Progress in etched facet technology for GaN and blue lasers - art. no. 64731F
Abstract and Figures
We report recent progress in chemically assisted ion beam etching (CAIBE) of GaN/AlGaN materials leading to improved performance of 405nm blue lasers fabricated with etched mirrors. Using a proprietary Etched Facet Technology (EFT) designed for GaN, we have fabricated ridge lasers in conventional GaN/sapphire material. Typical 3mum ridge lasers with 600mum cavity lengths exhibit threshold currents of 150mA with high yield and cross wafer uniformity. This represents a factor of five reduction in threshold current over previous results. Additional processing (such as FIB) was not required to improve the mirror verticality and smoothness as in previous work. Continuing improvements in laser performance are anticipated with further optimization of facet smoothness, laser design, and improved epitaxial material. We are also investigating the benefits of shorter cavity lasers, made feasible by etching, to realize improvements in laser reliability and yield. The yield advantage is based on the concept that shorter cavity devices will intercept fewer defects per device. Combined with EFT advantages like low cost wafer-scale testing and monolithic integration, this is a promising approach for next generation blue lasers for optical storage applications.
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The success of silicon photonics is sparking widespread interest in photonic integrated circuits at visible light wavelengths using SiN and other waveguiding platforms. Compact active circuits desire the heterogeneous integration of GaN‐based laser diodes. Herein, the optimization of smooth and vertical facets is reported using a combination of inductively coupled plasma etching followed by wet etching with a tetramethylammonium hydroxide‐based solution. Facet quality for concave‐, flat‐, and convex‐shaped structures surrounding the mirror is compared. Convex‐shaped structures result in the highest facet quality due to the evolution of the crystal plane‐dependent etching. 2 μm‐wide ridge waveguide, etched facet Fabry–Pérot cavity lasers with length of 1.5 mm emitting at 457 nm are realized using the optimized process. The lasers deliver up to 28 mW of optical power at 250 mA under continuous wave with slope efficiency of 0.26 W A ⁻¹ and lasing threshold current density of 4.6 kA cm ⁻² .
Atomic force microscope images reveal a root-mean-square roughness Δd = 16 nm for InGaN/GaN double-heterostructure laser structures with cleaved a-plane facets. The c-plane sapphire substrate cleaves cleanly along both the a and m planes. A theoretical model is developed which shows that the power reflectivity of the facets decreases with roughness by a factor of e−16π2(nΔd/λ0)2, where n is the refractive index of the semiconductor and λ0 is the emission wavelength. Laser emission from the optically pumped cavities shows a TE/TM ratio of 100, an increase in differential quantum efficiency by a factor of 34 above threshold, and an emission line narrowing to 13.5 meV. © 1998 American Institute of Physics.
Current-injection InGaAlN heterostructure laser diodes grown by metalorganic chemical vapor deposition on sapphire substrates are demonstrated with mirrors fabricated by chemically assisted ion beam etching. Due to the independent control of physical and chemical etching, smooth vertical sidewalls with a root-mean-squared roughness of 4–6 nm have been achieved. The diodes lased under pulsed current-injection conditions at wavelengths in the range from 419 to 423 nm. The lowest threshold current density was 25 kA/cm <sup> 2 </sup> . Lasing was observed in both gain-guided and ridge-waveguide test diodes, with cavity lengths from 300 to 1000 μm; and output powers of 10–20 mW were achieved. Laser performance is illustrated with light output-current and current–voltage characteristics and with a high-resolution optical spectrum.© 1998 American Institute of Physics.
The story of Shuji Nakamura and the blue laser diode is remarkable. It is clear from this book that he enjoys this fact and wishes his readers to become familiar with his success. Nakamura was a little known researcher at a small but successful Japanese company, Nichia Chemical, on Shikoku, one of Japan's four main islands. One of their successful lines was phosphors for fluorescent lights. In 1989, Nakamura was given a few million dollars by the company's Chairman Nobuo Ogawa. Nakamura chose to research into blue light emitters using gallium nitride, a material that had been studied by Pankove at RCA some 20 years earlier and largely written off by the conventional semiconductor industry. In spite of many factors against progress, this second edition of The Blue Laser Diode testifies to the success of this gamble.
