Lithium Gallate Research Articles

Sharp-to-Broad Band Energy Transfer in Lithium Aluminate and Gallate Phosphors for SWIR LED

Sharp-to-Broad Band Energy Transfer in Lithium Aluminate and Gallate Phosphors for SWIR LED

Exploring the luminescence of tin‐incorporated nickel‐sensitized lithium gallate short‐wave‐infrared phosphors

AbstractA series of Ni2+‐sensitized LiGa5O8 nanocrystals doped with varying amounts of Sn4+ were synthesized via a high‐temperature solid‐state method. X‐ray diffraction and scanning electron microscopy were employed to investigate the microstructure of the LiGa5O8 host, and optical spectroscopy was used to examine the effects of Sn4+ addition on the emission spectra and persistent luminescence (PersL) properties, which indicated a homogeneous distribution of the constituent elements in the phosphor. The Sn4+ incorporation led to an approximately sixfold enhancement of photoluminescence intensity at room temperature and decrease in the energy transition of Ni2+(3T2(3F)→3A2(3F)), which resulted in ∼65 nm bathochromic shift in the photoluminescence emission maxima. However, the addition of Sn4+ dopant led to a decrease in the quantum yield of luminescence compared to that of the original phosphor. Temperature‐dependent thermoluminescence measurements revealed that doping of LiGa5O8 with Sn4+ interfered with the Ni2+ trap centers. Moreover, Ni2+‐sensitized Sn4+‐doped LiGa5O8 nanocrystals exhibited an afterglow effect that persisted for up to 300 s.

Effect of Eu3+ doping on the structural, optical, and photoluminescent properties of LiGa5O8 phosphor

This study reports on the synthesis and characterization of a series of lithium gallate (LiGa5O8) phosphors doped with varying concentrations of Eu3+ ions.

Tailoring grain boundary structures and chemistry of Li7La3Zr2O12 solid electrolytes for enhanced air stability

Solid-state batteries with inorganic solid electrolytes provide a fundamental solution for resolving safety concerns. Garnet-type Li7La3Zr2O12 (LLZO) is considered a promising candidate for solid electrolytes because of its high Li+ conductivity and superior chemical/electrochemical stability against metallic Li. However, when exposed to ambient air, LLZO electrolytes react with H2O and CO2 to form Li2CO3, resulting in significant degradation of Li+ conductivity. In this study, we propose a simple but effective approach to enhance air stability of LLZO via tailoring grain boundary structures and chemistry. The interfacial stability of the solid electrolytes is examined under accelerated durability test (ADT) conditions, where the concentrations of O2, H2O, and CO2 are precisely controlled to promote interfacial reactions. We show that Ga incorporation into Ta-doped LLZO (LLZTO) plays a crucial role in governing the grain growth behavior during the sintering process to modify the density, morphology, and composition of the grain boundaries. Furthermore, Ga-incorporated LLZTO (Ga-LLZTO) exhibits remarkably improved stability over LLZTO upon ADTs with high H2O and CO2 concentrations and enables stable cycling of metallic Li electrodes. The combined microstructural/compositional analyses and theoretical simulations suggest that the enhanced air stability of Ga-LLZTO can be attributed to the remarkably reduced grain boundary density with enlarged grains and segregation of H2O/CO2-tolerant lithium gallate (LiGaO2) in the grain boundaries. The findings of this study are critical for understanding the role of microstructural engineering in mitigating the degradation of Li+ conductivity and developing highly conductive and stable LLZO electrolytes.

Energy efficient microwave-assisted preparation of deep red/near-infrared emitting lithium aluminate and gallate phosphors

The development of impurity doped luminescent materials, or phosphors, have been continuously pursued for a variety of applications nowadays. However, the majority of phosphors undergo time-consuming and energy inefficient preparation, such as conventional high-temperature solid-state reaction (SSR). In the present work, a facile microwave-assisted solid-state (MASS) method is used to prepare a series of transition metal (Mn4+ or Cr3+) doped lithium aluminates and gallates. With a one-step 25-min MASS method, LiAlO2 is obtained in γ phase rather than α phase. Using the same method, followed by a short thermal treatment in air or two-step MASS preparation, inverse spinel LiAl5O8:Mn4+ phosphors with deep-red emission are obtained, while near-infrared emitting LiGa5O8:Cr3+ persistent phosphors are successfully prepared with a 40-min MASS process. Compared to the SSR method, LiGa5O8:Cr3+ achieved by MASS exhibits totally different trap depth and trap distributions, which is evidenced by the temperature dependency of persistent luminescence. The study of the detailed preparation of transition metal doped lithium aluminates and gallates may enable more widespread use of MASS method and facilitate the exploration of new luminescence properties involving defects.

