Comparative study of BeMgZnO/ZnO heterostructures on c-sapphire and GaN by molecular beam epitaxy

Abstract

Growth of ZnO directly on c-plane sapphire, with a large in-plane lattice mismatch of 18%, is relatively well understood for O-polar variety. However, the two-dimensional (2D) growth of Zn-polar ZnO on c-sapphire, needed for 2D electron gas formation, with low background electron density is in its infancy. While Zn-polar ZnO can be grown on GaN with the resulting small lattice mismatch (1.8%), the parallel conduction through GaN (bulk and/or surface) in fabricated BeMgZnO/ZnO heterostructure field effect transistors (HFETs) limits its utility for the time being. In this contribution, the authors report on the growth of high-quality Zn-polar BeMgZnO/ZnO HFET structures directly on sapphire substrates, in an effort to avoid the aforementioned parallel conduction, by employing an MgO and low-temperature ZnO buffer stack. The residual 2D equivalent concentration in the HFET structure on GaN is ∼2 × 1012 cm−2, which is detrimental, while highly resistive (>100 Ω.cm) Zn-polar ZnO layers with smooth surfaces on c-sapphire have been obtained via controlling the buffer stack growth conditions, which is vital to the realization of HFET device structures. Compared with the highest room temperature electron mobility of ∼250 cm2/V s in BeMgZnO/ZnO HFETs on GaN templates, a slightly lower electron mobility of ∼220 cm2/V s was achieved on c-sapphire due to a somewhat lower overall crystalline quality.

High‐Performance BeMgZnO/ZnO Heterostructure Field‐Effect Transistors

“…The growth details including polarity control to obtain highquality ZnO and the BeMgZnO barrier growth conditions can be found in previous reports. [17,18] The metal-oxide-semiconductor (MOS)-HFET devices were processed by conventional semiconductor photolithography techniques. First, the mesa etching was conducted in an inductively coupled plasma (ICP) etching system with a photoresist as the mask.…”

“…[ 23 ] It is clear that higher crystalline quality in ZnO films on GaN plays an important role in increasing the maximum applicable electric field and the drift velocity compared to that in films on c‐sapphire, but at the expense of a parallel‐conduction channel. [ 18 ] Although the HFETs in this study were grown on c‐sapphire substrates, improved quality was achieved in the ZnO channel layer by optimized Zn‐polar growth buffer technique and in the BeMgZnO barrier by BeO and MgO coalloying.…”

“…Nevertheless, a non‐negligible parallel conduction contribution with an equivalent sheet electron density of ≈2 × 10 12 cm −2 frustrates utilization of such HFET structures on GaN for the time being. [ 18 ] By adopting an MgO and low‐temperature (LT) ZnO buffer stack with an appropriate thickness and growth temperature, HFETs exhibiting an electron mobility of 220 cm 2 (Vs) −1 were grown directly on c‐sapphire, eliminating the parallel conduction channel. [ 18 ] In this study, we report the fabrication and resulting device performance of the Zn‐polar BeMgZnO/ZnO HFETs grown on sapphire substrates.…”

“…[18] By adopting an MgO and lowtemperature (LT) ZnO buffer stack with an appropriate thickness and growth temperature, HFETs exhibiting an electron mobility of 220 cm 2 (Vs) À1 were grown directly on c-sapphire, eliminating the parallel conduction channel. [18] In this study, we report the fabrication and resulting device performance of the Zn-polar BeMgZnO/ZnO HFETs grown on sapphire substrates. Figure 1a shows the cross-sectional schematic of the BeMgZnO/ZnO HFET structure.…”

Photoluminescent Enhancement by Effect of Incorporation Nickel in ZnO Films Grown

“…However, these results have been difficult to replicate in order to achieve the desired homojunction. By another hand, different morphological growths have been reported, such as microwires [8], microribbons [9], microspheres [9], and even nanometric structures as nanoparticles [10], nanorods [11], and nanowires [12] of ZnO with the aim of manufacture different types of devices as gas sensors [13], light-emitting diodes [14] photodetectors [15], high electron mobility transistors (HEMT) [16], [17], field-effect transistors (FET) [18] and thin-film transistors (TFT) [19]. In this work, microsphere and sea urchin-type structures were found; the last structure is obtained when ZnO films are doped with Nickel.…”