Nicholas Fichtenbaum

Optical and structural properties of GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells

GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells (MQWs) were fabricated by holographic lithography and subsequent reactive ion etching. Etch related damage of the nanostructures was successfully healed through annealing in NH3∕N2 mixtures under optimized conditions. The nanopatterned samples exhibited enhanced luminescence in comparison to the planar wafers. X-ray reciprocal space maps recorded around the asymmetric (101¯5) reflection revealed that the MQWs in both nanopillars and nanostripes relaxed after nanopatterning and adopted a larger in-plane lattice constant than the underlying GaN layer. The pillar relaxation process had no measurable effect on the Stokes shift typically observed in MQWs on c-plane GaN, as evaluated by excitation power dependent photoluminescence (PL) measurements. Angular-resolved PL measurements revealed the extraction of guided modes from the nanopillar arrays.

Recent progress in metal-organic chemical vapor deposition of

Progress in metal-organic chemical vapor deposition of high quality N-polar (Al, Ga, In)N films on sapphire, silicon carbide and silicon substrates is reviewed with focus on key process components such as utilization of vicinal substrates, conditions ensuring a high surface mobility of species participating in the growth process, and low impurity incorporation. The high quality of the fabricated films enabled the demonstration of N-polar (Al, Ga, In)N based devices with excellent performance for transistor applications. Challenges related to the growth of high quality N-polar InGaN films are also presented.

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A recessed slant gate processing has been used in AlGaN/GaN high electron mobility transistors (HEMTs) to mitigate the electric field, minimize the dispersion and increase the breakdown voltage. More than one order of magnitude of decrease in gate leakage has been observed by recessing the slant gate. For a 0.65 µm gate-length device, an extrinsic fT of 18 GHz and extrinsic fMAX of 52 GHz at a drain bias of 25 V were achieved. At 10 GHz, a state-of-the-art power density of 20.9 W/mm, with a power-added efficiency (PAE) of 40% at a drain bias of 83 V, was demonstrated.

N-polar group-III nitrides

It has been found that the reverse leakage current of AlGaN/GaN Schottky contacts can be significantly reduced by a CF4 plasma treatment prior to the Schottky metal evaporation. The data of electrical characterization suggest that the leakage reduction is related to the modification of the semiconductor surface by plasma treatment. The leakage reduction effect was also observed in GaN Schottky contacts. Capacitance-voltage characterization of the GaN Schottky contacts indicates that the Schottky barrier height was slightly increased by the plasma treatment. A two-step surface treatment procedure, consisting of a BCI3 plasma treatment followed by a brief CF4 plasma treatment, has been developed as an efficient approach to reduce the reverse leakage of the Schottky contacts, while avoiding side effects related to the CF4 plasma.

Recessed Slant Gate AlGaN/GaN High Electron Mobility Transistors with 20.9 W/mm at 10 GHz

We present a systematic study of the impact of CF4 plasma treatment on GaN. It was found that CF4 plasma etches GaN at a slow rate and yields a smooth etched surface. The effect of CF4 plasma on electrical characteristics of GaN metal-semiconductor field-effect-transistor structures shows that the CF4 plasma introduces acceptors into the near surface region of the GaN, which depletes mobile electrons. It was further demonstrated that leakage current of AlGaN/GaN (or GaN) Schottky diodes can be significantly suppressed by proper CF4 plasma treatment. These unique properties of CF4 plasma can be utilized for the advanced processing of GaN transistors.

Plasma Treatment for Leakage Reduction in AlGaN/GaN and GaN Schottky Contacts

The metalorganic chemical vapor deposition of AlN and GaN on C-face 6H–SiC was investigated. Similar to the procedure on Si-face SiC, GaN films were fabricated in a two-step process, where first a thin AlN base layer was deposited prior to the growth of the main GaN layer. Polarity conversion from the expected N-polar AlN and GaN to Al-polar AlN and Ga-polar GaN films was observed when the AlN base layers were deposited using a low ammonia to trimethylaluminum ratio of 250 during growth. The properties of the resulting Ga-face GaN-films were similar to those grown on Si-face SiC. Hexagonal surface features were seen on the N-polar AlN and GaN films obtained with a high V/III-ratio during AlN growth.

Impact of

Nitrogen- and Ga-polar GaN and InGaN/GaN multiple quantum-well (MQW) films were prepared via metal organic chemical vapor deposition and evaluated via photoluminescence at reduced temperatures in order to compare their optical characteristics. While N- and Ga-polar GaN films grown at standard high temperatures were comparable in terms of photoluminescence at tested temperatures, the N-polar InGaN MQW quality was inferior to their Ga-polar counterparts, confirmed with greater enhancement in luminescence intensity from 300 to 10 K and unobservable phonon replicas at 10 K for the N-polar InGaN MQW due to the broad emission peak. Additionally performed electroluminescence studies on N-polar light-emitting diode samples indicated that the poor luminescence of the N-polar samples was not related to electric field effects. Influence of residual impurities (C and O) was strongly suggested via secondary ion mass spectroscopy, leading us to conclude that the poor luminescence properties of the N-polar InGaN MQWs were predominantly caused by the elevated residual impurity concentrations in the N-polar (In,Ga)N layers grown at low temperatures.

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N-polar (000-1) GaN templates were patterned using holographic lithography to create nanopillar (NP) and nanostripe (NS) arrays. InGaN and GaN was subsequently regrown on the patterned wafers by metalorganic chemical vapor deposition (MOCVD). The impact of changes in V/III ratio and growth temperature were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL). Highly selective growth of InGaN and GaN on planes close to the {10-10} m-planes was observed over a wide range of growth conditions as well as different facet formation compared to regrowth on Ga-polar (0001) NP and NS arrays.

Plasma Treatment on GaN

Employing deeply recessed GaN/AlGaN/GaN high-electron mobility transistors, we experimentally demonstrate the correlation between the DC-RF dispersion and the gate leakage current. It was found that both the DC-RF dispersion and the gate leakage are strongly affected by surface charging. The impact of surface charging can be controlled by using GaN/AlGaN/GaN structures with varied GaN cap thickness. In the absence of field plates, the tradeoff between the DC-RF dispersion and the gate leakage can be compromised by choosing a proper GaN cap thickness. Our optimum epistructure design yields an output power density of 5.6 W/mm with an associated power added efficiency of 72% at 28-V bias and 4-GHz frequency. The gate leakage current is as low as 30 muA/mm at up to 40-V gate-drain bias.

Effect of the Nucleation Conditions on the Polarity of AlN and GaN Films Grown on C-face 6H-SiC

An n+ GaN cap layer has been applied on an AlGaN/GaN high electron mobility transistor (HEMT) to achieve a non-alloyed ohmic contact. Delta dopings were used at the heterointerface of the n+ GaN cap and AlGaN layer to lower the AlGaN potential barrier and to improve communication between the n+ GaN cap and the two-dimensional electron gas (2DEG) channel. Non-alloyed ohmic contact resistance as low as 0.2 Ωmm, and sheet resistance of 60 Ω/square were achieved. Selective etch and sidewall technology were applied during processing. The n+ capped device was fabricated and compared to a standard AlGaN/GaN HEMT. Lower on-resistance and access resistance, higher fτ and fMAX, and better linearity in terms of gm were achieved.