Development of tunable Y-junction laser diodes for trace-gas sensing applications

Abstract

Y-junction laser diodes are promising light sources needed for trace-gas sensing systems based on tunable diode-laser absorption spectroscopy. The merit of this laser structure is the fabrication simplicity and the ability for realization of tunable laser devices. This PhD dissertation presents successful development of widely tunable Y-junction laser diodes emitting in mid-infrared regime based on the optimization of laser growth and fabrication process. Antimonide-based mid-infrared laser structures have been grown using molecular beam epitaxy Carrier concentration in the doped cladding layers play a key role in the electrical properties of the laser diodes. Thus, optimization of carrier concentration is required with regards to dopant density. Incorporation of tellurium (Te) and beryllium (Be) dopants in Al0.9Ga0.1As0.06Sb0.94 cladding layers was investigated to enhance the electrical properties while minimizing the defect levels for better optical properties. The dependence of carrier concentration and Hall mobility on dopant density for both n- and p-type doped AlGaAsSb cladding layers has been precisely determined using Hall effect and secondary ion mass spectrometry depth profile measurements. Carrier concentration was found to have linear dependence on the dopant density for Be-doped Al0.9Ga0.1As0.06Sb0.94 for dopant density up to 2.9 × 1019 cm-3, whereas it gets saturated for Te-doped Al0.9Ga0.1As0.06Sb0.94 samples at 1.6 × 1017 cm-3 for dopant density values above 8.0 × 1018 cm-3. Deep level transient spectroscopy revealed the presence of deep trap levels (most likely DX-centers) as a cause of low doping efficiency in the Te-doped Al0.9Ga0.1As0.06Sb0.94 samples. The grown laser materials were processed into Fabry-Perot (FP) and Y-junction lasers with different configuration. Light confinement in a high loss waveguide such as the Y-junction structure requires a well-defined ridge waveguide structure. Such structure can be obtained from optimized photolithography processes and precise control of etching processes using in situ reflectance monitored inductively-coupled plasma reactive ion etching. The etch depth or the height of the ridge waveguide were precisely and repeatably controlled with an endpoint depth precision within ± 10 nm using this etching technique. As a result, significant improvement in the performance of Y-junction lasers has been achieved as compared to previous studies in our group. For high performance operation of the laser diodes, low ohmic contact resistance to GaSb is highly desirable to enhance the carrier injection and minimize the heat generation in the contact region, thus reducing the heat inside the laser structure. Such good ohmic contact requires an oxide-free surface of GaSb, which is in contact with the metallic layers. Prior to metallization, several plasma-assisted techniques were applied to remove the native oxide of GaSb. The effect of these techniques on the contact properties in comparison to the conventional chemical methods was evaluated via the specific contact resistivity between the metallic layers and ptype GaSb, obtained from transfer length method (TLM) and four-point probe measurements. Very low specific contact resistivities of less than 5 × 10-8 Ω cm2 (the limit for accurate TLM measurement results) were observed after pre-surface treatment by H2/Ar sputtering etching and low-ion-energy Ar irradiation. By eliminating sample exposure to air prior to metallization, in situ Ar irradiation is a promisingly reliable technique for fabrication of high performance GaSb-based semiconductor devices. Aluminum (Al)-based metallizations to n- and p-type GaSb were demonstrated as an alternative for gold (Au)–based contacts in laser applications. Measurement of specific contact resistivities between metallic layers and n-/p-type GaSb showed that the Al-based contacts performed as good as the conventional Au-based contacts. The contact property between Al and p-type GaSb was even better after annealing. The potential reliability problem of this type of contact revealed by reliability test when used in some devices, due to interdiffusion, can be reduced by using a diffusion barrier. The good performance of GaSb-based straight waveguide edge emitting lasers using Al-based contacts shows the applicability of this type of contact in GaSb-based semiconductor devices. Characterizations of FP laser diodes at room temperature (RT) and in continuous wave (cw) mode showed their good output performance in term of electrical, optical and spectral characteristics. The ridge waveguide lasers with 25 μm width and 1 mm length had maximum output power per facet of 20 mW, threshold current density of 200 A/cm2 (67 A/cm2 per quantum well), internal loss of 9.83 cm-1, internal quantum efficiency of 40.6 % and characteristic temperature of 84.3 K. The laser diodes exhibited low operation voltage of 1 V which helped to reduce the heat generation inside the laser structure. Single mode behavior emission with full width at half maximum of ~ 0.1 nm and side mode suppression ratio (SMSR) > 14 dB were obtained for 5 μm wide (and below) straight waveguide. To improve the output performance and long-term stability of the GaSb-based laser diodes, optical coatings on the laser facets have been demonstrated. Different optical coatings including high reflection (HR) coating, anti-reflection (AR) coating, and partial HR were characterized by simulation and experimental works. At the desired wavelength of 2.33 μm, the reflectivities of the HR coating, AR coating and partial HR coating were measured to be 99 %, 7 % and 50 %, respectively. The effect of optical coatings on the performance of the GaSb-based edge emitting laser diodes has been examined. Significant improvement of the output performance of the laser diodes in terms of optical power, threshold current and differential quantum efficiency has been observed after applying one facet with HR coating and the other with AR coating. Based on the optimization of laser growth and fabrication process, tunable Y-junction laser diodes of different configurations have been successfully fabricated and characterized at RT in cw mode. Y-junction structures with large bend radius and small length difference between the two cavities have been chosen to maximize the output power and wavelength tunability while having sufficient SMSR. Using a large bend radius of 1000 μm and a length difference of 20 μm between the two cavities, widely continuous tuning ranges up to 50 nm with the SMSR > 23 dB were achieved by adjusting the injected current ratio between different sections of the Yjunction lasers. The promising results show the applicability of the Y-junction lasers for tracegas sensing applications.