Signatures of Induced Superconductivity in NbTi Contacted InAs Quantum Wells*

We have studied electrical transport through InAs quantum wells grown by MBE with unannealed superconducting NbTi contacts deposited \textit{ex-situ} and patterned by optical photolithography. Characterization of the InAs 2DEG's without superconducting contacts yields typical mobilities greater than 100,000 cm$^{\mathrm{2}}$/Vs at a density of 4e11 cm$^{\mathrm{-2}}$. NbTi-InAs-NbTi (SNS) and NbTi-InAs (SN) devices with dimensions greater than 1 \textmu m are fabricated using optical lithography. Although the dimensions of the fabricated SNS devices are too large to observe a supercurrent, signatures of superconductivity induced in the InAs are present. We observe two superconducting critical temperatures: one of the NbTi leads (T$_{\mathrm{c\thinspace }}$\textasciitilde 8K), and a second (T$_{\mathrm{c}}$ \textless 4.5K) attributed to superconductivity induced in the InAs channel. dI/dV vs V spectroscopy on SNS junctions below the second critical temperature shows a conductance maximum at zero applied voltage while conductance minima appear at finite bias voltage which is attributed to the presence of an induced superconducting gap in the InAs quantum well.