Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.     

Property comparison of polarons in zinc-blende and wurtzite GaN/AlN quantum wells

本文详细比较闪锌矿和纤锌矿GaN/AlN量子阱中Fr?hlich电声子相互作用哈密顿量及极化子性质。采用LLP变分方法计算由界面、局域和半空间声子模导致的基态极化子能移。结果表明,若忽略z方向和x-y平面之间异性的影响,闪锌矿和纤锌矿哈密顿量是一致的。各向异性在窄阱情形对界面声子能移的影响较为明显,而在略宽阱时对局域声子能移影响明显。此外,内建电场对各支光学声子引起的能移的影响较大。     

Property comparison of polarons in zinc-blende and wurtzite GaN/AlN quantum wells

The properties of polarons in zinc-blende and wurtzite GaN/AlN quantum wells with Fr(o|¨)hlich interaction Hamiltonians are compared in detail.The energy shifts of polarons at ground state due to the interface(IF), confined(CO) and half-space phonon modes are calculated by a finite-difference computation combined with a modified LLP variational method.It is found that the two Fr(o|¨)hlich interaction Hamiltonians are consistent with each other when the anisotropic effect from the z-direction and the x-y plane is neglected.The influence of the anisotropy on the polaron energy shifts due to the IF phonon modes for a smaller well width or due to the CO phonon modes for a moderate well width is obvious.In addition,the built-in electric field has a remarkable effect on the polaron energy shifts contributed by the various phonon modes.     

Correlated electron-hole transitions in wurtzite GaN quantum dots:the effects of strain and hydrostatic pressure

Within the effective-mass and finite-height potential barrier approximation,a theoretical study of the effects of strain and hydrostatic pressure on the exciton emission wavelength and electron-hole recombination rate in wurtzite cylindrical GaN/Al_xGa_1-xN quantum dots（QDs） is performed using a variational approach.Numerical results show that the emission wavelength with strain effect is higher than that without strain effect when the QD height is large（> 3.8 nm）,but the status is opposite when the QD height is small（< 3.8 nm）.The height of GaN QDs must be less than 5.5 nm for an efficient electron-hole recombination process due to the strain effect.The emission wavelength decreases linearly and the electron-hole recombination rate increases almost linearly with applied hydrostatic pressure.The hydrostatic pressure has a remarkable influence on the emission wavelength for large QDs,and has a significant influence on the electron-hole recombination rate for small QDs.Furthermore,the present numerical outcomes are in qualitative agreement with previous experimental findings under zero pressure.     

纤锌矿GaN量子点中相关电子空穴的光跃迁：应变和流体静压力效应

在有效质量和有限高势垒近似下,变分研究了应变和流体静压力对纤锌矿GaN/AlxGa1-xN柱形量子点中激子发光波长和电子空穴复合率的影响。计算结果表明,在量子点高度较大情况下( >3.8 nm ),考虑应变后的发光波长比不考虑应变的发光波长大。而在量子点高度较小情况下( <3.8 nm )则相反。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5.5nm。发光波长随流体静压力的增大而线性减小,电子空穴复合率随流体静压力的增大而近线性增大。在量子点尺寸较大的情况下,流体静压力对发光波长的影响比较显著,而在量子点尺寸较小的情况下,流体静压力对电子空穴复合率的影响比较显著。此外,我们将零流体静压力下光跃迁能的理论计算值和实验值进行了比较,理论值和实验值相符合。     

The effect of differential temperatures on the latent heat in the nucleation of CdSe quantum dots

This work studied the effect of differential temperatures on the latent heat in the nucleation of CdSe quantum dots (QDs). The result showed that, by the formula of phase change, with increasing the reaction temperature, the latent heat in the nucleation of QDs reduced. CdSe QDs with the size-dispersion from 2.7 to 3.6 nm were synthesized via oleic acid-paraffin liquid system by controlling the reaction temperature from 180 to 220 ℃. Synthesized QDs were characterized by UV-vis absorption spectra and X-ray diffraction (XRD). The result of UV-vis absorption spectra showed that with increasing of reaction temperature, the first absorption peak was red-shifted and the size of QD increased. The result of XRD showed that the synthesized QDs were zinc-blende structure.     

