2.3 Limitations of the Barrier Material AlGaAs

As stated before it is favorable to maximize the Al content in the barriers to obtain a large conduction band offset. AlGaAs barriers can be used on both sides of the channel. But some constraints related to epitaxial growth have to be considered.

The optimum MBE growth temperature of AlGaAs is significantly higher than that of GaAs and InGaAs. During epitaxial growth the substrate temperature cannot be changed in arbitrary short times. Therefore either the growth has to be interrupted or part of the AlGaAs and/or InGaAs has to be grown with a non optimum temperature. Any case usually deteriorates the quality of the AlGaAs/InGaAs interface. Poor heterojunction interface quality decreases the velocity of the electrons in the channel and increases the noise figure. Therefore sometimes Al is avoided in the lower barrier for low noise and low power HEMTs.

The Al content in both barrier layers is limited by a severe increase in the concentration of donor-complex centers (DX­centers) above an Al content of 20 % [31]. This problem might be overcome in the future using Al free barriers. As shown in  Figure 2.2 In_0.49Ga_0.51P represents a lattice matched ternary semiconductor on GaAs with E_g = 1.88 eV, which has about the same E_g as Al_0.35Ga_0.65As. Layers containing phosphorous can be grown epitaxially by metal organic vapor phase epitaxy (MOVPE) [32]. So far HEMTs based on MOVPE grown layers show poor performance compared to HEMTs employing MBE layers. This is mainly due to steeper profiles possible with MBE although MOVPE growth has been improved substantially.

A few years ago gas source molecular beam epitaxy (GSMBE) was made possible to grow layers containing phosphorus [33]. In addition to a rather complicated and toxic system, the major physical drawback are transient problems at semiconductor heterojunctions due to the control of partial pressures of the group V elements arsenic and phosphorus during epitaxial growth. InGaP/InGaAs HEMTs are reported in [34]. Very recently the successful use of quaternary In_0.5(Al_0.3Ga_0.7)_0.5P barriers grown by GSMBE was presented [35].
 

   
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