Front-side Layer Research Articles

Thin Film Based Micro-Machined Sensor Development and Mitigation of Process Challenges

Realization of thin film based sensor employing bulk micro-machined process using wet chemistry is challenging due to lateral etching phenomena. This phenomenon of convex corner undercutting is associated with wet micromachining process where differential etching is required. The challenge of having differential etching to achieve varied depth in close vicinity imposes extra constraint on the process. The present requirement simultaneously etching of closely spaced tunnel and cavity from back side having varied depth keeping intact front side layers is the main bottleneck which was circumvented with modified process. Etching trials related combining wet and dry etching techniques for deeper etching are also carried out and critical aspects are presented in this article. Another aspect is to define process steps compatible with CMOS integration. Further study of piezo-electric layer and etch rate of various deposited layer are also presented. This article details process trials, challenges, mitigation techniques and realization aspects of the micro-machined based sensor.

Transparent silicon carbide/tunnel SiO2 passivation for c‐Si solar cell front side: Enabling Jsc > 42 mA/cm2 and iVoc of 742 mV

AbstractN‐type microcrystalline silicon carbide (μc‐SiC:H(n)) is a wide bandgap material that is very promising for the use on the front side of crystalline silicon (c‐Si) solar cells. It offers a high optical transparency and a suitable refractive index that reduces parasitic absorption and reflection losses, respectively. In this work, we investigate the potential of hot wire chemical vapor deposition (HWCVD)–grown μc‐SiC:H(n) for c‐Si solar cells with interdigitated back contacts (IBC). We demonstrate outstanding passivation quality of μc‐SiC:H(n) on tunnel oxide (SiO2)–passivated c‐Si with an implied open‐circuit voltage of 742 mV and a saturation current density of 3.6 fA/cm2. This excellent passivation quality is achieved directly after the HWCVD deposition of μc‐SiC:H(n) at 250°C heater temperature without any further treatments like recrystallization or hydrogenation. Additionally, we developed magnesium fluoride (MgF2)/silicon nitride (SiNx:H)/silicon carbide antireflection coatings that reduce optical losses on the front side to only 0.47 mA/cm2 with MgF2/SiNx:H/μc‐SiC:H(n) and 0.62 mA/cm2 with MgF2/μc‐SiC:H(n). Finally, calculations with Sentaurus TCAD simulation using MgF2/μc‐SiC:H(n)/SiO2/c‐Si as front side layer stack in an IBC solar cell reveal a short‐circuit current density of 42.2 mA/cm2, an open‐circuit voltage of 738 mV, a fill factor of 85.2% and a maximum power conversion efficiency of 26.6%.

Crystalline silicon solar cell with front and rear polysilicon passivated contacts as bottom cell for hybrid tandems

In this paper we analyze and model perovskite/c-Si tandem cells with front and rear polySi passivated contacts on the bottom cell. A high-efficiency tandem approach will benefit from the high Voc potential of a c-Si bottom cell with front and rear polySi passivated contacts while the combination with a high band gap, semi-transparent, perovskite top cell will largely diminish the UV-Vis parasitic absorption in a polySi front side layer on the c-Si cell. On the other hand since the Jsc is strongly reduced in a tandem bottom cell, free carrier absorption, to which both front and rear polySi layers contribute, will become a relatively more important loss mechanism. We investigate the trade-off between the optical gains and resistive losses from reducing the polySi thickness for cell configurations without transparent conductive oxide (TCO) and also consider the potential of the combination with TCOs. From our optical simulations we conclude that optical losses in the polySi layers of 100 nm and below are limited when applied on the bottom cell. Taking into account resistive losses in the polySi layers of varying thickness the optimal thickness is found to be 50 nm. In combination with the high Voc values resulting from the application of polySi passivating contacts this offers a promising route to establish a bottom cell with high efficiency. The combination of very thin polySi with highly transparent TCOs is likely to further improve bottom cell performance.

