Self-assembled cobalt (Co) nanocrystals on ultra-thin silicon dioxide layer were fabricated by in... more Self-assembled cobalt (Co) nanocrystals on ultra-thin silicon dioxide layer were fabricated by in situ annealing Co ultrathin films deposited with Co effusion cell in a molecular-beam-epitaxy chamber. The resultant nanocrystals obtained at the optimized annealing temperature are around 3-4 nm in diameter with dot density of about 1 • 10 12 cm À2 . The metal-oxide-semiconductor capacitors containing Co nanocrystals exhibit much longer retention times than a Si nanocrystal memory with the same tunneling oxide thickness. This study suggests that Co nanocrystal should be an excellent alternative to replace Si nanocrystal as floating gates for future nonvolatile flash-type memory application.
Fabrication and performance of a medium-dependent SiO_2/Si photonic heterostructure device
Optics Letters, May 15, 2003
An optical device based on a photonic bandgap heterostructure is designed, fabricated, and charac... more An optical device based on a photonic bandgap heterostructure is designed, fabricated, and characterized. The sample contains two sets of Si/SiO2 photonic crystals with different periods. When the device is working in air, it reflects omnidirectionally both TE and TM mode lights at the wavelength near 1.3 microm. The reflectivity measured in the bandgap is higher than 98% in an incident angle range 0 degrees-70 degrees. When the device is surrounded with silica (n > 1.33), it permits the total transmission for the TM mode but prevents the TE mode from propagating, thus behaving as a polarization splitter. The experimental extinction ratio of the reflected TE/TM is approximately 31 dB. The uniformity of the device performance over a large sample area is demonstrated.
The nitrogen-doped, p-type ZnO film was grown by plasma-assisted molecular beam epitaxy (P-MBE) o... more The nitrogen-doped, p-type ZnO film was grown by plasma-assisted molecular beam epitaxy (P-MBE) on c-plane sapphire (c-Al 2 O 3 ) using radical NO as oxygen and nitrogen sources. The activation energy of the nitrogen acceptor (AENA) was obtained by the temperature-dependent Hall effect and photoluminescence (PL) measurements, respectively. However, the values of AENA were rather different for the two methods, i.e., 75 meV for the Hall effect measurement and 145 meV for the photoluminescence measurement. In terms of the electrostatic interaction model, this energy difference was explained by the relationship between the Hall activation energy and the optical activation energy. According to these analyses, the reasonable value of the acceptor activation energy in nitrogen-doped p-type ZnO can be determined either by Hall experiments or by optical methods.
A silicon-based one-dimensional photonic crystal heterostructure is proposed, and its application... more A silicon-based one-dimensional photonic crystal heterostructure is proposed, and its applications to an omnidirectional reflector and a transverse electric/transverse magnetic (TE/TM) splitter are illustrated. The photonic crystal heterostructure consists of two sets of multilayers, each has six periods of bilayers of poly-silicon and silicon dioxide. When the incident light comes from a medium with refractive index lower than 1.16, there is a frequency range over which of both TE and TM polarizations will be totally reflected at any incident angle. In such a case, the photonic crystal heterostructure can be used as a low loss omnidirectional dielectric reflector. However, when the refractive index of the medium is higher than 1.16, the heterostructure will serve as a good TE/TM splitter of very low crosstalk within certain ranges of light frequency and incident angle.
The valence-band offset of ZnO / MgO ͑111͒ heterojunction has been directly measured by x-ray pho... more The valence-band offset of ZnO / MgO ͑111͒ heterojunction has been directly measured by x-ray photoelectron spectroscopy. Excluding the strain effect, the valence-band offset is determined to be 0.87Ϯ 0.20 eV, and the conduction-band offset ⌬E C is deduced to be -3.59Ϯ 0.20 eV, indicating that ZnO / MgO heterojunction has a type-I band alignment. The conduction-band and valence-band offset of MgO / ZnO is used to interpret the origination of p-type conduction in undoped Mg x Zn 1-x O alloy and deeper acceptor level in undoped and N-doped p-type Mg x Zn 1-x O alloy than in ZnO.
