Electrically Nonconductive Thermal Pastes with Carbon as the Thermally Conductive Component

Electrically nonconductive thermal pastes have been attained using carbon (carbon black or graphite) as the conductive component and ceramic (fumed alumina or exfoliated clay) as the nonconductive component. For graphite particles (5 μm), both clay and alumina are effective in breaking up the electrical connectivity, resulting in pastes with electrical resistivity up to 10¹³Ω·cm and thermal contact conductance (between copper surfaces of roughness 15 μm) up to 9 × 10⁴ W/m²·°C. For carbon black (30 nm), clay is more effective than alumina, providing a paste with resistivity 10¹¹ Ω·cm and thermal contact conductance 7 × 10⁴ W/m²·°C. Carbon black increases the thermal stability, whereas either graphite or alumina decreases the thermal stability. The antioxidation effect of carbon black is further increased by the presence of clay up to 1.5 vol.%. The addition of clay (up to 0.6 vol.%) or alumina (up to 2.5 vol.%) to graphite paste enhances the thermal stability.     

Comparative evaluation of thermal interface materials for improving the thermal contact between an operating computer microprocessor and its heat sink

Testing of the relative effectiveness of various thermal interface materials for improving the thermal contact between the well-aligned mating surfaces of an operating computer microprocessor (with an integrated heat spreader) and its heat sink shows that carbon black paste, whether by itself or as a coating on aluminum or flexible graphite, is more effective than silver paste (Arctic Silver), but is comparable in effectiveness to aluminum paste (Shin-Etsu). The carbon black paste by itself is as effective as the Shin-Etsu paste coated aluminum. The Shin-Etsu paste is more effective than Arctic Silver, whether by itself or as a coating. The relative performance is mostly consistent with that assessed by measuring the thermal contact conductance. The correlation is particularly strong for conductance below 3×10⁴ W/m²·°C. The discrepancy is attributed to the difference in surface roughness between computer and guarded hot plate surfaces. In the case in which the mating surfaces of microprocessor and heat sink are not well aligned, Shin-Etsu and Arctic Silver are more effective than carbon black.     

Carbon Nanotube Thermal Pastes for Improving Thermal Contacts

The use of 0.6 vol.% single-walled carbon nanotubes in a poly(ethylene glycol)-based dispersion gave a thermal paste that was as effective as solder for improving thermal contacts. A thermal contact conductance of 20 × 10⁴ W m⁻² K⁻¹ was attained. An excessive amount of nanotubes (e.g. 1.8 vol.%) degraded the performance, because of conformability loss. The nanotubes were more effective than hexagonal boron nitride particles but were less effective than carbon black, which gave a thermal contact conductance of 30 × 10⁴ W m⁻² K⁻¹.     

Nanoclay Paste as a Thermal Interface Material for Smooth Surfaces

A paste in the form of a polyol ester vehicle (liquid) containing 0.6 vol.% nanoclay is an effective thermal interface material. Nanoclay with a high conformability and hence a small bond line thickness is preferred, namely montmorillonite containing a quarternary ammonium salt organic modifier (dimethyl dehydrogenated tallow) at 125 meq/100 g clay, after exfoliation by using the vehicle. When it is used between smooth (0.009 μm) copper surfaces at a pressure of 0.69 MPa, the thermal contact conductance reaches 40 × 10⁴ W/m² K, in contrast to the corresponding values of 28 × 10⁴ W/m² K, 28 × 10⁴ W/m² K, 25 × 10⁴ W/m² K, and 24 × 10⁴ W/m² K previously reported for carbon black, fumed alumina, fumed zinc oxide, and graphite nanoplatelet pastes. Between rough copper surfaces (12 μm), the conductance provided by the nanoclay paste is slightly below those of the other pastes. The superiority of the nanoclay paste for smooth surfaces is attributed to the␣submicron bond line thickness; the inferiority for rough surfaces is due to the low thermal conductivity. The conductance provided by the nanoclay paste increases from 31 × 10⁴ W/m² K to 40 × 10⁴ W/m² K when the pressure is increased from 0.46 MPa to 0.92 MPa. This pressure dependence is stronger than that of any of the other pastes studied.     

