Thomas Clausen

Transport properties of low-resistance ohmic contacts to InP

Abstract The transport properties of conventional Au-based low-resistance ohmic contacts to n- and p-type InP have been investigated. For n-type InP, a good agreement between the minimum specific contact resistance and the bulk doping density is observed in accordance with an inverse bulk doping density dependence for the specific contact resistance. Drift and diffusion across a thermodynamically stable metalphosphide-InP junction is found to be the rate-limiting step of the low-resistance contacts to n-type InP. Tunneling is not observed, but rather a significantly effective Schottky barrier lowering is initially responsible for the low-resistance contacts. Significant Schottky barrier lowering is also observed for ohmic contacts to p-type InP, but no total transition from predominately thermionic emission to drift and diffusion is observed for low-resistance contacts to p-type InP, indicating that the contacts are not fully developed, and that further reduction of the specific contact resistance can be expected.

Metallurgical optimization for ohmic contacts to InP using conventional metallization schemes

Abstract Conventional Au-based metallization schemes for both n-InP (AuGeNi) and p-InP (AuZnNi) were studied as a function of annealing temperature. For n + -InP, a value of the specific contact resistance as low as 5 × 10 −8 Ω cm 2 , has been obtained by optimizing the contact metallurgy structure. For p + -InP a similar very low value, 7 × 10 −6 Ω cm 2 , has been obtained. Metallurgical investigations have shown that very low specific contact resistance in Au-based metallization schemes can be obtained by growing a certain AuIn binary phase (ϵ-phase; Au 3 In), thus maximizing the amount of P at the metal-InP surface for barrier-lowering metal-phosphide formations.

Improved current transport properties of post annealed Y1Ba2Cu3O7-x thin films using Ag doping

The influence of Ag doping on the transport properties of Y1Ba2Cu3O7−x thin films prepared by Y, BaF2, and Cu co‐evaporation and optimized ex situ post annealing has been investigated. Both undoped and Ag doped films have values of Tc above 90 K, but Jc (77 K) is highly dependent on the nominal thickness (tnom) of the as‐deposited film. For undoped films with tnom≤300 nm Jc (77 K) (≫106 A/cm2) decreases monotonically with increasing film thickness. Above 300 nm Jc (77 K) decreases rapidly to values below 5×105 A/cm2. Ag doped films with tnom≥200 nm have higher Jc (77 K) values than those of undoped films. Ag doped films have a maximum in Jc (77 K) around 250 nm. As for the undoped films, there is a large decrease in Jc (77 K) for Ag doped films with tnom≥300 nm. It was found that the higher values of Jc (77 K) for the Ag doped films were due to a better epitaxial growth of the YBCO compound. The low values of Jc (77 K) for both undoped and Ag doped single layer films with tnom≥300 nm were found to be due ...

Andreev reflections at interfaces between δ‐doped GaAs and superconducting Al films

By placing several Si δ‐doped layers close to the surface of a GaAs molecular beam epitaxy–grown crystal, we achieve a compensation of the Schottky barrier and obtain a good Ohmic contact between an in situ deposited (without breaking the vacuum) Al metallization layer and a highly modulation doped (n++) conduction layer embedded below the δ‐doped layers in the GaAs crystal. When cooled to below the critical temperature (≊1.2 K) of Al, superconductivity is induced in the conductive layer of the semiconductor. We have studied the current voltage (I–V) characteristics in a planar geometry where the Al has been removed in a thin stripe. We find a manifestation of the superconducting energy gap and a rich fine structure at injection energies both below and above the gap.

Transport mechanisms in low‐resistance ohmic contacts to p‐InP formed by rapid thermal annealing

Thermionic emission across a very small effective Schottky barrier (0–0.2 eV) are reported as being the dominant transport process mechanism in very low‐resistance ohmic contacts for conventional AuZn(Ni) metallization systems to p‐InP formed by rapid thermal annealing. The barrier modulation process is related to interdiffusion and compound formation between the metal elements and the InP. The onset of low specific contact resistance is characterized by a change in the dominant transport mechanism; from predominantly a combination of thermionic emission and field emission to purely thermionic emission.

