Two-Dimensional Electron Gas of Very High Mobility in Planar Doped Heterostructures B

Two-dimensional electron gas of very high mobility in planar doped heterostructures B. Etienne, E. Paris

To cite this version:

B. Etienne, E. Paris. Two-dimensional electron gas of very high mobility in planar doped heterostructures. Journal de Physique, 1987, 48 (12), pp.2049-2052. 10.1051/jphys:0198700480120204900. jpa- 00210651

HAL Id: jpa-00210651 https://hal.archives-ouvertes.fr/jpa-00210651 Submitted on 1 Jan 1987

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Classification Physics Abstracts 72.20F - 73.40K - 68.55B

Two-dimensional electron gas of very high mobility in planar doped heterostructures

B. Etienne and E. Paris

Laboratoire de Microstructures et de Microélectronique (L2M), Centre National de la Recherche Scientifique (CNRS), 196 Av. Henri Ravera, 92220 Bagneux, France (Regu le f4 aolit 1987, rivisi le 21 octobre 1987, accept le 26 octobre 1987)

Résumé.2014 Nous montrons comment optimiser la réduction de la diffusion par les impuretés ionisées introduites dans les hétérostructures à modulation de dopage. Ceci peut être obtenu par une nouvelle conception du dopage de la barrière : celle-ci comprend deux monocouches de dopage planaire séparées par une couche non dopée. Nous avons vérifié cette prédiction dans des hétérojonctions GaAs/GaAlAs et obtenu des mobilités électroniques très élevées atteignant 3, 7 106 cm2 V-1 s-1 pour une densité surfacique d’électrons de 1, 8 x 1011 cm-2.

Abstract.2014 We demonstrate how it is possible to optimize the reduction of remote ionized impurity scattering in modulation doped heterostructures. This can be obtained by a novel implementation of the doping in the barrier using two planar doped layers separated by a large spacer. We have verified this prediction in GaAs/GaAlAs heterojunctions and obtained in preliminary studies very high mobilities reaching a peak value of 3.7 106cm2V-1s-1 at a sheet electron density of 1.8 x 1011 cm-2.

