J. Phys. Chem. 1994,98, 3515-3581
3575
FEATURE ARTICLE Luminescence of Silicon Materials: Chains, Sheets, Nanocrystals, Nanowires, Microcrystals, and Porous Silicon+ Louis Brus AT& T Bell Laboratories, Murray Hill, New Jersey 07974
Received: December 2, 1993; In Final Form: January 25, 1994'
Both nanocrystal silicon and porous silicon thin films show efficient visible luminescence at room temperature, thus suggesting the possibility of introducing some form of optically active, silicon-based material into integrated circuit processing. In this context, I discuss and analyze the responsible electronic quantum size effects and excited-state photophysics. In a broader context I discuss silicon optical and electronic properties as a function of dimensionality and surface chemistry. As one progresses from trans-polysilane ( 1D-Si) through puckered sheet polysilyne (2D-Si) to diamond lattice silicon (3D-Si), there is a systematic progression from direct to indirect gap behavior. In 2D-Si the direct and indirect gaps are nearly degenerate, and the electronic properties can be tailored through surface chemical derivatization. It may be that a direct gap material can be found in the hydroxysiloxene materials. Bulk 3D-Si is remarkable for its slow rates of both radiative and nonradiative intrinsic excited-state decay. Nanocrystal Si and porous Si are indirect gap type materials with oscillator strengths that are not markedly increased with respect to bulk Si. Luminescence increases because spatial confinement keeps the electron and hole superimposed and because surface nonradiative rates are also extremely slow. Theory indicates that lowering of the diamond lattice symmetry via physical effects, such as strain or nanometer finite size, creates only a relatively minor perturbation of the 3D-Si luminescence. However, finite nanometer size can increase the indirect gap by as much as 1 eV. Microcrystalline silicon can be an efficient optical cavity.
I. Introduction Digital computing is carried out electronically in silicon integrated circuits, while digital communication is carried out optically on silica single-mode fibers. Each of these technologies is amazingly powerful and environmentally benign in use. Their success depends critically upon the specific material properties of SiOz, crystalline Si, and their common solid-state interface. While engineering and materials problems can be severe, the fundamental physical limits in each field will possibly be reached only several decades into the future. Both are "natural" technologies from several different perspectives. Barring an unexpected major discovery, it is hard to anticipate how and why they might be ultimately replaced, especially in view of the huge worldwide economic investment. It is the interconnection between these two technologies that is expensive and complex and hence a place where useful scientific discoveries and inventions might occur. Crystalline, diamond lattice silicon is an indirect gap semiconductor, and thus its luminescence is electron dipole forbidden, in the same way that the luminescence of highly symmetric molecules (for example, icosahedral C,) is sometimes dipole forbidden within the molecular point group. Silicon itself cannot be used to generate communication optical signals. However, if there were some way to essentially break the diamond lattice symmetry, either physically or chemically, such that silicon (or some material derived from silicon by a process compatible with VLSI technology) showed useful luminescence and optical gain, then optical This article is dedicated to Prof. h i m Henglein of the Hahn-Meitner Institute in Berlin, upon the occasion of his retirement in 1994. The physical chemistry of colloidal nanocrystals, both metals and semiconductors, has been reborn and transformedby his pioneering experiments over the past two decades. *Abstract published in Advance ACS Abstracts, March 1, 1994.
0022-3654/94/2098-3515~04.50/0
communication devices might be grown directly on the silicon computer chip. This Feature Article is an outgrowth of our recent work investigating whether silicon nanocrystals (sometimes called quantum crystallites or quantum dots) might be made to have useful luminescence. In order to describe the science involved more fully, I discuss silicon electrical and optical properties as a function of dimensionality, surface chemistry, and crystallite size. I also discuss several mostly hypothetical silicon-xygenbased materials that are intriguing, as well as a real and truly novel, naturally nanostructured "porous Si" phase which shows room temperature visible luminescence. I will try to emphasize things which are not known and are thus opportunities for research. The chemistry and physics of direct gap semiconductor nanocrystals such as CdSe have been described elsewhere.'
