D. Kapolnek

Role of threading dislocation structure on the x‐ray diffraction peak widths in epitaxial GaN films

In this letter we demonstrate that the anomalously low (002) x‐ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation (TD) geometry. Epitaxial GaN films were grown on c‐plane sapphire by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a horizontal flow reactor. Films were grown with (002) rocking curves (ω‐scans) widths as low as 40 arcsec and threading dislocation densities of ∼2×1010 cm−2. The threading dislocations in this film lie parallel to the [001] direction and within the limit of imaging statistics, all are pure edge with Burgers vectors parallel to the film/substrate interface. These TDs will not distort the (002) planes. However, distortion of asymmetric planes, such as (102), is predicted and confirmed in (102) rocking curve widths of 740 arcsec. These results are compared with films with (002) rocking curves of ∼270 arcsec and threading dislocation densities of ∼7×108 cm−2.

Very-high power density AlGaN/GaN HEMTs

Research work focusing on the enhancement of large-signal current-voltage (I-V) capabilities has resulted in significant performance improvement for AlGaN/GaN HEMTs. 100-150 /spl mu/m wide devices grown on SiC substrates demonstrated a record power density of 9.8 W/mm at 8 GHz, which is about ten times higher than GaAs-based FETs; similar devices grown on sapphire substrates showed 6.5 W/mm, which was thermally limited, 2-mm-wide devices flip-chip mounted on to AlN substrates produced 9.2-9.8 W output power at 8 GHz with 44-47% PAE. A flip-chip amplifier IC using a 4-mm device generated 14 W at 8 GHz, representing the highest CW power obtained from GaN-based integrated circuits to date.

Defect structure of metal‐organic chemical vapor deposition‐grown epitaxial (0001) GaN/Al2O3

Defect structures were investigated by transmission electron microscopy for GaN/Al2O3 (0001) epilayers grown by metal‐organic chemical vapor deposition using a two‐step process. The defect structures, including threading dislocations, partial dislocation bounding stacking faults, and inversion domains, were analyzed by diffraction contrast, high‐resolution imaging, and convergent beam diffraction. GaN film growth was initiated at 600 °C with a nominal 20 nm nucleation layer. This was followed by high‐temperature growth at 1080 °C. The near‐interfacial region of the films consists of a mixture of cubic and hexagonal GaN, which is characterized by a high density of stacking faults bounded by Shockley and Frank partial dislocations. The near‐interfacial region shows a high density of inversion domains. Above ∼0.5 μm thickness, the film consists of isolated threading dislocations of either pure edge, mixed, or pure screw character with a total density of ∼7×108 cm−2. The threading dislocation reduction in the...

Anisotropic epitaxial lateral growth in GaN selective area epitaxy

Epitaxial lateral mask overgrowth which occurs during GaN selective epitaxy has been studied using linear mask features. The lateral growth varies between its maximum and minimum over a 30° angular span and exhibits hexagonal symmetry. Vertical growth follows an opposite trend, with lateral growth maxima, and vertical growth minima occurring for lines parallel to the GaN 〈10•0〉. Large variations in the lateral growth are also obtained through variations in the growth temperature and NH3 flow. Under proper growth conditions, lateral to vertical growth rate ratios of up to 4.1 can be achieved, resulting in significant lateral mask overgrowth and coalescence of features without excessive growth times.

Very high breakdown voltage and large transconductance realized on GaN heterojunction field effect transistors

We report record high breakdown voltages up to 340 and 230 V realized on unintentionally doped (1.5 μm gate length) and Si doped (1 μm gate length) AlGaN/GaN modulation doped field effect transistors (MODFETs), respectively. The devices also have large transconductances up to 140 mS/mm and a full channel current of 150–400 mA/mm. The Si doped MODFET sample demonstrated a very high room temperature mobility of 1500 cm2/Vs. With these specifications, GaN field effect transistors as microwave power devices are practical.

Structural evolution in epitaxial metalorganic chemical vapor deposition grown GaN films on sapphire

The structural evolution of epitaxial GaN layers grown on basal plane sapphire has been studied by atomic force microscopy (AFM), x‐ray diffraction, and transmission electron microscopy (TEM). High‐temperature growth (1050–1080 °C) on optimized nucleation layers leads to clear, specular films. AFM on the as‐grown surface shows evenly spaced monatomic steps indicative of layer by layer growth. AFM measurements show a step termination density of 1.7×108 cm−2 for 5 μm films. This value is in close agreement with TEM measurements of screw and mixed screw‐edge threading dislocation density. The total measured threading dislocation density in the 5 μm films is 7×108 cm−2.

Influence of sapphire nitridation on properties of gallium nitride grown by metalorganic chemical vapor deposition

The properties of 1.2 μm thick GaN films were found to be significantly influenced by the duration of exposing the sapphire substrate to ammonia prior to the GaN growth initiation. The different nitridation schemes of sapphire strongly affect the dislocation structure of GaN films resulting in a decrease of the dislocation density from 2×1010 to 4×108 cm−2 for shorter NH3 preflow times. Room‐ and low‐temperature electron transport characteristics of these films are specifically affected by the dislocation structure. A 300 K electron mobility as high as 592 cm2/V s was obtained for a short ammonia preflow whereas a long nitridation caused the mobility to drop to 149 cm2/V s. Additionally, the photoluminescence quality deteriorates for samples with a long sapphire nitridation time.

Nucleation layer evolution in metal‐organic chemical vapor deposition grown GaN

The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600 °C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on c‐plane sapphire. The layers consist of predominantly cubic GaN (c‐GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600 °C) to the high temperatures (1080 °C) necessary for the growth of device quality GaN. Upon heating to 1080 °C the nucleation layer partially converts to hexagonal GaN (h‐GaN) while retaining a high stacking fault density. The mixed cubic‐hexagonal character of the nucleation layer region is sustained after subsequent high‐temperature GaN growth.

Bias dependent microwave performance of AlGaN/GaN MODFET's up to 100 V

1 /spl mu/m gate-length AlGaN/GaN modulation doped field effect transistors (MODFETs) have been fabricated on an insulating GaN buffer layer for better carrier confinement. These devices demonstrate simultaneously high current levels (>500 mA/mm), excellent pinch-off and high gate-drain breakdown voltages (220 V for 3 /spl mu/m gate-drain spacing). In contrast to their GaAs counterparts, the current-gain cutoff frequency of the AlGaN/GaN devices shows little degradation at high drain voltage biases. A power-gain cutoff frequency of 19 GHz is obtained at 100 V. ACW power density of 1.57 W/mm at 4 Ghz is also achieved when biased at 28 V and 205 mA/mm.

Growth and characterization of bulk InGaN films and quantum wells

InGaN bulk layers and single quantum wells were grown by atmospheric pressure metalorganic chemical vapor deposition on c‐plane sapphire. We have found that the incorporation efficiency of indium into InGaN epitaxial layers is strongly dependent on the growth rate of the films. Narrow and bright band edge related luminescence was observed for InGaN films up to an indium content of 20% grown at 700 °C. In0.16Ga0.84N single quantum wells with graded InxGa1−xN barriers showed intense luminescence, with an energy shift towards shorter wavelength with decreasing quantum well thickness. The photoluminescence full width at half‐maximum of the 50 A thick well was as low as 7.9 nm (59 meV) at 300 K.