- Email: [email protected]
Hot-electron induced electroluminescence and avalanche multiplication in hydrogenated amorphous silicon
ELSECVIER
Journal of Non-Crystalline
Solids 198-200
(1996) 198-201
Hot-electron induced electroluminescence and avalanche multiplication in hydrogenated amorphous silicon *, Kazuhiro
Toshihiko Toyama
Faculty
ofEngineering
Hiratsuka, Hiroaki Okamoto, Yoshihiro Hamakawa Science,
Osaka
Unicersity.
Toyonako,
Osaka
560, Japan
Abstract High electric field effects above 1 MV/cm in a-Si:H, have been investigated employing an ac-driven double insulating electroluminescent device structure. An emission tailing into energies above the optical energy gap has been observed, giving direct evidence for existence of hot-electrons in a-Si:H generated by an electric field. An analysis of the emission spectrum due to the lucky-drift model indicates the mean free path of I .O nm for hot-electrons. Electroabsorption measurements reveal that the internal electric field in a-Si:H saturates at N 1.5 MV/cm, which implies multiplication of charges transferred across the a-Si:H layer at this field strength. Avalanche multiplication would be the most consistent mechanism with the results of the hot-electrons induced light emission.
1. Introduction In recent years, intensive studies have been made to understand the high electric field effect in a hydrogenated amorphous silicon (a-Si:H) with an interest of seeking for a synopsis of the avalanche multiplication in a-Si:H and applying to an optoelectronic device [l-3]. Several groups reported the internal quantum efficiency exceeds unity employing a-Si:H based photodiodes which are deeply biased in the reverse direction [2]. They dealt with the photocurrent characteristics under the internal electric field less than 10” V/cm, since the dark current tends to grow steeply. A fundamental question then arises whether carriers become sufficiently ‘hot’ to induce the impact-ionization required for avalanche
* Corresponding author. Tel.: +81-6 850 6317; fax: 850 6316; e-mail: [email protected]. 0022-3093/96/$15.00 SSDI
0022.3093(95)0068
Copyright l-8
+81-6
multiplication under the electric fields at the level of 10” V/cm in a-Si:H. Though hot-electrons have been observed in a-Si:H in the optical studies [4], they were never observed in a high electric field. If hot-carriers are created by the field acceleration, they would be observed in electroluminescence extending into photon energies above the band-gap energy, as done in a crystalline Si (c-S8 device such as a metal-oxide-semiconductor field-effect-transistor (MOSFET) [5]. On amorphous silicon carbide (a-SiC:H) or amorphous carbon (a-C:H), the high field induced electroluminescence has been already reported using a double insulating electroluminescent (EL) device structure, although the spectra include only the sub-band gap emission [6]. In this report, utilizing the EL device structure, we report the direct observation of the hot-electrons in a-Si:H as the electroluminescence including the above gap energy emission and discuss the charge multiplication from an analysis of the field saturation
0 1996 Elsevier Science B.V. All rights reserved.
T. Toq’ama et al. / Journal
in a-Si:H surement.
monitored
by an electroabsorption
ofNon-CystallineSolids
198-200
il996)
IYY
198-201
mea3 .z
2. Experiments
’
100
10-l
a 3
The sample has a structure of glass/ITO/ SnOJa-SiN:H (250 nm)/a-Si:H (200-500 nm>/aSiN:H (250 nm)/Al. Amorphous Si:H was fabricated by rf (13.56 MHz) plasma chemical vapor deposition from SiH, diluted by H, to 10%. Nearstoichiometric amorphous silicon nitride (a-SiN:H) layers were deposited from a mixture of SiH, and NH,, both diluted by H, to 10%. The substrate temperature of 180°C and the gas pressure of 133 Pa (1 Torr) were maintained during the deposition. The aluminum top electrode was evaporated with an area of 0.033 cm*. The dielectric constants of a-Si:H and a-SiN:H were estimated to be 12.5 and 5.6, respectively. The EL devices were operated by a sinusoidal wave voltage at the frequency of 5 kHz. In this paper, both a voltage and an electric field are represented by the values at zero-to-peak, namely the maximum height of the wave.