Electrical transport properties in nitrogen-doped p-type ZnO thin film
Semiconductor Science and Technology, Sep 25, 2006
Electrical transport properties of p-type ZnO:N films grown by thermal activation of the nitrogen... more Electrical transport properties of p-type ZnO:N films grown by thermal activation of the nitrogen dopant were investigated via the temperature-dependent Hall effect. The Hall mobility increases with decreasing temperature. Varied scattering mechanisms have been analysed including lattice vibration scattering, ionized impurity scattering and dislocation scattering. A fit of the theory to temperature-dependent hole mobility experimental data in p-type ZnO:N films gives dislocation densities in the order of 1012 cm−2. The analysis shows dislocation scattering is indeed important for the p-type ZnO films grown on the mismatched substrate. The thermal ionization energy of the nitrogen acceptor is estimated to be 170 meV in terms of the temperature-dependent hole concentration. On the other hand, the emission related to the acceptors is observed in PL spectra.
Figure 1. (a) The schematic diagram of the p-channel Memory device using Ge/Si as floating gates.... more Figure 1. (a) The schematic diagram of the p-channel Memory device using Ge/Si as floating gates. (b) The energy band structure of the memory.
As one type of discrete storage nodes, Si nanocrystal has been extensively investigated as the fl... more As one type of discrete storage nodes, Si nanocrystal has been extensively investigated as the floating gate in nonvolatile MOSFET memory devices [1]. The discrete nature offers the immune of the charge storage from the local weak point in the tunnel oxide, which has dramatically driven the memory device toward the target of being smaller, faster and less power consuming. However, the defect-related traps play a dominant role in the long retention mechanism, which in other words means the non-reliable device performance. Although this issue can be solved by using metal nanocrystals, new issue is introduced: some metal would react with tunnel oxide in the high-temperature annealing process. In this work, we propose to use metallic silicide/Si hetero-nanocrystal floating gate and prove from both simulation and experiment that such hetero-nanocrystals can significantly improve retention without depressing the programming speed. The metal/oxide inter-diffusion is also minimized, leading to better endurance. We use CoSi2/Si hetero-nanocrystals as an example to demonstrate the concept. Figure 1 shows the band structure of the memory devices with CoSi2/Si heteronanocrystals. It is evident that the hetero-nanocrystal offers an additional barrier for the stored electron, which leads to longer retention time. Fig.2 and Fig.3 give the calculated retention and writing/erasing performance of the CoSi2/Si hetero-nanocrystal memory devices. Obviously the retention quality is orders of magnitude improved as compared to a defect-free Si nanocrystal memory. One notices that this improvement is not at the cost of the writing/erasing speed, as shown in Fig.3. The CoSi2/Si hetero-nanocrystals were fabricated with the self-aligned silicide technique [2]. First, Si nanocrystals were deposited in a LPCVD furnace on ~ 5 nm thermally grown SiO2. Then the wafer was covered with ~ 3 nm Co followed by rapid thermal annealing at 700 °C and 750 °C to form CoSi2/Si hetero-nanocrystals and CoSi2 nanocrystals, respectively. After the metal removal in a selective etchant, discrete CoSi2/Si hetero-nanocrystals or CoSi2 nanocrystals were left on the tunnel oxide. 15 nm oxide as control oxide was then deposited followed by aluminum electrodes formed by lift-off technique. The resultant devices are MOS structures with nanocrystals embedded. The capacitance-voltage measurements before and after the writing operation were performed to characterize the memory effect, as shown in Fig. 4 where writing was realized by bias the Al gate with 10 V for 1 s. It is found that the C-V shift due to the charge injection is almost the same for all three MOS devices with Si, CoSi2 nanocrystals and CoSi2/Si heteronanocrystals, respectively. The similar C-V shifts indicate the similar charge injection efficiency for these devices. This is further confirmed in Fig. 5 where the C-V shift, or memory window as a function of the writing time is plotted. One finds the similar charge injection speed for the three MOS memory devices. The retention performance was found to be significantly different for different devices, as given in Fig.6. In these first devices, the tunneling oxide quality is poor, leading to the fast decay within the first 100 seconds for all devices. Nevertheless, it is easily found that CoSi2 nanocrystal device possesses a much longer retention than that of the Si nanocrystal device. A 1.0 V flat-band voltage shift can be expected after 10 years for a CoSi2 nanocrystal memory device. It is also found that CoSi2/Si hetero-nanocrystal MOS memory exhibits the longest retention. A flat-band voltage shift of 1.5 V is still available after 10 years of retention at zero gate voltage. In summary, MOS memory with CoSi2/Si hetero-nanocrystal floating gate was proposed and investigated. Both theoretical simulation and practical device performance show that CoSi2/Si hetero-nanocrystal memory devices have much longer retention time while also with almost the same writing/erasing speed, as compared with its Si nanocrystal and metallic CoSi2 nanocrystal counterparts. References [1] S. Tiwari, et al. Appl. Phys. Lett., vol. 68, pp. 1377-1379, 1996. [2] J. P. Gambino, and E. G. Colgan, Mater. Chem. Phys. 52, 99 (1998)
