In-doped GaN Research Articles

Photoelectrochemical catalytic CO2 reduction enhanced by In-doped GaN and combined with vibration energy harvester driving CO2 reduction

Photoelectrochemical (PEC) technology seamlessly integrates and optimizes the merits of photocatalysis and electrocatalysis, facilitating charge separation and enhancing solar conversion efficiency. It stands out as a promising approach for CO2 treatment. GaN as III-Ⅴ semiconductor, has garnered substantial attention in the realm of PEC CO2 reduction reactions (RR). In this study, GaN and In/GaN micro-rods were prepared via straightforward hydrothermal synthesis. Attaining a current density of approximately 10 mA/cm2 and CO Faradaic Efficiency (FE) of ∼45 % at −0.75 VRHE (Reversible Hydrogen Electrode, RHE), In/GaN exhibited exceptional stability over a 2 h PEC CO2 RR. The introduction of In into GaN significantly augmented CO2 adsorption capacity and light harvesting. Additionally, Density Functional Theory (DFT) calculations elucidated that In-doped GaN can diminish the adsorption of intermediate CO, favoring subsequent CO desorption. Furthermore, the N-vacancy increased with In doping, resulting in a rise in the number of unpaired electrons, facilitating carrier transport. Herein, vibration energy harvester was introduced to drive CO2 RR, marking a significant advancement in development of PEC CO2 RR for future green energy applications.

Plasma-Assisted Halide Vapor Phase Epitaxy for Low Temperature Growth of III-Nitrides

Developing growth techniques for the manufacture of wide band gap III-nitrides semiconductors is important for the further improvement of optoelectronic applications. A plasma-assisted halide phase vapor epitaxy (PA-HVPE) approach is demonstrated for the manufacture of undoped and In-doped GaN layers at ~600 °C. A dielectric barrier discharge (DBD) plasma source is utilized for the low-temperature activation of ammonia. The use of the plasma source at a growth temperature of ~600 °C increases the growth rate from ~1.2 to ~4–5 µm/h. Furthermore, the possibility for the growth of InGaN at ~600 °C has been studied. Precursors of GaCl and InCl/InCl3 are formed in situ in the reactor by flowing HCl gas over a melt of metallic Ga and In, respectively. The In concentration was low, in the order of a few percent, as the incorporation of In is reduced by plasma due to the activation of chlorine-containing species that etch the relatively poorly bonded In atoms. Nevertheless, the approach of using plasma for ammonia activation is a very promising approach to growing epitaxial III-nitrides at low temperatures.

Coexistence of doping and strain to tune electronic and optical properties of GaN monolayer

Abstract GaN monolayer is a prospective two-dimensional (2D) graphene-like material due to its excellent physical and chemical properties. Band gap engineering plays a crucial role for extending potential applications of GaN-based optoelectronic devices. We performed first-principles calculations to study the structural, electronic and optical properties of GaN monolayers by Al and In atoms doping with different concentrations. We found that Al atom doping can increase the band gap and result in the blue-shift with the increase of doping concentration, while In atom doping presents the opposite results. However, the band gap characteristics remains indirect which is unfavourable for the optical transition and solar energy conversion. Next, biaxial strains within magnitude range of −10% to 13% are applied in Al and In-doped GaN monolayers. Coexistence of doping and strain to tune electronic and optical properties of GaN monolayer are systematically studied for the first time. The results show the band gap increases at first then decreases under compressive strain and decreases monotonically under tensile strain. Moreover, the band gap characteristics changes from indirect to direct in the certain compressive strain. For the optical property modification, compressive strain can cause blue-shift in the whole energy range, while tensile strain can result in red-shift in the visible energy range and blue-shift at first then red-shift in the ultraviolet energy range. Our results confirm coexistence of doping and strain induced in GaN monolayer is an effective method in band gap engineering for the applications in tunable nano-optoelectronic devices.

Synthesis and optical properties of In-doped GaN nanocrystals

Indium-doped GaN nanocrystals with 5% and 10% In have been prepared by a low temperature solvothermal method using hexamethyldisilazane as the nitriding reagent. The nanocrystals show Raman bands at lower frequencies compared to GaN. Photoluminescence spectra of the In-doped GaN nanocrystals exhibit an increase in the FWHM with the decrease in the PL band energy, the band energy itself decreasing with increase in the In content.

