极化效应对AlGaN/GaN异质结p-i-n光探测器的影响

基于等效薄层电荷近似模拟表征极化电荷的作用, 通过自洽求解Poisson-Schrödinger方程以及求解载流子连续性方程, 计算并且讨论了p-AlGaN层掺杂浓度和界面极化电荷对AlGaN/GaN异质结p-i-n紫外探测器能带结构和电场分布以及光电响应的影响. 结果表明, 极化效应与p-AlGaN层掺杂浓度相互作用对探测器性能有较大影响. 其中, 在完全极化条件下, p-AlGaN层掺杂浓度越大, p-AlGaN层的耗尽区越窄, i-GaN层越容易被耗尽, 器件光电流越小. 在一定掺杂浓度条件下, 极化效应越强, p-AlGaN层的耗尽区越宽, 器件的光电流越大. 最后还分析了该结构在不同温度下的探测性能, 证明了该结构可以在高温下正常工作.     

Optimization of a non-cryogenic quantum infrared detector

A study of the optimization of the detectivity of a mid-infrared double heterostructure photovoltaic detector is proposed. Simple approximate analytic expressions for the dark current are compared with full numerical calculations, and provide a physical insight into the mechanisms dominating the dark current. The analysis is performed step by step, from a simple p–n junction to the full double heterostructure. The influence of temperature, barrier band gap energy in a double heterostructure, doping density in the active region, on diffusion and generation–recombination mechanisms is analyzed. It is finally shown how the performances of a double heterostructure photovoltaic detector can be improved by controlled doping of the active region.     

Optimization, design and fabrication of a non-cryogenic quantum infrared detector

A study of the optimization of the detectivity of a mid infrared double heterostructure photovoltaic detector is proposed. Simple approximate analytic expressions for the dark current are compared with full numerical calculations, and give physical insight on the mechanisms dominating the dark current. The analysis is performed step by step, from a simple p–n junction to the full double heterostructure. The influence of temperature, barrier band gap energy in a double heterostructure, doping density in the active region, on diffusion and generation–recombination mechanisms is analyzed. It is shown how the performances of a double heterostructure photovoltaic detector can be improved by a controlled doping the active region. Nevertheless, its development is still limited by the difficulties occurring during device processing. For example, the use of dry etching for the processing of InAs_0.91Sb_0.09 p–i–n photovoltaic detectors induces a strong leakage current along the mesa edge. In this letter, we show an improvement of the R₀A characteristic by several orders of magnitude at low temperature by using an Ion Beam Etching (IBE) followed by a wet chemical etching. This optimized and reliable device processing allows us to demonstrate that the detector performance is actually limited by the diffusion current of holes. Finally, we discuss the ability of an n-type barrier made of InAs/AlSb super-lattice to avoid hole diffusion and to improve the R₀A characteristic of these detectors. To cite this article: B. Vinter et al., C. R. Physique 4 (2003).     

Improved thermoelectric power factor in metal-based superlattices

In this paper we present a detailed theory of electron and thermoelectric transport perpendicular to heterostructure superlattices. This nonlinear transport regime above barriers is also called heterostructure thermionic emission. We show that metal-based superlattices with tall barriers can achieve a large effective thermoelectric figure of merit (ZT > 5 at room temperature). A key parameter to achieving high performance is the nonconservation of lateral momentum during the thermionic emission process. Conservation of lateral momentum is a consequence of translational symmetry in the plane of the superlattice. We also discuss the use of nonplanar barriers and embedded quantum dot structures to achieve high thermoelectric conversion efficiency.     

Scanning tunneling microscopy and potentiometry at GaAs/AlGaAs heterojunctions

In certain problems of electrical transport through condensed matter, it is important to know the potential distribution with nanometer resolution, e.g., at interfaces (Schottky barriers) or pn junctions. Scanning tunneling potentiometry, a new application of scanning tunneling microscopy, is capable of providing this information. The tunnel current is used for simultaneously sensing probe-to-sample distance and local potential. We have used this technique for the localization of semiconductor junctions in cleaved MBE-grown AlGaAs-GaAs laser structures. Among other applications, the electron injection into the active GaAs layer of a heterostructure laser diode was readily observed.     

