Band Edge Discontinuity Research Articles

Sub-nanosecond acousto-electric carrier redistribution dynamics and transport in polytypic GaAs nanowires

The authors report on a combined structural, optical and acousto-electric study of polytypic GaAs nanowires. Two types of nanowires with different zincblende and wurtzite crystal phase mixing are identified by transmission electron microscopy and photoluminescence spectroscopy. The nanowires exhibit characteristic recombination channels which are assigned to different types of spatially direct recombination (electron and hole within the same crystal phase segment) and spatially indirect recombination (electron and holes localized in different segments). Contact-free acousto-optoelectric spectroscopy is employed to resolve spatiotemporal charge carrier dynamics between different recombination channels induced by a piezoelectric surface acoustic wave. The observed suppression of the emission and its dynamic temporal modulation shows unambiguous fingerprints of the local bandedge variations induced by the crystal phase mixing. A nanowire, which exhibits a variation from a near-pristine zinc blende crystal structure to a highly mixed crystal phase, shows a clear dependence on the propagation direction of the acoustic wave. In contrast, no pronounced directionality is found for a nanowire with an extended near-pristine zincblende segment. The experimental findings are corroborated by solving the drift and diffusion equations of electrons and holes induced by the surface acoustic wave. The key characteristics observed in our experimental data are well reproduced in the numerical simulations by assuming two general bandedge modulations and realistic parameters for the bandedge discontinuities and transport mobilities of electrons and holes. This evidences that even all relevant physical processes are accounted for in the model.

ZnO as an anti-reflective layer for GaAs based heterojunction solar cell

Currently, how to improve the efficiency of solar cells has attracted wide attention. ZnO film is one of the most effective films today, which can act as both emitter and anti-reflective coating of solar cells. In this paper, n-type ZnO/p-type GaAs solar cell is modeled by analyzing the band edge discontinuities, electric field distributions at the ZnO/GaAs interface and cell parameters with varying ZnO layer thickness, affinity values and carrier concentration. Moreover, in order to improve the band offset alignment at the heterojunction, Mg doped ZnO emitter is a possible alternative. Then, the thickness and carrier concentration of MgZnO emitter layer are studied and simulation results show stronger electric field, better fill factor and higher efficiency. After optimization of two solar cells by using Silvaco Atlas, it is observed that the conversion efficiencies of ZnO/GaAs and MgZnO/GaAs solar cells are 22.84% and 23.44% respectively.

Band Alignment and Interface Recombination in NiO/β-Ga2O3 Type-II p-n Heterojunctions

Engineering oxide interfaces with defined electronic band structures is of vital importance for designing all-oxide devices with controllable multifunctionality and improved performance. Here, we report the band alignment, band bending, and transport mechanism in the NiO/ $\beta $ -Ga2O3 p-n heterojunction (HJ) which exhibits high performances with a rectification ratio over 1011, a turn-on voltage of 1.87 V and specific ON-resistance of 10.2 $\text{m}\Omega \cdot $ cm2. A type-II band alignment is identified at NiO/ $\beta $ -Ga2O3 HJs with a valence band offset of 3.60 eV and a conduction band offset of 2.68 eV, respectively, determined from the depth-profiled X-ray photoelectron spectroscopic analysis. Besides band edge discontinuity, an additional built-in potential of 0.78 V is observed at the interface due to the charge transfer across the p-n-junction. In comparison, the NiO/ $\beta $ -Ga2O3 p-n HJ has lower leakage current and higher breakdown voltage than that of the Ni/Ga2O3 Schottky barrier diode. Capacitance–frequency analysis indicates the presence of interfacial states, and interface recombination is the dominant transport mechanism. The type-II NiO/Ga2O3 HJ provides favorable energetics for facile separation and transportation of photogenerated electrons and holes, which is important for all-oxide devices that require bipolar operation and power devices with higher conversion efficiencies.

Thallium quantum dot photodetectors

This work studies, theoretically, the thallium-based quantum dot photodetectors. The structures studied are $$In_{.93} Tl_{.07} Sb$$, $$In_{.33} Tl_{.67} P$$, $$In_{.85} Tl_{.15} As/GaAs$$, and $$In_{0.8} Tl_{0.2} N$$. Some parameters of the structures, like band-edge discontinuity, are not determined, yet. Structure parameters, energy subbands, absorption, and quantum efficiency (QE) spectra are calculated. High QE was obtained with the first structure which can be tuned at the required wavelength. High QE is obtained for the structures $$In_{.93} Tl_{.07} Sb$$ and $$In_{.33} Tl_{.67} P$$ when the junction depth is comparable to or less than the hole diffusion length $$(L_{p} )$$, while for the other two structures ($$In_{.85} Tl_{.15} As$$ and $$In_{0.8} Tl_{0.2} N$$) the junction depth must be larger than the hole diffusion length. The proposed photodetectors are covering the infrared range 3000–12000 nm which have an important applications.

