Simultaneous characterization of bulk impurities and interface states by photocurrent measurements

Applied Surface Science 235 (2004) 340–350

Simultaneous characterization of bulk impurities and interface states by photocurrent measurements M.L. Polignanoa,*, A.P. Caricatob a

b

ST Microelectronics, Via Olivetti 2, 20041 Agrate Brianza, Milan, Italy Department of Physics, INFM and University of Lecce, Lecce 73100, Italy Available online 28 July 2004

Abstract A new method for evaluating both surface recombination velocity and bulk minority carrier lifetime by photocurrent measurements is discussed and validated by comparison with capacitance–voltage measurements of interface state density. This method is an evolution of the measurement of surface recombination velocity by the Elymat technique, it does not require the oxide to be etched off and consists in measurements of surface recombination velocity under an applied surface bias. The application of a surface bias allows the control of the interface potential and the identification of the suitable interface condition so that surface recombination velocity can be considered as a measurement of interface state density. In addition, it is shown that surface recombination velocity is suppressed when the surface is under accumulation conditions, so the application of a surface bias provides the possibility of a surface passivation by driving the surface into accumulation. This passivation by surface polarization is about as effective as the chemical passivation by HF. Finally, the dependence of surface recombination velocity on the injection level is shown to be reversed when the interface changes from depletion to accumulation or inversion conditions. This technique does not require the formation of a capacitor structure, so it is suitable for the measurement of as-grown interface properties. For this reason, this technique was chosen for a systematic study of the nitridation process of oxide films. Surface recombination velocity was correlated with nitrogen concentration at the oxide–silicon interface. # 2004 Elsevier B.V. All rights reserved. Keywords: 73.20.
r; 73.20.Gr Keywords: Oxide–silicon interface; Interface states; Surface recombination velocity; Nitridation

1. Introduction Measurements of minority carrier recombination lifetime are affected by volume and surface recombination, so in principle these techniques can be used for the simultaneous measurement of bulk and surface properties, provided volume and surface contributions *

Corresponding author. Tel.: þ39-039-6035593; fax: þ39-039-6035358. E-mail address: [email protected] (M.L. Polignano).

are resolved. For what concerns volume properties, photocurrent measurements have been commonly used for a few years in the Elymat technique (electrolytic metal tracer) [1]. Recently, it was shown that this technique can be extended to the measurement of surface recombination velocity, in addition to recombination lifetime. A new, non-destructive method for evaluating both surface recombination velocity and minority carrier lifetime by photocurrent measurements was proposed and validated by comparison with capacitance–voltage

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.106

M.L. Polignano, A.P. Caricato / Applied Surface Science 235 (2004) 340–350

measurements of interface state density [2]. This method consists of measurements of surface recombination velocity under an applied surface bias. The application of a surface bias allows the control of the interface potential and the identification of the suitable interface condition so that the surface recombination velocity can be considered as a measurement of interface state density. This method allows the measurement of the oxide– silicon interface properties to be obtained without the formation of a capacitor structure. It is known [3] that the capacitor process can alter interface properties, so a method that does not require further processing after dielectric growth is suitable for the study of as-grown interfaces. For this reason in this work we use the photocurrent-based measurement of surface recombination velocity in the characterization of new technologies for dielectric growth, e.g. the rapid thermal oxidation (RTO) and nitridation (RTN) of the silicon surface. Surface recombination velocity (hence surface state density) is shown to reduce as nitridation proceeds, so surface recombination velocity data give information about interface nitridation. However this information is indirect, and for this reason we compare surface recombination velocity data with nitrogen concentration at the oxide–silicon interface obtained by SIMS measurements.

