Showing papers on "Heterojunction published in 2001"

Band parameters for III–V compound semiconductors and their alloys

Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Recent progress in quantum cascade lasers and applications

Abstract: Quantum cascade (`QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple - often several tens of - photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance. The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 µm wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid- to far-infrared wavelength range from 3.5 to 24 µm, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 µm. Three distinct designs of the active region, the so-called `vertical' and `diagonal' transition as well as the `superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.

Band Gap Variation of Size- and Shape-Controlled Colloidal CdSe Quantum Rods

Abstract: We report the band gaps of rodlike CdSe quantum dots with diameter varying from 3.0 to 6.5 nm and length from 7.5 to 40 nm. A qualitative explanation for the dependence of band gap on width and length is presented.

High efficiency solid-state photovoltaic device due to inhibition of interface charge recombination

Abstract: The performance of solid-state dye-sensitized solar cells based on spiro-MeOTAD was considerably improved by controlling charge recombination across the interface of the heterojunction. This was achieved by blending the hole conductor matrix with a combination of 4-tert-butylpyridine (tBP) and Li[CF3SO2]2N. Open circuit voltages Uoc over 900 mV and short circuit currents Isc up to 5.1 mA were obtained, yielding an overall efficiency of 2.56% at AM1.5 illumination. These values have been fully confirmed at the National Renewable Energy Laboratories for a device with an active area of 1.07 cm2, signifying a dramatic improvement compared to previously reported values for a similar device.

Exciton diffusion and optical interference in organic donor–acceptor photovoltaic cells

Abstract: The influence of the organic layer thickness on short-circuit photocurrent spectra and efficiency is investigated in heterojunction photovoltaic cells with the electron donor materials poly(p-phenylenevinylene) (PPV) and Cu-phthalocyanine (CuPc), respectively, together with C60 as electron acceptor material. The main process of photocurrent generation after light absorption, exciton generation, and exciton diffusion in the bulk of the absorbing material is given by the exciton dissociation at the donor–acceptor interface. We determined a strong dependence of the optimum layer thickness of the absorbing material on the exciton diffusion length by systematically varying the layer thickness of the electron donor material. Additionally, a significant photocurrent contribution occurred due to light absorption and exciton generation in the C60 layer with a subsequent hole transfer to PPV, respectively, CuPc at the dissociation interface. Using a simple rate equation for the exciton density we estimated the exci...

Lithium doping of semiconducting organic charge transport materials

Abstract: We study the effects of lithium (Li) incorporation in the cathodes of organic light-emitting devices. A thermally evaporated surface layer of metallic Li is found to diffuse through, and subsequently dope, the electron transporting organic semiconducting thin films immediately below the cathode, forming an Ohmic contact. A diffusion length of ∼700 A is inferred from analyses of the current–voltage and secondary ion mass spectrometry data. The conductivity of the Li-doped organic films is ∼3×10−5 S/cm. Photoemission spectroscopy suggests that Li lowers the barrier to injection at the organic/cathode interface, introduces gap states in the bulk of the organic semiconductor, and dopes the bulk to facilitate efficient charge transport.

Physical Structure and Inversion Charge at a Semiconductor Interface with a Crystalline Oxide

Abstract: We show that the physical and electrical structure and hence the inversion charge for crystalline oxides on semiconductors can be understood and systematically manipulated at the atomic level Heterojunction band offset and alignment are adjusted by atomic-level structural and chemical changes, resulting in the demonstration of an electrical interface between a polar oxide and a semiconductor free of interface charge In a broader sense, we take the metal oxide semiconductor device to a new and prominent position in the solid-state electronics timeline It can now be extensively developed using an entirely new physical system: the crystalline oxides-on-semiconductors interface

Quantum dot infrared photodetectors

Abstract: Self-assembled strained semiconductor nanostructures have been grown on GaAs substrates to fabricate quantum dot infrared photodetectors. State-filling photoluminescence experiments have been used to probe the zero-dimensional states and revealed four atomic-like shells (s,p,d,f) with an excitonic intersublevel energy spacing which was adjusted to ∼60 meV. The lower electronic shells were populated with carriers by n doping the heterostructure, and transitions from the occupied quantum dot states to the wetting layer or to the continuum states resulted in infrared photodetection. We demonstrate broadband normal-incidence detection with a responsivity of a few hundred mA/W at a detection wavelength of ∼5 μm.

