Deep traps in AlGaN/GaN heterostructures studied by deep level transient spectroscopy: Effect of carbon concentration in GaN buffer layers

Fang, Z-Q.; Claflin, B.; Look, David C.; Green, D. S.; Vetury, R.

doi:10.1063/1.3488610

Cited by 101 publications

(74 citation statements)

References 17 publications

(31 reference statements)

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“…Among these, photoionization spectroscopy (PS) 8,17 , deep level transient spectroscopy (DLTS), including minority carrier transient spectroscopy (MCTS) and photoinduced current transient spectroscopy (PICTS) [18][19][20][21][22][23][24] , deep level optical spectroscopy (DLOS) [18][19][20]25 and cathodoluminescence (CL) measurement 16,24 . DLTS, MCTS and PICTS have been primarily employed to detect trap levels close to the band edges (within ∼ 1.0 eV).…”

Section: -5mentioning

confidence: 99%

A first-principles study of carbon-related energy levels in GaN. I. Complexes formed by substitutional/interstitial carbons and gallium/nitrogen vacancies

Matsubara

Bellotti

2017

Journal of Applied Physics

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Various forms of carbon based complexes in GaN are studied with first-principles calculations employing Heyd-Scuseria-Ernzerhof hybrid functional within the framework of density functional theory. We consider carbon complexes made of the combinations of single impurities, i.e. CN − CGa, CI − CN and CI − CGa, where CN, CGa and CI denote C substituting nitrogen, C substituting gallium and interstitial C, respectively, and of neighboring gallium/nitrogen vacancies (VGa/VN), i.e. CN − VGa and CGa − VN. Formation energies are computed for all these configurations with different charge states after full geometry optimizations. From our calculated formation energies, thermodynamic transition levels are evaluated, which are related to the thermal activation energies observed in experimental techniques such as deep level transient spectroscopy. Furthermore, the lattice relaxation energies (Franck-Condon shift) are computed to obtain optical activation energies, which are observed in experimental techniques such as deep level optical spectroscopy. We compare our calculated values of activation energies with the energies of experimentally observed C-related trap levels and identify the physical origins of these traps, which were unknown before.

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Section: -5mentioning

confidence: 99%

A first-principles study of carbon-related energy levels in GaN. I. Complexes formed by substitutional/interstitial carbons and gallium/nitrogen vacancies

Matsubara

Bellotti

2017

Journal of Applied Physics

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“…9,[15][16][17] In this paper, we suggest both phenomena (i.e. temperature activated capture with no temperature activated current and the appearance of hole traps signals) can be attributed to multistage process involving deep traps in the AlGaN barrier emitting and subsequently capturing electrons, forming a step in the space charge moving toward the AlGaN/GaN interface.…”

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confidence: 99%

“…[12][13][14] In many cases, the current and capacitance relaxations in AlGaN/GaN HEMTs show not only electron-trap-like transitions natural for a majority carrier device such as a HEMT, but also hole-trap-like signals. 9,[15][16][17] In this paper, we suggest both phenomena (i.e. temperature activated capture with no temperature activated current and the appearance of hole traps signals) can be attributed to multistage process involving deep traps in the AlGaN barrier emitting and subsequently capturing electrons, forming a step in the space charge moving toward the AlGaN/GaN interface.…”

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confidence: 99%

Gate-Lag in AlGaN/GaN High Electron Mobility Transistors: A Model of Charge Capture

Polyakov

Smirnov

Shchemerov

et al. 2017

ECS J. Solid State Sci. Technol.

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Gate and drain current transients following the application of a step/pulse reverse bias to the gate of AlGaN/GaN HEMTs can be described by a superposition of 2-3 extended exponents with characteristic times of the main process having activation energy of 0.7-0.8 eV. The drain current relaxation is a consequence of charge trapping and movement in the AlGaN barrier. A qualitative model explains this process by movement of a step of charge trapped on deep states in the barrier and movement of the step toward the interface by multiple emission/capture hops. The process is assumed to be confined to channels in AlGaN (presumably dislocations) surrounded by potential barriers, preventing effective injection of the carriers into the channels with forward bias pulses. The model qualitatively explains the often reported appearance of hole-trap-like signals in deep level studies of AlGaN/GaN HEMT structures and the persistent changes in capacitance and current of these structures occurring upon pulsing and illumination. One of the most serious problems with III-Nitrides-based high electron mobility transistors (HEMTs) is the phase delay of the drain current upon pulsed change of the gate bias, so called "gate-lag". [1][2][3] This phenomenon causes increase of the switching time and long-term drift of the threshold voltage and the drain current. The generally accepted view is the gate-lag is caused by trapping of electrons flowing from the gate at high reverse voltage and subsequent capture of electrons by deep centers under the gate. Both the capture and emission process from these traps can be studied by analysis of gate and drain transients as a function of pulsing conditions and temperature by using current or capacitance deep level transient spectroscopy (CDLTS or DLTS) 2-5 or by direct measurements of individual transients.1,6-9These measurements provide the concentration and position of the traps involved and their capture cross sections. This is complicated by the presence of multiple traps, strong non-uniform electrical and strain fields, and the contribution of the band-like states related to extended defects and to surface or interface states. As a result, the drain and gate current relaxation curves are non-exponential. A common approach is the deconvolution of the actual relaxations into a sum of independent exponential relaxations and fitting the pre-exponential coefficients to experiment.6 Various methods to improve the convergence and reproducibility of the procedure exist. 7,8 Another popular approach is to build the derivative of the relaxation curve on the logarithm of time, find the peaks in such plots, and trace the peak position variation with temperature to extract the activation energy and capture cross section of the traps.2,3,9 The number of peaks in either approach is quite limited (typically, 2-3) in either method, 9 but the peaks are generally broad which requires invoking a large number of exponents to attain good fit in the basic method, 6 while providing no knowledge on the physical nature o...

