Large Conduction Band Offsets Research Articles

Impact of capacitive effect on the J-V hysteresis in MgZnO/CdTe solar cells

In this study, we demonstrate that the current–voltage (J-V) hysteresis of MgZnO (MZO)/CdTe solar cells mainly originates from the modification of J-V curves by nonsteady-state photocurrent, which is correlative with low-frequency capacitance corresponding to the MZO/CdTe interface. The hysteresis was significantly strengthened by introducing oxygen into the CdTe growth atmosphere, and became more pronounced as the MZO thickness increased. The electrical properties of MZO in CdTe solar cells were analyzed in detail by using thermomechanical lift-off technique. The results showed that oxygen during CdTe growth seriously deteriorated the conductivity of MZO by reducing the concentration of oxygen vacancy. Furthermore, a spike-like conduction band offset (CBO) greater than 0.25 eV was determined at MZO/CdTe interface due to the formation of Mg-rich surface layer. Thus, the low-frequency capacitance originating from electron accumulation at MZO/CdTe interface was increased due to the less probability of electron tunneling through the interface barrier. SCAPS simulations confirmed that the low-frequency capacitance was caused by substantially reduced conductivity of MZO and large CBO at MZO/CdTe interface. The experimental and simulation results indicate that the J-V hysteresis of MZO/CdTe cells can be suppressed by reducing low-frequency capacitance, which is achievable through enhanced electron transport at MZO/CdTe interface.

Structural characters and band offset of Ga2O3–Sc2O3 alloys

Alloy engineering is a promising approach to optimize the electronic properties and application of the ultrawide bandgap semiconductor Ga2O3. Here, the structural and electronic properties of (ScxGa1−x)2O3 alloys are studied using density functional theory with the Heyd–Scuseria–Ernzerhof hybrid functional. Hexagonal (ScxGa1−x)2O3 alloys show more negative formation enthalpies than (AlxGa1−x)2O3 alloys, and the increments in the positive formation enthalpies for monoclinic (ScxGa1−x)2O3 alloys are different from the (AlxGa1−x)2O3 alloys. (ScxGa1−x)2O3 alloys will undergo the compressive strain if grown on the Ga2O3 substrate. The bandgaps range from 4.78 to 5.44 eV for monoclinic (ScxGa1−x)2O3 and from 5.17 to 6.10 eV for hexagonal (ScxGa1−x)2O3. It is noted that Ga2O3/(ScxGa1−x)2O3 heterojunctions keep the type-II band alignments and whose conduction and valence band offsets can be significantly and negligibly enlarged by increasing Sc concentration, respectively. The large conduction band offsets for Ga2O3/(ScxGa1−x)2O3 heterojunctions allow (ScxGa1−x)2O3 alloys to be an electron blocking layer for the Ga2O3 device, and ease the problems of parasitic conduction in the field effect transistor.

Heterovalent semiconductor structures and devices grown by molecular beam epitaxy

Heterovalent structures consisting of group II-VI/group III-V compound semiconductors offer attractive properties, such as a very broad range of bandgaps, large conduction band offsets, high electron and hole mobilities, and quantum-material properties such as electric-field-induced topological insulator states. These properties and characteristics are highly desirable for many electronic and optoelectronic devices as well as potential condensed-matter quantum-physics applications. Here, we provide an overview of our recent studies of the MBE growth and characterization of zincblende II-VI/III-V heterostructures as well as several novel device applications based on different sets of these materials. By combining materials with small lattice mismatch, such as ZnTe/GaSb (Δa/a ∼ 0.13%), CdTe/InSb (Δa/a ∼ 0.05%), and ZnSe/GaAs (Δa/a ∼ 0.26%), epitaxial films of excellent crystallinity were grown once the growth conditions had been optimized. Cross-sectional observations using conventional and atomic-resolution electron microscopy revealed coherent interfaces and close to defect-free heterostructures. Measurements across CdTe/InSb interfaces indicated a limited amount (∼1.5 nm) of chemical intermixing. Results for ZnTe/GaSb distributed Bragg reflectors, CdTe/MgxCd1−xTe double heterostructures, and CdTe/InSb two-color photodetectors are briefly presented, and the growth of a rock salt/zincblende PbTe/CdTe/InSb heterostructure is also described.

GaN基多量子阱红外探测器研究进展（特邀）

Quantum well infrared photodetector (QWIP) is a new device utilizing the intersubband transition in conduction band or valance band, which has a very high free degree of device design. Due to the large conduction band-offset, the ultrafast electron relax time, the ultra-wide infrared transparency and the high energy LO-phonon, the GaN/Al(Ga)N multi-quantum wells (MQWs) has become a potential candidate for the infrared detector since the GaAs based MQWs. In this paper, the research progresses of intersubband transition absorption (ISBT) and corresponding photoresponse of GaN based MQWs were systematically reviewed. First, the operation principle and the selection rule of the quantum well infrared photodetector was explained. Then, the main research work was introduced including the ISBT absorption of polar, nonpolar and nanowire GaN based MQWs, from the near infrared to far infrared, even the THz range. Finally, the progress of GaN based QWIP and quantum cascade detectors (QCD) was reviewed including the photofresponse and the frequency response of the device. A conclusion and perspective was presented for the future research in GaN based QWIP and QCDs.

Band alignment of ScAlN/GaN heterojunction

The natural band alignments of ScxAl1−xN/GaN heterojunctions, with Sc-contents ranging from 0% to 25%, are investigated by first-principles density functional theory with the local density approximation. Type-I ScxAl1−xN/GaN heterojunctions with large conduction band offsets (CBOs) and valence band offsets (VBOs) are found. The band alignment of nearly lattice-matched ScAlN (x = ∼18.75%) with respect to GaN (CBO = 1.74 eV, VBO = 0.34 eV) is also calculated for future implementation in GaN-based quantum wells and power devices. Our findings provide useful band parameters necessary for enabling the implementation of ScAlN alloys in GaN-based power and optoelectronic devices.

Multiple graphene-layer-based heterostructures with van der Waals barrier layers for terahertz superluminescent and laser diodes with lateral/vertical current injection

We analyze terahertz superluminescent and laser diodes using the injection pumping in the multiple graphene-layer (GL)/van der Waals barrier layers (BLs) heterostructures. The operation of such terahertz radiation sources is associated with the interband transitions in the gapless GLs under the population inversion leading to the GL negative dynamic conductivity. These devices use the lateral injection of holes and vertical injection of electrons. Such an injection might have advantages over the lateral injection of the bipolar carriers. Due to relatively large conduction band offsets at the GL-BL interface, the population inversion at the vertical injection can be markedly hampered by the injection current heating of the two-dimensional electron-hole plasma in the GLs. We show that doping GLs and/or BLs can substantially diminish the carrier heating promoting the interband population. Numerical analysis assumed the BL material parameters of MoS2 and WSe2 and showed the feasibility of such terahertz radiation sources.

Tuning the high-κ oxide (HfO2, ZrO2)/4H-SiC interface properties with a SiO2 interlayer for power device applications

SiC based power devices are promising for the next generation energy-efficient power converters. But its device performance is always largely limited by the tradition thermal SiO2 gate dielectric. The high-κ dielectric gate oxides have been widely used in Si and III-V devices. However, high quality dielectric layer using high-κ oxide for SiC power device is still lacking sufficient theoretical investigation. In this paper, the properties of XO2/4H-SiC (X = Hf or Zr) interfaces with a SiO2 interlayer are systematically studied by density-functional supercell calculations. Reliable interface atomic structures without any gap states have been built according to the electron counting rule. Based on the models, the electronic structure of each interface is obtained by hybrid functional. The interfacial characteristics including the averaged potential and charge transfer analysis are calculated in detail. The calculated band edge line-ups are comparable with the experimental reports. In term of the band alignment, HfO2 and ZrO2 directly bonding to SiC are not suitable dielectric materials for SiC based power device, due to the relatively smaller conduction band offsets (CBO). While introducing the SiO2 interlayer can effectively tune the interface properties, especially the band alignment. With HfO2(ZrO2)/SiO2 stack as the dielectric layer for 4H-SiC, the interface has the large CBO (>1 eV) to efficaciously confine the channel carriers, indicating that the HfO2(ZrO2)/SiO2/4H-SiC stack is the promising strategy for SiC based power device applications.

