Investigation on high mobility nanocrystalline Si with crystalline Si heterostructure

A heterostructure of n-type hydrogenated nanocrystalline silicon (nc-Si:H) film with p-type crystalline silicon, i.e., (n)nc-Si:H/(p)C-Si, was fabricated to investigate carrier characteristics and transport. After electrical experiments, carrier information, such as hole and electron as well as 2-dimension electronic gas in the studied system, was identified respectively. The forward current conduction was analyzed while the reverse current transport was distinguished as different mechanisms within the different range of negative applied voltage. The performed study also leads us to ascribe the main origin of short transient times on the produced structure to a tunneling mechanism.     

硅锗量子阱结构在硅异质结太阳电池中应用的数值模拟

利用半导体工艺和器件仿真软件silvaco TCAD(Technology Computer Aided Design),模拟研究了采用硅/硅锗合金(silicon/silicon germanium alloy,Si/Si_(1-x)Ge_x)量子阱结构作为吸收层的薄膜晶体硅异质结太阳电池各项性能.模拟结果显示,长波波段光学吸收随锗含量的增加而增加,而开路电压则因Si_(1-x)Ge_x)层带隙的降低而下降.锗含量为0.25时,短路电流密度的增加补偿了开路电压的衰减,效率提升0.2%.氢化非晶硅/晶体硅(a-Si:H/c-Si)界面空穴密度以及Si_(1-x)Ge_x)量子阱的体空穴载流子浓度制约着空穴费米能级的位置,进而影响到开路电压的大小.随着锗含量增加,a-Si:H/c-Si界面缺陷对开压的影响降低,Si_(1-x)Ge_x)量子阱的体缺陷对开压的影响则相应增加.高效率含Si_(1-x)Ge_x)量子阱结构的硅异质结太阳电池的制备需要a-Si:H/c-Si界面缺陷的良好钝化以及高质量Si_(1-x)Ge_x)量子阱的生长.     

Inelastic scattering processes in GaAs/n-AlGaAs selectively doped heterojunctions with InGaAs quantum dots

Inelastic scattering processes of the two-dimensional electron gas (2DEG) in both normal and inverted n-AlGaAs/GaAs heterojunction FET structures have been studied, for the case where InGaAs dots are embedded in the vicinity of GaAs channel. By analyzing the magnetoresistance data, the inelastic scattering time τ_in is determined as a function of the concentration N_2D of 2D electrons and shown to be reduced by 10–40% by the presence of InGaAs dots. By investigating a GaAs/n-AlGaAs inverted heterojunction FET with embedded InGaAs dots, we have varied the percentage P_oc of charged dots filled with an electron and found that τ_in decreases as P_oc increases, indicating that the inelastic scattering rate of 2DEG by charged dots is higher than that by the neutral ones.     

Light-emitting structures based on nanocrystalline (Si/CaF2) multiquantum wells

Periodic nanocrystalline (Si/CaF₂) multilayers, deposited on (111) silicon by Molecular Beam Epitaxy (MBE) at room temperature, were used to fabricate simple light emitting structures and to study their electrical and optoelectronic properties. Photoluminescence (PL) and electroluminescence (EL) spectra from the same area of the devices are approximately the same, indicative of the same emission mechanism. Current–voltage characteristics reveal important phenomena in vertical carrier transport. Regions of negative differential resistance and current oscillations were observed and were tentatively attributed to resonant tunneling at high electric fields under field domain formation.     

界面缺陷态密度与衬底电阻率取值对硅异质结光伏电池性能的影响

在异质结前界面缺陷态密度Dit1和异质结背界面缺陷态密度Dit2均取不同值时,对p型单晶硅(c-Si(p))为衬底的硅异质结太阳电池的衬底电阻率ρ与电池性能的关系进行了数值研究.结果表明:衬底电阻率的最优值ρop取决于前界面缺陷态密度Dit1,且ρop随着Dit1的增大而增大;当ρρop时,背界面缺陷态密度Dit2对衬底电阻率的可取值范围具有较大影响,Dit2越大衬底电阻率的可取值范围越小.     

