Light-emitting porous silicon

ELSEVIER

Materials

Chemistry

and Physics

40 (1995) 253-259

Review

Light-emitting

porous

silicon *

U. Giisele a, V. Lehmann

b

Institute ofMicrostmcture Physics, Weinberg 2, D-06120 Ha&, Germany b Siemens AG, ZFE BT ACM 42, Otto-Hahn-Ring 6, D-81739 Munich, Germany

“MaxPlanck Received

5 January

1995; revised

16 January

1995; accepted

23 January

1995

Abstract

Although porous silicon has been known for more an increased bandgap and efficient room-temperature

than 35 years, only in 1990 was it recognized that porous silicon shows photoluminescence in the visible. This paper will give an overview of

porous silicon research, with special emphasis on the formation mechanism of microporous silicon in terms of a depletion of holes in the porous region due to quantum confinement and the understanding of the origin of the visible luminescence. The status of research on electroluminescent and other devices based on porous silicon will be discussed, as well as results for other

luminescent

Keywords:

Porous

forms silicon;

of nanocrystalline

silicon.

Photoluminescence

1. Introduction Silicon is a semiconductor with an indirect bandgap. Radiative band-to-band recombination of an electron-hole pair involves a phonon and is therefore a relatively slow process. Radiative recombination has to compete with generally much more efficient nonradiative recombination processes via deep level states associated with impurity atoms, intrinsic defects or surface states. As a consequence, crystalline bulk silicon is not an appropriate material for efficient light emitting devices based on electron-hole recombination. Since silicon is the base material for the overwhelming number of microelectronic devices, it would nevertheless be highly desirable to have light-emitting silicon available for either optical interconnects or for integrated displays. The observation by Canham [l] in 1990 that crystalline silicon, appropriately anodically etched in hydrofluoric acid, leads to a porous silicon structure (‘porous silicon’) that shows efficient photoluminescence in the visible at room temperature generated an enormous research effort in this and related materials. The research was sparked not only by the potential for technological applications but also by the claim that a quantum * ICEM invited paper. 0254-0584/95/$09.50 0 1995 Elsevier SSDI 0254-0584(95)01493-E

Science

S.A. AII rights

reserved

confinement effect was responsible for the fabrication of porous silicon [2] as well as for the photoluminescence [l] corresponding to an increased bandgap of 1.5 eV and more as compared with the bandgap of bulk crystalline silicon of about 1.1 eV. In the meantime, close to 1500 papers have been published on porous silicon and related nanostructures, so that it is impossible to deal with all available research results in this paper. For more information the reader is referred to the many conference proceedings [3-71, review papers [8-181 or a recent book 1191 on this subject. The present paper, which is partly based on a book chapter by the present authors [20], will first shortly mention the history of porous silicon, then deal with the formation mechanism, before turning to the mechanism of photoluminescence, other forms of nanocrystalline silicon and of porous non-silicon materials, porous silicon superlattices and finally to the present state of research on applications of porous silicon.

2. History

of porous

silicon

Porous silicon, which forms on the surface of caystalline silicon substrates in hydrofluoric acid (HF) under an appropriate anodic bias, was first observed by Uhlir

254

U. Gtisele,

V. Lehmann

/ Materials

[21] in 1956 and later investigated in detail by Turner [22]. As the name porous silicon indicates, the bulk crystalline silicon changes under the etching process into a kind of sponge structure with interconnected and hydrogen-covered silicon columns and pores, as indicated schematically in Fig. 1. The width of the pores and of the remaining silicon skeleton strongly depend on the specific doping and etching conditions, as well as on the illumination conditions during etching. Originahy, the interest in porous silicon concentrated on its fast oxidation rate and its application in a specific silicon-on-insulator technology, the so-called FIPOS technology [23,24]. Already in 1984, Pickering et al. [25] reported visible photoluminescence at 4 K, as shown in Fig. 2, but attributed the unusual optical

Chemistry and Physics 40 (1995) 253-259

properties to suboxides, partially amorphized silicon or impurities. In 1990, Canham [l] found strong visible roomtemperature photoluminescence in porous silicon which is much more efficient than the usual bandgap-related infrared luminescence of crystalline bulk silicon, as shown in Fig. 3. The present authors [2] independently observed an increase in the absorption edge of about 0.5 eV in porous silicon as compared with bulk crystalline silicon. Both groups attributed the apparent increase in the bandgap of porous silicon to quantum confinement effects due to the remaining thin crystalline silicon columns (‘quantum wires’) which are interconnected to form some kind of ‘quantum sponge’ [26]. Alternative explanations were suggested for the observed photoluminescence, ranging from contaminated amorphous silicon to radiative hydrogen-related polysilane surface centers and, most intriguing, the molecular compound siloxene (Si,O,H,) [27]. As discussed later in the paper, in the meantime it has been shown that nanocrystalline silicon fabricated in various ways different from anodic etching also showed efficient photoluminescence in the visible.

