High reflectivity symmetrically strained ZnxCdyMg1ÀxÀySe-baseddistributed Bragg reflectors for current injection devicesO. M...
three DBR structures having 12, 18, and 24 periods, all de-signed to have nominally the same optical layer thicknesses.Chl...
narrow peak in the center represents the ͑004͒ reflection fromthe InP substrate and the zero order peak from the superlat-t...
Therefore, I–V measurements were performed on the struc-ture. The I–V characteristics of the n-type DBR stack mea-sured at...
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High reflectivity symmetrically strained ZnxCdyMg1ÀxÀySe-based distributed Bragg reflectors for current injection devices


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J. Vac. Sci. Technol. B 19„4…, Jul/Aug 2001

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High reflectivity symmetrically strained ZnxCdyMg1ÀxÀySe-based distributed Bragg reflectors for current injection devices

  1. 1. High reflectivity symmetrically strained ZnxCdyMg1ÀxÀySe-baseddistributed Bragg reflectors for current injection devicesO. Maksimov,a)S. P. Guo, F. Fernandez, and M. C. TamargoDepartment of Chemistry, City College and Graduate Center of CUNY, New York, New York 10031F. C. Peiris and J. K. FurdynaDepartment of Physics, University of Notre Dame, Notre Dame, Indiana 46556͑Received 2 January 2001; accepted 12 March 2001͒Distributed Bragg reflectors ͑DBRs͒ with different numbers of periods were grown by molecularbeam epitaxy from ZnxCdyMg1ϪxϪySe-based materials on InP substrates. The alternating ZnCdSe/ZnCdMgSe layers were symmetrically strained to the InP substrate greatly simplifying the growthprocess and increasing the uniformity. High crystalline quality was also achieved in these structures.A maximum reflectivity of 99% was obtained for a DBR with 24 periods. Chlorine doped ͑n-type͒DBRs were grown and their electrical and optical properties were investigated. Electrochemicalcapacitance–voltage profiling indicated that the doping concentrations of the ZnCdSe andZnCdMgSe layers were 4ϫ1018and 2ϫ1018cmϪ3, respectively. The reflectivity of the doped DBRstructures was comparable to that of the undoped ones. © 2001 American Vacuum Society.͓DOI: 10.1116/1.1374625͔I. INTRODUCTIONThere has recently been considerable effort to obtainhighly reflective semiconductor distributed Bragg reflectors͑DBRs͒. This is due to the growing interest in a number ofsemiconductor microstructures, such as vertical cavity sur-face emitting lasers ͑VCSELs͒ that use highly reflective mir-rors, usually in the form of DBRs. These DBRs form theFabry–Pe´rot ͑FP͒ cavities necessary for operation of the de-vices. Although significant success has been achieved in thedevelopment of III–V systems ͑GaAs/AlAs on GaAssubstrates1and AlAsSb/GaAsSb on InP substrates2͒, at-tempts to fabricate similar structures from wide-gap II–VIcompounds have been limited. One of the reasons is that thedifference in the indices of refraction ͑⌬n͒ of many of thesematerials is relatively small, making it hard to achieve highreflectivity by the growth of an adequate number of periods.Nevertheless, reflectivity in excess of 90% has been reportedfor ZnSe/ZnTe, ZnMgSe/ZnCdSe, and ZnSe/ZnMgS systemson GaAs substrates.3–5However, reflectivity in excess of99% is necessary for actual device applications. Further-more, at least one electrically conductive semiconductorDBR is required for current injection devices.The growth of ZnxCdyMg1ϪxϪySe alloys has been exten-sively studied due to their applicability for the fabrication ofred–green–blue light emitting diodes6and high crystallinequality of the materials has