- Email: [email protected]
Preparation of boron nitride thin films by microwave plasma enhanced CVD, for semiconductor applications
MAWJRIAIS SCIENCE & ENGIREERING Materials Science and Engineering B46 (1997)
B
101-104
Preparation of boron nitride thin films by microwave plasma enhanced CVD, for semiconductor applications 0. Baehr *, P. Thkvenin, LICMjCLOES
A. Bath, A. Koukab,
Unirersity
of Metz,
2 rue E. Belbz,
E. Losson, B. Lepley
57078 Metz,
France
Abstract Thin films of boron nitride (BN) have beendepositedon silicon and indium phosphide(InP) substratesat low temperature ( z 300°C) using a microwave
plasma
CVD
system. The source material
is molten
borane-dimethylamine.
The vapour
was
decomposedin a microwave nitrogen and argon plasma. The index of refraction and the thickness of the films have been determinedby ellipsometry.FTIR spectroscopywasusedfor a fast phaseidentification. The compositionwas analyzed by X-ray photoelectron spectroscopy (XPS). The electrical properties of the films were evaluated by capacitance-voltage (C- v) measurements of metal/BN/semiconductor (MIS) structures. From these results a minimum interface state density of 3.5 10” cme2 eV- ’
and a dielectric constant of 5.2 have beendeduced.0 1997Elsevier ScienceS.A.
1. Introduction Among III-V semiconductors, indium phosphide (InP) is an attractive material for high frequency electronic as well as optoelectronic applications. This is because of a high mobility and direct band gap structures, respectively. Various dielectrics have been tested in several laboratories to develop a valuable field effect transistor on InP [l-5]. To avoid the presence of native oxides at the insulator-InP interface, which may be detrimental to the interface properties, nitrides can be used as the gate insulator. Boron nitride (BN) is a material which has high insulating properties and which can be useful for InP passivation. The deposition process should be performed at low temperature (below 350°C) to preserve the underlaying InP. Various plasma enhanced chemical vapour deposition (PECVD) apparatus for growth of BN have been presented in the literature [6-lo]. Previously we have presented results of BN deposited by a radio frequency (RF) plasma CVD technique [11,12]. In this paper, results obtained with a new microwave PECVD equipment are presented. The plasma is produced from an electromagnetic surface wave [13,14]. The optical properties of the deposited films were studied by ellipsometry and by infrared spectroscopy. * Corresponding author. 0921-5107/97/S17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5107(96)01976-9
Their composition was determined by X-ray photoelec-
tron spectroscopy (XPS). The capacitance-voltage characteristics (C-V) of Au/BN/InP MIS structures were used to assess the quality of the insulator-semiconductor interface. 2. Experiment The plasma is produced from a mixture of nitrogen and argon excited by a microwave generator at 2.45 GHz. The gases were introduced at the top of the quartz chamber. The maximum power is 1200 W. A schematic diagram of the apparatus is shown in Fig. 1. The boron source compound is borane-dimethylamine (BDMA) heated between 38 and 45°C. Its vapour is carried into the reaction chamber with an argon flow through a movable Pyrex injector.
Before deposition the InP samples were etched with a HCl solution (2 mol/l) for 5 min. Then the samples were rinsed in deionized water and stored in methanol. A load-lock substrate introduction chamber was used to transfer the samples into the deposition chamber. The substrate holder was located downstream of the
plasma zone and was heated with an internal resistance. The temperature was controlled with a K type thermocouple and a digital PID controller. It was maintained at 320°C during deposition. The distance between the ~~ plasma zone and the samples could be monitored to soni: extent.
102
0. Baehr
et al. /Materials
Science
and Engineering
B46 (1997)
101- 104
Once the samples are introduced into the reaction chamber, the different deposition parameters can be selected. A microwave plasma power of 500 W is then applied and stubs and short circuit are adjusted to minimize the reflected power. The total pressure is controlled by a MKS baratron gauge ranging from 0.01 to 2 Torr and is maintained constant (typically at 120 mTorr during the deposition) by an automatic pressure monitored gate valve. Ail the gas flow are regulated by electronic mass flow controllers. The flow rates were typically 20 and 100 cm3 min- ’ for nitrogen and argon, respectively, and 0.5 cm3 min - i for the carrier gas. Fig. 2. FTIR spectrum of BN film deposited on silicon.
