Optical and electrical properties of undoped and doped Ge nanocrystals
© Das et al; licensee Springer. 2012
Received: 30 November 2011
Accepted: 20 February 2012
Published: 20 February 2012
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.
The development of silicon-based optoelectronics has attracted a lot of attention over the past decade [1, 2]. The concept is based on integration of Si-based photonic components, in which light can be generated, waveguided, modulated, amplified, and detected with the advanced electronic components to realize monolithically integrated Si-based optoelectronic circuits. The study of Ge [3–5] and Si [6, 7] nanostructures is motivated by the prediction that quantum confinement of carriers leads to efficient luminescence despite the indirect nature of the energy gaps. Germanium nanocrystals [NCs] have been found to exhibit visible luminescence at room temperature [3–5, 8, 9]. However, the mechanism of visible luminescence from Si and Ge nanocrystals is still disputed. Rare earth-doped semiconductors also have been shown to be of remarkably important for combining electronic devices with optical elements . During the last several decades, the optical properties of erbium-doped semiconductor materials have been extensively studied due to the intra-4f 4I13/2 → 4I15/2 transition (first excited state to the ground state of Er3+ ion), which overlaps with the 1.54 μm wavelength of maximum transmission of silica-based optical fibers. Since Ge has higher electron and hole mobility, larger excitonic Bohr radius than Si  and is compatible with planar Si technology, efforts are being made to study the optical properties of Er-doped Ge nanostructures.
On the other hand, flash memory with nanocrystals floating gate has received much attention because of the high-speed write/erase operation, long retention time, and small device size . Ge with a smaller band gap compared to Si is expected to improve the memory characteristics by inducing a higher valence band offset between the Si substrate and nanocrystals [12, 13]. A thick tunnel barrier can guarantee a long retention time of the flash-memory device, but unfortunately, it slows down the programming speed. A thinner tunnel barrier will result faster programming speed but shorten the retention time. The use of a physically thicker high-permittivity oxide ensures good retention characteristics. On the other hand, thin-tunneling barriers due to the low equivalent oxide thickness allow high currents across the tunneling oxide at low control gate voltages during programming and erasing cycles [9, 14–16]. For Ge nanocrystals embedded in a high dielectric constant [high-k] material, the electrostatic energy is much higher due to the difference in the static dielectric constant of SiO2 and high-k oxides . In 2003, VARIOT structured tunnel oxide was reported by Govoreanu et al.  for the first time. Simulations and experimental results showed that a larger injected gate current density is possible for the memory devices with VARIOT structure tunnel barrier compared to memories with only a single-layered tunnel oxide [18, 19].
In this paper, we report the size- and host matrix-dependent photoluminescence [PL] and electroluminescence [EL] characteristics of Ge nanocrystals. The systematic study demonstrated the origin of visible luminescence due to the quantum confinement of carriers. The temperature-dependent characteristics of 1.54 μm emission from Er-doped Ge nanocrystals are also presented. An improved charge storage characteristic for the nanocrystal in trilayer structure is reported using high-k Al2O3 and HfO2, as compared to conventional SiO2. The experimental results showed that a VARIOT tunnel stack is attractive as a replacement for the traditional single-layer tunnel barrier for improving the data retention and programming speed simultaneously.
Details of various samples deposited by radio frequency magnetron sputtering system
Tunnel oxide (nm)
Middle layer (nm)
Cap oxide (nm)
(°C for 30 min in N2)
Annealed at 800
Annealed at 900
Annealed at 1,000
Ge + Al2O3: 15
Annealed at 900
Ge + HfO2: 15
Annealed at 900
HfO2: ~ 5
Annealed at 900
Results and discussions
TEM studies of Ge nanocrystals
Hence, the mixture of HfO2 and Ge has the lower Gibbs free energy in the co-sputtered film, resulting in the agglomeration of Ge atoms into nanocrystals.
Photoluminescence characteristics of undoped Ge nanocrystals
PL peak energy and nanocrystal size for SiO2 embedded Ge NCs
PL peak details
dNC (TEM) nm
Peak position (eV)
2.4 ± 0.8
5.3 ± 1.3
10 ± 3
Size of Ge nanocrystals
PL peak energy (eV)
dNC [confinement model] (nm)b
dNC (TEM) (nm)b
Emission characteristics of Er-doped Ge nanocrystals
I0 being the intensity at absolute zero temperature, E1 and E2 are the activation energies, and c1 and c2 are the corresponding coupling coefficients. At low temperatures (T < 75 K), the PL peak intensity is observed to be weakly temperature-dependent, with small thermal activation energy of 5.1 meV. With increase in temperature above 100 K, the PL peak intensity is observed to be quenched with large activation energy of 84.8 meV. It is suggested that the main energy transfer mechanism is the Förster mechanism , which is a nonoptical dipole-dipole interaction. Since the Förster mechanism is effective over several nanometres, it is likely that this mechanism is mainly responsible for the energy transfer from Ge nanocrystals to Er3+ ions.
Electroluminescence characteristics of undoped Ge nanocrystals
Memory characteristics of Ge nanocrystals in oxide matrices
Details of memory window and charge retention characteristics of different Ge NCs memory devices
ΔVFB at t= 0 (V)
Charge storage (cm-2)
ΔVFB after 10 years (V)
Charge loss after 10 years (%)
3.2 × 1010
7.6 × 1012
1.6 × 1013
2.1 × 1013
In conclusion, we have reported a systematic study on the size- and host matrix-dependent photoluminescence characteristics of Ge nanocrystals showing the origin of visible luminescence due to the quantum confinement of charge carriers. This is corroborated by the broad visible electroluminescence characteristics from devices with Al2O3 and HfO2, attributed to the radiative recombination from Ge nanocrystals and also from the defect states. A two-stage quenching process has been observed from 1.54 μm emission characteristics of Er-doped Ge nanocrystals due to the energy transfer process between Er and Ge following Förster's mechanism. A large memory window of 5.88 V and high retention time of 16% charge loss per 10 years have been obtained in MIS structures using Ge nanocrystals floating gate with VARIOT tunneling barrier.
full width at half maxima
high-resolution transmission electron microscopy
scanning electron microscopy
variable oxide thickness
The research at IIT Kharagpur is supported by DST-MBE, DRDO-FIR, and DST-ITPAR project grants, Government of India.
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