It can be therefore concluded that this band is not associated with the presenceofdoped QWsinthestructure under investi-gation.Thisbandcanbecausedbyresidualimpurities inthesubstrate or i
Trang 1St Petersburg Polytechnical University Journal: Physics and Mathematics 2 (2016) 281–286
www.elsevier.com/locate/spjpm
Peter the Great St Petersburg Polytechnic University, 29 Politekhnicheskaya St., St Petersburg 195251, Russian Federation
Available online 17 November 2016
Abstract
The paper presents the results of an experimental study of impurity-assisted photoluminescence in the far- (terahertz) and near-infrared spectral ranges in n-GaAs/AlGaAs quantum well structures with different well widths under interband photoexcitationofelectron–holepairs.Theopticalelectrontransitionsbetweenthefirstelectronsubbandanddonorgroundstate
aswellas betweenexcitedand grounddonor stateswererevealedin thefar-infraredphotoluminescencespectra.Observation
of theseopticalelectron transitionsbecame possible becauseof the depopulation of the donor ground statein the quantum wellduetothenon-equilibriumchargecarrierradiativetransitionsfromthedonorgroundstatetothefirstheavyholesubband Theopportunitytotune the terahertzradiationwavelengthin structures withdopedquantumwells bychanging the quantum wellwidthwasdemonstratedexperimentally
Copyright© 2016,St.Petersburg PolytechnicUniversity.Productionand hostingby ElsevierB.V
ThisisanopenaccessarticleundertheCCBY-NC-NDlicense.(http://creativecommons.org/licenses/by-nc-nd/4.0/ )
Keywords:Terahertz luminescence; Radiation; Quantum well; Spectrum; Nanostructure; Semiconductor.
Introduction
The task of developing effective semiconductor
sources of terahertz radiation (the wavelength range
of electromagnetic radiation is 30–300μm) is rather
important at present as these devices can be used
in diverse areas of science and technology, such as
medicine,environmentalmonitoring,security systems,
∗Correspondingauthor.
E-mail addresses:makhoviv@gmail.com (I.S Makhov),
pvyu@rphf.spbstu.ru (V.Yu Panevin), mvin@spbstu.ru
(M.Ya Vinnichenko), sofronov@rphf.spbstu.ru (A.N Sofronov),
dmfir@rphf.spbstu.ru (D.A Firsov), lvor@rphf.spbstu.ru
(L.E Vorobjev).
andcomputer science (see,for example, Refs.[1–3]) One of the most promising mechanisms for generat-ing terahertz radiation is based on optical transitions
of nonequilibrium charge carriers involving impurity statesinsemiconductorsandsemiconductor nanostruc-tures.Thismechanismisanalternativetothequantum cascade laser [4], since fabricating the latter requires very sophisticated techniques of high-quality growth
of semiconductor nanostructures
There are currently several known mechanisms for generating terahertz radiation, based on impurity-assisted transitions of charge carriers in semiconduc-tors and semiconductor nanostructures For example, terahertz radiation was observed during optical tran-sitions of nonequilibrium charge carriers involving
http://dx.doi.org/10.1016/j.spjpm.2016.11.006
2405-7223/Copyright © 2016, St Petersburg Polytechnic University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )
(Peer review under responsibility of St Petersburg Polytechnic University).