3. Results and discussion
The nature of the deposited film was first determined by using infrared spectroscopy. For sp2 bonded hexagonal BN phase (h-BN), there are two characteristic peaks near 1380 cm-’ and 800 cm - ’ associated with in plane B-N bond stretching and out plane B-N-B bond bending vibration, respectively. The cubic phase of BN exhibits a peak at 1065 cm-’ attributed to the sp3 bonding. Fig. 2 shows the typical FTIR spectrum of a h-BN film grown on silicon. No absorption at 3430 cm-l and 2530 cm-i (assigned to N-H and B-H vibrations, respectively) can be detected in this spectrum. The thickness and the refractive index were also evaluated on silicon substrates by using ellipsometry at 633 nm. Depending on the experimental parameters, the growth rate was found to vary from 24 to 72 rim/h and the refractive index from 1.64 to 1.75. The chemical composition of the deposited BN films was investigated using XPS. The binding energies of the
B,, and N,, and the full half-widths of the peaks suggest that B and N atoms are involved essentially in BN bonds. The oxygen concentration is low, whereas a more significant amount of carbon is observed. The carbon impurity originates probably from the BDMA precursor. The relative concentration [NJ/LB] is about 1, indicating that the film is close to stoichiometry. Finally, the electrical properties of the BN films were evaluated using Au/BN/InP MIS structures. Typical C-Y curves recorded at 10 KHz and 1 MHz, with a bias sweep rate of 50 mV/s, are reported in Fig. 3. The curves show a small amount of hysteresis and a small frequency dispersion in accumulation. The dielectric constant, calculated from the measured capacitance in accumulation and the insulator thickness, was found to be 5.2. Bias-temperature-stress (BTS) measurements were performed in order to evaluate the interface state density [15,16]. The MIS structure was first biased at - 5 V at room temperature. After 15 min the interface states are emptied and the sample is then cooled down and kept at 80 K (Fig. 4, step a). The C- 1’ plots were recorded by increasing Ys, with a sweep rate of 50 mV/s, up to the accumulation regime (step b). During
IOKFk
-5
4
-3
-2
-1
0
1
2
3
4
Y&O?
Fig. 1. Schematic diagram of the microwave PECVD apparatus.
Fig. 3. 10 kHz and 1 MHz C-I’ curves of a BN-InP MIS structure, at room temperature with a 50 mV s-’ bias sweep rate.
0. Baehr
et nl. /Materials
Science
and Engineering
B46 (1997)
101-104
cr Ez EC -#, ci “4” ET Vs=t3” ==y C” e,-p 9“;-iv @2p, ,
E"
Vs A +3"-------------
_---------15mn I II 1 1
0 -5V
t cm4
/ b) ( c) ( 4 (
T Wt
02
O,l
0,3
0,4
0,s
‘26
0,7
EC-E @V
Fig. 6. Interface state density Di, versus (E,- E) deduced from 1 MHz C- V curves in Fig. 3 using the Terman method. Fig. 4. Schematic diagram of the band alignment during the different steps of the BTS measurements: (a) bias at - 5 V during 15 min at 300 K and cooling at 80 K; (b) C- T’measurements from - 5 to + 3 V; (c) from + 3 to - 5 V; and (d) from - 5 to + 3 V.
