W. Düngen

Trapping of hydrogen in argon-implanted crystalline silicon

Crystalline silicon wafers are implanted with argon ions and subsequently hydrogenated by H-plasma treatments (Sample A). The control silicon samples are solely treated by the H-plasma under identical conditions (Sample B). Depth-resolved μ-Raman spectroscopy and cross-sectional transmission electron microscopy are carried out on both samples. In Sample A, two kinds of hydrogen molecules, namely H2(I) located in vacancies, and H2(II) located in platelets are observed. It is found that the depth profile of the H2(I) molecules is consistent with that of the vacancies created by the Ar implantation, but deeper than that of H2(II) molecules (∼0.4 versus ∼0.3μm). In Sample B, only the H2(II) molecules are observed. Its depth distribution extends much deeper than in Sample A (∼1 versus ∼0.3μm). These results indicate that the vacancies created during Ar implantation act as trap centers to block the hydrogen diffusion into deeper wafer regions during the subsequent H-plasma treatments.

Blistering of implanted crystalline silicon by plasma hydrogenation investigated by Raman scattering spectroscopy

Czochralski silicon wafers were implanted with H+ ions at a dose of 1×1016cm−2 followed by hydrogen plasma treatments at different temperatures. The minimum hydrogen implantation dose required for silicon surface exfoliation of 3×1016H+∕cm2 without further hydrogen incorporation was reduced to one-third by subsequent plasma hydrogenation. The corresponding local vibrational modes of hydrogen molecules, vacancy-hydrogen complexes, and Si–H bonds on surfaces have been analyzed by micro-Raman scattering spectroscopy to investigate blistering and platelet formation. The surface profile has been studied by atomic force microscopy and scanning electron microscopy. The plasma treated samples were annealed to investigate the mechanism and applicability of the induced exfoliation. ⟨111⟩-platelet formation occurred below plasma hydrogenation temperatures of 350°C. At temperatures above 450°C, ⟨100⟩-platelet nucleation induced blistering.

µ-Raman Investigations on Hydrogen Gettering in Hydrogen Implanted and Hydrogen Plasma Treated Czochralski Silicon

µ-Raman measurements were carried out on hydrogen implanted, plasma hydrogenated and subsequently annealed Cz Silicon samples, respectively. In comparison to as-implanted or asplasma treated samples, in consideration of the thermal evolution, the effects of the implanted and subsequently plasma treated samples were analyzed. An enhanced trapping of molecular hydrogen in multivacancies has been observed after hydrogen implantation and subsequent plasma hydrogenation. In comparison to as-implanted samples, the intensity of the local vibrational modes (LVM) of vacancy-hydrogen complexes and silicon-hydrogen bonds are increasing.

DLTS study on deep levels formed in plasma hydrogenated and subsequently annealed silicon

P-n junctions are created in p-type Czochralski silicon after a low temperature (270°C) hydrogen plasma exposure. This is attributed to the formation of hydrogen-related shallow donors. A deep level (E1) with an activation energy of about EC-0.12 eV is observed by DLTS measurement and assigned to a metastable state of the hydrogen-related shallow donors. At an annealing temperature of 340°C, the E1 centres disappear and oxygen thermal donors appear. The concentrations of the oxygen thermal donors are found typically to be 2-3 decades lower than that required for over-compensating the initial p-type doping and for contributing the excess free carriers.

On the Formation Kinetics of Thin Nanopatterned Layers on Silicon Wafers Created by Hydrogen Plasma Exposure

Czochralski silicon wafers which are treated by hydrogen plasma at ca. 260 °C are structured at the surface due to the high reactivity of atomic hydrogen. “Nanopatterned” (np) Si layers with average structure diameters below 100 nm are created. The thickness of the np-Si layer is on the order of 100 nm. The morphology of np-Si layers depend on the applied plasma power and the exposure time. The formation of np-Si layers is discussed in the frame of a combined etching/ redeposition mechanism. Annealing at T ≥ 800 °C causes a reconstruction of np-Si layers and the appearance of tensile stress in the wafers up to a depth of several micrometers.

