Structural and optical properties of germanium nanostructures on Si(100) and embedded in high-k oxides
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The structural and optical properties of Ge quantum dots (QDs) grown on Si(001) for mid-infrared photodetector and Ge nanocrystals embedded in oxide matrices for floating gate memory devices are presented. The infrared photoluminescence (PL) signal from Ge islands has been studied at a low temperature. The temperature- and bias-dependent photocurrent spectra of a capped Si/SiGe/Si(001) QDs infrared photodetector device are presented. The properties of Ge nanocrystals of different size and density embedded in high-k matrices grown using radio frequency magnetron sputtering have been studied. Transmission electron micrographs have revealed the formation of isolated spherical Ge nanocrystals in high-k oxide matrix of sizes ranging from 4 to 18 nm. Embedded nanocrystals in high band gap oxides have been found to act as discrete trapping sites for exchanging charge carriers with the conduction channel by direct tunneling that is desired for applications in floating gate memory devices.
KeywordsHfO2 Reflection High Energy Electron Diffraction Memory Window Dark Current Density Al2O3 Matrix
atomic force microscope
high-resolution transmission electron microscopy
molecular beam epitaxy
quantum dot infrared photodetector
reflection high energy electron diffraction
Germanium nanostructures have potential applications for electronic flash memories [1, 2, 3] and light emitters in visible  and near-infrared  wavelengths, making the indirect gap semiconductor attractive for novel electronic and optical devices. In comparison to bulk Ge, nanocrystals exhibit a tunable emission wavelength  and increased oscillator strength due to the quantum confinement of excitons. The confinement of charge carriers in these nanostructures allows one to increase the efficiency of the radiative recombination. The growth of Ge islands on Si substrates via Stranski-Krastanow growth mode has been extensively investigated as this opens up the possibility to integrate optoelectronics with planar Si technology. Most of the SiGe/Si structures are believed to exhibit a type-II heterointerface, where electrons and holes are spatially separated with a limited wave function overlap . Owing to the type-II band alignment, Ge quantum dots (QDs) themselves form a potential well only for holes, whereas the electrons are weakly confined in their vicinities, i.e., by the tensile and compressive strain fields in the Si cap induced by the strained QDs . This has led to the enhancement of PL quantum efficiency in planar Si/SiGe superlattices at elevated temperatures due to 3D carrier localization within the Ge QDs and presumably due to large energy barriers formed at the heterointerfaces between the Ge clusters and the surrounding Si matrix .
On the other hand, intersubband transitions in Ge/Si quantum dots (QDs) are attractive for quantum dot infrared photodetectors (QDIPs) in the wavelength range 5-10 μm. Ge QDIPs have an advantage that the absorption of normally incident infrared radiation by holes in the valence band is allowed, without the requirement of fabrication of gratings or any other optical coupling elements unlike for the conduction band of III-V semiconductors. Similarly, persistent efforts have been made to achieve efficient visible light emission from Si and Ge nanocrystals (NCs) embedded in oxide matrix . Even though Ge NCs embedded in the high band gap oxide matrix show efficient and tunable PL emission by varying their size, the origin of the light emission is still under debate [3, 6].
In this article, we report the structural and optical properties of Ge QDs grown on Si(001) by molecular beam epitaxy (MBE) as well as Ge nanocrystals embedded in high band gap oxide matrices. The observed infrared PL signal from Ge dots grown on Si(001) is influenced by island size and the intermixing of Si/Ge. The origin of photoresponse of the Ge islands in the mid-infrared (IR) wavelength range is discussed. The emission and charge trapping behavior of Ge nanocrystals embedded in different high band gap oxide matrices are also reported.
Ge QDs on Si(001) substrates were grown by solid source MBE (Riber Supra 32) system using an electron gun for the deposition of thin Si buffer layer (approximately 5 nm) with a growth rate of 0.4 Å/s and a Knudsen cell for Ge deposition, followed by the growth of a 3.0 nm Si cap layer. Growth temperature was varied from 500 to 600°C and Ge monolayer (ML) thickness was assorted from 6 to 20 ML. The growth was monitored in situ by reflection high energy electron diffraction (RHEED). On the other hand, Ge nanocrystals embedded in high-k HfO2 and Al2O3 matrix on Si(100) substrates were deposited by radio frequency (13.56 MHz) magnetron co-sputtering method in Ar + O2 ambient at an rf power of 50 W, similar to those reported earlier [2, 3]. The as-grown sample is defined as 'A-as' and 'F-as' for Ge embedded Al2O3 and HfO2, respectively. In order to grow Ge nanocrystals in high-k matrix, the sputter deposited film was thermally annealed in N2 gas ambient for 30 min at 800 and 900°C.