II. Silicon Polymers in One, Two, and Three Dimensions A. Band Structure. Consider three Si polymers that show sp3 hybridization: linear trans-polysilane (SiHZ), (hereafter 1D-Si), puckered sheet polysilyne (SiH), (2D-Si), and diamond lattice silicon (3D-Si). In each case the HOMO band is strongly delocalized (i.e., shows a large dispersion across the Brillouin zone) and is formed by S i S i Pu bonds with a maximum at the zone center r point, as shown in Figures 1 and 2. At I' the HOMO spatial degeneracy is equal to the S i S i bonding dimensionality, because of the Si P degeneracy. In ID-Si and 2D-Si there are occupied Si-H bands between the low-lying, very stable S i S i Su band and the HOMO S i S i Pu band. The S i S i bonding is strong in each polymer; for example, in 3D-Si the valence bands are 12 eV wide. I' represents K = 0 in the Bloch molecular orbital = $x(r)efK' where q5K(r) is a molecular wave function inside the unit cell, and the eiKr factor is a propagating wave. The valence to 0 1994 American Chemical Society
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3576 The Journal of Physical Chemistry, Vol. 98, No. 14, 199’4
1D-Si
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Figure 1. Band structures of ID-Si and 2D-Si. Adapted from ref 2. GaAs
3D-Si
Figure 3. Structures of various siloxenes.
6 6
mix strongly for propagation along (100) to give a P,S hybrid LUMO at 1.1 eV near the X point. 2 These band structures might be modified either physically-by - 0 0 -2 size effects, strain, or external fields-or by chemical substitution. -2 $ -4 In 3D-Si a drastic chemical substitution would be to replace IV-4 -6 IV Si by 111-V GaAs, for example. Figure 2 shows that, while -6 -6 -6 GaAs has essentially the same valence band structure as Si, the Sa* and Pa* conduction bands are reversed at r. In a situation -12 -12 somewhat analogous to 2D-Si, the LUMO shows local minima r * x U,K X U. K near X and L, as well as at I?. However, unlike 2D-Si, the r SILICON GALLIUM ARSENIDE direct gap in GaAs is lowest by about 0.3 eV, and this fact makes DIAMOND STRUCTURE ZINC BLENDE STRUCTURE GaAs into an extremely important diode laser material. Figure 2. Band structures of 3D-Si and GaAs from ref 4. In 2D-Si either Si or H atoms might be chemically substituted. Substitution by oxygen creates a generic family of “siloxenes” conduction band optical absorption momentum matrix element with remarkable calculated band structures. In what we might (olyleplcK’)is zero unless K = K! Therefore, in these Si polymers call “hydroxy 2D-Si”, H is replaced by OH on one side of the 2D the LUMO conduction band must have it minimum at I’ (that sheet in Figure 3a. In this structure, the oxygen lone pairs, which is, must be a direct gap semiconductor) for the band gap lie below the S i S i HOMO, interact with the HOMO to raise luminescence to be optically allowed. its energy. Also, the empty oxygen S orbital preferentially The LUMO conduction bands all show antibonding S i S i resonates with the conduction band sp orbital a t r, more than character. However, while the valence band shapes of the three with the sp3 LUMO at *. As a result, hydroxy 2D-Si has a polymers are quite similar, the LUMO conduction band shapes 1.7-eV direct gap, unlike the indirect gap parent 2D-Si.3~6The are quite different, with profound consequences for the optical strong dispersion of the HOMO and LUMO bands is retained. properties. 1D-Si shows a 3.89-eV direct band gap, with the This material might well be an optically active, useful semiconLUMO band minimum at r.2 In 2D-Si, the LUMO has two ductor, if practical methods of synthesis and doping can be found. local minima, with the direct 2.61-eV gap slightly higher than A linear chain siloxene in Figure 3b, with Si chains bridged the indirect 2.48-eV gap a t the * point near the zone b ~ u n d a r y . ~ ~ . ~together by oxygen, has a quasi-direct gap at 3.24 eV.6 In this In 3D-Si the direct gap is present as a 3.4-eV saddle point, while structure the HOMO is principally formed from oxygen lone the indirect gap is now fully 2.3 eV lower at 1.1 eve4 1D-Si pairs and has little dispersion; in this respect this material is like should show strong UV emission, while 3D-Si should show weak, SiOz. In Kautsky siloxene (Figure 3c), in which six-membered vibronically induced IR emission. Si rings are bridged by oxygen in a sheet structure, the HOMO This systematic progression from direct to indirect behavior is also formed by oxygen lone pairs. The 0.6-eV gap is quasireflects the relative energies of different types of antibonding direct in K space but forbidden by orbital symmetry within the states, ultimately traceable to the facts that (i) S-P hybridization unit cell. is a strong function of wavevector K ( 5 ) and (ii) the antibonding While a synthesis of hydroxy 2D-Si from CaSiz has been Pa* states come down in energy with respect to the Sa* states reported? none of the siloxenes have been rigorously characterized. as dimensionality increases. In 2D-Si at I’, for example, the They illustrate a research opportunity for solid-state