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Pseudomorphic two-dimensional electron-gas-emitter resonant tunneling devices
MicroelectronicEngineering15 (1991) 663-666 Elsevier
P s e u d o m o r p h i c Two-Dimensional R e s o n a n t Tunneling Devices
663
Electron-Gas-Emitter
H. Brugger 1, U. Meiners 1, C. W~lk 1, R. Deufell, A. Marten1, M. Rossmanith 2, K.v. Klitzing~, and R. Sauer 3 1 Daimler Benz AG, Forschungszentrum, P.O. Box 2360, D-7900 Ulm, Germany 2 Max-Planck-Institut f'tir FestkSrperforschung, D-7000 Stuttgart, Germany 3 U n i v e r s i ~ t Ulm, Abteilung Halbleiterphysik, D-7900 Ulm, Germany Abstract A resonant tunneling diode with the highest room temperature peak-to-valley current ratio (PVR) in the GaAs/A1GaAs system is described. Using a pseudomorphic InGaAs quantum well for carrier injection, room temperature PVR = 7.2 and liquid nitrogen temperature PVR = 27 were obtained with a reasonable peak current density of 104A/cm2. Experiments under hydrostatic pressure give evidence that X-like states in the barriers are strongly involved in the tunneling process. 1. I N T R O D U C T I O N Resonant tunneling structures with improved peak current density Jl~ and peak-to-valley current ratio (PVR) have potential for microwave aevice applications such as mixers, detectors, oscillators and frequency multipliers [1]. Resonant tunneling diodes (RTD) with an operating frequency of 420 GHz have been demonstrated [2]. The device performance (e.g. maximum frequency, power) is limited by the intrinsic structure (PVR, jp) and parasitic elements (contact resistance, series inductance). In the GaAs/A1GaAs system PVR = 5.9 with an InGaAs prewell layer [3] and PVR = 6.3 with an A1GaAs chair barrier [4] were achieved recently. We present the work on Alo.7oGao.30As/GaAs double barrier RTD where we have achieved a significant improvement of PVR at 300 K and 77 K by incorporation of an emitter-side InGaAs quantum well. 2. LAYERED STRUCTURE AND MATERIAL CHARACTERIZATION A schematic cross section of the RTD with a pseudomorphic InGaAs quantum well on an n+-GaAs substrate is shown in Figure 1. The structure was grown by molecular beam epitaxy at a substrate temperature of T~ = 630°C and a growth rate of r = 1.15 ~m/h without growth interruptions. The double barrier structure with the pseudomorphic InGaAs layer is intentionally undoped (i.u.) and sand0167-9317/91/$3.50 © 1991 - ElsevierSciencePublishersB.V. All fights reserved.
664
H. Brugger et aL / Resonant tunneling devices
) 40
n +÷
GaAs
n +
Ga~
5O
n-graded
GaAs
26
L U.
GaAs
LU.
In Ga As 0.15 8.B5
500
4.0 3.4
z.u.
A1 Ga As 0.70
4.0
1. U•
3.4
1.
19
C~
Ca/Is
5O
n-graded
Ga~
1000
n+
Ca/Is
(~m)
n+
GaAs substrate
,~,°
°'°" ,,"
Z
•
• o" ,,
o') 0
Ca/ks
z.u.
_~,.~,
o v
0.30
30
•~
E
18
substrate temperature
0'.s Depth (pm)
f
Figure 1. MBE layer sequence, substrate temperature profile during growth and measured doping profile of a RTD structure with emitter-side InGaAs QW.
wiched between GaAs spacer layers and graded doping regions. During the growth of these layers r and T~ were reduced to achieve reliable layer thickness control and to avoid In-desorp~ion. A 0.5 ~tm thick doped GaAs emitter contact layer and a thin highly doped capping layer was used to facilitate the formation of a low-resistance top-side ohmic contact. The doping profile was measured with a Polaron etch profiler and is also shown in Figure 1. To investigate the influence of the emitter-side QW we have also grown reference samples without an InGaAs QW but with an otherwise identical layer sequence. The high quality of the layered material is also reflected in the photoluminescence (PL) behaviour which is shown in Figure 2. Strong lowtemperature PL-signals are observed from different RTD-layers. A comparison of the measured transition energies with calculated values based on a quantum box model and on a transfer matrix method yields information about the In-content in the prewell (PW) layer and the energy of the first resonant level in the GaAs QW, respectively. 3. DEVICE F A B R I C A T I O N A N D T R A N S P O R T P R O P E R T I E S
Device structures consist of a front side (emitter) contact pattern and a large area backside (collector) contact. They are fabricated with optical photolithography and are isolated by wet chemical mesa etching or ion implantation techniques. Various emitter geometries with contact areas ranging from 4 pan2 up to 4000 ~m 2 were realized. For the ohmic contact metallization we have used
665
H. Brugger et a L / Resonant tunneling devices
InGaAs-PW
Ca) ¢-
.d lb--
tl:l >., o~
c'¢...
