Temperature dependence of visible photoluminescence from PECVD nanocrystallites embedded in amorphous silicon films

Solid Slave Communications.

Vol. 104, No. IO, pp. 603407. 1997 8 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-lO98/97 $17.00+.00

PII: s0038-1098(97)00391-8

TEMPERATURE DEPENDENCE OF VISIBLE PHOTOLUMINESCENCE FROM PECVD NANOCRYSTALLITES EMBEDDED IN AMORPHOUS SILICON FILMS+ Song Tong,” Xiang-na “National

Liu,” Ting Gao,O Xi-mao Baa,’ Yong Chang,b Wen-zhong

Shenb and Wen-guo

Tangb

Laboratory

of Solid State Microstructures/Department of Physics, Nanjing University, Nanjing, 210093, China bShanghai Institute of Technical Physics, Academic Sinica, Shanghai, 200083, China (Received

and acmpted

15 July 1997 by Z.Z. Can)

The temperature dependence of visible range photoluminescence (PL) properties of nanocrystallites embedded in silicon films deposited at the substrate temperature T, = 50-150°C by PECVD method was studied. It was found that there are many differences between them and that of the near-infrared range PL in otdinary a-Si : H films. Combining with the results of photo-absorption studies of these nanocrystalline samples, we discussed the mechanism of their visible range PL. We suggest that the light excitation occurs in both the nanocrystallites core and their surface regions; however, the radiative recombination can only occur between the electrons and holes in the localized band tail states of the crystallite surface regions, which are more disordered than ordinary a-Si : H with their defect states extending more deeply into the band gap. 0 1997 Elsevier Science Ltd Keywords: A. nanostructures, trapping, E. luminescence.

A. semiconductor,

1. INTRODUCTION In the field of silicon based room temperature light emitting materials, besides the porous silicon (PS) fabricated by anodized oxidation, other kinds of films fabricated by sputtering, gas vaporation, chemical vapor deposition (CVD) and so on were reported in recent ,years [l-5]. These films are of two phases structure, i.e. nanocrystallites embedded in amorphous m,atrix. They are more stable than PS in both mechanical and luminescent properties, so that they are more promising in device applications. Their luminescent mechanism is a subject of hot debate and three kinds of explanation have been proposed: (1) both excitation between the quantized cores [2, 41; _

and recombination events occur levels inside the nanocrystallite

’ This research was partially supported by the Nat ional Scientific Foundation of China under Grant No. 19474017. 603

D. recombination

and

(2) the excitation occurs between the quantized levels inside the nanocrystallite cores and the recombination occurs at the surfaces of nanocrystallites [3]; (3) the luminescence originates from some kinds of luminescent compounds or defect states and has no relation with quantum size effect (QSE) [l, 51. To investigate the luminescent mechanism, one important way is to study its temperature dependence, for which there have been many results reported in a-Si : H [6-lo] and also in PS [ 1 l-161, but a few in the films that contain nanocrystallites [5, 221. In our previous work, we have fabricated nanocrystallites embedded in a-Si : H films by plasma enhanced CVD (PECVD) and have observed visible photoluminescence (PL) and electroluminescence (EL) at room temperature without any._ post-processing [17-191. We have studied . the film deposition mechanism, the optimum deposition conditions and have proposed two prerequisitions for effective visible PL, i.e. average crystallites size not more than 3.4 nm and crystalline fraction not more than 30% [ 181. About the PL mechanism, we proposed

