Atomic structure of monolayer AlAs islands on GaAs and its anisotropy revealed by mobility study in island-inserted quantum wells

Surface Science 267 (1992) 187- ! 90 North-Holland

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a blstitute of b~dustrial Science, Unit'ersiO' of Tokyo, 7-22-1 RoppongL Minato-ku. Tokyo 106, Japan t, Research Center for Advanced Science and Technolo,o'. University of Tok~'o, 4-6-1 Komaba. Megltro-ku, Tokyo 153. Japan " Quantum Wave Project, JRDC, 4-3-24-302 Komaba, Meguro-ku, Tokyo 153. Japan Received 14 June 1991; accepted for publication 12 August 1991

We have studied the lateral structure of one-monolayer thick AlAs islands deposited on the fiat GaAs(001 ) surface by studying the mobility It of two-dimensional (2D} electrons in novel modulation doped quantum wells in which one-monolayer thick AIAs islands are randomly inserted in the central part of the MBE grown GaAs/AIAs well. It is found that It of 2D electrons in such island-inserted quantum wells is greatly reduced (by a factor of 5 .,. 10) by the i,m'nduction of AlAs islands. In addition. It measured along the (110) direction is found to be about half of It along the (~101 direction, indicating lhat the AlAs islands have anisotropic structures. By analyzing the mobility data, the correlation lengths .: of the AlAs islands are estimated to be 140 and 911 ,~ along the (1101 and (110) directions, respectively.

T h e atomic structure of heterointerfaces has a d o m i n a n t role in d e t e r m i n i n g the electronic properties of quantum wells ( Q W ' s ) and other quantum microstructures. In such systems, the interface roughness gives rise to in-plane r a n d e m potential V ( x , y ) and influences electrons [1,2] strongly, particularly when the lateral size of the roughness becomes c o m p a r a b l e with the wavelength of the electrons [3]. While the G a A s surface can easily be s m o o t h e d by growth interruption (GI), the u n d e r s t a n d i n g and control of AlAs diffusion is still a problem. We study the lateral structure of o n e - m o n o layer (1 ML) thick AlAs islands deposited on the fiat GaAs(001} surfaces by molecular beam epitaxy (MBE). For this purpose, we p r e p a r e d novel modulation doped G a A s / A I A s QW's in which one-monolayer thick AIAs islands are randomly inserted in the central part of the well (see the inset of fig. 1). The mobility ~ of two-dimensional (2D) electrons in such island-inserted Q W ' s (l-'Q W ' s ) is expected to be reduced by the r a n d o m potential associated with the AlAs islands. We have m e a s u r e d the mobilities as functions of electron concentration N, and analyzed them to de-

termine the lateral size of islands or their correlation lengths. We sho~ in the following that ~ in I2-QW's is decreased by a factor of 5---10 a.,, compared with ~ in an ordinary. QW, and that /a is about two times higher along the (-ll0) direction than along the (11{}) direction. From the detailed comparison of experiment with theory', we have d e t e r m i n e d successfully the correlation lengths A of AlAs islands, both along the (]101 and (110) directions. This concept of island-insertion in quantum wells is strongly related to the concept of grid-inserted q u a n t u m well ( G I - Q W ) structures, which we proposed previously to form quantum wires and in-plane superlattice states [4,5]. Note also that this 12-QW approach offers a unique method for the formation of quantum box s t r u d u r e s when the island potential V(x, v), either repulsive o~" attractive, confines electrons or cxcitons. This possibility and prcliminar3' experiments ~ill bc discussed elsewhere [6,7]. Sample,'; were grown on Cr-doped semi-insulating GaAs(001) substrates at 58{} --- {~(}{Io C, as shown in the inset of fig. 1. An 8000 A-thick ut~Ooped G a A s buffer was grown on the sub-

0039-6028/92/$05.00 c/~,1992 - Elsevier Science Publish,_rs B.V. and Yamada Science Foundation. All rights reserved

188

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Atomic structure of monolayer AIAs islands on GaAs

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Ns (¢m-2) Fig. 1. Mobilities ~ of 2D electrons as functions of electron concentration N, at 4.2 K. Open squares are V- in the reference OW, while closed and open circles are the data in island-inserted QW's (I2-OW's) with the coverage of 0.5 for the current flowing along the ( l l 0 ) and (ll0) directions. respectively. Solid and broken lines are theoretically calculated mobilities at 0 K when the correlation lengths are 140 and 9{I A ahmg the (110) and (110) directions, rcspcctiv-ly. Broken lines inch, de the contribution of the island scattering and also other scattering mechanisms present in the mobilit.~ data of the reference QW. The inset shows the structure of modulation doped I 2-OW's.

