K. Radhakrishnan

Reactive sputter deposition and characterization of tantalum nitride thin films

Abstract Electrical and structural properties of tantalum nitride films, deposited on GaAs substrates by a d.c. magnetron sputtering technique at different nitrogen partial pressures for use as resistors in integrated circuits, have been reported. Sheet resistivity measurements indicate that stoichiometric TaN films with stable resistivity values can be obtained if the nitrogen partial pressure is maintained between 10.5 and 20.6%. Structural properties studied using X-ray diffraction indicate the presence of pure Ta, TaN, Ta3N5 or a mixture of Ta–N phases in the films depending on the amount of nitrogen in the sputtering gas. The temperature coefficient of resistivity measured for the films deposited with the nitrogen partial pressure around 13% showed a stable value of −200 ppm K−1. Fabricated TaN thin-film resistors showed resistance values within ±10% of the designed value, suggesting the possibility of integrating these resistors with devices.

Grey scale structures formation in SU-8 with e-beam and UV

An experimental study of the possibility to fabricate grey scale optical elements and 3D structures in SU-8 resist was carried out. It was found that the negative CAR SU-8 has a contrast smaller than 1 for PEB at a temperature of 20-45 °C and a sensitivity of the order of 1 µC/cm2 to 20-keV e-beam and 100 mJ/cm2 to 365-nm UV radiation. Continuous surface relief 3D structures formation in SU-8 by UV exposure was demonstrated both with true grey scale photomasks made with a high-energy beam sensitive glass and binary coded grey scale photomasks. A new technology for 3D self-supporting structure formation in thick SU-8 resist layer was suggested for applications in microfluidics and bioscience. This technique includes anchor elements exposure with UV for the whole resist thickness and a subsequent exposure of the self-supporting fine structures in the upper resist layer only with a low-energy electron beam.

Metamorphic InP/InGaAs double-heterojunction bipolar transistors on GaAs grown by molecular-beam epitaxy

InP/InGaAs double-heterojunction bipolar transistor (HBT) structures were grown metamorphically on GaAs substrates by solid-source molecular-beam epitaxy. A linearly graded InxGa1−xP (x varying from 0.48 to 1) buffer layer was used to accommodate the strain relaxation. The crystallinity of the buffer layer and the HBT structure was examined by x-ray diffractometry. Devices with 5×5 μm2 emitter area showed a typical peak current gain of 40, a common-emitter breakdown voltage (BVCEO) higher than 9 V, a current gain cut-off frequency (fT) of 46 GHz, and a maximum oscillation frequency (fmax) of 40 GHz.

Reduction of Cu-rich interfacial layer and improvement of bulk CuO property through two-step sputtering for p-CuO/n-Si heterojunction solar cell

Copper-rich interfacial-layer (Cu-rich IL) is formed during sputter deposition of cupric oxide (CuO) layer on silicon (Si). It has significant impact on the performance of p-CuO/n-Si heterojunction solar cells. In this report, CuO films deposited on Si at different RF-power levels using single and two-step RF-sputtering techniques and p-CuO/n-Si heterojunction solar cells have been investigated. Systematic characterization using XPS, AFM, XRD, Raman, and HR-TEM reveal that two-step RF-sputtering technique offers better crystal quality CuO film with thinner Cu-rich IL layer. Photovoltaic (PV) properties with an open-circuit voltage (Voc) of 421 mV, short circuit current (Jsc) of 4.5 mA/cm2, and a photocurrent of 8.3 mA/cm2 have been achieved for the cells prepared using two-step sputtering method, which are significantly higher than that for the solar cells fabricated using a single-step sputtering. The PV properties were further improved by depositing CuO films at higher working pressure with nitrogen dop...

Thermal resistance of metamorphic InP-based HBTs on GaAs substrates using a linearly graded In/sub x/Ga/sub 1-x/P metamorphic buffer

Thermal properties of metamorphic InP-InGaAs heterojunction bipolar transistors (HBTs) on GaAs substrates using a linearly graded InGaP buffer have been investigated. Compared to the widely used InAlAs metamorphic buffer, InGaP offers better thermal properties resulting in a much smaller thermal resistance for the metamorphic HBTs (MHBTs). Theoretical calculations of the thermal resistance of devices have been made based on a simple constant heat-spreading model, and the results are shown to be consistent with experimental results. It has been made clear that the smaller thermal resistance measured from the MHBTs using a linearly graded InGaP buffer is due to the small bowing parameters and high thermal conductivity of the binary endpoints. Although the use of InGaP as a buffer may slightly degrade the devices thermal properties compared to one using InP directly on GaAs substrate, it gives more freedom to the growth optimization of metamorphic buffer by using compositional grading. With regards to the thermal conductivity and flexible growth optimization, InGaP metamorphic buffer could be considered as an important alternative to the existing InAlAs and InP schemes.

