第一原理研究界面弛豫对InAs/GaSb超晶格界面结构、能带结构和光学性质的影响

通过广义梯度近似的第一原理全电子相对论计算, 研究了不同界面类型InAs/GaSb超晶格的界面结构、电子和光吸收特性. 由于四原子界面的复杂性和低对称性, 通过对InAs/GaSb超晶格进行电子总能量和应力最小化来确定弛豫界面的结构参数. 计算了InSb, GaAs型界面和非特殊界面(二者交替)超晶格的能带结构和光吸收谱, 考察了超晶格界面层原子发生弛豫的影响.为了证实能带结构的计算结果, 用局域密度近似和Hartree-Fock泛函的平面波方法进行了计算. 对不同界面类型InAs/GaSb超晶格的能带结构计算结果进行了比较, 发现界面Sb原子的化学键和离子性对InAs/GaSb超晶格的界面结构、 能带结构和光学特性起着至关重要的作用.     

Four-component superlattice empirical pseudopotential method for InAs/GaSb superlattices

For the design of InAs/GaSb superlattice (SL) heterojunction infrared photodetectors with very low dark current we have extended the standard two-component superlattice empirical pseudopotential method (SEPM) and implemented a four-component model including interface layers. For both models, the calculated bandgap values for a set of SL samples are compared to bandgaps determined by photoluminescence measurements. While the bandgap resulting from the two-component model agrees well with experimental data for SL structures with individual layer thicknesses of 7 monolayers and more, we show that for SLs with thinner GaSb layers the four-component SEPM model is accurate, when the As-content in the interface and barrier layers is included in the model.     

Growth and electrical characterization of type II InAs/GaSb superlattices for midwave infrared detection

Herein, we report a type II InAs/GaSb superlattice structure (SLS) grown on GaSb(1 0 0) substrates by molecular beam epitaxy (MBE) and its electrical characterization for mid-wavelength infrared detection. A GaSb buffer layer was grown under optimized SLS growth conditions, which can decrease the occurrence of defects for similar pyramidal structures. The complications associated with these conditions include oxide desorption of the substrate, growth temperature of the SLS, the V/III ratio during superlattice growth and the shutter sequence. High-resolution X-ray diffraction (HRXRD) shows the sixth satellite peak, and the period of the SLS was 52.9 Å. The atomic force microscopy (AFM) images indicated that the roughness was less than 2.8 nm. High-resolution transmission electron microscopy (HRTEM) images indicated that the SLS contains few structural defects related to interface dislocations or strain relaxation during the growth of the superlattice layer. The photoresponse spectra indicated that the cutoff wavelength was 4.8 μm at 300 K. The SLS photodiode surface was passivated by a zinc sulfide (ZnS) coating after anodic sulfide.     

Interfaces as design tools for short-period InAs/GaSb type-II superlattices for mid-infrared detectors

The effect of interface anisotropy on the electronic structure of InAs/GaSb type-II superlattices is exploited in the design of thin-layer superlattices for mid-IR detection threshold. The design is based on a theoretical envelope function model that incorporates the change of anion and cation species across InAs/GaSb interfaces, in particular, across the preferred InSb interface. The model predicts that a given threshold can be reached for a range of superlattice periods with InAs and GaSb layers as thin as a few monolayers. Although the oscillator strengths are predicted to be larger for thinner period superlattices, the absorption coefficients are comparable because of the compensating effect of larger band widths. However, larger intervalence band separations for thinner-period samples should lead to longer minority electron Auger lifetimes and higher operating temperatures in p-type SLs. In addition, the hole masses for thinner-period samples are on the order the free-electron mass rather than being effectively infinite for the wider period samples. Therefore, holes should also contribute to photoresponse. A number of superlattices with periods ranging from 50.6 to 21.2 Å for the 4 μm detection threshold were grown by molecular beam epitaxy based on the model design. Low temperature photoluminescence and photoresponse spectra confirmed that the superlattice band gaps remained constant at 330 meV although the period changed by the factor of 2.5. Overall, the present study points to the importance of interfaces as a tool in the design and growth of thin superlattices for mid-IR detectors for room temperature operation.     

Interface effect on superlattice quality and optical properties of InAs/GaSb type-II superlattices grown by molecular beam epitaxy

We systematically investigate the influence of InSb interface (IF) engineering on the crystal quality and optical properties of strain-balanced InAs/GaSb type-II superlattices (T2SLs). The type-II superlattice structure is 120 periods InAs (8 ML)/GaSb (6 ML) with different thicknesses of InSb interface grown by molecular beam epitaxy (MBE). The high-resolution x-ray diffraction (XRD) curves display sharp satellite peaks, and the narrow full width at half maximum (FWHM) of the 0th is only 30-39 arcsec. From high-resolution cross-sectional transmission electron microscopy (HRTEM) characterization, the InSb heterointerfaces and the clear spatial separation between the InAs and GaSb layers can be more intuitively distinguished. As the InSb interface thickness increases, the compressive strain increases, and the surface "bright spots" appear to be more apparent from the atomic force microscopy (AFM) results. Also, photoluminescence (PL) measurements verify that, with the increase in the strain, the bandgap of the superlattice narrows. By optimizing the InSb interface, a high-quality crystal with a well-defined surface and interface is obtained with a PL wavelength of 4.78 μ, which can be used for mid-wave infrared (MWIR) detection.     

