北京邮电大学半导体低维量子结构材料研究组

半导体应变异质结构材料,如量子阱、量子线和量子点等在光电材料与器件中发挥着越来越重要的作用,晶格失配导致的应变对材料生长过程、形貌与物理性质具有非常重要的作用,弹性应变场能够改变半导体材料的能带结构,进而影响材料的光学和电学特性以及光电器件的性能。以应变工程为基础的低维量子结构材料的研究已经成为国际半导体材料与器件领域的重要研究方向。北京邮电大学半导体低维量子结构材料研究组在俞重远教授带领下,以连续弹性理论和原子势函数理论为基础,对于自组织量子结构材料的内部应变场、应变能、压电效应、电子与空穴的能级分…     

GGA U方法研究应变对纤锌矿ZnO能带结构的影响

基于第一性原理密度泛函理论和GGA U方法,以Zn1-xMgxO衬底的应变为例,计算了应变ZnO体材料的能带结构。同时研究了应力对ZnO材料的禁带宽度、价带分裂能以及电子和空穴有效质量的影响。研究结果表明,Mg组分不大于0.3时,ZnO/Znl-xMgxO材料禁带宽度随应力增大而增大,该结论与实验研究结果相符合。沿[00k] 和[k00]晶向,导带电子有效质量随应力增加而稍有增大,“场致分裂带”空穴有效质量随应力增大明显减小,而“轻空穴带”和“重空穴带”空穴有效质量几乎不随应力改变而变化。     

双轴压应力Ge材料价带物理参量计算

应变Ge材料载流子迁移率高,且与硅工艺兼容,在硅基CMOS中的应用潜力大。能带结构是深入研究应变Ge材料基本属性,设计高速/高性能CMOS器件的重要理论依据。为此,本文采用结合形变势理论的kp微扰法,分析了(001)双轴压应力Ge材料价带结构、轻重空穴带Γ点能级及轻重空穴带间分裂能等价带物理参量与应力的理论关系,获得有实用价值的相关结论。     

CdSe/ZnSe量子点的合成与荧光特性

采用低温成核生长与一步法相结合的方式合成了CdSe/ZnSe核壳结构量子点,并通过吸收光谱、荧光光谱、X射线衍射等分析手段证明了ZnSe壳层包覆成功.对加入空穴传输材料后CdSe/ZnSe量子点的荧光变化情况进行了深入的研究.稳态光谱结果表明.空穴传输材料对量子点发光有较强的淬灭作用;时间分辨光谱结果显示,随着空穴传输材料分子浓度的增加,量子点的荧光寿命明显缩短,其荧光淬灭过程可以解释为静态淬灭和动态淬灭过程.静态淬灭来源于量子点表面与空穴传输材料间的相互作用;而动态淬灭则来源于量子点到空穴传输材料的空穴转移过程.因此,量子点的壳层结构及空穴传输材料的种类对量子点的荧光淬灭起关键作用.     

670nm发光二极管材料的MOCVD外延生长

重点介绍了670nm LED材料的结构与制备方法,用MOCVD方法生长了较高压应变的670nm多量子阱。分析比较了670nm量子阱室温光荧光谱线宽度的影响因素,指出室温光荧光主要来源于带-带复合,荧光谱线宽度的减小是应变量子阱轻重空穴能级分离的结果,并不意味着量子阱界面质量的改进。同时介绍了二乙基锌(DEZn)的掺杂技术和掺杂浓度,通过优化掺杂条件和退火条件,p型AlInP材料获得了0.9×1018/cm3的空穴密度。外延材料制作成200μm×200μm尺寸的LED管芯,在20mA工作电流下亮度为22～24mcd。器件结果表明,用5个压应变量子阱的有源区并且采用DEZn掺杂可以制作出高亮度的670nm LED外延材料。     

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。     

InAs/GaAs量子点光致发光光谱多峰结构发光本质

研究了InAs/GaAs量子点光致发光光谱中出现的多峰结构. 观察到随着激发功率的增加光谱中发光峰的数目逐渐增多并且部分发光峰的峰位随激发功率的增加向高能量方向移动. 解释了各发光峰的来源并结合量子点能级结构的特点,计算了量子点中电子和空穴各子带间的能级间距.     

