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Photoluminescence and electroluminescence from electrochemically oxidized porous silicon layers
Journal of Luminescence 57 (1993) 283—292
LUMINESCENCE JOURNAL OF
Invited paper
Photoluminescence and electroluminescence from electrochemically oxidized porous silicon layers F. Muller, R. Herino, M. Ligeon, F. Gaspard, R. Romestain, J.C. Vial and A. Bsiesy Laboratoire de Spectromètrie Physique associk au CNRS (URA 08), Universith Joseph Fourier de Grenoble, BP 87, 38402 Saint Martin d’Hères Cedex, France
In this paper we review the luminescence properties of porous silicon layers formed on p-type silicon substrates and subsequently oxidized by anodic polarization in an aqueous electrolyte. The electrochemical oxidation of the porous material leads to a large increase in the photoluminescence intensity, accompanied by a blue shift ofthe emitted spectra. A bright visible electroluminescence is also observed during anodic treatment, with characteristics showing similar trends to that of the photoluminescence. The features ofthe emission are analyzed using a model that expresses the energy dependence ofthe emitted intensity. The model is developed on the hypothesis that the visible light emission originates in the confinement of charge carriers in the quantum-sized crystallites which form the material, and that its efficiency is determined by nonradiative processes, which involve the carrier escape from the confined zone where they are created (or injected) through a tunnelling mechanism. This model is shown to be well supported by the experimental results, and allows an understanding of the spectral shifts and the intensity variations ofboth photoluminescence and electroluminescence during electrochemical oxidation of the porous layers.
1. Introduction Since the discovery of the luminescence properties of porous silicon films [1], much interest has been devoted to the possibility of obtaining light emission from an electrical excitation, which may open the door to the development of silicon-based optoelectronic devices [2,3]. Bright visible electroluminescence of porous silicon films has been evidenced for the first time during the anodic oxidation of p-type porous layers in aqueous elctrolytes [4]. Since then, this new interesting property has been observed under other conditions involving either ‘dry’ contacts obtained by the metallization of the sample surface [2,3] or ‘wet’ contacts resulting from the immersion of the porous layer in a liquid electrolyte [5,6]. The most commonly used hypothesis to explain such light emission in the visible range from an Correspondence to: Dr. F. Muller, Laboratoire de Spectrometric Physique associé au CNRS (URA 08), Université Joseph Fourier de Grenoble, BP 87, 38402 Saint Martin d’Héres Cedex, France. 0022-2313/93/$06.00 © 1993 SSDI 0022-2313(93)EOl 18-H
—
indirect bandgap semiconductor is the confinement of charge carriers in the quantum-sized crystallites which are formed in the high-porosity porous silicon layers [1,7,8]. However, to get good emission efficiency requires an efficient passivation of the large specific surface of the material. Such passivation is readily obtained just after formation of the porous layer, and it is provided by the Si—H surface coverage which results from the anodic attack of the crystalline silicon in hydrofluoric acid (HF) solutions [9]. However, this passivation is not permanent, and the result is that photoluminescence degradation is often observed, for example, upon annealing [10] or under illumination. Various attempts have been made in order to stabilize the surface passivation of the material, and it has been shown that partial oxidation of the layer can be an efficient procedure. This can be obtained by the ‘ageing’ of the samples [11], which corresponds to the native oxidation of the silicon surface in air, but this process seems quite difficult to control accurately. Good results have also been obtained by rapid thermal oxidation [12] or by oxidation in diluted oxygen ambient. Another approach is
Elsevier Science Publishers B.V. All rights reserved
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F. Muller ci’ a!.
/ Luminescence properties
electrochemical oxidation of the porous layer, performed with the anodic polarization of the sample in an aqueous electrolyte [8]. This process is easy to control by the amount of exchange charge which is proportional to the amount of oxide formed. It is also interesting because it is accompanied by a bright visible electroluminescence. This emission is not permanent and is unlikely to lead to technological applications, but it can be used as a tool for studying the material properties. During anodic oxidation of the p-type porous material, large intensity variations and spectral shifts are observed for both photoluminescence and electroluminescence; this paper presents a detailed analysis of such emission characteristic changes. First, a phenomenological model based on the quantum confinement hypothesis is presented, which leads to an expression of the emitted intensity allowing us to expect efficiency variations and spectral shifts with the passivation provided by oxidation. It is then shown that the observed experimental trends in the modification of the characteristics of both photoluminescence and electroluminescence during electrochemical oxidation of p-type layers can be well described by the model,
2. Experimental This work was performed on porous silicon layers of different porosities, obtained by anodization of (100)-oriented, boron-doped p-type substrates of 2—S ~ cm resistivity. The samples were prepared according to the experimental procedure described in [13]. The electroluminescence was detected during electrochemical oxidation in an electrolytic solution composed of deionized water with 0.1 M KNO3 as the supporting electrolyte. The oxidation was performed under constant anodic 2 andcurthe rent density, in the range 0.5—100 mA/cm electroluminescence was detected by placing a detector near the surface of the electrolyte. The emitted intensity integrated over the whole spectrum can be continuously recorded as a function of time by using a silicon photodiode, or the emitted spectra can be recorded by using an optical multichannel analyzer (Princeton Instruments) coupled to a cooled CCD detector. Under these conditions,
01 porous silicon layers
a whole spectrum could be recorded in less than 5 x l0 2 s. Photoluminescence characteristics were obtained under excitation of the 365 nm UV line of a Hg arc or a 458 nm argon laser and the emitted light was detected through a 20 cm monochromator by a GaAs photomultiplier. The main characteristics of the electroluminescence observed during the electrochemical oxidation were described in detail in a previous paper [13]; only the main features are recalled here. During anodic oxidation of lightly p-doped porous silicon layers at constant current density, there is a first regime where the silicon electrode potential increases slightly, corresponding to the progressive in-depth oxidation of the porous layer. During this regime, light emission becomes noticeable after a delay that depends on the sample porosity. A large increase in the electroluminescence intensity is further obtained up to a maximum, and then the emission decreases to zero when the anodizing potential starts to increase very sharply. At this point, the total exchange charge is Qo. The oxidation level of the porous layer reached upon Qo is only about 50%, but the electrical contact between the partially oxidized silicon crystallites in the pore walls and the silicon substrate is broken. Consequently, on further anodization, because of the imposed galvanostatic conditions, the anodization potential increases quite rapidly and this second regime corresponds to further oxidation of the bulk silicon at the bottom of the pores.
3. Mechanisms of light emission 3.1. The confinement model for PL and EL The visible light emission of porous silicon is generally attributed the spatial confinement of charge carriers in thetonanometer-sized crystallites that are formed in the material [1,7,8]. Although other possible explanations have been proposed [14,15], the confinement origin seems now to be well supported by theoretical calculations of the energy levels in small size silicon clusters [16,17] and by several different experimental approaches attesting to the presence of quantum-sized crystallites in porous silicon layers [18—20].
