True Blue Inorganic Optoelectronic Devices

True Blue Inorganic OptoelectronicDevices**

By David A. Gaul and William S. Rees, Jr.*

Zinc selenide and gallium nitride have long been considered the leading candidates in the exploration of blue light-emittingdiodes and laser diodes. Optoelectronic devices of each composition operating near 425 nm are limited by the p-type doping ef-ficiency, which is the major hurdle to commercial production of ZnSe-based materials. A successful dopant precursor must de-liver the desired element to the electronically active lattice site, while balancing the molecule's volatility and vapor phase stabil-ity. While a comparable issue remains with GaN:Mg, one strategy to improve the quality of the p-type layer of ZnSe is discussedhere.

1. Introduction

Within the last decade, the realization of blue light-emittingoptoelectronic devices, e.g., light-emitting diodes (LEDs) andlaser diodes (LDs), has attracted much attention. LEDs re-quire one-quarter less energy compared to incandescent lightbulbs. By combining blue with yellow-green LEDs in a singleunit, an inexpensive long-lived white light source is producedthat could replace current devices.[1] In addition, the multicol-ored LEDs are applicable for use in large outdoor, as well assmaller, displays. Current optical compact disc (CD) technol-ogy incorporates a laser operating in the red region of the visi-ble spectrum; a typical 12.7 cm CD holds 650 megabytes. Stor-age density increases proportionally to the square of the lightsource wavelength, which would result in a four-fold improve-ment by switching to a blue laser. Laser printer resolutionwould also be enhanced by a comparable magnitude. Addi-tionally, blue LDs are needed to complete the set of basic col-ors to produce a full-color electroluminescent display. Re-

gardless of the application, research is focused on productionof a durable current-injected (low energy input, 3±5 mW)continuous wave (CW) true blue emitter operating at roomtemperature with a minimum requirement, for commerciali-zation purposes, of a lifetime greater than 10 000 h.[1±6]

2. Candidates

2.1. II±VI vs. III±V

The relationship between bandgaps and lattice constantsfor a variety of compounds is illustrated in Figure 1. In gener-al, the composition of group II and group VI elements, orII±VI, possesses larger bandgap energies when compared tothe III±V derivatives. Hence, II±VIs are well suited for opto-electronic devices emitting on the ultraviolet end of the visiblespectrum, while III±Vs are predisposed to infrared applica-tions. True blue has a characteristic wavelength of about425 nm, which corresponds to a bandgap of approximately2.4±2.5 eV. For a review of relevant topics pertaining to semi-conductors, interested readers should refer to Introduction toSolid State Physics by Kittel.[7]

To sustain a continuous emission, a p±n junction is required.When the p- and n-type semiconductors are placed in contact,the system equilibrates so that charge transfer occurs until thebands match in energy (Fig. 2).[8] If a voltage is applied to thissystem, an electron is promoted from the valence band of then-doped side to the conduction band, resulting in formation of

Adv. Mater. 2000, 12, No. 13, July 5 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/1307-0935 $ 17.50+.50/0 935

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The realization of blue light-emitting optoelectronic devices has attractedmuch attenion. Here the leading candidates (ZnSe, GaN, SiC, organic poly-mers) are examined and research towards the improvement of zinc selenide inparticular is reviewed. Deposition methods, precursors, and p-type dopants areall discussed. The Figure shows a p±n junction after equilibration.

±[*] Prof. W. S. Rees, Jr., Dr. D. A. Gaul

School of Chemistry and Biochemistry and School of Materials Scienceand Engineering and Molecular Design InstituteGeorgia Institute of TechnologyAtlanta, GA 30332-0400 (USA)E-mail: [email protected]

[**] This work was supported by the United States Office of Naval Research.William S. Rees, Jr. was the recipient of an Alexander von HumboldtAward during 1998±1999 with Professor Dr. H. Schumann at the Tech-nische Universität Berlin.

an electron±hole pair. If the electron can diffuse into thespace±charge region prior to recombination, it will be sweptacross the junction to the p-doped side. Relaxation back tothe valance band is accompanied by emission of a photon withenergy equal to the bandgap of the material.

Commercialization within recent years of blue LEDs hasshifted research efforts towards laser diodes. There are twoleading materials systems, II±VI and III±V, in the race for a CWtrue blue LD operating at room temperature. Both have provensuccessful; however, there exists a universal need to improvethe film quality regardless of the system to increase the lifetime

of these optical devices. Within the last decade,there have been numerous advances in II±VI andIII±V short-wavelength emitting device technology.These promising results have indicated that in thenear future blue lasers will also be commercialized.

2.2. ZnSe, a II±VI Candidate

The first zinc selenide±based laser diode was fab-ricated in 1991 by 3M.[9] The molecular beam epi-taxy (MBE) grown ZnCdSe/ZnSe/ZnSSe singlestrained quantum well (SQW) device operated un-der pulsed conditions at 77 K to emit at 490 nm.Two years later, Ishibashi and co-workers fromSony synthesized the first CW LD consisting of aZnCdSe/ZnSe/ZnMgSSe SQW structure emittingin the green region of the visible spectrum(524 nm).[10] By manipulating the cadmium and

magnesium content in the structure, the lattice constant was al-tered, thus changing the bandgap, which ultimately lowered theemitting wavelength to 490 nm.[11] Unfortunately, these earlydevices suffered from limited lifetimes. A collaboration be-tween Purdue and Brown universities produced the first pulsedblue laser (460 nm) operating at ambient temperature.[12] In1996, Sony synthesized a ZnCdSe quantum well arrangement,which emitted at 515 nm under CW conditions at ambient tem-perature for 101 h.[13] Still, the lifetime must be extended forcommercialization by improving the film quality via reductionof structural defects, i.e., stacking faults and point defects.

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D. A. Gaul, W. S. Rees, Jr./True Blue Inorganic Optoelectronic

Dr. David A. Gaul, born in 1971, began studying chemistry at James Madison University, wherehe received his BS in 1993. He earned his Ph.D. in 1998 under the direction of ProfessorWilliam S. Rees, Jr. at the Georgia Institute of Technology. He was a Molecular Design InstituteFellow for three years and received a Union Camp Fellowship for graduate research excellence.David won the 1999 Sigma Xi Award for the most outstanding Ph.D. dissertation at Georgia Tech.Currently he is serving as a post-doctoral research fellow at Georgia Tech with Dr. Suzanne B.Shuker. His research interests encompass explorations of diverse molecular interactions.

Professor William S. Rees, Jr. received his B.Sc. degree from Texas Tech University in 1980 andhis Ph.D. in 1986 from the University of California, Los Angeles. Following a postdoctoral fel-lowship at MIT, he accepted a joint appointment on the faculty of the Department of Chemistryand the Materials Research and Technology Center at the Florida State University, where he waspromoted to Associate Professor in 1993. He moved to Georgia Institute of Technology in Janu-ary 1994, with a joint appointment between the School of Chemistry and Biochemistry and theSchool of Materials Science and Engineering, and was named the first director of the MolecularDesign Institute in February 1995. In 1998 and 1999 he was an Alexander von Humbolt Fellowat the Technische Universität Berlin. Professor Rees' research interests are in the synthesis andcharacterization of inorganic and organometallic compounds for use in the preparation of elec-tronic materials.

Fig. 1. Comparison of bandgap energies and lattice constants (from [153]).