Photoluminescence Studies of In-Doped GaN:Mg Films

Photoluminescence (PL) studies of In-doped GaN:Mg films revealed that the Mg-related emission at 3.1 eV is enhanced by more than one order of magnitude on the shoulder of the broad band centered at 2.8 eV for GaN:Mg after an optimal In concentration was added into the films. This enhancement of the 3.1 eV band is believed to be associated with the reduction in the number of self-compensation centers. A slow decay in PL intensity evolution was also observed, which may be ascribed to a local energy barrier that impedes carriers that relax into the valence band. The temperature dependences of the decay time constants were measured and a barrier energy as high as ∼103±7 meV was obtained for In-doped GaN:Mg as compared with 69±8 meV for GaN:Mg.

A comparative study on In‐doping effects for MOVPE GaN films on Si(111) and sapphire substrates

This paper reports In-doping effects in GaN films grown on Si(111) and α-Al2O3(0001) substrates using N2 carrier gas. By using the three step (350 °C, 450 °C and 550 °C) buffer layer growth process, single-crystalline GaN is obtained on Si(111) as well as on sapphire. From X-ray diffraction and RBS measurements it is revealed that the In incorporation in GaN films grown on Si is about 0.5%, whereas the level in GaN on sapphire is only 0.1%. The crystalline quality and optical properties for In-doped GaN on Si are scarcely improved in spite of the high In content. In-doped samples on Si show a very low excitonic emission peak energy (3.36 eV) because of both InGaN alloy formation and a high tensile stress caused by the hardening effect. In spite of the high tensile stress, the In-doped sample on Si(111) has very few cracks on the surface because of the hardening effect of GaN by In doping. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of indium surfactant on the crystalline and optical properties of GaN during initial growth stage

In this article, we report the effects of indium doping on crystalline and optical properties of GaN grown by metalorganic chemical vapor deposition during initial growth stage. Atomic force microscopy observations revealed that the In doping enhanced the lateral growth while the c-face growth rate was reduced. X-ray diffraction (XRD) and micro-Raman scattering measurements showed that the epilayers during this growth stage are nearly strain free. From XRD measurements, we found that In doping has increased the full width at half maximum values in both (0002) and (202̄4) ω-scan. Room temperature photoluminescence measurements show that In doping has enhanced the band-edge related emission by an order of magnitude compared to that of undoped GaN. Raman spectra indicate that In doping suppressed the misorientation of crystallites. In addition, a Raman mode occurred near 710 cm−1 in the In-doped GaN and has been assigned as the Fröhlich vibration.

Electron capture behaviors of deep level traps in unintentionally doped and intentionally doped n-type GaN

In n-type GaN films grown on sapphire substrates by metal-organic chemical vapor deposition such as unintentionally GaN and intentionally Si-doped GaN and In-doped GaN, the electron capture behaviors were investigated by deep level transient spectroscopy with various filling pulse durations. Two distinct deep levels E1 and E2 were typically observed in unintentionally doped n-type GaN. After optimized growth of undoped GaN, deep level E1 disappears. With increasing Si doping, the trap concentration of deep level E2 is increased. However, In doping in n-type GaN growth was found to suppress the formation of deep level E2. The electrons captured at the traps E1 and E2 were found to depend logarithmically on the duration time of the filling pulse. From an analysis of a model involving barrier-limited capture rate, it can be concluded that deep level E1 is associated with linear line defects along dislocation cores while deep level E2 is related to point defects.

Growth and Characteristics of In‐Doped GaN Thin Films

In-doped GaN thin films were grown by metalorganic chemical vapor deposition (MOCVD) with different trimethylindium (TMIn) flow rates and those structural and electrical characteristics were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), Hall effect measurement and deep level transient spectroscopy (DLTS). The full width at half maximum (FWHM) of X-ray rocking curves and the pit density, which is estimated by AFM images, decrease with increasing TMIn flow rate. It seems that In doping has definite effects of reducing dislocation density in In-doped GaN films. Hall mobility increases with increasing TMIn flow rate. As an explanation, from an analysis of temperature-dependent Hall effect measurements based on a two-layer mixed conduction model, the mobility contribution of the epilayer compared to that of the interface layer between GaN and sapphire is increased due to reduction of dislocation density in In-doped GaN thin films. It is also observed that the reduction of deep level E2 is related to In-doping.

