Etching of elemental layers in oxide molecular beam epitaxy by O2-assisted formation and evaporation of their volatile suboxide: The examples of Ga and Ge

The delivery of an elemental cation flux to the substrate surface in the oxide molecular beam epitaxy (MBE) chamber has been utilized not only for the epitaxial growth of oxide thin films in the presence of oxygen but also in the absence of oxygen for the growth temperature calibration (by determining the adsorption temperature of the elements) and in-situ etching of oxide layers (e. g., Ga2O3 etched by Ga). These elemental fluxes may, however, leave unwanted cation adsorbates or droplets on the surface, which traditionally require removal by in-situ superheating or ex-situ wet-chemical etching with potentially surface-degrading effects. This study demonstrates a universal in-situ approach to remove the residual cation elements from the surface via conversion into a volatile suboxide by a molecular O2-flux in an MBE system followed by suboxide evaporation at temperatures significantly below the elemental evaporation temperature. We experimentally investigate the in-situ etching of Ga and Ge cation layers and their etching efficiency using in-situ line-of-sight quadrupole mass spectrometry (QMS) and reflection high-energy electron diffraction (RHEED). The application of this process is demonstrated by the in-situ removal of residual Ga droplets from a SiO2 mask after structuring a Ga2O3 layer by in-situ Ga-etching. This approach can be generally applied in MBE and MOCVD to remove residual elements with vapor pressure lower than that of their suboxides, such as B, In, La, Si, Sn, Sb, Mo, Nb, Ru, Ta, V, and W.


Introduction
2][3][4][5] Their growth as epitaxial thin films by molecular beam epitaxy (MBE) is beneficial for materials exploration and device applications, both requiring a high-degree of purity and crystallinity.
In the MBE growth of oxides possessing volatile suboxide such as Ga2O3, In2O3, SnO2, and GeO2 from elemental sources (Ga, In, Sn, and Ge), the provided elemental flux is oxidized via a first oxidization step to form suboxide (Ga2O, In2O, SnO, and GeO) on the substrate.The suboxide is further oxidized via a second oxidization step to form solid oxide thin film. 6,7The competing desorption of the intermediately formed suboxide (typically having a higher vapor pressure than its cation element) can decrease the thin film growth rate.While in an oxide MBE-growth chamber, suboxides were also found to form (and evaporate) readily from the elemental sources at a typical molecular O2 background pressure present during growth 8,9 , their oxidation into the stable oxide (e.g.Ga2O3, In2O3, SnO2, GeO2 and SiO2) required more reactive oxygen species, e.g.1][12][13] Solely, the growth of In2O3 at a low growth rate of 0.6 nm/min has been demonstrated using molecular O2. 14 Beyond the mere epitaxy, delivering an elemental cation flux to a substrate surface at absent anion flux in the vacuum of the MBE growth chamber has been used for substrate-temperature calibration purposes 15 or as an in-situ oxide removal technique to remove the native Ga2O3 from GaAs (or GaN) substrates by delivering a Ga flux ("Ga polishing") 16,17 .In the oxide-removal process, the provided element reacts with the oxide into a volatile suboxide, e.g., or which desorbs at elevated substrate temperature. 7,18The In-situ oxide removal is beneficial not only for preparing a clean substrate surface prior to growth but can speed up the MBE growth routine by regaining a fresh substrate surface after in-situ growth calibration or unsuccessful oxide layer growth, thus eliminating the need for unloading/loading of substrates and associated temperature ramps for each growth attempt.Meanwhile, it has even been used as damage-free etching to structure highly scaled vertical and lateral 3D Ga2O3-based devices. 19spite these beneficial applications, the elemental fluxes are prone to leave unwanted elemental adsorbates, layers, or droplets on the surface.The removal of these elemental layers, requires heating to the desorption temperature of the element, high-energy sputtering, or exsitu wet-chemical etching -all of which may create unacceptable degradation of the surface.
This work demonstrates a universal in-situ approach to remove the respective elemental layer from a substrate surface by exposure to molecular O2.The technique consists of heating the elemental layer to the desorption temperature of its volatile suboxide (typically well below that of the cation element), exposing it to O2 to induce suboxide formation, e.g., followed by suboxide desorption [20][21][22] as schematically shown in Fig. 1.We investigated the in-situ etching of Ga and Ge cation layer by an O2-flux experimentally on 2-inch Al2O3(0001) substrates in an MBE system and studied the etching efficiency (suboxide-flux/O2-flux).Our results indicate successful etching of Ga and Ge where ≈2.1% and ≈1.8% of the provided O2 contributed to their removal.Finally, we demonstrate the application scenario of Ga-droplet removal from a SiO2 mask after in-situ structuring of a Ga2O3 layer by Ga-etching.

