Origin of n type conductivity in wide gap semiconductors
studied by SR
K. Shimomuraa, R. Kadono a, K. Nishiyama a, I. Watanabe b, T. Suzuki b, F. Pratt c
K. Ohishi d, M. Mizuta e, M. Saito f, K. H. Chow g, B. Hitti h, R.L. Lichti i,
a
Muon Science Laboratory, High Energy Accelerator Research Organization (KEK)
Oho 1-1, Tsukuba, Ibaraki, 305-0801 Japan
b
Advanced Meson Science Laboratory, RIKEN, Saitama 351-0198, Japan
c
Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, U.K
d
Advanced Science Research Center, JAERI, Ibaragi 319-1195, Japan.
e
Nippon EMC LTD., Tokyo, 206-0001, Japan.
f
Graduate School of Natural Science and Technology, Kanazawa Univ., Ishikawa, 920-1192, Japan
g
Department of Physics, University of Alberta, Edmonton, Alberta, T6G 2J1, Canada
h
TRIUMF, Vancouver, V6T 2A3, Canada
Origin of n type conductivity in wide gap semiconductors is an important issue for the device
applications and still under serious discussions. Recently hydrogen is thought to be one of the
most interesting candidates to solve this problem. Progresses of the studies of isolated hydrogen
center in ZnO and GaN and by muon spin rotation methods are reported.
1.
Introduction
Hydrogen is a ubiquitous impurity in most
semiconductors, including elemental (e.g., Si),
compound (e.g., GaAs). In these systems, hydrogen
is known to be forming an acceptor level in n-type
and a donor level in p-type materials. In contrast,
hydrogen can lead to electron conduction in some
wide gap semiconductors such as ZnO [1,2]. These
observations raise the question of what is the basic
systematic at work here: if H can be incorporated
into some materials, which one will be doped by H
(i.e., become conductive) and which will not?
Currently considerable theoretical studies were
performed in this subject [3,4,5]. Muonium Spin
Rotation Methods is now considered to be one of the
most powerful tools to experimentally investigate
these theoretical predictions. The muonium center
(Mu; an analog of isolated hydrogen whose proton is
substituted by a positive muon) is readily observed
in a wide variety of semiconductors after positive
muon implantation, and has been serving as a unique
source of information on the electronic structure of
isolated hydrogen centers. While the dynamical
aspect (e.g., diffusion property) may be considerably
different between Mu and H due to the light mass of
Mu ( ~1/9), the local electronic structure of Mu is
virtually equivalent to that of H after a small
correction due to the difference in the reduced mass
( ~0.4%). In this contribution, our recent results of
isolated hydrogen center in several semiconductors
by muon spin rotation methods are reported.
2.
Radio Frequncy SR in ZnO
In ZnO, we have reported in the previous paper
[6], two species of Mu centers with extremely small
hyperfine (HF) parameters have been observed
below 40 K. Both Mu centers have an axialsymmetric HF structure along with the c axis,
indicating that they are located at the antibonding
and bond-center sites. It is inferred from their small
ionization energy (~6 and 50 meV) and HF
parameters (~10-4 times the vacuum value) that these
centers behave as shallow donors, strongly
suggesting that hydrogen is one of the primary
origins of n type conductivity in as-grown ZnO,
which is predicted by the theoretical studies.
However, the other group reported only a single
species of muonium existence in ZnO [7, 8]. For the
further studies of the existence of the second species,
we preformed muon spin resonance experiments,
which is similar to the normal NMR methods.
Because the weak radio frequency (RF) field (~1G)
measurements might achieve the better frequency
resolution to distinguish the signals from these
muoniums.
Fig. 1.
Diamagnetic muon and muonium
resonance sprectra in ZnO at 2.0 K with 50 MHz
of radio-frequency field and 0.35T of static field
parallel to [11-20] axis.
Fig.1 shows our preliminary results of RF
measurements in ZnO, where, 50.000 MHz RF and
longitudinal
magnetic
field
were
applied
perpendicular and parallel to [11-20] axis,
respectively. Center peak is corresponding to the
diamagnetic muon resonance. One satellite, which is
symmetrically located around the center peak, is
corresponding to the shallow muonium resonance.
Because 370 mT already corresponds to the highfield or Paschen–Back limit in which the muon–
electron hyperfine interaction is effectively
decoupled. The deduced hyperfine parameter at this
orientation is determined to 360 kHz. This value is
fairly consistent with the parameter of the muonium
which ionization energy is 6 mV in the previous
report [6].
Unfortunately, the preliminary analysis cannot
distinguish two species of muonium, might be due to
the inhomogenity of the applied static field. Further
improvements are required.
3.
Transverse Field SR in GaN
More interestingly, a shallow Mu center has been
discovered in GaN, while no such state has been
predicted in the latest theoretical studies. Above 22.5
K, only a single (diamagnetic) precession signal is
observed at the muon Larmor frequency The muon
spin rotation signal changes drastically below 22.5
K, which strongly suggests the shallow muonium is
formed. Figure 2 shows the angular and temperature
dependences of the frequency spectra obtained by
Fourier transform, in which one pair of satellite lines
is clearly seen with their positions situated
symmetrically around the central line, which
corresponds to the precession of diamagnetic muons.
The splitting of these satellites remained unchanged
when the applied field was increased from 1.5 up to
5 T [see Figs. 2(a) and 2(c)], a result that is
important in identifying the spectra as due to the
hyperfine interaction of a Mu0 center. The splitting
decreases when the [0001] axis is tilted with respect
to the external magnetic field as in Fig. 2(d).
Moreover, an equivalent frequency spectrum was
observed when the [11-20] axis was rotated by 90
degree around [0001], which was oriented at 35
dgree to the applied magnetic field. These
observations demonstrate the presence of a
paramagnetic muonium state in GaN. The resulting
hyperfine interaction is extremely small, about 10-4
times the vacuum value for a Mu atom, and is
axially symmetric with respect to [0001].
Due to the muonium electron polarization, the
satellite peak has an asymmetric feature, which is
shown in Fig.2 (a) and 2 (c). These results inform us
Fig. 2 Frequency spectra obtained for GaN at (a)
2.5K and (b)22.5K with 3.0T parallel to [0001]
axis, and with 1.5T at 2.5K, where [0001] is
parallel to B (c) or tiltrd by 35 degree from B (d).
the hyperfine constant is the positive for the
displayed orientations. For the detailed analysis, the
parallel and perpendicular component of the
hyperfine tensor is deduced to be +337(10) kHz and
–243(30) kHz, respectively and their ionization
energy is less than 14 meV [9].
The observed Mu center has an extremely small HF
parameter with an axially symmetric HF structure,
suggesting that it is located either near a nitrogenantibonding or a bond-centered site oriented parallel
to the c axis. Its small ionization energy (<14 meV)
and small HF parameter (~10-4 times the vacuum
value) indicate again that a shallow electronic state
is associated with the Mu center, suggesting the
possibility that the analogous hydrogen center could
be a source of n-type conductivity in as-grown GaN.
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