- Email: [email protected]
GaN MODFETs with high breakdown voltage
Journal of Crystal Growth 201/202 (1999) 327}331
MBE grown AlGaN/GaN MODFETs with high breakdown voltage A. Vescan *, R. Dietrich , A. Wieszt , H. Tobler , H. Leier , J.M. Van Hove, P.P. Chow, A.M. Wowchak Daimler Chrysler, Research and Technology, Wilhelm-Runge-Str. 11, 89081 Ulm, Germany SVT Associates, 7620 Executive Drive, Eden Prairie, MN 55344, USA
Abstract We report on the performance of AlGaN/GaN MODFETs grown by MBE. A record value of 410 V maximum drain voltage was measured for devices with current densities of 200 mA/mm. From the dependence of breakdown voltage with gate-drain separation a breakdown "eld of +1 MV/cm is estimated. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.H; 85.30.T Keywords: AlGaN/GaN; MBE; MODFET; Breakdown
1. Introduction In the past years great progress has been made in the development of III}V nitride-based optoelectronic devices and transistors. The main applications are, besides blue and green lasers and LEDs, devices for high-power and high-frequency operation. In this respect, impressive power density and cut-o! frequency results have already been published using epitaxial layers grown by both metal organic chemical vapor deposition (MOCVD) [1}3] and molecular beam epitaxy (MBE) [4,5]. Nevertheless, the present state-of-the-art results have been mainly reached using MOCVD material. Recently, 6.8 W/mm with high-power-added e$ciency
at 10 GHz have been reported for AlGaN/GaN HEMTs grown on SiC [1]. The highest cut-o! frequencies to date are f "67 GHz and R f "140 GHz for piezoelectric AlGaN/GaN
HEMTs grown on sapphire [2]. To achieve high levels of output power the fabricated devices must sustain a large maximum voltage swing. For 10 W/mm at least 80 V of maximum drain voltage are needed for a device with 1 A/mm maximum drain current density. In this paper, we present results on AlGaN/GaN MODFETs grown by MBE showing breakdown performance comparable to MOCVD grown devices and discuss the dependence of the electrical characteristics on device geometry. 2. Epitaxial layer structures
* Corresponding author. Tel.: #49-731-5052086; fax:#49731-5054102; e-mail: [email protected].
An atomic RF plasma source developed speci"cally for the growth of MBE nitrides was used for
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 4 1 - 4
328
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331
this work and has been described previously [6]. The basic concept is to create a plasma of nitrogen with the use of a RF "eld. RF energy (350 W) is fed into the gun through a water-cooled copper coil. A pyrolitic boron nitride (PBN) tube with a changeable nozzle is centered between the RF coils. Nitrogen is introduced to the tube and a plasma is created within the tube. Elemental Ga, Al and Si was supplied from e!usion cells. The group III #ux was measured in situ from the GaN or AlN growth rate under nitrogen rich growth conditions and at a substrate temperature where Ga evaporation was insigni"cant. Similarly, the nitrogen #ux was measured in situ from the GaN growth rate under Ga rich conditions. These measurements were done by optical interference of a re#ected HeNe laser beam from the growing GaN surface. The AlGaN/GaN MODFET structures were grown on sapphire (0 0 0 1). After nitridation of the
surface a low-temperature AlN (300 As ) bu!er layer was grown. This was followed by an 1 lm undoped GaN grown at 7503C under Ga rich growth conditions and "nally an AlGaN barrier/donor layer. Growth temperature was measured with an optical pyrometer which provided a reproducible but perhaps not accurate temperature value. A growth rate of 0.5}1 lm/h was used for all depositions. More details on the MBE growth parameters may be found in Ref. [7]. In situ cathodoluminescence measurements were used to determine the Al content of the AlGaN layer [8]. Fig. 1 shows the resulting spectrum indicating clearly the AlGaN layer and the underlying GaN bu!er region. The AlGaN layer con"gurations investigated in this study are summarized in Table 1 together with the results of R.T. Hall measurements. The basic structure consisted of an unintentionally doped (UID) AlGaN spacer, a Si-doped AlGaN donor layer (doping level in the 10 and 10 cm\ range) and "nally a UID AlGaN cap layer. The Al content in the AlGaN was 15%.