Trang 2electricfieldinmechanicallystrained p-Ge [5] andin
GaAs/GaAsN:Bemicrostructureswithbuilt-instresses
[6] Additionally, terahertz radiation was observed
frombulk silicondopedwithvariousimpuritiesunder
intrabandopticalexcitationof chargecarriers[7]
Ter-ahertz radiation under interband photoexcitation was
observed indoped bulk semiconductors such as GaN
[8],GaAs andGe [9]
Therearefewstudiesexaminingterahertzradiation
fromnanostructureswithdopedquantumwells(QWs)
Forexample,terahertzradiationinlongitudinalelectric
fields was observed in GaAs/AlGaAs quantum wells
doped with donor[10] andacceptor[11] impurities
Terahertzradiation fromnanostructureswithdoped
QWs under interband optical pumping was first
de-scribed inRef.[11].Thistypeof pumping entailsthe
generationofelectron–holepairsthataresubsequently
trapped in the QW At low crystal lattice
tempera-tures, donor impuritiesin the QWs are neutral
Elec-trons from donor ground states can recombine with
nonequilibrium holes, which is usually accompanied
by the emission of near-infrared photons The
impu-rity ground states depopulatedas aresult of this
pro-cess canbe filledwith nonequilibriumelectrons from
the first subband of size quantization Thiscan occur
withan emission of photons of the terahertz range
This study continues our previous studies on the
subject [11] and is dedicated to examining radiation
of the terahertz and near-IR ranges in nanostructures
withdonor-doped QWsof different widths
Samples and experimental procedure
Optical studieswere carried out for threesamples
Two of them were grown by molecular-beam
epi-taxy on a semi-insulating gallium arsenide substrate
and contained doped GaAs/AlGaAs QWs of
differ-ent widths The first sample contained 226 periods
of GaAs QW 16.1nm in width, separated by
4.8-nm-thick Al0.15Ga0.85As barriers The second sample
contained 50 periods of GaAs QWs 30nm in width,
separatedby7-nm-thickAl0.30Ga0.70Asbarriers
Struc-tureswithnarrowandwideQWshadGaAscaplayers
60 and 20nm thick, respectively The QWs in both
structureswere dopedwithsilicon(acting as adonor)
withasurfaceconcentrationn s=3·1010cm–2.A
semi-insulating GaAs substrate, similar to those on which
the nanostructures withdoped QWswere grown,was
used as the third referencesample
During optical measurements, the samples were
mounted into a Janis PTCM-4-7 closed-cycle optical
cryostatthatallowedmaintainingthesample’s temper-aturein the range from 4 to 320K The optical exci-tation of nonequilibrium charge carriers in the struc-tures was carried out through a fused-quartz window
byacontinuouswaveradiationof asolid-state diode-pumpedlaser(withthewavelengthof λ =532nmand the averageoutput power of P=8mW)
The photoluminescence (PL) spectra in the ter-ahertz spectral range were studied using a Bruker Vertex 80v vacuum Fourier transform infrared spec-trometer operating in a step-scan mode The output window of the optical cryostat was made of poly-methylpentene,theentrancewindowofthe spectrome-terwasmadefrompolyethylene.Thesematerialshave
a high degree of transparency in the terahertz spec-tral range The PL radiation of the sample was col-lected by an off-axis parabolic mirror of the Fourier spectrometer through a black polyethylene filter that prevented the penetration of scattered pumping radia-tion into the measurement section of the experimen-tal setup A liquid helium cooled silicon bolometer, which had a vacuum contact with the spectrometer, wasused asadetector ofterahertz radiation.The sig-nal of the bolometer photoresponse was measured by
an SR830 lock-in amplifier which was synchronized withthepumplaser.Laserradiationwasmodulatedby
achopperatafrequencyof87Hzwithadutycycleof 50%
We used two configurations of the optical system
of theFourier spectrometertoobtain the terahertzPL spectra
The first one consistedof acombination of a 0.5-mm-thick polyethylene filter at the entrance of the silicon bolometer and a 6-μm-thick multilayer My-lar beam splitter This optical configuration allowed
toperform measurements in the photon energy range from4 to40meV
The second configuration included a filter of crys-tallinequartzonthebolometeranda25-μm-thick My-larbeamsplitter;thisconfigurationallowedtoincrease theopticaltransmissionoftheFourierspectrometerin the photon energy range from2 to14meV
The PL spectra in the near infrared spectral range were measured by a Horiba Jobin Yvon FHR 640 grating monochromator witha 1200groves/mm holo-graphic grating The fused silica exit window of the closed-cycle cryostat, was used for measuring the PL spectra inthenear-IR range ThePL radiation passed through a red optical filter, transparent in the wave-length range 0.68–2.50μm and stopping the scat-teredpumpradiation, andwas focused byalensonto the entrance slit of the grating monochromator The
Trang 3Fig 1 Terahertz photoluminescence spectra of the GaAs/AlGaAs
sample with narrow QWs (1) and the semi-insulating GaAs substrate
(2), measured at 4.2 K.
