Frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts and hence are of great interest for integrated photonics. Here, an overview on the generation electrically pumped optical frequency combs on integrated platforms using semiconductor lasers.
This includes self-starting generation of frequency modulated combs in quantum cascade laser in the 8um and interband cascade lasers in the 3-4um wavelength region, respectively. Furthermore, we will discuss how to integrate efficient high-speed modulators in these devices in order to facilitate the generation of picosecond pulses.
Germanium is a nonpolar semiconductor with missing Reststrahlen band. In spite of other promising properties including low bandgap and small effective mass, its long, µs-scale recombination time has been prohibitive for applications as photoconductive THz emitters. Using Au-implantated Ge, with recombination times reduced to sub-ns values, we have demonstrated a broadband photoconductive THz emitter compatible with modelocked fibre lasers operating at wavelengths of 1.1 and 1.55 µm and with pulse repetition rates of 78 MHz. Reaching up to 70 THz bandwidth, this approach points towards the possibility of compact, high-bandwidth THz photonic devices compatible with Si CMOS technology.
The generation of short pulses with quantum cascade lasers (QCLs) remains challenging to date due to their ultrafast gain dynamics. Here, we report on active mode-locking of mid-infrared QCLs. For the first time we show, that picosecond pulses can be generated also at room-temperature using high-performance QCL material. Mounted epi-up, the QCLs emit a train of pulses as short as a 7ps with an average power of 100mW. The nonlinear autocorrelation shows reveals the famous 8:1 ratio, which proves unambiguously that the QCL operates in the mode-locked regime. This result is further verified using the beatnote spectroscopy technique SWITS.
Semiconductor quantum posts (QPs) - nanowire-like InGaAs heterostructures in a GaAs matrix - resemble many
properties of regular self-assembled quantum dots (QDs), to which they are closely related. Due to their increased size as
compared to QDs, QPs have proven to be suitable for very low threshold interband lasers. However, their well
controllable height makes them attractive for precise tuning of the interband energy spacing that in QDs can only be
achieved via post-growth annealing. Specifically, the 1s - 2p transition energy is expected to drop below LO-phonon
energies at post heights of more than 30 nm, making them attractive as frequency-agile structures at terahertz
frequencies. In the work presented here we explore the capture dynamics of QP structures after photoexcitation into the
GaAs matrix. While the combined electron-hole dynamics are studied using time-resolved photoluminescence
spectroscopy, optical pump - THz probe experiments were performed in order to solely study the electron dynamics. The
results of the THz experiment show that after ultrafast excitation, electrons relax within a few picoseconds into the
quantum posts, which act as efficient traps. The saturation of the quantum post states, probed by photoluminescence, was
reached at approximately ten times the quantum post density in the samples. Also studied was the presence of possible
electronic resonances after direct photoexcitation into QPs where a broad absorption around 1.5 THz was observed.
S. Winnerl, D. Stehr, M. Wagner, H. Schneider, M. Helm, W. Seidel, P. Michel, E. Zibik, B. Carpenter, N. Porter, M. Skolnick, L. Wilson, T. Grange, R. Ferreira, G. Bastard, M. Orlita, P. Plochocka, P. Kossacki, M. Potemski, M. Sprinkle, C. Berger, W. de Heer
The free-electron laser FELBE at the Helmholtz-Zentrum Dresden Rossendorf enables experiments with spectral,
temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz
pulses in a wavelength range from 4 μm to 280 μm. Here we review the potential of the laser and focus on two highlight
pump-probe experiments. In the first experiment, the relaxation dynamics in self assembled InGaAs quantum dots at
energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of
14 meV (3.4 THz), decreasing very strongly to ~ 2 ps at 30 meV (7 THz). The results are in very good agreement with
our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons. In the
second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 - 250 meV. For
excitations below the energy of the optical phonon (G mode), the relaxation times are more than one order of magnitude
longer as compared to the relaxation times observed for near infrared excitation.
