Short Carrier Lifetime Research Articles

Separated-transport-recombination p-i-n photodiode for high-speed and high-power performance

We demonstrate a novel p-i-n photodiode (PD) structure, the separated-transport-recombination PD, which can greatly relieve the tradeoffs among the resistance-capacitance bandwidth limitation, responsivity, and output saturation power performance. Incorporating a short carrier lifetime (less than 1 ps) epitaxial layer to serve as a recombination center, this device exhibits superior speed and power performance to a control PD that has a pure intrinsic photoabsorption layer. Our demonstrated structure can also eliminate the bandwidth degradation problem of the high-speed photodetector, whose active photoabsorption layer is fully composed of short lifetime (/spl sim/1 ps) materials, under high dc bias voltages.

Photovoltaic properties and technological aspects of In1−xGaxN/Si, Ge (0<x<0.6) heterojunction solar cells

Photovoltaic properties of n-In1−xGaxN/p-Si, Ge (IGN) heterostructures, covering the compositional range 0<x<0.6, have been evaluated by 1d device simulation, and are compared with the performance of c-Si homojunction thin film cells. Film morphology and physical properties were characterized by high-resolution transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS) and photoluminescence (PL). Best achievable cell performances under AM1.5 illumination conditions were 18% for p-Ge, and up to 27% for n-IGN/p-Si contacts, achievable under optimum cell design, materials and operation parameters. Pure InN bottom layers, exhibiting an intrinsic band gap of 0.7 eV, reveal a reduced efficiency of 2.5%. The cell efficiency is strongly affected by film quality, accounted for by variation of electron affinity, majority carrier mobility, minority carrier lifetime, film thickness and doping levels. The morphology of thin IGN and InN films deposited onto silicon and sapphire substrate material revealed granular growth, along with a high density of grain boundaries. TEM resolved the formation of a very thin homogeneous silicon nitride interlayer on silicon substrates. The electrically isolating layer almost completely suppresses the photovoltaic effect. Depth profiling of InN films deposited onto sapphire substrates by SIMS analysis indicated oxygen as the dominant material contamination. It accounts, among other effects, for a gradually increasing band gap throughout the film structure. Observed large photoluminescence broadening effects, and related short minority carrier lifetimes are most likely related to high levels of oxygen contamination and concentration of grain boundaries. Possible routes to overcome these problems are discussed.

Gain clamping in semiconductor optical amplifiers with second-order index-coupled DFB grating

A systematic investigation of gain-clamped semiconductor optical amplifiers (GC-SOAs) based on the second-order index-coupled DFB gratings is carried out by way of simulation. In particular, we focus on the main effects of the radiation loss caused by the first-order diffraction of the gratings on the amplifier performance. The magnitude of the total complex coupling coefficient is the main factor to determine the level of gain clamping. We demonstrate that a high-performance GC-SOA can be realized by using purely loss-coupled second-order DFB gratings with more relaxed tolerance on grating strength and period. It is shown that, in the presence of weak reflection-related coupling, the parasitic radiation loss associated with the second-order grating always helps to expand the linear amplification region and to reduce the longitudinal spatial hole burning along the cavity. Further, we demonstrate through comparison that the GC-SOAs have higher saturation power and much shorter carrier lifetime than the conventional SOAs. An improved design by longitudinal variation of the grating duty cycle is proposed such that the noise performance of the amplifier can be enhanced without much sacrifice on the linear amplification regime.

양성자 조사법에 의한 고속스위칭 사이리스터의 제조

A fast switching thyristor with a superior trade-off property between the on-state voltage drop and the turn-off time could be fabricated by the proton irradiation method. After making symmetric thyristor dies with a voltage rating of 1,600 V from 350 <TEX>$\mu$</TEX>m thickness of 60 <TEX>$\Omega$</TEX>ㆍcm NTD-Si wafer and 200 <TEX>$\mu$</TEX>m width of n-base drift layer, the local carrier lifetime control by the proton irradiation was performed with help of the HI-13 tandem accelerator in China. The thyristor samples irradiated with 4.7 MeV proton beam showed a superior trade-off relationship of <TEX>$V_{TM}$</TEX> = 1.55 V and <TEX>$t_{q}$</TEX> = 15 <TEX>$\mu$</TEX>s attributed to a very narrow layer of short carrier lifetime(～1 <TEX>$\mu$</TEX>s) in the middle of its n-base drift region. To explain the small increase of <TEX>$V_{TM}$</TEX> , we will introduce the effect of carrier compensation at the low carrier lifetime region by the diffusion current.ffusion current.t.

