Growth of high-quality 1.93-eV AIGaAs using metalorganic chemical vapor deposition

High-quality Al_xGa_1-xAs with a bandgap of 1.93 eV has been grown using metalorganic chemical vapor deposition (MOCVD). Levels exceeding 10₁₈ cm_-3 can be obtained for Se, Si and Zn dopants. In particular, incorporation of the re-type dopants at this bandgap is not appreciably less efficient than in lower-bandgap Al_xGa_1-xAs. The best material, as measured by photoluminescence intensity, is obtained using high V/III ratios (40 forp-type material and as high as 60 forn- type and low growth rates (300 Å/min). Purification of the arsine in situ with an Al/Ga/In eutectic bubbler to remove trace water and oxygen impurities is found to be essential for the growth of high-quality material, as indicated by photoluminescence intensity measurements. Solar cells fabricated from such material exhibit efficiencies as high as 12.1% under one-sun, airmass zero (AMO) conditions, with an open-circuit voltage of 1.38 V, short-circuit current density of 14.1 mA/cm², and fill factor of 0.84.     

Theoretical calculations of Debye length,built-in potential and depletion layer width versus dopant density in a heavily doped p-n junction diode

Theoretical calculations of Debye length, built-in potential, depletion layer width and capacitance as a function of dopant density in a heavily doped p-n junction diode are described in this paper. The heavy doping effects such as carrier degeneracy, dopant density-dependent dielectric constant and bandgap narrowing are accounted for by using the empirical approximation for the reduced Fermi-energy given by[1] and the dopant density dependent dielectric constant given by[2], as well as the bandgap narrowing model proposed by[3]. The results show that: (1) bandgap narrowing and carrier degeneracy have important effects on the junction built-in potential; (2) carrier degeneracy and dopant density-dependent dielectric constant are important to Debye length for the abrupt junction case, and (3) the dopant density-dependent dielectric constant is a key parameter which strongly affects the values of depletion layer width and depletion capacitance. These findings are important for modeling of heavily doped p-n junction devices in the VLSI applications.     

双极器件中硅基区和锗硅基区的禁带变窄量

采用一种新的方法计算双极器件中离子注B硅基区和原位掺B的锗硅基区禁带变窄量．在器件基区的少子迁移率、多子迁移率和方块电阻已知的情况下,应用这种方法只需测量室温和液氮温度下的电学特性就可以获得禁带变窄量．这种方法从双极晶体管的集电极电流公式出发,利用VBE做自变量,在室温和液氮温度下测量器件的Gummel图,选取lnIC随VBE变化最为线性的一部分读出VBE及相应的IC数值,获得两条VBE-lnIC直线,通过求解两条直线的交点可以计算出基区的禁带变窄量ΔEG．利用这种方法测试了硅双极器件和锗硅基区双极器件,其基区禁带变窄量分别为41meV和125meV,这个测量结果与文献中的数值符合较好．     

Electrical determination of bandgap narrowing in bipolartransistors with epitaxial Si, epitaxial Si1-xGex,and ion implanted bases

The apparent bandgap narrowing in bipolar transistors with epitaxial Si, epitaxial SiGe and ion implanted bases is measured from the temperature dependence of the collector current density J_c(T). A graph of InJ_c(T)/J_o(T) as a function of reciprocal temperature is plotted, and the apparent bandgap narrowing obtained from the slope. For epitaxial base transistors, in which the boron base profiles are abrupt, a linear J_c(T)/J _o(T) characteristic is obtained, which allows the unambiguous determination of the apparent bandgap narrowing. The measured values for epitaxial Si bases are in good agreement with the theoretical model of Klaassen over a range of base doping concentrations. For Si_0.88 Ge_0.12 and Si_0.87Ge_0.13 epitaxial base heterojunction bipolar transistors (HBT's), values of bandgap narrowing of 119 and 121 meV are obtained due to the presence of the Ge, which can be compared with theoretical values of 111 and 118 meV. For the implanted base transistor, the J_c(T)/J_o(T) characteristic is not linear, and its slope is larger at high temperatures than at low. This behaviour is explained by the presence of a tail on the ion implanted profile, which dominates the Gummel number of the transistor at low temperatures     

