- Email: [email protected]
SiO2 superlattices
Physica B 270 (1999) 104}109
In#uences of thicknesses of SiO layers on electroluminescence from amorphous Si/SiO superlattices C.L. Heng , B.R. Zhang , Y.P. Qiao , Z.C. Ma, W.H. Zong, G.G. Qin * Department of Physics, Peking University, Beijing 100871, People's Republic of China The National Laboratory for GaAs IC, 13th Institute of Ministry of Electronic Industry, Shijiazhuang 050051, People's Republic of China Department of Physics, Peking University, Beijing 100871, People's Republic of China International Center for Materials Physics, Academia Sinica, Shenyang, People's Republic of China Received 23 November 1998; received in revised form 9 March 1999
Abstract Amorphous Si/SiO superlattices, with four periods, have been grown using the two-target alternation magnetron sputtering technique. The thicknesses of Si layers in all the superlattices are 1.0 nm, and those of SiO layers in six types of the superlattices are 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 nm. Electroluminescence (EL) from the Au/(Si/SiO ) superlattice/p-Si samples has been observed at a forward bias about 5 V or larger. The in#uences on the EL spectra from the thicknesses of SiO layers in the amorphous Si/SiO superlattices and from input electrical power are studied systematically. 1999 Elsevier Science B.V. All rights reserved. Keywords: SiO ; Electroluminescence; Superlattice
1. Introduction The studies of Si-based luminescence have long been a subject of great interest due to the prospect of photoelectronics application. In 1984, Dimaria et al. [1] have studied electroluminescence (EL) from semitransparent Au "lm/SiO (500 As )/Si-rich SiO (200 As )/n-Si structures and attributed the EL to band-to-band recombination of electrons and holes in the nanoscale Si particles (NSPs) in the Si-rich SiO layer. In 1990, Canham et al. [2] reported the visible photoluminescence (PL) from porous silicon (PS) at room temperature, and * Corresponding author. Department of Physics, Peking University, Beijing 100871, People's Republic of China. Fax: #8610-6275-1615.
suggested that both photoexcitation and radiative recombination of electron-hole pairs occur within nanoscale silicon wires in PS, and their energy gaps become larger than that of the body Si due to the quantum con"nement e!ect. Both models are referred to as the quantum con"nement model, and have been used to interpret many Si-based luminescence experiment results later. In recent years, however, more and more experimental results show that the quantum con"nement model cannot interpret well the luminescence containing PL and EL from the nanoscale silicon systems. (For example, see Refs. [3}10]). In a previous research, we observed EL from the Au/amorphous Si/SiO superlattice (SSOSL)/p-Si structure, and studied the in#uence of the thicknesses of silicon layers on the EL spectra, and
0921-4526/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 1 7 0 - 2
C.L. Heng et al. / Physica B 270 (1999) 104}109
105
discussed the experimental results using the tunneling-quantum con"nement-luminescence center (TQCLC) process [11]. In this paper, we study systematically the in#uence of the thicknesses of SiO layers and the input electrical power on the EL spectra of the Au/SSOSL/p-Si structure, and explain the experimental results using also the TQCLC process. 2. Experiment Four-period a-Si/SiO superlattices have been deposited using the two-target alternation magnetron sputtering technique on (1 0 0) oriented, 6}9 ) cm p-type Si substrates in argon (&2 Pa). The thicknesses of the Si layers were 1.0 nm in all the superlattice samples, and the thicknesses of SiO layers in six types of SSOSL samples were 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm. The alternation targets are a pure SiO target and a n> type Si target with a resistivity of &10\ ) cm. Depositing thin Al "lms onto the back of Si substrates and annealing them in a N ambient at 5303C, good ohmic con tacts have been obtained. Just before the deposition of the SSOSLs, native oxide layers on Si substrates were removed using 5% hydro#uoric acid, then the Si substrates were put immediately into a magnetron sputtering device (ION Tech INC MPS-3000FC). The background vacuum of the device is lower than 10\ Pa. An argon-ion beam was used to remove the leftover oxide layers on the front surfaces of the Si substrates. Deposition rates of SiO and Si were determined by depositing separately thick SiO and Si layers and measuring their thicknesses using a surface pro"le meter. During depositing, the substrates were kept at 2003C. Finally, semitransparent Au "lms were deposited on the front surfaces as electrodes to form Au/SSOSL/p-Si structures. The Au electrodes are circles with diameters of 3 mm. The TEM observation indicates that both Si and SiO layers in the structure are amorphous. The same result has been obtained in Ref. [11]. 3. Experimental results All the Au/SSOSL/p-Si samples exhibit good rectifying junction behavior. Fig. 1 shows the I}<
Fig. 1. The I}< characteristic curves of three Au/SSOSL/p-Si samples with SiO layers having thickness of 1.0 nm (full squares), 2.0 nm (full circles) and 3.5 nm (full triangles).