3. Results Fig. 1 displays the EL emission spectra of the sample with the 200 nm thick a-Si:H layer, of which the schematic cross-sectional view is illustrated in the inset. The EL spectra measured at voltages of 156 to 240 V above the emission threshold voltage of about 150 V. The spectra were corrected for the wavelength dependence of the optical system as well as the optical absorption in a-Si:H. The EL intensity was converted to the number of emission photons per unit energy, 12rh, after division by the photon energy at every wavelength. The broad-band spectra include the emission above the optical energy gap of a-Si:H ( = I .79 eV>, while covering almost the whole visible region. Thus the light emission is detected by the naked eye, although it is fairly weak. The emission spectra have a near exponential lineshape superposed with an oscillation due to the optical interference which is confirmed by the dependence of the oscillation period on the a-Si:H thicknesses. The internal field in the a-Si:H layer, Fsi, has
rp 10-Z r 10.3
10.4’ 1.4
’
’
’
1.6
1.8
2.0
Photon
t
2.2
Energy
2.4
2.6
(eV)
Fig. I. Spectra of emission photon number per unit energy (PI,,) at different applied voltages (V,) (240 V, 0: 212 V, 0; 184 V. A; 156 V 0) and a schematic illustration of a cross-sectional view of the ac-driven double insulating EL device. Solid lines are fitting lines calculated from the electron distribution function of Eq. (3). (h w = SO meV: A = I .Onm.)
been measured on the EL devices of the a-Si:H thicknesses of 200 to 500 nm by the electroabsorption (EA) technique [7]. The wavelength of the probe light was chosen as 660 nm so as to detect the EA signal associated with the a-Si:H layer. In this wavelength region, the signal due to the a-SiN:H layers does not appear. The experimental data is shown in Fig. 2, in which the vertical axis is scaled to Fsi by the following procedure. Under the low and uniform electric field, the EA signal AS is written by ASaAaaFi,,
(1)
where ACY denotes the field-induced change in the absorption coefficient of the a-Si:H layer. Below the
iLz
0.5
100 Voltage
+ +
200nm
--o-
400nm
300nm
200 (V)
Fig, 2. Internal field of a-Si:H (Fs,)-voltage characteristics as a function of a-Si:H thickness cd,, ). cd,,: 200 nm. 0: 300 nm, A ; 400 nm, 0; 500 nm, V.) F,, is translated by Eqs. (1) and (2) from the EA signal analysis.
T. Toyama et al. / Journal of Non-Crystalline Solids 198-200 (1996) 198-201
200
emission threshold voltage, the square root of the EA signal intensity is proportional to the applied voltage, V,. In this region, the EL device will be modeled as two series capacitors, i.e., the a-Si:H capacitor and the other capacitor consisting of the two a-SiN:H layers in series. Thereby Fsi is expressed by Fsi
‘SiN = dSi(CSi
+
V
‘SiNI ”
where C = E,,E/~ denotes the capacitance per unit area; F the dielectric constant and d the thickness, the index refers to the either a-Si:H or a-SiN:H layers. Employing Eqs. (1) and (2), the EA signals are translated into the field strength, Fsi.