We present large-area emitters based on GaInAsN which show efficient THz emission for excitation wavelengths up to 1.35 μm. The substrate material consists of a 1000 nm
Ga1-yInyAs1-xNx (y = 0.11 and x = 0.04) layer grown by molecularbeam epitaxy on semi-insulating GaAs. On top there is an additional
GaAs/Al0.3Ga0.7As heterostructure with thicknesses of 5 nm for the GaAs and 60 nm for the AlGaAs layer, respectively. Transmission measurements with a Fourier
transform spectrometer reveal a bandgap corresponding to a wavelength of 1.5 μm. The resistance of a complete device
with an active area of 1 mm2 is 0.3 MΩThis allows operation with high bias fields (30 kV/cm) without being limited by
heating.
We present photoconductive emitters and detectors for radially and azimuthally polarized terahertz beams.
Microstructured electrodes consisting of concentric rings serve as antennas for radially polarized beams. Electrode
patterns consisting of radially arranged segments are employed for the antennas for azimuthally polarized beams. A
second periodic structure is used to prevent destructive interference of THz wavelets of opposite polarization. Beam
profiles are detected for divergent beams in proximity to the emitter as well as for refocused beams. The THz beams
have a donut-like intensity distribution and the beam profiles can be quantitatively described as lowest order Bessel-
Gauss modes. Furthermore we demonstrate that detection antennas of similar patterning are selective for detection of
particular modes.
KEYWORDS: Electrons, Picosecond phenomena, Absorption, Superlattices, Free electron lasers, Temperature metrology, Scattering, Monte Carlo methods, Quantum wells, Infrared radiation
In this work we investigate the miniband relaxation dynamics of electrons in doped GaAs/AlGaAs superlattices by twocolor
infrared pump-probe experiments using a free electron laser synchronized to a table top broadband IR source. In
contrast to single color experiments, by this technique we are able to separate the different contributions from inter- and
intraminiband relaxation to the transient behavior after an ultrafast excitation. In particular, the intraminiband relaxation
is studied for different miniband widths, below and above the optical phonon energy of GaAs. For minibands wider than
this critical value we find fast relaxation, nearly constant for different excitation intensities whereas for narrow
minibands, a strong temperature and intensity dependence of the relaxation is found. The results are in good agreement
with previously published Monte Carlo simulations.
Gaussian generation-recombination is accepted to be a dominant mechanism of current
noise source in quantum well systems biased by electric field normal to the layers. Recent
experiments in n-type and p-type multiple quantum wells have revealed an additional
pronouncedly non-Gaussian excess current noise with a low cut-off frequency in the kHz
range. The non-Gaussian noise has been attributed to metastable spatial configurations of
electric field. The metastability is originating from negative differential conductance
caused by intervalley scattering in n-type wells and heavy and light holes tunneling in p-type
wells. At a constant bias the system randomly switches between high resistivity state
with low current flow and low resistive state with high current. The non-Gaussianity of
the noise is more pronounced in p-type wells where the time traces of current fluctuations
resemble closely two-level random telegraph signal. In n-type wells the telegraph-like
fluctuations have not been straightforwardly observed. The non-Gaussianity of the noise
in n-type systems has been revealed by nonzero skewness. The differences between noise
properties of between n- and p-type systems have been attributed to small capture
probability of electrons in n-type wells, as opposed to very high capture probability of
holes in p-type wells. As a consequence the noise of any p-type multi-well system is
dominated by the tunneling from the wells while in the n-type the noise appears as a
superposition of many fluctuators associated with individual wells.
The two-photon QWIP approach involves three equidistant subbands, two of which are bound in the quantum well, and
the third state is located in the continuum. The intermediate subband induces a resonantly enhanced optical nonlinearity,
which is about six orders of magnitude stronger than in usual semiconductors. Temporal resolution is only limited by the
sub-ps intrinsic time constants of the quantum wells, namely the intersubband relaxation time and the dephasing time of
the intersubband polarization. Both properties make this device very promising for pulse diagnostics of pulsed midinfrared
lasers. We have performed autocorrelation measurements of ps optical pulses from the free-electron laser (FEL)
facility FELBE at the Forschungszentrum Dresden Rossendorf. Using a rapid-scan autocorrelation scheme at a scan
frequency of 20 Hz, high-quality quadratic autocorrelation traces are obtained, yielding ratios close to the theoretically
expected value of 8:1 between zero delay and large delay for interferometric autocorrelation, and 3:1 for intensity
autocorrelation. Thus, two-photon QWIPs provide an excellent new technique for online pulse monitoring of the FEL. In
addition, we have investigated the saturation mechanism of the photocurrent signal, which is due to internal space
charges generated in the detector.