Study of capping agent effect on the structural,optical and photocatalytic properties of zinc sulfide quantum dots

In this work, the effect of capping agent type on the structural, optical and photocatalytic properties of pure zinc sulfide (ZnS) quantum dots (QDs) has been investigated. ZnS QDs were prepared by a simple, fast and water based chemical precipitation method, in the presence of various capping agents including 2-mercaptoethanol, thiourea, and l-cysteine. The obtained QDs have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), and UV–visible absorption spectroscopy. The results revealed that the optical absorption band of ZnS nanostructures varied by capping agents. In the photocatalytic investigations, the prepared ZnS QDs were applied for the photodecolorization of crystal violet (CV) as a model molecule. Influence of affecting parameters on the decolorization efficiency of the capped ZnS QDs was studied and optimized. The results indicated that the prepared QDs can effectively remove different concentrations of CV dye at alkaline pH, in the presence of low concentrations of QDs. According to the photocatalytic results, the presented method can be considered as a green, quick and efficient strategy for photobleaching of organic pollutants based on the high performance photocatalytic behavior of ZnS QDs capped by different capping agents.     

MO-PPV/ZnSe量子点复合材料的制备及光致发光特性研究

采用水相法制备了颗粒尺寸为3.75nm的硒化锌(ZnSe)量子点,采用表面活性剂将ZnSe量子点转移到有机相聚(2-甲氧基-5-辛氧基)对苯乙炔(MO-PPV)中,获得了MO-PPV/ZnSe复合材料。通过对MO-PPV和ZnSe量子点的吸收光谱(ABS)和光致发光(PL)光谱的研究发现,随着ZnSe量子点掺杂浓度的提高,复合材料的发光强度明显增强,发光峰位置出现了蓝移。当ZnSe∶MO-PPV的质量比为1∶0.181时,发光峰位置蓝移10nm。结果表明,MO-PPV与ZnSe量子点之间存在着能量传递,这是导致MO-PPV/ZnSe量子点复合材料具有PL增强的重要原因。     

应变纤锌矿ZnO/MgxZn1-xO量子点中离子施主束缚激子的光学性质

Within the framework of the effective-mass approximation and the dipole approximation, considering the three-dimensional confinement of the electron and hole and the strong built-in electric field(BEF) in strained wurtzite Zn O/Mg0:25Zn0:75O quantum dots(QDs), the optical properties of ionized donor-bound excitons(D+, X)are investigated theoretically using a variational method. The computations are performed in the case of finite band offset. Numerical results indicate that the optical properties of(D+, X) complexes sensitively depend on the donor position, the QD size and the BEF. The binding energy of(D+, X) complexes is larger when the donor is located in the vicinity of the left interface of the QDs, and it decreases with increasing QD size. The oscillator strength reduces with an increase in the dot height and increases with an increase in the dot radius. Furthermore, when the QD size decreases, the absorption peak intensity shows a marked increment, and the absorption coefficient peak has a blueshift. The strong BEF causes a redshift of the absorption coefficient peak and causes the absorption peak intensity to decrease remarkably. The physical reasons for these relationships have been analyzed in depth.     

Observation of the quantum confined stark effect in ZnSe/ZnCdSe single quantum well systems

We report the observation of the quantum-confined Stark effect (QCSE) in ZnSe/ ZnCdSe single quantum wells grown by molecular beam epitaxy, using photoluminescence. In our experiments the electric field was applied via a reverse-biased Schottky barrier contact. To our knowledge, this is the first observation of the QCSE in any wide gap II-VI semiconductor heterostructure. Significant red shifts, typically 10–15 meV, are detected before quenching. An associated reduction in the transition intensity, consistent with the QCSE. is clearly observed. The dependence of these results will be discussed as a function of quantum well depth and thickness. Complete quenching of the luminescence is observed with applied voltages as low as 5 V. In addition, at lowest voltages, we also detect small blue shifts (up to 4 meV), which we attribute to the interaction between the externally applied electric field and the built-in field of the structure.     