Modelling, simulation, optimization of Si/ZnO and Si/ZnMgO heterojunction solar cells

Currently, crystalline and polycrystalline silicon wafer based solar cells dominate the photovoltaic market, which employs expensive manufacturing processes. Zinc oxide is one among the inexpensive alternatives for acting as emitter/anti-reflection coating layer which can be used in combination with widely available p-type silicon. Hence it becomes essential to gain in-depth knowledge about the charge transport mechanisms involved in the n-type ZnO/p-type silicon (Si) heterojunction devices. Therefore, Si/ZnO solar cell is modeled and the carrier transport mechanisms are carefully examined by analyzing the band edge discontinuities, electric field distributions at the Si/ZnO interface, carrier generation-recombination profile for varying ZnO thickness, carrier density and affinity values. The photo- generation rate degrades with the increase in ZnO emitter thickness owing to increased absorption in the blue region and is independent of ZnO affinity and donor concentration. The cliff like configuration with high conduction band discontinuity at the Si/ZnO interface for higher values of ZnO affinity has resulted in the formation of poor electric field and the lattice mismatch resulting in high defects formation (recombination centers) at Si/ZnO interface are the predominant causes for deficit in open circuit voltage and fill factor. Hence the conduction band offset alignment is further engineered by designing Si/MgZnO solar cell model and Mg doped ZnO emitter has significantly improved the band offset alignment at the p-n heterojunction when compared to ZnO emitter. Moreover, the simulation results revealed that the stronger electric field strength developed at the heterojunction is suggested to favor the carrier separation process by drift motion thereby improving the open circuit voltage and fill factor of the device. As a next step, both the models are optimized for varying emitter parameters like thickness, carrier density etc using Silvaco ATLAS simulator. Based on this investigation, the optimized Si/ZnO and Si/MgZnO designs anticipate improved conversion efficiencies of 11.57% and 14.46% respectively.

The Native Material Limit of Electron and Hole Mobilities in Semiconductor Nanowires.

Piezoelectric surface acoustic waves are employed to induce radio frequency spatiotemporal dynamics of photogenerated electrons and holes in the GaAs core of individual GaAs/AlGaAs core/shell semiconductor nanowires. Comparison of the time-dependent interband optical recombination to numerical simulations allow to determine the charge carrier transport mobilities of electrons, μe = 500–250+500 cm2/(V s), holes, μh = 50–30+50 cm2/(V s) and their ratio μe:μh = (20 ± 5):1. Our method probes carrier transport at low carrier density. Thus, the obtained values represent the native material limit of these nanowires, determined by their structural properties. We show that for near-pristine nanowires, individual twin defects do not significantly affect electrical transport, in strong contrast to polytypic nanowires. In the acoustoelectrically modulated emission, we observe unambiguous signatures of (i) hole localization within long wurtzite-rich segments and (ii) electrons in zinc blende regions being reflected at the interface to a wurtzite-rich region. The experimentally observed periodic emission bursts are faithfully reproduced by advanced numerical simulations which include static band edge discontinuities between a single wurtzite segment in an otherwise pure zinc blende nanowire. Otherwise using the same input parameters as for near-pristine zinc blende nanowires, we can deduce from our simulations a minimum conduction band offset of ΔEC ≈ 20 meV at the interface between the zinc blende part and the wurtzite-rich region. These results furthermore confirm that a single wurtzite segment with sufficiently large band offsets efficiently traps holes and blocks electron transport.

Direct Measurement of Band Edge Discontinuity in Individual Core–Shell Nanowires by Photocurrent Spectroscopy

Group III-V coaxial core-shell semiconducting nanowire heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic, and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in nanowires are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual nanowires. Here, we report on, in the GaAs/AlxGa1-xAs and GaAs/AlAs core-shell nanowire systems, how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets.

