Study on the nitridation of β-Ga_2O_3 films

Single-crystal GaN layers have been obtained by nitriding β-Ga_2O_3 films in NH_3 atmosphere. The effect of the temperature and time on the nitridation and conversion of Ga_2O_3 films have been investigated. The nitridation process results in lots of holes in the surface of films. The higher nitridation temperature and longer time can promote the nitridation and improve the crystal quality of GaN films. The converted Ga N porous films show the single-crystal structures and lowstress, which can be used as templates for the epitaxial growth of high-quality GaN.     

Synthesis and characterization of β-Ga_2O_3@GaN nanowires

In this work, we prepared the β-Ga_2O_3@GaN nanowires(NWs) by oxidizing GaN NWs. High-quality hexagonal wurtzite GaN NWs were achieved and the conversion from GaN to β-Ga_2O_3 was confirmed by x-ray diffraction, Raman spectroscopy and transmission electron microscopy. The effect of the oxidation temperature and time on the oxidation degree of GaN NWs was investigated systematically. The oxidation rate of GaN NWs was estimated at different temperatures.     

Investigation of β-Ga_2O_3 films and β-Ga_2O_3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201) β-Ga_2O_3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD). It is revealed that the β-Ga_2O_3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga_2O_3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga_2O_3 film and the β-Ga_2O_3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga_2O_3 film. Moreover, the energy band structure of β-Ga_2O_3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga_2O_3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga_2O_3/GaN heterostructures in microelectronic applications.     

Device topological thermal management of β-Ga_2O_3 Schottky barrier diodes

The ultra-wide bandgap semiconductor β gallium oxide(β-Ga_2 O_3) gives promise to low conduction loss and high power for electronic devices. However, due to the natural poor thermal conductivity of β-Ga_2 O_3, their power devices suffer from serious self-heating effect. To overcome this problem, we emphasize on the effect of device structure on peak temperature in β-Ga_2 O_3 Schottky barrier diodes(SBDs) using TCAD simulation and experiment. The SBD topologies including crystal orientation of β-Ga_2 O_3, work function of Schottky metal, anode area, and thickness, were simulated in TCAD, showing that the thickness of β-Ga_2 O_3 plays a key role in reducing the peak temperature of diodes. Hence, we fabricated β-Ga_2 O_3 SBDs with three different thickness epitaxial layers and five different thickness substrates. The surface temperature of the diodes was measured using an infrared thermal imaging camera. The experimental results are consistent with the simulation results. Thus, our results provide a new thermal management strategy for high power β-Ga_2 O_3 diode.     

Study on nitridation processes of β-Ga2O3 single crystal

Nitridated /3-Ga2Os (100) substrate was investigated as the substrate for CaN epitaxial growth.The effects of nitridation temperature and surface roughness of -Ga2O3 wafers on the formation of CaN were studied.     

Characterization of vertical Au/β-Ga_2O_3 single-crystal Schottky photodiodes with MBE-grown high-resistivity epitaxial layer

High-resistivity β-Ga_2O_3 thin films were grown on Si-doped n-type conductive β-Ga_2O_3 single crystals by molecular beam epitaxy(MBE).Vertical-type Schottky diodes were fabricated,and the electrical properties of the Schottky diodes were studied in this letter.The ideality factor and the series resistance of the Schottky diodes were estimated to be about1.4 and 4.6×10~6 Ω.The ionized donor concentration and the spreading voltage in the Schottky diodes region are about4×10~(18)cm~(-3) and 7.6 V,respectively.The ultra-violet(UV) photo-sensitivity of the Schottky diodes was demonstrated by a low-pressure mercury lamp illumination.A photoresponsivity of 1.8 A/W and an external quantum efficiency of8.7 ×10~2%were observed at forward bias voltage of 3.8 V,the proper driving voltage of read-out integrated circuit for UV camera.The gain of the Schottky diode was attributed to the existence of a potential barrier in the i-n junction between the MBE-grown highly resistive β-Ga_2O_3 thin films and the n-type conductive β-Ga_2O_3 single-crystal substrate.     