characteristic curves of three types of Au/SSOSL/ p-Si samples with SiO layers having thickness of 1.0, 2.0 and 3.5 nm. When the applied forward bias on the Au/SSOSL/p-Si samples is higher than about 4 V, the current increases fast with increasing forward voltage. When the reverse bias increases to 10 V, the reverse current increases to be several mA. Fig. 2 shows the current in six types of Au/ SSOSL/p-Si samples with SiO layers having thick ness of 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm under a forward bias of 9 V. When the thickness of SiO layers increases from 1.0 to 1.5 nm, the current decrease slightly but reaches maximum at a thickness of 2.0 nm, then it decreases monotonically when the thickness changes from 2.0 to 3.5 nm. Fig. 3 shows EL spectra from six types of Au/ SSOSL/p-Si samples with SiO layers having thick ness of 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm under a forward bias of 9 V. Visible light from the Au/ SSOSL/p-Si samples can be seen in the dark when the forward bias is about 5 V and becomes stronger with increasing forward bias. Under reverse bias,
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Fig. 2. The currents in six Au/SSOSL/p-Si samples with SiO layers having thickness of 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm under a forward bias of 9 V.
however, no EL emission can be observed. In all EL spectra of Au/SSOSL/p-Si samples there is a major peak being located between 620 and 655 nm and two shoulders around &510 nm and &820 nm. From EL spectra, we can see that increasing the thickness of the SiO layers, the EL peak wave length changes back and forth. The EL peak wavelength for an Au/SSOSL/p-Si sample with SiO layers having a thickness of 1.0 nm each is about 620$5 nm; When the thickness of SiO layers increases from 1.0 to 1.5 nm and then to 2.0 nm, the peak wavelength redshifts to 635$5 nm and then to 650$5 nm; Whereas the peak wavelength blueshifts to 635$5 nm, when the thickness of SiO layers increases to 2.5 nm; The peak wavelength redshifts to 655$5 nm as the thickness of SiO layers extends to 3.0 nm; Finally, the peak wavelength blueshifts again to 640$5 nm as the thickness of SiO layers extends to 3.5 nm. We "nd that when the thickness of SiO layers increases
Fig. 3. EL spectra from six Au/SSOSL/p-Si samples with Si layer thickness of 1.0 nm and SiO layer thickness of 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 nm under a forward bias of 9 V.
from 1.0 to 3.5 nm, the EL intensity also changes back and forth. The EL intensity of the Au/SSOSL/p-Si sample with the SiO layers hav ing thickness of 2.0 nm is higher than that with the SiO layers having thickness of 3.0 nm, and both are obviously higher than the samples with SiO layers having other thicknesses. We have also studied how the EL spectra of Au/SSOSL/p-Si samples change with the applied bias. Fig. 4 shows the EL spectra under a number of forward bias for an Au/SSOSL/p-Si sample with the SiO layer having thickness of 2.5 nm. The EL intensity becomes stronger with increasing bias, whereas, the EL peak wavelength pins at &635$ 5 nm almost independent of the bias applied. The measurement of EL power e$ciency has been conducted. The results show that the EL
C.L. Heng et al. / Physica B 270 (1999) 104}109
Fig. 4. EL spectra from an Au/SSOSL/p-Si sample with Si layer and SiO layer thickness of 1.0 nm and 2.5 nm, respectively, change with the forward bias and current.
power e$ciency of the Au/SSOSL/p-Si structure with the SiO layers having thickness of 3.5 nm attains a maximum value of about 1;10\ when the forward bias is 9 V.
4. Discussion Although the quantum con"nement e!ect does exist in nanoscale amorphous silicon layers [12,13] the above experimental results can hardly be explained using the quantum con"nement (QC) model. First, according to this model [1,2], the recombination emission occurs within the nanoscale Si layers, because all the six types of Au/ SSOSL/p-Si samples have the same Si layer thickness (1.0 nm), as a predication, EL energy of the six
107
types of samples should be almost the same. Second, when the applied forward bias increases, the junction temperature increases, according to the Vashni Equation: [14] E "E #a¹/(¹#b), where a and b are two constants, the band gap becomes narrow with increasing temperature, which will lead to a monotonical redshift of the EL peak. Both predications are in con#ict with the experimental results. The QC model fails to explain the experiments, but it does not mean the QC e!ect has no e!ect on EL. The energy levels of the electrons and holes in nanometer Si layers, which have been increased due to the QC e!ect, can a!ect the relative EL intensities of the component EL bands, and thus a!ect the whole EL intensity and the peak position, because the energy levels determine which type of luminescence center (LC) the electrons and holes will tunnel into. The experimental results can be interpreted using the following two processes: when a forward bias is applied on the Au/SSOSL/p-Si structure, electrons from the Au electrode and holes from the p-Si substrate tunnel into the LCs in the SiO layers of the SSOSL and recombine radiatively there. This process is called tunneling-luminescence center process, and the QC e!ect does not have any e!ect on the process. However, a more probable process is that the electrons and holes tunnel, respectively, into the excited electron states and excited hole states in the nanoscale Si layers of the SSOSL. The energies of all these excited states and ground states are larger than the energies of those states in bulk Si due to the QC e!ect. During the relaxation processes in the Si layers, some electrons and holes would tunnel into the LCs (defects or impurities or self-trapped excitons) in the neighboring SiO layers and recombine radiatively there rather than in the nanoscale Si layers due to the indirect gap structure of Si. This process is called the tunneling quantum con"nement luminescence center (TQCLC) process. A qualitative explanation to the experimental results of this paper is as follows: the carriers tunnel through a series of SiO quantum barriers and Si quantum wells under the applied electric "eld, the carriers tunneling into the exited states of nanoscale Si layers will relax to lower-energy levels;
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similar to the resonant tunneling in double barriers [15}18] and in multi-quantum wells [19,20] under an applied bias, when the positions of energy levels in our Si multi-quantum wells satisfy the resonant tunneling condition, the tunneling current will reach a maximum. In Fig. 2, the tunneling current reaches a maximum at a thickness of 2.0 nm. The EL intensity is proportional to the number of electron-hole pairs tunneling into the LCs in SiO layers per second and thus increases with the tunneling current in the structure. Therefore, the result that the tunneling current and the EL intensity arrive at their maxima simultaneously at a de"nite thickness of SiO layers can be realized. In addition, the EL intensity also depends on whether the positions of LC-energy levels satisfy the condition of resonant tunneling. So, the variation rule of EL intensity is more complicated than that of the tunneling current. If only one type of LCs is responsible for the major EL peak, the EL peak would pin at a de"nite position while the EL intensity changes back and forth with increasing thickness of SiO layers, which however, is in con#ict with the experimental results. So, we suppose that there are at least two types of LCs in SiO layers with a little di!erent luminescence wavelengths in or near the range of 620}655 nm. The numbers of carriers tunneling into the two or more types of LCs per second change back and forth with increasing SiO layer thickness with di!erent phases. As a result, the EL peak position, which is determined by the relative contributions from the two or more types of LCs in the SiO layer, as well as the EL strength, changes back and forth. As the experimental results shows in Fig. 4, EL peak energy does not depend on the applied bias, this can also be explained by the TQCLC process. When we increase the applied bias, although the energy gaps of Si layers become narrower due to the temperature e!ect, however, because carriers radiatively recombine via the LCs in the SiO layers rather than via nanoscale Si layers. Thus the EL peak position pins at a constant wavelength almost independent of the bias applied. A luminescence band located at 1.9 eV being common in SiO has been studied for two decades and its origin is attributed to the nonbridging oxy-
gen hole centers [21}23]. In our previous work, we have ascribed the origin of 650 nm (1.9 eV) EL band from Au/extra thin Si-rich oxide layer/p-Si structure to the nonbridging oxygen hole centers in the Si-rich silicon oxide layer [7]. We consider the type of LCs being responsible for the major EL peak in this paper is the nonbridging oxygen hole centers. We consider the LCs being responsible for the 510 nm shoulder are some type of impurities, because the intensity of the shoulder is very sensitive to the annealing temperature and to the oven in which the annealing is carried out. The type of LCs being responsible for the &820 nm luminescence shoulder is not clear now.