Band Alignments of Ternary Wurtzite and Zincblende III-Nitrides Investigated by Hybrid Density Functional Theory.

Band gaps and electron affinities of binary and ternary, wurtzite (wz-) and zincblende (zb-) III-nitrides are investigated using a unified hybrid density functional theory, and band offsets between wz- and zb- alloys are calculated using Anderson’s electron affinity model. A conduction (and valence) band offset of 1.85 (0.89) eV has been calculated for zb-GaN/InN heterojunctions, which is 0.25 eV larger (and 0.26 eV smaller) than that of the wz- counterpart. Such polarization-free zb-GaN/InGaN/GaN quantum well structures with large conduction band offsets have the potential to suppress electron leakage current and quantum-confined Stark effects (QCSEs). Contrarily, the conduction (and valence) band offset of zb-AlN/GaN heterojunctions is calculated to be 1.32 (0.43) eV, which is 1.15 eV smaller (and 0.13 eV larger) than that of the wz- case. The significant reduction in zb-AlN/GaN band offsets is ascribed to the smaller and indirect band gap of zb-AlN—the direct-to-indirect crossover point in zb-AlXGa1–XN is when X ∼ 65%. The small band gap of the zb-AlN barrier and the small conduction band offsets imply that electrons can be injected into zb-AlN/GaN/AlN quantum well heterostructures with small bias and less energy loss when captured by the quantum wells, respectively, i.e., loss as heat is reduced. The band gap of ternary III-nitrides does not linearly depend on alloy compositions, implying a nonlinear dependence of band offsets on compositions. As a result, the large bowing of the conduction band offset is identified and ascribed to the cation-like behavior of the conduction band minimum, while the linear dependence of the valence band offset on compositions is attributed to the anion-like character of the valence band maximum.

Band alignments of β-Ga2O3 with MgO, Al2O3 and MgAl2O4 measured by x-ray photoelectron spectroscopy

High-k and wide bandgap materials are necessary to work as gate dielectrics and surface passivation layers in β-phase gallium oxide (β-Ga2O3) based metal–oxide–semiconductor (MOS) devices. The band offsets between these materials and β-Ga2O3 play an important role in the properties of the devices. In this work, we grew β-Ga2O3 thin films on MgO, sapphire and MgAl2O4 single crystal substrates by the radio-frequency magnetron sputtering method, and used x-ray photoelectron spectroscopy to measure the β-Ga2O3/MgO, β-Ga2O3/Al2O3, and β-Ga2O3/MgAl2O4 heterointerfaces. The valence band offsets were determined to be −0.65(±0.17) eV (staggered gap, type II alignment), 0.57(±0.07) eV (straddling gap, type I alignment), and 0.64(±0.23) eV (straddling gap, type I alignment) for β-Ga2O3 on sapphire (0 0 0 1), MgO (100), and MgAl2O4 (1 0 0) substrates, leading to the large conduction band offsets of over 2 eV accordingly. The results suggested that these materials can be used to construct heterojunctions with β-Ga2O3 in MOS devices for their excellent electron confinement properties.

