I. W. Feng

Erbium-Doped AlInGaN Alloys as High-Temperature Thermoelectric Materials

The potential of Er-doped AlxIn0.1Ga0.9-xN quaternary alloys as high-temperature thermoelectric (TE) materials has been explored. It was found that the incorporation of Er significantly decreased the thermal conductivity (κ) of AlxIn0.1Ga0.9-xN alloys. The temperature-dependent TE properties were measured up to 1055 K for an Er and Si co-doped n-type Al0.1In0.1Ga0.8N alloy. The figure of merit (ZT) showed a linear increase with temperature and a value of about 0.3 at 1055 K was estimated. The ability to survive such high temperature with reasonable TE properties suggests that low-In-content Er and Si-doped AlInGaN alloys are potential candidate of high-temperature TE materials.

Enhancing erbium emission by strain engineering in GaN heteroepitaxial layers

Much research has been devoted to the incorporation of erbium (Er) into semiconductors aimed at achieving photonic integrated circuits with multiple functionalities. GaN appears to be an excellent host material for Er ions due to its structural and thermal stability. Er-doped GaN (GaN:Er) epilayers were grown on different templates, GaN/Al2O3, AlN/Al2O3, GaN/Si (111), and c-GaN bulk. The effects of stress on 1.54 μm emission intensity, caused by lattice mismatch between the GaN:Er epilayer and the substrate, were probed. The emission intensity at 1.54 μm increased with greater tensile stress in the c-direction of the GaN:Er epilayers. These results indicate that the characteristics of photonic devices based on GaN:Er can be optimized through strain engineering.

Correlation between the optical loss and crystalline quality in erbium-doped GaN optical waveguides

Erbium-doped GaN (GaN:Er) epilayers were synthesized by metal organic chemical vapor deposition. GaN:Er waveguides were fabricated based on four different GaN:Er layer structures: GaN:Er/GaN/Al2O3, GaN:Er/GaN/AlN/Al2O3, GaN:Er/GaN/Al(0.75)Ga(0.25)N/AlN/Al2O3, and GaN/GaN:Er/GaN/Al2O3. Optical loss at 1.54 μm in these waveguide structures has been measured. It was found that the optical attenuation coefficient of the GaN:Er waveguide increases almost linearly with the GaN (002) x-ray rocking curve linewidth. The lowest measured loss was ~6 dB/cm.

Photonic properties of erbium doped InGaN alloys grown on Si "001… substrates

Erbium doped InGaN alloys (InGaN:Er) were grown on Si (001) substrates using metal organic chemical vapor deposition. The growth of epitaxial films was accomplished by depositing InGaN:Er on GaN templates deposited on 7.3° off-oriented Si (001) substrates which were prepared by etching and subsequent selective area growth. X-ray diffraction measurements confirmed the formation of wurtzite InGaN (11¯01) epilayers, which exhibit strong photoluminescence emission at 1.54 μm. The observed emission intensity at 1.54 μm was comparable to that from similar alloys grown on GaN/AlN/Al2O3 templates. These results indicate the high potential for on-chip integration of erbium based photonic devices with complementary metal oxide semiconductor technology.

Optical excitation cross section of erbium in GaN.

Epilayers of erbium-doped GaN (GaN:Er) were synthesized by metal-organic chemical vapor deposition, and the optical excitation cross section (σ(exc)) of Er ions in this host material were determined. Photoluminescence (PL) measurements were made using laser diodes at excitation wavelengths of 375 and 405 nm, and the integrated emission intensity at 1.54 μm was measured as a function of excitation photon flux. Together with time-resolved PL measurements, values of σ(exc) of Er ions in GaN:Er were obtained. For excitation at 375 nm, the observed excitation cross section was found to be 4.6×10(-17) cm(-2), which is approximately three orders of magnitude larger than that using resonant excitation. Based on the present and previous works, the optical excitation cross section σ(exc) of Er ions in GaN:Er as a function the excitation wavelength has been obtained. The large values of σ(exc) with near-band-edge excitation makes GaN:Er attractive for realization of chip-scale photonic devices for optical communications.

