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Thermal stability of p-type doped amorphous silicon suboxides
Journal of Non-Crystalline Solids 266±269 (2000) 840±844
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Thermal stability of p-type doped amorphous silicon suboxides R. Janssen *, A. Janotta, M. Stutzmann Walter Schottky Institute, Technical University of Munich, Am Coulombwall, D-85748 Garching, Germany
Abstract We investigate the eects of thermal annealing on the optical and electrical properties of p-type doped amorphous silicon suboxide (a-SiOx :H) samples prepared by plasma enhanced chemical vapour deposition (PECVD). Changes in the microscopic structure of the amorphous network upon thermal annealing at low temperatures (<550 K) cause a dopant activation of the p-type samples. The resulting increase of the dark conductivity becomes smaller with increasing oxygen content, but still comprises almost 2 orders of magnitude for samples with 9 at.% oxygen. Thermal annealing at higher temperatures (>550 K) leads to an eusion of hydrogen, thereby reducing the optical bandgap, E04 , of the samples. The dependence of E04 on the hydrogen content for amorphous suboxides with dierent oxygen content is found to be similar to that of amorphous silicon. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Hydrogenated amorphous silicon suboxides (aSiOx :H) represent a material system suitable for the application in silicon based light emitting devices. The optical bandgap and stable room temperature photoluminescence can be controlled by varying the oxygen content of the ®lms [1,2]. Pand n-type doping is possible by incorporation of boron and phosphorus, and thus light emitting diodes (LED) can be fabricated [3,4]. As previously shown, thermal annealing at temperatures <300°C reduces the defect absorption of intrinsic suboxides and annealing of a-SiOx :H p±i±n structures increases the forward current densities and the total electroluminescence intensity [5]. This work was performed to better understand the eects of thermal annealing on p-type suboxide
* Corresponding author. Tel.: +49-89 2891 2768; fax: +49-89 2891 2737. E-mail address: [email protected] (R. Janssen).
layers with the aim to improve the performance and stability of a-SiOx :H p±i±n structures.
2. Experimental P-type a-SiOx :H layers were deposited by plasma enhanced chemical vapour deposition (PECVD) using SiH4 (with 1 vol.% B2 H6 ) and CO2 as source gases and H2 as dilution gas. The substrate temperature, deposition pressure and deposition power were nominally 250°C, 0.5 mbar and 1 W, respectively. The oxygen content and the optical gap E04 (energy at which the absorption coecient is 104 cmÿ1 ) were controlled by varying the CO2 partial pressure CO2 = SiH4 CO2 between 0 and 0.6. To study the thermal stability of the optical and electrical properties, the samples were annealed under high vacuum (p < 10ÿ6 mbar) or nitrogen atmosphere at temperatures between 200°C and 900°C. Additionally, hydrogen eusion spectra were measured by heating the ®lms inside a quartz
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 8 5 2 - 2
R. Janssen et al. / Journal of Non-Crystalline Solids 266±269 (2000) 840±844
glass tube with a heating rate of 20°C/min and measuring the H2 -partial pressure with a quadrupole mass spectrometer.
3. Results Hydrogen eusion measurements of p-type suboxides (1% B2 H6 diluted in SiH4 ) are shown in Fig. 1. The total amount of eused hydrogen increases with increasing oxygen content. The hydrogen content of samples with 0, 6, 9 and 18 at.% oxygen was 18, 20, 21 and 24 at.%, respectively. Up to 9 at.% oxygen, there is a shift of the onset of hydrogen eusion and of the main eusion peak to higher temperatures, suggesting that backbonded oxygen leads to stronger silicon±hydrogen bonding. We assume that all incorporated hydrogen is bonded to silicon, since IR spectroscopy gives no evidence for the existence of O±H bonds in our amorphous silicon suboxides. The broadening of the eusion peaks with increasing oxygen content is caused by chemical bonding disorder. In con-
Fig. 1. Hydrogen eusion of p-type suboxides (1 vol.% B2 H6 ) with oxygen contents of 0, 6, 9 and 18 at.% as a function of temperature (lower x-axis) or the annealing energy parameter Eth kT ln mH t (upper x-axis). T and t are annealing temperature and time and mH 1010 Hz is a frequency prefactor.
