Determination of band alignment between GaOx and boron doped diamond for a selective-area-doped termination structure

Qi-Liang Wang(王启亮) Shi-Yang Fu(付诗洋) Si-Han He(何思翰) Hai-Bo Zhang(张海波)Shao-Heng Cheng(成绍恒) Liu-An Li(李柳暗) and Hong-Dong Li(李红东)

1State Key Laboratory of Superhard Materials,Jilin University,Changchun 130012,China

2Shenzhen Research Institute,Jilin University,Shenzhen 518057,China

3Guangdong Juxin New Material Technology Co.,Ltd,Zhuhai 519000,China

Keywords: GaOx,boron-doped diamond,edge termination,band alignment

1. Introduction

Diamond is a promising material for next-generationpower electronic and optoelectronic device applications due to its excellent physical and electrical properties.[1–3]The extremely large critical breakdown field (10 MV/cm), high thermal conductivity (22 W/cm·K) and high carrier mobility(2200 cm2·V−1·s−1and 1800 cm2·V−1·s−1for electrons and holes, respectively) are beneficial to achieve a high Baliga’s figure of merit (BFOM). The p-type boron-doped diamondbased power rectifiers,including both Schottky barrier diodes(SBD) and PN diodes (PND), have attracted considerable attentions.[4]The SBD presents a typically low on-state voltage and no storage of minority carriers,which is a promising structure for high-efficiency rectifier application. However,the device’s performances, especially the breakdown electric field,are still far below the theoretical limit of diamond,which can be ascribed to the material quality, non-optimized device design, termination structure and so on. It was reported that the field-crowding effect at the junction periphery under a large reverse voltage increases the leakage current and leads to a reduced breakdown voltage. Recently, different kinds of termination structures have been proposed for diamond SBDs. Diamond Schottky-pn diodes (SPNDs) with a lightly nitrogen-doped layer realized a high forward current density of over 20 kA/cm2at 7 V and a high breakdown electric field of 3.3 MV/cm at room temperature.[5]Using boron implanted guard ring termination,the average breakdown voltage of the device with a 310 nm p−drift layer was significantly improved from 79 V to 125 V.[6]Using a floating metal guard ring structure,an SBD device with a breakdown electric field of 2.6 MV/cm was also fabricated.[7]

Generally, high-power switching devices encounter a trade-off between two contradicting properties, namely, high blocking voltage and low forward loss. From this point, the edge termination (ET, Fig. 1(a)) and junction barrier Schottky (JBS, Fig. 1(b)) diode, combining the Schottky contact with an area-selective pn-junction, are enable to benefit from both.[8,9]In the forward bias region, a Schottky contact with a low Schottky barrier height metal can realize small forward losses, while the lateral field of a buried pn-junction helps to shield the strong electric fields around the Schottky contact under high reverse bias. However,although p-type doping can be easily realized using a boron dopant,[10]effective n-type doping is still a challenge for diamond due to the large activation energy (e.g., 1.7 eV for nitrogen) or low doping efficiency (e.g., 50%–90% compensation ratio for phosphorusdoped (001) diamond).[11]In addition, the ion implantation technique degrades the crystalline quality and needs high temperature annealing.[12]

Alternatively,gallium oxide(GaOx),with a wide bandgap of approximately 4.8 eV, a high critical electrical field of 8 MV/cm and large BFOM is suitable for high-power applications.[13]Furthermore,GaOxcan obtain n-type doping easily with electron concentration ranges from 1016cm−3–1019cm−3. The structure, morphology and optical properties of GaOxthin films can be adjusted by the growth conditions during magnetron sputtering deposition, which enable the selective area deposition of n-type regions with easy controllability.[14]Previously, an n-GaOx/p-diamond heterojunction photodiode was demonstrated with a high rectification ratio and low reverse leakage current,which is beneficial to obtain high responsivity, a high photo-to-dark-current ratio,and fast response time due to the built-in electric field.[15]For power diodes, by introducing the GaOx/diamond pnheterojunction, a depletion region with extra ionized charge attracts the electric field, which is beneficial to shield the field-crowding effect.[16]However,the fabrication of diamond diodes with ET and/or JBS using n-GaOxhas not been extensively investigated. The oxide materials are usually used as gate dielectrics for semiconductor devices. Generally, the interface band offsets between the gate dielectric play a key role in device operation,which determines the mobility and transport properties, as well as the threshold voltage stability.[17]In addition,the band offsets are essential to predict the carrier barrier for pn heterojunction applications, which affects both the forward turn-on and reverse depletion region. Therefore,it is very important to measure the band offsets at the interfaces of oxides and semiconductors.[18,19]

In this report,GaOxthin films are deposited on a singlecrystal boron-doped diamond by RF magnetron sputtering to form a heterojunction. In addition, the band offsets and band configuration at the n-GaOx/p-diamond heterojunction interface are measured using x-ray photoelectron spectroscopy(XPS).

