Investigation of the quantum dot infrared photodetectors dark current

Quantum dot infrared photodetectors (QDIPs) are more efficient than other types of semiconductor based photodetectors; so it has become an actively developed field of research. In this paper quantum dot infrared photodetector dark current is evaluated theoretically. This evaluation is based on the model that was developed by Ryzhii et al. Here it is assumed that both thermionic emission and field-assisted tunneling mechanisms determine the dark current of QDIPs; moreover we have considered Richardson effect, which has not been taken into account in previous research. Then a new formula for estimating average number of electrons in a quantum dot infrared photodetector is derived. Considering the Richardson effect and field-assisted tunneling mechanisms in the dark current improves the accuracy of algorithm and causes the theoretical data to fit better in the experiment. The QDIPs dark current temperature and biasing voltage dependency, contribution of thermionic emission and field-assisted tunneling at various temperatures and biasing voltage in the QDIPs dark current are investigated. Moreover, the other parameter effects like quantum dot (QD) density and QD size effect on the QDIPs dark current are investigated.     

Physical model for the dark current of quantum dot infrared photodetectors

Dark current has attracted much attention in recent years due to its great influence on the performance of the QDIP. In this paper, a model for the dark current is proposed with the consideration of the influence of the nanoscale electron transport on the dark current based on the dark current model proposed by H.C. Liu. The model permits calculating the dark current as a function of the electric field, and it can further estimate the photocurrent, the current responsivity and the detectivity via the current equilibrium equation under the dark condition. The results obtained show a good agreement with the experimental results and manifest the validity of the proposed model.     

甚长波量子阱红外探测器的暗电流特性研究

基于载流子在量子结构中的输运理论研究了甚长波量子阱红外探测器(峰值响应波长15μm，量子阱个数大于40)的载流子的输运性质.研究结果表明，在甚长波量子阱红外探测器中，电流密度一般很低，暗电流主要来源于能量高于势垒边的热激发电子.通过薛定谔方程和泊松方程以及电流的连续性方程的自洽求解，发现外加偏压下电子浓度在甚长波器件各量子阱的分布发生较大变化，电场在整个器件结构上呈非均匀分布，靠近发射极层的势垒承担的电压远远高于均匀分布的情形.平带模型假定电压在器件体系上均匀分布，导致小偏压下的理论计算值远远低于实验值. 关键词： 甚长波量子阱红外探测器 量子波输运 暗电流     

Dark currents of GaAs/AlGaAs quantum-well infrared photodetectors

We study the dark current of the GaAs/AlGaAs quantum-well infrared photodetector (QWIP) by assuming a drift-diffusion carrier transport in the barriers where the electric fields are obtained by the current continuity condition and the self-consistent energy band structure. It has been shown that due to the current continuity condition, the dark currents across the QWIP devices are determined by the thermionic emission from the emitter to the multiple quantum well (MQW) region. The self-consistent calculation of the Schrödinger and Poisson equations shows a weak electric field in the barrier region connecting to the emitter (much smaller than the average field across the QWIP at low bias) due to the accumulation of carriers in the triangle quantum well formed at the emitter-MQW interface, which results in a very small dark current at low bias. The numerical results explain well our experimental observation.     

Electron transfer based voltage tunable two-color quantum-well infrared photodetectors

We present a detailed investigation of the temperature T dependence of photoresponse of voltage tunable two-color quantum-well infrared photodetectors (QWIPs) that are based on the transfer of electrons between coupled QWs under an applied bias V_b. For T40 K, the peak detection wavelength switches from 7.2 μm under positive bias to 8.6 μm under large negative bias as electrons are transferred from the right QW (RQW) to the left QW (LQW). For T50 K, the short wavelength peak is not only present for both bias polarities but also increases rapidly with T while the long wavelength peak decreases rapidly with T. We investigate this temperature dependence by extracting absorption coefficient and photoconductive gain g using corrugated QWIPs with different corrugation periods. The deduced absorption spectra indicate that the LQW population first increases and then decreases with increasing negative bias for T50 K. The deduced gain spectra show that short and long wavelength gain under negative bias exhibit a strong enhancement and reduction, respectively, with T above 50 K. We show that both these temperature dependences are caused by large thermal currents from the LQWs, which are designed for long wavelength detection and, therefore, have a significantly lower activation energy than the RQWs.     

