Recent understandings of elementary growth processes in MBE of GaAs

ELSEVIER

Tnin Solid

Films 343-341 i1999i 495499

Recent understandings of elementary growth processes in MBE of GaAs T. Nishinaga’“, A. Yamashiki

Abstract

Elementarygrowth processes invoIved in MBE of Ga4s is discussed basing on theexperiments done by a microprobe RHEED/SEM MBE system. The incorporation diffusion length of Ga, Alnc during MBE growthwas measured in situ on (001) and it was shovm thaf hins depends which suggests di&rent order of arsenicreactionis on thearsenic pressure in different manner for the different rangesof arsenicpressure. involved in the growth process.The direction of intersurfacediffusionbetween two facetsis studied and it is concluded that the ke! paramererwhich conk01 the direction is the incorporation lifetime. T,,~ of Ga. Its arsenic pressure and the surface reconstruction dependencies are found as the major factors which determine the direction of the intersurface diffusion. 0 1999 Elsevier Science 5%. All rights reserved. Kryvo~u’s:

Surface

diKusion;

MoIecuiar

beam Epitaxy;

G&s:

incorporation

1. Introduction There are various techniques to fabricate nanostructures in semiconductorsAmong them, epitaxial growth is one of the most hopeful techniquesbecauseit gives highly perfect nanostmctureswith high density. So far many techniquesof the epitaxial growth have been proposedto employ for the fabrications of nanostructure such as epitaxial growth on patterned substrate [i-5], selective area epitaxy [6-lo], self-assemblinggrowth [ 1l-151 and etc. To fabricate nanostructures, one should understand the elementary growth processso that one can precisely control the growth. In the present article, we discussthe recent understandings on elementary growth processesinvolved in molecular beam epitaxy (MBE) taking GaAs as an example. What is important for understandingthe growth processesis that the experiments to obtain the growth information should be carried out at growth temperaturein real time. In the present work, we employed a microprobe RHEED/SEM MBE [ 16 181by which we can conduct in situ observation of change in growth morphology [ 19,201as we11as the measurements of Iocal growth velocity in real time 121-251.

2. Incorporation

diffusion length

There are two kinds of diffusion length employed in the * Corresponding author. Tel.: + 81-3-3817-2111 ext: 6673: ias: + Sl-

diffusion

E-nmil 0ddre.m; tatau~ee.t,u-roh~o.ac.jp CT. Kishinzga) Q 1999 Elsevier

diffusion

theory of cry&I growth One is the average distance of a growing atom between arrival and evaporating points. However, in MBE of GaAs, Ga atomsare difficult to evaporate at usual growth temperature and stay until they are incorporated into the crystal. The other is the incorporation diEusion length which is defined as Ihe distance between arrival and incorporating points. The concept of the incorporation diffusion length is not cIear compared with the evaporation diffusion length becausethe former depends on the step density and rhe arsenic pressurein the case of GaAs MBE, while the latter is defined on an ideal surface where no step exists and hence is a simpIe function of the temperatureanddependson adsorptionand desorptionenergies. However, the incorporation diffusion length is a key parameter which characterizes the growth in nanostructure fabrication. The incorporation diffusion length can be determined experimentally by measuring the distribution of growth rate on a facet where the growth atoms diffuse lateraily from or to a next facet. In ,MBE of GaAs, Ga adatomscan diffuse in the distance on the order of l-10 m before the incorporation. It has been shown that or (001) surface the incorporation diffusion length of Ga dependson the arsenic pressure [23,34]. In our previous report. we showed tie diffusion Iength is inversely proportional to the As4 pressure. However. we found when the arsenic pressureis lou the diffusion length is inversely proportional to squareroot of As4 pressure.The dependenceof h;,,, on arsenicpressure on (001) surfaceis given in Fig. 1. Here the arsenicpressure

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0040~6090/99/$ - see front matter PII: s0040-6090198)01710-9

length; Lntersurface

Science S.A. All rights resemea.

~1

R(x)=&(x)

+ Rv

lllkd flu

Rv

:...