841
trast to the ®lms with smaller oxygen content, the sample with 18 at.% oxygen has an eusion spectrum with two maxima. The low temperature effusion peak accounts for most of the bonded hydrogen. Note that at approximately 350°C a spike appears in all eusion spectra indicating a sudden eusion of a speci®c subset of hydrogen atoms. For the upper axis in Fig. 1 we introduce the parameter, Eth kT ln mH t, as a measure for the eective thermal annealing energy, Eth . Here, t and T are the annealing time and temperature, k the Boltzmann constant and mH 1010 Hz a frequency prefactor commonly used for amorphous silicon [6]. In this case the frequency prefactor, mH , is describing an attempt-to-escape frequency characteristic for hydrogen eusion. The parameter kT ln mH t is useful to combine isothermal and isochronal annealing experiments and was calculated for the eusion spectra by approximating the linear heating rate of 20°C/min by subsequent annealing temperatures 5°C apart and corresponding annealing times of 15 s. We now consider the eects of thermal annealing on the optical properties of p-type amorphous suboxides. Fig. 2 shows the absorption
Fig. 2. Absorption coecient of a p-type suboxide with 9 at.% oxygen in the as-deposited state and after annealing steps at successively increasing annealing energies.
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R. Janssen et al. / Journal of Non-Crystalline Solids 266±269 (2000) 840±844
coecient measured by photothermal de¯ection spectroscopy (PDS) for a sample with 9 at.% oxygen in the as-deposited state and after consecutive annealing steps parameterized by Eth . For Eth 1:4 eV (corresponding to the observed spike in the eusion spectra) no reduction of the optical bandgap E04 2:02 eV and only a small increase of the subgap absorption was detected. Increasing the thermal annealing energy reduced the optical bandgap, broadened the absorption tails and increased the subgap absorption. The eusion of hydrogen atoms from the ®lm thus leads to a greater structural disorder (indicated by an increase of the Urbach energy from 125 meV for the as-deposited state to approximately 200 meV for Eth 2:4 eV and an increased defect density due to broken silicon±hydrogen bonds. The optical bandgap as a function of the thermal annealing energy is shown in Fig. 3 for p-type suboxides with dierent oxygen contents. The values of E04 in the as-deposited state were 1.95, 1.98, and 2.02 eV and after eusion of all hydrogen 1.6, 1.65, and 1.72 eV for 0, 6, and 9 at.% oxygen, respectively. With increasing oxygen content the onset of the reduction of E04 shifted to larger an-
nealing energies and spread over a wider energy range, in agreement with the hydrogen eusion spectra. We now concentrate on the electrical properties of p-type amorphous suboxides. Thermal annealing at temperatures (T < 550 K) irreversibly increased the dark conductivity of p-type suboxides. Fig. 4 shows the dark conductivity of p-type samples measured at room temperature after consecutive annealing steps at increasing temperatures. In this ®gure, mr is a frequency prefactor characteristic of the structural changes aecting the electrical properties upon thermal annealing. Generally, mr will be dierent from mH since different microscopic processes will be at the origin of hydrogen eusion and the variation of electrical properties of amorphous suboxides (mH 10 mr , see below). An increase of the dark conductivity of p-type samples was observed for annealing energies less than 1.6 eV. The increase was largest for the a-Si:H sample in which the dark conductivity increased by almost 3 orders of magnitude. For larger oxygen contents the doping activation is reduced, but still comprises more than an order of magnitude at 18 at.% oxygen. At annealing energies of
Fig. 3. Optical bandgap E04 of p-type amorphous suboxides with oxygen contents of 0, 6 and 9 at.% as a function of annealing energy. The dotted lines are guide to the eye.
Fig. 4. Room temperature dark conductivity of p-type suboxides with oxygen contents of 0, 6, 9 and 18 at.% as a function of annealing energy.
R. Janssen et al. / Journal of Non-Crystalline Solids 266±269 (2000) 840±844
approximately 1.5 eV, a sudden decrease occurs in the dark conductivity by about an order of magnitude for all p-type samples containing oxygen. After this decrease, the dark conductivity remains almost unchanged over a range of annealing energies for all samples. This invariance is quite remarkable to us in view of the fact that the majority of the incorporated hydrogen euses from the ®lms in this annealing regime (see Fig. 1), however apparently without an eect on the dark conductivity, although the defect density increases (see Fig. 2). For Eth P 2:2 eV an increase of the dark conductivity, indicating partial crystallization of the ®lms, was followed by an increase of several orders of magnitude due to total crystallization of the ®lms. Note that both the onset of crystallization and the total crystallization shifted to larger annealing energies with increasing oxygen content. Comparison with Fig. 1 shows that the onset of crystallization is detected only after all hydrogen has eused from the ®lms.