3. Device fabrication
Fig. 1. CL scan taken from AlGaN/GaN MODFET structure showing emission from both the AlGaN layer and GaN bu!er.
MODFETs with gate widths of 20 and 50 lm were processed. The device isolation was achieved by mesa etching (RIE) in a parallel reactor using a BCL /Ar gas mixture. Ohmic contacts were formed by alloying a Ti/Al/Ni/Au (20/200/40/50) metallization scheme at 9503C as described by Ref. [9]. Prior to the evaporation the samples were
Table 1 Barrier/donor layer con"guration under investigation and electrical data of the devices with ¸ "1.5 lm Sample T14
Sample T25
Sample T27
AlGaN cap-layer (nm) AlGaN Si-doped layer (nm) AlGaN barrier-layer (nm)
1.5 nm 5 nm 5 nm
1.5 nm 5 nm 5 nm
5 nm 15 nm 3 nm
k (cm/V s) & ** n (cm\) R () mm) I (mA/mm) " g (mS/mm)
< (V) 2&
450$50 (6$2);10
450$60 (9$1);10
360$50 (3.5$0.2);10
'10 160 70 2.8
1 470 110 +4.0
2 203 90 2.0
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331
rinsed in HCl and in situ cleaned by an Ar ion beam. This resulted in contact resistances of R "1}2 ) mm. These values may be further im proved by optimized pre-metallization treatment (i.e. short RIE exposure of the contact region) and better temperature control. Subsequent experiments yielded indeed signi"cantly lower contact resistances (R (0.5 ) mm). In comparison, Ti/Au contacts show slightly non-linear characteristics and signi"cant higher R were observed (sample T14). This was probably due to the balling up of the Ti/Au alloy upon annealing, leading to an inhomogeneous contact interface. For the gate contact standard Pt/Au (50/200 nm) was used, patterned by optical lithography, with gate lengths from 1 lm to 5 lm. For comparison between the di!erent samples only the nominal 1.5 lm (real+1.8 lm) devices will be discussed here in detail. For breakdown investigations various gatedrain spacings (1}6 lm) were also fabricated.
329
Fig. 2. Output characteristics of 1.5 lm MODFET on sample T25. The gate-source and gate drain spacings are 1.9 and 2.2 lm, respectively.
4. Device performance The results of the DC characterization of the various samples are also given in Table 1. The maximum drain current I and transconduc" tance g are for T25 and T27 in good agreement
with the sheet concentrations and mobilities from the Hall characterization. The in#uence of the poor ohmic contacts dominates the electrical performance of sample T14, leading to reduced currents and transconductances. The largest I and g are measured for sample "
T25, yielding 470 mA/mm and 110 mS/mm, respectively (Figs. 2 and 3). The device on layer T25 cannot be completely pinched-o!, as may be more clearly seen in the transfer characteristics in Fig. 3. Nevertheless, the extrapolated pinch-o! voltage of approximately 4.0 V "ts quite well with the measured n and the design of the epitaxial layer. Signif icant enhancement of the electrical data may be achieved if the gate-source spacing and the gate length are reduced. Details of the sub-lm device performance will be given elsewhere. The breakdown characterization of the samples was perfomed on a Tektronix curve tracer (see Fig. 4). The devices were biased into the subthres-
Fig. 3. Transfer characteristics and transconductance of AlGaN/GaN MODFETs for a nominal gate length of 1.5 lm.
hold regime. The breakdown voltage was de"ned as the < bias point at which the drain current in"1 creased to 1 mA/mm. The observed breakdown always occurred across the gate-drain region of the transistors, as evidenced by microscope analysis. The breakdown voltage < was determined for 0 several gate-drain spacings d and also for two di!erent gate widths (20 and 50 lm) as shown in
330
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331
Fig. 4. Curve tracer image, the high-voltage MODFET performance.