Physics of non-adiabatic transport and field-domain effect in quantum-well infrared photodetectors

A previous theory for studying the distribution of non-uniform fields in multiple-quantum-well photodetectors under an ac voltage is generalized by including non-adiabatic space-charge-field effects. Numerical calculations indicate that field-domain effects are only important at high temperatures or high voltages when both injection and sequential-tunneling currents are significant. On the other hand, it is found that the non-adiabatic effects included in this generalized theory become significant at low temperatures and low voltages when field-domain effects are negligible. In order to explain the non-adiabatic charge-density fluctuations quantum-statistically, a non-adiabatic differential equation is derived based on the self-consistent Hartree model by using a shifted Fermi–Dirac model for the local fluctuation of electron distributions. The non-adiabatic effect is found to cause an “equilibrium” state variation with time under an ac voltage.     

The transition mechanisms of quantum-dot/quantum-well mixed-mode infrared photodetectors

The transition mechanisms of a 10-period quantum-dot (QD)/quantum-well (QW) mixed-mode infrared photodetector is investigated in this paper. Both mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) responses are observed for the device. The lower normal incident absorption of the LWIR peak suggests that the QW intra-band transition is responsible for the response while the QD intra-band transition for the MWIR response. Due to the coexistence of MWIR and LWIR responses, the MWIR response should be resulted from one-photon transition while the LWIR response from the two-photon transition. To explain the transition mechanisms of the MMIP device, a model is proposed in this paper. The increases of both MWIR and LWIR responses with increasing measurement temperatures observed for the device are attributed to the increase of electrons in the QW ground state/wetting layer state resulted from the increase of one-photon absorption process with increasing temperatures.     

An FDTD approach to the simulation of quantum-well infrared photodetectors

A Finite Difference Time Domain approach is used to design and to optimize quantum-well based infrared photodetectors. Results showing the influence of some parameters on the performance of these devices are presented and discussed.     

Quantum mechanical scattering investigation of the dark current in quantum well infrared photodetectors (QWIPs)

This work focuses on the quantum mechanical evaluation of two components of the dark current in quantum well infrared photodetectors (QWIPs)––field induced emission (FIE) and thermionic emission (TE). The negligible value of the third component of the dark current––sequential tunnelling (ST)––was shown theoretically in previously published work. Calculations are on devices that cover the long wavelength- to very long wavelength-infrared (LWIR to VLWIR) region of the spectrum. The results prove theoretically for the first time various experimentally observed characteristics of these two emission components of the dark current.     

The role of sequential tunnelling in the dark current of quantum well infrared photodetectors (QWIPs)

A quantum mechanical approach is taken to investigate the contribution of sequential tunnelling as a component of the dark current in quantum well infrared photodetectors (QWIPs). Calculations are performed on three different experimentally reported QWIP devices made for different detection wavelengths. The results show that the sequential tunnelling component remains rather constant with different devices, however it is swamped by the thermionic emission components of the dark current at longer wavelengths. The lack of a local maximum in the dark current due to resonant LO phonon emission, which should be observed at short wavelengths, suggests that interface roughness and alloy disorder could be destroying the coherence of the electron wavefunctions between quantum wells.     

Review of current progress in quantum dot infrared photodetectors

Quantum dot infrared photodetectors (QDIPs) have made significant progress after their early demonstration about a decade ago. We review the progress made by QDIP technology over the last few years and compare QDIPs with quantum well infrared photodetectors (QWIPs). It is shown that the performance of QDIPs has significantly improved using novel architectures such as dots‐in‐a‐well designs, and large‐format (1 K × 1 K) focal plane arrays have been realized. However, even though there are significant reports of performance parameters better than QWIPs from single‐pixel devices, QDIP‐based focal plane arrays are still a factor of 3–5 worse in terms of noise equivalent temperature difference. We discuss the reasons for the performance gap and the key scientific and technological challenges that need to be addressed to achieve the full potential of QD‐based technology.     