Schottky barrier inhomogeneities in Au-Ni and Au-Cr contacts to InP-ohmic contact applications

Abstract The electrical transport properties of carriers (electrons) across the metal-semiconductor interface for Au/Ni/Au and Au/Cr/Au contacts to n-type InP have been compared. It is found that the interfacial properties are complex with many possible phases at the interface. The electrical properties of the contacts can be described in terms of Schottky barrier inhomogeneities at the interface and a parallel conduction model applies. From this model it is found that the observed reduction in the contact resistance is closely connected to a lowering of the effective Schottky barrier towards 0 eV.

Double-sided YBa2Cu3O7-x thin films for microwave applications

YBa2Cu3O7-x(YBCO) thin films with superconducting transition temperatures above 91 K were deposited on both sides of double-sided, polished LaAlO3(100) substrates. Using ex situ furnace annealing, the substrates could be kept at 45 degrees C during the codeposition of Y, Cu and BaF2. No O2 was necessary in the deposition chamber. The furnace annealing was carried out in a mixture of Ar, O2 and H2O vapour. The gas flow was 1000 sccm at an O2 partial pressure of 1.0 mbar. The maximum annealing temperature was 810 degrees C which was maintained for 1 h. Double-sided YBCO superconducting thin films may provide a good base material for microwave devices in which one superconducting side acts as a low-loss ground plane.

Enhanced Jc's of YBa2Cu3O7-x-Ag ex situ annealed coevaporated films on LaAlO3 (100) substrates

A 5× increase of the critical current density (Jc) at 77 K was obtained by coating a coevaporated 500 nm thick Y, BaF2, Cu film with 50 nm Ag prior to the ex situ annealing. Jc increased from 0.2 for uncoated samples to 1 MA/cm2 for the Ag‐coated sample without severely affecting the zero resistance transition temperature (Tc0). Scanning electron microscopy showed that the surface morphology was improved and that the normally observed trellislike structure was greatly reduced. By combining electron microscopy and sputter assisted Auger analysis it was found that the Ag nucleated in droplets on the surface of the superconductor with only small amounts of Ag in the superconductor matrix. X‐ray diffraction confirmed that the Ag‐coated film was highly c‐axis oriented. The increase in Jc is believed to be due to the improved surface properties of the superconductor, indicating that a larger amount of the film is c‐axis oriented or that the single‐crystalline grains are larger.

Electrical properties of nanosized non-barrier inhomogeneities in Zn-based metal-semiconductor contacts to InP

Abstract We have found that the electrical properties of carriers across the metal-semiconductor interface for alloyed Zn based metallizations to n- and p-InP are dominated by nanosized non-barrier inhomogeneities. The effective area covered by the nanosized regions is a small fraction of the contact area resulting in high values of the specific contact resistance to p-InP. For n − -InP, thermionic emission across nanosized inhomogeneities dominates the carrier flow when T ann 440°C.

Novel light trapping scheme for thin crystalline cells utilizing deep structures on both wafer sides [solar cells]

A new light trapping structure is presented with trapping capabilities comparable to or better than those of the perpendicular grooves structure. The new structure traps a larger fraction of rays for 8-80 passes than the perpendicular grooves structure. The average path length enhancement is about 62 times the average thickness. The structure consists of deep (-200 /spl mu/m) inverted pyramids on the front side and deep (-200 /spl mu/m) truncated pyramids with eight sides on the back. The structure is realized in crystalline silicon by wet chemical etching using potassium hydroxide (KOH) and isopropanol (IPA). A process for creating thin solar cells with this light trapping scheme is described. The process includes only two main photolithographic steps and features a self-aligned front metallization. The process uses 250 /spl mu/m wafers to create cells that on average are about 70 /spl mu/m thick.