GaAs---GaAlAs heterojunction transistor for high frequency operation

A bipolar transistor structure is proposed having application for either high frequency operation or integration with certain types of light emitting devices. The structure involves liquid phase epitaxially grown layers of GaAs for the collector and base regions, and of Ga_1−xAl_xAs for the heterojunction emitter. The high frequency potential of this device results primarily from the high electron mobility in GaAs and the ability to heavily dope the base region with slowly diffusing acceptors. The Ga_1−xAl_xAs emitter region provides a favorable injection efficiency and, because it is etched preferentially relative to GaAs, access to the base layer for making contact. Transistor action with d.c. common emitter current gains of 25 have been thus for observed. Calculations of the high speed capability of this transistor are presented.     

High performance Schottky contacts on Se-doped AlxGa1−xAs by cryogenic processing

Al_xGa_1−xAs samples were grown on Se-doped Al_xGa_1−xAs layer above GaAs substrate by metalorganic chemical vapor deposition (MOCVD). It was reported that the crystal quality of the re-growth Al_xGa_1−xAs layer can be improved significantly when an Se-doped Al_xGa_{1 − x}As layer was used. The surface carrier concentration, however, was increased as a result of the Se-doping. Therefore, it could be difficult to obtain a high performance Schottky contacts on such a structure for practical devices application. In this work, Schottky contacts to Al_xGa_1−xAs/Se-doped Al_xGa_1−xAs were formed by cryogenic processing. In such a processing, the Schottky metal deposition was carried out with the sample cooled to a low temperature (LT) of 77 K. The Schottky barrier height was increased significantly in a Au/Al_xGa_1−xAs/Se-doped Al_xGa_1−xAs contact. The leakage current, for the LT contact, was nearly 4 orders lower than that of contact made at room temperature. A current transport mechanism dominated by thermionic emission for the LT contacts was found from current-voltage-temperature (I-V-T) measurement.     

High conductance in random superlattices with correlated disorder

We study d.c. conductance of disordered GaAs-Ga_1−xAl_xAs superlattices where the disorder is intentional and short-range correlated. We consider Ga_1−xAl_xAs layers of the same thickness, where GaAs layers present two different thicknesses randomly distributed along the growth direction, with the constraint that one of them always appears in pairs. A set of almost unscattered electron states is observed in spite of the disorder, revealing itself through a distinct conductance maximum. Interestingly, this maximum is also detected in imperfect superlattices, where interface roughness is taken into account by allowing quantum-well thicknesses to fluctuate around their nominal values. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder, even in the presence of imperfections.     

Transparent self-electro-optic effect device based on Wannier-Stark localization in unstrained InxGa1−xAs/InxAl1−xAs superlattices on GaAs substrate

The first transparent-type Wannier-Stark localization self-electro-optic effect device made without substrate removal is reported. Unstrained 100-period In_xGa_1−xAs/In_xAl_1−xAs superlattice layers were successfully grown by molecular beam epitaxy on GaAs substrate by introducing specially designed buffer layers that consist of two sections. The first section is an In-composition-graded In_yGa_1−yAs, whose In content Y changes linearly from 0 to X, and the second is a strain-relaxed In_xGa_1−xAs layer. The introduction of the strain relaxation layer allows us to choose a wide range of In content X and an ample number of superlattice periods avoiding the critical layer thickness problem. When X = 0.17, the operating wavelength is 933 nm, which can penetrate through the GaAs substrate. The operating wavelength of 1064 nm is accomplished when X = 0.3, and clear bistability is demonstrated using a Nd:YAG laser as a light source.     

Light-activated switching based on light-induced band-structure modifications

We have investigated the mechanism of light-induced band-structure modifications to be employed in light-activated switching devices. This mechanism has been studied in a specially designed modulation doped GaAs/Al_xGa_1−xAs heterostructure by means of photoluminescence spectroscopy at low temperatures. It is related to the build-up of photogenerated carriers in the heterostructure, thereby changing the band structure. We show that this mechanism can be employed in a light-activated all-optical switch or a light-activated all-optical modulator with an expected contrast ratio of 27:1.     

