Room temperature photoluminescence from etched silicon surfaces: The effects of chemical pretreatments and gaseous ambients

ROOM T~MPE~T~R~ PH~T~LUMIN~~~~NC~ FROM ETCHED SILKON SURFACES: THE EF.‘FEZCTSOF CHEMICAL PRET~EATMENTS AND GASEOUS AM~IENTS LEIGH T. CANHAM W~e~~to~e Physics Laboratory, King’s Coffege London, Strand, London WCZR 2LS, UK.

Abs&wt-The mom temperature p~o~lumin~cenc~ intenaitty (RTPLI) from etched Si surf&es is shown to be very sensitive to both the chemicai pretreatment and the ~~oo~d~ng ambient. ft can be great& increased by chemical treatmenfs that leave the surf&a high:%hfy n-type or baby p-type, or through effective passivating t~trn~n~ such as thermal o~~dat~oo. The RTPLI from oo~~~~t~, scone Mype surfaces is severely d~rninis~~d by oxygen adsu~t~on and from strongly p-type spaces by water adsorption. Slow RTPtl transients with time coos~uts of the order of minutes are ~~dn~ by changes in either thts intensity of above-band-gap j~~urn~nat~ouor the gaseous ambient. The t~s~nts can exhibit a wide variety of forms and their reversibility is d~e~~n~ by the chemical activity of the j~~~rniua~ susace, with respect to both adsorption from the surrounding ambient and internal structural char~ges. The RTPLI is shown to be sensitive to both positive charging of the surf&e by water vapour and negative charging due to molecular axygen. Totally reversible transients induced by gaseous cycling tmder cf~nstant j~~urninatioo are interpreted using the Stevenson-Keyes expression for surface recom~natioo. ~ey~~o~~~~P~otolum~~e~~uce, silicon suffaces, slow RTPU transients, gasecms ambients, chemical pr& treatments, surface r~~ornb~nation.

Room temperature photolumine~nce m~uremen~ could protide a rapid, nondes~~tive and contactless means of monito~n~ the electronic quality of semiconductor wafers during their pr~e~~n~ in device fab~~~t~on. Studies of the room tern~m~ photoIurn~n~~en~ from direct gap semi~ond~i~to~ [l--10] have illustmted both the potential sensitivity of the technique and the need for a thyroid ~nde~~nd~ng of the various factors that can influence the intensity. It has been reported recently [9], for example, that the integrated l~~j~~~n~ intensity from n-type InP d& rectly reflects the density of interface states in the upper part of the gap. A number of studies have also shown that the PL intensity from both InP and GaAs is extremely sensitive to both surface preparation and the nature ofthe s~~ounding ambient [Z-10]. Similar studies have not previously been conducied on Si surfaces, probably because of the much lower lumin~~n~e efliciency ofindirect gap ~mi~ondu~o~. Neve~hel~s it has been claimed [ f t-14] that the degradation of RTPLI gives a reliable in~~a~o~ of the prest-nce ofcertain te~~noI~~a~l~-im~~nt && deF%ts in Si, namely, t~~~~ donors, swirl defects and oxidation-i~du~ stacking faults. The ir@uence of surface r~omhi~ation on RTPLI from Si therefore needs to be investigated. It has been recognised for some time that the electrophy&al parameters of etched Si surfaces can be markedly affected by the adsorbents arising from certain chemical treatments and gaseous ambients. Buck and MeKim [ 151first demonstrated that variations in the surface Fermi tevelposition of up to 0.7 eV, and

a wide range of sur&e recombination velocities (40lo4 cm/se& can be obtained in this manner. Since radiative combination processes are sensitive to both sutiace electric fields and surface r~ombination [XSf, different ad~r~~~ should exert considerable influence on the RTPLI ~~~ienc~ of similar; ~n~s~vat~ $i s&aces. The ~~~rnen~ rest&s reported here show that this is indeed the case.

The Si samples employed were all disl~~ti~~~f~ee, single-crysta! FYZmaterial, either p-type (0.2 and 20 $2 cm) or near-intrinsic (25 kS1cm) and had surfaces area% of 1 cm2 and thicknesses of 0.5-2.0 mm. A lap~i~~~ etching-polishing sequence (800 mesh C~~~ndu~ water slurry-WF:NNO~ etch-Silica Sol polish) was first used to remove saw damage and produce flat SWfaces. The RCA cleaning sequences based on H$& f 17) were then follows by a final light IWI-IN0~ (1: 101 etch. ~~~~i~~d water of 20 F&Qcm resisrivity was ~rn~~oy~din both the rapid qaench ( 10 swf oft& et&, and the:s~~~ue~t brief rinse f20 se@. The hy-

drophubic su&ces were dried by evaporation in air, or by drawing off excess water with filter paper, Any su~uent chemical treatments were either (a)scx&ing in aqueous WF (48%), (b) staining in aqueous 50: 1 H& NNOJ solution, or (c) boiling in aqueous sodium dichrom@ ( 1S-3). Treatment (a) markedly reduces the oxygen cove~~~~ and increases the fluoridation ofthe surface compared with that I& after HI?HNcl, etching (see section 4. f ).

