- Email: [email protected]
Amorphous silicon suboxide light-emitting diodes
Journal of Non-Crystalline Solids 227–230 Ž1998. 1151–1155
Amorphous silicon suboxide light-emitting diodes R. Janssen b
a,)
, U. Karrer a , D. Dimova-Malinovska b, M. Stutzmann
a
a Walter Schottky Institute, Technical UniÕersity of Munich, Am Coulombwall, D-85748 Garching, Germany Central Laboratory for Solar Energy and New Energy Resources, Bulg. Acad. Sci., Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria
Abstract Optical absorption was measured on undoped as deposited and annealed amorphous silicon suboxide Ža-SiO x :H. samples prepared by plasma enhanced chemical vapour deposition. The subgap absorption was reduced by one order of magnitude upon annealing. Annealing of a-SiO x :H p–i–n structures at 2508C increased the forward current densities and shifted the dominant electroluminescence peak from 1.1 eV to 1.3 eV, indicating a change in the recombination mechanism due to a lower defect density in the intrinsic layer. The total electroluminescence efficiency increased by a factor of 3 upon annealing and reached ; 10y3 %. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous silicon suboxide; Diodes; Optical absorption
1. Introduction Hydrogenated amorphous silicon suboxide ŽaSiO x :H. has been considered as a material 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 films w1–3x. P- and n-type doping is possible by incorporating boron or phosphorous atoms into the films, and thus, lightemitting diodes ŽLED. can be produced w4,5x. In this work we present electrical and spectrally resolved electroluminescence properties of a-SiO x :H p–i–n devices deposited by plasma enhanced chemical vapour deposition ŽPECVD..
)
Corresponding author. Tel.: q49-89-289-12768; fax: q4989-289-12737; e-mail: [email protected].
2. Experimental Hydrogenated amorphous silicon suboxide ŽaSiO x :H. was deposited by PECVD using SiH 4 and CO 2 diluted in H 2 as source gases. The substrate temperature and pressure of the reactive gases were nominally 2508C and 0.6 mbar, respectively. The oxygen content and the optical gap E04 of the samples can be controlled by varying the CO 2-partial pressure ŽCO 2rŽSiH 4 q CO 2 . s 0.4 to 0.85. and the deposition power Ž1 to 10 W.. Doping was realized by adding PH 3 and B 2 H 6 Ž1% diluted in SiH 4 . to the source gases. The oxygen content of the films was determined by energy dispersive X-ray spectroscopy ŽEDX. using a silicon wafer and a quartz plate as standard samples. P–i–n diodes were deposited on transparent conducting oxide using p- and n-doped layers with lower oxygen content Ž10 at.%, E04 s 2.0 eV. and thicknesses of 30 nm and 100 nm, respectively, for
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 4 9 - X
1152
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
an efficient injection of carriers into an i-layer with approx. 30 at.% oxygen Ž E04 s 2.3 eV. and variable thickness as the active luminescent layer. As back contacts, we used 40-nm Cr and 100-nm Au. The dark conductivities of the p-, n- and undoped layers are 10y7 Srcm, 10y4 Srcm and 10y1 2 Srcm. We measured the optical bandgap defined by E04 , which is the energy where the absorption coefficient equals 10 4 cmy1 . To study the effects of annealing on the optical properties, the samples were annealed under vacuum for 60 min. The annealing temperature was varied from 2508C to 9008C. Room temperature integral electroluminescence intensity ŽEL. was measured through the substrate using a silicon detector with a low energy detection limit of about 1.2 eV. Spectrally resolved EL was measured with a single grating monochromator and a liquid nitrogen cooled germanium detector.
