Fermi Level Effect Research Articles

Dopant diffusion and segregation in semiconductor heterostructures: Part 2. B in Ge x Si 1-x /Si structures

Si1-x/Si heterostructures have been obtained. Here the chemical effects seem to be of less importance. The Fermi-level effect determines the ionized boron solubilities in GexSi1-x and in Si, as well as the thermal equilibrium concentration of the singly-positively-charged crystal self-interstitials I+ which governs the boron diffusion process. The junction carrier concentration affects the concentration of I+ and solubility of B in the region and hence controls B diffusion across the heterojunction.

Optimization of p-dopant profiles for GaInAsP MQW laser structures in MOMBE

For industrial device fabrication gaseous dopant sources are preferred in metalorganic growth technologies. Both in metalorganic vapor phase epitaxy (MOVPE) and metalorganic molecular beam epitaxy (MOMBE/CBE) diethylzinc is used. For doping of InP based ridge waveguide laser structures the critical parts are the active region with GaInAsP confinement layers, MQW layers and the p-type region with InP spacer layer, GaInAsP etch stop layer and InP cladding layer. Growing Zn doped InP in between GaInAsP layers leads to a significant Zn diffusion into the adjacent GaInAsP layers. This effect is much more pronounced in MOMBE than in MOVPE. It is demonstrated how this effect is reduced by insertion of additional intermediate layers with appropriate dopant concentration, and by compensating Si co-doping in the spacer and adjacent layers taking advantage of the Fermi level effect. Applying these techniques, reduced threshold current densities and improved high temperature performance can be obtained. The results are discussed by models of dopant diffusion in MOMBE in comparison to MOVPE.

Ab initio pseudopotential calculations of dopant diffusion in Si

The ab initio pseudopotential method is used to study transient enhanced diffusion (TED) related processes. The electronic degrees of freedom are included explicitly, together with the fully self-consistent treatment of the electron charge density. A large supercell and a fine k-point mesh are used to ensure numerical convergence. Such method has been demonstrated to give quantitative description of defect energetics. We will show that boron diffusion is significantly enhanced in the presence of the Si interstitial due to the substantial lowering of the migrational barrier through a kick-out mechanism. The resulting mobile boron can also be trapped by another substitutional boron, forming an immobile and electrically inactive two-boron pair. Similarly, carbon diffusion is also enhanced significantly due to the pairing with Si interstitials. However, carbon binds to Si interstitials much more strongly than boron does, taking away most Si interstitials from boron at sufficiently large carbon concentration, which causes the suppression of the boron TED. We will also show that Fermi level effect plays an important role in both Si interstitial and boron diffusion.

Polymer/metal interfaces and the performance of polymer light-emitting diodes

Conjugated polymers are often treated as semiconductors with low doping concentrations. Unlike the traditional semiconductors which have a high density of surface states (mainly due to the dangling bonds), the nature of the metal/polymer interface, including barrier height and charge injection efficiency, is quite sensitive to the work function of the contact metal. In this article, we present evidence to show that the pinning of the surface Fermi level effect commonly observed at the silicon/metal interface can also be observed at the metal/polymer interface. It is achieved by controlling the doping level at the metal/polymer [poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) or MEH-PPV] interface. ITO/MEH-PPV/Al devices doped with 2 Å of calcium on the cathode side of the interfacial layer have the same device performance as the ITO/MEH-PPV/Ca devices. The heavily n-doped region pins the surface energy level, hence the polymer interface at the cathode side is no longer sensitive to the work function of the overcoated metal. It is believed that either the midgap bipolaron energy states created by the dopants or the sharp band bending at the interface is responsible for facilitating the electron injection. On the other hand, a p-doped region at the anode side, obtained by using a thin layer of an acid at the interface, pins the surface energy level and makes the contact insensitive to the work function of the anode. Therefore, an efficient polymer light-emitting diode with the p-i-n structure has been demonstrated without the matching of the work function of the metal electrodes.

