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 Bragg reﬂectors based on ZnCdMgSe lattice-matched to InP Maria C. Tamargoa,*, Shiping Guoa, Oleg Maksimova, Ying-Chih Chenb, Frank C. Peirisc, Jacek K. Furdynac a Department 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, USAb Department of Physics, Center for Advanced Technology on Ultrafast Photonic Materials and Applications, Hunter College of CUNY, New York, NY 10021, USA c Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USAAbstract 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 wereconstructed 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 thicknessof 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 andvertical 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 diodes1. Introduction ductor light emitters, such as light emitting diodes (LEDs) or lasers, three diﬀerent materials must be Semiconductor based red, green and blue (R– combined to make the three primary colors. ThisG–B) light emitters are of interest for applications can be cumbersome and costly, and can ultimatelyin optoelectronics, including full-color displays limit the possibility of miniaturization of theand white light sources. Currently, in order to device. We have recently proposed and investi-make full-color display elements from semicon- gated a family of wide bandgap II–VI semicon- ductors that have unique properties that make them very attractive for this application. *Corresponding author. Tel.: +1-212-650-6147; fax: +1- ZnCdMgSe materials lattice matched to InP212-650-6848. E-mail address: firstname.lastname@example.org substrates exhibit direct bandgaps that range from(M.C. Tamargo). 2.15 to 3.5 eV, overlapping with most of the visible0022-0248/01/$ - 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
M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716 711spectral range [1,2]. Fig. 1 shows the plot of the LEDs and the demonstration of monolithic (i.e.,bandgap versus lattice constant for this family of grown on a single substrate) R–G–B structures.materials. Instead of the actual lattice constant we We will also present results of high reﬂectivityhave plotted the lattice mismatch (Da=aÂ100) to distributed Bragg reﬂector (DBR) structures madeInP, since InP is the substrate of interest. Open from these materials that operate throughout thecircles indicate layers grown of various composi- visible range. We propose that by combining thesetions. It is clear from Fig. 1, that by combining results, novel devices, such as resonant cavityZnCdSe quantum well (QW) layers with LEDs and vertical cavity surface emitting lasersZnCdMgSe layers, lattice matched QW structures (VCSELs) may be achieved. Furthermore, becausecan be made whose emission energy will vary from the R–G–B light is obtained from nearly identical2.2 to 3.0 eV by only changing the QW thickness structures that diﬀer only in the QW thickness. In order to reach the red emission range and/or composition, the integration of the three(B2.0 eV), a strained ZnCdSe QW with excess Cd colors onto a single device element may be(solid triangle in Fig. 1) can be used, whose explored.thickness can be kept within the pseudomorphiclayer regime to avoid the formation of misﬁtdislocations. Also shown in Fig. 1 is the curve for 2. Experimental procedurethe ZnSeTe alloys. Lattice-matched ZnSeTelayers, as we will discuss below, can be highly The materials were prepared by molecular beamdoped p-type, and can serve as p-type ohmic epitaxy (MBE) using a dual chamber Riber 2300Pcontacts for the device structures based on MBE system that includes a chamber for theZnCdMgSe materials. growth of As-based III–V materials and another In this paper, we summarize the current status for the growth of wide bandgap II–VI compounds.of these materials, their application as R–G–B The two chambers are connected by ultra-high vacuum (UHV) transfer modules. Epi-ready InP (1 0 0) substrates were mounted on 200 moly-blocks and introduced in the MBE chamber, where they were deoxidized in the III–V chamber under an As ﬂux, at 450–4801C. An InGaAs buﬀer layer, lattice matched to InP, was then grown with an As- terminated surface having a (2 Â 4) surface recon- struction. The substrate was transferred to the II– VI chamber, and the InGaAs surface was exposed to a Zn ﬂux (Zn-irradiation) for a few seconds prior to II–VI deposition. Then a ZnCdSe inter- facial layer was grown at low temperature (1701C) during approximately 1 min. Growth was inter- rupted and the temperature adjusted to the optimum growth temperature of 250–2751C at which point the desired ZnCdMgSe-based layer orFig. 1. Bandgap versus lattice constant, given as % lattice structure was grown. These steps have beenmismatch to InP, for the (Zn, Cd, Mg)Se alloy. The ternary previously used to achieve a low defect densityZn(Se, Te) alloy relationship is also shown. Bandgap values are and high crystalline quality in these materials .given at 10 K. The solid line connecting the ZnSe and CdSe and Fe-doped semi-insulating InP and n-type Si dopedthe dashed line connecting the ZnSe and ZnTe binaries InP substrates were used for test samples or devicerepresent measured dependences of the bandgap and latticeconstant for these ternary materials. Other ternary lines are structures, respectively. Doped ZnCdMgSe layersassumed linear. Open circles and ﬁlled triangle represent data with n-type doping levels as high as 1018/cm3 werepoints for layers grown. obtained using Chlorine as the dopant, obtained
