J. Lammasniemi

Model of growth of single‐domain GaAs layers on double‐domain Si substrates by molecular beam epitaxy

Growth of single‐domain GaAs (100) layers on double‐domain Si (100) substrates by molecular beam epitaxy has been investigated. It has been shown that domain orientation of the top layer of GaAs depends on the surface structure of a buffer layer. The size of atomic step heights on the Si surface and the As‐Si interaction temperature before film growth are not important factors in controlling domain orientation. Suppression of an antiphase disorder is explained in terms of nonstoichiometric antiphase boundary annihilation operative during growth.

Reduction of surface defects in GaAs grown by molecular beam epitaxy

A method to reduce the density of oval defects originating from pregrowth surface particulates and other contaminants for GaAs layers grown by molecular beam epitaxy (MBE) is presented. It appears that if a thin GaAs buffer layer is deposited by alternately supplying Ga atoms and As4 molecules to a GaAs substrate, prior to further growth by MBE, the density of the oval defects in the final layer is reduced reproducibly by a factor of 7, from about 490 to 70 cm−2, when compared with that obtained using MBE alone under closely similar conditions. The improved surface morphology produced by the pulsed beam method is thought to be related to initial film growth which proceeds likely in a two‐dimensional layer‐by‐layer fashion.

Characteristics of indium phosphide solar cells bonded on silicon

Indium phosphide solar cells were bonded on silicon substrates by applying a eutectic bonding method. The InP solar cell structure was first grown homoepitaxially on an InP substrate. Gold layers were deposited on top of the InP cell structure and on the Si substrate. These two samples were then put in contact with each other and the structure was heated to 400-500/spl deg/C so that eutectic alloys formed between Au and the semiconductor materials. When cooling the structure, these two samples stuck together. The InP substrate was then removed by selective etching. After the bonding process the solar cell devices were processed by using the standard photolithographic methods. These cells exhibited low leakage currents, quantum efficiencies almost identical to homoepitaxial InP solar cells and conversion efficiencies of 12.9% under AMO illumination and 16.5% under AM1.5 illumination, which are relatively high in comparison to heteroepitaxial InP solar cells.<<ETX>>

Majority carrier traps in proton-irradiated GaInP

The majority carrier traps formed in p-GaInP following room temperature irradiation with 3.1 MeV protons have been investigated using deep level transient Fourier spectroscopy. The radiation damage consists of several closely spaced peaks, one of which may have existed in the as-grown material. Energy levels of three of these new traps are reported although in the presence of such closely spaced peaks the energy parameters could only be reliably measured after annealing was used to eliminate shoulder peaks. The spectrum and its annealing behavior are explainable in terms of GaP and InP levels being superimposed. Among the observed peaks, two of the radiation induced levels have been associated with a gallium vacancy defect and a phosphorous Frenkel.

GaInP/GaAs cascade solar cells grown by molecular beam epitaxy

First results for molecular beam epitaxy (MBE) grown Ga/sub 0.51/In/sub 0.49/P/GaAs cascade solar cells are presented. The structures were prepared by both solid-source (SS) MBE and gas-source (GS) MBE. The tunnel diodes between the subcells were grown by using the standard MBE dopants (silicon and beryllium) but dopant diffusion related degradation of the cell characteristics was not observed. For the best SSMBE structure, a conversion efficiency of 21.1% was measured at AM0 for a 1/spl times/1 cm/sup 2/ cell. For the GSMBE structures, the best AM0 conversion efficiency was 20.8% for a 2/spl times/2 cm/sup 2/ device. In addition, the first MBE results for advanced Al/sub 0.51/In/sub 0.49/P/Ga/sub 0.51/In/sub 0.49/P-based tunnel diodes are presented.