300 K operation of a GaAs-based quantum-cascade laser at λ≈9 μm

Abstract: The room-temperature (300 K), pulsed mode operation of a GaAs-based quantum-cascade laser is presented. This has been achieved by the use of a GaAs/Al0.45Ga0.55As heterostructure which offers the maximum Γ–Γ band offset (390 meV) for this material system without inducing the presence of indirect barrier states. Thus, better electron confinement is achieved, countering the loss of injection efficiency with temperature. These devices show ∼100 K increase in operating temperature with respect to equivalent designs using an GaAs/Al0.33Ga0.67As heterostructure. We also measure 600 mW peak power at 233 K a temperature readily accessible by Peltier coolers.

Sol-gel processed TiO2 films for photovoltaic applications

Abstract: The dye sensitized solar cells (DYSC) provides a technically and economically credible alternative concept to present day p-n junction photovoltaic devices. In contrast to the conventional systems where the semiconductor assumes both the task of light absorption and charge carrier transport the two functions are separated here. Light is absorbed by a sensitizer which is anchored to the surface of a wide band gap semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. The present concepts evolved in the context of research on mesoporous oxide semiconductor films prepared via a sol-gel process. The use of transition metal complexes having a broad absorption band in conjunction with oxide films of nanocrstalline morphology permits to harvest a large fraction of sunlight. Nearly quantitative conversion of incident photons into electric current is achieved over a large spectral range extending over the whole visible region. Overall solar (standard AM 1.5) to electric conversion efficiencies over 10% have been reached. There are good prospects to produce these cells at lower cost than conventional devices. The lecture will present the current state of the field. We shall discuss new concepts of the dye-sensitized nanocrystalline solar cell (DYSC) including solid heterojunction variants and analyze the perspectives for the future development of the technology into the next millennium.

High speed composite p-channel Si/SiGe heterostructure for field effect devices

Abstract: A method and a layered heterostructure for forming p-channel field effect transistors is described incorporating a plurality of semiconductor layers on a semiconductor substrate, a composite channel structure of a first epitaxial Ge layer and a second compressively strained SiGe layer having a higher barrier or a deeper confining quantum well and having extremely high hole mobility. The invention overcomes the problem of a limited hole mobility for a p-channel device with only a single compressively strained SiGe channel layer.

Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction

Abstract: By combining ultraviolet and x-ray photoelectron spectroscopy with inverse photoemission spectroscopy, we find that the conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction is flat (0.0±0.2 eV). Furthermore, we observe a valence-band offset of 0.8±0.2 eV. The electronic level alignment is dominated by (1) an unusually large surface band gap of the CuInSe2 thin film (1.4 eV), (2) by a reduced surface band gap of the CdS overlayer (2.2 eV) due to intermixing effects, and (3) by a general influence of the intermixing on the chemical state near the interface.

Metalorganic chemical vapor deposition of GaN on Si(111): Stress control and application to field-effect transistors

Abstract: Two schemes of nucleation and growth of gallium nitride on Si(111) substrates are investigated and the structural and electrical properties of the resulting films are reported. Gallium nitride films grown using a 10–500 nm-thick AlN buffer layer deposited at high temperature (∼1050 °C) are found to be under 260–530 MPa of tensile stress and exhibit cracking, the origin of which is discussed. The threading dislocation density in these films increases with increasing AlN thickness, covering a range of 1.1 to >5.8×109 cm−2. Films grown using a thick, AlN-to-GaN graded buffer layer are found to be under compressive stress and are completely crack free. Heterojunction field effect transistors fabricated on such films result in well-defined saturation and pinch-off behavior with a saturated current of ∼525 mA/mm and a transconductance of ∼100 mS/mm in dc operation.