signal was detected by a silicon CCD array, cooled
withliquidnitrogen
Results and discussion
Theterahertz PL spectra,detectedfor thestructure
withnarrow doped QWsand a semi-insulating GaAs
substrate,areshowninFig.1.Itcanbeseenthatboth
spectra exhibit the sameemission band inthe photon
energy range from 18 to27meV It can be therefore
concluded that this band is not associated with the
presenceofdoped QWsinthestructure under
investi-gation.Thisbandcanbecausedbyresidualimpurities
inthesubstrate or inbulklayersof the structurewith
QWs.Carbonisoneofsuchimpuritiesthathasa
bind-ing energy of 20meV andcan emergein the process
of growing bulk gallium arsenide by the Czochralski
method [12] or in the process of growing epitaxial
layersbymolecularbeamepitaxy[13].Thedifference
inthe width of terahertz radiation bands (the band is
wider for spectrum 1 in Fig 1) and the absence of
finebandstructure inspectrum1 (see.Fig.1)aredue
tothelowerspectral resolutionofthe PLspectrumof
thestructurewithnarrowQWs(spectrum1 inFig.1)
The fine line structure for spectrum 2 is associated
withartifactinterference inthe blackpolyethylene
fil-ter.Theinterferenceperiodisequaltoabout1.86meV,
whichcorrespondstotheactualthicknessof
polyethy-lene(about 100μm)
The emission band in the photon energy range of
8–16meV witha maximumnear 12meV is observed
onlyinthe terahertz PL spectrum of the sample with
narrow QWs, and, conversely, is not observed inthe
emissionspectrumof the GaAssubstrate(see Fig.1)
This indicates that the emission band can be caused
by the presence of QWs in the nanostructure under investigation
The binding energy of the silicon donor impurity
in narrow QWs can be estimated from the theoret-ical calculation of the QW energy spectrum taking intoaccounttheimpuritystates[14].Accordingtoour estimation, the binding energy of the donor impurity for a 16.1-nm wide QWis, about 10meV Therefore, the emission band with a maximum near the photon energy of 12meV may be associated with radiative transitions of nonequilibrium electrons from the first quantum-confinement subband of electrons e1 to the ground state of the ionized donor impurity 1s (indi-catedbythearrow inFig.1).Thespectral positionof this emission band is in a good agreement with our estimations for the ionization energy of the impurity
in the narrow QW of the studied sample, amounting
to about 10meV The emission band in question is considerablywide,possiblybecauseofthelow resolu-tionofthemeasuredterahertzPLspectrum,as wellas its inhomogeneousbroadening dueto inhomogeneous distribution of a substantial amount of impurities in the QWs
Inaccordancewiththeabove-describedmechanism
of radiation, the presence of e1→1s transitions, dis-covered in the terahertz PL spectrum of the sample with narrow QWs (see Fig 1), should be confirmed
by thebehavior of interband PL spectra relatedto ra-diativerecombination ofnonequilibrium electronsand holesthrough the ground donorstate inthe QW The experimental PL spectra in the near-IR range for the sample with narrow GaAs/AlGaAs QWs are shown
in Fig 2 The calculated value for the energy of the
e1→hh1 interband transition is markedby the arrow
at the bottom inFig.2
Usually, the interband PL spectra of doped nanos-tructures (if the spectra are obtained at low lattice temperature and low pumping levels) exhibit radia-tion peaksrelated toeitherradiativerecombination of heavy or light-hole excitons, or to radiative recom-bination of impurity-bound excitons, or to radiative recombination of electron-holepairs through impurity states (see, for example,Refs [15–17])
The position of the emission peak ata photon en-ergy of 1.528eV in Fig 2 differs from the calcu-lated value of the e1→hh1 optical transition energy
by 7meV We assume that this peak is due to radia-tiverecombinationof heavy-holefreeexcitonsformed
in the ground subbands (the Xe1→hh1 transition in Fig 2) The binding energy of a heavy-hole free ex-citon in bulk GaAs amounts to about 4.2meV [18],
Trang 4Fig 2 Near-infrared PL spectra for the sample with narrow
GaAs/AlGaAs QWs (T= 4.2 K), measured with different
integra-tion times: 1 s (curve 1) and 10 s (2); the scale of the curves on
the vertical axis is different Arrows indicate the positions of the
potential optical transitions; the calculated energy for thee1 →hh1
transition is also marked by an arrow;Mis the matrix.