Fermi-level effect on steady-state and transient photoconductivity in microcrystalline silicon

We show that steady-state and transient photoconductivity in microcrystalline silicon deposited by hot-wire chemical-vapor deposition depend strongly on the position of the Fermi level. The steady-state mobility-lifetime product increases significantly by shifting the Fermi level from around midgap towards the conduction or valence band. This increase corresponds to a slower decay in the transient photocurrent after pulsed excitation compared to the case with the Fermi level at midgap. We thus attribute the enhancement of the mobility-lifetime product to the increase of the lifetime of the majority carriers due to a change in the thermal occupation of defect centers by the shift in the Fermi level. The mobility-lifetime product information, often used as an indicator for material quality, should be complemented by the value of the dark conductivity, which monitors the Fermi level, in order to allow comparison between different samples. From our transient photoresponse data we deduce a ratio of 10:1 for the mobility of electrons to the mobility of holes.

Effect of column III vacancy on arsenic precipitation in low-temperature grown III–V arsenides

Separately grown p-type, intrinsic, and n-type GaAs at low temperatures as well as a combined p-i-n structure have been used to study the formation of As precipitates upon annealing at 800 °C. For the separate structures, least precipitates have been noticed in the n-type material. In contrast, the highest density of precipitates appears in the n region for the p-i-n structure. In addition, an obvious band depleted of precipitates, exists in the intrinsic region near the n-i interface. A general vacancy model, including Fermi level effect and crystal bonding strength (thermodynamic factor), has been developed to explain the current results as well as to predict As precipitation in various low temperature grown III–V heterostructures.

Investigation of Be diffusion in InGaAs using Kick-out mechanism

Be diffusion during post-growth annealing has been investigated in InGaAs epitaxial layers. Kick-out mechanisms considering species charges, build-in electric field and Fermi-level effect have been studied. Several forms of Kick-out mechanism have been implemented in our simulation programs. Experimental concentration profiles obtained by SIMS have been compared systematically with the results of simulations. We have deduced that the Kick-out mechanism Bei0 ⇔ Be− + IIII+ is the dominating diffusion mechanism in InGaAs under our experimental conditions (C0 = 3 × 1019cm−3).

Fermi-Level Effect, Electric Field Effect, and Diffusion Mechanisms in GaAs Based III-V Compound Semiconductors

ABSTRACTDiffusion mechanisms and point defects in GaAs and related III-V compounds are discussed. An understanding of the As sublattice situation has been arrived at fairly recently and is presently tentative. Understanding of the Ga sublattice situation has become more acceptable in that experimental results are consistently explained by the Fermi-level effect and the As4 pressure effect. On the Ga sublattice, though controversies still exist, some are readily resolved by noting the role of the electric field produced by semiconductor electrical junctions, physical junctions, and surfaces.

Fermi-Level effect and junction carrier concentration effect on p-Type dopant distribution in IlI-V Compound superlattices

AbstractThe pronounced segregation phenomenon in the distribution of p-type dopants Zn and Be in GaAs and related III-V compound heterostructures has been explained quantitatively by treating simultaneously the processes of dopant atom diffusion, segregation, and the effect of heterojunction carrier concentrations on these two aspects. Segregation of a dopant species between two semiconductor heterostructure layers is described by a model incorporating (i) a chemical effect on the neutral species; and (ii) in addition, a Fermi-level effect on the ionized species. The process of Zn and Be diffusion in GaAs and related compounds is governed by the doubly-positively-charged group III element self-interstitials whose thermal equilibrium concentration and hence also the Zn and Be diffusivities exhibit also a Fermi-level dependence, i.e., in proportion to p2.A heterojunction is consisting of a space charge region with an electric field, in which the hole concentration is different from those in the bulk layers. This influences the junction region concentrations of and of Zn− or Be−, which in turn influence the distribution of the ionized acceptor atoms. The overall process involves diffusion and segregation of holes, , Zn− or Be−, and an ionized interstitial acceptor species. The junction electric field also changes with time and position.

