Er -doped Nanophotonics : A Review on Growth, Characterization & Fabrication for 1.54 μm Emission

1. Er-doped Nanophotonics: A Review on Growth, Characterization & Fabrication for 1.54 μm Emission AntaraHomChowdhury, Abu Ashik Md. Irfan & Md. Nizam Sayeed *Supervisor: Prof. Zahid Hasan Mahmood Authors’ Affiliation: Department of Applied Physics, Electronics & Communication Engineering, University of Dhaka *Supervisor’s Affiliation: : Department of Applied Physics, Electronics & Communication Engineering, University of Dhaka • In this paper, we have reviewed the process and limitations of Erbium doped Silicon, Gallium Arsenide and Gallium Nitride growth techniques. Moreover, growth mechanisms and optimization of the GaN epilayers are discussed based on Er concentration growth temperature for Molecular Beam Epitaxy and MOCVD. The fabrication of GaN channel waveguides using standard lithography and etching is also reviewed, along with waveguide optical characterization. • Er doped Si & GaAs • Growth mechanism: MBE for Si. MBE & MOCVD for GaAs • Low Optical Cross Sections. • High doping concentrations is required to achieve adequate gain which introduce extremely short lifetime (Thermal Quenching Effect). • Presence of oxygen during growth is another dominant ingredient for the degradation of 1.54 µm emission. • Er doped GaN Since the thermal stability of the Er emission increases with an increase in energy bandgap and crystalline quality of host material; among the WBGS, GaN and its alloys appear to be excellent hosts for Er ions due to their crystal structure and the ability to fabricate efficient light emitting devices. • Temperature Insensitivity, hence much lower noise • Growth Mechanisms: Ion Implantation MBE MOCVD • Er:GaN - MOCVD The Er-doped GaN epilayers were synthesized by MOCVD in a horizontal reactor and grown on (0001) sapphire substrates. In all cases the growth of epilayers are usually begin with a thin GaN buffer layer, followed by GaN epilayer template and a Er-doped GaN layer. The growth temperature of the GaN template and Er-doped GaN layer was 1040°C. GaN:Erepilayers were also grown by MOCVD on (0001) AlN/Si templates. The Erepilayer grown on Si substrate began with a thin AlN buffer layer and AlN epi-template followed by an Er-doped GaN:Er layer grown at 760°C. • The challenge in producing Er-doped GaN by the MOCVD process is the low vapor pressures of the Er precursors and the large organic chains which act as parasitic elements against the quality of GaN. However several groups have come to succeed and report on the in situ incorporation of Er into GaN epilayers by MOCVD. Er: GaN Waveguide The heterostructure waveguide device consists of 0.5µm Al0.03Ga0.97N top cladding, 0.5 µm Er-doped GaN as an active guiding medium and 1.5 µm bottom cladding grown on c-plane sapphire substrate. The Er concentration in the active wave guide core medium was ~1021 cm−3. The strip waveguides were fabricated using standard lithography techniques and inductively coupled plasma dry etching. The etch process was followed by the deposition of 250 nm SiO2passivation layer by PECVD. Er:GaN - Molecular Beam Epitaxy in situ doping does not suffer from the damage effects and, hence, results in more efficient emission. However, in situ Er-doping requires a good understanding and control of the overall growth process and the fluxes of dopants and layer substrates. 3 most crucial factors for efficient emissions: • Er concentration (determined by the Erbium cell temperature) An optimum Er concentration for which maximum emission can be observed. Usually the upper doping limit is considered to be the Erbium solid solubility in the host material. • Growth Temperature Starting at low growth temperature, PL data and other structural characterization shows that the BIV (the largest current-normalized brightness obtained over the current-voltage range measured in each case) is found to increase almost exponentially with growth temperature, exhibits a maximum at 600°C and starts to decrease at higher range. Thus a conclusion can be made stating that 600°C is the optimum growth temperature for Er-doped GaN. • Stoichiometry i.e., V/III ratio of GaN(determined by the Gallium cell temperature). In light of recent publications the optimum growth condition for maximum Er-doped intensity peak in GaN films is found under slightly N-rich flux near the stoichiometric region. • Characterization Techniques :photoluminescence (PL), cathode luminescence (CL), and Electroluminescence (EL). The sharp luminescence lines near 1.54 µm arise for the intra-4f shell transitions of Er3+ where the lowest energy electrons are not spatially the outer most shells.Being a transition metal, Erbium forms stable compounds with partially filled inner electronic shells which are highly localized and screened by the filled outer shells of 5p and 5s electrons. Because of this, the optical transitions often maintain an atomic like nature in Erbium based ionic and covalent molecules. By a selective excitation source, which is usually a semiconductor laser at 980 nm wavelength, we can pump 4I15/2 ground state electrons to the 4I11/2 state. After the optical excitation, rapid relaxation to intermediate 4I13/2 excited state occurs. Depending on the ion parity basis, the subsequent transition back to the ground state is forbidden. Hence the spontaneous lifetime is very long for both solid and dilute solution which is an ideal situation for producing an inverted population. If photons of ~1.54 μm interact with the inverted population of this erbium ion, stimulated emission takes place and in consequence amplification. Conclusion Based on the characterization data of recent publications, MOCVD is the optimized growth technique for required emission with lowest quenching effect. Measurement techniques of amplification and optical loss are summarized and we conclude that such fiber amplifiers possess maximum excitation intensity and the advantage of chip size photonic integration which means they will introduce minimum cross-talk between different channels in wavelength division multiplexing networks. Finally, the potential and versatility of GaN:Er technology can be considered to be the key ingredient for next generation integrated optical communication. The amplification properties of the waveguides can be characterized by studying the relative change in transmitted signal intensity at 1.54 µm operating wavelength. Another essential parameter which distinguishes the practical feasibility of the system, The optical loss is mainly due to light scattering by etch side walls of the waveguide and can be minimized through wavelength selective coating as well as gentle wet etching following inductively coupled plasma etching. Thus essential condition to the development of waveguides by GaN:Er complexes is the ability to achieve etched surface with morphology that is compared to the smooth unetched surface. According to the characterization of Dahal et al, the peak signal wavelength shows an increase of relative signal intensity with increasing excitation of the 365 nm GaN LED. The measured relative signal enhancement is about 8 dB/cm for a 3-mm-long waveguide optically pumped by the source operating at 400 mA. Moreover, propagation loss have been measured at 1.54 µm using a 371 nm nitride laser beam and found to be 3.5 cm-1. Acknowledgement We are grateful to Shah Mohammad Bahauddin for his altruistic support in this reviewwork.