Growth and characterization of Ga2O3 and ZnMgO thin films for solar-blind ultraviolet applications (2020-2023)

Project leader Ramūnas Nedzinskas

Agreement No 1.1.1.2/16/I/001

Research application No 1.1.1.2/VIAA/3/19/442

Deep ultraviolet (UV) photodetectors based on wide-bandgap semiconductors can be used in a variety of applications in defense, astronomy, flame detection, biomedicine, chemical sensing water purification, etc. The concept of this project is to extensively verify the applicability of emerging wide bandgap (WBG) semiconductor Ga2O3 along with the novel pseudobinary system of ZnO-MgO for the needs of ultraviolet (UV) sensor applications, with a particular interest in p-type doping of both materials. Indeed, since a bandgap of beta-Ga2O3 is 4.6­­–4.9 eV, and a tunable bandgap of recently developed ZnMgO can be taylored within 3.3–7.8 eV, both materials are very attractive for their solar-blind nature.

In this project we seek to achieve new insights into the most perspective UV sensing materials by investigation of beta-Ga2O3 and ZnMgO semiconductor thin film structures, exclusively produced in Latvia, as a result of advanced technology transfer from Taiwan. The project includes the number of inbound and outbound training with further activities sweeping from technological growth, fundamental (optical and electrical) characterization, and theoretical modelling to sensor device fabrication and its prototype realization. We believe this project will be beneficial to the society of Baltic countries, owing to the advanced technology and novel engineering systems established.

The project will be implemented at the Institute of Solid State Physics, University of Latvia from 01.06.2020 until 31.05.2023. The total cost of the project is 133’805.88 EUR.


Project progress

08.06.2021.

On April 30th, 2021, the annual public event “Zinātnieku nakts” (Researchers‘ night) took place as a remote conference/session online (https://www.zinatniekunakts2021.lv). A poster presentation entitled “Zinc magnesium oxide wide-bandgap semiconductor for the demanded ultraviolet applications” was prepared and uploaded to the online platform, where it was available to the public during the session for discussion.

According to the Project plan, production of ZnMgO thin films using magnetron sputtering was carried out in Thin films laboratory, ISSP UL, Latvia. To calibrate the crucial growth parameters of ZnMgO thin film deposition, the initial growth of ZnO thin films on either soda-lime and fused-quartz substrates was performed. Owing to significantly better structural quality of thin films grown on quartz substrates, ZnMgO thin films were realized by reactive co-sputtering. Additionally, alternative novel material of Mg(Zn)NiO thin films were achieved in order to compare its structural and optical properties with the ones measured for ZnMgO. Indeed, it is known that ZnMgO suffers from dual phase coexistence, while MgNiO is believed to overcome this problem due to fundamental structural similarity between MgO and NiO binary compounds.

Currently, X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy structural measurements are being performed in Laboratory of Spectroscopy ISSP UL, Latvia. Simultaneously, optical absorption, spectroscopic ellipsometry and temperature-dependent photoluminescence data are being collected both in ISSP UL (Latvia) and FTMC (Lithuania).


10.03.2021.

According to the Project plan, (i) Advanced WBG semiconductor technology transfer from NSYSU (Taiwan) to USSP UL (Latvia) through training, and (ii) Epitaxial growth of the Zn(Mg)O and Ga2O3 thin films were to be considered.  For (i), alternative mobility actions/training was implemented in largest scientific research center FTMC in Vilnius, Lithuania, while for (ii), the in-depth literature study and test-growth of ZnO and MgO materials have been performed.

Owing to a limited scheduled time for cleanroom training and the previous outbound internships in Vilnius, Lithuania, the license for the cleanroom in ISSP was partly acquired in 17.11.2020. and starting from 19.11.2120, outbound mobility internship in FTMC started (duration: 1 month).

The main purpose of the secondment at FTMC was to gain a deeper knowledge on magnetron sputtering technique for oxide thin film growth and metal contact deposition. This is one of two main methods to be used for synthesizing oxide (ZnMgO and Ga2O3) thin films and depositing metal contacts for the actual sensor devices in ISSP UL, Latvia. The Angstrom Engineering EvoVac sputtering device in FTMC was used for training. Different operation regimes were explored (RF, DC and HiPIMS). 10-100 nm ultrathin metallization by Ni on Czochralski-grown crystalline Si (100) was implemented followed by surface treatment procedure by HF:H2O2:H2O chemical etching. Furthermore, ultrathin (up to 10 nm) oxide (TiO2) layers on Si substrate were grown maintaining different substrate temperature.

