Wire-like incorporation of dopant atoms during MBE growth on vicinal GaAs(001) surfaces

Solid-State Electronics Vol. 37. Nos 4-6. pp. 783-787, 1994 Copyright ~ 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1101/94 $6.00 + 0.00

Pergamon

WIRE-LIKE INCORPORATION OF DOPANT ATOMS DURING MBE GROWTH ON VICINAL GaAs(001) SURFACES L. DAWERITZ,C. MUGGELBERG,R. HEY, H. KOSTIALand M. HORICKE Paul-Drude-Institut ffir Festk6rperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany Abstract--The ordered incorporation of dopant atoms by combining lattice step growth on vicinal GaAs(001) surfaces and Si delta(6 )-doping has been studied by real-time reflection high-energy electron diffraction (RHEED) measurements. For Si deposition on 2° toward the (lll)Ga plane misoriented surfaces and 0.4° toward the (lll)As plane misoriented surfaces it is shown that the Si atoms arrange themselves preferentially along the step edges in a (3 × 2) structure consisting of an ordered array of Si dimers. For Si concentrations not exceeding substantially the amount expected to be attached at the step edges the GaAs growth can be continued at reduced substrate temperature without adverse effects on the growth front. By pulsed 6-doping an unusual high concentration of Si atoms can be incorporated as donors on Ga sites.

l. INTRODUCTION

Delta-doping of GaAs with Si by molecular beam epitaxy (MBE) is now a well established method to incorporate an electrical active impurity in a sheet of at most a few atomic layers in thickness[I,2]. The usually applied procedure consisting of a suspension of GaAs growth, evaporation of dopant atoms on the nongrowing surface while an arsenic flux is incident, and the growth of subsequent GaAs layers is assumed to result in a statistical in-plane distribution of the dopant atoms. The introduction of an in-plane order of the dopant atoms would allow to study the intriguing electronic properties expected for such ordered structures. In particular it has been proposed to combine the lattice step growth on vicinal surfaces with planar doping to create a one-dimensional system of socalled doping quantum wires[3]. First studies on the deposition of Si atoms on vicinal GaAs(001) surfaces by reflection high-energy electron diffraction (RHEED)[4,5] suggest that a preferential attachment at the edges of misorientation steps occurs provided the vicinal surface is well ordered and the Si migration is strong enough. At present, however, neither the underlying surface processes nor the structure and ordering of the "wire-like" incorporated dopant atoms are well understood. In this work, we use real-time RHEED measurements for a systematic study of the incorporation of Si atoms on various vicinal GaAs(001) surfaces. Additional information on the site occupancy of the incorporated dopant atoms is derived from Hall effect measurements.

2. EXPERIMENTAL

The undoped semiinsulating GaAs(001) substrates

misoriented 2 ° and 0.4 ° toward the ( l l l ) G a or (IT1)As plane were prepared in the usual way. A buffer layer of 0.5/zm thickness was grown and annealed at 580°C. The GaAs growth rate was 0.7ML (monolayers) s -I, and an As4:Ga beam equivalent pressure (BEP) ratio of 15 was used. At all substrate temperatures ranging between 550 and 610°C a clear (2 x 4) reconstruction was found. Before Si deposition, growth interruptions in the order of 1 h duration and the successive deposition of several monolayers GaAs with growth interruptions of 2 min were used to smooth the initial GaAs surfaces. Si was deposited with a continuous or interrupted flux of 1.08 x 10Hcm-:s -~ (1.7 x 10 -4 ML s-l). The subsequent GaAs layers on the Si modified surface were grown at a reduced substrate temperature of 540°C. The Si flux was calibrated by capacitance-voltage (CV) depth profiling of a dopant staircase structure. To assess the site occupancy of the Si atoms at high concentrations 300 K low field Hall effect measurements were performed by using chemical etching and a van der Pauw configuration. For real-time RHEED measurements a 15keV electron beam with an incidence angle close to the first out-of-phase condition for the diffraction at the stepped surface was used. The specular beam intensity was recorded by using an arrangement consisting of a collimator, optical fibre and photomultiplier. The incident beam was parallel to the step edges. With this geometry the specular beam intensity is sensitive to changes in step edge roughness and strain induced effects, adatom density and nucleation processes on the terraces[6,7]. In this azimuth the terraces are not shadowed by the steps. Therefore, Si induced changes in surface reconstruction near the step edges on the lower terrace can be detected from the very

783

L.D.~WERITZ et al.