5. Conclusion Six types of Au/SSOSL/p-Si samples with SiO layers in the SSOSLs having di!erent thickness have been deposited using the magnetron sputtering technique. For a de"nite forward bias greater than 5 V, the current arrives at a maximum when the thickness of SiO layers in the SSOSLs is 2.0 nm. Both the EL intensity and the peak position change back and forth with increasing thickness of SiO layers in the SSOSLs. However, the EL peak wavelength for a given Au/SSOSL/p-Si sample is independent of the input power. The EL experimental results have been discussed using the TQCLC process.
Acknowledgements This work was supported by the National Natural Science Foundation of China and State Key Laboratory for Integrated Optoelectronics.
References [1] D.J. DiMaria, J.R. Kirtley, E.J. Pakulis et al., J. Appl. Phys. 56 (1984) 401. [2] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [3] G.G. Qin, J.Q. Jia, Solid State Commun. 86 (1993) 559. [4] S.M. Prokes, Appl. Phys. Lett. 62 (1993) 3244.
C.L. Heng et al. / Physica B 270 (1999) 104}109 [5] A.J. Kontkiewicz, A.M. Konrkiewicz, J. Siejka, S. Sen et al., Appl. Phys. Lett. 65 (1994) 1436. [6] H. Tamura, M. Ruckscloss, T. Wirschem, S. Veprek, Appl. Phys. Lett. 65 (1994) 1537. [7] G.G. Qin, A.P. Li, B.R. Zhang, B.C. Li, J. Appl. Phys. 78 (1995) 2006. [8] D.W. Cooke, B.L. Bennett, E.H. Farnum et al., Appl. Phys. Lett. 68 (1996) 1663. [9] T. Inokuma, Y. Wakayama, T. Muramoto, R. Aoki et al., J. Appl. Phys. 83 (1998) 2228. [10] J. Yuan, D. Haneman et al., J. Appl. Phys. 83 (1998) 4385. [11] G.G. Qin, S.Y. Ma et al., Solid State Commun. 106 (1998) 329. [12] D.J. Lockwood, Z.-H. Lu, J.-M. Baribeau, Phys. Rev. Lett. 76 (1996) 539. [13] G. Allan, C. Delerue, M. Lannoo, Phys. Rev. Lett. 78 (1997) 3161.
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[14] Y.P. Varshni, Physica 34 (1967) 149. [15] L.L. Chang, L. Esaki, R. Tsu, Appl. Phys. Lett. 24 (1974) 593. [16] H. Kitabayashi, T. Waho, M. Yamamoto, J. Appl. Phys. 84 (1998) 1460. [17] K.F. Longenbach, L.F. Luo, W.I. Wang, Appl. Phys. Lett. 57 (1990) 1554. [18] E.E. Mendez, J. Nocera, W.I. Wang, Phys. Rev. B 45 (1991) 3910. [19] F. Capasso, K. Mohammed et al., Appl. Phys. Lett. 48 (1986) 478. [20] H.T. Grahn, R.J. Haug, W. Muller et al., Phys. Rev. Lett. 67 (1991) 1618. [21] A.R. Silin, L.N. Skuja, A.V. Shendrik, Fizkhim. stekla 4 (1978) 405. [22] L.N. Skuja, A.R. Silin, Physica Status Solidi A 56 (1979) K11. [23] L.N. Skuja, J. Non-Cryst. Solids 179 (1994) 51.
