Modulation Doped GaAs with Electron Mobilities Exceeding 10 7 cm 2 /V sec
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Modulation Doped GaAs with Electron Mobilities Exceeding 10 7 cm2/V sec
Loren Pfeiffer, K. W. We8t, H. L. Stormer, and K. W. Baldwin AT&T Bell Laboratories, Inc. Murray Hill, N.J. 07974
A modulation doped AI 35Ga 6 5As/GaAs single interface structure using Si delta doping setback from the two dimensional electron gas (2 DEG) by 700A, was measured in van der Pauw geometry at 0.35K and showed an electron mobility of 1.17X 107 cm 2 /V see at a carrier density of 2.2X 101 1 electrons/cm 2 . An Al30Ga.70As/GaAs single quantum-well 250Ak wide of similar structure with a 500A setback yielded a 2 DEG mobility of 5.1X 106 cm 2 /Vsec at 3.0X10" electrons/cm 2 under similar measurement conditions. These mobilities exceed those previously published by more than a factor of two for the single interface structure, and by nearly an order of magnitude for the quantum-well. The samples were grown by solid source MBE in a Varian Gen II modified with He closed-cycle cryogenic vacuum pumping, and in other ways, to achieve a base pressure of 1.5X 10-12 torr, as measured on a extractor-type ionization gauge that is not subject to the usual x-ray limited reading. The low temperature electron mobility of modulation doped1 GaAs is now the highest of any semiconductor. Such mobility measurements have become the benchmark of GaAs epitaxial quality. In this paper we report on improvements to our molecular beam epitaxy (MBE) system and to the modulation structure for electrons in GaAs that make possible new record mobilities for electrons in GaAs that exceed previous work by more than a factor of two for single interfaces at high two dimensional electron gas (2DEG) densities, 2 and by nearly an order of 3 2 magnitude for quantum wells and for low density single interfaces. Figure 1 shows the temperature dependence of the Hall mobility of our highest mobility modulation-doped single interface sample. It shows a low temperature saturated mobility of 11.7X 106 cm 2 /V see at 2.42 X 1011 electrons/cm 2 in the 2DEG after exposure to light at 0.35K. Also shown are similar data for a previous high mobility single interface sample grown by English et a12 together with some of the earlier landmark samples in the history of modulation doped GaAs.4 We will discuss these results in more detail below. Our samples were grown in a modified Gen II MBE System 5 that before2 modification had grown some of the previous record mobility GaAs samples. We modified the system as the follows: (i) all-metal valves were installed throughout to eliminate the outgas load from the Viton seals; (ii) the loadlock was redesigned with a new manifold and a 3000 e/sec He-closed-cycle cryopump that improved the loadlock pumping speed by a factor of 30 and lowered its transfer pressure after an overnight pump down from 10-s Torr to 10-11 Torr; (iii) the pumping speed in the growth chamber itself was increased by a factor Mat. Res. Soc. Symp. Proc. Vol. 145. 01989 Materials Research Society
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TEMPERATURE (K)
Fig. 1. Temperature dependence of the Hall m
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