Effect of the Buffer Layers on Lattice Polarity of GaN Epilayers Grown on the N‐Face of Bulk GaN Single Crystals by Molecular‐Beam Epitaxy

Lattice polarity of GaN epilayers grown on N-faces of bulk GaN single crystals has been investigated. The determinations of lattice polarity have been carried out by observation of surface reconstruction and surface morphology change by alkaline etching. As a result, it is found that GaN epilayers grown on N-faces using In-doped GaN buffer layers have Ga-polarity; on the other hand, GaN epilayers grown on N-faces using high-temperature (HT) AlN buffer layers have N-polarity. These results suggest that In-doping has the role of reversing the polarity of the GaN epilayers from N- to Ga-polarity, whereas HT-AlN buffer layers itself do not act as the layer reversing the polarity from N- to Al-polarity.

Electrical and crystalline properties of as-grown p-type GaN grown by metalorganic vapor phase epitaxy

P-GaN layer was grown by metalorganic vapor phase epitaxy at 1000°C with an LT-deposited AlN buffer layer. Bis(cyclopentadienyl) magnesium was used as a dopant and trimethyindium (TMIn) was simultaneously supplied. N 2 carrier gas was used during GaN growth. The contact of as-grown GaN:Mg sample was Ohmic only for In-doped GaN. Others showed Schottky behavior. The hole concentration was increased up to 5.0×10 17 cm −3 at room temperature for In-doped samples. Moreover, with increasing TMIn flow rate, the biaxial strain in as-grown p-GaN was reduced and accordingly the hole concentration was increased.

Secondary ion mass spectrometry analysis of In-doped p-type GaN films

SIMS was used to investigate the isoelectronic In-doped p-type GaN films. The growth rate of the p-type GaN film decreased with increasing Mg and In doping. The Mg saturation in GaN was 3.55×10 19 atoms/cm 3. The role of In as surfactant was evaluated by varying In concentrations and it was observed that the surface appeared smooth with increasing In incorporation. The Mg solubility in p-type GaN improved to 0.0025% molar ratio of the GaN with In incorporation. The In concentration results observed in neutron activation analysis (NAA) were found to be higher by a factor of 2.88 than that observed in SIMS and can be attributed to the difference in sensitivity of the two techniques. Good linearity in the results was observed from both techniques.

Non-Monotonous Behavior of In-Doped GaN Grown by MOVPE with Nitrogen Carrier Gas

This paper reports the In-doping effects in MOVPE grown GaN using a N 2 carrier gas. Electrical and optical properties of the In-doped GaN are found to show non-monotonous behavior, when the In source (TMI) supply is increased. Carrier concentration in the In-doped GaN is monotonously decreased with increasing TMI supply, while Hall mobility and PL intensity ratio between exciton emission and yellow deep emission (I EX /I YL ) are decreased in the region of low TMI supply (TMI/TEG <0.5). In this region, tensile stress in the grown GaN estimated from the exciton emission peak energy is also increased with increasing TMI/TEG ratio. For TMI/TEG ratio in the range of 0.5-1.5, on the other hand, they show very normal behavior (improvement by In doping). Such anomalous behavior for mobility, I EX /I YL and tensile stress is discussed from the viewpoints of impurity hardening of GaN, as well as stress relaxation by both crack formation and In doping.

Effect of Isoelectronic In Doping on Deep Levels in GaN Grown by MOCVD

We have investigated the effect of isoelectronic In doping on deep levels in GaN films grown by metalorganic chemical vapor deposition via deep level transient spectroscopy. Deep level E2 0.5 eV below the conduction band was observed in both undoped GaN and In-doped GaN. However, with increasing In mole flow rate, the trap concentration of E2 decreases sharply from 2.3 × 10 14 to 2.27 × 10 13 cm -3 , nearly an order of magnitude in reduction, comparing to undoped GaN. This might be due to the dislocation pinning and/or bending effect. Therefore, In doping in GaN growth can effectively decrease the dislocation density and suppress the formation of deep levels E2.

