Photocurrent spectroscopy of indirect transitions in Ge/Si multilayer quantum dots at room temperature

Lateral photoconductivity spectra of multilayer Ge/Si heterostructures with Ge quantum dots were studied in the work proposed at room temperature. The photocurrent with minimal energy 0.48-0.56 eV that is smaller than Ge band gap was observed from such structures at the geometry of waveguide excitation. Generation of the photocurrent with the limit energy 0.48-0.56 eV was explained by spatially indirect electron transitions from heavy hole states of SiGe valence band into Δ₂-valley of the conduction band of Si surrounding. It was found out that the limit energy of such transitions decreased, as the number of SiGe quantum dot layers increased.     

The studies of Ge quantum dots on strained Si0.7Ge0.3 layer by photoluminescence and deep level transient spectroscopy

Systematic studies of Ge quantum dots (QDs) grown on strained Si_0.3Ge_0.7 layer have been carried out by photoluminescence (PL) and deep level transient spectroscopy (DLTS). In PL measurements, two peaks around 0.7 eV are distinguished, which are assigned to two types of QDs observed by atomic force microscopy (AFM). Large blueshifts of the PL peaks from small QDs with the increase of excitation power are observed and attributed to the band bending effects typical for type-II band alignment. From DLTS measurements, the energy levels of holes in both types of QDs are derived, which shift with the change of the number of holes in QDs due to their charge energy. By comparing results from PL and DLTS measurements, further understanding of band alignment with the increase of the number of excitons in QDs is deduced.     

Photoluminescence and magneto-optical properties of multilayered type-II ZnTe/ZnSe quantum dots

Multilayered Zn–Se–Te structures grown by migration enhanced epitaxy are studied by temperature- and excitation-dependent photoluminescence (PL) as well as magneto-PL. The PL consists of two bands: a blue band, overlaid with band edge sharp lines, dominant at low temperatures and high excitation, and a green band, which appears at elevated temperature and low excitation. Upon varying excitation intensity by four orders of magnitude, the green band peak energy shifts by ∼60 meV, indicating recombination of excitons in type-II quantum dots (QDs); no significant shift is observed for the blue band. Therefore, the green emission is attributed to ZnTe/ZnSe type-II QDs, which co-exist with isoelectronic centers, responsible for the blue and band edge emissions. The existence of type-II ZnTe/ZnSe QDs is further confirmed by magneto-PL, for which the observed oscillations in the PL intensity as a function of magnetic field is explained in terms of the optical Aharonov–Bohm effect.     

p型掺杂Si/Ge量子点的电子拉曼散射(英文)

在本文中我们首次报道了p型掺杂的自组织Si/Ge量子点中空穴能级子带间的电子拉曼散射,此电子跃迁的能量为105meV。Si/Ge量子点Ge Ge模的共振拉曼散射表明此空穴能级间的电子拉曼散射与Γ点附近的E0(≈2.52eV)发生了共振,而E1的能量小于2.3eV.变温实验和偏振实验进一步证实了我们的指认。所有观测的实验数据与6 bandk·p能带结构理论的计算结果吻合得很好。     

High-Density Effects on the Exciton Systems and Their Dynamical Behavior in Al x Ga 1− x As/AlAs Quantum Wires

In Al x Ga 1 m x As/AlAs quantum wire (QWR) structures, the lower lying indirect exciton (IE) photoluminescence (PL) peak shows remarkable blue-shift under intense light-excitation contrary to the higher lying direct exciton (DE) PL band with very small blue-shift, although the two kinds of exciton states consist of the common hole state. In time-resolved PL spectra in the type-II QWR of x = 0.4, the DE PL band appears at an earlier stage without peak-shift and the excitons relax to the IE state making the IE PL peak dominant with time evolution. The blue-shift of the IE peak in a quasi equilibrium after the relaxation seriously depends on the excitation density. The origin of the blue-shift is explained in terms of many-body effects including band-bending effect due to the electric field induced by spatially separated electrons and holes in the QWR structures.     

掺杂对多层Ge/Si(001)量子点光致发光的影响

利用超高真空化学气相沉积设备, 在Si (001) 衬底上外延生长了多个四层Ge/Si量子点样品. 通过原位掺杂的方法, 对不同样品中的Ge/Si量子点分别进行了未掺杂、磷掺杂和硼掺杂. 相比未掺杂的样品, 磷掺杂不影响Ge/Si量子点的表面形貌, 但可以有效增强其室温光致发光; 而硼掺杂会增强Ge/Si量子点的合并, 降低小尺寸Ge/Si量子点的密度, 但其光致发光会减弱. 磷掺杂增强Ge/Si量子点光致发光的原因是, 磷掺杂为Ge/Si量子点提供了更多参与辐射复合的电子. 关键词： Ge/Si量子点 磷掺杂 光致发光     

Infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices

We report the study of infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices. The superlattices, which were grown on (001) oriented Si substrates by a solid source molecular beam epitaxy system, are composed mainly of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of them and of the uncapped Ge dots grown on the surfaces of some of them were tested by cross-sectional transmission electron and atomic force microscopes, respectively. It is found that the Ge quantum dots have flat lens-like shapes. Infrared absorption signals peaking in the mid-infrared range were observed using Fourier transform infrared and Raman scattering spectroscopy techniques. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions within the valence band of the Ge quantum dots in the superlattices. The fact that the intraband absorption is strongly polarized along the growth axis of the superlattices signifies that the Ge quantum dots with flat lens-like shapes perform as Ge/Si-based quantum wells. This study demonstrates the application potential of these kinds of Ge/Si quantum dot superlattices for developing mid-infrared photodetectors.     

