Shih-Hung Lin

Resonance-enhanced dipolar interaction between terahertz photons and confined acoustic phonons in nanocrystals

Taking advantage of the specific core-shell charge separation structure in the CdSe∕CdTe core-shell type-II nanocrystals, we experimentally observed and verified the existence of the resonance-enhanced dipolar interaction between terahertz photons and their corresponding confined acoustic phonons. From the measured terahertz transmission spectra, we found that the photon frequency of the terahertz resonant absorption is inversely proportional to the diameter D of the nanocrystals and agrees with that of dipolar active l=1, n=0 confined acoustic modes. The corresponding absorption cross section shows a D4 dependence, supporting a charged simple-harmonic-oscillator model. These facts verify the occurrence of dipolar interaction between terahertz photons and confined terahertz acoustic phonons.

InGaAlAs∕InGaAs strain-balanced multi-quantum-well laser/semiconductor optical amplifiers operating at excited transitions

We report the design, growth, and fabrication of strain-balanced n-type modulation-doped InGaAlAs∕InGaAs multiple quantum well laser/semiconductor optical amplifiers on InP. The quantum well contains a lattice-matched InGaAs core, a compressive-strained InGaAs padding, and a tensile-strained InGaAlAs spacer. Electroluminescence spectra show higher optical gain for the quantum well e1-hh2 transition at λ=1460nm than the e1-hh1 transition at λ=1550nm. Ridge-waveguide lasers of Fabry-Perot (FP) type and tilted-end-facet (TEF) type were fabricated. The FP laser shows a lasing peak of λ=1514nm at threshold. Additional lasing wavelengths at λ=1528 and 1545nm were observed sequentially as the injection current increased. For the TEF laser, only the emission at λ=1511nm was observed. These TE-polarized lasing wavelengths are consistent with the δ-like absorption peaks in photocurrent spectra. The lasing performance is attributed to optical transitions within quantum dots/wires which are formed by the strain-field...

Interface characterization of nanometer scale CdS buffer layer in chalcopyrite solar cell

The buffer layer of a chalcopyrite solar cell plays an important role in optical responses of open circuit voltage (V oc) and short circuit current (J sc). A CdS buffer layer is applicable on the nanometer scale owing to its high carrier concentration and n-type semiconductor behavior in chalcopyrite solar cells. The thin buffer layer also contributes to the passivation of the absorber surface to reduce defect recombination loss. Non-destructive metrological parameters such as photoluminescence (PL) intensity, external quantum efficiency (EQE), and depth-resolved photovoltage are used to characterize the interface quality of CdS/chalcopyrite. The defects and dangling bonds at the absorber surface will cause interface recombination and reduce the cell performance in build-in voltage distribution. Post annealing can improve Cd ion diffusion from the buffer layer to the absorber surface and reduce the density of defects and dangling bonds. After thermal annealing, the EQE, PL intensity, and minority carrier lifetime are improved.

Application of luminescence technology for solar PV industry

The luminescence technology is a non-destructive testing and can be applied for kinds of solar cells such as c-Si, thin film (α-Si, CIGS, and CdTe) and multi-junction solar cells. The luminescence intensity is correlated to the quality of absorber or junction and suitable to be developed as monitors of device property during process. Photoluminescence (PL) and Electroluminescence (EL) are most common luminescence metrologies and they can help to identify the band gap, defect level, defect density, radiative recombination coefficient, junction quality, and implied open circuit voltage (Voc). The 2D PL/EL image can help to evaluate the uniformity and physical defect information such as crack, contact disconnection, shunting points, serious resistance distribution, and impurity. The failure analysis of long-term reliability test (light illumination and thermal stress) by the combination of PL/EL technology can help to identify the key root cause. The qualitative analysis of PL/EL metrologies can apply for process correlation to stabilize the production line and/or further improve the efficiency.

Photovoltaic performance improvement of Si HIT solar cell by incorporating flower-like light trapping structures

We implemented the flower-like light trapping structures on the surface of HIT solar cell to achieve a great reflectance reduction and effectively improve the J_SC from 34.32 mA/cm² to 39.4 mA/cm² compared to the reference.

