Reproducible increased Mg incorporation and large hole concentration in GaN using metal modulated epitaxy

Abstract

The metal modulated epitaxy ͑MME͒ growth technique is reported as a reliable approach to obtain reproducible large hole concentrations in Mg-doped GaN grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates. An extremely Ga-rich flux was used, and modulated with the Mg source according to the MME growth technique. The shutter modulation approach of the MME technique allows optimal Mg surface coverage to build between MME cycles and Mg to incorporate at efficient levels in GaN films. The maximum sustained concentration of Mg obtained in GaN films using the MME technique was above 7 ϫ 10 20 cm −3 , leading to a hole concentration as high as 4.5ϫ 10 18 cm −3 at room temperature, with a mobility of 1. GaN,7 unintentional hydrogen and oxygen doping, 8,9 a significant compensation of Mg acceptors at high dopant concentrations, 1 and a drastic dependence of incorporation upon the growth regime or III-V ratio.10,11 As a consequence, there is a narrow window of growth conditions, which yield electrically active p-type GaN. Furthermore, even low or moderate hole concentrations are often not consistently obtained and are difficult to reproduce due to the Mg incorporation sensitivity to growth conditions. These complications result in large and varied resistances and mobilities in GaN-based devices that rely on p-type layers. If hole carrier concentrations were to be increased, and p-type doping of GaN standardized, these devices could benefit from better performance and better reliability.Despite the complications associated with Mg-doped GaN, current state-of-the-art reports show promising results and give a reference for comparison. Mg incorporation as determined by secondary ion mass spectroscopy ͑SIMS͒ has been reported to be as high as 8 ϫ 10 20 cm −3 for metal organic chemical vapor deposition 12 and 3.5ϫ 10 20 cm −3 for molecular beam epitaxy ͑MBE͒.13 However, above approximately ͑2 -3.5͒ ϫ 10 20 cm −3 , the Mg incorporation begins to decrease with increased Mg flux 13 and inverts Ga-polar films to N-polar. 14 M-plane ͑1010͒ oriented substrates have been used to achieve a hole concentration as high as p = 7.2 ϫ 10 18 cm −3 .15 Electrical characterization for current stateof-the-art films grown on c-plane substrates is shown in Table I. [16][17][18] Reasonably high hole carrier concentrations are in the range of ͑1-3͒ ϫ 10 18 cm −3 . However, hole concentrations have been reported to be as high as 6 ϫ 10 18 cm −3 , 18 although the carrier concentration was metastable and significantly reduced upon heating during measurement. The metal modulated epitaxy ͑MME͒ growth technique was developed to specifically address reproducibility issues. [19][20][21][22] The MME growth technique is characterized by a group-III ͑metal͒ flux much higher than the group-V source, which would normally lead to droplets. However, the metal shutter is modulated to avoid droplet buildup and therefore build and deplete the metal bilayer on the growth surface, yielding smoother surfaces, larger grain sizes, and better repr...

Metal modulation epitaxy growth for extremely high hole concentrations above 1019cm−3 in GaN

“…This technique has demonstrated dramatic improvements in hole concentrations, 4.5ϫ 10 18 cm −3 published previously 11 and over 1.5 ϫ 10 19 cm −3 in the present report. To understand why MME provides such a significant enhancement in hole concentration, we first must review Mg incorporation behavior and the formation of compensating donor-type defects.…”

“…In this work, both the Ga and Mg shutters are modulated ͓the Mg being delivered from a Veeco corrosive series valved cracker with a flux of ͑4-5͒ ϫ 10 −10 Torr beam equivalent pressure ͑BEP͔͒ at a growth temperature of 500°C. 11,18 The Ga fluxes used are large enough that droplets rapidly form when the Ga shutter opens and are subsequently depleted when the Ga shutter closes. Figure 2͑a͒ shows Mg concentrations in alternating Mg-doped/undoped GaN with different periodic ͑open/close͒ duty cycles of metal shutters while maintaining the same Ga fluxes.…”

Extremely high hole concentrations in c‐plane GaN

“…18 cm -3 [1][2][3][4]. This approach, when applied to higher temperatures or in a predominantly droplet rich regime, as indicated by arrow "a" in Figure 1, results in atomically smooth surfaces normally only found when growing in the Ga-droplet regime.…”

“…The Ga fluxes used are sufficiently large that droplets rapidly form when the Ga shutter opens and are subsequently depleted when the Ga shutter closes. Our previous references detail RHEED signatures useful in identifying the proper growth regimes [1][2][3][4], subsequent work has reduced noise and allowed new physical understanding of the kinetics of growth, summarized in Figure 2. RHEED intensity features have been determined to be related to; (a) the exchange of Ga with the N adsorbed layers on shutter opening, (b) the buildup of the mono/bilayer of Ga, (c) the accumulation of Ga droplets, (d) the consumption of droplets after shutter closing, (e) the consumption of the mono/bilayer, and finally the (f) adsorption of a Nitrogen adlayer.…”

“…Recently, Metal Modulation Epitaxy (MME) [1] was introduced. MME is a technique wherein only the metal fluxes (Al, Ga, In, and Mg) are modulated in a short periodic fashion in RF MBE system, while maintaining a continuous nitrogen plasma flux.…”