Development of III-nitride bipolar devices: avalanche photodiodes, laser diodes, and double-heterojunction bipolar transistors

This dissertation describes the development of III-nitride (III-N) bipolar devices for optoelectronic and electronic applications. Research mainly involves device design, fabrication process development, and device characterization for Geiger-mode gallium nitride (GaN) deep-UV (DUV) p-i-n avalanche photodiodes (APDs), indium gallium nitride (InGaN)/GaN-based violet/blue laser diodes (LDs), and GaN/InGaN-based npn radio-frequency (RF) double-heterojunction bipolar transistors (DHBTs). All the epitaxial materials of these devices were grown in the Advanced Materials and Devices Group (AMDG) led by Prof. Russell D. Dupuis at the Georgia Institute of Technology using the metalorganic chemical vapor deposition (MOCVD) technique. Geiger-mode GaN p-i-n APDs have important applications in DUV and UV single-photon detections. In the fabrication of GaN p-i-n APDs, the major technical challenge is the sidewall leakage current. To address this issue, two surface leakage reduction schemes have been developed: a wet-etching surface treatment technique to recover the dry-etching-induced surface damage, and a ledged structure to form a surface depletion layer to partially passivate the sidewall. The first Geiger-mode DUV GaN p-i-n APD on a free-standing (FS) c-plane GaN substrate has been demonstrated. InGaN/GaN-based violet/blue/green LDs are the coherent light sources for high-density optical storage systems and the next-generation full-color LD display systems. The design of InGaN/GaN LDs has several challenges, such as the quantum-confined stark effect (QCSE), the efficiency droop issue, and the optical confinement design optimization. In this dissertation, a step-graded electron-blocking layer (EBL) is studied to address the efficiency droop issue. Enhanced internal quantum efficiency (ɳi) has been observed on 420-nm InGaN/GaN-based LDs. Moreover, an InGaN waveguide design is implemented, and the continuous-wave (CW)-mode operation on 460-nm InGaN/GaN-based LDs is achieved at room temperature (RT). III-N HBTs are promising devices for the next-generation RF and power electronics because of their advantages of high breakdown voltages, high power handling capability, and high-temperature and harsh-environment operation stability. One of the major technical challenges to fabricate high-performance RF III-N HBTs is to suppress the base surface recombination current on the extrinsic base region. The wet-etching surface treatment has also been employed to lower the surface recombination current. As a result, a record small-signal current gain (hfe) > 100 is achieved on GaN/InGaN-based npn DHBTs on sapphire substrates. A cut-off frequency (fT) > 5.3 GHz and a maximum oscillation frequency (fmax) > 1.3 GHz are also demonstrated for the first time. Furthermore, A FS c-plane GaN substrate with low epitaxial defect density and good thermal dissipation ability is used for reduced base bulk recombination current. The hfe > 115, collector current density (JC) > 141 kA/cm², and power density > 3.05 MW/cm² are achieved at RT, which are all the highest values reported ever on III-N HBTs.PhDCommittee Chair: Shen, Shyh-Chiang; Committee Member: Dupuis, Russell; Committee Member: Jiang, Zhigang; Committee Member: Mukhopadhyay, Saibal; Committee Member: Yoder, Dougla

Growth Optimization of III-N Electronic Devices by Plasma-Assisted Molecular Beam Epitaxy

