Due to the advantages of both large size, low cost, and compatibility with existing CMOS processes, GaN based ratio frequency (RF) electronic materials and devices on Si substrates have become the next focus of attention in this field after power electronic devices. Due to the different mechanical properties of high-resistivity Si substrates, it is still difficult to control stress and suppress dislocations in the growth of GaN based epitaxial materials on high-resistivity Si substrates, as well as address serious RF loss issues that limit their applications in the field of RF electronics. This work briefly introduces the research status and challenges of GaN based RF electronic materials on Si substrates, focusing on the main research progress of the Peking University research team in the formation mechanism of RF loss, epitaxial growth of GaN on high-resistivity Si substrates with low dislocation density and low RF loss. Finally, the future development of GaN based RF electronic materials and devices on Si substrates is prospected.

Diamond has excellent material properties such as wide band gap (5.47 eV), high carrier mobility (3 800 cm2/(V·s) for holes and 4 500 cm2/(V·s) for electrons), high thermal conductivity (22 W·cm-1·K-1), high critical breakdown field strength (>10 MV/cm), and optimal Baliga’s figure of merit, which make diamond semiconductor devices ideal choices for the applications in extreme conditions such as high temperature, high frequency, high power, and strong irradiation. With the breakthroughs in diamond crystal growth by CVD techniques and p-type doping of diamond crystal, research on Schottky barrier diode (SBD) based on boron-doped diamond has been widely carried out. In this review, the working principles of the diamond SBD are introduced in detail. The growth processes of highly doped p-type thick films, p-type films of low doping drift region are investigated, and the conditions for the formation of ohmic contact and Schottky contact between different metals and diamond crystals are studied. Then the preparation processes of transverse, vertical and pseudo-vertical device structures and their effect on forward, reverse and breakdown characteristics of the SBDs are analyzed. The modulation of the internal electric field of SBDs by device structures like field plate, passivation layer and edge terminal, which strengthens the reverse breakdown voltage of the device, is illustrated. Finally, the application prospects and challenges of diamond SBDs are summarized.

A novel coupled barrier heterojunction structure (i.e. Al0.26Ga0.74N/GaN/Al0.20Ga0.80N/GaN) was proposed and prepared in this paper to improve transconductance linearity of GaN high electron mobility transistor. A double two-dimensional electron gas (2DEG) channel in the coupled barrier heterojunction structure is demonstrated by theoretically calculation and capacitance-voltage (C-V) measurement. 2DEG mobility of 1 510 cm2·V-1·s-1 and sheet density of 9.7×1012 cm-2 are obtained. Benefiting from the double channels, the transconductance profile of the device with coupled heterojunction barrier structure shows two peaks, and the voltage swing is 3.0 V, which is 1.5 times larger than that of normal structure. First-order and second-order derivative characteristics of transconductance indicate that the device of the coupled barrier structure has higher harmonic suppression capability. This study demonstrates a superiority of coupled barrier heterojunction in the area of high linearity application.

KOH etching is a convenient method with high efficiency for characterizing dislocations in SiC. However, the etching process is often affected due to the hydrolysis of KOH. To ensure stability and reproducibility of the etching process, frequent replacement of KOH is necessary, which leads to a large consumption of the etching materials. In this study, an improved method for KOH etching technique was proposed by directly injecting dry air into the molten KOH through a bubbler, in order to quickly remove moisture from the molten KOH and enhance dissolved oxygen. The effectiveness of the method is verified by etching lightly doped n-type epitaxial wafers and heavily doped n-type substrates. The experimental results show that introducing dry air into the molten KOH during the constant temperature stage before etching the SiC epitaxial wafer accelerate the evaporation of moisture in partially deliquescent KOH and achieve a faster etching rate of dislocations than fresh KOH. In the substrate etching experiment, an increase in the concentration of dissolved oxygen in the etchant is obtained by introducing dry air into the molten KOH, which promotes the etching effect through promoting the chemical reaction on SiC surface. As a result, an etching rate of dislocations similar to that of the mixed etchant of Na2O2 and KOH is achieved. This research provides a helpful method to improve the etching effect of SiC dislocations, which would have practical value in SiC production.

