Heterojunction Interfacial Workfunction Internal Photoemission Research Articles

High-temperature photon-type ultra-broadband detectors based on ratchet structure

Achieving a photodetector with high bandwidth, high sensitivity, high speed, and high operating temperature is a common goal for researchers, especially for infrared and terahertz regions due to its important application in various fields like military defense and biochemical analysis. Among photon-type terahertz (THz) detectors, heterojunction interfacial work function internal photoemission (HEIWIP) detectors demonstrate high bandwidth, high sensitivity, and high speed, but are limited by the low operating temperature. In this work, we developed an effective strategy for raising its operating temperature by introducing ratchet structures. By comparing with traditional HEIWIP, the ratchet HEIWIP exhibits a low dark current and high detectivity while retaining the ultra-broad response bandwidth. The operating temperature has been raised from 7 to 20 K in the terahertz region, which is meaningful progress in THz photon-type detection. The universality of this scheme makes it possible to be further migrated to general photon-type semiconductor detectors.

Ultra-broadband frequency up-converter based on AlGaAs\GaAs heterojunction detector

The development of high performance is essential for the realization of ultra-broadband imaging technology, which can be widely used in astronomy, medical health, defense and security, and many other applications. In this work, we demonstrate an ultra-broadband upconversion device based on a heterojunction interfacial work function internal photoemission (HEIWIP) detector and an AlGaAs/GaAs/In0.04Ga0.96 As quantum well LED. The up-converter exhibits an ultra-broad response covering from the terahertz to mid-infrared region (3–165 THz) due to free carrier absorption and inter-valence-band absorption in p-type based photodetectors. The peak responsivity in the terahertz and mid-infrared region is measured to be 7.2 A/W at 9.2 THz and 3.5 A/W at 28.8 THz, respectively, which is improved by an order of magnitude compared to the previous homojunction interfacial work function internal photoemission-based up-converter (0.5 A/W) in the THz region. The up-converter also demonstrates an improved upconversion efficiency of 1.5%. The noise equivalent power (NEP) of the imaging system consisting of the HEIWIP-LED up-converter and Si CCD is studied. The optimized NEP can achieve ∼10 pW/Hz1/2 in both terahertz and mid-infrared regions. The HEIWIP-LED up-converter exhibits excellent properties of high responsivity, ultra-broad response, and low noise, making it a great prospect for future applications in the field of ultra-broadband imaging.

Surface plasmon enhanced IR absorption: Design and experiment

Metal corrugated surfaces have the potential of enhancing optical absorption through surface plasmon (SP) excitation facilitated by light-metal interactions. The successful utilization of metal corrugation induced optical absorption can improve the response of free carrier absorption (FCA), based HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors. This article reports theoretical and experimental investigations of SP-induced infrared (IR) absorption in GaAs based structures, exhibiting absorption peak wavelength ( λ p ) in the 10–14 μm region. Moreover, substantial absorption and responsivity improvement using metallic corrugation on typical heterojunction detector is predicted. Proposed design and optimization approaches will be useful for improving the performance of IR detectors.

Effect of emitter thickness on the spectral shape of heterojunction interfacial workfunction internal photoemission detectors

Results are presented showing the effect of emitter layer thickness on the shape of the spectral response of heterojunction interfacial workfunction internal photoemission detectors. The results confirm that thicker emitters increase the response at shorter wavelengths. A model is developed to explain the experimentally observed blueshift in the peak wavelength with increased emitter thickness, using a combination of hot-cold carrier scattering and phonon emission processes. The study provides a tool for designing detectors exhibiting different peak responses, as demonstrated by evaluating the design parameters for the 8–14μm spectral range.

Operating temperature and the responsivity of split-off band detectors

A GaAs/AlGaAs heterojunction is used as a spin-split-off band IR detector operating at or around room temperature. This detector structure followed a similar layer architecture to the quantum well IR photo detectors (QWIP) and Heterojunction Interfacial Work function Internal Photoemission (HEIWIP) detectors. Compared to QWIPs, the emitter layer thickness is increased to avoid confinement. Unlike either the QWIPs or HEIWIPs, these detectors will have two energy gaps (barriers) to obtain the wavelength threshold which could be used to design detectors either for optimum operating temperature or optimum responsivity. The free carrier energy gap is determined by the Al fraction and the spin-split-off transition energy provides another handle on controlling the effective threshold of the detector. Unlike QWIPs, these will also detect normal incidence radiation. A preliminary detector showed a peak responsivity of 0.29 mA/W at 2.5 μm at room temperature.