Self-organization of quantum wire-like morphology on InxGa1 − xAs single quantum wells grown on (100)InP vicinal surfaces depending on the substrate misorientation, buffer mismatch and growth temperature

The appearance of a quantum wire-like morphology on the In_xGa_{1 − x}As single quantum well of High Electron Mobility Transistor structures grown by Molecular Beam Epitaxy (MBE) on (100)InP vicinal surfaces is reported. This quantum wire morphology in the InGaAs well is driven by the lateral modulation existing in the In_yAl_{1 − y} As tensile buffer of the heterostructure. The development of the lateral modulation in the buffer, related to In-rich or Alrich regions oriented along {133} or {122} planes, has been found to be favoured by the presence of steps at the interface of 4° off (100) towards (111)A misoriented InP substrates and also attributed to a thermally activated phenomenon. Furthermore, the results of Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD) reveal that the lateral composition modulation acts as a strain relieving mechanism that accommodates the tensile mismatch between the InAlAs buffer and the substrate.     

Hole mobility enhancement and Si cap optimization in nanoscale strained Si1−xGex PMOSFETs

P-channel metal-oxide-semiconductor field-effect-transistors (PMOSFETs) with a Si_1−xGe_x/Si heterostructure channel were fabricated. Peak mobility enhancement of about 41% in Si_1−xGe_x channel PMOSFETs was observed compared to Si channel PMOSFETs. Drive current enhancement of about 17% was achieved for 70 nm channel length (L_G) Si_0.9Ge_0.1 PMOSFETs with SiO₂ gate dielectric. This shows the impact of increased hole mobility even for ultra-small geometry of MOSFETs and modest Ge mole fractions. Comparable short channel effects were achieved for the buried channel Si_1−xGe_x devices with L_G=70 nm, by Si cap optimization, compared to the Si channel devices. Drive current enhancement without significant short channel effects (SCE) and leakage current degradation was observed in this work.     

Carrier density distribution in modulation doped GaAs-AlxGa1−xAs quantum well heterostructures

A theoretical model for the distribution of charged carriers at the interface of a doped Al_xGa_1−xAs-undoped GaAs heterostructure is evaluated. Assuming a triangular potential well at the interface, we obtain numerically, curves describing the Fermi energy, depletion width in the alloy, and average separation of impurities and carriers, and the electron density at the interface as functions of alloy layer composition and doping level. The results obtained can be applied to multiple heterointerface quantum well heterostructures and superlattices as well. Carrie concentration profile data on a 51-layer modulation doped QWH grown by MOCVD are presented and are discussed using the results of the model.     

Light-scattering determination of electron tunneling gaps in double quantum wells

Resonant inelastic light-scattering techniques have been used to measure directly the single-particle tunneling gap (Δ_SAS) in Al_xGa_1−xAs/GaAs double quantum wells. We have observed a systematic decrease in Δ_SAS with increasing height of the barrier in agreement with the Δ_SAS calculated self-consistently within a Hartree approximation.     

Composition induced design considerations for InP/GaxIn1−xAs heterojunction bipolar transistors

Several design principles based on compositional grading and heavy doping of the base region of a heterojunction bipolar transistor (HBT) have been presented. Physical and technological advantages underlying composition induced design criteria of InP/Ga_xIn_1−xAs HBTs have been discussed. A number of issues such as superlattice based grading in the base region, base resistance vs base region grading, the emitter–base junction design, tradeoffs between base region grading and the nonuniform doping of the base region, and the surface recombination at the external base region, have been articulated.     

Effect of the Ge-molefraction on the subthreshold slope and leakage current of vertical Si/Si1−xGex MOSFETs

The effect of the Ge-concentration on the subthreshold behaviour of vertical Si/Si_1−xGe_x pMOSFETs and of complementary Si_1−xGe_x/Si nMOSFETs is investigated by using an analytical model, which includes thermionic emission across the hetero-barrier. It is shown that inclusion of Si_1−xGe_x and strained Si in the source region of the pMOSFET and nMOSFET respectively, suppresses the subthreshold slope roll-up substantially and lowers the leakage current of even the smallest devices with channel lengths down to 50 nm.     

