Table Of ContentPhotoresponse dynamics in amorphous-LaAlO /SrTiO interfaces
3 3
Emiliano Di Gennaro,1,∗ Ubaldo Coscia,2 Giuseppina Ambrosone,1
Amit Khare,1 Fabio Miletto Granozio,1 and Umberto Scotti di Uccio1
1Dipartimento di Fisica, Univ. di Napoli Federico II and CNR-SPIN,
Compl. Univ. di Monte S. Angelo, Via Cinthia I-80126 Napoli (Italy)
2Dipartimento di Fisica, Univ. di Napoli Federico II and CNISM Unit´a di Napoli,
Compl. Univ. di Monte S. Angelo, Via Cinthia I-80126 Napoli (Italy)
(Dated: January 15, 2015)
5 The time-resolved photoconductance of amorphous and crystalline LaAlO3/SrTiO3 interfaces,
1 bothhostinganinterfacial 2-dimensionalelectron gas,isinvestigatedunderirradiation byvariable-
0 wavelengths, visible or ultraviolet photons. Unlike bare SrTiO3 single crystals, showing relatively
2 small photoconductance effects, both kinds of interfaces exhibit an intense and highly persistent
photoconductance with extraordinarily long characteristic times. The temporal behaviour of the
n
extra photoinduced conductance persisting after light irradiation shows a complex dependence on
a
interface type(whether amorphous or crystalline), sample history and irradiation wavelength. The
J
experimental results indicate that different mechanisms of photoexcitation are responsible for the
4
photoconductance of crystalline and amorphous LaAlO3/SrTiO3 interfaces undervisible light. We
1
propose that the response of crystalline samples is mainly due to the promotion of electrons from
the valence bands of both SrTiO3 and LaAlO3. This second channel is less relevant in amorphous
]
l LaAlO3/SrTiO3, where the higher density of point defects plays instead a major role.
e
- Keywords: Interfaces,2DElectronGas,Photoconductivity, Oxides
r
t
s
.
t The interface between the band insulators the oxygen vacancies are formed close to the interface,
a
LaAlO (LAO) and SrTiO (STO) can host a 2- duringsamplefabrication,becausethe oxidationofLAO
m 3 3
dimensional electron gas (2DEG). In the seminal partially occurs by taking O atoms from the STO sub-
-
paper by Ohtomo and Hwang,[1] an epitaxial LAO strate. [11] Furthermore, oxygen vacancies can produce
d
n film was grown on a single crystal (001) STO substrate a 2DEG even when an ER cannot take place, as for in-
o with single TiO termination. In this heterostructure, stanceintheinterfacebetweenoxygen-reducedSTOand
2
c the alternating LaO and AlO planes carry opposite vacuum.[12,13]Aspectaculardemonstrationofsuchbe-
2
[
charges, leading to a net polarization of the LAO film, haviorinLAO/STOinterfaceswasprovidedbythe work
1 in contrast with the non-polar state of STO. Such polar of Chen, et al., [14] showing that the interface between
v discontinuity was indicated as a source of instability, crystallineSTO andamorphousLAOcanbe conductive.
4 resulting in the injection of electrons from the topmost Recently,theroleofoxygenvacanciesininterfacesbased
7 LAO layers into the Ti3d states at the interface, thus on either a crystalline (c), or an amorphous (a) LAO
3
giving rise to the interfacial 2DEG. [1] The above layers on STO was further clarified by observing the ef-
3
described ”electronic reconstruction” (ER) model is fects of thermal and ion irradiation treatments on both
0
. confirmed by ab initio computations[2–4] and provides kinds of samples.[15, 16] The results suggest that in c-
1
a natural explanation of some well established exper- LAO/STO the conductivity can be ascribed to both ER
0
imental results, as the need of TiO -terminated STO and oxygen vacancies, depending on the sample growth
5 2
substrates and a LAO film critical thickness of 4 unit conditions, whereas in a-LAO/STO only oxygen vacan-
1
: cells to achieve interface conductivity.[5, 6] Experiments cies play a role.
