John C. C. Fan

Van der Waals bonding of GaAs on Pd leads to a permanent, solid‐phase‐topotaxial, metallurgical bond

Various forms of wafer bonding have now emerged as a serious competitor to heteroepitaxy for optoelectronic integration of dissimilar semiconductor materials. Among the types of wafer bonding, perhaps the most flexible is that which employs free‐standing III–V films as created by epitaxial liftoff. For some purposes, weak Van der Waals forces provide an adequate bond between the native oxides of the III–V film and its new substrate. If the substrate is coated by palladium however, a low temperature solid‐phase‐topotaxial reaction occurs, producing oriented Pd4GaAs under the GaAs film. In effect, the topotaxy comes about through mechanical contact alone. The resulting metallurgical bond is an ohmic contact, a thermal contact and a robust, permanent, adherent contact.

Formation of epitaxial Au/Ni/Au ohmic contacts to p-GaN

We have designed a promising contact scheme to p-GaN, where Au/Ni/Au layers are deposited on p-GaN and annealed in air for 30 min at 470 °C to produce low-resistivity ohmic contacts. The Au layer in contact with p-GaN grows epitaxially via domain matching epitaxy, which acts as a template for NiO growth via lattice matching epitaxy. The 〈111〉 oriented gold rotates 30° in the basal (0001) plane of GaN by 30° with the following orientation relationship: [111]Au//[0001]GaN; [112]Au//[2110]GaN. As a result, we can create epitaxial NiO–Au composite, where Au as well as NiO are in contact with p-GaN. This epitaxial composite structure is envisaged to be important in achieving low-resistivity ohmic contacts in p-GaN. We present the details of atomic structure, epitaxial relationship, chemistry, and electrical properties of ohmic contacts.

Effect of thickness variation in high-efficiency InGaN/GaN light-emitting diodes

InxGa(1−x)N/GaN multiquantum-well light-emitting diodes (LEDs) having periodic thickness variations (TVs) in InxGa(1−x)N active layers exhibit substantially higher optical efficiency than LEDs with uniform InxGa(1−x)N layers. In these nanostructured LEDs, the thickness variation of the active layers is found to be more important than the In composition fluctuation in quantum confinement of excitons (carriers). Detailed scanning transmission electron microscopy-atomic number Z contrast analysis, where image contrast is proportional to Z2 (Z being the atomic number), was carried out to investigate the variation in thickness as well as the spatial distribution of In. In the nanostructured LEDs, there are short-range thickness variations (SR-TVs) (3–4 nm) and long-range thickness variations (LR-TVs) (50–100 nm) in InxGa(1−x)N layers. It is envisaged that LR-TV is key to quantum confinement of the carriers and enhancement of the optical efficiency. We propose that the LR-TV is caused by two-dimensional strain ...

Characteristics of nucleation layer and epitaxy in GaN/sapphire heterostructures

We present the details of GaN nucleation layer grown on (0001) sapphire substrates below 600°C by metal organic chemical vapor deposition. These films have cubic (c-GaN) zinc blende structure which starts to transform into a hexagonal (h-GaN) wurtzite structure upon annealing around 650°C and above. The films deposited above 700°C by pulsed laser deposition directly on sapphire substrate showed the wurtzite structure. Both c-GaN and h-GaN films grow epitaxially on (0001) sapphire substrates via domain matching epitaxy, where integral multiples of planes match across the film-substrate interface. The c-GaN has the following epitaxial relationship: ⟨111⟩c-GaN‖⟨0001⟩sap, ⟨110⟩c-GaN‖⟨10-10⟩sap, and ⟨211⟩c-GaN‖⟨−2110⟩sap. In terms of planar matching, (220) planes of c-GaN match with (30-30) planes of sapphire, and 1∕3(4¯2¯2¯) planes of c-GaN match with (−2110) planes of sapphire in the perpendicular direction. The transformation from c-GaN into h-GaN involves the transformation of (220) planes of c-GaN into (−...

A high quantum efficiency GaInAs‐InP photodetector‐on‐silicon substrate

We report the first Ga0.47In0.53As‐InP photodetectors fabricated on silicon substrates, using the low‐pressure metalorganic chemical vapor deposition growth technique. Quantum efficiencies of 0.34, 0.88, and 0.92, A/W at wavelengths of 0.8, 1.3, and 1.55 μm, respectively, have been obtained for the devices without antireflection coating. A reverse leakage current of 100 μA at −10 V bias was measured for 250‐μm‐diam diodes.

Monolithic integration of a light‐emitting diode array and a silicon circuit using transfer processes

This letter reports the attainment of a monolithically integrated light‐emitting diode array on a silicon integrated circuit. The emitters are first formed epitaxially on a lattice‐matched substrate and are subsequently transferred to the silicon. Interconnections are made using thin‐film techniques between the 128 separately addressable light‐emitting diodes and the driver circuit. This work demonstrates attainment of a high level of optoelectronic/logic integration.

Room-temperature continuous operation of GaAs/AlGaAS lasers grown on Si by organometallic vapor-phase epitaxy

Graded‐index separate‐confinement heterostructure single‐quantum‐well GaAs/AlGaAs diode lasers exhibiting continuous (cw) operation at room temperature have been grown on a Si substrate by organometallic vapor‐phase epitaxy, without the use of molecular‐beam epitaxy. To improve the quality of the laser structure, a defect‐filtering layer was incorporated between this structure and a GaAs buffer layer about 1.5 μm thick grown on the substrate. Of four types of defect‐filtering layers investigated, the most effective was one grown with thermal cycling, which made it possible to obtain pulsed threshold current densities as low as 350 A/cm2 for broad‐stripe lasers with a cavity length of 500 μm. Ridge‐waveguide lasers with this type of defect‐filtering layer have exhibited cw threshold currents as low as 25 mA and a differential quantum efficiency of 55%.

Multijunction cells for space applications

Abstract Recent efforts to improve space solar cell efficiency have involved development of various types of multijunction tandem solar cells. This paper reviews the limits to the efficiency of present-day designs and the most recent results from many active investigations involving silicon, germanium, CuInSe2 and III–V materials.

Initial growth of gallium arsenide on silicon by organometallic vapor phase epitaxy

Thin films (less than 1000 nm) of heteroepitaxial GaAs on Si were grown by organometallic vapor phase epitaxy with the intent of comparing the initial stages of growth with the authors’ previous experience in examining films grown by molecular beam epitaxy (MBE). The films were found to be epitaxial after 10 nm of growth at 425 °C, and to uniformly cover the substrate completely, unlike the asymmetrical island nature of MBE films grown under comparable conditions. The relaxation of strain was found to be quite similar to the MBE case.

Thin-film solar cells with over 21% conversion efficiency

Abstract Thin-film solar cells have received a great deal of attention for use in terrestrial flat-plate systems. The large-area capability and effective use of solar absorbing materials gives these cells significant advantages over thin-film cells for low-cost systems. Our approach has been to use a separable film technology, the CLEFT process, to produce thin-film GaAs with state-of-the-art heterostructures capable of very high efficiencies. We have fabricated 4-cm 2 thin-film GaAs cells with conversion efficiencies as high as 21.5% one sun AM1.5 global. These results and our effort to increase cell area and reduce cell costs are discussed.