0.1 0.1

:.._

1

Fig. 3. Sch~mnric illusu,ltion of the posltion .v from ihe boundq belhrrn denore. respectively. rhe r.lleb of growrh thx by rhr direcr flux R.. which ih equal bound=?.

10

Arsenic pressure (1O“Pa) Fig. 1. Arsenic pressure dependence on the incorporation diffusion of Ga on (001) surface. Gro\
length were

is recalculated sothat the arsenicflux enter the (00 1) surface at right angle. A,,,, is given by well known formula as (1)

A!!;, = jD,,x. T,,,,

where D..,: and T::,; denote the diffusion coefficient and the incorporation lifetime of Ga adatoms. respectively. Since the growth rate of GaAs. Rc,,~,is proportional to the inverse of T,,,,.we get RG.A

(2)

= AI /T,o.

where A, is a constant. From Fig. 1 we get A!,, = A2Pi:’ 1

(3)

where AI is a constant.The variable 11takesthe valuesof 0.5

growth Me. RUI 3s a function of two f.~cer In the ligurc. RI ad R, which ib caux!d by lateral flux and 10 rhr rd!e 2~ the prim far from rhr

and 1 when PI,, is lower than 8 X IO-’ Pa and higher than this value. respectively. From Eqs. ( I l-(3) we get &ii\\

= A.3 ,A,,,(P,,, 2 8 x lo-

= A&,~(P,+

2 8 x 10-l Pa).

Pa)

(1) (5)

respectively for lower and higher As, pressures.Eq. C-1) shows that at lower .4sl pressureR(;,,,+ is proportional to P.I.& which means one As: molecule decomposesand gives active arsenic atoms or molecule5 for the growth. On the other hand. at high arsenic pressure.Eq. (5) shows two As, moleculesreact to give the active arsenicatomsor molecules.There is no information to determine the kind of the active arsenic atoms or molecules but 4s2 is the most probable candidate [37-291. Fig. 2 showsthe dependenceof A,!,; on arsenic pressure on ( 110) surface.The arsenicpressureis recalculated sothat the arsenicflux is incident on ( 1101at right angle. As seenin the figure. h!:,, is proportional to Pi::’ in low arsenic pressureregion and to Pi,!! in high pressureregion, which mean 7lib.is proportional to Pi,.: and Pi,.,, respectively. Arsenic pressuredependencyof A;!,, is exactly the sameas that on (001). 3. Intersurface diffusion

Substrate

$ -c

Temperature=

580%

I

1 I

0.1 Arsenic

1 Pressure

IO (1 Oe3Pa)

Fis. 2. Ar,enic pressure dependence on rhc incorporation difukion lengrh of Ga on ( I 101 xrface. Growth temperature and the side surfuce ivere 5SO’C and (001). respectively.

When two facet5 are generated side by side during the growth. Ga atoms diffuse from one facet to the other dependingon the gro\vth conditions. We call this as intersurface diffusion. If there is a difference in adatomconcentration. surface diffusion occurs assuming there is no potential difference nor barrier between the facets. Once intersurface diffusion occurs, one facet grows faster than the other. and the facet growing fast will disappearquickly [19.10]. Thus. during the growth many facet> appear and

T. ~Nshimpn,

A. Iimrnshiki

/ Thin Soizd Fi’hrs 343-314

1.4

1.2 1

2

m

6

0.8

0.6

0.4 0

1

2 3 4 5 6 Arsenic Pressure (lo” Pa)

7

disappearand only a few facets remain in the final stage of the growth. For the fabrication of nanostructure, one should know relative growth rate among various facets during the growth. However, the growth rate of facet dependson the growth conditions and hence one should understandwhat is the factor which determine the growth rate. By measuring the growth rate distribution on one facet near the boundary, onecan know the direction of the surface diffusion. Fig, 3 shows the principle. In the figure, x = 0 is the position of the boundary betweentwo facets. The growth rate distribution can be obtained by microprobe-RHEED by measuringthe intensity oscillation. As shown in the figure, if the growth rate decreasesasthe distanceis increased,one knows the lateral Aux occurs from the adjacentfacet ix < 0) to the facet ix > 0) on which the growth rate distribution is measured.While, if the growth rate increasesasthe distance is increased,one knows the direction is opposite We measuredthe distribution of the growth velocity on the (001) surfaceof GaAs. As the sidefacet, we have chosen