4. Discussion To correlate the optical bandgap measured after consecutive isochronal annealing steps (Fig. 3) with the hydrogen content of the ®lms, the amount of eused hydrogen at a certain Eth was calculated from the eusion spectra (Fig. 1). For all samples E04 increases approximately linearly with the hydrogen content. The explicit dependence of E04 on the hydrogen content [H] can be expressed in the form E04 H E04 O 0:02 eVH ; where [H] is given in at.% and E04 O is a function of the oxygen content, namely 1.60, 1.66 and 1.73 eV for 0, 6, and 9 at.% oxygen, respectively. A similar dependence is usually observed for undoped a-Si:H (E04 H 1:67 eV 0:02 eVH [e.g. 7]. The smaller E04 O for our samples with 0 at.% oxygen is due to the broadened bandtail of doped compared to undoped samples, leading to smaller values of E04 . With increasing oxygen content, a decrease of the dark conductivity of p-type amorphous sub-
843
oxides is detected in the as-deposited state (shown in Fig. 4 at Eth 1:1 eV for display purposes). This decrease can be explained by a decreased probability of the formation of active dopant sites (dopant atom surrounded by four silicon atoms) due to the incorporation of oxygen atoms, based on continuous random network statistics [8]. Moreover, the appearance of a void-rich microstructure for our p-type sample with 18 at.% oxygen indicated by the low temperature eusion peak (see Fig. 1) may also contribute to the observed decrease of the dark conductivity with increasing oxygen content [9]. Boron-doped a-SiOx :H ®lms with oxygen contents of 10 at.% E04 2:0 eV and dark conductivities of 10ÿ6 S/cm were reported by Ichikawa et al. [10]. The dark conductivities of our borondoped samples with 10 at.% oxygen are an order of magnitude less in the as-deposited state, whereas dark conductivity of 2 10ÿ6 S/cm for our p-type as-deposited a-Si:H ®lm is less by almost 2 orders of magnitude compared to conductivities for stateof-the-art p-type amorphous silicon [11]. A possible reason for this fact is the passivation of boron acceptors by hydrogen atoms by formation of boron±hydrogen complexes, which has been observed in boron-doped crystalline silicon [12]. Thermal annealing at low annealing temperatures increased the dark conductivity. IR-measurements have provided evidence that, in this annealing range, the eusion of hydrogen bound in Si±H2 or (Si±H2 )n con®gurations takes place [13]. The thermal conductivity activation is limited (conductivity decrease in Fig. 4) by the sudden eusion of a large quantity of weakly bound hydrogen (eusion spike in Fig. 1), causing a major reconstruction of the amorphous network. A comparison of Figs. 1 and 4 shows that the annealing energy at which the eusion spike occurs is less by approximately 0.12 eV compared to the annealing energy of the decrease in the conductivity. A correlation of the eusion spike and the conductivity decrease can however be achieved by assuming a ratio of the frequency prefactors mH =mr 10. This dierence in the frequency prefactors, mH and mr , can be explained by assuming that 1 out of 10 hydrogen atoms which are eused from the sample at smaller annealing energies
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causes a change in the electrical properties of the amorphous suboxides. In the range of thermal conductivity activation (i.e. kT ln mr t < 1:5±1:6 eV) approximately 2 at.% of weakly bound hydrogen atoms euse from the ®lms. We therefore assume that the activation of boron acceptors is either directly caused by removal of hydrogen atoms or is mediated by a small scale motion of hydrogen. The shift of the onset of dopant activation to larger annealing energies with increasing oxygen content (see Fig. 4) is thus possibly due to the stabilization of passivating hydrogen con®gurations by backbonded oxygen atoms. Figs. 3 and 4 show that an optimized thermal annealing process at smaller annealing temperatures increased the dark conductivity of p-type suboxides without aecting the optical bandgap. This result is important, as the low conductivity of the p-layers determines the series resistance of p±i± n light emitting devices [5]. Moreover, the bandgap discontinuity at a p±i interface governs the injection of holes into the i-layer which is of critical importance for the electroluminescence properties [14]. Thermal annealing of diode structures therefore caused an increase of the forward current density without inhibiting the carrier injection properties. 5. Conclusion We have proven that thermal annealing of ptype suboxides at smaller annealing temperatures leads to conductivity activation by eusion of weakly bound hydrogen or a small scale motion of hydrogen. This activation is smaller and shifts to larger annealing energies with increasing incorporation of oxygen, but still increases the dark conductivity at 300 K by almost 2 orders of magnitude for p-type ®lms with 9 at.% oxygen. At higher annealing temperatures the eusion of hydrogen in
Si±H con®gurations causes a decrease of the optical bandgap E04 and a linear dependence of E04 on the hydrogen content of the amorphous suboxides was found, similar to what is known for aSi:H. Optimized annealing thus is important to improve the dark conductivity of p-type amorphous silicon suboxides. This annealing has direct consequences for the fabrication of LEDs, as the series resistance of these diodes is dominated by the resistivity of the p-type layer [5].
