Toward a precise localization of Zinc in InP crystal lattice using novel nondestructive approach of X-ray Standing Wave Technique

Precise control of the doping in the p-type semiconductor layers is important for fabrication of integrated optoelectronic structures. Zinc is a common p-type dopant used in InP-InGaAsP and InP-InGaAlAs systems grown by MOVPE. For example, Zn-doped InP with the carrier concentration more than 18 3 10 cm is widely used as ptype cladding layer on top of the adjacent undoped active region of the vast majority of InP-based optoelectronic p i n structures. Compared to other acceptors (Cd, Mg, and Be), Zinc has several crucial advantages, such as high solubility and low ionization energy, the latter results in relatively high degree of electrical activity of Zn at room temperature. However, the unintentional Zn diffusion during MOVPE growth is very fast compared to the ntype dopants. As a result, the Zn profile distribution, which has tremendous impact on the device characteristics, is hard to predict and control. For example, uncontrolled diffusion of Zn can displace the desired position of the p i n junction and change the build-in electric field in the active region of the device that usually deteriorates the performance at the high-speed. Extensive studies of InP and related materials resulted in a common agreement that Zn diffusion during the growth can be generally described by the substitutional-interstitial mechanism. Thus, interstitial zinc ( ) i Zn + diffuses until it is captured at the vacancies of the In-sublattice In V − to form substitutional zinc ( ) s Zn − . The p-type carrier concentration is determined mostly by the substitutional fraction that behaves as a shallow acceptor, while the interstitial Zn does not contribute to the p-type carrier concentration and is not electrically active in this sense. In the attempt to suppress zinc diffusion keeping the conductivity and mobility of the InP layer at maximum, great care should be taken to control and minimize the interstitial fraction of Zn. The standard approach to control Zn incorporation and activation in InP-based structures grown by MOVPE is an arrangement of Secondary Ion Mass Spectroscopy (SIMS) and electrochemical capacity-voltage (CV) profiling. However this combined indirect approach is destructive by its nature and it does not provide sufficient spatial resolution for in situ device structure characterization and the measurements has to be taken from the “control squares” of the device wafers. New nondestructive approaches for precise localization of p-type impurities in the III V − semiconductor matrix are required and a possibility to utilize such techniques at the micron-scale would be a big advantage for novel integrated optoelectronic devices. In our recent experiments at the A2 beamline of CHESS we applied the nondestructive X-ray Standing Wave (XSW) technique to address incorporation of acceptor impurities in semiconductors [1]. Among investigated systems are Zn-doped InP, GaAs:Mn, Mg-doped and Er-doped GaN. Some of these measurements have been carried out with the in-plane spatial resolution of 10 μm using imaging capillary optics. In this paper we discuss the InP layers doped with Zn to the level of 18 3 10 cm grown epitaxially on InP (001) substrate and we also analyze the activation of Zn due to the post-growth rapid thermal annealing (RTA). The X-ray Standing Wave (XSW) technique is based on the generation in and above the crystal the interference field emergent due to the superposition of the incident and Bragg-diffracted x-ray waves. The periodicity of the standing wave is the same as for the diffraction planes hkl d . As the crystal is scanned through the