InAlN has received significant attention due to its great potential for electronic and optoelectronic applications. In particular, $In_{0.18}Al_{0.82}N$ presents the advantage of being lattice-matched to GaN and simultaneously exhibiting a high spontaneous polarization charge, making $In_{0.18}Al_{0.82}N$ attractive for use as the barrier layer in high-electron-mobility transistors (HEMTs). However, in the case of InAlN growth by plasma-assisted molecular beam epitaxy (PAMBE), a strong non-uniformity in the in-plane In distribution was observed for both N-face and metal-face $In_{0.18}Al_{0.82}N$. This compositional inhomogeneity manifests itself as a columnar microstructure with AlN-rich cores (5-10 nm in width) and InN-rich intercolumn boundaries. Because of the large differences between the bandgaps and polarization of InN and AlN, this non-uniformity in InAlN composition could be a source of scattering, leading to mobility degradation in HEMTs. In this work, the growth conditions for high quality lattice-matched InAlN layers on free-standing GaN substrates were explored by plasma-assisted molecular beam epitaxy (PAMBE) in the N-rich regime. The microstructure of N-face InAlN layers, lattice-matched to GaN, was investigated by scanning transmission electron microscopy and atom probe tomography. Microstructural analysis showed an absence of the lateral composition modulation that was previously observed in InAlN films grown by PAMBE. Using same growth conditions for InAlN layer, N-face GaN/AlN/GaN/InAlN high-electron-mobility transistors with lattice-matched InAlN back barriers were grown directly on SiC. A room temperature two-dimensional electron gas (2DEG) mobility of $1100\,cm^2V^{-1}s^{-1}$ and 2DEG sheet charge density of $1.9\times10^{13}\,cm^2$ was measured on these devices. However, the threading dislocation density (TDD) of GaN grown directly on SiC by PAMBE ($\approx2\times10^{10}\,cm^{-2}$) is two orders of magnitude higher than GaN grown by MOCVD on SiC or sapphire ($\approx5\times10^8\,cm^{-2}$). This high TDD can severely degrade the 2DEG mobility, especially at lower 2DEG sheet densities. Relatively low TDD ($\approx5\times10^8\,cm^{-2}$) on MOCVD-grown GaN substrates motivated us to study the growth of N-face GaN-based HEMT structures with InAlN backbarriers on such substrates. Since on-axis GaN-on-sapphire substrates with low threading dislocation density are not available in the N-face orientation, we explored the growth of InAlN on vicinal ($4^{\circ}$ miscut along GaN $10\bar{1}0$) GaN-on-sapphire substrates. The microstructure of $In_{0.18}Al_{0.82}N$ layers grown by PAMBE at different temperatures was studied using scanning transmission electron microscopy (STEM). The cross-sectional and plan-view STEM images revealed lateral variations in the InAlN composition along $10\bar{1}0$ (perpendicular to the step edges), in addition to step bunching in InAlN layers thicker than 10 nm. N-face HEMTs with lattice-matched InAlN backbarriers were then grown on these vicinal substrates with different InAlN thicknesses. Transmission line measurements showed that step bunching and lateral variation of InAlN composition degraded the 2DEG mobility in the directions parallel and perpendicular to the steps. A 2DEG charge density of $1.1\times10^{13}\,cm^{-2}$ and mobility of $1850\,cm^2V^{-1}s^{-1}$ were achieved on a GaN/AlN/InAlN/GaN structure with 7.5 nm thick $In_{0.18}Al_{0.82}N$. By designing a double backbarrier ($In_{0.18}Al_{0.82}N$(7.5 nm)/$Al_{0.57}Ga_{0.43}N$(7 nm)), a 2DEG charge density of $2\times10^{13}\,cm^{-2}$ and mobility of $1360\,cm^2V^{-1}s^{-1}$ were attained, which resulted in a sheet resistance of $230\,\Omega/\square$.Two good measures of the device quality concerning the power loss in power switch and high frequency switch applications are Huang material figure of merit , and Baliga high-frequency figure of merit, respectively, which shows that for any fixed material system, power loss reduces by increasing the mobility of the 2DEG. Therefore, it is very important to understand the source of scattering mechanisms which affect the 2DEG mobility. In this work, we studied effect of decreasing channel thickness or increasing gate reverse bias on charge density and 2DEG mobility in N-face HEMT structure. Our calculations showed that increasing the gate reverse bias and decreasing the channel thickness both reduce the 2DEG mobility. This trend has been observed by experiment as well. Previously, it was believed that increasing the gate reverse bias or decreasing the channel thickness in N-face GaN-based HEMT structures lead to deeper penetration of the 2DEG wavefunction into the barrier, and consequently, higher interface roughness and alloy scattering rates. Although this statement is true, our calculations revealed that the penetration of the 2DEG into the barrier and, therefore, 2DEG mobility limited by alloy and interface roughness scattering mechanisms do not vary significantly by increasing gate reverse bias or decreasing the channel thickness. therefore, these two scattering mechanisms are not enough to explain the significant drop in the 2DEG mobility observed in experiments. We believe that the charged trap states at the AlGaN-GaN interface, where the 2DEG forms, are responsible for this 2DEG mobility reduction