2. Surface recombination velocity versus interface state density

341

kB the Boltzmann constant and T the absolute temperature. Considering for instance p-type silicon, surface recombination velocity is then obtained as Us ¼ sdns, where dns is the electron excess at the surface. In order to understand the dependence to be expected for surface recombination velocity versus surface potential, let us consider low level injection conditions, i.e. dns/p0 ! 1 (where p0 is the equilibrium majority carrier–hole concentration), and a surface under depletion or inversion conditions. In the presence of a depletion region an effective surface recombination velocity seff at the edge of the space-charge region is defined [4,6], Us ¼ seffdn(xd), where xd is the depletion region edge. In addition, under low injection conditions it is commonly assumed that the electron and hole quasi-Fermi levels are approximately flat across the space-charge region at the surface, so the pn product is constant through the space-charge region, nsps ¼ n(xd)p(xd) ’ (n0 þ dn(xd))p0, as under low injection conditions the majority carrier concentration is not significantly varied. So, under these hypotheses: Z Ec s ¼ vth Ev



Dit ðET ÞdET ðns =p0 þ ni =p0 expðET
Ei Þ=kB TÞ=sp þðps =p0 þni =p0 exp
ðET
Ei Þ=kB TÞ=sn (2)

Surface recombination rate Us can be written in terms of interface state density according to Shockley– Read–Hall theory by analogy with bulk recombination [4–6]: Z Ec ðps ns
n2i ÞDit ðET ÞdET Us ¼ vth Ev ðns þ ni expðET
Ei Þ=kB TÞ=sP þ ðps þ ni exp
ðET
Ei Þ=kB TÞ=sn (1) where vth is the carrier thermal velocity, ns and ps the electron and hole concentrations at the surface, ni the intrinsic carrier concentration, Ei the intrinsic Fermi level, Dit(ET) the density of interface traps at the energy ET, ET the trap position in the energy gap, sn and sp the electron and hole capture cross sections,

This equation shows that surface recombination velocity is expected to decrease when either majority or minority carrier concentrations are increased with respect to depletion conditions, i.e. when approaching inversion or flat-band conditions. Generally speaking, in order to study the dependence of surface recombination velocity on surface potential as a function of the injection level a numerical solution of the Poisson, continuity, and driftdiffusion equation system is required. Recently these theoretical calculations were carried out [7] and confirmed the qualitative analysis based on Eq. (2), i.e. surface recombination velocity is predicted to reach its maximum value smax under depletion conditions, approximately when ns ¼ ps and to decrease when moving to accumulation or inversion conditions. When no carriers are injected, the equality between

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electron and hole surface concentrations implies that both quantities are equal to the intrinsic carrier concentration, i.e. ns,dark ¼ ps,dark ¼ ni, where ns,dark and ps,dark are the surface carrier concentrations in the dark. Under low injection conditions, injected carriers must be much less than the equilibrium majority carriers, but we can assume that the injected carrier concentration is large with respect to the intrinsic carrier concentration. So under low injection conditions and bias leading to band bending for which ns ¼ ps the surface carrier concentrations can be written in the form ns ¼ ni þ dns  dns. Therefore, provided measurements are carried out at the same injection level and Dit(ET) can be replaced by its average value over the energy gap hDiti, according to this rough analysis we expect that smax increases in proportion to hDiti. In the following sections surface recombination velocity is obtained from photocurrent measurements and experimentally studied as a function of applied surface bias. It is shown that the so-obtained surface recombination velocity data follow calculation predictions and are correctly related to the measured interface state density.

3. Experimental techniques 3.1. Surface recombination velocity from photocurrent measurements In the Elymat technique [1] carriers are injected by a laser beam at the wafer frontside and are collected in the space-charge region of a Schottky contact located at the wafer backside or at the wafer pffiffiffiffiffiffi frontside. Carrier diffusion length Ldiff ( Ldiff ¼ Dt, where D is the carrier diffusion coefficient and t is the carrier lifetime) is extracted from backside photocurrent (IBPC) measurements. Frontside photocurrent (IFPC) measurements are also possible, but they will not be used in this work for measuring carrier diffusion length. The frontside photocurrent is used here as a direct measurement of the total injected current (this identification is correct if the diffusion length is large enough). In this work a laser with a 820 nm wavelength (penetration depth 1/a ¼ 14 mm, where a is the absorption coefficient) was used. The Schottky contact for charge collection is provided by a suitable solution,