F. Muller et a!.
/
Luminescence properties of porous silicon layers
Following the confinement model, the observation of a simultaneous emission of light during the anodic oxidation of p-type porous layers requires that radiative recombination of carriers takes place within the quantum-sized crystallites in the porous layer. Because holes are supplied by the anodic polarization of the silicon electrode, this implies that electrons are simultaneously injected into the crystallites, from the electrolyte or from the interface. The origin of this electronic injection is not clear, as the involved anodization potentials remain below the oxidation potential of the species present in the electrolyte. As previously suggested [4], this charge injection might result from the oxidation of the Si—H bonds which cover the porous silicon surface after formation. Another possibility might be that this electron injection results from the silicon oxidation itself: in the overall process leading to the oxidation of surface silicon atoms, intermediate species can be formed with energy levels high enough to allow electronic injection into the confined energy levels [21]. In any case, both photoluminescence (PL) and electroluminescence (EL) can be analyzed in the frame of the quantum confinement model. This model has been extended to include nonradiative mechanisms which involve the carrier escape from the crystallite in which they are confined. We now briefly describe this model and then show that it provides a coherent explanation of the main characteristics of both types of emission.
285
quently, considering that the starting silicon substrate is of high quality, it seems quite unlikely to find volumic recombination centres in such crystallites. The emission efficiency is then determined by the surface passivation of the crystallites, and this is demonstrated by a large number of experimental observations (see, for example, Refs. [10,23]). As long as the size distribution of the crystallites is not determined experimentally, we can choose in a first approach to describe the energy distribution of the emitting crystallites N(E) by a simple Gaussian law as: N(E)
=
[
-iY-~_exp ~
2 —
(E
[
—
E0)
~2
1
(1)
j’
where E0 is the emitted energy corresponding to the most probable crystallite size. However, it must be emphasized that two types of emitting crystallites have to be considered. In the material some crystallites are electrically ‘disconnected’ from the substrate. This means that it is not possible to inject charge carriers in these crystallites by polarizing the silicon substrate. Consequently, these crystallites will only shine under optical excitation, and they will only contribute to the photoluminescence. On the other hand, other crystallites are still electrically ‘connected’ to the substrate, in which charge carriers can be injected according to the substrate polarization. For this population, the emission excitation can be either optical or electrical, so that it can contribute to both photoluminescence and electroluminescence.
3.2. Distribution of the emitting crystallites The relatively low quantum efficiencies of photoluminescence measured at room temperature strongly suggest that the radiative recombination of carriers in porous silicon is countered by nonradiative processes, which may involve either volume or surface recombination centres [22]. The high-porosity material which shows luminescent properties can be described as an assembly of small-size silicon crystallites which are highly interconnected by narrowed regions. The crystallite size distribution is generally not accurately known, but according to different experimental studies, typical sizes range between a few and 10 nm. Conse-
33. Nonradiative processes The luminescence spectrum is not the direct image of the crystallite size distribution, but is the result of the product of this distribution by the internal quantum efficiency of the crystallites. The emitted intensity can be written as:
1(E)
=
AON(E)
~(E) +
Wnr(E)’
(2)
where A0 is a proportionality constant which depends on the conditions of the charge generation in
286
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/ Luminescence properties
the crystallites, and f’J’(E) and Wnr(E) are the radiative and nonradiative rates, respectively. Time-resolved photoluminescence characterization gives useful information about this internal efficiency [22]. Photoluminescence decays following pulsed excitation show a strongly nonexponential behaviour, but an average decay time can be determined which corresponds to lifetimes in the range 1—100 jis, depending on the emitted wavelength. By studying samples of different oxida tion levels and the temperature dependence in the range 300—500 K of photoluminescence characteristics, lifetimes and intensities are found to be nearly proportional. Because the lifetime t is related to ~ and Wnr by t = 1/(Wr + ~4’~r)and the intensity is proportional to Wr/( Wr + Wnr), these results suggest that in the studied temperature range, the dominant mechanism for relaxation is nonradiative with I
=
of porous silicon layers
/~ Electrolyte
d
~
Silicon
D
~
1
(3)
=
(a)
Ue e H
—
_______
H
hv
—
,l~ Eh Uh
(b
Fig. 1. Schematic description of the electrolyte—porous silicon interface. (a) Idealized representation of a quantum size crystallite (diameter D) linked to a nonradiative silicon cell by a rowed region (length a, diameter d); (b) corresponding energetic diagram in the flat-band condition: E silicon gap energy; E, and Eh electron and hole confinement energy in the crystallite; U, and U 5 electron and hole confinement energy in the narrowed region; E = hv = Eg + E, + E5 energy of the emitted light; and E~= Eg + U, + U5 energy gap of the barrier.
W0T(E)
with:
T(E) c’ exp
)
_____
1 ~r
a
-________
1/Wnr.
Among the different mechanisms that can account for such lifetime variation we believe that a likely nonradiative mechanism can involve the escape of the carriers from the confined zone where they are generated towards more extended and less passivated neighbouring crystallites where nonradiative recombination can occur [22]. For this escape, the carriers have to experience the energy barrier which corresponds to the short narrowed regions that link the crystallites (see Fig. 1) and can be totally or partially oxidized. The exponential dependence of the nonradiative recombination rates with the emission energy [22] well supports a tunnelling mechanism for such nonradiative leaks. Then, the lifetime is expected to depend on the barrier transparency T(E) according to:
=
/
.
[
—
~~~/2m(Ue h
—
Ee)]
(4)
where E = E~+ Eh + Eg(Ee is the confinement energy of the electron, Eh is the confinement energy of the hole, and Eg is the bulk silicon gap, as shown in
Fig. 1), a and Ue are the barrier thickness and the barrier height, respectively, and m is the carrier effective mass. We consider here only the electron escape for which the transparency is greater; in fact, as soon as the electron has tunnelled through the barrier, the hole is attracted by Coulombic interaction and leaves the crystallite to recombine nonradiatively.
F. Muller et al.
3.4. Energy dependence intensity
of the
/ Luminescence
luminescence
Finally, taking into account this tunnelling model, the emitted intensity can be expressed as: 1(E)
=
Wr [2a AN(E).WexP [-~--\/2m(Ue
—
Ee)]
(5)
This expression can be approximated by considering that Ee is much lower than Ue:
properties of porous silicon layers
287
intensity is shifted towards lower energies when a increases or when E~decreases. It also appears that the intensity depends exponentially on the term a..,,/E~.On the other hand, the pre-exponential factors can also vary depending on the experimental conditions. For example, the number of emitting crystallites can vary with the passivation of the material, with the excitation wavelength of the photoluminescence, or with the applied voltage for the electroluminescence.
~UeEei(1~)~
with E E5
hv
=
=
4. Emission intensity oxidation
Eg + (1 + C)Ee, and
during
anodic
(6)
Here, we study only the first part of the anodic
where e is the ratio of the hole and of the electron confinement energy: (7)
oxidation where a very large increase in the emitted intensity is observed for both PL and EL signals. Fig. 2 shows that the photoluminescence intensity of 65% porosity layers is increased by more than three orders of magnitude after anodic oxidation. A similar behaviour is found for 70% porosity
(8)
ponds to as samples, an shown exponential in Fig. variation 3. This of regime the intensity correswith exchange Q. The same dependence is alsothefound for thecharge increase in the electroluminescence signal, and Fig. 4 shows that the same type of
=
=
Eg + (1 + e) Ue,
increase
Ei~,/Ee,
which expresses the effective mass ratio. Then ~2a 1(E)
=
N(E) exp
x exp
[ r
—
/~Ii~
+ 1 I 1 2m a h~c+1 ~
El.
If 2E~)>Eo
—
..~i—,
one can write the emitted intensity as follows: 1(E)
AN0Wr =
~_~exp
[2a /~~1 ____ hVe+l]
}
2 xex~[_ ~2 The energy E’[ (E—E’~) 0 is/ obtained by: ~2 2m a E’0 = E0 2~.Jh2(e + 1) —
~7.