Experimental details
For this study, Ge and Ga cation layers were grown in high vacuum (background pressure 10 −8 mbar) on 2-inch c-plane sapphire (Al 2 O 3 (0001)) wafers at a temperature of 400 • C by MBE.The rough backside of the single-side polished substrate was sputter-coated with titanium to allow for non-contact substrate heating by radiation from the substrate heater.The substrate temperature (Tsub) was measured with a thermocouple placed behind the substrate heater.
Standard shuttered effusion cells were used to evaporate Ge (7N purity) and Ga (7N purity) from pyrolytic BN crucibles.The beam equivalent pressure (BEP) of the cations and O2, proportional to the particle flux, were measured by a nude filament ion gauge positioned at the substrate location.The BEPs are given in units of mbar and are converted into the equivalent particle flux (atoms cm -2 s -1 ) by multiplying the measured growth rate of the GeO2 and Ga2O3 layer under conditions of full Ge and Ga incorporation by the cation number density of Ge (4.6 × 10 22 cm −3 ) and Ga (4.4 × 10 22 cm −3 ) and using kinetic gas theory in the case of O2.For the layer deposition, the used Ge and Ga -cell temperatures of 1300 • C 1 and 900 • C resulted in Ge and Ga-fluxes (Φ) of ΦGe = 4.6 × 10 14 cm −2 s −1 and ΦGa = 1.35 × 10 14 cm −2 s −1 impinging on the substrate.
Next, we provided molecular O2 to etch the deposited Ga and Ge layers at elevated substrate temperatures that allow the forming suboxides to desorb.For this purpose, a mass flow controller supplied molecular O2 from the research-grade O2 gas (6N purity) and the O2 flow was set as standard cubic centimeters per minute (sccm).The flux Φ of desorbing species from the layer surface was measured in-situ by line-of-sight quadrupole mass spectrometry (QMS, Hiden Analytical "HAL 511 3F").The QMS ionizer was run at an electron energy of 50 eV to obtain optimal sensitivity.Therefore, some of the measured signals might be affected by fragmentation of suboxide molecules into cation and oxygen atoms. 23To assess the surface coverage, the process was additionally in-situ monitored by reflection high-energy electron diffraction (RHEED).
As an application example, we demonstrated the in-situ removal of Ga-droplets from a SiO2 mask directly after in-situ patterning of Ga2O3 by Ga-etching.For this purpose, an MBE-grown, 1 Note, that this high temperature lead to a relatively fast degradation of the used standard effusion cell.

O 2 Suboxide
Substrate Cation ≈500 nm-thick Ga2O3 layer was covered by a ≈75 nm-thick SiO2 hard mask (deposited using sputtering and structured by contact lithography and CF4-based reactive ion etching) and subsequently loaded into the MBE growth chamber.Ga-etching was performed by exposure to a Ga flux of ΦGa = 7.8 × 10 14 cm −2 s −1 at Tsub =650 o C in the absence of O2 for a total etching time of 40 min, resulting in an etch depth of ≈140 nm (determined by profilometry measurement).Subsequently, we in-situ removed the Ga droplets that remained on the SiO2 mask by exposure to 1 sccm O2 for 90 min at the same Tsub.The untouched structured-Ga2O3 sample, the structured -Ga2O3 sample after Ga-etching, as well as a Ga-etched Ga2O3 sample after molecular O2 exposure were observed by top-view scanning electron microscopy (SEM).Next, we quantified and related the ΦGa 2 O measured by QMS and the impinging O2 flux, as presented in Fig. 3(b).All Ga layers were deposited with a fixed Ga flux of 1.35 × 10 14 cm −2 s −1 over a period of 780 s, resulting in a total surface Ga-atom coverage of DGa = 1.05 × 10 17 cm −2 .By numerically integrating the QMS signal Q(t) for Ga2O from the time when O2 is supplied (t = t0) to the time when the whole layer is removed (t = trem), an equivalence relationship of

Results and Discussion
was obtained and allowed us to determine the calibration factor α that converts the QMS-signal of the provided O2 species that can contribute to the removal of Ga, the resulting O2 flux ΦO 2 used at the different O2 flow rates is calculated based on kinetic gas theory 24 from the corresponding measured O2-BEP (PO 2 ) according to with the Avogadro constant NA, the molar mass M of O2, and O2 temperature T (298 K).The x 100% = 2.1% was obtained.x 100% = 1.8% representing removal efficiency of the O2 was obtained according to Eq. (4).
To better illustrate the application scope of our studies, we conducted experimental tests on the device structuring process by in-situ Ga-etching a SiO2-masked Ga2O3 layer.Fig. 5 showcases the top view SEM images of the masked and structured Ga2O3 samples.By comparing Fig. 5(a) with Fig. 5(b), as anticipated, we observed Ga droplets remaining on the SiO2 mask after structuring Ga2O3 by exposing it to the Ga flux.However, these droplets can be completely removed in-situ by providing O2 following the oxide etching process, as evidenced by a clean mask surface depicted in Fig. 5(c).

