GaAs Homojunctions Research Articles

Effect of dopant density on contact potential difference across n-type GaAs homojunctions using Kelvin Probe Force Microscopy

In this study, cross-sectional surface potential imaging of n+/semi-insulating GaAs junctions is investigated by using amplitude mode kelvin probe force microscopy. The measurements have shown two different potential profiles, related to the difference in surface potential between the semi-insulating (SI) substrate and the epilayers. It is shown that the contact potential difference (CPD) between the tip and the sample is higher on the semi-insulating substrate side than on the n-type epilayer side. This change in CPD across the interface has been explained by means of energy band diagrams indicating the relative Fermi level positions. In addition, it has also been found that the CPD values across the interface are much smaller than the calculated values (on average about 25% of the theoretical values) and increase with the electron density. Therefore, the results presented in study are only in qualitative agreement with the theory.

Electronic and thermoelectric properties of nanograting layers

Recently, new quantum features have been observed and studied in the area of nanostructured layers. Nanograting (NG) on the surface of the thin layer imposes additional boundary conditions on electron wave function and forbids some quantum states. Electrons, rejected from the forbidden quantum states, have to occupy states with higher energy. In the case of semiconductor materials, electrons rejected from the valence band have to occupy empty quantum states in the conduction band. Such increase in conduction band electron concentration n can be termed as geometry-induced doping or G-doping. G-doping is equivalent to donor doping from the point of view of the increase in n. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility. G-doping involves electron confinement to NG layer. Here, we investigate the system of multiple NG layers forming a series of homo junctions. Si and GaAs homojunctions were studied and G-doping levels of 1018-1019 cm−3 were obtained. We also study a system composed of NG layer and an additional top layer forming periodic series of p-n junctions. In such system, charge depletion region develops inside the NG and its effective depth reduces, becoming a rather strong function of temperature T. Consequently, T-dependence of chemical potential magnifies and Seebeck coefficient S increases. We investigate S in the system of semiconductor NG layer having abrupt p-n junctions on the top of the grating. Analysis made on the basis of Boltzmann transport equations shows dramatic increase in S. At the same time, other transport coefficients remain unaffected by the junctions. Calculations show one order of magnitude increase in the thermoelectric figure of merit ZT relative to bulk material.

Geometry-induced electron doping in periodic semiconductor nanostructures

Recently, new quantum features have been observed and studied in the area of nanostructured layers. Nanograting on the surface of the thin layer imposes additional boundary conditions on the electron wave function and induces G-doping or geometry doping. G-doping is equivalent to donor doping from the point of view of the increase in electron concentration n. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility with respect to the donor-doped layer. G-doping involves electron confinement to the nanograting layer. Here, we investigate the system of multiple nanograting layers forming a series of hetero- or homojunctions. The system includes main and barrier layers. In the case of heterojunctions, both types of layers were G-doped. In the case of homojunctions, main layers were G-doped and barrier layers were donor-doped. In such systems, the dependence of n on layer geometry and material parameters was analysed. Si and GaAs homojunctions and GaAs/AlGaAs, Si/SiGe, GaInP/AlGAs, and InP/InAlAs heterojunctions were studied. G-doping levels of 1018–1019cm−3 were obtained in homojunctions and type II heterojunctions. High G-doping levels were attained only when the difference between band gap values was low.

Enhanced light emission in nanostructures

The enhancement of light emissive processes in different quantum nanometric systems is presented in this review. The plasmonic enhanced photoluminescence (fluorescence) of metals and metal nanoparticles, molecules and semiconductor nanostructures, as well as surface-enhanced Raman scattering is initially considered. The enhancement of excitonic photoluminescence intensity in semiconductor confined systems such as quantum wells, quantum wires and quantum dots, and microcavities are then discussed. Finally, the experimental results of the enhanced exciton photoluminescence from GaAs homojunctions, δ-doped GaAs structures, GaAs/AlGaAs selectively doped and AlInN/GaN heterostuctures is presented. These results can be applied to enhance the emission of light-emitting diodes, as well as to increase the efficiency of solar cells.

