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This paper presents low-temperature measurements of magnetoresistivity in heavily doped n-type GaN grown by basic GaN growth technologies: molecular beam epitaxy, metal-organic vapor phase epitaxy, halide vapor phase epitaxy and ammonothermal. Additionally, GaN crystallized by High Nitrogen Pressure Solution method was also examined. It was found that all the samples under study exhibited negative magnetoresistivity at a low temperature (10 K < T < 50 K) and for some samples this effect was observed up to 100 K. This negative magnetoresistivity effect is analyzed in the frame of the weak localization phenomena in the case of three-dimensional electron gas in a highly doped semiconductor. This analysis allows for determining the phasing coherence time τφ for heavily doped n-type GaN. The obtained τφ value is proportional to T−1.34, indicating that the electron–electron interaction is the main dephasing mechanism for the free carriers.

We studied degradation mechanisms of ultraviolet InGaN laser diodes emitting in the UVA range. Short wavelength nitride devices are subjected to much faster degradation, under the same packaging and testing conditions, than their longer wavelength counterparts. Transmission electron microscopy analysis of the degraded laser diodes showed pronounced damage to facets in the area of the active layer (waveguide, quantum wells, and electron blocking layer). Energy-dispersive X-ray spectroscopy showed that the active layers were heavily oxidized, forming a compound close in composition to Ga2O3 with proportional addition of Al in the respective area. The oxidation depth was roughly proportional to the intensity of the optical field. We propose UV-light-induced water splitting on a semiconductor surface as a mechanism of the oxidation and degradation of these devices.

Gallium nitride (GaN) doped with germanium at a level of 1020 cm−3 is proposed as a viable material for cladding layers in blue- and green-emitting laser diodes. Spectral reflectometry and ellipsometry are used to provide evidence of a reduced index of refraction in such layers. The refractive-index contrast to undoped GaN is about 0.990, which is comparable to undoped aluminium gallium nitride (AlGaN) with an aluminium composition of 6%. Germanium-doped GaN layers are lattice-matched to native GaN substrates; therefore, they introduce no strain, cracks, and wafer bowing. Their use, in place of strained AlGaN layers, will enable significant improvements to the production process yield.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s2 1S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 − 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

Most optoelectronic devices share the same basic epitaxial structure – a stack of quantum wells (QWs) sandwiched between p- and n-doped layers. In nitride semiconductors, where holes have 20-times lower mobility than electrons, the holes are able to populate only the topmost 1–2 QWs. The inability to distribute the holes in a large-enough number of QWs is a cause of high Auger recombination in nitride LEDs. Lateral carrier injection is an alternative design, in which the doped regions are situated at the sides of the QW stack and the carriers diffuse horizontally into the QWs. Given that the carriers are injected into all available QWs, it finally makes sense to grow structures with a large number of QWs. We report the results of our computer simulations, which explore the advantages of LCI-based LEDs in terms of energy efficiency.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s2 1S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 – 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

For certain, growing, class of applications it is necessary to have better suiting combination of beam and spectral quality. This specific combination of beam quality and spectrally broad emission can be delivered by superluminescent diodes (SLEDs). Additionally, the expected advent of nitride integrated photonic circuits and visible light communication call for new active optical elements, such as optical semiconductor amplifiers (SOA). Within this work we will present the progress in the development of broad spectral emission (>15 nm and OSA amplifiers which reach the gain of 10-20 dB at violet-blue wavelength range.

Gallium nitride (GaN) laser diodes (LDs) are considered for visible light communications (VLC) in free space, underwater, and in plastic optical fibers (POFs). A review of recent results is presented, showing high-frequency operation of AlGaInN laser diodes with data transmission rates up to 2.5 Gbit/s in free space and underwater and high bandwidths of up to 1.38 GHz through 10 m of plastic optical fiber. Distributed feedback (DFB) GaN LDs are fabricated to achieve single-frequency operation. We report on single-wavelength emissions of GaN DFB LDs with a side-mode suppression ratio (SMSR) in excess of 35 dB.