In#uences of thicknesses of SiO layers on electroluminescence from amorphous Si/SiO superlattices C.L. Heng , B.R. Zhang , Y.P. Qiao , Z.C. Ma, W.H. Zong, G.G. Qin * Department of Physics, Peking University, Beijing 100871, People's Republic of China The National Laboratory for GaAs IC, 13th Institute of Ministry of Electronic Industry, Shijiazhuang 050051, People's Republic of China Department of Physics, Peking University, Beijing 100871, People's Republic of China International Center for Materials Physics, Academia Sinica, Shenyang, People's Republic of China Received 23 November 1998; received in revised form 9 March 1999
Abstract Amorphous Si/SiO superlattices, with four periods, have been grown using the two-target alternation magnetron sputtering technique. The thicknesses of Si layers in all the superlattices are 1.0 nm, and those of SiO layers in six types of the superlattices are 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 nm. Electroluminescence (EL) from the Au/(Si/SiO ) superlattice/p-Si samples has been observed at a forward bias about 5 V or larger. The in#uences on the EL spectra from the thicknesses of SiO layers in the amorphous Si/SiO superlattices and from input electrical power are studied systematically. 1999 Elsevier Science B.V. All rights reserved. Keywords: SiO ; Electroluminescence; Superlattice
1. Introduction The studies of Si-based luminescence have long been a subject of great interest due to the prospect of photoelectronics application. In 1984, Dimaria et al. [1] have studied electroluminescence (EL) from semitransparent Au "lm/SiO (500 As )/Si-rich SiO (200 As )/n-Si structures and attributed the EL to band-to-band recombination of electrons and holes in the nanoscale Si particles (NSPs) in the Si-rich SiO layer. In 1990, Canham et al. [2] reported the visible photoluminescence (PL) from porous silicon (PS) at room temperature, and * Corresponding author. Department of Physics, Peking University, Beijing 100871, People's Republic of China. Fax: #8610-6275-1615.
suggested that both photoexcitation and radiative recombination of electron-hole pairs occur within nanoscale silicon wires in PS, and their energy gaps become larger than that of the body Si due to the quantum con"nement e!ect. Both models are referred to as the quantum con"nement model, and have been used to interpret many Si-based luminescence experiment results later. In recent years, however, more and more experimental results show that the quantum con"nement model cannot interpret well the luminescence containing PL and EL from the nanoscale silicon systems. (For example, see Refs. [3}10]). In a previous research, we observed EL from the Au/amorphous Si/SiO superlattice (SSOSL)/p-Si structure, and studied the in#uence of the thicknesses of silicon layers on the EL spectra, and
0921-4526/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 1 7 0 - 2
C.L. Heng et al. / Physica B 270 (1999) 104}109
105
discussed the experimental results using the tunneling-quantum con"nement-luminescence center (TQCLC) process [11]. In this paper, we study systematically the in#uence of the thicknesses of SiO layers and the input electrical power on the EL spectra of the Au/SSOSL/p-Si structure, and explain the experimental results using also the TQCLC process. 2. Experiment Four-period a-Si/SiO superlattices have been deposited using the two-target alternation magnetron sputtering technique on (1 0 0) oriented, 6}9 ) cm p-type Si substrates in argon (&2 Pa). The thicknesses of the Si layers were 1.0 nm in all the superlattice samples, and the thicknesses of SiO layers in six types of SSOSL samples were 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm. The alternation targets are a pure SiO target and a n> type Si target with a resistivity of &10\ ) cm. Depositing thin Al "lms onto the back of Si substrates and annealing them in a N ambient at 5303C, good ohmic con tacts have been obtained. Just before the deposition of the SSOSLs, native oxide layers on Si substrates were removed using 5% hydro#uoric acid, then the Si substrates were put immediately into a magnetron sputtering device (ION Tech INC MPS-3000FC). The background vacuum of the device is lower than 10\ Pa. An argon-ion beam was used to remove the leftover oxide layers on the front surfaces of the Si substrates. Deposition rates of SiO and Si were determined by depositing separately thick SiO and Si layers and measuring their thicknesses using a surface pro"le meter. During depositing, the substrates were kept at 2003C. Finally, semitransparent Au "lms were deposited on the front surfaces as electrodes to form Au/SSOSL/p-Si structures. The Au electrodes are circles with diameters of 3 mm. The TEM observation indicates that both Si and SiO layers in the structure are amorphous. The same result has been obtained in Ref. [11]. 3. Experimental results All the Au/SSOSL/p-Si samples exhibit good rectifying junction behavior. Fig. 1 shows the I}<
Fig. 1. The I}< characteristic curves of three Au/SSOSL/p-Si samples with SiO layers having thickness of 1.0 nm (full squares), 2.0 nm (full circles) and 3.5 nm (full triangles).