Pressure electrical contact improved by carbon black paste

Pressure and pressureless electrical contacts were evaluated by measuring the contact electrical resistivity between copper mating surfaces. Pressure electrical contacts with a contact resistivity of 2×10⁻⁵ Ω·cm² have been attained using a carbon black paste of a thickness of less than 25 μm as the interface material. In contrast, a pressureless contact with silver paint as the interface material exhibits a higher resistivity of 3×10⁻⁵ Ω·cm² or above. A pressureless contact with colloidal graphite as the interface material exhibits the same high contact resistivity (1×10⁻⁴ Ω·cm²) as a pressure contact without any interface material. On the other hand, pressureless contacts involving solder and silver epoxy exhibit lower contact resistivity than carbon black pressure contacts.     

Large VLWIR Hg1−xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 μm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ_c >15 μm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 μm to 17 μmm spectral range. Recently, VLWIR (λ_c ∼ 17 μm at 78 K) photovoltaic large (1000 μm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 μm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 μm diameter, λ_c ∼ 17 μm at 78 K detectors have dark currents ∼160 μA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 μm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10¹⁷ to 3.5×10¹⁷ ph-cm⁻²sec⁻¹ range. This non-linearity is an order of magnitude better than for photoconductive detectors.     

Polyol-based phase-change thermal interface materials

Polyol-based phase-change thermal interface materials that exhibit high thermal contact conductance and thermal stability have been developed for microelectronic cooling. By using a diol (polycaprolactone or polyester diol in the form of 2-oxepanone) of molecular weight 1,000–2,000 amu, along with 4 vol.%-hexagonal boron nitride particles, this work attained thermal contact conductance (at 70°C, across copper surfaces) that is higher than that attained by using paraffin wax, polyether glycol, polyethylene glycol, or tetradecanol (in place of the diol) and that attained by commercial phase-change thermal interface materials. The thermal stability of the diol is superior to the other phase change materials mentioned above, although the heat of fusion is lower. Boron nitride is more effective than carbon black (also 4 vol.%) for enhancing the conductance, but carbon black diminishes the heat of fusion less than does boron nitride.     

Halogen lamp rapid thermal annealing of Si- and Be-implanted In0.53Ga0.47As

Halogen lamp rapid thermal annealing was used to activate 100 keV Si and 50 keV Be implants in In_0.53Ga_0.47As for doses ranging between 5 × 10¹²−4 × 10¹⁴ cm⁻². Anneals were performed at different temperatures and time durations. Close to one hundred percent activation was obtained for the 4.1 × 10¹³ cm⁻² Si-implant, using an 850° C/5 s anneal. Si in-diffusion was not observed for the rapid thermal annealing temperatures and times used in this study. For the 5 × 10¹³ cm⁻² Be-implant, a maximum activation of 56% was measured. Be-implant depth profiles matched closely with gaussian profiles predicted by LSS theory for the 800° C/5 s anneals. Peak carrier concentrations of 1.7 × 10¹⁹ and 4 × 10¹⁸ cm⁻³ were achieved for the 4 × 10¹⁴ cm⁻² Si and Be implants, respectively. For comparison, furnace anneals were also performed for all doses.     

Status of the MBE technology at leti LIR for the manufacturing of HgCdTe focal plane arrays

