Conduction Band Barrier Research Articles

Improved VBIC model for SiGe HBTs with an unified model of heterojunction barrier effects

An improved bipolar transistor model considering heterojunction barrier effect (HBE) in SiGe double heterojunction bipolar transistors is developed. The effect of barrier formation due to high level injection, which is related to the rapid degradations of the dc current gain (/spl beta/) and cutoff frequency (f/sub T/), is carefully investigated and analyzed. As the collector current becomes high, the conduction band barrier is induced and increased. It causes the saturation of collector current (J/sub C/) due to the blocking of carrier transport, the sharp increase of base transit time (/spl tau//sub B/) due to the additional charge storage, the increase of base current (J/sub B/) due to the increased recombination, and the decrease of intrinsic base resistance (R/sub bi/) due to the increased charge and base pushout. Those phenomena are included into a vertical bipolar intercompany model (VBIC) compact model by employing a unified model of the HBE on J/sub C/, J/sub B/, /spl tau//sub B/, and R/sub bi/. Furthermore, portions of /spl tau//sub B/ and R/sub bi/ from the Kirk effect itself are modeled according to the high current model description and the new formulation of widened base, respectively. A full extraction of parameters has been performed and the modified VBIC model is applied. The modeling accuracy is significantly improved at the high current region for the dc and RF characteristics.

CIGS J– V distortion in the absence of blue photons

Common buffer materials used with CuInGaSe 2 (CIGS) absorbers produce conduction-band barriers that may significantly distort the current–voltage ( J– V) curves, especially when short-wavelength photons are excluded from the illumination spectrum. Earlier work documented this effect for CuInSe 2 (CIS) absorbers (band gap near 1.0 eV) with CdS buffers. Higher band-gap (∼1.15 eV) CIGS absorbers show little or no distortion with CdS buffer layers. However, wider band gap (lower electron affinity) ZnS(O,OH) or InS(O,OH) buffers, prepared by chemical-bath deposition (CBD), clearly show the J– V distortion. The distortions have a turn-on time constant the order of a minute and turn-off time constant the order of a day, and they correlate with major variations in apparent quantum efficiency (QE) measured with varying intensity and spectral content of bias light. The results are consistent with a conduction-band spike barrier that increases with buffer band gap and is larger when the electron concentration in the buffer is small.

Chemical and electrical characterization of Gd2O3∕GaAs interface improved by sulfur passivation

We report the formation of a gate dielectric film on the sulfur passivated GaAs(001). The Gd2O3 films were deposited on the n-GaAs substrates using the electron-beam evaporation. The sulfur passivated GaAs metal-oxide-semiconductor diode exhibited the improved electrical properties compared to a similar GaAs diode that was cleaned with HCl including an enhanced conduction-band barrier and a reduced-leakage current. We investigated the relation between the interfacial bonds and the electrical properties by a systematic characterization using the photoemission spectroscopy. Sulfidation was shown to preserve a stable Gd2O3∕GaAs interface and to prevent an interfacial GaAs oxide formation. The stable interface and the passivated surface were caused by the bonding transition from As–S to Ga–S. The As-oxide was dissociated to form the interfacial Ga2O3 and to volatilize the elemental As in the Gd2O3 deposition, conducted without sulfidation. The interfacial oxides and excess As degraded the electrical properties and resulted in the decrease of the conduction-band offset. It was found that the Gd2O3 film was composed of two oxidation states: the amorphous Gd–O and the crystalline Gd2O3. The bonding transition with a substrate heating implies that the Gd2O3 film on the GaAs surface completely crystallizes above 400°C. Electrical properties were understood in relation to the band alignment and were estimated from the valence band and the core-level spectra analysis. The sulfidation introduced a shift of the Fermi level toward the conduction-band minimum on the GaAs surface and increased the conduction-band offset at the Gd2O3∕GaAs interface, followed by a saturated accumulation capacitance with a low-leakage current.

Device Level Characterization for Energy Bandgap of Strain-relaxed SiGe and Oxide/SiGe Barrier Height

From the interface state density with energy plot measured by capacitance-voltage characteristics, we have derived the energy bandgap of strain-relaxed (x: 0.4 and 0.7) for the first time at the device level, because the allowed states increases sharply near the conduction and valence bandedges. We find that the energy bandgap of SiGe is reduced from 0.90 to 0.83 eV as the Ge composition increases from to In contrast, similar conduction-band barrier heights of ∼3.1 eV have been obtained for both SiGe cases using Fowler-Nordheim tunneling. These results are in good agreement with published theoretical and experimental data from material level characterization. The device level characterization is especially important because the SiGe composition and material property may be altered after thermal cycles for device fabrication. © 2004 The Electrochemical Society. All rights reserved.

