Interband cascade lasers (ICLs) are highly efficient semiconductor lasers operating in the mid-infrared range. Their cascade structure of multiple quantum wells enables continuous operation at room temperature with low threshold current. This study explores the impact of tensile and compressive strain on W-QWs' electronic structure and carrier dynamics. Using transient absorption measurements on specific material structures, researchers investigated carrier lifetimes and fundamental transitions. Photoreflectance and photoluminescence measurements were also employed to study band structure and optical properties. The findings provide insights into optimizing ICL performance, improving their performance in application for gas sensing, spectroscopy, medical diagnostics, and communication.
Modern optical gas detection systems utilize the technique of tunable laser absorption spectroscopy for different applications in science, manufacture, or medicine. Superlattice structures composed of semiconductors from the 6.1 Å family enable type-II band alignments and have the potential to exceed state of the art figure of merits of widely used infrared detectors. In this study, InAs/GaSb and InAs/InAsSb type-II superlattices were grown using molecular beam epitaxy and characterized using Fourier-transform infrared spectroscopy and pump-probe transient absorption technique. Photoluminescence spectra were obtained for all samples in 10 to 300K temperature range and then complemented with photoreflectance measurements for characteristic temperatures to increase the sensitivity of the measurement for less optically active transitions. In addition, pump-probe measurements were performed to investigate the dynamics of carrier relaxation and recombination processes in proximity of transition energies observed in previous experiments.
We discuss state-of-the-art mid-infrared light emitters and detectors based on the so-called 6.1 Å family of semiconductors, i.e. InAs, GaSb, and AlSb. Via epitaxial design routines, heterostructures composed of their binary, ternary or quaternary alloys allow unique features such as optically active type-II superlattices enabling light emitters and detectors suitable for the mid-infrared wavelength region. Here we compare and discuss the design differences between interband cascade infrared detectors employing Ga free Type II superlattices and resonant tunneling diodes (RTD) employing the quaternary alloy GaInAsSb. We show that by substituting the standard InAs/GaSb superlattice for a Ga-free superlattice, i.e. InAs/InAsSb, one requires an inverted carrier extraction path. Here it is needed to form a hole-ladder in the electron-barrier, instead of an electron-ladder in the hole-barrier. At elevated temperatures, we observe seven negative-differential-conductance (NDC) regions due to electrons tunnelling through the electron barriers of the seven cascade stages. The detector operates in photovoltaic mode with a cut-off wavelength of 8.5 μm. The RTD photodetector on the other hand utilizes GaInAsSb absorbers that allow efficient operation in the 2-4 μm range with significant electrical responsivity of 0.97 A/W at 2 μm. Contrary to interband cascade infrared detectors, RTD PD operate only at finite voltages and hence these devices are Shot noise limited.
We present recent work on III-V semiconductor mid-infrared light emitters and detectors. The employed type-II broken bandgap alignment between InAs and GaxIn1-xSb allows for widely tunable emission and absorption wavelengths with energies below the individual material bandgaps. We demonstrate room temperature operation of GaSb-based interband cascade lasers (ICLs) emitting between 6.1 and 6.9 μm. Furthermore, we investigate ideal growth conditions for InAs/GaSb type-II superlattices (T2SL) for the implementation in interband cascade detectors (ICDs) with cut-off wavelengths up to 7.5 μm at room temperature. We focus on strain balancing different SL compositions for different cutoff wavelengths via Sb-soak and sub-monolayer (SML) growth of InSb. An ideal growth temperature of TSub=430 °C is found by comparing the quality of different sets of samples by means of high-resolution X-ray diffractometry (HRXRD) and room temperature photoluminescence (PL) measurements.
For many quantum-photonic applications highly efficient and fast single-photon detectors are of utmost importance. Resonant tunneling diode (RTD) photodetectors can be operated as low-noise and high-speed amplifiers of small optically generated electrical signals. For this purpose, RTD photodetectors exploit that the tunneling current is extremely sensitive to changes in the local electrostatic potential, which enables the detection of single photogenerated minority charge carriers, and hence the detection of single photons with the capability of photon-number resolution. Here, we present different RTD device geometries and operation schemes for enhanced quantum-efficiency and operation frequencies.
