Journal of Crystal Growth 227–228 (2001) 244–248
Oxygen-related deep level defects in solid-source
MBE grown GaInP
N. Xianga,*, A. Tukiainena, J. Dekkerb, J. Likonenc, M. Pessaa
a
Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland
b
Laboratory of Physics, Helsinki University of Technology, 02150 Espoo, Finland
c
Chemical Technology, Technical Research Centre of Finland, P.O. Box 1404, 02044 VTT, Finland
Abstract
We report the first observation of oxygen-related deep level defects in solid-source MBE-grown GaInP. Si-doped
GaInP samples were studied by deep level transient spectroscopy (DLTS), secondary-ion mass spectrometry (SIMS),
capacitance–voltage (C2V) profiles, and photoluminescence (PL). Different amounts of oxygen impurities were
introduced into GaInP epilayers by growing with different phosphorus cracking temperatures. Four traps were resolved
by DLTS from the GaInP samples. Among them, two traps, with thermal activation energies of 0.45–0.46 and 0.63–
0.82 eV, were found to be oxygen-related. # 2001 Elsevier Science B.V. All rights reserved.
PACS: 81.15.Hi; 71.55.Eq; 78.55.Cr
Keywords: A1. Characterization; A1. Defects; A1. Impurities; A3. Molecular beam epitaxy; B2. Semiconducting indium gallium
phosphide
1. Introduction
Epitaxial Ga0.5In0.5P (hereafter written as
GaInP), closely lattice matched to GaAs, has
applications in optoelectronic devices such as
semiconductor lasers, light emitting diodes and
solar cells. The presence of deep level defects in
GaInP can degrade device performance by creating non-radiative recombination centers and reducing the carrier lifetime [1]. Oxygen is a commonly
suspected residual impurity in epi-layers grown by
molecular beam epitaxy (MBE). It has been shown
that oxygen impurities in MBE-grown Si-doped
GaInP compensate silicon doping and degrade the
material optical properties [2]. Similar phenomenon was observed for MBE-grown InP [3]. Kwon
et al. studied the effect of oxygen on GaInP grown
by liquid phase epitaxy (LPE) [4] but did not find
oxygen-related deep traps. Oxygen-related deep
level defects have been reported in AlGaInP, InP,
GaInAs, and AlGaAs [5–9] but, to our knowledge,
not in MBE-grown GaInP. In this paper, we
report the first systematic study on oxygen-related
deep level defects in MBE-grown GaInP.
2. Experimental procedure
*Corresponding author. Tel.: +358 3 3652914; fax: +358 3
3653400.
E-mail address: [email protected].fi (N. Xiang).
Two sets of Si-doped GaInP samples were
grown on n+-GaAs substrates using solid-source
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 6 9 1 - 1
N. Xiang et al. / Journal of Crystal Growth 227–228 (2001) 244–248
MBE (SSMBE). Phosphorus was provided by a
three-zone valved cracker cell. Gallium, indium
and silicon were evaporated from the conventional
effusion cells. The thickness of GaInP epilayers
was 2 mm, the growth rate was kept at 1 mm/h, and
growth temperature for GaInP was (495 5)8C
(pyrometer reading). Samples in each set were
grown with same Si dopant cell temperature but
using different phosphorus cracking temperatures
(Tcr ). The incorporated oxygen and silicon impurity concentrations [NO] and [NSi], were determined by SIMS using ion-implanted standard
samples. The oxygen impurity is thought to
originate from phosphorus trioxide (P4O6) in the
phosphorus ingot [2]. More P4O6 may be cracked
at high Tcr leading to more [NO] in the epilayer.
The P2/III ratio was 13 (within 10% error) for
all the GaInP growths.
Deep level transient spectroscopy (DLTS) was
employed to detect the deep level defects. DLTS
samples were made by evaporating Ni/Au/Ge/Au
ohmic contacts at the bottom of n+-GaAs
substrates and Au–Schottky contacts on top of
the GaInP epilayers. The diodes showed good
current–voltage (I2V) characteristics with ideality
factors less than 1.2. Measurements were carried
out using a Bio Rad DL8000 DLTFS system at
2 V reverse bias. The pulse voltages were 0 V for
set 1 and 1 V for set 2, respectively. The pulse
duration was 0.1 s and the period width was 0.2 s.