Energy band structure of Ru/(Ba,Sr)TiO3/Si capacitor deposited by inductively-coupled plasma-assisted radio-frequency-magnetron plasma sputtering

Properties of barium strontium titanate (Ba,Sr)TiO3 (BST) thin films, deposited by inductively-coupled-plasma (ICP)-assisted rf magnetron-plasma sputtering, were investigated. It is found that the ICP-assisted rf-magnetron plasma improved the stoichiometry of the BST film. The energy band structure of the Ru/BST/Si system was investigated. It is found that the bandgaps of BST and interface SiO2 were 4.30 and 8.95 eV, respectively, and the work function of Ru was 4.97 eV. The valence band offsets for BST/Si and interface SiO2/Si are 3.55 and 4.48 eV, respectively. As a result, the conduction band barrier height of Ru against BST was found to be 1.40 eV.

Numerical modeling of parasitic barrier formation at the SiGe/Si heterojunction due to p–n junction displacement

The formation and characteristics of a parasitic conduction band barrier located at a SiGe/Si heterojunction have been investigated using a commercial numerical simulator and a simple, three-region model of a heterojunction with a nearby p–n junction. The barrier’s formation is examined as a function of the displacement of the p–n junction from the heterojunction, but also found to depend on the germanium concentration, junction doping and the applied bias. The phenomenon is of interest for understanding the performance of SiGe/Si heterojunction bipolar transistors, where the p–n junction is intentionally displaced from the heterojunction at either the emitter or collector junctions or where boron outdiffusion from the base produces p–n junction displacement. The barrier is found to scale with the germanium mole fraction and to be significantly larger when the heterojunction is displaced into the p-side of the p–n junction. Beyond some minimum separation of the junctions, the barrier height rises with junction displacement and saturates. For a given displacement, the barrier’s height can be suppressed with reverse bias or enhanced by forward bias of the p–n junction. The results of the numerical simulations are compared with those from a simple analytical model as an aid in understanding the barrier’s formation and characteristics.

MOCVD-grown broad area InGaAs/GaAs/InGaP laser diodes

A MOCVD technology for growth of InGaAs/GaAs/InGaP laser heterostructures on a modified Epiquip VP-50-RP installation was developed. Mesa stripe laser diodes with threshold current density Jth=100–200 A/cm2, internal optical loss αi=1.3–1.7 cm−1, and internal quantum efficiency ηi=60–70% have been fabricated. A CW output optical power of 5 W has been obtained for a single 100-µm-wide aperture mesa stripe laser diode emitting at 1.03 µm. It is shown that use of AlGaAs waveguide layers, which increase the conduction band barrier offset, lowers the temperature sensitivity of laser heterostructures within the temperature range 10–80°C.

Tetrahedral amorphous carbon–silicon heterojunction band energy offsets

Non-hydrogenated tetrahedral amorphous carbon (ta-C) has shown superior field emission characteristics. The understanding of the emission mechanism has been hindered by the lack of any directly measured data on the band offsets between ta-C and Si. In this paper results from direct in situ X-ray photoemission spectroscopy (XPS) measurements of the band-offset between ta-C and Si are reported. The measurements were carried out using a filtered cathodic vacuum arc (FCVA) deposition system attached directly to an ultra-high vacuum (UHV) XPS chamber via a load lock chamber. Repeated XPS measurements were carried out after monolayer depositions on in situ cleaned Si substrates. The total film thickness for each set of measurements was approximately 5 nm. Analysis of the data from undoped ta-C on n and p Si show the unexpected result that the conduction band barrier between Si and ta-C remains around 1.0 eV, but that the valence band barrier changes from 0.7 to 0.0 eV. The band line up derived from these barriers suggests that the Fermi level in the ta-C lies 0.3 eV above the valence band on both p and n+Si. The heterojunction barriers when ta-C is doped with nitrogen are also presented. The implications of the heterojunction energy barrier heights for field emission from ta-C are discussed.

