Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Status of HVPE Growth Background. Although our films had dislocation densities as low as 10 7 cm -2, they were rather conducting and had Si, C and O concentrations ~ 1 x cm -3. High concentrations of Si, C and O attributed to catalytic role of carbon on the SiO-SiO 2 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.

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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 ( cm -2 ) reduced by x 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

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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. 2  m 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

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB SIMS Analysis of HVPE GaN Films Use of nucleation layer reduces incorporation of impurities 50x reduction in O and 10-20x reduction in C in films grown on nucleation layer H C O CN Cl

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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.

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Spontaneous vs Induced Charge For Positive Piezoelectric coefficient

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Experimental Arrangement

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Strain-Induced Piezoelectric Charge in GaAs NB: Opposite response produced by GaAs crystals of opposite polarity. Stress of ~ 1 GPa. Longer time scale shows multiple stress wave reflections. (B orientation)

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB GaAs Strain-Induced Piezoelectric Charge Sign convention: The piezoelectric coefficient is positive when a positive charge is induced In the positive direction of the axis under an extensile stress. A-face (Ga) A-face B-face (As) [111] 

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Calibration on ZnO Single Crystal In Progress

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Effect of Free Carrier Screening Doped (111) B GaAs grown by MBE Schematic for + ve piezoelectric

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Re-cap: Observations on GaN 2.2  m MOCVD HVPE-GaN NB: Same sign of initial voltage swing as (111) A GaAs

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Comparison: GaAs and GaN GaAsGaN In terms of stress: Film coordinates

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Piezoelectric Coefficients of Sphalerite and Wurtzite Semiconductors A-B InSb CdTe CdS 0.35 InAs ZnSe ZnS 0.27 GaSb ZnS ZnO 1.57 GaAs BeO AlN 1.55 SphaleriteWurtzite Units: C/m 2. Data from Arlt and Quadflieg, Phys. Stat. Sol., 25, 323 (1968), and T. Ikeda, “Fundamentals of Piezoelectricity”

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Spontaneous vs Strain-induced Polarization SpontaneousStrain-induced GaN ZnO AlN GaAs Units: C/m (calc)- ?? (calc) (calc)

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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 e 33 has yet to be determined unambiguously.

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB GaN Materials Polished Growth MethodPolarity MBE, Mg-dopedN-face MOCVDN-face HVPEN-face MOCVD, low Ga fluxGa-face MOCVDGa-face HVPEGa-face

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Chemo-mechanical Polishing of GaN Before polishing After polishing

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Chemo-mechanical Polishing of MBE GaN:Mg Before polishingAfter polishing, pH=11.4

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Chemo-mechanical Polishing of GaN Colloidal silica in base solution pH extended using organic base Data for GaN:Mg grown by MBE pHRemoval rate (Microns / hour)

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 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)