3. Formation

2

Fig. 1. Schematic illustration of the structure of (1) the porous silicon layer on (2) crystalline bulk silicon.

1.8

I

1.4

Energy [ev] 1.0

The etching reaction that occurs during porous silicon formation by anodic etching in HF requires electronic holes which have to be supplied by the silicon [28,29]. The basic conditions for electrochemical pore formation in a homogeneous electrode are a passive state of the pore walls and an active state, which promotes dissolution, at the pore tips. A surface area that is depleted of holes will be passivated. Hole depletion will occur only if any hole that reaches the interface is immediately I

I

I

0.8

. II

I

mechanism

300

I

K

106 r 7 ? _: .z 2 Y c

1w-

102 2 1 0.6

1.4

1 .o Wavelength

1 .a

[pm] -

Fig. 2. Photoluminescence spectra of porous silicon at 4.2 K formed from p-type silicon with different resistivities leading to different porosities [25].

0.5

I

I

l1q_=2.6 eV 500 mW/cmz I

1.0

1.5

2.0

c-Si b

-

3.5

Photon Energy [eV] --t Fig. 3. Comparison of room-temperature photoluminescence spectra of (a) crystalline bulk silicon and (b) porous silicon [27]. Please note the logarithmic scale.

U. Giisele, E Lehmann

/ Materials

Chemistry

consumed in the dissolution reaction. This requires that the chemical reaction is not limited by mass transfer in the electrolyte. This condition is fulfilled if the current density is below a critical value Jps. Above Jps the reaction is limited by ionic mass transfer, which will lead to electropolishing. For current densities below Jps holes may be depleted at the surface either by the formation of a space charge region or by an increase in the bandgap due to quantum confinement. The first possibility leads to macro- and mesopores and will not be considered here [30]. The second possibility leads to the microporous light-emitting silicon and will therefore be described in more detail in the following sections. Both mechanisms are independent of each other and may coexist, resulting in a superposition of microporous and meso- or macroporous silicon structures. The proposed formation mechanism of microporous silicon by a quantum confinement effect [2] is shown schematically in Fig. 4. It is based on the fact that holes are necessary for the electrochemical dissolution process of Si. Since the bandgap in porous silicon is increased compared with bulk Si due to quantum confinement [1,2], holes need the additional energy A.!?, to penetrate into the porous layer. If AE, is larger than the bias-dependent energy of the holes, the porous layer becomes depleted of holes and therefore passivated against further dissolution. It is then energetically more favorable for a hole to enter the electrolyte directly (solid arrow) than via the porous structure (dashed arrow). Since AE, is a function of the size of the nanoparticles that form the porous skeleton, one can conclude that an increase in the formation bias (and therefore the formation current density) will result in a decrease in the particle size in the porous layer and an increased bandgap. An increase in the bandgap in

and Physics 40 (1995) 253-259

microporous silicon will produce a measurable blueshift of the optical absorption edge. A blueshift of the photoluminescence (PL) peak position due to an increased bandgap is also expected for an increased current density (provided the surface passivation layer remains the same) and has been found repeatedly, as shown, for example, in Fig. 5 [31]. The increase in the bandgap of porous silicon has been measured by various methods. Most notable are the results of Van Buuren et al. [32], who managed to measure the offset of the valence band A& and the conduction band AE, separately (Fig. 6). It turns out + 2.48

2.07

500

600

2

Si

a-d---- --+

V

C-

Enorgy [w] 1.77

F s

700

-

spectra of porous silicon for two different anodic etching (311.

0.2

-

J

0

tI

I ll’

1 I

1

w’ a

900

800 [nm] +

I 0.4

1.3E

1.55

Wavelength

Fig. 5. Photoluminescence current densities during

PS

255

-fp+=+-

I

I

--II

0

I

0.2

1

0.4

I

I

0.6

0.8

I

1

I

I

1.2

I .4

A Ev [eV] -+ Fig. 4. Schematic illustration of porous silicon fabrication and the corresponding band diagram for the siliconxlectrolyte transition at the pore tip and between bulk and porous silicon.

