Original
Paper
phys. stat. sol. (a) 205, No. 8 (2008)
2055
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
markedly decrease at increasing photon energies. Taking
into account that the edge of the absorption band of
GaSe(Cu) layer actually coincides with that of ∆U(hν)
spectral characteristics, as well as the magnitude of α, one
can consider that generation of non-equilibrium charge car-
riers takes place in an about 1–10 µm thick GaSe(Cu)
layer by Bi O /GaSe(Cu) interface. The slow decrease of
2
1
10000
1000
2
3
∆U at increasing α values indicates presence of short life-
time surface states; these are produced by atoms inside
Bi O layer, which form new valence bonds with Se atoms
8
7
6
5
100
2
3
3
4
3
2
1
by surface of Se–Ga–Ga–Se package.
If one consider that αd Ӷ 1, where the depth of gen-
n
eration of non-equilibrium charge carriers in GaSe(Cu)
Lp=1.7 mm
0
-2
0
2
4
6
8
10
1/a (mm)
10
layer by junction interface (d ) is lesser than hole diffusion
n
1.5
2.0
2.5
3.0
3.5
length (L ), and recombination rate at Bi O /GaSe(Cu) in-
p 2 3
hn (eV)
terface is independent on the exciting level (illumination
intensity), then the open-circuit voltage can be described
by a monotonic function of αJ /(αL + 1) [14], where J is
Figure 4 1 – Spectral characteristics of the absorption coeffi-
cient of single-crystalline GaSe(Cu) layers; 2 – Spectral charac-
teristics of the photoconductivity for GaSe(Cu) layers; 3 – Spec-
tral distribution of the open-circuit voltage for Bi2O3/GaSe(Cu)
structure. Thicknesses: Bi2O3 films – 130 nm; GaSe(Cu) layer –
12 µm. Doping content: 0.05 at% Cu. Inset: Plots of the open-
circuit voltage vs. inverse of absorption coefficient for the
Bi2O3/GaSe(Cu) junction.
0
p
0
the flux of the incident photons. By analyzing ∆U(1/α) de-
pendence (Fig. 4 Inset) hole diffusion length in GaSe(Cu)
layer by Bi O /GaSe(Cu) interface can be determined. By
2
3
extrapolating the linear part of this curve to the abscissa
axis, L ≈ 1.7 µm was found.
p
4 Conclusion RF reactive magnetron sputtering of Bi
targets onto GaSe(Cu) in an atmosphere Ar:O (2:1) leads
2
this behavior is more pronounced for smaller thickness
samples.
to deposition of polycrystalline diphasic (β- and δ-Bi O )
2 3
Figure 4 presents the absorption spectrum of single-
crystalline GaSe(Cu) layers (curve 1), their photoconduc-
tivity spectral characteristics (curve 2), as well as the spec-
tral dependence of the open-circuit voltage (∆U) for
Bi O /GaSe(Cu) junction illuminated through Bi O layer
bismuth oxide films.
The optical bandgap of bismuth oxide films onto amor-
phous quartz at temperature T = 293 K is equal to 3.2 eV.
As resulted from the analysis of the polarization ellipse
for the reflected light from Bi O /GaSe(Cu) junction, in the
2
3
2
3
2
3
(curve 3).
spectral range 400–800 nm both optical functions n(λ)
and k(λ) of bismuth oxide films onto single-crystalline
GaSe(Cu) decrease with increasing film thickness.
As can be observed from curves 1 and 2, at photon en-
ergies hν > 2.1 eV the photoconductivity of single-
crystalline GaSe(Cu) layers increases together with the ab-
sorption coefficient α. If one takes into account that actual
thickness of GaSe layer (about 12 µm) fulfills the con-
dition for complete photon absorption within sample,
αd ӷ 1, then one can consider that photoconductivity is
proportional to total number of absorbed photons, A:
Bi O /GaSe(Cu) junctions show photogeneration prop-
2 3
erties for photon energy range 1.5–3.0 eV.
Presence of Bi O film leads to formation of new va-
2
3
lence bonds at GaSe(Cu) crystal surface, resulting in in-
creasing density of short lifetime surface states.
By analyzing spectral characteristics α(hν) and ∆U(hν)
hole diffusion length in the Bi O /GaSe(Cu) junction layer
2
3
Dσ ~ A◊τ ,
(5)
was found as 1.7 µm.
where τ is lifetime of nonequilibrium charge carriers. For a
low-level excitation of sample, it can be considered to be
constant.
Acknowledgements This work was supported by National
Authority for Scientific Research (ANCS), Minister of Education
and Research, Romania, through CEEX grants no. 2/01.08.2006
(code 06-11-51-1) and FOTONTECH. Authors would like
to greatly thank Professor G.I. Rusu for many fruitful discus-
sions.
The linear increase of the absorption coefficient and
photoconductivity in the depth of the fundamental absorp-
tion band do attest small recombination rates of the non-
equilibrium charge carriers through surface states in
GaSe(Cu) layers.
References
When exciting Bi O /GaSe(Cu) structure (by illumi-
2 3
nating through Bi O film) with photons of energy
2 3
[1] L. Leontie, M. Caraman, and G. I. Rusu, J. Optoelectron.
Adv. Mater. 2, 385 (2000).
hν > 1.7 eV, separation of non-equilibrium charge carriers
within junction occurs. The open-circuit voltage (∆U)
reaches its maximum for the energy corresponding to Cu
impurity absorption band (hν ≈ 1.88 eV) and tends to
[2] L. Leontie, M. Caraman, A. Visinoiu, and G. I. Rusu, Thin
Solid Films 473, 230 (2005).
[3] L. Leontie, J. Optoelectron. Adv. Mater. 8, 1221 (2006).
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim