GaAs interfaces

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Vol. 12, No. 2, 1992

ATOMIC STRUCTURE OF (1001 ZnSe/GaAs

INTERFACES

A. Josiek’ ) , R. Enderlein’ 1, J. Neugebauer*) 11 Department of Physics, Humboldt-University Berlin, Invalidenstra@e 110, O-1040 Berlin, F.R.G. 21 Fritz-Haber-Institut der MPG, Faraday-Weg 4 - 6, 1000 Berlin 33, F.R.G. (Received 4 August 19921

We study the atomic structure of various monolayers as well as sequences of layers on (1001 GaAs substrates, consisting of Zn, Se, Ga, and As. Gaand As-rich substrate surfaces are distinguished. Total energies are obVarious tained by means of self-consistent tight binding calculations. surface reconstructions of ZnSe layers are found depending on substrate The formation of GasAs. is demonstracoverages and layer terminations. ted. No ZnzAss growth is observed.

1. Introduction Recently, considerable progress has been achieved in growing high quality ZnSe epitaxial layers on, (1001 GaAs substrates using MBE or MOCVD techniques . The atomic structures of such layers have been studied experimentally by various methods, among them Auger Electron SpectroscoI$y (AESl, Electron ,Energy Loss Raman Scattefing , TransmisSpectroscopy (EELS) and , Reeflection sion Electron Microsropy iTEM High Energy Electron Diffraction (RHEED) , . Interesting observations have been reported as, e.g., the formation of an interfacifl TzSes compound layer under Ga-rich conditions , , or an alternating 2x1 and ~(2x21 reconstruction of &Se surfaces de; pnding on their terminations either by Se or Zn , Theoretical results on the atomic structure of ZnSe layers on GaAs substrates are rare at present. Atomic and electronic structure calculations of (1101 (rather th2 ($0011 ZnSe monolayers have been perby using pseudopotential and density formed in , functional calculations. Within this frame difficulties arise from the 3d core electrons of Zn which are close in energy to the 4s-and 4p levels. The bandstructure of a single ZnSe 1aJer on (1001 GaAs by means of an substrate has been calculated in empirical tight binding method. A simple structural model of the ‘GazSes layer has been proposed in . self -consistent tight binding total energy Using calculations, the present authors have studied monol+ayers of Zn, Se, and ZnSe on (1001 GaAs substrates . For both Se layers on 1007. Ga-rich substrate, and Zn layers on 1007. As-rich substrate, surface structures have been found with the atoms of each second [Olll row forming dimers (2x2 reconstruction). The ZnSe monolayer exhibits Zn dimers in each IO111 row Pronounced recharging effects (2x1 reconstruction).

0749-6036/92/060225+06

$08.00/O

occur at the ZnSe/GaAs interfa e accompanied by $1 strong macroscopic electric fields tight In this paper we will use the seif-consistent in order to inbinding total energy method from vestigate the atomic structure of a variety of Zn and Se containing films on (1001 GaAs substrates in greater detail. Both Ga and As terminated surfaces will be considered, with different degrees of coverages in each case. 2. Method A short description of the self-consistent $ight binding total energy method has been given in It starts from the Hartree-Fock one particle Hamiltonian Hw. Correlation effects are incorporated in the empirical parameters. A structurally adapted from atomic s-and hybrid basis set is used, built The Hamiltonian matrix elements with p-orbitals. respect to this set are expressed,cin terms of uni, using generaliversal tight binding parameters z$ Slater dirpctional dependencies. and Harrisons corrections of . Orthogonalization d scaling diagonal Hamiltonian matrix elements give rise to repulsive forces necessary for the stabilization of structures against collapsing. The intra-atomic Coulomb matrix elements are expressed by single Uwhile inter-atomic elements are taken parameters, from a corrected point charge model. The total energy Etotal may be written as statistical average of HHF with respect to the one particle statistical operator P of the ground state, more strictly as

1

+ Ecc

(11

where VHF means the HF potential, and ECC the corecore interaction energy. For a semiconductor at T =