v
and theoretical calculations also agree on the bending Thesensitivityofc-LAO/STOtothelightwasdiscov-
i
X of the electronic bands of STO close to the interface, ered severalyears ago.[17–19] More recently, the investi-
r as an effect of the local electric field. Such bending gation of the transport properties of irradiated samples
a determines a triangular quantum well that confines the revealed its potential interest for both applicative and
2DEG within a few nanometers. [4, 7, 8] fundamental physics.[20–26] Photoconductivity was re-
It was soon realized that, beside the ER model, other ported to be moderate when samples were illuminated
mechanisms may explain the presence of free electrons by visible light, and it became considerably larger above
populating of the quantum well. In particular, each aphotonenergythresholdofabout3.25eV(382nm),[20]
oxygen vacancy introduced in STO acts as an electron correspondingtotheindirectbandgapofSTO.Astriking
donor, thus explaining the observed dependence of the featureofc-LAO/STOisthepersistenceofthephotocur-
carrier number on the oxygen partial pressure during rents after restoring the dark conditions; the character-
the LAO film growth.[9, 10] It was also suggested that istic times range from 10 to 104 seconds or even more,
depending on the sample. In presence of point defects,
bareSTOcanalsoshowpersistentphotoconductivity,[27]
resemblingtheclassicallattice-relaxationofdopedGaAs,
∗ Correspondingauthor: [email protected] CdZn,CdS,etc.[28]Thecaseofc-LAO/STOmayinstead
2
beassociatedwith,e.g.,GaAs/AlGaAsheterostructures,
TABLE I. Transport data at room temperature in dark con-
wherethepersistentphotoconductivityisascribedtothe
dition for all samples.
spatial separation of the electron-hole pair under the ef-
fect of the local electric field.[29, 30] Sample d (nm) V (V) Id (A) Rd (Ω)
Inthis paperweinvestigatethe photoresponseofboth C1 2.4 1 23.2 µ 43 k
a- and c-LAO/STO.Our aim is to provide a wider char- A1 2.4 1 15.6 µ 64 k
acterization of the interfaces based on amorphous LAO, A2 1.2 30 3.5 n 8.6 G
thatarecertainlylessexplored,andtocomparetheirbe-
S1 - 500 2 p 250 T
haviortothatofcrystallineinterfaces. Potentially,thisis
averyinterestingissue,becausethecrucialdifferencebe-
tween the two systems consists in the amount of oxygen
vacancies that, acting as point defects, may play a key
role in the photoresponse. In the following, we present
data regarding two samples of a-LAO/STO with differ-
ent thickness, one c-LAO/STO and one bare STO sub-
strate. We stress that, according to the mentioned atti-
tude of LAO to oxidize at the expense of STO, and due
to the differentgrowthconditions we adopted,the inves-
tigated a-LAO/STO samples contain much more point
defects, i.e., oxygen vacancies, than c-LAO/STO. Fur-
ther details on the fabrication are given in the Method
section. It turned out that the time evolution of the
photoconductance, during the exposition to radiation of
different wavelengths in the visible-ultraviolet (VIS-UV)
region, and after having turned the light irradiation off,
shows clear differences between different types of sam-
ples. Weanalyzethepossiblemicroscopicmechanismsof FIG. 1. Resistance vs. temperature plots of C1, A1 and A2
samples
carrier photogeneration, and propose that the different
behavior is ascribed to different band diagrams for both
structures. This is discussed in terms of naive diagrams
for a- and c-LAO/STO. R(T)plot)areobserved,inagreementwithliterature.[14,
15] Basically,A1 shares with the crystalline interface C1
quite similar transport properties in dark.
RESULTS AND DISCUSSION To measurethe samplesphotoresponsewe selectthree
different wavelengths: 365 nm (3.4 eV, above the indi-
rectbadgapofSTO);400nm(3.1eV,belowthe indirect
The four samples used in transport and photo-
badgap of STO); 460 nm (2.7 eV, well below the indi-
transport measurements are labelled as follows:
C1 - a conducting 6 u.c. (thickness d≃2.4 nm) c- rect badgap of STO). We define the photocurrent Iph(t)
as the difference between the current I(t), measured at
LAO/STO heterostructure;
time t during the light illumination, and the current in
A1 - a conducting a-LAO/STO interface, with an esti-
mated LAO thickness d≃2.4 nm (similar to C1); darkconditionsbeforetheirradiation,Id. Then,thepho-
A2 - an insulating a-LAO/STO interface, with an esti- toconductance is given by σph(t) = IphV(t), V being the
mated LAO thickness d≃1.2 nm; voltage drop across the two probes. The time depen-
S1 - a bare TiO -terminated STO single crystal. dent photoconductance of C1, A1, A2, and S1 at three
2
different wavelengths is reported in fig. 2.
