Papers by Keyword: Surface Passivation

Abstract: Amorphous/crystalline silicon heterojunction solar cells are commonly made by low temperature deposition of front and back side thin films on bare H-passivated Si wafers, obtained by HF last processes. This work discusses the impact of HF last step parameters on cell performance, considering textured and cleaned Si (100) wafers. A complete native oxide removal is mandatory and achieved in a short time (< 5 min) by HF concentration higher than 1% (by weight). Above 1%, surface passivation and cells performance slightly increases with the concentration. The best process time is found to be the minimum time to deoxidize textured wafers, as seen by a good dewetting. For [H > 2% this is less than 1 min. Longer process times slightly degrade surface passivation. Post rinse and drying, provided they do not reoxydize the surface, were seen to have no impact. The delay between the HF last and deposition steps is critical and depends on the efficiency of the cleaning before the HF last. With a high performance cleaning, leading to a very good surface passivation (< 10 cm/s surface recombination velocity), 30 min delay has no impact and 90 min leads to about 5% relative degradation of cell performance. Regarding the HF cleanliness, HCl spiking is an efficient way to enhance robustness of surface passivation keeping < 10 cm/s values when the metallic contamination, including Cu, is in the sub 50 ppb range.

Abstract: Evolution of interdiffused Gaussian-shape nanolayer of Au-Si, formed due to diffusion of Au into Si(111) substrate at ambient conditions, depends strongly on the Si surface pretreatment/passivation conditions. Negligible diffusion in the Au-OSi(111) sample, confirms the strong barrier action of the oxide-layer against diffusion, while large diffusion in the Au-HSi(111) sample compared to that in the Au-BrSi(111) sample suggests that the H-passivated Si(111) surface is more stable. This nature of the Au-Si(111) system is qualitatively similar to that of the Au-Si(001) system but it differs quantitatively. The size, electronegativity and bond-energy of the passivating elements and the number of dangling bonds on the Si surface influence the instability of the Si surface. This instability, parameterized by growth-time of oxide layer alone, can be utilized to tune the amount of diffusion into the sub-surface Si region. The distribution of growth-time and fractional passivated area, which are related to the improper Si surface passivation, are against such control and needs perfection.

Abstract: The set of quantum confinement levels in SiGe quantum wells (QW) was observed in the temperature range from 80 to 300 K by means of charge deep-level transient spectroscopy (Q-DLTS) and transport measurements. These observations proved possible due to a passivation of structure surface with organic monolayer deposition. Si/SiGe/Si structures with different Ge contents in SiGe layer were studied. The confined levels in passivated structures became apparent through DLTS measurements as various activation energies in temperature dependence of the rate of carrier emission from QW. It was found that the recharging of SiGe QWs and carrier emission accomplish due to thermo-stimulated tunneling. The steps in the current-voltage characteristics originated from direct tunneling via the confined states were found to determine the current flow at high fields.

Abstract: Direct metal thermocompression bonding is one of the key approaches used in creating interconnections in many heterogeneous devices. It has been reported that by coating a monolayer of alkanethiols on metallic surfaces such as gold or copper prior to bonding, the bonding temperature required for forming joints can be significantly reduced. In this paper, room temperature copper bonding is demonstrated successfully with the help of the organic monolayers. We also found that all alkanethiol- coated copper (CnH2n+1SH, at n = 6, 11, 18) exhibited superior bond strength (>25MPa) compared to that of the uncoated copper (<23MPa) at bonding temperatures from 25°C to 80°C. Further investigation shows that bond strength of copper joints increases with alkanethiol chain length (C18>C11>C6), which contradicts our previous finding in gold. We attribute this discrepancy to the difference in hardness between the two substrates.

Abstract: Surface passivation of 4H-SiC has been investigated for high current-gain bipolar junction transistors (BJTs). For the characterization of surface passivation, we have introduced the product “sp•Ls” of a surface recombination velocity (sp) and a surface diffusion length (Ls). The sp•Ls value was obtained by analyzing the I-V characteristics of pn diodes. Both BJTs and pn diodes were fabricated with several passivation methods. We have found clear correlation between the sp•Ls value and the current gain of the fabricated BJTs. Optimizing the surface passivation, we realized high performance BJTs with a current gain of 107 and a blocking voltage VCEO of 950 V.

Abstract: Phosphorous implanted n+/p diodes have been included in the masks for manufacturing n-MOSFET devices and processed in the same way of source/drain regions. The diode junctions were made by a P+ implantation at 300°C and a post implantation annealing at 1300°C. The diode emitter area was protected by 0.6 m thick CVD oxide during the processing of the MOSFET gate oxide. Three gate oxide processes were taken into account: two of them include a N implantation before a wet oxidation, while the third one was a standard oxidation. Considering the effect on the n+/p diodes, the main difference among the processes were the wet thermal oxidation time that ranged between 180 and 480 min at a temperature of 1100°C. The diode current-voltage characteristics show similar forward but different reverse curves in the temperature range of 25-290°C. Differences in reverse bias voltage as a function of the measurement temperature have been analyzed and are related to the different gate oxidation time. A correlation between the shortest oxidation time and the lower leakage current is presented.

Abstract: III-V compound semiconductors have been recognized among the potential options for continuing transistor power-performance scaling owing to their ultra high charge carrier mobility. In order to realize their potential in high performance and lower-power digital logic applications, there must be strong gate control and a high Ion-Ioff ratio, achieved by integrating a stable, ultra thin high-K dielectric between the semiconductor and the gate [1, 2]. Unlike Si, which has long benefited from its very stable native oxide, III-V materials suffer from their poor native oxides that cause charge traps and Fermi level pinning at the semiconductor-oxide interface. Attempts to deposit high-K directly on III-V often produce MIS structures with fast surface state and CV instability [3].

Abstract: We report a density functional theory study of N and NO passivation of surface dangling bonds at the Si-face of (0001) 4H-SiC. Results agree with many key experimental findings in nitrogen processed devices including: observed interface N coverage and characteristic changes in Dit. Dangling bonds at the SiC surface are fully passivated by 1/3 ML N or NO coverage. Upon passivation the surface is found to incur negligible strain and no reconstruction. Allowing atomic O to interact with the 1/3 ML N or NO passivated surface, we find oxygen prefers to incorporate into SiO2 rather than adsorb to the surface. This indicates the possibility of oxide deposition onto nitrogen passivated (0001) 4H-SiC surfaces.

Abstract: Low-temperature plasma-enhanced chemical vapor deposition of amorphous carbon (a-C:H) films was investigated for surface passivation of carbon-doped silicon oxide (SiOCH) films. The a-C:H films were deposited using CH4 and Ar gases at 40–65°C. FT-IR results showed that the deposited films are a-C:H which incorporates hydrocarbon groups. In current−voltage measurements, the a-C:H showed a low leakage current of ~10–10 A/cm2 in air, indicating that the a-C:H films have a potential as a surface passivation layer to prevent moisture absorption in air. The insulating properties of room-temperature deposited SiOCH covered by the a-C:H strongly depended on radio frequency (RF) power in the SiOCH deposition. In the SiOCH film deposited at high RF power of 200 W, the resistivity in air was improved by the a-C:H passivation.