Wide Bandgap Semiconductor Research at Mississippi State University

Transcription

1 Wide Bandgap Semiconductor Research at Mississippi State University Dr. Yaroslav Koshka Associate Professor Department of Electrical and Computer Engineering Mississippi State University

2 Presentation Outline 1. Why SiC? 2. History of SiC program at MSU 3. Advancements in SiC epitaxial growth 4. Recombination Induced Defect Reactions involving hydrogen in SiC.

3 SiC Material Properties (comparing to other semiconductors) Property\Material 4H-SiC 6H-SiC GaN Si GaAs Thermal conductivity [W/cm*K] Bandgap [ev] Saturation drift velocity [10 5 m/s] Low field electron mobility (300ºK, 1e16cm -3 )[cm 2 /V*s] Dielectric constant (Π) 865 ( ) (Π) 405 ( ) (peak) (pick) ~ (peak) (pick) Critical electric field [MV/cm] ~ 3 > 2.4 ~ Direct/Indirect Bandgap I I D I D Commercially available substrates 4 3 n/a 12 8

4 Drift region resistance T drift Schottky contact Heavily doped n-type substrate Drift region Source contact Source Gate contact P-type epitaxial layer Drift region T drift Heavily doped p-type substrate Drain contact Drain R drift sp = T n drift qµ N drift Cathode contact Body contact Anode contact T drift P-type anode Heavily doped n-type substrate Drift region Cathode contact R drift sp = qµ N n drift Tdrift ( µ n + µ p) J Fτa + T drift

5 Drift region blocking capability (1-D approach) Voltage drop on drift region is given by: To find Ex ( ), UBlocking solve the Poisson equation T Drift = assuming free carrier concentration to be negligible (no leakage current) for non- punch-through layer: 0 E( x) dx div( ε ϕ) = ρ, dϕ 1 1 E = = ρx = q NDx dx ε ε 1 E = q N T ε crit Drift Drift N N Drift 2 Drift 1 T drift 0 TDrift 1 T Drift 2

6 Drift region blocking capability (1-D approach) Voltage drop on drift region is given by: To find Ex ( ), UBlocking solve the Poisson equation T Drift = assuming free carrier concentration to be negligible (no leakage current) for non- punch-through layer: 0 E( x) dx div( ε ϕ) = ρ, dϕ 1 1 E = = ρx = q NDx dx ε ε E crit 1 1 E = q N T ε crit Drift Drift E crit 2 0 T Drift 1 N drift 2 << N drift 1 T drift T drift 2 >> T drift 1

7 Advantages of wide bandgap Specific on-resistance of SiC Specific On-Resistance (mž (mω cm cm 2 ) ) Silicon 400x 4H-SiC V B2 / B2 / R ON = ON constant Resistance-Area product of 4H-SiC is 400x lower than silicon at any given blocking voltage Blocking Voltage (V)

8 Advantages of wide bandgap Intrinsic carrier concentration Example: n-type, N D = cm -3

9 Advantages in Switching Characteristics 600 V diode recovery comparison Si pin diode 1.32 µs SiC Schottky diode 72 ns

10 Presentation Outline 1. Why SiC? 2. History of SiC program at at MSU 3. Advancements in SiC epitaxial growth 4. Recombination Induced Defect Reactions involving hydrogen in SiC.

11 History of Wide Band Gap Semiconductors at Mississippi State University Materials - Devices- Circuits - Systems Emerging Materials Research Laboratory EMRL ONR N C-0167 C. Wood SiC Epi at MSU Miss. Center for Advanced Semiconductor Prototyping MCASP AFRL/MDA F D-2103 J. Scofield SiC SBD Design Power Electronics Prototyping / Power Component Test Bed PEP/PCT SMDC DASG60-00-C-0074 L. Riley Modify HMMWV at MSU MESFET Fab at GE 0.3 µm n channel 0.55 µm p - buffer Semi-insulating substrate SBD Fab at MSU Prototype Power Electronics at MSU Testing at GE & MSU I DS (ma/mm) 800 SiC161, Wafer A 1x100 µm 600 Vg=0V, Vg step=-5v Commercial Packaging ADPC Sentinel Demo by MSU/Guard/ SMDC V DS (V)

12 Leveraging Basic Research into Production - Rapid, economical transfer of government sponsored technology development into systems Component Production Mississippi Small Business SBIR s MCASP Mississippi Center for Advanced Semiconductor Prototyping EMRL SiC CVD Epi Research AIXTRON SiC Reactor LAM 9400 Etcher Defect Engineering Materials Characterization Device Design Multi Wafer SiC Epi Sub-micron lithography Multi Wafer Plasma Etching Multi Wafer PECVD Multi Wafer Metal Deposition

13 Material Science Program at MSU ONR, AFRL, SBIR/STTR PL SEM DLTS FTIR SiH 4 Novel epitaxial growth techniques Traditional Material Characterization Si(g) Epitaxial Process Simulation Novel Non-Contact Material Characterization Techniques Defects and Impurities. Defect Engineering in WBG Semiconductors SiC for Nanotechnologies

14 Presentation Outline 1. Why SiC? 2. History of SiC program at MSU 3. Advancements in in SiC epitaxial growth 4. Recombination Induced Defect Reactions involving hydrogen in SiC.

