Session D25: Focus Session: Dopants and Defects in Semiconductors - Nitrides, SiC

Electrical properties of point and extended defects in indium nitride

The surface and bulk electrical properties of indium nitride (InN) are determined by point and extended native defects to an extent greater than in any other III-V semiconductor. The n-type behavior of undoped InN thin films and the surface accumulation layer of electrons reflecting surface Fermi level pinning high in the conduction band are now well-established experimentally and understood in terms of InN's large electron affinity, nearly 6 eV. However, the properties of Mg-doped material (is it really p-type?) and the effect on electron concentration and transport of the high density of extended defects was, until recently, less well understood. A coordinated experimental approach using a combination of electrical and electrothermal measurements will be described that allows definitive evaluation of hole transport in Mg-doped InN and, when combined with transport modeling based on solutions to the Boltzmann transport equation, a quantitative understanding of the crucial role of charged line defects in limiting electron transport in this material. The use of thermopower is especially noteworthy as it mitigates the effect of the ubiquitous surface accumulation layer which had prevented direct measurement of hole transport by Hall effect. The extension of the present transport measurement methodology to other systems including buried interfaces and heterojunctions will also be described.   

Optical Properties of GaN/ZnO solid solutions studied by Density Functional Calculations

Semiconducting alloys of GaN/ZnO are promising hosts for solar photo-catalysis. The aqueous interface has been shown by Domen and collaborators\footnote{K. Maeda {\it et al.}, Nature {\bf 440}, 295 (2006).} to catalyze water oxidation, a key half- reaction in water splitting. We calculate by DFT, the energetics of many (GaN)$_{1-x}$(ZnO)$_x$ supercell configurations. Results show that significant short-range order should be expected. A phase diagram is suggested from free energies calculated versus temperature and concentration. The mechanism of band gap bowing is examined. The DFT+U method and hybrid functionals are used to reduce the problems of band gap underestimation. Oscillator strengths near the band gap are calculated, and an ensemble-averaged dielectric function is constructed, with an aim to learn how to optimize solar light absorption.   

Theory of hot-carrier-induced phenomena in GaN high-electron-mobility transistors

It has long been known that GaN high-electron-mobility transistors can degrade significantly under hot electron stress. Meneghesso et al [1]. showed that GaN-based HEMTs are most prone to degradation in a state with high electric field and low current. More recently, an increase in the yellow luminescence was observed under similar stress conditions [2]. Hot electrons can provide sufficient energy to cause a pre-existing defect to convert into a metastable configuration, release a hydrogen atom, or cause migration of pre-existing defects. We show that, among the possible known defects in GaN, the hydrogenated Ga vacancy has the properties that are needed to account for both the electrical degradation and the luminescence data. \\[4pt] [1] G. Meneghesso, et al., IEEE Trans. Dev. Mater. Reliab. \textbf{8}, 332 (2008). \\[0pt] [2] C.-H. Lin, et al., Appl. Phys. Lett. \textbf{95}, 033510 (2009).   

Stress-assisted migration of vacancies in GaN HEMTs

GaN is widely used in the fabrication of High Electron Mobility Transistors (HEMTs), but limited understanding of degradation mechanisms still hampers applications. At moderate bias, an inverse piezoelectric effect in GaN induces stress that often leads to cracking. A possible role of stress-induced defects and defect migration remains elusive. Here we examine the possibility that lattice stress may favor vacancy formation and migration by strain relaxation as a possible precursor to crack formation and device failure. We report results of first-principles density functional calculations of the dependence of both Ga and N vacancy formation and migration energies under strain and electric field and assess the impact of these factors on degradation in GaN HEMTs. Migration barriers are calculated by a Nudged Elastic Band (NEB) method. The results will be assessed against experimental data.   

Mg-related EPR signal in high hole density GaN

Although GaN devices have successfully entered technology, continued development of nitride electronics is hampered by the limitations of p-type doping. For this reason, we have employed electron paramagnetic resonance (EPR) spectroscopy to study GaN:Mg grown with high Mg (1-4x10$^{20}$ cm$^{-3})$ and hole densities 1-40x10$^{18}$ cm$^{-3}$. EPR measurements are made in the dark and under illumination at 4 K. Consistent with measurements made on less heavily doped films, the Mg-related EPR signal exhibits a sample-dependent anisotropy which depends on the hole density. Unlike lower doped samples, however, the increased EPR signal intensity created by the high hole density reveals photo-induced changes which suggest direct defect-to-band transitions. Detailed stepped wavelength photo-EPR results indicate that the Mg-related defect may be ionized with photon energy below 1.2 eV, likely related to capture of an electron from the valence band. A second ionization near 2.3 eV remains to be understood.   

Gate Bias Dependent Two Layer Conduction in InAlN/AlN/GaN Heterostructures

We studied InAlN/AlN/GaN heterostructures with In compositions near 17{\%} grown by Organo-Metallic Vapor Phase Epitaxy. These structures are free of strain and provide the confinement needed for a relatively high density and high mobility two-dimensional electron gas. The cross-sectional TEM images indicate the presence of an inadvertent GaN layer formed between the InAlN and AlN layers during the growth. Using Shubnikov-de Haas and Hall measurements performed on gated Hall bar samples at 4.2 K, we find that this additional GaN layer acts as a parasitic conduction channel. The quantitative mobility spectrum analysis of our data indicates that this parasitic channel has a very low mobility, and can be depleted by the application of a negative gate voltage.   

