III-V compound semiconductor devices: Optical detectors

This article presents a review of the historical developments in optical detectors and discusses the motivations for interest in III-V semiconductors for optical-detector applications. Early device work in both depletion-mode photodiodes and avalanche photodiodes in III-V semiconductors is covered as well as the improvements that have been made in avalanche photodiode structures through work in silicon. Also, the results of ionization coefficient measurements on III-V compounds are summarized. Finally, several examples of recent avalanche photodiodes that utlilize the unique properties of heterostructures are presented.     

Design and analysis of micromechanical tunable interferometers forWDM free-space optical interconnection

This paper proposes a new concept of high density chip-to-chip optical interconnection based on wavelength division multiplexing (WDM), which can be realized as a microelectromechanical system (MEMS) device. The output signals from a preprocessing integrated circuit (IC) chip (N channels) are converted to optical signals of different wavelengths by using wavelength tunable laser diodes and projected onto an array of wavelength-selective photodiodes. Channel connection is made by wavelength matching between the light sources and detectors, by using micromechanical Fabry-Perot interferometers. In this scheme arbitrary 1 to 1 connection as well as 1 to N or N to 1 connection is available. Since the input/output connections are made in an optical manner, the switching state can be reconfigured and the whole switching system can be integrated in a compact space. We have investigated design principle of the micromechanical Fabry-Perot interferometers for tunable photodiodes to maximize the switching contrast and channel density in a given wavelength range. The preliminary interferometer arrays are implemented by silicon-based surface micromachining     

The impact of a large bandgap drift region in long-wavelengthmetamorphic photodiodes

A comparative study of two types of metamorphic double heterojunction long-wavelength photodiodes on GaAs substrates is performed in terms of their bandwidths and responsivities. A p-i-i-n heterostructure with a large bandgap drift layer (I-InAlAs) at the cathode end of the photoabsorption region (i-InGaAs) is compared experimentally and theoretically to a p-i-n structure without a drift layer. Both types of photodiodes were fabricated using an InGaAs-InGaAlAs-InAlAs double heterostructure design to simultaneously achieve high bandwidths and high responsivities. The inclusion of an I-InAlAs drift region resulted in p-i-i-n photodiodes with larger bandwidths than p-i-n photodiodes with the same areas, or conversely a p-i-i-n photodiode can be made larger than a comparable p-i-n photodiode, but achieve the same bandwidth. Therefore, p-i-i-n photodiodes provide larger optical fiber alignment tolerances and better coupling efficiency than p-i-n photodiodes with the same bandwidths, p-i-i-n photodiodes with 10-μm-diameter optical windows typically exhibited low dark currents of 500 pA at 5-V bias, responsivities of 0.6 A/W, and a -3-dB bandwidth of 38 GHz for 1.55-μm operation     

Investigation of manufacturing variations of planar InP/InGaAs avalanche photodiodes for optical receivers

Planar InP/InGaAs avalanche photodiodes are widely used for high-speed optical receivers in optical fiber communication systems. Even though these avalanche photodiodes offer the excellent characteristics in high-speed operation, the performance metrics are affected by manufacturing parameter variations considerably. In this paper, the effects of manufacturing variations on the device performance are investigated. In order to build a photodiode model, the test structures were fabricated and the measured current-voltage characteristics were compared with the simulated data to verify the model. After the model verification, the variations of the breakdown voltage and punch-through voltage according to the different manufacturing parameters such as multiplication layer width and charge sheet density are examined. Based on the results, the manufacturability of the avalanche photodiodes can be improved by analyzing the manufacturing variations.     

Photocurrent multiplication in GaAs Schottky photodiodes

Many attempts have been made to build fast, sensitive photodetectors which offer simple fabrication and ease of integration. Regarding this view, GaAs Schottky barrier photodiodes seem to be ideally suited for use in the near-infrared region. The fabrication of GaAs Schottky photodiodes and the investigation of their properties (mainly photocurrent multiplication) are presented in this paper. A photocurrent gain of 10⁴ was achieved and dependences of gain on incident power level and position were observed.     

