A. Maaßdorf

Advanced thin film technology for ultrahigh resolution X-ray microscopy

Further progress in the spatial resolution of X-ray microscopes is currently impaired by fundamental limitations in the production of X-ray diffractive lenses. Here, we demonstrate how advanced thin film technologies can be applied to boost the fabrication and characterization of ultrahigh resolution X-ray optics. Specifically, Fresnel zone plates were fabricated by combining electron-beam lithography with atomic layer deposition and focused ion beam induced deposition. They were tested in a scanning transmission X-ray microscope at 1.2 keV photon energy using line pair structures of a sample prepared by metal organic vapor phase epitaxy. For the first time in X-ray microscopy, features below 10nm in width were resolved.

Carbon doping for the GaAs base layer of Heterojunction Bipolar Transistors in a production scale MOVPE reactor

In this work different approaches for carbon doping of GaAs in MOVPE are compared with respect to their growth-and device-related material properties. Doping levels up to 6 x 10 19 cm -3 and smooth surface morphologies are achieved with either intrinsically (TMG and AsH 3 or TMAs) or extrinsically (CBr 4 ) doped layers. Despite comparable structural and majority carrier properties differences in GaInP/GaAs-HBT device performance depending on base doping conditions are obtained. Devices with an intrinsically doped base layer (TMG + AsH 3 ) show superior transistor performance with a current gain to base sheet resistance ratio (β/R sb ) exceeding 0.5 for base thicknesses as large as 120 nm. The use of either CBr 4 or TMAs as base growth precursors results in reduced current gains (β/R sb ≤ 0.3). It is shown that the achieved HBT current gain is directly related to recombination centers in the heavily doped base layer depending on doping method.

975-nm high-power broad area diode lasers optimized for narrow spectral linewidth applications

Many pumping and direct diode applications of high power diode lasers require sources that operate within a narrow (< 1nm) temperature stable spectral line. The natural linewidth of high power broad area lasers is too wide (4-5nm) and varies too quickly with temperature (0.3-0.4nm/K) for such applications. The spectrum can be narrowed by introducing gratings within the diode laser itself or by the use of an external stabilization via a Volume Bragg Grating, VBG. For optimal loss-free, low cost wavelength stabilization with a VBG, the narrowest possible far field angles are preferred, provided power and efficiency are not compromised. Devices that contain internal gratings are potentially the lowest manufacturing cost option, provided performance remains acceptable, as no external optics are required. Therefore, in order to address the need for high power with narrow linewidth, three different diode laser device designs have been developed and are discussed here. For VBG use, two options are compared: (1) devices with high conversion efficiency (68% peak) and reasonable far field (45° with 95% power content) and (2) devices with extremely small vertical far field angle (30° with 95% power content) and reasonable conversion efficiency (59% peak). Thirdly, the latest performance results from broad area devices with internal distributed feedback gratings (DFB-BA Lasers) are also presented, constructed here using buried overgrowth technology. DFB-BA lasers achieve peak conversion efficiency of 58% and operate with < 1nm linewidth operation to over 10W continuous wave at 25°C. As a result, the system developer can now select from a range of high performance diode laser designs depending on the requirements.

Reliable operation of 976nm high power DFB broad area diode lasers with over 60% power conversion efficiency

Diode lasers that deliver high continuous wave optical output powers (> 5W) within a narrow, temperature-stable spectral window are required for many applications. One technical solution is to bury Bragg-gratings within the semiconductor itself, using epitaxial overgrowth techniques to form distributed-feedback broad-area (DFB-BA) lasers. However, such stabilization is only of interest when reliability, operating power and power conversion efficiency are not compromised. Results will be presented from the ongoing optimization of such DFB-BA lasers at the Ferdinand-Braun- Institut (FBH). Our development work focused on 976nm devices with 90μm stripe width, as required for pumping Nd:YAG, as well as for direct applications. Such devices operate with a narrow spectral width of < 1nm (95% power content) to over 10W continuous wave (CW) optical output. Further optimization of epitaxial growth and device design has now largely eliminated the excess optical loss and electrical resistance typically associated with the overgrown grating layer. These developments have enabled, for the first time, DFB-BA lasers with peak CW power conversion efficiency of > 60% with < 1nm spectral width (95% power content). Reliable operation has also been demonstrated, with 90μm stripe devices operating for over 4000 hours to date without failure at 7W (CW). We detail the technological developments required to achieve these results and discuss the options for further improvements.