this recording, the interface states have been filled, resulting in a stretchout of the C- V curve (Fig. 5, curve a). The third step consist in a bias sweep from + 3 to - 5 V, still at 80K (Fig. 4, step c). In this case, the electron thermal emission is frozen for all traps deeper than approximately 0.15 eV below the conduction band. The corresponding data are visualized in Fig. 5, curve b. A last bias sweep from - 5 to + 3 V (Fig. 4, step d) shows that no more hysteresis appears (Fig. 5, curve b), traps remaining frozen in the filled state. The interface state concentration r\ii, (per unit area) filled during the first forward biasing is evaluated by the shift AV, from curve a to curve b in Fig. 5: N,
=
-- ‘iA’s
-
4
x
1011
cm-2
It 4
/
I
I
3co t +-
250 -
i
200 150 -
where Ci is the capacitance of the insulating BN film and q the electronic charge. The interface state energy distribution D, (E) (Fig. 6) is deduced from the Terrnan analysis applied to the 1 MHz C-V curve of Fig. 3. This method gives for our sample a minimum interface state density of 3.5 101’ cm-” eV- l between 0.4-0.65 eV below the conduction band. This value is consistent with the ail value reported just above.
4. Conclusion
Boron nitride thin films were deposited at low temperature onto Si and InP substrates with a new microwave plasma enhanced CVD equipment. The gases were N, and Ar, and BDMA was used as the boron source. The IR spectra and XPS results indicate that the films consist of nearly stoichiometric h-BN. C- V measurements were used to evaluate the electrical properties of the Au/BN/InP MIS structures. The small hysteresis and the small frequency dispersion in accumulation indicate a rather good quality of the insulator/semiconductor interface. The interface state distribution shows a minimum of 3.5 10” crnp2 eV-’ between 0.4-0.65 eV below the conduction band, which is consistent with the value obtained using BTS measurements. The dielectric constant of the deposited film was found to be 5.2.
loo 50 r -6
I
I
4
-2
0
I
I
2
4
References [1] L. Messick, J. Appl. Phys., 47 (1976) 4949. [2] Y. Hirota, T. Kobayashi and Y. Furukawa, Jpn. J. Appl. Phys.,
Fig. 5. C- V curves at 1 MHz recorded at 80 K with a 50 mV s- i bias sweep rate. First measurement (curve a) from Vg= - 5 V to V, = + 5 V after 15 min biasing at - 5 V at room temperature. Second measurement (curve b) from Jk = + 3 V to V, = - 5 V, and reverse.
22 (1983)
385.
M. Armand, D.V. Bui, .I. Chevrier and N.T. Linh, Electron. Lett., 19 (1983) 433. [4] Y. Iwase. F. Arai and T. Sugano, Appl. Phys. Left., 52 (1988) 1437. [3]
104
0. Bnehr
et al. /Materials
Science
Y.H. Jeong, SK. Jo, B.H. Lee, and T. Sugano, IEEE Electron Device Lett., 16 (1995) 109. [6] W. Schmolla and H.L. Hartnagel, Solid-State Electron., 26 (1983) 931. [7] E. Yamaguchi and M. Minakata, J. Appl. Phys., 55 (1984) [5]
3098. [8]
D.C. Cameron, M.Z. Karim Films,
236 (1993)
and MS. Hashmi, Thin Solids
96.
Sot.,
141 (1994)
1633.
[IO] C.R. Eddy Jr. and B.D. Sartwell, J. Vat. Sci. Technol, (1995)
2018.
Al3
B46 (1997)
IOl-
104
[l l] A. Bath, 0. Baehr, M. Barrada. B. Lepley, P.J. van der Put and J. Schoonman, Thin Solids Films, 241 (1994) 27% [I21 A. Bath, P.J. van der Put, J.G.M. Becht, J. Schoonman and B. Lepley, J. Appl. Piys., 70 (1991) 4366. [13] J. Musil, Vncuwz, 36 (1986) 161. [14] M. Moisan, C. M. Ferreira, Y. Hajlaoui, D. Henry, J. Hubert, R, Pantel, A. Ricard and Z. Zarzewski, &rue Phys. Appl., I7 (1982)
[9] S.V. Nguyen, T. Nguyen, H. Treichel and 0. Spindler, J. Electrochem.
and Engineering
707.
[IS] P. Klopfenstein, G. Bastide, M. Rouzeyre, M, Gendry and J. Durand, J. Appl. Phys, 63 (1988) 150. [16] A. Koukab, A. Bath and E. Losson, Solid-State Electron, (to be published).
