Current gain and external quantum efficiency modeling of GeSn based direct bandgap multiple quantum well heterojunction phototransistor

This paper aims to provide the performance characteristics of proposed, strain balanced direct band gap multiple quantum wells (MQWs) hetero phototransistor (HPT) made of SiGeSn/GeSn alloys grown on Si substrate which is compatible with recent CMOS fabrication technology. This also presents a comprehensive comparison of this proposed structure with the existing HPT structure made of indirect gap Ge/SiGe MQWs. Alloys of Ge and Sn grown on Si platform shows about tenfold increase in absorption over Ge at C and L-bands due to direct nature of band gap in GeSn. Initial work begins the solution of continuity equation to solve the different terminal current densities and optical gain of the multiple quantum well structure. Main analysis was concentrated on finding the external quantum efficiency depending on the doping variations of emitter and base, base width etc. Finally the photocurrent density variations are estimated for the structure and compared with existing indirect band gap HPT. The calculated values for direct band gap GeSn HPT device are found to be comparable with those for indirect band gap SiGe device to flourish as a potential candidate of photo detectors for the present day telecommunication network.     

Optical band gap of Si nanoclusters

We discuss the nature of the optical transitions in porous silicon and in Si nanoclusters in the light of recent theoretical calculations. The accuracy of the different techniques used to calculate the band gap of Si nanoclusters is analyzed. We calculate the electronic structure of crystallites in the Si-III (BC8) crystalline phase which is known to have a direct gap and we examine the effect of quantum confinement on clusters of SiGe alloy and amorphous silicon. The comparison with the experiments for all the systems suggests the possibility of different channels for the radiative recombination.     

Impurity optical absorption in parabolic quantum well

Optical absorption in GaAs parabolic quantum well in the presence of hydrogenic impurity is considered. The absorption coefficient associated with the transitions between the upper valence subband and donor ground state is calculated. The impurity ground state wave function and energy are obtained using the variational method. Dependence of the absorption spectra on impurity position in quantum well was investigated. It is shown, that along with quantum well width decrease the absorption threshold shifts to higher frequencies. Results obtained within frames of parabolic approximation are compared with results for rectangular infinite-barrier quantum well case. The acceptor state → conduction band transitions considered as well.     

Modulation of the absorption coefficient at 1.3 μm in Ge/SiGe multiple quantum well heterostructures on silicon

We report modulation of the absorption coefficient at 1.3?μm in Ge/SiGe multiple quantum well heterostructures on silicon via the quantum-confined Stark effect. Strain engineering was exploited to increase the direct optical bandgap in the Ge quantum wells. We grew 9?nm-thick Ge quantum wells on a relaxed Si0.22Ge0.78 buffer and a contrast in the absorption coefficient of a factor of greater than 3.2 was achieved in the spectral range 1290-1315?nm.     

第一性原理研究B掺杂Mn_4Si_7的电子结构和光学性质

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及B掺杂Mn_4Si_7的电子结构和光学性质进行理论计算.研究结果表明,未掺杂Mn_4Si_7是间接带隙半导体,其禁带宽度为0.786 eV,B掺杂后其禁带宽度下降为0.723 eV. B掺杂Mn_4Si_7是p型半导体材料.未掺杂Mn_4Si_7在近红外区的吸收系数达到10~5 cm~(-1),B掺杂引起Mn_4Si_7的折射率、吸收系数、反射系数及光电导率增加.     

Electroluminescence study on electron hole plasma in strained SiGe epitaxial layers

The electroluminescence (EL) of thick fully strained SiGe layers is investigated in order to clarify the recombination mechanisms. In the investigated temperature range of 20–80 K and for SiGe thickness of 70–450 nm an electron–hole plasma (EHP) is observed even at low current densities of 1 Acm⁻². In SiGe-based quantum devices the EHP condition is expected to be attained at even lower injection levels. We used the band filling model for EHP to extract the renormalized gap of SiGe in dependence on the plasma density by performing a line shape analysis of EL spectra. The results were compared with the theoretical prediction. Based on this analysis as well as on measurements and modelling of the spectral photocurrent and the external quantum efficiency, we were able to evaluate parameters of recombination transitions for EHP in SiGe. Above 200 K there is an important contribution to EL from the silicon regions. For a better evaluation of the SiGe contribution, we compared EL of SiGe diodes with EL of pure silicon diodes.     