andtheexcitonbindingenergy forsufficientlynarrow
QWs must be higher than the bulk energy [15] We
similarly detected radiative transitions of heavy- and
light-hole free excitons bound to the corresponding
subbands Theseoptical transitions are also indicated
by Xe1→lh1, Xe2→hh2, Xe2→lh2, Xe3→hh3
ar-rowsin Fig.2
Theemissionpeakataphotonenergyof1.5257eV
differs in energy from the Xe1→hh1 transition by
2.1meV and is associated with radiative
recombina-tion of donor bound excitons According to the data
from Ref [15], the binding energy of adonor bound
exciton is2meV for 20-nm-wide QWs, whichagrees
well withourresults
A shoulder observed in the PL spectrum near
the 1.523eV photon energy (see Fig 2), is
posi-tioned 12meV awayfrom the calculated value of the
e1→hh1 radiativetransitionandisassociatedwith
ra-diativerecombination ofnonequilibriumelectrons and
holes via the ground state of the Si donor impurity
innarrowQWsofthe sampleunder investigation(the
1s→hh1transition inFig.2).Recallthat anemission
band with a peak near the photon energy of 12meV,
associated withthe radiativetransitions of
nonequilib-rium electrons between the ground electron subband
e1andthegrounddonorstate 1s,wasobservedinthe
terahertzPLspectrumofthesamplewithnarrowQWs
(see Fig.1) In addition,the energy of the donor
im-purityin a20-nm-thickGaAs/AlGaAs QWamounted
to 11.6meV [15], whichis also in agood agreement
withour results
Fig 3 Terahertz PL spectra of the sample with wide GaAs/AlGaAs QWs, measured at 4.4 K (1) and 10 K (2) Arrows indicate the cal-culated energies of the optical electron transitions.
Awide emission band in the photon energy range from 1.485 to 1.510eV, marked as M (matrix) in Fig 2, may be associated with the band→impurity opticaltransitions inthe bulk layersof the structure Thus, a line related to radiative recombination of nonequilibriumelectronsandholesthroughtheground donorstate (the1s→hh1transition inFig.2)was de-tectedinthe interbandPL spectrafor thesamplewith narrowdopedQWs Theterahertz PLspectrumof the same sample exhibited an emission band caused by opticaltransitions of electrons between the first elec-tronsubbandandthe grounddonorstate (thee1→1s
transition inFig.1)
The second sample with QWs had the same QW doping level;howeverits doped QWswerewider In-creasing the QW’s width should lead to a reducing the binding energy of the donor impurity inthe QW [19,20] This should in turn affect the terahertz PL spectra shifting the emission peak related to optical electrontransitions to thedonor impurity inthe QWs towardsthe long-wave region
The terahertz PL spectra for the sample withwide doped QWs, measuredat differentcrystallattice tem-peratures, are showninFig.3 Thesecond configura-tionof theopticalsystemofthe Fourierspectrometer, described above, was used in these measurements A
PLpeakcorrespondingtothephoton energyof8meV can be seen in the graph The position of this peak
in the spectrum is in a good agreement with the re-sultsofcalculationoftheQWenergyspectrumtaking intoaccountthe presenceofimpurity states[10].This peak may be associated with an optical transition of
Trang 5Fig 4 Near-infrared PL spectrum of the structure with wide
GaAs/AlGaAs QWs (T= 4.2 K) Arrows indicate the positions of
the presumed optical transitions.