This paper presents recent developments that have been made in Leti Infrared Laboratory in the field of molecular beam epitaxy (MBE) growth and fabrication of medium wavelength and long wavelength infrared (MWIR and LWIR) HgCdTe devices. The techniques that lead to growth temperature and flux control are presented. Run to run composition reproducibility is investigated on runs of more than 15 consecutively grown layers. Etch pit density in the low 10⁵ cm⁻² and void density lower than 10³ cm⁻² are obtained routinely on CdZnTe substrates. The samples exhibit low n-type carrier concentration in the 10¹⁴ to 10¹⁵ cm⁻³ range and mobility in excess of 10⁵ cm²/Vs at 77 K for epilayers with 9.5 μm cut-off wavelength. LWIR diodes, fabricated with an-on-p homojunction process present dynamic resistance area products which reach values of 8 10³ Ωcm² for a biased voltage of −50 mV and a cutoff wavelength of 9.5 μm at 77 K. A 320 × 240 plane array with a 30 μm pitch operating at 77 K in the MWIR range has been developed using HgCdTe and CdTe layers MBE grown on a Germanium substrate. Mean NEDT value of 8.8 mK together with an operability of 99.94% is obtained. We fabricated MWIR two-color detectors by the superposition of layers of HgCdTe with different compositions and a mixed MESA and planar technology. These detectors are spatially coherent and can be independently addressed. Current voltage curves of 60 × 60 μm² photodiodes have breakdown voltage exceeding 800 mV for each diode. The cutoff wavelength at 77 K is 3.1 μm for the MWIR-1 and 5 μm for the MWIR-2.     

Rheological Behavior of Thermal Interface Pastes

Polyol-ester-based thermal pastes containing carbon black, fumed alumina or nanoclay exhibit Bingham plastic behavior with shear thinning. Carbon black gives double yielding, but fumed alumina and nanoclay give single yielding. The plastic viscosity increases with the solid content. Antioxidants increase the plastic viscosity and yield stresses. Nanoclay (1.0 vol.%) gives low shear moduli, high critical shear strain, and high loss tangent, thus resulting in low bond-line thickness and high thermal contact conductance for smooth (0.009 μm) proximate surfaces. Carbon black (Tokai, 8.0 vol.%) gives high moduli, low critical strain, and low loss tangent, thus resulting in high bond-line thickness, though the high thermal conductivity due to the high solid content results in high thermal contact conductance for rough (15 μm) proximate surfaces. Antioxidants enhance the solid-like character, increase the yield stress, plastic viscosity, and bond-line thickness, and decrease the thermal contact conductance.     

Mid-wavelength infrared p-on-n Hg1−xCdxTe heterostructure detectors: 30–120 kelvin state-of-the-Art performance

We report on Hg_1−xCd_xTe mid-wavelength infrared (MWIR) detectors grown by molecular-beam epitaxy (MBE) on CdZnTe substrates. Current-voltage (I-V) characteristics of HgCdTe-MWIR devices and temperature dependence of focal-plane array (FPA) dark current have been investigated and compared with the most recent InSb published data. These MWIR p-on-n Hg_1−xCd_xTe/CdZnTe heterostructure detectors give outstanding performance, and at 68 K, they are limited by diffusion currents. For temperatures lower than 68 K, in the near small-bias region, another current is dominant. This current has lower sensitivity to temperature and most likely is of tunneling origin. High-performance MWIR devices and arrays were fabricated with median R_oA values of 3.96 × 10¹⁰ Ω-cm² at 78 K and 1.27 × 10¹² Ω-cm² at 60 K; the quantum efficiency (QE) without an antireflection (AR) coating was 73% for a cutoff wavelength of 5.3 μm at 78 K. The QE measurement was performed with a narrow pass filter centered at 3.5 μm. Many large-format MWIR 1024 × 1024 FPAs were fabricated and tested as a function of temperature to confirm the ultra-low dark currents observed in individual devices. For these MWIR FPAs, dark current as low as 0.01 e⁻/pixel/sec at 58 K for 18 × 18 μm pixels was measured. The 1024 × 1024 array operability and AR-coated QE at 78 K were 99.48% and 88.3%, respectively. A comparison of these results with the state-of-the-art InSb-detector data suggests MWIR-HgCdTe devices have significantly higher performance in the 30–120 K temperature range. The InSb detectors are dominated by generation-recombination (G-R) currents in the 60–120 K temperature range because of a defect center in the energy gap, whereas MWIR-HgCdTe detectors do not exhibit G-R-type currents in this temperature range and are limited by diffusion currents.     