Phonons in Ge/Si superlattices with Ge quantum dots

Ge/Si superlattices containing Ge quantum dots were prepared by molecular beam epitaxy and studied by resonant Raman scattering. It is shown that these structures possess vibrational properties of both two-and zero-dimensional objects. The folded acoustic phonons observed in the low-frequency region of the spectrum (up to 15th order) are typical for planar superlattices. The acoustic phonon lines overlap with a broad emission continuum that is due to the violation of the wave-vector conservation law by the quantum dots. An analysis of the Ge and Ge-Si optical phonons indicates that the Ge quantum dots are pseudoamorphous and that mixing of the Ge and Si atoms is insignificant. The longitudinal optical phonons undergo a low-frequency shift upon increasing laser excitation energy (2.54–2.71 eV) because of the confinement effect in small-sized quantum dots, which dominate resonant Raman scattering.     

Tailoring of the Wave Function Overlaps and the Carrier Lifetimes in InAs/GaAs1−xSbx Type-II Quantum Dots

Effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs_1−xSb_x layer are investigated by photoluminescence (PL) and time-resolved PL measurements. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAs_1−xSb_x layer. We find that the type-II QD structure can sustain thermal annealing up to 850 °C. In particular, we find that it is possible to manipulate between type-I and type-II recombinations in annealed QDs by using different excitation powers. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAs_1−xSb_x type-II QDs.     

Dependency of photoluminescence from SiO_2 thin films containing Si_(1-x)Ge_x quantum dots on Ge/Si doping ratio

SiO2 thin films containing Si1-xGex quantum dots （QDs） are prepared by ion implantation and annealing treatment. The photoluminescence （PL） and microstructural properties of thin films are investigated. The samples exhibit strong PL in the wavelength range of 400-470 nm and relatively weak PL peaks at 730 and 780 nm at room temperature. Blue shift is found for the 400-nm PL peak, and the intensity increases initially and then decreases with the increase of Ge-doping dose. We propose that the 400-470 nm PL band originates from multiple luminescence centers, and the 730- and 780-nm PL peaks are ascribed to the Si=O and GeO luminescence centers.     

Optical properties of Stranski–Krastanov grown three-dimensional Si/Si1−xGex nanostructures

Detailed Raman and photoluminescence (PL) measurements are reported for Si/Si_1−xGe_x nanostructures grown by molecular beam epitaxy under near Stranski–Krastanov (S–K) growth mode conditions. In samples with x ranging from 0.096 to 0.53, we observe that an increase in the Raman signal related to Ge–Ge vibrations correlates with (i) a red shift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that for x>0.5 Ge atoms form nanometer size clusters with a nearly pure Ge core surrounded by a SiGe shell. Time-resolved PL measurements reveal a stretched-exponential long-lived PL component that is associated with compositional and dimensional fluctuations in the SiGe dots.     

InN分凝的InGaN薄膜的光致发光与吸收谱

我们用低压MOCVD在蓝宝石衬底生长了InGaN/GaN外延层.用X射线衍射(XRD),光致发光谱(PL),光吸收谱等测量手段,研究了InGaN的辐射发光机制.In组分利用Vegard定理和XRD测量得到.我们发现随着In组分的增加,在光吸收谱上发现吸收边的红移和较宽的Urbach带尾;PL谱中低能端的发射渐渐成为主导,并且在PL激发谱中InGaN峰也变宽.我们认为压电效应改变了InGaN的能带结构,从而影响了光学吸收特性.而在InN量子点中的辐射复合则是InGaN层发光的起源.     

Resonant Raman scattering of discrete hole states in self-assembled Si/Ge quantum dots

We report the first resonant electronic Raman spectroscopy study of discrete electronic transitions within small p-doped self-assembled Si/Ge quantum dots (QDs). A heavy hole (hh) to light hole (lh) Raman transition with a dispersionless energy of 105 meV and a resonance energy of the hh states to virtually localised electrons at the direct band gap of 2.5 eV are observed. The hh–lh transition energy shifts to lower values with increasing annealing temperature due to significant intermixing of Si and Ge in the QDs. Structural parameters of the small Si/Ge dots have been determined and introduced into 6-band k·p valence band structure calculations. Both the value of the electronic Raman transition of localised holes as well as the resonance energy at the E₀ gap are in excellent agreement with the calculations.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

Spin states of holes in Ge/Si nanowire quantum dots

We investigate tunable hole quantum dots defined by surface gating Ge/Si core-shell nanowire heterostructures. In single level Coulomb-blockade transport measurements at low temperatures spin doublets are found, which become sequentially filled by holes. Magnetotransport measurements allow us to extract a g factor g approximately 2 close to the value of a free spin-1/2 particle in the case of the smallest dot. In less confined quantum dots smaller g factor values are observed. This indicates a lifting of the expected strong spin-orbit interaction effects in the valence band for holes confined in small enough quantum dots. By comparing the excitation spectrum with the addition spectrum we tentatively identify a hole exchange interaction strength chi approximately 130 microeV.     