Modeling of a quantum well infrared detector with a dark current discriminator

A model is developed to provide estimates of the performance of quantum well intersubband infrared photodetectors. By introducing an energy filter to reduce the dark current, the quantum well device performance is improved. The amount of reduction in dark current depends upon the height of the energy filter barrier which can be varied by bias voltage. The energy distributions of the dark current and photocurrent electrons are discussed. Calculated results are presented for detectors with different quantum well widths, including the cases of bound-to-bound and bound-to-continuum transitions. The reduction in dark current results in a higher detectivity, and a substantial improvement over traditional designs can be obtained for some well widths.     

Intersubband transitions in quantum well mid-infrared photodetectors

A study of intersubband transitions in quantum well infrared detectors working at high temperatures has been reported. This study allows a greater tunability in the device designs, with the ability to control the peak wavelength, the absorption coefficient, the dark current, the quantum efficiency and the detectivity of the modeled structure operating around 3.3 μm wavelength. The detection energy and absorption coefficient dependences with an applied electric field are given. Then, the electro-optic performances of the modeled mid-infrared detector are estimated, the dark current dependence with the applied voltage and temperature as well as the quantum efficiency and the detectivity are investigated and discussed. High detectivities were found at high temperatures revealing the good performances of the designed photodetector, especially at 3.3 μm wavelength.     

Resonant-cavity-enhanced photodetectors and LEDs in the mid-infrared

In this paper we outline the use of resonant-cavity enhancement for increasing the exterior coupling efficiency of photodetectors and light-emitting diodes (LEDs) in the mid-infrared (MIR) spectral region. This method is potentially very important in the MIR because encapsulation is not presently feasible due to the lack of suitable materials. Among other potential applications, resonant-cavity-enhanced (RCE) photodetectors and LEDs could be particularly suitable for greenhouse gas detection because of their ‘pre-tunable’ spectrally narrowed resonantly enhanced peaks. We also present the optical characterization of an InAs RCE photodetector aimed at the detection of methane gas (λ≈3.3 μm), and an InAs/InAs_0.91Sb_0.09 resonant-cavity LED (RCLED) aimed at carbon dioxide gas (λ≈4.2 μm). The high peak responsivity of the RCE photodetector was 34.7 A/W at λ=3.14 μm, and the RCLED peaked at λ=3.96 μm. These are among the longest operating wavelengths for III–V RCE photodetectors and RCLEDs reported in the literature.     

Characteristics analysis of quantum wire infrared photodetectors under both dark and illumination conditions

This paper presents a theoretical analysis for the characteristics of quantum wire infrared photodetectors (QRIPs). Mathematical model describing this device is introduced. Maple 4 software is used to device this model. The developed model is used to investigate the behavior of the device with different values of performance parameters such as number of quantum wire layers, lateral characteristic size, and temperature. The modeling results are validated against experimental published work and full agreements are obtained. Several performance parameters are tuned to enhance the performance of these quantum photodetectors through the presented modeling. The resultant performance characteristics and comparison among both quantum well infrared photodetectors (QWIPs) and QRIPs are presented in this work. From the obtained results we notice that the total dark current in the QRIPs can be significantly lower than that in the QWIPs. Moreover, main features of the QRIPs such as the large gap between the induced photocurrent and dark current open the way for overcoming the problems of quantum dot infrared photodetectors (QDIPs).     

