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The influence of the underlying nucleation layer on the properties of semipolar $$(11{\bar{2}}2)$$ GaN grown on m-plane sapphire by metalorganic vapor-phase epitaxy has been investigated. $$(11{\bar{2}}2)$$ GaN epilayers of ~ 1 μm thickness were grown using four different initiating sequences: low-temperature AlN and GaN, and high-temperature AlN buffer layers, and directly (high-temperature GaN). The choice of nucleation layer had a pronounced effect on the surface morphology and crystal quality of the overlying GaN epilayer. In comparison, direct growth of $$(11{\bar{2}}2)$$ GaN without any buffer layer provided the best crystal quality with a rocking-curve $$\omega $$ full-width at half-maximum (FWHM) value of 720 arcsec along the $$[11{\bar{2}}{\bar{3}}]$$ direction and relatively enhanced near-band-edge photoluminescence emission, thus showing this direct growth process to be a simple route for synthesis of semipolar $$(11{\bar{2}}2)$$ GaN layers.

In this work, the optical properties of thin-film α-Ga2O3 are examined and contrasted with those of β-Ga2O3 thin films grown on similar substrates. Thin films of α-Ga2O3 were synthesized employing mist chemical vapor deposition (mist CVD) on double-side polished c-plane sapphire. Optical properties were studied through absorbance, photoluminescence (PL), and photoluminescence excitation (PLE). Absorbance measurements showed spectra consistent with that previously reported in the literature, with Eg ~ 5.19 eV for α-Ga2O3. Despite an absorption edge laying at higher energies, the PL spectra in α-Ga2O3 films yield similar features to those of metal-organic chemical vapor deposition (MOCVD) grown β-Ga2O3 thin-films. In both cases, when excited above the band gap, the PL spectra consist of UV, UV′, blue and green broad bands. As the excitation photon energy was varied, the relative intensity of these PL bands changed. This is attributed to a non-uniform defect concentration distribution across the sample depth. With smaller concentration of defects associated with blue and green emission in the bulk compared to the surface in the α-phase films.

A pulsed atomic-layer epitaxy growth technique has been introduced to substantially diminish the induced strain and oxygen incorporation on aluminium nitride films grown at standard pressure by metal organic chemical vapour deposition. The qualities of the as-deposited aluminium nitride films were studied by varying the aluminium nitride nucleation layer growth temperature at 700°C, 800°C, 900°C, 1000°C and 1100°C, respectively. The compressive strain inside the as-deposited aluminium nitride films, induced by the hetero-epitaxial growth on sapphire, was investigated through Raman spectroscopy by focusing on the evolution of E2 (high) peak frequency, where almost stress-free aluminium nitride films were attained at nucleation layer growth temperature of 1100°C. Then, the correlation between luminescence defect and level of foreign impurities respective to the varied nucleation layer growth temperatures were also systematically analysed through photoluminescence spectroscopy and x-ray photoelectron spectroscopy, respectively.

High-quality β-Ga2O3 films were grown on (0001) sapphire substrates at various substrate temperature by pulsed laser deposition (PLD) in a high vacuum chamber. Low pressure (1.6 mPa) and high reactivity oxygen plasma was introduced as reaction gas. The films were used to fabricate metal–semiconductor–metal (MSM) solar-blind photodetectors by electron-beam evaporation. The β-Ga2O3 -based photodetector exhibited high performance with a low dark current (about 40 pA), fast response speed (τrise: 0.17 s, τdecay: 0.03 s) and high responsivity (0.35 A/W). These results represent the high quality of β-Ga2O3 films and excellent performance for PLD-grown β-Ga2O3 -based MSM solar-blind photodetectors.