In dark, S1 exhibits a very high resistance (250 TΩ at
roomtemperature,closetothelimitofourmeasurement The photoresponsedynamics of allthe measuredsam-
setup), characteristic of fully oxidized, non doped STO plesshowarelativelysteepincrease,followedbyaslower
crystals. The deposition of a very thin amorphous LAO transient behavior. However, the intensity is dramati-
overlayer (sample A2) determines a noticeable change: cally different for each case. In table II we report the
the room temperature resistance in dark is lower by 4 value of photoconductance after 300 s of illumination,
orders of magnitude with respect to S1 (Table I). σpmhax , for each sample and wavelength. At 365 nm,
σmax is of the order of µS for A1 and C1 samples,
However, A2 still shows an insulating behavior, i.e., a ph
while it is 3 and 6 orders of magnitude smaller for A2
negative slope in the R(T) plot (fig. 1), suggesting that
and S1, respectively. Conversely, the photosensitivity
thefabricationprocessintroducedsomeoxygenvacancies
in the structure, but their amount is still insufficient to Ψph(t) = σpσhd(t) = IpIhd(t) is maximum in S1, lower in
determine the formation of a mobile 2DEG. When the A2 and even lower in A1 and C1.
amorphous LAO film is thicker (A1), a dramatic dropof These preliminary observations allow to outline the
resistance and a metallic behavior (positive slope in the general framework. A set of donor states (possibly dif-
3
tions,thesestatesarecompletelyfilledintheconducting
interfaces,sothatallthe electronspromotedbythelight
1.0 C1 365 nm a) must contribute to the conduction. Instead, they are (at
0.8
S) 400 nm leastpartially)empty inA2. Therefore,a fractionofthe
0.6 460 nm
(ph 0.4 photo-generated electrons may occupy these states and
may not contribute to the electrical conduction of A2,
0.2
i.e., it may not be detected in the photoresponse mea-
0.0
surements.
1.2 A1 b) Since the same photon flux is adopted at all wave-
S) 1.0
length, and considering that each process of photo-
(ph00..68 excitation generates one free carrier, it follows that the
0.4 variation of photoconductance with wavelength just re-
0.2 flects the variation of photogeneration efficiency. The
0.0 observation that the maximum photocondutance σmax
ph
6.0 A2 c) systematicallyincreasesatdecreasingtheradiationwave-
nS) 4.5 length is then easily understood, as higher energy pho-
(ph3.0 tonscanexcitedeeperstates;and,inparticular,photons
at 365 nm can, in principle, promote electrons from the
1.5
STO valence band (VB) to the conduction band (CB).
0.0 The ratio between the values of σmax at 365nm and
ph
1.0 S1 d) 460nmishigherintheinsulatingsamples(S1,A2),than
S) 0.8 in the conducting samples (A1, C1). Still in the same
p
(ph0.6 spirit of a na¨ıve interpretation, this suggests that the
0.4 trapstatesformanarrowband,closetothevalenceband
0.2 edge, in S1 and in A2, so that low energy photons can
0.0 hardlypromoteelectronstotheCB.Thetrapstatesform
0 250 500 750 1000 1250 1500 instead a broad band in A1, and still broader in C1. In
Time (s) other terms, the effective optical gap at the interface is
FIG. 2. a), b), and c) Photoconductance vs. time of C1, A1 always smaller than the band gap of bulk STO; and it
andA2interfaces;d)photoconductancevs. timeofbareSTO issmallerinc-LAO/STOinterfacesthanina-LAO/STO
(sampleS1). Thephotonenergy at 365nm(3.4 eV)isabove ones.
the indirect STO gap; at 400 nm (3.1 eV) and 460 nm (2.7
eV) it is below this threshold. All samples were illuminated
for the same time (for 300 s) at equal photon flux (∼ 2x1014 TABLEII.Photoconductanceandphotosensitivityvaluesaf-
photons cm−2s−1) for each wavelength. ter300 s of illumination for thethree different wavelengths.