(InAs)1/(GaSb)1超晶格原子链的第一原理研究

利用第一原理平面波赝势法, 对(InAs)₁/(GaSb)₁超晶格原子链的原子结构、力学特性、电子能带结构、 声子结构和光学特性进行研究, 并结合密度泛函理论数值原子轨道赝势法和非平衡格林函数法计算量子输运特性. 与二维层结构的(InAs)₁/(GaSb)₁超晶格相比, (InAs)₁/(GaSb)₁超晶格原子链的能带结构有明显不同, 在某些情况下表现为金属能带特性. 对理想条件下(InAs)₁/(GaSb)₁ 超晶格原子链的力学强度计算表明, 该结构可承受的应变高达 ε=0.19. 通过对声子结构的完整布里渊区分析, 研究了(InAs)₁/(GaSb)₁超晶格原子链的结构稳定性. 对两端接触电极为Al纳米线的InAs/GaSb超晶格原子链的电子输运特性计算表明, 电导随链长和应变的改变而发生非单调变化.光吸收谱的计算结果表现出在红外波段具有陡峭吸收边, 截止波长随超晶格原子链的结构而变化.预计InAs/GaSb超晶格原子链可应用于红外光电子纳米器件, 通过改变超晶格原子链的结构来调节光电响应波段.     

Influence of the p-type doping on the radiometric performances of MWIR InAs/GaSb superlattice photodiodes

In this paper, quantum efficiency (QE) measurements performed on type-II InAs/GaSb superlattice (T2SL) photodiodes operating in the mid-wavelength infrared domain, are reported. Several comparisons were made in order to determine the SL structure showing optimum radiometric performances: same InAs-rich SL structure with different active zone thicknesses (from 0.5 μm to 4 μm) and different active zone doping (n-type versus p-type), same 1 μm thick p-type active zone doping with different SL designs (InAs-rich versus GaSb-rich and symmetric SL structures). Best result was obtained for the p-type doped InAs-rich SL photodiode, with a 4 μm active zone thickness, showing a QE that reaches 61% at λ = 2 μm and 0 V bias voltage.     

Performance of dual-band short-or mid-wavelength infrared photodetectors based on InGaAsSb bulk materials and InAs/GaSb superlattices

In this paper,we demonstrate bias-selectable dual-band short-or mid-wavelength infrared photodetectors based on In_(0.24)Ga_(0.76)As_(0.21)Sb_(0.79)bulk materials and InAs/GaSb type-II superlattices with cutoff wavelengths of 2.2μm and 3.6μm,respectively.At 200 K,the short-wave channel exhibits a peak quantum efficiency of 42%and a dark current density of5.93×10~(-5)A/cm~2at 500 mV,thereby providing a detectivity of 1.55×10~(11)cm·Hz~(1/2)/W.The mid-wave channel exhibits a peak quantum efficiency of 31%and a dark current density of 1.22×10~(-3)A/cm~2at-300 mV,thereby resulting in a detectivity of 2.71×10~(10)cm·Hz~(1/2)/W.Moreover,we discuss the band alignment and spectral cross-talk of the dual-band n-i-p-p-i-n structure.     

Modeling and simulation of long-wave infrared InAs/GaSb strained layer superlattice photodiodes with different passivants

Current–voltage characteristics of long-wave infrared (LWIR) InAs/GaSb strained layer superlattice photodiodes (cut-off wavelength ∼10 μm), passivated with different surface passivants, have been modeled and simulated using ATLAS software from SILVACO. The simulated results are fitted to previous experimental results obtained on unpassivated devices and those passivated by silicon-dioxide (SiO₂), silicon nitride (Si_xN_y) and zinc sulfide (ZnS). Surface parameters in terms of surface recombination velocity, shunt resistance and interface trap density are extracted for different passivants. The performance of silicon-dioxide passivated diode is solely dominated by a shunt leakage path with a shunt resistance value of 0.56 Ω-cm². Extracted electron and hole surface recombination velocities have values of 10⁵ cm/s and 10⁷ cm/s for unpassivated, 10³ cm/s and 10⁵ cm/s for Si_xN_y passivated and 10² cm/s and 10³ cm/s for ZnS passivated devices. Interface trap density follows a similar trend with values of 10¹⁵ cm⁻², 8.5 × 10¹⁴ cm⁻² and 10¹⁰ cm⁻² for unpassivated, Si_xN_y passivated and ZnS passivated devices respectively. The suitability and limitations of the simulation tool are discussed.     