We have grown nonpolar nitrogen (N) doped a-plane zinc oxide (ZnO) films on r-plane sapphire substrates in order to eliminate the self-polarization component in the growth direction, which decreases the doping efficiency of N acceptors. Nonpolar a-ZnO:N films were grown by plasma-assisted molecular beam epitaxy (PA-MBE) using Zn metal and a plasma source of O2 and NO mixed gas. It was confirmed by reflection high-energy electron diffraction and x-ray diffraction that single phase a-plane ZnO:N films were grown on the r-plane sapphire substrates. After the PA-MBE growth, the post-annealing was performed in an oxygen atmosphere. Photoluminescence experiments showed donor–acceptor pair emissions increase with increasing the annealing temperature (≤ 700°C). AC magnetic field Hall effect measurements revealed that n-type conduction of the as-grown films clearly changed to the p-type at the annealing temperature of 650°C. The resistivity, hole concentration, and mobility were ρ = 3.4 Ω cm, p = 8.0 × 1017 cm−3, and μ = 2.3 cm2/Vs, respectively.

Smooth 200 nm thick N-polar InGaN films were grown by metal–organic chemical vapor deposition (MOCVD) on sapphire using a digital approach consisting of a constant In, Ga, and N precursor flow with pulsed injection of H2 into the N2 carrier gas. Using this growth scheme, the H2 injection time was altered and the effect on the morphology and indium incorporation in the films observed. The effect of periodic insertion of additional GaN inter-layers on the surface morphology of the InGaN layers was also studied.

(GaIn)2O3 (IGO) films with a Ga content of about 30% were deposited on sapphire substrates by magnetron sputtering followed by thermal annealing. Annealing temperatures from 600°C to 1000°C showed little influence on the structural and optical properties of the films. All the films showed smooth surfaces and high transmittances in the visible range. X-ray diffraction patterns identified the mixed phase structure of the obtained films. Only one absorption edge was identified for all the films, suggesting these films are suitable for fabricating high responsivity ultraviolet detectors.

In this work, hexagonal barium ferrite films were deposited on polished sapphire (0001) substrates by pulsed laser deposition. The dependencies of morphology, crystallinity and magnetic properties on film thickness were investigated in detail. The results from measurements revealed that the degree of orientation and surface roughness of films gradually deteriorated with increasing film thickness. Additionally, as the film thickness increased, the magnetic properties showed an obvious degradation, especially in terms of the saturation magnetization and magnetic anisotropy. Consequently, the results of this study further prove that the deposition of high-quality BaM films with large thickness is significant, while difficult.

Irregular silica nanoparticles were prepared through polymer templating and their application on chemical mechanical polishing (CMP) of sapphire substrates was performed. Irregular silica nanoparticles were obtained by the chain structure of PEG200 and many oxygen functional groups. Silica seed particles can be rapidly coated through hydrogen bonding and also overlapped for two or more silica seed particles. The results show that the material removal rate of irregular silica nanoparticles increases by 58.7% compared to that of spherical silica nanoparticles. The surface roughness of the sapphire substrates measured by an Ambios Xi-100 surface profiler is 1.584 nm. Energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy revealed that some solid-state chemical reactions occurred during the CMP process. Possible mechanical effects of irregular silica nanoparticles in CMP process were also discussed.

Single-crystalline ZnO films with good crystal quality were grown by plasma-assisted molecular beam epitaxy (MBE) technique on c-plane sapphire substrates and implanted with fixed energy of 180-keV P and 100-keV O ions at 460°C. The implanted single-crystalline ZnO films exhibit p-Type characteristics with hole concentration in the range of 5.3 × 1017–1.5 × 1018 cm−3, hole mobilities between 1.4 cm2V−1 s−1 and 2.1 cm2V−1 s−1, and resistivities in the range of 0.672–1.832 Ωcm, as confirmed by Hall-effect measurements. The x-ray diffraction pattern of the implanted single-crystalline ZnO films exhibits (002) orientation (c-plane), with no other secondary phase appearing after ion implantation and dynamic annealing. It is deduced from x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy results that P ions were effectively implanted and formed acceptor complex PZn–2VZn, acting predominantly in all acceptors and achieving the goal of increasing the density of acceptors. Raman spectra and XPS results reflect that the enhanced solubility and stability of acceptor complexes in implanted single-crystalline ZnO films are related to the reduction of the concentration of oxygen vacancies by O ion implantation, achieving the goal of restraining the density of oxygen vacancies. It is concluded that the method of combining the increased density of acceptors and the restrained density of oxygen vacancies is meaningful and feasible, and afforded excellent p-type characteristics.

The effects of oxygen and nitrogen pressure on the structural and electrical properties of crystalline β-Ga2O3 films grown by pulsed laser deposition were investigated. We found that varying the oxygen pressure from 1 × 10−4 Pa to 1 × 10−1 Pa changed the resistivity from the order of 102–103 Ω cm. Adjusting the nitrogen pressure from 1 × 10−4 Pa to 1 × 10−1 Pa increased the resistivity from the order of 104–105 Ω cm. Oxygen and nitrogen pressures in the range from 1 × 10−4 Pa to 1 × 10−1 Pa had no obvious influence on the crystal quality, surface morphology, or transmittance of the β-Ga2O3 films grown at 500°C on sapphire substrates. All the films showed a (− 201) oriented monoclinic structure with smooth surfaces and high transmittance. The obtained β-Ga2O3 films have potential for applications in thin film resistors for monolithic microwave integrated circuits.

Cupric oxide thin films were deposited on silicon and sapphire substrates by reactive radio frequency magnetron sputtering at different substrate temperatures. The results showed that the CuO films were composed of different sizes of CuO nano-grains and the CuO films deposited on Si substrates showed a more dense and uniform surface than that deposited on Al2O3 substrates. It was noted that both the CuO films deposited on Si and Al2O3 substrates revealed only CuO related diffractions and the preferred orientation of the CuO films changed from (002) to (111) as the substrate temperature increased. Moreover, the carrier concentration was 1.141 × 1018 cm−3 and the mobility was 0.401 cm2/v s at 450°C substrate temperature. The controllable electrical properties of the films can be achieved by the variation of crystal quality arising from the substrate temperature.

Indium nitride (InN) films were grown on Si (111), bulk GaN, quartz and sapphire substrates by radio frequency magnetron sputtering. Prior to the film deposition, a zinc oxide (ZnO) buffer layer was deposited on all the substrates. The x-ray diffraction patterns of InN films on ZnO-buffered substrates indicated c-plane-oriented films whereas the Raman spectroscopy results indicated A1 (LO) and E2 (high) modes of InN on all the substrates. The crystalline quality of InN was found to be better on sapphire and quartz than on the other substrates. The surface roughness of InN was studied using an atomic force microscope. The results indicated higher surface roughness of the film on sapphire as compared to the others; however, roughness of the film was lower than 8 nm on all the substrates. The electrical properties indicated higher electron mobility of InN (20.20 cm2/Vs) on bulk GaN than on the other substrates. The optical band gap of InN film was more than 2 eV in all the cases and was attributed to high carrier concentration in the film.

Preparation of non-polar ZnO ( $$ 11\overline{2} 0 $$ ) films on single-crystal NdGaO3 (NGO) (001) substrates was successfully achieved by the radio frequency (RF) sputtering method. Orientation, deposition rate, and surface roughness of ZnO films strongly depend on the working pressure. Characteristics of ZnO films deposited on single-crystal NGO (001) substrates were compared with those of ZnO films deposited on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates. An x-ray diffraction peak of the ZnO ( $$ 11\overline{2} 0 $$ ) plane was observed on ZnO films deposited on single-crystal NGO (001) substrates under working pressure of less than 0.5 Pa. On the other hand, uniaxially oriented ZnO ( $$ 11\overline{2} 0 $$ ) films on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates were observed under working pressure of 0.1 Pa. The mechanism by which the diffraction angle of the ZnO ( $$ 11\overline{2} 0 $$ ) plane on single-crystal NGO (001) substrates was shifted is discussed on the basis of anisotropic stress of lattice mismatch. The deposition rate of ZnO films decreased with an increase in working pressure, and the deposition rate on single-crystal NGO (001) substrates was larger than that on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates. Root mean square (RMS) roughness of ZnO films increased with an increase in working pressure, and RMS roughness of ZnO films on single-crystal NGO (001) substrates was smaller than that of ZnO films on single-crystal sapphire ( $$ 01\overline{1} 2 $$ ) substrates even though the film thickness on single-crystal NGO (001) substrates was greater than that on sapphire substrates. It is thought that a single-crystal NGO (001) substrate is useful for deposition of non-polar ZnO ( $$ 11\overline{2} 0 $$ ) films.