3. Results With the oxygen content increasing from 0 to 50 at.%, the optical bandgap, E04 , and the Urbach energy, E0 , increased from 1.9 eV to 3.0 eV and from 70 meV to 200 meV, respectively, indicating an increasing disorder of the amorphous network with increasing incorporation of oxygen. Similar results for E04 and E0 as a function of the oxygen content were obtained by Carius et al. w2x for amorphous SiO x alloy systems produced by glow discharge of SiH 4 and N2 O, while dc magnetron sputtering using a crystalline silicon target and water vapour as the oxygen and hydrogen source results in samples with smaller optical bandgap Žprobably because of a smaller hydrogen content in the films. and lower Urbach energies w6x. The subgap absorption at 1.3 eV shows a maximum for oxygen contents of 30 at.% Ž E04 s 2.3 eV. for our samples. Electron spin resonance ŽESR. measurements confirm that the dangling bond density of a-SiO x :H samples with 30 at.% oxygen amounted to about 10 19 cmy3 and decreased for both higher and lower oxygen concentrations by about an order of magnitude. Thus, the absorption coefficient at 1.3 eV is a convenient measure of the defect density of suboxide films with oxygen contents F 50 at.%.
Annealing the sample with 30 at.% oxygen at 2508C for 60 min reduces a Ž1.3 eV. by about a factor of 5 without affecting the Urbach edge and the optical bandgap ŽFig. 1.. Annealing at 3008C further reduced the defect absorption, but also reduced the bandgap due to a beginning effusion of hydrogen from the sample. Annealing at higher temperatures again increases the absorption at 1.3 eV and the Urbach energy from 120 meV to more than 180 meV. In this regime, dangling bonds are no longer saturated by H-atoms, and after annealing at 9008C, all hydrogen has effused from the sample. A minimum of the defect density was detected for annealing temperatures of about 3008C by both ESR and PDS measurements. Fig. 2 shows the current–voltage Ž I–V . characteristics of a series of suboxide p–i–n diodes with identical p- and n-layers as described above Ž E04 s 2.0 eV.. The optical gap of the i-layer was 2.3 eV and the i-layer thickness was varied between 0 nm and 800 nm. The rectification ratio of thin suboxide diodes exceeds 5 orders of magnitude, and a significant current density under reasonable forward bias is reached for diodes with an i-layer thickness less than 200 nm. High voltage stress for several hours was used to certify the stability of the devices for i-layer thicknesses larger than 100 nm.
Fig. 1. Influence of annealing on the optical absorption of undoped a-SiO x :H with 30 at.% oxygen. The absorption coefficient can be calculated from the PDS-signal intensity data for values below f10 4 cmy1 Žindicated by the dashed line..
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
Fig. 2. I – V characteristics of suboxide p–i–n diodes with different i-layer thicknesses at 300 K. The dashed line shows an amorphous silicon p–i–n diode for comparison.
As annealing of undoped suboxide samples reduces the defect density significantly, we also annealed the p–i–n structures at 2508C for 60 min. Fig. 3 compares the I–V characteristics of as deposited and annealed suboxide diodes. While for as deposited thick suboxide diodes the I–V curve is dominated by the high resistivity of the undoped layer, rectifying behaviour over 4 orders of magnitude is detected after annealing at 2508C. This rectification
Fig. 3. Comparison of the I – V characteristics of as deposited and annealed diodes with i-layer thicknesses of 100 nm and 800 nm.
1153
allows current densities exceeding 10y3 Arcm2 necessary for detectable EL intensity as shown below. The EL intensity at room temperature was measured with a conventional silicon detector, in order to demonstrate the usefulness of the p–i–n diodes for Si-integrated optoelectronics ŽFig. 4.. An EL signal appears at current densities exceeding 10y3 Arcm2 for as deposited samples and increases linearly with the injection current density over more than 2 orders of magnitude. The EL intensity at a given current density also increases with increasing i-layer thickness, and the best EL performance was detected for an i-layer thickness of 150 nm. No EL is detected under reverse bias. Annealing of the suboxide diodes at 2508C increases the maximum integral EL intensity by almost a factor of 10 for all i-layer thicknesses. Moreover, EL is now detected for thick diodes as well, however, the EL intensity remains constant for i-layer thicknesses larger than 100 nm. Spectrally resolved EL measurements at 300 K using a cooled Ge-detector ŽFig. 5. show a defect related luminescence peak at 0.85 eV for all diodes, being most prominent for thinner samples in the as deposited state. As deposited diodes show an additional luminescence peak at 1.1 eV, which dominates the spectrum for an i-layer thickness of 100 nm and shifts towards lower photon energies for thicker ilayers. Annealing the diodes at 2508C results in a
Fig. 4. EL intensity measured by a Si-photodetector vs. injection current density for as-deposited and annealed suboxide diodes.
