Study of AlGaN/GaN HEMT Uniformity with Various Buffer and Barrier Structures
In the past years, great progress has been demonstrated in improving and sustaining good wafer uniformity for production growth of AlGaN/GaN based HEMTs on SiC substrates at IQE RF. The average sheet resistance (Rsh) uniformity has been from 1% to 2.5% for products with various barrier and buffer structures, and the growth was performed on both 4H and 6H substrates. The uniformity of the HEMT wafers has been gauged by measures of standard deviations of three parameters: Rsh tested by a Lehighton Eddy current system, the AlGaN barrier composition calculated from peak separation on (006) reflection XRD scans, total barrier thickness determined by Hg-probe C-V, and pinch-off voltage also determined by Hg-probe C-V.
All growth has been carried out in two Veeco E300 reactors, with the substrate diameter ranging from 50 mm to 100 mm. Different substrate sizes and polytypes (4H and 6H) result in minimal variations in uniformity for all four parameters, which indicate that these differences have not lead to deterioration of barrier uniformity and in turn the uniformity of two-dimensional electron gas (2DEG).
Same barrier structures have been used on top of two types of back barriers, namely AlGaN and InGaN, whereas the AlGaN back barrier is essentially a buffer, and the InGaN back barrier is realized by inserting a thin InGaN layer between the GaN buffer and a GaN channel. Similar uniformities have been demonstrated from these structures with different configurations prior to formation of the 2DEG. Although the back barrier may help in term of free carrier confinement, Rsh uniformity is not shown to be better, which could be possibly due to the fact that increased complicity in growth and structure may have compensated the improving free carrier confinement.
However, difference in uniformity has been observed with different barrier/cap structures after the 2DEG formation. The best average uniformity and controllability have been obtained from structures with a single layer AlGaN barrier with fixed composition, independent of whether or not a back barrier is used prior to the 2DEG. More complex structures with an AlN interlayer and/or a GaN cap layer all show slightly worse average uniformity and wider upper control limit. One potential reason for the drop off in uniformity is that: the multi-stake barrier structures might increase variation of polarization conditions of the free carrier supplying AlGaN layer as well as the AlN interlayer if it is used. Other reasons may come from change in interface and surface conditions, such as the potential interface charge resulted from an un-depleted thick GaN cap.