1
Scientific RepoRts | 7: 7457 | DOI:10.1038/s41598-017-07568- z
www .nature.com/scientificreports
Radiative recombination of
confined electrons at the MgZnO/
ZnO heterojunction interface
Sumin Choi 1 , David J. R ogers 2 , Eric V . Sandana 2 , Philippe Bove 2 , Ferechteh H. T ehe rani 2 ,
Christian Nenstiel 3 , Axel Hoffmann 3 , Ryan McClintock 4 , Manijeh Razeghi 4 , David Look 5 ,
Angus Gentle 1 , Matthew R. Phillips 1 & Cuong T on- That 1
W e investigate the optical signature of the interface in a single MgZnO/ZnO heterojunction, which
exhibits two orders of magnitude lower resistivity and 10 times higher electron mobility compared
with the MgZnO/Al 2 O 3 film grown under the same conditions. These impressive transport properties
are attributed to increased mobility of electrons at the MgZnO/ZnO heterojunction interface. Depth-
resolved cathodoluminescence and photoluminescence studies re veal a 3.2 eV H -band optical emission
from the heterointerface, which exhibits ex citonic properties and a localization energy of 19.6 meV .
The emission is attributed to band-bending due to the polarization discontinuity at the interface, which
leads to formation of a triangular quantum well and localized excitons by electrostatic coupling.
Oxide heter ojun c tion s are a s ubject of stro ng cur r ent in teres t b ecau s e they pr ovide a wealth of n ovel function -
alities tha t can be exploit ed in a broad ran ge of curren tly emerging t e chn ologies inv olving ultra fast electronics,
high-sensitivi ty chemical s enso rs and q uant um technology 1 – 4 . The electric al trans port pro p erties of the MgZnO/
ZnO interface ar e part icularly ex citing fo l lo wing the obser vation o f two dimensio nal elec tro n gas (2DEG) fo r -
ma tion at MgZnO/ZnO hetero interfaces d ue to a stron g built-in pot ential arising fro m p ola rization misma tch 5 .
These 2DEG sam ples sho w remar kably high electron m obilities (in exces s of 10 6 cm 2 /V·s a t 40 mK 6 ) and exhi bit
the frac tion a l q uant um Hall effect 5 . While the electronic tran sport pro p erties of MgZnO/Z nO heter ojun c tion s
ha ve been investiga ted by va rious gro ups 7 , 8 , few studies ha ve focused on its o ptical pro per ties. T o date, o nly op t i-
cal inv estigation s of indir ec t exci ton s hav e b een carried out in MgZnO/ZnO quan tum wells 7 , 9 , 10 . H owever , op ti -
cal characteriza tion of the MgZnO/ZnO interface using s uch double h eteroj unction system s has two significan t
disadvan tages. First, the r ecomb ination o f elect r ons fro m sub-bands in exci ted quan tum wells us ua lly take place
because of sho rt recomb inatio n lifetimes, leading to exci ton bl ue shift and w ell size-dep enden t electron-o ptic
effects 11 . Se co nd, the electron mob ility in quan tum w ells is typically lower tha n in single hetero junctions.
An op tical interface emissio n (calle d the H -ba nd) has been obser ved in AlGaAs/GaA s 12 and AlGaN/GaN 13 – 15
single heter ostruc tur es at lo w tem peratur es (T < 20 K), which h as be en a ttribu ted to radiativ e recom bina tion
between phot o exci ted holes an d elect r ons co nfined at the in terface. Similarly , d ue to its lar ge band o ffs et an d
stro ng strain ind uced piezoelec tric field, the MgZnO/Z nO in terface is expe cted to p roduce a 2DEG a nd an asso-
ciat ed H -b a nd emission tha t is mor e rob ust at higher t emperat ures because of the high exci ton b inding energy
in ZnO . In this wo rk, MgZnO/Z nO single heter ostr uctur es grown b y Pulsed L aser Dep ositio n (PLD) ar e studied
using co rrelati ve luminescence an d elec trical characteriza tion techniques. N otably , dep th-resolved ca tho dol u -
minescence (CL) spect r oscopy was em plo yed to in vestiga te the optical emissio n phenom enon a t the MgZ nO/
ZnO interface. The o ptical pro per ties of MgZnO/ZnO core-she ll nanowir es hav e b een in vestiga ted previous ly by
other wo rkers 16 , 17 . The au thors r ep o r ted an enha ncemen t of the near -band-edge (NBE) l uminescence; how ever ,
the exact cause of the incr ease could not be identified unam biguous ly due to the difficulty of s patially resolving
the int erface geometr y in nan owire co re-she ll str uctur es. Iden tificatio n of op tica l signa ture for MgZnO/Z nO
heter ostruc tur es cons titu tes a seminal step in the understan ding of localized excito ns a t the oxide in terface and
can ha ve ma jor imp lication s for the in terpreta tion of o ptical and electrica l measur ements.
1 School of Mathematical and Physical Science, University of T echnology Sydney , Broadway , PO Box 123, NSW 2007,
Australia. 2 Nanovation, 8 Route de Chevreuse, 78117, Châteaufort, France. 3 Institut für Festk örperphysik, T echnische
Universität Berlin, 10623, Berlin, German y . 4 Center for Quantum Devices, ECE Department, Northwestern
University , Evanston, IL, 60208, USA. 5 Semiconductor R esearch Centre, Wright State University , Dayton, OH, 45435,
USA. Correspondence and requests for materials should be addressed to C. T .- T . ( email: [email protected] )
R eceived: 31 October 2016
A ccepted: 28 June 2017
P ublished: xx xx xxxx
OPEN