4. Discussion Figs. 1 and 2 indicate the existence of hot-electrons in a-Si:H from the electroluminescence with the high energy tailing above the optical energy gap exceeding the threshold electric field strength of 1 MV/cm. We now try to interpret the electron energy distribution from an analysis of the emission mechanism. Intraband transition of hot electrons is the most likely emission mechanism in a-Si:H for the following reasons; (i) Radiative electron-hole recombination is hardly possible since photoluminescence experiments shows the recombination is almost completely quenched either above the electric field of lo5 V/cm or at the room temperature [8]. (ii) Intraband electron transition process in conduction bands is proposed in recent experimental and theoretical studies about hot-electron induced luminescence in a c-Si MOSFET with a field over lo5 V/cm [5]. In the case of intraband transition, the decay of the spectra in Fig. 1 should reflect the electron energy distribution. According to the lucky-drift model in which the parabolic bands and the energy-independent mean free path, A, are assumed, the number of electrons per unit energy n(E) is approximately expressed by n(E)
=
exp( - E/eFA,)
where E denotes the electron
energy,
A, =
(4)
where fi w denotes the optical phonon energy, k, the Boltzman constant and T the lattice temperature. The solid lines in Fig. 1 are plotted in accordance with Eq. (3), in which the mean free path of 1.0 nm and the optical phonon energy of 50 meV are used at fields of 1.0 to 1.5 MV/cm. These parameters lead to an approximate average energy Eayg= eFA, of 0.13 eV at a threshold field of the light emission Fs, = 1.0 MV/cm. Finally, we will consider the field saturation observed at voltages over 150 V in Fig. 2. We first suppose that the transferred charge per unit area, Q,, which is accumulated at the interface between a-Si:H and a-SiN:H, is swept out by Fsi and transferred to the opposite side of the interface across the a-Si:H layer. Now, Q, is written by
Qt = SOES~Fsi - CO&SINSin. For the case of finite formed as
’
(3)
F the electric
(5)
Q,
Eq. (2) must be re-
Csi,Vo- Qr
Fs,=
(6) dSi(CSi
+
‘SiN)
Fig. 3 shows the relation between Q, and Fsi, which is replotted from Fig. 2 employing Eqs. (5) and (6). At any thicknesses of a-Si:H, Q, sharply increases at Fs, exceeding > 1.2 MV/cm. While, under the same field above 1.2 MV/cm, the multiplication of Q, depends on the thickness of a-Si:H -0.5 -0.4
GE -0.3 E 0 5 -0.2
d
-0.1 0 1.0
1.1
1.2
F a,
- exp( -E/&A)
eF( A, - A)
field, e the electronic charge and A, the energy relaxation length [9]. A, is given as a function of A,
1.3
1.4
1.5
(MVlcm)
Fig, 3. Internal field of a-Si:H (Fs, )- transferred charge (Q,) characteristics replotted from Fig. 2 according to Eq. (7). cd,,: 200 nm, ? ?; 300 nm, a ; 400 nm, 0; 500 nm, v .) The inset shows an estimation of the impact-ionization rate 01.
T. Toyama et al. /Journal
of Non-Crystalline
layer. Two possible mechanisms for the multiplication are raised: one is the field assisted current injection from the external electrodes, and the other is the avalanche multiplication. However, the former possibility would be ruled out by another EA experiment employing the samples of which the a-SiN:H thicknesses were varied 200 to 500 nm. The result exhibits the saturation field strength on the a-Si:H is almost constant among the samples, namely the observed charge multiplication is independent of the field strength on the a-SiN:H. Thus the injection current would be excluded as the key mechanism of the multiplication. Consequently, the avalanche multiplication is the most acceptable mechanism from which the dependence of the multiplication of Q, on the a-Si:H thicknesses would be explained. The occurrence of the avalanche multiplication is consistent with the electroluminescence results which show the existence of hot-electrons possessing energies greater than the optical energy gap. The threshold field strength of > 1.2 MV/cm is good agreement with the expectation in the study of the reverse biased a-Si:H p-i-n diode [3]. Given in the inset of Fig. 3., the electron ionization rate (Y is estimated under the assumptions: (i) the hole transport in a-Si:H is negligible, (ii) the multiplication uniformly takes place through out the a-Si:H. Setting the initial transferred charge at 0.005 j-K/cm’, the ionization rate is found to be represented in the form of LY= 3.5 X 10' exp( - 1.1 X 10e7/Fsi)being independent of a-Si:H thickness.
