Antimonide-based mid-infrared laser structures: Growth and characterization
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Antimonide-based mid-infrared laser structures have been grown by molecular beam epitaxy (MBE). Epitaxially grown laser-related semiconductor materials have been characterized by X-ray diffraction (XRD), Hall effect measurement, photoluminescence, secondary ion mass spectrometry (SIMS) and spectroscopic ellipsometry. Edge emitting semiconductor diode lasers were fabricated and tested. Firstly, the (AlGaIn)(AsSb)-based semiconductor materials were characterized with regards to structural properties, optical properties and doping characteristics. Aluminum containing antimonide-based semiconductor alloys, latticematched to GaSb, are very important for fabrication of cladding region for midinfrared quantum well lasers, and the lattice constant of AlSb plays a crucial role in determining the lattice matching condition at growth temperature for these alloys. Temperature dependent XRD measurements have been performed on epitaxially grown AlSb layer on GaSb (001) substrate. Temperature dependence of the lattice constant and Poisson’s ratio have been determined between 32 and 546 0C. For temperatures above 67 0C, the lattice constant of AlSb was found to be larger than previously published linear extrapolation values. Poisson’s ratio was found to be constant up to 300 0C and decreasing above. Tellurium (Te) and beryllium (Be) doped Al0.9Ga0.1As0.06Sb0.94 layers with Al0.3Ga0.7As cap layer were grown by MBE. Room temperature Hall effect measurements were performed on Hall bar samples with six-contact 1-2-2-1 geometry with photoresist sidewall passivation to determine the carrier concentration and Hall mobility values. Te and Be dopant densities were calculated from the SIMS depth profile of the doped Al0.9Ga0.1As0.06Sb0.94 samples. Carrier concentration was found to have a linear dependence on the dopant density for Be-doped Al0.9Ga0.1As0.06Sb0.94 for dopant density up to 2.9 × 1019 cm-3 and carrier concentration 3.7 × 1019 cm-3, whereas it gets saturated for Tedoped Al0.9Ga0.1As0.06Sb0.94 samples at dopant density value of 8.0 × 1018 cm-3 and carrier concentration 1.6 × 1017 cm-3. Hall mobilities for both Te- and Bedoped samples were much lower than that of GaAs. Low doping efficiency in Te-doped Al0.9Ga0.1As0.06Sb0.94 samples were further investigated by deep level transient spectroscopy (DLTS). Evidence of deep level traps suggests that the low doping efficiency could be due to presence of DX-centers. Dispersive refractive index of epitaxially grown AlGaInAsSb pentenary/quinary and AlGaAsSb quaternary alloys with different compositions were studied using spectroscopic ellipsometry. Reflection high-energy electron diffraction (RHEED) and XRD measurements were performed to determine the compositional variations. The refractive index for AlGaAsSb quaternary alloys was found to decrease with increase in Al content in the wavelength range 1-5 μm, suggesting the use of high Al containing AlGaAsSb quaternary alloys lattice-matched to GaSb as cladding layers in GaInAsSb/AlGaAsSb-based laser structures to achieve high optical confinement in the core. But for each GaSblattice- matched composition of AlGaAsSb, the refractive index was not constant above 2 μm and decreased slowly with longer wavelength. For AlGaInAsSb pentenary alloys, the refractive index was found to decrease with increase in Al and In content, in the 1-5 μm wavelength range. A change in As content in the AlGaInAsSb pentenary alloy, while keeping the group III composition fixed, had little effect on the measured refractive index in the 1-5 μm wavelength range. Since AlGaInAsSb pentenary alloy layers are used as separate confinement layer (SCL) in the core of the diode laser, low Al and In containing pentenary alloys should be preferred to get higher refractive index for better optical confinement in the core. Dependence of the refractive index on dopant density and carrier concentration for both Te- and Be-doped Al0.90Ga0.10As0.06Sb0.94 quaternary alloys were studied using spectroscopic ellipsometry in the 0.05 - 2.1 eV energy range (0.59 - 4.8 μm wavelength range). Composition of the quaternary alloy was determined using RHEED and XRD and doping variations were confirmed by previously performed carrier concentration calibration curves and Raman spectroscopic measurements. Carrier induced change in refractive index was observed in Be-doped Al0.9Ga0.1As0.06Sb0.94 quaternary alloys for carrier concentrations above 3.0×1017 cm-3. But due to low doping efficiency in Tedoped Al0.90Ga0.10As0.06Sb0.94 quaternary alloys, the carrier concentration gets saturated at 1.6×1017 cm-3 and hence the carrier concentration was found to be too small to induce a change in refractive index in this wavelength range. Refractive index versus carrier concentration calibration curve for Be-doped Al0.90Ga0.10As0.06Sb0.94 quaternary alloy can be used for better design of waveguides for (AlGaIn)(AsSb)-based diode lasers. Secondly, laser structures were grown by MBE and edge emitting diode lasers were fabricated using conventional UV-lithography, inductively-coupled plasma reactive ion etching (ICP-RIE) and e-beam metal deposition processes. AlGaAsSb ridge waveguides with high-precision etching very close to the core of the laser were demonstrated using reflectance monitoring during ICP-RIE. Several plasma-assisted techniques were investigated to remove the oxide on ptype GaSb prior to metallization and the metal contacts were characterized using the transfer length method (TLM) and four-point probe measurements. Very low contact resistivities of <5×10-8 Ω.cm2 (i.e. below the limit for accurate TLM measurement results) were achieved after surface pre-treatment by H2/Ar sputter etching and low-ion-energy Ar+ irradiation. This process recipe was used for low contact resistance metal contacts to p-type GaSb cap layer of the laser structures. Straight waveguide as well as Y-junction waveguide diode lasers were fabricated. (AlGaIn)(AsSb)-based laser material with GaInAsSb quantum wells and AlGa(In)AsSb barriers with emission wavelength up to 3.05 μm were grown using MBE. All the lasers were characterized by optical power-current-voltage (P-I-V) characteristics and spectral measurements. For 25 μm wide and 1 mm long ridge waveguide lasers, the threshold current density was found to be 2×106 A.m-2. The maximum power per facet from these lasers was found to be 20 mW at 2.334 μm emission wavelength with injected current density of 2×107 A.m-2. Wide tuning of 50 nm in emission wavelength with multimode emission was achieved in electrically tuned (AlGaIn)(AsSb)-based Y-junction laser emitting at 2.32 μm. Aluminum-based metal contacts to both p-type and n-type GaSb epilayers as an alternative to gold (Au)-based contacts were demonstrated for application on the antimonide-based straight waveguide edge emitting lasers. The specific contact resistivity of the contact between Al and p-type GaSb was found to be lower than that of the contact between Au and p-type GaSb. For the n-type GaSb, the Al-based contact was found to be as good as the Au-based contact. The good performance of GaSb-based laser diode using Al-based contacts shows the applicability of this type of contact in GaSb-based devices.