www .nature.com/scientificreports/
2
Scientific RepoRts | 7: 7457 | DOI:10.1038/s41598-017-07568- z
R esults and Discussion
Enhanced conductivity in the MgZnO/ZnO bilayer . The fo ur-po int r esistivity o f t he MgZnO/ZnO/
sap phire b ilayer is 0.006 Ω .cm, which is mo re than tw o order s of magnit ude lower tha n that o f the consti tuent
MgZnO fi lm an d an or der of magnit ude lower tha n t ha t of the con stituen t ZnO ( ρ = 3.0 Ω .cm and 0.1 Ω .cm for
the single-lay er MgZ nO and ZnO films, r espe ctivel y). This resistivi ty value is exceptio nally low in absol ute term s
fo r a Z nO film withou t inten tional shallow do nor do ping 18 . The rep roducib ility of the measur ed resistivity value
was confirmed b y collect ing da ta at several separat e areas of sa mple wi th various fo r ward a nd reverse in je ction
curren ts and repea ting the analysis in thr ee differen t labora tories. This enhan ced conduction is a ttrib uted to
electron co nfinement a t t he smooth MgZnO/ZnO interface. The surface r oughness of the ZnO film was meas -
ured to be in the ran ge 0.3–0.9 nm ov er an at omic fo rce microsco py (AFM) scan ar ea of 5 × 5 μ m 2 (shown in
Su pplemen tar y Fig.S1(b)). Figur e 1 sho ws tem peratur e-dependent H all mobility a nd carrier concen tration f or
each of the films. All sam ples sho w n-typ e beha vior . Fo r t he ZnO single lay er , the mob ility is about 20 cm 2 /V·s
a t room tem perat ure (R T) and decreases linearl y with tem peratur e to a ppr oxima tel y 16 cm 2 /V·s at 77 K. The
corr esponding carrier concen tratio n ris es fro m a R T value of 4.0 × 10 18 cm − 3 to a p latea u of 4.4 × 10 18 cm − 3 f rom
180 K t o 77 K. F or the MgZnO single layer , the R T mobili ty of 3.5 cm 2 /V·s is significan tly low er than that o f the
ZnO single layer . In a similar ma nner to the ZnO layer , the MgZnO mobility decreases linearl y with tempera ture
to ~0.8 cm 2 /V·s at 77 K. The corres p o nding carrier concen tratio n is an or der of magni tude low er than for the
ZnO and rises f ro m a R T value of 1.2 × 10 18 cm − 3 to 1.5 × 10 18 cm − 3 a t 77 K. A t 42 cm 2 /V·s, the R T mobility f or
the MgZnO/Z nO bila yer is significantly higher than f or the ZnO or MgZnO single layers. A s the tem p era ture
decreases, the mobili ty of the bilay er remain s fairly co nstan t down to ~200 K, after which it dr ops to 31 cm 2 /V·s
a t 77 K. This tem peratur e-dependent beha vior is differ ent fro m those of the Z nO o r MgZ nO single la yers, which
exhibi t mono tonic decr eases in mobility wi th de cr easing tem p era tur e. This could be caused b y the presence o f
highly mobile electro ns and in terface roughness sca ttering. The carrier concen tratio n of the bilay er is higher than
fo r t he ZnO single lay er and dro ps from a R T va l ue of 5.0 × 10 18 cm − 3 to a minim um value of 4.8 × 10 18 cm − 3 at
180 K.
Luminescence band originating from the he terostructure interface. Figur e 2 sho ws photo lumi -
nescence (PL) spectra for the ZnO , MgZnO , and MgZnO/Z nO sam ples. The ZnO spe ctrum exhibits a do minan t
dono r -b oun d excito n (D 0 X) pea k a t 3.365 eV , a ttribut ed to an I 3 ionized don or , whi le a weaker peak on the right
sho ulder at 3.385 eV is d ue to free exci ton tran sitions ( FX) 19 , 20 . The pea ks a t 3.295 eV and 3.221 eV ar e the first a nd
second or der longi tudinal op tic al (LO) ph onon r eplicas of the FX, r espec tive ly , with a cons tant spacin g e q ua l to
the ZnO LO phon on energy of 72 m eV . The pr esence of the pho non r eplica pea ks indica tes high optical quali ty
of the film. The b road NBE emission o f the MgZ nO an d MgZ nO/MgO films a t 3.570 eV a re bl ue shifted with
respect to tha t of the ZnO due to the enlarged bandga p c a used by Mg substi tutio n on Zn sites 21 . The b roadening
Figure 1. T emperat ure dependen t Hall effect measurem ents co nducted on the MgZnO/Z nO b il a yer and
con stituen t single-lay er Z nO and MgZnO films on c-sa pphir e substra te, which yield electron mo bility an d
concen tratio n. A t R T , t he b ilayer exhib its two o rders o f magnitude lo wer resi stivity and 10 times higher electro n
mob ility com pared with the MgZnO/sapp hire film gro wn under the s am e conditio ns.

