Transport Of Photoexcited Carriers Research Articles

Lateral transport of photoexcited carriers in semiconductor superlattices

I investigate the lateral transport of photoexcited carriers in semiconductor superlattices by microphotoluminescence imaging. Tuning the excitation excess energy, I find that the transport length of carriers shows pronounced and regular oscillation feature with a period of about 5 meV. Such a behavior is not observed from quantum-well structure under similar conditions, confirming it as an intrinsic property of carriers in superlattices. A qualitative explanation is proposed based on folding of the Brillouin zone and of the phonon dispersion. The experimental result reveals the complication of lateral transport process and implies a coupling between lateral and vertical transports of carriers in superlattice.

Photoconductivity in organic thin films: From picoseconds to seconds after excitation

We present a detailed study, on time scales from picoseconds to seconds, of transient and continuous wave (cw) photoconductivity in solution-grown thin films of functionalized pentacene (Pc), anthradithiophene (ADT), and dicyanomethylenedihydrofuran (DCDHF). In all films, at temperatures of 285–350 K, we observe fast carrier photogeneration and nonthermally activated charge transport on picosecond time scales. At ∼30 ps after photoexcitation at room temperature and at applied electric field of 1.2×104 V/cm, values obtained for the product of mobility and photogeneration efficiency, μη, in ADT-tri-isoproplysilylethynyl-(TIPS)-F, Pc-TIPS, and DCDHF films are ∼0.018–0.025, ∼0.01–0.022, and ∼0.002–0.004 cm2/V s, respectively, depending on the film quality, and are weakly electric field dependent. In functionalized ADT and Pc films, the power-law decay dynamics of the transient photoconductivity is observed, on time scales of up to ∼1 μs after photoexcitation, in the best samples. In contrast, in DCDHF amorphous glass, most of the photogenerated carriers are trapped within ∼200 ps. Transport of photoexcited carriers on longer time scales is probed by cw illumination through an optical chopper, with a variable chopper frequency. In contrast with what is observed on picosecond time scales, charge carriers on millisecond and longer time scales are predominantly localized, and are characterized by a broad distribution of carrier lifetimes. Such carriers make the principal contributions to dc photoconductivity.

Modeling of THz - Electro-Optical Sampling Measurements

Recent experiments on ultrafast high-field transport of photoexcited carriers in superlattices and their interpretation [1, 2] have stimulated a critical discussion [3] upon the possibility to determine the spectrum of the small-signal conductivity of the Bloch oscillating electrons by using the THz–electro-optical (THz–EO) sampling technique in the time domain. The THz–EO technique allows one to measure directly the time response of the THz electric field ETHz(t) ∼ dj(t) dt (1)

Fermi level splitting and thermionic current improvement in low-dimensional multi-quantum-well (MQW) p-i-n structures

Photo-excitation and subsequent thermionic currents are essential components of photo-excited carrier transport in multi-quantum-well photovoltaic (hetero-PV) structures. p-i-n multi-quantum structures are useful probes for a better understanding of PV device properties. Illumination of the intrinsic region of p-i-n multi-structures causes carrier trapping in any of the quantum wells, and subsequent carrier recombination or thermal escape is possible. At the vicinity of a quantum well, we find that the (quasi) Fermi levels undergo an upward split by a small, but non-negligible, energy amount Δ E F in the order of 12 meV. We conclude this fact by comparing the photo-excited carriers trapped in a quantum well, under illumination, to the carrier concentrations under dark. Based on such a prediction, we subsequently relate thermionic current density dependence on Fermi level splitting, concluding that excess thermal currents may increase by a factor of the order of 2. We conclude that illumination causes (a) Fermi level separation and (b) an apparent increase in thermionic currents.

High electric field response of wide bandgap a-Si:H photodiodes probed by transient current measurements

Diode devices n/i/Pt and n/i/p(Sb 2S 3) were fabricated using high-quality wide bandgap a-Si:H (bandgap ∼1.85 eV) for the intrinsic photoactive layers to investigate high electric field transport of photoexcited carriers. Time-resolved photoresponse was examined by a transient current method. It was shown that primary and secondary photocurrents were distinguished by the transient photocurrent method. The n/i/Pt diode device showed a photocurrent gain as large as 10 3 but its response was slow, 5 ms, which is associated with secondary photocurrent. The use of p-type Sb 2S 3 for electron blocking layer instead of Pt successfully suppressed the secondary photocurrent. Fast photoresponse with a response time <50 μs was observed in this device. The mechanism of the dark current leak was associated with shallow defect states at the i/p(Sb 2S 3) interface through a thermally-excited process.