5. Conclusions High electric field effects in a-Si:H have been investigated employing ac-driven double insulating
So1id.s 198-200
(19961 I 98p201
201
EL devices. EL emission involving spectral component above the optical energy gap has been observed at the a-Si:H field > 1 MV/cm. The EL spectra reflect the electron energy distribution according to the lucky-drift model, leading to an estimate of the mean free path of 1.0 nm and the average electron energy of 0.13 eV. The internal electric field saturates at the fields of 1.2 to 1.5 MV/cm as inferred from the EA measurements which is consistent with the multiplication of the transferred charge swept out from the a-Si:H/a-SiN:H interfaces. The avalanche multiplication mechanism is the most likely explanation of the carrier multiplication, supported by the observation of the hot-electrons in the EL spectra.
References [II T. Toyama, K. Hiratsuka, H. Okamoto and Y. Hamakawa,
J. Appl. Phys. 77 (1995) 6354. Dl K. Sawada, C. Mochizuki, S. Akata and T. Ando, Appl. Phys. Lett. 65 (1994) 1364. [31 J.B. Chtvrier and B. Equer, J. Appl. Phys. 76 (1994) 7415. [41 M. Wraback and J. Taut, Phys. Rev. Lett. 69 (1992) 3682. [51 A. Toriumi, M. Yoshimi, M. Iwase, Y. Akiyama and K. Taniguchi, IEEE Trans. Electron. Dev. ED-34 (1987) 1501. [61 Y. Hamakawa, D. Kruangam, T. Toyama, M. Yoshimi, S.M. Paasche and H. Okamoto, Optoelectronics-Device and Technol. 4 (1989) 281. J. Non-Cryst. 171 H. Okamoto, K Hattori and Y. Hamakawa, Solids 137 and 138 (1991) 627. and Semimetals, Vol. 21B, [81 R.A. Street, in: Semiconductors ed. J.I. Pankove (Academic Press, New York, 1984) p, 197. 191 E. Bringuier, Phys. Rev. B49 (1994) 7974.
Journal of Non-Crystalline
Solids 198-200
(1996) 198-201
Hot-electron induced electroluminescence and avalanche multiplication in hydrogenated amorphous silicon *, Kazuhiro
Toshihiko Toyama
Faculty
ofEngineering
Hiratsuka, Hiroaki Okamoto, Yoshihiro Hamakawa Science,
Osaka
Unicersity.
Toyonako,
Osaka
560, Japan
Abstract High electric field effects above 1 MV/cm in a-Si:H, have been investigated employing an ac-driven double insulating electroluminescent device structure. An emission tailing into energies above the optical energy gap has been observed, giving direct evidence for existence of hot-electrons in a-Si:H generated by an electric field. An analysis of the emission spectrum due to the lucky-drift model indicates the mean free path of I .O nm for hot-electrons. Electroabsorption measurements reveal that the internal electric field in a-Si:H saturates at N 1.5 MV/cm, which implies multiplication of charges transferred across the a-Si:H layer at this field strength. Avalanche multiplication would be the most consistent mechanism with the results of the hot-electrons induced light emission.