(9)
~10
-
~ io~
-
,~ —
10~
-
/Porosity
(10)
10i I
0
Following this schematic phenomenological model one can derive expressions that show how the emitted intensity is expected to depend on the barrier characteristics. One can see that the peak
: 65%
0.6 0.8 Relative exchanged charge Q/Q0 0.2
0.4
1
Fig. 2. Variations in the PL peak intensity as a function of the exchange charge during anodic oxidation of a 2~tm1). 65% porosity sample (anodic current density 0.5 mAcm
288
F. Muller et al. 1
/ Luminescence properties of porous silicon
~ ••••
~
•••
•
/.
10~
5?
• •7,///’
*
•
Porosity
lOi 0
0.2
0.4
70%
I
I
0.6
0.8
1
Relative exchanged charge Q/Q Fig. 3. Variations in the PL peak intensity as a function of the exchange charge during anodic oxidation of a 70% porosity 21.tm sample (anodic current density 0.5 mA cm -
layers
barrier modifications are proportional to the exchange charge Q, which seems quite reasonable. If we recall that the energy barrier is attributed to the silicon narrowed regions that link the crystallites corresponds to much smaller dimensions than itthat of the crystallite. Then, the oxide growth can more significantly affect the sizes of these narrowed regions. The oxidation of the surface of these narrowings will induce an increase in the energy E~, by arising the confinement energy related to these restricted regions. If we suppose that they can be fully oxidized, the energy E~may increase towards the limit of 9eV which is the energy gap of silica. The modification of E~is also supported by the strong simultaneous increase in the photoluminescence decay time [24]. It is interesting to notice heredilute that the chemical oxidation of the porous layer by aqueous HNO 3 solutions leads to the same kind of behaviour [25].
exponential increase is obtained whatever the anodizing current density. This large increase can be related to the improvement of the barrier efficiency towards the nonradiative leaks due to anodic oxidation. From Eq. (9), the emitted intensity is expected to increase exponentially with a..JE5. It is of course difficult to express how this quantity is modified as a function of Q, but the experimental results suggest that the
100
j=15 m&cm2m~ 1 2.5
10
0.5 I
5?
0
200
/
:
5. Spectral shifts resulting from anodic oxidation The second exponential term in Eq. (9) leads us to expect spectral shifts upon barrier modifications. In particular, an increase in E~will induce a spectral blue shift. Such an increase will reduce the barrier transparency to carriers, and thus the car-
rier sensitive escape in the probability, crystallites, resulting inwhich an have the enhancement. Itsmallest is clearenergy that this effect willefficiency be more highest confinement (the ‘blue’ emitting
.
I~ 1.25
5?
Another contribution to the intensity increase which has to be taken into account is the increase in the number of emitting crystallites during electrochemical treatment. A discussion of this effect will be published elsewhere [26].
crystallites), Fig. 5 shows and the willvariations result in a in spectral the PL blue andshift. EL
.
/i~,:~
400 600 800 1000 Exchanged Charge (mC)
1200
Fig. 4. Logarithmic plot of the )~-integratedEL light intensity as a function of the exchange charge upon anodic oxidation at different current densities. Porosity 70%; layer thickness 2 .im.
peak wavelengths as a functionoxidation of the exchange charge of 65% porosityduring layers.electrochemical Quite large reduction in peak wavelength are observed, attesting for the blue shift of the spectra with the oxidation level, It is interesting to notice that very similar variations were obtained for both kinds of emission, suggesting that the same crystallite distribution is involved in both phenomena.
F. Muller ci a!.
/ Luminescence properties of porous silicon
850
layers
289
900 Po
:
•
EL
70
%
~800 800
_
\~orositY:
850
~750
.
5.
..
CS
5. CS
•~g ‘~°~ 5?
PL~N\~~
.~ 5?
•
650
0.2 0.4 0.6 0.8 Relative exchanged charge
• 700
650
11111
0
• • . • PL
750
1
1.2
QIQQ
Fig. 5. Variations in the peak wavelengths of PL and EL during anodic oxidation of a 65% porosity2~sm sample i)~ (anodic current density 0.5 mA cm
According to the confinement model, such blue shifts in the emitted wavelengths should also be explained by a decrease in the size of the emitting crystallites. The first proposed explanation for this effect was to assign these shifts to the thinning of the structure that results from the oxide growth on the crystallite surface. This consumes silicon atoms and reduces the dimensions of the crystalline material [4]. If we assume that the oxide growth is homogeneous over the whole porous layer, we can calculate from the specific surface thestudied mater2/cm3area forofthe ial (in the range 500—600 m porosities) that the total exchange charge Qo at the end of the process only corresponds to an oxidation of about 3 A silicon thickness. Following the hypothesis that the confinement energy varies with the crystallite diameter d as 1/d2, it can be calculated that a spectral shift AE should correspond to a radius reduction Ar as Ar = r.AE/2(E Eg). The spectral shift observed after oxidation at Qo, should correspond to a 7 A size reduction for a 20 A radius crystallite. More accurately, a closer analysis of the experimental results (Fig. 5) shows, for PL and EL, that the effect of oxidation on the spectral shift is much more important in the first part of the process. It can thus be concluded that the observed blue shift of the peak intensity does not result only from the crystallite size reduction due to anodic oxidation; we believe that the in-
~
0
I
0.2 0.4 0.6 0.8 1 Relative exchanged charge Q/Q 0
1.2
Fig. 6. Variations in the peak wavelengths of PL and EL during anodic oxidation density of a 70% 0.5 mA porosity cm - 21.tm sample 1) (anodic current
crease in barrier height (Eq. 10) can contribute to it in a large part. Fig. 6 shows that similar results are obtained on 70% porosity layers. However, in that case, in the first part of the anodic treatment, there is a large difference in the peak wavelengths of PL and EL at a given oxidation level. For example, Fig. 7 gives the PL and EL spectra at the beginning of oxidation, for the exchange charge equal to 0.2Q 0. This feature indicates that different populations of emitting crystallites are involved. All connected and disconnected contribute crystallites to the photoemission whencrystallites only the connected are
‘ ~ ~
Porosity 70%
—
‘~
450
550
650 750 Wavelength (nm)
850
950
Fig. 7. Comparison of the PL and EL spectra ofa 70% porosity sample oxidized at O.2Qo.
290
F. Muller et a!.
/ Luminescence
properties of porous silicon layers
~
850
750,
I~\PorosN\Porosit~:8o%
0
0.2 0.4 0.6 0.8 1 Relative exchanged charge Q/Q
1.2
150
185 Time (s)
220
Fig. 8. Variations in the peak wavelengths of PL and EL during anodic oxidation of a 80% porosity sample (anodic current 2~.sm ~ density 0 5 mA cm -
Fig. 9. Variations in the peak intensity and peak wavelength of the EL during anodic oxidation upon a sudden change of the anodizing current (I and 2 mA cm - 2j.tm - i) Notice that the instantaneous intensity is proportional to the current.
involved in the electroluminescence. At the beginning of the electrochemical oxidation, the anodic polarization is quite low, and acccording to the Lehmann model [7], holes are mostly supplied towards the less confined crystallites (the ‘red’ emitting ones). On further oxidation, the anodic polarization increases, and holes can be also injected into smaller crystallites, leading to a spectral blue shift as well as the barrier enhancement provided by the oxide growth. In the second part of the electrochemical treatment, the PL and EL spectra superpose well: all the connected crystallites are similarly excited for both phenomena, and the contribution of disconnected crystallites to the photoluminescence does not provoke major differences in the spectra. A crystallite qualifies as a ‘connected crystallite’ when a current can flow across it. This depends, for a given voltage, on the confinement and on the oxide coating of the crystalline surface, Fig. 8 shows the spectral shifts recorded for 80% porosity layers. In that case, it is only at the end of the anodic treatment that a good spectral agreement is observed for both types of emission. These differences account for the evolution of the material structure with the porosity: in the 80% porosity layers, the population of disconnected crystallites is likely more important, and then, the PL spectra are almost determined by this kind of crystallites. This feature might also explain why the electrochemical
treatment of 80% porosity layers only increases the PL intensity by a factor of 5 (after exchange of Qo). Indeed, the barrier enhancement can only concern the connected crystallites which seem to be present in much lower proportions than the disconnected ones in these very high porosity layers.
