I. P. Smorchkova

POLARIZATION-INDUCED CHARGE AND ELECTRON MOBILITY IN ALGAN/GAN HETEROSTRUCTURES GROWN BY PLASMA-ASSISTED MOLECULAR-BEAM EPITAXY

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51u200a700 cm2/Vu200as (T=13u200aK) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012u200acm−2.

AlN/GaN and (Al,Ga)N/AlN/GaN two-dimensional electron gas structures grown by plasma-assisted molecular-beam epitaxy

We report on an extensive study of the two-dimensional electron gas (2DEG) structures containing AlN layers. It is shown that the presence of large polarization fields in the AlN barrier layer in AlN/GaN heterostructures results in high values of the 2DEG sheet density of up to 3.6×1013u2002cm−2. Room-temperature sheet resistance of 180u2002Ω/□ is demonstrated in the AlN/GaN structure with a 35 A AlN barrier. As a result of reduced alloy disorder scattering, low-temperature electron mobility is significantly enhanced in AlN/GaN heterostructures in comparison to AlGaN/GaN structures with similar values of the 2DEG sheet density. The growth of GaN cap layers on top of AlN/GaN structures with relatively thick (∼35 A) AlN barriers is found to lead to a significant decrease in the 2DEG sheet density. However, inserting a thin (∼10 A) AlN layer between AlxGa1−xN and GaN in the AlxGa1−xN/GaN (x∼0.2–0.45) 2DEG structures does not affect the 2DEG sheet density and results in an increase of the low-temperature electron mob...

Optimization of the surface morphologies and electron mobilities in GaN grown by plasma-assisted molecular beam epitaxy

The morphology and electrical properties of homoepitaxial GaN layers grown by molecular beam epitaxy at 720u200a°C were investigated as a function of Ga/N ratio. GaN films grown with low Ga/N ratios (N-stable regime) are semi-insulating and have heavily pitted morphologies. GaN films grown with higher Ga/N ratios (intermediate regime) have fewer pits with areas of atomically flat surface. The room-temperature electron mobilities in samples grown in the intermediate regime are greater than 800 cm2/Vu200as and increase with Ga/N ratio. At the highest Ga/N ratios (Ga-droplet regime), Ga droplets formed on the surface during growth. Although the surface morphology is free of pits and atomically flat for films grown within the Ga-droplet regime, the mobility decreases significantly compared to films grown in the intermediate regime. Room-temperature electron mobilities as high as 1191 cm2/Vu200as were measured in a GaN film grown with the highest Ga/N ratio within the intermediate regime.

Gallium nitride based transistors

An overview is presented of progress in GaN electronic devices along with recent results from work at UCSB. From 1995 to 2001, the power performance of AlGaN/GaN high electron mobility transistors (HEMT) improved from 1.1 to 11 W mm-1, respectively. The disadvantage of the low thermal conductivity of the sapphire substrate was mitigated by flip-chip bonding onto AlN substrates, yielding large periphery devices with an output power of 7.6 W. A variety of HEMT amplifier circuits have been demonstrated. The first AlGaN/GaN heterojunction bipolar transistor (HBT) was demonstrated in 1998, with a current gain of about 3. By developing the technique of emitter regrowth, a current gain of 10 was achieved in both GaN BJTs and AlGaN/GaN HBTs. A common emitter current gain cutoff frequency of 2 GHz was measured. Critical issues involved in the growth of high quality AlGaN/(AlN)/GaN heterostructures and GaN:Mg by metal-organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) and the device fabrication are discussed.

Two-dimensional electron-gas AlN/GaN heterostructures with extremely thin AlN barriers

Plasma-assisted molecular-beam epitaxy is used to grow a set of two-dimensional electron-gas AlN/GaN structures with AlN barrier thicknesses varied between 24 and 50 A. The density of the two-dimensional electron gas formed at the GaN/AlN interface increases from 1.51×1013u200acm−2 for the AlN barrier width of 24 A to 3.65×1013u200acm−2 for the AlN barrier width of 49 A. The increase in the electron sheet density is accompanied by a decrease in electron mobility related to tensile strain relaxation and enhanced interface roughness scattering. It is shown that room-temperature sheet resistances below 200 Ω/□ can be achieved in AlN/GaN high electron mobility transistor structures with 35–45 A AlN barriers.

High mobility two-dimensional electron gas in AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

High quality AlGaN/GaN heterostructures have been grown by radio-frequency plasma-assisted molecular beam epitaxy on n-type GaN templates grown on sapphire by metal organic chemical vapor deposition. The unintentionally doped Al0.12Ga0.88N/GaN heterostructure exhibits a 77 K Hall mobility of 14u2009500 cm2/Vs and a 12 K mobility of 20u2009000 cm2/Vs (ns=5.0×1012u2002cm−2). A room temperature mobility of 1860 cm2/Vs (ns=4.8×1012u2002cm−2) was calculated for the two-dimensional electron gas channel using a two layer model from the measured mobility for the whole structure (template plus heterostructure). Magnetoresistance measurements at 4.2 K showed well-resolved Shubnikov–de Haas oscillations, which began at 2.6 T.

Mg doping of GaN layers grown by plasma-assisted molecular-beam epitaxy

GaN:Mg layers grown by plasma-assisted molecular-beam epitaxy at 650u200a°C are investigated. Secondary-ion-mass-spectroscopy measurements reveal uniform Mg doping profiles with very sharp boundaries. The amount of incorporated Mg atoms changes approximately linearly with incident Mg flux. Hall measurements on p-type GaN:Mg layers show that about 1%–2% of all Mg atoms are ionized at room temperature. The hole mobility depends strongly on the hole concentration, varying from μp=24u200acm2/Vu200as for p=1.8×1017u200acm−3 to μp=7.5u200acm2/Vu200as for p=1.4×1018u200acm−3. GaN p–n diodes with molecular-beam-epitaxy-grown p regions are analyzed using current–voltage measurements.

Charge control and mobility in AlGaN/GaN transistors: Experimental and theoretical studies

In this article we report on two dimensional sheet charge and mobility in GaN/AlGaN heterostructure field effect transistors. Both experimental and theoretical results are presented. Experimental results are reported on samples grown by metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Theoretical studies are done to examine how spontaneous polarization and piezoelectric effect control the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier and phonon scattering influence mobility. We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface compared to the MOCVD samples. By carefully fitting the experimental data we present results on interface roughness parameters for MBE and MOCVD samples.

GaN–GaN junctions with ultrathin AlN interlayers: Expanding heterojunction design

The formation of a two-dimensional electron gas (2DEG) was observed at GaN–GaN junctions, when an AlN layer of a thickness greater than or equal to 0.5 nm was inserted, and the GaN cap layer was modulation-doped with silicon. No 2DEG was found for undoped samples. When the AlN interlayer thickness was increased from 0.5 to 1 nm, the electron mobility increased from 720 to 1250 cm2/Vs at 300 K and from 6400 to 12u200a000 cm2/Vs at 77 K. The electron mobility was also affected by the Si-spike doping concentration with the highest electron mobility of 13u200a800 cm2/Vs at 77 K measured for a doping of 5×1012u2009cm−2. The formation of the 2DEG was attributed to the polarization-induced discontinuity in the conduction band caused by the extremely strong polarization fields in the pseudomorphically strained AlN interlayers.

Characterization of an AlGaN/GaN two-dimensional electron gas structure

An AlxGa1−xN/GaN two-dimensional electron gas structure with x=0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. X-ray diffraction maps of asymmetric reciprocal lattice points confirmed that the thin AlGaN layer was coherently strained to the thick GaN layer. Methods for computing the aluminum mole fraction in the AlGaN layer by x-ray diffraction are discussed. Hall effect measurements gave a sheet electron concentration of 5.1×1012u200acm−2 and a mobility of 1.9×104u200acm2/Vu200as at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov–de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006u200am0 based on the temperature dependence of the amplitude of Shubnikov–de Haas oscillations. The quantum lifetime was about one...