Lithium and gallium vacancies in LiGaO2 crystals

Lithium gallate (LiGaO2) is a wide-band-gap semiconductor with an optical gap greater than 5.3 eV. When alloyed with ZnO, this material offers broad functionality for optical devices that generate, detect, and process light across much of the ultraviolet spectral region. In the present paper, electron paramagnetic resonance (EPR) is used to identify and characterize neutral lithium vacancies (VLi0) and doubly ionized gallium vacancies (VGa2−) in LiGaO2 crystals. These S = 1/2 native defects are examples of acceptor-bound small polarons, where the unpaired spin (i.e., the hole) is localized on one oxygen ion adjacent to the vacancy. Singly ionized lithium vacancies (VLi−) are present in as-grown crystals and are converted to their paramagnetic state by above-band-gap photons (x rays are used in this study). Because there are very few gallium vacancies in as-grown crystals, a post-growth irradiation with high-energy electrons is used to produce the doubly ionized gallium vacancies (VGa2−). The EPR spectra allow us to establish detailed models for the two paramagnetic vacancies. Anisotropy in their g matrices is used to identify which of the oxygen ions adjacent to the vacancy has trapped the hole. Both spectra also have resolved structure due to hyperfine interactions with 69Ga and 71Ga nuclei. The VLi0 acceptor has nearly equal interactions with Ga nuclei at two Ga sites adjacent to the trapped hole, whereas the VGa2− acceptor has an interaction with Ga nuclei at only one adjacent Ga site.

Stabilization of orthorhombic phase in single-crystal ZnSnN2 films

We report on the crystal structure of epitaxial ZnSnN2 films synthesized via plasma-assisted vapor deposition on (111) yttria stabilized zirconia (YSZ) and (001) lithium gallate (LiGaO2) substrates. X-ray diffraction measurements performed on ZnSnN2 films deposited on LiGaO2 substrates show evidence of single-crystal, phase-pure orthorhombic structure in the Pn21a symmetry [space group (33)], with lattice parameters in good agreement with theoretically predicted values. This Pn21a symmetry is imposed on the ZnSnN2 films by the LiGaO2 substrate, which also has orthorhombic symmetry. A structural change from the wurtzite phase to the orthorhombic phase in films grown at high substrate temperatures ∼550°C and low values of nitrogen flux ∼10−5 Torr is observed in ZnSnN2 films deposited on YSZ characterized by lattice contraction in the basal plane and a 5.7% expansion of the out-of-plane lattice parameter.

High-gain Zn1-xMgxO-based ultraviolet photodetectors on Al2O3and LiGaO2substrates

We report the fabrication and characterization of highly responsive ZnMgO-based ultraviolet (UV) photodetectors in the metal–semiconductor–metal (MSM) configuration for solar-blind/visible-blind optoelectronic application. MSM devices were fabricated from wurtzite Zn1–xMgx O/ZnO (x ∼ 0.44) thin-film heterostructures grown on sapphire (α-Al2O3) substrates and w-Zn1–xMgx O (x ∼ 0.08), grown on nearly lattice-matched lithium gallate (LiGaO2) substrates, both by radio-frequency plasma-assisted molecular beam epitaxy (PAMBE). Thin film properties were studied by AFM, XRD, and optical transmission spectra, while MSM device performance was analyzed by spectral photoresponse and current–voltage techniques. Under biased conditions, α-Al2O3 grown devices exhibit peak responsivity of ∼7.6 A/W at 280 nm while LiGaO2 grown samples demonstrate peak performance of ∼119.3 A/W, albeit in the UV-A regime (∼324 nm). High photoconductive gains (76, 525) and spectral rejection ratios (∼103, ∼104) were obtained for devices grown on α-Al2O3 and LiGaO2, respectively. Exemplary device performance was ascribed to high material quality and in the case of lattice-matched LiGaO2 films, decreased photocarrier trapping probability, presumably due to low-density of dislocation defects. To the best of our knowledge, these results represent the highest performing ZnO-based photodetectors on LiGaO2 yet fabricated, and demonstrate both the feasibility and substantial enhancement of photodetector device performance via growth on lattice-matched substrates. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Evidence of CO2 Chemisorption at High Temperature in Lithium Gallate (Li5GaO4)

Abstract Li5GaO4 was tested as a possible CO2 captor. Li5GaO4 was synthesized by solid-state reaction, structurally characterized, and then thermally treated under a CO2 flow, from 30 to 900 °C, having the highest CO2 chemisorption at around 709 °C. The results clearly showed that Li5GaO4 is able to trap CO2 chemically in two different steps. The CO2 quantity trapped was equal to 8.9 mmol g−1, which is considerably high in comparison to other ceramics.

