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
31.05.2023.
Temperature-dependent photoluminescence of amorphous and crystalline thin films of non-stoichiometric ZnGa2O4 (zinc gallium oxide), prepared by reactive magnetron co-sputtering from liquid/solid Ga/Zn targets, has been carried out. Thin films were grown in a wide temperature (200-800 oC) and Ga:Zn atomic ratio range (≈ 5.7–0.3).
The emission spectra of ZnGa2O4 samples show a peak value in a blue spectral range around 3.1 eV, and a low photon energy tail pronounced especially for samples grown at lowest temperatures (200-300 oC). It has been established that PL spectra from Ga2O3 are typically dominated by a broadened optical transitions centred near 3.108 eV (399 nm), which have been ascribed to oxygen-vacancy related transitions. Also, a strong effect due to thermal annealing was recorded for the sputtered ZnGa2O4 thin films: the broad low photon energy tail for thin film sputtered at room temperature can be significantly reduced by annealing at 700 oC temperature. However, simple analysis of the PL spectra suggests the optimal sputtering can be performed within 400-600 oC temperature range mainly. Furthermore, sputtering under different Ga-to-Zn ratio impact strongly on optical properties of ZnGa2O4 thin films grown at 600 oC. Although the emission peak located around 3.1 eV is not shifted if sputtering is performed under non-stoichiometric conditions (Ga:Zn = 1.3 or 4.7), the unwanted low photon energy tail manifest itself obviously, compared to the ZnGa2O4 thin film grown using decent stoichiometric ratio of Ga:Zn = 2.0.
Currently, a manuscript regarding the study on ZnGa2O4 thin films grown by reactive magnetron co-sputtering from liquid/solid Ga/Zn targets is being prepared.
Postdoc mobility visit (duration: 1 month) to prof. Liu-wen Chang research group in Department of Materials and Optoelectronic Science, National Sun Yat-sen University (NSYSU, Taiwan) has taken place between 20.03.2023 – 21.04.2023. During the visit a new growth method for a p-type delafossite compound of CuAlO2 (which is very similar to p-CuGaO2) has been explored. Indeed, elaborating recently the growth of n-type b-Ga2O3 thin films using different growth methods (magnetron sputtering, MOCVD, PLD) requires also finding optimal p-type semiconductor counterpart to realize optoelectronic devices based on p-n junction, operating in the deep ultraviolet spectral range.
Additionally, electrodeposition of p-Cu2O epitaxial thin films and the subsequent structural investigation using XRD, XPS and SEM microscopy has been performed, aiming to find optical growth conditions for the epitaxial growth. Indeed, p-Cu2O semiconductor is very promising to be used together with ZnMgO of n-type.
The collaborating research group in NSYSU, Taiwan has succeeded growing CuAlO2 as nanowires, oriented with their c-axis parallel to the substrate to obtain a mixture of a 3-R phase and a 2-H phase. The optical and electrical properties of the 2-H phase has seldom been reported. Also, a successful epitaxial growth of Cu2O thin films on different metallic substrates allows a realization of ZnMgO/Cu2O heterostructures, to be further elaborated. Therefore, in ISSP LU, Latvia and Vilnius University, Lithuania a subsequent study of the anisotropy of optical and electrical properties of Cu2O and CuAlO2 (platelet vs. wire, and 2-H versus 3-R or related issues) will be performed, as the continuation of the activities.
A final phase of the Project was devoted to writing the research publication manuscripts. In total 4 publications are to be delivered:
- R. Nedzinskas, A. Suchodolskis, L. Trinkler, G. Krieke, W.-Y. Yao, C.-C. Chang, L. Chang, M. M.-C. Chou, Optical Characterization of High-Quality ZnO (0002) / Cu (111) Epilayers Grown by Electrodeposition. Opt. Mater. 138, 113650 (2023). DOI: 10.1016/j.optmat.2023.113650
- L. Trinkler, D. Dai, L. Chang, M. M. C. Chou, T.-Y. Wu, J. Gabrusenoks, D. Nilova, R. Ruska, B. Berzina, R. Nedzinskas, Luminescence properties of epitaxial Cu2O thin films electro–deposited on metallic substrates and Cu2O single crystals. Mater. Phys. (in press).
