22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Features of the reactive nitrogen source based on ECR discharge sustained by
24 GHz radiation for metal organic vapour phase epitaxy
A. Vodopyanov, D. Mansfeld, I. Dubinov and A. Sidorov
Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia
Abstract: In this paper, we describe a source of reactive nitrogen based on ECR discharge
plasma, sustained by technological gyrotron radiation. Measurements of atomic nitrogen
flux were conducted by mass-spectrometric analysis of the products of the reaction with
nitrogen monoxide. Vibrational temperature of N 2 molecules was measured by plasma
emissive spectroscopy. Electron temperature and density was measured by Langmuir
probe.
Keywords: active nitrogen, indium nitride, ECR plasma, MOVPE, microwaves, gyrotron
1. Introduction
In recent years, A3B5 compounds have become very
popular owing to their unique and promising properties.
Heteroepitaxial films of these compound semiconductors
have attracted great interest from researchers because of
the possibility of using them for effective optoelectronic
devices and microwave electronic devices [1-4]. The
most challenging among these compounds is indium
nitride. The predicted properties of this material are
extremely tempting [5]; however, it is still difficult to
realize them in practice. Since growing indium nitride
from the melt is impossible [6], the only way to grow InN
is layer-by-layer deposition.
The best heteroepitaxial layers can be obtained by the
plasma-assisted molecular beam epitaxy (PA MBE)
method. However, in this method also, it is necessary to
conduct crystal growth at temperatures insufficient for
high-quality growth.
The main restriction is the
decomposition of indium nitride at high temperatures [7].
Increasing the growth temperature leads to an enhanced
degradation of the material, which consequently requires
an increase in the flow of reactants, particularly, active
nitrogen. It was shown that increasing the flow of
reactive nitrogen makes it possible to grow InGaN films
at higher temperatures, resulting in a better crystalline
quality [9]. Indeed, from the data presented in Ref. 9, one
can find that for InN growth at 700 °C it is necessary to
provide at least 1017 cm-2s-1 of active nitrogen flux to
compensate for material decomposition. In this paper, we
propose a new source of atomic nitrogen based on the
plasma of electron cyclotron resonance (ECR) discharge,
which is sustained by microwave radiation at a frequency
of 24 GHz for the growth of InN and InGaN with high
indium contents.
Nitrogen activation in plasma has obvious advantages.
In nitrogen-activation-based methods, the rate of active
nitrogen supply in the growth zone is determined by
plasma parameters and is independent of substrate
temperature (in contrast to NH 3 -based methods). The use
of a technological gyrotron as a source of microwave
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radiation, with high frequency and power provides
additional capacity to adjust the flux of activated nitrogen.
This gives grounds to expect a significant increase in
active nitrogen flux. We propose the use of the ECR
discharge produced and sustained by CW gyrotron
radiation with power up to 5 kW and a frequency of
24 GHz for the growth of heteroepitaxial indium nitride
films and InGaN films with high In contents. Heating at a
high (compared with the conventional source: magnetron
2.45 GHz) frequency can significantly increase the
plasma density in the discharge, which provides an
increased rate of molecular nitrogen dissociation and
excitation. In addition, a high average power of gyrotron
radiation provides a high specific power absorbed in the
plasma [10], which can be up to 100 W•cm-3.
2. Setup
The scheme of the setup is presented in Fig. 1. The
microwave radiation (1) of the technological gyrotron (2)
is injected into the plasma chamber (3) through the
window. The magnetic coil (4) provides the conditions
for ECR discharge. Molecular nitrogen (5) goes to the
plasma discharge zone, where it is dissociated upon
electron impact. After exiting the reactor, the flow of
activated nitrogen enters the growth chamber, where the
substrate is placed on a heated substrate holder (6).
trimethylindium is injected near the substrate (7).
Thermal decomposition of trimethylindium on the
substrate surface in a stream of active nitrogen provides
the indium nitride film growth. Langmuir probes were
placed at the centre of the reactor at the position of the
substrate holder to measure plasma density. A gas line
(7) was used for NO supply for atomic flux
measurements.
3. Atomic flux measurements
Measurements of atomic nitrogen flux of the ECR
plasma discharge were carried out by titration reaction
[11, 12]. The method consists of carrying out the
reduction reaction of nitrogen monoxide (NO), which is
1
To measure plasma parameters, we used the Langmuir
probe placed along the system’s axis in the reactor part of
the vacuum chamber 23 cm from the center of the
4
5
7
2
1
3
8
6
Fig. 1. Setup scheme. 1: microwave radiation,
2: gyrotron, 3: nitrogen plasma, 4: magnetic coil,
5: nitrogen supply line, 6: substrate holder,
7: trimethylindium or NO supply line, 8: pumping port.
added at a known amount. Atomic nitrogen restores the
monoxide by the following reaction: N + NO -> N 2 + O.
