Maximum Electron Concentration Research Articles

Modeling of Chlorine Related Defects and Complexes in ZnMgSe

ABSTRACTWe have used the ab-initio full potential LMTO method to model native defects and chlorine-impurity-related defects in ZnSe and ZnxMg1−xSe. Our results show that there is a strong tendency for formation of a defect complex between a chlorine impurity at the Se site and a vacancy at the neighboring Zn site. The formation energies of this complex and other chlorine related defects decrease in the presence of magnesium. However, the maximum achievable electron concentration in the presence of magnesium is lower because of the increase in the band gap.

Nitrogen-induced enhancement of the free electron concentration in sulfur implanted GaNxAs1−x

We have achieved a large increase of the activation efficiency of sulfur atoms implanted into GaNxAs1−x thin films. For thin films with only 0.8% N content, we find a maximum free electron concentration of >6×1018 cm−3 for implanted S concentration higher than 1019 cm−3, about 20 times larger than that in semi-insulating GaAs implanted and annealed under the same conditions. This large increase of the free electron concentration can be quantitatively explained with the recently developed band anticrossing model.

Nitrogen-induced increase of the maximum electron concentration in group III-N-V alloys

The maximum free-electron concentration is observed to increase dramatically with the nitrogen content $x$ in heavily Se-doped ${\mathrm{Ga}}_{1\ensuremath{-}3x}{\mathrm{In}}_{3x}{\mathrm{N}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ $(0l~xl0.033)$ films. For example, an electron concentration of $7\ifmmode\times\else\texttimes\fi{}{10}^{19} {\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ was observed at $x=0.033;$ a value more than 20 times larger than that observed in GaAs films grown under similar conditions. It is shown that the increase is caused by a combination of two effects: (1) a downward shift of the conduction band and (2) an increase of the electron effective mass caused by flattening of the conduction-band minimum. Both these effects are due to modifications to the conduction-band structure caused by an anticrossing interaction of a localized $N$ state and the conduction band of the III-V host.

Heavy Si doping in AlGaAs near modulation-doped heterointerfaces

We propose that the local Fermi-level modulation that occurs at heterojunctions enables the achievement of a higher carrier concentration in a limited thin layer than that can be achieved in a thick layer. We studied heterojunctions consisting of Si-doped AlGaAs (Al=20%) (AlGaAs:Si)/InGaAs (In=20%) grown by metalorganic chemical vapor deposition to demonstrate effectiveness of our doping method. The thickness and doping concentration of the AlGaAs:Si were designed carefully not to form a neutral region in the structure. We obtained a maximum electron concentration Nd=2.4×1019 cm−3 at 300 K in AlGaAs:Si; this value is about seven times higher than that reported for AlGaAs:Si so far. We believe that the AlGaAs/InGaAs heterojunction positions the Fermi level in the AlGaAs:Si (near the heterojunction) far from the conduction-band minimum, and that this effectively suppresses creation of compensation centers in AlGaAs:Si, then all the Si donors in AlGaAs:Si are activated.

Chlorine doping of cubic CdS and ZnS grown by compound source molecular-beam epitaxy

We report on the chlorine doping of ZnS and CdS grown by compound source molecular-beam epitaxy. The maximum free electron concentrations achieved were 1×10 18 and 8×10 19 cm −3, respectively. The conductivity of the cubic CdS epilayers turned out to be anisotropic with respect to the 〈1 1 0〉 crystallographic directions due to an anisotropic lattice defect structure.

Thermoelectric transport properties of n-doped and p-doped Bi0.91Sb0.09 alloy thin films

In order to understand the doping behavior of extremely narrow band gap materials and to optimize their characteristics for use in a thermoelectric module, we performed n- and p-type doping experiments on semiconducting Bi0.91Sb0.09 alloy thin films using the group VI(IV) element Te(Sn) as donor (acceptor). Thermoelectric power (TEP), electrical resistivity, and Hall effect were studied in the range of temperatures 5–300 K. Increased Sn doping causes the TEP to change sign (from negative to positive) and the maximum in the TEP can be controlled with the dopant concentration. Increased Te doping causes the TEP to decrease. The maximum Te-doped electron concentration was about 5×1020 cm−3 and the highest Sn-doped hole concentration was about 1×1021 cm−3. Highly Sn- and Te-doped samples show degenerate behavior in the electrical resistivity, TEP and Hall measurements.

