- Email: [email protected]
pn diode
A v-groove Schottky/pn diode Simon S. Ang
Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72 701, U.S.A. V-groove Schottky/pn diodes were fabricated and their current-voltage characteristics were investigated. These diodes were found to have low cut-in voltages as in conventional Schottky diodes. However, their reverse currant-voltage characteristics were found to resemble those of conventional pn diodes. 1. Introduction Fast switching diodes are desirable for a variety of high frequency switching applications. Historically, Schottky diodes have been used where fast switching is desirable. However, the low reverse breakdown and high reverse leakage current of Schottky diodes usually limit their practical usage to below 50 V. For high voltage switching applications, the p-i-n diode is the only choice. Conventional p-i-n diodes are relatively slow switching devices due to their long reverse recovery time caused by the large amount of stored charge in the i region. Gold-doped p-i-n diodes are faster than conventional p-i-n diodes but they suffer from a higher forward voltage drop and larger reverse leakage currents. High power switching diodes demand low forward voltage drop to reduce wasteful power dissipation. The reverse blocking of Schottky diodes is a major problem at high voltage and high temperature. The high reverse leakage current of Schottky diodes is caused by injection of carriers across the Schottky barrier and is predominantly controlled by the barrier height of the Schottky junction. The soft breakdown characteristic of Schottky diodes is due to barrier height lowering during reverse bias [1]. A merged Schottky/p-i-n diode has been analysed by Baliga [2] recently. In this work a vgroove Schottky/pn diode is fabricated for the first time. This diode has a low cut-in voltage as in a conventional Schottky diode but with a lower reverse leakage current and thus a higher breakdown voltage. Both titanium nitride (TIN) [3] and aluminium (A1) were used to form the Schottky electrodes. 2. Experimental All the devices were fabricated in a single run with splits for the conventional pn diodes, v-groove TiN Schottky/pn diodes and v-groove A1 Schottky/pn diodes. The structural view of the v-groove Schottky/pn diode is shown in Fig. 1. The active area of the diode, excluding the surrounding moat, is 4 mm 2. The starting material was a 10 lam, 2×10 ]~ cmr-3, phosphorus-doped epitaxial layer on a 3× 10,'8 cm -3 phosphorus-doped [100] silicon substrate. After an intial 10 min H202 and H2SO4 (1:2 by volume) clean, a 2000-/~ thick silicon dioxide layer was thermally grown in an oxidation furnace at 1000°C to serve as an etch mask for the surrounding moat. The anisotropic etching was accomplished at 85°C using a potassium hydroxide solution diluted with propanol. A 9 pm v-groove moat surrounding the active area was etched to isolate the active area from the periphery to minimise edge leakage. The remaining silicon dioxide mask was etched away after the anisotropic etch and a 7000-:k thick silicon dioxide layer was thermally grown in pyrogenic steam at 1000°C after a H202 and H~SO4 clean. The active anode area was then formed by etching away the oxide where a subsequent BBr3 deposition at 1050°C was performed. A 2500-A silicon dioxide layer was grown during the boron diffusion step. The v-groove Schottky contact was then photolithographically defined and the silicon dioxide was etched in dilute hydMICROELECTRONICS JOURNAL Vol. 20 No. 5 © 1989 Elsevier Science Publishers Ltd., England
7
2,5u~ 3oron
~IF~u~e~ r
!~yer
SIUcon
dioxide
Anode
3xlO ~m.~ "i0~2 Phost~horu_~-5omecl SILicon Suos"zr~.e
/~/
.f
~f
I i.,////,///? ~ "~> "/////~ V / / 2 ;/""///////////"I'/I~ Y / / / / / / Z .C/~.;"/ Y / /i"/// //.4
Atu~inu,,-,
i ] o
C~.'~ode
Fig. I Structural view of the v-groove Sebottky/pn diode
rofluoric acid. V-groove etching was accomplished using propanol-diluted potassium hydroxide (KOH) at a temperature of about 85°C. Preferential etching occurred in the [100] direction on silicon. Since the [111] planes were 54.7 ° oblique to the [100] surface, the etching proceeded in the vertical direction under the silicon dioxide openings until the [111[ planes emanating from the edges of the oxide opening intersected. When the crystallographic etch fronts intersected, all silicon etching effectively ceased since only the [ 111] planes were exposed. The depth into the silicon at which the crystallographic etch fronts intersected was strictly determined by the size of the oxide openings. Thus, it was possible to control accurately the depth of the v-groove in a process-insensitive manner