Yong K. Kim, Judith E. Baker, Andrew A. Allerman*, and Kent D. Choquette
University of Illinois at Urbana-Champaign, Micro and Nanotechnology Laboratory, The Frederick Seitz Materials Research Laboratory, Urbana, IL 61801, USA
*Sandia National Laboratories, Albuquerque, NM
Vertical cavity surface emitting lasers (VCSELs) have emerged as an important optoelectronic source, principally for 850-nm high-speed local area networks such as 10-Gigabit Ethernet. The epitaxial structure of a VCSEL is quite complex, requiring as many as 100 distinct layers with stringent tolerances on thickness, composition, and doping concentration. Optical reflectance characterization can provide a wealth of information regarding the thickness and composition, but the sophisticated doping profiles required to enable low series resistance in VCSELs are difficult to determine. We show that depth profile analysis by secondary ion mass spectrometry (SIMS) can be useful as a diagnostic method for VCSEL epiwafers, and that the doping of a few or even one layer can dramatically impact the laser performance. The basic device structure of 850-nm VCSELs consist of an active cavity containing GaAs multiple quantum wells, and p- and n-type doped distributed Bragg reflectors (DBR), all of which are grown by metalorganic vapor phase epitaxy (MOVPE). DBR mirrors are comprised of AlxGa1-xAs/GaAs layers doped with C and Si for p- and n- type dopants, respectively. In order to achieve confinement of both the optical mode and current, high Al composition layers adjacent to the active region are incorporated which form buried oxide apertures produced by wet oxidation. Thus, material properties of the aperture layers can affect electrical and optical performance, especially since all the current is funneling through these layers into the active region. Doping concentrations of p- and n- type dopants in VCSEL wafers were measured by SIMS. The SIMS analyses were performed by using a CAMECA IMS-5f with a 14.5 KeV Cs+ primary beam scanning over an area of 150x150 µm2. In order to obtain precise depth resolution and avoid mass interferences among Si, Al-H, and C-O, high resolution mode measurements were carried out. The raw counts data from the measurement were quantified by using standard reference samples and relative sensitivity factors (RSF). The measurement results were compared with the electronic carrier concentrations measured by Hall measurements and Polaron C-V measurements. These SIMS depth profiles were correlated to device characteristics such as light output versus current and voltage. We have determined that self-pulsating VCSEL behavior [1] of the light output can result from unexpected doping profiles in the oxide aperture layers. For example, we have measured the Si concentration in the n-type oxide layer of a self-pulsating VCSEL to be approximately equal to the C concentration in that layer. The background carbon in the n-type layers is formed by the MOVPE gas source precursors. This implies the Si concentrations are insufficient to compensate for the background carbon. We also find that the Si doping concentration in the n-type DBRs can be extremely high (~1x1019). Such high doping concentrations can have reliability implications. Thus, we show that high resolution SIMS can accurately determine the doping profiles in VCSEL structures, which in turn can dramatically influence the resulting device performance.
[1] K. D. Choquette, H. Q. Hou, K. L. Lear, H. C. Chui, K. M. Geib, A. Mar, and B. E. Hammons, Electron. Lett., 1996, 32, pp. 459-460.