A. Torabi

Molecular beam epitaxial growth and device performance of metamorphic high electron mobility transistor structures fabricated on GaAs substrates

Single and double pulse doped metamorphic high electron mobility transistor (MHEMT) structures have been grown on GaAs substrates by molecular beam epitaxy. A linear indium graded buffer layer was used to expand the lattice constant. Transmission electron microscopy cross sections showed planar interfaces. Threading dislocations were not observed along both cleavage directions. For a single pulse doped MHEMT structure with an In0.56Ga0.44As channel layer, the mobilities (10u200a030 cm2/Vu200as at 292 K; 32u200a560 cm2/Vu200as at 77 K) and sheet density (3.2×1012u200acm−2) were nearly equivalent to values obtained for the same structure grown on an InP substrate. Secondary ion mass spectroscopy measurements of a double pulse doped structure indicated no measurable migration of the silicon doping pulses. MHEMT devices with 0.15 μm gates were fabricated, tested, and compared to GaAs pseudomorphic HEMT devices of the same geometries. Above 9 GHz, the MHEMT devices exhibited lower noise figure. From 3 to 26 GHz, the associated ga...

High indium metamorphic HEMT on a GaAs substrate

Metamorphic growth of device structures on GaAs substrates has advanced rapidly in recent years. High quality electronic and optical devices have been demonstrated. Also long-term reliability has been achieved with low noise MHEMT devices. Most of the development emphasis has been with structures conventionally grown on InP substrates. This work is motivated by the lower cost, larger diameter, and greater robustness of GaAs substrates compared to InP substrates. However an important characteristic of metamorphic growth is the degree of freedom in choosing the In/sub x/(GaAl)/sub 1-x/As composition and consequently the lattice constant between GaAs and InAs. Consequently new device structures can be achieved which are not possible by pseudomorphic growth on either GaAs or InP substrates. In this effort, solid source MBE was used to grow metamorphic HEMT structures with high indium content. For the conventional MHEMT, the indium concentration is graded to In/sub 0.52/Al/sub 0.48/As to expand the lattice constant to that of InP. Here the indium content was graded to In/sub 0.64/Al/sub 0.36/As to achieve a larger lattice constant than InP. The resulting surface roughness was examined by AFM. For a 25 /spl mu/m x 25 /spl mu/m area, the RMS roughness was 12/spl Aring/ which is very similar to the roughness present in the conventional MHEMT with less indium content.

Properties of metamorphic materials and device structures on GaAs substrates

In this work, the structural, optical, and electrical properties of metamorphic films are examined and compared to non-metamorphic films. Results for electrical and optical devices are presented. Finally the reliability of metamorphic HEMTs is examined.

Growth and characterization of metamorphic Inx(AlGa)1−xAs/InxGa1−xAs high electron mobility transistor material and devices with X=0.3–0.4

High electron mobility transistor structures containing Inx(AlGa)1−xAs and InxGa1−xAs device layers with X=0.3–0.4 were grown on metamorphic buffer layers on GaAs substrates. The structures exhibited good flatness with a root mean square roughness of 9 A. Cross-sectional transmission electron micrographs indicated that the threading dislocations from metamorphic growth were contained in the graded buffer layer. For double pulse doped structures, sheet densities up to 4×1012u200acm−2 were readily achieved. Room temperature mobilities of 8600–8800 cm2/Vu200as were obtained using In0.42Ga0.58As channel layers. Photoluminescence measurements of the InGaAs channel layer in metamorphic and pseudomorphic structures showed no significant reduction in room temperature luminescence intensity due to metamorphic growth. Transistor structures of the same device geometry were fabricated from both metamorphic and pseudomorphic GaAs high electron mobility transistor structures. At 25 GHz, the metamorphic device produced a 3 dB h...

Metamorphic heterojunction bipolar transistors and P–I–N photodiodes on GaAs substrates prepared by molecular beam epitaxy

Metamorphic heterojunction bipolar transistor (M-HBT) structures and metamorphic P–I–N (M-PIN) photodiode structures were grown on GaAs substrates. A compositionally graded AlGaInAs buffer layer was used to expand the lattice constant to that of InP. Cross-sectional transmission electron micrographs of the M-HBT structure showed that the dislocations from compositional grading were predominantly localized in the buffer layer and that the device layers possessed planar interfaces. Secondary ion mass spectroscopy depth profiles of the 4×1019u200acm−3 beryllium-doped In0.53Ga0.47As base layer exhibited sharp doping interfaces, indicating that metamorphic growth was not causing enhanced beryllium diffusion. The current gain of large emitter M-HBT devices approached the current gain for the same device structure grown on an InP substrate. A P–I–N photodiode structure was also grown metamorphically on a GaAs substrate and lattice matched on an InP substrate. Both types of photodiodes showed almost identical respons...

