Charge carrier transport in a structure with silicon nanocrystals embedded into oxide matrix

June 16, 2017 | Autor: Leonid Tsybeskov | Categoria: Condensed Matter Physics, Quantum Physics, Semiconductors, Experimental Data
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ISSN 1063-7826, Semiconductors, 2006, Vol. 40, No. 9, pp. 1052–1054. © Pleiades Publishing, Inc., 2006. Original Russian Text © Yu.V. Ryabchikov, P.A. Forsh, E.A. Lebedev, V.Yu. Timoshenko, P.K. Kashkarov, B.V. Kamenev, L. Tsybeskov, 2006, published in Fizika i Tekhnika Poluprovodnikov, 2006, Vol. 40, No. 9, pp. 1079–1081.

SEMICONDUCTOR STRUCTURES, INTERFACES, AND SURFACES

Charge Carrier Transport in a Structure with Silicon Nanocrystals Embedded into Oxide Matrix Yu. V. Ryabchikova^, P. A. Forsha, E. A. Lebedevb, V. Yu. Timoshenkoa, P. K. Kashkarova, B. V. Kamenevc, and L. Tsybeskovc aFaculty

of Physics, Moscow State University, Vorob’evy gory, Moscow, 119899 Russia ^e-mail: [email protected] bIoffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia cDepartment of Electrical and Computer Engineering, New Jersey Institute of Technology, 07102 Newark, New Jersey, USA Submitted January 11, 2006; accepted for publication January 24, 2006

Abstract—Current–voltage characteristics of Al/SiO2/c-Si structures with silicon nanocrystals (nc-Si) in the oxide layer are studied in a wide temperature range. The analysis based on the experimental data has shown that thermostimulated tunneling via electron states in nc-Si is a most probable mechanism of charge carrier transport in these structures. PACS numbers: 73.60. Bd; 72.80. Jc; 73.40. Gk DOI: 10.1134/S1063782606090119

Recently, structures with silicon nanocrystals embedded in a dielectric matrix have attracted substantial interest in view of the prospects of the fabrication on their basis of electronic and optoelectronic devices, in particular, light-emitting diodes, single-electron transistors, and memory devices [1–4]. To fabricate devices on the basis of structures containing silicon nanocrystals, information on their optical and electric properties is necessary. At present, in most studies, the optical properties of such structures are considered (see, e.g., [5–7]). Charge-carrier transport mechanisms in the systems containing nc-Si have been investigated to a much lesser degree. Up to now, the effects of resonant tunneling and Coulomb blockade in nc-Si structures have been the focus of most studies in this field [8–11].

structure of the investigated samples is schematically shown in the inset in Fig. 1. The area of the top contact was S ≈ 10–2 cm2. The electrical characteristics were measured in the temperatures range of T = 100–350 K. The obtained structures had symmetric current– voltage (I–V) characteristics. When a negative voltage was applied, the current was substantially greater than the current for the opposite bias. This fact can indicate the presence of a potential barrier between the c-Si substrate and the SiO2 layer. In Fig. 1, I–V characteristics measured in the forward direction at different tempera-

We study the current–voltage characteristics of Al/SiO2/c-Si structures with silicon nanocrystals in the SiO2 layer in a wide temperature range; this study provided information on the charge transport mechanism in these structures. The samples under study were deposited on an ntype c-Si substrate using high-temperature crystallization of the a-Si/SiO2 layers [12]. The obtained samples contained one layer of silicon nanocrystals located in the oxide matrix. The diameter of these silicon nanocrystals was approximately 5 nm and the oxide layer thickness was about 10 nm as confirmed by Raman spectroscopy, tunneling microscopy, and electron diffraction data. To study charge-carrier transport in such structures, aluminum contacts were sputtered on the surface. The bottom electrode was formed by depositing InGa paste on the rear side of the Si substrate. The 1052

I, A

Al nc-Si SiO2 c-Si

10–4

10–7

10–10

10–13

4 3 2 1 10–2

10–1

100 U, V

Fig. 1. The forward portion of the current–voltage characteristic of the sample under study. The numbers correspond to different temperatures T: (1) 107, (2) 190, (3) 260, and (4) 317 K. Curve 2 is approximated by the straight lines corresponding to the three characteristic regions in the current– voltage characteristic. In the inset, the structure of the samples under study is shown.

