Paul R. Berger

Role of strain and growth conditions on the growth front profile of InxGa1−xAs on GaAs during the pseudomorphic growth regime

Theoretical and experimental studies are presented to understand the initial stages of growth of InGaAs on GaAs. Thermodynamic considerations show that, as strain increases, the free‐energy minimum surface of the epilayer is not atomically flat, but three‐dimensional in form. Since by altering growth conditions the strained epilayer can be grown near equilibrium or far from equilibrium, the effect of strain on growth modes can be studied. In situ reflection high‐energy electron diffraction studies are carried out to study the growth modes and surface lattice spacing before the onset of dislocations. The surface lattice constant does not change abruptly from that of the substrate to that of the epilayer at the critical thickness, but changes monotonically. These observations are consistent with the simple thermodynamic considerations presented.

Room temperature operation of epitaxially grown Si/Si0.5Ge0.5/Si resonant interband tunneling diodes

Resonant interband tunneling diodes on silicon substrates are demonstrated using a Si/Si0.5Ge0.5/Si heterostructure grown by low temperature molecular beam epitaxy which utilized both a central intrinsic spacer and δ-doped injectors. A low substrate temperature of 370 °C was used during growth to ensure a high level of dopant incorporation. A B δ-doping spike lowered the barrier for holes to populate the quantum well at the valence band discontinuity, and an Sb δ-doping reduces the doping requirement of the n-type bulk Si by producing a deep n+ well. Samples studied from the as-grown wafers showed no evidence of negative differential resistance (NDR). The effect of postgrowth rapid thermal annealing temperature was studied on tunnel diode properties. Samples which underwent heat treatment at 700 and 800 °C for 1 min, in contrast, exhibited NDR behavior. The peak-to-valley current ratio (PVCR) and peak current density of the tunnel diodes were found to depend strongly on δ-doping placement and on the annea...

Nanometer-period gratings in hydrogen silsesquioxane fabricated by electron beam lithography

Hydrogen silsesquioxane (HSQ) is a high-resolution negative-tone inorganic resist for electron beam lithography. Investigations on the smoothness of the surfaces of thin films (less than 100 nm thick) have been conducted for nanolithography applications. It is demonstrated that films at thicknesses down to 25 nm have very low rms roughness and are defect free. Using 50 kV electron beam lithography, we demonstrate the achievement of isolated 6-nm-wide lines and 27 nm period gratings in 30 nm HSQ films on silicon substrates. These results are superior to those achieved with poly(methylmethacrylate) resist and demonstrates the versatility of HSQ for nanolithography.

4.8% efficient poly(3-hexylthiophene)-fullerene derivative (1:0.8) bulk heterojunction photovoltaic devices with plasma treated AgOx/indium tin oxide anode modification

We report here an improved efficiency, up to 4.8% with a high fill factor of ∼63% under AM 1.5G spectral illumination and 100mW∕cm2 intensity, for poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester bulk heterojunction photovoltaic (PV) devices with a 1:0.8 weight ratio using surface modifications to the indium tin oxide (ITO) anodes through plasma oxidized silver. Here, an enhanced short-circuit current density was achieved without significant loss in the open-circuit voltage (>0.6V) nor the fill factor (>63%), leading to an efficiency jump from 4.4% in the control devices to 4.8% with the surface modified ITO anode. The enhanced short-circuit density is attributed to an interface energy step between the ITO and the polymer hole transporting layer. It has been theorized that the introduction of an interface energy step could alter the charge collection efficiency, resulting in an improved overall efficiency in PV devices. In our study, the current density–voltage characteristics under d...

Photoresponsivity of polymer thin-film transistors based on polyphenyleneethynylene derivative with improved hole injection

The photoresponse of polymer field-effect transistors (PFETs) based on the 2,5-bis(dibutylaminostyryl)-1,4-phenylene-b-alkyne-b-1,4-bis(2-ethylhexyl)benzene terpolymer (BAS-PPE) is investigated. BAS-PPE is a photoluminescent conducting polymer with a band gap of 2.25eV. The BAS-PPE PFETs were fabricated using an open coplanar configuration and light is illuminated onto the top side of the PFETs with no shadowing present. A sweep of VDS demonstrates that IDS saturation is suppressed during illumination, which suggests that pinch-off cannot be reached since the injected photogenerated carriers continue unabated. Also, with incident light, the channel cannot be turned off, even at high positive gate biases, due to the accumulation of photogenerated carriers. A sweep of VDS shows that BAS-PPE can act as a p-type polymer and favors hole injection and transport. A sweep of VGS shows an increase in IDS with different light intensities. The Ilight∕Idark ratio reaches as high as about 6000 at an incident light int...

