Lifetime Of Photoexcited Carriers Research Articles

Mechanism of photoconductivity in intrinsic epitaxial CVD diamond studied by photocurrent spectroscopy and photocurrent decay measurements

We report experimental results on photocurrent spectra and the transient decay of photocurrent in intrinsic CVD diamond films grown homoepitaxially on natural, (100) oriented, single crystal type IIa diamond substrates. The detectors fabricated from these layers were developed in the frame of the LYRA project (the LYman-alpha RAdiometer onboard the European satellite PROBA-2) and will be used in space for UV solar monitoring. The typical photosensitivity is about 10 −2 A/W at 200 nm and 10 −6 A/W at 300 nm; the typical response time well below 0.1 ms. The enhance in photosensitivity, related to defects in CVD diamond layer and accompanied by long, non-exponential tails in photocurrent transients (persistent photocurrent), is observed in some detectors. The enhancement in photosensitivity also correlates with a long effective lifetime of photo-excited carriers. New defects at about 3.8 eV and another at about 4.3 eV are observed in the photocurrent spectrum of the sample with enhanced photosensitivity.

Photo-Hall effect measurements in P, N and B-doped diamond at low temperatures

Resistivity and Hall effect measurements are applied to understand the electronic properties of homoepitaxially grown n- and p-type CVD diamond in the wide temperature range 10–900 K. To overcome the high dark resistivity at low temperature, carrier concentration is enhanced by photo-excitation. It is found that the lifetime of photo-excited carriers decreases significantly at low temperature whereas their mobility increases. It is suggested that the short lifetime and the continuous photo-excitation can lead to the non-equilibrium stationary distribution of the photo-excited carriers at low lattice temperature. The enhanced average kinetic energy of the photo-excited carriers can effectively reduce ionized impurity scattering hence increasing the carrier mobility.

Photoluminescence of small-size semiconductor quantum dots

The results of photoexcited carrier dynamics and optically detected microwave resonance (ODMR) spectroscopy of smallsize InAs/GaAs quantum dots (QDs) are presented. Very long lifetimes (1–10 ns) of photoexcited carriers were observed in this system by time-resolved photoluminescence measurements. This feature could be exploited to develop high-sensitivity infrared photodetectors based on small-size quantum dots. The ODMR spectra of InAs/GaAs QDs were detected via modulation of the total intensity of the QDs emission induced by 95 GHz microwave excitation, and the exciton fine structure was studied. The microwave-induced signal at magnetic field of 1.6 T was attributed to magnetic resonance transitions between spin states of holes confined in the dots. The obtained low field (at 0.18 T) signal is ascribed to cyclotron resonance of the electron in a two-dimensional wetting layer, corresponding to an effective electron mass of 0.053m0.

Photoluminescence of GaN deposited on single-crystal bulk AlN with different polarities

Photoluminescence (PL) properties of two GaN epilayers grown in identical conditions on substrates of Al face and N face of bulk single-crystal AlN are studied in the temperature range from 8 to 300 K under weak cw excitation and strong pulsed excitation up to the intensities when electron–hole heating and stimulated emission are observed. At low temperatures and pump intensities, PL of GaN on Al-face AlN is consistent with that of homoepitaxial Ga-face GaN, while GaN on N-face AlN exhibits features indicating the existence of tail localized states. At carrier densities high enough for band-to-band transitions to dominate, the room-temperature PL of GaN on N-face AlN is higher than that in GaN on Al-face AlN due to longer effective lifetime of photoexcited carriers.

Formation of Mn-Related Defect Band in InP

Electron paramagnetic resonance, optical absorption, luminescence and electrical studies of InP highly doped with Mn were performed. Electron paramagnetic resonance revealed presence of manganese in Mn 2 + (d 5 ) configuration. In optical absorption, systematic reduction of InP band gap was observed with increase in Mn content. This was correlated with increase in photoionization-type absorption band starting at 0.2 eV. Time-resolved photoluminescence measurements showed decrease in photoexcited carrier lifetime and shortening of donor-acceptor pair recombination time with increase in Mn content. Moreover, photoluminescence band was shifted to lower energies, similarly to optical band gap. In electrical transport two mechanisms of conductivity were observed. Valence band transport dominated at higher temperatures, above 160 K, and activation energy of free-hole concentration was determined as about 0.20 eV. At lower temperatures hopping conductivity, clearly related to Mn defect band, was present. All these results were consistent with assumption of creation of Mn-related defect-band at 0.2 eV above InP valence band. It was found that Mn centers responsible for this band were in configurations of either d 5 or d 5 plus a hole localized about 7 A around corresponding Mn core.