The opportunity of using InGaAsN/AlGaAs quantum wells for extended short-wavelength infrared photodetection

We propose and demonstrate a novel concept to address high-performance, extended short wavelength (e-SWIR) photodetectors. Our approach is based on shifting the well-developed GaAs quantum-well infrared photodetector (QWIP) technology to e-SWIR wavelengths. In order to increase the available conduction band offsets (CBOs), we suggest incorporating nitrogen (N) atoms into the quantum well material. The incorporation of N atoms into III-Vs results in dilute-nitride highly mismatched alloys with lower bandgaps and higher CBOs. In our work, we demonstrate CBO values reaching up to ∼1 eV in InGaAsN/AlGaAs QWIPs. This large CBO makes these structures suitable for e-SWIR detection. The large CBO reduces the dark current dramatically and allows efficient detection at room temperature.In our study, we devised two similar InGaAsN/AlGaAs QWIP devices with twofold, 1% and 2% N composition. Based on the measured dark current data, we extracted activation energy barriers of 780 meV and 580 meV for the 1% and 2% N devices, respectively. The dark current and photocurrent spectral response behave in correlation to the change in the barriers’ height. The photocurrent response of the 1% N device peaks at ∼2.25 μm and spans at a spectral range between 1.3 and 2.95 μm. The photocurrent response of the 2% N device is blue shifted to ∼1.42 μm and spans at a spectral range between 1.1 and 2.2 μm. The 2% N device exhibits lower dark currents and a strong photoresponse at room temperature, whereas the 1% N device exhibits a clear response only at temperatures below 150 K.The detection mechanism in the InGaAsN/AlGaAs QWIP devices is based on optical excitation of carriers from the quantum wells into highly-localized nitrogen-related E+ defect-like states located energetically above the conduction band edge. For this reason, the photoresponse is insensitive to the radiation polarization and exhibits low absorption efficiency on the order of 0.15% per quantum well. However, at the same time, the responsivity and photocurrent gain are very high due to the long lifetime of the highly localized excited states.The peak responsivity of the 2% N device is ∼70 A/W at room temperature. Significant responsivity is also available at the response tail at longer wavelengths, i.e., ∼15 and 1 A/W for 1.65 μm—the peak wavelength of the night glow emission and 2 μm—at the e-SWIR wavelengths, respectively. Corresponding detectivity values are ∼1010 and ∼5 × 108 cm·Hz1/2/W at 1.65 μm and 2.0 μm, respectively. These values are similar to those of other developing technologies. In-reach potential detectivity is estimated based on the measured data and can arrive easily to values as high as ∼2 × 1012 and ∼1 × 1011 cm·Hz1/2/W for ∼1.65 μm and 2.0 μm, respectively.The presented characteristics and potential performance indicate that InGaAsN/AlGaAs quantum wells are most suitable for efficient e-SWIR photodetection.

Outstanding memory characteristics with atomic layer deposited Ta2O5/Al2O3/TiO2/Al2O3/Ta2O5 nanocomposite structures as the charge trapping layer

A charge trapping memory device using Ta2O5/Al2O3/TiO2/Al2O3/Ta2O5 (TATiAT) nanocomposite high-k dielectrics as charge trapping layer (CTL) and amorphous Al2O3 as the tunneling and blocking layers has been investigated, in which all the high-k dielectric films were fabricated by atomic layer deposition (ALD) technique. A large memory window of 10.0 V with higher charge trap density of 1.10 × 1013 cm−2 at gate voltage of ±10 V has been achieved. The better storage characteristic is attributed to the inter-diffusion between nanocomposite high-k oxides and the formation of more oxygen vacancies in CTL. The device shows the fast P/E speed and excellent endurance characteristics. A 70% initial memory window can be maintained after 10 years of retention by using the extrapolation method. The large conduction band offsets of 1.02 eV between TATiAT and Al2O3 is beneficial to retention characteristic due to the deep trap level.

Nearly-zero valence band and large conduction band offset at BAlN/GaN heterointerface for optical and power device application