2. Experiment

The p-type single crystalline boron-doped diamond(BDD) was grown on a single-crystal seed using the microwave plasma chemical vapor deposition system. The type-Ib(100)seeds were commercially available with a dimension of 3 mm×3 mm×1 mm. During growth, H2/CH4/boron flow rates of 200/2/4 sccm were adopted in the chamber to reach a reactor pressure of approximately 8 kPa. To realize p-type doping, liquid B(OCH3)3(kept at 25◦C) was chosen as the boron dopant. The B(OCH3)3was carried by bubbling H2with a flow rate of 4 sccm. The thickness of the p-diamond layer was approximately 1 µm when the growth duration was adjusted to 30 min. The electrical properties of the BDD were examined using Hall-effect measurements.The hole concentration and mobility of the sample were approximately 1.39×1019cm−3and 30.4 cm2·V−1·s−1,respectively.

To determine the band discontinuities at the GaOx/diamond interface, diamond samples with 4 nm and 40 nm-thick GaOxwere deposited by radio frequency magnetron sputtering (as shown in Fig. 1(c)). The deposition conditions of GaOxwere similar to previous work.[14]After growth, the boron-doped diamond samples were treated by H2SO4:H2O2(3:1) solution and organic cleaning to form an oxygen termination.The GaOxtarget(99.99%purity)was employed above the diamond substrate with a spacing of 10 cm.The oxygen and argon gas mixture,with a flow rate of 2 sccm and 40 sccm, was introduced during deposition. The sputtering power and growth ratio were 160 W and 0.33 nm/min,respectively. The GaOxthin film presents a(201)texture with a bandgap of approximately 5.0 eV deduced using the Tauc method.[14]

Fig.1. A schematic view of the vertical diamond diode with GaOx edge termination(a)and junction barrier Schottky(b)structures. (c)The samples for XPS measurements.

The XPS apparatus (Thermo Fisher Scientific) was calibrated before measurements by C 1s with a binding energy at approximately 284.80±0.08 eV.A monochromatized AlKα(1486.6±0.08 eV)was chosen as the radiation source with a spot size of approximately 500 µm. The standard deviations of the core level energy and valence band are±0.08 eV and±0.2 eV,respectively.Based on the extensively adopted Kraut method,[20–22]the valence band offset(VBO,∆EV)can be calculated from the formula

3. Results and discussion

Firstly,the crystalline quality of BDD and GaOx(40 nm)films is evaluated using an x-ray diffraction system (XRD,Rigaku D/MAX-RA with CuKαradiation). For the diamond,only a strong peak appears at approximately 119.7◦, which can be assigned to the diffraction of the (004) crystal plane(Fig. 2(a)). However, there are no obvious peaks that can be assigned to the GaOxfilm (Fig. 2(b)), implying the amorphous structure of the sputtered film on diamond. The surface morphologies of the BDD and GaOxfilms are characterized at three different positions using an atomic force microscope(AFM, Bruker Dimension Edge). The scan dimension is set to 5µm×5µm with a tapping-mode. The typical morphologies measured at the in the central area are shown in the insets of Fig. 2. It demonstrates that the BDD surface presents many small pits with a root mean square roughness(RMS)of 0.83 nm (insets of Fig. 2(a)), which may be ascribed to the acid solution treatment. As shown in Fig. 2(b), the surface RMS is approximately 0.71 nm after the deposition of GaOxfilm(40 nm).

Fig.2. AFM images of BDD(a)and GaOx(b)films scanned in 5µm×5µm.