0 1992 Academic

Press Limited

226

Superlattices

0 with either completely states cpi, one has

occupied or empty HF eigen-

I N/Z)

P=ZC

I’p,)((P,I

(2)

!=1

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preparation, partial pressures of Ga and As. as well as on temperature, one has Ga-rich or As-rich surfaces with different percentages of coverages in each reconstructions. Experimentafl~ case, and different p’je finds 2x1, 2x4, c(4x4), and other structures , For a 100% Ga-rich surface our total energy cal-

where i runs upon all occupied states, and N means the total number of valence electrons. We exploit the invariance of the ground state statistical operator P against unitary transformations U within the ground state space , i.e. P

=

u-‘PU

(3)

Owing to this property we need not to calculate the HF eigenfunctions explicitly, a certain basis set which spans the ground state space suffices. This set is formed from generalized bonding orbitals. The gneralization makes that all Hamiltonian matrix elements between bonding and antibonding orbitals are zero in first order perturbation theory. The statistical operator P built of generalized bonding orbitals depends on P itself via the HF Hamiltonian. Thus P has to be calculated self-consistently. If it is known the total energy Etotat is obtained by means of equation (1). This method allows us to treat supercells of 50 atoms in a few seconds on a workstation. Thus a big variety of structural models may be studied in reasonable time. This is just what one needs for simulating epitaxial growth of ZnSe on GaAs substrate. The growth simulation proceeds as follows. We start and a reservoir of with the uncovered substrate, atoms to be deposited on it. In a first step we remove the top layer of substrate and put its Ga or As atoms into the reservoir. Then a film consisting of N monolayers of atoms of the reservoir is deposited (we take N = 2 or 3). There are various possibilities for the chemical compositions and geometrical structures of N-monolayer films. We calculate their total energies and find out the most stable one. Of this N-monolayer film we remove the uppermost (N-1) layers. The removed atoms are put back to the reservoir. The lowest monolayer is kept on the substrate. It forms the first epitaxially grown layer. Then the same procedure is applied again. Various N-monolayer films of reservoir atoms are deposited, and estimated with respect to their total energies in order to get the most advantageous one. Its first (N - 1) monolayers are removed, only the lowest is kept. It represents the second grown monolayer, etc. This procedure allows us to avoid ad hoc assumptions on the chemical composition of the epitaxial film which will grow from given reservoir atoms. Chemical compositions are calculated simultaneously with geometrical structures. Owing to this opportunity the question may be addressed if GazSes or ZnsAsz will grow under certain conditions.

Fig.1 Graphical symbols of atoms. Small circles are used in order to vizualize atoms of the first plane below the drawing plane. Crystallograhic directions as used in the drawings are also shown.

/ h

3. Results 3.1

Clean (100) GaAs surfaces

As is well-known ses of the (100)

‘s, I3 a variety of different phaGaAs surface exists. Depending on

Fig. 2 Structure of lOO%Ga-rich (a) and 1007. Asrich (100) GaAs surfaces as obtained from selfconsistent tight binding total energy calculations.

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culations yield two stable structures (see Fig.Za), one with 2x2 and one with 2x4 reconstruction. Graphical symbols for atoms as well as crystallographic directions used in this and other drawings are shown in Fig.1. In Fig.Za all Ga atoms are dimerized in rows parallel to [Olll, with an alternation of symmetrical and asymmetrical dimers in neighboring rows. In asymmetrical dimers one of the two atoms are shifted up by .l R . Their dangling bonds are doubly occupied. The other three atoms are shifted down by .8 A. Their dangling bonds are empty. The lateral displacements amount to .8 R for the two atoms of asymmetrical dimers. and, respectively, .7 or 1.0 8, for the two atoms of symmetrical dimers. The 2x2 and 2x4 reconstruction mode!% differ only by the relative positions of dimers in neighboring parallel IO111 rows. They have equivalent locations in the 2x2 case, and are displaced with respect to each other parallel to IO111 by one surface lattice constant in the 4x2 case. For 100% As-rich surfaces a 4x1 reconstruction turns out to be energetically most favorable (see Fig.2b). Atoms in rows parallel to IO111 form asymmetric dimers. Two non-equivalent dimers occur in each row, one dimer tilted to the right is followed by another tilted to the left. The two inner atoms of neighboring non-equivalent dimers form x-bonds hosting two electrons each. The two outer atoms have dangling bonds which are also completely occupied. Units of two inequivalent dimers along IO111 have equivalent positions in neighboring [Olll rows. This means 4x1 reconstruction. The vertical displacements of the four atoms in sequence amount to -.3, -.9, -.9, and -.3 6(, and the lateral displacements to .8, 1.0, 1.0, and .8 b. Because of the weakness of x-binding the As-rich surface possessesI a certain metallic character as noticed already in 3.2 Growth on Ga-rich