High-speed Si-based metal-semiconductor-metal photodetectors with an additional composition-graded i-a-Si1−xGex:H layer

The response-speed of Si-based metal-semiconductor-metal (MSM) photodetectors was improved by depositing a composition-graded intrinsic hydrogenated amorphous silicon–germanium (i-a-Si_1−xGe_x:H) layer on crystalline silicon (c-Si). In contrast to the non-composition-graded one (using intrinsic hydrogenated amorphous silicon (i-a-Si:H) layer), the full width at half maximum (FWHM) and fall time of the photodetector transient response were improved from 145.2, 404.6 to 107.6, 223.4 ps respectively. The experimental results showed that the device responsivity and quantum efficiency were increased from 0.329 (A/W) and 0.492 to 0.414 and 0.619 respectively by the employed composition-graded technique. We propose that this enhancement is due to a smoother barrier that is formed at the c-Si and i-a-Si_1−xGe_x:H interface. A lower deposition temperature of i-a-Si_1−xGe_x:H layer could be used to further reduce the fall time of the device transient response from 315.6 (250 °C) to 97.6 (180 °C) ps. To improve the contact properties between Cr electrode and i-a-Si_1−xGe_x:H layer, an annealing technique in hydrogen ambient was employed. The device knee voltage, which is the applied voltage at which the device current start to enter the saturation region in its current (log-scale) versus applied voltage characteristics, could be reduced to around 3.5 V after annealing.     

Incorporation behaviour of carbon and silicon on (100) and (111) surfaces during growth of GaAs/GaAs and Ga0.47In0.53As/InP by molecular beam epitaxy

Silicon and carbontetrabromide were used as dopant sources in the growth of GaAs/GaAs and Ga_0.47In_0.53As/InP structures. We studied the incorporation behaviour of these group IV atoms on (100) and {111} surfaces as a function of growth temperature. The free carver concentrations determined by Hall measurements for Si-doped GaAs and Ga_0.47In_0.53As layers are independent of growth temperature on all surface orientations studied. Silicon acts fundamentally as a donor except, as expected, for doped layers on (111)A GaAs substrates, where it acts as an acceptor. Carbon incorporation in GaAs and Ga_0.47In_0.53As always results in a p-type conduction independent of the surface orientations (100)/{111} or the growth temperatures we used. In contrast to the results on GaAs, carbon shows a strong temperature-dependent activation in Ga_0.47In_0.53As grown on (100) and (111)B surfaces. Carbon-doped Ga_0.47In_0.53As on (111)A and carbon-doped GaAs layers on (100)/{111} GaAs surfaces exhibit only a very weak dependence of the carrier concentration on the growth temperature. A significant amphoteric behaviour of carbon was not observed in any of the materials investigated.     

Fabrication of strained Si channel PMOSFET on thin relaxed Si1−xGex virtual substrate

A novel MBE-grown method using low-temperature Si technology is introduced into the fabrication of strained Si channel PMOSFETs. The thickness of relaxed Si_1−xGe_x epitaxy layer on bulk silicon is reduced to approximate 400 nm (x=0.2). The Ge fraction and relaxation of Si_1−xGe_x film are confirmed by DCXRD (double crystal X-ray diffraction) and the DC characteristics of strined Si channel PMOSFET measured by HP 4155B indicate that hole mobility μ_p has an maximum enhancement of 25% compared to similarly processed bulk Si PMOSFET.     

InxGa1−xAs/InP quantum well structures grown on [111]B InP

We report growth and characterization details of lattice matched and coherently strained In_xGa_1−xAs/InP quantum well structures grown on misoriented [111]B InP substrates. Photoluminescence from a range of such structures, grown on substrates with optimum misorientation, show linewidths as good or better than equivalent [100] layers. Multiquantum well (MQW) samples with relatively modest compressive strain show X-ray diffraction data characteristic of highly uniform pseudomorphic quantum wells. With increased strain (x = 0.63), relaxation is evident through a degradation of the diffracted peak widths and through the observation of defects in the surface morphology. Fabricated strained p-i(MQW)-n diode structures exhibit low reverse leakage current densities (e.g. j = 6 μA/cm²). Room temperature photocurrent measurements on these devices show a strong excitonic blue shift (15 nm) with applied bias, as a consequence of the built-in piezoelectric field. The rate of peak shift, up to 8 nm/V, demonstrates excellent potential for low voltage optical modulator applications at around 1.55 μm wavelength.     

