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Photoelectric properties of Bi2O3/GaSe
heterojunctions
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Applied Physics Letters
· February 2009
DOI: 10.1063/1.3035854
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Available from: L. Leontie
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Photoelectric properties of Bi 2O3/GaSe heterojunctions
L. Leontie,1,a/H20850I. Evtodiev,2V. Nedeff,3M. Stamate,3and M. Caraman3
1Al. I. Cuza University, 11 Carol I Boulevard, RO-700506 Iasi, Romania
2Moldova State University, 60 A. Mateevici Str., Chisinau MD-2009, Republic of Moldova
3University of Bacau, 157 Calea Marasesti, RO-600115 Bacau, Romania
/H20849Received 18 June 2008; accepted 3 November 2008; published online 18 February 2009 /H20850
Photoelectrical characteristics and photoluminescence of n-Bi2O3/p-GaSe structures have been
investigated. They show photosensitivity in the photon energy range of 1.85–3.10 eV. Duringthermal treatment of the heterojunction, Bi and O atoms diffuse into the GaSe layer, forming twoimpurity levels located at 0.101 and 0.429 eV above the valence-band top of GaSe. © 2009
American Institute of Physics ./H20851DOI: 10.1063/1.3035854 /H20852
Gallium selenide /H20849GaSe /H20850is a typical representative of
layered III-VI semiconductors, whose specific characteristicsrecommend them for optoelectronic device applications.
1
The anisotropy of chemical bonds within stratified packages/H20849of ionic-covalent nature /H20850, as well as between packages /H20849of
Van der Waals type /H20850,
2allows single crystal cleavage in plan-
parallel plates, atomically smooth and optically homoge-neous, which can serve as active element of photovoltaicstructures.
3
The possibility of obtaining GaSe-based heterojunctions
by Van der Waals epitaxy was demonstrated in Refs. 4and5.
Due to low surface-state density of GaSe /H20849/H110211010cm−2/H20850,6
semiconductor structures with even nanometric fullerene C 60
layers can be prepared.7
The photoelectrical characteristics of indium tin oxide
/H20849ITO /H20850/p-GaSe heterojunctions, irradiated with neutrons,
have been studied in more detail in Ref. 8. High energy
radiation excitation leads to the increase of defect concentra-tion in the cationic subnetwork of GaSe by breaking therather weak Se-Ga bonds.
9
In this letter the photoelectrical properties of GaSe-based
heterojunctions in which the ITO transparent electrode wassubstituted by a heavy metal oxide /H20849Bi
2O3/H20850are investigated.
The optical properties of Bi 2O3/GaSe /H20849Cu/H20850structures, as well
as the influence of the oxide film on GaSe surface, were
examined in our previous paper.10
GaSe single crystals have been grown by Bridgmann
technique from its component elements, Ga and spectrallypure Se /H20849both of 99.999% purity /H20850. Their electrical conductiv-
ity at 293 K was of 3.8 /H1100310
−3/H9024−1cm−1. Singlecrystalline
p-type GaSe plates, 30-50 /H9262m thick, with hole concentration
of 6.2 /H110031014cm−3, have been obtained by splitting single
crystals in the /H208510001 /H20852direction and served as substrates in the
n-Bi2O3/p-GaSe heterojunctions.
Onto freshly cleaved surfaces of p-GaSe plates, heated
at 470 K, thin Bi 2O3films with thickness in the range of
95–125 nm were deposited by rf magnetron sputtering in anAr:O /H2084975:25 /H20850atmosphere. For the Bi
2O3film, 125 nm thick,
the transmission coefficient at 500 nm wavelength was 68%,while its surface resistance was 160 /H9024//H17040. A thin Ni film
/H20849/H11011300 nm thick /H20850, thermally vacuum evaporated onto the lat-
eral face of GaSe plate, was used as Ohmic contact. Its con-tour was strengthened by an In film /H20849also vacuum evapo-
rated /H20850with thickness of 2
/H9262m. Thin In films were depositedonto the outer Bi 2O3surface, in form of tapes, 0.5 mm
broad, equidistanced one from another at 5 mm.
The spectral characteristics of the photocurrent through
GaSe and Bi 2O3/GaSe heterojunctions in the range of
300–700 nm have been recorded by a photometric equip-ment including a monochromator with diffraction grating/H20849600 mm
−1/H20850with an energy resolution of 2 meV. The spectra
were normalized to the incident photon flux. X-ray diffrac-
tion analysis /H20849a DRON-2 apparatus, Cu K/H9251radiation, and /H9261
=1.5418 Å /H20850revealed two main diffraction maxima located at
27.60° and 33.37° /H208492/H9258angles /H20850, attesting that /H9251-Bi2O311poly-
morph is predominant in actual sputtered films.