784

beginning. To determine the full symmetry of the Si induced structures complementary measurements with the R H E E D beam incident in the orthogonal (110) azimuths have been performed.

3.

RESULTS

AND DISCUSSION

Si on GoAs (001) misoriented toward (111)As •

Ga

~l

o As

(3x2) (2x4) o o C)-Q

~ Si

3. I. Structure o f the vicinal GaAs(O01) surfaces The first step in the synthesis of doping quantum wires is the preparation of a regularly stepped vicinal surface with smooth terraces. Using the transition from an oscillating to a constant R H E E D intensity as a function of the GaAs growth temperature[8] the critical temperature Ten, for a change from a twodimensional nucleation and step propagation mechanism to the desired step flow mechanism has been estimated[5]. For the given growth parameters and the 2 ° and 0.4 ° misorientation toward the ( l l l ) G a

i i,.~.~

I l,~l-l,,.d

. . . .

:¥'O',O'~

o c)(]

I

a)

I

Si on GaAs (001) misoriented toward (111)Ga •

(2x4)

Oa

b)

I

~1 ( 3 x 2 ) O A S s i

I

I

yl / a)

c) Fig. 2. Schematic model of the (2 x 4) reconstructed vicinal GaAs(001) surface misoriented toward (ITI)As with Si atoms attached in a (3 x 2) symmetry along the step edge in top (a) and side view (b); (c) is a schematic representation of the step edge roughness derived from STM studies[10].

b)

' -200A

zl

[110]

c) Fig. 1. Schematic model of the (2 x 4) reconstructed vicinal GaAs(001) surface misoriented toward (lll)Ga with Si atoms attached in a (3 x 2) symmetry along the step edge in top (a) and side view (b); (c) is a schematic representation of the step edge roughness derived from STM studies[10].

plane it amounts to about 560 and 640°C, respectively. For the corresponding misorientations toward the ( I l l ) A s plane these values are lower by about 20 ° . This is consistent with a recent first-principle calculation on microscopic processes of Ga adatom diffusion on the (2 x 4) reconstructed surface according to that the Ga adatoms diffuse on the surface by passing through the missing dimer rows[9]. As can be seen in Figs 1 and 2, this preferential diffusion path is normal to the As-terminated steps for the misorientation toward the (l]'l)As plane but parallel to the Gaterminated steps for the misorientation toward the (1 ! l ) G a plane. Figures 1 and 2 show also schematically the different behaviour of the two different step types. F r o m scanning tunneling microscopy (STM) studies it is known that Ga-terminated steps are relatively straight whereas As-terminated steps are very ragged[10].

Incorporation of dopant atoms during MBE growth

"~

.... ~ SliIon

GaAs (001) 2 ° toward (111) Ga Jsi = 1,08 x 101i c m - 2 s -1

I ~ _ ~ ~Ga o,-''~

T =550"C '

~,

~ ~ 1/2-order •, ! spots T = 580"C | • . s --

"~

1/3-order~

spots

OCrit

I

:

0

I

I

[

0.1

0.2

0.3

Si coverage (ML)

Fig. 3. RHEED intensity recorded in the [TI0] azimuth during continuous Si deposition at various suhstrate temperatures on GaAs(001) 20 misoriented toward (lll)Ga. The crosses mark the appearance of half-order and asymmetric third-order spots, respectively. 8=n,is equivalent to the density of Ga step-edge sites. As a consequence of the growth mechanism the terraces of the vicinal surfaces grown above Tom are expected to be smooth whereas on those grown below Tent a certain concentration of holes and islands will exist even after annealing. 3.2. Si attachment Figure 3 shows the time evolution of the RHEED intensity during Si deposition on the 2° toward Si on

GoAs (001) 2' toword (111)Go

Ts =

58o*c

Jsi: 1.08 x IOTIcm-2s'l growth go s ---~

interrupt=on

180s

¢-

L

" - ' ~ S i off

,w .1o ¢. t~

on

ol "rL~

¢-.

--~

¢,-

si off ~~=~

191. . . . . . v, 0

//

. .~

5 10 Time (min)

.

( b ) ~ - $i dimer [110] -- m0ssmg dimer

Fig. 4. RHEED intensity recorded in the [TI0] azimuth during interrupted Si deposition at 580°C on a vicinal GaAs(001) surface 2° misoriented toward (lll)Ga (a) and the related incorporation model (b). The cross marks the appearance of half-order spots.