Wurtzite BAlN alloy with large bandgap is an emerging material system for UV optoelectronic and power devices. In this study, the BAlN/GaN heterojunction with a sharp interface and uniform distribution of the elements was formed by careful control of epitaxial conditions to avoid GaN desorption and parasitic reactions whose band alignment was then measured for the first time. The valence band offset (VBO) was found to be nearly-zero, i.e. −0.2 ± 0.2 eV, in the B0.14Al0.86N/GaN heterojunction by X-ray photoelectron spectroscopy. Additionally, the conduction band offset (CBO) is estimated at 2.1 ± 0.2 eV, which is the largest reported CBO among epitaxial GaN-based heterojunctions. In comparison to the type-I Al0.75Ga0.25N/GaN heterojunction (both Al0.75Ga0.25N and B0.14Al0.86N alloy have the same bandgap of 5.7 eV), the CBO and VBO of the B0.14Al0.86N/GaN heterostructure are significantly larger and smaller, respectively. The nearly-zero VBO and the very large CBO of the B0.14Al0.86N/GaN heterojunction could potentially lead to considerable performance enhancement for GaN optoelectronics and power electronics devices.

Band alignment at β-(AlxGa1−x)2O3/β-Ga2O3 (100) interface fabricated by pulsed-laser deposition

High-quality β-(AlxGa1−x)2O3 (x = 0–0.37) films were epitaxially grown on β-Ga2O3 (100) substrates by oxygen-radical-assisted pulsed-laser deposition with repeating alternate ablation of single crystals of β-Ga2O3 and α-Al2O3. The bandgap was tuned from 4.55 ± 0.01 eV (x = 0) to 5.20 ± 0.02 eV (x = 0.37), where bowing behavior was observed. The band alignment at the β-(AlxGa1−x)2O3/β-Ga2O3 interfaces was found to be type-I with conduction- and valence-band offsets of 0.52 ± 0.08 eV (0.37 ± 0.08 eV) and 0.13 ± 0.07 eV (0.02 ± 0.07 eV) for x = 0.37 (0.27), respectively. The large conduction-band offsets are ascribed to the dominant contribution of the cation-site substitution to the conduction band.

AlInAsSb/GaSb staircase avalanche photodiode

Over 30 years ago, Capasso and co-workers [IEEE Trans. Electron Devices 30, 381 (1982)] proposed the staircase avalanche photodetector (APD) as a solid-state analog of the photomultiplier tube. In this structure, electron multiplication occurs deterministically at steps in the conduction band profile, which function as the dynodes of a photomultiplier tube, leading to low excess multiplication noise. Unlike traditional APDs, the origin of staircase gain is band engineering rather than large applied electric fields. Unfortunately, the materials available at the time, principally AlxGa1−xAs/GaAs, did not offer sufficiently large conduction band offsets and energy separations between the direct and indirect valleys to realize the full potential of the staircase gain mechanism. Here, we report a true staircase APD operation using alloys of a rather underexplored material, AlxIn1−xAsySb1−y, lattice-matched to GaSb. Single step “staircase” devices exhibited a constant gain of ∼2×, over a broad range of applied bias, operating temperature, and excitation wavelengths/intensities, consistent with Monte Carlo calculations.

The band alignment of Cu2O/ZnO and Cu2O/GaN heterostructures

Using photoelectron spectroscopy, we investigate the band alignments of the Cu2O/ZnO heterointerface and compare the findings with the corresponding values for Cu2O/GaN. While for Cu2O/ZnO, we find a valence band offset (VBO) of 2.17 eV and a conduction band offset (CBO) of 0.97 eV, both values are considerably reduced for Cu2O/GaN where the numbers are 1.47 eV (VBO) and 0.24 eV (CBO), respectively. The large CBO between ZnO and Cu2O will very likely result in low photovoltaic power conversion efficiencies as is the current status of Cu2O/ZnO solar cells.

Atomic and electronic structures at ZnO and ZrO2 interface for transparent thin‐film transistors

AbstractIn this paper, we report the studies of atomic and electronic structures at ZnO and ZrO2 interface. The epitaxial heterostructures were grown by laser molecular beam epitaxy and the interface atomic structure was determined by using high‐resolution transmission electron microscopy (TEM). Band alignment for high‐k ZrO2 layer on ZnO was investigated by in situ X‐ray photoemission spectroscopy (XPS) characterization and first‐principles calculations based on density functional theory (DFT). The valence and conduction band offsets (CBOs) were found to be 0.27 ± 0.05 eV and 2.16 ± 0.05 eV, respectively. The results are in good agreement with values from theoretical calculations. The large CBO and small lattice mismatch between ZnO and ZrO2 suggest potential for ZrO2 to be used as a gate dielectric in ZnO‐based transparent electronic devices.

