Low Temperature Photocurrent Research Articles

The Low-Temperature Photocurrent Spectrum of Monolayer MoSe2: Excitonic Features and Gate Voltage Dependence.

Two-dimensional transition metal dichalcogenides (2D-TMDs) are among the most promising materials for exploring and exploiting exciton transitions. Excitons in 2D-TMDs present remarkably long lifetimes, even at room temperature. The spectral response of exciton transitions in 2D-TMDs has been thoroughly characterized over the past decade by means of photoluminescence spectroscopy, transmittance spectroscopy, and related techniques; however, the spectral dependence of their electronic response is still not fully characterized. In this work, we investigate the electronic response of exciton transitions in monolayer MoSe2 via low-temperature photocurrent spectroscopy. We identify the spectral features associated with the main exciton and trion transitions, with spectral bandwidths down to 15 meV. We also investigate the effect of the Fermi level on the position and intensity of excitonic spectral features, observing a very strong modulation of the photocurrent, which even undergoes a change in sign when the Fermi level crosses the charge neutrality point. Our results demonstrate the unexploited potential of low-temperature photocurrent spectroscopy for studying excitons in low-dimensional materials, and provide new insight into excitonic transitions in 1L-MoSe2.

Inter-level carrier dynamics and photocurrent generation in large band gap quantum dot solar cell by multistep growth

In this work we present a solar cell structure where the concept of intermediate band is exploited by a high energy barrier AlGaAs material with embedded InAs-based quantum dots via a multistep growth approach. In this way the intrinsic issues related to different surface kinetics of involved species (Ga, In and Al adatoms) and affecting crystal quality are successfully overcome. With respect to energy band engineering of the cell, this growth approach introduces a two-dimensional quaternary layer and consequently an additional energy band, between the host junction and the dot energy levels. This band results strongly related to the quantum dot states by thermal transferring and inter-level filling processes. Moreover, low temperature (up to 100K) photocurrent generation via additional infrared absorption is promoted by the employed band engineering, thus representing an effective method to extend intermediate band solar cell design flexibility.

Hole trap state analysis in SrTiO3

We have analyzed the temperature and time dependences of the photoconductivity response of SrTiO3 single crystals and thin films. We observe a sharp increase in photoconductivity at 35 K and infrared quenching of the photocurrent below this temperature. We use a simple two-level sensitization model to show that the low-temperature photocurrent is controlled by hole trapping at shallow sensitizing centers within 100 meV of the valence band top. We assign these defect sites to intrinsic Sr vacancies in SrTiO3. This work shows that photoconductivity measurement is an effective tool for determining the location and density of acceptor-type defects close to the top of the valence band. This technique is useful for analyzing the effectiveness of p-type doping in wide band gap oxide semiconductors for developing hydrogen evolution photocatalysts that have visible light response.

Stark shifts in mid-infrared type II quantum well transitions

We have studied electric field induced (Stark) shifts in mid-infrared (IR) transitions that occur in type II AlSb/InAs/GaSb quantum wells. Because of the spatial separation of the electron and hole wave functions in the type II system, the potential drop between the layers dominates the shift in the real-space-indirect transition energies when an external electric field is applied. This can result in either a redshift or a blueshift, depending on the ordering of the quantum well layers within the intrinsic region of a p-i-n diode. The case in which a reverse bias on the diode yields a blueshift in the transition energy is of particular interest for IR electro-optic device applications. The modulator section of an integrated source/waveguide modulator would strongly absorb at zero bias and could be biased into transparency, and bistable optical switches could be made more efficient than with redshifting devices. We have used low temperature current-voltage, capacitance-voltage, and photocurrent measurements to characterize a type II quantum well structure that exhibits a blueshift in the lowest energy transitions that is roughly linear with applied bias and is comparable to the potential drop across the structure.

Selective optical doping to predict the performance and reveal the origin of photocurrent peaks in quantum dots-in-a-well infrared photodetectors

Resonant optical pumping across the band gap was used as artificial doping in InAs/In 0.15Ga 0.85As/GaAs quantum dots-in-a-well infrared photodetectors. Through efficient filling of the quantum dot energy levels by simultaneous optical pumping into the ground states and the excited states of the quantum dots, the response was increased by a factor of 10. Low temperature photocurrent peaks observed at 120 and 148 meV were identified as intersubband transitions emanating from the quantum dot ground state and the quantum dot excited state, respectively by a selective increase of the electron population in the different quantum dot energy levels.

Optical pumping as artificial doping in quantum dots-in-a-well infrared photodetectors

Resonant optical pumping across the band gap was used as artificial doping in InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well infrared photodetectors. A selective increase in the electron population in the different quantum dot energy levels enabled the low temperature photocurrent peaks observed at 120 and 148 meV to be identified as intersubband transitions emanating from the quantum dot ground state and the quantum dot excited state, respectively. The response was increased by a factor of 10 through efficient filling of the quantum dot energy levels by simultaneous optical pumping into the ground states and the excited states of the quantum dots.

