Mg-doped Gallium Nitride Research Articles

Resonant Lasing Emission in Undoped and Mg-Doped Gallium Nitride Thin Films on Interfacial Periodic Patterned Sapphire Substrates.

In this work, low-threshold resonant lasing emission was investigated in undoped and Mg-doped GaN thin films on interfacial designed sapphire substrates. The scattering cross-section of the periodic resonant structure was evaluated by using the finite difference time domain (FDTD) method and was found to be beneficial for reducing the threshold and enhancing the resonant lasing emission within the periodic structures. Compared with undoped and Si-doped GaN thin films, p-type Mg-doped GaN thin films demonstrated a better lasing emission performance. The lasing energy level system and defect densities played vital roles in the lasing emission. This work is beneficial to the realization of multifunctional applications in optoelectronic devices.

Self-powered, low-noise and high-speed nanolayered MoSe2/p-GaN heterojunction photodetector from ultraviolet to near-infrared wavelengths

Integration of nanolayered metal chalcogenides with wide-bandgap semiconductors forming pn heterojunction leads to the way of high-performance photodetection. This work demonstrates the fabrication of a few nanometer thick Molybdenum diselenide (MoSe2)/Mg-doped Gallium Nitride (p-GaN) heterostructure for light detection purposes. The device exhibits low noise broadband spectral response from ultraviolet to near-infrared range (300–950 nm). The band-alignment and the charge transfer at the MoSe2/p-GaN interface promote self-powered photodetection with high photocurrent to dark current ratio of 2000 and 1000 at 365 nm and 640 nm, respectively. A high responsivity of 130 A W−1, detectivity of 4.8 × 1010 Jones, and low noise equivalent power of 18 fW/Hz1/2 at 365 nm is achieved at an applied bias of 1 V. Moreover, the transient measurements reveal a fast rise/fall time of 407/710 μsec for the fabricated device. These outcomes exemplify the viability of MoSe2/p-GaN heterostructure for high-speed and low-noise broadband photodetector applications.

Mechanism of Photocurrent Degradation and Contactless Healing in p-Type Mg-Doped Gallium Nitride Thin Films.

Light-induced degradation (LID) phenomenon is commonly found in optoelectronics devices. Self-healing effect in halide lead perovskite solar cells was investigated since the electrons and holes in the shallow traps could escape easily at room temperature. However, the degradation in the semiconductors could not easily recover at room temperature, and many of them needed annealing at temperatures in the several hundreds, which was not friendly to the integrated optoelectronic semiconductor devices. To solve this problem, in this work, LID effect of photocurrent in p-type Mg-doped gallium nitride thin films was investigated, and deep defect and vacancy traps played a vital role in the LID and healing process. This work provides a contactless way to heal the photocurrent behavior to its initial level, which is desirable in integrated devices.

Optical Transitions and Magnetism in Mn-Implanted Gallium Nitride for Three-Level Magnetooptic Devices

A method of preparing passive materials suitable for magnetooptic interactions is shown using manganese implantation into gallium nitride (GaN) epitaxial layers, which establishes both dilute ferromagnetism and a three-level optical system with persistence to over 300 K. A sweep of thermal anneal parameters for a high implant dose into Mg-doped p-type GaN films is presented, and the materials are tested for both their magnetization and photoluminescence (PL). The optimal anneal process at 825 °C for 5 min maintains ferromagnetism with $T_{C}$ > 305 K, confirming magnetic alignment at room temperature with a coercivity of ~100 Oe. PL and spectrophotometry of the optimally prepared materials show the effects of the mid-gap defect state on the material's optical characteristics. The anneal process returns the real part index to its baseline dispersion while retaining an onset of absorption starting at the defect level $E_{A}$ = 1.8 eV, signifying a stable mid-gap energy transition with a measured state lifetime of $τ _{PL}$ = 2.7 ns. The scalability of this process for producing three-level transition magnetic materials suggests passive optical or magnetooptic devices can be constructed that interconnect photonic and spintronic effects for emerging system designs and potential applications in quantum information.