Characterization of beryllium-doped molecular beam epitaxial grown GaAs by photoluminescence

We report a detailed investigation of the Be-doped layers grown at a high AsGa flux ratio as a function of hole concentration using room and low temperature photoluminescence (PL). It has been found that the change of PL spectra with doping differs from that observed in Zn-doped layers grown by liquid phase epitaxy (LPE). The luminescence intensity increased monotonically with free hole concentration. The PL full widths at half maximum (FWHM) at room temperature and 4 K are proportional to the log of doping level and can be used to estimate free hole concentration in the range 1017 < p < 2.25 × 1020 cm−3. In addition, the PL peak energy EM at room temperature was found to drop slowly with hole concentration up to 2.25 × 1020 cm−3 while EM at 4 K did not change significantly up to 3 × 1018 cm−3 and dropped drastically thereafter. The former could be explained by band tail transitions, and the latter, we suggest, is most likely a result of band-acceptor transitions.

Studies on the Degradation of InP/InGaAs/InP Double Heterojunction Bipolar Transistors Induced by Silicon Nitride Passivation

The effect of silicon nitride passivation on the electrical characteristics of InP/InGaAs heterojunction bipolar transistors (HBTs) has been investigated comprehensively. The major degradations of I–V characteristics identified in our InP/InGaAs HBTs are: (1) the decrease of current gain due to a significant increase in the forward base leakage current and (2) large increase of base-collector (B-C) and base-emitter (B-E) reverse leakage currents. We found that different physical origins should be attributed to these two degradation behaviors.

Demonstration of aluminum-free metamorphic InP/In/sub 0.53/Ga/sub 0.47/As/InP double heterojunction bipolar transistors on GaAs substrates

We report, for the first time, the successful fabrication of aluminum-free metamorphic (MM) InP/In/sub 0.53/ Ga/sub 0.47/ As/InP double heterojunction bipolar transistors (DHBTs) on GaAs substrates with a linearly graded In/sub x/Ga/sub 1-x/P buffer grown by solid-source molecular beam epitaxy (SSMBE). Devices with 5/spl times/5 /spl mu/m/sup 2/ emitters display a peak current gain of 40 and a common-emitter breakdown voltage (BV/sub CE0/) higher than 9 V, a current gain cut-off frequency (f/sub T/) of 48 GHz and a maximum oscillation frequency (f/sub max/) of 42 GHz. A minimum noise figure of 2.9 dB and associated gain of 19.5 dB were measured at a collector current level of 2.6 mA at 2 GHz. Detailed analysis suggests that the degradation of the base-emitter heterojunction interface and the increase of bulk recombination are the most probable causes for the poorer device performance of current metamorphic HBTs compared with lattice-matched HBTs.

Optical bandgap widening and phase transformation of nitrogen doped cupric oxide

The structural and optical properties of sputter deposited nitrogen (N) doped CuO (CuO(N)) thin films are systematically investigated. It is found that the incorporation of N into CuO causes an enlargement of optical bandgap and reduction in resistivity of the CuO(N) films. Furthermore, a gradual phase transformation from CuO to Cu2O is observed with the increase in N concentration. The effects of annealing temperature on the structural properties of CuO (N) and its dependence on N concentration are also investigated. It is observed that the phase transformation process from CuO to Cu2O significantly depends on the N concentration and the annealing temperature. Heterojunction solar cells of p-type CuO(N) on n-type silicon (Si) substrate, p-CuO(N)/n-Si, are fabricated to investigate the impact of N doping on its photovoltaic properties.

Demonstration of AlGaN/GaN high-electron-mobility transistors on 100 mm diameter Si(111) by plasma-assisted molecular beam epitaxy

AlGaN/GaN high-electron-mobility transistor structures grown on 100 mm high-resistivity Si(111) substrates using plasma-assisted molecular beam epitaxy are reported. The two-dimensional electron gas (2DEG) formation in the heterostructures was realized by the growth optimization of two-step low temperature and high temperature AlN layers and GaN buffer layer. High-electron mobility of 1100 cm2/V s with a sheet carrier density of 9×1012 cm−2 was achieved. The presence of 2DEG in the AlGaN/GaN interface was confirmed by temperature dependent Hall measurements and capacitance-voltage carrier profiling. The fabricated 1.5 μm gate length high electron mobility transistor exhibited a maximum drain current density of 530 mA/mm and a peak extrinsic transconductance of 156 mS/mm.