~,
GaAs'~s,'Q2~~
x 20
xlf
.-I
13.
I
|
1.35
I
,
1.55
1.45
I
1.65
1.75
1.85
Energy (eV) Figure 2. Low temperature PL-spectrum and band diagram of RTD structure. a standard AuGe alloy. Electrical connection is made either by whiskers or by bond wires. DC I N curves at different sample temperatures are shown in Figure 3 for forward bias which corresponds to electron injection from the emitter-side InGaAs QW into the double barrier structure. A well pronounced negative differential resistance region with PVR = 7.2 (300 K) and PVR = 27 (77 K) and peak current densities of jv = 10 kA/cm2 (300 K) and jp = 12 kA/cm2 (77 K) is observed. For backward bias we found PVR = 3.6 (17) and jv = 3 kA/cm2 (4 kA/cm2) at 300 K (77 K), which is similar to measured values on reference s~mples without InGaAs QW. The presented results demonstrate how to improve
50 3OO K ......
40 <
77K
/!
fiE30 t(1) "-" I b - - 20
O
10 0
0
/.,,, x.i /: , , 1 2 d /3 4 Voltage (V)
Figure 3. DC I/V curves of RTD with pseudomorphic InGaAs QW.
666
H. Brugger et a L / Resonant tunneling devices
I
'
6 A
'
I
'
'
"
I
'
'
T=300K
I
"
"
"
-
6 kbar
l+
2
0
.
0
i
0,5
.
i
•
.
,
,
1 1.5 Volfage (V)
.
|
.
2
Figure 4. DC current/voltage curves of a RTD reference sample without InGaAs quantum well under applied hydrostatic pressure. the DC performance of RTD by inserting an appropriate QW layer. We obtained an improvement of PVR by a factor of 2 and an increase ofjp by a factor of 6. On reference samples without InGaAs QW we performed experiments under hydrostatic gas pressure. Room temperature IN-curves are shown in Figure 4. The pressure is increased from 0 to 6 kbar in steps of 0.2 kbar. Due to the similar pressure coefficient of the F-minima in GaAs and A1GaAs [5] the F - F conduction band profile remains nearly unchanged under pressure, whereas the F - X barrier height decreases linearly with -12.5 meV/kbar [6]. This mainly influences the valley current which increases,exponentially with pressure. The results give evidence that X-like states in the barriers are strongly involved in the tunneling process and it is expected that their energetic seperation from the emitter level sensitively influences the transport performance of GaAs-based RTD devices. This work was partially supported by the Bundesministerium fiir Forschung und Technologie (Bonn) under contract numbers NT 2754 2 and NT 2718 A3. 4. R E F E R E N C E S
1 2 3 4 5 6
For an overview, "Physics of Quantum Electron Devices", Springer Series in Electronics and Photenics, ed. F. Capasso, vol. 28, Springer, Berlin,.1990 E.R. Brown, T.C.L.G. Sollner, C.D.Park, W.D. Goodhue, and L.C. Chen, Appl. Phys. Lett. 55, 1777 (1989) H. Riechert, D. Bernklau, J.P. Reithmaier, and R.D. Schnell, Electronics Lett. 26, 340 (1990), and references therein V.K. Reddy, A.J. Tsao, and D.P. Neikirk, Electronics Lett. 26, 1742 (1990) M. Rossmanith, K. Syassen, E. BSckenhoff, K. Ploog, and K.v. Klitzing, Phys. Rev. B (1991), in press, and references therein M. Holtz, R. Cingolani, K. Reimann, R. Muralidharan, I~ Syassen, and K. Ploog, Phys. Rev. B41, 3641 (1990)
P s e u d o m o r p h i c Two-Dimensional R e s o n a n t Tunneling Devices
663
Electron-Gas-Emitter
H. Brugger 1, U. Meiners 1, C. W~lk 1, R. Deufell, A. Marten1, M. Rossmanith 2, K.v. Klitzing~, and R. Sauer 3 1 Daimler Benz AG, Forschungszentrum, P.O. Box 2360, D-7900 Ulm, Germany 2 Max-Planck-Institut f'tir FestkSrperforschung, D-7000 Stuttgart, Germany 3 U n i v e r s i ~ t Ulm, Abteilung Halbleiterphysik, D-7900 Ulm, Germany Abstract A resonant tunneling diode with the highest room temperature peak-to-valley current ratio (PVR) in the GaAs/A1GaAs system is described. Using a pseudomorphic InGaAs quantum well for carrier injection, room temperature PVR = 7.2 and liquid nitrogen temperature PVR = 27 were obtained with a reasonable peak current density of 104A/cm2. Experiments under hydrostatic pressure give evidence that X-like states in the barriers are strongly involved in the tunneling process. 