The current bottleneck for zinc selenide devices is the incor-poration of a high concentration of electronically active p-type extrinsic dopants to overcome self-compensation effects,which will be discussed later. Other difficulties, such as prob-lems dealing with substrate, the contacts on the p-type layer,and the cladding layer, each have been, to differing degrees,overcome. The ideal situation would involve deposition onbulk zinc selenide substrate; however, poor quality hamperedusing this substrate. Although in recent years progress hasbeen achieved to increase purity and improve structural qual-ity, the limited availability of bulk, single-crystal, large-diam-eter zinc selenide fails to offset the advantages.[14] Sincehomoepitaxial growth is difficult, heteroepitaxial growth istypically used on gallium arsenide. The lattice mismatch isabout 3 % along the c-axis,[3] which can result in creation ofstacking faults that propagate through the structure, thus lim-iting device lifetime. Differing thermal expansion coefficientsfor ZnSe and GaAs subsequently create additional strain oncooling to ambient temperature from film deposition values.Growing a buffer layer of zinc sulfide, followed by grading thesulfur content until the lattice match is obtained, can reducethese defects. An alternative approach incorporates InGaAsas the substrate, and indium and arsenic levels are manipu-lated to obtain a lattice match (3.6 % InAs).[15] ZnSe mayexist in both cubic and hexagonal phases, which also can resultin defects during growth. It is generally regarded that lowergrowth temperatures are required to minimize imperfections.Incorporation of graded contacts involving zinc telluride haslowered the device resistance, which has been attributed to in-creases in product lifetime.[16,17] A closely related material,ZnTe, has characteristics that render it easier to p-type dope(vs. ZnSe), while the achievement of substantial n-type dopinglevels is rather difficult. The incorporation of cadmium andmagnesium produces a suitable cladding layer, which im-proves the carrier confinement.[18,19] Lower confinementequates to higher operating voltages.

Zinc selenide is more ionic than III±V materials, thus thelattice is considered softer, thereby allowing dislocation mo-bility, which limits lifetimes. Beryllium telluride can be intro-duced to strengthen the lattice (by increasing the covalency),as well as serving as a pseudo grading, to improve the p-typecontact layer.[20,21] The perceived toxicity of beryllium may

pose some considerable challenge to acceptance ofthis material.[22]

2.3. GaN, a III±V Candidate

Gallium nitride also is useful in blue-emitting op-toelectronic devices. In 1992, Nakamura[23±25] wasfirst to synthesize an LED consisting of a InGaN/AlGaN double heterostructure (DH) with an inten-sity over one candela. Since then, further research

has resulted in increasing this luminous intensity to over twocandelas.[26] The emission color was tunable based on theamount of incorporated indium. The logical extension sparkedextreme interest in the possibility of III±Vs serving as LDs.[27]

In 1995, Akasaki et al. produced the first ultraviolet laser(376 nm) utilizing a GaN/AlGaN DH.[28] Nakamura, in thesame year, made the first ambient-temperature GaN LD oper-ating under pulsed conditions.[29,30] Similar to II±VI materials,the device lifetime reported in these initial studies greatly lim-ited the potential of the product for commercialization.

Analogous to II±VIs, it is a challenging task to synthesize thep-type layer, as well as developing its ohmic contact. There ex-ists no available substrate for homoepitaxial growth of GaN(Fig. 1), thus heteroepitaxial growth is typically used involvingsapphire. The lattice mismatch of this combination is 13.8 %along the c-axis,[3] which results in numerous defects, men-tioned earlier. One alternative, silicon carbide, has an im-proved lattice match; however, producing large wafers of the6H polytype is difficult. Incorporating a buffer layer betweenthe sapphire substrate and the device can alleviate defect prob-lems due to the lattice mismatch. This is achieved by growth ofan amorphous region of gallium nitride, prior to a high-temper-ature growth region, on which the final device is deposited.The GaN materials also suffer from poor cleavability and resis-tance to etch chemicals, which are needed to form mirrors re-quired for LD operation. When compared to zinc selenide, III±Vs are less ionic and, therefore, can be considered harderÐre-sulting in impeded dislocation movement. Regarding LEDfabrication, the available range of bandgaps of the AlGaIn ma-terial spans the spectrum (6.2±1.95 eV) and is advantageousfor producing a multicolor emitting device with a uniform com-position.[3] Recently, Nakamura solved these issues to extendthe device lifetime of a violet LD to an estimated 10 000 h, thegoal set for commercialization.[31] Work is continuing to in-crease the lifetime of a LD operating in the true blue region.

While great success has come to Nichia Chemical Companysince Nakamura's initial report of III±N based blue-emittingmaterials, his recent departure to take up a professorship atthe University of California, Santa Barbara, serves to furtherstrengthen the university's present strong position among aca-demic locations involved in exploration of devices fabricatedfrom these materials systems.


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Fig. 2. A p±n junction a) before and b) after equilibration.

2.4. Other Potential Candidates

There exist other technologies that have shown potential.Porous silicon nanoparticles and silicon carbide have eachbeen shown to produce blue emission from an indirect band-gap. It is thought that the quantum size effect is responsiblefor the emission from porous silicon.[32] However, reproduci-bility and low intensity are problems that plague this system.A more promising material, SiC, has the advantage over pre-vious systems, since it can be readily doped both p- and n-type.[33±35] Increasing temperature results in shrinking of thebandgap. For the II±VIs and III±Vs, this effect is observed bythe emission shifting from blue to green. The bandgap in SiCis wide enough not to pose a problem, thus making this mate-rials system the leading candidate for blue optoelectronic de-vices operating at high temperature and high power. Due to alimited supply of large high-quality 6H SiC wafers, this tech-nology has not yet appealed to manufacturers. LDs requirefast and efficient generation of conducting species, in whichdirect bandgap materials have been favored over their indi-rect counterparts.

Another approach involves the use of second-harmonicgeneration, which is a doubling of the frequency of the sourcelaser light.[36±38] Vertical-cavity surface-emitting laser diodesare often employed in this situation, due to the high intensityintracavity light produced, and the large second-order opticalnonlinearity of this design. Advantages related to low thresh-old current and ability to manufacture high volumes, however,remain insufficient to offset the low output power.

Another class of compounds, organic polymers, has provento emit blue light with an applied voltage. These devices arecomposed of the active polymers sandwiched between twoelectrodes, in which one is required to be transparent. Stim-ulated green emission was witnessed when poly(p-phenylene-vinylene)[39,40] and the blend of poly(2-methoxy-5-(2¢-ethyl-hexyloxy)-1,4-phenylenevinylene)[40,41] backbone structureswere employed. Incorporating poly(p-phenylene) or poly-fluorene as the backbone constituent produced photolumines-cence emissions in the range of 420±450 nm.[40] Although notas structurally robust as the inorganic counterparts, polymersystems are advantageous due to the ease of processing theselightweight flexible materials.[42] By inverting the classical ar-rangement by replacing the brittle indium tin oxide with atransparent polyaniline anode, in conjugation with an alumi-num cathode substrate, novel device shapes have beenachieved.[43] Production of larger devices is possible, becausethey also are capable of covering wide areas. Low driving volt-ages (<10 V) and high efficiencies (up to 4 %), combined withhigh luminance (1 cd/cm2), are additional advantages for em-ploying polymers in LED fabrication. Like their inorganic rel-atives, undesired side reactions and contaminates must beminimized. Chemical vapor deposition (CVD) has been em-ployed in growth of poly(p-phenylenevinylene) on an alumi-num-coated silicon wafer using dichloro-p-xylene mono-mers.[44] A polymeric device based on parahexaphenyl wassynthesized and emitted in the blue, green, and red regions.[45]

It relied on a bright blue emission (0.03 cd/cm2 pulsed and0.0035 cd/cm2 CW), which could be converted to any othervisible color using dyes and filters. Simplicity warrants the re-quest of the manufacturers for a single material that can pro-duce multicolored LEDs for a flat panel display. Approachingthis goal, one polymer system, involving 2,5-bis{4-[bis(4-methylphenyl)amino]phenyl}thiophene, has been reported toproduce colors from blue to orange, by varying the conjuga-tion length of the thiophene linkage.[46] Additionally, there isevidence that an organic LED can be tuned based on voltage(or current) applied, thereby leading to a simplistic design,composed of a single material able to achieve all necessarycolors.[47±49]

3. Deposition Techniques

Of all the materials previously mentioned, ZnSe and GaNsystems are considered the front-runners for commercializa-tion; however, there are other possibilities (i.e., CdS) thathave not been pursued with the same vigor, due to lack of asuitable substrate. Another important facet in the productionof a LD to consider is the film deposition method. The tech-nique employed will dictate the characteristics of the precur-sor required. Interested readers should refer to CVD of Non-Metals edited by Rees,[50] CVD of Compound Semiconductorsedited by Jones and O'Brien,[8] and Organometallic VaporPhase Epitaxy: Theory and Practice by Stringfellow[51] for ad-ditional details concerning the various growth techniques.