B
101-104
Preparation of boron nitride thin films by microwave plasma enhanced CVD, for semiconductor applications 0. Baehr *, P. Thkvenin, LICMjCLOES
A. Bath, A. Koukab,
Unirersity
of Metz,
2 rue E. Belbz,
E. Losson, B. Lepley
57078 Metz,
France
Abstract Thin films of boron nitride (BN) have beendepositedon silicon and indium phosphide(InP) substratesat low temperature ( z 300°C) using a microwave
plasma
CVD
system. The source material
is molten
borane-dimethylamine.
The vapour
was
decomposedin a microwave nitrogen and argon plasma. The index of refraction and the thickness of the films have been determinedby ellipsometry.FTIR spectroscopywasusedfor a fast phaseidentification. The compositionwas analyzed by X-ray photoelectron spectroscopy (XPS). The electrical properties of the films were evaluated by capacitance-voltage (C- v) measurements of metal/BN/semiconductor (MIS) structures. From these results a minimum interface state density of 3.5 10” cme2 eV- ’
and a dielectric constant of 5.2 have beendeduced.0 1997Elsevier ScienceS.A.
1. Introduction Among III-V semiconductors, indium phosphide (InP) is an attractive material for high frequency electronic as well as optoelectronic applications. This is because of a high mobility and direct band gap structures, respectively. Various dielectrics have been tested in several laboratories to develop a valuable field effect transistor on InP [l-5]. To avoid the presence of native oxides at the insulator-InP interface, which may be detrimental to the interface properties, nitrides can be used as the gate insulator. Boron nitride (BN) is a material which has high insulating properties and which can be useful for InP passivation. The deposition process should be performed at low temperature (below 350°C) to preserve the underlaying InP. Various plasma enhanced chemical vapour deposition (PECVD) apparatus for growth of BN have been presented in the literature [6-lo]. Previously we have presented results of BN deposited by a radio frequency (RF) plasma CVD technique [11,12]. In this paper, results obtained with a new microwave PECVD equipment are presented. The plasma is produced from an electromagnetic surface wave [13,14]. The optical properties of the deposited films were studied by ellipsometry and by infrared spectroscopy. * Corresponding author. 0921-5107/97/S17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5107(96)01976-9
Their composition was determined by X-ray photoelec-
tron spectroscopy (XPS). The capacitance-voltage characteristics (C-V) of Au/BN/InP MIS structures were used to assess the quality of the insulator-semiconductor interface. 2. Experiment The plasma is produced from a mixture of nitrogen and argon excited by a microwave generator at 2.45 GHz. The gases were introduced at the top of the quartz chamber. The maximum power is 1200 W. A schematic diagram of the apparatus is shown in Fig. 1. The boron source compound is borane-dimethylamine (BDMA) heated between 38 and 45°C. Its vapour is carried into the reaction chamber with an argon flow through a movable Pyrex injector.
Before deposition the InP samples were etched with a HCl solution (2 mol/l) for 5 min. Then the samples were rinsed in deionized water and stored in methanol. A load-lock substrate introduction chamber was used to transfer the samples into the deposition chamber. The substrate holder was located downstream of the
plasma zone and was heated with an internal resistance. The temperature was controlled with a K type thermocouple and a digital PID controller. It was maintained at 320°C during deposition. The distance between the ~~ plasma zone and the samples could be monitored to soni: extent.
102
0. Baehr
et al. /Materials
Science
and Engineering
B46 (1997)
101- 104
Once the samples are introduced into the reaction chamber, the different deposition parameters can be selected. A microwave plasma power of 500 W is then applied and stubs and short circuit are adjusted to minimize the reflected power. The total pressure is controlled by a MKS baratron gauge ranging from 0.01 to 2 Torr and is maintained constant (typically at 120 mTorr during the deposition) by an automatic pressure monitored gate valve. Ail the gas flow are regulated by electronic mass flow controllers. The flow rates were typically 20 and 100 cm3 min- ’ for nitrogen and argon, respectively, and 0.5 cm3 min - i for the carrier gas. Fig. 2. FTIR spectrum of BN film deposited on silicon.