Capacitance–voltage (C2V) profile was utilized
for checking the compensation. Photoluminescence (PL) and X-ray diffraction (XRD) were
used to characterize the optical and crystalline
properties of the grown samples.
245
3. Results and discussion
All the GaInP samples were mirror-like and
lattice-matched to GaAs within Da/a40.1%.
Table 1 lists [NO], [NSi], and net doping levels
[Nnet] (measured by C2V) in each set of the
samples grown with different Tcr . It can also be
seen in Table 1 that [Nnet] decreases when [NO]
increases, while [NSi] is not reduced. This may
indicate that oxygen can compensate Si doping in
GaInP. Room-temperature PL spectra from these
samples are shown in Fig. 1. In both sets, PL
intensity was found to decrease as [NO] was
increased. This can be an indication that oxygen
impurities have created non-radiative recombination centers in GaInP.
The DLTS spectra obtained from set 1 and 2 are
both shown in Fig. 2. Four deep levels, marked as
T1, T2, T3 and T4 are resolved. The Arrhenius
plots are shown in the insets. The evaluation
results are summarized in Table 2, where Ea is the
thermal activation energy and s the capture crosssection of a trap. [NT1], [NT2] and [NT3] are the
concentrations of traps T1, T2 and T3. Of the four
traps, T1 has been identified as either a DX center
caused by Si doping [1,10–13] or a native defect
caused by phosphorus vacancies [14,15]. T2 in
samples D and E strongly overlaps with T3 so its
accurate evaluation is difficult, while T3 has
relatively high intensity in all the samples. T4 is
located in the high temperature range and is not
well resolved in any of the samples making
accurate evaluation difficult. We have also observed peaks in this temperature range in other
MBE-grown GaInP samples. This peak may in
Table 1
SIMS and C–V results from Si-doped GaInP samples
Samples
(Set 1)
A
B
(Set 2)
D
E
F
Tcr (8C)
[NO ] (cm3)
[NSi ] (cm3)
[Nnet ] (cm3)
800
900
4.5 1017
6.7 1017
9.6 1016
9.9 1016
5.18 1016
4.65 1016
800
850
900
5.1 1017
5.9 1017
8.7 1017
2.9 1017
3.3 1017
3.3 1017
1.85 1017
1.78 1017
1.65 1017
246
N. Xiang et al. / Journal of Crystal Growth 227–228 (2001) 244–248
Fig. 1. Room-temperature PL spectra taken from Si-doped
GaInP samples containing different oxygen impurity concentrations. Si concentrations are 1 1017/cm3 and 3 1017/cm3 in
sets 1 and 2, respectively.
Fig. 2. DLTS spectra measured from samples of set 1 (dashed
lines), and set 2 (solid lines). Both sets were obtained using a
2 V reverse bias, a 0.1 s pulse, and a 0.2 s period. Pulse
voltages were 0 and 1 V for sets 1 and 2, respectively. The
Arrhenius plots used to calculate the activation energies for the
trap emission processes are shown in the insets. Nc is the density
of states in the conduction band, vth is the thermal velocity of
electrons, and t is the time constant of the capacitance
transient.
fact contain two or more peaks and at this stage
the nature of T4 is unidentified. However, [NT2]
and [NT3] both increase clearly with [NO] indicating that T2 and T3 are oxygen-related.
The difference in Ea for T3 between set 1 and 2
can be due to the different doping levels and may
indicate that T3 is a charged defect [16]. The
electric field around a charged defect can be
affected by doping so that Ea changes. Note that
the Arrhenius plots for T3 in set 1 are perfectly
overlapping, indicating that the nature of T3 in
samples A and B is same. The differences in T3
trap signatures among the samples in set 2 may be
due to such effects as different configurations
around the defects, peak broadening, or the
influence of neighboring peaks. For example, in
sample F, T3 is very broad and the neighboring
peaks, T2 and T4, are stronger than in samples D
and E which may explain why the value of s in
sample F is much smaller than that in samples D
and E.