1154
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
Fig. 5. Spectrally resolved electro-luminescence as a function of i-layer thickness for as-deposited and annealed diodes.
shift of the dominant luminescence peak from 1.1 eV to about 1.3 eV Žboth at 300 K.. The total EL efficiency was estimated to be 10y3 % for annealed diodes with i-layer thicknesses of 100 nm Ž E04 Ži-layer. s 2.3 eV., which is a factor of 3 higher than the efficiency detected for as deposited samples.
4. Discussion The discussion of the data is limited by the lack of understanding of recombination and transport mechanisms in amorphous silicon suboxides. Moreover, band-offsets at the p–i and i–n interfaces and carrier injection mechanisms into the intrinsic layer are not fully understood. Modelling the I–V characteristics with a basic circuit diagram for nonideal diodes gives values of 10 9 V for the shunt resistance influencing the current density at small forward voltages and under reverse bias ŽFig. 2.. The ideality factor describing the exponential increase of the current density at intermediate forward voltages increases from 3 for an i-layer thickness of 30 nm to about 20 for thicker i-layers. A plausible explanation for this experimental result is still lacking. At high forward bias, the I–V curves of thin diodes are
dominated by the diodes’ series resistance of about 800 V, which is due to the smaller conductivity of the p-doped layer. The temperature dependence of the forward bias I–V properties have shown that at the lowest applied fields carrier transport through the junction is determined by thermionic emission over the energy barriers at the p–i and i–n interfaces, while tunneling injection dominates the current flow at larger fields w7x. Annealing of the suboxide diodes leaves series and shunt resistances unchanged, while the onset of the exponential increase of the current density shifts to smaller voltages and the ideality factor is reduced ŽFig. 3.. Reduction of the defect density in the i-layers of the diodes by annealing at 2508C thus improves the performance of the diodes significantly. The detailed influence of annealing on contacts, interfaces and doped layers still needs to be investigated. The EL intensity measured by a silicon detector ŽFig. 4. shows a power law dependence on the injected current density, I EL ; jg. For as deposited suboxide diodes, g equals 1, whereas annealed diodes show g values in the range of 1.2 to 1.4. A similar result was reported by Han and Wang w8x for EL studies on hydrogenated amorphous silicon p–i–n diodes. At 300 K, they found two emission bands at 0.85 eV Ždefect recombination. and 1.2 eV Žbandto-band recombination. with g values of 1.0 and about 1.5, respectively. Thus, as deposited suboxide diodes also seem to show a predominance of radiative recombination at defects, whereas annealed diodes exhibit an increasing contribution of band-toband recombination. This is in qualitative agreement with the spectrally resolved EL measurements ŽFig. 5.. All diodes show an EL peak at 0.85 eV assigned to defect recombination at the p–i interface w8x. The origin of the EL peak at 1.1 eV detected for as deposited diodes is most likely due to recombination of electrons at neutral dangling bond defects ŽD 0 . in the i-layer. Photoluminescence at 1.1 eV has been detected in amorphous silicon–oxygen alloys with similar oxygen content, and has also been assigned to oxygen related defects w9x. With increasing i-layer thickness, the carriers thermalize deeper into the bandtails of the i-layer and thus, the EL is observed at lower photon energies.
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
As confirmed by PDS and ESR measurements, the defect density in the i-layer decreases significantly upon annealing. Thus, the probability of recombination at neutral D 0 defects will be reduced in favour of band-to-band recombination, shifting the EL peak to 1.3 eV. Thus, the EL peak energy of band-to-band recombination appears to occur about 1.0 eV below the absorption bandgap of the intrinsic layer. A similar result was obtained by Kruangam et al. w10x for a-SiC:H p–i–n thin film light emitting diodes.