il999j

-N-l99

497

(ill) B and ( 1IO) surfaces.Instead of measuringthe whoIe distribution; we measuredgrowth rate at two points. One is on the faceI and close to the boundary and the other is the point on the facet 1:ery far from the boundary. We de&e each growth rate as R,,,,, and R,;,,,,. respectively. Fig. 4 showsthe resultsfor a combination of (11 I) B-1001) facets. In the figure, closed circle and open triangle denote R$$J Rg,& andR$id;,“iR$&‘. respectively. Here, we define these normalized growth rates as R, and !?a, respectively. When the arsenicpressureis low, R;, is larger than unity. As easily understood with Fig. 3, this indicates Ga adatomsdiffuse from (ill) B to (001). On the other hand, Ra is lower rhan unity which simukaneousipindicates Ga adatoms diti%se from il I1 1 B to (001) being consistent with the direction given by RA. As the arsenic pressure is increased, RA decreasesand crossesthe Lineof unity at the arsenicpressure of 1.4 X lo-’ Pa. which meansthe direction of the Iateral flow is reversed.namely, from (001) to ill1 ) B. At the same arsenicpressure.RB also crossthe Lineof unity from lower side to the higher side. This is very important, sinceif this doesnot happen, one can not assumea pure two face intersurface diffusion. As the arsenicpressureis increased,both R,Aand RB keep aimost constant values but as the arsenic pressureis further increased, they again cross the line of unity which means the direction of the difikion is again reversed. The direction of the diffusion for each arsenic pressurerange is given on the top of the figure. Fig. 5 shows the amount of Iaterai fIus crossing the boundary between (001) and (,llO) facets. As seenin the figure, the direction of Ga flux is from (1 IO) to (001) u.hen the arsenicpressureis low. On the other hand as the arsenic pressureis increased,the direction of the Ga flux is reversed. In the whole range of the arsenic pressurein Fig. 5, the reconstructions of (0013 and ( 110) were kept at (2 X 4) and (1 x I), respectively.

4. Etemedary

growth processes

Basing on the experimental resultsgiven in the above, we discusselementary gron!th processesinvolved in the MBE of GaAs.

-6 J 0

1

Arsenic

2

Pressure

3

(10” Pa)

Fig. 5. The arsenic pressure dependence of the lateral flux iit the boundary berv;een iOO1) 2nd (110) for the grmvth ternperature~ of 580 and 600°C. The pnsitive flux me8115that the direction of the flux is toward ,001) from (110).

There are three major modesof the growth in MBE of IIIV compounds. which are of ?D nucleation, of step flow (steps are supplied mostly from misoriented surface) and of the growth on the atomically rough surface. The rough surface is prepared when one use largely misoriented surface from a singuIar surface such as (1 ll), (OOl), (1 IO) and etc. Among them, we discussthe modeof 2D nucleation in which the MBE of III-V compoundsis most popularly conducted. In Section 3. w-e showed Ga adatoms diffuse from one facet to the other. In Fig. 3, the growth rate distribution on one facet was schematically shown. With this distribution,

one can get experimentally the diffusion length of incorporation which has been shown in Figs. 1 and 3 as a function of arsenic pressure. Here. the esperimcntal growth rate was measured by RHEED intensity oscillation so that the growth can be understood as being conducted in 2D nucleation mode. This means birth and spread of 2D nucleation are continually happening during the growth and Ga adatoms diffuse bet\veen and over the 2D nucleation. The incorporation lifetime of Ga. T,,( which was defined in the Section 1. should be the function of available number of incorporation sites. Hence. in this situation. the number changes periodically with the same periodicity but in opposite phase of RHEED intensity oscillation. Nevertheless, we can define the time average of h,,, and T:,,..which we have measured in the Section 2. The average distance of the adatom incorporation is on the order of 1 p-m which means under arsenic pressure Ga adatoms can diffuse crossing over many steps supplied from 2D nucleation. Hence. the sticking coeflicient of Ga adatoms at the step edges is much less than unity. 4.2. Chntlge in the dir-ection of irltetwifcm