Dopant Migration-Induced Interface Dipole Effect in n-Doped GaAs/AlGaAs Terahertz Detectors

A heterojunction interfacial workfunction internal photoemission (HEIWIP) terahertz detector with ~1times1018cm-3 n-type doped GaAs emitters in a multilayer GaAs/Al0.13Ga0.87 As heterostructure is presented. The detection mechanism is based on free carrier absorption with a broad response extending to ~ 5.26 THz (57 mum), corresponding to an effective workfunction of ~ 21.8 meV, which is much smaller than the offset expected for an Al fraction of x = 0.13 at a 1times1018 cm-3 doping. This is attributed to a reduction of the conduction band offset by interface dipole formation between the accumulated negative charges at the interface states and migrated positively charged donors in the barrier. The device has a peak responsivity of 0.32 A/W at ~ 26 mum at 5 K. It is demonstrated that the dopant migration-induced interface dipole effect can be used to extend the zero response threshold frequency (f 0) of n-type HEIWIP detectors.

Enhanced light absorption in GaN∕AlGaN midinfrared detectors and application for pixel-less upconversion imaging

Based on the experimental responsivity of single-period GaN∕AlGaN midinfrared (MIR) heterojunction interfacial work-function internal photoemission (HEIWIP) detectors, we have optimized in detail the single- and multiperiod GaN∕AlGaN MIR HEIWIPs to enhance the light absorption in the detectors. The yielded parameters of emitter, intrinsic, and bottom contact layers could achieve large improvement of the MIR peak absorption efficiency in the multiperiod GaN∕AlGaN MIR HEIWIPs. Employing the concept of photon frequency upconversion, we have further investigated the realization of MIR pixel-less upconversion imaging through the integration of GaN∕AlGaN MIR HEIWIP with a GaN∕AlGaN violet light-emitting diode (LED). Under the optimized device structures, good imaging characteristics and high upconversion efficiency could be expected in the GaN∕AlGaN HEIWIP-LED MIR upconverter.

Effects of a p–n junction on heterojunction far infrared detectors

HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) far infrared detectors based on the GaAs/AlGaAs material system have shown promise for operation at wavelengths up to a few hundred microns. HEIWIP detectors with GaAs emitters have been shown to operate out to 92μm. Recent modifications to use AlGaAs emitters have extended the zero response threshold out to 128μm. Extension to longer wavelengths will require reducing the dark current in the devices. An approach using the addition of a p–n junction in the detector, which was shown to work in QWIP and homojunction detectors is considered here. Differences between the predicted and observed threshold behavior could be explained by the presence of space charge within the device. The band bending from this space charge produces the observed variation in the threshold. The space charge can also be used to explain anomalous conduction observed at low biases. When the device is forward biased, the current is expected, to be small until the bias voltage is similar to the bandgap of 1.4eV, above which the current should increase rapidly. Dark current was observed for biases significantly less than the bandgap. The threshold bias decreased with temperature, and was as low as 0.25V for a temperature of 300K. This is much lower than could be explained by thermal effects alone.

Quantum mechanical effects in internal photoemission THz detectors

The variation in spectral shape around the threshold frequencies between model and experimental responsivity spectra in heterojunction interfacial workfunction internal photoemission (HEIWIP) infrared detectors was investigated. This is attributed to the loss of photoexcited carriers, within the escape cone, prior to photoemission. The energy dependent transmission of excited carriers is incorporated to the existing photoemission model to show that emission around the threshold frequency is reduced considerably by quantum mechanical reflection of photoexcited carriers as observed in experimental results. In the new model, the photoemission of carriers become bias dependent.

Si doped GaAs/AlGaAs terahertz detector and phonon effect on the responsivity

Terahertz detection capability of an n-type heterojunction interfacial work function internal photoemission (HEIWIP) detector is demonstrated. Threshold frequency, f0, of 3.2THz (93μm) was obtained by using n-type GaAs emitter doped to 1×1018cm−3 and Al0.04Ga0.96As single barrier structure. The detector shows a broad spectral response from 30 to 3.2THz (10–93μm) with peak responsivity of 6.5A/W at 7.1THz under a forward bias field of 0.7kV/cm at 6K. The peak quantum efficiency and peak detectivity are ∼19% and ∼5.5×108Jones, respectively under a bias field of 0.7kV/cm at 6K. In addition, the detector can be operated up to 25K.