The effect of growth temperature and precursor flows on the doping level and surface morphology of Ge-doped GaN layers was researched. The results show that germanium is more readily incorporated at low temperature, high growth rate and high V/III ratio, thus revealing a similar behavior to what was previously observed for indium. V-pit formation can be blocked at high temperature but also at low V/III ratio, the latter of which however causing step bunching.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for quantum sensors such as optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s2 1S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 – 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

In this work we investigate the role of threading dislocations in nitride light emitters with different indium composition. We compare the properties of laser diodes grown on the low defect density GaN substrate with their counterparts grown on sapphire substrate in the same epitaxial process. All structures were produced by metalorganic vapour phase epitaxy and emit light in the range 383–477 nm. We observe that intensity of electroluminescence is strong in the whole spectral region for devices grown on GaN, but decreases rapidly for the devices on sapphire and emitting at wavelength shorter than 420 nm. We interpret this behaviour in terms of increasing importance of dislocation related nonradiative recombination for low indium content structures. Our studies show that edge dislocations are the main source of nonradiative recombination. We observe that long wavelength emitting structures are characterized by higher average light intensity in cathodoluminescence and better thermal stability. These findings indicate that diffusion path of carriers in these samples is shorter, limiting the amount of carriers reaching nonradiative recombination centers. According to TEM images only mixed dislocations open into the V-pits, usually above the multi quantum wells thus not influencing directly the emission.

Quantum based devices offer distinct advantages over conventional technology, such as improved sensitivity for sensing applications or enhanced accuracy for metrology. To utilize this potential, a number of technical requirements must be met, such as the cooling and trapping of neutral atoms for their use as quantum systems. We present our work on InGaN-based semiconductor cooling lasers for a variety of atomic species such as strontium, magnesium and ytterbium whos target wavelength was met by quantum-well composition engineering. Results on growth-epitaxy, facet coating as well as different configurations such as ECDL and MOPAs are presented, depending on the requirement of the application.

Our goal is to fabricate a laser diode 2D array which combines the properties of both VCSEL and edge emitting laser. Proposed light emitter will have a horizontal cavity with 450 deflectors. The role of these deflectors would be to deflect light perpendicular to the cavity, achieving vertical out-coupling. The most challenging part of this project is the fabrication of the micro-mirrors which act as both as beam deviating mirrors and cavity forming mirrors. Owing to the excellent thermal conductivity of GaN substrates the properties of such a 2D array should be better than of conventional nitride laser diode arrays, not even mentioning nitride-based stacked bars systems. In this paper I will describe our new device design and processing, giving insight to its possible applications and advantages over simple light emitting laser diode.

We studied mechanisms of recombination in InGaN quantum wells in polar and semipolar structures. Photoluminescence measurements show that the optical emission linewidths for polar and semipolar structures are almost identical suggesting the same level of indium fluctuations in quanutm wells. Their “peak-energy-versus-temperature” relations demonstrate very pronounced “s-shape” effect. Emission linewidth measured by cathodoluminescence does not depend on area from which the light is collected meaning that the fluctuations are smaller that 100 nm. The time scale of recombination process are of the order of 80 ns for polar and 2 ns for semipolar. Energy dispersion of the recombination time is strong in polar structures and very weak in semipolar ones which can be interperted in terms of electric field influence on photoluminescence lifetime energy dispersion. At room temparture emmission is dominated by Schockley-Hall-Read recombination and does not show any dispersion. Rate equation analysis of photoluminescence transients show domination of excitonic recombination in the case of polar samples (low temperature) and bimolecular in the case of semipolar ones. Both types of quantum wells, polar and semipolar look similar from the point of view of localization but differ in their radiative recombination mechanisms.

In this work, we investigate the role of the electron blocking layer (EBL) in laser diodes based on a graded index separate confinement heterostructure. We compare two sets of devices with very different EBL aluminum composition (3% and 12%) and design (graded and superlattice). The results of electro-optical characterization of these laser diodes reveal surprisingly modest role of electron blocking layer composition in determination of the threshold current and the differential efficiency values. However, EBL structure influences the operating voltage, which is decreased for devices with lower EBL and superlattice EBL. We observe also the differences in the thermal stability of devices – characteristic temperature is lower for lasers with 3% Al in EBL.

