Cory Lund

Indium segregation in N-polar InGaN quantum wells evidenced by energy dispersive X-ray spectroscopy and atom probe tomography

Energy dispersive X-ray spectroscopy (EDX) in scanning transmission electron microscopy and atom probe tomography are used to characterize N-polar InGaN/GaN quantum wells at the nanometer scale. Both techniques first evidence the incorporation of indium in the initial stage of the barrier layer growth and its suppression by the introduction of H2 during the growth of the barrier layer. Accumulation of indium at step edges on the vicinal N-polar surface is also observed by both techniques with an accurate quantification obtained by atom probe tomography (APT) and its 3D reconstruction ability. The use of EDX allows for a very accurate interpretation of the APT results complementing the limitations of both techniques.

Relaxed c-plane InGaN layers for the growth of strain-reduced InGaN quantum wells

A fully relaxed In0.1Ga0.9N layer was grown by plasma-assisted molecular beam epitaxy on c-plane GaN using a grading technique. The growth of the graded InGaN layer in the intermediate regime enabled a smooth surface without the accumulation of In droplets. Transmission electron microscopy images show that the relaxation occurs through the formation of a high density of threading dislocations (TDs). Despite the presence of these TDs, relaxed InGaN films were then successfully used as a pseudo-substrate for the growth of InGaN/GaN quantum wells which luminesced at room temperature.

Comparing electrical performance of GaN trench-gate MOSFETs with a-plane and m-plane sidewall channels

GaN trench-gate MOSFETs with m- and a-plane-oriented sidewall channels were fabricated and characterized. The trench-gate MOSFET performance depended strongly on the sidewall-MOS-channel plane orientation. The m-plane-oriented MOS channel devices demonstrated higher channel mobility, higher current density, lower sub-threshold slope, and lower hysteresis with similar threshold voltage and on–off ratio compared to a-plane MOS channel devices. These results indicate that orienting trench-gate MOSFET toward the m-plane would allow for better on-state characteristics while maintaining similar off-state characteristics.

First report of scaling a normally-off in-situ oxide, GaN interlayer based vertical trench MOSFET (OG-FET)

GaN lateral transistors (HEMTs) continue to penetrate the power electronics market demonstrating excellent performance in the medium power applications. However, for power applications 10kW and higher, vertical GaN devices are preferred over lateral one, since the former offers higher current and power densities. To date, several different vertical transistor structures have been proposed and reported, such as in-situ oxide based vertical trench MOSFET with an undoped GaN interlayer as a channel (OGFET) [1, 2], current aperture vertical electron transistors (CAVETs) [3, 4], junction field effect transistors (JFETs) [5, 6] and MOSFETs [7, 8]. Gupta et al. have demonstrated the high performance OGFET with low specific on-state resistance (Ron, sp) recently [1]. This study presents the large device scaling of the OGFET to realize high output current.

Metal-organic chemical vapor deposition of high quality, high indium composition N-polar InGaN layers for tunnel devices

In this study, the growth of high quality N-polar InGaN films by metalorganic chemical vapor deposition is presented with a focus on growth process optimization for high indium compositions and the structural and tunneling properties of such films. Uniform InGaN/GaN multiple quantum well stacks with indium compositions up to 0.46 were grown with local compositional analysis performed by energy-dispersive X-ray spectroscopy within a scanning transmission electron microscope. Bright room-temperature photoluminescence up to 600 nm was observed for films with indium compositions up to 0.35. To study the tunneling behavior of the InGaN layers, N-polar GaN/In0.35Ga0.65N/GaN tunnel diodes were fabricated which reached a maximum current density of 1.7 kA/cm2 at 5 V reverse bias. Temperature-dependent measurements are presented and confirm tunneling behavior under reverse bias.

Optimization of a chlorine-based deep vertical etch of GaN demonstrating low damage and low roughness

The dry etching of GaN to form deep vertical structures is a critical step in many power device processes. To accomplish this, a chlorine and argon etch is investigated in detail to satisfy several criteria simultaneously such as surface roughness, crystal damage, and etch angle. Etch depths from 2 to 3.4 μm are shown in this paper. The authors investigate the formation of etch pits and its contributing factors. In addition, a nickel hard mask process is presented, with an investigation into the causes of micromasking and a pre-etch to prevent it. The authors show the results of optimized etch conditions resulting in a 2 μm deep, 0.831 nm rms roughness etch, with a 7.6° angle from vertical and low surface damage as measured by photoluminescence.

III-N heterostructure devices for low-power logic

Future generations of ultra-scaled logic may require alternative device technologies to transcend the limitations of Si CMOS; in particular, power dissipation constraints in aggressively-scaled, highly-integrated systems make device concepts capable of achieving switching slopes (SS) steeper than 60 mV/decade especially attractive. Tunneling field effect transistors (TFETs) are one such device technology alternative. While a great deal of research into TFETs based on Si, Ge, and narrow band gap III-Vs has been reported, these approaches each face significant challenges. An alternative approach based on the use of III-N wide band gap semiconductors in conjunction with polarization engineering offers potential advantages in terms of drain current density and switching slope. In this talk, the prospects for III-N based TFETs for logic will be discussed, including both simulation projections as well as experimental progress.

High Spatial Resolution Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography study of Indium segregation in N-polar InGaN Quantum Wells

N-polar grown III-nitrides are very interesting materials for the fabrication of heterostructures devices such as transistors, photodetectors, solar cells or optoelectronic devices. In GaN/(In,Ga)N/GaN heterostructures with thin (In,Ga)N layers, polarization engineering allows to achieve interband tunneling. Thereby the tunneling probability is proportional to the indium concentration in the InGaN layers. Indium incorporation is higher for N-polar InGaN films than for the typically grown Ga-polar ones. N-polar III nitride films are often grown on misoriented substrates enabling the growth of smooth, high quality layers [1]. The crystal misorientation leads to the formation of surface steps, and misorientation angles of 4°-5° can result in up to 3-4 unit cell high steps. The growth of N-polar InGaN films is further complicated by the necessary reduced growth temperatures and the required absence of hydrogen in the growth ambient. In quantum well structures, however, hydrogen, which acts as surfactant and promotes the growth of smooth layers, can be introduced during GaN barrier growth, allowing the deposition of thick multiple quantum well (MQW) stacks. Ga-polar InGaN films have been extensively studied and are known for their local fluctuations in the indium composition [2]. Not much is known about the uniformity of N-polar InGaN layers.

Novel III-N heterostructure devices for low-power logic and more

Future ultra-scaled logic and low-power systems require fundamental advances in semiconductor device technology. Due to power constraints, device concepts capable of achieving switching slopes (SS) steeper than 60 mV/decade are essential if scaling of conventional computational architectures is to continue. Likewise, ultra low power systems also benefit from devices capable of maintaining performance under low-voltage operation. Towards this end, tunneling field effect transistors (TFETs) are one promising alternative. While much work has been devoted to realizing TFETs in Si, Ge, and narrow-gap III-V materials, the use of III-N heterostructures and the exploitation of polarization engineering offers some unique opportunities. From physics-based simulations, performance of GaN/InGaN/GaN heterostructure TFETs appear capable of delivering average SS approaching 20 mV/decade over 4 decades of drain current, and on-current densities exceeding 100 μA/μm in aggressively scaled nanowire configurations. Experimental progress towards realizing III-N based TFETs includes demonstration of GaN/InGaN/GaN backward tunnel diodes by both MOCVD and MBE, and nanowires grown selectively by MBE and used as the basis for device fabrication.

Metal-organic chemical vapor deposition of N-polar InN quantum dots and thin films on vicinal GaN

N-polar InN layers were deposited using MOCVD on GaN-on-sapphire templates which were miscut 4° towards the GaN m-direction. For thin layers, quantum dot-like features were spontaneously formed to relieve the strain between the InN and GaN layers. As the thickness was increased, the dots elongated along the step direction before growing outward perpendicular to the step direction and coalescing to form a complete InN layer. XRD reciprocal space maps indicated that the InN films relaxed upon quantum dot formation after nominally 1 nm thick growth, resulting in 5–7 nm tall dots with diameters around 20–50 nm. For thicker layers above 10 nm, high electron mobilities of up to 706 cm2/V s were measured using Hall effect measurements indicating high quality layers.N-polar InN layers were deposited using MOCVD on GaN-on-sapphire templates which were miscut 4° towards the GaN m-direction. For thin layers, quantum dot-like features were spontaneously formed to relieve the strain between the InN and GaN layers. As the thickness was increased, the dots elongated along the step direction before growing outward perpendicular to the step direction and coalescing to form a complete InN layer. XRD reciprocal space maps indicated that the InN films relaxed upon quantum dot formation after nominally 1 nm thick growth, resulting in 5–7 nm tall dots with diameters around 20–50 nm. For thicker layers above 10 nm, high electron mobilities of up to 706 cm2/V s were measured using Hall effect measurements indicating high quality layers.