chemistry, conduction band minimum is a P,S hybrid orbital on the opposite in the search for new optical materials compatible with silicon sideof the Si atom from the Si-H bond.3 The doubly degenerate, processing. In any material made from silicon and oxygen alone, antibonding S i S i Pa* band lies about 1S eV higher. However, the thermodynamic tendency to phase separate into Si02 and as K increases toward the S point, these bands mix so that the 3D-Si is very strong. However, with a third atom X present-H, (lowest) LUMO state at * is close to a sp3 hybrid directed into a halogen, or a metal-it may be possible to stabilize new structures a puckered Si ring. and phases. Since 3D-Si and Si02 are of such technological importance, it seems worthwhile to explore SiO,X, phase In 3D-Si at the r point the situation is reversed: the triply diagrams, and possible metastable new phases, somewhat more degenerate Pa* conduction band is lowest (at 3.4 eV), with the systematically. antibonding S conduction band about 1 eV higher. These states 4
4
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z
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Feature Article
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3511
6
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,w
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300 Wavelength
300 (nm)
400 500 Wavelength (nm)
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630nm emission,
Figure 4. Optical absorption and emission spectra of 1D-Si and 2D-Si
Bulk optical absorption
samples from ref 16a.
B. Luminescence. Band structure incorporates translational symmetry into simple molecular orbital theory. Electron-hole interaction and excited-state structural change are not considered. This proves to be an excellent approximation for 3D-Si and a moderately poor approximation for 1D-Si. 1D-Si, predicted to be direct gap, does emit strongly in the UV, with little Stokes shift and radiative lifetimes on the order of 0.3 ns.8 The absorption spectrum is discrete as shown in Figure 4. A comparison of one- and two-photon spectra shows that electron-hole Coulomb attraction is quite strong, on the order of 1 eV, producing discrete exciton states observed in absorption and emission.9 These states are confined to 1D-Si segments of about 20 Si atoms,1° apparently by solvation-induced conformational deviations from all-trans planar structure. Experimental absorption and emission spectra are inhomogeneous, in the sense that segments of different lengths and different exciton energies absorb and emit. Recent theory predicts that the UV emitting excitons in gas phase (unsolvated) 1D-Si should in fact localize (i.e., self-trap) without barrier on one S i S i bond, leading to fast molecular dissociation with high quantum yield." It seems then that condensed-phase solvation of 1D-Si is essential in stabilizing thedelocalized emitting state. This fact, and the strong electronhole interaction, reveal "molecular" aspects of excited-state dynamics in 1D-Si. In agreement with calculated band structure, 3D-Si is an indirect gap semiconductor with weak, continuous visible absorption extending to the indirect 1.1-eV gap. This absorption is vibronically induced principally by a TO (transverse optical) phonon at 458 cm-I. There is also intense ultraviolet absorption due to direct gaps at 3.4 and 4.2 eV, as shown in Figure 5 . In 3D-Si, small effective masses and screening of the electronhole Coulombic interaction (q = 13) decrease the electron-hole interaction energy nearly 100-fold in comparison with 1D-Si. The Rydberg for hydrogenic exciton electron-hole states is 0.01 47 eV.IZ As a result, electron-hole pairs thermally dissociate a t 23 OC. The rigid, 3D-Si lattice shows very little conformational relaxation (Le., almost no electron-phonon interaction) for free carriers or the low-temperature hydrogenic exciton. The weakness of electron-phonon coupling for both holes and electrons allows the associated mobilities to be large enough so that 3D-Si shows useful conductivity when doped. This fact, along with the extreme stability of the strongly bonded 3D lattice against current-induced defect formation, allows stable electronic device performance. Both radiative and nonradiative electron-hole recombination in perfect 3D-Si crystals are extremely slow. As far as is known, nonradiative recombination, when it occurs, is always catalyzed by a defect or trap state which effectively increases electronphonon coupling. At 23 OC, optically created free electrons and holes have been observed to live as long as 40 ms, a t very low density in nearly perfect Si wafers with surfaces passivated by
00 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5
excitation energy (ev)
Figure 5. (right-handscale) Square root of the 3D-Si optical absorption
cross secaon (arbitraryunits) for a IO-nm sphereversusopticalabsorption energy, as calculated from Mie theory. (left-hand scale) Square root of 630-nm Si nanocrystal emission intensity versus excitation energy (20 K). The organic glass sample is optically thin. Adapted from ref 24.
