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Preparation of BaF2 thin films for the application to MIS tunnel emitter
Synthetic
ELSEVIER
Preparation
Metals 71 (1995) 2267-2268
of BaF2 thin films for the application
K.Usami,
I.Takahashi,
M.Moriya,
X.Y.Cai,
to MIS tunnel
T.Kobayashi
emitter
and T.Goto
The University of Electra-Communications, Chofu-shi, Tokyo, 182 ,Japan Abstract Ultrathin BaFz home of leas than 2Onm thickness
on the Si(100) substrates were deposited by ion beam sputtering technique. The crystallographic quality and surface morphology of the f&us were investigated by X-ray diffractometer and scanning electron microscope, respectively. The MIS diode type tunnel emitter was fabricated on the n-type Si substrate using this film as a tunnel barrier insulator. The electrical properties of the tunnel emitter such as I-V characteristics and electron emission current into the vacuum were measured. F’ic~ampere/mm* order electron emission current density was obtained for typical samples. These results were systematically investigated in relation to the BaFr deposition conditions.
l.INTRODUCTION
electrode and semiconductor, diode characteristics measured using four terminal method.
Tunnel emitter, an electron emission source to the vacuum using a tunnel phenomena, was expected as a miniature cold cathode in a micron scale vacuum electron tube which operate at very high speed or which can operate correctly in high temperature or radiometric environment. Some researchers have been reported the fabrication and operation of MIM (Metal-humlatorMetal) or MIS (Metal-Insulator-Semiconductor) type tunnel emitter so far [1][2][3]. But in these cases, amorphous oxide insulator such as anodic oxidized aluminum or thermal oxidized silicon films were commonly used as the tunnel barrier layer. On the other hand, alkalineearth fluoride materials such as CaFz or BaFa can be hetero-epitaxially grown on Si substrate and they have superior electrical properties as an insulator lllm [4]. In this experiment, epitaxial BaFz film on a n-type Si( 100) substrate was examined as a barrier insulator of MIS diode
type
tunnel
emitter.
The
electrical
properties
of MIS diode,
such as I-V characteristics and electron emission into the vacuum were investigated as a parameter of BaFz lilm crystallinity on the Si substrate. 2.EXPERIMENTALS The BaFz film was deposited on a chemically cleaned n-type Si(lO0) substrate with a resistivity of a few ohmcm for various substrate temperature by ion beam sputtering technique. The crystal structure and stoichiometory of BaFz films were investigated by X-ray difIractometer(XRD) and X-ray photoemission spectroscopy (XPS) respectively. A 1Onm thick and lmm width Au electrode was deposited across the lmm width BaFz barrier strip which was trimmed by SiOz insulator, and hence a cross type MIS diode which have lmmxlmm emission area was fabricated as shown in Fig-l. In order to reject the contact resistance between metal 0379-6779/95/$09.50
SSDI
0
1995 Elsevier
0379-6779(94)03253-3
Science
S.A.
Ail rights reserved
were
The electron emission current into the vacuum was measured in a high vacuum metal chamber which was evacuated to less than lO-“Pa by a turbo-molecule pump. Schematic diagram of electron emission measurement was shown in Fig.2. Where the electron collector voltage was 55V and the space between tunnel emitter and electron collector electrode was 2mm. 3.RESULTS
AND
DISCUSSIONS
The crystallinity of the BaF3 film on Si(100) substrate improved as a increase of substrate temperature Ts during deposition in the range from room temperature to 700°C. The (111) orientated crystal growth were observed above Ts=400” C by X-ray diffractometer measurement as shown in Fig.3. But SEM morphology showed that the smoothness of film surface deteriorated as a increase of substrate temperature. The I-V characteristics of MIS diodes for various BaFz deposition temperature (film thickness d=lOnm for all samples) were shown in Fig.4. Diode current break voltage decreased with increasing BaFa deposition temperature Ts. If the break voltage (calculated in eV) of the diode is less than Au work function (4.7eV’), tunnel electrons through the insulator cannot emit to the vacuum. In addition, the surface of BaFz film deposited at higher substrate temperature was rough and not uniform in film thickness. Hence the tunnel current through the junction was not uniform and it concentrate at thinner portion of the insulator layer. The electron emission of the tunnel emitter which was fabricated on the smooth and having good crystallmity BaFz film deposited at Ts=500”C was measured. PicoAmpere/mm2 order electron emission current density into the vacuum (where, BaFz thickness was 1Omn and diode voltage Vd=lOV) was obtained as shown in Fig.5.
