Red–green–blue light emitting diodes and distributed Bragg reflectors based on ZnCdMgSe lattice-matched to InP

Journal of Crystal Growth 227–228 (2001) 710–716

Red–green–blue light emitting diodes and distributed Braggreflectors based on ZnCdMgSe lattice-matched to InP

Maria C. Tamargoa,*, Shiping Guoa, Oleg Maksimova, Ying-Chih Chenb,Frank C. Peirisc, Jacek K. Furdynac

aDepartment of Chemistry, Center for Advanced Technology on Ultrafast Photonic Materials and Applications,

Center for Analysis of Structures and Interfactes (CASI), City College of CUNY, New York, NY 10031, USAbDepartment of Physics, Center for Advanced Technology on Ultrafast Photonic Materials and Applications, Hunter College of CUNY,

New York, NY 10021, USAcDepartment of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

Abstract

Wide bandgap II–VI materials and structures based on ZnCdMgSe alloys lattice matched to InP substrates have beeninvestigated. Quantum well (QW) structures designed to emit throughout the entire visible spectral range were

constructed by varying the QW layer thickness and/or composition. Light emitting diode (LED) structures were grown,fabricated and tested. Electroluminescence emission in the blue, green, yellow and red regions of the visible spectralrange were obtained from lattice-matched or pseudomorphic structures that varied only in the composition or thickness

of the QW layer. Lattice-matched distributed Bragg reflector (DBR) structures with reflectivity maxima in the red,green and blue were also grown and characterized. Since the LED and DBR structures are made from the samematerials it should be possible to combine these into a single structure for the design of high performance LEDs and

vertical cavity surface emitting lasers that operate in the entire visible range. r 2001 Elsevier Science B.V. All rightsreserved.

Keywords: A3. Distributed Bragg reflectors; A3. Molecular beam epitaxy; A3. Quantum wells; B3. Light emitting diodes

1. Introduction

Semiconductor based red, green and blue (R–G–B) light emitters are of interest for applicationsin optoelectronics, including full-color displaysand white light sources. Currently, in order tomake full-color display elements from semicon-

ductor light emitters, such as light emitting diodes(LEDs) or lasers, three different materials must becombined to make the three primary colors. Thiscan be cumbersome and costly, and can ultimatelylimit the possibility of miniaturization of thedevice. We have recently proposed and investi-gated a family of wide bandgap II–VI semicon-ductors that have unique properties that makethem very attractive for this application.ZnCdMgSe materials lattice matched to InP

substrates exhibit direct bandgaps that range from2.15 to 3.5 eV, overlapping with most of the visible

*Corresponding author. Tel.: +1-212-650-6147; fax: +1-

212-650-6848.

E-mail address: [email protected]

(M.C. Tamargo).

0022-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 0 8 - 9

spectral range [1,2]. Fig. 1 shows the plot of thebandgap versus lattice constant for this family ofmaterials. Instead of the actual lattice constant wehave plotted the lattice mismatch (Da=a�100) toInP, since InP is the substrate of interest. Opencircles indicate layers grown of various composi-tions. It is clear from Fig. 1, that by combiningZnCdSe quantum well (QW) layers withZnCdMgSe layers, lattice matched QW structurescan be made whose emission energy will vary from2.2 to 3.0 eV by only changing the QW thickness[3]. In order to reach the red emission range(B2.0 eV), a strained ZnCdSe QW with excess Cd(solid triangle in Fig. 1) can be used, whosethickness can be kept within the pseudomorphiclayer regime to avoid the formation of misfitdislocations. Also shown in Fig. 1 is the curve forthe ZnSeTe alloys. Lattice-matched ZnSeTelayers, as we will discuss below, can be highlydoped p-type, and can serve as p-type ohmiccontacts for the device structures based onZnCdMgSe materials.In this paper, we summarize the current status

of these materials, their application as R–G–B

LEDs and the demonstration of monolithic (i.e.,grown on a single substrate) R–G–B structures.We will also present results of high reflectivitydistributed Bragg reflector (DBR) structures madefrom these materials that operate throughout thevisible range. We propose that by combining theseresults, novel devices, such as resonant cavityLEDs and vertical cavity surface emitting lasers(VCSELs) may be achieved. Furthermore, becausethe R–G–B light is obtained from nearly identicalstructures that differ only in the QW thicknessand/or composition, the integration of the threecolors onto a single device element may beexplored.