H atoms.13 (The lifetime is a strong function of electron-hole density because of three-body Auger nonradiative recombifiation.) At 23 OC the apparent radiative lifetime of a free electron and hole is orders of magnitude longer than the 6 X le5s vibronic emission radiative lifetime estimated for a free, hydrogenic exciton a t low temperat~re.1~ This occurs because free carriers at low density do not interact with each other except in rare scattering events. Near 4 K temperature, excitons trapped at impurities show millisecond lifetimes with quantum yields of a few percent.15 Trapped excitons have wave functions significantly distorted from hydrogenic free excitons. While some impurities do increase luminescence at low temperature, generally the exciton binding energies are less than kT and there is no luminescence a t 23 OC. Little spectroscopic and dynamical data are available for 2DSi and associated siloxenes. Single crystals have not been made. Alkyl-derivatized fragments of 2D-Si puckered sheets show weakly structured UV absorption spectra and self-trapped, broad visible luminescence (see Figure 4), with a large Stokes shift and multiexponential kinetics characteristic of slow recombination from separately localized carriers.16 It remains to be seen under what conditions, if any, these materials (including direct gap hydroxy 2D-Si) are conformationally stable after creation of an electron-hole pair. Hydroxy 2D-Si "siloxene" preparations show strong green emission and excitation spectra more structured than those of alkyl 2D-Si fragments." This encouraging result justifies further work on purified, structurally characterized siloxenes. However, at present rigorous evidence in siloxenes for a direct gap, fast radiative recombination rate, or carrier delocalization is not available. 111. Passivated Silicon Nanocrystals and Nanowires
-.
A. Theory of Size-Induced Luminescence. 1 . Nanocrystals. * transition, near the X point at 1.1 eV, is dipole forbidden due to translational symmetry. Each value of K is a different irreducible representation, with K = 0 being totally symmetric. Any physical effect which breaks this symmetry may make the transition formally allowed by mixing different K values. If a finite size nanocrystal, a superposition of K values is necessary to physically localize the wave function inside the nanocrystal, by simple Fourier transform. As shown in Figure 6 , the tail of the electron distribution around the * point will overlap the tail of the holedistribution around r,thuscreating In 3D-Si the indirect gap r
3578 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
x
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*
x
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kX
Figure 6. (upper) Schematic 3D-Si band structure diagram showing
vibronic luminescence mechanism. (lower) Wavevector distributions for confined electron (e) and hole (h) in 2.5-nm Si nanocrystals. qdapated from ref 20. CUBE SIZE L 10-21
10.61
(A)
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0.2 0.5 1.0 BAND GAP SHIFT AE (eV)
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Figure 7. Electronic radiative lifetimes (dots) and vibronic radiative lifetimes (line) as a function of Si nanocrystal size (upper axis). The lower axis shows the band gap blue shift for the indicated size. For a given size, several electronic lifetimes are possible depending upon shape. An effective medium electromagnetic theory has been applied to p-Si; this lengthens the lifetimes by about a factor of 5 with respect to emission in a homogeneous Si medium. Adapted from ref 20.