References [1] M. Zacharias, H. Freistedt, F. Stolze, T.P. Dr usedau, M. Rosenbauer, M. Stutzmann, J. Non-Cryst. Solids 164±166 (1993) 1089. [2] R.A. Street, J.C. Knights, Philos. Mag. B 42 (4) (1980) 551. [3] M.C. Rossi, M.S. Brandt, M. Stutzmann, Electrochem. Soc. Proc. 95-25 (1995) 445. [4] W. Boonkosum, D. Kruangam, B. Ratwises, T. Sujaridchai, S. Panyakeow, S. Fujikake, H. Sakai, J. Non-Cryst. Solids 198±200 (1996) 1226. [5] R. Janssen, U. Karrer, D. Dimova-Malinovska, M. Stutzmann, J. Non-Cryst. Solids 227±230 (1998) 1151. [6] R.A. Street, M. Hack, W.B. Jackson, Phys. Rev. B 37 (8) (1987) 4209. [7] M. Stutzmann, C.E. Nebel, in: Silicon, Amorphous, Encyclopedia of Applied Physics, vol. 18, 1997, p. 151. [8] H.R. Phillip, J. Non-Cryst. Solids 8±10 (1972) 627. [9] W. Beyer in: B. Adler, H. Fritzsche (Eds.), TetrahedrallyBonded Amorphous Semiconductors, Plenum, New York, 1985, p. 129. [10] Y. Ichikawa, S. Fujikake, H. Ohta, T. Sasaki, H. Sakai, in: Proc. 22th IEEE Photovoltaik Conf., Las Vegas, 1991, p. 1296. [11] W.E. Spear, P.G. Le Comber, Solid State Commun. 17 (1975) 1193. [12] J.I. Pankove, P.J. Zanzucchi, C.W. Maggee, G. Lucovsky, Appl. Phys. Lett. 46 (1985) 421. [13] R. Janssen, A. Janotta, D. Dimova-Malinovska, M. Stutzmann, Phys. Rev. 60 (9) (1999) 13561. [14] D. Kruamgam, M. Deguchi, T. Toyama, H. Okamoto, Y. Hamakawa, IEEE Trans. Electron Devices 35 (7) (1988) 957.
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Thermal stability of p-type doped amorphous silicon suboxides R. Janssen *, A. Janotta, M. Stutzmann Walter Schottky Institute, Technical University of Munich, Am Coulombwall, D-85748 Garching, Germany
Abstract We investigate the eects of thermal annealing on the optical and electrical properties of p-type doped amorphous silicon suboxide (a-SiOx :H) samples prepared by plasma enhanced chemical vapour deposition (PECVD). Changes in the microscopic structure of the amorphous network upon thermal annealing at low temperatures (<550 K) cause a dopant activation of the p-type samples. The resulting increase of the dark conductivity becomes smaller with increasing oxygen content, but still comprises almost 2 orders of magnitude for samples with 9 at.% oxygen. Thermal annealing at higher temperatures (>550 K) leads to an eusion of hydrogen, thereby reducing the optical bandgap, E04 , of the samples. The dependence of E04 on the hydrogen content for amorphous suboxides with dierent oxygen content is found to be similar to that of amorphous silicon. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Hydrogenated amorphous silicon suboxides (aSiOx :H) represent a material system suitable for the application in silicon based light emitting devices. The optical bandgap and stable room temperature photoluminescence can be controlled by varying the oxygen content of the ®lms [1,2]. Pand n-type doping is possible by incorporation of boron and phosphorus, and thus light emitting diodes (LED) can be fabricated [3,4]. As previously shown, thermal annealing at temperatures <300°C reduces the defect absorption of intrinsic suboxides and annealing of a-SiOx :H p±i±n structures increases the forward current densities and the total electroluminescence intensity [5]. This work was performed to better understand the eects of thermal annealing on p-type suboxide
* Corresponding author. Tel.: +49-89 2891 2768; fax: +49-89 2891 2737. E-mail address: [email protected] (R. Janssen).
