VIA-4 high-responsivity long-wavelength (λ = 10 µm) GaAs/AlxGa1-xAs multiquantum well superlattice...
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2382 IEEE TRLtNSACTIONS ON ELECTRON DEVICES, VOL. ED-34, NO. 11, NOVEMBER 1987 V I A 3 Resonant-Tunneling Hot-Electron Transistors (RHET’s) Using InGaAs/In ( AlGa) As Heterostructure--I< , Imamura, S. Muto, H. Ohnishi, T. Fujii, and N. Yokoyama, Fu-- jitsu Limited, 10-1, Morinosato-Wakamiya, Atsugi 243-01, Japan. This paper describes a resonant-tunneling hot-electron transistor (RHET) using an InGaAs/In( A1Ga)As heterostructure. A corn- mon-emitter current gain of 10 and a collector currept peak-to-val- ley ratio of 9.6 were measured at 77 K €or a 1000-A base RHET , The peak current depsity was 3.14 X lo4 A/cmZ. The measuretl current gain of 250- A base RHET’s reached 28, which is the high. est value yet reported for any hot-electron transistors. In 1985, we proposed and fabricated a three-terminal resonant. tunneling device, named the RHET, which used a GaAs/AlGaAs heterostructure [l]. We demonstrated the various attractivefea- tures of RHETs for memory and/or logic applications [2]. How- ever, the common-emitter current gain in the GaAs/AlGaAe RHET was not high enough to enable practical use. On the other hand, we reported [3] that useof InGaAs for the base layer of hot electron transistors (HET) was effective to increase a current gain because of larger F-L valley separation energy (0.55 eV) than that of GaAs (0.30 eV) . This paper demonstrates that current gains and collec- tor peak-to-valley ratios of RHET’s are greatly improved by using an InGaAs /In ( A1Ga) As heterostructure . The devices have a quantuy well, which acts as the hot-electron injector, consisting of 38.1-A Ino,53Gao,47As sandwiched by two 44- A Ipo.53Alo,47As. The thickness of the base layers were 250 and 1000 A. The carrier concentration of the base layer was 1 X 10” ~ m - ~ . The thiskness of the tno.52 ( Alo.5G~,5 )o.4sAscollector barrier was 2000 A with a 60-A graded layer. Each layer was grown using pulsed molecular-beam methods. A common-emitter current gain of 10.0 was measured at 77 K at a collector-emitter voltage of 2 V and a base current of 1 mA. The increased current gain of InGaAs/InAlGaAs RHET’s is believed to be due to the large I’-L valley separation energy of InGaAs compared to that of GaAs. The collector current, measured as a function of the base- emitter voltage, exhibited a pronounced peak due to resonant-tun- neling at a base-emitter voltage of 1 V. The peak current density was 3.14 X lo4 A/cm2 and the collector current peak-to-valley ratio reached 9.6. The increase in the peak-to-valley ratio with a high peak collector current is due to the low electron effective mass of the InAlAs emitter barrier, compared to that of AlGaAs [4]. A common-emitter current gain of 28 was measured for 250-A base RHET’s at a collector-emitter voltage of 1.5 V and a base current of 0.2 mA. A common-emitter current gain of 28 is the highest value yet reported. The collector current peak-to-valley ra- tio was 4.54. These results indicate that the use of In- GaAs/ln( AlGa) As heterostmctures may well result in the high performance RHET’s. This work was supported by the MITI’s Project of Basic Technology for Future Industries. [I] N. Yokoyama et al., Japan. J. Appl. Phys., vol. 24, p. L8.53, 1985. [2] N. Yokoyama et al., 18th Conf. SSDM, p. 347, 1986. [3] K. Imamura et al., Electron. Lett., vol. 9, no. 22, p. 1148, 1986. [4] T. Inata et al., Japan. J. Appl. Phys., vol. 25, p. L983, 1986. VIA-4 High-Responsivity Long-Wavelength ( h = 10 pm) GaAs /Alx Gal - xAs Multiquantum Well Superlattice Tunnel- ing Detector-B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J . Malik, AT&T Bell Laboratories, Murray Hill, NJ 07974. With the objective of demonstrating long wavelength ( X = 10 pm) infrared sensitivity using 111-V materials, rather than the less technologically advanced Hg,Cd, -,Te alloys, we have experi- mentally and theoretically investigated a novel detector