electronsfromthe firstquantum-confinement subband
tothedonorground state(e1→1s,the photon energy
of 8.7meV),as wellas withthe 2p xy→1sintracenter
transition (6.6meV) The calculatedenergies of these
transitions areindicated by arrowsin Fig.3
The results we obtained are also in agood
agree-mentwiththephotoconductivity spectra under
excita-tion by p- or s-polarized light in a structure similar
tothe onewe examined, also containing wide doped
QWs [10], where a broad absorption line associated
with the 1s→e1 and 1s→2p xy transitions was also
observed
Comparing the terahertz PL spectra obtained for
the samples with the narrow and the wide wells one
can see that the emission band radiation caused by
impuritytransitionsof nonequilibriumelectronsinthe
QWshiftedtolonger wavelengthswithanincreasein
thewidthof theQW,whichispreciselywhatwehave
expected
It can be seen from comparing the terahertz PL
spectraobtainedforthesamplewithwidedopedQWs
at two temperatures (see Fig 3) that the intensity of
terahertz PL decreases withthe temperature increase
Thismay occur because the probability that an
elec-tron is trapped by the ionized donor also decreases
Thisbehaviorof terahertz luminescence with
increas-ing temperature has already been observed in bulk
semiconductorsby the authorsof Ref [21]
To confirm the suggested mechanism of terahertz
emission involving impurity states in QWs, we
mea-sured the interband PL spectrum (Fig 4), the same
as for the sample with the narrow QWs The arrows
inFig.4 indicate thespectral peculiaritieswhichmay
be associated with radiative recombination of heavy-hole free excitons bound to the ground electron and holesubbands(theXe1→hh1 transitioninFig.4),as well as with radiative recombination of donor bound excitons (theSi→X transition inFig.4).The above-described spectral peculiarities were identified based
on the experimental data and the calculation of the energy spectrum,andoncomparingtheestimationsof the impurity and the exciton binding energies in the QWs The broademission band in the photon energy rangefrom1.48to1.51eV,markedasM(matrix),may
be associated withthe band→impurity optical transi-tions in the bulk layers of the structure This band
is also observed in the interband PL spectra for the structure with narrow doped QWs (see Fig 2) The emissionline atthephotonenergy of1.528eV canbe attributedtoradiativerecombinationofnonequilibrium electrons andholesvia theground impurity state (the
1s→hh1transitioninFig.4),sinceitdiffersfromthe calculatedvalueofthee1→hh1radiativetransitionby
8meV.Thisresultisinagoodagreementwiththe cal-culated electron energy spectrum taking into account the impurity states[10],as wellas withthe resultsof theanalysisofterahertzPLspectraofthesamplewith wide QWs(see Fig.3)
Conclusion
The mechanism of terahertz emission under inter-bandphotoexcitationofnonequilibriumchargecarriers
in GaAs/AlGaAs quantum wells of different widths doped by silicon donor impurity is discussed This mechanism is supported by the experimental results obtained for interband photoluminescence spectra of the investigatedsamples.Thesespectrawereanalyzed
in detail and compared with the results of the theo-retical calculation we performed, as well as with the data from the literature It was confirmed that tuning the terahertzradiation wavelengthinnanostructuresis possible by changingthe width of the doped QWs The study was supported by a RFBR grant no 16-32-60085, a grant of the President of the Rus-sianFederationforyoungCandidatesofsciences MK-6064.2016.2, and by the Ministry of Education and Science of theRussian Federation (state assignment)
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