GaSe晶体的双光子吸收对太赫兹输出的影响

根据光整流效应,利用超快激光脉冲泵浦GaSe晶体实现了0.2~2.5 THz的宽带太赫兹辐射输出。禁带中的电子在两个800 nm光子的作用下激发到导带中形成自由载流子,进而吸收所产生的太赫兹辐射,最终导致太赫兹的输出随泵浦功率的增加而趋于饱和。为了研究双光子吸收对太赫兹输出的影响,测量了800 nm处的GaSe晶体的双光子吸收系数,结果为 0.165 cm/GW。通过对太赫兹输出实验数据的拟合,得到GaSe晶体中自由载流子对太赫兹输出的吸收截面为1×10^-15 cm²。本文的研究结果可用于优化GaSe晶体在强激光泵浦下的太赫兹转换效率。     

Synthesis and Application of Monomeric Chalcogenolates of 13 Group Elements

Utilization of the N,C,N‐chelating ligand L (L={2,6‐(Me₂NCH₂)₂C₆H₃}^?) in the chemistry of 13 group elements provided either N→In coordinated monomeric chalcogenides LIn(μ‐E₄) (E=S, Se) with unprecedented InE₄ inorganic ring or monomeric chalcogenolates LM(EPh)₂ (M=Ga, In). Complex LGa(SePh)₂ was selected as the most suitable single source precursor (SSP) for the deposition of amorphous semiconducting GaSe thin films using spin coating method.     

Enhancement of terahertz wave difference frequency generation based on a compact walk-off compensated KTP OPO

A compact, walk-off compensated dual-wavelength KTP OPO near the degenerate point of 2.128 μm pumped by a Nd:YAG pulsed laser is employed as the pump for terahertz (THz) source based on difference frequency generation (DFG) in a GaSe crystal. Coherent THz radiation that is continuously tunable in the range of 81-1617 μm (0.186-3.7 THz) is achieved. An enhancement of 76.7% in average for the THz energies at different wavelengths is realized using the walk-off compensated KTP OPO than the common one. Using a 8 mm-long GaSe crystal, the maximum output THz pulse energy is 48.9 nJ with the peak power of 11 W, corresponding to the energy conversion efficiency of 5.4 × 10^− 6 and the photon conversion efficiency of about 0.09%.     

Formation of native oxide crystallites on GaSe(0 0 1) surface

The process of GaSe native oxide formation was studied using atomic-force microscopy. It was found that the oxide film growth is accompanied by a work function increase. This increase saturates in several hours. The illumination by 1 mW laser at 650 nm stimulates the oxidation process. Continuous illumination changes the work function by 1 eV and that is 2 times higher than that without irradiation. It is supposed that the oxide formation occurs at edge dislocation lines.     

掺碲硒化镓晶体的光学性能

采用水平区熔法生长了碲（Te）掺杂浓度（质量百分比）分别为0.05%,0.1%,0.5%,1%,2%的硒化镓（GaSe）晶体,并分别对掺杂浓度为0.01%,0.07%,0.38%,0.67%,2.07%的GaSe∶Te晶体的光学性能进行了表征。首次研究了GaSe∶Te晶体中刚性层声子模式的转换。吸收光谱测试结果表明：当Te掺杂浓度小于0.38%时,振动中心位于0.59 THz附近的E＇（2）刚性模式吸收峰强度可达最大值,这一过程与GaSe∶Te晶体光学性能的提高密切相关。但Te掺杂浓度的进一步提高会导致E＇（2）刚性模式吸收峰强度逐渐减弱,当Te掺杂浓度为1%时,E＇（2）刚性模式吸收峰基本消失。这两个过程与GaSe∶Te晶体光学质量的下降密切相关。因此,E＇（2）刚性模式吸收强度达到最高时对应的掺杂浓度即是GaSe∶Te晶体中Te的最佳掺杂浓度,光整流产生太赫兹过程证实了此结论的正确性。     

Highly stable passivation of a Si(1 1 1) surface using bilayer-GaSe

As a stable and ‘epitaxial’ passivation of a Si surface, we propose the bilayer-GaSe termination of a Si(1 1 1) surface. This surface is fabricated by depositing one monolayer of Ga on a clean Si(1 1 1) surface and subsequent annealing in a Se flux at around 520 °C, which results in unreconstructed 1×1 termination of the Si(1 1 1) surface by bilayer-GaSe. We found by scanning tunneling microscopy observation that slow cooling of the clean Si(1 1 1) surface from 850 to 520 °C with simultaneous deposition of a Ga flux results in better termination of the Si(1 1 1) surface. It was also found that this surface is stable against heating around 400 °C in O₂ atmosphere of 3×10⁻³ Pa. By utilizing these properties of the bilayer-GaSe terminated surface, we have succeeded in fabricating ZnO quantum dots on this substrate.     

Numerical simulation of heat conduction for the growth of anisotropic layered GaSe crystals

In this report, we present the usage of a second rank cylindrical conductivity tensor which we derived to simulate the crystal growth processes of a layered compound GaSe in a cylindrical enclosure by directional solidification. Use of such a tensor is inevitable in the simulations of the growth of highly anisotropic crystals having layered structure, since the crystallographic orientation of the grown material is not necessarily aligned with the ampoule symmetry. Using the finite difference control volume approach in 3D, we solved transient heat conduction equation for a highly anisotropic solid in a cylindrical enclosure. We obtained sloped thermal fields and isothermal surfaces and the magnitudes of the slopes are strong functions of both azimuthal angle and growth orientation. The results showed that the orientation of the crystallographic axes of GaSe in the enclosure is quite effective in the steady and the transient fields, isotherms, and axial and radial temperature gradient within the material. Increase of Bi number decreases the magnitude of the slope of isothermal surface. Anisotropy of the conductivity seems to be effective in the orientation of the growth direction of the resulting crystal within the cylindrical ampoule. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optoelectronical properties of polycrystalline β ‐GaSe thin films

Polycrystalline β ‐GaSe thin films were obtained by the thermal evaporation of GaSe crystals onto glass substrates kept at 300 °C under a pressure of 10^–5 Torr. The transmittance and reflectance of these films was measured in the incident photon energy range of 1.1–3.70 eV. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct allowed transitions band gap of 1.83 eV. The data analysis allowed the identification of the dispersive optical parameters by calculating the refractive index in the wavelength region of 620–1100 nm. In addition, the photocurrent of the samples was studied as function of incident illumination‐intensity and temperature. The photocurrent is found to exhibit sublinear and supralinear character above and below 270 K, respectively. The temperature dependent photocurrent data analysis allowed the calculation of photocurrent activation energies as 603, 119 and 45 meV being dominant in the temperature regions of 250–300 K, 180–240 K and 80‐160 K, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Terahertz generation from electron- and neutron-irradiated semiconductor crystal surfaces

Terahertz generation from the InP, InSb, GaAs and GaSe crystal surfaces excitated by femtosecond laser pulses has been studied. The terahertz spectra emitted from the native crystals and the crystals previously irradiated by high-energy neutrons or electrons have been recorded. Also, a simulation of the terahertz emission process has been performed. A weak terahertz signal generated from the GaSe native surface has been registered. In the case of electron-irradiated GaSe, the signal is increased several fold because of increased laser radiation absorption.     

Annealing effect on electrical and photoconductive properties of Si implanted GaSe single crystal

GaSe single crystals grown by Bridgman method have been doped by ion implantation technique. The samples were bombarded in the direction parallel to c‐axis by Si ion beam of about 100 keV to doses of 1 × 10¹⁶ ions/cm² at room temperature. The effects of Si implantation with annealing at 500 and 600 °C on the electrical properties have been studied by measuring the temperature dependent conductivity and photoconductivity under different illumination intensities in the temperature range of 100–320 K. It is observed that Si implantation increases the room temperature conductivity 10⁻⁷ to 10⁻³ (Ω‐cm)⁻¹ depending on the post annealing temperature. The analysis of temperature dependent conductivity shows that at high temperature region above 200 K, the transport mechanism is dominated by thermal excitation in the doped and undoped GaSe samples. At lower temperatures, the conduction of carriers is dominated by variable range hopping mechanism in the implanted samples. Annealing of the samples at and above 600 °C weakens the temperature dependence of the conductivity and photoconductivity. This indicates that annealing of the implanted samples activates Si‐atoms and increases structural deformations and stacking faults. The same behavior was observed from photoconductivity measurements. Hence, photocurrent‐illumination intensity dependence in the implanted samples obeys the power low I_pc ∝ Φⁿ with n between 1 and 2 which is an indication of continuous distribution of localized states in the band gap. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)