Molecule and gas sensing is a key technology that is applied in multiple environmental, industrial and medical fields. In particular optical detection technologies enable contactless, nondestructive, highly sensitive and fast detection of even smallest concentrations of trace gases and molecules. During the past years, an increasing demand for mid-infrared (MIR) light sources suitable for, e.g. molecule or gas sensing applications, has driven the development and optimization of novel MIR lasers and light sources, such as quantum cascade lasers (QCL) or interband cascade lasers (ICL). Despite the progress on MIR light sources, there is still a lack in appropriate MIR detectors. Here, we present and discuss two promising and novel GaSb/InAs-based detector concepts. First, resonant tunneling diode (RTD) photodetectors as an alternative to avalanche photodetectors. In RTDs, amplification of photogenerated minority charge carriers is based on modulation of a majority charge carrier resonant tunneling current. Second, interband cascade photodetectors (ICD), in which a cascading scheme allows for fast carrier extraction and a compensation of the diffusion length limitation.
We present antimonide-based resonant tunneling photodetectors with GaSb/AlAsSb double barrier structures and pseudomorphically grown prewell emitter structures comprising the ternary compound semiconductors GaInSb and GaAsSb. Due to the incorporation of GaInSb and GaAsSb prewell emitters, room temperature resonant tunneling with peak-to-valley current ratios of up to 2.4 are shown. The room temperature operation is attributed to the enhanced Γ-Lvalley energy separation and consequently a re-population of the Γ-conduction band of the ternary compound emitter prewell with respect to bulk GaSb. By integration of a quaternary absorption layer, RTDs photodetectors with cut-off wavelengths up to 3 μm have been realized.
We have studied the photocurrent-voltage relation of resonant tunneling diode (RTD) photodetectors by means of electrooptical transport measurements. The investigated RTDs are based on an Al0.6Ga0.4As/GaAs double barrier resonant tunneling structure (RTS) with an integrated GaInNAs absorption layer for light sensing at the telecommunication wavelength of λ= 1.3 μm. Under illumination, photogenerated holes can be captured for accumulation in vicinity to the RTS and modulate the resonant tunneling current that is highly sensitive to changes in the local electrostatic potential. The resulting photocurrent-voltage relation is found to be a nonlinear function of the applied bias voltage, and governed by the interplay of the electronic transport properties of the RTS and the dynamics of photogenerated holes. Time-resolved photocurrent measurements were employed to analyze the dynamics of photogenerated holes. From the photocurrent-time traces the quantum-efficiency and mean lifetime of photogenerated holes can be separately determined. We found that the photoresponse is suppressed by a low quantum efficiency for bias voltages below V ≤ 1 V. In this regime, the built-in electric field prevents photogenerated holes from accumulation at the RTS. For voltages above V >1 V, the built-in field is compensated by the external bias, and η(V) takes on a constant value. In this regime, the RTD photoresponse is mainly determined by the lifetime of holes accumulated at the RTS. The lifetime is limited by thermionic carrier escape and was found to decrease exponentially with the applied bias voltage.
Gas sensing is a key technology with applications in various industrial, medical and environmental areas. Optical detection mechanisms allow for a highly selective, contactless and fast detection. For this purpose, rotational-vibrational absorption bands within the mid infrared (MIR) spectral region are exploited and probed with appropriate light sources. During the past years, the development of novel laser concepts such as interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) has driven a continuous optimization of MIR laser sources. On the other hand side, there has been relatively little progress on detectors in this wavelength range. Here, we study two novel and promising GaSb-based detector concepts: Interband cascade detectors (ICD) and resonant tunneling diode (RTD) photodetectors. ICDs are a promising approach towards highly sensitive room temperature detection of MIR radiation. They make use of the cascading scheme that is enabled by the broken gap alignment of the two binaries GaSb and InAs. The interband transition in GaSb/InAs-superlattices (SL) allows for normal incidence detection. The cut-off wavelength, which determines the low energy detection limit, can be engineered via the SL period. RTD photodetectors act as low noise and high speed amplifiers of small optically generated electrical signals. In contrast to avalanche photodiodes, where the gain originates from multiplication due to impact ionization, in RTD photodetectors a large tunneling current is modulated via Coulomb interaction by the presence of photogenerated minority charge carriers. For both detector concepts, first devices operational at room temperature have been realized.