Optimization of SiGe HBTs for operation at high current densities

A comprehensive investigation of the impact of Ge profile shape as well as the scaling of collector and base doping profiles on high-injection heterojunction barrier effects in SiGe HBTs has been conducted over the -73-85/spl deg/C temperature range. The onset of Kirk effect at high current densities is shown to expose the Si/SiGe heterojunction in the collector-base space charge region, thereby inducing a conduction band barrier which negatively impacts the collector and base currents as well as the dynamic response, leading to a premature roll-off in both /spl beta/ and f/sub T/. In light of this, careful profile optimization is critical for emerging SiGe HBT circuit applications, since they typically operate at high current densities to realize maximum performance. We first explore the experimental consequences and electrical signature of these barrier effects over the 200-358 K temperature range for a variety of Ge profiles from an advanced UHV/CVD SiGe HBT technology. We then use extensive simulations which were calibrated to measured results to explore the sensitivity of these barrier effects to both the Ge profile shape and collector profile design, and hence investigate the optimum profile design points as a function of vertical scaling.

Defect-Enhanced Electron Transport through Semiconductor Barriers

It has often been suggested that defects can enhance electron tunneling through barriers. Here, we derive a general expression of the current flowing through semiconductor barriers when induced by the presence of defects, taking into account tunneling and other possible mechanisms. Therefore, we calculate the capture probability of free electrons by defects located in the barrier and the subsequent emission probabilities of the captured electrons, by thermal emission or phonon-assisted tunneling into the barrier conduction band. The defect potential considered is composed of a δ-function localized on first neighbors and of a Coulomb tail, thus allowing to treat all types of defects from shallow to deep ones. The result of the calculation is then compared with experiment. The temperature and electric field dependences of the current fit well the experimental data obtained for undoped GaAlAs barriers embedded in n+ GaAs, if the defect considered is the DX center. The concentration of these native donor defects that accounts for the amplitude of the current is, as expected, of the order of 1016 cm—3.

Electron tunneling through ultrathin SiO 2

The tunnel current through 3.0–6.0 nm thick SiO 2 grown on Si(100) substrates has been compared with the theory of the WKB approximation. By using the measured conduction band barrier height, the electron effective mass, which is only the fitting parameter, is obtained to be (0.34 0.04)m 0 in the Fowler-Nordheim tunneling region and (0.29 0.02)m 0 in the direct tunneling region. It is also shown that the charge-to-breakdown for electron injection from n + poly-Si gates is not significantly deteriorated by decreasing the oxide thickness to 3.3 nm and even dramatically improved for the case of a 3.0 nm thick gate oxide. Quasi-breakdown current observed in ultrathin SiO 2 has been analyzed and a new dielectric degradation model is proposed.

Investigation of electric fields, interface charges, and conduction band offsets at ZnSe/GaAs heterojunctions with a novel photoreflectance technique

AbstractZnSe/GaAs heterojunctions were investigated by contactless electroreflectance and photoreflectance techniques. Negative surface charge densities on the order of 1012 cm-2 were observed for films grown on n-type GaAs indicating a large contribution to the conduction band barrier between the materials due to band bending. The conduction band offset was also measured using a new photoreflectance technique involving a tunable pump laser.

Role of defects on electron transport through semiconductor barriers

We describe the effect of electron irradiation on the current-voltage characteristics of nonintentionally doped GaAlAs barriers imbedded in n-type doped GaAs. It is found that below typically 100 K where the Fowler–Nordheim tunneling applies, tunneling switches from the lowest (Γ) band to a higher (X) one after a fluence of few 1016 electron cm−2. At high temperature a defect assisted process involving tunneling into defect sites followed by the thermal ionization of electrons into the barrier conduction band, dominates. This process is also modified by the irradiation, the native defects being replaced by the electron induced defects.

Electronic transport through semiconductor barriers

Electron transport across rectangular barriers, made of 200 AA thick GaAlAs layers embedded in GaAs, and triangular barriers at the (n+)GaAs-(n)GaInP interface has been studied. Current versus voltage and temperature characteristics have been analysed in order to extract the different mechanisms that induce this transport, and to determine the temperature and electric field range in which they apply. At low temperature and high field the current is driven by the Fowler-Nordheim regime. At higher temperatures the current is dominated by a defect-induced mechanism. This mechanism consists of the thermal emission of electrons into the barrier conduction band from defects lying in the barrier that can be refilled by tunnelling. The defect involved appears to be the deep state associated with the donor impurity, i.e. the DX centre. This study demonstrates that the apparent band offset depends strongly on the experimental conditions under which it is measured.

Carrier-induced localization in In-Ga-As/In-Ga-As-P separate-confinement quantum-well structures

We present a detailed analysis of carrier-induced modifications on the spatial band diagrams of In-Ga-As/In-Ga-As-P separate-confinement multiple-quantum-well structures and its consequences on optical properties. In the high-carrier-density regime, we observe experimentally a second peak in the spectra of the optical gain. This can be understood by a buildup of a space charge and band bending which is caused by a spatial separation between electrons and holes due to the different density of states of the conduction and valence bands. This band bending causes a localization of barrier conduction band states in the quantum-well region and an enhancement of the overlap integral of those states with quantized heavy hole states giving rise to substantial modifications in the optical matrix elements. These modified matrix elements are the origin of the new class of optical transitions observed in the optical gain spectra.

