ARTICLE IN PRESS

Journal of Crystal Growth 310 (2008) 5106–5110

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

VLS growth of GaAs/(InGa)As/GaAs axial double-heterostructure nanowiresby MOVPE

J. Bauer a,�, V. Gottschalch a, H. Paetzelt a, G. Wagner b

a Institute of Inorganic Chemistry, Semiconductor Chemistry Group, University of Leipzig, Johannesallee 29, D-04103 Leipzig, Germanyb Institute of Mineralogy, Crystallography and Materials Science, University of Leipzig, Scharnhorststrasse 20, D-04275 Leipzig, Germany

a r t i c l e i n f o

Available online 24 July 2008

PACS:

68.65.�k

81.15.Gh

81.16.�c

81.05.Ea

81.07.�b

Keywords:

A1. Nanostructures

A3. Metalorganic vapor phase epitaxy

B2. Semiconducting gallium compounds

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.07.059

esponding author. Tel.: +49 34136206; fax: +

ail address: jbauer@uni-leipzig.de (J. Bauer).

a b s t r a c t

We report on the gold-based vapor–liquid–solid (VLS) growth of GaAs/(InGa)As/GaAs axial double-

heterostructure nanowires using low-pressure metal-organic vapor phase epitaxy (MOVPE). Systematic

growth investigations were performed to study the influence of the growth time, the temperature and

the V/III ratio on the composition and the extent of the (InGa)As segments. A group-III precursor growth

interrupt prior to and after the (InGa)As growth showed the highest solid indium fraction and the

strongest composition gradients at the heterojunctions. The experimental results were discussed

considering the droplet composition as well as MOVPE and VLS kinetics.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor nanowires (NW) are of recent interest becauseof promising new application fields in electronics and optoelec-tronics.

We applied the vapor–liquid–solid (VLS) mechanism with goldseeds in combination with low-pressure metal-organic vaporphase epitaxy (LP-MOVPE) to achieve reproducible growth ofGaAs/(InGa)As/GaAs axial double-heterostructure NW with highgrowth rates. While the growth of heterojunctions with group-Vmaterial change has been successfully demonstrated [1], thegrowth of the desirable III-As heterostructures is problematic. Thekey to understand the differences lies in the droplet composition.The MOVPE growth of (InGa)As NW on GaAs substrate has beeninvestigated to get information about the growth determiningeffects [2] and the (InGa)As composition with specific precursorsupply [3]. Furthermore, structural and optical properties of GaAs/(InGa)As/GaAs axial double-heterostructure NW have been stu-died [3,4]. Frequently, heterostructure growth is accompanied bycharacteristic phenomena like kinking, increased twin density, theformation of diffuse heterojunctions with alloy crystal gradientsinstead of abrupt interfaces [3,5], and growth along a side facet ofthe already grown NW [6,7]. However, sharp axial InAs/GaAsheterojunctions have been reported [8], which motivates a more

ll rights reserved.

49 34136249.

detailed investigation of the composition of the alloy droplet andits influence on NW growth.

We investigated the growth and the structure of GaAs/(InGa)As/GaAs axial double-heterostructure NW. In particular,the composition and extent of the (InGa)As segment wasdetermined with respect to important growth parameters.Consequently, the experimental results were discussed consider-ing the droplet composition as well as MOVPE and VLS kinetics.

2. Experimental procedure

All samples were grown using LP-MOVPE (AIX200) at a totalpressure of 5 kPa and with a total flow of 7 slm in H2 atmosphere.Trimethylgallium (TMG), trimethylindium (TMI) and arsine servedas precursor gases. GaAs ð1 1 1ÞB substrate was used to achieveperpendicular NW growth. Prior to growth a 6 nm gold film wasdeposited by thermal evaporation. The temperature was increaseddirectly to growth temperature without any further annealing.Atomic force microscopy investigations at a comparable sampleannealed at 480 1C showed gold particles with a limited sizedistribution of ð68� 30Þnm. The usual particle density was about8� 109 cm�2. Fig. 1 shows the sample structure consisting of aGaAs NW bottom section followed by the (InGa)As NW segmentand a GaAs top section. Both GaAs NW sections were grown for2 min with TMG (0.36 Pa) and arsine (7.1 Pa). The growthtemperature was always held constant during GaAs/(InGa)As/GaAs growth. GaAs NW growth was investigated in detail