layers with the aim to improve the performance and stability of a-SiOx :H p±i±n structures.
2. Experimental P-type a-SiOx :H layers were deposited by plasma enhanced chemical vapour deposition (PECVD) using SiH4 (with 1 vol.% B2 H6 ) and CO2 as source gases and H2 as dilution gas. The substrate temperature, deposition pressure and deposition power were nominally 250°C, 0.5 mbar and 1 W, respectively. The oxygen content and the optical gap E04 (energy at which the absorption coecient is 104 cmÿ1 ) were controlled by varying the CO2 partial pressure CO2 = SiH4 CO2 between 0 and 0.6. To study the thermal stability of the optical and electrical properties, the samples were annealed under high vacuum (p < 10ÿ6 mbar) or nitrogen atmosphere at temperatures between 200°C and 900°C. Additionally, hydrogen eusion spectra were measured by heating the ®lms inside a quartz
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 8 5 2 - 2
R. Janssen et al. / Journal of Non-Crystalline Solids 266±269 (2000) 840±844
glass tube with a heating rate of 20°C/min and measuring the H2 -partial pressure with a quadrupole mass spectrometer.
3. Results Hydrogen eusion measurements of p-type suboxides (1% B2 H6 diluted in SiH4 ) are shown in Fig. 1. The total amount of eused hydrogen increases with increasing oxygen content. The hydrogen content of samples with 0, 6, 9 and 18 at.% oxygen was 18, 20, 21 and 24 at.%, respectively. Up to 9 at.% oxygen, there is a shift of the onset of hydrogen eusion and of the main eusion peak to higher temperatures, suggesting that backbonded oxygen leads to stronger silicon±hydrogen bonding. We assume that all incorporated hydrogen is bonded to silicon, since IR spectroscopy gives no evidence for the existence of O±H bonds in our amorphous silicon suboxides. The broadening of the eusion peaks with increasing oxygen content is caused by chemical bonding disorder. In con-
Fig. 1. Hydrogen eusion of p-type suboxides (1 vol.% B2 H6 ) with oxygen contents of 0, 6, 9 and 18 at.% as a function of temperature (lower x-axis) or the annealing energy parameter Eth kT ln mH t (upper x-axis). T and t are annealing temperature and time and mH 1010 Hz is a frequency prefactor.
841
trast to the ®lms with smaller oxygen content, the sample with 18 at.% oxygen has an eusion spectrum with two maxima. The low temperature effusion peak accounts for most of the bonded hydrogen. Note that at approximately 350°C a spike appears in all eusion spectra indicating a sudden eusion of a speci®c subset of hydrogen atoms. For the upper axis in Fig. 1 we introduce the parameter, Eth kT ln mH t, as a measure for the eective thermal annealing energy, Eth . Here, t and T are the annealing time and temperature, k the Boltzmann constant and mH 1010 Hz a frequency prefactor commonly used for amorphous silicon [6]. In this case the frequency prefactor, mH , is describing an attempt-to-escape frequency characteristic for hydrogen eusion. The parameter kT ln mH t is useful to combine isothermal and isochronal annealing experiments and was calculated for the eusion spectra by approximating the linear heating rate of 20°C/min by subsequent annealing temperatures 5°C apart and corresponding annealing times of 15 s. We now consider the eects of thermal annealing on the optical properties of p-type amorphous suboxides. Fig. 2 shows the absorption
Fig. 2. Absorption coecient of a p-type suboxide with 9 at.% oxygen in the as-deposited state and after annealing steps at successively increasing annealing energies.