High performance lateral Schottky diodes based on quasi-degenerated Ga_2O_3

Ni/β-Ga_2 O_3 lateral Schottky barrier diodes(SBDs) were fabricated on a Sn-doped quasi-degenerate n~+-Ga_2 O_3(201)bulk substrate. The resultant diodes with an area of 7.85 ×10~(-5) cm~2 exhibited excellent rectifying characteristics with an ideality factor of 1.21, a forward current density(J) of 127.4 A/cm2 at 1.4 V, a specific on-state resistance(R_(on,sp)) of1.54 mΩ·cm~2,and an ultra-high on/off ratio of 2.1 ×10~(11) at±1 V. Due to a small depletion region in the highly-doped substrate, a breakdown feature was observed at-23 V, which corresponded to a breakdown field of 2.1 MV/cm and a power figure-of-merit(VB2/R_(on)) of 3.4×10~5 W/cm~2. Forward current-voltage characteristics were described well by the thermionic emission theory while thermionic field emission and trap-assisted tunneling were the dominant transport mechanisms at low and high reverse biases, respectively, which was a result of the contribution of deep-level traps at the metal-semiconductor interface. The presence of interfacial traps also caused the difference in Schottky barrier heights of 1.31 eV and 1.64 eV respectively determined by current-voltage and capacitance-voltage characteristics. With reduced trapping effect and incorporation of drift layers, the β-Ga_2 O_3 SBDs could further provide promising materials for delivering both high current output and high breakdown voltage.     

Influence of annealing treatment on the luminescent properties of Ta:β-Ga_2O_3 single crystal

Ta~(5+)doped β-Ga_2O_3 single crystals were grown by using the optical floating zone method, and then annealed in the air and nitrogen gas at 1400℃ for 20 hours.The transmittance spectra, photoluminescence(PL), x-ray irradiation spectra, and PL decay profiles of the samples were measured at room temperature.The relevant results show that the optical transmittance of the samples annealed in the air or nitrogen gas was improved.By drawing the(ahv)~2–hv graph,it can be seen that the band gap decreased after being annealed in the air, but increased in nitrogen gas.The PL spectra and x-ray irradiation spectra show that the luminescent intensity of the sample annealed in the air increased substantially,while decreased for the sample annealed in nitrogen.The PL decay time of the Ta:β-Ga_2O_3 annealed in the air increased significantly compared with that of the Ta:β-Ga_2O_3 sample without annealing, but the tendency after annealing in nitrogen gas was opposite.     

β-Ga_2O_3 MOSFETs on the Si substrate fabricated by the ion-cutting process

β-Ga_2O_3 MOSFETs are demonstrated on heterogeneous Ga_2O_3-Al_2O_3-Si(GaOISi)substrate fabricated by ion-cutting process.Enhancement(E)-and depletion(D)-modeβ-Ga_2O_3 transistors are realized on by varying the channel thickness(T_(ch)).E-mode GaOISi transistor with a T_(ch)of 15 nm achieves a high threshold voltage V_(TH)of～8 V.With the same T increase,GaOISi transistors demonstrate more stable ON-current I_(ON)and OFF-current I_(OFF)performance compared to the reported devices on bulk Ga_2O_3 wafer.Transistors on GaOISi achieve the breakdown voltage of 522 and 391 V at 25°C and 200°C,respectively.     

Electronic structures and optical properties of Si-and Sn-doped β-Ga_2O_3: A GGA+U study

The electronic structures and optical properties of β-Ga_2O_3 and Si-and Sn-doped β-Ga_2O_3 are studied using the GGA + U method based on density functional theory. The calculated bandgap and Ga 3d-state peak of β-Ga_2O_3 are in good agreement with experimental results. Si-and Sn-doped β-Ga_2O_3 tend to form under O-poor conditions, and the formation energy of Si-doped β-Ga_2O_3 is larger than that of Sn-doped β-Ga_2O_3 because of the large bond length variation between Ga–O and Si–O. Si-and Sn-doped β-Ga_2O_3 have wider optical gaps than β-Ga_2O_3, due to the Burstein–Moss effect and the bandgap renormalization effect. Si-doped β-Ga_2O_3 shows better electron conductivity and a higher optical absorption edge than Sn-doped β-Ga_2O_3, so Si is more suitable as a dopant of n-type β-Ga_2O_3, which can be applied in deep-UV photoelectric devices.     