a weak electronic transition dipole.18 A similar effect happens in bulk 3D-Si at low temperature, when an exciton is bound to a shallow impurity. In the bulk crystal optical absorption and luminescence are vibronically induced by a T O phonon at 458 cm-l, of wave vector matching the difference between r and *. The vibronic intensity which appears in the r * transition at 1.1 eV is borrowed from the two allowed (vertical in K space) transitions near 3.5 eV in Figure 6. Allvibronicinteractions, with theexception of Frohlich coupling, should increase when wave functions become more compact in nanocrystals.19 Thus, the vibronic luminescence rate as well as the electronic luminescence rate should increase with decreasing size in nanocrystals. Figure 7 shows the predicted size dependencies for both mechanisms in passivated, near-cubic Si nanocrystals." For sizes larger than roughly 15 A, thevibronic luminescence dominates. The vibronic emission rate scales roughly as the inverse nanocrystal volume, while the electronic rate has a steeper dependence. These calculations assume perfectly passivated nanocrystals with nochangein latticeconstant from the bulk. The predicted 103-105-s-l radiative rates are slow with respect to the IO9-s-l rate of an allowed molecular transition or a direct gap semiconductor. Except for clusters significantly below 1.5 nm in size, they are actually not significantly faster than the ca. 2 X lo4 s-l vibronic emission rate of the hydrogenic exciton at
-
Brus low temperature. On the other hand, as we have seen, the nanocrystal nonradiative ratecan be extremely slow if the surface can be passivated to eliminate recombination sites. Thus, the luminescence quantum yield might be high. In this regard, nanocrystals have an advantage with respect to bulk crystals: a spatially confined, superimposed electron-hole pair in the nanocrystal cannot dissociate at higher temperature. In a good macroscopic crystal, dissociated free carriers are mobile over a volume equivalent to many nanocrystals. They can nonradiatively recombine at rare defect sites. Also, in the bulk crystal at finite densities they can nonradiativelyrecombine in three-body Auger processes. Thus, at 23 OC there are two effects that might increase luminescence in nanocrystals: ( 1) possible increase in the oscillator strength and (2) structural isolation of superimposed electronhole pairs. Figure 7 also shows the calculated band gap blue shift. This shift is smaller than in equivalently sized CdSe nanocrystals because Si has a larger electron effective mass. In Si the electron and hole have about equal confinement energies, while in CdSe the electron confinement energy is about 4 times the hole energy. 2. Nanowires. Crystalline Si wires of 2-4-nm width combine electrical conductivity along the wire, with the band gap blue shift and possible luminescence enhancement that result from quantum confinement along the diameter. Several recent calculations show that wires along (001) are in fact formally direct gap.Z1 However, as might be expected, the electronic oscillator strength is weak and decreases rapidly with diameter, in a somewhat similar fashion to the nanocrystal electronic oscillator strength. Wires are the structure of lowest dimension in which the distinction between bound excitons and free carriers still exists. In a 3-nm wire that has a band gap of 1.4 eV, the lowest exciton is calculated to have a 76-meV binding energy and 1.7 X 10-4 s electronicradiative lifetime.2'c This electroniclifetime is a factor of 3 or 5 shorter than that of a 3-nm nanocrystal in Figure 7, if both are compared assuming the same optical index. This result appears to reflect increased electron-hole correlation in the nanowire exciton. Calculations of vibronic emission in wires have not been done; it may be that the crossover from vibronic to electronic luminescence occurs at a larger size in nanowires than in nanocrystals. The fact that nanowires have excitons that are bound at 23 OC, and conductivity along the wire, suggests they might be useful structures for electroluminescent diodes, if practical methods of synthesis, diameter control, and doping can be found. In principle, they are far better luminescent structures than 3D-Si. They are better electroluminescent,and poorer photoluminescent,structures than nanocrystals. B. Experimental Luminescence. 1. Surface Oxidized Nanocrystals. We synthesize Si nanocrystals with a 10-1 5-A surface oxide layer by a high-temperature aerosol method, in order to create crystalline, annealed structures with a high-quality Si: Si02 interface.22 These nanocrystals have rather broad size distributions and are collected as an ethylene glycol colloid. A second oxidation step involves treating the nanocrystals with aqueous acidic hydrogen peroxide. Partial size separation is achieved using size exclusion chromatography and reversible size selective precipitation. While it is difficult in 2-3-nm nanocrystals to obtain a precise measure of the crystalline core diameter inside the Si02 layer, we find that nanocrystals of Si diameter near 2 nm emit near 700 nm, in rough agreement with theory. (The size dependence of the conduction band edge energy also roughly agrees with simple quantum theory, as observed in electron energy loss spectroscopy of single nanocrystals.23) Figure 8 shows luminescence from two fractions created by size selectiveprecipitation of a nanocrystal colloid initially emitting from 600 to 900 nm.24 The liquid chromatograms show that the 650-nm fraction contains only smaller, single nanocrystals. The