Fig. 5. Sample T25 showed basically no dependence of the breakdown voltage on gate-drain separation (< +50 V). This is due to the large gate leakage 0 current of the devices, manifested also in the incomplete pinch-o! behaviour, leading to evaporation of the gate contact under large drain bias. For samples T14 and T27 a clear dependence of the breakdown voltage with gate-drain separation is observed. However, this dependence is all but conclusive with respect to the breakdown behaviour. In the simplest case where avalanche breakdown is assumed, an increase of < with d is 0 expected, until the gate-drain depletion region becomes smaller than the gate-drain separation (a similar behaviour has been previously reported by Gaska et al. [10]). Beyond this value the breakdown voltage should become independent of d . However, this model does not take any surface or leakage current related e!ects into account, which would reduce the breakdown voltage independent of the breakdown "eld of the material. In our case the `ideala behaviour could not be identi"ed. We observe two regions with di!erent slopes. Nevertheless, it is clearly shown that the maximum breakdown voltage may be adjusted simply by choosing an appropriate gate-drain spacing. From the steepest slope of the plot the lower limit of the breakdown "eld is estimated in this model to +1 MV/cm. For a reasonable interpretation of the results a more detailed analysis of the breakdown behaviour is needed, to identify the dominating mechanisms.
Fig. 5. Breakdown voltage of AlGaN/GaN MODFETs as a function of gate-drain separation.
5. Conclusion We have analysed the dependence of breakdown voltage on gate-drain spacing for di!erent AlGaN/MODFETs grown by MBE on sapphire. The maximum achieved breakdown voltage was +410 V for devices with 200 mA/mm open channel current and 6.2 lm gate-drain spacing demonstrating the potential of MBE grown material for high-voltage operation.
References [1] S.T. Sheppard, K. Doverspike, W.L. Pribble, S.T. Allen, J.W. Palmour, L.T. Kehias, T.J. Jenkins, 56th Annual Device Research Conf. Digest, Charlottesville, VA, June 22}24, 1998. [2] L.F. Eastman et al., Materials Research Society Symp. Proc., April 13}17, San Francisco, California, 1998, to be published. [3] Y.-F. Wu, B.P. Keller, P. Fini, J. Pusl, M. Le, N.X. Nguyen, C. Nguyen, D. Widman, S. Keller, S.P. Denbaars, U.K. Mishra, Electron. Lett. 33 (20) (1997) 1742. [4] C. Nguyen, N.X. Nguyen, M. Le, D.E. Grider, Electron. Lett. 34 (3) (1998) 309. [5] Z. Fan, C. Lu, A.E. Botchkarev, H. Tang, A. Salvador, OG . Aktas, W. Kim, H. Morkoc, Electron. Lett. 33 (9) (1997) 814.
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331 [6] J.M. Van Hove, G.J. Cosimini, E. Nelson, A.M. Wowchak, P.P. Chow, J. Crystal Growth 150 (1995) 908. [7] J.M. Van Hove, P.P. Chow, A.M. Wowchak, J.J. Klaassen, R. Hickman II, C. Polley, J. Vac. Sci. Technol. B 16 (3) (1998) 1286. [8] J.M. Van Hove, P.P. Chow, A.M. Wowchak, J.J. Klaassen, M.F. Rosamond, D.R. Croswell, J. Crystal Growth 175/176 (1997) 79.
331
[9] Z. Fan, S.N. Mohammad, W. Kim, OG . Aktas, A.E. Botchkarev, H. Morkoc, Appl. Phys. Lett. 68 (12) (1996) 1672. [10] R. Gaska, Q. Chen, J. Yang, M.A. Khan, M.S. Shur, A. Ping, I. Adesida, Electron. Lett. 33 (14) (1997) 1255.
