ANALYSIS OF (AlAs) (GaAs) MONOLAYER SUPERLATTICES GROWN BY FLOW-RATE MODULATION EPITAXY

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ANALYSIS OF (AlAs) (GaAs) MONOLAYER SUPERLATTICES GROWN BY FLOW-RATE

MODULATION EPITAXY

L. Tapfer, N. Kobayashi, T. Makimoto, Y. Horikoshi

To cite this version:

L. Tapfer, N. Kobayashi, T. Makimoto, Y. Horikoshi. ANALYSIS OF (AlAs) (GaAs) MONOLAYER

SUPERLATTICES GROWN BY FLOW-RATE MODULATION EPITAXY. Journal de Physique

Colloques, 1987, 48 (C5), pp.C5-521-C5-524. �10.1051/jphyscol:19875111�. �jpa-00226694�

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JOURNAL DE PHYSIQUE

Colloque C5, suppll?ment au noll, Tome 48, novembre 1987

ANALYSIS OF (AlAs)(GaAs) MONOLAYER SUPERLATTICES G R O W B Y FLOW-RATE MODULATION EPITAXY

L. TAPFER'~), N. KOBAYASHI, T. M A K I M O T O and Y. HORIKOSHI NTT ECL, M u s ~ s ~ ~ ~ o - s h i , Tokyo 180, Japan

Nous decrivons la croissance de superreseaux (AlAs)n(GaAs)n avec n = 1,2,3,4 ainsi qu'une analyse structurelle detaillee. Les superr6seaux sont realises par une technique d' epitaxie en phase vapeur d' organomeetalliques modifiee, 1 ' dpitaxie par modulation de flux. Les proprietes structurelles sont etudiees par simple et double diffractometriede rayons X . Un modele de diffraction dynamique est utilisee pour analyser la forme des pics satellites. Nous trouvons que la rugosite d ' interface moyenne de nos superreseaux est inferieure a 0.05 nm.

This paper reports on the growth of (AlAs),(GaAs),, superlattices (SL) with n = 1,?,3,4 and gives a detailed investigation of their structures. The superlattice structures are s y n t h e s ~ z e d by a modified metallorganic chemical vapor deposition, flow-rat.e m o d ~ ~ l a t i o n epitaxy (FMF). The structural properties are studied by powder and double crystal X-ray diffractometry. A dynamical diffraction model is used t o analyle the shape of the SL satellite peaks. We found that the average interface roughness in our SL is C 0.05 nm.

Monolayer semiconductor superlattices represent a new class of materials with interesting optical and transport properties.

'

Recently, it has also been postulated that (AlAs)l(GaAs)l SL should be a good candidate for high quality spin-polarized photoelectron sources.' A prerequisite for a thorough examination of the theoretical predictions is high quality of the epitactic layers, i.e. the perfectly ordered arrangement of group-111-elements along the growth direction.

X-ray diffractometry is a very sensitive method to study the atomic arrangement in thin layered structures. Powder diffraction as well as high resolution double crystal diffraction experiments were carried out to analyze our (AlAs)l(GaAs)l SL.

Flow-rate modulation epitaxy (FME) is applied to grow (AlAs),(GaAs), SL ( n ~ 4 ) o n

<001, S1 GaAs ~ u b s t r a t e s . ' ~ In the FM€ method, in contrast to conventional MOCVII growth, source gases from an alternate supply are used which yields the desired layer by layer growth. The flow of triethyl gallium (TEG) o r triethyl aluminurn (TEA) and arsine (AsH3) is alternated with hydrogen carrier gas. A further important advantage of this method is the considerably reduced substrate

temperature. Our samples are grown at the GaAs substrate temperature of 550 "C. The mono-, bi-, tri-, and tetralayer SLs are composed of 900, 450, 300, and 225 SL unit cells, respectively, in order t o maintain the same total SL thickness for all SL (about 1 irm).

In order to detect the SL satellite peaks and to determine the SL period lengths the entire X-ray spectra between H g = S" and 50" is recorded by using a powder diffractometer with a post-sample curved graphite monochromator. A double-crystal diffractometer with a symmetrically cut (001) GaAs monochromator is used for a high-resolution recording of the shape of the main diffraction peaks and of the most intense satellite peaks. The X-ray powder diffraction patttrn are evaluated by using a kinematic diffraction model,+ while the double crystal diffraction pattern are analyzed by using the dynamical diffraction theory.'

'''present address : Max-Planck-Institut fur Festk6rperforschung. Heisenbergstr. 1, D-7000 Stuttgart 8 0 .

F.R.G.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875111

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C5-522 JOURNAL DE PHYSIQUE

the numbers nA1 and nGa are not Fig.1 Powder diffraction pattern of the mono-, integers this will lead to a bi-, tri-, and tetralayer (A1As)tGaAs) SL rouqh (or disordered)

The models are modified in order to consider the interface rouhgness and the Al-Ga interdiffusion at the heterointerface. The computation method is given in detail in ref. 6.

Figure 1 displays the X-ray diffraction pattern for the mono-, bi-, tri-, and tetralayer SLs recorded with the powder diffractometer. At the reciprocal lattice

heterointerf ace. Furthermore,

to' C n ~ l and n ~ a are not exactly

( A I As), (GoAsl,

I

equal. This accounts for the even order satellite peaks " - 6 " , " - 4 " ,

1 oO " - 2 " and "+2" around the (002)

points (relp) L = 2 and L=4 the main SL diffraction lines are not

separated from GaAs substrate peaks. The asymmetry of the peak L = 2 and the split of the line at L=4 are due to the Knl and the K02 doublett of the CuKn

-

line.

