Jacques I. Pankove

A GaN/4H-SiC heterojunction bipolar transistor with operation up to 300°C

We report on the fabrication and characterization of GaN/4H-SiC n-p-n heterojunction bipolar transistors (HBTs). The device structure consists of an n-SiC collector, p-SiC base, and selectively grown n-GaN emitter. The HBTs were grown using metalorganic chemical vapor deposition on SiC substrates. Selective GaN growth through a SiO 2 mask was used to avoid damage that would be caused by reactive ion etching. In this report, we demonstrate common base transistor operation with a modest dc current gain of 15 at room temperature and 3 at 300°C.

Photo-, cathodo-, and electroluminescence from erbium and oxygen co-implanted GaN

Efficient Er-related photo-, cathodo-, and electroluminescence at 1539 nm was detected from Er and O co-implanted n -type GaN on sapphire substrates. Several combinations of Er and O implants and postimplant annealing conditions were studied. The Er doses were in the range (0.01–5)×10 15 ions/cm 2 and O doses (0.1–1)×10 16 ions/cm 2 . GaN films implanted with 2×10 15 Er 2+ /cm 2 at 350 keV and co-implanted with 10 16 O + /cm 2 at 80 keV yielded the strongest photoluminescence intensity at 1539 nm. The annealing condition yielding the strongest Er-related photoluminescence intensity was a single anneal at 800 °C (45 min) or at 900 °C (30 min) in flowing NH 3 . The optimum O:Er ratio was found to be between 5:1 and 10:1. Co-implanting the GaN:Er films with F was also found to optically activate the Er, with slightly (20%) less photoluminescence intensity at 1539 nm compared to equivalent GaN:Er,O films. The Er-related luminescence lifetime at 1539 nm was found to depend on the excitation mechanism. Luminescence lifetimes as long as 2.95±0.15 ms were measured at 77 K under direct excitation with an InGaAs laser diode at 983 nm. At room temperature the luminescence lifetimes were 2.35±0.12, 2.15±0.11, and 1.74±0.08 ms using below-band-gap excitation, above-band-gap excitation, and impact excitation (reverse biased light emitting diode), respectively. The cross sections for Er in GaN were estimated to be 4.8×10 −21 cm 2 for direct optical excitation at 983 nm and 4.8×10 −16 cm 2 for impact excitation. The cross-section values are believed to be within a factor of 2–4.

Electroluminescence from erbium and oxygen coimplanted GaN

Room temperature operation of erbium and oxygen coimplanted GaN m‐i‐n (metal–insulator–n‐type) diodes is demonstrated. Erbium related electroluminescence at λ=1.54 μm was detected under reverse bias after a postimplant anneal at 800°C for 45 min in flowing NH3. The integrated light emission intensity showed a linear dependence on applied reverse drive current.

Cathodoluminescence study of erbium and oxygen coimplanted gallium nitride thin films on sapphire substrates

The cathodoluminescence(CL) of erbium and oxygen coimplanted GaN(GaN:Er:O) and sapphire (sapphire:Er:O) was studied as a function of temperature. Following annealing, the 1.54 μm intra‐4f‐shell emission line was observed in the temperature range of 6–380 K. As the temperature increased from 6 K to room temperature, the integrated intensity of the infrared peak decreased by less than 5% for GaN:Er:O, while it decreased by 18% for sapphire:Er:O. The observation of minimal thermal quenching by CL suggests that Er and O dopedGaN is a promising material for electrically pumped room‐temperature optical devices emitting at 1.54 μm.

Electrical characterization of GaN/SiC n-p heterojunction diodes

GaN/SiC heterojunction diodes have been fabricated and characterized. Epitaxial n-type GaN films were grown using metalorganic chemical vapor deposition (MOCVD) and electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE) on p-type Si-face 6H-SiC wafers. The I–V characteristics have diode ideality factors and saturation currents as low as 1.2 and 10−32 A/cm2, respectively. The built-in potential in the MOCVD- and ECR-MBE-grown n-p heterojunctions was determined from capacitance–voltage measurements at 2.90±0.08 eV and 2.82±0.08 eV, respectively. From the built-in potential the energy band offsets for GaN/SiC heterostructures are determined at ΔEC=0.11±0.10 eV and ΔEV=0.48±0.10 eV.

Optical characterization of GaN/SiC n-p heterojunctions and p-SiC

Optical characterization of GaN/SiC heterojunctions and p-SiC has been performed to explain the current–voltage (I–V) characteristics in GaN/SiC n-p heterojunction diodes. The I–V characteristics exhibit tunneling-assisted current with low forward “turn-on” voltages around 1.15 V as opposed to the expected drift/diffusion current with a turn on around 2.5 V. Electroluminescence (EL) measurements on these diodes revealed an infrared peak at 1.25 eV and a red peak at 1.75 eV. Photoluminescence (PL) measurements on p-SiC yielded peaks at 1.25 and 1.80 eV. Since the band gap of 6H–SiC is 3.03 eV, we attribute the EL and PL peaks to radiative transitions from the conduction band edge to a defect level and subsequently down to the valence band edge of p-SiC. This defect level is located 1.25 eV above the valence band edge.

Molecular doping of gallium nitride

Photoconductivity experiments were made on bulk GaN doped with Mg and O and grown using high pressures and high temperature. The bulk GaN:Mg,O was insulating, indicating compensation. The photoconductive response to photons above the energy band gap was comparable to that of epitaxially grown GaN:Mg samples. However, the UV-to-visible rejection ratio (solar blindness) was three orders of magnitude larger in the bulk GaN:Mg,O than for other epitaxially grown GaN samples. The dramatically improved visible rejection ratio is tentatively attributed to molecular doping by paired donors (O) and acceptors (Mg). Vacuum UV reflectance was performed to verify if MgO critical point transitions could be found in the GaN:Mg,O. A reflectance peak at 6.7 eV was found in both MgO and GaN:Mg,O.

Properties of Gallium Nitride

Gallium nitride has many useful properties that include a large direct gap, high electrical and thermal conductivities, and nearly the hardness of sapphire. GaN decomposes at -100°C, can sustain high electron velocities and exhibit acoustoelectric effects. But two challenges remain: to make it conducting p-type and to synthesize the cubic phase in a large single crystal instead of the usual hexagonal structure.

GaN/SiC heterojunction bipolar transistors

Abstract We report on the evolution of the fabrication and characterization of high-temperature and high-power GaN/SiC n–p–n heterojunction bipolar transistors (HBTs). The HBT structures consists of an n-type GaN emitter and a SiC p–n base/collector. Initially, the HBTs were fabricated using reactive ion etching (RIE) to define both the emitter and base areas. However, the poor etch selectivity between GaN and SiC made it difficult to stop at the thin base layer. Furthermore, the RIE caused damage at the heterojunctions, which resulted in large leakage currents. Selective area growth was therefore employed to form the n-GaN emitters. GaN/SiC HBTs were first demonstrated using the 6H-polytype. These transistors had an extraordinary high dc current gain of >10 6 at room temperature and were able to operate at 520°C with a current gain of 100. However, in more recent work, this performance could not easily be reproduced due to the presence of a parasitic deep defect level in the p-type 6H–SiC. The possibility of obtaining higher quality 4H–SiC than 6H–SiC, without this defect level, seemed promising since much of the materials development is focused on 4H–SiC, due to its larger energy band gap and superior electron mobility. GaN/4H–SiC HBTs are demonstrated with a modest dc current gain of 15 at room temperature and 3 at 300°C.

High-Power High-Temperature Heterobipolar TransistorWith Gallium Nitride Emitter

A new heterobipolar transistor was made with the wide bandgap semicon-ductors gallium nitride (GaN) and silicon carbide (SiC). The heterojunction allows high injection efficiency, even at elevated temperatures. A record current gain of ten million was obtained at room temperature, decreasing to 100 at 535°C. An Arrhenius plot of current gain vs 1/T yields an activation energy of 0.43 eV that corresponds to the valence band barrier blocking the escape of holes from the base to the emitter. This activation energy is approximately equal to the difference of energy gaps between emitter and base. This Transistor can operate at high power without cooling. A power density of 30 KW/cm 2 was sustained.