2. Experimental procedure

The materials were prepared by molecular beamepitaxy (MBE) using a dual chamber Riber 2300PMBE system that includes a chamber for thegrowth of As-based III–V materials and anotherfor the growth of wide bandgap II–VI compounds.The two chambers are connected by ultra-highvacuum (UHV) transfer modules. Epi-ready InP(1 0 0) substrates were mounted on 200 moly-blocksand introduced in the MBE chamber, where theywere deoxidized in the III–V chamber under an Asflux, at 450–4801C. An InGaAs buffer layer, latticematched to InP, was then grown with an As-terminated surface having a (2� 4) surface recon-struction. The substrate was transferred to the II–VI chamber, and the InGaAs surface was exposedto a Zn flux (Zn-irradiation) for a few secondsprior to II–VI deposition. Then a ZnCdSe inter-facial layer was grown at low temperature (1701C)during approximately 1min. Growth was inter-rupted and the temperature adjusted to theoptimum growth temperature of 250–2751C atwhich point the desired ZnCdMgSe-based layer orstructure was grown. These steps have beenpreviously used to achieve a low defect densityand high crystalline quality in these materials [4].Fe-doped semi-insulating InP and n-type Si dopedInP substrates were used for test samples or devicestructures, respectively. Doped ZnCdMgSe layerswith n-type doping levels as high as 1018/cm3 wereobtained using Chlorine as the dopant, obtained

Fig. 1. Bandgap versus lattice constant, given as % lattice

mismatch to InP, for the (Zn,Cd,Mg)Se alloy. The ternary

Zn(Se, Te) alloy relationship is also shown. Bandgap values are

given at 10K. The solid line connecting the ZnSe and CdSe and

the dashed line connecting the ZnSe and ZnTe binaries

represent measured dependences of the bandgap and lattice

constant for these ternary materials. Other ternary lines are

assumed linear. Open circles and filled triangle represent data

points for layers grown.

M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716 711

from a ZnCl2 source. A Nitrogen RF-plasmasource was used to introduce Nitrogen in thelayers for p-type doping. The growth of ZnCdSe,ZnCdMgSe and ZnSeTe layers was done at about1 mm per hour under group VI-rich conditions.The layers and the structures were characterizedusing single and double crystal X-ray diffraction,photoluminescence (PL) measurements, Hall ef-fect, capacitance–voltage (C2V) and current–voltage (I2V) measurements. Devices were fabri-cated by depositing Au dots on the top surface andattaching Au wires to the back n+InP substratecoated with In and the top Au dot. The Au dotwas B0.3mm2. No post growth annealing wasperformed. The surface electroluminescence (EL)spectra of the device structures under pulsedoperation at various current levels were measuredusing a microscope objective and focusing into anoptical multi-channel analyzer to record theirspectral characteristics. The refractive index ofsingle layers of ZnCdMgSe was measured using aprism coupler technique that has been previouslyreported [5]. Reflectivity spectra were measuredwith a Cary 500 UV-Visible Spectrophotometerwith a Variable Angle Specular Reflectanceaccessory. The data were calibrated using anAg-coated mirror of known reflectivity as refer-ence.

3. Results and discussion

The unique characteristics of these materials arebest illustrated by investigating QW structuresmade from them. The PL peak emission energy ofthree sets of samples with various QW thicknessesand composition are shown in Fig. 2. The mea-surements were made at 10K. The solid circlesrepresent the set of near-lattice-matchedZn0.5Cd0.5Se QWs with ZnCdMgSe barriers. Thebandgap energy of the barrier layer is in the rangeof B3.0 eV. The bandgap of a thick layer ofZn0.5Cd0.5Se of the same composition as that usedin the QW layers was 2.27 eV. A large range ofemission energies is obtained from these structuresby changing only the QW thickness, with valuescorresponding to emission ranging from blue toyellow. In order to achieve red emission it is

necessary to use slightly strained Cd-richZn0.3Cd0.7Se QW layers. Data from a set ofsamples with strained Zn0.3Cd0.7Se QWs areshown by the open diamonds in Fig. 2. In thiscase, the QW layer has a bandgap of about 2.0 eV.Emission in the red range is obtained with variousQW thicknesses. To avoid the formation of misfitdislocations due to relaxation, QW thickness lessthan 50 (A can be used. An additional set ofdata is shown in Fig. 2 as open triangles. Thesecorrespond to QWs with lattice-matchedZn0.3Cd0.4Mg0.3Se QW layers of bandgap ofB2.6 eV. These quaternary QWs produce blueemission from QW layers that are 40–50 (A, ratherthan the 20–25 (A layers required for blue emissionwhen near-lattice-matched Zn0.5Cd0.5Se QWs wereused (circles). Thicker QW layers result in nar-rower emission lines and higher luminescenceefficiencies from the structures, and are desirablefor low threshold diode lasers [6]. From these datawe conclude that red–green–blue (R–G–B) emis-sion is readily obtained from lattice-matched orpseudomorphic structures in which only in the QWthickness and/or composition are varied. This

Fig. 2. Photoluminescence emission energy (at 10K) as a

function of quantum well (QW) thickness for three sets of

ZnCdMgSe-based QW structures. Solid circles represent near

lattice matched Zn0.5Cd0.5Se wells, open diamonds represent

strained Zn0.3Cd0.7Se wells, and the open triangles represent

lattice matched quaternary Zn0.3Cd0.4Mg0.3Se wells. Dashed

lines are drawn to aid the eye.