CVD[8,50,51] encompasses all processes that are resultant infilm growth from a chemical reaction originating from vaporphase precursors. Organometallic vapor phase epitaxy(OMVPE) or metal±organic chemical vapor deposition(MOCVD) incorporate precursors that have characteristicmetal±organic bonds, and these terms, while not identical, fre-quently appear in the literature in interchangeable use. Theemployment of the process descriptor CVD does not imply anorientation of the film with respect to its underlying substrate.To differentiate, vapor phase epitaxy (VPE) is utilizeduniquely for processes that yield epitaxial orientation of theoverlayer, with respect to the coated substrate. Organometal-lic (OM) precursors are those, rigorously defined, that havedirect metal±carbon interatomic interaction. Metal±organic(MO) classified compositions consist of compounds with or-ganic-based ligands bound to the central metal atom, in theabsence of a direct metal±carbon interatomic interaction. Forblue optoelectronic applications, VPE is mandatory; it is pre-mature at the present juncture to determine if MO- or OM-based precursors will enjoy field dominance for device manu-facturing. Each has advantages and limitations.[50]

Advantages of OMVPE include the ability to synthesize auniform thickness and elemental composition onto any mor-phology; thus it is labeled a non-line-of-sight method. VPE isthe most versatile method with suitability for large-scaleproduction involving a relative simple reactor design (Fig. 3).Both low cost per unit and high throughput are additional at-


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tractive features appealing to production. The drawbacks in-clude the large number of parameters that must be controlledprecisely to ensure uniform reproducible film growth and therequired expensive precursors. There also exists a demand tominimize the use of hazardous materials. For further informa-tion, Chemical Vapor Deposition by Jensen[52] is an excellentsource reviewing the fundamentals.

There are three spin-offs of CVD, where the constituentsare introduced via plasma,[8,50] by a photo-degradation pro-cess,[50] or an aerosol.[53] First, the plasma-enhanced VPE ischaracterized by introducing the film components generatedby a plasma, which results in lower substrate temperatureswhen compared to conventional techniques. In this approachto film growth, the deposit accretes from precursors that un-dergo bond rupture by (predominantly) non-thermal pro-cesses. In this manner, the vapor phase homogeneous reactionis favored predominately over the more traditional VPE reac-tion of heterogeneous vapor±solid thermolytically driven pro-cesses. A disadvantage includes the difficulty in achievingcontrol of ultimate film properties, due to the plasma being anextraordinarily complex chemical and energetic mixture.Damage to the deposited film from the bombardment withenergetic neutral and charged particles is another drawback.

Secondly, photo-assisted CVD also operates at lower de-position temperatures when compared to the thermal CVDmethods. This technique relies on photo-degradation of theprecursors prior to deposition to strip the desired element ofits ligands. Reduced substrate damage is an advantage overplasma techniques. With the use of monochromatic radiation,deleterious side reactions may be minimized. Incorporating awell-defined radiation beam leads to the potential of masklessdeposition. High levels of unsaturation in the precursors are adisadvantage, due to the increased carbon incorporation ob-served in resultant films.

Lastly, precursors can be introduced via an aerosol that re-lies on fast solvent evaporation for vaporization. An advan-tage of this technique is that the precursor remains at ambienttemperature until it is required, which reduces the potentialfor premature reactions. Since materials are frequently trans-ported non-perpendicular to the substrate surface, difficultyin maintaining uniform step coverage results.

Liquid-phase epitaxy (LPE)[8,50,51] involves a combined meltof the matrix elements. In this procedure, high-purity compo-nentsÐfree from oxygen contaminatesÐare delivered, be-cause any oxides formed remain on the surface of the meltand create a protective barrier. Disadvantages include limitedscalability and flexibility, as well as non-uniformity issues

manifested in both short- and long-range end phase varia-tions. Additionally, interfaces may be difficult to synthesize.LPE had a traditional niche in the optoelectronic device mar-ket, due to it being a well-established technology; however,OMVPE has become a rival for LPE, and is often now a pre-ferred deposition method.[54]

MBE[8,50,51] operates by introducing elemental sources via aKnudsen cell into an ultrahigh vacuum reaction chamber. Thisrelatively simple method provides uniform growth, with sharpinterfaces without graded transition regions. Film growth isachieved often at lower temperatures than in conventionalCVD-related methods, constituting an additional advantage.The large capital outlay, high operating costs, and, particular-ly, low throughput of the ultrahigh vacuum assembly are dis-advantages that challenge the manufacturing utility of thistechnique.

OMVPE is the most cost-effective method to employ forthe synthesis of a blue LD. Depicted in Figure 3, depositionsystems consist of the following three variable hardware com-ponents: precursor entry system, reactor chamber, and sub-strate.[8,50] Heatable stainless-steel vessels, known as ªbub-blersº, are typically used in CVD techniques to vaporize thecondensed state precursors, which then are carried by an inertgas to the reaction area via heated transfer lines. Various reac-tor designs have been employed, ranging from horizontal,where material flow passes over the substrate, to vertical,where the material flow is perpendicular to the substrate. Re-gardless of the substrate setup, there are two main types: hot-and cold-walled reactors. A drawback of the hot-wall setup in-cludes film deposition on the reactor walls, which can breakloose and disrupt the growth process. A cold-wall reactor less-ens deposition on the walls; however, the isothermal conditionis compromised by the formation of convection currents fromthe heated substrate, which can result in non-uniform growth.Conductive substrates may be heated resistively, while non-conductive substrates can be heated by optical or thermalradiation techniques. Both may be heated indirectly by usingradio frequency induction.

4. ZnSe: Precursors and Dopants for OMVPE

4.1. Precursors for OMVPE

The nature of the precursors employed directly affects thedeposited film; thus knowledge of molecular properties anddecomposition mechanisms are integral in evaluating the utili-ty of a compound. The ideal precursor displays a high volatil-ity (higher than 10±1 torr at 100 �C) with vapor phase stabilityduring the evaporation and transport. Liquids are preferredover solids, due to the greater surface area available to aid inmaintaining a constant vapor flux to the reactor. Althoughsolids initially may be finely crushed powders, agglomerationoften occurs under conditions of use, thereby substantially re-ducing the surface area. High purity is essential in allmicroelectronics applications, since even traces of impurities


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Fig. 3. Generic representation of a CVD reactor.

introduce highly undesirable defects, which have detrimentaleffects on the operation of the device. Additionally, the pre-cursor requirements include clean decomposition to give thedesired material without added contamination from the li-gands or unwanted pre-reactions. In forming an efficient p±njunction, the resulting films must have dopant concentrationsgreater than 1018 cm±3 of either donors (n-type) or acceptors(p-type). The precursor should be synthesized easily at lowcost, while possessing a long shelf life. Finally, concentratingon safety, the reactantsÐalong with their by-productsÐshould exhibit a minimal toxicity, as well as being non-pyro-phoric and non-corrosive.[55,56]

4.2. Growth of ZnSe by OMVPE

In 1968, Manasevit and Simpson were the first to demon-strate the feasibility of II±VI semiconductor growth using or-ganometallic compounds.[57] OMVPE growth of zinc selenidehas been performed traditionally with dimethylzinc (ordiethylzinc) and hydrogen selenide.[58] This combination of re-actants suffers from severe premature gas phase reactions, re-sulting in non-uniform coverage. Attack from the hydride onthe selenium precursor eliminates an alkyl substituent fromzinc, via a presumed R2Zn±SeH2 intermediate, in these pre-reactions. The use of [R2ZnL] adducts (L = NR3; R = Me, Et)blocks the previously mentioned reaction, improvinggrowth.[59,60] This non-pyrophoric complex is simpler to purifyand has increased resistance to oxygen-containing impuritiesas compared to the dialkyl zinc sources. It is important to notethat neither the zinc±nitrogen adducts in this case, nor thepresence of NR3, result in formation of shallow acceptors.High-quality films of ZnSe have been obtained at low growthtemperatures (250±350 �C) using H2Se; however, H2Se is ex-tremely toxic, and produces large amounts of active hydrogenradicals during decomposition. These are believed to deacti-vate the p-type dopants, and will be discussed later. Alterna-tives include dialkylselenide,[61±63] which contributes lessactive hydrogen, but growth temperatures must rise to450±500 �C. Higher temperatures increase the occurrence ofnative defect complexes, and switch-ing to an alkyl substituent leads toincreased carbon incorporation.[64]

A comparison of several seleniumand zinc sources is given in Table 1,and the best optical properties ofZnSe were obtained employingMe2Zn:NEt3 and iPr2Se at 440 �C.[65]

Incorporating tert-butyl groups in-stead of isopropyl resulted in lower-ing of the growth temperature.[66]

Research continues in the search forimproved ZnSe main element pre-cursor combinations that utilize thelowest growth temperatures with aminimum toxicity.