The doping modulation of GaAs/GaAlAs het- sheet density ng = 3 x 1011 cm-2 at 4K by Har- erojunctions, i.e. the spatial separation between ris et at. [3] and u = 5 x 106 CM2V-lS-1 with the intentionally introduced impurities (in the ns = 1.5 x 1011 cm-2 at 1K by English et at. [4]. GaAlAs barrier) and the free charge carriers (at Second an analysis of the physical processes lim- the interface in the GaAs channel), has led to iting the mobility and a resulting optimization of the obtention of quasi two-dimensional electron the structure might also still lead to mobility im- gas of high mobility at low temperature [1]. Un- provement. We present in this letter a way of do- der transverse intense magnetic field fascinating ing this and the results of preliminary transport quantum physical properties of such nearly per- measurements. We obtain very repeatedly electron in excess of 2 x 106 at fect 2D electron gas have been observed (the mobilities cm2 V -1 s-1 4K with values 2.5 X 106 cm2V-1s-1 fractional quantum Hall effect) [2] or may be ex- peak of it = pected (the Wigner crystallization). It is there- with ns = 1.7 x 1011 cm-2 at 4K (JJ = 3.7 x 106 fore quite challenging to attempt to improve the cm2 V -1 s-1 with ns =1.8 x 1011 cm- 2 at 1. 5K in electron mobility in these structures, in which another heterojunction) in conditions of epitaxy we are more attainable than remarkable progress has already been obtained which think easily and in which the ultimate limit is surprisingly those reported in references [3] and [4] (concern- not accurately known now. ing the basic pressure of the growth chamber, the residual background doping of the layers and Further progress may be expected currently the number of growth required before the obtan- two directions. First the along steady improve- tion of heterostructures with very high electron ment of epitaxy conditions (higher purity of the mobility) products, better growth procedure) has allowed recently the obtention of record electronic mobil- In modulation doped heterostructures of ities : p = 3.1 x 10s cm2V-1s-1 with electron good quality the dependence of the electron mo-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198700480120204900 2050 bility p at low temperature with 2D electron den- mid forbidden gap [6], is much larger than the sity ns (y proportional to ns l2, see below) proves charge transferred into the 2D electron gas : typ- that u is limited by ionized impurity scattering ically = 6 x 1012 CM-2 if put at 200 as from the (refer to Fig.la for a sketch of the structure and surface for the former, versus * 2 x 1011 cm-2 for of its most important elements). When p > 10e the latter. Starting from the general expression cm2 IV sec was obtained, it was not so clear un- of the screened Coulomb interaction between Ni til recently whether the dominant mobility limi- ionized charge centres per surface unity situated tation came from background or remote ionized at distance di from an electron gas of Fermi wave impurity (i.e. either residual impurities in the vector kf [7], it can be deduced after some alge- channel or doping impurities in the barrier). The bra that, in the limit kF di > > 1, the electron mo- report by Harris et al. [5] of an increase of mobility is approximately proportional to k3 cPi IN¡ bility concomitant with unchanged transferred or equivalently to n. i INi (the electron den- charge density solely by increasing the thickness sity n. being related to the Fermi wave vector of the doped GaAlAs layer provided in our opin- kF by ns = k 2 /2H for a 2D system at OK). This ion strong evidence that : i) the mobility is lim- settles the figures in order to obtain a reduction ited rather by remote ionized impurity scatter- of the ionized impurity scattering : the donors ing even for quite large spacer thickness and low ionized for surface depletion, being roughly 30 residual doping of the GaAs channel, ii) the im- times more numerous than those needed for the purities ionized because of surface depletion can- formation of the 2D electron gas, the former not be in fast ignored in any realistic estima- should ideally be placed more than three times tion of total ionized impurity scattering. Higher farther away from the heterointerface than the electron mobility is therefore to be expected by latter. Therefore we chose to divide the usual putting them far away from the electron gas. one piece doped layer in the barrier (Fig.1a) into two parts : one close to the surface, the other close to the heterointerface. These two doped layers are thus separated by a second spacer d2 much larger than the well established spacer d1 between the electron gas and the nearest intentionally introduced donors. Next, as noticed by Stern [8], the transferred charge density ns is solely determined (for given barrier height) by the distance d between the heterointerface and the centre of the depleted doped layer (the result is in fact rigourously true only in the case of absence of electrical charge in the spacer). Using the planar doping technique [9], we are in this way able to obtain this distance d to correspond nearly completely to spacer d1. This helps again to increase the mobility because for a given n. and a given Ni in the doped layer transferring electrons to the 2D gas (Ni may be than ns in of overall neutral- Sketch of the conduction band larger spite charge Fig.l- energy profile because of of the heterostructure. (a) Conventional bulk doping ity possible impurity compensation) we can afford now a This of the barrier : d1 is the spacer, d is the distance from larger spacer dl. planar can be used as well for the the heterointerface to the middle point of the region doping technique depleted by charge transfer. (b) Planar doping in other doped layer because of the high carrier two layers bl and 82 separated by a spacer d2. The sheet density which can be obtained with this cap layer of GaAs is also shown.. growth technique at very close proximity to the surface of the sample [10]. This second point is most important once For all these reasons we believe that a nearly it has been realized that the electrical charge complete optimization of the barrier doping in needed for surface depletion, because of the pin- a modulation doped heterostructure can be ap- ning of the Fermi level at the surface at roughly proached by a design of the structure correspond- 2051 ing to figure 1b : the barrier comprises now two At present time our best values are 2.5 x 106 planar doped layers b1 (~ 2 x 1011 cm-2), 82 cm2V-1 s-1 for ns = 1.7 x 1011 cm-2 at 4K (~ 6 x 10’2 cm-2) and two spacers di and d2. and d1 = 1000 A and 3.7 x 106 cm2V-1 s-1 for = 1.8 x 1011 CM-2 at 1.5K and = 800 A. A The heterostructures are obtained by molec- ns d1 ular beam epitaxy. A detailed report of the figure of merit of modulation doped heterojunc- growth procedure will be given elsewhere. Our tions being given by the ratiou/n. 3/2 the interest of the new of the barrier system is a Riber 2300. The basic pressure of the implementation doping is clear. growth chamber, which is warmed up at room temperature every night, is 10-1° torr before In order to test more quantitatively the pro- growth. The structures studied here have been posed ideas concerning the minimization of re- obtained a short time after reloading of the As mote ionized impurity scattering, we have com- cells (they are between the 5th and the 20th epi- pared the following series of heterojunctions. layers). In references [3] and [4] it seems that They all have the same spacer thickness di of the continuous chamber cooling (resulting in a 750 A. Four heterojunctions have been grown lower basic pressure of the chamber) and the im- with the new structures (Fig.lb) and spacer provement of the quality of the epilayers after a thickness d2 varying between 0 and 1800 as, the large number of growths over an extended pe- fifth is grown with the conventional structure us- riod of time are major points. In our case the ing bulk doping (Fig.la). The thickness of the net residual doping of the GaAs is in the low doped GaAlAs layer of this last heterojunction 1014 cm-3 range with a compensation ratio of 3 is 500 A and chosen in order to have the same as estimated from the temperature dependance amount of Si atoms as in the planar doped layer of the electronic mobility of slightly Si doped 82 of the others. Slight variations of n. between 10 pm thick GaAs epilayers.Thus our growth the different heterojunctions have been corrected conditions are rather common and we estimate in the presentation of the results (Fig.2) in or- therefore that the increase of the electron mobil- der to compare the structures for the same 2D ity observed in this work is rather a matter of charge density of the 2D electron gas chosen to optimization of the structure. be ns = 1.5 x 1011 CM-2 . The large increase of electronic a factor of which is The Al composition of the barrier is around mobility (by 3.3) observed demonstrates the interest of the het- 32 % . This composition and the quality of the erostructures with a second in addition GaAs channel have been verified by photolumi- spacer d2 to dl. nescence at low temperature. The dopant is sil- icon. The carrier density and electron mobility are obtained by conventional four point Hall effect and Van der Pauw measurements between room and liquid He temperature on square pieces of samples. The ohmic contacts are obtained by In diffusion and very good quality is easily achieved in spite of the large spacer d2. Light il- lumination (at wavelength below the GaAs barrier band gap) is used to increase slightly the charge density and this effect is persistent in the dark. The corresponding increase of the mobil- ... ity is proportional to ng as expected from the relation given above between > and ns. Fig.2.- 4K electronic Hall mobilities (normalized at = 1.5 x 1011 for a series of With the conventional structures (Fig.la) ns cm-2) heterojunctions = 750 Left part conventional bulk doping of we obtain electronic mobilities at 4K around 106 (d1 A). 500 A of the barrier. Right part : effett of the increase cm2V-1s-1 for ns around 2.5 x 1011 cm-2 and of spacer d2 thickness in planar doped structures. for a spacer thickness d1 in the 400-600 k range. With the new structures (Fig.lb) we have obtained repeatedly mobilities in excess of 2 x 106 Thus we have put forward in this study how cm2V-’s-’ for ns around 1.7 x 1011 cm- 2 and a large increase of electron mobility at low tem- spacer thickness di in the 750-1000 A range. perature in modulation doped GaAs/GaAlAs he- 2052 terojunctions could be expected by optimizing is varied. The striking improvement demonstra- the doping of the barrier in order to reduce as ted here is expected to be fundamental in the at- much as possible the remote ionized impurity tempt to realizing nearly perfect 2D electron gas scattering. of very high Jl Ins /2 ratio needed for further pro- We have proposed a design of the GaAIAs gress in quantum transport studies. barrier doping that fits ideally well this analy- We would like to thank V. Thierry-Mieg who sis, using two planar doped layers separated by has been involved in the study of planar doping a large spacer. We have proved the effective in- of GaAIAs, Y. Lagadec, F. Mollot, R. Planel with terest of this a structure by study of the electron whom we are sharing the molecular beam expi- mobility in a series of heterojunctions in which taxy system and J. Rosiu for in contact studies the thickness of this new additional spacer layer and preliriiinary electrical characterization.

