Long wavelength infrared (LWIR) photodetection in a modulation doped thyristor

A modulation doped thyristor concept is described for LWIR photodetection based upon intersubband bound to continuum absorption. The intersubband absorption generates photocurrent from undoped quantum wells to modulation doped layers (MDL). Due to the lower dark current compared to conventional quantum well infrared photodetectors (QWIPs), the thyristor infrared detector operates with little or no cooling and with similar or better performance than QWIPs at low temperatures. The operating characteristics of absorption coefficient, quantum efficiency, responsivity, detectivity, infrared gain, and dark current are determined as a function of thyristor voltage and input power level in the range of 1 μW/cm².     

Electric field redistribution under IR radiation in quantum well infrared photodetectors as deduced from current noise measurements at low temperature and bias

Current noise has been investigated in AlGaAs/GaAs quantum well infrared photodetectors (QWIPs), having nominally the same design except the number of wells N. The experiments have been carried out in the dark and under infrared (IR) radiation in a wide range of currents and temperatures. We have found that the current noise scales as the inverse of the number of wells N in the dark condition. In the presence of IR radiation, the noise exhibits strong deviations from the simple 1/N behavior. These effects are still more evident when the photocurrent noise is measured at low biases and temperatures. Nonlinear effects in the QWIPs operation at high IR power, related to the potential redistribution in the interelectrodic region, are probably responsible for such anomalies. This conclusion is consistent with results of the steady-state responsivity obtained in the QWIPs in the same experimental conditions.     

Arguments for p-type Si 1 − xGe x/Si quantum well photodetectors for the far- and very-far (terahertz)-infrared

An analysis, by a carrier scattering approach, of the thermionic emission contribution to the dark current is carried out in conventional bound-to-continuum quantum well infrared photodetectors (QWIPs). It is found that the thermionic emission increases with increasing temperature or when extending the detection wavelength from mid- to far-infrared. Considering p-type instead of n-type material, however, the increased effective mass decreases the thermionic emission. Designs for mid- and far-infrared p-type QWIPs based on the Si _1 − xGe _x/Si system are discussed for both normal and non-normal incident geometries.     

Modulation-doping in 3–5 μm GaAs/AlAs/AlGaAs double barrier quantum well infrared photodetectors: an alternative to achieve high photovoltaic performance and high temperature detection

In this work we propose new detector designs, which allow achieving mid-infrared photovoltaic (PV) detection at temperatures as high as 180 K. The devices, which are grown by molecular beam epitaxy, are modulation-doped (MD) double barrier quantum well infrared photodetectors (QWIPs) based on AlGaAs/AlAs/GaAs. As the photocurrent spectra and I–V characteristics (in the dark and under infrared illumination) show that the dopant location is a relevant design parameter regarding the performance of PV QWIPs, we begin our work with a comparison of the performance of a set of MD samples (where we have varied the dopant location in the AlGaAs barriers) with respect to a well-doped sample of nominally the same structure. We find that the responsivity and detectivity of the MD devices seem to be higher than those of the well-doped detector, specially when the dopant is located in the substrate-sided barrier. Then, in order to improve the dark current-limited performance, we designed a new set of substrated-sided MD detectors that exhibit an extremely low dark current, even at high temperatures, otherwise no drop in the zero bias peak responsivity. Therefore, the association of the notable PV signal detection in the 3–5 μm range of these MD detectors together with the dark current reduction of the new structures has allowed us to achieve a 140 K zero bias peak responsivity of 0.015 A/W and a 180 K zero bias peak responsivity of 0.01 A/W at 4.4 μm.     

Characteristics and developments of quantum-dot infrared photodetectors

Quantum dots infrared photodetectors (QDIPs) theoretically have several advantages compared with quantum wells infrared photodetectors (QWIPs). In this paper, we discuss the theoretical advantages of QDIPs including the normal incidence response, lower dark current, higher responsivity and detectivity, etc. Recent device fabrication and experiment results in this field are also presented. Based on the analysis of existing problems, some approaches that would improve the capability of the device are pointed out.     

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.     

Photoluminescence and dark current of p-doped InGaAs/AlxGa1−xAs strained multiple quantum wells

We report the temperature dependence of the photoluminescence spectra and current-voltage (I_d-V) characteristics of p-doped In_0.15Ga_0.85As/AlGaAs quantum well infrared photodetectors (QWIPs) with different barrier heights grown by molecular beam epitaxy (MBE). The dark current at low temperatures is found to be about three orders of magnitude lower than that reported for the n- and p-doped QWIPs made of other material systems. The PL spectra show two emission peaks which correspond to an intersubband absorption and are tunable by changing the mole fraction of Al. The low energy emission peak of the In_0.15Ga_0.85As/Al_0.45Ga_0.55As QWIP is found to be much lower in intensity than that of the high one, due possibly to excess Al which may result in defects or imperfection at/or near well-barrier interfaces.     

