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We introduce a toolbox for modelling laser diode operation over a large temperature range, as is encountered in long-pulse hair removal, and in mobile applications such as LiDAR. Power-to-current characteristics and lifetime estimations are sought for arbitrary pulse patterns in quasi-continuous-wave (QCW) operation. Our model is based on (1) the Zth-representation of the package thermal transient, (2) a temperature-dependent family of diode characteristics replacing the insufficient T0,T1-approach, and (3) the assumption that the device lifetime depends on the maximum junction temperature. The simulated evolution of output power and temperature is experimentally verified. Using our model, we assess the influence of the package geometry on the diode temperature and on the efficiency of diode-pumped solid state lasers. We also re-assess lifetime data, and derive safe operating parameters for an arbitrary pulse length.

Vertical GaN nanowires are grown in a self-induced way on a sputtered Ti film by plasma-assisted molecular beam epitaxy. Both in situ electron diffraction and ex situ ellipsometry show that Ti is converted to TiN upon exposure of the surface to the N plasma. In addition, the ellipsometric data demonstrate this TiN film to be metallic. The diffraction data evidence that the GaN nanowires have a strict epitaxial relationship to this film. Photoluminescence spectroscopy of the GaN nanowires shows excitonic transitions virtually identical in spectral position, line width, and decay time to those of state-of-the-art GaN nanowires grown on Si. Therefore, the crystalline quality of the GaN nanowires grown on metallic TiN and on Si is equivalent. The freedom to employ metallic substrates for the epitaxial growth of semiconductor nanowires in high structural quality may enable novel applications that benefit from the associated high thermal and electrical conductivity as well as optical reflectivity.

We study the formation of nanoscale epitaxial Au islands on Si(001) below the eutectic point. Growth experiments were performed in molecular beam epitaxy, and plausible interface models were derived from electron diffraction and transmission electron microscopy. For these models, formation energies were obtained in density functional theory (DFT). In-situ electron diffraction indicates that during the deposition of the first two monolayers, the Si(001) surface mesh is preserved. Au(110) islands form at a coverage above three monolayers. DFT shows that the formation energy for an atomically flat interface is higher for this (110) orientation than for (001) growth. We propose an interface configuration that promotes Au(110) growth and is kinetically stabilized even though this is not the epitaxial orientation with the lowest mismatch strain. The proposed configuration implies a mixed interface layer containing both Au and Si atoms.

An analysis of the strain in an axial nanowire superlattice shows that the dominating strain state can be defined arbitrarily between unstrained and maximum mismatch strain by choosing the segment height ratios. We give experimental evidence for a successful strain design in series of GaN nanowire ensembles with axial InxGa1-xN quantum wells. We vary the barrier thickness and determine the strain state of the quantum wells by Raman spectroscopy. A detailed calculation of the strain distribution and LO phonon frequency shift shows that a uniform in-plane lattice constant in the nanowire segments satisfactorily describes the resonant Raman spectra, although in reality the three-dimensional strain profile at the periphery of the quantum wells is complex. Our strain analysis is applicable beyond the InxGa1-xN/GaN system under study, and we derive universal rules for strain engineering in nanowire heterostructures.

Electron irradiation of GaN nanowires in a scanning electron microscope strongly reduces their luminous efficiency as shown by cathodoluminescence imaging and spectroscopy. We demonstrate that this luminescence quenching originates from a combination of charge trapping at already existing surface states and the formation of new surface states induced by the adsorption of C on the nanowire sidewalls. The interplay of these effects leads to a complex temporal evolution of the quenching, which strongly depends on the incident electron dose per area. Time-resolved photoluminescence measurements on electron-irradiated samples reveal that the carbonaceous adlayer affects both the nonradiative and the radiative recombination dynamics., Comment: This is an author-created, un-copyedited version of an article accepted for publication/published in Nanotechnology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://dx.doi.org/10.1088/0957-4484/27/45/455706

Laser stacks emitting short light pulses are ideally suited for medical and cosmetic applications. Developing enhanced, stable and reliable assembly processes, Jenoptik is reaching for higher energy densities and lower manufacturing costs. In this paper an improved technology for actively cooled QCW stacks is presented. Based on simulations and experimental data, the impacts on the laser stack performance are described and shown as power-current and thermal impedance plots. We show that the bar-to-bar pitch can be reduced from 1.7 mm to 1.2 mm without detrimental thermal effects for pulse durations up to 100 ms.