Nonvolatile Si nanocrystal memory is fast approaching commercialization. In order to extend the s... more Nonvolatile Si nanocrystal memory is fast approaching commercialization. In order to extend the scaling limits, many kinds of nanocrystals or device architectures were used to replace Si nanocrystals. In this work, we report for the first time the experimental demonstration of a nonvolatile memory device using cobalt silicide nanocrystals as floating gates. The band diagram is shown in Fig. 1 (a). There are at least two advantage of using cobalt silicide nanocrystal over Si nanocrystal: first, cobalt silicide is a metallic material and its band edges for conduction and valence bands are all lower than those of the Si [1], which helps both electron and hole retentions. Second, cobalt silicide can be grown on Si nanocrystal plus a rapid thermal annealing (RTP) [2]. The self-aligned silicide remains while the unreacted metal cobalt is removed in selective etchant. The device fabrication begins with a thermal oxide growth at 1000 °C for 4.5 minutes on a 4-inch n-type Si substrate, which leads to a tunneling oxide thickness ofabout 3 nm. Si nanocrystals were formed in a LPCVD furnace followed by a deposition of ultra thin Co metal in an e-beam evaporator. RTP is then performed in nitrogen to form silicide. The unreacted cobalt metal was removed in a selective. The control oxide (40 nm) was grown in another LPCVD furnace. A MOS capacitor is finally obtained after depositing and patterning the aluminum electrodes on frond and back sides of the sample. An SEM image of the CoSi2 nanocrystals on SiO2 is shown in Fig.1 (b).
Threshold voltage shift of heteronanocrystal floating gate flash memory
Journal of Applied Physics, Jan 18, 2005
Simulations of threshold voltage shift of a p-channel Ge/Si heteronanocrystal floating gate memor... more Simulations of threshold voltage shift of a p-channel Ge/Si heteronanocrystal floating gate memory device were carried out using both a numerical two-dimensional Poisson–Boltzmann method and an equivalent circuit model. The results show that the presence of a Ge dot on top of a Si dot significantly prolongs the retention time of the device, indicated by the time decay behavior of the threshold voltage shift. Both methods lead to consistent results that an increase in the thickness of either the Si dot or Ge dot will result in a reduction of the threshold voltage shift. Additionally, the threshold voltage shift increases significantly as the heteronanocrystal density increases. Nevertheless, only a weak dependence of threshold voltage shift on the tunneling oxide thickness was found.
Transient processes of Ge/Si hetero-nanocrystal floating gate memories are simulated numerically.... more Transient processes of Ge/Si hetero-nanocrystal floating gate memories are simulated numerically. Compared with Si nanocrystal memories, Ge/Si hetero-nanocrystal memories show similar writing and erasing efficiency with a weaker writing saturation and markedly improved retention characteristics.
After the proposal of nanocrystal floating gate memory by Tiwari, tremendous effort has been made... more After the proposal of nanocrystal floating gate memory by Tiwari, tremendous effort has been made to improve the device performance of a memory containing nanocrystal floating gate, including semiconductor nanocrystals (Si, Ge et al.), metal dot (W, Ni, Au, Pt, Ag et al.), dielectric nanocrystals (Al2O3, HfO2 and Si4N3 et al.) and Ge/Si hetero-nanocrystals. In this work, we will report
Silicon-based optical waveguide polarizer using photonic band gap
Applied Physics Letters, Jul 2, 2002
Based on different photonic band structures of TE and TM polarization modes in periodic multilaye... more Based on different photonic band structures of TE and TM polarization modes in periodic multilayers, a method to realize the waveguide polarizer is proposed. The waveguide structure contains a SiO2 core layer sandwiched between two multilayers of alternately stacked poly-Si and SiO2, and the whole structure can be grown on a Si substrate. Its propagation characteristics are studied theoretically. High extinction ratio over 40 dB at a light wavelength of 1.3 μm is expected in the waveguide of only 40 μm long, accompanied with very low propagation loss of the passive TE mode. These characteristics are very suitable for the applications in integrated optics. The fabrication of this polarizer structure by using the magnetron sputtering method is demonstrated.