Spin split-off transition based IR detectors operating at high temperatures

GaAs/AlGaAs based Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors were used to demonstrate experimental split-off response that is based on hole transitions between light/heavy hole bands and the split-off band (spin-orbit). Preliminary results indicate that, this detection mechanism is more efficient than free carrier mechanism for NIR operation. An unoptimized, GaAs/AlGaAs detector with a free carrier threshold wavelength of ∼20μm showed a maximum operating temperature of 130K for split-off response in the range 1.5–5μm with a peak D∗ of 1.0×108Jones. By adjusting the free carrier threshold to match the split-off threshold, it should be feasible to further increase the operating temperature. Analysis indicates that practical devices with properly optimized parameters are capable of achieving room temperature operation with higher specific detectivity. The possible ways to tailor the threshold, for the split-off response to different wavelength rangers using different materials such as phosphides and nitrites are also discussed.

Quantum structures for multiband photon detection

AbstractThe work describes multiband photon detectors based on semiconductor micro-and nano-structures. The devices considered include quantum dot, homojunction, and heterojunction structures. In the quantum dot structures, transitions are from one state to another, while free carrier absorption and internal photoemission play the dominant role in homo or heterojunction detectors. Quantum dots-in-a-well (DWELL) detectors can tailor the response wavelength by varying the size of the well. A tunnelling quantum dot infrared photodetector (T-QDIP) could operate at room temperature by blocking the dark current except in the case of resonance. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunnelling, while the dark current is blocked by AlGaAs/InGaAs tunnelling barriers placed in the structure. A two-colour infrared detector with photoresponse peaks at ∼6 and ∼17 μm at room temperature will be discussed. A homojunction or heterojunction interfacial workfunction internal photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect near infrared (NIR) photons due to interband transitions and mid/far infrared (MIR/FIR) radiation due to intraband transitions. The threshold wavelength of the interband response depends on the band gap of the barrier material, and the MIR/FIR response due to intraband transitions can be tailored by adjusting the band offset between the emitter and the barrier. GaAs/AlGaAs will provide NIR and MIR/FIR dual band response, and with GaN/AlGaN structures the detection capability can be extended into the ultraviolet region. These detectors are useful in numerous applications such as environmental monitoring, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.

Terahertz absorption in AlGaAs films and detection using heterojunctions

HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors using AlGaAs as both the emitter and the barrier material with different Al fractions for the two layers are demonstrated. The extension of the HEIWIP concept to wavelengths longer than 110 μm in the GaAs/AlGaAs system requires the use of AlGaAs as the emitter material to reduce the barrier height. The p-type doping produces an offset in the valance band between doped and undoped material. The Al fraction difference then gives a valance band offset in the opposite direction, which reduces the total offset. The FIR absorption up to ∼400 μm for AlGaAs films with different Al fractions and doping are presented. The absorption in the films with low Al fraction (1%) shows little variation from comparable GaAs films while for 20% Al, the absorption is reduced. The spectral results on a device with 12% Al emitters and 11% Al barriers have shown a response of 0.6 A/W at 30 μm with D ∗ = 3 × 10 10 Jones measured at 5 K. The low responsivity is due to the reduced number (3) of emitters in the test device, and when scaled for the number of emitters this is comparable to results obtained from GaAs/AlGaAs HEIWIP detectors. Based on these results, a design for a 300 μm detector is presented and potential difficulties in growth and fabrication such as dopant migration are discussed.

AlGaAs emitter∕GaAs barrier terahertz detector with a 2.3 THz threshold

A heterojunction interfacial work function internal photoemission (HEIWIP) detector with a threshold frequency (f0) of 2.3 THz (λ0=128μm) is demonstrated. The threshold limit of ∼3.3THz (92 μm) due to the Al fraction being limited to ∼0.005, in order to avoid control and transition from alloy to isoelectronic doping behavior, was surpassed using AlGaAs emitters and GaAs barriers. The peak values of responsivity, quantum efficiency, and the specific detectivity at 9.6 THz and 4.8 K for a bias field of 2.0kV∕cm are 7.3A∕W, 29%, 5.3×1011 Jones, respectively. The background-limited infrared photodetector temperature of 20 K with a 60° field of view was observed for a bias field of 0.15kV∕cm. The f0 could be further reduced toward ∼1THz regime (∼300μm) by adjusting the Al fraction to offset the effect of residual doping, and/or lowering the residual doping in the barrier, effectively lowering the band bending.

