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X-02: Nitrides II
Monday, 19/Jun/2017:
4:00pm - 6:15pm

Session Chair: Frank Bertram, Otto von Guericke University
Session Chair: Arnab Bhattacharya, TIFR-Mumbai
Location: Rm 324

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4:00pm - 4:30pm

Alternative Doping of GaN by Germanium or Carbon


Otto-von-Guericke University, Germany

Doping of GaN during metalorganic vapor phase epitaxy still possesses limits arising from gas phase reactions of precursors and/or growth at non-optimum growth conditions. I will present two major advances using germane for n-type doping and propane for compensation of background donors.

Germanium doping mitigates SiNx-formation during MOVPE growth with high Si concentrations in the gas phase which leads to severe surface roughening. Usable Si doping concentrations for n-type doping are limited to the low 1019 cm-3 range. The lateral current spreading resistance is therefore an issue in vertical pn-junction device structures such as LEDs and VCSELs to be considered for improving device efficiency. In contrast, germanium at concentrations beyond 2x1020 cm-3 can be incorporated into GaN while smooth layer morphologies are still maintained. Maximum free electron concentrations of 2.7 1020 cm-3 have been measured and the associated Burstein-Moss effect alters the optical properties of such heavily doped GaN:Ge layers. Recently, we have investigated the use of such layers for LED applications by realizing a disitrbuted Bragg reflector solely out of GaN and by forming GaN:Mg/GaN:Ge tunnel junction on top of a InGaN/GaN LED.

Carbon doping using propane also allows for high structural quality of semi-insulating GaN layers through its compatibility with the required high-temperature and high V/III-ratio growth conditions. Compensation of background electron concentrations goes linear with incorporated carbon concentration up to [C]=1.2x1019 cm-3 resulting in highly resistive layers. Unlike iron doping in MOVPE, this dopant sopurce shows negligible memory effects as well as unaltered crystalline properties. As a return, leakage reduction by several orders of magnitude in GaN buffer layers grown on Si substrates are obtained and the lateral and vertical breakdown field strength of such buffer layers is increased to 2.5 MV/cm. Carbon-doping and Ge-doping were recently used to realize vertical pn-diodes for high-power electronics on Si.

4:30pm - 5:00pm

Epitaxial Growth of GaN-based Heterostructures on Si Substrates Using a Large Lattice-mismatch Induced Stress Control Technology

Bo SHEN, Xuelin YANG, Maojun WANG, Jianpeng CHEN, Panfeng JI, Jie ZHANG, Yuxia FENG, Anqi HU

State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, China

Recently, GaN-on-Si substrates have attracted much attention for next generation high power electronic devices. Despite the promising applications, GaN-on-Si technology is facing reproducibility and reliability issues, which are likely to be related to growth processes and crystalline quality. Although several complicated stress-control approaches such as LT-AlN, AlN/GaN superlattice, and compositionally graded AlGaN layer have been proposed, the crystalline quality (defects and residual stress), as well as uniformity issues still remain, especially for growth onto large diameter substrates. Therefore, it is still highly desirable to develop a simple and cost effective GaN-on-Si technology while maintaining high material quality. In this study, a large lattice-mismatch induced stress control technology with a single low Al content AlGaN layer has been used to grow high quality GaN layers on Si substrates by means of MOCVD. Strain relaxation and dislocation evolution mechanisms have been investigated. It is demonstrated that the large lattice mismatch between the low Al content AlGaN layer and AlN buffer layer could effectively promote the edge dislocation inclination with relatively large bend angles and therefore significantly reduce the dislocation density in the GaN epilayer. By further balancing the compressive stress induced and consumed during the growth, and the thermal tensile stress induced during the cooling down process, the use of this technology allows for high uniformity AlGaN/GaN heterostructures with electron mobility of 2240 cm2/Vs. The sheet resistance is 313 ± 4 Ω/□ with ± 1.3% variation. Our results demonstrate a promising approach to simplifying the growth processes of GaN-on-Si to reduce the wafer bow and lower the cost while maintaining high material quality.