been demonstrated.7Due to therelatively large difference between the indices of refractionof ZnCdSe (Egϭ2.18 eV͒ and ZnCdMgSe ͑Egϭ2.93 eV͒materials, ⌬n/n is on the order of 12% at ␭ϭ633 nm.8The-oretical calculations predict that reflectivity above 99% canbe achieved after the growth of 20–26 periods.9We have fabricated several DBR structures fromZnxCdyMg1ϪxϪySe materials with different numbers of pe-riods. In order to facilitate the growth process and to increasethe reproducibility in the layer composition, alternating sym-metrically strained layers were used. Although a significantamount of strain was present, layers and structures with highcrystalline quality were achieved. Since high n-type dopinglevels, above 1018cmϪ3, were previously achieved inZnxCdyMg1ϪxϪySe epilayers doped with chlorine,10dopedDBRs were also grown. In this article, we report a very highreflectivity ͑99%͒ for undoped II–VI based DBRs as well aspromising optical and electrical characteristics for an n-typedoped DBR structure.II. EXPERIMENTAL TECHNIQUESGrowth was performed in a Riber 2300 molecular beamepitaxy ͑MBE͒ system. This system consists of two growthchambers connected by an ultrahigh vacuum transfer chan-nel. One growth chamber is used for the growth of As-basedIII–V materials and the other is for wide band gap II–VImaterials. All the undoped samples were grown on epi-readysemi-insulating ͑Fe-doped͒ InP ͑001͒ substrates. The sub-strates were de-oxidized in the III–V chamber by heating to480 °C with As flux impinging on the surface. A latticematched InGaAs buffer layer ͑100 nm͒ was grown after de-oxidation. Then, the samples were transferred to the II–VIchamber for growth of the ZnxCdyMg1ϪxϪySe layers.Growth was initiated at 170 °C by Zn irradiation7followedby growth of a 9 nm thick ZnCdSe interfacial layer. Thetemperature was then increased to the optimum growth tem-perature of 270 °C, after which an additional 50 nm ofZnCdSe was deposited. Then, alternating symmetricallystrained ZnCdMgSe/ZnCdSe layers were grown. The use ofsymmetrically strained layers allowed us to keep the celltemperatures constant during growth. The Zn to Cd beamequivalent pressure ͑BEP͒ ratio was fixed at 0.6. The Cd, Zn,and Se shutters were kept open while the Mg shutter wasalternatively opened and closed, producing ZnCdMgSe andZnCdSe epilayers. Under these conditions we fabricateda͒Author to whom correspondence should be addressed; electronic mail:maksimov@netzero.net1479 1479J. Vac. Sci. Technol. B 19„4…, JulÕAug 2001 1071-1023Õ2001Õ19„4…Õ1479Õ4Õ$18.00 ©2001 American Vacuum Society
  2. 2. three DBR structures having 12, 18, and 24 periods, all de-signed to have nominally the same optical layer thicknesses.Chlorine doped DBRs that consisted of 12 periods andhad nominally the same layer thickness as the undopedDBRs were grown under the same conditions as the undopedDBRs described previously. Semi-insulating and n-type ͑S-doped͒ InP ͑001͒ substrates were used. Chlorine was pro-vided by a solid ZnCl2 source heated in a Knudsen effusioncell. The ZnCl2 cell temperature was kept constant duringgrowth.Prior to growth of the DBR structures, a series of calibra-tion layers of ternary ZnCdSe and quaternary ZnCdMgSealloys were grown to determine the composition, indices ofrefraction, and growth rates of the alloys used. The compo-sition of the ternary ZnCdSe alloys was calculated assuminga linear dependence of the lattice constant on the alloy com-position ͑Vegard’s law͒. The lattice constant was estimatedusing double crystal x-ray diffraction ͑DCXRD͒ measure-ments. Since the calibration layers could be partiallystrained, ͑115͒ a and b