Spin dynamics in semiconductors

This article reviews the current status of spin dynamics in semiconductors which has achieved a lot of progress in the past years due to the fast growing field of semiconductor spintronics. The primary focus is the theoretical and experimental developments of spin relaxation and dephasing in both spin precession in time domain and spin diffusion and transport in spacial domain. A fully microscopic many-body investigation on spin dynamics based on the kinetic spin Bloch equation approach is reviewed comprehensively.

The physical properties of detectors based on intraband optical absorption in quantum dots is described and examined in the interest of providing a competitive alternative infrared (IR) detector technology. These quantum dot detectors are an extension of quantum well infrared photodetectors and are expected to have a large performance advantage. A model is developed for quantum dot infrared photodetectors based on fundamental performance limitations enabling a direct comparison between IR materials technologies. A comparison is made among HgCdTe, quantum well, and quantum dot IR detectors, where quantum dots are expected to have the potential to outperform quantum wells by several orders of magnitude and compete with HgCdTe. In this analysis, quantum dots are expected to possess the fundamental ability to achieve the highest IR detector performance if quantum dot arrays with high size uniformity and optimal bandstructure may be achieved.

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Evaluation of the fundamental properties of quantum dot infrared detectors

BAE SYSTEMS has developed a laser-illuminated, gated imaging system for long range target identification which has generated bright images at ranges in excess of 10km from modest laser energies. The system is based on a short pulsewidth laser and a custom detector for sensing the return pulse. The source is a Nd YAG laser converted by an optical parametric oscillator (OPO) to 1571nm and producing 20ns pulses at 15Hz. The detector (named SWIFT) is a 320x256 array of HgCdTe photodiodes operating with high avalanche gain to achieve sensitivities as low as 10 photon rms. A custom silicon multiplexer performs the signal injection and temporal gating function, and adds additional electronic gain. Trials show that the current detectors have gate edges equivalent to 1.5m in range and complete extinction of signals outside of the gated range. The detector is encapsulated in an integrated-detector-cooler-assembly and utilises standard productionised thermal imaging electronics to perform non-uniformity correction and grey scale images. Imaging trials using the camera have shown little excess noise, crosstalk or non-uniformity due to the use of avalanching in the HgCdTe photodiodes up to gains of over 100. The images have shown high spatial resolution arising from the use of solid state focal plane array technology. The imagery, collected both in the laboratory and in field trials, has been used to explore the phenomenology unique to laser-illuminated targets and to verify system models.

A low-noise laser-gated imaging system for long-range target identification

Electron spin relaxation in bulk III-V semiconductors is investigated from a fully microscopic kinetic spin Bloch equation approach where all relevant scatterings, such as, the electron--nonmagnetic-impurity, electron-phonon, electron-electron, electron-hole, and electron-hole exchange (the Bir-Aronov-Pikus mechanism) scatterings are explicitly included. The Elliott-Yafet mechanism is also fully incorporated. This approach offers a way toward thorough understanding of electron spin relaxation both near and far away from the equilibrium in the metallic regime. The dependences of the spin relaxation time on electron density, temperature, initial spin polarization, photo-excitation density, and hole density are studied thoroughly with the underlying physics analyzed. We find that these dependences are usually qualitatively different in the nondegenerate and degenerate regimes. In contrast to the previous investigations in the literature, we find that: (i) In $n$-type materials, the Elliott-Yafet mechanism is less important than the D'yakonov-Perel' mechanism, even for the narrow band-gap semiconductors such as InSb and InAs. (ii) The density dependence of the spin relaxation time is nonmonotonic and we predict a peak in the metallic regime in both $n$-type and intrinsic materials. (iii) In intrinsic materials, the Bir-Aronov-Pikus mechanism is found to be negligible compared with the D'yakonov-Perel' mechanism. We also predict a peak in the temperature dependence of spin relaxation time which is due to the nonmonotonic temperature dependence of the electron-electron Coulomb scattering in intrinsic materials with small initial spin polarization. (iv) In $p$-type III-V semiconductors, the Bir-Aronov-Pikus mechanism dominates spin relaxation in the low-temperature regime only when the photoexcitation density is low. When the photoexcitation density is high, the Bir-Aronov-Pikus mechanism can be comparable with the D'yakonov-Perel' mechanism only in the moderate temperature regime roughly around the Fermi temperature of electrons, whereas for higher or lower temperature it is unimportant. The relative importance of the Bir-Aronov-Pikus mechanism decreases with the photoexcitation density and eventually becomes negligible at sufficiently high photoexcitation density. The effect of electric field on spin relaxation in $n$-type III-V semiconductors is also studied with behaviors very different from those in the two-dimensional case reported. Finally, we find good agreement of our calculation with the experimental results.