usually diluted HF, which also has the purpose of suppressing surface recombination [8] (see Fig. 1(a)). In the limit of a negligible surface recombination velocity, the collected photocurrent depends on bulk recombination only. Vice versa, if surface recombination is not suppressed by the HF solution, but determined by an oxide–silicon interface, the collected photocurrent IBPC depends both on bulk lifetime and on surface recombination velocity s. Therefore, a modification of the Elymat bulk technique allows surface recombination at an oxide–silicon interface to be evaluated from subsequent photocurrent measurements. 3.1.1. Standard method Photocurrent is measured with the frontside surface of the sample covered by an oxide. Obviously, in this measurement the wafer frontside is not immersed in the HF solution (see Fig. 1(b)) [9–11]. Under this condition, from the solution of the diffusion equation the backside photocurrent is IBPC ¼ I0

1 þ s=ðaDÞ cos hðd=Ldiff Þ þ sLdiff =D sin hðd=Ldiff Þ

(3)

where d is the sample thickness and I0 is the injected current when wafer frontside is not immersed in the HF solution. After this measurement, the oxide is etched off and the measurement is repeated with HF passivation, which is assumed to yield a surface with negligible surface recombination velocity. Under this hypothesis, Eq. (3) reduces to IBPC ¼ I0

1 cos hðd=Ldiff Þ

(4)

where I0 is the injected current when wafer frontside is immersed in HF, and is obtained by a measurement of IFPC. This is the Elymat bulk measurement which is generally used to evaluate carrier diffusion length. From Eq. (3) and (4) surface recombination velocity at the original oxide–silicon interface can also be calculated. For a correct evaluation of the surface recombination, it must be taken into account that the total injected current can be different if the wafer surface is covered by an aqueous solution (such as the HF solution, injected current I0) with respect to the case when there is no solution at wafer frontside (injected current I0 ). This difference can be due for instance to

M.L. Polignano, A.P. Caricato / Applied Surface Science 235 (2004) 340–350

343

backside photocurrent was measured with the front ). side surface exposed to air (measurement of IBPC Then, samples were immersed in the HF solution and IBPC was measured immediately, before the oxide could be etched by the HF solution. In so doing the injected current was changed from I0 to I0, but s remained unchanged, since it was still determined by the original silicon–oxide interface. So, according to Eq. (3) the ratio of these measurements yields the ratio Z ¼ I0 =I0 . By this method we obtained Z ¼ 0:9. 3.1.2. Modified method The standard Elymat measurement of surface recombination velocity does not allow surface potential to be controlled. In the proposed method, the wafer surface is immersed in a solution that does not etch the oxide, for instance an acetic acid solution. The electrode in the frontside chamber (generally used for FPC measurements) is now used for biasing the wafer surface, thus moving the oxide–silicon interface from accumulation to inversion conditions (see Fig. 1(c)). The backside photocurrent is collected as usual by a HF solution at the wafer backside. Bulk and surface contributions can be discriminated as in the standard method by eventually etching the oxide off and passivating the surface with the HF solution. However, under suitable bias conditions surface recombination velocity is strongly reduced, thus providing the possibility of a passivation by surface bias that does not require oxide etching. 3.2. Measurements of interface state density

Fig. 1. (a) Method usually employed for bulk lifetime measurements. (b) Method generally used for surface recombination velocity measurements (no surface bias is possible). (c) Proposed method for measurements of surface recombination velocity with surface bias.

the different reflection properties of the wafer surface/ solution and wafer surface/air interfaces, or to light absorption by the solution. A specific experiment was set up in order to accurately measure the ratio I0 =I0 ˚ ) were grown, [9,11]. Relatively thick oxides (200 A the oxide at wafer backside was stripped and the

