Delaunay meshing problem with stretched mesh

Hello, everyone. I have a meshing problem using Delaunay function. I have evenly scattered points (stored in x y and z vector) describing a half-sphere, see below. And I want to create triangle mesh for the surface. When I use delaunay(x,y,z) to mesh it, I got good mesh on the top, but the mesh on the bottom seems to be stretched and it might be caused by the almost vertical angle when it is close to the bottom. My code is as below:

tri = delaunay([x,y]); trisurf(tri,x,y,z);

Does anyone know what is the problem and how to solve it?

Thank you so much in advance!