Terahertz Quantum-well Photodetectors Research Articles

Optical coupling enhancement of multi-color terahertz quantum well detector

Multi-color terahertz (THz) detector has attracted much attention in various applications because of the ability to obtain more comprehensive information simultaneously. THz quantum well photodetectors (QWPs) have great advantages in realizing multi-color detection because of high speed, sensitivity, and mature technology. In this work, QWPs based on antenna coupled microcavity (AM-QWP) and etched antenna coupled microcavity (EAM-QWP) structures are proposed to realize multi-color THz detection. Thanks to the combination of the microcavity resonance and surface plasmon polariton mode, AM-QWP achieves a coupling efficiency of one order of magnitude higher than that of the conventional 45° edge facet coupler (45°-QWP) in multiple bands. The EAM-QWP only retains the active region where the effective photocurrent is generated so that the coupling light is highly localized in a small area, improving the optical coupling efficiency by two orders higher compared with 45°-QWP. It is theoretically estimated that the responsivity of AM-QWP and EAM-QWP at the temperature of 4 K is 9.6–24.0 A/W and 78.4–196.0 A/W while their noise equivalent power (NEP) is 5.4 × 10−4–1.1 × 10−3 pW/Hz1/2 and 1.7 × 10−5–3.5 × 10−5 pW/Hz1/2, and the specific detectivity is 4.4 × 1012–8.9 × 1012 and 6.9 × 1013–1.4 × 1014 cm Hz1/2/W, respectively. This work provides a guideline for the experimental realization of high-performance multi-color THz QWPs.

High responsivity random metal grating couplers for terahertz quantum well photodetectors

Terahertz quantum-well photodetectors (THz QWPs) have advantages in constructing THz high-speed communication and fast imaging systems. In this paper, GaAs/(Al,Ga)As THz QWPs with metal random-grating couples are designed, fabricated, and characterized. Both one-dimensional metal random grating devices and a contrast device with 45°-incident facet are fabricated, and their peak responsivities and photocurrent spectra are characterized with the samples soaked in the liquid helium. Compared with 45°-incident coupling, it is found that normal incidence with the random metal grating is an efficient method to improve the responsivity. The random metal grating THz QWP is ∼400% improved in the peak responsivity.

Two‐colour THz quantum well photodetectors

Two-colour terahertz (THz) quantum-well photodetectors are designed, fabricated, and characterised. To realise two-colour detection, a stacked structure is adopted by sequentially growing two one-colour detectors. It is found that the spectra of the stacked device are in accordance with those of the two individual detectors connected in series. The relatively broad spectral bandwidth may enable its use in a THz spectrometer.

Terahertz Quantum Well Photodetectors With Metal-Grating Couplers

In this paper, terahertz (THz) quantum well photodetectors (QWPs) with metal-grating couplers are presented for normal-incident operation. Both one-dimensional and two-dimensional grating samples were fabricated. The photocurrent spectra and the peak responsivity were characterized. The effects of the grating period, fill factor, and shape of the grating patch were investigated. It is found that metal gratings are simple but efficient couplers for THz QWPs, and compared with the 45°-incident facet sample, the peak responsivity of the THz QWP with the optimum grating coupler is improved by 146%.

Frequency Up-Conversion Photon-Type Terahertz Imager.

Terahertz imaging has many important potential applications. Due to the failure of Si readout integrated circuits (ROICs) and the thermal mismatch between the photo-detector arrays and the ROICs at temperatures below 40 K, there are big technical challenges to construct terahertz photo-type focal plane arrays. In this work, we report pixel-less photo-type terahertz imagers based on the frequency up-conversion technique. The devices are composed of terahertz quantum-well photo-detectors (QWPs) and near-infrared (NIR) light emitting diodes (LEDs) which are grown in sequence on the same substrates using molecular beam epitaxy. In such an integrated QWP-LED device, photocurrent in the QWP drives the LED to emit NIR light. By optimizing the structural parameters of the QWP-LED, the QWP part and the LED part both work well. The maximum values of the internal and external energy up-conversion efficiencies are around 20% and 0.5%. A laser spot of a homemade terahertz quantum cascade laser is imaged by the QWP-LED together with a commercial Si camera. The pixel-less imaging results show that the image blurring induced by the transverse spreading of photocurrent is negligible. The demonstrated pixel-less imaging opens a new way to realize high performance terahertz imaging devices.

Optical-phonon-mediated photocurrent in terahertz quantum-well photodetectors

Strong and sharp photocurrent peak at longitudinal optical (LO) phonon frequency (8.87 THz) is found in GaAs/(Al,Ga)As terahertz quantum-well photodetectors (QWPs). Two mesa-structure terahertz QWPs with and without one-dimensional metal grating are fabricated to investigate the behavior of such photoresponse peak. The experimental and simulation results indicate that the photocurrent peak originates from a two-step process. First, at the LO phonon frequency, a large number of non-equilibrium LO phonons are excited by the incident electromagnetic field, and the electromagnetic energy is localized and enhanced in the thin multi-quantum-well layer. Second, through the Frohlich interaction, the localized electrons are excited to continuum states by absorbing the non-equilibrium LO phonons, which leads to the strong photoresponse peak. This finding is useful for exploring strong light-matter interaction and realizing high sensitive terahertz photodetectors.