Sample λ(nm) σmax (nS) Ψmax
ph
365 991 0.043
ferentforS1,A1,A2andC1)canbeexcitedbyphotons. C1 400 524 0.023
The photoresponse is directly related to the number ∆n 460 267 0.012
of electrons promoted into the conduction band. In the 365 1285 0.082
steady-stateregime,∆nisdeterminedbythebalancebe- A1 400 285 0.018
tween the excitation and the recombination rates. The
460 75 0.005
enhancement of the photoconductance in the sequence
365 6.3 54
S1,A2,andC1/A1thensuggeststhattherecombination
A2 400 0.666 5.7
ratebecomeslowerandlowerwhentheLAOthicknessin-
460 0.296 2.5
creases. Underthispointofview,ourresultsallowtoex-
tendtothecaseofa-LAO/STOapreviousinterpretation 365 8.8·10−4 220
of the c-LAO/STO photoresponse.[23] The argument is S1 400 9.8·10−5 25
also qualitatively confirmed by the different duration of 460 1.2·10−5 3
the tails after turning the illumination off; in fact, the
recovery rate increases in the sequence C1/A1, A2, S1.
Inordertogetamoreaccuratedescriptionofthe pho-
However, this enhancement may also have an alterna- toresponse,itisnecessaryto discussthe dynamicbehav-
tive explanation. It hasbeen proposed[2] thatthe deep- ior of the signals. We first start addressing the issue of
est electronic states in the quantum well of c-LAO/STO the increase of photoconductance as a function of the
interfaces have low or null mobility. It is reasonable to illumination time (see fig. 2). The plots of σ (t) are
ph
assume that the same also holds for a-LAO/STO, since characterizedby a steep onset followedby a slowertran-
the electronic properties of the 2DEG are mainly deter- sient. Thisbehavioriswelldescribedbyabi-exponential
mined by the STO crystal in both cases. In dark condi- growthfunctionwithtwocharacteristicrisetimes τ and
1
4
τ , relatedto the fast and slow components respectively: of the fast and slow component of the signal, i.e., the A
2
parameter. This difference is marked by the dependence
σ (t)=σ A 1−exp − t + of A vs. radiation wavelength for the two samples (fig.
ph inf
(cid:26) (cid:20) (cid:18) τ (cid:19)(cid:21) 3c).
1
t
(1−A) 1−exp − (1)
(cid:20) (cid:18) τ2(cid:19)(cid:21)(cid:27) TABLEIII.Bestfittingparametersforthethethreedifferent
illumination wavelengths.
where σ is the saturation value of the photoconduc-
inf
tance and A represents the relative weight of the fast Sample λ(nm) σinf (nS) A τ1(s) τ2(s)
component. 365 990 0.78 0 82
C1 400 553 0.53 9 147
460 295 0.19 24 140
1.0 a) 365 1420 0.33 1.3 166
0.8 A1 400 322 0.56 4 228
inf0.6 460 74 1 40 –
/ph0.4 365 nm
400 nm In order to address this issue, we first remind that the
0.2
460 nm
0.0 C1 selected wavelengths correspondto photon energies that
span from slightly above to well below the indirect STO
1.0 b) gap. In sample C1, the fast component is progressively
0.8 reduced with increasing the wavelength, confirming the
inf0.6 previous observations for crystalline samples.[23] At 460
/ph0.4 nm, well below the gapthreshold, the fast channel is ac-
tually almost suppressedand the signalrise is veryslow.
0.2
A1 The opposite behavior is observed in sample A1, where
0.0 the relative amplitude of the fast component increases
0 50 100 150 200 250 300 at increasing wavelength. At 460 nm, the slow channel
Time (s) is totally suppressed, so that the signal is well described
by a simple exponential growth with one characteristic
1.0 c)
time τ . Such different response of C1 and A1 undoubt-
1
0.8
edlyindicatesadifferentunderlyingmechanismofphoto-
A 0.6 excitation, which distinguishes a- from c-LAO/STO.
