Silicon Epitaxial Layers with Non-Uniform Doping Profiles (I). The Model Parameters' Adjustment

The non-uniform doping profiles may prove useful for some semiconductor devices instead of the uniform ones. In the paper the doping mechanism of silicon epitaxial layers in the SiCl₄ H₂ PH₃ system is investigated. The epitaxial doping profile simulation program is developed based on the Wong-Reif trapping model and taking into account also the difference between dopant profile and the charge carrier profile. The three unknown parameters of the Wong-Reif model are found with the aid of the simulation program.     

On the deposition mechanism of boron in silicon CVD epitaxy

The deposition mechanism of boron doping in CVD silicon epitaxy has been investigated by exposing silicon substrates to B₂H₆ H₂ doping gas mixtures at epitaxy temperatures and examining the effect by dopant profile measuring in an afterwards intrinsically in-situ deposited epitaxial silicon layer. It has been shown that boron is deposited increasing its concentration on the surface linearly with prolonged exposition time and desorbed by purging the surface in pure hydrogen. In the latter case its content decreases linearly proportional to the predeposited concentration. The desorbed boron builds up a secondary doping source which maintains a parasitic boron flow for reincorporation during following layer growth.     

Crystal growth of boron monophosphide using a B2H6-PH3-H2 system

Boron monosphide (BP) with (100) orientation can be epitaxially grown on Si substrates with (100) orientation by thermal decomposition of B₂H₆ and PH₃ in hydrogen. In a horizontal CVD system, the growth rate was studied as a function of gas phase composition and temperature. The growth rate is independent of the PH₃ partial pressure in the region where the PH₃ is in excess. For low values of the B₂H₆ partial pressure the growth rate is proportional to the B₂H₆ partial pressure (a linear region) with an activation energy of 1.8 eV, and for high values of the B₂H₆ partial pressure the growth rate becomes constant (a saturation region) with an acivation energy of 3.0 eV. A simple adsorption-reaction model can be proposed to explain the experimental growth kinetics. The conductivity of the as-grown layer is determined by the PH₃ partial pressure. n-type BP can be obtained for high values of the PH₃ partial pressure and p-type BP for low values. Si doping during the growth and phosphorus anti-site donors are two possibilities to explain the results.     

Doping behavior of silicon in vapor-grown III–V epitaxial films

An expanded silicon-doping model, applicable for unintentionally doped vapor-grown III–V epitaxial materials, is presented based on additional experimental results of doping-level influences of AsH₃, PH₃, and HCl. This model retains the role of HCl as the silicon generation agent, as suggested in a recently reported silicon-doping model for GaAs. It also includes a surface kinetic factor for the silicon-incorporation efficiency in III–V materials as influenced by the relative vapor-phase concentration of As₂, As₄ and P₂, P₄ species. The present studies indicate a different degree of influence of the HCl and AsH₃ partial pressures on the suppression of the GaAs background doping level. The additive effect of both partial pressures is very similar to that found in the chloride system. Qualitatively, the same sensitivity of GaP doping levels to variations in HCl and PH₃ has been observed; however, the magnitudes of the doping-level changes are different. In all instances, the impurity levels in the epitaxial films are reduced by increases in the partial pressures of the various reactants, with the AsH₃ and PH₃ having a greater influence than the HCl.     

Silicon Epitaxial layers with non-uniform doping profiles (II). The non-uniform doping profile realization

In Part I of the paper the epitaxial doping profile simulation program was described based on the Wong-Reif trapping model for the SiCl₄ H₂ PH₃ system. This model is used in Part II to design the non-uniform profile epilayer processing procedure accounting also for the parasitic effect of autodoping. Several epilayers with exponential doping profiles are realized. The resulting profiles exhibit very close agreement with the target profiles.     

