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Amorphous silicon-hydrogen-fluorine-oxygen alloys
Journal of Non-Crysta111ne Solids 35 & 36 (1980) 309-312 ~North-Holland Publishing Company
AMORPHOUS
SILICON-HYDROGEN-FLUORINE-OXYGEN
ALLOYS
Bernard J. Feldman Department of Physics University of Missouri St. Louis, Missouri 63121 U.S.A.
Amorphous silicon-hydrogen-fluorine-oxygen alloys have been grown by the plasma deposition of a SiF4/H 2 gas mixture. The composition and optical properties of these alloys are very similar to those of Knights' amorphous silicon-hydrogen a~loys, but are significantly different from those of Ovshinsky and Madan's amorphous silicon-fluorine-hydrogen alloys. Plasma deposition of amorphous sSlicon-hydrogen alloys (a-Si-H) from silane gas was first developed by Chittick, Alexander, and Sterling; 1 using this technique, both p-n junctions 2 and solar cells 3 have been fabricated out of e-Si-H. Very recently Ovshinsky and Madan reported the growth of amorphous silicon-fluorine-hydrogen alloys (~-Si-F-H) from the plasma deposition of a SiF4/H 2 mixture; the approximate composition of their films was 95 at. % Si, 4 at. % F, and 0.5 at. % H. 4 I report the plasma deposition from a SiF4/H 2 feedstock of amorphous silicon-hydrogen-fluorine-oxygen alloys (e-Si-H-F-0), whose composition is 88 at. % Si, i0 at. % H, 1 at. % F and 1 at. % 0, and whose optical properties are comparable and potentially superior to the best silane deposited ~-Si-H films. My e-Si-H-F-0 films were grown in a glow discharge plasma reactor following the design of Knights 5, with internal stainless steel electrodes and the substrates mounted on the anode. The SiF 4 gas was supplied by Matheson, contained 0.3 molar % air, and is believed to be the source of the oxygen in the alloys. The films were fabricated under the following conditions: pressure = 1.4 Tort; SiF 4 flow rate = 29 sccm; H2 flow rate = 10 sccm; substrate temperature 200°C; and rf power ~ 15 Watts. The deposition rate was approximately 8 A/sec. The composition of the alloys was analyzed by secondary ion mass spectroscopy (SIMS). The system was calibrated with a series of ion-implanted crystalline silicon standards. The SIMS analysis gave the following composition: 88 at. % Si, I0 at. % H, 1 at. % F, and 1 at. % 0. Further confirmation of this analysis came from the infrared absorption spectrum of these films shown in Figure i. The spectrum was generated by subtracting the infrared absorption of the silicon substrate from that of the substrate with a 4~ film deposited upon it; differences in reflectivity between the two were not corrected for and gave rise to the small negative absorbance seen in parts of the spectrum. All the features of this spectrum are identical to those found in spectra of silane deposited films with substrate temperatures at 25 ° and 3000C. 6 The observed lines are the S-H rocking mode at 640 cm -1, the S-H stretching mode at 2000 cm-l; and
309
310
S.J. Feldman / Amorphous Silicon-Hydrogen-Fluorine-Oxygen Alloys
the SiH 2 bending modes at 850 cm -I and 900 cm-l. 6 The Si-F and St-0 stretching modes at 830 cm -I and 1000 cm -I respectively are below the sensitivity of our spectrometer, but are present in the e-Si-F-H films of Ovshinsky and Madan. 4
o:4
~
m
r
0,~
~
0 , 2
~0,1 0.0 -0.1
I
400
SO0
, I
I
~200
ENERGY Infrared Absorption
~800
,
I
I
2000
240O
(crn"I)
Figure i Spectrum of e-Si-H-F-O Alloys
The photoluminescence spectrum of the e-St-H-F-0 alloys is shown in Figure 2. The photoluminesce~ce was excited by a 0.02 Watt unfocused Ar ion laser beam (5145A). The films were grown on roughened quartz substrates and immersed in liquid nitrogen. The luminescence was collected from the rear surface, focused into a i/4-meter monochromator, and detected by a cooled germanium photodiode. The monochromator and detector sensitivity was calibrated by a tungsten lamp standard. Also shown for comparison in Figure 2 is the photoluminescence spectrum of one of Knights' strongest luminescing silane deposited films. 5 The film was grown under the following conditions: feedstock = 100% silane; rf.power = i Watt; and substrate temperature = 230°C. The composition of the film was 92 at. % Si and 8 at. % H. The luminescence spectra of the two films are strikingly similar; the e-St-H-F-0 spectrum has 26% greater integrated intensity and its frequency full width at half maximum is about 28% broader. As the temperature of the e-St-H-F-0 sample is increased from 77°K to 208°K, the luminescence integrated intensity decreased by two orders of magnitude in a non-exponential fashion, and the spectrum shifted to longer wavelengths. Very similar changes with temperature are observed in Knights' e-Si-H film. 7 For both films the luminescence decays following pulsed optical excitation at 8°K have almost identical non-exponential shapes. 7 And the cptical absorption in the energy region near the mobility gap (~2.0 eV) is very similar for the ~wo alloys.
