Carrier transport in III–V quantum-dot structures for solar cells or photodetectors

According to the well-established light-to-electricity conversion theory,resonant excited carriers in the quantum dots will relax to the ground states and cannot escape from the quantum dots to form photocurrent,which have been observed in quantum dots without a p–n junction at an external bias.Here,we experimentally observed more than 88% of the resonantly excited photo carriers escaping from In As quantum dots embedded in a short-circuited p–n junction to form photocurrent.The phenomenon cannot be explained by thermionic emission,tunneling process,and intermediate-band theories.A new mechanism is suggested that the photo carriers escape directly from the quantum dots to form photocurrent rather than relax to the ground state of quantum dots induced by a p–n junction.The finding is important for understanding the low-dimensional semiconductor physics and applications in solar cells and photodiode detectors.     

A method to extend wavelength into middle-wavelength infrared based on InAsSb/(Al)GaSb interband transition quantum well infrared photodetector

We present a method to extend the operating wavelength of the interband transition quantum well photodetector from an extended short-wavelength infrared region to a middle-wavelength infrared region. In the modified In As Sb quantum well, Ga Sb is replaced with Al Sb/Al Ga Sb, the valence band of the barrier material is lowered, the first restricted energy level is higher than the valence band of the barrier material, the energy band structure forms type-II structure. The photocurrent spectrum manifest that the fabricated photodetector exhibits a response range from 1.9 μm to 3.2 μm with two peaks at 2.18 μm and 3.03 μm at 78 K.     

Improvement of green InGaN-based LEDs efficiency using a novel quantum well structure

The green light emitting diodes(LEDs)have lower quantum efficiency than LEDs with other emission wavelengths in the visible spectrum.In this research,a novel quantum well structure was designed to improve the electroluminescence(EL)of green InGaN-based LEDs.Compared with the conventional quantum well structure,the novel structure LED gained 2.14times light out power(LOP)at 20-mA current injection,narrower FWHM and lower blue-shift at different current injection conditions.     

Absorption Enhancement of Silicon Solar Cell in a Positive-Intrinsic-Negative Junction

Absorption coefficient is a physical parameter to describe electromagnetic energy absorption of materials, which is closely related to solar cells and photodetectors. We grow a series of positive-intrinsic-negative(PIN) structures on silicon wafer by a gas source molecule beam epitaxy system and the investigate the absorption coefficient through the photovoltaic processes in detail. It is found that the absorption coefficient is enhanced by one order and can be tuned greatly through the thickness of the intrinsic layer in the PIN structure, which is also demonstrated by the 730-nm-wavelength laser irradiation. These results cannot be explained by the traditional absorption theory.We speculate that there could be some uncovered mechanism in this system, which will inspire us to understand the absorption process further.     

高效锂离子筛吸附剂MnO2·0.5H2O的软化学合成及吸附性能研究

以MnCl2·4H2O,LiOH·H2O等试剂为初始原料,采用溶胶-凝胶、水热处理、固化等软化学合成步骤制备了锂离子筛前驱体Li1.6Mn1.6O4,并经稀盐酸抽锂后得到了高选择性锂离子筛吸附剂MnO2·0.5H2O.着重对合成过程中锂锰比,氧化剂用量等因素影响进行了探讨,并对所制备吸附剂的吸附性能进行了研究.结果表明,经软化学合成步骤制备的锂离子筛对Li 有良好的吸附量和选择性,在未来从海水、卤水等液态锂资源富集或提取锂的应用中具有很大的潜力.     

