MOCVD生长GaAs高质量掺碳研究

利用低压金属有机化合物汽相淀积(LP-MOCVD)系统,通过调节生长参量和掺杂工艺,得到了晶格质量高、表面形貌好且空穴浓度从8×10¹⁷到4×10²¹可控的掺碳GaAs外延层,最后给出一应用实例-用重掺碳的GaAs材料做级联GaInP/GaAs/Ge太阳能电池的隧道结.     

n型掺杂GaAs中重空穴的飞秒动力学

本文采用fs脉冲饱和吸收光谱技术研究了室温下Si掺杂GaAs在电子激发态处于费密面附近时重空穴的超快弛豫特性。测量到重空穴的热化时间约为300fs,与理论计算结果一致,表明重空穴-光学声子散射是主要的热化途径,并得到光学形变势常数d₀为31eV.     

应变纤锌矿GaN/A10.15Ga0.85N柱形量子点的光学性质:流体静压力效应

在有效质量近似下,考虑强的内建电场和应变对材料参量的影响,变分研究了流体静压力对有限高势垒应变纤锌矿GaN/Al_0.15Ga_0.85N柱形量子点中重空穴激子的结合能、发光波长和电子空穴复合率的影响.数值结果表明,激子结合能和电子空穴复合率随流体静压力的增大而近线性增大,发光波长随流体静压力的增大而单调减小.在量子点尺寸较小的情况下,流体静压力对激子结合能和电子空穴复合率的影响更明显.由于应变效应,为了获得有效的电子-空穴复合过程,GaN量子点的高度必须小于5.5 nm.     

Zn0.8Cd0.2Se-ZnSe应变层超晶格的激子发光和受激发射

本文用MOCVD技术在GaAs衬底上成功地制备了具有波导结构的Zn0.8Cd0.2Se-ZnSe应变层超晶格样品,在77K温度的光致发光光谱中观测到n=1的重空穴和轻空穴激子的辐射复合。在光泵浦下,在波导结构的F-P腔中观测到具有多模结构的受激发射,受激发射谱中的不同模具有不同的阈值功率密度;时间延迟衰减曲线的半宽度越窄,阈值光强越大.     

量子限制受主远红外电致发光器件的制备与测量

采用分子束外延技术生长GaAs/AlAs三量子阱,并在中间的GaAs阱中δ-掺杂浅受主杂质Be原子,制作出量子限制受主远红外Teraherz原型电致发光器件.实验上测量得到4.5 K时器件的电致发光谱（EL）和电传输特性（I-V曲线）.在EL发射谱中清楚地观察到222 cm^-1处宽的尖峰,这来源于Be受主奇宇称激发态到其基态的辐射跃迁,而非辐射弛豫过程则使发射谱的信号很弱.另外在I-V曲线中072和186 V的位置出现两个共振隧道贯穿现象,分别对应于中间δ-掺杂量子阱受主能级1s_3/2(Γ₆+Γ₇)到左边非掺GaAs量子阱中HH带,及右边非掺杂GaAs量子阱中HH重空穴带到中间掺杂GaAs量子阱中Be受主杂质原子奇宇称激发态2p_5/2(Γ₆+Γ₇)能级的共振隧穿.     

钙钛矿晶体的电荷复合动力学研究

本文利用紫外吸收光谱和稳态荧光光谱技术结合理论模型,研究了钙钛矿材料CH₃NH₃PbI₃晶体在光激发过程中的电荷复合动力学行为,进而获得晶体的扩散长度. 电荷载体的扩散长度是判断光电材料的重要参数. 研究通过合成两种不同缺陷态浓度的CH₃NH₃PbI₃晶体,测量这两种晶体在0.019∽4.268 μJ/cm²的激光激发下的时间分辨荧光光谱,利用动力学模型对光谱进行拟合,可以获得每个晶体的掺杂浓度,空穴浓度以及电荷复合参数. 将这些参数结合已有公式,最终可获得每个晶体的电荷载体的扩散长度.     

微波法制备掺氮碳点及其用作探针检测铁离子

为了研究氮掺杂对纳米碳点荧光发射行为的影响和探索掺氮碳点的快速制备途径,以2-氨基对苯二甲酸为前驱体,与不同的修饰剂一起溶解于去离子水中,经微波辐射3 min,一步法合成了新型掺氮碳点。实验结果表明:制备的掺氮碳点水溶性好,发蓝色荧光,且发射行为不依赖于激发波长;颗粒近似为球形,尺寸5～8 nm,晶面间距为0.23 nm,接近石墨碳(100)面晶格结构;Fe~(3+)通过与掺氮碳点表面含氧基团的络合配位,可有效地猝灭其荧光,Fe~(3+)浓度在5～60μmol·L~(-1)的范围内与相对荧光强度呈现良好的线性关系,检出限约为1.01μmol·L~(-1),可以作为检测Fe~(3+)浓度的荧光探针。     

(110)取向的调制掺杂GaAs-AIGaAs单异质结的光致荧光谱

测量了含碳量高低不同的(110)取向的调制掺杂GaAs-Al_0.3Ga_0.7As单异质结在4.2K下的光致荧光谱。在含碳量高的样品中,出现了强的沟道二维电子到受主的复合荧光峰;而在含碳量很低的样品的光致荧光中则只出现体GaAs的荧光峰。     

FA离子重定向引起的电荷局域显著抑制了FAPbI3钙钛矿电子-空穴复合：时域从头算研究

本文利用从头算非绝热分子动力学结合时域密度泛函理论模拟,计算表明部分FA(FA=HC[NH₂]₂⁺)阳离子的重定向造成电子和空穴局域在不同位置,减小了非绝热耦合并抑制了原子运动,从而显著抑制了FAPbI₃的非辐射电子-空穴复合. 虽然慢的核运动同时增加了退相干时间,但是减小的非绝热耦合是影响电荷复合的主导因素,将电子-空穴复合的时间尺度延长至数纳秒,比原始的FAPbI₃激发态寿命长约3.9倍,与实验结果相符. 研究厘清了实验报导的FAPbI₃激发态寿命增加的机理,为设计高性能的钙钛矿太阳能电池和光电器件提供了合理的策略.     

γ辐照对掺Er硅酸盐玻璃吸收和发光特性的影响

用传统的高温熔融法熔制了一系列掺Er硅酸盐玻璃, 并测试了这些样品经5 kGy γ 射线辐照前后紫外至近红外的吸收和荧光光谱. 实验结果表明, 辐致暗化效应使得玻璃材料中形成了大量色心, 导致在400 nm附近出现强吸收带, 其尾端延伸至近红外区. 辐照产生的新能带增加了基质与Er³⁺ 特定能级(如²H_11/2, ⁴S_3/2及⁴F_9/2等) 之间的能量传递, 从而使辐照后的样品荧光寿命减小, 且在相同激发条件下荧光强度下降. 室温下辐照样品在荧光测试过程中出现了漂白现象.     

Properties of ion implanted silicon,sulfur, and carbon in gallium arsenide

Work is described in which chromium-doped semi-insulating gallium arsenide has been successfully doped n-type with ion implanted silicon and sulfur, and p-type with ion implanted carbon. A dilute chemical etch has been employed in conjunction with differential Hall effect measurements to obtain accurate profiles of carrier concentration and mobility vs. depth in conductive implanted layers. This method has so far been applied to silicon-and sulfur-implanted layers in both Cr-doped semi-insulating GaAs and high purity vapor grown GaAs. In the case of sulfur implants, a strong diffusion enhancement has been observed during the annealing, presumably due to fast-diffusing, implantation-produced damage. Peak doping levels so far obtained are about 8 × 10¹⁷ electrons/cm³ for silicon implants and 2 × 10¹⁷ electrons/cm³ for sulfur implants. Mobility recovery has been observed to be complete except in regions near the surface which are heavily damaged by the implantation.     

Pressure-dependent studies of the DX centre in Si- and Sn-doped n+GaAs

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the effect of hydrostatic pressure on the free electron concentration, mobility, and the occupancy of the DX centre in MBE grown n⁺GaAs heavily doped with either Si or Sn. The results show that the DX centre produces a resonant donor level between the Γ- and L- conduction band minima at a concentration comparable with the doping level. The position and occupancy of the DX centre are calculated using Fermi-Dirac statistics. For the Si-doped samples comparison with local vibrational mode measurements indicate that the DX level can be identified with Si_Ga.     

Laser-induced doping of GaAs

Thin layers of GaAs are heavily doped locally by laser induced Se or Zn diffusion. H₂Se or diethylzinc gases are used to provide Se or Zn dopant atoms. The surface is locally heated with 3 ns light pulses from a Q-switched frequency doubled Nd-YAG laser. The doping process is described in detail. Doping profiles and sheet carrier concentrations are measured as a function of substrate temperature, laser fluence and processing time. Dopant concentrations of more than 10²¹ cm^–3, with a thickness of the doped layer of less than 20 nm can be achieved.     

Investigation of gallium arsenide containing coppee as a material for photoresistor receivers

The properties of gallium arsenide doped with copper are investigated for different modes of diffusion, as a material for the sensitive elements of a photoresistor IR receiver. The procedure for obtaining specimens with the required parameters is established, and the material obtained is used to manufacture photoresistors. The threshold sensitivity P_t of a photoresistor receiver with microwave bias based on gallium arsenide doped with copper reaches the values obtained in photoreceivers based on germanium doped with gold, but the GaAs:Cu photoreceiver has a more rapid response. The detection capability D^*of a GaAs:Cu photoresistor receiver with microwave bias considerably exceeds the values of D^* of a photoreceiver with a constant bias using the same material.     

Lifetimes of high Rydberg states of methyl halides

The lifetimes of high Rydberg states of the methyl halides CH₃I, CH₃Br and CH₃Cl have been studied. A pulsed dye laser tuned near half the ionization energy was used, with preprogrammed field pulses, and the high Rydberg states of the compounds in selected ranges of n values were ionized after a variable delay. Plots of the electron signal versus delay yielded information about the Rydberg state lifetime. The clearest trend was found for around n = 120–160: the lifetimes of the states decreased by more than an order of magnitude from CH₃I (τ ≈12.3 μs) through CH₃Br to CH₃C1. Within the range studied, the theoretically predicted increase in lifetimes (n ⁵ or n ³) was not found, and in fact the highest group of states studied (with n > 215) exhibited for all three compounds relatively short lifetimes (≈2-3 μs), possibly due to collisional processes.     

Nanoscale order in GaAs:(B, Sb)

Nanoscale order caused by self-assembling of 1B4Sb and 4B10Sb clusters in GaAs:(B, Sb) is described. Self-assembling occurs in wide ranges of temperature and impurity concentration. Co-doping with boron and Sb isoelectronic impurities transforms GaAs into GaAs-rich B_xGa_1−xSb_yAs_1−y quaternary alloy. The self-assembling conditions are obtained from 0 to 800 °C with boron and Sb concentrations from x=1×10^–5 to x=2×10^–4 and from y=5×10^–4 to y=0.01, respectively. If Sb content is much larger than that of boron almost all boron atoms are in 1B4Sb clusters up to 800 °C and other boron impurities are isolated. If boron content is nearly equal or larger than that of Sb the formation of 4B10Sb clusters is preferential.     

The production of neutral kaons in Z0 decays and their Bose-Einstein correlations

Updated measurements of the B⁰ and B⁺ meson lifetimes are presented. From a data sample of 1.72 million hadronic Z⁰ decays recorded during the period 1991 to 1993, a sample of approximately 1000 semileptonic B meson decays containing a D⁰, D⁺ or D^*+ has been isolated. From the distribution of decay times in the different samples the lifetimes of the B⁰ and B⁺ mesons are determined to be 1.53±0.12±0.08 ps and 1.52±0.14±0.09 ps, respectively, where the first error is statistical and the second systematic. The ratio of the B⁺ to B⁰ lifetimes is measured to be 0.99±0.14 _–0.04 ^+0.05 , confirming expectations that the lifetimes are similar.     

Temperature accelerated dielectric breakdown of PECVD low-k carbon doped silicon dioxide dielectric thin films

The dielectric breakdown strength of carbon doped silicon dioxide thin films with thickness d from 32 nm to 153 nm is determined at 25 °C, 50 °C, 100 °C, 150 °C and 200 °C, using I–V measurements with metal-insulator-semiconductor (MIS) structures. It is found that the dielectric breakdown strength, E_B, decreases with increasing temperature for a given film thickness. In addition, a film thickness dependence of breakdown is also observed, which is argued to show an inverse relation to thickness d in the form of E_B∝(d-d_c)^-n. The exponential parameter n and critical thickness limit d_c also exhibit temperature dependent behavior, suggesting a temperature accelerated electron trapping process. The activation energy for the temperature acceleration was shown to be thickness dependent, indicating a thickness dependent conduction mechanism. It is thereafter demonstrated that for relatively thick films (thickness >50 nm), the conduction mechanism is Schottky emission. For relatively thin films (thickness <50 nm), the Schottky conduction mechanism was obeyed at low field region while FN tunnelling was observed as a prevail one in the high field region. PACS 73.40.Qv     

The efficiency and radiative lifetime concentration dependences for the direct band-gap GaAs-like semiconductors

Values of the radiative lifetimes in doped GaAs calculated within the framework of a simple quantum mechanical model are shown to be in good agreement with experimental values obtained for the active region material of the AlGaAs/GaAs heterostructures. Variations of the dominant recombination mechanism with the doping level are discussed. In particular, Auger nonradiative recombination mechanism is shown to play a prominent role in heavily doped p-type material.     

本征GaAs量子阱中电子自旋扩散输运的时-空分辨吸收光谱研究

发展了一种时-空分辨圆偏振光抽运-探测光谱及其理论,并用于本征GaAs量子阱中电子自旋扩散输运的实验研究.获得室温下本征GaAs量子阱中的“自旋双极扩散系数”为D_as=37.5±15 cm²/s.此结果比用自旋光栅法测量到的掺杂GaAs量子阱中电子自旋扩散系数小.解释为是由于“空穴库仑拖曳”效应减慢了电子自旋波包的扩散输运. 关键词： 时-空分辨抽运-探测光谱 电子自旋扩散 GaAs量子阱