Titanium oxide (TiO2) is a semiconducting oxide of increasing interest due to its chemical and thermal stability and broad applicability. In this study, thin films of TiO2 were deposited by pulsed laser deposition on sapphire and silicon substrates under various growth conditions, and characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption spectroscopy and Hall-effect measurements. XRD patterns revealed that a sapphire substrate is more suitable for the formation of the rutile phase in TiO2, while a silicon substrate yields a pure anatase phase, even at high-temperature growth. AFM images showed that the rutile TiO2 films grown at 805°C on a sapphire substrate have a smoother surface than anatase films grown at 620°C. Optical absorption spectra confirmed the band gap energy of 3.08 eV for the rutile phase and 3.29 eV for the anatase phase. All the deposited films exhibited the usual high resistivity of TiO2; however, when employed as a buffer layer, anatase TiO2 deposited on sapphire significantly improves the conductivity of indium gallium zinc oxide thin films. The study illustrates how to control the formation of TiO2 phases and reveals another interesting application for TiO2 as a buffer layer for transparent conducting oxides.

NiO thin films were deposited on sapphire and Corning 1737 glass substrates using the radio frequency magnetron sputtering system. The structures, optical and electrical properties of NiO films grown under different sputtering temperatures were thoroughly studied. The NiO films were composed of different-size NiO nano-grains with a strong (111) preferred orientation. The NiO grain size increased with the sputtering temperature increase. The band gap values decreased from 3.69 eV to 3.38 eV on the sputtering temperature increase from 30°C to 450°C. Moreover, the electrical property variations of the NiO samples were studied by the Hall effect in detail.

Plasma-assisted molecular beam epitaxy (PAMBE) was used to grow Ga2O3 films on oxidized GaN layers on nitrided sapphire substrates. The GaN layer was grown by PAMBE, and the in situ oxidation of the GaN layer was achieved through exposure to oxygen plasma, which resulted in the formation of monoclinic β-Ga2O3. Crystalline monoclinic β-Ga2O3 films were grown on the GaN layers, with and without oxidation. The orientation relationships were [\(11\overline{2} 0\)] Al2O3//[\(1\overline{1} 00\)] AlN//[\(1\overline{1} 00\)] GaN//[102] β-Ga2O3 and [\(1\overline{1} 00\)] Al2O3//[\(11\overline{2} 0\)] AlN//[\(11\overline{2} 0\)] GaN//[010] β-Ga2O3. The grown β-Ga2O3 films were not single-crystalline but showed rotational domains along the growth direction with three variations, which resulted in six-fold rotational symmetry instead of two-fold rotational symmetry. The surface roughness of the grown β-Ga2O3 film was closely reflected to that of as-grown GaN and oxidized GaN. By analyzing the x-ray omega rocking curves for the on-axis (\(\overline{2} 01\)) and off-axis (002) reflections, it was concluded that rotational domains dominantly affected the crystal quality of the β-Ga2O3 films.

In this paper, a microporous structure at the GaN/sapphire interface has been obtained by an electrochemical etching method via a selective etching progress using an as-grown GaN/sapphire wafer grown by metal organic chemical vapor deposition. The as-prepared GaN interfacial microporous structure has been used as a template for the following growth of thick-film GaN crystal by hydride vapor phase epitaxy (HVPE), facilitating the fabrication of a free-standing GaN substrate detached from a sapphire substrate. The evolution of the interfacial microporous structure has been investigated by varying the etching voltages and time, and the formation mechanism of interfacial microporous structure has been discussed in detail as well. Appropriate interfacial microporous structure is beneficial for separating the thick GaN crystal grown by HVPE from sapphire during the cooling down process. The separation that occurred at the place of interfacial microporous can be attributed to the large thermal strain between GaN and sapphire. This work realized the fabrication of a free-standing GaN substrate with high crystal quality and nearly no residual strain.

