Johannes Strassner

Precise in situ etch depth control of multilayered III−V semiconductor samples with reflectance anisotropy spectroscopy (RAS) equipment

Reflectance anisotropy spectroscopy (RAS) equipment is applied to monitor dry-etch processes (here specifically reactive ion etching (RIE)) of monocrystalline multilayered III–V semiconductors in situ. The related accuracy of etch depth control is better than 16 nm. Comparison with results of secondary ion mass spectrometry (SIMS) reveals a deviation of only about 4 nm in optimal cases. To illustrate the applicability of the reported method in every day settings for the first time the highly etch depth sensitive lithographic process to form a film lens on the waveguide ridge of a broad area laser (BAL) is presented. This example elucidates the benefits of the method in semiconductor device fabrication and also suggests how to fulfill design requirements for the sample in order to make RAS control possible.

Efficient Ga(As)Sb quantum dot emission in AlGaAs by GaAs intermediate layer

Ga(As)Sb quantum dots (QDs) are epitaxially grown in AlGaAs/GaAs in the Stranski-Krastanov mode. In the recent past we achieved Ga(As)Sb QDs in GaAs with an extremely high dot density of 9.8∙1010 cm-2 by optimization of growth temperature, Sb/Ga flux pressure ratio, and coverage. Additionally, the QD emission wavelength could be chosen precisely with these growth parameters in the range between 876 and 1035 nm. Here we report a photoluminescence (PL) intensity improvement for the case with AlGaAs barriers. Again growth parameters and layer composition are varied. The aluminium content is varied between 0 and 90%. Reflectance anisotropy spectroscopy (RAS) is used as insitu growth control to determine growth rate, layer thickness, and AlGaAs composition. Ga(As)Sb QDs, directly grown in AlxGa1-xAs emit no PL signal, even with a very low x ≈ 0.1. With additional around 10 nm thin GaAs intermediate layers between the Ga(As)Sb QDs and the AlGaAs barriers PL signals are detected. Samples with 4 QD layers and AlxGa1-xAs/GaAs barriers in between are grown. The thickness and composition of the barriers are changed. Depending on these values PL intensity is more than 4 times as high as in the case with simple GaAs barriers. With these results efficient Ga(As)Sb QD lasers are realized, so far only with pure GaAs barriers. Our index-guided broad area lasers operate continuous-wave (cw) @ 90 K, emit optical powers of more than 2∙50 mW and show a differential quantum efficiency of 54% with a threshold current density of 528 A/cm2.

Microfluidic Droplet Array as Optical Irises Actuated via Electrowetting

Initiated by a task in tunable microoptics, but not limited to this application, a microfluidic droplet array in an upright standing module with 3 × 3 subcells and droplet actuation via electrowetting is presented. Each subcell is filled with a single (of course transparent) water droplet, serving as a movable iris, surrounded by opaque blackened decane. Each subcell measures 1 × 1 mm ² and incorporates 2 × 2 quadratically arranged positions for the droplet. All 3 × 3 droplets are actuated synchronously by electrowetting on dielectric (EWOD). The droplet speed is up to 12 mm/s at 130 V (Vrms) with response times of about 40 ms. Minimum operating voltage is 30 V. Horizontal and vertical movement of the droplets is demonstrated. Furthermore, a minor modification of the subcells allows us to exploit the flattening of each droplet. Hence, the opaque decane fluid sample can cover each water droplet and render each subcell opaque, resulting in switchable irises of constant opening diameter. The concept does not require any mechanically moving parts or external pumps.

Monolithically integrated fourier-optical transverse-mode selector for broad area lasers

Ga(As)Sb/GaAs quantum dot broad area lasers (BALs) with an emission wavelength of λ= 930 nm are realised. For suppressing the typical multi-transverse-mode operation of the BALs transverse mode selection filters are monolithically integrated into the laser crystal. Two different approaches are reported, which can both be considered Fourier-optical. Following our spatial-frequency-filter concept with 4f setup from [1] BALs with a monolithically integrated film waveguide lens on one facet and spatial filter elements on the other facet supporting the fundamental mode are developed [2]. The resonator length is l = f = 2700 μm determined by the focal length f of the film lens, the ridge width of 100 μm, and the filter width of 30 μm.