HOT infrared photodetectors

At present, uncooled thermal detector focal plane arrays are successfully used in staring thermal imagers. However, the performance of thermal detectors is modest, they suffer from slow response and they are not very useful in applications requiring multispectral detection. Infrared (IR) photon detectors are typically operated at cryogenic temperatures to decrease the noise of the detector arising from various mechanisms associated with the narrow band gap. There are considerable efforts to decrease system cost, size, weight, and power consumption to increase the operating temperature in so-called high-operating-temperature (HOT) detectors. Initial efforts were concentrated on photoconductors and photoelectromagnetic detectors. Next, several ways to achieve HOT detector operation have been elaborated including non-equilibrium detector design with Auger suppression and optical immersion. Recently, a new strategies used to achieve HOT detectors include barrier structures such as nBn, material improvement to lower generation-recombination leakage mechanisms, alternate materials such as superlattices and cascade infrared devices. Another method to reduce detector’s dark current is reducing volume of detector material via a concept of photon trapping detector. In this paper, a number of concepts to improve performance of photon detectors operating at near room temperature are presented. Mostly three types of detector materials are considered — HgCdTe and InAsSb ternary alloys, and type-II InAs/GaSb superlattice. Recently, advanced heterojunction photovoltaic detectors have been developed. Novel HOT detector designs, so called interband cascade infrared detectors, have emerged as competitors of HgCdTe photodetectors.     

4.3 THz quantum-well photodetectors with high detection sensitivity

We demonstrate a high performance GaAs/AlGaAs-based quantum-well photodetector(QWP) device with a peak response frequency of 4.3 THz. The negative differential resistance(NDR) phenomenon is found in the dark current–voltage(I–V) curve in the current sweeping measurement mode, from which the breakdown voltage is determined. The photocurrent spectra and blackbody current responsivities at different voltages are measured. Based on the experimental data, the peak responsivity of 0.3 A/W(at 0.15 V, 8 K) is derived, and the detection sensitivity is higher than 10~(11) Jones,which is in the similar level as that of the commercialized liquid-helium-cooled silicon bolometers. We attribute the high detection performance of the device to the small ohmic contact resistance of ～ 2 ? and the big breakdown bias.     

Uncooled infrared photodetectors in Poland

The history and present status of the middle and long wavelength Hg_1-xCd_xTe infrared detectors in Poland are reviewed. Research and development efforts in Poland were concentrated mostly on uncooled market niche. Technology of the infrared photodetectors has been developed by several research groups. The devices are based on mercury-based variable band gap semiconductor alloys. Modified isothermal vapour phase epitaxy (ISOVPE) has been used for many years for research and commercial fabrication of photoconductive, photoelectromagnetic and other devices. Bulk growth and liquid phase epitaxy was also used. At present, the fabrication of IR devices relies on low temperature epitaxial technique, namely metalorganic vapour phase deposition (MOCVD), frequently in combination with the ISOVPE. Photoconductive and photoelectromagnetic detectors are still in production. The devices are gradually replaced with photovoltaic devices which offer inherent advantages of no electric or magnetic bias, no heat load and no flicker noise. Potentially, the PV devices could offer high performance and very fast response. At present, the uncooled long wavelength devices of conventional design suffer from two issues; namely low quantum efficiency and very low junction resistance. It makes them useless for practical applications. The problems have been solved with advanced 3D band gap engineered architecture, multiple cell heterojunction devices connected in series, monolithic integration of the detectors with microoptics and other improvements. Present fabrication program includes devices which are optimized for operation at any wavelength within a wide spectral range 1–15 μm and 200–300 K temperature range. Special solutions have been applied to improve speed of response. Some devices show picoseconds range response time. The devices have found numerous civilian and military applications. The paper presented there appears in Infrared Photoelectronics, edited by Antoni Rogalski, Eustace L. Dereniak, Fiodor F. Sizov, Proc. SPIE Vol. 5957, 59570K (2005).     

Modeling of intersubband transitions in quantum well infrared photodetectors with complex potential profiles

The subband energy dispersions and optical intersubband transitions in n-type InGaAs/Al _x Ga_1-x As quantum well infrared photodetector (QWIP) with linear-graded barriers are calculated using an 8-band k·p model combined with the envelope-function Fourier expansion. The relaxation of quantum confinement in the growth direction has been taken into reasonable consideration. This work is helpful for the analysis and the design of QWIPs with complex well and barrier structures.