4.3. Potential Dopants

A p±n junction is necessary for CW blue emission. The n-typelayer effectively injects electrons into the system by either sub-stituting a group 13 element on the zinc site or a group 17 ele-ment on the selenium site. It is important that the dopant incor-porates substitutionally, as opposed to interstitially, to beelectronically active. Undoped films frequently exhibit n-typeproperties due to the presence of donor defects, which resultfrom impurities or point defects. The n-type growth has beenachieved at donor levels exceeding 1018 cm±3. Group 13 candi-dates include boron, aluminum, gallium, indium, and thallium.The large size of Tl (Table 2) would disrupt the crystal lattice,and, therefore, limits its usefulness. Calculations preformed byKatayama-Yoshida and Yamamoto suggested that boron wouldform high resistive layers and deep level donors.[67] Aven andWoodbury in 1962 synthesized low resistive n-type ZnSe byeither growing in the presence of elemental Al, Ga, or In underzinc overpressure, or by diffusing the elements into as-grownfilm at high temperature, followed by annealing in zinc vapor.[68]

Stutius synthesized n-type ZnSe by employing triethylalumi-num, which removed the need for further treatment after filmgrowth.[69,70] The measured carrier concentrations ranged from1015 cm±3 to 1017 cm±3 in these results. Akimoto et al. reportedgallium doping to be equally as efficient (1017 cm±3).[71,72]

Another approach that has shown promise involves group17 elements such as fluorine, chlorine, bromine, iodine, andastatine. The large ionic radius of At (Table 2), coupled withits radioactivity, eliminates it as a possibility. Incorporation offluorine results in surface formation of Zn±F, which is rela-tively volatile, hindering dopant addition. The other alkyl ha-lides have achieved n-type doping in the range 1016±1019 cm±3

from various sources such as methyl iodide,[73] ethyl iodide,[74]

n-butyl iodide,[74] n-butyl chloride,[75] n-octyl chloride,[76,77]

and n-octyl bromide.[77] Although somewhat less effective,zinc dichloride also has been employed as a chlorinesource.[78] Successful high doping concentrations with chlorinehave made it the most used of the group 17 elements.

The realization of the p-type doped layer is a major obstaclefor ZnSe-based optoelectronic devices. Electronically active


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Table 1. Comparison of main element precursors for zinc selenide growth (from [54]).

[a] TMTZ = trimethyltriazine.

dopants inject a hole into the system by substituting an elementpossessing one less electron. The candidates include a group 1or group 11 atom on the zinc site or a group 15 element on theselenium site. Oxygen, which is isoelectric with selenium, hasbeen employed as a p-type dopant. Akimoto et al. synthesizeda LED incorporating O by either ion implantation[79] or usingZnO[71,72] to p-dope ZnSe. A blue emission was reported at77 K and ambient temperature, although low oxygen levelswere observed (1.2 ´ 1016 cm±3). A shift from 466 nm (77 K) to446 nm when operating the device at room temperature wasaccompanied by a reduction of photoluminescence intensity. Ithas been hypothesized that the origin of the emission is theelectronegativity of oxygen causing charge transfer, creating anelectron±hole pair. However, the substitutional configurationwhere O resides on a Se site is not believed to be responsiblefor producing the observed doping effect. Helms has suggestedinterstitial incorporation of oxygen, to form species such asZnO and ZnSeO3, to compensate for excess Se or Zn.[80]

Members of group 1 are lithium, sodium, potassium, rubid-ium, cesium, and francium. Only Li and Na are viable, sincethe remaining members of the group have large atomic radii

(Table 2) and their incorporation would disrupt the crystallattice. The first report of OMVPE growth of ZnSe:Li was in1988 by Yasuda et al., utilizing Li3N.[16,81] It was ambiguous ifthe doping level of 9 ´ 1016 cm±3 was attributable to lithium ornitrogen. Yoshikawa et al. used cyclopentadienyl lithium toproduce shallow acceptors.[82] The major disadvantage of thisligand is the observed increase in carbon incorporation intothe films. Skromme et al. studied the effects of carbon addi-tion on the luminescence properties, and reported that ele-vated levels resulted in degradation of crystallinity and mor-phology, with stimulation of lattice relaxation.[83] Workers atToshiba employed tert-butyl lithium to achieve similar levelsof doping (4.1 ´ 1016 cm±3) as Yasuda observed.[84] Other po-tential Li sources, such as lithium amides[85] and the pure met-al,[78] have yielded mixed successes. Lithium mobility is themajor drawback, coupled with the difficulty in obtaining, pref-erentially, substitutional over interstitial incorporation. So-dium doping has been attempted with the pure metal[78] andNa2Se.[86] Neither study exhibited any evidence for p-typedoping. Members of group 11 include copper, silver, and gold.The ineffectiveness of this group was indicated through stud-


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Table 2. Ionic, atomic, and covalent radii (after [7]).

ies by Dean, who reported formation of only deep level ac-ceptors by these elements.[87]

The elements of group 15 are nitrogen, phosphorous, ar-senic, antimony, and bismuth. The latter two have quite largeionic radii (Table 2); thus the crystallinity would be disrupted.N, P, and As have been ion-implanted into ZnSe.[88] This tech-nique causes extreme damage to the lattice. Annealing underzinc vapor, at temperature exceeding 550 �C, was inadequateto repair the defects. Akimoto et al. at Sony implanted nitro-gen, followed by a thermal annealing step using an infraredlamp.[89] Blue emission was observed at 77 K; however, theimplanted layer exhibited a high resistivity. Arsenic dopingwas studied by Tournie et al. involving As plasma[14] and alsoby Skromme et al. using Zn3As2

[90±92] as the source, but bothproved difficult to introduce into the films. Furthermore, bothAs and P are known to form deep emission centers instead ofshallow acceptor levels, and they fail to lower the resistivity ofthe grown layer.[93±95] Chadi posed an explanation that com-pares the calculated energies of two atomic states having tet-rahedral and three-fold symmetry.[96] Both As and P favor thethree-fold state that involves large lattice relaxation, and theresulting acceptor level is deep. The tetrahedral symmetry is alower energy state for nitrogen and is associated with shallowacceptor levels. This supports the conclusion that N is themost promising candidate for production of p-type ZnSe.

