Numerical study on strained InGaAsP/InGaP quantum wells for 850-nm vertical-cavity surface-emitting lasers

The physical and optical properties of compressively strained InGaAsP/InGaP quantum wells for 850-nm vertical-cavity surface-emitting lasers are numerically studied. The simulation results show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAsP quantum wells can be efficiently improved by employing a compressive strain of approximately 1.24% in the InGaAsP quantum wells. The simulation results suggest that the 850-nm InGaAsP/InGaP vertical-cavity surface-emitting lasers have the best laser performance when the number of quantum wells is one, which is mainly attributed to the non-uniform hole distribution in multiple quantum wells due to high valence band offset. PACS 42.55.Px; 78.20.-e; 78.20.Bh; 78.30.Fs     

Numerical study on optical and electrical properties of strained GaInSb/GaInAlSb mid-infrared quantum well laser

The optical and electrical properties of compressively strained GaInSb/GaInAlSb mid-infrared quantum well lasers are numerically studied solving one-dimensional Schrödinger equation using finite difference method. The simulation results demonstrate that band-mixing effects and effective mass of hole are reduced when the well is compressively strained. The strain-dependent optical and differential gains are evaluated for 0.6, 0.9, 1.21, and 1.52% compressively strained quantum well and found maximum when well is strained by 1.52%. The emission wavelength for the proposed laser can be tuned from 2.40 to 2.26 μm due to change in compressive strain from 0.60 to 1.52% at temperature 300 K. For the range of strain, the shift in wavelength is found from 2.38 to 2.24 μm at temperature 275 K. The results obtained from PSPICE simulation indicate that, the optical output power and threshold current are strongly depend on the number of wells and found to be almost constant for the number of wells three and above.     

Optical spectra in the region of exciton resonances in quantum wells and quantum dots of In0.3Ga0.7As/GaAs heterostructures

The transparency, reflection and luminescence spectra of In_0.3Ga_0.7As structures with 8 nm thickness and quantum wells limited by the barrier layer GaAs of a 9 nm (upper layer) and 100 nm (bottom layer) thickness had been studied in the region of photon energy 0.5–1.6 eV. Lines associated with the transitions hh,lh1-e1(1s,2s,3s), hh2,lh2-e2(1s,2s,3s), hh1,lh1-e2(1s) and hh3,lh3-e3(1s) had been revealed in reflection spectra. The shapes of the reflection and transparency lines had been calculated using a single oscillator model of dispersion relations and the Kramers–Kronig integrals. The binding energy of hh,lh1-e1 excitons, the effective mass m_hh^∗ and m_lh^∗ and the damping factor for the optical transitions to QW and QD had been determined. The lifetime of charge carriers on quantum dots varies in the range of 0.04–0.1 ps, while the radiative lifetime of excitons in quantum wells in the considered structure is around 2 ps.     

Theoretical modeling for quantum-confined Stark effect due to internal piezoelectric fields in GaInN strained quantum wells

Recent experimental investigations revealed that the biaxial stress in thin InGaN layers grown on thick GaN layer induces a large piezoelectric field along [0001] orientation that causes red-shift in optical transitions and reduction in oscillator strengths because of spatial separation of the electron and hole wave functions. In this Letter based on theoretical modeling we determined the well width z-dependent effect on red-shifted quantum-confined Stark effect (QCSE) in GaN/In_xGa_1 − xN (x=0.13) strained quantum well structures. Analyses are based on the solution of Schrödinger equation in a finite well including the internal piezoelectric electric field (F) due to the strained polarization as the perturbation potential. Our theoretical results show: (1) the red-shift in optical transition has a quadratic well-width form as it is for infinite wells (Davies, 1998) [1], (2) assuming the model based on a carrier effective mass dependence on the width of quantum wells, m^∗(z), fits the experimental data (Takeuchi et al., 1997) [2] much more accurate compare to the model with constant effective mass, m^∗.     

