Control of dark current mechanisms is essential to improving the performance of infrared photodetectors and many other electronic devices. Unipolar barriers can readily be applied to practically and efficiently filter out multiple dark current components exhibited by infrared photodetectors. Via careful placement of unipolar barriers in a standard photodetector architecture, effective suppression of dark currents due to surface leakage, direct band-to-band tunneling, trap-assisted tunneling, and Shockley-Read-Hall generation is demonstrated. We present unipolar barrier photodiodes exhibiting six orders of magnitude improvement in RoA and near Auger-limited device performance.

Tunneling currents and surface leakage currents are both contributors to the overall dark current which limits many semiconductor devices. Surface leakage current is generally controlled by applying a post-epitaxial passivation layer; however, surface passivation is often expensive and ineffective. Band-to-band and trap assisted tunneling currents cannot be controlled through surface passivants, thus an alternative means of control is necessary. Unipolar barriers, when appropriately applied to standard electronic device structures, can reduce the effects of both surface leakage and tunneling currents more easily and cost effectively than other methods, including surface passivation. Unipolar barriers are applied to the p -type region of a conventional, MBE grown, InAs based pn junction structures resulting in a reduction of surface leakage current. Placing the unipolar barrier in the n -type region of the device, has the added benefit of reducing trap assisted tunneling current as well as surface leakage currents. Conventional, InAs pn junctions are shown to exhibit surface leakage current while unipolar barrier photodiodes show no detectable surface currents.

The unipolar barrier is a new approach for control of dark currents in infrared photodetectors. First demonstrated in the nBn detector and then in the unipolar barrier photodiode, unipolar barriers have been shown to block surface leakage current. Unipolar barriers can also be implemented to filter out dark current components such as Shockley-Read-Hall current, direct band-to-band tunneling and trap-assisted tunneling, but are not useful for blocking diffusion currents. Current density-voltage characteristics of molecular-beam-epitaxy-grown InAs based unipolar barrier photodiodes are presented and analyzed, showing effective limiting of noise current mechanisms for different unipolar barrier photodiode architectures. RoA data shows near Auger-limited device performance and RoA values in excess of 1x107 Ω-cm2.

Excessive surface leakage currents, and their associated noise, deteriorate the performance of infrared photodetectors. The conventional approach to suppress surface leakage is post-epitaxy deposition of polycrystalline or amorphous passivation layers. Disadvantages of such passivation layers are the cost and complexity of the required additional processing steps, and the fact that they do not always work well. An alternative approach, presented here, is to design the photodetectors' epitaxial structures so that surface leakage currents are suppressed without the need for ex-situ deposition of passivation layers. Two examples of such epitaxial designs are the nBn detector and the unipolar barrier photodiode.

Surface leakage currents are one of the many causes of dark current exhibited by semiconductor devices and are a common limiting factor to device performance for many types of devices. Traditionally, these surface leakage currents are controlled through the post-epitaxial insertion of a surface passivation layer, but surface passivation is often expensive, time consuming, and not entirely effective. Unipolar barriers may be incorporated into electronic devices as a less expensive and more effective alternative way to control surface leakage currents. Unipolar barrier devices are demonstrated here using MBE-grown InAs nBn detectors and unipolar barrier photodiodes. A commercial InAs photodiode becomes surface leakage current limited at 220 K while the nBn detector shows no surface leakage current down to at least 135 K. The unipolar barrier photodiode shows no detectable surface leakage down to at least 130 K while a matching conventional photodiode is limited by surface conduction at a temperature of 150 K. This indicates the greatly improved performance of the unipolar barrier devices. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The performance of many electronic devices and materials is limited by surface leakage. These currents are conventionally inhibited by post-epitaxy deposition of a surface passivation layer. This work presents the use of epitaxial, unipolar barriers to limit surface leakage currents, which are a more effective and less expensive approach to control surface current than surface passivation. The concept is illustrated with molecular beam epitaxy-grown InAs photodiodes. The dark current in a conventional InAs photodiode is dominated by surface conduction at temperatures below 150 K, whereas the unipolar barrier InAs photodiode exhibits no detectable surface leakage.

Surface leakage current is a significant source of dark current in conventional narrow-gap p-n junction photodetectors. It is shown that the nBn photodetector, which was originally designed to eliminate dark current arising from generation-recombination mechanisms, also effectively eliminates surface leakage currents. The result is a measured dark current lower by over six orders of magnitude than that of the InAs p-n photodetector for device temperatures of 140 K.

