Selected problems in semiconductor physics and electronic devices

This book presents a set of solved examples on semiconductor device physics. Semiconductordevices is a core subject in electrical engineering and physics curricula. The level of the proposedexamples corresponds to a semester course at senior undergraduate or junior graduate level.Readers are expected to have a basic background on quantum and solid state physics, moreovera reasonable mathematical knowledge reaching differential equations is also assumed.2019/202

Origin of the negative differential resistance in the output characteristics of a picene-based thin-film transistor

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In this work, we have fabricated and studied p-type picene thin-film transistors. Although the devices exhibited good electrical performance with high field-effect mobility (up to 1.3 cm2/V¿s) and on/off ratios above 105, the output electric characteristics of the devices exhibited a Negative Differential Resistance for higher drain-source voltage. Finally, a possible explanation for this phenomenon is developed.Peer ReviewedPostprint (author's final draft

Origin of passivation in hole-selective transition metal oxides for crystalline silicon heterojunction solar cells

Transition metal oxides (TMOs) have recently demonstrated to be a good alternative to boron/phosphorous doped layers in crystalline silicon heterojunction solar cells. In this work, the interface between n-type c-Si (n-Si) and three thermally evaporated TMOs (MoO3, WO3, and V2O5) was investigated by transmission electron microscopy, secondary ion-mass, and x-ray photoelectron spectroscopy. For the oxides studied, surface passivation of n-Si was attributed to an ultra-thin (1.9–2.8 nm) SiOx~1.5 interlayer formed by chemical reaction, leaving oxygen-deficient species (MoO, WO2, and VO2) as by-products. Carrier selectivity was also inferred from the inversion layer induced on the n-Si surface, a result of Fermi level alignment between two materials with dissimilar electrochemical potentials (work function difference ¿¿ = 1 eV). Therefore, the hole-selective and passivating functionality of these TMOs, in addition to their ambient temperature processing, could prove an effective means to lower the cost and simplify solar cell processing.Postprint (author's final draft

Superior performance of V2O5 as hole selective contact over other transition metal oxides in silicon heterojunction solar cells

Transition metal oxides (TMOs) have recently been proved to efficiently serve as hole-selective contacts in crystalline silicon (c-Si) heterojunction solar cells. In the present work, two TMO/c-Si heterojunctions are explored using MoO3 (reference) and V2O5 as an alternative candidate. It has been found that V2O5 devices present larger (16% improvement) power conversion efficiency mainly due to their higher open-circuit voltage. While V2O5/c-Si devices with textured front surfaces exhibit larger short-circuit currents, it is also observed that flat solar cell architectures allow for passivation of the V2O5/n-Si interface, giving significant carrier lifetimes of 200 µs (equivalent to a surface recombination velocity of Seff ~140 cm s-1) as derived from impedance analysis. As a consequence, a significant open-circuit voltage of 662 mV is achieved. It is found that, at the TMO/c-Si contact, a TMO work function enhancement ¿FTMO occurs during the heterojunction formation with the consequent dipole layer enlargement ¿’=¿+¿FTMO. Our results provide new insights into the TMO/c-Si contact energetics, carrier transport across the interface and surface recombination allowing for further understanding of the nature of TMO/c-Si heterojunctions.Peer ReviewedPostprint (published version

Defect states assisted charge conduction in Au/MoO3¿x/n-Si Schottky barrier diode

Role of defect states of thermally evaporated molybdenum trioxide (MoO3-x) on electrical conductivity was investigated via low temperature current–voltage and capacitance–voltage measurements. To clarify the charge transport phenomena through MoO3-x, a 15 nm thin layer of MoO3-x film was used as an interface layer between gold and n-type Silicon (n-Si). The formation of an interface dipole between n-Si and MoO3-x exhibits a rectifying behaviour of Au/MoO3-x/n-Si Schottky barrier diode (SBDs). The rectifying nature of the SBDs shown up to 175 K due to proper electron extraction from valence band to conduction band via the defect states; however at =165 K the rectifying nature was not observed due to insulating behaviour of MoO3-x layer. Oxygen deficiency as a formation of defects was determined by x-ray photoelectron spectroscopy (XPS). Consequences of these defects as a function of current conduction across the MoO3-x was also confirmed by low temperature photoluminescence (PL)measurement.Peer ReviewedPostprint (author's final draft

A self-branched lamination of hierarchical patronite nanoarchitectures on carbon fiber cloth as novel electrode for ionic liquid electrolyte-based high energy density supercapacitors

