Niels Posthuma

Emitter Formation and Contact Realization by Diffusion for Germanium Photovoltaic Devices

Standalone germanium solar cells are under development for application in high-efficiency mechanically stacked multijunction solar cells and thermophotovoltaic systems. To realize a suitable device, a more fundamental research has been done on germanium doping, surface passivation, and contact formation. In this paper, emitter formation and contact realization are discussed in detail. Emitter formation is done by phosphorous diffusion from a spin-on dopant (SOD) source. Critical parameters are the diffusion time, diffusion temperature, and phosphorous content in the SOD. Front contact formation is done by an innovative method, where the contacting metal is diffused through the amorphous silicon passivation layer. The specific properties of the diffusing metal, diffusion temperature, and diffusion time are important. Using the developed process, a stand-alone germanium solar cell has been realized with a world-class AM1.5G energy conversion efficiency of 7.8%

Forward Bias Gate Breakdown Mechanism in Enhancement-Mode p-GaN Gate AlGaN/GaN High-Electron Mobility Transistors

In this letter, we studied the forward bias gate breakdown mechanism on enhancement-mode p-GaN gate AlGaN/GaN high-electron mobility transistors. To the best of our knowledge, it is the first time that the temperature dependence of the forward gate breakdown has been characterized. We report for the first time on the observation of a positive temperature dependence, i.e., a higher temperature leads to a higher gate breakdown voltage. Such unexpected behavior is explained by avalanche breakdown mechanism: at a high positive gate bias, electron/hole pairs are generated in the depletion region at the Schottky metal/p-GaN junction. Furthermore, at a high gate bias but before the catastrophic gate breakdown, a light emission was detected by a emission microscopy measurement. This effect indicates an avalanche luminescence, which is mainly due to the recombination of the generated electron/hole pairs.

Screen-Printed Aluminum-Alloyed

We demonstrate the use of industrial-orientated screen-printed aluminum-alloyed emitter for high-efficiency n-type interdigitated back-contact silicon solar cells. Different cell designs with various pitch sizes and emitter fractions have been studied. With an improved front surface field (FSF), short-circuit current densities up to 40 can be obtained. By combining the best cell design and the improved FSF, a high conversion efficiency of 19.1% with Czochralski n-type material has been achieved.

\hbox{P}^{+}

A homogeneous emitter designed for a high efficiency Si solar cell needs to fulfill specific requirements: (1) a low surface concentration of dopants and (2) a good contact resistance. The emitter formation is studied in detail for an optimized POCl3 diffusion scheme and for plasma doping.

Emitter on High-Efficiency N-Type Interdigitated Back-Contact Silicon Solar Cells

We demonstrate a straightforward and economical way to obtain smooth germanium layers of high quality on silicon. Thin amorphous germanium layers deposited by plasma enhanced chemical vapor deposition on Si(111) substrates are transformed into single crystalline and smooth layers by solid phase epitaxy in N2 atmosphere. The crystal orientation of the substrate has a clear influence on the crystal quality. This is most likely due to a different growth mode, namely, layer-by-layer for Si(111) and three-dimensional growth for Si(001). The amorphous germanium layer can roughen during annealing due to mobile atoms on the surface. This can be effectively suppressed by annealing in N2 ambient. Electrical measurements show high charge mobility.

Advanced phosphorous emitters for high efficiency Si solar cells

Low frequency direct plasma-enhanced chemical vapor deposited Si–SiNx interface properties with and without NH3 plasma pretreatment, with and without rapid thermal annealing (RTA) have been investigated with deep-level transient spectroscopy (DLTS) on both n- and p-type monocrystalline silicon samples. It is shown that four different defect states are identified at the Si–SiNx interface. Energy-dependent electron and hole capture cross sections were also measured by small-pulse DLTS. Samples with plasma NH3 pretreatment and RTA show the lowest DLTS signals, which suggest the lowest overall interface states density. Moreover, SiNx with RTA passivates interface states more efficiently in n-type Si compared with p-type Si; also the deep-level parameters change in n-type Si but not in p-type Si. The combination of plasma NH3 pretreatment and RTA is suggested for application in the solar cell fabrication.

Solid phase epitaxy of amorphous Ge on Si in N2 atmosphere

In order to relax the mechanical constraints of processing thin crystalline Si wafers into highly efficient solar cells, we propose a process sequence, where a significant part of the process is done on module level. The device structure is an interdigitated-back-contact cell with an amorphous silicon back surface field. The record cell reaches an independently confirmed efficiency of 18.4%. Although the device deserves further optimization, the result shows the compatibility of processing on glass with efficiencies exceeding 18%, which opens the door to a high-efficiency solar cell process where the potentially thin wafer is attached to a foreign carrier during the full processing sequence.

A deep-level transient spectroscopy study of silicon interface states using different silicon nitride surface passivation schemes

Deep Level Transient Spectroscopy (DLTS) has been applied to Metal-Oxide-Semiconductor (MOS) capacitors fabricated on crystalline silicon n- and p-type substrates, with a SiO 2 or a SiO 2 /SiN x passivation stack, covered by an Al gate. It is shown that similar interface state distributions are obtained in both cases, from which it is concluded that the SiN x deposition does not degrade the interface. It is also shown that a rather large density of dangling bond defects is present at the Si/SiO 2 interface, which is related to the absence of a post metallization forming gas annealing.

18% Efficiency IBC Cell With Rear-Surface Processed on Quartz

Stand‐alone germanium solar cells, intended for application as bottom cell in mechanically stacked solar cells, have been realized applying an innovative process where the front contact is diffused through the a‐Si passivation layer to establish the contact. The developed germanium solar cell process has resulted in world‐class energy conversion efficiencies above 8 percent (AM1.5G). To optimize germanium cells for use in TPV applications, where an increased cell response at high wavelengths is desirable, an innovative back‐side contacting mechanism has been developed. In order to increase the back surface reflection properties of the cell, laser fired contacts have been optimized and used. Using this contact, a germanium thermophotovoltaic cell with an AM1.5 energy conversion efficiency of 6.3 percent has been realized, showing an improved response at high wavelengths compared to the classical germanium solar cell. Cost estimations show that, by integrating this type of germanium photocells in a TPV syst...

A DLTS Study of SiO2 and SiO2/SiNx Surface Passivation of Silicon

In this letter, we report a detailed experimental investigation of the time-dependent breakdown induced by forward gate stress in GaN-based power HEMTs with a p-type gate, controlled by a Schottky metal/p-GaN junction. When a high stress voltage is applied on the gate, a large voltage drop and an electric field occur in the depletion region of the p-GaN close to the metal interface, promoting the formation of a percolation path. We have investigated the mechanisms underlying the gate breakdown by adopting different stress conditions, analyzing the influence of the temperature, and investigating the activation energy of the traps. In addition, thanks to this approach, the device lifetime has been evaluated and an original empirical model, representing the relationship between the gate leakage current and the time to failure, has been proposed.