Optical properties of p-type ZnO doped by lithium and nitrogen
Solid State Communications, Mar 1, 2007
ABSTRACT A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by R... more ABSTRACT A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al2O3 as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a LiZn–N complex acceptor level (eFA), radiative recombination of a donor–acceptor pair and recombination of the LiZn–N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The LiZn–N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.
High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer la... more High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 mm were grown at different growth temperatures of 350, 450 and 550 1C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 mm at 550 1C. A ZnO buffer layer with a growth temperature of 450 1C (450 1C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 1C-buffer sample (6.7 nm), the 550 1C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 1C-buffer sample was the narrowest as 0.2091, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 1C. Consistent with these results, the 450 1C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm À1 was found in Raman scattering spectra for all films with buffers, which is corresponding to the E 2 mode.
A nanocrystal memory using CoSi 2 /Si hetero-nanocrystals as floating gate was proposed. Numerica... more A nanocrystal memory using CoSi 2 /Si hetero-nanocrystals as floating gate was proposed. Numerical investigations on the writing, erasing and retention were performed. The hetero-structure provides an extra quantum well for the charge to achieve much longer retention time while maintains a writing/erasing speed similar to that of Si nanocrystal memory.
Transient capacitances were numerically investigated for Ge/ Si heteronanocrystal memories. Flatb... more Transient capacitances were numerically investigated for Ge/ Si heteronanocrystal memories. Flatband voltage shifts ͑⌬V fb ͒ were obtained. The results suggest that the Ge/ Si heteronanocrystal memories have significantly longer data retention compared with the memories embedding Si nanocrystals only. It is also found that larger heteronanocrystal leads to longer retention, larger device capacitance, and smaller ⌬V fb .
Self-assembled cobalt (Co) nanocrystals on ultra-thin silicon dioxide layer were fabricated by in... more Self-assembled cobalt (Co) nanocrystals on ultra-thin silicon dioxide layer were fabricated by in situ annealing Co ultrathin films deposited with Co effusion cell in a molecular-beam-epitaxy chamber. The resultant nanocrystals obtained at the optimized annealing temperature are around 3-4 nm in diameter with dot density of about 1 • 10 12 cm À2 . The metal-oxide-semiconductor capacitors containing Co nanocrystals exhibit much longer retention times than a Si nanocrystal memory with the same tunneling oxide thickness. This study suggests that Co nanocrystal should be an excellent alternative to replace Si nanocrystal as floating gates for future nonvolatile flash-type memory application.
Fabrication and performance of a medium-dependent SiO_2/Si photonic heterostructure device
Optics Letters, May 15, 2003
An optical device based on a photonic bandgap heterostructure is designed, fabricated, and charac... more An optical device based on a photonic bandgap heterostructure is designed, fabricated, and characterized. The sample contains two sets of Si/SiO2 photonic crystals with different periods. When the device is working in air, it reflects omnidirectionally both TE and TM mode lights at the wavelength near 1.3 microm. The reflectivity measured in the bandgap is higher than 98% in an incident angle range 0 degrees-70 degrees. When the device is surrounded with silica (n > 1.33), it permits the total transmission for the TM mode but prevents the TE mode from propagating, thus behaving as a polarization splitter. The experimental extinction ratio of the reflected TE/TM is approximately 31 dB. The uniformity of the device performance over a large sample area is demonstrated.
The nitrogen-doped, p-type ZnO film was grown by plasma-assisted molecular beam epitaxy (P-MBE) o... more The nitrogen-doped, p-type ZnO film was grown by plasma-assisted molecular beam epitaxy (P-MBE) on c-plane sapphire (c-Al 2 O 3 ) using radical NO as oxygen and nitrogen sources. The activation energy of the nitrogen acceptor (AENA) was obtained by the temperature-dependent Hall effect and photoluminescence (PL) measurements, respectively. However, the values of AENA were rather different for the two methods, i.e., 75 meV for the Hall effect measurement and 145 meV for the photoluminescence measurement. In terms of the electrostatic interaction model, this energy difference was explained by the relationship between the Hall activation energy and the optical activation energy. According to these analyses, the reasonable value of the acceptor activation energy in nitrogen-doped p-type ZnO can be determined either by Hall experiments or by optical methods.
