HAL Id: jpa-00245883
https://hal.archives-ouvertes.fr/jpa-00245883
Submitted on 1 Jan 1988
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Optical properties of EL2
M. Kaminska
To cite this version:
M. Kaminska. Optical properties of EL2. Revue de Physique Appliquée, Société française de physique / EDP, 1988, 23 (5), pp.793-802. �10.1051/rphysap:01988002305079300�. �jpa-00245883�
793
Optical properties of EL2
M. Kaminska
Institute of Experimental Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland
(Reçu le 15 juillet 1987, accepté le 12 février 1988)
Résumé. 2014 On donne d’abord une description des propriétés optiques reliées à EL2 et observées en
photocapacitance, en absorption optique, en dichroïsme circulaire magnétique et en photoluminescence qui permettent de placer les niveaux d’énergie associés à EL2 dans le schéma de niveau de GaAs. On présente
ensuite les informations obtenues sur l’état métastable de EL2 à partir des effets de métastabilité associés aux
propriétés optiques. Enfin, on montre comment des résultats spectroscopiques récents d’absorption et de piezo-absorption dans le proche infrarouge, obtenus principalement dans l’équipe de l’auteur, contribuent à
une description microscopique de EL2.
Abstract. 2014 In this paper is given first a description of the optical features related to EL2 observed in photo- capacitance, absorption, magnetic circular dichroism and photoluminescence which allow to determine a level scheme for the neutral and ionized charge states of EL2 in the energy band levels of GaAs. From the analysis
of the metastability effects of EL2 observed in the optical properties of GaAs is derived information on some
parameters of the metastable state. Finally, are reported and analysed optical experiments in which the authors has been thoroughly involved which can help to provide a microscopic description of the defect.
Revue Phys. Appl. 23 (1988) 793-802 MAI 1988,
Classification
Physics Abstracts
61.70 - 78.50G - 71.55
1. Photocapacitance studies.
Photocapacitance transient measurements in the infrared region made by Bois et al. in 1979 [1] were
the first measurements of EL2 optical properties.
They allowed to determine value and the spectral dependence of EL2 optical cross section both for the
photoionization to the conduction band o-0 and for
the electron capture from the valence band ug
(Fig. 1). The changes of the slope seen for u n 0 curve
were explained as corresponding to electron photo-
Fig. 1. - Spectral dependence of a§ and ug [2].
ionization to GaAs conduction band in the region of
X and L points of Brillouin zone in addition to
simple photoionization to r point [1, 2].
2. Absorption spectra.
In 1981 Martin [3] published the absorption spectrum in the near infrared, which he attributed to EL2. The spectrum, characteristic of n-type and semiinsulating (SI) GaAs, consisted of three structures with energy thresholds about 0.8 eV, 1.0 eV and 1.3 eV, and its shape was similar to o- n 0 (Fig. 2). Two years later it
was found that the central band of the spectrum begins with the fine structure consisting of the zero- phonon line (ZPL) of 1.039 eV energy followed by phonon replica of 11 meV energy [4] (Fig. 2) in
insert. The comparison of EL2 absorption spectrum with the corresponding photocapacitance [3] and photocurrent [4] spectra showed that the central part of the three spectra is distinctly weaker for the last
two. It was then possible to separate the absorption
spectrum in the energy range 1.0 - 1.3 eV and attri- bute it to EL2 intracenter transitions [4] (Fig. 2).
The rest of the absorption spectrum corresponds to
EL2 photoionization to the conduction band. The
change in the slope of absorption, photocurrent and
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01988002305079300
Fig. 2. - EL2 absorption and photocurrent spectra with intracenter absorption spectrum separated. The shape of
EL2 absorption spectrum was first published by Martin [3]
and the fine structure shown in the insert by Kaminska
et al. [4].
photocapacitance spectra at about 1.0 eV is con- nected with extra possible transitions to X and L
points of GaAs Brillouin zone [1, 2].
However, the connection of the above mentioned infrared absorption and its fine structure with EL2 defect have been questioned on several occasions [5]. Measurements carried out by Skowronski et al.
[6] explicitly showed direct proportionality of EL2
concentration from DLTS measurements both with the intensity of the absorption band taken for the maximum at 1.17 eV energy and with the intensity of
1.039 eV ZPL.
