Search

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 1018 and 2 × 1019 cm−3 and resistivities of ~1011 Ωcm and ~106 Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed. [ABSTRACT FROM AUTHOR]

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 1018 and 2 × 1019 cm−3 and resistivities of ~1011 Ωcm and ~106 Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed.

Metal modulation epitaxy (MME) is a technique in which metal beams (Al, Ga, In, and Mg) are switched on and off in short periods in an RF MBE system while a continuous nitrogen plasma beam is kept on. We systematically studied the effect of periodic duty cycling on the morphology, crystalline quality, Mg doping concentration, and electrical properties of GaN:Mg films grown by MME. When the metal shutter duty cycling is 20 s open/10 s close, the sample has smooth surface with clear steps even with Mg doping concentration higher than 1 × 1020 cm−3. The RMS roughness is about 0.5 nm. The FWHM of (002) XRD rocking curve is 230 arcsec and the FWHM of (102) XRD rocking curve is 260 arcsec. As result, a hole concentration of 5 × 1018 cm−3 and a resistivity of 1.5 Ω·cm have been obtained. The hole concentration increases due to the incorporation of surface accumulated Mg dopants into suitable Ga substitutional sites with minimal formation of compensatory defects. [ABSTRACT FROM AUTHOR]

Metal modulation epitaxy (MME) is a technique in which metal beams (Al, Ga, In, and Mg) are switched on and off in short periods in an RF MBE system while a continuous nitrogen plasma beam is kept on. We systematically studied the effect of periodic duty cycling on the morphology, crystalline quality, Mg doping concentration, and electrical properties of GaN:Mg films grown by MME. When the metal shutter duty cycling is 20 s open/10 s close, the sample has smooth surface with clear steps even with Mg doping concentration higher than 1 × 1020 cm−3. The RMS roughness is about 0.5 nm. The FWHM of (002) XRD rocking curve is 230 arcsec and the FWHM of (102) XRD rocking curve is 260 arcsec. As result, a hole concentration of 5 × 1018 cm−3 and a resistivity of 1.5 Ω·cm have been obtained. The hole concentration increases due to the incorporation of surface accumulated Mg dopants into suitable Ga substitutional sites with minimal formation of compensatory defects.

Flexible piezoelectric nanogenerators (PNGs) with high power-conversion efficiency are of great interest. Here, we propose wearable PNGs based on pristine magnesium (Mg)-doped GaN (GaN:Mg) and GaN:Mg/ZnO coaxial nanowires (NWs) exhibiting state-of-the-art high-voltage and -current outputs. The PNGs were designed to suppress internal screening by a reduction in the diameter of the NWs, successful incorporation of Mg in GaN NWs as a p-type dopant, and deposition of a ZnO shell on the GaN:Mg NWs. The Fermi-level pinning reduces free carriers in thin NWs (diameter up to ~30 nm). By tuning the size of the metal–alloy catalyst, we obtained NWs with diameters ranging from 30 to 50 nm. To deplete the NWs fully of free carriers, Mg is incorporated without activation, which enhanced the resistance of GaN:Mg NWs by forming inactive Mg–H complexes. The flexible PNG fabricated with pristine GaN:Mg NWs exhibited the maximum output voltage and current of 52 V and 23 µA, respectively. The junction current screening was suppressed by depositing a ZnO shell with an optimized thickness of 10 nm on GaN:Mg NWs. The PNG fabricated with GaN:Mg/ZnO coaxial NWs showed an enhanced output voltage and current of 66 V and 40 µA and demonstrated a state-of-the-art high power density of 170 µW/cm2 at an optimum load resistance of 2.5 MΩ. When mounted on the wrist as a wearable healthcare monitoring device, it successfully detected the movement of tendons and muscles as soon as the fingers were moved and exhibited a corresponding change in voltage response. Additionally, the charging of a capacitor is proven by the PNG under continuous foot tapping, demonstrating its potential to charge energy storage devices by walking or jogging. [Display omitted] • GaN:Mg/ZnO coaxial NWs exhibits state-of-the-art high power density of 170 µW/cm2. • Semi-insulating GaN:Mg NWs exhibited the highest output voltage of 52 V. • The p-n heterojunction atop of GaN:Mg/ZnO coaxial NWs enhanced the output voltage to 66 V. • The PNG exhibited high sensitivity by measuring slight movement of tendons of wrist. • It also demonstrated the charging of a capacitor under continuous foot taping. [ABSTRACT FROM AUTHOR]

