I. Daumiller

MOVPE growth of GaN on Si(1 1 1) substrates

Metalorganic chemical vapor phase deposition of thick, crack-free GaN on Si can be performed either by patterning of the substrate and selective growth or by low-temperature (LT) AIN interlayers enabling very thick GaN layers. A reduction in dislocation density from 10 10 to 10 9 cm -2 is observed for LT-AIN interlayers which can be further improved using monolayer thick Si x N y in situ masking and subsequent lateral overgrowth. Crack-free AlGaN/GaN transistor structures show high room temperature mobilities of 1590 cm 2 /V s at 6.7×10 12 cm -2 sheet carrier concentration. Thick crack-free light emitters have a maximum output power of 0.42 mW at 498 nm and 20mA.

High-sheet-charge–carrier-density AlInN∕GaN field-effect transistors on Si(111)

AlInN∕GaN heterostructures have been proposed to possess advantageous properties for field-effect transistors (FETs) over AlGaN∕GaN [Kuzmik, IEEE Electron Device Lett. 22, 501 (2001); Yamaguchi et al., Phys. Status Solidi A 188, 895 (2001)]. A major advantage of such structures is that AlInN can be grown lattice-matched to GaN while still inducing high charge carrier densities at the heterointerface of around 2.7×1013cm−3 by the differences in spontaneous polarization. Additionally, it offers a higher band offset to GaN than AlGaN. We grew AlInN FET structures on Si(111) substrates by metalorganic chemical vapor phase epitaxy with In concentrations ranging from 9.5% to 24%. Nearly lattice-matched structures show sheet carrier densities of 3.2×1013cm−2 and mobilities of ∼406cm2∕Vs. Such Al0.84In0.16N FETs have maximum dc currents of 1.33A∕mm for devices with 1μm gate length.

Piezoelectric GaN sensor structures

Free-standing GaN and AlGaN/GaN cantilevers have been fabricated on (111) silicon substrate using dry etching. On these cantilevers, a piezoresistor and a high-electron-mobility transistor (HEMT) structure have been realized, and the piezoresponse has been characterized. Cantilever bending experiments resulted in a Youngs modulus of approximately 250 GPa, a sensitivity of K/spl sim/90, and a modulation of the HEMT current of up to 50%. It is seen that the piezoresponse could be related to both the bulk properties and the properties of the heterostructure interface.

Transient characteristics of GaN-based heterostructure field-effect transistors

DC current-switching and power-switching transients of various GaN-based FET structures are investigated. Two different characteristics are compared, namely, thermal and electronic transients. While the thermal transients are mainly reflected in changes in channel carrier mobility, the electronic transients are dominated by charge instabilities caused by the polar nature of the material. The discussion of the electronic transients focuses, therefore, on instabilities caused by polarization-induced image charges. Three structures are discussed, which are: 1) a conventional AlGaN/GaN heterostructure FET; 2) an InGaN-channel FET; and 3) an AlGaN/GaN double-barrier structure. In structures 2) and 3), field-induced image charges are substituted by doping impurities, eliminating this source of related instability. This is indeed observed.

Unstrained InAlN/GaN HEMT structure

InAlN has been investigated as barrier layer material for GaN-HEMT structures, potentially offering higher sheet charge densities (Kuzmik, 2002) and higher breakdown fields (Kuzmik, 2001). Lattice matched growth of the barrier layer can be achieved with 17% in content, avoiding piezo polarization. In this configuration the sheet charge density is only induced by spontaneous polarization. First experimental results of unpassivated undoped samples realized on 111-Si substrate exceed a DC output current density of 1.8 A/mm for a gate length of 0.5 /spl mu/m. Small signal measurements yield a f/sub t/ = 26 GHz and f/sub max/ = 14 GHz, still limited by the residual conductivity of the Si-substrate. A saturated output power at 2 GHz in class A bias point yielded a density of 4.1W/mm at V/sub DS/ = 24 V.

Strains and Stresses in GaN Heteroepitaxy - Sources and Control

We present a study of the sources of strain in GaN heteroepitaxy by in- and ex-situ measurement techniques. With an in-situ curvature measurement technique the strain development can be directly correlated to the different layers and doping in simple and device structures. We show several solutions for strain reduction and control. High-quality devices grown on Si are demonstrated.

Evaluation of AlGaN-GaN HFETs up to 750/spl deg/C

GaN based FET structures offer the potential of high speed, high power and high temperature operation beyond that of GaAs based FET devices, which have been operated up to 500°C. Here, operation of a Si-doped channel AlGaN-GaN HFET up to 750°C in vacuum is evaluated. Specifically, three parts of the device structure were evaluated: (a) the Schottky contact, (b) the active channel region with its channel sheet charge, and (c) the buffer layer.

Analysis of surface charging effects in passivated AlGaN-GaN FETs using a MOS test electrode

Passivated AlGaN-GaN high electron mobility transistor (HEMT) structures are modified by adding a MOS test gate placed between gate and drain to identify surface charge phenomena by stress experiments. A new method is described to identify the vertical position of the charge centroid of charge injected from the gate. In the case investigated, this is within the passivation layer.

First diamond FET RF power measurement on diamond quasi-substrate

Diamond is an exceptional widegap material predestined for high power high frequency electronics. However, up to now devices could only be fabricated on chips cut from synthetic single crystal stones of small size, and device performance had been restricted to small signal measurements due to severe large signal instabilities However, diamond 100-oriented quasi-substrates of single crystal quality can be grown on ceramic substrates such as SrTiO/sub 3/ using a single crystal iridium buffer layer. In this experiment we have succeeded in fabricating the first surface channel FETs (using a hydrogen induced p-type channel) on such a diamond quasi-substrate. The FETs show high electrical stability, enabling large signal and power measurements for the first time, and thus demonstrating the feasibility of diamond microwave high power electronics.

GaN based piezo sensors

This work presents a technology which has been developed to fabricate free-standing GaN membrane and cantilever structures. First experiments have enabled us to verify the piezo response of these GaN based cantilever structures. Especially, the bulk polarization doping generated in the base layer is a new important contribution. GaN heterostructures grown on 111-oriented Si wafers have been used. Free standing cantilevers and membranes have been fabricated using RIE and ICP dry etching. Cantilevers have been etched from the rear side or from the surface. It is expected that this technology will enable new device concepts based on stress induced pn-junction effects.