Growth of polycrystalline phosphorous-doped CVD diamond layers

Microwave plasma-enhanced chemical vapour deposition (MW PE CVD) growth conditions for preparation of polycrystalline phosphorous-doped diamond layers are presented. The incorporation of substitutional phosphorous was confirmed by low temperature photocurrent (PC) and cathodoluminescence (CL) measurements. The topographical characteristics of the films and the relation between the substrate and P-doped film grain orientation were studied by scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD). The growth process for P-doped layers on (1 1 0) oriented polycrystalline diamond was optimised and the best set of parameters differs significantly from the standard conditions used for P-doping of single crystalline (1 1 1) oriented diamond surfaces.

Detailed studies on the origin of nitrogen-related electron traps in dilute GaAsN layers grown by liquid phase epitaxy

Dilute GaAsN layers are grown by liquid phase epitaxy from saturated melts containing polycrystalline GaN as the source of nitrogen. From photoluminescence measurements the nitrogen content in the material is obtained. Low temperature photocurrent and photocapacitance measurements reveal the presence of an electron trap with an ionization energy of 0.65–0.67 eV in the as-grown layers, whose origin is related to interstitial (N–N)As defects. High temperature annealing of the material almost removed the trap and new electron traps at 0.8 and 0.9 eV are produced. It is suggested that during the annealing process the (N–N)As defects, due to their lower energy of formation, are converted to more thermally stable (AsGa–NAs) or (AsN)As defects which might be the source of the new electron traps. High temperature treatment of the growth melt with erbium is found to remove nitrogen from the grown layer with complete annihilation of all the electron traps.

Highly sensitive photocurrent spectroscopy of resonant couplings between GaAs/AlAs superlattice Stark ladders and GaAs single quantum well states

We have investigated interactions between the localized states in a single quantum well (SQW) embedded in the center of a superlattice (SL) and the Wannier–Stark ladder states in the SL by low temperature photocurrent (PC) spectroscopy using numerical differentiation for an analysis of the measured PC spectra. It is found that the PC spectra show drastic changes of transition energies and oscillator strengths due to resonances between the SQW localized states and the extended SL Stark ladder states. We demonstrate high sensitivity of PC spectroscopy by introducing the differentiated spectra and gray-scale representation. Based on transfer matrix calculations of the eigen energies and wave function overlap integrals, the observed anticrossings and delocalization phenomena are rigorously explained.

Exciton resonance induced dips in photocurrent spectra of weakly coupled GaAs [formula omitted]AlAs superlattice p–i–n diodes: barrier thickness dependence

Low-temperature photocurrent (PC) spectra of 100-period GaAs /AlAs superlattice p–i–n diodes with narrow electron miniband widths show negative PC peaks (dips) at the exciton resonance wavelengths under low field conditions. These dips observed at 17–100 K in the positive PC background are caused by the resonance-induced changes of the photogenerated carriers distribution within the thick intrinsic region and by the transit time effects on the vertical tunneling transport which is reduced with increasing the barrier thickness. Enhancement of the dip appearance at higher temperatures (up to 60 K) consistently explains the causes of the PC dips in terms of slowing down of the tunneling transport due to the thermal saturation of the drift velocity.

Low temperature photocurrent spectra of ordered and disordered superlattices

We present low temperature photocurrent (PC) spectra of ordered and disordered GaAs/AlGaAs superlattices (SLs). The investigated disordered SLs were designed to provide both localized and extended states. Our results show a dramatic difference between the PC of ordered and disordered SLs. In the ordered case, the PC is observed to decrease abruptly on increasing the applied voltage V, as expected. In contrast, the total PC signal in the disordered SLs increases in steps as a function of the electric field. Our measurements indicate a transition from a conduction regime to Stark localization in the ordered case, and from the Anderson localization to a conductive regime in the disordered ones. Numerical results obtained by the transfer matrix method indicate the weak dependence and the red shift with V of the disordered SL miniband as the causes of the observed behavior.

Negative peaks in photocurrent spectra of thick barrier GaAs/AlAs multiple quantum wells

We have experimentally studied photocurrent (PC) spectral features of a relatively thick barrier multiple quantum well (MQW) p-i-n diode at 18 and 80 K as a function of axial electric field. It is found that PC spectra do not always reflect the photoabsorption spectral line shape under the low field condition and show negative peaks at the exciton resonance wavelengths. These PC dips are qualitatively explained by considering the distribution of photogenerated carriers within the intrinsic region and competition between carrier transit and recombination times. Furthermore, the dominance of electron-tunneling transport for the low temperature PC mechanism explains enhanced negative peaks in the PC spectra at 80 K when the tunneling assisted drift is reduced.

Selectivity control of Stark-ladder transitions in asymmetric double-well GaAs/AlAs superlattices by barrier sequence modulation

We have investigated field-induced features of optical transitions in asymmetric double-well superlattices (ADW-SLs) with and without barrier sequence modulation by low-temperature photocurrent (PC) spectroscopy. The difference between the heavy-hole confinement energies in the wide and narrow wells results in two types of Stark-ladder transitions, so that four ±1st-order spatially indirect transitions are expected to exist in the ADW-SL. The oscillator strength of these indirect transitions is affected by the strength and the direction of coupling between the wide and narrow wells. The introduction of the barrier sequence modulation into the ADW-SL realizes the control of these coupling mechanisms. This technique is a new method to modulate the oscillator strength of the indirect Stark-ladder transitions.