842
R. Janssen et al. / Journal of Non-Crystalline Solids 266±269 (2000) 840±844
coecient measured by photothermal de¯ection spectroscopy (PDS) for a sample with 9 at.% oxygen in the as-deposited state and after consecutive annealing steps parameterized by Eth . For Eth 1:4 eV (corresponding to the observed spike in the eusion spectra) no reduction of the optical bandgap E04 2:02 eV and only a small increase of the subgap absorption was detected. Increasing the thermal annealing energy reduced the optical bandgap, broadened the absorption tails and increased the subgap absorption. The eusion of hydrogen atoms from the ®lm thus leads to a greater structural disorder (indicated by an increase of the Urbach energy from 125 meV for the as-deposited state to approximately 200 meV for Eth 2:4 eV and an increased defect density due to broken silicon±hydrogen bonds. The optical bandgap as a function of the thermal annealing energy is shown in Fig. 3 for p-type suboxides with dierent oxygen contents. The values of E04 in the as-deposited state were 1.95, 1.98, and 2.02 eV and after eusion of all hydrogen 1.6, 1.65, and 1.72 eV for 0, 6, and 9 at.% oxygen, respectively. With increasing oxygen content the onset of the reduction of E04 shifted to larger an-
nealing energies and spread over a wider energy range, in agreement with the hydrogen eusion spectra. We now concentrate on the electrical properties of p-type amorphous suboxides. Thermal annealing at temperatures (T < 550 K) irreversibly increased the dark conductivity of p-type suboxides. Fig. 4 shows the dark conductivity of p-type samples measured at room temperature after consecutive annealing steps at increasing temperatures. In this ®gure, mr is a frequency prefactor characteristic of the structural changes aecting the electrical properties upon thermal annealing. Generally, mr will be dierent from mH since different microscopic processes will be at the origin of hydrogen eusion and the variation of electrical properties of amorphous suboxides (mH 10 mr , see below). An increase of the dark conductivity of p-type samples was observed for annealing energies less than 1.6 eV. The increase was largest for the a-Si:H sample in which the dark conductivity increased by almost 3 orders of magnitude. For larger oxygen contents the doping activation is reduced, but still comprises more than an order of magnitude at 18 at.% oxygen. At annealing energies of
Fig. 3. Optical bandgap E04 of p-type amorphous suboxides with oxygen contents of 0, 6 and 9 at.% as a function of annealing energy. The dotted lines are guide to the eye.
Fig. 4. Room temperature dark conductivity of p-type suboxides with oxygen contents of 0, 6, 9 and 18 at.% as a function of annealing energy.
R. Janssen et al. / Journal of Non-Crystalline Solids 266±269 (2000) 840±844
approximately 1.5 eV, a sudden decrease occurs in the dark conductivity by about an order of magnitude for all p-type samples containing oxygen. After this decrease, the dark conductivity remains almost unchanged over a range of annealing energies for all samples. This invariance is quite remarkable to us in view of the fact that the majority of the incorporated hydrogen euses from the ®lms in this annealing regime (see Fig. 1), however apparently without an eect on the dark conductivity, although the defect density increases (see Fig. 2). For Eth P 2:2 eV an increase of the dark conductivity, indicating partial crystallization of the ®lms, was followed by an increase of several orders of magnitude due to total crystallization of the ®lms. Note that both the onset of crystallization and the total crystallization shifted to larger annealing energies with increasing oxygen content. Comparison with Fig. 1 shows that the onset of crystallization is detected only after all hydrogen has eused from the ®lms.