785

(111)Ga misoriented surface for different substrate temperatures that were kept constant during GaAs growth and Si deposition. The intensity change during the deposition of only 0.04 ML Si is in the same order as that during the deposition of I ML GaAs. The intensity behaviour observed for substrate temperatures below and above Tcr~,is quite different. In a first approximation, it can be understood by considering the two extreme cases, namely a random attachment of Si atoms on the terraces with statistically distributed defects and a preferential attachment of Si atoms at the edges of ordered steps. At 550°C the intensity decreases linearly with the Si coverage. This suggests that the Si adatoms form a lattice gas of increasing density. The change of the slope of the intensity curve at a coverage of about 0.1 ML can be explained by the formation of islands with a (3 x 2) structure evidenced by the appearance of half-order spots in the [TI0] azimuth and of third-order spots in the [110] azimuth. At a coverage of about 0.24 ML asymmetric third-order spots are observed in the IT10] azimuth due to the formation of a distorted (1 x 3) structure. The intensity behaviour at 580°C and 590:C with a nonlinear intensity decrease, a minimum at a Si coverage of about 0.05 ML and the appearance of half-order spots at about the same coverage is considered to be typical for a wire-like attachment of the Si atoms at the step edges as will be discussed below in detail for the interrupted Si deposition. It should be noticed that compared to the case considered before the (3 x 2) structure develops at a lower coverage whereas the distorted (1 x 3) structure is formed at higher coverages. Figure 4(a) shows the RHEED intensity recorded during Si deposition on the 2° toward ( l l l ) G a misoriented surface in intervals of 90 s growth and 180 s growth interruption. The intensity decreases nonlinearly with the Si coverage and shows a slight recovery when the Si flux is interrupted. After reaching a coverage of 0.041 ML the intensity rises rapidly. At approximately the same coverage the (3 x 2) structure develops. These observations are consistent with the incorporation model schematically represented in Fig. 4(b) (cf. also Fig. 1). The nonlinear change in "kink" density and strain accompanying the attachment of migrating Si atoms as dimers at the step edges is reflected in a nonlinear intensity decrease. The recovery behaviour of the intensity suggests that this process including the ordering of the dimers can be completed by periodic interruptions of the Si flux. From symmetry and coverage arguments it is coneluded that the Si dimers arrange themselves in units with two dimers and one missing dimer (model I) or with one rimer and two missing dimers per unit mesh (model 2). The completion of (3 x 2) units along the step edge (to such extent that they can be detected by electron diffraction) and the attachment of a second row finally leads to the three-fold and twofold periodicity in the [TI0] and [110] direction,

786

L. DgWERITZet al. TabLe I. Si coverages 0 ,~,, corresponding to the minimum in the R H E E D intensity recording 0 ,~,,

Misorientation 2 toward (I 11)Ga 0 . 4 toward (1Tl)As 0.2 toward (ITI)As

0or,, (ML) 0.049 0.010 0.005

Experiment (atoms cm z)

Model I

Model 2

(ML)

4/3 0el,, =0.066 ML 20¢r. =0.020 ML 2 0c,. =0.010 ML

2 3 0on~ =0.033 ML I Ocf,t =0.010 ML t 0cr,L =0.005 ML

0.0410.075 0.021

(2.6-4.7} × 10 ~ 1.3 x 1012

0.0100.014

(6-.9) × 10 ~z Ref. [4].

Model 1 refers to a (3 x 2) unit mesh with two Si dimers, model 2 to a (3 x 2) unit mesh with one Si dimer. 0on~ is equivalent to the density of G a step-edge sites.