Highly ordered Ga(As)Sb quantum dots grown on pre-structured GaAs

Ga(Sb)As quantum dots (QDs) are usually grown on plane GaAs substrates by self-organization in the StranskiKrastanov mode. Here we report on Ga(As)Sb QD growth on a pre-structured GaAs substrate to achieve highly ordered QDs. The structure consists of a two-dimensional array of holes/troughs milled into the substrate (wafer with initial epitaxial buffer layer) with a gallium focused ion beam (Ga-FIB). Thus, the area density of the QDs can be controled. For exact positioning of the QDs in the milled holes it is important that the diameter of the dots equals the diameter of the milled holes. In a previous publication we have shown that we are able to change the diameter as well as the height of the QDs by controlled variation of growth temperature, Ga/Sb ratio, and nominal coverage. The diameter and depth of the milled holes as well as their separation are varied. Also, different milling techniques are examined to optimize milling time and procedure. The pre-structured GaAs substrate is overgrown in a second molecular-beam-epitaxial (MBE) step, first with another thin GaAs buffer layer, then with a QD layer. With the optimum of the milling and growth parameter sets the diameter of the QDs equals the size of the milled holes and the QDs can be grown highly ordered in the given pre-structured array. To the best of our knowledge this is the first report about exact positioning of Ga(As)Sb QDs on GaAs.

Atomic layer sensitive in-situ plasma etch depth control with reflectance anisotropy spectroscopy (RAS)

Reflectance anisotropy spectroscopy (RAS) allows for in-situ monitoring of reactive ion etching (RIE) of monocrystalline III-V semiconductor surfaces. Upon use of RAS the sample to be etched is illuminated with broad-band linearly polarized light under nearly normal incidence. Commonly the spectral range is between 1.5 and 5.5 eV. Typically the spectrally resolved difference in reflectivity for light of two orthogonal linear polarizations of light is measured with respect to time - for example for cubic lattices (like the zinc blende structures of most III-V semiconductors) polarizations along the [110] and the [-110] direction. Local anisotropies on the etch front cause elliptical polarization of the reflected light resulting in the RAS signal. The time and photon energy resolved spectra of RAS include reflectometric as well as interferometric information. Light with wavelengths well above 100 nm (even inside the material) can be successfully used to monitor surface abrasion with a resolution of some tens of nanometers. The layers being thinned out act as optical interferometers resulting in Fabry-Perot oscillations of the RAS-signal. Here we report on RAS measurements assessing the surface deconstruction during dry etching. For low etch rates our experimental data show even better resolution than that of the (slow) Fabry-Perot oscillations. For certain photon energies we detect monolayer-etch-related oscillations in the mean reflectivity, which give the best possible resolution in etch depth monitoring and control, i.e. the atomic scale.

Fundamental Transverse Mode Selection (TMS#0) of Broad Area Semiconductor Lasers with Integrated Twice-Retracted 4f Set-Up and Film-Waveguide Lens

Previously we focused on fundamental transverse mode selection (TMS#0) of broad area semiconductor lasers (BALs) with two-arm folded integrated resonators for Fourier-optical spatial frequency filtering. The resonator had a round-trip length of 4f, where f is the focal length of the Fourier-transform element (FTE), that is, a cylindrical mirror in-between the orthogonal resonator branches. This 4f set-up can be called “retracted once” due to the reflective filter after 2f; that is, the 2f path was used forwards and backwards. Now the branches are retracted once more resulting in a compact 1f long linear resonator (called “retracted twice”) with a round-trip length of 2f. One facet accommodates the filter, while the other houses the FTE, now incorporating a film-waveguide lens. The BAL facet with the filter represents both the Fourier-transform plane (after 2f, i.e., one round-trip) as well as the image plane (after 4f, two round-trips). Thus filtering is performed even after 4f, not just after 2f. Experimental results reveal good fundamental TMS for pump currents up to 20% above threshold and a one-dimensional beam quality parameter = 1.47. The BALs are made from AlGaInAsSb, but the concept can equally well be employed for BALs of any material system.