Dynamics of unidirectional phonon-assisted transport of photoexcited carriers in step-gradedInx(Al0.17Ga0.83)1−xAs/Al0.17Ga0.83Asmultiple quantum wells

The dynamics of perpendicular transport of photoexcited carriers assisted by phonon scattering is investigated in a novel step-graded ${\mathrm{In}}_{x}({\mathrm{Al}}_{0.17}{\mathrm{Ga}}_{0.83}{)}_{1\ensuremath{-}x}\mathrm{As}$ quantum-well heterostructure by measuring the temperature dependence of spectrally and temporally resolved photoluminescence (PL). When builtin potential gradients are present in the quantum-well heterostructure due to variations in the In mole fraction (x) in the well, carriers that are thermally released by the particular well move unidirectionally from shallower to deeper wells. That is, asymmetric unidirectional motion of photoexcited carriers is possible via phonon-assisted activation above the barrier band-edge state. We have directly measured this perpendicular motion of photoexcited carriers by monitoring the transient PL signals from the different wells, which are spectrally separated. A rate equation analysis rigorously explains the dynamical changes of the PL signal intensities from the quantum wells as a function of lattice temperature. Our study of PL dynamics proves the asymmetric perpendicular flow of photoexcited carriers and the capture by the deeper quantum wells, providing firm evidence for the dynamical carrier flow and capture processes in the novel heterostructure.

気相合成ダイヤモンド単結晶薄膜における短パルスレーザー励起キャリアの輸送特性

In this study, we have measured photocurrents and time-resolved photoluminescence spectra of high-quality CVD diamond UV detectors using a short pulse UV laser (wavelength : 220 nm). Photo-excited carrier transport properties measured for CVD diamond films under electric field (E) demonstrate that the photocurrent peak intensity at the shortest period after the pulse excitation proportionally increased with increasing E in the range of E below ≈1 × 103 V/cm while it was proportional to E1/2 in the range of E above ≈ 1 × 103 V/cm. This well corresponds to changes of carrier drift speeds theoretically obtained for sufficiently high quality diamond. On the other hand, the photocurrent intensity integrated in a constant time period (below 360 ns) is almost proportional to E in the whole E regions measured (to 1× 104 V/cm). On the contrary, photoluminescence (PL) intensity only slowly decreased with increasing E while the PL decay constants were independent of E. Furthermore, the non-radiative relaxation processes of photo-excited carriers were almost independent of E in the examined field region.

High-sensitivity modulation-doped quantum dot infrared photodetectors

We have designed and fabricated quantum dot (QD) infrared photodetectors which utilize photoionization of self-assembled InAs QDs and lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional channels (modulation-doped quantum-dot infrared photodetectors; MD-QDIPs). A broad photocurrent signal has been observed in the mid-infrared range. A very large photoconductive gain of the order of 105-6 is achieved by long lifetimes as well as high mobilities of photoexcited carriers in the modulation-doped conduction channels.

Dynamical transport of photoexcited carriers between a narrow and a wide quantum well embedded in a GaAs/AlAs superlattice

The temperature dependence of the emission properties in a composite quantum well structure consisting of a wide and a narrow GaAs quantum well (QW) separated by and embedded in a GaAs/AlAs short-period superlattice (SPS) has been studied by steady-state and time-resolved photoluminescence (PL) measurements. At low temperatures (20 K) , distinct PL peaks originating from the QWs and SPS are observed. When the temperature is increased to 60 K , the PL intensity of the wide QW with strongly confined states significantly increases, while the ones of the narrow QW and the SPS gradually decrease. Above 100 K , however, the PL intensity of the wide QW decreases and the others increase. The temperature dependence of the PL dynamics indicates that the PL properties of the composite QWs are connected to Bloch-type transport of photoexcited carriers in the SPS between the wide and narrow QW.

A quantum dot infrared photodetector with lateral carrier transport

In this contribution we present a normal-incidence quantum dot (QD) infrared detector structure. It is based on intra-conduction band transitions between the p-states of the QD and the wetting layer subband and lateral transport of photoexcited carriers in a channel next to the quantum dot layers. The photoresponse is peaked at 6.5 μm (186 meV ) and reaches several A/W up to 60 K . The intrinsic detector response time is determined to be about 0.8 ms . Temperature and frequency dependence of the detector structure are discussed.

The role of interfaces in thin-film CdTe solar cells

AbstractThin-film CdTe solar cells are very promising for future cost-effective photovoltaics. The photovoltaic effect in these cells is based on the extraction of photoexcited carriers by the field provided by the CdTe/CdS heterojunction. An additional interface with non-rectifying characteristics is needed to close the external circuit on the other side of the CdTe thin film. Finally, the transport of photoexcited carriers is influenced by the presence of grain boundaries (GBs). In this contribution, we investigate several aspects of these interfaces and their effects on the operation of CdTe solar cells by electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements.

