Single Crystal Growth and Characterization of Superconducting LiFeAs
Growth and characterization of BGaAs and BInGaAs epilayers on GaAs by MOVPE
Transcript of Growth and characterization of BGaAs and BInGaAs epilayers on GaAs by MOVPE
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: [email protected]
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
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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 %.