Ti silicide electrodes low contact resistance for undoped AlGaN/GaN structure

Transcription

1 222nd ECS meeting 11 Oct Ti silicide electrodes low contact resistance for undoped AlGaN/GaN structure K. Tsuneishi, J. Chen, K. Kakushima, P. Ahmet, Y. Kataoka, A. Nishiyama, N. Sugii, K. Tsutsui, K. Natori, T. Hattori, H. Iwai Tokyo Institute of Technology Tokyo Institute of Technology 1

2 GaN for next power electronic substrates Promising semiconductors GaN & 4H-SiC High E BD Large ν sat Large κ power devices with low loss AlGaN/GaN on Si sub. E g (ev) bandgap µ e (cm 2 /Vs) mobility ν sat (cm/s) saturated velocity E BD (V/cm) breakdown field κ (W/cmK) thermal conductivity ρ C (Ωcm 2 ) Reported specific contact resistance HEMT structure with high µ e Low wafer cost (mass production capability) Si 4H-SiC GaN ~2000 HEMT 1.0x x x x x x ~10-8 ~10-5 ~10-5 High contact resistance (~10-5 Ωcm 2 ) Contact resistance should be reduced for high device performance 2

3 Common contact process for AlGaN/GaN Dislocation assisted diffsuion of Ti atoms with Au/metal/Al/Ti Au/Mo/Al/Ti AlGaN dislocation anneal AuAl,Mo,TiN TiN TiN GaN L. Wang, APL, 87, (2005) Diffused Ti atoms form TiN and directly contact to 2DEG layer Drawbacks Non-uniform contact of TiN and 2DEG Local conduction Limited process temperature due to reaction of TiN formation Small window for optimized process, Extinction of 2DEG High quality epi-wafers may degrade the contact resistance Stable contact process with large process window is required 3

4 Au/Mo/Al/Ti AlGaN GaN Si layer insertion for low ρ C for AlGaN/GaN Si or SiN x anneal N -rich AlN formation N AlGaN GaN -rich AlN B. Daele, APL, 89, (2006) AlGaN+Al -rich AlGaN + -rich AlN (need 800 o C for this reaction) Insertion of Si to form eutectic Al-Si to prevent out diffusion of Al atoms Over 800 o C, Al react with AlGaN to extract N atoms and form -rich AlN Low ρ C of ~10-5 Ωcm 2 was reported with this process Still local conduction through may give limit for low ρ C Uniform extraction process of N atoms from AlGaN layer is preferable for further low ρ C 4

5 Our approach for uniform formation in AlGaN Guideline: Inhibition of Ti atoms to diffuse into dislocations Ti compound metal; TiB 2, TiC, TiSi 2, Formation Substance - H 298 (kj/mol) Selection Easy to form by semiconductor process Large enthalpy of formation (- H) Al 0.25 Ga 0.75 N TiN TiSi TiC TiB Si 3 N Expected reaction from thermodynamics 3 Al Ga 0.75N = Al + Ga TiSi 2 3 TiN 11 Attempt to use TiSi 2 as a contact electrode 2 + Si3N4 8.6kJ/mol 11 TiSi 2 can decompose AlGaN by forming Si 3 N 4 5

6 Sample preparation for IV measurements i-al 0.25 Ga 0.75 N(26nm)/i-GaN(1.3µm) on buffer/si(111) TiN(15nm)/Ti(60nm) or TiN(45nm)/TiSi 2 (20nm) Chemical cleaning (SPM, HF) Device isolation (RIE with Cl 2 ) Oxide passivation (plasma-teos) Contact opening (Buffered HF) Metal deposition TiN(15nm)/Ti(60nm) [reference] TiN(45nm)/TiSi 2 (20nm) Metal patterning (RIE with Cl 2 ) Annealing in N 2 Current-voltage measurements d SiO 2 AlGaN GaN Buffer layer Si sub. XPS sample Ti(10nm) or TiSi 2 (10nm) AlGaN GaN Buffer layer Si sub. 6

7 Metal/AlGaN interface reaction Ga 2p 3/2 spectra Ga-N 900 o C, 1min in N 2 (hν=7940ev, TOA=80 o ) BE=0.16eV Metal Al 0.25 Ga 0.75 N GaN Intensity (a.u.) Ti (10nm) TiSi 2 (10nm) Ti-Ga E f Ga 2p 3/ Binding energy (ev) Suppressed reaction at metal/algan with TiSi 2 uniform interface Creation of n + -AlGaN fromation BE shift indicates band bending in AlGaN/GaN (Also formation of TiN and SiN are confirmed) Uniform extraction of N can be expected with TiSi 2 metal 7

8 (ma) Current (ma) IV curves on annealing temperature RTA:N 2 for 1min 750 o C 900 o C 800 o C 700 o C 850 o C 950 o C Ti TiN/Ti Voltage (V) Ti Ohmic non-linear IV non-linear IV TiSi Temperature ( o C) Ohmic Current (ma) Ti electrode RTA:N 2 for 1min 700 o C 800 o C TiSi 2 electrode 950 o C 900 o C 850 o C 750 o C TiSi 2 TiN/TiSi Voltage (V) Ohmic behavior at 750 o C Higher temperature degrades the contact Non-linear curves below 900 o C Ohmic behavior at 950 o C 80µm 300µm 150µm 8

9 Stability of contact on annealing time Current (ma) Ti: 750 o C TiSi 2 : 950 o C 5min 80µm 300µm Voltage (V) 60min 150µm (ma) TiSi 2 Ti (950 o C) Ohmic in all range Ohmic (750 o C) non-linear IV Annealing time (min) Ti electrode with 750 o C annealing Ohmic contact can be achieved between 2 to 5 minutes Longer anneling degrades the contact TiSi 2 electrode with 950 o C annealing Ohmic contact can be achived in all range of annealing time 9

10 Specific contact resistance (ρ C ) for Ti and TiSi ρ c (Ωcm 2 ) o C TiSi 2 950o C Ohmic for Ti Annealing time (min) ρ c decreases with longer annealing time with TiSi 2 Further reduction in ρ c can be expected with TiSi 2 10

11 Possible model for contact formation Ti electrode High temperature excess time for annealing small current TiSi 2 electrode Extraction of N atoms from AlGaN over 950 o C V VN N TiN Ti TiN Ti TiSi 2 TiSi 2 VN VN Disappearance of 2DEG due to consumption of AlGaN layer Contact senstive to annealing temperature and time Stable interface properties High temperature is needed, but stable contact can be obtained 11

12 Summary TiSi 2 layer is examined for AlGaN/GaN contact metal Al and Ga atoms in AlGaN layer remain intact with TiSi 2 Uniform interface can be obtained with TiSi 2 Binding energy of Ga and Al core level shift to higher energy TiSi 2 effectively extract N atoms from AlGaN layer Ohmic contact has been obtained with TiSi 2 at 950 o C annealing Longer annealing further reduces the specific contact resistance Thermally stable ohmic contact can be obtained Further reduction in contact resistance can be exptected with TiSi 2 contact for AlGaN/GaN We would like to thank Prof. Nohira with Tokyo City University for XPS measurement 12