A photoluminescence technique for characterizing the GaInAsP channeled substrate buried heterostructure wafer for lasing
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I. INTRODUCTION A Ga^In, _x As^P, _y alloy semiconductor grown lattice-matched on InP substrate is the material of choice for making lasers in the wavelength range 1.31.55 jum that are employed in the present-day fiber optic communication systems. Of many laser structures based on this material, the channeled substrate buried heterostructure (CSBH) lasers grown by liquid phase epitaxy (LPE)isone. 1 A problem specific to the growth of GalnAsP by LPE is that the In-P solution dissolves the quaternary layer for compositions corresponding to A > 1.50 /urn.2 s Against this problem, the growth of active layers for X > 1.50 /urn is achieved by growing, immediately after the active layer growth, what is known as an antimelt back (AMB) layer. The AMB layer, which is also a quaternary layer of A ~ 1.30-1.40/inn, mitigates the dissolution of the longer wavelength active layer.6-7 Often in the process of specific development, the lasing wavelength needs to be changed because of some modifications in the system requirements. The wavelength of the CSBH laser is determined by the composition of the GalnAsP active layer in the channel. It normally requires experimentation with several growth runs to determine the right LPE solution compositions to grow the active layer and the AMB layer latticematched to InP. Even after the selection of the proper LPE solution compositions, reproducible growth of CSBH structures with wavelengths within a specified range is hindered by the possibility of contamination of the solutions with P, due, for example, to solution carryover or P vapor leaking from sources used to prevent thermal decomposition of the InP substrate. The contamination problem is particularly severe for the growth of the longer wavelength (1.55 fim compared to 1.30 fira) GalnAsP active layer since slight perturbations in the P mole fraction in the growth solution cause large J. Mater. Res. 3 (2), Mar/Apr 1988
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changes in wavelength. Given these difficulties, it is absolutely necessary to characterize the as-grown wafer for wavelength so that growth conditions can be kept optimized and wavelength reproducibility can be achieved. The latter is particularly important since small interwafer variation in wavelength ensures high wafer yield and hence low cost. Photoluminescence spectroscopy has been used quite extensively in the past to characterize the GalnAsP/InP heterostructures.8"10 In this paper we describe a photoluminescence technique to characterize the CSBH wafer for lasing wavelength. The technique involves photopumping9 a piece from the as-grown CSBH wafer to lase at 80 K and determining the lasing wavelength at 300 K from that at 80 K after correcting for the wafer-dependent temperature shift of the bandgap. By comparing the optically determined lasing wavelength of the wafer against the value measured on the device, we find that the photopumping technique can screen wafers for lasing wavelength to an accuracy of + 20 nm. This accuracy is found to be satisfactory for providing fast feedback
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