G. von Winckel

Three-color (λp1∼3.8 μm, λp2∼8.5 μm, and λp3∼23.2 μm) InAs/InGaAs quantum-dots-in-a-well detector

We report a three-color InAs/InGaAs quantum-dots-in-a-well detector with center wavelengths at ∼3.8, ∼8.5, and ∼23.2 μm. We believe that the shorter wavelength responses (3.8 and 8.5 μm) are due to bound-to-continuum and bound-to-bound transitions between the states in the dot and states in the well, whereas the longer wavelength response (23.2 μm) is due to intersubband transition between dot levels. A bias-dependent activation energy ∼100 meV was extracted from the Arrhenius plots of the dark currents, which is a factor of 3 larger than that observed in quantum-well infrared photodetectors operating at comparable wavelengths.

Theoretical modeling and experimental characterization of InAs∕InGaAs quantum dots in a well detector

Theoretical modeling and experimental characterization of InGaAs∕GaAs quantum dots-in-a-well (DWELL) intersubband heterostructures, grown by molecular beam epitaxy are reported. In this heterostructure, the self-assembled dots are confined to the top half of a 110A InGaAs well which in turn is placed in a GaAs matrix. Using transmission electron microscopy, the quantum dots are found to be pyramidal in shape with a base dimension of 110A and height of 65A. The band structure for the above mentioned DWELL heterostructure was theoretically modeled using a Bessel function expansion of the wave function. The energy levels of the three lowest states of the conduction band of the quantum dot are calculated as a function of the electric field. Intersubband n‐i‐n detectors were fabricated using a ten layer DWELL heterostructure. The spectral response of the detector is measured at a temperature between 30 and 50 K and compared with the prediction of our theoretical model.

Two color InAs/InGaAs dots-in-a-well detector with background-limited performance at 91 K

Normal incidence long wave infrared (λc∼9 μm) InAs/In0.15Ga0.85As dots-in-a-well detectors with background limited performance at 91 K, under f#1.7 300 K background irradiance, are reported. Two distinct peaks (λp1∼4.2 μm and λp2∼7.6 μm) are observed in the spectral response, which could possibly be due to a bound-to-continuum transition and a bound-to-bound transition, respectively. The operating wavelength of the detector can be varied by changing the width of the quantum well surrounding the quantum dots. Using calibrated blackbody measurements, the peak responsivity of the detector is measured to be 0.73 A/W (Vb=−1.7 V at T=60 K).

Normal-incidence InAs/In0.15Ga0.85As quantum dots-in-a-well detector operating in the long-wave infrared atmospheric window (8-12 μm)

Normal incidence InAs/In0.15Ga0.85As dots-in-a-well (DWELL) detectors are reported in which the peak operating wavelength was tailored from 7.2 to 11 μm using heterostructure engineering of the DWELL structure. Using an optimized design, a detector with a spectral response spanning the long-wave infrared atmospheric window (8–12 μm) is obtained. Spectral response peaks were observed at λp=10.3 μm and 11.3 μm under positive and negative bias, respectively. These peaks are attributed to bound-to-bound transitions from the InAs quantum dot to the InGaAs well.

Optical characterizations of heavily doped p-type AlxGa1−xAs and GaAs epitaxial films at terahertz frequencies

The optical properties of p-type AlxGa1−xAs (x=0, 0.01, and 0.16) epitaxial films with different beryllium and carbon doping concentrations (1018–1019cm−3) were investigated by far-infrared reflectance spectroscopy in the 1.5–15‐THz frequency range. The dielectric response functions of the film samples were expressed using the classical Lorentz–Drude model. Optical properties were obtained using a three-phase model (air∕film∕substrate) which agrees with the experimental reflectance spectral data. The effects of doping concentrations on the optical constants were studied in detail. The results indicate that the refractive index increases with the doping concentration in the low-frequency region (⩽5THz) where the free-carrier absorption plays an important role in the optical response. However, the extinction coefficient increases with the doping concentration in the entire frequency region. This indicates that the absorption coefficient increases with the doping concentration. The calculated plasma frequenc...

