C.-H. Lin

Low-temperature fabrication and characterization of Ge-on-insulator structures

Ge-on-insulator structures have been fabricated by wafer bonding and layer transfer techniques. Ultralow bonding temperatures of 150–300°C are employed in order to suppress hydrogen outdiffusion and to produce a low defect density, in an attempt to produce high photocurrent and photoresponse. Thus reducing the hydrogen outdiffusion results in decreased surface roughness. A low defect density is suggested by a low inversion-current leakage of the tunnel diodes. The photoresponse of the Ge-on-insulator detector is also found to increase with decreasing bonding temperature, indicating that defects caused by hydrogen implantation are passivated more effectively.

A PMOS tunneling photodetector

A metal/oxide/n-Si structure with ultrathin gate oxide is utilized as a photodetector. At inversion gate bias, the dark current and photocurrent are determined by both the minority carrier (hole) generation rate in the deep depletion region and the electrons tunneling from the gate electrode to n-type Si, while only the former component is significant in the NMOS photodetector. The electron tunneling current dominates the photocurrent at sufficiently large negative gate voltage, and the sensitivity of PMOS detectors is, therefore, enhanced by approximately one order of magnitude, as compared to NMOS detectors.

Electroluminescence from the Ge quantum dot MOS tunneling diodes

A Ge quantum dot (QD) light-emitting diode (LED) is demonstrated using a MOS tunneling structure for the first time. The oxide film was grown by liquid phase deposition at 50/spl deg/C to reduce the thermal budget. The infrared emission of /spl sim/1.5 /spl mu/m was observed from Ge QD MOS LEDs, similar to the p-type-intrinsic-n-type structure reported previously. At the negative gate bias, the electrons in the Al gate electrode tunnel to the Ge QD through the ultrathin oxide and recombine radiatively with holes to emit the /spl sim/1.5/spl mu/m infrared. The electrons also recombine with holes in the Si cap, and the band edge emission from Si is also observed.

Broadband SiGe∕Si quantum dot infrared photodetectors

The broadband absorption of metal-oxide-semiconductor SiGe∕Si quantum dot infrared photodetectors is demonstrated using boron δ doping in the Si spacer. The peak at 3.7–6μm results from the intersubband transition in the SiGe quantum dot layers. The other peak at 6–16μm mainly comes from the intraband transition in the boron δ-doping wells in the Si spacers. Since the atmospheric transmission windows are located at 3–5.3 and 7.5–14μm, broadband detection is feasible using this device. The δ doping in SiGe quantum dots and Si0.9Ge0.1 quantum wells is also investigated to identify the origin of the absorption.

Novel MIS Ge-Si quantum-dot infrared photodetectors

The metal-insulator-semiconductor (MIS) Ge-Si quantum-dot infrared photodetectors (QDIPs) are successfully demonstrated. Using oxynitride as gate dielectric instead of oxide, the operating temperature reaches 140 and 200 K for 3-10 and 2-3 /spl mu/m detection, respectively. From the photoluminescence spectrum, the quantum-dot structures are responsible for the 2-3 /spl mu/m response with high operation temperature, and the wetting layer structures may be responsible for the 3-10 /spl mu/m response. This novel MIS Ge-Si QDIP can increase the functionality of Si chip such as noncontact temperature sensing and is compatible with ultra-large scale integration technology.

Enhanced photocurrent gain and spectrum range based on the composite consisting of SnO2 nanowires and CdSe quantum dots

High sensitivity with additional spectral response based on the composite consisting of SnO2 nanowires (NWs) and CdSe quantum dots (QDs) has been demonstrated. The underlying mechanism is attributed to the spatial separation of photogenerated electrons and holes due to the charge transfer arising from type II band alignment between CdSe QD and SnO2 NW. This work shows that by selective decoration of suitable QDs, the photocurrent gain of NWs not only can be greatly enhanced, but also can be extended to a wider range photoresponse spectrum. Our result, therefore, provides a very useful guideline to create high efficiency photodetectors.

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.

Comparative study of InAs quantum dots with different InGaAs capping methods

The authors have used cross-sectional scanning tunneling microscopy to examine strain relaxation profiles of InAs quantum dots with In0.33Ga0.67As layers overgrown by three distinct capping methods. A statistical analysis of strain relaxation profile allowed them to infer that the long wavelength emission (>1.3μm) of InAs quantum dots capped with sequential GaAs∕InAs binary growth is mainly due to a weaker quantum confinement effect. This particular capping method is better than the traditional molecular beam epitaxy with simultaneous In∕Ga∕As deposition, and much better than a capping method with separated Ga deposition followed by As and InAs growth.

Novel methods to incorporate deuterium in the MOS structures

The deuterium concentration as high as 2/spl times/10/sup 20/ cm/sup -3/ can be incorporated in rapid thermal oxide layers by a process of deuterium prebake and deuterium post oxidation anneal. The deuterium distributed not only at Si/oxide interface but also in the bulk oxide. The deuterium incorporation shows the improvement on soft breakdown characteristics and the interface state density at SiO/sub 2//Si after stress. The addition of very high vacuum prebake process yields a deuterium concentration of 9/spl times/ 10/sup 20/ cm/sup -3/, but also leads to the formation of rough oxide.

FANO INTERFERENCE IN THE QUANTUM WELL–QUANTUM DOT SYSTEM

The interaction between the quantum dot (QD) level and the 2D continuum of a quantum well (QW) is investigated theoretically. Due to interference from these states, the Fano-like resonance states appear in the 2D continuum of the QW. Also, the free-electron energy band of the QW is changed because of QD potential influence. We have calculated the position and the broadening of Fano resonances, and also modified the band structure of free states in the QW. The obtained results are used further for research on tunneling from the QW to the QD.