References

[1] STÖRMER, H.L., Surf. Sci. 132 (1982) 519. [6] see for instance ILEGEMS, M. in The Tech- [2] TSUI, D.C., STÖRMER, H.L. and GOSSARD, nology and Physics of Molecular Beam Epi- A.C., Phys. Rev. Lett. 48 (1982) 1559. taxy, edited by E.H.C. Parker (Plenum Press, London) 1985, p.83 and references [3] HARRIS, J.J., FOXON, C.T., BARNHAM, therein. J. K.W.J., LACKLISON, D.E., HEWETT, ANDO, T., FOWLER, A.B. and STERN, F., and 61 [7] WHITE, C., J. Appl. Phys. (1987) Rev. Mod. Phys. 54 437. 1219. (1982) [8] STERN, F., Appl. Phys. Lett. 43 (1983) [4] ENGLISH, J.H., GOSSARD, A.C., STÖRMER, 974. H.L. and BALDWIN, K.W., Appl. Phys. [9] WOOD, C.E.C., METZE, G., BERRY, J. and Lett. 50 1826. (1987) EASTMAN, L.F., J. Appl. Phys. 51 (1980) [5] HARRIS, J.J., FOXON, C.T., LACKLISON, 383. D.E. and BARNHAM, K.W.J., Superlattices [10] PLOOG, K., J. Crystal Growth 81 (1987) 304 Microstructures 2 (1986) 563. and references therein.