Band gap engineering of thin-film electroluminescent devicesAlexey N. Krasnov Citation: Applied Physics Letters 78, 3223 (2001); doi: 10.1063/1.1374236 View online: http://dx.doi.org/10.1063/1.1374236 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/78/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electroluminescence thermal quenching in alternating-current thin-film electroluminescent devices J. Appl. Phys. 90, 2185 (2001); 10.1063/1.1385570 Electrical characterization of white SrS/ZnS multilayer thin-film electroluminescent devices J. Appl. Phys. 88, 2906 (2000); 10.1063/1.1286076 Pulsed laser annealing for high-efficiency thin film electroluminescent devices J. Appl. Phys. 88, 1606 (2000); 10.1063/1.373861 Improved brightness, efficiency, and stability of sputter deposited alternating current thin film electroluminescentZnS:Mn by codoping with potassium chloride Appl. Phys. Lett. 76, 1276 (2000); 10.1063/1.126007 Alkali metal coactivators in SrS: Cu,F thin-film electroluminescent devices Appl. Phys. Lett. 75, 1398 (1999); 10.1063/1.124706

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Band gap engineering of thin-film electroluminescent devicesAlexey N. Krasnova)

Luxell Technologies Incorporated, Mississauga, Ontario, Canada L4W 2S5

~Received 29 January 2001; accepted for publication 3 April 2001!

We introduce an alternative concept to increase the efficiency and brightness of thin-filmelectroluminescent~TFEL! devices. The method utilizes band gap engineering of the active layer ofthe device. The initial steps of our work using a ZnSxSe12x alloy are also presented to demonstratethe workability of the method. This letter discusses the related obstacles and future potentials of theband gap engineering for monochrome and color TFEL devices. ©2001 American Institute ofPhysics. @DOI: 10.1063/1.1374236#

The operational efficiency of today’s ac thin-film elec-troluminescent~ACTFEL! display depends on three mainfactors, which are the radiative efficiency, the outcouplingefficiency of the emitted photons, and the excitation effi-ciency of dopant centers.1 These three components of thetotal efficiency must be increased to meet the brightness re-quirements for full-color applications and to improve the op-erational parameters of a monochrome~typically, ZnS:Mnbased! display. The radiative efficiency is determined mainlyby the recombination cross section of the luminescent cen-ters. A straightforward increase of the dopant concentrationresults in a higher concentration quenching of theluminescence.2 The improvement of the photon outcouplingefficiency is mainly associated with the optical matching ofthe individual layers of the device. Difficulties in obtaining asignificant increase in the excitation efficiency of lumines-cent centers are generally attributed3,4 to the low efficiencyof the tunneled electrons, caused partially by space chargeformation and a lack of electron acceleration due to shortelectron mean-free path~the distances required for electronsto gain enough energy in ZnS to excite Mn centers and toionize the lattice are 16 and 40 nm, respectively!. This leadsto a decreasing impact ionization probability with the dis-tance from the insulator–semiconductor interface~ISI! to themiddle of the phosphor layer. As a result, only the phosphorregion adjacent to the cathodic ISI is efficiently involved inlight emission.5 All attempts to increase the impact ioniza-tion probability of the kinetic electrons through the phosphorband gapEg reduction have so far shown little success,mainly due to a decreased energy of the tunneled electrons ortemperature quenching problems.6

In this letter, we report on use of band gap engineeringto improve the ACTFEL device efficiency through increas-ing the impact ionization probability of kinetic electrons. Thedevice preparation was described in detail elsewhere.7

Briefly, two stacked dielectrics~normally, SiON as a barrierand Y2O3 or Al2O3-based insulator as an ‘‘injector’’! wereused to confine the current in the semiconductor active layer~ZnS doped with Mn in conventional devices!. Opaque Aland transparent polycrystalline indium–tin–oxide claddingelectrodes were used to ensure the matrix addressing of thedevice. Upon application of the external bipolar trapezoidal

wave form, the trapped electrons start tunneling from themomentary cathodic ISI into the bulk of the active layer,causing excitation of the luminescent impurity centers andionization of the crystal lattice.

To demonstrate the band gap engineering concept, aZnSxSe12x alloy doped with Mn has been chosen for theactive layer of the proposed device. The layer was engi-neered through automated thermal co-evaporation in such away as to provide a monotonic decrease of the band gapfrom both interfaces to its middle part with precise control ofthe dopant concentration, which was maintained at;0.7%.The presence of wide band gap ZnS at both ISIs ensures highenergy of the tunneled electrons. At the same time, the im-pact ionization threshold energy of the semiconductor latticeis expected to decrease with the band gap from the momen-tary cathodic interface to the middle part of the active layer.In fact, the ionization ratea i is defined as the number ofelectron–hole pairs generated by an electron per unit timeand is a function of the electric field and the ionizationthreshold energyEi .8

a i5S qF

FiDexpF2S F0

F D 2G , ~1!

where F5 f (Ei2) and F0 is a characteristic electric field.

Thus,a i is expected to increase dramatically with decreasingEi , whereEi is proportional to the band gap energy.