Ultrafast optical and magneto-optical studies of III–V ferromagnetic semiconductors

We use ultrafast optical techniques to investigate the dynamics of charge and spin carriers and coherent phonons as well as magnetic order in III-V ferromagnetic semiconductors. We observe a rich array of dynamical phenomena that are absent in traditional nonmagnetic semiconductors or metallic ferromagnets. Very short charge and spin lifetimes of the photoinjected carriers (∼2ps) and multi-level charge decay dynamics are observed, which are attributed to a large density of mid-bandgap states introduced during low temperature molecular beam epitaxy (LT-MBE) growth and highly p-type Mn doping. During the very short free carrier lifetime, the coercivity of the system is seen to be reduced. We attribute this photo-induced ‘softening’ to the transient modification of carrier-mediated ferromagnetic exchange coupling between Mn spins. After the photogenerated free electrons are trapped by defects, periodic oscillations appear in differential reflectivity due to the coherent generation of acoustic phonon wavepackets.

Growth of GaNxAs1−x atomic monolayers and their insertion in the vicinity of GaInAs quantum wells

The deposition of N-rich GaNAs atomic monolayers was investigated. Such layers were successfully grown while exposing a GaAs surface to a nitrogen plasma source during a growth interruption at 400 °C in a molecular beam epitaxy reactor. N accumulation was confirmed and evaluated by secondary ion mass spectroscopy. This process is compatible with regrowth, as in situ monitored by reflection high-energy electron diffraction. The crystal shows good structural quality, as displayed by transmission electron microscopy, that reveals that the accumulation occurred within 1 nm. In a series of samples, two of these ultrathin GaNAs layers were inserted in GaAs barriers, on each side of a GaInAs quantum well (QW). A drastic effect of the N-rich layers on the QW photoluminescence (PL) intensity was observed, as well as on the carrier recombination dynamics, with a strong influence of the spacer thickness between the QW and the N-rich layers. A time-resolved PL analysis of these samples evidenced nonradiative relaxation times in the range of a few ps. This very short carrier lifetime is attributed to the presence of nonradiative centres related to the N-rich layers close to the QW, and can be used to design ultrafast optical devices.

Dynamics of the pump-probe reflectivity spectra in GaAs and GaN

The pump-probe reflectivity (PPR) technique is a quick way to characterize the short carrier lifetime in materials which may be potentially good terahertz (THz) emitters or detectors. Here, we study the PPR signal in semiconductors theoretically in the frequency domain (at various energies above and below the band gap) as a function of pump-probe delay. We consider two conditions of carrier relaxation. In one, the carriers are assumed to form a hot, thermalized energy distribution during excitation itself and then to cool via phonon emission, as is expected in the case of high density excitation in GaAs. In the other case, the carriers essentially remain in a nonequilibrium, nonthermal state even as they relax. This can happen when the carrier-longitudinal optical phonon interaction is stronger than carrier–carrier scattering, as is likely in GaN even at moderately high densities. In addition, effects of carrier trapping and recombination determining the carrier lifetime are included. The calculation takes into account the effect of Sommerfeld factor and pump induced modulation of the probe reflectivity due to band filling (BF), band gap renormalization (BGR), and free carrier absorption. Signatures of carrier cooling and decay can be identified from the delay dependence of the PPR signal at high enough carrier densities (⩾1×1018 cm−3) when the carrier cooling rate is comparable with the decay rate. In that case, carrier cooling shows up in the reflectivity signal as a rise in the time evolution whereas the signal decay is mainly related to carrier decay, albeit in a nonexponential way. However, at lower densities, the signal evolution with delay is rather complex. There, it is not possible to identify the signature of carrier cooling and the decay of the signal is not governed simply by the carrier decay rate. We point out that in general, the magnitude and signature of the PPR signal at different delays are governed by an interplay between the BGR and BF effects. The delay dependence of the signal is a very sensitive function of the form of BGR used to describe its density dependence at low densities. We find that the delay and frequency dependence of the PPR signal is different for a thermalized, cooling distribution from that for the relaxing, nonthermalized distribution. Thus, PPR experiments may be able to distinguish rapid carrier relaxation via a cascade emission of longitudinal optical phonons due to stronger Fröhlich coupling in GaN from cooling of hot, thermalized carriers in GaAs.