Microfluidic Optical Shutter Flexibly - Actuated via Electrowetting-on-Dielectrics with <20 ms Response Time

Tunable microoptics deals with devices of which the optical properties can be changed during operation without mechanically moving solid parts. Often a droplet is actuated instead, and thus tunable microoptics is closely related to microfluidics. One such device/module/cell type is an optical shutter, which is moved in or out of the path of the light. In our case the transmitting part comprises a moving transparent and electrically conductive water droplet, embedded in a nonconductive blackened oil, that is, an opaque emulsion with attenuation of 30 dB at 570 nm wavelength over the 250 μm long light path inside the fluid (15 dB averaged over the visible spectral range). The insertion loss of the cell is 1.5 dB in the “open shutter” state. The actuation is achieved via electrowetting-on-dielectrics (EWOD) with rectangular AC voltage pulses of  V peak-to-peak at 1 kHz. To flexibly allow for horizontal, vertical, and diagonal droplet movement in the upright x-y plane, the contact structures are prepared such that four possible stationary droplet positions exist. The cell is configured as two capacitors in series (along the axis), such that EWOD forces act symmetrically in the front and back of the 60 nl droplet with a response time of <20 ms.

Growth control of Ga(As)Sb quantum dots (QD) on GaAs with reflectance anisotropy spectroscopy (RAS)

Ga(As)Sb quantum dots (QDs) are grown on GaAs substrate in the Stranski-Krastanov mode. The molecular beam epitaxial (MBE) growth is monitored by reflectance anisotropy spectroscopy (RAS). For certain photon energies of the light used for RAS, the RAS signal values for GaAs layers, GaSb layers, and Ga(As)Sb QD surface morphologies can clearly be distinguished. The finding verifies that RAS is a valuable tool to identify growth of these QDs.

Ga(As)Sb/GaAs quantum dots for emission around 1300 nm

Summary form only given. We report on Ga(As)Sb/GaAs quantum dots (QDs) for use in efficient QD lasers. The emission wavelength can be chosen with the variation of the growth temperature and the Sb/Ga V/III partial pressure flux ratio. As can be seen from the experimental photoluminescence (PL) results the emission wavelength can be shifted in a wide range between 876 and 1035 nm. The nominal coverage of 3 monolayers (ML) is constant. A higher Sb flux causes bigger quantum dots with a higher Sb concentration within the dots. This results in a red shift of the emission wavelength. In contrast, a higher growth temperature leads to a (here) even larger blue shift, caused by an increase of the As diffusion into the QDs. A higher growth temperature also increases dot size, but As and Sb intermixing is the dominating process causing the resulting (net) blue shift. With these growth parameters we could not achieve an emission wavelength beyond 1100 nm. To achieve emission around 1300 nm, two different approaches and parameter sets are used: On one hand a higher Sb/Ga ratio of 2/1 and a growth temperature of 561°C are chosen, because this way the red shift of the higher Sb/Ga ratio should exceed the blue shift resulting from the higher growth temperature. The PL measurement results are shown in the inset of Fig. 1a. An emission wavelength near the preferable value of 1300 nm is achieved, although PL intensity is still relatively low. The QDs have a density around 3·1010 cm-2 and a highly uniform size distribution (a diameter of 28±2 nm and a height of 4±1 nm). On the other hand the nominal coverage is reduced to 2 ML and an additional growth interruption of 10 s is scheduled after the deposition of the QDs. The other growth parameters are an Sb/Ga ratio of 1/1 and a growth temperature of 530°C. During the growth interruption the QDs are stabilized with an Sb flux. The PL spectrum is shown in Fig. 1b revealing an emission wavelength of 1225 nm. So far a disadvantage of both variations is a lower PL intensity resulting from a lower radiative recombination rate. By combining the new growth parameters with the growth interruption we are confident to achieve stronger PL signals and an emission wavelength closer to 1300 nm.