Growth and characterization of BGaAs and BInGaAs epilayers on

GaAs by MOVPE

Philippe Rodriguez*, Laurent Auvray, Hervé Dumont, Jacques Dazord and Yves Monteil

Laboratoire des Multimatériaux et Interfaces, UMR CNRS 5615, Université Claude Bernard – Lyon 1

43 Boulevard du 11 Novembre 1918, 69622 VILLEURBANNE Cedex

*Corresponding Author.

E-Mail address: Philippe.Rodriguez@univ-lyon1.fr

Abstract

BGaAs and BInGaAs alloys have been grown by metalorganic vapour phase epitaxy. The

epitaxial layers were grown on (001) GaAs substrates misoriented 1° towards [110] using

diborane, trimethylindium, triethylgallium and arsine as precursors. The influence of diborane

flow rate on the growth mode of BGaAs layers was highlighted. We also studied the influence

of boron gas-phase concentration and growth temperature on the incorporation of boron and

indium into BGaAs and BInGaAs alloys.

BInGaAs/GaAs single quantum wells were grown and quantum well emission was

observed by photoluminescence at room and low temperature. At 300 K, emission wavelength

was up to 1.07 µm.

Keywords: A3. Metalorganic vapour phase epitaxy; B1. BGaAs; BInGaAs; B2.

Semiconducting III-V materials

1. Introduction

The incorporation of boron in the ternary alloy InGaAs is a promising way to develop

solar cells devices or active materials for 1.3 µm laser diodes. Incorporating boron in

InGaAs/GaAs quantum wells may extend the emission wavelength towards 1.3 µm by

reducing the compressive strain. There were few reports on BGaAs and BInGaAs alloys since

the pioneering work of Geisz et al. [1]. BGaAs alloy has been prepared by molecular beam

epitaxy [2, 3] and MOVPE [4-6]. The growth of lattice-matched BInGaAs-layers on GaAs has

also been studied by Gottschalch et al. [4] and solar cells structure with BInGaAs as base

layer have been grown and successfully tested [7].

In this paper, we present the growth behaviour of BGaAs and BInGaAs alloys on GaAs by

MOVPE using diborane with other typical organometallic sources. Influence of boron gas-

phase concentration and growth temperature on epilayers morphology and boron and indium

incorporation is discussed. We also report on the growth and first optical characterizations of

BInGaAs/GaAs single quantum well structures.

2. Experimental procedure

The growth of BGaAs and BInGaAs layers has been performed by atmospheric-pressure

MOVPE in a T-shape horizontal reactor. The layers were deposited on (001) GaAs substrates

misoriented 1° towards [110] direction. Diborane (B2H6), triethylgallium (TEG),

trimethylindium (TMI) and arsine (AsH3) were used as boron, gallium, indium and arsenic

sources, respectively. Hydrogen was used as carrier gas. AsH3 flow rate was kept constant,

leading, for BGaAs layers, to V/III ratios ranging between 120 and 460.

X-ray diffraction (XRD) was used to evaluate the boron concentration and the crystalline

quality of the BGaAs layers and the perpendicular lattice mismatch ((∆a/a)⊥) of the BInGaAs

layers. X-ray photoelectron spectroscopy (XPS) was used to quantify In compositions in the

quaternary alloy and surface morphologies were observed by atomic force microscopy

(AFM).

3. Results and discussion

3.1. BGaAs

Boron incorporation and epilayer morphology are strongly influenced by B2H6 flow rate

(Fig. 1). We observed that B incorporation (x), determined by XRD, linearly increased with

boron gas-phase concentration, quantified by the initial molar flow rate ratio:

Xv=2[B2H6]/(2[B2H6]+[TEG]). However, boron incorporation efficiency (x/Xv) is very low

(about 2.5 % at 580°C). This low value for boron incorporation efficiency is probably due to

the formation of higher boranes [8] and parasitic gas-phase reactions between B2H6 and AsH3

[9].

We highlighted the influence of boron gas-phase concentration on the growth mode of

BGaAs epilayers. For layers grown at 580°C, a transition from a surface with indistinct

terraces (2D nucleation) to a bunched step/terrace structure was evidenced by AFM when

increasing B2H6 flow rate (Fig. 1a-c). At high Xv, a morphological and structural breakdown

(no x-ray peak of the layer) was systematically observed (Fig. 1d), probably related to a phase

separation (formation of B13As2 phase).

At higher temperature (610°C), step-bunching is strongly enhanced, step height and

terrace width increasing with B2H6 flow rate. Step height increases from 6 monolayers for Xv

= 39 % to 20 monolayers for Xv = 62 %. The origin of this strong influence of B2H6 flow rate

on step-bunching is not clarified. We believe that it could be related to the boron surface

segregation evidenced in our BGaAs epilayers [10]. B segregation may affect surface

diffusion kinetics and energy barriers for adatom incorporation at step-edges.

