type-II Band Lineup Research Articles

(Invited) Low Power Tunneling FET Technologies Using Ge/III-V Materials

Low power consumption is one of the most important requirements for present and future integrated systems. Here, reduction in supply voltage is most effective in power reduction. Low supply voltage operation of charge-based logic switch devices can be achieved by steep slope devices with lower sub-threshold swing (S.S.) than CMOS. For this purpose, tunnel FETs (TFETs) have recently stirred a strong interest [1, 2], because of the high compatibility with CMOS platform. Here, materials with small and/or direct band gap such as III-V and Ge are preferred to enhance Ion of TFETs. In addition, type-II hetero-structures are known to be effective in boosting the TFET performance [1-3]. One of the critical technological challenges of pn-junction-based TFETs is the source junction engineering allowing us to form the source junctions with steep impurity profiles and low defect density for optimal Ge/III-V and/or hetero-interface materials. In addition, another key factor is low defect densities in the channel/junction regions and at the MOS interfaces. The MOS interface defects can include fast interface states, slow states and tail states near the band edges, while the reduction in the defect densities is not easy for other materials than Si. Thus, the channel and gate stack engineering for realizing defect-less structures are also quite important. From the above viewpoints, we are currently working for the following material systems and source doping technologies [2-4], (1) InGaAs TFET with Zn diffusion source (2) GaAsSb (source)/InGaAs (channel) TFETs with Be in-situ doping (3) Ge (source)/strained Si (channel) TFETs with B in-situ doping. Here, Zn diffusion into InGaAs can automatically form pn junctions with steep Zn profile, because of the diffusion constant of Zn strongly dependent on Zn concentration [5, 6]. We experimentally confirmed the steep Zn profile and lower leakage current of InGaAs p-n junctions by solid phase diffusion from SOG films, attributed to the defect-less junction formation. Quantum-well InGaAs/InAs(3nm)/InGaAs n-TFETs with the Zn-diffused source regions and W/HfO2/Al2O3 gate stacks have exhibited the minimum S.S. value of 55 mV/dec and I60 (maximum source/drain current with S.S. less than 60 mV) of 0.7 nA/um at room temperature [3, 6]. In comparison with InGaAs homo-junction TFETs, III-V TFETs composed of type-II hetero-structures, GaSb/InAs and GaAsSb/ InGaAs, are expected to provide superior electrical characteristics, because of the smaller effective bandgap. We fabricated vertical n-TFETs using in-situ Be-doped p+-GaAs0.51Sb0.49/ In0.53Ga0.47As grown on InP by MOMBE [3, 7]. The steep Be profile of 11 nm/dec. is obtained, thanks to the in-situ doping of Be during the GaAsSb growth. We confirmed the operation of GaAsSb/InGaAs TFETs through observation of the negative differential resistance under forward bias condition. However, high leakage current attributable to defects in the source junction significantly degrades the performance at 297 K. The characteristics at 20 K with S.S. of 82 mV/dec. are regarded as the TFET performance dominated only by tunneling. Another type of type-II hetero-interface n-TFETs has been realized by p+Ge/strained SOI structures [3, 4, 8, 9]. Here, higher valence band edge of the source due to Ge and lower conduction band edge of the channel due to bi-axial tensile strain are combined to form the type-II band line-up. p+Ge/SOI with tensile strain of 0.8 and 1.1 % were fabricated by using bi-axial strained SOI substrates and in-situ B doping during Ge MBE growth. The higher Ion and lower S.S.min were obtained by applying tensile strain. The unstrained, 0.8 and 1.1 % strained SOI TFETs after 400 oC PMA [9] yield S.S.min of 69, 48 and 29 mV/dec. and Ion/Ioff of 4.4, 2.2 and 3.7x107, respectively, at room temperature. This work has partly been supported by JST-CREST. The authors would like to thank Drs. M. Yokoyama, O. Ichikawa, H. Yamada and T. Yamamoto in Sumitomo Chemical Corporation, Drs. M. Mitsuhara, H. Sugiyama, Hoshi and H. Yokoyama in NTT for their collaborations.