Photoluminescence spectroscopy study of excited states in In<SUB align="right">xGa<SUB align="right">1−xAs-capped InAs quantum dots

This study reports on photoluminescence (PL) and PL excitation (PLE) spectroscopy studies of excited state transitions in InxGa1–xAs-capped InAs quantum dots (QDs). InAs self-assembled QDs were grown on GaAs (001) wafers using 2.5 mono layer (ML) InAs deposition via molecular beam epitaxy. GaAs-capped QDs showed PL peak position at 1.182 μm at 78 K with a narrow full width at half maximum of 23 meV, indicating a highly uniform QD ensemble. The PLE spectrum of GaAs-capped QDs showed four peaks related to the QD excited state transitions at 1.117, 1.140, 1.192 and 1.224 eV at 8 K. Both ground state and excited state interband transitions were red-shifted when the first 30 ML of GaAs capping layer were replaced by InxGa1–xAs layer. The PL peak of In0.2Ga0.8As-capped QDs reached 1.246 μm (1.335 μm) at 78 K (296 K). Energy differences between the excited and ground states became smaller with increasing In composition of the InxGa1–xAs capping layers because of increasingly lower InAs to InxGa1–xAs band edge discontinuity as a result of the chemical and strain relieving effects. In addition, PLE spectra clearly showed wetting layer and quantum well states caused by the InxGa1–xAs capping layer for GaAs- and InxGa1–xAs-capped QDs, respectively. Based on the current PL, PLE, and the calculated transition energies available in literature, the lowest electron state levels of ~60 meV and ~112 meV above the electron ground state and the lowest hole state levels of ~23 meV and ~55 meV above the hole ground state were extracted.

Characterization of conduction band edge discontinuity for InAlAs/GaAsSb using InP/GaAsSb heterojunction bipolar transistors

AbstractIn this work, the temperature‐dependent DC performance of InP/GaAsSb double heterojunction bipolar transistors (DHBTs) with an InP/InAlAs composite emitter was characterized. The current transport mechanisms in DHBTs with a type‐I InAlAs/GaAsSb emitter–base junction interface and a type‐II GaAsSb/InP base–collector were studied. The experimental results reveal that electron injection at emitter–base junction could be affected by conduction barrier limited carrier transport. A conduction band edge discontinuity of 9.5 meV for InAlAs/GaAsSb heterojunction was experimentally estimated.

Improvement of breakdown characteristics in AlGaN/GaN/AlxGa1−xN HEMT based on a grading AlxGa1−xN buffer layer

AbstractTo improve the breakdown characteristics of an AlGaN/GaN based high electron mobility transistor (HEMT) for high voltage applications, AlGaN/GaN/AlxGa1−xN double heterostructure (DH‐HEMTs) were designed and fabricated by replacing the semi‐insulating GaN buffer with content graded AlxGa1−xN (x = x1 → x2, x1 > x2), in turn linearly lowering the Al content x from x1 = 90% to x2 = 5% toward the front side GaN channel on a high temperature AlN buffer layer. The use of a highly resistive AlxGa1−xN epilayer suppresses the parasitic conduction in the GaN buffer, and the band edge discontinuity limits the channel electrons spillover, thereby reducing leakage current and drain current collapse. In comparison with the conventional HEMT that use a semi‐insulating GaN buffer, the fabricated DH‐HEMT device with the same size presents a remarkable enhancement of the breakdown voltage.

Avalanche characteristics of thin GaAs/Al 0.6Ga 0.4As heterojunction avalanche photodiodes

The mean multiplication gain and excess noise factor of thin GaAs/Al 0.6Ga 0.4As heterojunction avalanche photodiodes (HAPDs) are simulated. The ionization coefficients of electron and hole in bulk GaAs and Al 0.6Ga 0.4As are used in this model to study the role of heterojunction in reducing excess noise. The band-edge discontinuities at the conduction and valence bands are included in our model which may influence the number of carrier crossing the heterojunction and hence modifies the dead space in the HAPDs. The mean multiplication gain and excess noise factor with electron- and hole-initiated multiplication for 0.1 and 0.2 μm multiplication lengths in GaAs/Al 0.6Ga 0.4As HAPDs are shown. By considering the dead space effect, our model demonstrated a small noise mainly due to the localization of carrier ionization and the limited carrier feedback ionization at heterointerface. In our model, most of the ionizations occur in the first-initiated multiplication layer which reduces the randomness of carrier ionization and noise.

Simulation and optimization of GaN-based metal-oxide-semiconductor high-electron-mobility-transistor using field-dependent drift velocity model

Undoped GaN-based metal-oxide-semiconductor high-electron-mobility-transistors (MOS-HEMTs) with atomic-layer-deposited Al2O3 gate dielectrics are fabricated with gate lengths from 1 μm up to 40 μm. With a two-dimensional numerical simulator, we report simulation results of the GaN-based MOS-HEMTs using field-dependent drift velocity model. A developed model, taking into account polarization-induced charges and defect-induced traps at all of the interfaces and process-related trap levels of bulk traps measured from experiments, is built. The simulated output characteristics are in good agreement with reported experimental data. The effect of the high field at the drain-side gate edge and bulk trap density of GaN on the output performance is discussed in detail for the device optimization. AlGaN/GaN/AlN quantum-well (QW) MOS-HEMTs have been proposed and demonstrated based on numerical simulations. The simulation results also link the current collapse with electrons spreading into the bulk, and confirm that a better electron localization can dramatically reduce the current collapse for the QW-MOS-HEMTs. Due to the large band edge discontinuity and effective quantum confinement of the AlGaN/GaN/AlN quantum well, the parasitic conduction in the bulk is completely eliminated.