The influence of growth temperature on boron incorporation has also been studied. It was

observed that boron incorporation increased linearly while decreasing growth temperature

from 610 down to 500°C. This shows that low growth temperatures promote the incorporation

of boron. The decrease of boron incorporation efficiency at high temperatures has been

related to the temperature-dependent kinetics of the reactions [11]. It should be noticed that

BGaAs layers with good surface morphology and crystal quality could be grown at

temperature as low as 500°C, with boron content up to 4.6 %.

Arsine partial pressure (V/III ratio) is also an important factor for controlling the surface

morphology and stabilizing the alloy. High V/III ratios (> 100) are required in order to inhibit

the formation of B-B bonds in the alloy and thus avoid phase separation [11]. Nevertheless,

we observed that V/III ratio has no significant influence on the epilayers B content.

In this study, we also observed that TEG flow rate has a major influence on boron

incorporation efficiency. Indeed, at constant diborane flow rate, B incorporation efficiency

significantly increased with increasing TEG flow rate. As the growth rate increases with TEG

flow-rate, the enhanced B incorporation efficiency could be related to a decrease in the rate of

B-species desorption from the surface or to an enhanced B solubility (growth occurring

farther from equilibrium).

3.2. BInGaAs

Experiments were carried out to study the influence of diborane flow rate and growth

temperature on the incorporation of indium into the quaternary alloy. Compressively strained

BInGaAs epilayers have been obtained with perpendicular lattice mismatch ((∆a/a)⊥) ranging

between 0.6 and 2.6 %. (∆a/a)⊥ values were determined from x-ray diffraction patterns.

As shown in Fig. 2, the perpendicular lattice mismatch steeply decreases when increasing

diborane flow rate, quantified by the initial boron molar flow rate ratio:

Xv(B)=2[B2H6]/(2[B2H6]+[TEG]+[TMI]). This suggests a strong reduction of In incorporation

efficiency in the presence of diborane as confirmed by XPS analyses. Indeed, XPS results

indicate that the addition of diborane reduced the indium incorporation by about 22 % while

incorporating less than 1 % boron. Similar trends have been reported by Geisz et al. [11]. This

inhibition of In incorporation is probably due to strong gas-phase interactions between B2H6

and TMI.

Concerning the boron content of BInGaAs epilayers, Geisz et al. only noticed a slight

decrease of the average boron composition when adding TMI to the gas-phase [11]. Thus, in

this study, we assumed that B content in BInGaAs epilayers was unchanged compared to

BGaAs epilayers grown using the same experimental conditions.

In order to optimize indium incorporation efficiency, the influence of growth-temperature

has been studied for BInGaAs layers grown with a boron gas-phase composition Xv(B) = 25

%. A significant increase in both (∆a/a)⊥ and indium content was observed when decreasing

the growth temperature from 640°C down to 500°C (Fig. 3). This shows that low growth

temperatures not only promote boron incorporation but also indium incorporation. High

growth temperatures may favour parasitic gas-phase reactions between B2H6 and TMI.

Thus, growing the quaternary alloy at low growth temperature seems a key point to

increase both boron and indium incorporation.

3.3. BInGaAs/GaAs quantum wells

Single quantum wells (SQW) were grown at 580°C with a boron gas-phase concentration

Xv(B) ranging between 18 and 28 %. The well thickness varied from 4 to 6 nm. The surface

roughness of the top GaAs barrier was low, with RMS values between 3 and 9 Å on 5x5 µm2

areas. Assuming that boron incorporation is the same in bulk-like structures and SQW

structures, boron composition ranged between 0.8 and 1.4 %. For photoluminescence (PL)

measurements, the excitation was provided by an Ar+ laser and the PL signal was detected by

a Ge photo-diode.

Room-temperature PL studies evidenced SQW emission wavelength (fundamental

emission) ranging between 0.94 and 1.07 µm, depending on the well composition and

thickness. Fig. 4 shows the PL spectrum for the sample with the highest emission wavelength.

Quantum well emission is observed at 1.16 eV (1.07 µm), the peak at 1.42 eV corresponding

to GaAs. To the best of our knowledge, it is the first time that BInGaAs/GaAs quantum wells

emission is reported. We noticed that boron incorporation strongly decreases PL intensity, as

already observed with nitrogen incorporation [12]. The full width at half maximum (FWHM)

of the PL band increases with the emission wavelength, ranging from 50 meV (λ=0.94 µm) to

65 meV (λ=1.07 µm).

In the view to improve SQW emission characteristics, rapid thermal annealing (RTA)

effects are under investigation. First results indicate that high RTA temperatures (700-750°C)

degrade the PL intensity and increase the FWHM.