Application specific spectral response with CMOS compatiblephotodiodes

Several methods are presented for realizing photodiodes with independent spectral responses in a standard CMOS integrated circuit process. Only the masks, materials, and fabrication steps inherent to this standard process were used. The spectral responses of the photodiodes were controlled by (1) using the SiO₂ and polycrystalline Si as thin-film optical filters, (2) using photodiodes with different junction depths, and (3) controlling the density of the interfacial trapping centers by choosing which oxide forms the Si/SiO ₂ interface. Also presented is an example method for constructing photo-spectrometers using these spectrally-independent photodiodes. This method forms weighted sums of the photodiodes' outputs to extract spectrographic information     

Low dark current InGaAs PIN photodiodes grown by molecular beam epitaxy

High-quality InGaAs PIN photodiodes have been made from high-purity layers grown on InP substrate by molecular beam epitaxy. The diodes are top-illuminated mesa-type passivated and planarised by polyimide. The devices exhibit dark current densities as low as 2.3 × 10?5 A/cm2 at ?10 V with a breakdown voltage of ?80 V. These values are comparable with those obtained by other more conventional growth techniques, and are the best so far reported by MBE.     

Silicon photodiode with a grid <Emphasis Type="Italic">p-n</Emphasis> junction

Silicon photodiodes with a grid-structured p-region were studied. Analytical expressions for the capacitance of such photodiodes were derived. The influence of the cell sizes and diffusion length of minority carriers on the sensitivity of the silicon grid photodiode was analyzed in a spectral range of 0.6–1.0 μm. The experimental characteristics of photodiodes with a grid p-n junction with cell sizes of 50 and 110 μm are given. The factors controlling the spectral-characteristic features of these photodiodes are discussed.     

Extended One-Dimensional Analysis to Effectively Derive Quantum Efficiency of Various CMOS Photodiodes

This paper proposes an extended 1-D analysis to derive quantum efficiency of various commonly used CMOS photodiodes. The theoretical model of the CMOS photodiode with the n-/p-epitaxial/p + substrate (n-/p-epi/p + sub) structure is established from steady-state continuity equations, where most existing boundary conditions are applied. In particular, the minority carrier and current densities are continuous across the interface between two layers with the same dopant type. Models of the other commonly used CMOS photodiodes are also examined. Three CMOS photodiodes with n-/p-substrate (n-/p-sub), p⁺/n-/p-substrate (p⁺/n-/p-sub), and n-/p-epi/p + sub structures are fabricated and characterized to validate the proposed model. Additionally, the surface recombination velocity is adequately determined by fitting the simulated quantum efficiency to the measured value. The simulated quantum efficiency of the proposed model for these three photodiodes is quite consistent with the measured values, revealing the feasibility and effectiveness of the proposed model in characterizing various CMOS photodiodes.     

Metamorphic graded bandgap InGaAs-InGaAlAs-InAlAs doubleheterojunction p-i-I-n photodiodes

High-speed metamorphic double heterojunction photodiodes were fabricated on GaAs substrates for long-wavelength optical fiber communications. The high quality linearly graded quaternary InGaAlAs metamorphic buffer layer made possible the growth of excellent InGaAs-InGaAlAs-InAlAs heterostructures on GaAs substrates. The use of a novel double heterostructure employing an InGaAlAs optical impedance matching layer, a chirped InGaAs-InAlAs superlattice graded bandgap layer (SL-GBL), and a large bandgap i-InAlAs drift region enabled photodiodes to achieve a low dark current of 500 pA, a responsivity of 0.6 A/W, and a -3 dB bandwidth of 38 GHz at -5 V reverse bias for 1.55 μm light. The effect of accumulated charges at the InGaAs-InAlAs heterointerface was examined through a comparison of the dark currents of InGaAs-InAlAs and InGaAs-InP abrupt single heterojunction photodiodes; to photodiodes with chirped InGaAs-InAlAs SL-GBLs. The charge accumulation effects observed in abrupt heterojunction devices were suppressed by including a chirped InGaAs-InAlAs SL-GBL between the InGaAs absorption layer and InAlAs drift layer. The effect of passivation techniques was evaluated by comparing dark currents of unpassivated photodiodes and photodiodes passivated with either polyimide or SiN_x. The enhancement of photodiode bandwidth through the inclusion of a transparent large bandgap I-InAlAs drift region was verified by comparing the bandwidths of the P-i-I-N photodiodes that have I-InAlAs between i-InGaAs photoabsorption layer and N⁺ InAlAs cathode to conventional P-i-N photodiodes without a drift region     