Fermi-Level Effect and Junction Carrier Concentration Effect on Boron Distribution in GexSil−x/Si Heterostructures

AbstractDopant segregation mechanism in general involves the chemical effect, the Fermi-level effect, and the effect of the junction carrier concentrations. Satisfactory fits of available B distribution profiles in GexSil−x/Si heterostructures have been obtained using such a model, but with the chemical effect not important. The Fermi-level effect determines the difference in the ionized B solubilities in GexSil−x and Si. The singly-positively charged crystal self-interstitials I+ governs the boron diffusion process. The junction carrier concentration affects the concentration of I+ and solubility of B in the region and hence controls B diffusion across the heterojunction.

Point defects and diffusion in thin films of GaAs

Abstract Impurity diffusivity in thin films of GaAs is affected by native defect concentrations which were grown into the film, and which enter the film from both the substrate and the external ambient. Variations in measured diffusivity are related to native defects whose concentrations gradually relax to equilibrium values via different kinetic pathways. The time required for native defects to equilibrate depends upon the experimental design which in turn determines which region of the phase diagram that an annealing experiment begins and ends in. The Gibbs phase rule is applied to several commonly used experimental designs to show why the equilibrium native defect concentrations are generally not defined solely by temperature. The necessary conditions which define an equilibrium state, and the sufficient conditions which determine whether that equilibrium state can be approximated within a short time, are explicitly discussed. Experiments begun far from equilibrium are often associated with unusually high or low time-dependent diffusivities, but the results are often irreproducible as the native defect concentrations drift for extended periods of time. Experiments begun close to equilibrium are generally associated with reproducible diffusivities because the native defect concentrations reach constant values in a time which is short compared with the anneal period. Some diffusion results can be adequately described by equilibrium models, and the so-called ‘Fermi-level effect’ model is shown to apply as a special case of solid-vapor equilibrium. Near-equilibrium, results from several groups provide strong evidence that a Ga vacancy, with a charge of −1, controls group III element marker diffusion in n-type, intrinsic, and p-type GaAs at T > 800 °C. By relating an anneal ambient to a region of the ternary phase diagram, it becomes clear why the group II-related diffusion and other variables are extremely sensitive to the experimental design. Pinning of the Fermi energy at the surface during vapor phase epitaxy appears to explain why nonequilibrium concentrations of dopants and charged point defects can be grown into GaAs and affect diffusion in post-growth anneals. The relatively large random measurement noise often accompanying interdiffusion appears to be largely associated with the presence of Al in GaAs. We conclude that this behavior is associated with a residual contaminant, most likely oxygen. It also appears likely that small amounts of an oxidant have been present in many closed ampoule anneals and affected the reported interdiffusion results. We conclude that poorly understood metallurgical reactions are probably responsible for the enormous range of interdiffusion observed in device structures annealed under SiO 2 and Si 3 N 4 glass caps.

Suppression of superlattice intermixing by p-type doping

We report a systematic investigation of the effect of n- and p-type doping on the AlGaAs GaAs superlattice (SL) intermixing using photoluminescence. We find that the SL intermixing is not only enhanced by Si doping but also suppressed by Be doping. Although previous models have suggested the contribution of triply negatively charged Ga vacancies, V 3 − Ga , in n-type and intrinsic materials and of doubly positively charged Ga interstitials, I 2 + Ga , in p-type materials, we find that at 900°C the Al-Ga interdiffusion is mediated by singly negatively charged Ga vacancies, V − Ga , in both p- and n-type materials for n < 1 × 10 18 cm − 3 . These results are shown to be explained consistently in terms of the Fermi-level effect. Our results demonstrate that the thermal stability of AlGaAs GaAs heterostructures can be controlled, i.e., weakened or strengthened, by appropriate tuning of the Fermi level.

The Fermi level effect in III–V intermixing: The final nail in the coffin?

We have shown that doping InGaAs/GaAs quantum well materials with 1019 Si/cm3 causes a time and temperature dependent diffusion process, which can be correlated with group III vacancy formation. This process can be modeled and shown to accurately fit other data in the literature. Samples with silicon doping concentrations below this value have no enhanced interdiffusion, in contradiction to the results of the Fermi level model. These results are shown to be comparable to data for AlGaAs/GaAs interdiffusion with doping concentrations between 5×1017 cm−3 and 1018 cm−3. We have shown that the position of the Fermi level plays no role in III–V intermixing.

Effect of p and n doping on neutral impurity and dielectric cap induced quantum well intermixing in GaAs/AlGaAs structures

We report the diffusion and quantum well intermixing (QWI) effects of the neutral impurities fluorine and boron in p- and n-type AlGaAs in the GaAs/AlGaAs system. Boron was found to show significant diffusion in p-doped AlGaAs material but only exhibited a very small amount of diffusion in n-doped material after furnace annealing. Boron was found to retard intermixing in p-AlGaAs. This may be due to the existence of interstitial B, which may reduce the native group III interstitial concentration. The mechanism of boron impurity-induced disordering (IID) in n-AlGaAs was proposed to be attributed to both the diffusion of point defects generated during ion implantation and the Fermi level effect from the deep acceptor . Extremely fast fluorine diffusion rates, which may be correlated to an interstitial diffusion mechanism, have been observed. Results from fluorine IID suggest the possibility of the ionization of fluorine which gives rise to a higher electron concentration during annealing, leading to higher degrees of intermixing in n-AlGaAs. The intermixing rate of impurity free vacancy disordering (IFVD) using an cap was found to be higher in n-type and intrinsic AlGaAs than in p-type AlGaAs, which suggests that the diffusion of the group III vacancy is inhibited in p-type material which has a higher group III interstitial concentration. These results imply that n-i-p structures should be more effective than conventional p-i-n structures for the IFVD process.

Defects and Diffusion issues for the Manufacturing of Semiconductors in the 21st Century

ABSTRACTWithin the past decade, process simulation has become an essential part of new technology development in the silicon IC industry. The use of TCAD (technology computer aided design) tools has been driven by the enormous cost of purely experimental approaches to technology development. Yet the power of these tools and their predictive capability are still greatly limited by the models they use. TCAD models for doping processes are universally based today on point defects. These models have evolved considerably in the past decade to incorporate additional understanding. The state-of-the-art today includes concentration dependent diffusion through Fermi level effects on defect concentrations, full coupling between defects and dopants which allows prediction of non-local diffusion effects, basic models for the effects of ion implantation damage (the +1 model), surface and interface effects (through effective recombination velocities and segregation), and full 2D and 3D simulations.As devices continue to shrink, better models will certainly be required. Challenges for the future include more detailed information about damage resulting from ion implantation, better understanding of point defect properties (equilibrium populations, diffusivities, transient response to temperatures changes), better models for point defect behavior at interfaces, and finally, development of accurate methods to actually measure 2D and 3D dopant profiles. This paper will attempt to describe where we are and where we need to be in the future.

Diffusion of Gold into Heavily Boron-Doped Silicon

ABSTRACTDiffusion of Au into dislocation-free and highly dislocated Si with high B-background doping levels has been investigated with the aid of neutron activation analysis in conjunction with mechanical sectioning. The high B-doping level causes extrinsic conditions, i.e., the hole concentration exceeds the intrinsic carrier concentration even at diffusion temperatures between 900°C and 1100°C. All profiles are accurately described on the basis of the kick-out diffusion model and a mechanism which takes into account segregation of Au at dislocations. Our analysis provides solubility data of Au in Si and effective diffusion coefficients related to interstitial Au and Si self-interstitials I. The dependence of these quantities on the B-background doping level is well described by the Fermi-level effect. This analysis supports singly positively charged states in p-type Si of Au on interstitial (Aui) and substitutional (Aus) sites and of Si self-interstitials. Successful fitting of additionally requires an acceptor level of Aus. The electrical properties deduced for Aui, Aus and I are summarized in Table 2. Au profiles in highly dislocated Si obtained especially after diffusion at 900° C give evidence of Au trapped at dislocations. From our preliminary experimental results we determine an enthalpy difference of 2.7 eV between Au on substitutional sites and Au captured at dislocations.

The effect of the surface Fermi level pinning on the properties of δ-doped systems

Electronic potential profile, energy levels, and their respective occupations are self-consistently calculated for δ-doped semiconductor films, taking into account the charge depleted by the surface. This effect is considered by requiring the Fermi level at the surfaces to be at some fixed value relative to the band gap edges. The electron concentration in the potential well is calculated for different values of the film thickness and of the surface chemical potential. The results show that these calculations can be helpful in determining the Fermi level pinning at the surface by fitting Hall concentrations of δ-doped samples.

The Role of Vacancies and Interstitials in Transient Enhanced Diffusion of Arsenic Implanted into Silicon

ABSTRACTBoron and antimony doped superlattices (DSLs) were implanted with arsenic at 40 keV to doses of 2×1014 cm−2, 5×1015 cm−2 and 2×1016 cm−2. Increasing the arsenic dose above 5×1015 cm−2 resulted in a reduction in the extent of arsenic transient enhanced diffusion (TED) following annealing at 700°C, 16 hr. Concurrent with this reduction in TED was a reduction in the number of free interstitials beyond the end-of-range, as measured by the boron diffusion enhancement in the doped superlattices. No enhancement in antimony diffusivity was observed in this region, indicating that vacancies play no direct role in the diffusion of arsenic in this region, although an indirect role for vacancies as recombination centers for mobile interstitials is not precluded by these experiments. We conclude that interstitials dominate arsenic diffusion in the end-of-range region and beyond. Interpretation of the DSL data in the projected range region is complicated by Fermi level and segregation effects and no definitive conclusion can be reached about the point defect populations in this region.

AB Initio Pseudopotential Calculations of Dopant Diffusion in SI

ABSTRACTThe ab initio pseudopotential method is used to study transient-enhanced-diffusion (TED) related processes. The electronic degrees of freedom are included explicitly, together with the fully self-consistent treatment of the electron charge density. A large supercell and a fine k-point mesh are used to ensure numerical convergence. Such method has been demonstrated to give quantitative description of defect energetics. We will show that boron diffusion is significantly enhanced in the presence of the Si interstitial due to the substantial lowering of the migrational barrier through a kick-out mechanism. The resulting mobile boron can also be trapped by another substitutional boron, forming an immobile and electrically inactive two-boron pair. Similarly, carbon diffusion is also enhanced significantly due to the pairing with Si interstitiels. However, carbon binds to Si interstitials much more strongly than boron does, taking away most Si interstitials from boron at sufficiently large carbon concentration, which causes the suppression of the boron TED. We will also show that Fermi level effect plays an important role in both Si intersititial and boron diffusion.

Simulation of Under- and Supersaturation of Gallium Vacancies in Gallium Arsenide During Silicon in- and Outdiffusion

ABSTRACTThe diffusivity of Si in GaAs shows a dependence on the cubic power of its concentration or the concentration of electrons n under both in- and outdiffusion conditions. Hence, the diffusion of Si in GaAs is consistent with the Fermi-level effect model invoking the triply-negatively-charged Ga vacancies, , as the point defect species responsible for diffusion to occur on the Ga sublattice under n-doping condition. However, the Si diffusivity values of the indiffusion cases is several orders of magnitude smaller than those of the outdiffusion cases at the same Si concentrations. This means that the two apparent Si diffusivity values under intrinsic conditions will contain also the same discrepancy, which has been previously assessed to be due to a undersaturation in indiffusion cases and a supersaturation in outdiffusion cases. In this study we have calculated the under- and supersaturation values using the known Si diffusivities. We found that the GaAs surface states play a key role in the development of the under- and supersaturations.