Local distribution of deep centers in GaP studied by infrared
cathodoluminescence
F. Domlnguez-Adame, J. Piqueras, and P. Fernhdez
Departamento de fisica de Materiales, Facultad de Ciencias Fisicas, Universidad Complutense,
28040 Madrid, Spain
(Received 15 June 1990; accepted for publication 23 October 1990)
Near-infrared cathodoluminescence (CL) in the scanning electron microscope has been used
to characterize GaP:S. Spectra of as-grown crystals show a broadband at about 1240
nm, probably related to Pea antisite defects. This emission has been found to be higher at
dislocations giving a CL image opposite to the visible CL image.
Due to the use of GaP in the field of light-emitting
diodes, its visible luminescence, with bands in the green
and red spectral regions, has been often investigated. One
of the techniques used was cathodoluminescence (CL) in
the scanning electron microscope which provides informa-
tion about the spatial distribution of the recombination
centers. On the other side deep recombination centers in
Gap, emitting in the near infrared (NIR) have been stud-
ied by photoluminescence and optical detected magnetic
resonance (OMDR) 1-4
and several models to explain the
appearance of luminescence bands in the range 1000-1300
nm have been given. As in the case of visible luminescence,
space-resolved CL techniques could give additional infor-
mation on the defects involved in the NIR emission. How-
ever, to our knowledge, such techniques have not been
applied to the study of deep centers in Gap. In the present
work NIR CL in the scanning electron microscope has
been used to study the nature and distribution of radiative
centers in liquid-encapsulated Czochralski (LEC) GaP:S.
The samples used were cut from a (100) oriented
S-doped LEC GaP wafer with a free-carrier concentration
n of 3-4X10” cmp3. The samples were observed
in the emissive and CL modes in a Hitachi S-2500 or a
Cambridge S4-10 scanning electron microscope at an
accelerating voltage of 30 keV and beam currents of 10 - ‘-
10 - 6 A. For the obtention of panchromatic NIR CL im-
ages an optical lens was used to concentrate the light on a
cooled North Coast EO-817 germanium detector attached
to a window of the microscope. In some cases CL images
for wavelengths above 1000 nm were recorded by adapting
a cut-on optical filter at the detector entrance. To record
spectra a light guide feeding the light to an Oriel 78215
computer-controlled monochromator was used. In cases of
low signals, spectra representing the average of a high
number of measurements are readily obtained. The spectra,
covering the range 800-l 800 nm, were corrected to include
the system spectral response. Due to the low emission at
room temperature, all CL measurements were carried out
at 170 K. Besides as-grown samples, samples which had
received the following treatments were investigated: (a)
annealing in argon atmosphere at 1000 K for times ranging
from 1 to 5 h, (b) irradiation with 2.8 MeV electrons to a
dose of 8X lOI e- cm-‘,
(c) electron irradiation, as in
(b), followed by annealings at 700 and 930 K for 1 h.
Visible CL of the same samples used in this work has been
previously investigated.5-7
Figure 1 (a) shows the CL spectrum of an as-grown
sample with a broad (full width at half maximum of 360
nm) band centered at about 1240 nm ( 1.0 eV). This spec-
trum has been recorded under the normal observation con-
ditions of the scanning electron microscopy, with the elec-
tron beam focused on the sample. With a defocused beam
the spectrum of Fig. 1 (b) showing the tail of an intense red
band and a composite broad IR band with a peak at about
1320 nm (0.94 eV) is recorded.
The 1240 nm band (obtained with focused beam) is
strongly reduced by annealing as-grown samples above 830
K. Annealing the as-grown samples at 1000 K for 1 h
induces only minor changes in the spectra obtained with a
defocused beam. The spectra show a small emission peak at
1100 nm ( 1.13 eV) that increases with annealing time.
After 5 h annealing at 1000 K, the spectrum of Fig. 2 is
obtained. No spectra could be recorded with a focused
beam. In the electron-irradiated samples, the NIR lumi-
nescence appears to be quenched, as previously reported
for the visible range.6 Partial recovery of the emission is
observed after annealing at 700 K and luminescence fur-
ther increases by annealing at 920 K for 1 h. The spectrum
recorded after this treatment is shown in Fig. 3.