Interface state density Dit was measured on most samples by comparing the low and high frequency C– V characteristics [12] of the capacitor under investigation. The method is based on the fact that time constants for trapping and detrapping of carriers at interface states are usually long enough so that these mechanisms do not take place when measurement frequency is higher than a few hundred kHz. At high frequency and in depletion condition the MOS capacitance is thus given only by the oxide capacitance COX in series with the depletion layer capacitance Cd. At low frequency, trapping and detrapping of carriers at interface states give rise to another capacitance term in parallel with that of the depletion layer. The capacitance Cit associated with interface states is

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simply proportional to their density, Cit ¼ qDit , where q is the positive unity charge. By comparing the low frequency Clf and high frequency Chf capacitance, the term Cit can be deduced, once the capacitance term is known. The relationship between gate voltage and band bending—and thus interface state position in the energy gap—can be determined by means of the Berglund technique [12]. High frequency C–V curves were measured at 500 kHz while for the low frequency the quasi static method was used [12]. 3.3. Nitrogen profiling by SIMS SIMS depth profiles were carried out by a magnetic sector mass spectrometer (CAMECA 4f) equipped with O2 and Cs beams. A Csþ primary beam with an impact energy of 2.25 keV and an incidence angle of 608 were used. MCsþ secondary ion species were monitored to reduce matrix effects [13]. In a previous work it was shown that the nitrogen profile shape depends on the primary beam impact energy [14]. In particular, by reducing the impact energy of the primary beam, the nitrogen profile shape becomes shallower, and the peak position shifts towards the SiO2/Si interface. Nevertheless, the ratio between the nitrogen peak concentrations in different samples is not changed, indicating that the nitrogen profile peak is a reliable parameter for comparing different nitridation processes. As a consequence, we consider the quantified maximum in the nitrogen profile as representative of the effective nitridation at the SiO2/Si interface. Nitrogen relative sensitivity factor (RSF) in silicon dioxide and silicon was measured by using nitrogen-implanted samples. To obtain a better quantification of nitrogen in the SiO2/Si interface region, the RSF was applied as a function of the oxygen signal variation, as reported elsewhere [14]. Under our analytical conditions the depth resolution was limited to about 2.6 nm.

4. Sample preparation 4.1. Validation of the surface recombination velocity measurement method P-type, 10 O cm resistivity wafers were used for surface recombination velocity measurements. If a

good sensitivity is required for surface recombination, bulk recombination must obviously be kept as low as possible. Therefore, wafers with a high lifetime (low contamination level and low level of potential oxygen precipitates) must be used. For this reason, magnetic Czochralski (MCZ) material was chosen for measuring surface recombination velocity. The same wafers were also used for capacitor tests. In addition, capacitors were also built on n-type wafers (Czochralski wafers with 8 O cm resistivity). Wafers for surface recombination velocity and for capacitor tests were simultaneously oxidized by three different oxidation cycles: A dry, high temperature annealed oxide: 950 8C, dry O2, with a 1000 8C annealing in N2O. Oxide ˚. thickness was 115 A A wet, high temperature annealed oxide: 770 8C, wet O2/H2, with a 1000 8C annealing in N2O. ˚. Oxide thickness was 115 A A wet, low temperature annealed oxide: 770 8C, wet O2/H2, with a 900 8C annealing in N2O. Oxide ˚. thickness was 110 A These cycles are expected to yield oxides with rather different surface state density, because of the different oxidation environments and annealing temperatures. In a previous experiment [11], it was shown that an aluminum layer deposited on top of the oxide produced an effective interface state reduction even if no specific annealing was carried out, possibly during the aluminum deposition at 150 8C. It has long been known [15] that a silicon nitride layer screens the SiO2/Si interface against the aluminum effect of interface state reduction by preventing the diffusion of ˚) hydrogen to the interface. For this reason a thin (80 A nitride layer was deposited after the oxidation on some samples from each oxidation cycle. An Al:Si layer was then deposited on the dielectric layer (whether SiO2/ Si3N4 or SiO2 only) both on samples for C–V measurements and on samples for surface recombination velocity measurements. This was done with the purpose that the reducing effect of the aluminum layer may affect both sets of samples. Finally the metal layer was defined and etched for producing capacitors, while it was removed from the whole surface of wafers for surface recombination velocity measurements. In capacitor tests, the contact to the substrate was obtained by backside lapping and gold deposition.

