Light emitting silicon, recent progress

] O U R N A L OF

NON-CRYS ILLmESOL ELSEVIER

Journal of Non-Crystalline Solids 198-200 (1996) 857-862

Section 16. Microcrystalline and porous silicon

Light emitting silicon, recent progress J. Ko~ka *, I. Pelant, A. Fejfar Institute of Physics, Czech Academy of Sciences, Cukrol~arnick6 10, 162 O0 Prague 6, Czech Republic

Abstract Recent results concerning selected optical and mainly transport properties of light-emitting silicon are critically reviewed. A short discussion is devoted to the question of the nature of the optical gap in porous silicon (direct/indirect). Special attention is given to the transport mechanism in strongly luminescent nanoporous silicon. It is concluded that the existence of tail states is crucial for the transport and a three-layer model of silicon nanoparticles is presented. Prospects tbr electroluminescent devices based on light-emitting silicon are evaluated from the point of view of their efficiency and stability.

1. Introduction The discovery [1] of strong room temperature photoluminescence (PL) of porous silicon (PS), prepared by electrochemical anodization of crystalline silicon (c-Si), opened a new field of research. Recently many new ways of preparation of a wide class of silicon based materials, commonly called 'light emitting silicon' (LES) have been presented (for a review see Ref. [2]). The microscopic model for the strong PL of LES is still a matter of controversy. In addition to the originally suggested quantum confinement model [1] many other models (siloxene, surface states . . . . ), have been proposed [2-4]. Progress in understanding the microscopic origin of PL is independently reviewed in Ref. [5]. Although 'standard' PS has found many other applications [6] one of the important targets is the preparation of LES based electroluminescent (EL)

* Corresponding author. Tel.: +42-2 2431 1137; fax: +42-2 312 3184; e-mail: [email protected].

devices [2,7]. To increase the rather low LES-EL efficiency, the detailed understanding of transport properties is the crucial step. That is why after brief comments on the selected LES optical properties (Section 2) the main part of this review (Section 3) is devoted to transport properties of LES and comments on the prospects of LES-EL devices (Section

4). 2. Selected optical properties The low efficiency of light emission of c-Si is due to its indirect-gap. Is the strong PL of PS due to its direct-like gap or is PS gap still indirect as in c-Si? Fig. 1 shows the dynamics of differential absorbance in a pump and probe experiment [8] which has been well fitted only by a simple model, based on bimolecular recombination of the photocarriers. The parameters obtained by the fit are in the inset of Fig. 1. The value of the bimolecular recombination coefficient, B = 4 × 10 - l ° c m 3 S - l , corresponds very well to the radiative recombination constant for

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 0 7 0 - 1

J. Kog'ka et al. / Journal ()f Non-Co,smlline Solids 198-200 (1996) 857-862

858

direct-gap semiconductors (B = 2 × 10 to c m 3 s- 1 for GaAs while B = 1.8 × 10 -~5 cm 3 s -1 for indirect-gap c-Si)! The value of the capture cross-section ( ~ c c ) is similar for all - PS, c-Si and a-Si:H [8]. Further support for the direct-gap nature of PS comes from the initial blue shift of the PS red PL band with hydrostatic pressure increasing up to 22 kbar [9,10] because the direct-gap in diamond or zincblende semiconductors increases with pressure, while the indirect F - X gap energy of c-Si decreases. On the other hand the indirect-gap nature of PS could be supported by the observed fine phonon structure in low temperature PL [11]. Also the lack of bleaching in ultrafast pump-and-probe experiments (no saturation of the absorption transitions across the gap) [12] is used as a proof of an indirect gap of PS. We believe that the solution of the above (direct-indirect?) controversy is that it is a wrongly posed question. If the long-range order is lost for whatever reason (disorder, geom. confinement) the k-vector and the concept of a direct-indirect gap no longer have any precise meaning. The unifying word is localisation as supported by the similarity between the properties of anodically etched a-Si:H and PS [13]. For acceptance or rejection of a quantum confinement model of PL it is important to evaluate the size of the particles. This is often done with the help of Raman spectroscopy (RS). How dangerous it can be is illustrated in Fig. 2. The loss of the long-range order leads also to the partial relaxation of the selection rules in RS. In addition to symmetry-allowed LO phonons in c-Si, in PS also TO phonons

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can contribute. Then instead of fitting the PS-RS in Fig. 2 by distribution of particle sizes (and LO mode) the measured RS of PS can be perfectly fitted (see Fig. 2) with one particle size and a mixture of LO and TO modes [14]. This sitting seriously questions the size distributions of PS, evaluated from Raman spectroscopy.

