No references generated

Question

I'm pretty new to latex but I'm having a real problem generating references.

Here's my tex file

\documentclass[11pt a4paper]{report}

\begin{document}

This is the first reference \cite{Shaklee1971}
This is the second reference \cite{Valenta2002}
This is the third reference \cite{Vehse2002}

\bibliographystyle{unsrt}
\bibliography{PhD_Library.bib}


\end{document}

My bib file is in the same folder and is as follows:

@article{Shaklee1971,
   author = {Shaklee, K. L.  and Leheny, R. F.},
   title = {Direct determination of optical gain in semiconductor crystals},
   journal = {Applied Physics Letters},
   volume = {18},
   number = {11},
   pages = {475-477},
   keywords = {Variable Stripe Length, Gain},
   year = {1971}
}

@article{Valenta2002,
   author = {Valenta, J. and Pelant, I. and Linnros, J.},
   title = {Waveguiding effects in the measurement of optical gain in a layer of Si nanocrystals},
   journal = {Applied Physics Letters},
   volume = {81},
   number = {8},
   pages = {1396-1398},
   keywords = {Silicon nanocrystals, gain, variable stripe length, Shifting excitation spot
},
   year = {2002}
}

@article{Vehse2002,
   author = {Vehse, M. and Meinertz, J. and Lange, O. and Michler, P. and Gutowski, J. and Bader, S. and Lell, A. and Härle, V},
   title = {Analysis of gain saturation behavior in GaN based quantum well lasers},
   journal = {Physica Status Solidi (c)},
   number = {1},
   pages = {43-47},
   keywords = {Gain, Semiconductors, GaN},
   year = {2002}
}

When I compile it I just get [?] for the references and title bibliography with nothing under it.

.aux and .bbl files are generated but the .bbl file looks like this:

\begin{thebibliography}{}

\end{thebibliography}

Please help I'm not sure what is going on here.