3. Results and discussion
The nature of the deposited film was first determined by using infrared spectroscopy. For sp2 bonded hexagonal BN phase (h-BN), there are two characteristic peaks near 1380 cm-’ and 800 cm - ’ associated with in plane B-N bond stretching and out plane B-N-B bond bending vibration, respectively. The cubic phase of BN exhibits a peak at 1065 cm-’ attributed to the sp3 bonding. Fig. 2 shows the typical FTIR spectrum of a h-BN film grown on silicon. No absorption at 3430 cm-l and 2530 cm-i (assigned to N-H and B-H vibrations, respectively) can be detected in this spectrum. The thickness and the refractive index were also evaluated on silicon substrates by using ellipsometry at 633 nm. Depending on the experimental parameters, the growth rate was found to vary from 24 to 72 rim/h and the refractive index from 1.64 to 1.75. The chemical composition of the deposited BN films was investigated using XPS. The binding energies of the
B,, and N,, and the full half-widths of the peaks suggest that B and N atoms are involved essentially in BN bonds. The oxygen concentration is low, whereas a more significant amount of carbon is observed. The carbon impurity originates probably from the BDMA precursor. The relative concentration [NJ/LB] is about 1, indicating that the film is close to stoichiometry. Finally, the electrical properties of the BN films were evaluated using Au/BN/InP MIS structures. Typical C-Y curves recorded at 10 KHz and 1 MHz, with a bias sweep rate of 50 mV/s, are reported in Fig. 3. The curves show a small amount of hysteresis and a small frequency dispersion in accumulation. The dielectric constant, calculated from the measured capacitance in accumulation and the insulator thickness, was found to be 5.2. Bias-temperature-stress (BTS) measurements were performed in order to evaluate the interface state density [15,16]. The MIS structure was first biased at - 5 V at room temperature. After 15 min the interface states are emptied and the sample is then cooled down and kept at 80 K (Fig. 4, step a). The C- 1’ plots were recorded by increasing Ys, with a sweep rate of 50 mV/s, up to the accumulation regime (step b). During
IOKFk
-5
4
-3
-2
-1
0
1
2
3
4
Y&O?
Fig. 1. Schematic diagram of the microwave PECVD apparatus.
Fig. 3. 10 kHz and 1 MHz C-I’ curves of a BN-InP MIS structure, at room temperature with a 50 mV s-’ bias sweep rate.
0. Baehr
et nl. /Materials
Science
and Engineering
B46 (1997)
101-104
cr Ez EC -#, ci “4” ET Vs=t3” ==y C” e,-p 9“;-iv @2p, ,
E"
Vs A +3"-------------
_---------15mn I II 1 1
0 -5V
t cm4
/ b) ( c) ( 4 (
T Wt
02
O,l
0,3
0,4
0,s
‘26
0,7
EC-E @V
Fig. 6. Interface state density Di, versus (E,- E) deduced from 1 MHz C- V curves in Fig. 3 using the Terman method. Fig. 4. Schematic diagram of the band alignment during the different steps of the BTS measurements: (a) bias at - 5 V during 15 min at 300 K and cooling at 80 K; (b) C- T’measurements from - 5 to + 3 V; (c) from + 3 to - 5 V; and (d) from - 5 to + 3 V.