by patterning the planar surface with normal lithographic techniques followed by an operatorinsensitive wet etch [4]. The depth of the Schottky v-groove was chosen to be 5 pm. After a post-KOH clean, contacts were photolithographically defined and the silicon dioxide was etched to expose the contact openings. Titanium nitride or aluminium was used to form the Schottky contact as well as the anode contact. The titanium nitride was sputtered in a direct current sputtering machine in a mixture of argon and nitrogen gases using a high purity titanium target. A 5000-/k aluminium film was evaporated on top of the titanium nitride film to reduce its resistance since only a 1500-/k TiN layer was sputtered. The aluminium was deposited in an e-beam evaporator. Conventional pn diodes were fabricated by skipping the Schottky v-groove etch step. Contact to the cathode was formed b y evaporating aluminium on the backside of the substrate. A 450°C hydrogen anneal was performed to ensure low contact resistance as well as to anneal the radiation effects on the junction passivation silicon dioxide film. Some of the fabricated diodes were encapsulated in a plastic package for ease of measurement. Current-voltage measurements of the diodes were performed using a HP4145 semiconductor parameter analyser interfaced with a Texas Instruments personal computer for rapid processing of data. For temperature measurements, the encapsulated devices were placed in a computer controlled oven and current-voltage measurements were performed after a 15 min temperature stabilisation period. 3. Results and discussion The v-groove Schottky/pn diodes were evaluated for their forward and reverse conduction characteristics. The forward current-voltage characteristics of typical diodes are shown in Fig. 2. The v-groove TiN Schottky/pn diode has the lowest cut-in voltage while the conventional pn diode has a cut-in voltage of about 0.6 V. The lower cut-in voltage of the
/ /
1.0
~
0.8 <
0.6
/
R
Qc
o
0.4
0-2
0.0 0
0.1
0.2
f : 0.3
0.4
0.5
0.6
0.7
0.8
0.9
FORWARO BIAS. V
Fig. 2 Forward current-voltage characteristics of typical v-groove TiN Schottky/pn diode (A), v-groove AI Schottky/pn diode (B) and conventional pn diode (C) at 295 K
TiN Schottky/pn diode as compared with the A1 Schottky/pn diode is due to the lower TiN barrier height on silicon. The cut-in voltages of v-groove Schottky/pn diodes are similar to conventional Schottky diodes. The same data is shown in Fig. 3 plotted on a semilogarithmic scale. As shown, the semi-logarithmic forward current-voltage characteristics of all the diodes at low forward voltages are straight lines with unity diode quality factors. At higher forward voltages, series resistance of the epitaxial layer dominates the forward conduction as in a conventional Schottky diode. The forward voltage drop of these diodes can be reduced by increasing the doping concentration and/or reducing the thickness of the epitaxial layer. The shorter path of the majority carrier conduction due to the v-groove Schottky diode also reduces the series resistance effects at high current density. The buried v-groove Schottky electrodes ensure that the dominant conduction mechanism at low forward voltage is by majority carriers. As the forward bias increases, the pn diode starts to contribute to the carrier conduction. At high current injection, conductivity modulation of the lightly doped epitaxial layer beneath the pn diode becomes significant. This conductivity modulation helps to reduce the series resistance effects of the lightly doped epitaxial layer. The forward current-voltage characteristics of these diodes were also measured as a function of temperature. Figure 4 shows the reverse saturation current versus reciprocal temperature of these diodes. Barrier heights of 0.5 eV and 0.7 eV are determined for the titanium nitride and aluminium Schottkyjunctions, respectively. This implies that the predominant conduction mechanism of these diodes at low forward bias is due to Schottky effects. A built-in voltage of 0.72 eV is found for the pn diode. The saturation current of the pn diode is at least two orders of magnitude smaller than those of the v-groove Schottky/pn diodes. The reverse current-voltage characteristics of typical diodes are shown in Fig. 5. The conventional pn diodes have a very sharp characteristic with a breakdown voltage of about 80 V. The reverse breakdown characteristics of the v-groove Schottky/pn diodes resemble those of conventional pn diodes except that they are somewhat softer. However, the reverse leakage currents of these diodes are much lower than conventional Schottky diodes of
10"
10-~
I0-~
10 ~ Z
!