15 Lower-temperature epitaxial growth of 4H-SiC using CH 3 Cl carbon gas precursor. Huang-De Lin, Galyna Melnychuk, Yaroslav Koshka 1 Mississippi State University, Box 9571, Mississippi State, MS 39762, USA

16 The hot-wall CVD reactors. Traditional Precursors: SiH 4 and C 3 H 8 Growth temperatures > C Lower temperatures => morphology degradation => polycrystalline

17 Simulated kinetics of chemical reactions of SiC epitaxial growth in the cross-section of the CVD reactor Silane decomposition SiH 4 Chloromethane decomposition CH 3 Cl SiH 2 CH 4 Si<g> C 2 H 4 Cl Our new model vapor-phase formation of Si droplets (clusters)

18 Growth at Lower Temperatures View of the susceptor from the rear-port window C Dense silicon-droplet cloud Si vapor condensation (cloud) is detrimental for epi quality and growth rate.

19 Surface morphology of the C growth Low Si/C ratio High Si/C ratio Optimized 2µm 50 µm >2 µm/hr J. of Crystal Growth Patent Pending

20 Growth rate dependence on silane 0 C 2.5 T Growth = C The saturation of the growth rate at higher SiH 4 flows is due to silicon vapor condensation in clusters reducing the amount of Si available for epitaxial growth Growth rate, 1/hr Exponential fitting Degraded morphology CH 3 Cl flow R( < < SiH 4 > SiH 4 > ) 1 exp τ SiH4 4 SiH 4 flow, sccm J. of Crystal Growth

21 Arhenius temperature dependences: (a) exponential rate coefficient of the SiH 4 flow dependence compared to (b) the growth rate temperature dependence. Exponential rate coefficient (sccm) τ SiH4 (sccm) R ( < < SiH 4 > SiH 4 > ) 1 exp τ τ SiH4 (T) 4 (b) E A = ev /T, K -1 K -1 Growth rate R (µm/hr) Growth rate, um/hr 10 1 (a) E A = ev /T, K -1 K -1 The value of E A is the same for R and τ SiH4 => the growth rate is determined by silicon vapor condensation.

22 HCl experiment: Rear view of the glowing susceptor during C epitaxial growth (a) No HCl (b) With HCl added Dense cloud of Si clusters Strongly reduced cloud

23 Selective epitaxial growth of 4H-SiC with SiO 2 mask. Bharat Krishnan, Hrishikesh Das, Huang-De Lin, Galyna Melnychuk, Yaroslav Koshka. 1 Mississippi State University, Box 9571, Mississippi State, MS 39762, USA Patent Pending

24 Pattern formation by Etching Plasma Plasma Mask Semiconductor Semiconductor

25 Pattern formation by Selective Epitaxial Growth CVD CVD Mask Semiconductor Semiconductor

26 SiO 2 mask for low-temperature selective epitaxial growth of 4H-SiC (LTSEG) SiC substrate SiO 2 mask SiO 2 Severely degrade at regular growth temperatures ( C) Survives at C of our novel low-temperature growth method

27 Low-temperature SEG at C: 4H-SiC mesas SiO 2 mask 4H-SiC substrate 4H-SiC mesas 2 µm (a) with SiO 2 mask >3 µm / hr 2 µm (b) SiO 2 mask removed Patent Pending

28 SEM image of the mesa-lines selectively grown in SiO 2 window at C. Lines oriented in different directions reveal the orientation-dependent defect generation. 5 µm Edge defects [ 1120] (off-axis direction) Patent Pending

29 Cross-sectional SEM of 30-µm-wide mesa line selectively grown in SiO 2 window at C. [ 1120] defective region SiO 2 SiO 2 Edge facet 1 µm 3 µm Patent Pending

30 Presentation Outline 1. Why SiC? 2. History of SiC program at MSU 3. Advancements in SiC epitaxial growth 4. Recombination Induced Defect Reactions involving hydrogen in in SiC. SiC.

31 Plasma hydrogenation. Experimental Reactive Ion Etching (RIE) system Hydrogen Plasma. Processing in new Inductively Coupled Plasma (ICP) system enabled further improvement of hydrogen incorporation in comparison to RIE hydrogenation.

32 SiC subjected to Plasma Hydrogenation. Incorporated hydrogen forms stable complexes with defects and impurities. Hydrogen concentration profiles repeat profiles of acceptor concentration. [1] S. Janson, A. Hallén, M. K. Linnarsson, and B. G. Svensson, Phys. Rev. B 64, (2001) Hydrogen plasma Defect or impurity Mobile hydrogen atom Stable hydrogen-defect complex in SiC

33 Effect of plasma hydrogenation on the concentration of active acceptors (Al). Passivation of Al acceptors down to cm -3 and to the depth in access of 2 µm was achieved after 2 hrs of hydrogenation. Ec 1E Al (SIMS) Ev Al-H H Al Concentration, cm E E E+16 Acceptors before hydrogenation (CV) Acceptors after hydrogenation (CV) E depth, µm

34 Simple Diffusion = x H D x t H x ] [ ] [ H hydrogen concentration t HT x H D x t H x = ] [ ] [ ] [ ] [ ] ][ [ ] [ HT T H K t HT ν = T concentration of empty traps. HT concentration of traps filled with hydrogen. K trapping constant. ν dissociation frequency Trap-Limited Diffusion H at interstitial sites H trapped by defect or impurity

35 PL spectra before and after plasma hydrogenation of epilayer moderately doped with Al. 7 4Al R 0 S 0 15 K PL intensity, arb.un Initial H 3 Hydrogenated Wavelength, Å Efficient trapping of hydrogen by Al acceptors is confirmed by reduction of Al-BE PL; Al acceptors are not the only trapping centers for hydrogen. H-related PL lines indicate simultaneous formation of H complex with Si vacancy (V Si -H ). Y. Koshka, M. S. Mazzola, Appl. Phys. Lett, 79(6), 752 (2001)

36 PL spectra of B-doped 4H-SiC epilayer: affect of plasma hydrogenation: A 4B 0 peak previously associated with a bound exciton at the neutral boron is in fat related to a hydrogen-defect complex (possibly, B-H). p = 8-9 x cm -3 t epi = 5 µm Y. Koshka, Appl. Phys. Lett., 82, 3260 (2003).

37 Phenomenon of Recombination-Induced Passivation (RIP) Optically-stimulated passivation of defects with hydrogen.

38 Schematic illustration of Recombination-Induced Passivation. Certain amount of the incorporated hydrogen does not form stable complexes with defects (so called free hydrogen ). Recombination-induced defect migration causes formation of various kinds of stable defect complexes. Above band-gap light T = 15 K T < ~ 250 K Al-H B-H 4B 0 V Si -H Mobile hydrogen atom Defect or impurity Stable hydrogen-defect complex in SiC

39 Photoluminescence spectra after Regular Hydrogenation and after Recombination Induced Passivation. Al-BE PL additionally quenches and disappears under optical excitation at low T in hydrogenated samples. 16 p = 6-8 x cm t epi = 2 µ m Al-BE Q 0 Contribution from the substrate Contribution from the substrate 4H-SiC 15 K 8 6 initial hydrogenated before excitation after excitation with above band-gap after excitation with above bandgap light Wavelength, Å

40 Changes in 4B 0 PL caused by optical excitation at 15K Recombination-Induced formation of defects responsible for 4B 0 PL (possibly Boron-Hydrogen complex) K B-H H 1 PL intensity, arb.un Al Q 0 (3) 2 hrs of Optical Excitation (2) 10 min of Optical Excitation (1) Hydrogenated before optical excitation Wavelength, Å

41 Optically induced changes of V Si -H (H 1 line). The optically induced growth of H 1 can not be masked by the concurren metastable quenching PL intensity (arb.units) H1 PL intensity Time of excitation (s) 3916 Å room T 4 hrs of optical excitation as-hydrogenated Wavelength (Å) H 1 H 1 H 1

42 Before Optical Excitation After Optical Excitation Recombination-induced formation of a strong radiative recombination channel (V Si -H emission)

43 Deuterium concentration in plasma-deuterated B-doped SiC epitaxial layer. The spot subjected to an optical excitation at 15K shows much deeper deuterium penetration => recombination-induced athermal migration. 1E+19 SIMS profiles --- No optical excitation Concentration, cm -3 1E+18 1E+17 ~ N B --- After short excitation --- After prolonged excitation with above-bandgap light at 15 K 1E Depth, um

44 Inversion of the conductivity type under optical excitation in hydrogenated p-type epilayers: strong recombination-induced passivation of acceptors. 1.2E Capacitance, pf 1E E E E E-11 0 after excitation n = 3-4 x cm -3 as-hydrogenated p = 1-2 x cm bias, V n p+

45 Hypothetical paths for H migration via Bourgoin- Corbett athermal migration mechanism. T C Hex T Si H + H + Capture e - Capture h + H 0 H 0

46 Recombination-induced formation of Hydrogen-Defect Complexes: Recombination energy => release of hydrogen from the trapping cites near the surface, athermal migration and trapping by more stable sites (e.g., Al and B acceptors, silicon vacancies, etc.) RIP effect (Above-bandgap light at 4-250K) Hydrogen trapped at the surface after plasma hydrogenation Al-H B-H 4B0 V Si -H Remaining questions The exact path for H migration? The energy position of the hydrogen state responsible for the recombination-induced migration? Device applications?