MBE grown AlGaN/GaN MODFETs with high breakdown voltage A. Vescan *, R. Dietrich , A. Wieszt , H. Tobler , H. Leier , J.M. Van Hove, P.P. Chow, A.M. Wowchak Daimler Chrysler, Research and Technology, Wilhelm-Runge-Str. 11, 89081 Ulm, Germany SVT Associates, 7620 Executive Drive, Eden Prairie, MN 55344, USA
Abstract We report on the performance of AlGaN/GaN MODFETs grown by MBE. A record value of 410 V maximum drain voltage was measured for devices with current densities of 200 mA/mm. From the dependence of breakdown voltage with gate-drain separation a breakdown "eld of +1 MV/cm is estimated. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.H; 85.30.T Keywords: AlGaN/GaN; MBE; MODFET; Breakdown
1. Introduction In the past years great progress has been made in the development of III}V nitride-based optoelectronic devices and transistors. The main applications are, besides blue and green lasers and LEDs, devices for high-power and high-frequency operation. In this respect, impressive power density and cut-o! frequency results have already been published using epitaxial layers grown by both metal organic chemical vapor deposition (MOCVD) [1}3] and molecular beam epitaxy (MBE) [4,5]. Nevertheless, the present state-of-the-art results have been mainly reached using MOCVD material. Recently, 6.8 W/mm with high-power-added e$ciency
at 10 GHz have been reported for AlGaN/GaN HEMTs grown on SiC [1]. The highest cut-o! frequencies to date are f "67 GHz and R f "140 GHz for piezoelectric AlGaN/GaN
HEMTs grown on sapphire [2]. To achieve high levels of output power the fabricated devices must sustain a large maximum voltage swing. For 10 W/mm at least 80 V of maximum drain voltage are needed for a device with 1 A/mm maximum drain current density. In this paper, we present results on AlGaN/GaN MODFETs grown by MBE showing breakdown performance comparable to MOCVD grown devices and discuss the dependence of the electrical characteristics on device geometry. 2. Epitaxial layer structures
* Corresponding author. Tel.: #49-731-5052086; fax:#49731-5054102; e-mail: [email protected].
An atomic RF plasma source developed speci"cally for the growth of MBE nitrides was used for
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 4 1 - 4
328
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331
this work and has been described previously [6]. The basic concept is to create a plasma of nitrogen with the use of a RF "eld. RF energy (350 W) is fed into the gun through a water-cooled copper coil. A pyrolitic boron nitride (PBN) tube with a changeable nozzle is centered between the RF coils. Nitrogen is introduced to the tube and a plasma is created within the tube. Elemental Ga, Al and Si was supplied from e!usion cells. The group III #ux was measured in situ from the GaN or AlN growth rate under nitrogen rich growth conditions and at a substrate temperature where Ga evaporation was insigni"cant. Similarly, the nitrogen #ux was measured in situ from the GaN growth rate under Ga rich conditions. These measurements were done by optical interference of a re#ected HeNe laser beam from the growing GaN surface. The AlGaN/GaN MODFET structures were grown on sapphire (0 0 0 1). After nitridation of the
surface a low-temperature AlN (300 As ) bu!er layer was grown. This was followed by an 1 lm undoped GaN grown at 7503C under Ga rich growth conditions and "nally an AlGaN barrier/donor layer. Growth temperature was measured with an optical pyrometer which provided a reproducible but perhaps not accurate temperature value. A growth rate of 0.5}1 lm/h was used for all depositions. More details on the MBE growth parameters may be found in Ref. [7]. In situ cathodoluminescence measurements were used to determine the Al content of the AlGaN layer [8]. Fig. 1 shows the resulting spectrum indicating clearly the AlGaN layer and the underlying GaN bu!er region. The AlGaN layer con"gurations investigated in this study are summarized in Table 1 together with the results of R.T. Hall measurements. The basic structure consisted of an unintentionally doped (UID) AlGaN spacer, a Si-doped AlGaN donor layer (doping level in the 10 and 10 cm\ range) and "nally a UID AlGaN cap layer. The Al content in the AlGaN was 15%.