The performance characteristics of green light-emitting diodes (LEDs) grown by metal-organic chemical-vapor deposition were investigated to study the dependence of the device performance on the materials and the growth conditions of p-type layer grown after the InGaN multiple-quantum-well active region. The electrical and structural qualities of Mg-doped p-In0.04Ga0.96N and p-GaN layers grown under different growth conditions were studied to optimize the growth conditions of p-type hole injection layers of green LEDs. A free-hole concentration of p=1.6times1018 cm-3 of with a resistivity of 0.33Omegamiddotcm was achieved for p-GaN:Mg layers grown at 1040degC. Lower hole concentrations and mobilities and rough surfaces were obtained when the growth temperature was decreased to 930degC in H2 ambient. In the case of p-In0.04Ga0.96N grown at 840degC in N2, a significant improvement of the hole concentration was achieved due to the reduced ionization activation energy of Mg acceptors in InGaN. Also we observed that as-grown p-GaN layers grown in N2 ambient showed p-type properties without Mg dopant activation. The electrical and optical properties of In0.25Ga0.75 N/GaN multiple-quantum-well green LEDs with such different p-layers were investigated. The electroluminescence intensity was improved for the LEDs with p-In0.04Ga0.96N layers grown at 840degC as compared to the LEDs with p-GaN layers grown at higher temperatures due to the reduced thermal damage to the active region, high hole injection, and low piezoelectric field induced in the active region. p-InGaN layers are very attractive candidates for the p-layer in green LED structures. The low temperature and N2 ambient used during the growth of InGaN layers are beneficial to protect the InGaN active region containing high-indium composition quantum-well layers in a- ddition to the advantage of providing a higher hole concentration. However, the LEDs with p-In0.04Ga0.96 N layer showed a slightly higher turn on voltage which could originate from the potential barrier for hole transport at the interface of the p-InGaN layer and the last GaN quantum-well barrier. to reduce this problem, we designed and characterized an LED structure having a graded indium composition in the p-In0.04Ga0.96N layer in order to improve hole transport into the active region. Optimized LEDs with p-InGaN layers grown in a N2 ambient showed much brighter electroluminescence due to low damage to the active region during p-InGaN layer growth [ABSTRACT FROM PUBLISHER]

Abstract: Small sized self-assembled inverted hexagonal pyramids consisting of GaN:Mg and InGaN/GaN multi-quantum-well (MQW) structures were formed using photoelectrochemical wet etching. Lateral etching, bottom-up etching, and anisotropic etching are the formation mechanism of the pyramids during the etching process sequentially. The dimension of these inverted hexagonal pyramids was measured as 245 nm in width and 184 nm in height, and the angle between the top GaN:Mg surface and the pyramid sidewall was calculated at about 56.3°. Due to the strain relief in the nano-disk MQW structure we induced an emission peak of photoluminescence at the tip of the inverted hexagonal pyramid which had a strong blue shift of 244 meV at 100 K. [Copyright &y& Elsevier]

Si ion implantation into p-type GaN followed by rapid thermal annealing (RTA) in N2 has been performed. X-ray diffraction analyses indicate that ion-implanted damage remains even with 1050°C, 60 s RTA. By varying implantation and postimplantation annealing conditions, we could convert carrier concentration from p-type 3 × 1017 cm-3 into n-type 2 × 1017 cm-3 ∼2 × 1019 cm-3. It was found that typical activation energies of Si implants in p-GaN are lower than 10 meV. Such activation energies are smaller than those observed from epitaxially grown Si-doped GaN films. A deep donor level with activation energy of 60 meV was also found from some samples. Photoluminescence studies show that the peak appears at 372 nm might be related to implantation-induced defects. It was also found that a green emission band could be observed from Si-implanted GaN. It was shown that such a green emission is related to the yellow band observed from epitaxially grown Si-doped GaN. The transport properties of these Si-implanted samples were also studied. [ABSTRACT FROM PUBLISHER]