This is the peer reviewed version of the following article: Ramu, M., Chellan, J. R., Goli, N., Joaquim, P., Cristobal, V., Kim, B. C., A Self‐Branched Lamination of Hierarchical Patronite Nanoarchitectures on Carbon Fiber Cloth as Novel Electrode for Ionic Liquid Electrolyte‐Based High Energy Density Supercapacitors. Adv. Funct. Mater. 2019, 1906586. https://doi.org/10.1002/adfm.201906586, which has been published in final form at https://doi.org/10.1002/adfm.201906586. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.The developments of rationally designed binder-free metal chalcogenides decorated flexible electrodes are of paramount importance for advanced energy storage devices. Herein, binder-free patronite (VS4) flower-like nanostructures are facilely fabricated on a carbon cloth (CC) using a facile hydrothermal method for high-performance supercapacitors. The growth density and morphology of VS4 nanostructures on CC are also controlled by varying the concentrations of vanadium and sulfur sources along with the complexing agent in the growth solution. The optimal electrode with an appropriate growth concentration (VS4-CC@VS-3) demonstrates a considerable pseudocapacitance performance in the ionic liquid (IL) electrolyte (1-ethyl-3-methylimidazolium trifluoromethanesulfonate), with a high operating potential of 2 V. Utilizing VS4-CC@VS-3 as both positive and negative electrodes, the IL-based symmetric supercapacitor is assembled, which demonstrates a high areal capacitance of 536 mF cm-2 (206 F g-1) and excellent cycling durability (93%) with superior energy and power densities of 74.4 µWh cm-2 (28.6 Wh kg-1) and 10154 µW cm-2 (9340 W kg-1), respectively. As for the high energy storage performance, the device stably energizes various portable electronic applications for a long time, which make the fabricated composite material open up news for the fabrication of fabrics supported binder-free chalcogenides for high-performance energy storage devices.Peer ReviewedPostprint (author's final draft

Analysis of temperature dependent current-voltage and capacitance-voltage characteristics of an Au/V2O5/ n -Si Schottky diode

Electronic properties of Au/V2O5/n-Si Schottky device have been investigated by temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements ranging from 300 K to 150 K. Ideality factor (n) and barrier height (ø) for the Schottky device were obtained from I-V characteristics as 2.04 and 0.83 eV at 300 K and 6.95 and 0.39 eV at 150 K respectively. It was observed that in presence of inhomogeneity at metal-semiconductor interface, the ideality factor increases and barrier height decreases with the decrease of temperature. The Richardson constant value was estimated as 137 A-cm-2-K-2 from modified Richardson plot, which is closer to the known theoretical value of n-Si where mean value of barrier height (øb0), and its standard deviation (s0) were estimated using double Gaussian distribution (DGD) analysis. Different device parameters, namely, built-in potential, carrier concentration, image force lowering and depletion width were also obtained from the C-V-T measurements. First time use of V2O5 thin-film as an interfacial layer (IL) on Au/V2O5/n-Si Schottky diode was successfully explained by the thermionic emission (TE) theory. The interesting result obtained in this present work is the V2O5 thin-film reduced its conducting capability with decreasing temperature, while it shows a totally insulating behaviour below 150 K.Peer ReviewedPostprint (author's final draft

Thin silicon films ranging from amorphous to nanocrystalline obtained by Hot-Wire CVD

In this paper, we have presented results on silicon thin films deposited by hot-wire CVD at low substrate temperatures (200 °C). Films ranging from amorphous to nanocrystalline were obtained by varying the filament temperature from 1500 to 1800 °C. A crystalline fraction of 50% was obtained for the sample deposited at 1700 °C. The results obtained seemed to indicate that atomic hydrogen plays a leading role in the obtaining of nanocrystalline silicon. The optoelectronic properties of the amorphous material obtained in these conditions are slightly poorer than the ones observed in device-grade films grown by plasma-enhanced CVD due to a higher hydrogen incorporation (13%)

Optimization of laser processes in n+Emitter formation for c-Si solar cells

Punctual phosphorus diffused emitters were achieved by laser patterning phosphorus doped a-SiCx:H films deposited by PECVD as a doping source. Two different lasers at wavelengths of 1064 nm and 532 nm were used. Phosphorus diffusion was confirmed by Secondary Ion Mass Spectroscopy. We explored the effect of pulse energy and number of pulses per diffused point. The results show that a fine tune of the energy pulse is critical while the number of pulses has minor effects. Scanning Electron Microscopy (SEM) pictures and optical profilometry showed a laser affected area where the c-Si is melted, ejected and solidified quickly again. Typically, the diameter of the affected area for 1064 nm laser is between two and four times greater than for 532 nm laser. Optimum parameters for both lasers were determined to obtain best J-V curves nearly to ideal diode behavior. Comparing best J-V results, lower emitter saturation current density (Jo) and contact resistance are obtained with 532 nm laser. The improvement in Jo can be related mainly to the smaller affected areas observed by SEM while lower contact resistance can be attributed to that 532 nm laser has a more superficial action resulting in higher phosphorus concentration at the surface. The expected open voltage circuit for finished solar cells using these emitters is in the range of 640 mV for 532 nm laser and 620 mV for 1064 nm one.Postprint (published version

N-type emitters passivation through antireflective phosphorus doped a-SiCxNy:H(n) stacks

This paper studies the passivation of industrially textured deep silicon emitters using amorphous silicon carbonitride layers in stack configuration, deposited by plasma enhanced chemical vapor deposition. With this technique, emitter saturation current density can be decreased to values around 250 fA middot cm-2. As a consequence, open circuit voltages can be increased 25 mV achieving values around 640 mV.Postprint (published version