3. Transport properties First of all it is important to note that not only LES but even 'standard' PS represent a wide class of materials, with wide range of microstructures, which is a function of preparation conditions, substrate conductivity and doping and, moreover, can even change along the PS thickness. LES transport properties are even more complicated due to typical longtime constants, ageing and contact effects [15]. The first question in which we were interested was how the microstructure of PS influences the movement of non-equilibrium carriers. We have used the time of flight (TOF) technique [16], estimated that the drift mobility (/x D) in p-type nanoporous PS is extremely low (/x D = 10 -~' c m 2 / V s [17]), and evaluated the so-called mobility-lifetime product,

J. Ko(ka et al. / Journal of Non-Cr~'stalline Solids 198-200 (1996) 857-862

~D'rO ~ 1 0 - 9 c m 2 / V . The physical meaning of this product is evident from the definition of the drift length L = ~t)~'D F (where F is electric field), which defines the distance which non-equilibrium carriers move before their loss (by trapping, recombination, ...). Even for the highest fields ( F = 105 V / c m ) , L ~ 0.1 to 1 Ixm. This length means that the transport 'through' many Si particles (of size 2 - 4 rim) is possible, but for efficient EL, L is rather low [15]. In a PL experiment there is no applied field, internal fields are probably low ( F < 103 V / c m ) and L = 1 to 10 nm. The carriers can just reach the surface of the same particle in which they were generated and that is why PL is so strong and EL so weak. Recent findings [18,19] of similar values of the ( / ~ - ) product in the range 10 -s t o 10 - 9 c m Z / V on mesoporous PS (in which quantum confinement is negligible) clearly indicate the important role of the surface on transport in both meso and nano-porous PS. With few exceptions [2,18,19] (for a review see Ref. [20]) the transport properties of n + or p+ PS, which often form a part of EL devices, are usually evaluated from the function of these devices. In many aspects (namely optical properties [18]) these materials are qualitatively similar to c-Si. The transport properties of strongly PL nanoporous p-type PS represent a rather difficult subject, studied only by a few groups and that is why we focus on it in the remaining part of this section. The basic question, important for construction of

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Fig. 4. Three-layer model of nanoporous silicon. Possible transport paths are indicated.

band diagrams of possible EL devices, is what is the transport mechanism (movement above the band edge, hopping .... ) and where (at which energy) is the transport path? This question is related to the question of why the typical activation energy of el. current (conductivity) E a ~- 0.5 eV is much smaller than the optical gap of PS, typically E~ ~ 1.8 eV? One attempt to solve this problem is (with great simplification) based on the idea [21] that the 'electrical (transport) gap' is actually given as a product of E A ( ~ 0.5 eV) and diode ideality factor (m ~ 4) and so practically equal to Eg. However, it has been convincingly illustrated [22] that the forward current of diode-like I - V characteristics of a metal/p-type P S / c - S i heterostructure is controlled by the P o o l e Frenkel like [23] voltage dependence of PS conductivity and often evaluated ideality factors (very high m ~ 100 for very thin PS [22]) are misleading. Recently the ac conductivity has been used [24,25] to expand our knowledge about PS transport. We agree with Ref. [25] that there is hopping at the Fermi level prevailing at low T a n d / o r high frequencies. However, the claims that this is the only mechanism in PS (based on the un-substantiated scaling arguments) and that E A has no relation to carrier thermal generation [25] are difficult to accept. In an effort to discover the true transport properties of PS we have concentrated on the self-supporting PS, measured in a special sample holder [17,26]. The need to use self-supporting PS samples is evident especially for photocurrent [27], for which the dominant depletion layer inside the c-Si [22] and the photocarriers generated in the c-Si substrate represent serious complications (compare Fig. 4 of Ref. [22] and Fig. 1 of Ref. [27]).