We demonstrate a cavity-enhanced photodetector at the telecommunication wavelength of λ = 1.3 μm based on a resonant tunneling diode (RTD). The cavity-enhanced RTD photodetector consists of three integral parts: First, a Ga0.89In0.11N0.04As0.96 absorption layer that can be grown lattice-matched on GaAs and which is light-active in the near infrared spectral region due to its reduced bandgap energy. Second, an Al0.6Ga0.4As/GaAs double barrier resonant tunneling structure (RTS) that serves as high gain internal amplifier of weak electric signals caused by photogenerated electron-hole pairs within the GaInNAs absorption layer. Third, an optical distributed Bragg reflector (DBR) cavity consisting of five top and seven bottom alternating GaAs/AlAs mirror pairs, which provides an enhanced quantum efficiency at the resonance wavelength. The samples were grown by molecular beam epitaxy. Electro-optical properties of the RTDs were studied at room temperature. From the reflection-spectrum the optical resonance at λ = 1.29 μm was extracted. The current-voltage characteristics were studied in the dark and under illumination and a wellpronounced photo-response was found and is attributed to accumulation of photogenerated holes in the vicinity of the RTS. The maximum photocurrent was found at the optical resonance of 1.29 μm. At resonance, a sensitivity of S = 3.97 × 104 A/W was observed. From the sensitivity, a noise equivalent power of NEP = 1.18 × 10-16 W/Hz1/2, and a specific detectivity of D∗ ≅ 6.74 × 1012 cm Hz1/2/W were calculated. For a single absorbed photon a photocurrent of ISP = 50 pA was determined.
AlGaAs/GaAs/AlGaAs double-barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby, lattice-matched GaInNAs absorption layer. RTD mesas with ring contacts and an aperture for optical excitation of charge carriers were fabricated on the epitaxial layers. Electrical and optical properties of the RTDs were investigated for different thicknesses of a thin GaAs spacer layer incorporated between the tunnel AlGaAs barrier adjacent to the GaInNAs absorption layer. Illumination of the RTDs with laser light of 1.3 µm wavelength leads to a pronounced photo-effect with a sensitivity of around 1000 A/W.
Novel nanostructured III-V semiconductor devices are investigated for light detection in the near infrared
spectral region. Single-electron memories based on site-controlled InAs quantum dots embedded in a GaAs/AlGaAs
quantum-wire transistor were fabricated and studied. By using a nanohole structure template on a modulation-doped
GaAs/AlGaAs heterostructure, two single InAs quantum dots were centrally positioned in a quantum-wire transistor so
that pronounced shifts of the transistor threshold occur by charging of the QDs with single electrons. Single-electron
read and write functionalities up to room temperature were observed and the memory function can be also controlled by
light with a wavelength in the telecommunication range. Furthermore, AlGaAs/GaAs/AlGaAs double barrier resonant
tunneling diodes (RTD) with an embedded GaInNAs absorption layer have been fabricated for telecom wavelength light
detection at room temperature. The absorption layer was lattice matched grown within the GaAs system of the RTD.
We demonstrate that the devices exhibit typical RTDs characteristic and they are light sensitive at the telecom
wavelength 1.3 μm in the order of just a few nW. Routes to further reduce the detection limit are discussed whereas the
envisaged devices have prospects to deliver sensitivities approaching the quantum limit.
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