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Supercurrent is observed in a superconductor-semiconductor-superconductor (S-Sm-S) junction using a two-dimensional-electron-gas system in an InAs-inserted-channel Inp»Alp 48As/Inp 536ap 47As heterostructure. The temperature dependence of the supercurrent cannot be explained by a simple exponential expression, but is here explained in terms of the clean-limit theory, taking account of the higher-order effect of the coherence length. The differential resistance of the junction shows clear subharmonic energy-gap structures due to multiple Andreev reflection since the phase coherence is maintained during the Andreev-reflection process. The Cooper pair transport explained by the clean-limit theory is attributed to the fact that the mean free path is longer than the coherence length and the length between the superconducting electrodes.In recent years, much attention has been devoted to the superconductor-semiconductor-superconductor (S-Sm-S} system for device application' as well as for the study of mesoscopic Josephson effects such as a superconducting quantum point contact (SQPC). ' In the SQPC, Andreev reflection at the S-Sm interface plays an important role. Furusaki and Tsukada have shown that the dc Josephson current is given by the Andreev-reflection amplitude and is carried by the discrete excitation bound states in the Sm region. It is a challenging problem to combine the ballistic, phase-coherent transport in a twodimensional electron gas (2DEG) with superconductivity. So far, the superconducting transport properties for S-Sm-S systems using an Si, InAs inversion layer and InGaAs (Ref. 7} have been explained in terms of the dirty-limit theory. Epitaxial InAs-coupled superconducting junctions are in the intermediate regime between the clean and dirty limits. ' High supercurrent density was reported in an S-Sm-S junction using the A1Sb/InAs quantum well system.However, it has not been well verified that the supercurrent obeys the clean-limit theory. It is important to study the transport properties of the clean S-Sm-S systems to understand the bal1istic transport coupled with superconductivity. An InAs-inserted-channelInp 52Alp 4gAs/ Inp 53GaQ 47As heterostructure is attractive for an S-Sm-S system because 2DEG in the channel has both high electron density and high electron mobility, which provide the long coherence length and the long mean free path. Moreover, a good S-Sm contact is expected because there is no Sehottky barrier between the superconducting electrodes and the InAs channel. This paper reports on the transport properties both in the superconducting state and in the finite voltage state of S-Sm-S junctions using the 2DEG in the InAs-inserted-channel Inp 52Alp 48As/Inp 53Gap 47As heterostructure.The schematic structure of the fabricated junction is Nb undoped-lno. 52Alo. 4sAs Nb~u ndoped-ln0. 53Ga0.47As A

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The mobility of two-dimensional electrons in an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure improved by inserting an InAs quantum well into the InGaAs channel. This letter addresses the main cause of this mobility improvement. By optimizing the thickness of the InAs quantum well, its distance from the underlying InAlAs spacer layer, and the InAlAs spacer-layer thickness, maximum mobilities of 16 500 cm2/V s at 300 K and 155 000 cm2/V s at 10 K are attained. The improvement in mobility is attributed to a decrease in scattering caused by ionized impurities, interface-roughness, and trap impurities. This decrease is a result of the superior confinement of two-dimensional electron gas in the InAs quantum well.