Structural Characterization of Multi‐Quantum Wells in Electroabsorption‐Modulated Lasers by using Synchrotron Radiation Micrometer‐Beams

Abstract

Advanced optoelectronic devices, the backbone of modern communication technology, require the monolithic integration of different functions at chip level. An example of devices fulfilling this requirement are multi-quantum well (MQW) electroabsorption-modulated lasers (EMLs) employed in long-distance, high-frequency optical fiber communications technology. Such devices are realized by using the selective area growth (SAG) technique. Optimization of the growth parameters is carried out by empirical approaches as direct structural characterization of the MQW is not possible with laboratory X-ray sources, owing to the micrometer-variation of composition and thickness inherent to the SAG technique. Micrometer-resolved X-ray (m-X-ray) beam available at third generation synchrotron radiation sources, such as the ID22 at the European Synchrotron Radiation Facility (ESRF), allowed us to directly measure the determinant structural parameters of MQW EML structures such as well and barrier widths and mismatches with a 2 mm spatial resolution.In more detail, optoelectronic devices, widely used in the generation and detection of optical signal for telecom and datacom applications, consist of different III-V semiconductors alloys deposited on suitable substrates. The deposition can be performed by different techniques that are based on the crystal rebuilding of a semiconductor used as substrate, such as metal organic vapor phase epitaxy (MOVPE). [1,2] The reaction takes place between free atoms and the substrate's surface with the rebuilding of the crystal lattice. During the growth different layers are sequentially and uniformly deposited on the entire substrate, resulting in the aimed structure relative to the specific device function (e.g., LED, laser, amplifier, modulator, photodiode, waveguide, etc.).As an example, uncooled distributed feedback (DFB) lasers, operating in the second window of telecommunications at 1310 nm, are MQW semiconductor lasers directly modulated at high frequency (up to 10 Gb s À1) and are a key element for connections typically up to 10 km. For longer distances a directly modulated laser cannot be used. The devices operate at 1550 nm (third-window) to take advantage of the limited fiber attenuation. However, in the presence of chromatic-dispersion on a standard single-mode fiber at this wavelength, the high positive chirp [3] of the optical pulses, generated by directly modulated DFB, severely limits the propagation span ($10 km at 10 Gb s À1). For 40-80 km propagation of 10 Gb s À1 optical signals, external modulation is needed. An external MQW electroabsorption modulator (EAM) can produce near-zero or even negatively chirped pulses, allowing propagation spans of more than 80 km. An EML, [4,5] obtained by monolithic integration of an EAM with a constant-injected DFB laser (Fig. 1b), is often the most convenient and compact solution. The modulation of the laser emission is ensured by the switching between an opaque and transparent state of the EAM due to the Stark effect [6] (Fig. 1c) induced by a...

μ‐EXAFS, μ‐XRF, and μ‐PL Characterization of a Multi‐Quantum‐Well Electroabsorption Modulated Laser Realized via Selective Area Growth

“…Conversely, when the growth parameters of a new monolithic device (III–V fluxes, growth temperature, stripe composition and geometry, etc.) have to be optimized, the characterization procedure reported in the present study and in two previous reports41, 42 will provide fundamental information on how to converge faster to the ideal growth parameters for the desired device. This work represents a clear example of how academic and industrial research efforts can merge constructively 106…”

“…Comparing the PL spectra along the Y‐line, a progressive relative degradation of the crystalline quality of the MQW is appreciable by moving from FIELD to SAG, where the PL intensity is lower and the full width at half maximum (FWHM) is larger (see Figure 2). The phenomenon is related to a lack of strain compensation, to a higher growth rate, and to the interference of precursors fluxes in the SAG region and has recently been highlighted on similar SAG heterostructures grown adopting a narrower SiO 2 stripes (20 μm) 41…”

“…Since the Ga/As ratio is lower in the SAG region than in the FIELD, we can assert that the average Ga content of the MQW structure progressively increases along the Y‐line by moving from SAG to FIELD. This is due to the SAG effect that exploits the shorter diffusion length of In ( D / k , where D is the diffusion coefficient and k is the rate of adsorption on the surface) with respect to that of Ga and Al 24, 28, 41, 79…”

“…Although focused on a very specific class of materials (SAG‐grown III–V heterostructures for optoelectronic devices37–42), the present contribution highlights the important improvements achieved in both beam focusing and beam stability that could be successfully applied in several frontier topics, in materials science, geology, archaeometry, environmental sciences, biology, neuroscience, and medicine. Without any presumption of exhaustivity, a small list of selected applications in different fields follows: •Materials science: Determination of doping profiles and doping local environments in semiconductor films;43–47 spatial determination of complex intermetallic phases in multicrystalline silicon doped with transition metals;48 study of composition and Cu oxidation‐state gradient in Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi‐2212) superconducting whiskers;49, 50 spatial determination of residual strain gradients in composite materials51, 52 or in externally bent semiconductors;53 investigation of the local strain tensor of different heteroepitaxial layers with submicrometer resolution in oxide films grown on metal substrates 54…”

5. Structural and electronic characterization of nanosized inorganic materials by X-ray absorption spectroscopies

“…In recent years space-resolved characterization at the (sub-)micrometric scale was made possible by the exponential increase in X-ray brilliance at III generation synchrotron radiation sources and by the impressive improvements in X-ray focusing devices [110]. In the field of materials characterization, they are becoming a key tool for the space-resolved determination of structural (XRD) [111][112][113][114] and electronic (XANES/EXAFS) properties [115,116] and for chemical speciation (XRF) [117][118][119][120] of nanostructured or composite materials. In particular, synchrotron radiation micro-/nano-beams can also be successfully employed to study devices based on nanostructured inorganic materials.…”