www .nature.com/scientificreports/
3
Scientific RepoRts | 7: 7457 | DOI:10.1038/s41598-017-07568- z
of the MgZnO emission i s due to poten tial fluctuatio n effects, arising from va riation in the Mg co mposition a nd
lat tice disorder in the MgZnO alloy 22 . The a bsorption edges in R T optical transmis sion spectra (Sup plemen tar y
Fig.S3) co rresponded to a ban dgap energy of 3.61 eV for the MgZnO/Z nO . This corres ponds to an Mg co nt ent
of 16.4 a t% 23 , 24 , which is 2.7 a t% more tha n the nominal com position o f the MgZ nO PLD tar get. Such a n Mg
enhancemen t com pared to the tar get concen t ra tion has been reported elsewhere fo r PLD growth o f MgZ nO an d
was a ttribut ed to a differen ce in t he va por pr essur es of Mg and Zn 18 . Th e two sharp pea ks a t 3.671 eV and 3.743 eV ,
located a t 72 meV and 144 meV from the laser exci tation line , are E 1 (L O) and 2E 1 (LO) Raman modes o f Z nO . The
most significan t feat ure in Fig. 2 is the em ergence of a b road emissio n band at ~3.2 eV (denoted H -ban d) in t he
MgZnO/Z nO bila yer . This luminescence band is absen t in both the single Z nO and MgZnO films, suggestin g t ha t
it o rig ina tes fro m t he het eroj unction int erface.
Depth-resolved CL microanalysis of the MgZnO/ZnO interface H -band. T o verif y the origin of t he
3.2 eV band, depth-r es ol ved CL was cond uc ted o n e ach o f the three films. In this measur ement the electro n b eam
power a nd hence the electron-h ole (e-h) pair genera tion ra te in the sam ple was kept co nstan t as the CL exci tation
depth was incr eas ed b y raising the electron beam ener g y . The Z nO an d MgZ nO films exhib it iden tic al CL spect ra
a t a ll accelera ting vol tages confirming the in-depth ho mogenei ty of the layer (S up plementa r y Fig.S3a). T o obtain
depth-dependent dependences o f the CL in the MgZ nO/ZnO bilayer , M on te Carlo modeling was carried ou t
using the CAS INO simula tion packag e 25 to determine the electron ener g y loss pr ofile with the electron beam
exci tation ener g y . Figure 3a sho ws the energy loss pro files of electro ns in the bilay er for acce leration v oltages
between 2 kV and 10 kV ob tained from the CAS INO simula tions. These ener g y loss pr ofiles are co mpa rable with
those obtained fo r ZnO by other wo rkers 26 , 27 . The M ont e Carlo simula tion of the b il a yer reveals tha t the elec tr on
beam reaches the 260 nm deep Z nO lay er at 5 kV and starts penetrating in to the s a pphir e substra te from 7 kV .
The depth-r esolved CL measur emen ts of the bila yer (S upp lementary Fig.S3b) ar e consis tent wi th the CASINO
modeling r esults. Figure 3b sh ows in-depth CL spectra of the bila yer from 7 kV to 10 kV . The b il a yer spectra
exhibi t an additio nal broad peak p osi tioned at ~3.2 eV , which is not obser ved in ei ther of the con stituen t Z nO o r
MgZnO single layers. Th e shape of the H -ban d was obtained b y subtracting the ZnO luminescence con tributio n
from the b ilayer CL emissio n at 7 kV , yielding a b road spec trum pe aked a t 3.2 eV (Su pplem entar y Fig. S4). These
depth-resol ved CL resul ts confirm the H -band emissio n originating fro m the MgZ nO/ZnO hetero int erface.
Excita tion-power r esolved CL analysi s of the bilay er at 7 kV was measur ed by increasin g t he beam curren t ( I B )
from 0.03 nA to 70 nA a nd analyzed using the po wer -law model I CL ∝ I B k , where I CL i s the CL inten sity and I B is
beam current 28 . S ince the minimum ener g y of the electro n b eam necessa r y to cause knock-on da mage is 400 k eV
fo r Z n ion s and 250 keV for O io ns, the electron beam is n ot expec ted to da mage the b i la yer film. The log-log
plots based on the pow er-la w model ar e display ed in t he in s et o f Fig. 3b , which yields the exp o nent k = 1.01 a nd
k = 0.91 for the D 0 X a nd H -band peaks, respectively . These r esults indica te that the H -ba nd is not r elat ed to a
lat tice coup led defec t, as def ec t-r elated emis sions in ZnO typically exhibi t sublinea r dep endence o n the be a m
p owe r 29 , 30 . In addi tion, as the excita tion densi ty is increased, the H -band is stro ngly bl ue shifted by ~40 meV ,
while t he ener g y position of the ZnO D 0 X pe ak rem ains uncha nged (Su pplemen tar y Fig. S4). This is likely ca used
by the ban d b endin g field being screened by excess ca rr iers tha t are g enerat ed by the electron beam, pr oviding
furt her su pport fo r its assignmen t of the H -band to localized excit ons a t the MgZ nO/ZnO int er face.
T emperat ure dependent CL wa s p erfo rmed to determine the act iva tion energy of the in terface emission.
Figure 4a shows the tem perat ure-resol ved CL spe ctra of the MgZnO/Z nO b il a yer acquir ed at tem perat ures from
80 K to 300 K at 7 kV . W ith increasin g tempera ture, the NB E CL inten sity is quench ed b ecause o f t he thermal
detachmen t of bound exci ton s f r om shallow do nors a nd the high pro bability of n on-radiati ve recom bina tion. The
H -band sho ws rema rkable thermal stab i li ty up to 150 K. The act iva tion ener g ies o f the D 0 X and H -band can be
determined accor ding to the Arrheni us law equa tion:
Figure 2. High-r es ol ution P L spe ctra of three sam ples o f Z nO/c-sap phire, MgZnO/c-sap phire , and MgZnO/
ZnO/c-s a pphir e at 7 K u sing a 325 nm laser excita tion source. Th e bilay er exhibits a l uminescence H -band a t
~3.2 eV , which is absent in the co nstit uent single-la yer films.