InAs/GaAs量子点光致发光光谱多峰结构发光本质

研究了InAs/GaAs量子点光致发光光谱中出现的多峰结构.观察到随着激发功率的增加光谱中发光峰的数目逐渐增多并且部分发光峰的峰位随激发功率的增加向高能量方向移动.解释了各发光峰的来源并结合量子点能级结构的特点,计算了量子点中电子和空穴各子带间的能级间距.     

P型张应变Si/SiGe量子阱红外探测器的能带设计

提出P型张应变Si/SiGe量子阱红外探测器(QWIP)结构,应用k·P方法计算应变Si/SiGe量子阱价带能带结构和应变SiGe合金空穴有效质量.结果表明量子阱中引入张应变使轻重空穴反转,基态为有效质量较小的轻空穴态,因此P型张应变Si/SiGe QWIP与n型QWIP相比具有更低的暗电流;而与P型压应变或无应变QWIP相比光吸收和载流子输运特性具有较好改善.在此基础上讨论了束缚态到准束缚态子带跃迁型张应变p-Si/SiGe QWIP的优化设计.     

P型张应变Si/SiGe量子阱红外探测器的能带设计

提出P型张应变Si/SiGe量子阱红外探测器(QWIP)结构,应用k·P方法计算应变Si/SiGe量子阱价带能带结构和应变SiGe合金空穴有效质量.结果表明量子阱中引入张应变使轻重空穴反转,基态为有效质量较小的轻空穴态,因此P型张应变Si/SiGe QWIP与n型QWIP相比具有更低的暗电流;而与P型压应变或无应变QWIP相比光吸收和载流子输运特性具有较好改善.在此基础上讨论了束缚态到准束缚态子带跃迁型张应变p-Si/SiGe QWIP的优化设计.     

Lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots

Spectra of lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots, fabricated by molecular-beam epitaxy are studied. The photoresponse caused by optical transitions between hole levels of quantum dots and Si electronic states was observed in the energy range of 1.1–0.3 eV at T = 78 K. It was shown that the electronic states localized in the region of Si band bending near the Ge/Si interface mainly contribute to lateral photoconductivity. The use of the quantum box model for describing hole levels of quantum dots made it possible to understand the origin of peaks observed in the photoconductivity spectra. A detailed energy-level diagram of hole levels of quantum dots and optical transitions in Ge/Si structures with strained Ge quantum dots was constructed.     

On the process of hole trapping in Ge/Si heterostructures with Ge quantum dots

Admittance spectroscopy is used to determine the cross sections and energy levels of holes in Ge/Si heterostructures with Ge quantum dots. The structures are grown by molecular-beam epitaxy. It is established that, in layers of quantum dots produced at low growth temperatures T _g ≤ 450°C, the capture cross section for hole trapping into quantum dots exponentially increases with increasing hole binding energy (the Meyer-Neldel rule), with the same characteristic energy ～25 eV independent of T _g . It is shown that the Meyer-Neldel rule is violated in structures grown at higher temperatures or in samples treated in hydrogen plasma. In the case of nanoclusters synthesized at low temperatures, the experimental results suggest that charge-carrier trapping into Ge quantum dots proceeds via the electron-phonon mechanism with the participation of structural defects.     

Exciton states in semiconductor spherical nanostructures

The results of theoretical studies of the energy spectra of excitons moving in semiconductor spherical quantum dots are described. The contributions of the kinetic electron and hole energies, the energy of the Coulomb interaction between an electron and hole, and the energy of the polarization interaction between them to the energy spectrum of an exciton in a quantum dot with a spherical (quantum dot)-(insulator medium) interface is analyzed.     