4H-SiC single crystal is a typically difficult material to process. The density and depth of surface damage after grinding directly affect the quality and efficiency of subsequent polishing operations. Grinding using traditional cast iron discs always results in significant scratches, broken edges, and an unstable removal rate for SiC substrates. In this experiment, a polyurethane pad was used to reduce post-grinding scratches, improve post-grinding surface quality, and achieve precise grinding of SiC substrates. The grinding experiments were carried out by changing the particle size of diamond abrasives, the rotating speed of discs, and the pressure added to the carrier disc. Our results show that the removal rate of SiC substrates increases with the increase of the rotating speed of discs, and the corresponding surface roughness after grinding decreases first and then increases. The removal rate of the SiC substrate increases with the increase of the particle size of diamond abrasives, but the surface roughness continues to increase. As the pressure added to the carrier disc increases, both removal rate and surface roughness after grinding increase, however, the increasing rate for the former would decrease, and the increasing rate for the latter would increase. Based on the experiment results, the optimal grinding conditions are as follows: the mass ratio of diamond in the slurry is 3%, the particle size of diamond abrasives is 1 μm, the supply rate for slurry is 5 mL/min, the pressure added on the carrier disc is 47 kPa, and the rotating speed of the disc is 35 r/min. Under this condition, a removal rate of 0.7 μm/h and surface roughness after grinding of 24 nm are achieved.

Silicon photonics is the core technology in the post-Moore’s era, characterized by the deep integration of optoelectronics and microelectronics. Silicon photonics can leverage the existing complementary metal-oxide-semiconductor (CMOS) infrastructure to fabricate low power consumption, high integration density, fast transmission speed, and high-reliability silicon photonic chips which are widely employed in data centers and communication systems. At present, most optoelectronic devices like Si-based photodetectors and Si-based optical modulators have realized on-chip integration except for the Si-based lasers as essential light sources. The directly epitaxial III-V materials on silicon substrates is recognized as one of the most promising solutions to achieve low-cost and large-size monolithic integration of Si-based lasers, still facing many significant challenges. In this paper, the research progress of Si-based light sources is presented from the aspects of directly epitaxial on-axis III-V/Si (001) substrates, on-axis Si-based laser materials, epitaxy technology and monolithic integration at first. Then the achievements in Si-based directly epitaxial quantum well lasers and quantum dot lasers in our group are reported in detail, including the growth of antiphase domains-free GaAs/Si (001) substrates, epitaxial materials of InGaAs/AlGaAs quantum well lasers and InAs/GaAs quantum dot lasers, and fabrication of novel coplanar electrode structures of silicon photonic chips in parallel mode.

Achieving AlGaN epitaxial layer with high Al content and excellent electrical properties is one of the most important aspects in the preparation of deep ultraviolet optoelectronic devices. In this work, a series of Si-AlxGa1-xN epitaxial layers with high Al content (x＞0.60) were obtained on AlN/sapphire substrates using molecular beam epitaxy (MBE) system, based on the growth method of periodic thermal desorption. Al content was modulated by changing the size of Al source supply, and n-type doping was realized with Si. The physical properties of the epitaxial layers were characterized. The results show that: Al content of the epitaxial layers shows a linear relationship with the Al flux size, which lays the foundation for the growth of AlGaN epitaxial layers with precise content. The AFM measurements reflect that the surface morphology of the AlGaN epitaxial layers strongly depend on the Ga supply. The electrical properties of Si-AlGaN epitaxial layers were measured based on the Vanderbilt method, and the good performance was confirmed. The free electron concentration, electron mobility and resistivity of the sample with Al content of 0.93 reach 8.9×1018 cm-3, 3.8 cm2·V-1·s-1 and 0.18 Ω·cm respectively at room temperature.

Epitaxial growth of InN films on GaN (0001) by metal organic chemical vapor deposition (MOCVD) was investigated with adding an InGaN bedding layer. It is found that pure InN can be obtained on the InGaN bedding layer with quality higher than on bare GaN surface. By adopting proper In composition (In0.23Ga0.77N in this work) in the InGaN bedding layer, indium droplet formation can be totally suppressed during the growth of InN. The structure and optical properties of InN were investigated by using optical microscopy, high-resolution X-ray diffraction (HR-XRD), transmission electron microscopy (TEM), photoabsorption and photoluminescence at room temperature. From the HR-XRD measurements, the ω and ω-2θ scans show the elimination of In diffraction signal by using the InGaN bedding layer and the ω scan gives a linewidth of 0.23° for 150 nm InN film. From the selective area electron diffraction pattern in the TEM, the InN is found to be almost strain free. Both photoabsorption and strong photoluminescence of InN film at room temperature indicate that the energy band gap of the InN is about 0.74 eV. Abnormal excitation-dependent photoluminescence behavior of the InN was preliminarily investigated too. It is proved that the surface effect of InN materials has a strong effect on the radiation recombination.