The role of setback layers on the breakdown characteristics of AlGaAs/GaAs/GaN HBTs

AbstractBreakdown voltages (VCBO) of AlGaAs/GaAs/GaAs and AlGaAs/GaAs/GaAs‐setback/GaN HBTs have been compared both theoretically and experimentally with respect to setback layer thicknesses and the doping type. VCBO was calculated using a non‐local energy model. The hard breakdown voltage was measured on as‐grown GaAs homojunctions and AlGaAs/GaAs/GaN HBTs formed by direct wafer fusion. The calculation showed an increase of VCBO from ∼ 20 V to ∼ 325 V by replacing the GaAs collector with GaN, and VCBO ∼90 V was indeed measured. The smaller than predicted VCBO is attributed to the large leakage current currently present in the fused junctions. Insertion of a lightly doped GaAs setback layer has resulted in improved current gain of the fused HBTs, but it also degrades the breakdown characteristics. 20‐30 nm setback layers were found to maintain VCBO significantly higher than that of GaAs homojunctions and likely reasonable current gains. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cross sectional studies of buried semiconductor interfaces by means of photoemission microscopy

An important application of photoemission spectromicroscopy would be to measure heterostructures and semiconductor devices in cross section to directly determine band offsets and band bending. We present here studies of p-n GaAs homojunctions and Al/GaAs Schottky junctions fabricated by molecular-beam epitaxy. Our results suggest that a minimum experimental uncertainty of about 0.15 eV will effect band offset determination. In general, useful quantitative information on the junction electrostatics can be obtained provided that the experimental data are analyzed to substract the diffuse photon background and take into account the intensity profile of the photon spot.

PHOTOEMISSION MICROSCOPY INVESTIGATION OF BURIED p–n GaAs HOMOJUNCTIONS AND Al/n-GaAs SCHOTTKY BARRIERS

A natural application of the emerging technique of photoemission microscopy to the study of semiconductor interfaces involves measuring a device in cross section to directly determine heterojunction parameters. We present here results on p–n GaAs homojunctions, which served as a prototype system to demonstrate the applicability of this technique to buried semiconductor interfaces. We also describe preliminary measurements of the electrostatic potential profile across Al/GaAs Schottky junctions.

Internal photoemission studies of artificial band discontinuities at buried GaAs(100)/GaAs(100) homojunctions

Internal photoemission phototransport measurements revealed 0.27±0.04 eV conduction- and valence-band discontinuities induced by a Si intralayer at p-GaAs(100)/n-GaAs(100) homojunctions. The interface dipole originating from the heterovalent character of the Si—GaAs bonds raises the bands of the GaAs overlayer above that of the GaAs substrate.

Artificial band discontinuities at GaAs homojunctions.

Valence- and conduction-band offsets can be induced at GaAs(100) polar homojunctions by means of ultrathin Si intralayers. The microscopic interface dipole responsible for the creation of such offsets can be varied by changing the Si intralayer thickness; the consequently variable band discontinuities represent a tunable parameter with potential applications in band-gap engineering. Even though the existence and the direction of these discontinuities are in agreement with theoretical predictions, the existing models overestimate the amount of the offset and cannot explain its dependence on the intralayer thickness. The presence of Si outdiffusion can be a possible explanation for this partial disagreement.

Comparative study of the substrate-film interfaces of GaAs grown by two molecular beam epitaxial methods

Carrier concentration profiles in the region of a substrate-film homojunction for n- and p-type GaAs were studied using a capacitance-voltage carrier profiling technique. The GaAs films were grown on GaAs substrates by molecular beam epitaxy (MBE) and by gas-source molecular beam epitaxy (GSMBE). The GaAs homojunctions grown by MBE exhibited a much larger reduction in carrier concentration than those prepared by GSMBE. The highest quality interface, without an observable depletion region, was obtained when an additional hydrogen plasma was used during heat treatment prior to GSMBE growth.