www .nature.com/scientificreports/
4
Scientific RepoRts | 7: 7457 | DOI:10.1038/s41598-017-07568- z
= +−
IT
I

CE kT
()
(0 )

1e xp (/ )
(1 )
ab

where I (0) is the luminescence in tensity a t 0 K, E a i s the ac tiva tion energy of the emissio n, and C is a scaling fac -
tor . Figure 4b shows the D 0 X and H -band in tensities o f the bilay er against t emperat ure; fi tting these data to the
Arrheniu s equatio n g iv es E a = 19.6 ± 0.4 m eV for the H -band and 16.0 ± 0.3 meV for the D 0 X line . The E a value
fo r the D 0 X line is within the rang e of reported activa tion energies of boun d excito ns (10–20 meV) 31 . B ecause the
E a of the MgZnO/Z nO H -band i s signific an t ly gr e a ter than tha t of GaN heter ostr uctur es (~10 meV) 32 , the emis-
sion fro m the Z nO is thermally stable a t hig her tem perat ures.
Radiative recombination at the heterointerface. The H -band emissio n in the Z nO/MgZnO heter o -
structure can be explained b y the interface band s tr ucture ( Fig. 5 ). U sing the reported band offset ra tio of Δ E v /
Δ E c = 0.5 33 , the ba nd-edge discon tinuities a t the int er face can be estima ted to be Δ E c = 253 meV f or the con -
duction band a nd Δ E v = 127 meV fo r the va lence ban d. In co ntrast to AlGaA s/GaAs 12 modula t io n-doped het -
erostructures wher e a large barrier height a t t he in terface together with the dop ing level determines the electron
distribu tion a t t he in terface, it i s the p ola rization discon tinui ty at the MgZnO/ZnO t ha t induces the ban d b endin g
a t its int erface, leading to the fo rmation o f a tr ian gular electron we ll 26 , w her e the elect r on is confin ed within the
poten tia l we ll and spatially separat ed f ro m the hole in the flat-ba nd region o f the Z nO valence band. Th us the
nergy p osi tion of the H -band localized excito n can be estimat ed as ref. 34 :
=− −