5. Conclusions Optical subgap absorption of undoped a-SiO x :H Žw O x s 30 at.%, E04 s 2.3 eV. prepared by PECVD is reduced by about 1 order of magnitude upon annealing at 2508C. This reduction improves the I–V properties of suboxide diodes with i-layer thicknesses between 30 nm and 800 nm. Spectrally resolved EL measurements show a luminescence peak at 0.85 eV assigned to defect related recombination at the p–i interface and an additional peak at 1.1 eV for as deposited diodes, shifting towards 1.3 eV upon annealing. The total EL efficiency of annealed sub-
1155
oxide diodes is increased by a factor of 3 to 10y3 %. Because this EL is easily detected by standard c-Si photodiodes, suboxide p–i–n LEDs are interesting active components for Si-based optoelectronics. References w1x M. Zacharias, H. Freistedt, F. Stolze, T.P. Drusedau, M. ¨ Rosenbauer, M. Stutzmann, J. Non-Cryst. Solids 164–166 Ž1993. 1089. w2x R. Carius, R. Fischer, E. Holzenkampfer, J. Stuke, J. Appl. ¨ Phys. 52 Ž6. Ž1981. 4241. w3x R.A. Street, J.C. Knights, Philos. Mag. B 42 Ž4. Ž1980. 551. w4x M.C. Rossi, M.S. Brandt, M. Stutzmann, Electrochem. Soc. Proc. 95–25 Ž1995. 445. w5x W. Boonkosum, D. Kruangam, B. Ratwises, T. Sujaridchai, S. Panyakeow, S. Fujikake, H. Sakai, J. Non-Cryst. Solids 198–200 Ž1996. 1226. w6x M. Zacharias, D. Dimova-Malinovska, M. Stutzmann, Philos. Mag. B 73 Ž5. Ž1996. 799. w7x M.C. Rossi, D. Dimova-Malinovska, M.S. Brandt, M. Stutzmann, in: R.H. Mauch, H.E. Gumlich ŽEds.., Inorganic and Organic Electroluminescence, Wissenschaft and Technik Verlag, Berlin, 1996, p. 189. w8x D. Han, K. Wang, J. Non-Cryst. Solids 190 Ž1995. 74. w9x R.A. Street, J.C. Knights, D.K. Biegelsen, Phys. Rev. B 18 Ž4. Ž1978. 1880. w10x D. Kruangam, T. Sujaridchai, K. Chirakawikul, B. Ratwises, S. Panyakeow, 227–230 Ž1998. 1146.
Amorphous silicon suboxide light-emitting diodes R. Janssen b
a,)
, U. Karrer a , D. Dimova-Malinovska b, M. Stutzmann
a
a Walter Schottky Institute, Technical UniÕersity of Munich, Am Coulombwall, D-85748 Garching, Germany Central Laboratory for Solar Energy and New Energy Resources, Bulg. Acad. Sci., Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria
Abstract Optical absorption was measured on undoped as deposited and annealed amorphous silicon suboxide Ža-SiO x :H. samples prepared by plasma enhanced chemical vapour deposition. The subgap absorption was reduced by one order of magnitude upon annealing. Annealing of a-SiO x :H p–i–n structures at 2508C increased the forward current densities and shifted the dominant electroluminescence peak from 1.1 eV to 1.3 eV, indicating a change in the recombination mechanism due to a lower defect density in the intrinsic layer. The total electroluminescence efficiency increased by a factor of 3 upon annealing and reached ; 10y3 %. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous silicon suboxide; Diodes; Optical absorption
1. Introduction Hydrogenated amorphous silicon suboxide ŽaSiO x :H. has been considered as a material 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 films w1–3x. P- and n-type doping is possible by incorporating boron or phosphorous atoms into the films, and thus, lightemitting diodes ŽLED. can be produced w4,5x. In this work we present electrical and spectrally resolved electroluminescence properties of a-SiO x :H p–i–n devices deposited by plasma enhanced chemical vapour deposition ŽPECVD..
)
Corresponding author. Tel.: q49-89-289-12768; fax: q4989-289-12737; e-mail: [email protected].
