Charge modification in InAs/Al xGa 1− xSb HEMT structures

Abstract

Intrinsic sheet charge levels in unintentionally doped AlSb/InAs/AlSb quantum wells are reproducibly ∼10 12/cm 2. While these levels are suitable for depletion-mode operation, realizing enhancement-mode devices ultimately depends on the ability to significantly reduce sheet charge. In an effort to limit carrier accumulation in the InAs quantum well, we have studied the impact of the addition of a p-type, Be, doping plane between the channel and the cap and the modification of defect densities present in the films. Molecular beam epitaxy (MBE) process conditions including substrate temperature, Be-doping density, and doping plane spacing above the quantum well were varied during growth of InAs–Al x Ga 1− x Sb high electron mobility transistors (HEMTs) structures. Utilizing a 2 3-full factorial statistical experimental design plus eight additional runs, the ranges for Be-doping (0–2e12/cm 2), substrate temperature (350–420°C), and separation above the quantum well (60–80 Å) were examined. Relationships between MBE process conditions and transport properties in Be-doped InAs–Al x Ga 1− x Sb HEMT structures were established. Among the process parameters investigated, substrate temperature during Be-doping had the most significant impact on 2DEG concentration. Substrate temperature and sheet charge exhibited an inversely proportional relationship, where increasing substrate temperature resulted in decreasing sheet charge values. The lowest charge with corresponding mobility was 6.6×10 10 at 6000 cm 2/V/s.