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L. T. CANHAM

This is known to shift the surface potential in the direction of more positive values [ 181.The very thin surface layer (5- 10 A) promotes strong downward bandbending, giving rise to inversion layers on p-type material for example [19]. Treatment (b) forms much thicker surface films, typically > 1000 A for visible interference colours [20]. Composed of amorphous suboxides, rich in hydrogen and fluorine, the films are very reactive with respect to water vapour, oxygen and illumination [21-221. They too produce strongly ntype surfaces. After treatment (c) the etched surface has a thick adsorbed salt film covering its oxide layer. The high electron affinity of CP+ ions promotes negative charging of the surface [23] and so, in contrast to the previous treatments, there is strong upward band bending, producing accumulation layers on p-type material, for example. For RTPLI studies in various ambients, samples were mounted within a fused quartz tube, through which gases at around atmospheric pressure and of known humidity, were passed at constant flow rates. The “dry” ambients had been passed through liquid Nz cold traps, the “wet” ambients had been bubbled through deionized water at room temperature. For all of the data reported here, room temperature photoluminescence was generated with the 647.1 nm ( 1.9 18 eV) line from a Spectra Physics 165 Kr ion laser, any additional plasma radiation being removed by a narrow band pass interference filter. The signal was analyzed with a Spex 314 m monochromator fitted with a 600 grooves/mm grating, detected with a North Coast EO-8 17 cooled Ge detector, amplified with a cosmic muon pulse suppression unit [24] and displayed on a

chart recorder. The samples were illuminated at 45’ SO that both the focussed image of the luminescence from the Si surface and the reflected laser beam fell on the entrance slit ofthe monochromator. The latter was attenuated by IO4 relative to the luminescence by an infrared transmitting filter. This geometry facilitated optical alignment and minimized the influence of selfabsorption of luminescence when comparing RTPLI values for different surfaces. Quoted excitation densities therefore refer to the average photon flux absorbed within an illuminated elliptical area of 1.73 mm2, for an unfocussed beam (I/$ diameter of 1.25 mm) with reflection losses of 35%.

3. EXPERIMENTAL

OBSERVATIONS

Figure 1 shows typical room temperature photoluminescence spectra from a single sample that has received markedly different surface treatments. In each case the surface was illuminated in ambient air (70% RH) for 50 min, prior to the spectrum being recorded. At the fixed excitation density shown the integrated intensity of the broad band varied by more than 2 orders of magnitude. In contrast, the widely differing surfaces have little effect on the spectral shape of the band, any changes in FWHM being attributable to slightly different levels of self-absorption on the high energy side of the band. There is apparently no evidence to suggest, therefore, that these large changes in RTPLI arise from the growth or decline of specific radiative recombination centres that emit light of similar energies. Such an effect is observed with hydrogenated amorphous silicon, for example, where laser irradiation

bl thermally

oxadized

-

1,2 1.0 1.1 PHOTON ENERGY (eV) Fig. 1. Room temperature photoluminescence from a Si sample given different surface treatments. The 20 Q cm p-type material initially had its front surface covered by a 6000A thick thermal oxide: spectrum (a). This surface was subsequently given a 150 set HF dip, spectrum (c); a 150 set HF:HNO:, (1:lO) etch, spectrum (d); and finally a 150 set boil in NazCr207 soln., spectrum (b). All spectra were taken in air ambient. The RTPLI from the thermally oxidized surface is insensitive to the choice of ambient, whereas the intensity of spectrum (b) would be raised by storage and illumination in dry 02, spectra (c) and (d) by storage and illumination in dry N2 or dry He.

Room temperature photoluminescence generates new radiative centres, emitting at about 0.8 eV [25]. For all the treatments indicated, above-band-gap illumination initiated slow changes in the PL efficiency and, for etched surfaces, more than 30 minutes of continuous illumination were required to “stabilize” the RTPLI. Figure 2 shows typical RTPLI transients observed from a freshly HF:HNOs etched Si surface that had been exposed to air for varying times before the onset of illumination at time t = 0. In all cases a full transient is observed; that is, the PL intensity initially increases before becoming irreversibly degraded to a value lower than that initially recorded. For a given excitation density and period of exposure to air, the precise shape of the transient was found to be sensitive to small variations in the quenching and rinsing operations that terminate the etching process. Nevertheless, a universal trend evident from Fig. 2 is that the magnitude of the degradation decreases with the “age” of the surface, following exposure to air for longer and longer periods. Since a freshly etched Si surface is in a chemically unstable state, it is first important to determine whether laser irradiation is totally or only partly responsible for these observed changes in PL efficiency with time. The sample was thus given the same chemical pretreatment, but then illuminated at a much lower excitation density and for only 2 set every 5 min. In this way the instantaneous RTPLI from the freshly etched surface was monitored during its first hour of exposure to air whilst in the dark for >99% of the time, so that any slow laser-activated effects were negligible. In contrast with the continuously illuminated surface, a monotonic decay of RTPLI was observed. The intensity, initially measured 5 min after the etch quench, fell by 50% after 25 min exposure to air, but by only a further 10%