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J. Bauer et al. / Journal of Crystal Growth 310 (2008) 5106–5110 5107

previously [9]. To create the heterostructures TMI was suppliedfor a certain time with the partial pressure ratio TMG=TMI ¼2 ðxv

In ¼ 0:33Þ in addition to the unchanged TMG flow. Usually, thearsine partial pressure from GaAs growth was maintainedresulting in a V/III ratio of 13. Systematic growth investigationswere carried out to investigate the influence of the growthparameters on the composition and extent of the (InGa)Assegments (Table 1). The time dependence was studied at 450 1Cfor 2 min, 1 min and 5 s (samples 1–3). In sample 4, the V/IIIratio was set to 40 by increasing the arsine partial pressure(21.4 Pa) during (InGa)As growth. The effect of temperaturewas analyzed using the growth conditions of sample 2 with adifferent temperature of 490 1C (sample 5). In sample 5, thesecond GaAs section was finished after only a few seconds insteadof 2 min. However, the (InGa)As section was not affected by thisshorter GaAs growth period. Note in the first five samples thegrowth was not interrupted during heteroepitaxy. In contrast insample 6, the influence of a 1 min group-III precursor interruptprior to and after the (InGa)As segment growth was investigated.The arsine supply was maintained during the interrupt.Apart from the interrupts all growth parameters were similar tosample 1.

The samples were inspected by scanning electron microscopy(SEM) and transmission electron microscopy (TEM). SEM wasapplied with a FEI Nova 200 NanoLab. TEM investigations wereaccomplished at a Philips CM-200 with 200 kV accelerationvoltage and double-tilt low-background sample holder. For TEMthe samples were prepared by the ‘‘top-down’’ technique. TheTEM apparatus was connected to an EDAX analysis system forenergy dispersive X-ray (EDX) experiments. The spot size for EDXpoint analysis was about 5–10 nm. The measurement error for thedetermined compositions was about 2 at%. Furthermore, thesystem allowed to perform EDX line scans along individual NWwithout intensity calibration.

Fig. 1. SEM image of GaAs/(InGa)As/GaAs double-heterostructure NWs on GaAs

ð1 1 1ÞB substrate. The sample is tilted by 201 to the normal.

Table 1Results from TEM–EDX investigations for the different (InGa)As segment samples

(InGa)As segment growth conditions NW

Sample 1 2 min, V=III ¼ 13, 450 1C 46

Sample 2 1 min, V=III ¼ 13, 450 1C 6

Sample 3 5 s, V=III ¼ 13, 450 1C –

Sample 4 1 min, V=III ¼ 40, 450 1C 27

Sample 5 2 min, V=III ¼ 13, 490 1C 40

Sample 6 2 min, V=III ¼ 13, 450 1C with growth interrupts 60

3. Results and discussion

The SEM (Fig. 1) and TEM (Fig. 2a) investigations showed anincrease in the diameter in the medial region of the NW.TEM–EDX mapping (Fig. 2b) demonstrated that the (InGa)Assegment existed exactly at the position of the diameter change ineach NW. All NW had a straight morphology without kinks.

Intensity calibrated TEM–EDX linescans by point analysis alongsingle heterostructure NWs (Fig. 3a) revealed the (InGa)Assegment composition as well as the alloy droplet composition

segment xsIn (at%) Segment extent (nm) Alloy droplet Ga:In:As:Au

ð110� 12Þ 8:24:7:62

ð70� 20Þ 8:22:6:63

– 26:4:24:46

ð136� 5Þ 13:20:9:58

ð170� 21Þ 10:21:6:63

ð103� 5Þ 18:19:12:51

Fig. 2. (a) Bright-field TEM image of GaAs/(InGa)As/GaAs double-heterostructure

NWs on GaAs ð1 1 1ÞB-substrate (sample 6). (b) TEM–EDX mapping of the same

sample position. The dark regions in the center of the NW represent the (InGa)As

segments.

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70

60

50

40

30

20

10

00 100 200 300 400 500 600 700 800 900

distance from substrate (nm)

atom

ic-%

GaInAsAu

segment

segment

alloydroplet

alloydroplet

In

Ga

As

Au

ED

X in

tens

ity (a

.u.)

0 100 200 300 400 500 600 700distance from substrate (nm)

Fig. 3. (a) Intensity calibrated TEM–EDX linescans by point analysis along a NW of sample 1. (b) Automated non-calibrated TEM–EDX linescan at an NW of sample 6.