1. Introduction In recent years, intensive studies have been made to understand the high electric field effect in a hydrogenated amorphous silicon (a-Si:H) with an interest of seeking for a synopsis of the avalanche multiplication in a-Si:H and applying to an optoelectronic device [l-3]. Several groups reported the internal quantum efficiency exceeds unity employing a-Si:H based photodiodes which are deeply biased in the reverse direction [2]. They dealt with the photocurrent characteristics under the internal electric field less than 10” V/cm, since the dark current tends to grow steeply. A fundamental question then arises whether carriers become sufficiently ‘hot’ to induce the impact-ionization required for avalanche
* Corresponding author. Tel.: +81-6 850 6317; fax: 850 6316; e-mail: [email protected]. 0022-3093/96/$15.00 SSDI
0022.3093(95)0068
Copyright l-8
+81-6
multiplication under the electric fields at the level of 10” V/cm in a-Si:H. Though hot-electrons have been observed in a-Si:H in the optical studies [4], they were never observed in a high electric field. If hot-carriers are created by the field acceleration, they would be observed in electroluminescence extending into photon energies above the band-gap energy, as done in a crystalline Si (c-S8 device such as a metal-oxide-semiconductor field-effect-transistor (MOSFET) [5]. On amorphous silicon carbide (a-SiC:H) or amorphous carbon (a-C:H), the high field induced electroluminescence has been already reported using a double insulating electroluminescent (EL) device structure, although the spectra include only the sub-band gap emission [6]. In this report, utilizing the EL device structure, we report the direct observation of the hot-electrons in a-Si:H as the electroluminescence including the above gap energy emission and discuss the charge multiplication from an analysis of the field saturation
0 1996 Elsevier Science B.V. All rights reserved.
T. Toq’ama et al. / Journal
in a-Si:H surement.
monitored
by an electroabsorption
ofNon-CystallineSolids
198-200
il996)
IYY
198-201
mea3 .z
2. Experiments
’
100
10-l
a 3
The sample has a structure of glass/ITO/ SnOJa-SiN:H (250 nm)/a-Si:H (200-500 nm>/aSiN:H (250 nm)/Al. Amorphous Si:H was fabricated by rf (13.56 MHz) plasma chemical vapor deposition from SiH, diluted by H, to 10%. Nearstoichiometric amorphous silicon nitride (a-SiN:H) layers were deposited from a mixture of SiH, and NH,, both diluted by H, to 10%. The substrate temperature of 180°C and the gas pressure of 133 Pa (1 Torr) were maintained during the deposition. The aluminum top electrode was evaporated with an area of 0.033 cm*. The dielectric constants of a-Si:H and a-SiN:H were estimated to be 12.5 and 5.6, respectively. The EL devices were operated by a sinusoidal wave voltage at the frequency of 5 kHz. In this paper, both a voltage and an electric field are represented by the values at zero-to-peak, namely the maximum height of the wave.
3. Results Fig. 1 displays the EL emission spectra of the sample with the 200 nm thick a-Si:H layer, of which the schematic cross-sectional view is illustrated in the inset. The EL spectra measured at voltages of 156 to 240 V above the emission threshold voltage of about 150 V. The spectra were corrected for the wavelength dependence of the optical system as well as the optical absorption in a-Si:H. The EL intensity was converted to the number of emission photons per unit energy, 12rh, after division by the photon energy at every wavelength. The broad-band spectra include the emission above the optical energy gap of a-Si:H ( = I .79 eV>, while covering almost the whole visible region. Thus the light emission is detected by the naked eye, although it is fairly weak. The emission spectra have a near exponential lineshape superposed with an oscillation due to the optical interference which is confirmed by the dependence of the oscillation period on the a-Si:H thicknesses. The internal field in the a-Si:H layer, Fsi, has
rp 10-Z r 10.3
10.4’ 1.4
’
’
’
1.6
1.8
2.0
Photon
t
2.2
Energy
2.4
2.6
(eV)
Fig. I. Spectra of emission photon number per unit energy (PI,,) at different applied voltages (V,) (240 V, 0: 212 V, 0; 184 V. A; 156 V 0) and a schematic illustration of a cross-sectional view of the ac-driven double insulating EL device. Solid lines are fitting lines calculated from the electron distribution function of Eq. (3). (h w = SO meV: A = I .Onm.)