M.L. Polignano, A.P. Caricato / Applied Surface Science 235 (2004) 340–350

4.2. Study of the nitridation process

5. Experimental results 5.1. Validation of the surface recombination velocity measurement method Fig. 2 shows surface recombination velocity versus bias applied at the surface in oxides obtained by three different oxidation cycles. It is immediately observed that these data of surface recombination velocity behave as expected from numerical calculations based on the SRH theory [7], i.e. data of surface recombination velocity versus applied voltage show a maximum (at a voltage Vs,max), which according to the theory corresponds to the condition ns ¼ ps . As data in Fig. 2 refer to a p-type substrate, when the applied voltage Vs is decreased below Vs,max the interface condition is

4

10

770 C wet + 900 C N2O + Si3N4 3

10 s (cm/sec)

P-type (1 0 0) Czochralski grown, 2 O cm resistivity wafers were used in this study. Oxides were grown by a rapid thermal oxidation and nitridation (RTO/ RTN) system (AMAT Centura). Both dry and wet oxides were annealed (nitrided) in N2, N2O and NO atmospheres. Unannealed oxides were not considered because of the lack of a technological interest. Wet atmospheres were obtained by in situ steam generation (ISSG). Oxides with thickness in the range 3–7 nm, annealing temperatures in the range 1000–1100 8C and annealing durations in the range 30–90 s were used. Finally, various technologies used to obtain thin nitrided oxide layers were compared for what concerns the interface properties of the oxide. RTO/RTN oxides, oxides grown and nitrided in a conventional furnace and high temperature deposited (HTO) nitrided oxide layers were included in this comparison. The conventional furnace oxides were grown in a diluted wet or a fully wet environment and nitrided in N2O or NO environment at temperatures in the range 850–1000 8C. The final oxide thickness was in the ˚ to 120 A ˚. range 80 A ˚ HTO layers were deposited by chemical 150 A vapor deposition with SiH2Cl2/N2O ratios from 1 to 3, or by a SiH4/N2O mixture at two different temperatures. Sample were then nitrided in NO atmosphere.

345

770 C wet + 1000 C N2O + Si3N4 2

10

950 C dry O2 + 1000 C N2O + Si3N4 1

10

-0.5

0.0

0.5

1.0

1.5

Vs (V) Fig. 2. Measured surface recombination velocity vs. surface potential.

changed from depletion to flat-band and then to accumulation conditions. Under accumulation condition, surface recombination velocity is reduced because minority carriers are swept away from the surface by the electric field [6]. When moving to Vs larger than Vs,max surface changes from depletion to inversion conditions, and surface recombination velocity decreases again because under this condition minority carrier concentration is very large and trap saturation occurs [7]. According to Eq. (2), surface recombination velocity would be expected to continuosly decrease when the surface is accumulated, because of the large increase in ps value. Vice versa Fig. 2 shows some saturation (though to very low levels) of surface recombination velocity in accumulation. As data in Fig. 2 are average values over wafer surface, it is possible that this saturation value is actually due to just a few points on the wafer where the surface is not accumulated yet. Besides, it must not be forgotten that Eq. (2) was deduced by assuming depletion conditions, and should not be used in accumulation. Fig. 3 reports the maximum s(Vs) values versus the average interface state density in p-type wafers as measured by C–V curves. From these data, smax

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M.L. Polignano, A.P. Caricato / Applied Surface Science 235 (2004) 340–350

10

4

0.6

Vmax (V)

smax (cm/sec)

0.5

0.4

0.3 0.2

10

3

Slope=1

0.1

0.0

-5

-4

-3

-2

-1

0 11

1 -2

Fixed oxide charge (10 cm ) 11

10 -2

12

-1

Average Dit (cm eV ) Fig. 3. Surface recombination velocity vs. average interface state density obtained from C–Vmeasurements.