Zinc telluride (ZnTe) epilayers were grown on S-plane (\(10\bar{1}1\)) sapphire substrates by molecular beam epitaxy, and the epitaxial relationships between the two were compared with data previously obtained for layers grown on c-plane (0001), m-plane (\(10\bar{1}0 \)) substrates, and r-plane (\(1\bar{1}02 \)). The crystallographic relationship between the (111) plane of the ZnTe layer and (0001) plane of the substrate was studied using x-ray diffraction pole figure measurements. It was confirmed that two kinds of {111} oriented domains were formed on the S-plane substrate, and the dominant domain was (111)-oriented. Layers grown on S-plane substrate and on m-plane substrate exhibited the same epitaxial relationship, while the epitaxial relationship of the layer grown on the c-plane substrate exhibited a 60° rotation. These findings would be applicable to control the orientation of ZnTe epilayer surface for various device applications and for various physical property characterizations.

In this work, GaN epilayer in situ doped with Eu3+ ions was deposited on the top of an undoped n-GaN/LT-GaN/sapphire structure by organometallic vapor-phase epitaxy. A set of different ohmic and Schottky contacts on GaN:Eu3+ surface was fabricated by electron-beam evaporation for performing deep level transient spectroscopy (DLTS) measurement. The deep defect energy levels in GaN:Eu3+ epilayer were assessed by standard DLTS and high resolution Laplace DLTS (L-DLTS). Three dominant DLTS peaks were observed in the temperature range from 35 K to 400 K. The calculated activation energies of these defect energy levels were 0.108 ± 0.03 eV (Trap A), 0.287 ± 0.04 eV (Trap B) and 0.485 ± 0.06 eV (Trap C) below conduction band edge, respectively. High resolution L-DLTS conducted for the GaN:Eu3+ epilayer revealed at least four closely spaced defect energy levels associated with Trap B. It is proposed that these defect energy levels correspond to the selected optically active Eu3+ ion centers in GaN host previously identified by optical studies in this material (Fujiwara and Dierolf in Jpn J Appl Phys 53:05FA13, 2014).

To facilitate the design of future heterostructure devices employing two-dimensional (2D) materials such as molybdenum disulphide (MoS2) and hexagonal/sp2 boron nitride (BN), x-ray photoelectron spectroscopy (XPS) has been utilized to determine the valence band offset (VBO) present at interfaces formed between these materials. For MoS2 grown on a pulsed laser-deposited amorphous BN (a-BN) layer with sp2 bonding, the VBO was determined to be 1.4 ± 0.2 eV. Similarly, the VBO between the a-BN layer and the aluminum oxide (Al2O3) substrate was determined to be 1.1 ± 0.2 eV. Using the bandgaps established in the literature for MoS2, h-BN, and Al2O3, the conduction band offsets (CBOs) at the MoS2/a-BN and a-BN/Al2O3 interfaces were additionally calculated to be 3.3 ± 0.2 and 1.7 ± 0.2 eV, respectively. The resulting large VBOs and CBOs indicate BN and Al2O3 are attractive gate dielectrics and substrates for future 2D MoS2 devices.

We have studied the growth of CeO2 thin films by molecular beam epitaxy on r-cut sapphire substrates. The oxidation state of the substrate surface controls the growth direction of CeO2. Oxygen pre-annealed substrates favor (001) growth, while oxygen vacancies lead to a mixed (001) and (111) orientation. Combining pre- and post-annealing, it is possible to achieve single- oriented CeO2 in both growth directions. Furthermore, post-annealing results in a dramatic increase of crystallinity with a rocking curve width of the (002) reflection as small as 0.004°. We provide a consistent growth model involving oxygen vacancies at the substrate to thin film interface.