Different Effects of Mg and Si Doping on the Thermal Transport of Gallium Nitride

Mg and Si as the typical dopants for p- and n-type gallium nitride (GaN), respectively, are widely used in GaN-based photoelectric devices. The thermal transport properties play a key role in the thermal stability and lifetime of photoelectric devices, which are of significant urgency to be studied, especially for the Mg- and Si-doped GaN. In this paper, the thermal conductivities of Mg- and Si-doped GaN were investigated based on first-principles calculations and phonon Boltzmann transport equation. The thermal conductivities of Mg-doped GaN are found to be 5.11 and 4.77 W/mK for in-plane and cross-plane directions, respectively. While for the Si-doped GaN, the thermal conductivity reaches the smaller value, which are 0.41 and 0.51 W/mK for in-plane and cross-plane directions, respectively. The decrease in thermal conductivity of Mg-doped GaN is attributed to the combined effect of low group velocities of optical phonon branches and small phonon relaxation time. In contrast, the sharp decrease of the thermal conductivity of Si-doped GaN is mainly attributed to the extremely small phonon relaxation time. Besides, the contribution of acoustic and optical phonon modes to the thermal conductivity has changed after GaN being doped with Mg and Si. Further analysis from the orbital projected electronic density of states and the electron localization function indicates that the strong polarization of Mg-N and Si-N bonds and the distortion of the local structures together lead to the low thermal conductivity. Our results would provide important information for the thermal management of GaN-based photoelectric devices.

Photochemical CO2 conversion on pristine and Mg-doped gallium nitride (GaN): a comprehensive DFT study based on a cluster model approach

Photochemical reduction of carbon dioxide into methanol on bare and Mg-doped gallium nitrides. A detailed DFT study to shed light on the photocatalytic activity of this promising material.

Fabrication of a thermostable Ga-face GaN template on a molybdenum substrate via layer transfer

Gallium nitride (GaN) films on high-thermal-conductivity substrates have attracted considerable attention for their applications in high-power light-emitting diodes and electronic devices. Herein, a 2-inch 8-µm-thick thermostable GaN/Mo template with Ga-face was fabricated via two consecutive layer transfer technique. The full-widths at half-maximum for the x-ray rocking curves of GaN (002) and (102) plane were 314 and 325 arcsec, respectively. Atomic force microscopy revealed that the surface had step-and-terrace structures with a root-mean-square value of 0.397 nm. Five periods of In0.15Ga0.85N/GaN multiple-quantum-wells and Mg-doped p-type GaN layers were regrown on the GaN/Mo template, which exhibited blue light emission without distinct degradation.

Ultrafast spatiotemporal photocarrier dynamics near GaN surfaces studied by terahertz emission spectroscopy

Gallium nitride (GaN) is a promising wide-bandgap semiconductor, and new characterization tools are needed to study its local crystallinity, carrier dynamics, and doping effects. Terahertz (THz) emission spectroscopy (TES) is an emerging experimental technique that can probe the ultrafast carrier dynamics in optically excited semiconductors. In this work, the carrier dynamics and THz emission mechanisms of GaN were examined in unintentionally doped n-type, Si-doped n-type, and Mg-doped p-type GaN films. The photocarriers excited near the surface travel from the excited-area in an ultrafast manner and generate THz radiation in accordance with the time derivative of the surge drift current. The polarity of the THz amplitude can be used to determine the majority carrier type in GaN films through a non-contact and non-destructive method. Unique THz emission excited by photon energies less than the bandgap was also observed in the p-type GaN film.

Impact of Surface Chemistry and Doping Concentrations on Biofunctionalization of GaN/Ga‒In‒N Quantum Wells.

The development of sensitive biosensors, such as gallium nitride (GaN)-based quantum wells, transistors, etc., often makes it necessary to functionalize GaN surfaces with small molecules or even biomolecules, such as proteins. As a first step in surface functionalization, we have investigated silane adsorption, as well as the formation of very thin silane layers. In the next step, the immobilization of the tetrameric protein streptavidin (as well as the attachment of chemically modified iron transport protein ferritin (ferritin-biotin-rhodamine complex)) was realized on these films. The degree of functionalization of the GaN surfaces was determined by fluorescence measurements with fluorescent-labeled proteins; silane film thickness and surface roughness were estimated, and also other surface sensitive techniques were applied. The formation of a monolayer consisting of adsorbed organosilanes was accomplished on Mg-doped GaN surfaces, and also functionalization with proteins was achieved. We found that very high Mg doping reduced the amount of surface functionalized proteins. Most likely, this finding was a consequence of the lower concentration of ionizable Mg atoms in highly Mg-doped layers as a consequence of self-compensation effects. In summary, we could demonstrate the necessity of Mg doping for achieving reasonable bio-functionalization of GaN surfaces.