2. Experimental Hydrogenated amorphous silicon suboxide ŽaSiO x :H. was deposited by PECVD using SiH 4 and CO 2 diluted in H 2 as source gases. The substrate temperature and pressure of the reactive gases were nominally 2508C and 0.6 mbar, respectively. The oxygen content and the optical gap E04 of the samples can be controlled by varying the CO 2-partial pressure ŽCO 2rŽSiH 4 q CO 2 . s 0.4 to 0.85. and the deposition power Ž1 to 10 W.. Doping was realized by adding PH 3 and B 2 H 6 Ž1% diluted in SiH 4 . to the source gases. The oxygen content of the films was determined by energy dispersive X-ray spectroscopy ŽEDX. using a silicon wafer and a quartz plate as standard samples. P–i–n diodes were deposited on transparent conducting oxide using p- and n-doped layers with lower oxygen content Ž10 at.%, E04 s 2.0 eV. and thicknesses of 30 nm and 100 nm, respectively, for
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 4 9 - X
1152
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
an efficient injection of carriers into an i-layer with approx. 30 at.% oxygen Ž E04 s 2.3 eV. and variable thickness as the active luminescent layer. As back contacts, we used 40-nm Cr and 100-nm Au. The dark conductivities of the p-, n- and undoped layers are 10y7 Srcm, 10y4 Srcm and 10y1 2 Srcm. We measured the optical bandgap defined by E04 , which is the energy where the absorption coefficient equals 10 4 cmy1 . To study the effects of annealing on the optical properties, the samples were annealed under vacuum for 60 min. The annealing temperature was varied from 2508C to 9008C. Room temperature integral electroluminescence intensity ŽEL. was measured through the substrate using a silicon detector with a low energy detection limit of about 1.2 eV. Spectrally resolved EL was measured with a single grating monochromator and a liquid nitrogen cooled germanium detector.
3. Results With the oxygen content increasing from 0 to 50 at.%, the optical bandgap, E04 , and the Urbach energy, E0 , increased from 1.9 eV to 3.0 eV and from 70 meV to 200 meV, respectively, indicating an increasing disorder of the amorphous network with increasing incorporation of oxygen. Similar results for E04 and E0 as a function of the oxygen content were obtained by Carius et al. w2x for amorphous SiO x alloy systems produced by glow discharge of SiH 4 and N2 O, while dc magnetron sputtering using a crystalline silicon target and water vapour as the oxygen and hydrogen source results in samples with smaller optical bandgap Žprobably because of a smaller hydrogen content in the films. and lower Urbach energies w6x. The subgap absorption at 1.3 eV shows a maximum for oxygen contents of 30 at.% Ž E04 s 2.3 eV. for our samples. Electron spin resonance ŽESR. measurements confirm that the dangling bond density of a-SiO x :H samples with 30 at.% oxygen amounted to about 10 19 cmy3 and decreased for both higher and lower oxygen concentrations by about an order of magnitude. Thus, the absorption coefficient at 1.3 eV is a convenient measure of the defect density of suboxide films with oxygen contents F 50 at.%.
Annealing the sample with 30 at.% oxygen at 2508C for 60 min reduces a Ž1.3 eV. by about a factor of 5 without affecting the Urbach edge and the optical bandgap ŽFig. 1.. Annealing at 3008C further reduced the defect absorption, but also reduced the bandgap due to a beginning effusion of hydrogen from the sample. Annealing at higher temperatures again increases the absorption at 1.3 eV and the Urbach energy from 120 meV to more than 180 meV. In this regime, dangling bonds are no longer saturated by H-atoms, and after annealing at 9008C, all hydrogen has effused from the sample. A minimum of the defect density was detected for annealing temperatures of about 3008C by both ESR and PDS measurements. Fig. 2 shows the current–voltage Ž I–V . characteristics of a series of suboxide p–i–n diodes with identical p- and n-layers as described above Ž E04 s 2.0 eV.. The optical gap of the i-layer was 2.3 eV and the i-layer thickness was varied between 0 nm and 800 nm. The rectification ratio of thin suboxide diodes exceeds 5 orders of magnitude, and a significant current density under reasonable forward bias is reached for diodes with an i-layer thickness less than 200 nm. High voltage stress for several hours was used to certify the stability of the devices for i-layer thicknesses larger than 100 nm.
Fig. 1. Influence of annealing on the optical absorption of undoped a-SiO x :H with 30 at.% oxygen. The absorption coefficient can be calculated from the PDS-signal intensity data for values below f10 4 cmy1 Žindicated by the dashed line..