Design and characterisation of millimetre wave planar Gunn diodes and integrated circuits

Heterojunction planar Gunn devices were first demonstrated by Khalid et al in 2007. This new design of Gunn device, or transferred electron device, was based on the well-established material system of GaAs as the oscillation media. The design did not only breakthrough the frequency record of GaAs for conventional Gunn devices, but also has several advantages over conventional Gunn devices, such as the possibility of making multiple oscillators on a single chip and compatibility with monolithic integrated circuits. However, these devices faced the challenge of producing high enough RF power for practical applications and circuit technology for integration. This thesis describes systematic work on the design and characterisations of planar Gunn diodes and the associated millimetre-wave circuits for RF signal power enhancement. Focus has been put on improving the design of planar Gunn diodes and developing high performance integrated millimetre-wave circuits for combining multiple Gunn diodes. Improvement of device design has been proved to be one of the key methods to increase the signal power. By introducing additional δ-doping layers, electron concentration in the channel increases and better Gunn domain formation is achieved, therefore higher RF power and frequency are produced. Combining multiple channels in the vertical direction within devices is another effective way to increase the output signal power as well as DC-to-RF conversion efficiency. In addition, an alternative material system, i.e. In0.23Ga0.77As, has also been studied for this purpose. Planar passive components, such as resonators, couplers, low pass filters (LPFs), and power combiners with high performance over 100 GHz have been developed. These components can be smoothly integrated with planar Gunn diodes for compact planar Gunn oscillators, and therefore contribute to RF power enhancement. In addition, several new measurement techniques for characterising oscillators and passive devices have also been developed during this work and will be included in this thesis

Advanced 3-V semiconductor technology assessment

Components required for extensions of currently planned space communications systems are discussed for large antennas, crosslink systems, single sideband systems, Aerostat systems, and digital signal processing. Systems using advanced modulation concepts and new concepts in communications satellites are included. The current status and trends in materials technology are examined with emphasis on bulk growth of semi-insulating GaAs and InP, epitaxial growth, and ion implantation. Microwave solid state discrete active devices, multigigabit rate GaAs digital integrated circuits, microwave integrated circuits, and the exploratory development of GaInAs devices, heterojunction devices, and quasi-ballistic devices is considered. Competing technologies such as RF power generation, filter structures, and microwave circuit fabrication are discussed. The fundamental limits of semiconductor devices and problems in implementation are explored

Compound Semiconductor-Based Thin-Film and Flexible Optoelectronics.