strate, which was followed by the successive formation of a superlattice (SL) buffer layer consisting of 15 °periods of undo0ed AlAs(40 ~,) and GaAs(40 A,, a 21 ML thick undoped AIAs barrier, an undoped GaAs QW in which 1 ML thick AlAs with the coverage 0 (0 = 0.25, 0.5 and 0.75) was inserted at the center, 2! .kMLthick ,,ndoped AlAs spacer, an 800 7X thick S'-doped A I , 3 G a , 7 As with the donor Si density N D of 7 × !t} ~-' cm =~, and finally a 100 A thick undoped GaAs capping layer. The growth was interrupted for 60 s prior to tim deposition of At atoms to smooth the bottom GaAs surface and for 30 s after the deposition to form the AlAs islands. The growth

was also interrupted during the formation of the SL buffer by waiting for 30 s prior to the deposition of tbe 40 A AlAs layer. The total well width Lw LSset to be 41 ML to reduce the influence of interface roughness at the bottom and top interfaces of the QW's. The GaAs substrate wafer has flat (001) orientation within 0.5 °, for which the average terrace width is larger than 320 ,~,. Typical growth rates are 0.59, 0.25 and 0 . 8 4 / x m / h for GaAs, AlAs and Alo.3Gao.7As, respectively. To evaluate the', ionized impurity and other scattering mechanisms, we prepared also a reference QW without AlAs islands which was prepared with GI of 90 s at the center of the QW. Electron mobilities were measured at 4.2 K as functions of N~ and plotted in fig. 1. For the reference sample, tz shown by open squares are more than 105 c m 2 / V s, indicating that the mobility in the sample is dominated by the ionized Jr'purity scattering from remote donors doped in the AIGaAs. Measured mobilities in I:-QW's are plotted in fig. 1 by closed and open circles for the current flowing along the (110) and (110) directions, res0ectively. Note that they are far lower than that of the reference QW, indicating that the AlAs islands act as dominant scatterers. In addition, the presence of clear anisotropy in suggests that the AlAs islands have an anisotropic structur~. To h terpret the data, we have formulated a theory of electron scattering by islands, following the theory of interface roughness scattering [1,2] and calculated # as a function of N,. Solid lines in fig. 1 are the results of calculations when A of islands is assumed to be 140 A along (110) and 90 along (110). Note that the N,, dependence of is dominated mostly by el [1,2]. Broken lines in fig. 1 are the predicted values when we take into account the contribution of the island scattering as well as other scattering mechanisms that are present in the reference sample. From the excellent agreement with experiment, we can conclude that the islands have of 140 and 90 A along the (-il0) and (110) directions, respectively. This is the first report on the lateral structure of AlAs islands and its anisotropy when grown on the flat GaAs surfaces. These values are close to the correlation length of the roughness of GaAs-on-

189

T. Noda et al. / Atomic structure of monotayer AlAs islands on GaAs

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AlAs interface prepared on :.kick AlAs [1,3]. Note that when the island scattering dominates, the N, dependence of g is mostly determined by .I, whereas the absolute magnitude of ~, which depends on the strength of scatt,'rers, can be evaluated from the matrix element M,. The matrix element .a4r is expressed as I,;n x /,l,a(A), where V~n is an effective potential associated with the introduction of the AlAs layer and Aq(A) is a Fourier component of the thickness A(r) of islands. To explain the data, we find that V~,, is ~ 6 meV. In contrast, the change in the quantized energy E. caused by the insertion of a 1 ML AlAs layer at the center of a 40 ML thick GaAs QW is ~ 30 meV for which V~n is estimated to be ~ 15 meV. The origin of this dis,-repancy is not clear at present. One possible reason is that some portions of AlAs may form much smaller clusters, which play a negligible role in the scattering process. In order to elucidate the AlAs island structures, we have also examined p, in 12-QW's with the AlAs coverage of 0.25 and 0.75. Fig. 2a shows the mesured mobilities plotted as functions of N,

at 4.2 K for the sample with an AlAs coverage of 0.25 ML. The N~ dependence of g is slighvly steeper, suggesting that the effecti~,," correlation length is enhanced. Fig. 2b shows the result k)r the case when the coverage is 0.75. The N, dependence of # is less steep, implying that ~ ,gel,, shorter. In both samples, anisotropic transport is observed with g being higher along (]10) than that along (110), which is consistent with the case of the coverage of 0.5 ML. A quantitative analysis on these data will be discussed elsewhere. In conclusion, we have studied the scattering of two-dimensional electrons by random AIAs islands in novel island-inserted quantum wells and successfully determined the structures of AlAs islands on the fiat GaAs(001) surfaces. The correlation lengths of AlAs islands are found to be 14{1 and 90 A along the (110} and (ll0) directions, respective[y when the covcragc of AlAs is O.5. This work is supported by the Grant in Aid from the Ministry of Educatiop and by the Research Development Corporation of Japan

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T. Noda et al. / A t o m i c structure of monolayer AlAs islands on GaAs

through the ERATO program for Quantum Wave Project.

[3] [4]

References [5] [1] H. Sakaki, T. Noda, K. Hirakawa, M. Tanaka and T. Matsusue, Appl. Phys. Lett. 51 (1987) 1934; K. Hirakawa, T. Noda and H. Sakaki, Surf. Sci. 196 (1988) 365. [2] A. Gold, Z. Phys. B 74 (1989) 53;

[6] [7]

R. Gottinger, A. Gold, G. Abstreiter, G. Weimann and W. Schlapp, Europhys. Letl. 6 (1988) 183. T. Noda, M. Tanaka and H. Sak ki, Appl. Phys. t.ett. 57 (1990) 1651. M. Tanaka and H. Sakaki, Appi. Phys. Lett. 53 (1989) 1326. J. Motohisa, M. Tanaka and H. Sakaki, Appl. Phys. Lett. 55 (1989) 1214. H. Sakaki, unpublished (Patent Application filed in 1989); T. Noda and H. Sakaki, in preparation. T. Noda, J. Motohisa and H. Sakaki, Appl. Phys. Lett., to be s~bmitted.