increases in proportion to hDiti, just as expected according to the previous discussion. This result validates the measurement of surface recombination velocity for mapping interface state density. It is worth noting that by choosing a suitable voltage in Fig. 2, the dependence of s on Dit could be varied in a wide range from less than linear to more than linear. In addition, in the standard method of measuring surface recombination velocity surface potential is not controlled, so charges in the oxide or at the oxide surface may affect the surface recombination velocity data. On the contrary, the present method allows us to select the measurement condition where surface recombination velocity is linearly related to interface state density, whatever charges may alter the potential in the unbiased sample. On the other hand, the position of the peak along the Vs axis depends on charges in the oxide or at the interface. As an example, Fig. 4 reports the peak voltage Vs,max versus oxide charge measured from C–V curves for the oxide–nitride stack. As expected, the shift of the peak in the s(Vs) curve results to be related to the presence and the amount of such charges.

Fig. 4. Peak voltage in s(Vs) curves vs. oxide charge obtained from C–V measurements for the oxide–nitride stack.

accumulated. This fact suggests that an accumulating applied bias can be used for a passivation of the surface, thus allowing both surface recombination velocity and bulk lifetime to be evaluated without etching the oxide. Fig. 5 compares surface recombination velocity data obtained by passivating the surface by polarization and by chemical passivation (i.e. by oxide etching and HF passivation). These data are in good agreement, indicating that the passivation by surface polarization provides an efficient method for 10

s (cm/s) (passivation by bias)

10

10

5

slope=1

4

sdestructive=spassivation by bias 10

3 3

10

5.1.1. Surface passivation by polarization Fig. 2 also shows that surface recombination velocity is reduced to a minimum value when the surface is

10

4

10

5

s (cm/s) (destructive) Fig. 5. Surface recombination velocity obtained by the destructive method and by the passivation by surface bias.

M.L. Polignano, A.P. Caricato / Applied Surface Science 235 (2004) 340–350

s (cm/s)

5.1.2. Surface recombination velocity versus injection level According to SRH theory, carrier recombination depends on the injection level too. In the case of bulk recombination, the dependence of recombination lifetime on the injection level is uniquely determined by the properties (energy levels and capture cross sections) of dominant recombination centers, and this feature was exploited [16] for the identification of bulk contaminants. The injection level was estimated from the injected current and from the measured lifetime by an analytical model. The case of surface recombination is more complex because carrier drift by the electric field near the surface cannot be neglected. Therefore, the injection level at wafer surface can be calculated by numerical simulations only, and is expected to depend on surface bias. As a consequence, the dependence of surface recombination velocity on the injected current can vary with surface bias. Fig. 6 shows that the dependence of surface recombination velocity on the injected current changes from a decreasing to an increasing behaviour when surface moves from

10

5

10

4

10

10

depletion (s=smax)

Vs=0 inversion

3

accumulation

2

-2

10

10

-1

10

0

10

1

10

2

Injected current (mA) Fig. 6. Surface recombination velocity vs. injection level under different surface conditions.

depletion to accumulation or inversion conditions. The decreasing behaviour of surface recombination velocity versus injected current under depletion conditions can be explained by the saturation of surface states as the SRH recombination centers. Data in Fig. 6 qualitatively agree with the numerical calculations in [7]. 5.2. Nitrided oxide–silicon interfaces Fig. 7 shows surface recombination velocity as a function of the applied surface bias for various durations of the NO nitridation. Here and in the following, surface recombination velocity data are fitted by Gaussian functions in the region of the maximum to achieve a more accurate determination of the maximum value and location. These data show that as the duration of the nitridation process increases surface recombination velocity (hence surface state density) decreases. In addition, curves of surface recombination velocity versus surface bias (Fig. 7) show that Vmax is shifted to smaller values. According to the discussion in Section 2 and to the results of the numerical simulations in Section 6, these data show that a positive charge is developed by oxide nitridation. Surface recombination velocity s (cm/sec)

separating bulk and surface recombination without oxide etching. This method is also used with corona electrostatic discharge before mPCD measurements [15].

