C.R. Wronski

Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultraviolet: Applications in thin film photovoltaics

We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (e1,e2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (e1,e2) can be simulated assuming a single ...

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H2 to SiH4 that can be sustained while avoiding the a→(a+μc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition.

OPTIMIZATION OF HYDROGENATED AMORPHOUS SILICON P-I-N SOLAR CELLS WITH TWO-STEP I LAYERS GUIDED BY REAL-TIME SPECTROSCOPIC ELLIPSOMETRY

Hydrogenated amorphous silicon (a-Si:H) p–i–n solar cell performance has been optimized using a two-step i-layer growth process. This effort has been guided by real-time spectroscopic ellipsometry (RTSE) studies of the nucleation and growth of a-Si:H films by plasma-enhanced chemical vapor deposition at 200 °C using a variable H2-dilution gas flow ratio R=[H2]/[SiH4]. RTSE studies during film growth with R>15 reveal a transition from the amorphous to microcrystalline (a→μc) phase at a critical thickness that decreases with increasing R. From such results, the optimum two-step process was designed such that the initial stage of the i layer (∼200 A) is deposited at much higher R than the bulk to ensure that the film remains within the amorphous side of the a→μc phase boundary, yet as close as possible to this boundary at low i-layer thicknesses.

The effect of surface treatment on the electrical properties of metal contacts to boron-doped homoepitaxial diamond film

Both doped and undoped homoepitaxial diamond films were fabricated using microwave plasma-enhanced chemical vapor deposition (CVD). The conductivity of the diamond film is strongly affected by the surface treatment. In particular, exposure of film surface to a hydrogen plasma results in the formation of a conductive layer which can be used to obtain linear (ohmic) I-V characteristics of the Au/diamond contacts, regardless of the doping level. It is shown how the proper chemical cleaning of the boron-doped homoepitaxial diamond surface allows the fabrication of Au-gate Schottky diodes with excellent rectifying characteristics at temperatures of at least 400 degrees C.<<ETX>>

Evolutionary phase diagrams for plasma-enhanced chemical vapor deposition of silicon thin films from hydrogen-diluted silane

Real-time optical studies have been applied to develop phase diagrams that characterize plasma-enhanced chemical vapor deposition (PECVD) of silicon thin films at low temperature (200 °C). The deposition phase diagrams describe regimes over which predominantly amorphous and microcrystalline Si phases are obtained as a function of the accumulated thickness and the hydrogen-to-silane gas flow ratio R=[H2]/[SiH4] in the PECVD process. The diagrams for different substrates provide insights into optimization of amorphous Si materials and solar cells.

Electrical characteristics of Schottky diodes fabricated using plasma assisted chemical vapor deposited diamond films

Schottky diodes were fabricated using gold and aluminum contacts to thin diamond films obtained by a microwave plasma assisted chemical vapor deposition process. The current‐voltage and capacitance‐voltage‐frequency characteristics of these devices are similar to those fabricated on a crystalline diamond base formed by traditional ultrahigh pressure process.

Enhanced conversion efficiencies for pillar array solar cells fabricated from crystalline silicon with short minority carrier diffusion lengths

Radial n+–p+ junction solar cells composed of densely packed pillar arrays, 25-μm-tall and 7.5 μm in diameter, fabricated from p-type silicon substrates with extremely short minority carrier diffusion lengths are investigated and compared to planar cells. To understand the two times higher AM 1.5 efficiencies of the pillar array cells, dark and light I-V characteristics as well as spectral responses are presented for the two structures. The higher pillar array cell efficiencies are due to the larger short-circuit currents from the larger photon absorption thickness and the shorter carrier collection length, with a significant additional contribution from multiple reflections in the structure.

Correlation of real time spectroellipsometry and atomic force microscopy measurements of surface roughness on amorphous semiconductor thin films

We have correlated the results of real time spectroellipsometry (SE) and ex situ atomic force microscopy (AFM) measurements of surface roughness on amorphous semiconductor thin films. Roughness layer thicknesses deduced from real time SE, using a conventional approach based on the Bruggeman effective medium theory, closely obey a relationship of the form: ds(SE)≊1.5 drms(AFM)+4 A, for 10≤dS(SE)<100 A, where drms(AFM) is the root‐mean‐square roughness from AFM. The slope and intercept of this relationship provide insights into the origin and interpretation of the optically deduced roughness layer thicknesses.

The barrier height of Schottky diodes with a chemical-vapor-deposited diamond base

A barrier height of 1.13±0.03 eV was measured for Al and Au rectifying contacts to p‐type chemical‐vapor‐deposited diamond thin films using the internal photoemission technique. The results are compared with experimental data reported for Schottky barriers on single‐crystal diamond.

Optimization of open circuit voltage in amorphous silicon solar cells with mixed-phase "amorphous+nanocrystalline… p-type contacts of low nanocrystalline content

Both the origins of the high open circuit voltages VOC in amorphous silicon solar cells having p layers prepared with very high hydrogen dilution and the physical structure of these optimum p layers remain poorly understood topics, with several studies offering conflicting views. This work attempts to overcome the limitations of previous studies by combining insights available from electronic measurements, real time spectroscopic ellipsometry, atomic force microscopy, and both high-resolution transmission electron microscopy TEM and dark field TEM of cross sections of entire solar cells. It is found that solar cells fabricated with p layers having a low volume fraction of nanocrystals embedded in a protocrystalline Si:H matrix possess lower recombination at the i/p interface than standard cells and deliver a higher VOC. The growth of the p layers follows a thickness evolution in which pure protocrystalline character is observed at the interface to the i layer. However, a low density of nanocrystallites nucleates with increasing thickness. The advantages offered by the protocrystalline character associated with the amorphous phase of the mixed-phase amorphous+nanocrystalline p layers prepared with excess H2 dilution account for the improved VOC of the optimum p layers. In this model, the appearance of a low volume fraction of nanocrystals near the top transparent conductor interface is proposed to be incidental to the high VOC .© 2007 American Institute of Physics. DOI: 10.1063/1.2714507