this recording, the interface states have been filled, resulting in a stretchout of the C- V curve (Fig. 5, curve a). The third step consist in a bias sweep from + 3 to - 5 V, still at 80K (Fig. 4, step c). In this case, the electron thermal emission is frozen for all traps deeper than approximately 0.15 eV below the conduction band. The corresponding data are visualized in Fig. 5, curve b. A last bias sweep from - 5 to + 3 V (Fig. 4, step d) shows that no more hysteresis appears (Fig. 5, curve b), traps remaining frozen in the filled state. The interface state concentration r\ii, (per unit area) filled during the first forward biasing is evaluated by the shift AV, from curve a to curve b in Fig. 5: N,
=
-- ‘iA’s
-
4
x
1011
cm-2
It 4
/
I
I
3co t +-
250 -
i
200 150 -
where Ci is the capacitance of the insulating BN film and q the electronic charge. The interface state energy distribution D, (E) (Fig. 6) is deduced from the Terrnan analysis applied to the 1 MHz C-V curve of Fig. 3. This method gives for our sample a minimum interface state density of 3.5 101’ cm-” eV- l between 0.4-0.65 eV below the conduction band. This value is consistent with the ail value reported just above.
4. Conclusion
Boron nitride thin films were deposited at low temperature onto Si and InP substrates with a new microwave plasma enhanced CVD equipment. The gases were N, and Ar, and BDMA was used as the boron source. The IR spectra and XPS results indicate that the films consist of nearly stoichiometric h-BN. C- V measurements were used to evaluate the electrical properties of the Au/BN/InP MIS structures. The small hysteresis and the small frequency dispersion in accumulation indicate a rather good quality of the insulator/semiconductor interface. The interface state distribution shows a minimum of 3.5 10” crnp2 eV-’ between 0.4-0.65 eV below the conduction band, which is consistent with the value obtained using BTS measurements. The dielectric constant of the deposited film was found to be 5.2.
loo 50 r -6
I
I
4
-2
0
I
I
2
4
References [1] L. Messick, J. Appl. Phys., 47 (1976) 4949. [2] Y. Hirota, T. Kobayashi and Y. Furukawa, Jpn. J. Appl. Phys.,
Fig. 5. C- V curves at 1 MHz recorded at 80 K with a 50 mV s- i bias sweep rate. First measurement (curve a) from Vg= - 5 V to V, = + 5 V after 15 min biasing at - 5 V at room temperature. Second measurement (curve b) from Jk = + 3 V to V, = - 5 V, and reverse.
22 (1983)
385.
M. Armand, D.V. Bui, .I. Chevrier and N.T. Linh, Electron. Lett., 19 (1983) 433. [4] Y. Iwase. F. Arai and T. Sugano, Appl. Phys. Left., 52 (1988) 1437. [3]
104
0. Bnehr
et al. /Materials
Science
Y.H. Jeong, SK. Jo, B.H. Lee, and T. Sugano, IEEE Electron Device Lett., 16 (1995) 109. [6] W. Schmolla and H.L. Hartnagel, Solid-State Electron., 26 (1983) 931. [7] E. Yamaguchi and M. Minakata, J. Appl. Phys., 55 (1984) [5]
3098. [8]
D.C. Cameron, M.Z. Karim Films,
236 (1993)
and MS. Hashmi, Thin Solids
96.
Sot.,
141 (1994)
1633.
[IO] C.R. Eddy Jr. and B.D. Sartwell, J. Vat. Sci. Technol, (1995)
2018.
Al3
B46 (1997)
IOl-
104
[l l] A. Bath, 0. Baehr, M. Barrada. B. Lepley, P.J. van der Put and J. Schoonman, Thin Solids Films, 241 (1994) 27% [I21 A. Bath, P.J. van der Put, J.G.M. Becht, J. Schoonman and B. Lepley, J. Appl. Piys., 70 (1991) 4366. [13] J. Musil, Vncuwz, 36 (1986) 161. [14] M. Moisan, C. M. Ferreira, Y. Hajlaoui, D. Henry, J. Hubert, R, Pantel, A. Ricard and Z. Zarzewski, &rue Phys. Appl., I7 (1982)
[9] S.V. Nguyen, T. Nguyen, H. Treichel and 0. Spindler, J. Electrochem.
and Engineering
707.
[IS] P. Klopfenstein, G. Bastide, M. Rouzeyre, M, Gendry and J. Durand, J. Appl. Phys, 63 (1988) 150. [16] A. Koukab, A. Bath and E. Losson, Solid-State Electron, (to be published).