MOVPE growth of InPSb/InAs heterostructures for mid-infrared emitters

We investigated the growth of InPSb on GaSb or InAs by low pressure (20 mbar) metalorganic vapor phase epitaxy (MOVPE). Trimethylindium, triethylantimony, and phosphine were used as starting materials. High resolution x-ray diffraction, photoluminescence at 10K, Hall measurements at 300 and 77K as well as scanning electron microscopy and scanning tunneling electron microscopy investigations were carried out to verify the layer properties. Lattice-matched InPSb layers on InAs substrate grown at 520°C show mirror-like surfaces and sharp x-ray peaks. N-type doping of InP0.69Sb0.31 was carried out with H2S and p-type doping was achieved with DEZn. Maximum electron concentrations of 2×1019 cm−3 and hole concentrations exceeding 1018 cm−3 were obtained after annealing in N2 ambient. The thermal stability of InPSb was studied during annealing experiments carried out at 500°C up to 30 min. The compositional integrity of the lattice proves to be stable and the InAs/InPSb interface can be improved. Multiple quantum well structures, pn-junction diodes and the two-dimensional electron gas at the InPSb/InAs/InPSb quantum wells were investigated to demonstrate the properties of the material.

Ion implantation in 6HSiC

In this study we have performed N, P, Al, B, V, Si, and C implantations to obtain p-type, n-type, and semi-insulating regions in 6HSiC crystals. Post-implantation annealings were performed in either a conventional ceramic processing furnace or using microwaves in the temperature range of 1200–1700°C. The material was characterized by secondary ion mass spectrometry, Hall, Rutherford backscattering via channeling, and two-probe I–V measurements. N-type regions with a maximum room temperature electron concentration of 2 × 10 19 cm −3 were obtained using N and P implantations. Though p-type regions were obtained by Al and B, the room temperature hole concentrations were low due to the deep acceptor levels associated with these impurities. The Si and C bombardment gave regions with resistivities as high as 10 9 Ω cm compared to 1 Ω cm in the starting material.

Si and Si/P implants in In0.5Ga0.5P and In0.5Al0.5P

Si and Si/P ion implantation doping of In0.5Ga0.5P and In0.5Al0.5P has been studied for several Si and P doses at energies of 90 and 100 keV, respectively. For single Si implants in InGaP a maximum Hall sheet electron concentration of 1.33×1013 cm−2 is achieved for a Si dose of 5×1013 cm−2. When an optimum dose (2.5×1013 cm−2) P coimplant is performed this electron concentration is increased by 65%. The same dose Si implants in InAlP show a maximum effective activation of 3.9% with no P coimplantation and 5.2% with a P-implant dose 1.5 times the silicon dose. The apparent donor ionization energies are estimated from variable temperature Hall measurements to be 2–5 meV for InGaP and ∼80 meV for InAlP. The deeper level in InAlP is attributed to the DX level found in the Al-containing material. The reduction in sheet resistance associated with the use of Si/P coimplantation in InGaP containing heterostructures should contribute to significant performance enhancements in HFETs and lasers.

Elevated temperature nitrogen implants in 6H-SiC

Elevated temperature (700°C) N ion implantations were performed into 6H-SiC in the energy range of 50 keV-4 MeV. By analyzing the as-implanted depth distributions, the range statistics of the N+ in 6H-SiC have been established over this energy range. Annealing at 1500 and 1600°C for 15 min resulted in Rutherford backscattering spectrometry scattering yields at the virgin crystal level, indicating a good recovery of the crystalline quality of the material without any redistribution of the dopant. A maximum electron concentration of 2 × 1019 cm−3, at room temperature, has been measured even for high-dose implants. The p-n junction diodes made by N ion implantation into a p-type substrate have a forward turn-on voltage of 2.2 V, an ideality factor of 1.90, and a reverse breakdown voltage of 125 V with nA range leakage current for -10 V bias at room temperature. By probing many devices on the same substrate we found uniform forward and reverse characteristics across the crystal.

