M.L. Timmons

Correlation of vapor pressure equation and film properties with trimethylindium purity for the MOVPE grown III–V compounds

Abstract The purity of trimethylindium (TMI) has improved significantly during the past few years, as a result of improvements in its synthesis and purification. However, consistent high purity and batch-to-batch variation remain as primary concerns. In the present study, the impurity concentrations in commercial TMI samples were analyzed at part per billion levels using Fourier transform-nuclear magnetic resonance spectroscopy, inductively coupled plasma-optical emission spectrometry, and inductively coupled plasma-mass spectrometry techniques. The impurity profiles of TMI were compared with the electrical characterization of grown InP layers, e.g. electron mobility, carrier concentration, and glow discharge mass spectrometry analyses of grown layers in order to establish a correlation. The vapor pressure equation for TMI was also re-evaluated using (a) dynamic concentration measurements by Epison™ monitor and (b) the direct measurement of vapor pressures at various temperatures using a solid-state Baratron™ capacitance manometer. The resultant equations are reported along with a novel delivery system (UNI-FLO™ cylinder) that provides consistent, reproducible delivery of TMI in the vapor phase. The influence of deleterious impurities on the optoelectronic properties is discussed along with the synthesis and purification strategies for the consistent manufacture of high-purity TMI.

An alternative Mg precursor for p-type doping of OMVPE grown material

Abstract A new Mg precursor, bis(methylcyclopentadienyl)magnesium has been studied. This source, used as a liquid, produced hole concentrations of 5×10 19 to 2×10 19 cm ×3 in GaAs. Surface morphologies in this doping range for GaAs layers are excellent. Electrical and optical evaluations indicate low compensation and material comparable to Mg-doped GaAs using other precursors. Some dopant tailing has been observed.

18.2% (AM1.5) efficient GaAs solar cell on optical-grade polycrystalline Ge substrate

In this work, the authors present GaAs material and device-structure optimization studies that have led to achieve a open-circuit voltage of /spl sim/1 volt and a best solar cell efficiency of 18.2% under AM1.5G illumination, for a 4 cm/sup 2/ area GaAs cell on commercially-available, cast, optical-grade polycrystalline Ge substrate. This V/sub /spl infin// is almost 70 mV higher than on their previously-reported best GaAs cell on similar substrates. They discuss the growth of high-quality GaAs-AlGaAs layers, across the various crystalline orientations of a polycrystalline Ge substrate, important for obtaining good device performance. Optimization studies of the minority-carrier properties of GaAs layers on poly-Ge substrates have revealed that lifetime-spread across various grains can be reduced through the use of lower doping for the Al/sub 0.8/Ga/sub 0.2/As confinement layers. The cell-structure optimization procedures for improved V/sub /spl infin// and cell efficiency, include the use of thinner emitters, a spacer layer near the p/sup +/-n junction and an improved window layer. An experimental study of dark currents in these junctions, with and without the spacer, as a function of temperature (77 K to 288 K) is presented indicating that the spacer reduces the tunneling contribution to dark current.

High‐quality eutectic‐metal‐bonded AlGaAs‐GaAs thin films on Si substrates

Device quality GaAs‐AlGaAs thin films have been obtained on Si substrates, using a novel approach called eutectic‐metal‐bonding (EMB). This involves the lattice‐matched growth of GaAs‐AlGaAs thin films on Ge substrates, followed by bonding onto a Si wafer. The Ge substrates are selectively removed by a CF4/O2 plasma etch, leaving high‐quality GaAs‐AlGaAs thin films on Si substrates. We have obtained a minority‐carrier lifetime of 103 ns in a EMB GaAs‐AlGaAs double heterostructure on Si, which is nearly forty times higher than the state‐of‐the‐art lifetime for heteroepitaxial GaAs on Si, and represents the largest reported minority‐carrier lifetime for a freestanding GaAs thin film. In addition, a negligible residual elastic strain in the EMB GaAs‐AlGaAs films has been determined from Raman spectroscopy measurements.