Dark current mechanism of terahertz quantum-well photodetectors

Dark current mechanisms of terahertz quantum-well photodetectors (THz QWPs) are systematically investigated experimentally and theoretically by measuring two newly designed structures combined with samples reported previously. In contrast to previous investigations, scattering-assisted tunneling dark current is found to cause significant contributions to total dark current. A criterion is also proposed to determine the major dark current mechanism at different peak response frequencies. We further determine background limited performance (BLIP) temperatures, which decrease both experimentally and theoretically as the electric field increases. This work gives good description of dark current mechanism for QWPs in the THz region and is extended to determine the transition fields and BLIP temperatures with response peaks from 3 to 12 THz.

Terahertz quantum well photo-detectors: grating versus 45° facet coupling

We demonstrate two GaAs/AlGaAs-based terahertz (THz) quantum well photo-detectors (QWPs), with one-dimensional (1D) metal grating and 45° facet light coupling configurations. The two THz QWPs have the same multi-quantum well structure parameters. The different light coupling configurations do not affect the peak response frequency (5.85 THz). The background limited infrared performance temperatures and responsivities of the two devices are measured. The effects of different light coupling configurations on the main features of the devices are investigated in detail. The experimental results show that the performance of the THz QWP with 1D metal grating is a little better but there is still a lot of room for improvement.

Terahertz quantum-well photodetectors: Design, performance, and improvements

Theoretical studies and numerical simulations on design, performance, and improvements of terahertz quantum-well photodetector (THz QWP) are presented. In the first part of this paper, we discuss the device band structure resulting from a self-consistent solution and simulation results. First, the temperature dependence of device characteristics is analyzed. Next, we deduce the condition of optimal doping concentration for maximizing dark current limited detectivity Ddet* when QWP is lightly doped. Accordingly, unlike in previously published reports, doping concentration is not fixed and is selected by the above condition. In the second part of this paper, we propose two schemes for improving operation temperature. The first is to incorporate an optical antenna which focuses incident THz wave. Numerical results show that the QWP with peak frequency higher than 5.5 THz is expected to achieve background-noise-limited performance at 77 K or above when employing a 106 times enhancement antenna. The second scheme is to use a laser as the signal source to achieve photon-noise-limited performance (PLIP) at high temperatures. Simulations show that when operating below critical temperature QWPs in the range of 1 ∼ 7 THz can reach PLIP under practical illumination intensities.

Negative differential resistance induced by thermalization of two-dimensional electrons in terahertz quantum-well photodetectors

Negative differential resistance (NDR) behavior existing in dark current-voltage (IV) curves of terahertz quantum-well photodetectors (QWPs) is theoretically investigated. Due to electron-electron scattering, the localized two-dimensional (2D) electrons in terahertz QWPs are thermalized. In steady state, the effective temperature of the 2D electrons is found to be higher than that of lattice. A self-consistent model is used to simulate the dark IV curves of terahertz QWPs, taking into account the thermalization effect of the 2D electrons. The NDR behavior is qualitatively reproduced. The periodic structures of electric-field domain and 2D electron occupation are formed in the NDR regime. The improved self-consistent model is useful for further understanding of the electron transport properties and improving the performance of terahertz QWPs.

Recent Progress in Terahertz Quantum-Well Photodetectors

Terahertz quantum-well photodetectors (QWPs) represent a new and emerging photon-type detector in a terahertz region. Recent progress in the development of terahertz QWPs is reviewed. We first discuss the many-particle effects on the accurate design of terahertz QWPs. Second, three types of light couplers for terahertz QWPs are introduced. At resonant coupling frequencies, the polarization of light field is effectively changed by the light couplers to fulfill the selection rule of intersubband transition. Meanwhile, the electric field intensities in the active multiquantum-well region of terahertz QWPs are enhanced. The performance of terahertz QWPs with these light couplers is improved significantly. Finally, terahertz-QWP-based wireless communication and imaging are demonstrated.

THz Imaging Using a Quantum-Well Photodetector with Background Limited Performance

Terahertz (THz) quantum well photodetectors (QWPs) are an extension of quantum well infrared photodetectors in the THz region. We construct an imaging system based on a THz QWP with a narrow response range from 3 THz to 6 THz. The peak responsivity of the THz QWP having background-limited performance is about 0.5 A/W, and the corresponding detectivity reaches 1011 cm·Hz1/2/W at temperature of 4.2 K. We obtain the images of a concealed object by the imaging system and prove that THz QWPs have the potential for imaging applications.