Doping limitations in wide gap II–VI compounds by Fermi level pinning

The maximum electron and hole concentrations in wide gap II–VI materials are quantitatively described by a model that assumes a Fermi level pinning at defect levels whose position is constant in absolute energy, namely 120 meV above the ZnSe conduction band edge for ndashdoping, and 130 meV above the ZnSe valence band edge for p-doping. The main conclusions from the model are that device-relevant p-doping levels in ZnSe 1 − x S x will be restricted to S contents below 15%, and that the emission energy of II–VI diode lasers with p- and ndashdoping levels above 10 17 cm −3 will probably not exceed 3.3 eV.

The sunspot cycle and “auroral” F layer irregularities

The use of the word “aurora” for many different observations at high latitudes has limited the concepts involved; this is particularly true for F region irregularities. Observations setting the position of the auroral oval (Starkov and Fel'dshtein, 1970) were made using primarily the 555.7‐nm green line, which is emitted predominantly at E layer heights. These observations have shown that the change in position of the auroral oval for low values of Kp as a function of sunspot cycle is of the order of 1° to 2° between sunspot maximum and sunspot minimum. However, irregularities in the F region show much larger solar cycle variations in the locations of the equatorward boundary, typically 10°. A review of scintillation data indicates that at a given auroral latitude, the scintillation activity increases with sunspot number. In addition, for a constant scintillation intensity, the equatorward boundary moves to lower latitudes as sunspot maximum is approached. We review existing spread F studies and show that for quiet geomagnetic conditions, there is lower occurrence during years of low sunspot numbers than during years of high sunspot numbers. However, the spread F index, related to Δ ƒ/ƒ0F2, is higher during years of low sunspot number than during years of high sunspot number. We demonstrate that this apparent dichotomy can be reconciled by using a new method of normalizing the spread F index by the maximum electron concentration of the F layer. We briefly discuss the possible explanations for the observed solar cycle variations of irregularity occurrence in terms of the absolute values and gradients of electron concentration and the E region conductivity.

Electrical and optical properties of selectively doped Al0.25Ga0.75As/InyGa1−yAs (0.25≤y≤0.45) pseudomorphic heterostructures grown by molecular-beam epitaxy

Electrical and optical properties of highly strained selectively doped Al0.25Ga0.75As/InyGa1−yAs (0.25≤y≤0.45) pseudomorphic heterostructures grown by molecular-beam epitaxy are investigated. At a low growth temperature of 400 °C, the In mole fraction exceeds than 0.30 without degrading crystalline quality. The maximum mobility and sheet electron concentration at room temperature reach 6560 cm2/(V s) (y=0.34) and 2.94×1012 cm−2 (y=0.425), respectively. Photoluminescence measurements confirm the In mole fraction. This work suggests that InyGa1−yAs channels (y≳0.3) grown at 400 °C improve the performance of high-electron-mobility transistors.

Diffusion and doping of Si into GaAs from undoped SiOx/SiN film

The diffusion and n-type doping of Si into GaAs from a novel diffusion source consisting of an undoped SiOx/SiN double-layered film were achieved by rapid thermal annealing at 860–940 °C. The film properties of the double-layered films employed as Si diffusion sources are experimentally presented. The characteristics of the Si diffused layers were investigated by secondary ion mass spectrometry, capacitance-voltage measurement, and the Hall method. The carrier profiles exceeded 2×1018 cm−3 and featured an abrupt diffusion front, while a maximum electron concentration of 6×1018 cm−3 was obtained at 940 °C. The diffused Si profiles were consistent with the SiGa+−VGa− pair diffusion model.

Characterization of vapor phase epitaxial ZnSe films

Properties of ZnSe films doped with donor impurities were investigated. The ZnSe films were grown at 350°C by using metallic zinc and selenium as the source materials; their vapors were transported separately by H2 gas under atmospheric pressure. Iodine-doped ZnSe films were grown using CH3I (1000 ppm, diluted in helium) as a dopant source. However, it was necessary to stop this dopant flow during the film growth to obtain epitaxial films. HC1 gas etching and evacuation of the reaction apparatus before the film growth began were employed to obtain epitaxial films and to avoid redistribution of impurities without heat-treatment at higher temperature. Secondary ion mass spectroscopy analysis indicated that both chlorine and gallium were included in the layers, as well as iodine, because of residual HC1 gas. Optically high-quality and rather highly conductive n-type ZnSe films were obtained. Maximum electron concentration was 3.3 × 1017 cm−3.

Erbium doping of molecular-beam epitaxially grown InSb on InP

The transport properties of molecular-beam epitaxially grown InSb films doped with Er (atom densities NEr from 4.3×1016 cm−3 to 3.6×1020 cm−3) are reported at temperatures between 40 and 400 K. Er is a donor in InSb films grown under Sb to In flux ratios smaller than 1.1. At a flux ratio Sb/In=1.06, 25 atoms of Er give one extrinsic electron. The maximum extrinsic electron concentration achievable is ∼1.8×1017 cm−3, obtained for NEr≳4×1018 cm−3. Magnetoresistance measurements at 4.2 K show evidence for spin–disorder scattering of the electrons. The low-temperature (T<150 K) electron mobility increases with doping concentration up to rare-earth densities of 1×1018 cm−3; at higher rare-earth concentrations, the mobility decreases again. At room temperature, the mobility decreases monotonically with increasing Er concentration. We note the analogy between these results and the observations made on HgSe:Fe, a system in which the carrier mobilities might be enhanced by impurity charge ordering.

Heavily Si-doped GaAs grown by low-pressure metalorganic chemical vapor deposition using tertiarybutylarsine and silane

Heavy Si doping was studied for low-pressure metalorganic chemical vapor deposition of GaAs by using tertiarybutylarsine (tBAs) as a group-V source and silane (SiH4) as a doping source gas. The Si doping efficiency was higher by one order of magnitude when tBAs was used instead of arsine (AsH3). The maximum electron concentration was 9.0×1018 cm−3, which is slightly higher than that obtained for AsH3 (5.9×1018 cm−3). The slight increase of the maximum concentration is considered to be due to a reduction of the carrier compensating center generated in high SiH4 partial pressure conditions. Generation of the electrical compensating center is assigned to be related with the carbon incorporation from alkylsilanes during growth.

Rapid isothermal processing of Si/sup +//P/sup +/ and Mg/sup +//P/sup +/ co-implantations into InP

The authors present the results of investigations on electrical properties of Si/sup +//P/sup +/ and Mg/sup +//P/sup +/ co-implanted InP wafers subjected to rapid isothermal processing. It was found that a significant improvement of activation of Si and Mg in InP can be obtained by P/sup +/ co-implantations and rapid isothermal processing. The annealing cycle was at temperatures from 825 to 875 degrees C for 5 and 14 s. The maximum dopant activation and peak electron concentration for Si/sup +//P/sup +/ coimplants at doses of 1*10/sup 15//cm/sup 2/ were 70% and 5*10/sup 19/ cm/sup 3/. A high hole concentration of 1.1*10/sup 19//cm/sup 3/ was obtained by Mg/sup +//P/sup +/ coimplantations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Gas source molecular-beam epitaxy of Si and SiGe using Si2H6 and GeH4

Gas source molecular-beam epitaxy (MBE) has been studied as a low-temperature Si and SiGe epitaxial method. Specular single-crystalline silicon films were successfully grown by the gas source Si MBE technique at a substrate temperature of about 650 °C. N-type doping was carried out using PH3 gas as a dopant and the maximum electron concentration of 2.9x1019cm−3 was obtained. Furthermore, p-type doping using B2H6 gas was carried out for the first time and the maximum hole concentration of 9.5×1017cm−3 was successfully achieved. The SiGe alloy layer was also grown by the gas source MBE technique using GeH4 as a Ge source gas. The Ge concentration of the sample could be precisely controlled by the molar fraction of Ge in supply and it was increased up to 0.30 with increasing the gas ratio of GeH4 to Si2H6.

Heavily P-doped (≥1021 cm-3) Si and SiGe films grown by photo-CVD at 250° C

A novel photo-CVD process has been successfully applied to a low-temperature heavy-doping technique using SiH4 + SiH2F2 + H2 or SiH4 + SiH2F2 + GeH4 + H2 gas mixture at a very low-temperature of 250° C with PH3 as a doping gas. A maximum electron concentration of 2.3 × 1021 cm-3 and 1.3 × 1021 cm-3 was achieved for Si and Si0.9Ge0.1 epitaxial layer on a single-crystal silicon substrate. The resistivity of the sample monotonously decreased with decreasing the sample temperature, which implies that the carriers were not frozen out in low temperature regions (4K) and that the obtained samples were “metallic” semiconductors.