Degradation of β-Ga_2O_3 Schottky barrier diode under swift heavy ion irradiation

The electrical characteristics and microstructures of β-Ga_2 O_3 Schottky barrier diode(SBD) devices irradiated with swift heavy ions(2096 Me V Ta ions) have been studied. It was found that β-Ga_2 O_3 SBD devices showed the reliability degradation after irradiation, including turn-on voltage Von, on-resistance Ron, ideality factor n, and the reverse leakage current density Jr. In addition, the carrier concentration of the drift layer was decreased significantly and the calculated carrier removal rates were 5 × 10~6–1.3 × 10~7 cm~(-1). Latent tracks induced by swift heavy ions were observed visually in the whole β-Ga_2 O_3 matrix. Furthermore, crystal structure of tracks was amorphized completely. The latent tracks induced by Ta ions bombardments were found to be the reason for the decrease in carrier mobility and carrier concentration. Eventually,these defects caused the degradation of electrical characteristics of the devices. In terms of the carrier removal rates, theβ-Ga_2 O_3 SBD devices were more sensitive to swift heavy ions irradiation than Si C and Ga N devices.     

Preparation and NO_2-gas sensing property of individual β-Ga_2O_3 nanobelt

Monoclinic gallium oxide (β-Ga_2O_3) nanobelts are synthesized from gallium and oxygen by thermal evaporation in an argon atmosphere and their NO_2 sensing properties are studied at room temperature.Electron microscopy studies show that the nanobelts have breadths ranging from 30 to 50 nm and lengths up to tens of micrometers.Both the x-ray diffraction (XRD) and the selected are electron diffraction (SAED) examinations indicate that β-Ga_2O_3 nanobelts have grown into single crystals.Room temperature NO_2 sensing tests show that the current of individual β-Ga_2O_3 nanobelt decreases quickly,and then gently when the NO_2 concentration increases from low to high.It is caused by the NO_2 molecule chemisorption and desorption processes in the surface of β-Ga_2O_3 nanobelt.     

Quantum dots-templated growth of strain-relaxed GaN on a c-plane sapphire by radio-frequency molecular beam epitaxy

We investigated the quantum dots-templated growth of a(0001) GaN film on a c-plane sapphire substrate.The growth was carried out in a radio-frequency molecular beam epitaxy system.The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images,as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer.The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra.The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves.Meanwhile,the threading dislocation density in the quantum dots-templated film was estimated to be 7.1×107cm-2,which was significantly suppressed compared with that of the AlN-buffered GaN film.The roomtemperature Hall measurement showed an electron mobility of up to 1860cm2 /V·s in the two-dimensional electron gas at the interface of the Al 0.25Ga0.75 N/GaN heterojunction.     

GaP layers grown on GaN with and without buffer layers

The growth of GaP layer on GaN with and without buffer layers by metal-organic chemical vapour deposition (MOCVD) has been studied. Results indicate that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature (HT) GaP growth. These sites can promote the two-dimensional (2D) growth of HT GaP and reduce the surface roughness. A GaP single crystal layer grown at 680°C is obtained using a 40-nm thick GaP buffer layer. The full-width at half-maximum (FWHM) of the (111) plane of GaP layer, measured by DCXRD, is 560 arcsec. The GaP layer grown on GaN without low temperature GaP buffer layer shows a rougher surface. However, the FWHM of the (111) plane is 408 arcsec, which is the indication of better crystal quality for the GaP layer grown on GaN without a low temperature buffer layer. Because it provides less nucleation sites grown at high growth temperature, the three-dimensional (3D) growth is prolonged. The crystalline quality of GaP is lightly improved when the surface of GaN substrate is pretreated by PH3 , while it turned to be polycrystalline when the substrate is pretreated by TEGa.     

Electrochemical liftoff of freestanding GaN by a thick highly conductive sacrificial layer grown by HVPE

Separation technology is an indispensable step in the preparation of freestanding GaN substrate. In this paper, a largearea freestanding GaN layer was separated from the substrate by an electrochemical liftoff process on a sandwich structure composed of an Fe-doped GaN substrate, a highly conductive Si-doped sacrificial layer and a top Fe-doped layer grown by hydride vapor phase epitaxy(HVPE). The large difference between the resistivity in the Si-doped layer and Fe-doped layer resulted in a sharp interface between the etched and unetched layer. It was found that the etching rate increased linearly with the applied voltage, while it continuously decreased with the electrochemical etching process as a result of the mass transport limitation. Flaky GaN pieces and nitrogen gas generated from the sacrificial layer by electrochemical etching were recognized as the main factors responsible for the blocking of the etching channel. Hence, a thick Si-doped layer grown by HVPE was used as the sacrificial layer to alleviate this problem. Moreover, high temperature and ultrasonic oscillation were also found to increase the etching rate. Based on the results above, we succeeded in the liftoff of ～ 1.5 inch GaN layer. This work could help reduce the cost of freestanding GaN substrate and identifies a new way for mass production.     