Investigation of GaAsBi/GaAsN Type-II Staggered Heterojunction TFETs with the Analytical Model

We investigate GaAs 1-x Bi x /GaAs 1-y N y type-II staggered heterojunction tunneling field-effect transistors (TFETs) with the analytical model. GaAs 1-x Bi x /GaAs 1-y N y exhibits a remarkable type-II band lineup and the effective bandgap ${E} _{{{G,{\mathrm{ eff}}}}}$ at interface is significantly reduced with the increasing of bismuth (Bi) and nitrogen (N) compositions. Theoretical characterization shows that the band-to-band tunneling probability and current of TFETs are boosted with the decreasing of ${E} _{{{G,\text {eff}}}}$ at the GaAs 1-x Bi x /GaAs 1-y N y staggered tunnel-ing junction by increasing Bi and N compositions. It is demonstrated by the analytical calculation that GaAs 1-x Bi x /GaAs 1-y N y heterostructure is a potential candidate for high-performance TFET.

Theoretical investigation of GaAsBi/GaAsN tunneling field-effect transistors with type-II staggered tunneling junction

We investigate GaAsBi/GaAsN system for the design of type-II staggered hetero tunneling field-effect transistor (hetero-TFET). Strain-symmetrized GaAsBi/GaAsN with effective lattice match to GaAs exhibits a type-II band lineup, and the effective bandgap EG,eff at interface is significantly reduced with the incorporation of Bi and N elements. The band-to-band tunneling (BTBT) rate and drive current of GaAsBi/GaAsN hetero-TFETs are boosted due to the utilizing of the type-II staggered tunneling junction with the reduced EG,eff. Numerical simulation shows that the drive current and subthreshold swing (SS) characteristics of GaAsBi/GaAsN hetero-TFETs are remarkably improved by increasing Bi and N compositions. The dilute content GaAs0.85Bi0.15/GaAs0.92N0.08 staggered hetero-nTFET achieves 7.8 and 550 times higher ION compared to InAs and In0.53Ga0.47As homo-TFETs, respectively, at the supply voltage of 0.3 V. GaAsBi/GaAsN heterostructure is a potential candidate for high performance TFET.

Type-II recombination dynamics of tensile-strained GaP quantum dots in GaAs grown by droplet epitaxy

We use droplet epitaxy to create tensile-strained GaP quantum dots in a GaAs matrix. A strong biaxial tensile strain leads to the formation of a type-II band lineup with a transition energy lower than the bulk GaAs band gap. The luminescence transients exhibit highly non-exponential decay behavior with an average time constant of 11 ± 2 μs, which is more than three orders of magnitude longer than the lifetime of standard type-I quantum dots. The prolonged luminescence decay time for the GaP/GaAs dots confirms the formation of the type-II band alignment associated with the tensile strain.

Band alignment of HfO2/AlN heterojunction investigated by X-ray photoelectron spectroscopy

The band alignment between AlN and Atomic-Layer-Deposited (ALD) HfO2 was determined by X-ray photoelectron spectroscopy (XPS). The shift of Al 2p core-levels to lower binding energies with the decrease of take-off angles θ indicated upward band bending occurred at the AlN surface. Based on the angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔEV of 0.4 ± 0.2 eV at HfO2/AlN interface was determined by taking AlN surface band bending into account. By taking the band gap of HfO2 and AlN as 5.8 eV and 6.2 eV, respectively, a type-II band line-up was found between HfO2 and AlN.

Material and device characteristics of InAs/GaAsSb sub-monolayer quantum dot solar cells

We have studied the material and photovoltaic characteristics of InAs/GaAsSb sub-monolayer quantum dot solar cells (QDSCs) with different Sb contents of 0%, 5%, 15%, and 20%. All QDSCs exhibit an extended external quantum efficiency (EQE) response in the wavelength range of 960–1000 nm that corresponds to sub-bandgap photon absorption. As Sb content increases from 5% to 20%, the cutoff wavelength in the EQE extends towards longer wavelength whilst the EQE in the wavelength region of 300–880 nm is lowered due to increased defect density. Compared to the QDSC (Sb 0%), an Sb incorporation of 5% enhances the short-circuit current density from 20.65 to 22.15 mA/cm2 induced by Sb surfactant effect. Since the open-circuit voltage and fill factor of the QDSC (Sb 5%) are comparable to those of the QDSC (Sb 0%), an enhancement in solar cell efficiency (10.5%) of the QDSC (Sb 5%) is observed. Further increasing Sb content to 15% and 20% results in the degradation of solar cell performance due to increased nonradiative recombination and large valence band offset in a type-II band line-up.