Relaxation and diffusion of photoexcited carriers in ZnO epitaxial thin films

Relaxation and diffusion of carriers in an electron-hole plasma (EHP) in ZnO epitaxial thin films were studied by femtosecond transient grating spectroscopy. By performing several experiments of different fringe spacing, the lifetime and diffusion coefficient of the EHP state are determined to be 23.8 ps and 10.8 cm2/s, respectively. The observed lifetime is in good agreement with the previous values obtained by pump-probe and time-resolved optical Kerr gate luminescence measurements. Based on a simple diffusion equation, we conclude that the carrier diffusion in the EHP state does not contribute to reduction of the carrier population within such a short carrier lifetime of a few tens picosecond. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement of Commercial Silicon Solar Cells with N+-P-N+Structure using Halogenic Oxide Passivation

This paper describes the effect of halogenic gettering during oxide passivation of commercial solar cell with the <TEX>$N^{+}$</TEX>-P-<TEX>$N^{+}$</TEX> structure. In order to study the effect of halogenic gettering on <TEX>$N^{+}$</TEX>-P-<TEX>$N^{+}$</TEX> structure mono-crystalline silicon solar cell, we performed conventional POCl<TEX>$_3$</TEX> diffusion for emitter formation and oxide passivation in the presence of HCl vapors. The <TEX>$N^{+}$</TEX>-P-<TEX>$N^{+}$</TEX> structure based silicon solar cells were found to have higher short circuit current and minority carrier lifetime. Their performance was also found to be superior than the conventional <TEX>$N^{+}$</TEX>-P-<TEX>$N^{+}$</TEX> structure based mono-crystalline silicon solar cell. The cell parameters of the <TEX>$n^{+}$</TEX>-p-<TEX>$p^{+}$</TEX> and <TEX>$n^{+}$</TEX>-p-<TEX>$n^{+}$</TEX> structure based cells, passivated by HCl assisted oxidation were measured. The improvement in <TEX>$I_{sc}$</TEX> was attributed to the effect of the increased diffusion length of minority carriers, which came from the halogenic gettering effect during the growth of passivating oxide. The presence of chlorine caused gettering of the cells by removing the heavy metals, if any. The other advantage of the presence of chlorine was the removal of the diffusion induced (in oxygen environment) stacking faults and line defects from the surfaces of the silicon wafers. All these effects caused the improvement of the minority carrier lifetime, which in-turn helped to improve the quality of the solar cells.

Swept away [quantum-well infrared photodetectors

The ultra high-speed capability of quantum-well infrared photodetectors (QWIPs) is well known, thanks to their intrinsic short carrier lifetime (∼5 ps). This obviously makes QWIPs well suited for high-speed and high-frequency applications. In the past, for thermal infrared imaging where devices are optimized to have the highest possible detectivity, the absorption quantum efficiency has been low (<10%). For high-speed applications where lasers are usually used, the most important parameter is the absorption efficiency. We show that high absorption (∼100%) can be easily achieved by simply changing some of the device parameters. We also review the experimental demonstrations of the high-speed capability. At present, QWIPs hold the unique position of having high-speed/-frequency capability and high absorption for the thermal infrared region. There are no competitive alternatives.

Ultrafast carrier dynamics in ferromagnetic InGaMnAs

We have carried out an ultrafast time-resolved differential reflectivity study of a ferromagnetic semiconductor InGaMnAs and made a systematic comparison with low-temperature grown and high-temperature grown InGaAs reference films. Very short carrier lifetimes (∼2 ps) were observed in InGaMnAs and the low-temperature grown InGaAs film, but not in the high-temperature grown InGaAs film. We attribute the short lifetimes to carrier trapping by mid-gap states introduced during low-temperature MBE growth. Furthermore, at long times, we observed periodic oscillations in the differential reflectivity signal with period ∼20 ps, which we interpret as coherent acoustic phonons.