Considering the pandemic covid-19 situation, it was decided to schedule the project implementation works of early 2021 to the in-depth literature study, especially on Zn(Mg)O material. On February 17, an online ISSP UL seminar presentation, entitled “ZnMgO thin films for deep ultraviolet applications: structural and optical approach”, was given. Further details of the presentation can be found at:https://www.cfi.lu.lv/en/events/issp-ul-workshops/2020/2021/february-17-ramunas-nedzinskas/ 

Currently in Thin films laboratory, ISSP UL, after the test-phase of the binary oxides ZnO, MgO have been performed, thin films of ZnO and MgO are being sputtered to examine the effect of carrier concentration and conductivity type stability on different dopants used.


09.12.2020

According to the implementation plan of the Project, in the 3rd quarter of 2020, a WP2 was set:
Advanced WBG semiconductor technology transfer from National Sun-Yat Sen University, (NSYSU; Kaoshiung, Taiwan) to ISSP UL (Latvia) through training. In particular, an extensive outbound training in the cleanroom course, epitaxial growth, and structural characterization of wide bandgap semiconductor thin films was planned in the NSYSU laboratories.

However due to the pandemic covid-19 situation, the training in NSYSU is postponed to the earliest possible time window. Accordingly, alternative mobility actions/training were and are being implemented in the largest scientific research centers in Vilnius, Lithuania - Center for Physical Sciences and Technology (FTMC; Partner institution) and Vilnius University (VU; non-Partner institution)

The main purpose of the secondment at FTMC (20.09.2020.-03.10.2020) was to gain new knowledge on optical sample characterization techniques to be used once the ZnMgO and Ga2O3 thin films will be grown. The techniques learned-to-operate: optical microscopy (Olympus SC50) for microstructural observation of the surface quality of the samples, FTIR spectroscopy (Nicolet 8700) to determine the properties of phonon system, optical absorption spectroscopy (Shimadzu UV-3600) to investigate the characteristic bandgap of the material.

The main purpose of the short secondment at VU (04.10.2020.-10.10.2020) was to gain new technological knowledge on the maintenance and programming of the MOCVD reactor (Aixtron MOCVD CCS 3x2 FT), which is similar to the MOCVD reactor (AIX-200RF) at ISSP UL (the latter is currently under maintenance). The routine-cleaning of the shower-head was performed and a general set of the programming commands was learned.

Currently in Thin films laboratory, ISSP UL, after the Mg target was purchased recently, thin films are being sputtered first as pure ZnO and MgO to obtain the critical growth parameters. Afterwards, different ZnxMg1-xO thin films with diverse material compositions will be grown and studied using the optical and electrical characterization equipment in ISSP UL, and if necessary, additionally in FTMC (Lithuania).
Furthermore, since first attempts to sputter Ga2O3 were not successful due to nearly room temperature (~30 oC) melting point of Ga and poor Ga adhesion to stainless steel target holder, a new design of target holder was implemented and is being produced.


10.09.2020

The initial period of the post-doctoral (PD) literature study of beta-Ga2O3 (beta-GO) and ZnMgO wide-bandgap materials. In particular, gallium-oxide is a rather new topic for a PD fellow, an extensive literature overview was carried out. Currently the presentation slides are being prepared for the seminar in ISSP UL (the seminar time was postponed to the 4th month due to a remote work). Optical low-temperature photoluminescence and modulation spectroscopy setup at Partner institution FTMC (Center for Physical Sciences and Technology, Vilnius,Lithuania) was adjusted for the UV-A (315-400 nm),UV-B (280-315 nm) and UV-C (200-280 nm) spectral ultraviolet (UV) windows, relevant for the beta-GO and ZnMgO (ZMO) thin film structures to be studied. Moreover, UV-VIS-NIR (ultraviolet-visible-near infrared) spectroscopy setup at FTMC is learned-to-operate. Currently, an extensive inbound training (supervised by dr. Mārtiņš Zubkins) on magnetron sputtering of WO(3-x) and Ga2O3 materials in Thin films laboratory, ISSP UL and the subsequent characterisation by X-Ray Fluoresence Spectrometer in Laboratory of spectroscopy,ISSP UL is being carried out.