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GaAs pseudomorphic high electron mobility transistor utilizing a strained superlattice spacer
Solid-Sfate Ekctronics Vol. 36, No. 8, pp. 1117-l 119, 1993 Printed in Great Britain. All rights reserved
$6.00 + 0.00 0038-l lOI/ Copyright 0 1993 Pergamon Press Ltd
A NEW 6 -DOPED InGaAs/GaAs PSEUDOMORPHIC HIGH ELECTRON MOBILITY TRANSISTOR UTILIZING A STRAINED SUPERLATTICE SPACER H. M. Department
SHIEH,
of Electrical
W. C. Hsu, M. J. KAO, C. L. WV and
T. S. Wu
Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China
1, University
Road,
(Received 21 October 1992; in revised form 24 December 1992) Abstract-A new d-doped In,,,Ga,,,,As/GaAs pseudomorphic high electron mobility transistor (HEMT) utilizing an In, ,BGa, ,,As/GaAs strained superlattice spacer grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) has been prepared. For a gate length of 5pm, the present structure reveals superior saturation current density (180 mA/mm at 300 K and 230 mA/mm at 77 K) and extrinsic transconductance (103 mS/mm at 300 K and 142 mS/mm at 77 K). The current-voltage characteristics are superior to those of reported similar InGaAs/GaAs structure with gate length of 2 pm grown by molecular beam epitaxy (MBE). Also, because of the undoped cap layer grown on the top of the d-doped GaAs, a breakdown voltage of 1OV has been achieved.
1. INTRODUCTION
Due to lower electron effective mass, higher low-field mobility, higher doping capability and better highfield transport properties than those of GaAs, the InGaAs alloys have been widely used as channel materials[ 11. By carefully controlling the epi-layer thickness, lattice mismatch between InGaAs and GaAs can be accommodated by elastic strain rather than by generating misfit dislocation[2]. Consequently, high quality pseudomorphic structures can be grown for application to high speed and/or optoelectronic devices[3]. Furthermore the use of 6doped GaAs has received considerable interest as a means of obtaining high density and high mobility quasi-2DEG systems[4]. Thus, a a-doped InGaAs/ GaAs pseudomorphic HEMT can possess high mobility and superior current--voltage characteristics[5]. However, the interface traps at heterojunctions may significantly influence the device characteristics. In this work, to improve the properties of the 6doped InGaAs/GaAs pseudomorphic HEMT, a strained In,,,,Ga,,,As/GaAs superlattice spacer was employed for replacing the conventional undoped GaAs spacer for the first time. The present structure manifests superior current-voltage characteristics as compared with those of similar InGaAs/GaAs pseudomorphic HEMTs without superlattice spacer[3,6].
2. DEVICE STRUCTURE
AND
FABRICATION
The device material was grown by a low-pressure metal organic chemical vapor deposition (LPMOCVD). The new structure in this work as shown in Fig. 1, consists of a 0.5 pm undoped GaAs buffer
layer grown on a semi-insulating GaAs (100) substrate, a 90 A undoped In,,,,Ga,,,,As conducting channel, an undoped 15 8, GaAs/20 8, In,,,, Gd,,,, As/ 3OA GaAs/20 A In,,, Ga, ,,As/45 8, GaAs graded supperlattice spacer, a n + 6 -doped GaAs layer, and finally a 400 A undoped GaAs cap layer. The low pressure growth (80 torr) was carried out in a horizontal quartz reactor. Trimethylindium (TMI), triethylgallium (TEG), arsine (ASH,), and silane (SiH,) were used as the In, Ga, As sources and n-type dopant, respectively. The growth temperature was 650°C. The growth rates of GaAs and In,,, Ga,,,, As were controlled at 158 and 230 Ajmin respectively. To achieve a sharp S-doping profile, the Ga source was shut-off while keeping Si and As sources open during the growth of d-doped GaAs layer. The 3-D concentration of the S-doping GaAs layer was set at 1 x 10’9cm-‘. Standard photolithography and lift-off techniques were employed for device fabrication[7]. Alloyed AuGe was used for source and drain contacts, onto which Ag was evaporated to reduce the bonding resistance. Au was used as the Schottky contact for the gate. The gate area was 5 x 250pm*, while the distance between the drain and source electrodes was 30 /*m. 3. RESULTS
AND
DISCUSSION
Figure 2 shows the three-terminal current-voltage characteristics at room temperature. The maximum extrinsic trdnsconductances at 300 and 77 K are 103 and 142 mS/mm respectively which, are comparable with those of In,,,Ga,,,As/GaAs single quantum well HEMT grown by MBE with conventional GaAs 1117
1118
H. M.
i-GaAs Cap layer 400 ,&
SHIEH et al.