Using High Speed Phase Change Material for ${\rm DVD}+{\rm RW}$ 2.4x Dual Layer

The authors succeeded in developing an innovative high-speed phase change material applied for DVD+RW dual layer recording media. The high transmittance of the front-side recording layer combining the stacking material and thin film structure was found. This front-side recording layer is a key functional layer structure for the dual layer phase change optical disk. The same writing powers can be used for both recording layers and the same reflection of 7% is obtained. The transmittance for the front-side layer is 58.1% and 59.4% in amorphous and crystalline state, respectively. The jitter after 10 direct-overwriting cycles is 6.5% and 7% for the front-side and back-side recording layer, respectively

High-Transmittance Phase-Change Media with TiO2 Film for Rewritable Dual-Layer Blu-ray Discs

We have developed a semitransparent layer structure that is ideally suited to the laser-incident-side (front-side) recording layer of a rewritable dual-layer Blu-ray Disc (BD-RE). A TiO2 top film and a memory film of GeTe–Sb2Te3 characterize this layer structure. The TiO2 film has a large refractive index, which works to increase the transmittance of the entire recording layer to more than 50%. It simultaneously increases the reflectivity contrast, defined by (Rcry-Ramo)/(Rcry+Ramo), where Rcry and Ramo (Rcry>Ramo) are the reflectivities of the optical disc in the crystalline and amorphous parts, respectively. The GeTe–Sb2Te3 memory film has optical constants that are appropriate for balancing the transmittance between the crystalline part (Tcry) and the amorphous part (Tamo) in the laser wavelength region of approximately 405 nm. Our experimental dual-layer disc shows high transmittances of Tcry=51.6% and Tamo=52.0% and a high reflectivity contrast of 0.71 in the laser-incident-side layer. Dynamic examination showed that both recording layers have a good jitter value, satisfying the BD-RE specifications; more specifically, the laser-incident-side layer jitter value is 7.4% (spec.: 8.5%) and the back-side layer jitter value is 6.4% (spec.: 6.5%) through the front-side layer.

Area-Selective Formation of Macropore Array by Anisotropic Electrochemical Etching on an n-Si(100) Surface in Aqueous HF Solution

A photoassisted anodization process to fabricate arrays of uniform and straight macropores at selected areas of a Si wafer surface was developed. The front- and backside surfaces of n-type Si(100) wafers were coated with a thin Si(3)N(4) layer, and the frontside layer was micro-patterned using photolithography and reactive ion etching to form an array of microscopic openings at selected areas. The inverted pyramid-shape micropits were formed at these openings by anisotropic etching using aqueous KOH solution; these pits act as the initiation sites for the anodization to form macropores. The electrochemical etching was carried out in aqueous HF solution under illumination from the backside of the wafer, on which Au/Cr electric contact was formed following removal of the Si(3)N(4) layer. To improve the uniformity of the formation condition of the macropores at the selected area, holes were area-selectively generated by controlling the illumination condition during the anodization. For this, micropatterns were formed on the Au/Cr layer at the backside surface, which were aligned to those at the frontside surface. The parameters, such as HF concentration, current density, and wafer thickness, i.e., hole diffusion length, were optimized, and the arrays of uniform and high-aspect-ratio macropores were formed at the selected area of the domain at the silicon surface.

Quantum Yield of GaN and (Ga, Al)N Band-Gap Graded Ultraviolet p–n Detectors

Calculation of the quantum yield has been carried out for an AlxGa1—xN(n)–GaN(p) photodiode as an ultraviolet detector with energy band-gap graded (EBGG) AlxGa1—xN(n) layer. The analytical solution of the one-dimensional continuity equation was used in the calculation. The spatial dependence of the material properties, such as the absorption coefficient and the energy band-gap of the n-type layer of the photodiode is included in the solution. The band-gap grading due to a reduced absorption coefficient at the surface region (where recombination occurs) and the built in electric field results in an increase of the generated minority carrier collection efficiency. Within the range of small values of EBGG there is a substantial rise in the detector quantum yield with increasing grading. In the case of surface recombinationless AlxGa1—xN(n) front side layer, the further increase of the grading does not result a significant rise of quantum yield, especially in the spectral region near to the cutoff. Much more pronounced increase in quantum yield with a grow of the grading has neem found when the surface recombination was represent.