6. Electroluminescence to charge injection
spectral
shifts
related
During electroluminescence, other spectral shifts can be observed that are not related to the barrier modification provided by anodic oxide growth and which can occur without major modification of the material. Fig. 9 shows that when the anodizing current is suddenly changed during electrochemical oxidation, there is an instantaneous and reversible shift of the peak wavelength: if the anodic current is increased from 1 to 2 mA cm -2 a blue shift of about 20 nm is immediately observed, when no noticeable changes in the oxidation level are induced. Changes in the anodic current modify the charge injection conditions: the resulting change in the polarization of the substrate induces modifications in the potential distribution at the electrolyte—crystallite interface and through the energy barrier. The analysis of the potential variations in the structure is quite complicated and is far beyond
F. Muller ci a!.
/ Luminescence properties
the scope of this article. However, we can assume in a first step that an increase in anodic polarization allows the injection of electrons of higher energies, thus involving more confined crystallites and resulting in a blue shift of the emission that is unrelated to any material modification. In the same way, a decrease in the current (associated with a decrease in anodic polarization) results in an instantaneous red shift because the smallest crystallites are no longer addressed. A similar voltage tuneable electroluminescence has also been observed under cathodic bias of a n-type porous layer and is described elsewhere [27]. The important role of the charge injection conditions upon the spectral position of the electroluminescence is also demonstrated by a different experiment. If we consider the EL spectrum of a 70% porosity layer after exchange of a charge Q equal to Qo/2, it is very similar to the corresponding photoluminescence spectrum and shows a peak intensity for a wavelength of 740 nm. If the anodic oxidation is halted at this step, and the anodic oxide is dissolved tn HF the EL spectrum which is recorded when restarting the electrochemical oxidation peaks at 860nm, when the PL spectrum exhibits only a slight red shift of about 20 nm (Fig. 10). The oxide dissolution mostly results in a quite large red shift of the electroluminescence. First, this behaviour confirms that the observed blue shift during anodic oxidation does not result from a thinning effect due to oxide growth because it should be irreversible upon oxide dissolution. On the other hand, it confirms that the oxide growth modifies the population of crystallites that luminesce. At the beginning of anodic oxidation, the current flows more easily through the largest crystallites because of their lower confinement energy: they are oxidized first and then they present a large resistance to electric current because of their oxide coating. On further anodization, the polarization increases and the current also flows through more confined crystallites, resulting in a blue shift of the emission. If at a given step, the oxide is removed by HF etching, the current can flow again through the largest crystallites, and the etching results in an instantaneous large red shift of the electroluminescence, while the photoluminescence is not seriously affected because the HF treatment
of porous silicon layers
HF etching
291
before
after I
—~
‘~ ~
~20
~ ~
/ before
~.
HF etching
..
~
~
x 20
,~‘
after
: -
500
I
I
I
I
600
700
800
900
1000
Wavelength (nm) Fig. 10. Comparison of the PL and EL spectra of a 70% porosity sample oxidized at Qo/2, before and after HF etching.
does not modify the structure of the crystalline material.
7. Conclusions The photoluminescence and the electroluminescence observed during the first part of the electrochemical oxidation of porous layers formed in p-type silicon substrate have been studied in detail. It is shown that both types of emission result from the same origin: the radiative recombination of carriers within crystallites of quantum size, the emission efficiency at room temperature being limited by the nonradiative losses. For the first time, a phenomenological model has been developed on the hypothesis that the nonradiative recombination is governe-id by the escape of carriers from the
292
F. Muller ci a!.
/
Luminescence properties of porous silicon layers
confined regions through a tunnelling mechanism. An expression for the intensity as a function of the emitted energy has been derived, which accounts well for the spectral variations and the intensity enhancement observed during electrochemical OX~ dation of the porous layer. This analysis shows that this behaviour is not the simple result of the crystallite size reduction due to oxide growth as previously suggested, but that it mostly involves the modification of the barrier efficiency and of the surface passivation, and the condition of the charge injection into the crystallites in the case of electroluminescence. We believe that the variations in PL and EL efficiency with anodic oxidation are caused mainly by the increase in the height of the barriers between passivated and unpassivated crystallites. In this paper we only analyze the recombination mechanisms; the next step is to take into account quantitatively the charge injection into the crystallites which is necessary to observe the electroluminescence. This requires a more thorough study of the problems of the charge exchange between the confined crystallites and the electrolytic solution.
Acknowledgement This work was supported in part by a DRET contract (9 1-126).
Refrences [I] L.T. Canham, AppI. Phys. Lett. 57 (1990) 1046. [2] A. Richter, W. Lang, P. Steiner, F. Kozlowski and H. Sandmaier, in Mater. Res. Soc. Fall Mg LTC, eds. S.S. Iyer and R.T. Collins (MRS, Boston, MA, 1991) vol. 256. [3] N. Koshida and H. Koyama, AppI. Phys. Lett. 60 (1992) 347. [4] A. Halimaoui, G. Bomchil, C. Oules, A. Bsiesy, F. Gaspard, R. Hero, M. Ligeon and F. Muller, AppI. Phys. Lett. 59 (1991) 304. [5] S. Billat, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain and J. C. Vial, in Mater. Res. Soc. Fall Mg LTC, eds. S.S. lyer and R.T. Collins (MRS, Bos125. ton, MA, 1991) vol. 256, p.
[6] L.T. Canham, WY. Leong, M.l.J. Beale, TI. Cox and L. Taylor, AppI. Phys. Lett. 61(1992) 2563. [7] V. Lehmann and H. FoIl, J. Electrochem. Soc. 137 (1990) 653. [8] A. Bsiesy, J.C. Vial, F. Gaspard, R. Herino, M. Ligeon, F Muller, R. Romestain, A. Wasiela, A. Halimaoui and G. Bomchil, Surf. Sci. 254 (1991) 195. [9] A. Venkateswara Rao, F. Ozanam and J.N. Chazaiviel, J. Electrochem. Soc. 138 (1991) 153. [10] C. Tsai, K.H. Li, J. Sarathy, S. Shih, iC. Campbell, BK. Hance and J.M. White, AppI. Phys. Lett. 59 (1991) 2814. [11] L.T. Canham, WY. Leong, TI. Cox, M.1.J. Beale, K.J. Nash, C.P.D. Brumhead, L.L. Taylor and K.J. Marsh, Invited paper presented at 21st ICPS, Beijing China, AugustPetrova-Koch, 1992. [12] V. T. Muschik, A. Meyer Kux, BK. and F. Koch, AppI. Phys. Lett. 61(1992) 943. [13] M. Ligeon, F. Muller, R. Herino, F. Gaspard, J.C. Vial, R. Romestain, S. Billat and A. Bsiesy, J. AppI. Phys. 74(1993) 1265. [14] SM. Prokes, Ui. Glembocki, V.M. Bermudez, R. Kaplan, L.E. Friedersdorf and P.C. Searson, Phys. Rev. B 45(1992) 13788. [15] M. Stutzmann, MS. Brandt, M. Rosenbauerandi. Weber, Phys. Rev. B, in press. [16] J.P. Proot, C. Delerue and G. Allan, AppI. Phys. Lett. 61 1948.Ri. Needs, K.J. Nash, L.T. Canham, P.D.J. [17] (1992) A.J. Read, Calcott and A. Qteish, Phys. Rev. Lett. 69 (1992) 1232. [18] AG. Cullis and L.T. Canham, Nature 353 (1991) 335. [19] H. Munder, C. Andrezejak, MG. Berger, U. Klemradt, H. Luth, R. Henna and M. Ligeon, Thin Solid Films 221 (192) 27. [20] MW. Cole, iF. Harvey, R.A. Lux, D.W. Eckart and R. Tsu, AppI. Phys. Lett. 60 (1992) 2800. [21] L.M. Peter, AM. Borazio, Hi. Leverenz and J. Stumper, J. Electroanal. Chem. 290 (1990) 229. [22] iC. Vial, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F.