2268
K. Usumi et al. I Synthetic
Metals 71 (1995) 2267-2268
4.CONCLUSION Hetero-epitaxiallg grown BaFz thin films on the ntype Si substrate were investigated as the barrier insulator of MIS type tunnel emitter. Electron emission into the vacuum was obtained from the MIS tunnel emitter which was fabricated on the film grown at the substrate temperature of 500°C. Authors would like to acknowledge Dr.Ono and Dr.Yokoo of Touhoku University for useful discussion and suggestions. 5.
References C.A.Mead
, J. Appl.
Phys.
, 4 (1961) 646
TXusunoki, M.Suauki, S.Sasaki, T.Yaguchi and T.Aida, Jpn. J. Appl. Phys. , 11B (1993) 1695 Y.Yokoo, H.Tanaka, J. Vat. Sci. Technol
SSato, J.Murota B, 2 (1993) 429
L.Pfeiffer, J.M.Phillips, W.M.Augustyniak and K.W.West, Lett. 46 (1985) 947
and S.Ono,
T.P.Smith, Appl. Phys.
Fig.3 X-ray diffraction patterns various deposition temperature.
of RaF, films for
X
2V/div. Y ZmA/div
T,=4OO”C Fig.4 I-V characteristics of MIS tunnel emitter for various BaF, deposition temperature.
Fig.1 Schematic emitter.
diagram
of
MIS
type
tunnel
0.1
Fig.2 Schematic diagram of electron emission current measurement. Where, diode voltage was 0-15V and electron collector voltage was 55v. 0
r $1
I_Ll__L-.I.__LL 10
Diode Voltage
P 14
I Ii I
Fig.5 Diode current and electron as a function of diode voltage. and d=lOnm.
emission Where,
current
T,=500"C
ELSEVIER
Preparation
Metals 71 (1995) 2267-2268
of BaF2 thin films for the application
K.Usami,
I.Takahashi,
M.Moriya,
X.Y.Cai,
to MIS tunnel
T.Kobayashi
emitter
and T.Goto
The University of Electra-Communications, Chofu-shi, Tokyo, 182 ,Japan Abstract Ultrathin BaFz home of leas than 2Onm thickness
on the Si(100) substrates were deposited by ion beam sputtering technique. The crystallographic quality and surface morphology of the f&us were investigated by X-ray diffractometer and scanning electron microscope, respectively. The MIS diode type tunnel emitter was fabricated on the n-type Si substrate using this film as a tunnel barrier insulator. The electrical properties of the tunnel emitter such as I-V characteristics and electron emission current into the vacuum were measured. F’ic~ampere/mm* order electron emission current density was obtained for typical samples. These results were systematically investigated in relation to the BaFr deposition conditions.
l.INTRODUCTION
electrode and semiconductor, diode characteristics measured using four terminal method.
Tunnel emitter, an electron emission source to the vacuum using a tunnel phenomena, was expected as a miniature cold cathode in a micron scale vacuum electron tube which operate at very high speed or which can operate correctly in high temperature or radiometric environment. Some researchers have been reported the fabrication and operation of MIM (Metal-humlatorMetal) or MIS (Metal-Insulator-Semiconductor) type tunnel emitter so far [1][2][3]. But in these cases, amorphous oxide insulator such as anodic oxidized aluminum or thermal oxidized silicon films were commonly used as the tunnel barrier layer. On the other hand, alkalineearth fluoride materials such as CaFz or BaFa can be hetero-epitaxially grown on Si substrate and they have superior electrical properties as an insulator lllm [4]. In this experiment, epitaxial BaFz film on a n-type Si( 100) substrate was examined as a barrier insulator of MIS diode
type
tunnel
emitter.