Abstract: High p-type conductivity of GaN – commonly achieved via doping with Mg – is a challenge to be faced on the way to the realization of a GaN-based diluted magnetic semiconductor showing ferromagnetism at room temperature. In this work we present a study of GaN:Mg films grown in a metalorganic vapor phase epitaxy process where the Mg atoms are incorporated in a δ-doping fashion. The effect of a post-growth annealing step on the diffusivity of magnesium is studied by means of atomic force microscopy and photoluminescence measurements in the ultraviolet regime. A comparison of the presented results with transmission electron microscopy studies leads to the conclusion that a considerable amount of Mg is transported towards the surface where it is thermally removed during the annealing step. [Copyright &y& Elsevier]

In this study, we used the selective ring-region activation technique to restrain the surface leakage current and to monitor the luminescence characteristics of InGaN-GaN multiple quantum-well blue light-emitting diodes (LEDs). To access the current blocking region after forming a periphery high-resistance ring-region of the Mg-doped GaN layer and to reduce the degree of carrier trapping by the surface recombination centers, we deposited a titanium film onto the Mg-doped GaN epitaxial layer to form a high-resistance current blocking region. To characterize their luminescence performance, we prepared LEDs incorporating titanium films of various widths of the highly resistive current blocking layer. The hole concentration in the Mg-doped GaN epitaxial layer decreased from 3.45times1017 cm-3 to 3.31times1016cm-3 after capping with a 250-nm-thick layer of titanium and annealing at 700 degC under a nitrogen atmosphere for 30 min. Furthermore, the luminescence characteristics could be improved by varying the width of the highly resistive region of the current blocking area; in our best result, the relative electroluminescence intensity was 30% (20 mA) and 50% (100 mA) higher than that of the as-grown blue LEDs [ABSTRACT FROM PUBLISHER]

InGaN-GaN multiple-quantum-well metal-semiconductor-metal photodiodes (PDs) with in situ grown 40-nm-thick unactivated semi-insulating Mg-doped GaN cap layer were successfully fabricated. The dark leakage current of this PD was comparably much smaller than that of conventional PD without the semi-insulating layer, because of a thicker and higher potential barrier of semi-insulating cap layer, and also a smaller number of surface states involved. For the PDs with the semi-insulating Mg-doped GaN cap layers, the responsivity at 380nm was 0.372A/W when biasing at 5 V. In short, incorporating a semi-insulating Mg-doped GaN cap layer into the PDs beneficially leads to the suppression of dark current and a corresponding improvement in the ultraviolet-to-visible rejection ratio [ABSTRACT FROM PUBLISHER]

A long‐standing goal of GaN device research has been the development of a reliable, well‐controlled process for p‐GaN formation by ion implantation. Results to date have indicated an activation of 1% or less using high‐temperature rapid thermal annealing (RTA) techniques and coimplantation. Although Mg is a relatively deep acceptor, this is still much less than the theoretically achievable value (8.2% based on the 160 meV acceptor level). A multicycle RTA process is presented that is capable of achieving up to 8% activation of the Mg‐implanted GaN. This approaches the theoretical value, and represents a significant step in GaN device research. [ABSTRACT FROM AUTHOR]

Nitride-based metal-insulator-semiconductor (MIS)-like photodiodes (PDs) with in situ grown 30-nm-thick unactivated semiinsulating Mg-doped GaN cap layers were fabricated. The authors found that the reverse leakage current of the aforementioned PD was comparably much smaller than that of conventional PD without the semiinsulating layer due to the facts that inserting a semiinsulating layer would result in a thicker and higher potential barrier, and also less amounts of interface states introduced. To sum up, it was determined that the benefits of incorporating a semiinsulating Mg-doped cap layer into the PD would encompass a larger photocurrent-to-dark-current contrast ratio and larger ultraviolet-to-visible rejection ratio [ABSTRACT FROM PUBLISHER]