[23]
[24]
[25] [26] [27]
Muller, R. Romestain and MacFarlane, Phys. Rev. B 45 (1992) 14171. BA. Khan, R. Pinker, K. Shahzad and B. Rossi, in Mater. Res. Soc. Fall Mg LTC, Vol. 256, eds. S.S. Iyer and R.T. Collins (MRS, Boston, MA, 1991) 143. iC. Vial, S. Billat, A. Bsiesy, G. Fishman, F. Gaspard, R. Herino, M. Ligeon, F. Madeore, I. Mihalcescu, F. Muller and R. Romestain, Physica B 185 (1993) 593. G. Mauckner, T. Walter, T. Baier, K. Thonke and R. Sauer, Mater. Res. Soc. Symp. 283 (1993). I. Mihalcescu, M. Ligeon, F. Muller, R. Romestain and iC. Vial, i. Lumin. 57 (1993) lii. A. Bsiesy, F. Muller, M. Ligeon, F. Gaspard, R. Herino, R. Romestain and iC. Vial, Phys. Rev. Lett. 71 (1993) 637.
LUMINESCENCE JOURNAL OF
Invited paper
Photoluminescence and electroluminescence from electrochemically oxidized porous silicon layers F. Muller, R. Herino, M. Ligeon, F. Gaspard, R. Romestain, J.C. Vial and A. Bsiesy Laboratoire de Spectromètrie Physique associk au CNRS (URA 08), Universith Joseph Fourier de Grenoble, BP 87, 38402 Saint Martin d’Hères Cedex, France
In this paper we review the luminescence properties of porous silicon layers formed on p-type silicon substrates and subsequently oxidized by anodic polarization in an aqueous electrolyte. The electrochemical oxidation of the porous material leads to a large increase in the photoluminescence intensity, accompanied by a blue shift ofthe emitted spectra. A bright visible electroluminescence is also observed during anodic treatment, with characteristics showing similar trends to that of the photoluminescence. The features ofthe emission are analyzed using a model that expresses the energy dependence ofthe emitted intensity. The model is developed on the hypothesis that the visible light emission originates in the confinement of charge carriers in the quantum-sized crystallites which form the material, and that its efficiency is determined by nonradiative processes, which involve the carrier escape from the confined zone where they are created (or injected) through a tunnelling mechanism. This model is shown to be well supported by the experimental results, and allows an understanding of the spectral shifts and the intensity variations ofboth photoluminescence and electroluminescence during electrochemical oxidation of the porous layers.
1. Introduction Since the discovery of the luminescence properties of porous silicon films [1], much interest has been devoted to the possibility of obtaining light emission from an electrical excitation, which may open the door to the development of silicon-based optoelectronic devices [2,3]. Bright visible electroluminescence of porous silicon films has been evidenced for the first time during the anodic oxidation of p-type porous layers in aqueous elctrolytes [4]. Since then, this new interesting property has been observed under other conditions involving either ‘dry’ contacts obtained by the metallization of the sample surface [2,3] or ‘wet’ contacts resulting from the immersion of the porous layer in a liquid electrolyte [5,6]. The most commonly used hypothesis to explain such light emission in the visible range from an Correspondence to: Dr. F. Muller, Laboratoire de Spectrometric Physique associé au CNRS (URA 08), Université Joseph Fourier de Grenoble, BP 87, 38402 Saint Martin d’Héres Cedex, France. 0022-2313/93/$06.00 © 1993 SSDI 0022-2313(93)EOl 18-H
—
indirect bandgap semiconductor is the confinement of charge carriers in the quantum-sized crystallites which are formed in the high-porosity porous silicon layers [1,7,8]. However, to get good emission efficiency requires an efficient passivation of the large specific surface of the material. Such passivation is readily obtained just after formation of the porous layer, and it is provided by the Si—H surface coverage which results from the anodic attack of the crystalline silicon in hydrofluoric acid (HF) solutions [9]. However, this passivation is not permanent, and the result is that photoluminescence degradation is often observed, for example, upon annealing [10] or under illumination. Various attempts have been made in order to stabilize the surface passivation of the material, and it has been shown that partial oxidation of the layer can be an efficient procedure. This can be obtained by the ‘ageing’ of the samples [11], which corresponds to the native oxidation of the silicon surface in air, but this process seems quite difficult to control accurately. Good results have also been obtained by rapid thermal oxidation [12] or by oxidation in diluted oxygen ambient. Another approach is
Elsevier Science Publishers B.V. All rights reserved
284
F. Muller ci’ a!.
/ Luminescence properties
electrochemical oxidation of the porous layer, performed with the anodic polarization of the sample in an aqueous electrolyte [8]. This process is easy to control by the amount of exchange charge which is proportional to the amount of oxide formed. It is also interesting because it is accompanied by a bright visible electroluminescence. This emission is not permanent and is unlikely to lead to technological applications, but it can be used as a tool for studying the material properties. During anodic oxidation of the p-type porous material, large intensity variations and spectral shifts are observed for both photoluminescence and electroluminescence; this paper presents a detailed analysis of such emission characteristic changes. First, a phenomenological model based on the quantum confinement hypothesis is presented, which leads to an expression of the emitted intensity allowing us to expect efficiency variations and spectral shifts with the passivation provided by oxidation. It is then shown that the observed experimental trends in the modification of the characteristics of both photoluminescence and electroluminescence during electrochemical oxidation of p-type layers can be well described by the model,
2. Experimental This work was performed on porous silicon layers of different porosities, obtained by anodization of (100)-oriented, boron-doped p-type substrates of 2—S ~ cm resistivity. The samples were prepared according to the experimental procedure described in [13]. The electroluminescence was detected during electrochemical oxidation in an electrolytic solution composed of deionized water with 0.1 M KNO3 as the supporting electrolyte. The oxidation was performed under constant anodic 2 andcurthe rent density, in the range 0.5—100 mA/cm electroluminescence was detected by placing a detector near the surface of the electrolyte. The emitted intensity integrated over the whole spectrum can be continuously recorded as a function of time by using a silicon photodiode, or the emitted spectra can be recorded by using an optical multichannel analyzer (Princeton Instruments) coupled to a cooled CCD detector. Under these conditions,
01 porous silicon layers
a whole spectrum could be recorded in less than 5 x l0 2 s. Photoluminescence characteristics were obtained under excitation of the 365 nm UV line of a Hg arc or a 458 nm argon laser and the emitted light was detected through a 20 cm monochromator by a GaAs photomultiplier. The main characteristics of the electroluminescence observed during the electrochemical oxidation were described in detail in a previous paper [13]; only the main features are recalled here. During anodic oxidation of lightly p-doped porous silicon layers at constant current density, there is a first regime where the silicon electrode potential increases slightly, corresponding to the progressive in-depth oxidation of the porous layer. During this regime, light emission becomes noticeable after a delay that depends on the sample porosity. A large increase in the electroluminescence intensity is further obtained up to a maximum, and then the emission decreases to zero when the anodizing potential starts to increase very sharply. At this point, the total exchange charge is Qo. The oxidation level of the porous layer reached upon Qo is only about 50%, but the electrical contact between the partially oxidized silicon crystallites in the pore walls and the silicon substrate is broken. Consequently, on further anodization, because of the imposed galvanostatic conditions, the anodization potential increases quite rapidly and this second regime corresponds to further oxidation of the bulk silicon at the bottom of the pores.