Electron-spin relaxation in bulk III-V semiconductors from a fully microscopic kinetic spin Bloch equation approach

HgCdTe 2D arrays are needed in both medium (MW) and long (LW) wavebands for imaging, search, and track and guidance applications. Often the detector is the performance-limiting component in the system, and it is necessary to use detectors with very low excess noise and few defective pixels. Normally the detector is cooled sufficiently to freeze-out thermally generated leakage currents, so the main interest is to understand the mechanisms that determine the general detector performance and the cause of defective pixels. This paper describes the detector technology and the ion beam junction-forming process. The fundamental performance limits of homojunction HgCdTe technology and the doping levels needed to produce a detector with impact-ionization limited performance are discussed. Extensive studies have been made on defective pixels in long wavelength arrays and some technologies for reducing them are described here. Defective pixels have been found to be associated with material dislocations crossing the p-n junction and a model has been proposed for the noise-generating mechanism.

Summary of HgCdTe 2D array technology in the U.K.

A new metal-organic vapor-phase epitaxial (MOVPE) reactor-cell design has been developed to grow on 3-in.-diameter substrates. This was required to produce uniform, fully doped heterostructures needed for array producibility and wafer-scale processing compatibility. The reactor has demonstrated epitaxial growth of HgCdTe (MCT) with good morphology onto both GaAs and GaAs on Si wafers. The density of surface-growth defects, typical of MOVPE growth, has been reduced to <5 cm−2 at a sufficient yield to make the production of low cluster-defect, two-dimensional (2-D) arrays possible. The new horizontal reactor cell uses substrate rotation to achieve improved uniformity and is able to incorporate substrates up to 4-in. diameter. Good compositional and thickness uniformity was achieved on epilayers grown on 3-in.-diameter, low-cost GaAs and GaAs on Si wafers. Sufficient uniformity has been achieved to produce 12 sites of full-TV format 2-D arrays per slice. To yield the benefits of heterostructure design, the MCT epilayers also needed to demonstrate efficient and uniform activation of both arsenic (acceptor) and iodine (donor) dopants. Secondary ion mass spectrometry (SIMS) and Hall assessment showed that the uniformity of As and I doping was ±10%. Fully doped heterostructures have been grown to investigate the device performance in the 3–5 µm and 8–12 µm infrared bands. The 2-D array performance has shown that at 180 K near-background-limited performance (BLIP) diodes have been produced in the 3–5 µm band.

Metal-organic vapor-phase epitaxial growth of HgCdTe device heterostructures on three-inch-diameter substrates

We have investigated the negative luminescent properties of a HgCdTe device, fabricated from a 1 mm diameter array of photodiodes having peak emission at a wavelength of 8.5 μm. This long-wavelength luminescence is of sufficient efficiency and area to be useful in device applications.

Long-wavelength HgCdTe negative luminescent devices

We have made direct pump-probe measurements of spin lifetimes in long-wavelength narrow-gap semiconductors at wavelengths between 4 and 10 \ensuremath{\mu}m and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes ${\ensuremath{\tau}}_{\mathrm{s}}\ensuremath{\sim}300\mathrm{ps}$ even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that ${\ensuremath{\tau}}_{\mathrm{s}}$ is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of ${\ensuremath{\tau}}_{\mathrm{s}}$ in nondegenerate lightly n-type ${\mathrm{Hg}}_{0.78}{\mathrm{Cd}}_{0.22}\mathrm{Te}$ of approximately the same bandgap as InSb and find that ${\ensuremath{\tau}}_{\mathrm{s}}$ varies from 356 ps at 150 K to 24 ps at 300 K. In this material lattice scattering dominates giving a ${T}^{\ensuremath{-}3/2}$ dependence for the mobility, and we expect a strong temperature dependence of ${\ensuremath{\tau}}_{\mathrm{s}}.$ There are two main models that have been invoked for describing spin relaxation in narrow-gap semiconductors: the Elliott-Yafet (EY) model which gives a ${T}^{\ensuremath{-}7/2}$ dependence of ${\ensuremath{\tau}}_{\mathrm{s}}$ in this limit and the D'yakonov-Perel model which gives a ${T}^{\ensuremath{-}3/2}$ dependence. Our results, both in magnitude and temperature dependence of ${\ensuremath{\tau}}_{\mathrm{s}},$ imply that the EY model dominates in these materials.

Electron spin lifetimes in long-wavelength Hg 1-x Cd x Te and InSb at elevated temperature

Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including use as thermal radiation shields in IR cameras, and as IR sources in gas-sensing systems. For many of these applications a large area (>1 cm2) device which displays as large as possible apparent temperature range is required. However, under reverse bias, significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. We have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. To fabricate the concentrators, spherical resist masks are first produced, which are ∼10 μm high and ∼53 μm wide, by resist reflow at 120°C. Inductively coupled plasma (ICP) etching is ...

C. D. Maxey

Papers

Summary of HgCdTe 2D array technology in the U.K.

Metal-organic vapor-phase epitaxial growth of HgCdTe device heterostructures on three-inch-diameter substrates

Long-wavelength HgCdTe negative luminescent devices

Electron spin lifetimes in long-wavelength Hg 1-x Cd x Te and InSb at elevated temperature

Micromachined optical concentrators for IR negative luminescent devices