In-plane anisotropic strain of elastically and plastically deformed III-nitrides on lithium gallate

We have investigated both elastically and plastically deformed GaN films on lithium gallate, LiGaO 2, by molecular beam epitaxy. The in-plane lattice parameters were determined from high resolution X-ray diffraction and indicated two different groups of in-plane lattice parameters, influenced by the a- and b-axis of LiGaO 2. The measured in-plane lattice parameters indicate that there exist both compressive and tensile strains of in-plane GaN along the a- and b-axis of LiGaO 2, respectively. This anisotropic strain in GaN films forms a slight distortion of the basal-plane hexagonal structure of GaN films, leading to a different critical thickness of 4.0 ± 0.17 and 7.8 ± 0.7 nm along the a- and b-axis of LiGaO 2, respectively.

Heat Dissipation in High-Power GaN Electronics on Thermally Resistive Substrates

The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was investigated. The thermal conductivity of single-crystal LGO was measured utilizing the 3/spl omega/ technique for temperatures ranging from 100 K-500 K. For the GaN layer, the thermal conductivity was estimated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junction temperature of 500 K, it was found that devices with a power dissipation of 1 W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.

The surface modification and reactivity of LiGaO 2 substrates during GaN epitaxy

The chemistry and kinetics of lithium gallate (LGO) substrates during nitridation are investigated. Nitridation experiments have been carried out using two remote nitrogen RF plasma sources: in an MBE system and in a remote plasma MOCVD system. The difference between the two nitrogen sources is the pressure. The experiments were run in parallel to demonstrate that the same heterogeneous chemistry applies during LGO nitridation in both MBE and MOCVD environments, provided that the same species is produced in the gas phase. Surface analysis techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE), show that an optimal temperature of about 600°C and an optimal time that depends on the incident nitrogen density exists that results in the formation of ∼5 Å of GaN on LGO. The nitridation process competes with lattice damage that is enhanced by the presence of hydrogen.

Challenges and potential payoff for crystalline oxides in wide bandgap semiconductor technology

While growth of wide bandgap semiconductor materials on crystalline oxides (sapphire, lithium gallate, lithium aluminate, zinc oxide and others) has become routine, growth of crystalline oxides on wide bandgap materials remains challenging and minimally explored. The potential payoff in terms of enhanced device performance, increased functionality and reliability warrants examining this option. This presentation aims at targeting key areas, where crystalline oxides could improve wide bandgap semiconductor device performance. Some of these include the use of ferroelectric oxides for power switching applications, oxides with anisotropic dielectric constants for high voltage termination and oxides with large electric flux density near breakdown. Unique polarization engineered structures are described that are enabled by using lithographically defined poled regions in a ferroelectric substrate. The desired crystalline oxide properties, potential implementation challenges and potential pitfalls will be discussed.

Synthesis, properties and photolysis of new iodonium tetrakis(pentafluorophenyl)gallate photoinitiators and comparison with their indate and aluminate analogs

A series of new iodonium tetrakis(pentafluorophenyl)gallates were synthesized in high yield via an improved preparation of the lithium gallate, their photoproducts analyzed by UV, 19F NMR and GC–MS spectra and a photolysis mechanism postulated. The stability of these photoinitiators was also compared with their borate, aluminate and indate analogs.

The heterogenous integration of GaN thin-film metal-semiconductor-metal photodetectors onto silicon

The heterogeneous integration of GaN thin-film metal-semiconductor-metal (MSM) photodetectors onto a host substrate of SiO/sub 2/-Si is reported. Thin-film GaN photodetectors were separated from the lithium gallate (LiGaO/sub 2/) growth substrate using selective etching, and contact bonded onto a SiO/sub 2/-Si host substrate. The thin-film MSMs exhibited a dark current of 13.36 pA and an UV photoresponse at 308 nm of 0.11 A/W at a reverse bias voltage of 20 V. This first demonstration of GaN thin-film device integration onto SiO/sub 2/-Si using a low-temperature integration process, combined with the advances in GaN material quality on LiGaO/sub 2/ substrates, enables the integration of GaN devices with Si circuitry for heterogeneously integrated systems.

