Possible lifetime-limiting defect in 6H SiC
N. T. Son,
a)
E. So
¨
rman, W. M. Chen, O. Kordina, B. Monemar, and E. Janze
´
n
Department of Physics and Measurement Technology, Linko
¨
ping University, S-581 83 Linko
¨
ping, Sweden
͑Received 14 June 1994; accepted for publication 19 September 1994͒
We reveal and investigate a possible lifetime-limiting defect in as-grown 6H SiC by optical
detection of magnetic resonance ͑ODMR͒. This defect is shown to be a deep level center ͑with an
energy level at about E
c
Ϫ1.1 eV͒, evident from the related deep photoluminescence emission and
a photo-excitation spectrum of the ODMR signal. The fact that this defect has been observed in both
bulk crystals and epilayers, regardless of their doping type, indicates that this must be a common
and basic defect in 6H SiC. © 1994 American Institute of Physics.
SiC has recently attracted much attention as one of the
most promising materials in applications for high-power,
high-temperature, and high-frequency devices.
1
The progress
in applications of SiC-based bipolar high-power devices has,
however, so far been severely hindered by the rather short
carrier lifetime in the material ͓typically shorter than 50 ns
͑Ref. 2͔͒. This is true even in areas free of crystalline imper-
fections due to extended defects such as micropipes, and is
generally believed to be due to the presence of point defects
in the material. Up to now very little is known about deep
level defects in SiC, which play important roles in carrier
recombination. Among them, the dominant recombination
center ͑i.e., the lifetime-limiting defect͒, despite its essential
importance, is still unknown. This is mainly due to the lack
of suitable experimental techniques which not only are sen-
sitive to carrier recombination but also provide detailed in-
formation about the electronic and geometrical structure of
the defects.
Optical detection of magnetic resonance ͑ODMR͒ has in
the past decade proven to be a very powerful technique,
3
which combines highly sensitive optical spectroscopy with
microscopically informative magnetic-resonance techniques.
In this work we have carried out a systematic and compre-
hensive ODMR study of 6H SiC, in an attempt to reveal and
investigate the lifetime-limiting defect in the material. The
reason why ODMR is a suitable technique in studying this
defect is that the radiative carrier recombination processes
are directly monitored in the experiments. A magnetic-
resonance enhanced carrier recombination via the dominant
recombination center will result in a corresponding decrease
in carrier recombination via other channels.
4
In other words,
photoluminescence ͑PL͒ emission directly related to the
dominant recombination channel, if it is partly radiative, will
increase upon the magnetic-resonance condition. Other PL
emissions not related to the dominant recombination center
will consequently decrease in intensity.
The samples studied include 6H bulk SiC crystals grown
by the modified Lely method and epilayers grown by chemi-
cal vapor deposition ͑CVD͒. They are either n-orp-type
doped. The carrier recombination processes are monitored
here by photoluminescence emissions from the materials.
The ODMR experiments were performed with the aid of a
modified Bruker ER-200D X-band ͑9.23-GHz͒ electron spin
resonance ͑ESR͒ spectrometer, equipped with a TE
011
micro-
wave cavity with optical access in all directions. The sample
temperature could be varied from room temperature down to
4 K in an Oxford-Instruments cryostat. The UV multilines
͑351.1–363.8 nm͒ of an Ar
ϩ
-ion laser was used as the optical
excitation source. PL emissions from the samples were first
filtered by proper optical filters or dispersed by a Jobin–
Yvon 0.25-m grating monochromator for spectral studies,
and were then collected by a GaAs photomultiplier ͑for vis-
ible PL emissions͒ or a cooled Ge detector ͑for near-infrared
PL emissions͒. They were finally synchronously detected
with a lock-in amplifier in phase with the amplitude-
modulated microwave field. The ODMR signal so obtained
was recorded in a computer for further analysis. In photo-
excitation experiments, a tunable Ti:sapphire laser was used
as a second excitation source.
In all these samples studied, an ODMR signal at around
Bϭ0.33 T at X-band can be observed via all the PL emis-
sions from the samples. As an example we show in Fig. 1
such an ODMR spectrum from Al-doped p-type 6H SiC
͑with the concentration of Al and residual N both in the order
of 10
17
cm
Ϫ3
͒. The ODMR signal arises from an unpaired
electron spin Sϭ1/2 of a paramagnetic defect. The resonance
line is rather broad ͑about 20 G͒ and probably contains un-
resolved hyperfine structure and unresolved resonances from
different inequivalent lattice sites of the defect. Conse-
quently, no definite conclusion could be reached on the sym-
metry and the chemical identity of the defect, from an angu-
lar dependence study of the ODMR signal. The averaged g
value is deduced to be 2.01Ϯ0.01 from an analysis of the
experimental data by a spin-1/2 spin Hamiltonian
Hϭ
B
BgS.
5
Here, S denotes the effective electronic spin of
the defect, and B is the external static magnetic field applied.
B
is the Bohr magneton.
The spectral dependence study shows that this ODMR
signal corresponds to an enhancement in the intensity of the
PL band peaking at about 1.68 eV and a decrease in the
intensity of other PL emissions ranging from band edge shal-
low bound excitons to near-infrared deep PL bands. In Fig. 2
we show, as an example, such spectral dependence spectra of
the ODMR signal from the same Al-doped p-type 6H SiC as
shown in Fig. 1. It can clearly be seen that the shallow N
donor–Al acceptor pair recombination is decreased at reso-
nance, accompanied by a corresponding increase in the
a͒
Permanent address: Department of Physics, University of Hanoi, 90
Nguyen Trai, Hanoi, Vietnam.
2687Appl. Phys. Lett. 65 (21), 21 November 1994 0003-6951/94/65(21)/2687/3/$6.00 © 1994 American Institute of Physics