3. Mechanisms of light emission 3.1. The confinement model for PL and EL The visible light emission of porous silicon is generally attributed the spatial confinement of charge carriers in thetonanometer-sized crystallites that are formed in the material [1,7,8]. Although other possible explanations have been proposed [14,15], the confinement origin seems now to be well supported by theoretical calculations of the energy levels in small size silicon clusters [16,17] and by several different experimental approaches attesting to the presence of quantum-sized crystallites in porous silicon layers [18—20].
F. Muller et a!.
/
Luminescence properties of porous silicon layers
Following the confinement model, the observation of a simultaneous emission of light during the anodic oxidation of p-type porous layers requires that radiative recombination of carriers takes place within the quantum-sized crystallites in the porous layer. Because holes are supplied by the anodic polarization of the silicon electrode, this implies that electrons are simultaneously injected into the crystallites, from the electrolyte or from the interface. The origin of this electronic injection is not clear, as the involved anodization potentials remain below the oxidation potential of the species present in the electrolyte. As previously suggested [4], this charge injection might result from the oxidation of the Si—H bonds which cover the porous silicon surface after formation. Another possibility might be that this electron injection results from the silicon oxidation itself: in the overall process leading to the oxidation of surface silicon atoms, intermediate species can be formed with energy levels high enough to allow electronic injection into the confined energy levels [21]. In any case, both photoluminescence (PL) and electroluminescence (EL) can be analyzed in the frame of the quantum confinement model. This model has been extended to include nonradiative mechanisms which involve the carrier escape from the crystallite in which they are confined. We now briefly describe this model and then show that it provides a coherent explanation of the main characteristics of both types of emission.
285
quently, considering that the starting silicon substrate is of high quality, it seems quite unlikely to find volumic recombination centres in such crystallites. The emission efficiency is then determined by the surface passivation of the crystallites, and this is demonstrated by a large number of experimental observations (see, for example, Refs. [10,23]). As long as the size distribution of the crystallites is not determined experimentally, we can choose in a first approach to describe the energy distribution of the emitting crystallites N(E) by a simple Gaussian law as: N(E)
=
[
-iY-~_exp ~
2 —
(E
[
—
E0)
~2
1
(1)
j’
where E0 is the emitted energy corresponding to the most probable crystallite size. However, it must be emphasized that two types of emitting crystallites have to be considered. In the material some crystallites are electrically ‘disconnected’ from the substrate. This means that it is not possible to inject charge carriers in these crystallites by polarizing the silicon substrate. Consequently, these crystallites will only shine under optical excitation, and they will only contribute to the photoluminescence. On the other hand, other crystallites are still electrically ‘connected’ to the substrate, in which charge carriers can be injected according to the substrate polarization. For this population, the emission excitation can be either optical or electrical, so that it can contribute to both photoluminescence and electroluminescence.
3.2. Distribution of the emitting crystallites The relatively low quantum efficiencies of photoluminescence measured at room temperature strongly suggest that the radiative recombination of carriers in porous silicon is countered by nonradiative processes, which may involve either volume or surface recombination centres [22]. The high-porosity material which shows luminescent properties can be described as an assembly of small-size silicon crystallites which are highly interconnected by narrowed regions. The crystallite size distribution is generally not accurately known, but according to different experimental studies, typical sizes range between a few and 10 nm. Conse-
33. Nonradiative processes The luminescence spectrum is not the direct image of the crystallite size distribution, but is the result of the product of this distribution by the internal quantum efficiency of the crystallites. The emitted intensity can be written as:
1(E)
=
AON(E)
~(E) +
Wnr(E)’
(2)
where A0 is a proportionality constant which depends on the conditions of the charge generation in
286
F. Muller ci’ a!.
/ Luminescence properties
the crystallites, and f’J’(E) and Wnr(E) are the radiative and nonradiative rates, respectively. Time-resolved photoluminescence characterization gives useful information about this internal efficiency [22]. Photoluminescence decays following pulsed excitation show a strongly nonexponential behaviour, but an average decay time can be determined which corresponds to lifetimes in the range 1—100 jis, depending on the emitted wavelength. By studying samples of different oxida tion levels and the temperature dependence in the range 300—500 K of photoluminescence characteristics, lifetimes and intensities are found to be nearly proportional. Because the lifetime t is related to ~ and Wnr by t = 1/(Wr + ~4’~r)and the intensity is proportional to Wr/( Wr + Wnr), these results suggest that in the studied temperature range, the dominant mechanism for relaxation is nonradiative with I
=
of porous silicon layers
/~ Electrolyte
d
~
Silicon
D
~
1
(3)
=
(a)
Ue e H
—
_______
H
hv
—
,l~ Eh Uh
(b
Fig. 1. Schematic description of the electrolyte—porous silicon interface. (a) Idealized representation of a quantum size crystallite (diameter D) linked to a nonradiative silicon cell by a rowed region (length a, diameter d); (b) corresponding energetic diagram in the flat-band condition: E silicon gap energy; E, and Eh electron and hole confinement energy in the crystallite; U, and U 5 electron and hole confinement energy in the narrowed region; E = hv = Eg + E, + E5 energy of the emitted light; and E~= Eg + U, + U5 energy gap of the barrier.
W0T(E)
with:
T(E) c’ exp
)
_____
1 ~r
a
-________
1/Wnr.
Among the different mechanisms that can account for such lifetime variation we believe that a likely nonradiative mechanism can involve the escape of the carriers from the confined zone where they are generated towards more extended and less passivated neighbouring crystallites where nonradiative recombination can occur [22]. For this escape, the carriers have to experience the energy barrier which corresponds to the short narrowed regions that link the crystallites (see Fig. 1) and can be totally or partially oxidized. The exponential dependence of the nonradiative recombination rates with the emission energy [22] well supports a tunnelling mechanism for such nonradiative leaks. Then, the lifetime is expected to depend on the barrier transparency T(E) according to:
=
/
.
[
—
~~~/2m(Ue h
—
Ee)]
(4)
where E = E~+ Eh + Eg(Ee is the confinement energy of the electron, Eh is the confinement energy of the hole, and Eg is the bulk silicon gap, as shown in
Fig. 1), a and Ue are the barrier thickness and the barrier height, respectively, and m is the carrier effective mass. We consider here only the electron escape for which the transparency is greater; in fact, as soon as the electron has tunnelled through the barrier, the hole is attracted by Coulombic interaction and leaves the crystallite to recombine nonradiatively.