concept, using Al,Ga, -,As superlattices [l]. The structure consists of 50 periods of 72-A GaAs quantum wells (doped n = 1.5 X 10’’ ~ m - ~ ) separated by Alo.ssGao,6zAs barriers, with n’ ohmic contacts on both sides. The quantum well is designed to contain only two bound states separated in energy by Ez - El = 124 meV (corresponding to X = 10 pm). We have shown that the ground state of one well aligns with the excited state of the neighboring well due to sequential resonant tunneling [2]. This band alignment ensures that the incident infrared radiation is strongly absorbed by the intersubband resonance [3] (which has a large absorption coefficent 01 = lo4 cm-’ ) and photoexcites an electron which rapidly and efficiently tunnels out of the well, trav- els a hot-electron mean free path L and thereby produces a large photocurrent. We have theoretically analyzed in detail the voltage dependence of the tunneling probability, and the physics of the hot-electron transport and achieve an excellent fit of the experimental data over three orders of magnitude in responsivity. The high responsivity, R = 1.9 A/W, i? four times larger than our previous results [l] while L = 4500 A is twice as large. This significant improvement is a consequence of the thicker and higher tunneling barriers, which lowers the dark current and thus allows the useof higher bias volt- ages. These detectors have an advantageous narrow bandwidth (Ah/ h = 10 percent) and an estimated high-speed response of over 10 GHz. In conclusion, we have demonstrated a new concept in far in- frared detectors, and improved our earlier sensitivity by a factor of four. These devices which show high responsivity and high speed, have been used to measure novel hot-electron superlattice transport physics. Because of their compatibility with GaAS IC technology, arrays of such detectors should be possible. By simply varying the superlattice parameters it is expected that the wavelength of oper- ation can be tuned from less than 5 pm to over 100 pm. [I] B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J. Malik, [2] L. Eskai and L. L. Chang, Phys. Rev. Lett., vol. 33, p. 495, 1974. [31 L. C. West and S. 1. Eglash, Appl. Phys. Lett., vol. 46, p. 1156, 1985. Appl. Phys. Lett., to be published. VIA-5 Long-Wavelength Infrared Detectors Made from Strained Type-I1 Superlattices-D. L. Smith, Los Alamos Na- tional Laboratory, Los Alamos, NM 87545, and C. Mailhiot, Xe- rox Wehster Research Center, 800 Philips Road, 01 14-41D, Web- ster, NY 14580. We show that infrared detectors made from strained type I1 su- perlattices can have favorable properties for long-wavelength ( X, > 10 pm) applications. We specifically consider superlattices of InAs-Gal -,In,Sb with x - 0.4. This is a type I1 superlattice be- cause the conduction band of InAs is lower in energy than the va- lence band of Ga, -,In,Sb. In a type I1 superlattice such as InAs- GaSb there is a trade-off between bandgap and absorptivity because photoexcitation occurs across the superlattice layers. Reducing the layer thickness increases the optical matrix element, but the re- sulting quantum confinement separates the energy bands. Using theoretical techniques, we show that the small bandgaps required for long-wavelength detection can be achieved for much thinner layers by taking advantage of lattice mismatch induced strain in the InAs-Gal -,Sb superlattice. This eliminates the principal difficulty 111 in using type I1 superlattices for infrared detectors. The optical absorption of the InAs-Gal -,In,Sb superlattice is calculated and found to compare favorably with that of the Hg, -.Cd,Te alloy of rhe same bandgap. The electron effective mass of the superlattice 1,s found to be nearly isotropic and much larger than that for the HgCdTe alloy (m: /m - 0.04 at X, - 10 pm). This larger value of the electron effective mass leads to very much reduced tunneling currents in reverse biased diodes used for infrared detection. As a result leakage currents are less o€ a problem in diodes made from the superlattice.
