Jinning Liu

High-performance uncooled distributed-feedback quantum cascade laser without lateral regrowth

We demonstrate, uncooled, room-temperature continuous-wave (cw) operation of single-mode distributed-feedback (DFB) quantum cascade lasers (QCLs) emitting around 4.6 μm without lateral regrowth. The effects of cavity length on device performance are studied. A record low threshold electrical power consumption of 2.3 W for the entire laser system with a 1.5-mm-long cavity is realized. For the 2-mm-long laser, high cw output power of 125 mW and very low threshold current density of 0.86 kA/cm2 are obtained. Our devices represent an important step towards using uncooled DFB QCLs in mid-infrared spectral range for practical applications.

Temperature dependent photoluminescence of an In(Ga)As/GaAs quantum dot system with different areal density

We have systematically studied the temperature dependent photoluminescence of a self-assembled In(Ga)As/GaAs quantum dot (QD) system with different areal densities from similar to 10(9) to similar to 10(11) cm(-2). Different carrier channels are revealed experimentally and confirmed theoretically via a modified carrier equation model considering a new carrier transfer channel, i.e. continuum states ( CS). The wetting layer is demonstrated to be the carrier quenching channel for the low-density QDs but the carrier transfer channel for the high-density QDs. In particular, for the InGaAs/GaAs QDs with a medium density of similar to 10(10) cm(-2), the CS is verified to be an additional carrier transfer channel in the low temperature regime of 10-60 K, which is studied in detail via our models. The possible carrier channels that act on different temperature regimes are further discussed, and it is demonstrated that density is not a crucial factor in determining the carrier lateral coupling strength.

Interface processing of amorphous-crystalline silicon heterojunction prior to the formation of amorphous-to-nanocrystalline transition phase

Two effective interface optimizations, i.e., hydrogen-plasma treatment (HPT) and oxygen incorporation (OIn), have been investigated to optimize the interface structure and promote the formation of hydrogenated amorphous silicon (a-Si:H) with an amorphous-to-nanocrystalline transition phase based on the surface passivation of crystalline silicon. Both multistep HPT and intentional OIn are capable of producing a compact interface and a transition phase in the case of initial a-Si:H films deposited under different conditions. An appropriate multistep HPT is responsible for modifying the a-Si:H into a transition phase, whereas an effective OIn forms a disordered and more compact interface, as well as a crystallite-isolated and more transparent structure.

Role of the buffer at the interface of intrinsic a-Si:H and p-type a-Si:H on amorphous/crystalline silicon heterojunction solar cells

We investigate the influence of the different buffer at the interface between the intrinsic a-Si:H and p-type a-Si:H layers on amorphous/crystalline silicon heterojunction (SHJ) solar cells performance. It is demonstrated that the ultrathin buffer at interface of intrinsic a-Si:H and p-type a-Si:H, obtained by H-rich plasma treatment on the initial intrinsic a-Si:H passivation layer, can significantly enhance the minority carrier lifetime and decrease the emitter saturation current density. Spectroscopic ellipsometry and Fourier transform infrared spectroscopy analyses indicate that the initial intrinsic a-Si:H films become dense and less defected as a result of the relaxation and reconstruction when they are treated during the H-rich plasma environment. Based on this finding combined with the optimization of surface texturization of the silicon wafer, this work allows us to reach very high Voc values over 730 mV without losses on fill factor, the 100 μm, 125 × 125 mm2 SHJ solar cells were fabricated with...

Investigation of positive roles of hydrogen plasma treatment for interface passivation based on silicon heterojunction solar cells

The positive roles of H2-plasma treatment (HPT) have been investigated by using different treatment procedures in view of the distinctly improved passivation performance of amorphous-crystalline silicon heterojunctions (SHJs). It has been found that a hydrogenated amorphous silicon thin film and crystalline silicon (a-Si:H/c-Si) interface with a high stretching mode (HSM) is detrimental to passivation. A moderate pre-HPT introduces atomic H, which plays an effective tuning role in decreasing the interfacial HSM; unfortunately, an epitaxial layer is formed. Further improvement in passivation can be achieved in terms of increasing the HSM of a-Si:H film treated by appropriate post-HPT based on the a-Si:H thickness. The minority carrier lifetime of crystalline wafers can be improved by treated films containing a certain quantity of crystallites. The microstructure factor R and the maximum intensity of the dielectric function e 2max have been found to be critical microstructure parameters that describe high-quality a-Si:H passivation layers, which are associated with the amorphous-to-microcrystalline transition phase induced by multi-step HPT. Finally, the open circuit voltage and conversion efficiency of the SHJ solar cell can be improved by implementing an effective HPT process.