AgGaTe2, AgAlTe2, and Ag(Ga,Al)Te2 layers were grown by the closed-space sublimation method on c-plane sapphire substrates. The crystallographic properties of the AgGaTe2 and AgAlTe2 layers were then analyzed by x-ray diffraction (XRD). Very strong 112 diffraction peaks were observed in the XRD spectra of both layers, and it was clear the (112) orientation was predominant in both layers. The orientation and domain structure of the AgAlTe2 layer were carefully analyzed by XRD pole-figure measurement, and the effect of substrate surface atomic arrangement on the crystallinity of the AgAlTe2 layer was investigated. On the basis of the pole-figure image, six types of {112} domain were confirmed in the layer, and [ $$ \bar{1} $$ 10] in one domain was regarded as aligned with the sapphire’s a-axis when the layer was formed on the c-plane sapphire substrate surface. In addition to crystallographic characterization, the optical properties of the Ag(Ga,Al)Te2 layer were evaluated on the basis of transmittance measurements. The optical bandgap, derived from the transmittance spectrum, was approximately 2.0 eV. This value was between the bandgap values of AgGaTe2 and AgAlTe2. These results showed that an alloy of AgGaTe2 and AgAlTe2 had been grown successfully.

High-quality single-domain (ZnO[11–20]//Al2O3[10–10]) ZnO heteroepitaxial growth on c-face sapphire [Al2O3(0001)] substrate at low temperature was investigated by vanadium (V) doping at growth temperatures between 150°C and 450°C using radiofrequency (RF) magnetron sputtering. In low-temperature growth at 150°C and 200°C, 30° twisted domains (ZnO[10–10]//Al2O3[10–10]) were eliminated and good in-plane crystal orientation was obtained for V doping of about 2 at.% to 3 at.%. Single domains were grown from the early stage. From the changes of surface morphology and growth rate, it was considered that migration during the growth was enhanced due to the increase of the diffusion energy of adatoms, while random nucleation was suppressed.

Patterned sapphire substrate light-emitting diodes display obvious negative capacitance (NC) at large forward biases. This is measured using a method based on a small signal alternating current together with direct I–V plots. The NC in patterned sapphire substrate LEDs grows exponentially with the forward applied voltage. This observation is unexpected and in contrast with Shockley’s p–n junction theory, which only includes an increasing diffusion capacitance and not a NC. However, this result is in good agreement with conventional sapphire substrate LEDs. Furthermore, the negative terminal capacitance confirmed the prediction of Laux and Hess’ theory. The ideal factor of a patterned sapphire substrate LED is about 5, greatly exceeding the traditional theoretical value. The capacitance increased to a maximum and then gradually decreased, which was similar to the results for a p–n junction. Patterned sapphire substrate LEDs can withstand higher voltages than conventional sapphire substrate LEDs. This work could further confirm the existence of NC.

Hydride vapor phase epitaxy (HVPE) is used for the growth of low-defect GaN. We have grown undoped films on sapphire and investigated them using steady-state and time-resolved photoluminescence (PL). One of the dominant PL bands in high-quality GaN grown by HVPE is the green luminescence (GL) band with a maximum at 2.4 eV. This PL band can be easily recognized in time-resolved PL measurements due to its exponential decay even at low temperatures (

Gallium oxide is a wide-bandgap semiconductor material which is being developed for a range of electronic and electrooptic device applications. Commercial implementation of these devices will require production-scale technology for Ga2O3 film deposition. This work demonstrated deposition of uniform Ga2O3 films on both 50-mm-diameter (0001) sapphire and 200-mm-diameter (100) silicon substrates, using a rotating disc metalorganic chemical vapor deposition reactor. The source reactants were trimethylgallium and oxygen. The resulting Ga2O3 films were smooth, optically transparent, and highly insulating and had excellent thickness uniformity. Ga2O3 films deposited on (0001) sapphire at temperatures of at least 600°C and pressures of at least 45 Torr consisted of ( $$\overline{ 2} 0 1$$ )-oriented β-Ga2O3 in the as-deposited state. The β-crystal structure was shown to be stable on annealing to 800°C, in either air or nitrogen atmosphere. A Ga2O3 film deposited at a lower temperature was shown to crystallize to a similar ( $$\overline{ 2} 0 1$$ )-oriented β-Ga2O3 structure on postdeposition annealing at 800°C.