M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716712

unique characteristic makes this set of materialsideally suited for the fabrication of semiconductorbased full-color displays and white light sources.Furthermore, it should be possible to makemonolithic three-color structures, grown on asingle substrate. We recently used shadow maskselective area epitaxy to demonstrate this possibi-lity [7]. To further demonstrate the potential forintegration of the three-color components onto asingle substrate we have grown a structure withthree stacked QWs designed to emit one in the red,one in the green and one in the blue spectralregions. The PL spectrum for this structuremeasured at room temperature using the 325 nmline from a He–Cd laser is shown in Fig. 3. Theinset in Fig. 3 shows the schematic of the structure.The first QW grown was the red QW, whichconsists of a strained ZnCdSe ternary layernominally 4.5 nm thick. The green and blue QWswere made of near-lattice-matched ZnCdSe mate-rial, nominally 2.0 and 1.0 nm thick, respectively.The wells were separated by 20 nm thick barrierlayers. The top and bottom barrier layers were 500and 100 nm, respectively. Barrier layers were allZnCdMgSe lattice-matched quaternaries with abandgap of 2.9 eV. The entire structure waspseudomorphic to the InP substrate. Three sharp

emission lines at 495, 537 and 603 nm were clearlyobserved. The lower PL intensity obtained fromthe red QW is due to the fact that many of thecarriers are captured by the blue and green QWs,which are closer to the surface.In order to fabricate devices, the material

quality is of utmost importance and the possibilityof bipolar doping is essential. Excellent materialquality has been demonstrated using the optimumgrowth conditions described above. Narrow X-rayrocking curves (less than 50 arcsec full widths athalf maximum), strong and sharp PL spectraexhibiting only bandedge emission, and lowstacking fault densities of 5� 104/cm2 are typicalcharacteristics [4]. We have also demonstrated n-type doping up to about 1� 1018/cm3 for quatern-aries with bandgaps as high as 3.0 eV [8]. Theselevels are fully compatible with practical lasers andLEDs. p-type doping of the quaternary layers hasalso been achieved, but net acceptor levels arepresently limited to 1� 1016/cm3 [9]. Althoughthese doping levels are sufficient for LED applica-tions, higher levels are required for injection laseroperation. Finally, a p+ doping level of 2� 1019/cm3 for ZnSeTe lattice matched to InP has beendemonstrated [10]. This material can be used as ap-type ohmic contact for the LEDs withoutintroducing defects due to lattice mismatch.Combining these results LED structures were

fabricated from these materials. The structuresconsist of: (1) a quaternary ZnCdMgSe barrier(500 nm thick) layer with a bandgap of B2.9 eVdoped n-type to a level of 1� 1018/cm3, (2) aZnCdSe QW layer whose thickness and composi-tion were varied to achieve various electrolumines-cence (EL) emission wavelengths, (3) a p-typeZnCdMgSe barrier layer (100 nm thick) with abandgap also B2.9 eV doped with Nitrogen to alevel of B1� 1016/cm3, and (4) a p+ contact layer(B20 nm thick) lattice matched ZnSeTe dopedwith Nitrogen toB1019/cm3. Four LED structureswere grown with different QW thickness and/orcomposition. The QW parameters used are listedin Table 1. Au contacts were deposited on thestructures and the devices were forward biased foroperation. Good I2V characteristics were ob-served, with low operating voltages estimated at 4–5V at typical LED current densities of 5A/cm2.

Fig. 3. Room temperature PL for a structure consisting of

three stacked QWs designed to emit in the red, green and blue

regions of the visible spectral range. The inset is a schematic of

the structure. Details of layer thickness and compositions are

discussed in the text.


This indicates good ohmic contacts and reasonabledoping levels for the materials [9]. The EL spectraare shown in Fig. 4. EL emission in the red, yellow,green and blue regions of the visible spectral rangewere obtained. Narrow widths, with less than20 nm of full width at half maximum (FWHM),were achieved for all but the red LED. The widerEL emission peak (27 nm FWHM) from the redLED is believed to be due to the partially relaxed10 nm thick QW used for the device structure. Useof a pseudomorphic well (4–5 nm) should improvethe EL characteristics of the red LED. Recently,Faschinger et al. [11] reported blue–green LEDsmade from the same alloy materials and a similarstructure to the ones we have used. His devices