consumption and financial burden of the multiple light sources required for such systems. The AlGaInN material system allows for single transverse mode laser diodes to be fabricated with optical powers up to 100 mW over a wide range from ~380 nm up to ~530 nm. By tuning the indium content and thickness of the GaInN quantum well, we have developed a range of AlGaInN diode-lasers targeted to meet the wavelength and power requirements suitable for optical clocks and atom interferometry systems. One of the major limiting factors in nitride laser diode development has been the lack of a suitable low defectivity and uniform GaN substrate. Recently, single crystal growth of large area, very low dislocation-density and uniform GaN substrates are grown using a combination of high temperature and high pressure enabling a range of AlGaInN laser technology to be developed. This direct light generation at the required wavelength is crucial to reduce complexity and size of the overall system, and to ensure a high wall-plug efficiency that is critical for space and mobile applications. We will present our development of GaN based, low SWaP, frequency-stabilised external-cavity seed and tapered amplifiers to operate at 461nm for first stage strontium cooling. This includes growth of custom optimised GaN epitaxy for operation at 461 nm, a robust ECDL geometry, a novel tapered amplifier design and important work in characterising the optical performance and minimising surface reflectivity to identify suitable working parameters.

Systems with the ability to observe and manipulate individual quantum states have been brought to applications that include among others satellite-free navigation and high-precision gravimetric sensing. Fundamentally, the applicability of quantum technology is limited by the complexity and financial burden of light sources required for such systems. These sources need to feature high optical power combined with compromised beam quality and frequency-stabilized narrow-linewidths. These parameters directly influence the performance of the quantum technology measurement system. Semiconductor devices are able to provide high brightness over broad spectral regions through band-gap engineering. InGaN-based laser sources can be engineered to operate from 380nm to 530 nm. This aligns well with the transitions of atomic species such as strontium, magnesium and ytterbium. However, a challenge remains to offer the narrow-linewidths ( 100 mW) required for many of these applications. We will present our development of GaN based narrow-linewidth seed and tapered amplifiers to operate at 461nm for first stage strontium cooling. This includes growth of custom optimised GaN epitaxy for operation at 461 nm, a robust ECDL geometry, a novel tapered amplifier design and important work in characterising and minimising the surface reflectivity to identify suitable working parameters. A comprehensive characterization of the device will be presented.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s21S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2-6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

In this work, we study how an epitaxial laser-like (or superluminescent diode-like) structure is modified by intentional changes of the substrate misorientation in the range of 0.5°–2.6°. The 40 μm × 40 μm test structure with misorientation profiling was fabricated using multilevel photolithography and dry-etching. The local structural parameters were measured by synchrotron radiation microbeam X-ray diffraction, with the sampling area of below 1 μm × 1 μm . We directly obtained the relation between the misorientation and indium content in the quantum well, changing from 9% to 18%, with a high resolution (small misorientation step). We also show a good agreement of local photoluminescence emission wavelength with simulation of transition energy based on synchrotron radiation microbeam X-ray diffraction (SR-XRD) data and estimated Stokes shift. We observe that the substrate misorientation influences also the InGaN waveguide and AlGaN cladding composition. Still, we showed through simulation of the optical confinement factor of a full laser diode structure that good light guiding properties should be preserved in the whole misorientation range studied here. This proves the usefulness of misorientation modification in applications like broadband superluminescent diodes or multicolor laser arrays.

We report a thorough study of InGaN quantum wells spatially modified by varying the local misorientation of the GaN substrate prior to the epitaxial growth of the structure. More than 25 nm shift of emission wavelength was obtained, which is attributed to indium content changes in the quantum wells. Such an active region is promising for broadening of the emission spectrum of (In,Al,Ga)N superluminescent diodes. We observed that the light intensity changes with misorientation, being stable around 0.5° to 2° and decreasing above 2°. This relation can be used as a base for future device designing.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for defence applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and cold-atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s21S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 – 6p2P3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

InGaN/GaN multiple quantum well structures were grown on bulk GaN and on sapphire substrates using the metalorganic vapor-phase epitaxy in order to study the influence of hydrogen during the growth of the GaN barriers. This hydrogen flow had the following effects on the structures: (i) the thickness of the QWs was reduced, (ii) the indium concentration in the QWs was decreased and (iii) the growth rate of the quantum barriers was increased.