Resolving degradation mechanisms in ultra-high performance N-p-n heterojunction bipolar transistors

The conduction band barrier caused by base dopant outdiffusion is investigated for MBE-grown Al/sub x/Ga/sub 1-x/As/GaAs N-p-n linearly graded heterojunction bipolar transistors (HBTs). The change of the B-E heterojunction conduction band barrier can be directly revealed by a novel technique based on the diffusion-thermionic emission current model. This is accomplished by measuring the inverted collector current ratio at two different heterojunction reverse biases. In addition, this ratio is found to correlate with an anomalous base current component measured at low temperature. The heterojunction potential barrier and the anomalous base current component are attributed to beryllium redistribution during MBE growth and forward current stress. This suggests that the diffusion and incorporation of beryllium dictate V/sub BE/ uniformity and long-term reliability of HBTs. >

Current transport mechanism in GaInP/GaAs heterojunction bipolar transistors

GaInP/GaAs heterojunction bipolar transistors (HBTs) and both graded and abrupt AlGaAs/GaAs HBTs were fabricated. A total of 20 wafers were analyzed. Comparisons of the experimental results establish that the dominant carrier transport mechanism in GaInP/GaAs HBTs is the carrier diffusion through the base layer. This suggests that the conduction-band barrier across the GaInP/GaAs emitter-base junction is so small that the barrier spike does not affect the carrier transport. This result differs from other published results which, by studying device structures other than HBTs, determined the conduction band barrier to be as large as approximately 50% of the bandgap difference. The findings of the present investigation, however, agree well with another published work which also examined an HBT structure. The difference between these works is discussed. >

Direct and exchange-correlation carrier interaction effects in a resonant tunnel diode

Results are presented of a density functional calculation of the effects due to both electron-electron direct and exchange-correlation interactions on the effective, single-electron potential, the conduction electron density profile, the device transmission coefficient, and the current-voltage characteristics of a GaAs-AlGaAs quantum-well tunnel diode. The calculation proceeds by self-consistently solving the Kohn–Sham and Poisson equations, accounting for the strong, electric-field-induced nonequilibrium effects by employing a spatially varying chemical potential which is essential for maintaining net charge neutrality. The exchange potential used is the Dirac local-density approximation and the correlation potential is determined from the Wigner interpolation formula. The results show a number of interesting effects: it is found that the direct, electrostatic repulsion due to the buildup of charge within the spacer layers extends far beyond the device, well into the contact layers. Furthermore, the depression of carrier density within the quantum well leads to a reduction in the exchange-correlation screening of the direct interaction. Consequently, the total band bending in the vicinity of the device is enhanced by the exchange-correlation interaction, a result in contradiction with a previous Hartree–Fock calculation [K. Bandara and D. Coon, Appl. Phys. Lett. 53, 1865 (1988)]. In the case of a resonantly biased quantum well, the formation of an exchange-correlation hole due to the population of the resonant level within the quantum well is also observed. Finally, elastic scattering effects have been modeled by including random noise in the barrier conduction band. This causes a broadening of the resonant level within the quantum well, which in turn leads to an enhanced current flow through the overbiased device, although this enhancement is not as large as that seen experimentally. This suggests that the enhanced current flow in the overbiased device is likely to be due to inelastic scattering processes that are not included in this model.

Parasitic energy barriers in SiGe HBTs

Parasitic energy barriers can easily be introduced during processing. Measurements and calculations of experimental n-p-n HBTs (heterojunction bipolar transistors) are presented, showing that a parasitic conduction-band barrier at the base-collector junction reduces the collector current and the cutoff frequency. A simple analytical model explains the f/sub T/ degradation, caused by the reduction of the collector current and a pileup of minority carriers in the base. With the model the effective height and width of the barrier can also be derived from the measured collector current enhancement factor I/sub C/(SiGe)/I/sub C/(Si). >

Electrical properties of GaAs/GaN/GaAs semiconductor-insulator-semiconductor structures

We report on the current-voltage (IV) characteristics of GaAs/GaN/GaAs semiconductor-insulator-semiconductor structures as a function of temperature. IV measurements show a strong temperature dependence indicating a thermal distribution of carriers flowing over a barrier. From these data we deduce an effective conduction band barrier of 0.9 eV between GaAs and GaN.