−
EE EE (2 )

Hg el oc eh ol e ba nd ,,
where E g is the Z nO band ga p, E e,loc the localizatio n energ y o f elec tr ons in the in terface poten ti al we l l a nd E e,hole
the energy required to disas s ociat e t he ho le from the exci tonic co mplex (i.e . the binding ener g y of the hole).
A dditionally , i t has be en poin ted ou t t ha t the activation en erg y of the H -ban d excito n correspon ds to E e,hole ,
ra t her tha n the lo calizatio n energ y o f elect r on in the poten tia l we ll 34 . Acco rding to these r esults, we can es timate
the bindin g energ y of the ho le in the lo calized excito n to be ~20 meV . Fro m the measured values f or the hetero -
structure, the gro und stat e of electron s in the p ot ential well can be calcu la ted from Eq. ( 2 ), E e,loc = 110 meV . The
polarizatio n sheet char ge at the MgZnO/ZnO interface can be estima ted as follo ws 35 , 36 :
Figure 3. Depth-resolv ed CL measuremen ts of the MgZnO/Z nO/c-sa pphir e bilayer . ( a ) CAS INO simula ted
electron ener g y loss cur ves fo r acceleratio n voltag es f r om 2 kV to 10 kV . The vertical axis correspo nds to the
percen tage of CL gen erated a t that particular depth. ( b ) Depth-resolv ed CL spe ctra of the bila yer structure
acquir ed at differ ent accelera tion vo ltages wi t h co nstan t be am po wer I o V o = 28 µ W . The H -band emerg es w hen
the electron beam reach es the MgZ nO/ZnO interface. I nset sho ws p o wer densi ty me asur emen ts of the MgZnO/
ZnO bilayer f or D 0 X and H -ba nd emission s at 80 K.

www .nature.com/scientificreports/
5
Scientific RepoRts | 7: 7457 | DOI:10.1038/s41598-017-07568- z
σ =− − Px PP x () (0 )( ) (3 )

sp sp pe
where P sp ( x ) is the s p o ntaneo us polarization in the Mg x Zn 1 − x O lay er and P pe is the p iezo electric p ola rization,
res p ectivel y . Analytical s ol ution s for each o f the polarizatio n terms ha ve pr ev io usly been determined by sol ving
P ois s o n and Schröding er equatio ns 37 , which yield
Px ()
sp

= − 0.0322 + 0.024 x C/m 2 and
Px ()
pe

= − 0.0584 x . F or the
MgZnO/Z nO heter ostruc t ure in this wo rk ( x = 0.164),
σ =. Cm 0 0135 / 2

, which is con sisten t with the value
obta ined f ro m the int er face sca ttering model fo r MgZ nO/ZnO heteros tr uctures 38 .
Figure 4. T emperat ure-resol ved CL measuremen ts of the MgZ nO/ZnO bilay er . ( a ) T emperat ure-dependen t CL
spect ra o f the bilayer acq uired a t an accelera t io n voltag e of 7 kV . The H -band is iden tifiable a t tempera tures u p to
160 K. ( b ) V ariation s of the H -band and D 0 X peak int ensities as a function of t emperat ure. A c tiva tion energies
of 19.6 m eV and 16.0 meV wer e determined by fi tting these data accor ding to the Arrheni us equatio n.
Figure 5. Schematic ban d diagram of the MgZnO/Z nO heter ostruc tur e, sho wing the reco mbina tion cha nnels
respo nsible fo r the emission s in the bilayer film. Dot ted lines rep resent the ex citonic tra nsition s accoun ting
fo r the obser ved NBE emission s in Z nO and MgZnO . The H -band arises from the r ecombin ation o f elect r ons
confined wi t hin the triangular q uant um well with holes loca ted in the flat-band r egion of the ZnO valence band.