5:00pm - 5:15pm

Si Doped Al0.3Ga0.7N Grown on 4-inch Si (111) Substrate by Plasma Assisted Molecular Beam Epitaxy

Yi ZHENG1, Dharmarasu NETHAJI2, Manvi AGRAWAL2, Radhakrishnan K1,2

1Centre for Micro-/Nano-electronics (NOVITAS), School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore; 2Temasek Laboratories @NTU, Nanyang Technological University, Singapore

AlGaN alloys find wide applications in high density optical storage devices, ultraviolet emitters, light-emitting diodes, and photodetectors due to their superior properties such as wide and direct bandgap, high breakdown voltage and high thermal conductivity. Well controlled doping in AlGaN is essential for obtaining high performance devices for these applications. Moreover, development of such a technology on Si allows low cost and opens the prospect of integration of III-nitride with Si technology. In this work, Si doped AlGaN was grown on AlN/Si (111) template using Plasma-Assisted Molecular Beam Epitaxy (PA-MBE). The growth was optimized to obtain 300 nm thick crack free n+-Al0.3Ga0.7N epilayer with sub-nanometer RMS roughness. The Si doped Al0.3Ga0.7N layers exhibited free carrier concentration of 1×1019, 5×1019, 1×1020 and 2×1020 cm-3, for the Si cell temperatures 1190°C, 1250°C, 1280°C and 1310°C, respectively. The carrier concentration increases with increasing Si cell temperature. This is the highest free carrier concentration level achieved in Si doped Al0.3Ga0.7N layer on Si substrate by PA-MBE to the best of our knowledge. The activation energy of highest doped n+-Al0.3Ga0.7N layer is ~0.14meV. For the doping levels 1×1020 and 2×1020 cm-3, the XRD ω-2θ Al0.3Ga0.7N (004) peak shows split into peaks while no split was observed for the layers with lower Si doping levels. The splitting of the peak can be attributed to the stress in n+-Al0.3Ga0.7N layer induced by high Si doping. The lower Si doped layers were found to be 0.24% tensile strained. The two peaks with highly doped n+-Al0.3Ga0.7N layers corresponded to a residual compressive strain of 0.25% and a tensile strain of 0.24%.

5:15pm - 5:30pm

Open-gate AlGaN/GaN HEMT Based MSM Gas Sensor Grown on Si(111) by MOCVD

Akhil RANJAN1, Yi ZHENG1, Karthikeyan Giri SADASIVAM2, Dharmarasu NETHAJI2, Manvi AGRAWAL2, Radhakrishnan K.1,2

1Centre for Micro-/Nano-electronics (NOVITAS), School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore; 2Temasek Laboratories @NTU, Nanyang Technological University, Singapore

Metal-semiconductor-metal (MSM) structures offer various advantages such as simplicity of planar-fabrication and low capacitance leading to higher response speed. Superior properties of GaN such as high breakdown voltage, high electron mobility and good thermal stability make it suitable for sensing applications. AlGaN/GaN HEMTs gas-sensor have been demonstrated mostly on sapphire substrates. However, AlGaN/GaN HEMT grown on Si has unique advantages such as large wafer sizes, low cost and integration with existing Si-technology. To the best of our knowledge, AlGaN/GaN HEMT sensor with MSM has not been reported yet. In this study, CO2, NO2 and NH3 sensing have been successfully demonstrated with open-gate AlGaN/GaN HEMT grown on Si by MOCVD. The AlGaN/GaN HEMT epiwafer exhibited 2DEG properties with sheet carrier density of 1.3×1013 cm-2 and mobility of 1200 cm2/V.s. MSM devices were fabricated using interdigitated electrode masks consisting of two interdigitated comb-like back-to-back Ohmic contact electrodes separated by semiconductor. Thus, HEMTs connected in parallel lead to enhancement in sensitivity. Length, width and finger spacing were varied 100-400 µm, 2-10 µm and 2-40 µm, respectively, to study the sensitivity effect. Output DC-characteristics with and without presence of these gases were measured for wide range of temperature from 273 K to 673 K. The CO2 was not detected at 273K. However, resistance was found to increase when temperature was raised to 425 K. For a given temperature, the resistance change showed a direct relationship with gas concentration variation which confirms the CO2 detection by the device. Similar trend was observed for NO2 and NH3 confirming detection of these gases. Sensitivity increases with decreasing the finger spacing. These preliminary results demonstrated a maximum sensitivity of about 1.3%, which can be further enhanced using functionalization layers such as Platinum, Starch, SnO2 in open-gate region.

