Thermal Stability of GaAs/InGaP and InGaP/(In)GaAs Interfaces
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InGaP barrier
Gate (Au/WSiN) GaAs cap
Source
isolation region
(In)GaAs n-channel
buried-p-laye• S.1 GaAs substrat
Fig. 1. Cross section of a self-aligned gate InGaP/(In)GaAs heterostructure MESFET. 37
Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
the channel surfaces. Therefore, these interfaces should retain sufficient conduction band offsets after the activation annealing. EXPERIMENT Epitaxial Growth The substrates used were three-in-diameter, semi-insulating, (100) GaAs wafers grown by the liquid-encapsulated Czochralski (LEC) method. The two structures shown in Fig. 2 were grown in a low pressure (60 Torr), metalorganic chemical vapor deposition (MOCVD) reactor at 500 'C. Structure (a) is for PL measurements and structure (b) is for SIMS measurements. Trimetylindium
(TMI), Trimetytlgallium (TMG), and AsH3 were used for (In)GaAs, and TMI, Trietylgallium (TEG), and PH3 were used for InGaP as source gases. The group-Ill gas pre-switch-on method [7] was used to prevent As/P intermixing at the heterointerfaces. In this method, TMG and TMI are released for a short time after PH3 purging at InGaP to (In)GaAs interfaces, and TEG and TMI are released for a short time after AsH3 purging at (In) GaAs to InGaP interfaces to cover the grown film surfaces with the group-Ill atoms.
GaAs
50A
InGaP
1,000 A
GaAs
14 A
InGaP
5oo A
GaAs
28 A
InGaP
sooA
GaAs
57A
InGaP
500 A
GaAs
iooA
GaAs substrate
(a)
GaAs
3,000
5oo A
InGaP
GaAs
A
10,000
A
GaAs substrate
(b)
Fig. 2. Grown wafer structures for (a) photoluminescence measurements and (b) SIMS measurements.
Annealing Isochronal annealings were carried out for 10 min with 1,000-A-thick Si02 cap films on both sides of the wafers in ambient nitrogen gas at a temperature range from 600 to 900 °C. Short time annealings were performed at 900 'C for 0.1 sec with a 4,000-A-thick WSiN cap film. WSiN is a refractory metal and was used as a gate-metal. These conditions are identical to those for interfaces under the gate electrode in the actual device fabrication process. Measurements The PL spectrum was measured at 4.2 K between the wavelength range of 0.6 and 1.0 jAm. Illumination was provided by an Ar-ion laser at a wavelength of 0.5145 Am with an intensity of 100 mW. The PL was dispersed in a 0.64-m monochromator and detected with a cooled S 1-type photomultiplier. The depth profiles of Ga, In, P, and As in the epilayers were measured by secondary-ion-mass spectroscopy (SIMS). The primary ion was 02+ with a 2.0-keV ion energy and a 25-nA ion current. The secondary ions, Ga-, In-, As-, and P-, were extracted using a quadrapole mass spectrometer. The lattice images were observed by transmission electron microscopy (TEM) operated at 300 kV. [110] cross-sectional view samples were cleaved, polished, and Ar-ion milled to electron transparency. RESULTS AND DISCUSSIONS The PL spectrum of InGaP/GaAs/InGaP SQWs after annealing is shown in Fig. 3 compared with that of the as-grown wafer. PL peaks are clearly shown corresponding to the wel
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