Influence of capping layer thickness on the polarization of photoluminescence of CdSe/ZnSe quantum dots grown by metalorganic chemical vapor phase deposition

The polarization of the photoluminescence (PL) of self-assembled CdSe quantum dots (QDs), grown by metalorganic chemical vapor phase deposition, was measured. From the (001) surface, the PL was found preferentially polarized in the direction, while from the cleaved surface in the [001] direction. The polarization of PL depends strongly on the ZnSe capping layer thickness and the PL energy. With an increase in ZnSe coverage, the intensity ratio was found to increase first, then decrease after the coverage is thicker than a critical value. Moreover, such a critical thickness is smaller for larger QDs (lower PL energies). Possible origins of the PL polarization are discussed. We suggest that besides the quantum confinement effects, the strain field in the QDs also plays an essential role in the observed polarization of PL.     

Theoretical study of InAs/GaAs quantum dots grown on [11k] substrates in the presence of a magnetic field

Eight-band k.p theory including strain and piezoelectric effects are employed to calculate the strain distribution and electron and hole energy levels of InAs/GaAs quantum dots grown on [11k] substrates in the presence of an external magnetic field. Height of the dot determines how the increasing of k influences isotropic part of the strain tensor while biaxial part of the strain tensor is always reduced with increasing k. Because of the reduced symmetry of high index surfaces, influence of piezoelectric effect on the electronic structure becomes more dominant with increasing k. Electron energy levels are influenced by the isotropic part of the strain compared to the hole energy levels, where strong heavy hole–light-hole mixing is observed. For the dots grown on the [11k] surfaces magnetic field has smaller influence on the electron and hole energy levels as compared to the referent case.     

自旋对半导体量子点中强耦合磁极化子性质的影响

在考虑电子自旋的情况下,应用么正变换和线性组合算符法研究了半导体量子点中强耦合磁极化子振动频率、基态能和基态结合能的性质.数值计算结果表明:电子自旋使磁极化子基态结合能分裂为二Eb(1/2)和Eb(-1/2),其间距与外磁场B成线性关系.当回旋共振频率小于和大于10倍声子振动频率时,Eb(1/2)随B的增大分别缓慢和迅速减小.当回旋共振频率小于和大于20倍声子振动频率时,Eb(-1/2)随B的增大分别迅速和缓慢增大.当回旋共振频率为440倍声子振动频率时,Eb(-1/2)取最大值为44倍声子能量.之后Eb(-1/2)随B的增大逐渐减小.随着磁场的加强,电子自旋影响增大.当回旋共振频率超过声子振动频率的5.97和820倍时,电子自旋能量已分别大于电子所受束缚势和电子与LO声手之间的诱生势.     

Optical and electrical simulation of hybrid solar cell based on conjugated polymer and size-tunable CdSe quantum dots: Influence of the QDs size

The photovoltaic performance of hybrid solar cell based on poly(3-hexylthiophene) (P3HT) and size-tunable CdSe quantum dots is analyzed by combination of optical and electrical simulations. The employed optical and electrical models describe the dependency of solar cell characteristics on CdSe QDs diameter and active layer thickness. The device performance improvement is observed by increasing CdSe QDs diameter from 2.3 nm to 8.3 nm. The short circuit current density (Jsc) shows significant ascending trend by QDs diameter which is due to electron mobility (μ_n) and absorption range enhancement. However, the maximum achievable open circuit voltage (V_OC) decreases by QDs size growth, V_OC shows ascending trend with CdSe QDs diameter increase, because of the higher dissociation probability and lesser recombination rate for larger nanocrystals. As the electron mobility proportionally increases by CdSe QDs size, the performance dependency on the charge mobility is studied. Results show that by growing the size of CdSe QDs, charge extraction is dominant in the competition between recombination rate increase and charge extraction increase.     