104,
10-'.
10--
Fig. 3 Semi-logarithmic plot of forward current-voltage characteristics of typical v-groove TiN Schottky/pn diode (A), v-groove A! Schottky/pn diode (B) and conventional pn diode (C) at 295 K
I0-'
°
|
10-~ ?.7
2-8
~.l
2.O
&l
32
3.1
3A
100OFT. ~-*
Fig. 4 Saturation current versus I000/T plot of typical v-groove TiN Schottky/pn diode (A), v-groove Ai Schottky/pn diode (B) and conventional pn diode (C) 10
0.0
-0.4
-0.8
-1.2
-1.6
-~0 0
~ -10
-20
, -30
j
-40
i
-60
-60
-70
-80
REVERSE BIAS. V
Fig. 5 Reverse breakdown characteristics of typical v-groove TiN Schottky/pn diode (A), v-groove AI Schottky/pn diode (B) and conventional pn diode (C) at 295 K
similar active area. Typical breakdown voltages of these v-groove Schottky/pn diodes are about 50-60 V. The higher reverse breakdown voltage of the AI Schottky/pn diode is probably due to the larger barrier height of aluminium on silicon. Optimisation of the thickness and doping density of the epitaxial layer will lead to an even higher breakdown voltage [5]. A u-shaped groove can also be used as the Schottky junction to reduce the electric field at the apex of the groove during reverse bias to increase the reverse breakdown voltage. Under reverse bias, the junction depletion layers created by the pn junctions expand and create a potential barrier under the v-groove Schottky interface. This junction-induced potential barrier is used to shield the Schottky barrier during reverse bias as shown in Fig. 6. The reverse breakdown voltage of these v-groove Schottky/pn diodes increases with temperature to 60-70 V at 370 K as in conventional p-i-n diodes. p*n diode p*n c/iOde 1 1 At Or ALIT;N Scho~ky diode electrode I
I /.///_///:,//////:'.'////d
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/
r
{~
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'
l
C~':hooe Fig. 6 Structural
view
of the v-groove Sehottky/pn diode
during reverse b i a s 11
4. Conclusion V-groove Schottky/pn diodes have been investigated. These diodes have a low cut-in voltage similar to conventional Schottky diodes. They have a lower reverse leakage current compared to conventional Schottky diodes and thus, can be used to block a higher reverse voltage than conventional Schottky diodes. The reverse breakdown characteristic of these diodes is predominantly controlled by the reverse-biased pn junctions. 5. Acknowledgement The author would like to express his gratitude to Professors William Brown and Jerry Yeargan of the University of Arkansas for their fruitful discussions and continuing support. 6. References [1] Sze, S.M., Physics of Semiconductor Deviccs, Wiley-Interscience, pp. 245-270, 1981. [2] Baliga, B.J., "Analysis of a high-voltage merged p-i-n/Schottky (MPS) rectifier", IEEE Electron. Device Lett., vol. EDL-8, no. 9, pp. 407--409, 1987. [3] Ang, S.S., "Titanium nitride films with high oxygen concentration", J. Electron. Mater., vol. 17, no. 2, pp. 95-100, 1988. [4] Curran, P.A. and Ang, S.S., "Nonplanar multiple-epitaxy bipolar power integratedcircuit process", 1EEE Trans. Electron. Devices, vol. ED-34, no. 8, pp. 1023-1030, 1987. [5] Williamowski, B.M., "Schottky diodes with high breakdown voltages", Solid-State Electron., vol. 26, no. 5, pp. 491-493, 1983.