P-doped μc-Si:H window layers prepared by hot-wire CVD for amorphous solar cell application

We report on boron-doped μc-Si:H films prepared by hot-wire chemical vapor deposition (HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in the range of 1–10 (Ω cm) −1 . The open circuit voltage V oc of the solar cells was found to decrease from 840 mV at low hydrogen dilution H-dil=91% to 770 mV at high H-dil =97% during p-layer deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent band edge discontinuity between μc-Si:H p- and amorphous i-layer. The short circuit current density J sc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for amorphous solar cells with and without the seed layer are presented. The I–V parameters for the best p–i–n solar cell obtained so far are J sc =13.95 mA/cm 2 , V oc =834 mV, FF=65% and η =7.6%, where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847 mV could be achieved at higher TMB concentrations.

Doubling of conductance steps in Si∕SiO2 quantum point contact

We have calculated the effect of the oxidation-induced strain on the ballistic conductance in a Si∕SiO2 quantum point contact. The strain-induced deformation potential was calculated semiempirically using a viscoelastic continuum model. The charge carriers are confined to the corners of the waveguide by both the strain-induced deformation potential and the Si∕SiO2 band edge discontinuity. As a consequence nearly degenerate symmetric and antisymmetric transverse states are formed for the Si [001] minima. This additional degeneracy within the Landauer-Büttiker formalism leads to doubling of conductance steps for electrons in the [001] minima which govern the conductance near the cutoff energy. Due to the additional strain-induced confinement, the effective channel width of the quantum point contact is smaller and therefore the conductance steps are sharper.

Improvement of current gain of C-doped GaAsSb-base heterojunction bipolar transistors by using an InAlP emitter

Large conduction band edge discontinuity (ΔEc) at the emitter/base interface in InP∕GaAs0.51Sb0.49∕InP heterojunction bipolar transistor (HBT) is one of the possible reasons that the recombination process in the emitter/base depletion region dominates the characteristics of this HBT. We fabricate an InAlP emitter∕GaAs0.51Sb0.49base∕InP collector HBT for reducing the ΔEc at the emitter/base interface. It is demonstrated that a HBT with an InAlP emitter shows a relatively lower recombination current than one with an InP emitter, resulting in the higher current gain. It is also found that the decrease of recombination current depends on the Al content of InAlP emitter. Additionally, the ideality factor of the emitter-base current is smallest at the Al content of 0.15 in the InAlP emitter. These results indicate that using an InAlP emitter is effective for improving the current gain of GaAsSb-base HBTs.

Analysis of size distributions of type II ZnTe/ZnSe quantum dots

AbstractThe photoluminescence (PL) spectrum of the ZnTe/ZnSe quantum dot (QD) structure which has a type II band alignment was investigated. A broader structure peaking at 2.180 eV together with the features of the ZnSe buffer layer located at 2.820 eV in the PL spectrum at 10 K were observed. The broadness of the PL attributes to the spatial inhomogeneity of the QD sizes. The PL spectrum was a normal distribution with a variance of 0.002 eV2. However, the dot size distribution was a deformed Gaussian. Using only one set of data of dot size distribution measured by the Atomic Force Microscopy (AFM), the statistical estimates with standard errors were evaluated using the bootstrap methodology. The probability distribution was inferred by calculating the means, percentiles and correlation coefficients of the base diameter and the height of the QDs. We concluded that the dot distributions were indeed a deformed Gaussian and the correlation coefficient of the diameter and the height of the QDs was 0.49 ± 0.01. The uniformity of the dot size distribution was poor. To infer the dot size distribution from the PL spectrum, we numerically solved the Schrödinger equation by elaborating on the orthogonal periodic functions (OPF) approximation for the type II lens shaped QDs. The convergence of the probability density was discussed in a great detail. We found that the band edge discontinuities inside the dot acts as a barrier with ΔEc = 315 meV for the conduction band and as a well with ΔEv = 735 meV for the valence band. In the AFM measurement, the offset in dot height is 14 Å and the aspect ratio is 0.04. We employed the Schrödinger equation to correlated the PL spectrum (intensity versus optical energy) to the AFM data (number of QDs versus dot size) and the agreement was excellent. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Measurement of polarization charge and conduction-band offset at InxGa1−xN/GaN heterojunction interfaces