F. Muller et al.
3.4. Energy dependence intensity
of the
/ Luminescence
luminescence
Finally, taking into account this tunnelling model, the emitted intensity can be expressed as: 1(E)
=
Wr [2a AN(E).WexP [-~--\/2m(Ue
—
Ee)]
(5)
This expression can be approximated by considering that Ee is much lower than Ue:
properties of porous silicon layers
287
intensity is shifted towards lower energies when a increases or when E~decreases. It also appears that the intensity depends exponentially on the term a..,,/E~.On the other hand, the pre-exponential factors can also vary depending on the experimental conditions. For example, the number of emitting crystallites can vary with the passivation of the material, with the excitation wavelength of the photoluminescence, or with the applied voltage for the electroluminescence.
~UeEei(1~)~
with E E5
hv
=
=
4. Emission intensity oxidation
Eg + (1 + C)Ee, and
during
anodic
(6)
Here, we study only the first part of the anodic
where e is the ratio of the hole and of the electron confinement energy: (7)
oxidation where a very large increase in the emitted intensity is observed for both PL and EL signals. Fig. 2 shows that the photoluminescence intensity of 65% porosity layers is increased by more than three orders of magnitude after anodic oxidation. A similar behaviour is found for 70% porosity
(8)
ponds to as samples, an shown exponential in Fig. variation 3. This of regime the intensity correswith exchange Q. The same dependence is alsothefound for thecharge increase in the electroluminescence signal, and Fig. 4 shows that the same type of
=
=
Eg + (1 + e) Ue,
increase
Ei~,/Ee,
which expresses the effective mass ratio. Then ~2a 1(E)
=
N(E) exp
x exp
[ r
—
/~Ii~
+ 1 I 1 2m a h~c+1 ~
El.
If 2E~)>Eo
—
..~i—,
one can write the emitted intensity as follows: 1(E)
AN0Wr =
~_~exp
[2a /~~1 ____ hVe+l]
}
2 xex~[_ ~2 The energy E’[ (E—E’~) 0 is/ obtained by: ~2 2m a E’0 = E0 2~.Jh2(e + 1) —
~7.
(9)
~10
-
~ io~
-
,~ —
10~
-
/Porosity
(10)
10i I
0
Following this schematic phenomenological model one can derive expressions that show how the emitted intensity is expected to depend on the barrier characteristics. One can see that the peak
: 65%
0.6 0.8 Relative exchanged charge Q/Q0 0.2
0.4
1
Fig. 2. Variations in the PL peak intensity as a function of the exchange charge during anodic oxidation of a 2~tm1). 65% porosity sample (anodic current density 0.5 mAcm
288
F. Muller et al. 1
/ Luminescence properties of porous silicon
~ ••••
~
•••
•
/.
10~
5?
• •7,///’
*
•
Porosity
lOi 0
0.2
0.4
70%
I
I
0.6
0.8
1
Relative exchanged charge Q/Q Fig. 3. Variations in the PL peak intensity as a function of the exchange charge during anodic oxidation of a 70% porosity 21.tm sample (anodic current density 0.5 mA cm -
layers
barrier modifications are proportional to the exchange charge Q, which seems quite reasonable. If we recall that the energy barrier is attributed to the silicon narrowed regions that link the crystallites corresponds to much smaller dimensions than itthat of the crystallite. Then, the oxide growth can more significantly affect the sizes of these narrowed regions. The oxidation of the surface of these narrowings will induce an increase in the energy E~, by arising the confinement energy related to these restricted regions. If we suppose that they can be fully oxidized, the energy E~may increase towards the limit of 9eV which is the energy gap of silica. The modification of E~is also supported by the strong simultaneous increase in the photoluminescence decay time [24]. It is interesting to notice heredilute that the chemical oxidation of the porous layer by aqueous HNO 3 solutions leads to the same kind of behaviour [25].
exponential increase is obtained whatever the anodizing current density. This large increase can be related to the improvement of the barrier efficiency towards the nonradiative leaks due to anodic oxidation. From Eq. (9), the emitted intensity is expected to increase exponentially with a..JE5. It is of course difficult to express how this quantity is modified as a function of Q, but the experimental results suggest that the
100
j=15 m&cm2m~ 1 2.5
10
0.5 I
5?
0
200
/
:
5. Spectral shifts resulting from anodic oxidation The second exponential term in Eq. (9) leads us to expect spectral shifts upon barrier modifications. In particular, an increase in E~will induce a spectral blue shift. Such an increase will reduce the barrier transparency to carriers, and thus the car-
rier sensitive escape in the probability, crystallites, resulting inwhich an have the enhancement. Itsmallest is clearenergy that this effect willefficiency be more highest confinement (the ‘blue’ emitting
.
I~ 1.25
5?
Another contribution to the intensity increase which has to be taken into account is the increase in the number of emitting crystallites during electrochemical treatment. A discussion of this effect will be published elsewhere [26].
crystallites), Fig. 5 shows and the willvariations result in a in spectral the PL blue andshift. EL
.
/i~,:~
400 600 800 1000 Exchanged Charge (mC)
1200
Fig. 4. Logarithmic plot of the )~-integratedEL light intensity as a function of the exchange charge upon anodic oxidation at different current densities. Porosity 70%; layer thickness 2 .im.
peak wavelengths as a functionoxidation of the exchange charge of 65% porosityduring layers.electrochemical Quite large reduction in peak wavelength are observed, attesting for the blue shift of the spectra with the oxidation level, It is interesting to notice that very similar variations were obtained for both kinds of emission, suggesting that the same crystallite distribution is involved in both phenomena.
F. Muller ci a!.
/ Luminescence properties of porous silicon
850
layers
289
900 Po
:
•
EL
70
%
~800 800
_
\~orositY:
850
~750
.
5.
..
CS
5. CS
•~g ‘~°~ 5?
PL~N\~~
.~ 5?
•
650
0.2 0.4 0.6 0.8 Relative exchanged charge
• 700
650
11111
0
• • . • PL
750
1
1.2
QIQQ
Fig. 5. Variations in the peak wavelengths of PL and EL during anodic oxidation of a 65% porosity2~sm sample i)~ (anodic current density 0.5 mA cm
According to the confinement model, such blue shifts in the emitted wavelengths should also be explained by a decrease in the size of the emitting crystallites. The first proposed explanation for this effect was to assign these shifts to the thinning of the structure that results from the oxide growth on the crystallite surface. This consumes silicon atoms and reduces the dimensions of the crystalline material [4]. If we assume that the oxide growth is homogeneous over the whole porous layer, we can calculate from the specific surface thestudied mater2/cm3area forofthe ial (in the range 500—600 m porosities) that the total exchange charge Qo at the end of the process only corresponds to an oxidation of about 3 A silicon thickness. Following the hypothesis that the confinement energy varies with the crystallite diameter d as 1/d2, it can be calculated that a spectral shift AE should correspond to a radius reduction Ar as Ar = r.AE/2(E Eg). The spectral shift observed after oxidation at Qo, should correspond to a 7 A size reduction for a 20 A radius crystallite. More accurately, a closer analysis of the experimental results (Fig. 5) shows, for PL and EL, that the effect of oxidation on the spectral shift is much more important in the first part of the process. It can thus be concluded that the observed blue shift of the peak intensity does not result only from the crystallite size reduction due to anodic oxidation; we believe that the in-
~
0
I
0.2 0.4 0.6 0.8 1 Relative exchanged charge Q/Q 0
1.2
Fig. 6. Variations in the peak wavelengths of PL and EL during anodic oxidation density of a 70% 0.5 mA porosity cm - 21.tm sample 1) (anodic current
crease in barrier height (Eq. 10) can contribute to it in a large part. Fig. 6 shows that similar results are obtained on 70% porosity layers. However, in that case, in the first part of the anodic treatment, there is a large difference in the peak wavelengths of PL and EL at a given oxidation level. For example, Fig. 7 gives the PL and EL spectra at the beginning of oxidation, for the exchange charge equal to 0.2Q 0. This feature indicates that different populations of emitting crystallites are involved. All connected and disconnected contribute crystallites to the photoemission whencrystallites only the connected are
‘ ~ ~
Porosity 70%
—
‘~
450
550
650 750 Wavelength (nm)
850
950
Fig. 7. Comparison of the PL and EL spectra ofa 70% porosity sample oxidized at O.2Qo.