Effect of varying thermal annealing temperatures on the surface and electrical properties of Mg-doped GaN

Gallium nitride (GaN)-based light emitting diodes (LEDs) have been widely used to produce blue light that, with phosphor, is converted into white light for every-day applications. Despite its high efficiency, the performance of GaN based LEDs is limited by the low electrical conductivity of magnesium (Mg)-doped GaN due to the high activation energy of Mg (140 – 200 meV) as well as the presence of magnesium-hydride (Mg-H) complex, which may passivate Mg as an acceptor. In this study, the efficacy of thermal annealing treatment at temperatures in the range of 550 - 850°C was investigated to activate metal-organic chemical vapor deposition (MOCVD) grown Mg-doped GaN. Changes in the roughness and surface morphology was observed between the varied annealing temperatures by atomic force microscopy (AFM) while X-ray diffraction (XRD) data acts as supporting information for structural quality. Hall effect characterization was conducted to determine electrical properties of the annealed samples. In this study, a correlation between the resistivity and surface roughness trends was observed through the acquisition of an insight into the lattice distortion affecting the material conductivity that occurred specifically at temperatures above 750 °C.

Electronic surface, optical and electrical properties of p – GaN activated via in-situ MOCVD and ex-situ thermal annealing in InGaN/GaN LED

Electronic surface properties, optical and electrical characteristics of p-type Mg-doped Gallium Nitride (p-GaN) activated under different thermal annealing condition were investigated. In this work, p-GaN samples were subjected to in-situ and ex-situ thermal annealing process at 650 °C in Nitrogen (N2) rich condition. In-situ annealing process took place in Metal Oxide Chemical Vapor Deposition (MOCVD) chamber while ex-situ annealing process was carried out in the conventional oven. X-Ray and Ultraviolet Photoemission Spectroscopy (XPS/UPS) were used to observe the energy alignment of the p-GaN surface. From PES spectra, the sample subjected to in-situ thermal annealing shown to exhibit lower surface bend bending of 0.28 eV as compared to ex-situ thermal annealing activation with 0.45 eV band bending. This result is in agreement with specific contact resistance measurement that shows in-situ sample exhibits lower resistance resulted in better carrier injection from metal to p-GaN. Photoluminescence (PL) spectra elucidates that in-situ sample has a good surface quality with less nitrogen related vacancies (VN) formed on the p-GaN surface. All these results are further proved with the higher output power from LED with in-situ annealing. At 20 mA, in-situ sample shown an increment of ~14% light output power compared to ex-situ sample. Results obtained in this work suggest that the thermal activation condition for p-GaN activation process plays an active role on the surface quality as well as the energy alignment of the film surface and shows the potential of in-situ p-GaN activation for tunnel junction structure.

UV Photodetector Based on Mg-Doped GaN Thin Films Prepared by Sol-Gel Spin Coating

In this work, sol-gel spin coated of magnesium (Mg) doped gallium nitride (GaN) thin films grown on AlN sapphire substrate was reported. The structural, lattice vibrational, and electrical properties of the deposited films were investigated and compared. X-ray diffraction results show that the deposited films composed of wurtzite structure with preferred orientation of GaN(002). The Raman active phonon modes correspond to the E2(high) and A1(LO) at 568 cm-1 and 733 cm-1 phonon modes of the hexagonal GaN were observed, while a broad peak attributed to the Mg-related lattice vibrational mode was detected at 669 cm-1. The Raman phonon modes were detected by using Raman spectroscopy. Hall effects results show that the resistivity, carrier concentration, and hall mobility of the Mg-doped GaN film was 0.1397 Ω cm, 1.77 × 1018 cm3, and 6.04 cm2/Vs, respectively. Besides, the characteristics of the ultraviolet (UV) photoresponse of the fabricated detector were investigated. The current-voltage characteristics of the Mg-doped GaN UV photodetector exhibits Schottky behaviour. The current-voltage measurements were carried out at room temperature with a computer-controlled integrated Source Meter (Keithley 2400). Lastly, the ideality factor and Schottky barrier heights were calculated using thermionic emission theory.