QWIPs are nowadays considered as rather good candidates for multispectral imaging. Indeed, their narrow spectral bandwidth can be tailored by band gap engineering, and they rely on the use of the mature GaAs technology. However, a light coupling scheme, such as 2D-grating for example, is required to obtain normal incidence absorption. The geometrical parameters of this 2D-grating (grating period, etch depth of the grating) need to be optimized. This optimization is particularly difficult for dual-band pixels, since a compromise is required between the two wavelengths. This paper deals with the influence of the etch depth of the grating on the angular response of quantum well infrared photodetectors. We will first present the experimental setup which was specifically developed at ONERA to perform angular response measurements on infrared photodetectors from different technologies. The results obtained on "low-noise" QWIP samples provided by Fraunhofer-IAF will be presented and analyzed, with emphasis on the effect of the etch depth of the grating on the angular response.
The 3rd generation of infrared (IR) detection modules is expected to provide advanced features like higher resolution 1024x1024 or 1280x720 pixels and/or new functions like multicolor or multi band capability, higher frame rates and better thermal resolution. This paper is intended to present the current status at AIM on quantum well (QWIP) and antimonide superlattices (SL) detection modules for ground and airborne applications in the high performance range. For spectral selective detection, a QWIP detector combining 3-5μm (MWIR) and 8-10μm (LWIR) detection in each pixel with coincident integration has been developed in a 384x288x2 format with 40 μm pitch. Excellent thermal resolution with NETD < 30mK @ F/2, 6.8 ms for both peak wavelengths (4.8 μm and 8.0 μm) has been achieved. Thanks to the well established QWIP technology, the pixel outage rates even in these complex structures are below 0.5% in both bands. QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected. For rapidly changing scenes-like e.g. in case of missile warning applications for airborne platforms-a material system with higher quantum efficiency is required to limit integration times to typically 1ms. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The SL technology provides-similar to QWIP's-an accurate engineering of sensitive layers by MBE with very good homogeneity and yield. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized. IAF and AIM last year managed to realize first most promising SL based detectors. Fully integrated IDCA's with a MWIR SL device with 256x256 pixels in 40µm pitch have been integrated and tested. The modules exhibit excellent thermal resolution of NETD<10mk @ F/2 and 5ms. Product improvement meanwhile allowed to reduce pixel outage rates below 1% i.e. down to a level as required for the military use of such detectors. Presently under development is therefore a dual color MWIR device based on SL technology and the existing 384x288 read out circuit (ROIC) used in the dual band QWIP device. This detector is primarily intended for the use in missile approach warning systems where the dual color capability significantly improves suppression of false alarms. Details of the modules and results of the electrooptical performance will be presented for the different items mentioned above.
Quantum well infrared photodetectors (QWIPs) have gained maturity for large focal plane arrays (FPA) with excellent thermal resolution, low 1/f noise, low fixed-pattern noise, and high pixel operability. Due to their spectrally narrow absorption, QWIPs are particularly suitable for thermal imaging applications involving several atmospheric transmission bands or several colors within the same band. We report on our progress on dual-band QWIP FPAs with pixel-registered, simultaneous integration in both bands. The arrays with 384x288 pixels and 40 μm pitch are based on a photoconductive QWIP for the 3-5 μm regime (MWIR) and a photovoltaic "low-noise" QWIP for 8-12 μm (LWIR). Excellent noise-equivalent temperature differences of only 20.6 mK (LWIR) and 26.7 mK (MWIR) have been achieved at 6.8 ms integration time and f/2 aperture. In addition, we have investigated test devices with different gratings, and discuss their dual-band coupling efficiencies.
