Status of HVPE GaN Growth and The Piezoelectric Coefficient of GaN and Chemo-mechanical Polishing

1. Status of HVPE GaN Growth and The Piezoelectric Coefficient of GaN and Chemo-mechanical Polishing Phil Tavernier, Ed Etzkorn and David Clarke Materials Department, College of Engineering University of California, Santa Barbara

2. Status of HVPE Growth • Background. Although our films had dislocation densities as low as 107 cm-2, they were rather conducting and had Si, C and O concentrations ~ 1 x 10-18 cm-3. • High concentrations of Si, C and O attributed to catalytic role of carbon on the SiO-SiO2 vapor equilibrium causing incorporation of Si, O and C. • Decision made to re-design HVPE reactor. • Redesign of HVPE reactor to remove all graphite structures and create an all-silica reactor completed. • Construction now complete and initial growths begun. • Emphasis will be on cantilever growth on sapphire using cantilevers produced by pulsed excimer laser ablation.

3. Summary of Two-step Growth Process • Demonstrated a two-step, nucleation and growth HVPE process for growth of high-quality GaN on sapphire. • No buffer or template film is required prior to HVPE growth, enabling the growth to be accomplished in a single chamber. • Dislocation densities (107-108 cm-2) reduced by 10-100x compared to typical planar MOCVD growth. • Patent application filed. • Two-step process does not reduce wafer curvature but this can be compensated by growth on both sides of the wafer. • Refinement and control of the two-step process continuing

4. HVPE Growth Activities • Development of greater process control for HVPE growth • Reactor redesign to create an all-quartz reactor • Establish a better understanding of cracking in GaN films • To break the critical thickness barrier for cracking (eg. 2mm on SiC) • Development of a LEO process using 2-step growth process • To achieve still lower dislocation densities • To control nucleation stage and hence cracking propensity • HVPE Films as Substrates for MURI program • Utilization of MBE and MOCVD growth on HVPE films to take advantage of lower dislocation densities in HVPE films • In the first three years one focus was the measurement of the thermal conductivity of GaN. Now discontinued

5. SIMS Analysis of HVPE GaN Films O CN C Cl Cl H • Use of nucleation layer reduces incorporation of impurities • 50x reduction in O and 10-20x reduction in C in films grown on nucleation layer

6. Strain-Induced Polarization of GaN • Objective: • Measure the strain-induced polarization of GaN • Approach: • Measure the charge induced by shock-wave loading of GaN films. • Pulsed Nd;YAG laser rapidly heats backside of sample generating a uniaxial strain that propagates through the crystal generating a one-dimensional stress shock wave. • The stress wave can be measured, in a non-contact way, by monitoring the surface displacement of the front-side. • Induced charge can be measured from the voltage generated across the piezoelectric film.

7. Spontaneous vs Induced Charge For Positive Piezoelectric coefficient

8. Pulsed Laser Induced Strain Wave Generation Doppler interferometer signal Derived stress pulse * Laser pulse applies a uniaxial strain wave that creates a propagating stress wave pulse

9. Experimental Arrangement

10. Strain-Induced Piezoelectric Charge in GaAs Longer time scale shows multiple stress wave reflections. (B orientation) NB:Opposite response produced by GaAs crystals of opposite polarity. Stress of ~ 1 GPa.

11. GaAs Strain-Induced Piezoelectric Charge A-face (Ga) [111] e B-face (As) A-face Sign convention: The piezoelectric coefficient is positive when a positive charge is induced In the positive direction of the axis under an extensile stress.

12. Calibration on ZnO Single Crystal In Progress

13. Effect of Free Carrier Screening Doped (111) B GaAs grown by MBE Schematic for + ve piezoelectric

14. Re-cap: Observations on GaN HVPE-GaN 2.2 mm MOCVD NB: Same sign of initial voltage swing as (111)A GaAs

15. Comparison: GaAs and GaN GaAs GaN Film coordinates In terms of stress:

16. Piezoelectric Coefficients of Sphalerite and Wurtzite Semiconductors Sphalerite Wurtzite Units: C/m2.Data from Arlt and Quadflieg, Phys. Stat. Sol., 25, 323 (1968), and T. Ikeda, “Fundamentals of Piezoelectricity”

17. Spontaneous vs Strain-induced Polarization Spontaneous Strain-induced - 0.034 (calc) - ?? GaN - 0.05 (calc) ZnO 0.96 - 0.09 (calc) AlN 1.46 - 0.185 GaAs 0.0 Units: C/m2

18. Summary of Findings • HVPE and MOCVD (0001) GaN films grown on sapphire have the same sign of piezoelectric strain coefficient as (111)A GaAs. • This implies that the piezoelectric coefficient is of opposite sign to the II-VI wurtzite crystals, BeO and ZnO. • The same piezoelectric sign for (111)A GaAs and (0001) GaN would be consistent if both are terminated with Ga atoms. • The numerical value of the e33has yet to be determined unambiguously.

19. Consequences for GaN devices • Opposite sign of strain-induced polarization to that of spontaneous polarization requires that both coherency stresses and thermal expansion mismatch stresses must be taken into account when computing interface charge. • Depending on stress state, the strain-induced polarization charge may enhance or detract from spontaneous polarization charge. • Example: • AlGaN on GaN vs InGaN on GaN

20. Chemo-mechanical Polishing of GaN • To fabricate devices on laser-lift off surfaces or MBE Mg:GaN need method of create a smooth surface • Diamond or alumina polishing produces scratches • Previous chemical recipes, ie hot KOH did not work • Siton based chemo-mechanical polishing

21. GaN Materials Polished

22. Chemo-mechanical Polishing of GaN Before polishing After polishing

23. Chemo-mechanical Polishing of MBE GaN:Mg Before polishing After polishing, pH=11.4

24. Chemo-mechanical Polishing of GaN • Colloidal silica in base solution • pH extended using organic base • Data for GaN:Mg grown by MBE 8.5

25. Summary: Chemo-mechanical Polishing • Colloidal silica in base solution can be used to polish GaN • Polish is most effective on the B (Nitrogen) face of GaN • pH range can be extended using an organic base • Max. polishing rate ~ 1.1 micron / hour • Polishing action is chemo-mechanical since no polishing • occurs in the absence of pressure • (Chemo-mechanical polishing also effective on sapphire)