2
Fig. 1. The schematic diagram of the present new B-doped InGaAs/GaAs pseudomorphic HEMT structure.
0.0
-0.5
-1.0
-2.0
-1,s
Gate voltage
spacer for gate length of 2 pm[3]. Figure 3 showa the extrinsic transconductance vs gate voltage. Figure 4 illustrates the saturation current densities vs gate voltage at room temperature. High saturation current density up to 180 mA/mm at room temperature (230 mA/mm at 77 K) is reached. The saturation current densities are significantly superior to those of Ref. [3]. Meanwhile, when compared with those of similar pseudomorphic HEMT structure and similar gate geometry[6], the present structure reveals much better extrinsic transconductances and saturation current densities. Table 1 lists the comparison between this work and Refs [3] and [6] for similar InGaAs/GaAs pseudomorphic HEMT structures. In addition to the a-doped GaAs can supply more electrons, the strained superlattice spacer may also play an important role on the enhancement of I-V characteristics for the present structure. Unlike the homogeneously doped HEMT, the cap layer of the present new HEMT is undoped and hence increases the breakdown voltage (- 10 V in this work). The high saturation current density, high breakdown voltage, and high transconductance imply
-2.5
(V)
Fig. 3. The extrinsic transconductances 300 K.
vs gate voltage at
that the present structure possesses the potential for high performance applications. The properties-enhancement effect from using a strained InGaAs/GaAs superlattice spacer in this work may be attributed partly to an improvement in the crystalline quality at the heterojunctions and a possible reduction in the elastic stress at the junction that results from the different lattice constants of InGaAs and GaAs[8]. Also, the thin In,.,,G%.,,As layer (2OA) in the strained superlattice spacer of the present structure may provide a means of reducing the parallel
3OOK vd,=x.w
45.54
7 E r/l n
4.554 idiv
VDS
1 .OOO/div
(V)
Fig. 2. The three-terminal current-voltage characteristic at 300 K. The top curve is V,. = 0 V, and V,, = -0.5 V/step.
0.0
-0.5
-1.0
Gate
-1.5
voltage
-2.0
-2.5
-3.0
(V)
Fig. 4. The saturation current densities vs gate voltage.
HEMT
with strained superlattice spacer
Table 1. Comparison between this work and similar pseudomorphic structures This work Gate length (rm) 300K Maximum extrinsic 77 K transconductance (mS/mm) Maximum saturation current density (mA/mm)
300K 77 K
Ref. 3
Ref. 6
5
2
5
103
90
56
142
140
18
180
100
II6
230
140
140
1119
ance as well as high breakdown voltage (- 1OV) make th HEMT structure with strained superlattice spacer possess a potential for high speed and,/or high power applications. Acknowledgements-The authors appreciate the helpful discussions with Professor S. S. Li, University of Florida, and Dr P. Ho, General Electric Co. This work was sponsored by the National Science Council, Republic of China, under contract no. NSC 81-0404-E006-104. REFERENCES
conduction. Further work on the enhancement A,echanism of superlattice spacer is in progress. 4. CONCLUSIONS
A new 6 -doped InGaAs/GaAs pseudomorphic high electron mobility transistor utilizing a strained InGaAs/GaAs superlattice spacer grown by LPMOCVD has been studied for the first time. For gate length of 5 pm, the present new structure reveals superior extrinsic transconductances of 103 (142 mS/mm) and high saturation current densities 180 (230mA/mm) at 300 (77 K) which are significantly higher than those of similar pseudomorphic structures for gate length of 2 pm. The high perform-
1. I. J. Fritz, P. L. Gourley, L. R. Dawson and J. E. Schirber, Appl. Phys. Letr. 53, 1098 (1988). 2. I. J. Fritz, S. T. Picraux, L. R. Dawson, T. J. Drummond, W. D. Ladig, and N. G. Anderson, Appl. Phys. LJW. 46, 967 (1985). 3. J. J. Rosenberg, M. Benlamri, P. D. Kirchner, J. M. Woodall and G. D. Pettit, IEEE Electron Device L-err. EDM, 491 (1985). 4. K. Ploog, J. Crysr. Growth 81, 304 (1987). 5. W. Lin, W. C. Hsu, T. S. Wu, S. 2. Chang, C. Wang and C. Y. Chang, Appl. Phys. Letr. 58, 2681 (1991). 6. W. C. Hsu and H. M. Shieh, Solid-St. Electron. 35,635 (1992). 7. W. T. Masselink, A. A. Ketterson, J. Klem, W. Kopp and H. Morkoc, Electron Let?. 21, 937 (1985). 8. T. J. Drummond, J. Klem, D. Arnold, R. Fischer, R. E. Throne, W. G. Lyons and H. Morkoc, Appl. Phys. Letr. 42, 615 (1983).
$6.00 + 0.00 0038-l lOI/ Copyright 0 1993 Pergamon Press Ltd
A NEW 6 -DOPED InGaAs/GaAs PSEUDOMORPHIC HIGH ELECTRON MOBILITY TRANSISTOR UTILIZING A STRAINED SUPERLATTICE SPACER H. M. Department
SHIEH,
of Electrical
W. C. Hsu, M. J. KAO, C. L. WV and
T. S. Wu
Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China
1, University
Road,
(Received 21 October 1992; in revised form 24 December 1992) Abstract-A new d-doped In,,,Ga,,,,As/GaAs pseudomorphic high electron mobility transistor (HEMT) utilizing an In, ,BGa, ,,As/GaAs strained superlattice spacer grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) has been prepared. For a gate length of 5pm, the present structure reveals superior saturation current density (180 mA/mm at 300 K and 230 mA/mm at 77 K) and extrinsic transconductance (103 mS/mm at 300 K and 142 mS/mm at 77 K). The current-voltage characteristics are superior to those of reported similar InGaAs/GaAs structure with gate length of 2 pm grown by molecular beam epitaxy (MBE). Also, because of the undoped cap layer grown on the top of the d-doped GaAs, a breakdown voltage of 1OV has been achieved.
1. INTRODUCTION
Due to lower electron effective mass, higher low-field mobility, higher doping capability and better highfield transport properties than those of GaAs, the InGaAs alloys have been widely used as channel materials[ 11. By carefully controlling the epi-layer thickness, lattice mismatch between InGaAs and GaAs can be accommodated by elastic strain rather than by generating misfit dislocation[2]. Consequently, high quality pseudomorphic structures can be grown for application to high speed and/or optoelectronic devices[3]. Furthermore the use of 6doped GaAs has received considerable interest as a means of obtaining high density and high mobility quasi-2DEG systems[4]. Thus, a a-doped InGaAs/ GaAs pseudomorphic HEMT can possess high mobility and superior current--voltage characteristics[5]. However, the interface traps at heterojunctions may significantly influence the device characteristics. In this work, to improve the properties of the 6doped InGaAs/GaAs pseudomorphic HEMT, a strained In,,,,Ga,,,As/GaAs superlattice spacer was employed for replacing the conventional undoped GaAs spacer for the first time. The present structure manifests superior current-voltage characteristics as compared with those of similar InGaAs/GaAs pseudomorphic HEMTs without superlattice spacer[3,6].
2. DEVICE STRUCTURE
AND
FABRICATION
The device material was grown by a low-pressure metal organic chemical vapor deposition (LPMOCVD). The new structure in this work as shown in Fig. 1, consists of a 0.5 pm undoped GaAs buffer
layer grown on a semi-insulating GaAs (100) substrate, a 90 A undoped In,,,,Ga,,,,As conducting channel, an undoped 15 8, GaAs/20 8, In,,,, Gd,,,, As/ 3OA GaAs/20 A In,,, Ga, ,,As/45 8, GaAs graded supperlattice spacer, a n + 6 -doped GaAs layer, and finally a 400 A undoped GaAs cap layer. The low pressure growth (80 torr) was carried out in a horizontal quartz reactor. Trimethylindium (TMI), triethylgallium (TEG), arsine (ASH,), and silane (SiH,) were used as the In, Ga, As sources and n-type dopant, respectively. The growth temperature was 650°C. The growth rates of GaAs and In,,, Ga,,,, As were controlled at 158 and 230 Ajmin respectively. To achieve a sharp S-doping profile, the Ga source was shut-off while keeping Si and As sources open during the growth of d-doped GaAs layer. The 3-D concentration of the S-doping GaAs layer was set at 1 x 10’9cm-‘. Standard photolithography and lift-off techniques were employed for device fabrication[7]. Alloyed AuGe was used for source and drain contacts, onto which Ag was evaporated to reduce the bonding resistance. Au was used as the Schottky contact for the gate. The gate area was 5 x 250pm*, while the distance between the drain and source electrodes was 30 /*m. 3. RESULTS
AND
DISCUSSION
Figure 2 shows the three-terminal current-voltage characteristics at room temperature. The maximum extrinsic trdnsconductances at 300 and 77 K are 103 and 142 mS/mm respectively which, are comparable with those of In,,,Ga,,,As/GaAs single quantum well HEMT grown by MBE with conventional GaAs 1117
1118
H. M.
i-GaAs Cap layer 400 ,&
SHIEH et al.
2
Fig. 1. The schematic diagram of the present new B-doped InGaAs/GaAs pseudomorphic HEMT structure.