Excitonic field screening and bleaching in InGaN/GaN multiple quantum wells

Photoinduced carrier dynamics in a sequence of InGaN/GaN multiple quantum wells (MQWs) are studied by employing steady state and ultrafast spectroscopy at room temperature. Time-resolved photoluminescence (PL) measured short carrier lifetimes of ∼140 ps at room temperature. Steady state differential transmission was used to measure the in-well field screening due to the photoinjected carriers. The observed offset in emission energy from excitonic screening energies is consistent with the emission of carriers through localized states slightly below the excitonic resonance energy. Furthermore, time-resolved differential transmission with amplified pulses, where significant carrier densities can be optically generated, provides evidence of both excitonic bleaching and field screening in these InGaN quantum wells (QWs). The comparison of the time-resolved differential absorption spectra at various carrier densities allows us to identify different carrier recombination dynamics in the InGaN well and to separate the field screening from the bleaching effects. Finally, the extreme prolongation of the carrier recombination lifetime up to ∼4 μs suggests the spatial separation between electrons and holes under the large in-well fields.

QWIPs DESIGNED FOR HIGH ABSORPTION AND HIGH OPERATING TEMPERATURE

For the majority of applications involving detection of weak signals or thermal imaging, the quantum well infrared photodetector (QWIP) is designed to have the highest possible detectivity and operating temperature. The device parameters, such as the doping density, are chosen accordingly. In a different direction, the intrinsic short carrier lifetime (~ 5 ps) makes QWIPs well suited for high speed and high frequency applications. In such cases, since lasers are normally used, a high dark current can be tolerated. The most important parameter is then the absorption efficiency. For system simplicity and potential wide use, room temperature or near room temperature (reachable by thermo-electric cooling) operations are desirable. This paper discusses the QWIP design for high absorption and elevated temperature operation, and present a systematic experimental study on a set of GaAs/AlGaAs QWIPs with different doping densities. High absorption (~ 100%) and up to room temperature operation are achieved in devices having high doping densities and 100 quantum wells.

Photoconductivity of highly oriented and randomly oriented diamond films for the detection of fast UV laser pulses

Diamond is a wide band-gap material that, depending on purity may exhibit a short carrier lifetime and because of its strong lattice bonding it generally exhibits a high damage threshold. For these reasons it is considered to be suitable for fast and solar blind UV detectors. The use of highly oriented diamond (HOD) films should be advantageous over randomly oriented diamond (ROD) films because of the lower grain boundary density that should lead to an increase in the carrier mobility. In the present study, we compare the photoresponse of UV detectors made of HOD and ROD films using an ArF excimer pulsed laser and we measure the UV-visible discrimination factor of both types of diamond films using a tunable laser. Because the ROD film available for this study was considerably thinner than the respective HOD film, some of the conclusions of this paper are specific to this particular thickness ratio.

High absorption (&amp;gt;90%) quantum-well infrared photodetectors

The intrinsic short carrier lifetime (∼5 ps) makes the quantum-well infrared photodetector (QWIP) well suited for high speed and high frequency applications. In such cases, since lasers are commonly used, a high dark current can be tolerated. The most important parameter is then the absorption efficiency. For system simplicity and potential wide use, room temperature operation is desirable. Using GaAs/AlGaAs QWIPs, high absorption (∼100%) and up to room temperature operation are achieved in devices having high doping densities and 100 quantum wells.

Time-Resolved Photoluminescence Study of InGaN MQW with a p-Contact Layer

To clarify the influence of diffused Mg impurities on the carrier recombination in InGaN multiple quantum wells (MQWs), we have performed a systematic study using time-resolved photoluminescence (PL) measurements. It was found that the MQWs with a p-contact layer and the MQWs with a nondoped GaN layer had almost the same carrier lifetime and PL intensity below 200 K. However, the MQWs with the p-contact layer had shorter carrier lifetime and lower PL intensity than the MQWs with the nondoped GaN layer above 200 K. This degradation of the PL for the MQWs with the p-contact layer can be attributed to nonradiative recombination caused by the diffusion of Mg impurities from the p-contact layer into MQWs.