The other absorption spectrum, which can be attributed to EL2 was found in SI GaAs by means of magnetic circular dichroism (MCD) method (Fig. 3) [7]. The method together with accompanying techniques based on electron and nuclear spin reso-
nances will be described in other part of the present book (see Sect. 5. - Meyer, ENDOR spectroscopy
of EL2 and also [8]). However, for the completeness
of EL2 optical absorption properties we have to
mention here the MCD spectrum. The observed two MCD absorption bands with peak positions at
1.05 eV and 1.29 eV were originally interpreted as
connected with As’ defect intracenter transitions
[7]. Simultaneously, several experiments were per-
Fig. 3. - Integrated magnetic circular dichroism of as
grown SI GaAs and its decomposition into two Gaussian bands [7].
formed in order to prove that the defect responsible
for MCD absorption spectrum is the same as EL2 defect seen in capacitance and infrared absorption
measurements mentioned above [9]. They showed
that MCD spectrum can be attributed to singly
ionized EL2 defect, EL2+ , and the two MCD bands
can be then interpreted as EL2+ intracenter tran- sitions.
3. Luminescence properties.
EL2 luminescence properties are not entirely clear.
For SI crystals the luminescence with Gaussian
shape and peak position changing in the energy range 0.62 eV - 0.68 eV was observed [10-28]. Re-
cent studies of that photoluminescence and especially
the measurements of its excitation spectrum [27]
shed some new light on its origin. It turned out that
the luminescence consists of two bands peaked at
0.63 eV and 0.67 eV (Fig. 4). The 0.63 eV lumi-
Fig. 4. - Photoluminescence (PL) spectra of undoped SI
LEC GaAs crystal as a function of excitation photon
energy [27].
nescence spectrum appeared for the exciting light of
the energy higher than 1.4 eV, whereas 0.67 eV band was observed when the excitation light from
the energy range 0.8 eV -1.4 eV was used. The excitation spectrum of 0.67 eV luminescence showed
two bands structure peaked at about 1.0 eV and
1.3 eV (Fig. 5) [27].
It was also found that 0.67 eV luminescence band
begins with the fine structure consisting of ZPL at
0.758 eV and its phonon replicas of 11 meV energy
[28] (Fig. 6). That luminescence fine structure is of the same pattern as the fine structure of EL2 intracenter absorption [4]. The phonon of 11 meV
energy does not correspond to any phonon from the
maximum of GaAs phonon state density [29], so it is
a local phonon characteristic for EL2. Tajima et al.
795
Fig. 5. - 0.67 eV photoluminescence (PL) and photolumi-
nescence excitation (PLE) spectra of GaAs. The spectra
were taken before photoquenching. For exciting light of
0.8 eV - 1.4 eV energy 0.67 eV luminescence band was
observed, whereas 0.63 eV luminescence appeared for exciting light of energy higher than 1.4 eV [27].
Fig. 6. - Spectrum of vibrational structure of 0.67 eV luminescence band [28].
[27, 28] interpreted the luminescence band 0.67 eV
as electron transitions from EL2 level to the valence band. However, our point of view on that lumi-
nescence origin is different - we will return to this
problem in section 4.
For SI GaAs another luminescence spectrum with maximum around 0.8 eV was also reported [13, 20, 30-32] (Fig. 7). Though several authors attributed it
to EL2, no opinion about 0.8 eV luminescence
origin was presented.
Fig. 7. - Photoluminescence spectrum of LEC SI GaAs
[31].
For n-type GaAs Tajima et al. [26] observed
luminescence band with the maximum varying in the
energy range 0.61 eV - 0.68 eV depending on the
crystal (Fig. 8). However, for temperature T = 77 K
the maximum of the band was always around
0.63 eV. That luminescence was attributed to elec- tron transitions from the conduction band to EL2 level [26].
Fig. 8. - Photoluminescence spectra of various n-type LEC GaAs crystals at (a) 4.2 K an 0 (b) 77 K [26].
4. EL2 energy levels corresponding to optical tran-
sitions.
On the basis of optical experiments it is possible to
determine EL2 energy levels corresponding to both
neutral and singly ionized charge states of that
defect.
We will start from determining energy levels of neutral EL2 defect, EL2°. From DLTS measure- ments it is possible to establish the position of EL2°
ground state within GaAs band structure at about 0.75 eV below the conduction band [33, 34]. The
energy 1.039 eV of EL2 intracenter absorption ZPL gives the energy position of EL2° excited state in
relation to the ground one and places it in the conduction band.
Together with placing EL2° levels within GaAs
band structure the symmetry of the levels was
determined. The key optical experiments which
allowed this were the measurements of 1.039 eV ZPL of EL2 intracenter absorption in magnetic field
and under uniaxial stress [35]. By means of those experiments it was possible to determine EL2 ground and excited terms in its intracenter absorp-
tion process as 1 AI and IT 2 respectively [35]. Spin singlets of EL2 terms were deduced from the lack of
any changes (no splitting and no energy shift) of the
ZPL in the magnetic field. On the other hand, the pattern of splitting for the ZPL under uniaxial stress
a along [111 ], [100 ] and [110 ] directions (including [001 ] and [110 ] inequivalent propagation directions
of Poynting vector 9 for [110 ] stress) (Fig. 9) was in
the agreement with the number of components expected for ZPL corresponding to Ai - T2 tran-
sition in the static crystal field theory. According to
Runciman [36], who made a classification of stress
spectra for different types of centers, it was imposs-
Fig. 9. - Experimental (points) dependence of energy of EL2 intracenter absorption ZPL on uniaxial stress cr.