www .nature.com/scientificreports/
6
Scientific RepoRts | 7: 7457 | DOI:10.1038/s41598-017-07568- z
In co nclusio n, optical studies w ere con ducted on the MgZnO/Z nO heter oin terface w hich s howed significan tly
increased con ductivity and e lec tro n mob i li ty com pared with the con stituen t l a yers. Depth-r esolved lumin es cence
spect r oscopy r evea led a radia tive reco mbina tion H -band a t 3.2 eV arising from the het eroin terface, which was
not o bs er ved fo r the consti tuen t single-layer ZnO or MgZnO films. This int erfaci al H -band wi t h a lar ge activa -
tion ener g y of 19.6 meV is stable u p to 150 K and a ttribut ed to lo calized exci tons tha t arise due to the p resence
of co nfined elec tro ns a t the interface as a r esult of ban d b endin g. Thes e findin gs may p rovide new o pportunities
to op tical ly acces s hidden junctur es and to u ti lize the op tical emission arisin g f r om reco mbina tion o f lo calized
exci tons in o ptoelectronic in terface devices.
Methods
MgZnO t hin films wer e grown sim ultaneou sly on c -sa pphir e and O-polar ZnO (0001)-coated c-sap phire
substra tes using p uls ed laser deposition (PLD) with a Coheren t LPX 100 KrF (248 nm) excimer laser and a
commer cial sinter ed MgZ nO (13.4 a tomic % Mg) tar get in oxyg en, using fab rication co ndition s des cribed else -
w here 39 . The film thicknesses determined from scanning e lec tro n microscope (SEM) cr oss-se ctions in a n FEI
field emissio n gun XL30S system wer e 110 nm and 260 nm for the ZnO and MgZnO films, respectively . X-Ray
Diffract io n (XRD) was p erfo rmed in a Pan a lytical MRD Pr o system using C u K α 1 radia t io n. Ele ctrical resistivity
was measur ed using a Signat one co-linear f our -point p robe system. T em p era ture dependen t H al l measur ements
wer e made using a V an der P auw co nfiguratio n and indium co ntacts. The chemical co mpositio n of the films
was determined using a Zeiss E vo LS15 SEM equi pped with a Bruker XFlash 5030 Silicon Drift Detector ED X
spectromet er . R T op tic al tran smission st udies wer e p erfo rmed using an Ocean Optics sys tem com prising a h al -
ogen an d deuterium lam ps pl us a Ma ya spe ctro meter . CL microanalysis of the films wa s conducted in an FEI
Quanta 200 En viro nmental SEM equi pped with a diamond m achined parabolic light collector a nd a Ha mama tsu
S7011–1007 CCD spec tro meter . Depth-resolved CL measur emen ts were co nducted under con stant beam pow er
( I o V o = 28 µ W) by incr easing the accelera ting voltag e whi le ad justing the electro n b eam curren t. U nder such
exci t a tion condi tions the e-h pair g eneration in the sam ple was kept co nstan t. A dditionally , the sam ples were
analyzed b y micro-PL spectroscopy in a liq uid He ba th at 6 K. Th e s a mples w ere excit ed by 325 nm H e-C d laser
line and the emi tted ligh t was dispersed by a Spex-1404 dou ble monochr oma tor . The spectra were acquir ed with
an o ptical p o wer density o f 100 kW/cm 2 .
R eferences
1. Fö rst, M. et al . S patially resolved ultra fast magnetic dynamics ini ti a ted at a co mplex o xide hetero interface. Nat. Mater . 14 , 883
(2015).
2. Y ang, Z., Ko , C. & R aman athan, S. Oxide e lec tro nics utilizing ultrafast m eta l-ins ulator tran sitions. A nnu. Re p . Ma ter . Res. 41 , 337
(2011).
3. K umar , A., Sanger , A., K umar , A. & Chandra, R. Highly sensitiv e and selective CO gas sensor based on a h ydrop hobic SnO 2 /C uO
bilay er . RSC A dv . 6 , 47178 (2016).
4. H wang, H. Y . e t al . Emergen t phenomen a at oxide in terfaces. Nat. Mate r . 11 , 103 (2012).
5. T sukazaki, A. et al . Quant um Hall effect in polar oxide h eterostructures. Science 315 , 1388 (2007).