High temperature stability of chromium boride ohmic contacts to p-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC using CrB₂. Two hundred nanometer thick films were sputter-deposited on substrates of doping concentration 1.3×10¹⁹ cm⁻³ in a system with a base pressure of 3×10⁻⁷ Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, and transmission electron microscopy. The as-deposited CrB₂ contacts exhibited rectifying characteristics and contained oxygen as a major contaminant. Ohmic behavior with linear current-voltage characteristics was observed following short anneals at 1100°C for 2 min at a pressure of 5×10⁻⁷ Torr. The oxygen in the CrB₂ films was removed by the annealing process, and the lowest value of the specific contact resistance (r_c) measured at room temperature was 8.2×10⁻⁵ Ω-cm². Longer anneals at 1100°C for 3.5 h and 1200°C for 2 h reduced the room temperature values of r to 1.4×10⁻⁵ Ω-cm². A thin reaction region has been identified at the CrB₂/SiC interface; however, the interface remains essentially stable. Thermal stressing at 300°C in vacuum for over 2200 h produced only a slight increase in the specific contact resistance. The low value of the specific contact resistance and the excellent high temperature stability of the CrB₂/SiC interface make this contact a candidate for high power/high temperature SiC device applications.     

A study on the key factors affecting the electronic properties of monocrystalline silicon solar cells

The model of monocrystalline silicon solar cells is established,and the effects of wafer parameters,such as the p-Si（100） substrate thickness,the defect density,and the doping concentration,on the electronic properties of monocrystalline silicon solar cells are analyzed.The results indicate that the solar cells with an Al back-surface-field will have good electronic properties when the wafers meet the following three conditions：（i） the defect density is less than 1.0×1011 cm^-3;（ii） the doping concentration is from 5×10^15 cm^-3 to 1×10^17 cm^-3,i.e.the bulk resistivity is from 0.5 Ω·cm to 10 Ω·cm;（iii） the cells substrate thickness is in the range of 100 μm to 200 μm.     

Ta/Au ohmic contacts to n-type ZnO

Ta/Au ohmic contacts are fabricated on n-type ZnO (∼1 × 10¹⁷ cm⁻³) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). After growth and metallization, the samples are annealed at 300°C and 500°C for 30 sec in nitrogen ambient. The specific contact resistance is measured to be 3.2×10⁻⁴ Ωcm² for the as-deposited samples. It reduces to 5.4×10⁻⁶ Ωcm² after annealing at 300°C for 30 sec without significant surface morphology degradation. When the sample is annealed at 500°C for 30 sec, the specific contact resistance increases to 3.3 × 10⁻⁵ Ωcm². The layer structures no longer exist due to strong Au and Ta in-diffusion and O out-diffusion. The contact surface becomes rough and textured.     

Minimization of leakage current of recessed gate AlGaN/GaN HEMTs by optimizing the dry-etching process

The backward current of Schottky contacts on unintentionally doped GaN samples prepared by different dry-etching methods was investigated. It was found that an ion beam etching (IBE) process with an accelerating voltage of 250 V under an angle of 20 degrees to minimize channeling achieves the best results. The backward current in this case is 4 × 10⁻¹⁰ A/μm² compared to the backward current of the unetched sample of 1 × 10⁻⁷ A/μm² at −100 V. With this process, recessed gate HEMTs on AlGaN/GaN heterostructures grown by low pressure MOVPE were fabricated and compared to HEMTs without recess. The applied gate recess etching technique improves the leakage current by nearly a factor of two. The maximum transconductance is improved from 40 mS/mm to 60 mS/mm at a gate length of 4 μm.     