This letter presents a type of infrared detector named the nBn detector. The nBn design essentially eliminates Shockley-Read-Hall generation currents. The result is greatly reduced dark current and noise, compared to other midwave infrared detectors, such as p-n photodiodes. This enables the nBn to operate at background-limited infrared photodetection conditions at significantly higher temperatures than conventional midwave infrared detectors and have greater detectivity near room temperature. The nBn is demonstrated in InAs and InAsSb materials, exhibiting cutoff wavelengths of 3.4 and 4.2μm, respectively.

InAs bipolar junction transistors (BJTs), grown by molecular beam epitaxy, are reported with common emitter current gains (β's) as large as 400. The factors affecting the common emitter current gain have been studied by estimating the magnitudes of the base transport factor (αT) and emitter injection efficiency (γ). This has been accomplished by studying a sequence of InAs BJTs with varying emitter doping densities, NE. Minority carrier diffusion length in the base (LB), αT, and γ have been extracted from measured electrical characteristics. The results of the study of these InAs BJTs are as follows: LB≈0.4μm, αT≈98% and γ ranges from 92% to nearly 100% depending on NE. This knowledge of the magnitudes of the injection efficiencies suggests when it would be useful to move from the simple BJT structure to the more advanced heterojunction bipolar transistor (HBT) structure. Lower γ BJTs would be improved, however high-γ BJTs would benefit little, by the use of the widegap emitters of HBTs. The method developed here to estimate γ, αT and LB is not specific to InAs BJTs, but should be useful for study of BJTs and HBTs in any material system.

In this work we reporrt on strained controlled InP/InGaAs and strained compensated InP/InGaAs/InGaP quantum well IR photodetectors (QWIPS) that cover the spectral bands 8-11 μm and 4-6μm respectively. Two different approaches were demonstrated in this work. i) We explore the effect of introducing non lattice-matched InGaAs well in InP/InGaAs QWIP structures. We show that this enables to extend the QWIP operating wavelength up to 11.5 μm taking advantage of strain as a bandgap engineering design parameter. State of art QWIPs with peak detectivity at 8.5 μm of 8×109 cmHz1/2/W at 80K with 2pi FOV was demonstrated. ii) A high detectivity mid-IR QWIPs were demonstrated. These devices are based on strain compensated INGaAs/InP/InGaP multiquantum well structures, where the InGaP conduction band offset extends the wavelength detection range ino the mid-IR. Photodetectors with background-limited performance with detectivity of D*λ (BLIP)=3.2*1010 cmHz1/2/W up to 110 K, were implemented. We conclude that InP/InGaAs material system is suitable fo multicolro QWIPs applications, particularly for two color QWIPs operating in the 8-12 and 3-5 microns range simultaneously.

The factors affecting the common emitter current gain (β) in InAs bipolar junction transistors (BJTs) have been studied by estimating the base transport factor (αT) and the emitter injection efficiency (γ). This has been accomplished by employing a specially designed sequence of InAs npn BJTs. αT, γ, and the minority carrier (electrons) diffusion length in the base (LB) are extracted from measured electrical characteristics by using a simple method. When the emitter doping density is large, the current gain is limited by the base transport factor αT. The value of LB in these BJTs is found to be 0.44 μm, which indicates high quality epitaxial material. The analysis developed is a general technique, also applicable to BJTs and heterojunction bipolar transistors in other material systems.

The large electron mobility and high saturation velocity of InAs make it a promising material for the operation of high speed electronic devices. As semiconductor scaling continues to reduce device size, the smaller bandgap III-Vs, such as InAs, play a more prominent role in the design of new low power devices. The present work involves materials and device investigations related to InAs-based bipolar transistors grown by molecular beam epitaxy (MBE).

Large mobilities and electron saturation velocity make InAs a promising material for high speed devices. Investigations into materials characteristics of doped InAs show nonideal behavior with standard molecular beam epitaxy dopants, silicon, and beryllium. Critical thicknesses for cracking of AlxIn1−xAs on InAs were empirically determined as a function of x. Mesa pn junctions in InAs show no effects of surface Fermi level pinning and exhibit good rectification with low reverse leakage. Bipolar junction transistor and heterojunction bipolar transistor devices are presented, along with their dc electrical characteristics. Common emitter current gains of 100 have been achieved in these bipolar devices.