VISIBLE PHOTOLUMINESCENCE

604

FROM PECVD NANOCRYSTALLITES

that the excitation occurs between the quantized levels inside the nanocrystallites core under the QSE and the recombination occurs between the defect states at the crystallite surfaces. One important fact supporting this proposition is that with the substrates temperature T,< decreasing from 250°C to 50°C, the average crystallites size decreases from 3.2 nm to 2.6 nm and the PL peak position shifts from 760 nm to 680 nm. Besides, we have found that with decreasing T,y, the intensity of Si-H bonds in IR spectra increases monotonously [ 181, but the PL intensity does not behave like this and also, the incorporation of oxygen in the films quenches the PL obviously. So, we can exclude the possibility of PL originating from SiH, or Si-H-O related component or defect states. From our previous studies [20], we found that in the films fabricated by PECVD, hydrogen is especially abundant at the crystallite surface regions in the form of -SiH* and -SiH3, which will inevitably influence the microstructure and the distribution of energy states there. In this paper, we investigated the temperature dependence of PL, comparing with that of a-Si : H and PS and obtained many important conclusions about the visible PL mechanism, mainly the effects of structural and- chemical fluctuations in the crystallite surfaces region on the radiative recombination processes. 2. EXPERIMENTAL The films were fabricated by PECVD. We used strong hydrogen diluted silane as the reactant gas and applied a proper negative bias to the sample substrates during deposition (for detailed description, see [ 17. 181). The three important deposition parameters of our sample series are controlled as follows: (1) the substrates temperature T,q, 50-300°C; (2) the negative bias -Vb, O-200 V; (3) the gas ratio rg (SiH4/SiH4 + HZ), 0.9-60/c. The optimum deposition parameters for the most intensive PL was found to be T, = lOO-150°C -V, = 125-200 V, rK - 2.2% [ 181. Deposition parameters of the three representative samples in this paper are shown in Table 1. The average grain size d of samples are measured by both high resolution electron microscopy (HREM) and Raman scattering and the crystallinity X,. (volume fraction of crystalline phase) are

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measured by Raman scattering [ 17, 19, 231. The PL are excited by the 5 14.5 nm line of an Ar+ laser and detected by a photoamplifier of 9684QB type at 233 K.

3. RESULTS

AND DISCUSSION

(1) All our samples deposited at Ts between 250°C to 300°C with their d and X, values 3.2-3.4 nm and 30-45%, respectively, show PL spectra only in the infrared range, peaking at 720-920 nm in wavelength. These infrared PL peaks are very intensive at low temperature of 77 K, which quench quickly with increasing temperature while their peak positions red shift. All these PL behavior are very similar with that of a-Si : H films. Wenguo Tang er al. [22] have also observed the similar PL peak at the PECVD nanocrystalline films prepared at similar conditions. It is reasonable to contribute these infrared PL to the radiative transmissions of electrons and holes in the band tail states of the amorphous tissue [7, 91. However, it is important to note that the PL intensity in visible range of these T, = 250-300°C samples is very weak and hardly to be detected, no matter in low or room temperature. An explanation of this experimental fact has been given in our previous studies which is consistent with the QSE model, i.e. (a) the mean crystallites size might be too large to deduce a detectable QSE; (b) being the crystallinity approaches -5O%, the distances between neighboring crystallites are only l-3 atomic spacings, carriers can tunnel through the amorphous region and the QSE might not be valid [ 17, 181. Other samples which were deposited at T, = 50-150°C with their d and X, -2.6-2.8 nm and values -5- 16%, respectively, exhibit PL spectra at visible range, peaking at 680720 nm. The intensity of these PL peaks are very strong at low temperature and also detectable even at room temperature, which quench slowly with increasing temperature. We should stress here that these samples of T, = 50-150°C do not exhibit any infrared PL. The samples of T, = 200°C however, exhibit both infrared and visible range PL at low temperature, but only exhibit very weak visible PL at room temperature. Now, a problem arises, why the samples of T, 5 150°C do not exhibit infrared PL, though a considerable amount (-85-95%) of amorphous component exists. The answer might be that the very high density of defect

Table 1 Sample

T, (“C)

-vb

625 611 511

100 100 50

200 125 200

(v)