Nowadays, with the rapid development of electronic information technology, solar-blind ultraviolet (UV) detectors having large anti-interference ability and higher sensitivity are attracting much attention. Possessing ultra-wide band gap, high optical absorption coefficient, good thermal conductivity, and high breakdown field strength, hexagonal boron nitride (h-BN) has become a promising material for solar-blind UV detectors. Additionally, high mechanical strength and optical transparency of h-BN make it a potential candidate for flexible detector. However, the h-BN films prepared at low temperatures often contain many defects, which greatly limit their performance as a detector. In this work, high-quality h-BN films were prepared on sapphire and Si substrates using B as growth source at room temperature by reactive magnetron sputtering method, and then a high-performance solar-blind UV detector was fabricated based on h-BN films. The prepared h-BN solar-blind UV detector has extremely low dark current (0.07 pA), high responsivity (1.37 μA/W) and detectivity (2.73×1010 Jones) at 3 V. These results indicate that preparation of high-quality h-BN film and its solar-blind UV detector at room temperature are feasible. The study also provides a reference for the application of h-BN detector working at room temperature.

In this study, a large area laser lift-off (LLO) of GaN-based micro light-emitting diodes (Micro LED) was achieved using an excimer laser at 248 nm. The critical laser energy density required for separation of devices is 800~835 mJ·cm-2. The separated device is intact with a residual stress of 0.071 4 GPa and a mean square roughness of 0.597 nm, which is much lower than that of the LLO method reported so far. This study provides a promising idea for the fabrication of GaN-based Micro LED chips with high quality and high efficiency, which is of great significance for the fabrication of flexible GaN-based devices.

Micro light emitting diode display (Micro-LED) has a wide range of application prospect as a new display technology and has been developed rapidly in recent years. However, as the size decreases, the luminous characteristics efficiency of Micro-LED decreases sharply, which is due to the influence of sidewall damage. In this paper, the Micro-LED mesa structures of different sizes were fabricated by photoetching and inductively coupled plasma (ICP) etching. The surface physical damage and enrichment of impurity elements caused by etching on Micro-LED were analyzed. Tetramethylammonium hydroxide (TMAH) was used to repair the sidewall damage and cathodoluminescence (CL) was used to characterize the optical properties. The results show that with the decrease of the size, the impact of sidewall damage becomes more serious. After passivation with TMAH, the sidewall can be effectively repaired and the luminescence intensity of Micro-LED can be effectively improved.

Three-junction 905 nm vertical-cavity surface-emitting laser (VCSEL) was designed and fabricated for medium and long range sensing applications such as lidar. Through simulations using PICS3D software, the optimal adjacent active region spacing and number of P-type distributed Bragg reflectors (DBR) in the multi-junction VCSEL were determined, resulting in a design featuring a 2λ adjacent active region spacing and 14 pairs of P-type DBR. Using this design, a 100-element three-junction VCSEL array was grown and fabricated with an oxide aperture of 15 μm. The resulting array achieves a maximum peak power of 24.7 W and a peak power density of 182 W/mm2 under narrow pulse conditions (with a pulse width of 100 ns and a duty cycle of 0.05%).

As an ultrawide band gap semiconductor, two-dimensional (2D) hexagonal boron nitride (h-BN) has attracted considerable research interest because of its unique properties such as excellent electrical insulation, high breakdown field, high thermal conductivity, and good chemical inertness, which make it a promising candidate as dielectric and substrate layers for devices based on other 2D materials. The synthesis of high-quality and large-area h-BN layers with few defects is strongly desirable for these applications. In this review, the methods and progress of the epitaxial growth of 2D h-BN on transition metal, h-BN films on dielectric and semiconductor substrates are described in detail. The high-quality 2D h-BN layers can be epitaxially grown on the transition metal substrates with catalytic activity including Cu, Ni, Fe, Pt, etc. However, it is a great challenge to directly grow h-BN single crystal films on insulating dielectric or semiconductor substrates. Sapphire is a preferred substrate for the growth of h-BN for its good thermal stability and chemical stability. The growth of h-BN thin films on sapphire substrate has been widely reported by various techniques, such as chemical vapor deposition, molecular beam epitaxy, ion beam sputtering deposition, metal-organic vapor phase epitaxy, high-temperature post-annealing and so on. Based on these techniques, high-quality 2D h-BN has been prepared on sapphire substrates, it can also be integrated into the epitaxial growth processes of some existing III-V compound semiconductors, laying the foundation for the large-scale application of h-BN. In addition, the growth of h-BN single crystal films have also been attempted on semiconductor substrates like graphene, silicon and germanium, which provide an attractive strategy for the fabrication and application of h-BN-based heterojunctions.