CHARGE CARRIER TRANSPORT IN A STRUCTURE

tures are shown on the log–log scale. We can recognize three characteristic regions in the current–voltage curves. In the first region (at low voltages, U < 0.1 V), the voltage dependence of the current is almost linear, corresponding to Ohm’s law. The second region, for voltages 0.1 V < U < 0.6 V, corresponds to a nonlinear I(U) dependence. In the voltage range 0.3 V < U < 0.6 V, the voltage dependence of the current can be described by a power law I ∝ Un with an exponent n > 2. The exponent depends heavily on temperature. Finally, the third region is observed at voltages U > 0.6 V and is described by the I ∝ Un dependence with an exponent n ≈ 2. It should be noted, however, that in this region, at low temperatures (T < 130 K), a deviation of the value of the exponent from two is observed. It is well-known that a quadratic I–V dependence is typical of the space-charge-limited currents (SCLCs) [13]. In the case of SCLCs where quadratic current– voltage dependence is observed, we can estimate the charge carrier drift mobility using the formula [13] 3

JL µ = --------------, 2 εε 0 U

(1)

where µ is the drift mobility, L is the sample thickness, ε is the sample permittivity, ε0 = 8.85 × 10–12 F/m is the permittivity free space, and J is the current density equal to I/S. Assuming that the current in the oxide layer is space-charge limited, we obtain the room temperature mobility of about 10–8 cm2 V–1 s–1. Our estimations have shown that there are two activation regions in the temperature dependence of the calculated mobility: the high-temperature region (T > 230 K) with activation energies of Eµ ≈ 0.12 eV and the temperature region T < 230 K, where Eµ ≈ 0.04 eV. We can assume that the charge-carrier transport across the SiO2 layer at T > 230 K is due to hopping via localized electron states. For hopping transport, the drift mobility is defined by the formula 2

E R eνR µ = ------------ exp ⎛ – -----µ- – ---⎞ , ⎝ kT r ⎠ 6kT

(2)

where ν is the phonon frequency, R is the hopping distance, and r is the localization radius. For the obtained values of the mobility and its activation energy, the preexponential factor is about 10–6 cm2 V–1 s–1, i.e., it is three to four orders of magnitude lower than the value corresponding to hopping over an interatomic distance [14, 15]. Such a small value may be due to a large ratio of the hopping distance to the localization radius. The hopping distance, which is almost half of the oxide layer thickness, becomes comparable to the localization radius so that the possibility of the interpretation of the I–V characteristics using hopping transport and space-charge-limited currents is doubtful. In this respect, the transport of charge carriers from the Si substrate into electronic states in nc-Si by thermally activated tunneling and then further to the aluminum elecSEMICONDUCTORS

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I, A 10–4 5 4

10–6

3 10–8 2 10–10 1 10–12

4

6

8

10 1000/T, K–1

Fig. 2. Temperature dependence of the current of the samples under study. The numbers correspond to the different voltages applied to the sample: (1) 0.1, (2) 0.3, (3) 0.4, (4) 0.6, and (5) 0.9 V. Straight lines show the regions with the different activation energies for curve 4.

trode (as considered in [16]) appears to be more probable. In this case, the deviation of the I–V characteristic from the shape typical of the tunneling effect may be due to the effect of the potential barrier at the boundary between the Si substrate and the oxide layer. Apparently, this potential barrier gives rise to a nonlinear I(U) dependence in the second region (0.1 V < U < 0.6 V) as well. For investigated Al/SiO2/c-Si (with nc-Si in SiO2) structures, the temperature dependences of the current measured at different applied voltages in the forward direction are shown in Fig. 2. In the temperature dependence, we can recognize two exponential regions, where the current is described by the expression E I = I 0 exp ⎛ – ------I ⎞ , ⎝ kT ⎠

(3)

where EI is the current activation energy and I0 is the preexponential factor. In both regions, the energy EI decreases with increasing applied voltage. When the voltage increases from 0.1 to 1 V, the activation energy decreases from EI ≈ 0.14 eV to EI ≈ 0.04 eV at low temperatures and from EI ≈ 0.33 eV to EI ≈ 0.12 eV at high temperatures. Such behavior of EI can be related to the dependence of the height and width of the potential barrier at the c-Si/SiO2 boundary on the applied voltage. Thus, studies of the Al/SiO2/c-Si structures with silicon nanocrystals in the oxide layer show that, at voltages U > 0.6 V, the current depends quadratically on the applied voltage. A quadratic voltage dependence of the current is characteristic of SCLCs. From the analysis of the I–V characteristics, under the assumption of SCLCs and hopping charge carrier transport across the SiO2 layer, we have shown that the charge carrier hopping

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distance becomes comparable to the thickness of the oxide layer. This circumstance raises doubts about the possibility of explaining I–V characteristics assuming hopping transport and SCLCs. In this context, chargecarrier transport by thermally activated tunneling via electronic states of silicon nanocrystals is more probable. REFERENCES 1. F. Koch and V. Petrova-Koch, J. Non-Cryst. Solids 198– 200, 840 (1996). 2. H. Hanafi, S. Tiwari, and I. Khan, IEEE Trans. Electron Devices 43, 1553 (1996). 3. S. Tiwari, F. Rana, H. Hanafi, et al., Appl. Phys. Lett. 68, 1377 (1996). 4. I. Kim, H. Han, H. Kim, et al., in Proceedings of International Electron Devices Meeting (San Francisco, CA, 1998), Vol. 98, p. 111. 5. M. L. Brongersma, A. Polman, K. S. Min, et al., Appl. Phys. Lett. 72, 2577 (1998). 6. K. S. Zhuravlev, A. M. Gilinsky, and A. Yu. Kobitsky, Appl. Phys. Lett. 73, 2962 (1998).

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Translated by I. Zvyagin

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