0.4 We consider now the delicate issue of the microscopic
0.2 A1 description of carriers photo-excitation. The modelling
C1 of the dynamics in terms of band diagrams is neither
0.0
straightforward nor unambiguous; however, we believe
360 380 400 420 440 460 that the main physics is captured by the naive sketches
(nm) of fig. 4. The right panel (fig. 4b) is based on published
FIG. 3. a) Photoresponse of C1 and b) photoresponse of A1 diagramsfor c-LAO/STO;[23,26,31] the left panel(fig.
at 365 nm (black), 400 nm (red), and 460 nm (green). The 4a)describesinsteada-LAO/STO.Therightsidesofthe
data are normalized to the asymptotic value σ ; solid lines band diagramsfor a-LAO/STOand c-LAO/STOdeeply
inf
are fit curves. c) Dependence of A vs. radiation wavelength differ the one from each other and are in agreement
λ for both samples. with those already reported in [16]. For a-LAO/STO,
we draw, as an ansatz, a scheme qualitatively similar
We focus now the attention on the metallic samples, to the one for bulk LAO crystal. For c-LAO/STO, in-
C1 andA1 samples. The fitting parametersarereported stead, we adopt the standard ER model, with the bands
in Table III, and the fits are shown in fig. 3. In our ex- of LAO bent upwards by the macroscopic electric field.
perimental conditions, the maximum photoconductance Theleftpartofthediagramsofbotha-andc-LAO/STO
scales with light intensity, i.e., the optical pumping is shows the quantum well that is formed at the interface,
below the threshold at which the response falls in the within STO, and that confines the mobile electrons in
saturationregime. This circumstance allows the connec- the 2DEG. The point defects, which we mainly identify
tionbetween the dynamics ofthe photoconductance and with oxygen vacancies, add to the bands a distribution
the photogeneration mechanism of carriers. On this ba- of trap states, forming a broad in-gap band. Such states
sis, we interpret the elongation of the fast time τ at are mainly localized in a region close to the interface,
1
increasingwavelengthasaneffectofthedecreaseofpho- because, as previously mentioned, they are formed dur-
togeneration efficiency. This mechanism is common to ing the film growth step.[15] Due to the different fab-
both C1 and A1. Instead, a striking difference between rication procedure, the density of vacancies is lower in
the samples is found in terms of the relative amplitude c-LAO/STO, and as a first approximation we neglect
5
it. The sketches show some possible channels of pho- 2. In c-LAO/STO, the excitation by low energy pho-
tons is a slow process, that promotes to the quan-
tum wellelectrons fromthe LAO VB;the fastpro-
cess requires, instead, high energy photons, and
consistsintheionization(possiblyphononassisted)
of STO VB states.
3. In a-LAO/STO, on the contrary, the excitation
by low energy photons is a fast process, promot-
ing to the quantum well electrons from the in-gap
trap states and the slow process (promoting elec-
tron from the LAO VB) requires high energy pho-
tons. Furthermore, the direct excitation of STO
VB states in a-LAO/STO may trigger a complex
energycascadewithinthe broadin-gapband. This
effectwouldalsoindirectlycontributetoslowdown
FIG.4. Sketchofthebandstructureofa)a-LAO/STOandb)
thedynamicsofexcitationbyhighenergyphotons.
c-LAO/STO. The grey dotted region depicts the in-gap trap
states of STO. For clarity, the bending of the STO bands is
We stress that our data would be hardly explained by
emphasizedinbothpicturesand,inc-LAO/STO,thebandof
assuming that all the relevant transitions are due to the
in-gapstatesisomitted. Theslow(s)andfast(f)mechanisms
excitation of one type of trap states, i.e., oxygen vacan-
of photogeneration are represented byarrows.
cies only. In particular, this would be at odds with the
higherphotoconductanceinducedbylowenergyphotons
togeneration, labeled as s or f to indicate slow and fast
in c-LAO/STO than in a-LAO/STO (see, e.g., data of
dynamics, respectively. The main channel of fast exci-
σmax in table II). In fact, the oxygenvacancy states cer-
tation is the local (on-site) promotion of electrons from ph
tainly form a broader band in a-LAO/STO than in c-
initial states with O-character to final states of the CB
LAO/STO.