365

from etched silicon surfaces

during the next 35 min. Slow laser-activated processes are thus totally responsible for the upward part of the transients and significantly enhance the downward trend. It is also important to establish whether the thermal effects of laser irradiation could be influential in determining the RTPLI transients observed, thereby helping to distinguish between thermally activated and photoactivated components. The data of Fig. 3 demonstrate how the temperature of the surface responsible for the RTPLI transients of Fig. 2 can vary with time following the onset of illumination at time t = 0. The surface temperatures indicated correspond to a fixed point sufficiently distant from the illuminated area to avoid direct heating of the thermocouple junction by backscattered light from the etched Si surface. Transients (a)-(c) were obtained when the sample was loosely clamped in a teflon holder to simulate a thermally isolated mode of mounting, where the dominant heat loss mechanism is the convective cooling by the surrounding air, resulting in a fairly uniform temperature rise over the sample surface. In contrast transient (d) was obtained when the sample’s backsurface was held in good thermal contact with a large polished brass heat sink by a thin layer of 50% Ga: 50% In eutectic. This heat sink mode of mounting enabled efficient heat flow via conduction through the sample into the brass. Clearly at the excitation densities employed here the laser-induced temperature rises depend critically upon the way in which the fairly small samples are mounted, but are nonetheless quite small for surfaces in air at room temperature. The maximum temperature rise, A T,,,,, occursat the centre of the illuminated region on the surface, but an accurate direct measurement of this is generally not feasible. Large surface temperature rises at the beam

i-iF:l-lNO-~etched Si surface 6.2 *lo19 photons cmS2 sY1 air ambient at r.t.p. and 70%R.H. previously exposed to air for : (a)5 mins. (bI50 mins. I~)‘00 mins.

0

5

10 15 TIME (minutes)

20

is

30

Fig. 2. RTPLI transients from a freshly etched and previously unilluminated Si surface, after exposure to air for varying times. The 0.2 Q cm ptype material had been given an HF:HNO, (1: 10) etch in each case. (see section 2), but then exposed to the surrounding air in the dark for the different times shown. The PL intensities have been normalized so that their initial values after the first 5 set of illumination all equal 10 units. In reality, these instantaneous RTPLI values are lower the greater the “age” of the etched surface; that is, the initial RTPLI from (a) is greater than that from (b) which is in turn greater than that from (c).

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TIME

(minutes)

Fig. 3. Laser-induced temperature transients from a freshly etched Si sample, for different excitation densities and different types of mounting. The 0.2 Q cm p-type material had been given an HF:HNOs (1:iO) etch and was illuminated in air at room temperature. The temperature of the surface at a distance of 3 mm from the centre of the illuminated area (measured from enlarged photographs of varying exposure) was monitored with an Iron-Constantan thermocouple, and displayed on a chart recorder.

centre may be estimated using Raman scattering [26]; for illumination at low excitation densities and for a sample in good thermal contact with a heat sink, the calculations of Lax [27] provide a simple means of dete~ining temperature rises within the illuminated area from temperature measurements on other parts of the surface. When the product of beamwidth, w, and absorption coefficient, N, is greater than about 25, the laser-induced lateral temperature distribution is not significantly different from that occurring for light absorbed in an infinitesimally thin layer at the surface. For the unfocussed 647 nm Kr line excitation employed in this study, NU’ = 264. Thus from Fig. 6 of Ref. [27] the measured tem~rature rise of 0.2 K [Fig. 3(d)] at R = 3 corresponds to 1/5th of that occurring at the beam centre. Continuous illumination with an unfocussed beam of power 500 mW resulted in a maximum temperature rise of only 1 K, in good agreement with that predicted from the expression 1271,

A T,,,

= P/(2n”‘Kw)