Photocurrent and photoluminescence measurements in the near-band-edge region of 6H GaN

The optical transitions near the band edge in molecular-beam epitaxy grown 6H GaN films on sapphire have been studied by photocurrent (PC) and photoluminescence (PL) measurements. The low-temperature PC spectra show well-resolved free-exciton transitions, which allow us to determine the A, B, and C free-exciton energies and the corresponding direct band gaps. PL measurements were performed to check the assignment of the excitonic transitions and to derive strain-dependent excitonic parameters from the energetic position of the free A exciton transition, which serve as an input to the fit function for the PC spectra. Variation of the sample temperature between 5 and 300 K allows us to extract the temperature dependence of the band gap.

Characterization of deep centers in undoped semi-insulating GaAs substrates by normalized thermally stimulated current spectroscopy: Comparison of 100 and 150 mm wafers

A well-established characterization method for investigating deep traps in semi-insulating (SI) GaAs is thermally stimulated current (TSC) spectroscopy; however, TSC is not considered to be a quantitative technique because it involves carrier mobility, lifetime, and geometric factors, which are either unknown or poorly known. In this paper, we first show how to quantify a TSC spectrum, by normalizing with infrared (hv = 1.13 eV) photocurrent, and then apply this method (called NTSC) to study the lateral uniformity of the main deep centers across the diameters of undoped SI GaAs wafers. The wafers used in the study include both the standard 100 mm sizes and the new 150 mm variations, and are grown by both the low and high pressure liquid encapsulated Czochralski techniques. The results reveal that the 150 mm wafers have a worse NTSC uniformity for the main traps and a higher degree of compensation, as compared these parameters for the 100 mm wafers. In addition, nonuniformities related to the electric field effects on both the TSC spectrum and the low temperature photocurrent are found in the 150 mm wafer grown by the low pressure technique.

Dependence on the In concentration of the piezoelectric field in (111)B InGaAs/GaAs strained heterostructures

A series of (111)B InxGa1−xAs/GaAs multiple quantum well p-i-n structures have been investigated via low temperature photocurrent and photoluminescence spectroscopies. The indium mole fraction was in the range of 0.07–0.23. Evolution of the experimental blueshifts of the transition energies, with the external bias, agreed very well with the theoretical calculations. This allowed us to obtain precise information about the piezoelectric constant, e14, for the various In compositions. For the range of x investigated, we have found e14(x) to be linear with x but significantly lower than predicted by a simple linear interpolation of the accepted values for GaAs and InAs.

Band discontinuity of strained-layer GaInAs/GaInAsP heterostructures

We performed low temperature photoluminescence excitation and photocurrent measurements on tensile and compressively strained GaxIn1−xAs quantum-well layers grown on GaInAsP, which was lattice matched to the InP substrate. From these measurements the excitonic transitions in the unstrained and strained samples can be identified. We determine the effective masses in growth direction GaxIn1−xAs (0.3<xGa<0.6) and the band offsets of the GaxIn1−xAs/GaInAsP heterojunction (0.4<xGa<0.6).

Optical transitions in GaAs/AlAs superlattices with different miniband widths

Band edge optical transitions and their external electric field effects are systematically studied in GaAs/AlAs superlattices with different miniband widths (2Δ) by low temperature photocurrent spectroscopy. Distinct Π excitons related with saddle-type M 1 critical points at the miniband top are clearly observed both for heavy-hole and light-hole excitons. Under the applied field strength F, discrete transitions due to Wannier-Stark localization are observed and it is found that their intensities of nth order Stark ladder transitions with negative indices show F −2 n and Δ 2 dependences in agreement with the tight binding theory.

Optical Investigation of Strained-Layer GaInAs/GaInAsP Heterostructures

AbstractWe performed low temperature photocurrent and photoluminescence excitation spectroscopy on tensile and compressively strained GaxIn1-xAs/GaInAsP quantum well layers to determine the band offset of the heterojunction (0.3 <XGa < 0.7). The ratio of the conduction band discontinuity to the heavy hole discontinuity has been obtained from well to barrier transitions and is found to be about 35/65 for gallium contents between 0.4 and 0.6. We obtained the effective heavy hole mass by comparison of PLE transition energies with calculations of the subband levels. We observe that the effective heavy hole mass increases with the gallium content from 0.3 m0 for XGa = 0.31 to about 0.45m0 for XGa = 0.55.

Optical spectroscopy of electrons and minority holes tunnelling in double-barrier diodes under operation

The low-temperature photocurrent, steady-state and time-resolved photoluminescence of a double-barrier diode under operation is studied. The observation of luminescence, with exciting light above (as well as below) the quantum well absorption edge, indicates that the tunnelling of holes plays an important role. Furthermore the authors have seen evidence of resonant tunnelling of minority holes when the double-barrier diode is illuminated. These holes are provided by a quantised level on the collector side. They recombine with collector electrons, which provides a signature of this level. The strong Stark shift of this new crossed transition gives information on the potential distribution in the diode.