Mechanism of Si doping in plasma assisted MBE growth of β-Ga2O3

Abstract

We report on the origin of high Si flux observed during the use of Si as a doping source in plasma assisted MBE growth of -Ga2O3. We show on the basis of secondary ion mass spectroscopy (SIMS) analysis that Si flux is not limited by the vapor pressure of Si but by the formation of volatile SiO. The low sublimation energy of SiO leads to weak dependence of the SiO flux of Si cell temperature and a strong dependence on the background oxygen pressure. Extended exposure to activated oxygen results in reduction of SiO flux due to the formation of SiO2 on the Si surface. The work reported provides key understanding for incorporating Si into future oxide-based semiconductor heterostructure and device MBE growth.The high breakdown voltage [1] and availability of bulk substrates grown from melt [2-4] makes -Ga2O3 promising for various applications, including power switches [5,6], high frequency amplifiers [7,8] and high temperature electronics [9,10].High quality epitaxial growth with low defect density and a wide range of controllable n-type doping [2][3][4]11] are critical enablers for these applications. Variety of growth techniques like molecular beam epitaxy [12,13], metal organic chemical vapor deposition [14], halide vapor phase epitaxy [15] and low pressure chemical vapor deposition [16] have been utilized to realize high quality epitaxial layers of -Ga2O3. Since the conduction band of -Ga2O3 is largely made up of Ga s oribitals, group II elements like Si (30 meV), Ge (30 meV) and Sn (60 meV) provide shallow donor levels [9, 17, 18,] and a detailed understanding of the doping process of each one of them is critical in realizing the full potential of -Ga2O3 based devices.Si is the preferred n-type shallow donor in many III-V materials like GaN (20 meV) and GaAs (6 meV) due to its low activation energy and compatibility with epitaxial growth processes. In the case of molecular beam epitaxy, elemental high-purity silicon is typically used, with the effusion cell maintained at high temperatures (typically 1000 °C -1300 °C) required for sublimation of the solid Si. This typically provides excellent control of doping density up to 10 20 cm -3 [19,20].Plasma and ozone-based molecular beam epitaxy growth have been used to obtain Si-doped n-type -Ga2O3 with excellent transport properties [9,21]. However, the cell temperatures necessary to achieve Si doping have been found to be significantly lower [9,22,23] than that used typically for other non-oxide material systems. Since Si has a high sticking coefficient, this

β-Ga2O3 material properties, growth technologies, and devices: a review

“…desorption and oxidization processes. Intentional n-type doping has been demonstrated by using Si, Sn, and Ge as donor dopants [41][42][43][44]. However, it is difficult to control a low doping density on the order of 10 16 cm −3 or less, because the surface of the dopant source in an effusion cell is easily oxidized during the MBE growth by background O species, resulting in the dopant flux hardly controlled.…”

β‐(Al0.17Ga0.83)2O3/Ga2O3 Delta‐Doped Heterostructure MODFETs with an Ultrathin Spacer Layer and a Back‐Barrier Layer: A Comprehensive Technology Computer‐Aided Design Analysis

“…[1][2][3][4] Recently, β-(Al x Ga 1-x ) 2 O 3 /β-Ga 2 O 3 modulation-doped field-effect transistors (MODFETs) have become an emerging technology for high-power and high-frequency device applications because of the demonstration of a 2D electron gas (2DEG) channel with mobility as high as 180 cm 2 Vs À1 at their heterointerface. [2,[4][5][6][7][8] Demonstration of 2DEG formation in these heterostructures showed that the mobility in β-Ga 2 O 3 -based devices can be higher than the reported mobility range for β-Ga 2 O 3 -based MOSFETs and MESFETs, which are in the range of 50-90 cm 2 Vs À1 . [5,6] In δ-doped β-(Al x Ga 1-x ) 2 O 3 /β-Ga 2 O 3 heterostructures, the band offset in the conduction band energy between β-(Al x Ga 1-x ) 2 O 3 and β-Ga 2 O 3 causes carrier confinement of the electrons in the channel located at the heterointerface, which results in higher mobility.…”

“…[2,[4][5][6][7][8] Demonstration of 2DEG formation in these heterostructures showed that the mobility in β-Ga 2 O 3 -based devices can be higher than the reported mobility range for β-Ga 2 O 3 -based MOSFETs and MESFETs, which are in the range of 50-90 cm 2 Vs À1 . [5,6] In δ-doped β-(Al x Ga 1-x ) 2 O 3 /β-Ga 2 O 3 heterostructures, the band offset in the conduction band energy between β-(Al x Ga 1-x ) 2 O 3 and β-Ga 2 O 3 causes carrier confinement of the electrons in the channel located at the heterointerface, which results in higher mobility. [5,6] In an early study on β-(Al x Ga 1-x ) 2 O 3 / Ga 2 O 3 heterostructure-based FETs, a 2DEG channel with a sheet carrier density of 3 Â 10 12 cm À2 was obtained at the interface of the heterojunction, which resulted from a Si-δ doping layer separated by a thin spacer layer from the heterojunction interface.…”

“…[5,6] In δ-doped β-(Al x Ga 1-x ) 2 O 3 /β-Ga 2 O 3 heterostructures, the band offset in the conduction band energy between β-(Al x Ga 1-x ) 2 O 3 and β-Ga 2 O 3 causes carrier confinement of the electrons in the channel located at the heterointerface, which results in higher mobility. [5,6] In an early study on β-(Al x Ga 1-x ) 2 O 3 / Ga 2 O 3 heterostructure-based FETs, a 2DEG channel with a sheet carrier density of 3 Â 10 12 cm À2 was obtained at the interface of the heterojunction, which resulted from a Si-δ doping layer separated by a thin spacer layer from the heterojunction interface. [7] In another study, a heterostructure FET with a thicker spacer layer was demonstrated.…”

N-type doping of low-pressure chemical vapor deposition grown β-Ga2O3 thin films using solid-source germanium

“…Growth of β-Ga2O3 has been realized using a variety of techniques such as MBE (Molecular beam epitaxy) 21 , MOCVD (Metal-organic chemical vapor deposition) [5][6][7]22,23 , HVPE (Hydride vapor phase epitaxy) 4,24 , PLD (Pulsed laser deposition) 25 and LPCVD (Low-pressure chemical vapor deposition) [26][27][28][29] . Growth of β-Ga2O3 has already been studied using MBE for a variety of film orientations (010), (-201), ( 001) and (110) and dopants(Si, Sn and Ge) [30][31][32][33][34] . A variety of lateral FETs have been fabricated using MBE-grown β-Ga2O3 films 2,18 .…”