4.4. Nitrogen Precursors for p-Type Doping

Stutius was the first to report nitrogen incorporation intozinc selenide, using ammonia as the source.[69] In this study, ahigh total concentration of nitrogen in the film was achieved;however, both low net carrier concentration and high resistiv-ity were observed. These results are explained by hydrogenpassivation of the nitrogen. The typical N±H binding energy is93.4 kcal/mol, therefore high growth temperatures are re-quired to ensure cleavage of all bonds in ammonia. Incom-plete decomposition, leading to introduction of NH or NH2

radicals into the films, at growth temperatures of 550 �C, ex-plains the low net acceptor concentration (1015 cm±3).[97] In-creasing temperature aids precursor decomposition at the ex-pense of raising the defect concentration. Myers et al.reported that hydrogen levels were tied closely to the nitrogenincorporation, based on infrared absorptions in their MBEstudy, using cracked precursors.[98] In their study, levels of1019 cm±3 of nitrogen were accompanied by 1020 cm±3 of hy-drogen. The excess hydrogen was suggested to reside in thevacancies, following from the results of Tatarkiewicz et al.,[99]

who demonstrated the presence of Zn±H (1315 cm±1) andSe±H (2150 cm±1) absorption bands. Yasuda et al. used MBEto illustrate that annealing a moderately doped sample in ahydrogen plasma reduced the number of active nitrogen ac-ceptors.[100] In response to ammonia providing a high concen-tration of nitrogen in the films, without achieving high net ac-ceptor levels, precursor development was directed towardsreducing the hydrogen substituents with alkyl groups.

Attempts using amines, ranging from primary to tertiary,have resulted in varying degrees of success. Nippon Steel Cor-poration noticed substantial reduction of hydrogen passiva-tion by employing tert-butylamine in photo-assisted MOCVDat growth temperatures between 330 and 390 �C and achievednitrogen levels greater than 1018 cm±3.[101] However, the freecarrier concentration was only 8.3 ´ 1017 cm±3, suggesting con-tinued passivation effects. Prior attempts with cracked tert-bu-tylamine in migration-enhanced epitaxy, akin to MBE, alsofailed to produce high active doping levels.[102] Methylamineonly produced layers of high resistivity.[103] In search of lowergrowth temperatures, Hahn et al. focused on the dissociationenergies of select amines, and concluded that allylamine andphenylhydrazine both required less energy to disassemblethan tert-butylamine.[104] Both were used in photo-assistedOMVPE, with growth temperatures between 300 and 340 �C,and produced improved results ([N] = 1018±1019 cm±3). De-composition of the phenyl hydrazine yields two nitrogen radi-cals, H2N and HN(phenyl). It was hypothesized that NH2,which is also a product in the decomposition of ammonia,would render only electronically inactive species. However,the HN(phenyl) fragment was proposed to further degrade toa nitrogen radical and benzene. Melas et al. observed higherincorporation levels of N, despite the low net acceptor con-centrations, utilizing decreased phenylhydrazine partial pres-sures, which was an improvement over ammonia.[105]

Since replacing a single hydrogen of ammonia yielded im-proved results, secondary amines were predicted to exhibitfurther improvements. Ishibashi et al. at Sony experimentedwith diisopropylamine as a dopant source for photo-assistedgrowth.[106] Similar to the primary amines (unfortunately), thenitrogen concentrations were approximately 1018 cm±3, butthe low net acceptor levels (1.4 ´ 1016 cm±3) were disappoint-ing. Fujita was unsuccessful utilizing tert-butyl-ethylamine inconjugation with photo-assisted methods.[107] Schumann et al.investigated tert-butyl-isopropylamine, diisopropylamine, di-isobutylamine, and hexamethyldisilazane; however, poor dop-ing results were produced using OMVPE.[108,109] Experimentsemploying tertiary amines, such as triallylamine,[110] trimethyl-amine,[103] and triethylamine,[107] displayed no nitrogen incor-poration. Overall, the best results were achieved employingphoto-assisted methods utilizing primary and secondaryamines; however, the net acceptor levels remained unaccepta-bly low.

Other nitrogen-containing precursors have been tried, suchas NF3, H2N2, EtN3, and trimethylsilyl azide (TMSN3). Nodaet al. reported a hole concentration of 1016 cm±3 employingNF3 in radical-assisted MOCVD.[111] The major limitation ofexercising fluorinated precursors is potential Zn±F formation,as discussed earlier. Yoshikawa et al. reported that dopingwith hydrazine resulted in low active acceptor levels due topassivation.[112] Ethylazide[113] also formed compensatedlayers, but extreme instability has limited its use. Replacingthe ethyl with trimethylsilyl greatly improves the stability ofthe azide; consequentially, only low levels of nitrogen incor-poration were observed.[114]


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N2 gas can be used as the carrier gas, and is not believed tocontribute to the active doping species, as Park et al. docu-mented.[115] However, Hishida et al. observed contrary effectsand published N2 gas doping experiments (without activation)in which nitrogen levels of 1020 cm±3 and net acceptor concen-trations of 1017 cm±3 were obtained.[116] For comparison, filmsincorporating activated N were deposited, and similar photo-luminescence results were observed. Insufficient sticking abil-ity of nitrogen gas on the surface generally produces no accep-tor incorporation. Hishida et al. hypothesized that elevatedgas pressure proportionally increased the sticking coefficient,thereby explaining the doping effect.

There have been numerous reports of doping of ZnSe withnitrogen radicals via a plasma. Park et al.[117] and Ohkawa etal.[118] were the first to publish this technique associated withMBE. They respectively achieved a net acceptor concentra-tion of 1017 cm±3 and 1015 cm±3. The plasma can be generatedusing a radio frequency,[117] electron cyclotron resonance mi-crowaves,[119] or direct current.[110] Marginal success was ob-tained by introducing the plasma into low-pressureOMVPE.[102,120] High resistivity and low net acceptor levels(1015 cm±3) were attributed to defects and passivation. Heu-ken et al. converted the carrier gas from H2 to N2 to eliminatethe potential of hydrogen incorporation through reaction withthe activated nitrogen.[121] The maximum achievable net ac-ceptor level incorporating a plasma was 1018 cm±3.

There have been multiple theories put forward to explainthe apparent limit of the net acceptor concentration inZnSe:N. The early belief was that native defects (e.g., Zn self-interstitials) were compensating the acceptor centers. If thisassumption indeed were true, then p-type doping would be ex-tremely difficult to accomplish. Laks and Van de Walle[122,123]

exercised ab initio total energy calculations to investigatethese defects and concluded that only for extremely Zn- andSe-rich materials did appreciable compensation effects occur.Recently, two alternative proposals have been debated in theliterature that focus on solubility of the dopants in the lattice,and self-compensation effects. First, thermodynamic limits ex-ist because two phases of the same material can only reside inequilibrium if the chemical potentials are the same for bothphases. The solubility limit is reached when the total energy ofadding another dopant atom exceeds the lowest free energydefect. The calculations of Laks and Van de Walle involvedLi, Na, and N, and relied on Li2Se, Na2Se, Zn3N2, and N2, asthe lowest energy defects. Nitrogen had the highest calculatedsolubility, followed by Li and then Na. These results agreedwell with experimental values previously mentioned. Sec-ondly, self-compensation is characterized by formation of de-fects, due to the introduction of nitrogen into the zinc selenideframework. Several authors[124±128] have noticed that increas-ing nitrogen concentrations resulted in formation of a newdeep center 45±55 meV below the conduction band. Many po-tential species have been proposed, ranging from selenium va-cancies[129,130] to larger donor complexes,[131±135] e.g., N±N and(VSe±Zn±NSe)+. Lengthy examinations comparing free ener-gies for diverse assemblies of potential defects were published,

though multiple mechanisms are likely to be responsible forthe compensation effects observed. It is widely accepted thatformation energies of the defect complexes are favorable atelevated doping levels, thus restricting the net acceptor con-centration to 1018 cm±3. In another approach, Faschinger sug-gests focusing on the relative position of the band edges, withrespect to the Fermi energy, to determine the maximum dop-ability.[136] To increase the active doping levels, the use ofsuperlattices to design the optimal configuration was pro-posed. Overall, the exact nature of the compensation is ambig-uous, but it is evident that a new approach is necessary to ac-complish high levels of electronically active nitrogen doping.