ZnS ~room temperature band gapEg53.7 eV! andZnSe(Eg52.68 eV) are known to ideally match in bothsingle crystalline and polycrystalline phases and to producean amenable ZnSxSex21 compound with a linear dependenceof almost all of its properties on the alloy composition@Fig.1~a!#. ZnS, ZnSe, and their alloys also have very similarelectronic structures.9 The Mn emission curve peaks at 585nm for any ZnSxSex21 composition and corresponds to a4 T1–6 A1 transition within the Mn 3d5 configuration.

X-ray diffraction measurements were used to calibratethe alloy composition. The diffraction peak shifts linearlyfrom 28.6° for ZnS~111! to 27.3° for ZnSe~111! @Fig. 1~b!#.For a quick estimation of the ZnSxSex21 composition, thethreshold voltage of the flat-band device can also be used. Toverify the band profile of the active layer, we used the sec-ondary electron emission~SEE! contrast profile techniquedeveloped by Krasnov.10a!Electronic mail: akrasnov@luxell.com

APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 21 21 MAY 2001

32230003-6951/2001/78(21)/3223/3/$18.00 © 2001 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.209.6.50

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It is known that for some compounds of the II–VI fam-ily, including ZnS, ZnSe, and their alloys, the energy levelposition of a particular native or impurity defect remainsroughly the same in respect to the vacuum level.11 Changesin the alloy composition, however, determine the position ofthe energy level in respect to the conduction and valencebands. This, in turn, determines the space charge distributionand the dynamics of the carrier transfer inside the activelayer, favoring the positive space charge formation closer tothe ISIs and not in the bulk of the active layer.

It should be noted at this point that the ZnSxSe12x :Mnalloy is used in this study only in order to demonstrate theconcept and may not be the best choice for a band gap en-gineered TFEL device. The reasons for that are as follows.The impact cross sections for Mn is 10216cm22 in ZnScompared to 5310217cm22 in ZnSe; the reduction ins de-creases the excitation efficiency of the impurity center. Theluminescence decay time of Mn centers has been found to be1 and 0.1ms for ZnS and ZnSe, respectively, which signifi-cantly decreases the integral brightness of the device; thenegative impact of this effect is less pronounced if the deviceis driven at high frequencies. Finally, the transmittance of thealloy decreases linearly from 0.9 to 0.7 with the Se contentincrease from 0 to 1. Therefore, in parallel with the ioniza-tion rate increase, the absorption coefficient increase alsotakes place. This is taken into account during the active layerdesign. Figure 2 schematically shows the band profile of theproposed device, which is expected to enhance carrier mul-tiplication and increase the total efficiency of the device. Itshould be pointed out that the classical understanding of themultiplication factor12 is applicable in this particular caseonly to a half of the device between the momentary cathodicinterface and the region of the active layer with a minimumband gap. Also demonstrated in Fig. 2 is an example of aSEE contrast profile of the active layer. Taking into accountthat the refractive index is 2.89 for ZnSe and 2.35 for ZnS,the region with the minimum band gap was placed closer tothe top~Al side! interface of the structure, thus providing abetter outcoupling of the emitted photons. The optimal alloy

composition of the middle part of the active layer was foundto be ZnS0.6Se0.4 with the band gap of;3.3 eV.

Despite all the listed drawbacks of the ZnSxSe12x :Mnalloy, an;2 times the brightness increase of the alternativedevice compared to a nonband-gap-engineered structure hasbeen achieved. The explanation is evident from Fig. 3, whichdemonstrates the results of the probe layer analysis, indicat-ing a much less significant brightness decrease of the probelayer ~a thin Mn-doped layer in the otherwise undopedsample! with a distance from the momentary cathodic inter-face of the alternative device. This, in turn, suggests higherelectron excitation efficiencies; more luminescent centers canbe excited by the increased number of electrons at the sameamount of the delivered energy. Poor Mn emission in ZnSe,therefore, is compensated with a higher excitation efficiencyof the kinetic electrons.

Theoretical modeling and preliminary experimentsshowed the combination of other materials such as CaS, SrS,MgS, BaS, and ZnO along with ZnS may be preferable to

FIG. 1. Dependence of the parameters on the ZnSxSe12x alloy composition.

FIG. 2. Band profile of the band gap engineered device~a! and its SEEcontrast profile pattern~b!.

FIG. 3. Luminance intensities of the conventional and band gap engineereddevices depending on the probe layer position.

3224 Appl. Phys. Lett., Vol. 78, No. 21, 21 May 2001 Alexey N. Krasnov

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maximize the ‘‘band gap’’ effect. For these materials, asteeper variation of the ionization rate with the electric field~and, in turn, withEi! is expected.13 Also, in addition to Mn,experiments to use band gap engineering for the efficiencyincrease of alternate dopants, such as Ho, Ce, Pr, etc., werestarted. The goal of these experiments is to achieve accept-able brightness and efficiency of the device for white andfull-color applications. The related results will be discussedin separate reports.

In conclusion, the concept of a TFEL device utilizing theband gap engineering has been demonstrated. The suggestedactive layer band gap profile allows one to significantly in-crease the number of electrons to become hot enough to sus-tain carrier multiplication in the bulk of the phosphor layer.Despite some of the inherent drawbacks of theZnSxSe12x :Mn system, an;2 times the brightness increasehas been achieved in the proposed device. The proposed con-cept is likely to have far-reaching technological and com-

mercial effects for both monochrome and full-color TFELdevices.

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3225Appl. Phys. Lett., Vol. 78, No. 21, 21 May 2001 Alexey N. Krasnov

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