Laser drivers are an enabling factor to inertial confinement fusion, because laser diodes must be used instead of flash lamps. We discuss the limitations of laser diode arrays and show what steps the industry is taking. The pump power requirements of large-scale projects such as LIFE or HiPER are within reach of semiconductor laser diode assemblies. Pulsed light output powers per laser bars have been around 300 W per bar, as in the Jenoptik 940 nm bars previously used for pumping the Yb:YAG slabs in the DiPOLE project. By redesigning the semiconductor laser structures 500 W per bar is now commercially available for 808, 880 and 940 nm pump wavelengths. The construction of one inertial fusion power plant will require an amount of semiconductor laser chips in excess of the current annual production by two orders of magnitude. This adds to the engineering task of improving the device characteristics a challenge to production capacity. While the industry benefits from the recent boost in solid-state lighting that acts as a technology driver, cooperation between manufacturers will be imperative, and to this end we propose standardization efforts.

We investigate the formation of the ternary alloy InxGa1–xN in the growth of self-induced nanowires by plasma-assisted molecular beam epitaxy. A series of samples grown at different temperatures were analyzed in situ by line-of-sight quadrupole mass spectrometry, and the predicted composition was verified by X-ray diffraction on as-grown nanowire ensembles. The InxGa1–xN composition is determined by the thermally activated loss of In due to InN decomposition and In desorption, similar to the composition of planar layers. The convolution of decomposition, reincorporation, and desorption is described by an apparent activation energy of about 2.5 eV. We show the feasibility of in situ control of the InxGa1–xN nanowire composition by line-of-sight quadrupole mass spectrometry.

High-energy class laser systems operating at high average power are destined to serve fundamental research and commercial applications. System cost is becoming decisive, and JENOPTIK supports future developments with the new range of 500 W quasi-continuous wave (QCW) laser diode bars. In response to different strategies in implementing high-energy class laser systems, pump wavelengths of 880 nm and 940 nm are available. The higher power output per chip increases array irradiance and reduces the size of the optical system, lowering system cost. Reliability testing of the 880 nm laser diode bar has shown 1 Gshots at 500 W and 300 μs pulse duration, with insignificant degradation. Parallel operation in eight-bar diode stacks permits 4 kW pulse power operation. A new high-density QCW package is under development at JENOPTIK. Cost and reliability being the design criteria, the diode stacks are made by simultaneous soldering of submounts and insulating ceramic. The new QCW stack assembly technology permits an array irradiance of 12.5 kW/cm². We present the current state of the development, including laboratory data from prototypes using the new 500 W laser diode in dense packaging.

A new high-power semiconductor laser diode module, emitting at 760 nm is introduced. This wavelength permits optimum treatment results for fair skin individuals, as demonstrated by the use of Alexandrite lasers in dermatology. Hair removal applications benefit from the industry-standard diode laser design utilizing highly efficient, portable and light-weight construction. We show the performance of a tap-water-cooled encapsulated laser diode stack with a window for use in dermatological hand-pieces. The stack design takes into account the pulse lengths required for selectivity in heating the hair follicle vs. the skin. Super-long pulse durations place the hair removal laser between industry-standard CW and QCW applications. The new 760 nm laser diode bars are 30% fill factor devices with 1.5 mm long resonator cavities. At CW operation, these units provide 40 W of optical power at 43 A with wall-plug-efficiency greater than 50%. The maximum output power before COMD is 90 W. Lifetime measurements starting at 40 W show an optical power loss of 20% after about 3000 h. The hair removal modules are available in 1x3, 1x8 and 2x8 bar configurations.

(In,Ga)N insertions embedded in self-assembled GaN nanowires are of current interest for applications in solid state light emitters. Such structures exhibit a notoriously broad emission band. We use cathodoluminescence spectral imaging in a scanning electron microscope and micro-photoluminescence spectroscopy on single nanowires to learn more about the mechanisms underlying this emission. We observe a shift of the emission energy along the stack of six insertions within single nanowires that may be explained by compositional pulling. Our results also corroborate reports that the localization of carriers at potential fluctuations within the insertions plays a crucial role for the luminescence of these nanowire based emitters. Furthermore, we resolve contributions from both structural and point defects in our measurements., Comment: 10 pages, 7 figures

We study the influence of surface and polarization potentials on the electronic properties of axial InxGa1-xN/GaN nanowire heterostructures. Our simulations indicate nontrivial, competing influences of both these potentials on the spatial separation of electrons and holes, which are well suited to explain previous experimental observations.