The
electrical
properties
of MIS diode,
such as I-V characteristics and electron emission into the vacuum were investigated as a parameter of BaFz lilm crystallinity on the Si substrate. 2.EXPERIMENTALS The BaFz film was deposited on a chemically cleaned n-type Si(lO0) substrate with a resistivity of a few ohmcm for various substrate temperature by ion beam sputtering technique. The crystal structure and stoichiometory of BaFz films were investigated by X-ray difIractometer(XRD) and X-ray photoemission spectroscopy (XPS) respectively. A 1Onm thick and lmm width Au electrode was deposited across the lmm width BaFz barrier strip which was trimmed by SiOz insulator, and hence a cross type MIS diode which have lmmxlmm emission area was fabricated as shown in Fig-l. In order to reject the contact resistance between metal 0379-6779/95/$09.50
SSDI
0
1995 Elsevier
0379-6779(94)03253-3
Science
S.A.
Ail rights reserved
were
The electron emission current into the vacuum was measured in a high vacuum metal chamber which was evacuated to less than lO-“Pa by a turbo-molecule pump. Schematic diagram of electron emission measurement was shown in Fig.2. Where the electron collector voltage was 55V and the space between tunnel emitter and electron collector electrode was 2mm. 3.RESULTS
AND
DISCUSSIONS
The crystallinity of the BaF3 film on Si(100) substrate improved as a increase of substrate temperature Ts during deposition in the range from room temperature to 700°C. The (111) orientated crystal growth were observed above Ts=400” C by X-ray diffractometer measurement as shown in Fig.3. But SEM morphology showed that the smoothness of film surface deteriorated as a increase of substrate temperature. The I-V characteristics of MIS diodes for various BaFz deposition temperature (film thickness d=lOnm for all samples) were shown in Fig.4. Diode current break voltage decreased with increasing BaFa deposition temperature Ts. If the break voltage (calculated in eV) of the diode is less than Au work function (4.7eV’), tunnel electrons through the insulator cannot emit to the vacuum. In addition, the surface of BaFz film deposited at higher substrate temperature was rough and not uniform in film thickness. Hence the tunnel current through the junction was not uniform and it concentrate at thinner portion of the insulator layer. The electron emission of the tunnel emitter which was fabricated on the smooth and having good crystallmity BaFz film deposited at Ts=500”C was measured. PicoAmpere/mm2 order electron emission current density into the vacuum (where, BaFz thickness was 1Omn and diode voltage Vd=lOV) was obtained as shown in Fig.5.
2268
K. Usumi et al. I Synthetic
Metals 71 (1995) 2267-2268
4.CONCLUSION Hetero-epitaxiallg grown BaFz thin films on the ntype Si substrate were investigated as the barrier insulator of MIS type tunnel emitter. Electron emission into the vacuum was obtained from the MIS tunnel emitter which was fabricated on the film grown at the substrate temperature of 500°C. Authors would like to acknowledge Dr.Ono and Dr.Yokoo of Touhoku University for useful discussion and suggestions. 5.
References C.A.Mead
, J. Appl.
Phys.
, 4 (1961) 646
TXusunoki, M.Suauki, S.Sasaki, T.Yaguchi and T.Aida, Jpn. J. Appl. Phys. , 11B (1993) 1695 Y.Yokoo, H.Tanaka, J. Vat. Sci. Technol
SSato, J.Murota B, 2 (1993) 429
L.Pfeiffer, J.M.Phillips, W.M.Augustyniak and K.W.West, Lett. 46 (1985) 947
and S.Ono,
T.P.Smith, Appl. Phys.
Fig.3 X-ray diffraction patterns various deposition temperature.
of RaF, films for
X
2V/div. Y ZmA/div
T,=4OO”C Fig.4 I-V characteristics of MIS tunnel emitter for various BaF, deposition temperature.
Fig.1 Schematic emitter.
diagram
of
MIS
type
tunnel
0.1
Fig.2 Schematic diagram of electron emission current measurement. Where, diode voltage was 0-15V and electron collector voltage was 55v. 0
r $1
I_Ll__L-.I.__LL 10
Diode Voltage
P 14
I Ii I
Fig.5 Diode current and electron as a function of diode voltage. and d=lOnm.
emission Where,
current
T,=500"C