The Journal of Physical Chemistry. Vol. 98. No. 14. 1994 3579
Feature Article PHOTOLUMINESCENCE
EMISSION WAVELENGTH (om)
TIME (min)
Mgwe8. (Ieft-handside)SpfftrallycorratcdSi nanocrystalluminaccnce spctra in organic glass (350-nm excitation. 20 K). (right-hand side)
Corresponding high-pressure liquid chromatograms with approximate logarithmicsizecalibration. Relative intensitiesare arbitrary. Adapted from ref 24. other fraction contains larger nanocrystals and aggregates and emits a t 800 nm. Absolute quantum yield measurements of the smaller particle emission indicate 5.8% quantum yield a t 293 OC increase to about 50% below 50 K. Detailed analysis of decay dynamicsindicates that someabsorbing particles are‘dark” while others appear to have 100% quantum yield a t low temperature. While the quantum yields are high, the measured radiative rates are low-near IO3 s-1-and temperature dependent. Thus, as discussed in section IIIA, passivated Si nanocrystals do show a band gap shift to red wavelengths, relatively high quantum yields, and low radiative rates. However, we have no direct evidence for the nature of the wave function. Is it influenced-more than just confined-by the Si:Si02 inferface? Isthereelectron-phonon couplingt~thepolarvibrationsofSiO~? Is there a significant static electric field in the first few Si layers due to the po1ar:nonpolar interface? In my opinion we do not have a real understanding yet of the forces which operate on electrons and holes near this interface. The lowest envelope function 1s-1s state is nanocrystal Si has an orbital and spin degeneracy of 48: the highest conduction band hole is a J = )I2state, and the lowest electron state has a 6-fold valleyorbit and 2-fold spindegeneracy. If we add in other nearby envelope function states and consider Coulomb interaction, what does the eigenspeetrum look like? Is the spectrum so dense that structure is absent? Also, can we identify the slow radiationless transition that decreases the luminescencequantum yield a t 300 K? Is electron-phonon coupling really larger in nanocrystals than in bulk Si? At present, rather low resolution we see no structure due to confinement in nanocrystal luminescence excitation spectra. In Figure 4 the 630nm nanocrystal excitation spectrum is indirect gap type with band gap 2.0 eV, corresponding to the emission wavelength. Even in the 3.4-eV region of the Si direct gap, there is no size-related structure. This situation contrasts markedly with the structured absorption spectrum of CdSe nanocrystals.’ This difference is caused by the difference in conduction bands: CdSe is direct gap with an Ss*conduction band. The electron has a very small mass with no spatial degeneracy. The density of quantum confined conduction band states is sparse in CdSe, and structure is observed in absorption. 2. Porous Si Thin Films. Porous Si ( p s i ) is a family of materials made of surface passivated, connected nanometer Si crystallites and wires.25 p s i films on Si crystals are formed by anodic electrochemical etching in H F solution. It appears now that both the initial synthesis and the final morphology and electronic structure are dominated by electronic quantum size
Fi9. Schematic diagram of psi elarochemical cell. prow morphology. and ~ l ~ ~ t r ojunction n i c slructure. Adapted from ref 26. effects. There is a close correspondence between psi luminacence and the passivated Si nanocrystal IumincSccnce described above. Figure 9 shows schematically psi structure and electronic structure.26 Underanodicpolarization,andat low current density. holes arriving at the Si:HF solution interface etch the Si lattice by forming SiF. molecules. In the process a passivated, H atom terminated SI surface is left. Etching creates a rough surface. When the size of small roughness features reaches a few nanometers. the quantum confinement energy for a hole inside such a feature becomes substantial. It is then endoenergetic for a hole, initially at the bulk Si band edge energy, to enter the nanometer feature. Instead. the hole etches Si at the bottom of a pore. thus lengthening the feature into an irregular nanometer wire. The film thicknesscan be microns, withasponge-or treelike structure of Si wires with varying diameters. This proposed mechanism for psi formation is a naturally self-limitingprocssfornanometerstructureformation.”Further structural control can be achieved via wavelength in light-assisted electrochemical etching. The bluer the light. the smaller the feature in the psi film that can absorb light and etch due to the photogenerated hole. psi films show an indirect optical band gap larger than that of bulk 3D-SI. in an analogous fashion to the nanocrystals described above. I n the absence of HF. the H atom terminated surfaces are passivated against nonradiative electron-hole recombination. psi showsvisible,roomtemperatureluminescence whose spectra, quantum yield, and decay dynamics are similar to that of nanocry~tals.~’A brief, high-temperature oxidation replaces surface H with SiOl: this form of psi also shows strong luminescence.” Upon resonant 60&750-nm laser excitation below 50 K, p s i shows TO vibronic luminescence thresholds; this result, as well as the indirect gap type excitation spectrum. indicates that vibronic luminescence dominates purely electronic l~minescence?