B.J.
Feldman / Amorphous Silicon-Hydrogen-Fluorine-Oxygen Alloys
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ENERGY
(eV)
Figure 2 Luminescence Spectra of e-Si-H-F-0 and u-Si-H Alloys
The optical properties of e-Si-H-F-0 and e-Si-H alloys are virtually identical. Why? Obviously, the similarity of their composition must be part of the story. The infrared spectra suggest that the hydrogen bondings in both films are very similar. But what role does the 1 at. % F and 1 at. % H play? Why is the composition of my films so different from that of Ovshinsky and Madan? And most important, how suitable is e-Si-H-F-0 as a device material? I can only begin to answer these questions. First, the difference in composition between my films and those of Ovshinsky and Madan is probably due to different operating conditions. I have observed that the fluorine concentration increases and the hydrogen concentration decreases as the SiF4/H 2 ratio of the feedstock is increased. Ovdhinsky and Madan used a SiF4/H 2 ratio of i0 while I used a matio of 3. Street, Knights, and Biegelson have shown that the luminescence integrated intensity in e-Si-H films correlates with a decrease of localized defect states. 5 The large integrated intensity of the e-Si-H-F-0 samples strongly suggests that their density of defect
312
S.J. Feldman / Amorphous
Silicon-Hydrogen-Fluorine-Oxygen Alloys
states is comparable to that of the best ~-Si-H films. Given the fact that the SiFt gas -- unlike silane gas -- was no% of semiconductor purity, this is a remarkable result. The hydrogen and probably the fluorine and oxygen atoms are bonding to silicon defect states in these films. Finally, the a-St-H-F-0 system may have some advantages over the m-Si-H system. First, one can only guess at the quality of e-StH-F-0 films if semiconductor purity gases were used. Second, the ~-Si-H-F-0 alloy has extra degrees of freedom not available with e-Si-H films; namely, the SiF4/H 2 ratio can be varied and possibly F~ and/or 0 2 can be added to the feedstock, so that the concentratlon of the various constituents can be changed. These two advantages may make it possible to fabricate a-Si-H-F-O films with electronic properties superior to those of the best e-Si-H films. I want to acknowledge the indispensible help of J. C. Knights and R. A. Street, the technical assistance of G. Bischoff, and the numerous discussions with G. Johnson. REFERENCES (i) R. C. Chittick, J. H. Alexander, chem. Soc. i_~16, 77 (19.S9).
and H. F. Sterling,
(2) W. E. Spear, P. G. LeCember, S. Kinmond~ Appl. Phys. Lett. 28, 105 (1976). (3) D. E. Carlson,
IEEE Trans.
(5) R. A. Street, J. C. Knights, 18, 1880 (1978).
(6) J. C. Knights, 3.7.7, 467
G. Lucovsky,
and M. H. Brodsky,
Electron Dev. ED-24,
(4) S. R. 0vshinsky and A. Madan, Nature 276,
J. Electro~
482
449 (1977).
(1978).
and D. K. Biegelson,
Phys. Rev. B
and R. J. Ne~.anich, Phil. Mag.
(1978).
(7) C. Tsang and R. A. Street,
Pt]ys. Rev. B 19,
3027 (1979).