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.

Tunneling competition of photoexcited carriers in a system of monolithically integrated dual multiple InGaAs/AlGaAs and GaAs/AlGaAs quantum wells

Vertical transport of photoexcited carriers has been studied in a p–i–n diode whose intrinsic layer contains two different multiple quantum wells (MQW), GaAs/Al0.15Ga0.85As (MQW1) and strained In0.15Ga0.85As/Al0.15Ga0.85As (MQW2) isolated by a thick Al0.15Ga0.85As barrier. Pseudo-negative photocurrent (PC) peaks are observed at exciton resonance wavelengths of MQW1 located far from the n-electrode under low electric fields, while the normal positive excitonic peaks recover with increasing the electric field. Moreover, the PC intensity of MQW1 as a function of inverse electric field shows linear dependence due to Fowler–Nordheim-type tunneling with a slope change. The observed PC intensity crossover is rigorously explained by tunneling probability calculations, because of differences in the thickness and height of the transport limiting tunneling barriers, assuming dominance of electron tunneling transport for the PC responses.

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.

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.

Vertical transport of photo-excited carriers for excitonic recombinations in modulation doped [formula omitted] heterojunctions

We propose a vertical transport of the photo-excited carriers under the influence of the two dimensional carrier gas (2DCG) in GaAs Ga 1−x Al x As heterojunctions before excitonic recombination. The vertical transport is responsible for a long photoluminescence (PL) rese time of the excitonic transitions as well as for the PL intensity oscillations with magnetic field. The vertical transport is proved for the first time by resonant PL intensity variation at the cyclotron resonance condition of the 2DCG, by which the effective masses of two dimensional electrons as well as two dimensional holes in the n- and p-type heterojunctions are determined.

Spin diffusion of optically oriented electrons and photon entrainment in n-gallium arsenide

An experimental and theoretical study of spin transport in the n-GaAs semiconductor is reported. Transport of average electron spin from the photoexcited crystal surface is shown to be determined by the spin diffusion process. At the same time the transport of photoexcited carriers takes place primarily through photon entrainment, which transfers nonequilibrium carriers into the bulk of the semiconductor to distances considerably in excess of the electron spin diffusion length. A comparison of the experimental results with theory permits one to determine the average-spin diffusion length and electron-spin relaxation time.

Photoexcited perpendicular transport through InGaAs/lnP multiple quantum well p-i-n heterostructures

A study of the perpendicular transport of photogene rated carriers through InGaAs/InP quantum wells grown by chemical beam epitaxy is presented and p-i-n diodes with a mesa structure containing several quantum wells within the intrinsic region have been investigated. Photocurrent spectra were obtained as afunction of temperature, intensity of light, and electric field. The excitonic peaks, identified by considering absorption measurements and interband transition energies, were observed up to room temperature. At low temperatures, afull bleaching of excitonic features associated with a blue shift of the long wavelength spectrum edge and a red shift (Stark effect) of the hh–e1 exciton peak were observed at low and highfield, respectively. Capacitance and photocurrent steps detected with increasing reverse bias are explained by considering afield dependence of the escape time of photogenerated carriers in a regime of non-resonant tunnelling. Thermally activated (thermionic emission over the barrier) and thermally assisted (phonon assisted tunnelling) processes were found to dominate the transport of photoexcited carriers at high and low temperature, respectively.MST/3299

Exciton transitions in InGaAs/InP quantum wells investigated by photocurrent spectroscopy

A detailed analysis of the exciton resonances dominating the intersubband transitions of InGaAs/InP multi-quantum wells grown by chemical beam epitaxy is presented. Pin diodes with a mesa structure, containing several quantum wells within the intrinsic region, were investigated. Photocurrent spectra were detected as a function of temperature and the exciton peaks HH-E 1, LH-E 1 and HH-E 2, identified by a comparison with calculations, were observed up to room temperature. Full bleaching of the exciton features, induced by exciton localization effects, was reached at low temperature. In agreement with optical absorption data, the temperature dependence of the exciton peaks was studied by lineshape fitting: a homogeneous broadening contribution ascribed to the interaction with LO phonons was found. Capacitance-voltage measurements at variable temperature (10–300 K) were performed: the temperature dependence of the space charge region width and impurity density profile was studied. The photocurrent corresponding to the ( n = 1) heavy hole to electron exciton transition (HH-E 1) was measured as a function of the reverse bias, temperature and illumination intensity. At zero bias, strong thermal activation of the photocurrent was measured, while, as a function of the reverse bias, some shoulders were observed in the low-temperature characteristics. Thermionic emission over the barriers and phonon-assisted tunnelling were identified as the dominant contributions to the perpendicular transport of photoexcited carriers.