347

8000 30s wet+NO

60s

6000

90s

4000

2000

0

-1.0

-0.5

0.0

0.5

1.0

Surface bias Vs (V) Fig. 7. Surface recombination velocity vs. applied voltage in NO nitrided samples for different nitridation durations.

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SIMS profiles on nitrogen concentration (CN) of 7 and 5 nm thick oxides treated in N2, N2O and NO were obtained [17]. As expected, no nitrogen accumulation is observed in the N2 annealed oxide, while in N2O and NO treated oxides a nitrogen concentration profile is observed. Nitrogen profile peaks are located at the oxide–silicon interface, thus indicating that interface nitridation is responsible for the observed variations of surface recombination velocity upon nitridation. Fig. 8 reports surface recombination velocity versus the peak nitrogen concentration (CN,peak) obtained from SIMS profiles. In this figure RTO oxides are compared to deposited nitrided oxides (HTO layers) to conventional furnace oxides. In all cases surface recombination velocity is correlated to the nitrogen concentration at the interface, showing that interface nitridation is actually responsible for interface state reduction. However, it is worth noting that a different correlation is obtained by varying the technology used to obtain the oxide layers. In other words, the efficiency of the nitridation process in interface state reduction also depends on the oxidation technology. Specifically, HTO layers always have the highest surface recombination velocity for a given nitrogen concentration. Deposited oxides usually have a larger density of dangling bonds at the interface than thermal oxides, and the surface recombination velocity results can be

105

smax (cm/sec)

HTO + NO RTO + NO RTO + N2O

104

Furnace, N2O Furnace, NO

explained by admitting that the nitridation process partially reduces but does not suppress such interface states. Fig. 8 also shows that RTO/RTN oxides generally have higher surface recombination velocities than conventional furnace oxides. Finally, in conventional furnace oxides NO nitrided interfaces have lower surface recombination velocity than N2O nitrided oxides. It is well-known that during the N2O treatment a reoxidation of the interface takes place along with the nitridation [18], so that two phenomena (nitridation and oxidation) are in competition during the N2O treatment. Actually, both SIMS and XPS (X-ray Photoelectron Spectrometry) showed [19] that in NO treated samples the maximum nitrogen concen˚ from the oxide–silicon tration is located within 10 A interface whereas in N2O treated oxides the peak ˚ nitrogen concentration is located at about 14–18 A from the interface. So, in N2O treated oxides the peak nitrogen concentration is shifted inside the oxide layer ˚ with respect to the peak position of NO by about 4–8 A treated oxides. This fact is due to the reoxidation of the interface during the N2O treatment, and results in a reduced nitridation efficiency of the N2O treatment in interface state reduction. It is interesting to remark that the difference between NO and N2O nitridation is significant when the process is carried out in a conventional furnace. Vice versa in RTO/RTN samples the difference between these treatments is not so evident. Actually, SIMS measurements showed [17] that the peak position is approximately the same in RTO/RTN oxides treated in N2O or NO environments. Tentatively, this fact can be explained by admitting that rapid thermal processes are too fast to allow a significant reoxidation to take place during the N2O treatment.

6. Modeling surface recombination velocity versus voltage

103

10

20

10

21

10

22

-3

CN,peak (cm ) Fig. 8. Surface recombination velocity vs. peak nitrogen concentration from SIMS profiles.

In the previous discussion we implicitly assume that the voltage Vmax is only related to the fixed oxide charge, not to the interface state charge, and, on the other hand, that smax is only affected by interface state density and does not depend on the fixed oxide charge. Indeed, interface states are commonly reported [20,21] to be donors when below the intrinsic Fermi