N极性GaN/AlGaN异质结二维电子气模拟

通过自洽求解薛定谔方程和泊松方程,较系统地研究了GaN沟道层、AlGaN背势垒层、Si掺杂和AlN插入层对N极性GaN/AlGaN异质结中二维电子气(2DEG)的影响,分析表明,GaN沟道层厚度、AlGaN背势垒层厚度及Al组分变大都能一定程度上提高二维电子气面密度,AlGaN背势垒层的厚度和Al组分变大也可提高二维电子气限阈性,且不同的Si掺杂形式对二维电子气的影响也有差异,而AlN插入层在提高器件二维电子气面密度、限阈性等方面表现都较为突出,在模拟中GaN沟道层厚度小于5nm时无法形成二维电子气,超过20nm后二维电子气面密度趋于饱和,而AlGaN背势垒厚度超过40nm后二维电子气也有饱和趋势,对均匀掺杂和delta掺杂而言AlGaN背势垒层Si掺杂浓度超过5×10~(19)cm~(-3)后2DEG面密度开始饱和,而厚度为2nmAlN插入层的引入会使2DEG面密度从无AlN插入层时的0.93×10~(13)cm~(-2)提高到1.17×10~(13)cm~(-2)。     

Normal distribution profile for doping concentration in multilayer tunnel junction

In this paper, the effect of doping concentration and layer thickness on the performance of tunnel junctions (TJs) is studied. We investigate the behavior of single, double and triple layer structures of TJs. Triple layer structure shows better performance in comparison with the other structures and can reach the higher tunneling current besides lower voltage drop. Also, the behavior of the triple layer TJ with different doping concentration profiles is studied. We propose a new normal distribution profile for doping concentration in multilayer TJs which shows better performance in comparison with the linear and graded doping concentration profiles. The higher $\upalpha $ parameters in normal distribution enhance the device performance with increasing the smoothness of doping variations in the center and edge of the TJ. Finally, we examine different thicknesses of triple layer TJ in order to achieve the optimum structure.     

Effects of lanthanides doping on the optical properties of graphene/WSe2 heterostructure based on ab-initio calculations

Based on the density functional theory, we discussed the electronic and optical properties of graphene/ WSe₂ (GW) heterostructure after lanthanides doping. Red shift appears and the optical parameter values are improved in the low energy region after the lanthanides are doped. Different doping types are also discussed. In the case of single doping, substitute Yb atom on W site will improve the peak values of the optical parameters greatly. In the case of co-doping, it is found that the effect will be more obvious when the two doped lanthanide atoms are located in the second neighboring positons. These results suggest that lanthanides doping does adjust the electronic structure and improve the optical properties of GW heterostructures, which providing useful guidance for the design of novel optical nanodevices based on two-dimensional materials.     

An iterative solution of the three-colour problem on a random lattice

We study the generalisation of Baxter's three-colour problem to a random lattice. Rephrasing the problem as a matrix model problem we discuss the analyticity structure and critical behaviour of the resulting matrix model. Based on a set of loop equations we develop an algorithm which enables us to solve the three-colour problem recursively.     

Two-dimensional modelling of HgCdTe photoconductive detectors

The performance of n-type HgCdTe mid-wavelength infrared (MWIR) photoconductors has been investigated using two-dimensional (2-D) device modelling. A comparison has been made between a practical detector structure with planar contacts on the upper HgCdTe surface, and a structure commonly used in one-dimensional (1-D) device modelling with end contacts to the photo-absorbing slab of semiconductor. This comparison highlights differences in detector responsivity, and spatial distribution of both the applied electric field and photogenerated minority carriers. The practical device structure, where 2-D effects are most evident, provided a good fit to experimental results for frontside-illuminated n-type HgCdTe photoconductive detectors with n⁺/n blocking contacts without needing to include S, the contact recombination velocity, which is commonly employed in 1-D models as a fitting parameter. Instead, only the n⁺ doping density (1 × 10¹⁶ cm^–3) and n⁺ doping region (depth of 3 μm), were used to account for the partial blocking of minority carriers by the contact region. In addition, the 2-D model was used to examine the influence of n⁺ blocking contact geometry and doping density on n-type HgCdTe photoconductor responsivity performance.     

