K.P. Lee

Fabrication and performance of GaN electronic devices

Abstract GaN and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.

GaN electronics for high power, high temperature applications

A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

Electrical characterization of GaN metal oxide semiconductor diode using Sc2O3 as the gate oxide

GaN metal oxide semiconductor diodes were demonstrated utilizing Sc 2 O 3 as the gate oxide. Sc 2 O 3 was grown at 100°C on MOCVD grown n-GaN layers in a molecular beam epitaxy system, using a scandium elemental source and an electron cyclotron resonance oxygen plasma. Ar/Cl 2 based discharges were used to remove Sc 2 O 3 , to expose the underlying n-GaN for ohmic metal deposition in an inductively coupled plasma system. Electron beam deposited Ti/Al/Pt/Au and Pt/Au were utilized as ohmic and gate metallizations, respectively. An interface trap density of 5 X 10 1 1 eV - 1 cm - 2 was obtained with the Terman method. Conductance-voltage measurements were also used to estimate the interface trap density and a slightly higher value was obtained as compared to the Terman method. Results of capacitance measurements at elevated temperature (up to 300°C) indicated the presence of deep states near the interface.

Electrical effects of N2 plasma exposure on dry-etch damage in p- and n-GaN Schottky diodes

Abstract Exposure of dry-etch damaged p- and n-GaN to a N 2 plasma at elevated temperatures (175–350°C) is found to produce partial recovery of the electrical properties of the near-surface region. The recovery is due to two mechanisms – an annealing effect and a nitridation of the initially N 2 deficient surface. A degree of surface smoothing was also obtained with N 2 plasma exposure. The extent of the damage recovery is less than complete for both conductivity types of GaN.

Magnetic Properties of Mn and Fe-Implanted p-GaN

The structural and magnetic properties of p-GaN implanted with high doses of Mn + or Fe + (0.1-5 at%) and subsequently annealed at 700-1000 °C were examined by transmission electron microscopy, selected-area diffraction patterns, X-ray diffraction and SQUID magnetometry. The implanted samples showed paramagnetic behavior on a large diamagnetic background signal for implantation doses below 3 at% Mn or Fe. At higher doses the samples showed signatures of ferromagnetism with Curie temperatures <250 K for Mn and <150 K for Fe implantation. The structural analysis of the Mn-implanted GaN showed regions consistent with the formation of Ga x Mn 1-x N platelets occupying ∼5% of the implanted volume. An estimate of ∼(5.5 ± 1.9) μ B per Mn was obtained, consistent with the expected value (5.0) for a half-filled shell. The formation of secondary phases such as Mn x Ga y or Mn x N y was excluded by careful diffraction analysis. The implantation process may have application in forming selected-area contact regions for spin-polarized carrier injection in device structures and in enabling a quick determination of the Curie temperatures in dilute magnetic semiconductor host materials.

Inductively coupled high-density plasma-induced etch damage of GaN MESFETs

Abstract The fabrication of a wide variety of GaN-based photonic and electronic devices depends on dry etching, which typically requires ion-assisted removal of the substrate material. Under conditions of both high plasma flux and energetic ion bombardment, GaN etch rates greater than 0.5 μm min −1 and anisotropic etch profiles are readily achieved in inductively coupled plasma (ICP) etch systems. Unfortunately, under these conditions, plasma-induced damage often occurs. Attempts to minimize such damage by reducing the ion energy or increasing the chemical activity in the plasma often result in a loss of etch rate or profile control which can limit dimensional control and reduce the utility of the process for device applications requiring anisotropic etch profiles. It is therefore necessary to develop plasma etch processes which couple anisotropy for critical dimension and sidewall profile control and high etch rates with low-damage for optimum device performance. In this study, we report changes in source resistance, reverse breakdown voltage, transconductance, and drain saturation current for GaN MESFET structures exposed to an Ar ICP plasma. In general, plasma-induced damage was more sensitive to ion bombardment energies as compared to plasma flux.

Dry etching mechanism of copper and magnetic materials with UV illumination

Abstract To better understand the dry etching of copper and magnetic materials with UV illumination an etch mechanism is proposed based on the subprocesses occurring in an inductively coupled plasma (ICP) reactor. The photo-dissociation of Cl 2 provides chlorine-enriched environment near the surface, and UV photons enhance the surface reaction, leading to fast formation of metal chlorides on the surface with low activation energy. The overall etching process of metals with UV illumination is limited by absorption capacity of UV radiation by reaction products. The proposed etch mechanism also showed the potential gaseous etch products are CuCl, CuCl 2 , Cu 2 Cl, Cu 2 Cl 2 , Cu 2 Cl 3 , Cu 3 Cl 2 and Cu 3 Cl 3 , while the dominant gas species are CuCl 2 and Cu 2 Cl 3 with UV illumination. The ICP etching of magnetic materials in Cl 2 /Ar discharges with UV illumination showed no enhancement in etch rate for NiFe, but a substantial enhancement for NiFeCo mainly due to the lower binding energy of NiFeCo relative to NiFe.

Self-aligned process for emitter- and base-regrowth GaN HBTs and BJTs

Abstract The development of a self-aligned fabrication process for small emitter contact area ( 2×4 μ m2) GaN/AlGaN heterojunction bipolar transistors and GaN bipolar junction transistors is described. The process features dielectric-spacer sidewalls, low damage dry etching and selected-area regrowth of p-GaAs(C) on the base contact or n-GaN/AlGaN on the emitter contact. Series resistance effects are still found to influence the device performance.

Development of chemically assisted dry etching methods for magnetic device structures

There is a strong need for advanced pattern transfer methods for magnetic devices such as magnetic random access memories, sensors for avionics and mine detection, and read/write heads for high density information storage. As the critical dimensions in these devices are decreased, the use of ion milling for pattern transfer presents major obstacles, including sidewall redeposition (which degrades magnetic performance) and poor mask selectivity. Most magnetic materials do not form volatile etch products in conventional reactive ion etching. We have recently found that high density plasmas provide efficient ion-assisted desorption of metal chloride etch products, provided that the etch production formation and removal are balanced by correct choice of ion/neutral ratio. We have completed the survey of plasma chemistries for etching of giant magnetoresistance (GMR) (NiFe, NiMnSb) and collossal magnetoresistance (CMR) (LaCaMnO3,LaSrMnO3,PrBaCaMnO3) materials. The optimum choices are Cl2/Ar for CMR oxides, SF6...

Comparison of the effects of H2 and D2 plasma exposure on AlGaAs/GaAs high electron mobility transistors

Abstract The dc and rf performance of AlGaAs/GaAs high electron mobility transistors were measured after exposure to low pressure (5 mTorr) inductively coupled plasmas of H 2 or D 2 for various source (100–400 W) and chuck powers (10–100 W) and durations (1–4 min). The plasma exposure decreases drain–source current, increases gate leakage and increases the gate ideality factor, even at very low plasma powers and for short process durations. A number of physical and chemical effects are found to be responsible for the observed behavior, including preferential loss of As from the surface, creation of deep trap states by energetic ion bombardment and passivation of Si donors by formation of Si–H or Si–D neutral complexes.