Role of Electrochemical Reactions in the Degradation Mechanisms of AlGaN/GaN HEMTs

Transcription

1 Role of Electrochemical Reactions in the Degradation Mechanisms of AlGaN/GaN HEMTs Feng Gao 1,2, Bin Lu 2, Carl V. Thompson 1, Jesús del Alamo 2, Tomás Palacios 2 1. Department of Materials Science and Engineering, Massachusetts Institute of Technology 2. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology This project is partially funded by the ONR DRIFT MURI program.

2 Introduction AlGaN/GaN HEMTs unique properties Wide bandgap E critical >3MV/cm Polarization 2DEG density >10 13 /cm 2 High electron mobility: >1500 cm 2 /V. s High electron peak velocity: 2.1x10 7 cm/s AlGaN E f GaN E c 2DEG E v Excellent for High Power RF & Power Electronics Applications Wireless Base Station Energy Conversion Automotive Electronics 2

3 Motivation Reliability Bottleneck Device reliability is one of the greatest obstacles for AlGaN/GaN HEMTs: various degradation modes [Meneghesso et. al., IEEE Transactions on Device and Materials Reliability, vol.8, no.2, p.332, 2008] 3

4 Motivation Permanent Degradation Overview of Reliability Phenomena Surface Pitting Electrical Degradation U. Chowdhury et al., IEEE Electron Dev. Lett, vol. 29, no.10, J. Joh et al., IEEE Electron Dev. Lett., vol. 29, no. 4, P. Makaram et al., Appl. Phys. Lett., 96, ,

5 Motivation Overview of Reliability Mechanisms Physical mechanism in permanent degradation Inverse Piezoelectric Effects Defect Percolation J. Joh et al., IEDM IEEE International, 2007, pp M. Meneghini et al., Appl. Phys. Lett., vol. 100, no. 3, p , Mass-Transport F. Gao et al., Appl. Phys. Lett., vol. 99, no. 22, pp ,

6 Goals of This Work Water-Assisted Electrochemical Mass Transport in Permanent Degradation Electrochemical Reaction Mechanism: - Holes generation - Water Diffusion 6

7 AlGaN/GaN HEMTs Under Study Device provided by industrial collaborator 7

8 Experimental Setup High Vacuum and High Temperature Probe Station Key features: Vacuum chamber of Torr. Thermal chuck up to 600 C. Four probes for electrical contact. Two gas lines that can supply the chamber with air, O 2,N 2, Ar, He and H 2. Acknowledgement to Prof. Harry L. Tuller 8

9 Impact of Atmosphere on Surface Pitting Ambient and Vacuum Control-Group Off-state stress test: V ds = 43 V, V gs = -7 V for 3000s in darkness at RT SEM Top View TEM Cross Section AFM Depth Profile Stressed in ambient air Stressed in vacuum of Torr Surface pitting caused by OFF-state electrical stress is significantly reduced in vacuum. 9

10 Role of Oxygen in Surface Pitting Ambient and Vacuum Control-Group TEM TEM EDX Stressed in ambient air Stressed in vacuum of Torr Higher concentration of oxygen (O) is found inside the surface pits for AlGaN/GaN HEMTs stressed in ambient air. 10

11 Influence of Ambient Air Off-state stress Ambient air more pits with deeper depther Ambient air increase of oxygen in pitting area Ambient air has N 2, O 2, H 2 O, CO 2 What gas in the ambient air causes: surface pitting? increasing of oxygen concentration in pitting area? 11

12 Impact of Moisture on Surface Pitting Wet and Dry Control-Group wet/dry Ar; wet/dry O 2 ; wet/dry N 2 ; wet/dry CO 2 ; wet/dry Air SEM Top View TEM Cross Section Stressed in watersaturated gas (Ar) Stressed in dry gas (Ar) Water has a major impact on surface pitting & source of O in pitting area 12

13 Mass Transport TEM EDX mapping: Ga and Al found in the gate region TEM EDX Mapping Al Unstressed Ga, Al migration Stressed in ambient for 3000s Al Stressed in vacuum for 3000s 13

14 Permanent Electrical Degradation By Off-State Stress Off-state stress: V gs =-7V,V ds = 43V, 3000s at RT in dark 1min UV illumination & 12hr at rest after Off-state to recover trapping transient I dss and I g measured at V gs =0,V ds =5V Stressed in ambient Stressed in vacuum 14

15 Permanent Electrical Degradation Surface Pitting Time Evolution Stressed in ambient 15

16 Permanent Electrical Degradation I d Degradation Time Evolution Time Evolution of Drain Current Degradation: Id measured at V gs = 0 and V ds = 5 V Drain current degradation evolves with the growth of the surface pits over stress time. 16

17 Summary 1 H 2 O surface pitting and source of oxygen in pitting area Ga, Al migration accompanies surface pitting Surface pitting & Ga, Al migration permanent electrical degadation Al What is the physical mechanism behind the impact of H 2 O on the surface pitting? 17

18 Water-assisted Electrochemical Reactions Corrosion in AlGaN/GaN HEMTs Reduction of water: 2H 2 O+2e - = 2OH - + H 2 Anodic oxidation of AlGaN: 2Al x Ga 1-x N+6h + = 2xAl (1-x)Ga 3+ +N 2 An electrochemical cell formed at the drain edge of the gate. e - H 2 O OH - 2xAl (1-x)Ga OH - = xal 2 O 3 + (1-x)Ga 2 O 3 + 3H 2 O Complete Reduction-oxidation (redox) electrochemical reactions: h + 2Al x Ga 1-x N + 3H 2 O = xal 2 O 3 + (1-x)Ga 2 O 3 + N 2 + H 2 18

19 Water-assisted Electrochemical Reactions Ga, Al out diffusion Stressed in ambient for 3000s Al Gate AlGaN Al Ga h + h Ga + 3+ Al 3+ GaN h + h + Two necessary conditions: 1. Holes are generated and accumulated at the AlGaN surface. 2. Ambient water diffuse through the SiN x passivation and reach the AlGaN surface. 19

20 Source of Holes Photo-Generated Holes Stressed under 254-nm UV illumination in ambient air at V ds = 43 V V gs = -7 V for 3000s at RT Increasing surface pitting SEM Top View TEM Cross Section 20

21 Source of Holes Impact Ionization vs Inter-band Tunneling j h Ei exp( ) E max Lateral impact ionization j g j h E 2 max E exp( E max Vertical inter-band tunneling t ) Keldysh, Soviet Phys. JETP, vol. 33, no.4, p763,

22 Source of Holes Quantitative Analysis: E-field Silvaco simulation of Electric Field in AlGaN Layer E max 22

23 Source of Holes Relationship of Surface Pits and Holes 2Al x Ga 1-x N+6h + = 2xAl (1-x)Ga 3+ +N 2 j h 3 d N Mt A + q d Quantitative Analysis: j h Average pits depth measured by AFM J h vs E max 23

24 Source of Holes Surface Pitting and Holes Data match with inter-band tunneling equation Data do not match with impact ionization equation Inter-band tunneling Impact ionization Too small 24

25 Source of Holes Trap-assited inter-band Tunneling Traps assist inter-band tunneling causing a lower E t Traps assist inter-band tunneling 25

26 Trap Assisted Inter-Band Tunneling TCAD Simulations TCAD simulation of trap-assisted inter-band tunneling Donor trap states w. E c E t = 0.45eV, N t = 5x10 18 /cm 3 V ds = 43 V, V gs = -7 V Hole Concentration Vertical Cut at Gate-Drain Edge Metal Gate Passivation Nitride 3nm GaN Cap 14nm Al 0.28 Ga 0.72 N Barrier GaN Buffer Acknowledgement to our collaborators: Hiu-Yung (Hugh) Wong, Ph.D et al at Synopsys, Inc. 26

27 Last Question: Diffusion of Water Water Vapor Transmission Rate Water Vapor Transmission Rate (WVTR) of PECVD SiN 2Al x Ga 1-x N + 3H 2 O = xal 2 O 3 + (1-x)Ga 2 O 3 + N 2 + H 2 Degradation of SiN Passivation WVTR 3 d M 2 Mt H 2 O g/m 2 /day Estimated WVTR is consistent with the reported value for PECVD SiN ( g/m 2 /day) in literature [1-2]. Stressed in wet air Stressed in dry air [1] A.S. da Silva Sobrinho, et al. J. Vac. Sci. Technol. A, vol. 16, no. 6, pp , [2] D.S. Wuu, et al. Surf. Coat. Technol., vol. 198, no.1-3, pp , Stressed in ambient air Stressed in vacuum Defects are created in SiN during off-state stress to accelerate water diffusion. 27

28 Conclusion Mechanisms of Permanent Degradation A Water-assisted Field-induced Electrochemical Process 2Al x Ga 1-x N + 3H 2 O = xal 2 O 3 + (1-x)Ga 2 O 3 + N 2 + H 2 Cause surface pitting & Ga, Al migration Cause permanent I d drop Traps assist inter-band tunneling 28

29 God made the bulk; surfaces were invented by the devil. Wolfgang Pauli 29

30 Is There Impact Ionization? Holes are swept to the source-edge of the gate Lateral impact ionization 30

31 Why is there still pits in devices stressed in vacuum? Water desorption requires vacuum annealing with T > 200 o C In-situ XPS Analysis

32 Source of Holes Quantitative Analysis:Pits Formation vs V ds Stressed in Air 1000s Vgd = 10V to 60V 32