The 3rd generation of infrared (IR) detection modules is expected to provide advanced features like higher resolution 1024x1024 or 1280x720 pixels and/or new functionalities like multicolor or multi band capability, higher frame rates and better thermal resolution. This paper is intended to present the current status at AIM on the Mercury Cadmium Telluride (MCT), quantum well (QWIP) and antimonide superlattices (SL) detection modules for ground and airborne applications in the high performance range. For high resolution a 1280x720 MCT device in the 3-5μm range (MWIR) is presently under development. For spectral selective detection, a QWIP detector combining MWIR and 8-10μm (LWIR) detection in each pixel has been developed in a 384x288x2 format with 40 μm pitch, NETD < 35mK @ F/2, 6,8 ms for both peak wavelengths (4.8 μm and 8.0 μm). The device provides synchronous integration of both bands for temporal and spatial coincidence of the events observed. QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected. For rapidly changing scenes - like e.g. in case of missile warning applications for airborne platforms - a material system with higher quantum efficiency is required to limit integration times to typically 1ms. For this case, several companies work on molecular beam epitaxy (MBE) of MCT to have access to double or multi layer structures. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The SL technology provides -- similar to QWIP's -- an accurate engineering of sensitive layers by MBE with very good homogeneity and yield. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized. Just recently, IAF and AIM managed to realize first most promising SL based detectors. Fully integrated IDCAs with a MWIR SL device with 256 x 256 pixels in 40 μm pitch have been integrated and tested. The modules exhibit excellent thermal resolution of NETD > 12 mk @ F/2 and 5 ms. The next step will now be to stabilize the technology and to start the development of a dual color MWIR device based on SL technology and the existing 384 x 288 read out circuit (ROIC) used in the dual band QWIP device.
We report on our QWIP focal plane array (FPA) developments for the
8 - 12 μm and 3 - 5 μm regimes. In the long-wavelength infrared, we have realized several types of QWIP FPAs with array sizes from 256 × 256 to 640 × 512 pixels and with different active regions, giving rise to photoconductive and photovoltaic operation, respectively. Best thermal resolution in the 8 - 12 μm regime is obtained with low-noise QWIP FPAs which are based on a photovoltaic QWIP structure. Special emphasis is given to our work on a 640 × 512 mid-wave QWIP FPA, which is based on strained InGaAs/AlGaAs quantum wells lattice matched to a GaAs substrate. By optimizing the carrier concentration and the geometry of the two-dimensional grating with 1.65 μm period, a high quantum efficiency of more than 10% in the long-wavelength part of the 3 - 5 µm regime is achieved, resulting in an excellent thermal resolution of only 14.3 mK.
For applications like missile warning and automatic target recognition, AIM is presently launching its new 3rd generation high speed dual-color module. The focal plane array (FPA) is a mercury cadmium telluride (MCT) 192x192 56micrometers pitch device in a dual-color mid wave (MWIR) design. The module provides spectral selection with temporal and spatial coincidence for both colors using a new AIM proprietary technology. The spectral bands presently selected are 3.4-4 and 4.2-5micrometers with a full frame rate of 870Hz. Prior to the new devices, a sequential multicolor MCT camera with broadband detector and spectral selection using a rotating filter wheel was developed and evaluated. Results are shown to demonstrate the capabilities of spectral selective detection specifically for clutter and false alarm suppression in missile warning applications. A new algorithm was developed to allow highly sensitive detection of missile plumes without any need for non-uniformity correction for long-term stable operation and maximum dynamic range. An outlook is given on new activities at AIM on dual-band devices. The dual-band approach combining mid wave (MWIR) and long wave (LWIR) detection is specifically useful in automatic target recognition. The application, existing devices and the design goal of the new dual-band device are discussed together with experimental results.
Low NETD's, coupled with other improvements in camera design and manufacturing, helps to further enable a new class of very demanding imaging applications in medicine and medical research.. The evolution of QWIP FPA over the past five years, with their low NETD, detector uniformity, and high pixel yield, along with improvements in camera control and processing electronics, represents key technical innovations responsible for the reemergence of medical infrared imaging through the development of a new infrared medical imaging technique called Dynamic Infrared Imaging or DIRI. The QWIP's high thermal and spatial resolution coupled with very fast data acquisition capabilities fill the essential requirements of DIRI. Other features required by DIRI applications are the need for stable operation with drifts in the image below a few mK, which allow longer data collecting time. Longer data collection time provides the camera the capability to detect the functional behavior of the autonomic nervous system which operates on a time scale of 0.1 to 0.2Hz.