A silicon-based one-dimensional photonic crystal heterostructure is proposed, and its application... more A silicon-based one-dimensional photonic crystal heterostructure is proposed, and its applications to an omnidirectional reflector and a transverse electric/transverse magnetic (TE/TM) splitter are illustrated. The photonic crystal heterostructure consists of two sets of multilayers, each has six periods of bilayers of poly-silicon and silicon dioxide. When the incident light comes from a medium with refractive index lower than 1.16, there is a frequency range over which of both TE and TM polarizations will be totally reflected at any incident angle. In such a case, the photonic crystal heterostructure can be used as a low loss omnidirectional dielectric reflector. However, when the refractive index of the medium is higher than 1.16, the heterostructure will serve as a good TE/TM splitter of very low crosstalk within certain ranges of light frequency and incident angle.
The valence-band offset of ZnO / MgO ͑111͒ heterojunction has been directly measured by x-ray pho... more The valence-band offset of ZnO / MgO ͑111͒ heterojunction has been directly measured by x-ray photoelectron spectroscopy. Excluding the strain effect, the valence-band offset is determined to be 0.87Ϯ 0.20 eV, and the conduction-band offset ⌬E C is deduced to be -3.59Ϯ 0.20 eV, indicating that ZnO / MgO heterojunction has a type-I band alignment. The conduction-band and valence-band offset of MgO / ZnO is used to interpret the origination of p-type conduction in undoped Mg x Zn 1-x O alloy and deeper acceptor level in undoped and N-doped p-type Mg x Zn 1-x O alloy than in ZnO.
Electrical transport properties in nitrogen-doped p-type ZnO thin film
Semiconductor Science and Technology, Sep 25, 2006
Electrical transport properties of p-type ZnO:N films grown by thermal activation of the nitrogen... more Electrical transport properties of p-type ZnO:N films grown by thermal activation of the nitrogen dopant were investigated via the temperature-dependent Hall effect. The Hall mobility increases with decreasing temperature. Varied scattering mechanisms have been analysed including lattice vibration scattering, ionized impurity scattering and dislocation scattering. A fit of the theory to temperature-dependent hole mobility experimental data in p-type ZnO:N films gives dislocation densities in the order of 1012 cm−2. The analysis shows dislocation scattering is indeed important for the p-type ZnO films grown on the mismatched substrate. The thermal ionization energy of the nitrogen acceptor is estimated to be 170 meV in terms of the temperature-dependent hole concentration. On the other hand, the emission related to the acceptors is observed in PL spectra.
Figure 1. (a) The schematic diagram of the p-channel Memory device using Ge/Si as floating gates.... more Figure 1. (a) The schematic diagram of the p-channel Memory device using Ge/Si as floating gates. (b) The energy band structure of the memory.