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
Fig. 2. I – V characteristics of suboxide p–i–n diodes with different i-layer thicknesses at 300 K. The dashed line shows an amorphous silicon p–i–n diode for comparison.
As annealing of undoped suboxide samples reduces the defect density significantly, we also annealed the p–i–n structures at 2508C for 60 min. Fig. 3 compares the I–V characteristics of as deposited and annealed suboxide diodes. While for as deposited thick suboxide diodes the I–V curve is dominated by the high resistivity of the undoped layer, rectifying behaviour over 4 orders of magnitude is detected after annealing at 2508C. This rectification
Fig. 3. Comparison of the I – V characteristics of as deposited and annealed diodes with i-layer thicknesses of 100 nm and 800 nm.
1153
allows current densities exceeding 10y3 Arcm2 necessary for detectable EL intensity as shown below. The EL intensity at room temperature was measured with a conventional silicon detector, in order to demonstrate the usefulness of the p–i–n diodes for Si-integrated optoelectronics ŽFig. 4.. An EL signal appears at current densities exceeding 10y3 Arcm2 for as deposited samples and increases linearly with the injection current density over more than 2 orders of magnitude. The EL intensity at a given current density also increases with increasing i-layer thickness, and the best EL performance was detected for an i-layer thickness of 150 nm. No EL is detected under reverse bias. Annealing of the suboxide diodes at 2508C increases the maximum integral EL intensity by almost a factor of 10 for all i-layer thicknesses. Moreover, EL is now detected for thick diodes as well, however, the EL intensity remains constant for i-layer thicknesses larger than 100 nm. Spectrally resolved EL measurements at 300 K using a cooled Ge-detector ŽFig. 5. show a defect related luminescence peak at 0.85 eV for all diodes, being most prominent for thinner samples in the as deposited state. As deposited diodes show an additional luminescence peak at 1.1 eV, which dominates the spectrum for an i-layer thickness of 100 nm and shifts towards lower photon energies for thicker ilayers. Annealing the diodes at 2508C results in a
Fig. 4. EL intensity measured by a Si-photodetector vs. injection current density for as-deposited and annealed suboxide diodes.
1154
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
Fig. 5. Spectrally resolved electro-luminescence as a function of i-layer thickness for as-deposited and annealed diodes.
shift of the dominant luminescence peak from 1.1 eV to about 1.3 eV Žboth at 300 K.. The total EL efficiency was estimated to be 10y3 % for annealed diodes with i-layer thicknesses of 100 nm Ž E04 Ži-layer. s 2.3 eV., which is a factor of 3 higher than the efficiency detected for as deposited samples.
4. Discussion The discussion of the data is limited by the lack of understanding of recombination and transport mechanisms in amorphous silicon suboxides. Moreover, band-offsets at the p–i and i–n interfaces and carrier injection mechanisms into the intrinsic layer are not fully understood. Modelling the I–V characteristics with a basic circuit diagram for nonideal diodes gives values of 10 9 V for the shunt resistance influencing the current density at small forward voltages and under reverse bias ŽFig. 2.. The ideality factor describing the exponential increase of the current density at intermediate forward voltages increases from 3 for an i-layer thickness of 30 nm to about 20 for thicker i-layers. A plausible explanation for this experimental result is still lacking. At high forward bias, the I–V curves of thin diodes are
dominated by the diodes’ series resistance of about 800 V, which is due to the smaller conductivity of the p-doped layer. The temperature dependence of the forward bias I–V properties have shown that at the lowest applied fields carrier transport through the junction is determined by thermionic emission over the energy barriers at the p–i and i–n interfaces, while tunneling injection dominates the current flow at larger fields w7x. Annealing of the suboxide diodes leaves series and shunt resistances unchanged, while the onset of the exponential increase of the current density shifts to smaller voltages and the ideality factor is reduced ŽFig. 3.. Reduction of the defect density in the i-layers of the diodes by annealing at 2508C thus improves the performance of the diodes significantly. The detailed influence of annealing on contacts, interfaces and doped layers still needs to be investigated. The EL intensity measured by a silicon detector ŽFig. 4. shows a power law dependence on the injected current density, I EL ; jg. For as deposited suboxide diodes, g equals 1, whereas annealed diodes show g values in the range of 1.2 to 1.4. A similar result was reported by Han and Wang w8x for EL studies on hydrogenated amorphous silicon p–i–n diodes. At 300 K, they found two emission bands at 0.85 eV Ždefect recombination. and 1.2 eV Žbandto-band recombination. with g values of 1.0 and about 1.5, respectively. Thus, as deposited suboxide diodes also seem to show a predominance of radiative recombination at defects, whereas annealed diodes exhibit an increasing contribution of band-toband recombination. This is in qualitative agreement with the spectrally resolved EL measurements ŽFig. 5.. All diodes show an EL peak at 0.85 eV assigned to defect recombination at the p–i interface w8x. The origin of the EL peak at 1.1 eV detected for as deposited diodes is most likely due to recombination of electrons at neutral dangling bond defects ŽD 0 . in the i-layer. Photoluminescence at 1.1 eV has been detected in amorphous silicon–oxygen alloys with similar oxygen content, and has also been assigned to oxygen related defects w9x. With increasing i-layer thickness, the carriers thermalize deeper into the bandtails of the i-layer and thus, the EL is observed at lower photon energies.