The book is subtitled `The complete story'. This is an unlikely epithet for the book because there is still a long way to go. The book is written with a mixture of academic integrity and commercial trumpet blowing. There is too often a lack of detail and logical order. There is inadequate discussion of the case for and against other materials such as ZnSe. One feels that the commercial pressure not to give away all the answers about gallium nitride has triumphed over the wish for scientific disclosure to enable results to be repeated.
The book clearly reports the two most significant difficulties faced by gallium nitride. First, it appeared from Pankove's work that it would not to be easy to find an appropriate p-type dopant that could make suitable p-n junctions. Two chapters consider this problem, starting with low energy electron beam irradiation and then in the second chapter considering thermal annealing in nitrogen. The writing and detail suggest that it is still a technology rather than science (or, perhaps more unkindly, cook-book recipes of time and temperature).
The second important difficulty is that gallium nitride has too many dislocations for long-life laser action. Growth on sapphire with appropriate buffer layers is described as an initial step in reducing the dislocations. Later in the book, it is recognized that InxGa1-xN offers greater versatility, and this is considered in more detail along with InGaN/AlGaN double heterostructures. Regrettably it is not easy though to dig out from this book all the details of lattice matching that are required and how successful lattice matching has been in removing dislocations and increasing lifetime. Clearly the general trend of longer lifetimes means that there has been useful success. Blue laser diodes are now claimed to be commercially available with lifetimes measured in thousands of hours while blue light emitting diodes, with their lower current densities, are said to have lifetimes measurable in years.
The book has a little for everyone. Applications are noted briefly as well as blow-by-blow accounts of the manufacturing technology of double heterostructure, multi-quantum well lasers and progress to room temperature operation. Applications range from the mundane traffic light, through full colour displays to 15-20 Gbyte optically read data storage discs. Interestingly it is the mundane applications that may have the biggest financial impact. A statistic that appears on the Internet is that if all the traffic signals in Japan could be switched to suitable LEDs then one could save the construction of at least one nuclear power plant.
Although Nakamura is an admirer of Pankove's work, the writing and scientific style does not match that of Pankove. Nevertheless the book records a thorough solid achievement and as such there should be a similar solid basis for many readers in materials science and laser technology wishing to read this book.
The book regrettably gives no indication why Nakamura has left Nichia for a Professorship at Santa Barbara after such magnificent early support by Nichia. Nor does the book explain why Nakamura's co-author Gerhard Fasol is undercutting the joint venture by selling for $15 over the web a 28 page summary about blue laser diodes using gallium nitride. However, if price is no consideration, there is also advertised on the web a 222 page report SC-23 from Strategies Unlimited entitled `Gallium Nitride 2000 - Technology Status, Applications, and Market Forecast' for a modest $3950. Clearly the present book cannot be `the complete story'. That will run for quite a time yet.
John Carroll
Double heterostructure lasers were fabricated in which one of the laser facets was produced by a hybrid wet and reactive‐ion‐etching technique. This technique is suitable for GaAs/GaAlAs heterostructure lasers and utilizes the selectivity of the plasma in preferentially etching GaAs over GaAlAs. Lasers fabricated by this technique are compatible with optoelectronic integration and have threshold currents and quantum efficiency comparable to lasers with both mirrors formed by cleaving. The technique enables the use of relatively higher pressures of noncorrosive gases in the etch plasma resulting in smoother mirror surfaces and further eliminates the nonreproducibility inherent in the etching of GaAlAs layers.
GaAs double heterostructure lasers compatible with monolithic intergration of optical devices have been fabricated by wet chemical etching. Results for a variety of laser orientations are reported. External differential quantum efficiencies as high as 18% have been achieved, as well as threshold current densities as low as 4.2 kA/cm<sup>2</sup>. An analysis for the evaluation of etched‐mirror properties is presented. Using this analysis, it was found that etched mirrors had reflectivity R?0.22±0.05 and scattering losses S=0.3±0.1.
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