CuI films can be prepared by the iodization of Cu metal films. In order to optimize the photoelectric properties of iodized CuI film, the CuI films were prepared with layer by layer iodization method, and then the CuI films were treated by post annealing and layer by layer annealing process. The structure, surface morphology and photoelectric properties of the samples by different annealing process were analyzed. The crystallinity of the CuI film is improved significantly after layer by layer annealing process. The surface density increases under the post annealing process, which can be ascribed to the movement of CuI grains along the two-dimensional plane. The transmittance of the CuI film increases to 80%~90% after post annealing process, and the resistivity of the CuI film is optimized to 0.034 Ω·cm after layer by layer annealing.

Chemical vapor deposition (CVD) is an effective method for the growth of large scale two-dimensional materials. However, high density of vacancy defects are inevitably generated in the CVD process, which affects the photoelectric properties of the materials. In this work, millimeter-scale and atomic-thick WS2 membranes were directly grown on p-type Si (111) substrates using an alkali metal halide-assisted CVD method. Er-doped WS2 films (WS2(Er)) were achieved by adding saturated ErCl3 powders in the tungsten sources. Optical microscopy, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, photoluminescence and Raman spectroscopy techniques were used to characterize the materials systematically. The results show that the fluorescence intensity of WS2(Er) films increase by an order of magnitude compared to pristine WS2, and the central wavelength is significantly red-shifted. Fluorescence tests show that the fluorescence characteristics of WS2 grown on Si substrates show a significant charge transfer effect compared to that of WS2 on SiO2 substrates. The photoelectric test results of WS2 and WS2(Er) field effect transistors based on SiO2 substrate show that the optical responsiveness of WS2(Er) field effect transistors is 4.015 A/W, and the external quantum efficiency is 784%, both of which are more than 2 000 times that of pristine WS2 devices under the same conditions. This work has significance for research of rare earth doping in 2D materials.

The excellent physical properties of diamond make it one of the most promising semiconductor materials for the next generation. At present, heteroepitaxy based on microwave plasma chemical vapor deposition may be the best method for preparing large-scale single crystal diamond in the future. In the past three decades, some progress has been made in the heteroepitaxial growth of single crystal diamond on iridium-based composite substrates, especially in recent years, the growth of large-scale single crystal diamond of more than 2 inch has been realized. This paper summarizes the various substrates used for diamond heteroepitaxy, briefly introduces bias-enhanced nucleation on heterogeneous substrates, and details the most successful iridium/oxide, iridium/oxide layer/silicon composite substrates. In the end, the problems existing in diamond heterogeneous substrates and heteroepitaxy are summarized, and some possible solutions are given.

In this paper, conventional continuous epitaxial growth and metal modulated epitaxial (MME) growth of AlN were investigated with the plasma-assisted molecular beam epitaxy (PA-MBE) system. It is difficult to control the growth mode of the conventional continuous epitaxial growth method, in which excessive Al-rich and N-rich growth modes easily occur, and also a slightly Al-rich growth mode is accompanied by the appearance of some pits, leading to rough surface morphology. However, the growth mode of AlN films is easier to control by MME method, by which AlN films with good morphology can be fabricated by adjusting the supply time of Al and N sources. The optimized MME solution is proposed as follows: firstly, opening the Al source shutter for 30 s, then opening the Al and N source shutters for 60 s, and finally opening the N source shutter alone for 72 s, the ratio of Al source shutter opening time to N source shutter opening time of a single cycle is 0.7. Almost pits-free AlN films with a low roughness of 0.3 nm (2 μm×2 μm) are fabricated after 40 cycles.

α-MoO3 is a typical layered semiconductor transition metal oxide, and its unique electronic structure and lattice structure have made it widely studied in recent years. Compared with bulk α-MoO3, α-MoO3 film has excellent optical, electrical and mechanical properties due to its two-dimensional geometric limitations. However, the epitaxial growth of single-layer defect-free α-MoO3 film has not been realized so far. In this paper, molecular beam epitaxy was used in ultra-high vacuum, and a single-layer defect-free semiconductor α-MoO3 film was prepared by van der Waals epitaxial on highly oriented pyrolytic graphite (HOPG) for the first time, and linear defects were generated by hydrogen reduction. The microstructure and apparent energy gap of the defect-free single-layer film and the defect were studied by scanning tunneling microscope (STM). The results show that, high-quality single-layer α-MoO3 films can be prepared on HOPG substrates by accurately controlling the substrate temperature. The film thickness and cell size conform to the characteristics of single-layer α-MoO3, and the apparent energy gap of 1.7 eV is determined by the scanning tunnel spectrum. The high quality of the grown film can be further confirmed by the moire pattern on the substrate HOPG. By introducing hydrogen, bright line defects perpendicular to the moire pattern can be obtained on the surface of the film, and the local apparent energy gap of the line defects is 0.4 eV, that is, a narrowband semiconductor channel is realized inside a broadband semiconductor.