Short-Wavelength Intersubband Light Emission from Optically Pumped GaN/AlN Quantum Wells

AbstractDue to their large conduction-band offsets, GaN/Al(Ga)N quantum wells are currently the subject of extensive research efforts aimed at extending the spectral range of intersubband optoelectronic devices towards shorter and shorter wavelengths. Here we report our recent measurement of optically pumped intersubband light emission from GaN/AlN quantum wells at the record short wavelength of about 2 μm. Nanosecond-scale optical pulses are used to resonantly pump electrons from the ground states to the second excited subbands, followed by radiative relaxation into the first excited subbands. The intersubband origin of the measured photoluminescence is confirmed via an extensive study of its polarization properties and pump wavelength dependence.

Midinfrared intersubband absorption in wide band gap II-VI ZnxCd1−xSe multiple quantum wells with metastable zincblende MgSe barriers

The authors report the observation of midinfrared intersubband (ISB) absorption in MgSe∕ZnxCd1−xSe multiple quantum wells (MQWs) grown on (001) InP substrates by molecular beam epitaxy. The in situ reflection high energy electron diffraction shows that zincblende MgSe∕ZnxCd1−xSe MQWs can be grown with the introduction of ZnxCd1−xSe spacer layers. For quantum wells with the nominal well width of 11–15 ML (3.2–4.4nm) ISB absorption in the wavelength range of 3.3–4.9μm is observed. The conduction band offset (CBO) of the MgSe∕ZnxCd1−xSe heterostructure is estimated to be 1.2eV. Using the estimated CBO, the theoretically calculated interband and ISB transition energies agree well with photoluminescence and Fourier-transform infrared spectroscopy measurements. With such a large CBO, ISB transitions in this material system could be extended to the near infrared region.

Band offset of high efficiency CBD-ZnS/CIGS thin film solar cells

The band offset at the CBD-ZnS/Cu(In,Ga)Se 2 (CIGS) interface and solar cells performance were investigated by means of X-ray photoelectron spectroscopy (XPS). The valence band maximum and valence band offset at the chemical bath deposition (CBD)-ZnS/CIGS interface region was directly measured using a XPS spectral multiplot. We found that the conduction band offset (CBO) at the CBD-ZnS/CIGS (ordered vacancy compound, OVC) interface was smaller than that of the evaporated ZnS/CIGS (OVC) interface. However, the CBO for the CBD-ZnS/CIGS (OVC) interface was still quite large for fabrication of high efficiency devices, although, in fact, high efficiency CBD-ZnS/CIGS devices were fabricated with good run-to-run reproducibility. The bandgap dependence of cell performance for CBD-ZnS/CIGS devices (large CBO) showed a tendency similar to that of CBD-CdS/CIGS devices (small CBO). Therefore, the observed decrease in cell performance for high bandgap devices is not interpreted in terms of the CBO. These results suggest that the band offset is not the predominant factor in achieving high efficiency CBD-ZnS/CIGS thin film solar cells. This finding suggests that the recombination mechanism of those wide-gap alloys is a bulk recombination.

Near-infrared laser pumped intersubband THz laser gain in InGaAs–AlAsSb–InP quantum wells

We investigate the possibility of using InGaAs–AlAsSb–InP coupled quantum wells to generate THz radiation by means of intersubband optical pumping. We show that large conduction band offsets of these quantum wells make it possible to use conventional near-infrared diode lasers around 1.55 μm as pump sources. Taking into account the pump–probe coherent interaction and the optical nonlinearity for the pump field, we calculate the THz gain of the quantum well structure. We show that resonant Raman scattering enhances the THz gain at low and moderate optical pumping levels. When the pump intensity is strong, the THz gain is reduced by pump-induced population redistribution and pump–probe coherent interactions.