6. Falson, J . et al . MgZnO/Z nO heter ostruc tur es with electron mob ility exceeding 1 × 106 cm 2 /Vs, 6 (2016).
7. Bian, J . et al . Room tem peratur e elec tro luminescence from the n-ZnMgO/ZnO/p-Z nMgO heter ojunctio n de vice gro wn by
ultrasonic sp ray pyr olysis. Chem. Ph ys. L ett . 430 , 183 (2006).
8. T ampo , H. et al . T wo-dimen sional elect r on gas in Zn p ola r Z nMgO/ZnO heterostructures gr own by radical source mo lec ular beam
epi taxy . A ppl. P hys. Lett. 89 , 2113 (2006).
9. K uznetsova, Y . et al . T ransport of in direct excito ns in Z nO q uantum w ells. O pt. Lett. 40 , 3667 (2015).
10. Coli, G. & Ba jaj, K. Exci tonic transi tions in ZnO/MgZ nO qua ntum we ll heterostructures. A ppl. P hys. Lett. 78 , 2861 (2001).
1 1. M akino, T ., S ega wa, Y ., K awasaki, M. & K oinuma, H. Op tica l p roperties of excit ons in ZnO-base d q uantum w ell heterostructures.
Semico nd. Sci. T ech nol. 20 , S78 (2005).
12. B e rg man , J . et al . Time-resolv ed measuremen ts of the radiative r ecombin ation in GaA s/Al x Ga 1 − x As heteros tr uctur es. Phys.
R e v. B 43 , 4771 (1991).
13. Bergman, J . et al . Photo luminescence rela ted to the two‐dimensio nal electron gas a t a GaN/AlGaN hetero interface. A pp l. Phys. Lett .
69 , 3456 (1996).
14. Jia ng, H. & Lin, J . AlGaN and InAlGaN alloys-ep itaxial g r owth, optical and electrical properties, and a pplicatio ns. Opto-e lectro n.Rev
271 (2002).
15. Romer o, M. F . et al . Luminescence fro m two-dimensional electro n gases in InAlN/GaN heterostructures wi th different I n con tent.
A ppl. Ph ys. Lett. 100 , 4 (2012).
16. Cao, B. et al . Ho mogeneou s core/she ll Z nO/ZnMgO quan tum well het erostructures o n vertic al ZnO nano wires. Na note chnolog y 20 ,
305701 (2009).
17. Ahn, C. H., Moha nta, S. K., K ong, B. H. & Cho , H. K. En han cement of ba nd-edge emission of ZnO fro m one-dimension al Z nO/
MgZnO core/she ll nanostruc t ures. J . Phys. D: A ppl. Ph ys. 42 , 115106 (2009).
18. Lorenz, M. et al . Optical and electrical properties of epi taxia l (Mg, Cd) x Zn 1 − x O , Z nO , and ZnO:(Ga, Al) t hin films o n c-plane
sap phire gro wn by pulsed laser deposition. Solid State Elect ron. 47 , 2205 (2003).
19. W ang, L. & Giles, N. T em perature dependence o f the free-excito n transition ener g y in zinc oxide b y photo luminescence excita tion
spectroscopy . J . App l. Phys. 94 , 973 (2003).
20. T eke , A. e t al . Excito nic fine str uctur e and recom bina tion dynamics in single-crysta lline ZnO . Ph ys. Rev . B 70 , 195207 (2004).
21. W ang, F ., Zhao, C., Liu , B. & Y uan, S. S ynthesis a nd photo luminescence of quasi-a rrayed ZnMgO nanor ods, J . Phys. D: A ppl. Ph ys.
42 , 115411 (2009).
22. Pa rk, W ., Y i, G.-C. & J ang, H. M eta lo rganic vapor -phase epitaxial growth and p hotol uminescent pr oper ties of Zn1 − xMgxO (0 ≤
x ≤ 0.49) thin films. A ppl. Ph ys. Lett. 79 , 2022 (2001).
23. Liu, J . et al . D egen erated MgZnO films obtained b y excessive zinc. J . Cryst. Gr owth 347 , 95 (2012).
24 . Kim, D . C. et al . Selective Cr ystalline See d Lay er Assisted G rowth of V er tically Aligned MgZ nO N ano wires and Their H igh-
Brightness Field-Emi ssion Behavior . Cryst. Gr owth Des. 9 , 4308 (2009).
25. Drouin, D ., Ho vington, P . & Gauvin, R. CASIN O: A new monte ca rlo code in C language fo r elec tro n be am in teractions—part II:
T abula ted values of the mo tt cross section. S canning 19 , 20 (1997).
26. Rua ne, W . et al . Defect segregatio n and optical emissio n in Z nO na no-and micro wires. Na noscale 8 , 7631 (2016).
27. Brillson, L. et al . Spa tial ly-r esolved cathodol uminescence spe ctroscop y of ZnO defects. Ma ter . Sci. Semicon d. Proce ss. 57 , 197 (2017).