Investigation of optical emission spectroscopy in diamond chemical vapor deposition by Monte Carlo simulation

The optical emission spectra(atomic hydrogen(H_α,H_β,H_γ),atomic carbon C(2p3s→2p~2:λ=165.7 nm)and radical CH(A~2Δ→X~2Π:λ=420-440 nm))in the gas phase process of the diamond film growth from a gas mixture of CH4 and H_2 by the technology of electron-assisted chemical vapor deposition (EACVD)have been investigated by using Monte Carlo simulation.The results show that the growth rate may be enhanced by the substrate bias due to the increase of atomic hydrogen concentration and the mean temperature of electrons.And a method of determining the mean temperature of electrons in the plasma in-situ is given.The strong dependence on substrate temperature of the quality of diamond film mainly attributes to the change of gas phase process near the substrate surface.     

Investigation of optical emission spectroscopy in diamond chemical vapor deposition by Monte Carlo simulation

The optical emission spectra(atomic hydrogen(Hα,Hβ,Hγ),atomic carbon C(2p3s→2p2:λ=165.7 nm) and radical CH(A2△→X2П:λ=420-440 nm))in the gas phase process of the diamond film growth from a gas mixture of CH4 and H2 by the technology of electron-assisted chemical vapor deposition (EACVD)have been investigated by using Monte Carlo simulation.The results show that the growth rate may be enhanced by the substrate bias due to the increase of atomic hydrogen concentration and the mean temperature of electrons.And a method of determining the mean temperature of electrons in the plasma in-situ iS given.The strong dependence on substrate temperature of the quality of diamond film mainly attributes to the change of gas phase process near the substrate surface.     

Cluster size and substrate temperature affecting thin film formation during copper cluster deposition on a Si （001） surface

The soft deposition of Cu clusters on a Si(001) surface was studied by molecular dynamics simulations.The embedded atom method,the Stillinger-Weber and the Lennar-Jones potentials were used to describe the interactions between the cluster atoms,between the substrate atoms,and between the cluster and the substrate atoms,respectively.The Cu13,Cu55,and Cu147 clusters were investigated at different substrate temperatures.We found that the substrate temperature had a significant effect on the Cu147 cluster.For smaller Cu13 and Cu55 clusters,the substrate temperature in the range of study appeared to have little effect on the mean center-of-mass height.The clusters showed better degrees of epitaxy at 800 K.With the same substrate temperature,the Cu55 cluster demonstrated the highest degree of epitaxy,followed by Cu147 and then Cu13 clusters.In addition,the Cu55 cluster showed the lowest mean center-of-mass height.These results suggested that the Cu55 cluster is a better choice for the thin-film formation among the clusters considered.Our studies may provide insight into the formation of desired Cu thin films on a Si substrate.     

Electrical characteristics of AlInN/GaN HEMTs under cryogenic operation

Electrical properties of an AlInN/GaN high-electron mobility transistor (HEMT) on a sapphire substrate are investi-gated in a cryogenic temperature range from 295 K down to 50 K. It is shown that drain saturation current and conductance increase as transistor operation temperature decreases. A self-heating effect is observed over the entire range of temperature under high power consumption. The dependence of channel electron mobility on electron density is investigated in detail. It is found that aside from Coulomb scattering, electrons that have been pushed away from the AlInN/GaN interface into the bulk GaN substrate at a large reverse gate voltage are also responsible for the electron mobility drop with the decrease of electron density.     

Evaluation of thermal resistance constitution for packaged A1GaN/GaN high electron mobility transistors by structure function method＊

The evaluation of thermal resistance constitution for packaged A1GaN/GaN high electron mobility transistor （HEMT） by structure function method is proposed in this paper. The evaluation is based on the transient heating measurement of the A1GaN/GaN HEMT by pulsed electrical temperature sensitive parameter method. The extracted chip-level and package-level thermal resistances of the packaged multi-finger A1GaN/GaN HEMT with 400μm SiC substrate are 22.5 K/W and 7.2 K/W respectively, which provides a non-invasive method to evaluate the chip-level thermal resistance of packaged A1GaN/GaN HEMTs. It is also experimentally proved that the extraction of the chip- level thermal resistance by this proposed method is not influenced by package form of the tested device and temperature boundary condition of measurement stage.