respectively. The final intensity rise is attributed to changes in strain and reconstruction accompanying the Si attachment. For an ideal wire-like attachment of Si dimers the first strip of (3 x 2) units will be completed at a coverage of 4/3 0on, (0.066 ML) and 2/3 0ont (0.033 ML), respectively, for model 1 and model 2, respectively, where the critical coverage 0on, is given by the number of Ga sites at the step edges. The value of 0.041 ML found in the experiment discussed lies in between. The data compiled in Table l for a series of experiments suggest, however, that the dimer arrangement according to model 1 dominates. The scatter in the data can be explained by taking into account that in the real experiment a second row of (3 x 2) units will start to grow before the first one is completed and that the step edges are kinked. The latter effect that would shift the critical coverage to higher values will be, however, not very important since Ga-terminated steps are relatively straight. The considered deviations from the idealized case can also explain why the half-order spots belonging to the (3 x 2) structure do not necessarily appear exactly at the coverage corresponding to the RHEED intensity minimum but often at slightly lower or higher coverages. Similar as for the 2° misorientation toward ( l l l ) G a also for the 0.4 ° misorientation toward (1TI)As a clear RHEED intensity minimum and at approximately the same coverage the development of a (3 x 2) structure was observed. A comparison with the Ga step-site density (cf. Table l) reveals that the coverage of 0.021 ML Si corresponds nearly exactly to 2 0ont. The data reported by Wood[4] for a 0.2 ° misorientation toward (1]'l)As satisfy the same relation. This is consistent with the model for the Si attachment presented in Fig. 2. The intensity rise and the appearance of the superstructure reflections coincide with the beginning of the formation of a second strip of (3 × 2) units with two filled and one missing dimer rows that are now parallel to the step edge. A slight shift of the intensity minimum from the expected coverage to higher values can be attributed to the pronounced meandering of As-terminated steps. The conclusions on a wire-like incorporation of dopant atoms drawn above from the intensity behaviour are supported by the quite different intensity versus Si coverage plots found for Si deposition on

the 0.4 ~ toward ( l l l ) G a misoriented surface. As a consequence of the competition between step-edge incorporation of dopant atoms and their clustering on the larger terraces, the coverage given by the RHEED intensity minimum deviates now strongly from the value expected for a preferential attachment at step edges, even at a substrate temperature of 610°C[5]. This is explained by the limited migration across the terraces due to the large activation energy for diffusion along the [110] direction[9]. 3.3. GaAs overgrowth

To study the effect of a high local Si concentration at the step edges on the subsequent GaAs growth, for the 2 ° toward (I I 1)Ga misoriented surface RHEED intensity recordings for GaAs growth at 540°C after Si deposition of 2 x 10~3atoms cm--' and without the preceding Si deposition have been compared. The similar intensity behaviour shows that the Si modified surface can be overgrown by GaAs without adverse effects on the growth front morphology, although the reduced intensity indicates a reduced surface order. To confirm the results of the RHEED measurements on the ordered Si incorporation Raman scattering by plasmon excitation was used. Difference spectra for light polarized parallel to the [Tl0] and [110] direction, respectively, showed a much stronger asymmetry for misoriented samples grown at conditions favourable for a wire-like Si incorporation than for reference samples grown on the perfectly oriented GaAs(001) substrate[ll]. The well-ordered (3 x 2) structure observed during interrupted Si deposition on the appropriately misoriented GaAs(001) surface has been explained by the ordered incorporation of Si atoms on Ga sites. The Hall effect measurements of samples overgrown by GaAs confirmed that by pulsed 6-doping Si can be incorporated in an unusual high concentration as donor. A sheet carrier concentration as high as 8 x 10~3cm -2 has been realized for the deposition of 0.61 ML Si on a 2° toward ( l l l ) G a misoriented GaAs(001) surface at 590°C. Using these data it can be concluded that the distorted (l x 3) structure observed at very high Si coverages is due to the incorporation of Si atoms in the As plane. For continuous 6-doping this is observed at much lower Si coverages.

Incorporation of dopant atoms during MBE growth

787

4. CONCLUSION

REFERENCES

In conclusion we have shown that at appropriate conditions, in particular for interrupted deposition, Si atoms arrange themselves on vicinal GaAs(001) surfaces in a (3 x 2) structure along the step edges. This structure arises from an ordered array of filled and missing Si dimer rows parallel to the [110] direction. The critical terrace width for preferential attachment of the Si atoms at the step edges can be realized as much larger for a misorientation toward (1TI)As than for a misorientation toward (111)Ga. This shows that as for Ga adatoms also for Si adatoms the preferential diffusion path is along the [TI0] direction. F r o m Hall effect measurements it is evident that nearly all Si atoms despite their high local concentration at the step edges are incorporated as donors on Ga sites. This Si modified surface can be overgrown by G a A s without adverse effects on the growth front morphology.

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Acknowledgements--The authors wish to thank K. Ploog for stimulating discussions and K. Hagenstein for technical assistance.