R. K. Singha

Improved charge injection characteristics of Ge nanocrystals embedded in hafnium oxide for floating gate devices

Metal-oxide-semiconductor capacitors with a trilayer structure consisting of Ge+HfO2 layers sandwiched between HfO2 tunnel and cap oxides were fabricated on p-Si substrates. Ge nanocrystals embedded in SiO2 were also studied for comparison. Cross-sectional transmission electron micrographs revealed the formation of spherical shaped Ge nanocrystals. The optical and charge storage characteristics of trilayer structures were studied through photoluminescence spectroscopy and capacitance-voltage measurements, respectively. An enhancement of the charge injection capability into nanocrystals was observed for the device with HfO2 as tunnel and cap oxide. The optical emission characteristics support the carrier confinement in Ge nanocrystals embedded in oxide matrices.

Room temperature infrared photoresponse of self assembled Ge/Si (001) quantum dots grown by molecular beam epitaxy

We report on the observation of intraband near infrared (∼3.1 μm) and mid infrared (∼6.2 μm) photocurrent response at room temperature using Ge/Si self-assembled quantum dots grown by molecular beam epitaxy. Due to the bimodal size distribution and SiGe intermixing, distinguishable photoluminescence transitions are observed at 10 K, below and above the optical band gap of bulk Ge. The observed redshift in photocurrent with increasing temperature has been explained by the excitonic electric field originated due to infrared excitation at low temperatures. A good correlation between the spectral photocurrent response and photoluminescence of the quantum dots has been established.

Optical and electrical properties of undoped and doped Ge nanocrystals

Size-dependent photoluminescence characteristics from Ge nanocrystals embedded in different oxide matrices have been studied to demonstrate the light emission in the visible wavelength from quantum-confined charge carriers. On the other hand, the energy transfer mechanism between Er ions and Ge nanocrystals has been exploited to exhibit the emission in the optical fiber communication wavelength range. A broad visible electroluminescence, attributed to electron hole recombination of injected carriers in Ge nanocrystals, has been achieved. Nonvolatile flash-memory devices using Ge nanocrystal floating gates with different tunneling oxides including SiO2, Al2O3, HfO2, and variable oxide thickness [VARIOT] tunnel barrier have been fabricated. An improved charge storage characteristic with enhanced retention time has been achieved for the devices using VARIOT oxide floating gate.

Electroluminescence and charge storage characteristics of quantum confined germanium nanocrystals

Quantum confined Ge nanocrystals embedded in high bandgap and high-k Al2O3 dielectric matrix have been synthesized to demonstrate dual functional devices using Si-compatible fabrication technology. Transmission electron microscopy has shown the formation of Ge nanocrystals of varying diameter from 2.5 to 7.5 nm, much lower than the excitonic Bohr radius of Ge. A broad visible electroluminescence band at room temperature has been observed, which is attributed to the recombination of injected electrons and holes in Ge nanocrystals. An anti-clockwise hysteresis in the capacitance-voltage measurement of these devices indicates the charge storage in nanocrystals, useful for floating gate memory devices.

Evolution of strain and composition of Ge islands on Si (001) grown by molecular beam epitaxy during postgrowth annealing

Self-assembled Ge islands have been grown using a Stranski–Krastanov growth mechanism on Si (001) substrates by solid source molecular beam epitaxy. We performed time varying annealing experiments at a representative temperature of 650 ° C to study the shape and size evolution of islands for a relatively high Ge coverage. Islands are found to coarsen due to heat treatment via structural and compositional changes through continuous strain relaxation. Different island morphologies, namely, “pyramids,” “transitional domes,” and “domes” are found during the annealing sequence. The dominant coarsening mechanisms for the temporal evolution of islands of as-grown and annealed samples are explained by the comprehensive analysis of Rutherford back scattering, Raman spectroscopy, high-resolution x-ray diffraction, and atomic force microscopy. A correlation of the morphological evolution with the composition and strain relaxation of grown islands is presented.