According to the common understanding, the 3rd generation of infrared (IR) detection modules is expected to provide advanced functionalities like more pixels, multicolor or multiband capability, higher frame rates and better thermal resolution. This paper is intended to present the present status at AIM on such technologies. A high speed device with 256 X 256 pixels in a 40μm pitch is designed to provide up to 800 Hz full frame rate with pixel rates as high as 80 Mpixels/s. The read out circuit is designed to stare while scan in a flash integration mode to allow nearly full frame integration for even 800 Hz frame rate. A miniaturized command and control electronics with 14 Bit deep digital output and a non uniformity correction board capable to take into account non linear self learning scene based correction models are developed together with the integrated detector cooler assembly (IDCA). As working horse for dual color/band capabilities, AIM has developed a sequential multi color module to provide customers with a flexible tool to analyze the pros and cons of spectral selective detection. The module is based on a 384x288 mercury cadmium telluride (MCT) detector available in the mid wave (MWIR) or long wave spectral band (LWIR). A rotating wheel with 4 facets for filters or microscanner plates provides spectral selectivity. AIM's programmable MVIP image processing is used for controlling the detector and for non uniformity correction. The MVIP allows set the integration time and NUC coefficients individually for each filter position for comparable performance to accurately evaluate the pay off of spectral selectivity in the IR. In parallel, a dual color detector FPA is under development. The FPA is realized as a MCT MWIR device, LWIR, however, is also doable. Dual color macro cells are realized with 192x192 pixels in a pitch of effectively 56 μm. The cell design provides, that both colors detect radiation from target points identical within the limited resolution of the optics to ensure coincident detection plus compensates the significant variation in photon flux of the different colors to output the analog signal at approximately the same level for good thermal resolution and correctability. The photovoltaic device is realized using AIM's mature liquid phase epitaxy. Since quantum well (QWIP) technology has proven state of the art results based on a well established material system, AIM is heading for QWIP devices for most affordable solutions in the MWIR/LWIR dual band applications. A summary of state of the art results achieved so far as basis for a QWIP dual band detector is presented.
We experimentally compare the peak responsivity R, gain g, quantum efficiency, and detectivity of GaAs/AlGaAs-QWIPs with devices based on the competing material system InGaAs/GaAs. For this purpose we use a typical n-type GaAs/AlGaAs-QWIP and three n-type InGaAs/GaAs-QWIPs with varying doping densities. R and g of the GaAs/AlGaAs-QWIP show a typical negative differential behavior, while both quantities grow monotonously with increasing bias voltage in the case of the InGaAs/GaAs-QWIPs. For identical nominal doping densities and similar cutoff wavelengths between 8.9 micrometers and 9 micrometers , InGaAs/GaAs-QWIPs show much higher responsivities than GaAs/AlGaAs-QWIPs. The ratio between these responsivities is 2.5 at the bias voltage where the GaAs/AlGaAs-QWIP has its maximum. By making use of the different bias dependence of the responsivity in both types of QWIPs a further enhancement of this factor is achieved. Nevertheless, both types of QWIPs show comparable detectivities. This is due to the fact that the gain has a negligible influence on the detectivity. In conclusion, InGaAs/GaAs-QWIPs are promising if high responsivities and short integration times are required.
We report on novel low-noise QWIP focal plane arrays (FPAs) which allow us to improve the thermal resolution of infrared sensors in the long-wavelength infrared (LWIR) atmospheric window. Our concept uses detector structures with a small photoconductive gain in order to achieve simultaneously a high internal quantum efficiency and a small responsivity. In comparison to conventional QWIPs where each period consists of a quantum well and a thermionic barrier, our approach involves additionally a combination of a narrow quantum well and a tunnel barrier. Due to these additional layers, a high emission probability of the photoexcited carriers and an efficient capture into the ground subband of the subsequent period are simultaneously achieved. FPA cameras using these detectors show an extremely low noise- equivalent temperature difference (NE(Delta) T) and a high dynamic range. In particular, NE(Delta) Ts of only 7.2 mK and 5.2 mK (at 20 ms and 40 ms integration time, respectively) are observed for a 256x256 FPA camera system which we have realized using low-noise QWIPs. This value is the best temperature resolution ever obtained for thermal imagers operating in the LWIR.