Growth of high material quality InAs/GaSb type-II superlattice for long-wavelength infrared range by molecular beam epitaxy

By optimizing the V/III beam-equivalent pressure ratio, a high-quality InAs/GaSb type-II superlattice material for the long-wavelength infrared (LWIR) range is achieved by molecular beam epitaxy (MBE). High-resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectrometer are used to characterize the material growth quality. The results show that the full width at half maximum (FWHM) of the superlattice zero-order diffraction peak, the mismatching of the superlattice zero-order diffraction peak between the substrate diffraction peaks, and the surface roughness get the best results when the beam-equivalent pressure (BEP) ratio reaches the optimal value, which are 28 arcsec, 13 arcsec, and 1.63 Å, respectively. The intensity of the zero-order diffraction peak is strongest at the optimal value. The relative spectral response of the LWIR detector shows that it exhibits a 100% cut-off wavelength of 12.6 μm at 77 K. High-quality epitaxial materials have laid a good foundation for preparing high-performance LWIR detector.     

Mini-gaps and novel giant negative magnetoresistance in InAs/GaSb semimetallic superlattices

We have studied the temperature dependence of the magnetoresistance of semimetallic InAs/GaSb structures with magnetic field applied parallel to the layers. We present the first unambiguous evidence for the presence of a mini-gap at the crossing point between the electron and hole dispersion relations. The resistivity is found to change from semiconductor-like behaviour with a strong temperature dependence at low parallel magnetic fields to that of a semimetal with a weak temperature dependence at high field. Furthermore, the magnetoresistance, for intrinsic samples, is found to decrease with field by as much as 70% at low temperatures. As the parallel magnetic field is increased the centres of the electron and hole dispersions are shifted apart in k-space and at sufficiently high field the mini-gap is destroyed and the bands overlap fully. Finally, a theoretical model allows us to estimate that the mini-gap is of order 7 meV.     

InAs/GaSb量子阱的能带结构及光吸收

采用八能带K-P理论以及有限差分方法,研究了沿[001]方向生长的InAs/GaSb二类断带量子阱体系的能带结构、波函数分布和对[110]方向线性偏振光的吸收特性.研究发现,通过改变InAs或GaSb层的厚度,可有效调节该量子阱体系的能带结构及波函数分布.计算结果表明,当InAs/GaSb量子阱的导带底与价带顶处于共振状态时,导带基态与轻空穴基态杂化效应很小,且导带基态与第一激发态的波函数存在较大的重叠,导带基态与第一激发态之间在布里渊区中心处的跃迁概率明显大于导带底与价带顶处于非共振状态时的跃迁概率.研究结果对基于InAs/GaSb二类断带量子阱体系的中远红外波段的新型级联激光器、探测器等光电器件的设计具有重要意义.     

N,Sb和单分子层数对GaAs/GaInNAsSb超晶格性能的影响

用Keating的价力场 (valenceforcefield)模型和蒙特卡罗方法计算了GaAs GaInNAsSb超晶格中键的分布、原子的精确位置以及应变 .用折叠谱法 (foldedspectrummethod)结合Williamson经验赝势法计算了GaAs GaInNAsSb超晶格应变条件下的电子结构 .讨论了N和Sb原子以及超晶格单分子层数对电子结构的影响 .发现导带底电子态在N原子周围的局域化减小了光跃迁矩阵元 ,从而影响了该超晶格的发光性能 .计算并讨论了超晶格的电子和空穴的有效质量 .     

稀土元素Sc、Tm掺杂GaSb的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算方法研究了本征GaSb以及稀土元素Sc、Tm掺杂后的GaSb的电子结构和光学性质.计算结果表明：Sc、Tm掺杂后GaSb仍为直接带隙的P型半导体. Sc掺杂后GaSb在导带中诱导了浅能级缺陷态,带隙变窄. Tm掺杂后GaSb在导带中诱导了深能级缺陷态,带隙变宽. Sc、Tm掺杂后使能量损失减小并且均增强了GaSb对中红外波段光子的吸收,其中Sc掺杂效果更佳,在4.3μm(0.28 eV)处存在吸收峰,峰值为1.7×10~5 cm^-1,且直到波长为6.8μm(0.18 eV)时,光吸收系数仍能达到1×10~5 cm^-1.计算结果为拓展GaSb基半导体材料在红外探测器、红外半导体激光器等领域的应用提供理论参考.     