surfaces

Two degrees of coverages are considered, 1007. and 507.. The two substrate surfaces are exposed to Zn and Se atoms at the same time. In both cases we find an exchange of top Ga atoms against Zn atoms to be unvavourable. Thus growth will start from surfaces having the same Ga content as without exposure to Zn and Se. For 50% Ga coverage vacant Ga sites are filled with Zn. 1007. Ga-rich Se atoms of the first monolayer dimerize in each second row parallel to IOlll, resulting in a 2x2 reconstruction (Fig.3a). The 6 dangling bonds of the four atoms of a surface unit cell are completely occupied. The first Zn monolayer on top of the first Se monolayer dimerizes in a similar way as in the case of a clean 1007. Ga-rich GaAs surface, i.e. complete dimerization with alternation of symmetrical and asymmetrical Zn dimers in neighboring [Olll rows (Fig.3b). All dangling bonds are empty except for the ones at atoms raised above the surface. The reconstruction is 2x2, as for the first Se monolayer. The lateral shifts of the 4 top Se atoms are (in dimer sequence) 1.0, 1.0, .O, and .O fi. and of the 4 top Zn atoms .8, 1.0, .8, .8 fi. The vertical shifts

Fig.3 Se surface layer on 1007. Ga-rich (100) GaAs (a). Zn surface layer on top of the Se layer from (a) amount to -.4, -.4, -.9, -.9 II for Zn.

-.l,

-.l

61 for

Se,

and .3,

-.6,

507. Ga-rich The first Se monolayer dimerizes completely in symmetrical dimers along IO111 rows (Fig.4a). The dimer positions in neighboring rows are equivalent, thus a 2x1 reconstruction follows. All dangling Se bonds are occupied. The lateral shifts amount to 1.0 , and the vertical to -.4 61. The first monolayer of Zn on top of the first Se monolayer exhibits the two reconstruction states 2x1 and ~(2x2) (Fig.4b) with almost identical total energies. In the 2x1 state Zn atoms are arranged in a similar way as Se atoms of the 2x1 reconstructed Se monolayer, i.e. symmetrical dimers occur in equivalent positions in all IO111 rows. The ~(2x2) reconstruction may formally be obtained from the 2x1 state by displacing dimers in neighboring IO111 I%WS by one surface lattice constant with respect to each other. For both 2x1 and ~(2x2) reconstructions the lateral displacements of the two Zn atoms in a dimer amount to .8. and the

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Fig.4 Se surface layer on 50% Ga-rich (100) GaAs (a). Zn surface on top of the Se monolayer from (a) vertical to -.9 A. All dangling bonds are empty. Displaced dimers in neighboring rows resulting in ~(2x21 reconstruction have also been considered in the case of a Se surface monolayer. Its total energy is clearly above that of the 2x1 reconstructed state with non-shifted dimers. 3.3 Growth on As-rich

surfaces

We consider surfaces with 507. As covet-ages at least. The surfaces are exposed to Se and Zn at the same time. The exposure to Se causes As atoms to be replaced by Se atoms until the As:Se ratio equals 1:l. Thus growth proceeds on a surface consisting of 507. As and 507. Se. The first Zn monolayer is deposited in a way similar to that of a 507. Ga-rich surface after the first Se monolayer has grown, i.e. complete dimerization takes place with symmetrical dimers in neighboring IO111 rows in equivalent or displaced positions resulting in 2x1 and ~(2x2) reconstructions, respectively (Fig.Sa). Again the energy difference between the two reconstructions is small. The first Se monolayer shows complete dimerization

and Microstructures,

Fig.5 Se surface on a Zn lAs/lSe monolayer of As-rich

Vol. I2, No

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layer on top of (100) GaAs (a).