712 M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716from 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 B0.3 mm2. No post growth annealing wasperformed. The surface electroluminescence (EL)spectra of the device structures under pulsed Fig. 2. Photoluminescence emission energy (at 10 K) as aoperation at various current levels were measured function of quantum well (QW) thickness for three sets ofusing a microscope objective and focusing into an ZnCdMgSe-based QW structures. Solid circles represent nearoptical multi-channel analyzer to record their lattice matched Zn0.5Cd0.5Se wells, open diamonds representspectral characteristics. The refractive index of strained Zn0.3Cd0.7Se wells, and the open triangles representsingle layers of ZnCdMgSe was measured using a lattice matched quaternary Zn0.3Cd0.4Mg0.3Se wells. Dashed lines are drawn to aid the eye.prism coupler technique that has been previouslyreported . Reﬂectivity spectra were measuredwith a Cary 500 UV-Visible Spectrophotometerwith a Variable Angle Specular Reﬂectance necessary to use slightly strained Cd-richaccessory. The data were calibrated using an Zn0.3Cd0.7Se QW layers. Data from a set ofAg-coated mirror of known reﬂectivity as refer- samples with strained Zn0.3Cd0.7Se QWs areence. shown by the open diamonds in Fig. 2. In this case, the QW layer has a bandgap of about 2.0 eV. Emission in the red range is obtained with various3. Results and discussion QW thicknesses. To avoid the formation of misﬁt dislocations due to relaxation, QW thickness less The unique characteristics of these materials are ( than 50 A can be used. An additional set ofbest illustrated by investigating QW structures data is shown in Fig. 2 as open triangles. Thesemade from them. The PL peak emission energy of correspond to QWs with lattice-matchedthree sets of samples with various QW thicknesses Zn0.3Cd0.4Mg0.3Se QW layers of bandgap ofand composition are shown in Fig. 2. The mea- B2.6 eV. These quaternary QWs produce bluesurements were made at 10 K. The solid circles ( emission from QW layers that are 40–50 A, ratherrepresent the set of near-lattice-matched ( than the 20–25 A layers required for blue emissionZn0.5Cd0.5Se QWs with ZnCdMgSe barriers. The when near-lattice-matched Zn0.5Cd0.5Se QWs werebandgap energy of the barrier layer is in the range used (circles). Thicker QW layers result in nar-of B3.0 eV. The bandgap of a thick layer of rower emission lines and higher luminescenceZn0.5Cd0.5Se of the same composition as that used eﬃciencies from the structures, and are desirablein the QW layers was 2.27 eV. A large range of for low threshold diode lasers . From these dataemission energies is obtained from these structures we conclude that red–green–blue (R–G–B) emis-by changing only the QW thickness, with values sion is readily obtained from lattice-matched orcorresponding to emission ranging from blue to pseudomorphic structures in which only in the QWyellow. In order to achieve red emission it is thickness and/or composition are varied. This
M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716 713unique characteristic makes this set of materials emission lines at 495, 537 and 603 nm were clearlyideally suited for the fabrication of semiconductor observed. The lower PL intensity obtained frombased full-color displays and white light sources. the red QW is due to the fact that many of theFurthermore, it should be possible to make carriers are captured by the blue and green QWs,monolithic three-color structures, grown on a which are closer to the surface.single substrate. We recently used shadow mask In order to fabricate devices, the materialselective area epitaxy to demonstrate this possibi- quality is of utmost importance and the possibilitylity . To further demonstrate the potential for of bipolar doping is essential. Excellent materialintegration of the three-color components onto a quality has been demonstrated using the optimumsingle substrate we have grown a structure with growth conditions described above. Narrow X-raythree stacked QWs designed to emit one in the red, rocking curves (less than 50 arcsec full widths atone in the green and one in the blue spectral half maximum), strong and sharp PL spectraregions. The PL spectrum for this structure exhibiting only