Conclusion
In conclusion, we have successfully demonstrated a process in which molecular O2 is utilized to remove elemental Ga and Ge layers in an MBE growth chamber through formation and desorption of their volatile suboxides at temperatures lower than those required for elemental desorption.Under the investigated (non-optimized) conditions about 1.8% -2% of the provided O2 contributed to cation removal.We showcased the application of this process for the in-situ removal of the residual Ga droplets from the SiO2 mask directly after structuring a Ga2O3 layer by in-situ etching using a Ga atomic flux.The O2-assisted cation removal process can be generally applied in-situ within an oxide MBE or MOCVD system to remove residual elemental layers that may occur after exposure to the cation fluxes during in-situ oxide etching or substrate temperature calibration, and is generally applicable for elemental layers whose suboxide exhibits a higher vapor pressure than the respective elements, such as B, In, La, Si, Sn, Sb, Mo, Nb, Ru, Ta, V, and W. 9,25

Fig. 1 .
Fig. 1.Schematic describing the in-situ etching of cation layer at elevated substrate temperature by an O2-flux, including the physisorption, dissociation, and chemisorption of O2, followed by the desorption of the formed volatile suboxide.

Fig. 2 .
Fig. 2. Ga deposition and its O2-assisted removal.The upper figure shows the measurement of the desorbing flux of 69.1 Ga and 156.1 Ga2O by QMS.Three stages are depicted: the deposition of the Ga layer on the c-plane sapphire substrate, the increase of the substrate temperature (Tsub) to enable suboxide desorption, and the subsequent in-situ etching of the already grown Ga layer.The corresponding Ga shutter opening and closing, Tsub as well as period of O2 supply are marked.The inset shows the mass spectrum of Ga2O detected by QMS.The arrows point to images of the RHEED pattern during different stages of the experiments.

Fig. 2
Fig. 2 presents the QMS signal of Ga and Ga2O (proportional to the desorbing flux o f ΦGa and ΦGa 2 O ) and the corresponding RHEED images at different stages during the elemental layer deposition and its subsequent etching by O2.When the Ga shutter was opened, th e elemental desorption slightly increased and then rapidly faded, corresponding to an almost full adsorption of the provided flux.After closing the Ga shutter, Tsub was immediately increased to 650 • C at 0.5 • C/s to facilitate the Ga2O desorption in the following etching process.The elevated Tsub did not result in detectable desorption of the already grown cation layers (the QMS signal before supplying O2 is negligible), while the disappeared streaky RHEED pattern (middle) clearly

Fig. 3 .
Fig. 3. Ga-deposition/O2-assisted etching cycles using six different decreasing O2 fluxes.(a) Detected Ga2O flux by QMS as a function of time tmeas during metallic Ga layer deposition and its O2-assisted etching.(b) Calibrated desorbing Ga2O flux during etching as a function of provided O2 flux in Fig. 3(a).

measured PO 2
as a function of O2 flow can be seen in Fig. S1 in supplementary material.Fig. 3(b) shows the peak ΦGa 2 O = αQ(t) max observed at the beginning of each etching-cycle as a function of the corresponding ΦO 2 .Based on Eq. (3), an average etching efficiency (ƞ) of ƞ= ΦGa2O 2*ΦO2

Fig. 4 .
Fig. 4. Ge deposition and its O2-assisted removal.The upper figure shows the measurement of the desorbing flux of 74 Ge and 90 GeO by QMS.Three stages are depicted: the deposition of the Ge layer on the c-plane sapphire substrate, the increase of the Tsub to enable suboxide desorption, and the subsequent in-situ etching of the already grown Ge layer.The corresponding Ge shutter opening and closing, Tsub as well as period of O2 supply are marked.The inset shows the mass spectrum of Ge and GeO detected by QMS.The arrows pointed images show the evolution of the RHEED pattern during different stages of the experiments.

Fig. 5 .
Fig. 5. SEM top view images of (a) the SiO2 mask on Ga2O3 thin film, (b) the Ga2O3 thin film etched by a Ga flux with residual Ga droplets on top of the SiO2 mask, and (c) after O2-assisted in-situ removal of the Ga droplets from the SiO2 mask.