Recent progress in atomic layer epitaxy of III–V compounds

Abstract Atomic layer epitaxy (ALE) has been recently established as a new growth technique which allows control of the growth process at the monolayer level through a self-limiting growth mechanism. We report here on recent progress and current problems facing this technology. Side wall growth by ALE has been demonstrated with deposited structures that differ from conventional chemical vapor deposition growth. Also ALE shows promise in the growth of GaAs on nonpolar substrates such as Ge. The problem of background doping in ALE films will be addressed.

Visible light emission from quantized planar Ge structures

Visible photoluminescence has been observed near 1.9 eV at 300 K from quantized planar Ge structures. This is the first observation of luminescence in Ge and is similar to the recently reported luminescence from porous Si. The quantum structures are prepared from bulk Ge substrates, and both n‐ and p‐type Ge produce luminescence at room temperature. These structures are fabricated by plasma‐assisted etching using a CF4/O2 gas mixture.

An inverted-growth approach to development of an IR-transparent, high-efficiency AlGaAs/GaAs cascade solar cell

An approach for inverted-grown AlGaAs/GaAs cascade cells, where the AlGaAs top cell is grown first at high temperatures, placing the surface to be illuminated nearest to the substrate, is presented. Following the growth of the top cell, the GaAs tunnel interconnect and the bottom cell are grown at lower temperatures. After the inverted growth, the AlGaAs/GaAs cascade structure is selectively removed from the parent substrate, Ge in this case. Advantages of the inverted-growth approach are discussed.<<ETX>>

Measurement of AlGaAs/AlGaAs interface recombination velocities using time‐resolved photoluminescence

Time‐resolved photoluminescence has been used to examine AlxGa1−xAs/AlyGa1−yAs interfaces, focusing on the recombination velocity. For an Al0.08Ga0.92As/Al0.88Ga0.12As interface, important for solar cells, recombination velocities are about 104 cm/s with the growth conditions used in this study. Several types of interface passivation were attempted, but the most successful was the insertion of thin Al0.14Ga0.86As layers between the other two alloys. Using this technique, a 16‐fold increase (to ∼20 ns) of the minority‐carrier lifetime was measured in a 0.8‐μm‐thick Al0.08Ga0.92As layer in which interface recombination would normally have limited the lifetime to about 1–2 ns. Compositional grading was found to be ineffective at passivating the interfaces.

Photoluminescence of porous silicon buried underneath epitaxial GaP

Recent observations of visible, room‐temperature photoluminescence in porous Si have stimulated research aimed at the realization of efficient, Si‐based electroluminescent devices. To achieve electroluminescence, it may be beneficial to generate carriers with sufficient energy to populate the states of the quantum‐confined Si structures. A viable method to accomplish this is to utilize a wide‐band‐gap heterojunction injector, such as GaP. Toward that end, we report the successful formation of porous Si buried underneath GaP islands, and we demonstrate that the buried porous Si layer exhibits strong photoluminescence (λ≊7000 A).

High quality GaAs on Si using Si0.04Ge0.96/Ge buffer layers

Abstract High-quality epitaxial growth of GaAs on Si has been achieved using Si0.04Ge0.96/Ge buffer layers. GaAs layers, approximately 1.3 μm thick, have been grown on Si using interfaces of Ge/Si0.04Ge0.96 layers to confine the majority of misfit dislocations that are generated by the 4% lattice mismatch between Ge and Si. The GaAs layers, grown by organometallic vapor phase epitaxy (OMVPE), have background carrier concentrations of ∼2×1015 cm-3. Transmission electron microscopy (TEM) indicates dislocation densities as low as 107 cm-2 in the GaAs layers. A photoluminescence-decay measurement on an AlGaAs/GaAs double heterojunction (DH), grown on a (100)-oriented Si substrate and the Si0.04Ge0.96/Ge buffers, yields a minority-carrier hole lifetime of ∼2.5 ns which is state-of-the-art for heteroepitaxial GaAs on Si. This value represents a significant development for a novel approach in the heteroepitaxy of GaAs on Si.