No abstract

We describe a new type of intersubband GaAs/AlGaAs infrared detector consisting of three stacks of quantum wells; the quantum wells in a given stack-are identical, but are different from stack to stack. Each stack is designed to yield an absorption and a photoresponse at a different peak wavelength. The resulting device is an infrared detector which can operate in a number of modes. Among the features of this device are a wide-band detection domain, a tunable response and excellent responsivities and noise figures. The tunable operation includes a sharp peak-switching response which follows the formation, expansion, and readjustment of electric field domains within the multiquantum well region.Intersubband quantum well detectors have recently been the subject of a considerable research effort;ld properties important for many applications are a wide spectral response,7 preferably over the atmospheric window 8-12 pm and tunability of the peak wavelength.s-"We report here on the operation of a new type of bound-to-continuum GaAs infrared detector, consisting of three different stacks of quantum wells arranged in series. All the wells in a given stack are identical, but each stack is designed for absorption and detection at a different wavelength, featuring distinct well widths and barrier heights.The detector can operate in one of a number of modes. At forward and low bias voltages, the response peaks at a single wavelength ( -1400 cm-') and the device functions as a standard bound-to-continuum infrared detector."'3 When exceeding a temperature-dependent critical applied voltage, the detector's spectral response switches to a different peak wavelength ( -1140 cm -'), while the detection at the previous peak is significantly reduced. At a reverse bias the detection is again centered on the higher energy peak ( -1400 cm-') up to a specific applied voltage; for moderately higher voltages, two peaks yield a significant photoresponse. At still higher values of the reverse voltage a third response peak appears, which results in operation as a wide-band detector. These features are accompanied by good responsivity and detectivity figures.The structure was grown by molecular beam epitaxy on a semi-insulating GaAs substrate. The superlattice, clad by two n-doped contact layers, consisted of three stacks of 25 quantum wells each; the first 25 wells were 3.9 nm wide and were separated by Alo,3sGac62As barriers; the second stack consisted of 25 quantum wells 4.4 nm wide with Alo,30Ga,,70As barriers; the last stack had 25 wells 5.0 nm wide and Alo.,,Gac76As barriers. All the barriers were 44 nm long; the wells and the contacts were uniformly doped with Si to n=4X lOI cmA3.The absorption at zero field and room temperature is shown in Fig.

Negative transconductance resonant tunneling field effect transistors and monolithically integrated resonant tunneling diodes

We describe the observation of phase conjugation at 10.6 μm in a GaAs/AlGaAs multi-quantum-well-doped structure. The responsible nonlinear susceptibility χ(3) is due to a nearly resonant intersubband transition. The magnitude of χ(3) is 7×10−5 esu and the phase conjugate reflectivity is a few tenths of a percent.

We report on the first observation of third-order intersubband nonlinearities in a quantum well structure. We have measured the dc Kerr effect in a symmetric quantum well and found that the Kerr coefficients due to intersubband transitions are six orders of magnitude larger than that of bulk GaAs. To our best knowledge this is the largest value ever measured for the third-order susceptibility. By including dc screening effects and evaluating the internal electric field in the well, a good agreement between the calculated coefficients and the experimental ones was found.

The formation, expansion, and readjustment of electric field domains in multiquantum well stacks is described and explained in terms of sequential resonant tunneling. These effects are used to control the multiband spectral response in IR detector applications of these structures.The formation of electric field domains (EFD) was first observed in bulk GaAs and is mostly known as the cause of Gunn oscillations.' It is explained in terms of the negative differential resistance (NDR) which occurs because of the electron transfer from the r to the X or L valleys. Esaki and Chang2 first observed the formation of static EFDs in multiquantum wells (MQW); this phenomenon was attributed to the NDR which arises due to sequential resonant tunneling (SRT) between subbands in adjacent wells.3aRecently, we demonstrated the operation of a tunable quantum well infrared detector which was based on the formation of EFDs in a MQW device.' In this letter, we report on an investigation designed to determine the parameters which govern EFD formation and expansion; We show theoretically and experimentally how the proper choice of well widths, heights, and doping determines the electric field domain profile.First, we discuss EFDs in the three-stack MQW device presented in Ref. 7. In this device the superlattice clad by two n-doped contact layers, consisted of three stacks of 25 QW each; the first 25 wells were 3.9 nm wide ,and were separated by AlxGa,...&s (x=0.38) barriers; the second stack consisted of 4.4 nm wide wells with (x=0.3) barriers; the last stack had 5.0 nm wide wells and (x=0.24) barriers. All the barriers were 44 nm wide; the wells and the contacts were uniformly doped with Si to n=4~ 10's cmm3.

Tunneling mechanism in zero- and one-dimensional quantum structures is studied. Several new results, peculiar to low dimensions, are predicted. We find that subband mixing and multichannel tunneling induce the appearance of new tunneling channels with unusual interference patterns, and allow for longer lifetime of the resonances at higher energies in various channels. It is shown that in low dimensions, there exists a critical size of the structure below which the resonance nature of the tunneling process is diminished. In zero and one dimensions, there exists a critical magnitude of the confinement potential, below which there are no resonances in the transmission function for any size of the well. Negative differential resistance and other phenomena related to the resonance characters of the tunneling will not appear in this case. We also develop a generalized transfer matrix method that takes into account subband mixing; this formalism can be used to describe any transport problem in low dimensions.

The operation of ultralow threshold current GaAs and InGaAs quantum well lasers at cryogenic temperatures has been studied. In particular the threshold current Ith and lasing wavelength of GaAs and strained InGaAs lasers have been measured as a function of temperature from 300 down to 5 K. Ith can in both lasers be characterized by a linear function of temperature up to 200 K, with a significantly (2.5×) larger dIth/dT for the GaAs laser. We measured a minimum threshold current of 120 μA for the GaAs laser and 165 μA for the InGaAs laser at 5 K. We derive a simple expression for the transparency carrier density as a function of temperature and effective masses to explain our results.