M.L. Polignano, A.P. Caricato / Applied Surface Science 235 (2004) 340–350

104 11

-2

10

-2

Q=-2 10 cm 10 -2 Q=-5 10 cm Q=0

103

s (cm/s)

level and acceptors when above the intrinsic Fermi level. Though there is no generally accepted model for the interface state microscopic structure, there is wide agreement about this experimental finding. As a consequence of it, in principle interface states do not contribute any charge when the interface is in intrinsic conditions, and no effect of interface state charge on Vmax is expected. For what concerns smax, according to the theory this quantity depends on interface state density, and not on the fixed oxide charge. However, it is hard to design an experiment to check these facts, as in real life oxides with different interface state density may also have different oxide charges, making the separation of effects difficult. So, we used numerical simulations in order to check whether our measurements correctly discriminate between the effect of interface state density and of oxide fixed charges. A numerical solver of the Poisson, continuity and driftdiffusion equation system taking into account both majority and minority carrier transport (DESSIS [22], in the GENESIS framework) was used for these calculations. Surface recombination velocity was calculated as a function of the applied surface bias for different values of the interface state density and zero oxide fixed charge (Fig. 9), and no significant shift in Vmax was observed. In addition, no effect of the oxide

349

102

101 Q=+5 10 cm

100

-1.0 -0.8

-0.6

-0.4

11

Q=+2 10 cm

-0.2

0.0

-2

0.2

Vs (V) Fig. 10. Calculated curves of surface recombination velocity vs. applied surface bias for different values of the oxide fixed charge.

fixed charge on smax is expected from the calculations (see Fig. 10, where 1:3  1011 cm
2 eV
1 interface state density is assumed). These results confirm that Vmax shifts are due to variations of oxide charges, whereas smax is only related to interface state density, so this technique is able to discriminate between interface state and fixed charge effects.

104

103

s (cm/s)

7. Conclusions

Dit=4.4 1010 cm-2eV-1 Dit=6.9 1011cm-2eV-1

Dit=1.3 1011cm-2eV-1

102

101

0

10

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Vs (V) Fig. 9. Calculated curves of surface recombination velocity vs. applied surface bias for different values of the interface state density.

A recently proposed method for evaluating both surface recombination velocity and bulk minority carrier lifetime by photocurrent measurements was discussed and validated by comparison with C–V measurements of interface state density. This method is an evolution of the measurement of surface recombination velocity by the Elymat technique, it consists in measurements of surface recombination velocity under an applied surface bias and does not require oxide etching. The application of a surface bias allows the control of the interface potential and the identification of the suitable interface condition so that surface recombination velocity can be considered as a measurement of interface state density. In addition, it was shown that surface recombination velocity is suppressed when the surface is under accumulation conditions, so the application of a sur-

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M.L. Polignano, A.P. Caricato / Applied Surface Science 235 (2004) 340–350

face bias provides the possibility of a surface passivation by driving the surface into accumulation. It was shown that this passivation by surface polarization is about as effective as the chemical passivation by HF. Finally, the dependence of surface recombination velocity on the injection level was shown to be reversed when the interface changes from depletion to accumulation or inversion conditions. This method does not require the formation of a capacitor structure, so it is suitable for the measurement of as-grown interface properties. For this reason, this method was used for a systematic study of the nitridation process of thin oxides. Nitrided silicon– silicon dioxide interface were studied. It was shown that as the interface nitridation proceeds, the interface state density is reduced and a positive charge is developed in the oxide. Surface recombination velocity (hence surface state density) is directly related to nitrogen concentration at the Si/SiO2 interface as obtained by SIMS.

Acknowledgements The authors wish to thank Dr. P. Eichinger for discussions about surface recombination velocity measurements and for providing us with the equipment modification required for this work. Thanks are also due to Drs. A. Giussani and P. Bacciaglia for performing some surface recombination velocity measurements, to Drs. R. Zonca and B. Crivelli (ST Microelectronics CR\&D, Agrate, Italy) for providing the samples for this characterization, to Drs. M. Bersani and M. Sbetti (ITC-IRST, TN, Italy) for SIMS measurements. References [1] V. Lehmann, H. Fo¨ ll, J. Electrochem. Soc. 135 (1988) 2831. [2] M.L. Polignano, A.P. Caricato, A. Modelli, R. Zonca, J. Electrochem. Soc. 147 (2000) 76.

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