4.5. Novel Dopant Introduction Approach

The following research discussion involves the introductionof the nitrogen dopant pre-bonded to the zinc, i.e., formationof a ªZn±Nº fragment from designed precursors. This conceptcapitalizes on the lack of an appreciable anti-site defect den-sity of zinc, which results in positioning the nitrogen on theelectronically active selenium site. Promising results employ-ing Zn{N[Si(CH3)3]2}2 (Fig. 4), first synthesized by Wannagatet al.[137] in 1965, to p-type dope ZnSe warranted further re-search into zinc bis(amide) compounds.[138,139] The first spec-trum in Figure 4 was collected at ambient temperature, anddisplays features attributed to impurities (DL = iodine in thediethylzinc and BE = oxygen) in the source materials. The ad-ditional peak in the second spectrum is assigned to the elec-tronically active incorporation of nitrogen into the film.

Frankland reported the first preparation of a molecule con-taining a zinc±nitrogen bond in 1856.[140] Since then, therehave been numerous reports of alkylzinc amides in the litera-ture;[141,142] however, any Zn±C linkage tends to increase car-bon incorporation into the films. Early compounds weresynthesized from ligands such as dimethylamine[143] anddiethylamine,[140,144] which were probably polymeric in nature.Noltes and Boersma followed by making Zn[N(phenyl)2]2,which resides as a dimer in benzene.[145] In 1983, the molecu-lar structure of Zn{N[Si(CH3)3]2}2 was determined by gasphase electron diffraction.[146] The synthesis and first solid-state characterization of a homoleptic zinc bis(amide) com-pound was reported by Rees et al. in 1992 involving the tert-butyl-trimethylsilylamine ligand.[147,148] The research groupsof Power et al.[149] and Raston et al.[150] synthesized and char-acterized Zn[N(SiMePh2)2]2 and Zn[N(8-quinolyl)(SiMe3)]2,respectively. Recently, other dialkylamines[151,152] were used toproduce novel amides, e.g., diisopropylamine,[103,142] diisobu-tylamine,[103,142] N,N-dimethylaminoethylmethylamine,[103]

and bis-N,N-diethylaminoethylamine.[103]

5. Future Directions

Research continues into exploring the OMVPE precursorissues involved in improving the p-type doping efficiency to


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ultimately produce a true blue laser diode. The dopant issuecan be addressed through designing precursors that limit thelevels of contaminants, including either residual atoms fromligand decomposition, or failure in the placement of the ac-ceptor in the electronically active site. Gaining enlightenmentabout the chemical and physical properties of the potentialprecursors will provide insight into balancing volatility andvapor phase stability, to result in a method of assessing theutility of the molecules. Future publications will describe acomprehensive study of a series of zinc bis(amide) com-pounds, as p-type MOCVD dopants for ZnSe:N.[152b]

Utilizing the identity of the precursors and the by-productsexiting the reactor, logical conversions are formulated to ex-plain the potential reactions amidst the substrate. Further re-search is needed to better understand the chemistry asso-ciated with reactions that take place near the substratesurface. This information could lead to a new generation ofdesigned precursors, in which the decomposition pathways ofprecursors could be tailored to ensure proper incorporation ofthe desired atoms with minimal side reactions.

Received: April 3, 2000

±[1] G. Fasol, Science 1996, 272, 1751.[2] G. Zorpette, Sci. Am. 1997, Sept., 36.[3] T. Matsuoka, Adv. Mater. 1996, 8, 469.[4] S. Nakamura, MRS Bull. 1997, Feb., 29.[5] R. Gunshor, A. Nurmikko, MRS Bull. 1995, July, 15.[6] S. Itoh, S. Taniguchi, T. Hino, R. Imoto, K. Nakano, N. Nakayama, M.

Ikeda, A. Ishibashi, Mater. Sci. Eng. 1997, B43, 55.[7] C. Kittel, Introduction to Solid State Physics, Wiley, New York 1996.[8] A. Jones, P. O'Brien, CVD of Compound Semiconductors, VCH, Wein-

heim 1997, Ch. 1.[9] M. Haase, J. Qiu, J. DePuydt, H. Cheng, Appl. Phys. Lett. 1991, 59,

1272.[10] N. Nakayama, S. Itoh, T. Ohata, K. Nakano, H. Okuyama, M. Ozawa,

A. Ishibashi, M. Ikeda, Y. Mori, Electron. Lett. 1993, 29, 1488.[11] N. Nakayama, S. Itoh, H. Okuyama, M. Ozawa, T. Ohata, K. Nakano,

M. Ikeda, A. Ishibashi, Y. Mori, Electron. Lett. 1993, 29, 2194.[12] D. Grillo, M. Ringle, G. Hua, J. Han, R. Gunshor, A. Nurmikko, J. Vac.

Sci. Technol. A 1995, 13, 681.[13] S. Taniguchi, T. Hino, S. Itoh, K. Nakano, N. Nakayama, A. Ishibashi,

M. Ikeda, Electron. Lett. 1996, 32, 552.[14] E. Tournie, C. Morhain, C. Ongaretto, V. Bousquet, P. Brunet, G. Neu, J.

Faurie, R. Triboulet, J. Ndap, Mater. Sci. Eng. 1997, B43, 21.[15] M. Bevan, H. Shih, H. Liu, A. Syllaios, W. Duncan, J. Cryst. Growth

1997, 170, 467.

[16] T. Honda, S. Lim, K. Inoue, K. Hara, H. Munekata, H. Kukimoto, F.Koyama, K. Iga, J. Cryst. Growth 1997, 170, 503.

[17] Y. Fan, J. Han, L. He, J. Saraie, L. Gunshor, G. Hua, N. Otsuka, Appl.Phys. Lett. 1992, 61, 3160.

[18] H. Okuyama, K. Nakano, T. Miyajima, K. Akimoto, J. Cryst. Growth1992, 117, 139.

[19] W. Meredith, III±Vs Rev. 1996, 9, 56.[20] A. Waag, F. Fisher, H. Lugauer, T. Litz, T. Gerhard, J. Nurnberger, U.

Lunz, U. Zehnder, W. Ossau, G. Landwehr, B. Roos, H. Richter, Mater.Sci. Eng. 1997, B43, 65.

[21] H. Lugauer, F. Fisher, T. Litz, A. Waag, U. Zehnder, W. Ossau, T. Ger-hard, G. Landwehr, C. Becker, R. Kruse, J. Geurts, Mater. Sci. Eng.1997, B43, 88.

[22] W. S. Rees, Jr., in Encyclopedia of Inorganic Chemistry, Vol. 1 (Ed:R. B. King), Wiley, New York 1994, pp. 67±87.

[23] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 1994, 64, 1687.[24] S. Nakamura, J. Vac. Sci. Technol. A 1995, 13, 705.[25] S. Nakamura, Nikkei Electron. 1994, 602, 93.[26] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 1994, 64, 1687.[27] See for example, the cover articles from the following: Adv. Mater. 1999,

11(16); Adv. Mater. 1998, 10(12); Adv. Mater. 1998, 10(1); Adv. Mater.1997, 9(9); Adv. Mater. 1997, 9(1); Adv. Mater. 1995, 7(9).

[28] I. Akaski, S. Sota, H. Sakai, T. Tanaka, M. Koike, H. Amano, Electron.Lett. 1996, 32, 1105.

[29] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Mat-sushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 1996, 35, L74.

[30] S. Nakamura, III±Vs Rev. 1996, 9, 22.[31] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Matsushita, T. Mu-

kai, Appl. Phys. Lett. 2000, 76, 22.[32] X. Zhao, O. Schoenfeld, J. Kusano, Y. Aoyagi, T. Sugano, Jpn. J. Appl.

Phys. 1994, 33, L649.[33] L. Hoffman, G. Ziegler, D. Theis, C. Weyrich, J. Appl. Phys. 1982, 53,

6962.[34] Y. Matsushita, T. Nakata, T. Uetani, T. Yamaguchi, T. Niina, Jpn. J.

Appl. Phys. 1990, 29, L343.[35] K. Koga, T. Yamaguchi, Prog. Cryst. Growth Charact. 1991, 23, 127.[36] N. Yamada, Y. Kaneko, D. Mars, Compd. Semicond. 1996, 3, 41.[37] T. Yokogawa, S. Yoshii, A. Tsujimura, Y. Sasai, J. Merz, Jpn. J. Appl.