been measured on the EL devices of the a-Si:H thicknesses of 200 to 500 nm by the electroabsorption (EA) technique [7]. The wavelength of the probe light was chosen as 660 nm so as to detect the EA signal associated with the a-Si:H layer. In this wavelength region, the signal due to the a-SiN:H layers does not appear. The experimental data is shown in Fig. 2, in which the vertical axis is scaled to Fsi by the following procedure. Under the low and uniform electric field, the EA signal AS is written by ASaAaaFi,,
(1)
where ACY denotes the field-induced change in the absorption coefficient of the a-Si:H layer. Below the
iLz
0.5
100 Voltage
+ +
200nm
--o-
400nm
300nm
200 (V)
Fig, 2. Internal field of a-Si:H (Fs,)-voltage characteristics as a function of a-Si:H thickness cd,, ). cd,,: 200 nm. 0: 300 nm, A ; 400 nm, 0; 500 nm, V.) F,, is translated by Eqs. (1) and (2) from the EA signal analysis.
T. Toyama et al. / Journal of Non-Crystalline Solids 198-200 (1996) 198-201
200
emission threshold voltage, the square root of the EA signal intensity is proportional to the applied voltage, V,. In this region, the EL device will be modeled as two series capacitors, i.e., the a-Si:H capacitor and the other capacitor consisting of the two a-SiN:H layers in series. Thereby Fsi is expressed by Fsi
‘SiN = dSi(CSi
+
V
‘SiNI ”
where C = E,,E/~ denotes the capacitance per unit area; F the dielectric constant and d the thickness, the index refers to the either a-Si:H or a-SiN:H layers. Employing Eqs. (1) and (2), the EA signals are translated into the field strength, Fsi.
4. Discussion Figs. 1 and 2 indicate the existence of hot-electrons in a-Si:H from the electroluminescence with the high energy tailing above the optical energy gap exceeding the threshold electric field strength of 1 MV/cm. We now try to interpret the electron energy distribution from an analysis of the emission mechanism. Intraband transition of hot electrons is the most likely emission mechanism in a-Si:H for the following reasons; (i) Radiative electron-hole recombination is hardly possible since photoluminescence experiments shows the recombination is almost completely quenched either above the electric field of lo5 V/cm or at the room temperature [8]. (ii) Intraband electron transition process in conduction bands is proposed in recent experimental and theoretical studies about hot-electron induced luminescence in a c-Si MOSFET with a field over lo5 V/cm [5]. In the case of intraband transition, the decay of the spectra in Fig. 1 should reflect the electron energy distribution. According to the lucky-drift model in which the parabolic bands and the energy-independent mean free path, A, are assumed, the number of electrons per unit energy n(E) is approximately expressed by n(E)
=
exp( - E/eFA,)
where E denotes the electron
energy,
A, =
(4)
where fi w denotes the optical phonon energy, k, the Boltzman constant and T the lattice temperature. The solid lines in Fig. 1 are plotted in accordance with Eq. (3), in which the mean free path of 1.0 nm and the optical phonon energy of 50 meV are used at fields of 1.0 to 1.5 MV/cm. These parameters lead to an approximate average energy Eayg= eFA, of 0.13 eV at a threshold field of the light emission Fs, = 1.0 MV/cm. Finally, we will consider the field saturation observed at voltages over 150 V in Fig. 2. We first suppose that the transferred charge per unit area, Q,, which is accumulated at the interface between a-Si:H and a-SiN:H, is swept out by Fsi and transferred to the opposite side of the interface across the a-Si:H layer. Now, Q, is written by
Qt = SOES~Fsi - CO&SINSin. For the case of finite formed as
’
(3)
F the electric
(5)
Q,
Eq. (2) must be re-
Csi,Vo- Qr
Fs,=
(6) dSi(CSi
+
‘SiN)
Fig. 3 shows the relation between Q, and Fsi, which is replotted from Fig. 2 employing Eqs. (5) and (6). At any thicknesses of a-Si:H, Q, sharply increases at Fs, exceeding > 1.2 MV/cm. While, under the same field above 1.2 MV/cm, the multiplication of Q, depends on the thickness of a-Si:H -0.5 -0.4
GE -0.3 E 0 5 -0.2
d
-0.1 0 1.0
1.1
1.2
F a,
- exp( -E/&A)
eF( A, - A)
field, e the electronic charge and A, the energy relaxation length [9]. A, is given as a function of A,
1.3
1.4
1.5
(MVlcm)
Fig, 3. Internal field of a-Si:H (Fs, )- transferred charge (Q,) characteristics replotted from Fig. 2 according to Eq. (7). cd,,: 200 nm, ? ?; 300 nm, a ; 400 nm, 0; 500 nm, v .) The inset shows an estimation of the impact-ionization rate 01.
