Spectrally Resolved Cathodoluminescence Determination of Dopant Diffusion In InP/InGaAsP Based Multi Quantum Well Fabry-
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C.ZANOTTI-FREGONARA*, C.FERRARI*, L.LAZZARINI* and G.SALVIATI* M.MELIGA**, D.BERTONE**, R.Y.FANG**, G.MORELLO**, R.PAOLETTI** *CNR-MASEPC Institute, Parco Area delle Scienze, 37A, 43010 Parma, Italy **OTC-CSELT Laboratory, Via G Reiss Romoli 274, 10148 Torino, Italy ABSTRACT
Low temperature monochromatic cathodoluminescence (CL) spectral analyses and imaging were used to determine the widths of resistive regions (due to Fe diffusion) in multi-quantumwell (MQW) InP-based laser devices and to detect the different amount of damage induced by alternative In-situ Etching (ISE) and Reactive Ion Etching (RIE) techniques. The widths of the resistive regions were estimated by comparing the 5 K CL emission width from the MQW and the actual width as obtained by SEM investigations. Monochromatic CL also did not reveal any emission from the InP:Sn layer between semi-insulating material (Fe-doped lnp) and p-type layer (Zn-doped InP), indicating interdiffusion of Fe and Zn laterally the MQW, and the presence of substantial Sn diffusion (up to 2500 nanometers) into the substrate. INTRODUCTION
Semiconductor lasers represent key components in optical systems and new technological processes have to be developed in order to reduce the number of steps and to improve the process yield. In particular the etching of the active stripes and the following selective regrowth represent two crucial technological steps for the realisation of buried structure lasers. Generally the etching step is performed by using RIE (Reactive Ion Etching) technique. This etching is based on a plasma effect, which can damage the quality of the interface between the nearly vertical wall of the stripe and the regrown InP layer. This effect is more evident in the case of semi-insulating InP regrowth, generally used for the fabrication of high speed and low threshold lasers (i.e. SI-BH Semi Insulating - Buried Heterostructure lasers). In fact a damaged interface can promote interdiffusion of the doping species and can affect the process yield and the device characteristics. To overcome these problems we have developed a new simple dry chemical selective etch performed inside the MOCVD reactor, called ISE -In Situ Etching [1]. In principle ISE technique allows to avoid any damage due to plasma effect and to air exposure of the sample after the active stripe etching. The active stripe size of such opto-electronic devices requires investigation techniques having high spatial resolution and depth resolution. In addition, the use of different dopants to increase electrical confinement requires the use of investigation techniques effectively for the selective dopant identification. The high spatial resolution of the CL technique (- 0.2 pm) was exploited in order to provide information from small regions of the grown devices. The dopant sensitivity of the CL technique [2-5] combined with the high spatial resolution was exploited in order to distinguish between sub-micron scaled regions of the device containing different impurities (Zn, Fe, Sn and S) and especially in th
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