Terahertz investigation of high quality indium nitride epitaxial layers

Article abstract of DOI:10.1002/pssa.200460434

We report on the optical characterization of InN layers in the THz range and magnetic fields up to 13 T. The results are interpreted using the dielectric function formalism, with contributions of cyclotron resonance, phonons, plasmons and helicon wave exc

Full text of DOI:10.1002/pssa.200460434

phys. stat. sol. (a) 202, No. 4, 590–592 (2005) / DOI 10.1002/pssa.200460434
Terahertz investigation
of high quality indium nitride epitaxial layers
Y. M. Meziani*, 1, B. Maleyre1, M. L. Sadowski2, S. Ruffenach1, O. Briot1, and W. Knap1
1
Université Montpellier II, Groupe d’Etude des Semiconducteurs. CC 074, Place Eugéne Bataillon,
34095 Montpellier, France
2
Grenoble High Magnetic Field Laboratory – CNRS 25, av des Martyrs 38042 Grenoble, France
Received 28 May 2004, revised 10 July 2004, accepted 1 September 2004
Published online 1 March 2005
PACS 72.30.+q, 76.40.+b, 78.20.Ls, 78.30.Fs
We report on the optical characterization of InN layers in the THz range and magnetic fields up to 13 T.
The results are interpreted using the dielectric function formalism, with contributions of cyclotron reso-
nance, phonons, plasmons and helicon wave excitations. We show how THz radiation transmission meas-
urements can provide an optical contactless method of determining the quality (carrier density and mo-
mentum scattering rate) in the InN layers.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
InN is of great potential interest for the realization of hyperfrequency/terahertz devices since it has been
theoretically established that its electron velocity peaks at a higher value than GaAs and GaN, under rather
moderate electric fields. Interest in this material has been recently renewed by the discovery that its band
gap was more probably around 0.7 eV [1, 2] than close to 1.9 eV, as earlier studies reported [3]. It appears
now that this material has a huge potential for optoelectronic devices, where a band gap around 0.7 eV
would allow for applications at the telecommunications wavelength of 1.55 µm, but also in hyperfre-
quency devices, since calculations have predicted a hot electron velocity peak at a much higher value than
in GaN and GaAs [4, 5]. In the present work, we investigate transmission and reflectivity of Indium Ni-
tride (InN) in THz range. Transmission measurement at low temperature (2 K) and high magnetic fields
(up to 13 T) was interpreted in the formalism of complex Dynamic Dielectric Function (DDF).
2 Results
The growth of InN layers is extremely difficult, due to both a low dissociation temperature (around
700 °C) and the lack of a matched substrate. A lot of progress has been made by Molecular Beam Epi-
taxy [1, 2], but recent advances in Metal organic Vapor Phase Epitaxy (MOVPE) [6] have allowed us to
grow mirror-like layers on 2 inches diameter sapphire substrates, with a reproducible mobility of
800 cm2/V.s, the highest value obtained by MOVPE up to date. The growth is realized under nitrogen
carrier gas, using Trimethyl Indium (TMIn) and ammonia (NH3) as precursors. We found that the use of
a low V/III molar ratio of 5000 is essential for the obtention of high quality material, and the growth
pressure is 200 mBar. The growth temperature is 550 °C. The layers were characterized by Atomic Force
Microscopy (AFM), and they exhibit a two dimensional growth mode, with a surface constituted of bi-
monolayer steps, resulting in a rms roughness of 0.6 nm. Table 1 present characteristics of our samples.
* Corresponding author: e-mail: yahya.meziani@ges.univ-montp2.fr, Phone: +33 467 143 217, Fax: +33 467 143 791
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
Paper
phys. stat. sol. (a) 202, No. 4 (2005) / www.pss-a.com
Table 1 Characteristics of samples.
591
samples
structure
377
390
InN(0.732)/GaN(0.063)/sapphire
InN(0.535) doped Si(7.1 × 10–2 µmol/min)/InN/sapphire
The measurements were performed using a Bruker 113V FIR Fourier transform spectrometer
(1–50 THz) connected via an outcoupling optic to a superconducting 13 T magnet. All measurements
were done in the Faraday geometry with unpolarized light, using a bolometer detector mounted close to
the sample.
Figure 1 present reflectivity spectra of all samples at ambient temperature and reflectivity for sample
377 at two temperatures T = 290 K and T = 20 K. It shows the existence of a plasma edge and invariance
of reflectivity as a function of temperature.
Obtained transmission data for sample 377 at two values of magnetic field are shown in Fig. 2. At low
energy, we observe increase of transmission as function of magnetic field.
3 Analysis and discussion
The far infrared transmission was analysed using a complex dynamic dielectric function in the form:
2
p
2
TO
2
2
(κ0 −κ∞ ) (ω −ω )
2
ω
κ∞ω
ω
(κ0 −κ∞ ) γω
−
2
TO
TO
ε(ω) = κ∞ +
+ i
.
ω(ω ω + iΓ )
C
(1)
2
2
2
2
(ω −ω ) + γ ω
2
2
2
2
2
(ω −ω ) + γ ω
TO
TO
Here κ0 and κ∞ are the static and optical dielectric constants, ωTO is the frequency of the transverse optical
phonon, γ is the phonon damping parameter, ωp is the plasma frequency, ωC is the cyclotron resonance
frequency and Г is the inverse of the electronic scattering time. The second and third term are the real
and imaginary parts of the lattice dielectric function, while the fourth corresponds to electronic contribu-
tions. The plus/minus sign depends on the sign of the circular polarisation of the incident radiation, giv-
ing the responses of the cyclotron resonance inactive (CRI) and cyclotron resonance active (CRA) po-
larisations, respectively. For a review of this formalism see e.g. [7]. The transmission is then calculated
in terms of plane waves
iη(ω ) x
2
where ε(ω) = |η(ω)| .
Ψ = e
,
The changes in transmission caused by the magnetic field will be given by the last term of Eq. (1),
where the magnetic field enters through the cyclotron resonance frequency ωC = eB/m0m*, where e is
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Sample 377 (T=290K)
Sample 390 (T=290K)
Sample 377 (T=20K)
2.0
1.8
1.6
1.4
1.2
1.0
B = 13 T
B = 5.5 T
Sample 377
T = 2 K
1000
2000
3000
4000
5000
100
150
200
250
300
Wave Number (cm-1)
Wave Number (cm-1)
Fig. 2 (online colour at: www.pss-a.com) THz
spectroscopic measurement performed on sample 377
at T = 2 K and for different magnetic field values.
Fig. 1 Reflectivity measurements performed on all sam-
ples at T = 290 K and B = 0 T.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

