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Improved mobilities and concentrations in double quantum well InGaAsGaAs pseudomorphic HFETs using multi-coupled δ-doped GaAs
Solid-Sfare Ekcrronics Vol. 38, No. 6. pp. ll71-1173, 1995 Copyright 0 1995 ElsevierScienceLtd
Pergamon
003%1101(!400226-6
Printed in Great Britain. All rights reserved 0038-I101/95 $9.50+ 0.00
IMPROVED MOBILITIES AND CONCENTRATIONS IN DOUBLE QUANTUM WELL InGaAs/GaAs PSEUDOMORPHIC HFETs USING MULTICOUPLED 6 -DOPED GaAs M. J. KAO, W. C. HSUt, H. M. SHIEH and T. Y. LIN of Electrical Engineering, National Cheng Kung University, I University Road, Tainan, Taiwan 70101, R.O. China
Department
(Received
I I August 1994; in revised form 28 September
1994)
Ahstraet-In this work, we report the improvement of mobilities and concentrations using multi-coupled Si-S-doped GaAs layers in double quantum well (DQW) GaAs/InGaAs/GaAs pseudomorphic heterostructures. The improved mobilities and sheet carrier densities are 5200 (21,000) cm*/% and 7.6 (5.0) x 10” cm-* at 300 (77) K, respectively, which are significantly superior to those of single quantum well structures. The transconductance and saturation current density are as high as 24OmS/mm and 750 mA/mm, respectively, at 300 K with a gate length of 1.5 pm.
1.
2. EXPERIMENTAL
INTRODUCTION
Over the past several years, the heterojunction devices for high speed digital and microwave applications have been received with much interest. Most of the reported heterojunctions field effect transistors (HFET) utilize Al,Ga, _ .rAs/GaAs heterojunctions in which the electrons are transferred from the heavily doped Al,Ga,_,As to the narrow bandgap GaAs. Although this material system has shown some excellent properties[l,2], the difficulties for growing high quality Al,Ga,_,As epilayers and the DX centers generated in high Al mole fraction (20.2) would degrade the device performance. Besides, the limitation in two-dimensional electron gas (ZDEG) concentration to less than 10’2cm-2 of the HFET structures leads to fairly high sheet resistances which may further limit the device performance[3]. In the previous work, we have studied single quantum well (SQW) GaAs/InGaAs/GaAs pseudomorphic heterostructure field effect transistors (PHFETs)[4] with one B-doped GaAs layer. In this work, for further improving the mobility and concentration, we investigate the effects of multi-coupled 6 -doped GaAs in double quantum well (DQW) GaAs/ InGaAs/GaAs PHFETs. The enhancement in mobilities and concentrations are due in part to the fact that more carriers distribute at the center of the two a-doped GaAs wells. Such a carrier transport is spatially separated from ionized impurity scattering and consequently enhances the electron mobilities. Meanwhile, the effect of the coupled 2DEGs was studied. tTo whom all correspondence
should be addressed. I171
The epilayers of the DQW GaAs/InGaAs/GaAs PHFETs used for this study were grown on (lOO)oriented semi-insulting GaAs substrates by a computer-controlled low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The chamber pressure was kept at 80 torr. Trimethylindium (TMI), triethylgallium (TEG), arsine (ASH,) and silane (SiH,) were used as the In, Ga, As sources and n-type dopant, respectively. The substrate temperature was set at 650°C. The V/III ratio was fixed at 75 and 80 for GaAs and InGaAs epilayers, respectively. During the growth of the d-doped GaAs layer, Ga source was switched off while keeping silane and arsine sources flowing in the chamber to achieve a very sharp doping profile. The sample structure of the proposed DQW 6doped GaAs/InGaAs/GaAs PHFETs, denoted by sample A, is shown in Fig. 1. It consists of a 0.5 pm undoped GaAs buffer, two 90 A In,,,,Ga,,,As channels, three d-doped GaAs layers, three 100 A GaAs spacers, and finally a 400 %,undoped GaAs cap layer. A p --doped GaAs layer was also grown on the buffer to improve the device performance[6]. The 2DEG concentrations and mobilities were characterized by Hall measurements under 5000 G at 300 and 77 K. Standard photolithography and lift-off techniques were employed for device fabrication. Alloyed Au/Ge/Ni metal was used for source and drain contacts, onto which Ag was evaporated to reduce the contact resistance. Au was evaporated as the Schottky contact metal. The gate dimensions were 1.5 x 100pm2.
1172
M. J. Kao et al. 75.00 Sample A 300 K
b-dopi=
&doping
0
6.000 Vos (O.bVktiv)
Fig. 1. Schematic structure of the multi-coupled ddoped DQW
GaAs/InGaAs/GaAs
Fig. 2. Room temperature I-V characteristic of sample A. The top curve is 1.5 V, VGs = -0.5 V/step.
HFET.
3. RESULTS AND DISCUSSION
Hall measurements performed on the samples are shown in Table 1. Sample B is a SQW GaAs/InGaAs structure by b-doping GaAs layer on both sides of the channel. Sample C is a SQW GaAs/InGaAs structure by h-doping GaAs layer on upper side of the channel. The sheet carrier densities of the multiple-coupled a-doped structure, i.e. sample A and sample C, are significantly higher than those of single a-doped structure (sample C). In addition, the DQW PHFET reveals significantly superior mobilities and sheet carrier concentrations to those of conventional SQW PHFETs, i.e. samples B and C. Therefore, the double quantum well and the multiple a-doped GaAs layers play an important role on achieving high sheet carrier density (n) and high mobility (p.). For samples A and B, the coupled B-doped GaAs layers make the electron wave functions interact between the delta layers, and then results in more carriers distributing towards the center of the two a-doped GaAs; i.e. the center of In,,,,Ga,,75As channel. A coupled 2DEG will be formed at the center of the InGaAs channel by the two 2DEGs generating from the two h-doped GaAs layers in both sides of the channel. Consequently, owing to the reduction of the interface scattering, the electron mobility in the InGaAs channel may further be improved. This may be the reason why samples A and B manifest much higher sheet carrier densities than sample C without significantly degrading the mobility. On the other hand, the double quantum wells in
sample A can provide more carrier collection in the channels, leading to an extremely high sheet carrier density without sacrificing the electron mobility. The mobility and sheet carrier density for sample A at 300 (77) K are 5200 (21,000) cm*/vs and 7.6 (5.0) x 1Ou cme2, respectively. Because the electron mobility measured by Hall measurement is an average value of the GaAs barriers and InGaAs channels, more carriers collected in the InGaAs channel will contribute a higher mobility than conventional FETs. Current-voltage characteristics of sample A at 300 K is shown in Fig. 2. Good saturation characteristics were obtained. Figure 3 shows the saturation current density and extrinsic transconductance vs gate voltage at room temperature for sample A. An z 250 E ;j 5 200 8 3 ;; 250 z : 3 200 3 b .I! so z ‘Z
r=l o-
-2.5
-1.5
-0.5
0.5
1.5
Gate voltage (V) Fig. 3. Extrinsic transconductance and saturation current density under the V, of 4.5 V for sample A.
Table I. Sheet carrier densities and mobilitics of the single and multi-coupled 6dope.d HFETs “P” (x lo’61/vs)
Sample A Sample B (Ref. [S]) gZJmt,k C (Ref. 151)
2.5
300K
77 K
300K
77 K
3OOK
77 K
7.6 6.2 2.0
5.0 4.1 1.8
5200 4600 5600
21,000 19,ooo 22.Ooa
3.95 2.85 1.12
10.5 7.79 3.96
Improved mobilities in pseudomorphic
extremely high saturation current density as high as 750 mA/mm was obtained. This high current driving capability is attributed to the very high sheet carrier density and mobility. As compared to the SQW structures, the DQW PHFET in this work also reveals higher device performance.
4.
CONCLUSIONS
Superior characteristics in saturation current density and transconductance of a double-quantumwell GaAs/In,,,, Gq,,5 As/GaAs pseudomorphic HFET has been demonstrated. The improved device performance is attributed to the increased 2DEG sheet density along with enhanced mobility by using the multi-coupled 6 -doped GaAs.
HFETs
1173
Acknowledgements-This
work was supported by the National Science Council, Republic of China, under contract no. NSC 83-0404-EOO6-051. REFERENCES
I. J. J. Berenz, Microwave and Millimeter-wave Monolithic Circuits Symp., p. 93 (1984).
2. A. A. Ketlerson, W. T. Masselink, J. S. Gedymin, J. Klem, C. K. Peng, W. F. Kopp, H. Morkoc and K. R. Gleason, IEEE Trans. Electron Devices 33, 564 (1986).
3. H. Hirakawa, H. Sakaki and Y. Yoshimo, Appl. Phys. L&r. 45, 253 (1984).
4. W. C. Hsu, H. M. Shieh, M. J. Kao, R. T. Hsu and Y. H. Wu, IEEE Trans. Electron Devices 40, 1630 (1993).
5. H. M. Shieh, W. C. Hsu, C. L. Wu and T. S. Wu, Jap. J. appl. Phys. 32, L303 (1993).
6. M. J. Kao, W. C. Hsu and H. M. Shieh, Jap. J. appl. Phys. 32, L1503 (1993).
Pergamon
003%1101(!400226-6
Printed in Great Britain. All rights reserved 0038-I101/95 $9.50+ 0.00
IMPROVED MOBILITIES AND CONCENTRATIONS IN DOUBLE QUANTUM WELL InGaAs/GaAs PSEUDOMORPHIC HFETs USING MULTICOUPLED 6 -DOPED GaAs M. J. KAO, W. C. HSUt, H. M. SHIEH and T. Y. LIN of Electrical Engineering, National Cheng Kung University, I University Road, Tainan, Taiwan 70101, R.O. China
Department
(Received
I I August 1994; in revised form 28 September
1994)
Ahstraet-In this work, we report the improvement of mobilities and concentrations using multi-coupled Si-S-doped GaAs layers in double quantum well (DQW) GaAs/InGaAs/GaAs pseudomorphic heterostructures. The improved mobilities and sheet carrier densities are 5200 (21,000) cm*/% and 7.6 (5.0) x 10” cm-* at 300 (77) K, respectively, which are significantly superior to those of single quantum well structures. The transconductance and saturation current density are as high as 24OmS/mm and 750 mA/mm, respectively, at 300 K with a gate length of 1.5 pm.
1.
2. EXPERIMENTAL
INTRODUCTION
Over the past several years, the heterojunction devices for high speed digital and microwave applications have been received with much interest. Most of the reported heterojunctions field effect transistors (HFET) utilize Al,Ga, _ .rAs/GaAs heterojunctions in which the electrons are transferred from the heavily doped Al,Ga,_,As to the narrow bandgap GaAs. Although this material system has shown some excellent properties[l,2], the difficulties for growing high quality Al,Ga,_,As epilayers and the DX centers generated in high Al mole fraction (20.2) would degrade the device performance. Besides, the limitation in two-dimensional electron gas (ZDEG) concentration to less than 10’2cm-2 of the HFET structures leads to fairly high sheet resistances which may further limit the device performance[3]. In the previous work, we have studied single quantum well (SQW) GaAs/InGaAs/GaAs pseudomorphic heterostructure field effect transistors (PHFETs)[4] with one B-doped GaAs layer. In this work, for further improving the mobility and concentration, we investigate the effects of multi-coupled 6 -doped GaAs in double quantum well (DQW) GaAs/ InGaAs/GaAs PHFETs. The enhancement in mobilities and concentrations are due in part to the fact that more carriers distribute at the center of the two a-doped GaAs wells. Such a carrier transport is spatially separated from ionized impurity scattering and consequently enhances the electron mobilities. Meanwhile, the effect of the coupled 2DEGs was studied. tTo whom all correspondence
should be addressed. I171
The epilayers of the DQW GaAs/InGaAs/GaAs PHFETs used for this study were grown on (lOO)oriented semi-insulting GaAs substrates by a computer-controlled low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The chamber pressure was kept at 80 torr. Trimethylindium (TMI), triethylgallium (TEG), arsine (ASH,) and silane (SiH,) were used as the In, Ga, As sources and n-type dopant, respectively. The substrate temperature was set at 650°C. The V/III ratio was fixed at 75 and 80 for GaAs and InGaAs epilayers, respectively. During the growth of the d-doped GaAs layer, Ga source was switched off while keeping silane and arsine sources flowing in the chamber to achieve a very sharp doping profile. The sample structure of the proposed DQW 6doped GaAs/InGaAs/GaAs PHFETs, denoted by sample A, is shown in Fig. 1. It consists of a 0.5 pm undoped GaAs buffer, two 90 A In,,,,Ga,,,As channels, three d-doped GaAs layers, three 100 A GaAs spacers, and finally a 400 %,undoped GaAs cap layer. A p --doped GaAs layer was also grown on the buffer to improve the device performance[6]. The 2DEG concentrations and mobilities were characterized by Hall measurements under 5000 G at 300 and 77 K. Standard photolithography and lift-off techniques were employed for device fabrication. Alloyed Au/Ge/Ni metal was used for source and drain contacts, onto which Ag was evaporated to reduce the contact resistance. Au was evaporated as the Schottky contact metal. The gate dimensions were 1.5 x 100pm2.
1172
M. J. Kao et al. 75.00 Sample A 300 K
b-dopi=
&doping
0
6.000 Vos (O.bVktiv)
Fig. 1. Schematic structure of the multi-coupled ddoped DQW
GaAs/InGaAs/GaAs
Fig. 2. Room temperature I-V characteristic of sample A. The top curve is 1.5 V, VGs = -0.5 V/step.
