D. S. Green

Impact of deep levels on the electrical conductivity and luminescence of gallium nitride codoped with carbon and silicon

The impact of C incorporation on the deep level spectrum of n-type and semi-insulating GaN:C:Si films grown by rf plasma-assisted molecular-beam epitaxy (MBE) was investigated by the combination of deep level transient spectroscopy, steady-state photocapacitance, and transient deep level optical spectroscopy. The deep level spectra of the GaN:C:Si samples exhibited several band-gap states. A monotonic relation between systematic doping with C and quantitative trap concentration revealed C-related deep levels. A deep acceptor at Ec−2.05eV and a deep donor at Ec−0.11eV are newly reported states, and the latter is the first directly observed deep level attributed to the CGa defect. A configuration-coordinate model involving localized lattice distortion revealed strong evidence that C-related deep levels at Ec−3.0eV and Eν+0.9eV are likely identical and associated with the yellow luminescence in C-doped GaN films. Of the deep levels whose trap concentration increase with C doping, the band-gap states at Ec−3....

Realization of wide electron slabs by polarization bulk doping in graded III–V nitride semiconductor alloys

We present the concept and experimental realization of polarization-induced bulk electron doping in III–V nitride semiconductors. By exploiting the large polarization charges in the III–V nitrides, we are able to create wide slabs of high-density mobile electrons without introducing shallow donors. Transport measurements reveal the superior properties of the polarization-doped electron distributions than comparable shallow donor-doped structures, especially at low temperatures due to the removal of ionized impurity scattering. Such polarization-induced three-dimensional electron slabs can be utilized in a variety of device structures owing to their high conductivity and continuously changing energy gap.

Memory Effect and Redistribution of Mg into Sequentially Regrown GaN Layer by Metalorganic Chemical Vapor Deposition

Mg redistribution into a subsequently regrown GaN epilayer by metalorganic chemical vapor deposition (MOCVD) is studied. Dopant profiles from secondary ion mass spectrometry (SIMS) on n–p–n GaN samples have been analyzed. The regrowth study in a Mg-free reactor reveals that a Mg-rich film is present on MOCVD as-grown GaN:Mg base layers and can be removed by an acid etch, and that a slow Mg decay into the sequentially regrown GaN results from this Mg-rich surface film. We believe the commonly seen Mg memory effect in MOCVD causes the accumulation of Mg on the surface. From a MOCVD regrowth on n–p–n GaN grown by molecular beam epitaxy (MBE), the Mg diffusion constant is calculated to be about 3 ×10-15 cm2/s at 1160°C for Mg concentrations between 5 ×1017 cm-3 and 1 ×1019 cm-3. The roles of memory effect, surface segregation, and diffusion associated with Mg are addressed.

Power performance of AlGaN-GaN HEMTs grown on SiC by plasma-assisted MBE

We report AlGaN-GaN high electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) on SiC substrates with excellent microwave power and efficiency performance. The GaN buffers in these samples were doped with carbon to make them insulating. To reduce gate leakage, a thin silicon nitride film was deposited on the AlGaN surface by chemical vapor deposition. At 4 GHz, an output power density of 6.6 W/mm was obtained with 57% power-added efficiency (PAE) and a gain of 10 dB at a drain bias of 35 V. This is the highest PAE reported until now at 4 GHz in AlGaN-GaN HEMTs grown by MBE. At 10 GHz, we measured an output power density of 7.3 W/mm with a PAE of 36% and gain of 7.6 dB at 40-V drain bias.

Carbon doping of gan with CBr4 in radio-frequency plasma-assisted molecular beam epitaxy

Carbon tetrabromide (CBr4) was studied as an intentional dopant during rf plasma molecular beam epitaxy of GaN. Secondary ion mass spectroscopy was used to quantify incorporation behavior. Carbon was found to readily incorporate under Ga-rich and N-rich growth conditions with no detectable bromine incorporation. The carbon incorporation [C] was found to be linearly related to the incident CBr4 flux. Reflection high-energy electron diffraction, atomic force microscopy and x-ray diffraction were used to characterize the structural quality of the film’s postgrowth. No deterioration of structural quality was observed for [C] from mid 1017 to ∼1019u200acm−3. The growth rate was also unaffected by carbon doping with CBr4. The electrical and optical behavior of carbon doping was studied by co-doping carbon with silicon. Carbon was found to compensate the silicon although an exact compensation factor was difficult to extract from the data. Photoluminescence was performed to examine the optical performance of the film...

High conductivity modulation doped AlGaN/GaN multiple channel heterostructures

A methodology for the design of modulation doped AlGaN/GaN multiple channel heterostructures is presented. Doping is utilized in conjunction with polarization effects to achieve high carrier mobility and high sheet carrier density in each channel, while maintaining a low energy barrier for majority carrier transfer between channels. Several eight-period Si-doped Al0.22Ga0.78N/GaN heterostructures were grown by metalorganic chemical vapor deposition, according to the methodology. Sheet electron densities around 7.7×1013u2009cm−2, and room temperature electron mobilities as high as 1200 cm2/Vu200as were measured. Applications for the structure include lateral current spreading layers in III-nitride visible light and UV emitters and detectors, and high conductance source and drain access regions in AlGaN/GaN high electron mobility transistors.

Blue GaN-based light-emitting diodes grown by molecular-beam epitaxy with external quantum efficiency greater than 1.5%

We have grown blue (480 nm) nitride semiconductor light emitting diodes (LEDs) by plasma-assisted molecular beam epitaxy (MBE) on GaN templates. Packaged devices exhibited output powers up to 0.87 mW at 20 mA forward current. The corresponding external quantum efficiency was 1.68%. Utilizing a combination of direct current (dc) and pulsed electroluminescence measurements it has been demonstrated that at low (<20u2009mA) dc conditions the emission from these devices is governed by the combined effects of bandfilling and screening of electrostatic fields. However, at larger currents device heating dominates the emission properties.

Polarity control during molecular beam epitaxy growth of Mg-doped GaN

Mg doping has been found in some situations to invert growth on Ga-face GaN to N-face. In this study, we clarified the role the Ga wetting layer plays in rf plasma molecular beam epitaxy of GaN when Mg doping, for [Mg] from ∼2×1019 to ∼1×1020u2009cm−3 corresponding to the useful, accessible range of hole concentrations of p∼1017–1018u2009cm−3. Structures were grown in the N-rich and Ga-rich growth regime for single Mg doping layers and for multilayer structures with a range of Mg concentrations. Samples were characterized in situ by reflection high-energy electron diffraction and ex situ by atomic force microscopy, transmission electron microscopy, convergent beam electron diffraction, and secondary ion mass spectroscopy. Growth on “dry” surfaces (without a Ga wetting layer) in the N-rich regime completely inverted to N-face upon exposure to Mg. No reinversion to Ga-face was detected for subsequent layers. Additionally, Mg was seen to serve as a surfactant during this N-rich growth, as has been reported by others...

Impact of substrate temperature on the incorporation of carbon-related defects and mechanism for semi-insulating behavior in GaN grown by molecular beam epitaxy

The electrical conductivity and deep level spectrum of GaN grown by molecular beam epitaxy and codoped with carbon and silicon were investigated for substrate temperatures Ts of 650 and 720°C as a function relative carbon and silicon doping levels. With sufficiently high carbon doping, semi-insulating behavior was observed for films grown at both temperatures, and growth at Ts=720°C enhanced the carbon compensation ratio. Similar carbon-related band gap states were observed via deep level optical spectroscopy for films grown at both substrate temperatures. Due to the semi-insulating nature of the films, a lighted capacitance-voltage technique was required to determine individual deep level concentrations. Carbon-related band gap states underwent substantial redistribution between deep level and shallow acceptor configurations with change in Ts. In light of a Ts dependence for the preferential site of carbon incorporation, a model of semi-insulating behavior in terms of carbon impurity state incorporation ...

Micro-Raman thermometry in the presence of complex stresses in GaN devices

Raman thermometry is often utilized to measure temperature in gallium nitride (GaN) electronics. However, the accuracy of the technique is subject to errors arising from stresses which develop during device operation as a result of both thermoelastic and inverse piezoelectric effects. To assess the implications of these stresses on Raman thermometry, we investigate the use of the Stokes peak position, linewidth, and Stokes to anti-Stokes intensity ratio to estimate the temperature of GaN devices during operation. Our results indicate that only temperature measurements obtained from the intensity ratio method are independent of these stresses. Measurements using the linewidth, meanwhile, were found to correspond well with those obtained from the intensity ratio through the use of a reference condition which accounted for the stress dependency of this spectral component. These results were then compared to a three dimensional finite element model which yielded a correlation to within 5% between the computat...