Fabrication and electrical characterization of nanocrystalline ZnO/Si heterojunctions

Nanocrystalline zinc oxide (nc-ZnO) films were prepared by a sol-gel process on p-type single-crystalline Si substrates to fabricate nc-ZnO/p-Si heterojunctions. The structure and morphology of ZnO films on Si substrates, which were analyzed by X-ray diffraction (XRD) spectroscopy and atomic force microscopy (AFM), showed that ZnO films consisted of 50-100 nm polycrystalline nanograins with hexagonal wurtzite structure. The electrical transport properties of the nc-ZnO/p-Si heterojunctions were investigated by temperature-dependent current-voltage (I-V) measurements and room temperature capacitance-voltage measurements. The temperature-dependent I-V characteristics revealed that the forward conduction was determined by multi-step tunneling current, and the activation energy of saturation current was about 0.26 eV. The 1/C²-V plots indicated the junction was abrupt and the junction built-in potential was 1.49 V at room temperature.     

Alloy disorder scattering limited mobility of two-dimensional electron gas in the quaternary AlInGaN/GaN heterojunctions

Nitride heterojunction field effect transistors (HFETs) with quaternary AlInGaN barrier layers have achieved remarkable successes in recent years based on highly improved mobility of the two-dimensional electron gases (2DEGs) and greatly changed AlInGaN compositions. To investigate the influence of the AlInGaN composition on the 2DEG mobility, the quaternary alloy disorder (ADO) scattering to 2DEGs in AlInGaN/GaN heterojunctions is modeled using virtual crystal approximation. The calculated mobility as a function of AlInGaN alloy composition is shown to be a triangular-scarf-like curved surface for both cases of fixed thickness of AlInGaN layer and fixed 2DEG density. Though the two mobility surfaces are quite different in shape, both of them manifest the smooth transition of the strength of ADO scattering from quaternary AlInGaN to ternary AlGaN or AlInN. Some useful principles to estimate the mobility change with the Al(In,Ga)N composition in Al(In,Ga)N/GaN heterojunctions with a fixed 2DEG density are given. The comparison between some highest Hall mobility data reported for Al_xGa_1−xN/GaN heterojunctions (x=0.06~0.2) at very low temperature (0.3~13 K) and the calculated 2DEG mobility considering ADO scattering and interface roughness scattering verifies the influence of ADO scattering. Moreover, the room temperature Hall mobility data of Al(In,Ga)N/AlN/GaN heterojunctions with ADO scattering eliminated are summarized from literatures. The data show continuous dependence on Hall electron density but independence of the Al(In,Ga)N composition, which also supports our theoretical results. The feasibility of quaternary AlInGaN barrier layer in high conductivity nitride HFET structures is demonstrated.     

a-Si:H/c-Si 异质结太阳电池 J-V 曲线的 S-Shape 现象

通过异质结界面分析与 AMPS 模拟计算研究了 a-Si:H/c-Si 异质结太阳电池在低温工作、a-Si:H 层低掺杂、高价带补偿以及高界面态时光态 J-V 曲线出现 S-Shape 现象的物理过程,总结了 S-Shape 现象的物理原因.分析结果表明,当空穴输运受到界面势垒的限制时,空穴在 c-Si 界面附近聚集,能带重新分配,c-Si 耗尽区的电场减小,更多的电子从 c-Si 准中性区反转至 c-Si 界面及耗尽区与空穴复合,复合速率显著增大,光电流的损失显著增大,光态 J-V< 关键词： 模拟 异质结太阳电池 a-Si:H/c-Si 异质结     