Comparative study between different quantum infrared photodetectors

This paper presents a theoretical analysis for the dark current characteristics of different quantum infrared photodetectors. These quantum photodetectors are quantum dot infrared photodetectors (QDIP), quantum wire infrared photodetectors (QRIP), and quantum well infrared photodetectors (QWIP). Mathematical models describing these devices are introduced. The developed models accounts for the self-consistent potential distribution. These models are taking the effect of donor charges on the spatial distribution of the electric potential in the active region. The developed model is used to investigate the behavior of dark current with different values of performance parameters such as applied voltage, number of quantum wire (QR) layers, QD layers, lateral characteristic size, doping quantum wire density and temperature. It explains strong sensitivity of dark current to the density of QDs/QRs and the doping level of the active region. In order to confirm our models and their validity on the practical applications, a comparison between the results obtained by proposed models and that experimentally published are conducted and full agreement is observed. Several performance parameters are tuned to enhance the performance of these quantum photodetectors through the presented modeling. The resultant performance characteristics and comparison among them 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.     

Dark current modeling of InP based SWIR and MWIR InGaAs/GaAsSb type-II MQW photodiodes

This paper reports the dark current characteristics of SWIR and MWIR p–i–n photodiodes with type-II InGaAs/GaAsSb multiple quantum wells as the absorption region. A bulk based model with the effective band gap of the type-II quantum well structure has been used. We investigated the dark current contributing mechanisms that are limiting the electrical performance of these photodiodes. The quantitative simulation of the I–V characteristics shows that the performance of InGaAs/GaAsSb photodiodes is dominated by generation-recombination component at the temperature between 200 and 290 K for reverse biases below 5 V. Trap-assisted tunneling current and direct tunneling current begin to dominate when the reverse bias is higher than 10 V.     

Nonparabolic tendency of electron effective mass estimated by confined states in In0.53Ga0.47As/In0.52Al0.48As multi-quantum-well structures

We observed that a series of peaks, which were clearly extracted from a photocurrent difference spectrum, corresponded to interband optical transitions of an In_0.53Ga_0.47As /In_0.52Al_0.48As multi-quantum wells structure. The nonparabolic tendency of the electron effective mass was suggested from eigenenergies of conduction subbands. The effective mass estimated in a direction normal to the InGaAs quantum well plane was heavier than the effective mass of the bulk band edge and was 0.07 m₀at the top of a quantum potential well.     

Time-resolved electron transport studies on InGaAs/GaAs-QWIPs

Due to the short internal response time, quantum-well infrared photodetectors (QWIPs) are interesting for high-speed applications such as heterodyne spectroscopy or laser pulse monitoring. We studied the photocurrent transients of InGaAs/GaAs-QWIPs after irradiation with infrared laser pulses of 250 fs duration. The excitation wavelength of about 9 μm matches the peak wavelength of the QWIP structure. The photocurrent transient consists of two different dynamical components, representing the fast photoionization in the quantum-wells and the slow injection current that compensates the remaining space charge. The investigations of the different components as a function of temperature and bias voltage were performed on a nanosecond time-scale. The experimental separation of the two photocurrent contributions allows us to determine the photoconductive gain. The Fourier transform of the photocurrent transient was compared with other experimental methods including heterodyne detection and microwave rectification. The quantitative agreement between these different measurement techniques is excellent.     

Photoelectrochemistry of monoclinic ZnP2: A promising new solar cell material

For the first time monoclinic p-type ZnP₂ has been used as photocathode material in a photoelectrochemical cell. In lM NaOH a saturation photocurrent of 8mA/cm² is observed (I = 120 mW/cm²) while the dark current is negligible. The pH dependence of the photocurrent is similar to that of p-GaAs. Using the Cr³⁺/Cr²⁺ EDTA redox system a photovoltage of 0.5 V is obtained from the shift of the photocurrent-potential curve with respect to the current-potential curve on a reversible Hg electrode. The spectral dependence of the photocurrent yields a value of 1.45 eV for the band gap in accordance with absorption measurements. These results show that an efficient liquid junction solar cell is feasible based on this material.     