The growth of Ge islands using MBE was studied using Veeco, Nanoscope-IV atomic force microscope (AFM). High-resolution transmission electron microscopy (HRTEM) was carried out using a JEM 2100F (JEOL) field emission system with an operating voltage of 200 kV to probe the formation of Ge nanocrystals in the oxide matrix. Raman spectra of the grown samples were obtained with a Renishaw Raman microscope equipped with a He-Ne laser excitation source emitting at a wavelength of 632.8 nm and a Peltier cooled (-70°C) charge-coupled device (CCD) camera. PL spectra of samples were recorded using a He-Cd laser as an excitation source, operating at 325 nm with an output power density of 1.3 W/cm2 and a TRIAX 320 monochromator fitted with a photomultiplier or an InGaAs detector. The photocurrent (PC) spectra were investigated under monochromatic light dispersed from a glowbar source by grating spectrometer and chopped at a frequency of 233 Hz. PC signals were detected by a standard lock-in amplifier technique. Aluminum was deposited on top of the sample by masking and was rapid thermal annealed at 200 C for 10 min to form a good ohmic contact. At backside, native oxide was etched by HF followed by Al deposition to form the ohmic contact. The electrical properties of the grown structures were measured using a Keithley semiconductor parameter analyzer (4200-SCS).
Results and discussion
Growth and optical properties of Ge nano-islands on Si(001)
Raman peak for relaxed (phonon confinement model) and embedded (experimental) Ge nanocrystals and estimated hydrostatics stress
Ge-Ge phonon peak position
Experimental (embedded) (cm-1)
Phonon confinement (relaxed) (cm-1)
3.5 × 1011
2.6 × 1011
5.8 × 1012
1.3 × 1012
where ΔVFB is the flat-band voltage shift, q is the magnitude of the electronic charge; tCO and εCO are the thickness and relative permittivity of the control oxide; tNC and εNC are the diameter and relative permittivity of the nanocrystal; and εO is the permittivity of the free space. The calculated stored charge densities for the A-800, A-900, F-800, and F-900 devices are 1.3 × 1012, 7.1 × 1012, 4.5 × 1012, and 5.4 × 1012 cm-2, respectively. Comparing with the nanocrystal density of the above samples presented in Table 1, it is evident that the numbers of charges stored per nanocrystal are around 4, 27, 1, and 4 for the samples A-800, A-900, F-800, and F-900, respectively. For sample F-900, with average nanocrystal diameter 3.9 nm, the charge stored per nanocrystal is one due to prominent Coulomb blockade effect in small clusters. Whereas for other samples with larger diameter, there are more than one electron per nanocrystal due to reduced Coulomb repulsion. The number of stored charges per cluster is highest for the sample A-900 with largest size (13 nm). The memory window and stored charge density is found to be significantly enhanced on increasing the annealing temperature (900°C) for Ge nanocrystals embedded in Al2O3 matrix as compared to that of HfO2, making it attractive for nanocrystal flash memory applications.
We have presented the structural and optical characteristics of Ge islands grown on Si(100) by MBE. The observed infrared PL signal at 10 K from Ge islands is associated with the radiative recombination of holes confined in the Ge islands and electrons localized in the Si buffer layer. The temperature and bias dependent PC spectra of a capped Si/SiGe/Si(001) QDIP photodetector device are presented. We have also grown Ge nanocrystals (4-18 nm in diameter) embedded in high-k Al2O3 and HfO2 matrices for applications in floating gate memory devices. The analysis of Ge-Ge phonon vibration using Raman spectroscopy has shown the formation of compressively stressed Ge nanocrystals in high-k matrix. The observed shift in flat-band voltage for C-V curves has been attributed to electron trapping in embedded Ge nanocrystals.
This study was supported in part by a sponsored research grant (FIR project) from DRDO, Government of India.
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