Full video format focal plane array (FPA) modules with up to 640 X 512 pixels have been developed for high resolution imaging applications in either mercury cadmium telluride (MCT) mid wave (MWIR) infrared (IR) or platinum silicide (PtSi) and quantum well infrared photodetector (QWIP) technology as low cost alternatives to MCT for high performance IR imaging in the MWIR or long wave spectral band (LWIR). For the QWIP's, a new photovoltaic technology was introduced for improved NETD performance and higher dynamic range. MCT units provide fast frame rates > 100 Hz together with state of the art thermal resolution NETD < 20 mK for short snapshot integration times of typically 2 ms. PtSi and QWIP modules are usually operated in a rolling frame integration mode with frame rates of 30 - 60 Hz and provide thermal resolutions of NETD < 80 mK for PtSi and NETD < 20 mK for QWIP, respectively. Due to the lower quantum efficiency compared to MCT, however, the integration time is typically chosen to be as long 10 - 20 ms. The heat load of the integrated detector cooler assemblies (IDCAs) could be reduced to an amount as low, that a 1 W split liner cooler provides sufficient cooling power to operate the modules -- including the QWIP with 60 K operation temperature -- at ambient temperatures up to 65 degrees Celsius. Miniaturized command/control electronics (CCE) available for all modules provide a standardized digital interface, with 14 bit analogue to digital conversion for state to the art correctability, access to highly dynamic scenes without any loss of information and simplified exchangeability of the units. New modular image processing hardware platforms and software for image visualization and nonuniformity correction including scene based self learning algorithms had to be developed to accomplish for the high data rates of up to 18 M pixels/s with 14-bit deep data, allowing to take into account nonlinear effects to access the full NETD by accurate reduction of residual fixed pattern noise. The main features of these modules are summarized together with measured performance data for long range detection systems with moderately fast to slow F-numbers like F/2.0 - F/3.5. An outlook shows most recent activities at AIM, heading for multicolor and faster frame rate detector modules based on MCT devices.
KEYWORDS: Quantum well infrared photodetectors, Thermography, Staring arrays, Image processing, Mid-IR, Sensors, Digital signal processing, Analog electronics, Nonuniformity corrections, Cameras
AIM has developed a family of 2D IR detection modules providing high-speed with frame rates > 1 kHz together with state of the art thermal resolution with an NETD as low as NETD < 7 mK based on either mercury cadmium telluride, platinum silicide, or quantum well infrared photodetector technology to fit for various applications and budgets in research and development.
Quantum well infrared photodetectors (QWIPs) are very promising for ultrafast photodetection in the 8 - 12 micrometers infrared regime. We report on time-resolved studies of the intersubband photocurrent in GaAs/AlGaAs (QWIPs). The photocurrent is excited by sub-picosecond infrared pulses, which are obtained by difference frequency mixing of the signal and idler waves of an optical parametric oscillator. Using a particular detector geometry with < 900 micrometers 2 device area, the measured electrical pulses have a full- width at half-maximum of only 16 ps and a 10% - 90% rise time of 13 ps. For practical applications, in particular for heterodyne detection, it is desirable to reduce the noise floor by using a cooled low-noise preamplifier. We have performed experiments with a low-temperature hybrid circuit consisting of a QWIP in connection with a GaAs transimpedance amplifier based on HEMT technology. This configuration yields a rise time of 19 ps at an amplifier gain of approximately 35 dB.
