GaN Active Region Research Articles

Effect of biasing voltage on quantum confinement in GaN/AlxGa1-xN nanowire structure

Effect of biasing voltage on the quantum confinement has been studied to analyse performance of GaN/AlGaN nanowire structure. Here, rectangular model of quantum nanowire with GaN active region surrounded by AlxGa1-xN barrier region has been considered. Schrodinger equation in two dimensions has been solved for this model to study electron confinement. The solving approach has been considerately utilized for sufficient accuracy and efficiency. Moreover, wave function intensity and FWHM for different biasing voltages has been obtained along with the potential profile in quantum nanowire structure. Biasing voltage was increased from 1V to 2.5V. The confinement factor has been increased from 0.81 to 0.838 with an increase in bias of 1.0V to 2.5V at Al mole fraction of 0.25. Our analysis shows that increasing biasing voltage increases the confinement of carriers in the wire region which will be advantageous for lasing action. Significant decrease in escape time of the carriers with biasing voltage has been obtained.

Photoluminescence enhancement by localized surface plasmons in AlGaN/GaN/AlGaN double heterostructures

Double heterostructures AlGaN/GaN/AlGaN grown by hydride vapor phase epitaxy and designed for use as light emitting diodes for 360 nm wavelength were patterned by shallow nanoholes and injected with Ag/SiO2 or Al nanoparticles. A 1.8 times increase in the photoluminescence and microcathodoluminescence signal from the GaN active region was observed for 100 nm diameter Al nanoparticles, the efficiency decreased compared to the reference planar samples for small Al nanoparticles of 30–40 nm diameter, and a moderate increase of 1.2 times was detected for Ag/SiO2 nanoparticles. The observed phenomena are explained by the GaN emitter coupling with localized surface plasmons produced by metallic nanoparticles. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Zn2GeO4 Nanowires As Efficient Electron Injection Material for Electroluminescent Devices

Pure phase Zn2GeO4 nanowires (NWs) were grown by the chemical vapor transport method on p-GaN: Mg/Al2O3 substrate. The as-grown Zn2GeO4 NWs exhibited n-type characteristic due to native defects and formed a p-n heterojunction with the p-GaN substrate. The unique energy level of Zn2GeO4 NWs promotes electron injection into GaN active region while suppressing hole injection into Zn2GeO4 NWs. The device exhibited an emission centered at 426 nm and a low turn-on voltage around 4 V. Zn2GeO4 NWs are first reported in this paper as promising electron transport and injection material for electroluminescent devices.

Terahertz dual-wavelength quantum cascade laser based on GaN active region

In this paper a novel terahertz (THz) quantum cascade laser (QCL) based on GaN/AlGaN quantum wells has been proposed, which emits at two widely separated wavelengths 33 and 52 μm simultaneously in a single active region. The large LO-phonon energy (∼90 meV), the ultrafast resonant phonon depopulation of the lower radiative levels, suppression of the electrons that escape to the continuum states and selective carrier injection and extraction all together lead to a considerable enhancement in the operating temperature of the structure. All calculations have been done at a temperature of 265 K. Moreover, similar behavior of the output optical powers is another remarkable feature, which makes both wavelengths useful for special applications.

Polarization of light emission by 460nm GaInN∕GaN light-emitting diodes grown on (0001) oriented sapphire substrates

Measurements on the polarization of top- and side-emitted light as a function of direction are performed for 460nm GaInN unpackaged and packaged light-emitting diode (LED) chips with a multiquantum well (MQW) GaInN∕GaN active region grown on (0001) oriented sapphire substrates. Side emission is found to be highly polarized with the electric field in the plane of the MQW. Intensity ratios for in-plane to normal-to-plane polarization reach values as high as 7:1, while the total intensity for the in-plane polarization is more than twice as large compared to the normal-to-plane polarization. Despite these measured polarization characteristics, conventional packaged LEDs are found to be virtually unpolarized due to packaging.