Transcript of VIA-4 high-responsivity long-wavelength (λ = 10 µm) GaAs/AlxGa1-xAs multiquantum well superlattice...
2382 IEEE TRLtNSACTIONS ON ELECTRON DEVICES, VOL. ED-34, NO. 11, NOVEMBER 1987
VIA3 Resonant-Tunneling Hot-Electron Transistors (RHET’s) Using InGaAs/In ( AlGa) As Heterostructure--I< , Imamura, S. Muto, H. Ohnishi, T. Fujii, and N. Yokoyama, Fu-- jitsu Limited, 10-1, Morinosato-Wakamiya, Atsugi 243-01, Japan.
This paper describes a resonant-tunneling hot-electron transistor (RHET) using an InGaAs/In( A1Ga)As heterostructure. A corn- mon-emitter current gain of 10 and a collector currept peak-to-val- ley ratio of 9.6 were measured at 77 K €or a 1000-A base RHET , The peak current depsity was 3.14 X lo4 A/cmZ. The measuretl current gain of 250- A base RHET’s reached 28, which is the high. est value yet reported for any hot-electron transistors.
In 1985, we proposed and fabricated a three-terminal resonant. tunneling device, named the RHET, which used a GaAs/AlGaAs heterostructure [l]. We demonstrated the various attractive fea- tures of RHETs for memory and/or logic applications [2]. How- ever, the common-emitter current gain in the GaAs/AlGaAe RHET was not high enough to enable practical use. On the other hand, we reported [3] that use of InGaAs for the base layer of hot electron transistors (HET) was effective to increase a current gain because of larger F-L valley separation energy (0.55 eV) than that of GaAs (0.30 eV) . This paper demonstrates that current gains and collec- tor peak-to-valley ratios of RHET’s are greatly improved by using an InGaAs /In ( A1Ga) As heterostructure .
The devices have a quantuy well, which acts as the hot-electron injector, consisting of 38.1-A Ino,53Gao,47As sandwiched by two 44- A Ipo.53Alo,47As. The thickness of the base layers were 250 and 1000 A . The carrier concentration of the base layer was 1 X 10” ~ m - ~ . The thiskness of the tno.52 ( Alo .5G~,5 )o.4sAs collector barrier was 2000 A with a 60-A graded layer. Each layer was grown using pulsed molecular-beam methods. A common-emitter current gain of 10.0 was measured at 77 K at a collector-emitter voltage of 2 V and a base current of 1 mA. The increased current gain of InGaAs/InAlGaAs RHET’s is believed to be due to the large I’-L valley separation energy of InGaAs compared to that of GaAs. The collector current, measured as a function of the base- emitter voltage, exhibited a pronounced peak due to resonant-tun- neling at a base-emitter voltage of 1 V. The peak current density was 3.14 X lo4 A/cm2 and the collector current peak-to-valley ratio reached 9.6. The increase in the peak-to-valley ratio with a high peak collector current is due to the low electron effective mass of the InAlAs emitter barrier, compared to that of AlGaAs [4].
A common-emitter current gain of 28 was measured for 250-A base RHET’s at a collector-emitter voltage of 1.5 V and a base current of 0.2 mA. A common-emitter current gain of 28 is the highest value yet reported. The collector current peak-to-valley ra- tio was 4.54. These results indicate that the use of In- GaAs/ln( AlGa) As heterostmctures may well result in the high performance RHET’s.
This work was supported by the MITI’s Project of Basic Technology for Future Industries. [I] N. Yokoyama et al., Japan. J . Appl. Phys., vol. 24, p. L8.53, 1985. [2] N. Yokoyama et al., 18th Conf. SSDM, p. 347, 1986. [3] K. Imamura et al., Electron. Lett., vol. 9, no. 22, p. 1148, 1986. [4] T. Inata et al., Japan. J . Appl. Phys., vol. 25, p. L983, 1986.
VIA-4 High-Responsivity Long-Wavelength ( h = 10 pm) GaAs /Alx Gal - xAs Multiquantum Well Superlattice Tunnel- ing Detector-B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J . Malik, AT&T Bell Laboratories, Murray Hill, NJ 07974.