Using continuum elasticity theory and an eight-band k·p formalism, we study the electronic properties of GaN nanowires with axial InxGa1-xN insertions. The three-dimensional strain distribution in these insertions and the resulting distribution of the polarization fields are fully taken into account. In addition, we consider the presence of a surface potential originating from Fermi level pinning at the sidewall surfaces of the nanowires. Our simulations reveal an in-plane spatial separation of electrons and holes in the case of weak piezoelectric potentials, which correspond to an In content and layer thickness required for emission in the blue and violet spectral range. These results explain the quenching of the photoluminescence intensity experimentally observed for short emission wavelengths. We devise and discuss strategies to overcome this problem.

GaN nanowire ensembles with axial In(x)Ga(1-x)N multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures.

Light emitting diodes (LEDs) were fabricated using ensembles of free-standing (In,Ga)N/GaN nanowires (NWs) grown on Si substrates in the self-induced growth mode by molecular beam epitaxy. Electron beam induced current analysis, cathodoluminescence as well as biased $\mu$-photoluminescence spectroscopy, transmission electron microscopy, and electrical measurements indicate that the electroluminescence of such LEDs is governed by the differences in the individual current densities of the single-NW LEDs operated in parallel, i.e. by the inhomogeneity of the current path in the ensemble LED. In addition, the optoelectronic characterization leads to the conclusion that these NWs exhibit N-polarity and that the (In,Ga)N quantum well states in the NWs are subject to a non-vanishing quantum confined Stark effect., Comment: This is an author-created, un-copyedited version of an article accepted for publication in Nanotechnology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at doi:10.1088/0957-4484/23/46/465301

This paper discusses some of the advantages of nanowire structures for use in LEDs as well as the challenges that need to be overcome towards the realisation of real-world devices. Our experimental results pertain to group-III nitride nanowire structures grown by MBE. We present clear evidence that the catalyst-free growth approach on Si yields best results with respect to structural and optical material properties. We elucidate the mechanism of nanowire nucleation and the factors determining the initial nanowire diameter, discuss the issue of InGaN growth in small-diameter nitride nanowires and review the results reported for nanowire-based group-III nitride LEDs reported so far.

an underestimation of the In content by approximately 30% of the given values, otherwise our findings remain valid. We derive here the correct expression that replaces Eq. (2) of the original paper. Symmetric Bragg x-ray diffraction probes the vertical (normal to the layers) lattice constants of strained layers in a heterostructure. For coherent epitaxial growth without plastic relaxation, all layers attain a common in-plane lattice parameter. In case of a planar heterostructure on a bulk substrate, all layers have the same lateral lattice parameter as the substrate. The resulting average vertical lattice parameter of the heterostructure is governed by the Poisson effect, and the well-known result is given below in Eq. (9) .I n a nanowire, however, the heterostructure is free to relax laterally, and the in-plane lattice parameter is obtained from a balance between layers. If the requirement of the stressfree side surface is satisfied on average, the Poisson effect is compensated and the average vertical lattice parameter is given by Eq. (11) below. To derive this not obvious result, let us consider a periodic superlattice stack made from two materials, 1 and 2. Material 1 is taken as a reference for all strain values and material 2 possesses an eigenstrain e0 with respect to it. We take the lateral and the vertical eigenstrain components equal to each other. Thus, the lattice constants c 0 and c 0 in the fully relaxed state are related by e0 ¼ð c 0 � c 0 Þ=c 0 . For the coherent growth considered here, the in-plane lattice parameter can be approximated to assume a constant value throughout the whole stack, so that the in-plane total strain e1xx ¼ e2xx ¼ exx. The same condition holds for the yy components, the axes x and y are in the lateral plane. For the lateral relaxation of the SL, we distinguish two cases

Self-induced GaN nanowires are grown by plasma-assisted molecular beam epitaxy, with InxGa1−xN quantum wells inserted to form an axial superlattice. From the ω−2θ scans of a laboratory x-ray diffraction experiment, we obtain the superlattice period, the thickness of the quantum wells, and the In content in this layer. The axial growth rate of the InxGa1−xN quantum wells is significantly enhanced, which we attribute to increased Ga diffusion along the nanowire sidewalls in the presence of In.