GaAs-based, single-stage, intersubband devices with active regions composed of deep quantum wells (i.e., In 0.3 Ga 0.7 As) and high AlGaAs barriers display strong room-temperature emission at = 4.7 m. The structures are grown by metalorganic chemical vapor deposition. The large energy barriers ͑ϳ360 meV͒ for electrons in the upper energy level of the active region strongly suppress both the carrier leakage as well as the tunneling escape rate out of the wells. As a result, the ratio of emissions at 80 and 300 K is as low as 2.0, and thus there is considerably less need for a Bragg mirror/transmitter-type region. Devices with virtually 100% tunneling injection efficiency have been realized, and their room-temperature spectra are narrow: 25 meV full width at half maximum. In the quest to achieve efficient, room-temperature (RT), continuous-wave (cw) laser operation in the mid-infrared (IR) wavelength range (i.e., 3 -5 m) one proposed approach is the use of two-dimensional arrays of unipolar quantum boxes 1 (QBs), with each QB incorporating a singlestage, intersubband-transition structure. In previous work on single-stage, unipolar devices RT intersubband emission has been reported only from InP-based structures 2 at a wavelength of 7.7 m. For 30-to 40-stage GaAs-AlGaAs quantum-cascade (QC) lasers at RT, intersubband emission wavelengths shorter than 8 m cannot be achieved, since at higher transition energies the active-region upper level is apparently depopulated via resonant tunneling between the X valleys of the surrounding AlGaAs barriers. 3 We present here the realization of RT mid-IR electroluminescence emission from single-stage intersubband devices. The RT output power is of the same order of magnitude as that of InP-based QC structures of approximately the same wavelength. The RT emission linewidth is narrow [ϳ25 meV full width at half maximum (FWHM)] and the 80" 300 K emission ratio is very low ͑ϳ2͒.Optimization studies of GaAs-based devices 4 have shown that for thin barriers between the injector region and the active region, two effects occur which cause significant decreases in the upper-(energy) state injection efficiency: (1) a diagonal radiative transition from the injector-region ground state, g, to an active-region lower state, and (2) severe carrier leakage from the state g to the continuum. Here we show that by using GaAs-based devices with very deep active quantum wells (QWs), In 0.3 Ga 0.7 As active layers sandwiched between Al 0.8 Ga 0.2 As barriers, we can virtually suppress carrier leakage to the continuum. Furthermore, since GaAs/ AlGaAs superlattices do not need to be used on both sides of the active region, resonant tunneling cannot occur between X valleys at high transition energies, and thus RT emission in the mid-IR range becomes possible for GaAs-based devices.The material used in the devices is grown by lowpressure metalorganic chemical vapor deposition (MOCVD) at 700°C and is essentially a single-stage structure embedded in a plasmon-enhanced n-GaAs waveguide, 3,4 which gives an op...
Peak current densities two times higher than the best values reported for GaAs-based resonant tunneling diode ͑RTD͒ structures have been obtained from metal-organic chemical-vapor deposition ͑MOCVD͒-grown deep-quantum-well strained-layer In 0.3 Ga 0.7 As/Al 0.8 Ga 0.2 As RTDs. By growing on nominally exact ͑100͒ ϩ/Ϫ0.1°GaAs substrates, we have been able to obtain smooth interfaces between the strained-layer In 0.3 Ga 0.7 As quantum well and Al 0.8 Ga 0.2 As barriers, which, in turn, enabled us to benefit from resonant tunneling through the second resonant energy level of In 0.3 Ga 0.7 As/Al 0.8 Ga 0.2 As structures. Peak current densities in excess of 300 kA/cm 2 , and peak-to-valley current ratios as high as 3:1, at 300 K, have been obtained from structures with 14-Å-thick barriers and a 57-Å-thick well. © 1997 American Institute of Physics. ͓S0003-6951͑97͒02821-0͔Currently, resonant tunneling diodes ͑RTDs͒ are the widest bandwidth semiconductor devices with gain, which have been used to build microwave oscillators with oscillation frequencies in excess of 700 GHz, 1 trigger circuits operating up to 110 GHz, 2 and a wide range of high-speed logic and switching circuits. 3-5 For the above applications the peak current density ͑PCD͒ and the current peak-to-valley ratio ͑PVR͒ are the two major figures of merit. 