InGaAs/GaAs应变量子阱激光器线宽展宽因子的理论研究

详细地介绍了计算线宽展宽因子(α因子)的理论基础及推导过程, 建立了α因子的简便模型. 该模型分别考虑了带间跃迁、带隙收缩和自由载流子效应对α因子的影响, 利用不同载流子浓度下的增益曲线得到光子能量随载流子浓度的变化速率以及微分增益, 进而对α因子进行近似计算. 模拟计算了InGaAs/GaAs量子阱激光器的增益曲线及α 因子的大小, 计算结果与文献报道的实验值相符. 进一步讨论了InGaAs/GaAs量子阱阱宽及In组分对α 因子的影响. 结果表明, α 因子随In组分和阱宽的增加而增加.     

SiGe superlattice nanocrystal pure and doped with substitutional phosphorus single atom: Density functional theory study

Ab initio density functional theory is used to simulate electronic structure of hydrogenated SiGe nanocrystal superlattice pure and doped with substitutional P single atom. The results of electronic structure calculations are compared to the same size silicon and germanium nanocrystals. The comparison reveals that the energy gap of the three kinds of nanocrystals is nearly the same in non-relativistic and relativistic limits. Because of large width of gap in the present small nanocrystals the relativistic corrections are not as much important as in the case of bulk crystals. The doping of SiGe nanocrystal with P single atom introduced an impurity level at 4 eV below original conduction band edge. This result is much larger than comparable silicon bulk and nanocrystal doping with P atoms. Results also show that the deep internal angles and bonds in SiGe nanocrystals reach approximately the angles and structure of bulk crystals after nearly three surface layers. A double positively charged layer is located at the Ge terminated surface of SiGe nanocrystal. This layer is enhanced and is accompanied with a large increase of the dipole moment of the nanocrystal in the case of P doped nanocrystal. Due to oscillatory lattice potential in SiGe superlattice, density of states show that bands are broken up to sub-bands in comparison with silicon nanocrystal density of states especially at the conduction band.     

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.     

半导体超晶格子带间跃迁光吸收理论研究

从理论上研究了半导体超晶格子带间跃迁的光吸收性质，以GaAs/AlxGa1－xAs超晶格为例进行数值计算，分析了该材料的吸收系数随入射光光子能量、光场强度和超晶格结构参量（阱宽，垒宽，势垒高）的变化关系计算表明：随着入射光光子能量的变化，出现非对称的吸收峰；光强只改变吸收系数大小；超晶格结构参量会改变吸收谱的谱宽和吸收峰所对应的入射光频率随着超晶格阱宽（垒宽）的增大，吸收谱由宽变窄，吸收峰红移；随着超晶格Al组分变大，吸收谱变窄.     

B、N单掺杂3C-SiC电子结构和光学性质的第一性原理研究

采用第一性原理的密度泛函理论赝势平面波方法,计算了未掺杂与B、N单掺杂3C-SiC的电子结构和光学性质.结果表明:掺杂改变了3C-SiC费米面附近的电子结构;B掺杂使得禁带宽度减小,价带顶上移,费米能级进入价带,形成p型半导体;N掺杂使得禁带宽度减小,导带底下移,费米能级进入导带,形成n型半导体.B、N掺杂均提高了3C-SiC在低能区的折射率、消光系数和吸收系数,增强了对红外光谱的吸收.     

Negative photoconductivity of selectively doped SiGe/Si: B heterostructures with a two-dimensional hole gas in the middle-infrared range

The spectra of lateral photoconductivity in selectively doped SiGe/Si: B heterostructures with a two-dimensional hole gas are analyzed. It is revealed that the lateral photoconductivity spectra of these heterostructures exhibit two signals opposite in sign. The positive signal of the photoconductivity is associated with the impurity photoconductivity in silicon layers of the heterostructures. The negative signal of the photoconductivity is assigned to the transitions of holes from the SiGe quantum well to long-lived states in silicon barriers. The position of the negative photoconductivity signal depends on the composition of the quantum well, and the energy of the low-frequency edge of this signal is in close agreement with the calculated band offset between the quantum-confinement level of holes in the quantum well and the valence band edge in the barrier.     