Effects of growth pressure on erbium doped GaN infrared emitters synthesized by metal organic chemical vapor deposition

Er doped GaN (GaN:Er) p-i-n structures were prepared by metal organic chemical vapor deposition. Effects of growth pressure on the optical performance of GaN:Er p-i-n structures have been investigated. Electroluminescence measurements revealed that the optimal growth pressure window for obtaining strong infrared emission intensity at 1.54 µm is around 20 torr, while the greater amount of Ga vacancies or non-raditive transitions were observed from the ones grown at lower or higher pressure. Our results point to possible applications in optical communications using current injected optical amplifiers based on GaN:Er p-i-n structures.

Formation energy of optically active Er3+ centers in Er doped GaN

Erbium doped GaN (GaN:Er) and low In-content InxGa1−xN (x∼0.05) epilayers were synthesized by metal organic chemical deposition. The 1.54 μm PL emission intensity was monitored for GaN:Er epilayers grown at different growth temperatures and utilized to establish a value of 1.8 ± 0.2 eV for the formation energy (EF) of the optically active Er3+ centers in GaN. The optically active Er+ centers are presumably Er and nitrogen vacancy (Er-VN) complexes. The experimentally measured value of the EF of the optically active Er3+ centers is about 0.98 eV larger than the calculated formation energy of Er ions at Ga sites; however, it is 1.1–2.2 eV lower than the formation energy of VN in GaN. Due to the large EF values, relatively high growth temperatures are required to improve the 1.54 μm emission efficiency in GaN:Er.

SiO2/TiO2 distributed Bragg reflector near 1.5 μm fabricated by e-beam evaporation

The authors report on the fabrication and characterization of SiO2/TiO2 distributed Bragg reflector (DBR) mirrors operating at the eye safe and optical communication wavelength window, λ = 1.5 μm. Our experimental results demonstrated that SiO2/TiO2 DBR mirrors with reflectivity exceeding 95% at λ = 1.5 μm can be achieved using e-beam evaporation in conjunction with postdeposition thermal annealing process in ambient air. It was found that the postdeposition annealing process transformed the crystal structure of the as-deposited TixOy to TiO2, leading to a significant reduction in optical absorption. Erbium doped III-nitride semiconductors incorporating DBR mirrors at 1.5 μm emission may open up many novel applications, including infrared emitters, optical amplifiers, and high power infrared lasers.

Emission and absorption cross-sections of an Er:GaN waveguide prepared with metal organic chemical vapor deposition

We repost the characterization of emission and absorption cross-sections in an erbium-doped GaN waveguide prepared by metal organic chemical vapor deposition. The emission cross-section was obtained with the Fuchtbauer–Ladenburg equation based on the measured spontaneous emission and the radiative carrier lifetime. The absorption cross-section was derived from the emission cross-section through their relation provided from the McCumber’s theory. The conversion efficiency from a 1480 nm pump to 1537 nm emission was measured, which reasonably agreed with the calculation based on the emission and absorption cross-sections.

Enhanced magnetization in erbium doped GaN thin films due to strain induced electric fields

The ferromagnetic properties of erbium-doped GaN (GaN:Er) epilayers grown by metal-organic chemical vapor deposition were studied. It is found that the different tensile strains produced by the respective lattice mismatch for different substrates used (GaN/Al2O3, AlN/Al2O3, GaN/Si (111), and c-GaN bulk) correlate well with the observed room-temperature saturation magnetization. Under application of a magnetic field, the photoluminescence of the erbium dopant, which causes the ferromagnetism, indicates that the magnetic states of the ions are coupled to the electronic states of the host. These results hold promise for the use of strain to control the magnetic properties of GaN:Er films for spintronic applications.