Formation of luminescent chemically etched porous silicon

Solid State Communications, Vol. 96, No. 8, pp. 579-581, 1995 Elsevier Science Ltd Printed in Great Britain

Pergamon

0038-1098/95

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003%1098(95)00450-5 FORMATION

OF LUMINESCENT

CHEMICALLY

ETCHED

POROUS SILICON

G. Di Francis ENEA-CRIF loc.Granatello, I-80055 Portici, Italy P. Maddalena and D. Ninno Dipartimento di Scienze Fisiche, Universitii di Napoli “Federico II” Pad. 20, Mostra d’oltremare, 80125, Napoli, Italy (Received 3 April 1995; accepted

14 June

1995 by G, Bastard)

We report a systematic study on the formation of photoluminescent chemically etched norous silicon. Samoles have been fabricated in HF/I-iNO? based solution’s for various etching’times. The growth of the luminescent l
The sample porosity E has been determined by the gravimetric method, under carefully controlled conditions of room temperature and relative humidity, measuring ml. the sample mass, m2. the mass after the etching, and mg, the mass after the PS layer has been completely removed by a 1N NaOH solution:

Following the discovery that room temperature light emission from the infrared to the ultraviolet region can be induced by electrochemically etching nanometer scale structures onto silicon wafer to form Porous Silicon1-2, a great deal of work has been world-wide dedicated to the comprehension of the phenomenon. Recently several’ authors3-6 have reported on the fabrication of luminescent Porous Silicon obtained bv the simple chemical dissolution in HF/HN03 based solutions. The etching reaction yielding the formation of chemically etched porous silicon (CEPS) has been already satisfactorily explained as a localized electrochemical process in which anode and cathode randomly switch from one site to another of the silicon surface7-8. This material is similar in structure to porous films fabricated electrochemically so that its characterization could be of importance to give information about the fundamental mechanism governing the light emission process5. In this study, the formation of CEPS for increasing etching times is followed. The films porosity, their thickness and the photoluminescence spectra at room temperature (PL) have been measured and correlated to the fabrication parameter. A mechanism for the porous layer formation is proposed. In order to produce CEPS, the starting n-type, 0.01 Qcm, , 2” Silicon wafers have been etched using a HlVHN03 solution (reagents were all electronic-grade 49% HF and 70% HN03). To better follow the porous film formation a HN03 concentration in HF of only O.Olmoles/liter has been used. The etch solution has been activated as described elsewhere 3, waiting for 2 min. Wafers have been then etched in room light, by immersion in the solution, for times ranging from 10 s to 960 s. After the etching all the wafers have been rinsed in deionized water and dryed using a Nitrogen flow. Under the light of a Wood lamp ail the samples appeared quite homogeneous and reddish luminescent.

E= (ml-m2)/(mt-m3)

(1)

The porosity values have an accuracy better than 2 % except for the first four samples corresponding to t = 10 s, t = 20 s, t= 30 s and t =45 s where, due to the low etched mass, samples accuracy is 30%, 25%, 15% and 5% respectively. In order to measure the porous layer thickness xt, a second series of samples has been fabricated in the same etching condition as above, starting with similar silicon substrates, after masking some parts of the wafer with photoresist. Thickness measurements of the crater left by the porous layer removal in the NaOH solution, have been then performed using a mechanical profiler. Resolution was better than 0.2 nm. The photoluminescence spectra have been recorded at 300 K, using as excitation source the light of a 250W Mercury lamp filtered by a 10 nm bandpass interference filter, centered at 400 nm. The collection spot size was in the order of 1 cm in diameter. The emission, filtered by means of a set of 10 nm bandpass interference filters ranging from 550 nm to 900 nm with step of 50 nm, was detector. Integrated measured by a silicon photoluminescence has been measured by filtering the light emitted at all the wavelengths by means of a long pass filter, with a cut-off at 550 nm. PL data, corrected for the apparatus response, have an accuracy of better than 1%. In Fig. 1 samples thickness and porosity are reported as a function of the etching duration. Let us first of all observe that the rate, r of film formation (rdx@t) is remarkably different for times greater (region II) or lower (region I) than t* (t*= 450 s). In region I, r=10.5 h/s to decrease 579

580

LWMINJZSCENT CHEMICALLY ETCHED POROUS SILICON

OY

0

I

I

I

I

200

400

600

800

I

to&

t, Etching time (s)

Fig. 1. The variation of porosity, E, and film thickness, xt, of n-type, 0.01 Rem, , Silicon samples in a O.Olmoles/Iiter solution of HN03 in I-IF, vs. the reaction time. The rate of film formation is remarkably different for times greater (region II) or lower (region I) than t* (t*- 4.50 s). Solid lines are not best fit.

more than ten times in region II (rs0.95 A/s). For electrochemically etched porous silicon (ECPS) it has been suggested that pore initiation is modified by the presence of defects on the silicon surface which are otherwise well known to increase the dissolution rate also in simple chemical etching9-11. If we assume that surface defects play the same role also in the case of CEPS then the starting enhanced dissolution at surface defects can readily explain the higher dissolution rate in region I. For time increasing in region 1, porosity too, rapidly increases with the time attaining a value of about 75% for a time nearly equal to t*, that is when the etching rate changes. However, while in region II xt further slowly increases with the time, E remains almost constant, indicating a saturation effect of the porosity with the etching duration.

