CHARACTERISATION OF GAP1-XNX LAYERS BY RAMAN SPECTROSCOPY
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APCOM 2003, 9th International Workshop on Applied Physics of Condensed Matter, June 11-13,
2003, Mala Lucivna, Slovak Republic
Characterisation of GaP1-xNx layers by Raman spectroscopy
J. Kovac 1, R. Srnanek1, L. Peternai1, M. Kadlecikova1 V. Gottschalch2, J. Wagner3,
J. Geurts3, G. Irmer4
(1) Microelectronic Department, Faculty of Electrical Engineering and Information
Technology, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia,
E-mail: jkovac@elf.stuba.sk
(2) Faculty of Chemistry and Mineralogy, University of Leipzig, D-04103 Leipzig, Germany
(3) Physikalisches Institut, University of Würzburg, D-97074 Würzburg, Germany
(4) Technische Universität, Bergakademie Freiberg, D- 09596 Freiberg, Germany
Introduction
Recently GaP1-xNx alloys have attracted considerable interest as candidates
for the realisation of devices emitting light (LED’s) in green-red range of the visible
spectrum. GaP:N has not only a large doping range for the impurity limit (1015–1020 cm-3) but
also a relatively high-critical concentration for the formation of the impurity band, which
makes it a unique system for investigating the impurity band effects that results from heavy
isoelectronic doping. On the other hand, a host semiconductor with isoelectronic impurities
is typically viewed as an alloy. Incorporation of only a few percent of N in GaNxP1-x alloys
leads to direct band-gap behavior due to the strong interaction among N-related bound states,
which exhibit a quasi-direct nature in the optical transition, and produces strong
photoluminescence red emission at room temperature and many other interesting
phenomena [1]. The basic information about the structural properties of such alloy system,
can be obtained by Raman scattering spectroscopy [2]. In this paper the properties
of GaP1-xNx layers grown by MO VPE are investigated by Raman spectroscopy using He-Ne
and Ar+ ion laser for excitation.
Experimental
The GaNxP 1-x layers were grown on GaP substrates at growth temperature 650 °C
using low-pressure metal-organic-vapor–phase epitaxy (LP MO VPE) at University
of Leipzig. Trimethylgallium, phosphine and dimethylhydrazine were used as precursors.
The investigated samples had following structure: N+ doped GaP (100) substrate, 100 nm
thick GaP undoped buffer layer and 360 nm thick undoped GaP1-xNx layer. The contentAPCOM 2003, 9th International Workshop on Applied Physics of Condensed Matter, June 11-13,
2003, Mala Lucivna, Slovak Republic
of nitrogen in GaNxP 1-x alloy varied in the range of 0.61 to 2.3 % for measured samples [3].
For investigations of GaNxP 1-x layers properties the micro-Raman spectroscopy in back-
scattering method was used with polarisation of the light parallel to [011] direction.
The excitation was performed by using He-Ne laser (633nm line) at Microelectronics Dept.,
STU Bratislava and Ar+ ion (514nm line) laser at Physikalisches Institut, University
of Würzburg. The diameter of laser spot at the sample surface was adjusted for all
measurements between 1- 4 µm.
Results and discussion.
Figure 1a shows typical Raman spectra of GaP substrate and GaNXP1-X alloys with
different nitrogen concentration by using He-Ne laser (633nm line) excitation. Optical
GaP-like phonons spectrum, which are marked as LO (longitudinal optic) and TO
(transverse optic) peaks in the Raman shift range from 300 to 600cm-1 have been investigated.
30000
LO1 a LO1 2.3 % N
Raman intensity (a. u.)
Raman intensity (a.u)
30000
GaP
20000
X GaNP
20000
10000 TO1
10000
2.3 % N TO1 X
1.35 % N
0
substrate
0
300 400 500 600 350 360 370 380 390 400 410 420 430 440 450
-1 -1
Raman shift ( cm ) Raman shift (cm )
Fig.1a/ Raman spectra of GaP substrate and GaNXP1-X structures with different
N concentration b/ detail Raman spectrum of the structure containing 2.3 % N
The dominant LO1 at around 400 cm-1 and TO1 phonon peaks at 368 cm-1 are clearly
resolved. The shifted zero level of Raman spectra corresponds to the increased
photoluminescence (PL) intensity due to the band gap reduction of GaNXP1-X alloy with
increased N content. In the same time for GaNXP1-X with x value higher than about 1.0 %
an additional Raman mode peak near 388 cm-1 (labelled as X) can be detected similarly
as found in [2]. This peak is connected with N - induced disorder in GaNXP1-X layer due to,
e.g. clustering of N atoms and the amplitude increase with N concentration. After detailed
analysis of LO1 phonon peak of GaP a second peak from GaNXP1-X layer (red shifted) appears
as shown in Fig.1b. This effect could be explained by the low absorption coefficientAPCOM 2003, 9th International Workshop on Applied Physics of Condensed Matter, June 11-13,
2003, Mala Lucivna, Slovak Republic
of the He-Ne laser light in GaNXP1-X layer and strong resonance effect of He-Ne light energy
(1.94 eV) with energy gap of GaNXP1-X layer. The approximated linear dependence
of LO1 phonon peak frequency shift from GaNXP1-X layers is shown in Fig.2a with evaluated
decreasing tendency of 1.3 cm–1 /(x) %. This decreasing is a little higher than that
(1.0 cm-1 /(x) % ) measured by He-Cd laser [2]. Similarly the dependence of PL intensity
increasing with increased N concentration is drawn on Fig.2b. This behaviour is connected
406
a 0.20
b
PL intensity ( a.u.)