Compound semiconductors are the basis of modern optoelectronics due to their intrinsically superior optical and electronic properties compared with elemental semiconductors. However, their applications remain limited due to a prohibitive substrate cost. This limitation has driven the development of epitaxial lift-off (ELO) technology that separates the thin-film epitaxial layer from the substrate by selectively removing a sacrificial layer between them. However, ELO has its own limitations including a long process time, complicated transfer to a secondary, low cost host substrate, and wafer surface degradation which prevents wafer recycling. In this thesis, we address all of these limitations by developing a new, non-destructive ELO (ND-ELO) process. When combined with adhesive-free cold-weld bonding of the wafer directly to a plastic substrate, ND-ELO provides an approximately 100 times reduction in process time, and a considerably simplified transfer process compared with conventional ELO. Furthermore, it allows indefinite wafer reuse by employing the epitaxial protection layers, eliminating surface degradation of the parent wafer encountered in conventional ELO. We demonstrate the feasibility and generality of this process by applying it to optoelectronic devices including photovoltaic cells, LEDs, MESFETs and photodetectors on two compound semiconductor systems, InP and GaAs. Furthermore, we present an approach that can achieve an estimated cost of only 3% that of conventional GaAs solar cells using an accelerated ND-ELO wafer recycling process, and integrated with lightweight, thermoformed plastic, truncated mini-compound parabolic concentrators (CPC) that avoid the need for active solar tracking. Using solar cell/CPC assemblies, without daily solar tracking, the annual energy harvesting is increased by 2.8 times compared with planar solar cells. This represents a drastic cost reduction in both the module and balance of systems costs compared with heavy, rigid conventional modules and trackers that are subject to wind loading damage and high installation costs. The demonstration of cost-efficient and high performance compound semiconductor-based flexible thin-film optoelectronics is a critical step toward allowing their widespread deployment in mainstream state-of-the-art applications including wearable, flexible and conformal devices.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111479/1/kyusang_1.pd

Characterization of Charge Trapping Phenomena in GaN-based HEMTs

This dissertation reports on charge-trapping phenomena and related parasitic effects in AlGaN/GaN high electron mobility transistors. By means of static and pulsed I-V measurements and deep-level transient spectroscopy, the main charge-trapping mechanisms affecting the dynamic performance of GaN-based HEMTs devoted to microwave and power switching applications have been comprehensively characterized, identifying the nature and the localization of the deep-levels responsible for the electrically active trap-states. A high-voltage measurement system capable for double-pulsed ID-VD, ID-VG and drain-current transient spectroscopy has been successfully designed and implemented. A characterization methodology, including the analysis of static I-V measurements, pulsed I-V measurements, and deep-level transient spectroscopy, has been developed to investigate the impact of voltage, current, and temperature on the parasitic effects of charge-trapping (threshold voltage instabilities, dynamic on-resistance increase, and transconductance reduction), and on trapping/detrapping kinetics. Experimental results gathered on transistor structures are supported by complementary capacitance deep-level transient spectroscopy (C-DLTS) performed on 2-terminal diode (FATFET) structures. Two main case-studies have been investigated. Schottky-gated AlGaN/GaN HEMTs grown on silicon carbide substrate employing iron and/or carbon doped buffers devoted to microwave applications, and MIS-gated double-heterostructure AlGaN/GaN/AlGaN HEMTs grown on silicon substrate devoted to power switching applications. The devices under test have been exposed to the complete set of current-voltage regimes experienced during the real life operations, including off-state, semi-on-state, and on-state. The main novel results are reported in the following: • Identification of a charge-trapping mechanism promoted by hot-electrons. This mechanism is critical in semi-on-state, with the combination of relatively high electric-field and relatively high drain-source current. • Identification of a positive temperature-dependent charge-trapping mechanism localized in the buffer-layer, potentially promoted by the vertical drain to substrate potential. This mechanism is critical in high drain-voltage off-state bias in high temperature operations. • Identification of deep-levels and charge-trapping related to the presence of doping compensation agents (iron and carbon) within the GaN buffer layer. • Identification of charge-trapping mechanism ascribed to the SiNX and/or Al2O3 insulating layers of MIS-gated HEMTs. This mechanism is promoted in the on-state with positive gate-voltage and positive gate leakage current. • Identification of a potential charge-trapping mechanism ascribed to reverse gate leakage current in Schottky-gate HEMTs exposed to high-voltage off-state. • The characterization of surface-traps in ungated and unpassivated devices by means of drain-current transient spectroscopy reveals a non-exponential and weakly thermally-activated detrapping behaviour. • Preliminary synthesis of a degradation mechanism characterized by the generation of defect-states, the worsening of parasitic charge-trapping effects, and the degradation of rf performance of AlGaN/GaN HEMTs devoted to microwave operations. The evidence of this degradation mechanism is appreciable only by means of rf or pulsed I-V measurements: no apparent degradation is found by means of DC analysis