As one type of discrete storage nodes, Si nanocrystal has been extensively investigated as the fl... more As one type of discrete storage nodes, Si nanocrystal has been extensively investigated as the floating gate in nonvolatile MOSFET memory devices [1]. The discrete nature offers the immune of the charge storage from the local weak point in the tunnel oxide, which has dramatically driven the memory device toward the target of being smaller, faster and less power consuming. However, the defect-related traps play a dominant role in the long retention mechanism, which in other words means the non-reliable device performance. Although this issue can be solved by using metal nanocrystals, new issue is introduced: some metal would react with tunnel oxide in the high-temperature annealing process. In this work, we propose to use metallic silicide/Si hetero-nanocrystal floating gate and prove from both simulation and experiment that such hetero-nanocrystals can significantly improve retention without depressing the programming speed. The metal/oxide inter-diffusion is also minimized, leading to better endurance. We use CoSi2/Si hetero-nanocrystals as an example to demonstrate the concept. Figure 1 shows the band structure of the memory devices with CoSi2/Si heteronanocrystals. It is evident that the hetero-nanocrystal offers an additional barrier for the stored electron, which leads to longer retention time. Fig.2 and Fig.3 give the calculated retention and writing/erasing performance of the CoSi2/Si hetero-nanocrystal memory devices. Obviously the retention quality is orders of magnitude improved as compared to a defect-free Si nanocrystal memory. One notices that this improvement is not at the cost of the writing/erasing speed, as shown in Fig.3. The CoSi2/Si hetero-nanocrystals were fabricated with the self-aligned silicide technique [2]. First, Si nanocrystals were deposited in a LPCVD furnace on ~ 5 nm thermally grown SiO2. Then the wafer was covered with ~ 3 nm Co followed by rapid thermal annealing at 700 °C and 750 °C to form CoSi2/Si hetero-nanocrystals and CoSi2 nanocrystals, respectively. After the metal removal in a selective etchant, discrete CoSi2/Si hetero-nanocrystals or CoSi2 nanocrystals were left on the tunnel oxide. 15 nm oxide as control oxide was then deposited followed by aluminum electrodes formed by lift-off technique. The resultant devices are MOS structures with nanocrystals embedded. The capacitance-voltage measurements before and after the writing operation were performed to characterize the memory effect, as shown in Fig. 4 where writing was realized by bias the Al gate with 10 V for 1 s. It is found that the C-V shift due to the charge injection is almost the same for all three MOS devices with Si, CoSi2 nanocrystals and CoSi2/Si heteronanocrystals, respectively. The similar C-V shifts indicate the similar charge injection efficiency for these devices. This is further confirmed in Fig. 5 where the C-V shift, or memory window as a function of the writing time is plotted. One finds the similar charge injection speed for the three MOS memory devices. The retention performance was found to be significantly different for different devices, as given in Fig.6. In these first devices, the tunneling oxide quality is poor, leading to the fast decay within the first 100 seconds for all devices. Nevertheless, it is easily found that CoSi2 nanocrystal device possesses a much longer retention than that of the Si nanocrystal device. A 1.0 V flat-band voltage shift can be expected after 10 years for a CoSi2 nanocrystal memory device. It is also found that CoSi2/Si hetero-nanocrystal MOS memory exhibits the longest retention. A flat-band voltage shift of 1.5 V is still available after 10 years of retention at zero gate voltage. In summary, MOS memory with CoSi2/Si hetero-nanocrystal floating gate was proposed and investigated. Both theoretical simulation and practical device performance show that CoSi2/Si hetero-nanocrystal memory devices have much longer retention time while also with almost the same writing/erasing speed, as compared with its Si nanocrystal and metallic CoSi2 nanocrystal counterparts. References [1] S. Tiwari, et al. Appl. Phys. Lett., vol. 68, pp. 1377-1379, 1996. [2] J. P. Gambino, and E. G. Colgan, Mater. Chem. Phys. 52, 99 (1998)
Nonvolatile Si nanocrystal memory is fast approaching commercialization. In order to extend the s... more Nonvolatile Si nanocrystal memory is fast approaching commercialization. In order to extend the scaling limits, many kinds of nanocrystals or device architectures were used to replace Si nanocrystals. In this work, we report for the first time the experimental demonstration of a nonvolatile memory device using cobalt silicide nanocrystals as floating gates. The band diagram is shown in Fig. 1 (a). There are at least two advantage of using cobalt silicide nanocrystal over Si nanocrystal: first, cobalt silicide is a metallic material and its band edges for conduction and valence bands are all lower than those of the Si [1], which helps both electron and hole retentions. Second, cobalt silicide can be grown on Si nanocrystal plus a rapid thermal annealing (RTP) [2]. The self-aligned silicide remains while the unreacted metal cobalt is removed in selective etchant. The device fabrication begins with a thermal oxide growth at 1000 °C for 4.5 minutes on a 4-inch n-type Si substrate, which leads to a tunneling oxide thickness ofabout 3 nm. Si nanocrystals were formed in a LPCVD furnace followed by a deposition of ultra thin Co metal in an e-beam evaporator. RTP is then performed in nitrogen to form silicide. The unreacted cobalt metal was removed in a selective. The control oxide (40 nm) was grown in another LPCVD furnace. A MOS capacitor is finally obtained after depositing and patterning the aluminum electrodes on frond and back sides of the sample. An SEM image of the CoSi2 nanocrystals on SiO2 is shown in Fig.1 (b).