Gallium nitride based high power heterojunction field effect transistors: process development and present status at UCSB

Abstract: The development of GaN based devices for microwave power electronics at the University of California, Santa Barbara (UCSB), is reviewed. From 1995 to 2000, the power performance of AlGaN/GaN-on-sapphire heterojunction field effect transistors improved from 1.1 W/mm to 6.6 W/mm, respectively. Compensating the disadvantages of the low thermal conductivity of the sapphire substrate through heat management via flip chip bonding onto AlN substrates, large periphery devices with an output power of 7.6 W were demonstrated. UCSB also fabricated the first GaN based amplifier integrated circuits. Critical issues involved in the growth of high quality AlGaN/GaN heterostructures by metal-organic chemical vapor deposition and the device fabrication are discussed.

Electrical spin injection into semiconductors

Abstract: We present the results of a theoretical model describing electrical spin injection from a spin-polarized contact into a nonmagnetic semiconductor. The model includes the possibility of interface resistance due, for example, to a tunnel barrier at the contact/semiconductor heterojunction, and shows that such interface resistance can be critical in determining spin injection properties. With no interface resistance spin injection is very weak for contacts with typical metallic resistivities. For higher bulk resistivity contacts, such as doped semiconductors, or for completely spin-polarized contacts, strong spin injection is possible without significant interface resistance. However the spin polarization must be extremely close to complete for contacts with metallic resistivities. A tunnel barrier with spin-dependent interface resistance can greatly enhance spin injection. An insulating tunnel barrier with a spin-polarized contact, and a ferromagnetic insulator tunnel barrier, both have spin-dependent interface resistance, and provide two promising approaches to achieve significant electrical spin injection. The model is consistent with a variety of experimental observations, identifies the basic physics problems that must be addressed to achieve a high degree of spin injection, and suggests systematic strategies to achieve strong spin injection.

Influence of electron-phonon interaction on the optical properties of III nitride semiconductors

Abstract: The electronic band structures of III nitride semiconductors calculated within the adiabatic approximation give essential information about the optical properties of materials. However, atoms of the lattice are not at rest; their displacement away from the equilibrium positions perturbs the periodic potential acting on the electrons in the crystal, leading to an electron-phonon interaction energy. Due to different ways that the lattice vibration perturbs the motions of electrons, there are various types of interaction, such as Frohlich interaction with longitudinal optical phonons, deformation-potential interactions with optical and acoustic phonons and piezoelectric interaction with acoustic phonons. These interactions, especially the Frohlich interaction, which is very strong due to the ionic nature of III nitrides, have a great influence on the optical properties of the III nitride semiconductors. As a result of electron-phonon interaction, several phenomena, such as phonon replicas in the emission spectra, homogeneous broadening of the excitonic line width and the relaxation of hot carriers to the fundamental band edge, which have been observed in GaN and its low dimensional heterostructures, are reviewed.

Large spontaneous spin splitting in gate-controlled two-dimensional electron gases at normal In0.75Ga0.25As/In0.75Al0.25As heterojunctions

Abstract: Amounts of spontaneous spin splittings were estimated from low-temperature magnetoresistances in two-dimensional electron gases created at In0.75Ga0.25As/In0.75Al0.25As heterojunctions under a gate bias. Typical sheet electron densities and mobilities in the raw wafers were ∼1.0×1012/cm2 and 2–5×105 cm2/V s at 1.5 K, respectively. A maximum spin-orbit coupling constant αzero of ∼30(×10−12 eV m) was obtained for the van der Pauw sample. In gated Hall-bar samples, a decrease in the αzero value with decreasing gate voltage (Vg) was first confirmed in a normal heterojunction. The main origin for such a large αzero, which is a few times larger than any previously reported, was found to be a structure-dependent so-called interface contribution in the Rashba term.