Surface Plasmon-Enhanced Absorption in Metal Grating Coupled Terahertz Quantum Well Photodetectors

Effects of surface plasmon polaritons (SPPs) on the absorption efficiencies of metal-grating-coupled terahertz quantum well photodetectors (QWPs) are theoretically investigated. The thin top contact layers of the devices are utilized to support the SPPs, and the metal gratings couple p-polarized incident light into SPP modes. The electric fields related to the SPP modes have large components in the quantum well growth direction and are mostly localized in the multi-quantum-well regions of the terahertz QWPs. The dependences of the resonant absorption behaviors on the device parameters are systematically studied. A maximum value of about 30-fold enhancement of inter-sub-band absorption efficiency is achieved in comparison with that of a traditional 45-degree facet coupling scheme. Our proposed metal-grating-coupled terahertz QWPs is expected to significantly improve the device performance.

Numerical Study on Metal Cavity Couplers for Terahertz Quantum-Well Photodetectors

Metal cavities composed of metal gratings, layered semiconductor structures, and bottom metal layers are proposed as light couplers for terahertz quantum-well photodetectors (QWPs). Due to a strong waveguide effect, the coupling properties of the metal cavities are significantly changed in comparison with the case of metal-grating-coupled terahertz QWPs. Numerical calculations show that there are two resonant coupling peaks for the sub-wavelength metal cavity couplers, with one located at the lower side of the first grating diffraction mode and the other at the upper side. A simple ray propagation picture explains the behaviors of the two resonant coupling peaks on a qualitative level. Our numerical data show that the coupling efficiencies of metal cavity couplers are several tens of times higher than that of the 45-degree facet coupling scheme.

Coupling efficiency of lamellar gratings for terahertz quantum-well photodetectors

One-dimensional (1D) etched gratings coated with a perfect metal are used to couple light into terahertz (THz) quantum-well photodetectors (QWPs). The coupling efficiencies of these reflective gratings are simulated by using the finite element method (FEM). The results show that simple optics arguments used in the design of gratings for quantum well infrared detectors (QWIPs) are not suitable in the THz region. The near-field effect is important in the coupling, and a “standing-wave effect” is found in the grating stripes. Optimizations of the gratings are also discussed.

Near field and cavity effects on coupling efficiency of one-dimensional metal grating for terahertz quantum well photodetectors

The Modal Method is employed to simulate the coupling between the terahertz wave and one-dimensional (1D) transmissive metal grating on the top of terahertz quantum well photodetectors (THzQWPs). Electrical field patterns and behaviors of 1D grating at different frequencies and device thicknesses are systematically studied. The results show that, the coupling efficiency is not only determined by the grating parameters, multiple reflections in the device and the subsequent multiple diffractions at the grating also play an important role. Different diffracted modes interact with each other, and near field effect caused by the evanescent waves are essential in evaluating the coupling efficiency especially when the frequencies are below the cutoff of the gratings. The optimization conditions of the performance of 1D metal grating coupled THzQWPs are also discussed.

Photocurrent spectra of heavily doped terahertz quantum well photodetectors

Terahertz (THz) quantum well photodetectors (QWPs) are an important candidate for THz imaging and THz free space communication. Low absorption efficiency of THz QWPs is the main factor limiting the performance of this kind of THz detectors. To increase the absorption efficiency, three heavily doped THz QWPs were fabricated. The band structure calculations show that the second subband falls into the quantum well with increasing Si doping concentration, which decreases the escape probability of the photon-excited electrons in the second subband. This effect is responsible for the blueshift and broadening of the photocurrent peak with increasing Si doping.

Emission spectra of terahertz quantum-cascade lasers based on the terahertz quantum-well photodetectors

The emission of a 41 THz quantum cascade laser (THz QCL) has been measured by a spectrally-matching terahertz quantum-well photodetector (THz QWP). The measured emission spectra have been analyzed with respect to the spectral shape and peak position. The relative light power of the laser under different drive currents have been estimated from the emission spectra and then the lasing domain and threshold current density of the laser has been determined. The temperature-dependent response of THz QWP to the laser has also been investigated. The analyzed results indicate that the THz QWPs could be a good detector for characterizing the emission properties of the THz QCLs and may be the receivers in the terahertz communications.

Temperature dependence of current–voltage characteristics of terahertz quantum-well photodetectors

We have performed current–voltage (I–V) measurements on a terahertz quantum-well photodetector (QWP) at different temperatures and employed an emission-capture model to simulate the I–V curves. A temperature-dependent vertical electron drift mobility has been used to fit the curves from 7 K to 20 K. Photocurrents caused by 300 K background radiation have also been measured at different temperatures and a background-limited infrared performance (blip) temperature of 12 K for this terahertz detector has been determined. The current–temperature (I–T) curves derived from the measured dark I–V curves indicate that the thermionic emission process is the major mechanism for dark current in this terahertz detector.

Effect of optical phonons on the spectral shape of terahertz quantum-well photodetectors

Abstract We calculate the spectral response of a terahertz (THz) quantum-well photodetector designed for 7 THz detection peak frequency. We incorporate the effect of GaAs optical phonons which give rise to a strong absorption in the region of 34–36 meV and result in an increase in reflection due to the large refractive index ( n ) change around this region. Comparing the calculated spectral shape with experiments, we show the improvement over the standard expressions.