纳米硅(nc-Si:H )/晶体硅(c-Si)异质结太阳电池的数值模拟分析

运用美国宾州大学发展的AMPS程序模拟分析了n-型纳米硅（n+-nc-Si:H）/p-型晶体硅(p-c-Si)异质结太阳电池的光伏特性.分析表明，界面缺陷态是决定电池性能的关键因素，显著影响电池的开路电压（VOC）和填充因子（FF），而电池的光谱响应或短路电流密度（JSC）对缓冲层的厚度较为敏感.对不同能带补偿（bandgap offset）的情况所进行的模拟分析表明，随着ΔEc的增大，由于界面态所带来的开路电压和填充因子的减小逐渐被消除，当ΔEc达到05eV左右时界面态的影响几乎完全被掩盖.界面层的其他能带结构特征对器件性能的影响还有待进一步研究.最后计算得到了这种电池理想情况下（无界面态、有背面场、正背面反射率分别为0和1）的理论极限效率ηmax=3117% (AM15，100mW/cm2，040—110μm波段).     

Structural and magnetic behaviour of aluminium doped barium hexaferrite nanoparticles synthesized by solution combustion technique

Nanocrystalline M-type Al³⁺ substituted barium hexaferrite samples having generic formula BaFe_12−xAl_xO₁₉ (where x=0.00, 0.25, 0.50, 0.75, 1.00) were synthesized by the solution combustion technique. The precursors were prepared using stoichiometric amounts of Ba²⁺, Fe³⁺ and Al³⁺ nitrate solutions with citric acid as a chelating agent. The barium nitrate to citric acid ratio was taken as 1:2 and pH of the solution was kept at 8. The sintered samples were characterized by XRD, EDAX, SEM, TEM and VSM techniques. Pure barium hexaferrite shows only single phase hexagonal structure while samples at 0.25≤x≥1.00 show α-Fe₂O₃ peaks with M-phase of barium hexaferrite in the X-ray diffraction pattern. The lattice parameters (a and c) obtained from XRD data decreases with increase in aluminium content x. The particle size obtained from X-ray diffraction data is in the nanometer range. The magnetic behaviour of the samples was studied using vibrating sample magnetometer technique. The saturation magnetization (M_s) and magneton number (n_B) decrease from 38.567 to 21.732 emu/g and from 7.6752 to 4.2126μ_B, respectively, with increase in Al³⁺ substitution x from x=0.0 to 1.0.     

Structure and optoelectronic properties of Si/O superlattice

We have carried out structural study of the Si/O semiconductor atomic superlattices (SAS) with up to 18 Si/O layers fabricated by molecular beam epitaxy and in situ oxygen exposure on both Sb-doped and undoped Si buffer layers, and correlated the results with our photoluminescence, electroluminescence (EL) and I–V data. The Si/O SAS is a new type of superlattice, where monolayers of oxygen are sandwiched between the Si layers. High-resolution cross-sectional transmission electron microscopy (TEM) study has confirmed the presence of the superlattice and shown epitaxy in the Si/O superlattices. The high structural quality of the layers grown on the undoped Si buffer layers with low density of stacking faults—less than 10⁷/cm²—was established by TEM. Although structure perfection is very important allowing this new class of superlattices to be extended to other systems, it is important to point out that a 9-period SAS-based EL device with emission of light in green has been life-tested with stable output for over 1 year of continuous operation. The Si/O superlattice also serves as an epitaxially grown insulating layer as possible replacement of silicon-on-insulator. Together with the tailor-made effective band gap, this epitaxially grown superlattice may serve as future silicon-based three-dimensional integrated circuits.     