= 1.03K

for a beam power P of 500 mW, a l/r beamwidth of 0.088 cm and a thermal conductivity K of 1.56 W cm-’ Km’ for Si at room temperature [28]. For all the RTPLI transients reported here the type of mounting was generally inte~~iate between the two opposing extremes of Fig. 3 and thus any temperature rises were ~20 K for similar excitation densities. For Si at room temperature, a rise of 20 K produces a < 10% drop in the intrinsic radiative recombination probability [29]. The observed PL transients must therefore be dominated by slow photoactivated processes rather than thermal effects. Figure 4 demonstrates that, all other factors being equal, the relative magnitudes of the upward and

downward parts of the RTPLl transients from etched surfaces can be controlled by the nature of the surrounding gaseous ambient. Once again the precise shapes of the transients are sensitive to small variations in the chemical pretreatment of the surface but overall trends are clear. Whereas the downward part of the transient relies on the presence of either oxygen molecules or water vapour in the gaseous ambient, the upward part is observed, at least to some extent, for all these ambients. Within flowing dry Nz gas only the upward part of the transient is observed, after which the RTPLI is stable. Water vapour alone can induce RTPLI degradation on a freshly etched, illuminated surface, but a dry O2 ambient produces the lowest PL intensities at these excitation densities. The data shown in Fig. 5 confirm that the RTPLI from an etched surface stabilized in dry N2 gas, is lowered on exposure to dry O2 gas. and that the greater the excitation density, the greater the rate of degradation. For a given excitation density, the magnitude of the transient also depends on the etched surface’s previous exposure to oxygen, as demonstrated in Fig. 6. The RTPLI from an etched surface with very low levels of oxygen coverage can fall by a factor of 50 within 10 min illumination in dry 02 gas. The photoactivated processes responsible for the downward parts of the transients of Figs. 2,4, 5 and 6 appear to be dominated by the photoenhan~ed adsorption of oxygen. To investigate the reversibility of these “oxidation”related PL transients, two surfaces of widely differing initial oxygen coverage were exposed to dry Nz-dry O2 cycling under continuous excitation, as shown in Fig. 7. Although surfaces with high affinity towards oxidation are irreversibly degraded, it does appear that freshly etched Si has some active adsorption sites that reversibly accept oxygen. Depending on the surround-

Room temperature photoluminescence

5

10

15

from etched silicon surfaces

20

25

367

30

TIME (minutes) Fig. 4. RTPLI transients from a freshly etched and previously unilluminated Si surface, in gaseous ambients containing different concentrations of Na, Oa, and water vapour. The 0.2 a cm p-type material had been given an HF:HNO, (I: 10) etch in each case,but then stored and illuminated in the various ambients indicated. The PL intensities have been normalized so that their “instantaneous” values all equal 10 units. In each case gas flow rate was 0. I l./min.

ing ambient, either photoenhanced adsorption or desorption of oxygen can occur from a given surface [30]. In contrast, the initial rise in RTPLI observed from previously unilluminated etched surfaces (see Figs. 2 and 4) occurs irrespective of the nature of the gaseous ambient. The laser-on/laser-off cycling data of Fig. 8 reveal a steady decrease in the magnitude of the transient, for a surface experiencing equal periods in the dark and under illumination. However, a significant part of the rise is reversible, and hydration in the dark

almost restores the large transient observed from the previously unilluminated surface. The RTPLI transients of Fig. 9 were taken from an etched surface that had been continuously illuminated in both dry and wet air until the oxidation-related degradation had effectively saturated. The gaseous-cyclinginduced transients observed during continuous illumination are compared with those obtained after periods of gas exchange in the dark, to distinguish between ambient-activated and photoactivated compo-

TIME (minutes) Fig. 5. Oxidation-induced fall in RTPLI from a freshly etched Si surface, for different excitation densities. The 0.2 fi cm p-type material had been given an HF:HNOI (1: 10) etch in each case, but then illuminated at different excitation rates in dry Nz until the RTPLI was stable (see Fig 4) before exposure to dry 4. Gas flow rate was 0.1 l./min and the PL intensities have been normalized so that they are all equal in dry Nz. PCS 47:4-c

368

L. T. CANHAM I ,.

I

I

I

I

N2 ; d;;C+

d;

I

I

I

I

I

I

-

HF:HNOJ etched Si surfaces bl previasly .illuminoted in 02 (tOmirks) lb) previously exposed to O2 ItOmi~~tes) ICI pfwlously unexposed to O2 Ld)Wscaked and prwlously unexposed “02-

a’ I 0

0

I

I

I

k)I (d)

_ I

I

I

I

l-

5

10

TIME (minutes) Fig. 6. Oxidation-induced fall in RTPLI from a freshly etched Si surface, for different initial oxygen coverages. All 4 transients were observed from 0.2 51cm p-type material under continuous illumination at the rate of 2.5 X lOI photons/cm’/sec. Following an HF:HNOs (1: 10) etch, transient (a) was obtained at?er the surface had previously been through 1 dry Nz-dry Oz-dry Nz cycle under illumination; transient (b) the same dry N&y Or cycle in the dark and then dry Nz under illumination; transient (c) dry N2 under illumination alone. Transient (d) was obtained after the same sample had been soaked in aqueous HF for 1 hr, given a 10 set water rinse and rapidly placed in dry N2 where it was illuminated for 50 min. The PL intensities have been normalized so that they are all equal in dry N1, the gas flow rate was 0.1 l./min.

nents. The presence of water vapour in air can clearly produce a reversible increase in the RTPLI from an etched (n-type) surface, but under these excitation rates, the rise in air of saturated humidity is greatly diminished by photodesorption of its adsorbed water layer (see section 4.3.).

