World Journal Of Engineering
INFLUENCE OF THE V/III RATIO ON THE GROWTH OF GROUP III NITRIDE QUANTUM DOTS
DEPOSITED ON SILICON SUBSTRATE BY PLASMA ENHANCED CHEMICAL VAPOR
DEPOSITION
Zakaria Bouchkoura, Pascal Tristanta, Christelle Dublanche-Tixiera, Cédric Jaoula, Xavier Landreaua, Elsa
Thuneb, René Guinebretièreb
SPCTS UMR CNRS 6638, Université de Limoges, Ecole Nationale Supérieure d’Ingénieurs Limoges, 16 rue
Atlantis 87068 Limoges, France.
b
SPCTS UMR CNRS 6638, Ecole Nationale Supérieure de Céramique Industrielle, CEC, 12 rue Atlantis 87068
Limoges, France.
a
Introduction
Group III nitride semiconductors have attracted
considerable attention due to their applications in shortwavelength optoelectronic systems. More precisely,
aluminium nitride (AlN) has a number of desirable
characteristics for photonic applications such as its high
direct bandgap of 6.2 eV [1] at room temperature, which
corresponds to a very short wavelength of 200 nm and
allows reaching deep UV. The incorporation of quantum
dots (QD) in the active layer of nitride based photonic
devices improves their efficiency [2].
The growth of AlN QD on Si(111) by Plasma Enhanced
Chemical Vapor Deposition (PECVD) has been carried out
in a previous work [3]. The AlN thin films were obtained
using trimethylaluminium (TMA) ((CH3)3-Al) as Al
precursor and N2 as plasma gas.
The purpose of this study is to investigate the impact of
different growth conditions on the growth mode of AlN
deposits grown on Si(111). A correlation between the
precursors ratio and the microwave power applied in the
reactor is identified and a way to tune the AlN growth
mode is then highlighted.
Fig. 1 Experimental set-up.
Apparatus and procedures
The surface morphology of the deposits has been analysed
by Atomic Force Microscopy (AFM) used in tapping mode
under ambient atmosphere at room temperature. The AFM
images were taken with a Multimode AFM and a silicon
cantilever with an integrated pyramidal tip of typical
radius of curvature smaller than 20 nm (manufacturer’s
data).
In order to check the AlN presence and hypothetical
impurity apparition by changing the growth conditions,
FTIR spectra were obtained with a Varian Scimitar 800
equipment at the normal incident angle in transmission
between 400 cm-1 and 4000 cm-1 with 1 cm-1 step at 300 K.
The substrate Si(111) was used as background. To collect
enough signal, the analysis have been performed on
samples grown during 15 minutes.
Experimental
Materials
The PECVD reactor used is shown fig. 1. This technique is
based on a chemical reaction of the activated AlN
precursors adsorbed in the substrate surface. The
aluminium precursor used was liquid trimethylaluminium
(TMA) contained in a bubbler with argon as carrier gas. N 2
was used as plasma gas and nitrogen precursor. These
precursors are activated by a microwave source operating
at 2.45 GHz. The power of the microwaves exciting the
precursors can be controlled.
The experimental conditions employed to grow AlN
quantum dots have been presented in a previous work [3].
The substrate temperature was set to 800°C and the
deposition time was 1 minute for all samples. To examine
the V/III ratio effects, the N2 flow rate was adjusted from
10 to 125 sccm while keeping the TMA flow rate fixed at
0.4 sccm, resulting in a variation of the V/III ratio from 25
to 312.5 keeping the pressure at 1 Pa by injecting argon
gas in complement of N2. To highlight the impact of the
microwave power, it has been fixed at 1000 W and 1600
W for each ratio. The samples were grown on Si (111)
substrates of 20 mm x 10 mm previously cleaned with
ethanol and acetone and placed in the reactor.
Results and discussion
The different samples were grown in the same conditions
with various N2/Al ratio of 25, 150 and 312.5. Fig. 2
exhibits three dimensional AFM images of these samples.
It can be seen that the growth mode changes with the
N2/Al ratio, from 2D to 3D growth mode.
For a microwave power of 1000 W, the AFM images and a
statistical study of the surface topography (Fig. 3) show
that, for the ratio of 25 a narrow height distribution under
2-3 nm corresponding to a multidomed-like surface is
observed, which is in agreement with the observation of an
AlN continuous layer topography [3]. For the higher ratios,
the presence of quantum dots is clearly exhibited on the
surface, with a wider height distribution of a maximum
height of 7 and 11 nm for a ratio of 150 and 312.5
respectively. This trend is in agreement with the
observations of GaN/AlN growth by molecular beam
epitaxy [4].
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The FTIR spectra exhibit the presence of both wurtzite
AlN vibration modes E1(TO) and A1(TO) at 675 cm-1 and
610 cm-1. The presence of the A1(TO) contribution (Fig. 4)
reveals the loss of (001) texturation by increasing the
N2/Al ratio [5]. The more mixed orientation of AlN grains
is present, the more the contribution of the A1(TO) mode
is important.
Fig. 2 Three dimensional AFM images of the surface evolution
increasing the N2/Al ratio for a microwave power of 1000 W
(left) and 1600 W (right).
On the contrary, for the highest power (1600 W) the
growth behavior seems to follow an opposite trend with
the V/III ratio. Indeed, quantum dots are obtained for low
N2/Al ratios. For the highest ratio, a continuous 2D layer is
grown. The height distribution follows the same opposite
trend with a narrow distribution of low islands
corresponding to the multidomed-like surface for a ratio of
312.5 and a wider distribution of higher islands
corresponding to isolated quantum dots for lower ratios.
As a result, for a fixed microwave power, the growth mode
of AlN on Si( 111) can be of Frank van der Merwe (FM)
or Stranski-Krastanow (SK) type, enabling the growth of
quantum wells or quantum dots by a simple variation in
the N2/Al ratio value.
Furthermore, the microwave power greatly influences the
growth mode too. For a N2/Al ratio of 25, the growth mode
is completely modified, from FM to SK growth when the
power passes from 1000 W to 1600 W and conversely for
a ratio of 312.5. At first view, as the microwave power
plays a primordial role in the precursor activation, it is
reasonable to think that the growth conditions inside de
reactor could be the same in terms of amount of activated
material with a high N2/Al ratio and 1000 W, and low ratio
at 1600 W.
Fig. 4 FTIR spectra of AlN deposits grow at microwave power of
1000W and 1600W for different N2/Al ratios.
Conclusion
The impact of V/III ratio and microwave power on the
growth mode of aluminium nitride quantum dots has been
investigated. It was shown that the growth mode can be
controlled by varying the N2/Al ratio to obtain either a
continuous layer or a growth of 3D quantum dots.
The microwave power appears to have a similar impact on
the growth of AlN on Si(111). These observations suggest
that the effect of these two parameters are correlated.
To confirm this, optical emission spectroscopy of the
reacting plasma will be realized and presented elsewhere.
References
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nano-dots prepared by plasma enhanced chemical vapour
deposition on Si(111)”, Surf. Coat. Technol. (2011),
Fig. 3 Height distribution of the surface topography for each
V/III ratio for a microwave power of 1000W (left) and 1600 W
(right).
doi:10.1016/j.surfcoat.2011.01.011.
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