with Ti-character; this process may actually involve in-
termediate states, such as excitons (i.e., localized Ti3+ We finally add some comments on the bending
of c-LAO bands (fig. 4b). In contrast to first-
states),[32] which rapidly decay into the CB. The slow
principle computations, [2–4] the bending was not
processis here identifiedwith the promotionofelectrons
observed in some x ray photoemission spectroscopy
from LAO states with O-character to STO states with
(XPS) measurements.[34–36] In other XPS works (e.g.,
Ti-character. These transitions are important, because
[37]), as well as in cross-sectional scanning tunnelling
they can involveinitial and final states with non negligi-
spectroscopy,[38]itappearedwithreducedslopewithre-
bleoverlap. Inthecaseofc-LAO,forinstance,theclosest
spect to expectations. In [35], the authors propose two
OatomofLAOisactuallyintheapicalsiteofaTiatom
effects (that in our view may concur) to explain the dis-
of the STO side. A similar consideration can be applied
crepancy between theory and experiment: a) during the
also to the case of a-LAO. However, there is a relevant
XPS experiment, the sample is heavily far from equilib-
difference between the twocases. All the VB-edge states
riumduetoxrayphotoexcitation;b)thesurfacecharge-
of LAO share the same energy in a-LAO/STO. Instead,
trapstatesactasdonors,sothatevenatequilibriumthe
as the sketch in fig. 4b qualitatively shows, the upturn
bandbending ofc-LAOis attenuated. Bothmechanisms
of the bands in c-LAO is so steep, that the VB edge is
alreadyliftedby≃0.2-0.3eVattheapicalsites. Thefar- mayalsoaffectthephotoresponseandgivequantitatively
relevant corrections. However, we believe that the sim-
thertheinitialstatesarelocated,thelargeristheenergy
plequalitativepicturethatweproposewouldnotchange
lift.
also taking into account these extra channels.
The sketch makes no claim to completeness; more ex-
We turnnow to the issue ofthe signaldecayafter illu-
citation mechanisms can be at play and also be quan-
mination (fig. 5). As a first comment, we note that this
titatively relevant (e.g., the transitions from the defect
process is affected by the sample history. Depending on
states of STO in c-LAO/STO, such as photoexcitations
the durationandonthe intensity ofpreviousexpositions
of carriers from defect-induced deep-level traps as sug-
tothelight,aswellasonthewavelength,weobservedif-
gested in [33], or from any other kind of surface trap
ferentvaluesofthefinalbaseline,indicativeofmetastable
state). However,the minimum set of transitions that we
states. The further relaxation toward the initial equilib-
showinthediagramsissufficienttoqualitativelyexplain
rium state in dark may be extremely long. We did not
several experimental observations:
exploresystematicallythisfeatureinthiswork. However
1. The reduction of the optical gap in a-LAO/STO it appears that the recoverytowardthe metastable state
(withrespecttotheindirectSTOgap)isexplained is always a process longer than the signal rise one.
as an effect of oxygen vacancies; in c-LAO/STO, As mentionedbefore,the increaseofthe characteristic
instead, the reduction of the optical gapis a result decay time with respect to the rise transient is indica-
of the band bending of LAO. tive of a hysteretic behavior that can be attributed to
6
LAO/STO.
a)
) 1 365 nm C1 The transient photoconductance of all samples shows
0
(t 400 nm a bi-exponential growth,under illumination, followedby
h
p 460 nm a long decay when the light is turned off. In spite of
ph the qualitative similarities, the photoconductance dy-
namics reveals a deep difference between a-LAO/STO
andc-LAO/STO.Inc-LAO/STO,thefastchannelofthe
0,1 photoresponse becomes progressively less relevant at in-
b) creasing photon wavelength; the reverse takes place in
) 1 A1
(t0 a-LAO/STO, where it is the slow channel loosing rela-
ph tive weight at increasing wavelength.
/h We propose a different mechanism of excitation for a-
p
LAO/STOandc-LAO/STO,whichnaturallystemsfrom
the different band diagrams of the two structures. In-
deed,theopticalexcitationofa-LAO/STOinvolvestran-
0,1
sitions from either in-gap trap states due to oxygen va-
300 500 700 900 1100 1300 1500
cancies (fast channel) or the LAO VB (slow channel).
