Onur S. Koksaldi

Ge doping of β-Ga2O3 films grown by plasma-assisted molecular beam epitaxy

The Ge doping of β-Ga2O3(010) films was investigated using plasma-assisted molecular beam epitaxy as the growth method. The dependences of the amount of Ge incorporated on the substrate temperature, Ge-cell temperature, and growth regime were studied by secondary ion mass spectrometry. The electron concentration and mobility were investigated using Van der Pauw Hall patterns. Hall measurement confirmed that Ge acts as an n-dopant in β-Ga2O3(010) films. These results were compared with similar films doped by Sn. The Hall data showed an improved electron mobility for the same electron concentration when Ge is used instead of Sn as the dopant.

Demonstration of β-(Al x Ga1− x )2O3/β-Ga2O3 modulation doped field-effect transistors with Ge as dopant grown via plasma-assisted molecular beam epitaxy

β-(Al x Ga1− x )2O3/β-Ga2O3 heterostructures were grown via plasma-assisted molecular beam epitaxy. The β-(Al x Ga1− x )2O3 barrier was partially doped by Ge to achieve a two-dimensional electron gas (2DEG) in Ga2O3. The formation of the 2DEG was confirmed by capacitance–voltage measurements. The impact of Ga-polishing on both the surface morphology and the reduction of the unintentionally incorporated Si at the growth interface was investigated using atomic force microscopy and secondary-ion mass spectrometry. Modulation doped field-effect transistors were fabricated. A maximum current density of 20 mA/mm with a pinch-off voltage of −6 V was achieved on a sample with a 2DEG sheet charge density of 1.2 × 1013 cm−2.

Dielectric stress tests and capacitance-voltage analysis to evaluate the effect of post deposition annealing on Al2O3 films deposited on GaN

Systematic stress tests that help to evaluate the stability and dielectric performance of Al2O3 films under DC bias conditions are reported. Capacitance-voltage (C-V) curves were monitored for changes after subjecting the dielectric film to constant forward and reverse bias stress. Stress tests, along with C-V analysis, are used to evaluate the effect of post deposition annealing on Metal-Organic Chemical Vapor Deposition) Al2O3 films deposited on GaN. The individual benefits and drawbacks of each film and anneal condition were identified. These suggest that the anneals can be tailored to the unannealed film characteristics to achieve desired improvements in performance. It is found that post deposition annealing in forming gas improves performance under reverse bias stress by reducing the fixed charge and the field in the oxide but does not improve performance under forward bias.

Metalorganic chemical vapor deposition and characterization of (Al,Si)O dielectrics for GaN-based devices

In this paper, we report on the growth and electrical characterization of (Al,Si)O dielectrics grown by metalorganic chemical vapor deposition (MOCVD) using trimethylaluminum, oxygen, and silane as precursors. The growth rates, refractive indices, and composition of (Al,Si)O films grown on Si(001) were determined from ellipsometry and XPS measurements. Crystallinity and electrical properties of (Al,Si)O films grown in situ on c-plane GaN were characterized using grazing incidence X-ray diffraction and capacitance–voltage with current–voltage measurements, respectively. Si concentration in the films was found to be tunable by varying the trimethylaluminum and/or oxygen precursor flows. The Si incorporation suppressed the formation of crystalline domains, leading to amorphous films that resulted in reduced interfacial trap density, low gate leakage and ultra-low hysteresis in (Al,Si)O/n-GaN MOS-capacitors.

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.

1 kV field plated in-situ oxide, GaN interlayer based vertical trench MOSFET (OG-FET)

In recent years, GaN trench MOSFETs have been actively investigated to achieve low on-resistance and high breakdown voltage [1-8]. The absence of a JFET region makes the trench MOSFET a favorable device structure to reduce the on-resistance. However, poor (electron) channel mobility in GaN trench MOSFETs lead to increased channel resistance. This could potentially result in reliability issues and/or high on-resistance as a large gate bias is needed to reduce the channel resistance. In our previous works, we demonstrated a novel device design (OG-FET), where enhanced channel mobility was obtained by inserting a MOCVD-regrown GaN interlayer between the trenched structure and the in-situ gate dielectric [7, 8]. The breakdown performance of OG-FETs reported in previous work was limited due to the absence of edge termination [8]. In this work, OG-FETs were fabricated with field plate based edge termination which resulted in an enhanced breakdown from 600 V (EBR ∼ 1.5 MV/cm) to 1000 V (EBR ∼ 2 MV/cm).

Quality and reliability of in-situ Al2O3 MOS capacitors for GaN-based power devices

This work reports on the electrical characterization of Al₂O₃/GaN MOS capacitors grown by means of metal-organic chemical vapor deposition. Novel results for 25-nm-thick Al₂O₃ show (i) an interface-state density measured by means of UV-assisted C-V technique lower than 7x10¹¹ cm^-2; (ii) flat-band voltage shift lower than 100 mV up to a gate voltage of 4 V (1.6 MV/cm); (iii) a breakdown strength of 8.8 MV/cm; (iv) timeto-breakdown of 20 years for electric field smaller than 3.7 MV/cm at room temperature, and charge to breakdown of 13.6 C/cm² if measured at 10 mA/cm² with an electric field of ~5.6 MV/cm.

Maskless regrowth of GaN for trenched devices by MOCVD

Blanket regrowth studies were performed on GaN trenches with varying widths and optimized for two types of devices—those that required the profile of the trench to be maintained and those that required the complete filling of trenches, i.e., a planar surface after regrowth. Low temperature Al0.22Ga0.78N growth was optimized and used as the marker layer for SEM. GaN deposition at a medium temperature of 950 °C and using N2 as carrier gas resulted primarily in growth on the (0001) plane, while the growth on the sidewalls was governed by the formation of slow growing semi-polar planes. This gave a conformal profile to the regrown GaN—useful for regrown GaN interlayer based vertical trench MOSFETs. In contrast, high temperature (1150 °C) growth in H2 resulted in high lateral growth rates. The planar surface was achieved under these conditions—a very promising result for CAVET-type devices.