InAs自组织生长量子点的空穴俘获势垒

成功地用深能级瞬态谱（ＤＬＩＳ）研究了ｐ 型ＩｎＡｓ 自组织生长的量子点的电学性质,测得２．５ 原子层ＩｎＡｓ 量子点空穴基态能级在ＧａＡｓ 价带底上约０．０９ｅＶ,该量子点在荷电状态发生变化时需要克服一个势垒,俘获势垒高度为０．２６ｅＶ．本工作首次利用ＤＬＴＳ测定了量子点空穴的基态能级和俘获势垒,相信对增加量子点性质的理解会起到有益的帮助     

Differential Gain and Gain Compression in Quantum-Dot Lasers

The dynamics of optical gain in semiconductor quantum dots (QDs) is investigated. Simple analytical expressions are derived, which directly connect the laser dynamical response to capture and intradot relaxation rates. The effect of hole spreading in the valence band and spectral hole burning in the QD ensemble is also quantitatively assessed. The analysis shows that intradot relaxation constitutes the main limitation in the dynamics and points to possible routes towards the improvement of QD lasers     

Photoconductivity of Si/Ge multilayer structures with Ge quantum dots pseudomorphic to the Si matrix

Longitudinal photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots grown pseudomorphically to the Si matrix are studied. Lines of optical transitions between hole levels of quantum dots and Si electronic states are observed. This allowed us to construct a detailed energy-level diagram of electron-hole levels of the structure. It is shown that hole levels of pseudomorphic Ge quantum dots are well described by the simplest “quantum box” model using actual sizes of Ge islands. The possibility of controlling the position of the long-wavelength photosensitivity edge by varying the growth parameters of Si/Ge structures with Ge quantum dots is determined.     

Nonradiative recombination in GaN quantum dots formed in the AlN matrix

The mechanisms of temperature quenching of steady-state photoluminescence are studied for structures with hexagonal GaN quantum dots embedded in the AlN matrix. The structures are grown by molecular beam epitaxy. The study is conducted for structures with differently sized quantum dots, for which the peak of the photoluminescence band is in the range from 2.5 to 4.0 eV. It is found that the activation energy of thermal quenching of photoluminescence varies from 27 to 110 meV, as the quantum-dot height is decreased from 5 to 2 nm. A model is suggested to interpret the results. According to the model, the photo-luminescence signal is quenched because of the transfer of charge carriers from energy levels in the quantum dots to defect levels in the matrix.     

半导体量子点最低导带态的量子限制效应

采用最新计算方法和半导体体材料传统量子计算结果,系统研究了14种半导体(Si,Ge,Sn,AlSb,GaP,GaAs,GaSb,InP,InAs,InSb,ZnS,ZnSe,ZnTe,CdTe)的立方量子点,得到了最低导带态的量子限制效应结果,我们把量子点对尺寸的依赖关系分为三类并详细讨论了它们的差别。     

Photoelectric spectroscopy of InAs/GaAs quantum dot heterostructures in a semiconductor/electrolyte system

The photovoltaic effect in the semiconductor/electrolyte junction is an effective method for investigation of the energy spectrum of InAs/GaAs heterostructures with self-assembled quantum dots. An important advantage of this method is its high sensitivity. This makes it possible to obtain photoelectric spectra from quantum dots with high barriers for the electron and hole emission from quantum dots into the matrix even if the surface density of the dots is low (～10⁹ cm^?2). In a strong transverse electric field, broadening of the lines of optical transitions and emission of electrons and holes from quantum dots into the matrix directly from the excited states are observed. The effect of the photovoltage sign reversal was detected for a sufficiently high positive bias across the barrier within the semiconductor. This effect is related to the formation of a positive charge at the interface between the cap layer and electrolyte and of the negative charge on impurities and defects in the quantum dot layer.     

Role of Cooper pairs for the generation of entangled photon pairs from single quantum dots

Generation of entangled photon pairs from semiconductor quantum dots (QDs) is highly desirable for realizing practical solid-state photon sources for quantum information processing and quantum cryptography. However, the energy splitting of exciton states in QDs almost prevent the generation of entangled photon pairs. This paper discusses the new possibility with the injection of electron as well as hole Cooper pairs into QDs.