As described above, only the spectra of as-grown sam-
ples could be recorded when a focused beam is used. In all
other cases the beam has to be defocused to get enough
emission to record spectra. Consequently, NIR CL images
were obtained only from the as-grown crystals. Figure
600 1000
1400
“aveld:gcn (nm)
1600
FIG. 1. NIR CL spectra from as-grown samples with (a) focused elec-
tron beam and (b) defocused beam.
257
Appl. Phys. Lett. 58 (3), 21 January 1991
0003-6951/91/030257-03$02.00 @ 1991 American Institute of Physics
257
Copyright ©2001. All Rights Reserved.
800
1000 1200
1400
1600
Wavelength (nm)
FIG. 2. NIR CL spectrum (defocused beam) of a sample annealed at
1000 K for 5 h.
4(a) shows a representative CL image obtained by using
an optical filter transmitting wavelengths above 1000 nm.
The corresponding image of visible CL with dot and halo
contrast associated with the presence of dislocations5 is
shown in Fig. 4(b), Comparison of both figures reveals
that contrast in the NIR CL image is about inverse to that
of the visible CL. Most of the dark dislocation points in
Fig. 4(b) appear bright in Fig. 4(a).
An infrared band very close to the 1240 nm band ob-
served in this work in as-grown crystals has been previ-
ously reported. Killoran et
al.’
concluded from their
ODMR experiments that Pea antisite takes part in an elec-
tron capture process which competes with the visible pho-
toluminescence emission. In particular, they assign emis-
sions at 1130 nm (1.1 eV) and 1275 nm (0.97 eV) to
different recombination processes involving the PC, an-
tisite. Yang
et aL2
have also detected the 1275 nm emission
and have related it to the presence of PGa. We suggest that
the NIR band of our as-grown crystals is the antisite-
related band of Refs. 1 and 2. Since in these crystals only
one band is observed in the NIR region, the CL image
would provide information about the PGa defect distribu-
tion in the sample, in particular around dislocations. The
CL images indicate that the NIR emitting centers concen-
trate near the dislocation core and other crystal regions,
leaving a denuded zone around dislocations.
The
fact that
the 1240 nm band disappears by annealing would be a
consequence of the annealing out of antisite defects. In
GaAs, thermal annealing of AsGa has been found to start at
780 K,* which is about the temperature causing a signifi-
cant reduction of the 1240 nm band in this work. On the
other hand, annealing the as-grown samples causes the ap-
pearance of the 1100 nm band shown in
Fig. 2, Since such
4
800
1000
1200
14ou
1600
Wavelength (nm)
FIG. 3. NIR CL spectrum (defocused beam) of a sample irradiated and
annealed at 925 K.
258
Appl. Phys. Lett.. Vol. 58, No. 3, 2‘1 January 1991
04
FIG. 4. (a) NIR (wavelength above 1000 nm) and (b) visible CL images
of the same area of an as-grown sample. Arrows indicate two emergence
points of dislocations.
thermal treatment causes a selective loss of phosphorus,’
vacancies in the P sublattice would be involved in the
emerging band. In F!ef. 2, VP have been also related to a
1100 nm luminescence band.
The quenching of NIR-CL by high-energy electron ir-
radiation has been also found in the visible spectral rangeh6
An
irradiation experiment was aimed to check if radiation-
induced defects act as recombination centers in the infra-
red region quenching the visible luminescence. The results
indicate that irradiation creates mainly nonradiative re-
combination centers quenching both the visible and NIR
luminescence.
This work was supported by the Comisibn Interminis-
terial de Ciencia y Tecnologia (Project PB86-015 1) . The
authors thank Wacker-Chemitronic (Dr. K. Liihnert) for
providing the samples and Dr. P. Moser (C.E.N.G.
Grenoble) for the electron irradiation.
Dominguez-Adame, Piqueras, and Ferndndez
258
Copyright ©2001. All Rights Reserved.
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Appl. Phys. Lett., Vol. 58, No. 3, 21 January 1991
Dominguez-Adame, Piqueras, and Ferngndez
259
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