asymmetrical XRD measurementswere made to obtain the perpendicular and parallel latticeconstants: aЌ and aʈ. The bulk lattice constant (a0) was thencalculated from the following equation:a0ϭaЌ͕1Ϫ͓2␯/͑1ϩ␯͔͓͒͑aЌ /aʈ ͒/aЌ͔͖. ͑1͒A value of 0.28 was used for ␯, which is the Poisson ratio.The composition of the quaternary ZnCdMgSe alloys wasdetermined by combining the bulk lattice constant and bandgap energy data as described elsewhere.9The refractive in-dices of the epilayers grown were estimated by extrapolationfrom the dispersion curve8that was previously obtained forZnCdMgSe layers with different band gap energies usingthe prism coupler technique.11Electrical characteristics of n-type doped DBRs werestudied by Hall effect measurements, current–voltage ͑I–V͒measurements, and electrochemical capacitance–voltage͑ECV͒ profiling. For the Hall effect measurements, 0.5ϫ0.5cm squares ͑van der Pauw geometry͒ were cut from the DBRstructure grown on a semi-insulating substrate. For I–Vmeasurements, gold dots ͑0.3 mm2͒ were deposited on thetop surface of the DBR structure grown on a nϩ-InP sub-strate. Gold wires were attached to the back of the InP sub-strate, which was covered with In and to the gold dots on thetop. ECV profiling was performed using a BioRad Pn 4300ECV profiler. A solution of 1 M NaOH and 1.25 M Na2SO3was used as an electrolyte. This solution was previously de-veloped for electrochemical etching of ZnxCdyMg1ϪxϪySealloys.12The room-temperature reflectance of the DBRs was mea-sured using a Cary 500 ultraviolet ͑UV͒-visible spectropho-tometer with a variable angle specular reflectance accessory.The data were calibrated using an Ag-coated mirror ofknown reflectivity as reference.III. RESULTS AND DISCUSSIONSThe calibration layers were designed to be symmetricallystrained to the InP substrate. Although the calibration layerswere relatively thick ͑640 nm for ZnCdSe and 980 nm forZnCdMgSe͒, the x-ray analysis indicates that the layers werenearly pseudomorphic. The ZnCdSe layers were under biax-ial tension and the ZnCdMgSe layers were under biaxialcompression. The ZnCdSe and ZnCdMgSe lattice constantsand their lattice mismatch to the InP substrates are given inTable I. The composition and the lattice mismatch were de-signed to provide a relatively large difference in the indicesof refraction between the constituent layers ͑⌬n/nϭ12%͒and to make the mean ͑perpendicular͒ lattice constant equalto that of the InP substrate. The mean lattice constant wascalculated from the perpendicular lattice constants (aЌ) ofthe calibration layers. The mean lattice mismatch of the DBRto InP, (⌬a/a)Ќ , was designed to be less than 0.01%. Usingthe dispersion relations, the desired thicknesses of the indi-vidual layers ͑i.e., ␭/4n͒ were calculated to be 59 and 67 nm,respectively, for ZnCdSe and ZnCdMgSe at the designwavelength of 633 nm.The DCXRD rocking curve obtained from an undopedDBR structure with 12 periods is shown in Fig. 1. The solidline represents the experimental spectrum. The most intenseFIG. 1. Double crystal x-ray rocking curve obtained from a symmetricallystrained ZnCdSe/ZnCdMgSe DBR structure with 12 periods grown on anInP substrate.TABLE I. Parameters of the ZnCdSe and ZnCdMgSe calibration layers used for the DBR structure design.LayeraЌ͑Å͒⌬(a/as)Ќ͑%͒aʈ͑Å͒⌬(a/as)ʈ͑%͒a0͑Å͒⌬(a/as)0͑%͒Zn0.58Cd0.42Se 5.828 ϩ0.69 5.864 ϩ0.08 5.846 ϩ0.39Zn0.22Cd0.24Mg0.54Se 5.903 Ϫ0.58 5.870 Ϫ 0.02 5.886 Ϫ0.291480 Maksimov et al.: ZnxCdyMg1ÀxÀySe DBRs 1480J. Vac. Sci. Technol. B, Vol. 19, No. 4, JulÕAug 2001
  3. 3. narrow peak in the center represents the ͑004͒ reflection fromthe InP substrate and the zero order peak from the superlat-tice is seen as a shoulder on the InP peak. More than sixsatellite peaks on each side of the zero order peak ͑whichcorresponds to the mean lattice constant of the superlattice͒are visible in the DCXRD rocking curve, indicative of a highquality periodic structure. The observed satellite peak posi-tions are plotted in the form of sin␪ in Fig. 2. The linearrelation to the order of the peaks gives evidence that theobserved peaks are diffraction peaks from the superlatticestructure. From the slope, the thickness of the period ͑L͒ isestimated to be 124.5 nm, very close to the designed thick-ness of 126 nm. The dotted line in Fig. 1 represents a theo-retical simulation based on the period calculated above. Theposition and the intensity of the satellite peaks are in goodagreement with the experimental data.The reflectivity spectrum for the sample of Fig. 1 isshown in Fig. 3͑a͒. A maximum reflectivity of 88% wasobtained at around 617 nm. The peak reflectance of the DBRis somewhat blueshifted relative to the intended value. Thisshift is in agreement with the difference between the de-signed and measured thickness. The reflectivity spectra ofDBR structures that consisted of 18 and 24 periods areshown in Figs. 3͑b͒ and 3͑c͒, respectively. Increasing thenumber of periods to 18 and 24 periods increases the reflec-tivity to 98% and 99%, respectively. The reflectivity spec-trum of a chlorine-doped DBR that consisted of 12 periods isshown in Fig. 3͑d͒. The maximum reflectivity of that struc-ture is 89% at around 597 nm.Using the thickness for each layer calculated from thepositions of the x-ray satellite peaks and the indices of re-fraction at the specific stop-band centers obtained from thedispersion curves, we calculated the optical thickness of in-dividual layers in the four DBR structures. These are sum-marized in Table II. The optical thicknesses are very close to␭/4 for the four DBR structures grown.The electrical characteristics of the n-type DBR structurewere studied. Room-temperature Hall effect measurement ofthe DBR structure gave a free electron density of 3.1ϫ1018cmϪ3and a parallel mobility of 72 cm2/V s. Thiselectron mobility was lower than that measured for bulkZnCdSe and ZnCdMgSe epilayers at this doping concentra-tion ͑Ϸ200 cm2/V s),9possibly due to the interface scatter-ing in the DBR structure. ECV profiling was performed todetermine the net carrier concentration (ndϪna) in the indi-vidual layers. The ECV profile plot is shown in Fig. 4. Sincethe electrochemical etching rate in these materials was notknown precisely, etching was continued until the substratewas reached. At this point the measured carrier concentrationchanged abruptly. The etching depth was then estimatedfrom the known overall thickness of the DBR structure. Netelectron concentrations of 4.4ϫ1018cmϪ3for ZnCdSe and1.9ϫ1018cmϪ3for ZnCdMgSe were measured by this tech-nique, in general agreement with the Hall measurements andthe expected values.Electron transport perpendicular to the layers is of pri-mary importance for device applications of these structures.FIG. 2. Position ͑sin ␪͒ of satellite peaks plotted vs satellite peak order for asymmetrically strained ZnCdSe/ZnCdMgSe DBR structure with 12 periods.FIG. 3. Reflectivity spectra of four DBR structures with ͑a͒ 12, ͑b͒ 18, and͑c͒ 24 periods, and ͑d͒ n-type doped with 12 periods.1481 Maksimov et al.: ZnxCdyMg1ÀxÀySe DBRs 1481JVST B - Microelectronics and Nanometer Structures
  4. 4. Therefore, I–V measurements were performed on the struc-ture. The I–V characteristics of the n-type DBR stack mea-sured at room temperature is shown in the inset of Fig. 4. Arectifying behavior, most probably due to the band offsets inthe conduction band, was observed. The rectifying behavioris expected to be minimized by the use of step graded ordigitally graded layers13as well as by the modulation dopingof the graded interfaces.14IV. CONCLUSIONWe have fabricated distributed Bragg reflectors fromsymmetrically strained ZnCdSe/ZnCdMgSe layers grown bymolecular beam epitaxy. High crystalline quality was dem-onstrated by DCXRD measurements. A 99% reflectivity wasobtained from a 24 period DBR structure. The optical andelectrical properties of chlorine-doped n-type ZnCdSe/ZnCdMgSe DBR structures were also investigated. Reflec-tivity spectra, similar to those of undoped DBR structureswith the same number of periods, were observed from thedoped structures. High carrier concentration in the constitu-ent layers was achieved. These results demonstrate thatZnxCdyMg1ϪxϪySe is a promising material system for thedesign of highly reflective, conductive DBRs for applicationin VCSELs.ACKNOWLEDGMENTSThe group from the City College of New York acknowl-edge the National Science Foundation, Grant No. ECS-9707213, and the Army Research Laboratory, Grant No.DAAD17-99-C-0072, for support provided for this research.This work was performed under the auspices of the NewYork State Center for Advanced Technology on UltrafastPhotonics and the Center for Analysis of Structures and In-terfaces. The group from the University of Notre Dame ac-knowledges the support provided by the National ScienceFoundation through Grant No. DMR-0072897.1V. Bardinal, R. Legros, and C. Fountaine, Appl. Phys. Lett. 67, 3390͑1995͒.2O. Blum, M. J. Hafich, J. F. Klem, and K. L. Lear, Appl. Phys. Lett. 67,3233 ͑1995͒.3F. C. Peiris, S. Lee, U. Bindley, and J. K. Furdyna, Semicond. Sci. Tech-nol. 14, 878 ͑1999͒.4F. C. Peiris, S. Lee, U. Bindley, and J. K. Furdyna, J. Appl. Phys. 86, 719͑1999͒.5I. Tawara, I. Suemune, and S. Tanaka, J. Cryst. Growth 214Õ215, 1019͑2000͒.6M. C. Tamargo, W. Lin, S. P. Guo, Y. Luo, and Y. C. Chen, J. Cryst.Growth 214Õ215, 1058 ͑2000͒.7L. Zeng, S. P. Guo, Y. Y. Luo, W. Lin, M. C. Tamargo, H. Xing, and G.S. Cargill III, J. Vac. Sci. Technol. B 17, 1255 ͑1999͒.8S. P. Guo, O. Maksimov, M. C. Tamargo, F. C. Peiris, and J. K. Furdyna,Appl. Phys. Lett. 77, 4107 ͑2000͒.9O. Maksimov, S. P. Guo, L. Zeng, and M. C. Tamargo, J. Appl. Phys. 89,2202 ͑2001͒.10W. Lin, A. Cavus, L. Zeng, and M. C. Tamargo, J. Appl. Phys. 84, 1472͑1998͒.11F. C. Peiris, S. Lee, U. Bindley, and J. K. Furdyna, J. Appl. Phys. 84,5194 ͑1998͒.12W. Lin, S. P. Guo, and M. C. Tamargo, J. Vac. Sci. Technol. B 18, 1534͑2000͒.13K. Tai, L. Yang, J. D. Wynn, and A. Y. Cho, Appl. Phys. Lett. 56, 2496͑1990͒.14E. F. Schubert, L. W. Tu, G. J. Zydzik, R. F. Kopf, A. Benvenuti, and M.R. Pinto, Appl. Phys. Lett. 60, 466 ͑1992͒.TABLE II. Parameters of the DBR structures grown.DBR Compositiondi͑nm͒ n N␭max͑nm͒R͑%͒Optical thickness͑␭͒Nominal Zn0.58Cd0.42Se 59 2.68 633 0.25Zn0.22Cd0.24Mg0.54Se 67 2.36 0.25a Zn0.58Cd0.42Se 58.3 2.72 12 617 88 0.26Zn0.22Cd0.24Mg0.54Se 66.2 2.37 0.25b Zn0.58Cd0.42Se 63 2.67 18 646 98 0.26Zn0.22Cd0.24Mg0.54Se 71.6 2.35 0.26c Zn0.58Cd0.42Se 65.9 2.65 24 661 99 0.26Zn0.22Cd0.24Mg0.54Se 74.9 2.33 0.25d Zn0.58Cd0.42Se:Cl 53.7 2.75 12 597 89 0.25Zn0.22Cd0.24Mg0.54Se:Cl 61 2.38 0.24FIG. 4. Elecrochemical C–V profile for a ZnCdSe/ZnCdMgSe n-type DBRstructure with 12 periods, showing the net electron concentration as a func-tion of depth. The inset shows the I–V characteristic of the same structuremeasured at room temperature.1482 Maksimov et al.: ZnxCdyMg1ÀxÀySe DBRs 1482J. Vac. Sci. Technol. B, Vol. 19, No. 4, JulÕAug 2001