Peak Tunneling Current Research Articles

High performance p++-AlGaAs/n++-InGaP tunnel junctions for ultra-high concentration photovoltaics.

A p++-AlGaAs: C/n++-InGaP: Te tunnel junction with a record peak tunneling current density of 5518 A/cm2 was developed. This was achieved by inserting a 6.6 Å undoped GaAs quantum well at the junction interface, and the numerical model demonstrated that trap-assisted tunneling contributes to the high peak tunneling current. Furthermore, we found that the p++-AlGaAs: C/n++-InGaP: Si + Te tunnel junctions have lower resistance and better stability than p++-AlGaAs: C/n++-InGaP: Te tunnel junctions in the operating temperature range of the multijunction solar cells, and the peak tunneling current density of the p++-AlGaAs: C/n++-InGaP: Si + Te tunnel junctions excess 3000 A/cm2 with a voltage drop of 7.5 mV at 10000 suns.

N-polar GaN/AlN resonant tunneling diodes

N-polar GaN/AlN resonant tunneling diodes are realized on a single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current–voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8 ± 0.8 kA/cm2 at a forward bias of ∼8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ∼−4 V. These resonant and threshold voltages are well explained with the polarization field, which is opposite to that of the metal-polar counterpart, confirming the N-polarity of the resonant tunneling diodes (RTDs). When the device is biased in the NDC-region, electronic oscillations are generated in the external circuit, attesting to the robustness of the resonant tunneling phenomenon. In contrast to metal-polar RTDs, N-polar structures have the emitter on the top of the resonant tunneling cavity. As a consequence, this device architecture opens up the possibility of seamlessly interfacing—via resonant tunneling injection—a wide range of exotic materials with III-nitride semiconductors, providing a route towards unexplored device physics.

Delta-doped quantum wire tunnel junction for highly concentrated solar cells

We propose a novel structure for tunnel junction based on delta-doped AlGaAs/GaAs quantum wires. Higher spatial confinement of quantum wires alongside the increased effective doping concentration in the delta-doped regions extremely increase the peak tunneling current and enhance the performance of tunnel junction. The proposed structure can be used as tunnel junction in the multijunction solar cells under the highest possible thermodynamically limited solar concentration. The combination of the quantum wire with the delta-doped structure can be of benefit to the solar cells’ advantages including higher number of sub-bands and high degeneracy. Simulation results show a voltage drop of 40 mV due to the proposed tunnel junction used in a multijunction solar cell which presents an extremely low resistance to the achieved peak tunneling current.

Building a Six-Junction Inverted Metamorphic Concentrator Solar Cell

We propose practical six-junction (6J) inverted metamorphic multijunction (IMM) concentrator solar cell designs with the potential to exceed 50% efficiency using moderately high quality junction materials. We demonstrate the top three junctions and their monolithic integration lattice matched to GaAs using 2.1-eV AlGaInP, 1.7-eV AlGaAs or GaInAsP, and 1.4-eV GaAs with external radiative efficiencies >0.1%. We demonstrate tunnel junctions with peak tunneling current >400 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> that are transparent to <;2.1-eV light. We compare the bottom three GaInAs(p) junctions with bandgaps of 1.2, 1.0, and 0.7 eV grown on InP and transparent metamorphic grades with low dislocation densities. The solution to an integration challenge resulting from Zn diffusion in the GaAs junction is illustrated in a five-junction IMM. Excellent 1-sun performance is demonstrated in a complete 6J IMM device with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 5.15 V, and a promising pathway toward >50% efficiency at high concentrations is presented.

Highly strained Esaki tunnel diodes on InP substrate with type-III band alignment

The first of a kind InP-based Esaki tunnel diodes with type-III (broken gap) band alignment are presented. This type is expected to have the highest tunneling current densities at equal doping concentration and structure geometry, compared to homo-, type-I or type-II junctions. The broken gap alignment is achieved by highly strained n-Ga0.2In0.8As/p-GaAs0.2Sb0.8 junction layers. Samples were fabricated by molecular beam epitaxy, the junction doping was varied between 2 × 1018 cm−3 and 2 × 1019 cm−3. Peak current densities up to 34.0 kA cm−2, for a doping of 2 × 1019 cm−3, are reached. The peak tunneling current shows a low dependence on the doping level for the type-III aligned junction layers. For example, highly strained diodes with a doping concentration of 2 × 1018 cm−3 can reach peak tunnel current densities of 3.6 A cm−2, whereas, with the lattice-matched junction, one order of magnitude higher doping is required.

High performance as-grown and annealed high band gap tunnel junctions: Te behavior at the interface

The performance of n+-InGaP(Te)/p+-AlGaAs(C) high band gap tunnel junctions (TJ) is critical for achieving high efficiency in multijunction photovoltaics. Several limitations for as grown and annealed TJ can be attributed to the Te doping of InGaP and its behavior at the junction interface. Te atoms in InGaP tend to get attached at step edges, resulting in a Te memory effect. In this work, we use the peak tunneling current (Jpk) in this TJ as a diagnostic tool to study the behavior of the Te dopant at the TJ interface. Additionally, we used our understanding of Te behavior at the interface, guided by device modeling, to modify the Te source shut-off procedure and the growth rate. These modifications lead to a record performance for both the as-grown (2000 A/cm2) and annealed (1000 A/cm2) high band gap tunnel junction.