Heterojunction Band Offset Limitations on Open-Circuit Voltage in <roman>p</roman>-Z<roman>n</roman>T <roman>e/n</roman>-Z<roman>n</roman>S<roman>e</roman> Solar Cells

Limitations on the open-circuit voltage of p-ZnTe/n-ZnSe heterojunction solar cells are studied via current-voltage (I-V) measurements under solar concentration and at variable temperature. The open-circuit voltage reaches a maximum value of 1.95 V at 77 K and 199 suns. The open-circuit voltage shows good agreement with the calculated built-in potential of 2.00 V at 77 K. These results suggest that the open-circuit voltage is limited by heterojunction band offsets associated with the type-II heterojunction band lineup, rather than the bandgap energy of the ZnTe absorber material.

Effect of band offset on carrier transport and infrared detection in InP quantum dots/Si nano-heterojunction grown by metalorganic chemical vapor deposition technique

Epitaxy of III-V semiconductors on Si gets recent interest for next generation system on heterogeneous chip on wafer. The understanding of band offset is thus necessary for describing the charge transport phenomenon in these heterojunctions. In this work, x-ray photoemission spectroscopy has been used to determine the band offsets in a heterojunction made of InP quantum dots on Si. The valence and conduction band offset was found to be 0.12 eV and 0.35 eV, respectively, with a type-II band lineup. Deviation from theoretical prediction and previously published reports on quasi similar systems have been found and analyzed on the basis of the effect of strain, surface energy, shift in the electrostatic dipole and charge transfer at the interface. The carrier transport mechanisms along with different device parameters in the heterojunction have been studied for a temperature range of 180–300 K. This heterojunction is found to behave as an efficient infrared photodetector with an ON/OFF ratio of 21 at a reverse bias of 2 V. The corresponding rise and decay time was found to be 132 ms and 147 ms, respectively.

(Invited) Narrow Linewidth, Highly Efficient, and Integrated Light Emitting Diodes Based on Ge Quantum Dots in Optical Microcavities

Ge self-assembled quantum dots (QDs) grown on Si are promising solution for Si-based light sources due to their emission wavelength in the telecommunication band, capability of electrical injection and CMOS process compatibility. However, as indirect band gap material, their light emission efficiency is still rather low. By embedding Ge QDs into optical microcavities with large Q-factor and small mode volume, the spontaneous emission rate can be enhanced due to the Purcell effect and the light extraction efficiency can also be improved due to the modified cavity mode fields. We have demonstrated strong and sharp resonant photoluminescence peaks from Ge QDs in photonic crystal (PhC) nanocavities and microdisk resonators [1]. For practical application, light emission by electrical injection is desirable. In this abstract, we will review some of our results on electrical injected light emitting diodes (LEDs) based on Ge QDs in microcavities. Electrical injected LEDs were firstly demonstrated by integrating vertical PIN diodes with H3-type PhC cavities [2]. Room-temperature electroluminescence (EL) was successfully observed, but with relatively broad emission peaks. This is due to the large cavity loss induced by the electrical structure. By using lateral PIN diode structure, in which the P+ and N+ regions are located laterally on the PhC slab, while the cavity region is left intrinsic, we achieved intense EL peaks around 1.3 µm with Q-factor of about 260, corresponding to a linewidth of 5 nm [3]. After optimization of PhC nanocavities and electrical structures, the Q-factor of emission peaks increased to 800-1500. The required current for occurrence of resonant peaks was reduced by one order, to 50 µA [4]. The schematic diagram and EL spectra of the device are shown in Fig. 1(a) and 1(b), respectively. Moreover, at injected current of 3 mA, we obtained a measurable output power on the pW order. We also confirmed that the output power was limited by the fabrication process and could be increased further. In order to realize monolithic integration with other photonic devices, waveguide-coupled LEDs are necessary. Waveguide-coupled microdisks containing Ge QDs were fabricated with vertical PIN diodes structures [5], as shown in Fig. 1(c). When there was current injection, light emission from the microdisk was coupled into the waveguide. Sharp resonant peaks around 1.55 µm with Q-factor over 5000 were obtained in the EL spectra collected through the waveguide, as shown in Fig. 1(d). The coupling between the waveguide and microdisk was confirmed by comparing the EL and optical transmission spectra along the waveguide. The LEDs thus can be easily integrated with other waveguide devices. Interestingly, the devices also can be operated as resonant-cavity enhanced photodetectors under reverse bias. Wavelength selective photo-response, with high responsivity and ultra-low dark current around 1.55 µm was also realized [6]. The light emission efficiency of Ge QDs themselves is fundamental to obtain higher intensity. Due to the type-II band lineup of Si/Ge heterostructure, only holes can be well confined in Ge QDs, and there is very weak confinement for electrons. By using n-type doping, extrinsic electrons can be provided and the radiative recombination is expected to be enhanced. In the talk, we will also present some results about light emission enhancement by phosphorus delta-doping between Ge QDs and Si spacers.

