IV-VI Compound Semiconductor Mid-Infrared Vertical Cavity Surface Emitting Lasers Grown by MBE
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W. W. Bewley, C. L. Felix, 1.Vurgaftman, and J. R. Meyer Naval Research Laboratory, Washington, DC 20375 ABSTRACT Mid-infrared vertical cavity surface emitting lasers (VCSELs) using PbSe as the active material and broadband high reflectivity Pbl,.SrxSe/BaF 2 distributed Bragg reflectors (DBR) as bottom and top mirrors were grown by molecular beam epitaxy. By pulsed optical pumping, this first IV-VI semiconductor VCSEL operated up to 290K at a wavelength of 4.5 gm. Further optimization of such VCSELs could lead to room temperature continuos wave operation. INTRODUCTION Due to the large market potential, recent years have seen a world-wide increase in mid-IR laser diode research. Most of that interest derives from the prospects for ultra-high-sensitivity chemical detection' and IR countermeasures. Desired performance requirements that are not yet available include continuous wave (cw) operation at room temperature, 4 2 3 spectral purity, and high output powers with good beam quality. Currently, IV-VI lead salts, ' quantum cascade (QC) ,5 and type-II QW diode lasers6' 7 are the leading approaches being pursued to meet the application needs. All have demonstrated above-room-temperature operation in pulsed mode. The highest cw temperatures are from lead salt lasers for electrical pumping (223K) and type-II QW lasers for optical pumping (290K). Lead salt diode lasers are the only ones that have been commercially available for a number of years. One major advantage of the lead salt materials is that the inter-band Auger coefficient in IV-VI structures is more than an order of magnitude smaller than in other semiconductors with the same energy gap8 ' 9 and therefore will not prevent cw room temperature operation. On the other hand, low thermal conductivity is the main factor limiting cw operation at high temperatures. In addition, problems of multi-mode operation and mode hopping in IV-VI lasers need to be solved. Vertical cavity surface emitting lasers (VCSELs) on BaF 2 (111) substrates are expected to have significantly improved heat dissipation, and for small spots also dynamic single longitudinal and lateral mode operation. In this work, we report preliminary lasing results for the first MBE-grown IV-VI VCSELs. EXPERIMENTS The optical cavity in a VCSEL is usually created by surrounding the active region with two quarter-wavelength DBR mirrors. Since the desired center wavelength for our DBR is in the 3-5 pim range, the thickness of each period in the quarter-wave stack significantly exceeds that in near-IR devices. It is thus important to choose materials for the alternating layers with as large a refractive index ratio as possible. In addition, both components of the mirror must be lattice- and thermal-expansion-matched to the underlying substrate. In this regard, Pbl.-Sr.Se and BaF 2 are an excellent material pair. Adding a small amount of Sr to PbSe increases the bandgap 181 Mat. Res. Soc. Symp. Proc. Vol. 607 @2000 Materials Research Society
significantly, which is why Pbl.xSr0 Se has been used as the confinement
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