Effects of microwave activation power on the structural properties of sol-gel spin coated magnesium doped gallium nitride thin films

Abstract In this paper, the effects of microwave (MW) activation power on the structural properties of sol-gel spin coated magnesium (Mg) doped gallium nitride (GaN) thin films grown on sapphire (001) substrates were reported. The activation processes were carried out using different microwave powers (i.e., 100 W, 200 W, 300 W and 450 W) for a duration of 10 minutes. X-ray diffraction results indicate that the Mg-doped GaN thin films exhibit hexagonal wurtzite structure with (002) preferential orientation. Mg-doped GaN thin film activated at 450 W has the highest dislocation density, δ, implies that it has the largest amount of nitrogen vacancies compared to all the other samples, and consequently the poorest crystalline quality as proved by the drastic decrease in intensities of the XRD peaks of the film. Since nitrogen vacancies are favorably formed only upon the removal of hydrogen from p-type GaN, it can be concluded that the activation process of Mg dopants is most efficient at 450 W. Tensile strain along c-axis of the film activated at 450 W further validates this statement. In summary, 450 W is the best power for the activation process of Mg dopant, but yet it is not the most ideal microwave power because it deteriorates the crystalline quality of the films.

Efficient Electrochemical Potentiostatic Activation Method for GaN-Based Green Vertical-LEDs

An efficient electrochemical potentiostatic activation (EPA) method was designed and performed for hydrogen atoms removal from inside a Mg-doped gallium nitride (GaN) layer of green vertical light-emitting diodes (V-LEDs). The EPA method was conducted at the potentiostatic conditions of 2, 3, and 5 V for 5 mins. The role of the applied voltage value in the breaking of the Mg-H complexes and in increasing the holes concentration inside the p-GaN layer in terms of LED device performances was investigated. The internal quantum efficiency (IQE) of the green V-LEDs behaved inversely with the applied voltage. The IQE of green V-LEDs EPA processed with a low voltage of 2 V realized an improvement of about 6% and only about 2.5% for green V-LEDs EPA processed with a high voltage of 5 V at an injection current of 100 mA compared to the conventional rapid thermal annealing method. The light output power achieved the highest enhancement of about 10.5% at 100 mA when applying the lowest voltage of 2 V, which originated from an improvement of the IQE. The forward voltage was also reduced after the EPA process. The developed EPA method was proven to effectively improve the external quantum efficiency of green V-LEDs.

Mg-compensation effect in GaN buffer layer for AlGaN/GaN high-electron-mobility transistors grown on 4H-SiC substrate

The present study investigated the Mg doping effect in the gallium nitride (GaN) buffer layers (BLs) of AlGaN/GaN high-electron-mobility transistor (HEMT) structures grown on semi-insulating 4H-SiC substrates by metal organic chemical vapor deposition. When the Mg concentration was increased from 3 × 1017 to 8 × 1018 cm−3, the crystal quality slightly deteriorated, whereas electrical properties were significantly changed. The buffer leakage increased approximately 50 times from 0.77 to 39.2 nA at −50 V with the Mg doping concentration. The Mg-compensation effect and electron trapping effect were observed at Mg concentration of 3 × 1017 and 8 × 1018 cm−3, respectively, which were confirmed by an isolation leakage current test and low-temperature photoluminescence. When the BL was compensated, the two-dimensional electron gas (2DEG) mobility and sheet carrier concentration of the HEMTs were 1560 cm2 V−1 s−1 and 5.06 × 1012 cm−2, respectively. As a result, Mg-doped GaN BLs were demonstrated as a candidates of semi-insulating BLs for AlGaN/GaN HEMT.

Synthesis of ZnO Nanowire Array Film on Mg-Doped Gallium Nitride Substrate by Simple Hydrothermal Method

ZnO nanowire array films, composed of well-aligned ZnO nanowires with about 200 nm in diameter and 1 μm in length, were successfully synthesized on Mg-doped gallium nitride by hydrothermal method. The films possess quite flattened surface. In the synthesized process, there was no catalyst used. Growth conditions are comprehensively discussed in the process of aqueous solution method. It was found that the length of ZnO nanowires and the thickness of the film could be tunable by altering solution concentration and growth time. Such ZnO film assembled of vertically aligned nanowire may have potential applications as UV light-emitting diodes.