Nitrogen monoxide was supplied through a tube of
stainless steel, whose outlet was at the center of the
atomic nitrogen flux and was directed downstream of the
gas flowing from the plasma chamber (see Fig. 1). The
gas mixture containing 10% NO and 90% N 2 was used in
the experiments. To analyze the composition of the
mixture of reacting gases in an exhaust outlet after the
turbo pump and before the inlet of the fore vacuum pump,
a specimen was taken. Subsequent analysis of the
specimen was performed by quadrupole massspectrometry. The dependences of nitrogen atomic flux
on microwave power and molecular nitrogen flow are
shown in Figs. 2 and 3, respectively. The dependence of
atomic nitrogen flux on microwave power exhibits
saturation, which is achieved at 500 W and a molecular
nitrogen flow of 730 sccm. The dependence of atomic
nitrogen flow on input power indicates that the electron
temperatures of 1.6 – 1.8 eV are sufficient for the
effective excitation of rotational and vibrational degrees
of freedom [13] of the N 2 molecule. Starting from these
values, a significant fraction of the energy deposited in
the discharge goes into gas heating by the excitation of
rotational and vibrational degrees of freedom and the
following collisions. Hot gas reduces the concentration of
molecules at a fixed flow, which leads to a decrease in the
rate of dissociation of the molecules. The dependence of
atomic nitrogen flux on N 2 flux through the source is
almost linear in the saturation region of power. The linear
dependence of atomic nitrogen flux on the total gas flow
through the plasma is further indicative of the heating and
expansion of the gas in this mode. Consequently, greater
gas flow through the plasma prevents the heating of the
gas, thereby increasing the efficiency of dissociation. The
maximum atomic nitrogen flux of 5.4x1018 atoms/s is
achieved at a higher heating power of 950 W (see Fig. 3).
4. Plasma parameters
2
Fig. 2. Atomic nitrogen flow versus microwave power.
Fig. 3. Atomic nitrogen flow versus N 2 flow through the
plasma.
discharge where a sample is placed usually. In this area of
the discharge parameters, the electron temperature is
usually weakly depends on the microwave power.
Electron temperature is determined by the gas used and
the pressure in the discharge. Figs. 4 to 7 show the
measured plasma parameters in the region of the
substrate. At this point, the plasma is already much
decayed and has no strong influence on the growth
processes.
5. Vibrational temperature
An important parameter that determines the reactivity of
molecular nitrogen is the degree of excitation of vibrational
degrees of freedom of the molecule. A significant part of
the energy input into the nitrogen plasma of this pressures
range falls on excitation of vibrational degrees of freedom
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[13]. The degree of excitation is described by the
vibrational
temperature
of
molecular
nitrogen.
Determination of the vibrational temperature was carried
out by the analysis of the optical plasma emission spectra
by the method described in detail in [14].
Fig. 4. Electron temperature versus microwave power
coupled to the plasma. N 2 flow through plasma was
1000 sccm.
Fig. 7. Plasma density versus N 2 flow through the plasma,
microwave power coupled to the plasma was 180 W.
Vibrational temperature of molecular nitrogen can be
determined based on the second positive system of a
nitrogen emission spectrum. This implies the radiation
transition between C and B electronic states:
N 2 (C3Π u ) →N 2 (B3Π g ) + hν. The vibrational temperature
determination method is based on experimentally
measured emission intensity of the radiative transition
between two energy levels with the vibrational numbers
v’and v’’, see Fig. 8.
Fig. 5. Electron temperature versus N 2 flow through the
plasma, microwave power was 180 W.
Fig. 8. Nitrogen emission peaks used for the vibrational
temperature determination. Corresponding vibrational
numbers v’–v’’ are indicated for each peak.
Fig. 6. Plasma density versus microwave power coupled
to the plasma. N 2 flow through plasma was 1000 sccm.
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Figs. 9 and 10 show the results of measurements of the
vibrational temperature, depending on the plasma
parameters. It is seen that the increase in microwave
power coupled to the plasma increases the vibrational
temperature, and increasing the gas flow through the
plasma leads to a decrease in the degree of vibrational
excitation of nitrogen molecules.
3
7. Acknowledgements
The work was supported by the Russian Foundation for
Basic Research; Grant No. 13-08-01313.
Fig. 9. Vibrational temperature of the molecular nitrogen
versus microwave power coupled to the plasma. N 2 flow
through plasma was 1000 sccm.
6. Conclusion
A new source of active nitrogen based on the plasma of
the ECR discharge, which is sustained by microwave
radiation at a frequency of 24 GHz for the growth of InN
and InGaN with a high indium content, is proposed.
Fig. 10. Vibrational temperature of the molecular
nitrogen versus N 2 flow through plasma, microwave
power coupled to the plasma was 180 W.
The maximal atomic nitrogen flux is 5x1018 atom/s.
The vibrational temperature of the molecular nitrogen
varies from 6000 to 10000 K. High amount of active
nitrogen flow enables the growth of indium nitride at
previously inaccessible rates and temperatures, offsetting
the high rate of decomposition of indium nitride.
4
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