InP掺杂超晶格的MOCVD生长与表征

用常压MOCVD技术生长了InP掺杂超晶格,所用源材料为TMIn和PH3.InP的载流子浓度低为8×1013cm-3,300K迁移率高达5744cm2/V·s.n型掺杂剂是DETe,p型为DMZn,两者在InP中的掺杂浓度均可达~1019cm-3.用不同激发强度下的低温PL谱,时间衰减,时间分辨PL谱和光反射(PR)谱技术表征了InP掺杂超晶格.文中给出了在不同激发强度下InP掺杂超晶格的4KPL谱,该样品由六层(n·p)组成,每层厚度200Å,掺杂浓度分别是1×1018cm-3和2×1018cm-3.发光峰值能量明显低于InP的禁带宽度,当激发强度增加时峰值能量移向高能边,这是真实空间中的间接跃迁特征.总PL信号衰减分成几步,每步都按指数规律,在4K情况下,衰减常数从6×10-8秒到7×10-4秒,具有明显间接带隙材料特征.InP掺杂超晶格中的量子尺寸效应用PR谱技术进行了研究.为描述InPnipi结构中的量子尺寸效应,引进了抛物型势阱模型,实验结果与根据引进模型所进行的计算相符.     

Electron-mobility enhancement and electron-concentration enhancement in δ-doped n-GaAs at T=300K

The electron-mobility of δ-doped GaAs grown by molecular-beam epitaxy is determined for two-dimensional doping concentrations ranging from 10¹⁰cm⁻² to 10¹³cm⁻². A mobility-enhancement (of up to a factor of four) of δ-doped epitaxial layers over their corresponding homogeneously doped layer is found in highly doped layers at T=300K. Temperature-dependent Hall-measurements are performed to clarify the physical origin of the mobility-enhancement. The electron-mobility is roughly constant in the range 4K ≤ T ≤300K. The Hall-mobility measurements confirm that (i) high degeneracy of the two-dimensional electron-gas, (ii) spatial electron-donor separation in n = 1,3,… subbands, as well as (iii) screening are the origins of high room-temperature mobility of δ-doped GaAs. Extremely high free-electron-concentrations of n_2DEG = 2×10¹³cm⁻² can be obtained in δ-doped GaAs, corresponding to three-dimensional concentrations of 9×10¹⁹ cm⁻³. This electron-concentration enhancement is assumed to be due to a reduced Si-autocompensation.     

Optimization of ZnSe film growth conditions for p-type doping

Wide bandgap semiconductors such as ZnSe and ZnO have attracted great interest due to their applications in solar cells, light emitting diodes, and lasers. However, these wide bandgap semiconductors are frequently difficult to be doped to heavy concentrations, greatly limiting their application. A substrate holder with a natural temperature gradient was developed for batch growth of films at different deposition temperatures, in order to investigate ZnSe film growth and doping challenges. Thin ZnSe films were grown by pulsed laser deposition and characterized using X-ray diffraction, optical transmission and reflection, Raman spectroscopy, and Energy Dispersive X-ray analysis. Deposition temperature and film stoichiometry (Zn:Se) are shown to be significant factors affecting ZnSe growth and doping. ZnSe films with improved crystallinity have been obtained by enriching with selenium and depositing at an optimized temperature. Heavily p-type ZnSe films with hole concentrations of ~2.7 × 10¹⁹ cm^?3 and resistivities of ~0.099 Ohm cm have been obtained (compared with previous reports of ~1 × 10¹⁸ cm^?3 and ~0.75 Ohm cm). The results, which are consistent with previous theoretical prediction of compensating defects in ZnSe films, can help to optimize ZnSe growth conditions and understand doping challenges in wide bandgap semiconductors.     

Quantum efficiency and radiative lifetime in degenerate n-type GaAs

The hole lifetime, the radiative lifetime, the non-radiative lifetime, and the internal quantum efficiency in degenerate n-type GaAs crystals have been investigated with a simplified model of degenerate semiconductors, in which the recombination constant B is approximately proportional to the$\text{?13}\text{8}$ power of the electron density.In n-type GaAs at 77 K, the radiative lifetime reaches a minimum equal to 4 × 10^?9 sec. at 6 × 10¹⁷ cm^?3, and the internal quantum efficiency exhibits a maximum equal to 50% at 8 × 10¹⁷cm^?3, in good agreement with the theoretical prediction of Dumke and the experiments of Cusano. At high impurity concentrations, the polytropy effect is taken into account in the case of GaAs crystals doped with tellurium and selenium. Finally, it is suggested that, for a given high concentration, the internal quantum efficiency increases with increasing temperature, in accordance with the observed results.     