Raman characterization of the structural evolution in amorphous and partially nanocrystalline hydrogenated silicon thin films prepared by PECVD

Hydrogenated silicon (Si:H) thin films were obtained by plasma‐enhanced chemical vapor deposition (PECVD). Raman spectroscopy was used to investigate the structural evolution in phosphor‐doped n‐type amorphous hydrogenated silicon thin films, which were prepared under different substrate temperatures and gas pressures. Meanwhile, the effect of nitrogen doping on the structure of P‐doped thin films was also investigated by Raman spectroscopy. Moreover, the transition from the amorphous state to the nanocrystalline state of undoped Si:H films deposited through low argon dilution was studied by Raman spectroscopy, X‐ray diffraction, and transmission electron microscopy. The results show that Raman spectroscopy can sensitively detect the structural evolution in hydrogenated silicon thin films deposited under different conditions in a PECVD system. Copyright © 2010 John Wiley & Sons, Ltd.     

Two sub-band conductivity of Si quantum well

We report on two sub-band/bi-layer transport in double gate SiO₂–Si–SiO₂ quantum well with 14 nm thick Si layer at 270 mK. At symmetric well potential the experimental sub-band spacing changes monotonically from 2.3 to 0.3 meV when the total electron density is adjusted by gate voltages between 0.7×10¹⁶–. The conductivity is mapped in large gate bias window and it shows strong non-monotonic features. At symmetric well potential and high density these features are addressed to sub-band wave function delocalization in the quantization direction and to different disorder of the top and bottom interfaces of the Si well. In the gate bias regimes close to second sub-band/bi-layer threshold the non-monotonic behavior is interpreted to arise from scattering from the other electron sub-system with localized or low mobility states.     

Effect of doping on structural,optical and electrical properties of nanostructure ZnO films deposited onto a-Si:H/Si heterojunction

We investigated the structural; optical and electrical properties of ZnO thin films as the n-type semiconductor for silicon a-Si:H/Si heterojunction photodiodes. The ZnO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of doping on device performance. The results show that the X-ray diffraction (XRD) spectra revealed a preferred orientation of the crystallites along c-axis. SEM images show that all films display a granular, polycrystalline morphology and the ZnO:Al exhibits a better grain uniformity. The transmittance of the doped films was found to be higher when compared to undoped ZnO. A low resistivity of the order of 2.8 × 10⁻⁴ Ω cm is obtained for ZnO:Al using 0.4 M concentration of zinc acetate. The photoluminescence (PL) spectra exhibit a blue band with two peaks centered at 442 nm (2.80 eV) and 490 nm (2.53 eV). It is noted that after doping the ZnO films a shift of the band by 22 nm (0.15 eV) is recorded and a high luminescence occurs when using Al as a dopant. Dark I–V curves of ZnO/a-Si:H/Si structure showed large difference, which means there is a kind of barrier to current flow between ZnO and a-Si:H layer. Doping films was applied and the turn-on voltages are around 0.6 V. Under reverse bias, the current of the ZnO/a-Si:H/Si heterojunction is larger than that of ZnO:Al/a-Si:H/Si. The improvement with ZnO:Al is attributed to a higher number of generated carriers in the nanostructure (due to the higher transmittance and a higher luminescence) that increases the probability of collisions.     

Electron spin resonance of the two-dimensional metallic state and the quantum Hall state in a Si/SiGe quantum well

We report electrically detected electron spin resonance (ESR) measurements of a high mobility two-dimensional (2D) electron system formed in a Si/SiGe quantum well, with millimeter wave in a high magnetic field . The negative ESR signal observed under an in-plane magnetic field gives direct evidence that the spin polarization leads to a resistance increase in the 2D metallic state. Suppression of spin decoherence was observed in the quantum Hall state at the Landau level filling factor ν=2. Strength of the nuclear magnetic field in the resonance is evaluated to be less than , much smaller than that reported for GaAs/AlGaAs heterostructures.     