I

I

I

I

-idry N2

drymIdryl=

lo-

On a p-type surface however, water adsorption produces the opposite effect, namely a reversible drop in RTPLI, as demonstrated in Fig. 10. The PL intensities at t = 20 min, after gas exchange in the dark, show that for this particular chemical pretreatment, the RTPLI is initially more than 10 times lower in a humid

HF:HNq

etched Si swfoces

b) previcusly exposed to 9 120minutesJ M ;F pked and previasly unexpad

0,

I 0

5

IO

15

XI

25

-

30

TIME (minutes) Fig. 7. RTPLI transients due to oxygen adsorptiondesorption cycling on freshly etched Si surfaces. Both transients were observed from 0.2 Q cm p-type material under continuous illumination at the rate of 6.2 X lOi photon/cm*/sec. Following an HF:HN03 (1: 10) etch, transient (a) was obtained from a surface previously exposed to dry O2 for 20 min in the dark, transient (b) from a surface HF soaked for 1 hr. Both surfaces had been subsequently illuminated in dry N2 until stabilized RTPLI was obtained, before commencing the gaseous cycling shown. In practice the RTPLI of(b) is more than 10 times that of (a) at t = 0, but the intensities have been normalized so that they are equal before illumination in dry 02. The gas flow rate was 0.1 l./min.

Room tem~rature

~hotoiumj~~nce

from etched silicon surfaces

369

He atmosphere than in a dry He atmosphere. Once again, laser irradiation photodesorbs an appreciable fraction of the adsorbed water vapour, resulting in the photoactivated rise in RTPLI to the level reached under continuous ilIumination. The higher the level of excitation, the larger the photoactivated part ofthe transient and the smaller the difference between the FL intensity in dry and wet ambients. Figure 11 emphasizes the opposite effects that water vapour adsorption can have on the RTPLI from a Si surface, depending on the sign of its surface conductivity. The PL efficiency of an n-type surface is raised, whilst for a p-type surface it is lowered. Finally, Fig. 12 demonstrates that for a surface of given conductivity-type, in this case n-type, oxygen and water vapour produce RTPLI transients of opposing sign. For an n-type surface water vapour raises the RTPLI whilst oxygen molecule adsorption lowers it and vice-versa for a p-type surface. 4. DISCUSSION 1 / /

I

:!---I

TIME imk-wtes) Fig. g. Partially reversible RTPLI transients from a freshly etched and previously u~ilIuminat~ Si surface, due to laseron/laser-off qycling. The 20 Q cm p-type material had received a 90 set HF dip and 30 set water rinse prior to storage and illumination in dry Nr. The excitation rate was 5.0 X 10” photons/cmz/sec and the gas flow rate 0.1 Ljmin. The arrows indicate the initial RTPLI values after the first 5 see of illumination, and in this figure, as well as Figs. 9 and 10, have been chosen to represent the beginning of the slow photoactivated transients.

4.1 Etched si s~rf~ce~ At this stage a brief discussion of the structure and composition of the etched Si surfaces under study is warranted, since it illustrates their chemical complexity and indicates the principal species that could be involved in the photoactivated processes observed. Prior to illumination, an HF:HNO~ etched Si surface, rinsed in deionized water and exposed to the surrounding air at atmospheric pressure, is already covered by a thin and heavily con~minat~ oxide layer. Being permeable to water, the oxide film is fairly hydrated so it has the porous colloidal structure typical of silicon hydroxide [3 I]. Its chemical composition and thickness

20

33

TIME (minutes) Fig. 9. Completely reversible RTPLI transients on an n-type Si surface due to water adsorption-photodesorption. The 0.2 fl cm p-type material had been given an HF:HNO, (1: 10) etch and then exposed to dry air/wet air cycling under illumination until the completely reversible transients shown were obtained after 1 hr. Excitation rate was 6.2 X 10” photons/cm$%ee and gas Sow rate, 0. I I./min. The dotted line refers to the PL intensity for the surface continuously excited; the solid lines to the PL transients observed after the excitation had been interrupted during the periods of changing humidity.