bandedge emission, and lowmeasured at room temperature using the 325 nm stacking fault densities of 5 Â 104/cm2 are typicalline from a He–Cd laser is shown in Fig. 3. The characteristics . We have also demonstrated n-inset in Fig. 3 shows the schematic of the structure. type doping up to about 1 Â 1018/cm3 for quatern-The ﬁrst QW grown was the red QW, which aries with bandgaps as high as 3.0 eV . Theseconsists of a strained ZnCdSe ternary layer levels are fully compatible with practical lasers andnominally 4.5 nm thick. The green and blue QWs LEDs. p-type doping of the quaternary layers haswere made of near-lattice-matched ZnCdSe mate- also been achieved, but net acceptor levels arerial, nominally 2.0 and 1.0 nm thick, respectively. presently limited to 1 Â 1016/cm3 . AlthoughThe wells were separated by 20 nm thick barrier these doping levels are suﬃcient for LED applica-layers. The top and bottom barrier layers were 500 tions, higher levels are required for injection laserand 100 nm, respectively. Barrier layers were all operation. Finally, a p+ doping level of 2 Â 1019/ZnCdMgSe lattice-matched quaternaries with a cm3 for ZnSeTe lattice matched to InP has beenbandgap of 2.9 eV. The entire structure was demonstrated . This material can be used as apseudomorphic to the InP substrate. Three sharp p-type ohmic contact for the LEDs without introducing defects due to lattice mismatch. Combining these results LED structures were fabricated from these materials. The structures consist of: (1) a quaternary ZnCdMgSe barrier (500 nm thick) layer with a bandgap of B2.9 eV doped n-type to a level of 1 Â 1018/cm3, (2) a ZnCdSe QW layer whose thickness and composi- tion were varied to achieve various electrolumines- cence (EL) emission wavelengths, (3) a p-type ZnCdMgSe barrier layer (100 nm thick) with a bandgap also B2.9 eV doped with Nitrogen to a level of B1 Â 1016/cm3, and (4) a p+ contact layer (B20 nm thick) lattice matched ZnSeTe doped with Nitrogen to B1019/cm3. Four LED structures were grown with diﬀerent QW thickness and/or composition. The QW parameters used are listed in Table 1. Au contacts were deposited on theFig. 3. Room temperature PL for a structure consisting of structures and the devices were forward biased forthree stacked QWs designed to emit in the red, green and blueregions of the visible spectral range. The inset is a schematic of operation. Good I2V characteristics were ob-the structure. Details of layer thickness and compositions are served, with low operating voltages estimated at 4–discussed in the text. 5 V at typical LED current densities of 5 A/cm2.
714 M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716Table 1Parameters for the four LED structures investigatedLED Color QW QW thickness EL (RT) EL (RT) EL (RT) Da=a (%) ( (A) wavelength (nm) energy (eV) FWHM (nm)Blue o0.2 15 500 2.480 19Green o0.2 20 520 2.385 17Yellow o0.2 60 552 2.244 18Red 1.2 100 630 1.967 27This indicates good ohmic contacts and reasonable exhibit greatly reduced degradation rates com-doping levels for the materials . The EL spectra pared to the degradation of blue–green LEDsare shown in Fig. 4. EL emission in the red, yellow, made from the ZnCdSe/ZnMgSSe structures ongreen and blue regions of the visible spectral range GaAs. The InP-based LEDs similar to the ones wewere obtained. Narrow widths, with less than propose in this work did not exhibit dark-line20 nm of full width at half maximum (FWHM), defects (DLDs), a typical defect often observed inwere achieved for all but the red LED. The wider other II–VI structures. We have also observed thatEL emission peak (27 nm FWHM) from the red our QW and LED structures do not form DLDsLED is believed to be due to the partially relaxed when subjected to e-beam induced degradation10 nm thick QW used for the device structure. Use . The absence of DLDs and the longer deviceof a pseudomorphic well (4–5 nm) should improve lifetimes suggest that the materials proposed inthe EL characteristics of the red LED. Recently, this work may be less prone to degradation thanFaschinger et al.  reported blue–green LEDs other wide bandgap II–VI materials.made from the same alloy materials and a similar The operating characteristics of the LEDs, suchstructure to the ones we have used. His devices as luminescence quantum eﬃciency, could be further improved by incorporating distributed Bragg reﬂector (DBR) mirrors in the device structure. Bragg mirrors may be used to reduce loss due to III–V substrate absorption, and they would also allow the design of vertical cavity surface emitting lasers (VCSELs) and resonant- cavity LEDs. For these reasons we have also grown DBR structures made from these ZnCdMgSe materials. Due to the large bandgap range available from the ZnCdMgSe layers lattice matched to InP used in our devices (see Fig. 1), a large variation of the index of refraction is expected from these materials. Recently, we have measured the refractive index, at several wave- lengths, for a set of quaternary layers ranging in bandgap from 2.2 to 2.9 eV using the prism coupler technique . With these data, we have designed stacks of alternating ZnCdSe and ZnCdMgSe layers lattice matched to InP havingFig. 4. Electroluminescence spectra measured at room tem-perature for four LED structures based on lattice matched 10–16 periods. The periods were designed suchZnCdMgSe on InP, designed to emit in the red, yellow, green that the DBRs would operate in the red region.and blue regions of the visible spectral range. For operation in the blue–green, it was necessary
M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716 715Table 2Parameters of DBR structuresDBRs Composition di a Eg (77 K) n Dn=n N lmax (nm) R (%) (nm) (eV) (%)DBR (a) Zn0.52Cd0.48Se/ 59.7 2.16 2.72 12 10 685 80 Zn0.30Cd0.28Mg0.42Se 68.0 2.86 2.39DBR (b) Zn0.52Cd0.48Se/ 56.1 2.16 2.72 12 16 605 95 Zn0.30Cd0.28Mg0.42Se 63.9 2.86 2.39DBR (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 a Nominal individual layer thickness.to use stacks of two quaternary layers, since the optical thickness of the individual layers to one-ZnCdSe low bandgap layer absorbs in that quarter of the desired stop-band wavelengthwavelength range. Table 2 summarizes the para- produced DBRs with reﬂectivity maxima at 685,meters of three DBR structures. Their reﬂectivity 605 and 507 nm, in the red to blue–green range.spectra are shown in Fig. 5. Adjustment of the Reﬂectivity of 95% for DBR structures having 16 periods and 80% for structures with 10 periods were observed. Calculations based on the phase transfer method  for structures having compar- able index of refraction steps (Dn=n) to the structures grown predict that reﬂectivity of nearly 100% could be achieved if 25–26 periods were used. Such high reﬂectivity is needed for practical VCSEL design. 4. Conclusions In summary, we have investigated a set of wide bandgap II–VI materials and structures based on ZnCdMgSe alloys lattice matched to InP sub- strates. We have made QW structures in which only the QW layer thickness and/or composition was varied. These QW structures could be designed to emit throughout the entire visible spectral range. To illustrate the potential for integration of the R–G–B colors onto a single device, a pseudomorphic structure with three stacked QWs that emit in the red, green and blue regions was grown. LED structures were also grown, fabricated and tested. EL spectra in the red, yellow, green, and blue emission regions wereFig. 5. Reﬂectivity spectra for three lattice-matched obtained from lattice matched or pseudomorphicZnCdMgSe-based distributed Bragg reﬂector (DBR) structures structures that varied only in the composition ordesigned to operate in the visible spectral range. thickness of the QW. Lattice matched DBR
716 M.C. Tamargo et al. / Journal of Crystal Growth 227–228 (2001) 710–716structures designed to operate in the red, green and  T. Morita, A. Kikuchi, I. Nomura, K. Kishino, J.blue were also grown and characterized. Since the Electron. Mater. 25 (1996) 425.  A. Cavus, L. Zeng, M.C. Tamargo, N. Bambha, F.LEDs and the DBR structures are made from the Semendy, A. Gray, Appl. Phys. Lett. 68 (1996) 3446.same materials it should be possible to combine  L. Zeng, S.P. Guo, Y.Y. Luo, W. Lin, M.C. Tamargo, H.these into a single structure for the design of high Xing, G.S. Cargill III, J. Vac. Sci. Technol. B 17 (1999)performance LEDs and VCSEL that operate in the 1255.entire visible range.  F.C. Peiris, S. Lee, V. Bindley, J.K. Furdyna, J. Appl. Phys. 84 (1998) 5194.  S.P. Guo, L. Zeng, M.C. Tamargo, Appl. Phys. Lett. 78 (2001) 1.Acknowledgements  Y. Luo, S.P. Guo, O. Maksimov, M.C. Tamargo, V. Asnin, F.H. Pollak, Y.C. Chen, Appl. Phys. Lett. 77 (2000) 4259. The authors would like to acknowledge the  W. Lin, A. Cavus, L. Zeng, M.C. Tamargo, J. Appl. Phys.support of the National Science Foundation 84 (1998) 1472.(NSF) through grant number ECS9707213 and  M.C. Tamargo, W. Lin, S.P. Guo, Y. Luo, Y. Guo, Y.C.the Army Research Laboratory through grant Chen, J. Crystal Growth 214/215 (2000) 1058.number DAAD17-99-C-0072. Two of us (FP  W. Lin, B.X. Yang, S.P. Guo, A. Elmoumni, F. Fernandez, M.C. Tamargo, Appl. Phys. Lett. 75 (1999)and JKF) would also like to acknowledge the 2608.support of the NSF through grant number .  W. Faschinger, J. Nurnberger, Appl. Phys. Lett. 77 (2000)DMR0072897. 187.  L.L. Chao, H. Xing, G.S. Cargill, III, L. Zeng, M.C. Tamargo, Presented at the APS March Meeting, Los Angeles, 1998.References  F.C. Peiris, J.K. Furdyna, S.P. Guo, M. C. Tamargo, J. Appl. Phys. 89 (2001) 3748. N. Dai, A. Cavus, R. Dzakpasu, M.C. Tamargo, F.  O. Maksimov, S.P. Guo, L. Zeng, M.C. Tamargo, Semendy, N. Bambha, D.M. Hwang, C.Y. Chen, Appl. F.C. Peiris, J.K. Furdyna, J. Appl. Phys. 89 (2001) Phys. Lett. 66 (1995) 2742. 2202.