- E. Butanovs, M. Zubkins, R. Nedzinskas, V. Zadin, B. Polyakov, Comparison of two methods for one-dimensional Ga2O3 - ZnGa2O4 core-shell heterostructure synthesis. J. Cryst. Growth (in press).
- M. Zubkins, E. Strods, V. Vibornijs, A. Sarakovskis, R. Nedzinskas, R. Ignatans, J. Purans, A. Azens, Deposition and photoluminescence of ZnGa2O4 thin films prepared by reactive magnetron co-sputtering from liquid/solid Ga/Zn targets (manuscript in preparation).
28.02.2023.
Temperature- and excitation-dependent photoluminescence along with optical absorption spectroscopy has been carried out for Ga2O3-ZnGa2O4 core-shell nanowires (NW) samples, realized by annealing of a sacrificial few-nm-thick ZnO coating (deposited via atomic layer deposition) at high temperature to enable solid state reaction between ZnO and Ga2O3. Optical measurements allowed establishing the optical bandgap (Eg) values and emission features associated with Ga2O3 (Eg = 4.674 eV) and ZnGa2O4 (Eg = 5.013 eV). In particular, the high energy PL features of pure- Ga2O3 and Ga2O3 - ZnGa2O4 NWs match around 3.4 eV. It has been established that PL feature at 3.444 eV (360 nm) should be attributed to self-trapped excitons. However, at lower energies the asymmetric PL shape of pure Ga2O3 sample indicate a broad blue-band centred at 3.016 eV (411 nm), associated with the recombination of a trapped electron in a donor with a trapped hole in an acceptor. While for the Ga2O3 - ZnGa2O4 core-shell NWs sample, a prominent low energy PL feature was established at 2.806 eV (442 nm), which is related to the self-activation centre of the octahedral Ga-O group in the spinel ZnGa2O4 -shell lattices.
Experimentally-derived Kubelka-Munk absorption aK-M spectra for the pure-Ga2O3 NWs along with Ga2O3 - ZnGa2O4 core-shell NWs and the corresponding sigmoid-Boltzmann function fit gave reasonable results of the bandgap energy. For pure Ga2O3 NW sample the estimated Eg = 4.821 eV show good agreement with the established direct energy bandgap value for β-Ga2O3 (Eg = 4.9 eV). Whilst for Ga2O3 - ZnGa2O4 core-shell NW sample, the absorption onset of a(E) spectrum is clearly composed of two different slopes, owing to the absorption in Ga2O3 (Eg = 4.674 eV) and ZnGa2O4 (Eg = 5.013 eV) parts of the NWs - ensemble, respectively.
Currently, a manuscript regarding the study on Ga2O3-ZnGa2O4 core-shell nanowires is being prepared.
Room temperature photoluminescence of amorphous and crystalline thin films of non-stoichiometric ZnGa2O4 (zinc gallium oxide), prepared by reactive magnetron co-sputtering from liquid/solid Ga/Zn targets, has been carried out and temperature-dependent emission measurements has been started.
30.11.2022.
The previous room temperature absorption and emission spectroscopy results demonstrated a feasibility to realise β-Ga2O3 thin films on sapphire (Al2O3) substrate using magnetron sputtering, metal organic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD) techniques in Thin films laboratory, ISSP UL, Latvia. However, due to a weak, and strongly Stokes-shifted room temperature photoluminescence (PL) signal, temperature- and excitation-dependent PL spectroscopy of a set of β-Ga2O3 thin films we carried out.
Low temperature PL for magnetron sputtered samples showed a less intensive and even broader spectral response as compared to room temperature PL data, with a PL peak value at 2.8 eV (442 nm) corresponding to the recombination of Ga-O vacancy pair. Moreover, as the temperature was increased, energy blue-shift towards 3.1 eV (400 nm; owing to oxygen-vacancy related transitions), emission band narrowing and intensity enhancement was observed, suggesting the possibility of the thermoluminescence effects (to be investigated in the next quarter of the Project).
