794 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 4, JULY/AUGUST 1998 Quantum-Well Intermixing for Fabrication of Lasers and Photonic Integrated Circuits Daniel Hofstetter, Bernd Maisenh¨ older, and Hans P. Zappe Abstract— Various applications of quantum-well intermixing, ranging from multiwavelength lasers to complex photonic integrated circuits, are described. The fabrication of these GaAs–AlGaAs-based devices relies on the postgrowth definition of regions with varying bandgap, enabling the manufacture of wavelength shifted modulators and lasers, as well as the integration of transparent waveguides with absorbing lasers and detectors. The impurity-free vacancy-enhanced disordering technique employed, and its integration with existing process technologies, will be presented, and examples of multicolor lasers, wavelength shifted modulators and integrated optical interferometers are shown. These applications yield high-optical functionality using relatively simple process and integration technology. Index Terms— Interferometers, monolithic integration, multi- wavelength lasers, photonic integrated circuits, quantum-well intermixing. I. INTRODUCTION O VER THE PAST two decades, quantum-well-inter- mixing (QWI) techniques have matured from a level of rather academic interest into a versatile tool for the fabrication of numerous optoelectronic devices, such as high- power semiconductor lasers or photonic integrated circuits. Starting with impurity-induced layer disordering [1], a variety of methods, such as vacancy-enhanced disordering (VED) [2], ion-implantation-induced intermixing [3], and laser-assisted disordering [4] have been investigated in many laboratories. While the former is probably the most popular, and has already lead to several commercial products, most of the other techniques are not yet employed in large-scale optoelectronic device manufacturing. The present article will review some of our recent results using VED for the fabrication of lasers, optoelectronic components and photonic integrated sensor circuits, thereby indicating that the technology may be approaching an industrially attractive level of maturity. The next section will outline some of the intermixing techniques developed and studied in the past and will discuss their potential device applications. We follow this with an in-depth look at one of these methods, vacancy-enhanced Manuscript received December 2, 1997; revised April 30, 1998. D. Hofstetter was with the Paul Scherrer Institute, 8048 Z¨ urich, Switzerland (now Centre Suisse d’Electronique et de Microtechnique SA, Z¨ urich). He is now with the Xerox Palo Alto Research Center, Palo Alto, CA 94304 USA. B. Maisenh¨ older was with the Paul Scherrer Institute, 8048 Z¨ urich, Switzer- land (now Centre Suisse d’Electronique et de Microtechnique SA, Z¨ urich). He is now with Balzers Thin Films, 9496 Balzers, Liechtenstein. H. P. Zappe is with the Centre Suisse d’Electronique et de Microtechnique SA, 8048 Z¨ urich, Switzerland. Publisher Item Identifier S 1077-260X(98)05852-3. disordering, which we have developed and employed for the fabrication of numerous optoelectronic devices and circuits. We conclude with a look at the design, fabrication and performance analysis of multicolor lasers, wavelength shifted modulators and two monolithically integrated optical sensor circuits. The latter structures, complete monolithic sensor microsystems, may be of particular industrial interest as they are small, robust, and capable of being produced by mass- fabrication techniques. II. SUMMARY OF IMPORTANT INTERMIXING TECHNIQUES The combination of lasers and transparent waveguides on a single epitaxially grown substrate for the fabrication of photonic integrated circuits requires the definition of regions with different bandgap energies. The means to solve this prob- lem can be divided into growth and intermixing approaches. The most popular among the growth methods are selective area growth [5], [6] and etch-and-regrowth on a patterned substrate [7], [8]. The first allows the simultaneous epitaxy using different growth rates, and, therefore, the growth of quantum wells with different thicknesses. In contrast, the second uses subsequent growth of material with different QW thicknesses. An alternative approach relies on selective partial intermix- ing of the QW using impurities [9] or vacancies [10]. During a high-temperature anneal, the QW material intermixes with that of the barrier material, resulting in a change in QW shape and thus transition energies. Key to the applicability of a QWI approach is the ability to selectively define the regions which are to be intermixed and those that are not. The primary advantage of intermixing methods is that they require no epitaxial regrowth and are thus potentially simpler and more cost-effective. In the following, we describe several QWI approaches and means of patterning intermixed and nonintermixed regions. The first QW intermixing technique to be demonstrated was impurity-induced layer disordering (IILD). In 1981, Laidig et al. [1], [11], demonstrated the disordering of an AlAs–GaAs superlattice using Zn as the active species. In these early experiments, a thermal anneal of several hours at tempera- tures of 600 C was used. As a result of this process, they found, depending on the anneal conditions, different grades of intermixing in a superlattice. In 1983, lasers with blue-shifted emission wavelengths were fabricated using this technique [12]–[14]. In 1984, stripe geometry QW laser devices using IILD to laterally define the waveguide of a buried heterostruc- ture became available [15]–[17], and one year later, the first 1077–260X/98$10.00  1998 IEEE