G. Targowski

Visible light communications using a directly modulated 422 nm GaN laser diode

Visible light communications using a Gallium-nitride (GaN) laser diode is reported. Devices, which are cased in TO packages, show modulation bandwidths of up to 1.4 GHz. We demonstrate error-free data transmission, defined as transmission of 1×10(-9) bits without any errors, at 2.5 Gbit/s with a sensitivity of 11.5 dBm.

Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes

The low-temperature breakdown of the electroluminescence intensity (ELI) of blue/violet InGaN-based light-emitting diodes (LEDs) is shown to be independent of the structural details of the LED active region. Instead, the presence of an electron blocking layer (EBL) plays a decisive role. The authors attribute the ELI collapse to the low-temperature hole-blocking properties of the EBL. However, removing the EBL leads to a much reduced ELI because of a disproportional increase of electron overflow processes, which shows that the presence of an EBL in blue/violet InGaN-based LEDs is still essential. Optimization of the EBL by means of Mg doping is discussed.

Elimination of AlGaN epilayer cracking by spatially patterned AlN mask

The inherent problem in III-nitride technology is the cracking of AlGaN layers that results from lattice mismatch between AlGaN and GaN. In case of thin substrates (30–90μm), such as, bulk GaN grown by the high-pressure/high-temperature method, the bowing of AlGaN∕GaN strained structures becomes an additional problem. To eliminate cracking and bowing, AlGaN layers were grown on GaN substrates with an AlN mask patterned to form 3–15μm wide windows. In the 3μm window, the AlGaN layer was not cracked, although its thickness and Al composition exceeded critical values for growth on nonpatterned substrates. Dislocation density in the windows was of 5×106∕cm2.

High-Optical-Power InGan Superluminescent Diodes with "j-shape" Waveguide

We demonstrate blue-violet InGaN superluminescent diodes (SLDs) with bent-waveguide (j-shape) geometry, emitting optical power exceeding 200 mW for SLD with 1-mm-long waveguide. The adopted j-shape geometry prevents devices from lasing, even at the highest applied currents (450 mA). However, high-resolution measurements reveal the appearance of spectral ripples at currents on the order of 250 mA. The maximum output power increases with the waveguide length.

Strain-compensated AlGaN∕GaN∕InGaN cladding layers in homoepitaxial nitride devices

One of the most important problems in III-nitride violet laser diode technology is the lattice mismatch between the AlGaN cladding layers and the rest of the epitaxial structure. For efficiently working devices, it is necessary to have both a high Al content and thick claddings. This leads, however, to severe sample bowing and even cracking of the upper layer. In this work, we propose a cladding structure of strain-compensated AlGaN∕GaN∕InGaN superlattice grown by metal-organic vapor phase epitaxy on bulk GaN substrates. Various thicknesses and compositions of the layers were employed. We measured the radius of bowing, lattice mismatches, aluminum and indium contents, and densities of threading dislocations. The proposed cladding structures suppress bowing and cracking, which are the two parasitic effects commonly experienced in laser diodes with bulk AlGaN claddings. The suppression of cracking and bowing is shown to occur due to modified strain energy distribution of the superlattices structure.

Graded-index separate confinement heterostructure InGaN laser diodes

We demonstrate graded-index-separate-confinement-heterostructure InGaN laser diodes (GRINSCH) grown by metal organic vapor phase epitaxy. In this type of structure, the optical mode is confined close to the active region by cladding layers with linearly changing Al content. The virtue of this structure lies in simultaneous improvement of injection efficiency through the funneling effect and mode confinement. After a careful modeling we demonstrate that GRINSCH InGaN laser indeed possess very encouraging properties. Low threshold current density of around 3.5 kA/cm2 accompanied by high differential gain (dG/dJ = 14.93 cm kA−1) make these devices an interesting alternative for the classical step-index structure.

AlGaInN laser diode technology for defence, security and sensing applications

The latest developments in AlGaInN laser diode technology are reviewed for defence, security and sensing applications. The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., i.e, 380nm, to the visible, i.e., 530nm, by tuning the indium content of the laser GaInN quantum well. Advantages of using Plasma assisted MBE (PAMBE) compared to more conventional MOCVD epitaxy to grow AlGaInN laser structures are highlighted. Ridge waveguide laser diode structures are fabricated to achieve single mode operation with optical powers of <100mW in the 400-420nm wavelength range that are suitable for telecom applications. Visible light communications at high frequency (up to 2.5 Gbit/s) using a directly modulated 422nm Gallium-nitride (GaN) blue laser diode is reported. High power operation of AlGaInN laser diodes is demonstrated with a single chip, AlGaInN laser diode ‘mini-array’ with a common p-contact configuration at powers up to 2.5W cw at 410nm. Low defectivity and highly uniform GaN substrates allow arrays and bars of nitride lasers to be fabricated. GaN laser bars of up to 5mm with 20 emitters, mounted in a CS mount package, give optical powers up to 4W cw at ~410nm with a common contact configuration. An alternative package configuration for AlGaInN laser arrays allows for each individual laser to be individually addressable allowing complex free-space and/or fibre optic system integration within a very small form-factor.or.

Emission wavelength dependence of characteristic temperature of InGaN laser diodes

We have determined thermal stability of various InGaN laser diodes emitting in the spectral range of 390-436 nm. Their characteristic temperature T0 increases steeply with the increasing emission wavelength, reaching the value of around 230 K. Our analysis of current-light characteristics and activation energy of electroluminescence proves that this behavior is predominantly related to the thermal escape of electrons from quantum wells, which depends critically on their depth.

AlGaInN laser diode technology for GHz high-speed visible light communication through plastic optical fiber and water

Abstract. AlGaInN ridge waveguide laser diodes are fabricated to achieve single-mode operation with optical powers up to 100 mW at ∼420  nm for visible free-space, underwater, and plastic optical fiber communication. We report high-frequency operation of AlGaInN laser diodes with data transmission up to 2.5 GHz for free-space and underwater communication and up to 1.38 GHz through 10 m of plastic optical fiber.

InGaN laser diodes with reduced AlGaN cladding thickness fabricated on GaN plasmonic substrate

We demonstrate InGaN laser diodes with substantially reduced thickness of their bottom AlGaN cladding grown on plasmonic GaN substrate. The electron concentration in plasmonic substrate grown by ammonothermal method was of the order of 1020 cm−3, which corresponds to the refractive index reduction by 0.7% when comparing to undoped GaN. We were able to reduce the thickness of AlGaN bottom claddings from the initial 800 nm down to 400 nm without any worsening of laser threshold current or near-field patterns. The results are in agreement with the optical modeling of the InGaN laser diode structure fabricated on plasmonic GaN substrate.