In case of MOCVD grown sample (e.g. s943), a similar energy blue-shift from 2.8 eV to 3.1 eV was observed with temperature increased. However, a typical emission intensity quenching along with band broadening here occurred.
For the PLD-grown sample (e.g. s4), the expected temperature-dependent PL behaviour was established (energy red-shift, emission broadening and PL intensity quenching with T increased), with peak energy value at 2.8 eV (at low temperature).
Comparing the growth techniques and the corresponding PL yield, best optical response was obtained from PLD (2 mV signal), then magnetron sputtering (1 mV), and MOCVD being the worst (0.5 mV). Overall optical response, however was moderate-to-low, as signal to noise ratio is approx. 100:1 for the 1 mV signal.
In all of the samples grown using different techniques, excitation-dependent optical investigation showed no emission saturation or E-shift as laser power density (LPD) was increased by 2 orders of magnitude (8-800 kW/cm2). Furthermore, the absence of excitonic feature(s), a high Stokes energy shift, and low values of characteristic k-parameter (k=0.78 – magnetron sputtering; k=0.64 – MOCVD; k=0.67 – PLD) indicate that main path for the radiative optical transitions is due to deep defect (oxygen vacancy; Ga-O vacancy pair) states.
According to the Project plan, manuscript entitled “Optical properties of high-quality ZnO / SiO2 thin films grown by magnetron sputtering” was written and to be submitted to Q2-Q3 scientific journal early next quarter. The manuscript is based on previously grown and optically studied ZnO thin films magnetron sputtered on fused quartz substrate.
31.08.2022.
Two poster presentations (enlisted below) were given in the International conference of Functional materials and nanotechnologies (FMNT), which took place in Riga, Latvia, on July 3-6, 2022
1. Optical spectroscopy of rock-salt Zn(x)Mg(1-x)O thin films with high (up to 85%) Zn content
2. Optical properties of high-quality ZnO / SiO2 thin films grown by magnetron sputtering
Furthermore, temperature- and excitation-dependent photoluminescence (PL) experimental setup is prepared and adjusted for optical spectroscopic measurements of β-Ga2O3\Al2O3 thin films, grown using magnetron sputtering, metal organic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD) techniques in Thin films laboratory, ISSP UL, Latvia. Preliminary room temperature optical emission measurements showed weak and significantly broad signal of PL maximum. Absence of excitonic transitions at room temperature, also a high Stokes shift, ranging from 1.7 eV (magnetron sputtered samples) up to 2 eV (MOCVD- and PLD-grown samples), indicate that finding the bandgap energy of the β-Ga2O3 thin films investigated should be based either on low-temperature PL or on absorption spectra.
31.05.2022.
Following the plan of the Project, a set of b-Ga2O3 thin films on sapphire (Al2O3) substrates were realized using magnetron sputtering, metal organic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD) techniques in the ISSP UL, Latvia. Furthermore, an in-depth analysis of room temperature absorption, along with temperature- and excitation-dependent spectroscopic photoluminescence (PL) results of ZnO and ZnMgO thin films was carried out.
Based on the recent scientific findings, oral presentation “The effect of growth temperature on optical properties of ZnO/SiO2 thin films” was delivered in the 38th Scientific Conference of the Institute of Solid State Physics, University of Latvia (within the framework of the 79th Scientific Conference of the University of Latvia) on 24 February, 2022.
Furthermore, two poster presentations (enlisted below) were accepted and are to be given in the International conference of Functional materials and nanotechnologies (FMNT), which will take place in Riga, Latvia, on July 3-6, 2022
- Optical spectroscopy of rock-salt Zn(x)Mg(1-x)O thin films with high (up to 85%) Zn content
- Optical properties of high-quality ZnO / SiO2 thin films grown by magnetron sputtering
Currently, conference presentations are being prepared and experimental setup for optical spectroscopic measurements of b-Ga2O3 thin films is being adjusted, aiming to record a significantly weak emission signal. The required optical bandpass filters along with laser line filter will be purchased using the support of the Project.
30.11.2021.
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.
03.09.2021.
The initial period of the post-doctoral (PD) programme is devoted to 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.
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.