Phys. 1995, 34, L751.[38] J. Zarrabi, P. Gavrilovic, S. Singh, Appl. Phys. Lett. 1995, 67, 2439.[39] N. Tessler, G. Denton, R. Friend, Nature 1996, 382, 695.[40] F. Hide, M. Diaz-Garcis, B. Schwartz, M. Andersson, Q. Pei, A. Heeger,

Science 1996, 273, 1833.[41] D. Moses, Appl. Phys. Lett. 1992, 60, 321.[42] See for example, the cover article from the following: Adv. Mater. 1996,

8(8).[43] E. Westerweele, P. Smith, A. Heeger, Adv. Mater. 1995, 7, 788.[44] K. Varth, K. Jensen, Adv. Mater. 1997, 9, 490.[45] S. Tasch, C. Brandstatter, F. Meghdadi, G. Leising, G. Froyer, L.

Athouel, Adv. Mater. 1997, 9, 33.[46] T. Noda, H. Ogawa, N. Noma, Y. Shirota, Adv. Mater. 1997, 9, 720.[47] M. Berggren, O. Inganas, G. Gustafsson, J. Rasmusson, M. Anderson, T.

Hjertberg, O. Wennerstrom, Nature 1994, 372, 444.[48] A. Fujii, M. Yoshida, Y. Ohmori, K. Yoshimo, J. Appl. Phys. 1995, 34,

499.


REVIE

WS


Fig. 4. Photoluminescence data of ZnSe:N employing Zn{N(Si(CH3)3)2]2 as the dopant source (from [138]).

[49] S. Tasch, A. Niko, G. Leising, U. Scherf, Appl. Phys. Lett. 1996, 68, 1090.[50] CVD of Non-Metals (Ed: W. S. Rees, Jr.), VCH, Weinheim 1996.[51] G. Stringfellow, Organometallic Vapor Phase Epitaxy: Theory and Prac-

tice, Academic, Boston, MA 1989.[52] M. Hitchman, K. Jensen, Chemical Vapor Deposition: Principles and

Applications, Academic, London 1993.[53] M. Hampden-Smith, T. Kodas, Chem. Vap. Deposition 1995, 1, 8.[54] R. Moon, J. Cryst. Growth 1997, 170, 1.[55] F. Maury, Chem. Vap. Deposition 1996, 2, 113.[56] a) W. S. Rees, Jr., A. R. Barron, Adv. Mater. Opt. Electron. 1993, 2, 271.

b) W. S. Rees, Jr., A. R. Barron, Mater. Sci. Forum 1993, 137±139, 473.[57] H. Manasevit, W. Simpson, J. Electrochem. Soc. 1968, 118, 644.[58] A. Jones, P. O'Brien, CVD of Compound Semiconductors, VCH, Wein-

heim 1997, Ch. 4.[59] P. Wright, B. Cockayne, A. Williams, A. Jones, E. Orrell, J. Cryst.

Growth 1987, 84, 552.[60] A. Jones, P. Wright, B. Cockayne, Chemtronics 1988, 3, 35.[61] H. Mitsuhashi, I. Mitsuishi, H. Kukimoto, Jpn. J. Appl. Phys. 1985, 24,

1864.[62] S. Srithoran, K. Jones, J. Cryst. Growth 1984, 66, 231.[63] K. Giapis, K. Jensen, J. Cryst. Growth 1990, 101, 111.[64] S. Patrick, K. Jensen, K. Giapis, J. Cryst. Growth 1991, 107, 390.[65] K. Jensen, ICMOVPE VI, Boston, MA 1992.[66] M. Heuken, in Materials Science Forum, (Eds: H. Heinrich, J. B. Müllen)

Trans Tech, Linz, Austria 1995, 182±184, 389.[67] H. Katayama-Yoshida, T. Yamamoto, Phys. Status Solidi B 1997, 202,

763.[68] M. Aven, H. Woodbury, Appl. Phys. Lett. 1962, 1, 53.[69] W. Stutius, J. Cryst. Growth 1982, 59, 1.[70] W. Stutius, J. Appl. Phys. 1982, 53, 284.[71] K. Akimoto, T. Miyajima, Y. Mori, Jpn. J. Appl. Phys. 1989, 28, L531.[72] K. Akimoto, T. Miyajima, Y. Mori, J. Cryst. Growth 1990, 101, 1009.[73] T. Muranoi, S. Onizawa, M. Sasaki, J. Cryst. Growth 1994, 138, 255.[74] N. Shibata, A. Ohki, A. Katsui, J. Cryst. Growth 1988, 93, 703.[75] H. Stanzl, K. Wolf, B. Hahn, W. Gerbhardt, J. Cryst. Growth 1994, 145,

918.[76] A. Kamata, T. Uemoto, M. Okajima, K. Hirahara, M. Kawachi, T. Bep-

pu, J. Cryst. Growth 1988, 86, 285.[77] T. Chu, S. Chu, G. Chen, J. Britt, C. Ferekides, C. Wu, J. Appl. Phys.

1992, 71, 3865.[78] H. Cheng, J. Depuydt, J. Potts, M. Haase, J. Cryst. Growth 1989, 95, 512.[79] K. Akimoto, T. Miyajima, Y. Mori, Phys. Rev. B 1989, 39, 3138.[80] C. Helms, Jpn. J. Appl. Phys. 1993, 32, 1558.[81] T. Yasuda, I. Mitsuishi, H. Kukimoto, Appl. Phys. Lett. 1988, 52, 57.[82] A. Yoshikawa, S. Muto, S. Yamaga, S. Kasi, Jpn. J. Appl. Phys. 1988, 27,

L260.[83] B. Skromme, W. Liu, K. Jensen, K. Giapis, J. Cryst. Growth 1994, 138, 338.[84] A. Yahata, H. Mitsuhashi, K. Hirahara, T. Beppu, Jpn. J. Appl. Phys.

1990, 29, L4.[85] A. Yoshikawa, S. Muto, S. Yamaga, S. Kasi, J. Cryst. Growth 1983, 93,

697.[86] T. Yao, T. Taguchi, in Proc. 13th Int. Conf. on Defects in Semiconductors

(Eds: L. C. Kimerling, J. M. Parsley), Metallurgical Society of AIME,Warrendale, PA 1984, p. 1221.

[87] P. Dean, B. Fitzpatrick, R. Bhargava, Phys. Rev. B 1982, 26, 2016.[88] H. Wolf, A. Burchard, M. Deicher, T. Filz, A. Jost, S. Lauer, R. Magerle,

V. Ostheimer, W. Pfeiffer, T. Wichert, Mater. Sci. Forum 1995, 196±201,309.

[89] K. Akimoto, T. Miyajima, Y. Mori, Jpn. J. Appl. Phys. 1989, 28, L528.[90] S. Shibli, B. Tamargo, B. Skromme, S. Schwarz, C. Schwartz, R. Nahory,

R. Martin, J. Vac. Sci. Technol. B 1990, 8, 187.[91] Y. Zhang, B. Skromme, S. Shibili, M. Tamargo, Phys. Rev. B 1993, 48,

10 885.[92] Y. Zhang, W. Liu, B. Skromme, H. Cheng, S. Shibili, M. Tamargo, J.

Cryst. Growth 1994, 138, 310.[93] R. Watts, W. Holton, M. de Witt, Phys. Rev. B 1971, 3, 404.[94] M. Okajima, M. Kawachi, T. Sato, K. Hirahara, A. Kamata, T. Beppu, in

Extended Abstracts 18th Conf. on Solid State Devices and Materials,Tokyo, August 20±22 1986, p. 635.

[95] Y. Kawakyu, M. Sasaki, T. Uemoto, T. Beppu, in Extended Abstracts19th Conf. on Solid State Devices and Materials, Tokyo, August 25±271987, p. 259.