ZnSe/BeTe Ⅱ型量子阱中界面结构对发光特性的影响

报道了具有特殊界面结构(界面包含三个Zn—Te或Te—Zn化学键)的非掺杂ZnSe/BeTe II 型量子阱在低温(5—10 K)条件下的空间间接光致发光(PL)光谱的实验结果. PL光谱显示了一个较弱的双峰结构和较低的线性偏振度,并且这两个峰的线性偏振度相反. 此外,这个PL光谱也强烈地依赖于一个外加电场的变化. 这些结果表明样品的两个发光峰是分别来自两个界面的发光跃迁,并且特殊界面结构降低了空间间接PL的发光效率和线性偏振性,以及界面附近的内秉电场. 随着激发强度的增加,PL谱的高能端发光峰显示了一个 关键词： Ⅱ型量子阱 光致发光 界面结构     

Carrier dynamics in type-II GaAsSb/GaAs quantum wells

Time-resolved photoluminescence (PL) characteristics of type-II GaAsSb/GaAs quantum wells are presented. The PL kinetics are determined by the dynamic band bending effect and the distribution of localized centers below the quantum well band gap. The dynamic band bending results from the spatially separated electron and hole distribution functions evolving in time. It strongly depends on the optical pump power density and causes temporal renormalization of the quantum well ground-state energy occurring a few nanoseconds after the optical pulse excitation. Moreover, it alters the optical transition oscillator strength. The measured PL lifetime is 4.5 ns. We point out the critical role of the charge transfer processes between the quantum well and localized centers, which accelerate the quantum well photoluminescence decay at low temperature. However, at elevated temperatures the thermally activated back transfer process slows down the quantum well photoluminescence kinetics. A three-level rate equation model is proposed to explain these observations.     

Anomalous temperature dependence of photoluminescence peak energy in InAs/InAlAs/InP quantum dots

We have observed an unusual temperature sensitivity of the photoluminescence (PL) peak energy for InAs quantum dots grown on InAs quantum wires (QDOWs) on InP substrate. The net temperature shift of PL wavelength of the QDOWs ranges from 0.8 to −4 Å/°C depending upon the Si doping concentration in the samples. This unusual temperature behavior can be mainly ascribed to the stress amplification in the QDOWs when the thermal strain is transferred from the surrounding InAs wires. This offers an opportunity for realizing quantum dot laser devices with a temperature insensitive lasing wavelength.     

Optical properties of InGaN quantum dots

We present time-resolved and spatially-resolved photoluminescence (PL) measurements of InGaN inclusions in a GaN matrix. The structures were grown by metal-organic chemical vapor deposition on sapphire and Si(111) substrates. Nonresonant pulsed excitation yields a broad PL peak, while resonant excitation into the nonresonant PL intensity maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under nonresonant excitation. Observation of a resonantly excited narrow PL line gives clear proof of the quantum dot (QD) nature of luminescence in InGaN–GaN samples. Cathodoluminescence (CL) and micro-PL measurements demonstrate sharp emission lines from single QD states. The recombination dynamics of single QD’s and the whole QD ensemble were investigated. Monoexponential decay was observed for the PL of single QD’s. For similar transition energies different time constants were obtained. Therefore the nonexponential decay observed for the whole ensemble is attributed to the coexistence of QD’s having similar ground-state transition energies, but significantly different electron–hole overlap.     

Characteristics of a type-II n-MoS2/p-Ge van der Waals heterojunction

A few-atomic-layer molybdenum disulfide (MoS₂) film on Si/SiO₂ substrates grown by metal-organic chemical vapor deposition was investigated. The few-atomic-layer MoS₂ film was subsequently transferred onto a (100) p-Ge substrate to build a van der Waals n-p heterojunction. The as-grown few-atomic-layer MoS₂ film and the MoS₂/Ge heterostructure were characterized atomic force microscopy, spectroscopic ellipsometry, high-resolution scanning transmission electron microscopy, Raman spectroscopy analyses, photoluminescence (PL) measurements at room temperature (RT, 300 K), and type-II band alignment of the heterostructure determined by ultraviolet photoelectron spectroscopy. The RT-PL measurements showed dominant peaks at 1.96 and 1.8 eV for the as-grown MoS₂ and red-shifted PL peaks for that transferred onto Ge. We examined the electrical characteristics of the few-atomic-layer MoS₂ by forming a type-II band alignment van der Waals heterojunction with a highly doped p-Ge. The heterojunction solar cell exhibited an open-circuit voltage of 0.15 V and a short-circuit current density of 45.26 μA/cm². The external quantum efficiency measurements showed a spectral response up to approximately 500 nm owing to the absorption by the few-atomic-layer MoS₂ film.