Fig. 4. Automated EDX-linescans of the indium signal along NW with similar diameter (80–87 nm) of the different samples.

J. Bauer et al. / Journal of Crystal Growth 310 (2008) 5106–51105108

after solidification in AsH3/H2 atmosphere (see Table 1). The alloydroplet composition values are average values from 2 to 12 EDXmeasurements each. The (InGa)As segment composition valuesare maximum values. Note, the obtained alloy droplet composi-tions may differ strongly from the actual liquid droplet composi-tion during NW growth. In particular, the liquid alloy dropletduring growth is expected to contain only a marginal amount ofarsenic [10]. During solidification in AsH3/H2 atmosphere wesuggest the formation of stable Au–Ga and Au–In phases and(InGa)As from excess group-III material. Hence, the measuredhigh arsenic values in Table 1 are not representative for thesituation at growth.

The alloy particle state of aggregation during NW growth isrecently under controversial discussion. The eutectic tempera-tures of the binary systems Au–Ga and Au–In are 334 and 454 1C,respectively [10]. However, during the (InGa)As growth step nopure Au–In alloy, but a Au–In–Ga ternary alloy had to beconsidered with a lower eutectic temperature than 454 1C. Hence,the used growth temperatures (450 and 490 1C) are situated well

above the eutectic temperature of the alloy particles. As a result,we predict a liquid alloy particle state during our experiments.

The segment length and position inside the NW were obtainedby automated non-calibrated TEM–EDX linescans (Fig. 3b). For thedetermination of the segment lengths 4–7 linescans wereevaluated and the length values averaged (see Table 1). Thelinescan in Fig. 3b revealed that there is not only an indiumintensity increase in the (InGa)As segment, but also in the goldalloy droplet on top of the NW. In contrast, the gallium as well asthe arsenic content decreased inside the droplet. This observationwas common for all NW examined. Hence, in Fig. 4 the indiumsignal was used to illustrate the position and the extent of the(InGa)As segment and the gold alloy particle.

The NW growth rate is basically determined by two influencesin MOVPE, the Gibbs–Thomson effect and the source materialdiffusion to the gold alloy droplet [9]. Both effects show a strongdependence on the droplet diameter and may differentiate for thevarious precursor materials. Hence, the droplet diameter mayaffect the (InGa)As composition and the segment length extent.

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However, our investigations covered a diameter range from 56 to117 nm. A systematic dependence could not be proven.

The results for the samples 1–3 in Table 1 reflect the timedependence of (InGa)As growth. After 5 s of growth (sample 3) thedroplet contained a small amount of indium. In the NW no(InGa)As segment could be found. After 1 min (sample 2) theindium fraction in the droplet arised to over 20 at%. This value didnot change for 2 min (sample 1). In contrast, the indium content inthe (InGa)As segment increased from 6 at% after 1 min to 46 at%after 2 min of growth. The length extent of the segments increasedwith time as well.

The influence of the V/III ratio could be considered comparingsamples 2 and 4. With increasing V/III ratio more indium wasincorporated into the (InGa)As segment. The droplet compositionshowed no recognizable change, but the extent of the (InGa)Assegments markedly increased.

The comparison of samples 1 and 5 showed that the indiumcontent inside the (InGa)As segment slightly decreased and theindium fraction in the alloy droplet was kept constant in thetemperature range of 450–490 1C. However, the segment lengthincreased with temperature.

Finally, the influence of a 1 min growth interrupt prior to andafter (InGa)As growth was studied by sample 6 with respect tosample 1. The segment length kept unchanged, but the indiumcontent in the (InGa)As segment was increased to 60 at%. Incontrast, the indium fraction in the alloy droplet decreased.Moreover, the linescans in Fig. 3 provide information aboutinterface sharpness. All investigated NW from sample 6 showedthe steepest flanks in the indium composition at the hetero-interfaces of the (InGa)As segments.