exhibit greatly reduced degradation rates com-pared to the degradation of blue–green LEDsmade from the ZnCdSe/ZnMgSSe structures onGaAs. The InP-based LEDs similar to the ones wepropose in this work did not exhibit dark-linedefects (DLDs), a typical defect often observed inother II–VI structures. We have also observed thatour QW and LED structures do not form DLDswhen subjected to e-beam induced degradation[12]. The absence of DLDs and the longer devicelifetimes suggest that the materials proposed inthis work may be less prone to degradation thanother wide bandgap II–VI materials.The operating characteristics of the LEDs, such

as luminescence quantum efficiency, could befurther improved by incorporating distributedBragg reflector (DBR) mirrors in the devicestructure. Bragg mirrors may be used to reduceloss due to III–V substrate absorption, and theywould also allow the design of vertical cavitysurface emitting lasers (VCSELs) and resonant-cavity LEDs. For these reasons we have alsogrown DBR structures made from theseZnCdMgSe materials. Due to the large bandgaprange available from the ZnCdMgSe layers latticematched to InP used in our devices (see Fig. 1), alarge variation of the index of refraction isexpected from these materials. Recently, we havemeasured the refractive index, at several wave-lengths, for a set of quaternary layers ranging inbandgap from 2.2 to 2.9 eV using the prismcoupler technique [13]. With these data, we havedesigned stacks of alternating ZnCdSe andZnCdMgSe layers lattice matched to InP having10–16 periods. The periods were designed suchthat the DBRs would operate in the red region.For operation in the blue–green, it was necessary

Table 1

Parameters for the four LED structures investigated

LED Color QW

Da=a (%)

QW thickness

( (A)

EL (RT)

wavelength (nm)

EL (RT)

energy (eV)

EL (RT)

FWHM (nm)

Blue o0.2 15 500 2.480 19

Green o0.2 20 520 2.385 17

Yellow o0.2 60 552 2.244 18

Red 1.2 100 630 1.967 27

Fig. 4. Electroluminescence spectra measured at room tem-

perature for four LED structures based on lattice matched

ZnCdMgSe on InP, designed to emit in the red, yellow, green

and blue regions of the visible spectral range.


to use stacks of two quaternary layers, since theZnCdSe low bandgap layer absorbs in thatwavelength range. Table 2 summarizes the para-meters of three DBR structures. Their reflectivityspectra are shown in Fig. 5. Adjustment of the

optical thickness of the individual layers to one-quarter of the desired stop-band wavelengthproduced DBRs with reflectivity maxima at 685,605 and 507 nm, in the red to blue–green range.Reflectivity of 95% for DBR structures having 16periods and 80% for structures with 10 periodswere observed. Calculations based on the phasetransfer method [14] for structures having compar-able index of refraction steps (Dn=n) to thestructures grown predict that reflectivity of nearly100% could be achieved if 25–26 periods wereused. Such high reflectivity is needed for practicalVCSEL design.

4. Conclusions

In summary, we have investigated a set of widebandgap II–VI materials and structures based onZnCdMgSe alloys lattice matched to InP sub-strates. We have made QW structures in whichonly the QW layer thickness and/or compositionwas varied. These QW structures could bedesigned to emit throughout the entire visiblespectral range. To illustrate the potential forintegration of the R–G–B colors onto a singledevice, a pseudomorphic structure with threestacked QWs that emit in the red, green and blueregions was grown. LED structures were alsogrown, fabricated and tested. EL spectra in thered, yellow, green, and blue emission regions wereobtained from lattice matched or pseudomorphicstructures that varied only in the composition orthickness of the QW. Lattice matched DBR

Table 2

Parameters of DBR structures

DBRs Composition dia

(nm)

Eg (77K)

(eV)

n Dn=n(%)

N lmax (nm) R (%)

DBR (a) Zn0.52Cd0.48Se/ 59.7 2.16 2.72 12 10 685 80

Zn0.30Cd0.28Mg0.42Se 68.0 2.86 2.39

DBR (b) Zn0.52Cd0.48Se/ 56.1 2.16 2.72 12 16 605 95

Zn0.30Cd0.28Mg0.42Se 63.9 2.86 2.39

DBR (c) Zn0.34Cd0.33Mg0.33Se/ 51.0 2.65 2.60 9 10 507 77

Zn0.23Cd0.18Mg0.59Se 55.8 3.10 2.37

aNominal individual layer thickness.

Fig. 5. Reflectivity spectra for three lattice-matched

ZnCdMgSe-based distributed Bragg reflector (DBR) structures

designed to operate in the visible spectral range.


structures designed to operate in the red, green andblue were also grown and characterized. Since theLEDs and the DBR structures are made from thesame materials it should be possible to combinethese into a single structure for the design of highperformance LEDs and VCSEL that operate in theentire visible range.

Acknowledgements

The authors would like to acknowledge thesupport of the National Science Foundation(NSF) through grant number ECS9707213 andthe Army Research Laboratory through grantnumber DAAD17-99-C-0072. Two of us (FPand JKF) would also like to acknowledge thesupport of the NSF through grant numberDMR0072897.

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