Non-Gaussian noise in quantum wells infrared photodetectors

Non-Gaussian dark current noise has been observed in quantum wells infrared photo detectors. The non-Gaussian component of the noise was ascribed to fluctuations of spatial distribution of electric field in the device. Non-Gaussian noise was found in both n- and p-type QWIPs, however, it was significantly less pronounce. In n-type devices non-Gaussian noise manifests itself only as randomly distributed excess current bursts. In p-type QWIPs the non-Gaussian noise takes form of bias dependent random telegraph-like fluctuations with a finite time of transition between the levels. The lifetime at both levels is Poisson distributed and the average lifetime, together with the level spacing, strongly depend on bias voltage. At low voltages the system stays predominantly in the low current level while at higher voltages the average lifetime of the high current level is longer. The transient time of passing between the states has been related to the charging time constant of the system determined by QWIP capacitance and contacts resistance.     

QWIP detectors and thermal imagers

Standard GaAs/AlGaAs QWIPs (Quantum Well Infrared Photodetector) are now well established for long wave infrared (LWIR) detection. The main advantage of this technology is the duality with the technology of commercial GaAs devices. The realization of large FPAs (up to 640×480) drawing on the standard III–V technological process has already been demonstrated. The second advantage widely claimed for QWIPs is the so-called band-gap engineering, allowing the custom design of the quantum structure to fulfill the requirements of specific applications such as multispectral detection. QWIP technology has been growing up over the last ten years and now reaches an undeniable level of maturity. As with all quantum detectors, the thermal current, particularly in the LWIR range, limits the operating temperature of QWIPs. It is very crucial to achieve an operating temperature as high as possible and at least above 77 K in order to reduce volume and power consumption and to improve the reliability of the detection module. This thermal current offset has three detrimental effects: noise increase, storage capacitor saturation and high sensitivity of FPAs to fluctuations in operating temperature. For LWIR FPAs, large cryocoolers are required, which means volume and power consumption unsuitable for handheld systems. The understanding of detection mechanisms has led us to design and realize high performance ‘standard’ QWIPs working near 77 K. Furthermore, a new in situ skimmed architecture accommodating this offset has already been demonstrated. In this paper we summarize the contribution of THALES Research & Technology to this progress. We present the current status of QWIPs in France, including the latest performances achieved with both standard and skimmed architectures. We illustrate the potential of our QWIPs through features of Thales Optronique's products for third thermal imager generation. To cite this article: E. Costard et al., C. R. Physique 4 (2003).     

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).     

QWIP focal plane arrays on InP substrates for single and dual band thermal imagers

Alternative material systems on InP substrate provide certain advantages for mid-wavelength infrared (MWIR), long-wavelength infrared (LWIR) and dual band MWIR/LWIR quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). While InP/InGaAs and InP/InGaAsP LWIR QWIPs provide much higher responsivity when compared to the AlGaAs/GaAs QWIPs, AlInAs/InGaAs system facilitates completely lattice matched single band MWIR and dual band MWIR/LWIR FPAs.We present an extensive review of the studies on InP based single and dual band QWIPs. While reviewing the characteristics of InP/InGaAs and InP/InGaAsP LWIR QWIPs at large format FPA level, we experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by only changing the quantum well (QW) width at the lattice matched composition. The cut-off wavelength can be shifted up to ～5.0 μm with a QW width of 22 Å in which case very broad spectral response (Δλ/λ_p = ～30%) and a reasonably high peak detectivity are achievable leading to a noise equivalent temperature difference as low as 14 mK (f/2) with 25 μm pitch in a 640 × 512 FPA. We also present the characteristics of InP based two-stack QWIPs with wavelengths properly tuned in the MWIR and LWIR bands for dual color detection. The results clearly demonstrate that InP based material systems display high potential for dual band MWIR/LWIR QWIP FPAs needed by third generation thermal imagers.     

Some physical properties of the TiO2 semiconductor electrode

Some physical properties of n-TiO₂ single crystals have been studied by electrochemical and photoelectrochemical techniques. X-ray diffractometry confirmed the TiO₂ basic rutile type structure. For the TiO₂/0.5 N NaOH system in the dark a Tafel slope of 0.11 V/decade was observed between 0.50 and 0.61 V versus SCE, suggesting a one-electron process. The potential, at which the gaseous oxygen evolution was confirmed, was well over 2.0 V versus SCE. On the other hand the potential for oxygen evolution was markedly decreased to −0.75 V versus SCE, when the TiO₂ was illuminated. From these facts and the observation of passivation, transpassivation and current saturation an oxygen-pumping mechanism is proposed for the polarization behaviour of the TiO₂ electrode in the dark. The spectral responses of photocurrent generated from TiO₂ at 0.0 V bias versus SCE showed that the peak illumination current at 360 nm is larger than the dark current by three orders of magnitude. The band gap of TiO₂ was determined as 3.1 eV by assuming that the photoelectrolysis of water involves an indirect energy transition. An anomalous photoresponse was observed at 1.8 eV, which is due to the presence of surface states at the TiO₂/electrolyte interface.