4. Discussion To correlate the optical bandgap measured after consecutive isochronal annealing steps (Fig. 3) with the hydrogen content of the ®lms, the amount of eused hydrogen at a certain Eth was calculated from the eusion spectra (Fig. 1). For all samples E04 increases approximately linearly with the hydrogen content. The explicit dependence of E04 on the hydrogen content [H] can be expressed in the form E04 H E04 O 0:02 eVH ; where [H] is given in at.% and E04 O is a function of the oxygen content, namely 1.60, 1.66 and 1.73 eV for 0, 6, and 9 at.% oxygen, respectively. A similar dependence is usually observed for undoped a-Si:H (E04 H 1:67 eV 0:02 eVH [e.g. 7]. The smaller E04 O for our samples with 0 at.% oxygen is due to the broadened bandtail of doped compared to undoped samples, leading to smaller values of E04 . With increasing oxygen content, a decrease of the dark conductivity of p-type amorphous sub-
843
oxides is detected in the as-deposited state (shown in Fig. 4 at Eth 1:1 eV for display purposes). This decrease can be explained by a decreased probability of the formation of active dopant sites (dopant atom surrounded by four silicon atoms) due to the incorporation of oxygen atoms, based on continuous random network statistics [8]. Moreover, the appearance of a void-rich microstructure for our p-type sample with 18 at.% oxygen indicated by the low temperature eusion peak (see Fig. 1) may also contribute to the observed decrease of the dark conductivity with increasing oxygen content [9]. Boron-doped a-SiOx :H ®lms with oxygen contents of 10 at.% E04 2:0 eV and dark conductivities of 10ÿ6 S/cm were reported by Ichikawa et al. [10]. The dark conductivities of our borondoped samples with 10 at.% oxygen are an order of magnitude less in the as-deposited state, whereas dark conductivity of 2 10ÿ6 S/cm for our p-type as-deposited a-Si:H ®lm is less by almost 2 orders of magnitude compared to conductivities for stateof-the-art p-type amorphous silicon [11]. A possible reason for this fact is the passivation of boron acceptors by hydrogen atoms by formation of boron±hydrogen complexes, which has been observed in boron-doped crystalline silicon [12]. Thermal annealing at low annealing temperatures increased the dark conductivity. IR-measurements have provided evidence that, in this annealing range, the eusion of hydrogen bound in Si±H2 or (Si±H2 )n con®gurations takes place [13]. The thermal conductivity activation is limited (conductivity decrease in Fig. 4) by the sudden eusion of a large quantity of weakly bound hydrogen (eusion spike in Fig. 1), causing a major reconstruction of the amorphous network. A comparison of Figs. 1 and 4 shows that the annealing energy at which the eusion spike occurs is less by approximately 0.12 eV compared to the annealing energy of the decrease in the conductivity. A correlation of the eusion spike and the conductivity decrease can however be achieved by assuming a ratio of the frequency prefactors mH =mr 10. This dierence in the frequency prefactors, mH and mr , can be explained by assuming that 1 out of 10 hydrogen atoms which are eused from the sample at smaller annealing energies
844
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causes a change in the electrical properties of the amorphous suboxides. In the range of thermal conductivity activation (i.e. kT ln mr t < 1:5±1:6 eV) approximately 2 at.% of weakly bound hydrogen atoms euse from the ®lms. We therefore assume that the activation of boron acceptors is either directly caused by removal of hydrogen atoms or is mediated by a small scale motion of hydrogen. The shift of the onset of dopant activation to larger annealing energies with increasing oxygen content (see Fig. 4) is thus possibly due to the stabilization of passivating hydrogen con®gurations by backbonded oxygen atoms. Figs. 3 and 4 show that an optimized thermal annealing process at smaller annealing temperatures increased the dark conductivity of p-type suboxides without aecting the optical bandgap. This result is important, as the low conductivity of the p-layers determines the series resistance of p±i± n light emitting devices [5]. Moreover, the bandgap discontinuity at a p±i interface governs the injection of holes into the i-layer which is of critical importance for the electroluminescence properties [14]. Thermal annealing of diode structures therefore caused an increase of the forward current density without inhibiting the carrier injection properties. 5. Conclusion We have proven that thermal annealing of ptype suboxides at smaller annealing temperatures leads to conductivity activation by eusion of weakly bound hydrogen or a small scale motion of hydrogen. This activation is smaller and shifts to larger annealing energies with increasing incorporation of oxygen, but still increases the dark conductivity at 300 K by almost 2 orders of magnitude for p-type ®lms with 9 at.% oxygen. At higher annealing temperatures the eusion of hydrogen in
Si±H con®gurations causes a decrease of the optical bandgap E04 and a linear dependence of E04 on the hydrogen content of the amorphous suboxides was found, similar to what is known for aSi:H. Optimized annealing thus is important to improve the dark conductivity of p-type amorphous silicon suboxides. This annealing has direct consequences for the fabrication of LEDs, as the series resistance of these diodes is dominated by the resistivity of the p-type layer [5].
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