Development of Superlattice Infrared Photodetectors

Infrared (IR) detectors play a critical role in both military and civilian applications and have been widely researched in recent decade. Because the atmospheric transparent windows for the IR radiation exist within the spectral ranges of 3−5 and 8−12 μm, and the spectrum of the black-body radiation at room temperature has a peak at 10 μm, the detectors with the 8-12 μm detection spectra are helpful to identify the heat radiation from a target at room temperature. Such detectors are the research focus in this chapter. The employment of intersubband transitions for the infrared radiation detection has drawn much attention. The transition is completed by the electrons which absorb photons with the appropriate energy equal to the suband energy difference to transit from the low subband to the high one. The intersubband photodetectors can be made from semiconductor heterostructures of multiple quantum wells (West & Eglash, 1985) (Harwit & Harris Jr., 1987) ( Levine et al., 1987) or superlattices as shown in Fig. 1. Infrared detection will be done by the intersubband transitions between two quantum states in the multiple quantum wells or two minibands in the superlattices. The wells are sandwiched by thick barriers in the multiple quantum well structure. Therefore electron wavefunctions in the wells would not interact with each others and discrete quantum states are formed. Contrarily, the adjacent wells in the superlattice are separated by thin barriers. Minibands are formed in the superlattice region by the coupling of electron wavefunctions. As shown in Fig. 1, in comparison with the quantum well infrared photodetectors (QWIPs), superlattice infrared photodetectors (SLIPs) have three different characteristics. The first one is the low operational bias. The electrons in the miniband of the superlattice (SL) are conductive while those in the quantum states of the multiple quantum wells (MQWs) are confined. The SL hence becomes a low resistance structure and thus no externally applied bias drops on the SL under low bias range. Therefore, the current blocking layer is needed to decrease the dark current in SLIPs and can determine the operational bias range. 6

Double-barrier superlattice infrared photodetector combined with quantum well infrared photodetector for operation at high temperature and low bias

We have designed a double-barrier superlattice infrared photodetector combined with quantum well infrared photodetector which has a superlattice (SL) sandwiched between the thick and graded barriers then multiple quantum wells(MQWs) are inserted in the right side of them . The two barriers can let the photoelectrons to bounce back and forth then the photoelectrons inject through the MQWs which are used as noise filter. The detector shows high-temperature operation (∼110K) at low bias. The associated detectivity is also optimized so this device is the promising candidate of a pixel in the focal plane array.

Double-Barrier Superlattice Infrared Photodetectors

We have designed a double-barrier superlattice infrared photodetector (SLIP) which has a superlattice (SL) sandwiched between the thin and thick barriers. Photoelectrons can bounce back and forth between the two barriers and inject through the thin barrier to enhance the photocurrent. However, the supply of electrons is limited by the thick barrier and thus we have to fabricate the emitter contact on the SL. In comparison with the single-barrier SLIP, this structure shows at least one-order higher magnitude of photocurrent at low bias and the associated detectivity is also increased for more than one order. The dramatic increment of the photocurrent is consistent with our design in the detailed analysis. Because it has the optimized performance at low bias, this double-barrier SLIP is suitable for low power consumption applications. Our detector can be operated at 100 K by blocking barriers incorporated into the structure to reduce the dark current.

Resonant-enhanced dipolar interaction between THz-photons and confined acoustic phonons in nanostructures

By using a frequency-controlled narrow band THz source, a Fourier Transform Infrared (FTIR) spectroscopy system, and a frequency-controlled terahertz (THz) emitter, for the first time, we studied the THz photon absorption related to the THz confined acoustic vibrations in semiconductor nanocrystals. Through a specific charge separation in the CdSe/CdTe type-II nanocrystals and a piezoelectric coupling in the wurtzite CdSe nanocrystals, the THz photons can be resonantly coupled with (l=1) dipolar modes and the (l=0) breathing modes, respectively. Our results provide new mechanisms for low dimensional systems to convert a THz photon into a phonon of the same frequency.