Electronic structure of graphene/Y2C heterostructure and related doping effect

Until now, many attempts have been made to dope graphene in various ways, but each method turned out to have pros and cons. In this study, to overcome the limitations of doping methods, yttrium hypocarbide (Y₂C) is investigated as one prospective material to dope graphene, using density functional theory calculations. In monolayer Y₂C, the anionic electrons localized away from Y atomic layers are confirmed to contribute to occupied states near the Fermi level. Next, we investigate the electronic structure of graphene in heterojunction with Y₂C. Anionic electrons of Y₂C occupy the empty states of graphene in graphene/Y₂C heterostructure, which makes the Dirac cone of graphene located at about 1.7 eV below the Fermi level. Such charge transfer of anionic electrons to graphene and the flatness of electric cloud of anionic electrons leads to evenly n-doped graphene in graphene/Y₂C heterostructure. This suggests that Y₂C is a good candidate to dope graphene.     

Novel carbon nanotube field effect transistor with graded double halo channel

A novel carbon nanotube field effect transistor with symmetric graded double halo channel (GDH–CNTFET) is presented for suppressing band to band tunneling and improving the device performance. GDH structure includes two symmetric graded haloes which are broadened throughout the channel. The doping concentration of GDH channel is at maximum level at drain/source side and is reduced gradually toward zero at the middle of channel. The doping distribution at source side of channel reduces the drain induced barrier lowering (DIBL) and the drain side suppresses the band to band tunneling effect. In addition, broadening the doping throughout the channel increases the recombination of electrons and holes and acts as an additional factor for improving the band to band tunneling. Simulation results show that applying this structure on CNTFET enhances the device performance. In comparison with double halo structure with equal saturation current, the proposed GDH structure shows better characteristics and short channel parameters. Furthermore, the delay and power delay product (PDP) analysis versus on/off current ratio shows the efficiency of the proposed GDH structure.     

Interfacial defect engineering and photocatalysis properties of hBN/MX2 (M = Mo,W, and X = S,Se heterostructures

Van der Waals (VDW) heterostructures have attracted significant research interest due to their tunable interfacial properties and potential applications in many areas such as electronics, optoelectronic, and heterocatalysis. In this work, the influences of interfacial defects on the electronic structures and photocatalytic properties of hBN/MX₂ (M = Mo, W, and X=S, Se) are studied using density functional theory calculations. The results reveal that the band alignment of hBN/MX₂ can be adjusted by introducing vacancies and atomic doping. The type-I band alignment of the host structure is maintained in the heterostructure with n-type doping in the hBN sublayer. Interestingly, the band alignment changed into the type-II heterostructrue due to V_B defect and p-type doping is introduced into the hBN sublayer. This can conduce to the separation of photo-generated electron-hole pairs at the interfaces, which is highly desired for heterostructure photocatalysis. In addition, two Z-type heterostructures including hBN(Be_B)/MoS₂, hBN(Be_B)/MoSe₂, and hBN(V_N)/MoSe₂ are achieved, showing the decreasing of band gap and ideal redox potential for water splitting. Our results reveal the possibility of engineering the interfacial and photocatalysis properties of hBN/MX₂ heterostructures via interfacial defects.     

Effects of doping and free carriers on the refractive index of direct-gap semiconductors

A model for the doping and current dependence of the refractive index of direct-gap semiconductors has been developed and applied to GaAs in order to explain certain properties of single heterostructure injection lasers. The model involves the concept of an effective energy gap which takes account of the effects of doping and free carriers via the Burstein shift, the exchange interaction, and the average screened Coulomb potential of the impurities. This effective gap, together with empirical results for the energy dependence of the refractive index, facilitates the calculation of refractive index changes with doping and current. Numerical results are given for GaAs, and the model is applied particularly to the substrate and active regions of single heterostructure injection lasers. In these devices the refractive index values are particularly important in determining the dielectric waveguide properties. With the aid of our model, this feature can be discussed in detail, with special reference to the breakdown of the waveguide effect which occurs (a) for substrate doping levels below a critical value, and (b) for currents well above threshold ('saturation').     