Polarization selection rules are indicated. For 0, il [110]
polarization rules are given for two inequivalent directions
of Poynting vector 9. Theoretical (curves) dependence
was obtained for Ai - T2 transitions assuming T2 state coupled with T Jahn-Teller mode [35].
ible to explain the experimental data by any other type of electronic degeneracy, orientational degener-
acy or electronic plus orientational degeneracy.
Some quantitative disagreements between exper- imental piezoabsorption results and the static crystal
field theory for Ai - T2 transition (like bending of
the middle ZPL component for a Il [110] and bend- ing of the lower ZPL component for au Il [111 ]) were explained by including the T-mode Jahn-Teller effect for the T2 state [35].
Figure 10 is the summary of the above discussed results and it shows EL2° levels within GaAs band structure.
Simultaneously, from foto-EPR [37] and inte- grated MCD [7] spectra one can establish the energy
position of AsGa defect levels. We believe that these levels correspond to the energy positions of singly
ionized EL2+ defect levels. The foto-EPR exper- iment placed As’ ground level at about 0.5 eV above the valence band [37]. Thermal energy of
Asca excited levels in respect to the ground one must
Fig. 10. - EL2° levels within GaAs band structure.
be taken as the low energy thresholds of separated
bands of the integrated MCD (Fig. 3) [7]. This
means that two excited AsGa states are about 0.8 eV
and 1.15 eV above the ground one. On the other hand the excitation spectrum of EL2 0.67 eV lumi-
nescence [27] (Fig. 5) is very similar to the integrated
MCD spectrum [7] (Fig. 3). One can thus conclude that 0.67 eV luminescence is excited by AsGa excited
states. In our opinion 0.67 eV luminescence corres-
ponds to the intracenter transitions within the
Asca defect. The energy 0.76 eV of the 0.67 eV luminescence ZPL [28] could therefore be regarded
as the energy distance between AsGa ground and first
excited state. This energy distance is determined with higher accuracy than if taken from the 0.8 eV threshold of the integrated MCD curve [7]. Figure 11
shows the energy levels of AsGa defect or singly
ionized EL2 defect. The symmetry of the states is taken from paper [7].
Fig. 11. - As’. levels within GaAs band structure. (We
believe that they correspond to EL2+ levels).
5. EL2 metastability observed in optical experiments.
The most characteristic feature of EL2 defect is its
metastability occurring in low temperatures under the influence of illumination with the light of 1.0 eV-
1.3 eV energy [16, 38, 39]. The return to the ground
state is possible by heating the crystal up to the temperature about 130 K for SI and about 50 K for n-type GaAs (Fig. 12) [40].
Fig. 12. - Thermally activated recovery of EL2 absorption
taken at 1.17 eV for n-type (n -1016 cm-3) and SI GaAs after previous quenching by illumination with 1 pLm light at
T = 10 K [40].
797
EL2 metastability could be observed in photo-
current [41], photocapacitance [42-44], absorption [3, 45] and luminescence [14-16, 23] measurements
through the quenching of the respective spectra - for example see the quenching of absorption
spectrum (Fig. 13). The first report about the ex- traordinary feature of GaAs crystals, which was the quenching of the photocurrent spectrum as a result of illumination, appeared in 1976 [41], so even
before the introducing of EL2 name by Martin [46].
Fig. 13. - Optical absorption spectra for the same undoped SI GaAs crystal. Curve a : after cooling in the dark, curves b and c : after white light illumination for 1 and 10 min respectively [3].
The first detailed description of photocapacitance
transient corresponding to ionization and EL2 defect transition to its metastable state came from the paper of Bois and Vincent [38]. In figure 14 such photocapacitance changes under illumination with 1.06 ktm light are shown. The photocapacitance changes, its initial quick rise and subsequent slow
decrease to the almost initial value without further
light sensitivity, show that electrons from EL2
ground state freeze at EL2 metastable state. The return from the metastable state to the ground one
takes place with the thermally activated rate deter-
mined independently by Mittoneau and Mircea
[43] : 1
Fig. 14. - Photocapitance quenchig for EL2 level in GaAs
versus time after a direct electric pulse under 1.17 eV illumination 0. Temperature T = 77 K [38].
and by Vincent et al. [39] :
Mittoneau and Mircea [43] also found that the
recovery process from the metastable state is
strongly accelerated by the presence of free elec- trons, so they proposed two mechanisms responsible
for reaching EL2 ground state :
(i) thermal deexcitation through the barrier of
about 0.3 eV high ;
(ii) Auger like deexcitation.