~An unidentified blue emitting species in psi. probably a passivated Si cluster of lens of atoms, may show stronger electronic luminescence. An optical material. be it psi, a siloxene. or something else, grown on wafer 3D-Si would have to show electroluminsccnce (not photoluminesccnce) to the useful. In essence, the emitting material would form a diode in which electrons and holes could be injected from opposite sides. In psi one electrical contact is naturally formed by theexpitaxial interfacewithunderlying wafer Si. Making efficient electrical contact to the other side proves to be a difficult problem. Liquid contacts. in which an aqueous electrolyte containing persulfate redoxcarrier penetrates the pore structure, have shown efficiencies of a few percent.I0 Most remarkably, the color of the emitted light can be tuned via the applied voltage.)’ This tunability appears to be related to the
Brus
3580 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 wire and nanocrystal size distribution. In p-Si, as in the colloidal Si nanocrystals, the photoluminescence is inhomogeneous: smaller crystallites emit to higher energy than larger crystallites. In electroluminescence it appears that the Fermi level can reversibly sweep across the crystallite and wire distribution, yielding sizeselective emission. The emitting nanocrystals are H atom passivated to prevent nonradiative recombination, yet they remain in electrical equilibrium with the underlying Si wafer. The fundamental transport and charge injection mechanisms in this device are not quantitatively understood. A practical device would require a conducting solid replacement for the sacrificial persulfate electrolyte and long-term stability. Very recently, pyrrole has been electropolymerized inside the pore structure to give efficient electroluminescence without a liquid junction.32 Such a system might make a useful display. However, the purely radiative lifetimes appear to be too long for possible use in optical communication at gigahertz rates. This area contains novel physics and deserves further basic and device research.
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IV. Physical Effects in 3D-Si: Strain and Microcavities A. Thin Strained 3d-Si Layers. It is possible to grow thin Si expitaxial layers on a substrate that has a larger lattice constant. These layers exist under lateral dilation; the diamond lattice unit cell undergoes a tetragonal distortion. This strain markedly increases the in-plane mobility of Si ele~trons.3~ However, the increase in the electronic interband transition dipole is quite weak in such strained layers, as well as in similar physical systems such as short period, strained layer Si:Ge super lattice^.^^ In short period superlattices, the perturbing, potential can be designed to specifically, yet weakly, mix the r and * points-such “zone folding” bears a partial analogy to confinement mixing in nanocrystals and n a n ~ w i r e s . ~ ~ B. Microcrystalsas Microcavities. Small 2-nm Si nanocrystals emitting near 700 nm interact with the electromagnetic field as molecules. However, as a nanocrystal grows into a microcrystal of bulk Si, it becomes a material object of high index of refraction and extremely high ultraviolet absorption cross section. For isolated crystallites, this transformation is described by the electromagnetic Mie equations. In general, microcrystal optical properties show intense resonances when size matches one-half an internal optical wavelength. In the ultraviolet where the optical index is very large, this occurs at microcrystal diameters of 6090 nm. In essence, the microcrystal becomes a microcavity with its own optical modes. Figure 10 shows calculated Mie optical extinction (Le., the sum of absorption and Raleigh scattering) cross sections for bulk 3D-Si spheres as a function of diameter. Below 25-nm diameter, the spectral shape is independent of size, and the magnitude varies as the microcrystal volume. As diameter increases beyond 25 nm, the electric and magnetic dipole resonances appear and shift to longer wavelength; at these sizes quantum confinement effects are absent. For smaller sizes the resonances are principally in the absorption cross section, while for larger sizes the resonances are principally in the scattering cross section. A scattering resonance corresponds to a cavity mode in which the internal electromagnetic field, and associated optical processes such as spontaneous Raman scattering, are significantly enhan~ed.3~ These effects are enhanced because silicon is an indirect gap material. In Figure 10,the resonances become pronounced when they shift to lower energy than the 3.4-eV direct gap. In this region, internal absorption is weak because of the indirect type transition, and the cavity Q can be high. In comparison, germanium absorbs strongly in this region, and cavity effects are much less pronounced. These ideas suggest another approach to making a luminescent material: microcavity construction from 3D-Si into which a luminescent impurity ion has been implanted. The microcavity
200
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Wavelength (nm) Figure 10. Logarithm of 3D-Si Mie optical extinction cross section versus size for a single isolated sphere, calculated using dielectric constant data from: Aspnes, D. E.; Studna, A. A. Phys. Rev.B 1983,27,985. The data have been interpolated between experimental points.