Defining a parameter of plasma-enhanced CVD to characterize the effect of silicon-surface passivation in heterojunction solar cells

The hydrogen content (CH) and microstructure of a hydrogenated amorphous-silicon (a-Si:H) layer fabricated by plasma-enhanced chemical vapor deposition (PECVD) were analyzed to determine the effect of surface passivation on crystalline silicon (c-Si). The ratio of radio-frequency power to gas pressure (Cpp in WPa−1) of the PECVD system is defined as a characterization parameter. It was found that CH and the passivation of a-Si:H layers were sensitively affected by Cpp. However, CH remained almost constant at the same Cpp even though the PECVD power and pressure were widely varied. We determined that an optimal region exists in the range of 0.75 < Cpp < 1.25, where a high implied open-circuit voltage of 732 mV was obtained from a passivated a-Si:H/c-Si/a-Si:H structure, indicating that Cpp was a useful parameter for characterizing the surface passivation effect of a a-Si:H layer on c-Si.

Stable single-mode distributed feedback quantum cascade laser with surface metal grating

A design of single-mode distributed feedback quantum cascade lasers (DFB-QCLs) with surface metal grating is described. A rigorous modal expansion theory is adopted to analyse the interaction between the waveguide mode and the surface plasmon wave for different grating parameters. A stable single-mode operation can be obtained in a wide range of grating depths and duty cycles. The single-mode operation of surface metal grating DFB-QCLs at room temperature for lambda = 8.5 mu m is demonstrated. The device shows a side-mode suppression ratio of above 20 dB. A linear tuning of wavelength with temperature indicates the stable single-mode operation without mode hopping.

Characterization of transparent conductive oxide films and their effect on amorphous/crystalline silicon heterojunction solar cells

Three different dopant indium oxide thin films were fabricated at low temperatures by reactive plasma deposition and sputtering. The optical and electrical characteristics of these films were analyzed as a function of the Hall electron concentration. Furthermore, these films were applied to amorphous/crystalline silicon heterojunction solar cells as transparent electrodes. Consequently, it was demonstrated that the high Hall mobility, high refractive index, and low extinction coefficient of transparent conductive oxide (TCO) films contribute to the high product of short-circuit current density and fill factor and conversion efficiency. Furthermore, it was found that the solar cell with a finger spacing of 1.9 mm on a 125 × 125 mm2 Si wafer is highly tolerant to TCO film resistivity when the electron concentration is less than 4.0 × 1020 cm−3.

Effective interface pretreatment for amorphous-crystalline silicon heterojunction solar cells

Effective interface pretreatment has been applied to investigate the role of oxygen incorporation between hydrogenated amorphous silicon (a-Si:H) passivation layer and crystalline silicon (c-Si). Oxygen content in the a-Si:H/c-Si interface can be cut down with a thermal pretreatment process of c-Si. High stretching mode in the intrinsic a-Si:H layer increases with the reduction of oxygen content. Both the interface with less oxygen and the a-Si:H layer with high HSM contribute to achieve a high minority carrier lifetime of more than 8 ms on the czochralski-grown c-Si wafer passivated by the intrinsic a-Si:H.

Impact of surface treatments on the passivation effect for n-type crystalline silicon in heterojunction solar cells

In a-Si:H/c-Si heterojunction solar cells, we developed two methods to improve the passivation effect at the a-Si:H/c-Si interface. (i) Chemical polish, etched c-Si wafer with HF and HNO3 mixtures to smooth the peaks and valleys of pyramids after the texturization with alkali; (ii) SiOx interlayer, formed ultrathin SiOx layers with a thickness of about 2 nm on c-Si surface after the chemical polish and standard RCA cleaning. This thin layer was formed by chemical oxidization in various hot solutions. The results demonstrated that these two methods improved the quality of a-Si:H/c-Si interface and the effective carrier lifetime. When they were applied to heterojunction solar cells, gains in Voc and Jsc were successfully achieved. The simplicity of these methods suggested the possible applications to the industry production.