The crystalline quality of BDD is also evaluated using Raman spectroscopy(Renishaw inVia Raman microscope,using a 514-nm Ar+laser as the excitation source). As shown in Fig.3(a),only a sharp peak around 1333 cm−1is observed in the spectrum, implying high purity, and the non-diamond(or sp2)phase is almost eliminated. Moreover, Fano interference as well as the broad satellite peaks(around 480 cm−1and 1190 cm−1) are not obvious.[23]The bandgap energy (Eg) of the GaOxfilm on diamond is an important parameter used to calculate the conduction band offset (∆Ec). From the photoelectron energy loss peak on the O 1s core-level spectrum of the GaOxbulk sample (as shown in Fig. 3(b)), theEgis deduced from the energy difference between the O 1s core level and the threshold energy of the photoelectron energy loss peak.The threshold energy is determined to be 537.00±0.08 eV by extrapolating a linear fit of the leading edge for the photoelectron energy loss peak to the baseline.[24]Then, theEgis calculated to be approximately 4.85±0.08 eV.

Fig. 3. The Raman spectroscopy of the film excited by a 514 nm laser (a),and the O 1s core-level spectrum of GaOx (b). The inset is the photoelectron energy loss peak used to determine the bandgap.

Fig.4. The surface morphology(a)and the corresponding elements distributions[(b)and(c)]of the diamond coated with 4 nm GaOx.

Fig.5.The valence band and the C 1s spectrum of bulk BDD.The energy difference( −)is calculated to be 283.21±0.2 eV by fitting the curves.

Fig.6.The valence band and C 1s spectra of the bulk GaOx on diamond.The energy difference of (−)is calculated to be 1115.12±0.2 eV by fitting the curves.

Fig. 7. The C 1s and Ga 2p spectrum for the 4 nm GaOx-coated diamond.The corresponding−)is deduced to be 833.19±0.08 eV.

Fig.8.The band configuration of the GaOx/diamond pn heterojunction based on the XPS results.

Table 1 summarizes the fitting results of the XPS and the corresponding energy difference; the ∆EVbetween GaOxand diamond is calculated to be 1.28±0.2 eV according to Eq.(1). Using the room temperature band gaps of GaOxand diamond(5.50 eV),we can calculate the ∆Ecof approximately 1.93±0.2 eV. The band structure in the GaOx/diamond interface is a type-II straddling band configuration, as shown in Fig. 8. In addition, Table 2 summarizes the band alignments between diamond and other materials reported in previous works for comparison. Both the electron and hole can contribute in the conduction without accumulation in the type-II staggered band configuration, which helps to decrease the on-resistance and power loss. It is worth noting that the ∆EVcalculated from the VBM discrepancy should be 1.66±0.2 eV,implying that some effects decrease the ∆EVof 0.38±0.2 eV at the interface. It is well known that the adsorption of hydrogen atoms on a diamond surface(C–H bonds)shifts the energy band of diamond to a negative electron affinity of 1.5 eV.[29]In addition, the C–H generates a high concentration of twodimensional hole gas(2DHG)on the surface,resulting in upward band bending.[30]Then, the C–H at the interface will increase the band offsets. On the other hand, the adsorption of oxygen atoms(C–O bonds)gives a positive electron affinity of approximately 0.5 eV and turns the surface into nearly insulator, resulting in decreasing band offsets. Therefore, the variation of ∆EVfrom the theoretical value is attributed to the increasing C–O at the interface.

Table 1. A summary of the fitting results of XPS and the corresponding energy differences.

Table 2. A summary of the band alignments between diamond and other materials.

4. Conclusion and perspectives

In conclusion, a GaOx/diamond pn heterojunction is deposited by RF magnetron sputtering for band alignment measurements. GaOxthin film deposited on the single-crystal boron-doped diamond presents a small surface roughness of 0.71 nm and a band gap of approximately 4.85 eV.Then,XPS is used to determine the band alignment of the GaOx/diamond pn heterojunction. The VBM values confirm that the GaOxand diamond are n-and p-type conduction, respectively. The VBO and CBO are calculated to be 1.28 eV and 1.93 eV,respectively. The variation of ∆EVfrom the theoretical value is attributed to the increasing C–O at the interface.By combining the material and interface characteristics, the wide bandgap and the type-II staggered band configuration,the GaOx/diamond heterojunction is promising for diamondpower electronic device applications.

Acknowledgment

Project supported by the Key-Area Research and Development Program of Guangdong Province, China (Grant No.2020B0101690001).