CVD diamond for radiation detection devices

CVD diamond is a remarkable material for the fabrication of radiation detectors. Radiation hardness, chemical resistance and high temperature operation capabilities of diamond explain its use in the fabrication of devices operating in hostile environments such as that encountered in the nuclear industry and in high energy physics. For this purpose, we have investigated the growth of high quality chemically vapour deposited (CVD) polycrystalline diamond as well as specific material and device processing. CVD diamond films were grown using the microwave plasma enhanced technique. Deposition processes were optimised according to the application requirements. This includes the synthesis of films with high sensitivity, with weak optical absorption in the UV-VIS domain or with short carrier lifetime. One inherent problem with diamond is the presence of defect levels altering the detection characteristics: these may be the cause of an observed instability of the device responses. We have found, however, that it was possible to moderate these trends through the fine-tuning of the growth conditions and of the device preparation steps. Films with thicknesses ranging from 5 to 500 μm have been used for detector fabrication. The role of post-growth treatments and the contact formation procedure was also extensively studied, leading to significant improvements of the detector characteristics. We present recent developments studied at CEA for material optimisation towards its use for specific applications, including radiation hard counters; X-ray intensity, shape and beam position monitors; solar blind photo-detectors, and high dose rate gamma-meters.

The effects of InP grown by He-plasma assisted epitaxy on quantum-well intermixing

He-plasma assisted InP (He*-InP) layers grown by gas source molecular beam epitaxy (GSMBE) have been employed to enhance quantum well (QW) intermixing induced by rapid thermal annealing in a 1.5 /spl mu/m InGaAsP QW laser structure. Inserting a 40 nm He*-InP layer just above the active region enhances the blue-shift for anneal temperatures larger than 680/spl deg/C, and a 42 nm additional blue-shift is obtained at 750/spl deg/C for samples with the He*InP layer, compared to samples with normal InP replacing the He*-InP. This is accompanied by a reduction in the photoluminescence (PL) intensity for anneal temperatures greater than 600/spl deg/C and is attributed to the migration of nonradiative defects from the He*-InP layer into the QWs. Insertion of a thin InGaAs layer between the He*-InP layer and the QW blocks the diffusion of these nonradiative defects into the QW. The results indicate that the He*-InP material could prove useful in QW intermixing to achieve integrated optoelectronic devices, in particular for high-frequency devices which require short carrier lifetimes.

Simulation study of on-state losses as function of carrier life-time for a GaN/SiC high power HBT design

SiC has several properties that makes it more suitable than silicon for high power devices. One problem with SiC bipolar devices is the short carrier life times, and this problem becomes more severe when designing devices for high voltage applications since the dimensions are larger. This work investigates how the Shockley-Read-Hall lifetime influences the on-state characteristics of a HBT or BJT switch in 4H-SiC. The on-state characteristics were simulated with varying SRH lifetimes in the base and drift region. Comparisons were made at 100 A/cm2 collector current density, JC, and at the base current density, JB, where the total on-state power loss of the design is at minimum. The SRH lifetime in the drift region is the dominant parameter for on-state performance, whereas the SRH lifetime in the base is of much less importance. The simulations showed that to reach an acceptable JC/JB-ratio of 100 at power minimum a SRH lifetime of at least 100 µs in the drift region was needed for the HBT design. This lifetime is far from the experimental values reported for 4H-SiC. The advantages of the heterojunction in comparison to ordinary BJTs decreases with shorter SRH lifetimes, but an improvement could always be seen.

High-speed low-temperature-grown GaAs p-i-n traveling-wave photodetector

We report a novel type of p-i-n traveling-wave photodetector utilizing low-temperature-grown GaAs (LTG-GaAs). The devices show a record impulse response time (530-fs fullwidth at half-maximum, /spl sim/560 GHz -3-dB bandwidth) which agrees with theoretical estimates. The effects of various limiting factors on the device performance were analyzed theoretically and compared with measurements obtained by electrooptic characterization of our devices. Calculations indicate that the device speed is dominated by the short carrier lifetime. DC external quantum efficiencies as high as 8% were obtained.