In order to establish in a first approximation the mecha-
nism of minority charge carriers transfer through then-Bi
2O3/p-GaSe heterojunction, current-voltage characteris-
tics at 293 K have been studied. Figure 1shows the J-V
characteristics as well as used electrode geometry. As can beseen, these characteristics are asymmetric diode shaped witha rectification factor of 350 at 1.4 V. For low applied volt-ages /H20849V/H110210.5 V /H20850they are adequately described by
12
J=J0/H20875exp/H20873eV
nkT/H20874−1/H20876, /H208491/H20850
where J0is the current density determined by the contact
potential, independent of the applied voltage and constant atfixed temperature, eis the elementary charge, kis Boltz-
mann’s constant, Tis the absolute temperature, and nis the
diode quality factor, equal to 1.4–1.7 and increasing togetherwith Bi
2O3film thickness. For n/H110221 the total current density
/H20849Jt=Jd+Jr/H20850through the junction is determined by both diffu-
sion and recombination currents.13For applied forward volt-
ages V/H110220.6–0.7 V, the J-Vdependences are determined by
the series resistance of the heterojunction. In the case ofheterojunctions containing thinner Bi
2O3films /H2084995 nm thick /H20850
the total sheet resistance was about 1200 /H9024//H17040; therefore the
relatively large series resistance of heterojunction is mainlydetermined by the Bi
2O3oxide film as well as interface layer.
The reverse current density through the heterojunction /H20851Fig.
1/H20849b/H20850/H20852is described by a power law J/H11011Vm. For applied bias
V/H110210.8 V, the J-Vcharacteristics are sublinear with a power
coefficient m=0.76–0.80. These dependences are deter-
mined in a first approximation by leakage currents. For V
/H110220.9 V, the current-voltage characteristics are supralinear
with m=2.2–2.6. The last values for mcannot yet be ad-
equately explained only from single-temperature measure-ments. Further work is to be done in order to elucidate this.
a/H20850Electronic mail: lleontie@uaic.ro.APPLIED PHYSICS LETTERS 94, 071903 /H208492009 /H20850
0003-6951/2009/94 /H208497/H20850/071903/3/$25.00 © 2009 American Institute of Physics 94, 071903-1
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Figure 2illustrates the spectral characteristics of the
photocurrent /H20849divided by the number of incident photons /H20850
under heterojunction illumination in the direction parallel tothe GaSe symmetry axis C
6. The photocurrent is seen to
increase by more than three orders of magnitude in a narrowenergy range /H208491.8–2.0 eV at T=293 K /H20850at both temperatures
of 78 and 293 K, which is in good agreement with the profileof GaSe absorption spectrum.
14A monotonic photocurrent
rise together with the photon energy, at T=293 K, is also
emphasized in the spectral region h/H9263/H110222.2 eV, where the ab-
sorption coefficient is seen to increase due to the increasingcombined density of states of electrons in the valence andconduction bands. At the temperature 78 K the photocurrentin the depth of the fundamental absorption band /H20849h
/H9263
/H110222.7 eV /H20850shows a monotonic decrease tendency. Therefore,when decreasing the sample temperature, the surface-state
density increases and, consequently, the recombination rateis prevailing on the generation rate of the nonequilibriumcharge carriers at 78 K. Photosensitivity maxima, located at2.04 eV at 293 K and at 2.120 eV at 78 K, are determinedby the nonequilibrium charge carrier /H20849electrons, in GaSe /H20850
generation by means of excitons. As a result of photon ab-sorption with energy
h
/H9263n=1=Eg−Rex, /H208492/H20850
where Egis the semiconductor band gap and Rexis the bind-
ing energy of the electron-hole pair in the state n=1, a part of
excitons dissociates forming nonequilibrium charge carriers;besides, another part annihilate, leading to the formation ofthe exciton photoluminescence /H20849PL/H20850band.
Figure 3shows spectral characteristics of the short-
circuit photocurrent divided by the number of incident pho-tons and normalized to unity for the untreated and thermallytreated Bi
2O3/GaSe heterojunction.
As can be observed by comparison of Figs. 2and3, the
photosensitivity edge of GaSe is well correlated with that ofphotocurrent through the heterojunction. Thus, generation ofnonequilibrium charge carriers takes place within the GaSeinterface layer. Besides /H20849Fig. 2, curve 3 /H20850, the absorption co-
efficient at T=293 K increases more than three times in the
energy range of 2.1−2.5 eV, while the current densitythrough junction monotonically decreases. Since the recom-bination current density rises together with the absorptioncoefficient, one can admit that Bi
2O3layer creates new short-
lifetime surface states onto the GaSe surface.