Study on hydrophilic property of hydro-oxygenated amorphous TiO x:OH thin films

This paper is on hydrophilicity of amorphous TiO x thin films, which were prepared by plasma enhanced chemical vapor deposition, including a large amount of OH groups. The as-deposited film showed a good hydrophilicity (water contact angle below 5°) that deteriorated after long time storage in a dark ambient air (water contact angle of 59°). By UV light irradiation, the film surface recovered its initial hydrophilicity. Atomic force microscopic measurements of tip–sample adhesion force showed homogeneous distribution on the film surface before and after UV light irradiation. We suppose that the OH groups have a strong effect on the film hydrophilicity. Measurements of dark and photo-excited currents suggested that the deposited films have high photo-efficiency and very long lifetime of photo-excited carriers. Hence, we refer to them as hydro-oxygenated amorphous TiO x (a-TiO x :OH) films in analogy with a-Si:H. Furthermore, it was observed that the conductivity of a-TiO x :OH films was sensitive to the surrounding gases, especially to electron-withdrawing oxygen. In ambient air, it is deduced that the photo-excited electrons diffuse toward the film surface and react with oxygen to generate O 2 − . Considering the good hydrophilicity of the as-deposited a-TiO x :OH film and the strong oxidation power of O 2 − , it is concluded that the hydrophilicity of the a-TiO x :OH film was recovered by the photo-catalytic decomposition of hydrophobic adsorbates on the film surface.

Lifetime of photoexcited carriers in modulation-doped quantum dot infrared photodetectors

We have systematically investigated the lifetimes of photoexcited carriers in order to clarify the relationship between the performance and the structural parameters of recently proposed modulation-doped quantum dot infrared photodetectors (MD-QDIPs). It is found that the lifetime as well as the responsivity depends exponentially on the distance between the heterointerface channel and the QD layer. A lifetime of photoexcited carriers as long as 3 ms and a photoconductive gain of ∼10 6 were observed at T=77 K . The extremely large photoconductive gains enable a high-sensitivity operation and demonstrate the superiority of MD-QDIPs.

Normal incidence infrared photoconductor of self-assembled InAs quantum dots in modulation doped AlGaAs/GaAs heterostructures

We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the AlGaAs/GaAs two-dimensional (2D) channels. A photocurrent signal has been observed in the range of 9.5–11 μm with a peak at 10.5 μm due to the bound-to-continuum intersubband absorption of the normal incidence radiation in the self-assembled InAs quantum dots. The peak detectivity was measured to be 1×10 9 cm Hz 1/2/W at 220 K. The high detectivity is realized mainly by low dark current, high mobility and a long lifetime of photoexcited carriers in the modulation doped 2D channels.

Optical nonlinearities and photo-excited carrier lifetime in CdS at 532 nm

Bound-electronic and free-carrier optical nonlinearities, and relaxation of photo-excited free carriers in CdS have been investigated by the use of a single-beam Z-scan technique at 532 nm. Under pulsed radiation of 35-ps duration with the input irradiances up to 4.8 GW/cm 2, the two-photon absorption coefficient, the bound-electron nonlinear refractive index, the free-carrier absorption cross-section, and the change in the refractive index per unit carrier density are determined to be 5.4±0.8 cm/GW, −(5.3±0.8)×10 −13 cm 2/W, (3.0±0.5)×10 −17 cm 2 and −(0.8±0.1)×10 −21 cm 3, respectively. By using these values in the open-aperture Z-scans conducted with 7-ns laser pulses, the carrier recombination time is extracted to be 3.6±0.7 ns. The measured parameters are compared to theoretical calculations.

