T.-H. Cheng

Strain-enhanced photoluminescence from Ge direct transition

Strong enhancement of Ge direct transition by biaxial-tensile strain was observed. The reduction in band gap difference between the direct and indirect valleys by biaxial tensile strain increases the electron population in the direct valley, and enhances the direct transition. The band gap reduction in the direct and indirect valleys can be extracted from the photoluminescence spectra and is consistent with the calculations using k⋅p and deformation potential methods for conduction bands and valence bands, respectively.

Surface passivation of Cu(In,Ga)Se2 using atomic layer deposited Al2O3

With Al2O3 passivation on the surface of Cu(In,Ga)Se2, the integrated photoluminescence intensity can achieve two orders of magnitude enhancement due to the reduction of surface recombination velocity. The photoluminescence intensity increases with increasing Al2O3 thickness from 5 nm to 50 nm. The capacitance-voltage measurement indicates negative fixed charges in the film. Based on the first principles calculations, the deposition of Al2O3 can only reduce about 35% of interface defect density as compared to the unpassivated Cu(In,Ga)Se2. Therefore, the passivation effect is mainly caused by field effect where the surface carrier concentration is reduced by Coulomb repulsion.

Competitiveness between direct and indirect radiative transitions of Ge

Both direct and indirect transitions of photoluminescence and electroluminescence are observed in a Ge n+p diode. The relative intensity of direct radiative recombination with respect to indirect radiative recombination increases with the increase in the optical pumping power, injection current density, and temperature. The increase in electron population in the direct valley is responsible for the enhancement. The spectra can be fitted by the combination of direct and indirect transition models. The direct radiative transition rate is ∼1600 times of the indirect transition, estimated by electroluminescence and photoluminescence spectra near room temperature.

Infrared emission from Ge metal-insulator-semiconductor tunneling diodes

The Ge light-emitting diode with ∼1.8μm strong infrared emission is demonstrated using a metal-insulator-semiconductor tunneling structure. The intensity of a Ge device is one order of magnitude stronger than a similar Si device. At the positive gate bias, the holes in the Al gate electrode tunnel to the n-type Ge through the ultrathin oxide and recombine radiatively with electrons. An electron-hole-plasma model can be used to fit all the emission spectra from room temperature down to 65K. From the measurement temperature range, the extracted band gap is ∼40meV lower than the reported band gap data, and the linewidth drops from 70to25meV. The longitudinal acoustic phonon (∼28meV) and/or the band gap renormalization at high carrier density are proposed to be responsible for the reduction of photon energy. The band gap reduction on the mechanically strained n-type Ge and Si is also investigated experimentally and theoretically.

2.0 μm electroluminescence from Si/Si0.2Ge0.8 type II heterojunctions

A metal-oxide-semiconductor tunneling diode is used to emit electroluminescence from a Si/Si0.2Ge0.8 heterojunction. Besides the 1.1 μm and 1.6 μm infrared emission from the band edges of Si and SiGe, respectively, 2 μm infrared emission is also observed due to the radiative recombination between the electrons in the Si conduction band and the holes in the SiGe valence band. This type II recombination can emit photons whose energy is below the SiGe band gap to extend the emission range of Si/Ge-based light-emitting devices. The emission line shape can be fitted by the electron-hole-plasma recombination model.

Biaxial tensile strain effects on photoluminescence of different orientated Ge wafers

The enhanced photoluminescence of direct transition is observed on (100), (110), and (111) Ge under biaxial tensile strain. The enhancement is caused by the increase in electron population in the Γ valley. The shrinkage of energy difference between the lowest L valleys and the Γ valley is responsible to the population increase on (100) and (110) Ge. For (111) Ge, the energy difference increases under biaxial tensile strain but the strain decreases energy difference between the electron quasi-Fermi level and the Γ valley due to the small density of state of the lowest L valleys, and thus enhances direct recombination.

Ge-on-glass detectors

A single crystalline thin film of Ge on glass is fabricated using wafer bonding and smart cut. A simple metal-insulator-semiconductor detector is demonstrated for visible light and telecommunication wavelength. The implantation damage of separated Ge film bonded on glass is removed by chemical etching, and the surface roughness is reduced from 14to4nm. The defect removal reduces the dark current by a factor of 30 and increases the responsivity by a factor of 1.85 at visible wavelength. The responsivity of 0.27A∕W at 1.3μm wavelength for an unetched device does not increase after damage removal due to the decrease of the absorption layer thickness.

Digital communication using Ge metal-insulator-semiconductor light-emitting diodes and photodetectors

Both Ge light-emitting diodes and photodetectors are demonstrated by using the same metal-insulator-semiconductor (MIS) tunneling structure. A Ge MIS tunneling diode biased at the accumulation region is used as a light-emitting device and a Ge MIS tunneling diode biased at the inversion region is used as a photodetector. The ultrathin gate oxide film used in the MIS tunneling diode was grown by liquid phase deposition at 50 °C to lower the thermal budget. A Ge light-emitting diode has a higher quantum efficiency than a similar Si device (at least one order of magnitude stronger) due to the higher radiative recombination coefficient. With the detection of the Ge MIS photodetector, the data communication in free space is reported and demonstrated for the first time.

Electrically pumped Ge Laser at room temperature

The stimulated emission from (110) Ge was observed in the metal-insulator-semiconductor (MIS) laser diode with a simple Fabry-Perot cavity by current injection at room temperature. The lasing characteristics consist of (1) sudden increase of efficiency in the light-out current-in curve, (2) the transverse electrical mode polarization, (3) the strong directivity of the far field pattern, (4) the narrow line width in the emission spectra above the threshold, and (5) the population inversion confirmed by fitting the spontaneous emission spectrum near the threshold current. The lasing wavelength is suitable for Si photonics due to the emission energy lower the Si bandgap.

Electroluminescence from monocrystalline silicon solar cell

The band edge emission with the peak at 1.15 μm is observed at room temperature from monocrystalline silicon solar cell at forward bias. The electroluminescence spectra can be fitted by electron hole plasma recombination model. The temporal response of electroluminescence is used to characterize the minority carrier lifetime by fitting the time evolution of radiative recombination using the Shockley–Read–Hall, radiative, and Auger recombination models. The minority carrier lifetime is almost constant (1.8 ms) for excess carrier density lower than 4×1015 cm−3, and then decreases at higher concentration.