3. Device fabrication
Fig. 1. CL scan taken from AlGaN/GaN MODFET structure showing emission from both the AlGaN layer and GaN bu!er.
MODFETs with gate widths of 20 and 50 lm were processed. The device isolation was achieved by mesa etching (RIE) in a parallel reactor using a BCL /Ar gas mixture. Ohmic contacts were formed by alloying a Ti/Al/Ni/Au (20/200/40/50) metallization scheme at 9503C as described by Ref. [9]. Prior to the evaporation the samples were
Table 1 Barrier/donor layer con"guration under investigation and electrical data of the devices with ¸ "1.5 lm Sample T14
Sample T25
Sample T27
AlGaN cap-layer (nm) AlGaN Si-doped layer (nm) AlGaN barrier-layer (nm)
1.5 nm 5 nm 5 nm
1.5 nm 5 nm 5 nm
5 nm 15 nm 3 nm
k (cm/V s) & ** n (cm\) R () mm) I (mA/mm) " g (mS/mm)
< (V) 2&
450$50 (6$2);10
450$60 (9$1);10
360$50 (3.5$0.2);10
'10 160 70 2.8
1 470 110 +4.0
2 203 90 2.0
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331
rinsed in HCl and in situ cleaned by an Ar ion beam. This resulted in contact resistances of R "1}2 ) mm. These values may be further im proved by optimized pre-metallization treatment (i.e. short RIE exposure of the contact region) and better temperature control. Subsequent experiments yielded indeed signi"cantly lower contact resistances (R (0.5 ) mm). In comparison, Ti/Au contacts show slightly non-linear characteristics and signi"cant higher R were observed (sample T14). This was probably due to the balling up of the Ti/Au alloy upon annealing, leading to an inhomogeneous contact interface. For the gate contact standard Pt/Au (50/200 nm) was used, patterned by optical lithography, with gate lengths from 1 lm to 5 lm. For comparison between the di!erent samples only the nominal 1.5 lm (real+1.8 lm) devices will be discussed here in detail. For breakdown investigations various gatedrain spacings (1}6 lm) were also fabricated.
329
Fig. 2. Output characteristics of 1.5 lm MODFET on sample T25. The gate-source and gate drain spacings are 1.9 and 2.2 lm, respectively.
4. Device performance The results of the DC characterization of the various samples are also given in Table 1. The maximum drain current I and transconduc" tance g are for T25 and T27 in good agreement
with the sheet concentrations and mobilities from the Hall characterization. The in#uence of the poor ohmic contacts dominates the electrical performance of sample T14, leading to reduced currents and transconductances. The largest I and g are measured for sample "
T25, yielding 470 mA/mm and 110 mS/mm, respectively (Figs. 2 and 3). The device on layer T25 cannot be completely pinched-o!, as may be more clearly seen in the transfer characteristics in Fig. 3. Nevertheless, the extrapolated pinch-o! voltage of approximately 4.0 V "ts quite well with the measured n and the design of the epitaxial layer. Signif icant enhancement of the electrical data may be achieved if the gate-source spacing and the gate length are reduced. Details of the sub-lm device performance will be given elsewhere. The breakdown characterization of the samples was perfomed on a Tektronix curve tracer (see Fig. 4). The devices were biased into the subthres-
Fig. 3. Transfer characteristics and transconductance of AlGaN/GaN MODFETs for a nominal gate length of 1.5 lm.
hold regime. The breakdown voltage was de"ned as the < bias point at which the drain current in"1 creased to 1 mA/mm. The observed breakdown always occurred across the gate-drain region of the transistors, as evidenced by microscope analysis. The breakdown voltage < was determined for 0 several gate-drain spacings d and also for two di!erent gate widths (20 and 50 lm) as shown in
330
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331
Fig. 4. Curve tracer image, the high-voltage MODFET performance.