--o-.-.x-. ..f..

Vol. 96. No.

In Fig.2 we report the samples PL data together with their best fit in the energy range from 1.4 to 2.4 eV, with the etching time t as the family parameter. Curve fits have been obtained using a Gaussian lineshape. According to several authors this kind of simmetrical lineshape reflects a gaussian distribution of silicon nanocrystallites resulting from the etching process12-13. PL spectra for samples corresponding to etching times less than 45 s have not been reported as the signal was too weak and almost completely covered by noise. It is worthwhile to observe that the PL energy peak, E,, has the same value, within the experimental error, for all the samples (Ep=1.85 eV.). If we assume that the quantum confined model for the Iight emission is still true for CEPS we are then forced to admit that, for all the samples, PL spectra are due to similar distribution of classes of nanometer in size structuresl4-15 centered at the same energy peak Ep, although with different relative weight of a given class from one sample to another. In Fig.3 the integrated PL intensity, IPL, is reported vs. the etching time. For the sake of clarity samples thickness are shown again. Also in this case we can recognize a different behaviour in region I and II. In region I a sharp increase of IpL vs.t is observed until t* is reached: fort >t*, IPL only slightly increases with the time with a slope remarkably simihar to that of xt. Our findings can be explained if we assume that once initiated, the preferential etching at some anodic site on the silicon surface stops only when a critical average pore width, We, is reached. For time increasing in region I, both the etching rate and the porosity rapidly increase with the increase of pore numbers and average width. As far as the critical pore width We is reached all over the surface, porosity attains its saturation value indicating that neither pores number nor their widths further increase. On the contrary a slow increase in the film thickness with the time is still observable in region II connected to the silicon

dissolution normally to the surface and then to an increase of porous layer thickness. The existence of a critical width We completely explains the PL features reported in Fig. 2 and 3. Due to our assumption,

emission

arises from quantum nanostrucures

t=96os t=720s t=4sos t=JM)s

t=18os -.&. t=5Jos --*- t=45s

Photon energy (eV)

t. Etching time (s)

Fig.2. Photoluminescence data together with their best fit in the energy range from 1.4 to 2.4 eV, with the etching time, t, as the family parameter. Curve fits have been obtained using Gaussian statistics. While data are not calibrated in an absolute sense, the relative heights of the spectra are correct.

Fig.3. Integrated PL intensity, IpL, and film thickness, xt, vs. the etching duration. In region I a sharp increase of IPL vs.t is observed until t* is reached: for t It*, IPL only slightly increases with the time with a slope remarkably similar to that of xt.

Vol. 96, No. 8

LUMINESCENT

CHEMICALLY

strictly determined by Wc thus originating Ep values almost eoual for all the samoles. As far as the PL intensitv is concerned, it will increase with the pores number ih region I until the saturation value is reached. The further slow 1pL increase in region II is almost completely due to an increase of the emitting material connected to the further slow dissolution rate in region II. We have at present no direct evidence of Wc. It is however interesting to observe that the saturation value of the porosity, observed in fig.1, is nearly equal to the percolation porosity value estimated by M. Woos and coworkers16. If this were the case, pore percolation, inhibiting any further chemical dissolution, would define the critical pore width. In conclusion, we have studied the formation mechanism of luminescent chemically etched porous silicon

ETCHED POROUS SILICON by correlating measurements of porosity, thickness and integrated photoluminescence to the etching duration. We have shown that a limiting value for the porosity exists. Moreover, while the integrated PL intensity monotonically increases with the time, PL spectra are similar in shape for all the samples at the different etching times. Our data are explained assuming the existence of a critical pore width, W,: once the anodic dissolution initiates in some site of the sillcon surface, the preferential etching will then stop only when W, is reached. For pores of width WC covering the whole surface, the etching rate will tend to decrease and the porosity to saturate. The PL features are all determined by the remaining silicon network defined by Wc: thus the PL energy peak is almost equal for all the samples while the integrated photoluminescence increases with the increase in the number and volume of the emitting centers.

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