Effect of trap tunneling on the performance of long-wavelength Hg1-xCdxTe photodiodes

The effect of trap tunneling on the detector performance of long cutoff wavelength (λco) Hg1-xCdxTe photodiodes was investigated with the use of a parametric model. The development of this model follows closely the formulation by Sah for treating the case of excess currents in gold-doped narrow silicon junctions. The trap tunneling limitedR_{0}A's for long-wavelength Hg1-xCdxTe photodiodes with different p- and n-side doping concentrations and at different temperatures were calculated using this model as a function of p-side trap density, trap location, and junction impurity concentration gradient. The calculated results are in agreement with those measured from actual photodiodes. In particular, the somewhat unexpected temperature dependence of the measuredR_{0}Aproduct at low-temperatures can be satisfactorily accounted for. The present tunneling model also adequately explains the observed soft reverse breakdown characteristics for these devices and their behavior as a function of temperature.     

Photoelectric and optical properties of Schottky-barrier photodiodes based on IrSi–Si

The basic properties in the Schottky-barrier photodiodes (SBPDs) can be reduced to photon absorption in the silicide layer and internal photoemission of charge carriers from a metal to a semiconductor. Therefore, the quantum efficiency and photoresponse of these photodiodes (PDs) are primarily determined by electronic and optical processes in the metal silicide rather than in the semiconductor. This implies that, to a first approximation, the SBPD photoresponse is independent of semiconductor parameters such as the doping level, degree of compensation of impurities, and minority-carrier lifetime. The main reasons for photoresponse nonuniformity in multielement detector arrays are thereby ruled out.     

Ultranarrow Bandwidth Organic Photodiodes by Exchange Narrowing in Merocyanine H‐ and J‐Aggregate Excitonic Systems

Ultranarrowband organic photodiodes (OPDs) are demonstrated for thin film solid state materials composed of tightly packed dipolar merocyanine dyes. For these dyes the packing arrangement can be controlled by the bulkiness of the donor substituent, leading to either strong H‐ or strong J‐type exciton coupling in the interesting blue (H‐aggregate) and NIR (J‐aggregate) spectral ranges. Both bands are shown to arise from one single exciton band according to fluorescence measurements and are not just a mere consequence of different polymorphs within the same thin film. By fabrication of organic thin‐film transistors, these dyes are demonstrated to exhibit hole transport behavior in spin‐coated thin films. Moreover, when used as organic photodiodes in planar heterojunctions with C₆₀ fullerene, they show wavelength‐selective photocurrents in the solid state with maximum external quantum efficiencies of up to 11% and ultranarrow bandwidths down to 30 nm. Thereby, narrowing the linewidths of optoelectronic functional materials by exciton coupling provides a powerful approach to produce ultranarrowband organic photodiodes.     

45-GHz bandwidth-efficiency resonant-cavity-enhanced ITO-Schottkyphotodiodes

High-speed Schottky photodiodes suffer from low efficiency mainly due to the thin absorption layers and the semitransparent Schottky-contact metals. We have designed, fabricated and characterized high-speed and high-efficiency AlGaAs-GaAs-based Schottky photodiodes using transparent indium-tin-oxide Schottky contact material and resonant cavity enhanced detector structure. The measured devices displayed resonance peaks around 820 nm with 75% maximum peak efficiency and an experimental setup limited temporal response of 11 ps pulsewidth. The resulting 45-GHz bandwidth-efficiency product obtained from these devices corresponds to the best performance reported to date for vertically illuminated Schottky photodiodes     

Performance improvement of CCD image sensor arrays

In conventional arrays, charges are read out from photodiodes through vertically placed charge-coupled registers. In an array presented here, reading out is performed through vertical diffusion lines. As shown, such an array is more sensitive, since the photodiodes occupy a larger space. Also, this array shows a reduced level of cross-coupling, because extra charges leave the photodiodes, and a reduced level of intrinsic back-ground, because charges generated exterior to the photodiodes are removed.     