5:45pm - 6:00pm

Simulation and Electrical Characterization Of Si3N4 Passivated AlGaN/GaN High Electron Mobility Transistor on Si Substrate

Adarsh NIGAM, Mahesh KUMAR

Indian Institute of Technology Jodhpur, India

The III-Nitride semiconductor devices such as AlGaN/ GaN High Electron Mobility Transistor (HEMT) has wide band gap energy, large breakdown electric field and high thermal stability which make it promising material for high power, high frequency and high temperature applications. Since high cost and lack of availability of GaN substrates other foreign materials like Si, SiC, and Sapphire has been used for the AlGaN/GaN HEMT fabrication. A HEMT has been fabricated on Si substrate with Si3N4 as a passivation to overcome the effects generated by surface traps like gate leakage and virtual gate problem. This process of passivation enhances device performance. The electrical characteristics of the device have been measured by I-V measurement. The DC Simulation of the AlGaN/ GaN HEMT device has been performed by Sentaurus Technology Computer Aided Design (TCAD) simulation tool to analyze the behavior of the device. Various models have been used for simulation for the calculation of energy bandgap, mobility, high field saturation, piezoelectric polarization and surface traps between gate to drain and gate to source etc. The corresponding I-V characteristics from simulation are measured to analyze device threshold, transconductance, sheet carrier concentration etc. The sheet carrier concentration of the device 1.1x1013 cm-2, electron density at 2DEG is 6´ 1019cm-3 and mobility at 2DEG interface is 1500cm2/Vs obtained. The device results also compared with unpassivated device and significant improvement has been observed.

6:00pm - 6:15pm

Capacitance-voltage Spectroscopy of Al2O3/InAlN/GaN-on-Si HEMT

Sandeep KUMAR1, Nayana REMESH1, Surani Bin DOLMANAN2, Sudhiranjan TRIPATHY2, Srinivasan RAGHAVAN1, R. MURALIDHARAN1, Digbijoy N. NATH1

1Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore, India; 2Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*STAR), Singapore

We have investigated Al2O3/InAlN/GaN HEMT interface grown on 200 mm diameter Si using capacitance dispersion technique. The partial depletion of 2DEG from ~2x1013 cm-2 to ~5x1012 cm-2 was observed after gate deposition, forming gas anneal and electrical stress. Low-temperature growth necessity of InAlN gives rise to high trap density at InAlN/GaN interface and activation of these traps by electrical and/or thermal stress can deplete the 2DEG partially as evident from 1D-Schrodinger-Poisson simulation supported by experimental data. Conductance method was used for quantitative estimation of trap density (Dit) and trap time constant (Tit) at Al2O3/InAlN (accumulation) and InAlN/GaN (depletion) interfaces. Traps at Al2O3/InAlN and InAlN/GaN interface were found to be in the range of 0.4-7x1013 eV-1cm-2. While trap time constant for Al2O3/InAlN interface was around 2.2 μs, almost constant time constant at the dielectric-InAlN interface confirms the robust dielectric-semiconductor interface.

The work is funded by Ministry of Electronics and IT (MeitY) under its National Mission on Power Electronics Technology (NAMPET) program.

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