size can in principle be matched to the desired emission wavelength. (“Thumbtack” micron-sized disc GaAs quantum well lasers utilizing surface optical modes already exist.37) Long-distance communication systems operate at 1.3- and 1.5-pm wavelengths, below the band gap of 3D-Si. Thus, one might implant some species that would catalyze radiative recombination at these specific wavelengths. Initial studies on implanted Er3+ ion for 1-5-pmemission have shown the importanceof the local chemical bonding with oxygen in controlling the photophysics; however, the efficiencies of these materials are quite low.38 This is an area for further solid-state chemistry research; major advances in materials and understanding are required. V. Conclusion and Outlook
A. Science. Experimentally, both “red” emitting Si nanocrystals and p-Si are indirect gap materials, of vibronic radiative rates not markedly higher than that of the free hydrogenic exciton in 3D-Si. Quantum confinement shifts the luminescence to higher energy than the bulk 3D-Si 1.1-eV band gap. The room temperature luminescence quantum yield is high, however, because of physical isolation of superimposed electron-hole pairs at 23 OC in nanostructures and because of remarkably efficient suppression of nonradiative recombination. In silicon the nanometer quantum size effect is primarily kinetic, while in the previously studied 11-VI and 111-V materials the effect is primarily spectroscopic. In CdSe, for example, the primary observation is appearance of discrete transitions in the optical spectra. In silicon, while the band gap does shift blue with decreasing size, the primary effect is isolation of electronhole pairs from each other. Isolation supresses Auger recombination at moderate excitation densities and confines the effect of a rare defect to nonradiative quenching in just the one nanocrystal containing the defect. As a result, luminescence increases. The breaking of the diamond lattice symmetry by physical effects-strain and the nanosize effect-creates only a minor perturbation in the radiative rate because the Sa* direct gap at 4.2 eV, which carries a large oscillator strength, is far away in energy and wave vector from the indirect gap at 1.1 eV. The perturbation-induced mixing between these transitions is weak.
Feature Article In indirect gap Ge, which has direct and indirect gaps at almost the same energy (similar to 2D-Si), larger mixing can occur. Ge is predicted to become direct gap for moderate tensile lateral ~train.9~ B. Technology. I have discussed a range of subjects, all in the context of introducingluminescence and optical gain functionality into silicon integrated circuits. Three areas show promise that suggests further research and invention: (i) Porous Si films may make useful displays if a conductive solid contact material inside the pore structure can be found and if long-term stability can be demonstrated. This naturally nanostructured material contains novel physics and electrochemistry. The idea of voltage tuned color is especially integruing and may lead to unique display functionality. However, the oscillator strengths are low, and it does not appear that p-Si will make a good gigahertz optoelectronic device. If independent methods for making and connecting Si nanowires could be found, many interesting basic and device experiments would be possible. (ii) A new direct gap, dopable, semiconducting material-perhaps hydroxysiloxene itself-may be found somewhere in the SiO,X, family. The band structure calculations show that 2D-Si systems have optical properties that can be fundamentally changed by chemicalderivatization. With such material, simple methods of growing gigahertz light emitting diodes for communication, directly on a silicon computer chip, might then be envisioned. (iii) The science and technology of electrically active dopants in silicon is thoroughly explored; this work makes VLSI circuits possible. Perhaps it is time to explore the solid-state chemical physics of electroluminescent "dopants", such as the erbiumoxygen complex. Whatever method ultimately works, science will benefit from the process of setting and achieving such a difficult technological goal, because of the basic scientific discoveries that have and will occur when electrochemists, laser spectroscopists,and materials chemists are motivated to work in entirely new areas.
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