290
F. Muller et a!.
/ Luminescence
properties of porous silicon layers
~
850
750,
I~\PorosN\Porosit~:8o%
0
0.2 0.4 0.6 0.8 1 Relative exchanged charge Q/Q
1.2
150
185 Time (s)
220
Fig. 8. Variations in the peak wavelengths of PL and EL during anodic oxidation of a 80% porosity sample (anodic current 2~.sm ~ density 0 5 mA cm -
Fig. 9. Variations in the peak intensity and peak wavelength of the EL during anodic oxidation upon a sudden change of the anodizing current (I and 2 mA cm - 2j.tm - i) Notice that the instantaneous intensity is proportional to the current.
involved in the electroluminescence. At the beginning of the electrochemical oxidation, the anodic polarization is quite low, and acccording to the Lehmann model [7], holes are mostly supplied towards the less confined crystallites (the ‘red’ emitting ones). On further oxidation, the anodic polarization increases, and holes can be also injected into smaller crystallites, leading to a spectral blue shift as well as the barrier enhancement provided by the oxide growth. In the second part of the electrochemical treatment, the PL and EL spectra superpose well: all the connected crystallites are similarly excited for both phenomena, and the contribution of disconnected crystallites to the photoluminescence does not provoke major differences in the spectra. A crystallite qualifies as a ‘connected crystallite’ when a current can flow across it. This depends, for a given voltage, on the confinement and on the oxide coating of the crystalline surface, Fig. 8 shows the spectral shifts recorded for 80% porosity layers. In that case, it is only at the end of the anodic treatment that a good spectral agreement is observed for both types of emission. These differences account for the evolution of the material structure with the porosity: in the 80% porosity layers, the population of disconnected crystallites is likely more important, and then, the PL spectra are almost determined by this kind of crystallites. This feature might also explain why the electrochemical
treatment of 80% porosity layers only increases the PL intensity by a factor of 5 (after exchange of Qo). Indeed, the barrier enhancement can only concern the connected crystallites which seem to be present in much lower proportions than the disconnected ones in these very high porosity layers.
6. Electroluminescence to charge injection
spectral
shifts
related
During electroluminescence, other spectral shifts can be observed that are not related to the barrier modification provided by anodic oxide growth and which can occur without major modification of the material. Fig. 9 shows that when the anodizing current is suddenly changed during electrochemical oxidation, there is an instantaneous and reversible shift of the peak wavelength: if the anodic current is increased from 1 to 2 mA cm -2 a blue shift of about 20 nm is immediately observed, when no noticeable changes in the oxidation level are induced. Changes in the anodic current modify the charge injection conditions: the resulting change in the polarization of the substrate induces modifications in the potential distribution at the electrolyte—crystallite interface and through the energy barrier. The analysis of the potential variations in the structure is quite complicated and is far beyond
F. Muller ci a!.
/ Luminescence properties
the scope of this article. However, we can assume in a first step that an increase in anodic polarization allows the injection of electrons of higher energies, thus involving more confined crystallites and resulting in a blue shift of the emission that is unrelated to any material modification. In the same way, a decrease in the current (associated with a decrease in anodic polarization) results in an instantaneous red shift because the smallest crystallites are no longer addressed. A similar voltage tuneable electroluminescence has also been observed under cathodic bias of a n-type porous layer and is described elsewhere [27]. The important role of the charge injection conditions upon the spectral position of the electroluminescence is also demonstrated by a different experiment. If we consider the EL spectrum of a 70% porosity layer after exchange of a charge Q equal to Qo/2, it is very similar to the corresponding photoluminescence spectrum and shows a peak intensity for a wavelength of 740 nm. If the anodic oxidation is halted at this step, and the anodic oxide is dissolved tn HF the EL spectrum which is recorded when restarting the electrochemical oxidation peaks at 860nm, when the PL spectrum exhibits only a slight red shift of about 20 nm (Fig. 10). The oxide dissolution mostly results in a quite large red shift of the electroluminescence. First, this behaviour confirms that the observed blue shift during anodic oxidation does not result from a thinning effect due to oxide growth because it should be irreversible upon oxide dissolution. On the other hand, it confirms that the oxide growth modifies the population of crystallites that luminesce. At the beginning of anodic oxidation, the current flows more easily through the largest crystallites because of their lower confinement energy: they are oxidized first and then they present a large resistance to electric current because of their oxide coating. On further anodization, the polarization increases and the current also flows through more confined crystallites, resulting in a blue shift of the emission. If at a given step, the oxide is removed by HF etching, the current can flow again through the largest crystallites, and the etching results in an instantaneous large red shift of the electroluminescence, while the photoluminescence is not seriously affected because the HF treatment
of porous silicon layers
HF etching
291
before
after I
—~
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~ ~
/ before
~.
HF etching
..
~
~
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,~‘
after
: -
500
I
I
I
I
600
700
800
900
1000
Wavelength (nm) Fig. 10. Comparison of the PL and EL spectra of a 70% porosity sample oxidized at Qo/2, before and after HF etching.
does not modify the structure of the crystalline material.
7. Conclusions The photoluminescence and the electroluminescence observed during the first part of the electrochemical oxidation of porous layers formed in p-type silicon substrate have been studied in detail. It is shown that both types of emission result from the same origin: the radiative recombination of carriers within crystallites of quantum size, the emission efficiency at room temperature being limited by the nonradiative losses. For the first time, a phenomenological model has been developed on the hypothesis that the nonradiative recombination is governe-id by the escape of carriers from the
292
F. Muller ci a!.
/
Luminescence properties of porous silicon layers
confined regions through a tunnelling mechanism. An expression for the intensity as a function of the emitted energy has been derived, which accounts well for the spectral variations and the intensity enhancement observed during electrochemical OX~ dation of the porous layer. This analysis shows that this behaviour is not the simple result of the crystallite size reduction due to oxide growth as previously suggested, but that it mostly involves the modification of the barrier efficiency and of the surface passivation, and the condition of the charge injection into the crystallites in the case of electroluminescence. We believe that the variations in PL and EL efficiency with anodic oxidation are caused mainly by the increase in the height of the barriers between passivated and unpassivated crystallites. In this paper we only analyze the recombination mechanisms; the next step is to take into account quantitatively the charge injection into the crystallites which is necessary to observe the electroluminescence. This requires a more thorough study of the problems of the charge exchange between the confined crystallites and the electrolytic solution.
Acknowledgement This work was supported in part by a DRET contract (9 1-126).
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