0.0
-0.5
-1.0
-2.0
-1,s
Gate voltage
spacer for gate length of 2 pm[3]. Figure 3 showa the extrinsic transconductance vs gate voltage. Figure 4 illustrates the saturation current densities vs gate voltage at room temperature. High saturation current density up to 180 mA/mm at room temperature (230 mA/mm at 77 K) is reached. The saturation current densities are significantly superior to those of Ref. [3]. Meanwhile, when compared with those of similar pseudomorphic HEMT structure and similar gate geometry[6], the present structure reveals much better extrinsic transconductances and saturation current densities. Table 1 lists the comparison between this work and Refs [3] and [6] for similar InGaAs/GaAs pseudomorphic HEMT structures. In addition to the a-doped GaAs can supply more electrons, the strained superlattice spacer may also play an important role on the enhancement of I-V characteristics for the present structure. Unlike the homogeneously doped HEMT, the cap layer of the present new HEMT is undoped and hence increases the breakdown voltage (- 10 V in this work). The high saturation current density, high breakdown voltage, and high transconductance imply
-2.5
(V)
Fig. 3. The extrinsic transconductances 300 K.
vs gate voltage at
that the present structure possesses the potential for high performance applications. The properties-enhancement effect from using a strained InGaAs/GaAs superlattice spacer in this work may be attributed partly to an improvement in the crystalline quality at the heterojunctions and a possible reduction in the elastic stress at the junction that results from the different lattice constants of InGaAs and GaAs[8]. Also, the thin In,.,,G%.,,As layer (2OA) in the strained superlattice spacer of the present structure may provide a means of reducing the parallel
3OOK vd,=x.w
45.54
7 E r/l n
4.554 idiv
VDS
1 .OOO/div
(V)
Fig. 2. The three-terminal current-voltage characteristic at 300 K. The top curve is V,. = 0 V, and V,, = -0.5 V/step.
0.0
-0.5
-1.0
Gate
-1.5
voltage
-2.0
-2.5
-3.0
(V)
Fig. 4. The saturation current densities vs gate voltage.
HEMT
with strained superlattice spacer
Table 1. Comparison between this work and similar pseudomorphic structures This work Gate length (rm) 300K Maximum extrinsic 77 K transconductance (mS/mm) Maximum saturation current density (mA/mm)
300K 77 K
Ref. 3
Ref. 6
5
2
5
103
90
56
142
140
18
180
100
II6
230
140
140
1119
ance as well as high breakdown voltage (- 1OV) make th HEMT structure with strained superlattice spacer possess a potential for high speed and,/or high power applications. Acknowledgements-The authors appreciate the helpful discussions with Professor S. S. Li, University of Florida, and Dr P. Ho, General Electric Co. This work was sponsored by the National Science Council, Republic of China, under contract no. NSC 81-0404-E006-104. REFERENCES
conduction. Further work on the enhancement A,echanism of superlattice spacer is in progress. 4. CONCLUSIONS
A new 6 -doped InGaAs/GaAs pseudomorphic high electron mobility transistor utilizing a strained InGaAs/GaAs superlattice spacer grown by LPMOCVD has been studied for the first time. For gate length of 5 pm, the present new structure reveals superior extrinsic transconductances of 103 (142 mS/mm) and high saturation current densities 180 (230mA/mm) at 300 (77 K) which are significantly higher than those of similar pseudomorphic structures for gate length of 2 pm. The high perform-
1. I. J. Fritz, P. L. Gourley, L. R. Dawson and J. E. Schirber, Appl. Phys. Letr. 53, 1098 (1988). 2. I. J. Fritz, S. T. Picraux, L. R. Dawson, T. J. Drummond, W. D. Ladig, and N. G. Anderson, Appl. Phys. LJW. 46, 967 (1985). 3. J. J. Rosenberg, M. Benlamri, P. D. Kirchner, J. M. Woodall and G. D. Pettit, IEEE Electron Device L-err. EDM, 491 (1985). 4. K. Ploog, J. Crysr. Growth 81, 304 (1987). 5. W. Lin, W. C. Hsu, T. S. Wu, S. 2. Chang, C. Wang and C. Y. Chang, Appl. Phys. Letr. 58, 2681 (1991). 6. W. C. Hsu and H. M. Shieh, Solid-St. Electron. 35,635 (1992). 7. W. T. Masselink, A. A. Ketterson, J. Klem, W. Kopp and H. Morkoc, Electron Let?. 21, 937 (1985). 8. T. J. Drummond, J. Klem, D. Arnold, R. Fischer, R. E. Throne, W. G. Lyons and H. Morkoc, Appl. Phys. Letr. 42, 615 (1983).