Molecular beam epitaxy grown GaInP top cells and GaAs tunnel diodes for tandem applications

Ga/sub 0.51/In/sub 0.49/P solar cells and GaAs tunnel diodes were grown by gas-source molecular beam epitaxy. The effect of various window layer materials, such as Al/sub 0.51/In/sub 0.49/P, Al/sub 0.8/Ga/sub 0.2/As and ZnSe were studied for n-on-p Ga/sub 0.51/In/sub 0.49/P cells. The best carrier collection was obtained with Al/sub 0.51/In/sub 0.49/P window and with graded doping in emitter and base layers. A total-area AM0 efficiency of 14.0% for 2/spl times/2 cm/sup 2/ area has been measured for this cell. The GaAs tunnel diodes were grown with Be-doping for p-type and Si-doping for n-type material. Specific resistance of 0.09 m/spl Omega/cm/sup 2/ and peak tunneling current of 200 A/cm/sup 2/ were obtained for the best GaAs tunnel diodes. In addition, tunneling effect in a n++Ga/sub 0.51/In/sub 0.49/P/p++GaAs diode was observed.

An X-ray diffraction study of the effects of rapid thermal annealing on GaAs layers on Si substrates

Abstract The rapid thermal annealing method was applied to improve the crystalline quality of GaAs layers grown by molecular beam epitaxy (MBE) on Si (100) substrates. The annealed GaAs-on-Si samples exhibited remarkably better crystal structure than did the as-grown samples, as revealed by X-ray diffraction (XRD). The smallest XRD linewidth was 145 arc sec for a GaAs layer of 4 μm in thickness. The optimum temperature and time of annealing were found to be 930°C and 10 s for the layers having 1 μm in thickness but longer annealing times at this temperature were needed for thicker layers. Growth of a buffer layer by migration-enhanced epitaxy, prior to growing the final layer by MBE, also improved the crystalline quality when compared to that of the unannealed layers grown by the two-step MBE procedure.

Proton irradiated MBE grown GaInP/GaAs single junction and tandem solar cells

Degradation characteristics for MBE grown Ga/sub 0.51/In/sub 0.49/P and GaAs single junction and Ga/sub 0.51/In/sub 0.49/P/GaAs tandem solar cells irradiated with 3 MeV and 10 MeV protons with fluences of 10/sup 10/- 10/sup 13/ cm/sup -2/ are reported. The cell degradation was characterized with illuminated current-voltage (I-V) and spectral response measurements. Minority carrier diffusion length damage coefficients for the GaAs cells for 3 MeV and 10 MeV protons were calculated as a function of fluence. The results were compared to the InP damage coefficients. In addition, photo-annealing recovery effect at temperatures of 35-60/spl deg/C under 1-Sun AM0 illumination on the Ga/sub 0.51/In/sub 0.49/P cells are presented.

Bonding of indium phosphide layers on silicon substrates

Indium phosphide layers were bonded on silicon substrates by indium antimonide and gold based eutectic alloys. Although the thermal expansion coefficient of InSb is the same as that of InP, the bonding did not succeed with this material. When using Au based eutectic alloys, the bonding was successful. Large areas could be bonded homogeneously, and InP solar cells were processed on these structures with good characteristics. Because the solar cell is a minority carrier device, its characteristics are significantly worsened by the dislocations which reduce the minority carrier diffusion length. The bonding method has been applied for transferring InP layers on Si to avoid the formation of misfit dislocations at the interface.<<ETX>>

High interface recombination velocity caused by spatially indirect quantum well transition in Al0.55In0.45As/InP heteroface solar cells

The effect of a strained Al0.55In0.45As window layer on p/n InP solar cell performance was studied. In comparison to homojunction InP solar cells, decreased quantum efficiency in the short wavelength region of the spectrum was observed in cells having the window layer. Photoluminescence measurements of the heterojunction and light emission measurements of the solar cell under forward bias revealed an intense radiative transition, which is related to the enhanced recombination of the carriers that are photogenerated in the emitter layer. This recombination occurs between the energy levels of the triangular quantum wells formed at the type II Al0.55In0.45As/InP heterojunction, and prevents effective carrier collection in the solar cell.