositionally uniform. Interestingly, the Au alloy nano-clusters created in the MgxZn1-xO:Ga layer during annealing at 500°C may also enhance the properties of the electrode by acting as a conducting bridge and a nano-sized mirror. Based on studies of the external quantum efficiency of blue LED devices, the proposed electrode scheme combined with an optimized annealing treatment suggests a potential alternative to ITO. Open image in new window

Thermal stability of post-growth-annealed Ga-doped MgZnO films grown by the RF sputtering method

ABSTRACTA radio-frequency magnetron sputtering technique and subsequent rapid thermal annealing (RTA) at 600, 700, 800, and 900 °C were implemented to grow high-quality Ga-doped MgxZn1-xO (GMZO) epi-layers. The GMZO films were deposited using a radio-frequency magnetron sputtering system and a 4 inch ZnO/MgO/Ga2O3 (75/20/5 wt %) target. The Hall results, X-ray diffraction (XRD), and transmittance were determined and are reported in this paper. The Hall results indicated that the increase in mobility was likely caused by the improved crystallization in the GMZO films after thermal annealing. The XRD results revealed that MgxZn1-xO (111) and MgO2 (200) peaks were obtained in the GMZO films. The absorption edges of the as-grown and annealed GMZO films shifted toward the short wavelength of 373 nm at a transmittance of 90%. According to these results, GMZO films are feasible for forming transparent contact layers for near-ultraviolet light-emitting diodes.

Transparent conductive Ga-doped MgZnO/Ag/Ga-doped MgZnO sandwich structure with improved conductivity and transmittance

This study presents the advantages of combining Ga-doped MgZnO (GMZO) transparent conductive oxide (TCO) with a silver (Ag) layer to form a GMZO/Ag/GMZO composite structure using a radio-frequency magnetron sputtering and electron beam system. The proposed structure improves the electrical properties of a single GMZO bulk and allows a low processing temperature. This study also investigates the effects of the silver thickness and post-annealing process on the structural, electrical, and optical properties of the GMZO/Ag/GMZO composite sandwich structure. The optimal GMZO/Ag/GMZO structural thickness was 40/20/60nm, which yielded a resistivity of 5.8×10−5Ωcm with an average transmittance of 81.0% at a wavelength range of 400–800nm. The figure of merit (ΦTC) for the as-deposited sandwich structure was calculated as 2.1×10−2Ω−1. This is approximately two times higher than the best results obtained from a single GMZO bulk layer with an annealing temperature of 600°C (1.1×10−2Ω−1). This remarkable improvement of the GMZO/Ag/GMZO sandwich structure is a highly promising candidate for optoelectronic devices, and has a low processing temperature than a single TCO bulk layer.

Phase separation in Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy

We report on the phase separation of Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy. Based on X-ray diffraction, low-temperature (10 K) photoluminescence, and reflection high-energy electron diffraction observations, it is possible to classify the phase of Ga-doped MgZnO layers into three regions depending on the incorporated Ga concentration ([Ga]). Single-phase Mg 0.1Zn 0.9O layers are grown when [Ga] is less than 1×10 18 cm −3. For [Ga] between 1×10 18 cm −3 and 1×10 20 cm −3, ZnO and Mg 0.2Zn 0.8O coexist, where electron transport is considered to be via two-channel conduction. When [Ga] exceeds 1×10 20 cm −3, the Ga-doped MgZnO layers become polycrystalline, where carrier compensation takes place presumably due to grain boundaries.