Far-infrared Detectors Research Articles

Radiative recombination characteristics in GaAs multilayer n+−i interfaces

In this communication, we have carried out a detailed investigation of radiative recombination in n-GaAs homojunction far-infrared detector structures with multilayer emitter (n+)-intrinsic (i) interfaces by temperature-dependent steady-state photoluminescence measurements. The observation of the emitter-layer luminescence structures has been identified from their luminescence characteristics, in combination with high density theoretical calculation. A photogenerated carrier transferring model has been proposed, which can well explain the dependencies of the luminescence intensities on the laser excitation intensity and temperature. Furthermore, the obtained radiative recombination behavior helps us to offer a proposal to improve the operating temperature of the detector.

Carrier dynamics in self-organized quantum dots and their application to long-wavelength sources and detectors

Carrier dynamics in self-organized quantum dots have been studied using temperature-dependent differential transmission spectroscopy and room temperature high-frequency electrical impedance measurements on quantum dot lasers. These results suggest the existence of a long relaxation time (∼100 ps) for the excited state carriers at higher temperatures with the dominant scattering mechanism being electron–hole scattering. The long relaxation time is exploited to realize far-infrared sources and detectors based on intersubband transitions in quantum dots. Quantum dot detectors with large detectivity ( D ∗ =(9–10) ×10 9 cm Hz 1/2/W) and responsivity ( R =100 mA/W ) have been reported at T =40 K . A unique unipolar intersubband quantum dot laser (13.3 μm) has also been reported at T =283 K , using the long intersubband relaxation time and the short interband recombination time to achieve population inversion between the ground and the excited states.

Suppression of electron–hole-pair recombination in edge states in quantum Hall regimes

Far-infrared (FIR) photoconductivity induced by cyclotron resonance in edge states of quantum Hall devices are studied by carrying out time-resolved measurements. It is found that the recombination of electron–hole pairs excited via cyclotron resonance in edge states is significantly suppressed, yielding a surprisingly long recombination lifetime (∼10ms). This is in marked contrast to much shorter recombination lifetimes (∼1ms) in bulk states. The experimental results lead us to suggest that the edge-state FIR photoconductivity is induced by an effect of electric polarization. This implies also that edge states can be a promising candidate for application as a FIR detector.

Highly sensitive and tunable detection of far-infrared radiation by quantum Hall devices

We studied far-infrared (FIR) response due to cyclotron resonance (CR) of two-dimensional electron gas systems in GaAs/AlGaAs heterostructures by using cyclotron radiation from n-InSb devices as the illumination source. We examined the dependence of the FIR response on different experimental parameters, including the aspect ratio of Hall bars, electron mobility, bias current, illumination intensity, magnetic field, and lattice temperature. A strong photoresponse emerges only in the vicinity of integer quantum Hall effect (IQHE) regimes. Time-resolved measurements show that the recombination lifetime of excited carriers depends largely on the electron mobility, ranging from 5 μs to 1 ms at 4.2 K. The temporal decay of photoresponse is nonexponential in higher-mobility samples, whereas it is exponential with a single time constant in lower-mobility samples. This, together with the relatively large time constants, suggests that the FIR response is induced through multitrapping processes, in which excited carriers in Landau levels are repeatedly captured by localized states and reexcited to delocalized states. This multitrapping process is suggested to be promoted by the CR-induced heating of the electron system. Theoretical calculation based on the electron heating model reasonably reproduces characteristic dependence of the photoresponse on the magnetic field in the vicinity of IQHE plateaus. The IQHE Hall bars serve as a high-sensitive narrow-band FIR detector, where the highest sensitivity reaches ∼108 V/W. Tunability of the detector is demonstrated by varying the electron density. We discuss briefly the design of high-sensitive FIR detectors using the IQHE Hall bars.

Free-carrier absorption in Be- and C-doped GaAs epilayers and far infrared detector applications

Far infrared (FIR) absorption, reflection, and transmission in heavily doped p-GaAs multilayer structures have been measured for wavelengths 20–200 μm and compared with the calculated results. Both Be (in the range 3×1018–2.6×1019 cm−3) and C (1.8×1018–4.7×1019 cm−3)-doped structures were studied. It is found that the observed absorption, reflection, and transmission are explained correctly by the model with a dominant role of free-carrier absorption in highly doped regions. High reflection from heavily doped thick layers is attractive for the resonant cavity enhanced FIR detectors.

Far-infrared photoresponse of the magnetoresistance of the two-dimensional electron systems in the integer quantized Hall regime

We have investigated the far-infrared (FIR) photoinduced resistance change of the two-dimensional electron systems in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{GaAs}$ heterojunctions in the integer quantized Hall regime. Sensitive photoinduced resistance change $\ensuremath{\Delta}{R}_{\mathrm{xx}}$ is observed only in the vicinity of the quantum Hall states. It is found that the magnitude and polarity of $\ensuremath{\Delta}{R}_{\mathrm{xx}}$ strongly depend on the Landau-level filling factor and the bias current. We have shown that not only a bolometric effect (i.e., electron heating) but also an electronic process due to edge channel transport is responsible for the observed behavior of $\ensuremath{\Delta}{R}_{\mathrm{xx}}.$ The observed photoresponse can be used for realizing very sensitive, narrow-band FIR detectors.