370

L. T. CANHAM dry tie

10

a” ,t /

_,

Wet’He

f

drv’He

7

I

0

~-

-4-- --.-- aI

I

33 TIME (minutes)

t Lo

Fig. 10. RTPLI transients on a strongly p-type Si surface due to water adsorption-photodesorption. The 0.2 Q cm ptype material had been treated in hot sodium dichromate solution (see section 2.0). Upon withdrawal the residual adsorbed salt film had been allowed to dry upon the surface in ambient air. After previous exposure to 1dry He-wet He-dry He cycle under illumination, transient (a) then transient (b) were recorded consecutively. The dotted lines refer to the PL intensities for the s&ace continuously excited throughout the gas cycle; the solid lines to the PL transients observed after the excitation had been interrupted during the periods of changing humidity. The gas flow rate of 5.0 I./min ensured that the solid lines represent photoactivated PL intensity changes under the constant humidities indicated. In practice the RTPLI of (a) is more than 10 times that of(b) at t = 0 but the intensities have been normalised so that they are equal before illumination in wet He.

(typicalty lo-250 & depend markedly on many factors such as the strength and purity of the etching soiution, the way in which the etching is terminated, the purity and duration of the water rinse, the degree of exposure to surrounding air, and so on. Nevertheless, provided metallic contamination is minimized, the major contaminants of the oxide coating will be fluorine, carbon and hydrogen. Fluoridation of the Si surface is intimately connected with the etching process itself. Within the etching so-

lution the surface is covered with oxyfluoride groups, chemisorb~ fluorine and physically adsorbed HF molecules. A brief deionized water rinse is effective in removing the weakly adsorbed fluorine but the removal of chemisorbed fluorine by hydrolysis is painfully slow due to the high strength of the B-F bond (5.3 eV). It has been suggested that about 100 hours of rinsing is required before a surface practically free of electrically active fluorine can be obtained 1321. Carbon con~mination can have a number of origins.

._~.__~_~_. i--. ------ws-,---dry He

1 wet He -

dry He

----ii / wet He 1 dry -

He

( wet He 1 dry

He

I

Fig. 11.Completely reversible RTPLI transients from n-type and p-type Si surfaces due to water adsorptiondesorption cycling. Both surfaces were prepared from 25 kQ cm near-intrinsic material. Transient (a) is from a sample stained in aqueous 5O:l HF:HNO, solution for 10 min; transient (b) from a sample boiled in aqueous NazCrzO? for 10 min. Both samples had been given a 30 set water rinse, 5 min exposure to air, and then illuminated during dry He/wet He cycling, until the completely reversible transients shown were obtained after 2 hr. Excitation rate was 6.2 X lOI photons/cm2/sec and rapid changes in humidity were achieved with a flow rate of 5.0 l./min. The PL intensities from the two surfaces have been normalized so that both are equal in a dry He atmosphere.

Room temperature photoluminescence 50_1_I dry N2

I -

IdryjwNZ_(dry /I

I

;:‘I

I

I 1dry 02

n-type surface

TIME (minutes)

Fig. 12. Completely reversible RTPLI transients from an ntype Si surface due to oxygen adsorption-desorption and water adsorption-desorption. The 25 kfi cm near-intrinsic material had been stained in 50: 1 HF:HNO, solution, given a 30 set water rinse, 5 min exposure to air, and then illuminated during dry OJwet N2 cycling, until the completely reversible transients were obtained after 1 hr. Excitation rate was 6.2 X lOI photons/cm2/sec and gas flow rate 5.0 I./min.

Henderson [33] has proposed that carbon is codeposited with the oxide phase during the etching process itself. In addition, covalent organic impurities in the deionized water in trace quantities will be preferentially adsorbed onto the fluoridated hydrophobic surface during rinsing. Even the rinsed surface can act as an effective getter of carbon-bearing species from the air. Chang [34] found that a relatively bare Si surface exposed to ambient air for several minutes showed an increase in surface carbon of 2 X lOI cm-‘. Apart from the hydrocarbon contamination mentioned above, hydrogen will also be present in the form of chemisorbed water, both on the surface (hydroxyl groups and coordination-bound water) and trapped within the pores of the oxide (solvated impurities and so on). Si surfaces soaked in aqueous HF, rinsed in deionized water and briefly exposed to air are covered by much thinner surface layers (typically 5-10 A), and it is an impurity film rather than an oxide film. The oxygen content corresponds to only 2-3 A and even this is probably in the form of oxidized impurities rather than native oxide [35]. The levels of fluoridation and carbon contamination are both higher, but the film is less hydrated.