B
AMORPHOUS
SILICON-HYDROGEN-FLUORINE-OXYGEN
ALLOYS
Bernard J. Feldman Department of Physics University of Missouri St. Louis, Missouri 63121 U.S.A.
Amorphous silicon-hydrogen-fluorine-oxygen alloys have been grown by the plasma deposition of a SiF4/H 2 gas mixture. The composition and optical properties of these alloys are very similar to those of Knights' amorphous silicon-hydrogen a~loys, but are significantly different from those of Ovshinsky and Madan's amorphous silicon-fluorine-hydrogen alloys. Plasma deposition of amorphous sSlicon-hydrogen alloys (a-Si-H) from silane gas was first developed by Chittick, Alexander, and Sterling; 1 using this technique, both p-n junctions 2 and solar cells 3 have been fabricated out of e-Si-H. Very recently Ovshinsky and Madan reported the growth of amorphous silicon-fluorine-hydrogen alloys (~-Si-F-H) from the plasma deposition of a SiF4/H 2 mixture; the approximate composition of their films was 95 at. % Si, 4 at. % F, and 0.5 at. % H. 4 I report the plasma deposition from a SiF4/H 2 feedstock of amorphous silicon-hydrogen-fluorine-oxygen alloys (e-Si-H-F-0), whose composition is 88 at. % Si, i0 at. % H, 1 at. % F and 1 at. % 0, and whose optical properties are comparable and potentially superior to the best silane deposited ~-Si-H films. My e-Si-H-F-0 films were grown in a glow discharge plasma reactor following the design of Knights 5, with internal stainless steel electrodes and the substrates mounted on the anode. The SiF 4 gas was supplied by Matheson, contained 0.3 molar % air, and is believed to be the source of the oxygen in the alloys. The films were fabricated under the following conditions: pressure = 1.4 Tort; SiF 4 flow rate = 29 sccm; H2 flow rate = 10 sccm; substrate temperature 200°C; and rf power ~ 15 Watts. The deposition rate was approximately 8 A/sec. The composition of the alloys was analyzed by secondary ion mass spectroscopy (SIMS). The system was calibrated with a series of ion-implanted crystalline silicon standards. The SIMS analysis gave the following composition: 88 at. % Si, I0 at. % H, 1 at. % F, and 1 at. % 0. Further confirmation of this analysis came from the infrared absorption spectrum of these films shown in Figure i. The spectrum was generated by subtracting the infrared absorption of the silicon substrate from that of the substrate with a 4~ film deposited upon it; differences in reflectivity between the two were not corrected for and gave rise to the small negative absorbance seen in parts of the spectrum. All the features of this spectrum are identical to those found in spectra of silane deposited films with substrate temperatures at 25 ° and 3000C. 6 The observed lines are the S-H rocking mode at 640 cm -1, the S-H stretching mode at 2000 cm-l; and
309
310
S.J. Feldman / Amorphous Silicon-Hydrogen-Fluorine-Oxygen Alloys
the SiH 2 bending modes at 850 cm -I and 900 cm-l. 6 The Si-F and St-0 stretching modes at 830 cm -I and 1000 cm -I respectively are below the sensitivity of our spectrometer, but are present in the e-Si-F-H films of Ovshinsky and Madan. 4
o:4
~
m
r
0,~
~
0 , 2
~0,1 0.0 -0.1
I
400
SO0
, I
I
~200
ENERGY Infrared Absorption
~800
,
I
I
2000
240O
(crn"I)
Figure i Spectrum of e-Si-H-F-O Alloys
The photoluminescence spectrum of the e-St-H-F-0 alloys is shown in Figure 2. The photoluminesce~ce was excited by a 0.02 Watt unfocused Ar ion laser beam (5145A). The films were grown on roughened quartz substrates and immersed in liquid nitrogen. The luminescence was collected from the rear surface, focused into a i/4-meter monochromator, and detected by a cooled germanium photodiode. The monochromator and detector sensitivity was calibrated by a tungsten lamp standard. Also shown for comparison in Figure 2 is the photoluminescence spectrum of one of Knights' strongest luminescing silane deposited films. 5 The film was grown under the following conditions: feedstock = 100% silane; rf.power = i Watt; and substrate temperature = 230°C. The composition of the film was 92 at. % Si and 8 at. % H. The luminescence spectra of the two films are strikingly similar; the e-St-H-F-0 spectrum has 26% greater integrated intensity and its frequency full width at half maximum is about 28% broader. As the temperature of the e-St-H-F-0 sample is increased from 77°K to 208°K, the luminescence integrated intensity decreased by two orders of magnitude in a non-exponential fashion, and the spectrum shifted to longer wavelengths. Very similar changes with temperature are observed in Knights' e-Si-H film. 7 For both films the luminescence decays following pulsed optical excitation at 8°K have almost identical non-exponential shapes. 7 And the cptical absorption in the energy region near the mobility gap (~2.0 eV) is very similar for the ~wo alloys.