Hydrogenated Amorphous Carbon-Silicon Alloys [a-Si x C 1-x (n):H y ] used as Emitters of Heterojunction Solar Cells

This work focuses on hydrogenated amorphous carbon-silicon alloys [a-SixC1-x(n):Hy] used as emitters of heterojunction solar cells. The a-SixC1-x(n):Hy alloys are deposited on p-type silicon (100) substrates via plasma enhanced chemical vapor deposition (PECVD) using a gas mixture of silane (SiH4), phosphine (PH3), methane and hydrogen (CH4), respectively. For characterization of the heterojunction solar cells, produced as mentioned above, I-V measurements, spectral response and reflection measurements as well as spectroscopic ellipsometry (SE) have been applied. Particularly, we focused on several effects, such as widening of the optical band gap of the emitter within the a-Six C1-x(n):Hy film, the thickness of the emitter and the I-V characteristics of the solar cell. It was confirmed that the band gap EG can be tailored by using an appropriate gas mixture during the decomposition. The optical band gap as well as the thickness of the emitter were determined by spectroscopic ellipsometry. The investigations have shown that the band gap increases with increasing carbon and hydrogen concentration in the feed stock during deposition. Although the addition of carbon (C) degrades the photoelectronic properties in the emitter layer, this deterioration can be minimized by H dilution. We will show that the efficiency of the prepared heterojunction solar cells can be improved with an appropriate addition of carbon and hydrogen in the emitter layer

Evolution of Hydrogen Related Defects in Plasma Hydrogenated Crystalline Silicon under Thermal and Laser Annealing

Boron doped [100]-oriented Cz Si wafers are hydrogenated with a plasma enhanced chemical vapor deposition setup at a substrate temperature of about 260 °C. In-situ Raman spectroscopy is applied on samples under thermal and laser annealing. It is found that different Si-H species have different stabilities. The most stable one is the Si-H bond at the inner surfaces of the platelets. The dissociated energy of Si-H bonds is deduced based on the first order kinetics. It is found that the hydrogen atoms which are released during annealing are trapped again by the platelets and passivate the silicon dangling bonds at the inner surfaces of the platelets or form H2 molecules in the open platelet volume, possibly relating to the basic mechanism of the hydrogen-induced exfoliation of the silicon wafer and the socalled “smart-cut” process.

P-N Junction Diodes Fabricated Based on Donor Formation in Plasma Hydrogenated P-Type Czochralski Silicon

A rather large amount of shallow donors is created in p-type Czochralski silicon (Cz Si) wafers after a hydrogen plasma exposure at ∼270 °C (substrate temperature) and a subsequent annealing in the temperature range of 350-450 °C. This two-step process has been used for the fabrication p-n junction diodes at low temperatures. Current-voltage characteristics show that the breakdown voltages of these diodes are higher than 100 V. The diode leakage is found to be improved after slow ramp annealing at temperatures up to 250 °C. Deep level transient spectroscopy measurements reveal that the oxygen related thermal donor is not the dominant doping species as expected before.

Void Formation in Hydrogen Implanted and Subsequently Plasma Hydrogenated and Annealed Czochralski Silicon

By μ-Raman spectroscopy the formation of hydrogen related defects (vacancy-hydrogencomplexes, hydrogen saturated silicon dangling bonds, H 2 molecules in multi-vacancies andvoids/platelets) has been investigated in H-implanted and subsequently H-plasma exposed andannealed Czochralski (Cz) silicon wafers. Annealing was done either in air or in an ambientcontaining hydrogen (forming gas). The investigations were applied under conditions, which arerelevant for ion-cut processes and layer exfoliation in Cz Si for SOI-wafer fabrication at reducedimplantation doses (as compared to standard procedures like the smart-cut® process).