Based on these results, we suggest the following attitude forNW heteroepitaxy by group-III element change. Starting pointprior to heteroepitaxy was (except for sample 6) the steady-stateGaAs NW growth. The time dependence showed that first aconsiderable amount of indium was accumulated inside thedroplet. In this period only a small fraction of the suppliedindium was incorporated into the NW. Another situation ispresent, if a growth interrupt has been introduced prior to indiumsupply. During the interrupt with arsine crystal growth continuedwith the gallium reservoir apparent inside the alloy droplet. Thegallium fraction in the droplet was decreased. As a consequence,with the start of the (InGa)As growth step both indium andgallium had to be accumulated into the droplet to reach a steady-state NW growth. In this situation the crystallization of higheramounts of indium was possible right from the start of (InGa)Asgrowth. As a result, the first heterojunction became narrower(Fig. 3). The same argumentation is suggested for the secondheterojunction, when the indium supply was switched off.Without interrupt the excess indium from the steady-state NWgrowth was lowered gradually accompanied by further (InGa)Asalloy crystal growth. In contrast, due to the interrupt the indiumand the gallium excess concentrations were reduced simulta-neously over a short crystallization region. However, in bothsituations a considerable amount of indium remained inside thedroplet also during further GaAs growth.

Growth investigations of single crystalline GaAs NW [9] haveshown that the crystallization rate at the liquid–solid interfacebetween droplet and NW increases with the arsenic partialpressure. Hence, with increased V/III ratio the segment extentincreased for the same growth time. Furthermore, the dropletalloy composition changed. The time dependence showed that inthe initial period of (InGa)As growth the supplied indium wasaccumulated inside the droplet. The additional, sudden growthrate increase at higher V/III ratio resulted in an enhancedconsumption of gallium from the droplet reservoir. As a result,the Ga/In ratio in the alloy decreased faster in this initial period.

Hence, the (InGa)As solid composition exhibited a significantincrease in the indium concentration. However, a second effecthas to be considered, too. An increase in the arsenic vaporconcentration would increase the arsenic liquid alloy fraction. Aslight increase in the (already marginal) arsenic liquid alloyfraction is expected to change the equilibrium droplet composi-tion toward a higher indium fraction [11]. As a result, the indiumsolid concentration had to increase supporting our observationfrom a second point of view.

For the influence of the growth temperature the environmentalMOVPE conditions have to be considered. NW growth is situatedin the kinetically limited growth regime, i.e. the precursordecomposition is the slowest step in crystal growth. Thedecomposition temperatures of arsine and TMG are situatedbetween 400 and 500 1C [12]. In the actual case with a V/III ratio41 the arsine decomposition temperature is higher than the TMGdecomposition temperature. As a result, the effective V/III ratioincreased with temperature in our experiments. In accordance tothe expected growth rate increase the extent of the (InGa)Assegment enlarged with temperature. The decomposition tem-perature of TMI amounts to 300–350 1C [13]. Since the investi-gated growth temperature range is evidently higher, TMI can beconsidered to be fully decomposed and keeps constant. However,the TMG decomposition is enhanced with temperature. Hence, theeffective Ga/In gas-phase ratio is increased for sample 5 comparedto sample 1. It has to be emphasized that due to an increase in theeffective V/III ratio in the gas phase the indium content in the(InGa)As segment is expected to increase strongly (like in sample4). Furthermore, the indium distribution coefficient for (InGa)Ascrystallization increases with temperature [11]. Actually, theexperiments revealed a slight drop of the indium content.Consequently, the increase in the gallium supply seems to affectthe droplet composition stronger than the increase in arsenic andindium distribution coefficients for (InGa)As crystallization.

4. Conclusion

The VLS growth of GaAs/(InGa)As/GaAs axial double-hetero-structure NW was investigated. The (InGa)As composition washighest, when a group-III precursor interrupt was performed priorto and after the (InGa)As growth. As expected the growthinterrupts influenced the droplet composition toward lowergroup-III element fractions. As a result, the strongest compositiongradient were found at the heterojunctions. An increased V/IIIratio during (InGa)As growth caused a higher solid indiumfraction and an increased extent of the (InGa)As segment. Theeffects can be understood considering the common VLS behaviorwith an increased crystallization rate, which is accompanied by anenhanced drop of the gallium fraction in the alloy droplet duringthe initial period of (InGa)As growth. Furthermore, a higherequilibrium indium fraction in the droplet can be expected withincreased V/III ratio. The influence of the growth temperaturebetween 450 and 490 1C was discussed considering the precursordecomposition kinetics. Since the effective V/III ratio increasedthe extent of the (InGa)As segment increased. Similarly, theeffective Ga/In gas-phase ratio increased resulting in a slightlydecreased indium fraction in the (InGa)As segment.

Acknowledgments

We thank M. Peter for the TEM sample preparation. The‘‘Deutsche Forschungsgemeinschaft’’ is acknowledged for finan-cial support.

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