Fig. 6. Energy of conduction band edge vs. the energy of the valence band edge for a series of differently etched silicon samples, relative to bulk Si, indicated by the open triangle [32].

LJ. Giisele, K Lehmann

256

++

a

1 Materials

I

Fig. 7. Photons of an energy larger than EPs will be absorbed in the topmost region of a porous silicon layer and will generate an electron-hole pair. The electron will move to the bulk if an anodic bias is applied, whereas the hole will enter the electrolyte and promote further thinning of the porous skeleton. The figure shows this process in the porous skeleton (upper part) and in the band diagram (lower part) [ZO].

that AE, is approximately 2AE,. If additional holes are generated in the porous layer (e.g., by illumination) while the anodic bias is applied, they will promote further thinning of the porous skeleton, as visualized in Fig. 7: An electron-hole pair is generated by a photon in the microporous silicon top region; while the electron moves to the bulk owing to the applied bias, the hole enters the electrolyte and promotes further dissolution. This additional thinning due to illumination produces a significant blueshift of the optical properties in the light-absorbing region. The effect described can be expected only for photon energies that are in excess of the gap energy in the microporous silicon layer as actually observed experimentally. An example is shown in Fig. 8 [20]. Inhomogeneous illumination will promote etching proportional to the light intensity for n-type Si and etching inversely proportional to the light intensity, due to cathodic protection, on p-type samples. The dependence of the size of structural elements of porous silicon on the current density has recently been used to produce superlattices of porous silicon layers of different structural sizes (and therefore different bandgaps and different optical properties) by simply varying the current density periodically with time [33]. An example of such a superlattice is shown in Fig. 9.

4. Photoluminescence The most controversial area in porous silicon research is that concerning the physical origin of the observed efficient room-temperature photoluminescence in the

Chemistry and Physics 40 (1995) 253-259

Photon+neqy

(eV)

Fig. 8. Photoluminescence (PL) spectra of microporous silicon samples (p-Si, 1 Cl cm, (loo), 50 mA cm-‘) anodized in the dark and under illumination using the setup sketched in the upper right-hand corner of the figure. A blueshift of the PL peak position and an increase in PL intensity was found for short wavelergth illumination compared with the samples prepared in the dark or under long-wavelength illumination [20].

Fig. 9. TEM cross section of a porous silicon superlattice fabricated by periodically changing the current density for anodic etching between 31 and 69 mA cm-‘, which results in porosities of 65 and 75%, respectively [33].

visible range. In the meantime, almost all conceivable experimental techniques have been applied to characterize the structural, compositional, optical and electrical properties of luminescent microporous silicon; for recent overviews see Refs. [3-191. It now appears well established that microporous silicon consists essentially of an interconnected, crystalline, highly elastically distorted silicon skeleton that is almost isotropic and not mainly columnar, as originally supposed and shown in Fig. 1. Two main questions have to be answered: (1) Why is the energy of the peak of the photoluminescence higher than the bandgap energy of 1.1 eV of crystalline bulk silicon? (2) Why is the efficiency of the room-temperature photoluminescence much higher than that of typical infrared bandgap photoluminescence (Fig. 3)? The first question can naturally be answered in terms of quantum confinement effects, which also appear to

U. Giisele, I/ Lehmann

I Materials Chemistry

be responsible for the formation of microporous silicon and lead to an increased bandgap of the remaining silicon skeleton. The effect of quantum confinement on the bandgap of small silicon crystallites has been dealt with in a series of theoretical papers (for references see Ref. [34]) and appears to be of the right order for the observed crystallite size in the 2-3 nm range. In addition, the surface coverage has a clear-cut influence on the peak position of the photoluminescence [35]. The essentials of the requirements for a model based on quantum confinement effects have been presented by the group of Koch 1361. Within this model, light is absorbed in the silicon quantum sponge (or the tiny silicon crystallites that form this structure, especially after oxidation processes which may influence the interconnectivity) with a correspondingly high bandgap. The resulting electron-hole pairs may either radiatively recombine within these crystallites with an energy corresponding to that of the bandgap (e.g., in the green-blue range), be trapped at surface states (which depend on the surrounding medium and the passivation by hydrogen or SiO,) and radiatively recombine there with a lower energy (e.g., in the red-orange range), or be trapped at actual defects in the crystallites and then recombine radiatively with an even smaller energy in the infrared range. In addition, most of the electron-hole pairs still recombine nonradiatively at defects probably located at the surface of the silicon crystallites. The only presently remaining serious alternative answer to question (1) is the suggestion that the photoluminescence comes from a molecular compound related to siloxene that covers the surface of the porous silicon crystallites and has photoluminescence properties which are very close to those of porous silicon [27]. Near-edge and extended X-ray absorption fme structure measurements indicate that siloxene is not a main constituent of the luminescence of porous silicon [37]. On the contrary, the remarkable similarity in the photoluminescence of annealed siloxene and porous silicon might rather indicate that during annealing of siloxene small silicon crystallites could develop which in turn might then be responsible for the luminescence behavior of annealed siloxene. The second question, concerning the high efficiency of the observed photoluminescence in porous silicon, has mainly been discussed in terms of a quantum confinement induced changeover of the silicon band structure from an indirect bandgap to a pseudo-direct bandgap. Although such an effect most likely exists, the main effect appears to be a combination of reduced diffusion possibilities of the charge carriers in the small silicon crystallites combined with an effective surface passivation by hydrogen or oxide, which together lead to a drastic reduction of nonradiative recombination processes. Consequently, under steady-state illumination conditions an efficient photoluminescence, even

