N. Medelci

InP-based quantum well solar cells grown by chemical beam epitaxy

We present the first experimental investigation of InP based-multiquantum well (MQW) solar cells. Lattice matched In/sub 0.47/Ga/sub 0.53/As/InP and strained InAs/sub x/P/sub 1-x//InP (x=0.3 to 0.7) multiquantum wells were introduced in the intrinsic region of a p-i(MQW)-n InP solar cell. All device structures (MQW and InP p and n layer) were grown by chemical beam epitaxy.

Metal–insulator–semiconductor Schottky barrier structures fabricated using interfacial BN layers grown on GaN and SiC for optoelectronic device applications

Metal–insulator–semiconductor Schottky barrier structures on GaN and 6H-SiC using highly stable interfacial BN layers have been investigated. As reported earlier, such structures fabricated with Ti contacts are capable of withstanding up to 350 and 600 °C when based on 6H-SiC and GaN, respectively. In this work we fabricated diode structures using interfacial BN layers and optically transparent Au contacts. Visible–blind photosensitive structures on GaN and 6H-SiC and prebreakdown light-emitting diode (LED) structures on 6H-SiC have been characterized. The radiant and spectral sensitivities of the photosensitive structures were measured in the range of 200–400 nm. The potential barrier heights determined from photoresponse measurements were 2.7 and 2.88 eV for GaN- and SiC-based samples, respectively. The spectrum of 6H-SiC and p-GaN-based prebreakdown LED structures measured through transparent Au electrodes extended to the ultraviolet region. The optical emission power of the 6H-SiC-based LED structures...

Photoenhanced reactive ion etching of III–V nitrides in BCl3/Cl2/Ar/N2 plasmas

Gallium nitride (GaN) and boron nitride (BN) are known as superior semiconductor materials for UV optoelectronic and high power, high temperature applications. As a result of their high molecular bond strength these materials are extremely difficult to etch. In this article, reactive ion etching (RIE) tests were performed on GaN and BN thin films using respectively BCl3/Cl2/Ar and Cl2/Ar chemistries. In order to improve the etch rates at lower rf powers and thus reduce ion bombardment-induced damage, a photoassisted RIE process was investigated. The same plasma chemistries in combination with a xenon arc lamp were utilized. In an attempt to minimize surface nitrogen depletion, N2 was used instead of Ar as dilution gas. Photoenhancement was observed for both GaN and BN etching. As expected, the etch rate increased with rf power for both unassisted and photoassisted etching conditions. The combination of illumination and N2 led to a nitrogen-rich surface for GaN. In the case of BN, photoassisted etching in ...

Very high peak current CBE grown InGaAs tunnel junction for InP/InGaAs tandem cells fabricated on InP, GaAs, and Si substrates

For the first time high quality In/sub 0.53/Ga/sub 0.47/As tunnel junctions were grown at relatively low temperatures (450/spl deg/C to 530/spl deg/C) using chemical beam epitaxy (CBE). p/sup ++n/sup ++/ In/sub 0.53/Ga/sub 0.47/As tunnel junctions were realized on InP, GaAs, and GaAs/Si substrates. Homoepitaxial devices (grown on InP) exhibit excellent room temperature I-V characteristics. Peak current density exceeding 1000 A.cm/sup -2/ with specific resistivities in the range of 10/sup -4/ /spl Omega/.cm/sup 2/ were obtained. Heteroepitaxial devices fabricated on GaAs (4% lattice mismatch) and Si (8% lattice mismatch) exhibit peak current densities similar to their homoepitaxial counterparts. Finally no degradation of the tunnel device performance is observed after the subsequent CBE growth of state of the art InP solar cells.<<ETX>>

Fabrication and characterization of 2.3eV InGaN photovoltaic devices

In this work we present the design, fabrication and characterization of 2.3 eV InGaN-based solar cells confirming the feasibility of high indium content III-Nitride materials for photovoltaics. Growth of single phase InxGa1-xN for x up to 0.4 is achieved using Molecular Beam Epitaxy (MBE) with flux modulation for active species. The material is characterized by x-ray diffraction and photoluminescence to quantify the layer quality and indium content, while the optical properties are characterized by studying spectral response and absorption. The fabricated devices are then studied for photo-response under AM0 and intense UV spectral conditions to evaluate solar cell characteristics. The dark and illuminated I–V results indicate the existence of significant shunt and series resistances. The causes of these behaviors are known and ultimately resolvable.