Intersubband transitions in an asymmetric double quantum well

The intersubband optical absorption in an asymmetric double quantum well for different barrier widths and the right well widths are theoretically calculated within the framework of effective mass approximation. The results obtained show that the intersubband transitions and the energy levels in an asymmetric double quantum well can be importantly modified and controlled by the barrier width and the well width. The sensitivity to the barrier and well widths of the absorption coefficient can be used in various optical semiconductor device applications.     

Superradiance of a degenerate exciton gas in semiconductors with indirect fundamental absorption edge

It is predicted that superradiant states can be formed in a degenerate exciton gas in a semiconductor with an indirect fundamental absorption edge. The superradiance results from four-particle recombination processes and occurs at photon energies approximately twice as high as the band gap energy. Experimental results supporting the possibility of the observation of superradiance from SiGe/Si quantum wells are presented.     

Features of the fine structure of the fundamental absorption edge in low-doped semiconductor quantum wells with allowance for the Coulomb gap

Influence of the Coulomb gap on the fine structure of the fundamental absorption edge in quantum wells is studied. Using the interpolation formula, which adequately describes the effect of the Coulomb gap in quantum wells under conditions of low doping regardless of the compensation degree, an explicit expression is obtained for the absorption coefficient related to transitions from the valence level of size quantization to the impurity band. The dependence of the absorption coefficient of a quantum well on the levels of doping and compensation is also studied.     

Nano-laser on silicon quantum dots

A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states in gap produced from the surface bonds of nano-clusters. Here we report the experimental demonstration of nano-laser on silicon quantum dots fabricated by nanosecond pulse laser. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of QD (d < 3 nm) can make the localized states into band gap. The emission energy of nano-laser will be limited in the range of 1.7-2.3 eV generally due to the position of the localized states in gap, which is in good agreement between the experiments and the theory.     

Photoacoustic operation modes for determination of absorption spectra of SiGe mixed crystals

This paper presents comparison of two photoacoustic modes of determination of optical absorption spectra of semiconductors illustrated with the results obtained for SiGe crystals. Experimental transmission and absorption photoacoustic spectra of SiGe crystals as well as appropriate models for determination of optical absorption coefficient spectra are given. The idea and experimental set-up of the analyzed methods are presented too. From the fitting procedure of theoretical characteristics to experimental transmission and absorption photoacoustic spectra and after computations of the optical absorption coefficient spectra, three components of the optical absorption coefficient spectra of SiGe crystals were identified, i.e., band to band transitions, Urbach tail and free carriers absorption. Their parameters are given and discussed in the paper. At the end, the advantages and disadvantages of both methods are discussed. To the best of our knowledge, such a comparison of the two PA methods of determination of the optical absorption spectra has not been done before.     

锯齿型硅纳米管不同手性下的电学与光学性质

本文用第一性原理中的局域密度近似方法,计算了锯齿型单壁硅纳米管(single-walled silicon nanotubes,SWSiNTs)的能带结构、态密度、吸收谱及反射谱.计算结果表明当n=6～9时,带隙为0,该组SWSiNTs具有金属性;当n=10～21时,能带图出现带隙,该组SWSiNTs具有半导体性;当n=13～21时,该组SWSiNTs的带隙以3组手性指数为周期减小;并在吸收谱和反射谱都会在一些相似频率值附近产生峰值.     

Free-carrier absorption in a parabolic quantum well with consideration of scattering on ionized impurities

Intra-subband transitions caused by light absorption in a parabolic quantum well is considered taking into account the scattering by ionized impurity centers. To calculate the scattering matrix element, the Born approximation is used and the interaction with the impurity is described by the Coulomb potential. An analytical expression for the absorption coefficient of processes with the initial absorption of photon and further scattering by an ionized impurity center is obtained. For absorption coefficient the frequency characteristics and dependence on the width of quantum well are examined.     

Infrared absorption in SiGe/Si multiple quantum wells

Infrared absorption has been used to investigate the subband structures in SiGe/Si quantum wells. The quantum wells are prepared using RRH/VLP-CVD and consist of 20 periods of and 60 periods of . The good periodical and interface sharpness of the SiGe/Si quantum wells have been shown by Auger Electron Spectroscopy (AES). The absorption peaks due to transitions between the hole subbands and the conduction band have been observed in infrared absorption spectra. The transverse photocurrent spectrum parallel to the growth plane have also shown absorption peaks due to transitions between the heavy and light hole band states and the conduction band states in quantum wells.