金属有机化学气相外延生长1310nm偏振无关混合应变量子阱半导体光放大器研究

采用低压金属有机化学气相外延设备进行了1.3μm压应变量子阱材料、张应变量子阱材料和混合应变量子阱材料的生长研究.通过x射线双晶衍射和光致发光谱对生长材料进行测试和分析.基于四个压应变量子阱和三个张应变量子阱交替生长的混合应变量子阱（4CW3TW）结构有源区，并采用7°斜腔脊型波导结构以有效抑制腔面反射，经蒸镀减反膜后，半导体光放大器光纤光纤小信号增益达21.5dB，在1280—1340nm波长范围内偏振灵敏度小于0.6dB. 关键词： 偏振无关 应变量子阱 半导体光放大器 减反膜     

Tensile-strained 1.3 μm InGaAs/InGaAlAs quantum well structure of high temperature characteristics

A new tensile strained InGaAs/InGaAlAs quantum well structure in the 1.3 μm wavelength region is proposed for high temperature characteristics via quantum well band structure and optical gain calculations. To obtain such features, a tensile-strained InGaAs/InGaAlAs quantum well structure, which emits light dominated by TM polarization, is considered. This proposed structure has very high temperature characteristics (T ₀ > 130 K) due to its high density of state at the first transition edge. This results clearly show the potential of tensile strained quantum well structure usage for the high temperature operation of quantum well semiconductor lasers.     

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.     

(001)和(111)取向的张应变量子阱光学特性的比较

以(001)和(111)价带Lutinger-Kohn哈密顿量为基础,讨论了这两种取向的量子阱的能带结构、态密度(DOS)、跃迁矩阵元和增益特性。由于材料结构的各向异性,(111)取向的量子阱在平行生长面内有较小的重空穴有效质量,无应变和压应变激光器可以很好地利用这一点。而(001)取向的量子阱虽有较小的平面内轻空穴有效质量,但并不比(111)量子阱的小多少;加上(001)量子阱的价带耦合效应强,使其DOS比(111)量子阱的大,在相同载流子注入下,张应变(111)量子阱的增益系数比相应(001)量子阱的要高。所以张应变(111)量子阱激光器仍然比相应的(001)量子阱激光器性能要好。可以认为匹配和压应变的(111)激光器优异的性能不仅来自小的面内有效质量,也来自于弱的价带耦合效应。     

光纤光栅的温度增敏实验

用热膨胀系数较大的聚合物材料对光纤光栅进行封装处理,极大地提高了光纤光栅的温度灵敏度。我们将光纤光栅封装于两种不同的聚合物材料中,其温度灵敏度分别提高6倍和23倍之多。这是迄今有文献报道的最大的光纤光栅温度灵敏度。     

Electronic structures of the zinc-blende GaN/Ga1−xAlxN compressively strained superlattices and quantum wells

The electronic structures of the zinc-blende GaN/Ga_0.85Al_0.15N compressively strained superlattices and quantum wells are investigated using a 6×6 Hamiltonian model (including the heavy hole, light hole and spin-orbit splitting band). The energy bands, wavefunctions and optical transition matrix elements are calculated. It is found that the light hole couples with the spin-orbit splitting state even at thek=0 point, resulting in the hybrid states. The heavy hole remains a pure heavy hole state atk=0. The optical transitions from the hybrid valence states to the conduction states are determined by the transitions of the light hole and spin-orbit splitting states to the conduction states. The transitions from the heavy hole, light hole and spin-orbit splitting states to the conduction states obey the selection rule Δn=0. The band structures obtained in this work will be valuable in designing GaN/GaAlN based optoelectronic devices.     

Low-density band-filling in strained InAs quantum wells

We study the low-temperature photoluminescence (PL) of strained InAs single quantum wells (SQWs) embedded in a Ga_0.47In_0.53As matrix grown on InP substrates by modified solid-source molecular beam epitaxy. The spectra are interpreted in the frame of a two-level rate equation model describing the carrier dynamics in the structures. We show that band-filling occurs in these QWs for an excitation power as low as 30 Wcm^–2. Moreover, the spectra reveal that the band-filling results from the rapid population of the hole subband. This observation highlights the low in-plane heavy-hole mass in the compressively strained film. Our results therefore demonstrate the high potential of InAs/Ga_0.47In_0.53As QW nonlinear optical devices operating in the mid-IR wavelength range.     

Hydrostatic pressure dependence of excitonic optical transitions in strained wurtzite GaN/AlN quantum disks

In the framework of the effective mass approximation, the effects of hydrostatic pressure on optical transitions associated with the excitons confined in strained wurtzite (WZ) GaN/AlN quantum disks (QDisks) with the confinement potential of finite depth are investigated by using a variational technique, with considering the influences of the built-in electric field (BEF) and the biaxial strain dependence of material parameters. The Schrödinger equation via the proper choice of the exciton trial wave function is solved. The behaviors of the excitonic optical transition are examined at different pressures for different QDisk sizes. In our calculations, the effective masses of electron and hole, dielectric constants, phonon frequencies, energy gaps, and piezoelectric polarizations are taken into account as functions of biaxial strain and hydrostatic pressure. Numerical results show that the hydrostatic pressure and the QDisk size have a remarkable influence on exciton states. The calculated pressure coefficient of optical transition energy shows a negative value if the QDisk height L > 3.2 nm, in contrast with the positive pressure coefficient of the GaN band gap. The peculiar pressure behavior is related to the pressure-induced increase of the built-in electric field. For a fixed pressure, the optical transition energy has a red-shift if the QDisk height and radius increase and QDisk height has a more obvious influence on E_ph than QDisk radius. Furthermore, the relationship between the radiative decay time and hydrostatic pressure (QDisk height) is also investigated. It is found that the radiative decay time increases with pressure and the increment tendency is more prominent for the large height QDisks. The radiative decay time strongly increases by three orders of magnitude reaching microsecond order if the QDisk height increases from 1 nm to 3 nm.     

Comparison of two kinds of active layers for high-power narrow-stripe distributed feedback laser diodes

Usually GaAs/AlGaAs is utilized as an active layer material in laser diodes operating in the spectral range of 800--850 nm. In this work, in addition to a traditional unstrained GaAs/AlGaAs distributed feedback (DFB) laser diode, a compressively strained InGaAlAs/AlGaAs DFB laser diode is numerically investigated in characteristic. The simulation results show that the compressively strained DFB laser diode has a lower transparency carrier density, higher gain, lower Auger recombination rate, and higher stimulated recombination rate, which lead to better a device performance, than the traditional unstrained GaAs/AlGaAs DFB laser diode.     

Band gap and mass renormalisation in GaInP/AlGaInP quantum wells

We report magneto-luminescence studies of dense electron–hole plasmas in compressively strained GaInP/AlGaInP quantum wells. At carrier densities of order 10¹³ cm⁻² many-body effects are investigated. The band gap is reduced by up to 36 meV, much less than expected from conventional random phase approximation calculations. However, much better agreement is obtained on comparison with recent density functional calculations which include the Hartree energy. The reduced mass is found to increase by up to 40% over values from low-excitation measurements. Comparison with other work suggests that the mass renormalisation is substantial in materials with wide band gaps.     

Spectral properties of Nd:Ca2Nb2O7 crystal

This paper reports the spectral properties of Nd³⁺:Ca₂Nb₂O₇. The spectral parameters of Nd³⁺ in Nd³⁺:Ca₂Nb₂O₇ crystal have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained. The parameters of line strengths Ω_λ are Ω₂=4.967×10⁻²⁰ cm², Ω₄=5.431×10⁻²⁰ cm², Ω₆=5.693×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 122 μs, 103 μs and 84.4%, respectively. The fluorescence branch ratios calculated: β₁=0.425, β₂=0.479, β₃=0.091, β₄=0.004. The emission cross section at 1068 nm is 6.204×10⁻²⁰ cm².     

Numerical study on gain and optical properties of AlGaInAs, InGaNAs, and InGaAsP material systems for 1.3-μm semiconductor lasers

In reference to real devices fabricated in laboratories, the optical properties of AlGaInAs, InGaNAs, and InGaAsP semiconductor material systems for 1.3-μm semiconductor lasers are systematically studied. Simulation results show that both the AlGaInAs/InP and InGaNAs/GaAs material systems have better gain performance and smaller transparency carrier density than the InGaAsP/InP material system. For the AlGaInAs/InP material system, the characteristic temperature is improved by using compensating tensile strain in barrier. Specifically, for a 250-μm-long short-cavity AlGaInAs/InP laser, when the barrier is with a compensating tensile strain of 0.39%, the characteristic temperatures in 290-330 K and 330-350 K can be enhanced to 121.7 K and 58.9 K, respectively. For the InGaNAs/GaAs material system, simulation results suggest that the laser performance can be significantly improved when the laser is with strain-compensated GaNAs barriers.     

The luminescence and optoelectrical properties of ITO films prepared by a sputter type negative metal ion deposition

Transparent conducting indium tin oxide (ITO) thin films were prepared on glass substrates by a magnetron sputter type negative ion source which requires cesium (Cs) vapor injection for surface negative ionization on the ITO target surface. Although the film was prepared at 70 °C, it attained high optical transmittance, 88% and low resistivity, 2.03 × 10⁻⁴ Ω cm, at an optimized Cs partial pressure of P_Cs = 1.7 × 10⁻³ Pa. The as-deposited ITO films have a poly-crystalline structure with (2 1 1), (2 2 2), (4 0 0), (4 1 1) and (4 4 0) reflections.Also, ITO films prepared at P_Cs = 1.7 × 10⁻³ Pa were post-deposition vacuum annealed at 300 °C for 30 min. The films had a resistivity of 1.8 × 10⁻⁴ Ω cm and a transparency of 89.2%. The post-deposition vacuum annealed ITO film was used as an anode for a transparent organic light emitting diode (TOLED). A maximum luminance of 19,000 cd/m² was obtained.     

Annealing effects on the bonding structures, optical and mechanical properties for radio frequency reactive sputtered germanium carbide films

The effects of thermal annealing in vacuum on the bonding structures, optical and mechanical properties for germanium carbide (Ge_1−xC_x) thin films, deposited by radio frequency (RF) reactive sputtering of pure Ge(1 1 1) target in a CH₄/Ar mixture discharge, are investigated. We find that there are no significant changes in the bonding structure of the films annealed below 300 °C. The fraction of Ge-H bonds for the film annealed at temperatures (T_a) above 300 °C decreases, whereas that of C-H bonds show a decrease only when T_a exceeds 400 °C. The out-diffusion of hydrogen promotes the formation of Ge-C bonds at T_a above 400 °C and thus leads to a substantial increase in the compressive stress and hardness for the film. The refractive indices and optical gaps for Ge_1−xC_x films are almost constant against T_a, which can be ascribed to the unchanged ratios of Ge/C and sp²-C/sp³-C concentrations. Furthermore, we also find that the excellent optical transmission for an antireflection Ge_1−xC_x double-layer film on ZnS substrate is still maintained after annealing at 700 °C.     

The structure and optical properties of evaporated Samarium fluoride films

Samarium fluoride (SmF₃) films have been deposited on quartz, silicon and germanium substrates by vacuum evaporation method. The crystal structure of the films deposited on silicon substrate is examined by X-ray diffraction (XRD). The films deposited at 100 °C, 150 °C and 250 °C have the (1 1 1) preferred growth orientation, but the film deposited at 200 °C has (3 6 0) growth orientation. The surface morphology evolution of the films with different thickness is investigated with optical microscopy. It is shown that the microcrack density and orientation of thin film is different from that of thick film. The transmission spectrum of SmF₃ films is measured from 200 nm to 20 μm. It is found that this material has good transparency from deep violet to far infrared. The optical constants of SmF₃ films from 200 nm to 12 μm are calculated by fitting the transmission spectrum of the films using Lorentz oscillator model.