Non-Monotonous Behavior of In-Doped GaN Grown by MOVPE with Nitrogen Carrier Gas

Non-Monotonous Behavior of In-Doped GaN Grown by MOVPE with Nitrogen Carrier Gas

In situ annealing treatment and In-doping of GaN epilayers grown by MOVPE

The effects of in situ annealing treatment in the initial growth stage and In-doping during growth of the GaN on the material properties were investigated. GaN was grown by LP-MOVPE. In situ annealing reduced the full-width at half-maximum (FWHM) of X-ray rocking curves and reduced etch pit density of GaN films. It improved the optical properties of the epilayer. Undoped and In-doped GaN films of initial growth stage were investigated. It was found that morphology and optical properties were improved in In-doped samples.

Strain relief by In-doping and its effect on the surface and on the interface structures in (Al)GaN on sapphire grown by metalorganic vapor-phase epitaxy

We have performed a growth technique that involves isoelectronic In-doping into GaN and AlGaN in order to reduce defects. The films were grown by atmospheric metalorganic vapor-phase epitaxy (MOVPE) at 950°C in a H 2 or N 2 carrier gas (denoted below by H 2-(Al)GaN and N 2-(Al)GaN, respectively) with a low-temperature-deposited AlN buffer layer. By using this technique, we were able to control the strain in the films; with increasing trimethylindium (TMIn) flow, the strain in GaN was decreased; accordingly, the tilting and twisting components of crystalline mosaicity (tilt and twist, respectively) were also decreased. The Raman shift and photoluminescence (PL) emission peak energy were shifted in accordance with the strain in GaN. The PL linewidth decreased with the decrease in the strain in GaN. Strong enhancement of the excitonic PL intensity of In-doped GaN and AlGaN was observed. The surface morphology was also dramatically improved on In-doping, and the growth pits on the films were distinguished. These results are not due to the surfactant effect of In because we confirmed from the secondary-ion mass spectroscopy (SIMS) results that In was incorporated in the films. The above results could be explained by considering the solid-solution hardening effect.

Time-resolved photoluminescence study of isoelectronic In-doped GaN films grown by metalorganic vapor-phase epitaxy

Time-resolved photoluminescence spectra were used to characterize isoelectronically doped GaN:In films. Our results indicate that the recombination lifetime of the donor-bound-exciton transition of undoped GaN exhibits a strong dependence on temperature. When In is doped into the film, the recombination lifetime decreases sharply from 68 to 30 ps, regardless of the measured temperature and In source flow rate. These observations might be related to the isoelectronic In impurity itself in GaN, which creates shallow energy levels that predominate the recombination process.

The Effect of Isoelectronic In-Doping on the Structural and Optical Properties of (Al)GaN Grown by Metalorganic Vapor Phase Epitaxy

We have studied the effect of isoelectronic In-doping on the structural and optical properties of GaN and Al0.05Ga0.95N. The films were grown by atmospheric metalorganic vapor phase epitaxy at 950°C in H2 or N2 carrier gas (hereafter referred to as H2-(Al)GaN and N2-(Al)GaN, respectively) with a low-temperature deposited AlN buffer layer. With the technique, we were able to control the strain in the films. Indeed, with increasing trimethylindium flow, the strain in GaN decreased, and accordingly, the tilting and twisting components of the crystalline mosaicity were also decreased. The photoluminescence (PL) emission peak energy shifted in accordance with the strain in GaN. Strong enhancement of the excitonic PL intensity of In-doped GaN and AlGaN was observed. In all the samples, room temperature PL mapping for linewidth and peak wavelength showed that with the addition of In, the uniformity of the PL improved significantly.

Optical and electrical investigations of isoelectronic In-doped GaN films

The optical and electrical properties of isoelectronic In-doped GaN films grown by metalorganic vapor phase epitaxy (MOVPE) were investigated by X-ray, photoluminescence (PL), Hall and Raman measurements. As a result, adequate In-doping quantity causes not only a reduction of yellow luminescence and unintentional background concentration, but an enhanced mobility and decrease in the widths. The improved crystalline and optical qualities of GaN films may be attributed to the decrease in defects.