高速光检测器和高速光接收机进展

本文综述了近年来微波光电子学中高速光检测器和高速光接收机的进展，分析并对比了各种光电二极管的响应速度、噪声及暗电流特性；指出光波导与ＭＳＭ、ＰＩＮ为代表的高速光电二极管以及ＨＥＭＴ、ＨＢＴ等低噪声微波器件的光电集成，是高速光接收机的发展趋势，行波光检测器作为高速分布参数光电器件，光波导与光电二极管的融合，光场效应晶体管以及掺铒光纤放大器的应用，都是值得重视的发展动态。     

Noise and dynamical gain studies of GaAs photoconductive detectors

Noise measurements on N-type GaAs planar photoconductive detectors have been made over the 10 MHz?1.5 GHz frequency range. The dynamical gains of the devices were calculated from noise data and compared with the values obtained using picosecond measurements. In the gigahertz frequency domain, the photodetectors have an internal current gain as observed in the avalanche photodiodes, but no excess noise factor has been found.     

Infrared detectors and applications

A large variety of methods for the detection of infrared radiation have been developed primarily over the last ten to twenty years. These include thermal detection using bolometers, thermocouples and pyroelectric detectors, photon detection by extrinsic and intrinsic semiconductor photoconductors, photodiodes and photoemitters, and infrared imaging using these devices. These methods also include the use of devices with internal gain such as avalanche photodiodes and infrared photomultipliers as well as various unconventional detector schemes. General considerations of infrared imaging and high speed detection are discussed and many of these methods are reviewed and compared, with particular attention given to their applications and limitations. This work was sponsored by the Department of the Air Force.     

Correlation between visual defects and increased dark current in large-area Hg1−xCdxTe photodiodes

The Cross-Track Infrared Sounder (CrIS) program [an instrument on the National Polar-Orbiting Operational Environmental Satellite System (NPOESS)] requires photodiodes with spectral cutoffs denoted by short-wavelength infrared [γ_c(98 K) ∼5.1 μm], midwavelength infrared [γ_c(98 K) ∼9.1 μm], and long-wavelength infrared (LWIR) [γ_c(81 K) ∼15.5 μm]. The CrIS instrument also requires large-area (850-μm-diameter) photodiodes with state-of-art performance. Molecular beam epitaxy (MBE) is used to grow n-type short-wavelength infrared, midwavelength infrared, or LWIR Hg_1−xCd_xTe on latticematched CdZnTe. Detectors with p-type implants 7 μm in diameter are used to constitute the 850-μm-diameter lateral collection diodes (LCDs). The photodiode architecture is the double-layer planar heterostructure architecture. Quantum efficiency, I-V, R_d-V, and 1/f noise in photovoltaic Hg_1−xCd_xTe detectors are critical parameters that limit the sensitivity of infrared sounders. These are some of the parameters used to select photodiodes that will be part of the CrIS focal plane module (FPM). During fabrication of the FPM, the photodiodes are subject to a significant amount of handling while transitioning from part of newly processed Hg_1–xCd_xTe wafers to individual photodiodes mounted in a CrIS FPM ready to be flown on NPOESS. Quantum efficiency, I-V, noise, and visual inspections are performed at several steps in the detector’s journey. Initial I-V and visual inspections are conducted at the wafer level followed by I-V, noise, and quantum efficiency after dicing and mounting the photodiodes in leadless chip carriers (LCCs). A visual inspection is performed following removal of the detectors from the LCCs. Finally, the individual photodiodes are precision mounted on an FPM base, and I-V, noise, quantum efficiency, and visual inspections are performed again. Each step in the FPM fabrication process requires handling and environmental conditioning that can result in detector dark current and noise increase. Some photodiodes on the first flightlike FPMs fabricated exhibited an increase in dark current and noise characteristics at the FPM level as compared to the measurements performed when the photodiodes were in LCCs prior to integration into the FPM. The degradation observed resulted in an investigation to discern the cause of the performance degradation (baking at elevated temperatures, mechanical handling, electrical stress, etc.). This paper outlines the results of the study and the corrective actions that led to the successful manufacture of LWIR large detectors from material growth to insertion into flight FPMs for the CrIS program.