Modeling of Thermoelectric Properties of Magnesium Silicide (Mg2Si)

Thermoelectric (TE) materials based on alloys of magnesium (Mg) and silicon (Si) possess favorable properties such as high electrical conductivity and low thermal conductivity. Additionally, their abundance in nature and lack of toxicity make them even more attractive. To better understand the electronic transport and thermal characteristics of bulk magnesium silicide (Mg₂Si), we solve the multiband Boltzmann transport equation within the relaxation-time approximation to calculate the TE properties of n-type and p-type Mg₂Si. The dominant scattering mechanisms due to acoustic phonons and ionized impurities were accounted for in the calculations. The Debye model was used to calculate the lattice thermal conductivity. A unique set of semiempirical material parameters was obtained for both n-type and p-type materials through simulation testing. The model was optimized to fit different sets of experimental data from recently reported literature. The model shows consistent agreement with experimental characteristics for both n-type and p-type Mg₂Si versus temperature and doping concentration. A systematic study of the effect of dopant concentration on the electrical and thermal conductivity of Mg₂Si was also performed. The model predicts a maximum dimensionless figure of merit of about 0.8 when the doping concentration is increased to approximately 10²⁰?cm^?C3 for both n-type and p-type devices.     

A new dielectric isolation method using porous silicon

A new dielectric isolation technology is proposed. In the new structure, single crystalline Si islands are separated from the silicon substrate by oxidized porous silicon. It is based on the following characteristics of the porous silicon oxide formation: (1) p-type Si is more easily changed to porous silicon than n-type Si; (2) porous silicon is formed along the anodic reaction current flow line; (3) the change in volume of porous silicon after oxidation is relatively small; (4) thick porous silicon films (10 μm) can be obtained easily. In this method, a p-type isolated layer is obtained by proton implantation used for an n-type layer formation. Lateral p-n junctions fabricated in such isolated silicon layers show lower leakage current than those reported in SOS technology.     

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.     

Dislocation Analysis in (112)B HgCdTe/CdTe/Si

High-quality (112)B HgCdTe/Si epitaxial films with a dislocation density of ??9 × 10⁵ cm^?2 as determined by etch pit density (EPD) measurements have been obtained by thermal cyclic annealing (TCA). The reduction of the dislocation density by TCA has led to a simple rate-equation-based model to explain the relationship between dislocation density and TCA parameters (time, temperature, and number of anneals). In this model, dislocation density reduction is based on dislocation coalescence and annihilation, assumed to be caused by dislocation motion under thermal and misfit stress. An activation energy for dislocation motion in n-type (112)B HgCdTe/Si of 0.93 ± 0.1 eV was determined. This model with no adjustable parameters was used to predict recent TCA annealing results.     

Heavy doping effects in silicon

The different mechanisms causing bandgap narrowing in heavily doped silicon are reviewed. A distinction is made between many-body effects and the effects due to random impurity distribution. The values of bandgap narrowing, calculated using a theoretical model, are compared with the experimental results. Recombination in heavily-doped silicon is discussed and the different recombination mechanisms, present at high doping levels, are explained. Experimental values for the minority-carrier lifetime as a function of the doping level are given. Surface recombination at the heavily doped Si/SiO₂ interface is discussed, the trnasport equations in the case of a position dependent bandgap are derived, and finally the influence of heavy-doping effects on the performance of several devices is discussed.     

High-frequency, 6.2 Å pN heterojunction diodes

Sb-based pN heterojunction diodes at 6.2 Å, consisting of narrow bandgap p-type In_0.27Ga_0.73Sb and wide bandgap n-type In_0.69Al_0.41As_0.41Sb_0.59, have been fabricated and measured. These diodes show excellent electrical characteristics with an ideality factor of 1.2 and high current density. S-parameter measurements and subsequent analysis show that these diodes have RC-cutoff frequencies over 1 THz, making these diodes excellent choices for high-frequency applications, such as sub-harmonic mixers for frequency conversion.     

Base resistance and effective bandgap reduction in n-p-nSi/Si1-xGex/Si HBTs with heavy base doping

This paper presents a comprehensive study of the effects of heavy doping and germanium in the base on the dc performance of Si/Si_1-x Ge_x/Si npn Heterojunction Bipolar Transistors (HBTs). The lateral drift mobility of holes in heavily doped epitaxial SiGe bases affects the base sheet resistance while the effective bandgap is crucial for the vertical minority carrier transport. The devices used in this study were Si_1-xGe_x npn HBTs with flat Ge and B profiles in the base grown by Rapid Thermal Chemical Vapor Deposition (RTCVD). Hall and drift lateral hole mobilities were measured in a wide range of dopings and Ge concentrations. The drift mobility was indirectly measured based on measured sheet resistivity and SIMS measurements, and no clear Ge dependence was found. The Hall scattering factor is less than unity and decreases with increasing Ge concentration. The effective bandgap narrowing, including doping and Ge effects, was extracted from the room temperature collector current measurements over a wide range of Ge and heavy doping for the first time. We have observed bandgap narrowing due to heavy base doping which is, to first order, independent of Ge concentration, but less than that observed in silicon, due to the effect of a lower density of states. A model for the collector current enhancement with respect to Si devices versus base sheet resistance is presented     

Electric Current Dependence of a Self-Cooling Device Consisting of Silicon Wafers Connected to a Power MOSFET

A self-cooling device has been developed by combining a commercial n-channel power metal–oxide–semiconductor field-effect transistor (MOSFET) and single-crystalline Sb-doped n-type or B-doped p-type silicon wafers in order to improve the heat removal or cooling quantitatively. The electric current dependence of the temperature distribution in the self-cooling device and the voltage between the source and drain electrodes have been measured to estimate the Peltier heat flux. We found that the average temperature is decreased for a power MOSFET in which an electric current of 50 A flows. In particular, the average temperature of the power MOSFET was decreased by 2.7°C with the n-type Si wafer and by 3.5°C with the p-type Si wafer, although an electric current of 40 A makes little difference. This certainly warrants further work with improved measurement conditions. Nonetheless, the results strongly indicate that such n-type or p-type silicon wafers are candidate materials for use in self-cooling devices.     

A method for determining the state of the silicon-sapphire boundary in thin silicon-on-sapphire layers

A simple method for determining the state of the silicon-sapphire boundary in thin silicon-on-sapphire layers, which is based on measuring a saturation photo-emf and volt-ampere characteristics in Au/Si diode structures is proposed. It is shown that in silicon-on-sapphire layers that are obtained by the low-temperature molecular beam epitaxy a p-type conduction layer is formed at the silicon-sapphire boundary.     

Solid-State Synthesis and Thermoelectric Properties of Al-Doped Mg2Si

The electronic transport and thermoelectric properties of Al-doped Mg₂Si (Mg₂Si:Al _m , m?=?0, 0.005, 0.01, 0.02, 0.03) compounds prepared by solid-state synthesis were examined. Mg₂Si was synthesized by solid-state reaction (SSR) at 773?K for 6?h, and Al-doped Mg₂Si powders were obtained by mechanical alloying (MA) for 24?h. Mg₂Si:Al _m were fully consolidated by hot pressing (HP) at 1073?K for 1?h, and all samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased significantly with increasing Al doping content, and the absolute value of the Seebeck coefficient decreased due to the significant increase in electron concentration from 10¹⁶ cm^?3 to 10¹⁹ cm^?3 by Al doping. The thermal conductivity was increased slightly by Al doping, but was not changed significantly by the Al doping content due to the much larger contribution of lattice thermal conductivity over electronic thermal conductivity. Mg₂Si:Al_0.02 showed a maximum thermoelectric figure of merit of 0.47 at 823?K.     

Measurements of bandgap narrowing in Si bipolar transistors

Theory predicts appreciable bandgap narrowing in silicon for impurity concentrations greater than about 10¹⁷ cm^?3. This effect influences strongly the electrical behaviour of silicon devices, particularly the minority carrier charge storage and the minority carrier current flow in heavily doped regions. The few experimental data known are from optical absorption measurements on uniformly doped silicon samples. New experiments in order to determine the bandgap in silicon are described here. The bipolar transistor itself is used as the vehicle for measuring the bandgap in the base. Results giving the bandgap narrowing (ΔV_g0) as a function of the impurity concentration (N) in the base (in the range of 4.10¹⁵–2.5 10¹⁹ cm^?3) are discussed. The experimental values of ΔV_g0 as a function of N can be fitted by:

$\text{δV}{}_{\mathrm{g0}}\text{= V}{}_1\text{ln}\text{N}\text{N}{}_0\text{+}\text{ln}{}^2\text{N}\text{N}{}_0\text{+C}$

where V₁, N₀ and C are constants.It is also shown how the effective intrinsic carrier concentration (n_ie) is related with the bandgap narrowing (ΔV_g0).     

A contribution to the theory of the open-circuit photovoltage of an abrupt p-n junction

The theory for calculating the open-circuit photovoltage of an abrupt p-n junction, as originally given by Parrott [5], is generalized to include degeneracy and heavy doping effects. A useful approximation is given which simplifies the general theory considerably. Results are presented for silicon assuming rigid bands with parabolic density of states functions and assuming the free-carrier screening model for the bandgap narrowing.     

Stress-sensitive properties of silicon-gate MOS devices

Stress-sensitive properties were measured on both p- and n-channel silicongate MOS devices fabricated on (100) Si at room temperature. Stress-induced variations in drain currents for both enhancement- and depletion-mode MOS transistors with various channel-dopings were measured over a wide range of gate biases. In addition to piezoresistance effect, remarkable drain-current variations were observed at weak-inversion and explained theoretically in terms of changes in minority carrier densities due to energy band shifts by stresses. Elastoresistance shear-constants for polycrystalline-silicon gate layers were also obtained and compared with coefficients for source-drain diffused layers. Further, the elastoresistance of p-type polycrystalline-silicon films was investigated on doping-concentration dependences. A theoretical model for polycrystalline-silicon elastoresistance was developed based on the barrier model for conductivity in polycrystalline-silicon. Results obtained from the model were compared with the experimental results and found to be in good agreement at higher doping-concentrations than trap density.     

Influence of bandgap narrowing on the performance of silicon n-p solar cells

This paper deals with the incorporation of bandgap narrowing in the modelling of n⁺-p solar cells. First, the physcial model on which the computations are based, is explained. Second, the technology used to fabricate the solar cells, and the measurement technique for the minority carrier lifetime are commented. Then a detailed comparison between measured and computed results of I_sc, V_oc and P_max follows and the importance of inclusion I72 of the bandgap narrowing is illustrated. The theoretical case where Auger—recombination is the only recombination process is also treated. Finally computed results obtained for solar cells with different surface concentrations are shown.     

Transient isothermal generation at the silicon-silicon oxide interface and the direct determination of interface trap distribution

The theory of transient isothermal generation through the interface traps at the semiconductor-insulator interface is presented. The generation current (I_g) vs time (t) characteristic is obtained in terms of the interface trap distribution throughout the bandgap. It is shown that a plot of I_et vs log_et is a direct image of the energy distribution of the traps in the upper-half of the bandgap (in the case of an n-type semiconductor) and a plot of I_gt vs log_et is a direct image of the trap distribution in the lower-half of the bandgap.     

Convenient Melt-Growth Method for Thermoelectric Mg2Si

We have succeeded in growing single-crystalline-like n-type Mg₂Si bulk crystals by a convenient melt-growth method that requires no vacuum or inert gas. The Sb-doped, n-type Mg₂Si crystals had a density equivalent to the theoretical ideal of 1.99 g cm³ to 2.00 g cm^?3 and well-developed crystalline grains. Powder x-ray diffraction measurements and scanning electron microscopy observations confirmed the single-phase Mg₂Si nature of the grown crystals, with no MgO or unreacted Si and Mg observed. The crystals had high Hall mobility and power factor compared with Sb-doped sintered Mg₂Si crystals. The achieved ZT values were 0.10 at 300 K and 0.36 at 600 K for 0.317 at.%Sb-doped Mg₂Si.