Effect of interface states on negative capacitance characteristics in GaAs homojunction far-infrared detectors

Bias-, frequency- and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. The origin of this effect is believed to be due to the carrier capture and emission at interface states, and has been confirmed by a comparison study of capacitance characteristics on p-GaAs HIWIP detectors with different interface state densities. A fitting data based on charging–discharging current and the inertial conducting current model show good agreement with the experimental observations.

Development of far-infrared Ge:Ga photoconductor having a longitudinal configuration

In order to efficiency obtain a two-dimensional (2D) image in the 3 THz range, it is important to develop a 2D far-infrared detector array. We have developed a gallium-doped germanium (Ge:Ga) far-infrared photoconductor that has a longitudinal configuration. This structure is suitable for a large format monolithic 2D array. In this letter, we show that a transparent electrode that is responsive in the far-infrared range can be formed by ion implantation, and this layer contributed to increasing quantum efficiency. We obtained a high responsivity (16.2 A/W) and good noise equivalent power (2.6×10−17 W/Hz1/2) of Ge:Ga photoconductor in longitudinal configuration by combination of ion-implanted layers and Ge:Ga bulk material.

Structure variation of intersubband electron–acoustic phonon scattering rate in coupled quantum wells

Intersubband electron–acoustic phonon scattering rates are calculated for asymmetric coupled AlGaAs/GaAs quantum wells (QW) at low temperature. Emphasis is placed on the dependence of the scattering rate on the thickness and position of the AlGaAs barrier in the QW. Strong variation of the scattering rate between the second and the first subbands is obtained for small barrier displacement from the center of the QW. These results show that intersubband scattering rates can be engineered to optimize the performances of far-infrared detectors, lasers, and photovoltaic devices. Moreover, this study shines new light on the discrepancy among recent experimental results for these structures.

Room temperature far infrared (8/spl sim/10 μm) photodetectors using self-assembled InAs quantum dots with high detectivity

Room temperature operation of far-infrared detectors made of self-assembled quantum dots embedded in the channel region of modulation-doped heterostructures is demonstrated. At room temperature, the detector shows a low dark current ranging in the nano-amperes at a bias voltage of 10 V. After the optimization of the separation between the quantum dot region and the 2DEG, a peak responsivity of 5.3 A/W is obtained at 9.0 /spl mu/m. The high detectivities of 6/spl times/10/sup 8/ and 5/spl times/10/sup 10/ cmHz/sup 1/2//W are obtained at room temperature and 80 K, respectively.

Performance and spectral response of Pb1−xSnxTe(In) far-infrared photodetectors

We have compared directly the performance of a Pb1−xSnxTe(In) photodetector with that of two other state-of-the-art far-infrared detectors: a Si(Sb) blocked impurity band (BIB) detector and a Ge(Ga) photoconductor in an integrating cavity. The Pb1−xSnxTe(In) photodetector has current responsivity SI several orders of magnitude higher than the Si(Sb) BIB at wavelength λ=14.5 μm. Persistent photoresponse with SI∼103 A/W at 40 mV bias and 1 s integration time at the wavelengths λ=90 and 116 μm has also been observed in the Pb1−xSnxTe(In) photodetector. This is larger by a factor of ∼100 than the responsivity of the Ge(Ga) photoconductor in the same conditions.

Electron beam bunch length characterizations using incoherent and coherent transition radiation on the APS SASE FEL project

The Advanced Photon Source (APS) injector linac has been reconfigured with a low-emittance RF thermionic gun and a photocathode (PC) RF gun to support self-amplified spontaneous emission (SASE) free-electron laser (FEL) experiments. One of the most critical parameters for optimizing SASE performance (gain length) is the electron beam peak current, which requires a charge measurement and a bunch length measurement capability. We report here initial measurements of the latter using both incoherent optical transition radiation (OTR) and coherent transition radiation (CTR). A visible light Hamamatsu C5680 synchroscan streak camera was used to measure the thermionic RF gun beam's bunch length (σ∼2–3 ps) via OTR generated by the beam at 220 MeV and 200 mA macropulse average current. In addition, a CTR monitor (Michelson Interferometer) based on a Golay cell as the far-infrared (FIR) detector has been installed at the 40-MeV station in the beamline. Initial observations of CTR signal strength variation with gun α-magnet current and interferograms have been obtained. Progress in characterizing the beam at these locations and a comparison to other bunch length determinations will be presented.

GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared detectors

The recent development of p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (>40 μm) detectors is briefly reviewed. The emphasis is on the detector performance, which includes responsivity, quantum efficiency, bias effects, cut-off wavelength, uniformity, noise, and negative capacitance characteristics. Promising results indicate that p-GaAs HIWIP detectors have great potential to become a strong competitor in far-infrared applications.

Effect of interface states on the performance of GaAs p+-i far-infrared detectors

Interface states have been shown to have an appreciable effect on the performance of p-GaAs multilayer (p+-i-p+-i-…) homojunction interfacial work function internal photoemission (HIWIP) far-infrared detectors. In this article, a comparison of detector performance was made of p-GaAs HIWIP detectors with different interface state densities, with the emphasis on the detector’s dark current, noise, and capacitance characteristics.

Recent Progress in Mid- and Far-Infrared Semiconductor Detectors

The recent developments of semiconductor infrared detectors in extending the wavelength coverage and improving the focal plane array (FPA) performance are reviewed. The emphasis is on the GeSi/Si heterojunction infrared photoemission detectors (HIPs), GaAs/AlGaAs quantum well infrared photodetecots (QWIPs), Si, Ge and GaAs blocked impurity band detectors (BIBS), and Si and GaAs homojunction interfacial work-function internal photo-emission (HIWIP) far-infrared (FIR) detectors. The advantages, current status, and potential limitations of these infrared detectors have also been discussed.

Recognition of Space Weather Impact on the Isophot Detectors

The effects of changes in the space environment on the ISOPHOT photoconductivedetectors over the whole ISO mission were studied using the complete setof responsivity check measurements taken after the curing of the detectors.We found that the responsivity of the Ge-based, low bias voltage far-infrared detectors (P3, C100, and C200) is sensitive to the conditions of the Space Weather.We present evidence that an increased responsivity level (20% – 50%) after curing of the detectors is linked to the onset of geomagnetic storms. TheSi-based, high bias voltage detectors P1, P2 and PHT–SS show only small changesin their responsivity. An exception is the PHT–SL array which shows a similar,but less pronounced behaviour as the FIR detectors. While these relationshave been demonstrated by our study, a detailed physical understanding is still outstanding. The Space Weather dependent scatter of the responsivity,being the photometric scaling factor (conversion from measured photo currentto inband power on the detector), justifies the observing mode design to include frequent monitoring of its actual level.

Application of Free Carrier Absorption for Far-Infrared Detection

Far-infrared (FIR) free-carrier absorption characteristics for epitaxially grown p-type Si and GaAs thin films over a range of carrier concentrations have been investigated, both experimentally and theoretically, and employed to analyze the experimental results of FIR detection. The free-hole absorption is found to be almost independent of the wavelength and a linear regression relationship between the absorption coefficient and the carrier concentration has been obtained, which can be applied to well explain the behavior of GaAs FIR detectors with different number of layers, different applied biases, as well as different layer concentrations.

The effect of growth temperature on pulsed laser deposited (Ba, Sr)TiO3 thin films designed for pyroelectric far-infrared detector applications

Pulsed laser deposited (Ba, Sr)TiO3 (BST) thin ferroelectric films, developed for use in pyroelectric detectors, have been grown on single crystal SrTiO3 substrates at temperatures ranging from 580 to 780°C. Temperature-dependent capacitance-voltage measurements reveal Curie temperature shifts of more than 50 K. However the Curie temperature does not shift monotonically with growth temperature. With increasing deposition temperature, the BST films transform from amorphous to epitaxial. Within the epitaxial growth regime, an increase in deposition temperature results in an increase in crystalline column size. These structural changes alter both the type and magnitude of film stress, resulting in shifts in Curie temperature and, consequently, the optimal pyroelectric detector operating temperature.

The susceptibility of incoherent detector systems to cryocooler microphonics.

Mechanical cryocoolers will be used to provide 4 K cooling for the HFI bolometer instrument in the Planck Surveyor mission for measurement of the anisotropies in the cosmic microwave background. They may also find application in future Earth-observing satellites. We describe an experimental programme to investigate the effects of microphonic vibrations on infrared detectors. Our initial tests have demonstrated the feasibility of operating far-infrared photoconductive detectors in conjunction with a 4 K cryocooler for Earth observing applications. The microphonic susceptibility of a bolometer at 100 mK has also been investigated, using a modified variation of the Planck Surveyor zero gravity flight dilution system. The results of these tests gives us confidence in the proposed use of sensitive bolometers with mechanical cryocoolers for space missions. We also describe development of the bolometers, the readout systems and temperature regulators for improved systems level performance of such instruments.

Negative capacitance of GaAs homojunction far-infrared detectors

Bias, frequency and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. Unlike in other devices, even up to 1 MHz in HIWIP, the negative capacitance value keeps increasing with frequency, giving a stronger effect. The origin of this effect is believed to be due to the carrier capture and emission at interface states. Fitting data based on charging-discharging current and the inertial conducting current model show good agreement with the experimental observations.