4.2 Oxygen adsorption on etched Si surfaces The excitation-density-dependent degradation of RTPLI observed in Fig. 5 suggests that laser irradiation significantly enhances the adsorption of oxygen at room temperature. The irreversible downward trend in RTPLI (see Fig. 7) could be associated with the chemisorption of atomic oxygen; that is, incipient oxide formation, whilst a more weakly bound form of adsorbed oxygen is responsible for the reversible component, e.g. ionosorbed 0; molecules.

from etched silicon surfaces

371

The effect of dry oxygen on the surface conductivity of freshly etched but unilluminated Si surfaces was studied by Yamin and Liberman [36]. An HF:HN03 etched surface, after being heated in dry N2. was also subjected to dry 02-dry N2 gaseous cycling at room temperature. Upon exposure to dry 02 a rapid decrease in surface conductance occurred, but which could be partly recovered when the ambient was switched back to dry NS. Restoring the oxygen ambient caused the drop in conductance to resume. Their electricalmeasurements thus showed that a fluoridated Si surface can reversibly adsorb oxygen, just as the optical data of Fig. 7 suggest. Indeed the close similarity between the RTPLI transients observed here and their surface conductivity transient is indicative of a strong correlation between the photosensitivity of Si surfaces and their electrical conductivity. Both high surface conductivity and high RTPLI are the manifestation of a low surface recombination velocity (see section 4.4). The laser-assisted growth of thin oxide layers on Si surfaces is receiving increased attention [37]. A number of studies have been conducted at temperatures in the range 600- 1300K using C02, Ar and Kr lasers with excitation densities of 100 W/cm’, where both the thermal and optically activated effects of irradiation contribute to the accelerated growth. In contrast, at room temperature and with the lower excitation rates employed in this study (~20 W/cm*), photonic enhancement of oxidation would be totally dominant. The high temperature oxidation studies have stressed the importance of photoinduced increases in the broken bond density at the surface [38] and indeed weak UV illumination has been shown [39] to induce important bond rearrangements at room temperature, whereby weakly bonded oxygen species can become incorporated into ultra-thin oxides. Nevertheless, it is clear from the previous discussion (see section 4.1) that for etched surfaces, the role of coadsorbed impurities cannot be ignored. Weak UV illumination of etched surfaces in air for example, greatly enhances their level of carbon contamination [35]. The study of laserirradiated etched Si surfaces with chemography [40] and ellipsometry [4 l] would confirm if the irreversible RTPLI degradation is indeed a consequence of ultrathin oxide growth.

4.3 Water adsorption on etched Si surfaces The transients of Figs. 9- 12 clearly demonstrate that the RTPLI from unpassivated Si surfaces are markedly sensitive to their level of hydration, as well as their level of oxidation. It has been recognized for some time that the adsorption of water vapour can induce significant changes in the electrical properties of Si surfaces [42], and that it generally acts as a surface donor, causing a net injection of electrons into the space charge layer and positively charging the surface. Water is chemisorbed on Si and SiOZ surfaces in the form of both the hydroxylic coating and molecules coordination-bound by Si04 tetrahedra. A fully hydrated Si02 film would be covered with - 10” silanol groups and - lOI coordination-bound water molecules per

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L.T.

CANHAM

cm2 of surface 1431. It is the outwardly disposed hydroxyls that at high humidities (>40% RH) effectively support a continuous film of physically adsorbed water through hydrogen bonding. In ambients saturated with water vapour this polymol~ular film can reach 4-5 monolayers thickness, and the outer water molecules will be quite mobile [42]. The freshly etched Si surfaces under study here however, have a much lower total capacity for water adsorption 141f, their hydrophobicity arising from fluorination, since Si-F groups do not form hydrogen bonds with water molecules and so the physical adsorption of water is restricted to residual surface hydroxyls [44]. Nevertheless, the concentration and chemical activity of coordination-bound water, (H,O),, is raised. Water molecules attached to the Si atom of oxyfluoride groups are more strongly bound, and more protonized than those attached to the Si atom of SiO, tetrahedra 1441. Janstch [45] first proposed that coordination-bound water created a system of slow surface states at the SiSiOz interface and the extensive studies of Kiselev and coworkers [46] have shown that this form of chemisorbed water is largely responsible for the positive charge observed on HF etched Si surfaces. Ambients saturated with water vapour typically induce a positive charge equivalent to 10’2-10’3 holes/cm’ on unilluminated surfaces at room temperature [47], the lifetime of coordination-bound water molecules ( 1o”- 10’ set) being sufficiently long to completely block the most active adsorption sites. Above-band-gap illumination will markedly reduce their lifetime as a result of photocarrier trapping. Kiselev et al. [44] have proposed that photodesorption of (H,O), occurs via a heterolytic dissociation process, induced by capture of a photohole. In Bowing humid ambients the continuous laser irradiation employed in this study will hence provide a continual supply of atomic hydrogen and oxygen to the etched surfaces. These active particles can either interact with the multitude of defects responsible for the fast surface states and recombination centres or recombine with one another to form molecular species that are subsequently desorbed. The chemical instability of the surface with respect to oxidation and hydrogenation will determine which channels dominate. With the surface exhibiting the completely reversible transients of Fig. 11, photodissociation of water must result in minimal chemical transformations of the surface structure, whereas with freshly etched, previously unilluminated surfaces, a sign~~cant fraction of the dissociation products become chemisorbed, thereby irreversibly changing the RTPLI (see Fig. 4). In flowing dry ambients, illumination slowly depletes etched surfaces of their chemisorbed water, thus leading to a gradual decline in the reversible photocharging with laser-on/laser-off cycling (see Fig. 8). 4.4 ~e~ers~b~e changes in the RTPLI ~hem~ca[Iy-treated Si surfaces