a

with dimers in equivalent positions in neighboring (0111 rows (Fig.5b). This means 2x1 reconstruction. Altogether the same reconstruction types are obtained for As-rich substrates and 507. Ga-rich. This observation suggests these reconstructions to be bulk rather than interface properties. We have checked this by depositing monolayers of Zn and Se on top of (100) ZnSe. The same reconstructions were obtained or 507. Ga-rich GaAs substrates as on As-rich (Fig.6). The 1007. Ga-rich substrate forms an exclusion since it reconstructs 2x2. 3.4 Exchange of Zn against Ga during growth Another question common to Ga-and As-rich surfaces concerns the possibility of an exchange between substrate and layer atoms during growth. For Ga-rich substrates we find such exchange to be energetically favorable between Ga atoms of the third layer and Zn atoms of the first fFig.7). For As-rich substrates the analogous exchange between Se atoms of the first layer and As in the third turns out to be unfavorable, mainly due to the lower dangling bond energy

Superlattices and Microstructures, Vol. 12, No. 2, 1992

Fig.6 Se (al and Zn (bl surfaces ZnSe

on (1001

Fig.7 Exchange between first layer Ga during ZnSe growth

layer

Zn and third

of Se as compared to As. For ZnSe films with more than two ZnSe monolayers on Ga-rich substrate, an exchange between Zn atoms at the surface and Ga atoms at the uppermost substrate layer is not advantageous. This is partially due to the fact that dangling Zn bonds are empty, thus no total energy gain results from the lower dangling bond energy of Ga replacing Zn. We conclude that intermixing is expected within the first two monolayers of ZnSe on Ga-rich substrate only. No intermixing is expected between Se and As on As-rich substrates. 3.5 Formation of Ga-Se and Zn-As compounds If a Ga-rich substrate surface is exposed to Se only instead of Se plus Zn as supposed before, a Ga-Se compound could be formed. Alternately, an As-rich surface exposed to Zn only could result in the growth of a Zn-As compound. Here we address the question whether this happens or not, and, if yes, which Ga-Se or Zn-As structures will be formed. We

Fig.8 Simulation of growth on 1007. Ga-rich GaAs exposed to Se and Ga only

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only the results are different. After growth simulation one ends up with a does not allow for further deposition (see Fig.91. We conclude that Zn-As unlikely to grow on As-rich substrate are supplied only.

three steps of surface which of Zn and As compounds are if As and Zn

Acknowlegement - This work was partially supported by the Deutsche Forschungsgemeinschaft (grant number En 239/1-l).

Fig.9 Simulation of growth exposed to Zn and As only

on 1007. As-rich

GaAs

assume that besides Se, also Ga is supplied in the case of a Ga-rich substrate, and also As besides Zn in the As-rich case. A 2x2 supercell will be considered. The structure obtained as the result of total energy growth simulation is shown in Fig.8. The GaSe structure consists of 6 layers, 3 with Ga and 3 with Se. In 2 of the 3 Ga layers 2 of the 4 atoms are missing per 2x2 cell. Altogether there are 24 Se and 16 Ga atoms within the 6 layers per 2x2 cell, i.e. a GazSes compound has grown. The top Ga layer of the structure in Fig.9 is equivalent to the topmost Ga layer of the substrate. Thus Gasses may grow further if Ga and Se are supplied continuously. The growth will stop if either the supply of Ga is interrupted or Zn is supplied in addition to Se. In the latter case ZnSe will start to grow on top of GazSes. For As rich surfaces exposed to Zn and As

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