Microstructural characteristics and phonon structures in luminescence from surface oxidized Ge nanocrystals embedded in HfO2 matrix

Ge nanocrystals embedded in HfO2 matrices were prepared by rf magnetron sputtering technique. Transmission electron micrographs reveal the formation of spherical shape Ge nanocrystals of 4–6 nm diameters for 800 °C and 6–9 nm for 900 °C annealed samples. X-ray photoelectron spectroscopy confirms the formation of surface oxidized Ge nanocrystals. Embedded Ge nanocrystals show strong photoluminescence peaks in visible and ultraviolet region even at room temperature. Spectral analysis suggests that emission in 1.58 and 3.18 eV bands originate from TΣ(TΠ)→S0, and TΠ′→S0 optical transitions in GeO color centers, respectively, and those in the range 2.0–3.0 eV are related to Ge/O defects at the interface of the oxidized nanocrystals. Temperature dependent photoluminescence study has revealed additional fine structures with lowering of temperature, the origin of which is attributed to the strong coupling of electronic excitations with local vibration of germanium oxides at the surface.

Effect of growth temperature and post-growth annealing on luminescence properties of molecular beam epitaxy grown single layer Ge quantum dots

We report the detailed structural and luminescence properties of Si-embedded single layer Ge nanoislands grown via Stranski-Krastanov mechanism using molecular beam epitaxy. The luminescence energy peak positions correlated with structural properties have been studied as a function of the growth temperature and post-growth annealing. The photoluminescence peak for the annealed sample (500 °C grown) is blue shifted with increasing anneal temperature compared to the as grown sample due to intermixing of Si/Ge. Electroluminescence from the single layer Ge islands in the wavelength range 1.53 to 1.67 μm has been demonstrated using a metal-insulator-semiconductor structure, making it attractive for quantum dot light emitting devices.

Characteristics of Ge Nanocrystals Grown by RF Magnetron Sputtering

We report the self-assembled growth of Ge islands of different shapes and sizes on p-Si (001) by r.f. magnetron sputtering by varying the r.f. power, growth temperature and postdeposition annealing condition. The well known Stranski-Krastanov growth mechanism due to lattice mismatch between Si & Ge leads to the formation of Ge islands, similar to a more sophisticated MBE growth, albeit at a much higher pressure in our study. Ge nanocrystals embedded in SiO2 matrix have also been grown. Optical properties of nanocrystals exhibiting visible luminescence at room temperature are presented.

Electroluminescence from metal-insulator-semiconductor tunneling diodes using compressively strained Ge on Si 0.5 Ge 0.5 virtual substrates

Direct band gap optical transition in compressively strained Ge film is demonstrated for the first time under current injection through a metal-insulator-semiconductor diode structure. The compressively strained Ge layer is grown on the relaxed Si0.5Ge0.5 substrate by solid source molecular beam epitaxy. The electroluminescence of direct band gap emission from strained Ge film and TO phonon assisted transition in Si and SiGe from the virtual substrate is observed under different current injections. The signature of heavy hole and light hole splitting in valence band is observed in the electroluminescence spectra from strained Ge layer. The temperature dependent electroluminescence characteristics have been studied over a temperature range of 10-300 K. AC frequency modulation for the Ge direct band electroluminescence has been studied to improve the emission efficiency over the DC bias.

Preferential ordering of self-assembled Ge islands on focused ion-beam patterned Si(100)

We report the growth of Ge islands on Si (001) substrates with lithographically defined two-dimensionally periodic pits using focused ion-beam patterning and molecular beam epitaxy. The formation of circularly ordered Ge islands has been achieved by means of nonuniform strain field around the periphery of the holes due to ion bombardment. Lateral ordering of the Ge islands have been controlled by both the pit size and pit separation. Preferential growth at the pit sites has also been achieved by using appropriate pattern shape and size.