Fig. 5. Sample T25 showed basically no dependence of the breakdown voltage on gate-drain separation (< +50 V). This is due to the large gate leakage 0 current of the devices, manifested also in the incomplete pinch-o! behaviour, leading to evaporation of the gate contact under large drain bias. For samples T14 and T27 a clear dependence of the breakdown voltage with gate-drain separation is observed. However, this dependence is all but conclusive with respect to the breakdown behaviour. In the simplest case where avalanche breakdown is assumed, an increase of < with d is 0 expected, until the gate-drain depletion region becomes smaller than the gate-drain separation (a similar behaviour has been previously reported by Gaska et al. [10]). Beyond this value the breakdown voltage should become independent of d . However, this model does not take any surface or leakage current related e!ects into account, which would reduce the breakdown voltage independent of the breakdown "eld of the material. In our case the `ideala behaviour could not be identi"ed. We observe two regions with di!erent slopes. Nevertheless, it is clearly shown that the maximum breakdown voltage may be adjusted simply by choosing an appropriate gate-drain spacing. From the steepest slope of the plot the lower limit of the breakdown "eld is estimated in this model to +1 MV/cm. For a reasonable interpretation of the results a more detailed analysis of the breakdown behaviour is needed, to identify the dominating mechanisms.
Fig. 5. Breakdown voltage of AlGaN/GaN MODFETs as a function of gate-drain separation.
5. Conclusion We have analysed the dependence of breakdown voltage on gate-drain spacing for di!erent AlGaN/MODFETs grown by MBE on sapphire. The maximum achieved breakdown voltage was +410 V for devices with 200 mA/mm open channel current and 6.2 lm gate-drain spacing demonstrating the potential of MBE grown material for high-voltage operation.
References [1] S.T. Sheppard, K. Doverspike, W.L. Pribble, S.T. Allen, J.W. Palmour, L.T. Kehias, T.J. Jenkins, 56th Annual Device Research Conf. Digest, Charlottesville, VA, June 22}24, 1998. [2] L.F. Eastman et al., Materials Research Society Symp. Proc., April 13}17, San Francisco, California, 1998, to be published. [3] Y.-F. Wu, B.P. Keller, P. Fini, J. Pusl, M. Le, N.X. Nguyen, C. Nguyen, D. Widman, S. Keller, S.P. Denbaars, U.K. Mishra, Electron. Lett. 33 (20) (1997) 1742. [4] C. Nguyen, N.X. Nguyen, M. Le, D.E. Grider, Electron. Lett. 34 (3) (1998) 309. [5] Z. Fan, C. Lu, A.E. Botchkarev, H. Tang, A. Salvador, OG . Aktas, W. Kim, H. Morkoc, Electron. Lett. 33 (9) (1997) 814.
A. Vescan et al. / Journal of Crystal Growth 201/202 (1999) 327}331 [6] J.M. Van Hove, G.J. Cosimini, E. Nelson, A.M. Wowchak, P.P. Chow, J. Crystal Growth 150 (1995) 908. [7] J.M. Van Hove, P.P. Chow, A.M. Wowchak, J.J. Klaassen, R. Hickman II, C. Polley, J. Vac. Sci. Technol. B 16 (3) (1998) 1286. [8] J.M. Van Hove, P.P. Chow, A.M. Wowchak, J.J. Klaassen, M.F. Rosamond, D.R. Croswell, J. Crystal Growth 175/176 (1997) 79.
331
[9] Z. Fan, S.N. Mohammad, W. Kim, OG . Aktas, A.E. Botchkarev, H. Morkoc, Appl. Phys. Lett. 68 (12) (1996) 1672. [10] R. Gaska, Q. Chen, J. Yang, M.A. Khan, M.S. Shur, A. Ping, I. Adesida, Electron. Lett. 33 (14) (1997) 1255.