As can be seen in Fig. 3/H20849curve 2 /H20850, the thermal treatment
in vacuum results in a more pronounced photocurrent de-crease in the spectral region h
/H9263/H110222.1 eV. Therefore, it leads
GaSeBi2O3In
Ni
Bi2O3GaSe
In
GaSe
Bi2O3
Ni
In In-V +V
(c)0 . 20 . 40 . 60 . 81 . 01 . 21 . 41 . 61 . 82 . 010-710-610-510-410-3
(a)n=1.4
n=1.5n=1.7321J (A/cm2)
V( V )d=9 5n m
d=1 0 8n m
d=1 2 5n m
0.01 0.1 110-810-710-610-5
(b)21m=2.20
m=0.76m=2.60
m=0.80d = 95 nm
d = 125 nmJ (A/cm2)
V( V )
FIG. 1. Dark current–voltage characteristics of Bi2O3/GaSe heterojunctions; /H20849a/H20850forward bias, /H20849b/H20850reverse bias, and /H20849c/H20850sample geometry. dis the thickness of
Bi2O3film.
1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40 3.6010-410-310-210-1100
321
293 K
78 K
293 K
hν(eV)Photocurrent (arb. units )
1021031041052.120 eV2.04 eV
α(cm-1)
FIG. 2. Spectral characteristics of photocurrent through GaSe at tempera-tures of 293 K /H20849curve 1 /H20850and 78 K /H20849curve 2 /H20850and absorption spectrum of
singlecrystalline GaSe layers at 293 K /H20849curve 3 /H20850.071903-2 Leontie et al. Appl. Phys. Lett. 94, 071903 /H208492009 /H20850
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

to the enhancement of Bi and O diffusion, which contributes
to the increase of the short-lifetime /H20849electron /H20850surface states.
Figure 4shows PL spectra of the Bi 2O3/GaSe structure,
without metal electrodes, upon the excitation by radiationfrom the fundamental absorption threshold region /H20849/H9261
=546.1 nm /H20850. The PL spectrum from outer GaSe surface
/H20849curve 1 /H20850is composed of a narrow band peaked at 2.092 eV
and can be ascribed to radiative annihilation of the freeexcitons.
15Moreover, the thermal treatment in vacuum prac-
tically does not change the structure of the PL spectrum. Incontrast with the previous spectrum, the structure of the PLspectrum from the Bi
2O3/GaSe heterojunction interface/H20849curve 2 /H20850is much more complex. It is composed of three
bands: two of them, with maxima at 2.081 eV and 2.038 eV,merge in a broad band with a halfwidth of 120 meV; thethird PL band lays in the range of 675–850 nm with maxi-mum at 1.710 eV. The PL band peaked at 2.081 eV is pro-duced by luminescent emission of localized excitons with thebinding energy of /H110117 meV. The other PL bands /H20849located at
2.038 and 1.710 eV /H20850are of impurity nature. As can be seen
from curves 2 and 3, the thermal treatment in vacuum leadsto the diminution of localized exciton PL, unaffecting thestructure of bands located at 2.038 and 1.710 eV. Thereforeone can assume that during the thermal treatment, Bi and Odiffusion takes place into the heterojunction interface layer,creating two recombination levels for the nonequilibriumelectrons. Taking into account that the band gap of GaSe, at78 K, is 2.139 eV,
16then the energy of the recombination
levels, through which the impurity PL bands, located at2.038 and 1.710 eV, are formed, can be determined as 0.101and 0.429 eV, respectively, relative to the valence-band top.
In conclusion, n-Bi
2O3/p-GaSe heterojunctions show
photosensitivity in the photon energy range of 1.85–3.10 eV. Nonequilibrium charge carriers /H20849electrons /H20850are gen-
erated within the GaSe interface layer. During the prepara-tion procedure and as a result of the thermal treatment ofheterojunctions, the diffusion of Bi and O atoms occurs, cre-ating recombination states for electrons and two luminescentrecombination levels located at 0.101 and 0.429 eV abovethe valence-band top.
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21
T=293 KShort- Circuit Photocurrent (rel. units )
hν(eV)
FIG. 3. Spectral distribution of the short-circuit photocurrent /H20849divided by the
number of incident photons and normalized to unity /H20850for Bi2O3/GaSe
heterojunction untreated /H20849curve 1 /H20850and thermally treated for 60 min in
vacuum at 630 K /H20849curve 2 /H20850.
2.10 2.05 2.00 1.95 1.80 1.600.00.20.40.60.81.0
PLPLGaSeBi2O3
λ=546,1 nm
(curves1 and 4) (curves 2 and 3)I II
4132
1.710 eV2.038 eV
2.081 eV2.092 eVPL (rel. units)
hν(eV)1
2
3
4
FIG. 4. PL spectra of Bi2O3/GaSe heterojunction without electrodes at tem-
perature of 78 K: /H208491/H20850from outer GaSe surface, /H208492/H20850from untreated hetero-
junction interface, /H208493/H20850from thermally treated /H2084960 min length, 630 K /H20850hetero-
junction interface, and /H208494/H20850from outer GaSe surface after thermal treatment
/H2084960 min length, 630 K /H20850in vacuum.071903-3 Leontie et al. Appl. Phys. Lett. 94, 071903 /H208492009 /H20850
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp