The excition tunneling in ZnCdSe/ZnSe asymmetric double quantum well

Photoluminescence spectra of asymmetric double-quantum-well structure are studied in this paper. We show the excitation power dependence of exciton tunneling. Due to the different tunneling time of electrons and holes, space-charge effect is observed.     

Exciton spectroscopy on single CdSe/ZnSe quantum dot photodiodes

We have investigated the properties of neutral and charged excitons in single CdSe/ZnSe QD photodiodes by μ-photoluminescence spectroscopy. By applying a bias voltage, we have been able to control the number of electrons in a single QD by shifting the energy levels of the QD with respect to the Fermi level in the back contact. Also the quantum-confined Stark effect was observed as a function of the applied electric field.     

Magnetic resonance studies of InGaN-based quantum well diodes

Photoluminescence (PL) based optically detected magnetic resonance (ODMR) studies as well as electroluminescence detected and electrically detected magnetic resonance (ELDMR and EDMR, respectively) measurements of In_xGa_1−xN quantum wells were performed. In the ODMR, two PL-enhancing resonances were observed: an electron resonance and a hole resonance. The electron resonance is consistent with expectations for the g value in bulk In_xGa_1−xN for x ≈ 0.4 but deviates significantly in an x≈0.3 sample. Possible reasons for this include the effects of strain and confinement. The hole resonance is qualitatively similar to observations in Mg-doped GaN, but more isotropic in the x ≈ 0.3 diode than in the x ≈ 0.4 sample. We measure relatively long radiative lifetimes (as long as ∼0.2 ms) in the ODMR which facilitate the observation of the resonances and indicate that the electron and hole are spatially separated either by potential fluctuations within the quantum well or by the trapping of the hole at an acceptor in the player of AlGaN whch serves as one of the confining barriers. In the EDMR and ELDMR experiments, the signal is primarily due to a reduction in the nonradiative recombination at resonance. While the ODMR is alwyas emission-enhancing, the ELDMR is luminescence-quenching, supporting the notion that techniques are probing different centers.     

MBE Growth of lattice-matched ZnCdMgSe quaternaries and ZnCdMgSe/ZnCdSe quantum wells on InP substrates

We report the growth and characterization of a new wide bandgap II-VI alloy, Zn_xCd_yMg_1-x-ySe, grown lattice-matched to InP. High quality quaternary layers with bandgaps ranging from 2.4 to 3.1 eV were grown by molecular beam epitaxy. The bandgaps and lattice constants were measured using photoluminescence and single crystal Θ-2Θ scans. Quantum well structures with quaternary barriers and ZnCdSe wells were also grown, entirely lattice matched to InP. Their photoluminescence properties suggest that these materials are suitable for the design of visible semiconductor lasers spanning the blue, green, and yellow regions of the visible range. The absence of strain in these heterostructures is expected to improve the reliability of the materials in device applications.     

Prospects and challenges of silicon/germanium on-chip optoelectronics

On-chip optoelectronics allows the integration of optoelectronic functions with microelectronics. Recent advances in silicon substrate fabrication (silicon-on-insulator (SOI)) and in heterostructure engineering (SiGe/Si) push this field to compact (chipsize) waveguide systems with high-speed response (50-GHz subsystems realized, potential with above 100 GHz). In this paper, the application and requirements, the future solutions, the components and the physical effects are discussed.A very high refractive index contrast of the waveguide Si-core/SiO2-cladding is responsible for the submicron line widths and strong bendings realized in chipsize waveguide lines and passive devices. The SiGe/Si heterostructure shifts the accessible wavelength into infrared up to telecommunication wavelengths 1.30-1.55 μm. Germanium, although also an indirect semiconductor as silicon, offers direct optical transitions which are only 140 meV above the dominant indirect one. This is the basic property for realizing high-speed devices for future above 10 GHz on-chip clocks and, eventually, a laser source monolithi-cally integrated on the Si substrate.     

Bright CdSe quantum dot inserted in single ZnSe nanowires

We report the evidence of CdSe quantum dot (QD) insertion in single defect-free ZnSe nanowire. These nanowires have been grown by molecular beam epitaxy in vapour-liquid-solid growth mode catalysed with gold particles. We developed a two-step process allowing us to grow very thin (from 15 to 5 nm) defect-free ZnSe nanowire on top of a nanoneedle, where all defects are localised. The CdSe QDs are incorporated to the defect-free nanowires part. Owing to the extraction efficiency of the nanowires and the reduced number of stacking fault defects in the two-step-process nanowires, a very efficient photoluminescence is observed even on isolated single nanowire. Time-resolved photoluminescence and correlation photon give evidences that the bright photon emission is related to the CdSe QD.     

Two electron transitions of the exciton bound at the Si Donor confined in GaAs/AlxGa1-xAs quantum wells

The exciton bound to the shallow Si-donor confined in a 100A wide GaAs quantum well has been studied in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. The transition from the ground state, ls(Γ₆), to the first excited state, 2s(Γ₆), of the confined Si donor has been observed via two-electron transitions (TETs) of the donor bound exciton observed in SPL for the first time to the best of our knowledge. The interpretation of the TET peaks is confirmed by PLE measurements. Further, from Zeeman measurements, the magnetic field dependence of the donor ls(Γ₆)-2s(Γ₆) transition energy has been determined.     

耦合GaN/AlxGa1-xN量子点中的激子特性

在有效质量近似下，运用变分方法，考虑到量子点内电子和空穴的三维束缚以及由压电极化和自发极化所引起的内建电场，对圆柱型耦合GaN量子点的光学性质及激子态做了研究。给出了激子结合能Eb、量子点发光波长λ、电子-空穴复合率和量子点高度L^GaN以及势垒层厚度L^AlGaN之间的函数关系。结果表明，量子点高度LG^GaN、势垒层厚度L^AlGaN的增加将导致激子结合能、电子-空穴复合率的降低，耦合量子点发光波长的增加。     

Optical properties and structure of InAs quantum dots in near-infrared band

The InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are studied as a function of growth temperature at a specific InAs coverage of 2.7 ML. The QDs density is significantly reduced from 8.0 × 1010 to 5.0 × 109 cm-2 as the growth temperature increases from 480℃ to 520℃, while the average QDs diameter and height becomes larger. The effects of the growth temperature on the evolution of bimodal QDs are investigated by combining atomic force microscopy (AFM) and photoluminescence (PL). Results show that the formation of the bimodal QDs depends on the growth temperature: at a growth temperature of 480℃,large QDs result from the small QDs coalition; at a growth temperature of 535℃, the indium desorption and InAs segregation result in the formation of small QDs.     

Photoluminescence study on coarsening of self-assembled InAlAs quantum dots on GaAs (001)

Red-emission at ∼640 nm from self-assembled In_0.55Al_0.45As/Al_0.5Ga_0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy (MBE) has been demonstrated. We obtained a double-peak structure of photoluminescence (PL) spectra from quantum dots. An atomic force micrograph (AFM) image for uncapped sample also shows a bimodal distribution of dot sizes. From the temperature and excitation intensity dependence of PL spectra, we found that the double-peak structure of PL spectra from quantum dots was strongly correlated to the two predominant quantum dot families. Taking into account quantum-size effect on the peak energy, we propose that the high (low) energy peak results from a smaller (larger) dot family, and this result is identical with the statistical distribution of dot lateral size from the AFM image.     

Determination of the direction of piezoelectric field in InGaN/GaN multiple quantum-well structures grown by metal-organic chemical vapor deposition

The direction of the piezoelectric field in InGaN/GaN multiple quantum-well (MQW) structures grown by metal-organic vapor deposition (MOCVD) was determined using excitation-power-density variable photoluminescence (PL). By comparing the excitation-power-density dependence of the shift of the PL peak and the change of the full-width at half-maximum (FWHM) of the peak from an InGaN/GaN MQW structure and an InGaN MQW-based light-emitting diode (LED), the piezoelectric field in the InGaN/GaN MQW structures was unambiguously determined to be pointing toward the substrate. This result helps to identify the surface polarity of the LED wafer as Ga-faced.     

MBE growth of tensile-strained Ge quantum wells and quantum dots

Germanium (Ge) has gained much interest due to the potential of becoming a direct band gap material and an efficient light source for the future complementary metal-oxide-semiconductor (CMOS) compatibl...     

Deep level electronic structure of ZnSe/GaAs heterostructures

We performed 1—2 keVcathodoluminescence measurements and He-Ne and HeCd excited photoluminescence studies of ZnSe/GaAs( 100) heterostructures grown by molecular beam epitaxy. Our goal was to investigate the deep level electronic structure and its connection with the heterojunction band offsets. We observed novel deep level emission features at 0.8, 0.98, 1.14, and 1.3 eV which are characteristic of the ZnSe overlayer and independent in energy of overlayer thickness. The corresponding deep levels lie far below those of the near-bandedge features commonly used to characterize the ZnSe crystal quality. The relative intensity and spatial distribution of the deep level emission was found to be strongly affected by the Zn/Se atomic flux ratio employed during ZnSe growth. The same flux ratio has been shown to influence both the quality of the ZnSe overlayer and the band offset in ZnSe/GaAs heterojunctions. In heterostructures fabricated in Se-rich growth conditions, that minimize the valence band offset and the concentration of Se vacancies, the dominant deep level emission is at 1.3 eV. For heterostructures fabricated in Zn-rich growth conditions, emission by multiple levels at 0.88,0.98, and 1.14 eV dominates. The spectral energies and intensities of deep level transitions reported here provide a characteristic indicator of ZnSe epilayer stoichiometry and near-interface defect densities.     

InP-based multiple quantum well structures grown with tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP): Effects of growth interruptions on structural and optical properties

In this paper, we investigate the effect of interfacial layers on GalnAs(P)/GalnAsP and GalnAs/InP multiple quantum well structures with x-ray diffraction and photoluminescence. We observe a decrease in the room temperature and low temperature photoluminescence intensity as the number of periods is increased which we attribute to the interfaces. Furthermore, different growth interruption schemes show that decomposed As species from TBA have an effect on the structural and optical quality of these structures at both the lower and upper interfaces due to As carry-over. The effect of this carry-over is shown in structural measurements and laser diode results.     

GaAlN/GaN量子阱中电子的激发态极化及其压力效应

考虑了纤锌矿结构材料的各向异性造成的内建电场的作用以及各向异性造成的应变张量和静压形变势与各向同性材料的差别.在此基础上计算了GaN/GaAIN量子阱内电子的激发态极化.研究了压力(应变)对电子激发态极化的影响.结果表明,电子势垒高度、电子有效质量和电子激发态极化均随压力线性下降,但由于内建电场的作用造成电子波函数高度局域化,上述变化的幅度不大.     

用PL谱评测多量子阱红外探测器外延材料

对GaAs/AlGaAs多量子阱红外探测器外延材料进行了光致荧光谱(PL)测量,结合理论计算,由材料吸收峰位置得到势垒高度以及势阱基态位置,并由此推算出相应的红外探测响应波长。推算结果与器件光电流实验值的对比表明,由光致荧光谱(PL)测量结果计算得到的响应波长与实际器件的响应波长有良好的一致性。     

Rapid thermal annealing effects on the photoluminescence properties of molecular beam epitaxy-grown GaIn(N)As quantum wells with Ga(N)As spacers and barriers

This article describes the effects of rapid thermal annealing (RTA) on the photoluminescence (PL) emission from a series of GaIn(N)As quantum wells. Indium compositions of both 20% and 32% were examined with nominal N compositions of 1% or 2%. The N location was varied within our quantum structure, which can be divided into three regions: (1) quantum well, (2) Ga(N)As spacer layers at the barrier-to-well interface and well-to-barrier interface, and (3) barriers surrounding each quantum well. Eight combinations of samples were examined with varying In content, Ga(N)As spacer layer thickness, N content, and N location in the structure. In the best cases, the presence of these Ga(N)As spacer layers improves the PL properties, due to annealing, with a reduction in the emission wavelength blueshift by ～400 Å, a reduction of the decrease in the full-width at half-maximum (FWHM) by ～5 meV, and a threefold reduction of the increase in integrated intensity. It was also observed that relocating N from the quantum wells to the barriers produces a comparable emission wavelength both before and after annealing. Our results further show that the composition of incorporated N in the material is most influential during the stages of RTA in which relatively small amounts of thermal energy is present from our lower annealing times and temperatures. Hence, we believe a low thermal-energy anneal is responsible for the recovery of the plasma-related crystal damage that was incurred during its growth. However, the In composition in the quantum well is most influential during the latter stages of thermal annealing, at increased times and temperatures, where the wavelength blueshift was roughly independent of the amount of incorporated N. As a result, our investigations into the effects of RTA on the PL properties support other reports that suggest the wavelength blueshift is not due to N diffusion.     

Contactless electroreflectance study of a GaAIAs/lnGaAs/ GaAs/GaAIAs step quantum well structure

Using contactless electroreflectance at 300 and 77K, we have studied the inter-subband transitions from a GaAlAs/InGaAs/GaAs/GaALAs step quantum well structure (small well inside a large well) consisting of two layers A (In_xGa_1−xAs) and B (GaAs) with widths L_A and L_B, respectively, bounded by two thick barrier regions of Ga_x Al_yAs. By comparison of the observed spectral features with an envelope function calculation, including the effects of strain, we have been able to characterize the potential profile of the structure, i.e., L_A, L_B, x, and y. There is very good agreement between experiment and the intended materials param-eters. Such configurations are of considerable importance since (a) they form the basis for pseudomorphic high electron mobility transistors, and (b) also have applications in optoelectronics due to their large Stark shifts.     

Improvement of GaAs/AlGaAs quantum well laser diodes by rapid thermal annealing

Post-growth annealing is shown to improve the laser diode quality of GaAs/AlGaAs graded-index separate confinement heterostructure quantum well laser diode structures grown at a nonoptimal substrate temperature lower than 680°C by molecular beam epitaxy. Reduction by a factor of up to three in the threshold current was accompanied by a reduction in the interface trap density. The reduced threshold current is still higher than that of laser diodes grown at the optimal temperatures which are between 680 and 695°C. The improvement in laser diode performance is ascribed to the reduction of interface nonradiative recombination centers.