Remote epitaxy is an emerging technology for producing single-crystalline, free-standing thin films and structures. The method uses 2D van der Waals materials as semi-transparent interlayers that enable epitaxy and release of epitaxial layers at the 2D layer interface. The use of single-layer graphene as the interlayer for heterogeneous remote epitaxial GaN nucleation layer and GaN film on the sapphire substrate was studied. The result shows that GaN nucleation island has a good orientation. Through parameter adjustment, a dense GaN nucleation layer was obtained. AFM observation and XRD detection confirm that GaN thin film has lower surface roughness and dislocation density compared with film that grow directly on sapphire substrate under the same conditions. The existence of graphene after growing GaN film was confirmed by Raman spectra.

With the widespread application of gallium nitride (GaN) in the high-power field, the heat dissipation of GaN based devices has become the main factor restricting the power density. Therefore, it is crucial to develop novel thermal management solutions. Diamond substrates with high thermal conductivity can be used to improve the heat dissipation of GaN devices. However, due to the natural lattice mismatch between diamond and GaN, the direct epitaxy of GaN on diamond substrates remains an insurmountable problem. This work achieved van der Waals epitaxy of single crystal GaN films on polycrystalline diamond substrates using a two-dimensional material/Al gradient AlGaN heterostructure as the nucleation layer between the substrate and the epitaxial layer. Among them, two-dimensional materials can effectively shield against the adverse effects caused by lattice mismatch between the substrate and epitaxial layer, while the Al component gradient AlGaN buffer layer can achieve orderly migration of Ga and N atoms, thereby accurately controlling the growth of GaN thin films. This work provides a novel approach for high-quality growth of nitrides on heterogeneous substrates. The experimental results indicate that the introduction of nucleation layers effectively eliminates the impact of lattice mismatch, thereby breaking the bottleneck of the difficulty in directly epitaxial single crystal GaN films on diamond substrates. This work provides a foundation for further improving the power density of GaN based devices.

Tungsten disulfide (WS2) has a wide application prospect in the field of optoelectronic devices due to its properties such as tunable band gap, strong light-matter interaction and high carrier mobility and so on. Dendritic WS2/monolayer WS2 was grown on SiO2/Si substrate by atmospheric pressure chemical vapor deposition (CVD). Sulphur powder and transition metal oxides were used as precursors. The morphological evolution of the samples on the substrate was divided into four regions: superimposed growth region (Ⅳ), dendritic WS2 growth region (Ⅲ), hexagonal WS2 growth region (Ⅱ) and no obvious morphological region (Ⅰ). The differences in the number, morphology, structure and properties of the prepared dendritic WS2/monolayer WS2 on the substrates were systematically compared by optical microscopy, atomic force microscopy, Raman spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, scanning electron microscopy and other testing methods. It is found that the morphology of the dendritic WS2 affects the actual defect concentration, thus affects the position of the Raman characteristic peaks. The growth mechanism of the morphological evolution was explained by the atomic adsorption model and the variation of the S and W vapor ratio. In addition, back-gated field effect transistor (FET) based on dendritic WS2/monolayer WS2 homojunction with a responsivity of 46.6 mA/W is prepared, and its response and recovery time are in the microsecond range, showing its superior performances than the monolayer WS2-FET prepared by CVD. This work will help to further understand the controlled growth of two-dimensional thin film materials and contribute to the preparation of large-area, high-quality dendrite structures.

High-speed wafer rotation vertical hot-wall chemical vapor deposition (CVD) was employed to conduct home-epitaxial growth on n-type 〈1010〉 4° off-angel 4H-SiC wafer. With temperature of 1 650 ℃, pressure of 250 mbar and rotation rate of 600 r/min, growth rate of over 40.44 μm/h was achieved, epi-layer with thickness uniformity of 1.37% and doping concentration uniformity of 2.79% was obtained. AFM testing indicates that the surface roughness is 0.11 nm. Leica microscope indicates that the surface of epilayer is smooth and no macro-step existing, the sharp Raman lines show typical features of 4H-SiC, and defect density is very low. Comprehensive analysis shows that high-quality SiC epilayer is obtained with high growth rate by national high-speed wafer rotation vertical hot-wall CVD equipment. This research provides some guidance for current SiC epitaxial industry development and equipment localization.