The full formula for the rate of EL2 recovery from its metastable state can in general be written as :
where n is free electron concentration in cm- 3 and vn is thermal electron rate in cm/s for temperature T.
Studies of EL2 recovery from its metastable state
were also carried out by means of absorption
measurements [40]. The kinetics of thermally acti-
vated recovery of EL2 absorption was observed after
prior quenching of the spectrum (Fig. 12). The analysis of that kinetics for different n-type and SI GaAs samples allowed to determine the rate of EL2 recovery as [40] :
The results are consistent with the model taking into
account two ways of EL2 recovery [43] and are
consistent with the value of parameters obtained by
means of photocapacitance experiments described
above.
EL2 recovery from its metastable state can be then accelerated by the presence of electrons in the conduction band. One can change the number of
free electrons using light irradiation either corre-
sponding to band to band transition or - impurity
level to band transition. In the presence of optically
created electrons in the conduction band it is possible
to lower significantly the temperature of EL2 recov- ery in SI GaAs. Such an effect accounts for so called
optically-assisted thermal anneal of EL2 defect like in experiments reported by Parker and Bray [47].
The studies of EL2 metastability allowed also to find the excitation spectrum for the process of EL2 transition to its metastable state. It tumed out that the excitation spectrum [16, 39, 48, 49] is identical with EL2 intracenter absorption spectrum (Fig. 15) including its fine structure (Fig. 16). That proves that the intracenter transition 1 AI -+ IT 2 is the first
step to reach EL2 metastable state.
No optical experiment has shown the energy
position of EL2 metastable state. Usually it is placed
in the upper part of GaAs energy gap [38, 39, 50].
Fig. 15. - EL2 intracenter absorption spectrum (solid line) [4] together with excitation spectra of luminescence
quenching (0) [16] and photocapacitance quenching (e) [39].
Fig. 16. - Spectrum of quenching efficiency of EL2 absorption band (broken line) in the vicinity of EL2
intracenter absorption fine structure (solid line) [49, 53, 59].
From the experiments studying EL2 return from its
metastable state to the ground one only the value of
thermal barrier between EL2 metastable and ground
states of about 0.3 eV energy has been found [39, 40, 43].
6. EL2 related optical properties of GaAs.
In 1985 Jimenez et al. [51, 52] reported on the peculiar behaviour of GaAs photocurrent after long
time illumination at 80 K with photons ranging from
1 to 1.35 eV. (That energy region corresponds to
EL2 intracenter transitions, which as it was described above cause EL2 transfer to its metastable state.).
Recording the photocurrent signal during the illumi- nation, the authors observed quenching of the signal
for a short time of light irradiation. That quenching corresponded to EL2 transition to its metastable state. On the other hand, after long excitation a
rather unexpected metastable increase of the signal
was noticed (Fig. 17).
The results of the same kind were reported by
means of thermally stimulated current (TSC) tech-
Fig. 17. - Photocurrent (1.31 eV) vs. time at 80 K [51].
nique [53]. In those measurements the sample was
cooled down to helium temperature in darkness.
1 itm illumination of two characteristic periods of
time was then applied : (A) when EL2 reached its metastable state, (B) few minutes longer. The result-
ing TSC spectra are shown in figure 18. For long
time illumination (B) the TSC spectrum exhibited peak at 85 K and p-type conductivity was found to
be connected with it. That structure was not ob- served for (A) type illumination.
Fig. 18. - TSC measured after short A (dotted curve) and long B (full curve) illumination time. In insert : monitoring photocurrent measurement vs. time ; arrows show short A and long B time of illumination [53].
The third report referring to the same phenome-
non comes from electronic Raman scattering studies
of nonequilibrium holes in SI GaAs [54]. The results
are presented in figure 19. They showed the presence of bound holes on GaAs shallow acceptors at 15 K.
The bound hole population decreased with growing
temperature and simultaneous increase in free hole concentration was observed - seen as a background
in figure 19 for 80 K. Finally, at 145 K (the tempera-
ture corresponds to EL2 full recovery) both the
bound and free holes appeared to have vanished.
The model of all those three effects is schemati-
cally drawn in figure 20. Figure 20a shows EL2
defect in SI GaAs prior to 1 >m illumination. Since the Fermi level is close to the middle of GaAs energy gap in such crystals, one can expect some of EL2
centres in neutral charge state and some of them in