Time Resolved Optical Studies of InGaN Layers Grown on LGO

ABSTRACTRadiative recombination processes in bulk InGaN grown by molecular beam epitaxy (MBE) on lithium gallate (LGO or LiGaO2) substrates were investigated using microscopic PL and time-resolved photoluminescence (TRPL). The improved structural quality resulting from a better lattice match of the LGO substrate to III-V nitride materials simplifies these investigations because well-defined composition phases can be analyzed for both homogeneous and phased separated InGaN samples. Epilayers of InGaN intentionally grown with and without indium segregation were studied. X-ray diffraction measurements showed that the homogeneous epilayer was high quality In0.208Ga0.702N and the segregated epilayer exhibited peaks corresponding to both In0.289Ga0.711N and In0.443Ga0.557N indicating the presence of higher In concentration regions in this sample. Spatially resolved photoluminescence spectra confirm the existence of these regions. The photoluminescence intensity decay is non-exponential for both samples and a stretched exponential fit to the decay data confirms the existence of local potential fluctuations in which carriers are localized before recombination.

Recent Advances in III-Nitride Devices Grown on Lithium Gallate

Recent advances in the growth of III-nitride materials and devices include: (i) The reduction of the near-surface threading dislocation density in GaN on lithium gallate (LGO) to ≈2 × 107 cm—2. (ii) The demonstration of GaN, 50 × 130 μm, metal–semiconductor–metal (MSM) photodiodes with extremely low leakage current, 0.11 pA at 2 V and 7.9 pA at 60 V, and UV photoresponse at 308 nm and 20 V of 0.105 A/W. (iii) State of the art MSM devices have been successfully removed from the LGO substrate and attached to silicon wafers with no degradation in current characteristics. (iv) Demonstration of very thin, 0.7 μm HFET structures, grown at a rapid rate of 0.9 μm/h, with near state of the art room temperature 2DEG mobilities of 1365 cm2/Vs at a sheet charge of 9 × 1012 cm—2. (v) The elimination of substrate impurity diffusion by inclusion of gettering buffers has also been demonstrated.

Recent Advances in III-Nitride Devices Grown on Lithium Gallate

Recent advances in the growth of III-nitride materials and devices include: (i) The reduction of the near-surface threading dislocation density in GaN on lithium gallate (LGO) to ≈2 × 107 cm—2. (ii) The demonstration of GaN, 50 × 130 μm, metal–semiconductor–metal (MSM) photodiodes with extremely low leakage current, 0.11 pA at 2 V and 7.9 pA at 60 V, and UV photoresponse at 308 nm and 20 V of 0.105 A/W. (iii) State of the art MSM devices have been successfully removed from the LGO substrate and attached to silicon wafers with no degradation in current characteristics. (iv) Demonstration of very thin, 0.7 μm HFET structures, grown at a rapid rate of 0.9 μm/h, with near state of the art room temperature 2DEG mobilities of 1365 cm2/Vs at a sheet charge of 9 × 1012 cm—2. (v) The elimination of substrate impurity diffusion by inclusion of gettering buffers has also been demonstrated.

The status and promise of compliant substrate technology

Compliant substrates offer significant promise as a new approach for strain management in semiconductors. The primary application is to produce device-quality highly mismatched materials on dissimilar substrates. Various implementations and processes for achieving substrate compliancy have been proposed and demonstrated. These include the use of twist-, glass-, and metal-bonds, and the use of engineered templates realized with laterally oxidized Al-containing alloys. A recent focus in our work has been on the growth of GaN on a novel and easily removable substrate — lithium gallate — for regrowth on a bonded GaN template.

Interfacial Structure and Defects in GaN/AlGaN Heterojunction Epitaxially Grown on LiGa02 Substrate by Molecular Beam Epitaxy

Abstract The performance of III-Nitride based Light Emitting Diodes (LEDs), LASERs, GaN/AlGaN MODFETs (Modulation-doped Field Effect Transistors) and HEMTs (High Electron Mobility Transistors) have been improved dramatically over the past few years [1,2], despite the relatively poor material quality of GaN, as compared to GaAs, for example. The intrinsic properties of the materials system make it extremely well suited to both optoelectronic and microwave power transistor applications. Typically, GaN is grown on substrates such as GaAs, Al2O3 (sapphire) or SiC with large lattice mismatch. This has usually resulted in an extremely high defect density in the GaN layer. The growth of GaN on lithium gallate LiGaO2 (LGO) affords many advantages compared to all other available substrates. LGO offers the smallest average lattice mismatch of any available substrate for the Ill-nitrides. This facilitates the growth of high quality GaN directly on Lithium Gallate without the need for a defective buffer to decouple the strain associated with the large lattice mismatch of other substrates [3].