Transparent Oxide Thin-Film Transistors Using n-(In2O3)0.9(SnO2)0.1/InGaZnO4 Modulation-Doped Heterostructures

We investigated transparent oxide thin-film transistors (TFTs) using n-(In2O3)0.9(SnO2)0.1/InGaZnO4 (n-ITO/IGZO) modulation-doped heterostructures, which are effective in achieving high carrier mobilities. From transmittance measurements and UV photoemission spectroscopy, n-ITO/IGZO modulation-doped heterostructures are expected to realize the type-II energy band lineup, in which both the conduction band minimum and the valence band maximum of n-ITO are higher in energy than those of IGZO. Van der Pauw Hall measurements revealed Hall mobility enhancement and two-dimensional behavior of electrons at the n-ITO/IGZO interface. Using the n-ITO/IGZO modulation-doped heterostructures, we obtained TFTs with higher electron mobility than that of IGZO TFTs. We consider that modulation doping is a promising method for performance improvements of TFTs using transparent oxide semiconductors.

Long-wavelength infrared quantum-dot based interband photodetectors

We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley–Read–Hall generation–recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal–organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 × 10 11 cm −2, 1 × 10 12 cm −2 and 3 × 10 10 cm −2 were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6–12 μm. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II interband transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.

E-beam-evaporated Al 2O 3 for InAs/AlSb metal–oxide–semiconductor HEMT development

Considerable on-state impact ionization and off-state tunneling leakages are the two principal drawbacks of InAs/AlSb HEMTs, which have a small bandgap and type-II band lineup. This work introduced a wide-bandgap high- k Al 2O 3 between the gate metal and semiconductor surface and successfully demonstrated DC and RF performance of the InAs/AlSb metal–oxide–semiconductor HEMTs (MOS-HEMTs). An MOS-HEMT device with a 2.0 μm gate length yields DC performance of I DSS = 286 mA/mm and G m = 495 mS/mm and RF performance of f T = 10.1 GHz and f MAX = 19.9 GHz. Compared with a conventional HEMT, gate leakage is reduced by one order and the marked dependence of drain current on gate bias in the deep subthreshold region is largely alleviated.

Wide-bandgap Semiconductor Devices using Group-III Nitride/SiC Heterointerface

This paper describes group-III nitride (III-N)/SiC heterointerface and its device applications. Heteroepitaxial growth of III-N on SiC opens new opportunity of SiC-based heterojunction devices such as heterojunction bipolar transistors (HBTs). The authors developed growth methods to grow high-quality III-N on SiC by molecular-beam epitaxy. Fabricated GaN/SiC heterojunction exhibited type-II band-lineup. By using AlN/GaN short period superlattice as a quasi AlGaN alloy, the authors successfully controlled the band-lineup to be type-I and demonstrated common-emitter-mode operation of III-N/SiC HBTs.

Type-II behavior in wurtzite InP/InAs/InP core-multishell nanowires

We study optical transitions from a periodic array of InP/InAs/InP core-multishell nanowires (CMNs) having a wurtzite crystal structure by using photoluminescence (PL) and PL excitation (PLE) spectroscopy. Observing a large Stokes shift between PL and PLE spectra, a blueshift of the PL peak with a cube-root dependence on the excitation power and a slow and nonexponential decay of PL with an effective decay time of 16 ns suggest a type-II band alignment. Band-offset calculation based on the “model-solid theory” of Van de Walle [Phys. Rev. B 39, 1871 (1989)] supports type-II band lineup if the InAs layer in the wurtzite CMNs is assumed to sustain compressive strain in all directions.

Coexistence of type-I and type-II band lineups in Cd(Te,Se)∕ZnSe quantum-dot structures

The authors report on transmission electron microscopy, cathodoluminescence, and time resolved photoluminescence studies of thin Cd(Te,Se) layers in a ZnSe matrix, grown by molecular beam epitaxy. All observations confirm strain-induced self-assembly of quantum dots (QD’s), induced primarily by the 14% lattice mismatch between CdTe and ZnSe. The emission spectrum of the structure is the superposition of a relatively narrow luminescence line originating from CdSe-enriched type-I QD’s and a broad band attributed to the emission of an ultrathin ZnTeSe∕ZnSe layer with type-II band lineup, formed in between the QD’s.