characteristic curves of three types of Au/SSOSL/ p-Si samples with SiO layers having thickness of 1.0, 2.0 and 3.5 nm. When the applied forward bias on the Au/SSOSL/p-Si samples is higher than about 4 V, the current increases fast with increasing forward voltage. When the reverse bias increases to 10 V, the reverse current increases to be several mA. Fig. 2 shows the current in six types of Au/ SSOSL/p-Si samples with SiO layers having thick ness of 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm under a forward bias of 9 V. When the thickness of SiO layers increases from 1.0 to 1.5 nm, the current decrease slightly but reaches maximum at a thickness of 2.0 nm, then it decreases monotonically when the thickness changes from 2.0 to 3.5 nm. Fig. 3 shows EL spectra from six types of Au/ SSOSL/p-Si samples with SiO layers having thick ness of 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm under a forward bias of 9 V. Visible light from the Au/ SSOSL/p-Si samples can be seen in the dark when the forward bias is about 5 V and becomes stronger with increasing forward bias. Under reverse bias,
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Fig. 2. The currents in six Au/SSOSL/p-Si samples with SiO layers having thickness of 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 nm under a forward bias of 9 V.
however, no EL emission can be observed. In all EL spectra of Au/SSOSL/p-Si samples there is a major peak being located between 620 and 655 nm and two shoulders around &510 nm and &820 nm. From EL spectra, we can see that increasing the thickness of the SiO layers, the EL peak wave length changes back and forth. The EL peak wavelength for an Au/SSOSL/p-Si sample with SiO layers having a thickness of 1.0 nm each is about 620$5 nm; When the thickness of SiO layers increases from 1.0 to 1.5 nm and then to 2.0 nm, the peak wavelength redshifts to 635$5 nm and then to 650$5 nm; Whereas the peak wavelength blueshifts to 635$5 nm, when the thickness of SiO layers increases to 2.5 nm; The peak wavelength redshifts to 655$5 nm as the thickness of SiO layers extends to 3.0 nm; Finally, the peak wavelength blueshifts again to 640$5 nm as the thickness of SiO layers extends to 3.5 nm. We "nd that when the thickness of SiO layers increases
Fig. 3. EL spectra from six Au/SSOSL/p-Si samples with Si layer thickness of 1.0 nm and SiO layer thickness of 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 nm under a forward bias of 9 V.
from 1.0 to 3.5 nm, the EL intensity also changes back and forth. The EL intensity of the Au/SSOSL/p-Si sample with the SiO layers hav ing thickness of 2.0 nm is higher than that with the SiO layers having thickness of 3.0 nm, and both are obviously higher than the samples with SiO layers having other thicknesses. We have also studied how the EL spectra of Au/SSOSL/p-Si samples change with the applied bias. Fig. 4 shows the EL spectra under a number of forward bias for an Au/SSOSL/p-Si sample with the SiO layer having thickness of 2.5 nm. The EL intensity becomes stronger with increasing bias, whereas, the EL peak wavelength pins at &635$ 5 nm almost independent of the bias applied. The measurement of EL power e$ciency has been conducted. The results show that the EL
C.L. Heng et al. / Physica B 270 (1999) 104}109
Fig. 4. EL spectra from an Au/SSOSL/p-Si sample with Si layer and SiO layer thickness of 1.0 nm and 2.5 nm, respectively, change with the forward bias and current.
power e$ciency of the Au/SSOSL/p-Si structure with the SiO layers having thickness of 3.5 nm attains a maximum value of about 1;10\ when the forward bias is 9 V.