Doping process control in silicon epitaxy (II). Calculation of optimum control

A procedure is developed to obtain desired dopant profile in epitaxial layer growth. Based an the results of system identification, linear-quadratic optimal control theory is used to determine the optimal input PH₃ concentration as a function of time. In the performance index an auxiliary weighting coefficient must be incorporated. It is discussed how to select this weighting coefficient.     

Doping process control in silicon epitaxy (I). System identification

The transition behaviour of the phosphorus incorporation in silicon epitaxial layers grown in a CVD reactor has been investigated, considering the reactor as a linear control system with u = lg p⁰_PH3 (t) as the input and y = lg N(t) as the output. The response of system to both upward and downward step inputs has been studied experimentally, using SiH₄ and PH₃ sources. The dopant system of a horizontal silicon epitaxial reactor has been identified and a mathematical model relating to the transient behaviour of the system has been found. The parameters of the model have been estimated from layer growth experiments. The step response functions found can be approximated by an exponential function relating to n time constants T, all equal to each other. It was found for the system investigated that the second order model is of sufficient accuracy for the optimal control calculations described in the next part of this series.     

New gaseous impurity and its effect on the purity of LPE GaAs

The present work describes a new gaseous impurity source of GaAs grown by the conventional liquid phase epitaxy. A large quantity of impurity gas of mass number 16 was detected in the LPE atmosphere even when H₂O vapour and O₂ residual gas were exhausted almost completely. The impurity gas was only detected in the presence of H₂ and a graphite boat at high temperature, and above 850°C this was exaggerated. From comparison of the mass-spectrum of standard gas, the impurity gas was identified as methane. An amount of 130 ppm of methane in H₂ resulted in a carrier concentration of LPE GaAs higher than 10¹⁶ cm^-3.     

Influence of Predeposition on the Properties of GaN

Growth experiments with GaN in the system Ga/HCl/NH₃/He or H₂ were carried out in a reactor with two deposition zones. The extent of the reaction in the first zone, called the predeposition zone, is controlled by the NH₃ flow rate. In the second zone GaN is deposited epitaxially onto sapphire substrates. The variation of the carrier concentration of these epilayers indicates the dominating effect of impurities especially oxygen in opposition to the widely accepted vacancy model. Due to the high incorporation ratio of donor impurities into the solid a predeposition reduces the impurity content in the vapour phase. A reduction of the free carrier concentration in the epitaxial layers could be achieved. Different behaviour in the two carrier gas systems could be established.     

Untersuchungen an Si-Epitaxieschichten auf Mg-Al-Spinell-Einkristallen

In Verneuil-grown MgAl-Spinels etch pit densities with non-uniform distribution between 10⁴ and 10⁷ cm⁻² were found. X-ray measuements of orientations showed deviations of the growth direction from the rod axis up to 8 deg. Berg-Barrett photographs point to distinctly marked substructures. Si epitaxial layers on MgAl-spinel were prepared according to the pyrolytic reductive reaction of SiCl₄ in H₂. Nucleation, etching reaction on the substrate and the quality of the Si layers are discussed. Measurements of the carrier mobility in the doping range from 10¹⁶ to 10¹⁷ · cm⁻³ resulted in 60 to 80 p.c. of the volume mobility of the silicon. Problems of segregation with annealing are dealt with. The use of Si layers on insolating substrates is discussed.     

MOVPE Growth and Characterisation of Zn‐Doped (GaIn)P Layers with Respect to Surface Structure and Ordering

(GaIn)P grown on (001)GaAs substrates by metal‐organic vapour phase epitaxy (MOVPE) is highly ordered material. In this work the effect of Zn doping on the epitaxial crystal growth, the ordering behaviour and the surface morphology is investigated. Zn‐doped (GaIn)P layers, grown with phosphine (PH₃), tertiarybutylphosphine (TBP) and ditertiarybutylphosphine (DitBuPH) as phosphorous precursors, reveal a strong drop of the binary growth rate of InP r_InP with increasing Zn/III ratio, whereas r_GaP remains nearly constant. The Zn doping efficiency and the ordering behaviour are observed to be dependent on the misorientation of the substrates. Finally, the surface morphology as a function of the different parameters was analysed by atomic force microscopy (AFM), and no considerable change of the growth mechanism was found for Zn‐doped layers in comparison to undoped layers.     