有源区掺杂浓度对GaAs/GaAlAsDH-ELED器件性能的影响

本文阐明了影响GaAs/GaAlAs双异质结边发光管特性的因素,其中着重讨论了有源区的掺杂浓度和掺杂类型对DH-ELED光功率和调制能力的影响.结果表明:掺Si或轻掺Ge的LEDs具有较高的功率和慢的响应,这些LED只可用于低码速和较高功率的系统.有源区重掺Ge导致器件的响应快和功率较低,这种LED可用于高速系统.通过最佳掺杂,可获得高速和大功率的DH-ELED.     

Tight-binding investigation of electron tunneling through ultrathin SiO2gate oxides

We investigate electron tunneling through ultrathin gate oxides using scattering theory within a tight-binding framework. We employ Si[100]/SiO₂/Si[100] model junctions with oxide thicknesses between 7 and 18 Å. This approach accounts for the three-dimensional microscopic structure of the model junctions and for the three-dimensional nature of the corresponding complex energy bands. The equilibrium positions of the atoms in the heterostructure are derived from first-principles density-functional calculations. We show that the present method yields qualitative and quantitative differences from conventional effective-mass theory.     

InAlAs浓度对In_(0.83)Al_(0.17)As/In_(0.83)Ga_(0.17)As红外探测器特性的影响北大核心CSCD

红外探测器的性能受内部结构各层掺杂浓度的影响,而倍增层掺杂浓度会明显改变器件的性能。为了降低暗电流,提高器件性能,采用三元化合物In_(0.83)Al_(0.17)As作为倍增层材料,借助仿真软件Silvaco详细研究了In_(0.83)Al_(0.17)As/In_(0.83)Ga_(0.17)As红外探测器的倍增层掺杂浓度对器件电场强度、电流特性和光响应度的影响规律。结果表明,随着倍增层掺杂浓度的增加,器件倍增层内的电场强度峰值增加,同时,器件的暗电流与光响应度减小。进一步研究发现,当倍增层掺杂浓度为2×10^(16) cm^(−3)时,器件获得最优性能,暗电流密度为0.62144 A/cm^(2),在波长为1.5μm时,光响应度和比探测率分别为0.9544 A/W和1.9475×10^(9) cmHz^(1/2)W^(−1)。     

双氮原子掺入二氧化钛电子结构的第一原理研究

采用基于第一性原理的平面波超软赝势方法计算研究了双N原子掺入金红石相TiO_2的几何结构和电子结构.通过比较三种可能的掺杂方式的总能发现,两个氮原子占据两个相邻的B原子位置时具有最稳定的结构.电子结构分析表明,双N原子掺杂TiO_2出现了杂质能级,三种结构的能带间隙均减小,其中杂质原子最近邻占位时,带隙最小,随着两个杂质原子的距离增大,带隙会逐渐变大.     

Superlattice doping interfaces

The effect of background doping on current transport in quantum well structures is investigated by the use of n⁺ -n junctions superimposed on superlattices. Data are presented which show that the applied bias appears across only lightly doped and depleted regions. Current vs. voltage characteristics are observed which are consistent with expected electron barriers for these superlattice structures. Negative differential resistance effects and conductance oscillation effects are also observed, and are shown to originate from movement of bias-dependent depletions.     

Deep-levels in silicon waveguides: a route to high yield fabrication

In this paper we outline two recent results which demonstrate the utility of deep-level engineering in silicon photonics. We describe the integration of silicon waveguide p-i-n photo-detectors in a ring (or race-track) resonator structure. The detectors are made sensitive to wavelengths around 1,550?nm via the introduction of deep-levels into the intrinsic volume of the waveguide detector. By exploiting the multiple-pass of the optical signal through the detector, we are able to significantly decrease the size of the detector structure (relative to straight waveguide detectors) while maintaining excellent responsivity on resonance. We also describe the use of deep-levels for optical modulation. Preliminary results show that thallium doped silicon waveguides may be switched between a dark and transparent condition through the variation of phosphorus doping. It is suggested that active devices may be fabricated in such a way as to vary the occupancy of the thallium level through field mediated modulation. The straightforward fabrication methods described lend themselves to a high-volume, high yield manufacturing process which should find general applicability in wavelength division multiplexing systems.