Szu-Lin Cheng

Strained germanium thin film membrane on silicon substrate for optoelectronics

This work presents a novel method to introduce a sustainable biaxial tensile strain larger than 1% in a thin Ge membrane using a stressor layer integrated on a Si substrate. Raman spectroscopy confirms 1.13% strain and photoluminescence shows a direct band gap reduction of 100meV with enhanced light emission efficiency. Simulation results predict that a combination of 1.1% strain and heavy n(+) doping reduces the required injected carrier density for population inversion by over a factor of 60. We also present the first highly strained Ge photodetector, showing an excellent responsivity well beyond 1.6um.

Electroluminescence from strained germanium membranes and implications for an efficient Si-compatible laser

We demonstrate room-temperature electroluminescence (EL) from light-emitting diodes (LED) on highly strained germanium (Ge) membranes. An external stressor technique was employed to introduce a 0.76% bi-axial tensile strain in the active region of a vertical PN junction. Electrical measurements show an on-off ratio increase of one order of magnitude in membrane LEDs compared to bulk. The EL spectrum from the 0.76% strained Ge LED shows a 100nm redshift of the center wavelength because of the strain-induced direct band gap reduction. Finally, using tight-binding and FDTD simulations, we discuss the implications for highly efficient Ge lasers.

Direct band Ge photoluminescence near 1.6 μm coupled to Ge-on-Si microdisk resonators

We fabricate and optically characterize germanium microdisks formed out of epitaxial germanium grown on silicon. Resonators coupled to fiber tapers display clear whispering gallery modes in transmission and photoluminescence with quality factors limited by germanium’s material absorption. Continuous wave pumping of the cavities resulted in a dominant heating effect for the cavity modes in both transmission and photoluminescence. Pulsed optical pumping proved to be more effective in minimizing heating, but was not sufficient to observe material gain or lasing. We believe that significantly higher doping levels are critical in order to achieve lasing at reasonable pump conditions.

Germanium In Situ Doped Epitaxial Growth on Si for High-Performance

We demonstrate an abrupt and box-shaped n+/p junction in Ge with a high level of activation of n-type-dopant phosphorus (P) using in situ doping during epitaxial growth. The temperature dependence of dopant activation was investigated associated with the shallower and abrupt junction formation. In addition, we have fabricated high-performance Ge n+/p-junction diodes at 400degC-600degC, based on the in situ doping technique. Excellent diode characteristics having a 1.1 times 104 on/off ratio and a high forward current density (120 A/cm2 at 1 V) are obtained in an n+/p diode at 600-C in situ doping.

\hbox{n}^{+}/\hbox{p}

Highly activated n-type dopant is essential for n⁺ /p germanium diodes which will be in use for source/drain regions in Ge n-MOSFETs as geometry scaling proceeds. Rapid thermal annealing of coimplanted P and Sb in Ge has provided n-type dopant activation beyond 1 × 10²⁰ cm^-3. However, there are limited reports on the electrical characteristics of these junctions. This letter has investigated the temperature-dependent diode I-V characteristics and contact resistance of metal-n⁺ Ge contacts. Well-behaved n⁺ /p Ge diodes (I_on/I_off >; 10⁵ and η <; 1.2) and significantly reduced contact resistance (ρ_c ~ 8 × 10^-7 Ω · cm²) have been demonstrated.

-Junction Diode

Plasmonic gratings of different periodicities are fabricated on top of silicon nanocrystals embedded in silicon dioxide. Total enhancements of up to 2 were observed, which matches the value from simulations. Plasmonic enhancements are observed for the first three orders of the plasmonic modes, with the peak enhancement wavelength varying with the periodicity. Biharmonic gratings are also fabricated to extract the enhanced emission from the first order plasmonic mode, resulting in enhancements with quality factors of up to 16.

Electrical Characteristics of Germanium

Germanium-on-insulator (GOI) is desired for high performance metal-oxide-semiconductor transistors and monolithically integrated optoelectronics. We demonstrate a promising approach to achieve single-crystal defect-free GOI by using lateral over-growth through SiO2 window. The dislocations due to the lattice mismatch are effectively terminated and reduced in SiO2 trench by selective area heteroepitaxy combined with hydrogen annealing. Low defect density of 4×106 cm−2 and low surface roughness of 0.7 nm (root-mean-square) on GOI are confirmed by plan-view transmission electron microscopy and atomic force microscopy analysis. In addition, the excellent metal-semiconductor-metal diode electrical characteristics fabricated on this GOI confirm Ge crystal quality. The selectively grown GOI structure can provide the monolithic integration of SiGe based devices on a Si very large scale integration (VLSI) platform.

\hbox{n}^{+}/ \hbox{p}

One dimensional nanobeam photonic crystal cavities are fabricated in silicon dioxide with silicon nanocrystals. Quality factors of over 9 x 10^3 are found in experiment, matching theoretical predictions, with mode volumes of 1.5(lambda/n)^3 . Photoluminescence from the cavity modes is observed in the visible wavelength range 600-820 nm. Studies of the lossy characteristics of the cavities are conducted at varying temperatures and pump powers. Free carrier absorption effects are found to be significant at pump powers as low as a few hundred nanowatts.

Junctions Obtained Using Rapid Thermal Annealing of Coimplanted P and Sb

Efficient silicon (Si)-compatible emitters can realize inexpensive light sources for a variety of applications. In this paper, we study both photonic crystal (PC) and plasmonic nanocavities that enhance the emission of Si-compatible materials. In particular, we examine the coupling of silicon nanocrystals (Si-NCs) to silicon nitride PC cavities and Si-NCs in silicon dioxide to plasmonic gratings, both for enhancement of emission in the visible wavelengths. In addition, we also observe the enhancement of the 1530 nm emission from erbium-doped silicon nitride films coupled to Si PC cavities. Finally, we analyze the loss mechanisms associated with the hybrid silicon nitride/silicon system, and propose advancements in the designs of PC and plasmonic cavities for the emitters described in this paper.

Plasmonic enhancement of emission from Si-nanocrystals

Supercritical fluid (SCF) technology is employed at low temperature as a postgate dielectric treatment to improve gate SiO(2)germanium (Ge) interface in a Ge-based metal-oxide-semiconductor (Ge-MOS) device. The SCF can transport the oxidant and penetrate the gate oxide layer for the oxidation of SiO(2)Ge interface at 150 degrees C. A smooth interfacial GeO(2) layer between gate SiO(2) and Ge is thereby formed after SCF treatment, and the frequency dispersion of capacitance-voltage characteristics is also effectively alleviated. Furthermore, the electrical degradation of Ge-MOS after a postgate dielectric annealing at 450 degrees C can be restored to a extent similar to the initial state.