Diffusion barrier cladding in Si/SiGe resonant interband tunneling diodes and their patterned growth on PMOS source/drain regions

Si/SiGe resonant interband tunnel diodes (RITDs) employing /spl delta/-doping spikes that demonstrate negative differential resistance (NDR) at room temperature are presented. Efforts have focused on improving the tunnel diode peak-to-valley current ratio (PVCR) figure-of-merit, as well as addressing issues of manufacturability and CMOS integration. Thin SiGe layers sandwiching the B /spl delta/-doping spike used to suppress B out-diffusion are discussed. A room-temperature PVCR of 3.6 was measured with a peak current density of 0.3 kA/cm/sup 2/. Results clearly show that by introducing SiGe layers to clad the B /spl delta/-doping layer, B diffusion is suppressed during post-growth annealing, which raises the thermal budget. A higher RTA temperature appears to be more effective in reducing defects and results in a lower valley current and higher PVCR. RITDs grown by selective area molecular beam epitaxy (MBE) have been realized inside of low-temperature oxide openings, with performance comparable with RITDs grown on bulk substrates.

Current-voltage characteristics of high current density silicon Esaki diodes grown by molecular beam epitaxy and the influence of thermal annealing

We present the characteristics of uniformly doped silicon Esaki tunnel diodes grown by low temperature molecular beam epitaxy (T/sub growth/=275/spl deg/C) using in situ boron and phosphorus doping. The effects of ex situ thermal annealing are presented for temperatures between 640 and 800/spl deg/C. A maximum peak to valley current ratio (PVCR) of 1.47 was obtained at the optimum annealing temperature of 680/spl deg/C for 1 min. Peak and valley (excess) currents decreased more than two orders of magnitude as annealing temperatures and times were increased with rates empirically determined to have thermal activation energies of 2.2 and 2.4 eV respectively. The decrease in current density is attributed to widening of the tunneling barrier due to the diffusion of phosphorus and boron. A peak current density of 47 kA/cm/sup 2/ (PVCR=1.3) was achieved and is the highest reported current density for a Si-based Esaki diode (grown by either epitaxy or by alloying). The temperature dependence of the current voltage characteristics of a Si Esaki diode in the range from 4.2 to 325 K indicated that both the peak current and the excess current are dominated by quantum mechanical tunneling rather than by recombination. The temperature dependence of the peak and valley currents is due to the band gap dependence of the tunneling probability.

Tri-state logic using vertically integrated Si-SiGe resonant interband tunneling diodes with double NDR

A vertically integrated npnp Si-based resonant interband tunneling diode (RITD) pair is realized with low-temperature molecular beam epitaxy by stacking two RITDs with a connecting backward diode between them. The current-voltage characteristics of the vertically integrated RITD pair demonstrates two sequential negative differential resistance regions in the forward-biasing condition. Tri-state logic is demonstrated by using the vertically integrated RITDs as the drive and an off-chip resistor as the load.

Low defect densities in molecular beam epitaxial GaAs achieved by isoelectronic In doping

We have studied the effects of adding small amounts of In (0.2–1.2%) to GaAs grown by molecular beam epitaxy. The density of four electron traps decreases in concentration by an order of magnitude, and the peak intensities of prominent emissions in the excitonic spectra are reduced with increase in In content. Based on the higher surface migration rate of In, compared to Ga, at the growth temperatures it is apparent that the traps and the excitonic transitions are related to point defects. This agrees with earlier observations by F. Briones and D. M. Collins [J. Electron. Mater. 11, 847 (1982)] and B. J. Skromme, S. S. Bose, B. Lee, T. S. Low, T. R. Lepkowski, R‐Y. DeJule, G. E. Stillman, and J. C. M. Hwang [J. Appl. Phys. 58, 4702 (1985)].

Room-temperature negative differential resistance in polymer tunnel diodes using a thin oxide layer and demonstration of threshold logic

Conjugated polymers, with π molecular orbitals delocalized along the polymer chain, are useful organic semiconductors that provide the possibility of molecular electronics for low-power organic-based memory and logic. Quantum functional devices based upon carrier tunneling processes open vistas into very efficient and low-power consumption circuitry that would be ideal for these applications. We demonstrate here strong room temperature negative differential resistance (NDR) for poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) polymer tunnel diodes (PTD) using a thin TiO2 tunneling layer (∼2–8nm) sandwiched between the MEH-PPV and the indium tin oxide anode. A key advantage is the pronounced NDR using a thick polymer layer with a large active area, circumnavigating the need for molecularly-sized junctions. Current-voltage measurements show large and reproducible NDR with a PVCR as high as 53 at room temperature. We also demonstrate basic logic circuit operation using a pair of these PT...