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J. Ko6ka et al. / Journal of Non-Crystalline Solids 198-200 (1996) 857-862

Fig. 3 shows the current-voltage (I-V) characteristics measured on self-supporting p-type PS [15] at different temperatures (T). In agreement with [23] we observe superlinear behaviour. However, the EA(V) first increases with V and only for high V starts to decrease, as expected for a Poole-Frenkel model [23]. To explain the initial EA(V) increase we have to modify the 'pure' Poole-Frenkel model or to use an alternative (we believe more probable) explanation, based on the model of space-charge-limited currents (SCLC). The SCLC model explains the similar EA(V) increase in other materials [28] by insufficiently injecting contacts. In PS, instead of 'bad' contacts it means 'a lot of different barriers' (interfaces, grain boundaries, etc.). The large internal surface of PS is responsible for the observed sensitivity of electrical conductivity to vapours of different chemicals (methanol [29], water [30]). Whether this 'surface channel' is due to direct conductivity via the PS internal surface states [31] or due to field-effect action of charge of trapped molecules (see Fig. 4), is not clear at present. In this context interesting results are shown on another set of 3 self-supporting p-type PS samples [26] in Fig. 5. Sample No. 1 has a very different activation energy ( E A = 0.38 eV instead of E a = 0.67 eV for samples No. 2 and 3) and at the same time an opposite ratio of Sill (Sill 2) related peaks in the IR spectra (see inset of Fig. 5). This indicates a relation between E A

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and the chemical composition of the internal PS surface. There is another message in Fig. 5. Extrapolation to ( l / T ) = 0 gives us the so called conductivity prefactors (o-0), the values of which hold information about the transport mechanism. The observed values o-0 = 10 -6 t o 1 0 - 3 ~-~ 1 cm i clearly indicate some kind of hopping and exclude transport above the mobility edge (for which o-0 = 103 ~ ] cm-= should be observed). The observed E A, 0.38 to 0.67 eV, again raises the question, where is the hopping transport path? Although the optical properties of PS are still not fully understood [5], their comparison with c-Si and a-Si:H, namely the observed wide 'Urbach edge' of PS (see Fig. 6(a) [4] and Ref. [32]) indicate in

J. Ko(ka et al. / Journal of Non-Crystalline Solids 198-200 (1996) 857-862

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amorphous semiconductor terminology a wide region of 'tail states'. Their existence is supported by featureless TOF transients and very low values of drift mobility [15,17]. Then a quite natural transport path is (see Fig. 6(b)) at the bottom of the conduction band tail or top of the valence band tail and this explains why E A = 0.5 eV << Eg. The inevitable existence of tail states (distorted Si-Si bonds) even within a single Si nanoparticle leads us to the 3-layer model of such a nanoparticle - c-Si core, Si - tail states and chemical compounds on its outer surface (see Fig. 4).

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4. Prospects for electroluminescence Because of the limited space we present just a few comments on EL. What are the targets and expected applications? First of all, light emitting diodes (LED) and large area flat displays. For LEDs, integrated into Si-ICs, the speed is critical. The blue PL band seems quick enough. For displays the critical quantity is luminosity ( > 50 cd/m2). This luminosity can be achieved only with a rather high current and in this context we want to note that often the EL efficiency as a function of current displays a maximum [33,34]. EL efficiency decreasing for high current could be related to heating [33], irreversible chemical changes [34] a n d / o r trapping [35] and this could be a serious problem for displays. Is it possible to construct an LES-EL device with efficiency comparable to PL, i.e. a few % ? Recent results [34,35] illustrate that a comparable internal EL efficiency (4% [35]) can be achieved under pulsed operation (ixs-ms), during which degradation is negligible. Due to loss by light emission into a large solid angle and loss by absorption in contacts, a max. external efficiency of 0.2% has been demonstrated [35]. The EL of these devices [35], the structure of which is sketched in Fig. 7(a), degrades on a time scale of 100 ms [35]. Part of this degradation is probably irreversible, related to coverage of the PS internal surface, as clearly evidenced independently in Ref. [34] and as illustrated by the dependence of EL (t) on exposure to different gases-(He versus air, see Fig. 7(b)). Part of the degradation has been attributed to trapping, reversible just by voltage

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switch-off [35]. Parallel measurement of the EL and current time dependences could help us to understand 'reversible' changes. Note in Fig. 7(c) the quick and slow components of I and different EL (t) and l(t) behaviour. It is evident that as in a-Si:H, also for LES the study of the origin and minimization of degradation will become the central problem. Finally, we should keep in mind that LEDs with comparable EL efficiency have been prepared from a-Si:C:H and a-Si:N:H [36], the advantage of which is easy large-area preparation and better controlled microstructure (homogeneity, stability?). In summary, some problems of LES transport and optical properties have been discussed and prospects of LES based EL illustrated. Many problems remain

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J. Ko(ka et al. / Journal of Non-Crystalline Solids 198-200 (1996) 857-862

to be solved but there is still ample opportunity for future improvements.

Acknowledgements This work was partially supported by the ECPECO 7839 and ASCR A1010528 grants. The authors wish to thank Dr. Linnros for supplying the EL samples and P. Knfipek for experimental assistance.

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