In this work, significant reduction of the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates via metalorganic chemical vapor deposition has been obtained by insertion of thin intermediate-temperature interlayers (IT-ILs). The growth temperature of the IT-ILs ranged from 750°C to 950°C after the initial growth at high temperature of 1200°C. Detailed characterizations were performed to understand the mechanisms of the reduction in dislocation density. It is found that the relatively low growth temperature of the IT-ILs can modify the originally two-dimensional (2D) growth mode to a three-dimensional (3D) growth process and creates a high density of small islands at the interface. During the subsequent growth of the high-temperature AlN layer, the AlN islands initially coalesce to form larger grains as the growth proceeds, and some TDs present in the previous AlN epilayers are found to bend near the interlayer, leading to dislocation merging and annihilation.

We report on a multichannel approach for AlGaN/GaN high-electron-mobility transistors (HEMTs) to increase the carrier mobility in the low and high sheet carrier concentration regimes. Between the AlGaN barrier and GaN buffer, alternating thin layers of AlN and GaN were inserted to create multiple two-dimensional electron gas (2DEG) channels. All samples were grown by metalorganic vapor-phase epitaxy (MOVPE) on sapphire substrates and compared with conventional single-channel HEMT with the same sheet carrier concentration. For low total 2DEG carrier concentrations ( 8 × 1012 cm−2), a mobility improvement is attributed to distribution of the sheet charge into multiple 2DEG channels. This has been found to reduce the carrier concentration per individual channel, which leads to larger distance between 2DEG and heterointerfaces and therefore less impact of interface roughness and alloy scattering. With increasing number of channels, phonon scattering is also reduced due to lowering of the three-dimensional (3D) electron density, which results from the added volume of the multiple 2DEGs. With this approach, the HEMT mobility was increased from 1620 cm2 V−1 s−1 to 1960 cm2 V−1 s−1 for a triple-channel device at sheet carrier concentration of 7.5 × 1012 cm−2.

We have investigated the thermoelectric properties of 2% Al-doped ZnO (AZO) thin films depending on the postdeposition cooling atmosphere [in oxygen pressure (AZO-O) or vacuum (AZO-V)]. Thin films were grown by pulsed laser deposition on sapphire (\(\hbox {Al}_2\hbox {O}_3\)) substrates at various deposition temperatures (\(400^{\circ }\hbox {C}\) to \(600^{\circ }\hbox {C}\)). All films were c-axis oriented. The electrical conductivity of AZO-V thin films was higher than that of AZO-O thin films across the whole temperature range from 300 K to 600 K, due to the optimal carrier concentration (\(10^{20}\,\hbox {cm}^{-3}\)) of AZO-V samples. Furthermore, the thermoelectric performance of AZO-V films increased with the deposition temperature; for instance, the highest power factor of \(0.87\,\times \,10^{-3}\,\hbox{W} \, \hbox{m}^{-1} \, \hbox{K}^{-2}\) and dimensionless figure of merit of 0.07 at 600 K were found for AZO-V thin film deposited at \(600^{\circ }\hbox {C}\).

AgGaTe2 and AgAlTe2 layers were grown on a-plane sapphire substrates by closed-space sublimation. These compounds replace Cd in CdTe with group I and III elements, and are, hence, expected to be ideal novel candidate materials for solar cells. The grown layers were confirmed to be stoichiometric AgGaTe2 and AgAlTe2 by x-ray diffraction (XRD). The AgAlTe2 layers had strong preference for the (112) orientation. The XRD spectrum of the AgGaTe2 layer was different from that of the AgAlTe2 layer, and strong peaks were observed for (103) and (110) diffraction. The variation in orientations of the grown layers was analyzed in detail by use of XRD pole figures, which revealed that the AgGaTe2 layers had an epitaxial relationship with the a-plane sapphire substrates.