We also observed that the concentration of peak
T3 increased following exposure to air, as shown
in Fig. 3. A duplicate of sample B, labeled B*, was
processed two weeks after the first. The DLTS
spectra of the two samples were unchanged except
for the amplitude of T3 which increased significantly. We attribute this to oxygen being incorporated at the surface. Another possible explanation
might be phosphorous outgassing leading to
phosphorous vacancies. Chae et al. has reported
an interface state with Ea ¼ 0:73 eV [17], possibly
due to a reaction involving the phosphorus
vacancy. Therefore, the increase in T3 may be
due to the formation of phosphorus vacancies or
the incorporation of additional oxygen, or a
complex of the two. T3 might be expected to have
higher concentration near the surface. This is
different from traps due to oxygen incorporated
during growth, which should be nearly uniform
throughout the layer.
It is interesting to compare our results of GaInP
with that of AlGaInP. Kondo et al. discovered two
oxygen-related traps, D2 and D3, in AlGaInP with
Ea 0.46 and 1.0 eV, respectively [6]. Ea for D2
was almost the same as Ea for T2 in our samples.
Ea for D3 was a bit higher than Ea for T3. The trap
concentrations of D2 ([ND2 ]) and D3 ([ND3 ]) were
found to increase linearly with oxygen concentration, and [ND3 ]/[ND2 ] ratio was almost constant. It
was suggested that D2 and D3 levels were a pair of
charge-state dependent multiple levels caused by
247
N. Xiang et al. / Journal of Crystal Growth 227–228 (2001) 244–248
Table 2
Evaluation results of deep level traps from Si-doped GaInP samples
Samples
(Set 1)
A
B
(Set 2)
D
E
F
T1
T2
T3
Ea (eV)
[NT1 ] (cm3)
s (cm2)
Ea (eV)
[NT2 ] (cm3)
s (cm2)
Ea (eV)
[NT3 ] (cm3)
s (cm2)
0.29
}
3.2 1013
}
2.3 1014
}
}
0.45
}
1.9 1014
}
2.2 1015
0.79
0.82
5.8 1013
2.1 1014
8.8 1014
2.5 1013
}
}
}
}
}
}
}
}
}
}
}
0.46
}
}
2.1 1015
}
}
1.5 1015
0.73
0.69
0.63
1.4 1015
2.1 1015
2.4 1015
6.0 1014
1.1 1014
5.9 1016
4. Conclusions
Oxygen-related deep level defects in solid-source
MBE grown Si-doped GaInP have been studied
using DLTS, SIMS, C2V profile and PL. Four
traps are resolved by DLTS. Among them, traps
T2 and T3, with thermal activation energies of
0.45–0.46 and 0.63–0.82 eV, are found to be
oxygen-related. T3 may also relate to phosphorus
vacancy, or phosphorus vacancy–oxygen complex.
This is the first report to indicate the presence of
oxygen-related deep level defects in MBE-grown
GaInP.
Fig. 3. Comparison of DLTS spectra from samples B and B*.
These two samples were from the same grown wafer and were
nominally processed with the same steps. However, sample B*
was processed 2 weeks later than sample B. The measurement
condition was the same as for set 1 mentioned in Fig. 2.
the off-center substitutional oxygen defect (VP–O).
In our GaInP samples, [NT2 ] and [NT3 ] also
increase with [NO ] but [N3 ]/[N2 ] ratio seems to
change with [NO ]. Due to the difficulties in
evaluation, precise [NT3 ]/[NT2 ] ratio is impossible
to obtain. These results may also be compared
with results obtained from As-based materials.
For example, oxygen has been observed to give
rise to a double acceptor in Al0.10Ga0.90As [18]
while first principal calculations of GaAs have
shown that oxygen interstitials or oxygen on As
sites can give rise to defects with multiple charges,
depending on the configuration of the defect [19].
More studies are needed to further clarify the roles
that oxygen plays in the formation of deep levels in
GaInP.
Acknowledgements
This work is supported, partly, by the Academy
of Finland within the EMMA MACOMIO Project
no. 46784.
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