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Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72 701, U.S.A. V-groove Schottky/pn diodes were fabricated and their current-voltage characteristics were investigated. These diodes were found to have low cut-in voltages as in conventional Schottky diodes. However, their reverse currant-voltage characteristics were found to resemble those of conventional pn diodes. 1. Introduction Fast switching diodes are desirable for a variety of high frequency switching applications. Historically, Schottky diodes have been used where fast switching is desirable. However, the low reverse breakdown and high reverse leakage current of Schottky diodes usually limit their practical usage to below 50 V. For high voltage switching applications, the p-i-n diode is the only choice. Conventional p-i-n diodes are relatively slow switching devices due to their long reverse recovery time caused by the large amount of stored charge in the i region. Gold-doped p-i-n diodes are faster than conventional p-i-n diodes but they suffer from a higher forward voltage drop and larger reverse leakage currents. High power switching diodes demand low forward voltage drop to reduce wasteful power dissipation. The reverse blocking of Schottky diodes is a major problem at high voltage and high temperature. The high reverse leakage current of Schottky diodes is caused by injection of carriers across the Schottky barrier and is predominantly controlled by the barrier height of the Schottky junction. The soft breakdown characteristic of Schottky diodes is due to barrier height lowering during reverse bias [1]. A merged Schottky/p-i-n diode has been analysed by Baliga [2] recently. In this work a vgroove Schottky/pn diode is fabricated for the first time. This diode has a low cut-in voltage as in a conventional Schottky diode but with a lower reverse leakage current and thus a higher breakdown voltage. Both titanium nitride (TIN) [3] and aluminium (A1) were used to form the Schottky electrodes. 2. Experimental All the devices were fabricated in a single run with splits for the conventional pn diodes, v-groove TiN Schottky/pn diodes and v-groove A1 Schottky/pn diodes. The structural view of the v-groove Schottky/pn diode is shown in Fig. 1. The active area of the diode, excluding the surrounding moat, is 4 mm 2. The starting material was a 10 lam, 2×10 ]~ cmr-3, phosphorus-doped epitaxial layer on a 3× 10,'8 cm -3 phosphorus-doped [100] silicon substrate. After an intial 10 min H202 and H2SO4 (1:2 by volume) clean, a 2000-/~ thick silicon dioxide layer was thermally grown in an oxidation furnace at 1000°C to serve as an etch mask for the surrounding moat. The anisotropic etching was accomplished at 85°C using a potassium hydroxide solution diluted with propanol. A 9 pm v-groove moat surrounding the active area was etched to isolate the active area from the periphery to minimise edge leakage. The remaining silicon dioxide mask was etched away after the anisotropic etch and a 7000-:k thick silicon dioxide layer was thermally grown in pyrogenic steam at 1000°C after a H202 and H~SO4 clean. The active anode area was then formed by etching away the oxide where a subsequent BBr3 deposition at 1050°C was performed. A 2500-A silicon dioxide layer was grown during the boron diffusion step. The v-groove Schottky contact was then photolithographically defined and the silicon dioxide was etched in dilute hydMICROELECTRONICS JOURNAL Vol. 20 No. 5 © 1989 Elsevier Science Publishers Ltd., England
7
2,5u~ 3oron
~IF~u~e~ r
!~yer
SIUcon
dioxide
Anode
3xlO ~m.~ "i0~2 Phost~horu_~-5omecl SILicon Suos"zr~.e
/~/
.f
~f
I i.,////,///? ~ "~> "/////~ V / / 2 ;/""///////////"I'/I~ Y / / / / / / Z .C/~.;"/ Y / /i"/// //.4
Atu~inu,,-,
i ] o
C~.'~ode
Fig. I Structural view of the v-groove Sebottky/pn diode
rofluoric acid. V-groove etching was accomplished using propanol-diluted potassium hydroxide (KOH) at a temperature of about 85°C. Preferential etching occurred in the [100] direction on silicon. Since the [111] planes were 54.7 ° oblique to the [100] surface, the etching proceeded in the vertical direction under the silicon dioxide openings until the [111[ planes emanating from the edges of the oxide opening intersected. When the crystallographic etch fronts intersected, all silicon etching effectively ceased since only the [ 111] planes were exposed. The depth into the silicon at which the crystallographic etch fronts intersected was strictly determined by the size of the oxide openings. Thus, it was possible to control accurately the depth of the v-groove in a process-insensitive manner by patterning