The spectra exhibit several weaker peaks reflecting the periodic arrangement of the AI and Ga atoms in the (001, growth direction. The SL period length is determined using the angular distance between the satellite peaks, while the average mole fraction is found using the angular distance between the (004) GaAs substrate peak and the main SL peak ("0"-peak).

Figure 2 shows the double-crystal diffraction pattern around the (004)-CuKnl reflex. The

Pendellosung fringes indicate the high homogeneity of the average chemical composition and of the total SI thickness. Therefore individual AlAs and GaAs layer thicknesses can be determined.

We find for the tetralayer SL that the SL unit cell is composed of 3.8 AlAs and of 3.9 GaAs

1 , G A S x3

1 :

relp. The relative intensity

m ' - between the first order satellite

2 peaks around the main (002) and

16- (004) diffraction peaks are in

W

J very good agreement with the

U

W theoretical values. There is,

16'- however, a discrepancy between

the experimental and theoretical

16' - absolute intensity values. The

- 6 - 4 -2 0 2 4 diffraction pattern with the

ANGLE (10~secofarc) first order satellite peak around

the (002) relp recorded with the monolayers on an average. Since

a X X )

A

double crystal diffractometer is Flg.2 (004) double-crystal diffraction pattern shown in Fig.3.

of the (AlAs)3.8(GaAs)3.9 SL The Fwhm o f the "0"-SL peak (40 sec of arc) and of the (002) GaAs peak are much larger than the theoretical values given by the dynamical diffraction theory.

X m

A

~ A 1 4 s I , ~ ~ G o 4 ~ 1 , ~ X 3

x j g rJOO

-

A

A .l ^Y

^,

^A

^,,L ^A^{x x a}^,

,.

"7 Z Y +

Z ,.

( ~ 1 4 s 1 ~ ~ ( G ~ A S I ~ ~

I

^,

_I(

^, ^~

-1

ZY)

4 . m _{A ,}

^r3M

lAl~l,(GoArl,,

~3

t 2

L

(4)

eg (degree)

"0"-SL peak

Substrate

W l-

A N G L E (sec of arc

Fig.3 (002) diffraction pattern with the first order satellite peaks of the (AlAs)3.8(GaAs)3.9 SL

This broadening is due to the wavelength dispersion caused by the not exactly parallel setting of the monochromator crystal ((004)-reflection) and of the sample

((002)-ref lection)

.'

However, the first order sate1 lite peaks are much broader;

This cannot be attributed to the wavelength dispersion, but is caused by the rough heterointerface. The interface roughness parameter is calculated to be 0.05 nm.

Figure 4 shows the double crystal diffraction pattern around the (002)-GaAs reflection and the "-1" and " - 3 " satellite peaks for the C(AlAs)l.8(GaAs)l.g]450 SL. The intensity of the "-3" satellite peak is much higher than the "-1" peak as was theoretically calculated. The average interface roughness in this sample is O.u3 nm.

-3"-SL peak

U) '-~'-sL p a k

W

z

-100 -50 0 M

ANGLE (sec of arc)

F i g . 4 (007) diffraction pattern with the "-1" and "-3" satellite peaks of the

I(AlAs)l.8(GaAs)l.g13~

The appearance of the "-2" satellite peak in the powder diffraction pattern also indicates that the thicknesses of the AlAs and GaAs bilayers are not exactly equal.

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C5-5 24 JOURNAL

DE

PHYSIQUE

The powder diffraction spectrum of the monolayer SL [(AlAs)l.l(GaAs)0.glg00 is displayed in Fig. la. In this sample the SL period length corresponds to the lattice parameter of the A10.5Ga0.5As crystal. Therefore, the satellite peaks are observed exactly at L=l and L=3 and are not split. The effect of the broadening of the satellite peaks and consequently of the decrease of the satellite peak

intensity becomes more pronounced with a shorter SL period. The satellite peak L=l of a monolayer SL (lpm thick) with an interface roughness of 0.01 nm will be broadend by 1000 seconds of arc (21 sec of arc for a perfect SL ! ) and the

intensity will decrease by two orders of magnitude. Unfortunately, due to the very weak intensities it was not possible to detect satellite peaks with a double crystal diffractometer. We estimated the interface roughness by extrapolating from the powder diffraction data to be 0.03 nm. Nevertheless, the interface

inhomogeneities of our mono-, bi-, tri-, and tetralayer SL are very small and may explain the observed narrow linewidths of the photoluminescence spectra in FME grown single quantum wells. The small interface disorder is probably caused by the unequal thickness of the AlAs and GaAs layers and/or the slight deviation of the average mole fraction from the ideal value of X-0.50.

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Epitaxy" Symp. D e c . 2-4 ( 1 9 8 5 ) , Boston, U S A , e d . J. M . Gibson, G. C. Osborn and R. M. Tromp, vol. 56 (1986) p. 19

2. F . Ciccaci, E . Molinari, and N . E. Christensen, Solid State Comm. 62, 1 (IY87!

3 . N . Kobayashi, T . Makimoto and Y . Horikoshi, Jap. J. Appl. Phys. 24, 19h2 (1985)

4. J . Kervarec et. al., J. Appl. Cryst.

17,

1 9 b (1984)

5. 5 . W. Wi.lso;i, Acta Lryst,

a$,

³⁴³(1918)

h. l . T a p t a r and K. Ploog, P h y s . Rev. 833, 5565 (1986)

7. N. Kobayashi and Y . Horikoshi, Appl. Phys. Lett. 50, 909, (1987)