Determination of the LO-phonon and Γ→L intervalley scattering time in GaAs from hot electron luminescence spectroscopy

Both the LO-phonon scattering time and the Γ→L intervalley scattering time for electrons in the conduction band of GaAs are of fundamental importance, and they are needed for the modelling of devices. We measure the steady state distribution of hot electrons in lightly p-doped bulk GaAs under carrier densities of 10¹³–10¹⁴cm⁻³, which is orders of magnitude lower than in pulsed laser experiments. Using a 16×16 k.p Hamiltonian and taking into account the transition matrix elements in a dipole model, we determine the hot electron lifetime from comparison with the experimentally found lifetime broadening. For electrons with a kinetic energy of 100meV to 300meV we obtain τ_LO=(132±10)fs. We also determine the Γ→L intervalley separation as E_ΓL=(300±10)meV. We find Γ→L scattering times around 150fs to 200fs, corresponding to a value for the associated deformation potential of D_ΓL=(9.4±1.5)×10⁸eV/cm.     

Interplay between the hot phonon effect and intervalley scattering on the cooling rate of hot carriers in GaAs and InP

The influence of hot phonon effect and intervalley scattering on the hot carrier cooling rate was investigated using femtosecond time‐resolved photoluminescence spectroscopy in bulk GaAs and InP, two electronically similar but vibrationally distinct semiconductors. In both materials, a broad photoluminescence signal that extends from the band gap energy to values larger than the pump pulse energy was observed during the first few picoseconds after photoexcitation, for different excitation energies (1.7, 1.88, and 2.4 eV) at high carrier densities (>10¹⁹ cm⁻³). Different hot carrier relaxation times were observed in GaAs and InP for different excitation energies, demonstrating the influence of intervalley scattering phenomena in GaAs. When electrons were not energetic enough to access satellite valleys, longer decay transients were observed for InP compared with GaAs. This provides experimental evidence of the hot phonon effect in InP. Temperature transients were calculated by analyzing the topography of the two‐dimensional spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

强THz场下GaAs和InSb中瞬态谷间散射和碰撞电离

运用系宗蒙特卡罗法计算了强THz场作用下, n型掺杂的GaAs和InSb中随时间变化的散射机制以及载流子非线性动力学演变, 获取了电子散射至卫星谷并弛豫回原能谷的时间信息, 并追踪描绘了载流子瞬态增加的过程, 结果同时显示了强场作用下谷间散射是GaAs中的主要散射机制, 而碰撞电离则是InSb中的关键因素.此外进一步讨论了这两种机制对于相关物理量: 平均动能、平均速度、材料的电导率的影响, 结果说明这两种机制导致了非线性效应并在两种材料中起到相反的作用, InSb中碰撞电离的响应时间比GaAs中谷间散射的响应时间更长.该研究结果在THz调制领域有一定的指导意义.     

The intervalley X6→Γ6, L6 scattering time in GaAs measured by ultrafast pump-probe infrared absorption spectroscopy

A direct measurement of the dynamics of electrons in the X₆ valley for a GaAs crystal by time-resolved absorption spectroscopy is reported for the first time. IR picosecond probe pulses were used to monitor the growth and decay of the population in the X₆ valley subsequent to excitation by a 527 nm pump pulse. The intervalley X₆→Γ₆, L₆ scattering time t_x of 0.70 ± 0.50 ps is determined and the crossection for the X₆→X₇ transition is estimated to be 1.8 × 10⁻¹⁶ cm².     

Time-resolved raman scattering measurement of electron-optical phonon intersubband relaxation in GaAs quantum wells

We have measured the lifetime of electronic intersubband excitations by emission of longitudinal optical phonons in GaAs---AlGaAs quantum wells using time-resolved anti-Stokes Raman scattering. We find the lifetime of electrons in the N = 2 level of a 146Å well to be 1ps which is in excellent agreement with theoretical calculations of two-dimensional electron-photon interactions.     

Picosecond photoluminescence measurements of hot carrier relaxation and auger recombination in GaSb

We have measured hot carrier relaxation and recombination of photoexcited carriers in GaSb using picosecond time-resolved photoluminescence. We find that these processes are greatly modified compared to other III–V materials by the conduction band structure which permits a large electron population at the L-minima. Rapid carrier cooling observed at early times is explained by efficient relaxation of electrons within the L-valleys. For carrier densities 10¹⁹cm⁻³ we find that the dominant recombination process at low temperatures (4K) is radiative, but at hihg temperatures Auger recombination becomes dominant. This is manifest in a dramatic reduction of luminescence intensity, faster recombination and very slow energy relaxation.     

Full dynamic screening calculation of hot electron scattering rates in multicomponent semiconductor plasmas

The total scattering rate of a single energetic electron in the conduction band of GaAs in the presence of a non-equilibrium electron-hole plasma is calculated using different approximations for the screening effects of the free carriers. The best agreement with recent experimental results is obtained when full dynamic screening is taken account of (exactly within the random phase approximation), including the lattice polarizability.     

Rapid capless annealing of28Si,64Zn,and9Be implants in GaAs

We report the use of tungsten-halogen lamps for rapid (−10 s) thermal annealing of ion-implanted (100) GaAs under AsH₃/Ar and N₂ atmospheres. Annealing under flowing AsH₃/Ar was carried out without wafer encapsulation. Rapid capless annealing activated implants in GaAs with good mobility and surface morphology. Typical mobilities were 3700–4500 cm²/V-s for n-layers with about 2×10¹⁷cm⁻³ carrier concentration and 50–150 cm²/v-s for 0.1–5xl0¹⁹ cm⁻³ doped p-layers. Rapid thermal annealing was performed in a vertical quartz tube where different gases (N₂, AsH₃/H₂, AsH₃/Ar) can be introduced. Samples were encapsulated with SiO when N₂ was used. Tungsten-halogen lamps of 600 or 1000 W were utilized for annealing GaAs wafers ranging from 1 to 10 cm² in area and 0.025 to 0.040 cm in thickness. The transient temperature at the wafer position was monitored using a fine thermocouple. We carried out experiments for energies of 30 to 200 keV, doses of 2×10¹² to 1×10¹⁵ cm⁻², and peak temperatures ranging from 600 to 1000‡C. Most results quoted are in the 700 to 870‡C temperature range. Data on implant conditions, optimum anneal conditions, electrical characteristics, carrier concentration profiles, and atomic profiles of the implanted layers are described. Presented at the 25th Electronic Materials Conference, Burlington, VT, June 22, 1983.     

Quantitative luminescence mapping of Cu(In,Ga)Se2 thin‐film solar cells

We investigate photoluminescence and electroluminescence (PL and EL) emission images from Cu(In,Ga)Se₂‐based solar cells by means of a Hyperspectral Imager. Using the generalized Planck's law, maps of the effective quasi‐Fermi level splitting Δμ_eff in absolute values are obtained. A good agreement is found between the spatially averaged splitting in PL and the global open‐circuit voltage. However, from a local carrier transport discussion, we conclude that the equality does not hold locally. The spatial variations are rather attributed to local depth variations of the quasi‐Fermi level splitting due to material properties spatial fluctuations. By comparing PL and EL emissions, we discuss qualitatively the local effective lifetimes and collection efficiencies. Copyright © 2014 John Wiley & Sons, Ltd.     

Femtosecond studies of intervalley scattering in GaAs and AlxGa1-xAs

Results are presented from ensemble Monte Carlo simulations of the relaxation of photoexcited electrons and holes. The results are compared directly with three types of femtosecond optical experiments: transient absorption saturation, pump and continuum probe, and tunable pump-probe experiments. For these experiments we find that intervalley scattering has a dominant effect for the first several hundred femtoseconds, and that electron-electron scattering is only important at later times.     

Growth methods,enhanced photoluminescence,high hydrophobicity and light scattering of 4,4′-bis(1,2,2-triphenylvinyl)biphenyl nanowires

4,4′-bis(1,2,2-triphenylvinyl)biphenyl (BTPE) nanowires have been facilely grown by either post-annealing its vacuum-sublimed amorphous film or slowly evaporating its chloroform solution. The morphology and density of the nanowires can be easily tuned by changing the growth conditions. For example, isolated single nanowires were readily obtained by thermally depositing the BTPE on a heated substrate, while bundles of nanotubes can be achieved by slowly evaporating a droplet of the BTPE solution. The self-assembled BTPE nanowires show enhanced and blue-shifted photoluminescence compared with that of its amorphous film. The nanowires exhibit a highly hydrophobic surface with a water contact angle of 126°. Also, the BTPE nanowires scatter the light effectively due to the random orientation of the wires. By depositing the BTPE nanowires on the backside of the glass substrate as a scattering media for organic light-emitting diodes, a 31.5% efficiency improvement has been achieved.     

Photoluminescence and raman scattering characterization of silicon-doped In0.52Al0.48As grown on InP (100) substrates by molecular beam epitaxy

Growth of In_0.52Al_0.48As epilayers on InP (100) substrates by molecular beam epitaxy at different silicon doping levels is carried out. The doped samples show an inverted S-shaped dependence of the PL peak energy variation with the temperature which weakens at high doping levels due to a possible reduction in the donor binding energy. There is a reduction in both the AlAs-like and InAs-like longitudinal-optic (LO) phonon frequencies and a broadening of the LO phonon line shape as the doping level is increased. The PL intensity also showed in increasing degrees at higher doping levels, a temperature dependence which is characteristic of disordered and amorphous materials.     

Microstructural Aspects of Nucleation and Growth of (In,Ga)As-GaAs(001) Islands with Low Indium Content

Molecular beam epitaxy growth of multilayer In _x Ga_1-x As/GaAs(001) structures with low indium content (x = 0.20–0.35) was studied by X-ray diffraction and photoluminescence in order to understand the initial stage of strain-driven island formation. The structural properties of these superlattices were investigated using reciprocal space maps, which were obtained around the symmetric 004 and asymmetric 113 and 224 Bragg diffraction, and ω/2θ scans with a high-resolution diffractometer in the triple axis configuration. Using the information obtained from the reciprocal space maps, the 004 ω/2θ scans were simulated by dynamical diffraction theory and the in-plane strain in the dot lattice was determined. We determined the degree of vertical correlation for the dot position (“stacking”) and lateral composition modulation period (LCM) (lateral ordering of the dots). It is shown that initial stage formation of nanoislands is accompanied by LCM only for [110] direction in the plane with␣a period of about 50 to 60 nm, which is responsible for the formation of a quantum wire like structure. The role of In _x Ga_1-x As thickness and lateral composition modulation in the formation of quantum dots in strained In _x Ga_1-x As/GaAs structures is discussed.     

YPO4:Ln3+(Ln=Eu,Tb)球形荧光粉的合成及其发光性能优化

采用水热法结合高温退火处理制备了YPO₄:Ln³⁺(Ln=Eu,T b)荧光粉。通过X射线 粉末衍射(XRD)、扫描电镜(SEM)和荧光光谱(PL)对样品的结构、形貌和发光性能进行 表征。结果 表明:在水热条件下合成了含结晶水六角相结构的YPO₄·0.8H₂O :Eu³⁺和YPO₄·0.8H₂O:Tb³⁺前躯体;经过 800 ℃高温烧结2h后,前躯体 失去结晶水后得到球形、尺寸均一、表面光滑、四方锆石结构的YPO₄:Eu³⁺和 YPO₄:Tb³⁺荧光粉,颗粒平均粒径约为200 nm。在396 nm波长激发下,YPO₄:Eu³⁺荧光粉可以获得Eu³⁺离 子的跃迁能级⁵D₀→⁷FJ(J=1－4)特征发射,以磁偶极跃迁⁵D₀→⁷F₁(596 nm)的发光强度最强,观察到橙 红色发射,且Eu³⁺的最佳掺杂摩尔分数为11%。同时,YPO₄: Tb³⁺荧光粉在372 nm的光激发下,在548 nm 处的⁵D₄→⁷F₅跃迁具有最高的荧光强度,观察到绿光发 射,且Tb³⁺最佳掺杂摩尔分数为7%。     

Raman scattering study of the immiscible region in InGaAsP grown by LPE on (100) and (111) GaAs

We have studied the asymmetric broadening of the Raman spectra of In_xGa_1-xAs_yP_1-y grown on GaAs substrates by means of the spatial correlation model. The broadening phenomena was found to be enhanced in the region of immiscibility, in agreement with PL observations. In Raman measurements, the broadening is more enhanced in the samples grown on (100) substrates than in the samples grown on (111) substrates. The enhancement is attributed to the immiscibility included in the samples grown on (100) substrate.     

Low energy ion scattering: surface preparation and analysis of Cu(In,Ga)Se2 for photovoltaic applications

Cu(In,Ga)Se₂ (CIGS) single‐crystal epitaxial films have been analyzed by low energy ion scattering, which is sensitive to exactly the outermost surface atomic layer, to determine the surface chemistry as a function of preparation conditions. The samples were grown by a hybrid sputtering and evaporation method on cation (A) or anion (B) terminated (111) GaAs substrates and had smooth surfaces. The samples were exposed to excited atomic oxygen or hydrogen beams or were sputtered with 500 eV Ar⁺ ions. Atomic O* treatment resulted in an otherwise clean, oxidized surface including all film constituents. Atomic H* resulted in strong enhancement of the surface Ga population, probably due to a preexisting Ga native oxide in the outermost atomic layer. Sputtering produced a clean surface that was closest to the bulk composition of the film as measured by energy‐dispersive spectroscopy. Copyright © 2014 John Wiley & Sons, Ltd.     

Polarized Optoelectronics of CsPbX3 (X = Cl,Br, I) Perovskite Nanoplates with Tunable Size and Thickness

Low dimensional semiconductor nanomaterials have shown their tailorable properties for a variety of promising applications in decades. Here a general strategy to synthesize all‐inorganic CsPbX₃ (X = Cl, Br, I or their mixture) perovskite 2D nanoplates by introducing additional metal halides MX'₂ or MX'₃ (M = Cu, Zn, Al or Pb, etc.; X' = Cl, Br or I) is reported. These CsPbX₃ perovskite nanoplates have uniform thickness and tunable size, which can be feasibly controlled by the component and ratio of the metal halides, temperature, time, and ligands. The well‐defined morphology of the nanoplates makes them ideal building blocks for the self‐assembly in the face‐to‐face and column‐by‐column arrangement. Compared to the optically isotropic CsPbX₃ nanocubes, the 2D CsPbX₃ nanoplates exhibit remarkable polarized UV–vis absorption and photoluminescence not only in liquid solvent and solid resin matrix, but also in self‐assembled films. An optoelectronic photodetector sensitive for linear polarized light is fabricated to demonstrate the proof‐of‐concept.     

Low‐Temperature Absorption,Photoluminescence, and Lifetime of CsPbX3 (X = Cl,Br, I) Nanocrystals

The absorption and photoluminescence, both steady‐state and time‐resolved, of CsPbX₃ (X = Cl, Br, I) nanocrystals are reported at temperatures ranging from 3 to 300 K. These measurements offer a unique window into the fundamental properties of this class of materials which is considered promising for light‐emitting and detection devices. The bandgaps are shown to increase from low to high temperature, and none of the examined cesium‐based perovskite nanocrystals exhibit a bandgap discontinuity in this temperature range suggesting constant crystal phase. Time‐resolved measurements show that the radiative lifetime of the band‐edge emission depends strongly on the halide ion and increases with heating. The increasing lifetime at higher temperatures is attributed primarily to free carriers produced from exciton fission, corroborated by the prevalence of excitonic character in absorption. The results particularly highlight many of the similarities in physical properties, such as low exciton binding energy and long lifetime, between CsPbI₃ and hybrid organic–inorganic plumbotrihalide perovskites.     

Optical properties and thermal transport of carriers in (Zn,Cd)Se-ZnSe heterostructures

We report a detailed optical study of ZnSe-based graded index separate confinement heterostructures. These structures were grown by metalorganic vapor phase epitaxy and are composed of either one or two Zn_0.79Cd_0.21Se central well(s) embedded between two ZnCdSe barriers which cadmium composition varies linearly from 5% near the wells to 0% at the end of the barriers. 2K photoreflectance and reflectivity experiments allow the observation of excitonic transitions involving the third electron and heavy hole confined states. The temperature dependence of the photoluminescence lines under in-well resonant excitation conditions (E_exc = 2.661 eV) shows that the thermal quenching of the photoluminescence line is ruled by nonradiative recombinations on defects localized at the heterointerfaces at low temperature and by the thermal escape of the minority carriers at higher temperatures. Under above-barrier excitation conditions (E_exc=3.814 eV), the temperature dependence of the photoluminescence line from the well shows a strong influence of the mechanism of diffusion of the carriers from the barriers to the well.