the planar surface with normal lithographic techniques followed by an operatorinsensitive wet etch [4]. The depth of the Schottky v-groove was chosen to be 5 pm. After a post-KOH clean, contacts were photolithographically defined and the silicon dioxide was etched to expose the contact openings. Titanium nitride or aluminium was used to form the Schottky contact as well as the anode contact. The titanium nitride was sputtered in a direct current sputtering machine in a mixture of argon and nitrogen gases using a high purity titanium target. A 5000-/k aluminium film was evaporated on top of the titanium nitride film to reduce its resistance since only a 1500-/k TiN layer was sputtered. The aluminium was deposited in an e-beam evaporator. Conventional pn diodes were fabricated by skipping the Schottky v-groove etch step. Contact to the cathode was formed b y evaporating aluminium on the backside of the substrate. A 450°C hydrogen anneal was performed to ensure low contact resistance as well as to anneal the radiation effects on the junction passivation silicon dioxide film. Some of the fabricated diodes were encapsulated in a plastic package for ease of measurement. Current-voltage measurements of the diodes were performed using a HP4145 semiconductor parameter analyser interfaced with a Texas Instruments personal computer for rapid processing of data. For temperature measurements, the encapsulated devices were placed in a computer controlled oven and current-voltage measurements were performed after a 15 min temperature stabilisation period. 3. Results and discussion The v-groove Schottky/pn diodes were evaluated for their forward and reverse conduction characteristics. The forward current-voltage characteristics of typical diodes are shown in Fig. 2. The v-groove TiN Schottky/pn diode has the lowest cut-in voltage while the conventional pn diode has a cut-in voltage of about 0.6 V. The lower cut-in voltage of the
/ /
1.0
~
0.8 <
0.6
/
R
Qc
o
0.4
0-2
0.0 0
0.1
0.2
f : 0.3
0.4
0.5
0.6
0.7
0.8
0.9
FORWARO BIAS. V
Fig. 2 Forward current-voltage characteristics of typical v-groove TiN Schottky/pn diode (A), v-groove AI Schottky/pn diode (B) and conventional pn diode (C) at 295 K
TiN Schottky/pn diode as compared with the A1 Schottky/pn diode is due to the lower TiN barrier height on silicon. The cut-in voltages of v-groove Schottky/pn diodes are similar to conventional Schottky diodes. The same data is shown in Fig. 3 plotted on a semilogarithmic scale. As shown, the semi-logarithmic forward current-voltage characteristics of all the diodes at low forward voltages are straight lines with unity diode quality factors. At higher forward voltages, series resistance of the epitaxial layer dominates the forward conduction as in a conventional Schottky diode. The forward voltage drop of these diodes can be reduced by increasing the doping concentration and/or reducing the thickness of the epitaxial layer. The shorter path of the majority carrier conduction due to the v-groove Schottky diode also reduces the series resistance effects at high current density. The buried v-groove Schottky electrodes ensure that the dominant conduction mechanism at low forward voltage is by majority carriers. As the forward bias increases, the pn diode starts to contribute to the carrier conduction. At high current injection, conductivity modulation of the lightly doped epitaxial layer beneath the pn diode becomes significant. This conductivity modulation helps to reduce the series resistance effects of the lightly doped epitaxial layer. The forward current-voltage characteristics of these diodes were also measured as a function of temperature. Figure 4 shows the reverse saturation current versus reciprocal temperature of these diodes. Barrier heights of 0.5 eV and 0.7 eV are determined for the titanium nitride and aluminium Schottkyjunctions, respectively. This implies that the predominant conduction mechanism of these diodes at low forward bias is due to Schottky effects. A built-in voltage of 0.72 eV is found for the pn diode. The saturation current of the pn diode is at least two orders of magnitude smaller than those of the v-groove Schottky/pn diodes. The reverse current-voltage characteristics of typical diodes are shown in Fig. 5. The conventional pn diodes have a very sharp characteristic with a breakdown voltage of about 80 V. The reverse breakdown characteristics of the v-groove Schottky/pn diodes resemble those of conventional pn diodes except that they are somewhat softer. However, the reverse leakage currents of these diodes are much lower than conventional Schottky diodes of
10"
10-~
I0-~
10 ~ Z
!