Structure and defects at the SiC:SiO$_{2}$ interface

The pervasion of defects at the SiC:SiO$_{2}$ interface has limited the performance and commercializing of SiC based transistors. While the defects are believed to be related to excess carbon in the interfacial region, no compelling microscopic models exists. Here we report the generation of microscopic interfaces models for the SiC:SiO$_{2}$ interface. These models include a 1 nm amorphous oxide and several layers of crystalline SiC. Defect and defect reactions are explored. For instance, the 3-fold bonded carbon defect is calculated to have an acceptor level at Ev + 1.4 eV close to the value found experimentally and encouraging confidence in the methods employed. The recently discovered beneficial effect of sodium ions motivates our examination of the basic electrochemistry of the sodium ion interactions with the ideal and defected interfaces. A comparison between microscopic defect results and experiment will be presented.   

Carbon-Rich Silicon Carbide

The application of SiC in electronic devices is currently hindered by low carrier mobility at the the SiO$_2$/SiC interfaces. Recently, it was found that thermally grown SiO$_2$/4H-SiC interface can have a transition layer on the SiC side with C/Si ratio as high as 1.2\footnote{T. Zheleva, et. al. APL 93, 022108 (2008)} and the channel mobility of the fabricated SiC MOSFETs decreases as the thickness of the transition layer increases.\footnote{T. L. Biggerstaff, et. al. APL 95, 032108 (2009)} However, the atomic structure of the transition layer is unknown. Here we present the results of first-principles calculations for silicon carbide with 20$\%$ excess carbon. Both static density functional theory calculations and quantum molecular dynamics simulations were performed. We reveal the structure that forms when a large amount of excess carbon is incorporated into the lattice. In addition we explore pairing and cluster formation. The overall results will be assessed against experimental data. This work is supported in part by NSF GOALI grant DMR-0907385.   

Carbon defects at the SiC-SiO$_{2}$ interface and the effects of hydrogen and fluorine

The channel mobilities in SiC-based metal-oxide-semiconductor field-effect transistors are significantly reduced by the interface defects that produce states in the band gap of the SiC-SiO$_{2}$ interface. Therefore, it is of great importance to investigate the nature of the interface defects and the ways for passivating such defects. We used first-principles quantum-mechanical calculations to study the interface defects due to excessive carbon atoms. We report the results about the atomic configurations of the defects and the associated electronic structures, as well as the effects of hydrogen and fluorine in passivating such interface defects.   

Strong hot electron reflection from subsurface 8H-SiC inclusion in 4H-SiC: Ballistic Electron Emission Microscopy (BEEM) study

The electrical properties of planar 8H stacking-fault inclusions (SFIs) formed during epilayer growth on an 8 degree miscut n-type 4H-SiC substrate were characterized using nm-resolution BEEM. A $\sim $0.39 eV lower Schottky barrier was measured along the line where the SFI intersect a Pt metal overlayer, confirming that 8H SFIs are electron quantum wells (QWs). Interestingly, an asymmetry of the BEEM current amplitude was observed around the intersection line, which is believed to be caused by strong hot electron reflection from the subsurface 8H inclusion. We will discuss our modeling to explain this strong hot electron reflection and the lower bound of hot electron attenuation length in 4H-SiC estimated from the measured BEEM current profile. Work supported by ONR.   

First principles study of p-type conductivity in wide band gap Cu$_3$TaQ$_4$ (Q=S,Se,Te) semiconductors

A recent study of Cu$_3$TaQ$_4$ (Q=S or Se) has shown that these materials exhibit several favorable optoelectronic properties including large optical band gap, tunable visible photoemission, and p-type conductivity.\footnote{P.F. Newhouse et al, Thin Solid Films {\bf 517}(2009) 2473} Cu$_3$TaQ$_4$ is unique among wide band gap p-type semiconductors in that it crystallizes in a cubic structure and is expected to show isotropic optical and electronic properties. The origin of p-type conductivity in these materials has been investigated using density functional theory in the GGA approximation. The structure and energetics of point defects have been determined using a supercell approach. We find Cu vacancies to be the most likely origin of free hole carriers. Compensation by donor like defects such as chalcogen vacancies is estimated to be negligible because of high formation enthalpies. Our study suggests that low overall defect concentrations are achievable in Cu$_3$TaQ$_4$, raising the potential that these materials could be used for p-type channel transparent transistors.   

Origin of p-type conductivity in wide band gap BaCuQF (Q=S,Se,Te) semiconductors

Bariumcopperchalcogenideflouride, BaCuQF (Q=S,Se,Te), is a family of p-type wide band gap semiconductors with excellent transparency over the whole visible part of the spectrum. Optical properties can be continuously tuned in thin film solid solutions of BaCuQF. Hole carrier concentration and mobility in BaCuQF decreases from Q = Te$\rightarrow$Se$\rightarrow$S. Understanding and control of the p-type conductivity in these materials is crucial for potential applications in photovoltaics and transparent electronics. We have investigated the origin of p-type conductivity using ab initio density functional theory in the GGA approximation. The structure and energetics of point defects have been determined using a supercell approach. Cu vacancies are the most likely origin of free hole carriers. Donor like defects such as chalcogen vacancies lead to strong compensation of charge carriers in BaCuSF and BaCuSeF but not BaCuTeF. Hole concentrations obtained from a self-consistent thermodynamic model reproduce the experimental trends. The potential for charge carrier control through intentional doping will be discussed.