Hamed Simchi | Pennsylvania State University (original) (raw)

Papers by Hamed Simchi

Cu(In,Ga)Se2 thin film solar cells have attracted a lot of interest because they have shown the h... more Cu(In,Ga)Se2 thin film solar cells have attracted a lot of interest because they have shown the highest achieved efficiency (21%) among thin film photovoltaic materials, long-term stability, and straightforward optical bandgap engineering by changing relative amounts of present elements in the alloy. Still, there are several opportunities to further improve the performance of the Cu(In,Ga)Se2 devices. The interfaces between layers significantly affect the device performance, and knowledge of their chemical and electronic structures is essential in identifying performance limiting factors. The main goal of this research is to understand the characteristics of the Cu(In,Ga)Se2-back contact interface in order to design ohmic back contacts for Cu(In,Ga)Se2-based solar cells with a range of band gaps and device configurations. The focus is on developing either an opaque or transparent ohmic back contact via surface modification or introduction of buffer layers in the back surface. In thi...

Journal of Applied Physics

Low-resistance and thermally stable Ohmic contacts are essential for radio frequency switches bas... more Low-resistance and thermally stable Ohmic contacts are essential for radio frequency switches based on the unique phase change properties of GeTe. Herein, Mo-based Ohmic contacts to p-type GeTe are reported, including the effect of pre-metallization surface preparation and annealing on Mo/Ti/Pt/Au contacts. In-situ Ar + plasma treatment resulted in a very low contact resistance of 0.004 ± 0.002 Ω.mm (5 ± 3 × 10-9 Ω.cm 2), which could not be achieved using ex-situ surface treatments, highlighting the need for oxide-free interfaces to obtain very low contact resistance using Mo-based contacts. Experiments aimed at creating a more Ge-or Te-rich interface yielded higher contacts resistances in both cases. The contact resistance increased for short term annealing (30 min) above 200 o C and for long term annealing (1 week) at 200 o C. No solid-state reaction between Mo and GeTe was observed using transmission electron microscopy with energy dispersive spectroscopy. However, Te migrated from the GeTe after annealing at 200 o C for a week, resulting in the formation of platinum telluride within the contact.

Crystal Growth & Design

Tungsten disulfide (WS2) films were grown on c-plane sapphire in a cold-wall gas-source chemical ... more Tungsten disulfide (WS2) films were grown on c-plane sapphire in a cold-wall gas-source chemical vapor deposition system to ascertain the effect of the chalcogen precursor on the film growth and properties. Tungsten hexacarbonyl (W(CO)6) was used as the tungsten source, and hydrogen sulfide (H2S) and diethyl sulfide (DES-(C2H5)2S) were the chalcogen sources. The film deposition was studied at different temperatures and chalcogen-to-metal ratios to understand the effect of each chalcogen precursor on the film growth rate, thickness, coverage, photoluminescence, and stoichiometry. Larger lateral growth was observed in films grown with H2S than DES. The reduced lateral growth with DES can be attributed to carbon contamination, which also quenches the photoluminescence. Thermodynamic calculations agreed well with the experimental observations, suggesting formation of WS2 with both sulfur precursors and additional formation of carbon when deposition is done using DES.

Inorganic Chemistry

Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO3 targets an... more Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO3 targets and evaluated for their suitability as catalysts for the oxygen evolution reaction (OER). Scanning electron microscopy (SEM) revealed a compact morphology for the as-deposited films and the formation of nanorodlike features on the surfaces after annealing at 600 °C. X-ray-photoelectron-spectroscopy analysis showed that the surface oxidation state is close to +III (as in Mn2O3) for the as-deposited films and increases slightly to a mixture of III and IV after annealing occurs in dry air at 400-600 °C. Glancing-incidence X-ray diffraction (GIXRD) suggested that the CaMnxOy films are amorphous even when heated to 600 °C. However, transmission electron microscopy (TEM) showed that there is actually a polycrystalline component of the film, which best matches Mn3O4 (hausmannite with the average Mn oxidation state of ∼+2.7) but may have a slightly expanded unit cell because of the incorporation of Ca. Electrochemical analyses revealed that the as-deposited CaMnxOy films were OER-inactive. In contrast, annealing at 400 or 600 °C resulted in an increase of ∼15-fold in the current densities, which reached j ≅ 1.5 mA·cm-2 at OER overpotentials of η ≈ 550 mV in cyclic voltammetry (CV) sweeps. For the same η, annealed CaMnxOy electrodes also showed good electrochemical stabilities during 2 h of electrolysis, as rather constant steady-state current densities of j ≅ 0.4-0.5 mA·cm-2 were observed. The thicknesses and surface morphologies of the CaMnxOy films did not change during the electrochemical measurements, indicating that corrosion was negligible. In comparison with a previous study in which Ca-free thin layers of MnOx were evaluated, the results demonstrate that Ca2+ incorporation can enhance the OER activity of MnOx electrocatalysts prepared by sputter deposition. This work provides guidance for designing new electrodes for water oxidation on the basis of the abundant and nontoxic elements manganese and calcium.

Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

Understanding oxidation of layered chalcogenide semiconductors is important for device processing... more Understanding oxidation of layered chalcogenide semiconductors is important for device processing, as oxidation can be both an intentional and unintentional result of processing steps. Here, the authors investigate chemical and morphological changes in mechanically exfoliated few-layer MoS2 in oxidizing and inert environments using different microscopies (optical, scanning electron, and atomic force) and spectroscopy (Raman, x-ray photoelectron, and Auger electron) techniques. The environments studied were oxygen, oxygen and water vapor, argon, argon and water vapor, and ultraviolet-generated ozone at temperatures from 25 to 550 °C. Oxidation at low temperatures resulted in the formation of a condensed molybdenum oxide phase and sulfur trioxide gas. At sufficiently elevated temperatures, all the products of oxidation volatilize, resulting in a vapor-phase etch. The kinetics of oxidation and etching depended upon the annealing gas, temperature, time, and the number of layers of MoS2. Conditions can be sele...

Journal of Materials Science

Sulfidation of selected transition metal thin films (Mo, W, Re, Nb, Ta) was combined with thermod... more Sulfidation of selected transition metal thin films (Mo, W, Re, Nb, Ta) was combined with thermodynamic calculations to study the synthesis of transition metal dichalcogenides (TMDCs) and understand variations among the metals as well as processing atmosphere. Metal seed layers were prepared by DC magnetron sputtering and sulfidized using sulfur vapor and H2S. Surface chemistry, structure, and morphology of the films were investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM), respectively. XPS analysis revealed that after treatment with sulfur vapor (p (S2) = 1–10 Torr), Mo, W, and Re films were transformed into MoS2, WS2, and ReS2, respectively. However, Nb and Ta films changed little, and Nb2O5 and Ta2O5 remained the predominant components. Alternatively, conversion of Nb and Ta films to NbS2 and TaS2 was feasible under H2S. Raman spectroscopy also revealed improved crystallinity for Mo, W, and Re sulfidized under H2S. Isobaric and isothermal stability diagrams were calculated to identify feasible processing conditions (sulfur partial pressure and temperatures) for the sulfidation of all of the metals, and our findings were in good agreement with the XPS and Raman results. It was found that for Mo, W, and Re a p (S2) = 10−5 bar is sufficient for the metals to be converted to sulfide phases at 750 °C. On the other hand, due to very high stability of Nb2O5 and Ta2O5, even at very low p (O2), a sulfur partial pressure of 103–104 bar is required to make NbS2 and TaS2, respectively. Nevertheless, thermodynamic calculations confirmed that Nb and Ta could be transformed to NbS2 and TaS2 under 760 Torr H2S. AFM analysis revealed very smooth films for MoS2, WS2, and NbS2 films, but dewetting of TaS2, and ribbons for ReS2. These results provide guidance for designing new processes for synthesizing 2D TMDCs.

Sustainable Energy Fuels

Physical vapour deposition of Mn metal followed by annealing in air is a promising route to prepa... more Physical vapour deposition of Mn metal followed by annealing in air is a promising route to prepare MnOx-anodes for water-oxidation.

ACS Applied Materials & Interfaces, 2016

Surfaces of polycrystalline α-GeTe films were studied by X-ray photoelectron spectroscopy (XPS) a... more Surfaces of polycrystalline α-GeTe films were studied by X-ray photoelectron spectroscopy (XPS) after different treatments in an effort to understand the effect of premetallization surface treatments on the resistance of Ni-based contacts to GeTe. UV-O3 is often used to remove organic contaminants after lithography and prior to metallization; therefore, UV-O3 treatment was used first for 10 min prior to ex situ treatments, which led to oxidation of both Ge and Te to GeOx (x &amp;lt; 2) and TeO2, respectively. Then the oxides were removed by deionized (DI) H2O, (NH4)2S, and HCl treatments. Additionally, in situ Ar(+) ion etching was used to clean the GeTe surface without prior UV-O3 treatment. Ar(+) ion etching, H2O, and (NH4)2S treatments create a surface richer in Ge compared to the HCl treatment, after which the surface is Te-rich. However, (NH4)2S also oxidizes Ge and gradually etches the GeTe film. All treated surfaces showed poor stability upon prolonged exposure to air, revealing that even (NH4)2S does not passivate the GeTe surface. The refined transfer length method (RTLM) was used to measure the contact resistance (Rc) of as-deposited Ni-based contacts to GeTe as a function of premetallization surface preparation. HCl-treated samples had the highest Rc (0.036 ± 0.002 Ω·mm), which was more than twice that of the other surface treatments. This increase in Rc is attributed to formation of the Ni1.29Te phase at the Ni/GeTe interface due to an abundance of Te at the surface after HCl treatment. In general, treatments that resulted in Ge-rich surfaces offered lower Rc.