With the objective of demonstrating long wavelength ( X = 10 pm) infrared sensitivity using 111-V materials, rather than the less technologically advanced Hg,Cd, -,Te alloys, we have experi- mentally and theoretically investigated a novel detector concept, using Al,Ga, -,As superlattices [l] .
The structure consists of 50 periods of 72-A GaAs quantum wells (doped n = 1.5 X 10’’ ~ m - ~ ) separated by Alo.ssGao,6zAs barriers, with n’ ohmic contacts on both sides. The quantum well is designed to contain only two bound states separated in energy by Ez - El = 124 meV (corresponding to X = 10 pm). We have shown that the ground state of one well aligns with the excited state of the neighboring well due to sequential resonant tunneling [2]. This band alignment ensures that the incident infrared radiation is strongly absorbed by the intersubband resonance [3] (which has a large absorption coefficent 01 = lo4 cm-’ ) and photoexcites an electron which rapidly and efficiently tunnels out of the well, trav- els a hot-electron mean free path L and thereby produces a large photocurrent.
We have theoretically analyzed in detail the voltage dependence of the tunneling probability, and the physics of the hot-electron transport and achieve an excellent fit of the experimental data over three orders of magnitude in responsivity. The high responsivity, R = 1.9 A/W, i? four times larger than our previous results [ l] while L = 4500 A is twice as large. This significant improvement is a consequence of the thicker and higher tunneling barriers, which lowers the dark current and thus allows the use of higher bias volt- ages. These detectors have an advantageous narrow bandwidth ( A h / h = 10 percent) and an estimated high-speed response of over 10 GHz.
In conclusion, we have demonstrated a new concept in far in- frared detectors, and improved our earlier sensitivity by a factor of four. These devices which show high responsivity and high speed, have been used to measure novel hot-electron superlattice transport physics. Because of their compatibility with GaAS IC technology, arrays of such detectors should be possible. By simply varying the superlattice parameters it is expected that the wavelength of oper- ation can be tuned from less than 5 pm to over 100 pm. [I] B. F. Levine, K. K. Choi, C. G . Bethea, J. Walker, and R. J. Malik,
[2] L. Eskai and L. L. Chang, Phys. Rev. Lett., vol. 33, p. 495, 1974. [31 L. C. West and S. 1. Eglash, Appl. Phys. Lett., vol. 46, p. 1156, 1985.
Appl. Phys. Lett., to be published.
VIA-5 Long-Wavelength Infrared Detectors Made from Strained Type-I1 Superlattices-D. L. Smith, Los Alamos Na- tional Laboratory, Los Alamos, NM 87545, and C. Mailhiot, Xe- rox Wehster Research Center, 800 Philips Road, 01 14-41D, Web- ster, NY 14580.
We show that infrared detectors made from strained type I1 su- perlattices can have favorable properties for long-wavelength ( X, > 10 pm) applications. We specifically consider superlattices of InAs-Gal -,In,Sb with x - 0.4. This is a type I1 superlattice be- cause the conduction band of InAs is lower in energy than the va- lence band of Ga, -,In,Sb. In a type I1 superlattice such as InAs- GaSb there is a trade-off between bandgap and absorptivity because photoexcitation occurs across the superlattice layers. Reducing the layer thickness increases the optical matrix element, but the re- sulting quantum confinement separates the energy bands. Using theoretical techniques, we show that the small bandgaps required for long-wavelength detection can be achieved for much thinner layers by taking advantage of lattice mismatch induced strain in the InAs-Gal -,Sb superlattice. This eliminates the principal difficulty 111 in using type I1 superlattices for infrared detectors. The optical absorption of the InAs-Gal -,In,Sb superlattice is calculated and found to compare favorably with that of the Hg, -.Cd,Te alloy of rhe same bandgap. The electron effective mass of the superlattice 1,s found to be nearly isotropic and much larger than that for the HgCdTe alloy (m: / m - 0.04 at X, - 10 pm). This larger value of the electron effective mass leads to very much reduced tunneling currents in reverse biased diodes used for infrared detection. As a result leakage currents are less o€ a problem in diodes made from the superlattice.