Nonequilibrium carriers in GaAs grown by low-temperature molecular beam epitaxy

Electronic properties of GaAs grown by low-temperature molecular beam epitaxy are analyzed using a model that assumes a semiconducting matrix with embedded semimetallic As clusters. The static and high-frequency conductivity, Hall effect, lifetime of photoexcited carriers, current-voltage characteristic, and the screening phenomena are all considered. Model results are compared with existing experimental data and a proposal is given for new experiments that could determine currently unknown properties of the material.

Modulation-doped quantum dot infrared photodetectors using self-assembled InAs quantum dots

We have designed and fabricated a new quantum dot infrared photodetector which utilizes lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. The peak responsivity was as high as 2.3 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels.

Ultrafast response of multi-energy proton-bombarded GaAs photoconductors

We have investigated the ultrafast optical and optoelectronic characteristics of multi-energy proton-bombarded GaAs (GaAs:H+) material and devices in some detail. Photo-excited carrier lifetimes of GaAs:H+ were observed to be as low as 350 ± 50 fs. Photoconductive switches (PCS) fabricated on GaAs:H+ were found to exhibit lower dark currents (15 nA at a bias of 10 V) and higher breakdown voltage (> 100 kV/cm) than PCS's fabricated on semi-insulating (S.I.) GaAs. The temporal response of the GaAs:H+ PCS was about 2 ps at full-wave half minimum. Optically excited terahertz (THz) radiation from GaAs:H+ was reported for the first time to our knowledge. The temporal response and spectral bandwidth of the emitted THz radiation were 0.7 ps and 1.25 THz, respectively. The field strength of the THz signal was about 20 mV/cm. From the THz data, we are able to deduce that the effective carrier mobility of GaAs:H+ was less than 1 cm2/V-sec.

Bound-to-continuum intersubband photoconductivity of self-assembled InAs quantum dots in modulation-doped heterostructures

We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to the bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 4.7 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, it is found that the observed photosensitivity survives up to 190 K.

Mid-Infrared Photodetector Using Self-Assembled InAs Quantum Dots Embedded in Modulation-Doped GaAs Quantum Wells

AbstractWe have designed and fabricated a quantum dot infrared photodetector which utilizes lateral transport of photoexcited carriers in the modulation-doped A1GaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 2.3 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, we found that this device has high operation temperature and very high photoconductive gain.

Material and ultrafast optoelectronic properties of furnace-annealed arsenic-ion-implanted GaAs

Structural, electrical, and ultrafast optical properties of furnace-annealed arsenic-ion-implanted GaAs (GaAs:As/sup +/) has been investigated for its applications in ultrafast optoelectronics. From these studies, we determine that GaAs substrates implanted with 200-keV arsenic ions at 10/sup 10/ ions/cm/sup 2/ and furnace-annealed at 500/spl deg/C-600/spl deg/C would have recovered its crystallinity, be highly resistive, and exhibit picosecond photo-excited carrier lifetimes. The duration of the electrical pulses generated by photoconductive switches (PCSs) fabricated on the optimized material was /spl ap/4 ps. The risetime (10%-90%) and l/e falltime were, respectively, /spl ap/2 and 3 ps. These results were measurement-system limited. We estimated the actual response to be /spl ap/2 ps, consistent with a photo-excited carrier lifetime of /spl ap/1.8 ps. The peak responsivity was /spl ges/4/spl times/10/sup -3/ A/W. The dark current for the GaAs:As/sup +/ PCS biased at 40 V was as low as 5 nA. The breakdown field was higher than 150 kV/cm. These characteristics are comparable to those of state-of-the-art photoconductors such as LT-GaAs.

Picosecond responses of low-dosage arsenic-ion-implanted GaAs photoconductors

Ultrafast optoelectronic characteristics of GaAs implanted with 200 keV arsenic ions at dosage as low as 1013 ions/cm2 are reported. Ultrashort photoexcited carrier lifetimes of 0.23±0.02, 0.87±0.02, and 3±0.2 ps were determined for as-implanted, rapid thermal annealing (RTA) (600 °C for 30 s) and furnace-annealed (600 °C for 30 min) arsenic-ion-implanted GaAs or GaAs:As+, respectively. The switching response of photoconductive switches (PCSs) fabricated on the latter two materials were ∼3 and 8 ps, respectively. The leakage current of the PCS fabricated on furnace-annealed sample was as low as 30 nA. The breakdown fields exceeded 150 (instrument-limited) and 90 kV/cm for furnace- and RTA-annealed samples, respectively. The optical responsivity of low-dosage arsenic-implanted GaAs photoconductive switch can be as high as 0.01 A/W. This also compared favorably with the same device made on state-of-the-art photoconductor such as molecular-beam epitaxy grown GaAs at low temperatures.