High performance In/sub 0.47/Ga/sub 0.53/As tunnel junctions grown by chemical beam epitaxy on InP and GaAs and Si substrates

High performance In/sub 0.47/Ga/sub 0.53/As tunnel junctions were successfully realized using chemical beam epitaxy on InP, GaAs and GaAs/Si substrates. The achievement of high performance tunnel junctions, besides extremely high hole and electron concentrations requires low interdiffusion of dopant species in the narrow space-charge region of the junction during the device growth procedure. Therefore a relatively low temperature growth process and a good control of interface properties is required. Using chemical beam epitaxy (CBE) high quality InP and InGaAs can be grown at temperatures substantially lower than those used in more conventional liquid phase epitaxy and metalorganic vapor phase epitaxy techniques rendering CBE specially attractive for such tunnel junction fabrication. The peak currents obtained on samples grown on InP substrates are the highest ever reported for such tunnel junctions. These results demonstrate that CBE is perfectly suited for fabrication of InP/InGaAs tandem solar cells.<<ETX>>

Boron carbon nitride materials for tribological and high temperature device applications

We have used ion (or neutral) and electron cyclotron resonance assisted physical vapor deposition to produce high quality BN/CN thin films on Si and sapphire substrates. We have already demonstrated deposition of films containing a high fraction of the metastable c-BN phase as determined by FTIR spectroscopy and transmission electron microscopy (TEM). Atomic force microscopy (AFM) measurements have shown our films to have an excellent rms roughness ∼10.5 A (which is better than the best CVD diamond thin films). Results from frictional force microscopy (FFM) from BN and BCN films show a direct correlation with surface N content. Preliminary results show friction properties superior to that of TiN (the standard in the thin film coatings industry). Hardness measurements on the same films yielded Knoop hardness (KH) values of ∼3350 kg/mm2, close to that of ceramic c-BN (3500 kg/mm2). In this paper we will present our results on the synthesis of these materials and discuss their hardness and tribological prope...

Single and multijunction InP‐based photovoltaic devices for space applications

InP‐based multijunction tandem solar cells show great promise for high conversion efficiency and high radiation resistance. In this work, we present results on an InGaAs solar cell with a very high current density, 60 mA/cm2 (the highest ever reported), and an efficiency (η) of 10.2% under natural sunlight, and InP solar cell with η=18% under AM1 simulator, and InGaAs tunnel junctions with peak current densities exceeding 1000 A/cm2. Preliminary results on an InP/InGaAs tandem solar cell show the possibility of no interconnect voltage loss and that a patterned tunnel junction is necessary to allow current matching in the tandem by avoiding photons absorption in the InGaAs tunnel junction. Finally, our preliminary investigation on new multiquantum well InP cells indicates the strength of this alternative approach.

First time demonstration of InP p/sup ++//n/sup ++/ tunnel junction

For the first time an InP p/sup ++//n/sup ++/ tunnel junction is demonstrated. InP tunnel diodes with peak current densities up to 1600 A/cm/sup 2/ and maximum specific resistivities (Vp/1p-peak voltage to peak current ratio) in the range of 10/sup -4/ /spl Omega/.cm/sup 2/ were obtained. This high peak current density is comparable to the highest results previously reported for their lattice matched In/sub 0.53/Ga/sub 0.47/As counterparts. This achievement is very important for InP/InGaAs tandem solar cell design due to the very high peak current density which allows concentrator applications and also because the tunnel junction material being the same as the tandems top solar cell avoids the optical losses observed when using In/sub 0.53/Ga/sub 0.47/As tunnel junctions. In this paper we discuss the device characteristics and the influence of the growth conditions on its performance.

High-efficiency p/n In/sub 0.53/Ga/sub 0.47/As solar cell for tandem applications

InP-based multijunction tandem solar cells show great promise for high conversion efficiency (/spl eta/) and high radiation resistance. InP and its related ternary and quaternary compound semiconductors such InGaAs and InGaAsP offer desirable combinations of energy bandgap values which are very suitable for multijunction tandem solar cell applications. In this work we present results on an InGaAs solar cell with a very high current density 60 mA/cm/sup 2/ (the highest ever reported) with a /spl eta/=10.2% under natural sunlight, an InP solar cell with /spl eta/=13% under natural sunlight without anti-reflection coating, and preliminary results about a tandem InP/InGaAs solar cell. It is shown that a patterned tunnel junction is necessary to allow current matching in the tandem by avoiding photons absorption in the InGaAs tunnel junction.<<ETX>>