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“…Incorporation of defect characterization techniques capable of S3098 ECS Journal of Solid State Science and Technology, 6 (11) S3093-S3098 (2017) extracting trap parameters related to all the defect levels encountered in the GaN band-gap will be necessary. [24][25][26][27] An accurate modeling of current relaxation curves, and consequently current collapse behavior thus entails comprehensive integration of all the tools in addition to TCAD simulation capability.…”

Section: Discussionmentioning

confidence: 99%

Simulation of Deep-Level Trap Distributions in AlGaN/GaN HEMTs and Its Influence on Transient Analysis of Drain Current

Mukherjee

Patrick

Law

2017

ECS J. Solid State Sci. Technol.

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AlGaN/GaN high electron mobility transistors for high efficiency power switching applications are susceptible to charge trapping by deep-levels in the GaN buffer resulting in current collapse during gate and drain voltage swings. Although drain-current transient based methodologies have been used consistently to extract trap parameters, the factors affecting the non-exponential nature of the transient are still not well understood. No effort at accurately replicating multiple deep-levels in the GaN buffer to simulate transient response of GaN based power devices has been reported previously. In this work, we present numerical simulation results of HEMTs having multiple discrete trap states as well as band-like distribution of traps in the GaN band-gap. GaN based High Electron Mobility Transistors (HEMT)s, which leverage wide-band-gap related properties, are established as ideal candidates for high-power, high-efficiency and high-frequency applications ranging from RF communication to power conversion. 1,2However, the most unremitting technological challenge limiting the wide-spread adoption of GaN process technology is understanding and predicting their reliability and degradation modes therein. Many concur that most of the reliability-limiting mechanisms and issues are related to the presence of defect states in the forbidden band-gap of GaN, and the subsequent performance degradation associated with such defects. [3][4][5][6][7][8] Drain-current transient analysis experiments have helped shed light on the degradation observed in switching performance, often manifesting in AlGaN/GaN HEMTs as gate lag, drain lag and current collapse.9,10 The collapse is attributed to electron trapping comprised of lateral and vertical components. The lateral component due to tunneling of electrons from the gate to surface states at the AlGaN/insulator interface is facilitated by large bias difference between the gate and drain (under off-state and high drain bias).11-13 However, this phenomenon has been observed to be suppressed by improved passivation techniques.13 Under similar bias conditions, a vertical trapping component, which deteriorates dynamic performance, dominates in devices with improves passivation. The mechanism is expected to involve injection of 2DEG electrons deeper into the buffer and consequent trapping for several buffer designs. [8][9][10][11][12][13] If the traps directly underneath the gate dominate, a shift in dynamic threshold is observed. However, if the traps in the drain access region dominate, they end up depleting the 2DEG in this region thereby increasing the dynamic on-resistance.Much of experimental work identifying GaN buffer traps responsible for deterioration of dynamic performance utilize thermal dependence of the relaxation time constants to extract trap activation energy.11,14 Current relaxation time constants are obtained by taking the derivative of such transients and typically indicate the presence of multiple trapping centers. The extracted derivatives assume an asymmetric shape, with ...

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Deep traps in AlGaN/GaN heterostructures studied by deep level transient spectroscopy: Effect of carbon concentration in GaN buffer layers

Cited by 101 publications

References 17 publications

A first-principles study of carbon-related energy levels in GaN. I. Complexes formed by substitutional/interstitial carbons and gallium/nitrogen vacancies

A first-principles study of carbon-related energy levels in GaN. I. Complexes formed by substitutional/interstitial carbons and gallium/nitrogen vacancies

Gate-Lag in AlGaN/GaN High Electron Mobility Transistors: A Model of Charge Capture

Simulation of Deep-Level Trap Distributions in AlGaN/GaN HEMTs and Its Influence on Transient Analysis of Drain Current

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