rR

(%I

2.26 1.1 0.9

To

WI

34.4 33 59

dEL/dT (meV K-‘) f0.058 +0.14 +0.32

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FROM PECVD NANOCRYSTALLITES

Fig. 1. A comparison between the absorption spectrum (solid line) and PL spectrum (dashed line) of sample 625. The deposition parameters of which are listed in Ta.ble 1. states in the amorphous tissue resulting from the rather low deposition temperature should be responsible for killing totally the infrared radiative transitions there. From the experimental results above, the following conclusion can be deduced: crystallites in the salmples of T, 5 150°C are embedded in an amorphous tissue which has higher density of defect states than ordinary a-Si : H films deposited at T, = 250-350°C. Besides, the PL spectra observed can only originate from the following two parts: (a) the inside of crystallite cores (we will deny this possibility in the next paragraph); (b) the crystallite surface regions. Though high density of shallow gap defect states exist in the crystallite surface regions, the abundant hydrogen content there might partially compensate the deep gap defect states, which are mainly dangling bond states and thus cause the radiative recombination possible. (2) Figure 1 shows both the photoabsorption spectrum and the PL spectrum of sample 625 (T, = 150°C). Because the samples are of a complex structure containing the crystalline phase (c-S& the amorphous phase (a-Si) and the crystallite surface regions, we cannot obtain its optical energy gap value from the above absorption spectrum (Fig. 1) according to some theoretical formula (for example, the Taut formula). But, Fig. 1 indicates distinctly that the energy of radiative photons are much smaller than that of the absorbed photons. It is reasonable to consider that the photoabsorption contributing to the visible range PL originates from both the quantized energy levels of the nanocrystallites and their surface regions, however, the radiative recombination can only take place at the surface regions. The possibility of band-to-band direct recombination inside the nanocrystallite cores can be excluded.

605

Temperature (K) Fig. 2. Temperature samples: A, sample 5 11. The deposition listed in Table 1.

dependence of PL intensity of 625; 0, sample 611; Cl, sample parameters of these samples are

(3) Figure 2 presents the temperature dependence of PL intensity of samples of T, I 100°C (see Table 1). In each curve of Fig. 2, there is a turning temperature T,, corresponding to the maximum of PL intensity. In the range of T < T,, PL intensity increases with increasing temperature, whereas in that of T > T,, PL intensity decreases and goes to be vanished. This PL temperature dependency is similar to that of a-Si : H and PS. However, the T,. values of a-Si : H and PS are of 40-60 K [7, IO] and 100-150 K [12-141, respectively and that of our samples are of 75- 120 K. It is recognized generally that the present of a maximum point in PL intensity in changing temperature implies that the radiative and nonradiative recombinative processes, which are competitive with each other, dominate the PL behavior in the different temperature ranges respectively. For a further illustration of the increase of PL intensity when T > T,, the model proposed by Onsager [6,24] can be used as below: at very low temperature (e.g. T < 10 K), the electrons and holes at band-tail states, which are combined by the coulomb interaction force, can be seen as “frozen” and thus have no contribution to the luminescence. With increasing temperature, the carriers start to move towards each other first in the range T < T,. In result, the geminate recombination increases the intensity of PL. For further incheasing temperature, the dissociation of e-h pairs by thermal activation and then recombination nonradiatively through defects dominates the whole transition process when T > T,. In the crystallite surface regions, the photoexcited electrons and holes might be trapped in a more highly localized energy states than a-% : H (we

VISIBLE PHOTOLUMINESCENCE

606

’

,611

FROM PECVD NANOCRYSTALLITES

.

3. . -

.

2I

F F

‘-

-1

100

//./.:

.

/

150

200

250

1

300

Temperature(K) Fig. 3. The temperature dependence of In [IO/I(T) - I] of sample 611 (see Table 1 for its deposition parameters). will discuss this point of view further in the next paragraph) and in consequence, the temperature Tc of which the nonradiative recombination starts to be dominant might be higher than a-Si : H. (4) The quenching of PL in temperature of T > T, can be expressed by a general function P,.l(P,,r + P,) = I(T)llo, in which, P, and P,, are the possibility of radiative and nonradiative recombination, respectively, I and lo are the PL intensity at temperature T and T,., respectively. Figure 3 is a plot of the dependence of In (PJP,) w T, from which, one can find that the increase of nonradiative recombination in our samples is a kind of thermalized process, similar to that in a-Si : H samples [7. lo]:

pttdpr = half

- 1 a exp (T/To),

(1)

in which To is a characteristic temperature. From the slope of the linear relationship of Fig. 1, we obtained the To values, which are listed in Table 1. According to the Street’s PL model of band-tail states, the To values in a-Si : H should be proportional to the width of exponential band tail states. The typical To in a-Si : H is -23 K, from which, the width of band tail states -0.40 eV can be obtained [ 10,251. The higher To reflects deeper extending of the band tail states towards the middle of band gap and the slowing down slope of the exponential decay in density of states. This corresponds to a more disordered material and its weaker temperature dependence of PL quenching. Though we do not know the specific distribution of energy states in the crystallites surface regions, we suppose from the obvious increase of To that the structure of these regions are more disordered than that of ordinary a-Si : H. This is likely because the deposition temperature is rather low and what is special in the case of crystallite surface region is that the elastic stress due to the lattice mismatch between the two phases, c-Si and a-Si, might induce more