104,
10-'.
10--
Fig. 3 Semi-logarithmic plot of forward current-voltage characteristics of typical v-groove TiN Schottky/pn diode (A), v-groove A! Schottky/pn diode (B) and conventional pn diode (C) at 295 K
I0-'
°
|
10-~ ?.7
2-8
~.l
2.O
&l
32
3.1
3A
100OFT. ~-*
Fig. 4 Saturation current versus I000/T plot of typical v-groove TiN Schottky/pn diode (A), v-groove Ai Schottky/pn diode (B) and conventional pn diode (C) 10
0.0
-0.4
-0.8
-1.2
-1.6
-~0 0
~ -10
-20
, -30
j
-40
i
-60
-60
-70
-80
REVERSE BIAS. V
Fig. 5 Reverse breakdown characteristics of typical v-groove TiN Schottky/pn diode (A), v-groove AI Schottky/pn diode (B) and conventional pn diode (C) at 295 K
similar active area. Typical breakdown voltages of these v-groove Schottky/pn diodes are about 50-60 V. The higher reverse breakdown voltage of the AI Schottky/pn diode is probably due to the larger barrier height of aluminium on silicon. Optimisation of the thickness and doping density of the epitaxial layer will lead to an even higher breakdown voltage [5]. A u-shaped groove can also be used as the Schottky junction to reduce the electric field at the apex of the groove during reverse bias to increase the reverse breakdown voltage. Under reverse bias, the junction depletion layers created by the pn junctions expand and create a potential barrier under the v-groove Schottky interface. This junction-induced potential barrier is used to shield the Schottky barrier during reverse bias as shown in Fig. 6. The reverse breakdown voltage of these v-groove Schottky/pn diodes increases with temperature to 60-70 V at 370 K as in conventional p-i-n diodes. p*n diode p*n c/iOde 1 1 At Or ALIT;N Scho~ky diode electrode I
I /.///_///:,//////:'.'////d
\\
4/
/
r
{~
3xi0:~=.~ -3 ,~00~ ,=~-c~:r~orus-?~o~ec~
J
SJ[IC3R
/
SL,~S~'~e
" / / / / Z . ~,Y A / / Y//,/~ /.,Y/ . <~/ / / / / I/IX~l/, ?;/~";///i Y / / / / / / / / / / H
'
l
C~':hooe Fig. 6 Structural
view
of the v-groove Sehottky/pn diode
during reverse b i a s 11
4. Conclusion V-groove Schottky/pn diodes have been investigated. These diodes have a low cut-in voltage similar to conventional Schottky diodes. They have a lower reverse leakage current compared to conventional Schottky diodes and thus, can be used to block a higher reverse voltage than conventional Schottky diodes. The reverse breakdown characteristic of these diodes is predominantly controlled by the reverse-biased pn junctions. 5. Acknowledgement The author would like to express his gratitude to Professors William Brown and Jerry Yeargan of the University of Arkansas for their fruitful discussions and continuing support. 6. References [1] Sze, S.M., Physics of Semiconductor Deviccs, Wiley-Interscience, pp. 245-270, 1981. [2] Baliga, B.J., "Analysis of a high-voltage merged p-i-n/Schottky (MPS) rectifier", IEEE Electron. Device Lett., vol. EDL-8, no. 9, pp. 407--409, 1987. [3] Ang, S.S., "Titanium nitride films with high oxygen concentration", J. Electron. Mater., vol. 17, no. 2, pp. 95-100, 1988. [4] Curran, P.A. and Ang, S.S., "Nonplanar multiple-epitaxy bipolar power integratedcircuit process", 1EEE Trans. Electron. Devices, vol. ED-34, no. 8, pp. 1023-1030, 1987. [5] Williamowski, B.M., "Schottky diodes with high breakdown voltages", Solid-State Electron., vol. 26, no. 5, pp. 491-493, 1983.
12