Some Properties of Group-III Nitride Thin Films Directly Grown on Non-Single-Crystalline Substrates by Using a Molecular Beam Epitaxy Apparatus

Gallium nitride (GaN) and indium-gallium nitride (InxGa1-xN) thin films were directly grown on several non-single-crystalline substrates such as quartz glass and amorphous-carbon-coated graphite. The films were grown by using a molecular beam epitaxy apparatus having single or dual nitrogen radio-frequency plasma cells, and in addition, germanium (Ge) or magnesium (Mg) doping to the films was also attempted. Crystallinity, photoluminescence (PL) property, and electrical property of the obtained films were investigated. Highly c-axis oriented GaN and InxGa1-xN thin films were obtained on the non-single-crystalline substrates. Near-band-edge emissions were observed in their PL spectra and the intensities were strongly enhanced by Ge doping. Ge doping was also effective on reducing resistivity of the GaN thin films grown on the non-single-crystalline substrates. Electrochemical capacitance-voltage measurements were carried out on the Mg-doped GaN thin films; and p-type conduction in the films was confirmed.

Characterization of Zn-doped GaN grown by metal–organic vapor phase epitaxy

The point defects and photoluminescence (PL) spectra of gallium nitride (GaN) epilayers with Mg, Zn, and unintentional doping were investigated in this study. The concentration of point defects (Ga vacancy and its related complexes) in the Zn-doped GaN is consistent with that in the Mg-doped GaN, but lower than that in undoped GaN. It is suggested that Zn (Mg) atoms occupy Ga sites and suppress the formation of Ga vacancies. Comparing the blue luminescence (BL) band intensity of GaN:Zn with that of GaN:Mg, a factor of 10 strong PL intensity demonstrates that a moderate incorporation of Zn to GaN is likely to improve the structural quality of GaN. Detailed studies on 2.93 eV BL band for GaN:Zn reveal that the Zn related BL band behaves as a donor–acceptor pairs character. For the acceptor level, isolated ZnGa with the activation energy of 0.386 eV above the valence band is obtained from temperature-dependent PL measurements, whereas the deep donor defect responsible for the 2.93 eV band is deduced to be 164 meV below the conduction band. An ON–H complex model is suggested to explain the deep donor origin.

Novel Technique for Improving the Signal-to-Background Ratio of X-ray Absorption Near-Edge Structure Spectrum in Fluorescence Mode and Its Application to the Chemical State Analysis of Magnesium Doped in GaN

A novel measurement technique for an X-ray absorption near-edge structure (XANES) for magnesium (Mg) doped in gallium nitride (GaN) has been developed. XANES spectra from Mg at very low concentrations of 1 ×1018/cm3 doped in GaN have successfully been obtained by optimizing the region of interest (ROI) and by using highly brilliant synchrotron radiation X-rays of SPring-8. The ROI is the limited energy region from an X-ray fluorescence spectrum to elicit signals of particular atoms. Using this new technique, we have investigated the effect of the annealing process for Mg-doped GaN on the XANES spectra. It has been found that the XANES spectra of Mg significantly changed as the annealing temperature increased. This indicates that the local structure around Mg atoms in GaN was modified by the annealing process.

The growth of axially modulated p–n GaN nanowires by plasma-enhanced chemical vapor deposition

Due to the n-type characteristics of intrinsic gallium nitride, p-type gallium nitride (GaN) is more difficult to synthesize than n-type gallium nitride in forming the p–n junctions for optoelectronic applications. For the growth of the p-type gallium nitride, magnesium is used as the dopant. The Mg-doped GaN nanowires (NWs) have been synthesized on (111)-oriented n+-silicon substrates by plasma-enhanced chemical vapor deposition. The scanning electron microscope images showed that the GaN NWs were bent at high Mg doping levels, and the transmission electron microscope characterization indicated that single-crystalline GaN NWs grew along <0001> orientation. As shown by energy dispersive spectroscopy, the Mg doping levels in GaN NWs increased with increasing partial pressure of magnesium nitride, which was employed as the dopant precursor for p-GaN NW growth. Photoluminescence measurements suggested the presence of both p- and n‐type GaN NWs. Furthermore, the GaN NWs with axial p–n junctions were aligned between either two-Ni or two-Al electrodes by applying alternating current voltages. The current–voltage characteristics have confirmed the formation of axial p–n junctions in GaN nanowires.