1. I N T R O D U C T I O N Resonant tunneling structures with improved peak current density Jl~ and peak-to-valley current ratio (PVR) have potential for microwave aevice applications such as mixers, detectors, oscillators and frequency multipliers [1]. Resonant tunneling diodes (RTD) with an operating frequency of 420 GHz have been demonstrated [2]. The device performance (e.g. maximum frequency, power) is limited by the intrinsic structure (PVR, jp) and parasitic elements (contact resistance, series inductance). In the GaAs/A1GaAs system PVR = 5.9 with an InGaAs prewell layer [3] and PVR = 6.3 with an A1GaAs chair barrier [4] were achieved recently. We present the work on Alo.7oGao.30As/GaAs double barrier RTD where we have achieved a significant improvement of PVR at 300 K and 77 K by incorporation of an emitter-side InGaAs quantum well. 2. LAYERED STRUCTURE AND MATERIAL CHARACTERIZATION A schematic cross section of the RTD with a pseudomorphic InGaAs quantum well on an n+-GaAs substrate is shown in Figure 1. The structure was grown by molecular beam epitaxy at a substrate temperature of T~ = 630°C and a growth rate of r = 1.15 ~m/h without growth interruptions. The double barrier structure with the pseudomorphic InGaAs layer is intentionally undoped (i.u.) and sand0167-9317/91/$3.50 © 1991 - ElsevierSciencePublishersB.V. All fights reserved.
664
H. Brugger et aL / Resonant tunneling devices
) 40
n +÷
GaAs
n +
Ga~
5O
n-graded
GaAs
26
L U.
GaAs
LU.
In Ga As 0.15 8.B5
500
4.0 3.4
z.u.
A1 Ga As 0.70
4.0
1. U•
3.4
1.
19
C~
Ca/Is
5O
n-graded
Ga~
1000
n+
Ca/Is
(~m)
n+
GaAs substrate
,~,°
°'°" ,,"
Z
•
• o" ,,
o') 0
Ca/ks
z.u.
_~,.~,
o v
0.30
30
•~
E
18
substrate temperature
0'.s Depth (pm)
f
Figure 1. MBE layer sequence, substrate temperature profile during growth and measured doping profile of a RTD structure with emitter-side InGaAs QW.
wiched between GaAs spacer layers and graded doping regions. During the growth of these layers r and T~ were reduced to achieve reliable layer thickness control and to avoid In-desorp~ion. A 0.5 ~tm thick doped GaAs emitter contact layer and a thin highly doped capping layer was used to facilitate the formation of a low-resistance top-side ohmic contact. The doping profile was measured with a Polaron etch profiler and is also shown in Figure 1. To investigate the influence of the emitter-side QW we have also grown reference samples without an InGaAs QW but with an otherwise identical layer sequence. The high quality of the layered material is also reflected in the photoluminescence (PL) behaviour which is shown in Figure 2. Strong lowtemperature PL-signals are observed from different RTD-layers. A comparison of the measured transition energies with calculated values based on a quantum box model and on a transfer matrix method yields information about the In-content in the prewell (PW) layer and the energy of the first resonant level in the GaAs QW, respectively. 3. DEVICE F A B R I C A T I O N A N D T R A N S P O R T P R O P E R T I E S
Device structures consist of a front side (emitter) contact pattern and a large area backside (collector) contact. They are fabricated with optical photolithography and are isolated by wet chemical mesa etching or ion implantation techniques. Various emitter geometries with contact areas ranging from 4 pan2 up to 4000 ~m 2 were realized. For the ohmic contact metallization we have used
665
H. Brugger et a L / Resonant tunneling devices
InGaAs-PW
Ca) ¢-
.d lb--
tl:l >., o~
c'¢...