Design and optimization of GaAs∕AlGaAs heterojunction infrared detectors

Design, modeling, and optimization principles for GaAs∕AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) infrared detectors for a broad spectral region are presented. Both n-type and p-type detectors with a single emitter or multiemitters, grown on doped and undoped substrates are considered. It is shown that the absorption, and therefore responsivity, can be increased by optimizing the device design. Both the position and the strength of the responsivity peaks can be tailored by varying device parameters such as doping and the thickness. By utilizing a resonant cavity architecture, the effect of a buffer layer on the response is discussed. Model results, which are in good agreement with the experimental results, predict an optimized design for a detector with a peak response of 9A∕W at 26μm with a zero response threshold wavelength λ0=100μm. For a λ0=15μm HEIWIP detector, background limited performance temperature (BLIP temperature), for 180° field of view (FOV) is expected around 80K. For a λ0=70μm optimized design, a highly doped n-type substrate could increase the peak detectivity from 1.7×1010to3.4×1010Jones at a FOV=180° operated at temperatures below T<TBLIP=13K. Intrinsic response times on the order of picoseconds are expected for these detectors.

20 μm cutoff heterojunction interfacial work function internal photoemission detectors

Results are reported on Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors designed for operation up to 20 μm. The peak response of 100 mA/W at 12.5 μm with a D* of 2×1011 Jones was observed with a cutoff wavelength of ∼20 μm. The BLIP temperature for the devices was 40 K at 1.5 V bias. While the peak response remained almost constant (∼95 mA/W) up to 40 K, the D* reduced to 5×109 Jones due to the increased dark current. The response increased with doping while the dark current did not change significantly. Hence, higher responsivity and D* can be expected for designs with higher doping. Designs utilizing increased reflection from the bottom contact are suggested to improve the resonant cavity enhancement for optimizing the detectors, which should lead to higher D* and BLIP temperature.

Resonant cavity enhancement in heterojunction GaAs/AlGaAs terahertz detectors

The room-temperature absorption and reflection spectra in the range of 5–100 μm (3–60 THz) for multilayer heterojunction interfacial work function internal photoemission (HEIWIP) GaAs/AlGaAs far-infrared (FIR) detectors are presented. Calculated results based on the free carrier absorption and interaction with optical phonons are found to be in good agreement with the experimental results. Experimental responsivity spectra demonstrate the expected maxima from the absorption measurements due to resonant cavity effects. It is shown that the resonance cavity architecture enhances the performance of the FIR HEIWIP detectors and further improvement is proposed through the use of n++ and p++ bottom contact layers or doped substrates.

Heterojunction interfacial workfunction internal photoemission detectors for use at 8–20 [micro sign

Heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors have already been demonstrated for use in the FIR range with cutoff wavelengths (λc) up to 85 µm. An interesting feature of these detectors is their strong response in the 8–20 µm range. Initial results are reported on HEIWIP detectors operating in the 8–20 µm range. The unoptimised detectors had a peak responsivity of 1.8 A/W at 15.2 µm at 4.2 K. HEIWIPs could be an alternative to already available detectors especially when optimised for higher temperatures. λc can be tailored by changing the Al concentration in the barrier. Responsivity of HEIWIPs can be selectively optimised by using the cavity mechanism and adjusting the Al fraction. It should also be possible to enhance the responsivity by changing the doping concentrations of the detectors.

Cutoff tailorability of heterojunction terahertz detectors

Heterojunction interfacial work function internal photoemission (HEIWIP) detectors provide an interesting approach to the development of quantum detectors for the terahertz range. In this letter, the cutoff frequency/wavelength variation of HEIWIP detectors having different Al fractions in AlGaAs/GaAs structures is experimentally verified, and a model is presented for designing the structures. A key feature of HEIWIP responsivity is the ability to cover a broad frequency range in a single detector with cutoff tailorability by adjusting the Al fraction in the barrier regions. Extending the response to lower frequencies by the use of AlGaAs emitters and GaAs barriers is also discussed.

QUANTUM STRUCTURES FOR FAR-INFRARED DETECTION

FIR photon detector development starting from the extrinsic detectors for LWIR to FIR wavelengths are presented. Several other types of IR detectors, including the cut-off wavelength extension into the FIR range for quantum well infrared photodetectors (QWIPs), are summarized. Efforts in developing p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared detectors and the most reason developments on GaAs/AlGaAs Heterojunction interfacial workfunction internal photoemission (HEIWIP) far-infrared detectors are presented.