Cavity characteristics of selectively oxidized vertical-cavity lasers

Abstract

We show that a buried oxide layer forming a current aperture in an all epitaxial vertical-cavity surface emitting laser has a profound influence on the optical and electrical characteristics of the device. The lateral index variation formed around the oxide current aperture leads to a shift in the cavity resonance wavelength. The resonance wavelength under the oxide layer can thus be manipulated, independent of the as-grown cavity resonance, by adjusting the oxide layer thickness and its placement relative to the active region. In addition, the electrical confinement afforded by the oxide layer enables record low threshold current densities and threshold voltages in these lasers. © 1995 American Institute of Physics.Vertical-cavity surface emitting lasers ͑VCSELs͒ are of interest from both scientific and technological points of view. The distributed Bragg reflector ͑DBR͒ mirrors used to provide longitudinal optical confinement have enabled the study of microcavity optical phenomena and are critical to VCSEL performance. In the transverse direction, optical and electrical confinement is commonly achieved using etched air-post index-guided structures, 1 or ion implanted ''gain-guided'' devices. 2 Recently, oxides formed from AlGaAs layers have been implemented into both edge emitting 3 and verticalcavity laser diodes. 4 -7 We report optical and electrical properties of VCSELs fabricated using selective lateral oxidation to produce a buried oxide layer which forms a current aperture within an all semiconductor DBR mirror. We show that the buried oxide layer has profound implications, leading to new optical phenomena in the cavity as well as significant advances in VCSEL performance.The selectively oxidized VCSEL structure 6 is shown in Fig. 1. VCSEL diodes employing an oxide layer under a hybrid top dielectric DBR mirror have also been reported. 4,5 The VCSELs described herein are grown by metalorganic vapor phase epitaxy and employ three InGaAs quantum wells designed to emit near 980 nm with parabolically graded mirror interfaces in the epitaxial DBRs. 8 The high oxidation selectivity of AlGaAs alloys 6 is exploited to form a single buried oxide layer between the active region and the top DBR as depicted in Fig. 1. Square mesas are reactive ion etched to expose the edge of the quarter-wavelength thick AlGaAs layer for lateral oxidation done at 425-450°C in water vapor. The AlGaAs layer is partially oxidized laterally into the interior of the mesa to form a current aperture. The oxide current aperture thus minimizes current spreading and eliminates sidewall nonradiative recombination or implantation damage present in air-post or implanted structures, respectively. The smaller refractive index of the oxide layer 9 also induces index-guided optical confinement, but in a planar VCSEL configuration amenable to efficient current flow and heat extraction. Greater optical and electrical confinement is achieved by employing current aperture oxide layers on both sides of the active region. Finally contrary to other reports...

Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers

“…The lower index oxide surrounding the unoxidized current aperture also provides strong index guiding to the laser field [74], [94], [95]. The induced effective index difference between the cavity and the surrounding region containing the oxide layer [96] can be controlled through the thickness of the oxide layer and the position of the oxide layer(s) relative to the optical cavity [97]. Positioning the oxide layers directly adjacent to or even inside the optical cavity produces the strongest index confinement.…”

“…The shorter wavelength emission corresponds to lasing modes at the periphery of the current aperture ("oxide" modes) arising from carriers diffusing in the QW's under the oxide layer, while the cavity modes inside the aperture emit at the as-grown resonance wavelength. The lasing emission of the oxide modes can be tailored through the thickness and position of the buried oxide aperture(s) [97]. For example, using quarterwave-thick oxide layers on each side of the optical cavity [90] sufficiently shifts the resonance of these modes off of the DBR stopband to inhibit them from lasing.…”

Experimental demonstration of oxide-mode influence in a dual-purpose oxide-confined vertical-cavity surface-emitting laser/resonant detector

“…This mismatch, which is detrimental to a dual-purpose application, can be reduced to about 3-4 nm by minimizing the thickness of the oxidized AlAs layer and by placing it at a node of the intracavity optical field as it has been already proposed for oxide-mode suppression for laser emission. 9,10 However, for photodetection, these oxide modes will not be totally suppressed because there is no gain selectivity. A perfect spectral overlapping between emission and detection on the whole active surface can only be achieved by locally degrading the finesse of the cavity.…”

Selectively oxidized vertical-cavity laser performance and technology

“…Nevertheless, ion implanted VCSELs are presently being manufactured by several U.S. companies and deployed within new commercial products. A recent innovation for transverse optical and electrical confinement is to employ selective oxidation of buried AlGaAs layers [4] to form oxide apertures within the VCSEL [5,6] The improved electrical confinement arising from the oxide apertures is manifest as reduced threshold current and voltage for broad area lasers as compared to other VCSEL structures [7]. In Fig.…”