cliffusiorl

Intersurface diffusion occurs if there is a difference in Ga adatom concentrations between two facets. Here. we assume there is no potential difference nor barrier between these faces. But. there is no evidence for this assumption. The adatom concentration of Ga. /zoa is proportional to incident flux of Ga. .Ic4 and T,,.,;.as follows ‘I(,,; = JG L,

facets. In our previous paper. we extrapolated I’:\, dependency of line on (111) B but a, described in SectiQo 2 recently we found there is a region in the lower arsenic pressure side where T,,, shows the dependency of Pi,+ SO that up to now. our previous conclusion for Pi,, dependency of line which is responsible for the directional reversal probably should be changed to Pi,: dependency If this is the case, we should assure P,i,\ dependency for (00 I ) surface and Pi,, dependency for (11 I ) B surfsce which allows the crossing of line on ( 1I 1) B and line on (00 I ) at the arsenic pressure of around I.1 X IO’ Pa. The directional reversal of intersurface diffubiun between (001) and (I 10) facets was given in the Section 2. The arsenic pressure dependence of A.,, is given in Fig. 2 in which one saw there are two regions of different arsenic pressure dependency. In the rage of low arsenic pressure. line is proportional to PA,, but in the range of high arsenic pressure it is proportional to Py,:. The crossing of these two lines happens at lower arsenic pressure than that on (001) surface. Although, the incident angle of As flux on (1 I!) is smaller than that on (001) when the intersurface diffusion experiment was conducted. The growth on (1 IO) is done in the range of T,,.(-Pi,: dependency while on (001) it is conducted in the range of T,,,,--P,:,.,. This situation again gives the crossing of the lines in r,,,, VS. P,r2 chart and at this point the direction of intersurface diffusion is reversed as we saw in Fig. 5. 5. Conclusion

(6)

As we have discussed. T,.,, is inversely proportional to the available number of Ga sites, in other words. to the step density. The step density depends on the number of 3D nucleation and their sizes. Hence. T,,,;depends on nucleation rate and the energy barrier for Ga adatoms to enter and to leave the kink site of the step. There are almost no information for their energies so that we should satisfy at this moment with qualitative discussions. In Section 3. we showed the esperimental observation for the change in the diffusion direction. These experimental findings also give us information of elementary growth processes. Fig. 1 showed the direction of the intersurface diffusion changes twice as the arsenic pressure is increased. At the second point of directional reversal (higher arsenic pressure side) the reconstruction of ( 111) B changes from ( J I9 x ,/ 19)to(? x 2 )as arsenic pressure is increased while the reconstruction of (001) is unchanged and keeps (2 X 1‘). As it is known the (2 X 2) reconstruction consists of arsenic trimer ivhich is rather difficult to be decomposed. Hence. once (2 X 2) reconstruction is formed the formation of 3D nucleation might be more difficult Lvhich causes the increase of 7+ and hence the increase of Ga adatom concentration. As for the first point of directional reversal occurring at I .4 x IO’ Pa. we have explained in terms of difference in arsenic pressure dependence of line on (001) and ( 111) B

The elementary growth processes of GaAs molecular beam epitaxy was discussed basing on the observation of incorporation diffusion length. The experiments show that the arsenic pressure dependence of the incorporation diffusion length is different for the different ranges of the arsenic pressure. From the dependencies it was concluded that As: molecules decompose to give active arsenic elements for the growth at lower arsenic pressure while at high arsenic pressure two arsenic molecules react to give active arsenic elements, which agrees with the model given by Foxon and Joyce. Intersurface diffusion was studied and it is concluded the factor governing the direction of the intersurface diffusion is the incorporation lifetime of Ga which depends on the available number of growth sites and the arsenic pressure. The former depends on the step density and hence on the nucleation rate. which is influenced by the kind of surface reconstructions.

Acknowledgements The present work was supported by JSPS Research for the Future Program in the Area of Atomic Scale Surface and Interface Dynamics under the Project of ‘Self-assembling of Nanostructures and Its Control’.

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