4. Conclusion

We have clarified the growth mode of BGaAs layers and studied the effect of growth

temperature and boron gas-phase concentration on boron incorporation and epilayers

morphology. We also studied the quaternary alloy BInGaAs on GaAs. We highlighted that

indium incorporation strongly depends on boron gas-phase composition and growth

temperature.

The first optical studies on BInGaAs/GaAs quantum wells are promising. The preliminary

work on BGaAs and BInGaAs epilayers indicates that low growth temperatures seem a key

point in order to increase both boron and indium incorporation into the alloys. It should be a

way to extend the emission wavelength of BInGaAs/GaAs SQW towards 1.3 µm. Low

growth temperatures should also have the advantage to kinetically increase the critical

thickness for both plastic relaxation and 2D-3D transition, as already observed for

InGaAs/GaAs quantum wells [13].

Acknowledgements

The authors would like to thank Catherine Bru-Chevallier (LPM – INSA Lyon1) for the

access to the photoluminescence characterization and Anthony Favier (LMI – UCB Lyon 1)

for the XPS analyses.

References

[1] J.F. Geisz, D.J. Friedman, J.M. Olson, S.R. Kurtz, R.C. Reedy, A.B. Swartzlander, B.M.

Keyes, A.G. Norman, Appl. Phys. Lett. 76 (2000) 1443.

[2] V.K. Gupta, M.W. Koch, N.J. Watkins, Y. Gao, G.W. Wicks, J. Electron. Mater. 29

(2000) 1387.

[3] M.E. Groenert, R. Averbeck, W. Hösler, M. Schuster, H. Riechert, J. Cryst. Growth 264

(2004) 123.

[4] V. Gottschalch, G. Leibiger, G. Benndorf, J. Cryst. Growth 248 (2003) 468.

[5] H. Dumont, J. Dazord, Y. Monteil, F. Alexandre, L. Goldstein, J. Cryst. Growth 248

(2003) 463.

[6] D.A. Pryakhin, V.M. Danil'tsev, Y.N. Drozdov, M.N. Drozdov, D.M. Gaponova, A.V.

Murel, V.I. Shashkin, S. Rushworth, Semiconductors 39 (2005) 11.

[7] G. Leibiger, C. Krahmer, J. Bauer, H. Herrnberger, V. Gottschalch, J. Cryst. Growth 272

(2004) 732.

[8] H. Fernández, J. Grotewold, C.M. Previtali, J. Chem. Soc. Dalton Trans. 20 (1973)

2090.

[9] F.G.A. Stone, A.B. Burg, J. Am. Chem. Soc. 76 (1954) 386.

[10] H. Dumont, D. Rutzinger, C. Vincent, J. Dazord, Y. Monteil, Appl. Phys. Lett. 82

(2003) 1830.

[11] J.F. Geisz, D.J. Friedman, S. Kurtz, J.M. Olson, A.B. Swartzlander, R.C. Reedy, A.G.

Norman, J. Cryst. Growth 225 (2001) 372.

[12] L. Auvray, H. Dumont, J. Dazord, Y. Monteil, J. Bouix, C. Bru-Chevallier, J. Cryst.

Growth 221 (2000) 475.

[13] H.H. Tan, P. Lever, C. Jagadish, J. Cryst. Growth 274 (2005) 85.

Figure captions

Fig. 1. AFM images of BGaAs/GaAs epilayers (0.25µm thick) grown at 580°C.

Fig. 2. Perpendicular lattice mismatch and indium composition evolutions as a function of

boron gas-phase concentration for BInGaAs layers grown at 580°C. The data were

respectively derived from XRD and XPS analyses.

Fig. 3. Perpendicular lattice mismatch and indium composition evolutions as a function of

growth temperature for BInGaAs layers grown with a boron gas-phase concentration Xv(B) =

25 %.

Fig. 4. Room-temperature PL spectrum from a compressively strained BInGaAs/GaAs

quantum well with Xv(B) = 18 %.

Fig. 1. AFM images of BGaAs/GaAs epilayers (0.25µm thick) grown at 580°C.

Xv = 0.30 x = 0.008

RMS = 5.3 Å

a)

d) c)

b)

Xv = 0.52 x = 0.0115

RMS = 6.9 Å

Xv = 0.81 No x-ray peak

Xv = 0.62 x = 0.0175

RMS = 11.3 Å

Fig. 2. Perpendicular lattice mismatch and indium composition evolutions as a function of

boron gas-phase concentration for BInGaAs layers grown at 580°C. The data were

respectively derived from XRD and XPS analyses.

Fig. 3. Perpendicular lattice mismatch and indium composition evolutions as a function of

growth temperature for BInGaAs layers grown with a boron gas-phase concentration Xv(B) =

25 %.

Fig. 4. Room-temperature PL spectrum from a compressively strained BInGaAs/GaAs

quantum well with Xv(B) = 18 %.