4. Discussion Although the quantum con"nement e!ect does exist in nanoscale amorphous silicon layers [12,13] the above experimental results can hardly be explained using the quantum con"nement (QC) model. First, according to this model [1,2], the recombination emission occurs within the nanoscale Si layers, because all the six types of Au/ SSOSL/p-Si samples have the same Si layer thickness (1.0 nm), as a predication, EL energy of the six
107
types of samples should be almost the same. Second, when the applied forward bias increases, the junction temperature increases, according to the Vashni Equation: [14] E "E #a¹/(¹#b), where a and b are two constants, the band gap becomes narrow with increasing temperature, which will lead to a monotonical redshift of the EL peak. Both predications are in con#ict with the experimental results. The QC model fails to explain the experiments, but it does not mean the QC e!ect has no e!ect on EL. The energy levels of the electrons and holes in nanometer Si layers, which have been increased due to the QC e!ect, can a!ect the relative EL intensities of the component EL bands, and thus a!ect the whole EL intensity and the peak position, because the energy levels determine which type of luminescence center (LC) the electrons and holes will tunnel into. The experimental results can be interpreted using the following two processes: when a forward bias is applied on the Au/SSOSL/p-Si structure, electrons from the Au electrode and holes from the p-Si substrate tunnel into the LCs in the SiO layers of the SSOSL and recombine radiatively there. This process is called tunneling-luminescence center process, and the QC e!ect does not have any e!ect on the process. However, a more probable process is that the electrons and holes tunnel, respectively, into the excited electron states and excited hole states in the nanoscale Si layers of the SSOSL. The energies of all these excited states and ground states are larger than the energies of those states in bulk Si due to the QC e!ect. During the relaxation processes in the Si layers, some electrons and holes would tunnel into the LCs (defects or impurities or self-trapped excitons) in the neighboring SiO layers and recombine radiatively there rather than in the nanoscale Si layers due to the indirect gap structure of Si. This process is called the tunneling quantum con"nement luminescence center (TQCLC) process. A qualitative explanation to the experimental results of this paper is as follows: the carriers tunnel through a series of SiO quantum barriers and Si quantum wells under the applied electric "eld, the carriers tunneling into the exited states of nanoscale Si layers will relax to lower-energy levels;
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similar to the resonant tunneling in double barriers [15}18] and in multi-quantum wells [19,20] under an applied bias, when the positions of energy levels in our Si multi-quantum wells satisfy the resonant tunneling condition, the tunneling current will reach a maximum. In Fig. 2, the tunneling current reaches a maximum at a thickness of 2.0 nm. The EL intensity is proportional to the number of electron-hole pairs tunneling into the LCs in SiO layers per second and thus increases with the tunneling current in the structure. Therefore, the result that the tunneling current and the EL intensity arrive at their maxima simultaneously at a de"nite thickness of SiO layers can be realized. In addition, the EL intensity also depends on whether the positions of LC-energy levels satisfy the condition of resonant tunneling. So, the variation rule of EL intensity is more complicated than that of the tunneling current. If only one type of LCs is responsible for the major EL peak, the EL peak would pin at a de"nite position while the EL intensity changes back and forth with increasing thickness of SiO layers, which however, is in con#ict with the experimental results. So, we suppose that there are at least two types of LCs in SiO layers with a little di!erent luminescence wavelengths in or near the range of 620}655 nm. The numbers of carriers tunneling into the two or more types of LCs per second change back and forth with increasing SiO layer thickness with di!erent phases. As a result, the EL peak position, which is determined by the relative contributions from the two or more types of LCs in the SiO layer, as well as the EL strength, changes back and forth. As the experimental results shows in Fig. 4, EL peak energy does not depend on the applied bias, this can also be explained by the TQCLC process. When we increase the applied bias, although the energy gaps of Si layers become narrower due to the temperature e!ect, however, because carriers radiatively recombine via the LCs in the SiO layers rather than via nanoscale Si layers. Thus the EL peak position pins at a constant wavelength almost independent of the bias applied. A luminescence band located at 1.9 eV being common in SiO has been studied for two decades and its origin is attributed to the nonbridging oxy-
gen hole centers [21}23]. In our previous work, we have ascribed the origin of 650 nm (1.9 eV) EL band from Au/extra thin Si-rich oxide layer/p-Si structure to the nonbridging oxygen hole centers in the Si-rich silicon oxide layer [7]. We consider the type of LCs being responsible for the major EL peak in this paper is the nonbridging oxygen hole centers. We consider the LCs being responsible for the 510 nm shoulder are some type of impurities, because the intensity of the shoulder is very sensitive to the annealing temperature and to the oven in which the annealing is carried out. The type of LCs being responsible for the &820 nm luminescence shoulder is not clear now.
