Journal of Electronic Materials

In a recent publication [J. Electron. Mater., 46, 585 (2017)], a number of formulae are presented for the effective conductivity of a bipolar semiconductor sandwiched between two metal contacts. However, the results are shown to be nonphysical, and the explanation is traced to errors appearing in previous literature on the subject.

In the present study, bulk samples of chalcogenide (Se80Te20)94-xGe6Sbx(x = 1, 2, 4, 6, 8 and 10) alloys were prepared using the melt-quenching technique. The amorphous nature of the as-primed samples was confirmed via x-ray diffraction. Current–voltage (I–V) measurements were carried out at room temperature as well as at elevated temperatures up to 373 K. I–V characteristics point toward ohmic behaviour at a low electric field, whereas non-ohmic behaviour is observed at a higher electric field. The chalcogenide glass with x = 8 shows maximum conductivity in the composition range under investigation. The temperature dependence of conductivity is also determined. The variation of ln(σDC) against the reciprocal of temperature exhibits Arrhenius behaviour. From the slope and intercept, ΔΕ and σo were calculated. The Meyer–Neldel (MN) rule between pre-exponential factor and activation energy is also observed. Conduction mechanism is concluded to be in the band tails of localized states responsible for electrical conduction in as-prepared glasses.

We report new results on metalorganic chemical vapor deposition (MOCVD)in situ growth of long wavelength infrared (LWIR) P-on-n and medium wavelength infrared (MWIR) n-on-P HgCdTe heterojunction photodiodes using the interdiffused multilayer process (IMP). The n-type regions are doped with iodine using the precursor ethyl iodide (El). I-doped HgCdTe using El has mobilities higher than that obtained on undoped background annealed films and are comparable to LPE grown In-doped HgCdTe. The p-type layers are doped with arsenic from either tertiarybutylarsine (TBAs) or a new precursor,tris-dimethylaminoarsenic (DMAAs). The substrates used in this work are lattice matched CdZnTe oriented (211)B or (100)4°→«110». Junction quality was assessed by fabricating and characterizing backside-illuminated arrays of variable-area circular mesa photodiodes. This paper presents four new results. First, carrier lifetimes measured at 80K on arsenic doped single HgCdTe layers are generally longer for films doped from the new precursor DMAAs than from TBAs. Second, we present data on the first P-on-n HgCdTe photodiodes grownin situ with DMAAs which have R0A products limited by g-r current at 80K for λco = 7–12 μm, comparable to the best R0A products we have achieved with TBAs. Third, we report the first experimental data on a new HgCdTe device architecture, the n-on-P heterojunction, with a wide gap p-type layer which allows radiation incident through the substrate to be absorbed in a narrower gap n-type layer, thereby eliminating interface recombination effects. With the n-on-P architecture, MWIR photodiodes were obtained reproducibly with classical spectral response shapes, high quantum efficiencies (70-75%) and R0A products above 2 x 105 ohm-cm2 for λco = 5.0 μm at 80K. Fourth, we report 40K data for LWIR P-on-n HgCdTe heterojunction photodiodes (using TBAs), with R0A values of 2 x 104 ohm-cm2 for λco = 11.7 μm and 5 x 105 ohm-cm2 for λco - 9.4 μm. These are the highest R0A values reported to date for LWIR P-on-n heterojunctions grownin situ by MOCVD.

A bottom-gate, top-contact transparent thin film transistor (TFT) based on c-axis oriented, vertically aligned ZnO nanorod arrays was fabricated on glass substrates via solution processing, in which ZnO nanorod arrays were synthesized on ZnO seed layers through a simple hydrothermal route. This TFT used SiO2 and indium tin oxide as the gate insulator and gate electrode, respectively. The source and drain electrodes were formed by radio frequency sputtered Au through a shadow mask. This ZnO TFT exhibited n-channel enhancement behavior with a field effective mobility of 3.86 cm2 V−1 s−1, a current on-to-off ratio of 65.5 and a threshold voltage of 1 V. Moreover, the ZnO TFT has a high transmittance of 80% in the visible spectrum. Our results demonstrate that hydrothermally grown, vertically aligned ZnO nanorod arrays are very promising for the fabrication of cost effective and high performance transparent thin-film transistors.

