Ga1-xAlxAs/GaAs光阴极光谱响应向短波延伸的机理分析和实验研究

为了实现光阴极光谱响应向短波延伸，分析了Ga1-xAlxAs/GaAs三代微光光阴极光谱响应向短波延伸的机理，提出了准禁带宽度的概念，讨论了Ga1-xAlxAs的禁带宽度随铝组份x值的增加而逐渐变宽的变化规律，计算了不同铝组份下光阴极光谱响应的短波限。通过提高光阴极材料的铝组份，做出了宽光谱响应像管，并给出了相应的光谱响应曲线。通过与标准三代微光像管光谱响应对比，发现：所得光谱响应曲线不仅向短波方向得以延伸，而且理论设计与实验结果吻合很好。     

掺Yb3+氧化镧钇透明激光陶瓷的光谱特性

采用传统陶瓷烧结工艺,在无压还原气氛下低温制备出透明性良好的掺Yb3+氧化镧钇透明激光陶瓷,测试了其在室温下的吸收光谱、发射光谱和荧光寿命.结果表明,掺Yb3+氧化镧钇透明激光陶瓷的吸收系数随着Yb3+掺杂浓度的增加而增大,最强吸收峰974nm处的吸收截面为0.90～1.12×10－20 cm2;主发射峰1 032 nm和1 075 nm处的发射截面分别为1.05×10－20 cm2和0.87×10－20 cm2; Yb3+掺杂浓度为5at.%时荧光寿命为1.38 ms,并随Yb3+掺杂浓度的增加而减小;当Yb3+掺杂浓度超过10at.%时,样品中存在严重的浓度猝灭.产生浓度猝灭的原因是高掺杂时离子间存在合作上转换和能量转移.     

显微光谱研究半绝缘GaAs带边以上E_0 Δ_0光学性质

通过显微光致发光技术和显微拉曼(Raman)技术研究了半绝缘GaAs(SI-GaAs)晶体的带边附近的发光.在光荧光谱中,观察到在高于GaAs带边0.348eV处有一个新的荧光峰.结合Raman谱指认此发光峰来源于GaAs的E0 Δ0能级的非平衡荧光发射.同时,通过研究E0 Δ0能级的偏振、激发光强度依赖关系,以及温度依赖关系说明E0 Δ0能级与带边E0共享了共同的导带位置Γ6,同时这也说明在GaAs中主要是导带的性质决定了材料的光学行为.同时,通过与n-GaAs和δ掺杂GaAs相比较,半绝缘GaAs晶体的E0 Δ0能级的发光峰更能反映GaAs电子能级高临界点E0 Δ0的能量位置和物理性质.研究结果说明显微光致发光技术是研究半导体材料带边以上能级光学性质的一种非常有力的研究工具.     

Kinetic stabilization of Fe film on GaAs(1 0 0): An in situ X-ray reflectivity study

We study the growth of Fe films on GaAs(1 0 0) at a low temperature, 140 K, by in situ X-ray reflectivity (XRR) using synchrotron radiation. The XRR curves are well modeled by a single Fe layer on GaAs both at the growth temperature and after annealed at the room temperature. We found that the surface became progressively rougher during the growth with the growth exponent, β_S = 0.43 ± 0.14. The observed β_S is attributed to the restricted interlayer diffusion at the low growth temperature. The change of the interface width during growth was minimal. When the Fe film was annealed to room temperature, the surface smoothed, keeping the interface width almost unchanged. The confinement of the interface derives from that the diffusion of Ga and As proceeds via the inefficient bulk diffusion, and the overlying Fe film is kinetically stabilized.     

AFM nanooxidation process - Technology perspective for mesoscopic structures

We study the technology of local anodic oxidation (LAO) by the AFM tip applied to semiconductor heterostructures with two-dimensional electron gas. The aim is to design mesoscopic rings with persistent current and one subband occupied. For this purpose the need is to oxidize narrow lines that represent energy barriers high enough. Using the electrostatic model, we explain the electric field distribution in the system tip-sample just before LAO starts. We study the influence of the conductivity of the cap layer on LAO and explain the origin of the saddle-like profile lines, observed in the experiment. Using Monte Carlo simulation we show that the carrier redistribution in the system with LAO energy barriers effectively lowers the barrier height. In the experimental part we have grown InGaP/AlGaAs/GaAs heterostructures by organometalic vapor phase epitaxy with an active layer only 31 nm below the surface. We have prepared oxide lines on the heterostructures by LAO and characterized them by the temperature-dependent transport measurement.     

Interfacial differences in enhanced schottky barrier height Au/n-GaAs diodes deposited at 77 K

The use of cryogenic temperatures (∼77 K) during Au Schottky contact deposition onto n-GaAs produces an increase in barrier height from 0.73 eV for room temperature diodes to 0.82 eV. Not all Schottky metals show this enhancement—for example Pt and Ti do not show any significant change in barrier height whereas Au, Pd and Ni show increases between 7 and 18%. We used X-ray reflectivity to show that the main difference between Au deposited at 77 K and room temperature is a decreased metal roughness while the interfacial roughness between the Au and GaAs is basically the same. As the diodes are annealed to 300 °C both the difference in barrier height and interfacial roughness is lost. This is a simple method with potential for improving the performance of GaAs metal-semiconductor-field-effect-transistors (MESFETs).     

Surface electronic properties of sulfur-treated GaAs determined by surface photovoltage measurement and its computer simulation

The Kelvin method together with the simulations of surface photovoltage has been used to determine the surface electronic properties, i.e. the surface band bending (qV_S), surface state density (N_SS0) and surface fixed charge (Q_Fx) of S₂Cl₂-treated GaAs (100) surfaces. The measured values of surface photovoltage (SPV) do not show saturation at high photon flux densities in contradiction to the simple theory of SPV. This behavior of SPV agrees very well with the rigorous computer simulations and can be explained in terms of the Dember effect. Moreover, the SPV values become insensitive to surface states at moderate photon flux densities. On this basis, the surface band bending of untreated (0.79 eV) and S₂Cl₂-treated (0.60 eV) GaAs surfaces was determined. The band diagrams summarizing the obtained results proved the influence on the potential variations not only from the ionized surface states and surface fixed charge but also from the surface dipole layer on the S₂Cl₂-treated GaAs surface. The dipole arises most probably due to the S-Ga bonding on the surface. The presented results offer an alternative explanation for increased PL commonly observed after the sulfidation in the absence of substantial reduction in the band bending.     

Phonon-assisted tunneling process in amorphous silicon nanostructures and GaAs nanowires

Experimental results on the current–voltage (I–V) characteristics of amorphous Si nanostructures reported by Irrera et al. [A. Irrera, F. Iacona, I. Crupil, C.D. Presti, G. Franzo, C. Bongiorno, D. Sanfilippo, G. Di Stefano, A. Piana, P.G. Fallica, A. Canino, F. Priolo, Nanotechnology 17 (2006) 1428] are reinterpreted in terms of a phonon-assisted tunneling model. It is shown that temperature dependence of current can be caused by the temperature dependence of electron tunneling rate from traps in the metal–semiconductor interface to the conduction band of the semiconductor. A good fit of experimental data with the theory is achieved in all measured temperature range from 30 to 290 K using for calculation the effective mass of 0.5m_e, and for the phonon energy the value of 12 meV. An advantage of this model over that of Irrera et al. used model is the possibility of describing the behavior of I–V data measured at both high and low temperatures with the same set of parameters characterizing this material. The temperature-dependent I–V data by Schricker et al. [A.D. Schricker, F.M. Davidson III, R.J. Wiacek, B.A. Korgel, Nanotechn. 17 (2006) 2681.] of GaAs nanowires, are also explained on the basis of this model.     

Raman study of the segregation of GaAs/AlAs heterostructures grown by molecular beam epitaxy

The Raman spectra of the optical confined phonons in the GaAs/AlAs ultra-thin layer superlattices grown with different growth conditions were used to determine the compositional profiles and to study the process of segregation at the heterointerfaces. A modified kinetic model was developed in order to calculate the compositional profiles in the samples under investigation. The comparison between the experimentally obtained compositional profiles and those calculated by the kinetic model allowed us to determine the parameters characterizing the segregation. It was shown that the increase of pressure of As acts equivalently to the decrease of the growth temperature, resulting in a more abrupt compositional profile.     

Time-resolved photoluminescence of delta-doped AlGaAs/GaAs heterostructures

The photoluminescence (PL) at low temperature of three delta-doped AlGaAs/GaAs heterostructures is investigated under continuous and pulsed excitations. The PL under continuous excitations allows the identification of the trapping centres and probes the carrier's transfer to the GaAs channel. The time-resolved photoluminescence (TRPL) inquires about carrier dynamics and gives radiative lifetimes of different levels. The DX level shows two time constants of the intensity decay relating the splitting of the valence band under impurity strains which reduce the crystal symmetry. The two time constants evolve with temperature and exhibit an increase near T=50 K.