ACS applied materials & interfaces, Jan 16, 2018

Germanium telluride (GeTe) is a phase change material (PCM) that has gained recent attention beca... more Germanium telluride (GeTe) is a phase change material (PCM) that has gained recent attention because of its incorporation as an active material for radio frequency (RF) switches, as well as memory and novel optoelectronic devices. Considering PCM-based RF switches, parasitic resistances from Ohmic contacts can be a limiting factor in device performance. Reduction of the contact resistance ( R) is therefore critical for reducing the on-state resistance to meet the requirements of high-frequency RF applications. To engineer the Schottky barrier between the metal contact and GeTe, Sn was tested as an interesting candidate to alter the composition of the semiconductor near its surface, potentially forming a narrow band gap (0.2 eV) SnTe or a graded alloy with SnTe in GeTe. For this purpose, a novel contact stack of Sn/Fe/Au was employed and compared to a conventional Ti/Pt/Au stack. Two different premetallization surface treatments of HCl and deionized (DI) HO were employed to make a Te...

$Research paper thumbnail of Fatigue fracture of nano-alumina reinforced copper matrix nanocomposite$

Molybdenum oxide (MoO 3 ) and tungsten oxide (WO 3 ) are considered as transparent back contact b... more Molybdenum oxide (MoO 3 ) and tungsten oxide (WO 3 ) are considered as transparent back contact back contacts for (AgCu)(InGa)Se 2 thin film solar cells. MoO 3 and WO 3 films were deposited by reactive rf sputtering at room temperature in an Ar/O 2 ambient on (AgCu)(InGa)Se 2 layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios. Determination of the valence band offsets by XPS showed that the Ag-alloying at the interface changes the energy band alignment with MoO 3 and WO 3 contacts. This produces a primary contact with lower valence band offset compared to Cu(InGa)Se 2 counterparts. The effect is less significant in films with Ga > 0.5 and Ag > 0.5 probably due to the different nature of ordered vacancy compounds forming near the surface phases.

Ieee Journal of Photovoltaics, Jun 1, 2014

A backwall superstrate device structure that outperforms conventional substrate Cu(In,Ga)Se 2 dev... more A backwall superstrate device structure that outperforms conventional substrate Cu(In,Ga)Se 2 devices for thin absorbers is described. The backwall structure of glass/ITO/MoO 3 /Cu(In,Ga)Se 2 /CdS/i-ZnO/Ag utilizes a MoO 3 transparent back contact to allow illumination of the device from the back. The device performance has been improved by modifying the Cu(In,Ga)Se 2 , including alloying with Ag to form (AgCu)(InGa)Se 2 absorber layers. In addition, sulfized back contacts including ITO-S and MoS 2 are compared. Interface properties are discussed based on the XPS analysis and thermodynamics of reactions.

Journal of Alloys and Compounds, 2014

Transparent back contacts can be used in thin film solar cells facilitating their potential appli... more Transparent back contacts can be used in thin film solar cells facilitating their potential application in tandem cells, bifacial devices and solar windows. In this study, tungsten oxide (WO 3 ) thin films were deposited by Radio Frequency (RF) reactive sputtering in Ar + O 2 ambient. The effects of post deposition anneals in air on the structural, optical, and surface properties of the deposited films were investigated using x-ray diffraction, UV/Vis/NIR spectrophotometry, and x-ray photoelectron spectroscopy, respectively. As-deposited films exhibited amorphous structures with no change after annealing at 300 °C. Samples annealed at 400 and 500 °C were crystallized and identified as pure monoclinic WO 3 phase with (002) preferred orientation, respectively, determined by XRD fiber texture analysis. Scherrer analysis of excess broadening indicated a coherency length of 50 and 65 nm for the 400 and 500 °C annealed films, respectively. High resolution XPS studies showed the presence of W 6+ (WO 3 ) oxidation states at the surface of the as-deposited and the 300 °C annealed films. Annealing at 400-500 °C led to an oxygen deficient surface with a sub-stoichiometric WO 3-x phase.