~,
GaAs'~s,'Q2~~
x 20
xlf
.-I
13.
I
|
1.35
I
,
1.55
1.45
I
1.65
1.75
1.85
Energy (eV) Figure 2. Low temperature PL-spectrum and band diagram of RTD structure. a standard AuGe alloy. Electrical connection is made either by whiskers or by bond wires. DC I N curves at different sample temperatures are shown in Figure 3 for forward bias which corresponds to electron injection from the emitter-side InGaAs QW into the double barrier structure. A well pronounced negative differential resistance region with PVR = 7.2 (300 K) and PVR = 27 (77 K) and peak current densities of jv = 10 kA/cm2 (300 K) and jp = 12 kA/cm2 (77 K) is observed. For backward bias we found PVR = 3.6 (17) and jv = 3 kA/cm2 (4 kA/cm2) at 300 K (77 K), which is similar to measured values on reference s~mples without InGaAs QW. The presented results demonstrate how to improve
50 3OO K ......
40 <
77K
/!
fiE30 t(1) "-" I b - - 20
O
10 0
0
/.,,, x.i /: , , 1 2 d /3 4 Voltage (V)
Figure 3. DC I/V curves of RTD with pseudomorphic InGaAs QW.
666
H. Brugger et a L / Resonant tunneling devices
I
'
6 A
'
I
'
'
"
I
'
'
T=300K
I
"
"
"
-
6 kbar
l+
2
0
.
0
i
0,5
.
i
•
.
,
,
1 1.5 Volfage (V)
.
|
.
2
Figure 4. DC current/voltage curves of a RTD reference sample without InGaAs quantum well under applied hydrostatic pressure. the DC performance of RTD by inserting an appropriate QW layer. We obtained an improvement of PVR by a factor of 2 and an increase ofjp by a factor of 6. On reference samples without InGaAs QW we performed experiments under hydrostatic gas pressure. Room temperature IN-curves are shown in Figure 4. The pressure is increased from 0 to 6 kbar in steps of 0.2 kbar. Due to the similar pressure coefficient of the F-minima in GaAs and A1GaAs [5] the F - F conduction band profile remains nearly unchanged under pressure, whereas the F - X barrier height decreases linearly with -12.5 meV/kbar [6]. This mainly influences the valley current which increases,exponentially with pressure. The results give evidence that X-like states in the barriers are strongly involved in the tunneling process and it is expected that their energetic seperation from the emitter level sensitively influences the transport performance of GaAs-based RTD devices. This work was partially supported by the Bundesministerium fiir Forschung und Technologie (Bonn) under contract numbers NT 2754 2 and NT 2718 A3. 4. R E F E R E N C E S
1 2 3 4 5 6
For an overview, "Physics of Quantum Electron Devices", Springer Series in Electronics and Photenics, ed. F. Capasso, vol. 28, Springer, Berlin,.1990 E.R. Brown, T.C.L.G. Sollner, C.D.Park, W.D. Goodhue, and L.C. Chen, Appl. Phys. Lett. 55, 1777 (1989) H. Riechert, D. Bernklau, J.P. Reithmaier, and R.D. Schnell, Electronics Lett. 26, 340 (1990), and references therein V.K. Reddy, A.J. Tsao, and D.P. Neikirk, Electronics Lett. 26, 1742 (1990) M. Rossmanith, K. Syassen, E. BSckenhoff, K. Ploog, and K.v. Klitzing, Phys. Rev. B (1991), in press, and references therein M. Holtz, R. Cingolani, K. Reimann, R. Muralidharan, I~ Syassen, and K. Ploog, Phys. Rev. B41, 3641 (1990)