Electronic Properties of Ga(In)NAs Alloys

Abstract: A brief review on the present knowledge of the electronic properties of the Ga(In)NAs ternary and quaternary alloys is given mainly from an experimental perspective. The discussion is focused on Ga(In)NAs with low N composition (< 10 %), where a large amount of experimental work has been done. Important fundamental electronic properties of the material system are analyzed with the emphasis on the nature of the giant band gap bowing in the alloy and nitrogen-induced modifications of the electronic structure of the conduction band. The current knowledge of the key material parameters, relevant for the device applications, such as electron effective mass, recombination processes and band alignment in Ga(In)NAs/GaAs heterostructures, is also reviewed.

Fabrication and characterization of ultraviolet-emitting diodes composed of transparent p-n heterojunction, p-SrCu2O2 and n-ZnO

Abstract: An ultraviolet light-emitting diode (UV-LED) was realized using a p-n heterojunction composed of the transparent oxide semiconductors p-SrCu2O2 and n-ZnO. A Ni/SrCu2O2/ZnO/ITO multilayered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. SrCu2O2 (112) was preferentially grown on ZnO (0001) at 350°C, while the preferential plane was changed into the (100) when the temperature was increased to 600 °C. The grown films were processed by conventional photolithography followed by reactive ion etching to fabricate heterojunction diodes. The resulting devices exhibited rectifying I-V characteristics inherent to p-n junctions. A relatively sharp electroluminescence band centered at 382 nm, attributed to transitions associated with exciton-exciton collision or electron-hole plasma in ZnO, was generated by applying a forward bias voltage greater than the turn-on voltage of 3 V. UV-LED performance characteristics such as threshold current and conversion efficie...

Four-terminal resistance of a ballistic quantum wire

Abstract: The electrical resistance of a conductor is intimately related to the relaxation of the momentum of charge carriers. In a simple model, the accelerating force exerted on electrons by an applied electric field is balanced by a frictional force arising from their frequent collisions with obstacles such as impurities, grain boundaries or other deviations from a perfect crystalline order1. Thus, in the absence of any scattering, the electrical resistance should vanish altogether. Here, we observe such vanishing four-terminal resistance in a single-mode ballistic quantum wire. This result contrasts the value of the standard two-probe resistance measurements of h/2e2≈ 13 kΩ. The measurements are conducted in the highly controlled geometry afforded by epitaxial growth onto the cleaved edge of a high-quality GaAs/AlGaAs heterostructure. Two weakly invasive voltage probes are attached to the central section of a ballistic quantum wire to measure the inherent resistance of this clean one-dimensional conductor.

Electronic band structure of single-crystal and single-layer WS 2 : Influence of interlayer van der Waals interactions

Abstract: The valence band structure of the layered transition metal dichalcogenide ${\mathrm{WS}}_{2}$ has been determined experimentally by angle resolved photoelectron spectroscopy and theoretically by augmented spherical wave band structure calculations as based on density functional theory. Good agreement between experimental and calculated band structure is observed for single crystal ${\mathrm{WS}}_{2}.$ An experimental band structure of a single layer was determined from an electronically decoupled film prepared on a single crystalline graphite substrate by metal-organic van der Waals epitaxy. The polarization dependent photoemission selection rules of the single layer film are appropriate for a free standing film. The experimental single layer band structure shows some differences compared to band structure calculations using bulk atomic positions within the layer. We conclude that relaxation of the single layer occurs as a consequence of the missing interlayer interactions leading to close agreement between electronic structure of the single layer and single crystal. As a consequence of the missing interlayer interactions the valence band maximum for the single layer is located at the K point of the Brillouin zone.