from

Etched Si surfaces, with their disordered and contaminated boundary layers, exhibit quasicontinuous

distributions of surface states throughout the bandgap that are of poorly understood origin [48]. All the same, the rate of surface recombination, 5’, can be usefully described by the phenomenological Stevenson-Keyes expression [49, 501 for surfaces with unspecified distributions of traps of type x;

where Q andp, are the equilibrium bulk concentrations of electrons and holes that have thermal velocities V, and v,, and each trap on the surface has an areal density N:, a capture cross-section for electrons, of;, and for holes, a;. The surface densities are denoted by n, and ps, and n$ and p+ are the corresponding values if the surface Fermi level were located at the particular trap’s energy level. The above expression shows that all else being equal, the most effective recombination states are those for which the denominators are the smallest, that is, those whose energy levels lie near the centre of the gap, since then n% - p$. Also, as their ability to capture both electrons and holes diminishes with increasing separation from the Fermi level, minimum surface recombination should occur when the surface Fermi level lies near either band edge. Although the precise dependence of S on surface potential, Y, varies according to the density, energy levels, distribution and carrier capture cross-sections of all the surface states, S(Y) curves for etched Si surfaces [5 l] do generally exhibit bell-shaped forms where suffi~entiy strong band bending can significantly lower surface recombination. The Stevenson-Keyes expression shown above provides a useful basis for interpreting the completely reversible RTPLI transients of Figs. 11 and 12, namely that the observed changes in photoluminescence intensity are due to changes in surface potential, not changes in the density of active recombination centres. Water vapour is effective at pushing the surface Fermi level towards the conduction band edge and consequently fowers surface recombination on a surface that is already strongly n-type, thereby raising the RTPLI. On a strongly ptype surface however, similar positive charging pushes the surface Fermi level position nearer midgap, thereby increasing surface recombination and lowering the RTPLI. In contrast to water, oxygen is known to induce negative charging of etched Si surfaces [52, 531. Thus on a strongly n-type surface it will increase carrier loss by decreasing the surface bandbending, and the RTPLI will be lowered as observed. 5. CONCLUSIONS Laser irradiation of freshly etched Si surfaces results in a full and slow RTPLI transient in ambients containing either water vapour or oxygen. The initial reversible rise in photoluminescence intensity is attributed to the trapping of photohol~ [54] at slow states formed by water already chemisorbed on the surface. This “photocharging” of the already n-type surface pushes the quasi-Fermi level positions at the surface

Room temperature photoluminescence nearer the conduction band edge, thereby lowering surface recombination and raising the RTPLI. This changing surface potential effect, is however, competing with an irreversible fall in PL efficiency due to a photoenhanced oxidation of the surface, which produces new recombination-active surface states, as well as negative charging of the surface due to ionosorbed oxygen. The nature of the surrounding ambient, the excitation density, and the level of hydration and oxidation of the surface prior to illumination all influence which of the two competing processes dominate. It is of interest to note that for cleaved GaAs surfaces, laser irradiation in ambient air also produces full RTPLI transients, but that these have been assigned to a photon-induced oxygen desorption-adsorption process [8]. Unpassivated, etched Si surfaces invariably contain a large density of surface states, but a significant improvement in their room temperature PL efficiency can be obtained by specific chemical treatments and ambient storage that preserve high surface conductivity [ 151. The RTPLI from highly n-type and highly ptype surfaces is sensitive to ambient-induced reversible shifting of the surface potential, the direction of the effect being that predicted by the Stevenson-Keyes model [49] for either positive charging with a surface donor (water) or negative charging with a surface acceptor (oxygen). For chemically treated Si surfaces under these excitation rates, changes in surface recombination have the major influence on RTPLI, rather than the “dead layers” that can arise from surface electric fields [55]. The magnitude of the observed changes in PL efficiency with changes in the state of the Si surface, indicate that strict control over the latter is essential if RTPLI degradation does indeed give a reliable indication of bulk defects such as thermal donors, as has been reported [ 111. Acknowledgements-The

author is indebted to E. C. Lightowlers for many helpful comments and to Mullard Ltd, Southampton for providing thermally oxidized material. This work was supported by the S.E.R.C.

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