The first InAlAs-InAs heterojunction bipolar transistors are reported. A current gain of 100 at room temperature is measured. The base-collector junction is an InAs p-n homojunction, which is optimised to have low reverse leakage at room temperature. The emitter is pseudomorphic AlInAs, linearly graded to increase the barrier which inhibits hole injection into the emitter.

Photoconductive infrared spectra due to intraband transitions in InAs/InAlAs/InP quantum dots show several peaks in the range of 100-400 meV. These peaks depend differently on bias, polarization and illumination configuration. Of particular interest is the polarized peak around 150 meV, which is observed only in wedge configuration. It is highly dependent on bias, i.e. due to bound to bound transition (followed by tunneling), and eventually becomes the dominant line. However, its oscillator strength is not large since is not observed in absorption measurements, which show a single peak at 100 meV. We associate this large signal with confinement of the radiation in the dot plane due to abrupt change in refractive index of the highly doped (similar to 8x10(18) cm(-3)) InGaAs contact layers.

Multicolor quantum dot infrared photodetectors with normal incidence background limited performance (BLIP) at 77 K were implemented. The devices are composed of self-assembled InAs dots grown on InAlAs barrier layers, lattice matched to InP substrate. These dots are formed in a shape of flattened parallelepipeds. The photoconductive spectra were observed at all polarizations. Normal incidence photocurrent spectra reveal several polarized peaks in the range of 100-400 meV due to intersubband transitions. The detector responsivity at normal incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in quantum well infrared photodetectors (QWIP's). The transition energies, the polarization selection rules, and relative intensities of the peaks were tentatively interpreted in terms of a three-dimensional separation of variables.

Infrared detectors were implemented on InAs self-assembled quantum dots fabricated using Stranski–Krastanov growth mode on InAlAs matrix, lattice matched to InP (0 0 1) substrates. These dots grow with a shape of small elongated boxes, with their long axis along the [1̄10] direction, and with a high concentration of 7×1010cm−2. Photoconductive measurements were performed in all three polarizations. Rich spectra in the range of 50–500 meV, with different polarization selection rules were observed. The bias dependence of peak intensity of the intraband transitions serves as an additional tool to identify their origin. Some of the peaks, which increase linearly with bias, are attributed to bound-to-continuum transitions. Others, which appear only at larger biases, and increase superlinearly, are due to bound-to-bound transitions. The magnitude of detector responsivity at normal-incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in QWIPs. BLIP conditions prevail at 77 K for integral photocurrent response at F#1. The effect of unintentional doping is discussed. It is shown that this doping can be destructive for detector operation unless the density of dots is large.

Infrared photoconductive measurements on self-assembled quantum dots structure of InAs/InAlAs/InP are reported for the first time. The infrared photoconductive signal is due to intersubband transitions. Normal incidence responsivity is comparable to that achieved in QWIPs in perpendicular polarization. The InAs dots were fabricated using Stranski-Krastanov growth made on InAlAs matrix lattice matched to InP (001) substrate. The dots have a shape of a flattened "cigar", with its long axis along the [(1) over bar 10] direction, and with a high concentration of 10(11) cm(-2). Some of the peaks are polarized solely along the [110]direction, a feature never observed before. Based on polarization and bias dependence, the infrared bands are identified and assigned to intersubband transitions.

Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate.

Theoretical predictions show that thermal generation rate in quantum dots can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. Quantum Dots Infrared Photodetectors (QDIP) were implemented in a GaAs/InAs material system. The device is composed of 10 layers of self assembled InAs dots grown on GaAs substrate. The illumination was introduced through a wedge in order to investigate polarization effects. A clear spectral response was obtained at 13K, with two peaks. The first, at 6.5 mu m, is polarized parallel to the growth axis, and is attributed to excitation from the first confined level to the continuum. The second, centered around 15 mu m, is unpolarized, and is due to excitation from the second confined level to the continuum. The fact that a photoconductive signal was recorded even though no absorbance was observed, indicates a very large gain due to a very long lifetime, consistent with theoretical predictions. It is estimated that once a device with quantum efficiency comparable to that of Quantum Well Infrared Photodetectors (QWIPs) is realized, it will be possible to operate QDIPs at temperatures higher than QWIPs.