Enhanced magnetic anisotropy and high hole mobility in magnetic semiconductor Ga1-x-yFexNiySb

(Ga,Fe)Sb is a promising magnetic semiconductor (MS) for spintronic applications because its Curie temperature (T_C) is above 300 K when the Fe concentration is higher than 20%. However, the anisotropy constant K_u of (Ga,Fe)Sb is below 7.6 × 10³ erg/cm³ when Fe concentration is lower than 30%, which is one order of magnitude lower than that of (Ga,Mn)As. To address this issue, we grew Ga_1-x-yFe_xNi_ySb films with almost the same x (≈24%) and different y to characterize their magnetic and electrical transport properties. We found that the magnetic anisotropy of Ga_0.76-yFe_0.24Ni_ySb can be enhanced by increasing y, in which K_u is negligible at y = 1.7% but increases to 3.8 × 10⁵ erg/cm³ at y = 6.1% (T_C = 354 K). In addition, the hole mobility (µ) of Ga_1-x-yFe_xNi_ySb reaches 31.3 cm²/(V∙s) at x = 23.7%, y = 1.7% (T_C = 319 K), which is much higher than the mobility of Ga_1-xFe_xSb at x = 25.2% (µ = 6.2 cm²/(V∙s)). Our results provide useful information for enhancing the magnetic anisotropy and hole mobility of (Ga,Fe)Sb by using Ni co-doping.     

Theoretical calculation and experimental evidence of the real and apparent bandgap narrowing due to heavy doping in p-type Si and strained Si1−xGex layers

Based on an analytical approach developed by Jain and Roulston, the different contributions to the bandgap narrowing at T = 0 K due to heavy doping in highly p-type doped Si and strained Si_1-xGe_x-layers are calculated for x < 0.3. The valence band in Si and in strained Si_1-xGe_x layers is not parabolic, it is highly distorted. To take the non-parabolicity into account a dopant concentration-dependent density of states effective mass is defined. within the framework of this formalism we find that the bandgap narrowing (BGN) in Si is not appreciably affected due to the band distortion. The situation for strained Si_1-xGe_x layers is quite different, in that the BGN increases significantly at doping levels exceeding 10¹⁹ cm⁻³. In the earlier published work, BGN of the Si_1−xGe_x layers was either slightly smaller or about the same as in Si. Now at high doping levels, BGN becomes considerably higher than in Si. We will show that the effect of the strain on the Fermi energy is much larger than on the BGN, which will cause a large change in the effective valence band offset. Comparison will be made then between our theoretical calculations and experimental results obtained on two different device structures. The modified effective valence band offset that we have calculated is in very good agreement with the experimental value derived from the published work on long-wavelength optical detectors. The apparent bandgap narrowing in strained p-type Si_1-xGe_x-layers is also calculated and compared with the experimental results on Heterojunction Bipolar Transistors fabricated in our laboratory.     

Demonstration of GaN MIS diodes by using AlN and Ga2O3(Gd2O3) as dielectrics

GaN MIS diodes were demonstrated utilizing AlN and Ga₂O₃(Gd₂O₃) as insulators. A 345 Å of AlN was grown on the MOCVD grown n-GaN in a MOMBE system using trimethylamine alane as Al precursor and nitrogen generated from a SVT RF N₂ plasma. For the Ga₂O₃(Gd₂O₃) growth, a multi-MBE chamber was used and a 195 Å oxide was E-beam evaporated from a single crystal source of Ga₅Gd₃O₁₂. The forward breakdown voltage of AlN and Ga₂O₃(Gd₂O₃) diodes are 5 and 6 V, respectively, which are significantly improved over 1.2 V from that of a Schottky contact. From the C–V measurements, both kinds of diodes showed good charge modulation from accumulation to depletion at different frequencies. The insulator/GaN interface roughness and the thickness of the insulator were measured with X-ray reflectivity.     

Residual strain in Si-Si1−xGex quantum dots

Quantum dots of 50–60 nm diameter fabricated from both Si-Si_1−xGe_x (x = 0.1−0.3) strained layer superlattices and a strain symmetrized Si₉-Ge₆ superlattice were investigated by photoluminescence and Raman scattering. It was found that the residual strain in 50 nm etched quantum dots can be as large as 50% of a corresponding pseudomorphic structure. In addition, both the photoluminescence intensity and quenching temperature of these dots increase as compared to the corresponding as-grown superlattices. These improved optical properties may be due to a combined effect of lateral quantum confinement and a possible indirect-direct bandgap transition in the dots induced by the change of strain field during the nanofabrication process.