Electroluminescent properties of erbium-doped III–N light-emitting diodes

We report on the synthesis of Er-doped III–N double heterostructure light-emitting diodes (LEDs) and their electroluminescence (EL) properties. The device structures were grown through a combination of metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE) on c-plane sapphire substrates. The AlGaN layers, with an Al concentration of ∼12%, were prepared by MOCVD and doped with Si or Mg to achieve n- and p-type conductivity, respectively. The Er+O-doped GaN active region was grown by MBE and had a thickness of 50 nm. The Er concentration was estimated to be ∼1018 cm−3. The multilayer n-AlGaN/GaN:Er/p-AlGaN structures were processed into LEDs using standard etching and contacting methods. Several different LEDs were produced and EL spectra were recorded with both forward and reverse bias conditions. Typically, the EL under reverse bias was five to ten times more intense than that under forward bias. The LEDs displayed a number of narrow emission lines representative of the GaN:Er system (green: 539 nm, 559 nm; infrared: 1000 nm, 1530 nm). While some current crowding was observed, green emission was visible under ambient room conditions at 300 K. At cryogenic temperatures, the emission lines increased in intensity and had a narrower linewidth. EL spectra were recorded down to 10 K and the L-I characteristics were systematically measured. The power output of the brightest LEDs was approximately 2.5 W/m2 at 300 K.

Dynamics of photoexcited carriers in AlxGa1−xN/GaN double heterostructures

We present results of a time-resolved photoluminescence study of the dynamics of photoexcited carriers in AlxGa1−xN/GaN double heterostructures (DHs). The carrier dynamics including generation, diffusion, spontaneous recombination, and nonradiative relaxation were studied by examining the time decay of photoluminescence associated with the spontaneous recombination from the samples. The temporal evolution of the luminescence from the GaN active layers of the DH samples was found to be governed by a carrier–diffusion dominated capture process. The determination of the capture time for the carriers drift and diffusion into the GaN active region, in addition to the effective lifetimes of the spontaneous recombination for carriers in the AlGaN cladding layers and the GaN active region, allows an estimation of the diffusion constants for the minority carriers in the AlxGa1−xN cladding layers of the DHs. Our results yield a diffusion constant of 2.6 cm2/s for Al0.03Ga0.97N and 1.5 cm2/s for Al0.1Ga0.9N at 10 K.

An optically pumped GaN–AlGaN vertical cavity surface emitting laser

An optically pumped GaN-based vertical cavity surface emitting laser (VCSEL) is demonstrated. Laser emission near 363 nm is observed at room temperature from the surface of a VCSEL structure optically pumped along a cleaved sample edge by focused light from a nitrogen laser. The VCSEL structure, which was grown on a sapphire substrate by metalorganic vapor phase epitaxy, consists of a 10 μm GaN active region sandwiched between 30-period Al0.40Ga0.60N–Al0.12Ga0.88N Bragg reflectors. At optical pump intensities above ∼2.0 MW/cm2, a narrow (<5 Å) laser mode at 363.5 nm emerges from the GaN photoluminescence spectrum. This mode becomes the dominant feature of the spectrum at higher pump powers, and additional modes appear ∼1.3 nm above and below this mode at 362.1 nm and 364.8 nm. The ∼1.3 nm mode spacing corresponds closely with the 1.1 nm spacing predicted from an electromagnetics model of the VCSEL structure.

Optically Pumped GaN-AlGaN Double-Heterostructure Lasers Grown by ECR-GSMBE and HVPE

ABSTRACTWe have observed laser emission with well-defined cavity modes in optically pumped GaN-Al0.1Ga0.9N double-heterostructure (DH) lasers. The laser structures were grown using an electron-cyclotron-resonance nitrogen-discharge source and gas-source molecular beam epitaxy (ECR-GSMBE) on thick (≥10 µm) GaN buffers grown by hydride vapor-phase epitaxy (HVPE) on c-plane sapphire. Transversely pumped cavities using a 337.1 nra nitrogen laser pump source exhibit a threshold pump fluence ranging from 0.15 to 0.3 mJ/cm2 at 77 K, a linear light output above threshold, a lasing wavelength of 358 nm, and an estimated differential quantum efficiency of 1%. The room-temperature threshold is about 1.7 times higher. Longitudinal mode structure has been resolved in a shorter-cavity (23 µn) device at 77 K. The measured mode spacing of 0.56 nm corresponds to a group index of 5.0. Far-field measurements in a plane perpendicular to the plane of the heterostructure indicate a double-lobed pattern for a 1000 Ǻ thick GaN active region, and a single lobe with a FWHM of 60° for a 4000 Ǻ active region. The thick HVPE GaN buffer layer provides for a lattice-matched growth and results in improved nucleation in MBE, as indicated by a high-quality reflection-electron-diffraction pattern of the as-loaded wafers. The surface morphology of the MBE layers on the HVPE buffer shows improved optical smoothness as compared to layers grown directly on sapphire using a low-temperature, MBE-grown GaN buffer. Laser facets were formed either by saw cutting or cleaving of the GaN buffer and epilayer along crystal planes. Details of the material development and laser performance are described.