Direct observation of the carrier transport process in InGaN quantum wells with a pn-junction

A new mechanism of light-to-electricity conversion that uses InGaN/GaN QWs with a p-n junction is reported.According to the well established light-to-electricity conversion theory,quantum wells(QWs) cannot be used in solar cells and photodetectors because the photogenerated carriers in QWs usually relax to ground energy levels,owing to quantum confinement,and cannot form a photocurrent.We observe directly that more than 95% of the photoexcited carriers escape from InGaN/GaN QWs to generate a photocurrent,indicating that the thermionic emission and tunneling processes proposed previously cannot explain carriers escaping from QWs.We show that photoexcited carriers can escape directly from the QWs when the device is under working conditions.Our finding challenges the current theory and demonstrates a new prospect for developing highly efficient solar cells and photodetectors.     

红外波长上转换器件中载流子阻挡结构的研究

复杂半导体材料结构中的载流子分布特性对器件性能有重要影响. 本文针对一种新型的波长上转换红外探测器, 研究了载流子阻挡结构对载流子分布和器件特性的影响. 论文通过自洽求解薛定谔方程、泊松方程、电流连续性方程和载流子速率方程分析了不同器件结构中的空穴分布. 同时, 生长了相应结构的外延材料, 并通过电致荧光谱分析了载流子阻挡结构对器件特性的影响. 结果表明, 2 nm厚的AlAs势垒层既能有效阻挡空穴又不影响电子输运, 有利于制作波长上转换红外探测器. 此外, 论文分析了阻挡势垒层的厚度和高度以及工作温度对载流子分布的影响. 本文研究结果亦可应用于其他载流子非均匀分布的半导体器件.     

利用表面微结构提高波长上转换红外探测器效率

波长上转换红外探测器具有实现大面阵焦平面的优势, 但光提取效率是制约器件整体效率的关键因素之一. 本文主要研究利用表面微结构来提高波长上转换红外探测器的效率. 首先通过仿真计算研究了表面微结构参数对光提取效率的影响, 然后基于优化设计的参数, 采用聚苯乙烯纳米球掩膜刻蚀的方法制作了具有圆台型表面微结构的波长上转换红外探测器. 测试结果表明, 具有表面微结构的器件的光提取效率比无表面微结构的器件提高了130%. 本文制作表面微结构的方法可以实现波长上转换红外探测器件整体效率的提高.     

Visualizing light-to-electricity conversion process in InGaN/GaN multi-quantum wells with a p-n junction

Absorption and carrier transport behavior plays an important role in the light-to-electricity conversion process, which is difficult to characterize. Here we develop a method to visualize such a conversion process in the InGaN/GaN multiquantum wells embedded in a p-n junction. Under non-resonant absorption conditions, a photocurrent was generated and the photoluminescence intensity decayed by more than 70% when the p-n junction out-circuit was switched from open to short. However, when the excitation photon energy decreased to the resonant absorption edge, the photocurrent dropped drastically and the photoluminescence under open and short circuit conditions showed similar intensity. These results indicate that the escaping of the photo-generated carriers from the quantum wells is closely related to the excitation photon energy.     

Numerical and Experimental Study on the Device Geometry Dependence of Performance of Heterjunction Phototransistors

Heterojunction phototransistors(HPTs)with scaling emitters have a higher optical gain compared to HPTs with normal emitters.However,to quantitativel.y describe the relationship between the emitter-absorber area ratio(A_e/A_a)and the performance of HPTs,and to find the optimum value of A_e/A_a for the geometric structure design,we develop an analytical model for the optical gain of HPTs.Moreover,five devices with different A_e/A_a are fabricated to verify the numerical analysis result.As is expected,the measurement result is in good agreement with the analysis model,both of them confirmed that devices with a smaller A_e/A_a exhibit higher optical gain.The device with area ratio of 0.0625 has the highest optical gain,which is two orders of magnitude larger than that of the device with area ratio of 1 at 3 V.However,the dark current of the device with the area ratio of 0.0625 is forty times higher than that of the device with the area ratio of 1.By calculating the signal-to-noise ratios(SNRs) of the devices,the optimal value of Ae/Aa can be obtained to be 0.16.The device with the area ratio of0.16 has the maximum SNR.This result can be used for future design principles for high performance HPTs.