Electron transport in low temperature grown GaAs

GaAs grown at low temperature exhibits features, such as high resistivity, large breakdown voltage and short photoexcited carrier lifetime, that make it interesting for several device applications. Despite detailed studies of its electrical properties, the mechanism for electron transport has not been definitively determined and thus the debate on the origin for its high resistivity is not settled. Consequently, we have undertaken an impedance spectroscopy study, monitoring the admittance Y versus the pulsation ω of the electric field (from 5 to 13 MHz) and temperature (in the range 300–450 K), in materials grown at various temperatures and submitted to annealing treatments. The data obtained clearly demonstrate that electron transport is the result of a hopping process (the admittance varies linearly with ω) occurring in a partially filled band. The thermal activation of electrons into the conduction band gives the energetical location in the gap, from which we deduce that the EL2 defect is at the origin of the band in which hopping occurs. Moreover, the complete variation Y(ω) is characteristical of a percolation regime which demonstrates that conduction occurs around insulating regions.

Micro through nanostructure investigations of polycrystalline CdTe: Correlations with processing and electronic structures

This paper provides first-time correlations of the nanoscale physical structure with the macroscale electronic and optical properties of CdTe/CdS thin films for several standard deposition techniques. Atomic force microscopy (AFM) was used to determine the micro and nanostructures of polycrystalline CdTe thin films used in photovoltaic (PV) cell fabrication. Photoluminescence (PL) was used to determine band gap, relative defect density, and photoexcited carrier lifetime. Cross-sectional scanning tunneling microscopy (STM) was used to determine the nanoscale electronic properties. Nanostructural features (nanograins), beyond the spatial resolution of conventional scanning electron microscopy (SEM), were observed and characterized in as-deposited CdTe. The correlations of the proximal probe measurements of the physical and electronic structure with the optically determined electronic properties were used to show the effects of the chemical and heat processing, directly and conclusively. A particularly striking effect with important implications for PV applications is the diffusion of sulfur across the CdTe/CdS interface during heat treatment.

Subpicosecond carrier lifetimes in arsenic-ion-implanted GaAs

We have investigated photoexcited carrier lifetimes in arsenic-ion-implanted semi-insulating GaAs by time-resolved reflectivity measurements. Subpicosecond carrier lifetimes (220 to 550 fs) which do not exhibit apparent dosage dependence for samples bombarded with 200 keV arsenic ions at increasing dosages in the range of 1012 and 1016 ions/cm2 are reported. The shortest carrier lifetime was observed for the sample irradiated at 1013 ions/cm−2. These are the shortest lifetimes ever observed for ion-damaged GaAs and comparable to those of low-temperature molecular beam epitaxially grown GaAs, which is also nonstoichiometric with excess-arsenic-related, deep-level defects.

GaAs Ultrafast Photoconductive Switches and Their Application to Ultrafast Photonic Sampling Technology.

GaAs ultrafast photoconductive switches are reviewed from the point of view of their application to ultrafast photonic sampling of high-speed electron devices. The photoconductive switches are interdigitated metal-semiconductor-metal diodes fabricated on low-temperature grown GaAs which reveals femtosecond lifetime of photoexcited carriers. On the basis of ultrafast voltage transients of photoconductive switches, it is pointed out that the dispersion of signal transmission lines and the RC time constant due to electrode capacitance limit the ultrafast performance of the photoconductive switches. Monolithic integration technology is developed for the ultrafast photonic sampling. In this technology, photoconductive switches are connected with an electron device by high-speed waveguides. Fabrication of a nanometer-scale transistor integrated with photoconductive switches is described and ultrafast characteristics of the transistor is presented.