The Au/n-Si Schottky barrier diodes (SBDs) with 200-µm (sample D200) and 400-µm (sample D400) bulk thicknesses have been fabricated. The ideality factor and the barrier height have been calculated from the forward-bias current-voltage (I-V) characteristics of D200 and D400 SBDs. The energy distribution of the interface states and relaxation time are found from the capacitance-frequency (C-f) characteristics. The density of interface state and relaxation times have a (nearly constant) slow exponential rise with bias in the range of Ec −0.77 and Ec −0.47 eV from the midgap toward the bottom of the conductance band. Furthermore, the energy distribution of the interface states obtained from C-f characteristics has been compared with that obtained from the forward-bias I-V characteristics.

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs1−xNx layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.

The ultrasonic wire bonding (UWB) process has been examined using transmission electron microscopy (TEM) and standard wire pull testing techniques. Al-0.5 wt.% Mg wires 75 μm in diameter were bonded to pure and alloyed Al substrates. The bonding parameters, surface roughness, and surface contamination levels were variables in the experiments. Cross-section TEM specimens were made from these samples. TEM analysis was conducted on the wire, wire/substrate interface and substrate. Pull tests showed that for the Al substrates the surface roughness or the presence of contamination did not effect the bond strength, whereas for contaminated stainless steel substrates, a three μm surface finish resulted in the highest bond pull strength. The TEM observations revealed features such as low-angle grain boundaries, dislocation loops and the absence of a high dislocation density, indicating that the wire and substrate were dynamically annealed during bonding. Based on the width of a zone near a grain boundary in the wire which was depleted of dislocation loops, it was estimated that local heating equivalent to a temperature of 250° C for 90 msec was achieved in the wire during bonding. No evidence was found for melting along the bond interface, indicating that UWB is a solid-state process. Based on the TEM observationsof the bond interface and the pull tests, it is concluded that the ultrasonic vibrations clean the surfaces to be joined to the extent that a good bond can be obtained by intimate metal-metal contact in the clean areas.

In this paper, single- and multi-junction thin-film solar cells based on copper tin sulfide (CTS) are proposed. The proposed single-junction cell consists of copper zinc tin sulfide (CZTS) and CTS, as a bilayer absorber. The double-junction structure is made of CZTS and CTS absorber layers for the top and bottom cells, respectively. Furthermore, a CZTS/(CZTSe)/CTS triple-junction cell, which is based on earth-abundant and non-toxic elements, is investigated. The performance of the proposed cells in the presence of tin sulfide layers as the back surface field to reduce recombination is also examined. In order to reach the maximum efficiency, the thickness of layers is varied and optimized. The simulated efficiency of the triple-junction structure, including CZTS, CZTSe, and CTS with the optimal thickness of 0.4 μm, 0.7 μm, and 0.7 μm, respectively, was as high as 34.1%.

The influence of the underlying nucleation layer on the properties of semipolar $$(11{\bar{2}}2)$$ GaN grown on m-plane sapphire by metalorganic vapor-phase epitaxy has been investigated. $$(11{\bar{2}}2)$$ GaN epilayers of ~ 1 μm thickness were grown using four different initiating sequences: low-temperature AlN and GaN, and high-temperature AlN buffer layers, and directly (high-temperature GaN). The choice of nucleation layer had a pronounced effect on the surface morphology and crystal quality of the overlying GaN epilayer. In comparison, direct growth of $$(11{\bar{2}}2)$$ GaN without any buffer layer provided the best crystal quality with a rocking-curve $$\omega $$ full-width at half-maximum (FWHM) value of 720 arcsec along the $$[11{\bar{2}}{\bar{3}}]$$ direction and relatively enhanced near-band-edge photoluminescence emission, thus showing this direct growth process to be a simple route for synthesis of semipolar $$(11{\bar{2}}2)$$ GaN layers.