Phenomena in silicon photodiodes doped with Zn and S

The electrophysical and photoelectrical properties of Si〈S〉 were investigated and the noise and threshold characteristics of photoresistors made of this material were considered. Such photoresistors had detectivity D* (5.3; 570.1)= 5.15 × 10¹⁰ and 2.0 × 10⁻¹¹cm Hz W⁻¹, near to the background limited performance for a field of view 40° and 10° respectively, at a temperature 78 K and an electrical field of 300 V cm⁻¹. Paraphotosensitivity of a diode made of Si doped with Zn was investigated. Some peculiarities of the current-voltage characteristics of p⁺nn⁺ and p⁺np⁺ Si〈Zn〉 structures were observed. Generation-recombination noise spectral density calculations are given for the case of doping the semiconductors with double-charged deep centres using the “impedance field” method.     

Role of beryllium doping in strain changes in II-type InAs/GaSb superlattice investigated by high resolution X-ray diffraction method

We have studied the influence of the beryllium doping on strain in the II-type InAs/GaSb superlattices (SLs) by means of high-resolution X-ray diffraction (HRXRD). Three analyzed superlattices were grown by molecular beam epitaxy. One of the examined superlattices was undoped. Two others structures, called doped SLs, composed of two superlattices: Be-doped and undoped which were grown one on the top of each other. The doping concentration was determined by secondary ion mass spectroscopy. The doping level was 1×10¹⁷ cm^?3 and 2×10¹⁹ cm^?3. For doped superlattices, the HRXRD measurements showed splitting of satellite (ST) peaks. Furthermore, the separation of ST peaks increase with doping level. In contrast, for undoped superlattice, the splitting of the ST peaks was not observed. Sometimes the separation of ST peaks can be caused by change of thickness period of superlattice or partial relaxation of the structure. However, we claim that in our experiment the splitting is caused by another mechanism: The presence of Be atoms in SL causes the change of average lattice constant of the superlattice. The influence of Be dopant on lattice parameter of superlattice was confirmed by theoretical simulations. Furthermore, the change of the lattice constant (Δa/a) of the GaSb:Be buffer was examined. The reduction of lattice parameter of GaSb was noticed. It was caused by the presence of Be doping and unintentionally incorporated As-atoms in the GaSb layer. It is very important to know that even very small Be concentration (1×10¹⁷ cm^?3) causes the change of average lattice parameter of SL.     

Mg2Si的电子结构和热电输运性质的理论研究

利用全势线性缀加平面波法,对Mg₂Si的几何结构和电子结构进行了计算,得到了稳定的晶格参数以及能带和电子态密度.能带结构表明,Mg₂Si为间接带隙半导体,禁带宽度为020 eV.在此基础上利用玻尔兹曼输运理论和刚性带近似计算了材料的电导率、Seebeck系数和功率因子.结果表明,在温度为700 K时p型和n型掺杂的Mg₂Si功率因子达到最大时的最佳载流子浓度分别为7749×10¹⁹ cm^-3和 关键词： 2Si')" href="#">Mg₂Si 全势线性缀加平面波法 热电输运性质     

Electrical properties of n-type vapor-grown gallium nitride

Hall measurements are reported for undoped and Zn-doped vapor-grown single crystal GaN on (0001) Al₂O₃ layers with 298 K carrier concentrations (n-type) between 1·4×10¹⁷cm^?3 and 9×10¹⁹ cm^?3. Then n～10¹⁷ cm^?3 crystals (undoped) have mobilities up to μ～440 cm²/V sec at 298 K. Their conduction behavior can be described by a two-donor model between 150 and 1225 K and by impurity band transport below 150 K. Crystals with n≥8×10¹⁸ cm^?3 show metallic conduction with no appreciable variation in n or μ between 10 and 298 K.Results of mass spectrographic analyses indicate that the total level of impurities detected is too low to account for the observed electron concentration at the n～10¹⁹ cm^?3 level, and suggest the presence of a high concentration of native donors in these crystals. No significant reduction in carrier concentration was achieved with Zn doping up to concentrations of ～10²⁰ cm^?3 under the growth conditions of the present work, and no evidence was found to indicate that high conductivity p-type behavior may be achieved in GaN. The influence of factors such as growth rate, crystalline perfection and vapor phase composition during growth on the properties of the layers is described.     

Temperature-dependent luminescence of GaAs doping superlattices

The photon energy of the luminescence between spatial separated n- and p-type doped layers of GaAs doping superlattices shifts with excitation density. At room temperature we observe a shift of the luminescence maximum in samples with high doping concentrations (n=4×10¹⁸cm⁻³), whereas for n=1×10¹⁸cm⁻³ this shift is observed at temperatures below 150K only. Temperature-dependent measurements between 4 and 700K show that the tunability disappears near a critical temperature, which is proportional to the doping concentration. A simple model including thermally activated direct transitions and tunable luminescence describes this temperature dependence.     

Deep levels in gallium arsenide by capacitance methods

The three capacitance methods, i.e., TSCAP, PHCAP, and transient capacitance measurements, are applied to determine electronic properties of deep levels inn-GaAs. In the boat-grown wafer detected are the 0.30 eV electron trap withN _T =3.6×10¹⁶ cm^?3 andS _n =2.4×10^?15 cm², and the 0.75 eV electron trap withN _T =2.0×10¹⁶ cm^?3 andS _n =1.2×10^?14 cm². In the epitaxial wafer, the 0.45 eV hole trap is detected withN _T >1.5×10¹³ cm^?3 andS _p =1.4×10^?14 cm² as well as the 0.75 eV electron trap withN _T =2.4×10¹³ cm^?3.     

Temperature dependence of the tunable luminescence of GaAs doping superlattices

Doping superlattices show tunable optical and electrical properties due to the space charge induced separation of photoexcited or electrically injected carriers. We have investigated the tunable luminescence in GaAs doping superlattices of doping levels n=1×10¹⁸cm⁻³ and n=4×10 ¹⁸cm⁻³ as function of excitation density and sample temperature. The temperature dependence of the tunability was investigated in the range between 4 and 700K, and we found the critical transition temperatures T₀ at 90 and 460K for the low and high doped samples, respectively. The results verify the theoretical prediction concerning the transition temperature at which the luminescence changes from full to zero tunability.     

流体静压力对InSb输运性质的影响

在室温和流体静压力达12500公斤/厘米²下,测量了不同掺杂浓度的n型(5×10¹³—2.3×10¹⁸厘米^-3和p型(2.6×10¹⁴—6.0×10¹⁷厘米^-3)InSb的霍耳系数和电导率。分析结果发现,霍耳系数公式中散射因子随压力而减小,禁带宽的压力系数不是常数;得出禁带宽、载流子浓度、电子和空穴迁移率与压力的关系,并对压力下的载流子散射机构作了初步讨论。     

Preparation and semiconducting properties of Th3As4

The resistivity, thermoelectric power and Hall constant in the temperature range of 78–830 K were determined for polycrystalline Th₃As₄ samples obtained by annealing thin thorium slabs in arsenic vapour. The samples examined were n-type semiconductors with a carrier concentration ranging from 1.0 × 10¹⁸cm^?3 to 2.8 × 10¹⁸ cm^?3 for which the effective mass was found to be equal to 0.55–0.76m₀. The Hall mobility, about 450cm²V^?1s^?1 at room temperature, obeys a $\text{T}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}$ law at high temperatures. On the basis of the electrical measurements the forbidden gap of Th₃As₄ was found to be equal to 0.43 eV.     

Auger recombination of electron-hole drops

Phonon-assisted Auger recombination is calculated for indirect band gap semiconductors in the strongly degenerate case. It follows a reciprocal lifetime τ^?1=Cn² with C=7.19×10³¹ cm⁶ sec^?1 for Si and C=2.94× 10^?31 cm⁶ sec^?1 for Ge. These results are in good agreement with experimental values of the decay of electron-hole drops. Therefore one can conclude that phonon-assisted Auger recombination is the essential nonradiative recombination process in this case.     

Determination of relaxation times of ground electronic state vibrational levels of mixed molecular crystals

The stimulated emission spectra of mixed molecular crystals were investigated with the Nd glass lasser third harmonic excitation at 4.2 K. Napthalene crystals doped with β-naphthyl-n-biphenylyl ethylene and dibenzyl doped with naphthacene in various concentrations were studied. Relaxation times of the ground state vibrational levels in mixed molecules were obtained by use of the dependence of stimulated emission intensity upon excitation energy for naphthacene in dibenzyl τ₁ of the vibrational level at 317 cm^-1 was ～ 3 × 10^-9 s; for β-naphthyl-n-biphenylyl ethylene in naphthalene τ₁ of the 1629 cm^-1 vibrational level was ～ 10^-10 s.     

Hall effect in the Fe<Subscript>x</Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S magnetic semiconductors

The electrical properties and the Hall effect in the Fe_xMn_1?xS magnetic semiconductors (0<x≤0.5) have been experimentally studied in the range 77–300 K in magnetic fields of up to 15 kOe. The cation-substituted sulfides with 0.25≤x≤0.3 possessing colossal magnetoresistance (CMR) were established to be narrow-gap semiconductors with carrier concentrations n ～ 10¹¹–10¹⁵ cm^?3 and high carrier mobilities μ ～ 10²–10⁴ cm² V^?1 s^?1. It is believed that the CMR effect in these sulfides can be explained in terms of the model of magnetic and electron phase separation, which is analogous to the percolation theory in the case of heavily doped semiconductors.     

Carrier transport and luminescence properties of n-type GaN

The surface morphology, electrical properties and optical properties of Si doped n-type GaN were investigated. The intentional SiH₄ doped GaN films were grown by metal organic chemical vapor deposition with the electron concentration varying from 3×10¹⁶ cm⁻³ to 5.4×10¹⁸ cm⁻³. The surface morphology shows that the roughness and dislocation pits increase as the mass flow rate of SiH₄ increases, which indicates that the quality of GaN degrades gradually. The activation energy of Si in GaN with different n concentrations varies from 12 to 22 meV, which may originate from the interactions of donor wave functions. The carrier transport mechanism with increasing temperature from 100 to 420 K was concluded as the complex effect of both impurity scattering and phonon scattering. The position of the near band edge emission peak was determined by both renormalization of the band gap and B-M effect. The intensity variations of the yellow luminescence could be explained by the change of Ga vacancy concentration caused by Si doping. Supported by the National Basic Research Program of China (Grant No. 2006CB6049), the National Hi-Tech Research and Development Program of China (Grant No. 2006AA03A142), the National Natural Science Foundation of China (Grant Nos. 60721063, 60731160628 and 60676057), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050284004) and the Natural Science Foundation of Jiangsu Province of China (Grant No. BK2005210)     

Transition probabilities for Kr I lines from wall-stabilized arc measurements

Transition probabilities for 23 Kr I lines between 431.9 and 892.9 nm pertaining to the 5s-5p and 5s-6p transition arrays were obtained from wall-stabilized arc measurements analyzed under the assumption of a Saha-Boltzmann relationship between the electron density and the population densities of the excited Kr I levels. The electron density (n_e=5×10¹⁵−2.2×10¹⁷cm^-3) was calculated from the measured emission coefficient for continuum radiation at 456.1 nm. The electron temperature (T=9500−17 000 K) was determined from measurements of the Kr I 811.3 nm center line intensity by using Kirchhoff's law. The experimental transition probabilities decrease in the direction of larger electron densities. For n_e ⩽ 2×10¹⁶cm^-3 there is agreement with published theoretical transition probabilities and lifetime measurements.     

Effect of the mutual dragging of electrons and phonons on the thermomagnetic effects in HgSe

The longitudinal and transverse Nernst-Ettingshausen (NE) effects were measured in HgSe samples doped with various concentrations of gallium, 0.6×10¹⁸<N(Ga)<4.7×10¹⁸ cm^?3. The NE measurements were performed for the classical interval of magnetic field strengths (0–40 kOe) in the temperature range from 20 to 60 K. It was established that the low-temperature NE coefficients change sign with increasing Ga concentration or the applied magnetic field strength. It is shown that all unusual features of the NE effects observed in HgSe crystals can be attributed to the effect of mutual dragging of electrons and phonons. This implies that the effect of mutual electron-phonon dragging can be experimentally detected in semiconductors with high concentration of conduction electrons.     

Transport and magnetic properties of multiwall carbon nanotubes before and after bromination

Bulk samples of oriented carbon nanotubes were prepared by electric arc evaporation of graphite in a helium environment. The temperature dependence of the conductivity σ(T), as well as the temperature and field dependences of the magnetic susceptibility χ(T, B) and magnetoresistance ρ(B, T), was measured for both the pristine and brominated samples. The pristine samples exhibit an anisotropy in the conductivity σ_∥(T)/σ_⊥>50, which disappears in the brominated samples. The χ(T, B) data were used to estimate the carrier concentration n ₀ in the samples: n _0ini ～3×10¹⁰ cm^?2 for the pristine sample, and n _0Br～10¹¹ cm^\t—2 for the brominated sample. Estimation of the total carrier concentration n=n _e+n _p from the data on ρ(B, T) yields n _ini=4×10¹⁷ cm^?3 (or 1.3×10¹⁰ cm^?2) and n _Br=2×10¹⁸ cm^?3 (or 6.7×10¹⁰ cm^?2). These estimates are in good agreement with one another and indicate an approximately fourfold increase in carrier concentration in samples after bromination.