Using metalorganic chemical vapor deposition, we heteroepitaxially grew undoped gallium nitride epilayers on sapphire. Assessing the epilayers at different growth stages, we investigated changes in epilayer strain and the lifetime of minority nonequilibrium charge carriers. The in-plane compressive strain was evaluated by x-ray diffraction and bandgap photoluminescence. The epilayer thickness ranged from 200 nm (islets) to 3.5 μm (continuous structure). The carrier lifetimes, measured using a light-induced transient grating technique, revealed a correlation between strain and the density of edge-type threading dislocations. This dislocation density was 109 cm−2 to 1011 cm−2, corresponding to the dominant mechanism for nonradiative carrier recombination. How the carrier lifetime depended on the growth stage differed between the surface and interfacial measurements. On the surface side, the carrier lifetime increased monotonically up to ~500 ps with thickness; on the interface side, the lifetime changed little with thickness, except in the thickest sample, where the carrier lifetime increased with thickness. We attributed this behavior to defect healing aided by long-term annealing, leading to mutual lateral motion and annihilation of mixed threading dislocations.

The properties of ZnO thin films codoped with lithium and phosphorus have been characterized. The films were deposited from high-purity ZnO and Li3PO4 solid targets onto c-plane sapphire substrates by radiofrequency (RF) magnetron sputtering. A substrate temperature of 900°C was determined as optimum for depositing undoped ZnO films with background electron concentration of 9.9 × 1015 cm−3 as the buffer layer on the sapphire substrate. Postdeposition annealing was carried out using rapid thermal processing in O2 at temperatures ranging from 500°C to 1000°C for 3 min. Analyses performed using low-temperature photoluminescence spectroscopy measurements revealed luminescence peaks at 3.356 eV, 3.307 eV, 3.248 eV, and 3.203 eV at 12 K for the codoped samples. X-ray diffraction 2θ-scans showed a single peak at about 34.4° with full-width at half-maximum of about 0.09°. Hall-effect measurements revealed initial p-type conductivities, but these were unstable and toggled between p-type and n-type over time with Hall concentrations that varied between 2.05 × 1013 cm−3 and 2.89 × 1015 cm−3. The fluctuation in the carrier type could be due to lateral inhomogeneity in the hole concentration caused by stacking faults in the films. An additional cause could be the small Hall voltages in the measurements, which could be significantly impacted by even small spikes in signal noise inherent in the measurements.

A comparative assessment of AlGaN/GaN high-electron-mobility transistors (HEMTs) grown by molecular beam epitaxy on silicon and sapphire substrates has been carried out. Large-area power GaN HEMTs with identical device dimensions were fabricated on both substrates. A thicker AlN buffer layer was used for the GaN HEMT on silicon to achieve similar quality and uniformity of GaN epitaxy for rational comparison with that grown on sapphire. Direct-current analysis and physical characterization were carried out to understand the performance of the devices. Mathematical measurement of the instability of the current–voltage (I–V) characteristic at high applied drain bias was carried out to evaluate the performance of both devices. An improved two-dimensional (2D) analysis of the I–V characteristic was performed from a thermal perspective including appropriate scattering effects on the 2D electron gas mobility. The experimental and analytical studies were correlated to reveal the effects of temperature-sensitive scattering phenomena on the mobility as well as on the I–V characteristic at high drain bias in terms of lattice thermal heating. It is observed that the HEMT on Si has improved stability compared with sapphire due to its weaker scattering phenomena at high drain bias, associated with its thermal conductivity. Simulation of 2D thermal mapping was also carried out to distinguish the hot-spot regions of the devices. The comparable electrical performance of these devices illustrates the viability of AlGaN/GaN HEMTs on Si(111) to achieve low-cost stable devices with better thermal power handling for high-voltage applications.