592
Y. M. Meziani et al.: Terahertz investigation of high quality InN epitaxial layers
2,0
1,8
1,6
1,4
1,2
1,0
Fig. 3 (online colour at: www.pss-a.com) Theo-
7 T
13 T
retical results of transmission for sample 377 at
T = 2 K and for two values of magnetic fields. The
results were obtained by using DDF formalism.
100
150
200
250
300
Wavenumber (cm-1)
the electron charge, B is the magnetic field, and m0m* is the electron effective mass. Figure 3 shows
simulated changes of transmission with magnetic field, T(B)/T(0) as function of the measured values
(Fig. 2). Thelow energy transmission of the sample is seen to increase when a magnetic field is applied;
this effect is well reproduced in the simulation. Given the metallic character of the sample (carrier con-
centration n ~1019 cm–3, plasma frequency ωp ~ 1000 cm–1), it is expected to be opaque to electromagnetic
radiation for frequencies ω such that ωp > ω > ωC. When a magnetic field is applied, however, we ap-
proach the so-called helicon limit:
2
p
ω
ꢀ ω ꢀ ω
C
ω
where CRA radiation may propagate. Such helicon waves are well known from studies of metals as well
as strongly doped semiconductors (e.g. [8]). The simulations shown in Fig. 1 were performed with the
following sample parameters: m* = 0.15, ωTO = 460 cm–1, κ0 = 15.3, κ∞ = 8.4, ωp = 951 cm–1, γ = 20 cm–1
and Г = 60 cm–1. The large values of the damping parameters γ and Г make it difficult to use the present
measurements to determine sample parameters such as the effective mass.
4 Conclusion
Results of transport and optical experiments performed in a wide temperature range (2–300 K) show that
the InN layers have a metallic character with carrier density between 5 × 1018 cm–3 –5 × 1019 cm–3 that
stays constant even at lowest temperatures and highest magnetic fields. In the THz experiments, we ob-
serve an increasing of transmission as function of magnetic field. This effect is well reproduced in the
simulation using complex dynamic dielectric function. We have demonstrated the importance of helicon
waves in InN.
References
[1] V. Yu. Davydov, A. A. Klochikhin, R. P. Seisyan, V. V. Emtsev, S. V. Ivanov, F. Bechstedt, J. Furthmüller,
H. Harima, A. V. Mudryi, J. Aderhold, O. Semchinova, and J. Graul, phys. stat. sol. (b) 229, R1 (2002); phys.
stat. sol. (b) 230, R4 (2002).
[2] J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager, E. E. Haller, Hai Lu, William J. Schaff, Yoshiki Saito, and
Yasushi Nanishi, Appl. Phys. Lett. 80, 3967 (2002).
[3] T. Tansley and C. P. Fowley, J. Appl. Phys. 59, 3241 (1986).
[4] B. E. Foutz, S. K. O’Leary, M. S. Shur, and L. F. Eastman, J. Appl. Phys. 85, 7727 (1999).
[5] C. G. Rodrigues, V. N. Freire, A. R. Vasconcellos, and R. Luzzi, Appl. Phys. Lett. 76, 1893 (2000).
[6] B. Maleyre, S. Ruffenach, O. Briot, and B. Gil, to be published in J. Cryst. Growth.
[7] E. D. Palik and J. K. Furdyna, Rep. Prog. Phys. 33, 1193 (1970).
[8] M. Hausenblas, A. M. Witowski, A. Wittlin, A. Mycielski, and P. Wyder, Semicond. Sci. Technol. 9, 77
(1994).
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim