Design and simulation of AlN-based vertical Schottky barrier diodes∗

Chun-Xu Su(苏春旭) Wei Wen(温暐) Wu-Xiong Fei(费武雄) Wei Mao(毛维) Jia-Jie Chen(陈佳杰)Wei-Hang Zhang(张苇杭) Sheng-Lei Zhao(赵胜雷) Jin-Cheng Zhang(张进成) and Yue Hao(郝跃)

1Key Laboratory of Wide Band-Gap Semiconductors and Devices,School of Microelectronics,Xidian University,Xi’an 710071,China

2China Electronic Product Reliability and Environmental Testing Research Institute,Guangzhou 510610,China

3Shanghai Academy of Spaceflight Technology,Shanghai 201109,China

Keywords: aluminum nitride,Schottky barrier diodes,specific on-resistance Ron,sp,breakdown voltage VBR

1. Introduction

Owing to high breakdown voltage, ultralow conduction loss and high switching speed,GaN SBDs,have aroused great research interest in recent years.[1–6]Edge termination techniques, such as guard rings, field plate (FP) and ion implantation treatment, are introduced in vertical GaN SBDs to minimize field crowding and increase the reverse breakdown voltage(VBR).[7–10]TheVBRof vertical GaN SBDs has been enhanced from 300 V to 1650 V by using argon ion implantation.[11]For quasi-vertical GaN-based SBDs, on/off current ratio of 1010and average breakdown electric field of 1.48 MV/cm were obtained by improving material design and device fabrication process.[12,13]Due to the limited thickness of GaN drift layer,VBRis difficult to exceed 2000 V,and vertical GaN SBDs cannot be used in ultra-high voltage applications.

Aluminum nitride(AlN)materials possess high bandgap energy of 6.1 eV and high critical electric field of up to 12 MV/cm, demonstrating great potential in power electronics.[14]The improvement in quality of bulk AlN material has promoted the development of AlN SBDs. The lateral AlN SBDs withVBRof 1 kV has been reported, but the current density is below 1 A/cm2.[15]Compared with lateral AlN SBDs,vertical AlN SBDs have the advantage of high current density. TheVBRranged from 550 V to 770 V for vertical AlN SBDs grown by hydride vapor phase epitaxy.[16]There are only a few studies on vertical AlN SBDs so far. The reportedVBRand average breakdown electric field of AlN SBDs are all below 1 kV and 1 MV/cm respectively,which are far below the ideal material limit. Therefore,it is necessary to study the breakdown mechanism and device design of AlN SBDs.

In this paper, the vertical AlN SBDs was simulated by Silvaco-ATLAS. The thickness and doping concentration of the drift layer were employed to modulateVBRandRon,spof the SBDs. This paper focus on the optimization of DLC and DLT to improve theI–Vcharacteristics. TheVBRis increased by 70%, which a FP is introduced at a specific concentration and thickness of drift layer for comparison.

2. Device structure

The simulated device of vertical AlN SBD as shown in Fig. 1. The Ni metal is utilized to form the Schottky contact electrode. The FP with a length of 0.5-µm are designed as shown in Fig. 1, which can increase the breakdown voltage by changing the degree of bending near the boundary of the depletion layer of the anode edge and changing the electric field distribution in the depletion layer. The SBD structure includes a 1-µm n+AlN substrate and a drift layer with variable thickness. The substrate’s concentration is assumed to be 1020cm−3, and its thickness is 1-µm. The drift layers with different DLTs(4 µm, 5µm, 6µm, 8µm, 11µm)are simulated. In the structure with a variable DLC,we change it from 1×1015cm−3to 3×1016cm−3,while the DLT is 11-µm.The material of SiN is used for passivation layer. In the structure of vertical GaN SBD, the DLC is 1016cm−3, the thickness of drift layer is 11-µm, other parameters are consistent with AlN SBDs. The SBDs were simulated by Silvaco-ATLAS.The ionized charge in the drift layer was assumed to be uniform.

Fig.1. The device structure of the vertical AlN-based SBD with FP.

3. Results and discussion

As shown in Fig. 2(a), the vertical AlN and GaN SBDs are simulated. TheVonis defined at the point when the current density is 1 A/cm2. Compared with GaN SBDs, the forward turn-on voltage (Von) of AlN SBDs is increased from 0.5 V to 3.1 V due to the lower electron affinity energy. A relation about the height of Schottky barrier (ΦB) in n-type semiconductor material is[17]

whereΦmis the work function of contact metal,χsis the electron affinity energy. Thus, theqΦBof AlN device is larger when the same contact metal is used that lead to the increase ofVon. As shown in Fig.2(a),the extracted ideal factor(n)of AlN SBD and GaN SBD are 1.08 and 1.01,respectively. The extracted Schottky barrier heights form theI–Vforward curve of the AlN SBD and GaN SBD are 3.45 eV and 0.74 eV,which are consistent with height extracted from the structure diagram in Silvaco tonyplot. The critical electric filed and breakdown voltageVBRare significantly improved for AlN material. TheVBRis defined as the anode voltage at which the peak electric field reaches 12 MV/cm. As shown in Fig.2(b),the critical electric field is improved from 3.3 MV/cm to 12 MV/cm,and the corresponding breakdown voltage is improved from−300 V to−2300 V.