Low-loss smile-insensitive external frequency-stabilization of high power diode lasers enabled by vertical designs with extremely low divergence angle and high efficiency

Broad area lasers with narrow spectra are required for many pumping applications and for wavelength beam combination. Although monolithically stabilized lasers show high performance, some applications can only be addressed with external frequency stabilization, for example when very narrow spectra are required. When conventional diode lasers with vertical far field angle, ΘV 95% ~ 45° (95% power) are stabilized using volume holographic gratings (VHGs), optical losses are introduced, limiting both efficiency and reliable output power, with the presence of any bar smile compounding the challenge. Diode lasers with designs optimized for extremely low vertical divergence (ELOD lasers) directly address these challenges. The vertical far field angle in conventional laser designs is limited by the waveguiding of the active region itself. In ELOD designs, quantum barriers are used that have low refractive index, enabling the influence of the active region to be suppressed, leading to narrow far field operation from thin vertical structures, for minimal electrical resistance and maximum power conversion efficiency. We review the design process, and show that 975 nm diode lasers with 90 μm stripes that use ELOD designs operate with ΘV 95% = 26° and reach 58% power conversion efficiency at a CW output power of 10 W. We demonstrate directly that VHG stabilized ELOD lasers have significantly lower loss and larger operation windows than conventional lasers in the collimated feedback regimes, even in the presence of significant (≥ 1 μm) bar smile. We also discuss the potential influence of ELOD designs on reliable output power and options for further performance improvement.

Critical issues of growth optimization for Ga0.5In0.5P/GaAs heterojunction bipolar transistors

This work reports on optimization of MOVPE growth procedures for high-quality, highly uniform (4 inch) and highly reliable Ga0.5In0.5P/GaAs-HBTs in a commercial multiwafer reactor. Appropriately to this application the main focus of research have been improvements in base-layer transport properties and their implications for HBT device characteristics and reliability. Different carbon-doping techniques are compared with regard to GaAs:C material quality and GaInP/GaAs-HBT device performance. Base-doping homogeneity and hydrogen incorporation were further aspects of optimization. The obtained differences in HBT device performance and reliability depending on base-growth conditions confirm the importance of optimization of this particular step in the growth process.

Assessing the influence of the vertical epitaxial layer design on the lateral beam quality of high-power broad area diode lasers

GaAs-based high-power broad-area diode lasers deliver optical output powers Popt > 10W with efficiency > 60%. However, their application is limited due to poor in-plane beam parameter product BPPlat=0.25×Θ95%×w95% (Θ95% and w95% are emission angle and aperture, 95% power content). We present experimental investigations on λ = 9xx nm broad area lasers that aim to identify regulating factors of the BPPlat connected to the epitaxial layer design. First, we assess the thermal lens of vertical designs with varying asymmetry, using thermal camera images to determine its strength. Under study are an extreme-double-asymmetric (EDAS) vertical structure and a reference (i.e. more symmetric) design. The lateral thermal profiles clearly show that BPPlat increase is correlated to the bowing of the thermal lens. The latter is derived out of a quadratic temperature fit in the active region beneath the current injection of the laser device and depends on the details of the epitaxial layers. Second, we test the benefit of low modal gain factor Γg0, predicted to improve BPPlat via a suppression of filamentation. EDAS-based lasers with single quantum well (SQW) and double quantum well (DQW) active regions were compared, with 2.5x reduced Γg0, for 2.2x reduced filament gain. However, no difference is seen in measured BPPlat, giving evidence that filamentary processes are no longer a limit. In contrast, devices with lower Γg0 demonstrate an up to twofold reduced near field modulation depth, potentially enabling higher facet loads and increased device facet reliability, when operated near to the COD limit.

Strong amplitude-phase coupling in submonolayer quantum dots

Submonolayer quantum dots promise to combine the beneficial features of zero- and two-dimensional carrier confinement. To explore their potential with respect to all-optical signal processing, we investigate the amplitude-phase coupling (α-parameter) in semiconductor optical amplifiers based on InAs/GaAs submonolayer quantum dots in ultrafast pump-probe experiments. Lateral coupling provides an efficient carrier reservoir and gives rise to a large α-parameter. Combined with a high modal gain and an ultrafast gain recovery, this makes the submonolayer quantum dots an attractive gain medium for nonlinear optical signal processing.

In situ etched gratings embedded in AlGaAs for efficient high power 970 nm distributed feedback broad-area lasers

We report optical nanostructuring technology, developed for distributed feedback gratings, broadly useable for many applications. The nanostructure is pre-structured into aluminum-free layers on top of AlGaAs then etched inside the epitaxy reactor and overgrown with AlGaAs. Oxygen contamination at the grating-interface is ∼3 × 1011 cm−2. These gratings introduce no extra internal optical loss and series resistance in broad-area lasers. Distributed feedback broad-area lasers using this technology achieve optical power >12 W, peak efficiency >60%, wide spectral locking range in current and heatsink temperature (over at least ∼30 °C) and operate at 10 W for >5000 h in a preliminary reliability test.

High-power, high-brightness 100W QCW diode laser at 940nm

We demonstrate 940nm diode lasers with more than 100W QCW output power having an aperture width 5 to 10 times smaller than commonly used 10mm bars. We used a super-large vertical waveguide structure to reduce the facet load. The waveguide design results in a very small vertical divergence of only 14° FWHM (24° including 95% of power). The threshold current of a device with 1mm wide aperture is about 8A and the slope efficiency is above 70%. The lateral far field width is below 10°, including 95% of power, and the wall plug efficiency is around 50% at 100W output power.