Chemistry and electrical properties of surfaces of GaN and GaN/AlGaN heterostructures

Abstract: Chemical and electrical properties of the surfaces of GaN and GaN/AlGaN heterostructures were systematically investigated by x-ray photoelectron spectroscopy (XPS), capacitance–voltage, and current–voltage measurements. From in situ XPS study, relatively smaller band bending of 0.6 eV was seen at the GaN (2×2) surface grown by radio frequency-assisted molecular beam epitaxy on the metalorganic vapor phase epitaxy GaN template. After exposing the sample surface to air, strong band bending took place at the surface. The surface treatment in NH4OH solution and N2 plasma was found to reduce the surface Fermi level pinning. Surface passivation process of GaN utilizing SiNx film by electron-cyclotron-resonance assisted plasma chemical vapor deposition (ECR–CVD) achieved low interface state density, 2×1011cm−2 eV−1. No pronounced stress remained at the SiNx/GaN interface, which was confirmed by Raman spectroscopy. The present NH4OH/ECR–N2 plasma treatment was also found to be effective in realizing well-ordered ...

Current instabilities in GaN-based devices

Abstract: Current dispersion effects have been experimentally investigated in a variety of AlGaN/GaN heterostructure FETs with large signal and switching measurements including HEMTs with doped and undoped barrier layer. A range of dispersion frequencies from 10/sup -3/ Hz to 10 GHz were observed, where the output current amplitude is drastically reduced. Through this effect the full channel charge of an AlGaN/GaN heterostructure FET may be completely depleted under specific bias conditions. This indicates that this phenomena cannot be related to deep traps alone, but is also connected to piezorelated charge states and conduction to these states.

Molecular Beam Epitaxy of Group‐III Nitrides on Silicon Substrates: Growth, Properties and Device Applications

Abstract: We report on the growth and properties of GaN films grown on Si(111) substrates by molecular beam epitaxy using ammonia. The properties of the layers show that our growth procedure is very efficient in order to overcome the difficulties encountered during the growth of nitrides on silicon substrates: first, no nitridation of the silicon substrate is observed at the interface between the AIN buffer laver and the silicon surface: second. there is no Si autodoping coming from the substrate and resistive undoped GaN layers are obtained; and, also, strain balance engineering allows one to grow thick GaN epilayers (up to 3 mum) without formation of cracks. The optical, structural and electrical properties of these films are studied. In order to evaluate the potentialities of III-V nitrides grown on silicon substrates, we have grown heterostructures to realize light emitting diodes (LEDs), photodetectors and high electron mobility transistors (HEMTs).

Effect of polarization fields on transport properties in AlGaN/GaN heterostructures

Abstract: We have calculated transport properties of unintentionally doped n-type AlGaN/GaN heterostructures. Using a thermodynamic model of defect formation, we have modeled the charge transfer process in such heterostructures, obtaining good agreement with experiment. The large polarization fields in the heterostructure dramatically lower the formation energy of the surface defects, leading to the observed extremely large two-dimensional electron gas concentrations. Calculations of the low temperature mobilities were also performed, showing that alloy disorder and, in some cases, interface roughness, are the dominant low-temperature carrier scattering mechanisms. At low temperatures a maximum intrinsic mobility of about 105 cm2/V s is predicted for these heterostructures.

SiGe-on-insulator prepared by wafer bonding and layer transfer for high-performance field-effect transistors

Abstract: SiGe-on-insulator material was fabricated by wafer bonding and hydrogen-induced layer transfer techniques. The transferred SiGe layer is strain relaxed and has a Ge content ranging from 15% to 25%. High-quality strained Si layers were grown on the SiGe-on-insulator substrates by the UHV/chemical vapor deposition process at 550 °C. An electron mobility of 40 000 cm2/V s in a modulation-doped Si/SiGe heterostructure was achieved at 30 K on a SiGe-on-insulator substrate.

High-performance p-i-n Ge on Si photodetectors for the near infrared: from model to demonstration

Abstract: We have investigated the integration of Ge p-i-n and n-i-p heterojunction photodiodes on Si. Recognizing the crucial role of interface defects at the Ge-Si interface on the performance of photodetectors, we have designed and fabricated high-performance n-i-p Ge photodiodes on p/sup +/-Si substrates. These photodiodes exhibit short-circuit responsivities of 0.3 and 0.2 A/W at 1.3 and 1.55 /spl mu/m, respectively, reverse dark currents of 20 mA/cm/sup 2/ and response times of 800 ps.