Time (s)
Thefirstchannelisactivatedbyphotonsinawide range
FIG.5. Signaldecayafterswitchingofftheilluminationfora)
ofenergy,whilethesecondisrestrictedtoabove-gappho-
C1andb)A1samples. Thesamedataoffig. 2arenormalized
tons. The excitation of c-LAO/STO also involves two
to thecorresponding σmax values and plotted in log scale.
ph channels. ThefirstincludesthetransitionsfromtheSTO
VB (fast channel) and is restricted to high energy pho-
tons; the second includes the transitions from the bent
the following mechanisms: a) the activation of a slow
LAO VB band (slow channel), and is activated even by
energy cascade through intermediate defect states, and
low energy photons.
b) the drift of photo-generatedholes under the action of
Finally, a-LAO/STOandc-LAO/STOalsohavea dif-
the macroscopic electric field in the region close to the
ferent behavior of permanent photoconductance when
interface. As previously discussedwhen commenting the
lightis turned off, indicating that the physicaldifference
signal rise dynamics, the former mechanism may have
between the structures also determines different dynam-
comparativelylarger weight in a-LAO/STOafter UV ir-
ics of relaxation.
radiation. Inthatcasethedeeplevelsinthebroadin-gap
band are excited, and part of them might be long-lived.
The latter mechanismmay instead affect the recoveryof
METHOD
c-LAO/STO after visible light irradiation, if some holes
createdintheLAOregionarepushedtowardthe sample
surface by the electric field that acts in the polar layer. The LAO thin films are grown by a RHEED-assisted
Pulsed Laser Deposition technique on TiO -terminated
2
(001) STO single crystal substrates, resorting to a KrF
CONCLUSION excimerlaserbeamwithafluence of1.5J/cm2 ontarget
and a repetition rate of 1 Hz. The crystalline sample
◦
We investigated the transient photoconductance of C1 is grown at 720 C with a partial pressure of oxygen
amorphous-LAO/STO interfaces, under monochromatic PA(O2)=1x10−2mbar. Afterthedeposition,thesample
radiationin the UV and Vis region, and comparedthese iscooledin1htoroomtemperatureinthesamepressure
systems with a crystalline-LAO/STO structure and a condition. TheamorphoussamplesA1andA2aregrown
bare STO crystal. In all the samples, the photoconduc- at room temperature with a PA(O2) = 1x10−5mbar.
tance assumes the highest value at the shortestexplored Allthesamplesarecontactedbyultrasonicallybonded
wavelength (365 nm, corresponding to above-gap exci- aluminiumwires. Theresistanceasafunctionofthetem-
tation). At longer wavelengths, the photoconductance perature R(T) is recorded in dark condition in a closed
significantly decreases in c-LAO/STO. The reduction is cyclerefrigerator,usingthe standardfourprobe Vander
even stronger in conducting a-LAO/STO, and dramatic Paw configuration for C1 and A1, and a two probe con-
in the insulating a-LAO/STO sample and in the bare figuration for A2 and S1, due to their high resistance
STO crystal. This observation indicates that the opti- values. In all the photoconductance measurements, the
cal gap of the interfaces is smaller than the band gap current across the samples is measured at fixed voltage
of bulk STO; and that it is the smallest in the case inthetwoprobeconfiguration,byresortingtoaKeithley
of c-LAO/STO. This effect would be hardly explained 6487 picoammeter. All the values of applied voltage V,
by assuming that the photogeneration takes place in all dark current Id, and resistance in dark Rd, are reported
samples by exciting the same states (e.g. in-gap states in Table I.
due to oxygen vacancies), since vacancies certainly gen- In order to investigate the time evolution of the pho-
erate a broader defects band in a-LAO/STO than in c- toconductance, the whole sample surface (∼5×5 mm2)
7
is uniformly illuminated for 300 s with monochromatic is measured by a radiometer (Laser Precision RK-5720
radiation obtained by means of a Xenon lamp and of Power Radiometer) equipped with a silicon probe. The
band-pass interference filters (FWHM = 10 ± 2 nm). light intensity was attenuated by means of neutral den-
Threewavelengthsareselected: 365nm(3.4eV);400nm sity filters in order to obtain a photon flux of ∼ 2x1014
(3.1 eV); 460 nm (2.7 eV). The power density radiation photons cm−2s−1 for the three wavelengths.
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band-offset profiling of nanostructured LaAlO3/SrTiO3
heterojunctions probed by element-specific resonant
spectroscopies. Phys. Rev. B 90, 035124 (2014). A.K. and E.D.G. prepared the samples, E.D.G. per-
formed the cryogenic transport measurements, U.C.
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functional theory study of the internal electric field in and G.A. carried out the phototransport measurements,
SrTiO3/LaAlO3 oxide heterostructures. Phys. Rev. B U.S.d.U. and F.M.G. wrote the main manuscript text.
87, 085128 (2013). All authors reviewed the manuscript.