[96] D. Chadi, N. Troullier, Physica B 1993, 185, 128.[97] H. Goto, T. Tanoi, M. Takemura, T. Ido, Jpn. J. Appl. Phys. 1995, 34,

L827.[98] Z. Yu, S. Buczkowski, L. Hirsch, T. Meyers, J. Appl. Phys. 1996, 80,

6425.

[99] J. Tararkiewicz, A. Breitschwerdt, A. Witowski, H. Hartmann, SolidState Commun. 1988, 68, 1081.

[100] T. Yasuda, T. Yasui, B. Zhang, Y. Segawa, J. Cryst. Growth 1996, 159,1168.

[101] Y. Fujita, T. Terada, T. Suzuki, Jpn. J. Appl. Phys. 1995, 34, L1034.[102] S. Zhang, N. Kobayashi, Jpn. J. Appl. Phys. 1992, 31, L666.[103] K. Morimoto, T. Fujino, Appl. Phys. Lett. 1993, 63, 2384.[104] B. Hahn, M. Deufel, M. Meier, M. Kastner, R. Blumberg, W. Gebhardt,

J. Cryst. Growth 1997, 170, 472.[105] S. Akarm, I. Bhat, A. Melas, J. Electron. Mater. 1994, 23, 259.[106] A. Toda, F. Nakamura, K. Yanashima, A. Ishabashi, J. Cryst. Growth

1997, 170, 461.[107] S. Fujita, in Abstracts 7th Intl. Conf. Met. Org. Vap. Phase Epitaxy (Eds:

Y. Horikoshi, S. Minagawa), Yokohama, Japan, May 31±June 3 1994.[108] U. Pohl, S. Freitag, J. Gottfriedsen, W. Richter, H. Schumann, J. Cryst.

Growth 1997, 170, 144.[109] U. W. Pohl, J. Gottfriedsen, H. Schumann, unpublished.[110] G. Prosch, K. Hellig, R. Beyer, A. Schneider, H. Burghardt, W. Taudt,

M. Heuken, D. Zahn, J. Cryst. Growth 1997, 170, 537.[111] Y. Noda, T. Ishikawa, M. Yamabe, Y. Hara, Appl. Surf. Sci. 1997, 113/

114, 28.[112] A. Yoshikawa, S. Matsumoto, S. Yamago, H. Kasai, J. Cryst. Growth

1990, 101, 305.[113] A. Kamata, J. Cryst. Growth 1994, 145, 557.[114] W. Taudt, S. Lampe, F. Sauerlander, J. Sollner, H. Hamadeh, M. Heu-

ken, A. Jones, J. Rushworth, P. O'Brien, M. Malik, J. Cryst. Growth1996, 196, 243.

[115] R. Park, H. Mar, N. Salansky, J. Appl. Phys. 1985, 58, 1047.[116] Y. Hishida, T. Yoshie, K. Yagi, K. Yodoshi, T. Niina, Jpn. J. Appl. Phys.

1995, 35, 1415.[117] R. Park, M. Troffer, C. Rouleau, J. DePuydt, M. Haase, Appl. Phys. Lett.

1990, 57, 2127.[118] K. Ohkawa, T. Karasawa, T. Mitsuyu, Jpn. J. Appl. Phys. 1991, 30,

L152.[119] T. Ohtsuka, K. Horie, Jpn. J. Appl. Phys. 1993, 32, L233.[120] K. Nishimura, Y. Nagao, K. Sakai, J. Cryst. Growth 1994, 138, 114.[121] W. Taudt, A. Hardt, S. Lampe, H. Hamadeh, M. Heuken, J. Cryst.

Growth 1997, 170, 491.[122] D. Laks, C. Van de Walle, Physica B 1993, 185, 1993.[123] D. Laks, C. Van de Walle, G. Neumark, S. Pantelides, Appl. Phys. Lett.

1993, 63, 1375.[124] K. Ohkawa, A. Tsujimura, S. Hayashi, S. Yoshii, T. Mitsuyu, Physica B

1993, 185, 112.[125] I. Hauksson, J. Simpson, S. Wang, K. Prior, B. Cavenett, Appl. Phys.

Lett. 1992, 61, 2208.[126] B. Murdin, B. Cavenett, C. Pidgeon, J. Simpson, I. Hauksson, K. Prior,

Appl. Phys. Lett. 1993, 63, 2411.[127] Z. Zhu, K. Takebayashi, K. Tanaka, T. Ebisutani, J. Kawamata, T. Yao,

Appl. Phys. Lett. 1994, 64, 91.[128] P. Boyce, J. Davies, D. Wolverson, K. Ohkawa, T. Mitsuyu, Appl. Phys.

Lett. 1994, 65, 2063.[129] S. Setzler, M. Moldovan, Z. Yu, T. Myers, N. Giles, L. Halliburton, Appl.

Phys. Lett. 1997, 70, 2274.[130] K. Prior, W. Meredith, G. Brownlie, Z. Zhu, P. Thompson, J. Milnes, I.

Hauksson, G. Horsburgh, T. Steele, S. Wang, B. Cavenett, Mater. Sci.Eng. 1997, B43, 9.

[131] S. Poykko, M. Puska, R. Korhonen, R. Nieminen, Mater. Sci. Eng. 1997,B43, 1.

[132] M. Moldovan, S. Setzler, T. Myers, L. Halliburton, N. Giles, Appl. Phys.Lett. 1997, 70, 1724.

[133] M. Moldovan, S. Setzler, Z. Yu, T. Myers, L. Halliburton, N. Giles, J.Electron. Mater. 1997, 26, 732.

[134] T. Yao, T. Matsumoto, S. Sasaki, C. Chung, Z. Zhu, F. Nishiyama, J.Cryst. Growth 1994, 138, 290.

[135] B. Cheong, K. Chang, Mater. Sci. Forum 1995, 196±201, 303.[136] W. Faschinger, J. Cryst. Growth 1996, 159, 221.[137] H. Burger, W. Sawodny, U. Wannagat, J. Organomet. Chem. 1965, 3,

113.[138] W. Rees, Jr., D. Green, T. Anderson, E. Bretschneider, B. Pathangey, C.

Park, J. Kim, J. Electron. Mater. 1992, 21, 361.[139] W. Rees, Jr., D. Green, T. Anderson, E. Bretschneider, Mater. Res. Soc.

Proc. 1992, 242, 281.[140] E. Frankland, Proc. R. Soc. 1857, 8, 502.[141] M. Lappert, P. Power, A. Sanger, R. Srivastava, Metal and Metalloid

Amides, Ellis Horwood, Chichester, UK 1980.[142] M. Malik, P. O'Brien, Polyhedron 1997, 16, 3593.[143] G. Coates, D. Ridley, J. Chem. Soc. 1965, 1870.


REVIE

WS


[144] T. Cuvigny, H. Normant, Comptes Rend.. 1969, 268C, 834.[145] J. Noltes, J. Boersma, J. Organomet. Chem. 1969, 16, 345.[146] A. Haaland, K. Hedberg, P. Power, Inorg. Chem. 1984, 23, 1972.[147] W. Rees, Jr., D. Green, W. Hesse, Polyhedron 1992, 11, 1697.[148] W. Rees, Jr., D. Green, W. Hesse, T. Anderson, B. Pathangey, Mater.

Res. Soc. Proc. 1992, 282, 63.[149] P. Power, K. Ruhlandt-Senge, S. Shoner, Inorg. Chem. 1991, 30, 5013.[150] L. Englehardt, P. Junk, W. Patalinghug, R. Sue, C. Raston, B. Skelton,

A. White, J. Chem. Soc., Chem. Commun. 1991, 930.

[151] H. Schumann, J. Gottfriedsen, S. Dechert, F. Girgsdies, Z. Anorg. Allg.Chem. 2000, 626, 747.

[152] a) D. Gaul, Ph.D. Dissertation, Georgia Institute of Technology 1998.b) D. Gaul, O. Just, W. S. Rees, Jr., unpublished results.

[153] T. Matsuoka, A. Ohki, T. Ohno, Y. Kawaguchi, J. Cryst. Growth 1994,138, 727.


REVIE

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