Free carrier absorption in Be-doped epitaxial AlGaAs thin films

Free hole absorption in doped AlxGa1−xAs films, grown by molecular-beam epitaxy on semi-insulating GaAs substrates, was investigated. Free carrier absorption for three different hole concentrations with the same Al fraction and for two different Al fractions with the same doping concentration was studied. Experimental absorption coefficients were obtained from the data using a model that includes multiple reflections in the substrate wafer. In the 100–400μm range, (3,5,8)×1018cm−3 Be-doped Al0.01Ga0.99As films have absorption coefficients of ∼(3,3.5,5)×103cm−1, respectively, where the magnitude of the absorption is found to be almost independent of the wavelength. This allows replacing doped GaAs emitters in heterojunction interfacial work function internal photoemission far-infrared (HEIWIP) detectors with p‐AlxGa1−xAs layers with x<0.017 facilitating the extension of the threshold wavelength of HEIWIP detectors beyond the 92μm limit due to the practical Al fraction growth limit of 0.005 in molecular-bea...

Spectral element modeling of semiconductor heterostructures

We present a fast and efficient spectral method for computing the eigenvalues and eigenfunctions for a one-dimensional piecewise smooth potential, as arises in the case of epitaxially grown semiconductor heterostructures. Many physical devices such as quantum well infrared photodetectors and quantum cascade lasers rely upon transitions between bound and quasi-bound or continuum states; consequently it is imperative to determine the resonant spectrum as well as the bound states. Instead of trying to approximate radiation boundary conditions, our method uses a singular mapping combined with deforming the coordinate system to a contour in the complex plane to construct semi-infinite elements of perfectly matched layers. We show that the PML elements need not be based on a smooth contour to absorb outward-propagating waves and that the resonant eigenvalues can be computed to machine precision. A fast means of computing inner products and expectations of quantum mechanical operators with quadrature accuracy in the spectral domain is also introduced.

Single and Multi Emitter Terahertz Detectors Using n-Type GaAs/AlGaAs Heterostructures

Terahertz detection is demonstrated using GaAs/Al_xGa_1-xAs n-type heterojunction interfacial work function internal photoemission (HEIWIP) detectors. A smaller work function (Delta) needed for terahertz detection can be achieved by using n-doped GaAs emitter and undoped Al_xGa_1-xAs barrier. A single emitter and a multi emitter n-type GaAs/Al_xGa_1-xAs HEIWIP detectors were designed, fabricated and characterized. In both designs, 1times10¹⁸ cm^-3 n-type doped GaAs was used as the emitter while Al_xGa_1-xAs with x = 0.04 for the single emitter detector and x=0.13 for the multi emitter detector was used as the barrier. The threshold frequency of 3.2 THz (93 mum ) with peak responsivity of 6.5 A/W at 7.1 THz at 6 K was successfully demonstrated for the single emitter detector while 5 THz (60 mum) threshold frequency and 0.32 A/W peak responsivity was observed for the multi emitter detector at 5 K. In addition, the peak quantum efficiency of ~19% and peak detectivity of ~5.5times10⁸ Jones under a bias field of 0.7 kV/cm at 6 K were obtained for the single emitter detector.

Effect of doped substrate on GaAs-AlGaAs interfacial workfunction IR detector response through cavity effect

In this paper, results are reported showing response enhancement in GaAs-AlGaAs IR detectors using a doped substrate to increase reflection, enhancing the resonant cavity effect. Responsivity for heterojunction interfacial workfunction detectors grown on semi-insulating (SI) and doped substrates are compared. For a device grown on an SI substrate, a 9-/spl mu/m resonance peak had a response of 1.5 mA/W while a similar device on an n-doped substrate showed 12 mA/W. Also, the difference between response under forward and reverse bias (3 versus 12 mA/W) for the sample grown on the doped substrate, as well as calculated results confirm that the increased response is due to the resonant enhancement. An optimized design for a 15-/spl mu/m peak (24 /spl mu/m 0 response threshold) detector grown on a doped substrate could expect a peak response of 4 A/W with a 50% quantum efficiency and D/sup */ /spl sim/ 2 /spl times/ 10/sup 10/ Jones at the background limited temperature of 50 K.