Dependence of the electronic parameters on the InyGa1−yAs quantum well width in modulation-doped AlxGa1−xAs/InyGa1−yAs/GaAs strained single quantum wells

The variation of the electronic parameters in the subband as a function of the In_yGa_1−yAs quantum well width in modulation-doped strained Al_xGa_1−xAs/In_yGa_1−yAs/GaAs single quantum wells were investigated by means of Shubnikov-de Haas (S-dH) and Van der Pauw Hall-effect measurements. The fast Fourier transform (FFT) of the S-dH oscillations and the Hall-effect data showed that the carrier density and the mobility of the two-dimensional electron gas (2DEG) occupied in the subband increased as the quantum well width increased. The increase in the 2DEG density with increasing the In_yGa_1−yAs well width originated from an increase in the energy difference between the energy level of the electronic subband and Fermi energy, and the increase in the 2DEG mobility is attributed to a decrease of the scattering source. The electronic subband energies, the corresponding wavefunctions, and the Fermi energies in the Al_xGa_1−xAs/In_yGa_1−yAs/GaAs single quantum wells were calculated by a self-consistent method taking into account the exchange-correlation effect together with the strain and nonparabolicity effects. These results indicate that the electronic parameters in Al_xGa_1−xAs/In_yGa_1−yAs/GaAs strained single quantum wells are significantly dependent on the quantum well width.     

Determination of conduction band edge characteristics of strained Si/Si1-xGex

The feature of conduction band （CB） of Tensile-Strained Si（TS-Si） on a relaxed Si1-xGex substrate is systematically investigated, including the number of equivalent CB edge energy extrema, CB energy minima, the position of the extremal point, and effective mass. Based on an analysis of symmetry under strain, the number of equivalent CB edge energy extrema is presented; Using the K.P method with the help of perturbation theory, dispersion relation near minima of CB bottom energy, derived from the linear deformation potential theory, is determined, from which the parameters, namely, the position of the extremal point, and the longitudinal and transverse masses （m1^＊ and mt^＊）are obtained.     

掺杂Si/SiO_2界面电子结构与光学性质的第一性原理研究

采用基于密度泛函理论的第一性原理方法,在局域密度近似(LDA)下研究了B掺杂Si/SiO_2界面及其在压强作用下的电子结构和光学性质.能带的计算结果表明:掺杂前后Si/SiO_2界面均属于直隙半导体材料,但掺B后界面带隙由0. 74 eV减小为0. 57 eV,说明掺B使材料的金属性增强;对B掺杂Si/SiO_2界面施加正压强,发现随着压强不断增大,Si/SiO_2界面的带隙呈现了逐渐减小的趋势,并且由直隙逐渐转变为间隙.光学性质的计算结果表明:掺B对Si/SiO_2界面在低能区(即红外区)的介电函数虚部、吸收系数、折射率以及反射率等光学参数有显著影响,且在红外区出现新的吸收峰;对B掺杂Si/SiO_2界面施加正压强,随着压强增大,红外区的吸收峰逐渐消失,而在紫外区出现了吸收峰.上述结果表明,对Si/SiO_2界面掺B及施加正压强均可调控Si/SiO_2界面的电子结构与光学性质.本文的研究为基于Si/SiO_2界面的光电器件研究与设计提供一定的理论参考.     

1.54 μm Light emission from Er/O and Er/F doped Si p–i–n diodes grown by molecular beam epitaxy

Various light emitting devices (LED) have been processed using Er/O- and Er/F-doped Si layered structures grown by molecular beam epitaxy (MBE) at low temperature. A comparative study has been carried out in order to provide more understanding of the electroluminescence (EL) excitation and de-excitation mechanisms in particular at a high injection current regime. Comparing the experimental results with model calculations the values of excitation cross section, σ_ex, and effective Auger coefficient, C_A, have been determined for various devices operated at different biases. Time-resolved EL measurements of these Er/O- and Er/F-doped MBE Si structures, using an experimental set-up with a time response of 200 ns, have been performed with different excitation conditions. Besides the spontaneous Er emission (700 μs), some fast EL decay processes associated with the Auger energy transfer via free carriers (4 μs), and the hot carrier effects (200 ns) have been identified.