Growth and annealing effect of high-quality ZnSe:N/ZnSe by MOCVD

The ZnSe : N epitaxial layers were grown on (1 1 0) ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas, and using ammonia as a dopant source. In order to obtain highly doped ZnSe : N epitaxial layers, the optimum growth and doping conditions were determined by studying the photoluminescence (PL) spectra from the ZnSe epitaxial layers grown at different ammonia flux and VI/II flux ratio. Furthermore, in order to enhance the concentration of active nitrogen in ZnSe epitaxial layer, a rapid thermal anneal technique was used for post-heat-treating. The results show that the annealing temperature of over 1023 K is necessary. Beside, a novel treatment method to obtain a smooth substrate surface for growing high quality ZnSe epitaxial layers is also described.     

On the kinetics of nitrogen incorporation in GaP LPE layers using NH3 vapour doping

The partial substitution of N for P in GaP has a considerable effect on the luminescence quantum efficiency of GaP epitactic layers. The incorporation of N in GaP liquid phase epitactic layers using NH₃ vapour doping has been studied at different H₂ partial pressures in argon gas. It is shown that the incorporation of N in different ambients is conveniently described by just one parameter. The reaction of NH₃ with Ga is found to be an equilibrium process. The N concentration in the layer is directly proportional to the GaN concentration in the melt. From the correlation of the dissociation of NH₃ with the formation of GaN it is concluded that the NH₃ pressure, as measured in the exit gas, is not representative of the NH₃ pressure at the melt. The dissociation of NH₃ is found to be strongly retarded by H₂ owing to adsorption of H₂ on the surface of reactor materials which catalyse the dissociation.     

Preparation of InN epitaxial layers in InCl3?NH3 system

The epitaxial growth of InN by the open tube flow method using the interaction of InCl₃ and NH₃ is discussed. The influence of various technological parameters on the process and structure perfection of the epitaxial layers, grown on the single crystal (0001)-oriented sapphire substrates is considered. The main kinetic dependences of the process are plotted and discussed. InN epitaxial layers were mosaic and n-type with electron concentration 2 · 10²⁰–8 · 10²¹ cm⁻³ and mobilities 50-35 cm²/V · s, respectively.     

Crystal and molecular structures of potassium cobalt hydrogen phthalate K2[Co(H2O)6](C8H5O4)4 ⋅ 4H2O and mechanism of Co2+ impurity trapping in KAP crystals

The single-crystal X-ray diffraction analysis of K₂[Co(H₂O)₆](C₈H₅O₄)₄ ? 4H₂O has been carried out. The K₂[Co(H₂O)₆](C₈H₅O₄)₄ ? 4H₂O single crystals are obtained in attempting to grow the KAP crystals with the maximum possible content of Co²⁺ impurity cations. The crystals are isostructural to the earlier-studied similar crystals with Ni(II). The structure is formed by double layers of biphthalate anions and the Co²⁺ and K⁺ cations in between. The Co²⁺ cations are coordinated only by water molecules, whereas the coordination of the K⁺ cations involves both the biphthalate anions and water molecules. A detailed crystal chemical analysis, together with the data on the growth kinetics of KAP crystals in the presence of Co²⁺ and the mass-spectrometric data obtained earlier for the KAP crystals, leads to the conclusion that the Co²⁺ impurity cations should be located in the form of the [Co(H₂O)₆]²⁺ cationic complexes in the interblock layers of the KAP crystals.     

Effect of illumination on hydrogenated amorphous silicon thin films doped with chalcogens

Hydrogenated amorphous silicon thin films doped with chalcogens (Se or S) were prepared by the decomposition of silane (SiH₄) and H₂Se/H₂S gas mixtures in an RF plasma glow discharge on 7059 corning glass at a substrate temperature 230 °C. The illumination measurements were performed on these samples as a function of doping concentration, temperature and optical density. The activation energy varied with doping concentration and is higher in Se-doped than S-doped a-Si:H thin films due to a low defect density. From intensity versus photoconductivity data, it is observed that the addition of Se and S changes the recombination mechanism from monomolecular at low doping concentration films to bimolecular at higher doping levels. The photosensitivity (σ_ph/σ_d) of a-Si, Se:H thin films decreases as the gas ratio H₂Se/SiH₄ increased from 10^?4 to 10^?1, while the photosensitivity of a-Si, S:H thin films increases as the gas ratio H₂S/SiH₄ increased from 6.8 × 10^?7 to 1.0×10^?4.     

Thermo-optic properties of B2O3 doped Li2O-Al2O3-SiO2 glass-ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li₂O-Al₂O₃-SiO₂ glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The dS/dT value of 2.6 mol% B₂O₃-doped glass-ceramic was 12.5  × 10⁻⁶/°C, which was 0.9 ×  10⁻⁶/°C smaller than that of B₂O₃-free glass-ceramic. On the other hand, reduction in dS/dT through B₂O₃ doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass-ceramic through B₂O₃ doping is caused by the reduction in ?. The reduction in ? from B₂O₃ doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B₂O₃-doped glass-ceramic crystallized under 196 MPa was 11.7 ×  10⁻⁶/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass.     

Doping dependence of crystallization growth rate in a-Si CVD films

Crystallization growth rates (V_g) on boron and phosphorus doped a-Si C VD films are obtained using conductivity measurements during isothermal annealings at temperatures 510<T_A<650°C. For boron doping, the associated activation energy of V_g is equal to 2.9 eV in the whole doping range (up to 2 × 10^?3 B₂H₆), whereas V_g increases by a factor 4 in the range 0 ? 7 × 10^?6 and remains almost constant for higher doping. In this low range, the neutral dangling bonds became positively charged and non paramagnetic by electronic compensation with the acceptors, and the E.S.R. signal decreases from 10¹⁹ to 10¹⁷ cm^?3. These results indicate clearly that dangling bonds and their charge state play an important role in the growth rate process.     

Dependence of doping element incorporation on temperature in the chemical vapour deposition of epitaxial silicon (III). Empirical incorporation characteristic of the Si-P-H system

The shape of the empirical incorporation characteristic of phosphorus in epitaxial silicon, deposited from silane-phosphine-hydrogen mixtures, shows two branches with different incorporation dependences on temperature (incorporation enthalpies). In the lower phosphorus-concentration range (N < 10¹⁸) the experimentally determined value of incorporation enthalpy can be explained as a complex quantity, including the enthalpies of the two phosphorus hydrides PH₃ and PH₂, the latter of which is formed by the partial decomposition of phosphine (PH₃) at deposition temperatures. — In the upper phosphorus-concentration range (N > 10¹⁸) the incorporation equilibrium of the dimeric phosphorus molecules, formed by the nearly complete decomposition of phosphine, is reflected in the incorporation enthalpy of the empirical incorporation characteristic.     

Band tail conduction in Zn-doped GaAs

The electrical parameters of Zn-doped GaAs liquid phase epitaxial layers are determined in the temperature range from 77 to 300 K by Hall effect and resistivity measurements. An analysis of the experimental data yields an ionization energy of 30.4 meV for the Zn acceptor in GaAs in the dilute limit of acceptor concentration. For the Mott transition a critical hole concentration of about 6 · 10¹⁷ cm⁻³ is estimated. It is found that the temperature dependence of the electrical parameters is essentially influenced by band tail conduction effects at doping levels above the Mott transition.