Threshold voltage shift of heteronanocrystal floating gate flash memory
Journal of Applied Physics, Jan 18, 2005
Simulations of threshold voltage shift of a p-channel Ge/Si heteronanocrystal floating gate memor... more Simulations of threshold voltage shift of a p-channel Ge/Si heteronanocrystal floating gate memory device were carried out using both a numerical two-dimensional Poisson–Boltzmann method and an equivalent circuit model. The results show that the presence of a Ge dot on top of a Si dot significantly prolongs the retention time of the device, indicated by the time decay behavior of the threshold voltage shift. Both methods lead to consistent results that an increase in the thickness of either the Si dot or Ge dot will result in a reduction of the threshold voltage shift. Additionally, the threshold voltage shift increases significantly as the heteronanocrystal density increases. Nevertheless, only a weak dependence of threshold voltage shift on the tunneling oxide thickness was found.
Transient processes of Ge/Si hetero-nanocrystal floating gate memories are simulated numerically.... more Transient processes of Ge/Si hetero-nanocrystal floating gate memories are simulated numerically. Compared with Si nanocrystal memories, Ge/Si hetero-nanocrystal memories show similar writing and erasing efficiency with a weaker writing saturation and markedly improved retention characteristics.
After the proposal of nanocrystal floating gate memory by Tiwari, tremendous effort has been made... more After the proposal of nanocrystal floating gate memory by Tiwari, tremendous effort has been made to improve the device performance of a memory containing nanocrystal floating gate, including semiconductor nanocrystals (Si, Ge et al.), metal dot (W, Ni, Au, Pt, Ag et al.), dielectric nanocrystals (Al2O3, HfO2 and Si4N3 et al.) and Ge/Si hetero-nanocrystals. In this work, we will report
Silicon-based optical waveguide polarizer using photonic band gap
Applied Physics Letters, Jul 2, 2002
Based on different photonic band structures of TE and TM polarization modes in periodic multilaye... more Based on different photonic band structures of TE and TM polarization modes in periodic multilayers, a method to realize the waveguide polarizer is proposed. The waveguide structure contains a SiO2 core layer sandwiched between two multilayers of alternately stacked poly-Si and SiO2, and the whole structure can be grown on a Si substrate. Its propagation characteristics are studied theoretically. High extinction ratio over 40 dB at a light wavelength of 1.3 μm is expected in the waveguide of only 40 μm long, accompanied with very low propagation loss of the passive TE mode. These characteristics are very suitable for the applications in integrated optics. The fabrication of this polarizer structure by using the magnetron sputtering method is demonstrated.
Optical properties of p-type ZnO doped by lithium and nitrogen
Solid State Communications, Mar 1, 2007
ABSTRACT A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by R... more ABSTRACT A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al2O3 as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a LiZn–N complex acceptor level (eFA), radiative recombination of a donor–acceptor pair and recombination of the LiZn–N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The LiZn–N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.
High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer la... more High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 mm were grown at different growth temperatures of 350, 450 and 550 1C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 mm at 550 1C. A ZnO buffer layer with a growth temperature of 450 1C (450 1C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 1C-buffer sample (6.7 nm), the 550 1C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 1C-buffer sample was the narrowest as 0.2091, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 1C. Consistent with these results, the 450 1C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm À1 was found in Raman scattering spectra for all films with buffers, which is corresponding to the E 2 mode.
A nanocrystal memory using CoSi 2 /Si hetero-nanocrystals as floating gate was proposed. Numerica... more A nanocrystal memory using CoSi 2 /Si hetero-nanocrystals as floating gate was proposed. Numerical investigations on the writing, erasing and retention were performed. The hetero-structure provides an extra quantum well for the charge to achieve much longer retention time while maintains a writing/erasing speed similar to that of Si nanocrystal memory.
Transient capacitances were numerically investigated for Ge/ Si heteronanocrystal memories. Flatb... more Transient capacitances were numerically investigated for Ge/ Si heteronanocrystal memories. Flatband voltage shifts ͑⌬V fb ͒ were obtained. The results suggest that the Ge/ Si heteronanocrystal memories have significantly longer data retention compared with the memories embedding Si nanocrystals only. It is also found that larger heteronanocrystal leads to longer retention, larger device capacitance, and smaller ⌬V fb .
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