2,6 Also, the choice of material system is of great importance. GaAsbased RTDs are favored over GaSb-and InP-based structures for practical applications due to the maturity of growth and material processing techniques as well as the possibility of integration with other high-speed devices. 7 The best reported PCD value for GaAs-based RTDs 8 is 140 kA/cm 2 , achieved from a Schottky-collector AlAs/GaAs structure having a PVR of 2:1. As for RTDs grown by the metal-organicchemical-vapor deposition ͑MOCVD͒ technique, the PCD values reported 9 are at best 96 kA/cm 2 . Here we report on MOCVD-grown GaAs-based RTDs operating at room temperature with PCDs in excess of 300 kA/cm 2 and PVRs as high as 3:1, made possible by resonant tunneling through the second energy level of a deep quantum well in a strainedlayer structure with smooth quantum-well interfaces.The initial measurements done by Broekaert et al., 10 show that, in an In 0.53 Ga 0.47 As/AlAs/InAs structure grown on InP substrate, the PCD for the resonant tunneling through the second energy level is almost ten times greater than the PCD associated with the first energy level for the same structure. However, the peak voltage for the second resonant energy level is so high (ϳ4.1 V) that it makes the use of tunneling through the second level highly impractical for any application. A later report by Mehdi et al. 6 shows that by using a deep quantum well and adjusting the second resonant energy level to the bottom of conduction band, it is possible to obtain higher PCD values at low peak voltages compared to the conventionally designed RTDs. However, PVRs measured for such structures did not exceed 2:1 at room temperature. Reports on GaAs-based strained-layer I...
Madison, WI 53706, Tel: (608) 265-4643, Fax: (608) 262-1267By taking advantage of resonant tunneling through the second energy level of a deep quantum well in a strained-layer structure, we have achieved more than 2 times higher peak current densities (PCD's) than the ones reported for GaAs-based' resonant tunneling diodes (RTD's) and more than 3 times higher PCD values than those obtained for any type of MOCVD-grown2 RTD's. PCD values higher than 300 kA/cm2, peak voltages as low as 1.2 volts, and peak-to-valley current ratios (PVR's) of 3: 1 at 300 K are obtained from structures with l4A-thick Alo.gG~.zAs barriers and a 57A-thick 1~.3Ga07As well. Fig. 1 and 2 show a schematic view of the grown structure including a 300 A intrinsic GaAs spacer layer and a cross-sectional view of the device, respectively. The thickness of various layers were measured by TEM.GaAs-based RTD's are favored over GaSb-and InP-based structures for practical applications due to the maturity of growth and material processing techniques as well as the possibility of integration with other high-speed devices. However, having a lower peak current density, which is basically the crucial figure of merit for high-speed RTD switching applications3, has been a major drawback for GaAs-based RTD's. Previously reported data4, show that the peak current density associated with the second resonant energy level of a RTD is almost 10 times greater than the one resulting from the first energy level for the same structure, however, attainable only at high peak voltages (-4 volts). In our structure, by using a lowbandgap material, such as I&,~G%.~As, for the well, the second resonant energy level has been welladjusted to the edge of conduction band of GaAs, and thus provides a relatively low peak voltage. Results of our calculations, Fig. 3, show that for an In0.3Gao.~As well, l4A-thick barriers, and a 300A-thick spacer layer, one can obtain very high PCD values together with a relatively low peak voltage, while the well thickness stays below the critical value for 1~.3G%.7As grown on GaAs substrate (-70 A). Fig. 4 shows the measured I-V curve for a 6*6 pm2 device. Circuit simulations5 show that t h~s device, if fabricated with minimal parasitic capacitance, could switch 1 volt in less than 3 ps. A more optimized spacer layer structure that minimizes intrinsic capacitance6 could reduce the switching time by a factor of two or more, possibly resulting in picosecond switching speed. The relatively good peak-to-valley current ratio may be attributed to the use of deep quantum well7, good ohmic contact made by a 2-step process (shown in Fig. 2), and growth on exact-orientation (100)-GaAs substrates, which results in smooth interfaces between the strained-layer quantum well and barriers*.
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