T. Toyama et al. /Journal
of Non-Crystalline
layer. Two possible mechanisms for the multiplication are raised: one is the field assisted current injection from the external electrodes, and the other is the avalanche multiplication. However, the former possibility would be ruled out by another EA experiment employing the samples of which the a-SiN:H thicknesses were varied 200 to 500 nm. The result exhibits the saturation field strength on the a-Si:H is almost constant among the samples, namely the observed charge multiplication is independent of the field strength on the a-SiN:H. Thus the injection current would be excluded as the key mechanism of the multiplication. Consequently, the avalanche multiplication is the most acceptable mechanism from which the dependence of the multiplication of Q, on the a-Si:H thicknesses would be explained. The occurrence of the avalanche multiplication is consistent with the electroluminescence results which show the existence of hot-electrons possessing energies greater than the optical energy gap. The threshold field strength of > 1.2 MV/cm is good agreement with the expectation in the study of the reverse biased a-Si:H p-i-n diode [3]. Given in the inset of Fig. 3., the electron ionization rate (Y is estimated under the assumptions: (i) the hole transport in a-Si:H is negligible, (ii) the multiplication uniformly takes place through out the a-Si:H. Setting the initial transferred charge at 0.005 j-K/cm’, the ionization rate is found to be represented in the form of LY= 3.5 X 10' exp( - 1.1 X 10e7/Fsi)being independent of a-Si:H thickness.
5. Conclusions High electric field effects in a-Si:H have been investigated employing ac-driven double insulating
So1id.s 198-200
(19961 I 98p201
201
EL devices. EL emission involving spectral component above the optical energy gap has been observed at the a-Si:H field > 1 MV/cm. The EL spectra reflect the electron energy distribution according to the lucky-drift model, leading to an estimate of the mean free path of 1.0 nm and the average electron energy of 0.13 eV. The internal electric field saturates at the fields of 1.2 to 1.5 MV/cm as inferred from the EA measurements which is consistent with the multiplication of the transferred charge swept out from the a-Si:H/a-SiN:H interfaces. The avalanche multiplication mechanism is the most likely explanation of the carrier multiplication, supported by the observation of the hot-electrons in the EL spectra.
References [II T. Toyama, K. Hiratsuka, H. Okamoto and Y. Hamakawa,
J. Appl. Phys. 77 (1995) 6354. Dl K. Sawada, C. Mochizuki, S. Akata and T. Ando, Appl. Phys. Lett. 65 (1994) 1364. [31 J.B. Chtvrier and B. Equer, J. Appl. Phys. 76 (1994) 7415. [41 M. Wraback and J. Taut, Phys. Rev. Lett. 69 (1992) 3682. [51 A. Toriumi, M. Yoshimi, M. Iwase, Y. Akiyama and K. Taniguchi, IEEE Trans. Electron. Dev. ED-34 (1987) 1501. [61 Y. Hamakawa, D. Kruangam, T. Toyama, M. Yoshimi, S.M. Paasche and H. Okamoto, Optoelectronics-Device and Technol. 4 (1989) 281. J. Non-Cryst. 171 H. Okamoto, K Hattori and Y. Hamakawa, Solids 137 and 138 (1991) 627. and Semimetals, Vol. 21B, [81 R.A. Street, in: Semiconductors ed. J.I. Pankove (Academic Press, New York, 1984) p, 197. 191 E. Bringuier, Phys. Rev. B49 (1994) 7974.