5. Conclusion Six types of Au/SSOSL/p-Si samples with SiO layers in the SSOSLs having di!erent thickness have been deposited using the magnetron sputtering technique. For a de"nite forward bias greater than 5 V, the current arrives at a maximum when the thickness of SiO layers in the SSOSLs is 2.0 nm. Both the EL intensity and the peak position change back and forth with increasing thickness of SiO layers in the SSOSLs. However, the EL peak wavelength for a given Au/SSOSL/p-Si sample is independent of the input power. The EL experimental results have been discussed using the TQCLC process.
Acknowledgements This work was supported by the National Natural Science Foundation of China and State Key Laboratory for Integrated Optoelectronics.
References [1] D.J. DiMaria, J.R. Kirtley, E.J. Pakulis et al., J. Appl. Phys. 56 (1984) 401. [2] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [3] G.G. Qin, J.Q. Jia, Solid State Commun. 86 (1993) 559. [4] S.M. Prokes, Appl. Phys. Lett. 62 (1993) 3244.
C.L. Heng et al. / Physica B 270 (1999) 104}109 [5] A.J. Kontkiewicz, A.M. Konrkiewicz, J. Siejka, S. Sen et al., Appl. Phys. Lett. 65 (1994) 1436. [6] H. Tamura, M. Ruckscloss, T. Wirschem, S. Veprek, Appl. Phys. Lett. 65 (1994) 1537. [7] G.G. Qin, A.P. Li, B.R. Zhang, B.C. Li, J. Appl. Phys. 78 (1995) 2006. [8] D.W. Cooke, B.L. Bennett, E.H. Farnum et al., Appl. Phys. Lett. 68 (1996) 1663. [9] T. Inokuma, Y. Wakayama, T. Muramoto, R. Aoki et al., J. Appl. Phys. 83 (1998) 2228. [10] J. Yuan, D. Haneman et al., J. Appl. Phys. 83 (1998) 4385. [11] G.G. Qin, S.Y. Ma et al., Solid State Commun. 106 (1998) 329. [12] D.J. Lockwood, Z.-H. Lu, J.-M. Baribeau, Phys. Rev. Lett. 76 (1996) 539. [13] G. Allan, C. Delerue, M. Lannoo, Phys. Rev. Lett. 78 (1997) 3161.
109
[14] Y.P. Varshni, Physica 34 (1967) 149. [15] L.L. Chang, L. Esaki, R. Tsu, Appl. Phys. Lett. 24 (1974) 593. [16] H. Kitabayashi, T. Waho, M. Yamamoto, J. Appl. Phys. 84 (1998) 1460. [17] K.F. Longenbach, L.F. Luo, W.I. Wang, Appl. Phys. Lett. 57 (1990) 1554. [18] E.E. Mendez, J. Nocera, W.I. Wang, Phys. Rev. B 45 (1991) 3910. [19] F. Capasso, K. Mohammed et al., Appl. Phys. Lett. 48 (1986) 478. [20] H.T. Grahn, R.J. Haug, W. Muller et al., Phys. Rev. Lett. 67 (1991) 1618. [21] A.R. Silin, L.N. Skuja, A.V. Shendrik, Fizkhim. stekla 4 (1978) 405. [22] L.N. Skuja, A.R. Silin, Physica Status Solidi A 56 (1979) K11. [23] L.N. Skuja, J. Non-Cryst. Solids 179 (1994) 51.