R. Janssen et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1151–1155
As confirmed by PDS and ESR measurements, the defect density in the i-layer decreases significantly upon annealing. Thus, the probability of recombination at neutral D 0 defects will be reduced in favour of band-to-band recombination, shifting the EL peak to 1.3 eV. Thus, the EL peak energy of band-to-band recombination appears to occur about 1.0 eV below the absorption bandgap of the intrinsic layer. A similar result was obtained by Kruangam et al. w10x for a-SiC:H p–i–n thin film light emitting diodes.
5. Conclusions Optical subgap absorption of undoped a-SiO x :H Žw O x s 30 at.%, E04 s 2.3 eV. prepared by PECVD is reduced by about 1 order of magnitude upon annealing at 2508C. This reduction improves the I–V properties of suboxide diodes with i-layer thicknesses between 30 nm and 800 nm. Spectrally resolved EL measurements show a luminescence peak at 0.85 eV assigned to defect related recombination at the p–i interface and an additional peak at 1.1 eV for as deposited diodes, shifting towards 1.3 eV upon annealing. The total EL efficiency of annealed sub-
1155
oxide diodes is increased by a factor of 3 to 10y3 %. Because this EL is easily detected by standard c-Si photodiodes, suboxide p–i–n LEDs are interesting active components for Si-based optoelectronics. References w1x M. Zacharias, H. Freistedt, F. Stolze, T.P. Drusedau, M. ¨ Rosenbauer, M. Stutzmann, J. Non-Cryst. Solids 164–166 Ž1993. 1089. w2x R. Carius, R. Fischer, E. Holzenkampfer, J. Stuke, J. Appl. ¨ Phys. 52 Ž6. Ž1981. 4241. w3x R.A. Street, J.C. Knights, Philos. Mag. B 42 Ž4. Ž1980. 551. w4x M.C. Rossi, M.S. Brandt, M. Stutzmann, Electrochem. Soc. Proc. 95–25 Ž1995. 445. w5x W. Boonkosum, D. Kruangam, B. Ratwises, T. Sujaridchai, S. Panyakeow, S. Fujikake, H. Sakai, J. Non-Cryst. Solids 198–200 Ž1996. 1226. w6x M. Zacharias, D. Dimova-Malinovska, M. Stutzmann, Philos. Mag. B 73 Ž5. Ž1996. 799. w7x M.C. Rossi, D. Dimova-Malinovska, M.S. Brandt, M. Stutzmann, in: R.H. Mauch, H.E. Gumlich ŽEds.., Inorganic and Organic Electroluminescence, Wissenschaft and Technik Verlag, Berlin, 1996, p. 189. w8x D. Han, K. Wang, J. Non-Cryst. Solids 190 Ž1995. 74. w9x R.A. Street, J.C. Knights, D.K. Biegelsen, Phys. Rev. B 18 Ž4. Ž1978. 1880. w10x D. Kruangam, T. Sujaridchai, K. Chirakawikul, B. Ratwises, S. Panyakeow, 227–230 Ž1998. 1146.