B.J.
Feldman / Amorphous Silicon-Hydrogen-Fluorine-Oxygen Alloys
• J",
,
,
,
e m)-
,
,
,
//\ \
,oL
o,ok
,
31 1
//\\ .
il
\l
o~.0 -
//
//
\ \
//
\\
\\ 12
ENERGY
(eV)
Figure 2 Luminescence Spectra of e-Si-H-F-0 and u-Si-H Alloys
The optical properties of e-Si-H-F-0 and e-Si-H alloys are virtually identical. Why? Obviously, the similarity of their composition must be part of the story. The infrared spectra suggest that the hydrogen bondings in both films are very similar. But what role does the 1 at. % F and 1 at. % H play? Why is the composition of my films so different from that of Ovshinsky and Madan? And most important, how suitable is e-Si-H-F-0 as a device material? I can only begin to answer these questions. First, the difference in composition between my films and those of Ovshinsky and Madan is probably due to different operating conditions. I have observed that the fluorine concentration increases and the hydrogen concentration decreases as the SiF4/H 2 ratio of the feedstock is increased. Ovdhinsky and Madan used a SiF4/H 2 ratio of i0 while I used a matio of 3. Street, Knights, and Biegelson have shown that the luminescence integrated intensity in e-Si-H films correlates with a decrease of localized defect states. 5 The large integrated intensity of the e-Si-H-F-0 samples strongly suggests that their density of defect
312
S.J. Feldman / Amorphous
Silicon-Hydrogen-Fluorine-Oxygen Alloys
states is comparable to that of the best ~-Si-H films. Given the fact that the SiFt gas -- unlike silane gas -- was no% of semiconductor purity, this is a remarkable result. The hydrogen and probably the fluorine and oxygen atoms are bonding to silicon defect states in these films. Finally, the a-St-H-F-0 system may have some advantages over the m-Si-H system. First, one can only guess at the quality of e-StH-F-0 films if semiconductor purity gases were used. Second, the ~-Si-H-F-0 alloy has extra degrees of freedom not available with e-Si-H films; namely, the SiF4/H 2 ratio can be varied and possibly F~ and/or 0 2 can be added to the feedstock, so that the concentratlon of the various constituents can be changed. These two advantages may make it possible to fabricate a-Si-H-F-O films with electronic properties superior to those of the best e-Si-H films. I want to acknowledge the indispensible help of J. C. Knights and R. A. Street, the technical assistance of G. Bischoff, and the numerous discussions with G. Johnson. REFERENCES (i) R. C. Chittick, J. H. Alexander, chem. Soc. i_~16, 77 (19.S9).
and H. F. Sterling,
(2) W. E. Spear, P. G. LeCember, S. Kinmond~ Appl. Phys. Lett. 28, 105 (1976). (3) D. E. Carlson,
IEEE Trans.
(5) R. A. Street, J. C. Knights, 18, 1880 (1978).
(6) J. C. Knights, 3.7.7, 467
G. Lucovsky,
and M. H. Brodsky,
Electron Dev. ED-24,
(4) S. R. 0vshinsky and A. Madan, Nature 276,
J. Electro~
482
449 (1977).
(1978).
and D. K. Biegelson,
Phys. Rev. B
and R. J. Ne~.anich, Phil. Mag.
(1978).
(7) C. Tsang and R. A. Street,
Pt]ys. Rev. B 19,
3027 (1979).
B