Hybrid mocvd/mbe epitaxial growth of high-efficiency lattice-matched multijunction solar cells

Abstract

Semiconductor devices and methods of fabricating semiconductor devices having a dilute nitride layer and at least one semiconductor material overlying the dilute nitride layer are disclosed. Hybrid epitaxial growth and the use of aluminum barrier layers to minimize hydrogen diffusion into the dilute nitride layer are used to fabricate high-efficiency multijunction solar cells.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a divisional of U.S. patent application Ser. No. 16 / 018,917 filed Jun. 26, 2018, and published on Jan. 10, 2019 as U.S. Patent Publication No. US-2019-0013429-A1 which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 62 / 529,214, filed on Jul. 6, 2017, which are incorporated by reference in their entirety.FIELD[0002]The present invention relates to semiconductor devices and to methods of fabricating semiconductor devices having a dilute nitride layer and at least one semiconductor material overlying the dilute nitride layer. Particularly, the present invention relates to hybrid epitaxial growth of high-efficiency multijunction solar cells.BACKGROUND[0003]Epitaxial growth of III-V materials is a cornerstone technology for the wireless, optical and photovoltaic industries. Structures such as pseudomorphic high electron mobility transistors (PHEMTs), heterojunction bipolar transistors (HBTs),...

AI Technical Summary

Benefits of technology

This approach minimizes surface contamination and defects, resulting in high-efficiency semiconductor devices with improved material quality and reliability, particularly in multijunction solar cells, by maintaining the integrity of dilute nitride layers and reducing hydrogen-related defects.

Problems solved by technology

It is difficult to incorporate a sufficient mole fraction of nitrogen by MOCVD into the lattice of epilayers.

MBE, on the other hand, involves longer maintenance periods and has setup variability limitations, for example, when growing phosphorous-containing epitaxial layers.

Use of sacrificial layers as protective or cap layers to be etched away prior to subsequent growth steps is inefficient, especially in high-volume production.

However, there are additional defects created by Sb and therefore it is desirable that the total concentration of Sb be limited to no more than 20 percent of the Group V lattice sites.

An undoped material can have a concentration of dopants that are intrinsic to the deposition process and can result, for example, from impurities in the materials being deposited, background contaminants in the system, or dopants that are undesired artifacts of the deposition process.

In addition, there will be lower series resistance losses in these multijunction solar cells compared with other solar cells due to the lower operating currents.

At higher concentrations of sunlight, the reduced series resistance losses become more pronounced.

Solar cells containing metamorphic buffer layers may have reliability concerns due to the potential for dislocations originating in the buffer layers to propagate over time into the subcells, causing degradation in performance.

It should be appreciated, however, that because of the complex interdependence of the GaInNAsSb material composition and the processing parameters it cannot necessarily be determined which combinations of materials and processing conditions will produce suitable high efficiency subcells having a particular band gap.

Light absorbed by tunnel junctions is not converted into electricity by the solar cell, and thus if the tunnel junctions absorb significant amounts of light, it will not be possible for the efficiencies of the multijunction solar cells to exceed those of the best multijunction junction solar cells.

However, at both of these operating points, the power from the solar cell is zero.

Fewer solar devices are then needed to generate the same amount of output power, and higher output power with fewer devices leads to reduced system costs, such as costs for mounting racks, hardware, wiring for electrical connections, etc.

Lighter weight and smaller solar modules are always preferred because the lifting cost to launch satellites into orbit is super expensive.

One important problem to solve in making hybrid epitaxy viable is the potential oxidation or contamination of exposed interface layers as epitaxial growth is interrupted and epi-wafers are moved from one reactor to another.

Any imperfections at the interface where growth resumes will result in poor overgrown epitaxial material.

Thermal treatment can damage the surface morphology of a dilute nitride J3 subcell, which has to be of sufficient quality for additional epitaxial growth in the MOCVD reactor.

An increase in haze post-thermal treatment is not uncommon and is indicative of structural defects.

These defects can propagate throughout the device structure and thereby decrease device performance.

Although it is common practice to liberate absorbed hydrogen by applying thermal treatment post-epitaxial growth (FIGS. 1B-1D, and 10), doing so can further worsen the doping profiles already altered by hydrogen diffusion, resulting in even poorer device performance.

Hydrogen diffusion from MOCVD growth is also known to cause effects such as dopant passivation-compensation, introduction of isolated donors, and may cause other defects such as complex defects of nitrogen and hydrogen.

These effects can change the doping profile of the dilute nitride material, resulting in degradation of the electrical and optical performance of a sub-cell.

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