KEYWORDS: Sensors, Staring arrays, Quantum well infrared photodetectors, Cameras, Gallium arsenide, Quantum wells, Diodes, Thermography, Long wavelength infrared, Chemical elements
The paper reviews the development of IR detectors for the 8 - 12 micrometer wavelength range based on GaAs/AlGaAs quantum well structures and InAs/(GaIn)Sb short-period superlattices (SPSLs) at the Fraunhofer-Institute IAF. Photoconductive GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are used for the fabrication of starring IR cameras for thermal imaging in the third atmospheric window. The long wavelength infrared (LWIR) camera, devleoped in cooperation with AEG Infrarot-Module (AIM), consists of a two-dimensional focal plane array (FPA) with 256 X 256 detector elements, flip- chip bonded to a read-out integrated circuit (ROIC). The technology for the fabrication of FPAs, electrical and optical properties of single detector elements in the two-dimensional arrangement and the properties of the LWIR camera system are reported. A noise equivalent temperature difference (NETD) below 10 mK has been measured at an operation temperature of T equals 65 K with an integration time of 20 ms. More than 99.8% of all pixels are working and no cluster defects are observed. InAs/(GaIn)Sb SPSLs with a broken gap type-II band alignment are well suited for the fabrication of IR detectors covering the 3 - 12 micrometer spectral range. Due to the lattice mismatch of the InAs/(GaIn)Sb SPSL with respect to GaSb, tight control of thickness and composition of the layers and a controlled formation of the chemical bonds across the interface in the SPSLs are used for strain compensation. Photodiodes with a cut-off wavelength (lambda) c equals 8 micrometer and a current responsivity R(lambda ) equals 2 A/W exhibit a dynamic impedance of R0A equals 1k(Omega) cm2 at T equals 77 K. This leads to a Johnson- noise limited detectivity in excess of D* equals 1 X 1012 cm(Hz)1/2/W for these type of detectors.
We report on the dynamics of the transport processes which determine the photoresponse of quantum well intersubband IR detectors. Immediately after intersubband excitation, coherent transport is important. This process can be identified via interference effects between photoexcited carriers. The carriers are re-captured subsequently within a few picoseconds. These fast transport processes give rise to space charges, which induce an additional, slow component of the photocurrent in the ns to microsecond(s) regime.
We investigated the intersubband photocurrent as a function of the bias voltage and the incident wavelength in n-type photovoltaic GaAs/AlAs/Al0.3Ga0.7As double-barrier quantum well (DBQW) infrared detectors. The significant photovoltaic behavior of the detectors arises from a segregation of the dopant during the growth process. For an externally applied bias voltage, which compensates the internal space-charge field, the photocurrent exhibits a multiple sign change for varying incident wavelengths. This observation can be understood in the context of resonant coupling between the excited state in the GaAs quantum well and states, which are confined in the Al0.3Ga0.7As-region. This coupling leads to an enhancement of the tunneling rates through the AlGaAs barriers and to a partial localization of the above barrier states in the GaAs region.
Electroreflectance (ER) spectroscopy has been used to study the influence of an electric field on the optical properties of a strongly coupled GaAs-AlAs superlattice. At certain electric field strengths, anticrossings of the fundamental transitions for heavy- and light-holes are observed, which can be assigned to resonant coupling of the electron wave functions over one, two, and three superlattice periods. The lineshapes of the ER-spectra are discussed for a wide range of temperatures and field strengths.
We have studied the electroabsorption properties of GaAs/AlAs superlattices with a 1 monolayer (ML) and 2 ML wide AlAs barriers. At large electric fields F parallel to the growth direction, Wannier-Stark localization of the carrier states is observed, even for the case of 1 ML AlAs barriers. In the low-field regime (up to 60 kV/cm), Franz-Keldysh oscillations are found. These oscillations are associated with the miniband dispersion of the superlattice. The oscillation period shows an F2/3 behavior, as theoretically expected. Numerical calculations on electroabsorption in strongly coupled superlattices are also presented.
Recent experimental results on the physics of coherent and incoherent resonant tunneling in superlattices
with an electric field perpendicular to the layers are discussed. For the case of weak coupling between
the wells, we observe a decrease of the time constant for electron transport if different subbands of adjacent
wells are at resonance. In this case) transport is incoherent and sequential. This leads to an efficient
population of higher conduction subbands by non-thermal carriers which can be probed by photoluminescence
experiments. For the case of strong inter-well coupling, on the other hand, coherence gives rise to
resonance-induced delocalization phenomena which have been studied by photocurrent spectroscopy.
We report on the investigation of the miniband transport regime in GaAs-AlAs superlattices by electrical
time-of-flight experiments. The temperature dependence of the low-field drift mobility is used to obtain
information about the underlying transport mechanisms. The photocurrent as a function of the applied
field can be fitted over a wide temperature range with a modified Kazarinov-Suris model.
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