HFET.
3. RESULTS AND DISCUSSION
Hall measurements performed on the samples are shown in Table 1. Sample B is a SQW GaAs/InGaAs structure by b-doping GaAs layer on both sides of the channel. Sample C is a SQW GaAs/InGaAs structure by h-doping GaAs layer on upper side of the channel. The sheet carrier densities of the multiple-coupled a-doped structure, i.e. sample A and sample C, are significantly higher than those of single a-doped structure (sample C). In addition, the DQW PHFET reveals significantly superior mobilities and sheet carrier concentrations to those of conventional SQW PHFETs, i.e. samples B and C. Therefore, the double quantum well and the multiple a-doped GaAs layers play an important role on achieving high sheet carrier density (n) and high mobility (p.). For samples A and B, the coupled B-doped GaAs layers make the electron wave functions interact between the delta layers, and then results in more carriers distributing towards the center of the two a-doped GaAs; i.e. the center of In,,,,Ga,,75As channel. A coupled 2DEG will be formed at the center of the InGaAs channel by the two 2DEGs generating from the two h-doped GaAs layers in both sides of the channel. Consequently, owing to the reduction of the interface scattering, the electron mobility in the InGaAs channel may further be improved. This may be the reason why samples A and B manifest much higher sheet carrier densities than sample C without significantly degrading the mobility. On the other hand, the double quantum wells in
sample A can provide more carrier collection in the channels, leading to an extremely high sheet carrier density without sacrificing the electron mobility. The mobility and sheet carrier density for sample A at 300 (77) K are 5200 (21,000) cm*/vs and 7.6 (5.0) x 1Ou cme2, respectively. Because the electron mobility measured by Hall measurement is an average value of the GaAs barriers and InGaAs channels, more carriers collected in the InGaAs channel will contribute a higher mobility than conventional FETs. Current-voltage characteristics of sample A at 300 K is shown in Fig. 2. Good saturation characteristics were obtained. Figure 3 shows the saturation current density and extrinsic transconductance vs gate voltage at room temperature for sample A. An z 250 E ;j 5 200 8 3 ;; 250 z : 3 200 3 b .I! so z ‘Z
r=l o-
-2.5
-1.5
-0.5
0.5
1.5
Gate voltage (V) Fig. 3. Extrinsic transconductance and saturation current density under the V, of 4.5 V for sample A.
Table I. Sheet carrier densities and mobilitics of the single and multi-coupled 6dope.d HFETs “P” (x lo’61/vs)
Sample A Sample B (Ref. [S]) gZJmt,k C (Ref. 151)
2.5
300K
77 K
300K
77 K
3OOK
77 K
7.6 6.2 2.0
5.0 4.1 1.8
5200 4600 5600
21,000 19,ooo 22.Ooa
3.95 2.85 1.12
10.5 7.79 3.96
Improved mobilities in pseudomorphic
extremely high saturation current density as high as 750 mA/mm was obtained. This high current driving capability is attributed to the very high sheet carrier density and mobility. As compared to the SQW structures, the DQW PHFET in this work also reveals higher device performance.
4.
CONCLUSIONS
Superior characteristics in saturation current density and transconductance of a double-quantumwell GaAs/In,,,, Gq,,5 As/GaAs pseudomorphic HFET has been demonstrated. The improved device performance is attributed to the increased 2DEG sheet density along with enhanced mobility by using the multi-coupled 6 -doped GaAs.
HFETs
1173
Acknowledgements-This
work was supported by the National Science Council, Republic of China, under contract no. NSC 83-0404-EOO6-051. REFERENCES
I. J. J. Berenz, Microwave and Millimeter-wave Monolithic Circuits Symp., p. 93 (1984).
2. A. A. Ketlerson, W. T. Masselink, J. S. Gedymin, J. Klem, C. K. Peng, W. F. Kopp, H. Morkoc and K. R. Gleason, IEEE Trans. Electron Devices 33, 564 (1986).
3. H. Hirakawa, H. Sakaki and Y. Yoshimo, Appl. Phys. L&r. 45, 253 (1984).
4. W. C. Hsu, H. M. Shieh, M. J. Kao, R. T. Hsu and Y. H. Wu, IEEE Trans. Electron Devices 40, 1630 (1993).
5. H. M. Shieh, W. C. Hsu, C. L. Wu and T. S. Wu, Jap. J. appl. Phys. 32, L303 (1993).
6. M. J. Kao, W. C. Hsu and H. M. Shieh, Jap. J. appl. Phys. 32, L1503 (1993).