Journal of Telecommunications and Information Technology, 2004, nr 1

kwartalni

A novel AlGaN/GaN based enhancement-mode high electron mobility transistor with sub-critical barrier thickness

Power-switching devices require low on-state conduction losses, high-switching speed, high thermal stability, and high input impedance. Using gallium nitride (GaN) based field-effect transistors, these properties for switching devices can be satisfied. GaN-based High Electron Mobility Transistors (HEMTs) are emerging as promising candidates for high-temperature, high-power (power electronics) and radio-frequency (RF) electronics due to their unique capabilities of achieving higher current density, higher breakdown voltage, higher operating temperatures and higher cut-off frequencies compared to silicon (Si). Conventional GaN HEMTs with an aluminium gallium nitride (AlGaN) barrier are of depletion-mode (d-mode) or normally-on which require a negative polarity power supply to turn off. On the other hand, enhancement-mode (e-mode) or normally-off AlGaN/GaN HEMTs are attracting increasing interest in recent years because no negative gate voltage is necessary to turn off the devices. This leads to the advantage of simple circuit design and low stand-by power dissipation. For power electronics applications, power switches which incorporate e-mode devices provide the highly desirable essential fail-safe operation. In this research, a new high performance normally-off GaN-based metal-oxide-semiconductor (MOS) high electron mobility transistor (HEMT) that employs an ultrathin sub-critical 3nm Al_0.25Ga_0.75N barrier layer and relies on an induced two dimensional electron gas (2DEG) for operation was designed, fabricated and characterized. The device consists of source and drain Ohmic contacts nominally overlapped by the gate contact and employs a gate dielectric. With no or low gate-to-source voltage (V_GS), there is no two dimensional electron gas (2DEG) channel at the AlGaN/GaN interface to allow conduction of current between the drain and source contacts as the AlGaN barrier thickness is below the critical thickness required for the formation of such channel. However, if a large enough positive bias voltage V_GS is applied, it causes the formation of a quantum well at the AlGaN/GaN interface into which electrons from the source and drain Ohmic regions are attracted (by the positive gate voltage), effectively creating a 2DEG channel, and so the structure is a normally-off field effect transistor. Normally-off GaN MOS-HEMT devices were fabricated using plasma enhanced chemical vapour-deposited (PECVD) silicon dioxide (SiO_2) as the gate dielectric. They demonstrated positive threshold voltages (V_th) in the range of +1V to +3 V, and very high maximum drain currents (I_DSmax) in the range of 450mA/mm to 650mA/mm, at high gate voltage (V_GS) of around 6 V. The devices also exhibited breakdown voltages in the range of 9V and 17V depending on the gate dielectric thickness, making them suitable for realising high current low voltage power devices required, for instance, for buck converters for mobile phones, tablets, laptop chargers, etc

Electronic Nanodevices

The start of high-volume production of field-effect transistors with a feature size below 100 nm at the end of the 20th century signaled the transition from microelectronics to nanoelectronics. Since then, downscaling in the semiconductor industry has continued until the recent development of sub-10 nm technologies. The new phenomena and issues as well as the technological challenges of the fabrication and manipulation at the nanoscale have spurred an intense theoretical and experimental research activity. New device structures, operating principles, materials, and measurement techniques have emerged, and new approaches to electronic transport and device modeling have become necessary. Examples are the introduction of vertical MOSFETs in addition to the planar ones to enable the multi-gate approach as well as the development of new tunneling, high-electron mobility, and single-electron devices. The search for new materials such as nanowires, nanotubes, and 2D materials for the transistor channel, dielectrics, and interconnects has been part of the process. New electronic devices, often consisting of nanoscale heterojunctions, have been developed for light emission, transmission, and detection in optoelectronic and photonic systems, as well for new chemical, biological, and environmental sensors. This Special Issue focuses on the design, fabrication, modeling, and demonstration of nanodevices for electronic, optoelectronic, and sensing applications