962 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 5, MAY 2005 Novel Passivation Process for the Mirror Facets of Al-Free Active-Region High-Power Semiconductor Diode Lasers Peter Ressel, Götz Erbert, Member, IEEE, Ute Zeimer, Karl Häusler, Gert Beister, Bernd Sumpf, Andreas Klehr, and Günther Tränkle, Member, IEEE Abstract—A novel process for the passivation of mirror facets of Al-free active-region high-power semiconductor diode lasers is presented. Designed for technological simplicity and minimum damage generated within the facet region, it combines laser bar cleaving in air with a two-step process consisting of 1) removal of thermodynamically unstable species and 2) facet sealing with a passivation layer. Impurity removal is achieved by irradiation with beams of atomic hydrogen, while zinc selenide is used as the pas- sivating medium. The effectiveness of the process is demonstrated by operation of 808-nm GaAsP-active ridge-waveguide diode lasers at record optical powers of 500 mW for several thousand hours limited only by bulk degradation. Index Terms—Atomic beams, life estimation, optical films, pas- sivation, semiconductor lasers. I. INTRODUCTION R ELIABILITY is a key issue in fabricating high-power semiconductor diode lasers. Often, it limits device per- formance in areas, where the advantages of diode lasers as compactness, wide choice of wavelengths, or the possibility to couple the beam into optical fibers, are otherwise com- pelling. For single edge-emitters as multimode broad-area or single-mode ridge-waveguide (RW) lasers, the main factor limiting device reliability and maximum optical power is the stability of the mirror facets. A number of facet passivation processes have been developed in the past with E2 [1] to name the probably most influential one. Based on cleaving the laser bars in ultrahigh vacuum (UHV), the process requires expensive and complicated equipment and, in its original version, is limited to laser wavelengths 800 nm. Recently, the process has been modified to yield operation of 800-nm RW lasers at 400 mW for at least 1000 h [2]. Thus further, mostly proprietary, processes have been created during the last decade. They concentrate on the elimination of facet degradation or catastrophic optical mirror damage (COMD) by decreasing optical absorption and/or reducing and stabilizing the interface state density in the facet region. For this purpose, various technological means have been ap- plied as, for instance, the increase of the interfacial bandgap by Manuscript received September 30, 2004; revised January 19, 2005. This work was supported within the MDS Project of the German Ministry for Ed- ucation and Research under Grant BMBF-13N7371/7. The authors are with the Ferdinand-Braun-Institut für Höchstfrequen- ztechnik, Berlin D-12489, Germany (e-mail: ressel@fbh-berlin.de). Digital Object Identifier 10.1109/LPT.2005.846750 impurity-induced disordering of the quantum well(s), thus cre- ating a so-called window-mirror structure [3], the removal of native oxide from the facets by irradiation with low-energetic ( 50 eV) Ar ions or plasma [4] or, more recently, with low-en- ergetic and reactive nitrogen ions [5]. Advantageous is that these techniques allow cleaving the laser bars in air. Irradiation of the facet region with energetic particles, however, is problematic generally since lattice defects in semiconductors can be generated at energies as low as 10 eV. We, thus, have recently introduced a novel facet passivation process [6] that combines bar cleaving in air with a purely chemical nonkinetic method of facet cleaning prior to facet sealing with a passivating layer. In this letter, the process is detailed and results are presented on the performance of thus treated high-power diode lasers. II. FACET PASSIVATION A. Concept Process development has been guided by several principles as follows. First, technical simplicity was mandatory implying cleaving the bars in air and the use of standard semiconductor technology. Second, facet cleaning has to be accomplished by a purely chemical process without employing energetic parti- cles potentially damaging the facet region. Cleaning has to con- centrate on the removal of thermodynamically unstable species as arsenic, arsenic oxides, adsorbed water, etc. Their presence spurs gradual facet degradation, eventually leading to COMD. Third, the cleaned facets are coated in situ with a passivating layer preserving the clean state of the facets. The following im- plementation, as stated first in [7], has been investigated. Purely chemical cleaning of III–V semiconductor surfaces is most easily accomplished by irradiation with atomic hydrogen. The technique is used routinely for epitaxial overgrowth of pat- terned GaAs or InP surfaces in molecular beam epitaxy. Atomic hydrogen can be generated in vacuum by cracking molecular hydrogen on hot filaments or by extracting it as neutral atomic beams from electron cyclotron resonance (ECR) plasma dis- charges. In any case, the kinetic energy of the atoms is below 1 eV, i.e., well below threshold for damage creation. Cleaning temperatures are below 400 C making the process compatible with the employment of standard metallizations for contacting diode lasers. Al O , however, which is part of the native oxide of AlGaAs-based semiconductor regions, cannot be eliminated 1041-1135/$20.00 © 2005 IEEE