Electrical properties of GaAs homojunctions grown by MOCVD on GaAs and Si substrates

Several techniques, including DLTS, I- V- T, C- V- T, C- T and photoreflectance spectroscopy were used to investigate the electrical properties of GaAs multilayer heteroface p- n junctions epitaxially grown by MOCVD on GaAs or Si substrates in order to study the influence of heteroepitaxy on device properties. A deep level (EL2) was detected for both GaAs/GaAs and GaAs/Si structures. For GaAs/Si, a continuous distribution of trap levels was also observed which could be ascribed to misfit defects. I- V- T data revealed the saturation current of GaAs/Si diodes to be four orders in amplitude higher than for GaAs/GaAs ones. Different temperature dependences were found in saturation current density, barrier height and capacitance between the two structures. A multistep tunneling model is utilized in explaining the results for GaAs/Si. C- T measurements indicated that the free carriers are not frozen-out for GaAs/Si at the observed freeze-out temperature of GaAs/GaAs. Defect-induced bandgap tailing might be responsible for this effect.

Effective minority-carrier hole confinement of Si-doped, n+-n GaAs homojunction barriers

The electrical performance of Si-doped n+-n GaAs homojunction barriers grown by molecular-beam epitaxy (MBE) is characterized and analyzed. We employed a successive etch technique to study hole injection currents in GaAs n+-n-p+ solar cells. The results of the analysis show that minority-carrier holes in our MBE-grown material have a mobility of 293 cm2/V s for an n-type Si-doping level of 1.5×1016 cm−3 at 300 K. The interface recombination velocity for these homojunction barriers is estimated to be less than 1×103 cm/s, and it appears to be comparable to that recently observed for Si-doped n+-n GaAs homojunction barriers grown by metalorganic chemical vapor deposition. We present evidence that these n+-n GaAs homojunctions, unlike p+-p GaAs homojunctions, are almost as effective as AlGaAs heterojunctions in minority-carrier confinement, and that their electrical performance is not degraded by heavy doping effects.

Deep levels and minority carrier lifetime in MOVPE p-type GaAs

Deep levels in p-type GaAs have direct impact on the performance of many devices, such as the hetero-junction transistor (HBT). We have studied the influences of dopant elements (Mg or Zn), growth temperature, and doping concentration on the characteristics of the deep levels in p-GaAs via n+-p GaAs homojunctions grown by metalorganic vapor phase epitaxy (MOVPE). Over various growth conditions, four hole traps and an electron trap with activation energies in the range of 0.18—0.79 eV were measured in GaAs: Mg. Only a single hole trap, which is also detected in GaAs: Mg, has been measured in GaAs: Zn. The characteristics of the deep levels presented in GaAs: Mg depend mainly on the doping concentration of Mg. The possible origins of several of these traps have been identified. Overall, the total concentration in the GaAs: Mg decreases rapidly with doping concentration for p>4×1017 cm−3. Upon comparison, the total deep trap concentration in GaAs: Mg is always greater than in GaAs: Zn, indicating that Zn is a preferred p-type dopant for MOVPE GaAs.

GaAs-AlGaAs tunnel junctions for multigap cascade solar cells

The growth and characterization of tunneling GaAs homojunctions and GaAs-AlGaAs heterojunctions by molecular beam epitaxy for use as optically transparent interconnects in GaAs and AlGaAs solar cells is described. GaAs tunneling interconnects have been achieved with conductances of 300 A/cm2-V at 0.050 V, and p-AlxGa1−x As/n-GaAs heterojunction structures with x up to 0.4 have been grown which have characteristics comparable to GaAs interconnects. Thin interconnects, 1000 Å total thickness, have also been fabricated with conductances comparable to thicker junctions. The effect of dopant diffusion was found to be minimal at the 580 °C junction growth temperature, but annealing these tunnel junctions at 650 °C for several minutes caused diffusion of about 100 Å distance.

Observation of dislocations in GaAs epitaxial layers

Dislocations in GaAs epitaxial layers grown by the temperature difference method of liquid phase epitaxy were observed using X-ray topography and a chemical etching technique. Dislocations in the substrates seem to react with those propagating along the two orthogonal 〈110〉 directions at the interface. The propagation of dislocations from the substrates into the epitaxial layers and the interface lattice distortion of the GaAs homojunctions were investigated using X-ray topography of cleavage surfaces.