Evidence of free carrier concentration gradient along the c-axis for undoped GaN single crystals

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional d oping by oxygen. We show, by the shift of the plasma edge (infrared reflect ivity measurements), that the free electron concentration is always higher on the (0 0 0 1)N face of the GaN single crystal than on the (0 0 0 I)Ga fa ce. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1 mum. Micro-Raman expe riments give a quantitative information on the free carrier concentration v ia the longitudinal optical phonon-plasmon (LPP) coupling modes. Thus, by s tudying the behavior of the LPP mode along the c-axis, we found the presenc e of a gradient of free electrons. We suppose that this gradient of electro ns is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains q ualitatively the incorporation of oxygen during the growth of GaN crystal u nder high pressure of nitrogen. (C) 2001 Elsevier Science B.V. All rights r eserved.