Effect of compensation doping on the electrical and optical properties of mid-infrared type-II InAs/GaSb superlattice photodetectors

This paper presents a theoretical study on the electrical and optical properties of mid-infrared type-II InAs/GaSb superlattices with different beryllium concentrations in the InAs layer of the active region. Dark current, resistance-area product, absorption coefficient and quantum efficiency characteristics are thoroughly examined. The superlattice is residually n-type and it becomes slightly p-type by varying beryllium-doping concentrations, which improves its electrical performances. The optical performances remain almost unaffected with relatively low p-doping levels and begin to deteriorate with increasing p-doping density. To make a compromise between the electrical and optical performances, the photodetector with a doping concentration of 3 × 1015 cm-3 in the active region is believed to have the best overall performances.     

量子局域效应和应力对GaSb纳米线电子结构影响的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法, 研究了不同晶体结构和尺寸的GaSb纳米线能带结构特性和载流子的有效质量, 以及单轴应力对GaSb纳米线能带结构的调控. 研究结果表明: 闪锌矿结构[111]方向和纤锌矿结构[0001]方向的小尺寸GaSb纳米线均出现间接带隙的能带结构, 并可通过单轴应力来实现纳米线能带结构由间接带隙到直接带隙的转变, 其中, 闪锌矿结构[111]方向GaSb纳米线仅在受到单轴拉伸应力时才发生能带由间接带隙到直接带隙的转变, 而纤锌矿结构[0001]方向GaSb纳米线无论受单轴拉伸还是压缩应力的作用均可实现能带由间接带隙到直接带隙的转变; [111]和[0001]方向GaSb纳米线的带隙和载流子有效质量与纳米线直径呈非线性关系, 并随纳米线直径的减小而增大; 同一方向和尺寸的GaSb纳米线, 其空穴有效质量要小于电子有效质量, 这表明小尺寸GaSb纳米线有利于空穴载流子输运.     

第一性原理研究3C-SiC/Mg复合材料界面的电子结构

基于密度泛函理论的第一性原理平面波赝势方法对4种3C-SiC(111)/Mg(0001)界面模型进行研究.界面间距和粘附功的计算表明,结构优化之后的界面模型只在z轴方向发生了移动,界面间距发生了不同程度的缩短;中心型模型的稳定性强于顶位型模型,C终端结构的稳定性强于Si终端结构,中心型C终端的界面模型具有最大的粘附功(2.5834 J/m~2)和最小的界面间距(1.7193?),是4种模型中最稳定的结构. Mulliken电荷、电荷密度分布、差分电荷密度和态密度的计算表明,中心性结构的Si终端和C终端模型界面处存在共价键、离子键和金属键.     

LaAlO3/SrTiO3界面的电子结构及光学性质的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,结合局域密度近似（LDA）研究了钙钛矿结构氧化物LaAlO3 /SrTiO3界面的电子结构及光学性质。能带结构分析表明当形成(AlO2)-/(TiO2)0界面时其禁带宽度为1.888 eV,呈现绝缘体的性质,当形成(LaO)+/(SrO)0界面时其禁带宽度为0.021 eV,呈现半导体或半金属性质。同时,对不同界面的光学性质也进行了研究,结果表明纯相的LaAlO3和SrTiO3的吸收系数、反射系数及能量损失谱强度明显高于由这两种单质形成不同界面的强度。     

LaAlO3/SrTiO3界面的电子结构及光学性质的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,结合局域密度近似(LDA)研究了钙钛矿结构氧化物LaAlO3/SrTiO3界面的电子结构及光学性质.能带结构分析表明当形成(AlO2)-/(TiO2)0界面时其禁带宽度为1.888 eV,呈现绝缘体的性质,当形成(LaO)+/(SrO)0界面时其禁带宽度为0.021eV,呈现半导体或半金属性质.同时,对不同界面的光学性质也进行了研究,结果表明纯相的LaAlO3和SrTiO3的吸收系数、反射系数及能量损失谱强度明显高于由这两种单质形成不同界面的强度.     

Very long wavelength infrared focal plane arrays with 50% cutoff wavelength based on type-Ⅱ In As/GaSb superlattice

A very long wavelength infrared(VLWIR) focal plane array based on In As/Ga Sb type-Ⅱ super-lattices is demonstrated on a Ga Sb substrate. A hetero-structure photodiode was grown with a 50% cut-off wavelength of 15.2 μm, at 77 K.A 320×256 VLWIR focal plane array with this design was fabricated and characterized. The peak quantum efficiency without an antireflective coating was 25.74% at the reverse bias voltage of-20 mV, yielding a peak specific detectivity of 5.89×1010cm·Hz~(1/2)·W~(-1). The operability and the uniformity of response were 89% and 83.17%. The noise-equivalent temperature difference at 65 K exhibited a minimum at 21.4 mK, corresponding to an average value of 56.3 mK.