405 0.16
LO1 shift ( cm )
-1
404 0.12
0.08
403
0.04
402
0.00
0.0 0.4 0.8 1.2 1.6 2.0 2.4 0.0 0.4 0.8 1.2 1.6 2.0 2.4
N concentration ( % ) N concentration ( % )
with decreasing value of the GaNXP1-X energy gap for higher N content [4].
Fig. 2a/ Dependence of LO1 phonon peak shift and b/photoluminescence intensity increasing
for different N concentrations of GaNXP1-X alloy
For comparison the Raman measurements were performed by using Ar+ laser
excitation and the Raman spectrum of GaNXP1-X sample with 2.3 % N is shown in Fig.3a.
The main difference between this spectrum and those presented before (He-Ne excitation)
is a lower X band intensity and failure second peak from GaNXP1-X layer on the phonon peak
(LO1) of GaP. This was an unexpected result, because the light of Ar+ ion laser is more
3000
404,68
2.3 % N 2.3 % N TO1 386,94 LO1
80000
LO1
PARALLEL
Raman intensity (a.u.)
Raman intensity (a.u.)
2400 [010]
X
40000
1800
TO1
365,42
1200
X
0
a b
320 340 360 380 400 420 440 320 340 360 380 400 420 440
-1 -1
Raman shift (cm ) Raman shift ( cm )APCOM 2003, 9th International Workshop on Applied Physics of Condensed Matter, June 11-13,
2003, Mala Lucivna, Slovak Republic
Fig. 3a/ Raman spectra measured by Ar+ ion laser excitation with polarization in [011]
direction and b/ [010] direction
absorbed in GaNXP1-X layer than that one of He-Ne laser and therefore one wait band X
of higher intensity. These discrepancies are possible to explain by high resonance effect
of He-Ne light energy (1.94 eV) with energy gap of GaNXP1-X layer near composition
of 2.0 % N and therefore higher phonon intensities [5]. The increasing of X band peak
intensity was possible by the changing of sample orientation. When the laser light was
polarised in [010] direction the LO modes are not allowed due to selection rules and
the X band can be better resolved as shown in Fig. 3b. LO2 and TO2 phonon modes were
detected only by He-Ne excitation, but their intensities were very low. By Ar+ laser excitation
these modes were not possible to detect. This is caused by very low absorption in GaNXP1-X
layer and therefore very low intensities of these phonon modes.
Conclusion
GaP1-xNx epitaxial layers were characterised by Raman spectroscopy by using two
excitation laser lines at 633nm and 514 nm. For the measured samples prepared by MO VPE
the obtained Raman spectra are comparable to those prepared by MBE growth [2]. It was
found that the energy of He-Ne laser which is very close to resonant conditions with GaP1-xNx
energy gap can be useful for analysis of GaP1-xNx layer properties because the intensities
of LO1 and band X peaks are higher than those obtained by using of Ar+ ion laser excitation.
The concentration of N in the GaNXP1-X alloys can be determined either from the LO1 peak
shift or from increasing of PL intensity signal superimposed on the Raman spectra by using
He-Ne laser.
Acknowledgements
This work was supported by the grant IST-2001-32793 VGF GAP-LED’s,
VEGA grant 1/0152/03, project of Germany/Slovakia co-operation SVK01/001 and
by NATO grant PST.CLG. 978729.
References
[1] J.N. Baillargeon, et al., Appl. Phys. Lett. 60 (1992) 2540
[2] I. A. Buyanova, et al., Appl. Phys. Lett. 78 (2001) 3959
[3] G.Leibiger, et al., Phys.Rev.B, 65 (2002) 245207APCOM 2003, 9th International Workshop on Applied Physics of Condensed Matter, June 11-13, 2003, Mala Lucivna, Slovak Republic [4] G. Yu. Rudko, et al., Solid - State Electronics 47 (2003) 493 [5] W. Shan, et. al., Appl. Phys. Lett. 76 (2000) 3251
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