沟道电场分布对AlN/GaN HFETs极化库仑场散射的影响

Based on the measured capacitance–voltage(C–V) curves and current–voltage(I–V) curves for the prepared differently-sized Al N/Ga N heterostructure field-effect transistors(HFETs), the I–V characteristics of the Al N/Ga N HFETs were simulated using the quasi-two-dimensional(quasi-2D) model. By analyzing the variation in the electron mobility for the two-dimensional electron gas(2DEG) with the channel electric field, it is found that the different polarization charge distribution generated by the different channel electric field distribution can result in different polarization Coulomb field(PCF) scattering. The 2DEG electron mobility difference is mostly caused by the PCF scattering which can reach up to 899.6 cm2/(V s)(sample a), 1307.4 cm2/(V s)(sample b),1561.7 cm2/(V s)(sample c) and 678.1 cm2/(V s)(sample d), respectively. When the 2DEG sheet density is modulated by the drain–source bias, the electron mobility for samples a, b and c appear to peak with the variation of the 2DEG sheet density, but for sample d, no peak appears and the electron mobility rises with the increase in the2 DEG sheet density.     

The effect of built-in electric field in GaN/AlGaN quantum wells with high AIN mole fraction

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/Al_xGa_1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al_0.8Ga_0.2N quantum wells.     

外加电场对GaN/AlxGa1-xN双量子阱中性施主束缚能的影响

在有效质量包络函数理论下,利用变分法计算了未加电场以及加入电场后 双量子阱中施主杂质各种情况下的束缚能,讨论了双量子阱中间势垒高度、施主杂质位置对杂质束缚能的影响。给出了加入电场后施主位置不同时的束缚能和波函数,以及量子阱宽度不同时的束缚能,并且计算了未加电场和加入电场后中间势垒高度变化以及宽度不同时的束缚能。当双量子阱中间垒宽一定时,束缚能随着阱宽的变化会出现一个峰值。在阱宽一定时,随着中间垒宽度的增加,束缚能逐渐减小,并在垒宽增加到一定宽度时双量子阱情况与单量子阱情况相似,束缚能不再明显变化。计算结果对设计和研究 量子阱发光和探测器件有一定的参考价值。     

边欧姆工艺对AlN/GaN 异质结场效应晶体管极化梯度库仑场散射的影响

Using the measured capacitance–voltage and current–voltage characteristics of the rectangular Al N/Ga N heterostructure field-effect transistors(HFETs) with the side-Ohmic contacts, it was found that the polarization Coulomb field scattering in the Al N/Ga N HFETs was greatly weakened after the side-Ohmic contact processing, however, it still could not be ignored. It was also found that, with side-Ohmic contacts, the polarization Coulomb field scattering was much stronger in Al N/Ga N HFETs than in Al Ga N/Al N/Ga N and In0:17Al0:83N/Al N/Ga N HFETs, which was attributed to the extremely thinner barrier layer and the stronger polarization of the Al N/Ga N heterostructure.     

The effect of substrate misorientation on the optical, structural, and electrical properties of GaN grown on sapphire by MOCVD

We report the growth and characterization of unintentionally doped GaN on both exact and vicinal (0001) sapphire substrates. The GaN heteroepitaxial layers are grown by metalorganic chemical vapor deposition on c-plane A1₂O₃ substrates either on-axis or intentionally misoriented 2° toward the a-plane (1120) or 5 or 9° toward the m-plane (10 10). The samples are characterized by 300K photoluminescence, cathodoluminescence, and Hall-effect measurements as well as by triple-axis x-ray diffractometry to determine the effect of the misorientation on the optical, electrical, and structural properties of heteroepitaxial undoped GaN. Ten different sample sets are studied. The data reveal enhanced photo-luminescence properties, increased electron mobility, a reduced n-type background carrier concentration, and a somewhat degraded surface morphology and crystalline quality for the misoriented samples compared to the on-axis samples.