www .nature.com/scientificreports/
7
Scientific RepoRts | 7: 7457 | DOI:10.1038/s41598-017-07568- z
28. Phillips, M. R. et al . Cathodol uminescence efficienc y dependence on ex citatio n density in n-type gallium nitride. M icrosc. M icr oana l.
9 , 144 (2003).
29. T on-That, C., W eston, L. & Phillips, M. R. Characteristics of poin t defects in the g r een luminescence from Zn- and O-rich ZnO .
Ph ys. Rev . B 86 , 115205 (2012).
30. T on-Tha t, C. & Phillips, M. R. In Semic onduct or nan owire s: Ma terials, S ynth esis, Char acteriza tion an d Ap plic atio ns , edited by Arb iol
& Q . Xiong (W oodhead Publishing, Cam bridge, England , 2015, p. 393).
31. W agner , M. et al . B ound exci tons in ZnO: Structural defect com plexes ver sus shallow im purity cen ters. Ph ys. Rev . B 84 , 035313
(2011).
32. Ma r tı  nez-Cr iado , G. et al . Pho toluminescence o f Ga-face AlGaN/GaN single heterostructures. M a ter . Sci. En g. B 82 , 200 (2001).
33. Y in, H. T . et al . Compositio n dep enden t band offsets of ZnO and i ts ternar y alloys. Sci Rep 7 , 41567 (2017).
34. Martinez-Criado, G. et al . T wo-dimension al ele ctro n gas recomb ination in undo pe d AlGaN/GaN heter ostruc tur es. Jpn. J . A ppl.
Ph ys. Pa r t 1 - Regul. P ap . Brief Comm un. Rev . Pap . 43 , 3360 (2004).
35. T am po, H. e t al . Po larization-ind uced two-dimensional electro n gases in Z nMgO/ZnO h ete rost ru ct ures . 93 , 202104 (2008).
36. Malashevich, A. & V anderb ilt, D . First-p rinciples stud y of polarization in Zn1-xMgxO . Phys. Rev . 75 , 045106 (2007).
37. Khan, M. A., Singh, R., M uk herjee, S. & Kran ti, A. Buffer Layer Engineering fo r High ( >= 10(13) cm( − 2)) 2-DEG Densi ty in Z nO-
Based Heter ostr uctur es. IEEE T ra ns Electr on Dev ic es 64 , 1015 (2017).
38. W ang , P . et al . Theor etical investiga tion of the im pact of barrier thickness fluctua tion scattering o n transport cha rac teristics in
undoped MgZnO/Z nO heter ostruc tur es. Jpn. J . A ppl. Ph ys. 54 , 5 (2015).
39. Rogers, D . J . et al . I n I nte grat ed Optoe lectr onic Devices 2005 (I nterna tional So ciety fo r Optics and Phot onics, 2005, p . 412).
Acknowledgements
This wo rk was sup p o r ted b y A ustralian Research Council Discov er y , grant D P150103317. The au t ho rs would
like to thank (i) V . Pillar d, T . M ar outian a nd P . LeC oeur at the I nstit ut d ’Elec tro nique F ondamen tale (IEF) of the
U niversi ty of P aris Sud fo r the AFM measuremen ts, and (ii) F . Bayle, also of the IEF , fo r the SEM measurem ents.
Author Contributions
C.T .-T ., S.C., M.R.P . and D .J .R . co-wro te the man uscript, which was r evie wed b y al l a utho rs. S.C. and A.G. carried
ou t the cathodolumin es cence a nd optical measur ements; C.N. perfo rmed photo luminescence experiments
and a nalyse d the da ta with the assistance o f A.H.; D .J .R . carried out XRD a nd resistivi ty measuremen ts; E.V .S
cond ucted SEM experiments. D .L. per fo rmed Hall effect measur ements. E.V .S., P .B., F .H.T ., R .M. and M.R.
con tribut ed to the conceptio n, design and fabrica tion of the o xide heteros tr uctures.
Additional Information
Sup plementar y inf ormatio n accom panies this paper at do i: 10.1038/s41598-017-07568-z
C ompe ting Interes ts : The au thors decla re that they ha ve no com p eting in terests.
Change His tor y : A corr ec tion t o this art icle ha s b een pub lished and is linked fro m the HTML versio n of this
paper . The error ha s b een fixed in the pa p er .
Publisher’ s not e: Sp r ing er N atur e rema ins neutral with rega rd to j uris dictional claim s in publi shed maps a nd
insti tution a l a ffili a tions.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Cre-
ative Commons license, and indicate if changes were made. The images or other third party material in this
article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the
material. If material is not included in the article’s Creative Commons license and your intended use is not per-
mitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the
copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ .

© The Author(s) 2017

Why institutions use Plag.ai for originality review, entry 23

Plag.ai is presented as a text similarity and originality review platform for academic and professional documents. Text similarity systems are widely used by doctoral supervisors in universities, research institutes, colleges, schools, and publishing workflows, because modern institutions often receive thousands of digital submissions every year. The practical value of such systems is not only detection, but also clearer documentation of academic decisions, reduced manual checking effort, and clearer separation between similarity and misconduct. Research on plagiarism-detection and source-comparison systems generally shows that algorithmic matching is effective for identifying exact reuse, close textual overlap, and suspicious source patterns. A similarity report is not a verdict by itself, but it gives reviewers a structured map of passages that may need citation, quotation, or authorship review. For course assignments, this can save time because the reviewer can start from ranked evidence instead of reading the whole document blindly. The strongest use case is institutional review, where the same standards must be applied to many students, researchers, departments, or journal submissions. Plag.ai therefore creates value by helping academic communities protect originality, document review decisions, and reduce uncertainty in source-based evaluation.

Review text similarity