Vol. 104, No. 10

localized states in the gap than the a-Si : H tissue. Furthermore, the abundant hydrogen atoms accumulated near the surfaces could induce some other kinds of defects. The behavior of sample 5 11, which was deposited at the lowest temperature T, = 5O”C, shows the highest To value and the weakest PL quenching (see Table 1 and Fig. 2), confirms our assumption. (5) We found that the temperature coefficient of the PL peak position in energy oL(dE,,ldT) of our samples is absolutely different from that of a-Si : H. In a-Si : H, the sign of oL is negative and is 10e3 eV K-’ of the order of magnitude (the red shift of band edge is -10m4 eV K-‘) [IO, 251. According to the literature, the sign of CY~in porous silicon are random: it may be negative [14], or positive [ 13, 151 and Zheng et al. [ 121 even found that the sign of oL may be different in the different sampling points of the same specimen. The oL of all our samples with 7’, > 200°C are negative, whereas that of all with T, < 100°C are positive and their absolute values are lo-“- lo-’ eV K-’ of order of magnitude (see Table 1). As to the behavior of 01~ in a-Si : H, there have been several explanations reported. Street et at. suggested that carriers trapped in the band tail states move towards the middle of band gap with increasing temperature, thus causes the emission energy red shift [6, 101. However, Fischer er al. proposed experimental evidence [9]. indicating that besides the main PL peak at -1.2 eV, there exists a secondary PL peak at -0.8 eV and the more rapid thermal quenching of the main one should be responsible for the PL peak red shift. Austin et al. [26] found in a-SiN, : H that the increase of nitrogen content has the effect of weakening the temperature dependence of PL as well as changing the sign of its cyLfrom negative to positive. They explained the phenomenon by means of a two-bands model concerning in nitrogen content. Being lack of experimental evidence, we can not give reasonable explanation to the abnormal behavior of oL in our sample series. We suspect that some changes in microstructure induced by the lattice mismatch and the accumulation of abundant hydrogen atoms at the crystallite surface regions might be responsible for the abnormal behavior of oL. 4. SUMMARY In this paper, we studied the temperature dependence of PL properties in nanocrystalline silicon films deposited at 50-300°C by PECVD method. From those samples which are deposited at lower temperature of 50- 100°C and show light emissions in the visible range at room temperature, we draw conclusions as below: (1) Though the amorphous tissue takes the main part in the volume of these samples, the near infrared range PL, which is characteristic in a-Si : H, is undetectable.

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This is because that the density of deep gap defects in the amorphous tissue is so high due to the low temperature deposition that the photoexcited carriers recombine totally through the nonradiative centers. (2) In view of the fact that the photon energy of absorption is much higher than that of the light emission, we conclude that the photo-excitation contributing to the visible PL occurs in both the inside of nanocrystallite cores due to QSE and their surfaces region, while the radiative recombination can only take place between the localized states of the crystallites surfaces, rather than a direct transition between the quantized levels inside the crystallite cores. (3) Comparing the temperature dependence behavior of our samples to that of a-Si : H, we found there are many differences between them: (a) The temperature To corresponding to the maximum of PL intensity is higher in our sample series. (b) The slope of the exponential thermal quenching of PL intensity is obviously more gentle in our samples. Furthermore, samples deposited at lower temperatures exhibit weaker temperature dependence of their PL quenching. (c) With increasing temperature, the energy position of PL peak blueshifts in our samples, while that in a-Si : H redshifts. (3) We attribute the visible light emission in our samples to the radiative recombination between electrons and holes trapped at the band tail states in the nanocrystallite surfaces regions, which are more disordered in structure than ordinary a-Si : H with their near band defect states extending more deeply into the energy

Acknowledgements-The

authors express their thanks to Feng Qing Hai for his help in sample deposition. REFERENCES

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