Nonalloyed Al ohmic contacts to MgxZn1−xO

Al nonalloyed ohmic contacts were fabricated and characterized on Mg_xZn_1−xO (0≤×≤0.34) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). Specific contact resistances were evaluated by the transmission line method (TLM). A specific contact resistance of 2.5×10⁻⁵ Ωcm² was obtained for Al contact to ZnO with an electron concentration of 1.6×10¹⁷ cm⁻³. The current flow mechanism was studied by investigating the dependence of specific contact resistances on electron concentration and on temperature. For Al contact to Mg_0.34Zn_0.66O, specific contact resistance values are two orders of magnitude larger than that of Al ohmic contacts to ZnO.     

Investigation of HgCdTe p-n device structures grown by liquid-phase epitaxy

The microstructure of p-n device structures grown by liquid-phase epitaxy (LPE) on CdZnTe substrates has been evaluated using transmission electron microscopy (TEM). The devices consisted of thick (∼21-μm) n-type layers and thin (∼1.6-μm) p-type layers, with final CdTe (∼0.5 μm) passivation layers. Initial observations revealed small defects, both within the n-type layer (doped with 8×10¹⁴/cm³ of In) and also within the p-type layer but at a much reduced level. These defects were not visible, however, in cross-sectional samples prepared by ion milling with the sample held at liquid nitrogen temperature. Only isolated growth defects were observed in samples having low indium doping levels (2×10¹⁴/cm³). The CdTe passivation layers were generally columnar and polycrystalline, and interfaces with the p-type HgCdTe layers were uneven. No obvious structural changes were apparent in the region of the CdTe/HgCdTe interfaces as a result of annealing at 250°C.     

Fabrication and characterization of two-color midwavelength/long wavelength HgCdTe infrared detectors

High-performance 20-μm unit-cell two-color detectors using an n-p⁺-n HgCdTe triple-layer heterojunction (TLHJ) device architecture grown by molecular beam epitaxy (MBE) on (211)-oriented CdZnTe substrates with midwavelength (MW) infrared and long wavelength (LW) infrared spectral bands have been demonstrated. Detectors with nominal MW and LW cut-off wavelengths of 5.5 μm and 10.5 μm, respectively, exhibit 78 K LW performance with >70 % quantum efficiency, reverse bias dark currents below 300 pA, and RA products (zero field of view, 150-mV bias) in excess of 1×10³ Ωcm². Temperature-dependent current-voltage (I–V) detector measurements show diffusion-limited LW dark current performance extending to temperatures below 70 K with good operating bias stability (150 mV ± 50 mV). These results reflect the successful implementation of MBE-grown TLHJ detector designs and the introduction of advanced photolithography techniques with inductively coupled plasma (ICP) etching to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. These detector improvements complement the development of high operability large format 640×480 and 1280×720 two-color HgCdTe infrared focal plane arrays (FPAs) to support third generation forward looking infrared (FLIR) systems.     

Pd-Ge contact to n-GaAs with the TiW diffusion barrier

Pd-Ge based ohmic contact to n-GaAs with a TiW diffusion barrier was investigated. Electrical analysis as well as Auger electron spectroscopy and the scanning electron microscopy were used to study the contact after it was subjected to different furnace and rapid thermal annealing and different aging steps. All analyses show that TiW can act as a good barrier metal for the Au/Ge/Pd/n-GaAs contact system. A value of 1.45 × 10⁻⁶ Ω-cm² for the specific contact resistance was obtained for the Au/TiW/Ge/Pd/n-GaAs contact after it was rapid thermally annealed at 425°C for 90 s. It can withstand a thermal aging at 350°C for 40 h with its ρ_c increasing to 2.94 × 10⁻⁶Ω-cm² and for an aging at 410°C for 40 h with its ρ_c increasing to 1.38 × 10⁻⁵ Ω-cm².     

A silicon carbide LOCOS process using enhanced thermal oxidation by argon implantation

A process is described for creating local oxidation of silicon structure (LOCOS) structures in silicon carbide using enhanced thermal oxidation by argon implantation. Thicker oxides were created in selective regions by using multiple energy argon implants at a dose of 1 × 10¹⁵ cm⁻² prior to thermal oxidation. Atomic force microscopy was used to analyze the fabricated LOCOS structure.