Reduction of oxygen contamination in InGaP and AlGaInP films grown by solid source molecular beam epitaxy

Oxygen contamination has been observed in In0.5Ga0.5P and (Al0.23Ga0.77)0.5In0.5P films grown by solid source molecular beam epitaxy with elemental phosphorus. Using a conventional P4 cracking zone temperature of 950u2009°C, spike contamination levels as high as 1×1019u2009cm−3 were observed at growth interrupted interfaces with the resultant deactivation of silicon doping pulses. By reducing the phosphorus cracking temperature to 700u2009°C, the oxygen level in InGaP was reduced to below the secondary ion mass spectrometry background level of 3×1016u2009cm−3. No measurable accumulation of oxygen was observed at growth interrupted interfaces and efficient silicon pulse doping was obtained. InGaP films grown at the lower cracking temperature exhibited improved mobilities and enhanced photoluminescence intensities. An oxygen level in (Al0.23Ga0.77)0.5In0.5P of less than 1.5×1017u2009cm−3 was obtained with good mobilities and luminescence. Efficient silicon pulse doping in AlGaInP was demonstrated. The oxygen contamination is i...

Thermodynamic analysis of cation incorporation during molecular beam epitaxy of nitride films using metal-rich growth conditions

The conventional approach to growth of the nitride films GaN, AlN, InN, and their alloys by rf plasma molecular beam epitaxy uses metal-rich surface conditions due to improved material quality compared to nitrogen-rich conditions. The surface metal may incorporate into the growing film, act as a surfactant, and/or react with the underlying film or substrate. Using a simple chemical exchange reaction model and tabulated thermodynamic data at molecular beam epitaxy growth temperatures the predicted preferential incorporation series on the column III site under metal-rich conditions is found to be Al>B,Be,Si, Mg>Ga>In,Fe. This series is consistent with the observed ternary growth behavior and surfactant order. The series is also consistent with silicon migration in AlN but not GaN, sharper beryllium transitions in GaN than AlN, the significant migration of iron in GaN, and the reactivity of AlN nucleation layers with SiC surfaces. The model is used to predict boron incorporation under metal-rich conditions i...

Influence of AlN nucleation layer on the epitaxy of GaN/AlGaN high electron mobility transistor structure and wafer curvature

Significant wafer curvature has been observed for AlGaN/GaN high electron mobility transistor (HEMT) structures grown on SiC substrates by rf plasma molecular-beam epitaxy. The curvature is caused by residual compressive strain in the films, due primarily to the lattice mismatch between substrate and epilayer. The wafers exhibit more bow when an AlN nucleation layer is used, than when GaN/AlGaN is grown directly on SiC. However, in test structures, AlN nucleation layers are found to impart tensile strain in the wafer that is small due to the AlN thickness. Using high resolution x-ray diffraction with reciprocal space maps, thin GaN films are found to relax more readily when grown directly on SiC substrates than on AlN buffer layers. The compressive strain in the thick GaN buffer layer grown on AlN bows the wafer and increases the substrate x-ray diffraction (XRD) linewidth. The GaN buffer, despite its thickness, does not relax fully but retains some residual strain.

Material properties and performance of metamorphic optoelectronic integrated circuits grown by molecular beam epitaxy on GaAs substrates

Solid source molecular beam epitaxy was used to deposit in a continuous process an integrated metamorphic high electron mobility transistor (HEMT) and PIN photodiode structure. A metamorphic buffer layer was first grown on a GaAs substrate to expand the lattice constant to that of In0.53Ga0.47As used in the device layers. The HEMT layers were subsequently grown followed by the PIN diode structure. Cross-sectional and plan-view transmission electron micrographs showed planar layer interfaces and a dislocation density in the device layers of 1×106u2009cm−2. The device characteristics of the HEMT transistors were not adversely affected by growth of the PIN structure on top. Also the bandwidth and responsivity of the metamorphic PIN photodiode were comparable to an InP PIN photodiode with similar dark currents. The integrated HEMT/PIN diode circuit had a 3 dB bandwidth 20% greater than a hybrid combination of devices due to a decrease in parasitic losses from device interconnects. The frequency performances of ci...

Reaction of molecular beam epitaxial grown AlN nucleation layers with SiC substrates

GaN high electron mobility transistor (HEMT) structures containing AlN nucleation layers were grown on SiC substrates by molecular beam epitaxy. Deleterious charge is observed near the GaN∕AlN interface when the AlN layer is grown using aluminum-rich growth conditions which promote AlN material quality. The unwanted charge is correlated with nondestructive mercury probe buffer leakage measurements and degraded capacitance-voltage profiles. Secondary ion mass spectrometry measurements on a HEMT structure with a thick AlN layer grown aluminum rich confirm that the unintentional dopant is silicon which rapidly migrates through the AlN layer to the GaN buffer layer. Leakage current measurements on aluminum-rich AlN layers indicate that the conduction is in the initial GaN layers near the GaN∕AlN interface. It is proposed that under aluminum-rich conditions the excess aluminum present on the growth surface in the liquid state is reacting with the substrate surface resulting in dissolved silicon that rapidly tr...