The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from UV, ~380 nm, to the visible ~530 nm, by tuning the indium content of the laser GaInN quantum well. This makes nitride laser diodes suitable for a vast range of applications, but most of them require not only the proper wavelength emission, but also high optical power and good beam quality. The typical approach - wide ridge waveguide - often suffers from spatial multimode emission (low beam quality). We report our initial results with tapered GaN lasers to increase the maximum optical power of the device with a good beam profile. This combination opens new possibilities for GaN laser diode technology in quantum technologies including optical atomic clocks and quantum gravity sensors.

GaN laser diodes fabricated from the AlGaInN material system is an emerging technology for high power, optical integration and quantum applications. The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the visible ~530nm, by tuning the indium content of the laser GaInN quantum well, giving rise to new and novel applications including displays and imaging systems, free-space and underwater telecommunications and the latest quantum technologies such as optical atomic clocks and atom interferometry.

The recombination rate coefficients (RRCs) A, B, and C in MOVPE-grown single-quantum-well light emitting diodes spanning the entire blue-green spectral range are determined by fitting efficiency curves and differential carrier lifetimes. The results show definite trends for each of the RRCs: A tendentially decreases with increasing wavelength, B definitely decreases, and C remains approximately constant. Therefore, the increase of the droop with increasing wavelength (the green gap problem) is rather due to the decrease of B than an increase of C. The determined values of C are shown to be similar to what has been predicted by others with first-principles computer simulations accounting for phonon-assisted Auger recombination. Samples grown on sapphire and silicon substrates are compared and show significant differences only for the RRC A, presumably due to the difference in threading dislocation density.

We investigated lateral charge carrier transport in indium gallium nitride InGaN/GaN multi-quantum wells for two different samples, one sample emitting green light at about 510 nm and the other emitting cyan light at about 470 nm. For the cyan light emitting sample we found a diffusion constant of 1.2 cm2/s and for the green light emitting sample 0.25 cm2/s. The large difference in diffusion constant is due to a higher point defect density in the green light emitting quantum wells (QWs) as high indium incorporation tends to reduce material quality.

In this work the effect of external electric field on the broadening of optical transitions in a triple polar InGaN/GaN quantum well is studied. Experimental investigation using photoluminescence and electroreflectance show that the reduction of the internal electric field by an external voltage reduces the broadening of the transitions. This is direct evidence that the broadening of photoluminescence in InGaN/GaN quantum wells is enhanced by the built-in electric field. This conclusion is supported by theoretical modelling within the random quantum well model. Additionally, we show that the exciton–phonon coupling can be controlled by an external electric field.

In this work, we fabricated green-light-emitting laser structures on a (20-21) semipolar GaN substrate. Using cathodoluminescence mapping, x-ray diffraction, and transmission electron microscopy, we revealed the formation of relaxation defects within InGaN waveguides and AlGaN claddings. The observed defects in the AlGaN layers are stripe-like and extend along the a axis, but in the InGaN layers they form a characteristic checkered pattern. We demonstrate that using the selective area growth method we can effectively suppress the formation of both types of defects, thus enabling the fabrication of defect-free green laser structures on semipolar GaN substrates.

The microstructural evolution of InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor phase epitaxy was studied as a function of the growth temperature of the GaN quantum barriers (QBs). We observed the formation of basal stacking faults (BSFs) in GaN QBs grown at low temperature. The presence of BSFs terminated by stacking mismatch boundaries (SMBs) leads to the opening of the structure at the surface into a V-shaped trench loop. This trench may form above an SMB, thereby terminating the BSF, or above a junction between the SMB and a subsequent BSF. Fewer BSFs and thus fewer trench defects were observed in GaN QBs grown at temperatures higher than 830 °C. Further increase in the growth temperature of the GaN QBs led to the suppression of the threading dislocation opening into V-pits.

We report on a green light-emitting device, in which the light of an efficient blue 1 mm2 GaInN/GaN light-emitting diode (LED) is converted into green light by an optically pumped GaInN/GaN multiple quantum well structure. This solution reached an efficacy of 127 lm/W, i.e., higher than that of state-of-the-art 1 mm2 GaInN/GaN LEDs emitting directly at the target wavelength, at 350 mA current and 535 nm peak wavelength. Optically pumped converters overcome the design limitations of typical multiple quantum well LEDs, where carrier transport issues limit the maximum number of functioning wells and might help to solve the problem of the green gap.