Extremely high-mobility two dimensional electron gas: Evaluationof scattering mechanismsV. Umansky,a)R. de-Picciotto, and M. HeiblumDepartment of Condensed Matter Physics, Braun Center for Submicron Research,Weizmann Institute of Science, Rehovot 76100, Israel~Received 16 April 1997; accepted for publication 31 May 1997!We report on the characterization of selectively doped GaAs/AlGaAs heterostructures, grown by anextremely clean molecular beam epitaxy system, which exhibit a Hall mobility of a two dimensionalelectron gas exceeding 103 106cm2/Vs for a wide range of undoped spacer layer thickness ~50–100 nm!. A maximum electron mobility of 14.43106cm2/Vs was measured at 0.1 K in a structurewith a 68 nm spacer thickness and an areal carrier density of 2.43 1011cm2 2. This is the highestelectron mobility ever reported, leading to a momentum relaxation mean-free path of ;120mm. Wepresent experiments that enable us to distinguish between the main scattering mechanisms. We findthat scattering due to background impurities limits electron mobility in our best samples, suggestingthat further improvement in structure quality is possible. © 1997 American Institute of Physics.@S0003-6951~97!01231-X#Since the first fabrication of a high-mobility two dimen-sional electron gas ~2DEG!,1embedded in a GaAs/AlGaAsheterostructure, employing the concept of spatial separationbetween ionized ~parent! donor impurities and the resultant2DEG, has led to a wide range of structures with electronmobility exceeding 13 106cm2/Vs.2Utilizing extremelyclean molecular beam epitaxy ~MBE! systems led to succes-sive improvements, with recent low temperature mobilityvalues exceeding 13107cm2/Vs ~Refs. 3–5!@with a recordmobility of 11.73 106cm2/Vs ~Ref. 3!#.A number of theoretical works calculated the mobility insuch structures from the first principles accounting for a va-riety of scattering mechanisms.2,6–11Most models qualita-tively describe the experimental results well, however, theyusually fail quantitatively. Moreover, issues such as themaximum achievable mobility and the weight of variousscattering mechanisms, are still under discussion. For ex-ample, in contrast to Pfeiffer’s et al.3optimistic predictionthat the mobility could rise by orders of magnitude in ex-tremely pure structures, Saku et al.11have recently statedthat a mobility limit of about 23 107cm2/Vs must alwaysexist due to remote ionized donor impurities scattering—when a realistic carrier concentration and spacer thicknessare being employed.In this letter, we present results of a systematic study oflow temperature 2DEG mobility in high purity selectivelydoped GaAs/AlGaAs heterostructures. We discuss both tech-nological factors and structure design that determine the mo-bility. By varying structure parameters we distinguish be-tween the different scattering mechanisms. Optimizing thestructure leads to a record 2DEG mobility of 14.43 106cm2/Vs at 0.1 K; the highest value published thus far.Our MBE system is a Riber 32 with a 3 in. diam sub-strate holder. The system is pumped with two cryopumpsand valved with all metal valves. One cryopump is speciallydesigned to have three stages, with the coolest one at 4.5 K.The chamber, continuously cooled by liquid nitrogen,reached ~after baking it for two weeks at some 200 °C! apressure of 13102 12Torr under idle conditions of all theeffusion cells. GaAs ~100! 2 in. substrates were indium-freemounted onto a 3 in. diam high purity Molybdenumholders.12Our experience taught us that the chamber’s pres-sure, though being of great importance, does not limit theultimate material purity in our system. Instead, impuritiesemanating from the substrate heater and source materialsduring epitaxy determine background impurity concentra-tion. Figure 1 summarizes the evolution of the 4.2 K mobil-ity as the As cell is being depleted for a 2 in. and 3 in. heaterdiameters. The 2 in. diam heater was inserted into the 3 in.holder in order to reduce the overall outgassing of the ma-nipulator; consuming indeed some 30% less power at typicalgrowth temperatures. One notices that the mobility increasessteadily as the As source is being depleted, with the maxi-mum 4.2 K mobility some 40% higher for the 2 in. heater.a!Electronic mail: [email protected]. 1. Mobility of 2DEG, measured in the dark after illumination, as afunction of the total thickness of grown material for two different substrateheaters.683Appl. Phys. Lett. 71 (5), 4 August 1997 0003-6951/97/71(5)/683/3/$10.00 © 1997 American Institute of PhysicsDownloaded 17 Sep 2002 to 129.2.94.235. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jspMoreover, in contrast to the 3 in. heater, where the mobilitysaturates after approximately 130mm of GaAs is grown, the2 in. heater provides a weak but continuous increase of themobility until complete As cell depletion. These results con-firm that contamination from the As ingot are as important asthose resulting from the substrate heater. Yet, our record mo-bility obtained with the 3 in. diam heater, being 4.73 106cm2/Vs at 4.2 K and 8.33 106cm2/Vs at 0.3 K, is thehighest reported for such a configuration. Hereafter, we de-scribe only the 2DEG structures fabricated using the 2 in.diam heater.We studied the effect of spacer thickness ~undoped Al-GaAs layer separating the doped region from the 2DEG! onthe mobility and density of the 2DEG ~see Fig. 2!. All struc-tures have a buffer layer composed of a 30 periodGaAs/Al0.37Ga0.63As superlattice followed by a 1.5mm un-doped GaAs layer grown at ;645 °C using a growth rate of0.5mm/h near the GaAs/AlGaAs interface. AnAl0.37Ga0.63As layer, some 30–300 nm thick, follows, withone or twod-doped Si sheets inserted into it. TheAs4-to-Ga beam equivalent pressure ratio was ;10. The de-pendence of the 2DEG density on spacer thickness agreeswith previous works3,5and fits well Poisson’s equation solu-tion using the conduction band discontinuityDEc'0.67DEg. The mobility increases monotonously asthe samples are cooled to about 0.8K and saturates thereafter,exceeding 107cm2/Vs for a spacer thickness in the range of50–100 nm. The highest mobility (14.46 0.2)3106cm2/Vs, was measured at 0.1 K in a singled-doped structure with a doping concentration of 6.33 1011cm2 2, a 68 nm spacer thickness, a 250 nm totaldepth of the 2DEG from the surface and a 2DEG density of2.43 1011cm2 2, measured after illumination. Note that thecarrier concentration was