Vertical Nanowire Heterojunction Devices Based on a Clean Si/Ge Interface

Different vertical nanowire heterojunction devices were fabricated and tested based on vertical Ge nanowires grown epitaxially at low temperatures on (111) Si substrates with a sharp and clean Si/Ge interface. The nearly ideal Si/Ge heterojuctions with controlled and abrupt doping profiles were verified through material analysis and electrical characterizations. In the nSi/pGe heterojunction diode, an ideality factor of 1.16, subpicoampere reverse saturation current, and rectifying ratio of 10(6) were obtained, while the n+Si/p+Ge structure leads to Esaki tunnel diodes with a high peak tunneling current of 4.57 kA/cm(2) and negative differential resistance at room temperature. The large valence band discontinuity between the Ge and Si in the nanowire heterojunctions was further verified in the p+Si/pGe structure, which shows a rectifying behavior instead of an Ohmic contact and raises an important issue in making Ohmic contacts to heterogeneously integrated materials. A raised Si/Ge structure was further developed using a self-aligned etch process, allowing greater freedom in device design for applications such as the tunneling field-effect transistor (TFET). All measurement data can be well-explained and fitted with theoretical models with known bulk properties, suggesting that the Si/Ge nanowire system offers a very clean heterojunction interface with low defect density, and holds great potential as a platform for future high-density and high-performance electronics.

Double quantum-well tunnel junctions with high peak tunnel currents and low absorption for InP multi-junction solar cells

Lattice matched InAlGaAs tunnel junctions with a 1.18 eV bandgap have been grown for a triple-junction solar cell on InP. By including two InGaAs quantum wells in the structure, a peak tunnel current density of 113 A/cm2 was observed, 45 times greater than the baseline bulk InAlGaAs tunnel junction. The differential resistance of the quantum well device is 7.52 × 10−4 Ω cm2, a 15-fold improvement over the baseline device. The transmission loss to the bottom cell is estimated to be approximately 1.7% and a network simulation demonstrates that quantum well tunnel junctions play a key role in improving performance at high sun-concentrations.

High 5.2 peak-to-valley current ratio in Si/SiGe resonant interband tunnel diodes grown by chemical vapor deposition

Si/SiGe resonant interband tunnel diodes were fabricated using chemical vapor deposition (CVD) on 200-mm diameter p-doped silicon wafers. The resonant interband tunnel diode structure consists of a p+-i-n+ diode that incorporates vapor phase doped δ-doping to enhance quantum mechanical tunneling probability. The tunneling barrier thickness is varied from 2 nm to 8 nm, and a record peak-to-valley current ratio of 5.2 for a CVD process is reported for a 6 nm barrier thickness with a room temperature peak tunneling current of 20 A/cm2. The current density increases exponentially with spacer thickness reduction with a maximum value of 280 A/cm2 for a 2 nm barrier.

Modeling of tunnel junctions for high efficiency solar cells

Ultrahigh efficiency, in the range of 40%, can be achieved in multijunction solar cells operating at high solar concentrations, larger than 100 suns. Critical to this approach are high band gap tunnel junctions that serve as electrically low loss interconnections between the cells. The purpose of this work is to theoretically model such wide band gap tunnel junctions and to explore the advantages of a staggered band line up for improving the peak tunnel current. Theoretical results are calculated for heterojunction diodes made of n+-InGaP/p+-AlGaAs over a range of doping levels. The results illustrate the advantage of a conduction band discontinuity in achieving low interconnect resistance for multijunction solar cells.

Transverse magnetic field dependence of the current-voltage characteristics of double-barrier quantum well tunneling structures

We report the effects of a transverse magnetic field (J⊥B) on the conductivity of quantum well tunneling structures based on AlGaAs/GaAs/AlGaAs quantum wells. The current-voltage characteristics in the positive differential resistance regime show negative magnetoconductance for all values of B. The peak bias voltage increases monotonically with increasing B. For B&amp;lt;6 T there is a decrease in the peak tunneling current, but then it increases for B&amp;gt;6 T. The data also show dramatic magnetic field induced changes in the negative differential resistance (NDR) features. The behavior of the NDR changes from sharp hysteretic bistable-like transitions to astable NDR transitions. Both the valley current and its bias voltage position increase with increasing magnetic field. This behavior is described by a simple model that includes magnetic field effects across the barriers.

A stability criterion for tunnel diode interconnect junctions in cascade solar cells

We have studied the degradation due to high-temperature annealing of the peak current of p +- n + tunnel junctions in GaAs. Such junctions are intended for use as inter-cell ohmic contacts in cascade solar cells. The current degradation is a consequence of the broadening of the tunnel junction space charge caused by dopant diffusion. We show that the peak current is correlated with the dopant concentration profile of the linearly-graded junction. To avoid serious degradation of the peak tunnel current, the product of the larger diffusion coefficient of donor or acceptor and the time at high temperature must be less than ∼ 1 × 10 −12 cm 2 , and the impurity diffusion length shorter than ∼ 10 nm. This criterion is also applicable to materials other than GaAs.

Degradation in Zinc-Doped GaAs Tunnel Diodes

The effects of the voltage swing, peak current-to-capacity ratio (IP/Cjv), zinc concentration, and lithium on the degradation of the peak tunnel current are examined in zinc-doped GaAs tunnel diodes.