257

and Physics 40 (1995) 253-259

for an indirect transition involving phonons [38], can occur. A number of groups have managed to grow small crystalline silicon particles covered by hydrogen or oxide by methods different from anodic etching. These silicon crystallites also show efficient photoluminescence in the orange-red range at room temperature. Especially noteworthy are the results of the group of Brus [39], in which a comparatively wide distribution of crystallite sizes is separated into two narrower ones of larger and smaller crystallites. As expected within the quantum confinement model and shown in Fig. 10, the peak luminescence of the smaller crystallites is located at a smaller wavelength, corresponding to an increased bandgap. Comparison of the luminescence of porous silicon with that of well-characterized silicon nanocrystallites indicates that the structural dimensions of the luminescent silicon should be on the order of 10 8, [40]. Besides isolated particles and porous silicon, other forms of luminescent silicon have been prepared by various kinds of chemicalvapor deposition, laser ablation or spark erosion [41]. Most intriguing is also the recent report that silicon pillars and walls fabricated by lithography and etching show photoluminescence in the red [42]. Spark erosion has also been used to produce photoluminescent Ge and GaAs particles (Fig. 11). Anodically etched Sic shows a porous structure and modified luminescence properties [43,44]. Normal porous silicon luminesces in the orange-red, with lifetimes in the microsecond range, and therefore cannot easily be used for optical interconnects which require much smaller lifetimes [19]. In contrast, porous silicon after boiling or rapid thermal oxidation shows photoluminescence in the blue and lifetimes in the nanosecond range. The origin of this luminescence is presently highly controversial and is probably associated with defects in the silicon dioxide or at the oxide-silicon interface [45].

500

700

Wavelength

900 [nm]

+

Fig. 10. Photoluminescence of silicon nanocrystallites at 20 K. Curve 1 corresponds to the initial broad distribution, curve 2 to the smaller particles, and curve 3 to the larger particles [39].

U. Giisele,

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/ \

-2 4 0.6 . .E ,r 0.4 z ;

0.2 -

425

500

575

650

Wavelength [nm] + Fig. 11. Room-temperature photoluminescence (PL) spectra of sparkprocessed Ge, GaAs and Si, normalized with respect to the Ge signal [41].

5. Applications The driving force for investigating light-emitting porous silicon is the hope of being able to fabricate efficient light-emitting diodes for either silicon-based optical interconnects or integrated displays. Porous silicon contacted by an electrolyte under anodic [13-151 or cathodic [9,10] bias exhibits efficient electroluminescence. Unfortunately, the efficiency of light-emitting diodes with solid state contacts [46-48] is orders of magnitude lower (less than O.Ol%), which presently prevents their use in actual applications. On the positive side, besides the usual red-orange luminescence, green and blue electroluminescence have also been demonstrated [49]. In addition, a possible increase in the efficiency to values close to 0.1% has been observed [50] but not yet been verified independently. Other possible applications of porous silicon are photodetectors and optical logic gates [51].

6. Outlook In conclusion, the formation mechanism as well as the properties of luminescent porous silicon may best be described within the concept of a surface passivated quantum sponge formed by a self-limited etching process. This self-limited process allows the microstructure to be influenced in a controlled manner. The efficiency of electroluminescent devices with solid state contacts based on porous silicon has increased significantly over the last few years, but is presently still too low for practical applications.

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