Band-edge alignment ofSiGe∕Siquantum wells andSiGe∕Siself-assembled islands

We report on the energy band gap and band lineup of $\mathrm{SiGe}∕\mathrm{Si}$ heterostructures either in the case of coherently strained quantum wells or in the case of $\mathrm{SiGe}∕\mathrm{Si}$ self-assembled islands. We take into account the strain field and the quantum confinement effects through an accurate description of the conduction band including the $\ensuremath{\Delta}$ and $L$ bands. The strain field is calculated using a microscopic valence force field theory. The conduction-band diagram and energies are obtained from a 30-band $\mathbf{k}∙\mathbf{p}$ Hamiltonian accounting for the strain through the Bir-Pikus Hamiltonian. The band-edge description is first given for biaxially strained pseudomorphic $\mathrm{SiGe}$ layers. In $\mathrm{SiGe}$ quantum wells grown on relaxed silicon, the band line-up switches from type I to type II depending on the value of the average valence band offset. Applying the 30-band formalism to the case of heterostructures grown on relaxed silicon germanium buffer layers indicates that a better agreement with experimental data is obtained for a valence-band offset value $\mathrm{\ensuremath{\Delta}}{E}_{v}=0.54x$ where $x$ is the Ge composition. For this parameter, a type-II band lineup is thus expected for all compositions of pseudomorphic $\mathrm{SiGe}$/relaxed Si heterostructures. For $\mathrm{GeSi}∕\mathrm{Si}$ islands, we take into account the strain relaxation in the surrounding Si matrix. A type-II band lineup is predicted for all Ge compositions. The near-infrared interband recombination energy of the islands is calculated as a function of their $\mathrm{SiGe}$ composition.

Ultrathin type-II GaSb/GaAs quantum wells grown by OMVPE

Heterostructures containing monolayer (ML) and submonolayer GaSb insertions in GaAs were grown using organometallic vapour phase epitaxy. At the GaAs-on-GaSb interface, strong intermixing occurs due to the surface segregation of Sb. To form structures with relatively abrupt interfaces, a flashoff growth sequence, in which growth interruptions are employed to desorb Sb from the surface, was introduced. Reflectance-difference spectroscopy and high-resolution X-ray diffraction data demonstrate that interfacial grading is strongly reduced by this procedure. For layer structures grown with the flashoff sequence, a GaSb coverage up to 1 ML can be obtained in the two-dimensional (2D) growth mode. For uncapped GaSb layers, on the other hand, atomic force microscope images show that the 2D–3D growth mode transition occurs at a submonolayer coverage between 0.3 and 0.5 ML. Low-temperature photoluminescence spectra of multiple quantum well samples grown using the flashoff sequence show a strong quantum well-related peak which shifts to lower energies as the amount of Sb incorporated increases. The PL peak energies are consistent with a type-II band lineup at the GaAs/GaSb interface.

Spatially direct and indirect transitions observed for Si/Ge quantum dots

The optical properties of Ge quantum dots embedded in Si were investigated by means of photoluminescence, with temperature and excitation power density as variable parameters. Two different types of recombination processes related to the Ge quantum dots were observed. A transfer from the spatially indirect to the spatially direct recombination in the type-II band lineup was observed with increasing temperature. A blueshift of the spatially indirect Ge quantum-dot-emission energy with increasing excitation power is ascribed to band bending at the type-II Si/Ge interface for high carrier densities. Comparative studies were performed on uncapped Ge dot structures.

Photoluminescence study of Si/Ge quantum dots

Ge quantum dots embedded in Si are studied by means of photoluminescence (PL). The temperature dependent PL measurements show two different types of recombination processes related to the quantum dots. We ascribe a peak near 0.80 eV to the spatially indirect recombination in the type-II band lineup where the electron is located in the surrounding Si close to the interface and the hole in the Ge dot. Furthermore, a peak near 0.85 eV is attributed to the spatially direct recombination. We observe a transition from the spatially indirect to the spatially direct recombination as the temperature is increased. The measurements also show an up-shift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is primarily ascribed to an enhanced confinement of the electron associated with the increased band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results, derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.

Luminescence study of Si/Ge quantum dots

We present a photoluminescence (PL) study of Ge quantum dots embedded in Si. Two different types of recombination processes related to the Ge quantum dots are observed in temperature-dependent PL measurements. The Ge dot-related luminescence peak near 0.80 eV is ascribed to the spatially indirect recombination in the type-II band lineup, while a high-energy peak near 0.85 eV has its origin in the spatially direct recombination. A transition from the spatially indirect to the spatially direct recombination is observed as the temperature is increased. The PL dependence of the excitation power shows an upshift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is ascribed to band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.