As shown in Fig.3,vertical AlN SBDs with various DLTs are simulated. Figure 3(a)shows the forwardI–Vcharacteristics of the SBDs with different DLTs.We can see that the lager the DLT is,the smaller the current density is. The decrease of current density is primarily due to the reduction of available charge carriers in the thick drift-layer thus resulting in the increased series resistance.[18]As shown in Fig. 3(b), the peak electric field decreases with the increase of DLT at the same voltage of−1800 V. Figure 3(c) shows the electric field distribution in vertical direction at the voltage of−1800 V.From Fig. 3(d), we can see theVBRandRon,spincreased with the increase of DLT.

Fig.2. Comparison of vertical AlN SBDs and GaN SBDs without FP.(a)The diagram of forward bias I–V characteristics. (b)The diagram of electric field distribution at 10 nm below the anode.

The impact of DLC on forward and reserveI–Vcharacteristic is shown in Fig. 4. Figure 4(a) shows the forwardI–Vcharacteristic which become better with the increase of the DLC.From Fig.4(b),we can see that theVBRdecreases which from−3.37 kV to−1.1 kV with the increased DLC.TheVBRis inversely proportional to the doping concentration. The electric field decreases to zero at the boundary of drift layer in the SBDs with four gradients of DLC from 1×1015cm−3to 1×1016cm−3,nevertheless in DLC of 3×1016cm−3,the electric field rapidly fell to zero at the distance of 5µm away from anode, that means the thickness of depletion layer is about 5µm.

Fig.3.The diagram of(a)forward I–V characteristics of vertical AlN SBDs with different DLTs, (b) electric field distribution at 10 nm below the anode,(c)electric field distribution along the vertical direction for the devices with different DLTs,(d)VBR and Ron as a function of DLT.

At the boundary of the depletion region,the electric field is zero,the electric field distribution function of the depletion region is[17]

From Fig.4(b),we can see the maximum of electric fieldEmappears at the interface of the metal and semiconductor. The potential distribution equation

The potential at the boundary of the depletion region has the following relationship with the applied reverse bias:

Thus, the maximum electric field isEm=2qNDVa/εS. WhenEmis equal to critical electric field of AlN(12 MV/cm),Vais inversely proportional toND, so theVBRdecreased with the DLC increase. FromWD=2εSVa/qND,we can see theWDdecreased with the DLC increase. And the diagram of electron concentration distribution is shown in Fig. 4(c), which indicated that the drift layer is not completely depleted because of the high DLC.

Fig.4. The diagram of(a)forward I–V characteristics of vertical AlN SBDs with different DLCs,(b)electric field distribution along the vertical direction for the devices with different DLCs,(c)electron concentration schematics of the device with DLC of 3×1016 cm−3.

Fig.5. The diagram of(a)electric field without and(b)with FP,(c)electric field distribution at 10 nm below the anode(Vanode=−2300 V),(d)VBR and Ron as a function of DLC.

Figure 5 shows the comparison between the structure with and without FP.From Figs.5(a)and 5(b),we can see that the electric field crowding is reduced in the structure with FP.At the same anode voltage of−2300 V.The electric filed peak is reduced from 12 MV/cm to 8 MV/cm by utilizing field plate which is shown in Fig. 5(c). Figure 5(d) is the dual coordinate diagram of A and B about DLC,theVBRof AlN SBDs is further increased from−2300 V to−4100 V.

As shown in Fig.6(a),the value of BFOM increases from 397 MW/cm2to 914 MW/cm2with the increase of DLT from 4µm to 11µm(the DLC is 1×1015cm−3). From Fig.6(b),we can see that BFOM increases for DLC＜1×1016cm−3and decreases for DLC＞1×1016cm−3. The maximum BFOM can be obtained from a simulation ofVBRas a function of DLC.In this paper,we obtain a maximum BFOM of 12.3 GW/cm2in the simulation of the structure with FP,which is 3 times of the BFOM compared with the structure without FP under the same DLC and DLT.

Fig. 6. BFOM as a function of (a) DLT and (b) DLC include the structure with FP.

4. Conclusion

In summary,the evaluations of drift layer’s thickness and doping concentration onI–Vcharacteristics for vertical AlN Schottky barrier diodes are investigated by simulation. The lager DLT is essential to improve theVBRand BFOM effectively. For the SBD with DLT of 11-µm, theVBRcan reach−3400 V. The forwardI–Vcharacteristics can be improved by increasing the drift layer’s doping concentration.By increasing the DLC, theRon,spis further decreased from 12.64 mΩ·cm2to 0.5 mΩ·cm2, but the effect of this method to improveVBRis adverse. The devices with lager doping concentrations are more likely to cause carrier multiplication when they are subjected to reverse bias. Thus, the combined effect ofVBRandRon,spshould be considered comprehensively to obtain a higher BFOM value. The vertical AlN SBD with FP also been simulated to improveVBRin this paper. Besides these methods, many other devices with termination structures are used to improveI–Vcharacteristics in vertical SBDs,which can be introduced in our paper.

At present,the existing experimental level of the AlN vertical SBD had a lot of material defects,[16]resulting in a small forward current density, and the ideal factor (n) was about 8. The high value ofnindicated that the transport mechanism deviates from the thermionic emission (TE) model,which is mainly caused by the nonuniform contact at the metal/semiconductor interface due to material defects. The calculated resistivity of 2.3×102Ω·cm, suggesting that the carrier concentration of the active region was low. As a result,it is highly desired to decrease the defect density of AlN epitaxial layers. In this paper,the simulated AlN vertical SBD suggests that high quality of material as a precondition to great potential of vertical AlN power devices.