The spontaneous and piezoelectric polarization fields in group-III nitride semiconductors lead to the presence of large electrostatic sheet charge densities at nitride semiconductor heterojunction interfaces. Precise quantitative knowledge of these polarization-induced charge densities and of the band-edge discontinuities at nitride heterojunction interfaces is therefore essential in nitride semiconductor device design and analysis. We have used capacitance–voltage profiling to measure the conduction-band offset and polarization charge density at InxGa1−xN/GaN heterojunction interfaces with x=0.054 and x=0.09. We obtain conduction-band offsets ΔEC=0.09±0.07 eV for x=0.054 and ΔEC=0.22±0.05 eV for x=0.09, corresponding to an averaged conduction-to-valence-band offset ratio ΔEC:ΔEV of 58:42. Our measurements yield polarization charge densities of (1.80±0.32)×1012 e/cm2 for x=0.054 and (4.38±0.36)×1012 e/cm2 for x=0.09. These values are smaller than those predicted by recent theoretical calculations, but in good agreement with values inferred from a number of optical experiments.

Lattice Temperature Model and Temperature Effects in Oxide-Confined VCSEL’s

A lattice temperature model is derived for oxide-confined vertical cavity surface emitting lasers (VCSELs) based on carrier transport and the conservation of energy. Peltier heat is caused by the bandedge and quasi-Fermi level discontinuities at a heterojunction. However, considering the device size, Peltier heat needs to be distributed and is not just generated at the interface, otherwise, an anomolous spike in temperature will occur. We have developed a novel treatment to model the Peltier heat at a heterojunction by use of a Monte Carlo simulation. Peltier heat is found to be a major heat contributor, and it results in a rapid and high temperature rise in the separate confinement heterostructure (SCH) region of the laser diode. We have also shown that the carrier thermal conductivities for materials with high mobilities must be included at high carrier densities because they contribute to additional spreading of the thermal energy. Subsequently, this lattice temperature model is coupled self-consistently to electronic and optical solvers to form a complete simulator for VCSELs. Self-heating causes a fast temperature rise when the VCSEL is operated under continuous wave conditions, causing resonant wavelength changes and an eventual thermal rollover. The resonant wavelength shift has been shown to be caused mainly by the heating of the distributed Bragg reflectors even though the peak temperature occurs within the SCH region. Possible physical factors causing the thermal rollover have also been examined with our complete simulator. The Auger recombination process is found to be one of the main factors causing the thermal rollover in 980 nm oxide-confined VCSELs while the photon lifetime is a factor in determining the position of the thermal rollover. We have also achieved a very good match between our simulated results and experimental data.

Optical properties of InGaAs/AlAsSb type I single quantum wells lattice matched to InP

InGaAs/AlAsSb quantum well structures have been grown by molecular beam epitaxy nominally lattice matched to InP substrates and characterized by photoluminescence. The exciton transitions from the n=1 electron to heavy-hole subbands were investigated and the band-edge discontinuity of an InGaAs/AlAsSb heterostructure was evaluated using an envelope function method. The type I band lineup with a band-edge discontinuity was estimated to be about 1.6 eV for As terminated samples, which exhibit the lowest compositional fluctuations across heterointerfaces. The dominant photoluminescence line-broadening mechanisms were found to be monolayer fluctuations in the well width and a random alloy compositional variation in sublayers, as well as exciton-optical phonon scattering. Using an optimized growth condition, short intersubband transitions in the 1.3–1.5 μm wavelength range were measured in the coupled double quantum well structures.

Photoluminescence study of InGaAs/AlAsSb heterostructure

InGaAs/AlAsSb quantum well structures have been grown by molecular beam epitaxy nominally lattice matched to InP substrates and characterized by photoluminescence. Growth interruptions at interfaces combined with selective group V species exposure were used. Our results indicate that interface quality: interface roughness as well as compositional variations involving group V sublattice species intermixing determine the nature of band alignment at heterointerfaces. The type I band lineup with band-edge discontinuity was estimated to be about 1.6 eV for As-terminated samples, which exhibit the lowest compositional fluctuations across heterointerfaces. The photoluminescence linewidths from InGaAs/AlAsSb quantum wells agree with linewidths calculated on the assumption of 1 ML fluctuations in well width. The inhomogeneous nature of the intermixed layers results in a large broadening of the luminescence spectra line shape for samples grown without termination or Sb termination.